research paper about recycled materials

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• Philos Trans R Soc Lond B Biol Sci
• v.364(1526); 2009 Jul 27

Plastics recycling: challenges and opportunities

Jefferson hopewell.

1 Eco Products Agency, 166 Park Street, Fitzroy North 3068, Australia

Robert Dvorak

2 Nextek Ltd, Level 3, 1 Quality Court, Chancery Lane, London WC2A 1HR, UK

Edward Kosior

Plastics are inexpensive, lightweight and durable materials, which can readily be moulded into a variety of products that find use in a wide range of applications. As a consequence, the production of plastics has increased markedly over the last 60 years. However, current levels of their usage and disposal generate several environmental problems. Around 4 per cent of world oil and gas production, a non-renewable resource, is used as feedstock for plastics and a further 3–4% is expended to provide energy for their manufacture. A major portion of plastic produced each year is used to make disposable items of packaging or other short-lived products that are discarded within a year of manufacture. These two observations alone indicate that our current use of plastics is not sustainable. In addition, because of the durability of the polymers involved, substantial quantities of discarded end-of-life plastics are accumulating as debris in landfills and in natural habitats worldwide.

Recycling is one of the most important actions currently available to reduce these impacts and represents one of the most dynamic areas in the plastics industry today. Recycling provides opportunities to reduce oil usage, carbon dioxide emissions and the quantities of waste requiring disposal. Here, we briefly set recycling into context against other waste-reduction strategies, namely reduction in material use through downgauging or product reuse, the use of alternative biodegradable materials and energy recovery as fuel.

While plastics have been recycled since the 1970s, the quantities that are recycled vary geographically, according to plastic type and application. Recycling of packaging materials has seen rapid expansion over the last decades in a number of countries. Advances in technologies and systems for the collection, sorting and reprocessing of recyclable plastics are creating new opportunities for recycling, and with the combined actions of the public, industry and governments it may be possible to divert the majority of plastic waste from landfills to recycling over the next decades.

1. Introduction

The plastics industry has developed considerably since the invention of various routes for the production of polymers from petrochemical sources. Plastics have substantial benefits in terms of their low weight, durability and lower cost relative to many other material types ( Andrady & Neal 2009 ; Thompson et al. 2009 a ). Worldwide polymer production was estimated to be 260 million metric tonnes per annum in the year 2007 for all polymers including thermoplastics, thermoset plastics, adhesives and coatings, but not synthetic fibres ( PlasticsEurope 2008 b ). This indicates a historical growth rate of about 9 per cent p.a. Thermoplastic resins constitute around two-thirds of this production and their usage is growing at about 5 per cent p.a. globally ( Andrady 2003 ).

Today, plastics are almost completely derived from petrochemicals produced from fossil oil and gas. Around 4 per cent of annual petroleum production is converted directly into plastics from petrochemical feedstock ( British Plastics Federation 2008 ). As the manufacture of plastics also requires energy, its production is responsible for the consumption of a similar additional quantity of fossil fuels. However, it can also be argued that use of lightweight plastics can reduce usage of fossil fuels, for example in transport applications when plastics replace heavier conventional materials such as steel ( Andrady & Neal 2009 ; Thompson et al. 2009 b ).

Approximately 50 per cent of plastics are used for single-use disposable applications, such as packaging, agricultural films and disposable consumer items, between 20 and 25% for long-term infrastructure such as pipes, cable coatings and structural materials, and the remainder for durable consumer applications with intermediate lifespan, such as in electronic goods, furniture, vehicles, etc. Post-consumer plastic waste generation across the European Union (EU) was 24.6 million tonnes in 2007 ( PlasticsEurope 2008 b ). Table 1 presents a breakdown of plastics consumption in the UK during the year 2000, and contributions to waste generation ( Waste Watch 2003 ). This confirms that packaging is the main source of waste plastics, but it is clear that other sources such as waste electronic and electrical equipment (WEEE) and end-of-life vehicles (ELV) are becoming significant sources of waste plastics.

Table 1.

Consumption of plastics and waste generation by sector in the UK in 2000 ( Waste Watch 2003 ).

Because plastics have only been mass-produced for around 60 years, their longevity in the environment is not known with certainty. Most types of plastics are not biodegradable ( Andrady 1994 ), and are in fact extremely durable, and therefore the majority of polymers manufactured today will persist for at least decades, and probably for centuries if not millennia. Even degradable plastics may persist for a considerable time depending on local environmental factors, as rates of degradation depend on physical factors, such as levels of ultraviolet light exposure, oxygen and temperature ( Swift & Wiles 2004 ), while biodegradable plastics require the presence of suitable micro-organisms. Therefore, degradation rates vary considerably between landfills, terrestrial and marine environments ( Kyrikou & Briassoulis 2007 ). Even when a plastic item degrades under the influence of weathering, it first breaks down into smaller pieces of plastic debris, but the polymer itself may not necessarily fully degrade in a meaningful timeframe. As a consequence, substantial quantities of end-of-life plastics are accumulating in landfills and as debris in the natural environment, resulting in both waste-management issues and environmental damage (see Barnes et al. 2009 ; Gregory 2009 ; Oehlmann et al. 2009 ; Ryan et al. 2009 ; Teuten et al. 2009 ; Thompson et al. 2009 b ).

Recycling is clearly a waste-management strategy, but it can also be seen as one current example of implementing the concept of industrial ecology, whereas in a natural ecosystem there are no wastes but only products ( Frosch & Gallopoulos 1989 ; McDonough & Braungart 2002 ). Recycling of plastics is one method for reducing environmental impact and resource depletion. Fundamentally, high levels of recycling, as with reduction in use, reuse and repair or re-manufacturing can allow for a given level of product service with lower material inputs than would otherwise be required. Recycling can therefore decrease energy and material usage per unit of output and so yield improved eco-efficiency ( WBCSD 2000 ). Although, it should be noted that the ability to maintain whatever residual level of material input, plus the energy inputs and the effects of external impacts on ecosystems will decide the ultimate sustainability of the overall system.

In this paper, we will review the current systems and technology for plastics recycling, life-cycle evidence for the eco-efficiency of plastics recycling, and briefly consider related economic and public interest issues. We will focus on production and disposal of packaging as this is the largest single source of waste plastics in Europe and represents an area of considerable recent expansion in recycling initiatives.

2. Waste management: overview

Even within the EU there are a wide range of waste-management prioritizations for the total municipal solid waste stream (MSW), from those heavily weighted towards landfill, to those weighted towards incineration ( figure 1 )—recycling performance also varies considerably. The average amount of MSW generated in the EU is 520 kg per person per year and projected to increase to 680 kg per person per year by 2020 ( EEA 2008 ). In the UK, total use of plastics in both domestic and commercial packaging is about 40 kg per person per year, hence it forms approximately 7–8% by weight, but a larger proportion by volume of the MSW stream ( Waste Watch 2003 ).

Rates of mechanical recycling and energy recovery as waste-management strategies for plastics waste in European nations ( PlasticsEurope 2008 b ).

Broadly speaking, waste plastics are recovered when they are diverted from landfills or littering. Plastic packaging is particularly noticeable as litter because of the lightweight nature of both flexible and rigid plastics. The amount of material going into the waste-management system can, in the first case, be reduced by actions that decrease the use of materials in products (e.g. substitution of heavy packaging formats with lighter ones, or downgauging of packaging). Designing products to enable reusing, repairing or re-manufacturing will result in fewer products entering the waste stream.

Once material enters the waste stream, recycling is the process of using recovered material to manufacture a new product. For organic materials like plastics, the concept of recovery can also be expanded to include energy recovery, where the calorific value of the material is utilized by controlled combustion as a fuel, although this results in a lesser overall environmental performance than material recovery as it does not reduce the demand for new (virgin) material. This thinking is the basis of the 4Rs strategy in waste management parlance—in the order of decreasing environmental desirability—reduce, reuse, recycle (materials) and recover (energy), with landfill as the least desirable management strategy.

It is also quite possible for the same polymer to cascade through multiple stages—e.g. manufacture into a re-usable container, which once entering the waste stream is collected and recycled into a durable application that when becoming waste in its turn, is recovered for energy.

(a) Landfill

Landfill is the conventional approach to waste management, but space for landfills is becoming scarce in some countries. A well-managed landfill site results in limited immediate environmental harm beyond the impacts of collection and transport, although there are long-term risks of contamination of soils and groundwater by some additives and breakdown by-products in plastics, which can become persistent organic pollutants ( Oehlmann et al. 2009 ; Teuten et al . 2009 ). A major drawback to landfills from a sustainability aspect is that none of the material resources used to produce the plastic is recovered—the material flow is linear rather than cyclic. In the UK, a landfill tax has been applied, which is currently set to escalate each year until 2010 in order to increase the incentive to divert wastes from landfill to recovery actions such as recycling ( DEFRA 2007 ).

(b) Incineration and energy recovery

Incineration reduces the need for landfill of plastics waste, however, there are concerns that hazardous substances may be released into the atmosphere in the process. For example, PVC and halogenated additives are typically present in mixed plastic waste leading to the risk of dioxins, other polychlorinated biphenyls and furans being released into the environment ( Gilpin et al. 2003 ). As a consequence primarily of this perceived pollution risk, incineration of plastic is less prevalent than landfill and mechanical recycling as a waste-management strategy. Japan and some European countries such as Denmark and Sweden are notable exceptions, with extensive incinerator infrastructure in place for dealing with MSW, including plastics.

Incineration can be used with recovery of some of the energy content in the plastic. The useful energy recovered can vary considerably depending on whether it is used for electricity generation, combined heat and power, or as solid refuse fuel for co-fuelling of blast furnaces or cement kilns. Liquefaction to diesel fuel or gasification through pyrolysis is also possible ( Arvanitoyannis & Bosnea 2001 ) and interest in this approach to produce diesel fuel is increasing, presumably owing to rising oil prices. Energy-recovery processes may be the most suitable way for dealing with highly mixed plastic such as some electronic and electrical wastes and automotive shredder residue.

(c) Downgauging

Reducing the amount of packaging used per item will reduce waste volumes. Economics dictate that most manufacturers will already use close to the minimum required material necessary for a given application (but see Thompson et al. 2009 b , Fig 1 ). This principle is, however, offset against aesthetics, convenience and marketing benefits that can lead to over-use of packaging, as well as the effect of existing investment in tooling and production process, which can also result in excessive packaging of some products.

(d) Re-use of plastic packaging

Forty years ago, re-use of post-consumer packaging in the form of glass bottles and jars was common. Limitations to the broader application of rigid container re-use are at least partially logistical, where distribution and collection points are distant from centralized product-filling factories and would result in considerable back-haul distances. In addition, the wide range of containers and packs for branding and marketing purposes makes direct take-back and refilling less feasible. Take-back and refilling schemes do exist in several European countries ( Institute for Local Self-Reliance 2002 ), including PET bottles as well as glass, but they are elsewhere generally considered a niche activity for local businesses rather than a realistic large-scale strategy to reduce packaging waste.

There is considerable scope for re-use of plastics used for the transport of goods, and for potential re-use or re-manufacture from some plastic components in high-value consumer goods such as vehicles and electronic equipment. This is evident in an industrial scale with re-use of containers and pallets in haulage (see Thompson et al. 2009 b ). Some shift away from single-use plastic carrier bags to reusable bags has also been observed, both because of voluntary behaviour change programmes, as in Australia ( Department of Environment and Heritage (Australia) 2008 ) and as a consequence of legislation, such as the plastic bag levy in Ireland ( Department of Environment Heritage and Local Government (Ireland) 2007 ), or the recent banning of lightweight carrier bags, for example in Bangladesh and China.

(e) Plastics recycling

Terminology for plastics recycling is complex and sometimes confusing because of the wide range of recycling and recovery activities ( table 2 ). These include four categories: primary (mechanical reprocessing into a product with equivalent properties), secondary (mechanical reprocessing into products requiring lower properties), tertiary (recovery of chemical constituents) and quaternary (recovery of energy). Primary recycling is often referred to as closed-loop recycling, and secondary recycling as downgrading. Tertiary recycling is either described as chemical or feedstock recycling and applies when the polymer is de-polymerized to its chemical constituents ( Fisher 2003 ). Quaternary recycling is energy recovery, energy from waste or valorization. Biodegradable plastics can also be composted, and this is a further example of tertiary recycling, and is also described as organic or biological recycling (see Song et al . 2009 ).

Table 2.

Terminology used in different types of plastics recycling and recovery.

It is possible in theory to closed-loop recycle most thermoplastics, however, plastic packaging frequently uses a wide variety of different polymers and other materials such as metals, paper, pigments, inks and adhesives that increases the difficulty. Closed-loop recycling is most practical when the polymer constituent can be (i) effectively separated from sources of contamination and (ii) stabilized against degradation during reprocessing and subsequent use. Ideally, the plastic waste stream for reprocessing would also consist of a narrow range of polymer grades to reduce the difficulty of replacing virgin resin directly. For example, all PET bottles are made from similar grades of PET suitable for both the bottle manufacturing process and reprocessing to polyester fibre, while HDPE used for blow moulding bottles is less-suited to injection moulding applications. As a result, the only parts of the post-consumer plastic waste stream that have routinely been recycled in a strictly closed-loop fashion are clear PET bottles and recently in the UK, HDPE milk bottles. Pre-consumer plastic waste such as industrial packaging is currently recycled to a greater extent than post-consumer packaging, as it is relatively pure and available from a smaller number of sources of relatively higher volume. The volumes of post-consumer waste are, however, up to five times larger than those generated in commerce and industry ( Patel et al. 2000 ) and so in order to achieve high overall recycling rates, post-consumer as well as post-industrial waste need to be collected and recycled.

In some instances recovered plastic that is not suitable for recycling into the prior application is used to make a new plastic product displacing all, or a proportion of virgin polymer resin—this can also be considered as primary recycling. Examples are plastic crates and bins manufactured from HDPE recovered from milk bottles, and PET fibre from recovered PET packaging. Downgrading is a term sometimes used for recycling when recovered plastic is put into an application that would not typically use virgin polymer—e.g. ‘plastic lumber’ as an alternative to higher cost/shorter lifetime timber, this is secondary recycling ( ASTM Standard D5033 ).

Chemical or feedstock recycling has the advantage of recovering the petrochemical constituents of the polymer, which can then be used to re-manufacture plastic or to make other synthetic chemicals. However, while technically feasible it has generally been found to be uneconomic without significant subsidies because of the low price of petrochemical feedstock compared with the plant and process costs incurred to produce monomers from waste plastic ( Patel et al. 2000 ). This is not surprising as it is effectively reversing the energy-intensive polymerization previously carried out during plastic manufacture.

Feedstock recycling of polyolefins through thermal-cracking has been performed in the UK through a facility initially built by BP and in Germany by BASF. However, the latter plant was closed in 1999 ( Aguado et al. 2007 ). Chemical recycling of PET has been more successful, as de-polymerization under milder conditions is possible. PET resin can be broken down by glycolysis, methanolysis or hydrolysis, for example to make unsaturated polyester resins ( Sinha et al. 2008 ). It can also be converted back into PET, either after de-polymerization, or by simply re-feeding the PET flake into the polymerization reactor, this can also remove volatile contaminants as the reaction occurs under high temperature and vacuum ( Uhde Inventa-Fischer 2007 ).

(f) Alternative materials

Biodegradable plastics have the potential to solve a number of waste-management issues, especially for disposable packaging that cannot be easily separated from organic waste in catering or from agricultural applications. It is possible to include biodegradable plastics in aerobic composting, or by anaerobic digestion with methane capture for energy use. However, biodegradable plastics also have the potential to complicate waste management when introduced without appropriate technical attributes, handling systems and consumer education. In addition, it is clear that there could be significant issues in sourcing sufficient biomass to replace a large proportion of the current consumption of polymers, as only 5 per cent of current European chemical production uses biomass as feedstock ( Soetaert & Vandamme 2006 ). This is a large topic that cannot be covered in this paper, except to note that it is desirable that compostable and degradable plastics are appropriately labelled and used in ways that complement, rather than compromise waste-management schemes (see Song et al . 2009 ).

3. Systems for plastic recycling

Plastic materials can be recycled in a variety of ways and the ease of recycling varies among polymer type, package design and product type. For example, rigid containers consisting of a single polymer are simpler and more economic to recycle than multi-layer and multi-component packages.

Thermoplastics, including PET, PE and PP all have high potential to be mechanically recycled. Thermosetting polymers such as unsaturated polyester or epoxy resin cannot be mechanically recycled, except to be potentially re-used as filler materials once they have been size-reduced or pulverized to fine particles or powders ( Rebeiz & Craft 1995 ). This is because thermoset plastics are permanently cross-linked in manufacture, and therefore cannot be re-melted and re-formed. Recycling of cross-linked rubber from car tyres back to rubber crumb for re-manufacture into other products does occur and this is expected to grow owing to the EU Directive on Landfill of Waste (1999/31/EC), which bans the landfill of tyres and tyre waste.

A major challenge for producing recycled resins from plastic wastes is that most different plastic types are not compatible with each other because of inherent immiscibility at the molecular level, and differences in processing requirements at a macro-scale. For example, a small amount of PVC contaminant present in a PET recycle stream will degrade the recycled PET resin owing to evolution of hydrochloric acid gas from the PVC at a higher temperature required to melt and reprocess PET. Conversely, PET in a PVC recycle stream will form solid lumps of undispersed crystalline PET, which significantly reduces the value of the recycled material.

Hence, it is often not technically feasible to add recovered plastic to virgin polymer without decreasing at least some quality attributes of the virgin plastic such as colour, clarity or mechanical properties such as impact strength. Most uses of recycled resin either blend the recycled resin with virgin resin—often done with polyolefin films for non-critical applications such as refuse bags, and non-pressure-rated irrigation or drainage pipes, or for use in multi-layer applications, where the recycled resin is sandwiched between surface layers of virgin resin.

The ability to substitute recycled plastic for virgin polymer generally depends on the purity of the recovered plastic feed and the property requirements of the plastic product to be made. This has led to current recycling schemes for post-consumer waste that concentrate on the most easily separated packages, such as PET soft-drink and water bottles and HDPE milk bottles, which can be positively identified and sorted out of a co-mingled waste stream. Conversely, there is limited recycling of multi-layer/multi-component articles because these result in contamination between polymer types. Post-consumer recycling therefore comprises of several key steps: collection, sorting, cleaning, size reduction and separation, and/or compatibilization to reduce contamination by incompatible polymers.

(a) Collection

Collection of plastic wastes can be done by ‘bring-schemes’ or through kerbside collection. Bring-schemes tend to result in low collection rates in the absence of either highly committed public behaviour or deposit-refund schemes that impose a direct economic incentive to participate. Hence, the general trend is for collection of recyclable materials through kerbside collection alongside MSW. To maximize the cost efficiency of these programmes, most kerbside collections are of co-mingled recyclables (paper/board, glass, aluminium, steel and plastic containers). While kerbside collection schemes have been very successful at recovering plastic bottle packaging from homes, in terms of the overall consumption typically only 30–40% of post-consumer plastic bottles are recovered, as a lot of this sort of packaging comes from food and beverage consumed away from home. For this reason, it is important to develop effective ‘on-the-go’ and ‘office recycling’ collection schemes if overall collection rates for plastic packaging are to increase.

(b) Sorting

Sorting of co-mingled rigid recyclables occurs by both automatic and manual methods. Automated pre-sorting is usually sufficient to result in a plastics stream separate from glass, metals and paper (other than when attached, e.g. as labels and closures). Generally, clear PET and unpigmented HDPE milk bottles are positively identified and separated out of the stream. Automatic sorting of containers is now widely used by material recovery facility operators and also by many plastic recycling facilities. These systems generally use Fourier-transform near-infrared (FT-NIR) spectroscopy for polymer type analysis and also use optical colour recognition camera systems to sort the streams into clear and coloured fractions. Optical sorters can be used to differentiate between clear, light blue, dark blue, green and other coloured PET containers. Sorting performance can be maximized using multiple detectors, and sorting in series. Other sorting technologies include X-ray detection, which is used for separation of PVC containers, which are 59 per cent chlorine by weight and so can be easily distinguished ( Arvanitoyannis & Bosnea 2001 ; Fisher 2003 ).

Most local authorities or material recovery facilities do not actively collect post-consumer flexible packaging as there are current deficiencies in the equipment that can easily separate flexibles. Many plastic recycling facilities use trommels and density-based air-classification systems to remove small amounts of flexibles such as some films and labels. There are, however, developments in this area and new technologies such as ballistic separators, sophisticated hydrocyclones and air-classifiers that will increase the ability to recover post-consumer flexible packaging ( Fisher 2003 ).

(c) Size reduction and cleaning

Rigid plastics are typically ground into flakes and cleaned to remove food residues, pulp fibres and adhesives. The latest generation of wash plants use only 2–3 m 3 of water per tonne of material, about one-half of that of previous equipment. Innovative technologies for the removal of organics and surface contaminants from flakes include ‘dry-cleaning’, which cleans surfaces through friction without using water.

(d) Further separation

After size reduction, a range of separation techniques can be applied. Sink/float separation in water can effectively separate polyolefins (PP, HDPE, L/LLDPE) from PVC, PET and PS. Use of different media can allow separation of PS from PET, but PVC cannot be removed from PET in this manner as their density ranges overlap. Other separation techniques such as air elutriation can also be used for removing low-density films from denser ground plastics ( Chandra & Roy 2007 ), e.g. in removing labels from PET flakes.

Technologies for reducing PVC contaminants in PET flake include froth flotation ( Drelich et al. 1998 ; Marques & Tenorio 2000 )[JH1], FT-NIR or Raman emission spectroscopic detectors to enable flake ejection and using differing electrostatic properties ( Park et al. 2007 ). For PET flake, thermal kilns can be used to selectively degrade minor amounts of PVC impurities, as PVC turns black on heating, enabling colour-sorting.

Various methods exist for flake-sorting, but traditional PET-sorting systems are predominantly restricted to separating; (i) coloured flakes from clear PET flakes and (ii) materials with different physical properties such as density from PET. New approaches such as laser-sorting systems can be used to remove other impurities such as silicones and nylon.

‘Laser-sorting’ uses emission spectroscopy to differentiate polymer types. These systems are likely to significantly improve the ability to separate complex mixtures as they can perform up to 860 000 spectra s −1 and can scan each individual flake. They have the advantage that they can be used to sort different plastics that are black—a problem with traditional automatic systems. The application of laser-sorting systems is likely to increase separation of WEEE and automotive plastics. These systems also have the capability to separate polymer by type or grade and can also separate polyolefinic materials such as PP from HDPE. However, this is still a very novel approach and currently is only used in a small number of European recycling facilities.

(e) Current advances in plastic recycling

Innovations in recycling technologies over the last decade include increasingly reliable detectors and sophisticated decision and recognition software that collectively increase the accuracy and productivity of automatic sorting—for example current FT-NIR detectors can operate for up to 8000 h between faults in the detectors.

Another area of innovation has been in finding higher value applications for recycled polymers in closed-loop processes, which can directly replace virgin polymer (see table 3 ). As an example, in the UK, since 2005 most PET sheet for thermoforming contains 50–70% recycled PET (rPET) through use of A/B/A layer sheet where the outer layers (A) are food-contact-approved virgin resin, and the inner layer (B) is rPET. Food-grade rPET is also now widely available in the market for direct food contact because of the development of ‘super-clean’ grades. These only have slight deterioration in clarity from virgin PET, and are being used at 30–50% replacement of virgin PET in many applications and at 100 per cent of the material in some bottles.

Table 3.

Comparing some environmental impacts of commodity polymer production and current ability for recycling from post-consumer sources.

a CO 2 -e is GWP calculated as 100-yr equivalent to CO 2 emissions. All LCI data are specific to European industry and covers the production process of the raw materials, intermediates and final polymer, but not further processing and logistics ( PlasticsEurope 2008 a ).

b Usage was for the aggregate EU-15 countries across all market sectors in 2002.

c Typical values for water and greenhouse gas emissions from recycling activities to produce 1 kg PET from waste plastic ( Perugini et al. 2005 ).

A number of European countries including Germany, Austria, Norway, Italy and Spain are already collecting, in addition to their bottle streams, rigid packaging such as trays, tubs and pots as well as limited amounts of post-consumer flexible packaging such as films and wrappers. Recycling of this non-bottle packaging has become possible because of improvements in sorting and washing technologies and emerging markets for the recyclates. In the UK, the Waste Resource Action Programme (WRAP) has run an initial study of mixed plastics recycling and is now taking this to full-scale validation ( WRAP 2008 b ). The potential benefits of mixed plastics recycling in terms of resource efficiency, diversion from landfill and emission savings, are very high when one considers the fact that in the UK it is estimated that there is over one million tonne per annum of non-bottle plastic packaging ( WRAP 2008 a ) in comparison with 525 000 tonnes of plastic bottle waste ( WRAP 2007 ).

4. Ecological case for recycling

Life-cycle analysis can be a useful tool for assessing the potential benefits of recycling programmes. If recycled plastics are used to produce goods that would otherwise have been made from new (virgin) polymer, this will directly reduce oil usage and emissions of greenhouse gases associated with the production of the virgin polymer (less the emissions owing to the recycling activities themselves). However, if plastics are recycled into products that were previously made from other materials such as wood or concrete, then savings in requirements for polymer production will not be realized ( Fletcher & Mackay 1996 ). There may be other environmental costs or benefits of any such alternative material usage, but these are a distraction to our discussion of the benefits of recycling and would need to be considered on a case-by-case basis. Here, we will primarily consider recycling of plastics into products that would otherwise have been produced from virgin polymer.

Feedstock (chemical) recycling technologies satisfy the general principle of material recovery, but are more costly than mechanical recycling, and less energetically favourable as the polymer has to be depolymerized and then re-polymerized. Historically, this has required very significant subsidies because of the low price of petrochemicals in contrast to the high process and plant costs to chemically recycle polymers.

Energy recovery from waste plastics (by transformation to fuel or by direct combustion for electricity generation, use in cement kilns and blast furnaces, etc.) can be used to reduce landfill volumes, but does not reduce the demand for fossil fuels (as the waste plastic was made from petrochemicals; Garforth et al. 2004 ). There are also environmental and health concerns associated with their emissions.

One of the key benefits of recycling plastics is to reduce the requirement for plastics production. Table 3 provides data on some environmental impacts from production of virgin commodity plastics (up to the ‘factory gate’), and summarizes the ability of these resins to be recycled from post-consumer waste. In terms of energy use, recycling has been shown to save more energy than that produced by energy recovery even when including the energy used to collect, transport and re-process the plastic ( Morris 1996 ). Life-cycle analyses has also been used for plastic-recycling systems to evaluate the net environmental impacts ( Arena et al. 2003 ; Perugini et al. 2005 ) and these find greater positive environmental benefits for mechanical recycling over landfill and incineration with energy recovery.

It has been estimated that PET bottle recycling gives a net benefit in greenhouse gas emissions of 1.5 tonnes of CO 2 -e per tonne of recycled PET ( Department of Environment and Conservation (NSW) 2005 ) as well as reduction in landfill and net energy consumption. An average net reduction of 1.45 tonnes of CO 2 -e per tonne of recycled plastic has been estimated as a useful guideline to policy ( ACRR 2004 ), one basis for this value appears to have been a German life-cycle analysis (LCA) study ( Patel et al. 2000 ), which also found that most of the net energy and emission benefits arise from the substitution of virgin polymer production. A recent LCA specifically for PET bottle manufacture calculated that use of 100 per cent recycled PET instead of 100 per cent virgin PET would reduce the full life-cycle emissions from 446 to 327 g CO 2 per bottle, resulting in a 27 per cent relative reduction in emissions ( WRAP 2008 e ).

Mixed plastics, the least favourable source of recycled polymer could still provide a net benefit of the vicinity of 0.5 tonnes of CO 2 -e per tonne of recycled product ( WRAP 2008 c ). The higher eco-efficiency for bottle recycling is because of both the more efficient process for recycling bottles as opposed to mixed plastics and the particularly high emissions profile of virgin PET production. However, the mixed plastics recycling scenario still has a positive net benefit, which was considered superior to the other options studied, of both landfills and energy recovery as solid refuse fuel, so long as there is substitution of virgin polymer.

5. Public support for recycling

There is increasing public awareness on the need for sustainable production and consumption. This has encouraged local authorities to organize collection of recyclables, encouraged some manufacturers to develop products with recycled content, and other businesses to supply this public demand. Marketing studies of consumer preferences indicate that there is a significant, but not overwhelming proportion of people who value environmental values in their purchasing patterns. For such customers, confirmation of recycled content and suitability for recycling of the packaging can be a positive attribute, while exaggerated claims for recyclability (where the recyclability is potential, rather than actual) can reduce consumer confidence. It has been noted that participating in recycling schemes is an environmental behaviour that has wide participation among the general population and was 57 per cent in the UK in a 2006 survey ( WRAP 2008 d ), and 80 per cent in an Australian survey where kerbside collection had been in place for longer ( NEPC 2001 ).

Some governments use policy to encourage post-consumer recycling, such as the EU Directive on packaging and packaging waste (94/62/EC). This subsequently led Germany to set-up legislation for extended producer responsibility that resulted in the die Grüne Punkt (Green Dot) scheme to implement recovery and recycling of packaging. In the UK, producer responsibility was enacted through a scheme for generating and trading packaging recovery notes, plus more recently a landfill levy to fund a range of waste reduction activities. As a consequence of all the above trends, the market value of recycled polymer and hence the viability of recycling have increased markedly over the last few years.

Extended producer responsibility can also be enacted through deposit-refund schemes, covering for example, beverage containers, batteries and vehicle tyres. These schemes can be effective in boosting collection rates, for example one state of Australia has a container deposit scheme (that includes PET soft-drink bottles), as well as kerbside collection schemes. Here the collection rate of PET bottles was 74 per cent of sales, compared with 36 per cent of sales in other states with kerbside collection only. The proportion of bottles in litter was reduced as well compared to other states ( West 2007 ).

6. Economic issues relating to recycling

Two key economic drivers influence the viability of thermoplastics recycling. These are the price of the recycled polymer compared with virgin polymer and the cost of recycling compared with alternative forms of acceptable disposal. There are additional issues associated with variations in the quantity and quality of supply compared with virgin plastics. Lack of information about the availability of recycled plastics, its quality and suitability for specific applications, can also act as a disincentive to use recycled material.

Historically, the primary methods of waste disposal have been by landfill or incineration. Costs of landfill vary considerably among regions according to the underlying geology and land-use patterns and can influence the viability of recycling as an alternative disposal route. In Japan, for example, the excavation that is necessary for landfill is expensive because of the hard nature of the underlying volcanic bedrock; while in the Netherlands it is costly because of permeability from the sea. High disposal costs are an economic incentive towards either recycling or energy recovery.

Collection of used plastics from households is more economical in suburbs where the population density is sufficiently high to achieve economies of scale. The most efficient collection scheme can vary with locality, type of dwellings (houses or large multi-apartment buildings) and the type of sorting facilities available. In rural areas ‘bring schemes’ where the public deliver their own waste for recycling, for example when they visit a nearby town, are considered more cost-effective than kerbside collection. Many local authorities and some supermarkets in the UK operate ‘bring banks’, or even reverse-vending machines. These latter methods can be a good source of relatively pure recyclables, but are ineffective in providing high collection rates of post-consumer waste. In the UK, dramatic increases in collection of the plastic bottle waste stream was only apparent after the relatively recent implementation of kerbside recycling ( figure 2 ).

Growth in collection of plastic bottles, by bring and kerbside schemes in the UK ( WRAP 2008 d ).

The price of virgin plastic is influenced by the price of oil, which is the principle feedstock for plastic production. As the quality of recovered plastic is typically lower than that of virgin plastics, the price of virgin plastic sets the ceiling for prices of recovered plastic. The price of oil has increased significantly in the last few years, from a range of around USD 25 per barrel to a price band between USD 50–150 since 2005. Hence, although higher oil prices also increase the cost of collection and reprocessing to some extent, recycling has become relatively more financially attractive.

Technological advances in recycling can improve the economics in two main ways—by decreasing the cost of recycling (productivity/efficiency improvements) and by closing the gap between the value of recycled resin and virgin resin. The latter point is particularly enhanced by technologies for turning recovered plastic into food grade polymer by removing contamination—supporting closed-loop recycling. This technology has been proven for rPET from clear bottles ( WRAP 2008 b ), and more recently rHDPE from milk bottles ( WRAP 2006 ).

So, while over a decade ago recycling of plastics without subsidies was mostly only viable from post-industrial waste, or in locations where the cost of alternative forms of disposal were high, it is increasingly now viable on a much broader geographic scale, and for post-consumer waste.

7. Current trends in plastic recycling

In western Europe, plastic waste generation is growing at approximately 3 per cent per annum, roughly in line with long-term economic growth, whereas the amount of mechanical recycling increased strongly at a rate of approximately 7 per cent per annum. In 2003, however, this still amounted to only 14.8 per cent of the waste plastic generated (from all sources). Together with feedstock recycling (1.7 per cent) and energy recovery (22.5 per cent), this amounted to a total recovery rate of approximately 39 per cent from the 21.1 million tonnes of plastic waste generated in 2003 ( figure 3 ). This trend for both rates of mechanical recycling and energy recovery to increase is continuing, although so is the trend for increasing waste generation.

Volumes of plastic waste disposed to landfill, and recovered by various methods in Western Europe, 1993–2003 ( APME 2004 ).

8. Challenges and opportunities for improving plastic recycling

Effective recycling of mixed plastics waste is the next major challenge for the plastics recycling sector. The advantage is the ability to recycle a larger proportion of the plastic waste stream by expanding post-consumer collection of plastic packaging to cover a wider variety of materials and pack types. Product design for recycling has strong potential to assist in such recycling efforts. A study carried out in the UK found that the amount of packaging in a regular shopping basket that, even if collected, cannot be effectively recycled, ranged from 21 to 40% ( Local Government Association (UK) 2007 ). Hence, wider implementation of policies to promote the use of environmental design principles by industry could have a large impact on recycling performance, increasing the proportion of packaging that can economically be collected and diverted from landfill (see Shaxson et al. 2009 ). The same logic applies to durable consumer goods designing for disassembly, recycling and specifications for use of recycled resins are key actions to increase recycling.

Most post-consumer collection schemes are for rigid packaging as flexible packaging tends to be problematic during the collection and sorting stages. Most current material recovery facilities have difficulty handling flexible plastic packaging because of the different handling characteristics of rigid packaging. The low weight-to-volume ratio of films and plastic bags also makes it less economically viable to invest in the necessary collection and sorting facilities. However, plastic films are currently recycled from sources including secondary packaging such as shrink-wrap of pallets and boxes and some agricultural films, so this is feasible under the right conditions. Approaches to increasing the recycling of films and flexible packaging could include separate collection, or investment in extra sorting and processing facilities at recovery facilities for handling mixed plastic wastes. In order to have successful recycling of mixed plastics, high-performance sorting of the input materials needs to be performed to ensure that plastic types are separated to high levels of purity; there is, however, a need for the further development of endmarkets for each polymer recyclate stream.

The effectiveness of post-consumer packaging recycling could be dramatically increased if the diversity of materials were to be rationalized to a subset of current usage. For example, if rigid plastic containers ranging from bottles, jars to trays were all PET, HDPE and PP, without clear PVC or PS, which are problematic to sort from co-mingled recyclables, then all rigid plastic packaging could be collected and sorted to make recycled resins with minimal cross-contamination. The losses of rejected material and the value of the recycled resins would be enhanced. In addition, labels and adhesive materials should be selected to maximize recycling performance. Improvements in sorting/separation within recycling plants give further potential for both higher recycling volumes, and better eco-efficiency by decreasing waste fractions, energy and water use (see §3 ). The goals should be to maximize both the volume and quality of recycled resins.

9. Conclusions

In summary, recycling is one strategy for end-of-life waste management of plastic products. It makes increasing sense economically as well as environmentally and recent trends demonstrate a substantial increase in the rate of recovery and recycling of plastic wastes. These trends are likely to continue, but some significant challenges still exist from both technological factors and from economic or social behaviour issues relating to the collection of recyclable wastes, and substitution for virgin material.

Recycling of a wider range of post-consumer plastic packaging, together with waste plastics from consumer goods and ELVs will further enable improvement in recovery rates of plastic waste and diversion from landfills. Coupled with efforts to increase the use and specification of recycled grades as replacement of virgin plastic, recycling of waste plastics is an effective way to improve the environmental performance of the polymer industry.

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Home > Books > Environmental Management in Practice

The Effects of Paper Recycling and its Environmental Impact

Submitted: 24 November 2010 Published: 05 July 2011

DOI: 10.5772/23110

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Author Information

Iveta čabalová *.

• Technical University in Zvolen,Faculty of Wood Sciences and Technology, Slovakia

František Kačík

Anton geffert *, danica kačíková *.

*Address all correspondence to:

1. Introduction

It is well known the paper production (likewise the other brands of industry) has enormous effects on the environment. The using and processing of raw materials has a variety of negative effects on the environment.

At the other hand there are technologies which can moderate the negative impacts on the environment and they also have a positive economical effect. One of these processes is the recycling, which is not only the next use of the wastes. The main benefit of the recycling is a double decrease of the environment loading, known as an environmental impact reducing. From the first view point, the natural resources conserves at side of the manufacturing process inputs, from the second view point, the harmful compounds amount leaking to the environment decreases at side of the manufacturing process outputs.

The paper production from the recycled fibers consumes less energy; conserves the natural resources viz. wood and decreases the environmental pollution. The conflict between economic optimization and environmental protection has received wide attention in recent research programs for waste management system planning. This has also resulted in a set of new waste management goals in reverse logistics system planning. Pati et al. (2008 ) have proposed a mixed integer goal programming (MIGP) model to capture the inter-relationships among the paper recycling network system. Use of this model can bring indirectly benefit to the environment as well as improve the quality of waste paper reaching the recycling unit.

In 2005, the total production of paper in Europe was 99.3 million tonnes which generated 11 million tonnes of waste, representing about 11% in relation to the total paper production. The production of recycled paper, during the same period, was 47.3 million tonnes generating 7.7 million tonnes of solid waste (about 70% of total generated waste in papermaking) which represents 16% of the total production from this raw material ( CEPI 2006 ).

The consumption of recovered paper has been in continuous growth during the past decades. According to the Confederation of European Paper Industries (CEPI), the use of recovered paper was almost even with the use of virgin fiber in 2005. This development has been boosted by technological progress and the good price competitiveness of recycled fiber, but also by environmental awareness – at both the producer and consumer ends – and regulation that has influenced the demand for recovered paper. The European paper industry suffered a very difficult year in 2009 during which the industry encountered more down-time and capacity closures as a result of the weakened global economy. Recovered paper utilisation in Europe decreased in 2009, but exports of recovered paper to countries outside CEPI continued to rise, especially to Asian markets (96.3%). However, recycling rate expressed as “volume of paper recycling/volume of paper consumption” resulted in a record high 72.2% recycling rate after having reached 66.7% the year before ( Fig. 1 ) ( Hujala et al. 2010 ;CEPI 2006; European Declaration on Paper Recycling 2010; Huhtala& Samakovlis 2002 ; CEPI Annual Statistic 2010).

European paper recycling 1995-2009 in million tonnes (European Declaration on Paper Recycling 2006 – 2010, Monitoring Report 2009 (2010) (www.erpa.info)

Recycling is not a new technology. It has become a commercial proposition since Matthias Koops established the Neckinger mill, in 1826, which produced white paper from printed waste paper. However, there were very few investigations into the effect of recycling on sheet properties until late 1960's. From then until the late 1970's, a considerable amount of work was carried out to identify the effects of recycling on pulp properties and the cause of these effects ( Nazhad 2005 ; Nazhad& Paszner 1994 ). In the late 1980's and early 1990's, recycling issues have emerged stronger than before due to the higher cost of landfills in developed countries and an evolution in human awareness. The findings of the early 70's on recycling effects have since been confirmed, although attempts to trace the cause of these effects are still not resolved ( Howard &Bichard 1992 ).

Recycling has been thought to reduce the fibre swelling capability, and thus the flexibility of fibres. The restricted swelling of recycled fibres has been ascribed to hornification, which has been introduced as a main cause of poor quality of recycled paper ( Scallan&Tydeman 1992 ). Since 1950's, fibre flexibility among the papermakers has been recognized as a main source of paper strength. Therefore, it is not surprising to see that, for over half a century, papermakers have supported and rationalized hornification as a main source of tensile loss due to drying, even though it has never been fully understood ( Sutjipto et al. 2008 ).

Recycled paper has been increasingly produced in various grades in the paper industry. However, there are still technical problems including reduction in mechanical strength for recycled paper. Especially, chemical pulp-origin paper, that is, fine paperrequires a certain level of strength. Howard & Bichard (1992 ) reported that beaten bleachedkraft pulp produced handsheets which were bulky and weak in tensile and burst strengthsby handsheet recycling. This behaviour could be explained by the reduction in re-swelling capability or the reduction in flexibility of rewetted pulp fibers due to fiber hornification and, possibly, by fines loss during recycling processes, which decrease both total bondingarea and the strength of paper ( Howard 1995 ; Nazhad&Paszner 1994 ; Nazhad et al. 1995 ; Khantayanuwong et al.2002 ; Kim et al. 2000 ).

Paper recycling is increasingly important for the sustainable development of the paper industry as an environmentally friendly sound. The research related to paper recycling is therefore increasingly crucial for the need of the industry. Even though there are a number of researches ascertained the effect of recycling treatment on properties of softwood pulp fibres ( Cao et al. 1999 ; Horn 1975 ; Howard&Bichard 1992 ; Jang et al. 1995 ), however, it is likely that hardwood pulp fibres have rarely been used in the research operated with recycling treatment. Changes in some morphological properties of hardwood pulp fibres, such as curl, kink, and length of fibre, due to recycling effects also have not been determined considerably. This is possibly because most of the researches were conducted in the countries where softwood pulp fibres are commercial extensively ( Khantayanuwong 2003 ). Therefore, it is the purpose of the present research to crucially determine the effect of recycling treatment on some important properties of softwood pulp fibres.

2. Alterations of pulp fibres properties at recycling

The goal of a recycled paper or board manufacturer is to make a product that meets customers΄ specification and requirements. At the present utilization rate, using recycled fibres in commodity grades such as newsprint and packaging paper and board has not caused noticeable deterioration in product quality and performance ( Čabalová et al. 2009 ). The expected increase in recovery rates of used paper products will require a considerable consumption increase of recycled fibres in higher quality grades such as office paper and magazine paper. To promote expanded use of recovered paper, understanding the fundamental nature of recycled fibres and the differences from virgin fibres is necessary.

Essentially, recycled fibres are contaminated, used fibres. Recycled pulp quality is, therefore, directly affected by the history of the fibres, i.e. by the origins, processes and treatments which these fibres have experienced.

McKinney (1995) classified the history into five periods:

fibre furnish and pulp history

paper making process history

printing and converting history

consumer and collection history

recycling process history.

To identity changes in fibre properties, many recycling studies have occurred at laboratory. Realistically repeating all the stages ofthe recycling chain is difficult especially when including printing and deinking. Some insight into changes in fibre structure, cell wall properties, and bonding ability is possible from investigations using various recycling procedures, testing methods, and furnishes.

Mechanical pulp is chemically and physically different from chemical pulp then recycling effect on those furnishes is also different. When chemical fibres undergo repeated drying and rewetting, they are hornified and can significantly lose their originally high bonding potential ( Somwand et al. 2002 ; Song & Law 2010 ; Kato & Cameron 1999 ; Bouchard & Douek 1994 ; Khantayanuwong et al. 2002 ; Zanuttini et al. 2007 ; da Silva et al. 2007 ). The degree of hornification can be measured by water retention value (WRW) ( Kim et al. 2000 ). In contrast to the chemical pulps, originally weakermechanical pulps do not deteriorate but somewhat even improve bonding potential during a corresponding treatment. Several studies( Maloney et al. 1998 ; Weise 1998 ; Ackerman et al. 2000 ) have shown good recyclability of mechanical fibres.

Adámková a Milichovský (2002 ) present the dependence of beating degree ( SR –Schopper-Riegler degree) and WRV from the relative length of hardwood and softwood pulps. From their results we can see the WRV increase in dependence on the pulp length alteration is more rapid at hardwood pulp, but finally this value is higher at softwood pulps. Kim et al. (2000 ) determined the WRV decrease at softwood pulps with the higher number of recycling (at zero recycling about cca 1.5 g/g at fifth recycling about cca 1.1 g/g).Utilisation of the secondary fibres to furnish at paper production decrease of the initial need of woody raw (less of cutting tress) but the paper quality is not significantly worse.

2.1. Paper recycling

The primary raw material for the paper production is pulps fibres obtaining by a complicated chemical process from natural materials, mainly from wood. This fibres production is very energy demanding and at the manufacturing process there are used many of the chemical matters which are very problematic from view point of the environment protection. The suitable alternative is obtaining of the pulp fibres from already made paper. This process is far less demanding on energy and chemicals utilisation. The paper recycling, simplified, means the repeated defibring, grinding and drying, when there are altered the mechanical properties of the secondary stock, the chemical properties of fibres, the polymerisation degree of pulp polysaccharidic components, mainly of cellulose, their supramolecular structure, the morphological structure of fibres, range and level of interfibres bonds e.g.. The cause of above mentioned alterations is the fibres ageing at the paper recycling and manufacturing, mainly the drying process.

At the repeat use of the secondary fibres, it need deliberate the paper properties alter due to the fiber deterioration during the recycling, when many alteration are irreversible. The alteration depth depends on the cycle’s number and way to the fibres use. The main problem is the decrease of the secondary pulp mechanical properties with the continuing recycling, mainly the paper strength ( Khantayanuwong et al. 2002 ; Jahan 2003 ; Hubbe & Zhang 2005 ; Garg & Singh 2006 ; Geffertová et al. 2008 ; Sutjipto et al. 2008 ). This decrease is an effect of many alterations, which can but need not arise in the secondary pulp during the recycling process. The recycling causes the hornification of the cell walls that result in the decline of some pulp properties. It is due to the irreversible alterations in the cells structure during the drying ( Oksanen et al. 1997 ; Kim et al. 2000 ; Diniz et al. 2004 ).

The worse properties of the recycled fibres in comparison with the primary fibres can be caused by hornification but also by the decrease of the hydrophilic properties of the fibres surface during the drying due to the redistribution or migration of resin and fat acids to the surface ( Nazhad& Paszner 1994 ; Nazhad 2005 ). Okayama (2002 ) observed the enormous increase of the contact angle with water which is related to the fiber inactivation at the recycling. This process is known as „irreversible hornification“.

Paper recycling saves the natural wood raw stock, decreases the operation and capital costs to paper unit, decrease water consumption and last but not least this paper processing gives rise to the environment preservation (e.g. 1 t of waste paper can replace cca 2.5 m 3 of wood).

A key issue in paper recycling is the impact of energy use in manufacturing.Processing waste paper for paper and board manufacture requires energy that isusually derived from fossil fuels, such as oil and coal. In contrast to the productionof virgin fibre-based chemical pulp, waste paper processing does not yield a thermalsurplus and thus thermal energy must be supplied to dry the paper web. If,however, the waste paper was recovered for energy purposes the need for fossil fuelwould be reduced and this reduction would have a favourable impact on the carbondioxide balance and the greenhouse effect. Moreover, pulp production based onvirgin fibres requires consumption of round wood and causes emissions of air-pollutingcompounds as does the collection of waste paper. For better paper utilization, an interactive model, the Optimal Fibre Flow Model, considersboth a quality (age) and an environmental measure of waste paper recycling was developed ( Byström&Lönnstedt 1997 ).

2.1.1. Influence of beating on pulp fibres

Beating of chemical pulp is an essential step in improving the bonding ability of fibres. The knowledge complete about beating improves the present opinion of the fibres alteration at the beating. The main and extraneous influences of the beating device on pulps were defined.The main influences are these, each of them can be improve by the suitable beating mode, but only one alteration cannot be attained. Known are varieties of simultaneous changes in fibres, such as internal fibrilation, external fibrilation, fiber shortening or cutting, and fines formation ( Page 1989 ; Kang & Paulapuro 2006a ; Kang & Paulapuro 2006c ).

Freeing and disintegration of a cell wall affiliated with strongswelling expressed as an internal fibrilation and delamination. The delamination is a coaxial cleavage in the middle layer of the secondary wall.It causes the increased water penetration to the cell wall and the fibre plasticizing.

External fibrillation and fibrils peeling from surface, which particularly or fully attacks primary wall and outside layers of secondary walls.Simultaneously from the outside layers there arecleavage fibrils, microfibrils, nanofibrils to the macromolecule of cellulose and hemicelluloses.

Fibres shortening in any place in any angle-wise across fibre in accordance with loading, most commonly in weak places.

Concurrently the main effects at the beating also the extraneous effects take place, e.g. fines making, compression along the fibres axis, fibres waving due to the compression. It has low bonding ability and it influences the paper porosity,stocks freeness ( Sinke&Westenbroek 2004 ).

The beating causes the fibres shortening, the external and internal fibrillation affiliated with delamination and the fibres plasticizing. The outside primary wall of the pulp fibre leaks water little, it has usually an intact primary layer and a tendency to prevent from the swelling of the secondary layer of the cell wall. At the beating beginning there are disintegrated the fibre outside layers (P and S1), the fibrilar structure of the fibre secondary layer is uncovering, the water approach is improving, the swelling is taking place and the fibrillation process is beginning. The fibrillation process is finished by the weaking and cleavaging of the bonds between the particular fibrils and microfibrils of cell walls during the mechanical effect and the penetration into the interfibrilar spaces, it means to the amorphous region, there is the main portion of hemicelluloses.

Češek& Milichovský (2005 ) showed that with the increase of pulp beating degree the standard rheosettling velocity of pulp decreases more at the fibres fibrillation than at the fibres shortening.

Refining causes a variety of simultaneous changes in the fiber structure, such as internal fibrillation, external fibrillation and fines formation. Among these effects, swelling is commonly recognized as an important factor affecting the strength of recycled paper ( Kang & Paulapuro 2006d ).

Scallan & Tigerstrom (1991 ) observed the elasticity modulus of the long fibres from kraft pulp during the recycling. Flexibility decrease was evident at the beating degree decrease ( SR), and also with the increase of draining velocity of low-yield pulp.

Alteration of the breaking length of the paper sheet drying at the temperature of 80, 100 a 120°C during eightfold recycling

The selected properties of the pulp fibres and the paper sheets during the process of eightfold recycling at three drying temperatures of 80, 100, 120°C.

From the result on Fig. 2 we can see the increase of the pulp fibres active surface takes place during the beating process, which results in the improve of the bonding and the paper strength after the first beating. It causes also the breaking length increase of the laboratory sheets. The secondary fibres wear by repeated beating, what causes the decrease of strength values ( Table 1 ).

The biggest alterations of tear index ( Fig. 3 ) were observed after fifth recycling at the bleached softwood pulp fibres. The first beating causes the fibrillation of the outside layer of the cell wall, it results in the formation of the mechanical (felting) and the chemical bonds between the fibres. The repeated beating and drying dues, except the continuing fibrillation of the layer, the successive fibrils peeling until the peeling of the primary and outside secondary layer of the cell wall. It discovers the next non-fibriled layer S2 (second, the biggest layer of the secondary wall) what can do the tear index decrease. The next beating causes also this layer fibrillation, which leads to the increase of the strength value ( Fig. 3 , Tab. 1 ).Paper strength properties such as tensile strength and Scott bond strength were strongly influenced by internal fibrillation; these could also be increased further by promoting mostly external fibrillation ( Kang & Paulapuro 2006b ).

The course of the breaking length decrease and the tearing strength increase of the paper sheet is in accordance with the results of Sutjipto et al. (2008 ) at the threefold recycling of the bleached (88% ISO) softwood pulps prepared at the laboratory conditions, beated on PFI mill to 25 SR.

Tear index alteration of the paper sheets drying at the temperature of 80, 100 a 120°C, during eightfold recycling

Song & Law (2010 ) observedkraft pulp oxidation and its influence on recycling characteristics of fibres, the found up the fibre oxidation influences negatively the tear index of paper sheets.Oxidation of virgin fibre prior to recycling minimized the loss of WRV and sheet density.

The beating causes the fibres shortening and fines formation which is washed away in the large extent and it endeds in the paper sludges. This waste can be further processed and effective declined.

Within theEuropean Union several already issued and other foreseendirectives have great influence on the waste managementstrategy of paper producing companies. Due to the large quantities ofwaste generated, the high moisture content of the wasteand the changing composition, some recovery methods,for example, conversion to fuel components, are simplytoo expensive and their environmental impact uncertain.The thermal processes, gasification and pyrolysis, seem tobe interesting emerging options, although it is still necessaryto improve the technologies for sludge application.Other applications, such as the hydrolysis to obtain ethanol,have several advantages (use of wet sludge and applicabletechnology to sludges) but these are not welldeveloped for pulp and paper sludges. Therefore, at thismoment, the minimization of waste generation still hasthe highest priority ( Monte et al. 2009 ).

2.1.2. Drying influence on the recycled fibres

Characteristic differences between recycled fibres and virgin fibres can by expected. Many of these can by attributed to drying. Drying is a process that is accompanied by partially irreversible closure of small pores in the fibre wall, as well as increased resistance to swelling during rewetting. Further differences between virgin and recycled fibres can be attributed to the effects of a wide range of contaminating substances ( Hubbe et al. 2007 ). Drying, which has an anisotropic character, has a big influence on the properties of paper produced from the secondary fibres.During the drying the shear stress are formatted in the interfibrilar bonding area. The stresses formatted in the fibres and between them effect the mechanical properties in the drying paper. The additional effect dues the tensioning of the wet pulp stock on the paper machine.

During the drying and recycling the fibres are destructed. It is important to understand the loss of the bonding strength of the drying chemical fibres. Dang (2007 ) characterized the destruction like a percentage reduction of ability of the water retention value (WRV) in pulp at dewatering.

Hornification = [(WRV 0 -WRV 1 )/WRV 0 ]. 100 [%],

WRV 0 –is value of virgin pup

WRV 1 –the value of recycled pulp after drying and reslushing.

According to the prevailing concept, hornification occurs in the cell wall matrix of chemical fibres. During drying, delaminated parts of the fiber wall, i.e., cellulose microfibrils become attached as Fig. 4 shows ( Ackerman et al. 2000 ).

Changes in fiber wall structure ( Weise &Paulapuro 1996 )

Shrinkage of a fiber cross section ( Ackerman et al. 2000 )

Hydrogen bonds between those lamellae also form. Reorientation and better alignment of microfibrils also occur. All this causes an intensely bonded structure. In a subsequent reslushing in water, the fiber cell wall microstructure remains more resistant to delaminating forces because some hydrogen bonds do not reopen. The entire fiber is stiffer and more brittle ( Howard 1991 ). According to some studies ( Bouchard &Douek 1994 ; Maloney et al. 1998 ), hornification does not increase the crystallinity of cellulose or the degree of order in the hemicelluloses ofthe fiber wall.

The drying model of Scallan ( Laivins&Scallan 1993 ) suggests that hornification prevents the dry structure in A from fully expanding to the wet structure in D. Instead, only partial expansion to B may be possible after initial drying creates hydrogen bonds between the microfibrils( Kato & Cameron 1999 )

Weise & Paulapuro (1996 ) did very revealing work about the events during fiber drying. They studied fiber cross section of kraft fibers in various solids by Confocal Laser Scanning Microscope (CLSM) and simultaneously measured hornification with WRV tests. Irreversible hornification of fibers began on the degree of beating. It does not directly follow shrinkage since the greatest shrinkage of fibers occurs above 80 % solids content. In Figs. 4 and 5 , stage A represented wet kraft fiber before drying. In stage B, the drainage has started tocause morphological changes in the fiber wall matrix at about 30 % solids content. The fiber wall lamellae start to approach each other because of capillary forces. During this stage, the lumen can collapse. With additional drying, spaces between lamellae continue shrinking to phase C where most free voids in the lamellar structure of the cell wall have already closed. Toward the end of drying in stage D, the water removal occurs in the fine structure of the fiber wall. Kraft fiber shrink strongly and uniformly during this final phase of drying, i.e., at solid contents above 75-80 %. The shrinkage of stage D is irreversible.

At a repeated use of the dried fibres in paper making industry, the cell walls receive the water again. Then the opposite processes take place than in the Fig. 4 and 5 . It show Scallan´s model of the drying in Fig. 6 .

The drying dues also macroscopic stress applied on paper and distributed in fibres system according a local structure.

2.1.3. Properties of fibres from recycled paper

The basic properties of origin wet fibres change in the drying process of pulp and they are not fully regenerated in the process of slushing and beating.

The same parameters are suitable for the description of the paper properties of secondary fibres and fibres at ageing as well as for description of primary fibres properties. The experiences obtained at the utilisation of waste paper showed the secondary fibres have very different properties from the origin fibres. Next recycling of fibres causes the formation of extreme nonhomogeneous mixture of various old fibres. At the optimum utilisation of the secondary fibres it need take into account their altered properties at the repeated use. With the increase number of use cycles the fibres change irreversible, perish and alter their properties. Slushing and beating causes water absorption, fibres swelling and a partial regeneration of properties of origin fibres. However the repeated beating and drying at the multiple production cycles dues the gradual decrease of swelling ability, what influences a bonding ability of fibres. With the increase of cycles number the fibres are shortened. These alterations express in paper properties. The decrease of bonding ability and mechanical properties bring the improving of some utility properties. Between them there is higher velocity of dewatering and drying, air permeability and blotting properties improve of light scattering, opacity and paper dimensional stability.

The highest alterations of fibres properties are at the first and following three cycles. The size of strength properties depends on fibres type ( Geffertová et al. 2008 ).

Drying influences fibres length, width, shape factor, kinks which are the important factors to the strength of paper made from recycled fibres. The dimensional characteristics are measured by many methods, known is FQA (Fiber Quality Analyser), which is a prototype IFA (Imaging Fiber Analyser) and also Kajaani FS-200 fibre-length analyser. They measure fibres length, different kinks and their angles. Robertson et al. (1999 ) show correlation between methods FQA and Kajaani FS-200. A relatively new method of fibres width measurement is also SEM (Scanning Electron Microscope) ( Bennis et al. 2010 ). Among devices for analyse of fibres different properties and characteristics, e.g. fibres length and width, fines, various deformations of fibres and percentage composition of pulp mixture is L&W Fiber Tester (Lorentzen & Wettre, Sweden). At every measurement the minimum of 20 000 fibres in a sample is evaluated. On Fig. 7 there is expressed the alteration of fibres average length of softwood pulps during the eightfold recycling at the different drying temperature of pulp fibres.

Influence of recycling number and drying temperature on length of softwood pulps

Influence of recycling number and drying temperature on width of softwood pulps

The biggest alteration were observed after first beating (zero recycling), when the fibres average length decrease at the sheet drying temperature of 80°C about 17%, at the temperature of 100°C about 15.6% and at the temperature of 120°C about 14.6%.

After the first beating the fibres average width was markedly increased at the all temperatures dues to the fibrillation influence. The fibres fibrillation causes the fibre surface increase. Following markedly alteration is observed after fifth recycling, when the fibres average width was decreased. We assume the separation of fibrils and microfibrils from the cell walls dues the separation of the cell walls outside layer, the inside nonfibriled wall S2 was discovered and the fibres average width decreased. After the fifth recycling the strength properties became worse, mainly tear index ( Fig. 3 ).

The softwood fibres are longer than hardwood fibres, they are not so straight. The high value of shape factor means fibres straightness. The biggest alterations of shape factor can be observed mainly at the high drying temperatures. The water molecules occurring on fibres surface quick evaporate at the high temperatures and fibre more shrinks. It can result in the formation of weaker bonds between fibres those surfaces are not enough near. At the beginning of wet paper sheet drying the hydrogen bond creates through water layer on the fibres surface, after the drying through monomolecular layer of water, finally the hydrogen bond results after the water removal and the surfaces approach. It results in destruction of paper and fibre at the drying.

Chemical pulp fines are an important component in papermaking furnish. They can significantly affect the mechanical and optical properties of paper and the drainage properties of pulp ( Retulainen et al. 1993 ). Characterizing the fines will therefore allow a better understanding of the role of fines and better control the papermaking process and the properties of paper. Chemical pulp fines retard dewatering of the pulp suspension due to the high water holding capacity of fines. In the conventional method for characterizing the role of fines in dewatering, a proportion of fines is added to the fiber furnish, and then only the drainage time. Fines suspension is composed of heterogeneous fines particles in water. The suspension exhibits different rheological characteristics depending on the degree of interaction between the fines particles and on their hydration ( Kang & Paulapuro 2006b ).

From Fig. 9 we can see the highest formation of fines were after seventh and eight recycling, when the fibres were markedly weakened by the multiple using at the processes of paper making. They are easier and faster beating (the number of revolution decreased by the higher number of the recycling).

Influence of recycling process and drying temperature on pulp fines changes

The macroscopic level (density, volume, porosity, paper thickness) consists from the physical properties very important for the use of paper and paperboard. They indirectly characterize the three dimensional structure of paper ( Niskanen 1998 ). A paper is a complex structure consisting mainly of a fibre network, filler pigment particles and air. Light is reflected at fibre and pigment surfaces in the surface layer and inside the paper structure. The light also penetrates into the cellulose fibres and pigments, and changes directions. Some light is absorbed, but the remainder passes into the air and is reflected and refracted again by new fibres and pigments. After a number of reflections and refractions, a certain proportion of the light reaches the paper surface again and is then reflected at all possible angles from the surface. We do not perceive all the reflections and refractions (the multiple reflections or refractions) which take place inside the paper structure, but we perceive that the paper has a matt white surface i.e. we perceive a diffuse surface reflection. Some of the incident light exists at the back of the paper as transmitted light, and the remainder has been absorbed by the cellulose and the pigments. Besides reflection, refraction and absorption, there is a fourth effect called diffraction. In other contexts, diffraction is usually the same thing as light scattering, but within the field of paper technology, diffraction is only one aspect of the light scattering phenomenon. Diffraction occurs when the light meets particles or pores which are as large as or smaller then the wavelength of the light, i.e. particles which are smaller than one micrometer (μm). These small elements oscillate with the light oscillation and thus function as sites for new light sources. When the particles or pores are smaller than half of the light wavelength the diffraction decreases. It can be said that the light passes around the particle without being affected ( Pauler 2002 ).

The opacity, brightness, colouring and brilliance are important optical properties of papers and paperboards. For example the high value of opacity is need at the printing papers, but opacity of translucent paper must be lower. The paper producer must understand the physical principles of the paper structure and to determine their characteristics composition. It is possible to characterize nondirect the paper structure. The opacity characterizes the paper ability to hide a text or a figure on the opposite side of the paper sheet. The paper brightness is a paper reflection at a blue light use. The blue light is used because the made fibers have yellowish colour and a human eye senses a blue tone like a white colour.The typical brightness of the printing papers is 70 – 95% and opacity is higher than 90% ( Niskanen 1998 ).

3. Paper ageing

The recycled paper is increasingly used not only for the products of short term consumption (newspaper, sanitary paper, packaging materials e.g.), but also on the production of the higher quality papers, which can serve as a culture heritage medium. The study of the recycled papers alterations in the ageing process is therefore important, but the information in literature are missing.

The recycling is also another form of the paper ageing. It causes the paper alterations, which results in the degradation of their physical and mechanical properties. The recycling causes a chemical, thermal, biological and mechanical destruction, or their combination ( Milichovský 1994 ; Geffertová et al. 2008 ).The effect of the paper ageing is the degradation of cellulose, hemicelluloses and lignin macromolecules, the decrease of low molecular fractions, the degree of polymerisation (DP) decrease, but also the decline of the mechanical and optical properties ( El Ashmawy et al. 1974 ; Valtasaari & Saarela 1975 ; Lauriol et al. 1987a ,b,c; Bansa 2002 ; Havermans 2003 ; Dupont & Mortha 2004 ; Kučerová & Halajová, 2009 ; Čabalová et al. 2011 ).Cellulose as the most abundant natural polymer on the Earth is very important as a renewable organic material. The degradation of cellulosebasedpaper is important especially in archives and museums where ageing in various conditions reduces the mechanical properties and deteriorates optical quality of stored papers, books and other artefacts. The low rate of paper degradation results in the necessity of using accelerating ageing tests. The ageing tests consistin increasing the observed changes of paper properties, usually by using different temperature, humidity, oxygen content and acidity, respectively. Ageing tests are used in studies of degradation rate and mechanism. During the first ageing stages—natural or accelerated—there are no significant variations in mechanical properties: degradation evidence is only provided by measuring chemical processes. Oxidation induced by environmental conditions, in fact, causes carbonyl and carboxyl groups formation, with great impact on paper permanence and durability, even if mechanical characteristics are not affected in the short term ( Piantanida et al. 2005 ). During the degradation two main reactions prevail – hydrolysis of glycosidic bonds and oxidation of glucopyranose rings. As a result of some oxidation processes keto- and aldehyde groups are formed. These groups are highly reactive; they are prone to crosslinking, which is the third chemical process of cellulose decay ( Bansa 2002 , Calvini & Gorassini 2006 ).

At the accelerated paper ageing the decrease of DP is very rapid in the first stages of the ageing, later decelerates. During the longer time of the ageing there was determined the cellulose crosslinking by the method of size exclusion chromatography (SEC) ( Kačík et al. 2009 ). The similar dependences were obtained at the photo-induced cellulose degradation ( Malesic et al. 2005 ).

An attention is pay to the kinetic of the cellulose degradation in several decades, this process was studied by Kuhn in 1930 and the first model of the kinetic of the cellulose chains cleavage was elaborated by Ekenstam in 1936.This model is based on the kinetic equation of first-order and it is used to this day in modifications for the watching of the cellulose degradation in different conditions. Hill et al. (1995 ) deduced a similar model with the

Alterations of DP (degree of polymerisation) of cellulose fibres due to recycling and ageing at the pulp fibres drying temperature of 80°C, 100°C a 120°C.

contribution of the zero order kinetic. Experimental results are often controversial and new kinetic model for explanation of cellulose degradation at various conditions was proposed ( Calvini et al. 2008 ). The first-order kinetic model developed by these authors suggests that the kinetics of cellulose degradation depends upon the mode of ageing. An autoretardant path is followed during either acid hydrolysis in aqueous suspensions or oven ageing, while the production of volatile acid compounds trapped during the degradation in sealed environments primes an autocatalytic mechanism. Both these mechanisms are depleted by the consumption of the glycosidic bonds in the amorphous regions of cellulose until the levelling-off DP (LODP) is reached.

At the accelerated ageing ofnewspaper ( Kačík et al. 2008 ), the cellulose degradation causes the decrease of the average degree of polymerisation(DP). The DP decrease is caused by two factors in accordance with equation

DP = LODP + DP01.e -k1.t + DP02.e -k2.t ,

where LODP is levelling-off degree of polymerisation. There is a first factor higher and quickdecreasing during eight days and a second factor is lower and slow decreasing and dominant aftereight days of the accelerating ageing in the equation. The number of cleavaged bonds can be welldescribed by equation

DP 0 /DP t – 1 = n 0 .(1-e -k.t ),

where n 0 is an initial number of bonds available for degradation. The equation of the regression function is in accordance with Calvini et al. (2007 ) proposal, the calculated value (4.4976) is in a good accordance with the experimentally obtained average values of DP 0 a DP 60 (4.5057). The DP decreased to cca 38% of the initial value and the polydispersity degree to 66% of the initial value. The decrease of the rate constant with the time of ageing was obtained also by next authors ( Emsley et al. 1997 ; Zervos & Moropoulou 2005 ; Ding & Wang 2007 ). Čabalová et al. (2011 ) observed the influence of the accelerated ageing on the recycled pulp fibres, they determined the lowest decrease of DP at the fibres dried at the temperature of 120°C ( Fig. 10 ).

The simultaneous influence of the recycling and ageing has the similar impact at the drying temperatures of 80°C (decrease about 27,5 %) and 100°C (decrease about 27.6%) in regard of virgin pulp, lower alterations were at the temperature of 120°C (decrease about 21.5%). The ageing of the recycled paper causes the decrease of the pulp fiber DP, but the paper remains good properties.

4. Conclusion

The recycling is a necessity of this civilisation. The paper manufacturing is from its beginning affiliated with the recycling, because the paper was primarily manufactured from the 100 % furnish of rag. It is increasingly assented the trend of the recycled fibers use from the European and world criterion. The present European papermaking industry is based on the recycling.

The presence of the secondary fibres from the waste paper, their quality and amount is various in the time intervals, the seasons and the regional conditions. It depends on the manufacturing conditions in the paper making industry of the country.

At present the recycling is understood in larger sense than the material recycling, which has a big importance from view point of the paper recycling. Repeatedly used fibres do not fully regenerate their properties, so they cannot be recycled ad anfinitum. It allows to use the alternative possibilities of the paper utilisation in the building industry, at the soil reclamation, it the agriculture, in the power industry.

The most important aim is, however, the recycled paper utilisation for the paper manufacturing.

Acknowledgments

This work was financed by the Slovak Grant Agency VEGA (project number 1/0490/09).

• 11. CEPI (Confederation of European Paper Industries). 2006 Special Recycling 2005 Statistics- European Paper Industry Hits New Record in Recycling. 27.02.2011, Available from: http://www.erpa.info/images/Special_Recycling_2005_statistics.pdf
• 12. CEPI (Confederation of European Paper Industrie). 2010 Annual Statistic 2009. 27.02.2011, Available from: http://www.erpa.info/download/CEPI_annual_statistics%202009.pdf
• 18. European Declaration on Paper Recycling 2006 2010 , Monitoring Report 2009 (2010), 27.02. 2011, Available from: http://www.erpa.info/images/monitoring_report_2009.pdf

© 2011 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike-3.0 License , which permits use, distribution and reproduction for non-commercial purposes, provided the original is properly cited and derivative works building on this content are distributed under the same license.

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JSmol Viewer

Chemical recycling of silicones—current state of play (building and construction focus).

Graphical Abstract

1. Introduction

2. applications of silicones in the building and construction industry and the importance of recycling for this sector, 3. challenges for companies in the silicone recycling business.

• Diversity of industries, applications, and geographies;
• Diversity of product forms;
• Chemical complexity due to wide variety of formulations;
• Durability of silicone materials.

3.1. Diversity of Industries and Applications

3.2. diversity of product forms, 3.3. chemical complexity due to large variety of formulations, 3.4. durability, 4. dynamics of the regulatory framework, 5. value chain of silicones (pdms)—sources and sinks, 6. silicone recycling methods, 6.1. mechanical (physical) recycling, 6.2. chemical recycling.

• It can be applied to mixed, heterogeneous waste streams and does not require the same level of careful sorting and cleaning because it allows for the removal of contaminants.
• It enables product upcycling—the transformation of silicone waste into higher-value products (as defined in [ 52 ])—because there are no restrictions on the new, value-added products that can be created through chemical recycling.

7. Characteristics of an Ideal Process for the Chemical Recycling of Silicone

• High yield;
• Easy scalability from laboratory to pilot to commercial scale.
• High quality output (product is free of SiH groups, multifunctional (T and Q) units, and hexamethyldisiloxane (MM)).
• High degree of adaptability (in terms of the types of silicones present in the waste).
• Low cost of reactor components (reaction components are non-corrosive or only slightly corrosive).
• Low cost of catalyst materials (in terms of purchase price, reuse, and quantity used).
• Maximum recyclability (ability to recycle most byproducts, such as fillers or catalysts, in separate processes), resulting in less waste.
• Good environmental compatibility and low health risk of the chemicals used and the resulting wastes (not corrosive, toxic, or highly flammable).
• Mild reaction conditions (ideally at room temperature and normal pressure), but fast conversion (low energy input).

8. Silicone Depolymerization Mechanisms

9. depolymerization chemistries, 9.1. depolymerization by acidolysis with protonic acids, 9.2. depolymerization by strong inorganic bases, 9.3. depolymerization by aminolysis, 9.4. depolymerization by alcoholysis, 9.5. depolymerization by halogen-containing cleavage agents, 9.6. depolymerization by acid anhydrides, 10. current industrial practice of chemical silicone recycling, 11. examples of silicone recycling and upcycling, 11.1. silicone-coated fabrics, 11.2. silicone sealants, 12. implications of potential regulation of silicone cyclomers d 4 to d 6 as persistent organic pollutants (pops) under the stockholm agreement, 13. market situation: silicone recycling at an inflection point.

• Investment de-risking: Corporate investment decisions often favor known risks (optimization of existing processes or business units) over unknown risks (new capabilities and processes); by partnering with SMEs, disruptive business models can be developed and tested with low financial risk.
• Increased profitability and flexibility: The inability of large corporations to internally develop cost-effective services and low-volume start-up operations (waste collection, sorting, and recycling) due to high labor and overhead costs and organizational inflexibility makes partnering with SMEs that offer low-cost structure and attractive organizational flexibility.

14. Key Challenges of Scaling Up the Recycling of Silicone Chemicals

15. outlook, 16. conclusions, author contributions, acknowledgments, conflicts of interest.

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Wolf, A.T.; Stammer, A. Chemical Recycling of Silicones—Current State of Play (Building and Construction Focus). Polymers 2024 , 16 , 2220. https://doi.org/10.3390/polym16152220

Wolf AT, Stammer A. Chemical Recycling of Silicones—Current State of Play (Building and Construction Focus). Polymers . 2024; 16(15):2220. https://doi.org/10.3390/polym16152220

Wolf, Andreas T., and Andreas Stammer. 2024. "Chemical Recycling of Silicones—Current State of Play (Building and Construction Focus)" Polymers 16, no. 15: 2220. https://doi.org/10.3390/polym16152220

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Comparison of reinforcement fibers in 3D printing mortars using multi-criteria analysis

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• Published: 05 August 2024

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• Sara Alonso-Cañon   ORCID: orcid.org/0000-0003-0080-4888 1 ,
• Elena Blanco-Fernandez   ORCID: orcid.org/0000-0002-7010-2649 1 ,
• Daniel Castro-Fresno   ORCID: orcid.org/0000-0001-5658-3901 1 ,
• Adrian I. Yoris-Nobile   ORCID: orcid.org/0000-0001-9332-2372 1 &
• Laura Castanon-Jano   ORCID: orcid.org/0000-0002-5968-2726 1  

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3D concrete printing (3DCP) has developed rapidly in recent years, with a relatively high amount of mortars emerging apt for 3D printing. Some of these mortars include fibers to improve their strength. Despite mechanical properties having been quite well studied, there still is missing information on cost, printability, and environmental impacts. The objective of this research is to select the best mortars with fibers considering four criteria: printability, mechanical strength, and economic and environmental impact applying a multi-criteria decision-making analysis (MCDMA). Seven types of fibers with different dosages were assessed in the reinforced mortars: zylon, aramid, carbon, glass, cellulose, textile, and polypropylene. AHP method and equal weights were used as ponderation techniques of the criteria while WASPAS and TOPSIS methods were used to calculate the rankings of the MCDMA. Printability was measured through rheological tests using a rotational rheometer, mechanical strength through flexural tests at 28 days based on EN 196–1, and cost just considering the materials and environmental impact through a life cycle assessment (LCA). The results showed that 13-mm-long glass fibers with a content of 0.1% were the best alternative, closely followed by the mortar with 6 mm cellulose fibers with a content of 0.05%. For the best option (G13;0.1), the increments in the printability index, flexural strength, cost, and LCA were − 14.37%, 16.70%, 5.88%, and 2.86%, respectively. It can also be concluded that high elastic modulus fibers (zylon and aramid), although able to increase significantly the flexural strength (up to 30% in the case of zylon), prevent them from being an optimal solution due to their high cost.

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1 Introduction

Engineering and construction are in continuous development seeking to improve their productivity, incorporating new technologies and techniques. In recent years, the process of additive manufacturing (AM) or 3D concrete printing (3DCP) has had a strong development. This construction system has some advantages over traditional construction, such as the production of complex shapes, the automation of the process, the reduction of waste materials, and the reduction of construction times.

The incorporation of fibers, which are already used in traditional construction, has begun to be tested in the field of concrete 3D printing by some authors using steel [ 1 , 2 ], carbon [ 3 , 4 ], glass [ 5 , 6 ], PVA [ 7 , 8 ], or polypropylene [ 9 , 10 ], fibers with the aim of improving the mechanical properties (flexural strength, toughness, impact resistance, plastic shrinkage cracking, etc.), and/or rheological behavior (dynamic viscosity, shear stress, etc.). However, there are no studies using recycled fibers (textile, cellulose) or ultra-high tensile modules (aramid, zylon). In the particular case of cellulose, some authors have already studied their potential application in precast concrete, because of its low cost and environmental impact, adequate mechanical performance, and sufficient durability even when exposed to weathering conditions such as sewage water [ 11 ] or carbonation and freeze–thaw cycles [ 12 ]; however, there are not yet studies on 3DCP.

Mechanical strength has been largely studied in fiber-reinforced 3DCP by several authors, as reported by Alonso-Cañon in a review paper [ 13 ]. The conclusion of this study revealed that adding fibers can increase up to 30% of the flexural strength of 3DCP with respect to a 3DCP without fibers while there are no clear conclusions regarding advantages or disadvantages in compressive strength.

In relation to cost, several studies have attempted to estimate and compare costs associated with reinforced 3D concrete printing (3DCP). The scarcity of cost information and inconsistencies in reporting make it challenging to draw definitive conclusions. Noteworthy findings include Inozemtcev and Duong’s comparison, suggesting potential savings of 30–50% in building costs with 3DCP due to reduced material and machinery hours [ 14 ]. Kreiger et al. compared various construction technologies, but their estimate of concrete costs at $144 USD/m 3 lacked detailed calculations [ 15 ]. García de Soto et al. demonstrated advantages for 3DCP in building non-standard shapes, with costs varying between straight and curved layouts [ 16 ]. Nerella et al. reported a 70% higher cost for 3DCP compared to conventional methods using high-performance concrete [ 17 ]. Otto et al. assumed a 30% higher cost for 3DCP without explicit calculations [ 18 ]. In Han et al.’s study, the environmental and cost analysis undertaken indicated higher costs for 3DCP in building a silo compared to conventional concrete [ 19 ]. Abdalla et al. carried out an environmental and economical analysis of a house using conventional methods vs 3DCP; however, concrete cost per m 3 data was not shown, hindering a direct comparison between conventional and 3DCP methods [ 20 ]. Weng et al. assumed a consistent concrete price for both methods in an environmental and productivity assessment of a concrete bathroom unit [ 21 ], despite 3DCP generally having a larger amount of cement, disregarding also differences in compressive strength. Yoris-Nobile et al. compared different mortars for 3D printing, providing cost per ton of material, with costs ranging from 44.80 to 184.18 €/T, depending on the type of mortar used; however, none of the mortars included fibers in the mix [ 22 ].

Environmental performance has been considered a key advantage for concrete 3D printing technologies by many authors, basically based on the topological optimization of the shapes and the elimination of formworks. In [ 23 ], a multi-criteria decision-making analysis (MCDMA) applying the MIVES and Delphy techniques is proposed and applied to the first 3DCP bridge in the world built in 2016 by Acciona to assess its sustainability performance. The material used was a concrete with 500 kg cement/m 3 concrete that incorporated steel fibers and used D-Shape 3D printing technology, and also has post-tensed cables. In this study, greenhouse emissions, energy consumption, and material consumption were considered part of environmental indicators assigning weights to each of them (among other factors) with the Delphy technique. However, greenhouse emissions and energy consumption are somehow correlated. Besides, it is more adequate to consider the global warming potential to deal with greenhouse gas emissions. In [ 19 ], an environmental and economic assessment on 3D-printed buildings was carried out. The aim was to compare 3 types of 3DCP vs the other 3 types of concrete for mold casting. In the 3DCP, around 440–455 kg of cement per m 3 of concrete was added, while in the mold casting, they added between 320 and 345 kg/m 3 . In this study, fibers were not added to the concrete. Furthermore, mechanical strength was not considered either, despite the differences in cement content between the C3DP and standard concrete. Instead, it was assumed that both had 30 MPa of nominal compressive strength at 28 days (C30). Thus, environmental comparison of structural concretes without considering their strength is not fully adequate, since the rationale “the more the strength, the least amount of concrete required, the least the cost, the less the environmental impact” should be assumed. In the study of Alhumayani et al. [ 24 ], a comparison of 4 alternatives (3DCP, cob-printing, concrete pour casting, cob blocks) of building a structural wall was carried out, without using fibers in any case. The wall thickness and amount of material used of each alternative changed; nevertheless, no data about the bearing capacity of the wall was provided. Therefore, the conclusions regarding the LCA are not fully comparable, since each alternative could have a different bearing capacity. In [ 25 ], similarly to [ 24 ] also performed a comparison of a bearing wall using 4 alternatives (1 with standard concrete, 3 with 3DCP with or without rebars and different mixes), using fibers in some of the concrete mixes. Again, wall-bearing capacity was not provided; therefore, LCA results are not fully comparable.

The rheological properties play a crucial role in the constructability and printability of cement mortars. To assess these properties, various authors have conducted tests using rotational rheometers, enabling the determination of both yield stress and viscosity values. Jayathilakage conducted a comparative analysis of different rheometers, confirming that torque rheometers yield accurate rheological property values [ 26 ]. In another study, Chen introduced retarder dosages into mortar mixtures and observed a decrease in yield stress values as the dosage increased, resulting in yield stress values ranging from 450 to 750 Pa [ 27 ]. Additionally, Kolawole and Banfill investigated the impact of superplasticizer incorporation, revealing a significant reduction in yield stress values (ranging from 550 to 750 Pa), notably lower than the 1100 Pa observed in the control mixture [ 28 , 29 ]. Conversely, Chen, in his research involving the addition of bentonite to mixtures, found that an increase in bentonite content led to an elevation in yield stress values [ 30 ]. Finally, Russel, in a rheological analysis focused on printable concretes, emphasized the critical need to minimize initial yield stress values for these materials [ 31 ].

Overall, comparative analyses that consider not only mechanical properties of mortars including fibers, but also their suitability to be printed with a real 3D printer, cost, and environmental impact at the same time, have not yet been carried out so far. This comparison is fundamental because it would provide practical conclusions for 3DCP users who aim to include fibers in their mixes, and that need to take a decision regarding the most adequate type of fiber in order to avoid excessive costs, undesired environmental impact, or printability problems, and what is more, just decide not to use fibers, not to mention new fibers such as aramid, zylon, cellulose, and recycle textile.

2 Methodology

2.1 overall procedure.

The objective of this paper, based on the gaps found in the state of the art as previously stated, is to determine the best reinforcement fibers and dosages in 3D-printed mortars through a multi-criteria decision-making analysis (MCDMA). These mixtures will be a total of 36 (35 with fibers plus a control mortar without them), in which different types of fibers and percentages of fibers will be combined. Four criteria were selected for the MCDMA: printability, mechanical strength, material cost, and environmental impact, for which different laboratory tests were carried out and a life cycle assessment (LCA) was performed. The Analytic Hierarchy Process (AHP) method and equal weights were used to assign the criteria weights, and the Technique for Order of Preference by Similarity to Ideal Solution (TOPSIS) and Weighted Aggregated Sum Product Assessment (WASPAS) methods were used to assess the MCMDA scores and, thus, rankings. Figure  1 represents the graphical abstract of the whole process carried out.

Graphical description of the work methodology

Firstly, for this purpose, the rheological and mechanical characteristics of the mortars will be analyzed in the laboratory. Secondly, a multi-criteria decision-making analysis will be performed, employing firstly two weighing scenarios (AHP method and equal weights) and two ranking methods (WASPAS and TOPSIS).

Aramid, carbon, glass, cellulose, textile, polypropylene, and zylon fibers were used. In the laboratory, the fiber content that helped to maintain good printability was determined for each type of fiber. Steel fibers were discarded since preliminary tests show that they could get clogged in the screw and, as a result, damage the 3D printer. As the number of fibers increases, workability is lost until the filament starts to get choppy or plugs are produced in the extrusion nozzle. Therefore, the percentages used vary between 0.05 and 0.3. Then, the laboratory tests of flexural strength, compression, and rheology were carried out. In the second part of the study, an MCDMA analysis was proposed, for which two methods were used: WASPAS and TOPSIS. The AHP method and equal weights were used to calculate the relative weights of the criteria. This made it possible to rank and obtain the best fibers for 3D printing. Figure  1 shows a graphical description of the methodology carried out.

2.2 Materials and dosages

This study aims to compare and obtain the best 3D printing dosages reinforced with fibers, based on 4 different criteria, such as resistance, cost, environmental impact and printability. Therefore, a total of 54 samples were prepared. A Cement type III/B 32.5 N-SR, which has a clinker content of 31% and 66% blast furnace slag was used. As a fine aggregate, sands of natural origin were used. As a fine material, fly ash was used, with a degree of crystallinity of 35% and a loss on ignition of 3.4%. To add cohesion to the mixture, a small percentage of kaolin was added as an addition. A workability modifier has also been used to give the mixtures the optimum consistency for printing. The selected superplasticizer was Mastersure 950 from BASF.

Finally, seven different types of fibers have been incorporated into the mixtures, whose properties are shown in Table  1 . These fibers are aramid, carbon, cellulose, polypropylene, textile, glass, and zylon. Some of these fibers have been supplied in different lengths, varying from 3 to 30 mm.

In terms of the percentages of fibers that have been incorporated, in conventional concrete are below 1%, because a high percentage of fibers significantly reduces the workability of the mixtures [ 32 , 33 ]. The 3D printing concrete should also maintain low percentages of fibers, so that the mixture can maintain its workability and no plugs are produced in the extrusion nozzle. In this study, the fibers were incorporated in different percentages (0.05; 0.075; 0.1; 0.2; and 0.3) depending on the printability of the mixtures. These percentages vary due to the fact that some of the mixtures with higher ratios begin to cut or to remain blocked in the extrusion nozzle. With the above-mentioned types of fiber and their different percentages, the 35 alternatives that were later used for the multi-criteria analysis were proposed. The fibers used in the study are shown in Fig.  2 .

Fibers used: a aramid fibers non-bundled short cut. b Aramid fibers bundled dipped chopped. c Glass fibers. d Zylon fibers. e Polypropylene fibers. f Carbon fibers. g Textile fibers. h Cellulose fibers

For the study, a reference mixture was used, composed of the aforementioned materials, cement, fly ash, limestone aggregate, kaolin, and superplasticizer, in which the different types of fibers were incorporated in different proportions.

Table 2 shows the dosages of the mixtures according to the percentage of fibers. All dosages are ratios of the cement weight, except the fiber content, which is reported in a fraction of the mixture.

2.3 Mixture preparation

A planetary mixer with a capacity of 30 l was used to prepare the mixtures. It has three rotation speeds: 142 (slow), 234 (medium), and 429 rpm (fast), but in our production, we only used the first two. First, the dry materials, cement, fly ash, kaolin, and aggregates, were mixed for 15 s at slow speed. Then, the water was gradually added and mixed for 2 min with the dry materials. Subsequently, to achieve the right consistency, the plasticizer was added and mixed for one and a half minutes. Later, the fibers were added gradually, to obtain a good dispersion and mixed for half a minute. Finally, to complete the mixing process, the mixer speed was changed to medium and it started to mix for two more minutes.

For the testing of the study, a Delta-type printer has been used, model “WASP 3MT,” which is based on EMS technology and that helps to develop the elements by extrusion. This model has two different parts: the control panel and the printing system. The control panel allows to adjust the different variables, such as the printing speed, height, and coordinates where the nozzle starts printing and the diameter of the layers. The printing system, in turn, is composed of three articulated arms disposed in a triangular configuration corresponding to the x, y, and z axes. It also has a hopper, inside which is an endless screw, connected to an electric motor, which allows the material to move to the nozzle while it is turning. The nozzle, which was modified and made of TPU, has a circular section of 20 mm in diameter.

2.4 Laboratory tests

For the preparation of this study, the mechanical and rheological characterization of the mixtures was carried out.

2.4.1 Preliminary printing trials

3D printing by extrusion requires specific rheological properties of the mixtures in the fresh state. These must have good workability and flowability, allowing the material to be extruded through the screw and nozzle of the 3D printer placing a continuous filament. But once the layers have been extruded, they must have sufficient self-supporting capacity to maintain their shape and support the weight of the upper layers. Thus, preliminary printing trials were performed in order to realize visual analyses of how certain mortars could be more or less suitable for printing purposes (Fig.  3 ). The printing parameters used for carrying out the preliminary tests of the mortars were the same: 20 mm nozzle diameter (16 mm software input to force overlap), 8 mm layer height, 70 mm/s maximum head speed, 300 rpm screw speed. As it can be seen in Fig.  3 , two parallel filaments were printed to form the vertical walls.

Preliminary printing trials

2.4.2 Rheological tests

A correlation between printability (based on visual observations during the printing trials) and the parameter yield stress*viscosity is proposed, the so-called printability index. Therefore, good printability (which means a combination of continuity of the extruded filament and self-bearing capacity of the filaments) is correlated, somehow, with a low value of the printability index.

For rheological characterization, a torque rheometer was used. It consists of three parts: an agitator, a cylindrical container, and a cross-shaped paddle. The agitator, with a range of rotation speeds between 10 and 2000 rpm, is connected to a computer to collect the torque values corresponding to the different speeds. A cross-shaped paddle with a height of 5 cm and a radius of 2.5 cm will be placed on the vertical axis of the agitator. Lastly, a cylindrical steel container with an inner diameter of 8 mm and outer diameter of 8.8 cm was adjusted to the characteristics of the materials used [ 34 , 35 , 36 , 37 ].

The rheological behavior of fresh concrete is considered to correspond to the Bingham model, governed by the following equation: \(\tau ={\tau }_{\text{o}}+\upeta \dot{\upgamma }\) , where \({\tau }_{\text{o}}\) is the yield stress and \(\eta\) refers to the dynamic viscosity of the mixture. In order to perform the test, speeds, which are gradually decreasing, are set, while the torque data obtained are recorded. With these data, the following equations are applied to obtain the shear rate-shear stress parameters [ 38 , 39 ].

To obtain the shear rate, \(\dot \gamma\) :

where M is the torque, Ω is the rotational speed, \({\tau }_{b}\) and \({\tau }_{\text{c}}\) are the shear stresses on the inner and outer cylinders, Rc is the radius of the cylinders, and Rb and h are respectively the radious and height of the blade.

To calculate the shear stress, \(\uptau\) :

With the obtained parameters of shear stress and shear rate, a curve is plotted, which gives the values of yield stress and viscosity.

2.4.3 Mechanical tests

For the mechanical characterization, flexural and compression tests were carried out, using prismatic specimens of 40 mm × 40 mm × 160 mm following the EN 196–1 standard (Fig.  4 ).

Prismatic samples after flexural test (left) and compression test (right). Mortar with PP fibers

Toughness was not studied, although the influence that fibers have in making concrete tougher (more energy at failure with respect to concrete without fibers) is well known, since it is believed that there will be somehow a correlation between toughness and flexural strength. Another reason of not measuring toughness is that there is no clear consensus on how to measure the area under the stress–strain curve, either in a tensile test or in a flexural test. In [ 40 ], toughness is measured through the so-called performance level which represents the energy absorbed up to the peak load in a tensile test. In [ 41 ], toughness is measured in a flexural test until a deformation that represents 1/150 of the span. In [ 42 ], a withdrawn standard with no replacement, toughness is measured through the “toughness index,” which is a relative measure of an area defined for different values of deformation. Therefore, depending on how toughness is measured, results could change. For the purpose of carrying out an MCDMA, it was considered more simple and robust to take only the flexural resistance of prismatic samples according to EN 196–1 [ 43 ].

It was also considered relevant to measure the compression strength of fiber-reinforced mortars because some authors have pointed out that the use of fibers might have a negative influence on their compressive strength while others state that it could be the same or even improve [ 44 , 45 ]. Since there is no general consensus, it was considered relevant to measure it in this research.

Even though some studies pointed out that the printing process might affect the anisotropy [ 46 ], not all authors agree with the degree of influence of these input variables in the final strength, since it depends on the printing path, the ratio between nozzle size and fiber length, the rheological properties of the fresh mortar, and the setting time of mortar, among others. Since it is quite common in 3D printing practice that the path pattern alternates printing directions layer by layer, anisotropy loses its influence somehow. This effect of bonding filaments in the same 3D printing plane takes more importance especially when using more fluid mixes, where the extruded filaments can bond one to another without leaving large gaps between them which is the main responsibility of anisotropy. Furthermore, if mortar has a sufficient setting time window, bonding between layers can occur without generating anisotropy between the printing plane and its perpendicular direction, or even between 3D-printed samples and cast samples [ 47 ]. The further anisotropy that fibers might introduce in the 3D-printed mortar also depends mainly on the ratio between nozzle size and fiber length, being the nozzle diameter dependent on each particular 3D printer used.

Therefore, in order to simplify the mechanical tests, prismatic samples were fabricated with cast mortar. It is assumed then that even though the mechanical properties of 3D-printed samples might vary a little bit with respect to cast samples, the relative differences between dosages would not be substantially altered. To calculate the flexural strength, the three-point loading method is used, with a loading rate of 50 N/s until failure, following the EN 196–1 standard. Once the maximum force \({F}_{\text{f}}\) is measured, the flexural strength is calculated with the following equation:

where \({F}_{\text{f}}\) is the flexural strength (MPa), l is the distance between supports (mm), and b is the side of the square section of the prism (mm).

To perform the compression test, the half-prisms broken in the flexural strength test are used. A load is applied uniformly at a loading rate of 2400 N/s, during the whole time of application of the load until the breakage. Once the maximum load to failure is obtained Fc, the compressive strength is calculated with the following equation:

2.5 Life cycle assessment (LCA)

An environmental analysis of the different mixtures developed was carried out, using the LCA methodology, based on ISO 14040:2006 [ 48 ] and 14044:2006 [ 49 ]. A gate-to-gate approach was selected by taking only into account the material production stage. The reference unit was 1 t of mortar. Calculations were performed using the SimaPro software.

Inventory values for the electricity of the mixture and 3D printer, cement, limestone aggregate, superplasticizer, kaolin, and water were collected from the Ecoinvent v3.10 cut-off database [ 50 ] as listed in Table  3 . Concerning fly ash, as it is considered a waste material resulting from the electricity production in coal power plants, no environmental burdens have been considered [ 51 ]. However, for their use in mortar dosages, they need milling and grinding, drying, and transport processes, which involve energy consumption. As there is no data available on the consumption of these processes, an impact factor of 0.027 kg CO2-e/kg proposed by Turner [ 52 ] was applied. As for carbon, glass, cellulose, textiles, and polypropylene fibers were also found in the Ecoinvent database. On the other hand, aramid and zylon fibers were not found in that database. The aramid was extracted from the GaBi V9.1 database, while in the case of zylon fibers, Cao et al. [ 53 ] presented in their study the components of the material. Finally, the electricity consumption during the mixing and printing processes was measured at the laboratory, resulting in 9.4 kWh and 18.5 kWh.

The EF 3.0 method was selected to transform the resources and the emissions obtained during the inventory phase into impacts. This method has been developed by the European Commission, to establish a common European methodology for the development of LCA. The categories of impacts and their units are shown in Table  4 .

The normalization factors used in this work are those included in the Environmental Footprint (EF) method that are based on the study by [ 54 ], where global emissions and resource uses were collected and characterized. For the calculation of the normalization factors (NFs), the characterization factors used in the International Reference Life Cycle Data System (ILCD) and the EF methods were applied.

As for the weighting factors, the Joint Research Center (JRC) recommended a set of weighting factors for the European Footprint method [ 55 ]. For the development of these weighting set, two different methods were adopted by JRC. The first one consisted in a panel-based approach, involving two target groups: general population and experts in the LCA field. The second method involved a hybrid evidence-based and expert-judgement approach. A final weighting set was recommended by the combination of the results from both methods and incorporating robustness factors.

However, the results of the LCA must be fed into the multi-criteria analysis, so we need to transform these category indicator results into a single value of environmental impact for each dosage analyzed. For this, two more steps are carried out: normalization and weighting. Normalization helps to convert the results of the environmental categories into neutral global units, to determine the relative importance of the results by dividing by a reference value. Meanwhile, weighting converts, by means of numerical factors, the normalized values so that they can be added together to obtain a single result. The normalization and weighting values used are those proposed in the EF 3.0 method and are shown in Table  4 .

Another criterion established for the MCDM analysis was the total cost of each mixture (in euro/ton). This was computed for each alternative by summation of individual cost of cement, aggregates, fly ash, kaolin, superplasticizer, and fibers. The individual costs of each of the materials that make up the different mixes are shown in Table  5 . The costs of each of the fibers were provided by the different suppliers, with the exception of the textile fibers, which are considered free of cost since they are a waste from the textile industry, which would be put to a new use.

2.7 Multi-criteria decision-making analysis (MCDMA)

The selection of the optimal fibers and percentages to incorporate into 3D printing mixtures involves several factors, that can be contradictory in many cases, such as printability and strength. The MCMD allows rational decisions to be made on the basis of the different criteria that are important for the choice of fibers. In this study, we use two different methods, WASPAS and TOPSIS, to obtain the ranking of alternatives. For assigning weighs, two methods were considered: AHP and equal weighs.

The four criteria considered were (1) printability, (2) mechanical resistance, (3) economic, and (4) environmental impact. The indicators that have been used to assess those criteria were, respectively, (1) yield stress*dynamic viscosity (Pa^2*s), (2) flexural strength (MPa), (3) cost of materials (€/T), and (4) points of an LCA.

2.7.1 WASPAS method

The first method to be used for decision-making will be the Weighted Aggregated Sum Product Assessment (WASPAS). This has been developed by Zavadskas et al. [ 56 ] and is a combination of other two MCDMs known, namely the Weighted Product Model (WPM) and the Weighted Sum Model (WSM). By combining the two models, it has become a very robust method [ 57 ], which has been employed in numerous engineering fields. The steps of the WASPAS method are described below:

Step 1: Definition of the decision-making problem and obtaining the decision matrix. The matrix has the following form:

\(X=\left[x_{ij}\right]=m\times n\) , where m represents the defined alternatives, and n the selected criteria.

Step 2: Normalization of the decision matrix. For these, the comparative importance of the criteria is defined, identifying beneficial and non-beneficial criteria, using the following equations:

Step 3: Determine the total relative importance of each alternative. For this purpose, the Weighted Sum Model (WSM) is used, defined by the following equation:

where \({w}_{j}\) is the weight of each criteria, which is obtained with the weighing method (in our case AHP method or equal weights).

Step 4: Determine the total relative importance of each alternative, using now the Weighted Product Model (WPM):

Step 5: Apply a joint criterion. For this purpose, both methods are combined by means of the following equation:

where \(\lambda\) is a coefficient that linearly combines both models. The value that is usually used and that will be used in this study is 0.5, since it gives the same weight to both models, although this can be varied.

2.7.2 TOPSIS method

The TOPSIS method, developed by Hwang et al. [ 58 ], is one of the most common and widely used methods for multi-criteria decision-making. This is based on the distances of each of the alternatives to the ideal solutions, both positive and negative [ 59 ]. The positive ideal solution maximizes the benefit response and minimizes the cost response, while the negative ideal solution is the other way around. Therefore, the best alternative will be the one closest to the ideal positive solution and furthest from the ideal negative solution. The steps of the TOPSIS method are described below:

Step 1: Establish the decision-making matrix, which has the following form X  = [

\({x}_{ij }]\) = m  ×  n , as in the WASPAS method.

Step 2: Normalize the decision matrix with the following equation:

where \({r}_{ij}\) is the normalized criteria rating.

Step 3: Obtain the weighted normalized decision matrix, with the following equation:

where [ \({v}_{ij}\) ] is the weighted normalized matrix and \({w}_{j}\) is the weightage of each criterion.

Step 4: Calculate the positive (PIS) and negative ideal solutions (NIS). For this purpose, the following equations are used:

where \(J\) is a beneficial criterion and \(J'\) is a non-beneficial criterion.

Step 5: Calculate the Euclidean distance of each alternative to the positive ideal solution and to the negative ideal solution, using the following equations:

Step 6: Calculate the relative closeness of each alternative to the ideal solution, using the following equation:

2.7.3 AHP method

When using multi-criteria decision-making methods, calculating and assigning the relative weights of the criteria is an important part of the process to obtain an optimal result. For this purpose, this study applied the AHP method [ 60 ], based on subjective weighting, which allows obtaining information based on the knowledge and experience of experts. Criteria weighting was performed using pairwise comparison matrices that are based on the experts judgement, for which a survey is designed that is sent individually to each expert to collect their opinion. The data collected per each expert was converted into a pairwise comparison matrix form [ X \(]\) = [ \({x}_{\text{ij }}]\) = n  ×  n , where n corresponds to the number of assessment criteria considered and satisfying the expression \({x}_{\text{jk}}\) × \({x}_{\text{kj}}\)  = 1, where \({x}_{\text{jk}}\) is one of the entries of the matrix and \({x}_{\text{kj}}\) its reciprocal value. The numerical scale from one to nine was used to measure the relative importance between two criteria.

The consistency of each judgment was checked by calculating the Consistency Ratio (CR):

R.C.I. is the Random Consistency Index, whose values are defined in Table  6 . C.I. is the consistency index, which is calculated with the following equation:

where \({\lambda }_{\text{max}}\) is the maximum eigenvalue of the matrix, and when \({\lambda }_{\text{max}}\) = n the matrix is consistent and starts to be inconsistent when this value decreases.

3 Results and discussion

3.1 rheological tests.

The yield stress values of the different mixtures range between 100 and 700 Pa and those of viscosity between 15 and 25 Pa*s. Table 7 shows the values of yield stress, viscosity, and printability index.

In relation to yield stress, it has been found that both fiber type and fiber content significantly affect yield stress (Table  8 (a)). These findings are based on an ANOVA test to identify which of these 3 input variables (fiber type, fiber length, fiber dosage) have a significant influence on the response (yield stress). In the case of fiber type the p -value was 0.005 and for fiber dosage the p -value was 0.000. Fiber length did not have a significant influence over yield stress ( p  = 0.608). Regarding viscosity, the same ANOVA test was carried out finding that the three variables had significant influence over the viscosity, resulting in p -values of 0.001, 0.017, and 0.003 for fiber type, fiber length, and fiber dosage, respectively. For the printability index, defined as the yield stress*viscosity, the ANOVA test detects significant influence on fiber type ( p -value:0.000) and fiber dosage ( p -value:0.001), but not on the fiber length ( p -value:0.160).

As for the yield stress values, it can be observed that they increase as the fiber content increases in all the mixtures analyzed (Table  8 (b)). Therefore, the force to initiate flow that the printer endless screw has to perform becomes higher as the fiber content increases. This was also observed in the laboratory during the preliminary printing trials, since as the fiber content increased, the mixtures printed in worse conditions, in some cases beginning to cut or to remain blocked in the nozzle. The higher fiber contents that were analyzed and of which the rheological analysis was carried out were at the limit that allowed the mixtures to print correctly, without the problems mentioned above. Regarding the viscosity value, this has a tendency opposite to that of yield stress, since as the fiber content increases, the mixtures reduce the viscosity value. These variations in viscosity are not as noticeable as in the case of yield stress for each type of fiber. As a result, the printability index increases with the increase of dosage, as it can be seen in the regression model generated.

3.2 Mechanical tests

Figure  5 (top) shows the flexural strength results obtained at 28 days for the mixtures with the different types and percentages of fibers. Values ranged from 9.49 to 11.70 MPa, while the control sample (no fibers) achieved 9.04 MPa, showing that flexural strength significantly increases with the addition of fibers, reaching a maximum increase of 30% with zylon fibers. Other fibers, such as carbon, glass, or polypropylene, also obtained high strength increases. On the other hand, recycled textile fibers were the worst performers in terms of strength.

Regarding the flexural strength values, it can be concluded that the best results have been obtained with the incorporation of 1% of zylon fibers, with an increase over the control mixture of 30%. This was followed by mixtures incorporating 0.1% of carbon fibers, both 6 and 25 mm in length, which achieved increases of 24%. In addition, both glass fibers with a content of 0.3% and polypropylene fibers with 0.05% have reached increases exceeding 20%. Finally, textile fibers with any of their contents are the worst performers, as they did not achieve an increase of 10%.

Hambach [ 61 ] performed a flexural test in 3D-printed samples reinforced with carbon fibers by applying a force in the normal direction of printing planes, reporting 13.9 MPa of flexural strength vs 11.4 MPa of control samples without fibers, which represents a 22% increment. The 3D printing pattern used were lays where the 3D-printed filaments direction was changed 90° degrees between one lay and the next one (what he called “Print path B”), since it is a quite common practice in 3D printing. These results are aligned with the results obtained in this paper for carbon fibers, which prove, at a certain extent, that assessing flexural strength on cast samples for 3D printing applications could be an acceptable approach to save testing time if printing patterns are changed 90° between layers.

The results of the compressive strength at 28 days of the different mixes analyzed in this study are shown in Fig.  5 (bottom). The compressive strength values obtained in the reference sample were 52.5 MPa and with the incorporation of the fibers the ranges varied between 49.38 and 56.92 MPa, yielding increments ranging from − 6 to 8%. Therefore, fibers in reinforced mortar show no clear advantage or disadvantage, as pointed out by different authors and summarized in [ 45 ].

Average of flexural (top) and compressive (bottom) strength at 28 days, standard deviation about mean is represented by the bars

In terms of the influence on length, dosage, strength, and elastic modulus of the fibers in relation to the flexural strength, an ANOVA test was carried out to find out correlations and only the elastic modulus of the fibers has a significant influence on the flexural strength with p -values of 0.514 (length), 0.334 (strength), 0.904 (dosage), and 0.015 (elastic modulus). Results of mortars including textile fibers and polypropylene fibers were omitted from the ANOVA since the manufacturers did not provide the values of fiber strength and/or elastic modulus.

In Table  9 , the ANOVA test results and the regression model generated for the flexural strength are summarized. This finding, the correlation between flexural strength and elastic modulus of fibers, has been reported by [ 62 ]. The lack of correlation with fiber length and dosage could be due to the fact that these factors could be beneficial or disadvantegous depending on how homogenous is the mixing process. For example, sometimes, large amounts and/or very long fibers could make the batch not to as homogenous as desired, producing voids that might affect flexural strength. In other occasions, on the contrary, large amounts of fibers and/or length could have the opposite effect, if mixing is homogeneous, might produce more resistant mortars. Alternatively, other factors from what we have no data, like the mortar-fiber bonding, could also have an influence, as mentioned in [ 62 ].

Finally, in order to verify that these increases in flexural strength with the incorporation of fibers are indeed significant, a statistical analysis was carried out. The results obtained are shown in Table  10 , where it can be seen that all the p -values obtained were lower than 0.05. These indicate that the incorporation of fibers significantly affects the increase in flexural strength. Only in the case of the 20-mm-long aramid fibers with a content of 0.3%, it was observed that this p -value was 0.326, not meeting the requirement to significantly affect. These fibers were the worst performers in flexural strength with an increase of only 5%. In addition, the statistical analysis of the compressive strength results was also carried out, proving that the results obtained for the p -value were greater than 0.05, demonstrating that the fibers do not significantly influence in this case.

With these results obtained in the statistical analysis, it was demonstrated that the fibers in the case of compressive strength had no major effect (see Table 10 ), but on the contrary, significantly impacted the flexural strength, so this was selected as a criterion for the MCMDA analysis.

3.3 Life cycle assessment (LCA)

As the last criterion for the MCDM analysis, the environmental impact was selected. The results obtained through the EF 3.0 method of the mixtures with the different types and percentages of fibers are shown in Table  11 . The results obtained were quite similar for the different dosages, since they only modify the type and percentage of fibers, which account for a very small amount of the mixtures, as it can be seen in Fig. 6 (for the particular case of polypropylene fibers). Although these variations are not very large, it can be observed that the textile fibers get the worst results in the LCA analysis, followed by the aramid fibers. In contrast, cellulose and polypropylene fibers were the best performers. These results of the indicators for each category have been converted, by normalization and weighting, into single results for each of the mixtures (Fig.  7 ), which can be entered in the MCMDA.

Contribution of unit processes to the total impact for the mixture with 0.1% polypropylene fiber

LCA points of each mixture

Table 12 shows the cost of each of the 36 mixes together with the control (no fibers). The cost of fibers has a fairly high weight in the total mix and is highly variable depending on the type. This is why the costs of the different mixtures vary so much, going from 49.13 €/t for textile mixes to 249.11€/t for mixes incorporating the highest content of zylon.

3.5 Multi-criteria decision-making analysis (MCDMA)

The weighing factors are shown in Table  13 while the values of the 4 indicators for the decision matrix are summarized in Table  14 .

In relation to the weighing factors, in the AHP method, printability criteria was the highest, followed by mechanical resistance, environmental, and economic criteria. A second scenario was proposed in which equal weight is assigned to each of the criteria (25%).

The classification of the mixtures was carried out with the two proposed multi-criteria analysis methods, WASPAS and TOPSIS, to verify that the results with both methods order the 36 alternatives (35 + control) in a similar way. The ranking of the two scenarios, with the two multi-criteria analysis methods, is shown in Fig.  8 . The CC and JPS score values range between 0 and 1 for the TOPSIS and WASPAS methods, respectively, with the best alternatives having the highest values.

Ranking (bars) and score (dots) of the alternatives with TOPSIS and WASPAS methods. a Scenario 1 (AHP weights); b Scenario 2 (equal weights)

The results obtained with both methods and in the two scenarios were very similar showing the same mixtures in the first and last places. The dosages were G13;0.1, Cell6;0.05, and C25;0.05. This is due to the fact that these dosages present good increases in resistance, which, although not the highest, oscillate between 15 and 20%, and have good printability values. Also, since these are the dosages that incorporate the lowest percentages of fibers and the type of fibers are also more economical, they have the lowest costs, since fibers constitute a very important part of the total price of the mixture. On the other hand, it has been obtained that the dosages incorporating both zylon, aramid of 20 mm, and textile fibers are the ones presenting the worst results. In the first case, in spite of presenting the best values of increase in flexural strength, these mixtures present very high costs in comparison with the rest of the fibers. In the second case, in addition to the high prices of the mixtures, they show some of the lowest increases in flexural strength. Finally, the textile fibers are also in the last positions, since they present the lowest increases in flexural strength and the worst printability values, as it could also be verified in the laboratory, since they were the fibers that presented the most problems when printing.

In addition, it is also obtained that the LCA presents very similar values in all the dosages because the only difference they present is the type of fibers used and in very low quantities. Therefore, both in Scenario 1 (weight, 21.83%) and Scenario 2 (weight, 25%), the results of the LCA have a very low impact. Furthermore, the increase of fiber content in all types of dosages produces an increase in the cost and a decrease in the workability (low workability is related to a high printability index: yield stress*viscosity), so that the mixtures with lower fiber content usually present the best results in the analysis, since it is difficult for the increase in flexural strength to be high enough as to compensate with the other two criteria punctuation.

In summary, mortars with glass, cellulose, and carbon fibers are those that occupy the first positions in the ranking. Those incorporating zylon, aramid of 20 mm, and textile fibers occupy the last positions. Control samples (no fibers) rank in a relatively good position, between 4 (Scenario 2, WASPAS) to 10 (Scenario 1, TOPSIS), proving that that the use of certain type of fibers would be even worse that not using any fibers.

4 Conclusions

In this study, an evaluation of fibers, which are incorporated into 3D printing mortar dosages, was carried out. The fibers used were aramid, glass, carbon, cellulose, textile, zylon, and polypropylene, in five different contents: 0.05; 0.075; 0.1; 0.2; and 0.3. These contents of fiber might appear quite low; however, some authors have also determined that optimal values could be 0.1% in PVA, 0.25% in PP, or 0.5% in basalt fibers as sumarized in [ 13 , 63 ].

Firstly, a certain range of values of fiber percentages added to the mortars was selected in order to avoid clogging or printing problems on the nozzle of the 3D printer based on preliminary printing trials. Then, a series of laboratory experiments were carried out on rheology, flexural, and compressive strength. Once the laboratory tests had been carried out, a life cycle and statistical analysis was performed. With all the data obtained, a multi-criteria MCMDA was carried out using the WASPAS and TOPSIS methods to select the best alternatives. In addition, these methods were combined with the AHP method and equal weights of assigning weights, to give the relative importance of all the responses involved in the MCMD. The main conclusions obtained in this study are the following:

With the incorporation of the fibers in the dosages used for 3D printing, increases in flexural strength of up to 30% were achieved, which matches previous studies. Compressive strength on fiber-reinforced mortars did not show significant changes with respect to control sample (no fibers).

Only significant correlations between flexural strength and elastic modulus of fibers have been demonstrated through a regression model. No correlations with amount of fibers or length could be demonstrated in this study.

As far as rheology is concerned, there is a good correspondence between the results obtained in the tests and what was shown in the laboratory when the printability tests were carried out. As the fiber content increased, the mixtures showed worse workability characteristics, to the point that with certain amounts of fibers the filaments began to break or block at the nozzle and the mortars were no longer suitable for printing. With the rheological results, as the fiber content increased, yield stress values increased significantly, as also reported in [ 45 ].

With respect to the LCA results, no very notable differences were obtained between the different dosages analyzed. This is due to the fact that the fibers represent a small part of the dosage and are not the part that has the greatest effect on the LCA result. However, a quantification of their impact is now available.

In relation to the AHP method of assigning weights to the different criteria, the greatest weight was given to printability (38.14%), followed by strength (27.17%), environmental impact (21.83%), and cost (12.86%). However, since the relative difference among alternatives in terms of environmental impact and strength was not that high, printability and cost were the criteria that affect more the MCDMA analysis.

The WASPAS and TOPSIS methods, in the two scenarios, yield the following ranking with the best 3 dosages: G13;0.1 (1st position always), Cell6;0.05 (2nd position in Scenario 2; 3rd position in Scenario 1), C25;0.05 (2nd position in Scenario 1; 3rd position in Scenario 2), showing that the dosages with lower fiber contents present better results. On the other hand, zylon and aramid fibers were ranked in the last positions mainly due to their high costs.

Mortars with textile fibers, even though their cost is null (it is a residue), were ranked in relatively bottom positions (21 in the best case: Scenario 2, “T20;0.5,” WASPAS), since their increase in flexible strength was quite low and its printability was also poor.

Control sample (no fibers) was ranked between 4 to 10 position, depending on weighting scenario and ranking method.

As an overall conclusion, it could be stated that certain low-medium cost fibers like glass, cellulose, or carbon could increase the flexural strength without compromising cost, printability, and environmental impact, by adding 0.05 to 0.1% in weight depending on the case. Besides, other highly resistant fibers such as aramid or zylon are not cost-effective solutions to increase flexural strength in 3D printing mortar. Also, the option of not using fibers should also be considered, since it was ranked in between 4 and 10th position, much better than highly resistant fibers.

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Acknowledgements

The authors would like to thank the following companies for their contributions: Cementos Portland Valderribas S.A for providing the cement, Basf Chemicals Ltd. for providing all the additives, Teijin Limited Ltd. for providing various types of fibers (aramid, carbon, and zylon), Fibratec Técnicas de la Fibra S.L for providing the glass fibers, and Textil Santanderina S.A. for providing the textile fibers.

Open Access funding provided thanks to the CRUE-CSIC agreement with Springer Nature. •  The work has received funding from the Spanish Ministry of Science and Innovation through three grants:

•  “Promotion of activity in R + D of GITECO and GCS groups of the University of Cantabria” (Ref: SSPJO1900I001723XV0)

•  “Fostering the circular economy and low CO2 technologies through the additive manufacturing -3DCircle-” (Ref: PID2020-112851RA-I00).

•  “Enhancing biodiversity in the Atlantic area through sustainable artificial reefs -EBASAR-” (Ref: TED2021-129532B-I00).

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Sara Alonso-Cañon, Elena Blanco-Fernandez, Daniel Castro-Fresno, Adrian I. Yoris-Nobile & Laura Castanon-Jano

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Elena Blanco-Fernandez, Daniel Castro, and Sara Alonso-Cañon contributed to the study conception and design. Material preparation and laboratory tests were mainly performed by Sara Alonso-Cañon with the help of Adrian I. Yoris-Nobile. Analysis of results and conclusions were performed by Sara Alonso-Cañon and Elena Blanco-Fernandez. Laura Castañon-Jano contributed mainly to the MCDM analysis. The first draft of the manuscript was mainly written by Sara Alonso-Cañon and all authors commented and improved previous versions of the manuscript. New versions of the manuscript during the revision process have been undertaken by Elena Blanco. All authors read and approved the final manuscript.

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Alonso-Cañon, S., Blanco-Fernandez, E., Castro-Fresno, D. et al. Comparison of reinforcement fibers in 3D printing mortars using multi-criteria analysis. Int J Adv Manuf Technol (2024). https://doi.org/10.1007/s00170-024-14126-1

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Received : 12 September 2023

Accepted : 09 July 2024

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DOI : https://doi.org/10.1007/s00170-024-14126-1

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