ICS Magazine

The Coloration of Wool

January 3, 2013
coloration of wool woman in field

I just returned from a trip to Ilkley and Otley, England to visit with my colleagues at WoolSafe®Headquarters. This part of England is where thousands of sheep are raised - much to my surprise, for food (mutton and lamb chops). However, a wonderful byproduct is the beautiful wool fleece which is used for carpet and rug manufacturing. One of the highlights of my trip was a day spent in Manchester visiting the British Wool Bureau, two wool processing plants and finally a yarn dyeing and weaving facility which manufactures wool yarn for Karastan®here in the U.S.

I believe there is a lot of misinformation about cleaning wool, especially related to dye stability, so I thought I’d give you an overview of the process, which you may find helpful when cleaning wool carpet or rugs and talking to your clients about your cleaning methods. 

Wool dyeing has been carried out for several thousand years and was an established industry in Rome around 750 B.C. Wool is one of the easiest textiles fibers to dye. It has a greater capacity to absorb and hold dyes than any other natural or synthetic fiber. It can be dyed in shades that range from pastelcolors up to very heavy depths. Early methods usedcoloring materials derived from natural sources, such as plants, insects and marine animals. Althoughnatural dyes are still used to some extent, particularlyby hand spinners and weavers, by far the greatest proportion of wool is now dyed with synthetic dyestuffs. Compared with dyes of natural origin, synthetic dyes offer the advantage of brighter shades and deeper/richer colors. They also have superior resistance to fading when exposed to light and are much faster to washing and dry-cleaning than natural dyes.

Wool is composed mainly of a large number of different proteins. These are made up of amino acids joined together to form long polymer chains. The amino acids contain side chains, some of which are acidic carboxyl groups or basic amino groups. The wool fiber surface consists of cuticle cells (scales) which overlap like tiles on a roof. The main part of the fiber (the cortex) is composed of spindle-shaped cells which overlap longitudinally. Between the cuticle and cortical cells is a region called the “cell membrane complex.” Although this represents only approximately 5% of the fiber, it is important because it is the only continuous phase in wool and provides a channel for the entry of dyes and other chemicals into the fiber.

Cuticle and cortical cells have different physical structures. They also differ chemically, as the various proteins in wool are not uniformly distributed throughout the fiber. Both the chemical and physical heterogeneity of wool are responsible for the way in which it takes up dyes.

Commercially, wool is dyed by batch methods from aqueous solutions, usually under acidicconditions. It can be dyed as loose (pre-scoured) fiber, or in sliver, yarn, fabric or garment form (post-scoured). In the case of fiber, sliver and yarn, the dye liquor is pumped through the stationary substrate. In fabric dyeing, the material may be transported through the liquor, or the liquor and fabric may be moved together through the machine. Simultaneous circulation of wool and liquor is also used in garment dyeing.

The majority of dyes used on wool contain groups (usually sulphonic acid) that give the molecules a negative charge. Commercial dyes are usually sold in the form of sodium salts, which are soluble in water. Dyestuffs also contain chemical groups (chromophores) that give the dye molecules their color.

The dyes used on wool range from compounds with relatively simple structures and low molecular weights to ones with larger, more complex molecules. As a general rule, low molecular weight dyes are more hydrophilic and are relatively easy to apply, but have poor wet fastness properties. Higher molecular weight dyes are usually more hydrophobic. They are more difficult to apply evenly, but have superior fastness to washing. Some dyes contain groups capable of forming chromium complexes when the dyed wool is treated with potassium dichromate. The chrome/dye complex has a higher molecular weight, and hence better fastness properties, than the parent dyestuff. Chrome dyes are relatively cheap, easy to apply, level dyeing and have excellent fastness properties, but may damage the fiber in the process. 

Over the past few years “reactive” dyes have become increasingly important for use on wool. As the name suggests, these dyes contain a reactive group that is capable of forming a covalent (chemical) bond with suitable groups in the wool fiber. This results in the dye molecules becoming locked into the fiber structure, which enables very high levels of washing fastness to be obtained.

A normal dyeing cycle commences by dissolving the dyes and chemical in the dye bath at around 105°F (40°C). The dye liquor is then heated to the boil, where the temperature is held for 30-90 minutes, depending on the dyes and depth of shade. It is important that the recommended hold time at the boil is used in order to ensure that the dye molecules diffuse throughout the whole fiber. The final step consists of rinsing to remove loose dye, and in some cases (for example with reactive dyes), a chemical after-treatment.

A small percentage of wool (about 1%) is colored by printing a design, using paste containing dyes and other chemicals, directly onto a fabric. The dyes are fixed by a steaming treatment. The presence of a layer of waxy, lipid material on the surface of the fibers makes it very difficult to wet untreated wool fabrics, even after scouring. In order to overcome this problem, wool fabrics are given a chlorination treatment prior to printing. This oxidizes the fiber surface and increases the “wettablity” of the fabric, thus promoting the spreading of the print paste and the penetration of the dyes into the fibers. This process results in an increase in color yield and brightness compared with a print produced on untreated wool.

OK. Bottom line, how does this discussion benefit you as a cleaner? Well, rest assured that if you’re cleaning wool carpet or rugs, the dyes are quite stable… as long as you use WoolSafe®-Approved products in your cleaning process, that is!