JPS6397267A - Precoat steel sheet having excellent workability and corrosion resistance at its worked part - Google Patents

Abstract

PURPOSE:To improve the workability and corrosion resistance at its worked part of steel sheet by coating an amino or mercapto silane coupling agent on the sheet, and then coating thereon a primer consisting of straight-chain high-mol.wt. polyester, epoxy resin, and a specified cross-linking agent. CONSTITUTION:The amino and/or mercapto silane coupling agents are coated on a layer formed by chemical conversion treatment. (A) The straight-chain high-mol.wt. polyester resin having >=5,000 number average mol.wt. and (B) the epoxy resin having >=2,500 epoxy equivalents, contg. >=3% hydroxyl, and having >=800 number average mol.wt. are mixed in 50/50-95/5 weight ratio of (A)/(B), and (C) one or >=2 kinds among melamine resin, urea resin, blocked isocyanate resin, and phenol resin are incorporated into the obtained mixture so that the (A)+(B)/(C) weight ratio is controlled to 60/40-95/5 to obtain the primer. The primer is coated on the coupling agent-coated layer. As a result, the precoat steel sheet with improved workability and corrosion resistance at its worked part can be produced.

Description

[Detailed description of the invention] <Industrial application field> The present invention improves workability and corrosion resistance of processed parts by applying a specific coupling agent as a pretreatment and then applying a specific brimer. This relates to prepainted steel sheets.

<Prior art and its problems> In recent years, steel sheets for home appliances have been changed from the current boss-coated steel sheets to so-called continuous coil coating in advance on the steel sheet supply side as part of the reduction of painting costs by omitting the painting process at the customer side and as part of pollution control measures. There is a shift towards the use of pre-painted steel sheets.

However, pre-coat paints that are applied before steel sheet processing are more likely to cause problems such as poor appearance and deterioration of corrosion resistance due to cracks in the paint film after bending or drawing, compared to post-coat paints that are applied after steel sheet processing. A high level of corrosion resistance and workability are required to avoid corrosion. Particularly recently, requirements for corrosion resistance of processed parts have become stricter.

The corrosion resistance of processed parts is greatly influenced by the processability of the overall coating film of the primer layer and top layer, and the adhesion of the primer layer. In other words, if the processability of the overall coating film is good and if the adhesion of the brimer is good, the corrosion resistance of the processed area will be good, but the top layer, which has a large effect on processability, has problems with surface hardness and contamination. Currently, it is limited to those that have only a certain degree of workability. Therefore, in order to improve the corrosion resistance of processed parts, the brimer must have high workability and high adhesion.

In response to these demands, polyester, which has high processability but has poor adhesion, is modified with epoxy resin that has good adhesion (Japanese Patent Application Laid-open No. 141768/1983), which has good adhesion. Modification of epoxy resins, which have difficulty in processability, with polymerized fatty acids (New Epoxy Resins, edited by Hiroshi Numauchi) and urethane (Japanese Unexamined Patent Publication No. 1-9446
No. 6).

In addition, it is common practice to control the crosslinking density by using block isocyanate, which has good workability, as a hardening agent, or by changing the blending ratio, but this method does not satisfy the workability and corrosion resistance required for pre-coated steel sheets. Performance has not yet been satisfied at the same time.

Therefore, current pre-painted steel sheets are used as high workability steel sheets, sacrificing corrosion resistance to some extent in processed areas, or are used as warm-worked steel sheets, with emphasis placed on corrosion resistance in processed areas. However, even when used as a steel plate for warm working, it cannot be said that the corrosion resistance of the processed parts is sufficient.

<Objective of the Invention> The present invention has been made in view of the above-mentioned points, and an object of the present invention is to provide a prepainted steel sheet that has high workability and excellent corrosion resistance in the processed portion.

<Problems to be Solved by the Invention> As mentioned above, the inventors of the invention found that when producing a pre-coated steel sheet with excellent workability and corrosion resistance in processed areas, there is a limit to the improvement made only by using a resin for brimer and a hardening agent. Therefore, the present invention was achieved by applying a coupling agent as a pretreatment and then applying a specific primer layer capable of reacting with the coupling agent.

That is, the present invention has a layer made of an amino-based and/or mercapto-based silane coupling agent on the chemical conversion treatment layer, and (A) a linear high-molecular-weight polyester resin having a number average molecular weight of 5000 or more. and (B) an epoxy resin having an epoxy equivalent of 2500 or less, a hydroxyl content of 3% or more, and a number average molecular weight of 800 or more, (A)/(B
)-50150 to 9515 by weight, and further one or more of (C) melamine resin, urea resin, block isocyanate resin, and phenol resin (A) + (B) / (C )=60/40~951
To provide a precoated steel sheet having excellent workability and corrosion resistance in processed parts, characterized by having a primer layer containing the primer layer in a weight ratio of 5 to 5.

<Structure of the invention> The present invention will be explained in detail below.

The steel sheets used in the present invention include cold-rolled steel sheets, hot-dip galvanized steel sheets, electrogalvanized steel sheets, alloyed hot-dip-plated steel sheets, zinc-nickel alloy-plated steel sheets, and other galvanized steel sheets.

Further, the chemical conversion treatment may be a general phosphate treatment using iron phosphate, zinc phosphate, etc., or a general chromate treatment using reactive chromate, coating type chromate, etc.

The silane coupling agents used in the present invention include amino-based γ-(2-aminoethyl)aminopropylmethyldimethoxysilane, γ-(2-aminoethyl)aminopropyltrimethoxysilane, etc., and mercapto-based silane coupling agents such as γ- Typical examples include -mercaptopropyltrimethoxysilane and γ-mercaptopropylmethyldimethoxysilane. These coupling agents can be used alone or in combination, and the silanol groups (-5i-OH) generated by hydrolysis react with the chemical conversion layer, while the amino groups (-NH2, -Nll-
) and mercapto group (-511) react with the resin, and as a result, the bond at the interface between the steel plate and the resin is strengthened.

The coating amount of these coupling agents is 0.01 to 1.0.
g/m2, and is usually used in an aqueous solution with a concentration of 10% or less. It may contain trace amounts of surfactants, stabilizers, thickeners, etc., and the solvent used is not necessarily water, but may be alcohol, ester, ketone, etc.

The specific brimer used in the present invention is component (A)
, component (B), and component (C) are essential components. Component A
), linear high molecular weight polyesters having a number average molecular weight of 5,000 or more are suitable for the purpose of the present invention, and more preferably 1oooo or more. That is, if the number average molecular weight is less than 5,000, sufficient processability cannot be obtained. As a linear high molecular weight polyester resin having a number average molecular weight of 5,000 or more, for example, commercially available product name Almatex manufactured by Mitsui Toatsu Chemical Co., Ltd.
HMP57-23. Also manufactured by Nippon Gosei Kagaku Co., Ltd. Product name Nichigo Polyester-TP-217, TP-27
Examples include 0. Component (B) has an epoxy equivalent of 25
Epoxy resins with a hydroxyl content of 3% or more and a number average molecular weight of 800 or more are suitable for the purpose of the present invention, and more preferably, epoxy resins with an epoxy equivalent of 2200 or less
Below, hydroxyl content of 4% or more - 1 number average molecular weight of 900 or more is good. If the molecular weight is less than 800, sufficient processability cannot be obtained, and if the hydroxyl content is less than 3.
If it is less than %, the reaction rate with the crosslinking agent is low, resulting in poor processability. Furthermore, if the epoxy equivalent is more than 2,500, the concentration of the epoxy group that should react with the amino group or mercapto group of the silane coupling agent is small, and sufficient adhesion cannot be obtained.

Component (B) in the present invention may be any epoxy resin as long as it satisfies the above range. For example, bisphenol A epoxy resin, bisphenol F epoxy resin, hydrogenated bisphenol A epoxy resin, phenol novolac epoxy resin, glycidyl ester epoxy resin, and bisphenol A epoxy resin modified with polymerized fatty acids, urethane, etc. However, bisphenol A type epoxy resin is the most suitable for its balanced performance.

For example, commercially available products manufactured by Yuka Shell Co., Ltd.
Product name Epicote 1001, Epicote 1004, Epicote 1007
, manufactured by Dainippon Ink and Chemicals Co., Ltd. under the trade name Epiclon 1050, Epiclon 4050, Epiclon 7050, and the like. The general formula of such an epoxy resin is as follows.

', 112 Here, for example, Epikote 1001 manufactured by Yuka Shell Chemical Co., Ltd. has a value of 2 out of n, Epikote 1004 has a value of 5 out of n, and Epicoat 1007 has a value of 9 out of n.

Components (A) and (B) are important elements in the present invention for the following reasons. That is, component (A) maintains high processability by using a linear high molecular weight polyester having a number average molecular weight of 5000 or more. Component (B) uses an epoxy resin with a certain content or more of epoxy groups and hydroxyl groups, but the role of the epoxy groups is to increase the adhesion of the primer by chemically bonding with the coupling agent; It maintains processability by reacting with a crosslinking agent.

The blending ratio of component (A) and component (B) is such that (A)/(B) is 50,150 to 9,515, which satisfies the purpose of the present invention, but more preferably (A)/(B) is 75/ 25-9
0/10 is good.

When the molecular weight of component (B') is small, the amount of component (B) to be blended is preferably small in order to prevent deterioration of processability. Also (B
When the epoxy equivalent of component (B) is large, the amount of component (B) to be blended should be large in order to maintain sufficient adhesion with the coupling agent.

Regarding component (C), urea resins, melamine resins, phenol resins, and blocked isocyanate resins are used as crosslinking agents. The blending ratio is (A) + (B)/(
C) is 60/40 to 9515, or more preferably 75/25 to 90/10.
There is a good gap between them. That is, (A) + (B)/ (C
) is greater than 60/40, the crosslinking density of the coating film will be too excessive, and if the content of (C) is less than 9515, the crosslinking density will be insufficient, and in either case, sufficient workability will not be obtained. I can't do it.

In addition to these components (A), (B), and (C), if necessary,
In addition to the anticorrosion pigments normally used in baking paints, such as chromates, basic lead compounds, and zinc compounds such as zinc phosphate, pigments for coloring and weighting, such as titanium white and kaolin clay, are used. Furthermore, various known and commonly used additives such as a thickener, an anti-settling agent, an anti-color separation agent, a wetting and dispersing agent, an antifoaming agent, and a curing accelerator can be added as appropriate.

The present invention is completed by applying a top coat on top of one layer of double brimer, but the top coat may be any of the commonly used polyester paints, acrylic paints, fluororesin paints, urethane resins, etc. Examples include paint, silicone resin paint, and chlorinated rubber paint.

In the present invention, processability is maintained by component (A) in the brimer, and corrosion resistance of processed parts is obtained by chemical bonding between component (B) and a coupling agent.

<Example> Hereinafter, the present invention will be explained in more detail with reference to Examples.

(Example 1) High molecular weight polyester [Product name: Polyester-TP-217, manufactured by Nippon Gosei Co., Ltd., number average molecular weight: 16,000]
90 parts (as solid content) Epoxy resin [manufactured by Yuka Shell Co., Ltd., trade name Epicoat 1001, number average molecular weight 90]
0, bitroxyl content 4%, epoxy equivalent 500
] 10 parts, melamine resin [manufactured by Mitsui Toatsu Co., Ltd., trade name
11.1 parts of Cynull 303], 56 parts of titanium oxide, 56 parts of strontium chromate, and 222 parts of thinner were added and kneaded in a ball mill to obtain a paint for brimer.

As a coupling agent solution, γ-(2-aminoethyl)
5 parts of aminopropyltrimethoxysilane [5H6020 manufactured by Toray Silicone Co., Ltd.] was diluted with 95 parts of water to prepare a 5% aqueous solution.

The iron phosphate treated steel plate was immersed in a coupling agent solution for 5 minutes and then dried at 120°C for 10 minutes, and the above-mentioned brimer was coated with a bar coater to a film thickness of 5 - and baked at a plate temperature of 200±10°C for 30 seconds.

For the top coat, apply a pre-coat polyester paint with a bar coater to a film thickness of 20 Ijm, and heat it at 220 ± 10°C for 30°C.
Burned in seconds.

Table 1 shows the coating system and performance results of this coated steel plate.

As shown in the data in the table, the workability and post-processing corrosion resistance of conventional pre-coated materials have been greatly improved.

(Example 2) The conditions were exactly the same as in Example 1 except that the coupling agent was changed to γ-mercaptopropyltrimethoxysilane [5H6062 manufactured by Toray Silicone Co., Ltd.].

(Example 3) The epoxy resin of component (B) was changed to Epicoat 1004 (trade name) manufactured by Yuka Shell Co., Ltd. [number average molecular weight 1600, hydroxyl content 5% epoxy equivalent weight 900],
The conditions were exactly the same as in Example 1 except that (A)/(B) was changed to 80/20.

(Example 4) The epoxy resin of component (B) was manufactured by Yuka Shell Co., Ltd. Product name Epicor 1-1007 [Number average molecular weight 2900,
The conditions were exactly the same as in Example 1, except that the hydroxyl content was changed to 6%, the epoxy equivalent weight was changed to 1900, and (A)/(B) was changed to 70/30.

(Comparative Example 1) Example 1 except that the epoxy resin of component (B) was changed to Epicoat 828 (trade name, manufactured by Yuka Shell Co., Ltd., number average molecular weight 380, hydroxyl content 0.6%, epoxy equivalent weight 190). The conditions are exactly the same.

(Comparative Example 2) Manufactured by Yuka Shell Co., Ltd. as the epoxy resin for component (B)
The product name was changed to Epicote 1009 [number average molecular weight 5500, hydroxyl content 6%, epoxy equivalent weight 3000], and (A)/(B) was changed to 70/30.
The conditions are exactly the same as in Example 1.

(Comparative Example 3) 180 parts of Epicoat 1001 and 180 parts of xylene were mixed into 8 parts.
The mixture was heated and dissolved at 0°C to obtain a 50% solution. Add 16.8 parts of hexamethylene diisocyanate to this in xylene +5]
, was added dropwise over 1 hour, and reacted at 80°C for 2 hours. The end point of the reaction was determined by an infrared spectrophotometer.
The point was set at which the absorption of isocyanate groups (2270c++r') disappeared. As a result, 528 g of a resin varnish (1) having a number average molecular weight of 2,000, a hydroxyl content of 1.8% and an epoxy equivalent of 500 was obtained. The conditions were exactly the same as in Example 1 except that resin varnish (2) was used as component (B).

(Comparative Example 4) The weight ratio (A)/(B) of component (A) and component (B) was 40
The conditions are exactly the same as in Example 3 except that the value was changed to 760.

(Comparative Example 5) The conditions were exactly the same as in Example 1 except that component (B) was not used.

(Comparative Example 6) Weight ratio (A)+ of component (A), component (B), and component (C)
The conditions were exactly the same as in Example 1 except that (B)/(C) was changed to 40/60.

(Comparative Example 7) Weight ratio (A)+ of (A) component, (B) component, and (C) component
The conditions were exactly the same as in Example 1 except that (B)/(C) was changed to 98/2.

(Comparative Example 8) Example 1 except that no coupling agent was applied.
The conditions are exactly the same.

(Comparative Example 9) The conditions were exactly the same as in Example 1 except that the component (A) was changed to a medium molecular weight polyester resin manufactured by Mitsui Toatsu Chemical Co., Ltd., trade name Almatex P645 [number average molecular weight 3000].

Separately, for comparison, polyester resins, epoxy-modified polyester resins, and epoxy resins, which are currently widely used, were each coated on the same iron phosphate treated steel plate as in Example 1 with a bar coater to a film thickness of 5p x 11, and then cured by heat. A similar performance evaluation was conducted using

The results are shown in Table 1.

Note 1) OT bending (180° crack occurrence or not)
(Confirmed with a 10x magnifying glass) ◎ No cracks at all ○ Very few cracks △ Slight cracks × Quite a lot of cracks Note 2) Seed water spray test after 2T bending After 200 hours, the processed part was peeled off with sellotape, and the degree of peeling was determined. Evaluation ◎ No peeling at all O Very slight peeling △ Slight peeling )
Straight chain linear high molecular weight polyester, (B) epoxy resin,
(C) A steel plate coated with a specific primer made of a specific crosslinking agent, and is a pre-coated steel plate with a well-balanced workability and corrosion resistance in processed areas.

The prepainted steel sheet of the present invention can be very effectively applied to home appliances.

Claims (1)

[Claims] (1) A layer consisting of an amino-based and/or mercapto-based silane coupling agent is provided on the chemical conversion treatment layer, and (A) a linear high-molecular-weight polyester resin having a number average molecular weight of 5,000 or more and (B) ) An epoxy resin having an epoxy equivalent of 2500 or less, a hydroxyl content of 3% or more, and a number average molecular weight of 800 or more is mixed at a weight ratio of (A)/(B) = 50/50 to 95/5, and further (C ) Contains one or more of melamine resin, urea resin, blocked isocyanate resin, and phenol resin in a weight ratio of (A) + (B) / (C) = 60/40 to 95/5. A pre-coated steel sheet with excellent workability and corrosion resistance in processed areas, characterized by having a primer layer of