JPH0257589B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a coating composition for precoated steel sheets. In recent years, large amounts of rock salt have been sprayed in the snowbelt zones of North America and Northern Europe to prevent roads from freezing during winter snowfall. For this reason, the degree of corrosion in automobiles is increasing year by year.
From the viewpoint of safety and economy, there is a rapidly increasing demand for improved corrosion resistance of automobiles. Furthermore, not only in the above-mentioned regions, but also for other products using steel sheets, rust prevention measures are being closely focused on from the viewpoints of resource saving and durability. As one of the countermeasures against this problem, a pre-coated steel plate has been developed in which the surface of the steel plate is coated with a chemical solution, paint, etc. This pre-painted steel sheet is manufactured through the following steps: (a) cold-rolled steel sheet → (b) pre-coating → (c) baking drying, but the pre-painted steel sheet manufactured in this way is actually used for automotive applications, etc. In this case, it is normal to go through the following steps: (d) Molding assembly → (e) Chemical treatment → (f) Electrodeposition painting (undercoating) → (g) Intermediate coating if necessary → (h) Finishing painting. Therefore, not only corrosion resistance but also weldability and moldability in step (d), adhesion with each paint in steps (f) to (h), etc. are required. Such pre-painted steel sheets are classified into two types: double-sided pre-painted steel sheets, in which both sides of the steel sheet are painted in advance in the process (b), and single-sided pre-painted steel sheets, in which only one side is painted. Because of this, single-sided pre-coated steel sheets, which are coated on only one side, are currently in practical use for automobile applications. However, when a single-sided pre-coated steel sheet is actually used, in the steps (F) to (H) above, especially in the step (F), both sides of the steel sheet, that is, the pre-coated side and the non-pre-coated side, are naturally separated. The (electrodeposition) paint is applied to both sides of the surface (bare surface), but it has been pointed out that the adhesion between the non-precoated surface (bare surface) and the (electrodeposition) paint decreases.
It has become a big problem. In view of the above circumstances, the present inventors have conducted intensive research to solve the above-mentioned problems, and as a result, the present inventors have solved the cause thereof, and have developed the coating composition for pre-coated steel sheets of the present invention as a means to solve the problem. This is what I discovered. First, the present inventors studied the heating conditions for ordinary cold-rolled steel sheets used for pre-coated steel sheets, and found that if the steel sheet surface is heated above a certain temperature range, the coating that is subsequently applied will It was found that the adhesion of In other words, if the bare surface of a cold-rolled steel sheet is exposed to an atmosphere exceeding 150°C, or if it is exposed to an atmosphere at a temperature close to or lower than this for more than 5 minutes, the bare surface may become excessively exposed. In this case, the phosphate coating crystals of the phosphate treatment performed in the chemical conversion treatment step (e) above are uniform,
It also becomes difficult to adhere to dense conditions. Therefore, it was confirmed that even if an (electrodeposition) paint was applied thereon in the subsequent step (f), the adhesion of the paint film was reduced. On the other hand, in the current manufacturing of pre-painted steel sheets, from the viewpoint of productivity and other reasons, the pre-painting in the step (b) above is performed by coil coating (roll coating).
A painting method is used. Therefore, this pre-coated coating has approximately 170
It is baked and hardened by heating at a high temperature of ℃~250℃. Therefore, in the case of a single-sided precoated steel sheet, when the precoated coating film is baked, the other surface, which is a bare surface that is not precoated, is also heated under the same conditions at the same time. Therefore, when actually using the single-sided pre-coated steel sheet manufactured in this way, the next steps (e) to (h) are applied on the bare surface which has already been heated to a high temperature. It was confirmed that because the paint is applied through a process, its adhesion deteriorates. Based on these confirmations, the present inventors have developed heating conditions that do not overheat the unprecoated bare surface, that is, do not worsen the adhesion state of the chemical conversion coating crystals, and do not cause the coating film to be coated on top of the heating conditions to deteriorate. We are conducting research to find a paint for pre-coated steel sheets that has not only corrosion resistance but also good weldability and formability even when baked and dried under heating conditions of 5 minutes or less at 150℃ or less without reducing the adhesion of the steel sheet. Ta. As a result, ordinary amino acrylic resin paints, amino alkyd resin paints, epoxy resin paints, urethane resin paints, etc., which are normally used on iron, zinc plating, etc. treated with phosphates, etc., and exhibit good performance, etc. However, when using the coating composition of the present invention as described below, curing was insufficient in both cases, resulting in poor physical performance and corrosion resistance. The inventors have now completed the present invention by discovering that even when baked and dried under heating conditions within 100 to 100%, the adhesive exhibits good corrosion resistance, weldability, and moldability without reducing the adhesion. That is, the coating composition for pre-coated steel sheets of the present invention comprises (A) 100 parts by weight of a linear polyether polyol with a hydroxy content of 6 to 9%, and epichlorohydrin-bisphenol type A epoxy with a hydroxy content of 2 to 5%. 30~ resin
It consists of a polyol resin composition containing 100 parts by weight and (B) a diphenylmethane diisocyanate prepolymer with an isocyanate content of 20 to 30%, and the components (A) and (B) are combined in an NCO/OH equivalent ratio of 1. /
Formulated with a mixing ratio of 1.2 to 1.5, which forms a cured coating film under heating conditions of 150°C or lower for 5 minutes, and the glass transition point of the coating film is 70°C or higher, and the spread rate is 120% or higher. It consists of a coating composition for pre-coated steel sheets. The present invention will be explained in detail below. Hydroxy content of component (A) used in the present invention: 6
~9% linear polyether polyol is a linear polyether polyol with a viscosity of 500-1500 cps (20°C) in the solvent-free state, and low-temperature curability when the hydroxy content is less than 6%. If it exceeds 9%, the coating film formed becomes too soft, and conversely, if it exceeds 9%, the coating film formed becomes hard and brittle, resulting in poor workability. In addition, epichlorohydrin-bisphenol A type epoxy resin with a hydroxy content of 2 to 5% is
It is a commonly commercially available compound consisting of a compound of epichlorohydrin and bisphenol A. If the hydroxyl content is less than 2%, the low-temperature curability will decrease, and if it exceeds 5%, the elongation of the coating will be poor. This results in a decrease in workability. The blending ratio of the above polyether polyol and epoxy resin is 100 parts by weight of polyether polyol and 30 to 100 parts by weight of epoxy resin. If the epoxy resin is less than 30 parts by weight, the corrosion resistance of the coating film will be insufficient. On the other hand, if it exceeds 100 parts by weight, the elongation of the coating film will be poor and processability will be reduced. Component (A) may include pigments, fillers,
Other resins and various additives can be blended. Next, the diphenylmethane diisocyanate prepolymer with an isocyanate content of 20 to 30% as component (B) used in the present invention has a viscosity of
50~200cps (20℃) polyisocyanate,
If the isocyanate content is less than 20%, low-temperature curability will decrease, and if it exceeds 30%, the coating will have poor elongation and processability. Since the coating composition of the present invention is a two-component reaction type, these components (A) and (B) are mixed during coating, and the mixing ratio is 1/1.2 to 1.5 in terms of NCO/OH equivalent ratio. If the OH equivalent ratio is less than 1.2, the adhesion of the coating will decrease, and if it exceeds 1.5, the corrosion resistance of the coating will be insufficient. Further, the coating composition of the present invention may contain a third component in addition to the components (A) and (B), if necessary. For example, if particularly excellent corrosion resistance is required, it is desirable to mix metal zinc powder and/or metal zinc alloy powder, and the mixing ratio should be the sum of component (A) and component (B).
It is preferably 400 to 900 parts by weight per 100 parts by weight.
If the amount of metallic zinc powder and/or metallic zinc alloy powder is less than 400 parts by weight, the cathodic corrosion resistance (corrosion resistance) which is the purpose of adding metallic zinc powder will be insufficient, and on the contrary,
If it exceeds 900 parts by weight, the relative blending amount of the resin component consisting of component (A) and component (B) will be too small, resulting in a decrease in the physical performance and processability of the coating film. The coating composition of the present invention consists of the above-mentioned components, and is produced by a known coating production method. In addition, coating methods include coating methods such as roll coating, flow coating, spray coating, and electrostatic coating while conveying the metal plate material on a conveyor line, but the coating film after coating is as described above.
The coating method, conveyor speed, heating temperature, etc. must be selected so that a cured coating can be formed under heating conditions of 5 minutes at a temperature of 150°C or less. Note that there is no particular lower limit to the heating conditions, but from a practical standpoint, it is desirable that the temperature be 80° C. or higher. Furthermore, the coating film obtained in this way will have insufficient adhesion and corrosion resistance if the glass transition point is less than 70°C, and if the elongation rate is less than 120%, the processability will be poor. It is necessary that the glass transition point is 70°C or higher and the elongation rate is 120% or higher. Although the explanation so far has been mainly directed to cold-rolled steel sheets, the coating composition of the present invention thus obtained can be applied not only to cold-rolled steel sheets but also to various types of galvanized steel sheets. . A pre-coated steel plate coated with such a coating composition exhibits excellent effects in adhesion, corrosion resistance, weldability, formability, etc. Hereinafter, the present invention will be explained in more detail with reference to Examples and Comparative Examples. Example 1 A coating composition of the present invention having the following formulation was prepared by a conventional coating manufacturing method. Next, this pre-coat paint composition was coated (pre-coated) on one side of a cold-rolled steel plate so that a dry coating film had a thickness of 15 μm, and baked at 150° C. for 2 minutes to produce a pre-coated steel plate. This prepainted steel plate was tested for weldability and workability. In addition, the pre-coated surface and bare surface (non-pre-coated surface) are treated with zinc phosphate, and
Dry film of commercially available epoxy cationic electrodeposition paint
Appearance, adhesion, and corrosion resistance were tested on steel plates that were electrodeposited to a thickness of 15μ and baked at 175°C for 20 minutes. The results of each test are shown in Table 1. Coating composition of the present invention (component A) Polyether polyol (Note 1) 40 parts by weight Epoxy resin (Ciel Chemical Co., Ltd. trade name Epicoat #1004) (Note 2) 20 〃 Water absorbing agent 5 〃 Toluene 20 〃 Xylene 15 〃 100 parts by weight (Component B) Diphenylmethane diisocyanate prepolymer (Note 3) 30 parts by weight (metallic zinc powder) 570 parts by weight of zinc powder with an average particle size of 1 to 5 μm Immediately before painting, remove component (A), component (B), and metal. A coating composition of the present invention was prepared by uniformly mixing and dispersing zinc powder. The mixing ratio of component (A) and component (B) is 1/1.33 in terms of NCO/OH equivalent ratio. Furthermore, the Tg of the pre-coated film formed was 83°C and the elongation rate was 140%. (Note 1) Polyether polyol is obtained by reacting trimethylolpropane with propylene oxide, has a hydroxyl content of 7.5%, a hydroxy equivalent of 228, an acid value of 0.3, and a viscosity of 1200 cps (20
℃) was used. (Note 2) Epoxy resin has a hydroxyl content
A 4.3% epichlorohydrin-bisphenol type A epoxy resin was used. (Note 3) Diphenylmethane diisocyanate prepolymer is a combination of modified MDI obtained by reacting 5 moles of 4,4'-diisocyanate diphenylmethane with 1 mole of tripropylene glycol, and 4,
A polyisocyanate mixed with 4'-diisocyanate diphenylmethane having an isocyanate content of 24%, an isocyanate equivalent weight of 173, and a viscosity of 150 cps (20°C) was used. Comparative Example 1 A test was carried out in the same manner as in Example 1 except that a pre-coat coating composition containing no epoxy resin of the coating composition of the present invention in Example 1 was used, and the results were compared to the first
Shown in the table. Comparative Example 2 A test was carried out in the same manner as in Example 1, except that a pre-coat coating composition containing 40 parts by weight of component (B) of the coating composition of the present invention in Example 1 was used, and the results were compared to the first example. Shown in the table. In addition, component (A) and (B) of this pre-coating paint composition
The blending ratio of the components is 1/1 in terms of NCO/OH equivalent ratio, and the elongation rate of the formed coating film is 112%.
Tg was 89℃. Comparative Example 3 A test was carried out in the same manner as in Example 1, except that a pre-coat coating composition containing 20 parts by weight of component (B) of the coating composition of the present invention in Example 1 was used, and the results were compared to the first example. Shown in the table. In addition, component (A) of this pre-coating paint composition and
The mixing ratio of component (B) is 1/2 in NCO/OH equivalent ratio, the elongation rate of the formed coating film is 124%, and the Tg is
It was 65℃.

【table】

[Table] Example 2 A coating composition of the present invention was prepared in the same manner as in Example 1 except that metallic zinc powder was not blended. Next, this pre-coat paint composition was applied to one side of a cold-rolled steel plate so that the dry coating film was 15μ, and the second
A prepainted steel plate was produced by baking under the heating conditions shown in the table. The coating film properties of this pre-coated steel sheet were as shown in Table 2. Furthermore, the bare surface (not pre-coated) of this steel plate was treated with zinc phosphate, and then a commercially available epoxy cationic electrodeposition paint was applied by electrodeposition to a dry film thickness of 15 μm at 175°C. An adhesion test was conducted on the steel plate baked for 20 minutes, and the results are shown in Table 2. Comparative Example 4 A precoated steel sheet was produced in the same manner as in Example 2, except that a one-component epoxy precoat coating composition having the following formulation was used. The coating film properties of this pre-coated steel sheet were as shown in Table 2. Epoxy resin (UCC product name: Phenoxy PKHH)
15 parts by weight Toluene 35 〃 Methyl ether ketone 35 〃 Ethyl cellosolve 15 〃 100 parts by weight Comparative example 5 A pre-coated steel sheet was produced in the same manner as in Example 2, except that a two-component epoxy pre-coat coating composition having the following composition was used. Ivy. The coating film properties of this pre-coated steel sheet were as shown in Table 2. (Main ingredient) Epoxy resin (Ciel Chemical Co., Ltd. trade name Epicote #1001) 50 parts by weight Toluene 20 〃 Methyl ether ketone 20 〃 Ethyl cellosolve 10 〃 100 parts by weight (Curing agent component) Polyamide resin (Henkel Co., Ltd. trade name Versamide #100) 30 parts by weight Isopropyl alcohol 10 Toluene 10 50 parts by weight Comparative Example 6 A precoated steel sheet was produced in the same manner as in Example 2, except that an acrylic precoat coating composition having the following formulation was used. The coating film properties of this pre-coated steel sheet were as shown in Table 2. Acrylic resin (Dainippon Ink Co., Ltd. trade name Acrydik 47-151) 80 parts by weight Toluene 10 〃 Methyl ether ketone 5 〃 Etel Cellosolve 5 〃 100 parts by weight Comparative example 7 An epoxy-melamine precoat coating composition with the following formulation was used. A prepainted steel plate was produced in the same manner as in Example 2 except for the above. The coating film properties of this pre-coated steel sheet were as shown in Table 2. Epoxy resin (Ciel Chemical Co., Ltd. trade name Epicote #1007) 30 parts by weight Melamine resin (Dainippon Ink Co., Ltd. trade name Betsukamine P-138) 10 〃 Toluene 30 〃 Methyl ether ketone 20 〃 Ethyl cellosolve 10 〃 100 parts by weight

【table】

Claims (1)

[Scope of Claims] 1 (A) 100 parts by weight of a linear polyether polyol with a hydroxy content of 6 to 9%, and 30 to 30 parts of an epichlorohydrin-bisphenol A type epoxy resin with a hydroxy content of 2 to 5%.
It consists of a polyol resin composition containing 100 parts by weight and (B) a diphenylmethane diisocyanate prepolymer with an isocyanate content of 20 to 30%, and the components (A) and (B) are combined in an NCO/OH equivalent ratio of 1. /
Formulated with a mixing ratio of 1.2 to 1.5, which forms a cured coating film under heating conditions of 150°C or lower for 5 minutes, and the glass transition point of the coating film is 70°C or higher, and the spread rate is 120% or higher. A coating composition for pre-coated steel sheets.