THE EFFECT OF TEFLON(TM) COATINGS IN POLYETHYLENE CAPILLARY EXTRUSION

Two LLDPE resins were used in this work to determine the critical cond itions for the occurrence of wall slip and melt fracture in capillary extrusion. It was found that the polymer-metal interface fails at a cr itical value of the wall shear stress of about 0.1 MPa and, as a resul t, slip occurs. At Values of wall shear stress of about 0.18 MPa the e xtrudate surface appears to be matte, while small amplitude periodic d istortions (sharkskin) appear on the surface of extrudates at wall she ar stresses above 0.25 MPa. Using a special slit die, the polymer-wall interface was coated with Teflon(TM) in order to examine the effect o f this coating on the processability of polyethylenes. It was found th at use of Teflon(TM) promotes slip, thus reducing the power requiremen t in extrusion and, most importantly, eliminates sharkskin at high ext rusion rates. (C) 1995 John Wiley and Sons, Inc.