Development of Poly(phenylene sulfide) as a Tribo-Material by means of Polymer Alloy

1. Introduction

@Poly(phenylene sulfide)(PPS) is a semicrystalline high performance thermoplastic with outstanding chemical and thermal durability ( melting point, 280�Ž ). This is one of the engineering plastics used widely in manufacturing automobiles, electric or electronic appliances and precision machine parts. We have succeeded in developing PPS based polymer blends with outstanding tribological properties (friction and wear), together with good balanced physical properties, such as impact strength, moldability, and thermal durability, etc. by applying polymer alloy technique. When using plastics to produce molded articles as substitutes for metal (industrial applications of engineering plastics), tribological properties are especially important from the viewpoint of durability, moreover, of energy saving. In other words, reducing friction coefficient leads to energy saving, or decreasing wear rates enhances the sustainability of machinery and increases productivity. In addition, self-lubricating materials are desired for ecological point of view.
@Polymer alloy (multi component polymer materials) is a technique to provide high performance polymeric materials by combining commodity polymers. This enables to produce polymer materials having properties unavailable in single polymer without large capital investment. Hence, the research on the development of the most effective blending has been carried out actively in order to apply its economical and time saving advantages in industrial environment. Besides, the research itself requires comparatively less time and cost.

2. Tribology of PPS/LDPE reactive blending and multiphase formation

@When low density polyethylene (LDPE) is blended into PPS at certain rations by melt blending at 300�Ž, the tribological properties of PPS is effectively improved. As it is indicated in Figure 1, both PPS and LDPE are poor in tribological properties, but by blending PPS/LDPE with 90/10 combination, both the coefficient of friction and the wear rate decreased to the lowest, and the tribological properties synergistically improved. The coefficient of friction is approximately 1/4 and the wear rate 1/100 of PPS. Moreover, when compared with the tribological properties of other typical engineering plastics listed in figure 1, both coefficient of friction and the wear rates are better for PPS/LDPE blend. However, this simple blending of PPS/LDPE give rise to the mold deposition (MD) on the mold surface in the cyclic operation of injection molding, which causes poor surface appearance of mold articles and leads to poor productivity.
@We overcame the problem of this molding process by applying reactive blending (the incorporation of glycidyl functional materials into the PPS/LDPEgMA blends), and at the same time, discovered the improvement in the physical properties. Figure 2 presents the scheme of PPS/LDPE reactive blend. The glycidyl functional materials work as coupling agents between the PPS matrix and the domains of LDPEgMA during melt mixing of the ternary reactive blend of PPS, low-density polyethylene grafted with maleic anhydride (LDPEgMA) and glycidyl functional materials. Consequently, the improvement in dispersion and interfacial adhesion between the PPS and the LDPE phases are achieved. As a result, the "mold deposition" (MD) is prevented, and notched izod impact strength is increased approximately by three folds stronger than that of PPS. The advantageous tribological properties of PPS/LDPE blend are maintained intact.
@Figure 3 is the transmission electron microscope (TEM) photographs of the above mentioned binary and ternary blend materials. The morphological observation on PPS/LDPE(90/10) binary blend indicates that the LDPE dispersion domain size is approximately 2ƒÊm, whereas it is reduced to smaller than 0.5ƒÊm in the reactive (ternary) blend. This suggests the improvement in dispersion. The detailed morphological characterizations of the phase formation of the blends are performed by electron spectroscopic imaging (ESI) and electron energy loss spectroscopy (EELS) on an energy-filtering transmission electron microscope (EFTEM). The application of ESI and EELS to the morphological analysis in our system allows us to create elemental distribution images (carbon, oxygen, and sulphur), which leads to the identification of the three phases in the blend systems. Figure 4 clearly presents that the LDPE domain sticks to the PE-GMA domain and dispersed in PPS. It is speculated that the preferable properties are achieved owing to the synchronization of the each three phase, i.e., PPS, LDPE and the glycidyl functional material's phases, by taking the "stuck phase formation." Namely, LDPE phase contributes to tribological properties, PE -GMA phase to enhance impact strength, chemical bonding between PPS and LDPE phases to improve moldability and PPS to heat resistance respectively.

@These materials can be applied to mold articles such as gears and bearings.
@The fundamental investigation is still being developed to explain the reason for the improvement of the tribological properties described here. It is presumed that LDPE forming thin film between PPS matrix functions as a lubricant, but its mechanism has not been made clear so far. Likewise, PPS, as presented in figure 2, being a simple unit polymer, chemical reactions taken place between glycidyl functional materials have to be characterized further in detail.