Effect of molecular structure on seal ability, flex crack and mechanical properties of linear low-density polyethylene films

Abstract

In this article, three linear low-density polyethylenes having different molecular structures were selected and films were produced from them using a semi-industrial cast film extrusion line. Rheological and gel permeation chromatography measurements were performed on the resins to assess their molecular structure. Mechanical, physical and sealability properties of the films were evaluated and the results were discussed with regard to the molecular structure of the resins. It was found that molecular weight and distribution of short-chain branching (comonomer) on the backbone of polyethylene chains are the main factors that control sealability, flexural cracking and mechanical properties. Placement of comonomer on medium length chains generated crystals with expanded unit cell that show lower melting peak. Sealing was controlled by crystal size distribution, chain diffusion and entanglement formation at the interface. linear low-density polyethylenes with lower melting point and boarder molecular weight distribution showed lower seal and hot tack initiation temperatures. Polydispersity along with molecular weight contributes to toughness and puncture resistance. Flexural cracking resistance was observed to be related to crystallinity, tie chain density and more importantly to the amorphous phase fraction. The amorphous part could absorb flexing energy and hinders crack initiation and propagation. Seal through contamination (caulkability) was found to be related to flowability and elasticity of the melt.