PREPARATIN FOR DYEING ACRYLIC FIBRES

Preparing the material:

Poor preparation of the goods is usually the major cause of poor quality dyeing and preparation should be of the highest quality consistent with the final price of the material. The preparation of acrylic fiber materials may involve desiring of woven materials, scouring and bleaching. Combined desiring and scouring are often possible since relatively soluble sizing materials such as modified starch.

Polyvinyl alcohol is normally used. Scouring with weakly alkaline solutions of ammonia or presidium pyrophosphate (Na3HP207) is common. A non-ionic detergent is essential. Cationic auxiliary products may have substantively for the anionic groups in the fibers and block dyeing sites whereas residues of anionic product will interact with and even precipitate the cationic dyes in the bath.
Acrylic materials sometimes have a slight yellow cast, usually a sign that drying was too severe. Bleaching is possible with sodium chlorite (NaCIO2) and formic acid or brightening with a fluorescent whitening agent. Some fluorescent whitening agents can be used in the presence of sodium chlorite, allowing combination of the two methods. Stabilizers that control chlorine dioxide emission (Scheme 18.3), such as borax or polyphosphates should be used. A corrosion inhibitor such as sodium nitrate is essential when using steel equipment. Some cationic dyes are very sensitive to traces of chlorine and will rapidly fade giving poor colour yields, particularly when dyeing pale shades. An anti-chlor treatment of fabric bleached with sodium chlorite may be necessary and small additions of sodium bisulphate or thiosulphate to the dyebath will avoid problems with cationic dyes sensitive to traces of chlorine in municipal water.

                                   5ClO2- + 2H+ = 4ClO2 + Cl- + 2HO-

Dyebath preparation:The dye powder is usually pasted with acetic acid and then mixed with boiling water. Cationic dyes with delocalized cationic charges are intensely coloured and it is essential to avoid dust escaping from the powders. Concentrated liquid dyes avoid this problem. Solid forms of these dyes are often not easy to dissolve because of their tendency to form gummy material. Preparation of a paste with methanol and addition of warm or hot water is sometimes a useful alternative. Some cationic dyes are not stable in boiling water. Many react with alkali to give colorless products such as the free amine from neutralization of an ammonium ion group (reverse of Scheme 18.1), or a carbine by reaction of the cationic group with hydroxide ion (Scheme 18.4). Dyeing with cationic dyes therefore invariably takes place in weakly acidic solution to avoid these problems.

                                      Dye+(aq) + HO- (aq) = Dye-OH(s)

Dyeing procedure:Acrylic fibers may contain a variety of different anionic groups. These include a limited number of terminal sulphate and sulphonate groups arising from the persulphate polymerization initiator (Figure 3.2). In other types there may be appreciable numbers of carboxyl ate groups from acrylic acid or similar commodores added to the acrylonitrile before polymerization (Section 4.4.1). These anionic groups are responsible for the substantively of cationic dyes for such fibers. Figure 18.2 compares equilibrium dye adsorption as a function of Ph for two types of acrylic fibres (A and B). Dyeing acrylic fibres with cationic dyes is carried
Equilibrium dye adsorption by acrylic fibres with only sulphate and sulphonate end groups (a) and with carboxyl ate group (b) Out in weekly acidic solution containing acetic acid and sodium acetate and a non ionic wetting agent and dispersant. A small amount of sequestrate ensures that heavy metals do not interfere with chemicals in the solution.The dye solution is often prepared by pasting with acetic acid, and a stable Ph OF 4.5 5.5 can be obtained by addition of sodium acetate to buffer the solution. Dyeing at around Ph 5 suppressed the dissociation of any carboxylic acid groups in the fibre and thus controls the dyeing rate. Note the increased dye uptake of the acrylic fibre with carboxylic acid groups (b) in figure 18.2) as the acid groups dissociate and become anionic above pH 6. The number of carboxylare groups in an acrylic fibre varies from one type to the next. Level dyeing requires strict control of the pof cationic retarding agents. The latter initially block the anionic sites in the fibre and are gradually replaced by the more. An addition of up to 2.5 g 1-1 of anhydrous sodium sulphate helps to offset the negative surface charge and sodium ions weakly block anionic sites in the fibre. Both effects decrease the initial rate of dye absorption. Sodium sulphate is not as effective as cationic retarding agents that have some substantively for the fiber. shows a typical dyeing procedure. If dye additions are needed to give the correct shade, the bath temperature is first slowly reduced to below 80 c Acrylic materials are quite thermoplastic. They easily form crack marks and creases and textures acrylic filaments also readily lose their characteristic Bulk.

After dyeing is complete, the bath is slowly cooled to 50-60 C to avoid these problems. Rapid cooling by addition of cold water to the dyebath cab can be disastrous as it causes immediate setting of creases in the goods. The material is finally rinsed and possibly given a mild scour with a non-ionic detergent and a little acetic acid plus a softening agent.

Problems in dyeing acrylic fibers with cationic dyes :

1. Cationic dyes rapidly adsorb on all available surface of the acrylic fibres because of the polymer’s negative surface potential in water, Once the fibre surfaces are saturated the rate of isothermal dyeing is inedependent of the bath concentration and of the lliquor ratio since the rate of diffusion of dye into the fibre is slow.
 
2. Addition of sodium supphate to the dyebath suppresses the rapid strike as sodium ions counteract the negative charge on the fiber surface. Above the dyeing transition temperature (TD) under the actual dyeing conditions, cationic dyes tend to exhaust very rapidly over a small range in temperature. Great care is needed at temperatures just above TD to avoid unlevel dyeing. The rate of diffusion of the cationic dyes into the acrylic fibre is very slow below TD because of the absence of the required polymer chain mobility. The rate of dyeing increases rapidly above TD and cab double for every 2.5-3.0 C increase in rapidly above TD and double for every 2.5-3.0 C increase in temperature. The corresponding increases in temperature needed to double the dyeing rates of nylon and polyester are typically 10 and 5 C respectively, but at higher temperatures for polyester. Once the acrylic fibre becomes accessible very careful temperature control is required. For this reason, once the bath tempetature reaches 70-75 C, the rate of heating Is usually significantly decreased .The careful temperature control required when dyeing acrylic fibres with cationic dyes is necessary to avoid unlevel dyeings. These dyes on acrylic materials have at best poor migration ability. Some newer types of more hydrophilic cationic dyes have low molecular weights and lower substantively. They migrate move readily but attempted leveling by extended heating or heating to a higher dyeing temperature is dangerous because of the thermo plasticity of the fibre.

If the acrylic fiber has a significant number of carboxylate groups present, the dyeing Ph will greatly influence the dyeing rate. The rate of exhaustion will increase with increase in the Ph as more carboxylic acid groups dissociate. Thus it is important to know the characteristics of the particular acrylic polymer in advance.