Dyeing with insoluble Azoic colours

Dyeing with insoluble Azoic colours 


Insoluble azoic colours are produced on cellulosics when a coupling component reacts with diazotised aromatic base under specific conditions. The lake thus formed in situ remains trapped in fibre pores and cannot come out resulting good colourfastness. However, a part of lake formed at the fibre surface remains embedded at the interior to cause poor rubbing fastness. These colours are water insoluble and brilliant in shades, specifically used to develop all shades of red, orange, yellow and maroon. 

Basic principle of application 
These colours are not manufactured tailor made colours like other dyes but these are lakes or ingrain dyes and are developed in situ cellulosics through chemical reaction between diazotised base and naphthol (coupling component). These are popularly known as naphthol dyes as the coupling component is mostly a naphthol (Figure. 1). The final azoic pigment is insoluble in water and so named ‘insoluble azoic colours’. Soluble azo colours essentially retain auxochrome while insoluble azoics do not possess the same. Dyeing of cellulose with these colours is basically a two-step process, viz. solubilisation of naphthol followed by its application on textile and treatment of this naphtholated textile with a diazotised base. Neither naphthol nor base possesses any such colour, but the pigment developed gives a specific hue depending on naphthol‑base combination. A part of pigment is also formed on fibre-solution interface causing poor rubbing fastness; coloured pigments are formed mainly on surface layers of cellulose due to poor affinity of naphtholates developing bright shades. 
Out of two essential components to develop these colours, viz. coupling component and base, the latter is an aromatic amine whereas the first one is either a naphthol or other products, e.g., Brenthols (Naphthol AS‑G) but are applied through same technique. 

Classification of coupling components
Based on affinity, these are commercially classified as 
(i) Low substantive naphthols: AS, AS‑G, AS‑OL, AS‑D etc.,  
(ii) Medium substantive naphthols: AS‑RL, AS‑BG, AS‑LT etc.. 
(iii) Higher substantive naphthols: AS‑ITR, AS‑BS, AS‑RS, AS‑E etc. and 
(iv) Very high substantive naphthols: AS‑S, AS‑LB, AS‑BT, AS‑SG, AS‑SR, AS‑BR 

Practical application 
The dyeing process consists of six main stages, viz. (i) solubilisation of naphthol, (ii) naphtholation of cotton, (iii) diazotisation of base, (iv) coupling or development, (v) soaping or steaming and (vi) washing.

Solubilisation of naphthol 
Naphthols are solubilised in two methods: hot and cold. 

1.Hot method 

Coupling component is pasted with little T R oil, hot water is added to it followed by slow addition of NaOH with constant stirring and boiled till a clear solution is obtained (Figure 2). Most of the naphthols on solubilisation develop yellow mustard oil like colour, while brenthols develop a turbid curdy colour. However, addition of excess NaOH precipitates naphthols from solution thus losing the same in solution (Figure 2). If the naphthol solution is to be used instantly or within a short time, no additives are added; otherwise the solution is cooled and HCHO is added for stabilisation. Addition of HCHO reduces affinity of solubilised naphthol and so it should not be added if naphtholation is not delayed too much. This method is more laborious than cold method but cost of production is too less, hence making this method commercially popular. Concentration of NaOH is to be controlled properly; if in excess, precipitates naphthols. 
A typical recipe for solubilisation of naphthol AS consists of naphthol AS (10 g/l), T R oil (a little), NaOH (6.5 g/l) at boil for 15 min. Concentration of NaOH depends on type of naphthol; it is around 60–65% with respect to weight of most of naphthols but for AS‑G and its derivatives, the dose is around 35%. The dose is not to be added at one time; rather 50% should be added first followed by slow addition with constant stirring till a clear solution is obtained.

2.Cold method 
Naphthol is added in methylated spirit and stirred with NaOH (at lower dose). Cold water is added to it when a clear naphthol solution is obtained. This solution is added in the naphtholating bath containing T R oil and NaOH. It is expensive, and cotton is to be naphtholated just after dissolution – as in open air, soluble naphthols show a tendency to be separated out from the solution.

Naphtholation of cotton 
Cellulose is treated in naphtholate solution in jigger at boil for 30 min followed by addition of 20 g/l NaCl and naphtholation is continued for further 1.5–2 hours. Heating up of bath reduces substantivity of naphthols. In semi‑continuous method, the solution is directly used for padding of cellulose followed by drying in a float drier or hot flue and stored for development in batches in jigger. In continuous method, cellulose is padded with solubilised naphthol at higher concentration and is continuously passed through a coupling bath for development of shade followed by open soaping and drying. If development is carried out just after naphtholation, no drying is required; otherwise padded cotton must be dried and are to be covered with polyethylene sheet to avoid contact with acid fumes which may separate out naphthol from cotton. Any water drop is to be prevented from coming on padded goods which removes a part of naphthol from the spot producing lighter shades. Most of commonly used naphthols possess lower affinity for cotton and so naphtholation through padding is preferred followed by either drying or passed through coupling bath for development. For lower substantive naphthols, subsequent drying is essential at controlled temperature. Adequate care is to be taken to avoid migration of naphthol. 

Diazotisation of base 
Fast bases are aromatic primary amines (ArNH2) and are marketed as either parent fast bases or their hydrochlorides. If the base is in its hydrochloride form, HCl is to be added first followed by NaNO2, because prior addition of NaNO2 will start diazotisation of base with the help of HCl already present, which is too little for the purpose. This will cause insufficient diazotisation, even diazotised base will react with undiazotised base resulting in loss of base and finally the colour value. Addition of HCl after applying NaNO2 will not improve the situation as diazotisation and coupling are too fast processes. In contrast, fast bases are diazotised by adding NaNO2 first followed by HCl, known as indirect method of diazotisation. There are two methods for diazotisation of bases, viz. direct and indirect. A specific base can only be successfully diazotised in a specific method only to achieve desired coupling. 
In direct method, base is mixed with five‑times hot water (owb) followed by addition of HCl. Ice is added to cool down the mixture to around 5°C. Sodium nitrite, separately dissolved in water (1:2) is slowly added to the acidified cold base and stirred well till all foams subside and is left till completion of reaction.  
In indirect method, base is pasted with cold water; sodium nitrite is added and is stirred till complete dissolution occurs. Cold HCl with sufficient ice is added to this dissolved base bath and stirred till foam subsides completely. The process is continued till 10 minutes for complete diazotisation of base. For a specific amount of naphthol, optimum amount of base is required. Diazotisation reaction occurs between hydrochloride of the base and HNO2. Presence of both HCl and NaNO2 should be tested by Congo red paper (turns blue if excess of HCl is present) and by starch–iodine paper (turns blue if excess of HNO2 is present). Deficiency in any of these components will result incomplete diazotisation. If HCl is insufficient, diazotised base reacts with undiazotised base. 
Ar-N N+Cl- + Ar-NH2 Ar-N N NH-Ar
The final pigment develops a colour different from that of the desired one. Successful diazotisation is indicated by the clarity of final solution and absence of froth. Some bases are marketed as their hydrochlorides and solubilisation of these bases in water favours backward reaction by hydrolysis. Excess HCl is added to favour the back reaction. 
ArNH3Cl → ArNH2 + HCl
The decomposition of diazotised base is generally accompanied by turbidity and formation of a scum. CH3COONa and CH3COOH are added in coupling bath just before development. Stability of diazotised bases vary at different extents, e.g., Red R/RC/T R, Scarlet R/RC and Blue BB possess good stability; Yellow GC, Scarlet G, Violet B. Red B, Boedeaux GP and Orange GC have medium stability while Blue B, Garnet GBC and Red KB possess poor stability necessitating use of it without further delay. A typical recipe of diazotisation may be formulated as follows: Fast Scarlet RC base – 10 g/l, NaNO2 – 3 g/l, HCl – 5 ml/l, Temperature ~ 5°C (with ice) Time – 10–15 min

Coupling or development 
Insoluble pigment is formed in situ fibre via reaction between coupling component (naphthol) and diazotised base which is an instantaneous and faster reaction. During coupling, the diazonium ion replaces a hydrogen ion located on the ring in a position ortho (-o) or para (-p) to the activating amine or –OH group based on whether the coupling component possesses a –NH2 or –OH group, i.e., the chemical structure of lake can be ascertained by attaching the N=N of diazotised base at the ‑o or -p position of naphthol. Coupling power of diazonium chloride gets reduced with fall in temperature: around 15–20°C is ideal for development. In order to prevent migration of coupling component in the liquor from impregnated fabric when dipped and before the reaction has taken place, NaCl (25 g/l) is added. Concentration of diazotised base should not go down beyond 0.5 g/l, otherwise rate of coupling will be slower. Addition of sulphated fatty alcohol or ethylene oxide condensate speeds up coupling and keeps in suspension any pigment formed in the liquor. 
Steaming or soaping 
One inherent problem associated with insoluble azoic colours is that pigments are formed in situ fibre, in bath and at the fibre–liquor interface; the last factor influences poor rubbing fastness. This happens, when naphthols, preferably those remain nearer to fibre surface and react with diazotised base at the fibre–bath interface. Soaping causes removal of superficial pigments and their dispersion in the bath, some sort of removal of pigments formed at the interface and promotes aggregation of pigments inside. High pressure steaming serves all the purposes as those with soaping, except the problem of superficial deposition. Thorough and repeated soaping can improve rubbing fastness.

Function of chemicals:
Coupling component A naphthol or brenthol, which develops the hue on reaction with diazotised base.

Naphthols are insoluble in water and have no affinity for cellulosics. NaOH converts these into sodium salt of naphthols, called naphtholates, which are soluble and show affinity for cellulose. When used in excess, causes precipitation of naphthols (Figure 2). Addition of excess NaOH produces disodium salts and the naphthol starts precipitating. Is is an exhausting agent and imparts substantivity to naphthols. It is safe to use Glauber’s salt (Na2SO4, 10H2O) as presence of magnesium and calcium salts in impure NaCl mostly used in industries may cause precipitation of naphthols in bath.

Exposure of naphtholates or treated fabric to acid fume or atmospheric CO2 causes separation of free naphthols from their sodium salts. Naphthol solutions – to be stored – require stabilisation, which is done by adding HCHO in bath to form methylol compounds (Figure 3). HCHO must be applied at room temperature, otherwise at higher temperature, methylol compounds are combined with a fresh naphthol molecule forming a methylene bridge with blocking of coupling position. Solubilised aromatic naphthols are practically stabilised with HCHO if so required. Naphtholated goods dried after padding or to be developed without any such delay do not require stabilisation with HCHO.  

Diazotisation and coupling 
It produces HNO2 after reacting with HCl, which in turn diazotises the base. 
NaNO2 + HCl → HNO2 + NaCl (at 5°C)
HCl It reacts with NaNO2 to produce HNO2 as well as converts free bases into water soluble salts.
Ar–NH2 + HCl → ArNH3+Cl
This hydrochloride of base reacts with HNO2 to produce diazonium salt. 
ArNH3+Cl− + HNO2 → Ar–N=N+–Cl− + 2H2O
Generally, a molar ratio of base, NaNO2 and HCl is maintained at 1:1:3. One molecule of base requires one molecule of HNO2 for diazotisation which comes from one molecule of NaNO2. Out of 2.5 molecules of HCl, one forms base hydrochloride, one reacts with NaNO2 to produce HNO2 while the half maintains pH during diazotisation.

Diazotisation reaction is an exothermic process requiring addition of ice to maintain temperature (0–5°C). If ice is not added, especially above 30°C, diazotised salt decomposes, react with undiazotised salt and water resulting loss of a part of base. Ar–N=N+–Cl− + Ar–NH2 → Ar–N=N–NH–Ar
Ar–N=N+–Cl− + H2O → Ar–N=N–OH

After diazotisation, the solution has a strong acidic pH (2–3) due to presence of excess HCl; for efficient coupling at pH ~ 5–6, the pH is to be raised by adding CH3COONa (3–4%), which converts inorganic HCl to organic CH3COOH, thus reducing acidity of bath. 
CH3COONa and CH3COOH combination act as buffer to maintain desired pH; however, some extra CH3COOH needs to be added to bath to maintain pH.

Alkali binding agent 
Naphthol treated fabric before development has strong alkaline pH and the coupling bath has a pH ~ 5–6. When naphtholated fabric is dipped in coupling bath for reaction, alkali attached with naphthol becomes free and migrates to the bath, neutralises acid and coupling is either retarded or stopped. Application of Al2(SO4)3 or more commonly used CH3COOH as alkali binding agent neutralises this NaOH and maintains acidity of bath required for coupling. 
Al2(SO4)3 + NaOH → Al(OH)3 + Na2SO4

It promotes aggregation of dye molecules within fibre and removes superficial pigments.

Precautions to be taken
• Falling of water drops on impregnated fabric before coupling leads to formation of lighter spots and it should be kept away from source of water.
• Sodium salts of coupling components (naphthols) are unstable in open air (CO2) and storing of impregnated wet goods or standing naphthol bath result in breaking down of salt linkages, reducing the coupling efficiency and resulting in loss in colour value. The problem may be minimised by drying naphtholated fibre or immediate coupling with diazotised base or HCHO may be added in naphthol solution for stabilisation. 
• Naphtholated cellulose should be dried at 80–90°C, otherwise a part of naphthol will be sublimised‑off. 
• Substantivity of coupling components is low and so a lower liquor ratio is preferred. • Rise in temperature hinders absorption of naphthols hence, room temperature impregnation is preferred. 
• Salt must be added in bath to enhance diffusion of naphthol for good rubbing fastness. 

• Diazonium salts are very unstable beyond 20°C necessitating addition of ice 
in bath, otherwise these react with water and get decomposed liberating nitrogen. • Use of metal vessels enhances decomposition of diazotised salts. 
• Diazotisation must be carried out in diffused light, as direct sunlight promotes decomposition. 
• Due to evolution of HNO2 during diazotisation, standing just before the bath should be avoided.

• Over‑soaping produces dull shades. 
• Soaping with 0.5–1 g/l Na2S2O4 on yellow combinations of AS‑G brings out true shades. 
• Some combinations should not be soaped beyond 65–70°C, e.g., combinations of Yellow GC base with naphthol AS, AS‑BO, AS‑SW, AS‑OL and combinations of Orange GC with naphthol AS, AS‑BO, AS‑SW.

Rubbing fastness
All insoluble azoic colours show poor to moderate rubbing fastness due to formation of pigments at the outer layer of cellulose, a part of which remains projected in bath/air and leave a part of colour on objects against which dyed cotton is rubbed. The problem can be handled in various ways: 
(a) Pretreatment of cellulose with metal salts followed by naphtholation results better diffusion causing better rubbing fastness.
(b) Minimising soluble naphthol deposition at the outer layer of cotton during naphtholation by allowing better diffusion of naphthols with application of NaCl at higher dose during naphtholation or running naphtholated fabric for shorter time in water containing NaCl at higher dose. 
(c) Thorough washing with severe agitation to detach pigments at the outermost layer. 
(d) Selecting high substantive naphthols in exhaust dyeing. 
(e) Low substantive naphthols must be applied only through padding under higher pressure for better penetration. (f) Migration of naphthols in coupling bath should be restricted using NaCl.
(g) Soaping must be carried out instead of steaming. 

(a) Build‑up of shade, i.e., application of naphthol or base at any stage for shade match is not possible. 
(b) Naphthol or base combination is difficult to exercise: (i) Different coupling components have affinity towards cellulose at varying extents, still combination of naphthols is somehow possible with only one base. (ii) Combination of base is difficult as coupling speed typically varies from one diazotised base to other. 
(c) Production of light shades is difficult as well as risky. 
(d) Shades are invariably bright. 
(e) Mainly restricted to red, maroon, orange and yellow colours. 
(f) Applied to products where rubbing fastness does not pose a problem. 
(g) Percent shade formation impossible as colouring materials such as dyes or pigments are not applied directly rather produced in bath.

Stripping of insoluble azoic colours is not possible but these can be discharged with Na2S2O4 in combination with NaOH at boil for 1 min.

Dyeing with insoluble Azoic colours, dyeing with azoic dyes
Dyeing with insoluble Azoic colours, Dyeing with azoic dyes

Dyeing with insoluble Azoic colours Dyeing with insoluble Azoic colours Reviewed by Suraj Gupta on April 25, 2020 Rating: 5

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