Quality and durability of recycled composite materials

12 Quality and durability of recycled composite materials K. L. P I C K E R I N G, University of Waikato, New Zealand, and M. D. H. B E G, University of Malaysia Pahang, Malaysia

Abstract: Polymer matrix composite products are manufactured on a mass scale that is increasing each year, along with consumer interest and government legislation driving the pressure for them to be recycled. A major factor determining the route for recycling of a polymer matrix composite is whether the matrix is a thermoplastic or a thermosetting plastic (thermoset); unlike thermosets, thermoplastics can be melted and therefore lend themselves more readily to recycling by being mechanically broken down for remoulding. Techniques for recycling thermoset and thermoplastic matrix composites are discussed including mechanical breakdown, thermal recycling as well as chemical recycling with available information on the related quality and where known, the durability of the recycled materials. Key words: thermoset and thermoplastic composites, mechanical recycling, chemical extraction, moisture resistance, thermal properties.

12.1

Introduction

It is estimated that globally, more than 6 million tonnes of polymer matrix composite (PMC) products are manufactured each year, largely dominated by glass fibre reinforced polymers (GFRPs) (Lester et al. 2004). Furthermore, the amounts used are increasing annually. For just sheet moulding compounds (SMCs), which are based on glass fibre reinforced unsaturated polyester resin and one of the most commonly used composite materials, waste increased from 0.1 million tonnes in 1984 to just less than 0.4 million tonnes in 2000 (Perrin et al. 2008). Markets include automotive, leisure, electronics, aerospace and construction industries. On a more modest level, an estimate of the worldwide production of carbon fibre reinforced plastic (CFRP) waste is of the order of only 5000 tonnes per year (Ogi et al. 2005). However, encouraged by price reductions, the use of carbon fibre, once largely the domain of aerospace and sporting goods, is now being adopted by mainstream automotive and construction industries and is also increasing. This increase is estimated in Europe, for example, to be occurring at the rate of about 10% per annum (Lester et al. 2004). In addition to 303

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end-of-life components, waste CFRP in the form of off-cuts is produced as a by-product of the manufacture of CFRP products. Increased PMC utilisation has led to increasing pressure to resolve issues relating to composite waste. Consumer interest and government legislation are now drivers to encourage recycling of polymer matrix composites. In Europe where the automotive sector is one the largest users of composite materials, the End-of-Life Vehicle (ELV) Directive (European Commission, 2000) was the earliest major legislation regarding recycling of materials (Perrin et al. 2008). However, legislation is being put into place to cover other industries. This includes the Waste Electrical and Electronic Equipment (WEEE) Directive (European Commission, 2003) and the impending directive on ‘Construction and Demolition Waste’. However, by their very multi-component nature, and the variety of materials used, composite recycling raises many challenges dependent on the composite type. One major factor determining the route for recycling of a PMC is whether the matrix is thermoplastic or a thermoset plastic. Thermoplastics can be melted and therefore lend themselves more readily to recycling by reshaping. Indeed, there is much interest in the area of thermoplastic matrix composites, particularly in the automotive area, due to the readiness with which they are expected to be recyclable. Thermoset matrix composites are less obviously recyclable because of the inability to remould them. However, increased adoption of these materials, along with increased cost of landfill and the use of more expensive fibres than are commonly used in thermoplastics, in particular carbon fibre, has driven interest in reuse of these materials (DeRosa et al. 2005). Potential techniques that could be in use for these materials include mechanical breakdown, thermal recycling as well as chemical recycling, which are discussed in the following sections. Although these techniques could also be used for thermoplastic matrix composites, energy considerations would suggest remoulding to be a much more desirable option. Owing to the fundamental differences in their treatment, recycling of thermoset and thermoplastic matrix composites are described separately within this chapter.

12.2

Recycling thermoset matrix composites

12.2.1 Performance using mechanically broken down material The use of mechanical means for recycling thermoset waste has received the most attention. Research has been carried out to assess the reuse of mechanically broken down thermoset matrix composites containing glass,

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carbon and Kevlar® fibre. However, glass, particularly contained within SMCs or bulk moulding compounds (BMCs) has had the most interest, largely reflecting its dominance in the market place. Products using virgin SMCs or BMCs, for example include baths and sinks, car bumpers, car panels, car headlamp housings, door panels on aircraft and circuit boards (DeRosa et al. 2005). Composite material is commonly mechanically recycled in three stages. Initially, coarse material of particulate diameter around 50–100 mm is produced by cutting or crushing, followed by grinding to give a diameter of 50 μm to 10 mm, which is finally separated into grades of materials of different size (Pickering 2006). Typically, separation leads to a number of particulate grades and also more fibrous grades with higher aspect ratios. The particulate grades have been regarded as a potential replacement for fillers, but it is hoped that separation of the more valuable fibre fraction for recycling is likely to give the best returns on recycling effort (Kouparitsas et al. 2002). Industrial-scale mechanical separation of thermoset matrix composites has been demonstrated by a number of companies including ERCOM, Mecelec and Valcor in Europe and R.J. Marshal, Premix and Phoenix Fiberglas in North America (Pickering et al. 2000; DeRosa et al. 2005; Pickering 2006), although Phoenix Fiberglas stopped operating in 1996. All of these companies have focused on SMC and BMC waste. Toyota Motor Corporation has also incorporated SMC recycling in-house (DeRosa et al. 2005). ERCOM, one of the largest companies in terms of recycling throughput, has a capacity of 6000 tonnes per annum. Particulate grade materials produced by mechanical separation have been investigated for reuse in composite production, with some success where the focus has been on replacement of filler. Substitution of up to 88 wt% of the CaCO3 filler in SMC by fine particulate recyclate has resulted in materials with comparable tensile and flexural strengths and moduli (DeRosa et al. 2005) to that containing only virgin material. Toyota has also recycled finely ground SMC for a filler in SMC up to 20 vol% and achieved performance similar to virgin SMC (Inoh et al. 1994). Of particular note, improvement of strength of 30% has been obtained where very finely ground (15–20 μm) recycled PMC produced using a specialised grinding method, was recycled into its own production stream as a replacement for calcium carbonate filler (Kojima and Furukawa 1997). The strength of epoxy resin has also been found to increase 16 and 20% with addition of 1% of finely ground moulding/pre-preg scrap containing Kevlar and carbon fibre respectively (Anonymous 2008). In the same study, Kevlar recyclate was found to increase the bending strength of polyurethane foam. In addition, non-mechanical improvement in the form of damping capacity has been obtained in plates and beams using sieved recyclate (Thomas et al.

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2000). Improvement in mechanical performance with particulate grade materials has been found to be heavily dependent on particulate size, with finer grades generally giving the greatest benefit. Addition of 17.5 wt% particulate recyclate in BMC has been found to lead to a modest improvement or a 15% reduction in tensile strength for fine and coarse grade particulate grades respectively (DeRosa et al. 2005). Success, even with fine grade recyclate, however, has been mixed. Fine grade recyclate has commonly led to a penalty in mechanical performance (Butler 1991; Jutte and Graham 1991; Pickering et al. 2000). Although improvement of flexural strength was obtained in one study by adding 15 wt% of fine recyclate particles to BMC, this was coupled with a reduction in tensile strength (Bledzki and Goracy 1993). Commonly, use of particulate recyclate has been found to reduce flexural modulus (DeRosa et al. 2005). Another major issue for reuse as a filler in SMC and BMC is that for the quantities of recyclate required in moulding compounds to improve economic viability, viscosity becomes too high for standard processing to be used (Pickering et al. 2000). In addition, delay of curing has also been considered to be due to a reduction in thermal conductivity (Petterson and Nilsson 1994). Another potential limit to automotive uptake is the effect of surface finish, although this is not affected when up to 10 wt% recyclate is added (DeRosa et al. 2005). Making use of coarser and more fibrous recyclate SMC/BMC has proved to be more of a challenge. Fibrous recyclate added to BMC has resulted in reductions of flexural modulus, tensile strength and impact strength (Bledzki and Goracy 1993). For example, replacement of virgin fibre and half of the filler by fibrous recyclate in BMC reduced tensile and flexural strength by 20% and 54% respectively (DeRosa et al. 2005). Complete replacement in BMC of glass and filler by granulated SMC recyclate has been shown to lead to a general reduction of mechanical properties of the order of around 30% (Curcuras et al. 1991). However, it has been shown to be possible to maintain flexural strength of BMC with up to 15 wt% replacement of fibrous filler (Bledzki and Goracy 1993; DeRosa et al. 2005). It is believed that properties have been held back by poor interfacial bonding between the recyclate and its new matrix material (Pickering et al. 2000). Pettersen and Nilsson (1994) have achieved improvement in flexural strength of 15% for SMC using a combination of particles and more fibrous recyclate added at 10 wt% with a slightly reduced flexural modulus, but with no penalty in tensile strength. Compensation for reduced performance has also been achieved by combining longer lengthed recyclate and virgin glass fibre than was to be originally used in materials (DeRosa et al. 2004). Mechanically recycled SMC has found industrial application in SMC/ BMC car spoilers (at 15 wt%) and spare wheel covers (at 10 wt%) along with other automotive parts (DeRosa et al. 2005). Products using finely

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ground recyclate have demonstrated the ability to reduce virgin glass fibre utilisation slightly, coupled with weight reduction. Further potential for such recyclates includes incorporation into materials for drain covers and wood chip particleboard used for domestic flooring where improvement in properties has been shown (Conroy et al. 2004). Overall, however, it is currently not possible to ‘close the recycling loop’ due to the limit at which they can be added to materials without compromising performance (DeRosa et al. 2005). As an alternative to incorporation in thermoset matrix composites, mechanically broken down thermoset-based PMC has been assessed for use in other materials including concrete and thermoplastics. Mechanically broken down CFRP has been used in concrete to give modest improvements in compressive and flexural strength as well as more significant improvements in work of fracture (Ogi et al. 2005). These improvements have also been found to be strongly dependent on the size of the added CFRP pieces. The advantage of using carbon fibre in this form was expressed relating to dispersion, such that when contained in a polymer, the clumping that would normally be expected to occur with fibre alone, is overcome, although reduction of bonding was observed. Improvement of mechanical performance of polypropylene (PP) has also been obtained by the addition of coarse and fine SMC recyclate, including more than a doubling of Young’s modulus and increased notched impact energy, flexural strength and modulus, whilst maintaining tensile strength, although unnotched impact energy reduced (Jutte and Graham 1991). One study showed the addition of BMC and woven glass reinforced phenolic fragments to be able to improve tensile strength of PP by 10 and 134%, as well as Young’s modulus by 72 and 183% respectively, dependent on silanation of the recyclate along with maleic anhydride modification of the PP, although reduction of charpy impact strength was found particularly with BMC (Bream et al. 1997; Bream and Hornsby 2000). Use of scrap prepreg consisting of carbon in epoxy has been investigated as reinforcement for thermoplastic (Blizard 1998). Good increase was found with flexural properties and creep resistance. One study looked at mechanical separation via grinding and sieving of glass from polyester and Kevlar® and carbon from epoxy resin (Kouparitsas et al. 2002). The fibre recovered still retained some of the matrix material such that the fibre did not separate into individual fibres. Strengths and stiffnesses of thermoplastic (PP or an ethylene/ methacrylic acid copolymer) reinforced using these recycled fibre-rich materials were found to be similar to that reinforced with the equivalent virgin fibre of similar average fibre length for glass or Kevlar®, although appreciably lower for carbon. Unfortunately, although not compared in the paper, improvement compared with that expected of matrix-only materials appears limited, particularly for strength.

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Potential products using recyclate as a reinforcement in thermoplastics include materials that can replace wood that can be drilled and cut in moderate load-bearing structures such as groynes, footbridge foundations and jetties. Improvement in asphalt for bridge decking and concrete have also been identified as a potential use of these materials (Conroy et al. 2004).

12.2.2 Performance using thermally extracted fibre The main techniques with which it is possible to thermally extract fibre include pyrolysis, catalytic conversion and fluidised bed processing. One general advantage of thermal extraction is that a range of material can be handled along with some contamination (DeRosa et al. 2005). Pyrolysis involves heating material in the absence of oxygen, and for polymers leads to the production of oils and gases which all have potential for recycling into other chemicals or used as fuels (Williams 2003; Williams et al. 2005). For thermoset matrix composites, it is generally carried out at less than 500 °C. Pyrolysis has been investigated for recycling mixed polyester/styrene composite waste (Williams et al. 2005) using a fixed bed reactor at 450 °C. Here, the final separation of the fibre involved oxidation in a muffle furnace followed by sieving. Relatively clean fibre was produced that maintained its pre-pyrolysis length, but only approximately half of its strength. When this glass fibre was substituted for 25 wt% of the fibre in polyester matrix composites, reductions in flexural strength, flexural modulus and impact strengths of 7, 19 and 26% respectively occurred compared with composite reinforced with only virgin fibre. Better fibre strength retention has been achieved through a similar route, but by using mechanical means rather than oxidation to separate the fibre, resulting in fibre strength of over 60% that of the virgin fibre, although fibres still retained some char on their surfaces (Cunliffe et al. 2003). Pyrolysis followed by milling has also been assessed for recycling SMC material as a filler for further SMC production (DeRosa et al. 2005). The particles obtained have been found to give similar, or improvement of performance, for a range of mechanical properties when replacing 20% of the CaCO3 filler in SMC. However, less success was achieved when trying to replace virgin fibre with recycled fibre, such that again reduction of properties occurred. Pyrolysis, however, has been shown to be self-sustaining in terms of energy release during processing (DeRosa et al. 2005). The company Milled Carbon Ltd has set up industrial-scale pyrolysis facilities in the UK with an interest in recycling carbon fibre from epoxybased carbon fibre off-cuts including prepreg and are also investigating markets for their fibre where virgin quality fibre is not needed (Anonymous 2006). However, this company also seems to be investigating fluidised bed processing (Rush 2007) including energy recovery from the matrix. Products identified for short recycled carbon fibre include cellular phones,

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laptop computers as well as BMCs, SMCs and injection moulding compounds (Rempes 2003). Catalytic conversion involves pyrolysis at low temperatures (as low as 200 °C) in the presence of a catalyst to similarly form hydrocarbons which could be used as chemical feedstock or fuel (Lester et al. 2004) and enable extraction of fibre and other inorganics from PMCs. An initial feasibility study demonstrated that SMC, mixed plastic car body parts (as well as thermoplastic matrix composites) containing glass fibre can be recycled using catalytic conversion enabling reuse of fibres as well as fillers, metal inserts and the matrix polymer as chemical feedstocks (Allred and Busselle 2000). Processing at 300 °C resulted in relatively clean fibre. This has been proposed as an economically viable alternative to using landfill (Allred et al. 1997). Optimised processing has resulted in glass fibre with approximately half the strength of virgin glass (Allred et al. 1997). Carbon fibre has been recovered from woven carbon fibre in epoxy matrix composites with similar surface chemistry and only 9% loss of strength compared with virgin fibres, for which the strength loss could largely be explained by the weaving process which is known to commonly reduce strength by 5–10% (Allred and Coons 1996). Epoxy residue was observed to be in the form of small particles less than a tenth of a micron in diameter still attached to fibres. However, the fibre recovered was considered as suitable for use in moulding compounds or for milling feedstock. It has also been shown that release plies on waste pre-preg can be left on for this process (Allred 1996). Adherent Technologies Inc. (ATI) in the US have developed a prototype system for a catalytic conversion process which is relatively flexible in terms of feedstock, that can extract carbon fibre from composites into a milled or chopped fibre form (Rush 2007). ATI has worked with Boeing to assess recovery processes as part of a life cycle analysis for the Boeing 787 Dreamliner (Rempes 2003). Dr Jan-Michael Gosau, the Environmental/Engineering Manager at ATI, has expressed the importance of this relationship, in that previously there has been insufficient continuous supply of scrap for recycling for commercial viability. He expects that due to Boeing 787 Dreamliner production, commercial level recycling of CFRP will be occurring in 2009, aiming at a throughput of approximately 500 tonnes per annum producing milled carbon fibre. The use of fluidised bed processing for recycling thermoset matrix composites has been under investigation for more than ten years. In this process, combustion allows retrieval of fibres as well as inorganic fillers and the potential for energy recovery from the matrix. Agitation caused by gas in the bed assists separation of fibres from fillers, which can be removed by washing in dilute detergent, followed by filtration (Kennerley et al. 1998). Early work investigated the extraction of glass fibre as well as fillers from unsaturated polyester matrix composites including SMC, wound pipe and

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sandwich panels (Pickering et al. 2000). The strength of fibre was found to be dependent on processing temperature. At 450 °C, the minimum temperature considered to give an adequate processing rate, strength was approximately half that of virgin fibre, which reduced further at higher processing temperatures, although Young’s modulus remained unaffected. The content of the recyclate after final washing and filtration was found to be 92 wt% fibre (Kennerley et al. 1998). A limit of about 10 mm on size of scrap pieces was used to ensure dwell times such as would minimise fibre damage. Preliminary assessment of the economic feasibility of this process suggested economic viability for a plant with a throughput of about 9000 tonnes per annum (Pickering et al. 2000). A pilot-scale assessment has also been carried out and showed that replacement of 50% of the virgin fibre by recycled material, in a standard formulation for a compression moulded headlamp surround, led to a material with generally similar processing and mechanical performance, although a slight reduction in impact performance was observed. Further investigation at higher substitution levels gave no reduction in Young’s or flexural modulus; however, they brought about a reduction of tensile and flexural strength as well as impact strength and a darkening of mouldings (Pickering et al. 2000). Reductions of properties were up to around 40, 50 and 75% for tensile strength, flexural strength and impact strength respectively for full substitution of virgin fibres by recycled fibre. Fluidised bed processing has been investigated for its potential to recycle carbon fibre. In one study (Jiang et al. 2008) shredded CFRP scrap was oxidised in a fluidised bed at 550 °C to remove the matrix and release the fibre. The interfacial strength was found to be unaffected compared with virgin fibres when used. Better strength retention than for glass fibre has been observed with values in the order of 80% along with full retention of stiffness (Pickering et al. 2000) compared with virgin fibre. Microwave heating has been assessed for the recovery of long carbon fibre from CFRP (Lester et al. 2004). Microwaving at 3 kW for 8 seconds was found to volatilise the epoxy resin matrix leaving only trace amounts of matrix material of around 2% w/w of fibre in the form of nodules, retaining approximately 80% of the original fibre strength and 87% of the Young’s modulus. The commercial viability of this process, however, remains to be proven.

12.2.3 Performance using chemically extracted fibre This is arguably the least mature of the potential techniques for recycling thermoset matrix composites. Hydrolysis, glycolysis, solvolysis and acid digestion have been considered for chemical recycling of thermoset matrix composites (DeRosa et al. 2005).

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Currently, hydrolysis appears to be of limited use for recycling of thermoset matrix composites. For hydrolysis to be effective it appears to be necessary to separate waste into that of different matrix polymers (Perrin et al. 2008). In addition, material has had to be ground down to a fine particle size, such that replacement of filler would be the best achievable outcome expected and further problems arise due to the waste stream produced. Interest in glycolysis has been demonstrated by the registering of a patent by the Miyaso Chemical Company in Japan involving degradation of polyester resin with glycol such that it can be used in further synthesis of unsaturated polyester or polyurethane, but with no reference to the effect on the fibre present (Shizu et al. 1997). Solvolysis with ethylamine has been shown to be capable of producing visually clean fibres (Winter et al. 1995). One research study involved investigation of different routes of chemical recycling for SMCs (Winter et al. 1995). One solvolysis treatment with a mixture of ethanol and potassium hydroxide, although successful at solubilising the matrix, involved a subsequent neutralisation that formed ‘a large stream of waste chemicals’. In the same study, solvolysis of SMC with ethanolamine followed by washing with ethanol enabled recovery of fibres and filler along with a polymeric residue. Substitution of half the virgin fibres of a standard BMC formulation with recycled glass was found to not significantly reduce Young’s modulus or flexural strength, although impact strength appeared to be lower. Some success of extracting aramid (TwaronTM) fibre from epoxy by solvent swelling was achieved using a solution of dimethyl sulphoxide (DMSO) and toluene in equal parts, although fibre damage was apparent without full removal of epoxy (Buggy et al. 1995). Solvent extraction of SMC using acetone, dichloromethane, trichloromethane and combined trichloromethane/benzene was shown in one study to be largely ineffective (Patel et al. 1993). Acid digestion raises the challenge of using hazardous chemicals and conditions as well as, similar to the other chemical processes, the resulting complex hydrocarbon/acid mixture (Allred 1996; Rempes 2003). Recently, however, extraction of initially shredded SMC waste has been carried out using orthophosphoric acid to dissolve calcium carbonate (generally of the order of 50–55 wt% of SMC) (Perrin et al. 2008). This process was found to dissolve about half of the calcium carbonate giving a recyclate product enriched in fibre with potential for use as reinforcement/filler in thermoplastics. Scale-up was investigated enabling batches of up to 200 kg to be processed. Recognition of the potential for this process occurred by way of receipt of the Innovation Techniques 2006 Waste Category prize from the ADEME (French national agency for the environment).

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12.3

Recycling thermoplastic matrix composites

Owing to the ability to melt thermoplastics, mechanical breakdown into granules for use in the original processing stream is the most obvious technique for recycling fibre reinforced thermoplastics and indeed has been the area of greatest focus. However, for fibre reinforced thermoplastics, fibre breakage induced by grinding and subsequent processing (e.g. injection moulding) leads to reduction of material properties. For these reasons, recycling by dissolution of the polymer matrix, often at elevated temperatures, has also been considered.

12.3.1 Effects of recycling on mechanical properties Performance using mechanically broken-down material The simplicity of mechanical recycling and its relatively low cost give it the greatest potential, particularly for short fibre reinforced thermoplastics, for which fibre breakage during reprocessing has a lower impact on reinforcing properties than for long fibre reinforced plastics (Bernasconi et al. 2007). Although no post-consumer based recycling studies have been carried out on glass fibre reinforced thermoplastic matrix composites, there are some studies on reprocessing. Eriksson and Albertsson (1996) assessed the effects of reprocessing of short glass fibre reinforced polyamide injection moulded composites containing 30 wt% of glass fibres, by comparing composites made from virgin fibre and polyamide with those produced from mechanically broken-down composites. This study evidenced the role of fibre breakage induced by successive injection moulding processes. The tensile strength was found to reduce from 192 MPa for virgin composites to 132 MPa for composites reprocessed eight times. A similar trend was found by Bernasconi et al. (2007) when reprocessing 35 wt% glass fibre reinforced polyamide composites. Chu and Sullivan (1996) obtained lower tensile strength, but similar Young’s modulus and improved impact strength for reprocessed glass fibre poly(butylene terephthalate) composites compared with virgin composites. As for synthetic fibre reinforced thermoplastic composites, no postconsumer based recycling studies have been carried out on agro-based fibre composites (Rowell et al. 1997), but again there have been a number of studies on reprocessing. Agro-based fibres are less brittle and softer than glass fibres and are therefore more likely to retain properties during recycling. Bourmaud and Baley (2007) compared reprocessing of hemp and sisal with glass fibre reinforced polypropylene (PP) composites. They observed reduction of both tensile strength (TS) and Young’s modulus (YM) for all composites, with greater reduction found for glass fibre

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reinforced PP (about 52% for TS and 40% for YM) than PP reinforced with sisal (17% for TS and 10% for YM) or hemp (13% for TS and 1% for YM) after being reprocessed seven times. Walz et al. (1994) studied reprocessing of 50 wt% kenaf fibre reinforced PP composites. Both tensile and flexural properties were found to decrease (by about 20%) with increased number of times the materials were reprocessed (up to nine times). A similar trend was shown by Joseph et al. (1993) for 20 wt% sisal fibre/LDPE matrix composites. However, Youngquis et al. (1994) reported that the mechanical properties and dimensional stability of second-generation wood fibre reinforced polyethylene panels were equivalent to, or better than, properties obtained from first-generation panels due to better encapsulation of fibre matrix. Beg and Pickering (2008a) carried out an extensive study on reprocessing of wood fibre reinforced PP composites. Composites were produced with 40 or 50 wt% fibre and reprocessed by repeated pelletising and injection moulding and the trend for mechanical performance was found to depend on fibre content. For 40 wt% fibre, TS and YM of composites decreased with increased number of times the materials were reprocessed in a linear fashion (Figs 12.1 and 12.2) leading to a reduction of 25% for TS and 17% for YM after reprocessing eight times. For 50 wt% fibre composites, TS of virgin composites was lower than for 40 wt% fibre content composites (Figs 12.1 and 12.3). This was explained to be due to the limited dispersion of fibre in composites at higher fibre content due to the increase in composite viscosity as indicated by the reduction of melt flow index (melt flow index of 40 wt% fibre composites was 0.9 g/10 min and 50 wt% fibre composites was 0.14 g/10 min). However, TS and YM of 50 wt% fibre composites increased with the first two reprocessing cycles (Figs 12.3 and 12.4), which was considered to be due to improved fibre 45

y = –1.36x + 41.16

Tensile strength (MPa)

40

Composite

PP

35 30 25 20

c

15 10 5 0

Virgin

1

2

3

4

5

6

7

8

Number of times reprocessed

12.1 Tensile strength of virgin and reprocessed composites (40 wt%) fibre and PP (Beg and Pickering 2008a).

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Young's modulus (MPa)

6000

Composite

y = –78.08x + 4553

5000

PP

4000 3000 2000 1000 0

Virgin

1

2

3

4

5

6

7

8

Number of times reprocessed

Tensile strength (MPa)

12.2 Young’s modulus of virgin and reprocessed composites (40 wt%) fibre and PP (Beg and Pickering 2008a). 50 45 40 35 30 25 20 15 10 5 0

Virgin

1

2

3

4

5

6

7

8

Number of times reprocessed

12.3 Tensile strength of virgin and reprocessed composites (50 wt%) fibre and PP (Beg and Pickering 2008a).

dispersion, although decreased with further reprocessing to give overall an 11% reduction of TS and a 6% increase in YM after reprocessing eight times, compared with the virgin composites. Similar to Walz et al. (1994) and Joseph et al. (1993), Beg and Pickering (2008a) also found flexural strength and flexural modulus to decrease with increased number of times the materials were reprocessed. A 30% reduction of flexural strength and 20% reduction of flexural modulus were found for 40 wt% fibre composites after reprocessing eight times compared with virgin composites. As observed for glass fibre reinforced thermoplastics, the reduction of mechanical properties with reprocessing has been linked to increased fibre damage (Harper et al. 2006; Beg and Pickering 2008a). Beg and Pickering (2008a) found the average fibre length to decrease from 2.36 mm for virgin

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Young's modulus (MPa)

8000 7000 6000 5000 4000 3000 2000 1000 0

Virgin

1

2

3

4

5

6

7

8

Number of times reprocessed

12.4 Young’s modulus of virgin and reprocessed composites (40 wt%) fibre and PP (Beg and Pickering 2008a).

Length (mm)

3.0 2.5

y = 3.4564e–0.3545x R 2 = 0.9802

2.0 1.5 1.0 0.5 0

Virgin Virgin 2 4 6 fibre composite Number of times reprocessed

8

12.5 Weighted average fibre length of virgin fibre and the fibre extracted from composites (Beg and Pickering 2008a).

fibre to 0.37 mm for fibre extracted from 40 wt% fibre composites reprocessed eight times (Fig. 12.5). In addition, the length distribution of fibres became narrower and reduced to shorter fibre lengths (Fig. 12.6) and the amount of fibre fines (fibre length less than 0.20 mm) was found to increase (from 9% for virgin composites to 80% for composites reprocessed eight times) with increased number of times the materials were reprocessed. Beg and Pickering (2008a) found that the change in fibre length with the number of times the composite material was reprocessed followed the empirical equation: lN = l0 e− bN

[12.1]

where lN is the average fibre length at any reprocessed composites, l0 is the length of virgin fibre, b is the slope of fibre length versus number of times

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15

Population (%)

Population (%)

Virgin fibre

10 5 0 0.2 1 1.8 2.6 3.4 4.2 5 Length (mm)

20 15 10 5 0 0.2 1 1.8 2.6 3.4 4.2 5 5.6 Length (mm)

5.6

8 times reprocessed composite Population (%)

Population (%)

4 times reprocessed composite 30 20 10 0 0.2 1 1.8 2.6 3.4 4.2 5 5.6 Length (mm)

60 40 20 0 0.2 1 1.8 2.6 3.4 4.2 5 5.6 Length (mm)

12.6 Fibre length distribution of virgin fibre and the fibre extracted from composites (Beg and Pickering 2008a).

reprocessed graph and N is the number of times the composites were reprocessed. Based on this equation it was found to be possible to correlate composite strength with the change in fibre length when fibre was evenly distributed as follows: a l σ N = σ 0 − ln ⎛⎜ 0 ⎞⎟ ⎝ b lN ⎠

[12.2]

Beg and Pickering (2008a) also studied the failure strain (FS) of composites during reprocessing and found that FS of both 40 and 50 wt% fibre composites increased exponentially with increased number of times the materials were reprocessed (Figs 12.7 and 12.8). Bourmaud and Baley (2007) also reported an increase in elongation at break after reprocessing for PP/hemp (22%), PP/sisal (9%) and PP/glass (34%) composites after reprocessing seven times. The significant increase was explained as due to the decrease in fibre length induced by reprocessing as discussed previously (Bourmaud and Baley 2007; Beg and Pickering 2008a). The more significant increase in elongation at break for PP/glass fibre composites was explained by the poor adhesion between glass fibres and matrix after seven cycles thus mobility of fibres was enhanced by easier debonding (Bourmaud and Baley 2007) but it would also be due to increased fibre breakage. Work by Beg and Pickering (2008a) also assessed the effect of reprocessing on impact strength and hardness of composites. The impact strength

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6 y = 1.6701e 0.1211x R 2 = 0.9761

Failure strain (%)

5 4 3 2 1 0

Virgin

1

2 3 4 5 6 Number of times reprocessed

7

8

12.7 Failure strain of virgin and reprocessed composites (40 wt% fibre) (Beg and Pickering 2008a).

4.5

Failure strain (%)

4.0 3.5 3.0 2.5

y = 1.051e 0.1366x R 2 = 0.9701

2.0 1.5 1.0 0.5 0 Virgin

1

2 3 4 5 6 Number of times reprocessed

7

8

12.8 Failure strain of virgin and reprocessed composites (50 wt% fibre) (Beg and Pickering 2008a).

has been shown to decrease from 6.2 kJ/m2 for virgin composites to 3.2 kJ/m2 for the composites reprocessed eight times. The reduction of impact strength has been suggested as being due to the reduction of molecular weight of PP during reprocessing and decrease in fibre length which increases the number of fibre ends that act as crack initiation. Hardness was found to increase with reprocessing such that a 35% increase in Vicker’s hardness number was obtained for composites reprocessed eight times compared with virgin composites. The increase in hardness has been considered to be due to the reduction of micro-voids and the increase in composite density with increased reprocessing.

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Performance with chemically extracted fibre Several studies have involved the use of solvents, including methanol, toluene and xylene, to extract fibre for recycling from composites. Papaspyrides et al. (1995) employed a solvent-based technique to recycle thermoplastic glass fibre composites. Toluene was used as a solvent for the low density polyethylene matrix. The glass fibre from the recycled route formed stronger second generation composites which was explained as being due to residual polymer on the fibre surfaces aiding fibre–matrix bonding in the recycled composites and also to new matrix being able to recrystallise to form a transcrystalline layer on the residual polymer around the fibres (Zafeiropoulos et al. 1999). Poulakis et al. (1997) also found that recovered glass fibre (from glass fibre/polypropylene composites by separating using xylene) composites provided higher tensile strength than for virgin composites. However, it has been identified that the handling of large amounts of solvents is associated with health, safety and environmental concerns (Baillie 2004). In addition, the use of solvent would be disadvantageous for natural fibre composites, as the fibre can be degraded by solvent and at high temperature.

12.3.2 Effects of recycling on thermal stability Beg and Pickering (2008a) studied thermal properties of composites using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The melting temperature (Tm) of PP in composites was found to be reduced slightly from 171 °C for virgin composites to 167 °C for composites reprocessed eight times which was considered to be due to reduced molecular weight as a consequence of thermo-mechanical degradation and chain scission (Ramírez-Vargas et al. 2004; Beg and Pickering 2008a). Three stages of decomposition have been observed for virgin and reprocessed composites (Fig. 12.9), starting with dehydration and decomposition of volatile components at around 250 °C, followed by rapid weight loss for oxidative decomposition and finally slow decomposition corresponding to formation of char as the temperature increased. Kinetic parameters for the various stages of thermal degradation were determined from the TGA graphs using the following equation, given by Broido (1969): E ⎛ 1⎞ ⎛ RZ ⎞ T 2 ln ⎜ ln ⎟ = − a + ln ⎜ ⎝ y⎠ ⎝ Ea β max ⎟⎠ RT

[12.3]

where y is the fraction of non-volatilised material not yet decomposed, Tmax is the temperature of maximum reaction rate, β is the heating rate, Z is the frequency factor and Ea is the activation energy. Initially, plots of lnln(1/y) versus 1/T for various stages of decomposition were drawn and

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found to be linear, suggesting good agreement with the Broido equation. The activation energies, Ea determined from the slopes of these plots are given in Table 12.1. Tmax and Ea for composites have been found to increase with increased number of times the materials were reprocessed. The positions of weight loss on the TGA traces of composites (Fig. 12.10) were also found to be shifted to higher temperatures with increased number of times Temp. diff./weight (°C/mg)

7 Virgin composites

6

8 times reprocessed composites

5 4 3 2 1 0

50

150

250

350

450

550

Temperature (°C)

12.9 DTA traces for virgin and reprocessed 40 wt% fibre composites (Beg and Pickering 2008a). Virgin composites

Weight (%)

100 80

8 times reprocessed composites

60 40 20 0

0

100

200 300 400 Temperature (°C)

500

600

12.10 TGA curves of virgin and reprocessed 40 wt% fibre composites (Beg and Pickering 2008a). Table 12.1 Thermal properties of composites (Beg and Pickering 2008)

Sample

Stage

Weight Temp. Activation energy loss (%) range (°C) Tmax (°C) Ea (kJ/mol)

Virgin composite

1st 2nd 3rd

61 31 7

226–351 351–436 436–508

285 371 455

85 68 60

8 times reprocessed 1st composite 2nd 3rd

30 52 15

230–340 340–447 470–512

289 412 470

87 71 81

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the materials were reprocessed, suggesting increased thermal stability. The increase in thermal stability has been considered to be due to an increase in crystallinity of PP resulting from molecular weight reduction.

12.3.3 Effects of recycling on rheology Viscosity is an important parameter which should be considered when processing composites. Bourmaud and Baley (2007) studied Newtonian limit viscosity (ηo) of reprocessed composites and found it to decrease with increased reprocessing cycles, which was proposed as relating to decrease in molecular weight. After six cycles, the decrease in ηo was 57% for the PP/glass fibre composite but higher for PP/vegetal fibre composite (72% for the PP/hemp composite and 65% for the PP/sisal composites). This difference was explained by the dispersion of fibres in different composites; the dispersion was better for PP/glass fibre composites whereas hemp/sisal fibres were seen to be aggregated. It was considered that the scission of aggregates was more efficient in decreasing viscosity than the scission of isolated fibres. Nair et al. (2000) also reported that fibre length has an important effect on viscosity of composites.

12.3.4 Durability of recycled composites The stability of composites over time is an important issue affecting their utilisation. Moisture penetration into composite materials occurs by three different mechanisms. The main process consists of diffusion of water molecules inside the microscopic gaps between polymer chains. The other mechanisms are capillary transport into the gaps and flaws at the interfaces between fibres and polymer due to incomplete wettability and impregnation, and transport by micro-cracks in the matrix, involving the flow and storage of water in the cracks, pores or small channels in the composite structure (Comyn 1985; Lin et al. 2002). Capillary transport is particularly significant when the interfacial adhesion is weak and when debonding of the fibres and the matrix has been initiated. Imperfections in the matrix can originate during the processing of the material, or due to environmental and service effects. Some conflicting results have been observed for recycled composites after hygrothermal ageing. Youngquist et al. (1994) studied wood fibre reinforced polyethylene composites and found moisture resistance of second-generation panels to be equivalent to, or better than, properties obtained from first-generation panels. This was explained as due to better encapsulation. However, Eriksson and Albertsson (1998) reported negative influence during accelerated ageing of glass fibre reinforced polyamide composites, indicating a lower resistance of recycled material toward

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oxidative degradation and hydrolysis compared with that of virgin material. Beg and Pickering (2008b) reported that exposure of wood fibre composites to hygrothermal ageing resulted in slight deterioration of the surface texture in the form of colour fading. Fibre became more discernible (from the matrix) as a consequence of hygrothermal ageing for virgin and reprocessed composites which was considered to be due to the reduction of interfacial bonding. This was less apparent for the reprocessed composites. Thickness swelling was found for virgin and reprocessed composites after hygrothermal ageing, the extent of which was found to decrease with reprocessing, such that after ageing, the virgin composites showed an increase in swelling by 3.7% which reduced down to 2.2% for composites reprocessed eight times. Moisture absorption increased with increased soaking time for virgin and reprocessed composites until saturation at about 5 months (Fig. 12.11). As no significant weight gain was found for PP during this period, it seemed likely that moisture only penetrated into the composites through the fibre and fibre–matrix interface. Both the equilibrium moisture content and diffusion coefficient decreased with increased number of times the materials were reprocessed respectively from 9.42% and 2.54 × 10−13 m2/s for virgin composites to 6.41% and 1.01 × 10−13 m2/s for composites reprocessed eight times. The decrease in moisture content and diffusion coefficient with increased number of times the materials were reprocessed was explained by a number of effects. As the fibre length decreased with increased number of times the materials were reprocessed (discussed in previous section), it was considered that it would have been more difficult to form finite clusters which serve as passages for water molecules to travel through the lattice from one side to another (Wang et al. 2006; Beg and Pickering 2008a, b). Also, reduction of micro-voids as evaluated by the increased density of composites with increased reprocessing would be expected to result in a

Moisture content Mt (%)

14 Virgin 4 times reprocessed 8 times reprocessed

12 10

2 times reprocessed 6 times reprocessed PP

8 6 4 2 0 0

50

100

150

200

250

300

Soaking time (days)

12.11 Moisture content versus soaking time of virgin and reprocessed composites and PP (Beg and Pickering 2008b).

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decrease in moisture content and diffusion coefficient. In addition, reprocessing increased the crystallinity of PP as a result of molecular weight reduction which was also considered to contribute to the reduction of moisture absorption. The effects on mechanical properties after hygrothermal ageing of recycled composites has been reported by Beg and Pickering (2008b). TS and YM have been observed to decrease after hygrothermal ageing for virgin and reprocessed composites. The percentage reduction in TS and YM due to ageing is presented in Fig. 12.12, and it can be seen that the extent of reduction in properties decreased with increased number of times the materials were reprocessed. After ageing, reductions in TS of 33% and YM of 40% were found for virgin composites compared with reductions for both TS and YM of 27% for composites reprocessed eight times. This was considered to be due to the equilibrium moisture content decreasing with increased number of times the materials were reprocessed, and therefore having less effect on behaviour. Failure strain and impact strength have been found to increase after hygrothermal ageing which was believed to be due to the water molecules acting as a plasticiser in the composite material (Beg and Pickering 2008b) as was also explained in another study carried out by Joseph et al. (2002). However, the extent of increase in failure strain and impact strength was generally found to decrease with increased number of times the materials were reprocessed (Beg and Pickering 2008b) which was also considered to be due to the reduction of equilibrium moisture content as explained above. Beg and Pickering (2008b) also studied the thermal stability of recycled composites after hygrothermal ageing. They found it to decrease after hygrothermal ageing both for virgin and reprocessed composites. The

Reduction of properties (%)

45

TS

40

YM

35 30 25 20 15 10 5 0 Virgin

2

4

6

8

Number of times reprocessed

12.12 Reduction in TS and YM of virgin and reprocessed composites after hygrothermal ageing (Beg and Pickering 2008b).

Quality and durability of recycled composite materials

323

reduction of thermal stability has been explained to be due to the loss of structural integrity, reduction of molecular weight and debonding of the fibre from the matrix, resulting from the development of shear stress at the interface due to absorbed moisture. However, the extent of reduction of thermal stability after hygrothermal ageing was found to be less for reprocessed composites than for virgin composites. This was also supported by crystallinity index where a 25% reduction of PP crystallinity was found for virgin composites and a 10% reduction was found for composites reprocessed eight times after hygrothermal ageing.

12.4

Conclusions

Currently, the desire is ahead of the ability to recycle plastic matrix composites and a number of different strategies are under investigation. For thermoset matrix composites mechanical breakdown of material is the most progressed, with a number of companies now carrying this out on an industrial scale. Although the finer grade particulates can be used to replace filler, leading to some modest performance improvement, the challenge still remains to avoid down-cycling within ideally the same processing stream or to find sufficient markets for the range of mechanically recycled grades. However, improvement of mechanical performance has been obtained by using mechanical recyclate in base resin systems, concrete and thermoplastics. Mechanical breakdown is also the most progressed recycling method for thermoplastic matrix composites. Indeed, here there is a greater potential than for thermoset composites recycling due to better property retention. Research based on reprocessing of thermoplastic matrix composites shows some reduction of tensile strength and Young’s modulus, with poor surface appearance but increased failure strain and better moisture resistance. Reductions of fibre length and polymer degradation have been highlighted to explain the change in properties of reprocessed composites. Overall, issues of contamination and separation will also need to be addressed for increased mechanical recycling and therefore the development of infrastructure and systems. Thermal recycling has shown great promise for thermoset matrix composites. Here there is less concern regarding contamination and the need for sorting. Industrial-scale thermal recycling is currently coming on-line, particularly for the recycling of the more expensive carbon fibre from composites. Fibre extracted using thermal recycling has shown greater potential to be used in primary production than that through mechanical recycling. Chemical recycling is the least developed method of those investigated for both thermoset and thermoplastic composites. Although it has been shown possible to extract fibre with acceptable properties, chemical

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Management, recycling and reuse of waste composites

recycling raises the problem of how to deal with the waste chemicals produced, as well as health, safety and environmental concerns and so currently could create more problems than it solves. As time progresses, increased landfill and petroleum prices along with increased legislation and technical progress would all be expected to change the balance in favour of increased recycling. In addition, companies are more likely to design with reuse in mind to enable easier recycling.

12.5

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