Process for the use of a re-tanning composition comprising rice husk

Abstract

The present invention provides a process for the use of a re-tanning composition comprising rice husk in the leather process that provides good re-tanning behaviour.

Description

[0001] Title: Process for the use of a re-tanning composition comprising rice husk

[0002] The present invention relates to a novel re-tanning agent selected from rice husk and a process for the use of rice husk as re-tanning agent or as a component in a re-tanning composition in the leather manufacturing process.

[0003] Leather is a durable, flexible material created via the tanning of animal rawhide and skin. The leather manufacturing process is divided into three fundamental sub-processes: preparatory stages, tanning and crusting.

[0004] The present invention relates to the sub-process of tanning, especially the re-tanning part thereof.

[0005] In the preparatory stages, hide or skin is prepared for tanning. After trimming, animal skin is soaked to remove salts and other solids, while restoring moisture when the skin was first dried. Then, the flesh side of the wet skin is scraped to remove any remaining traces of flesh or fat, and the skin is optionally dehaired. After an optional bating and pickling step, the skins are subjected to tanning. Other potential steps that may be part of the preparatory stages include preservation, liming, splitting, reliming, deliming, degreasing, frizzing, bleaching and depickling.

[0006] Tanning is the process of preserving the skins by converting the protein, via crosslinking the collagen fibres, of the raw hide or skin into a stable material that does not putrefy and provides tanned leathers with satisfactory properties, such as high shrinkage temperatures Ts, suppleness and suitability for subsequent processing such as neutralization, re-tanning, fatliquoring, dyeing, finishing.

[0007] Tanning is dominantly carried out by treatment of the hides with chromium sulphate (giving so-called wet-blue leather) or by the use of organic reactive tanning agent as aldehydes, especially glutaraldehyde (resulting in wet-white leather). In rare cases the tanning is carried out with vegetable tanning agents - as traditionally done - or by use of synthetic tanning agents (syntans), or other conventional techniques. The product prepared in this subprocess is an intermediate since it is not sufficient to obtain the desired characteristics specified by the customer. The tanned hides are therefore further treated with various products. This process is called re-tanning. Retanning affects the feel of the leather, the dyeability, fullness of the leather, the fineness of the grain and the stability of grain and other factors such as light fastness, to suit characteristics required for the final leather article - whether for automotive or aviation seating, footwear, garments or bags and leather goods. Re-tanning includes dyeing to give colour and fatliquoring to add softness, fullness and touch. Once re-tannage is complete, the leather is known as “crust”.

[0008] Vegetable tanning agents were the first tanning agents. They are now mostly used in the re-tanning, because of the nowadays wide acceptance of chromium sulphate or glutaraldehyde as tanning agents. Common vegetable tanning agents are Mimosa, obtained from the bark of the Acacia tree, and Tara, obtained from the fruit of the Tara bush. They can impose softness and limited filling of the collagen structures to leathers (Hans Herfeld, "Library of Leather; Volume 3: Tanning Agents, Tanning and Retanning", Frankfurt 1985, page 44). Usually, vegetable tanning agents lack fastness properties, such as resistance to light or resistance to heat induced ageing.

[0009] The term syntan refers to the range of synthetic tanning agents. The first syntans were made by condensation of phenol sulfonic acid and formaldehyde (E. Stiasny, 1911, Austrian Patent Nr. 58405). While these syntans were initially used as dispersers and auxiliaries for vegetable tannins, they could be applied to replace some or even all vegetable tannins after further development of their chemistry. US 1841840 describes the incorporation of urea into the polycondensation of phenol sulfonic acid and formaldehyde, by which such a further development was achieved, enabling obtaining leathers with increased technical requirements like fastness properties concerning light or heat induced ageing. Because of the wide acceptance of chromium sulphate or glutaraldehyde as tanning agents, syntans are now mainly used in the re-tanning process, where they help to structure and fill the crosslinked collagen fibres. Unfortunately, syntans can contain a residual amount of free formaldehyde or undesired side products as Bisphenol S or F, which means that they should be handled and used with care due to safety reasons.

[0010] In many applications syntans and vegetable tannins are applied together since the performance of vegetable tannins alone is considered insufficient. The syntans generally have higher fastness properties and beyond that have dispersing properties. This helps to support the even distribution of vegetable tannins and other leather chemicals like fillers, dyes, and fatliquors (DE 1142173).

[0011] There is currently a big driving force for companies, and the chemical industry in particular, for corporate responsibility and the use of sustainable or renewable sources of raw materials. It is of particular interest to use biobased raw materials that do not compete with usage of those biobased raw materials as food source, and hence the usage of biobased waste streams is particularly advantageous.

[0012] Rice husks are the hard protecting coverings of grains of rice. Another name for rice husks is rice hulls. Rice husks are part of the rice seed. The husk protects the grain during the growing season from pests. The husk is formed from hard materials and is hard to eat or swallow and mostly indigestible to humans because of its enriched fibre components. Using a rice hulling machine, during the milling processes, the husks are removed from the raw grain to reveal whole brown rice, which is then usually milled further to remove the bran layer, resulting in white rice. Rice husks are an inexpensive byproduct of human food processing and are used in several applications, such as pillow filling materials, insulating material, as component in low-density fibreboard or cardboard, and rice bran oil can be extracted from rice husks. Combustion of rice husks yields rice husk ash, which is a potential source of amorphous reactive silica, which has a variety of applications in materials science. Most of the rice husk ash is used in the production of Portland cement.

[0013] EP3257955B1 (TFL; 2016) describes a process for tanning, pretanning or re-tanning leather using an aqueous composition or a powder composition comprising at least one carboxymethyl cellulose and / or its salts.

[0014] JP2009067847 (Taneka; 2006) describes a re-tanning agent using a collagen powder comprising crosslinked regenerated collagen.

[0015] The present invention provides a re-tanning agent composition comprising rice husks. The invention further provides a process to use rice husk as a re-tanning agent or as a component in a re-tanning composition that provides good re-tanning behaviour.

[0016] It was found that rice husk can be used as re-tanning agent and are thus able to replace syntans or other re-tanning agents in re-tanning of leather.

[0017] The object of the present invention is to provide a re-tanning agent composition comprising rice husks and a process for the use of said re-tanning composition comprising rice husk in a leather process that is free of formaldehyde and bisphenols and that provides good re-tanning behaviour.

[0018] The inventors discovered that rice husk can be applied on leather, such as via an industry standard procedure, for re-tanning. Surprisingly, it was found that leathers treated with rice husk, gave similar or better properties to the leather than syntans or vegetable tannins, concerning softness, fullness, tightness, smoothness, bleaching, light fastness and heat fastness while creating no bisphenol or formaldehyde problem. An analysis of toxic formaldehyde or bisphenols is usually not even required for these re- tanning compositions, because none of the ingredients contains or could release formaldehyde or could form bisphenols. The use of rice husk represents a novel non-toxic, renewable alternative re-tanning agent, sourced from a readily available byproduct of the rice industry, compared to conventional re-tanning agents in the leather industry sourced from oil based phenol formaldehyde condensates or from vegetable sources that are less readily available, such as conventional Mimosa, obtained from the bark of the Acacia tree, and Tara, obtained from the fruit of the Tara bush.

[0019] A re-tanning composition comprising rice husk may also comprise other components, such as lignin, starch, chitin, kaolin, Tara, or proteins or conventional re-tanning agents. Preferably, the re-tanning composition comprising rice husk of the present invention comprises rice husk in an amount of between 20 and 100 weight percent based on the total (dry) weight of the re-tanning composition, and wherein the remaining amount may comprise other re-tanning agents. If these other re-tanning agents in a re-tanning composition are fossil-based, then an additional advantage of adding rice husk is that this adding of rice husk increases the biobased content of such a re-tanning composition, thereby positively influencing the transition from a linear to a circular economy.

[0020] In an advantageous embodiment, the re-tanning composition of the present invention is solid at ambient conditions. Such a solid composition may be in the form of a powder.

[0021] According to preferred embodiments, the re-tanning composition of the invention comprises milled rice husk wherein the rice husk is milled and optionally dried to obtain a rice husk powder having a moisture content of maximum 20 weight%, preferably maximum 10 weight % based on the total weight of the powder and an average particle size in the range from 50 gm up to 1000 jim, preferably in the range from 50 jim up to 750 jim as measured according to ISO 13320. According to preferred embodiments, the re-tanning composition of the invention is a composition wherein at least part of the re-tanning agents is selected from rice husk (powder) and the remaining part is selected from state of the art synthetic and / or vegetable re-tanning agents. The amount of rice husk in said re-tanning composition is in the range 10 up to 99 wt%, preferably in the range 20 up to 90 wt% and most preferred in the range 30 up to 80 wt% based on the total (dry) weight of the re-tanning composition. The amount of additional re-tanning agents in said re-tanning composition is in the range between 0 wt% up to 90 wt%, preferably between 0 wt% up to 80 wt%, more preferably between 0 wt% up to 70 wt% calculated on the total solid weight of the re-tanning composition. Preferably, the re-tanning composition comprises between 10 wt% up to 99 wt%, preferably between 20 up to 90 wt% and most preferred in the range 30 up to 80 wt% rice husk as re-tanning agents and between 1 up to 90 wt%, preferably in the range 10 up to 80 wt% and most preferred in the range 20 up to 70 wt% additional re-tanning agents based on the total (dry) weight of the re-tanning composition. A particular preferred re-tanning composition according to the invention comprises vegetable re-tanning agents selected from rice husk and tara thereby giving synergetic effects towards lighter colour of the leather. The ratio of rice husk to said additional vegetable retanning agents is in the range 10 / 90 to 90 / 10, preferably in the range 30 / 70 to 70 / 30.

[0022] According to preferred embodiments, the re-tanning composition of the invention is a state of the art re-tanning composition wherein rice husk according to the invention is added. The amount of rice husk added depends on the type of the state of the art re-tanning composition and the desired re-tanning effect. For example, rice husk can be added to the state of the art re-tanning composition to improve the degree of bleaching during the re-tanning. In the context of the present invention, between 2 and 15 weight percent, preferably between 4 and 12 weight percent and most preferably in between 6 and 10 weight percent, relative to the weight of the leather, of rice husk is used.

[0023] According to preferred embodiments, the re-tanning composition of the invention may further comprise additives selected from such as lignin, starch, chitin, kaolin,... preferably in a concentration between 0 weight percent and 50 weight percent based on the total weight of the re-tanning composition. An industry standard procedure for re-tanning comprises treating a tanned leather, such as wet blue, with water, sodium formate and sodium bicarbonate for a period of time resulting in a float with a pH of below neutral, after which the re-tanning composition is added followed by further turning of the tanning drum for a certain period of time, followed by fixation with an acid, such as, but not limited to formic acid, followed by discharging the float and washing with water. Afterwards the leather is dried, such as by hang drying at room temperature without vacuum, or in a vacuum chamber. The re-tanning composition is added in an amount of between 2% and 15%, and preferably in an amount of between 4% and 12%, and most preferably in an amount of between 6% and 10%, wherein the percentages refer to the weight percentage of the non-volatile part of the re-tanning composition compared to the weight of the leather.

[0024] The lightfastness of leather can be determined according to ISO 105-B02, in for example a sun tester. Re-tanned leathers are exposed to light for usually 24 to 72 hours. The resulting yellowing can be quantified in a spectral photometer, or assigned via the blue scale method, assigning numbers from 1 to 8 with increasing lightfastness, wherein a score of 6 or above is considered ‘good’.

[0025] Heat induced ageing of leather can be determined by exposing leathers to heat at various temperatures for various hours. Leather samples can be, after exposure to heat, evaluated according to the grey scale with numbers from 1 to 5, according to ISO 105-A02. Larger numbers indicate more stability against heat, wherein a score of 3.5 or higher is considered ‘good’.

[0026] Fullness is an aesthetic quality parameter, organoleptical characteristic, defined by the spacing between the fibres, suggesting (by the tactile sense) a larger or smaller amount of fibres per area. It is the result of a good filling of the interfibrillar spaces and an adequate lubrication of the fibres avoiding their agglomeration. In colloquial words: a full leather is the one that is round to the touch and an empty leather (contrary to full) is a flat, thin leather. Fullness of re-tanned leathers is dominantly determined via haptics.

[0027] Tightness is an aesthetic quality parameter, organoleptical characteristic, defined by the amount of wrinkling or creasing when the leather is flexed grain-inwards (by visual assessment), result of a good or bad adherence between the Grain layer to the underlying Dermis. Tightness can be influenced since the from the breed of the animal to the end of the retanning, passing by distribution of the products, mechanical action, drying, etc... and is desirable for all the articles, even than those which will be milled. The challenge is to keep the layers together without hardening the leather (Hard leathers = normally tight grain). The smoother the layer remains upon bending, the tighter the bended leather is. The more wrinkles observed, the lower the tightness.

[0028] Softness, fullness and tightness of re-tanned leathers are determined via haptics and are generally graded with a number, wherein a lower number is better.

[0029] Dry weight as referred herein refers to the mass of materials excluding the mass of any water, liquid or moisture present within the materials. Dry weight and (dry) solid weight refer to the same weight and have an equal meaning herein.

[0030] Average Particle Size as referred herein refers to the average particle size as measured with Laser Diffraction according to ISO 13320. The re-tanning composition comprising rice husk can be used to prepare leathers for all applications, for example shoe, furniture, car, clothing and bag leathers.

[0031] Any kind of leather which is conventionally treated is suitable to be treated by using a re-tanning composition comprising rice husk, particularly grain leather (e.g. nappa from sheep, goat or cow and box-leather from calf or cow), suede leather (e.g. velours from sheep, goat or calf and hunting leather), split velours (e.g. from cow or calf skin), buckskin and nubuck leather; further also woollen skins and furs (e.g. fur -bearing suede leather).

[0032] The leathers can be of various thicknesses, such as from 0.5 mm to 8 mm, thus, thin leathers, are suitable for garment leather or glove-leather (nappa); leather of medium thickness, is suitable for shoe upper leather, and handbags, or also thick leathers, are for shoe-sole leather, furniture leather, leather for suitcases, for belts and for sport articles; hair-bearing leathers and furs may also be used.

[0033] The leathers obtained by treating with a re-tanning composition comprising rice husk can subsequently be further processed, as is customary in the leather industry, by any of the processes of bleaching, colouring, dyeing, fatliquoring, hang drying, vacuum drying, toggling, conditioning, staking, milling, tumbling, buffing, pressing, embossing, ironing, finishing with a wax or a coating.

[0034] The re-tanning compositions comprising rice husk represent a novel non-toxic, renewable alternative re-tanning agent, sourced from a readily available side product of the rice industry, compared to conventional re-tanning agents in the leather industry sourced from oil based phenol formaldehyde condensates or from vegetable sources that are less readily available, such as conventional Mimosa, obtained from the bark of the Acacia tree, and Tara, obtained from the fruit of the Tara bush. The Acacia tree and Tara bush are harvested specifically for their conversion into re-tanning agents, whereas rice husk is available in abundance, because rice is cultivated in abundance, with rice husk being obtained as a side product.

[0035] The re-tanning compositions according to the invention comprising rice husk can be used as re-tanning agents for the treatment of leather giving softness, fullness, colour properties, light fastness and heat fastness thereby giving at least comparable or even improved results when compared to state of the art reference re-tanning agents. Further, rice husk is a material that is fully biobased and a side product from food industry that is often discarded as waste.

[0036] The present invention will be further elaborated by the following non-limiting working examples.

[0037] Examples

[0038] All percentages mentioned are based on leather weight before processing. This is true for all percentages in all examples, unless stated otherwise.

[0039] Example 1: Re-tanning Results

[0040] The re-tanning compositions comprising rice husk were tested as re-tanning agents on pelt that had been tanned with Granofin® Easy F -90 liq. (obtainable via Stahl Europe BV), which is a metal-free tanning agent. The re-tanning agents have the role to fill the collagen structures and impose characteristics like softness, grain tightness, and light and heat fastness properties.

[0041] Re-tanning was carried out starting with a wash of 200% of water and 0.1% of Eusapon® OC (a degreasing agent; obtainable via Stahl Europe BV) and 0.1% of formic acid for 30 minutes. Thereafter the re-tanning process is initiated by adding 200% of water to the drum, together with 2% of Densodrin® DP (a waterproofing agent; obtainable via Stahl Europe BV) and the drum was run for 20 minutes. Hereafter 1.5% of sodium formate, 2.0% of Coralon® NL liq. (a neutralization agent; available via Stahl Europe BV), and 10% of Re-Tanning agent were added. After 20 minutes of turning the drum, 5.0% of Relugan® RV (agent added to improve penetration; available via Stahl Europe BV) was added to the drum and the drum was run for 10 minutes. Hereafter 2.0% of Relugan® Soft AP (a fatliquoring agent; available via Stahl Europe BV), 2.0% of Lipoderm® Liquor LA (a natural fatliquoring agent; available via Stahl Europe BV) and 3.0% of Lipoderm® liquor A-l New (a natural fatliquoring agent; available via Stahl Europe BV) were added. After 10 minutes of turning the drum, 4.0% of Inoderme® Light Brown NG (a dye; available via Stahl Europe BV) were added to the drum and the drum was run for another 10 minutes. Still in the same float, another 10% of re-tanning agent was added to the drum and the drum was run continuously for 180 minutes. After 180 minutes, the drum was set to turn all night at an interval of 5 minutes turning and 55 minutes resting. After the overnight phase, 70% of water was added to the drum and the heating of the drum was set to 50°C. After the temperature reached 50°C, formic acid was added to the drum to fix all the products. The heat was run off and the float was discharged, and the leather was washed with 150% of water for 10 minutes. After the wash, 100% of water was added to the drum together with 0.5% of Coriagen® CR II New (a complexing agent; available via Stahl Europe BV), 1.5% of Densotan® A (a waterproofing agent; available via Stahl Europe BV) and 5.0% of Relugan® RV and the drum was run for 10 minutes. Next, 15% of re-tanning agent was added to the drum. After turning the drum for 40 minutes, 100% of water was added to the drum and the heating was run on to increase the temperature of the drum to 50°C. After turning the drum for 20 minutes, 1.5% of Catalix® 150 liq. (a fatliquoring agent; available via Stahl Europe BV), 6.0% of Relugan® Soft AP, 2.0% of Lipoderm® Liquor LA and 3.0% of Lipoderm® liquor A-l New were added to the drum and the drum was run for 120 minutes. Thereafter 1.4% of formic acid was added and the drum was run for 40 minutes to fix the products. The heating was run off and the float is drained. The leathers were washed with 150% of water for 10 minutes. Afterwards the leathers were set out and vacuum dried at 43 °C for 180 seconds. Thereafter the leathers were hung to complete the drying process. The tightness of the grain is an evaluation of the top layer of the leather once it is bent with two hands. The smoother the layer remains the tighter is the bent leather. The more wrinkles observed, the lower the tightness. Lower numbers indicate better tightness.

[0042] The smoothness I fineness of the surface was evaluated, in which levelness is considered as very good.

[0043] The softness of leathers was determined via haptics and are generally graded with a number, wherein a lower number is better.

[0044] Fullness of re-tanned leathers was determined via haptics. Lower numbers indicate better fullness.

[0045] The handle I touch of the re-tanned leathers is considered as very good if the superficial touch it is not greasy or too dry, and a silky touch is considered as perfect.

[0046] The uniformity of colour is about the dyeing levelness, in which a uniform dyeing coverage on the surface is considered good.

[0047] The shade is also a colour property and is considered good if the dyeing does not result in any shade on the surface. In this case the dyestuffs are penetrated, but the colour that the re-tanning agents has, affects the colour that the dyestuffs should bring to the leather. In the case of a white retanning agent, there is generally no influence on the shade, while some dark vegetable re-tanning agent may have a negative effect on the shade.

[0048] The degree of bleaching is also a colour property, in which a darker colour is considered as bad, and a lighter colour is considered as good. The darker colour is a result of re-tanning agents blocking the pores of the leather due to which the dyestuffs cannot penetrate the leather properly.

[0049] The degree of colouring in a cross section takes into account whether the dyestuffs are completely penetrated, so whether colouring is at the surface only or also deeper inside the leather, in which a full cross section colouring is considered as best. Leather specimens, first tanned using Granofin® Easy F-90 liq., were re-tanned with milled rice husk, with quebracho (a powder re-tanning agent derived from wood extracts) or with Synektan® VW (a powder retanning agent, obtainable from Stahl Europe BV) as Re-Tanning agent. The evaluation results are collected in Table 1.

[0050] Table 1: Re-tanning results on leather specimens, previously tanned using Granofin® Easy F-90 liq., were re-tanned with rice husk, with quebracho or with Synektan® VW. The scale goes from 1.0 (very good), 2.0 (good), 3.0 (average), 4.0 (below average), 5.0 (poor) to 6.0 (fail). A difference of 0.5 means a marginal difference which will need technical experience to differentiate, but a 1.0 difference means an obvious difference.

[0051] As can be seen in Table 1, the leathers re-tanned with rice husk give an average score that is comparable with the scores obtained with the reference re-tanning agent Quebracho and better than obtained with the reference retanning agent Synektan® VW. Most notable property that is better using rice husk as re-tanning agent is the bleaching, because a much lower value is obtained. It has thus been demonstrated that re-tanning using rice husk gives similar or slightly better properties to the leather as re-tanning using reference materials. The leather specimens were also used for measuring the light fastness. The leathers were exposed to light in a sun tester for 24, 48 or 72 hours according to ISO 105-B02, also known as the Xenotest. Leather samples were, after exposure to light, evaluated according to blue scale with numbers from 1 to 8. Larger number indicate less colour change and therefore more resistance to light induced ageing, wherein a score of 6 or above is considered ‘good’.

[0052] The leather specimens were also used for measuring the resistance to heat. The leathers were exposed to heat at 80°C for 72 h, or 100°C for 168 hours or 120°C for 168 hours, according to ISO 105-A02. Leather samples were, after exposure to heat, evaluated according to the grey scale with numbers from 1 to 5. Larger numbers indicate more stability against heat, wherein a score of 3.5 or higher is considered ‘good’.

[0053] Table 2: Light and heat fastness results of leathers after re-tanning; higher values are better.

[0054] As can be seen in Table 2, the leathers re-tanned with rice husk gave light fastness scores that were better than scores obtained with the reference retanning agent Quebracho and almost comparable as obtained with the reference re-tanning agent Synektan® VW. The heat fastness of the leathers re-tanned with rice husk was somewhat better than scores obtained with the reference re-tanning agent Quebracho and only little lower as obtained with the reference re-tanning agent Synektan® VW.

[0055] Example 2: Re-tanning results including color measurement

[0056] The re-tanning compositions comprising rice husk were tested as re-tanning agents on pelt that had been tanned with chromium sulphate or had been tanned with Granofin® Easy F-90 liq. (obtainable via Stahl Europe BV), which is a metal-free tanning agent. The re-tanning agents have the role to fill the collagen structures and impose characteristics like softness, grain tightness, and fastness properties.

[0057] Re-tanning was carried out starting with a first wash of 200% of water and 0.15% of Eusapon® OC (a degreasing agent; obtainable via Stahl Europe BV) and 0.5% of oxalic acid for 30 minutes at 40 °C and a second wash with 200% of water for 10 minutes. After draining the drum, the re-tanning process was initiated by adding 150% of water to the drum, together with 5% of Chromitan MSN (basic chromium sulphate, obtainable via Stahl Europe BV) and the drum was run for 120 minutes. Hereafter 2.0% of sodium formate and 2.0% of Coralon® NL liq. (neutralizing agents; available via Stahl Europe BV) were added and the drum was run for 20 minutes. Then 2.0% of sodium bicarbonate was added and the drum was run for another 60 minutes. Hereafter a washing step was performed with 200% of water at 50°C. After draining 150% of water was added at 50°C and 10.0% of Derminol® SO-31 (a fatliquoring agent; available via Stahl Europe BV) was added to the drum and the drum was run for 60 minutes. Hereafter 0.7% of formic acid was added and the drum was run for 20 minutes to fix the product. Hereafter a washing step was performed with 200% of water at 20°C. After draining the drum, 20% of water was added with 3.0% of Inoderme® Brown GOL (a dye; available via Stahl Europe BV) and the drum was run for 20 minutes. Next, 15% of Re-Tanning agent was added and the drum was run for 120 minutes. Then 150% of water was added at 50°C, together with 2.0% of formic acid and the products were left to fix to the leather for 40 minutes. The heating was turned off and the float was drained. The leathers were washed with 200% of water for 10 minutes. Afterwards the leathers were set out and vacuum dried at 50°C for 180 seconds. Thereafter the leathers were hung to complete the drying process.

[0058] Re-tanning of chrome tanned leathers was done with Inoderme® Brown GOL as dyestuff and without dyestuff. The resulting re-tanned leathers were brown, when the brown dyestuff had been used, or were blueish when no dyestuff was used. Re-tanning of Easywhite tanned leathers was done with the dyestuff Inoderme® Light Brown NG, and the resulting and retanned leathers were yellow to orange in colour.

[0059] Leather specimens, first tanned using Granofin® Easy F-90 liq. or chrome sulphate, were re-tanned with rice husk, with Tara, with Tanicor® RS-34-IT (a powder vegetable-based re-tanning agent; obtainable from Stahl Europe BV), with ‘Tanicor® RS-Ricehusk', in which the regular vegetable retanning components of Tanicor® RS-34-IT were replaced by rice husk, with a mixture of 30% rice husk and 70% Tara, or with a mixture of 60% rice husk and 40% of Synektan® VW (a powder re-tanning agent; obtainable from Stahl Europe BV) as Re-Tanning agent.

[0060] The colour of the resulting re-tanned leather specimens was measured using a Datacolor 600 reflection spectrophotometer in the spectral range of 360 nm to 700 nm. The colour curves were measured, and the colour differences were calculated versus one of the measured leather specimens, which was in this set of measurements the leather specimen that was retanned using the Tanicor RS-34-IT reference re-tanning agent. The differences in colour are expressed in CIELAB colour. AL is the lightness difference, with a positive value meaning a lighter overall colour. Ab is the yellow / blue difference, with a negative value meaning more blueish and a positive value being more yellowish. AC is the chroma difference, with a positive value meaning brighter and a negative value being duller. AH is the metric hue (colour) difference, with a negative value meaning less chromatic or less saturated colour.

[0061] The values were measured using D65 light at 10 degrees measurement angle. The D65 hght is corresponding to natural hght. The results are collected in Table 3.

[0062] Table 3: Colour of the re-tanned leather specimens, using the appropriate specimen re-tanned with Tanicor RS-34-IT as reference for the colour difference values expressed in CIELAB colour.

[0063] Using the ‘Tanicor RS-Rice’ instead of the normal Tanicor RS-34- IT on chrome tanned (blueish colour) leather showed a positive AL. Also using only rice husk as re-tanning agent showed a positive AL. This confirms the visual assessment that these leathers were lighter in colour than obtained using normal Tanicor RS-34-IT. The AL is also positive in the same manner for brown colour chrome tanned leather and for Granofin® Easy F-90 liq. tanned leather when using rice husk based re-tanning compositions.

[0064] Using the ‘Tanicor RS-Rice’ instead of the normal Tanicor RS-34- IT on chrome tanned (blueish colour) leather showed a negative Ab. Also using only rice husk as re-tanning agent showed a negative Ab. This confirms the visual assessment that these leathers were more blueish in colour. The Ab were less notable on brown colour chrome tanned leather and Granofin® Easy F-90 liq. tanned leather, which will probably because of the overall colour of these leathers, which are brown to brownish orange, albeit that the Tara retanned leather gave a large negative Ab value on brown colour chrome tanned leather, which corresponds to the visual observation that this leather specimen appears less reddish.

[0065] Using rice husk as re-tanning agent or as a part of the re-tanning agent composition results in lighter coloured leathers, and less reddish or more blueish coloured leathers, depending on the type of preceding tanning.

[0066] The properties tightness, smoothness, softness, fluffiness and touch were evaluated, in a similar manner as in Example 1, for the leather specimens that had been tanned with chrome sulphate and re-tanned using Tanicor® RS-34-IT or with ‘Tanicor® RS-Ricehusk', and which were either without or with dyestuff. The degree of whiteness is also a colour property, in which it is the effect of a product to produce a gradation of the dyeing. In this case no dyeing agents are used, but the properties of the re-tanning agent can affect the colour of the leather. A more white appearance is considered better, and is graded with a lower value, in a similar way as the other properties mentioned.

[0067] The results are collected in Table 4.

[0068] Table 4: Re-tanning results on leather specimens, previously tanned using chrome sulphate, were re-tanned with with Tanicor® RS-34-IT (a powder vegetable-based re-tanning agent; obtainable from Stahl Europe BV) or with ‘Tanicor® RS-Ricehusk', in which the regular vegetable retanning components of Tanicor® RS-34-IT were replaced by rice husk. The scale goes from 1.0 (very good), 2.0 (good), 3.0 (average), 4.0 (below average), 5.0 (poor) to 6.0 (fail). A difference of 0.5 means a marginal difference which will need technical experience to differentiate, but a 1.0 difference means an obvious difference.

[0069] As can be seen in Table 4, the leathers re-tanned with ‘Tanicor® RS-Ricehusk' give an average score that is comparable with the average score obtained with the reference re-tanning agent Tanicor® RS-34-IT. Most notable property that is better using ‘Tanicor® RS-Ricehusk' as re-tanning agent is the degree of whiteness, because a much lower value is obtained. Also, bleaching is better using ‘Tanicor® RS-Ricehusk' as re-tanning agent, but cross section is lower using ‘Tanicor® RS-Ricehusk' as re-tanning agent. It has thus been demonstrated that re-tanning using ‘Tanicor® RS-Ricehusk' gives similar or slightly better properties to the leather as re-tanning using reference material. The leather specimens were also used for measuring the lightfastness. The leathers were exposed to light in a sun tester for 24, 48 or 72 hours according to ISO 105-B02, also known as the Xenotest. Leather samples were, after exposure to light, evaluated according to blue scale with numbers from 1 to 8. Larger number indicate less colour change and therefore more resistance to light induced ageing, wherein a score of 6 or above is considered ‘good’.

[0070] The leather specimens were also used for measuring the resistance to heat. The leathers were exposed to heat at 80°C for 72 h, or 100°C for 168 hours or 120°C for 168 hours, according to ISO 105-A02. Leather samples were, after exposure to heat, evaluated according to the grey scale with numbers from 1 to 5. Larger numbers indicate more stability against heat, wherein a score of 3.5 or higher is considered ‘good’.

[0071] Table 5: Light and heat fastness results of leathers after re-tanning; higher values are better.

[0072] As can be seen in Table 5, the leathers re-tanned with rice husk or with ‘Tanicor® RS -Ricehusk' gave light fastness scores that were the same as scores obtained with the reference re-tanning agent Tanicor® RD-34-IT. The heat fastness of the leathers re-tanned with rice husk was better than scores obtained with ‘Tanicor® RS-Ricehusk' or with reference re-tanning agent Tanicor® RD-34-IT, wheres the scores for ‘Tanicor® RS-Ricehusk' and reference re-tanning agent Tanicor® RD-34-IT were similar, albeit not the same for each measurement.

Claims

CLAIMS1. A re-tanning composition for re-tanning of pre-tanned leather, tanned leather, pelts, skins, tanned hides, leather intermediate products or non-finished leather, said composition comprising rice husk as re-tanning agent wherein the amount of rice husk in said re-tanning composition is in the range 10 up to 99 wt%, preferably in the range 20 up to 90 wt% and most preferred in the range 30 up to 80 wt% based on the total solid weight of the re-tanning composition and wherein said re-tanning composition is further comprising additional re-tanning agents..

2. The re-tanning composition according to claim 1 wherein the retanning composition is solid at ambient conditions, preferably a powder and the rice husk is selected from milled rice husk having a particle size in the range 50 micron up to 1000 micron as measured according to ISO 13320.

3. The re-tanning composition according to any of foregoing claims wherein the amount of rice husk in said re-tanning composition is in the range 10 up to 99 wt%, preferably in the range 20 up to 90 wt% and most preferred in the range 30 up to 80 wt% based on the total solid weight of the re-tanning composition and wherein the re-tanning composition further comprises additional re-tanning agents in a concentration between 1 wt% up to 90 wt%, preferably between 10 wt% up to 80 wt%, more preferably between 20 wt% up to 70 wt% based on the total solid weight of the re-tanning composition .

4. The re-tanning composition according to any of foregoing claims wherein the re-tanning composition comprises additional re-tanning agents selected from vegetable re-tanning agents selected from tara, quebracho and / or mimosa and the ratio of rice husk to said additional vegetable retanning agents is in the range 10 / 90 to 90 / 10, preferably in the range 30 / 70 to 70 / 30.

5. Use of the re-tanning composition according to any of foregoing claims 1-4 in a process for re-tanning of pre-tanned leather, tanned leather,pelts, skins, hides, leather intermediate products or non-finished leather, wherein between 2 and 15 weight percent, preferably between 4 and 12 weight percent and most preferably in between 6 and 10 weight percent, relative to the weight of the leather, of rice husk is used.

6. Use according to claim 5 to improve the degree of bleaching during the re-tanning process.

7. Use according to claim 5 to improve the light fastness and / or heat fastness during the re-tanning process.

8. Re-tanned leather, obtained by a re-tanning process using the re- tanning composition according to any of foregoing claims 1-4 wherein between2 and 15 weight percent, preferably between 4 and 12 weight percent and most preferably in between 6 and 10 weight percent, relative to the weight of the leather, of rice husk is used.

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