A variety of methods produce wool fabrics that withstand repeated washing without shrinkage and felting. They are particularly important for knitted woollens and worsted fabrics. Two main principles are used:
(1) modification of the scale structure of the fibre cuticle to decrease the directional friction effect – this can be achieved by chemical treatments that either partially remove the scales or cover them with a smooth film of polymer;
(2) reduction of fibre mobility by adhesion of fibres and yarns at their points of contact and by decreasing fibre elasticity by means of intermolecular crosslinking.
Treatment of wool with chlorine attacks the scales of the cuticle and gives fibres with a smoother surface and a fabric with improved shrink resistance. This is done by treating a scoured wool fabric with dilute HCl solution at room temperature, draining, and then running the goods in water slowly adding sodium hypochlorite solution. The acid retained in the wool reacts with the hypochlorite liberating chlorine (Scheme 7.4). Alternatively, treatment of the moist wool with chlorine gas in a sealed chamber is also effective. After chlorination, treatment with a
WOOLPROCESSING
2 HCl + NaOCl Cl2 + H2O + NaCl Cl2 + H2O + NaHSO3 NaHSO4 + 2HCl Scheme 7.4
solution of the reducing agent sodium bisulphite removes residual traces of chlorine (Scheme 7.4). Both chlorination methods, however, require careful control to ensure uniformity of the treatment and to avoid imparting an overly harsh handle to the wool. The reaction of chlorine on the cuticle is very rapid. It is for this reason that the chlorine must be generated or added slowly.
The chlorination process increases the substantivity of acid dyes for the wool and level dyeing becomes more difficult. It will be impossible if the wool has been unevenly chlorinated, since chlorinated wool has a much higher rate of dye absorption. Unfortunately, chlorinated wool also gives much higher rates of dye desorption on washing. Acid dyes with good migration, which might be useful for dyeing unevenly chlorinated wool because of desorption from heavily dyed fibres and re-absorption by paler ones, also have rather poor washing fastness and are almost useless on chlorinated wool that will be washed regularly.
The problem of even chlorination can be partly overcome by use of a compound whose hydrolysis slowly liberates chlorine once it has been uniformly distributed in the wool. The sodium salt of dichloroisocyanuric acid is such a compound (Figure 7.5). Gradually increasing the acidity of the solution at room temperature, or increasing the temperature at a constant pH of about 4.5 to 5.0, controls the rate of chlorine evolution. Continuous processing is possible, but again a bisulphite anti-chlor treatment is needed to remove residual chlorine from the goods.
N N
N
N N
O N Cl
O
O Cl
H
O O
O H
Na Na
+ 2H2O + 2HOCl
Figure 7.5 Hydrolysis of dichloroisocyanuric acid
Wool CH2 S S CH2 Wool + 5Cl2 + 6H2O 2Wool CH2 SO3H+ 10HCl Scheme 7.5
The objective of the reaction with chlorine is to modify the scales of the cuticle rather than the cortex. The scales, particularly the tips, are attacked and their mutual friction is reduced thus decreasing the degree of shrinkage on washing.
Chlorination also oxidises disulphide crosslinks in the cuticle proteins to sulphonic acid groups (Scheme 7.5). These are strongly solvated by water and the cuticle
125 therefore swells and becomes softer. Chlorination also generates some water- soluble peptides from the proteins in the cuticle, which also contribute to softening the scales. The anti-shrink effect of chlorination may not be permanent if these proteins are gradually removed from the fibre surface on washing and the scales harden.
A number of other so-called subtractive or destructive wool shrink-resist processes are known. These also modify the scale structure. In particular, the process based on treatment of wool with permonosulphuric acid (H2SO5) merits mention. It will be of increasing importance as concern for pollution by organochlorine compounds increases.
The most successful, permanent anti-shrink treatments for wool combine mild chlorination with cationic polymer deposition on the fibre surface. The chlorine- Hercosett method for continuous treatment of wool tops (combed wool sliver) is particularly well known. The process is also applicable to wool garments. Mild pre- chlorination with hypochlorite and acid prepares the wool fibre surface for the cationic polymer. An anti-chlor treatment with sodium bisulphite eliminates residual chlorine and reacts with residual acid to form sulphurous acid. The Hercosett polymer is a polyamide with cationic azetidinium groups introduced by reaction of secondary amino groups with epichlorhydrin (3-chloropropene oxide) (Figure 7.6). The polymer is applied to wool from aqueous solution and is substantive to the anionic wool surface. During drying, the azetidinium groups are able to react with thiol groups in the wool and with free secondary amino groups in other parts of the polyamide chain. This insolubilises and fixes the resin to the fibre.
Hercosett-treated wool has different dyeing properties than wool and chlorinated wool. Under neutral or acidic conditions, the deposited polymer retains a pronounced cationic nature. This arises from residual azetidinium groups and the protonation of the tertiary amino groups from their hydrolysis (2,3- dihydroxypropanamine groups). Anionic acid dyes have higher substantivity for the treated wool and high initial rates of absorption because of their attraction to the cationic polymer coating. Level dyeing requires considerable care. Under washing conditions, these quaternary 2,3-dihydroxypropanamine groups lose their protons, becoming neutral, so that the ionic attraction holding the dye molecules is lost. There is therefore a substantial decrease in the washing fastness of most anionic dyes, except reactive dyes that have covalently bonded to the protein.
A variety of other polymer deposition processes is used for producing shrink- resistant wool. These act by coating the fibre scales and reducing the frictional
WOOLPROCESSING
effect, and by welding fibres together at their contact points. The polymer can be a polyamide produced by interfacial polymerisation, or a silicone or polyurethane coated onto the fibres using an organic solvent. A curing step is usually needed to fix the film of polymer onto the surface scales.
Several different testing procedures are used to evaluate the shrinkage of wool fabrics or articles. These involve evaluation of the relaxation and felting shrinkage in standardised washing and felting procedures. As with other chemical treatments of wool, shrink-resist processes cause some damage to the fibres. This can be assessed by dyeing and wool solubility tests. Damaged wool invariably gives a higher rate of dye absorption and is more readily soluble in dilute NaOH solution, or in solutions of urea and sodium bisulphite. The evaluation of fibre damage is important because damaged wool is less resistant to repeated washing and to abrasion [5].
NH CO(CH2)4 CONH (CH2)2 NH (CH2)2 NH CO (CH2)4 Polyamide CH2 CH CH2Cl
O +
(CH2)2 N (CH2)2 CH2 CH OH CH2Cl
(CH2)2 N (CH2)2 CH2 CH CH2O
(CH2)2 N (CH2)2 H2C
CH CH2 OH
(CH2)2 N (CH2)2 CH2 CH OH CH2OH H
Hydrolysis product Hercosett polymer
N CH2 CH CH2 S CH2 Wool OH
Polymer fixed to a wool thiol group
N CH2 CH CH2 NH Wool OH
Polymer fixed to a wool amino group n
Figure 7.6 Chemistry of the Hercosett process for shrink-proof wool