The starting point of carpet is the fiber, a fine thread-like unit that is converted into yarn then tufted or woven into carpet. There are only six primary fibers used in carpet today: Nylon, Polypropylene, Corterra^® (same as DuPont’s Sorona, Polyester, Acrylic and Wool.

Nylon fiber is created by “extrusion” - melting the polymer chips and then forcing the thick, viscous liquid (about the consistency of cold honey) through the tiny holes of a device called a spinnerette (also spelled “spinneret”) to form continuous filaments of semi-solid polymer. The spinnerettes used in the production of most manufactured fibers look very much like a bathroom showerhead. A spinnerette may have from one to several hundred holes. As the filaments emerge from the holes in the spinnerette, the liquid polymer is converted first to a rubbery state and then solidified. This process of extrusion and solidification of endless filaments is called spinning, not to be confused with the textile operation of the same name, where short pieces of staple fiber are twisted into yarn. The fibers in conventionally spun staple yarns must be held together by twisting them together into an endless yarn bundle.

Once exposed to air-cooling, the fiber strands harden to a single filament (much like fishing line). The fibers can be extruded in different cross-sectional shapes (round, trilobal, pentagonal, octagonal, and others). The original nylon carpet fibers were round, which creates a lens effect, magnifying soil. By adding chalk to the fiber (called delustering), the magnification of soil is somewhat lessened, but the colors are now dull. The irregular shape of trilobal-shaped fibers reduces the magnification effect and scatters more light to make soil less visible and give an attractive sparkle to textiles. Pentagonal-shaped and hollow fibers, when used in carpet, show less soil and dirt. Octagonal-shaped fibers offer glitter-free effects. Hollow fibers, such as DuPont’s Antron^®, provide better soil hiding and thermal insulation qualities.

The hardened fiber strands are then wound around a bobbin, stretched to a smaller diameter, twisted, and wound. While extruded fibers are solidifying, or in some cases even after they have hardened, the filaments may be drawn to impart strength. Drawing pulls the molecular chains together and orients them along the fiber axis, creating a considerably stronger yarn.

Synthetic fibers are usually produced in bulked continuous filament (BCF) for use in loop carpets and cut pile carpets and in staple form that is spun into yarn for use in cut pile carpets.

Nylon continues to hold over half the carpet market. Nylon, a polyamide polymer, has excellent resilience (the ability of a fiber to “spring back” to its original configuration), abrasion resistance, mildew resistance, and very good color retention. It can be acid dyed or solution dyed, which is discussed later. On the downside, it is melted by very strong acids such as hydrochloric acid toilet bowl cleaners, is easily stained by acid dyes (the dyes in most foods and drinks), and is bleached out by chlorine bleach. Nylon comes in two forms: Type 6,6 and Type 6.

Type 6,6, invented by Dr. Carothers of DuPont in 1938, is formed by the reaction between adipic acid, which has 6 carbon atoms, with hexamethylene diamine, which also has 6 carbon atoms. The advantage of Type 6,6 is that it has greater stain resistance than Type 6, is 12% harder than Type 6, and has a slightly higher melt point. This is the type of nylon used by DuPont and Solutia (formerly Monsanto).

Type 6, invented by Paul Schlack in Germany in the 1930’s, is made by co-polymerizing caprolactam, a 6-carbon molecule. This is the type of nylon used by Allied Signal and BASF. This type of nylon has a slightly lower melting point than Type 6,6 nylon, but it has superior light degradation, better dye-ability, elastic recovery, fatigue resistance, and thermal stability.

While these are the pros and cons of type 6,6 versus type 6 nylon, there really isn’t enough difference to matter very much - it’s more a matter of marketing than anything else. There have been tests done that indicate identical carpet performance when all aspects of the carpet construction such as yarn size, color, construction, etc. are exactly the same.

An article comparing type 6 versus type 6,6 nylon by Carey Mitchell of Shaw Industries: Are There Real Differences Between Type 6 and 6,6 Nylons

There are five generations in nylon fiber:

1. First generation: The first nylon fibers. These performed poorly because the soil could be seen through the fiber (unlike wool) and because, due to the round cross-section of the nylon fiber, the soil was actually magnified. As a result, apparent soiling was extremely rapid compared to wool. Later, delustering agents such as chalk were added to eliminate this problem, but the fibers had a dull appearance with this treatment.
2. Second generation: This is first generation nylon fiber with modified cross-section for soil hiding, such as tri-lobal. This eliminated the round shape problem causing the magnification problem.
3. Third generation: Anti-static qualities were added.
4. Fourth generation: Fluorochemical soil/stain protection was added.
5. Fifth generation: Acid dye stain blocker was added (stain-resist nylon). Note: Nylon stain resist warranties are most often covered under installations in owner-occupied residences.

The attached chart of carpet fiber characteristics gives a quick overall comparison of the primary carpet fibers.