Photocurable primer composition and coating method by use of the same

1. A photocurable primer composition containing (A) an acrylic resin having through an urethane linkage a polymerizable unsaturated group on its side chain and containing the polymerizable unsaturated group in an amount of 0.2 to 2.0 mol/kg in a solid content of the resin (A), (B) an urethane (meth)acrylate oligomer containing at least one polymerizable unsaturated group in one molecule and prepared by reacting a compound (e) containing at least two isocyanate groups in one molecule, a hydroxyl group-containing polymerizable unsaturated monomer (f) and a monohydric alcohol (g), (D) an extender pigment in an amount of 1 to 300 parts by weight per 100 parts by weight of a total resin solid content of the components (A), (B) and (C) in the primer composition, and (E) a photopolymerization initiator.

2. A photocurable primer composition as claimed in claim 1 , wherein the resin (A) is prepared by reacting a hydroxyl group-containing acrylic resin (a) and a compound (b) containing isocyanate group and polymerizable unsaturated group in one molecule.

3. A photocurable primer composition as claimed in claim 1 , wherein the resin (A) is prepared by reacting an isocyanate group-containing acrylic resin (c) and a compound (d) having hydrolxyl group and polymerizable unsaturated group in one molecule.

4. A photocurable primer composition as claimed in claim 1 , wherein said composition further contains a polymerizable unsaturated compound (C) other than the resin (A) and the urethane (meth)acrylate oligomer (B).

5. A photocurable primer composition as claimed in claim 4 , wherein a mixing amount of the components (A), (B) and (C) is such that the component (A) is in the range of 5 to 95% by weight, and the component (C) is in the range of 0 to 60% by weight based on a total solid content of the components (A), (B) and (C) respectively.

6. A photocurable primer composition as claimed in claim 1 , wherein the extender pigment (D) is used in combination with a resin particle.

7. A photocurable primer composition as claimed in claim 1 , wherein said composition has a thixotropy index value in the range of 5 or more.

8. A photocurable primer composition as claimed in claim 1 , wherein the compound (e) is a cyclic trimer of hexamethylene diisocyanate.

9. A photocurable primer composition as claimed in claim 1 , wherein said composition further contains a phosphate group-containing compound in an amount of 0.01 to 1.0 mmol/g as the phosphate group based on a total resin solid content of the components (A), (B) and (C) in the composition.

10. A coating method which comprises coating a photocurable putty composition onto a substrate to form an uncured coating film layer, irradiating light onto the uncured coating film layer to form a cured coating film layer, coating thereonto the photocurable primer composition as claimed in claim 1 to form an uncured coating film layer, and irradiating light onto the uncured coating film layer to form a cured coating film layer.

11. A coating method which comprises coating a photocurable putty composition onto a substrate to form an uncured coating film layer, coating thereonto the photocurable primer composition as claimed in claim 1 to form an uncured coating film layer, and irradiating light thereonto to cure both uncured coating film layers simultaneously.

12. A coating method as claimed in claim 11 , which method is followed by coating a colored base coating composition and a clear composition onto the photocurable primer coating film layer, and heating or irradiation light to cure.

13. A coating method as claimed in claim 12 , wherein both the colored base coating composition and the clear composition are a one-pack composition.

14. A coating method as claimed in claim 10 , which method is followed by coating a colored base coating composition and a clear composition onto the photocurable primer coating film layer, and heating or irradiating light to cure.

15. A coating method as claimed in claim 14 , wherein both the colored base coating composition and the clear composition are a one-pack composition.

16. A coating method as claimed in claim 15 , wherein the clear composition is a one-pack clear composition containing (h) a resin containing at least one functional group selected from siloxy group and isocyanate group, (i) a compound containing at least one group selected from siloxy group and isocyanate group, and reactive with a functional group contained in the resin (h), and (j) a siloxy group-dissociating catalyst.

17. A coating method as claimed in claim 15 , wherein the clear coating composition is a one-pack clear composition containing (h) a resin containing at least one functional group selected from siloxy group and isocyanate group, (i) a compound containing at least one group selected from siloxy group and isocyanage group, and reactive with a functional group contained in the resin (h), (j) a siloxy group-dissociating catalyst, (k) a polymerizable unsaturated group-containing compound, and (l) a photopolymerization initiator.

18. A coating method as claimed in claim 15 , wherein the clear composition is a one-pack coating composition containing (m) a resin having a polymerizable unsaturated group and containing at least one functional group selected from siloxy group and isocyanate group, (n) a compound containing at least one function group selected from siloxy group and isocyanate group and reactive with a functional group reactive with a functional group contained in the resin (m), (j) a siloxy group-dissociating catalyst, and (l) a photopolymerization initiator.