Chemical recycling of poly(p-phenylene sulfide) in high temperature fluids
Introduction
Poly(p-phenylene sulfide) (PPS) is the industrially important and an attractive engineering thermoplastic because of its high solvent resistance, high mechanical strength and high thermal stability [1]. Various PPS composites were prepared to achieve improvements of mechanical properties furthermore [2], [3], although PPS is a plastic endowed with excellent thermal and chemical resistance [4], [5]. Good flow properties and thermal stability in the molten state make PPS relatively easy to process by injection molding and extrusion. Long time operation is possible even at 240 °C, however, when heated to temperatures exceeding 300 °C, PPS undergoes cyclization, branching and cross linking processes. The degradation study on PPS is important since the processing in high temperature may induce changes that will affect the ultimate performance [6], [7], [8].
The idea of recycling of PPS has become important not only environmentally, but also economically since the production of PPS has increased firmly [9]. Mechanical recycling has been investigated as probable recycling method for engineering thermoplastic with high thermal stability such as PPS [10].
Recently, we have found that waste plastics including thermosetting resin such as phenol resin decomposed into their monomeric compounds in sub and supercritical water. The addition of basic compounds such as Na2CO3 was effective on the decomposition reaction. Furthermore, chemical participation of water on the reaction was suggested [11], [12], [13].
High temperature water has become the subject of wide ranging interests [14], [15], [16], [17], because of its low dielectric constant similar to organic solvents and high thermal stability at near and above its critical point (Tc = 374.2 °C, Pc = 22.1 MPa) [18], [19], [20].
In this study, to obtain information on the decomposition reaction of PPS in high temperature fluids, PPS was treated in high temperature water or methanol by using a 10 ml tubing bomb reactor.
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Materials
PPS was supplied kindly from Japanese Chemical Company and used after pulverizing.
The elemental composition of the PPS was found to be carbon 66.6%, hydrogen 3.8%, nitrogen 0.1% and sulfur 28.8%. Their values are similar to the theoretical composition of carbon 66.6%, hydrogen 3.7%, nitrogen 0.0% and sulfur 29.6%.
Methanol from Yamaichi Chemical Industries was used after distillation. Model compounds of PPS such as thioanisole from Thermo Fisher Scientific, thiophenol from Kanto Chemical Co.
Thermal properties of PPS
PPS is thermally stable and weight loss of PPS has started at 450 °C as shown in Fig. 1 although the residue reached to 37% even by the thermal treatment at 1000 °C. It suggested the occurrence of carbonization reaction. The amount of the recovered residue was similar to those reported [7] in which the residues taken at 800 °C from TG experiments under nitrogen of PPS were 37 and 42%.
The reaction of PPS in water
In the reaction of PPS in water alone, no conversion was attained by the treatment at 350 °C for 2 h as shown in
Conclusions
Effective decomposition reactions of PPS were confirmed in the high temperature fluids. In high temperature water, conversion reached to 50% was attained at 430 °C by adding the basic compound, perfect solubilization could not be attained even by the reaction for 2 h. Perfect solubilization was attained by the reaction in high temperature methanol at 430 °C. Even by the reaction at 370 °C, conversion reached to 75% in the reaction for 5 h.
Main products in the reaction of PPS in high temperature
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