REACTIVE PROCESSING OF LLDPES IN COROTATING INTERMESHING TWIN-SCREW EXTRUDER .2. EFFECT OF PEROXIDE TREATMENT ON PROCESSABILITY

Commercial ethylene-octene linear low-density polyethylenes (LLDPEs) w ere reactively extruded with low levels of a peroxide [2,5-dimethyl-2, 5-di(t-butylperoxy) hexane] to modify polymer molecular structure and processing properties. Peroxide levels were kept low to avoid crosslin king. This article reports the effects of this reactive extrusion on v iscoelastic properties. Rheological properties are more sensitive than are molecular structure characteristics to the changes produced by re actions of very low peroxide concentrations. Complex viscosity increas es are seen, especially at low frequencies. Shear-thinning behavior is also accentuated. The crossover between G' and G '' moves to lower fr equencies. A modified Cole-Cole presentation of these data shows that the elastic component is more predominant for extrusion-reacted materi als. Peroxide-modified materials all have higher Bagley end correction values than those of barefoot resins. The former exhibit lower power indices (more shear thinning). All these properties indicate more long -chain branching and higher melt elasticity. However, die swell decrea sed as a function of peroxide concentration. Peroxide treatment result s in an enhancement of elongational viscosity, both under isothermal a nd cooling conditions, along with a decrease in drawdown ability. The theological changes parallel those reported earlier in molecular chara cteristics but are more sensitive and suitable to evaluate the effects of reactive processing. The effectiveness of the reactive extrusion p rocess for improving processability of LLDPEs depends critically on th e extrusion conditions. (C) 1995 John Wiley & Sons, Inc.