How curing is important in pigment printing

A pigment has no affinity to fiber. It is insoluble in water. It needs binder for fixation onto fiber.  A binder is a prepolymer available in the form of aqueous emulsion. Chemically it is copolymer of UTYLACRYLATE-N-METHYLOL ACRYL AMIDE.  Mechanism of binding involves the following sequence: PRINT > DRY >CURE. During curing, the binder polymerises and forms a strong film.  The film embeds pigment color and also strongly adheres to the fiber. Curing is done at 150 degrees for 4-5 minutes. When curing is not proper the poor wash fastness and poor colorfastness will result. Assuming sufficient binder was added to the color paste, these problems are usually resolved by repeating the heat exposure ( Re curing)

Some Notes about Bleaching

Effect of Metals on Hypochlorite Bleaching

Copper and Iron catalyze the oxidaton of cellulose by Sodium hypochlorite degrading the fiber. Fabric must be free from rust spots and traces of metals otherwise bleaching will damage the fabric. 

Stainless stell equipments should be used and care must be taken that the water supply is free from metal and rust from pipes. Prescouring from chelating agents become an important step when bleaching with sodium hypochlorite. 

Weight Loss of Fabric in Bleaching

After bleaching operation the weight loss in the material takes place and it depends on different types of bleaching agents that are used. Due to the removal of coloring matters and fiber damage in the bleaching, textile material may lose considerable weight. In a study it was found that in plain weave fabrics, the weight loss was upto 11% for Sodium hypochlorite bleach and upto 8% in Hydrogen Peroxide bleach.

(Source:http://dspace.daffodilvarsity.edu.bd:8080/xmlui/bitstream/handle/123456789/519/Determination%20of%20weight.pdf?sequence=1)

 

 

 

Buying of Indian Ethnic Fabrics- Weaving Issues

Weaving flaws come naturally with Indian traditional fabrics. Some of these are inevitable and some of these are avoidable. Below are the details of the common weaving issues that come with the Indian traditional fabrics:

Fabric Weight

Weaving defects in the traditional fabrics arise due to techniques used in weaving them as well as the manual operations involved in it. They also occur as the quality of yarn used in warp and weft is compromised or the yarn itself is handspun. In Handloom fabrics, the usual defects are holes, mending, missing end and missing and broken picks. The fabric weight also varies as the fabric is getting woven with variable beat up depending upon the person and also varies during the course of the day of weaving. In the morning, the weaver is fresh, and the beat up is more compact. As the day progresses the picks may spread farther apart.

From the facts given above, it can be inferred. The usual method of finding GSM by using GSM cutter is no longer valid when evaluating handloom fabric. The best way to evaluate is to weight it thaan by thaan and average it out. 

Chira ( Missing End)

This effect occurs in all the fabrics, but it is more severe in powerlooms which are without warp stop motion. Chira is not prevalent in South looms as they are equipped with warp stop motion. However, for woven fabrics of north India, it is quite a common defect. 

Banding

Banding in the weft occurs due to particular contrast of colors, it is more visible in fabrics with cross colors. Also banding is visible when yarns are hand dyed and after the finish of one pirn, the next pirn contains different dyed lot of yarn. It also is visible in dyed fabrics when the count in the weft changes appreciably. It is more common in Khadis where the yarns of weft ( Amber) come in different lots. 

Tight End and Reed Mark

This forms a series of warp wise faint lines in the fabric. It occurs due to uneven tension in the warp beam which can happen when making the warp beam manually. It also occurs due to not cleaning, damaged heald wires or some problems in the reed. Often reed marks come in these fabrics. 

Holes

The main cause of holes is the pointed scale used all across the country to measure the length and fold the cloth. If a center point is used to hold the fabrics, and the point gets blunt it can cause appreciably visible holes and sometimes makes the whole fabric amenable to rejecting. 

Slippage of the Warp or weft ends

This takes place in almose all the fabrics loosly woven but it is more appreciable in silks and especially unions of silks with Viscose. The unions from Bhagalpur are more susceptible to this defect. To counter this the fabric after weaving is given a special starch finish, but that too is unsustainable and gives way in three or four washes. The cause of this defect is the smooth surface of viscose which can slip easily on silk. This damage leads to seam slippage which is easily noticeable in the stress areas of the garment(neck and arm whole) The solution is to improve the construction of the fabric or use a better quality viscose. To control this problem in garments at the nect, moon patches are applied

Specs

This defect is observed in handloom fabrics which use handspun yarn. Based on the quality of roving they are using these will contain foreign fibers which do not catch dyes leading to this defect. 

Rough appearance

Rough appearance occurs due to the nature of yarn. In most of the cases carded yarn is used, which contain short fibers which come at the top in the process of weaving. This also due to the uneven count of yarn and slubs present therein. This defect is not a defect as such rather than a mark of true ethnic fabrics. Moreover, hand feel of two garments made from identical fabrics will be different as they may be woven on different looms and subject to different treatments.

Pilling

Pilling in cotton fabric is observed in cross colors where one of the yarn is of dark color. The short fibers come to the surface and form a pill type structure. This defect is aggravated when one of the yarns is sulphur dyed. This is also present in Matka silk which is handspun and handwoven. In yarn dyed Matka, the problem is further aggravated. Silk Noil fabrics are also a victim to this defect as they by default contain short fibers.  

Buying of Indian Traditional Fabrics- Dyes

Every region of India comes with a characteristics of technique of textiles that has perfected one particular class of dyes. For a buyer it poses a significant challenge to maintain the quality of fabrics over time as each class of dyes has its own strength and limitations. 

Napthol Color

All over in south and in Bengal, Napthol colors are used to dye Ikats and Cottons. Generally vat colors are used to dye the dull shades. However, to achieve the required saturation in the darker shades, napthol colors are used. Due to process restrictions and the conditions when dyeing locally, the colorfastness to rubbing is a big problem when working with these colors. A case needs to be pointed out in this regard. When asked about the colorfastness issue for a certain sari from a vendor, it was found that even after washing the yarn after dyeing and washing the fabric after weaving, the colorfastness to rubbing was not improving for napthol dyes. Napthol colors are also being used in Maheshwari Saris for red and other dark colors.

Please see also the following links in this regard:

http://mytextilenotes.blogspot.com/2011/02/dyeing-of-ikat-orissa-yarn-with-vat-and.html

http://mytextilenotes.blogspot.com/2009/07/dyeing-some-practical-notes.html

http://mytextilenotes.blogspot.com/2011/01/types-of-dyes-used-in-handblock.html

Vat Colors

Vat colors are the most commonly used colors all across the country in dyeing traditional fabrics. Vat colors are easy to apply, the process can be done at a temperature achievable in the open furnaces. The colors are fast to rubbing and washing. The main issue is in the achieving of bright and saturated shades which vat colors cannot produce using ordinary condition.

Sulphur Colors

Sulphur dyes are often used to dye black. Cheaper and easy to apply, they have a very good colorfastness to washing. The drawback is that the fabric starts to tear after a prolonged storage.

Reactive Colors

Reactive colors are increasingly being used in woven yarn dyed stripes and dobbies, thanks to the chambers used in dyeing hank yarn. They have good colorfastness properties overall.

Direct Colors

Direct dyes are used extensively in the Indian traditional textile industry. These are easy to apply and cheap. Almost all the tie and dye fabrics whether, Bandhni, Lehariya, Mothra, Ikat and  Shibori have these dyes. These are also being used in the Tussar/Viscose blends in piece dyed form. The colorfastness to washing is good or acceptable but to that of rubbing is poor. A challenge for a bulk buyers of the fabric of these dyes is to convert the dyers to reactive or vat dyes.

Acid/Metal Complex Colors

These are used in pure silk and wool. They pose no problems for the buyer. These are colorfast to washing and stable to fading. 

Natural Dyes

Natural Dyes are obtained from plant extract. The problem with natural dyed fabrics is that the volumes cannot be obtained and quality is not consistent. Patchiness, tonal variation across the length and listing ( Center to Selvedge variations) are some of the defects that come naturally with natural dyes. Also the choice of colors is limited to a very restricted pallete; beige, black, maroon, mustard, rust, green and indigo are the colors that can be got in these dyes. Color fastness is a big issue with these dyes. These are often sold in the market with the disclaimer tag.