3.3.1 Physical treatments
There are many physical treatments currently employed in the textile industry to ameliorate the effects of dimensional instabilities. Treatments include heat setting, relaxation drying, compressive shrinkage, UV curing and corona discharge tech- nology, and autoclave finishing. These are discussed in more detail below.
Heat setting is a process used to enhance the dimensional stability of a fabric produced from thermoplastic fibres. The aim of the process is to obtain a stress-free textile product. The temperature required in heat setting must be below the melting point of the polymer fibres (Miles, 2003). Heat setting can be done on a pin stenter, which is a machine for fabric setting that completely controls both the width and length of fabrics using the overfeeding mechanism pioneered by Krantz (Lockett, 2003). In order to ensure good dimensional stability, it is essential that the fabric is cooled before it is removed from the pins. Woven fabrics are easier to stabilise compared with knitted fabrics because woven fabrics contain clearly defined crimp intersections and only small changes in yarn shape, causing minimal effects on fabric dimensions. A lower heat setting temperature may be used when processing woven fabrics, whereas the heat setting temperature required for processing knitted fabrics depends upon the loop shape for the desired dimensional stability (Miles, 2003).
Phillips et al. (2003) studied the effect of heat setting on the dimensional stability of poly(lactic acid) (PLA) fibres and found that heat setting improved the dimensional stability of the yarn, especially false-twist texturised yarn. Heat setting for between 15 and 45 s conferred almost as much stability as heat setting for 2 minutes. Therefore, there is no significant advantage in extending the time of heat setting during bulk processing beyond 30–45 s. The optimum time for heat setting of spun and false-twist texturised PLA yarns to reduce shrinkage during subsequent wet processing was reported to be in the region of 30–45 s at 130 °C (Phillips et al., 2003).
Fabric shrinkage can also be reduced by relaxation drying, known as ‘London shrinking’. During relaxation drying, the fabric is placed in contact with a dampened wrapping material and allowed to stand under tensionless conditions for a period of time (e.g. several days) and then the fabric is hung on a frame to dry (Lockett, 2003).
Compressive shrinkage is a technique which virtually eliminates the length shrinkage of finished fabrics. Two compressive shrinkage processes are com- monly employed: the Sanforizing process developed by Wrigley and Melville, and
Dimensional stability of fabrics 63 the Rigmel process (Lockett, 2003). Both depend on the cloth being held in contact with an elastic surface that is changed from an extended state to a contracted form under the action of a compressive force, so that the cloth is also subjected to the same compression (Lockett, 2003). In the original Sanforizing machine, the elastic surface was a thick felt blanket with a thickness varying between 0.275 and 0.45 inches, depending on the material to be treated. In the Rigmel system, a thick rubber belt is used instead of the felt blanket; current Sanforizing machines also use a rubber belt (Lockett, 2003). The fabric that is to undergo compressive shrinkage is first humidified and steamed. A short stenter is then employed to achieve the desired finished cloth width. The cloth is then processed through the rubber belt, and shrunk until the required length is achieved; finally, the cloth is dried on a drum dryer (Lockett, 2003).
For knitgoods stabilisation, modern machines such as the Sperotto-Rimar TS 150 (for tubular fabric) and TS 240 (for open-width goods), employ a combination of overfeed and tensionless drying/calendaring, while compressive shrinkage machines such as the Tube-Tex ‘Compactor’, have been developed specifically for knitted fabrics (Lockett, 2003). A variant of the compressive shrinkage technique, known as the ‘confining passage’ method, was developed specifically for knitgoods.
The Bestan machine, produced by Hunt and Moscrop, is an example of a confining passage method (Lockett, 2003).
The application of UV curing and corona discharge technology improves the shrink-resistance of wool with little physical damage. This is a dry process in which solventless UV-curable polymer systems are used on corona pre-treated and non- pretreated wool fabric. UV radiation curing can be defined as the use of radiant energy in the UV portion of the electromagnetic spectrum (200–400 nm) to convert a polymer coating from the liquid to solid state (Dodd and Carr, 1998).
Autoclave finishing is a physical treatment method used to reduce hygral expansion in piece-dyed fabrics. Baird and Shahkarami (1999) reported a signifi- cant reduction of hygral expansion at a pressure of 1 atm in pieced-dyed fabrics.
Since piece dyeing is a very severe setting treatment, the treated fabric will be more extensible with higher crimp after treatment – this will cause hygral expansion values to be high after wet finishing. Therefore, the application of mechanical tension in a further processing treatment such as autoclaving produces a decrease in yarn crimp that can result in a significant reduction in hygral expansion. The reverse scenario occurs in yarn-dyed fabrics, whereby hygral expansion values would increase if autoclave finishing was used as a setting treatment.
3.3.2 Chemical treatments
Chemical treatments employed to maximise dimensional stability in fabrics in- clude chlorination, dry cleaning, fabric softeners, silicone softeners, enzyme softeners and shrink-resistant finishes. All these treatments have associated advantages and disadvantages.
64 Understanding and improving the durability of textiles
The chlorination of wool fabric by the application of Hercosett polymer has the advantage of partially shrink-proofing the fabric (Dodd and Carr, 1998); complete shrink resistance has also been demonstrated. However, the use of this polymer leads to the presence of absorbable organohalogens in the treatment effluent. The development of low-effluent systems for wool is an important environmental issue requiring further research.
Dry cleaning involves appropriate solvents and agitation but the solvents are not absorbed by the fibres so they do not swell and the properties of the fibres are not affected. This reduces some of the problems that occur during wet cleaning processes (Saville, 1999).
Fabric softeners such as cationic and non-ionic surfactants are used in textile wet processing to improve fabric hand and mechanical properties. The effect of softeners is to coat the fibre surface with a thin film layer so as to lubricate the surface by reducing the friction between the fibres and yarns in the fabric substrate;
this reduction in friction improves the hand feel and dimensional stability of the fabric. For example, organofunctional silicone softeners, a mixture of aminofunctional and epoxyfunctional compounds with different compositions, have been used effectively to improve both dimensional stability and hand feel of wool fabrics. Tae and Min (2001) found that an increase in dimensional stability of wool fabric can be obtained with a 1:1 mixture of aminofunctional and epoxyfunctional silicone softeners because of their synergistic effect. The hygral expansion of wool fabrics treated with silicone softeners was less than that of untreated fabric. The reduced hygral expansion could be associated with the earlier onset of swelling shrinkage as a result of the silicone softener. As moisture regain increased, the separation of the yarn centres at crossover points increased, which led to a reduction in the spacing between adjacent threads and hence improved stability (Tae and Min, 2001).
Enzyme softening is also used as a chemical treatment of yarn. It has been shown to increase the dimensional stability of single jersey fabric to a remarkable level.
The use of cellulase on cellulosic fabrics of various constructions and fibre compositions exhibits commercially significant improvements in dimensional stability with no deterioration of fabric handle after subsequent washing and drying cycles. A controlled treatment of cotton fabric with cellulase enzyme can considerably reduce fibre collapsing during drying and rewetting, which results in improved dimensional stability. Cortez et al. (2001) showed that pad-batch treatments with low levels (1.0 to 5.0 mg/g) of selected endoglucanase (EG)-rich cellulases can provide excellent improvements in the dimensional stability of a wide range of cellulosic fabrics. The treatment resulted in more fully relaxed dimensions in treated fabrics. The successful results with cellulase enzyme treat- ments is thought to be due to the fact that the partial hydrolysis of fabrics with cellulase enzyme results in effective stress relaxation with minimum weight loss.
Minimum length-wise fabric shrinkage of 1.5% and width-wise shrinkage of 1%
after full relaxation is achieved with enzyme softening at the yarn stage. Changes
Dimensional stability of fabrics 65 in mass per unit area and thickness remain unaltered after wet relaxation treatments in enzyme-softened fabric (Dhurai and Natarajan, 2007).
Textile materials are also given shrink-resistant finishes to minimise dimen- sional changes during processing. Attempts have been made to control dimensional instability with resins such as urea formaldehyde and nitrogenous substances. Since the swelling of cellulosic fibres by moisture is the primary cause of shrinkage, self-crosslinking urea or melamine products can be used to reduce swelling (Schindler and Hauser, 2004). Resins can be applied using the durable or permanent press process. This process is one of the most important post-cure finishings for cotton fabrics in terms of improving dimensional stabil- ity by the restriction of cellulosic chain slippage. The cellulose chains of cotton fabrics are crosslinked by N-methylol-based resins with strong covalent bonds.
The three basic steps of the formation of crosslinks are: (i) the impregnating of resins (emulsion form) in cotton fibres; (ii) drying; and (iii) curing to enable condensation to occur, with the formation of crosslinking. However, conven- tional durable press finishing processes with resins and reactants involve the use of formaldehyde or formaldehyde precursors (such as dimethyl dihydroxy ethylene urea (DMDHEU), dihydroxy dimethyl imidazolidinone (DHDMI) and a polycarboxylic acid) which are carcinogenic (Sahin et al., 2009). Therefore, Sahin et al. (2009) proposed a formaldehyde-free durable press finishing proc- ess that used ionic crosslinking on cotton fabrics and showed beneficial effects on dimensional stability.