**Dyeing and Fastness Properties of Disperse Dyes on Poly(Lactic Acid) Fiber**

Jantip Suesat1,2 and Potjanart Suwanruji3

*1Department of Textile Science, Faculty of Agro-Industry, Kasetsart University, 2Center of Advanced Studies for Agriculture and Food, KU Institute for Advanced Studies, Kasetsart University 3Department of Chemistry, Faculty of Science, Kasetsart University, Thailand* 

#### **1. Introduction**

350 Textile Dyeing

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Poly(lactic acid) or PLA is an aliphatic polyester being considered as a green material due to its natural-based origin and biodegradability properties. Lactic acid obtained from the fermentation of sugar and vegetables e.g. corn and cassava is used as a monomer for PLA polymerization. Production of PLA polymer can be achieved by 2 major synthesis routes viz., direct condensation polymerization of lactic acid and ring-opening polymerization of lactide, a cyclic dimer of lactic acid, yielding poly(l-lactic acid), poly(d-lactic acid) or poly(d,l-lactic acid) depending on lactic acid isomers employed. The chemical structure of PLA is shown in fig. 1. PLA possesses desired properties required for packaging materials. Major market share of PLA therefore falls in the packaging industry. At the same time, its interesting properties have drawn attention from the textiles industry. An attempt to use PLA as a textile fiber has been pursued with the aim of replacing poly(ethylene terephthalate), PET, fiber with this green polyester fiber. PLA fiber can be produced by both melt and solution spinning processes (Gupta et al., 2007) but the former is used more regularly due to the more eco-friendliness and ease of processing. Thermal degradation of the PLA polymer during melt spinning can be prevented by addition of a thermal stabilizer. The processing of PLA fiber/yarn is one of the important parameters in controlling the properties of PLA. PLA yarns which are formerly passed through different yarn processing possess different physical properties and morphological characteristics, which subsequently influence the accessibility of the chemicals into the fiber during textile wet processing for example, dyeing and finishing (Suesat et al., 2003).

Fig. 1. Chemical structure of PLA

PLA fiber has superior elastic recovery and a slightly higher hydrophilicity as compared with PET. It also exhibits lower flammability and less smoke generation. One of the

Dyeing and Fastness Properties of Disperse Dyes on Poly(Lactic Acid) Fiber 353

Jamshidi et al. claimed that PLA was relatively sensitive to thermal degradation, especially at a temperature higher than 190oC. It was explained that the degradation reactions involved cleavage of the ester bonds on the main chain of the polymer. In addition, the presence of low molecular weight compounds e.g. water, monomers, oligomers, and catalysts in the polymer seemed to influence the reduction of the molecular weight at high temperatures

a) b) c)

b) and c) peroxide bleaching used for PLA/cotton blend

**2. Dyeing PLA fiber with disperse dyes** 

commercially infeasible (Phillips et al., 2003).

Fig. 3. SEM photographs of damage on PLA fiber caused by ; a) electron irradiation of SEM;

PLA is not only thermally sensitive but it is also sensitive to alkali. It can be destroyed more easily by alkaline hydrolysis than PET. Thus, it can be deteriorated by those using alkaline wet processing in textiles production. Under alkaline conditions, PLA can be damaged by an alkaline hydrolysis reaction. The fiber surface is eroded and its strength is impaired, especially at high temperatures. An example of the alkaline preparation process is peroxide bleaching used to whiten the cotton component in the PLA/cotton blend. The alkaline hydrolysis takes place and the fiber surface is eroded as depicted in Fig. 3.b) and c), resulting in a substantial reduction of the fiber strength (Phillips et al., 2004a). Therefore, the preparation, dyeing and finishing processes for PLA should be milder than those used for PET. It is suggested to process PLA fiber at lower alkalinity, processing temperature and

Although PLA fibers exhibit many attributes similar to other synthetic fibers, they are a new category that requires modified dyeing and finishing techniques to maximize their benefits. The dyeing properties of PLA have been investigated, especially in comparison with PET fiber. The dyeing of 100% PLA fiber has been intensively studied (Scheyer & Chiweshe, 1999; Nakamura et al., 2001; Phillips et al., 2003, 2004a, 2004b, 2004c). Owing to its relatively hydrophobic nature like PET, PLA can normally be dyed with disperse dyes. The optimum dyeing conditions for dyeing PLA are 110oC for 30 mins under an acidic pH (pH 5) (Fig. 4.) whereas PET dyeing is normally carried out at 20oC higher (130oC) under a more acidic condition (pH 4) (Phillips et al., 2004b). Disperse dyes which show good dyeing properties on PET do not always provide good dyeability on PLA. According to the study of DyStar (2004), the disperse dyes recommended for dyeing PLA fiber are the medium-energy azo dyes which exhibit a superior degree of exhaustion as compared with other dye types. The disperse dyes based on benzodifuranone structure are not recommended due to their low uptake and poor build-up on PLA, therefore, a heavy depth of shade seems to be

(Jamshidi et al., 1988).

time.

important properties influencing dyeing properties of PLA is claimed to be the effect of its lower refractive index. It was informed by NatureWorks, Co., Ltd. that refractive indices of PLA and PET were 1.35-1.45 and 1.50, respectively while Yang & Huda claimed that they were 1.45 and 1.58 for PLA and PET, respectively (Yang & Huda, 2003). The lower refractive index of PLA causes a deeper shade of the disperse dyes obtained on PLA at the same applied dye concentration (Lunt & Bone, 2001). Thermal properties of PLA were reported to be similar to that of polypropylene. The glass transition temperature (Tg) of PLA is 55-65oC. The melting temperature (Tm) of PLA, containing the L- or D-isomeric form alone, is between 171-180oC whereas that of the stereocomplex analogue is 220oC (Perepelkin, 2002). The Tm of PLA is dependent on the molecular weight, thermal history, and isomeric composition of the polymer (Södergård & Stolt, 2002). The most typically used PLA for textile application is poly(*l*-lactic acid) or PLLA. PLA has a lower melting temperature than PET. Fig. 2. shows the DSC scans of the fabrics derived from PLA and PET fibers. The melting temperature of PLA is at 170oC while PET melts at 260oC. This allows PLA to be processed at a lower temperature, for example disperse dyeing of PLA is done at 110oC while PET is dyed at 130oC, heat setting of PLA is carried out at 130oC whilst PET is heat set at 180oC (Phillips et al., 2003). These lower thermal properties are a cause of sensitivity of PLA fabric to high temperatures employed in textile processing and the conditions being experienced during its service life. Exposure to high temperatures could harm the fiber. Therefore, precaution is taken for the textile products obtained from PLA fiber to avoid ironing at high temperatures which can cause fiber damage. Alternatively, PLA is recommended for knitted goods rather than wovens in order to avoid such problems.

Fig. 2. DSC scans of the knitted fabrics derived from PLA and PET fibers

As PLA fiber is rather thermally sensitive, even the heat generated by scanning electron microscope (SEM) during a measurement performed at 15 kV could cause the fiber to melt and fuse together after being exposed to electron beam within a few seconds as seen in Fig. 3.a), while no damage was observed on PET fiber (Suesat, 2004). The same electron beam damage has also been found on the low melting point polymer such as polypropylene.

important properties influencing dyeing properties of PLA is claimed to be the effect of its lower refractive index. It was informed by NatureWorks, Co., Ltd. that refractive indices of PLA and PET were 1.35-1.45 and 1.50, respectively while Yang & Huda claimed that they were 1.45 and 1.58 for PLA and PET, respectively (Yang & Huda, 2003). The lower refractive index of PLA causes a deeper shade of the disperse dyes obtained on PLA at the same applied dye concentration (Lunt & Bone, 2001). Thermal properties of PLA were reported to be similar to that of polypropylene. The glass transition temperature (Tg) of PLA is 55-65oC. The melting temperature (Tm) of PLA, containing the L- or D-isomeric form alone, is between 171-180oC whereas that of the stereocomplex analogue is 220oC (Perepelkin, 2002). The Tm of PLA is dependent on the molecular weight, thermal history, and isomeric composition of the polymer (Södergård & Stolt, 2002). The most typically used PLA for textile application is poly(*l*-lactic acid) or PLLA. PLA has a lower melting temperature than PET. Fig. 2. shows the DSC scans of the fabrics derived from PLA and PET fibers. The melting temperature of PLA is at 170oC while PET melts at 260oC. This allows PLA to be processed at a lower temperature, for example disperse dyeing of PLA is done at 110oC while PET is dyed at 130oC, heat setting of PLA is carried out at 130oC whilst PET is heat set at 180oC (Phillips et al., 2003). These lower thermal properties are a cause of sensitivity of PLA fabric to high temperatures employed in textile processing and the conditions being experienced during its service life. Exposure to high temperatures could harm the fiber. Therefore, precaution is taken for the textile products obtained from PLA fiber to avoid ironing at high temperatures which can cause fiber damage. Alternatively, PLA is recommended for knitted goods rather than wovens in order to

Fig. 2. DSC scans of the knitted fabrics derived from PLA and PET fibers

As PLA fiber is rather thermally sensitive, even the heat generated by scanning electron microscope (SEM) during a measurement performed at 15 kV could cause the fiber to melt and fuse together after being exposed to electron beam within a few seconds as seen in Fig. 3.a), while no damage was observed on PET fiber (Suesat, 2004). The same electron beam damage has also been found on the low melting point polymer such as polypropylene.

avoid such problems.

Jamshidi et al. claimed that PLA was relatively sensitive to thermal degradation, especially at a temperature higher than 190oC. It was explained that the degradation reactions involved cleavage of the ester bonds on the main chain of the polymer. In addition, the presence of low molecular weight compounds e.g. water, monomers, oligomers, and catalysts in the polymer seemed to influence the reduction of the molecular weight at high temperatures (Jamshidi et al., 1988).

Fig. 3. SEM photographs of damage on PLA fiber caused by ; a) electron irradiation of SEM; b) and c) peroxide bleaching used for PLA/cotton blend

PLA is not only thermally sensitive but it is also sensitive to alkali. It can be destroyed more easily by alkaline hydrolysis than PET. Thus, it can be deteriorated by those using alkaline wet processing in textiles production. Under alkaline conditions, PLA can be damaged by an alkaline hydrolysis reaction. The fiber surface is eroded and its strength is impaired, especially at high temperatures. An example of the alkaline preparation process is peroxide bleaching used to whiten the cotton component in the PLA/cotton blend. The alkaline hydrolysis takes place and the fiber surface is eroded as depicted in Fig. 3.b) and c), resulting in a substantial reduction of the fiber strength (Phillips et al., 2004a). Therefore, the preparation, dyeing and finishing processes for PLA should be milder than those used for PET. It is suggested to process PLA fiber at lower alkalinity, processing temperature and time.
