Cleaning & Restoration Association News

Caring and Maintaining Wool Carpet, Part 1

September 13, 2000
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Exhibit 2


Wool as a floor covering textile has steadily gained in popularity in recent years. The use of this age-old natural fiber can be traced to the beginning of recorded history with references to sheep in the Old Testament. The oldest discovered rug is the Pazyryk Carpet, which was woven from wool in the 5th Century BC.

Currently, wool holds 5% of the U.S. floor covering market in terms of dollars (excluding imported area rugs). Based on yardage, this represents 2.5% of the total market. As a high-ticket textile, it’s imperative for the professional cleaner to become adept at cleaning this fiber. More than 50% of wool broadloom carpet is sold into the U.S. residential market. In addition, the growth of Oriental and specialty area rugs has brought wool into most homes today. With the recent trend toward natural fibers and materials, we have every reason to believe the use of wool will continue to grow.

Wool is a unique fiber as it’s taken from a living animal. It is comprised of 19 amino acids, and grows from follicles in the sheep’s skin. Like human hair, wool is made of the protein keratin.

Sheep are a 24-hour-a-day fiber factory, each fiber growing .008 inches per day. A merino sheep has 60,000 wool follicles per square inch of skin, which results in 100 million fibers in one fleece. One merino can produce nearly 5,500 miles of wool fibers in one year. The fibers of five merinos, joined end to end, would tie a bow around the world.

One of the major characteristics of wool fiber is its resilience. Wool, which grows permanently crimped, can be stretched up to 40% of its original length without breaking (wool can be bent 20,000 times without breaking; silk breaks after 1,800 bends; rayon breaks after 75). Wool’s natural crimp helps absorb noise and gives the fiber bulk, trapping air between the fibers providing natural insulation. The fiber is also resistant to abrasion and wears well in high traffic areas.

Wool fibers absorb up to 30% of their weight in moisture and still feel dry. This feature helps reduce static electricity in wool carpets as wool always strives to stay in balance with the surrounding moisture conditions. HVAC systems during the heating season may cause the drying of the environment, which will result in static problems. A topical treatment can correct this situation. Contract wool carpet is produced with conductive fibers to eliminate static if this feature is needed.

In comparison to synthetic fibers, wool carpet holds up well to cigarette burns or fireplace embers due to the high moisture and nitrogen content. The tips of wool pile will char, most of which can be brushed away, while thermoplastics tend to melt quickly leaving a permanent scar. Wool will self-extinguish when a flame is removed. This leaves a crumbly ash and smells like burning hair.

In a cross-section, (See Exhibit 1) each wool fiber consists of a two-part outer layer and an absorbent core. The outer layer has a thin protective membrane called the epicuticle, which gives wool its ability to shed liquids and absorb water vapor. The cuticle has tiny overlapping scales which look like unopened pinecones (See Exhibit 2). They hold dirt high in the carpet pile and enable wool carpet to release soil easily, responding well to vacuuming and cleaning.

A disadvantage of wool is its susceptibility to damage by the larvae of clothes moths and carpet beetles. Just-hatched larvae feed on the keratin as they mature. For wool carpets to receive the Fernmark (which replaces the Woolmark for carpets made from New Zealand wool) they must be treated with a mill applied mothproofing agent.

Wool does have an affinity for dyestuffs, which react chemically and combine with the protein in the cell structures, bonding with either acid, neutral or alkaline dye. However, if wool is exposed to sunlight for prolonged periods its chemical linkages break down and the fiber is weakened.

Exposure to sun or UV light can also result in photobleaching. This is when the natural yellow color component of white or light-colored wool is bleached. When a protected area is uncovered, such as under furniture, the carpet in that area will appear yellow. Exposing these areas to sunlight will ultimately cause the yellow to disappear.

Wool is very sensitive to alkalis and concentrated solutions of strong acids. Chlorine bleach will damage wool by yellowing it and can dissolve it completely. The fiber is resistant to treatment with dilute acids. Aggressive stain removal agents, which contain strippers or reducing agents, can damage the fiber and cause dye loss.

If you are a nomad making a felt cover for your tent, felting would be considered one of the great characteristics of wool. However, if you are a carpet cleaner using too aggressive a cleaning method, then this may be considered one of wool’s worst characteristics. Felting occurs when mechanical action is applied to wool fibers. The scales, as discussed above, cause the fibers to lock together in an irreversible tangle. Mechanical action (like aggressively rubbing the carpet) is accelerated when combined with soaps, moisture and lubricants.

The cleaner can find wool used in every style of carpet construction including woven, tufted, bonded, knitted or hand-tied. Wilton and Axminster are the two primary machine construction methods for woven carpets.

The Wilton method (See Exhibit 3) can include cut- and loop-pile, sculptured or embossed effects. As the backing is woven, pile yarns are introduced lengthwise and looped over “wires” that run across the machine. The wires may then be withdrawn leaving loop pile, or they may be fitted with projecting knives, which cut the pile as the knife is withdrawn. Up to five colors can be introduced on a Wilton loom. When any color is not visible on the surface of the carpet, the yarn is carried along inside of the backing.

With proper maintenance, Wilton carpets can last for many years. In a commercial environment, frequent power pile lifting along with daily vacuuming is crucial. Wiltons tend to have heavier face weight and are more absorbent because of the pattern yarns, which run inside the backing. It’s important to keep moisture from the cleaning method at a controlled level. When using the hot water extraction method, turn down the pressure and make several extra vacuum passes. Overwetting can result in shrinkage, rapid resoiling and mildew of natural fiber backing.

Axminster carpet (See Exhibit 4) is the weaving method used to produce highly patterned designs of many colors, always in cut pile carpets. Two main types of Axminster looms are the Gripper, which can use 6 to 12 colors and has a jacquard device to lift the required colors in the loom; and the Spool loom, which has unlimited colors and is used to produce Karastan® rugs.

Wilton and Axminster backings are woven on the loom as pile yarn is inserted. The length-wise yarns interlock with the width-wise yarns, which also secure the pile yarn into position. These interlocking yarns can be seen on the back of the carpet. The backing is generally cotton in the warp direction with polypropylene or jute in the weft.

Woven carpets are given a back coating of latex to improve tuft bind, increase dimensional stability and prevent carpet pilling. Over use of solvent spotters can adversely affect this backing material.

Wool is also used in tufted carpet. Currently the most popular style for residential use is textured loop (i.e. sisal pattern), characterized by a combination of earth and natural colors.

Next month, I’ll discuss cleaning guidelines for wool carpet and rugs.

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