Wool is the most useful fiber with outstanding properties.Its warmth, moisture absorbency, drapability, resiliency, flame retardancy makeit ideal for numerous applications in apparel and interior. Moreover, it isgreen being a renewable resource and ecofriendly.


It has been observed that in case of wool, dyeing is themost commonly used technique to enhance aesthetic appeal. Printing is not commonlyused either at small scale or large scale to produce beautiful attractivedesigns. It has been reported that hardly 2% of total wool in the world isprinted. This is because of physical and morphological nature of wool. Thehairy nature of woolen fabric operates against the production of fine, crispdesigns which may be printed on the smooth fabric. Wool does not normallyabsorb the print paste very well, which may be because of scales on itssurface.


Wool fiber consists of two morphological parts, cuticle andcortex. Cuticle is composed of overlapping epithelial cells or scales. Althoughfunction of these scales is to protect wool from weather, abrasion etc., it isthe main hindrance in efficient and uniform processing of wool. The scalesoffer hydrophobicity decreasing absorbency to dyes and chemicals. Any violationof this layer promotes dye absorbency of wool.


Chemical modification of wool can be done at mild processconditions. Chemical treatment brings about change in the surfacecharacteristics specially the scaly layer and in the chemical nature of woolfiber which makes fiber more receptive of dyes resulting in different colouryield, fixation and colour fastness properties.


Various chemicals and auxiliaries are used to bring aboutmodification in physical structure or to help in swelling of fiber ordissolution of dyes. Conventional procedure for chemical modification of woolis chlorination. It is a pretreatment which has dual advantage of opening thecuticle, the barrier in diffusion of dye into fiber, together with preventingthe felting shrinkage which may otherwise distort the printed designs. Chlorinationresults in toxic byproducts, hence it is considered non-ecofriendly. Studyconducted by Purwar and Pant has shown that proteolytic enzymes can be used asreplacement of chlorine treatment. The enzyme treatment improved printabilityand texture of wool but there was some loss in strength of wool.


Auxiliaries can be incorporated in printing paste to modifyprintability. Singh and Pant printed wool by incorporating monoethanol amineand benzyl alcohol in printing paste of acid dye. Both the chemicals improvedcolour depth of printed wool but monoethanol amine was more effective thanbenzyl alcohol. There was slight loss in strength after addition of amine. Fastnessof prints to light, rubbing, and ironing was not altered.


Presence of monoethanol amine in print paste leads toincrease in swelling of wool on steaming which provides more accessibility tothe dye molecules. There is better penetrability of dye inside the fiber. Onereason for the improvement in colour value on addition of benzyl alcohol ismodification of fiber structure which is disrupted by benzyl alcohol. Anotherreason may be that benzyl alcohol causes quicker swelling of fiber duringsteaming which facilitates diffusion of dye molecules in the fiber.


Redox system can also be used to enhance dye fixation andprinting performance. Purwar and Pant found modification in printability ofwool after incorporating urea/ammonium persulphate redox system in printingpaste. Moreover, strength of wool also improved. Redox system creates newactive sites on both fiber and dye structure thereby enhancing the extent ofdye fixation.


Diethylene glycol, an excellent solvent can also be used toimprove printing performance of wool. The dye instead of remaining asaggregates converts into molecular form and can enter in the fiber easily.


Brady studied effect of selected pretreatments such asmodified sirolan BPA finish and tetra ethylene pentamine. Pretreatmentsresulted in rapid, even deposition of dyes on wool fabric and produced goodcolour yields. Chen gave low plasma treatment to wool fabric before printingwith acid dye and found deeper colour yield with good even prints.

Thus printability of wool can be improved either by pretreatment with chemicals or by incorporating auxiliaries in printing paste.


References:


  1. Gupta S. (1991) Printing and dyeing of wool. The Indian Textile Journal. pp 40
  2. Purwar S.and Pant S. (2004). A study on effect of enzymatic pretreatment and selected specialty chemicals on wool printing. Masters thesis, Banasthali University.
  3. Singh N. and Pant S. (2004) Modifying the printing behavior of wool by chemical pretreatment. Masters thesis, Banasthali University.


About the Author:


The author is associated with Faculty of Home Science, Banasthali University


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