'Practical Guidelines for the Efficient Postbaking of Molded Phenolics'

摘要

Postbaking is a manufacturing step where molded parts are heated in an oven after being removed from the die. Many compound molders make perfectly good parts and never need to postbake. For many applications, postbaking is not needed or even recommended, however, postbaking can have a profound effect on the physical properties of a phenolic part. For applications requiring high dimensional stability or resistance to high temperature, it is well worth the time and effort. It is, therefore, equally well worth the effort to try understanding what actually happens when we postbake. Though it is not the topic of this paper, outgassing is one of the most useful effects of postbaking. Most phenolic compounds are cured with hexamethylenetetramine (hexa), which produces ammonia as a byproduct of the curing reaction. Similar situations can occur with other thermosets where remnants of crosslinkers or curing agents might remain in molded parts. Ammonia removal is simply a diffusion process. Raising the temperature will increase the diffusion rate, but the effect is not linear. One of the most important factors in promoting rapid diffusion is maintaining a concentration gradient. This means it is important to have good ventilation in the oven. Another big factor is the gas permeability of the compound itself. Environmental and other situation specific parameters tend to dominate any predictions of the rate of outgassing. Optimization of this type of process is therefore best handled on a case by case basis.