Light stabilization of dyed leathers

The instant invention relates to a dyed leather in which is incorporated an effective amount of a transparent inorganic pigment and which are optionally coated with a transparent polymer, or which are coated with a transparent polymer comprising an effective amount of an transparent inorganic pigment. The instant invention also relates to a process for the protection of dyed leathers against degradation and fading by the action of actinic radiation, which comprises the treatment of dyed leathers during the production of leather with an effective amount of an inorganic transparent pigment.

It is known that dyestuffs are subjected to a permanent actinic radiation in articles of daily use. The short wavelength radiation and particularly UV radiation leads to a continuous degradation of dyestuffs in dyed articles and therefore to a fading of color intensity. Leathers can be treated therefore with UV absorbers in the wet end or finishing processes during or after dyeing to impart some protection against decomposition of dyestuffs and fading. This kind of protection is considered as insufficient especially under the aspect of long term stabilization.

Transparent inorganic pigments and coating compositions containing said pigments for depositing layers on surfaces and their application as coloring materials are known since a long time. In US-A-2558302, US-A-2558303 and US-A-2558304 is described the preparation of various transparent iron oxide pigments and coating materials containing these pigments for use in transparent coatings and incorporated in amounts to obtain colored and decorative coatings. EP-A1 -0704498 discloses a process for the preparation of highly transparent yellow iron oxide pigments with increased color intensity for use in colored lacquers, paints and plastics. The use of inorganic transparent pigments as light stabilizers for dyestuffs or for surfaces colored with dyestuffs is not described in the prior art.

In WO 2005/061602 A1 is described a composition comprising an acid-free polymer on the basis of acrylic or methacrylic esters and which contains nanoparticulate zinc

oxide for use as coatings to protect material surfaces from degradation under UV radiation.

In US 2004/0099975 A1 are disclosed dispersions of surface modified inorganic oxide nano-sized particles, which can be used in the manufacture of inorganic- organic polymeric functional materials, especially by incorporation in thermoplastic polyesters to produce anti-UV polyester fibers.

It has been surprisingly found that inorganic transparent pigments are even in low amounts effective light stabilizers for colored substrates, such as furs and leather, which are dyed with organic dyestuffs, whereby the original aspect of the surface such as the color is substantially maintained. It has also been surprisingly found that the long term durability of the protection against degradation and fading is very high and bleaching is not observed for a longer time period. It was also found that color uniformity of dyed leathers can be improved with a coating containing transparent inorganic pigments.

A first object of the invention relates to a finished leather, which is colored with an organic dyestuff, characterized in that it comprises near its external surface a total amount of transparent inorganic pigment of at least 0.01 g/m ² , a) that transparent inorganic pigment being incorporated in the external surface of the leather and that surface optionally comprising a transparent polymer coating; or b) that transparent inorganic pigment being contained in a transparent polymer coating covering the external surface of the leather; or c) that transparent inorganic pigment being incorporated in the external surface of the leather and contained in a transparent polymer coating covering the external surface of the leather.

The polymer coatings according to b) and c) preferably do not contain a transparent ZnO pigment, when the polymer is an acid-free acrylic resin based on acrylic and methacrylic esters.

A finished leather means in the context of the invention a dyed leather ready for end use such as upholstery, where the leather is additionally treated with a transparent inorganic pigment during production processes of the leather.

The total amount of transparent inorganic pigment is preferably at least 0.05 g / m ² and more preferably at least 0.1 g / m ² . The transparent inorganic pigment is also applied in amounts which substantially maintain the surface aspect of the finished dyed leather substrate. The upper amount may be at most 2O g / m ² , preferably at most 15 g / m ² , more preferably at most 1O g / m ² , and especially preferred at most 5 g / m ² . Examples of preferred ranges are 0.01 to 20 g / m ² , more preferably 0.05 to 15 g / m ² , most preferably at most 0.05 to 10 g / m ² , especially preferred 0.1 to 5 g / m ² . When amounts of transparent inorganic pigments within the above upper values or even higher amounts are used, an interaction with the dyestuff may lead to mixed colors aside the light stabilization and which may be desirable to provide special coloring effects.

Particularly, the transparent inorganic pigment may be used in amounts of additives, which may be in a range of 0.1 to 3 g / m ² , preferably 0.1 to 2 g / m ² , providing light stabilization substantially without mixed color formation.

Transparent inorganic pigments are well known and described in the prior art and are commercially available. Transparent pigments are materials which are insoluble or have a low solubility in a matrix (such as solvents, binders or plastics) to be colored. Transparent pigments are defined either by a particle size of less than 0.1 μm or 100 nm (mean diameter) or by a specific surface area of at least 80 m ² /g (transparent), detected according to BET method, or at least 100 m ² /g (highly transparent), see EP-A1-0 704 498, page 2, lines 5 to 10. The specific surface is used as a more convenient definition, when the morphology of manufactured pigments provides needles or longer columns. In a preferred embodiment the particle size of the transparent inorganic pigments (mean diameter) is from 10 to 100 nm, especially preferred from 10 to 80 nm and in particular 10 to 60 nm. In another preferred

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embodiment of the invention at least 80 percent by weight of the transparent inorganic pigments comprises particle sizes in the range of 10 to 100 nm and more preferably, at least 80 percent by weight of the transparent inorganic pigments comprises particle sizes in the range of 10 to 60 nm.

Suitable transparent inorganic pigments may be prepared from natural or synthetic materials. Pigments with the required mean diameter or specific surface area can be obtained by dry or wet milling operations, using ball or bead mills. Another possibility is the direct manufacture of such small size transparent pigments with special precipitation processes as described in US-A-2558302, US-A-2558303, US-A-2558304 and EP-A1 -0704498 for the direct preparation of iron oxide and yellow Fe(O)OH pigments.

Suitable pigments are selected in particular from those which absorb or reflect UV- light. Inorganic pigments may be selected from carbon black, alumina silicates with carbon black, metal oxides, metal sulfides, basic metal oxides or sulfides, mixed metal oxides, mixed metal oxides and metal sulfides, metal salts (such as carbonates, sulfates, aluminates, silicates, titanates, chromates, phosphates, and metal complexes or metal complex salts. Some specific examples are zinc white, rutile, anastase, barites, chalk, bone white, kaolin, gypsum, lithopone, lead white, vine black, mineral black, bone black, ivory black, magnetite, vermillion (cinnabar), red ochre, realgar, molybdenum orange, molybdenum red, red lead (minium), litharge, chrome red, cadmium red, sienna (burnt), cadmium orange, ochre (goethite), zinc yellow, strontium yellow, tin disulfide, massicot, cobalt yellow (aureolin), chrom yellow, chrome yellow, cadmium yellow, lead tin yellow, barium yellow, orpiment, lead antimonite yellow, emerald green, basic copper(ll)arsenite, pseudo-malachite, phthalocyanine green, permanent green deep, malachite, cobalt green, verdigris (basic), green earth (a mix of celadonite and glauconite), viridian, chromium oxide, atacamite, ultramarines [e.g. lazurite (lapis lazuli)], smalt, Prussian blue, posnjakite, phthalocyanine blue, manganese blue, cobalt blue, verdigris, cerulean blue, Han Purpur and Han Blue (copper barium silicates), azurite and Egyptian blue.

In a preferred subgroup the transparent inorganic pigments are selected from colored pigments, more preferably from black, brown, red, orange and yellow pigments. Transparent iron oxide pigments, such as yellow, red or brown iron oxide pigments and carbon black are preferred in view of their excellent UV stabilizing effectiveness, availability and due to economic reasons.

Transparent polymers are known in the art in a large diversity. The coating may be formed from transparent addition polymers (such as polyurethanes) or condensation polymers (such as polyesters), or from polymerisates (such as poly(meth)acrylic acid esters). The coating may provide additionally other desired properties such as soiling resistance, water repellence and migration prevention.

Polymerisates may be composed from one or more than one olefinically unsaturated, unsubstituted or substituted monomer, such as ethylene, propylene, butylene, vinyl chloride, vinyl dichloride, tetrafluoroethene, vinyl ethers, vinyl esters, vinyl amides, styrene, (meth)acrylnitrile, (meth)acrylic acid esters or (meth)acrylic acid amides. The polymerisates may be additionnally composed from polyenes to provide crosslinked polymerisates, which possess in general higher mechanical stability. Suitable cross- linkers are for example butadiene, isoprene, divinyl styrene, esters from polyols such as diols, triols or tetrols and (meth)acrylic acid.

Suitable condensation polymers are polyesters, polyamides, polyethers and polycarbonates. Polyesters may be composed from at least one dicarboxylic acid, diol and lactone, including polylactons. Polyamides may be composed from at least one dicarboxylic acids, diamine and lactam, including polylactam. Polyesters and polyamides may be crosslinked with smaller amounts of at least trifunctional alcohols, amines or carboxylic acids. Polyethers may be composed from diols such as ethylene glycol, propylene glycol or butylenes glycol or corresponding alkylene oxides respectively (polyalkylene glykols). Polycarbonates may be composed of phosgene or carbonates and diols.

Suitable addition polymers are polyethers, polyurethanes and polyureas. Polyethers may be composed from at least one di-epoxides such as bisphenol-A diglycidyl ether and at least one glycol. Crosslinking may be realized with polyepoxides having more than two epoxide groups and/or triols or tetrols. Polyurethanes and polyureas may be composed from at least one diisocyanate and at least one diols or diamine or from mixtures of diols and diamines. Polyurethanes and polyureas may be crosslinked with small amounts of polyurethanes, alcohols or polyamines having at least three urethane, hydroxyl, amine or amidine groups.

It is possible to use mixtures of different polymers such as polymerisates and polyurethanes. In a preferred embodiment, the polymer is a polymerisate, a polyurethane or a mixture thereof.

The thickness of the coating may range from 1 to 100 μm, preferably 2 to 50 μm and in particular 3 to 20 μm.

Coating or coating layer means in the context of the invention a continuous film on the leather surface or a film on collagen fibers present on the surface of the leather, which substantially depends on the used amounts of coating materials.

Materials for the preparation of transparent polymer coatings are also well known in a large diversity, even commercially available or said materials may be prepared by analogous methods. It depends on the desired coating method, what types of materials and film forming chemicals are used.

The leather according to the invention, which is stabilized against light degradation and fading of color can be prepared by known leather treatment and coating techniques.

Another object of the invention relates to a process for the stabilization of finished leather, which is colored with an organic dyestuff, against light degradation and

fading of color, comprising applying near its external surface a total amount of transparent inorganic pigments of at least 0.01 g / m ² by a) treatment and thereby incorporating in said leather a transparent inorganic pigment in the external surface and optionally coating then said surface with a transparent polymer; or b) coating on the external surface of the leather a transparent polymer containing a transparent inorganic pigment; or c) treatment and thereby incorporating in said leather a transparent inorganic pigment in the external surface and coating the external d) surface with a transparent polymer containing a transparent inorganic pigment.

The polymer coatings according to b) and c) preferably do not contain a transparent ZnO pigment, when the polymer is an acid-free acrylic resin based on acrylic and methacrylic esters.

The treatment according to a) and according to c) can be carried out in preparing under stirring an aqueous dispersion of the desired amount of a transparent inorganic pigment optionally in the presence of a surfactant, which are well known in a big diversity. The pH value of the dispersion may be weakly acidic to neutral, for example from 3.5 to 7. The water may contain organic and water-miscible solvents such as alcohols and alcohol-ethers, e.g. methanol, ethanol, propanol, ethylene glycol, propylene glycol, butylenes glycol, ethylene mono-methyl ether and ethylene mono- ethyl ether. The dispersion is stable due to the very small particle size. The treating temperature may range from about room temperature to about 60 ⁰ C. The leather is placed in this dispersion and treated under moving the dispersion and leather e.g. in a turning drum for a time sufficient that pigment particles penetrate in the surface of the dyed leather. The treatment according to a) and according to c) can be carried out (1 ) as a separate process step during leather production after tanning or (2) in combination with wet end process steps such as washing, neutralization, pre- fatliquoring, retanning, dyeing, fixation, or fatliquring. The treated leather is then removed and dried at air, occasionally at elevated temperature. If desired, the

surface of the treated leather is coated then with a transparent polymer, applying polymer coating compositions and methods as described for b), but without transparent inorganic pigments. A desired coating is preferably carried out as a last step during leather production.

In a first coating method, a solution, emulsion or dispersion of a suitable polymer in an aqueous, organic-aqueous or organic solvent, optionally containing a transparent inorganic pigment, may be used and applied to the leather surface. The solvent is then evaporated at room or elevated temperature (e.g. up to 60 ⁰ C) and optionally under vacuum. The solid coating is formed either by continuous deposition of the dissolved polymer or deposition and coalescence of emulsified or dispersed polymer particles and drying of the deposition on the leather surface. Aqueous emulsions or dispersions are preferred systems in this coating method according to the invention.

In a second method, a solution, emulsion or dispersion of one or more polymers and additionally at least one cross-linker for the polymers in an aqueous, organic- aqueous or organic solvent, optionally containing a transparent inorganic pigment, may be used and applied to the leather surface either alone or in combination with an emulsified or dispersed solid binder. These systems are used when the formation of a solid coating on the leather surface is provided by deposition of a layer, drying to a solid layer and cross-linking of the polymers in said solid layer. This method is preferred in the process according to the invention. The type of used cross-linkers depends on the functional groups of the polymer and may be selected from the group of polyaziridines, carbodiimides, polyols with 2 to 4 hydroxyl groups, poly amines with 2 to 4 amine groups, or polyisocyanates with 2 to 4 isocyanate groups. Preferred are polyurethanes with pendent hydroxyl groups in the backbone or polymer chain ends and di- or triisocyanates as crosslinkers. Preferred binders are polymers from one or more unsubstituted or substituted ethylenically unsaturated monomers such as (meth)acrylic acid, (meth)acrylic acid esters or amides, vinyl ethers and esters, styrene and C2-C2o-olefins. Aqueous, optionally water-miscible solvent containing systems are preferred. Cross-linking may be initiated by thermal curing (e.g.

temperatures up to 100 ⁰ C) after evaporation of solvents and drying to a solid layer. Emulsions and dispersions can additionally contain surfactants.

A preferred emulsion or dispersion for the second method comprises an aqueous dispersion of polyurethanes with hydroxyl or isocyanate groups in the polymer backbone and/or polymer chain ends and cross-linkers containing isocyanate or hydroxyl groups respectively, and additionally an emulsion of dissolved poly(meth)acrylates as binder. The amount of polyurethane in the dispersion may be from 5 to 30 percent by weight, of the cross-linker 0.5 to 5 percent by weight, and 20 to 50 percent by weight of a poly(meth)acrylate emulsion.

The transparent inorganic pigment is dispersed in the polymer coating compositions for b) and c). The amount of pigment in the coating composition may range from 0.1 to 40 percent by weight, preferably 1 to 30 percent by weight, 2 to 20 percent by weight, referred to the weight of the composition.

Coating may be performed by various methods, such as dipping, painting, roller coating, curtain coating, transfer-pressing or spraying. Coating may be carried out in one step or stepwise applying the same or different compositions repeatedly. Drying of coated leathers and polymerization can be performed in tunnel ovens, optionally equipped with UV radiation sources.

The leather stabilized according the invention may be used for the manufacture of leather goods in general, for example furniture or automotive upholsteries for seats or car interior linings (such as soft trims or trim panels), garments, shoes, handbags, suitcases or briefcases.

Another object of the invention is the use of transparent inorganic pigments for the stabilization of finished leathers, dyed with organic dyestuffs, against light degradation and fading of color.

Transparent ZnO pigments are preferably not included in the above use of transparent inorganic pigments, when a transparent ZnO pigment containing polymer coating of an acid-free acrylic resin based on acrylic and methacrylic esters is used.

The following example explains the invention.

Example 1 : a) Preparation of dyed leather

The substrate is 12 kg of bovine wet blue, 1.8-1.9 mm. Values in % are based on shaved weight. Treatment is carried out in a turning drum.

a1 ) Wetting-back

200% water 35°C, 0.3% anionic surfactant based on sulphated fatty alcohols, 0.1 % formic acid (85%), 20 minutes, pH 3.5. Drain float.

a2) Rechroming

100% water 30 ⁰ C, 4% liquid chromium containing condensation product of phenolic sulphonic acids, 3% chrome pwd. 26/33, 2% emulsion based on a sulfosuccinic ester combined with natural and synthetic fatty substances, 40 minutes, pH 3,45; 3% neutralizing agent based on aromatic sulphonic and aliphatic dicarboxylic acids, 20 minutes, pH4; 1 % sodium formate, 0.5% sodium bicarbonate 60 minutes, pH 4.7; kept overnight. Drain float.

a3) Washing

200% water 30 ⁰ C, 10 minutes; drain float.

a4) Neutralization, pre-fat liquoring, re-tanning, fixation

Neutralization: 100% water, 35°C, 3% neutralizing agent based on aromatic sulphonic and aliphatic dicarboxylic acids, 20 minutes, pH 5.3, 1 % sodium formate,

1 % sodium bicarbonate, 60 minutes, pH 6.0.

Pre-fatliquoring: 2% combination of natural phospholipids and emulsifiers, 3% emulsion based on a sulfosuccinic ester combined with natural and synthetic fatty substances, 20 minutes.

Re-tanning: 3% amphoteric acrylic copolymer for retanning, filling and tightening effect, 4% retanning agent based on a condensation product of phenolic sulphonic acids and aromatic oxysulphones, 4% retanning agent based on a condensation product of sulphone and sulphonic acids of cyclic compounds, 30 minutes; 3% acrylic acid / acrylic acid alkylester copolymer for softening effect, 30 minutes; 10% tara pwd., 2% mimosa pwd. 60 minutes. Fixation: 1 % formic acid (85%) 30 minutes, pH 4; drain float.

a5) Washing (two times) 200% water 50 ⁰ C, 10 minutes.

a6) Fat liquoring and fixation

100% water, 50 ⁰ C, 8% emulsion based on a sulfosuccinic ester combined with natural and synthetic fatty substances, 2% combination of natural phospholipids and emulsifiers, 60 minutes, pH 4.3; 1 % formic acid (85%), 30 minutes, pH 3.5. Drain float.

a7) Washing

200% water, 20°C, 10 minutes.

a8) Dyeing and fixation

50% water, 20 ⁰ C, 2% liquid leveling agent based on a condensation product of aromatic sulphonic acids, free of formaldehyde, 10 minutes; 0.9 % dyestuff mixture of A BR 355, A YE 194 and A OR 162, 0.55% dyestuff A BR 452, 0.8% dyestuff A BR 432, 60 minutes; 150% water, 50°C, 10 minutes; 1.5% formic acid (85%), 15 minutes, 1.5% formic acid (85%), 15 minutes, pH 3.5; drain float and washing.

a9) Top dyeing

100% water, 40°C, 0.04 dyestuff mixture of A BR 355, A Ye 194 and A OR 162, 0.15% dyestuff A BR 432, 0.04 dyestuff A BR 425, 20 minutes; 100% water, 55°C, 10 minutes; 0.5% formic acid (85%), 20 minutes. Drain float, washing with cold water. The obtained brown leather is horsed up, set out, wet toggled, conditioned and staked.

b) Transparent inorganic pigment treatment of brown leather

Three coating composition are prepared in dispersing or emulsifying the components in water under high speed stirring and stirring after addition of all components for 10 minutes: a) 20 percent by weight of an agueous dispersion of 34 weight/% transparent inorganic pigment (a1 ) Red Iron Oxide SYN-OX HTR-870, (a2) Yellow Iron Oxide SYN-OX HTR-810 or (a3) Yellow Iron Oxide SYN-OX HTR-820 (Hoover Color Corporation, Hiwassee, VA. USA); b) 15 percent by weight of a dispersion of 40 weight/% aliphatic polycarbonate isophorone diisocyanate polyurethane in water; c) 35 percent by weight of an agueous emulsion of 40% poly acrylate; d) 1.5 percent by weight HDI poly isocyanate crosslinking agent; e) 5.0 percent by weight of an agueous polysiloxane emulsion (a feel additive); f) 23.5 percent by weight of distilled water.

The compositions are coated in two different amounts on the leather surface by atomized air spraying. The coated leather substrate is then dried at 60 ⁰ C in air and cross-linked at a temperature of between 20 to 28°C. The finished leather substrates contain a total amount of (a) 0.43 g/m ² and (b) 0.86 g/m ² transparent inorganic pigment on the surface.

The leather shows a high color uniformity and the original, natural appearance is maintained. Color fastness is determined according to standard method XENOTEST DIN 75202/2 at a BST temperature of 83 ⁰ C, and at a relative humidity of 20% for 83 hours. The color fastness is determined pursuant to ISO 105A03 GRAY SCALE. Result: The coated leathers are rated 6, which means no color change, whereas a blank of the used leather substrate is rated 2, meaning strong color change.