JP2002307811A - Cationic aqueous polyurethane resin coating agent composition for ink jet recording sheet made of plastic, coating agent using the composition, and ink jet recording sheet made of plastic coated with the coating agent - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cationic aqueous polyurethane resin coating agent composition, capable of obtaining an ink jet recording sheet made of a plastic which displays excellent adhesive properties to a plastic sheet of wide types and which has excellent water resistance of a printed ink. SOLUTION: The cationic aqueous polurethane resin coating agent composition for the ink jet recording sheet made of a plastic is obtained by dispersing a cationic urethane prepolymer obtained by neutralizing a part of a tertiary amino group of an urethane prepolymer containing (A) a polyisocyanate, (B) a polyester polyol and (C) a chain extender having a tertiary amino group in a molecule, with an acid or quaternizing the part of the tertiary amino group with a quaternizing agent, in water. In the composition, the amine value of the composition is set to a range of 1 to 40 (KOHmg/g).

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cationic aqueous polyurethane resin coating composition for a plastic ink jet recording sheet, a coating composition using the composition, and a plastic sheet using the coating composition. More specifically, a cationic aqueous polyurethane resin coating composition for a plastic ink jet recording sheet having excellent adhesion and water resistance of the ink, a coating composition using the composition, and a coating composition using the coating composition. It is related to a plastic sheet coated with a resin.

[0002]

2. Description of the Related Art In recent years, printers for ink-jet recording have become widespread, and printing targets have been expanded to not only paper but also plastic sheets. Since plastic sheets do not have water absorbency, they need to be treated with some kind of coating agent, but this coating agent has sufficient adhesion to various plastic sheets and the water resistance of the printed ink. Performance such as ensuring performance is required.

[0003] The performance of these coating agents depends on the performance of the resin constituting the coating agent. However, a solvent-based coating agent containing a polyurethane resin as a main component has conventionally been used for reasons such as adhesion to a plastic sheet. Is often used.

However, recently, from the viewpoints of environmental problems, resource saving, occupational safety and food hygiene, demands for water-based coating agents have been increasing. However, the water-based coating agent generally has insufficient adhesion to a plastic sheet, and in particular, PET (polyethylene terephthalate), PP
There is a problem that adhesion to (polypropylene) is not sufficient.

[0005]

SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a plastic ink jet recording sheet exhibiting excellent adhesion to a wide variety of plastic sheets and having excellent water resistance of the printed ink. Is to provide a cationic water-based polyurethane resin coating composition capable of obtaining the following. Another object of the present invention is to
An object is to provide a coating agent exhibiting such performance. Another object is to provide a plastic sheet exhibiting such performance.

[0006]

According to the present invention, a polyester polyol (B) is used as a polyol for performing a urethane reaction with a polyisocyanate (A), and a urethane is produced by using a chain extender (C) having a tertiary amino group. A prepolymer is prepared, and a part of the tertiary amino group is neutralized with an acid or quaternized with a quaternizing agent to give an amine value of 1 to 40 (KOHm
g / g), it has been found that a cationic water-based polyurethane resin coating composition for a plastic ink jet recording sheet capable of achieving the above object can be obtained.

That is, the cationic aqueous polyurethane resin coating composition for a plastic ink jet recording sheet of the present invention has a polyisocyanate (A), a polyester polyol (B) and a tertiary amino group in the molecule. A cationic urethane prepolymer obtained by neutralizing a part of the tertiary amino groups of the urethane prepolymer constituted by the chain extender (C) with an acid or quaternized by a quaternizing agent with water or a polyamine compound (D) A cationic water-based polyurethane resin coating composition which has been chain-extended by using an aqueous polyurethane resin coating composition, wherein the amine value of the water-based polyurethane resin coating composition is 1 to 40 (KO)
Hmg / g).

In the present invention, the chain extender (C) is prepared by preliminarily neutralizing a part of the tertiary amine group with an acid or quaternizing with a quaternizing agent, and then preparing the polyisocyanate (A) and the polyester polyol ( B) can also be reacted.

As described above, in the present invention, by using the polyester polyol (B) as the polyol, the adhesion to the plastic sheet is improved. Further, a part of the tertiary amino group introduced by the chain extender (C) having a tertiary amino group is neutralized with an acid or quaternized with a quaternizing agent to give an amine value of 1 to 40 (KOH mg / g). ), The adhesion to the plastic sheet is further improved. Further, by neutralizing the tertiary amino group introduced by the chain extender (C) with an acid or quaternizing with a quaternizing agent, the water resistance of the printed ink is improved.

[0010]

Embodiments of the present invention will be described below.

In the present invention, as the polyisocyanate (A), conventionally used polyisocyanates such as aliphatic, alicyclic, aromatic and araliphatic can be used.

Specific examples of the aliphatic polyisocyanate include, for example, tetramethylene diisocyanate, dodecamethylene diisocyanate, 1,4-butane diisocyanate, hexamethylene diisocyanate, 2,2,4
Trimethylhexamethylene diisocyanate, 2,
4,4-trimethylhexamethylene diisocyanate,
Lysine diisocyanate, 2-methylpentane-1,5
-Diisocyanate and 3-methylpentane-1,5-diisocyanate.

Specific examples of the alicyclic polyisocyanate include, for example, isophorone diisocyanate, hydrogenated xylylene diisocyanate, 4,4'-dicyclohexylmethane diisocyanate, 1,4-cyclohexane diisocyanate, methylcyclohexylene diisocyanate, 1,3- Bis (isocyanatomethyl) cyclohexane and the like can be mentioned.

Specific examples of the aromatic polyisocyanate include, for example, tolylene diisocyanate, 2,2'-diphenylmethane diisocyanate, 2,4'-diphenylmethane diisocyanate, 4,4'-diphenylmethane diisocyanate, and 4,4'-diphenyldimethyl. Methane diisocyanate, 4,4'-dibenzyl diisocyanate, 1,5-naphthylene diisocyanate, xylylene diisocyanate, 1,3-phenylene diisocyanate, 1,4-phenylene diisocyanate, and the like can be given.

Specific examples of the araliphatic polyisocyanate include dialkyldiphenylmethane diisocyanate, tetraalkyldiphenylmethane diisocyanate, α, α, α, α-tetramethylxylylene diisocyanate.

These polyisocyanates may be used alone or
A mixture of more than one species can be used.

As the polyester polyol (B) used in the present invention, those composed of various polycarboxylic acids and polyols can be used.
Dicarboxylic acids consisting of aliphatic dibasic acids and aromatic dibasic acids, and those composed of aliphatic glycols,
It is more preferable in that the adhesion becomes higher. As the aliphatic dibasic acid, for example, malonic acid, succinic acid, tartaric acid,
Oxalic acid, glutaric acid, adipic acid, pimelic acid, suberic acid, azelaic acid, sebacic acid, alkyl succinic acid, linoleic acid, maleic acid, fumaric acid, mesaconic acid, citraconic acid, itaconic acid, glutaconic acid, or these acids Examples thereof include reactive derivatives such as anhydrides, alkyl esters, and acid halides. These aliphatic dicarboxylic acids can be used alone or in combination of two or more. Further, as the aromatic dibasic acid, for example, phthalic acid, isophthalic acid, terephthalic acid, 1,4-naphthalenedicarboxylic acid, 2,5-naphthalenedicarboxylic acid,
Examples thereof include 2,6-naphthalenedicarboxylic acid, biphenyldicarboxylic acid, tetrahydrophthalic acid, and the like, and reactive derivatives thereof such as acid anhydrides, alkyl esters, and acid halides. These aromatic dicarboxylic acids can be used alone or in combination of two or more.

As the aliphatic glycol in the present invention, ethylene glycol, diethylene glycol, triethylene glycol, 1,3-propanediol, 1,1,
Examples thereof include 3-butanediol, 1,4-butanediol, neopentyl glycol, pentanediol, 1,6-hexanediol, and propylene glycol. These can be used alone or in combination of two or more.

The chain extender (C) having a tertiary amino group in the molecule used for introducing a tertiary amino group in the present invention includes N-methyldiethanolamine and N-methyldiethanolamine.
N-alkyl dialkanolamines such as -ethyldiethanolamine, N-methyldiaminoethylamine, N-
Examples thereof include N-alkyldiaminoalkylamines such as ethyldiaminoethylamine. These chain extenders (C) having a tertiary amino group can be used alone or in combination of two or more.

In the present invention, the amount of the chain extender (C) to be added is 5 to 20% by weight based on the total amount of the polyisocyanate (A), the polyester polyol (B) and the chain extender (C). Is more preferable. If the amount of the chain extender (C) is less than 5% by weight, the amount of tertiary amino groups introduced is reduced, and the adhesiveness and the water resistance of the ink tend to decrease. If the amount of the chain extender (C) is more than 20% by weight, the effect of improving the adhesiveness and the water resistance of the ink cannot be obtained. The urethane prepolymer with an increased amount of (1) tends to be difficult to prepare.

In the present invention, the terminal of the urethane prepolymer formed by the reaction of the polyisocyanate (A), the polyester polyol (B) and the chain extender (C) having a tertiary amino group in the molecule. Is more preferably in the range of 1 to 5% by weight. When the content of the terminal free NCO is less than 1% by weight, it becomes difficult to prepare a urethane prepolymer, which is not preferable. On the other hand, if the content of terminal free NCO is more than 5% by weight, the resulting aqueous polyurethane resin has too high a cohesive force, which is not preferable from the viewpoint of adhesion.

In the present invention, a cationic urethane prepolymer in which a part of the tertiary amino groups introduced by the chain extender (C) is neutralized with an acid or quaternized with a quaternizing agent is prepared. You. When neutralizing a part of the tertiary amino group with an acid,
Examples of the acid used for neutralization include organic acids such as formic acid, acetic acid, propionic acid, butyric acid, lactic acid, malic acid, malonic acid, and adipic acid, and inorganic acids such as hydrochloric acid, phosphoric acid, and nitric acid. These acids can be used alone or in combination of two or more.

When a part of the tertiary amino group is quaternized with a quaternizing agent, the quaternizing agent may be an alkyl halide such as benzyl chloride or methyl chloride, or a sulfuric acid such as dimethyl sulfate or diethyl sulfate. Esters and the like can be mentioned. These quaternizing agents can be used alone or in combination of two or more.

In the present invention, tertiary amino groups are not completely neutralized or quaternized as described above, and only some tertiary amino groups are neutralized or quaternized. The amount of tertiary amino groups remaining without being neutralized or quaternized is determined when the amine value of the aqueous polyurethane resin coating composition is 1 to 40 (KOH mg
/ G), and this amine value can be prepared by neutralization or quaternization with an acid.

The cationic urethane prepolymer obtained by neutralizing or quaternizing a part of the tertiary amino groups is then dispersed in water or, after dispersing in water, the chain is reacted with a polyamine compound (D). By performing the extension, the cationic aqueous polyurethane resin coating composition of the present invention is obtained.

Examples of the polyamine compound (D) include hydrazides such as ethylenediamine, propylenediamine, diethylenetriamine, hexylenediamine, triethylenetetramine, tetraethylenepentamine, isophoronediamine, piperazine, diphenylmethanediamine, hydrazine and adipic dihydrazide. And other compounds having two or more amino groups.

The aqueous polyurethane resin coating composition of the present invention comprises:
It is used by coating on a plastic ink jet recording sheet, and contains the above-mentioned aqueous polyurethane resin coating composition for a plastic ink jet recording sheet and a water-soluble polymer. Examples of the water-soluble polymer include oxidized starch, polyvinyl alcohol, gelatin, polyvinylpyrrolidone (PVP), polyacrylamide, sodium polyacrylate, casein, carboxymethylcellulose, and the like, and polyvinyl alcohol is preferred in terms of price.

The aqueous polyurethane resin coating composition of the present invention may further contain an inorganic water absorbing material. Inorganic water absorbing materials include light calcium carbonate, heavy calcium carbonate, kaolin, talc, calcium sulfate, barium sulfate, titanium oxide, zinc oxide, zinc sulfate, zinc carbonate, satin white, aluminum silicate, diatomaceous earth, calcium silicate, Examples thereof include regular silica, aluminum hydroxide, alumina, and lithobon. Amorphous silica is preferable from the viewpoint of printing speed and coloring.

In the present invention, a polyol having 3 or more hydroxyl groups may be added. When a polyol having three or more hydroxyl groups is added, the adhesion to a plastic sheet can be improved. However, it is necessary to use the polyurethane resin coating composition obtained within a range that does not impair the dispersibility in water. Such polyols include, for example, sorbitol, 1,2,3,6-hexanetetraol, 1,4-sorbitan, 1,2,4-butanetriol, 1,2,5-pentanetriol, glycerin, trimethylol Ethane, trimethylolpropane, pentaerythritol and the like can be mentioned, and these can be used alone or in combination of two or more.

The coating composition using the cationic water-based polyurethane resin coating composition of the present invention includes soft polyvinyl chloride,
Semi-rigid polyvinyl chloride, rigid polyvinyl chloride, PET,
It can be applied to a sheet of polypropylene, polyethylene or the like.

[0031]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below with reference to embodiments.

<Preparation of cationic water-based polyurethane resin coating composition> First, a cationic water-based polyurethane resin coating composition for a plastic ink jet recording sheet was prepared as follows.

Example 1 A polyester polyol comprising 1,6-hexanediol and ethylene glycol, isophthalic acid and adipic acid (molecular weight 1700,
Product name “Teslac 2477, Hitachi Chemical Polymer Co., Ltd.
300 parts by weight) and 10 parts of trimethylolpropane
Parts by weight, 48 parts by weight of N-methyl-N, N-diethanolamine, 190 parts by weight of isophorone diisocyanate, and 267 parts by weight of methyl ethyl ketone (MEK) are placed in a reaction vessel, and the reaction is carried out while maintaining the temperature at 70 to 75 ° C. And a urethane prepolymer. 38.1 parts by weight of dimethyl sulfate is added to the urethane prepolymer, and
The mixture was reacted at 60 ° C. for 30 to 60 minutes to obtain a cationic urethane prepolymer having an NCO content of 2.5%. Water 13 is added to the obtained cationic urethane prepolymer.
After 70 parts by weight were uniformly added and emulsified, MEK was recovered to obtain a cationic water-based polyurethane resin coating composition of Example 1 having an amine value of 10.3.

Example 2 Polyester polyol composed of neopentyl glycol and ethylene glycol, isophthalic acid and adipic acid (molecular weight: 2,000, trade name "Teslac 2508, manufactured by Hitachi Chemical Co., Ltd.)
And 238 parts by weight of trimethylolpropane.
Parts by weight, 38.1 parts by weight of N-methyl-N, N-diethanolamine, and 13 parts by weight of isophorone diisocyanate.
7.0 parts by weight and 210 parts by weight of MEK were placed in a reaction vessel and reacted while maintaining the temperature at 70 to 75 ° C. to obtain a urethane prepolymer. 30.3 parts by weight of dimethyl sulfate is added to this urethane prepolymer,
The mixture was reacted for 60 minutes to obtain a cationic urethane prepolymer having an NCO content of 2.1%. After uniformly adding 1155 parts by weight of water to the obtained cationic urethane prepolymer and emulsifying the mixture, MEK is recovered.
The cationic aqueous polyurethane resin coating composition of Example 2 having an amine value of 10.7 was obtained.

(Example 3) Polyester polyol composed of neopentyl glycol and ethylene glycol, isophthalic acid and adipic acid (molecular weight: 2000, trade name "Teslac 2508, manufactured by Hitachi Chemical Polymer Co., Ltd.)
And 238 parts by weight of trimethylolpropane.
Parts by weight, 38.1 parts by weight of N-methyl-N, N-diethanolamine, and 13 parts by weight of isophorone diisocyanate.
7.0 parts by weight and 210 parts by weight of MEK were placed in a reaction vessel and reacted while maintaining the temperature at 70 to 75 ° C. to obtain a urethane prepolymer having an NCO content of 2.1%. After adding 11.5 parts by weight of acetic acid as an acid to the obtained urethane prepolymer and uniformly stirring to obtain a cationic urethane prepolymer, 1155 parts by weight of water was uniformly added and emulsified, and then MEK was added. An amine value of 1
The cationic aqueous polyurethane resin coating composition of Example 3 of 7.1 was obtained.

Example 4 A polyester polyol comprising 1,6-hexanediol and ethylene glycol, isophthalic acid and adipic acid (molecular weight 1700,
Product name “Teslac 2477, Hitachi Chemical Polymer Co., Ltd.
300 parts by weight) and 10 parts of trimethylolpropane
Parts by weight, 48 parts by weight of N-methyl-N, N-diethanolamine, 190 parts by weight of isophorone diisocyanate, and 267 parts by weight of methyl ethyl ketone (MEK) are placed in a reaction vessel, and the reaction is carried out while maintaining the temperature at 70 to 75 ° C. And a urethane prepolymer. 38.1 parts by weight of dimethyl sulfate is added to the urethane prepolymer, and
The mixture was reacted at 60 ° C. for 30 to 60 minutes to obtain a cationic urethane prepolymer having an NCO content of 2.5%. Water 13 is added to the obtained cationic urethane prepolymer.
After uniformly adding 70 parts by weight and emulsifying, an ethylenediamine aqueous solution obtained by diluting 9.0 parts by weight of ethylenediamine with 36.0 parts by weight of water is added uniformly to completely react with the NCO group, and then MEK is added. By collecting, a cationic aqueous polyurethane resin coating composition of Example 4 having an amine value of 10.3 was obtained.

Comparative Example 1 238 parts by weight of polytetramethylene glycol (molecular weight: 2000, trade name: PTMG2000, manufactured by Mitsubishi Chemical Corporation), 5.95 parts by weight of trimethylolpropane, N-methyl-N, 38.1 parts by weight of N-diethanolamine, 137.0 parts by weight of isophorone diisocyanate and 210 parts by weight of MEK were placed in a reaction vessel, and the reaction was carried out while maintaining the temperature at 70 to 75 ° C. to obtain a urethane prepolymer. 30.3 parts by weight of dimethyl sulfate is added to the urethane prepolymer,
And react for 30 to 60 minutes with NCO content of 2.1%
Was obtained as a cationic urethane prepolymer. After 1155 parts by weight of water were uniformly added to the obtained urethane prepolymer and emulsified, MEK was recovered to obtain a cationic polyurethane resin coating composition of Comparative Example 1 having an amine value of 10.7. . (Comparative Example 2) Polyester polyol (Molecular weight 1700, trade name "Teslac 2477,
300 parts by weight of Hitachi Chemical Polymer Co., Ltd.), 7.5 parts by weight of trimethylolpropane, and N-methyl-N, N
-48 parts by weight of diethanolamine, 177.5 parts by weight of isophorone diisocyanate, and 267 parts by weight of MEK were placed in a reaction vessel, and the reaction was carried out while maintaining the temperature at 70 to 75 ° C to obtain a urethane prepolymer. All of the tertiary amino groups of the urethane prepolymer were replaced with dimethyl sulfate.
Quaternization was performed with 8 parts by weight to obtain a cationic urethane prepolymer having an NCO content of 2.2%. After 1370 parts by weight of water was uniformly added to the obtained cationic urethane prepolymer and emulsified, MEK was recovered to obtain a cationic aqueous polyurethane resin coating composition of Comparative Example 2 having an amine value of 0. Was.

<Preparation of Coating Agent Using Cationic Water-Based Polyurethane Resin Coating Composition> The cationic water-based polyurethane resin coating composition obtained in Examples 1 to 3 and Comparative Examples 1 and 2 was used. Then, a coating agent was prepared as described below.

To 50 parts by weight of each of the compositions obtained in Examples 1 to 3 and Comparative Examples 1 and 2, 50 parts of amorphous silica were added.
A coating agent for a plastic ink jet recording sheet was prepared by blending 200 parts by weight of a 20% aqueous polyvinyl alcohol solution.

<Preparation of Plastic Ink-Jet Recording Sheet Coated with Coating Agent> Each coating agent prepared as described above using each of the compositions of Examples 1 to 3 and Comparative Examples 1 and 2 was prepared as follows. Using a bar coater, the composition was applied onto the three types of plastic sheets shown in Table 1 so that the film thickness after drying was 50 μm, and dried at 120 ° C. for 3 minutes to obtain a plastic ink jet recording sheet.

<Evaluation of Water Resistance> The water resistance was determined by applying each coating agent to a plastic sheet of semi-rigid polyvinyl chloride (PVC), untreated polyethylene terephthalate (PET), and corona-discharge treated polypropylene as described above. An inkjet printer (Epson M)
ACHJETMJ-830C) was used for printing, and one drop of water was dropped with a dropper on the boundary between the printed portion and the non-printed portion, and the print bleeding was visually observed.
Table 1 shows the evaluation results. In addition, the evaluation results of water resistance are as follows:
Since the same result was obtained with the above three types of sheets, they are collectively shown in one column in Table 1.

The evaluation criteria for water resistance are as follows.

◎: No bleeding at all ：: Slight bleeding (printing is identifiable) ×: Bleeding (printing is not identifiable)

As a result of the evaluation of water resistance, as shown in Table 1,
In the plastic sheet coated with the coating composition using the cationic water-based polyurethane resin coating composition of Examples 1 to 3, no ink bleeding was observed at all, and the water resistance was very high. On the other hand, in the sheets of Comparative Examples 1 and 2, print bleeding was observed, and it was found that the sheets had poor water resistance.

[0045]

[Table 1]

<Evaluation of Adhesion> Adhesion was measured by applying an adhesive tape (Nichiban Co., Ltd.) to the coated surface of a plastic ink jet recording sheet obtained by applying each coating agent to the three types of plastic sheets shown in Table 1 as described above. , 12 mm width) and evaluated by observing the remaining amount of the coating layer when one end of the adhesive tape was rapidly peeled off in the direction perpendicular to the coating surface. Evaluation criteria The evaluation criteria are as follows.

A: The coating layer remains at 90% or more. O: The coating layer remains at 70% or more and less than 90%. Δ: The coating layer remains at 60% or more and less than 70%. X: The coating layer remains at less than 60%.

As a result of the evaluation of the adhesion, as shown in Table 1,
In the plastic sheet to which the coating agent using the cationic water-based polyurethane resin coating agent composition of Examples 1 to 3 was applied, 90% or more of the coating layer remained, indicating that the adhesion was extremely high. On the other hand, in the sheets of Comparative Examples 1 and 2,
It can be seen that large peeling of the coating layer occurred and adhesion was low.

[0049]

According to the cationic water-based polyurethane resin coating composition for a plastic ink jet recording sheet of the present invention, the tertiary amino group introduced into the urethane prepolymer by the chain extender (C) having a tertiary amino group. All of the groups are not neutralized or quaternized, and only some of them are neutralized or quaternized with an acid so that the amine value ranges from 1 to 40 (KOH mg /
g). By leaving the tertiary amino group in this way, the adhesion and the water resistance of the ink are greatly enhanced.
Further, polyester polyol (B) is used as the polyol.
By using, the adhesion to the plastic sheet is further improved.

Continuation of the front page (72) Inventor Shuichi Wada 1F F-term (reference) 2H086 BA31 BA35 BA37 BA41 4J034 BA08 CA01 CA12 CB08 DA01 DB03 DF16 DF20 DF21 DF22 HA01 HA07 HC03 HC12 HC13 HC22 HC46 HC52 HC61 HC64 HC67 HC71 HC73 JA42 RA07 4J038 CE022 DG111 DG261 MA08 PC08

Claims (9)

[Claims] 1. A part of a tertiary amino group of a urethane prepolymer composed of a polyisocyanate (A), a polyester polyol (B), and a chain extender (C) having a tertiary amino group in a molecule. A cationic urethane prepolymer obtained by neutralizing with an acid or quaternizing with a quaternizing agent a chain-extended cationic water-based polyurethane resin coating composition using water or a polyamine compound (D). The amine value of the polyurethane resin coating composition is 1 to
A cationic water-based polyurethane resin coating composition for a plastic inkjet recording sheet, wherein the composition is 40 (KOH mg / g). 2. The amount of the chain extender (C) to be added is 5 to 20 with respect to the total amount of the polyisocyanate (A), the polyester polyol (B) and the chain extender (C).
The cationic aqueous polyurethane resin coating composition for a plastic ink jet recording sheet according to claim 1, which is contained in an amount of 1% by weight. 3. The polyester polyol (B) comprises:
The aqueous polyurethane resin coating agent for a plastic inkjet recording sheet according to claim 1 or 2, which is a polyester polyol composed of a dicarboxylic acid composed of an aliphatic dibasic acid and an aromatic dibasic acid, and an aliphatic glycol. Composition. 4. The aqueous polyurethane for a plastic ink jet recording sheet according to claim 1, wherein the content of terminal free NCO groups in the urethane prepolymer is in the range of 1 to 5% by weight. Resin coating composition. 5. A water-based polyurethane for a plastic ink-jet recording sheet, comprising the water-based polyurethane resin coating composition for a plastic ink-jet recording sheet according to claim 1 and a water-soluble polymer. Resin coating agent. 6. The aqueous polyurethane resin coating composition for a plastic ink jet recording sheet according to claim 5, wherein the water-soluble polymer is polyvinyl alcohol. 7. The aqueous polyurethane resin coating composition for a plastic ink jet recording sheet according to claim 5, further comprising an inorganic water absorbing material. 8. The aqueous polyurethane resin coating composition for a plastic ink jet recording sheet according to claim 7, wherein the inorganic water absorbing material is amorphous silica. 9. A plastic ink jet recording sheet coated with the water-based polyurethane resin coating agent according to claim 5. Description: