An Article Explains The CFRP Atmospheric Pressure Recycling Technology Of Hitachi Chemical Co., Ltd

 

Abstract

At present, carbon fiber composite material (CFRP) recycling and reuse technology has not yet been commercialized. As a major carbon fiber manufacturing country, Japan is also at the forefront of CFRP recycling technology. This paper first introduces several CFRP recycling technologies currently being developed in Japan, and focuses on the atmospheric depolymerization recycling technology developed by Hitachi Chemical.

 

The atmospheric depolymerization recycling technology developed by Hitachi Chemical can not only realize CFRP recycling under atmospheric pressure, but also the recycling reagents used are harmless to the human body, and the recycled carbon fiber also has excellent mechanical properties. In addition, this technology can also realize the reuse of recycled carbon fiber and resin.

 

This paper introduces the characteristics and advantages of atmospheric depolymerization technology in detail, and uses examples to show the structure and properties of carbon fiber obtained after recycling tennis rackets, badminton rackets, etc. by using this technology. It further introduces the reuse technology of the material system after atmospheric depolymerization process recycling, as well as the life cycle evaluation of the recycling carbon fiber process.

 

 

1. CFRP Recycling Background And Main Technologies

 

Carbon fiber reinforced plastics (CFRP) have been rapidly and widely used in transportation fields such as aircraft and automobiles due to their advantages such as light weight and improved fuel efficiency. However, the CF manufacturing process involves a long and high-temperature heating process, which consumes considerable energy. The results of the life cycle assessment (LCA) show that CFRP is not an environmentally friendly material.

 

Using CFRP to reduce the weight of the transportation system can save fuel, but researchers at the University of Tokyo pointed out that weight reduction has only a slight impact on commercial transportation systems, so it is recommended that the best option is to reuse the CF recovered from the old transportation system in the same transportation system. At present, the technology for recycling carbon fiber from CFRP and reusing it has not been commercialized, but the technology is gradually developing.

 

Table 1 shows the CFRP recycling technology currently being developed in Japan. Toray, Teijin and Mitsubishi Rayon Corporation of Japan are studying thermal decomposition recycling technology, in which CFRP is decomposed at a processing temperature of 500°C-700°C. In 2010, they entrusted the recycling work to the recycling plant of Japan Coke Engineering Co., Ltd. in Omuta City, Fukuoka Prefecture, which can produce about 1,000 tons of recycled CFRP per year.

 

Table 1 CFRP recycling technology being developed in Japan

Takayasu Co., Ltd. has developed a technology to recycle carbon fiber with longer lengths, and also developed a technology to cut recycled CF into any length, and use dry and wet methods to produce high-quality non-woven fabrics. The facility has a capacity of 5 tons/month.

 

The Institute of Chemistry at Shizuoka University is conducting a research project sponsored by the National Electric Power Company of Japan to study the CFRP recycling technology using supercritical alcohol. Epoxy resin EP is used as the matrix. Carbon fiber is recovered by decomposing EP using supercritical methanol, and thermosetting resin can be further prepared by removing methanol from the decomposed resin and adding a curing agent.

 

Kumamoto University Researchers are studying the method of recycling CFRP using subcritical alcohol. By heating high-boiling alcohols such as benzyl alcohol to subcritical at 300℃~400℃ and then treating CFRP, the entire resin will decompose within an hour. The study found that when alkali metal salts are used as catalysts, the damage to CF is less. The advantage of this method is that the pressure of the processing environment is relatively low, only about 4mpa.

 

Different from the above methods, Hitachi Chemical has developed a normal pressure depolymerization technology, which can achieve the recovery of carbon fiber from CFRP in a low-cost and low-energy form with certain economic efficiency. This article analyzes this technology in detail.

 

2. CFRP Atmospheric Pressure Recovery Technology And Advantages

 

Average pressure depolymerization is achieved by using a treatment liquid composed of an alcohol solvent and an alkali metal salt (catalyst) to depolymerize and dissolve the cured resin. When this technology is applied to composite materials containing thermosetting resins such as unsaturated polyester resins (UP), the resin in the composite material depolymerizes and dissolves, and inorganic substances (such as metals, glass fibers and carbon fibers) can be separated and recovered.

 

When CFRP is treated with a treatment liquid using tricalcium phosphate (K3PO4) as a catalyst and benzyl alcohol (BZA) as a solvent, since both K3PO4 and BZA are recognized food ingredients, their safety to the human body is undoubtedly very high. When CFRP is treated with this treatment liquid at a conventional pressure of about 200°C, the cured EP is immediately depolymerized and dissolved, allowing the CF to be fully restored. The treatment time depends on the thickness of the CFRP, but the entire EP dissolves in about 10 hours.

 

Compared with other chemical recycling technologies, this atmospheric depolymerization method has the following advantages: processing under atmospheric pressure, the recycled resin does not need to be pre-treated such as crushing, the fundamental reason is to determine the best combination of catalysts and solvents to selectively destroy the special bonds in the resin. Processing under normal pressure means lower equipment costs. In addition, continuous processing contributes to more economical mass production.

 

The recycled resin can be reused as a high-value material when reconstructed. Since this method eliminates the pretreatment process, it does not generate fragments, dust and crushing processing costs, and can improve the applicability of recycled materials. When the resin is crushed, the length of the recycled fiber is often less than 1mm, which hinders the reuse of the fiber. In terms of safety, atmospheric depolymerization eliminates the risk of dust explosion and crushing pneumoconiosis.

 

3. Comparison Of The Performance Of Carbon Fiber Recycled by Atmospheric Pressure Of CFRP

 

Figures 1 and 2 show the pictures of tennis rackets and badminton rackets recycled by atmospheric pressure recycling technology, all of which are processed with fiber-reinforced plastics, and the recycled fibers are mainly carbon fibers, with a small amount of glass fibers.

1 Recycling Tennis Rackets and Recycled Carbon Fibers Using Atmospheric Pressure

 

From badminton rackets, aluminum frames, wooden grips, and carbon fibers for wooden poles can be recycled. Since the strings of tennis and badminton rackets are dissolved during the recycling process, it is estimated that they are made of polymer esters. From the experimental results, it can be seen that the complete recycling of aluminum and wood is one of the characteristics of this method.

 

Figure 2 Badminton racket recycled by atmospheric pressure, and the recycled aluminum frame, wooden handle, and carbon fiber obtained

 

Figure 3 shows the SEM photos of carbon fibers recovered from molded parts of tennis rackets and transportation equipment after atmospheric pressure recycling, and Table 2 shows the results of single fiber tensile test.

 

To increase comparability, the structure and properties of carbon fibers recycled by pyrolysis and fresh carbon felt are also shown in the chart. The surface profile of CF recovered from molded structural parts by atmospheric pressure depolymerization in Figure 3 is roughly equal to the surface profile of new CF used for carbon felt processing.

Figure 3 SEM images of different types of carbon fibers

 

The mechanical properties comparison in Table 2 shows that the carbon fibers used for tennis rackets and carbon felt should be T300 carbon fibers based on the wet process. The surface grooves of the carbon fibers recovered from tennis rackets after normal pressure treatment are obvious, and the fiber strength and modulus are significantly higher than those of carbon felt.

 

Further, by comparing the strength of the recovered fibers in the structural parts formed by the normal pressure depolymerization method, which is 4.393 GPa, is much higher than the 3.459 GPa of the pyrolysis-recovered fibers, it is shown that the fiber performance after normal pressure depolymerization has obvious advantages.

 

Table 2 Comparison of the performance of carbon fibers recovered by different methods

 

4. Application Development Of Recycled Carbon Fiber

 

The carbon fiber recovered from carbon fiber composite material (CFRP) is in a flocculated state, which may reduce the production efficiency of CFRP and hinder the production of high-quality CFRP. To solve this problem, Hitachi Chemical staff studied how to use recycled carbon fiber to produce non-woven fabrics.

 

Non-woven fabrics are produced by dry or wet methods, where a carding machine is used in the dry method and a paper press is used to produce floccules during wet production. Since CF is conductive, the processing equipment must be fully insulated. CF recycled by dry method can be made into non-woven fabrics. By using professional processing equipment, CF is opened and combed, and some thin CF is layered to produce non-woven fabrics, as shown in Figure 4.

 

Figure 4 Processing Recycled Carbon Fiber into Nonwoven Fabric Using a Carding Machine

 

Figure 5 shows a nonwoven fabric made from recycled carbon fiber. In this nonwoven fabric, CFRP is experimentally made by compression molding.

 

Figure 5 Nonwoven Fabric Processed From Recycled Carbon Fiber

 

In order to make the fibers in the nonwoven fabric in a better state, the nonwoven fabric was further processed, and the molded recycled carbon fiber nonwoven fabric produced by molding is shown in Figure 6.

 

Figure 6 Molded recycled carbon fiber cloth obtained by reprocessing recycled carbon fiber nonwoven fabric

 

The mechanical properties of CFRP processed by recycled carbon fiber nonwoven fabric from molded parts of tennis rackets and transportation equipment were compared with those of carbon fiber CFRP materials made from pyrolytic recycled carbon fiber and fresh carbon felt. Figure 7 shows the comparison of CFRP tensile strength and tensile modulus, while Figure 8 shows the comparison of CFRP flexural strength and flexural modulus.

 

Figure 7 Comparison of Tensile Properties of Several Recycled Carbon Fiber and New Carbon Fiber Composites

 

Figure 8 Comparison of Bending Properties of Several Recycled Carbon Fiber and New Carbon Fiber Composites

 

Compared with carbon fiber composites recycled by pyrolysis and fresh carbon felt, there is no obvious difference in the mechanical properties of carbon fiber composites recycled by atmospheric pressure, which also shows that carbon fiber recycled by different methods can be used for CFRP processing, and when the CF content in the four samples exceeds 25%, all properties will decrease, which may be due to imperfect combing, rather than machine defects.

 

5. Application and Development of Recycled Resins

 

The most commonly used epoxy resins (EP) in CFRP include amine-cured EP (EP/Am) and anhydride-cured EP (EP/Ah), with amine (Am) and anhydride (Ah) as curing agents, respectively. In the atmospheric pressure depolymerization technology, the ester exchange method is used to depolymerize the ester bond and solvent in the resin structure, that is, monoalcohol. Therefore, this technology is only applicable to EP/Ah CFRPs.

 

However, it was found in the study that this technology can also be used for EP/Am by setting the atmospheric pressure depolymerization process. At present, the depolymerization mechanism of EP/Am is still under analysis, but this method can recycle almost all CFRPs. The depolymerization mechanism of EP/Ah has been discovered, where the depolymerization is initiated by transesterification, and the depolymerization products are generated with benzyl esters or diols at the ends of their chemical chains. Figure 9 shows the estimated depolymerization reaction equation of EP/Ah. EP prepolymers can be regenerated by these products.

 

Figure 9 Reaction mechanism of epoxy resin and chemical reagents in atmospheric pressure depolymerization technology

 

6. Life Cycle Assessment Of Carbon Fiber Recycling Process

 

Tennis rackets made of CFRP with a carbon fiber volume content of 50% were used as samples for degradation. After atmospheric pressure depolymerization, all resins were completely dissolved within 10 hours, and then the EP depolymerization products and CF were recovered. Three types of recycling processes were set, 1000, 2000 and 17000 rackets/month, respectively, and the facilities and processing conditions suitable for these types were determined. The energy required for dissolution, cleaning and drying processes was calculated and summarized, and the total energy required for recycling CFs was finally determined. The energy required for 1000, 2000 and 17000 rackets/month was 91, 78 and 63MJ/kg, respectively (Figure 10).

Figure 10 Energy Consumption of CFRP Recycling by Atmospheric Pressure Depolymerization Technology

 

The energy of 17,000 rackets/month was further broken down and displayed, of which the distillation energy was 38MJ/kg, accounting for about 60% of the total energy. In order to save the energy required to recycle CFs, Hitachi Chemical plans to study a new clean regeneration method that consumes about 1/4 or less energy in the 17,000 rackets/month category compared to the energy required to make new CFs (286MJ/kg) (Figure 11).

 

Figure 11 Comparison of Energy Consumption of Recycled CFRP and New CFRP

 

7. Conclusion

 

Japanese companies have a considerable share of the carbon fiber and CFRP market, and Japan is also considered to be the country with the most advanced CFRP recycling technology. Although papers on chemical recycling technologies for carbon fiber composites such as supercritical propanol have begun to appear in countries around the world, almost all of them involve pyrolysis carbon fiber recycling technology.

 

Moreover, in large-scale projects in Europe, the United States and other countries where many researchers and engineers are involved in the development of carbon fiber recycling applications, recycling technology is only one of the goals of carbon fiber reinforcement. There is currently no national project specifically for the development of CFRP recycling technology.

 

In April 2012, Hitachi Chemical launched a commercial promotion project focusing on carbon fiber recycling. Commercial operations are currently being discussed with carbon fiber processing companies, carbon fiber reinforced plastic manufacturers, and carbon fiber reinforced plastic users. These technologies have been adopted and promoted in the regional new industry creation technology development subsidy program sponsored by the Cantor Bureau of Economy, Trade and Industry.