Coating structure, sheet-like product and its use

Abstract

A coating structure for a sheet-like substrate, which includes lignocellulosic fibres is disclosed. The structure includes at least one pre-coat layer including at least 70 weight-% of a first styrene (meth)acrylate copolymer polymerised in the presence of a stabiliser, having a glass transition temperature Tg 20 C. preferably 10 C., 30 weight-% of inorganic platy mineral particles and 0.01-2 weight-% of at least one first thickener. The coating structure further includes a top coat layer including at least 50 weight-% of a second styrene (meth)acrylate copolymer polymerised in the presence of a stabiliser having a glass transition temperature Tg 20 C. preferably 10 C., 30 weight-% of inorganic mineral particles, 0.01-5 weight-% of at least one second thickener and 0.5-30 weight-% of an antiblocking agent. Further disclosed is a sheet-like product coated with the coating structure.

Claims

1. A coating structure for a sheet-like substrate comprising lignocellulosic fibres, the structure comprising: at least one pre-coat layer comprising: (a) at least 70 weight-% of a first styrene (meth)acrylate copolymer polymerised in the presence of a stabiliser, said first styrene (meth)acrylate copolymer having a glass transition temperature Tg 20 C., preferably 10 C., (b) 30 weight-% of inorganic platy mineral particles, such as kaolin or talc, and (c) 0.01-2 weight-% of at least one first thickener; and a top coat layer comprising: (a) at least 50 weight-% of a second styrene (meth)acrylate copolymer polymerised in the presence of a stabiliser, said second styrene (meth)acrylate copolymer having a glass transition temperature Tg 20 C., preferably 10 C., (b) 30 weight-% of inorganic mineral particles, (c) 0.01-5 weight-% of at least one second thickener, and (d) 0.5-30 weight-% of an antiblocking agent.

2. The coating structure according to claim 1, wherein the pre-coat layer comprises: (a) 70-99 weight-%, preferably 70-95 weight-%, more preferably 73-90 weight-% of the first styrene (meth)acrylate copolymer, and/or (b) 0.1-30 weight-%, preferably 5-25 weight-%, more preferably 10-20 weight-% of inorganic platy mineral particles, and/or (c) 0.1-2 weight-%, preferably 0.1-1.5 weight-%, more preferably 0.2-1 weight-% of the at least one first thickener.

3. The coating structure according to claim 1, wherein the top-coat layer comprises: (a) 50-99 weight-%, preferably 60-95 weight-%, more preferably 63-85 weight-% of the second styrene (meth)acrylate copolymer, and/or (b) 0.1-30 weight-%, preferably 5-25 weight-%, more preferably 5-20 weight-% of inorganic mineral particles, and/or (c) 0.1-5 weight-%, preferably 0.1-3 weight-%, more preferably 0.1-2 weight-% of the at least one second thickener, and/or (d) 1-30 weight-%, preferably 1-20 weight-%, more preferably 1-15 weight-% of anti-blocking agent.

4. The coating structure according to claim 1, wherein the anti-blocking agent is selected from C16-C18 alkenyl ketene dimers, paraffin wax, caranauba wax, calcium stearate, polyglycerides or high density polyethylene.

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

6. 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 carboxylic acid.

7. The coating structure according to claim 1, wherein the first and/or the second styrene (meth)acrylate copolymer has a glass transition temperature in a range of 40-20 C., preferably 20-10 C., more preferably 10-10 C.

8. The coating structure according to claim 1, wherein the first styrene (meth)acrylate copolymer and the second styrene (meth)acrylate copolymer are identical to each other.

9. The coating structure according to claim 1, wherein the at least one first and/or the at least one second thickener are selected from a group comprising synthetic pH-triggered thickeners, such as alkali soluble/swellable emulsion (ASE) thickeners, hydrophobically-modified alkali soluble emulsion (HASE) thickeners, polyvinyl alcohols, ethylated polyvinyl alcohols, carboxymethyl cellulose, xanthan gum, guar gum or gum Arabic.

10. The coating structure according to claim 1, wherein the at least one first thickener and the at least one second thickener are different from each other, wherein the at least one first thickener is preferably selected from synthetic thickeners, preferably from synthetic pH-triggered thickeners and the at least one second thickener is preferably carboxymethyl cellulose.

11. The coating structure according to claim 1, wherein the pre-coat layer and/or the top coat layer comprises a cross-linker reacting with OH or COOH groups.

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

13. The coating structure according to claim 1, further comprising a polyethylene film applied on the top coat layer.

14. The coating structure according to claim 1, wherein the pre-coat layer has a coat weight of 2-30 g/m.sup.2, preferably 3-20 g/m.sup.2, more preferably 5-15 g/m.sup.2, and the top coat layer has a coat weight of 0.5-20 g/m.sup.2, preferably 0.5-15 g/m.sup.2, more preferably 0.5-10 g/m.sup.2.

15. A sheet-like product comprising: a substrate comprising lignocellulosic fibres, and having a first parallel large surface 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.

16. The product according to claim 15, 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.

17. The product according to claim 15, wherein the product has KIT test value of at least 8, mineral oil barrier HVTR value <100 g/m.sup.2/d, and/or water vapour barrier VWTR value <100 g/m.sup.2/d.

18. Use of a sheet-like product according to claim 15 for making a food service package.

Description

EXPERIMENTAL

[0059] Reference Sample 1

[0060] Reel-to-reel coating was done using a semi-pilot coater. 130 g/m.sup.2 liner board was used as substrate. Coating speed was 10 m/min and 20 m rod was used for coating. Used coating colour contained 50% styrene acrylate binder with Tg=20 C. and 50% talc pigment (Finntalc C15, used in all the following examples).

[0061] Sample substrate was double coated with the coating colour. Coat weight was determined in oven.

[0062] Following barrier properties were tested, used standard methods given in parenthesis: [0063] Oxygen, OTR (ASTM D3985, 23 C. 50% RH; 23 C. 80% RH) [0064] Water vapor, WVTR (ASTM E-96, D3985 & F1927, 23 C. 50% RH) [0065] Water, Cobb 300s (ISO 535, EN 20535 & TAPPI T 441) [0066] Grease and oil [0067] KIT test (TAPPI method T-559 m-96) [0068] Olive oil 40 C. (ISO 16235-2 & TAPPI 507 cm-99), 5 bar 170 h

[0069] The obtained results are given in Table 2. Visual result for creased and stained sample is given in FIG. 1.

[0070] Reference Sample 2

[0071] Reel-to-reel coating was done using a semi-pilot coater. 130 g/m.sup.2 liner board was used as substrate. Coating speed was 10 m/min and 14 m rod was used for coating. Sample substrate was coated with a pre-coat layer and a top coat layer. Compositions of used pre-coat and top coat coating colours are shown in Table 1. Used coating colours contained styrene acrylate binder with Tg=10 C.

TABLE-US-00001 TABLE 1 Compositions of pre-coat coating colour and top coat coating colour for Reference Sample 1. Pre-coat (%) Top coat (%) Binder, Tg = 10 C. 66.6 58.4 Talc 28.6 25 Polyvinyl alcohol (Poval 6-98) 4.8 8.3 AKD 8.3

[0072] The obtained results are given in Table 2. Visual result for creased and stained sample is given in FIG. 2.

TABLE-US-00002 TABLE 2 Obtained results for Reference Samples 1 and 2. Reference Reference Coating Property Sample 1 Sample 2 Coat weight (g/m2) 19.4 14.8 OTR 23 C. 50% RH 100 0.7 (cm.sup.3/m.sup.2, d, bar) WVTR 23 C. 50% RH 20 33.6 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 8.38 41 KIT 12 12 Olive oil test no grease 2% stained penetration HVTR (g/m.sup.2*d) 10

[0073] Coating Example 3

[0074] Used substrate was 265 g/m.sup.2 folding box board. One or two barrier pre-coats were applied on the uncoated substrate by using RK K Control Coater rod coating unit. For double coated samples the pre-coat was applied by using coating rod (3) with wire diameter of 24 m. For triple-coated samples the first pre-coat layer was applied by using coating rod (3) with wire diameter of 24 m and the second pre-coat layer by using smooth rod (0) with wire diameter of 0.05 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 3. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00003 TABLE 3 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 3. Pre-coat (%) Top Coat (%) Binder, Tg 5 C. 79.6 74 Talc 20 10 Synthetic thickener, 0.4 ASE type AKD 10 Cross-linker, 5 Potassium Zirconium carbonate CMC (Finnfix-10) 1

[0075] pH of the pre-coat coating colour was adjusted to 8.5 using NaOH.

[0076] Simple converting test was done for the coated samples. The test included sample creasing by using Cyklos CPM 450 creasing and perforation unit, and folding by using Cobb-roller to press the fold at constant pressure. Creasing and folding was done in both machine and cross directions. Staining test was done for the folded samples by using methyl red dissolved in ethanol. Water vapor barrier properties were measured using Systech Permeation Analyzers M7002 instrument. Grease barrier properties were tested using boiled chicken fat. Fat was placed on the barrier coated side of the coated and folded FBB sample and placed at 60 C. oven for 60 minutes and photographed for evaluation of grease penetration.

[0077] Visual result for creased and folded and stained sample, coat weight 13.8 g/m.sup.2, double-coated with a pre-coat layer by using rod 3 and a top coat layer by using rod 0, is given in FIG. 3a. Visual result for chicken fat grease barrier test for the similar sample, coat weight 13.8 g/m.sup.2, creased and folded, is given in FIG. 3b.

[0078] The obtained measured results for similar sample with coat weight 12.6 are given in Table 4.

[0079] Visual result for creased and folded and stained sample, coat weight 15.9 g/m.sup.2, triple-coated with two pre-coat layers by using rods 3 and 0 and a top coat layer by using rod 0, is given in FIG. 3c. Visual result for chicken fat grease barrier test for the similar sample, coat weight 15.9 g/m.sup.2, creased and folded, is given in FIG. 3d.

TABLE-US-00004 TABLE 4 Obtained results for double-coated sample of Coating Example 3. Coating Property Value Coat weight (g/m2) 12.6 WVTR 23 C. 50% RH 52.0 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 47

[0080] Coating Example 4

[0081] Used substrate was 265 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds.

[0082] Compositions of used coating colours are shown in Table 5. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00005 TABLE 5 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 4. Pre-coat (%) Top Coat (%) Binder, Tg 5 C. 79.6 73.2 Talc 20 10 Synthetic thickener, HASE type 0.4 AKD 9.9 Cross-linker, 5 Potassium Zirconium carbonate CMC (Finnfix-10) 1.9

[0083] pH of the pre-coat coating colour was adjusted to 8.5 using NaOH.

[0084] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0085] Visual result for creased and folded and stained sample, coat weight 13.8 g/m.sup.2, is given in FIG. 4a. Visual results for chicken fat grease barrier test for the sample, coat weight 13.8 g/m.sup.2, creased and folded, is given in FIG. 4b. The obtained measured results similar sample with coat weight 11.1 g/m.sup.2 are given in Table 6.

TABLE-US-00006 TABLE 6 Obtained results for double-coated sample of Coating Example 4. Coating Property Value Coat weight (g/m2) 11.1 WVTR 23 C. 50% RH 47.4 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 47

[0086] Coating Example 5

[0087] Used substrate was 265 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 7. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00007 TABLE 7 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 5. Pre-coat (%) Top Coat (%) Binder, Tg 5 C. 74.7 77.0 Talc 19.3 11 Synthetic thickener, 0.35 ASE type AKD 10.4 Cross-linker, 5.1 Potassium Zirconium carbonate CMC (Finnfix-10) 0.55 9.9 ethylated polyvinyl alcohol 0.6 (Exceval HR-3010)

[0088] pH of the pre-coat coating colour was adjusted to 8.5 using NaOH.

[0089] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0090] Visual results for creased and folded and stained sample, coat weight 12.5 g/m.sup.2, is given in FIG. 5a. Visual results for chicken fat grease barrier test for the sample, coat weight 12.5 g/m.sup.2, creased and folded, is given in FIG. 5b. The obtained measured results for similar sample, coat weight 10.6 g/m.sup.2, are given in Table 8.

TABLE-US-00008 TABLE 8 Obtained results for double-coated sample of Coating Example 5. Coating Property Value Coat weight (g/m.sup.2) 10.6 WVTR 23 C. 50% RH 55.0 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 50

[0091] Coating Example 6

[0092] Used substrate was 265 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 9. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00009 TABLE 9 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 6. Pre-coat (%) Top Coat (%) Binder, Tg 5 C. 89.6 64.1 Talc 10 19.9 Synthetic thickener, 0.4 ASE type AKD 9.9 Cross-linker, 4.9 Potassium Zirconium carbonate CMC (Finnfix-10) 1.2

[0093] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0094] Visual result for creased and folded and stained sample is given in FIG. 6a.

[0095] Visual result for chicken fat grease barrier test for the sample, creased and folded, is given in FIG. 6b. The obtained measured results for similar sample are given in Table 10.

TABLE-US-00010 TABLE 10 Obtained results for double-coated sample of Coating Example 6. Coating Property Value Coat weight (g/m.sup.2) 9.8 WVTR 23 C. 50% RH 60.6 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 49

[0096] Coating Example 7

[0097] Used substrate was 235 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 11. Used coating colours contained carboxylated styrene acrylate binder with Tg=10 C.

TABLE-US-00011 TABLE 11 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 7. Pre-coat (%) Top Coat (%) Binder, Tg 10 C. 79.6 74.1 Talc 20 9.9 Synthetic thickener, 0.4 ASE type AKD 9.9 Cross-linker, 4.9 Potassium Zirconium carbonate CMC (Finnfix-10) 1.2

[0098] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0099] Visual results for creased and folded and stained sample are given in FIG. 7a. Visual results for chicken fat grease barrier test for the sample, creased and folded, is given in FIG. 7b. The obtained measured results the same sample are given in Table 12.

TABLE-US-00012 TABLE 12 Obtained results for double-coated sample of Coating Example 7. Coating Property Value Coat weight (g/m.sup.2) 10.6 WVTR 23 C. 50% RH 55.0 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 50

[0100] Coating Example 8

[0101] Used substrate was 265 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 13. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00013 TABLE 13 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 8. Pre-coat (%) Top Coat (%) Binder, Tg C. 79.6 78.8 Talc 20 10 Synthetic thickener, 0.4 ASE type Carnauba wax 5 Cross-linker, 5 Potassium Zirconium carbonate CMC (Finnfix-10) 1.2

[0102] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0103] The obtained measured results the same sample are given in Table 14.

TABLE-US-00014 TABLE 14 Obtained results for double-coated sample of Coating Example 8. Coating Property Value Coat weight (g/m.sup.2) 12.2 WVTR 23 C. 50% RH 55.9 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 49

[0104] Coating Example 9

[0105] Used substrate was 265 g/m.sup.2 folding box board. Barrier pre-coat was applied on the uncoated substrate by using RK K Control Coater rod coating unit and coating rod (3) with wire diameter of 24 m. Top coat was applied using smooth rod (0) with wire diameter of 0.05 m and giving wet film thickness of 4 microns. Coating speed was set to 5 and drying was carried out using IR dryer for 60 seconds. Compositions of used coating colours are shown in Table 15. Used coating colours contained styrene acrylate binder with Tg=5 C.

TABLE-US-00015 TABLE 15 Compositions of pre-coat coating colour and top coat coating colour for Coating Example 9. Pre-coat (%) Top Coat (%) Binder, Tg 5 C. 74.5 70.2 Talc 20 9.2 Synthetic thickener, 0.4 ASE type AKD 9 Cross-linker, 5.1 Potassium Zirconium carbonate CMC (Finnfix-10) 1.4 PEG 300 4.95 5.1

[0106] Simple converting test was done for the samples in the same manner as described for Coating Example 3.

[0107] Visual result for creased and folded and stained sample is given in FIG. 8a.

[0108] Visual result for chicken fat grease barrier test for the sample, creased and folded, is given in FIG. 8b. The obtained measured results for similar sample are given in Table 16.

TABLE-US-00016 TABLE 16 Obtained results for double-coated sample of Coating Example 9. Coating Property Value Coat weight (g/m.sup.2) 14.5 WVTR 23 C. 50% RH 47.5 (g/m.sup.2, d) Cobb 300 s (g/m.sup.2) 44

[0109] In order to develop barrier coatings for fibre based packagings it is not sufficient to only look at properties of the coating as a flat surface. Barrier coated products will go through a converting process and therefore it is crucial for the coating to remain intact during the creasing and folding processes. The examples above show that the reference samples with good barrier properties cannot be commercially used as the barrier coating cracks at fold and loses the barrier properties it had as flat sample. The coating examples 3-9 show that different coating formulations according to the invention can provide a combination of good barrier and good converting properties. Product can be double or multilayer coated. The disclosed coating structures provide especially improved grease and water vapour barrier properties.

[0110] 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.