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This paper presents an overview of FRP/composite recycling activities in Europe, North America and the Pacific Rim area, where great efforts have been made recently. A green-label system and the European Composites Recycling Concept (ECRC) have been launched in the EU. Recycled materials (recyclate) are much more commonly used in North America. In Japan, the cement kiln process is being used with success, while chemical recycling technologies are being developed actively.
KANEMASA NOMAGUCHI, BOARD MEMBER OF JRPS (JAPAN REINFORCED PLASTICS SOCIETY), CHAIRMAN OF THE LNTERNATIONAL EXCHANGING COMMITTEE
TAKAHARU NAKAGAWA, GENERAL MANAGER, SUSTAINABLE PROCESS DEVELOPMENT GROUP, ADVANCED TECHNOLOGIES DEVELOPMENT LABORATORY, PANASONIC ELECTRIC WORKS CO., LTD.
This report first touches upon the general situation in the world’s three main areas, then goes on to describe the current FRP recycling situation in Japan in more detail.
FRP recycling in Europe, North America and the Pacific Rim area
FRP recycling has been described in many reports. In this paper, when we refer to FRP (Fibre Reinforced Plastics), we mean composites with thermosetting resins. Materials like glass-mat thermoplastics (GMT) and long-fibre thermoplastics (LFT), known as fibre-reinforced thermoplastics (FRTP), have more recently been recognized as composites.These composites consist of three types of materials: reinforcement fibres (e.g. glass fibre), matrix resins and, if needed, fillers (Figure 1).
These materials have been widely used in automotive applications, as shown in Table 1 and Table 2, because they are lighter than steel, and sometimes less problematic. Today, the lightness and safety properties of composites are taken into account more widely in automotive technology development. Some sample cases are shown below. SMC mouldings, which previously were considered as materials that are difficult to recycle, have been adopted successfully in automotive parts, particularly for exterior panels.
Composites like FRP and FRTP combine benefits such as mechanical strength with lightness, shape flexibility, weather/corrosion resistance and cost issues. For these reasons, FRP recycling technology is expected to develop. A proposed concept is shown in Figure 2.
To meet the recycling requirements, active research has been carried out in the European Union, North America and Japan. The first solution developed was the cement kiln process, which has already been used in Japan to recycle more than 1,000 metric tons of waste FRP, and in the EU, to recycle waste from FRP fishing boats, bathtubs and water tanks (Table 3).
FRP moulding shops traditionally have recycled some of the process scrap consisting of chips from the composites and rejects. However, this concerns no more than a few per cent of the total.
On the other hand, the amount of end-of-life waste FRP is almost equivalent to production. While this type of waste can be easily recycled using the cement kiln process, it doesn’t constitute “bumper to bumper” recycling (where the recycled material from a part is made into another equivalent part), which is our ultimate goal.
Progress in chemical recycling
The cement kiln process is the first (Phase α) solution to mandatory recycling requirements. The waste FRP to be recycled comes from automobiles, aircrafts, rolling stock, electronics and so forth. It includes valuable materials such as carbon fibres, resins and other components. Chemists have been trying to develop certain “degradation and rebuilding technologies” with synthetic resins from composites matrices, using supercritical or subcritical water systems. With these technologies, the "bumper to bumper recycling” dream will hopefully come true.
FRP recycling technology using subcritical water hydrolysis
Fibre-reinforced plastics are defined as composite materials made of a thermosetting resin and inorganic materials such as inorganic fillers or glass fibre. They are widely used for many products such as bathtubs or pleasure boats due to their lightness and high mechanical strength. They are used in automotive parts to save weight and thus reduce C02 emissions, and trials applying them for automotive body parts have also been conducted. However, their constituent thermosetting resin cannot be re-shaped after the final cure. In addition, they are very difficult to burn since their inorganic material content is about 50-70 wt.%. The total amount of FRP waste in Japan reaches 400,000 metric tons annually and most of it is landfilled, Ieading to a wasteful use of Iimited oil resources. To reduce waste amounts and utilize exhaustible oil resources more efficiently, these materials (including the thermosetting resins) should be recycled, which would be more conducive to a sustainable society. However, there is no effective method available to recycle FRP materials along with the thermosetting resin so, in spite of its weight-saving advantages, FRP is no longer widely accepted because improved recycling rates are now required for automotive parts.
Our research aims at establishing an effective horizontal recycling technology that includes thermosetting resin so as to solve the above-mentioned environmental issues.
We focused on subcritical water hydrolysis of thermosetting resins and were instructed by Professor Hiroyuki Yoshida of Osaka Prefecture University. Subcritical water is hightemperature, high-pressure water under the critical temperature (374°C). It is known to have a significant hydrolytic capability with high resin affinity due to its ion product content, which is 1,000 times higher than at ordinary temperature, and its low dielectric constant equal to organic solvents.
FRP recycling concept using subcritical water hydrolysis Figure 3 shows the concept for the subcritical water hydrolysis recycling process applied to the thermosetting resin in FRP. First, unsaturated polyester (UP) is obtained through a dehydration reaction with glycol and organic acid. It is cured with styrene as cross-linking agent, glass fibre and inorganic filler. During cure, styrene develops a styrene cross-linking moiety to form a lattice molecular structure of thermosetting polyester resin. The resin’s ester bonds can be hydrolysed using the significant hydrolytic capability of subcritical water to generate resin raw materials and styrene-fumaric acid copolymer (SFC). The same monomers of resin raw materials are obtained since the ester bonds were generated through dehydration.
Optimizing reaction conditions for subcritical water hydrolysis
Figure 4 shows the experimental results of subcritical water hydrolysis. The experiment started with a reaction temperature higher than 300°C. The reaction liquid collected was black at 360°C for 20 min. Only 20% of the resin can be recycled, and 70% of the decomposing organic ingredients appear to be substances derived from pyrolysis of the styrene cross-linking moiety, suggesting that hydrolysis could be dominant, ideally through suppressing the pyrolysis. It was reported that the pyrolysis of the styrene cross-linking moiety of the thermosetting polyester resin started at 230°C [I]. To suppress the pyrolysis, the reaction was conducted at 230°C and 2.8 MPa for 4 hours with calcium carbonate. Figure 8 shows the experimental results obtained at 230°C. The reaction liquid obtained was clear. No substances derived from pyrolysis of the styrene cross-linking moiety were observed. Several alkali catalysts were then examined under the same reaction conditions (230°C, 2.8 MPa for 4 hours) to improve the conversion. Table 4 shows that the highest conversion of 96.9% was achieved using potassium hydrate. The recovery rate of glycol and fumaric acid was 70.7% and 21.81% by weight, respectively.
Figure 5 shows the SFC extracted from the reaction liquid and its molecular formula. A molecular structure analysis of the SFC showed that the styrene/fumaric acid ratio was 2.2:1 and its molecular weight was approximately 30,000 . The weight of SFC was 75% of the weight of the initial thermosetting resin. Thus, combining with glycol and fumaric acid, it was shown that up to 96 wt.% of the initial resin existed in the reaction liquid as reusable ingredients. It was obvious that hydrolysis was ideally dominant and the pyrolysis was almost suppressed in these reaction conditions.
Horizontal recycling of UP resin
Figure 6 shows the experimental results of UP resin horizontal recycling from recovered glycol. The recovered glycol was polymerized into polyester with new resin raw materials to produce recycled UP resin with molecular weight higher than 4,000, that was used to produce an FRP board sample for strength-characteristic testing. The recovered glycol/new glycol ratio was 1:9. It showed strength characteristics equivalent to a new UP resin.
Enhanced recycling of the styrene-fumaric acid copolymer
Molecular structure analysis of the SFC suggested that it has a potential for various applications. In exploring the application, it was found that its molecular structure was similar to that of a commercial LPA (low profile additive) comprising polystyrene for FRP forming (Figure 7).
The LPA is a high value-added additive used to suppress shrinkage during FRP forming. The LPA stores styrene before forming and releases it during forming, keeping the space occupied by styrene vacant to suppress shrinkage. Therefore, solubility in styrene is needed to generate a shrinkage control effect. The SFC was not soluble in the styrene monomer due to its hydrophilic group. Hydrophobic modification of the SFC using benzyl chloride as modification agent and tetra-n-butyl ammonium bromide as phase transfer catalyst was tried to add solubility in styrene. Figure 8 shows the modification reaction experiment result. A modified SFC – SBFC (styrene-benzyl fumarate copolymer) – was obtained.
SMC (sheet moulding compound) sheets were produced with the resulting SBFC and the commercial LPA comprising polystyrene, inorganic filler and glass fibre to evaluate the shrinkage control effect. After cure of the SMC sheet, the shrinkage ratio of the FRP board sample was evaluated. FRP board samples were produced from hand-made SMC sheet, as shown in Figure 9. The blank sample without LPA showed crease and its shrinkage ratio was 4%. In the sample with the commercial LPA at 9 wt.% compounding ratio as solid content, the shrinkage ratio was 1.7%. The sample with 6 wt.% SBFC showed a 1.9% shrinkage ratio.
FRP board samples were then produced from SMC sheets made by a SMC test machine and were tested. The results showed that the SBFC’s shrinkage control effect was almost equivalent to the commercial LPA (Table 5). The market price of commercial LPA is 5 to 10 times higher than that of styrene. The results suggest that the SFC was regenerated to such high-value-added materials after recycling. Industrially speaking, the process can be called “enhanced recycling” because the materials can be recovered and renewed to a much greater extent than in more sophisticated applications such as LPA.
Bench test of the subcritical water hydrolysis process
In consideration of scale-up, a subcritical water hydrolysis bench plant with a capacity of 40 kg FRP per operation was built (Figure 10). The plant is a huge autoclave containing a reaction vessel with a 200- litre volume. A bench test was conducted using the plant. Although the test scale between the reaction pipe and the bench plant differed by a magnitude of 10,000, the test results of conversion, generation rate of the SFC and glycol were almost equivalent to that of the reaction pipe. The bench test successfully demonstrated the scale-up potential. Inorganic material separation was also bench tested and the recovery rate of the reaction liquid and inorganic materials was 90% and 95% respectively. In addition, the separation process of the SFC from the reaction liquid was also tested at the laboratory scale. The test result showed that the recovery rate of thermosetting resin was 70%.
Conclusion and future prospects
The reaction conditions for subcritical water hydrolysis were optimized to achieve the ideal reaction where hydrolysis is dominant. The recovery rate of thermosetting resin and entire FRP components reached 70% and 80% respectively. The recovered glycol was verified to be horizontally recycled to UP resin mixing with new resin raw materials. The recovered glycol/new glycol mixing ratio was 1:9. The SFC modified by benzyl chloride with tetra-n-butyl 1 ammonium bromide showed almost the same shrinkage control effect as a commercial LPA sold at a market price 5 to 10 times higher than styrene. This opens the way for enhanced recycling. In addition, the bench plant with a capacity of 40 kg FRP per operation for the subcritical water hydrolysis process and inorganic materials separation process was built and bench tested successfully.
Figure 11 shows the process flow of FRP horizontal recycling using subcritical water hydrolysis as mentioned above. First, thermosetting resin was hydrolysed to resin raw materials and SFC using subcritical water. These were dissolved into water. The reaction liquid became slurry with milled inorganic materials such as glass fibre and inorganic fillers. Inorganic materials and reaction liquid were then obtained from the slurry through a separation process. Resin raw materials such as glycol and SFC were separated from the reaction liquid. The recovered glycol was polymerized with new resin raw materials to produce the recycled UP resin. The SFC was modified to be enhanced to the recycled LPA which has a shrinkage control effect. These recycled inorganic materials, recycled UP resin and recycled LPA were mixed with new raw materials to produce recycled SMC sheets. The recyclates were horizontally recycled into FRP through hot-press curing of the recycled SMC sheets.
Figure 12 shows future prospects. We have been developing a technology designed to scale up the plant, reduce the cost of the SFC separation and modification process, and improve the quality of the recyclate. A pilot plant for subcritical water hydrolysis 10 times larger than the bench plant is under construction based on the experimental test results. The vessel volume was designed to test 400 kg of FRP per operation (20 bath tubs, 0.8 metric tons daily). Pilot testing will start soon. We plan to start treating FRP bathtub manufacturing waste so as to reach an FRP horizontal recycling objective of 200 metric tons annually by 2012. In the future, the potential of this technology will be explored for waste FRP bathtubs and other waste FRP products such as pleasure boats, tanks, automotive parts, etc. If the recycling issue is solved based on this technology, the use of FRP automotive parts will expand more rapidly. This should contribute to the prevention of global warming globally through lighter cars. Moreover, new applications for these unique functional polymers derived from waste FRP are also being explored.
This research was supported by the Ministry of Economy, Trade and Industry, and conducted with the International Centre for Environmental Technology Transfer. Professor Hiroyuki Yoshida of Osaka Prefecture University provided technical advice for subcritical water hydrolysis. We also cooperated with Showa High Polymer Co., Ltd. for the trial production of an SMC board sample. This article appeared once in Review of Automotive Engineering.