Structural, Rheological, and Mechanical Properties of Binary Compounds Based on High-Density Polyethylene and Linear Low-Density Polyethylene

Rheological, molecular, thermal, and mechanical properties of binary blends based on bimodal high-density polyethylene (HDPE; ethylene-1-hexene copolymer) and unimodal linear low-density polyethylene (LLDPE; ethylene-1-butene-1-hexene terpolymer) were studied. Six blends with different HDPE/LLDPE weight ratios, 80/20, 70/30, 60/40, 50/50, 40/60, and 20/80, were prepared by blending in a double-worm extruder. The correlation of the rheological properties of the blends, analyzed by the capillary and rotation methods, with the melt flow index and molecular characteristics was demonstrated. LLDPE with increased flowability (MFI = 3 g/10 min) as a polymer additive is an effective plasticizer allowing control of the melt viscosity in a wide range and choice of the optimum target value of the melt flow index of the blend. The activation energy of the melt flow was calculated, and the relationship between this quantity and the content of short-chain branchings in polyethylene blends was revealed. The blends obtained are homogeneous, but an increase in the content of LLDPE in HDPE leads to a change in the polyethylene structure, which is confirmed by a decrease in the crystallinity and in the lamella thickness. The strength values at the yield point and at break decrease with an increase in the fraction of LLDPE in HDPE. Changes in the properties of these blends in their processing into a film by flat-die extrusion were studied.