The Use of Thermoplastic Elastomers as Polymer Processing Aids in Processing of Linear Low Density Polyethylene

Commercially available thermoplastic elastomers based on block copolymers of diisocyanates and polyols and based on silicones have been reported by Kulikov et al. (2004 and 2006) to delay sharkskin in extrusion of Linear Low Density Polyethylene. In this work thermoplastic elastomers have been used as polymer processing additives in blown film extrusion of Linear Low Density Polyethylene. When a thermoplastic elastomer is added in small amounts to Linear Low Density Polyethylene it deposits at the die surface during extrusion and may postpone the onset of sharkskin enabling up to 20 times higher rate of extrusion. Many thermoplastic elastomers are certified for body and food contacts and could be a cost-effective substitution of fluorinated polymers in processing of Linear Low Density Polyethylene by extrusion. Oscillating and capillar rheometry have been used to reveal the mode of action of the additives. Results of capillar rheometry showed a decrease of apparent viscosity of the polymer melt when additive was added. Therefore Mooney method (Mooney, 1931) was applied to prove occurring slip inside of the die. Substantial delay of sharkskin was achieved also in conditions without slip of molten polyethylene inside the die by adding thermoplastic elastomer based on urethanes. Oscillating rheometry has been used to characterize elasticity of the materials. It could be shown that efficiency to delay sharkskin depends on elasticity of the specific thermoplastic elastomer at processing temperature. Surface tension of the solid materials was used to estimate the mutual affinity of the materials. Therefore a theoretical model of Rathod and Hatzikiriakos (2004) was used to evaluate the data. Finally a classification of polymer processing aids into slip inducers and flow enhancers by their mode of action was done. Ability of novel processing aids to postpone sharkskin was shown in blown film extrusion. Applicability of polymer processing aids in injection moulding could be proved by use of a mould with spiral cavity.