JPS6157076B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a highly formable precoated steel sheet using an electron beam curing method for curing a coating film. More specifically, the present invention relates to a method for producing a pre-coated steel sheet having excellent coating film surface properties and high workability by appropriately adjusting the accelerating voltage in the case of electron beam irradiation depending on the thickness, specific gravity, etc. of the coating film. It is something.

Conventionally, in order to manufacture highly formable pre-coated steel sheets, materials such as steel sheet pre-treatment agents, undercoat paints, top coat paints, etc., as well as baking conditions for heat-curing methods, have been considered, and the level of processing required is quite high. but,
It is difficult to obtain a product that has high processability and has good coating film hardness and stain resistance.

In other words, a coating film with high workability, for example elongation, lacks hardness, or a coating film that is too hard to be easily damaged by chemicals is generally brittle and cannot withstand processing. be.

The present invention was developed in order to improve these problems, and as a result of studies, the top layer of the coating film is sufficiently cured and has excellent hardness, stain resistance, and chemical resistance, and the bottom layer is flexible without being sufficiently cured. This is based on the experimental finding that it is possible to obtain a single-layer coating film that retains the In other words, when curing a paint film with an electron beam, the voltage for electron beam irradiation is set taking into consideration the film thickness and specific gravity, and as a result, the electron beam absorbed dose of the top layer and the bottom layer of the paint film is different. This method is characterized by irradiating an electron beam with an accelerating voltage that lowers the latter.

Conventionally, the lower layer of the coating is coated with a flexible resin coating system, and after curing, a hard coating is coated on the upper layer and cured. That is, the concept of the two-coat, two-cure method was well known. However, this method is disadvantageous in terms of cost due to the large number of steps, and there are restrictions on each paint system due to the compatibility of the top coat and undercoat paint systems.
Furthermore, since the two coatings are different, they tend to cause delamination, or even if they are good immediately after coating, they tend to peel off over time.

However, according to the present invention, (1) it is one coat, and (2) it is the same resin-based coating film that has a strong structure near the top layer and a flexible structure near the bottom layer, and (3) ) Can be manufactured with 1 cuyua. It has excellent features such as

The penetrating power of an electron beam through a substance varies depending on its accelerating voltage, and when the voltage is low, the penetrating power becomes small. Traditionally, the development of electron beam accelerators, especially for those used in industrial applications, has started with high voltages, such as those in the megavolt class, and has increased to 700KV.
500KV class, 300KV class, and gradually changed to low voltage, high current type. In the past two or three years, even lower voltage products such as 200KV and 150KV class products have become commercially available. This trend is due to the increasing use of electron beam irradiation for thin materials such as curing of paint films, coatings of paper and films, lamination of paper and metal foil, and the like. As is well known, the advantage of low-voltage accelerators is that thin films have good energy absorption efficiency and a simple shielding structure for protecting against secondary X-rays. The advantages of the former low-voltage accelerator are beginning to be recognized by accelerator manufacturers, and high-voltage equipment, such as 300KV~
Even in the 1MV class, accelerators that can freely change the voltage have been developed, making it easier to obtain the desired voltage setting.

Next, the present invention will be explained in detail. The "dose after passing through the paint film" shown in the drawings is an example of experimental data obtained by the inventors, but if we take an acceleration voltage of 170 KV as an example, the dry film thickness is 60μ (hardened film). If the top layer with a specific gravity of 1.65) is set to 1.0, the bottom layer is at point A, which indicates that about 0.4 of the electron beam was absorbed. If such a graph is created using coating film specific gravity, film thickness, and accelerating voltage as parameters, the electron beam absorption dose ratio of the uppermost layer and the lowermost layer of the coating film can be seen at a glance. Expressing this in a general formula, for example, in order to set the voltage (E) so that the absorbed dose is 80% in the bottom layer compared to the top layer, E = 1/0.97 (ρd-110) ρ: Coating film density (g/cm ³ ) d: Coating film thickness (μ) In the case of 70%, it is roughly expressed by the formula E=1/1.13(ρd−124). The reason for using these two equations as an example is that, although it depends on the type of coating film, when the absorbed dose ratio between the bottom layer and the top layer of the coating film is about 0.7 to 0.8 or less,
This is because it satisfies the requirements of the present invention in many cases.

The lower limit of the absorbed dose ratio between the bottom layer and the top layer of the coating film is about 0.05. If it is less than this, even if the coating surface is sufficiently cured, the bottom layer is almost unreacted and the underlying metal is This is not desirable as it may cause problems with adhesion or corrosion resistance (particularly if scratches occur) or other performance issues. In the method of the present invention, in order to set the appropriate voltage, it is most convenient to control the accelerating voltage with the accelerator itself, but in some cases, a metal A foil or film or the like may be inserted, resulting in a lower voltage to the irradiated object.
The latter method is effective, for example, when the minimum accelerating voltage of an electron beam accelerator can only be lowered to 200 KV, or when an even lower voltage is required. The metal foil used here is titanium, aluminum, iron, copper, tin, zinc, or the like, and the film is acrylic, vinyl chloride, polypropylene, polyethylene, or the like. One or more of these foils or films may be used to obtain a suitable thickness. Alternatively, the absorbed dose can be adjusted by appropriately changing the irradiation distance.

In the present invention, any commercially available industrial accelerator can be used as the electron beam accelerator used, and the voltage is 150 KV to 1 MV, preferably
About 150KV to 500KV is sufficient, but of course,
It depends on the coating thickness, specific gravity, and irradiation distance.

Incidentally, the above-mentioned formula holds true when the irradiation distance is approximately 3 to 20 cm.

Next, the electron beam curing type paint used in the present invention will be described. In principle, it is sufficient to contain one or more components having a C=C bond that is active in electron beams, but in the case of the present invention, when irradiated with electron beams,
It is better for the curability to be sensitive to the absorbed dose.

That is, this is because it is possible to apply a large dose to the top layer of the coating film to obtain a sufficiently cured film, and a small dose to the bottom layer to obtain a film with a flexible structure. Examples of such resin systems include those with a high density of C=C bonds in their molecules, or those in which methacrylic acid is used when introducing C=C bonds into molecules, such as:
ethylene glycol, polyethylene glycol,
These include (meth)acrylates such as polyhydric alcohols, polyester (meth)acrylate oligomers, epoxy (meth)acrylates, and diene oligomers. A coating system containing at least one of these components is desirable.

As the original plate to be coated in the present invention, hot-dip galvanized steel sheets, electrolytic galvanized steel sheets, iron sheets, steel sheets subjected to chemical conversion treatment, tinplate, stain-free steel, stainless steel, aluminum sheets, copper sheets, and other metal sheets are used. It is preferable to use a primer on these original plates in order to impart rust prevention properties, adhesion properties, etc. The primer may be of a thermosetting type, an ultraviolet curing type, or an electron beam curing type, but when applying an electron beam curing type primer and curing two coats, an electron beam is irradiated through the top coat. Because it is considerably attenuated, it must be a low-dose electron beam curable resin system. The primer film thickness may be about 1 to 20 μm.

In the present invention, the electron beam irradiation atmosphere is
It may be used in air, but if the physical properties of the coating surface are particularly important, an inert gas atmosphere is preferable, preferably an oxygen concentration of 1% or less, and more preferably 0.5%.
It is better to be below.

The thickness of the coating film of the present invention is preferably about 20 to 30 μm or more, although it depends on the irradiation distance.

Example 1 An acrylic copolymer with a number average molecular weight of about 80,000, consisting of ethyl acrylate, methyl methacrylate, 2-hydroxylethyl acrylate, and acrylic acid in a monomer ratio (mol%) of 70:24:3:3. 100 parts of polymer, trimethylolpropane trimethacrylate
Add 20 parts of titanium white 120 parts to the resin system dissolved in 120 parts of xylene-butyl acetate (equal weight mixture) solution.
Added parts and made it into paint.

Electrogalvanized steel sheet (0.6mm
(thickness) and the paint obtained earlier (dry coating specific gravity approximately 1.65).
was applied with a curtain flow coater to a dry film thickness of 60μ, and after setting it was passed through a solvent scattering furnace at 120°C for about 1 minute to scatter the solvent, and then the coated plate was placed in a nitrogen stream (oxygen concentration 0.1%). So, the accelerating voltage
The electron beam irradiation conditions were set to 150 KV and 25 mA current, and the coating was passed through the coating plate at a conveyor speed of 8 m/min to cure the coating film. The electron beam absorption dose ratio between the top layer and the bottom layer of the coating is 1/0.4.

The coating performance is as follows. (Good◎＞〇＞
△＞×Poor) Impact OT bending ◎ (No crack) Pencil hardness (surface) 2H Chemical resistance (24 hours) 5% sodium hydroxide ◎ 5% hydrochloric acid ◎ Xylene ◎ Stain resistance Lipstick ◎ Magic ink 〇 Comparative example 1 Implementation A coated plate was obtained under exactly the same conditions as in Example 1 except for the acceleration voltage. In this example, the accelerating voltage is 300KV.

The coating performance is as follows.

Impact OT bending △ (crack) Other performances are the same as in Example 1.

As can be seen from the drawings, the bottom layer of the coating film in Example 1 absorbed only about 40% of the dose in this example and had a flexible structure. In the case of , the workability is considered to be good.

Example 2 In Example 1, the acceleration voltage was set to 300 KV, and the conveyor speed at which the impact OT bending was ◎ was searched.
As a result, it was found that the conveyor speed should be approximately 20 m/min or higher. This means that the dose to the bottom layer of the coating is approximately the same as when the conveyor speed is 8 m/min at a voltage of 150 KV. However, other coating properties are as follows.

Pencil hardness (surface) F~H Chemical resistance (24 hours) 5% caustic soda 〇 5% hydrochloric acid ◎ Xylene 〇〜◎ Stain resistance Lipstick 〇～◎ Magic Ink ×〜△

[Brief explanation of the drawing]

The drawings show changes in the optical density of blue cellophane by peeling cured coatings of different thicknesses from the material, placing them on blue cellophane for dose measurement, and irradiating them with electron beams under various conditions. It is.

Claims (1)

[Claims] 1. When curing the paint film with an electron beam,
The voltage for electron beam irradiation is set in consideration of the coating thickness and specific gravity, and the accelerating voltage is set such that the electron beam absorption dose of the top layer and the bottom layer of a single layer of top coat is different, and the latter is lower. A method for producing a highly formable pre-coated steel sheet, the method comprising irradiating with an electron beam.