Recycling carbon fibres from aerospace scrap offers environmental and business benefits

Recycling carbon fibres from aerospace composite scrap has environmental and business benefits. This is leading to progress in a shift from pilot scale to industrial operations. Demand and consumption of virgin carbon fibre is estimated to cross the 100,000 tpa mark by 2018. About 3000 tpa of carbon fibre composite (CFRP) scrap is estimated to be generated in USA and Europe. By 2030, an estimated 6000 to 8000 commercial planes will reach end-of-life dismantlement leading to recycling the high-grade carbon fibre components onboard. Neither landfill nor incineration disposal of CFRP scrap is optimal. Also, environmental regulations may eventually lead to a ban on both. Currently, the industry is in its infancy, and processes are expensive and complicated mainly because high-performance engineered materials are well engineered. CFRP recycling involves formidable requirements: consistent scrap availability, appropriate size reduction technologies for the CFRP waste, established process parameters, the infrastructure for secondary operations such as material collection at a manufacturer�s site, and eventually, creation of standardised recyclate product properties. Recycled carbon fibres are perceived to be secondary or �lesser than� quality than VCF. While recyclate properties vary, leading R&D indicates that properties reduction of only 3-5% compared to VCF has been exhibited in reclaimed chopped and milled fibres used to make thermoplastic compounds such as bulk moulding compound (BMC).

Adherent Technologies Inc (ATI) based in Albuquerque has evolved a catalytic conversion technology centered around its batch-based carbon fibre recyclate processing, combining three different processes studied over the past decade, each with specific advantages and limitations. Vacuum pyrolysis, a dry process operated at around 500�C (932�F), recovers resins as marketable liquids and can be easily scaled up to multi-tonne capacity. At this temperature, however, fibre product may retain oxidation residue or char. The company�s low-temperature liquid process operates at 150�C (302�F), runs at less than 150 psi on standard equipment, and produces a market-ready fibre, but is not particularly tolerant of scrap contaminants (such as metal, wire, paint and sealants). The high temperature liquid option (around 300�C/572�F) produces clean fibres from most composites, but requires customised equipment and is currently not considered necessary for commercial recyclate production. Phenol has proven a good choice as initial heat transfer fluid for both wet processes; the breakdown products of the resin can be recycled into glue for the production of plywood. ATI has demonstrated its low-temperature wet process in combination with vacuum pyrolysis for the removal of insoluble contaminants in a pilot-scale reactor capable of processing 23 kg (50 lbs) an hour. Testing of its recycling technology on CFRP scrap with multiple resin chemistries indicates that the combination of dry and wet processes is the best way to maximise recyclate quality. Low temperature wet chemical processing removes the bulk of the resin and some contaminants, followed by thermal post-treatment through vacuum pyrolysis to eliminate remaining resin and produce 99% fibre purity.

Microwave pyrolysis is another form of CFRP recycling under development by companies and universities in the USA, UK and Germany. Generally, microwave energy absorbed by the conductive properties of carbon fibre heat the matrix resin internally rather than externally. The can result in more rapid resin decomposition and recovery of fibres without char formation, shorter overall processing time, and smaller scale equipment than is required for other pyrolysis methods. Over the past three years, research company Firebird Advanced Materials Inc in Raleigh, North Carolina, USA, has built a small pilot-scale installation to test its microwave recycling process, and this year, has begun implementation of commercialisation plans.

Aircraft OEMs play a critical role in making CFRP recycling viable, as do government agencies, industry organisations, and universities in enhancing R&D and project cost sharing. Leading ongoing recycling technology and demonstration programs underway internationally are briefly described below. Original equipment manufacturer (OEM) Boeing aims to boost the amount of recycled aircraft material from 70% today to 90% by 2016. The company began CFRP recycling from retired F-18A military planes in 2005, carried this through 777 composite components and even used 787 pre-production scrap from a fuselage test article to prototype a seat arm rest and composite lay-up tool.
Airbus Industries aims to enhance the eco-efficiency of new aircraft so that 85-95% of components and materials could be recycled, reused or recovered, and to establish new standards for green management in the disposal of end-of-life aircraft. In a separate consortia effort, Airbus is working with CFK-Valley Stade Recycling GmbH & Co KG of Stade, Germany to develop a 1000 tpa pyrolysis-based recycling plant for the recovery of carbon fibres from both CFRP manufacturing scrap and decommissioned Airbus aircraft.
University of Nottingham has a decade worth of R&D into carbon fibre recycling, particularly in the fluidised bed process and more recently, involving supercritical fluids such as propanol. Researchers have created nonwoven mats with CFRP recyclate as are developing a convergent flow slurry process to enhance fibre alignment and achieve higher fibre volumes. The university has also participated in the automotive-focused HIRECAR (High Value Composite Materials from Recycled Carbon Fibre) project (2005-2008) and is now involved, with partners, in the follow-on AFRECAR (Affordable Recycled Carbon Fibres) project with technology and application research inclusive of both automotive and aerospace industries.
(Source Courtsey: Vicki P. McConnell in Reinforced Plastics magazine)