Polyethylene and polypropylene are familiar to consumers who recycle as PP, HDPE, and LDPE in disposable plastic items. As fibers, the Federal Trade Commission classifies them as “olefins”; this is also the chemists’ term for the ethylene and propylene used to make them. Depending on the way the polymer is made, polyethylene melts at 110° to 135°C (240° to 275°F), while the usual polypropylene melts at around 165°C (330°F). For this reason, the vast majority of olefin fibers are based on polypropylene, and even then, the low melting point is a limitation. Gel-spun polyethylene fibers are distinctly different and are discussed below.
The lack of moisture absorption translates into “wickability,” and olefin fibers have thus been used for athletic and hiking socks, cold weather underwear, and diaper liners. In many instances, polyester, which also has a very low moisture retention but is dyeable, has taken over those end-uses. Low cost renders the material disposable, and olefin has been used for disposable surgical gowns. It has tended to replace cellulosic fibers such as jute in carpet backing and in sacking.
The technique of gel spinning has been used to produce polyethylene fibers in which the polymer chains are highly aligned along the length of the fiber. One commercial example is sold as Spectra. The excellent alignment gives the material a very high strength, some 3 to 4 times stronger than polyester, and of the same order as para-aramid fibers such as Kevlar. Like Kevlar, it is thus useful in cut protection, ballistic protection, and sailcloth. While the lower weight of olefin is an advantage, the low melting point may be considered a limitation.