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 sometime 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 ap¬ preciable loss of physical strength. For a fabric comprising cotton, bleaching can be preceeded by or combined with de¬ sizing and scouring, and for a yarn, bleaching can be pre¬ ceeded 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 conj uction with the bleaching chemical to the bleach bath. For goods to be dyed or printed, optimal whiteness is not necessary and 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 adven¬ titious 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 absorb¬ tivity.

SUBSTITUTESHE!

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 or sodium bromite depending on the sizing materi to be removed.

Scouring is applied to remove oils, waxes and fats from th cotton fibers by saponification and emulsification in orde to obtain good whiteness and absorbtivity or to pretreat t fibers for subsequent removal of these substances includin pectins, hemicellulose and protein materials. During scouring also motes, seedcoats and other fragments are pretreated for easier removal during the following bleachi 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 fabri 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 a the degree of polymerization of the cellulose should remain high.

The whiteness of the fabric should remain stable upo storage.

SUBSTITUTE SHEET

The oxidative chemicals used in bleaching are chlorine chemi cals (hypochlorite, chlorite, and chlorine dioxide), hydroge peroxide and sulfur derivatives.

Today, hydrogen peroxide is the most widely used bleaching agent for textiles, and over 85% of all fabrics are bleached with it!. H-ydrogen peroxide has a number of important advan¬ tages 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 solu¬ tions used.

Cotton husks can be effectively bleached.

Its reaction products are relatively nontoxic and it decomposes to oxygen and water, i. e. it is environmen¬ tally 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 catalytic decomposition of hydro¬ gen peroxide will cause degradation of cellulose, severe degradation can result in broken fibers and holes in the fabric.

SUBSTITUTESHEET

Stabilizers are added to control decomposition of hydrogen peroxide. Stabilizers based on silica precipi¬ tate on yarn or fabric under certain conditions and uneven dyeing will result.

Hydrogen peroxide is a comparatively expensive chemi¬ c l .

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 sub¬ jecting 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 pressent invention

One object of the present invention is to bleach goods comp¬ rising cotton fibers to optimal whiteness and absorbtivity 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 bleaching agent, such as hydrogen peroxide or sodium hypochlorite.

SUBSTITUTE SHEET

Further another object of the present invention is to obtain faster bleaching with hydrogen peroxide to a predetermined whiteness .

Still another object is to bleach goods containing cotton fibers using less hydrogen peroxide or sodium hypochlorite or none of these agents to obtain a whiteness which is suit¬ able for dyeing or printing fabric in a dark shade.

Still another object of the present invention is to reduce t 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 cotton containing goods.

Brief description of the invention

The present invention has regard to bleaching of goods comp¬ rising cotton fibers with a conventional bleaching agent by subjecting the goods to the action of an oxygen containing ga dissolved or dispersed in an alkaline treatment liquid at an elevated temperature. The oxygen containing gas consists of a 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 hydro¬ gen 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 not dissolved oxygen is collected above the liquid in the vessel, or by introducing oxygen gas into the headspace of an expansion vessel or treat

SUBSTITUTE SHEET

ment vessel of the bleaching unit, the treatment liquid bein agitated in order to dissolve or entrain oxygen gas. After such treatment with oxygen gas in the absence of any conven¬ tional bleaching agent the goods are treated in an alkaline bleaching liquid containing a bleaching agent, preferably hydrogen peroxide. The alkaline treatment with oxygen gas ca 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 presenc of a bleaching agent, i. e. in the alkaline bleaching liquid In this embodiment a conventional desizing step and a conven tional 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 subjecte to the action of oxygen gas after the bleaching step in the presence or absence of bleaching agent. A necessary conditio is that the treatment with oxygen is carried out in an alka¬ line liquid at elevated temperature.

Still another preferred embodiment comprises the use of ozon in admixture with the oxygen containing gas. In this case pa of the oxygen or the inert gas is replaced by ozone. The ozo 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 alka¬ line solution at a relatively high temperature is suitable a 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

SUBSTITUTE SHEET

degree of polymerization of the cotton fibers below an accep able level.

A prerequisite for obtaining a bleaching effect with gaseous oxidizing agents, such as oxygen and ozone, in the presence absence of a conventional bleaching agent, such as hydrogen peroxide, in an alkaline solution is that the gaseous oxidiz agent is " transported to the reaction sites on or in the fibe to be bleached. Oxygen can be transported from a gas phase directly to a thin liquid layer surrounding a cotton fiber, first dissolved in the bulk of a liquid and then by means of forced circulation of the liquid be transported to the cotto fibers .

Oxygen acts as a bleaching agent at elevated temperatures und alkaline conditions. Process conditions applied in bleaching with hydrogen peroxide can be applied in bleaching with oxyge Oxyge can thus be used together with hydrogen peroxide or hyp chlorite.

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 as whitenes increase versus bleaching time, thus is influenced by increas oxygen .partial pressure, increased concentration of oxygen in the treatment liquid, both physically dissolved and free oxyg in the liquid phase, and increased liquid circulation turbu¬ lence.

Bleaching equipment that can be pressurized is well suited fo application of oxygen bleaching, e g autoclaves, kiers, packa dyeing machines, hank dyeing machines, winches, jiggers and j 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 espe ally designed for dissolution of oxygen and which is connect to the bleaching apparatus.

Pressurizing the head space in a jet machine with oxygen is an example of direct addition of oxygen. The intensive liqui flow in a jet machine creates good contact between gas and liquid and hence oxygen is dissolved into the liquid. Since fabric is moving through the head space, oxygen will also be dissolved directly to the liquid layer on the cloth or fiber s rfaces.

Oxygen can also be introduced into a J box kier through whic wetted cloth is being moved. In a J box at least the upper p is void of liquid (except the liquid layer on the cloth). Ox in this upper part will diffuse into the liquid layer and th 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: 5 to 1:10; Temperature, degrees Celsius 60 to 130, preferably

75 to -115,- pH 9 to 13.7, preferably

9.5 to 12.5 in the presence on hydrogen peroxide and preferably 13 to 13.5 in the absence of hydrogen peroxide; Total pressure, bar (g) 1 to 10, preferably

3 to 10; Bleaching time, minutes 10 to 200, preferably

20 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 bleachin 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; and

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 wit hydrogen peroxide.

SUBSTITUTE SHEET

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 communi¬ cation with line 16 via line 15 having a valve 19. The botto 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. Li

20 connects line 8 to line 9. Further, the top of autoclave is via line 22 opening into line 21 connected to line 16. Li

21 is provided with a first valve 17 between line 22 and lin 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. When the desired temperature is reached oxygen is supplied to the expansion vessel 13 via

SUBSTITUTE SHEET

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 expan¬ sion 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 the from the inside of package 11 to the outside thereof the reverse flow direction is through line 6 into the autoclave 1 and through the yarn package 11 from the outside to the insid thereof. The reversal of flow direction is made relatively often in order to produce uniform bleaching of the yarn in th package 11. At predetermined intervals, e. g. every 5 minutes the expansion vessel 13 is vented to expell gas and more oxy¬ gen is then added. After completing the bleaching, the bleach ing liquid is drained. Then the yarn is neutralized, washed and dried.

Usually line 28 including valve 20 is not present in a labora tory 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 autoclav 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 fab¬ rics 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 autoclav 30 by pump 34 through line 49 and returned to the autoclave by-pass line 31 via pipe 40 and a j et nozzle 33. Bleaching chemicals are added to the bleaching liquid by pump 35 from container 36. An expansion vessel 43 is.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 connect ing an oxygen source (not shown) to the expansion vessel 43 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, lin 38 to the expansion vessel 43. From expansion vessel 43 the bleaching liquid is returned to pipe 44 and via jet 33 intro duced into by-pass line 31. Oxygen is supplied to the expan¬ sion 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 intr duce 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 wi a sintered body (not shown) to produce small bubbles. These bubbles are easy distributed in the liquid passing through p 44. In this way oxygen can continuously be introduced into t machine. The oxygen flow can be regulated by valve 48. When 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 or line 51 directly into the headspace of the auto¬ clave 30. The flow of oxygen gas may be controlled by valve 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 bleachin line. A fabric 61 to be bleached is entering a saturator 62 the left hand side of the Figure in order to wet chemicals o the fabric. These chemicals can include sodium hydroxide and hydrogen peroxide in aqueous solution. Before leaving the saturator 62 the fabric is passed through a pair of rolls 63 for removing excess liquid from the fabric. From the saturat 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, sever 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 indirect in order to keep the fabric at set a temperature. J box 64 i 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 solutio is 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 solu¬ tion 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 oxy¬ gen 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 1.3 bar (g) . The increase of whiteness with increas¬ ing pH is clearly noticeable.

SHEET

Examples of cotton bleaching conditions applied in bleaching of cotton and cotton-polyester yarns and fabrics are present below. These examples show that the charge of hydrogen perox ide can be reduced up to 80 % when the bleaching is reinforc with oxygen. The achieved effect of oxygen in bleaching of cotton yarns and fabrics is unexpectedly high considering th effect of oxygen in bleaching of paper pulps . In bleaching o pulp reinforcement with oxygen of an alkali extraction stage gives a reduction in the consumption of active chlorine by about 30 per cent.

Examples Example 1

Cotton fabric , scoured but not bleached , was treated in in¬ directly heated rotating autoclaves ( volume 750 ml) under ^" th 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 temper ature

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 to ISO 2470 standard method. Fluidity was determined according to S 650038 standard. Fluidity is an indirect method of determini the degree of polymerization of the cellulose fibers of the cotton. Low fluidity values indicate a high degree of

SUBSTITUTE SHEET

polymerization. The fluidity values in rhes should be less than 15 in order to retain an acceptable strength of the cel¬ lulose fibers.

Table 1. Whiteness before and after treatment with oxygen or nitrogen

SAMPLE Applied Pressure Whiteness Fluidity gas MPa (g) 467 nm, % rhe

A. Before treatment - -

B. After treatment nitrogen 0.2

C. After treatment oxygen 0.2

D. After treatment oxygen 0.4

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), % 0.1

Na ₂ SiO- (on fabric), % 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.

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Table 2. Hydrogen peroxide bleaching conditions in bleaching of fabric pretreated with nitrogen or oxygen and properties of the fabric after bleaching. For pre- treatment conditions refer to table 1. The charge o hydrogen peroxide is calculated as pure (100%) hydr gen peroxide on the weight of the fabric.

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 pretreat ent without oxygen.

The water absorption (suction height) of oxygen bleached fab ric after bleaching with reduced charge of hydrogen peroxide is as good as after bleaching with a higher charge of hydrog 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 min utes 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.

Example 2

Cotton fabric, scoured but not bleached, was treated in in¬ directly 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/l 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, % 0.25

MgSO. (on fabric), % 0.1

Na ₂ Si0_ (on fabric), % 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, % rhe ness (*)

E. Untreated 57.6 1.5 3.5

F. After 0 ₂ treatment 68.7 1.6 0-1

G. After H„0„ bleaching following O treatment 79.3 2.3 0

(*) Cleanliness was evaluated according to a standard method used by the Swedish Institute for Textile Research (TEFO) . (5 = poor; 0 = no impurities).

It was shown in example 1 that bleaching with 0.25 % hydroge peroxide following oxygen-alkali treatment gave the same whiteness as bleaching with 0.50 - 1.00 % 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 com lete 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 (weigh 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.

Trial Stabili- NaOH Hydrogen Oxygen Initial Final No zer, % % peroxide Pressure pH pH

% MPa

1.0 % 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 Celsi and drop.

4. Refill with water, 10 minutes wash at -71 degrees Celsi and drop.

5. Refill with water, add 0.25 % chelating agent and

0.25 % wetting agent, 10 minutes wash at 60 degrees Ce sius and drop.

6. Refill with water, add 1 % acetic acid and 1 % silicon softener, 10 minutes wash at 38 degrees Celsius and drop.

Exampl e 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 fabri were charged to the machine and the temperature raised to 95 degrees Celsius. The- bleaching time was- 30 minutes. Thereaft again 0.5 per cent hydrogen peroxide on fabric were charged the machine and bleaching was continued at the same tempera¬ ture for 30 minutes. An additional charge of 0.5 per cent hydrogen peroxide was then added and the bleaching of the fa ric continued for another 30 minutes. No oxygen gas was adde 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/l NaOH were charged to the machine and the tem erature 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 adde to the bleaching liquid. The bleaching time was 30 minutes. Then the fabric was bleached further for two bleaching cycle 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.

SUBSTITUTE SHEET

Exampl e 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 percent hydrogen peroxide on fabric were charged to the system and bleaching without oxygen present was continued fo 30 minutes. Then without any addition the bleaching was agai 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 90 minutes thus gave the same whiteness as bleaching in hydro gen 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/l NaOH were charged the machine. The treating liquid was heated to 95 degrees Celsius. Then oxygen was sup¬ plied to the head space of the expansion vessel to a pressure of 3.5 bar (g) . A samle was taken after 30 minutes and bleach ing 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- were excellent.

Six bleaching experiments on pure cotton were made on yarns . In each experiment six packages of yarn having a total weigh of about 4.9 kg were bleached. An apparatus similar to that shown i Figure 1 was used. However, a down-flow bubble conta tor was used for the introduction of oxygen. The liquid volu was about 120 liters, resulting in a ratio liquid to yarn of about 14:1. This is a greater value than normally used. Hydr gen peroxide and an optical whitener were added in proportio to the weight of yarn. The concentrations of other chemicals used, such as deairation agent, wetting agent, agent for con¬ trolling 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 the actual temperature. A great ratio of liquid to yarn cert¬ ainly implies a great amount of dissolved oxygen for each kg of yarn but a correctly installed bubble contactor can _. dis¬ solve 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.

Wl. Reference. Normal charge of hydrogen peroxide, 2 % on yarn, usual pH;

W2. With oxygen. Reduced charge of hydrogen peroxide, 1 % on yarn, usual pH;

W3. With oxygen. Reduced charge of hydrogen peroxide, 0.5 % on yarn, usual pH;

W4. Without oxygen. A 25 per cent reduction of hydrogen peroxide, 1.5 % on yarn, reduced pH;

W5. Without oxygen. A 75 per cent reduction of hydrogen peroxide, 0.5 % on yarn, reduced pH; and

W6. With oxygen. A 75 per cent reduction of hydrogen perox¬ ide, 0.5 % on yarn, reduced pH.

The yarn was introduced in the bleaching apparatus. Then wat and the chemicals were added in order. Thereafter the temper ture was raised from the starting temperature of about 30 de rees Celsius to the maximal temperature of about 95 degrees Celsius during about 20 minutes. This corresponds to a tempe ature increase of about 3 degrees Celsius per minute. In ex¬ periments with oxygen the gas was added already during the heating phase via the bubble contactor. When the maximum tem¬ perature 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 whiten er and hydrogen peroxide amounts are calculated on yarn. The results are summarized in table 6.

Table 5. Conditions for bleaching cotton yarn. Charge of hy¬ drogen peroxide and optical whitener are given as per cent on yarn.

Trial Yarn Bath H„0 NaOH optical 0 _? -pressure No kg 1 % 45% ml whitener % bar (g)

Table 6. Whiteness, whiteness index (CIE-lab) with and witho UV light and fluidity for bleached yarn.

(*) 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 strenght wer considered necessary.

The obtained whiteness of bleached yarn or fabric when using oxygen gas in an alkaline liquid in combination with a conve tional bleaching agent, such as hydrogen peroxide, was not expected. Although Mueller more than 70 years ago proposed t 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 pe oxide to water and oxygen be a harmful reaction which must b avoided. In many bleaching processes the amount of added hy¬ drogen peroxide is so great that the amount of oxygen being

formed when most of the peroxide decomposes cannot be held b the liquid. In spite of that fact the bleaching effect of ox gen has not been recognized. On the contrary, there are stat ments in the literature stating that oxygen produced by deco position 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 indi¬ cate a synergistic effect. Since the amount of hydrogen perox ide 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 silicon com¬ pounds of sodium -and chelating agent can be reduced or elimi¬ nated. 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 th 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 lesser than in case of bleachin fabric or yarn which have been bleached with hydrogen peroxid 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 solu-

tion as a foam. It is not necessary that a bulk liquid phase is present when bleaching is performed on a wetted fabric wi oxygen gas in the gaseous phase or contained in a foam.

SUBSTITUTE SHEET