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.
Olefin fibers are cheap. The polymer is melted for extrusion through a spinneret into fiber. Olefin production is a relatively simple operation that small companies can undertake. Most olefin fibers have a round cross-section. They have strength comparable to nylon and polyester with a fiber tenacity of 5-7g/d (grams per denier). If olefin fibers are stretched or crushed they bounce back well; they have good resilience and recovery properties. Olefin also doesn't absorb moisture, and the fiber is the lightest of all the common fibers. Its g/cc density is 0.92 (grams per cubic centimeter). This means that fabrics of a given bulk are lightweight, and olefin materials float in water. Weather resistance is limited, but stabilizers are added to render this deficiency unimportant in practice. The fiber is undyeable, and while much research has been undertaken to achieve dyeability, few of these modifications have proved commercially successful. For this reason, most colored olefin fibers are produced by the inclusion of pigment in the melt before spinning, in a process commonly called "solution-dyeing" (although it is technically neither a solution nor a dyeing process). The undyeability can also be viewed as inherent stain resistance, and together with the good resilience, abrasion resistance, low density (i.e., good cover for a given weight), and low cost make olefin a realistic alternative to nylon for carpet fiber, and olefin is widely used in upholstery fabrics for the same reasons. The strength is sufficient to make olefin ropes and cords useful, and coupled with low biodegradability and low cost, makes olefin fibers a good choice for geotextile applications.
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.
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