US2202331A - Treatment of textiles - Google Patents

Description

Patented May 28, 1940 UNITED STATES PATENT OFFICE TREATMENT OF TEXTILES No Drawing. Application June 21, 1938, Serial No. 215,062. In Great Britain June 28, 1937 2 Claims.

This invention relates to the treatment of textiles; and it comprises an improvement in methods of bleaching bast fibers such as li r ign iute, hemp and the like to obtain a maximum white iness i without a high solubility number, said method being a first treatment with a neutral hypochlorite solution and a second treatment with an alkaline solution of hydrogen peroxide, the improvement consisting in maintaining the alkalinity of the hydrogen peroxide as sodium carbonate alkalinity in the absence of either caustic alkalinity or of bicarbonate acidity; all as more fully hereinafter set forth and as claimed.

The usual methods for bleaching cellulose in its various forms, such as paper pulp and textile fibers vary considerably. They are different for the two classes of material. But they are based on combinations of treatments with alkaline solutions and h ,pochlorite solutions, sours, antichlors and washe med-attire various stages when they are considered to be necessary. Treatments with solutions containing peroxides and persalts have also been used in conjunction with 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 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 fibres.

The chemical degradation 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 the term solubility number is used. 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 a copending application Ser. No. 183,396 filed January 4, 1938, there is described a method of bleaching bast fiber such as linen to any degree up to a full white without undue attack on the cellulose as evidenced by the solubility number. In this method there is a two-stage operation, the unbleached bast fiber in the first stage being treated with a hypochlorite solution maintained within a substantially neutral pH range by means of a buffer compound. In the next stage it is treated with an alkaline hydrogen peroxide solution.

This method provides a process of bleaching bast fibers to obtain an optimum white with a minimum degradation of the cellulose. It gives a material with a better white in relation to the solubility number than has proved practicable with the processes of the prior art. It also gives a more uniform bleaching and there is a less loss in weight of the fiber.

In these methods the hydrogen peroxide is usually made by dissolving sodium peroxide in water. This gives a liquid containing caustic soda as well as hydrogen peroxide. Sometimes sodium peroxide is dissolved in weak acid and then brought to alkalinity by bicarbonate. In the first case there is caustic alkalinity; in the second case practically there is always an excess of bicarbonate. There is carbon dioxide acidity.

I have found that much better results are obtained in the described method if the second stage is conducted with an alkaline hydrogen peroxide solution in which the alkalinity is due to sodium carbonate alone; that is, a solution from which both bicarbonates and free caustic are absent.

As a further feature of the invention the goods are treated, priortothestreatment with the neu traLhyponhlorit'ej with a scouring solution containing hydrogen peroxide and sodium carbonate.

Conveniently, the liquor utilised for the scour is obtained from the liquor used for the peroxide bleaching step which may be diluted with water or strengthened in its alkali and/or its peroxide concentration, either or both, as may be found desirable.

These various steps which may be regarded as the basic features of the process are, of course, supplemented by intermediate and final rinses and washes with water and advantageously the hypochlorite treatment is succeeded by a sour in acid before the goods are bleached in the peroxide liquor.

In the form of the invention in which the material is not scoured with a liquor containing sodium carbonate and peroxide it is necessary that it should be treated so as to render it capable of being rapidly and uniformly wetted by the neutral hypochlorite solution when this is applied. This preliminary treatment can be carried out as is described in the aforementioned application Serial No. 183,396. That is to say it may be merely a thorough wetting out of the goods by means of a solution of a wetting agent or it may be an alkaline scour. Such a scour may vary in severity from case to case but in general I have found that satisfactory results can be obtained on linen yarn when using scours containing 5 per cent to 15 per cent of sodium carbonate on the weight of the goods and applied for 2 to 3 hours at 65 to C. With woven goods it is usually preferable to scour more severely, because of the physical condition of the material.

By replacing the sodium carbonate wholly or in part by a caustic alkali more severe scouring is obtained but this condition is also influenced by other factors. The severity of the scour as indicated by the weight loss sustained by the fibre can be adjusted approximately by control of the concentration of alkali (carbonate or hydroxide) ratio of liquor to fibre, temperature and time of treatment.

Thus where a low weight loss e. g. 8-10 per cent is desired the following alternatives are possible: scouring for three hours in liquor containing 1.0 per cent NazCOs at 65-70 C. using 12.5 parts of liquor for each part by weight of goods or scouring for four hours in liquor containing 0.25 per cent NaOI-I at boiling temperature using 5 parts of liquor to each part of material.

Advantageously I use a scour containing sodium carbonate and hydrogen peroxide as thereby I obtain uniformity of treatment and a good final white with a conservation of the Weight of the goods. Such a scouring liquor is readily prepared by adding hydrogen peroxide to a solution of soda ash, or by dissolving sodium peroxide in such a solution or in a solutionof sodium bicarbonate. I find it most convenient, however, to make use of a liquor recovered from the peroxide bleaching b'ath used on a previous batch of material. Such liquor usually contains residual hydrogen peroxide together with the sodium carbonate and need only be adjusted to the desired concentrations of alkali and peroxide. For example, the addition of sodium peroxide will increase the hydrogen peroxide and add sodium hydroxide. If the caustic alkalinity from the sodium peroxide is undesirably high it may be neutralised wholly or in part by an appropriate amount of acid or it may be converted to a carbonate alkalinity by adding sodium bicarbonate. Alternatively, concentrated hydrogen peroxide can be added to increase the peroxide content of the liquor. If an increased alkali content only is desired soda ash alone may be added to the used liquor.

The amount and concentration of the peroxide and alkali in the scour will be governed both by economic and technical considerations. From the technical aspect variations may be introduced to deal with difficult cases of goods and it may also be desirable in some cases to repeat the scour. When a double scour is used it is usual to use liquors of lower concentrations than when only one scour is used. In the bleaching of linen the amounts of the chemicals I have found to give satisfactory results are in the neighbourhood of 5 per cent to 15 per cent of sodium carbonate and 1 per cent of hydrogen peroxide on the weight of the goods. Greater amounts at alkali may be used even up to per cent on the weight of the goods or higher. Such amounts are, however, unnecessary and uneconomic and cause greater losses in weight than the preferred amounts. The amount of hydrogen peroxide may also be higher say up to 5 per cent on the weight of the goods but again such practice is not economic. In some cases the sodium carbonate may be replaced in part by caustic alkali, but in order to preserve the stability of the peroxide and to avoid high weight loss of the fibre I find it better to avoid the presence of caustic alkali. In any case I have found it advisable not to use liquors containing more than 2.5 per cent of NaOH on the weight of the goods. Other constituents may also be present in the scour, e. g. in a scour derived from a used peroxide bleaching bath, silicate may be present. Wetting agents may also be added if desired. The concentration of the chemicals in the scouring liquid will vary to some extent with the quantity of liquor used per unit weight of fibre. Usually from 5 to 20 parts by weight of liquor for one part by weight of material will cover all conditions.

A scour of the type described can be applied satisfactorily at temperatures from about 60 C. up to boiling point but it is better to work at 65 to 85 C. At such temperatures a scour of from 1 to 5 hours duration followed by a thorough wash is suificient to enable highly satisfactory bleaching to be obtained on normal quality of linen yarn by the steps described below. In some cases where a more severe scouring is required it is usually found desirable to give two or more treatments of reduced severity rather than to increase the severity of a single treatment. As in this case, when peroxide is absent from the scour the severity is adjusted by the appropriate control of the concentration of alkali, ratio of liquor to goods, temperature and time.

When the goods have been washed after scouring they are ready for immediate treatment with the neutral hypochlorite. The methods by which the neutral hypochlorite may be prepared are described in detail in the aforementioned application Serial No. 183,396. Advantageously there is added to a suitable hypochlorite solution, e. g. sodium or calcium hypochlorite, a solution of a salt having the required characteristics, more particularly sodium bicarbonate. The conditions of the use of such a hypochlorite solution are also described in detail in said application and it is stated that in order to obtain satisfactory bleaching while avoiding damage to the cellulose the available chlorine concentration should be within the limits 1.5 and 5.0 grams per litre. The amount of buffer salt added may be varied to some extent in accordance with the cencentration of available chlorine, but in general a liquor containing before use between 2.5 and 3.5 grams of available chlorine and 8 grams of sodium bicarbonate per litre, will be found applicable for most purposes. The source of the hypochlorite is to a large extent immaterial and in most cases it will be found that a solution prepared by the usual methods from bleaching powder is applicable. To avoid undue attack on the cellulose the hypochlorite solution should be applied with the substantial exclusion of actinic rays and at a temperature not higher than 20 C. and preferably not higher than 15 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, I find that half an hour should preferably not be uLLmulnivu \1 U LIlHJj FLUIU TREATtN-Etll & CHEltiiCAL MODIFI- its exceeded. The severity of the hypochlorite treatment should also be adjusted roughly inversely as the severity of the scour. Thus with a mild scour I may use hyochlorite containing up to 4.5 to 5.0 grams available chlorine per litre while on a very heavily scoured material I should use a hypochlorite solution containing 1.5 to 2.0 grams available chlorine per litre.

In preparing the neutral hypochlorite solution I have referred only to the use of sodium bicarbonate as the buffer salt having the required characteristics. I have done this as I find it advantageous to use this salt on account of its cheapness and general convenience. I may, however, use other substances for the adjustment of the liquor to the required stabilized condition of substantial neutrality, for example, alkali metal phosphates and borates. It is also to be noted that when I refer to substantial neutrality I do not restrict myself to the precise neutral point but to a region within approximately one unit above and below '7 on the pH scale. More particularly have I obtained excellent results in the neighbourhood of pH 6.5.

Following the hypochlorite treatment and prior to the peroxide bleaching I have found it 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 advantageously be applied cold and may be of any strength normally applied to cellulosic fibres. I have found satisfactory a liquor containing about 0.5 per cent HCl applied for half an hour. Sulphuric acid may be used but it is better 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 thoroughly washed and are then ready to be bleached in the peroxide bath containing sodium carbonate. One method of preparing such a bath is by the dilution of a strong solution of hydrogen peroxide, e. g. of 100 volume strength, to the appropriate strength for bleaching and making a direct addition of sodium carbonate, say 0.5 per cent to 1 per cent by Weight on the liquor. More conveniently however the liquor is prepared by dissolving sodium peroxide and sodium bicarbonate in cold water in substantially the proportions required by the equation That is to say, in the proportion by weight of '78 of sodium peroxide with 168 of sodium bicarbonate. It will, of course, be evident that variations from this proportion may be made whereby a degree of caustic alkalinity may be used if this is desirable. In dealing with many classes of goods, it is, however, advantageous to avoid causticity of the liquor, and in such cases a slight excess of the bicarbonate may be used.

It is to be noted in preparing a peroxide bleaching liquor of 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. My observations have indicated that such undue decomposition generally takes place when a soft water is used in the making UGiI bil 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 who have the general knowledge of their behaviour. I find it advantageous therefore to 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, I add to the Water a suitable small 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 the practical application, however, I have found that little useful purpose is served by increasing the strength of the bath above about A; volume, but strengths up to 1.5 volumes can be used without deleterious results, While I begin to get good results with as small a strength as 0.1 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 (Imperial 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 the above, both as regards whiteness and the avoidance of chemical degradation of the fibre, do depend to a certain extent on the time and temperature of the treatment. While I have stated above that a variation is possible in these factors, I have found that there is relationship 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, 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 maintained at a high standard. Subject to the observance of these limits I am able to produce quite a Wide range of variation in the whiteness of the material by varying the temperature. If for any reason it is found desirable to shorten the time of bleaching, say, to one or two hours I may use a higher temperature e. g. 85 C. 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 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 threequarter white. If the desired white has been attained 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 HUUI 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 I have described in considerable detail the special characteristics of the combination of steps comprising the invention. I 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 textile materials. Neither have I 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 the invention.

Example 1 570 lbs. of linen yarn were scoured for three hours at about65 C. in 800 gallons of liquor containing lbs. soda ash, and then washed. The hanks of yarn were then transferred to a reeling machine and treated for thirty minutes in a neutral hypochlorite solution at 15 C. in subdued light. For this operation 900 gallons of liquor were used containing initially 3.3 grams per litre available chlorine and having a pH value 6.5. The liquor was prepared by strengthening a liquor which had been previously used several times in similar operations. This used liquor already contained buffer salt and there was added a stock solution of bleaching powder to replenish the available chlorine to the desired amount, and a stock solution of sodium bicarbonate to maintain the bufier action. The ratio of these stock solutions was such that about equal weights of available chlorine and sodium bicarbonate were added. During the reeling the direction of rotation was reversed several times, and throughout the operation approximately 15 inches of each hank were immersed in the liquor and the linear speed at which the hanks were moving through the liquor was six feet per minute. At the conclusion of the operation the hypochlorite liquor was run off rapidly and the yarn was immediately rinsed in three changes of cold water in the same machine. It was then removed from the machin and soured for half an hour in cold 0.5% hydrochloric acid and washed.

A peroxide bleaching bath was prepared by dissolving in 700 gallons of water 18 lbs. of sodium peroxide and 39 lbs. sodium bicarbonate then adding 4 gallons of 84 Tw. sodium silicate solution. The yarn was entered into this bath at about 35 C. and the temperature raised to 65 C. in half an hour. After a further two and a half hours at this temperature the liquor was run off and the yarn thoroughly washed in water and dried. The dried yarn was a very good half white and had a solubility number 3.5.

Example 2 For this example the used liquor from a peroxide bleaching step as described in the previous example was recovered and 40 lbs. soda ash were added to the 700 gallons. The residual peroxide in the used liquor was 0.4 gram H202 per litre. 550 lbs. of 40s linen yarn was scoured in this liquor for 2 hours at 65 C. and then washed thoroughly. Th yarn was transferred to a reeling machine and treated for 30 minutes at 15 C. in a neutral hypochlorite prepared as in Example 1 but containing 2.5 grams available chlorine per litre. The yarn was then rinsed three times in cold water removed from the reel, soured in 0.5% H01 and washed well in cold water.

Following the washing the yarn was treated in a peroxide bleaching bath of the same composition and in precisely the same manner as in Example 1 and again washed well.

The yarn was then immersed in a neutral hypochlorite liquor containing 0.4 gram available chlorine per litre made up in a similar manner as before. This treatment was carried out at 15 C. for 30 minutes, following which the yarn was rinsed.

Another peroxide bleach was then given at the same temperature and for the same time as before but in a weaker liquor containing in 700 gallons only 9 lbs. peroxide, 20 lbs. sodium bicarbonate and 4 gallons of 84 Tw. sodium silicate solution with the addition of 25 lbs. soda ash. After thorough washing the yarn was again treated in neutral hypochlorite containing 0.3 gram available chlorine per litre treated with an antichlor and finally washed off and dried.

The color of the yarn was a full 4/4 white and its solubility number was 3.3.

In the above examples the process of the invention has been applied to linen because of the relatively high importance of this material in the textile industry. It will be realized however that the invention is not limited thereto and that the principles are applicable directly to other bast fibre materials such as hemp without appreciable modifications.

This invention is a valuable advance in the art as by it I can obtain a full white bleach with a solubility number as low as 3.3.

By the combination of the steps of the present invention I find that the operation of the process described in the prior specification Serial No. 183,396 is facilitated and that the uniformity of treatment can be improved. I also find that I am able to combine the uniform treatment with a notable conservation of weight while attaining high shades of white and avoiding the chemical degradation of the cellulose.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof, except as defined in the appended claims.

I claim:

1. In two-stage bleaching methods for bleaching linen and like relatively long bast fibers, wherein the first step is contacting the unbleached material with a substantially neutral hypochlorite 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 sufficient to substantially attack and remove most of the color and non-cellulosic constituents of said material but insufiicient to materially deteriorate the bast fibers and wherein the second step is contacting the treated material with an alkaline aqueous solution of hydrogen peroxide at a temperature between and 85 to obtain a bleached bast fiber mmatrtttitlftbUm- E AHCN 0F TEXTILES @L FIBERS,

having a solubility number of less than 5 and a whiteness of at least half white, the improvement which consists in maintaining the alkalinity of the hydrogen peroxide solution by sodium carbonate alone in the absence of both NaOH and NaHCOz.

2. The process of claim 1 wherein the goods prior to the said bleaching with neutral hypochlorite solution are scoured with an aqueous solution of sodium carbonate containing a little hydrogen peroxide. 5

ERNEST BU'ITERWORTH.