JPS62195057A - Coating composition for precoat steel plate - Google Patents

Abstract

PURPOSE:To provide the titled composition outstanding in processability, hardness, solvent resistance and weatherability, comprising a specific urethane polymer, copolyester resin having hydroxyl group and alkyletherified melamine resin. CONSTITUTION:The objective composition comprising (A) 10-80pts.wt. of such an urethane polymer as to contain in (i) a resin prepared by polycondensation between a copolyester resin having hydroxyl group and polyisocyanate compound (ii) 0.1-20wt% of urethane bond of formula, (B) 10-80pts.wt. of a copolyester resin having hydroxyl group, and (C) 5-40pts.wt. of an alkyletherified melamine resin.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a coating composition for precoated steel sheets that has excellent workability, hardness, solvent resistance, and weather resistance.

[Conventional technology and its problems]

Prepainted steel sheets are used in a wide range of applications, including home appliances, building materials, and furniture. Since pre-coated steel sheets are used by forming coated sheets, they are required to avoid coating defects such as cracks and scratches during processing.

Conventionally, polyester paints made of polyester resin and melamine resin are generally used as paints for pre-coated steel sheets. Polyester paint is 1. It has excellent weather resistance when used outdoors, but workability and hardness are contradictory factors; improving workability reduces hardness, and increasing hardness reduces workability. It was difficult to achieve a high level of balance.

An object of the present invention is to provide a coating composition for pre-coated steel sheets that has excellent performance in both processability and coating hardness and has good weather resistance.

[Means for solving problems]

That is, the present invention provides: a) ain-d-o in a resin obtained by polycondensing a copolyester resin having a hydroxyl group and a polyisocyanate compound;
) 10 to 80 parts by weight of a urethane polymer containing 0.1 to 20% by weight of urethane bonds, b) 10 to 80 parts by weight of a copolyester resin having hydroxyl groups, and C) 5 to 40 parts by weight of an alkyl etherified melamine resin. Part by weight (
provided that the total of a, b, and c is 100 parts by weight).

The urethane polymer used as component a) of the present invention is obtained by polycondensation of a copolyester resin having a hydroxyl group and a polyisocyanate compound.

Polycarboxylic acids constituting the copolymerized polyester resin having hydroxyl groups include oxalic acid, succinic acid, adipic acid,
Aliphatic polycarboxylic acids such as azelaic acid, aromatic polycarboxylic acids such as terephthalic acid, isophthalic acid, orthophthalic acid, 4,4'-diphenyldicarboxylic acid, and trimellitic anhydride; polyol components include ethylene glycol, propylene glycol, trimethylene glycol, 1
．． 3-phtadiol, 1,4-butadiol, 1,5-
Examples include bentanediol, 1,6-hexanediol, neopentyl glycol, trimethylolpropane, trimethylolethane, and penquerythritol.

The copolymerized polyester of the present invention has an equivalent ratio of carbonyl groups in the polycarboxylic acid to hydroxyl groups in the polyol component of 1.0.
It is preferable to blend hydroxyl groups in excess in the range of /1.001 to 1.0/2.0, and it can be obtained by a normal transesterification method or direct esterification reaction. A particularly preferred equivalent ratio is in the range of 1.0/1.01 to 1.0/1.5.

Examples of polyisocyanate compounds include hexamethylene diisocyanate, 2,4.4 or 2,2.4-) dimethylhexamethylene diisocyanate, dicyclohexylmethane-4,4'-diisocyanate, and 3-isocyanate methyl-3,5,5-diisocyanate. -) aliphatic polyisocyanates such as dimethylcyclohexyl isocyanate; 2.
4-) Luene diisocyanate, 2.6-) Luene diisocyanate, xylylene diisocyanate, 0-phenylene diisocyanate, m-phenylene diisocyanate, p-phenylene diisocyanate, diphenylmethane-4,4゛-diisocyanate, 3.3-dimethyldiphenylmethane-4 , 41-diisocyanate and the like, and at least one of these can be used in the present invention.

It is also possible to use polymerized forms of these polyisocyanate compounds, such as water adducts, polyol adducts, and isocyanurate forms.

The urethane polymer of the present invention is obtained by polycondensation of a polyester resin having a hydroxyl group and a polyisocyanate compound, and the urethane polymer contains 0.1 to 20% by weight of urethane bonds represented by (NH- and -0). It is essential that the

Urethane bonding is a necessary condition for satisfying both workability and hardness, but if the urethane bonding is less than 0.1% by weight, it will not be possible to satisfy both workability and hardness. If it exceeds it, the urethane polymer has poor solvent solubility, resulting in poor paint stability and easy coating defects such as rough skin, which is not practical. Furthermore, the amount of the urethane polymer as component a) is in the range of 10 to 80 parts by weight, when the total of a), b), and C) is 100 parts by weight. 10
If it is less than 80 parts by weight, processability such as bending will be poor, and if it exceeds 80 parts by weight, weather resistance will be poor.

The copolyester having a hydroxyl group used as component b) of the present invention can be obtained by a conventional transesterification method or direct esterification reaction of the above-mentioned polycarboxylic acid and polyol. If the hydroxyl value is less than 5, the solvent resistance and weather resistance will be poor, and if it is more than 200, the processability will be unfavorable. The amount of component b) ranges from 10 to 80 parts by weight. If it exceeds 80 parts by weight, it will not be possible to achieve both workability and hardness, and if it is less than 10 parts by weight, weather resistance will deteriorate, which is not preferable.

The alkyl etherified melamine resin used as component C) of the present invention includes formaldehyde or paraformaldehyde that has been alkyl etherified with an alkyl alcohol such as methanol, ethanol, isopropanol, n-butanol, isobutanol, urea, benzoguanamine, etc. , glycoluril, aminotriazine, etc., including methoxylated methylolurea, methoxylated methylolmelamine, methoxylated methylolbenzoguanamine, butoxylated methylolmelamine,
Examples include butoxylated methylol benzoguanamine.

The alkyl etherified melamine resin is a curing agent component that reacts with the urethane polymer as component a) and the copolymerized polyester resin as component b) during heating to form a crosslinked coating film. The amount of alkyl etherified melamine resin is 5~
It is necessary that the amount is in the range of 40 parts by weight. If the alkyl etherified melamine resin is less than 5 parts by weight, hardness and solvent resistance will be insufficient, and if it exceeds 40 parts by weight, processability will be insufficient, and both are not suitable for practical use.

The total amount of a), b), and C) in the coating composition is not particularly limited, but is usually 20 to 80% by weight.

The coating composition of the present invention is prepared by dissolving it in at least one solvent selected from toluene, xylene, Tsurpetz 1001 butyl acetate, ethyl acetate, butanol, Celsolve, butyl Celsopuru, methyl isobutyl ketone, cyclohexanone, isophorone, Cellosolve acetate, etc. done by doing.

Furthermore, it is possible to use pigments such as titanium oxide, phthalocyanine blue, phthalocyanine green, carbon blank, calcium carbonate, flow regulators, antifoaming agents, and acidic catalysts such as p-)luenesulfonic acid, if necessary.

Furthermore, if necessary, epoxy resin, vinyl chloride resin, cellulose derivative, and acrylic resin can also be used in combination.

The coating composition of the present invention can be manufactured using a mixer such as a roll mill, ball mill, or disper.

The coating composition of the present invention can be applied to cold-rolled steel sheets, galvanized steel sheets, chrome-plated steel sheets, aluminized steel sheets, and those subjected to chemical conversion treatments such as chromate treatment and phosphoric acid treatment.

At that time, it is preferable to coat the steel plate with an epoxy-based, polyester-based, etc. primer in order to improve the adhesion between the paint and the steel plate and to enhance rust prevention.

An example of a coating method for the coating composition of the present invention is coating with a commonly used high-speed roll coater and heating and curing in a hot air drying oven. At that time, the film thickness is about 5-30μ, the metal temperature is 150-300℃ during heat curing, and the time is 15-30℃.
An example is a condition of about 100 seconds.

〔Example〕

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but the present invention is not limited to these Examples. Parts in the recommended examples indicate parts by weight.

Production Example 1 332 parts of isophthalic acid and 296 parts of phthalic anhydride (J
, adipic acid 146 parts, neopentyl glycol 572 parts
The mixture was heated from 150°C to 230°C to perform a dehydration reaction to obtain copolymerized polyester resin A-1.

A-2 to A-3 shown in Table 1 were obtained in the same manner as A-1.

Production Example 2 450 parts of copolymerized polyester resin A-1, 50,495 parts of Tsurpetz 150,495 parts, and 55 parts of cellosolve acetate were placed in a four-bottle flask equipped with a thermometer, condenser, and stirrer, and after melting at 80°C, 25 parts of hexamethylene diisocyanate was added. The reaction mixture was kept at 80°C for 8 hours to obtain urethane polymer B-1. The number average molecular weight of the obtained urethane polymer B-1 by GPC was 100.
It was 00.

Thereafter, in the same manner, urethane polymer B- was prepared with the composition shown in Table 2.
2 to B-5 were obtained.

The results obtained are shown in Table 2.

Example 1 100 parts of urethane polymer B-1, knee pants 205
15 parts of E-60, 20 parts of Almatics P-64, and 44 parts of titanium oxide R-930 were dispersed using three rolls.
WC (pigment iyeight content)
t) Obtain 40% paint, apply it to ZINKOTE (manufactured by Nippon Steel ■, galvanized steel plate, 0.6 mm thickness) using a bar coater to a film thickness of 15 μm, and apply it at a metal temperature of 230°C for 60 seconds. A baked test plate was obtained and evaluated.

The results are shown in Table 3, showing excellent properties in hardness, bending workability, and resistance to pheasant rolls.

The test results were judged as follows.

(11 pencil hardness Indicates the hardness value of a pencil that leaves no scratches on the coating when rubbed.

(21 bending process Cracks were observed when bent to 180 mm.

(3) Resistance to Kijiroll After rubbing the coating 50 times with gauze impregnated with Kijiroll, the condition of the coating film was observed.

(4) Shows the gloss retention of the coating film after being run on a weather-resistant weather-o-meter for 1000 hours.

Examples 2 to 5 PWC40 was prepared in the same manner as in Example 1 with the composition shown in Table 3.
% paint was prepared, a test plate was obtained from this, and a test was conducted.

As shown in Table 3, the results are hardness, bending workability,
Shows excellent properties such as pheasant roll resistance and weather resistance.

Comparative Examples 1 to 3 Paints were prepared in the same manner as in Example 1 with the compositions shown in Table 3, test plates were obtained, and tests were conducted.

The results are shown in Table-3.

Comparative Example 4 An attempt was made to prepare a paint in the same manner as in Example 1 using urethane polymer B-5, but the solubility of the urethane resin was poor and precipitation occurred, making it impossible to obtain a paint.

〔Effect of the invention〕

As is clear from the above Examples and Comparative Examples, the coating composition for pre-coated steel sheets of the present invention has excellent workability, hardness, solvent resistance,
It is possible to provide a precoated steel sheet that has excellent weather resistance and excellent performance for applications such as home appliances, building materials, and furniture.

Claims (1)

[Claims] a) In the resin obtained by polycondensing a copolyester resin having a hydroxyl group and a polyisocyanate compound, there are ▲numerical formulas, chemical formulas, tables, etc. ▼The urethane bond represented by ▼ is 0.1~
10 to 80 parts by weight of urethane polymer containing 20% by weight and b) 10 to 8 parts by weight of copolyester resin having hydroxyl groups
c) 5 to 40 parts by weight of alkyl etherified melamine resin (
provided that the total of a, b, and c is 100 parts by weight).