ANALYSIS OF THE EFFECT OF PRESSURE ON COMPRESSION MOLDING OF UHMWPE

Ultra-high molecular weight polyethylene (UHMWPE) powder is effectivel y processed by compression moulding due to its very high melt viscosit y. Compression moulding involves application of temperature and pressu re as a function of time. The pressure applied during processing has a significant influence on the part properties. The effect of pressure applied during compression moulding was studied by moulding parts at d ifferent pressures. Increase in the applied pressure causes increase i n the melting and recrystallization temperatures. An increase in the p ressure applied at the melt temperature (similar to 140 degrees C) fro m 7.8 MPa to 15.6 MPa caused the crystallinity to increase from 54% to 61%, the stiffness of the moulded part to increase from 257 MPa to 43 5 MPa and oxidative index to increase from 0.055 to 0.059. Further inc rease in the pressure applied at the melt to 23 MPa caused the crystal linity to fall to 49%, the modulus to reduce significantly to 302 MPa and the oxidative index to change to 0.063. Increase in the pressure a pplied at the recrystallization temperature (similar to 91 degrees C) from 38 MPa to 78 MPa increased the crystallinity from 54% to 65%, inc reased the modulus from 257 MPa to 279 MPa and increased the oxidative index from 0.055 to 0.065. Further increase in the applied pressure t o 97 MPa, caused the crystallinity to drop slightly to 61% the modulus to reduce to 269 MPa, and the oxidative index to reduce to 0.057. The experiments showed that for obtaining maximum crystallinity and stiff ness, the applied pressure should be within a narrow range. The highes t recrystallization pressure (97 MPa) indicated the formation of exten ded-chain crystals in addition to the chain-folded crystals. The chang e in pressure applied at the melt temperature had a significantly grea ter effect on Young's modulus, as compared to change in pressure appli ed at the recrystallization temperature. Fourier transform infrared sp ectroscopy analysis of the samples moulded at different pressures reve aled that the increase in crystallinity and stiffness was accompanied by increase in oxidation within the part. By filling the die in a nitr ogen atmosphere instead of air, the oxidation level in the moulded par ts was reduced by almost 60%, without adversely affecting the crystall inity and the modulus. (C) 1998 Chapman & Hall.