The technical field of this invention includes a process for removing dark colored manganese dioxide stains from faded denim fabric or made-up garments to provide a clean, yet faded appearance.

Denim garments such as jeans, jackets and skirts are considered by many to be more fashionable once they have attained a faded, worn appearance. Accord- ingly, denim fabrics and/or garments are frequently subjected to a bleaching procedure during their manu¬ facture to give them a bleached, superbleached, rifled or whitewashed appearance. While such pre- bleached goods are a very marketable product, the bleaching procedures conventionally employed are relatively labor intensive, which adds significantly to the cost of the bleaching process.

U.S. Patent No. 4,218,220 discloses that it is sometimes desirable to prepare prefaded denim gar- ments uniformly faded, that is prefaded blue jeans free of streaks. Satisfactory, unstreaked, suitably faded blue jeans were hitherto obtained only by re¬ peated washings. The patent teaches subjecting the denim fabric to a washing cycle comprising an initial wash with detergent and emulsifier, a suitable inter¬ mediate rinsing operation, a bleaching operation in which the garments are subjected to the simultaneous action of bleach and a quaternary ammonium fabric softener optionally with the addition of a suitable amount of detergent, a further rinsing operation, and/or an optional final treatment with fabric softener and laundry sour. The patent teaches the use of a chlorine bleach typically sodium hypo- chlorite, trichloroisocyanuric acid or the like. U.S. Patent No. 4,852,990 teaches a modification

wherein denim garments are first desized, then con¬ tacted with an aqueous polyacrylic acid solution. A chlorine-type bleaching agent is subsequently added to provide a uniform bleached appearance. Subsequently, the trend has been away from uni¬ form fading and toward a look featuring random faded effects. One such manifestation of this trend is the practice of rocking or stone-washing, that is, tum¬ bling the damp denims with pumice stones. The effect sought by this method is one of natural fading, a "used" look, characterized by a contrast between light and dark areas; in made-up garments however, the effect tends to appear on and around the seams only, whereas the color of the remaining fabric re- mains substantially uniform.

U.S. Patent No. 4,740,213 discloses a process in which granules of a coarse, permeable material, such as pumice, are impregnated with a chlorine bleaching agent and are tumbled in a drum with denim fabric in a dry state. Traces of the chlorine bleaching agent are removed subsequently, optionally by an antichlor such as acidic hydrogen peroxide.

However, chlorine bleaching agents are known to be very destructive to cotton, consequently alterna- tive bleaching agents have been employed to produce the faded look. Potassium permanganate is very de¬ sirable for such an oxidative treatment. When ap¬ plied in a solution an even fading is obtained; when impregnated into an inert porous material by "rock- ing" a desired random uneven oxidation of colorbodies is obtained. Unfortunately, dark colored, insoluble manganese dioxide is deposited on the denim resulting in a dirty, stained appearance. The manganese di¬ oxide can be removed by a process called "neutraliz- ing", that is, reducing the manganese dioxide to soluble manganous salts with chemical reducing agents, usually sulfites, thiosulfate, hydroxylamine

and the like as disclosed in U.S. Patents 4,795,476; 534,450; 1,361,833 and 3,384,444. These reducing agents must be used in a large excess and at a pH of 2.5 to 3.0, causing damage to the cotton fibers. The excess reducing agent from the neutralizing step is very undesirable to dispose of because of its very high toxicity and high chemical oxygen demand of the effluents.

After neutralization the denim is frequently "brightened" or bleached to enhance the contrast between the dyed and the decolorized areas. A hypo- chlorite bleach or a sodium perborate bleach bath is usually employed for brightening.

Copending U.S. Application Serial No. 451,067 filed December 15, 1989 claims a process for fading and bleaching denim fabric by contacting washed fabric with potassium permanganate and subsequently neutralizing the decolorized denim fabric between pH 3.0 and 6.0 in the presence of a carboxylic acid chelating agent and hydrogen peroxide. The decolor¬ ized denim fabric is subsequently bleached with an alkaline bleaching solution of hydrogen peroxide. The process is very effective. However, some bleach plants are limited by their available equip- ment and would prefer not to require a separate acidic neutralizing step followed by an alkaline bleaching step. A further disadvantage is the cost of the chelating agent, and ultimately its disposal. The present invention overcomes the disadvantages of the prior art process for processing denim by the steps of desizing the denim fabric, washing the de- sized fabric, contacting the washed fabric with potassium permanganate to oxidize part of the color- bodies in the denim fabric to a form which is easily removed from the fabric surface thereby partially decolorizing the denim fabric, and neutralizing the decolorized denim fabric by removing residues of the

potassium permanganate and of the oxidized color- bodies, wherein the oxidized denim fabric is neutral¬ ized by contacting the fabric with an aqueous solu¬ tion containing about 1.5 to 30 parts by weight per- acetic acid per hundred parts by weight denim fabric. The aqueous solution also containing sufficient ace¬ tic acid and hydrogen peroxide to provide a peracetic:acetic acid:hydrogen peroxide weight ratio of at least 1:1:1.5. That is, the aqueous solution also contains sufficient acetic acid and hydrogen peroxide to provide at least 1 part by weight acetic acid and at least about 1.5 parts by weight hydrogen peroxide per part by weight peracetic acid. Commer¬ cially available peracetic acid is an equilibrium mixture of peracetic acid, acetic acid, hydrogen peroxide and water. In the absence of a catalyst a week or more may be required for equilibrium to take place. For the purpose of this invention the compo¬ sition of the solution will be reported as the weight or mole ratio peracetic acid:acetic acid: ydrogen peroxide.

Peracetic acid is known as a strong oxidizing agent which is capable of oxidizing manganous ions to permanganate. Surprisingly, commercial peracetic acid does not decolorize the colorbodies (chromo- phores) usually used to dye denim. Commercial 35% peracetic acid does not remove manganese dioxide stains from denim without added hydrogen peroxide to adjust the PAA:H2θ ₂ ratio to 1:1.5. Desirably, the aqueous solution will contain by weight either about 1 part by weight peracetic acid, 3 parts by weight of acetic acid and at least one part by weight hydrogen peroxide (a 1:3:1 weight ratio) , or at least one part by weight peracetic acid, one part by weight acetic acid and about 1.5 parts by weight hydrogen peroxide (1:1:1.5). A weight ratio greater than 1:5:3 is effective but may

not be economical. Preferably, the aqueous solution will contain a 1:3:1.5 weight ratio of peracetic acid:acetic acid:H ₂ 02. The denim fabric may be treated as a single batch by immersion, as a series of batch immersions or continuously by spraying or counter-current immersion or treatment.

One skilled in the art will readily recognize that the total amount of aqueous solution required will depend on the amount of manganese dioxide on the fabric and the degree of color removal required.

The process is effective over a wide temperature range, for example, from 0°C to 100°C. The time of contact decreases with an increase of temperature. The range of about 15° (room temperature) to 65° is generally desirable.

Optionally including 1% to 5% surfactant on the weight of the goods (owg) in the aqueous solution will increase its efficacy in removing manganese dioxide. Generally, any nonionic or anionic sur- factant which is inert to oxidizing agents is satisfactory.

Typical surfactants include alkyl sulfates, alkoxy sulfates, alkylaryl sulfonates, glycerol esters, polyoxyethylene alkylphenol; alcohols, ethers and esters of polyoxyethylene, polyoxypropylene or polyethylene glycol and the like, either alone or as blends.

The denim fabric may be treated in any convenient form such as uncut piece goods, as partially fabri- cated garments or as finished garments. Denim is conventionally woven with colored warp and white filling threads but the term also includes striped denim fabrics or denim fabrics woven with both warp and filling threads colored. Usually the denims are dyed with vat dyes such as indigo or sulfur dyes or the like. Denim fabrics may also be woven with mix¬ tures of cotton and synthetic fibers.

The process is particularly useful for producing a denim with a random faded pattern by the process of desizing the denim fabric, washing the desized fabric, contacting the washed fabric with potassium permanganate to oxidize part of the dye chromophores or colorbodies in the denim fabric to a form which is easily removed from the fabric surface, thereby de¬ colorizing the denim fabric, and neutralizing the decolorized denim fabric by removing residues of the potassium permanganate and of the oxidized colorbodies.

The denim fabric may be desized by contacting the denim fabric with an effective amount of a peroxygen compound and 0.2% to 3% surfactant owg (preferably 1% to 2%) at pH 7-12 (preferably 9-10) for a sufficient time (5-15 minutes, preferably 10-12 minutes) , there¬ by substantially removing sizing therefrom. An effective amount of a peroxygen compound for desizing is desirably 0.5% to 3% sodium persulfate owg or 2% to 6% hydrogen peroxide owg (100% basis) . This pro¬ cess has the advantage of counteracting BOD of the effluents compared to enzymatic design processes and of not discharging boron compounds to the environment as the sodium perborate desize processes do. Either sodium persulfate or hydrogen peroxide are particularly desirable to remove both starch and polyvinyl alcohol, the two types of sizing generally used for weaving denims.

As used herein all percentages are by weight; "owg" stands for on weight of goods.

The present invention is further illustrated by the following examples.

DECOLORIZATION PROCEDURE The sample preparation or decolorization process was simulated by placing a 4 inch square desized sample (approximately 5 grams) in a 500 mL Erlenmeyer

flask. 50 mL of 2% KMn0 ₄ solution was added to the flask and allowed to react 15 minutes at room temper¬ ature. The KMnθ4 solution was poured off and the denim sample rinsed with three 100 mL portions of deionized water.

In the neutralization process 100 mL of neutrali¬ zation liquor was added to the flask (Table I) . The flask was then agitated at room temperature for 10 minutes. The neutralization liquor was poured off and the sample rinsed with three 100 mL portions of deionized water. The degree of manganese removal (Mnθ2) was evaluated on an arbitrary scale of 10 (no decrease in color) to 1 (no perceptible residual Mnθ2 color) . For convenience a commercial 15% peracetic acid (15% PAA) , an equilibrium mixture, was employed as the source of the peracetic acid and part of the acetic acid and hydrogen peroxide. As used herein "15% PAA" stands for the equilibrium solution while "PAA" alone stands for the peracetic acid therein. For example, 200% owg 15% PAA provides 30% owg PAA. Additional acetic acid (HOAc) and hydrogen peroxide (H2O2) are added to provide the desired PAA:HOAc:H2θ ₂ weight or mole ratio. EXAMPLE 1

A series of seven runs were made in which the total amount of peracetic acid (PAA) was varied as presented in Table I.

The data indicate at least 10 parts by weight peracetic acid is required per 100 parts by weight goods (10% owg 15% PAA) to obtain a noticeable stain removal. The data also show that adding additional acetic acid to the equilibrium commercial 15% perace¬ tic acid enhances stain removal. In each of the formulations in Table I, the weight ratio of acetic acid to hydrogen peroxide is 1:1:5 or a 1:3.4 mole ratio.

EXAMPLE 2 A series of 3 runs, 2A, 2B and 2C were carried out in a similar manner to Example 1. Example 2 demonstrates it is desirable for the acetic acid to hydrogen peroxide molar ratio to be at least 1:1. As the molar ratio of acetic acid to hydrogen peroxide approached 1:1, the amount of Mn0 removal increase .

EXAMPLE 3 Effect of Temperature The procedure followed was the same as described in Example 1 with the exception that instead of agi¬ tating the sample by hand an oscillating bath was used. Because the oscillating bath did not provide as much mechanical action as manual agitation, a control sample was employed for the room temperature "control". The neutralization formulation used was 10% owg of peracetic acid (15% commercial peracetic) , or 10% owg 15% PAA, with an additional 2.5% owg acetic acid. The temperatures were 50°C and 65°C as well as room temperature (about 18°C) .

The sample produced at 50°C (Sample 3A) exhibited less nθ2 discoloration than the control sample (pro¬ duced at room temperature) and the sample produced at 65°C (Sample 3B) exhibited less discoloration than Sample 3A. As the temperature increased the amount of nθ2 discoloration decreased.

EXAMPLE 4 Effect of Surfactants The procedure of Example 1 and the formulation of Example 3 were used at 65°C to evaluate the effect of surfactants on the ability of the formulation to remove nθ2 discoloration. Concentration of each surfactant in the formulation was 1% owg. In each case, samples produced with formulations containing surfactants exhibited less Mnθ2 discoloration than the control sample produced without surfactant. All surfactants performed equally. The surfactants are

listed by trademark, manufacture, location and generic type.

4A. Vircoscour New-LF; Virkler Chemicals, Charlotte NC;anionic detergent. 4B. DowFax ^R 2A1; Dow Chemical, Midland MI; 47% dodecyl (sulfophenol) benzenesulfonic acid, disodium salt.

4C. Rapid Scour; Gist Brocades, Charlotte NC; proprietary formula. 4D. Silvatol PBS; Ciba Geigy, Greensboro NC; cryptoanionic ethylene oxide concentrate.

4E. Denwet; Denim Finishing Supplies Co., Mobile AL; proprietary formula.

4F. Lakol Scour CP-16; Dexter Chemical Corp., Bronx NY; phosphate ester/alkyl phenol ethoxylate, 8% hexylene glycol.

4G. Kieralon NB-OL Liquid; BASF Corp Chemicals Division, Parsippany NJ; mixture of anionic/nonionic surfactants, proprietary formula. EXAMPLE 5

Active Oxygen Content An experiment was conducted to determine if the performance of the peracetic acid formulation was due to the active oxygen content (AO) or to unique pro- perties of peracetic acid. The peracetic acid formu¬ lation was compared to an effective hydrogen peroxide formulation (Table III) . The procedure followed was that described in Example Three at 65°C. Formulation 5A was a peracetic acid formulation found to be effective in the laboratory. Formulation 5B was an hydrogen peroxide formulation used in a mill trial. Formulations 5C and 5D were modifications of Formula¬ tion 5B that increased the AO content to that of Formulation 5A. In Formulation 5C, the sample re- ceived two treatments whereas in 5D the sample received one treatment. The concentration of active ingredients added to each sample (5C and 5D) was the

sa e. Formulation 5E was identical to 5A, except the sample was treated in two steps instead of one.

The overall effectiveness of each treatment was determined by the amount of Mn0 discoloration that was removed. The results indicate that the greater the AO, more Mnθ ₂ discoloration was removed. How¬ ever, Formulations 5A, 5E, 5C and 5D had identical AO content yet 5A and 5E produced samples with the least Mnθ2 discoloration (a rating of 3) . Therefore, the peracetic acid present in these formulations was beneficial. The results also indicate that while the hydrogen peroxide formulations perform better in two steps, the peracetic acid formulations unexpectedly perform equally well in one or two steps. Eliminat- ing one step in production has the advantage of in¬ creasing throughput.

EXAMPLE 6 Peracetic Acid to Hydrogen Peroxide Molar Ratio This example demonstrates the importance of the molar (and therefore the weight) ratios of the com¬ ponents of the peracetic acid formulations. The procedure followed was that described in Example 3 at 65°C.

FMC peracetic acid solutions are produced in concentrations of 15% and 35%. These solutions are equilibrium mixtures of peracetic acid, acetic acid and hydrogen peroxide. The weigh ratios of each component in the 15% and 35% solutions differ and therefore the molar ratios of the components of each stock solution differ.

Formulations 6A through 6D were prepared so that the AO and acetic acid contents of the formulations were the same regardless of the stock solution used in preparation. Formulations 6A and 6B were the same with 6B delivered in two steps instead of one, this applies to Formulations 6C and 6D as well.

Formulations 6A and 6B produced samples that were almost entirely free of Mn0 ₂ discoloration, while Formulations 6C and 6D had no effect. As Table IV indicates, the difference in these formulations lies in the ratio of hydrogen peroxide to peracetic acid. In Formulations 6C and 6D, the molar ratio of hydro¬ gen peroxide to peracetic acid was less than one. Therefore for optimum efficacy the hydrogen peroxide to peracetic acid molar ratio should be at least 3 to 1.

Formulations 6E and 6F were reformulations of 6C modified to bring the hydrogen peroxide to peracetic acid molar ratio to 3 to 1. When the hydrogen per¬ oxide to peracetic acid ratio was changed, the formu- lation prepared from the 35% stock solution produced samples comparable to those produced by Formulations 6A and 6B.

Formulation 6C was unusual in that stain began to clear up and then both the solution and the sample turned purple indicating the formation of the per¬ manganate ion. The solution then cleared, but all of the manganese appeared to have been redeposited on the fabric. It is well known that peracetic acid is a powerful oxidant which will oxidize manganous ions to permanganate. It is also well known that per¬ acetic acid is an equilibrium mixture in which per¬ acetic acid and water are in equilibrium with acetic acid and H2O2.

■12-

TABLE I

WT RATIO OF MOLAR RATIO OF

COMPONENTS COMPONENTS

RUN FORMULATION (PAA:HOAc.H O ^ (PAA:HOAc.H Oo ^') RATING ⁴

1A 200% owg ¹ 15% PAA ² 1:1:1.5 1:1.4:3.4 2 1:2.5:3.4 1

IB 200% owg 15% PAA 1:2:1.5 30% owg HOAc ³

200% PAA 1:2:1.5 1:2.5:3.4

1C owg 15%

30% owg HOAc

2% owg chelating agent

1:2.5:3.4

ID 20% owg 15% PAA 1:2:1.5 3% owg HOAc 1:2.5:3.4

IE 20% owg 15% PAA 1:2:1.5

3% owg HOAc

(per each of 2 neutralization steps)

IF 40% owg 15% PAA 1:2:1.5 1:2.5:3.4

6% owg HOAc

(delivered in 1 neutralization step)

1G 10% owg 15% PAA 1:2:1.5 1:2.5:3.4 1.5% owg HOAc

1. owg - "on the weight of the goods"

2. PAA - Peracetic Acid

3. HOAc - Acetic Acid

4. Samples rated visually. 1 - no brown discoloration 10 - sample completely brown (untreated)

TABLE II

WT RATIO OF MOLAR RATIO OF

COMPONENTS COMPONENTS

RUN FORMULATION CPAA:HOAc^202 ^") IP __l__ ____H2^2l RATING ⁴

2A 10% owg ¹ 15% PAA ² 1:2:1.5 1:2.6:3.4 1.5% owg HOAc ³

2B 10% owg 15% PAA 1:2.7:1.5 1:3.5:3.4 2.5% owg HOAc

2C 10% owg 15% PAA 1:1:1.5 1:1.4:3.4

1. owg - "on the weight of the goods"

2. PAA - Peracetic Acid

3. HOAc - Acetic Acid

4. Samples rated visually.

1 - no brown discoloration 10 - sample completely brown

TABLE III

MOLAR RATIO WEIGHT RATIO

MOLES A.O. MOLES A.O. TOTAL MOLES PAA:HOAc: PAA:HOAc:

RUN TREATMENT FROM PAA FROM U2Q2 A.O. MOLES HOAc H20 H202 RATING 5A 10% owg 15% PAA 9.87xlO" ⁴ 3.38xl0" ³ 4,37xl0" ³ 3.41xl0" ³ 1:3.5:3.4 1:2.7:1.5 3

2.5% owg HOAc

(1 neutralization)

5B 1% owg H ₂ 0 ₂ (100%) 0.0 2.94x10" ³ 2.94x10" ³ 3.33xl0" ³ 0:1:0.88 0:1:0.5

2% owg HOAc

(per each of 2 neutralizations)

5C 1.5% owg H ₂ 0 ₂ (100% 0.0 4.37xl0" ³ 4.37xl0" ³ 4.50xl0" ³ 0:1:1 0:1:0.56

2.7% owg HOAc

(per each of 2 neutralizations) I

5D 3% owg H ₂ 0 ₂ (100%) 0.0 4.37xl0 ^"3 4.37xl0 ^"3 4.50x10 -3 0:1:1 0:1:0.56

5.4% owg HOAc

(1 neutralization)

5E 5% owg 15% PAA 9.87xl0" ⁴ 3.38xl0 ^"3 4.37xl0 ^"3 3.41xl0" ³ 1:3.5:3.4 1:2.7:1.5

1.5% owg HOAc

(per each of 2 neutralizations)

1. Samples rated visually. 1 - no brown discoloration 10 - sample completely brown

TABLE IV

MOLAR RATIO WEIGHT RATIO

MOLES A.O. MOLES A.O. TOTAL MOLES PAA:HOAc: PAA:HOAc:

RUN TREATMENT FROM PAA FROM H2Q2 A.O. MOLES HOAc H2Q H2Q2 RATING

6A 10% owg 15% PAA 9.87X10" ⁴ 3.38X10 ^"3 4.37X10 ^"3 6.36X10 ^"3 1:6.4:3.4 1:5:1.5 6% owg HOAc (1 neutralization)

6B 5% owg 15% PAA 9.87X10" ⁴ 3.38X10" ³ 4.37X10" ³ 6.36X10 ^"3 1:6.4:3.4 1:5:1.5 3% owg HOAc (per each of 2 neutralizations) J1

6C 13% owg 35% PAA 3.06xlO" ³ 1.31xlO" ³ 4.37X10" ³ 4.29xl0 ^"3 1:1.4:0.43 1:1:0.2 10* ' (1 neutralization)

6D 6.5% owg 35% PAA 3.06xl0" ³ 1.31xl0" ³ 4.37X10" ³ 4.29xl0 ^"3 1:1.4:0.43 1:1:0.2 (per each of 2 neutralizations)

6E 13% owg 35% PAA 3.06xl0" ³ 1.04xl0 ^"2 1.35xl0" ² 4.29xlO ^"3 1:1.4:3.4 1:1:1.5 6% owg H 0 (100%) (1 neutralization)

6F 13% owg 35% PAA 3.06xl0 ^"3 1.04xl0 ^"2 1.35xl0 ^"2 1.04xl0 ^"2 1:3.4:3.4 1:2.6:1.5 6% owg H ₂ 0 ₂ (100%) 7% owg HOAc (1 neutralization)

1.Samples rated visually. 1 - no brown discoloration 10

*Turns purple