The HT resin may be formulated with a large percentage of mineral filler.

I just saw that you posted a similar question 6 days ago, and received a similar response. What information do you need ?

Peter Farrington Will adding a filler change the chemical composition of the polymer? What I am particularly looking to know is what kind of changes in the chemical composition (functional groups maybe) can improve the thermal stability of a given polymer?

Hi, a lot of factors can cause increasing the glass transition temperature and/or melting point of a polymer. Without knowing exactly what type of resin you are dealing with, no specific answer can be given to your question. Generally speaking, the melting temperature of a polymer depends on the history of the specimen, particularly the temperature at which it was crystallized and the rate with which this transition happened! The thicker the crystallized regions, the higher the melting temperature. (For polymer, crystallization has a looser meaning than that for metals and ceramic, but we still use it since more molecular order (called Lamelae) can be found in those regions). Addition of impurities usually decreases the melting temperature. Although, since you're dealing with a resin, it may be a thermosetting polymer and cross-links between chains prevent the crystallization in most cases and this does not apply here. Glass transition temperature on the other hand, can be affected by addition of bulky side groups, polar groups or C=C double bonds and aromatic groups in the backbone of the chains which prevent rotation and increase glass transition (There are more possibilities..). As I mentioned earlier, without knowing more about the polymer, pinpointing the exact cause is relatively impossible.