KR101647301B1 - Fabricating method for wood adhesives composition - Google Patents

Abstract

Mixing polyvinyl alcohol, water, and fine powder to form a fine aqueous solution; And a step of adding an isocyanate compound to the aqueous solution of the fine particles to form an aqueous solution of the two components.

Description

FIELD OF THE INVENTION [0001] The present invention relates to wood adhesives,

TECHNICAL FIELD The techniques disclosed herein relate to methods of making adhesive compositions for wood, and more particularly to methods for making practical use of adhesive compositions for wood without formaldehyde components.

Most wood adhesives are synthesized from petroleum based materials due to many advantages such as water resistance, good adhesion and low cost. However, the use of formaldehyde-based adhesives such as urea-formaldehyde (UF) and phenol-formaldehyde (PF) has attracted a lot of attention recently due to environmental issues. However, due to the hydrolysis of the adhesive composition, formaldehyde may be generated and gradually released to the surrounding environment, which causes the indoor air quality to deteriorate. On the other hand, natural adhesives derived from biomaterials are cheap, safe and reproducible materials. In particular, polyurethane (PU) adhesives based on natural materials have been extensively studied due to their unique properties such as manageable curing speed, no formaldehyde emission, excellent weatherability and excellent adhesion. For example, a polyurethane adhesive made of potato starch / natural oil based polyol and toluene diisocyanate (TDI) for joining wood panels has been reported by Sandip et al. (Sandip D. Desai, Jigar V. Patel, Vijay Kumar Sinha, Polyurethane adhesive system from biomaterial-based polyol for bonding wood. International Journal of Adhesion & Adhesives , 23, 393-399, 2003 ).

However, the synthesis method thereof is complicated, costly, and requires a multi-step process such as a chemical reaction at a high temperature and a vacuum distillation process, so that it is difficult to be practically applied to wood-based panels. Generally, in the production of wood products, the cost of the adhesive is higher than that of wood, so a lower cost and improved process is required.

1. Sandip D. Desai, Jigar V. Patel, Vijay Kumar Sinha, 'Polyurethane adhesive system from biomaterial-based polyol for bonding wood.' International Journal of Adhesion & Adhesives, 23, 393-399, 2003. 2. R. Gallego, J.F. Arteaga, V. Valencia, J.M. Franco, " Rheology and thermal degradation of isocyanate-functionalized methyl cellulose-based oleogels. &Quot; Carbohydarate Polymers, 98, 152-160, 2013. 3. Aamer Saeed, Ghulam Shabir, 'Synthesis of thermally stable high-gloss water dispersible polyurethane / polyacrylate resins.' Progress in Organic Coatings, 76, 1135-1143, 2013. 4. Nairong Chen, Qiaojia Lin, Jiuping Rao, Qinzhi Zeng, " Water resistances and bonding strengths of soy-based adhesives containing different carbohydrates. Industrial Crops and Products, 50, 44-49, 2013.

According to one aspect of the present invention, there is provided a method for producing a water-soluble polyvinyl alcohol, which comprises mixing polyvinyl alcohol, water, and a fine powder to form a fine aqueous solution; And injecting an isocyanate compound into the aqueous fine particle solution to form a binary aqueous solution; And stirring the aqueous bicarbonate solution to form a final adhesive composition.

According to another aspect of the present invention, there is provided a method for producing a microcapsule, comprising: mixing polyvinyl alcohol, water, and a fine powder to form a fine aqueous solution; Mixing an aqueous solution of the fine particles with an isocyanate compound to obtain a urethane adhesive in the form of a binary aqueous solution; And applying the urethane adhesive to a wood substrate, and then joining the wood substrate and another wood substrate to each other by heating and pressing the other wood substrate in an odd numbered manner.

According to another aspect of the present invention, there is provided an adhesive composition for wood in the form of an aqueous bicarbonate solution comprising an aqueous solution of a fine powder and an isocyanate compound mixed with polyvinyl alcohol, water, and fine particles of 200 탆 or less, Wherein the polyvinyl alcohol is contained in an amount of 1 to 5% by weight, the fine powder is contained in an amount of 10 to 20% by weight, and the isocyanate compound is contained in an amount of 10 to 40% by weight.

1 is a process flow diagram illustrating a method of manufacturing an adhesive for wood according to an embodiment of the present invention.
Figure 2 is a schematic diagram showing the formation of chemical bonds between the pMDI and the wood structure as a carbohydrate source.
Figure 3 shows the specifications of test specimens according to KS F 3101.
Figure 4 shows the effect of the differential content on adhesive performance.

Various embodiments of the present invention will now be described in detail with reference to the drawings. The adhesive for wood according to one embodiment of the present invention uses a fine powder (rice powder, RP) as a carbohydrate raw material. The derivatives are cheap and have hydroxyl functional groups that can easily provide a chemical cross-linking reaction with isocyanates of polymeric methylene diphenyl-diisocyanate (pMDI).

A method for manufacturing an adhesive for wood according to an embodiment of the present invention can be manufactured in the following manner. First, polyvinyl alcohol (PVA), water, and fine powder are mixed to form a fine aqueous solution. Next, an isocyanate compound is added to the aqueous solution of the fine particles to form an aqueous solution of two components to obtain an adhesive composition for wood.

 1 is a process flow diagram illustrating a method of manufacturing an adhesive for wood according to an embodiment of the present invention. Referring to Fig. 1, polyvinyl alcohol (PVA), water, and fine powder are mixed in step S1 to form a fine aqueous solution. The aqueous solution of the fine powder is prepared by adding the PVA powder into water and stirring at a temperature elevated temperature of, for example, 40 to 70 ° C to form a PVA aqueous solution. The fine powder is slowly added to the aqueous PVA solution and then the fine powder is completely dissolved Lt; / RTI > and cooling at room temperature.

The polyvinyl alcohol is added to the adhesive composition to improve the lack of tackiness. The addition of the polyvinyl alcohol can increase the viscosity as well as the viscosity and stickiness of the polyurethane adhesive solution. As the polyvinyl alcohol, those having a degree of saponification of 70 to 97 mol% and an average degree of polymerization of about 500 to 3,000 can be used. When the average degree of polymerization of the polyvinyl alcohol is less than 500, even if the polyvinyl alcohol is crosslinked by isocyanate, it may be difficult to exhibit sufficient adhesion due to a low degree of polymerization. When the average degree of polymerization is more than 3,000, the problem of poor solubility in water Can be. If the degree of saponification is less than 70 mol%, the adhesive strength may deteriorate. If the degree of saponification is more than 97 mol%, the reaction time of a large amount of hydroxyl groups and the other isocyanate, life may be shortened.

The polyvinyl alcohol may be added in an amount of 1 to 30 parts by weight, preferably 5 to 20 parts by weight, based on 100 parts by weight of the isocyanate compound. If the addition amount of the polyvinyl alcohol is less than the above range, the effect of improving the tackiness is insignificant, and if it exceeds the above range, the water resistance of the adhesive composition may be deteriorated.

It is preferable that an appropriate amount of isocyanate compound is contained in order to impart strong initial adhesion and water resistance between woody materials during a short bonding time. The isocyanate compound is effectively crosslinked with the hydroxyl group of polyvinyl alcohol and the hydroxyl group of cellulose (fibrin) of wood to form a urethane bond, so that the adhesive for wood has water-resistant adhesion. The isocyanate compound may be contained in an amount of 10 to 40% by weight, preferably 15 to 30% by weight, based on the total weight of the adhesive for wood. When the content of the isocyanate compound is less than the above range, it is difficult to satisfy the water resistance for a short bonding time. If the content is above the above range, the pot life of the adhesive composition is deteriorated, Can be.

The fine powder used in the present invention is used as a natural polyol, and is made by pulverizing rice particles. For example, the fine powder used in the embodiment of the present invention to be described later is obtained by pulverizing 100% white rice, and the particle size is approximately 50 to 200 탆.

According to FAO statistics for 2012, rice accounted for more than 20% of world grain resource production, followed by sugarcane and corn. Rice contains about 80% of carbohydrates, about 7% of protein, and other lipids, ash and vitamins. Differences have higher carbohydrate content than other grains, for example, about 70% for corn and wheat. Since the carbohydrate structure contains many hydroxyl functional groups capable of forming crosslinks with isocyanate, it can be used as a raw material for forming the basic skeleton of the adhesive. Adhesiveness is due to amylopectin, which accounts for 70 to 80% of carbohydrates. Amylopectin is a mixture of main chains in which 25 to 30 glucose are linked by α-1,4-linkages and α- Amylopectin, which forms a 1,6-bond, is insoluble in cold water but melts in hot water and has high viscosity.

Since the fine powders used in the present invention are prepared in the form of a colloidal solution having a high viscosity and a semi-transparent state by heating to 60 ° C or higher, the size of the fine particles when pulverized, irrespective of the type of rice, It is preferable to mix it with isocyanate in terms of uniformity in coating.

In addition, the above-mentioned derivatives have advantages such as low price, low production cost due to a large amount of production compared to other grains, and diversity of cultivation areas. In addition, when making woody board materials, additives such as wheat flour and rice briquette which are 10-15% of total adhesive amount for viscosity increase or filling are used in addition to adhesives. By using fine powder as a raw material of adhesive, There is also an advantage that no additional use is required.

Before the development of synthetic adhesives in the past, rice was boiled and used as wallpaper for wallpapering, but rice (fine powder) itself has weak adhesive strength and weak water resistance. The present invention uses an isocyanate as a crosslinking agent to enhance such low adhesion.

The fine powder may be added in an amount of 10 to 150 parts by weight, preferably 20 to 120 parts by weight, more preferably 30 to 100 parts by weight, based on 100 parts by weight of the isocyanate compound. If the added amount of the fine powder is less than the above range, the viscosity is very weak, and it may flow down when it is applied or sprayed on the woody raw material, or may be absorbed into the woody raw material to cause adhesion failure. When the amount exceeds the above range, I can not do it. The polyvinyl alcohol may be contained in an amount of 1 to 5% by weight and the fine powder may be contained in an amount of 10 to 20% by weight based on the weight of the total fine aqueous solution in the aqueous solution of the fine particles.

Figure 2 is a schematic diagram showing the formation of chemical bonds between the pMDI and the wood structure as a carbohydrate source.

Referring back to FIG. 1, in step S2, an isocyanate compound is added to the aqueous solution of the fine particles to form an aqueous solution of two components. And then the binary aqueous solution is stirred in step S3 to form the final adhesive composition.

After the addition of the isocyanate compound, the mixture is stirred for a predetermined time, for example, 10 seconds to 1 minute to form a final adhesive composition in the form of a two-component aqueous solution. Hydrophobic polymeric isocyanate compounds are not soluble in water but can be dissolved once they are mixed with an aqueous solution of a fine powder. This is due to the formation of a hydrophilic adduct between the carbohydrate and the isocyanate compound.

The method for producing an adhesive composition for wood according to an embodiment of the present invention is implemented as a two-component system. If a urethane adhesive is manufactured using a one-component system, there is a problem that an isocyanate compound is sensitive to water, to be. In other words, if the isocyanate compound is quickly mixed with the aqueous solution of the microparticles immediately before applying the two-component urethane adhesive to the veneer, and then applied to the veneer and heated and pressed (pressed), the moisture in the polyurethane adhesive rapidly evaporates, Structure, the NCO functional group of the isocyanate reacts more well with the OH functional group of the derivative and the fibrin moiety of the single plate than with water to form a chemical crosslinking agent. Unless it is a two-component system, the isocyanates are sensitive to moisture and can form by-products such as polyurea and non-elliptical structures.

Examples of the isocyanate compound usable in the present invention include aliphatic diisocyanates such as tetramethylene diisocyanate, trimethylhexamethylene diisocyanate, 1,6-hexamethylene diisocyanate and lysine diisocyanate; Hydrogenated xylene diisocyanates such as 1,3-cyclohexylene diisocyanate and 1,4-cyclohexyl diisocyanate; Alicyclic diisocyanates such as isoholon diisocyanate and 1,5-tetrahydronaphthalene diisocyanate; 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, diphenylmethane-4,4'-diisocyanate, diphenylmethane- Aromatic diisocyanates such as 4-diisocyanate, diphenylmethane-2,2'-diisocyanate, 3,3'-dimethyl-4,4'-biphenylene diisocyanate and 1,4-naphthalene diisocyanate; Aromatic aliphatic diisocyanates such as xylene diisocyanate and tetramethyl xylene diisocyanate; Triisocyanates such as 2,4,6-triisocyanate toluene, 2,4,4'-triisocyanate diphenyl ether, triisocyanate phenyl methane and triisocyanate phenyl thiophosphite; A biuret type polyisocyanate derived from 3 moles of diisocyanate and 1 mole of water; Isocyanurate type polyisocyanates formed by trimerization of a diisocyanate; Poly methylene polyphenyl polyisocyanate by-product when the diphenylmethane-4,4'-diisocyanate is produced; Adduct type polyisocyanates with glycols, triols, polyester polyols or polyether polyols; And polyisocyanates of isocyanate prepolymer. Among these, it is preferable to use liquid isocyanates which are relatively stable in an aqueous solution and have low volatility. Diphenylmethane-4,4'-diisocyanate has a lower vapor pressure than other isocyanates and has a lower toxic level due to low occurrence of harmful vapors during handling, and is the most widely used, and is structurally simple compared to other isocyanates , Especially because the NCO functional group is exposed at position 4 of the linear structure and the NCO functional group is attached at the 2-position, or the other structure is attached, which is more reactive than other isocyanates which are sterically hindered . For example, polymeric methylene diphenyl-diisocyanate (pMDI), which is a polymeric form of diphenylmethane-4,4'-diisocyanate, is used as the isocyanate compound.

The adhesive composition for wood according to one embodiment of the present invention is also preferably substantially free of an organic solvent. In the context of the present invention, "substantially free of organic solvent" preferably means less than 5% by weight, particularly preferably less than 3% by weight, more particularly preferably less than 1% by weight, Means the weight percent, i. E. Does not contain an organic solvent, the amount given here being based on the total amount of the composition in each case.

According to one embodiment of the present invention, the composition is used as a coating material and an adhesive, particularly as an adhesive for joining wood substrates. In view of the present invention and according to DIN 16920, 06/1981, the adhesive is a non-metallic substrate which bonds the substrate parts by surface bonding and internal strength (adhesion). The wood substrate, which can be bonded with an adhesive according to the present invention, is a particularly wood-loaded structural element. The wood substrate may be selected from the group consisting of maple, apple, birch, pear, beech, bean, oak, serviceberry, duck, ash, spruce, , Gold lacquer, gold teak, as well as lime, walnut, poplar, sycamore, robinia, marronier, red birch, elm, Swiss pear, fir, elm, willow and plum (eg, aningeria, stone pine, bete, bubinga, ebony, hickory, iroko, kambala, kevazingo, limba, Macassar ebony, makore, mansonia, meranti, bog oak, mutenye, silver plum, olive ash, Oregon pine, rosewood, jasper, rhinoceros, rio, pencil jam, american cedar, Large maple trees, giant arborvitae, sapelli mahogany, sequoia, sen, sucupira, tchitola, teak, such as birdseye maple, wenge, and zebrano. < RTI ID = 0.0 > Beech, spruce, larch, dong, pine and rovinia are preferred.

The adhesive composition for wood made by the above-described production method can be used as an adhesive in which formaldehyde is not dispersed by using a recyclable raw material, a fine powder and an isocyanate compound as a crosslinking agent. The wood adhesive composition had an adhesive strength of 0.6 N / mm < ² > when tested according to KS F 3101 Or more tensile shear adhesive strength. For example, the wood adhesive has a dry tensile strength of 1.5 to 2.5 N / mm ² , a wet tensile strength of 0.7 to 1.5 N / mm ² , a water resistance tensile strength The cyclic boiled tensile strength may be 0.7 to 1.0 N / mm < ² >.

According to one embodiment of the present invention, a method of bonding a wood substrate using a urethane adhesive is provided. First, polyvinyl alcohol, water, and a fine powder are mixed to form a fine aqueous solution. Next, an isocyanate compound is mixed with the aqueous solution of the fine particles to obtain a urethane adhesive in the form of a two-component aqueous solution. Then, the urethane adhesive is applied to the wood substrate, and then the wood substrate and the other wood substrate are superimposed on each other in an odd number of layers so as to be orthogonal to each other. The urethane adhesive is preferably applied to the wood substrate immediately after preparation. For example, it is preferable to apply the two-component aqueous solution to the wood substrate within 1 to 20 minutes, preferably within 3 to 10 minutes after the preparation. The reason for this is that when 20 minutes have elapsed after the adhesive is mixed, the adhesive itself hardens, which makes it impossible to uniformly apply the adhesive to the wooden substrate, resulting in poor adhesion.

The urethane adhesive is applied to the at least one wood substrate in an amount of from 100 to 500 g / m ² , preferably from 120 to 350 g / m ² , and then the wood substrate is also selectively applied with the urethane adhesive according to the invention Bonded to at least one other wood substrate. Next, the heating conditions for heating and pressing them may be 130 to 200 ° C, preferably 150 to 180 ° C, and the pressing conditions may be 5 to 30 kgf / cm ² , preferably 10 to 20 kgf / cm ² .

The present invention will be described in more detail with reference to the following examples. Those skilled in the art will recognize that the scope of the present invention is not limited to these examples.

[Example]

Preparation of differential (RP) aqueous solution

30 g of PVA 500 (manufactured by OCI Corporation) was added to 850 g of water and stirred at about 60 캜. During heating, 150 g of a fine powder (manufactured by Ant Industries Ltd.) was slowly added to the hot PVA aqueous solution and stirred. The mixture was stirred for about 1 hour to completely dissolve the fine powder, and then the resulting fine powder aqueous solution was cooled to room temperature.

Manufacture of adhesives for wood mixed with fine powders and pMDI

The two-component solution of the aqueous solution of the fine powders and the pMDI was mixed for about 30 seconds to prepare a final wood adhesive. A two-component fine-particle-containing polyurethane adhesive solution (adhesive solution A-1 to A-5) was prepared by adding 150 g, 300 g, 400 g, 425 g and 450 g of pMDI on the basis of the differential 150 g.

Adhesion test

To evaluate the adhesiveness of the adhesive solutions A-1 to A-5 according to the ratio of pMDI / RP, each adhesive solution prepared was applied to a veneer according to the following test conditions, hot pressed to prepare a test specimen, and the test method of KS F 3101 , The adhesion test was carried out.

1) Non-internal tensile strength test (dry tensile strength test)

The test material used in the tensile shear adhesion test was a radiate pine veneer with a thickness of 2.7 mm and a moisture content of 8-10%. The aqueous solution of the above-mentioned pulverulent solution and the pMDI were quickly mixed for 30 seconds and then applied to the upper and lower surfaces of the middle plate of three veneers (300 mm x 300 mm x 2.7 mm) to allow an amount of 130 g / m ² to be adhered. The following plywood was placed in a high-temperature heater and cured at a temperature of 150 ± 2 ° C under a pressure of 981 kPa (10 kg / cm ² ) for about 5 minutes. The plywood was cut into specimens of 80 mm x 25 mm in size according to KS F 3101. Figure 3 shows the specifications of test specimens according to KS F 3101.

2) Wet tensile strength test

The adhesive strength of plywood specimens was measured by semi - internal tensile shear adhesive strength test. Ten specimens cut from each plywood panel were immersed in 60 ± 3 ° C hot water for 3 hours, then immersed in water at room temperature, taken out, and subjected to a water resistance tensile shear test.

3) Cyclic boiled tensile strength test

The water resistance of plywood specimens was measured by the tensile shear strength test. Ten specimens cut from each plywood panel were soaked in boiling water for 4 hours. The boiled samples were dried at 60 ° C for 20 hours. It was then boiled in boiling water for 4 hours and then soaked in water at room temperature. The wet tensile shear bond strength test was performed using the obtained wet specimens.

Table 1 shows the results of comparing the adhesiveness of the adhesive according to various pMDI / RP ratios.

No.
pMDI
(g)
PVA
(g)
RP
(g)
water
(g)
Tensile Shear Adhesion (N / mm ² ) Non-domestic Quasi-domestic Domestic consumption A-1 100 20 100 567 1.65 0 0 A-2 100 10 50 283 1.88 0.40 0.02 A-3 100 7.5 37.5 212.5 2.12 0.56 0.51 A-4 100 7.0 35 200 1.67 0.84 0.37 A-5 100 6.7 33 189 1.57 1.17 0.79 Comparative Example 100 6.7 33 189 1.45 0.78 0.67

* In Table 1, the comparative example is a result of applying flour instead of fine powder in the same manner as A-5

In the case of using pMDI only because the plywood is manufactured using a single plate, it is absorbed into the wood when the adhesive is applied on the surface of the single plate, although not shown in Table 1, Displacement may eventually lead to poor adhesion. Therefore, it is preferable to use the derivative in combination with pMDI. However, if the amount of the fine powder is too large, the OH functional group capable of reacting with the NCO functional group of pMDI becomes too large, and the adhesion of pMDI (quasi-in- or domestic-water) may be reduced. From the results of the above Table 1, it can be concluded that A-5 passes the KS standard for all three adhesive forces, and A-5 is the optimum condition.

On the other hand, in the case of the comparative example, the adhesive strength was tested under the condition of A-5, which showed the best result in the differential test using wheat flour instead of the fine powder. As a result, the adhesive strength was lower than that of the A-5 using the fine powder.

Figure 4 shows the effect of the differential content on adhesive performance. Referring to FIG. 4, as the weight ratio of the fine particles to the pMDI is decreased to a certain level, the adhesion performance is improved.

As a result of the tensile strength test, the tensile shear adhesive strength of the specimen having the highest performance in the non-water tensile shear adhesive strength test was 2.12 N / mm ² , and the specimen was an A-3 adhesive. Semi-water resistance Tensile shear strength and water resistance tensile shear adhesion are the most important properties in wood adhesives. Adhesive A-5 showed the best results with a quasi-internal tensile shear adhesive strength of 1.17 N / mm ² and a water resistance tensile shear adhesive strength of 0.79 N / mm ² . Both of the results showed a tensile shear adhesion strength of 0.6 N / mm ² or more, which is required for plywood made from coniferous wood in the quasi-domestic tensile shear adhesive strength and water resistance tensile shear adhesive strength test according to KS F 3101 of Korea Standard.

Generally, the isocyanate competitively reacts with hydroxyl functional groups and moisture in the adhesive. According to the adhering method of the present invention, water can be easily removed from the adhesive by the high temperature and pressure resistance of the plywood, so that the isocyanate has a greater chance of reacting with the hydroxyl functional group than water, and thus has excellent water resistance.

According to one embodiment of the present invention, an adhesive composition for wood and a method for producing the same provide a method for in-situ synthesis of an adhesive for wood based on an isocyanate compound and a fine powder. The fine powder is an excellent natural material used in an adhesive for wood which does not dissolve formaldehyde, and can be combined with an isocyanate compound such as pMDI to obtain an adhesive for wood having a very excellent water resistance.

Claims (9)

1. An adhesive composition for wood in the form of an aqueous two-component solution comprising a fine-particle adhesive mixed with polyvinyl alcohol, water and a fine powder and a polymeric isocyanate compound,
Wherein the polyvinyl alcohol is contained in an amount of 1 to 5% by weight, the fine particles in an amount of 10 to 20% by weight and the polymer isocyanate compound in an amount of 10 to 40% by weight based on the total weight of the wood adhesive,
Wherein the crosslinking of the hydroxyl functional group of the fine powder with the NCO group of the polymeric isocyanate compound is formed by stirring the two-component aqueous solution to form a basic skeleton of the adhesive. The method according to claim 1,
Wherein the polyvinyl alcohol has a degree of saponification of 70 to 97 mol% and an average degree of polymerization of 500 to 3,000. delete delete delete Mixing polyvinyl alcohol, water, and fine powder to form a fine aqueous solution;
Mixing the aqueous solution of the fine particles with a polymeric isocyanate compound to obtain a urethane adhesive in the form of a binary aqueous solution; And
Applying the urethane adhesive to a wood substrate, and then joining the wood substrate and another wood substrate to each other by heating and pressing the other wood substrate in an odd numbered manner,
Wherein the polyvinyl alcohol is contained in an amount of 1 to 5% by weight, the fine powder is contained in an amount of 10 to 20% by weight and the polymeric isocyanate compound is contained in an amount of 10 to 40% by weight based on the total weight of the urethane adhesive,
Wherein the crosslinking of the hydroxyl functional group of the fine powder with the NCO group of the polymeric isocyanate compound is formed by stirring the two-component aqueous solution, thereby forming the basic structure of the adhesive. The method according to claim 6,
Wherein the application of the urethane adhesive is performed after 1 to 20 minutes from the production of the two-component aqueous solution. delete The method according to claim 1,
Wherein the polymeric isocyanate compound is a polymeric methylenediphenyl-diisocyanate.

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