Rheological properties of low-density polyethylenes produced by tubular and vessel processes

Various rheological properties have been studied for two kinds of low-density polyethylenes (LDPEs): one is produced by tubular process (tubular LDPE) and the other by vessel process (vessel LDPE). The latter shows smaller g′-parameter, the ratio of intrinsic viscosity to that of linear polyethylene. It was found that both LDPEs exhibit marked strain hardening behavior, i.e. upturn departure from the low strain rate asymptote, in both uniaxial and biaxial elongational viscosities, with stronger strain hardening in uniaxial elongation. Further, the vessel LDPE exhibits larger upturn behavior than the tubular LDPE. In the stress relaxation measurement, the vessel LDPE shows larger damping function, while the tubular LDPE shows values similar to those predicted by the Doi–Edwards theory. Furthermore, there is a significant difference in the effect of the shear history on the melt strength. The melt strength of the vessel LDPE decreases more rapidly with the shear history than that of the tubular LDPE. All these differences between the two types of LDPEs are due to the difference in the branch structure. The complex, multi-branched structure of the vessel LDPE gives much more prominent elastic features.