A key requirement for automotive proton exchange membrane fuel cell stacks is improved performance and durability. Highperformance can be delivered using perfluorosulfonic acid ͑PFSA͒ membranes that are thin ͑30 m͒ and have low equivalent weight ͑920͒. Initial durability in a single cell confirmed edge protection films prevent premature membrane electrode assembly ͑MEA͒ failure. This allowed the true MEA durability to be assessed in 30-cell stacks. Based on physical leak tests and open-circuit voltage ͑OCV͒ decay rates a Johnson Matthey Fuel Cells ͑JMFC͒ reinforced membrane provided a sixfold improvement in durability compared to commercially available nonreinforced membranes. The improved durability is ascribed to increased dimensional stability during hydration/dehydration and an improved resistance to tear propagation. The commercial membranes fail mechanically, accelerated by chemical attack of the ionomer. The reinforced membrane is mechanically robust and fails due to removal of the PFSA ionomer from the reinforcement by chemical attack. Accelerated OCV hold tests with hydrocarbon membranes confirmed the chemical attack was from the radicals generated by H 2 and O 2 gas crossover through the membrane. Adding a H 2 O 2 decomposition catalyst to the JMFC reinforced membrane significantly extended the membrane lifetime at OCV, suggesting H 2 O 2 was the major source of the damaging radicals. The automotive durability target of 5500 h should be achievable using the JMFC reinforced membrane containing the H 2 O 2 decomposition catalyst.

The initial part of a comprehensive study into the investigation and validation of in-process measurements on a low density polyethylene (LDPE) compound during extrusion is reported. An unfilled, commercial grade of low density polyethylene was processed using fully instrumented, computer monitored, single and twin screw extruders in the Interdisciplinary Research Centre (IRC) laboratories, University of Bradford. Three in-process rheometers were used; a prototype Rosand capillary on-line rheometer (OLR), a four sensor slit die in-line rheometer (ILR) and a six sensor slit die ILR. Off-line comparisons were carried out using a Rosand RH twin bore capillary rheometer. All devices allowed shear ow characterisation at a range of processing rates, and the capillary rheometers also provided a measurement of entry pressure drop through the use of orifice dies. The six-sensor ILR also allowed measurement of entry pressure drop into the slit, although the differences in ow geometry meant these measurements were not directly comparable with the capillary devices, and exit pressures were extrapolated from ILR pressure drops. The in-process devices exhibited very good agreement with measurements made off-line, in both shear ow and entry pressures. In-line slit die measurements were very close to those made using capillary dies, despite the in-line tests being non-isothermal and employing a different geometry of measurement section. Differences in melt rheology between single and twin extrusion were observed, due to the amount of mechanical work input by each extruder. Overall, good agreement was observed from the four different routes to measurement, for the LDPE examined.

The second part of a comprehensive study into the investigation and validation of in-process measurements on a low density polyethylene compound during extrusion is reported. A commercial grade of branched low density polyethylene was used in the study, compounded with various levels of magnesium hydroxide¯ame retardant ®ller, up to 50 % by weight. In-line and on-line rheometry were used to monitor melt¯ow behaviour during single screw and twin screw extrusion; off-line capillary rheometry was performed on the compound before and after extrusion, to examine any change in rheology. Morphological observations were made using scanning electron microscopy, and molecular characterisations carried out using gel permeation chromatography. In-process rheometry was found to be able to detect the presence of ®ller at low and high loadings, but was unable to discriminate between intermediate levels. Offline rheometry indicated that twin screw extrusion altered the rheology of the compound, and capillary wall slip was found to increase after extrusion. Morphological observation indicated that ®ller in the compound was better dispersed after extrusion, especially at high volume fractions. Molecular weight and polydispersity of the LDPE were shown to increase after extrusion, more so in the case of low ®ller loadings. Overall, in-process measurements provided useful rheological data, and the compound was found to exhibit complex, process-dependent rheology.

An ultrasonic technique was used to determine whether diVerences in the degree of dispersion of carbon ller in high density polyethylene (HDPE) melts could be identi ed. Ultrasonic transit time measurements were used to identify diVerences between three grades of HDPE with diVerent degrees of ller dispersion (designated high, medium and low dispersion). Tests were carried out on static melts (oV line) over a range of temperatures and pressures, and during extrusion (inline).Tests carried out on samples of static melt showed a signi cant increase in ultrasonic transit time for the poorly mixed ( low dispersion) sample, compared with the medium and high dispersion samples. DiVerences measured during oV line tests were not observed during extrusion tests.

The relative eVects of changes in temperature, pressure and weight percentage ller on ultrasonic velocity through static samples of magnesium hydroxide lled low density polyethylene (LDPE) melt have been determined for a range of temperatures and pressures. Ultrasonic velocity was found to be most sensitive to change in temperature and least sensitive to change in pressure. Using these data and data obtained during extrusion of un lled LDPE, the amount of magnesium hydroxide present (weight percentage) in LDPE was determined. Values of weight percentage calculated from extrusion data were compared with values obtained from the thermogravimetric analysis of extrudate. Variations in weight percentage of ller between the two sets of data followed the same trend with time of extrusion.