New plastic foam insulation manufacturers or products encounter the problem that long age thermal performance data cannot be available, such as 5 years old. A method for estimating long-term thermal performance of extruded polystyrene foams based on computer curve fitting against a menu of classic techmcal equations is presented and compared with actual measurements at 5 years after insulation manufacture. The effect of closed cell content, deviation of the foam from ideal uniformity, and foam density is discussed. The method permits estimation of long age thermal performance using relatively short age measurements without detailed and accurate knowledge of gas permeabilities, foam cell morphology, and deviations from ideal foam uniformity.
In my paper presented at the 1959 Conference on Electrical Insulation l , it was reported that a sudden increase in the rate of weight loss of polychlorotrifluoroethylene, known as CTFE, was observed after heating had been conducted for a period of time. As shown in figure 1, the phenomenon was observed to occur between 840 and 1000 hours of heating at 235°C. A corresponding increase in the rate of molecular weight degradation was also apparent. Figure 2 illustrates the effect when zero-strength-time, known as the ZST test, is used as a measure of molecular weight. The same effect, as shown in figure 3, is observed if flow index is used to determine average molecular weight. In all cases, the commonly observed relationship of weight loss or molecular weight with time is interrupted by a sudden rate surge. The designation of lie levator effect II will be used throughout this paper when referring to this phenomenon. A similar observation has been reported more recently by Lee and 2 Hodges . Their thermal aging studies included a proprietary polybutadiene filled with silica flour. The weight loss vs. time curves are shown in figure 4. A definite increase in the rate of weight loss is observed after heating at 225°C has passed 400 hours. No such behavior is detected if the heating is conducted at 200°C or 250°C. Also, unfilled polybutadiene does not exhibit the elevator effect. Lee and Hodges suggest that the phenomenonis caused by the difference in coefficient of expansion between the resin and silica filler. Such a difference is claimed to cause separation of the resin and filler so that the resin surface exposed to thermal oxidation is incr,eas ed • However, such a condition may not be true, since the expans ion coefficient of the resin would be greater than that of the filler. Then it might be expected that the resin would press more tightly around the filler so that any possibility of the presence of oxygen at the resin-filler interface would be further reduced.
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