Method for using a retanning composition comprising rice hulls

Abstract

The present invention provides a method for using a retanning composition containing rice husk in leather processing, the retanning composition providing good retanning behavior.

Description

[0001] This invention relates to a novel retanning agent selected from rice husks and a method for using rice husks as a retanning agent or as a component in a retanning composition in a leather manufacturing process.

[0002] Leather is a durable and supple material made by tanning animal hides and skin. The leather manufacturing process consists of three basic sub-processes: preparation, tanning, and crusting.

[0003] This invention relates to a sub-process of tanning, and more particularly to its retanning portion.

[0004] In the preparation stage, the hide or skin is prepared for tanning. After trimming, the animal skin is soaked to remove salts and other solids while restoring the skin to its initial dry moisture. The fleshy side of the wet skin is then scraped to remove any remaining meat or fat, and optionally, the skin is dehaired. After optional softening and pickling steps, the skin is tanned. Other potential steps that may be part of the preparation stage include preservation, liming, flaking, reliming, deliming, degreasing, curling, bleaching, and deacidification.

[0005] Tanning is a process that preserves skin by converting proteins into a stable material through cross-linking the collagen fibers of raw hides or skin. This material does not rot and provides tanned leather with desirable properties such as a high shrinkage temperature (T0). S Its softness and suitability for subsequent processing such as neutralization, retanning, fatliquoring, dyeing, and finishing.

[0006] Tanning is primarily carried out by treating hides with chromium sulfate (resulting in so-called wet blue leather) or by using organic reactive tanning agents such as aldehydes, particularly glutaraldehyde (resulting in wet white leather). In rare cases, tanning is performed using vegetable tanning agents (as in traditional practices) or by using synthetic tanning agents (synthetic tanning agents) or other conventional techniques. The product prepared in this sub-process is an intermediate because it is insufficient to obtain the desired properties specified by the customer. Therefore, tanned hides undergo further treatment with various products. This process is called retanning. Retanning affects the leather's hand feel, dyeability, fullness, texture fineness and stability, as well as other factors such as light fastness, to suit the properties required for the final leather goods (whether for automotive or aircraft seats, footwear, clothing, or bags and leather goods). Retanning includes dyeing to impart color and fatliquoring to increase softness, fullness, and hand feel. Once retanning is complete, the leather is referred to as "hard leather."

[0007] Vegetable tanning agents are the primary tanning agents. They are now mainly used for retanning because chromium sulfate or glutaraldehyde are widely accepted as tanning agents today. Common vegetable tanning agents are acacia, obtained from the bark of the acacia tree, and tara, obtained from the fruit of the tara shrub. They provide the softness and limited filling of the collagen structure in leather (Hans Herfeld, "Library of Leather; Volume 3: Tanning Agents, Tanning and Retanning", Frankfurt 1985, p. 44). Generally, vegetable tanning agents lack fastness properties such as lightfastness or heat aging resistance.

[0008] The term "synthetic tanning agent" refers to the range of synthetic tanning agents. The first synthetic tanning agents were prepared by the condensation of phenolsulfonic acid and formaldehyde (E. Stiasny, 1911, Austrian Patent No. 58405). While these synthetic tanning agents were initially used as dispersants and auxiliaries for vegetable tanning agents, their chemical properties were further developed, allowing them to replace some or even all vegetable tanning agents. US1841840 describes this further development by incorporating urea into the condensation of phenolsulfonic acid and formaldehyde, enabling the production of leathers that meet higher technical requirements, such as fastness properties to light or heat aging. Since chromium sulfate or glutaraldehyde are widely accepted as tanning agents, synthetic tanning agents are now primarily used in retanning processes, where they help build and fill cross-linked collagen fibers. Unfortunately, synthetic tanning agents may contain residual amounts of free formaldehyde or undesirable byproducts (such as bisphenol S or F), meaning they should be handled and used with caution for safety reasons.

[0009] In many applications, synthetic tanning agents are used together with vegetable tanning agents because the performance of vegetable tanning agents alone is considered insufficient. Synthetic tanning agents generally have higher fastness properties, in addition to dispersing properties. This helps to support the uniform distribution of vegetable tanning agents and other leather chemicals such as fillers, dyes, and fatliquoring agents (DE1142173).

[0010] Currently, companies (especially in the chemical industry) are facing a strong drive to fulfill their corporate social responsibility and use sustainable or renewable raw material sources. The use of bio-based feedstocks is particularly important, as these feedstocks do not compete with bio-based feedstocks used as food sources, and therefore the use of bio-based waste streams is especially advantageous.

[0011] Rice husks are the hard, protective covering of rice grains. Another name for rice husks is the outer shell of rice. Rice husks are part of the rice seed. During the growing season, the husk protects the grain from pests. The husk is formed of a hard material that is difficult to eat or swallow and is largely indigestible for humans due to its high fibrous content. During the milling process, a rice husk removes the husk from the raw grain to expose the whole brown rice, which is then typically further milled to remove the bran layer, producing white rice. Rice husks are an inexpensive byproduct of human food processing and are used in several applications, such as pillow filling, insulation, as a component of low-density fiberboard or cardboard, and rice bran oil can be extracted from rice husks. Burning rice husks produces rice husk ash, a potential source of amorphous active silica, which has various applications in materials science. Most rice husk ash is used in the production of silicate cement.

[0012] EP3257955B1 (TFL;2016) describes a method for tanning, pretanning or retanning leather using an aqueous composition or powder composition comprising at least one carboxymethyl cellulose and / or its salt.

[0013] JP2009067847 (Taneka; 2006) describes a retanning agent using collagen powder containing cross-linked regenerated collagen.

[0014] This invention provides a retanning agent composition comprising rice husks. This invention further provides a method for using rice husks as a retanning agent or as a component in a retanning composition that provides good retanning behavior.

[0015] It has been found that rice husks can be used as a retanning agent and therefore can replace synthetic tanning agents or other retanning agents in the retanning of leather.

[0016] The object of the present invention is to provide a retanning composition containing rice husks and a method for using the retanning composition containing rice husks in leather processing, the retanning composition being free of formaldehyde and bisphenol and providing good retanning behavior.

[0017] The inventors discovered that rice husks can be applied to leather for retanning, for example, through standard industrial procedures. Surprisingly, leather treated with rice husks exhibits properties similar to or better than those of synthetic or vegetable tanning agents in terms of softness, fullness, firmness, smoothness, bleaching properties, light fastness, and heat fastness, without producing bisphenol or formaldehyde issues. These retanning compositions typically do not even require analysis for toxic formaldehyde or bisphenol, as none of the components contain or are likely to release formaldehyde or form bisphenol. Compared to conventional retanning agents in the leather industry, the use of rice husks represents a novel, non-toxic, and renewable alternative to conventional retanning agents derived from readily available byproducts of the rice industry, which are derived from oil-based phenol-formaldehyde condensates or less readily available plant sources such as conventional acacia obtained from arabess bark and tara obtained from the fruit of the tara shrub.

[0018] The rice husk-containing retanning composition may also contain other components, such as lignin, starch, chitin, kaolin, tara, or protein or conventional retanning agents. Preferably, the rice husk-containing retanning composition of the present invention comprises rice husk in an amount between 20% and 100% by weight based on the total (dry) weight of the retanning composition, and wherein the remaining amount may contain other retanning agents. If these other retanning agents in the retanning composition are fossil-based, an additional advantage of adding rice husk is that this addition increases the bio-based content of such retanning compositions, thereby positively influencing the transition from a linear economy to a circular economy.

[0019] In an advantageous embodiment, the retanning composition of the present invention is a solid under ambient conditions. Such a solid composition may be in powder form.

[0020] According to a preferred embodiment, the retanning composition of the present invention comprises milled rice husks, wherein the rice husks are milled and optionally dried to obtain rice husk powder having a moisture content of up to 20% by weight, preferably up to 10% by weight, based on the total weight of the powder, and an average particle size in the range of 50µm to 1000µm, preferably in the range of 50µm to 750µm, as measured according to ISO 13320.

[0021] According to a preferred embodiment, the retanning composition of the present invention is a composition in which at least a portion of the retanning agent is selected from rice husk (powder) and the remainder is selected from prior art synthetic and / or vegetable retanning agents. The amount of rice husk in the retanning composition is in the range of 10% to 99% by weight, preferably in the range of 20% to 90% by weight, and most preferably in the range of 30% to 80% by weight, based on the total (dry) weight of the retanning composition. The amount of additional retanning agent in the retanning composition, based on the total solid weight of the retanning composition, is between 0% and 90% by weight, preferably between 0% and 80% by weight, and more preferably between 0% and 70% by weight. Preferably, based on the total (dry) weight of the retanning composition, the retanning composition includes rice husk as a retanning agent in the range of 10% to 99% by weight, preferably between 20% and 90% by weight, and most preferably between 30% and 80% by weight, and additional retanning agent in the range of 1% to 90% by weight, preferably between 10% and 80% by weight, and most preferably between 20% and 70% by weight. A particularly preferred retanning composition according to the invention comprises a vegetable retanning agent selected from rice husk and tara, thereby producing a synergistic effect on the lighter color of the leather. The ratio of rice husk to the additional vegetable retanning agent is in the range of 10 / 90 to 90 / 10, preferably in the range of 30 / 70 to 70 / 30.

[0022] According to a preferred embodiment, the retanning composition of the present invention is a prior art retanning composition in which rice husks according to the present invention are added. The amount of rice husks added depends on the type of prior art retanning composition and the desired retanning effect. For example, rice husks can be added to a prior art retanning composition to improve the degree of bleaching during retanning. In the context of the present invention, rice husks are used at a rate between 2% and 15% by weight, preferably between 4% and 12% by weight, and most preferably between 6% and 10% by weight, relative to the weight of the leather.

[0023] According to a preferred embodiment, the retanning composition of the present invention may further comprise additives selected from lignin, starch, chitosan, kaolin, etc., at a concentration preferably between 0% and 50% by weight based on the total weight of the retanning composition. The standard industrial procedure for retanning involves treating tanned leather (such as wet blue leather) with water, sodium formate, and sodium bicarbonate for a period of time to produce a float with a pH below neutral; then adding the retanning composition; followed by further rotating the tanning drum for a period of time; then fixing with an acid (such as, but not limited to, formic acid); then removing the float and washing with water. The leather is then dried, such as by hanging to dry at room temperature without vacuum or in a vacuum chamber. The amount of the retanning composition added is between 2% and 15%, preferably between 4% and 12%, and most preferably between 6% and 10%, where the percentage refers to the weight percentage of the non-volatile portion of the retanning composition relative to the weight of the leather.

[0024] The light fastness of leather can be determined according to ISO 105-B02 using a daylight tester, for example. Retanned leather is typically exposed to light for 24 to 72 hours. The resulting yellowing can be quantified using a spectrophotometer or by ranking the light fastness on a scale of 1 to 8 using the blue scale method, where a score of 6 or higher is considered "good".

[0025] The thermal aging of leather can be determined by exposing the leather to heat at different temperatures for different numbers of hours. According to ISO 105-A02, after exposure to heat, leather samples can be evaluated using a gray scale of 1 to 5. The higher the number, the greater the heat resistance stability, with a score of 3.5 or higher considered "good".

[0026] Fullness is an aesthetic quality parameter (i.e., a sensory characteristic) defined by the spacing between fibers, and (through touch) indicates whether the amount of fibers in each area is large or small. This is the result of good filling of the spaces between fibers and sufficient lubrication of the fibers to prevent them from clumping together. In simpler terms: full leather is leather that feels round to the touch, while hollow leather (the opposite of full leather) is flat and thin. The fullness of retanned leather is primarily determined by touch.

[0027] Tightness is an aesthetic quality parameter (i.e., a sensory characteristic) defined by the amount of wrinkles or creases that form when leather bends inwards (as assessed visually), a result of good or poor adhesion between the grain layer and the underlying dermis. Leather tightness can be influenced by a multitude of factors, from animal breed to the completion of retanning, including product distribution, mechanical action, drying, etc., and is desirable for all products, even those that will be milled. The challenge lies in holding the individual layers together without hardening the leather (hard leather = typically tightly grained). The smoother the layers remain when bent, the tighter the leather. It has been observed that the more wrinkles there are, the lower the tightness.

[0028] The softness, fullness, and firmness of retanned leather are measured by touch and are usually graded with numbers, where the lower the number, the better.

[0029] Dry weight as used herein refers to the mass of the material, excluding the mass of any water, liquid, or moisture present within the material. Dry weight and (dry) solids weight refer to the same weight and have the same meaning herein.

[0030] The average particle size referred to in this article is the average particle size as measured by laser diffraction according to ISO 13320.

[0031] Retanning compositions containing rice husks can be used to prepare leather for all applications, such as shoe, furniture, automotive, clothing, and bag leather.

[0032] Any type of leather that has undergone conventional treatment is suitable for treatment using a retanning composition comprising rice husks, particularly grain leather (e.g., Nappa leather from sheep, goats, or cattle and box leather from calves or cattle), suede leather (e.g., velvet from sheep, goats, or calves, and hunting leather), suede (e.g., hides from cattle or calves), deerskin, and nubuck; in addition, sheepskin and fur (e.g., suede with fur).

[0033] Leather can come in various thicknesses, such as from 0.5mm to 8mm, so thin leather is suitable for clothing leather or glove leather (Nappa); medium-thickness leather is suitable for shoe uppers and handbags; or thick leather is also suitable for shoe soles, furniture leather, suitcase leather, belt leather and sporting goods leather; leather with hair and fur can also be used.

[0034] Leather obtained by treatment with a retanning composition containing rice husks can then be further processed according to the conventions of the leather industry, through any of the following processes: bleaching, coloring, dyeing, fatliquoring, hanging drying, vacuum drying, turning, conditioning, softening, milling, tumbling, polishing, pressing, embossing, ironing, waxing, or coating finishing.

[0035] Compared to conventional retanning agents used in the leather industry, retanning compositions containing rice husks represent a novel, non-toxic, and renewable alternative. They are derived from readily available byproducts of the rice industry, whereas conventional retanning agents are derived from oil-based phenol-formaldehyde condensates or less readily available plant sources, such as conventional acacia obtained from arabess bark and tara obtained from the fruit of the tara shrub. Acacia and tara shrubs are harvested specifically for conversion into retanning agents, while rice husks are abundantly available because rice is widely cultivated and rice husks are obtained as a byproduct.

[0036] The retanning compositions according to the invention, containing rice husks, can be used as retanning agents for treating leather to achieve softness, fullness, color properties, light fastness, and heat fastness, thereby exhibiting at least equivalent or even improved results compared to prior art reference retanning agents. Furthermore, rice husks are a completely bio-based material and a byproduct of the food industry, typically discarded as waste.

[0037] The present invention will be further illustrated by the following non-limiting working examples. Example

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

[0039] Example 1: Retanning Results

[0040] The retanning composition containing rice husks was used as a retanning agent in a mixture already treated with Granofin. ® EasyF-90liq. (available from Stahl Europe BV) was tested on tanned fur. This liquid is a metal-free tanning agent. Retanning agents have the function of filling collagen structures and achieving properties such as softness, texture firmness, and light and heat fastness.

[0041] Retanning begins with 200% water and 0.1% Euapon. ® Wash with OC (degreasing agent; available via Stahl Europe BV) and 0.1% formic acid for 30 minutes. Afterwards, rinse with 200% water and 2% Densodrin. ® DP (waterproofing agent; available via Stahl Europe BV) is added to the rollers to begin the retanning process, and the rollers are run for 20 minutes. Afterward, 1.5% sodium formate and 2.0% Coralon are added. ® NLliq. (neutralizing agent; available via Stahl Europe BV) and 10% retanning agent. After rotating the drum for 20 minutes, add 5.0% Relugan.® RV (a reagent added to improve permeability; available via Stahl Europe BV) is added to the roller, and the roller is run for 10 minutes. Afterward, 2.0% Relugan is added. ® SoftAP (fatting agent; available from Stahl Europe BV), 2.0% Lipoderm ® LiquorLA (a natural fatliquoring agent; available via Stahl Europe BV) and 3.0% Lipoderm ® Liquor A-1 New (a natural fatliquoring agent; available from Stahl Europe BV). After rotating the drum for 10 minutes, add 4.0% Inoderme. ® LightBrownNG (dye; available via Stahl Europe BV) is added to the drum, and the drum is run for 10 minutes. Still in the same float, another 10% of the retanning agent is added to the drum, and the drum is run continuously for 180 minutes. After 180 minutes, the drum is set to run overnight, with intervals of 5 minutes of running and 55 minutes of rest. After the overnight stage, 70% water is added to the drum, and the drum heating temperature is set to 50°C. Once the temperature reaches 50°C, formic acid is added to the drum to fix all products. The heat is released and the float is removed, and the leather is washed with 150% water for 10 minutes. After washing, 100% water with 0.5% Coriagen is added. ® CRIINew (a complexing agent; available via Stahl Europe BV), 1.5% Densotan ® A (waterproofing agent; available from Stahl Europe BV) and 5.0% Relugan ® Add RV together to the drum and run the drum for 10 minutes. Next, add 15% retanning agent to the drum. After running the drum for 40 minutes, add 100% water to the drum and continue heating to raise the drum temperature to 50°C. After running the drum for 20 minutes, add 1.5% Catalix. ® 150 liq. (fatting agent; available via Stahl Europe BV), 6.0% Relugan ® SoftAP, 2.0% Lipoderm ® LiquorLA and 3.0% Lipoderm ®Liquor A-1New is added to the drum, and the drum is run for 120 minutes. Afterward, 1.4% formic acid is added, and the drum is run for 40 minutes to fix the product. Heating is stopped, and the floats are emptied. The leather is washed with 150% water for 10 minutes. Then, the leather is spread out and vacuum-dried at 43°C for 180 seconds. Afterward, the leather is hung to complete the drying process.

[0042] The tightness of the leather grain refers to the evaluation of the top layer of leather when bent by hand. The smoother the layer, the tighter the leather will be when bent. It has been observed that the more wrinkles there are, the lower the tightness. A lower number indicates better tightness.

[0043] The smoothness / fineness of the surface was evaluated, with flatness being considered very good.

[0044] The softness of leather is measured by touch and is usually graded with numbers, where the lower the number, the better.

[0045] The fullness of retanned leather is determined by touch. The lower the number, the better the fullness.

[0046] If the surface feels neither oily nor too dry, the feel / touch of retanned leather is considered very good, and a smooth feel is considered perfect.

[0047] Color uniformity refers to the uniformity of dyeing, where uniform dye coverage on the surface is considered good.

[0048] Hue is also a color characteristic; if dyeing does not produce any hue on the surface, the hue is considered good. In this case, the dye penetrates, but the color of the retanning agent affects the color the dye should impart to the leather. In the case of white retanning agents, there is usually no effect on hue, while some dark vegetable retanning agents may have a negative impact on hue.

[0049] The degree of bleaching is also a color characteristic, with darker colors considered undesirable and lighter colors considered desirable. Darker colors result from retanning agents clogging the leather's pores, preventing dyes from properly penetrating the leather.

[0050] The degree of coloration across the cross section takes into account whether the dye has fully penetrated, thus considering whether the coloring is only on the surface or deeper inside the leather, with full cross-section coloring being considered the best.

[0051] Leather samples were first tested using Granofin. ® EasyF-90liq. is tanned, then separately with milled rice husks, white oak (a powdered retanning agent derived from wood extracts), or Synektan. ®VW (powdered retanning agent, available from Stahl Europe BV) was used as a retanning agent for retanning. The evaluation results are collected in Table 1.

[0052] Table 1: Using Granofin ® EasyF-90liq. After tanning, use rice husks, white oak, or Synektan. ® Retanning results of VW retanned leather samples. Grades are 1.0 (Very Good), 2.0 (Good), 3.0 (Average), 4.0 (Below Average), 5.0 (Poor), and 6.0 (Fail). A difference of 0.5 indicates a marginal difference that requires technical experience to distinguish, while a difference of 1.0 indicates a significant difference.

[0053]

[0054] As shown in Table 1, the average score of leather retanned with rice husks was comparable to that obtained using the reference retanning agent, white oak, and superior to that obtained using the reference retanning agent, Synektan. ® The average score obtained by VW. The most significant property of using rice husks as a retanning agent is bleaching, as the obtained value is much lower. Therefore, it has been demonstrated that retanning with rice husks imparts leather with similar or slightly better properties than retanning using reference materials.

[0055] Leather samples are also used to measure lightfastness. According to ISO 105-B02 (also known as the Xenotest), the leather is exposed to light in a daylight tester for 24, 48, or 72 hours. After exposure, the leather samples are evaluated using a blue scale from 1 to 8. Higher numbers indicate less color change and therefore greater resistance to light aging, with 6 or higher considered "good".

[0056] Leather samples were also used to measure heat resistance. According to ISO 105-A02, leather was exposed to heat at 80°C for 72 hours, or at 100°C for 168 hours, or at 120°C for 168 hours. After heat exposure, the leather samples were evaluated using a gray scale of 1 to 5. Higher numbers indicate greater heat resistance, with a score of 3.5 or higher considered "good".

[0057] Table 2: Results of light fastness and heat fastness of retanned leather; higher values ​​are better.

[0058]

[0059] As shown in Table 2, the light fastness score of leather retanned with rice husk is better than that obtained using the reference retanning agent white oak, and is comparable to that obtained using the reference retanning agent Synektan. ®The light fastness scores obtained by VW were almost identical. The heat fastness score of leather retanned with rice husk was slightly better than that obtained using the reference retanning agent, white oak, and only slightly lower than that obtained using the reference retanning agent, Synektan. ® The result obtained by VW.

[0060] Example 2: Retanning results, including color measurement

[0061] The retanning composition containing rice husks is used as a retanning agent in cases where the mixture has already been tanned with chromium sulfate or granofin. ® EasyF-90liq. (available from Stahl Europe BV) was tested on tanned fur; this liquid is a metal-free tanning agent. Retanning agents help fill collagen structures and achieve properties such as softness, texture firmness, and durability.

[0062] Retanning begins with 200% water and 0.15% Euapon. ® OC (degreasing agent; available from Stahl Europe BV) and 0.5% oxalic acid were washed at 40°C for 30 minutes, followed by a second wash with 200% water for 10 minutes. After draining the drum, the retanning process was initiated by adding 150% water and 5% Chromitan MSN (basic chromium sulfate, available from Stahl Europe BV) to the drum, and the drum was run for 120 minutes. Subsequently, 2.0% sodium formate and 2.0% Coralon were added. ® NLliq. (neutralizing agent; available via Stahl Europe BV), and run the drum for 20 minutes. Then add 2.0% sodium bicarbonate and run the drum for another 60 minutes. Afterward, perform a wash cycle with 200% water at 50°C. After draining, add 150% water at 50°C and 10.0% Derminol. ® SO-31 (fatting agent; available via Stahl Europe BV) was added to the drum, and the drum was run for 60 minutes. Afterward, 0.7% formic acid was added, and the drum was run for 20 minutes to fix the product. Following this, a washing step was performed with 200% water at 20°C. After draining the drum, 20% water and 3.0% inoderme were added. ®BrownGOL (dye; available via Stahl Europe BV) was added, and the drum was run for 20 minutes. Next, 15% retanning agent was added, and the drum was run for 120 minutes. Then, 150% water and 2.0% formic acid were added at 50°C, and the product was fixed to the leather for 40 minutes. The heating was stopped, and the floats were removed. The leather was washed with 200% water for 10 minutes. Afterward, the leather was spread out and vacuum-dried at 50°C for 180 seconds. The leather was then hung to complete the drying process.

[0063] The retanning of chrome-tanned leather uses Inoderme ® Brown Gol was used as a dye, and retanning was also performed without dye. When brown dye was used, the resulting retanned leather was brown; when no dye was used, it was blue. Inoderme dye was used... ® LightBrownNG retannes leather tanned with Easywhite, and the resulting leather, as well as the retanned leather, is yellow to orange in color.

[0064] First use Granofin ® EasyF-90liq. or chromium sulfate tanned leather samples were tested using rice husks, tara, and tannicor. ® RS-34-IT (plant-based powdered retanning agent; available via Stahl Europe BV), "Tanicor" ® RS-Ricehusk (where Tanicor) ® RS-34-IT's conventional plant-based retanning components consist of rice husk, a mixture of 30% rice husk and 70% tara, or 60% rice husk and 40% Synektan. ® VW (powdered retanning agent; a mixture that can be obtained via Stahl Europe BV) is used as a retanning agent for retanning.

[0065] The color of the retanned leather samples was measured using a Datacolor 600 reflectance spectrophotometer in the spectral range of 360 nm to 700 nm. Color curves were measured, and the color difference relative to one of the measured leather samples was calculated. In this set of measurements, the leather sample was retanned using Tanicor RS-34-IT reference retanning agent. Color differences are expressed in CIELAB color. ΔL is the lightness difference, where a positive value indicates a lighter overall color. Δb is the yellow / blue difference, where a negative value indicates more blue, and a positive value indicates more yellow. ΔC is the chromaticity difference, where a positive value indicates brighter, and a negative value indicates darker. ΔH measures the hue (color) difference, where a negative value indicates a color with less chromaticity or less saturation.

[0066] These values ​​were measured using D65 light at a 10-degree angle. D65 light corresponds to natural light. The results are collected in Table 3.

[0067] Table 3: Color of retanned leather samples, using appropriate samples retanned with Tanicor RS-34-IT as a reference for color difference values ​​expressed in CIELAB color.

[0068]

[0069] Using "Tanicor RS-Rice" instead of regular Tanicor RS-34-IT showed a positive ΔL on chrome-tanned (light blue) leather. Using rice husks alone as a retanning agent also showed a positive ΔL. This confirms the visual assessment that the color of these leathers is lighter than that obtained using regular Tanicor RS-34-IT. For brown chrome-tanned leather and for Granofin... ® EasyF-90liq. tanned leather is also positive in the same way when using rice husk-based retanning compositions.

[0070] Using "Tanicor RS-Rice" instead of the normal Tanicor RS-34-IT showed a negative Δb on chrome-tanned (light blue) leather. Using only rice husk as a retanning agent also showed a negative Δb. This confirms the visual assessment that these leathers are more blue. Δb is also observed in brown chrome-tanned leather and Granofin-tanned leather. ® The difference is less noticeable on EasyF-90liq tanned leather, likely because the overall color of these leathers is brown to brownish-orange, although the Tara retanned leather gives a large negative Δb value on brown chrome-tanned leather, which corresponds to the visual observation that the leather sample does not appear as red.

[0071] Depending on the type of tanning previously used, the use of rice husks as a retanning agent or as part of a retanning agent composition produces lighter-colored leather, as well as leather with less red or more blue.

[0072] For those already tanned using chromium sulfate and using Tanicor ® RS-34-IT or using "Tanicor" ® RS-Ricehusk leather was retanned (in both dye-free and dye-containing cases) and its properties, such as firmness, smoothness, softness, loft, and hand feel, were evaluated in a manner similar to that of Example 1. Whiteness is also a color characteristic, where it is the effect of the product producing a dyeing gradient. In this case, no dye is used, but the properties of the retanning agent affect the color of the leather. A whiter appearance is considered better and is graded with lower values, similar to the other characteristics mentioned.

[0073] The results are collected in Table 4.

[0074] Table 4: Leather samples previously tanned with chromium sulfate using Tanicor ® RS-34-IT (a plant-based powdered retanning agent; available from Stahl Europe BV) or using "Tanicor" ® RS-Ricehusk (including Tanicor) ® The retanning results of RS-34-IT (where the conventional plant retanning components were replaced with rice husks) were graded as follows: 1.0 (Very Good), 2.0 (Good), 3.0 (Average), 4.0 (Below Average), 5.0 (Poor), and 6.0 (Fail). A difference of 0.5 indicates a marginal difference that requires technical experience to distinguish, while a difference of 1.0 indicates a significant difference.

[0075]

[0076] As shown in Table 4, using "Tanicor" ® The average score of RS-Ricehusk retanned leather compared to the reference retanning agent Tanicor ® The RS-34-IT received a comparable average rating. It's best to use "Tanicor". ® The most notable characteristic of RS-Ricehusk as a retanning agent is its whiteness, as the resulting value is much lower. Furthermore, the use of Tanicor... ® RS-Ricehusk scores higher bleaching ratings when used as a retanning agent, but Tanicor... ® RS-Ricehusk showed a lower cross-sectional score when used as a retanning agent. Therefore, it has been demonstrated that using Tanicor... ® RS-Ricehusk retanning imparts to leather properties similar to or slightly better than retanning using reference materials.

[0077] Leather samples are also used to measure lightfastness. According to ISO 105-B02 (also known as the Xenotest), the leather is exposed to light in a daylight tester for 24, 48, or 72 hours. After exposure, the leather samples are evaluated using a blue scale from 1 to 8. Higher numbers indicate less color change and therefore greater resistance to light aging, with 6 or higher considered "good".

[0078] Leather samples were also used to measure heat resistance. According to ISO 105-A02, leather was exposed to heat at 80°C for 72 hours, or at 100°C for 168 hours, or at 120°C for 168 hours. After heat exposure, the leather samples were evaluated using a gray scale of 1 to 5. Higher numbers indicate greater heat resistance, with a score of 3.5 or higher considered "good".

[0079] Table 5: Results of light fastness and heat fastness of retanned leather; higher values ​​are better.

[0080]

[0081] As shown in Table 5, rice husks or "Tanicor" are used. ® The light fastness rating of RS-Ricehusk retanned leather and the use of Tanicor as a reference retanning agent ® The RD-34-IT received the same score. Leather retanned using rice husks scored better in heat fastness than that using "Tanicor". ® RS-Ricehusk or using the reference retanning agent Tanicor ® The RD-34-IT received a rating; while "Tanicor" ® RS-Ricehusk and reference retanning agent Tanicor ® The RD-34-IT scores are similar, although they are not entirely consistent across the various measurements.

Claims

1. A retanning composition for retanning pre-tanned leather, tanned leather, fur, skin, tanned hides, leather intermediates or semi-finished leather, said composition comprising rice husk as a retanning agent, wherein the amount of rice husk in said retanning composition is in the range of 10% by weight to 99% by weight, preferably in the range of 20% by weight to 90% by weight, and most preferably in the range of 30% by weight to 80% by weight, based on the total solid weight of said retanning composition, and wherein said retanning composition further comprises an additional retanning agent.

2. The retanning composition according to claim 1, wherein the retanning composition is a solid under ambient conditions, preferably a powder, and the rice husk is selected from milled rice husks with a particle size in the range of 50 micrometers to 1000 micrometers as measured according to ISO 13320.

3. The retanning composition according to any one of the preceding claims, wherein, based on the total solid weight of the retanning composition, the amount of rice husk in the retanning composition is in the range of 10% to 99% by weight, preferably in the range of 20% to 90% by weight, and most preferably in the range of 30% to 80% by weight, and wherein, based on the total solid weight of the retanning composition, the retanning composition further comprises an additional retanning agent at a concentration between 1% and 90% by weight, preferably between 10% and 80% by weight, and more preferably between 20% and 70% by weight.

4. The retanning composition according to any one of the preceding claims, wherein the retanning composition comprises an additional retanning agent selected from plant retanning agents, said plant retanning agent being selected from tara, white oak and / or acacia, and the ratio of rice husk to said additional plant retanning agent is in the range of 10 / 90 to 90 / 10, preferably in the range of 30 / 70 to 70 / 30.

5. Use of the retanning composition according to any one of claims 1 to 4 in a method for retanning pre-tanned leather, tanned leather, fur, skin, hides, leather intermediates or semi-finished leather, wherein rice husk is used at a rate between 2% and 15% by weight, preferably between 4% and 12% by weight, and most preferably between 6% and 10% by weight, relative to the weight of the leather.

6. The use according to claim 5, for increasing the degree of bleaching during the retanning process.

7. The use according to claim 5, for improving light fastness and / or heat fastness during the retanning process.

8. Retanned leather obtained by using a retanning method of the retanning composition according to any one of claims 1 to 4, wherein rice husk is used at a rate between 2% and 15% by weight, preferably between 4% and 12% by weight, and most preferably between 6% and 10% by weight, relative to the weight of the leather.

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