polypropylene; its made from small fuel petrolium capsules, smells like parafin when doing a burn test, poly also has a low melting point = 300' f, don't drag furniture across the carpet as friction burns cannot be removed, even skiding on the carpet can cause the same damage, not very resistant to oil, older polyprop's can have a poor 'memory' of resilience and recovery when crushed, only fibre to float on water, always pre-vac less chance off wick back "generally" clean without pre-vac forces the dirt to the back of the pile, naughty! naughty! C.C.S.