An interlaboratory comparison of the melt flow index: Relevant aspects for the participant laboratories

Melt flow index is an important parameter that is frequently used during commercial polymer processing. The lack of traceability and of reference standards for measuring melting flow index makes it difficult to validate laboratory measurements of this parameter. The comparison of interlaboratory analyses of the same material provides a useful means of establishing criteria for standard laboratory tests and quality control. However, it is incorrect to assess laboratory performances using samples taken from a batch of unknown homogeneity and quality. In this report, we describe a method to test the homogeneity of small samples of polymeric materials and its applicability to interlaboratory comparisons. Two lots of polypropylene and polystyrene were prepared and their homogeneity was verified by one-way ANOVA (packed samples). The homogenous samples were subsequently distributed to 24 laboratories and tested as described by the ASTM D 1238 procedure. The results were evaluated using the z-score statistical test, after correction with the outlier-test procedure, as described in the ISO 5725 instructions. Finally, the interlaboratory uncertainty was estimated so that the uncertainty attributable to sample heterogeneity could be distinguished from that attributable to uncertainties in the interlaboratory measurements of the samples.