Expanded Linear Low-Density Polyethylene Beads: Fabrication, Melt Strength, and Foam Morphology

A linear low-density polyethylene copolymerized with 1-hexene (H-LLDPE) was synthesized by gas-phase polymerization, using the metallocene catalyst systems. The HLLDPE exhibited longer ethylene average sequence length, narrower molecular weight distribution, and higher melt strength, compared with linear low-density polyethylene copolymerized with 1-butene (B-LLDPE). The expanded polyethylene (EPE) beads were prepared by a batch foaming process using above two LLDPEs as base resins. Both ethylene average sequence length and comonomer content were measured by solid-state nuclear magnetic resonance (NMR). Influences of melt strength and molecular structure on the cellular morphology of EPE are investigated using scanning electron microscopy (SEM) and a melt strength meter, respectively. The influence of saturation temperature and pressure on cell density and morphology are examined by homogeneous and heterogeneous nucleation theory. Results indicate that, with higher melt strength and longer ethylene average sequence length, the demonstrated H-LLDPE facilitates the entire batch-foaming process and yields improved cellular structure of EPE beads over that of B-LLDPE.