High-viscosity heat-resistant waterborne polyurethane for battery label and preparation method of high-viscosity heat-resistant waterborne polyurethane

Abstract

The invention relates to the field of polyurethane materials, in particular to high-viscosity heat-resistant waterborne polyurethane for a battery label and a preparation method of the high-viscosity heat-resistant waterborne polyurethane. The waterborne polyurethane is prepared from raw materials in percentage by weight as follows: 25%-35% of glycyrrhizic acid modified epoxidized soybean oil, 18%-20% of isocyanate, 0.2%-0.5% of a catalyst, 0.6%-1.2% of a chain extender, 10%-18% of modified nano filler, 6%-8% of a wetting agent, 2.2%-2.5% of a curing agent and the balance of a solvent. According to the present invention, with the formula, the prepared waterborne polyurethane has characteristics of high adhesion, lasting adhesion reaching more than 50 h, excellent high temperature resistance, biodegradability, and environmental protection.

Description

Technical Field

[0001] The present invention relates to the field of polyurethane materials, and particularly to a high-viscosity hot water-resistant polyurethane for battery labels and a preparation method thereof. Background Art

[0002] Polyurethane was developed by Otto Bayer in 1937 as an independent polymer class. As a kind of binder, polyurethane binder has a unique chemical structure, including soft segments and hard segments composed of isocyanate, chain extender, and crosslinking agent, among which there are polar bonds such as urethane bonds and urea bonds. This harmonious structure endows polyurethane binder with excellent adhesion and film-forming properties. Nowadays, this material has been widely used in a variety of different fields, such as foams, insulation materials, plastics, composites, films, plastics, composites, sealants, coatings, inks, and adhesives.

[0003] When polyurethane is applied as a binder resin to printing inks, its surface and overall properties after forming are affected by the crosslinking density, the movement of side reaction segments of polyurethane groups, the structure and interaction of polyols and isocyanates, and other components in the coating formulation. The formulation of inks consists of different components to enhance their performance. Polar groups in the surface chemical structure and hydrophilic components in the coating formulation are crucial for the water repellency of polyurethane inks, especially at high temperatures, otherwise it will lead to a decrease in hydrophobicity.

[0004] Printing inks containing polyurethane binder resin have the unique feature of exerting strong mechanical properties through intermolecular hydrogen bond interactions. However, due to its relatively hydrophilic nature, it is still challenging to design super-hydrophobic or ice-repellent polyurethane-based coatings with high durability, especially at low temperatures. During the use of polyurethane, when it comes into contact with moisture in the air, its performance may be affected. When the environmental temperature is high or dry, the moisture gradually decreases, and polyurethane begins to shrink, which leads to a further increase in internal stress. If polyurethane is in an alternating dry and humid environment for a long time, the internal stress will accumulate continuously. With the accumulation of internal stress, fatigue phenomena may occur in polyurethane materials. On the surface, problems such as bubbles, defects, and cracks may gradually appear. At the same time, due to the action of internal stress, the bonding strength between the polyurethane coating and the base material will decrease, and even peeling may occur.

[0005] Waterborne polyurethane, as an environmentally friendly and multifunctional material, has been used as a binder resin for printing inks. However, there are still defects in the molecular structure of waterborne polyurethane itself, such as uneven molecular weight, hydrophilic groups in the molecular chain, and relatively large molecular chain polarity, which result in performance problems such as poor mechanical properties, poor hydrophobic properties, and thermal instability of the material, making it difficult to meet specific application requirements. Summary of the Invention

[0006] The object of the present invention is to provide a high-viscosity and hot-water-resistant polyurethane for battery labels and a preparation method thereof. As a bonding resin, this polyurethane is applied to the printing ink of battery labels and has excellent adhesion and heat resistance.

[0007] In a first aspect, the present application provides a high-viscosity and hot-water-resistant polyurethane for battery labels, which is prepared from the following raw materials: 25-35% of glycyrrhizic acid-modified epoxy soybean oil, 18-20% of isocyanate, 0.2-0.5% of catalyst, 0.6-1.2% of chain extender, 10-18% of modified nano-filler, 6-8% of wetting agent, 2.2-2.5% of curing agent, and the remaining amount is solvent; wherein, the modified nano-filler is 1-allyl-3-methylimidazolium tetrafluoroborate surface-modified nano-alumina.

[0008] In some embodiments of the present invention, the preparation method of the glycyrrhizic acid-modified epoxy soybean oil comprises the following steps: sequentially adding epoxy soybean oil, glycyrrhizic acid, an organotin catalyst, and a polar solvent into a four-necked flask equipped with mechanical stirring, a thermometer, and a condenser, and heating and reacting under the protection of an inert gas; stopping the reaction when the acid value of the system no longer decreases and reaches stability; removing the solvent and water by vacuum drying to obtain the glycyrrhizic acid-modified epoxy soybean oil.

[0009] Further, the molar ratio of the epoxy groups contained in the epoxy soybean oil to the carboxyl groups contained in the glycyrrhizic acid is 0.8-1:1.1-1.2.

[0010] Further, the organotin catalyst is at least one of tetrabutyltin, dibutyltin dichloride, dibutyl dimethoxy tin, tetraphenyltin, diphenyltin dichloride, and dibutyltin dilaurate, and its addition amount accounts for 2-5% of the total mass of the epoxy soybean oil and glycyrrhizic acid.

[0011] Further, the polar solvent is at least one of trimethylamine, triethylamine, diethylamine, N-methylmorpholine, N,N-dimethylaniline, tetramethylethylenediamine, diisopropylamine, and dimethylacetamide.

[0012] By adopting the above technical solution, the glycyrrhizic acid-modified epoxy soybean oil prepared contains multiple aromatic rings in its molecular structure, which can improve the mechanical properties of the polyamide prepared therefrom. Coupled with the presence of ether bonds, as well as the remaining hydroxyl groups and partial carboxyl groups, it can endow the polyurethane with stronger viscosity.

[0013] In some embodiments of the present invention, the preparation method of the modified nano filler comprises the following steps: Take 1-allyl-3-methylimidazolium tetrafluoroborate and place it in an ultrasonic oscillator for ultrasonic treatment for 30 - 45 min. Then weigh nano-aluminum oxide and put it into a three-necked flask. Subsequently, add the 1-allyl-3-methylimidazolium tetrafluoroborate that has completed ultrasonic treatment, add an anhydrous alcohol solvent, heat to 60 - 80 °C and stir at a constant temperature for 2 - 5 h, and then dry it in a vacuum drying oven to obtain the modified nano filler.

[0014] Further, the mass ratio of the 1-allyl-3-methylimidazolium tetrafluoroborate to the nano-aluminum oxide is 2.5 - 3.2:100.

[0015] Further, the anhydrous alcohol solvent is at least one of anhydrous methanol, anhydrous ethanol, anhydrous isopropanol, and anhydrous n-propanol; its addition amount and the volume-mass ratio of the nano-aluminum oxide is 2 - 3 ml:1 g.

[0016] By adopting the above technical solution, the particle size of the nano filler is optimized, and the specific surface area of the nano filler is increased, so that the contact area between the modified nano filler and the polyurethane matrix is greatly increased, thereby enhancing the interfacial interaction and bonding force, contributing to the more stable dispersion of the nano filler in the polyurethane system and more effectively transmitting internal stress, thus further improving the viscosity of the waterborne polyurethane and its high-temperature resistance characteristics, and extending its service life.

[0017] In some embodiments of the present invention, the diisocyanate is any one of hexamethylene diisocyanate, isophorone diisocyanate, methylcyclohexyl diisocyanate, dicyclohexylmethylene diisocyanate, 2,2,4-trimethylhexane diisocyanate, xylylene diisocyanate, p-1,4-tolylmethylene diisocyanate, and dimer acid diisocyanate.

[0018] In some embodiments of the present invention, the catalyst is at least one of tetrabutyltin, dibutyltin dichloride, dibutyltin dimethoxide, tetraphenyltin, diphenyltin dichloride, and dibutyltin dilaurate.

[0019] In some embodiments of the present invention, the curing agent is an amine curing agent, specifically, it can be diethyltoluenediamine, dimethylthiotoluenediamine, 4,4'-methylenebis(2-chloroaniline), etc.

[0020] In some embodiments of the present invention, the solvent is at least one of an alcohol solvent and an ester solvent. Specifically, for example, ethanol, methanol, propanol, ethyl acetate, butyl acetate, and dimethyl carbonate, etc.

[0021] In some embodiments of the present invention, the chain extender is obtained by mixing dimethylthiotoluenediamine and ethanolamine in a weight ratio of 1-2:1; the wetting agent is at least one of nonylphenol polyoxyethylene ether and polyoxyethylene fatty alcohol ether.

[0022] Second, the present invention also provides a method for preparing the high-viscosity heat-resistant water-dispersible polyurethane, comprising the following steps:

[0023] S1: Under the protection of dry inert gas, put glycyrrhizic acid-modified epoxy soybean oil into a reaction vessel, heat up to 100-110 °C to remove water under reduced pressure, and then lower the system temperature to 60-80 °C;

[0024] S2: Add isocyanate and catalyst to the reaction vessel, mix evenly, and then use a solvent to adjust the viscosity of the mixture, and keep the reaction at 60-80 °C for 10-12 h;

[0025] S3: After the reaction in S2 is completed, add a chain extender for cross-linking reaction, stir, and then add a modified nano-filler, a wetting agent and a curing agent and stir to obtain the waterborne polyurethane.

[0026] Beneficial effects: Compared with the prior art, the waterborne polyurethane prepared by the present invention uses glycyrrhizic acid-modified epoxy soybean oil as a raw material and adds a self-made modified nano-filler: the aromatic ring and oxygen-containing six-membered ring contained in glycyrrhizic acid have high resonance energy, which can enhance the thermal stability of the waterborne polyurethane; the modified nano-filler is prepared by surface-modifying nano-alumina with 1-allyl-3-methylimidazolium tetrafluoroborate, which not only realizes the uniform distribution of the nano-filler in the polyurethane system, but also enables heat to be dispersed by the presence of the uniformly distributed nano-filler, thus showing excellent heat resistance.

[0027] In addition, because of the controllable partial residue of the hydrophilic group in glycyrrhizic acid when glycyrrhizic acid-modified epoxy soybean oil is used in the waterborne polyurethane, while improving the adhesion required for the polyurethane as a binder resin, the heat resistance is not damaged due to the presence of the hydrophilic group. Specific embodiments

[0028] The above content of the present invention will be further described in detail below through specific examples, but it should not be understood that the content of the present invention is limited to the following examples.

[0029] The preparation processes of the glycyrrhizic acid-modified epoxy soybean oil and the modified nano-filler used in the examples are exemplarily described below:

[0030] Glycyrrhizic acid-modified epoxy soybean oil #1

[0031] 136.6 g (0.14 mol) of epoxidized soybean oil, 304.5 g (0.37 mol) of glycyrrhizic acid, 5.04 g of tetrabutyltin, and 1500 ml of trimethylamine were successively added to a reaction vessel equipped with mechanical stirring, a thermometer, and a condenser, and the reaction was carried out by heating to 110 °C under nitrogen protection; when the acid value of the system no longer decreased and reached stability, the reaction was stopped; the solvent and water were removed by vacuum drying to obtain the glycyrrhizic acid-modified epoxidized soybean oil #1.

[0032] Glycyrrhizic acid-modified epoxidized soybean oil #2

[0033] 146.3 g (0.15 mol) of epoxidized soybean oil, 312.7 g (0.38 mol) of glycyrrhizic acid, 8.09 g of dibutyltin dichloride, and 1500 ml of diethylamine were successively added to a reaction vessel equipped with mechanical stirring, a thermometer, and a condenser, and the reaction was carried out by heating to 120 °C under nitrogen protection; when the acid value of the system no longer decreased and reached stability, the reaction was stopped; the solvent and water were removed by vacuum drying to obtain the glycyrrhizic acid-modified epoxidized soybean oil #2.

[0034] Glycyrrhizic acid-modified epoxidized soybean oil #3

[0035] 160.9 g (0.165 mol) of epoxidized soybean oil, 329.2 g (0.4 mol) of glycyrrhizic acid, 14.01 g of dibutyltin dimethoxide, and 1500 ml of N,N-dimethylaniline were successively added to a reaction vessel equipped with mechanical stirring, a thermometer, and a condenser, and the reaction was carried out by heating to 125 °C under nitrogen protection; when the acid value of the system no longer decreased and reached stability, the reaction was stopped; the solvent and water were removed by vacuum drying to obtain the glycyrrhizic acid-modified epoxidized soybean oil #3.

[0036] Glycyrrhizic acid-modified epoxidized soybean oil #4

[0037] 146.3 g (0.15 mol) of epoxidized soybean oil, 246.9 g (0.30 mol) of glycyrrhizic acid, 4.57 g of tetrabutyltin, and 800 ml of trimethylamine were successively added to a reaction vessel equipped with mechanical stirring, a thermometer, and a condenser, and the reaction was carried out by heating to 110 °C under nitrogen protection; when the acid value of the system no longer decreased and reached stability, the reaction was stopped; the solvent and water were removed by vacuum drying to obtain the glycyrrhizic acid-modified epoxidized soybean oil #4.

[0038] Modified nano filler #1

[0039] Take 2.5 g of 1-allyl-3-methylimidazolium tetrafluoroborate and place it in an ultrasonic oscillator for ultrasonic treatment for 30 min. Then weigh 100 g of nano-aluminum oxide and put it into a three-necked flask. Subsequently, add the ultrasonic-completed 1-allyl-3-methylimidazolium tetrafluoroborate, add 200 ml of anhydrous methanol, heat to 60 °C and stir at a constant temperature for 5 h, and then dry it in a vacuum drying oven to obtain the modified nano-filler #1.

[0040] Modified nano-filler #2

[0041] Take 2.8 g of 1-allyl-3-methylimidazolium tetrafluoroborate and place it in an ultrasonic oscillator for ultrasonic treatment for 30 min. Then weigh 100 g of nano-aluminum oxide and put it into a three-necked flask. Subsequently, add the ultrasonic-completed 1-allyl-3-methylimidazolium tetrafluoroborate, add 250 ml of anhydrous ethanol, heat to 70 °C and stir at a constant temperature for 3 h, and then dry it in a vacuum drying oven to obtain the modified nano-filler #2.

[0042] Modified nano-filler #3

[0043] Take 3.2 g of 1-allyl-3-methylimidazolium tetrafluoroborate and place it in an ultrasonic oscillator for ultrasonic treatment for 30 min. Then weigh 100 g of nano-aluminum oxide and put it into a three-necked flask. Subsequently, add the ultrasonic-completed 1-allyl-3-methylimidazolium tetrafluoroborate, add 300 ml of anhydrous isopropanol, heat to 80 °C and stir at a constant temperature for 2 h, and then dry it in a vacuum drying oven to obtain the modified nano-filler #3.

[0044] Modified nano-filler #4

[0045] Take 3.5 g of 1-allyl-3-methylimidazolium tetrafluoroborate and place it in an ultrasonic oscillator for ultrasonic treatment for 30 min. Then weigh 100 g of nano-aluminum oxide and put it into a three-necked flask. Subsequently, add the ultrasonic-completed 1-allyl-3-methylimidazolium tetrafluoroborate, add 300 ml of anhydrous isopropanol, heat to 80 °C and stir at a constant temperature for 2 h, and then dry it in a vacuum drying oven to obtain the modified nano-filler #4.

[0046] Example 1

[0047] S1: Under the protection of dry nitrogen, put 35% glycyrrhizic acid-modified epoxy soybean oil #1 into a reaction vessel, heat up to 100 °C to remove water under reduced pressure, and then lower the system temperature to 60 °C;

[0048] S2: Add 20% hexamethylene diisocyanate and 0.5% dibutyltin dilaurate to the reaction vessel, mix evenly, and use 17.7% ethanol to adjust the viscosity of the mixed solution, and keep reacting at 60 °C for 12 h;

[0049] S3: After the reaction of S2 is completed, 0.6% chain extender (the mass ratio of dimethylthiotolylenediamine to ethanolamine is 1:1) is added for crosslinking reaction, stirred, and then 18% modified nano-filler #1, 6% wetting agent (nonylphenol polyoxyethylene ether), and 2.2% diethyltoluenediamine are added and stirred to obtain the aqueous polyurethane.

[0050] Example 2

[0051] S1: Under the protection of dry nitrogen, 30% glycyrrhizic acid-modified epoxy soybean oil #2 is put into a reaction vessel, heated to 105 °C to remove water under reduced pressure, and then the system temperature is lowered to 70 °C;

[0052] S2: 19% isophorone diisocyanate and 0.3% diphenyldichlorotin are added to the reaction vessel. After mixing evenly, the viscosity of the mixed solution is adjusted with 26.5% methanol, and the reaction is carried out at 70 °C for 11 h;

[0053] S3: After the reaction of S2 is completed, 0.9% chain extender (the mass ratio of dimethylthiotolylenediamine to ethanolamine is 2:1) is added for crosslinking reaction, stirred, and then 14% modified nano-filler #2, 7% wetting agent (polyoxyethylene fatty alcohol ether), and 2.3% dimethylthiotolylenediamine are added and stirred to obtain the aqueous polyurethane.

[0054] Example 3

[0055] S1: Under the protection of dry nitrogen, 25% glycyrrhizic acid-modified epoxy soybean oil #3 is put into a reaction vessel, heated to 110 °C to remove water under reduced pressure, and then the system temperature is lowered to 80 °C;

[0056] S2: 18% methylcyclohexyl diisocyanate and 0.5% dibutyldichlorotin are added to the reaction vessel. After mixing evenly, the viscosity of the mixed solution is adjusted with 34.8% ethyl acetate, and the reaction is carried out at 80 °C for 10 h;

[0057] S3: After the reaction of S2 is completed, 1.2% chain extender (the mass ratio of dimethylthiotolylenediamine to ethanolamine is 2:1) is added for crosslinking reaction, stirred, and then 10% modified nano-filler #3, 8% wetting agent (a mixture of nonylphenol polyoxyethylene ether and polyoxyethylene fatty alcohol ether in a mass ratio of 1:1), and 2.5% 4,4'-methylenebis(2-chloroaniline) are added and stirred to obtain the aqueous polyurethane.

[0058] Example 4

[0059] The process is the same as that of Example 3, except that glycyrrhizic acid-modified epoxy soybean oil #4 is used to replace glycyrrhizic acid-modified epoxy soybean oil #3.

[0060] Example 5

[0061] The process was the same as that of Example 3, except that modified nano-filler #4 was used to replace modified nano-filler #3.

[0062] Comparative Example 1

[0063] The process was the same as that of Example 3, except that unmodified epoxy soybean oil was used to replace glycyrrhizic acid-modified epoxy soybean oil #3.

[0064] Comparative Example 2

[0065] The process was the same as that of Example 3, except that unmodified nano-aluminum oxide was used to replace modified nano-filler #3.

[0066] Comparative Example 3

[0067] The process was the same as that of Example 3, except that unmodified epoxy soybean oil was used to replace glycyrrhizic acid-modified epoxy soybean oil #3, and unmodified nano-aluminum oxide was used to replace modified nano-filler #3.

[0068] Performance Test

[0069] 180° Peel Strength: Measured using an electronic tensile machine in accordance with the standard of GB / T2792-2014;

[0070] High Temperature Resistance: A layer of glue with a thickness of 2 mm was applied to the surface of the label, pasted on a glass sheet, and placed at 100 °C for 30 min, and evaluated by observing whether bubbles appeared.

[0071] The test results are shown in Table 1:

[0072] Table 1 Performance Test Results of the Waterborne Polyurethanes Obtained from Examples 1-5 and Comparative Examples 1-2

[0073]

[0074] As can be seen from the data in Table 1, the waterborne polyurethane provided by the present invention has excellent adhesion and high temperature resistance. However, if a certain amount of carboxyl groups are not retained in the glycyrrhizic acid-modified epoxy soybean oil molecules, or if the amount of 1-allyl-3-methylimidazolium tetrafluoroborate used in the preparation of the modified nano-filler is excessive (Examples 4-5), the adhesion of the resulting waterborne polyurethane will decrease, and its heat resistance will also weaken. This may be because when the amount of 1-allyl-3-methylimidazolium tetrafluoroborate continues to increase, due to the similar structures between the organic alkane chains connected to the imidazole ring in the ionic liquid, interactions will occur, increasing the aggregation tendency between the modified nano-fillers, resulting in a slight increase in the actual particle size of the distributed modified nano-fillers, and ultimately leading to uneven distribution, which affects the overall performance of the resulting waterborne polyurethane. The results of Comparative Example 3 illustrate from the opposite side that there is a non-negligible synergistic effect between the glycyrrhizic acid-modified epoxy soybean oil and the modified nano-filler in improving the performance of the waterborne polyurethane in the present invention.

Claims

1. A high-viscosity and hot-water-resistant polyurethane for battery labels, characterized in that, Prepared from the following raw materials by weight percentage: 25-35% of glycyrrhizic acid-modified epoxy soybean oil, 18-20% of diisocyanate, 0.2-0.5% of catalyst, 0.6-1.2% of chain extender, 10-18% of modified nano filler, 6-8% of wetting agent, 2.2-2.5% of curing agent, and the balance is solvent; wherein, the modified nano filler is 1-allyl-3-methylimidazolium tetrafluoroborate surface-modified nano alumina.

2. The high-viscosity hot water-resistant polyurethane according to claim 1, wherein The preparation method of the glycyrrhizic acid-modified epoxy soybean oil comprises the following steps: successively adding epoxy soybean oil, glycyrrhizic acid, organotin catalyst and polar solvent into a reaction vessel equipped with mechanical stirring, thermometer and condenser tube, heating to 110-125 °C under inert gas protection for reaction; stopping the reaction when the acid value of the system no longer decreases and reaches stability; removing the solvent and water by vacuum drying to obtain the glycyrrhizic acid-modified epoxy soybean oil.

3. The high-viscosity hot water-resistant polyurethane according to claim 2, wherein The molar ratio of the epoxy groups contained in the epoxy soybean oil to the carboxyl groups contained in the glycyrrhizic acid is 0.8-1:1.1-1.

2.

4. The high-viscosity hot water-resistant polyurethane according to claim 2, wherein The organotin catalyst is at least one of tetrabutyltin, dibutyltin dichloride, dibutyltin dimethoxide, tetraphenyltin, diphenyltin dichloride, dibutyltin dilaurate, and its addition amount accounts for 2-5% of the total mass of epoxy soybean oil and glycyrrhizic acid.

5. The high-viscosity hot water-resistant polyurethane according to claim 2, wherein The polar solvent is at least one of trimethylamine, triethylamine, diethylamine, N-methylmorpholine, N,N-dimethylaniline, tetramethylethylenediamine, diisopropylamine, dimethylacetamide.

6. The high-viscosity hot water-resistant polyurethane according to claim 1, wherein The preparation method of the modified nano filler comprises the following steps: placing 1-allyl-3-methylimidazolium tetrafluoroborate in an ultrasonic oscillator for ultrasonic treatment for 30-45 min, then weighing nano alumina and putting it into a three-necked flask, then adding the ultrasonic-completed 1-allyl-3-methylimidazolium tetrafluoroborate, adding an anhydrous alcohol solvent, heating to 60-80 °C for constant-temperature stirring reaction for 2-5 h, and drying in a vacuum drying oven to obtain the modified nano filler.

7. The high-viscosity hot water-resistant polyurethane according to claim 6, wherein The mass ratio of the 1-allyl-3-methylimidazolium tetrafluoroborate to the nano alumina is 2.5-3.2:

100.

8. The high-viscosity hot-water resistant polyurethane according to claim 6, wherein, The anhydrous alcohol solvent is at least one of anhydrous methanol, anhydrous ethanol, anhydrous isopropanol, anhydrous n-propanol; its addition amount and the volume-mass ratio of the nano alumina is 2-3 ml:1 g.

9. The high-viscosity hot water-resistant polyurethane according to claim 1, wherein The chain extender is obtained by mixing dimethylthiotoluenediamine and ethanolamine in a weight ratio of 1-2:1; the wetting agent is at least one of nonylphenol polyoxyethylene ether and polyoxyethylene fatty alcohol ether.

10. The preparation method of the high-viscosity hot-water resistant polyurethane according to any one of claims 1-8, characterized in that, Comprising the following steps: S1: Under the protection of dry inert gas, put the glycyrrhizic acid-modified epoxy soybean oil into a reaction vessel, raise the temperature to 100-110 °C to remove water under reduced pressure, and then lower the system temperature to 60-80 °C; S2: Add isocyanate and catalyst to the reaction vessel, mix evenly, and then adjust the viscosity of the mixture with solvent, and keep the reaction at 60-80 °C for 10-12 h; S3: After the reaction of S2 is completed, a chain extender is added for cross-linking reaction, followed by stirring. Then, a modified nano filler, a wetting agent, and a curing agent are added and stirred to obtain the aqueous polyurethane.

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