Carpet and Rug Fiber Chemistry

   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. Also, very rarely, carpet may contain Cotton. (Yarn is a continuous strand of fibers.) The terms “carpet” and “rug” are sometimes used interchangeably. Rug generally means a textile floor covering that is not fastened down and that does not extend over the entire floor. Carpet usually refers to a floor covering that is installed and fastened down from wall to wall. In this section, we are primarily covering carpet fibers. Rug fibers would include these fibers and many others.

  Most synthetic fibers are 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 spinneretteSpinnerette (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 dyeability, 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.

   There are five generations in nylon fiber:

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.
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.
Third generation Anti-static qualities were added.
Fourth generation Fluorochemical soil/stain protection was added.
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.

  Polypropylene (olefin), a by-product of gasoline refining, continues to gain market share for two reasons: it costs significantly less than nylon and it is inherently stain resistant. Its inherent stain resistance arises from its lack of dye sites and the fact that it is chemically inert. In fact, chlorine bleach and even battery acid have no effect on it. It is inherently mold resistant. It is also resistant to fading from sunlight and is, therefore, the fiber of choice for outdoor use. Olefin’s biggest drawbacks are its lack of resilience, its strong attraction for oily soils, and its propensity to wick more than nylon resulting in more frequent complaints of “reappearing spots”, streaking, yellowing, and resoiling. Also, because it has a much lower melting point than nylon, friction from moving furniture or casters can permanently damage the fibers. Additionally, a broken vacuum cleaner belt will seriously and permanently damage an olefin rug or carpet, melting the rubber into the fiber. Fortunately, because it is so resistant to most chemicals, more aggressive cleaning agents can be used on olefin.

   Olefin is used in most Berber carpets or for use where a less expensive product is required and life expectancy and long-term appearance are unimportant. Olefin is the fiber of choice to use outdoors. Olefin should NEVER be used where the primary soil is oil; for example, olefin should never be used in a car showroom.

   To reduce wicking on olefin carpets and rugs, it is best to prevacuum, make extra drying passes, and use air movers to speed up drying. The new Advanced Teflon® is recommended for olefin. Olefin should never be treated with a solvent-based protector. The mill can apply protector to olefin, by essentially fusing it into the fiber.

   Polyester (PET) had been gaining market share, but is no longer. It had been gaining because it was easily recycled from 2-liter soda pop bottles (a good “talking point” to environmentally conscious consumers) and because it too is inherently stain resistant. Polyester carpets are soft to the touch. Like olefin, it has very poor resilience, which obviates its use in high traffic areas and it readily attracts oily soils.

   Corterra® (PolyTrimethylene Terephthalate – sometimes abbreviated PTT), made by Shell Chemical, was touted at one time as the eventual replacement for nylon as the primary fiber of choice in carpet, but that has not happened.

   Acrylic is very rarely used today because of its poor wear qualities and its poor resilience. Acrylic is a plastic fiber made from acrylonitrile units, frequently used in fake fur upholstery fabrics. Has a similar "hand" to wool but is seldom used in carpet today because of its lack of resiliency and its strong tendency toward pile reversal. Commonly used in upholstery. The burn test for acrylic is that it burns quickly with a large, orange flame and thick, black smoke to an irregular, rough ash that can readily be crushed. Usually wet cleaned.

   Natural fibers made from organic sources that have not been chemically altered include wool, cotton, silk, sea grass, jute, and hemp. Natural fibers require more drying time because they are very absorbent, are more easily damaged by chemicals, shrink, stain easily, water spot, fade, and are readily attacked by mold and insects.

  Silk is the only natural filament fiber. The fibers usually are about 1000 feet long. It is a protein fiber obtained by unreeling the cocoons of various silkworms. Characterized by a very high sheen, silk is very sensitive to alkali, sunlight, etc., and tends to yellow when exposed to alkaline detergents. Dye loss, yellowing, and ringing are common problems when cleaning silk. Silk is normally dry-cleaned. Wet cleaning is very risky because of the possibility of water spotting and watermarks. If silk is wet cleaned, only use a neutral detergent. The absorption of water breaks the bonds in the silk fiber and causes silk to lose approximately 20% of its strength. “Washable silk” results from a modification in the dye or a resin treatment to prevent fiber degradation. See the glossary for more detailed information.

   Wool is normally from fleece (sheep hair), but in antique rugs the wool can also come from other animals like goat hair, etc. At one time, wool was the primary fiber in carpet, but its cost resulted in steadily decreasing market share. Wool has several excellent qualities. It hides soil much better than synthetic fibers because it is not clear and, therefore, soil cannot be seen through it, plus it has scale edges to further hide microscopic particles of soil. It is also the preferred fiber where cigarette burns are a problem, because it does not melt. Wool's high moisture content and protein constituents provide natural flame resistance. Wool also “feels” softer to the touch. Wool carpets wear well and age beautifully and have a look and feel that is unmistakably their own. However, nylon and olefin carpets will last longer in high traffic areas.

   Wool cells come in two different types: the paracortex and the orthcortex, which lie on opposite sides of the fiber and grow at slightly different rates. This causes a three-dimensional corkscrew pattern of coiled springs much like shock absorbers, giving wool high elasticity and a “memory” that allows the fibers to recover and resume normal dimensions. Wool fibers can be stretched up to 30 percent without rupturing and still bounce back.

   However, wool is expensive, is easily stained by nearly everything, has very poor chemical resistance making spotting and stain removal much more difficult, mildews, is attacked by carpet beetles and moths, fades easily in direct sunlight, and attracts and stubbornly holds on to protein soils such as urine, blood, and meat juices.

   Use caution when using proteolytic enzyme spotters or deodorizers on wool – they are designed to attack proteins and wool is a protein.

   To protect wool rugs from moths, apply Steri-Fab® or Microban Formula X-580 and turn the rugs frequently.

   Per WoolSafe North America, wool carpets and rugs should always be neutralized after cleaning to a natural pH of 4.5-5.5. This is easily done with application of Brown Out or a fiber rinse such as Last Step.

   Cotton, a vegetable fiber obtained from the seed of the cotton plant, is occasionally found in carpet, but it is too easily and Cotton Flowerirreversibly stained and wears too poorly to be common. The most common use for cotton is in fringes in rugs. There, soiling and graying are a problem.

   Cotton is made into small area rugs, such as braided cloth rugs, that can be washed and dried easily. Some woven rugs have cotton as the warp fibers, (particularly antiques and imports from certain countries). Unlike silk, cotton is 15% stronger when wet!

   The choice of fiber to use depends upon cost considerations, desired life span, where it’s to be used, how much traffic it will be exposed to, and how much moisture and sunlight it will be exposed to.

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

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