A METHOD FOR BLEACHING COTTON

Abstract: Bleaching of natural fibers, such as cotton, wool and silk fibers and of such fibers in blends with other types of fibers is carried out in an alkaline solution through the action of an oxygen containing gas prior, simultaneously or after bleaching with a conventional bleaching agent, such as hydrogen peroxide or sodium hypochlorite. The oxygen gas treatment can be applied in continuous or batch processes. The gas can be dissolved in the treatment liquid and the liquid be brought in intimate contact with the fibers or introduced into a treatment vessel and allowed to diffuse through a liquid layer surrounding the fibers to the reaction sites.

Description: A METHOD FOR BLEACHING COTTON Field of the invention Bleaching of fibers of cotton, wool and silk and blends of such fibers and other textile fibers is a process stage in the preparation of yarns and fabrics and can be carried out at several stages of the production sequence. It is sometimes done on loose fibers, on yarns in hank or package form, on the fabric after singeing and desizing and before or after mercerization. The object of bleaching is to produce clean and white goods with good absorbtivity and without any appreciable loss of physical strength. For a fabric comprising cotton, bleaching can be preceeded by or combined with desizing and scouring, and for a yarn, bleaching can be preceeded by or combined with scouring only. The degree of desired whiteness of the bleached goods, article or product depends on whether it is to be dyed or not. If the goods after finishing are to be white, they must be bleached to an optimal whiteness. An optical whitener is normally added in conjuction with the bleaching chemical to the bleach bath. For goods to be dyed or printed, optimal whiteness is not necessary and the goods are subjected to a less intensive bleaching process commonly called pre-bleaching, which does not include an optical whitener. In the present description and claims the term bleaching is understood to include both the pre-bleaching and bleaching processes . Background art Fabrics and yarns are bleached to remove natural and adventitious impurities as well as spin-finishes, coning oils and for fabrics also desizing agents and moreover, to produce fabrics and yarns with satisfactory whiteness and absorbtivi ty. Treatment before bleaching may include desizing and scouring. In desizing, the sizing material is removed from the fabric as completely and uniformly as possible. The residual size can lead to uneven dyeing and stiff and variable hand. Desizing is done either in neutral solution using enzymes or in basic solution usually using sodium carbonate, sodium bromite or hydrogen peroxide depending on the sizing material to be removed. Scouring is applied to remove oils, waxes and fats from the cotton fibers by saponification and emulsification in order to obtain good whiteness and absorbtivity or to pretreat the fibers for subsequent removal of these substances including pectins, hemicellulose and protein materials. During scouring also motes, seedcoats and other fragments are pretreated for easier removal during the following bleaching stage. In the bleaching stage mostly oxidizing chemicals are applied in order to decolorize and remove impurities from the fabric. The bleaching processes aim at achieving several goals: A high and uniform absorbtivity of the yarn or fabric for water, dyestuffs and finishing agents. A sufficiently high and uniform degree of whiteness and sufficiently low content of particles (motes, seedcoats, fragments, funiculi, etc). The fibers, yarn and fabric should not be damaged and the degree of polymerization of the cellulose should remain high. The whiteness of the fabric should remain stable upon storage. The oxidative chemicals used in bleaching are chlorine chemicals (hypochlorite, chlorite, and chlorine dioxide), hydrogen peroxide and sulfur derivatives. Today, hydrogen peroxide is the most widely used bleaching agent for textiles, and over 85 X of all fabrics are bleached with it. Hydrogen peroxide has a number of important advantages over other bleaching agents: It produces a stable white color, and the bleached goods are highly hydrophilic since the fats and waxes are solubilized and removed by the hot alkaline solutions used. Cotton husks can be effectively bleached. Its reaction products are relatively nontoxic and it decomposes to oxygen and water, i. e. it is environmentally sound. It is compatible with most fibers. The number of operations and stages in the bleaching process can be reduced, e. g. scouring and hydrogen peroxide bleaching can be combined. Some disadvantages of hydrogen peroxide as a bleaching agent are: Catalytic decomposition of hydrogen peroxide can occur due to the presence of iron, nickel, copper and other heavy metal ions. The. catal tic decomposition of hydrogen peroxide in high concentration will cause degradation of cellulose. Severe degradation can result in broken fibers and holes in the fabric. Stabilizers are added to control decomposition of hydrogen peroxide. Stabilizers based on silica precipitate on yarn or fabric under certain conditions and uneven dyeing will result. Hydrogen peroxide is a comparatively expensive chemical. US patent specification No 1 163 438 to Mueller describes a bleaching process, which consists in immersing unbleached material to be bleached in an aqueous alkaline bath and subjecting the bath liquid to the action of molecular oxygen or air. Preferred embodiments comprise forcing oxygen or air into the alkaline bath under pressure, heating said alkaline bath and simultaneously forcing air or oxygen into the bath, and the presence in the bath of a non-alkaline compound of a metal, e. g. manganese carbonate. However, the time needed to obtain cotton material bleached to total whiteness is 16 hours when no heavy metal compound is present in the bath and 12 hours when for example manganese carbonate is present in an amount of 0.01 per cent. This time consuming process has been abandoned in favor of other more rapid bleaching processes, e. g. bleaching with hydrogen peroxide which now is the most utilized process. Objects of the present invention One object of the present invention is to bleach goods comprising cotton fibers to optimal whiteness and absorbtivit with a much reduced amount of conventional bleaching agents. Another object of the present invention is to bleach such goods to higher whiteness than previously obtainable by the use of either only- oxygen or only a conventional bleachin agent, such as hydrogen peroxide or sodium hypochlorite. Further another object of the present invention is to obtain faster bleaching with hydrogen peroxide to a predetermined whiteness. Still another object of the present invention is to reduce the amount of auxiliary agents, such as optical whiteners and stabilizers, in the bleaching process. Another object is to reduce the amount of chlorine chemicals in the effluent when using sodium hypochlorite in bleaching of cotton containing goods. Brief description of the invention The present invention has regard to bleaching of goods comprising cotton fibers with a conventional bleaching agent by subjecting the goods to the action of an oxygen containing gas dissolved or dispersed in an alkaline treatment liquid at an elevated temperature. The oxygen containing gas consists o at least 50 per cent oxygen, preferably at least 99 per cent oxygen, i. e. pure oxygen gas. The remainder of the gas is an inert gas such as nitrogen or a noble gas. This treatment wit oxygen gas can be performed in an alkaline treatment step in the absence of a conventional bleaching agent, such as hydrogen peroxide. Oxygen gas is dissolved in the treatment liquid by an injection device spreading or dispersing the gas as uniformly as possibly in the liquid, possibly supported by agitation of the liquid, whereby non dissolved oxygen is collected above the liquid in the vessel, or by introducing oxygen gas into the headspace of an expansion vessel or treatment vessel of the bleaching unit, the treatment liquid being agitated in order to dissolve or entrain oxygen gas. After such treatment with oxygen gas in the absence of any conventional bleaching agent the goods are treated in an alkaline bleaching liquid containing a bleaching agent, preferably hydrogen peroxide. The alkaline treatment with oxygen gas can be the scouring step after enzymatic desizing or a combined step of alkaline desizing and scouring. According to another preferred embodiment the treatment with oxygen gas in an alkaline liquid is performed in the presence of a bleaching agent, i. e. in the alkaline bleaching liquid. In this embodiment a conventional desizing step and a conventional scouring step can preceed the bleaching step or for certain applications and quality requirements one or both of these conventional pretreatment steps can be omitted. According still another embodiment the goods can be subjected to the action of oxygen gas after the bleaching step in the presence or absence of bleaching agent. A necessary condition is that the treatment with oxygen is carried out in an alkaline liquid at elevated temperature. Still another preferred embodiment comprises the use of ozone in admixture with the oxygen containing gas. In this case part of the oxygen or the inert gas is replaced by ozone. The ozone concentration may typically be in the range of about 6 per cent to about 0.5 per cent. It has been found that oxygen can be used to bleach cotton to a high degree of whiteness in a hot alkaline treatment step. This whiteness obtained by using oxygen in a hot alkaline solution at a relatively high temperature is suitable as a so called pre-bleaching prior to dyeing or printing the goods. By using a pH of up to about 13,7 and a temperature of at least 90 degrees Celsius pre-bleaching can be performed in e. g. 1 to 2 hours, without decreasing the degree of polymerization of the cotton fibers below an acceptable level. A prerequisite for obtaining a bleaching effect with gaseous oxidizing agents, such as oxygen and ozone, in the presence or absence of a conventional bleaching agent, such as hydrogen peroxide, in an alkaline solution is that the gaseous oxidizing agent is transported to the reaction sites on or in the fibers to be bleached. Oxygen can be transported from a gas phase directly to a thin liquid layer surrounding a cotton fiber, or first dissolved in the bulk of a liquid and then by means of forced circulation of the liquid be transported to the cotton fibers. Oxygen acts as a bleaching agent at elevated temperatures under alkaline conditions. Process conditions applied in bleaching with hydrogen peroxide can be applied in bleaching with oxygen. Oxygen can thus be used together with hydrogen peroxide or hypochlorite. The overall reaction rate for the oxygen bleaching reactions is influenced by the concentration of oxygen at the reaction sites. Increased bleaching effect, e. g. expressed a whiteness increase versus bleaching time, thus is influenced by increased oxygen partial pressure, increased concentration of oxygen in the treatment liquid, both physically dissolved and free oxygen in the liquid phase, and increased liquid circulation turbulence. Bleaching equipment that can be pressurized is well suited for application of oxygen bleaching, e g autoclaves, kiers, package dyeing machines, hank dyeing machines, winches jiggers and jet machines. Oxygen can be added either directly to an available sealed vessel in the bleaching apparatus where good contact between gas and liquid can be established or in a separate unit especially designed for dissolution of oxygen and which is connected to the bleaching apparatus. Purging and/or pressurizing the head space in a jet machine with oxygen is an example of direct addition of oxygen. The intensive liquid flow in a jet machine creates good contact between gas and liquid and hence oxygen is dissolved into the liquid. Since the fabric is moving through the head space, oxygen will also be dissolved directly to the liquid layer on the cloth or fiber surfaces. Oxygen can also be introduced into a J box kier through which a wetted cloth is being moved. In a J box at least the upper part is void of liquid (except the liquid layer on the cloth). Oxygen in this upper part will diffuse into the liquid layer and thus to the sites where bleaching reaction can take place. Oxygen treating (bleaching) process conditions Process conditions suitable for oxygen bleaching are as follows: Bath ratio (fibers to liquid) 1:1 to 1:50, preferably 1:1 to 1:10; Temperature, degrees Celsius 60 to 130, preferably 75 to 115; pH 9 to 13.7, preferably 10 to 12 in the presence on hydrogen peroxide and preferably 13 to 13.5 in the absence of hydrogen peroxide; Total pressure, bar (g) 0 to 10, preferably 0 to 6; Bleaching time, minutes 10 to 200, preferably 15 to 60. Oxygen can be used together with chemicals and agents that normally are added to the bleaching liquid, e. g. wetting agents, sequestrants, stabilizers, pH-buffers and optical whiteners as well as oxidizers like hydrogen peroxide and hypochlorite. Brief description of the drawings Figure 1 schematically shows a laboratory yarn package bleaching machine; Figure 2 schematically shows a jet bleaching machine being equipped with several means for introducing oxygen; Figure 3 schematically shows a part of a continuous bleaching line; Figure 4 is a graph showing the whiteness versus time for bleaching of cotton fabric with oxygen at different pH; Figure 5 is a graph showing the whiteness versus hydrogen peroxide charge for the bleaching of cotton fabric with and witout oxygen treatment; Figure 6 is a graph similar to Figure 5; Figure 7 is a graph showing the whiteness versus hydrogen peroxide charge for bleaching cotton fabric without oxygen treatment and with oxygen treatment before the treatment with hydrogen peroxide; and Figure 8 is a graph showing the whiteness versus time for bleaching cotton fabric with hydrogen peroxide in the presence of oxygen and in the absence of oxygen, i. e. in the presence of nitrogen. Preferred embodiments of the invention Figure 1 schematically shows a modified laboratory bleaching machine comprising an autoclave 10 for bleaching yarn packages 11. The autoclave 10 is via lines 6 and 7 connected to a reverse valve 4. An outlet 3 of valve 4 is connected to line 8 having two branch lines 20 and 28. Each of the lines 20 and 28 is provided with a valve 26 and 18, respectively. Line 28 is connected to line 16, one end of which opens into an expansion vessel 13. A source of oxygen 14 is in communication with line 16 via line 15 having a valve 19. The bottom of the expansion vessel 13 is via a line 9, a pump 5 and a line 25 connected to an inlet opening of reverse valve 4. Line 20 connects line 8 to line 9. Further, the top of autoclave 10 is via line 22 opening into line 21 connected to line 16. Line 21 is provided with a first valve 17 between line 22 and line 16 and a second valve 23 on the opposite side of line 22. The yarn package 11 comprises a central channel 24. One end of this channel 24 is put over a pipe socket 12 connected to pipe 7. The other end of channel 24 is closed. In operation the yarn package 11 is placed in the autoclave 10 and water and chemicals are charged to the bleaching machine. Then, upon startig pump 5 the liquid is circulated in a loop from the expansion vessel 13 through line 9, pump 5, line 25, reverse valve 4, line 7, channel 24 through the yarn package 11 in the autoclave 10. From the autoclave 10 the liquid is returned to the expansion vessel via line 6, reverse valve 4, lines 8, 28 and pipe 16. Valves 23, 17, and 26 are closed. Valve 17 can be a non-return valve allowing liquid flow only in the direction from line 22 to line 16. The circulating liquid is heated to the desired temperature by an heat exchanger (not shown), e. g. in line 9 between the pump 5 and line 20. Vhen the desired temperature is reached oxygen is supplied to the expansion vessel 13 via line 15 and 16 until a predetermined pressure is obtained. Thereafter the expansion vessel is vented to remove air, whereupon oxygen is supplied again. This procedure is repeated e. g. four times. The liquid from pipe 8 flowing into line 16 is then injected in the head space of the expansion vessel. Since there is an oxygen pressure prevailing in the head space oxygen is dissolved into the liquid. After a predetermined period of time the flow direction of the liquid to and from the autoclave 10 is reversed through switching of the reverse valve 4. Since this run was started having the liquid to flow through line 7 into the pipe socket 12 and then from the inside of package 11 to the outside thereof the reverse flow direction is through line 6 into the autoclave 10 and through the yarn package 11 from the outside to the inside thereof. The reversal of flow direction is made relatively often in order to produce uniform bleaching of the yarn in the package 11. At predetermined intervals, e. g. every 5 minutes the expansion vessel 13 is vented to expell gas and more oxygen is then added. After completing the bleaching, the bleaching liquid is drained. Then the yarn is neutralized, washed and dried. Usually line 28 including valve 20 is not present in a laboratory equipment. In such a case line 8 merges in line 20 and the liquid from the autoclave 10 leaving the outlet 3 of the reverse valve 4 is fed directly into line 9 and returned to valve 4 via pump 5. In order to introduce liquid containing oxygen from expansion vessel 13, pump 5, valve 4 and autoclave 10, a partial liquid flow is taken out of autoclave 10 via line 22 and fed to the expansion vessel 13 via lines 21 and 16. A corresponding liquid amount is drawn by pump 5 from the expansion vessel 13. Figure 2 shows schematically a jet machine for bleaching fabrics comprising an autoclave 30 having a by-pass conduit 31. The fabric is arranged in a closed loop and is continuously forced through the by-pass conduit 31 in the direction shown by the arrow 50. Bleaching liquid is drawn from the autoclave 30 by pump 34 through line 49 and returned to the autoclave by-pass line 31 via pipe 40 and a jet nozzle 33. Bleaching chemicals are added to the bleaching liquid by pump 35 from a container 36. An expansion vessel 43 may be connected to pipe 44 via an input line 40 and an output line 38. A valve 37 is placed between lines 38 and 40 in pipe 44. A line 41 connecting an oxygen source (not shown) to the expansion vessel 43 is equipped with a valve 42. In operation valve 37 is closed and the liquid drawn from the autoclave 30 via line 49 is transferred via pipe 44, line 38 to the expansion vessel 43. From expansion vessel 43 the bleaching liquid is returned to pipe 44 and via jet 33 introduced into by-pass line 31. Oxygen is supplied to the expansion vessel 43 from the oxygen source via pipe 41. Oxygen is supplied to the vessel 43 in the same way as described in connection with Figure 1. In Figure 2 there are also shown other possibilities to introduce oxygen gas into the bleaching process equipment. The oxygen source can be connected to line 47 having a valve 48. Line 47 opens into pipe 44 and can at its end be provided with a sintered body (not shown) to produce small bubbles. These bubbles are easy distributed in the liquid passing through pipe 44. In this way oxygen can continuously be introduced into the machine. The oxygen flow can be regulated by valve 48. Vhen oxygen gas is introduced this way valve 37 is open and valve 39 closed or neither expansion vessel 43 nor valve 37 may be present. Still other possibilities are to introduce oxygen gas via line 45 directly into the headspace of the autoclave 30 or line 51. The flow of oxygen gas may be controlled by valve 46 and 52 respectively. Line 51 is preferably provided with a sintered body at its end opening into vessel 30. Figure 3 schematically shows a part of a continuous bleaching line. A fabric 61 to be bleached is entering a saturator 62 on the left hand side of the Figure in order to wet chemicals on the fabric. These chemicals can include sodium hydroxide and hydrogen peroxide in aqueous solution. Before leaving the saturator 62 the fabric may pass through a pair of rolls 63 for removing excess liquid from the fabric. From the saturator 62 the fabric is transferred to a J box kier 64, in which bleaching is taking place. In the kier the residence time of the fabric is sufficient to bleach the fabric, if not, several saturators and kiers are connected in series. From the J box kier 64 the fabric is transferred to one or several washing stations 65. Usually a J box kier is heated by steam directly or indirectly in order to keep the fabric at a set temperature. J box 64 is heated by steam acting directly on the fabric 61 and being introduced via line 66. To the lower part or "heel" of the kier 64 a bleaching solution i. e. hydrogen peroxide solution may be added by pump 68 through pipe 69 from tank 67. Hydrogen peroxide solution is withdrawn from kier 64 via line 70. In some J box kiers there is no recirculation of bleaching solution to the lower part thereof. When the conventional bleaching with hydrogen peroxide is supplemented with oxygen bleaching oxygen is introduced through line 66 either alone or together with steam. The oxygen is distributed in the kier and diffuses into the liquid layer surrounding the fibers of the fabric. In Figure 4 graphs representing the whiteness versus time at different pH values are given for bleaching with oxygen. The temperature was 104 degrees Celsius and the total oxygen pressure was 1.3 bar (g). The increase of whiteness with increasing pH is clearly noticeable. Examples of cotton bleaching conditions applied in bleaching of cotton and cotton-polyester yarns and fabrics ar presented below. These examples show that the charge of hydrogen peroxide can be reduced up to 80 X when the bleachin is reinforced with oxygen. Examples Example 1 Cotton fabric, scoured but not bleached, was treated in indirectly heated rotating autoclaves (volume 750 ml) under the following conditions: Bath volume, ml 500 Fabric-to-bath ratio 1:50 Temperature, degrees Celsius 95 NaOH concentration, mol/1 0.1 Time, minutes 60 Pressure, at room temperature MPa (g) see table 1 The autoclave was pressurized with nitrogen and oxygen respectively. The volume of the head space was 1/3 of the volume of the autoclave. The effects of the treatments on whiteness and fluidity are given in Table 1. Whiteness in all experiments was determined according t ISO 2470 standard method. Fluidity was determined according t SIS 650038 standard. Fluidity is an indirect method of determining the degree of polymerization of the cellulose fibers of the cotton. Low fluidity values indicate a high degree of polymerization. The fluidity values in rhes should be less than 15 in order to retain an acceptable strength of the cellulose fibers. Table 1. Whiteness before and after treatment with oxygen or nitrogen SAMPLE Applied Pressure Whiteness Fluidity gas MPa (g) 467 nm, X rhe A. Before treatment - - 57.6 1.4 B. After treatment nitrogen 0.2 61.0 1.6 C. After treatment oxygen 0.2 72.4 2.3 D. After treatment oxygen 0.4 73.6 2.1 Samples B and D were then bleached with hydrogen peroxide. The bleaching conditions were as follows: Fabric-to-bath ratio 1:1 Tempetature, degrees Celsius 95 Magnesium sulfate (on fabric), X 0.1 Na2Si03 (on fabric), X 1.0 pH (adjusted with NaOH) 10.6-11.0 Time, minutes 60 Hydrogen peroxide charge see table 2 Whiteness and fluidity after bleaching with hydrogen peroxide are given in table 2. The results in table 2 show that treatment with oxygen and alkali before hydrogen peroxide bleaching reduces the need of hydrogen peroxide by 50 to 75 per cent to reach a given whiteness compared to pretreatment without oxygen. The water absorption (suction height) of oxygen bleached fabric after bleaching with reduced charge of hydrogen peroxide is as good as after bleaching with a higher charge of hydrogen peroxide after pretreatment without oxygen (cf. examples Dl and B2). Water absorption was evaluated as the distance a colored water solution moves vertically (suction height) after 5 minutes when the end of the cloth sample is in contact with the colored solution. Values on suction height in table 2 are given for individual samples. Table 2. Hydrogen peroxide bleaching conditions in bleaching of fabric pretreated with nitrogen or oxygen and properties of the fabric after bleaching. For pretreatment conditions refer to table 1. The charge of hydrogen peroxide is calculated as pure (100%) hydrogen peroxide on the weight of the fabric. Example 2 Cotton fabric, scoured but not bleached, was treated in indirectly heated rotating autoclaves (volume 750 ml) under the following conditions: Bath volume, ml 750 Fabric-to-bath ratio 1:50 Temperature, degrees Celsius 95 NaOH concentration, mol/1 0.1 Time, minutes 60 Pressure (at 95 degrees Celsius), kPa (g) 700 The autoclave was pressurized with oxygen. The volume of the head space was 1/3 of the volume of the autoclave. After oxygen-alkali treatment the fabric was bleached with hydrogen peroxide. The conditions for hydrogen peroxide bleaching were the following: Fabric-to-bath ratio 1:1 Temperature, degrees Celsius 95 Hydrogen peroxide on fabric, X 0.25 MgS04 (on fabric), X 0.1 Na2Si03 (on fabric), X 1.0 pH (adjusted with NaOH) 10.8 Time, minutes 60 The evaluated properties of the fabric after the different treatment stages are given in table 3. Table 3. The evaluated properties of the fabric after the different treatment stages in table 2. Sample Whiteness Fluidity Cleanli- 467 nm, X rhe ness (*) E. Untreated 57.6 1.5 3.5 F. After 02 treatment 68.7 1.6 0-1 G. After H202 bleaching following 02 treatment 79.3 2.3 (*) Cleanliness was evaluated according to a standard method used by the Swedish Institute for Textile Research (TEF0). (5 = poor; 0 = no impurities). It was shown in example 1 that bleaching with 0.25 X hydrogen peroxide following oxygen-alkali treatment gave the same whiteness as bleaching with 0.50 - 1.00 X hydrogen peroxide after alkali treatment. In the present example it is shown that oxygen-alkali treatment followed by hydrogen peroxide bleaching with reduced charge of hydrogen peroxide gives complete removal of particles, i. e. excellent cleanliness . Example 3 Bleaching of cotton-polyester yarn in a laboratory package machine (Figure 1). The package machine was charged with one yarn package (weight 1 kg). The bleach bath was pumped through the yarn package, inside out and outside in alternating. The yarn to liquid ratio was 1:10. The head space in the expansion vessel was pressurized with oxygen in order to dissolve oxygen into the bleach bath. Chemical charges and procedures are given in table 4. After final washing and drying, the oxygen reinforced bleached yarn had the same whiteness as the reference bleached yarn (bleached with only hydrogen peroxide). Table 4. Chemical charges expressed as weight on yarn. 1.0 X optical whitener was added in each trial. The procedure for experiments 1 and 2 was as follows: 1. Add chemicals hot in order. 2. Heat to 115 degrees Celsius, run 30 minutes and drop. 3. Refill with water, 10 minutes wash at 88 degrees Celsius and drop. 4. Refill with water, 10 minutes wash at 71 degrees Celsius and drop. 5. Refill with water, add 0.25 X chelating agent and 0.25 X wetting agent, 10 minutes wash at 60 degrees Celsius and drop. 6. Refill with water, add 1 X acetic acid and 1 X silicon softener, 10 minutes wash at 38 degrees Celsius and drop. Example 4 Bleaching experiments were made in a jet bleaching machine shown in Figure 2. Run 11. Valves 46 and 48 were closed. A cotton fabric, water, 0.024 mol/1 NaOH and 0.5 per cent hydrogen peroxide on fabric were charged to the machine and the temperature raised to 95 degrees Celsius. The bleaching time was 30 minutes. Thereafter again 0.5 per cent hydrogen peroxide on fabric wer charged to the machine and bleaching was continued at the same temperature for 30 minutes. An additional charge of 0.5 per cent hydrogen peroxide was then added and the bleaching of the fabric continued for another 30 minutes. No oxygen gas was added. After each bleaching period of 30 minutes samples were taken. Run 12. Valves 46 and 48 were closed. A cotton fabric, water and 0.024 mol/1 NaOH were charged to the machine and the temperature raised to 95 degrees Celsius. 3.5 bar (g) oxygen was charged to the head space of the expansion vessel 43 and the fabric was bleached for 30 minutes (Hydrogen peroxid was not present). Thereafter the oxygen was released from the head space and 0.5 per cent hydrogen peroxide on fabric were added to the bleaching liquid. The bleaching time was 30 minutes. Then the fabric was bleached further for two bleaching cycles of 30 minutes. 0.5 per cent hydrogen peroxide on fabric were added to the bleaching liquid before the start of each cycle. Figure 5 shows the whiteness at different bleaching times as a function of hydrogen peroxide charge for runs 11 and 12. Compared at whiteness of 89 per cent and a total bleaching time of 60 minutes oxygen pre-bleaching means a 50 per cent saving of hydrogen peroxide. Example 5 A further experiment was made with the jet machine of Figure 2. This experiment, run 13, was commenced in the same way as run 12 of example 4 during the first 30 minutes. Thereafter 0.8 per cent hydrogen peroxide on fabric were charged to the system and bleaching without oxygen present was continued for 30 minutes. Then without any addition the bleaching was again continued for 30 minutes. Samples were taken after every bleaching cycle. The graph in Figure 6 shows the whiteness versus hydrogen peroxide charge for run 11 of example 4 and run 13. Oxygen pre-bleaching followed by hydrogen peroxide bleaching, charge 0.8 hydrogen peroxide on fabric, and a total bleaching time of 90 minutes thus gave the same whiteness as bleaching in hydrogen peroxide only for the same time, the hydrogen peroxide charge being 1.5 to 1.6 per cent on fabric. Example 6 The jet machine of Figure 2 was used for this experiment. Run 14. Fabric, water, 0.8 per cent hydrogen peroxide on fabric and 0.024 mol/1 NaOH were charged the machine. The treating liquid was heated to 95 degrees Celsius. Then oxygen was supplied to the head space of the expansion vessel to a pressure of 3.5 bar (g). A samle was taken after 30 minutes and bleaching was continued for 30 minutes without further addition of hydrogen peroxide. Oxygen at the given pressure was present all the time. In Figure 7 the filled symbols represent run 11 of example 4. The open symbols represent run 14. Combined oxygen and hydrogen peroxide bleaching at a peroxide charge of 0.8 per cent for 60 minutes gave the same whiteness as a hydrogen peroxide charge of 1.5 per cent for 90 minutes. Absorption properties of all bleached samples of examples 4-6 were excellent. Six bleaching experiments on pure cotton were made on yarns. In each experiment six packages of yarn having a total weight of about 4.9 kg were bleached. An apparatus similar to that shown i Figure 1 was used. However, a down-flow bubble contactor was used for the introduction of oxygen. The liquid volume was about 120 liters, resulting in a ratio liquid to yarn of about 14:1. This is a greater value than normally used. Hydrogen peroxide and an optical whitener were added in proportion to the weight of yarn. The concentrations of other chemicals used, such as deairation agent, wetting agent, agen for controlling the water hardness, stabilizer for the hydrogen peroxide and sodium hydroxide, were equal in all experiments and in the prescribed concentration. The oxygen uptake by the liquid is limited to the solubility of oxygen in the liquid at the actual temperature. A great ratio of liquid to yarn certainly implies a great amount of dissolved oxygen for each kg of yarn but conventional means such as a bubble contactor can dissolve oxygen in the liquid in such an amount that the oxygen concentration is near its maximum value all the time. The following bleaching experiments were made. VI. Reference. Normal charge of hydrogen peroxide, 2 X on yarn, usual pH; W2. With oxygen. Reduced charge of hydrogen peroxide, 1 X on yarn, usual pH; W3. With oxygen. Reduced charge of hydrogen peroxide, 0.5 X on yarn, usual pH; W4. Without oxygen. A 25 per cent reduction of hydrogen peroxide, 1.5 X on yarn, reduced pH; W5. Without oxygen. A 75 per cent reduction of hydrogen peroxide, 0.5 X on yarn, reduced pH; and W6. With oxygen. A 75 per cent reduction of hydrogen peroxide, 0.5 X on yarn, reduced pH. The yarn was introduced in the bleaching apparatus. Then water and the chemicals were added in order. Thereafter the temperature was raised from the starting temperature of about 30 degrees Celsius to the maximal temperature of about 95 degrees Celsius during about 20 minutes. This corresponds to a temperature increase of about 3 degrees Celsius per minute. In experiments with oxygen the gas was added already during the heating phase via the bubble contactor. When the maximum temperature was reached this temperature was maintained for 45 minutes . The bleaching was stopped by dropping the liquid. Thereafter the yarn was washed with water. Concentrated acetic acid, 40 ml, was added to the washing water in order to neutralize the remaining alkali. Then the yarn was centrifugated and dried with air. The bleaching conditions are given in table 5. Optical whitener and hydrogen peroxide amounts are calculated on yarn. The results are summarized in table 6. Table 5. Conditions for bleaching cotton yarn. Charge of hydrogen peroxide and optical whitener are given as per cent on yarn. Trial Yarn Bath H202 NaOH optical 02-pressure No kg 1 X 45% ml whitener X bar (g) Table 6. Whiteness, whiteness index (CIE-lab) without UV light and fluidity for bleached yarn. Trial No Wl W2 W3 W4 W5 W6 (*) The fluidity was determined only for samples Wl and W2, being subjected to the roughest conditions in this test. According to table 6, the fluidity values were less than 2.5 rhe. Due to the low fluidity values no tests on strength were considered necessary. Example 7 In a jet machine having a cloth capacity of 160-180 kg bleach chemicals and cotton cloth were charged. In trial XI the normal amount of hydrogen peroxide was charged and atmospheric pressure of air was maintained. Normal batch time and temperature cycle was followed. Trial XI is a standard bleach process. The resulting whiteness as CIE without UV was 63.5 units. In trial X2 60 per cent of the amount of hydrogen peroxide of trial XI was charged. The machine was purged until all air was replaced by oxygen gas. A slight gauge pressure (about 1 bar (g)) of oxygen gas was maintained. Normal batch time and temperature cycle was followed. The resulting whiteness as CIE without UV was 63.7 units. The initial whiteness of the cloth in both trials was 20.4 units (CIE without UV). This example shows that the same whiteness can be obtained by replacing 40 per cent of the hydrogen peroxide necessary in the standard bleach process by oxygen. The oxygen consumption was 2.0 cubic meters. Example 8 In this example three trials were made in order to show the synergy effect of hydrogen peroxide and oxygen. These tests were performed in a modified single package laboratory machine at atmospheric pressure. The tests simulated continuous cloth bleaching. Dry cloth of pure cotton was saturated with bleaching liquor (approx. 8 grams of cloth in a 300 ml bath), squeezed in a uniform way and then placed in the kier which was already at run temperature. Steam and oxygen in trials B and C or only steam in trial A was supplied to the kier. The kier was open to the room via a valve. The run temperature in the kier in all trials was 93 degrees Celsius. Upon completing the bleaching after 30 minutes, the kier was opened and the sample removed for washing in water. The whiteness given in table 7 is the final whiteness minus the starting whiteness, measured as CIE index without UV light. The results are summarized in table 7. Table 7. Whiteness as CIE index for bleached cloth. Trial Peroxide cone No g/1 p_H Gas Whiteness (*) A 10 11.27 Air 47.68 B 0 11.28 Oxygen 2.74 _C 10 11.30 Oxygen 54.87 (*) Final whiteness minus starting whiteness Table 7 shows clearly that the whiteness obtained in trial No C is greater than the sum of whiteness for trials A and B. Example 9 These trials for bleaching of cotton were performed in order to show reduction in hydrogen peroxide, caustic and stabilizer consumption when oxygen is present while achieving higher whiteness than without the presence of oxygen. These trials were performed in vapor phase in a steamer box. In all trials the pH of the bleaching solution was 10.9 - 11.0. The pressure was slightly more than atmosperic pressure and the temperature in the steamer box was 93 degrees Celsius. The bleaching time was 30 minutes. The whiteness, CIE index without UV, is the final whiteness minus the starting whiteness. Table 8. Whiteness as CIE index for bleached cloth. Peroxide NaOH Stabilizer Whiteness g/1 g/1 g/1 Air Oxygen 57.8 57.3 60.0 59.3 58.5 58.4 59.2 Table 8 demonstrates that the amount of hydrogen peroxide can be reduced by at least 40 percent when oxygen is present instead of air. Example 10 The following trials were made in order to show that the objective of faster bleaching is met. The bleaching solution contained 37.5 g/1 of hydrogen peroxide, 7.0 g/1 of sodium hydroxide and 30 g/1 of stabilizer. The pH of this bleaching solution was 10.7 - 10.8. These tests simulate continuous cloth (fabric) bleaching. Dry cloth of cotton was saturated with the bleaching liquor and squeezed in a uniform way and then placed in a steamer box. The temperature within the steamer box was 93 degrees Celsius and the pressure of steam and gas (nitrogen or oxygen) was slightly higher than atmospheric pressure. Samples were taken out of the steamer box in regular time intervalls of 5 minutes. After washing and drying of the samples the whiteness was evaluated. The results are given in Figure 8 showing the whiteness, CIE index without UV, as a function of the bleaching time using a nitrogen and oxygen atmosphere, respectively, as the gaseous phase. Example 11 This example shows that the amount of hydrogen peroxide used to desize cloth can be reduced when oxygen is added. For continuous cloth this is done just like bleaching using a saturator and a J box kier. However, bath liquor is different. Oxygen is added in the same way as in bleaching. Results are shown in table 9. Table 9 Extractables measures the amount of size left on the cloth. Run No 1 is standard plant operation and result. Run No 2 shows that a reduced amount (1.0 g/1 instead of 1.7 g/1) can be used resulting in a better whiteness while leaving the same amount of extractables on the cloth. The obtained whiteness of bleached yarn or fabric when using oxygen gas in an alkaline liquid in combination with a conventional bleaching agent, such as hydrogen peroxide, was not expected. Although Mueller more than 70 years ago proposed the use of molecular oxygen his method is not in use. Today most goods comprising cotton fibers and cotton fibers in blends with other fibers are bleached by hydrogen peroxide. Mueller proposed the use of a heavy metal ion compound as a catalyst to enhance the bleaching reactions. The man skilled in the art considers the decomposition of peroxide to water and oxygen to be a harmful reaction which must be avoided. In many bleaching processes the amount of added hydrogen peroxide is so great that the amount of oxygen being formed when most of the peroxide decomposes cannot be held by the liquid. In spite of that fact the bleaching effect of oxygen has not been recognized. On the contrary, there are statements in the literature stating that oxygen produced by decomposition of hydrogen peroxide is inactive, not giving any bleaching effect. The results according to the present invention are excellent and it is assumed that the two agents, oxygen and hydrogen peroxide, act in a catalytic way. The excellent results indicate a synergistic effect. Since the amount of hydrogen peroxide can be reduced considerably by using oxygen as a second bleaching agent other advantages than cheaper bleaching costs are apparent. The amount of stabilizer such as silicate compounds of sodium and chelating agent can be reduced or eliminated. Since such agents mostly are conveyed to a recipient, such reduction of chemicals has a positive influence on the environment. Further the use of less silicate stabilizer give the bleached fabrics a better hand. It is further possible to bleach at lower pH values. Further, it has been found that the amount of hydroxide ions necessary to reach a given high pH value is less when the concentration of hydrogen peroxide and stabilizer is lower. Our observations seem to to indicate that there is some kind of promoting effect regarding the optical whitener when used on fabric or yarn bleached with oxygen. The amount necessary to reach a given whiteness is less than in the case of bleaching fabric or yarn which have been bleached with hydrogen peroxide only. Oxygen is an active bleaching agent also when supplied to goods comprising cotton in gaseous phase when said goods are wetted by an alkaline solution. Oxygen can also be supplied to goods comprising cotton fibers wetted by an alkaline solution as a foam. It is not necessary that a bulk liquid phase is present when bleaching is performed on a wetted fabric with oxygen gas in the gaseous phase or contained in a foam.

Claims: CLAIMS 1. A process for bleaching goods comprising fibers of cotton and such fibers in blends with other types of fibers, preferably in the form of filaments, threads, yarns, crochets, knitted and woven fabrics, characterized by introducing an oxygen containing gas into an alkaline treatment liquid of a separate alkaline treatment stage preceeding or following a bleaching stage and/or of an alkaline bleaching stage; and, if necessary, bringing the goods in intimate contact with said alkaline treatment liquid containing dissolved oxygen gas. 2. A process according to claim 1, characterized in that hydrogen peroxide or a hypochlorite is used as a conventional bleaching agent in the bleaching stage. 3. A process according to claim 1 or 2, characterized in that the oxygen containing gas is introduced into a treatment vessel containing the treatment liquid and dissolved into the treatment liquid by an injection means or introduced into a head space of said vessel and dissolved into said treatment liquid by agitation or circulation of the liquid. 4. A process according to claim 1 or 2, characterized in that the treatment liquid is recirculated to an expansion vessel and in that the oxygen containing gas is introduced into the expansion vessel containing the recirculated treatment liquid and dissolved therein by an injection means or introduced into a head space of said expansion vessel and dissolved into said treatment liquid by agitation or circulation of the liquid. 5. A process according to claim 1 or 2, characterized in that the oxygen containing gas is introduced into a treatment vessel containing the goods wetted with the alkaline treatment liquid and in that the oxygen is diffused into liquid layer of the wetted fibers. 6. A process according to one or several of claims 1-5, characterized in that the oxygen containing gas consists of at least 50 per cent by volume of oxygen, preferably at least 99 per cent by volu-πe of oxygen. 7. A process according to one or several of claims 1-5, characterized in that the oxygen containing gas comprises up to 6 per cent by volume of ozone. 8. A process according to one or several of claims 1-7, characterized in that the temperature of the treatment liquid containing dissolved oxygen gas is in the range of about 80 and about 130 degrees Celsius, preferably in the range of about 75 and about 115 degrees Celsius. 9. A process according to one or several of claims 1-8, characterized in that the pH of the treatment liquid containing dissolved oxygen gas is in the range of about 9 and about 13.7, preferably in the range of about 10 to about 12 when the treatment liquid contains hydrogen peroxide as the conventional bleaching agent, and in the range of about 13 and to about 13.5 when the treatment liquid is void of a conventional bleaching agent. 10. A process according to one or several of claims 1-9, characterized in that the time for treatment of the goods in the alkaline liquid containing dissolved oxygen gas is in the range of about 10 to about 200 minutes, preferably in the range of about 15 to about 60 minutes. 11. A process according to one or several of claims 1-10, characterized in that the bath ratio (fibers to liquid) of the treatment liquid containing oxygen gas is in the range of about 1:1 to about 1:50, preferably in the range of about 1:1 to about 1:10. 12. A process according to one or several of claims 3-5, characterized in that the partial oxygen pressure in the vessel to which the oxygen containing gas is supplied is in the range of about 0.1 to about 1.0 MPa, preferably in the range of about 0.1 to about 0.6 MPa. 13. A process according to one or several of claims 2-12, characterized in that the oxygen containing gas is supplied to the bleaching stage and that hydrogen peroxide is used as the conventional bleaching agent. 14. A process according to one or several of claims 1-13, characterized in that an optical whitener is added to the bleaching liquid containing a conventional bleaching agent. 15. A process according to one or several of claims 1-14, characterized in that the oxygen containing gas is intermittently or continuously introduced into the treatment stage. 16. A process according to one or several of claims 1-15, characterized in that the goods are continuously or intermittently fed to or removed from the treatment stage. 17. A textile product comprising fibers of cotton and such fibers in blends with other types of fibers, preferably in the form of filaments, threads, yarns, crochets, knitted and woven fabrics, characterized in that it is bleached by oxygen and a conventional bleaching agent.

Inventors: MALANDRUCCOLO ROBERT, PETERSSON MARTIN, SCHULZ ROBERT, SJOEBLOM KRISTER, THEXTON TIMOTHY, MALANDRUCCOLO, ROBERT, PETERSSON, MARTIN, SCHULZ, ROBERT, SJOEBLOM, KRISTER, THEXTON, TIMOTHY
Applicants: AGA AB
Application year: 1988
Main classification: D06L3
IPC codes: D06L3