JP2012135910A - Method for manufacturing of foamed urethane sheet - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a method for manufacturing an foamed urethane sheet which can manufacture a soft foamed urethane sheet having low density and a high foaming ratio with favorable thickness accuracy even if the foamed urethane sheet has a thin thickness.SOLUTION: The method for manufacturing the foamed urethane sheet comprises steps for: applying a composition containing a urethane prepolymer having an NCO group at the terminal of the molecule thereof to a base to form a sheet-like coating film 2; bringing the coating film into contact with a perforated mold-release base 1; and exposing the coating film 2 to water vapor while making the perforated mold-release base 1 contact with the coating film to foam and cure the coating film 2.

Description

  The present invention relates to a method for producing a flexible urethane foam sheet that is suitably used as a sealing material for impact absorption, dust prevention, light shielding and the like in various office automation equipment such as mobile phones, video equipment, personal computers, and thin televisions.

  Cellular phones and OA devices are required to be small and thin. Therefore, the glass substrate for liquid crystal has a thickness of, for example, 0.5 mm or less. Accordingly, the sealing material for fixing the periphery of the liquid crystal display screen with low stress is thin and low in thickness. It is required to be able to be fixed without being displaced even by stress, and to be further excellent in long-term recovery (that is, low compression set). In addition, since a dustproof property and a light shielding property are required, the sealing material needs to have fine cells. As a sealing material applied to such applications, products obtained by thinly slicing a soft or semi-rigid urethane foam have been used. I can no longer handle it. In order to meet such needs, the following technologies are being studied.

  In Patent Documents 1 and 2, in order to achieve a good sealing property and a low relative dielectric constant, a technology for setting a compressive load value in a specific range in a polyurethane-based seal member having a density in a predetermined range, and Patent Document 1 (See Patent Document 2).

  However, in these techniques, the sealing material is manufactured using a method called mechanical flossing, in which air or nitrogen is mechanically mixed with a urethane raw material, and a foamed liquid is thinly applied and cured. This technology uses special main raw materials and foam stabilizers for mechanical foaming, and special delay catalysts are used to prevent gelation and thickening until mechanically foamed and thinly applied. It has become a manufacturing method. Although it is true that a product with a small coating amount can be produced because a pre-foamed urethane stock solution is applied, the mixed urethane stock solution thickens in a short time, so the thickness of the coating is increased by thickening during continuous coating. Changes are likely to occur. Therefore, the thinner the product, the more the amount of catalyst must be reduced, and the product that is actually obtained tends to be a product with a low expansion ratio, that is, a product with a high density and a hard product. is there.

Furthermore, the following techniques are disclosed as techniques for manufacturing a thin foam sheet.
That is, for example, in Patent Document 3, as a method for producing a foamed polyurethane elastomer sheet, a prepolymer having an isocyanate group at a terminal and 2-pyrrolidone are mixed to form the mixture into a sheet, and then the sheet-like mixture is prepared. A method is disclosed in which water vapor is brought into contact with the foam and foamed and cured. Since this method is a non-reactive one-component stock solution containing a prepolymer, there is a possibility that a comma coater suitable for thin coating can be applied, and as a result, a thin foam may be produced. When the person reproduced the example, the sheet actually obtained had a low foaming ratio, a high compressive stress, and a poor appearance on the product surface.

  In Patent Document 4, as a method for producing a polyurethane foam sheet, a hot-melt urethane prepolymer having an isocyanate group at a molecular end that has been heated and melted and a compound having at least two active hydrogen atom-containing groups are mixed. A method is disclosed in which a liquid mixture obtained in the above manner is applied in a sheet form on a substrate, and water vapor is brought into contact with the liquid mixture of the obtained sheet-like substance to cause water foaming. This method is to improve the stability of foam during foaming by melting a urethane prepolymer of a normal temperature with an increased molecular weight and bringing it into contact with water vapor. There is a drawback that the compressive stress becomes high. Moreover, the apparatus used when mixing and applying the hot-melt raw material is an expensive apparatus that operates at a high temperature.

JP 2005-227392 A JP 2006-124578 A Japanese Patent Laid-Open No. 11-246695 JP 2004-216880 A

The manufacturing method of this invention WHEREIN: The schematic sectional drawing which shows one form of the state in which the perforated releasable base material is contacting the urethane coating film. In the manufacturing method of this invention, the schematic sectional drawing which shows the other form of the state in which the perforated releasable base material is contacting the urethane coating film.

  An object of the present invention is to provide a method for producing a urethane foam sheet, which can produce a urethane foam sheet having a low density and a high foaming ratio with high thickness accuracy even when the thickness is small. And

  Another object of the present invention is to provide a method for producing a urethane foam sheet capable of producing a urethane foam sheet having fine cells, excellent resilience, and good appearance.

For example, the present invention is as follows.
[1] Applying a composition containing a urethane prepolymer having an NCO group at the molecular end on a substrate to form a sheet-like coating film, and bringing the release coating substrate into contact with the coating film That is, a method for producing a urethane foam sheet comprising foaming and curing the coating film by exposing the coating film to water vapor in a state where the perforated release substrate is in contact.

  [2] The method according to [1], wherein a hole diameter of the holey release base material is in a range of 0.01 to 2.0 mm.

  [3] The perforated releasable base material in which a hole is formed in a resin film selected from an olefin resin film, a fluorine resin film, a silicone resin film, and a release-treated polyester film, a woven fabric, Or [1] or [2], which is either a net-like material.

  [4] The method according to any one of [1] to [3], wherein the urethane prepolymer is a prepolymer obtained by a reaction between a polyol having a functional group number exceeding 2 and an isocyanate.

  [5] The method according to [4], wherein the polyol has a functional group number of 2.2 to 3.0 and a hydroxyl value of 84 to 168.

  [6] The method according to [4] or [5], wherein the polyisocyanate is diphenylmethane diisocyanate.

  According to the present invention, it is possible to provide a soft urethane foam sheet having a low density and a high foaming ratio even when the thickness is small. In addition, the urethane foam sheet obtained by the present invention has fine cells, is excellent in resilience, and has a good appearance, so that it can be used in various applications as a sealing material that is required to have dust resistance, light shielding properties, restoration durability, etc. It can be preferably used.

The present invention will be described in detail below.
The method for producing a urethane foam sheet according to the present invention comprises applying a composition containing a urethane prepolymer having an NCO group at a molecular end on a substrate to form a sheet-like coating film, and forming a hole in the coating film. And bringing the releasable substrate into contact and foaming and curing by exposing the coating film to water vapor in a state where the perforated releasable substrate is in contact.
In the method of the present invention, foaming and curing of a composition containing a urethane prepolymer (hereinafter also referred to as “urethane stock solution”) proceeds by being exposed to water vapor in a sheet-shaped state. In that case, it is exposed to water vapor through a perforated releasable substrate.

  The perforated releasable base material used in the present invention does not include a base material having moisture permeability enough to allow water vapor to permeate, and needs to have predetermined holes. When a releasable substrate that does not have a predetermined hole and has only moisture permeability is used, foaming and curing are insufficient with high productivity because it is difficult for moisture to penetrate to the center in the width direction and thickness direction of the sheet. The desired urethane foam cannot be obtained.

  The perforated releasable substrate may be a releasable substrate having a predetermined hole (including a substrate subjected to a release treatment), and the predetermined hole is caused by the structure of the substrate. Alternatively, a predetermined hole may be formed in a base material having no hole. For example, a perforated resin film, a fiber woven fabric, a net-like material, and the like can be given.

  Examples of the resin film include mold release treatment using an olefin resin film (polymethylpentene, polypropylene, polyethylene, etc.), a fluorine resin film, a silicone resin film, or a silicone resin, which is a release material. The polyester film etc. which were made are mentioned, In this invention, what formed the predetermined hole in these is used suitably. The releasable resin film is preferable in terms of the following points as compared with paper having releasability, for example. In other words, when the release paper is exposed to humidity, it causes expansion and contraction and the product curls or peels, whereas the release resin film does not change its dimensions even when it absorbs moisture. , Paper defects can be eliminated.

As the woven fabric, a release agent such as a silicone resin is applied to a woven fabric formed using a release material such as polypropylene, polyethylene, or a fluorine resin, or a non-release material such as polyester or nylon fiber. An impregnated one is used, and these woven fabrics having a predetermined hole are preferably used.
Examples of the net-like material include those obtained by stretching an olefin-based resin film or an olefin-based resin film blended with an inorganic filler into a net-like shape, or those obtained by stretching after scratching the film. Preferably used.

The size of the hole can be determined as appropriate. In one embodiment, the diameter is in the range of 0.01 to 2.0 mm, more preferably in the range of 0.05 to 0.5 mm. Even if the hole is this size, the urethane stock solution does not leak from the hole because the wetness of the urethane stock solution to the release substrate is poor.
Moreover, the edge distance (pitch) between adjacent holes can be determined as appropriate. In one embodiment, from the viewpoint of uniform foaming properties, the pitch is preferably 1.4 mm or less, and more preferably 1.2 mm or less. Although a lower limit is not specifically limited, For example, you may be 0.01 mm or more. If the pitch is larger than 1.4 mm, a foamed product having voids between the holes (where the holes are not formed) may be obtained.

  The arrangement of the holes can also be set as appropriate, but for example, the ones having a uniform pitch, such as a staggered lattice, are preferable because no gaps are generated between the holes (where there are no holes).

  As described above, the present invention is characterized in that it is exposed to water vapor in a state in which a releasable substrate with holes is brought into contact with a sheet-like coating film (hereinafter also referred to as “urethane coating film”) made of a urethane stock solution. To do. Here, contacting the urethane coating film with a perforated releasable substrate means that in one embodiment, as shown in FIG. 1, a continuously formed sheet on the urethane coating film 2 formed continuously. It means that the pierced releasable base material 1 is laminated. Thus, by exposing the urethane coating film to water vapor through a perforated releasable substrate without directly exposing it to water vapor, cell defoaming in the coating film skin portion is suppressed, resulting in high foamability. A low density and soft polyurethane foam sheet is obtained. In the present invention, the foaming and curing of the urethane coating film when exposed to water vapor proceeds at substantially the same speed as compared with the case where the urethane coating film is exposed to water vapor without using a perforated releasable substrate.

  In the present invention, the urethane coating film can be exposed to water vapor by, for example, passing the laminate containing the urethane coating film shown in FIG. 1 through a constant temperature and high humidity bath or superheated steam bath containing water vapor. For example, it passes for 1 to 5 minutes through those tanks set to a temperature of 40 to 120 ° C. and a humidity of 30 to 95%. Then, you may perform a drying process and a postcure at 80-120 degreeC.

  The perforated releasable substrate can be easily peeled after foaming and curing of the urethane coating. The urethane foam sheet after peeling the perforated releasable base material has a self-skin formed on its surface. For this reason, the coefficient of friction with a surface to be sealed such as a liquid crystal panel material is increased and it is difficult to slip, and even if it is sealed with low stress, it is unlikely to be displaced.

  Moreover, as another form which makes a urethane-coated film contact a perforated releasable base material, the form which passes the continuous base | substrate with which the urethane coating film was formed, for example in the belt press 5 shown schematically in FIG. Can be mentioned. That is, in the belt press 5, the releasable base material 7 with a hole is wound around belt rolls 6 and 6 'having a function of releasing water vapor (not shown). By passing the laminate 8 in which the urethane coating film 9 is laminated on the continuous base material 10 in the X direction through the belt press 5 from which water vapor has been released, the urethane coating film is perforated and the releasable base material is brought into contact. In such a state, the urethane coating film is exposed to water vapor.

The urethane stock solution used in the present invention for forming a coating film contains a urethane prepolymer having an NCO group at the molecular end. In this urethane stock solution, the polyol used for the synthesis of the contained urethane prepolymer has previously reacted with the polyisocyanate and does not contain water as a blowing agent. That is, this urethane stock solution is non-reactive as it is, and is a so-called one-component urethane stock solution. Therefore, continuous coating with good thickness accuracy is possible without thickening.
A urethane prepolymer having an NCO group at the molecular end (hereinafter also referred to as “terminal NCO group”) is obtained by a reaction between a polyol and a polyisocyanate.
Examples of the polyol that can be used in the present invention include polyoxyalkylenes having a hydroxyl value of 30 to 200, polyesters, dimer acid esters, and polycaprolactones. In addition, special polyols such as polycarbonate, polybutadiene, polyolefin, and castor oil may be used for the purpose of improving strength and water resistance. The number of functional groups of these polyols is preferably more than 2 and more preferably more than 2 and less than 3.5 from the ease of prepolymer synthesis and the physical properties of the urethane foam obtained. In particular, a prepolymer obtained from a polyol having a hydroxyl value of 84 to 168 and a functional group number of 2.2 to 3.0 is preferable because of high terminal NCO%. Furthermore, since the prepolymer is difficult to defoam when foamed in contact with water vapor, it has a high foaming ratio and is a low rebound foam. Therefore, it is preferable because of low stress during compression and low compression set and stickiness. Can be used. Furthermore, since the urethane foam sealing material using these polyols is easily bonded to an untreated polyester film, it is preferable for obtaining a urethane foam sealing material integral with the polyester film as the base material.

Here, the number of functional groups in the present invention is the number of hydroxyl groups in one molecule, but the number of functional groups when a polyol is mixed is a value obtained by multiplying the mole fraction of each polyol by the number of functional groups.
As a polyol, a polyoxyalkylene type is preferable because it has a narrow molecular weight distribution and a narrow distribution of the number of functional groups, so that it is easy to synthesize a prepolymer of a terminal NCO group. Specifically, for example, propylene oxide, ethylene oxide, tetrahydrofuran or the like is added and polymerized using water, alcohols, amines or the like as an initiator. Examples of alcohols include ethylene glycol, propylene glycol, glycerin, trimethylolpropane, and pentaerythritol. Examples of amines include ethanolamine, diethanolamine, triethanolamine, ethylenediamine, and aniline. Furthermore, an alternating copolymer of propylene oxide and phthalic anhydride is also included. Moreover, the polyoxyalkylene type polyol containing vinyl polymer is also mentioned.

  Examples of polyester polyols include glycols such as ethylene glycol, diethylene glycol, 2-methyl 1.3 propylene glycol, glycerin and trimethylol propane, and condensation reactions of triols with dicarboxylic acids such as adipic acid, sebacic acid and phthalic acid. Can be obtained. The dimer acid polyol is a polyester polyol using dimer acid as a dicarboxylic acid. The caprolactone polyol is a polyester polyol obtained by adding caprolactone to glycol or triol.

  As the polyisocyanate that can be used in the present invention, known ones used for the production of polyurethane foams can be used. For example, tolylene diisocyanate (TDI), diphenylmethane diisocyanate (MDI), hexamethylene diamine. Examples thereof include isocyanate (HDI), and these carbodiimide-modified products, burette-modified products, allophanate-modified products, dimer-modified products, and three-body modified products may also be used. TDI and MDI are preferred from the viewpoint of easy synthesis of the terminal NCO group prepolymer and physical properties of the obtained urethane foam. In particular, MDI is most preferable because it has a fast foam curing speed, and therefore requires a small amount of catalyst addition, and further, there is no stickiness on the product surface.

  The method for synthesizing the prepolymer of the terminal NCO group is carried out by a usual method. NCO equivalent ratio of polyisocyanate to OH equivalent of polyol, NCO equivalent / OH equivalent = 1.8 to 3.0 is preferable. If it is 1.8 or less, the viscosity of the prepolymer increases, and gelation tends to occur due to moisture in the air. When 3.0 is exceeded, a crack will arise in the product of the urethane foam obtained by water vapor | steam foaming, or compression set will deteriorate. Most preferred is 2.0 to 2.6. In order to increase the hardness of the foamed sealing material, about 0.5% of water can be added to the polyol for the synthesis of the prepolymer. The prepolymer is liquid at room temperature, and it is easy to mix the raw materials, and since it can be left at a relatively low temperature, the amount of catalyst that can be blended is very flexible (the type and amount of catalyst is limited at high temperatures). In many cases, the temperature control of the construction apparatus can be lowered. The NCO% of the prepolymer is preferably about 3 to 11% because the foaming ratio can be increased and a soft foamed sealing material can be provided.

  In the present invention, a foam stabilizer generally used for the production of urethane foam can be used in the preparation of the urethane stock solution. When these are used, the foamed foam becomes fine and open, and a thick product can be obtained with a small coating amount. As the foam stabilizer, for example, polydimethylsiloxane alone or a silicone compound such as a block copolymer of polydimethylsiloxane and polyether or a graft copolymer can be preferably used. Moreover, a fluorine-type compound like a perfluoroalkyl ethylene oxide adduct can also be used.

  In this invention, the catalyst normally used for manufacture of a urethane foam can be used in preparation of a urethane stock solution. When an amine catalyst or a metal catalyst is used, bubbles can be made finer, a product with a large thickness can be obtained with a small coating amount, and the foam curing time can be shortened. As the amine catalyst, triethylenediamine (TEDA), N, N-dimethylcyclohexylamine (DMC), N-methyldicyclohexylamine (MDC), or dimethylaminoethylmorpholine (XDM) can be preferably used. In particular, DMC, MDC, and XDM are preferable because they do not increase in viscosity even when blended with a prepolymer. Examples of the metal catalyst include stannous catalysts such as stannous octate (SO), dibutyltin dilaurate (DBT), dioctyltin dilaurate (DOT), isopropyl titanate, and bismuth. Even if added, the thickening is small, so that continuous coating of thin materials for a long time is possible, the reaction rate is fast, the physical properties of the obtained steam foamed product are excellent, and it is effective in suppressing the stickiness of the product surface. Even if these catalysts are blended with terminal NCO group prepolymers, they can be applied continuously for a long time because they do not thicken even when heated to a high temperature, for example, 60 to 80 ° C., if they are dry for a long time. In addition, since a large amount of catalyst can be blended, the production speed can be increased.

As a method of applying the urethane stock solution onto the base material, it can be performed by a known method, but in order to obtain a thin coating film, a coating device such as a comma coater, a roll coater, a knife coater, or a die coater. Is preferably applied onto a substrate such as a release paper, a release film, a resin film, or paper. As the coating amount according to these coaters, it can be for example 30g / ^{m 2} ~300g / ^m 2 about a thin coating film formation. The urethane coating film thus thinly formed is foamed and cured by being exposed to water vapor.

  As a base material on which a urethane coating film is formed, for example, a polyester film that has been subjected to a release treatment with silicone may be used, or a urethane film integrated with a film using a polyester film or an olefin film that has been subjected to an untreated or easy adhesion treatment. A sealing material may be obtained.

  Examples The present invention will be described more specifically with reference to examples. However, the scope of the present invention is not limited to the following examples. In the following examples, “part” means part by mass.

<Synthesis of urethane prepolymer>
Polypropylene triol [T1500] (functional group number: 3, OH value: 112, trade name: MN1500; made by Mitsui Chemicals) 50 parts, polypropylene glycol [D1000] (functional group number 2, OH value 112, trade name: Diol1000; Mitsui Chemicals) 50 parts of toluene and 37.66 parts of toluene diisocyanate [T80] (Mitsui Chemicals Co., Ltd.) are mixed (NCO / OH equivalent ratio = 2.2), heated at 80 ° C. for 3 hours, and reacted with NCO. A prepolymer [TD1250T] having a terminal NCO group with a% of 7.3% was synthesized.

In the same manner, other prepolymers shown in Table 1 were synthesized.
In the prepolymer product numbers shown in Table 1, the leading T or D represents the polyol used, and represents polypropylene triol and diol, respectively. The intermediate number represents the molecular weight of the polyol. Terminal T and M indicate the polyisocyanate used, and each represents T80 and MDI (diphenylmethane diisocyanate).
The terminal M in polyolAM represents MDI. Polyol A is described in the note of Table 1.

<Releasable substrate>
Table 2 shows the releasable substrate used in each example.

Example 1
100 parts of prepolymer [TD1250T], 1.0 part of foam stabilizer SF2938F (manufactured by Dow Corning Toray), 0.3 part of a 10% solution of tricresyl phosphate of triethylenediamine (abbreviated as CO), octanoic acid A urethane stock solution containing 0.1 part of tin (abbreviated as SO) was temperature-controlled at 60 ° C. and applied onto a 75 μm release agent-treated polyester film using a 200 μm-thick bar coater. A urethane coating sheet was produced by applying a releasable substrate A with holes from above. This was put into a constant temperature and high humidity bath at 70 ° C. and 90% humidity for 5 minutes to foam and cure the urethane coating. Thereafter, the releasable substrate was peeled off to obtain a urethane foam sealing material. This sealing material was further dried at 100 ° C. for 10 minutes. The evaluation results of the physical properties of the obtained urethane foam sealing material are shown in Table 3.

(Example 2 to Example 11, Example 14)
A urethane foam sealing material was produced in the same manner as in Example 1 except that the type of prepolymer, the type and number of added foam stabilizers, and the releasable base material were changed to those shown in Table 3. The evaluation results of physical properties are shown in the same table.

(Example 12), (Example 13)
In Examples 12 and 13, a urethane foam sheet integrated with a film was produced using a pet film not subjected to a release treatment, instead of a polyester film subjected to a release treatment. The evaluation results of physical properties are shown in Table 3.

(Comparative Example 1)
A sheet was produced in the same manner as in Example 1 except that a 25 μm-thick polymethylpentene film B (trade name: TPX film) was used in place of the perforated moisture-permeable release substrate A in Example 1. The evaluation results of physical properties are shown in Table 3.

  Although the TPX film is known to have high moisture permeability, the foaming / curing speed is much slower than the perforated film, and the central part of the sheet does not permeate and does not foam and cure.

(Comparative Example 2) to (Comparative Example 5)
In Comparative Examples 2 to 5, sheets were produced under the same conditions as in Examples 1 to 4, respectively, except that a perforated release substrate was not used. The evaluation results of physical properties are shown in Table 3.

(Comparative Example 6)
This is an additional test of the invention described in JP-A-11-246695. After blending 2-pyrrolidone, it was exposed to water vapor, but it was not cured even after 5 minutes, and was cured by exposure for 10 minutes.

[Methods for measuring and evaluating physical properties]
[1] Application amount It was determined from the 10 cm square mass and area of the urethane foam sheet. (Unit: g / m ² )
[2] Foam thickness Urethane foam sheet thickness (unit: mm)
[3] Density
It was determined by dividing the 10 cm square mass of the urethane foam sheet by the area and thickness.

(Unit: g / cm ³ )
[4] obtained from foaming ratio _{V 0} ÷ V _1.

Here, V ₀ is the density of the uncured urethane stock solution, and V ₁ is the density of the foam.

[5] Number of cells The surface of the foam is observed with a 50 × microscope, and the number of cells per 3.3 mm is counted.

(Unit: pieces / 3.3mm)
[6] 50% compressive stress (Hs)
30 mm x 30 mm foam thin product is compressed at a compression speed of 1 mm / min,
A stress of 50% of the thickness is measured. (Unit: Mpa)
[7] 50% compression set (set)
A urethane foam sheet of 10 mm × 10 mm is compressed and fixed to 50% of the thickness before the set test, and left in an oven at 70 ° C. for 22 hours. After that, it is opened and the thickness after 30 minutes is measured and calculated by the following formula.

Compression set = (Thickness before test-Thickness after test) ÷ Thickness before test x 100 (Unit:%)
[8] Appearance The case where self-skin was present and gloss was evaluated as “excellent”, and the case where there was no self-skin and gloss was determined as “poor”. Moreover, the thing without a space | gap and the defoaming of a skin part was evaluated as "uniform."

DESCRIPTION OF SYMBOLS 1 ... Perforated release base material 2 ... Urethane coating film 3 ... Base material 4 ... Hole 5 ... Belt press 6, 6 '... Belt roll 7 ... Perforated release Mold member 8 ... Laminate 9 ... Urethane coating 10 ... Base material

Claims (6)

  Applying a composition containing a urethane prepolymer having an NCO group at the molecular end onto a substrate to form a sheet-like coating film, contacting the coating film with a holed releasable substrate, A method for producing a urethane foam sheet comprising foaming and curing the coating film by exposing the coating film to water vapor in a state where the perforated release substrate is in contact.   The method according to claim 1, wherein a diameter of a hole of the holey releasable base material is in a range of 0.01 to 2.0 mm.   The perforated releasable substrate is formed by forming holes in a resin film selected from an olefin resin film, a fluorine resin film, a silicone resin film, and a release-treated polyester film, a woven fabric, or a net shape The method according to claim 1 or 2, wherein the method is one of an object.   The method according to any one of claims 1 to 3, wherein the urethane prepolymer is a prepolymer obtained by a reaction between a polyol having a functional group number exceeding 2 and an isocyanate.   The method according to claim 4, wherein the polyol has a functional group number of 2.2 to 3.0 and a hydroxyl value of 84 to 168.   The method according to claim 4 or 5, wherein the polyisocyanate is diphenylmethane diisocyanate.

Images