Ionic Dyeability

Since the basic stmcture of the modified fiber is a copolymer, more rapid disperse dyeing is also gained with tiiese cat-dye fibers. Losses in fiber strength, temperature stabihty and increased hydrolytic degradation are the prices paid for the dyeability enhancement. 

Some efforts have been made to incorporate sites into PET that accept acid dyes, but most of these alkaline-containing additives cause degradation and discoloration of the polymer. No acid-dye PET polymer has yet been commercially successful. 

It is also possible to add PEG at the last stages of polymerization, so that much of the additive remains agglomerated in a separate phase. In this case, subsequent scouring of the fibers removes much of the soluble PEG, leaving microscopic 

---

As earlier noted, PET has no dye attachment sites for chemically active dyes. It is possible to add ionic dyeability by forming copolymers of PET with monomer species that possess active sites, for example, on a pendant side chain. The most common of these has been the incorporation of a sodium salt of a dicarboxylic acid, e.g. of 5-sulfoisophthalic acid (Figure 12.14). The acidic sulfo group allows the attachment of cationic dye molecules. If both the modified and the unmodified fibers are put into a dye bath containing a mixture of disperse and cat dyes, they will emerge with two different colors. This is useful in the creation of specialty fabrics, e.g. when two different dye types are woven into fabrics with a predetermined pattern. The multicolored pattern emerges upon dyeing. 

The modacrylic fibres have similar properties to those of acrylics and are flame resistant..Mostly this fibre is based on a 60/40 or 50/50 copolymer of acrylonitrile with vinylidine chloride (CH = CCy together with small proportion of ternary monomer to improve ionic dyeability or hydrophilicity. The better known modacrylic fibres have a ribbon-shaped or pea-nut shaped cross-section. One problem encountered with modacrylic fibre is loss of lustre at the boil [83, 84]. This fibre is used for apparel, home furnishing, wigs etc. 

Recently, nitrilases have been applied to polymer modification, specifically to the modification of polyacrylonitrile (PAN). Nearly 3 x 106 tons of PAN are produced per annum and used in the textile industry. However, there is a great need to improve moisture uptake, dyeability with ionic dyes, and feel of this acrylic fiber. The cyano moieties of PAN have been successfully modified to carboxylates with the commercial Cyanovacta nitrilase, thus enhancing the aforementioned properties of PAN [98]. Nitrilase action on the acrylic fabric was improved... 

Because of the repulsion of the cyanide groups the polymer backbone assumes a rod-like conformation. The fibers derive their basic properties from this stiff structure of PAN where the nitrile groups are randomly distributed about the backbone rod. Because of strong bonding between the chains, they tend to form bundles. Most acrylic fibers actually contain small amounts of other monomers, such as methyl acrylate and methyl methacrylate. As they are difficult to dye, small amounts of ionic monomers, such as sodium styrene sulfonate, are often added to improve their dyeability. Other monomers are also employed to improve dyeability. These include small amounts (about 4%) of more hydrophilic monomers, such as -vinyl-2-pyrrolidone (Equation 6.69), methacrylic add, or 2-vinylpyridine (Equation 6.70). 

Acrylic fibers are polymers with greater than 85% aerylonitrile content, while those containing 35 to 85% acrylonitrile are known as modaeryhe. Aeryhe fibers eontain minor amounts of other comonomers, usually methyl acrylate, but also methyl methaerylate and vinyl aeetate. These comonomers along with ionic monomers such as sodium styrene sulfonate are ineorporated to enhance dyeability with conventional textile dyes. Modaeryhes usually eontain 20% or more vinyl chloride (or vinylidene chloride) to improve fire retardancy. 

Fibers prepared from straight polyacrylonitrile are difficult to dye. To improve dyeabiUty, manufacturers invariably add to monomer feed minor amounts of one or two comonomers, such as methyl acrylate, methyl methacrylate, vinyl acetate, and 2-vinyl-pyridine. Small amounts of ionic monomers (sodium styrene sulfonate) are often included for better dyeability. ModacryKc fibers are composed of 35-85% acrylonitrile and contain comonomers, such as vinyl chloride, to improve fire retardancy. 

The modified cotton showed excellent dye exhaustion for all the dyes in the absence of salt. The treatment was foimd to improve the dye fixation also. The modification was also assessed through elemental analysis. This study may be further extended to viscose material after suitably modifying the treatment conditions. In another trial, 3-chloro-2-hydroxypropyltrimethylammonium chloride was used as a cationic agent to cationize cotton fabric by a pad-batch process. The cationized cotton samples were dyed with different reactive dyes containing various reactive groups. The dyeability of the cationized cotton samples with reactive dyes without salt was significantly improved due to an increase in the ionic attraction between the dye and cationized cotton. The... 

---