Hollow thermoexpandable particles or microspheres are provided that contain
hydrocarbon blowing agents and have a shell
polymer that can be softened at the onset of the expansion temperature and solidified at a higher temperature (thermoset) in an expanded state. Preferably, the microspheres have a shell of thermally expandable and thermally crosslinkable
polymer and a hollow interior that contains a
hydrocarbon liquid that boils at a temperature below the heat activated crosslinking temperature of the
polymer shell. The crosslinking of the shell polymer during or after expansion, which is heat activated at an elevated temperature, can solidify the shell polymer and, then, maintain the expanded volume of the microspheres. The thermoexpandable thermoset polymer particles are useful in insulation, packaging, for making foam materials such as
polyurethane or
polyisocyanurate rigid foams. The invented thermoset microspheres can substantially improve the close
cell percentage and
compressive strength of the foam materials and make the foam manufacturing less hazardous.