JP3358261B2 - Polyurethane coating composition and polyurethane adhesive composition - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a paint and an adhesive using a polyisocyanate curing agent having excellent low-temperature stability and compatibility with a polyhydric hydroxyl compound and a solvent.

[0002]

2. Description of the Related Art When various isocyanate compounds are used as a curing agent, for example, in the fields of metals, plastics, paints for this work, paints for magnetic recording media, and adhesives, for example, the drying property of a coating film is high. Reactivity as well as low temperature stability are important factors. Therefore, hexamethylene diisocyanate, than aliphatic isocyanates typified by isophorone diisocyanate, tolylene diisocyanate (hereinafter, abbreviated as TDI), diphenylmethane diisocyanate (hereinafter, abbreviated as MDI) aromatic isocyanates typified Mainly used.
Among these isocyanates, in the case of thick-film type paints such as heavy duty anticorrosion paints and anti-chipping paints for automobiles,
Alternatively, in the case of a hot-melt adhesive, MDI is generally used because MDI has higher reactivity of isocyanate groups than TDI.

[0003]

SUMMARY OF THE INVENTION However, MDI
Among them, typical 4,4′-MDI lacks low-temperature stability due to its strong crystallinity, and has a high viscosity upon various modifications such as urethanization and isocyanuration as described later, and There are limits to use in solvent systems. Currently available MDI-based curing agents having good low-temperature stability used for paints and adhesives include 4,4'-M
DI polymerization type, such as Millionate MR-100
(Nippon Polyurethane Industry) or 4,4'-MDI uretonimine carbodiimide type, for example, Millionate MTL (Nippon Polyurethane Industry Co., Ltd.). Therefore, foaming tends to occur at the time of thick film coating, and the appearance of the coating film tends to be inferior. On the other hand, as the TDI-based low-temperature stable curing agent currently available for use in paints and adhesives, there is a urethane oligomer based on a polyol of TDI, for example, Coronate L (manufactured by Nippon Polyurethane Industry). The reaction tended to be slower than that of the MDI system.

[0004]

Means for Solving the Problems The present inventors have studied the low-temperature stability of a curing agent, the compatibility with a polyhydric hydroxyl compound and a solvent, the viscosity when a isocyanate group is modified, the reactivity, and the like.
Furthermore, the appearance of the paint or adhesive film using it,
As a result of intensive studies and studies on MDI in order to improve the adhesiveness, etc., it was found that the improvement can be achieved by using a modified product of 2,4'-MDI and 4,4'-MDI in combination at a specific ratio. Reached the present invention.

That is, the present invention contains a modified polyisocyanate obtained by partially isocyanurating a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate. A polyurethane coating composition or a polyurethane adhesive composition comprising a polyisocyanate curing agent and a polyhydric hydroxyl compound.

According to the present invention, a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is isocyanurated and uretoniminated and / or carbodiimide. A polyurethane coating composition or a polyurethane adhesive composition comprising a polyisocyanate curing agent containing a modified polyisocyanate and a polyvalent hydroxyl compound.

[0007] The present invention relates to a modified polyisocyanate in which a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is urethanized and isocyanurated with a polyol. A polyurethane coating composition or a polyurethane adhesive composition comprising a polyisocyanate curing agent containing isocyanate and a polyhydric hydroxyl compound.

The present invention also relates to a method for urethanizing and isocyanurating a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate with a polyol. A polyurethane coating composition or a polyurethane adhesive composition comprising: a polyisocyanate curing agent containing a modified polyisocyanate that has been ureton-iminated and / or carbodiimidated; and a polyhydric hydroxyl compound.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION

In the production of the modified polyisocyanate contained in the polyisocyanate curing agent of the present invention, 2,4'-
MDI 10 to 90% by weight and 4,4'-MDI 90 to 10
% By weight of a mixture, preferably 2,4'-MDI 20-8
A mixture of 0% by weight and 80 to 20% by weight of 4,4'-MDI is used. It is considered that 2,4′-MDI has a non-linear structure, which makes it difficult to form an aggregated structure due to urethane bonds, and can improve low-temperature stability. Also,
Due to the steric hindrance, the isocyanuration reaction at the 2-position is slower than that at the 4-position, so that the isocyanuration reaction can be easily controlled, and the resulting modified polyisocyanate has a relatively slow reaction.
Is higher than the modified polyisocyanate obtained only from 4,4'-MDI because of the presence of the isocyanate group at the terminal. It is considered that the foaming due to the resin hardly occurs and the effect of improving the appearance is obtained. Therefore, 4,4'-MDI or 2,4'-
When the weight ratio of MDI is less than 10% by weight and 90% by weight
When it is larger, the influence of 4,4'-MDI or 2,4'-MDI becomes too large, and the polyisocyanate curing agent excellent in low-temperature stability intended in the present invention cannot be obtained. Further, the weight ratio of 2,4'-MDI is 9
0 wt% at greater than its aggregate structure is not easily taken up by hydrogen bonds of the urethane bonds for non-linear structure, insufficient hardness of the coating film. The weight ratio of 2,4'-MDI is 10
When the amount is less than 10% by weight, it is difficult to control the reaction due to the high reactivity of 4,4'-MDI, and the obtained curing agent has poor compatibility with the polyhydric hydroxyl compound and the solvent.

The conversion reaction of a mixture of 2,4'-MDI and 4,4'-MDI into a part of isocyanurate bond of an isocyanate group includes tetramethylammonium hydroxide, phenyltrimethylammonium hydroxide, β- Quaternary ammonium salts such as hydroxypropylmethylammonium formate; fatty acid alkali metal salts such as potassium octylate, potassium naphthenate and magnesium naphthenate; and 1,3,5-tris (dimethylaminopropyl) -S-hexahydro Triazine compounds such as triazine, further triethylamine,
A known isocyanuration catalyst such as an amine compound such as dimethyloctylamine and diazabicycloundecene can be used. It is preferable to use a co-catalyst in the isocyanurate-forming reaction. Examples of such a co-catalyst include phenolic hydroxyl compounds such as phenol and cresol, and alcoholic hydroxyl compounds such as ethanol and cycloethanol. The isocyanuration catalyst comprises 2,4'-MDI and 4,4 '
0.001 to 1.0% by weight based on the mixture with MDI
In particular, it is preferably used in an amount of 0.01 to 0.3% by weight.
It is preferably used in an amount of 0.1 to 0.3% by weight, particularly 0.1 to 0.3% by weight. The reaction temperature for isocyanuration is 50 to 120.
C. is preferred. 2,4'-MDI and 4,4'-MD
If the isocyanuration reaction of the mixture with I proceeds too much, the viscosity of the product increases, and a composition excellent in compatibility with a polyhydric hydroxyl compound or a solvent which is a feature of the present invention cannot be obtained . Progress isocyanate (NCO)
The isocyanuration reaction is stopped by adding a strong acid, such as phosphoric acid or sulfuric acid, when the target NCO content is reached.

The modified polyisocyanate contained in the polyisocyanate curing agent of the present invention may further have an isocyanurate bond if further low-temperature stability is desired.
Effective der introducing uretonimine bond and / or carbodiimide bond is, also, even more than
Do not expect compatibility with polyhydric hydroxyl compounds and solvents
Therefore, it is effective to introduce an isocyanurate bond.
You. In addition, modified polyisocyanates include isocyanurate
Uretonimine bond together with
And / or carbodiimide linkages,
Compatibility with multivalent hydroxyl compounds and solvents, low-temperature stability
It is even more preferred because you can get an excellent type
No. As the uretonimine / carbodiimidization catalyst,
Any substance capable of forming an uretonimine bond and / or a carbodiimide bond by causing an isocyanate group to undergo an aggregation reaction can be used. For example,
Phosphorene oxide derivatives such as 1-phenyl-3-methyl-2-phospholene-1-oxide and ethylmethylphospholene oxide are particularly preferred. The amount of the uretonimine / carbodiimidation catalyst used is 2,4'-MDI
And 0.004 to 4,4'-MDI,
1.0% by weight is preferred, especially 0.01 to 0.3% by weight
Is preferred. The reaction temperature of uretonimine / carbodiimidization is preferably about 40 to 120 ° C. 2,4'-M
Uretonimine in a mixture of DI and 4,4'-MDI
If the carbodiimidization reaction proceeds too much, the viscosity of the product increases, and a composition excellent in compatibility with a polyhydric hydroxyl compound or a solvent which is a feature of the present invention cannot be obtained .
The progress is monitored by measuring the NCO content, and when the target NCO content is reached, the uretonimine carbodiimidation reaction is stopped, for example by adding trichlorosilane.

In the production of the modified polyisocyanate contained in the polyisocyanate curing agent of the present invention, the polyol for introducing a urethane bond preferably has a molecular weight of 62 to 5,000. For example, ethylene glycol, propylene glycol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol, 3-methyl-1,5-pentanediol, 1,6
-Hexanediol, 2-ethyl-1,3-hexanediol, neopentyl glycol, 1,3,5-trimethyl-1,3-pentanediol, 2,2,4-trimethyl-1,3-pentanediol, , 2-diethyl-
1,3-propanediol, 2-n-butyl-2-ethyl-1,3-propanediol, diethylene glycol, 1,4-cyclohexanediol, 1,4-cyclohexanedimethanol, or ethylene oxide or propylene oxide of bisphenol A glycol, etc. adducts. The temperature of the urethanization reaction is usually selected from the range of 50 to 90 ° C. 2,4'-
The urethanization reaction of a mixture of MDI and 4,4'-MDI, especially the catalyst is not necessary for the, dibutyltin dilaurate optionally, a naphthoquinone Tex calcium or triethylamine, dimethyl octylamine, diazabicycloundecene, it also Ru effective der using a catalyst of amine compounds and the like. If the urethanization reaction of the mixture of 2,4'-MDI and 4,4'-MDI proceeds too much, the viscosity of the product increases, and the compatibility with the polyhydric hydroxyl compound and the solvent which is a feature of the present invention is excellent. The reaction progress can be used to measure the NCO content
Follow up and stop the reaction when the target NCO content is reached.

The urethanization rate of the modified polyisocyanate contained in the polyisocyanate curing agent of the present invention is preferably at most 20 mol%, particularly preferably at most 15 mol%, based on all isocyanate groups. However, this ratio is determined by the NCO content (mol) at the start of the reaction,
The value obtained by subtracting the content (mol) is the NCO content at the start of the reaction.
(Mol) and multiplied by 100. When the urethanization ratio exceeds 20 mol%, the amount of the isocyanurate bond formed thereafter becomes relatively small, and the feature cannot be fully utilized. Ureton iminization and / or
Alternatively, the carbodiimidization ratio is preferably 1 to 15 mol%, particularly preferably 2 to 10 mol%, based on all isocyanate groups.
This ratio is calculated by subtracting the NCO content (mol) at the end of the uretonimine and / or carbodiimidization reaction from the NCO content (mol) at the start of the reaction, and calculating the NCO content at the start of the reaction.
The value is 100 times divided by the CO content (mol) . The reason for limiting this ratio is the same as in the case of the urethane conversion ratio.
The isocyanuration rate is preferably 10 to 60 mol%, particularly preferably 15 to 55 mol% of all isocyanate groups.
If the isocyanurate conversion rate is lower than 10 mol%, unreacted MDI will remain, and there is a risk of storage stability, especially crystal precipitation due to aging at a low temperature. The molecular weight and the number of functional groups of the product are increased, and the solubility in a solvent and the compatibility with a polyvalent hydroxyl compound are deteriorated. The ratio is calculated by subtracting the NCO content (mol) at the start of the reaction from the NCO content (mol) at the start of the reaction, and calculating the NCO content at the start of the reaction.
The value is 100 times divided by the content (mol) . When the modified polyisocyanate has an isocyanurate bond, a urethane bond, and / or a uretonimine bond and / or a carbodiimide bond, the total modification ratio is preferably not more than 60 mol% of all isocyanate groups. No.

In these reactions, inert solvents commonly used in the polyurethane industry, such as aromatic solvents such as toluene and xylene, ketone solvents such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone, ethyl acetate,
Ester solvents such as butyl acetate and isobutyl acetate, ethylene glycol ethyl ether acetate, propylene glycol methyl ether acetate, 3-methyl-3
Glycol ether ester solvents such as -methoxybutyl acetate, ethyl-3-ethoxypropionate,
One or more ether solvents such as tetrahydrofuran and dioxane can be used in combination . By selecting the solvent to be used, the viscosity can be adjusted according to the use conditions.

As the polyhydric hydroxyl compound used in the coating composition and the adhesive composition of the present invention, those having a molecular weight of 500 to 100,000 having two or more hydroxyl groups in a molecule are preferable. For example, saturated or unsaturated polyester polyol, polycaprolactone polyol, saturated or unsaturated oil-modified or fatty acid-modified alkyd polyol, amino alkyd polyol, polycarbonate polyol, acrylic polyol, polyether polyol, epoxy polyol, polyurethane polyol,
Cellulose acetate butyrate polyol, fluorinated polyol and the like can be mentioned. Among them, particularly in terms of coating film performance (gloss, texture, hardness, flexibility, durability, etc.), workability (dryability, curability, etc.), cost, etc., saturated or unsaturated polyester polyols, saturated Alternatively, unsaturated oil-modified or fatty acid-modified alkyd polyols and acrylic polyols are preferred. In the coating composition of the present invention, the mixing ratio (molar ratio) of the polyisocyanate curing agent and the polyhydric hydroxyl compound is as follows: isocyanate group / active hydrogen group =
A range of 0.5 to 2.0 is preferred, a range of 0.7 to 1.5 is particularly preferred, and a range of 0.8 to 1.3 is most preferred. In the adhesive composition of the present invention, the mixing ratio (molar ratio) of the polyisocyanate curing agent and the polyvalent hydroxyl compound is as follows: isocyanate group / active hydrogen group = 0.5 to 1
A range of 0.0 is preferable, and a range of 0.8 to 9.0 is particularly preferable. The coating composition and the adhesive composition of the present invention contain various additives such as a curing accelerating catalyst, a retarder, an ultraviolet absorber, an antioxidant, and a leveling agent, and various pigments generally used in the lacquer industry. Is also good. In addition, this paint,
The adhesive can be applied by the usual coating method conventionally performed, and the coating is performed by an airless spray machine,
An air spray machine, an electrostatic coating machine, a dipping, a roll coating machine, a brush and the like can be used.

[0017]

The present invention comprises an MDI-based polyisocyanate curing agent and a polyhydric hydroxyl compound having relatively low viscosity, excellent in low-temperature stability, compatibility with polyhydric hydroxyl compounds and solvents, and reactivity. Polyurethane coating composition,
The polyurethane adhesive composition can be applied in one thick coating compared to conventional paints and adhesives, which improves the waki and sagging limits and has good coating performance and workability such as appearance of the coating. Or excellent adhesion. Therefore, it can be applied to a wide range of fields such as magnetic recording media such as metals, plastics, concrete, wood, audio tapes, video tapes, and floppy disks, and can widely contribute to industry.

[0018]

Next, the present invention will be further described with reference to Examples and Comparative Examples. “Parts” and “%” in Examples and Comparative Examples mean “parts by weight” and “% by weight” unless otherwise specified. Examples 1 to 9 and Comparative Examples 1 to 5 Various raw materials shown in Tables 1, 2, and 3 were charged into a four-necked flask equipped with a stirrer, a thermometer, and a reflux condenser. That is, in Examples 1 to 4, the isocyanate was dissolved in the solvent, and the isocyanurate-forming catalyst was added thereto to perform the isocyanurate-forming reaction. In Examples 5, 6 and 9, the isocyanate was dissolved in the solvent, the polyol was charged, and the urethanization reaction (50 to 90 ° C. for 2 to 5 hours) was performed until the target NCO content was reached.
As in Examples 1 to 4, an isocyanurate-forming catalyst was added to carry out an isocyanurate-forming reaction. In Example 7, after the isocyanate was dissolved in the solvent, the polyol was charged, and the urethanization reaction (50 to 90 ° C. for 2 to 5 hours) was performed until the target NCO content was reached, the uretonimine / carbodiimidation catalyst was added. Uretonimine carbodiimidization reaction (40 to 90 ° C for 1 to 5 hours)
After reaching the target NCO content, the reaction was stopped by adding the terminating agent, and then, as in Examples 1-4,
An isocyanurate-forming catalyst was added to perform an isocyanurate-forming reaction. In Example 8, the isocyanate was dissolved in a solvent, and a uretonimine / carbodiimidation catalyst was added to the mixture to prepare a uretonimine / carbodiimidation reaction (4).
(0 to 90 ° C. for 1 to 5 hours) until the target NCO content is reached, and the reaction is stopped by adding the terminating agent. Then, as in Examples 1 to 4, the isocyanuration catalyst is added. An isocyanuration reaction was performed. 50-60 ° C
When the NCO content reached the target NCO content for about 7 to 8 hours, the reaction was stopped by adding an isocyanurate terminator to obtain a pale yellow reaction product. Tables 1 and 2 show the properties of the reaction products. In Comparative Examples 1 to 5, the reaction was carried out in the same manner as in Examples 1 to 9 to obtain reaction products. However, in Comparative Example 2, the reaction system of the modified isocyanurated polyisocyanate was gelled.
It was impossible to synthesize. Table 3 shows the properties of these reaction products.
Shown in

[0019]

[Table 1]

[0020]

[Table 2]

[0021]

[Table 3]

Low-temperature stability The various reaction products obtained in Tables 1, 2 and 3 were converted to -2.
Judgment was made based on the appearance after leaving for 2 months at 0 ° C. The results are shown in Tables 4 and 5. Compatibility Table 1 with a polyvalent hydroxyl compound, Table 2, and various reaction products obtained in Table 3 Table 4
And a polyhydric hydroxyl compound described in Table 5 and a weight ratio of 1
/ 1 were mixed, after which the resulting mixture was applied to a glass plate, after full to scatter the solvent over 120 ° C. / 1 hr, was determined by the appearance of the resulting coating film. The results are shown in Tables 4 and 5. Tolerance 5 g of each reaction product obtained in Tables 1, 2 and 3
Then, various solvents shown in Tables 4 and 5 were added little by little with a burette, and the mixture was shaken well to obtain a turbid point, and the required number of ml at that time was determined. Then, the tolerance (solvent dilutability) corresponding to various solvents was determined by the following equation. Tolerance = required m1 number / sample amount (5 g) The results are shown in Tables 4 and 5.

[0023]

[Table 4]

[0024]

[Table 5] In Tables 4 and 5, Nipporan 133 is a polyester polyol manufactured by Nippon Polyurethane Industry. Epicoat 1001 is an epoxy polyol manufactured by Yuka Shell. Cellnova BTH1 / 2 is a nitrified cotton manufactured by Asahi Kasei Corporation.

Examples 10 to 18, Comparative Examples 6 to 9 The reaction products (modified polyisocyanate solutions) obtained in Examples 1 to 9 and Comparative Examples 1 to 5 as polyisocyanate curing agents, and Nippolane as polyvalent hydroxyl compound 13
3 (Polyester polyol manufactured by Nippon Polyurethane Industry,
Using a hydroxyl value of 200 and a solid content of 60%) and butyl acetate as a solvent, coating compositions and adhesive compositions having the compositions shown in Tables 6 and 7 were obtained.

[0026]

[Table 6]

[0027]

[Table 7]

Coating Film Test The coating compositions of Examples 10 to 18 and Comparative Examples 6 to 9 were degreased with trichlorethylene (JIS G3141 <3).
141-SB>, specifications: PF-1077, manufactured by Nippon Test Panel Kogyo; bond steel plate), 20 ° C, 65% R
It was left for 1 week in an environment of H to form a coating film having a dry film thickness of 40 to 50 μm. And the physical properties of the formed coating film
The test was performed according to the IS K5400 test method. Table 8 shows the results.
And Table 9 below. Adhesion: Performed by the cross-cut tape method. Flexibility: Measured at a diameter of the mandrel of 2 mmφ. Impact resistance: 1/2 inch, 1 kg.

[0029]

[Table 8]

[0030]

[Table 9]

Adhesion Test Using the adhesive compositions of Examples 10 to 18 and Comparative Examples 6 to 9, adhesion tests were performed as follows. That is, a polyurethane RIM (reactive injection molding) molded product having a thickness of 3 mm, FRP, ABS, and a bond steel plate are degreased with trichloroethylene, and the adhesive composition is applied thereon so that the dry film thickness becomes 40 to 50 μm. did. Then, it is pre-dried at 50 ° C. for 5 minutes to evaporate the solvent in the coating film, and the above-mentioned plates of the same type are overlapped so as to prevent air bubbles from entering, and pressed under 25 kg / cm ² .
It was left for one week in an environment of 20 ° C. and 65% RH to obtain a dried coating film. After that, cut the sample to 25mm width,
According to JIS K6854, tensile speed = 10 with Tensilon UTM-500 manufactured by Orientic Co., Ltd.
Bonding strength under the condition of 0 mm / min (unit: kg / 25 m
m) was measured. The results are shown in Tables 10 and 11.

[0032]

[Table 10]

[0033]

[Table 11] In Tables 10 and 11, A indicates material failure and B indicates cohesive failure.

Example 19, Comparative Example 10 In combination with Nipporan 3022 (a polyurethane resin manufactured by Nippon Polyurethane Industry Co., Ltd., hydroxyl value = 1.4), the curing agents of Example 6 and Comparative Example 3 were used.
In the same manner as in the above, an adhesive composition was prepared at a compounding ratio of Nipporan 3022 / curing agent = 10/1 (weight), and an adhesion test was performed (unit of adhesion strength: kg / 25 mm). When this compounding ratio (weight) is converted into a molar ratio, Example 6 and Comparative Example 3
Corresponds to isocyanate group of the curing agent / active hydrogen group of Nipporan 3022 = 8 (molar ratio). Table 12 shows the results.

[0035]

[Table 12]

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C08G 18/00-18/87 C09D 175/04-175/12 C09J 175/04-175/12

Claims (8)

(57) [Claims] 1. A polyisocyanate containing a modified polyisocyanate obtained by partially isocyanurating a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate. A polyurethane coating composition comprising an agent and a polyhydric hydroxyl compound. 2. A part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is isocyanurated and uretoniminated and / or carbodiimidated. A polyurethane coating composition comprising a polyisocyanate curing agent containing a modified polyisocyanate and a polyvalent hydroxyl compound. 3. A modified polyisocyanate obtained by partially urethanizing and isocyanurating a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate with a polyol. A polyurethane coating composition comprising: a polyisocyanate curing agent; and a polyvalent hydroxyl compound. 4. A part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is urethanized and isocyanurated with a polyol, and uretonimine is used. A polyurethane coating composition comprising: a polyisocyanate curing agent containing a modified and / or carbodiimidated modified polyisocyanate; and a polyhydric hydroxyl compound. 5. A polyisocyanate containing a modified polyisocyanate in which a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is isocyanurated. A polyurethane adhesive composition comprising an agent and a polyhydric hydroxyl compound. 6. A part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate was isocyanurated and uretoniminated and / or carbodiimidated. A polyurethane adhesive composition comprising: a polyisocyanate curing agent containing a modified polyisocyanate; and a polyhydric hydroxyl compound. 7. A modified polyisocyanate obtained by partially urethanizing and isocyanurating a part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate with a polyol. A polyurethane adhesive composition comprising a polyisocyanate curing agent and a polyhydric hydroxyl compound. 8. A part of a mixture of 10 to 90% by weight of 2,4'-diphenylmethane diisocyanate and 90 to 10% by weight of 4,4'-diphenylmethane diisocyanate is urethanized and isocyanurated with a polyol, and uretonimine is used. A polyurethane adhesive composition comprising a polyisocyanate curing agent containing a modified and / or carbodiimidated modified polyisocyanate and a polyvalent hydroxyl compound.