Polyethylenes in blown films: effect of molecular structure on sealability and crystallization kinetics

Three types of polyethylene (PE), with different molecular structure, were selected and used in the production of films using a semi-industrial blown film extrusion line. Carbon nuclear magnetic resonance and crystallization analysis fractionation tests were performed on the PE polymers to assess their molecular structure. Sealability properties of the films, as well as crystallization kinetics, were evaluated and the results are discussed according to the molecular structures of the polymers. It was found that distribution of short chain branching (average comonomer sequence nH or nO) on the backbone of the PE chains are the main factor controlling the beginning of sealability. The completely random placement of the comononer on the polymer backbone (monomer dispersity 100%) generated crystals with smaller size and a lower melting temperature. Sealing was controlled by crystal distribution, chain diffusion, and entanglement formation at the interface. PE with lower melting point and linear molecular structure showed greater hot tack strength. PE with long chain branching showed a delay on seal process leading to lower sealing speed.
INTRODUCTION Polyethylene is one of the most interesting polymers available in the marketplace. The variety of commercial grades is huge because of the vast number of variables like catalysts systems, [...]