JP2001123112A - Polyurethane coating agent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polyurethane coating agent excellent in properties such as adhesion to every substrate including a polyolefin-based substrate, abrasion resistance, blocking resistance, solvent resistance and weatherability and also excellent in low temperature stability without using a halogen-based raw material and injuring workability and the environment by a bad smell or the like with consideration of recent environmental problems. SOLUTION: The polyurethane coating agent features comprising a polyurethane resin obtained by reacting (A) an organic diisocyanate selected from the group consisting of an aliphatic diisocyanate and an alicyclic diisocyanate with (B) a polymeric polyol containing a polycarbonate-polyol having a number average molecular weight of 500-3,000 and the substantial average number of functional groups of 2 to 5 and (C) a low molecular aliphatic polyol having a 2-5C alkyl group and a molecular weight of <500 and, if necessary, (D) a low molecular alicyclic polyol having a molecular weight of <500. The addition of a polyisocyanate curing agent is more preferable.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001] The present invention relates to a polyurethane coating agent that can be applied to films of various materials.
More specifically, from environmental issues such as acid rain and global warming,
The present invention relates to a polyurethane coating agent which does not use a halogen component and has excellent low-temperature stability.

[0002]

2. Description of the Related Art Polyurethane resins are widely used in paints, adhesives, coating agents and the like because of their excellent abrasion resistance, adhesion and low-temperature properties. In particular, in the field of coating agents, studies are being made on polyurethane resins using polycarbonate polyols due to their required performance.

For example, Japanese Patent Application Laid-Open No. 5-202336 proposes a coating agent comprising a polycarbonate polyurethane emulsion and an oxazoline crosslinking agent. Also, Japanese Patent Application Laid-Open No. 7-41731 proposes a coating agent comprising a polyurethane resin using polytetramethylene carbonate. Further, JP-A-9-151233 proposes a polyurethane resin using a polycarbonate having a specific structure,
This polyurethane resin is said to be applicable to a coating agent.

[0004]

DISCLOSURE OF THE INVENTION The present invention does not use halogen-based raw materials in consideration of recent environmental problems, does not impair the workability such as odor and the environment, and provides a polyvinyl chloride. Adhesion to any substrate including olefin-based and polyolefin-based substrates, abrasion resistance, blocking resistance, solvent resistance,
An object of the present invention is to provide a polyurethane coating agent which is excellent in various physical properties such as weather resistance and also has excellent low-temperature stability.

[0005]

Means for Solving the Problems The present inventors have made intensive studies in view of the above circumstances, and as a result, have found that the above problems can be solved by using a specific low-molecular polyol in producing a polycarbonate-based polyurethane resin. They have found what they can do and have completed the present invention.

That is, the present invention provides the following (1) to
(3). (1) Organic diisocyanate (A) selected from aliphatic diisocyanate and alicyclic diisocyanate, number average molecular weight 500 to 3,000, substantial average number of functional groups 2 to 2
Polyurethane resin obtained by reacting a high molecular polyol (B) containing 5 polycarbonate polyols and a low molecular weight aliphatic polyol (C) having an alkyl group having 2 to 5 carbon atoms and a molecular weight of less than 500. To do,
A polyurethane coating agent characterized by the following.

(2) A high molecular polyol containing an organic diisocyanate (A) selected from aliphatic diisocyanates and alicyclic diisocyanates, and a polycarbonate polyol having a number average molecular weight of 500 to 3,000 and a substantially average number of functional groups of 2 to 5 (B) a polyurethane obtained by reacting a low-molecular aliphatic polyol (C) having an alkyl group having 2 to 5 carbon atoms and having a molecular weight of less than 500 and a low-molecular alicyclic polyol (D) having a molecular weight of less than 500 A polyurethane coating agent comprising a resin.

(3) A polyisocyanate curing agent is further added to the polyurethane resin (1) or (2).
A polyurethane coating agent characterized by being added and blended in an amount of up to 20% by mass.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION Raw materials of a polyurethane resin used in the polyurethane coating agent of the present invention will be described.

The organic diisocyanate (A) used in the present invention includes tetramethylene diisocyanate, hexamethylene diisocyanate, 2-methyl-1,5-pentane diisocyanate, decamethylene diisocyanate,
Aliphatic diisocyanates such as 3-methyl-1,5-pentane diisocyanate, lysine diisocyanate, isophorone diisocyanate, hydrogenated tolylene diisocyanate, hydrogenated diphenylmethane diisocyanate,
Hydrogenated xylylene diisocyanate, hydrogenated tetramethyl xylylene diisocyanate, alicyclic diisocyanates such as cyclohexyl diisocyanate, and a mixture of two or more thereof, urethane-modified, allophanate-modified, and urea-modified organic diisocyanates, It is selected from biuret-modified, uretdione-modified, uretoimine-modified, isocyanurate-modified, carbodiimide-modified and the like.

The organic diisocyanate (A) has
2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, 4,4'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 2,2'-diphenylmethane diisocyanate,
1,5-naphthylene diisocyanate, 1,4-naphthylene diisocyanate, p-phenylene diisocyanate, m-phenylene diisocyanate, o-xylylene diisocyanate, m-xylylene diisocyanate,
p-xylylene diisocyanate, tetramethyl xylylene diisocyanate, 4,4'-diphenyl ether diisocyanate, 2-nitrodiphenyl-4,4'-
Diisocyanate, 2,2'-diphenylpropane-
4,4'-diisocyanate, 3,3'-dimethyldiphenylmethane-4,4'-diisocyanate, 4,4 '
Aromatic diisocyanates such as -diphenylpropane diisocyanate and 3,3'-dimethoxydiphenyl-4,4'-diisocyanate, mixtures of two or more of these, urethane-modified organic diisocyanates,
By using an allophanate-modified product, a urea-modified product, a biuret-modified product, a uretdione-modified product, a uretoimine-modified product, an isocyanurate-modified product, a carbodiimide-modified product, etc., the mechanical strength of the urethane coating agent is improved.

In the present invention, the organic diisocyanate (A)
Those containing 50 to 100 mol% of an aliphatic diisocyanate and / or alicyclic diisocyanate therein, and particularly preferably 80 to 100 mol%. If the aliphatic diisocyanate and / or alicyclic diisocyanate is less than the lower limit, the film is likely to yellow, and,
Adhesion is also reduced. The organic diisocyanate (A) used in the present invention is preferably an alicyclic diisocyanate,
Particularly, isophorone diisocyanate and hydrogenated diphenylmethane diisocyanate are preferable.

The high molecular polyol (B) used in the present invention has a number average molecular weight of 500 to 3,000, preferably 700 to 2,800, and a substantial average number of functional groups of 2 to 5, preferably 2 to 5. 3 containing a polycarbonate polyol. When the number average molecular weight of the high molecular weight polyol (B) is less than the lower limit, the viscosity of the obtained polyurethane resin as a solution becomes too high and the workability is apt to deteriorate. Prone to worse. When the substantial average number of functional groups is out of this range, the mechanical strength of the film tends to be low. In the present invention, the “substantially average number of functional groups” is the theoretical number of functional groups calculated from the number of functional groups of the low-molecular-weight raw material polyol described below and the charging ratio of the low-molecular-weight raw material carbonate.

The polycarbonate polyol includes ethylene glycol, 1,2-propanediol,
1,3-propanediol, 1,2-butanediol,
1,3-butanediol, 1,4-butanediol,
1,5-pentanediol, 1,6-hexanediol, 1,8-octanediol, 1,9-nonanediol, 3-methyl-1,5-pentanediol, 2-ethyl-4-butyl-1,3 -Propanediol, diethylene glycol, dipropylene glycol, neopentyl glycol, cyclohexane-1,4-diol, cyclohexane-1,4-dimethanol, dimer acid diol, ethylene oxide or propylene oxide adduct of bisphenol A, bis (β- (Hydroxyethyl) benzene, xylylene glycol, glycerin, trimethylolpropane, pentaerythritol, etc., and one or more low molecular polyols and ethylene carbonate, diethyl carbonate, diphenyl carbonate, etc. It is obtained from Lumpur reaction. In the present invention, a preferable polycarbonate polyol is a polycarbonate polyol obtained from a low molecular diol having 2 to 10 carbon atoms, and a polycarbonate polyol derived from 1,6-hexanediol is particularly preferable because it is most easily available.

If the polymer polyol (B) has a number average molecular weight of 500 to 3,000 and a substantial average number of functional groups of 2 to 5, a polymer polyol other than the above polycarbonate polyol (B) Hereinafter, other polymer polyols are abbreviated), for example, polyester polyols, polyamide ester polyols, polyether polyols, polyether ester polyols, polyolefin polyols, animal and plant polyols, dimer acid polyols, hydrogenated dimer acid polyols, and the like. You may use together. The number average molecular weight is 500 to 3,000.
0, and those having an average of one or more active hydrogen groups in one molecule, epoxy resin, polyamide resin, polyester resin, acrylic resin, rosin resin, urea resin,
An active hydrogen group-containing resin such as a melamine resin, a phenol resin, a coumarone resin, and polyvinyl alcohol may be used in combination. The amount of the other polymer polyol used is preferably 30% by mass or less as the content in the polymer polyol (B).

The low-molecular aliphatic polyol (C) used in the present invention is an aliphatic compound having an alkyl group having 2 to 5 carbon atoms and having two or more alcoholic hydroxyl groups having a molecular weight of less than 500. When the alkyl group (C) has 1 or less carbon atoms, the obtained polyurethane coating agent tends to have insufficient low-temperature stability. When the number of carbon atoms is 6 or more, the reaction rate is reduced due to steric hindrance of the alkyl group during the urethanization reaction, and the productivity tends to be poor. In the present invention, in consideration of low-temperature stability, adhesion and the like, preferred low-molecular polyol (C) is 1
It is a low molecular weight diol having two or more alkyl groups having 2 to 5 carbon atoms in the molecule and having a molecular weight of less than 500.

As the low molecular weight aliphatic polyol (C), 2-ethyl-1,3-propanediol, 2,2-
Diethyl-1,3-propanediol, 2-ethyl-2
-Propyl-1,3-propanediol, 2-ethyl-
4-butyl-1,3-propanediol, 1,2-butanediol, trimethylolpropane, N-ethyl-diethanolamine, N-ethyl-dipropanolamine, and alkylene oxide adducts thereof.

The low molecular weight alicyclic polyol (D) used in the present invention is an alicyclic compound having a molecular weight of less than 500 and having two or more alcoholic hydroxyl groups. When (D) is an aliphatic compound, the film strength tends to be insufficient. In the case of an aromatic compound, low-temperature stability and adhesion tend to be insufficient. In the present invention, in consideration of the balance of film strength, low-temperature stability, adhesion, and the like, a preferred low-molecular polyol (D) is a low-molecular alicyclic diol having 6 to 10 carbon atoms in one molecule.

The low molecular weight alicyclic polyol (D) includes cyclohexane-1,4-diol, cyclohexane-1,4-dimethanol, cyclohexane-1,4-diol.
Examples include diethanol, hydrogenated bisphenol A, and alkylene oxide adducts thereof.

In the present invention, preferred (B) and (C)
The molar ratio of (B) :( C) = 1: 0.5 to 1:10,
Particularly preferably, (B) :( C) = 1: 1 to 1: 8. When (D) is used, (B): [(C) +
(D)] = 1: 0.5 to 1:10, particularly preferably (B): [(C) + (D)] = 1: 1 to 1: 8 (where (C) ≧ (D)) is there. If (B) is too small,
Adhesion tends to be insufficient. On the other hand, when (B) is too large, the film strength tends to be insufficient.

The low molecular polyol (hereinafter abbreviated as other low molecular polyol) other than the low molecular aliphatic polyol (C) and the low molecular alicyclic polyol (D) has a number average molecular weight of less than 500. If so, they may be used in combination. Other low molecular polyols include ethylene glycol, 1,2-propanediol, 1,3-
Propanediol, 1,3-butanediol, 1,4-
Butanediol, 1,5-pentanediol, 1,6-
Hexanediol, 1,8-octanediol, 1,9
-Nonanediol, 3-methyl-1,5-pentanediol, diethylene glycol, dipropylene glycol, neopentyl glycol, glycerin, pentaerythritol, bis (β-hydroxyethyl) benzene,
Xylylene glycol and the like. The amount of the other low molecular polyol to be used is preferably 30 mol% or less in the whole low molecular polyol.

In addition, a low molecular weight polyamine may be used if necessary. Examples of the low molecular weight polyamine include ethylenediamine, hexamethylenediamine, piperazine,
Isophoronediamine, diaminodiphenylmethane and the like.

[0023] A reaction terminator may be used if necessary. Examples of the reaction terminator include monoalcohols such as methanol and ethanol, monoamines such as ethylamine, propylamine, dibutylamine and morpholine, and amino alcohols such as monoethanolamine and diethanolamine.

As a reaction catalyst for synthesizing the polyurethane resin in the present invention, a known so-called urethane-forming catalyst can be used. Specific examples include organic metal compounds such as dibutyltin dilaurate and dioctyltin dilaurate; organic amines such as triethylenediamine and triethylamine; and salts thereof.

As a method for synthesizing the polyurethane resin in the present invention, a known method is used. That is, (1) a method in which an active hydrogen group component and an organic diisocyanate are reacted under a condition of an excess of active hydrogen groups until a predetermined molecular weight is reached;
(2) The polyol component and the organic diisocyanate are reacted under an isocyanate group excess condition to obtain an isocyanate group-containing prepolymer, and then the prepolymer is chain-extended with a low molecular weight polyol or the like and reacted until a predetermined molecular weight is reached. Method and the like.

In consideration of the ease of controlling the reaction and the work of applying the coating agent, in the present invention, it is preferable to use an organic solvent at the time of the urethanization reaction. This organic solvent is an aromatic hydrocarbon solvent such as toluene and xylene,
Aliphatic hydrocarbon solvents such as pentane, hexane, octane, etc., alicyclic hydrocarbon solvents such as cyclopentane, cyclohexane, methylcyclohexane, acetone, methyl ethyl ketone, methyl butyl ketone, methyl isobutyl ketone, diisobutyl ketone, cyclohexanone, methyl cyclohexanone Ketone solvents such as diethyl ether, 1,4-dioxane, ether solvents such as tetrahydrofuran, ethylene glycol dimethyl ether, ethylene glycol monomethyl ether acetate, propylene glycol dimethyl ether, cellosolve solvents such as propylene glycol monomethyl ether acetate, ethyl acetate, Monobasic acid ester solvents such as butyl acetate, disalts such as dimethyl adipate, dimethyl succinate and dioctyl phthalate Esters, dimethylformamide, dimethylacetamide, not particularly limited as long as it is inactive to the isocyanate group, such as N- methylpyrrolidone. Depending on the conditions, an alcoholic solvent such as isopropanol can be used. In the present invention, a ketone-based solvent having no active hydrogen group is preferable, especially in consideration of a residual solvent when applied to a polyolefin-based resin, low-temperature stability, a pot life when a polyisocyanate curing agent is blended, and the like.

The reaction device for the polyurethane resin in the present invention may be any device as long as the above-mentioned reaction can be achieved, such as a reaction vessel equipped with a stirring device, a kneader, or the like.
A mixing and kneading apparatus such as a single-screw or multi-screw extrusion reaction apparatus can be used.

The preferred method for producing the polyurethane resin in the present invention will be described in more detail. In producing a polyurethane resin, first, a raw material having an active hydrogen group is mixed and appropriately diluted with a ketone or ester having no active hydrogen group, a hydrocarbon solvent, or the like.

After adding an organic diisocyanate to the mixture of the active hydrogen group components and, if necessary, a urethanation catalyst, the reaction temperature is increased to 30 to 100 ° C., preferably 50 to 100 ° C.
React at 80 ° C. for several hours. At this time, the molar ratio (R value) of isocyanate group / active hydrogen group is 0.5 to 1.3.
Is preferred, and particularly preferably 0.8 to 1.0.

When the R value is less than the lower limit, the durability of the finally obtained polyurethane coating agent is reduced. If the R value exceeds the upper limit, gelation may occur during production.

The polyurethane coating composition of the present invention includes, in addition to the above-mentioned polyurethane resin, pigments, dyes, solvents, thixotropic agents, antioxidants, ultraviolet absorbers, defoamers, thickeners, dispersants, and surfactants. , Fungicides, antibacterial agents, preservatives, catalysts,
An additive such as a filler or a binder to which an auxiliary binder such as nitrified cotton is appropriately added can also be used.

In the present invention, the above-mentioned polyurethane resin is
By further adding a polyisocyanate curing agent,
This is preferable because the mechanical properties of the film are improved. The amount of the polyisocyanate curing agent to be added is 1 to 20% by mass based on the polyurethane resin. As the polyisocyanate curing agent, for example, commercially available from Nippon Polyurethane Industry Co., Ltd., hexamethylene diisocyanate-modified coronate HX, coronate HL, tolylene diisocyanate-modified coronate L, coronate 2030, Coronate 203
And the like, and these may be used alone or in combination of two or more.

The polyurethane coating agent of the present invention comprises
Excellent low-temperature stability, abrasion resistance, blocking resistance, and weather resistance compared to conventional polyurethane resin coating agents, and adheres to all types of plastic films and sheets, including polyvinyl chloride and polyolefin. It had excellent properties. The urethane coating agent of the present invention is suitable for building materials, and particularly suitable for a vinyl chloride coating agent for flooring.

[0034]

The present invention will be described below with reference to examples, but the present invention is not limited to these examples. In addition,
“%” In the examples indicates mass%.

Synthesis Example 1 141.4 g of polyol A, 67.9 g of DMH, and 10% of MEK were placed in a reactor having a capacity of 2,000 cm ^{3 and} a stirrer, a thermometer, an Aline cooling tube, and a nitrogen gas inlet tube.
0 g was charged and stirred uniformly. In this polyol solution,
116.1 g of H ₁₂ MDI, 24.6 g of IPDI, D
0.05 g of OTDL was charged, and the reaction was started at 70 ° C. As the reaction proceeded, the viscosity increased, and thus the mixture was diluted with MEK as needed. (The final MEK charge is 6
50 g. ) The reaction was carried out until the absorption peak of the isocyanate group disappeared in the infrared absorbance analysis to obtain a polyurethane resin solution PU-1. The solid content of PU-1 is 34.9.
%, The viscosity at 25 ° C. was 2,500 mPa · s, and the number average molecular weight was 30,000.

Synthesis Examples 2 to 6 and Synthesis Comparative Examples 1 to 5 Using the same apparatus and reaction method as in Synthesis Example 1, using the raw material amounts shown in Tables 1 and 2, the polyurethane resin solution PU-2 was used.
To 11 were obtained.

The obtained PU-1 to PU-11 were cooled at -10 ° C.
After storage for 0 days, the appearance was observed and the low-temperature stability was evaluated. The results are shown in Tables 1 and 2.

[0038]

[Table 1]

[0039]

[Table 2]

The raw materials in Synthesis Examples 1 to 6, Synthesis Comparative Examples 1 to 5, and Tables 1 and 2 are shown below. Polyol A: Polycarbonate diol composed of 1,6-hexanediol and diphenyl carbonate Number average molecular weight = 1,000 Polyol B: Polycarbonate diol composed of 1,6-hexanediol and diphenyl carbonate Number average molecular weight = 2,000 Polyol C : Polyester diol composed of adipic acid and 1,6-hexanediol Number average molecular weight = 1,000 DMH: 3,3-dimethylolheptane CHDM: Cyclohexane-1,4-dimethanol BD: 1,4-butanediol BHEB: Bis (β-hydroxyethyl) benzene NPG: neopentyl glycol H ₁₂ MDI: hydrogenated diphenylmethane diisocyanate IPDI: isophorone diisocyanate TDI: 2,4-tolylene diisocyanate DOTDL: Dioctyltin dilaurate MEK: Methyl ethyl ketone Method for measuring number average molecular weight of polyurethane resin: Gel permeation chromatography by polystyrene calibration curve Low temperature stability ○: No change in appearance. X: Change in appearance is observed. (Turbidity and suspended matter can be confirmed)

[Evaluation of Coating Agent] Example 1 PU-1 was mixed in a container having a capacity of 150 cm ³ at a ratio shown below to obtain a coating agent A. Adhesion, blocking resistance, abrasion resistance, solvent resistance,
The weather resistance was evaluated. Coronate HX: Nippon Polyurethane Industry Co., Ltd. product Isocyanurate-modified polyisocyanate of hexamethylene diisocyanate

(1) Adhesion The coating agent A was printed at a printing speed of 20 with a gravure printing machine.
m / min, and a thickness 1 so that the coating thickness after drying is 50 μm.
It is applied to a discharge treated surface of a 00 μm polyvinyl chloride (hereinafter abbreviated as PVC) film and a 15 μm-thick corona discharge treated stretched polypropylene (hereinafter abbreviated as OPP) film, and aged at 40 ° C. for 7 days. And a cellophane tape was applied thereto, and this was quickly peeled off. Evaluation ：: The coating film was not peeled off at all. ：: 80% to 100% of the coating film remained. Δ: 50% to 80% of the coating film remained. X: Only 50% or less of the coating film remained.

(2) Blocking resistance The coating agent A was applied to a PVC film having a thickness of 15 μm in the same manner as described above, and aged at 40 ° C. for 7 days. A blocking tester was applied with a load of 0.5 MPa to evaluate the blocking resistance. Evaluation ：: Peeled off without any resistance when peeled off, and the coating surface did not fall off. Δ: The resistance when peeling was slightly large, and some of the coating surface dropped off. X: The resistance at the time of peeling is large, and the coating surface has fallen off.

(3) Abrasion resistance The coating agent A was applied to a PVC film having a thickness of 100 μm in the same manner as described above, aged at 40 ° C. for 7 days, and then subjected to abrasion resistance according to JIS K-7204 (1995). Was evaluated. Wear test conditions Wear wheel: H-18 Load: 9.8 N Rotational speed: 62.5 times per minute Number of times: 1,000 times

(4) Solvent Resistance The coating agent A was applied to a PVC film having a thickness of 15 μm in the same manner as described above, aged at 40 ° C. for 7 days, allowed to stand at room temperature for 1 day, and then added to MEK. Alternatively, an absorbent cotton impregnated with isopropanol was rubbed and evaluated. Evaluation ：: 80% to 100% of the coating film remained. Δ: 50% to 80% of the coating film remained. X: Only 50% or less of the coating film remained.

(5) Weather Resistance The coating agent A was applied to a PVC film having a thickness of 15 μm in the same manner as described above, aged at 40 ° C. for 7 days, allowed to stand at room temperature for 1 day, and then subjected to Q- After setting the panel in a QUV tester manufactured by PANEL and repeating the following cycle 10 times, a cellophane tape was stuck to the coated surface and this was rapidly peeled off. Condition of one cycle by QUV tester: 70 ° C. × 8 hours (Dry) + 50 ° C. × 4 hours (Wet) Coating amount: 3 g / m ² evaluation by dry ○: 80% to 100% of coating film remained. Δ: 50% to 80% of the coating film remained. X: Only 50% or less of the coating film remained.

Examples 2 to 6, Comparative Examples 1 to 3 PU-1 in Example 1 was replaced with PU-2 to 7, 10, 11
A coating agent was prepared in the same manner as in Example 1 except that the coating agent was replaced with Example 1. In addition, PU-
8 and 9 were not evaluated as coating agents because the low-temperature stability was poor. Table 3 shows the evaluation results.

[0048]

[Table 3]

In Table 3, PVC: polyvinyl chloride film having a thickness of 100 μm OPP: stretched corona discharge treated polypropylene film having a thickness of 15 μm MEK: methyl ethyl ketone IPA: isopropanol

[0050]

As described above, the polyurethane resin for a coating agent of the present invention exhibits good adhesion to all kinds of plastic films including polyvinyl chloride and polyolefin films, and the winding after coating the film. It was found to be excellent in blocking resistance after removal, abrasion resistance when used as a building material, solvent resistance, weather resistance, and the like.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4J034 BA08 CB07 CC03 CC05 CC26 CC45 DA01 DB04 DC02 DF02 HA07 HA11 HB06 HB07 HB09 HC03 HC12 HC13 HC16 HC22 HC45 HC46 HC54 HC64 RA07 4J038 DG051 DG121 DG262 DG271 KA03 MA14

Claims (3)

[Claims] An organic diisocyanate (A) selected from an aliphatic diisocyanate and an alicyclic diisocyanate, and a polymer polyol containing a polycarbonate polyol having a number average molecular weight of 500 to 3,000 and a substantially average number of functional groups of 2 to 5 ( B) and a polyurethane resin obtained by reacting a low-molecular aliphatic polyol (C) having an alkyl group having 2 to 5 carbon atoms and a molecular weight of less than 500, comprising a polyurethane resin. 2. An organic diisocyanate (A) selected from an aliphatic diisocyanate and an alicyclic diisocyanate, and a high molecular weight polyol containing a polycarbonate polyol having a number average molecular weight of 500 to 3,000 and a substantially average number of functional groups of 2 to 5 ( B) a polyurethane resin obtained by reacting a low-molecular aliphatic polyol (C) having an alkyl group having 2 to 5 carbon atoms and having a molecular weight of less than 500, and a low-molecular alicyclic polyol (D) having a molecular weight of less than 500 And a polyurethane coating agent. 3. A polyurethane coating agent, characterized by further adding 1 to 20% by mass of a polyisocyanate curing agent to the polyurethane resin according to claim 1 or 2.