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Recycling of Engineering Polymers

By Oana Ghita and Mike Sloan, Exeter University

Established in 1997, Exeter Advanced Technologies (X-AT) is a team of industry focused multi skilled technical researchers based within the College of Engineering Mathematics and Physical Sciences at the University of Exeter. As a self funding organisation, X-AT specialises in applying its expertise to industrially focused grant funded research projects and commercial consultancy services. Its core expertise is in materials selection and characterisation and across its 19 current research projects, in which knowledge is applied to enhance the sustainability of materials and manufacture. One approach to sustainable manufacturing is to recycle materials.

The European (FP7) funded project EURECOMP aims to develop a novel, eco-friendly, and closed-loop route for recycling thermoset composites through solvolysis. It gathers partners from various fields of activity including universities, research centres, and industrial companies (from materials producers to end users). The water-based solvolysis process (Figure 1) separates composites into different components, such as fillers, fibres and matrix compounds, under specific temperatures and pressures for reuse in new applications. The recyclates, (thermosetting resin and reinforcing fibres), are subsequently investigated in order to optimise the process and the quality of the products.

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The technology is expected to solve the problems of recycling thermoset composites and overcome the drawbacks of other recycling techniques. Because solvolysis depolymerises the chemical bonds in thermoset resin, it enables the organic components of the composites to be reused. Furthermore, in comparison to mechanical granulation processes, solvolysis can maintain the length of the recovered reinforcing fibres so that they can be reused for specific industrial and technical applications.
Building upon the success of the Technology Strategy Board (TSB) funded recycling project RECCOMP (Recycling of thermoset composites for automotive applications), the team at X-AT contribute to EURECOMP by mechanical characterisation of the recyclate fibres obtained from the solvolysis process. Advanced fibre micro-testing facilities at X-AT are used to facilitate the investigation of single fibre mechanical properties (e.g. tensile and interfacial properties). In the past, it has been shown that the mechanical performance of recyclate fibres is strongly affected by recycling processes such as pyrolysis and mechanical granulation. To minimise degradation of fibre strength, EURECOMP will carefully optimise the solvolysis process through a detailed investigation of its various parameters and the products recovered. The project explores not only the fundamentals of solvolysis but also the practical implementation of the technology. In addition, the project collates information regarding the implications of solvolysis for upstream and downstream markets, economic efficiency, and life cycle assessment.

Building on the recycling theme of EURECOMP, the second approach taken by the team at Exeter is to reduce the waste of current manufacturing processes illustrated by the TSB funded CAPSCRAP project. Working with ten UK based polymer companies the team at Exeter aims to minimize and recycle the waste generated from injection moulding in-house. For the past two years, X-AT has helped partner companies Robinson Plastic Packaging and ALGRAM to clean the contaiments from their waste stream using novel separation techniques and convert it to a high value recylate stream. Much of this waste is caused by variability in the supply of raw materials and an inability to monitor their charaterastics online (i.e. whlist being processed). The project is developing a methodology to prevent scrap from being generated at source by close monitoring of the polymer melt during injection moulding. This is achieved by employing an in-line monitoring system developed at Exeter in conjunction with Colormatrix Europe Ltd. (the project leader).  The system uses two optical fibre probes that continuously monitor material as it flows through the injection nozzle. In this way, real-time spectroscopic information on the condition of the liquid polymer can be collected during the injection process. This offers a solution to materials related problems by providing additional process control based on the real-time condition of the material itself, rather than operator judgement.  Other consortium partners Boots plc., Becton Dickinson, Data Plastics, AAVF, and the British Plastics Federation are excited by the possibility of monitoring polymer properties such as mositure content and material degradation in this way. The team at X-AT are working with individual partners to design tailored solutions for each company (i.e. to exploit the in-line monitoring system in a manner that best suits their needs). It is estimated that 63% of the UK plastics sector could potentially benefit commercially from CAP-SCRAP technology.

In addition to recycling and reducing waste, researchers at X-AT are applying their skills to boost sustainability from the very first stages of manufacture. By replacing synthetic materials with sustainable ones, and reducing the net energy input into manufacture, the TSB funded ECOBRAKE project is developing novel brake pads for mass rail transit applications. Figure 2 shows a pair of rail brake pads, with a 50 pence coin for scale. Four of these pads are used on each disc.

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A conventional brake pad is a complex composite of more than 10 ingredients, bound together by a synthetic resin (bakelite).  Material formulations are developed empirically and manufactured into brake pads using technologies based around asbestos fibres, which have subsequently been replaced with aramid. In a conventional brake pad, neither the resin (its greatest single component), or the fibres (its most expensive component) are sustainably sourced. The manufacturing process is also incredibly energy intensive. The ECOBRAKE project is developing a new method of manufacture that requires considerably less energy and makes use of environmentally sustainable materials.  Synthetic phenol-formaldehyde resins have been replaced with ones derived from Cashew Nut Shell Liquid (CNSL), and aramid fibres have been (partially) replaced with hemp.

Found in the husk of the cashew nut shell, raw CNSL is a viscous substance that contains a variety of naturally occurring phenolic compounds.  It accounts for
18 – 27% of the raw nut weight and, since the shells are a waste product of the food industry, is in cheap and plentiful supply.  After extraction, the major component (~60%) of CNSL is ‘cardanol’, which can be used to synthesise phenolic resins. In the new manufacturing process, aramid can be completely replaced by hemp fibres – grown and extracted in the UK by Hemp Technology Ltd.  Hence, there is potential to extend this technology to produce a host of eco-friendly structural composites at a fraction of the energy consumed using current processes. Developing the friction materials using novel fibres, resins and a new manufacturing process requires a detailed understanding of the raw materials. Samples are analysed throughout the research to characterise their mechanical and thermal behaviour so that the manufacturing process may be tailored to optimise their performance.  Tested on a full-scale rail simulation inertia dynamometer, brake pads made with hemp fibres and CNSL-based resin exhibit significantly better wear performance than the current market material.  Supplementing the hemp fibres with a certain proportion of aramid enhances this performance by up to 40% and results in a brake pad that contains a significant proportion of sustainably sourced, lower cost materials and is produced by a manufacturing process that is quicker, cheaper, more reliable and uses less energy.

Sustainability is a core part of the material research taking place at the University of Exeter

for further details please contact:
Exeter Advanced Technologies;
Tel: + 44 (0)1392 723617
Web: www.x-at.co.uk