Removable, biodegradable coating

1. An aqueous coating composition for providing a removable functional coating, which coating is removable with a removing agent comprising a strong base wherein the strong base is an alkali metal hydroxide, the coating composition comprises a pigment and a polymeric binder, wherein the binder has a weight average molecular weight of from 2000 to 50000 g/mole, and an acid value of 40 to 250 mg KOH/g polymeric binder, wherein the binder is a polyester comprising a side group introduced by a Diels-Alder and/or pericyclic Ene-reaction, wherein the side group contains an ionic group.

2. The coating composition according to claim 1 , wherein the binder is obtained by at least the following steps:

(a) preparing an unsaturated polyester,

(b) effecting a Diels-Alder reaction and/or pericyclic Ene-reaction between the unsaturated polyester and an unsaturated compound containing an ionic group and/or ion-forming group to obtain a polymer with side groups containing ionic groups and/or ion-forming groups, and

(c) optionally converting at least part of the ion-forming groups present in the polymer to ionic groups.

3. The coating composition according to claim 2 , wherein the unsaturated compound is

a) a conjugated diene with carboxylic acid ionic groups and/or carboxylic acid ion forming groups; or

b) an unsaturated compound(s) with ion forming groups to obtain a polymer with side groups containing ion-forming groups; or

c) a conjugated diene with carboxylate forming groups to obtain a polymer with side groups containing carboxylic acid ion-forming groups.

4. The coating composition according to claim 2 , wherein the unsaturated polyester is a linear or substantially linear unsaturated polyester.

5. The coating composition according to claim 1 , wherein the binder has a weight average molecular weight of at least 5000 g/mole, and/or

wherein the binder has a weight average molecular weight of at most 40000 g/mole.

6. The coating composition according to claim 1 , wherein the acid value of the binder is from 60 to 160 mg KOH/g polymeric binder.

7. The coating composition according to claim 1 , wherein the glass transition temperature of the binder is from 10 to 80° C.

8. The coating composition according to claim 1 , wherein the polymeric binder is water-dispersible or water-soluble.

9. The coating composition according to claim 1 , wherein the side group of the polymeric binder is represented by any one of structures

or any isomer thereof, wherein:

R 1 is selected from the group of anionic groups, which group consists of carboxylate (—COO − ), sulphate (—OSO 3 31 ), phosphate (—OPO 3 31 ), sulphonate (—SO 2 O 31 ), phosphinate (—POO − ), phosphonate (—PO 2 O − ) and substituted alkyl, alkenyl, aryl and alkylaryl groups of the formula —R 12 -A − ;

wherein R 12 is linear or branched alkyl, alkenyl, aryl or alkylaryl moiety, which moiety has 1-12 carbon atoms;

wherein A − is an anionic group selected from carboxylate, sulphate, phosphate, sulphonate, phosphinate and phosphonate;

wherein R 2 -R 11 may each be individually chosen from either the group of anionic groups defined for R 1 or from the group consisting of hydrogen, alkyl, alkenyl, aryl and alkylaryl;

wherein * represents a carbon atom of the polyester to which the side group is attached, and;

wherein for structure (2), at least one of R 5 , R 6 , R 7 and R 8 must be an anionic group;

wherein for structure (3), at least one of R 5 , R 6 , R 8 R 9 and R 10 must be an anionic group;

wherein for structure (4), at least one of R 9 R 10 and R 11 must be an anionic group.

10. The coating composition according to claim 9 , wherein A − is a carboxylate and/or wherein R 2 -R 11 are each be individually chosen from either the group of anionic groups defined for R 1 or from the group consisting of H, alkyl and alkenyl.

11. The coating composition according to claim 1 , wherein the polymeric binder comprises at least one of the following structural units (5) and (6):

wherein R 1 is COO − , R 2 and R 3 are H; and R 4 is methyl; and

wherein R 11 is ethyl; R 9 is COO − ; and R 10 is H; and/or any isomer thereof.

12. The coating composition according to claim 1 , wherein the amount of binder is from 5 to 45 wt. % and the amount of pigment is from 55 to 95 wt. %, based on the dry weight of the aqueous coating composition.

13. The coating composition according to claim 1 , wherein the pigment is selected from the group consisting of calcium carbonate, titanium dioxide, a silicate, gypsum, baryte, and combinations thereof.

14. The coating composition according to claim 1 , wherein the coating composition further comprises

a) an adhesion promotor selected from the group consisting of silanes; and/or

b) a pigment divider; and/or

c) a thickener.

15. The coating composition according to claim 1 having a pH in the range of 8.0-12.0.

16. A method for forming a functional coating on an outside wall and/or roof of a greenhouse, wherein the method comprises (1) applying the coating composition according to claims 1 and (2) drying the coating composition.

17. The coating composition according to claim 1 , wherein the coating composition comprises a crosslinker in an amount of less than 1 wt. % based on the total weight of the coating composition.

18. The coating composition according to claim 1 , wherein the coating composition comprises no crosslinker.

19. The coating composition according to claim 1 , wherein the ionic group is selected from the group consisting of carboxylate (—COO − ), sulphate (—OSO 3 − ), phosphate (—OPO 3 − ), sulphonate (—SO 2 O − ), phosphinate (—POO − ) and phosphonate (—PO 2 O − ).

20. A functional coating removable with a removing agent comprising a strong base wherein the strong base is an alkali metal hydroxide, wherein the functional coating comprises a pigment and a polymeric binder, wherein the binder has a weight average molecular weight of from 2000 to 50000 g/mole, and an acid value of 40 to 250 mg KOH/g polymeric binder, wherein the binder is a polyester comprising a side group introduced by a Diels-Alder and/or pericyclic Ene-reaction, wherein the side group contains an ionic group.

21. A surface provided with a functional coating according to claim 20 .

22. The surface according to claim 21 , wherein the surface is an outside wall and/or roof of a greenhouse.

23. A method for removing a functional coating on an outside wall and/or roof of a greenhouse which coating is a coating according to claim 20 , wherein the method comprises treating the functional coating with a removing agent which comprises water, a strong base and optionally a complex former, the strong base being present in an amount of from 1 to 10 wt. %, and the complex former, if present, is present in an amount of from 2 to 10 wt. %, based on the weight of the removing agent, and wherein the strong base is an alkali metal hydroxide.