For over 30 years, one of the most common materials specified for acid waste piping systems in North America has been polypropylene. Polypropylene is widely resistant to a wide range of acids and bases, having a capability to withstand a pH range of less than 2 to more than 13. However, while polypropylene is an economical choice for standing up to a wide range of chemicals, it is combustible and, as such, building codes often do not permit combustible pipe to be exposed in certain areas including return air plenums.
Polypropylene is a polyolefin thermoplastic that is polymerized from an oil derivative known as propylene. Propylene, which is an olefin, is like most derivatives of crude oil in that the basic molecule consists mostly of carbon and hydrogen molecules (CH3 - CH3 = CH2). Like most carbon-based products, polypropylene is combustible. It often is blended with additives to make it "flame-retardent" (also referred to as "fire-retardant"). However, it is not possible to retard combustion in a carbon-based product without increasing smoke generation to some extent due to the production of partially combusted hydrocarbon byproducts.