Successful closed-loop recycling of thermoset composites

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Abstract

This study investigates closed-loop thermoset composite recycling through grinding and reincorporation. Contrary to many other studies, it is shown that by careful attention to the separation and reformulation procedure, the mechanical integrity of recycled composites can be improved, and the use of substantially larger volumes of recyclate is possible. Both factors are important aspects affecting the economic viability of regrinding as a possible future solution to the composites recycling problem.

Introduction

The difficult issue of recycling glass or carbon fibre reinforced thermoset composites, has been the subject of much investigation over the last 15 years. An excellent review of the subject, together with the status of current recycling methodologies has been provided by Pickering [1]. Sheet moulding compounds (SMC) and dough moulding compounds (DMC) have received more attention than most other types of composites because they are used in large quantities in the automotive sector. Automotive manufacturers are now under considerable pressure to meet new recycling legislation, particularly the End-of-Life Vehicle Directive (2000/53/EC), requiring the re-use and recovery of at least 95% of the average weight of a car by 2015. SMC/DMC recycling was the particular focus of a concurrent review by Bartl et al. [2]. This provided a detailed account of competing methodologies including pyrolysis, fluidised bed, chemical degradation (through hydrolysis, glycolysis, and solvolysis,) and mechanical grinding – the only method that has found its way into commercial processes conducted on an industrial scale. These operations have not, however, been able to maintain a positive cost balance, where the cost of the recycling and refining outweighs the market value of the recyclate produced. A characteristic of successful recycling operations is at least a neutral balance sheet. Successful commercialisation is hindered by the practice of using ground recyclate as a replacement for cheap filler materials and thereby representing only a modest cost saving to the moulder, leading to recyclate being regarded as a low value commodity. Manufacturers are also deterred by the tendency for these recyclate additions to negatively affect the mechanical properties of the finished composite. This study goes some way to addressing both these issues. Through careful separation and classification of recyclate a more intelligent replacement approach can be adopted, enabling recyclate materials to be used, not only for filler replacement, but also as part of the reinforcing component, thus adding value. In addition, it is shown that by careful attention to mixing times and reformulation practices, large percentages of recyclate can be used without affecting the mechanical performance of the finished composite. It is hoped that these findings could make thermoset recycling more commercially viable in the future.

Mechanical grinding and reincorporation as a method of recycling composites has been evident since the beginning of 1990s. Much of this work has focused on the granulation of waste SMC/DMC parts to a fine powder for reincorporation into new SMC or DMCs, as a replacement for the filler fraction. Studies carried out by Jutte and Graham [3] attempted to produce DMC incorporating recycled SMC regrind as a direct replacement for filler, exploring the use of several different grades over a range of loadings. The study reported a reduction in mechanical properties in all cases. These results were complicated by the finding that the addition of granulated SMC significantly increased the mix viscosity compared to conventional CaCO3 – this resulting in a “dry” mix lacking cohesiveness. At higher recyclate loadings the problem was so acute that the overall resin content had to be increased by a third. These factors would have influenced mechanical performance of the finished composite. Derosa et al. [4] was the first to take into consideration the actual material composition of the recyclate and adjust reformulations accordingly. Despite the more considered reformulations the study again showed a drop in the mechanical properties of the resulting DMC composites where, DeRosa reported a 70% reduction in flexural strength.

Bledzki et al. [5] conducted one of the few thorough investigations which takes into consideration all the issues of granulation and separation of long fibre recyclate to replace glass fibres as reinforcement, and reported positive results for mechanical properties. The researchers first attempted to replace the reinforcing component in DMC with recyclate materials from ground SMC, where three grades of fibrous recyclate (coarse, medium, and fine) were chosen for investigation. These three grades were then used in new DMC formulations by replacing the virgin fibres at 5, 10, and 30% by weight of the formulation, whilst retaining the overall glass content at 35%Wt. It was again found the mechanical properties suffered in all cases, with a reduction in flexural strength values compared to standard material. The behaviour was attributed to fibre damage, and poor bonding between the recyclate particles and the new resin matrix.

The above work was extended where the replacement of both fibre and filler fractions was attempted [6], and various combinations of recyclate grades and replacement percentages were explored and modelled [7]. The findings from this work were that, when used at 15% replacement, an increase in flexural strength and modulus was achievable, though in all cases, tensile and impact strengths could not be maintained. The improvements observed could have been a result of a raised fibre content in samples compared to the standard. Nevertheless this work did show that with careful replacement, mechanical properties need not be degraded in the finished composite, and also highlighted the importance of producing well separated and characterised recyclate fractions after grinding.

The grinding process itself, results in recyclate constituting a collection of small particles and fibres, including finely ground filler and resin particles less than 100 μm in size, together with large fibrous clumps up to approximately 5–10 mm in length, some plate-like particles and other singular fibres with very high aspect ratios. This material is then generally separated or classified by some means, though because there is no standard method of classifying these materials the methods investigated have varied greatly. Sieving classification is the most used in spite of issues such as the “fuzz-balling” – an effect created by the glass fibres and motion of the sieves [3]. Additionally, contamination of all grades with longer fibres is unavoidable as the high aspect ratio fibres are able to fall though successive sieve meshes. Combinations of air classifiers and cyclones are employed by industrial companies such as ERCOM, though final grades of materials are still determined by sieving. This approach is unsurprising considering that the approach taken by ERCOM (and others) is to grind the recyclate to a fine powder for use as filler to replace CaCO3 in new composites [5], [8], [9], where the retention of a significant fibrous fraction is not attempted.

The general conclusion of the foregoing work is that the characteristic drop in mechanical properties through using recyclate is most likely due to a combination of reduced recyclate fibre length caused by the granulation process [6], and poor bonding between the recyclate and the virgin matrix as another possible cause of weakness [3], [4]. The quantity of recyclate used to replace filler and the methods of reformulation have varied greatly between studies, however, the substitution with recyclate has typically been limited to replacements of 5–30%, by weight. This is due to the recyclate’s high resin absorption and the resulting increased processing difficulties, which frequently resulted in poor wetting, or a “dry” mix, which would again contribute to a drop off in mechanical properties in the finished composite [10], [11]. This is evidenced by the fact that even when fibrous grades have been used to replace basic filler materials, resulting in higher overall fibre content, the flexural, tensile, and impact properties have all degraded [10].

It is believed that a successful replacement of reinforcing fibres rather than just the filler fraction, would make the use of recyclate more competitive and financially viable. Therefore, the current study attempts to produce a coarse grade of recyclate from ground SMC which retains original glass fibre length. The recyclate is then further refined using a novel air separation technique and then reused as reinforcement in new DMCs. New reformulation and manufacturing methods (taking into consideration the recyclate content, the fibres morphology and the effect of mixing during sample preparation) will be investigated in an attempt to optimise mechanical properties in the reformulated composite.

Section snippets

Grinding method

Automotive front fenders made of glass fibre reinforced SMC were used as the feedstock (received from Mitras Automotive Ltd.). They were first cut down to 20 × 20 cm sections using a band saw and then fed into a TRIA Screen-classifier type hammer mill with an 8 mm classifier screen. This granulation method has been reported to produce long fibre recyclate materials that could be used as reinforcement [8]. Visual inspection of the recyclate showed that a smaller classifier screen produced too fine a

Materials

All DMC virgin ingredients were provided by Menzolit UK and used as received, based on a general purpose DMC formulation. The exact formulation is the intellectual property of Menzolit UK and therefore it will not be discussed in detail. The formulation was based on an ortho-phthalic polyester resin, calcium carbonate filler and E-glass reinforcing fibres as the main ingredients.

Standard DMC compounding

All DMC batches were mixed using a 1.5 L ‘Z’-blade mixer. The control DMC batch was prepared using standard

Manufacturing process

For each formulation 1.5 kg of DMC compound was produced so that four flat panels of dimensions 230 × 230 × 3.6 mm could be moulded. Compression moulding was performed with a hot press, where all formulations were cured at 145 °C and 4.1 MPa for 3 min, replicating the curing conditions of the standard composite.

Testing

For each of the different formulations, two batches were manufactured separately to minimise the effect of batch to batch variations. From each of the two batches, two of the moulded panels were selected randomly for mechanical testing. From each panel 16 test samples were prepared, 8 for flexural and 8 for impact testing, resulting in a total of 32 samples tested for each mechanical test for every different formulation.

Mixing times

The results of the mechanical tests on the formulations with different mixing times are shown in Fig. 7, Fig. 8, Fig. 9. This investigation clearly shows that the mixing time does affect the mechanical properties when reincorporating recyclate materials.

The optimum mechanical properties are obtained when the recyclate receives an extended mixing time of 8 min, showing a 10% increase in impact and a 25% increase in flexural strength, over the standard 4 min mixing procedure, for both the fine and

Conclusions

This investigation shows that the use of a novel zig–zag air-classifier separation technique allows refined grades of useful recyclate to be produced, which can be used to replace existing reinforcing fibres in a new composite, with minimal effect on the mechanical properties.

Although the mixing times used for standard DMC manufacturing are well established in industry, the incorporation of fibrous recyclate materials clearly requires more careful consideration. The results of mechanical tests

Acknowledgments

The authors would like to thank Ester Wegher at Menzolit UK Ltd. for technical support and supply of materials and Dr. Richard Hooper at Sims Group UK Ltd. for supply of recyclate materials.

This project is co-funded by the Technology Strategy Board’s Collaborative Research and Development programme, following an open competition. The Technology Strategy Board is a business-led executive non-departmental public body, established by the government. Its mission is to promote and support research

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