KR20040059131A - Coating Composition for Pre-Sealed Steel Sheet And Preparing Method of Pre-Sealed Steel Sheet Having Improved Corrosion Resistance, Electro-Painting Properties and Weldability - Google Patents

Abstract

PURPOSE: A resin composition for a pre-sealed steel sheet and a method for preparing a pre-sealed steel sheet by using the composition are provided, to improve corrosion resistance, electrodepositing property and weldability. CONSTITUTION: The resin composition comprises an epoxy resin having a number average molecular weight of 3,000-7,000 dispersed or emulsified in water; 5-20 phr of a melamine resin; 13-30 phr of a water-dispersive colloidal silica; 1-5 phr of a wax; and 80-300 phr of at least one metal powder having a particle size of 0.5-4 mm and selected from the group consisting of Al, Cu, Ni, Zn, Fe2P, Fe, Mn, Co, Ti and Sn. Preferably the wax is selected from the group consisting of polyolefin-based waxes, carnauba waxes, Teflon-based waxes and silicone-based waxes; and the metal powder is needle-shaped or plate-shape one having a length of major axis of 0.5-4 mm.

Description

Coating Composition for Pre-Sealed Steel Sheet And Preparing Method of Pre-Sealed Steel Sheet Having Improved Corrosion Resistance, Electro-Painting Properties and Weldability}

The present invention relates to a resin composition for pre-shielded steel sheet having improved corrosion resistance, electrodeposition coating, and weldability, and a method of manufacturing a pre-shielded steel sheet using the same. More specifically, the present invention relates to a resin composition for pre-shielded steel sheet having improved weldability and electrodeposition coating property, despite the thick resin film thickness of 2 mm or more for imparting corrosion resistance, and a method of manufacturing a pre-shielded steel sheet using the same.

Steel plate manufactured by steel company is subjected to phosphate treatment and painting after assembling the car body in automobile company. At this time, the overlapped parts of the steel plate is generated in the assembled body, for example, the hem (Hem) of the door of the car, for example. In this way, since the phosphate treatment solution or paint hardly penetrates the steel plate overlapping part of the vehicle body, the corrosion resistance becomes weak. In order to solve this problem, the automobile company is used to seal the overlapping parts of the steel sheet (Sealing) to block the corrosion factor access to the site. However, this causes a decrease in productivity and a rise in cost, and thus requires a surface treated steel sheet which can omit the sealing process.

Accordingly, European steel companies are galvanizing steel plates, chromate-processing them, and producing steel sheets coated with organic coatings having a thickness of about 4 to 10 mm on them. Since the site also ensures corrosion resistance, the sealing process in an automobile company can be omitted, and is therefore called a pre-sealed steel sheet. However, in the pre-shielded steel sheet, since a thick resin film is formed in order to secure corrosion resistance, the electric resistance weldability used in automobile companies is reduced. In order to solve this problem, a metal powder having excellent electrical conductivity is mixed and used in the resin film, but there is still a problem that electrodeposition paintability is poor due to the thick resin film.

Accordingly, an object of the present invention is to provide a resin composition for pre-shielded steel sheet to ensure excellent weldability and electrodeposition coating despite the increase of the resin film thickness.

Another object of the present invention is to provide a method of manufacturing a pre-shielded steel sheet excellent in corrosion resistance, weldability and electrodeposition coating properties.

1 is a side cross-sectional view of a free shield steel sheet,

2A and 2B are diagrams illustrating a specimen preparation method for corrosion resistance evaluation.

According to one aspect of the invention,

Epoxy resin with a number average molecular weight of 3,000 to 7,000 dispersed or emulsified in water, 5 to 20 phr of melamine resin, 12 to 30 phr of water-soluble colloidal silica, 1 to 5 phr of wax, and Al, Cu, Ni, Zn Water-soluble resin-coated resin sheet for resin composition comprising at least one metal powder of 80 to 300 phr having a particle size of 0.5 to 4 mm selected from the group consisting of P, Fe ₂ P, Fe, Mn, Co, Ti and Sn This is provided.

According to another aspect of the present invention,

Pre-shielded resin to be baked at 180-250 ° C. after coating the resin composition of the present invention on a zinc or zinc alloy plated steel sheet chromated with a chromium deposition amount of 30 to 200 mg / m ^{2 so} as to have a dry film thickness of 1.5 to 8 mm. A method for producing a coated steel sheet is provided.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The physical properties of the preshielded steel sheet are largely affected by the chromate and the resin layer.In particular, the resin in the upper portion of the steel sheet has a primary shielding effect against corrosion factors and prevents the rapid chromium elution of the lower layer chromate to prevent corrosion of the preshielded steel sheet. It has a role to secure.

If the corrosion resistance of the pre-shielded steel sheet is not secured, corrosion holes of the steel sheet may be generated along with corrosion of the vehicle body when applied to automobile bodies, and the vehicle may be damaged.

In addition, the resin film constituting the upper layer of the pre-shielded steel sheet should be formed of a rigid film having excellent alkali resistance and solvent resistance, as well as protecting the chromate layer of the lower layer. If the resin coating is not strong, such as corrosion resistance of the pre-shielded steel sheet due to peeling or damage because the resin coating cannot withstand the alkaline degreasing solution and the acidic phosphate solution in the degreasing and phosphate treatment process, which is applied to automobile vehicles. Physical properties are not achieved.

In addition, corrosion resistance should be exerted in the hemming area where electrodeposition coating is not applied among the automobile bodies to which pre-shielded steel is applied, and pre-shielding is performed smoothly in other parts like electrode plating steel plate or cold rolled steel plate. The resin must be suitable for automotive electrodeposition coating.

The vehicle body goes through the degreasing and phosphate treatment bath, and then enters the electrodeposition paint bath. In the electrodeposition paint bath, paint particles having the properties of the anode are attached to the vehicle body having the cathode. When the electrodeposition paint is initially attached to the steel sheet, the electrodeposition paint adheres to the steel sheet when the pH changes rapidly from neutral to alkaline due to the generation of hydrogen gas on the surface of the steel sheet. The electrodeposition coating film formed by this process is no longer adhered to the electrodeposition coating particles due to the increased electrical resistance as the thickness increases, the electrodeposition coating is terminated. However, if the preshield resin does not have an advantageous hydrophilic structure for electrodeposition coating, the exchange of water or ions does not occur actively during electrodeposition coating, resulting in excessive hydrogen gas generation at a specific site due to localized current concentration. As a result, the electrodeposited coating film formed on the surface of the coating film is hydrogen bubble not only rough surface, but also uneven shape is transferred as it is during the intermediate and top coat, which is a subsequent process, causing coating failure.

Therefore, in order for the resin film to have an advantageous structure for the electrodeposition coating, it is preferable that the structure of the resin constituting the resin film contains a structure having a hydrophilic group, that is, a hydroxyl group (-OH) and an amide group (-NH3). therefore,

In the present invention, an epoxy resin advantageous for electrodeposition coating is used as the main resin.

As the epoxy resin, an epoxy resin having an average molecular weight of 3000 to 7000 dispersed or emulsified in water, in particular an epoxy resin of diglycidyl ether type, may be used, wherein the resin dispersed or emulsified in water has a maximum particle size of 10 microns. It is preferable to make it become the following. If the average molecular weight of the epoxy resin is less than 3,000, it reacts with amine or amide rather than with the melamine resin, which is a curing agent.If the average molecular weight is over 7,000, the time for dissolving the epoxy resin in the solvent to make the water-soluble resin is increased. It is not preferable because it is difficult and the cost is increased, and even after synthesizing with a water-soluble resin, the viscosity is too high, which causes a problem of uniform coating.

In addition, the smaller the particle size of the epoxy resin dispersed or emulsified in water, the better. However, when the maximum resin particle size exceeds 10 microns, not only the reactivity with the melamine resin is reduced, but also the resin particles containing the metal powder are coated with large resin particles. Due to this, the coating becomes non-uniform, and therefore, incompletely coated spots occur, which is not preferable.

Melamine resin may be used as a curing agent in the epoxy resin. Melamine resin is added 5 ~ 20phr to the epoxy resin (topic). If the content of the curing agent is less than 5 phr, the curing reaction of the coating film is not enough to secure the desired physical properties. If it exceeds 20phr, there is no effect of increasing the curing reaction by additional addition, and there is a fear that the added amount of the curing agent lowers the solution stability and the physical properties of the resin film.

Furthermore, silica is added in an amount of 12-30 phr to the epoxy resin to improve corrosion resistance. In view of the fact that the resin is aqueous, silica is preferably added to the colloidal silica. Since the stability of colloidal silica is very sensitive to pH, it is necessary to use colloidal silica having an appropriate pH depending on the pH of the resin solution. Since the epoxy resin used in the present invention is alkaline in pH, it is preferable to use colloidal silica in the range of 8 to 10.5. If the colloidal silica has a pH of 8 or less or 10.5 or more, a solution stability problem may occur due to an inadequate pH of the resin solution.

The content of the colloidal silica is used in 12 ~ 30phr with respect to the epoxy resin. If it is less than 12 phr, the desired corrosion resistance effect is insufficient, and even if it exceeds 30 phr, there is no effect of improving the physical properties by the addition of silica.

In addition, wax may be used to improve lubricity of the steel sheet. As the wax, various kinds of waxes such as polyolefin, cannuber, teflon and silicone waxes may be used, but it is preferable to use a teflon wax having excellent effect even in a small amount. Teflon wax is added in an amount of 1 to 5 phr based on the epoxy resin. If the content is less than 1 phr, the lubricating performance is insufficient, and if it exceeds 5 phr, it is not preferable because it inhibits the adhesion with the electrodeposition coating film applied on the top.

In addition, the metal powder is added to improve the weldability. As the metal powder, any type of metal powder, such as spherical, needle or plate, may be used. Since the particle size of the metal powder should be smaller than the thickness of the resin film, it is preferable that the particle diameter is 0.5 to 4 μm in the case of spherical shape, and 0.5 to 4 μm in the major axis in the case of needle or plate shape.

If the particle size is less than 0.5㎛, it is difficult to form a structure in which metal powders are connected in the resin film, so that the current does not easily flow during welding. Not only is this damaged, but also causes various quality problems such as damage to the mold during processing, which is undesirable. When the metal powder is dispersed in the resin solution, it is preferable that the metal powder is plate or needle rather than spherical to prevent sedimentation in the resin solution. That is, in the case of a plate-like or needle-like structure, since the buoyancy of the resin is received under the same resin viscosity, the spherical shape is much better because the buoyancy acts much less than that of the bed or plate.

As the metal powder, a metal powder selected from the group consisting of Al, Cu, Ni, Zn, Fe ₂ P, Fe, Mn, Co, Ti, and Sn may be added alone or in combination.

The metal powder is preferably a hard metal rather than a soft metal. The reason why hard metals are advantageous for weldability is that when pressing force is applied during welding, the soft metals do not remain intact and are dispersed with the same metals or resins, while hard metals disperse current even when pressure is applied. This is because the efficiency of energizing the current by maintaining the shape is large.

The content of the metal powder is 80 ~ 300 phr compared to the epoxy resin content. If the content of the metal powder is less than 80 phr, the metal powder content is too small to weld. If it exceeds 300 phr, the metal powder particles are peeled off in the form of powder when the film is processed due to the large amount of metal in the resin film. Because it comes out.

Hereinafter, the manufacturing method of the pre-shielded resin coated steel sheet using the resin composition will be described in detail.

Reaction type and coating type chromate treatment is carried out on the plated steel sheet, and the resin composition of this invention is apply | coated on it, The resin coating steel plate for preshields of water-soluble resin excellent in corrosion resistance, electrodeposition coating property, and weldability are manufactured.

Galvanized steel sheet or zinc alloy coated steel sheet may be used as the plated steel sheet. Various methods such as electroplating, hot-dip plating, and vacuum deposition may be used as a method of plating a zinc alloy mixed with other metals such as zinc, zinc, iron, nickel, cobalt, or the like on a steel sheet.

The plated steel sheet is treated with a chromate solution for the purpose of improving corrosion resistance. As the chromate treatment, reactive or coated chromate treatment may be used, and coated chromate is preferable from the viewpoint of improving corrosion resistance. The coated chromate treatment solution is usually composed of chromic anhydride, phosphoric acid, silica and silane dissolved in water. However, it is preferable to use a two-component type of chromium solution and silane because of the stability of the chromate solution.

The amount of chromium deposition on the steel sheet during the chromate treatment is preferably 30 ~ 200mg / m ² level. When 30 mg / m ² is less than this it is because corrosion resistance is, the coating film adhesion and formability if mothamyeo insufficient, exceeds 200mg / m ² degradation.

The resin composition according to the present invention is coated on the chromate plated steel sheet so that the baked dry film thickness is 1.5 to 8.0 µm (resin amount of 2.0 to 10.0 g / m ² ). This is because when the dry coating thickness is less than 1.5 µm, physical properties such as corrosion resistance decrease, and when the thickness exceeds 8.0 µm, there is a problem in welding because of high resin thickness.

After apply | coating a resin composition, it bakes at the steel plate temperature of 180-250 degreeC. If the baking temperature is less than 180 ° C., the curing reaction of the resin is insufficient, and the physical properties of the coating film are lowered. If the baking temperature is higher than 250 ° C., the curing reaction does not proceed any more with the change of the material of the steel sheet, which consumes only heat. not. After baking, the pre-shielded steel sheet is manufactured by cooling by a conventional method such as water cooling or air cooling.

The water-soluble preshield resin coated steel sheet produced by the method according to the present invention is excellent in corrosion resistance, electrodeposition coating, and weldability. In addition, it is suitable for use in areas where phosphate coating or electrodeposition coating film is difficult to form, such as heme.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples do not limit the present invention.

(Example)

Example 1

In this example, the solution stability of the resin solution and the physical properties of the resin coating according to the content of the epoxy resin and the curing agent were evaluated.

Epoxy resins dispersed at a maximum particle size of 4 microns or less, and amide resins and melamine resins as hardeners were mixed in the amounts shown in Table 1 below, and water dispersed colloidal silica 15 phr, wax 3 phr, long axis of 1 to 3 microns ₂ P formulated to be composed of metal powder was prepared in the invention example and the comparative example resin composition.

Thereafter, solution stability, chemical resistance, and electrodeposition coating resistance of the resin composition were evaluated.

Solution stability was evaluated according to the presence or absence of gel formation in the resin composition after 30 days at room temperature in consideration of the sufficient time to actually use.

The case where a gel is formed is shown by (circle) and the case where a gel is not formed is shown by (triangle | delta).

Resin coated steel sheet for evaluation of chemical resistance and electrodeposition coating is coated on a zinc electroplated steel sheet coated with zinc with a coating weight of 30 g / m ^{2 on} a steel plate having a thickness of 0.7 mm and subjected to chromate treatment with a Cr coating amount of 50 mg / m ² . The resin coating was applied using a bar coater to obtain a dry coating thickness of 4.0 μm under the resin solution conditions shown in Examples and Comparative Examples of Table 1, followed by baking at a steel plate temperature of 200 ° C., followed by water cooling. Coated steel sheet specimens were prepared.

Preliminary degreasing of the resin-coated steel sheet prepared above to simulate the automobile body treatment process (Chemical name: Pyroclean 442, concentration: 5% by weight, treatment time: 150 seconds, temperature: 50 ℃, spray)-> Degreasing (Chemical name: Clean Gard Pyroclean 442, Concentration: 5% by weight, Treatment time: 150 seconds, Temperature: 50 ° C, Dipping) → Water washing (pure, Treatment time: 150 seconds, Temperature: Room temperature, spraying) → Surface Adjusting (spraying) (Parcolene Z. 45 ℃, 2 seconds) → Phosphate treatment (Chemical name: Bonderite 699D, Temperature: 45 ℃, Acidity: 20.5, Free acidity: 1.05, Acceleration: 2.5) → Electrodeposition coating (Chemical name: ED -1800 Gray, solid content: 20%, temperature: 28 ° C, treatment time: 180 seconds, voltage: 50V) → baking (165 ° C x 20 minutes).

Since the painting process of automobile is made by the above process, chemical resistance evaluation is processed to the degreasing (preliminary degreasing + main degreasing) process before electrodeposition coating, and then the degree of discoloration and peeling of the resin coating (coating film) is divided into five stages. Evaluated. Evaluation criteria are as follows.

(Double-circle): Color difference (relative to the color difference before processing) E1 or less, without peeling of the resin coating film

○: No peeling of the resin coating film, color difference (contrast color difference before treatment) E 2-3

□: No peeling of resin film, color difference (contrast color difference before treatment) E 5 or more

(Triangle | delta): Partial peeling of resin coating film (The peeling part is 30% or less of resin conductive cloth surface.)

×: The resin coating film is completely peeled off (the peeled portion is more than 50% of the resin conductive cloth surface)

Electrodeposition coating property was evaluated in three ways according to the surface state of electrodeposition coating film after the last process of the above treatment conditions. If the degree of roughness is high, the electrodeposition coating surface is '×', and the electrodeposition coating surface is visible. If it looks smooth and there is no blemish, '◎' and the middle is marked with '□'.

TABLE 1

When the amide resin is used as a curing agent as in Comparative Examples 1 to 7 of Table 1 or when the molecular weight of the main epoxy resin is less than 3000 phr as in Comparative Examples 8 to 10, the molecular weight of the melamine resin is 5 phr as the curing agent as in Comparative Example 11. If less than, it was found that the chemical resistance is insufficient, and in the case of more than 20 phr of the melamine resin of the curing agent as in Comparative Example 12, the chemical resistance and electrodeposition coating resistance is excellent, but the solution stability deterioration due to excessively added the curing agent appeared to be a problem. On the other hand, when the molecular weight of the main epoxy resin exceeds 7000 as in Comparative Example 13, it is excellent in chemical resistance, solution stability, and electrodeposition coating property, but as described above, it is difficult to prepare the epoxy resin as a water-soluble resin and synthesized as a water-soluble resin. Even after that, the viscosity is too high to cause uniform coating, which is not preferable.

Example 2

The physical properties of the resin composition were evaluated according to the change of metal powder, wax and silica content.

The resin coating solution is an epoxy resin (molecular weight: 4,000 phr) with a particle size of 0.5 mm emulsified in water, and methylated melamine formaldehyde 10 phr as a curing agent (manufactured by US Cytec, hereinafter referred to as melamine resin), metal powder (1 to 3 mm long) Plate), Teflon wax and silica were used in the amounts shown in Table 2 below using butyl cellosolve as a solvent to prepare the resin compositions of Comparative Examples 14-20 and 7-26.

The base plate coated with the resin coating was a galvanized steel sheet coated with zinc with a coating weight of 30 g / m ² on a 0.7 mm thick steel plate and subjected to chromate treatment with a Cr coating amount of 50 mg / m ² .

The resin compositions of Comparative Examples 14-20 and Inventive Examples 7-26 were coated on the steel sheet in the same manner as in Example 1, and then evaluated for corrosion resistance, processability, electrodeposition coating property, and weldability.

Corrosion resistance was cycled in the order of high temperature drying, saline immersion 2 hours, and natural drying 2 hours for 18 hours, 70 hours for 2 hours at 70 ^o using a salt spray tester (CCT: Cyclic Corrosion Tester). After the corrosion was investigated. Brine spraying conditions were carried out under JIS Z 2371 conditions.

Specimens for corrosion resistance were prepared in the form as shown in Figure 2a and 2b using the resin-coated specimens. At this time, the resin-coated surface was to be located on the side of the steel sheet overlap. The specimen prepared as described above was subjected to the phosphate treatment and electrodeposition coating treatment as in the automotive coating process simulation conditions of Example 1, and the composite salt spray test was performed.

After completion of the composite salt and water test, the overlapped steel sheets were separated and the thickness of the steel sheets reduced by corrosion at the overlapping steel sheets was measured. The evaluation criteria are as follows.

◎: Thickness reduction due to corrosion: 0 ~ 0.05mm

○: Thickness reduction due to corrosion: 0..05 ~ 0.1mm

□: Thickness decrease by corrosion: 0.1 ~ 0.15mm

△: thickness decrease by corrosion: 0.15 ~ 0.2mm

X: thickness reduction by corrosion: 0.2 mm or more

The workability was evaluated by the planar friction coefficient, and the conditions of evaluating the planar friction coefficient were 1m / min withdrawal speed and 0.5 kgf / mm ² with pressing force.

◎: friction coefficient is 0.15 or less

□: Friction Coefficient 0.15 ~ 0.2

×: coefficient of friction 0.2 or more

Electrodeposition coating property was evaluated in the same manner as in Example 1.

In order to evaluate the weldability, each specimen was welded using an AC pneumatic spot welding machine under a pressurization of 250 kgf and an energization time of 16 cycles at a welding current of 8 kA. RWMA Class II of Cu—Cr alloy was used as a welding electrode. Weldability was evaluated by continuous RBI. The specimens for evaluation were overlapped to two points of 80 mm in length and 20 mm in width to the point of 20 mm from the end, and then welded one point to the center of the overlapped area, and the number of spots in which the welded part fell by the tensile tester was examined. This RBI was evaluated as a continuous RBI, and the evaluation criteria are as follows.

◎: 1000 ~ 1500 consecutive RBI

○: 500-1000 consecutive RBIs

□: Number of consecutive RBIs 300 ~ 500

△: Number of consecutive RBIs 100-3000

×: No welding

TABLE 2

Example 3

In this embodiment, the properties of the baking temperature and the thickness of the dry coating film during the resin coating were evaluated.

The composition of the resin coating solution is epoxy resin having a molecular weight of 4,000 phr of 0.5 mm emulsified in water, 10 phr of melamine resin as a hardener, 120 phr of plate-shaped Fe ₂ P metal powder with a major axis of 1 to 3 mm, 1.5 phr of Teflon wax and 20 phr of silica. The resin was coated on the chromated galvanized steel sheet.

The base plate coated with the resin coating was a galvanized steel sheet coated with zinc with a coating weight of 30 g / m ² on a 0.7 mm thick steel plate and subjected to chromate treatment with a Cr coating amount of 50 mg / m ² .

Using a comparative material and an inventive material, the chromate-treated zinc alloy plated steel sheet was coated with resin at the baking temperature and the dry film thickness shown in Table 3, and then electrodeposition coating property, weldability and corrosion resistance were evaluated. It was.

TABLE 3

Comparative material 22, the baking temperature is out of the scope of the present invention exhibits excellent electrodeposition coating properties, weldability and corrosion resistance, but has a defect that the curing reaction does not proceed with the change in the material of the steel sheet.

The resin coating film coated on the galvanized steel sheet or the zinc alloy coated steel sheet according to the resin composition and processing conditions of the present invention exhibits excellent chemical resistance, corrosion resistance, weldability, electrodeposition coating property, and workability, and is excellent as a general steel plate for automotive and exterior plates. Not only does it show quality, but it is also suitable for use in areas where it is difficult to form phosphate coatings or electrodeposition coatings, such as those in the rear of automobiles or in heme (Hem) areas.

Claims (4)

Epoxy resin with a number average molecular weight of 3,000 to 7,000 dispersed or emulsified in water, 5 to 20 phr of melamine resin, 13 to 30 phr of water-soluble colloidal silica, 1 to 5 phr of wax, Al, Cu, Ni, Zn Water-soluble resin-based resin coated steel sheet resin comprising 80 to 300 phr of at least one metal powder having a particle size of 0.5 to 4 mm selected from the group consisting of Fe ₂ P, Fe, Mn, Co, Ti and Sn Composition. The resin composition according to claim 1, wherein the wax is selected from the group consisting of polyolefin-based, cannuber-based, Teflon-based and silicone-based waxes. The resin composition according to claim 1, wherein the metal powder has a needle or plate shape having a long axis of 0.5 to 4 mm. The resin composition according to claim 1 is coated on the zinc and zinc alloy plated steel plates chromated with a chromium deposition amount of 30 to 200 mg / m ^{2 so} as to have a dry film thickness of 1.5 to 8 mm, followed by baking at 180 to 250 ° C., followed by cooling. Method for manufacturing shielded resin coated steel sheet.