We have discussed HR's and a bit of the processing of making them on the  
Clar BBoard, www,woodwind.org.  All of this rubber chemistry of course  dates 
back to Goodyear's experimentation to make elastomers and harder solids  from the 
"juice" of Asian rubber trees, primarily isoprene [low polymers], now  using 
other dienes, in partic.,  butadiene, with co-monomer  styrene etc. A number 
of sulfur [et al] compounds [even elements] can be  used in the hardening 
process with temperature and duration of heat treatment  being variables leading to 
many varieties of HR.  That process is  essentially one of chemical 
crosslinking and leads to THERMOSET "resins" as  distinguished from THERMOPLASTIC 
resins, which melt, not decompose with  later application of heat.  I wasn't aware 
of blends of these two basic  types of what we "plastics", but am not 
surprised at specialities such as for  our mps.  I may have said more here than I 
really know, so will  request more up-to-date info, please HELP.  Don