Have a look at the technical sheet with this website -Tthermastat.
Thermastat , hollow core fibre, for extra warmth. Probably the Rolls Royce of thermal fibres.
The hollow core holds in more warmth, breathes, soft, and doesnt smell like polyprop after wearing.
Unlike polypropolene, this thermalite has a teflon coating of the fibre, and doesnt cut the body like polypropolene.