JPH0475068B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a method for forming a coating film of a powder coating material for pre-coating by baking at high temperature for a short time. [Prior art] In recent years, the practical application of pre-coating powder coatings has become popular, but since the baking process to form the coating film is carried out at 230°C for about 5 minutes, This is 20 to 80%, similar to conventional solvent-based paints.
It is desired to form a coating film by baking at a high temperature and for a short time on the order of seconds. This is done in as little space as possible in order to deal with the space constraints of equipment such as printing equipment, and to reduce equipment costs and running costs.
And keep energy costs as low as possible,
This is due to the intention of mass producing the product. By the way, as a powder coating for pre-coating,
Generally, epoxy resin powder coatings, polyester resin powder coatings, acrylic resin powder coatings, etc. are used, but these can be heated to temperatures of 100 to 130℃.
It is a type that melts at a temperature of 180 to 270℃ and crosslinks and hardens at a temperature of 180 to 270℃.As mentioned above, if you want to mass-produce products in as little space as possible and with as little energy cost as possible, this melting-crosslinking method is must be completed within a short period of time as mentioned above. Therefore, we initially considered raising the temperature to 250-260℃ in about 20 seconds with the idea of raising the temperature as quickly as possible to a crosslinking temperature suitable for high-temperature, short-time baking. When heated to a temperature of In cases where there are chips, the appearance is poor, and high workability and corrosion resistance of the machined parts are required in terms of performance, it is impossible to meet these demands, and the conventional baking method is used to improve both appearance and performance. It was inferior to other coatings. Then, as above, heat the temperature to 250-260℃ for about 20 seconds.
Even if the heating was continued after the temperature was raised to 100%, the above-mentioned drawbacks could not be solved. [Problems to be Solved by the Invention] As mentioned above, the present invention solves the problem of poor leveling property by high-temperature, short-time baking within 80 seconds, which meets the purpose of mass-producing products in as little space as possible and with as little energy cost as possible. The purpose of the present invention is to form a pre-coat powder coating film that prevents the occurrence of oxidation, has a good appearance, and has excellent performance such as high workability and corrosion resistance in processed parts. [Means for Solving the Problems] The present inventors have conducted various studies on coating film formation by high-temperature, short-time baking of powder coatings for pre-coat from the viewpoint of saving space and energy.
In order to obtain a coating film with good appearance and sufficient performance, it is not preferable to raise the temperature rapidly as mentioned above, even though it is baked for a short time. Increase temperature for up to 20 seconds
It has been found that it is appropriate to set the temperature to 100 to 180°C and to raise the temperature for up to 40 seconds to a temperature exceeding 180°C and not more than 270°C. In other words, when the temperature is rapidly raised to the crosslinking temperature as described above, the crosslinking reaction proceeds rapidly without sufficient intermolecular contact, resulting in the formation of many low molecular weight parts and a wide molecular weight distribution. However, as in the present invention, the powder coating is mainly melted for the first 20 seconds, and the initial stage at high temperature is After suppressing the reaction and increasing the initial melting period to increase fluidity and ensure sufficient contact between molecules, the temperature is increased over the next 40 seconds to reach the crosslinking temperature of the powder coating. At temperatures above 180℃
When the powder coating is crosslinked and polymerized by raising the temperature to below 270℃, the resulting coating film has a uniform molecular weight distribution, which improves the smoothness of the coating surface and improves impact resistance. Good results are achieved in factors that have an important influence on the post-processing of coating films, such as bendability and tensile strength, and coating films with excellent appearance and performance can be obtained. Powder coatings that can be used in the present invention include:
Examples include polyester resin powder paints, epoxy resin powder paints, acrylic resin disguise paints, and the like. Examples of polyester resin powder coatings include those of the blocked isocyanate curing type, those of the N-methylol etherified melamine resin curing type, and those using a glycidyl compound as a curing agent. Examples of epoxy resin powder coatings include those using BF ₃ complex as a curing agent, and those using acid anhydride adducts, dicyandiamide, modified dicyandiamide, etc. as a curing agent. Examples of acrylic resin powder coatings include those using melamine resin, blocked isocyanate, epoxy resin, and polycarboxylic acid as a curing agent, and self-crosslinking types. As a baking device, a high-velocity hot air furnace, an ultra-far infrared furnace, a high-frequency induction heating furnace, a near-infrared furnace, etc. are used singly or in combination of two or three types, and these are usually converted into a conveyor furnace to be suitable for continuous operation. ing. Electricity, city gas, natural gas, liquefied petroleum gas, etc. are used as the heat source. [Example] Next, the present invention will be explained in more detail by giving examples. Examples 1 to 6 and Comparative Examples 1 to 4 First, the composition, material, and equipment of the powder coating to be tested will be explained. Note that the number of parts is based on parts by weight. Composition of powder coating Polyester resin (OH value 40mg/g) 80 parts Blocked isocyanate curing agent (Isocyanate content 15%) 17 parts Epoxy resin (epoxy equivalent 950) 3 parts Benzoin (foaming inhibitor) 0.5 part Titanium white (R-930) (manufactured by Ishihara Sangyo Co., Ltd.) 43 parts Acronal 4F (manufactured by Bayer) (leveling agent)
0.5 parts dibutyltin dilaurate (hardening accelerator) 0.2 parts Material: Alloyed hot-dip galvanized steel sheet (silver alloy) manufactured by Nippon Steel Corporation, thickness 0.45 mm Pretreatment: Zinc phosphate coating coating machine: Gemate Blowing electrostatic gun baking furnace: Electric hot air circulation type conveyor furnace The powder coating of the above composition was applied onto zinc phosphate treated silver alloy using a Gema hand blowing gun, and then the powder coating was applied using an electric hot air circulation type conveyor furnace. A coated plate was obtained by baking under the baking conditions shown in the table. The average coating film thickness was 35 μm.

[Table] Further, using a fixed furnace preset at 250°C, the coated plate immediately after being taken out from the conveyor furnace was further kept warm in the fixed furnace to produce a coated plate.
The baking temperature conditions at that time are shown in Table 2. In addition, the plate surface temperature in Examples 5 and 6 when passing through the conveyor furnace for 20 seconds was 156° C. as in Example 1.

[Table] Table 3 shows the results of the coating film performance test of the coated plate prepared as described above. The test method and evaluation criteria are as follows. Glossiness: Using a gloss meter manufactured by Murakami Color Research Institute,
60°/60° specular gloss was measured. Smoothness of coated surface: Visual comparison was made with a coat formed by baking the same powder coating on a silver alloy using a conventional method at 230°C for 5 minutes. The evaluation criteria are as follows. ◎: Appearance equivalent to or better than the coating film made by the conventional method (Excellent) ○: Appearance almost the same as the coating film made by the conventional method (Good) □: Slightly inferior to the coating film made by the conventional method, but no problem in practical use (Acceptable) △: Very inferior to the coating film made by the conventional method (not possible) 20℃ bending workability: Bend the approximately center part of a 50 mm x 150 mm test piece by 180 degrees (length of the bent part 50 mm), and after bending, the bent part Tighten two test pieces of the same type in a vise with scissors (2T), tighten one test piece with a scissor vise (1T), 0
The sheets were tightened in a vise (0T) without pinching anything, and the degree of crack occurrence at each bend was visually observed. The evaluation criteria are as follows. ◎: No abnormalities (excellent) ○: 1 to 2 cracks, no other abnormalities (good) □: Several small cracks (acceptable) ◇: Significant cracks (unacceptable) Magick stain resistance: Magick ink At 20℃ after application
After leaving it for 24 hours, it was wiped off with a 50/50 mixed solvent of petroleum benzene/ethyl alcohol, and the degree of contamination was visually observed. The evaluation criteria are as follows. ◎: No contamination (excellent) ○: Almost no contamination (good) □: Slight contamination (acceptable) ◇: Considerable contamination (unacceptable) Corrosion resistance of processed parts: Flange bent part (bending length
The degree of blistering was observed after 500 hours of salt water spray test (300 mm). The evaluation criteria are as follows. ◎: No eyelash bulges ○: 1 point of blister occurrence □: 2 to 3 points of blister occurrence ◇: 4 to 5 points of blister occurrence △: 6 or more points of blister occurrence In addition, the same powder paint was applied to silver alloy with 230
Table 3 also shows the glossiness of the coating film formed by baking at 50°C for 5 minutes, the smoothness of the coated surface, the bending workability at 20°C, the stain resistance with magic, and the corrosion resistance of the processed part as a control example.

〔Effect of the invention〕

According to the present invention, the pre-coating material can be baked at a high temperature for a short period of time within 80 seconds to save space and energy, has a good appearance, and has excellent post-processability and corrosion resistance of the processed parts. A coat of powder paint was formed.

Claims (1)

[Claims] 1. When forming a pre-coat powder coating film on the object to be coated by baking it under high temperature and short time conditions of 80 seconds or less, the heating condition of the object to be coated shall not be increased for more than 20 seconds. A method for forming a coating film for a powder coating for pre-coating, characterized in that the temperature is increased from 100 to 180°C, and the temperature is increased over 180°C to 270°C or less for up to 40 seconds.