Varnish having a microstructure
The present invention relates to a waterborne paint composition, comprising a non-film-forming poly(styrene acrylate) having a glass transition temperature (T G ) of at least 80° C., a poly(styrene acrylate) resin having a weight average molecular weight (M W ) of up to 20 000 g/mol, and water. Such a paint composition is suitable as an overprint varnish or primer for substrates such as paper, paperboard and sheets. Due to its surface structure having micro-cracks the applied paint film can advantageously be printed by inkjet printing.
1. A waterborne paint composition, comprising:
45 to 65 wt.-% of a non-film-forming poly(styrene acrylate) having a glass transition temperature (T G ) of at least 100° C. in the form of an aqueous dispersion,
8 to 12 wt.-% of a polystyrene acrylate) resin having a weight average molecular weight (M W ) of up to 5 000 g/mol in the form of a neutralized solid,
10-14 wt.-% of a hydrophobic dispersing agent,
5 to 7 wt.-% of an inorganic cross-linking agent in solution,
4 to 7% wt-% of an emulsion of polyethylene wax having an amount of 32 wt.-% solids content,
1 to 3 wt.-% of glycerol,
0.1 to 2 wt.-% of surfactant or wetting agent,
0.1 to 1 wt.-% of a silicone defoamer,
ammonia, and
water,
wherein the percentages add to a total of 100 wt.-%.
2. The waterborne paint composition of claim 1 ,
wherein the ratio of non-film-forming poly(styrene acrylate) to poly(styrene acrylate) resin, each based on the weight of solids, is in the range of from 4:1 to 1.2:1, and
wherein when said paint composition is applied to a substrate, a microstructured paint film is formed on said substrate.
3. The waterborne paint composition according to claim 2 , wherein the microstructured paint film has numerous cracks having a width of at most 2 μm.
4. The waterborne paint composition according to claim 1 , wherein the ratio of the non-film-forming poly(styrene acrylate) to the poly(styrene acrylate) resin, each based on the weight of solids, is in the range of from 3:1 to 1.5:1.
5. The waterborne paint composition according to claim 1 , wherein the ratio of the non-film-forming poly(styrene acrylate) to the poly(styrene acrylate) resin, each based on the weight of solids, is about 2:1.
6. The waterborne paint composition according to claim 1 , wherein the poly(styrene acrylate) resin has a weight average molecular weight (M W ) of up to 2 000 g/mol.
7. The waterborne paint composition according to claim 1 , wherein the poly(styrene acrylate) resin has a glass transition temperature (T G ) of at least 55° C.
8. The waterborne paint composition according to claim 1 , wherein the poly(styrene acrylate) resin has an acid number of from 200 to 300.
9. The waterborne paint composition according to claim 1 , wherein the hydrophobic dispersing agent comprises a salt of a polycarboxylic acid.
10. The waterborne paint composition according to claim 9 , wherein the hydrophobic dispersing agent comprises an ammonium salt of a polycarboxylic acid.
11. The waterborne paint composition according to claim 1 , wherein the inorganic cross-linking agent comprises ammonium zinc carbonate.
12. The waterborne paint composition according to claim 1 , having a minimum film forming temperature of at least 50° C.
13. A method of preparing a microstructured paint film on a substrate, comprising applying the waterborne paint composition according to claim 1 to said substrate.
14. The method of claim 13 , wherein the microstructured paint film has numerous cracks having a width of at most 2 μm.
15. The method of claim 13 wherein said waterborne composition is applied as a primer and/or overprint varnish to a substrate selected from the group consisting of paper, cardboard, paperboard, and sheet.
16. The method according to claim 13 , further comprising printing with a solvent containing gravure printing ink on the microstructured paint film, thereby obtaining a coated substrate.
17. The method according to claim 16 , wherein the coated substrate has a gloss of from 40 gloss points to 60 gloss points.