COATING STRUCTURE, SHEET-LIKE PRODUCT AND ITS USE

Abstract

A coating structure for a sheet-like substrate is disclosed including cellulosic fibres and a sheet-like product. The structure includes at least one pre-coat layer and a top coat layer, where the pre-coat layer(s) and the top coat layer include 10-90 weight-% of a styrene (meth)acrylate copolymer which is polymerised in the presence of a stabiliser, and which has a glass transition temperature Tg20 C., preferably 15 C., more preferably 13 C. Furthermore, the pre-coat layer(s) includes a polysaccharide, such as starch, and the top coat layer includes 5-60 weight-% of at least one water-soluble coating binder, which is selected from polysaccharides and polyvinyl alcohols, as well as at most 5.0 weight-% of a cross-linker, which reacts with OH or COOH groups.

Claims

1. A coating structure for a sheet-like substrate comprising cellulosic fibres, the structure comprising at least one pre-coat layer and a top coat layer, where; the pre-coat layer(s) and the top coat layer comprise 10-90 weight-% of a styrene (meth)acrylate copolymer which is polymerised in the presence of a stabiliser, and which has a glass transition temperature Tg20 C., preferably 15 C., more preferably 13 C., the pre-coat layer(s) comprises a polysaccharide, such as starch, and the top coat layer comprises: 5-60 weight-% of at least one water-soluble coating binder, which is selected from polysaccharides and polyvinyl alcohols, and at most 5.0 weight-% of a cross-linker, which reacts with OH or COOH groups.

2. The coating structure according to claim 1, wherein the precoat layer(s) comprise 10-70 weight-%, preferably 15-50 weight-%, more preferably 40-60 weight-%, of styrene (meth)acrylate copolymer, and 30-90 weight-%, preferably 50-85 weight-%, more preferably 40-60 weight-%, of polysaccharide.

3. The coating structure according to claim 1, wherein the top coat layer comprises 30 weight-% of an inorganic mineral pigment.

4. The coating structure according to claim 3, wherein the inorganic mineral pigment is selected from kaolin, talc, calcium carbonate or any mixture thereof, preferably calcium carbonate.

5. The coating structure according to claim 1, wherein the styrene (meth)acrylate copolymer has a glass transition temperature in the range of 40-20 C., preferably 30-15 C., more preferably 20-13 C., even more preferably 0-13 C.

6. The coating structure according to claim 1, wherein the styrene (meth)acrylate copolymer is polymerised in the presence of a stabilizer, which is selected from starch or polyvinyl alcohol, preferably starch.

7. The coating structure according to claim 1, wherein the styrene (meth)acrylate copolymer has a weight average molecular weight <100,000 g/mol, preferably <75,000 g/mol.

8. The coating structure according to claim 1, wherein the styrene (meth)acrylate copolymer is a carboxylated copolymer, preferably obtained by polymerizing a monomer mixture comprising 1-5 weight-% of a carboxylic acid.

9. The coating structure according to claim 1, wherein the top coat layer comprises the cross-linker in an amount of 0.1-5 weight-%, preferably 0.1-4 weight-%, more preferably 0.5-3 weight-%.

10. The coating structure according to claim 1, wherein the cross-linker in the top coat layer is selected from ammonium zirconium carbonate, potassium zirconium carbonate, glyoxal or citric acid.

11. The coating structure according to claim 1, wherein the coating binder in the top coat layer is polyvinyl alcohol, which preferably has a molecular weight of 50,000 g/mol, preferably 13,000-50,000 g/mol.

12. The coating structure according to claim 1, wherein the coating binder in the top coat layer is starch, preferably degraded starch.

13. The coating structure according to claim 1, wherein the polysaccharide on the precoat layer is anionic or non-ionic starch, preferably degraded anionic starch.

14. The coating structure according to claim 1, wherein the styrene (meth)acrylate copolymers in the pre-coat layer(s) and the top coat layer are identical to each other.

15. The coating structure according to claim 1, wherein the structure comprises two or more pre-coat layers and one top-coat layer, which is applied on the outermost pre-coat layer.

16. The coating structure according to claim 1, wherein each pre-coat layer has a coat weight of 0.5-6 g/m.sup.2, preferably 1-5 g/m.sup.2, more preferably 1-4 g/m.sup.2, and the top coat layer has a coat weight of 1-10 g/m.sup.2, preferably 2-8 g/m.sup.2, more preferably 2-7 g/m.sup.2.

17. The coating structure according to claim 1, wherein the amount of styrene acrylate copolymer is higher in the top coat layer than in the pre-coat layer(s).

18. A sheet-like product comprising: a substrate comprising cellulosic fibres, and having a first and a second parallel large surface, and a coating structure according to claim 1 applied on at least one of the large surfaces of the substrate.

19. The product according to claim 18, wherein the substrate has a grammage of 25-800 g/m.sup.2, preferably 30-700 g/m.sup.2, more preferably 40-500 g/m.sup.2.

20. The product according to claim 18, wherein the product has KIT test value of at least 8, and/or mineral oil barrier HVTR value<100 g/m.sup.2/d, and/or water vapour barrier WVTR value<100 g/m.sup.2/d.

21. (canceled)

Description

EXAMPLE 1

[0059] Used pre- and top coat formulations are given in Tables 1 and 2. Styrene acrylate copolymer has high carboxylation degree. Applied coat weight for the pre-coat formulation was 2.0 g/m.sup.2.

TABLE-US-00001 TABLE 1 Tested coating formulations for pre-coat in example 1. P2 (%) Coating color starch 80 Styrene acrylate copolymer, Tg = 10 C., high carboxylation 20

[0060] Starch was cooked at 10% solids.

TABLE-US-00002 TABLE 2 Tested coating formulation for top coat in example 1. T3 (%) Styrene acrylate copolymer, Tg = 10 C., high carboxylation 47.5 Fully hydrolyzed polyvinyl alcohol, Mw 47000 g/mol 50 Potassium zirconium carbonate 2.5

TABLE-US-00003 TABLE 3 Test results for coating formulations in example 1. Tested coatings Total coat weight WVTR (pre-coat/top coat) (g/m.sup.2) KIT (g/m.sup.2*d) P2 + T3 6.9 8 61 P2 + T3 9.1 9 54

[0061] It can be seen from results in Table 3 that the created coating structure provided good barrier properties.

EXAMPLE 2

[0062] Used pre- and top coat formulations are given in Tables 4 and 5. Styrene acrylate copolymer has low carboxylation degree. Applied coat weight for pre-coat formulation P3c was 2.4 g/m.sup.2.

TABLE-US-00004 TABLE 4 Tested coating formulations for pre-coat in example 2. P3c Pre (%) 4 Coating color starch 60 40 Styrene acrylate copolymer, Tg = 10 C., low carboxylation 40 60

TABLE-US-00005 TABLE 5 Tested coating formulations for top coat in example 2. T3c T4b T5 T6 T7 (%) (%) (%) (%) (%) Styrene acrylate copolymer, 47.5 47.5 47.5 47.5 47.5 Tg = 10 C., low carboxylation Fully hydrolyzed polyvinyl alcohol, 50 30 Mw 47000 g/mol Partially hydrolyzed polyvinyl alcohol, 30 20 30 Mw 47000 g/mol Coating colour starch 20 10 Potassium zirconium carbonate 2.5 2.5 2.5 2.5 2.5 Coarse ground calcium carbonate 20 20 20

TABLE-US-00006 TABLE 6 Test results for coating formulations in example 2. Total Coat Tested coatings weight Cobb300 WVTR HVTR (pre-coat + top coat) (g/m2) KIT (g/m.sup.2) (g/m.sup.2*d) (g/m.sup.2*d) P3c + T3c 8.0 10 61.2 25 P3c + T4b 10.3 10 57 P3c + T7 8.6 11 16 P4 + P4 10.0 12 78 P4 + T5 7.1 12 45 P4 + T6 8.7 11 16

[0063] It is seen from Table 6 that improved KIT values were obtained as well as good hexane vapor and water vapor results.

[0064] Even if the invention was described with reference to what at present seems to be the most practical and preferred embodiments, it is appreciated that the invention shall not be limited to the embodiments described above, but the invention is intended to cover also different modifications and equivalent technical solutions within the scope of the enclosed claims.