Shrink resistance

Textile Finishing. Polyethyleneimine-A/-methylolurea derivatives improve the crease and wear resistance of cotton (429,430). The adhesion between individual wool fibers is improved by pretreatment with amines, which leads to improved shrink resistance (431). An antimicrobial finish can be appHed to cotton by using a combination of PEI and ureas to bind zinc pyrithione to the fabric (432). After wool has been provided with a flameproof finish using fluorozirconate or fluorotitanate, the wool can be neutralized with PEI (433). Conventional neutralizing agents caimot be used for this purpose since they impair the flameproof characteristics of the impregnated fabric. 

Some combing plants also produce shrink-resistant treated wool-top. The current process incorporates chlorination, and the appHcation of a shrinkproofing polymer. Chlorination is likely to be replaced soon by mote environmentally friendly treatments. 

The principal oxidi2ing agent used in degradative shrink-resist treatments is chlorine. Free chlorine reacts very rapidly with wool hence it is difficult... 

Fig. 8. Electron micrograph of Merino wool fibers in a fabric that have been treated with a typical shrink-resistance polymer, showing fiber—fiber bond...

The principal additive shrink-resist treatment uses the polymer Synthappret BAP (Bayer AG) which is a polypropylene oxide polyurethane containing reactive carbamoyl sulfonates (or isocyanate bisulfite adduct groups, —NHCOSO —Na" ). An aqueous solution of this polymer is padded onto woven fabrics, which are immediately dried. Other polymers may be appHed at the same time to modify the handle. 

To produce easy care wool—polyester blend garments, heat setting of the polyester component will impart set stability to repeated machine washing. A minimum of 20—30% polyester is needed for adequate stability. A fabric shrink-resist treatment such with Synthappret BAP may be necessary to give the required shrink-resistance for easy cate performance. If the polyester content is increased, particularly above 50%, and a suitable fabric constmction is used, heat setting alone will give easy care performance. 

It is more difficult to produce pure wool easy care garments. The problem is to obtain adequate durability of the setting rather than obtaining shrink-resistance, because some wool setting processes are not stable to machine washing. As of ca 1997, the best approach is to combine the Synthappret BAP fabric shrink-resist process with Siroset garment setting. 

Shrink-resistance processes compatible with Zirpro-treated wool have also been developed (166). 

Although the present situation and the way ahead appear uncertain, it is clear that enzyme treatment alone does not fulfil the technical requirements for shrink-resist finishing. Even with enzyme treatment, some degree of chlorination (with the attendant AOX problems) and/or application of a resin will still be required. Two-stage or even three-stage processes have been proposed [116] ... 

Enzymes can be used to modify the surface of wool fibres in order to improve lustre, softness, smoothness or warmth of the fabric. Since such processes involve attack on the cuticular scales of the fibre, there is clearly a resemblance to shrink-resist treatments and similar methods are used [116] ... 

Although chlorination with sodium dichloroisocyanurate is still by far the most commonly used method of shrink-resist finishing, there is considerable concern over the environmental influence of its AOX contribution. For this reason, its usefulness could decline in future and there has been considerable investigation of alternatives to this attractively cost-effective treatment. 

Enzymes have been proposed as a means of subtractive shrink-resist treatment. Their use has been discussed already in section 10.4-2. There are difficulties, however, in the commercially successful application of enzymes to wool at present. 

Chlorination treatments, of course, are invariably followed by a reductive aftertreatment. In the case of permonosulphuric acid, this is even more important as the sulphite treatment significantly enhances the shrink resistance (Figure 10.46). 

Permonosulphuric acid treatment confers only a modest shrink-resist effect which usually needs to be improved by a subsequent additive treatment. It has been suggested [300] that the most likely mechanism for inhibiting felting by permonosulphuric acid treatment is the removal of degraded protein from below the exocuticle, producing a modified surface with a reduced differential friction. The direct formation from cystine residues of low concentrations of Bunte salts has been confirmed, as indicated in Scheme 10.42. 

The treatment must impart maximum shrink resistance. 

High substantivity for wool. This is clearly linked with the foregoing requirement. These two factors are particularly important when application takes place after an AOX-free oxidative stage, since such treatments generally impart lower initial shrink resistance than chlorine-based subtractive treatments. Indeed, these two requirements may need to be fulfilled so effectively that the oxidative stage before polymer treatment can be omitted. 

No shrink-resist polymer developed so far meets all the above requirements [301]. There is clearly some similarity with easy-care finishing of cotton. Although effective crosslinking agents are readily available for application to cotton, the morphological complexity of the wool fibre is such that an equally effective polymer has yet to be identified for wool treatment [304]. 

Polysiloxanes as shrink-resist finishes have been developed from their traditional uses as softeners and water repellents as such their chemistry is discussed in section 10.10.3. This was a natural trend as many shrink-resist finishes tend to impart a harsh handle to wool. 

Polyquaternary compounds are useful in that as well as conferring shrink resistance they may also improve acid dye fastness to wet treatments. 

Table 10.33 Comparison of typical shrink-resist polymers of the non-AOX types for application to wool [301]...

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