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Dyeing with mineral colour
Dyeing with mineral colour
145
12 Dyeing with mineral colour
Abstract: Mineral colours are formed in situ cellulosics when water soluble metal salts are applied on fibre followed by conversion to insoluble metal oxides. No dye or pigments are applied rather colour of specific metal oxide alone or in combination shows the final hue. The metal oxides occupy free volume in fibre making it stiffer. Mineral colours are acid sensitive and less brilliant; invariably topped with vat or sulphur colours. Important member is mineral khaki for military uniforms. Keywords: Metal salt, oxidation, metal hydroxide, metal oxide
12.1
Introduction
Metal salts can be applied on cotton either alone or in combination followed by treatment with alkali to form metal hydroxides and finally to metal oxides through oxidation when coloured pigments are formed in situ. This can not be termed as dyeing, rather formation of coloured metal oxide complexes. Shades lack brightness, cheaper, dyed cotton develops harsh feel, gains excessive weight, fast to wash and light but sensitive to acid. Viscose is not dyed with mineral colours due to problem of loosing lustre. Metal oxides e.g. TiO2, ZnO, ZnS, etc. are used as white pigments, while Fe 2O 3 is red brown, etc. Water insoluble metal oxides, alone or in combination, can be formed at specific proportion for a specific hue. Metal salts used possess no affinity for cotton and a padding method is evident for thorough application. Mineral khaki, the most important colour in this category, is a mixture of chromium oxide and ferric oxide at right proportion for colouration of military uniforms (Kramrisch, 1986). It is developed through padding cotton with a solution of chromium sulphate and ferrous sulphate, dried and passed through alkali solution at boil to form respective hydroxides; on drying, chromium hydroxide gets oxidized to green chromium oxide (Cr 2O 3) and ferrous hydroxide to red ferric oxide (Fe 2O 3) finally to produce khaki. Insolubility of both these oxides in water develop
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Fundamentals and practices in colouration of textiles
excellent wash fastness; post-treatment with Na 2 SiO 3 enhances perspiration fastness too.
12.2
Fundamentals of application
Chromium oxide (Cr2O 3) and ferric oxide (Fe2O 3) are the final components required to form khaki shade. These can be produced from water soluble higher salts (-ic) of respective metal sulphates but higher cost of these has led to use of FeSO4 and Na2Cr2O 7 as starting materials. FeSO4 with alkali forms Fe(OH)2; the latter releases water to form FeO and then to Fe2O 3 through oxidation. Na2Cr2O 7 is reduced in presence of H 2 SO 4 and jaggery to Cr 2 (SO 4) 3, excess of acid is neutralized with stiochiometric Na2CO3 as little excess of it forms CrCO3 ; Cr2(SO4)3 in turn forms Cr(OH)3 with alkali and finally to Cr2O 3. In practice, Na2Cr2O 7 is reduced to Cr2(SO4) 3, the latter is mixed with FeSO4 and both are treated with alkali to form respective oxides. Cotton fabric, padded with mixture of chromium sulphate and ferrous sulphate solution is dried in a float drier or hot flue followed by treatment in a boiling solution of alkali, preferably combination of NaOH (10%) and Na2CO3 (5%) when corresponding metal hydroxides are precipitated in situ fabric, which is then aired for conversion of chromium hydroxide and ferrous hydroxide into chromium oxide and ferric oxide, respectively. Use of NaOH alone forms colloidal metal hydroxides which are difficult to oxidise; if soda ash is also present, metal hydroxide and carbonate are formed which makes conversion into oxides much easier. Following are the related chemical reactions (Shenai, 1993). (i) Conversion of Na2Cr2O 7 to Cr2(SO4) 3 8Na 2Cr 2O 7 + 3H2SO 4 + C12 H22 O 11 → H2SO 4 + Na 2 CO 3 →
8Na 2SO4 + 8Cr2 (SO4)3 + 43H2 O + 12CO2 Na2 SO4 + H2O + CO 2
(ii) Padding with FeSO4 and Cr2(SO4) 3 at 75–80% expression at a speed of 15–20 m/min. (iii) Passing of padded cotton through alkaline boiling bath for 30 min, when respective hydroxides are formed FeSO4 + 2NaOH → Fe(OH)2 + Na 2SO4 Cr 2(SO4 )3 + 6NaOH → 2Cr(OH) 3 + 3Na 2SO4
(iv) Oxidation of hydroxides to form oxides in situ Fe(OH)2 2FeO + O2 2Cr(OH) 3 Fe 2O 3 + Cr 2O 3
→ → → →
FeO + H2O Fe2 O 3 Cr 2O 3 + 3H2O Khaki shade
Dyeing with mineral colour
12.3
147
Practical application
12.3.1 Preparation of pad liquor Dichromate method Na2Cr2O 7 (80 kg) is solubilised in water (160 l) in lead lined wooden tank followed by gradual addition of H2SO4 (166°TW, 77 kg) with constant stirring; the solution is cooled, jaggery (20 kg) dissolved in 45 l of water is added with constant stirring. Complete oxidation of jaggery and conversion of Na2Cr2O7 to Cr2(SO4)3 are brought over a time of 24 h. Excess H 2SO 4 is neutralized with Na2CO3 (~4 kg) dissolved in water (15 l) when CO2 is evolved with vigorous effervescence. FeSO4 (28 kg) dissolved in of water (45 l) is added to the above solution. The solution is diluted depending upon depth of shade required (Doshi and Dixit, 1975). Chromium sulphate method A solution is prepared with 25% chromium sulphate [Cr2(SO4) 3] (35 kg), Na2CO3 (3 kg), 95% FeSO 4 (5.5 kg), CH3COONa (1 kg), dispersing agent (0.25 kg) and wetting agent (0.06 kg) dissolved in rest of water to make finally 100 l. Cr2(SO4)3 powder is stirred well to dissolve in cold water in a plastic drum. The solution is filtered through sieve or jute fabric and taken in a stainless steel container. Pre-dissolved Na2CO3 is added slowly to this Cr2(SO4)3 solution, which follows an exothermic reaction. Air is injected in this solution and allowed to stand overnight. FeSO4 is dissolved in boiled water, allowed to stand for 24 h followed by addition of it in the stainless steel container. Pre-dissolved CH3COONa, wetting agent and dispersing agent are added to this with injection of air and allowed to stand for a few hours; pH and specific gravity of the solution are checked (~44°T w). Dichromate method is cheaper to chromium sulphate method (Doshi and Dixit, 1975).
12.3.2 Padding of cotton Cotton fabric is padded with this solution for 75–80% expression, dried or covered with polyethylene sheet to avoid oxidation of selvedge and developed in a boiling solution of NaOH (40 g/l) and Na2CO3 (20 g/l) for 30 min, aired, washed in cold and hot water, treated with Na2SiO3 solution (2.5 g/l), washed thoroughly and dried. A greener or redder khaki is produced by adjusting chromium or iron content of pad liquor. To develop brilliancy of shade, dyeings are often topped with either vat olive green B, khaki GG or sulphur khaki dyes.
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Fundamentals and practices in colouration of textiles
12.3.3 Precautions to be taken ● ● ● ●
Well pretreated cotton fabric with equal moisture content through prolonged exposure in dye-house conditions is to be taken for padding. No water drops should fall on cloth till its development and padded fabric should be protected from acid fumes. Rate of drying on both sides of padded fabric should be uniform to avoid migration of metal salt, which otherwise forms uneven shade. Process conditions and concentration of chemicals must be controlled accurately to avoid variation from lot to lot.
12.4
Other mineral colours
Chrome yellow, chrome red, chrome orange, chrome green, manganese brown, prussian blue, iron oxide etc are the other examples (Shenai, 1993; Allen, 1971). Chrome colours are derivatives of lead chromate (PbCrO4) and so are invariably poisonous. These colours are of least textile importance and are mainly used in paints, printing inks etc.
12.4.1 Chrome yellow (PbCrO4, C I Pigment Yellow 34, CI 77603) 2(CH3 COO)2Pb + K2 Cr 2 O 7 + H2 O = 2PbCrO4 + 2CH 3COOK + 2 CH3 COOH
The yellow varies from lemon yellow to orange yellow ; fast to wash, light and acids. Alkali solubilises it with change to orange while H 2S turns it to brown due to formation of PbS. PbCrO4 + 4NaOH = Na2 PbO2 + Na2CrO4 + 2H2 O
Cotton is padded with lead nitrate or acetate, treated with Na2SO 4 to develop lead sulphate, oxidised with dilute solution of K2Cr 2O7 or Na2Cr 2O7 to form PbCrO4 ; depth of shade depends on strength of lead solution and quantity of lead oxide formed on fibre.
12.4.2 Chrome orange (C I Pigment Orange 21, C I 77601) It is a mixture of lead chromate (chrome yellow, PbCrO4) and basic lead chromate (chrome red, PbCrO4.PbO). It is produced by precipitating basic solution of basic lead salt with Na2Cr2O 7 or Na2Cr2O 4. 2PbNO 3(OH) + Na 2CrO4 = 4PbNO 3(OH) + Na 2 Cr 2 O 7 + Na2 CrO4 =
Pb2 CrO5 + 2NaNO 3 + H 2O Pb2 CrO5 + 2PbCrO 4 + 4NaNO 3 + 2H2 O
Dyeing with mineral colour
149
Practically chrome orange is developed by reacting lime with chrome yellow which is fast to alkali but not to acids.
12.4.3 Chromium oxide (Cr2O3, C I Pigment Green 17, C I 77288) It is produced by heating Na2Cr2O7 with sulphur or charcoal. Na 2Cr 2 O 7 + S → Na 2 SO4 + Cr2 O 3 Na 2Cr 2 O 7 + 2C → Cr 2O 3 + Na2 CO3 + CO
Green powder is produced with very high melting point (2000°C); used in oil and water based paints, tinting glass, distempers and painting porcelain. Fast to light, heat, acid and alkali. It is solublised by fusion with either of Na 2O 2 or KHSO4. Cr 2O 3 + 3Na 2O 2 + H2 O → → Cr 2O 3 + 6KHSO4
2Na 2CrO4 + 2NaOH Cr 2 (SO4)3 + 3 K2 SO4 + 3H 2O
Hydrated chromium oxide is called Guignet’s green (Cr2O 3, 2H2O). It is a bright pigment (C I Pigment Green 18, C I 77289) and is obtained by fusing K2Cr2O7 with boric acid.
12.4.4 Manganese brown (MnO.OH) MnCl 2 + 2NaOH = Mn(OH)2 + 2NaCl 2Mn(OH) 2 + O = 2MnO(OH) + H 2O
The brown is fast to light, soap, dilute acid and alkali. To enhance precipitation and oxidation simultaneously, NaOCl is mixed with NaOH solution. The brown is very readily discharged by reducing agents.
12.4.5 Iron oxides Hydrated ferric oxides are prduced by precipitating a ferrous or ferric salt with alkali; in case a ferrous salt being used, the resulting oxide is oxidised by airing or oxidising agents. 2FeSO 4 + 4NaOH = 2Fe(OH) 2 + 2Na 2SO 4 2Fe(OH) 2 + O = Fe2O 2 (OH)2 + H2 O Fe 2 (SO4)3 + 6NaOH = Fe 2O 2 (OH)2 + 3Na 2 SO4 + 2H 2O
Yellows (C I Pigment Yellow 42 & 43, C I 77492), reds (C I Pigment Reds 101 & 102, C I 77491), browns (C I Pigment Browns 6 & 7, C I 77491) and black (C I Pigment Black 11, C I 77499) shades can be produced by precipitating iron salts followed by oxidation. These are fast to light, soap, alkali; durable and chemically inert with low cost; brightest shades are obtained using ferric nitrate.
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Fundamentals and practices in colouration of textiles
12.4.6 Prussian blue (C I Pigment Blue 27, C I 77510 ) It is prepared by reacting FeSO4 with potassium ferrocyanide to give a complex ‘white paste’ which on further oxidation with acidic K2Cr2O7 forms blue pigments. FeSO4 + K4 Fe(CN) 6 → K2Fe.Fe(CN) 6 + H2SO 4 + O →
K2Fe.Fe(CN) 6 + K2 SO4 2KFe.Fe(CN)6 + K 2SO4 +H 2O
Bright blue shades with excellent fastness to light, stable upto 120°C but sensitive to alkali.
12.5
Stripping of mineral colours
Most of the minerals colours can be stripped out completely through treatment in 10% HCl at 70–80°C while NaOCl has no effect on these colours (Acharya and Venkataraman, 1994).
12.6
References
ACHARYA B S and VENKATARAMAN K (1994) ‘Determination of barium activity number of mineral khaki dyed cotton fabrics’, BTRA Scan, 25, 4, 16–17. ALLEN R L M (1971) Colour Chemistry, 1st Edition, London, Thomas Nelson & Sons Ltd. DOSHI S M and DIXIT M D (1975) ‘Dyeing of mineral khaki on cotton’, Colourage, 22, 1, January 9, 28–31. KRAMRISCH B (1986) ‘Pigment printing and dyeing of cotton’, Am. Dyestuff Rep., 75, 2, 13 & 43. S HENAI V A (1993) Technology of Dyeing, Mumbai, Sevak Publications.
145
12 Dyeing with mineral colour
Abstract: Mineral colours are formed in situ cellulosics when water soluble metal salts are applied on fibre followed by conversion to insoluble metal oxides. No dye or pigments are applied rather colour of specific metal oxide alone or in combination shows the final hue. The metal oxides occupy free volume in fibre making it stiffer. Mineral colours are acid sensitive and less brilliant; invariably topped with vat or sulphur colours. Important member is mineral khaki for military uniforms. Keywords: Metal salt, oxidation, metal hydroxide, metal oxide
12.1
Introduction
Metal salts can be applied on cotton either alone or in combination followed by treatment with alkali to form metal hydroxides and finally to metal oxides through oxidation when coloured pigments are formed in situ. This can not be termed as dyeing, rather formation of coloured metal oxide complexes. Shades lack brightness, cheaper, dyed cotton develops harsh feel, gains excessive weight, fast to wash and light but sensitive to acid. Viscose is not dyed with mineral colours due to problem of loosing lustre. Metal oxides e.g. TiO2, ZnO, ZnS, etc. are used as white pigments, while Fe 2O 3 is red brown, etc. Water insoluble metal oxides, alone or in combination, can be formed at specific proportion for a specific hue. Metal salts used possess no affinity for cotton and a padding method is evident for thorough application. Mineral khaki, the most important colour in this category, is a mixture of chromium oxide and ferric oxide at right proportion for colouration of military uniforms (Kramrisch, 1986). It is developed through padding cotton with a solution of chromium sulphate and ferrous sulphate, dried and passed through alkali solution at boil to form respective hydroxides; on drying, chromium hydroxide gets oxidized to green chromium oxide (Cr 2O 3) and ferrous hydroxide to red ferric oxide (Fe 2O 3) finally to produce khaki. Insolubility of both these oxides in water develop
145
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Fundamentals and practices in colouration of textiles
excellent wash fastness; post-treatment with Na 2 SiO 3 enhances perspiration fastness too.
12.2
Fundamentals of application
Chromium oxide (Cr2O 3) and ferric oxide (Fe2O 3) are the final components required to form khaki shade. These can be produced from water soluble higher salts (-ic) of respective metal sulphates but higher cost of these has led to use of FeSO4 and Na2Cr2O 7 as starting materials. FeSO4 with alkali forms Fe(OH)2; the latter releases water to form FeO and then to Fe2O 3 through oxidation. Na2Cr2O 7 is reduced in presence of H 2 SO 4 and jaggery to Cr 2 (SO 4) 3, excess of acid is neutralized with stiochiometric Na2CO3 as little excess of it forms CrCO3 ; Cr2(SO4)3 in turn forms Cr(OH)3 with alkali and finally to Cr2O 3. In practice, Na2Cr2O 7 is reduced to Cr2(SO4) 3, the latter is mixed with FeSO4 and both are treated with alkali to form respective oxides. Cotton fabric, padded with mixture of chromium sulphate and ferrous sulphate solution is dried in a float drier or hot flue followed by treatment in a boiling solution of alkali, preferably combination of NaOH (10%) and Na2CO3 (5%) when corresponding metal hydroxides are precipitated in situ fabric, which is then aired for conversion of chromium hydroxide and ferrous hydroxide into chromium oxide and ferric oxide, respectively. Use of NaOH alone forms colloidal metal hydroxides which are difficult to oxidise; if soda ash is also present, metal hydroxide and carbonate are formed which makes conversion into oxides much easier. Following are the related chemical reactions (Shenai, 1993). (i) Conversion of Na2Cr2O 7 to Cr2(SO4) 3 8Na 2Cr 2O 7 + 3H2SO 4 + C12 H22 O 11 → H2SO 4 + Na 2 CO 3 →
8Na 2SO4 + 8Cr2 (SO4)3 + 43H2 O + 12CO2 Na2 SO4 + H2O + CO 2
(ii) Padding with FeSO4 and Cr2(SO4) 3 at 75–80% expression at a speed of 15–20 m/min. (iii) Passing of padded cotton through alkaline boiling bath for 30 min, when respective hydroxides are formed FeSO4 + 2NaOH → Fe(OH)2 + Na 2SO4 Cr 2(SO4 )3 + 6NaOH → 2Cr(OH) 3 + 3Na 2SO4
(iv) Oxidation of hydroxides to form oxides in situ Fe(OH)2 2FeO + O2 2Cr(OH) 3 Fe 2O 3 + Cr 2O 3
→ → → →
FeO + H2O Fe2 O 3 Cr 2O 3 + 3H2O Khaki shade
Dyeing with mineral colour
12.3
147
Practical application
12.3.1 Preparation of pad liquor Dichromate method Na2Cr2O 7 (80 kg) is solubilised in water (160 l) in lead lined wooden tank followed by gradual addition of H2SO4 (166°TW, 77 kg) with constant stirring; the solution is cooled, jaggery (20 kg) dissolved in 45 l of water is added with constant stirring. Complete oxidation of jaggery and conversion of Na2Cr2O7 to Cr2(SO4)3 are brought over a time of 24 h. Excess H 2SO 4 is neutralized with Na2CO3 (~4 kg) dissolved in water (15 l) when CO2 is evolved with vigorous effervescence. FeSO4 (28 kg) dissolved in of water (45 l) is added to the above solution. The solution is diluted depending upon depth of shade required (Doshi and Dixit, 1975). Chromium sulphate method A solution is prepared with 25% chromium sulphate [Cr2(SO4) 3] (35 kg), Na2CO3 (3 kg), 95% FeSO 4 (5.5 kg), CH3COONa (1 kg), dispersing agent (0.25 kg) and wetting agent (0.06 kg) dissolved in rest of water to make finally 100 l. Cr2(SO4)3 powder is stirred well to dissolve in cold water in a plastic drum. The solution is filtered through sieve or jute fabric and taken in a stainless steel container. Pre-dissolved Na2CO3 is added slowly to this Cr2(SO4)3 solution, which follows an exothermic reaction. Air is injected in this solution and allowed to stand overnight. FeSO4 is dissolved in boiled water, allowed to stand for 24 h followed by addition of it in the stainless steel container. Pre-dissolved CH3COONa, wetting agent and dispersing agent are added to this with injection of air and allowed to stand for a few hours; pH and specific gravity of the solution are checked (~44°T w). Dichromate method is cheaper to chromium sulphate method (Doshi and Dixit, 1975).
12.3.2 Padding of cotton Cotton fabric is padded with this solution for 75–80% expression, dried or covered with polyethylene sheet to avoid oxidation of selvedge and developed in a boiling solution of NaOH (40 g/l) and Na2CO3 (20 g/l) for 30 min, aired, washed in cold and hot water, treated with Na2SiO3 solution (2.5 g/l), washed thoroughly and dried. A greener or redder khaki is produced by adjusting chromium or iron content of pad liquor. To develop brilliancy of shade, dyeings are often topped with either vat olive green B, khaki GG or sulphur khaki dyes.
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Fundamentals and practices in colouration of textiles
12.3.3 Precautions to be taken ● ● ● ●
Well pretreated cotton fabric with equal moisture content through prolonged exposure in dye-house conditions is to be taken for padding. No water drops should fall on cloth till its development and padded fabric should be protected from acid fumes. Rate of drying on both sides of padded fabric should be uniform to avoid migration of metal salt, which otherwise forms uneven shade. Process conditions and concentration of chemicals must be controlled accurately to avoid variation from lot to lot.
12.4
Other mineral colours
Chrome yellow, chrome red, chrome orange, chrome green, manganese brown, prussian blue, iron oxide etc are the other examples (Shenai, 1993; Allen, 1971). Chrome colours are derivatives of lead chromate (PbCrO4) and so are invariably poisonous. These colours are of least textile importance and are mainly used in paints, printing inks etc.
12.4.1 Chrome yellow (PbCrO4, C I Pigment Yellow 34, CI 77603) 2(CH3 COO)2Pb + K2 Cr 2 O 7 + H2 O = 2PbCrO4 + 2CH 3COOK + 2 CH3 COOH
The yellow varies from lemon yellow to orange yellow ; fast to wash, light and acids. Alkali solubilises it with change to orange while H 2S turns it to brown due to formation of PbS. PbCrO4 + 4NaOH = Na2 PbO2 + Na2CrO4 + 2H2 O
Cotton is padded with lead nitrate or acetate, treated with Na2SO 4 to develop lead sulphate, oxidised with dilute solution of K2Cr 2O7 or Na2Cr 2O7 to form PbCrO4 ; depth of shade depends on strength of lead solution and quantity of lead oxide formed on fibre.
12.4.2 Chrome orange (C I Pigment Orange 21, C I 77601) It is a mixture of lead chromate (chrome yellow, PbCrO4) and basic lead chromate (chrome red, PbCrO4.PbO). It is produced by precipitating basic solution of basic lead salt with Na2Cr2O 7 or Na2Cr2O 4. 2PbNO 3(OH) + Na 2CrO4 = 4PbNO 3(OH) + Na 2 Cr 2 O 7 + Na2 CrO4 =
Pb2 CrO5 + 2NaNO 3 + H 2O Pb2 CrO5 + 2PbCrO 4 + 4NaNO 3 + 2H2 O
Dyeing with mineral colour
149
Practically chrome orange is developed by reacting lime with chrome yellow which is fast to alkali but not to acids.
12.4.3 Chromium oxide (Cr2O3, C I Pigment Green 17, C I 77288) It is produced by heating Na2Cr2O7 with sulphur or charcoal. Na 2Cr 2 O 7 + S → Na 2 SO4 + Cr2 O 3 Na 2Cr 2 O 7 + 2C → Cr 2O 3 + Na2 CO3 + CO
Green powder is produced with very high melting point (2000°C); used in oil and water based paints, tinting glass, distempers and painting porcelain. Fast to light, heat, acid and alkali. It is solublised by fusion with either of Na 2O 2 or KHSO4. Cr 2O 3 + 3Na 2O 2 + H2 O → → Cr 2O 3 + 6KHSO4
2Na 2CrO4 + 2NaOH Cr 2 (SO4)3 + 3 K2 SO4 + 3H 2O
Hydrated chromium oxide is called Guignet’s green (Cr2O 3, 2H2O). It is a bright pigment (C I Pigment Green 18, C I 77289) and is obtained by fusing K2Cr2O7 with boric acid.
12.4.4 Manganese brown (MnO.OH) MnCl 2 + 2NaOH = Mn(OH)2 + 2NaCl 2Mn(OH) 2 + O = 2MnO(OH) + H 2O
The brown is fast to light, soap, dilute acid and alkali. To enhance precipitation and oxidation simultaneously, NaOCl is mixed with NaOH solution. The brown is very readily discharged by reducing agents.
12.4.5 Iron oxides Hydrated ferric oxides are prduced by precipitating a ferrous or ferric salt with alkali; in case a ferrous salt being used, the resulting oxide is oxidised by airing or oxidising agents. 2FeSO 4 + 4NaOH = 2Fe(OH) 2 + 2Na 2SO 4 2Fe(OH) 2 + O = Fe2O 2 (OH)2 + H2 O Fe 2 (SO4)3 + 6NaOH = Fe 2O 2 (OH)2 + 3Na 2 SO4 + 2H 2O
Yellows (C I Pigment Yellow 42 & 43, C I 77492), reds (C I Pigment Reds 101 & 102, C I 77491), browns (C I Pigment Browns 6 & 7, C I 77491) and black (C I Pigment Black 11, C I 77499) shades can be produced by precipitating iron salts followed by oxidation. These are fast to light, soap, alkali; durable and chemically inert with low cost; brightest shades are obtained using ferric nitrate.
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Fundamentals and practices in colouration of textiles
12.4.6 Prussian blue (C I Pigment Blue 27, C I 77510 ) It is prepared by reacting FeSO4 with potassium ferrocyanide to give a complex ‘white paste’ which on further oxidation with acidic K2Cr2O7 forms blue pigments. FeSO4 + K4 Fe(CN) 6 → K2Fe.Fe(CN) 6 + H2SO 4 + O →
K2Fe.Fe(CN) 6 + K2 SO4 2KFe.Fe(CN)6 + K 2SO4 +H 2O
Bright blue shades with excellent fastness to light, stable upto 120°C but sensitive to alkali.
12.5
Stripping of mineral colours
Most of the minerals colours can be stripped out completely through treatment in 10% HCl at 70–80°C while NaOCl has no effect on these colours (Acharya and Venkataraman, 1994).
12.6
References
ACHARYA B S and VENKATARAMAN K (1994) ‘Determination of barium activity number of mineral khaki dyed cotton fabrics’, BTRA Scan, 25, 4, 16–17. ALLEN R L M (1971) Colour Chemistry, 1st Edition, London, Thomas Nelson & Sons Ltd. DOSHI S M and DIXIT M D (1975) ‘Dyeing of mineral khaki on cotton’, Colourage, 22, 1, January 9, 28–31. KRAMRISCH B (1986) ‘Pigment printing and dyeing of cotton’, Am. Dyestuff Rep., 75, 2, 13 & 43. S HENAI V A (1993) Technology of Dyeing, Mumbai, Sevak Publications.