Decomposition Mechanism of Epoxy Resin in Nitric Acid for Recycling

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1 Decomposition Mechanism of Epoxy Resin in Nitric Acid for Recycling Weirong Dang I, Masatoshi Kubouchi I, Shurou Yamamoto I, Hideki Sembokuya I, Kazuyoshi Arai and Ken Tsuda Department of Chemical Engineering, Tokyo Institute of Technology okayama, Meguro-ku, Tokyo , Japan Department of Mechamical Engineering, Hosei University, Kajino-cho, Koganei-shi, Tokyo , Japan Abstract Bisphenol F type epoxy resin cured with 1,s-p-menthanediamine was completely decomposed in specified immersion time in nitric acid solution due to its poor corrosion resistance to strong acid. By repolymerizing the decomposed products, an approach to chemical recycling of epoxy resin was proposed. Organic decomposed products of the resin in nitric acid with the highest yield were extracted by ethyl acetate from neutralized solution. The extract was repolymerized to prepare recycled resin, mixed with bisphenol F type epoxy resin and curing agent of phthalic anhydride. The mechanical properties of original resin and recycled resins were compared, as indicated that the recycled resin showed higher strength when the neutralized extract was used to substitute for a part of resin rather than curing agent. Furthermore, in order to investigate these behaviors, the decomposed products were analyzed in detail. The results of size exclusion chromatograph (SEC) and FT-IR showed the extract was mainly nitrated compounds which retained the main chain of epoxy resin, as explained the recycling mechanism that it could prepare recycled resin as quasi-monomer in place of part of epoxy resin. 1. Introduction With widespread application of thermosetting resins, many environmental problems brought by polymer waste have caused the public concern [ Although landfill is an inefficient disposal method of materials from the ecological viewpoint, it is still a main way of waste disposal while only a very small amount of waste was reused or recycled [3]. As a part of the trend that technologies of polymer recycling have being explored and improved [4-51, chemical recycling of waste polymers was recognized as a promising approach to convert waste polymer to useful hydrocarbons in recent years [6-71. For purposes of chemical recycling of thermosets, chemical decomposition of network in these resins must be carried out. We have been investigating chemical resistance of organic materials for chemical equipments such as lining, coating and corrosion resistant FRP. In past researches, it was found that epoxy resin cured with amine had low resistance to acid solution, and it could even be completely decomposed in strong acid of high concentration [8-91. The phenomena aroused our interest in the possibility of recycling of thermosetting resins by treating their decomposed products. The decomposition of typical bisphenol A type epoxy resin cured with amine in acid solution has been studied [IO]. It was very easy to be corroded and applied to chemical recycling, however, bisphenol A was easy to break its main chain since A type possessed quaternary carbon atom, which was susceptible to be attacked by acid to produce tertiary positive ion. Consequently, it was difficult to retain the structure of main chain in strong inorganic acid, and therefore A type could not act as a good candidate for repolymerization /01 $ IEEE 980

2 This paper focused on bisphenol F type epoxy resin that possessed - CH2 - in the center of main chain. It was aimed at proposing an approach to chemical recycling of epoxy resin cured with amine by applying decomposed products. The cured epoxy resin was decomposed in nitric acid solution, and then the decomposed products were repolymerized with bisphenol F type epoxy resin and phthalic anhydride to prepare recycled resin. The mechanical properties of the recycled resins were measured. Furthermore, mechanism of decomposition and repolymerization was discussed. 2. Experimental 2-1. Materials Bisphenol F type epoxy resin (BPF) and curing agent, 1,8-p-menthanediamine (MDA), were used in this work, which was provided by Dow Chemical Company and Aldrich Chemical Company, respectively. In addition, phthalic anhydride (PA) was used as curing agent when the decomposed products were repolymerized to recycled resins. Figure 1 shows their chemical structure Decomposition experiments Epoxy resin was cured with MDA (BPFIMDA) in weight ratio of 100:22 at 80C for 2 hours, and then the post-curing was conducted at 150C for 3h. The BPFlMDA specimens cut into 60x25~2 mm were put into 30 mm diameter glass tubes, and then immersed BPF (Bisphenol F type epoxy resin) MDA (1,8-p-menthanediamine)!? in 70 ml of 4moU nitric acid solution. The immersion temperature was controlled at 80C in the water bath. It was observed that yellowish brown decomposed compounds with high viscosity, namely residue (RE), appeared in the upper and bottom of solution. After the residue and remaining specimens were removed from the yellow solution, three extractions with ethyl acetate were carried out and the extract was dried in vacuum under room temperature. Finally, brown high-viscosity product was obtained, referred to as extract (EX) Procedure of neutralizing extract In order to neutralize nitric acid contained in EX, it was dissolved in ethyl acetate, and specified amount of sodium carbonate solution was poured into the solution, with stirring. When ph of the mixture solution reached about 7, neutralized EX was extracted from ethyl acetate phase in the solution, and then was dried under room temperature at least for one day. Because water can partly dissolve into ethyl acetate, neutralized EX contained sodium nitrate. To remove the inorganic compounds, the neutralized EX was redissolved in ethyl acetate again, and then filtered and separated. At last it was dried in vacuum under room temperature, named neutralized extract (NT-EX) Repolymerization of neutralized extract Initially, solvent removal and preheating of NT-EX was carried out at 80C for about 1 hour. And then weighed epoxy resin (BPF), NT-EX, and PA were mixed. After degassing and solvent removal in vacuum oven at 115C, the mixture was cast to be plates. The curing reaction was carried out at 115C for 8 hours, and post-curing at 130C for 10 hours, and then recycled resin was prepared Analytical methods PA (phthalic anhydride) Figure 1 Chemical structures of epoxy resin and curing agents. 0 Mechanical strength of recycled resins and original resin was measured by the three-point bending and tensile test, which were carried out on a Instron Universal Testing Machine at room temperture. 98 1

3 The decomposition mechanism was investigated by applying size exclusion chromatograph (SEC) and FT-IR. SEC was used to determine the molecular weight distributions of EX. Tetrahydrofuran (THF) was used as mobile phase and the flow rate was maintained at O.lrnl/rnin. FT-IR was used to analyze their chemical structures. 3. Results and Discussion 3-1. Yields of decomposed products Figure 2 gives the change of yield of each decomposed product with increase of immersion time in 4rnol/f nitric acid solution. It can be seen that the resin was rapidly decomposed, and completely disappeared at the immersion time of about 100 hours. RE and EX appeared at the beginning of immersion, and increased gradually. The yields of RE and EX started to reduce after they reached their highest levels, respectively, at 50 hours for RE, and at 150 hours for EX after the resin was totally decomposed, being 6OwtY0 of initial resin. The high yield of EX was one of the advantages of the recycling approach proposed in this paper. h g 3 'F "b h 60 * F Immersion time Q Figure 2 Change of yields with BPF/MDA decomposed in 4moM nitric acid solution at 80C Properties of recycled resins The yield of RE was relatively low after 150 hours, and RE could continue to be decomposed if immersed in nitric acid with initial concentration again, as suggested that it was a mixture of intermediate decomposed products during decomposition process of cured resin. The decomposed product EX with the highest yield could be recovered by controlling the immersion conditions. The EX obtained at 150 hours immersion time was chosen as recycling material (see Figure 2). After further chemical treatment of neutralization, the NT-EX was cured to recycled resins together with original epoxy resin (BPF) and curing agent (PA) in various weight ratios. Besides curing reaction of epoxy resin (BPF) and curing agent (PA) towards the cross-linking structure, there might exist the reactivity of the neutralized extract (NT-EX) with both epoxy resin and curing agent. Therefore, two kinds of preparation methods of recycled resins were considered. In the first method, NT-EX was intended to replace a part of curing agent. The weight ratios of NT-EX to the sum of NT-EX and curing agent (PA) were selected as 0, 5, 10, 20 and 30% and the quantity of resin (BPF) was invariable. The flexural strength of the original resin (OYO) and recycled resins was evaluated by the three-point bending test (Figure 3). It """' IS wt% of NT-EX Figure 3 Flexural strength of original and recycled resins with change of ratios of NT-EX and curing agent. was obvious that the network of recycled resins was affected by content of NT-EX. When the content of NT-EX was less than 5%, the flexural strength of recycled resins increased with increase of its content. The strength reached maximum value at the level of 982

4 5-10%. When the content was beyond IO%, the strength gradually decreased. At the content of 30%, its strength was near that of original resin. In the second method, NT-EX was considered to take the place of a part of resin. The weight ratio of NT-EX to the sum of NT-EX and resin was varied from O(BPF/PA) to 30%, and the quantity of curing agent was kept constant. At the 30% adding level, cured resin could not be successfully moulded because of its high viscosity and short pot life. In the moulded recycled resin, there were many large voids that were formed in the process of degassing under vacuum. The reaction was so rapid that gas was prevented from escaping from the viscous resin. On the other hand, below 25%, casting was successfully done, and 2mm thick recycled resin plates without any void could be obtained. It was found that the gel time was accelerated by increasing NT-EX. Figure 4 showed their flexural strength. The flexural strength of recycled resins was higher than that of the original resin under the same curing conditions. At low content of NT-EX below 10%. the flexural strength rapidly increased with the increase of rn postiunng lsoh Wt% of NT-EX Figure 4 Flexural strength of original and recycled resins with change of ratios of NT-EX and epoxy resin. z h loo/ -[ Original resin containing 0 02% accelerator 40 t/ I.Recycledrezn] I wt% of NT-EX Figure 5 Tensile strength of original and recycled resins with change of ratios of NT-EX and epoxy resin. NT-EX. While, from 10% content of NT-EX, the increase of the flexural strength of recycled resins was slowed down. Note that the strength did not decrease, which was different from the first method (Figure 3). In order to sufficiently cure the original resin, post curing time was extended to 150 hours. Its strength increased, but was still lower than that of recycled resins with high content of NT-EX. Based on the above investigation, it concluded that NT-EX contributed to curing reaction as a role of resin, but not curing agent. That is, NT-EX could be reused to take a part of epoxy resin to react with curing agent to form the better network and might accelerate the conductibility of curing reaction. Therefore, in the first method, the strength decreased with increase of weight ratio of NT-EX more than 10% resulting from the decrease of curing agent. The curing reaction of acid anhydride can be accelerated if the Lewis base exists, which acts as initiator to the reaction [ll]. The NT-EX might contain tertiary amine that acts as the Lewis base, as resulted in the curing reaction of recycled resin was faster than that of original resin. It was because of the existence of tertiary amine that the recycled resin formed the higher density network than the original resin under the same curing condition. As a result, the flexural strength of the recycled resin was higher than that of the original one when NT-EX was used as epoxy resin (Figure 4). This 983

5 also explained the case in which NT-EX was added in place of a little of curing agent in the first method (Figure 3). To confirm this viewpoint, 0.02% tertiary amine was added into the original resin as an accelerator. As shown in Figure 4, under the same curing conditions the flexural strength was much higher than that of original resin and slightly higher than that of recycled resin containing 25% NT-EX. Furthermore, the tensile strength was examined and is shown in Figure 5. The tensile strength of recycled resin rapidly increased at the initial stage (<lo%), and then tended to be stable with the increase of NT-EX content. The strength was lower than but close to that of original resin added accelerator. The results well agreed with the observations in the bending test Mechanism analysis of decomposition As mentioned above, the decomposed products EX could be applied as a part of epoxy resin to prepare recycled resin with higher strength. It implied that the decomposed products might possess the similar structure as epoxy resin. By analysis of FT-IR and SEC, the chemical structure and molecular weight distribution were investigated. FT-IR spectra of original resin and EX are compared in Figure 6. The peak at 1350 cm-' in the IR region of EX corresponded to nitro compound. % B h,._. i., lurrvl Figure 7 Change of molecular weight distribution of EX for BPF/MDA decomposed in 4mob'l nitric acid solution. Aromatic nitro groups absorbing strongly at 1540 cm-' were also found in the IR spectrum of EX. The results demonstrated that the original resin was nitrified. Furthermore, the peaks of aliphatic ether groups at cm" and aromatic ether groups at 1040 cm-' appearing in spectrum of the original resin almost disappeared in that of EX, implying that the ether bonds between and within the molecules were modified due to the effect of the nitric acid. Finally, from the molecular weight distribution of EX, shown in Figure 7, it was found that the peaks appearing in high molecular region shifted to low molecular region with increase of immersion time. It was supposed that EX was mixture with lower molecular weight compounds. Based on these findings and the structure of the original material (see Figure I), it was concluded that EX was the nitrated mixture that retained the structure of main chain of original bisphenol type resin. It was because the main chain was not broken in EX, it could be reused as quasi-monomer for recycling the resin and substitute for part of resin to prepare the recycled resins IS Wave number (crn.l) Figure 6 FT-IR spectra of BPFMDA resin and decomposed products EX: (a) BPF/MDA resin before immersion; (b) EX immersed for 150h. 4. Conclusions Bisphenol F type epoxy resin cured with 1,8-~-menthanediamine could be decomposed in nitric acid solution. The decomposed products with highest yield were applied to prepare the recycled resins. The 984

6 decomposition mechanism was analyzed. The conclusions was shown as follows: (1) Bisphenol F type epoxy resin cured with 1,I-p-menthanediamine was decomposed by nitric acid to produce decomposed products, including residue and extract. (2) The neutralized extract was used to prepare recycled resin by adding it into epoxy resin, and being cured with phthalic anhydride (PA). (3) The mechanical properties of recycled resin were reported! When the neutralized extract was applied in place of a part of epoxy resin, the flexural and tensile strength of recycled resins was much higher than that of original one. (4) The residue was considered to be an intermediate product during the decomposition process, and the extract was mainly a nitrated mixture retaining the structure of main chain, equivalent to monomer or dimmer and could be applied to prepare recycled resin. Science: Part A: Pobmer Chemistry, Vol. 34, pp , 1996 [8] K. Tsuda, M. Kubouchi, T. Nishiyama, S. Ono and H. Hojo, Proc. Intern. Con$ Composite Mater., Whistler, Vol. VI, pp , 1995 [9] M. Kubouchi, K. Tsuda, T. Nishiyama and H. Hojo, Advanced Composites Letters, Vol. 4, No.1, pp.13-15, 1995 [IO] M. Kubouchi, H. Sembokuya, S. Yamamoto, K. Arai and K. Tsuda, Journal of the Society of Materials Science, Japan, Vo1.49 No.5, pp , 2000 [ll] S. Muroi and H. Ishimura, A Guide of Epoxy Resin, Polymer Publication, pp , Acknowledgements This research was partially supported by the Grant-in-Aid for Scientific Research (B), , The authors gratefully acknowledged the support from Dow Chemicals, Inc. References [l] S. J. Pichering and M. Benson, Plastics Recycling Meeting at Challenge, RPI yd International Conference on Rbcycling Plastics, London 13-14, March, 23, 1991 [2] 0. S. Woo, N. Ayala and L. J. Broadbelt, Catalysis Today, 55, pp ,2000 [3] C. Thomark, Resources, Conservation and Recycling, Vol. 33, NO. 2, pp , 2001 [4] J. Economy and A. G Andreopoulos, Polymers for Advanced Technologies, Vol. 7, pp , 1996 [5] M. Toselli and M. Impagnatiello, Polymer Recycling, Vol. 2, No. 1, pp , 1996 [6] K. H. Yoon, A. T. DiBenedetto and S. J. Huang, Polymer,Vol. 38, No.9, pp , 1997 [7] S. L. Buchwalter and L. L. Kosbar, Journal ofpolymer 985