Interlaboratory comparison of melt flow rate testing of moisture sensitive plastics

The current standard for melt flow rate testing, ISO 1133, does not adequately cover the measurement of materials that degrade or further polymerise at the melt flow rate test conditions. Materials such as PET, PBT and PA tend to degrade, sometimes rapidly, at melt temperatures if they contain absorbed water. This results in poor repeatability and reproducibility of melt flow rate results. The ISO Plastics Melt Rheology Working Group (TC61/SC5/WG9) is currently developing a modified melt flow rate test method for reliable measurement of moisture sensitive materials, thereby providing an alternative to intrinsic viscosity measurements. The modified method controls more tightly the time-temperature history experienced by the material during testing and, thus, the resultant variability in measured properties due to degradation. The results of an intercomparison of testing of moisture sensitive polymers using the melt volume flow rate (MVR) and melt mass flow rate (MFR) methods are reported. A polypropylene (PP), used as a benchmark material, and five moisture sensitive materials were tested by a number of laboratories measuring either MVR or MFR, or both. For standard testing of the PP material, the repeatability and reproducibility of measurements of MVR were up to 5% and 11%, respectively, and for MFR were 7% and 21%. respectively (95% confidence level, outliers excluded). For the moisture sensitive materials the repeatability and reproducibility of measurements varied significantly, with repeatability values up to 26% and reproducibility values up to 114% being obtained. By imposing moisture and temperature control criteria in accordance with the proposed testing standard, the repeatability and reproducibility limits were significantly reduced to approximately 10% and 60%, respectively. The results suggest that both the moisture and temperature criteria were important for improving the repeatability of MVR measurements. However, the moisture content criterion was more important for improving the reproducibility of MVR measurements. The need to tightly control the sample preparation, in particular the drying and subsequent sample handing procedures, and the time-temperature history of the sample throughout the test is considered critical to further improve the precision of measurements of such materials. (C) 2009 Published by Elsevier Ltd.