Modeling of internal residual stress in linear and branched polyethylene films during cast film extrusion: Towards a prediction of heat-shrinkability

The singular property of shrinkage is used in the packaging sector to wrap products. Polyethylene film products are mostly produced by either blown or cast film extrusion. During processing, under the effect of drawing, the molten polymer is stretched in one or two dimensions according to the cast or blown film extrusion. When the resulting solid film is subjected to a sudden increase in temperature, it shrinks as a result of the relaxation of polymer chains that were oriented under stress and frozen in a stretched state upon processing resulting in internal residual stresses. A general viscoelastic model in the framework of large strain has been used to predict the development of the internal residual stresses stored during the stretching stage of the process. Linear and branched polyethylenes (LLDPE, LDPE) were used as model matrices and additional blends (80/20%, 50/50% and 20/80% LDPE/LLDPE w/w) of pure resins were also studied. Moreover, as properties of stretched films are highly dependent on processing conditions, the influence of the draw ratio was also investigated. Contrasted mechanical response in strain of the LDPE and LLDPE films under stretching were observed and justified by differences in their rheological behavior due to long branch chains (LCB) in LDPE. The modeled ultimate stress stored in LDPE and LLDPE films was compared to the shrinkage stress measured by Dynamic Mechanical and Thermal Analysis (DMTA). Both stresses were found to be in the same order of magnitude which constitutes an interesting result for the prediction of heat-slcrinkability of stretched polymer films.