Effects of different types of bleaching

SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Effects of different types of bleaching PREPARED BY ©right Name : MAZADUL HASAN SHESHIR ID: 2010000400008 Batch: 13th Batch (Session 2009-2013) Department: Wet Processing Technology Email: mazadulhasan@yahoo.com Blog: www Textilelab.blogspot.com Southeast University Department of Textile Engineering SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Bleaching The process of decolorize the natural coloring matter present in the cloth treating with some oxidizing agent and reducing agent and ensure the permanent whiteness is called bleaching Objects A high uniform absorbency of fabric to water and dye stuffs Uniform degree of whiteness Fabric should not damage and DP should remain high Destruction of natural coloring matters from the fabric To ensure a level dyeing properties To make the textile materials suitable for subsequent processing (Dyeing, printing, etc.) Prof Engr Dr Ayub Nabi Khan Bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Types of bleaching agents Oxidative bleaching agent Hydrogen peroxide bleaching Sodium chlorite bleaching Hypochlorite bleaching Percretic acid Ozone Bleaching powder Reductive bleaching Zn dust Staneous chlorite Ferous sulphate Sulpher dioxide Sodium hydrosulphate Prof Engr Dr Ayub Nabi Khan Bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Reducing bleaching is a temporary bleaching process because after bleaching there is possibility that the oxygen in the air may reoxidize and it will return to its original state But oxidizing bleaching is permanent because it gives almost invariably a more permanent white Bleaching can be done in various process: Hypochlorite bleaching for 100% cotton (less used) Hydrogen peroxide bleaching for 100% cotton (more used) Sodium Chlorite bleaching only used for special sector and only for polyester Peracetic acid bleaching can also be done for the better improvement of the country commercially Prof Engr Dr Ayub Nabi Khan Bleaching and cotton blend process and this is not still used in our SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Hypochlorite bleaching Prof Engr Dr Ayub Nabi Khan Hypochlorite bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering In textile hypochlorite bleaching sodium hypochlorite [NaOCl] or calcium hypochlorite [Ca(OCl)2] may be used as hypochlorite bleaching agent Differences between Ca(OCl)2 and NaOCl bleaching Ca(OCl)2 NaOCl 1.It is unstable 1.It is stable 2.It produces CaCO3 precipitate It doesn’t produce any precipitate 3.It makes harsh feeling on the fabric 3.It doesn’t make harsh feeling on the fabric 4.Comperatively NaOCl bleaching Prof Engr Dr Ayub Nabi Khan cheaper than 4.Higher cost than Ca(OCl)2 bleaching Hypochlorite bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Sequence of cotton bleaching NaOCl Antichlore treatment Cold wash Ca(OCl)2 souring Antichlore treantmnet Cold was It is seen that souring is used only for Ca(OCl)2 but antichlore treatment is used for both Cause when Ca(OCl)2 is used, it reacts with atmospheric CO2 to give CaCO3 as white precipitate CaCO3 deposited on the fabric causing harsh handling and uneven dyeing, hence it has to separate and souring (acid treatment) is done to remove it Ca(OCl)2 + CO2 + HO2 Prof Engr Dr Ayub Nabi Khan CaCO3 + HOCl Hypochlorite bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Souring The treatment by which the fabrics after processing with alkali (scouring/bleaching) are treated with HCl or dilute H2SO4 for removing alkali or neutralization for alkali is called scouring This process is necessary in case of Ca(OCl)2 bleaching CaCO3 + 2HCl = CaCl2 + CO2 + H2 O CaCO3 + H2SO4 = CaSO4 + CO2 + H2 O Antichlore treatment In case of hypochlorite bleaching (OCl ) hypochlorous ion is produced, which will react with residual protein to form chloramines > NCl which is corrosive and toxic Cl2 is also produced which also unhygienic and irritate to skin Moreover >NCl reacts with moisture and gradually cotton becomes yellowish due to forming of HCl Prof Engr Dr Ayub Nabi Khan Hypochlorite bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering OCl - + resedual protein >NaCl + H2 O Prof Engr Dr Ayub Nabi Khan >NaCl HCl Hypochlorite bleaching 10 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering The results for the degree of whiteness and the color coordinates in Table reveal that the concentration of the bleaching agent is very important in exhaustion bleaching of cotton fabric with PAA As expected, with increasing concentration of PAA, the degree of whiteness was also increased The highest whiteness was obtained with 240 ml/l of Persan S15 (W = 94.7) and the lowest at ml/l of Persan S15 (W = 70.3) Along with the increase of the degree of whiteness, the lightness (L*) was also increasing and chroma (C*) was decreasing The hue (h) value shows that most samples had a yellow hue, and only the samples bleached with the highest concentration of PAA had a bluish hue At lower concentrations of Persan S15 (up to 60 ml/l), the whiteness was lower (up to W = 88.2), but still high enough to allow the dyeing in light, medium and dark shades Prof Engr Dr Ayub Nabi Khan Peracetic acid bleaching 29 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Exhaustion bleaching with PAA at 60 °C and an initial pH of 7.5 for 40 minutes gave whiteness values from W = 70 to W = 95, depending on the concentration of the bleaching agent The remarkable drop in DP occurred only at very high PAA concentrations In the cases where preparations with 15 % PAA (Persan S15) were used, the use of 15 to 25 ml/l of bleaching agent can be recommended to obtain a whiteness value of around W = 80 without any damage to the cotton fibers With 60 ml/l of the bleaching agent, values near W = 90 could be achieved, but with a small degree of damage to the fibers (DP ≈2070) Higher concentrations are not recommended Prof Engr Dr Ayub Nabi Khan Peracetic acid bleaching 30 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – There is no need of stabilizers, acetic acid, caustic soda, sequestrants and dispersants – Bleaching takes place at the temperature between 40 °C to 80 °C and at pH to – Neutralization is not required – Low energy comsumption Disadvantages – This is not used in commercially – More experiments are needed – Not available Prof Engr Dr Ayub Nabi Khan Peracetic acid bleaching 31 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Sodium chlorite bleaching Prof Engr Dr Ayub Nabi Khan 32 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Chlorine dioxide is a non-chlorinating reagent used to obtain a high degree of whiteness on cotton as well as synthetic fibers without degrading them properties of sodium chlorite: (a) At temperature of up to 150ºC sodium does not decompose (b) When heated to 200ºC for 30 less than 50% of sodium chlorite decomposed with the formation of sodium chlorate and chloride; less than 5% of the chlorine decomposed into sodium chloride and gaseous oxygen 3NaClO2 NaClO2 2NaClO3 + NaCl NaCl + O2 (C) Sodium chlorite is fairly highly soluble in water (d) Sodium chloride exists in two forms – NaClO2 (anhydrous) and NaClO2 3H2O (trihydrate) (e) Solid sodium chlorite has been found to be extremely stable It Prof Engr Dr Ayub Nabi Khan decomposes in the temperature range 180º – 220ºC Sodium chlorite bleaching 33 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering (f) Dilute solution of sodium chlorite containing 0.5% sodium hydroxide are of the laboratory stable for several days if protected from light (g) the chlorite decomposition reaction Increase in the concentration of sodium chlorite does not further accelerate Bleaching process: Semi-continuous bleaching Two semi-continuous processes are used for open-width bleaching of cotton fabrics In the first, called Pad-Roll, the fabric is saturated with a pad (80-100% delivery rate), placed in a steamer at 95-98°C and wound in a box in which steam is injected at a temperature of 95- 98°C to keep the air inside moist and warm The fabric is spun on its shaft for 1-3 hours In the second, called Pad-Batch, the fabric is impregnated with a pad, wound around a shaft at ambient temperature, wrapped in a sheet of plastic to avoid evaporation and spun for 15-20 hours The fabric then passes into the washer, where it is rinsed in hot then cold water and then dried Before being dried, it may be neutralized with acetic acid Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 34 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Fig: Pad - Roll process Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 35 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Fig: Pad – Batch process Prof Engr Dr Ayub Nabi Khan Hypochlorite bleaching 36 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Continuous bleaching Continuous bleaching lines generally utilize the Pad-Steam process and include continuous desizing and boiling steps Continuous processes are suitable for large amounts of fabric of the same, or at least similar, quality The machines used are a saturator (pad), a preheater and a steamer in the shape of a ‘U’ or ‘J’ (called a U-box or J-Box) The fabric travels through these chambers for a relatively long period of time (30 minutes on average) in folded (cuttled) rope form (open-width boxes may be used, but they are expensive and bulky) in a 100-102°C steam atmosphere Several units may be placed side by side so that the fabric may be desized, boiled and bleached once or twice without stopping A washer is placed at the end to rinse and neutralize the fabric Hydrogen peroxide is particularly suited to this type of process • Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 37 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Batch bleaching Batch processes are used for small amounts of fabric The fabric is bleached in machines called winches with a high liquor ratio (1:20) or jet or overflow machines with a low liquor ratio (1:5 to 1:10) If the latter are used, non-foaming auxiliary agents are necessary on account of the agitation Jiggers are best for crease-sensitive fabrics (heavy fabrics or those with a high thread /count) The liquor ratio is around 1:5 Kiers and beam machines are used more for delicate fabrics such as gauze and bunting as well as knitted fabrics The liquor ratio is nearly 1:10 Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 38 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Result Sample Concentratio Temp Time required for no n of ºC complete NaOCl2, decomposition of g/liter Remarks NaOCl2, 10 50 30 Not completely white, motes and shieves still present 20 60 30 Not completely white, motes and shieves still present 30 70 35 Fairly good white, shieves still present 40 80 40 Good white, almost free from shieves 50 90 40 Good white, almost free from shieves Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 39 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering The cloth is bleached with sodium chlorite solution (40 g/liter) at 50º C the yellowness persisted and so also motes and shieves But its feel was better than that of grey cloth When it was bleached at 60ºC there was slight improvement, but yellowness was not completely removed Yellowness was absent on cloth bleached at 70ºC but some motes and shieves still remained When bleached at 80ºC and 90ºC, the cloth acquired good whiteness with less no of shieves The sample bleached at the boil had the maximum whiteness and was almost free from motes and shieves When the bleaching process was repeated once again, samples bleached at all the temperatures had very good whiteness and were completely free from motes and shieves The feel was further improved by the second bleaching process and improved gradually as the temperature increased Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 40 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Advantages – Permanent whiteness – High brightness degree (especially for acrylic fibers) – Negligible degradation of fibers (1-2% weight loss for cellulosic fibers and no attack to the polymeric chains in the synthetic fibers) – Lower environmental impact of wastewaters (negligible level of COD) – Versatile bleaching agent for cellulosics, synthetics and blends, especially polycotton blends It is also safe for those synthetic fibers which are sensitive to alkali – Non sensitive to metal ions such as iron and Water hardness under acidic conditions Disadvantages – – – – It is expensive as compared to hydrogen peroxide It cannot be used for wool and silk Chlorine dioxide is highly corrosive to metals and toxic in nature Bleaching in neutral and acidic pH damage cotton Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 41 SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Conclusion: In textile factory before dyeing, pretreatment i e singeing, desizing, scouring and bleaching play an important role for the convenience in dyeing procedure A bleaching agent can whiten or decolorize a substance by reacting with the chromophores that are responsible for the color of the substance Depending on the nature of the chromophores, the bleaching agent will either be an oxidizing or reducing agent That is, the chromophore is either oxidized or reduced to produce a colorless or whitened substance As there is various types of bleaching agent, selecting the suitable one is the major task to conduct What may be the bleaching agent there are some parameters which have to be maintained strictly, they are: time, temperature and pH, otherwise the whole procedure would be faulty Prof Engr Dr Ayub Nabi Khan Sodium chlorite bleaching 42 ... printing, etc.) Prof Engr Dr Ayub Nabi Khan Bleaching SOUTHEAST UNIVERSITY School of Science and Engineering, Department of Textile Engineering Types of bleaching agents Oxidative bleaching agent... reducing greatly the effluent pollution of the bleaching plant – Weight of fabric after H2O2 bleaching is higher than that of hypochlorite bleaching – Bleaching and dyeing can be sometimes combined... UNIVERSITY School of Science and Engineering, Department of Textile Engineering Hypochlorite bleaching Prof Engr Dr Ayub Nabi Khan Hypochlorite bleaching SOUTHEAST UNIVERSITY School of Science and