The properties of a fabric are affected by durable press treatment. This is due mainly to the crosslinking of cellulose molecules and the chemicals applied in the treatment process. Durable press has many positive effects on fabrics. Owing to the crosslinking, swelling and shrinkage are reduced, and thus dimensional stability is achieved. Needless to say, wrinkle/crease recovery is greatly improved and retention of intentional creases or pleats is also enhanced. A smooth, wrinkle-free appearance after washing without ironing can be expected after a good DP treatment. Pilling is alleviated due to the reduction in tensile strength and abrasion resistance. Dye and pigment can be fixed by incorporating them into the crosslinked cellulose, and fastness is normally improved at the same time. Crosslinking of a water repellency film onto the fiber surface improves and extends water repellency during washing (Xu and Shyr, 2001).
Reduction in the elasticity and flexibility of the fabric is the most obvious negative effect of durable press. Loss of abrasion and tearing strength occurs in almost all kinds of durable press treatments. For instance, the strength loss of BTCA treated fabric would be up to 50% if no improving treatments were applied.
After durable press treatment, the rigidity of the polymer chains is too high and the mobility of the polymer chains change, which causes a noticeable reduction in fabric flexibility. Loss of tearing strength is caused mainly by depolymerization of the cellulose chains and intramolecular crosslinking. The durable press process normally requires acidic conditions in the bath: for example, N-methylol finishing uses Lewis acid, polycarboxylic acids and its catalyst sodium hypophosphite (SHP). This causes a high level of depolymerization in the cellulose chains.
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Further reasons for loss of tearing strength are, as stated the intramolecular crosslinking (Xu and Li, 2000a) and also crystal destruction in the cellulose (Xu, 2003). Both intermolecular and intramolecular crosslinking happen during dur- able press treatments; intermolecular crosslinking makes the polymer chains stronger and thus contributes to fabric strength, while intramolecular crosslinking affects the elasticity and rigidity of the polymer chains, which incurs a loss of strength. Different methods have accordingly been used to improve strength in durable press treated fabric. Ionic crosslinking can be used in order to change the acidic finishing conditions, which results in less strength loss than conventional methods (Sahin et al., 2009). Careful selection of curing temperature could also minimize strength loss (Xu and Li, 2000b). Short chain polycarboxylic acids have almost the same effect on strength loss as formaldehyde based reagents, but little to no strength loss occurs when long chain polycarboxylic acids are used (Wei et al., 1999). The so-called Sanforset®process uses liquid ammonia plus DMDHEU finishing to obtain better strength retention and wrinkle resistance. Mixed reagents have been used to treat fabric for a better DP performance; research showed the two-step treatment resulted in better winkle recovery properties, but strength retention decreased when curing time was increased (Xu et al., 2001). The one- step treatment could thus greatly increase strength retention (Li, 2008). Boric acid was also added to suppress the reactions during curing and the strength retention increased accordingly (Srichharussin et al., 2004). When pre-tension was exerted during treatment to prevent intramolecular crosslinking, the resulting fabric had 75% strength retention compared to 45% without pre-tension (Xu and Li, 2000a).
Further treatments, such as ammonia treatment, addition of silicones, and other auxiliaries such as polyvinyl acetate and polyurethane, may also improve the strength retention to some extent (Schindler and Hauser, 2004).
Yellowing of fabric is another disadvantage of durable press treatments, which is mainly due to chlorine retention by the fabrics. In urea-formaldehyde treatment, -NH groups can react with chlorine from the bleach and laundry bath, and the resulting hydrochloric acid causes yellowing of cellulose (Bajaj et al., 1984; Shin et al., 1989). Yellowing could be reduced slightly by selecting proper catalysts, and using fluorescent agents or developing chlorine free products for bleaching and laundering. Using cotton/polyester blend yarn may help solve the problem since durable press treatment has little yellowing effect on polyester (Pollack, 1993). In some cases, light fastness and color fastness are reduced by the durable press treatment, but this may be resolved by selecting proper dyestuffs, crosslinkers and catalysts.
Durable press treated fabric always exhibits a very stiff hand due to the self- crosslinking of the reagents, which greatly affects the flexibility of the fabric. In addition, crosslinking agents are film formers, so they may produce thermoset plastic materials on the surface of fibers during the treating processing. Softeners can be hydrocarbon or silicon based and must be used in the durable press finishing formulation to improve the harsh hand of the treated fabric.
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4.3.1 Assessment methods for fabric after durable press treatments
The physical properties of fabrics change greatly after durable press treatment, and show further changes after several laundering cycles. The appearance of the fabric, wrinkle recovery, abrasion resistance, chlorine retention, whiteness, and most importantly, strength loss, are the main aspects that require evaluation. Since formaldehyde is a serious issue in durable press finishing, specific evaluation methods also need to be applied to this issue.
The appearance of fabric after repeated home laundry can be evaluated by AATCC Test Method 124, in which the appearance of the treated fabric after washing and drying processes is compared with the reference standards. The standards are the AATCC three dimensional smoothness appearance replicas, which can be used to rate the appearance of the tested samples, from SA-1 (crumpled creased and wrinkled) to SA-5 (very smooth, pressed, finished) by comparison. AATCC Test Method 128 aims to evaluate the wrinkle recovery of a fabric using the appearance method. The fabric is wrinkled intentionally using specified apparatus, and the appearance of the fabric is then compared to the reference standards. AATCC Test Method 88C also rates the appearance of fabric after intentional wrinkling, but the fabric is subject to washing and drying procedures. AATCC Test Method 66 tests the wrinkle recovery angle of fabric;
the fabric is wrinkled in either a dry or wet state, and the final dry or wet recovery angle is measure to evaluate the ability to withstand wrinkle. ISO 2313 can also be used to evaluate wrinkle recovery by measuring the horizontal wrinkle recovery angle after the fabric is folded. AATCC Test Method 93 can be used to evaluate the abrasion resistance of fabric by the accelerator method, in which the weight loss of a fabric due to abrasion is tested. Chlorine retention can be measured by AATCC Test Methods 92 and 114, in which fabric is treated with a standard hypochlorite solution and exposed to high temperature;
the strength loss is then measured to assess the chlorine retention. Yellowing of fabric can be evaluated by measuring the whiteness of the fabric, usually ac- cording to AATCC Test Method 110. Strength loss of a fabric can be assessed either by measuring the strip strength on a universal tensile tester or by testing the tearing strength on the Elmendorf tear tester (ASTM D 1424). Details of strength testing can be found in reference Saville, 1999. Different countries have different limitations on formaldehyde usage in the textile industry. Various methods have also been used for determining the amount of free and released formaldehyde, such as AATCC 112–1993, Japan Law 112, Shirley I BS 6806, and DIN 54360 (Schindler and Hauser, 2004).
Formaldehyde content in the fabric can be quantitatively tested by measuring the amount of free and released formaldehyde. In one technique, the treated fabric is sealed on a cup of water for a certain period of time, so that the free and released formaldehyde dissolves into the water and its concentration can be measured.
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Alternatively, the free formaldehyde can be extracted by a test liquid and then measured accordingly.