Abstract
Polyethylene has a broad relaxation spectrum that makes it ideal for blown film and blow molding. In polymer processing, melt and thermal phenomena are superimposed to generate semi-crystalline morphology that delineates solid-state properties. A polymer melt is exposed to deformations varying in both time scale and strength. In many processes, melt stress simultaneously grows and decays so characterizing time-dependent stress relaxation is important. Stress-strain behavior in polymer processing is determined by the relaxation spectrum. Commercial polyethylenes have had narrow to broad molecular weight distributions; metallocene catalysis can give polyethylene with quite narrow molecular weight distributions. This affects their relaxation spectra, the breadth of which is quantified by the Relaxation Spectrum Index, RSI. This dimensionless index is a sensitive, reliable indicator of long-range melt state order. The RSI illustrates unique rheology of two metallocene polyethylene families (SSC-1 and SSC-2) used in polyethylene manufacture by the UNIPOL process. Their melt rheology is compared to competitive metallocene polyethylenes and other commercial resins. This chapter presents the melt behavior of SSC-1 and SSC-2 contributes to their processability, clarity, and toughness advantages compared to LLDPE and HP-LDPE.
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