RHEOLOGICAL MEASUREMENTS - SOME BASIC YET VITAL CONSIDERATIONS

This article addresses a seemingly trivial question-''Based on theolog ical measurements, when are two samples different beyond experimental error?''-and proceeds to give a non trivial answer. The preferred answ er requires the application of t statistics to multiple measurements o n each sample. The larger the number of multiple measurements made, th e lower is the uncertainty in conclusively discriminating between two samples. However, such multiple measurements on every sample are seldo m feasible in real life and then it becomes necessary to resort to the use of Z statistics. A stepwise procedure for the application of both , t and Z statistics is given. Knowledge of the measurement precision is a prerequisite for scientifically answering this seemingly trivial question. Hence, the precision of capillary rheometry measurements (ap parent viscosity, extrudate swell ratio, and entrance pressure loss) i s discussed in detail. The precision of these capillary rheometry meas urements improves as the magnitude of the variable measured (force/pre ssure or swell) increases. This implies that the precision of capillar y rheometry measurements can be improved by lowering the experimental temperature. Extensive numerical data on various high-density polyethy lenes generated using two different capillary rheometers are given and are compared with the ASTM round-robin data. (C) 1995 John Wiley & So ns, Inc.