The performance of durable press is largely decided by the reagent, and so most research into this technique focuses on the development of effective and environmentally friendly reagents. Early durable press reagents were mostly based on formaldehyde. Although the treated products showed very good DP perform- ance, they emitted formaldehyde during use. Since formaldehyde has been proven to be harmful to health and the environment (O’Quinn and Barrett Kennedy, 1965), attention has been drawn to low-formaldehyde or formaldehyde-free reagents. In addition to these covalent crosslinking treatments, the ability of ionic crosslinking to improve the wrinkle resistance properties of cellulose fabrics has been studied extensively.
4.2.1 Formaldehyde based reagents
Formaldehyde is the simplest crosslinker; it reacts easily with water to give formalin, which reacts with cellulose at high temperatures in the presence of a catalyst to form methylene bridges (Thomason, 2006). The resultant very strong crosslinking in the cellulose gives fabric very good DP performance.
Urea-formaldehyde (UF) treatment was first developed in the 1920s (Foulds et al., 1929) to treat cotton fabric using pad-dry-cure finishing in the presence of an acidic catalyst. The cellulose crosslinking precursor dimethylol urea (DMU) was formed from one mol of urea and two mols of formaldehyde. The methylol group on DMU reacts easily with the hydroxyl group on cellulose chains to form strong crosslinks. Even though UF treatment results in excellent crease recovery, the UF compounds can produce a variety of products and so it is hard to control the types of compounds formed by polymerization. In addition to these unpredictable reactions with cellulose, the quality of the finished fabric is often unsatisfactory (Li, 2008). The product also shows a stiff hand, due to the self-crosslinking of the reagent. Yellowing and tendering of the fabric has also been observed due to chlorine retention by the product (from bleaching and laundering processes). Since DMU is very reactive, the finish bath has to be used within a few hours. The amount of formaldehyde released by the UF finish is very high, but it gives the best elastic resilience to the treated fabric.
Durable press treatments 73
In melamine-formaldehyde (MF) treatment, each amino group in melamine is capable of reacting with two formaldehyde molecules, forming mono-, di-, tri-, tetra-, hexa-methylol melamine derivatives. Trimethylol melamine (TMM) and hexamethylol melamine (HMM) are the most used MF derivatives; they react with methanol to form more stable trimethoxymethyl and hexamethoxymethyl melamines, respectively (Schindler and Hauser, 2004). In the presence of a Lewis acid and heated to a suitable temperature, these two products easily react with the cellulose hydroxyl groups, or self-crosslink, forming covalent bonds. MF treat- ment shows better DP performance than UF treatment but is more expensive, and yellowing, worse tenacity, and lower tear strength are caused by chlorine retention by the treated product.
UF and MF derivatives, also known as amino plastics, not only crosslink with cellulose but also self-crosslink to form stiff films, and as a result the treated fabric shows a stiff hand. Cyclic urea can also react with formaldehyde to form crosslinking reagents, and it can react with cellulose without forming films. The well-known crosslinkers include dimethylol ethylene urea (DMEU), dimethylol propylene urea (DMPU) and dimethylol dihydroxy ethylene urea (DMDHEU). DMEU is formed from the reaction between one mol of ethylene urea and two mols of formaldehyde. It reacts with the hydroxyl groups of cellulose in the presence of an acid catalyst (Reinhardt and Harper, 1984). DMEU gives good DP performance, producing fabric with a softer hand than UF and MF, but it has the disadvantages of chlorine retention and formaldehyde release (>1000 mg/Kg fabric). DMPU is formed from the reaction between one mol of propylene urea and two mols of formaldehyde. It has the same crosslinking mechanism as DMEU, but it is more stable and has better chlorine resistance with no yellowing effect, though it is more expensive (Sarma et al., 1973). DMDHEU is formed from the reaction between one mol of 4,5-dihydroxy ethylene urea and two mols of formaldehyde. DMDHEU also reacts with the hydroxyl groups on cellulose to form a crosslinked structure between adjacent cellulose molecules. It offers a very good DP performance with excellent crease angle recovery, and is thus the most widely used DP finish reagent (Yang et al.,, 2001). It allows for a delayed curing step due to its lower reactivity, and the finish has a longer shelf and pad bath life compared with DMU and TMM.
It also shows better durability and lower formaldehyde release than DMEU, but causes fabric yellowing.
There are other formaldehyde based reagents that can be used in durable press, such as carbamates. Carbamates do not cause chlorine retention or yellowing, and offer good durability, but their disadvantages include low strength and high formaldehyde release in the treated fabric. Vapor phase formaldehyde is also applied in garment processing. Alkylated DMDHEU, in which DMDHEU is partially or fully methylated, causes less formaldehyde release than DMDHEU.
Ultra low formaldehyde release can also be achieved by crosslinking DMDHEU with diethylene glycol (Andrews et al., 2000).
74 Understanding and improving the durability of textiles
4.2.2 Non-formaldehyde reagents
More than 1 ppm of formaldehyde in the air can cause discomfort and is potentially hazardous to humans. Skin contact with textiles that contain a high level of formaldehyde also incurs an allergic reaction, and formaldehyde is now regarded as a human carcinogen. Concentration limits of formaldehyde in the workplace have been set, in different countries, that restrict the use of formaldehyde in the textile industry. Non-formaldehyde reagents are therefore more commonly used in modern durable press finishing. Dimethyl dihydroxyethylene urea and poly- carboxylic acids are the most important non-formaldehyde reagents.
1,3-Dimethyl-4,5-dihydroxyethylene urea (DMeDHEU) is synthesized from N,N'-dimethyl urea and glyoxal. It has a similar chemical structure to DMDHEU.
DMeDHEU has two –CH3 distributed groups while DMDHEU has two –CH2OH;
there are no methylol groups in the chemical structure of DMeDHEU (Geubtner, 1990). Crosslinking occurs between the hydroxyl groups of DMeDHEU and cellulose. DMeDHEU finishing is a formaldehyde free treatment, but the finishing efficiency is very low and the price is very high compared to DMDHEU. In order to reduce the cost and still avoid exceeding the formaldehyde limit, DMeDHEU is always mixed with DMDHEU (Schindler and Hauser, 2004).
Various types of polycarboxylic acid anhydrides have been used as durable press finishing reagents, with the most efficient being 1,2,3,4-butanetetracarboxylic acid (BTCA). Using sodium hypophosphite as catalyst, BTCA is first activated to give a dianhydride intermediate, and then the carboxylic groups on BTCA react with the hydroxyl groups on cellulose to form ester bonds (Welch and Peters, 1997). BTCA could be regarded as the most effective polycarboxylic acid for durable press finishing; it gives the fabric very good crease recovery but low durability, because the ester bonds can be hydrolyzed easily. Large amounts of the sodium hypophosphite catalyst are usually needed to bring about the reaction, and so the cost of BTCA finishing is very high, almost four times higher than that of DMDHEU (Thomason, 2006). Less expensive chemicals have therefore been used in DP finishing, such as citric acid, maleic acid anhydride copolymers, and tartaric acid (Shank, 2002; Welch and Peters, 1999; Wei and Yang, 2000). To make durable press more efficient and economical, a mixture of different acids can be used to achieve better performance, such as citric acid and maleic acid/itaconic acid (Choi and Welch, 1994), copolymer of maleic acid (Yang et al., 2003) acrylic acid and vinyl alcohol, and a copolymer of maleic and acrylic acids (Udomkichdecha et al., 2003). A two-step treatment with citric acid and BTCA has resulted in better DP performance than a mixture of the two reagents (Xu et al., 2001).
Other non-formaldehyde reagents have also been applied in durable press finishing. APCM (polycarboxylic acids modified by molecular incorporation of the phosphorus catalyst in the BTCA structure) shows good crosslinking proper- ties with minimal strength loss (Gelabert and Kyriazis, 1999). Chitosan citrate can also be used for DP finishing, where its antimicrobial properties are an additional
Durable press treatments 75
advantage (Aly et al., 2004; Chung et al., 1998). Clytaraldehyde (Zhang et al., 2011), epoxide (Cai and Qiu, 2003), and enzymes (Tzanov et al., 2002; Stamenova et al., 2003) are alternative durable press reagents.
4.2.3 Ionic crosslinking
Ionic crosslinking has been applied to durable press in recent years, where it offers an alternative to traditional covalent crosslinking. In the ionic crosslinking proc- ess, cellulose must undergo a carboxymethylation process to form anionic cellulose (Borsa et al., 1999), followed by a cationization process to form the crosslinking structure (Sahin et al., 2009). Ionic crosslinking can be carried out either by treating cellulose with sodium chloroacetate plus cationic glycerin/cationic chitosan, or by treating with 3-chloro-2-hydroxypropyl trimethyl ammonium chloride (CHTAC) plus BTCA (Hashem et al., 2003). Research on this topic has focused on different routes and different reagents for the cationization process (Hashem et al., 2005; Bilgen et al., 2006; Hauser and Tabba, 2001).