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/I2. 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.