Wool is the most useful fiber with outstanding properties. Its warmth, moisture absorbency, drapability, resiliency, flame retardancy make it ideal for numerous applications in apparel and interior. Moreover, it is green being a renewable resource and ecofriendly.
It has been observed that in case of wool, dyeing is the most commonly used technique to enhance aesthetic appeal. Printing is not commonly used either at small scale or large scale to produce beautiful attractive designs. It has been reported that hardly 2% of total wool in the world is printed. This is because of physical and morphological nature of wool. The hairy nature of woolen fabric operates against the production of fine, crisp designs which may be printed on the smooth fabric. Wool does not normally absorb the print paste very well, which may be because of scales on its surface.
Wool fiber consists of two morphological parts, cuticle and cortex. Cuticle is composed of overlapping epithelial cells or scales. Although function of these scales is to protect wool from weather, abrasion etc., it is the main hindrance in efficient and uniform processing of wool. The scales offer hydrophobicity decreasing absorbency to dyes and chemicals. Any violation of this layer promotes dye absorbency of wool.
Chemical modification of wool can be done at mild process conditions. Chemical treatment brings about change in the surface characteristics specially the scaly layer and in the chemical nature of wool fiber which makes fiber more receptive of dyes resulting in different colour yield, fixation and colour fastness properties.
Various chemicals and auxiliaries are used to bring about modification in physical structure or to help in swelling of fiber or dissolution of dyes. Conventional procedure for chemical modification of wool is chlorination. It is a pretreatment which has dual advantage of opening the cuticle, the barrier in diffusion of dye into fiber, together with preventing the felting shrinkage which may otherwise distort the printed designs. Chlorination results in toxic byproducts, hence it is considered non-ecofriendly. Study conducted by Purwar and Pant has shown that proteolytic enzymes can be used as replacement of chlorine treatment. The enzyme treatment improved printability and texture of wool but there was some loss in strength of wool.
Auxiliaries can be incorporated in printing paste to modify printability. Singh and Pant printed wool by incorporating monoethanol amine and benzyl alcohol in printing paste of acid dye. Both the chemicals improved colour depth of printed wool but monoethanol amine was more effective than benzyl alcohol. There was slight loss in strength after addition of amine. Fastness of prints to light, rubbing, and ironing was not altered.
Presence of monoethanol amine in print paste leads to increase in swelling of wool on steaming which provides more accessibility to the dye molecules. There is better penetrability of dye inside the fiber. One reason for the improvement in colour value on addition of benzyl alcohol is modification of fiber structure which is disrupted by benzyl alcohol. Another reason may be that benzyl alcohol causes quicker swelling of fiber during steaming which facilitates diffusion of dye molecules in the fiber.
Redox system can also be used to enhance dye fixation and printing performance. Purwar and Pant found modification in printability of wool after incorporating urea/ammonium persulphate redox system in printing paste. Moreover, strength of wool also improved. Redox system creates new active sites on both fiber and dye structure thereby enhancing the extent of dye fixation.
Diethylene glycol, an excellent solvent can also be used to improve printing performance of wool. The dye instead of remaining as aggregates converts into molecular form and can enter in the fiber easily.
Brady studied effect of selected pretreatments such as modified sirolan BPA finish and tetra ethylene pentamine. Pretreatments resulted in rapid, even deposition of dyes on wool fabric and produced good colour yields. Chen gave low plasma treatment to wool fabric before printing with acid dye and found deeper colour yield with good even prints.