THERMOSETS VS THERMOPLASTICS
Some Essential Differences
There is an essential difference in heat resistance between thermoplastics and thermosets. In thermoplastics, the absence of any appreciable cross-linking makes them subject to catastrophic failure almost immediately after the heat distortion point is reached. In a cross-linked THERMOSET, thermal degradation is slow (relatively) at temperatures as well above the heat distortion point. Thermal excursions have little or no effect on THERMOSETS.
When discussing thermosets more specifically, the maximum temperature to which moldings may be subjected without major physical change is determined by four factors:
(1) temperature environment at which the binder resin decomposes.
(2) temperature environment at which the fillers decomposes.
(3) thermal history of the molded parts.
(4) length of exposure time at maximum temperature. UL tests the various electrical properties for 50,000 to 100,000 hours in or out of ovens.
Not very much is known concerning temperature limits when exposure time is of the order of seconds or milliseconds, except that materials which normally would not be suitable for continuous service above 375°F, have withstood extremely high temperatures, perhaps up to 5000°F. We can produce electric fuse tubes that quench the arc flash of lighting 3 successive times without failing.
Long term or continuous exposure can be fairly well defined in terms of limiting constants of factors #1-3. For example, the maximum recommended temperature for exposure of cellulose-filled phenolics is approximately 300°F, and is determined by the decomposition of cellulose which occurs at approximately 300°F temperature in air. For inorganic-filled (mineral, glass, quartz, etc.) phenolics, the phenolic resin binder is the limiting factor, and such materials may be suitably used at temperatures at least 500°F and higher depending on factor #3. To the extent that thermal decomposition of organic materials is hastened by the presence of oxygen, the concentration of this gas in the environment can also be a determining factor. Thermosets will not melt under blowtorch temps, but they will turn red hot and combine with oxygen and char, called ABLATION.
The “curing” of thermosetting resins entails making larger and more cross-linked molecules out of smaller molecules. Curing is a process which is always accompanied by a shrinkage of the resin involved, the amount of shrinkage being dependent on the molecular changes involved and the completeness of cure. In addition, with some resin types, by-products are produced during cure. For example, the isocyanates liberate carbon-dioxide; while melamine, urea, and phenolic resins liberate water during the cure. If the volatiles are not entirely eliminated slowly during cure, the moldings may blister if suddenly subjected to thermal shock. Usually these volatile substances can be slowly expelled by slow heating, which also results in a greater degree of cure. This process is called POST CURING and is ramped slowly to eliminate the molded part from blistering.
It is a general rule that to assure resistance to thermal shock at any given temperature ( up to the decomposition temperature), the plastic part should previously have been exposed to that temperature. This may be accomplished by post-cure in circulating air oven in which the part will be exposed in service. The maximum permissible rate of oven-temperature rise in the post-cure schedule will depend on the rapidity with which volatiles are diffused from the molding. The rate of volatile escape is greatest along reinforcing fibers. In the absence of fiber orientation, the volatile substances will escape via the shortest path through the molding.