GB637244A - Process for bleaching natural cellulosic fibres - Google Patents

Abstract

Natural cellulosic fibres (e.g. flax, hemp, jute) are disincrusted and freed of lignin and pectin by an alkaline boil at a pH between 10.5 and 12 with a Ca and Mg sequestering agent such as Graham's salt, followed by treatment with active chlorine in an acid medium. The term "sequestering agent" is defined in Specification 601,645, [Group IV (c)], but is here limited to agents having a calcium value of over 14 g. of sequestered calcium per 100 g. of sequestering agent. The process may be applied in various ways: (1) after the alkaline boil (90-100 DEG C.), the fibres are passed into a bath where there is circulated chlorine water containing about 5 g. of active chlorine per litre at a pH about 2, fresh chlorine being added to replace that consumed; the material is then rinsed (and dried if desired) and treated with a further alkaline bath the pH of which should not exceed 11.5 at 70 DEG C. Final chlorination is effected with an alkali hypochlorite at pH 8.5 or above, and residual chlorine removed with a reducing agent (e.g. sodium bisulphite) at pH preferably below 5. In passing from the alkaline to the acid bath or vice versa, the process should be carried out rapidly to avoid damage to the fibre in the dangerous range of pH 6.5 to 8 when hypochlorite ions are present; (2) the initial alkaline boil is followed by treatment with an acidic buffer solution and the fibre is then treated with chlorine dioxide generated by the acidification of an alkali metal chlorite; the fibre is passed through the acid chlorite solution under conditions of concentration, pH and temperature such that the chlorine dioxide is used up as fast as it is generated (e.g. sodium chlorite 2 g./litre, pH 2.2 at 60 DEG C.); a wetting agent is preferably present, e.g. a polyglycol or polyglycerol ester or an alkyl sulphonate or sulphate. Rinsing, treatment with hypochlorite and dechlorination follow as in (1), the last-named being effected in alkaline solution; (3) the two previous methods may be combined by carrying out the first chlorination with chlorine water and acidified chlorite in succession; (4) the final treatment with hypochlorite and subsequent dechlorination may be dispensed with if the first chlorination is carried out with acidified chlorite at a temperature of at least 90 DEG C. In processes (1) to (3), the consumption of active chlorine in the first bath is always at least double the consumption in the hypochlorite bath. The initial alkaline boil may be carried out with sodium hydroxide, carbonate, phosphate or other alkaline salt. Examples are given of all 4 methods as applied to flax fibres.ALSO:Natural cellulosic fibres in the form of yarns or fabrics are bleached with active chlorine in an acid medium after having been subjected to an alkaline boil at a pH between 10.5 and 12 with a Ca and Mg sequestering agent such as Graham's salt. The process is applicable to flax, hemp, jute and other cellulosic fibres. The term "sequestering agent" is defined in Specification 601,645, but is here limited to agents having a calcium value of over 14 g. of sequestered calcium per 100 g. of sequestering agent. The process may be applied in various ways: (1) after the alkaline boil (90-100 DEG C.), the fibres are passed into a bath where there is circulated chlorine water containing about 5 g. of active chlorine per litre at a pH about 2, fresh chlorine being added to replace that consumed; the material is then rinsed (and dried if desired) and treated with a further alkaline bath, the pH of which should not exceed 11.5 at 70 DEG C. Final chlorination is p effected with an alkali hypochlorite at pH 8.5 or above, and residual chlorine removed with a reducing agent (e.g. sodium bisulphite) at pH preferably below 5. In passing from the alkaline to the acid bath or vice versa, the process should be carried out rapidly to avoid damage to the fibre in the dangerous range of pH 6.5 to 8 when hypochlorite ions are present; (2) the initial alkaline boil is followed by treatment with an acidic buffer solution and the fibre is then treated with chlorine dioxide generated by the acidification of an alkali metal chlorite; the fabric is passed through the acid chlorite solution under conditions of concentration, pH and temperature such that the chlorine dioxide is used up as fast as it is generated (e.g. sodium chlorite 2 g./litre, pH 2.2, at 60 DEG C.); a wetting agent is preferably present, e.g. a polyglycol or polyglycerol ester, or an alkyl sulphonate or sulphate. Rinsing, treatment with hypochlorite and dechlorination follow as in (1), the last-named being effected in alkaline solution; (3) the two previous methods may be combined by carrying out the first chlorination with chlorine water and acidified chlorite in succession; (4) the final treatment with hypochlorite and subsequent dechlorination may be dispensed with if the first chlorination is carried out with acidified chlorite at a temperature of at least 90 DEG C. In processes (1) to (3), the consumption of active chlorine in the first bath is always at least double the consumption in the hypochlorite bath. The initial alkaline boil may be carried out with sodium hydroxide, carbonate, phosphate or other alkaline salt. Examples are given of all four methods as applied to flax fibres.