Melt flow ratio: A way to identify the type of polyethylene

Abstract

Melt flow ratio (MFR or I₂₁/I₂) was measured for dozens of polyethylene (PE) in the melt index (MI or I₂) range of 1–10 for various types of PE. I₂₁/I₂ for samples of linear low density polyethylene (LLD) polymerized by a Ziegler-Natta catalyst (ZN-LLD) lie near I₂₁/I₂ = 25, whereas those for specimens of metallocene linear low density polyethylene (m-LLD) scatter close to I₂₁/I₂ = 18. The plot of I₂₁/I₂ vs I₂ for LLD showed that I₂₁/I₂ is almost independent on molecular weight and short chain branching (SCB), but strongly dependent on molecular weight distribution (MWD). The broader MWD, the larger I₂₁/I_2. This rule basically holds for both low density polyethylene (LDPE) and high density polyethylene (HDPE). However, at a certain I₂ around 6 a discontinuous jump of I₂₁/I₂ for HDPE samples exits, where I₂₁/I₂ for samples of I₂ less than 6 lie near those of LDPE and I₂₁/I₂ for ones having I₂ bigger than 6 disperse close to those of ZN-LLD. In addition, bimodal PE or PE with long chain branching has a bigger I₂₁/I₂ in some case. Further, the relationship between the viscosity and melt index was discussed, indicating that I₂₁/I₂ as well as MWD is characteristic of all types of PE. Consequently, I₂₁/I₂ can be used as a convenient and efficient approach to identifying the type of PE.