US2202335A - Treatment of textiles - Google Patents

Description

Patented May 28, 1940 UNITED STATES Search PATENT OFFICE TREATMENT OF TEXTILES tion of Great Britain No Drawing. Application July 26, 1938, Serial No. 221,422. In Great Britain July 30, 1937 Claims.

This invention relates to the treatment of textiles;. and it comprises an improvement in bleaching bast fibers such as linen, jute, hemp and the like to half white or better without material increase in the solubility number by a two-stage bleaching with-neutral,,hypochlorite solution' folldwd by an alkaline hydrogen peroxide solution, the hypochlorite solution being maintained neutral during the operation under the present invention by the presence of an insoluble carbonate, usually calcium carbonate, in the liquor; all as more fully hereinafter set forth and as claimed.

The usual methods for the bleaching of cel lulose, either in the form of paper pulp or textile fiber are based on combinations of treatments with alkaline solutions and hypochlorite solutions, sours, antichlors and washes being used at the various stages when they are considered to be necessary. Treatments with alkaline solutions containing peroxides and persalts have also been used both before and after hypochlorite solutions in combinations of treatments of the above type. These various methods have resulted very largely from an empirical development over a long period, and while they can be operated to give very good results as regards whiteness, they are usually somewhat lengthy and frequently give rise to considerable chemical degradation of the cellulose. This is particularly true in the bleaching of linen and other bast fibres, and for this reason the description hereinafter will be directed more especially to the bleaching of such fibers.

The chemical degradation of cellulose has an important influence in reducing the effective life of the bleached material, and therefore it is obviously desirable to avoid this type of attack on the goods being bleached. As a convenient means of expressing the degree of chemical degradation of linen we use the term solubility number. This number furnishes a measure of the amount of the material soluble in caustic soda solutions under closely specified conditions, and since it is mainly the chemically degraded cellulose that dissolves under these conditions, high solubility numbers are associated with severe degradation and low numbers with but slight attack. The conditions and method of determining solubility numbers are set out in a paper by C. R. Nodder in The Journal of the Textile Institute 1931, 22, T. 416.

In another and copending application Ser. No. 183,396 filed January 4, 1938 are described and claimed methods of subjecting bast fibers to a two-stage bleaching treatment wherein the first bleaching is with a hypochlorite solution maintained within a substantially neutral pH range by a soluble buffer salt, usually sodium bicarbonate and the second bleaching is with an alkaline solution containing hydrogen peroxide. In certain other copending applications, Ser. Nos. 213,076, filed June 10, 1938, 215,062 filed June 21, 1938 and 183,396 are described and claimed various improvements and modifications of the process as just described. By the use of these various methods it is possible to bleach to the highest shades of white without appreciable chemical degradation of the cellulose or undue loss in weight.

In the present invention the processes described in these various applications are somewhat simplified and made more economical, the improvement being in the method of maintaining the hypochlorite solution within the substantially neutral pH range desired.

We have found that we can maintain the hypochlorite solution within the substantally neutral pH range if we use as a buffer agent a compound which is normally insoluble in water but capable of being dissolved by dilute weak acids to form a substantially neutral solution.

Therefore according to the present invention a process for the bleaching of bast fibres, such as linen, includes the step of treating the materials with a substantially neutral hypochlorite solution to which has been added a compound normally insoluble in water but capable of being dissolved by dilute weak acids to form a substantially neutral solution, which step is followed by treatment with an alkaline solution and hydrogen peroxide.

These steps are almost invariably preceded and followed by other operations. For example, it is necessary that the material being bleached should be capable of being rapidly and uniformly wetted by the hypochlorite solution when this is applied, therefore at least one scour is usually given before the hypochlorite treatment. It is also necessary to wash the goods at various stages, and it is generally found that the best results are not obtained unless a sour in acid is introduced between the treatment with the neutral hypochlorite and the subsequent alkaline treatments.

The preliminary treatment of the goods prior to the hypochlorite treatment may be merely a thorough wetting out by means of a solution of a wetting agent. Advantageously, however, the goods are subjected to an alkaline scour which may be mild or severe, provided only that it renders the fibres readily wettable by the hypochlorite. It will be evident that preliminary treatments of widely differing degrees of severity may be used. We have found it possible to combine successfully all such differing treatments with the fundamental features of the invention, but in order to achieve the best results it is necessary to control the various stages of the process with reference to one another as will be explained more fully hereinafter. As a general guide it may be stated that satisfactory results have been consistently obtained on linen yarn by the complete process when using scours containing 5% to 15% of sodium carbonate on the weight of the goods applied for 2 to 3 hours at 65 to 85 C. With wovengoods it is usually preferable to scour moiseverely"because of the physical condition of the material.

Further methods of scouring involving the use of alkaline liquors containing hydrogen peroxide are described in the specification of ap plication Serial No. 215,062. In certain cases such liquors can be prepared from peroxide bleaching baths which have been used for a previous batch of goods. These alkaline scours containing hydrogen peroxide can also be usefully employed in the present invention when it is desired to obtain good bleaching with but a low loss in the weight of the goods. The characteristics and use of the scours are described in detail in the prior specifications, and all that need be said in the present case is that we find it advantageous to have present not more than about 1% of hydrogen perox de on the weight of the goods and to use the conditions of alkalinity, temperature and time indicated above as desirable for scours in which no peroxide is present.

In all of the above scours the severity can be controlled by variations of one or more of the following conditions:--concentration of alkali, ratio of liquor to goods, temperature and time of treatment.

Following the alkaline scours the goods should be well washed before the hypochlorite treatment.

The hypochlorite solution is conveniently made up from bleaching powder in known manner. This usually involves a thorough agitation of bleaching powder with water to make a strong solution which can subsequently be diluted with water for the preparation of the treating liquor. Depending on the precise method of preparation and on the bleaching powder itself, the .stock solution will contain in solution, varying amounts of lime. For the purposes of the present invention an appropriate amount of acid is added either to the stock solution or to the diluted liquor, so that the diluted liquor is substantially neutral. By substantially neutral we mean a region within approximately one unit above and below 7 on the pH scale. Very good results have been obtained in the lower part of this region and especially in the neighbourhood of pH 6.5.

When the dilute neutral liquor has been prepared a suitable water insoluble compound is added. It will, of course, be evident that the insoluble compound and its solution in weak acids must both be without deleterious action on the hypochlorites, either by reaction or by reason of a catalytic effect inducing decomposition. As a suitable compound we have used with success calcium carbonate, preferably in a finely divided condition. The amount of the addition is by no means critical, but so much is used as is needed to maintain a. proportion undissolved at the end of the treatment, conveniently 2 to 10 lbs. to each gallons (imperial gallons) of liquor may be used.

The properties of the neutral hypochlorite solutions prepared in the above manner toward bast fiber materials are essentially the same as those of the neutral hypochlorite solutions containing a soluble buffer compound. Therefore, the methods of application are similar to those described in copending application Serial No. 183,396. We do not propose to discuss in detail all the characteristic features which are referred to therein, but merely to set out the more advantageous conditions whereby satisfactory results may be attained. without causing any significant chemical degradation of the cellulose. It is best that the available chlorine concentration should be between the limits of 1.5 and 5.0 grams of available chlorine per litre, the solution should. be applied to the goods with the substantial exclusion of actinic rays, and at a temperature of about 15 C. and certainly not higher than 20 C. The time of treatment may be varied considerably, but for the solutions containing about 3.0 or more grams of available chlorine per litre mentioned above, we find that half an hour should not be exceeded. The severity of the hypochlorite treatment should also be adjusted roughly inversely as the severity of the scour. Thus with a mild scour we may use hypochlorite containing up to 4.5 to 5.0 grams available chlorine per litre, while on a very heavily scoured material we should use a hypochlorite solution containing 1.5 to 2.0 grams available chlorine per litre.

Following the hypochlorite treatment it is desirable to rinse the goods with cold water and then to sour. This latter step is of importance in the treatment of linen goods, and is of effect for dealing with the woody fragments of the plant remaining in the yarn after spinning and commonly known as sprit. The sour should preferably be applied cold and may be of any strength normally applied to cellulosic fibres. A liquor containing about 0.5% H01 applied for half an hour is satisfactory. Sulphuric acid may be used but it is preferable to use a hydrochloric acid sour when the hypochlorite bath has been made up from bleaching powder. The risk of calcium sulphate remaining in the fibre due to its relatively low solubility is thereby avoided.

After the sour the goods are washed Well and are then ready for treatment with the alkaline liquor.

It is advantageous to use an alkaline liquor containing peroxide. Numerous methods for the preparation of the peroxide liquor have been described in the specifications referred to. For example, it may be prepared by dilution of strong hydrogen peroxide solutions, or by dissolving sodium peroxide in water or dilute acid. As it is essential to use the peroxide in an alkaline condition, we prefer to prepare the liquor from sodium peroxide. Thereby We are able to make use of the alkalinity naturally occurring when sodium peroxide is dissolved in water. If it is permissible to use a caustic alkalinity with the goods being treated, we may dissolve the sodium peroxide in water or in an amount of dilute acid insufficient to neutralise completely the peroxide used. Alternatively, the sodium peroxide may be completely neutralised and a suitable alkali in the appropriate amount may then be added to the solution. We find, however, that it is advaniii-till) as all) lJLLrtuinnu u. U

'iliEAl i some. tastes.

an. CHEi fiJa ill YEXHLEEZ ii tageous to use a peroxide bath having a sodium carbonate alkalinity. A liquor of this type is very conveniently prepared by dissolving the sodium peroxide in water to which has been added the chemically equivalent quantity of sodium bicarbonate, and that is to say, in the proportion by weight of 78 parts of sodium peroxide to 168 parts of sodium bicarbonate. It is, of course, possible to make modifications in this proportion if it is found desirable. In the event of a slight amount of caustic alkali being desired, then a proportion of sodium peroxide to bicarbonate greater than that specified above may be used. On the other hand, if it is essential that caustic alkalinity be absent, an excess of bicarbonate may be used to prevent such a condition arising. In addition, we may add to the peroxide liquor other materials such as wetting agents, and in particular stabilisers, such as the commonly used sodium silicate.

It is to be noted in preparing a peroxide bleaching liquorof the above type from a solution of sodium peroxide and sodium bicarbonate, that the presence of sodium bicarbonate sometimes causes an unduly rapid decomposition of the hydrogen peroxide with the consequent loss of bleaching value. Our observations have indicated that such undue decomposition generally takes place when a soft water is used in the making up of the liquor. When hard water is used there is practically no useless decomposition of the peroxide in the absence of the known materials which catalyse the decomposition and which are normally excluded from peroxide liquors by those skilled in the art. We therefore advantageously use a water which has several degrees of hardness for making up the peroxide liquors for the present invention. In circumstances where a soft water has to be used of necessity, we add to the water a suitable amount of a soluble calcium or magnesium salt to produce an artificially hardened Water.

The strength of the peroxide liquor and the time and temperature of its application are capable of variation within somewhat wide limits. In practical application, however, we have found that little useful purpose is served by increasing the strength of the bath above about volume, and excellent results have been obtained by the use of a liquor obtained by dissolving 2 lbs. of sodium peroxide in each 100 gallons of water and subsequently 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 above, both as regards whiteness and the avoidance of chemical degradation of the fibre, depend to a certain extent on the time and temperature of the treatment. While we have stated above that a variation is possible in these factors, we have found that there is a relationship between them and that 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 the time 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, we have found that a convenient time is three hours. With this time of treatment Well till] it is not desirable to use temperatures lower than 55 0. nor higher than 75 C. if the quality of the finished material is to be maintained at a high standard. Subject to the observance of these limits we are 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 dependent to some extent on the conditions and on the goods, but usually it is found that one to one and a half gallons of liquor to each pound of material is convenient.

Treated in the above manner all normal qualities of linen yarn will be bleached to a degree which is better than the half-white stage and in many cases may be what is known as three-quarter white. tained the material will now be washed, finished as desired, and dried. If, however, higher shades of white are desired it is only necessary to apply one or more treatments with hypochlorite liquors and alkaline liquors alternately as required. These treatments may be carried out with neutral hypochlorite solutions and alkaline peroxide solutions in which case they will be on the same general principles as already set out but with reduced severity. On the other hand, alkaline hypochlorite liquors and alkali scalds may be used in accordance with well known practice.

In the foregoing we have described in considerable detail the special charac eristics of the combination of steps comprising the invention. We have not thought it necessary to describe methods whereby the particular liquors can be applied to textile materials, as these will be apparent to those having a knowledge of the processing of textile materials. Neither have we thought it necessary to draw attention to those elementary precautions which are well known to bleachers and others dealing with cellulosic fibres, and which are necessary to prevent damage when treating such fibres with acids and alkalies.

The following examples illustrate but do not limit our invention.

Example 1 A batch of linen yarn was scoured for three hours at between 65 and 70 C. in 15 times its weight of liquor containing 0.6% soda ash, and then washed well before being treated with the hypochlorite solution.

For the hypochlorite solution a strong stock solution of bleaching powder made in the ordinary way was diluted to a strength of about 3.7 grams available chlorine per litre, and sufficient dilute hydrochloric acid was added with thorough mixing to neutralise, approximately, the alkalinity. Chalk was then added in the proportion of about 2 lbs. per 100 gallons liquor. The pH value of the liquor was then 7.2. The scoured yarn was reeled in this liquor in subdued light for thirty minutes at about 15 C. and then rinsed in cold water. After the reeling the pH value of the liquor was 7.1. The yarn was next bleached in a peroxide liquor made by dissolving in each 100 gallons of water, 2.6 lbs. of sodium peroxide and 5.6 lbs. sodium bicarbonate, and then adding /2 gallon of 84 Tw. sodium silicate solution. This bleaching was carried out at approximately 65 C. for three hours and then the yarn was thoroughly washed oif and dried. The solubility number of the dried yarn was 3.9.

Example 2 If the desired white has been at" In this example the series of treatments and times and temperatures were the same as in Example 1, with the exception that the hypochlorite liquor was prepared from the used liquor from that example by adding sufficient of a neutralised stock solution of bleaching powder to bring the available chlorine content up to 3.5 grams per litre, after which 1 lb. of chalk for each 100 gallons was added. The pH value of the liquor was then 7.0. At the conclusion of the treatment it was 6.9.

Example 3 A batch of linen yarn was scoured for two and a half hours at 65 C. in ten times its weight of liquor containing 0.5% caustic soda and washed well.

A hypochlorite solution was prepared by diluting a clear stock solution of bleaching powder to a strength of 3.6 grams available chlorine per litre. Dilute hydrochloric acid was then added with thorough mixing until the pH value was reduced to about 6.5 when three pounds of strontium carbonate were added to each 100 gallons. A test of the solution at this stage showed the pH to be 6.4. The yarn was reeled in this liquor for 30 minutes as in Example 1 after which it was rinsed, soured, washed, bleached and washed off as in Example 1. After the reeling operation the pH of the hypochlorite liquor was 6.2. The solubility number of the dried bleached yarn was 3.4.

In the above examples the colour of the dried yarn was a half white. If it had been desired to bleach to a higher shade, the yarn instead of being dried could have been subjected to further series of milder alkaline treatments and bleaching steps, and a full white readily produced.

Instead of the calcium carbonate used in Example 1 we could have used any of the carbonates of the alkaline earth metals or the carbonates of aluminum, magnesium or zinc.

This invention represents another forward step in the art of bleaching as it provides a simple and efiective method of maintaining the requisite pH range in a manner more trouble-free than previously known methods.

As many apparently widely different embodiments of this invention may be made without dehypochlorite solution having a pH value between "'6'and'8 and containing between 1.5 and 2 grams per liter of available chlorine as well as a sufficient amount of a buffer compound to maintain the pH value within said range during the contact, contact being at temperatures not exceeding 20 C. and for a time sufiicient to substantially attack and remove most of the color and non-cellulosic constituents of said material but insuflicient to materially deteriorate thebast fibersand wherein the second step is contacting the so-treated material with an alkaline solution oj hydrogen peroxide to obtaifi'ableached ba'st"fiber having a'so"lubi-lity number of less than 5 and a whiteness of at least half white, the improvement in the first step thereof which consists in maintaining the neutral hypochlorite solution within said pH range during the hypochlorite bleaching by means of a water-insoluble carbonate capable of reacting with the dilute acids formed during such bleaching.

2. The process of claim 1 wherein said waterinsoluble carbonate is an alkaline earth metal carbonate.

3. The process of claim 1 wherein said waterinsoluble carbonate is calcium carbonate.

4. The process of claim 1 wherein the unbleached material, prior to said bleaching with neutral hypochlorite, is preliminarily secured with an alkaline bath containing hydrogen peroxide.

5. The process of claim 1 wherein the said neutral hypochlorite solution buffered with said water-insoluble carbonate is applied'to the unbleached bast fiber material with the substantial exclusion of actinic rays.

ERNEST BUTTERWORTH. JOSEPH ARTHUR MUSGRAVE WOODCOCK MITCHELL.