US2202332A - Bleaching of textiles - Google Patents

Description

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' BLEACHING F TEXTILES Ernest Butterworth, Ballymena, Northern Ireland, assignor to Imperial Chemical Industries Limited, a corporation of Great Britain No Drawing. Application July 26, 1938, Serial No. 221,423. In Great Britain August 4, 1937 Claims.

This invention relates'to improved methods for the bleaching of bast fibres, e. g. linen, hemp, jute and the like.

' In the bleaching of cellulosic materials it is "customary to use among a series oftreatments at leastone treatment with a hypochlorite liquor. Usually a liquor made up from blaRhing-powder and containing calcium hypochlorite is used, but occasionally a solution of sodium hypochlorite is used. As normally prepared, these solutions have initially an alkaline reaction and during use this alkalinity is gradually reduced and they frequently become acid in reaction. In this change of condition there is a serious risk of the hypochlorite liquor causing considerable damage by chemical degradation of the cellulose. So far as it has been possible to ascertain, this damage has been occasioned by the highly active condition of the liquors while they are in a substantially neutral condition.

In copending United States patent application SJN. 183,396 filed. January 4; 1938, hypochlorite solutions maintained in a substantially neutral condition by means of specially added bufier materials are utilised for the treatment of bast fibre materials. When such solutions are employed under the preferred conditions described in the said specifications, highly satisfactory bleaching results can be obtained without undue chemical degradation of the cellulose.

This invention has as an object to provide a new method of bleaching bast fibres. A further object is to devise a method of carrying out the h o lorite treatment in a bleaching pgo ee s fiifififie? is'bflififi'f salts and without serious chemical degradation of the cellulose. Further objects will appear hereinafter. These objects are accomplished by the following invention.

I have found that loan obtain a good bleaching effect with the hypochlorite treatment with out added buffer material calculated to maintain a substantially neutral condition and with no serious damage due to chemical degradation of the cellulose if I maintain the concentration of the hypochlorite solution between the limits of 1.5 and 5.0 grams of available chlorine per litre while the solution is substantially neutral, and maintain the temperature below 20 C., and carry out the process with the substantial exclusion of actinic rays, and only permit the material to be in contact with the liquor for a time which is dependent on the quantity of available chlorine,

the time varying from 30 minutes for liquors with the highest permitted available chlorine to 3 is slightly on the acid side and slightly on, the

alkaline side of the precise neutral point, and which for the present purpose may be taken to extend from about pH 5.8 to about pH 8.2.

In carrying out atreatment according to the invention I do not find it necessary to confine the initial and final values of the liquors strictly within the pH limits set out above. For example, in the usual method of preparing and using a hypochlorite solution in a bleaching process, a strong stock solution is made from bleaching powder and water and this strong solution is subsequently diluted for use,. or is added in the necessary amount to a used liquor to replenish the available chlorine content. As bleaching. powder itself is not a material of constant composition, and the ratio of bleaching powder to water used in making up the stock solution may vary somewhat, it is evident that varying concentrations of lime may be present in solution in both stock and treating solutions. men a treating solution is just made up it will probably have a pH value appreciably above 8.2, for example 9 to 9.5. In the continued use of the solution after one or more replenishments, the pH value will be lower and may be subject to considerable variation from one batch to another.

The present invention involves the usage of hypochlorite solutions prepared by the above and other known methods, combined with the determination of the pH value and of the available chlorine content before and during use, and the adjustment of the treating time as a result of such determinations. It will be evident that a process according to the invention can most easily be carried out when the pH value and the concentration of the solution are within the limits set out above before the material comes in contact with the solution. The process may, however, be carried out quite well when using a solution which is initially of a higher available chlorine concentration and higher pH value, be cause of the natural fall of these values during initial values should not greatly exceed'the values set out; for example, they maybe respectively 5.5 grams available chlorine per litre, and pH 9.0. When using a. solution of this type the pH value is tested from time to time after the material is introduced, and when it is found that the solution has entered the range which I regard. as

It is advantageous, however, that thesesearch one 8. BLEACHING a LlYtlNti; tLUlU TREATMENT 6i CHEFJHCM iriOBlFi- 'ciuctg titxmts t titres,

substantially neutral, its available chlorine concentration is determined. From this determination the maximum time of contact may then be specified. In this connection I have found that with available chlorine concentrations in the highest portion of the range set out, say 4.5 to 5.0 grams per litre, the time of treatment should not exceed about thirty minutes. With lower concentrations the time may be correspondingly increased until in the lowest portion of the range at 2.0 to 1.5 grams per litre the treatment may be for 2. /2 to 3 hours. These figures are specified particularly for bast fibres which have been subjected to a comparatively mild scour as evidenced by the weight lost by the fibres in the securing operation. A typical weight loss which would put linen in a condition for the treatments outlined above is 8% to 10%. If the cellulose is associated with less impurities, say in the case of linen which has been scoured to a weight loss of 15% to-20%, the conditions of the hypochlorite treatment should be made less severe. It must also be stated that the above conditions are given for the treatment of yarn by reeling. In this operation hanks of "yarn are suspended on rotating poles so that the hank constantly revolves with its lower end in the treating liquor; the greater part of the hank is thus not immersed. Other methods of treatment involve the. complete immersion of materials, and when this takes place the hypochlorite treatment should again be used in milder conditions than are specified above. The ternperatureof this treatment must be kept below 20 C. as previously specified, but I prefer to work at about 15 C. as it has been found that very slight differences of temperature have a pronounced effect on the amount of damage which may be caused by the hypochlorite solution in the range of substantial neutrality. For most purposes the substantial exclusion of actinic rays can be achieved by screening the vessels in which the treatment with the hypochlorite solution is being carried out, in such a manner as to prevent access of bright light, such as direct sunlight to the goods being treated. In many cases, for example,'when the goods are completely immersed in liquor, it is possible to carry out the treatment completely in the-dark.

It will, of course, be realised that while I have described in considerable detail the conditions of the hypochlorite treatment, this, in itself, is not a complete bleaching operation. Prior to the hypochlorite treatment it is desirable in practically all cases to subject the material to be treated to a preliminary scour or alkaline treatment. The conditions of such a treatment may vary considerably from case to case, and. any one such as is normally used for such a purpose can be used before the treatment according to the present invention. I have found that treating at70 C. to C. for 2% to 3 hours in liquor containing 7.5% to 10% of soda ash on the weight of thegoods, followed by thorough washing, will usually put linen yarn in a suitable condition for the hypochlorite treatment.

Subsequent to the treatment with the hypochlorite the material should be rinsed, soured in a known manner, and then treated in an alkaline hydrogen peroxide, such as is described in copend-- ing United States patent application S. N. 215,062, filed June 21, 1938. A very convenient method of making up such a bath of suitable strength is by dissolving 2 lbs. of sodium peroxide in each gallons (imperial gallons) of water and sub sequently adding 4.3 to 4.5 lbs. of sodium bicarbonate and 8 lbs. of a 60 Tw. solution of sodium silicate. The excellence of the results obtained with baths such as the above, both as regards whiteness and the avoidance of chemical degrathere is a relationship and certain maximum figures should not be exceeded if the-quality of the fibre is to be preserved. Thus, if experience has shown that certain qualities of the fibre are produced with a certain combination of time and temperature, equivalent results are to be expected within limits when using a lower temperature for a longer time; In practice, it is usually most convenient to standardise thetime of the treatment and to obtain such variations as are desired in the bleached material by variations in the temperature. Thus, in using a bath of the composition detailed above, I have found that a convenient time is three hours. With this time of treatment it is not desirable to use temperatures lower than 55 C. nor higher than 75 C. if the quality of the finished material is to be main-- tained at a high standard. Subject to theob servance of these'limitsI am'able to produce a wide range of Variation in the whiteness of the material by varying the temperature. The ratio of liquor to goods is de penolr-int to some extent on the conditions andon the goods, but usually it is foundthat, one to one and a half gallons of liquor to each pound of material is convenient.

Following the peroxide treatment the .goods are washed off and, if desired, given any appro priate auxiliary treatment, and dried'. The treatments which have been'outlined above will give onlinen a degree of bleaching which is known as half-white. "This isa very usual stage" to which bleaching'is carried out'on yarn, but iii the event of highe rshades being desired it'is only necessary instead of drying as indicated above, to continue the treatmentin knownimanner with alternating liquors of mildscouring and bleach ing characteristics,

, This invention reveals a new method whereby a good bleach can be obtained without serious attack on the cellulose and this is achieved. without recourse to the use of buffer salts and is, therefore a .valuablecontribution to the art.

As many apparently widelydifierent embodia ments of this inventionmay be made without de-.- parting from the spiritand scope thereof, .it is to be understood. that the invention is not lim{ ited to the specific embodiments thereof, except as defined in the appended claimse Iclaim: U

l. A method of bleaching bast fibre materials comprising treating said materials at a temperature not greater than 20 C.', while excluding, substantially all actinic rays, with a hypochlorite solution, determining the available chlorine content and pH value of said hypochlorite solution. and adjusting the time of treatment between '30 bed-ill?! W00??? minutes for treatment with a solution having an available chlorine content on the orderio'f 5.0 grams perlitre and a pH value within a substantially neutral range, and 3 hours for treatment with a solution having an available chlorine content on the order of 1.5 grams per litre and a pH value within a substantially neutral range, whereby bleached textile fibres of high quality are obtained.

2. A method as claimed in claim 1 which comprises a preliminary scour before the hypochlorite treatment.

3. A method as claimed in claim 1 which comprises a subsequent treatment with an alkaline liquor.

4. A method as claimed in claim 1 whichcomprises a subsequent treatment with an alkaline liquor containing hydrogen peroxide and sodium carbonate.

5. A method as claimed in claim 1 which comprises a subsequent treatment with an alkaline liquor containing hydrogen peroxide.

ERNEST BUTTERWORTH.