The worst PP material (polypropylene), the furniture industry uses recycled PP materials, such as plastic bags, wires, medical waste, etc. If burned, it will smell bad and the quality is very poor. Sihoo doesn't use it.
Conventional PA material (nylon, scientific name polyamide), good quality and long life. Can be brushed.
Some high-end furniture wheels even use zinc alloy for cup holders, and ABS engineering plastics for wheels (ABS resin scientific name acrylonitrile-butadiene-styrene plastic).
PU edging wheel (scientific name polyurethane), soft, wear-resistant, comfortable, but expensive.
(Scientific name polyvinyl chloride), comfort is slightly worse than PU edging caster and durable.