Dyeing of Acrylic

Dyeing of Acrylic


Absence of reactive sites in acrylic does not permit to dye with ionic dyes. In contrast, dyeing with disperse dye at high temperature produces only light shades and tendering of fibre. Addition of co-monomers in polymerisation bath develops amorphousness, flexibility and introduces reactive site enabling it to dyeable at boil. Generally acidic co-monomers are added due to which all acrylic fibres are popularly dyed with basic dyes. Rapid opening of fibre throughout its Tg may be handled by either adding retarding agent in bath or controlling rate of heating.

Problem in dyeing pure acrylic fibre
Acrylic fibre is synthesised through polymerisation of acrylonitrile (vinyl cyanide) and has the chemical name polyacrylonitrile (PAN) with the empirical formula where ‘n’ is the degree of polymerisation.
-[CH2 –CH ]-n


Melt spinning of acrylic is not possible as the polymer is thermo-setting in nature and does not melt at higher temperature rather gets degraded forming infusible mass. Wet or dry spun acrylic is produced by dissolving PAN in HNO3 (65%) or dimethyl formamide (DMF). Pure acrylic is tuff and compact, rod shaped, brittle, possesses poor abrasion resistance, lacks textile properties, Tg is very high (105°C), electrically inert and difficult to process chemically. 

Modification of pure acrylic
To impart textile value, a copolymer of PAN has been produced by adding 5–10% neutral comonomers, e.g., methyl acrylate, methyl methacrylate or vinyl acetate to reduce its Tg and to enable it to process in open bath. However, electrical inertness impaired level dyeing and was inferior in aesthetics. Modern PAN is a copolymer and is produced by adding electrically active ethylenic comonomers in polymerisation bath. Thus produced PAN has two nomenclatures: if share of co-monomer is ≤ 15%, it is named acrylic and ≥ 15%, is called modacrylic. This modern concept of introducing co-monomers in single or in combination at varying extents during polymerisation has given birth of varieties of PAN to cut demands of present day marke. Based on electrical nature of co-monomer, fibre acquires electrical charge, e.g., allyl sulphuric acid, acrylic acid, methacrylic acid, itaconic acid, sodium allyl sulphonate etc., carry negative charge while vinyl pyrazine, vinyl pyridine, ethylene imine, allylamine etc., carry positive charge. Basic dye is preferred in the frst case and acid dye in second case. The co-monomer may be either from vinyl chloride, vinyl bromide, vinylidene chloride (a halogen derivative) etc., if flame retardancy of PAN is to be improved; while co-monomers like 2-hydroxymethyl methacrylate, acrylamide etc., are used to improve hydrophilicity and related anti-static properties. The negative charge is also introduced by activator or initiator like peroxysulphate or thiosulphate during polymerisation and these remain as end groups at a concentration of 20–25 meq/g. Both these anions effectively take part in dyeing with basic dye. Cationic co-monomers are expensive, after-treated acrylic turns yellow on storage and that is why anionics are preferred. Introduction of co-monomers show plasticizing effect on PAN and lowers down its Tg from 105°C to 85°C. Cationic dyes show poor migration below 100°C; a reason why it is difficult to produce consistent well levelled shades on PAN in commercial dyeing because it migrates well above 110°C and higher the temperature the better the migration. As degradation of acrylic starts from 110°C onwards, dyeing temperature is to be kept little below or at 110°C. Cationic dyes are well stable at 110°C and reproduction of shades is excellent. Special cationic dyes have come up in the market and specification of some of these is still awaited. These dyes are modified basic dyes, costlier but dyeing process is too simple, bigger in structure and do not diffuse so readily. Rise in dyebath temperature is not important as these dyes do not react with PAN below boil because the cationic site of dye remains balanced with some other anionic molecules and the dye does not show affinity for fibre rather only gets deposited on surface. At boil, anionic halogen atom splits up, exposes cationic site of dye which then reacts with fibre. Retarders are not used, at least for medium and deep shades. With special cationic dyes, dyeing starts at 80°C with NaCl (5–10%), CH3COONa (0.5–1%) and CH3COOH (0.5–1%) at pH 4.5 for 5 min followed by addition of dye, raised to 95°C in 15 min and dyeing is carried out for 1–2 hours. However, their higher cost has forced many dyers to apply conventional basic dyes in commercial dyeing.

Dyeing with basic dye
Basic dyes are invariably applied on acrylics. It has some advantages over other dyeing methods, viz. (i) all deep shades can be developed with greater ease, (ii) shades are brighter and (iii) fastness properties are excellent when special cationic dyes are used. Maximum dye uptake is governed by number of acid groups in fibre though a small amount of dye is also held by dissolution process. Efficient dyeing of acrylic in yarn, package as well as hank form is also carried out extensively 

Troubleshoots in dyeing
Structure of PAN starts and completes opening at 80°C and 85°C, respectively. Below 80°C, no such dyeing or diffusion of basic dye occurs except some surface deposition whereas beyond 80°C due to fast opening of fibre structure dye molecules rush towards fibre, react where it is adsorbed, block passage for migration of other dye molecules causing unlevelled dyeing. To produce levelled shades, either slow heating of bath in this range is preferred for controlled opening as well as diffusion of dye (non-retarder method) or retarding agents are used to get levelled dyeing (retarder method); related problems are that while non-retarder method lengthens time of dyeing the retarder method shortens dyeing time but adds to cost of dyeing.

Practical application
In retarder as well as non-retarder methods, dye is first pasted with CH3COOH (0.5 g/l) and dissolved by adding boiling water. It is the dyeing temperature and concentration of retarder those decides maximum dye uptake but not the pH; change in pH only causes change in colour.

Non-retarder method
In non-retarder method, the bath is heated up in a controlled manner to cope up with opening of the fibre structure. Dyeing is started at 40°C, quickly raised to 70°C beyond which, rise in temperature is restricted to 1°C/min up to 85°C, raised to 95°C rapidly with further dyeing for 1–2 hours followed by soaping and washing (Figure 1); a better control over temperature, as shown in Figure 2, produces best levelled shades.

Retarder method
In cationic retarder method, the bath is set at pH 4.5 with CH3COOH, retarder (2%) is added at 60°C, PAN is treated in this bath for 10 min followed by addition of basic dye and dyeing is carried out in this bath at 95°C for 1–2 hours.
In anionic retarder method, a bath is set at 60°C with retarder (0.5–2%), non-ionic dispersing agent (2–3%) and CH3COOH in combination with CH3COONa to get a pH around 4.5. PAN is treated in this bath for some time followed by addition of dye. Bath is heated up to 85°C at a faster rate, then
slowly to boil for steady acceptance of complex and dyeing is further carried out for 1-2 hours followed by soaping and washing. This method has a unique feature that, as not the dye but the complex positively takes part in dyeing, rate of dyeing with any basic dye is somewhat same.
Polymeric retarders, due to bigger in size, are absorbed on surface and cannot diffuse inside; presence of multiple cationic charges on these retarders increase their affinity for PAN many-folds–resulting a better controlled process with levelled shade at even lesser concentration (0.1–0.2%).Basic dye possesses limited tendency to migrate throughout acrylic below 95°C. Absorption is slow up to 80°C but above 90°C rate of absorption increases suddenly and in spite of taking all precautions to control rate of dyeing, uneven dyeing is invariably obtained in case of medium and pale shades due to problem of migration. According to BASF, sources of unevenness in dyeing PAN may be better demonstrated as (i) yarn differences, i.e., faults in PAN yarn in manufacturing stage, which cannot be rectified, (ii) temperature difference at different points of bath (it is 30°C between bottom and top of the machine used) which causes bottom part to be dyed 30 times faster necessitating thermostatic control with maximum agitation (constant temperature method) and (iii) difference in dye concentration, i.e., at the time of addition top part gets maximum dye while bottom part receives less dye necessitating pumping of dye solution at multiple points with efficient circulation system.

Retarder and its types
Retarders are colourless chemicals and slow down rate of dyeing. Cationic retarders are smaller in structure possessing cation similar to that on dye but its mobility is faster than that of dye. Anionic retarders possess negative electrical charge. Polymeric retarders are polyquaternary ammonium compounds with high molecular weight of 1000–2000 compared to that of conventional cationic retarders 300–500 and carry multi-positive charges on structure. All retarders, irrespective of their electrical nature, reduce strike rate of dye to ensure level dyeing.

Mechanism of retarder action
Cationic retarders possess smaller molecular size and its mobility is faster than dye. During dyeing, these compete with dye, reaches to fibre surface first, occupy sites and neutralises fibre charge temporarily. As a result, dye molecules do not feel attraction for fibre causing slow and even deposition on fibre and displace retarders slowly as final affinity of dye for fibre is much higher than that of retarder. In this way, cationic retarders help positioning of dye molecules uniformly to produce level dyeings. Anionic retarder increases molecular weight of dye forming dye–retarder complex, reduces its mobility and hence the strike rate. Precipitation of such a complex during dyeing is avoided by applying a non-ionic dispersing agent. Dye–retarder complex formed at different parts of bath strikes fibre surface at different times. With heating up of bath, the complex breaks, dye is released from complex and diffuses inside fibre. Polymeric retarders are larger in size, possess multiple cationic charges and are attracted by fibre. These cannot diffuse inside fibre, rather remain at the surface and controls rate of surface deposition and diffusion of dye on fibre.

Matching of shades
In case a specific dye is to be added to bath to match shades, dyebath is to be cooled down below 80°C otherwise there is every possibility of rushing of dye molecules towards fibre at complete opening stage. After required addition, the temperature is again raised and dyeing is continued.

Dyeing with disperse dye
Mechanism of fixation of dye is similar to that with polyester but production of heavy shade is restricted as solubility of dye in polymer is low and fibre has lower saturation value though fastness properties are acceptable. Dye is pasted with dispersing agent (1–2 g/l), water is added followed by CH3COOH (0.5–1 ml/l) to maintain a pH of 4.5–5.5. PAN is entered in bath at 40°C, temperature is raised slowly to boil and dyeing is continued at boil for 60–90 min. Dyeing may also be carried out in HTHP method as that used for polyester with pressure 3.5 kg/cm2 at 125–130°C for 30 min. After dyeing, bath is cooled down and discharged followed by soaping and washing. Under practical situations, disperse dyes are never applied on PAN as medium and deep hades can never be built-up as well as excessive shrinkage occurs beyond 110°C. Shades are dull with poor sublimation but good light and wash fastness as well as gas fading pro.

Dyeing of Acrylic, dyeing of acrylic with basic dye
Dyeing of Acrylic, dyeing of acrylic with basic dye

Figure 2: Absorption of basic dye at Tg of PAN (non-retarder method)

Dyeing of Acrylic Dyeing of Acrylic Reviewed by Suraj Gupta on April 25, 2020 Rating: 5

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