A kind of waterborne polyurethane surface resin and its preparation method and application

Abstract

The present invention discloses a waterborne polyurethane surface layer resin, and its raw materials include, by mass percentage: 6.0-8.0% of diisocyanate, 12-15% of polyol compound, 0.8-1.6% of reactive hydroxy silicone oil, 1.6-2.5% of nonionic hydrophilic chain extender; 1.1-1.7% of small molecule chain extender, 0.002-0.004% of catalyst, 15-25% of organic solvent, 54-58% of water; wherein, the reactive hydroxy silicone oil is a diol hydroxyl mono-terminated silicone oil; the polyol compound is a composition of polytetrahydrofuran diol, polycarbonate diol, and diol containing carbon-carbon double bonds. The present invention also discloses a preparation method of the above-mentioned waterborne polyurethane surface layer resin and its application in semi-silicon synthetic leather. The present invention introduces carbon-carbon double bonds and Y-shaped structure silicone oil into polyurethane, which can greatly improve the bonding strength between semi-silicon surface treatment agent and polyurethane; it also has excellent hydrolysis resistance and heat resistance.

Description

Technical Field

[0001] The invention relates to the technical field of waterborne polyurethane synthetic leather, in particular to a waterborne polyurethane surface layer resin and a preparation method and application thereof. Background Art

[0002] Since the 21st century, synthetic leather products have been successfully used in sofas, clothing, shoes, electronic packaging, decoration and other fields with their excellent performance and gorgeous appearance. In recent years, synthetic leather products for sofas not only require excellent mechanical properties, color fastness, acid and alkali resistance, but also put forward higher requirements for personalized indicators such as feel, touch, and appearance.

[0003] In recent years, sofa leather products have been treated with silicone surface to prepare "semi-silicon" sofa leather products, which can achieve functional characteristics such as easy decontamination and skin-feeling surface, and are deeply favored by consumers. However, the water-based polyurethane surface resin used in the market has problems in combining with silicone surface treatment materials (organic silicone surface treatment materials for short). The sofa leather just after semi-silicon surface treatment is basically normal. After being made into a sofa, the silicone will delaminate, the surface gloss will disappear, and powder will fall off in about half a year.

[0004] At present, the silicone surface treatment materials used are usually based on vinyl-terminated polydimethylsiloxane polymers. Since polyurethane is a polar polymer material and silicone is a non-polar polymer material, the polarity difference between the two is large, making it difficult for the silicone surface treatment material to achieve a high degree of bonding with the polyurethane surface layer. Summary of the invention

[0005] Based on the technical problems existing in the background technology, the present invention proposes a water-based polyurethane surface resin and a preparation method and application thereof. The present invention introduces a carbon-carbon double bond and a Y-shaped structure of a diol hydroxyl mono-terminated silicone oil into the polyurethane, which can greatly improve the bonding strength between the semi-silicon surfactant and the polyurethane; and the present invention also has excellent hydrolysis resistance and heat resistance.

[0006] The invention provides a waterborne polyurethane surface layer resin, wherein the raw materials thereof include, by mass percentage, 6.0-8.0% of diisocyanate, 12-15% of polyol compound, 0.8-1.6% of reactive hydroxy silicone oil, 1.6-2.5% of nonionic hydrophilic chain extender, 1.1-1.7% of small molecule chain extender, 0.002-0.004% of catalyst, 15-25% of organic solvent and 54-58% of water;

[0007] Wherein, the reactive hydroxy silicone oil is a diol hydroxy mono-terminated silicone oil;

[0008] The polyol compound is a combination of polytetramethylene glycol, polycarbonate glycol and a diol containing a carbon-carbon double bond.

[0009] The above-mentioned diol hydroxyl mono-terminated silicone oil can be reactive hydroxyl silicone oil 8866, etc., which can be purchased from the market. The above-mentioned diol hydroxyl mono-terminated silicone oil has a Y-shaped structure, and the specific structure is as follows Figure 1 As shown, R is an alkyl group; the organic silicon segment is based on repeated Si-O bonds as the main chain, and an organic group is connected to the silicon atom, and the organic group can be hydrogen, alkyl, aryl, etc.

[0010] The present invention introduces a polyol compound containing a carbon-carbon double bond into the polyurethane soft segment. The carbon-carbon double bond is opened, and the polyol compound can react with the silicon-hydrogen bond of the organic silicon in the semi-silicon surfactant to form an appropriate amount of covalent bond, which can significantly improve the bonding strength between the semi-silicon surfactant and the polyurethane. In addition, a Y-shaped diol hydroxyl mono-terminated silicone oil is introduced into the polyurethane molecular chain, and the polyol compound can be combined with the organic silicon molecular chain in the semi-silicon surfactant in the form of a side chain anchor in the polyurethane molecular chain, which can significantly improve the bonding strength between the semi-silicon surfactant and the polyurethane. The above two effects cooperate with each other to greatly improve the bonding strength between the semi-silicon surfactant and the polyurethane.

[0011] Preferably, the diol containing carbon-carbon double bonds is at least one of polybutadiene diol and maleic anhydride adipic acid polyester diol.

[0012] The maleic anhydride adipic acid polyester diol is prepared by copolymerization of maleic anhydride, adipic acid and diol, and can be purchased from the market.

[0013] Preferably, the mass ratio of polytetrahydrofuran diol, polycarbonate diol and diol containing carbon-carbon double bonds is 2-3:3-4:1.

[0014] Preferably, the diisocyanate is an aliphatic diisocyanate.

[0015] Preferably, the aliphatic diisocyanate is at least one of isophorone diisocyanate and hexamethylene diisocyanate.

[0016] The invention selects a suitable mass ratio of polytetrahydrofuran diol, polycarbonate diol and diol containing carbon-carbon double bonds, and selects suitable types of diisocyanate and polyol compounds, so that the waterborne polyurethane surface layer resin is firmly combined with the semi-silicon surface treatment agent, and the waterborne polyurethane surface layer resin can also have excellent hydrolysis resistance and heat resistance.

[0017] Preferably, the nonionic hydrophilic chain extender is a diol hydroxyl mono-terminated polyether.

[0018] The above-mentioned diol hydroxyl mono-terminated polyether can be a non-ionic hydrophilic chain extender such as YmerN120, which can be purchased from the market. The above-mentioned non-ionic hydrophilic chain extender can improve the hydrophilicity of waterborne polyurethane and ensure the emulsifiable performance of polyurethane resin.

[0019] Preferably, the small molecule chain extender is a combination of a diol chain extender and a diamine chain extender.

[0020] Preferably, the weight ratio of the diol chain extender to the diamine chain extender is 0.8-1.2:0.3-0.5.

[0021] Preferably, the diol chain extender is at least one of ethylene glycol, 1,4-butanediol and neopentyl glycol.

[0022] Preferably, the diamine chain extender is at least one of ethylenediamine and isophoronediamine.

[0023] Preferably, the raw material of the waterborne polyurethane surface layer resin also contains an antioxidant.

[0024] Preferably, the mass percentage of the antioxidant is 0.016-0.03%.

[0025] The catalyst may be an organic bismuth catalyst MB20 or the like; the antioxidant may be pentaerythritol tetrakis(β-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate), 2,2'-methylenebis(4-methyl-6-tert-butylphenol) or the like.

[0026] The present invention also proposes a method for preparing the above-mentioned water-based polyurethane surface layer resin, comprising the following steps: taking reactive hydroxy silicone oil and diisocyanate, reacting in the presence of a catalyst; then adding a polyol compound, a non-ionic hydrophilic chain extender, and part of a small molecule chain extender to continue the reaction; then adding an organic solvent to reduce the viscosity, and adding water for emulsification and dispersion; then adding the remaining small molecule chain extender to continue the chain extension reaction, and removing the organic solvent to obtain the water-based polyurethane surface layer resin.

[0027] Preferably, in the presence of a catalyst, the reaction is carried out at a constant temperature of 100-105° C. for 2-3 hours.

[0028] Preferably, the reaction is continued at 80-85°C for 2-2.5 hours.

[0029] Preferably, when the temperature drops to 30-40° C., an organic solvent is added to reduce the viscosity.

[0030] Preferably, the reactive hydroxy silicone oil is dried before use.

[0031] The above organic solvent may be acetone or the like; the above water may be deionized water.

[0032] The invention also proposes the application of the waterborne polyurethane surface layer resin in semi-silicon synthetic leather.

[0033] Beneficial effects:

[0034] 1. The present invention introduces a polyol compound containing a carbon-carbon double bond into the polyurethane soft segment. By opening the carbon-carbon double bond, it can react with the silicon-hydrogen bond of the organosilicon in the semi-silicon surfactant to form an appropriate amount of covalent bond, which can significantly improve the bonding strength between the semi-silicon surfactant and the polyurethane;

[0035] 2. The present invention introduces a Y-shaped diol hydroxyl mono-terminated silicone oil into the polyurethane molecular chain, which will combine with the organic silicon molecular chain in the semi-silicon surfactant in the form of a side chain anchor in the polyurethane molecular chain, which can significantly improve the bonding strength between the semi-silicon surfactant and the polyurethane;

[0036] Through the cooperation of the above two effects, the bonding strength between the semi-silicone surfactant and polyurethane is greatly improved, solving the problem of poor bonding between water-based polyurethane surface resin and semi-silicone surfactant in the industry. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the Y-type structure diagram of diol hydroxyl single-terminated silicone oil.

[0038] Figure 2 It is the structural formula of reactive hydroxy silicone oil 6636. DETAILED DESCRIPTION

[0039] The technical solutions of the present invention are described in detail below through specific embodiments. However, it should be clearly stated that these embodiments are for illustration only and are not to be construed as limiting the scope of the present invention.

[0040] Raw material information: Polytetramethylene glycol (PTMG-2000) was purchased from Dalian Chemical Industry Co., Ltd., Taiwan, China; polycarbonate diol (PCD-5652) was purchased from Asahi Chemical Industry; polybutadiene glycol (HTPB) was purchased from Shandong Qilong Chemical; reactive hydroxy silicone oil 8866 was purchased from Shin-Etsu of Japan; reactive hydroxy silicone oil 6636 was purchased from Hangzhou Chongyao; hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI) were purchased from Wanhua Chemical; non-ionic hydrophilic chain extender YmerN120 was purchased from Pasto, Sweden; maleic anhydride butane adipate (PPAB) was purchased from Asahikawa Chemical (Suzhou) Co., Ltd.; ethylene glycol, ethylenediamine, and acetone were all in reagent bottles.

[0041] Example 1

[0042] A waterborne polyurethane surface layer resin, whose raw materials include: 60.2g of hexamethylene diisocyanate (HDI), 70.1g of isophorone diisocyanate (IPDI), 35g of polybutadiene diol (HTPB), 88g of polytetramethylene glycol (PTMG-2000), 122g of polycarbonate diol (PCD-5652), 886615g of reactive hydroxy silicone oil, 39.8g of non-ionic hydrophilic chain extender YmerN120, 19.8g of ethylene glycol, 7.8g of ethylenediamine, 0.05g of organic bismuth catalyst, 0.3g of antioxidant I-1010, 380g of acetone, and 1069g of deionized water.

[0043] The preparation method of the above-mentioned waterborne polyurethane surface layer resin comprises the following steps:

[0044] Weigh the raw materials according to the above formula, add reactive hydroxy silicone oil 8866 and antioxidant I-1010 into the reaction bottle, and dehydrate under reduced pressure at 110-120° C. for 1-2 hours to obtain a mixture;

[0045] The mixture is cooled to 50°C, and then HDI and IPDI are added, and the mixture is reacted at a constant temperature of 100-105°C for 2h, during which an organic bismuth catalyst is added; then PTMG-2000, PCD-5652, YmerN120, HTPB, and ethylene glycol are added and the mixture is reacted at 80-85°C for 2h, and then a sample is taken, and the NCO% is determined by di-n-butylamine titration method to be lower than the theoretical value, and the reaction is stopped; when the temperature is cooled to 30-40°C, acetone is added to reduce the viscosity, and deionized water is added to emulsify and disperse the mixture under high-speed stirring at 1000-1500rpm; then ethylenediamine is added and the mixture is stirred and mixed, and then the acetone is removed by reduced pressure distillation at 40-55°C to obtain a waterborne polyurethane surface resin.

[0046] Example 2

[0047] A waterborne polyurethane surface layer resin, whose raw materials include: 58.8g of hexamethylene diisocyanate (HDI), 72.1g of isophorone diisocyanate (IPDI), 33g of maleic anhydride butane adipate (PPAB), 90g of polytetramethylene glycol (PTMG-2000), 123g of polycarbonate diol (PCD-5652), 14g of reactive hydroxy silicone oil 886614g, 39.8g of non-ionic hydrophilic chain extender YmerN120, 19.8g of ethylene glycol, 8.2g of ethylenediamine, 0.05g of organic bismuth catalyst, 0.3g of antioxidant I-1010, 365g of acetone, and 1069g of deionized water.

[0048] The preparation method of the above-mentioned waterborne polyurethane surface layer resin comprises the following steps:

[0049] Weigh the raw materials according to the above formula, add reactive hydroxy silicone oil 8866 and antioxidant I-1010 into the reaction bottle, and dehydrate under reduced pressure at 110-120° C. for 1-2 hours to obtain a mixture;

[0050] The mixture is cooled to 50°C, and then HDI and IPDI are added, and the mixture is reacted at a constant temperature of 100-105°C for 2h, during which an organic bismuth catalyst is added; then PTMG-2000, PCD-5652, YmerN120, PPAB, and ethylene glycol are added and the mixture is reacted at 80-85°C for 2h, and then a sample is taken, and the NCO% is determined by di-n-butylamine titration method to be lower than the theoretical value, and the reaction is stopped; when the temperature is cooled to 30-40°C, acetone is added to reduce the viscosity, and deionized water is added to emulsify and disperse the mixture under high-speed stirring at 1000-1500rpm; then ethylenediamine is added and the mixture is stirred and mixed, and then the acetone is removed by reduced pressure distillation at 40-55°C to obtain a waterborne polyurethane surface resin.

[0051] Example 3

[0052] A waterborne polyurethane surface layer resin, whose raw materials include: 60g of hexamethylene diisocyanate (HDI), 72.1g of isophorone diisocyanate (IPDI), 10g of maleic anhydride butane adipate (PPAB), 25g of polybutadiene glycol (HTPB), 90g of polytetramethylene glycol (PTMG-2000), 113g of polycarbonate diol (PCD-5652), 886625g of reactive hydroxy silicone oil, 39.8g of non-ionic hydrophilic chain extender Ymer N120, 19.8g of ethylene glycol, 8.2g of ethylenediamine, 0.05g of organic bismuth catalyst, 0.3g of antioxidant I-1010, 365g of acetone, and 1069g of deionized water.

[0053] The preparation method of the above-mentioned waterborne polyurethane surface layer resin comprises the following steps:

[0054] Weigh the raw materials according to the above formula, add reactive hydroxy silicone oil 8866 and antioxidant I-1010 into the reaction bottle, and dehydrate under reduced pressure at 110-120° C. for 1-2 hours to obtain a mixture;

[0055] The mixture is cooled to 50°C, and then HDI and IPDI are added, and the mixture is reacted at a constant temperature of 100-105°C for 2h, and an organic bismuth catalyst is added during the reaction; then PTMG-2000, PCD-5652, YmerN120, PPAB, HTPB, and ethylene glycol are added and the reaction is continued at 80-85°C for 2h, and then a sample is taken, and the NCO% is determined by di-n-butylamine titration method to be lower than the theoretical value, and the reaction is stopped; when the temperature is cooled to 30-40°C, acetone is added to reduce the viscosity, and deionized water is added to emulsify and disperse under high-speed stirring at 1000-1500rpm; then ethylenediamine is added and the stirring and mixing are continued, and then the acetone is removed by reduced pressure distillation at 40-55°C to obtain a waterborne polyurethane surface resin.

[0056] Comparative Example 1

[0057] A waterborne polyurethane surface layer resin, wherein the formula does not contain reactive hydroxy silicone oil 8866 and polybutadiene glycol (HTPB), and contains 123 g of polytetramethylene glycol (PTMG-2000) and 137 g of polycarbonate diol (PCD-5652), and the rest is the same as in Example 1.

[0058] The preparation method of the above-mentioned waterborne polyurethane surface layer resin comprises the following steps:

[0059] According to the above formula, weigh the raw materials, add PTMG-2000, PCD-5652, YmerN120, ethylene glycol, and antioxidant I-1010 into a reaction bottle and dehydrate under reduced pressure at 110-120° C. for 1-2 hours to obtain a mixture;

[0060] The mixture is cooled to 50°C, and then HDI and IPDI are added, and the mixture is reacted at 80-85°C for 2h, during which an organic bismuth catalyst is added; then a sample is taken, and the NCO% is determined by di-n-butylamine titration method to be lower than the theoretical value, and the reaction is stopped; when the temperature is cooled to 30-40°C, acetone is added to reduce the viscosity, and deionized water is added to emulsify and disperse the mixture under high-speed stirring at 1000-1500rpm; then ethylenediamine is added, and stirring and mixing are continued, and then acetone is removed by reduced pressure distillation at 40-55°C to obtain a waterborne polyurethane surface resin.

[0061] Comparative Example 2

[0062] A water-based polyurethane surface layer resin, wherein the formula does not contain reactive hydroxy silicone oil 8866, and the content of polytetramethylene glycol (PTMG-2000) is 104 g, and the other contents are the same as those in Example 2.

[0063] The preparation method of the above-mentioned waterborne polyurethane surface layer resin comprises the following steps:

[0064] According to the above formula, weigh the raw materials, add PTMG-2000, PCD-5652, YmerN120, PPAB, ethylene glycol, and antioxidant I-1010 into a reaction bottle and dehydrate under reduced pressure at 110-120° C. for 1-2 hours to obtain a mixture;

[0065] The mixture is cooled to 50°C, and then HDI and IPDI are added, and the mixture is reacted at 80-85°C for 2h, during which an organic bismuth catalyst is added; then a sample is taken, and the NCO% is determined by di-n-butylamine titration method to be lower than the theoretical value, and the reaction is stopped; when the temperature is cooled to 30-40°C, acetone is added to reduce the viscosity, and deionized water is added to emulsify and disperse the mixture under high-speed stirring at 1000-1500rpm; then ethylenediamine is added, and stirring and mixing are continued, and then acetone is removed by reduced pressure distillation at 40-55°C to obtain a waterborne polyurethane surface resin.

[0066] Comparative Example 3

[0067] Replace "reactive hydroxy silicone oil 8866" with "reactive hydroxy silicone oil 6636", and the rest is the same as Example 3.

[0068] The structural formula of the reactive hydroxy silicone oil 6636 is as follows: Figure 2 As shown, the silicone segment is introduced into the main chain of polyurethane.

[0069] The waterborne polyurethane surface layer resins of Examples 1-3 and Comparative Examples 1-3 were prepared into working slurries with a viscosity of 3000-5000 CPS / 25° C. according to the proportions in Table 1.

[0070] Table 1 Working slurry formula

[0071] raw material Thickener Leveling agent Water-based black paste Waterborne polyurethane surface resin Quality Ratio 0.3-0.5 0.1-0.2 3 100

[0072] Take the working pulp of each group above, respectively coat it on release paper (the knife gap is controlled at 0.15mm), and then dry it at 110-130℃ (preferably the drying time is 3-5min) to obtain a water-based surface layer; then, stir the two-component solvent-free combination material XCNS-3007C-A or XCNS-3007C-B (purchased from Asahikawa Chemical (Suzhou) Co., Ltd.) and coat it on the water-based surface layer (the coating gap is controlled at 0.3mm), pre-cook it at 100℃ for 1-2min, stick it to the base fabric, and then bake it at 130-140℃ (preferably the baking time is 3-5min), then peel off the release paper to obtain the synthetic leather surface layer.

[0073] The prepared semi-silicon surface treatment agent is coated on the surface layer of the sampled synthetic leather by roller coating, and then placed in an oven at 130-140° C. for 5-7 minutes to obtain the semi-silicon synthetic leather.

[0074] The semi-siliconized synthetic leathers of each group were placed in a constant temperature and humidity chamber (temperature: 70°C, humidity: 95%) for 4, 5, 6 and 10 weeks, and then taken out to observe the surface appearance. The bonding strength between the semi-siliconized treatment agent and the polyurethane was determined by scratching with fingers, and whether there was delamination. The results are shown in Table 2 below.

[0075] Table 2 Test results

[0076]

[0077] It can be seen from the results in Table 2 that the waterborne polyurethane surface layer resin of the present invention has a high bonding strength with the semi-silicon treatment agent currently on the market, and the problem of delamination of the silicone surface treatment agent will not occur.

[0078] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any technician familiar with the technical field can make equivalent replacements or changes according to the technical scheme and inventive concept of the present invention within the technical scope disclosed by the present invention, which should be covered by the protection scope of the present invention.

Claims

1. A waterborne polyurethane surface layer resin, characterized in that: The raw materials include, by mass percentage, 6.0-8.0% diisocyanate, 12-15% polyol compound, 0.8-1.6% reactive hydroxy silicone oil, 1.6-2.5% nonionic hydrophilic chain extender, 1.1-1.7% small molecule chain extender, 0.002-0.004% catalyst, 15-25% organic solvent, and 54-58% water; the sum of the mass percentages of the raw materials is 100%; The reactive hydroxy silicone oil is a diol hydroxyl mono-terminated silicone oil with a Y-shaped structure; The polyol compound is a combination of polytetramethylene glycol, polycarbonate diol, and diol containing carbon-carbon double bonds; The diol containing a carbon-carbon double bond is at least one of polybutadiene diol and maleic anhydride adipic acid polyester diol.

2. The waterborne polyurethane surface layer resin according to claim 1, characterized in that: The mass ratio of polytetrahydrofuran diol, polycarbonate diol and diol containing carbon-carbon double bonds is 2-3:3-4:

1.

3. The waterborne polyurethane surface layer resin according to claim 1, characterized in that: The diisocyanate is an aliphatic diisocyanate.

4. The waterborne polyurethane surface layer resin according to claim 3, characterized in that: The aliphatic diisocyanate is at least one of isophorone diisocyanate and hexamethylene diisocyanate.

5. The waterborne polyurethane surface layer resin according to claim 1, characterized in that: The nonionic hydrophilic chain extender is a diol hydroxyl mono-terminated polyether.

6. The waterborne polyurethane surface layer resin according to claim 1, characterized in that: The small molecule chain extender is a combination of a diol chain extender and a diamine chain extender.

7. The waterborne polyurethane surface layer resin according to claim 6, characterized in that: The weight ratio of the diol chain extender to the diamine chain extender is 0.8-1.2:0.3-0.

5.

8. The waterborne polyurethane surface layer resin according to claim 6, characterized in that: The diol chain extender is at least one of ethylene glycol, 1,4-butanediol and neopentyl glycol.

9. The waterborne polyurethane surface layer resin according to claim 6, characterized in that: The diamine chain extender is at least one of ethylenediamine and isophoronediamine.

10. The waterborne polyurethane surface layer resin according to claim 1, characterized in that: The raw material of the waterborne polyurethane surface layer resin also contains an antioxidant.

11. The waterborne polyurethane surface layer resin according to claim 10, characterized in that: The mass percentage of the antioxidant is 0.016-0.03%.

12. A method for preparing the waterborne polyurethane surface layer resin according to any one of claims 1 to 11, characterized in that: The method comprises the following steps: taking reactive hydroxy silicone oil and diisocyanate and reacting them in the presence of a catalyst; then adding a polyol compound, a nonionic hydrophilic chain extender and part of a small molecule chain extender to continue the reaction; then adding an organic solvent to reduce the viscosity and adding water for emulsification and dispersion; then adding the remaining small molecule chain extender to continue the chain extension reaction and removing the organic solvent to obtain a waterborne polyurethane surface layer resin.

13. The method for preparing the waterborne polyurethane surface layer resin according to claim 12, characterized in that: In the presence of a catalyst, the reaction is carried out at a constant temperature of 100-105°C for 2-3 hours.

14. The method for preparing the waterborne polyurethane surface layer resin according to claim 12, characterized in that: The reaction was continued at 80-85°C for 2-2.5h.

15. The method for preparing the waterborne polyurethane surface layer resin according to claim 12, characterized in that: When the temperature drops to 30-40°C, add organic solvent to reduce viscosity.

16. The method for preparing the waterborne polyurethane surface layer resin according to claim 12, characterized in that: The reactive hydroxy silicone oil is dried before use.

17. Use of the waterborne polyurethane surface layer resin according to any one of claims 1 to 11 in semi-silicone synthetic leather.

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