Curable coating composition and coating film forming method

Abstract

The present invention is to provide a curable coating composition having much less of a volatile organic component, and being excellent in coating film performance such as durability and scratch resistance, and a coating film forming method using the above curable coating composition capable of suppressing generation of sagging and so on. The present invention is a curable coating composition which comprises (a) a component having an active methylene group and / or an active methine group, (b) a component having a methacrylate group and / or an acrylate group, (c) a Michael reaction catalyst and (d) a photo-polymerization initiator. The present invention also provides a coating film forming method which comprises a step (1) of applying a curable coating on a substrate to be coated and a step (2) including a stage of irradiating with ultraviolet ray and a stage of heating, said curable coating comprising the above curable coating composition.

Description

[0001] The present invention relates to a curable coating composition suitable as a top coating for an automotive and a coating film forming method using the same.PRIOR ART[0002] Recently, reduction of a volatile organic component (VOC) content in coatings, namely a super high solid coating, is strongly demanded in light of environmental issues. In this case, it is usually necessary to decrease a viscosity of a resin contained in a coating and for this reason, the method has been adopted in which a resin component in the coating is rendered to have low polarity or low molecular weight. However, when the resin component in the coating is rendered to have low polarity or low molecular weight, there arises a workability problem such that sagging tends to occur when it is coated on a vertical surface, for example.[0003] Moreover, improved durability of a coating film such as acid rain resistance and scratch resistance is further demanded, leading to a requirement for a curable system fo...

AI Technical Summary

Benefits of technology

[0068] Referring to the above resin, an acrylic monomer having no epoxy group is preferred from the viewpoint of storage stability.

[0072] In the curable coating composition of the present invention, in a case where the above component (c) comprises the above component (i), an amount of the above component (ii) as an epoxy group is preferably 1 to 30 equivalents, more preferably 3 to 20 equivalents relative to the onium salt of the above component (i). If it is less than 1 equivalent, the concentration of the epoxy group as a promoter is too low to proceed curing reaction sufficiently, while exceeding 30 equivalents, unreacted epoxy groups remain after curing to thereby deteriorate a durability quality such as chemical resistance and weather resistance.

[0091] The above stage of irradiating with ultraviolet ray and the stage of heating may be carried out simultaneously or sequentially, and there is no specific limitation in the order of the stages, however, in the step (2), it is important to perform both of the above stages therein. For example, in a case where workability such as sagging property is to be improved, or in a case where an appearance of the coating film obtained by wet-on-wet coating is to be improved, it is preferable to perform the two stages simultaneously or the stage of irradiating with ultraviolet ray ahead of the stage of heating. On the other hand, for improving durability of the coating film such as acid resistance and scratch resistance or for improving repair ability such as polishability, it is preferable to perform the stage of heating ahead of the stage of irradiating with ultraviolet ray.

[0092] The curable coating composition of the present invention, which has components described above, can make it possible to reduce the content of a volatile organic component to the lowest level. Moreover, since both of ultraviolet ray irradiation and heating are combinedly used, even if the coating composition contains much of such low molecular weight components, it has a good curability and excellent sagging property.EXAMPLES

Problems solved by technology

However, when the resin component in the coating is rendered to have low polarity or low molecular weight, there arises a workability problem such that sagging tends to occur when it is coated on a vertical surface, for example.

However, the .alpha.,.beta.-unsaturated carbonyl group and the active methylene group are tend to be comparatively affected by steric hindrance; therefore, sufficient crosslinking density is hard to be attained by a Michael reaction only.

Moreover, since the .alpha.,.beta.-unsaturated carbonyl group is highly reactive, the group, if being remained in a coating film, tends to deteriorate durability performance such as weather resistance.

In addition, the super high solid coating has not reached to a satisfactorily sufficient level in terms of workability such as sagging property.

If the number average molecular weight is less than 300, a hardness of the resulting coating film may become low and curability of the coating may be insufficient, thereby solvent resistance, water resistance and weather resistance of the coating film may possibly deteriorate.

On the other hand, if it exceeds 10,000, a viscosity of the component (a) itself becomes high, thereby a content of an organic solvent in the diluted coating upon application may be excessive.

If the above active hydrogen equivalent is less than 40, compatibility with other components described hereinafter may decrease and the resulting coating film may possibly be hard and brittle.

On the other hand, if it exceeds 2,000, the crosslinking density of the resulting coating film becomes small, thereby physical properties and performance of the coating film may possibly deteriorate.

If the above number average molecular weight is less than 200, the composition is evaporated away in heat curing after application, a hardness of the resulting coating film is decreased, or curability of the coating becomes insufficient, whereby solvent resistance, water resistance and weather resistance of the coating film may possibly deteriorate.

On the other hand, if it exceeds 10,000, a viscosity of the component (b) itself becomes high and a content of an organic solvent in the diluted coating upon application may become excessive.

If the double bond equivalent is less than 100, unreacted (meth) acrylate groups remain in the resulting coating film, whereby weather resistance of the coating film may deteriorate and the resulting coating film may possibly be hard and brittle.

If it exceeds 1,500, a crosslinking density of the resulting coating film becomes small, thereby physical properties and performance of the coating film may possibly deteriorate.

If the amount is less than 0.01% by equivalent, a catalyst concentration is excessively low, whereby a curing reaction may not proceed sufficiently, while if in excess of 10% by equivalent, it becomes difficult to uniformly mix the catalyst into the resin composition due to its hydrophilicity, and water resistance, moisture resistance and other performances of a cured coating film may possibly deteriorate.

If it is less than 50% by weight, the amount of a volatile organic component becomes high, thereby satisfactory effect for the recent environmental issues may not be obtained.

If the above film thickness is less than 10 .mu.m, a substrate cannot be covered sufficiently, while exceeding 60 .mu.m, inconveniences such as foaming or sagging may possibly occur upon application.

The above preheating step is not to cure the uncured base coating film, but to evaporate away moisture contained.