Composites Part A: Applied Science and Manufacturing
ReviewChemical recycling of glass fibre reinforced composites using subcritical water
Introduction
The total volume of end-of-life and production waste generated by the glass thermoset composites market in Europe is expected to reach 304,000 tonnes by 2015 according to the European Recycling Services Company (ECRC). Regarding respect for environment, the criterion of recyclability is today as important as performance or cost criteria for composite materials. The current non-recyclability of thermoset composites and the obligations imposed by the regulations can cause the manufacturers to prefer more recyclable materials [1]. The development and the sustainability of the composite industry could be well at stake [1]. The recycling of thermoset composite materials has thus been studied extensively for many years. Three methods have been researched: material recycling, thermolysis and chemical recycling. Material recycling involves grinding of glass fibre reinforced composites to a recyclate that can be used as fillers or partial reinforcement in other product but the incorporation rate is limited [1], [2]. Thermolysis involves the volatilisation of the resin releasing the fibres and, eventually, fillers from the composite. But in the case of glass fibre reinforced composites, the fibres are strongly degraded [3]. Chemical recycling operates at lower temperatures and uses a solvent to depolymerise the resin and release the fibres and, eventually, fillers. The interest is that valuable product can be recovered from the resin and the fibres can be reused. Water constitutes an interesting reaction medium since it is inexpensive, ecological and easy to handle. Hydrolysis of esters is known to take place in the presence of strong mineral acids or bases producing considerable amounts of waste acid and salt [4]. Thus hot compressed water can be successfully used to hydrolyse polyesters without adding further product, as its ionic product can be three orders of magnitude higher in the range 200–300 °C than in ambient conditions, so water can play the role of an acid or base catalyst [5].
Chemical recycling in supercritical conditions of water (T > 374 °C and P > 221 bars) and alcohols has been more applied to carbon composite materials because of the value of carbon fibres, but without any interest in the products issued from the resin (which is more often epoxy) [6], [7]. It has also been applied to glass fibre reinforced composite materials like sheet moulding compounds (SMCs) [8], [9], [10], [11], or to polyester resin model without fibres [12], [13]. Nakagawa [9] have developed a hydrolysis process at rather low temperature (230 °C) but that requires the use of alkali catalysts to realise the treatment in 4 h. They would recover a styrene–fumaric acid copolymer that could be reused as low profile additive after a complex chemical reaction, and glycols. The fibres were not considered. Other works [8], [10], [11] also used different solvents with catalysts and recovered essentially secondary products; the fibres were not analysed.
The objective of this paper is to present a more complete evaluation of recycling by subcritical hydrolysis of glass fibre–polyester composite materials, in terms of quality of recovered fibres, nature of recovered products and process efficiency using a commercial product.
Section snippets
Glass fibres reinforced composite material
The composite samples were taken from an infused plate. They contained 37.5% by weight resin. The resin used was Synolite 8488-G-2 developed by DSM©, made of an unsaturated polyester pre-polymer based on DCPD (dicyclopentadiene) in styrene monomer. The polymerisation was realised at ambient conditions in between 78 and 92 min. No post-curing was done. The reinforcement was a biaxial fabric (0°/90°) made of E-glass Advantex® fibres and standard E-glass fibres from Owens Corning©. Two plies of
Influence of the operating conditions on the yield of eliminated resin
The results of the two series of experiments, given in Table 1, Table 2, were treated with the statistical analysis software Statgraphics© Plus 4.0 to evaluate the influence of the parameters on the criterion yield of eliminated resin.
The temperature appeared to be the most influential parameter. An alkaline medium seemed to catalyse the hydrolysis reaction at 250 °C but not sufficiently and showed a very negative effect on the fibres at 350 °C, according to the results of the first series of
Conclusions
The study allowed an evaluation of the potential of the hydrolysis process to recycle glass fibres reinforced composites. The glass fibres were separated from the resin, but contaminated by a residual organic substance. A washing with an appropriate organic solvent was necessary and proved to be an important step of the process in batch conditions. A degradation of the mechanical properties of the glass fibres was observed and increased when the reaction temperature increased. The temperature
Acknowledgements
This study was carried out as part of the EURECOMP Project with funding received from the European Community’s Seventh Framework Programme (FP7/2007–2013) under grant agreement no. 218609. The authors wish to thank to all the partners of the Project and particularly Dr Keith R. Hallam and Dr Peter J. Heard from the Interface Analysis Centre of the University of Bristol (UK), who provided the ToF-SIMS, XPS and ESEM data, and Dr Oana Ghita and Dr Chih-Chuan Kao from the College of Engineering,
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