METHOD FOR FORMING A PVOH BASED BARRIER LAYER ON A SUBSTRATE

Abstract

There is provided a method for the production of a packaging material comprising a substrate and a gas barrier layer based on polyvinyl alcohol (PVOH), said method comprising the steps of—applying a coating composition of said PVOH dissolved in a first solvent onto said substrate to form a coating—subjecting the coating to a first drying step to form a dried PVOH-based coating on said substrate, contacting the dried PVOH-based coating with a crosslinking solution comprising a crosslinking agent in a second solvent, to effect crosslinking of the PVOH-based coating, and—subsequent to the contact with the crosslinking solution, subjecting the PVOH-based coating to a second drying step, forming the PVOH-based barrier layer on said substrate, with the proviso that if the crosslinking solution comprises PVOH, the amount of PVOH added by the crosslinking solution is less than 20% (by weight), such as less than 10% (by weight), of the amount of PVOH added by the coating composition.

Claims

1. A method for the production of a packaging material comprising a substrate and a gas barrier layer based on polyvinyl alcohol (PVOH), said method comprising the steps of applying a coating composition of said PVOH dissolved in a first solvent onto said substrate to form a coating subjecting the coating to a first drying step to form a dried PVOH-based coating on said substrate, contacting the dried PVOH-based coating with a crosslinking solution comprising a crosslinking agent in a second solvent, to effect crosslinking of the PVOH-based coating, and subsequent to the contact with the crosslinking solution, subjecting the PVOH-based coating to a second drying step, forming the PVOH-based barrier layer on said substrate, with the proviso that if the crosslinking solution comprises PVOH, the amount of PVOH added by the crosslinking solution is less than 20% (by weight), of the amount of PVOH added by the coating composition.

2. The method according to claim 1, wherein in case crosslinking solution comprises PVOH, the weight ratio of crosslinking agent to PVOH in the crosslinking solution is at least 2:3.

3. The method according to claim 1, wherein the dried PVOH-based coating formed after said first drying step is subjected to a heating step before the contact with the crosslinking solution, during which heating step said coating is heated to a temperature above the boiling temperature of said first solvent.

4. The method according to claim 1, wherein a second heating step is performed after said second drying step, during which heating step the barrier layer is heated to a temperature above the boiling temperature of said second solvent.

5. The method according to claim 1, wherein the crosslinking agent is chosen from glutaraldehyde, glyoxal, and borate derivatives.

6. The method according to claim 1, wherein the crosslinking solution that is contacted with the dried PVOH-based coating has a temperature of at least 40° C.

7. The method according to claim 1, wherein the degree of crosslinking is controlled by controlling the temperature of the crosslinking solution added to said PVOH-based coating.

8. The method according to claim 1, wherein no rinsing of the PVOH-based coating is carried out prior to the second drying step.

9. The method according to claim 1, wherein the coating composition further comprises an inter-polymer complexation (IPC) polymer having a weight average molecular weight (Mw) in an interval of 10 kg/mol to 10 000 kg/mol.

10. The method according to claim 9, wherein the IPC polymer is chosen from polyacrylic acid, polyvinyl pyrrolidone, non-ionic polyacrylamide or poly(methyl vinyl ether-alt-maleic acid).

11. The method according to claim 9, wherein the IPC polymer or polymers is present in the coating composition in an interval of about 0.5 to about 20 weight %, based on the weight of the PVOH component.

12. The method according to claim 1, wherein the coating composition further comprises a nanofiller.

13. The method according to claim 1, wherein the substrate is a fibre based substrate, such as a paper substrate, preferably a paper or paperboard comprising at least one fibre based layer.

14. A PVOH-based barrier layer, wherein said layer has an oxygen transmission rate (OTR) in the interval of 0.1 to 3 ml/m.sup.2 day atm, measured according to ASTM F1927-07 at a relative humidity (RH) of 50% and 23° C.

15. The PVOH-based layer according to claim 14, wherein said layer has an oxygen transmission rate (OTR) in the interval of 0.5 to 3 ml/m.sup.2 day atm measured according to ASTM F1927-07 at a relative humidity (RH) of 80% and 23° C.

16. The PVOH-based layer according to claim 14, wherein said layer has coat weight in the interval of 0.8 to 8.0 g/m.sup.2.

17. A packaging material comprising a PVOH-based barrier layer according to claim 14.

18. A package comprising a packaging material or substrate, having a PVOH-based layer according to claim 14.

19. A package comprising a packaging material or substrate, having a packaging material according to claim 17.

20. The method according to claim 13, wherein the fibre based substrate is chosen from a paper substrate or a paper or paperboard comprising at least one fibre based layer.

Description

EXAMPLES

[0104] The inventors have performed extensive experimental work using a non-modified PVOH layer as reference, obtaining improved barrier properties at high relative humidity.

Materials

[0105] PET-plastic films were used as the substrates for film forming in this experimental set-up.

[0106] KURARAY POVAL® 15-99 (Kuraray America Inc.) was used as the PVOH in the experiments. This polymer has a degree of hydrolysis of 99% and a viscosity of 12.5-17.5 mPa.Math.s (measured at 4% (w/w)).

[0107] Other components were: CLOISITE-Na+ (Southern Clay Products/BYK Additives & Instruments), which is a micro-granulated nanofiller. LUTENSOL® ON70 (BASF/BTC Europe GmbH) is a fatty alcohol ethoxylate. Zn(NO.sub.3).sub.2 was purchased from Fisher Scientific.

[0108] Glutaraldehyde (in the form of a 50% aqueous solution) and nonionic polyacrylamide with Mn of about 150 kg/mol corresponding to M.sub.w 400 kg/mol (NPA) were purchased from Sigma Aldrich.

[0109] Poly(methyl vinyl ether-alt-maleic acid) with Mn of about 80 kg/mol corresponding to M.sub.w 216 kg/mol (PMV.sub.low) was kindly provided by Ashland.

[0110] Tap water was used in all experiments.

Methods

1. Preparation of Polymer Solutions

[0111] The PVOH-PMV solutions were prepared by adding the polymers to water under continuous stirring at room temperature, prior to heating for one hour at 95° C.

[0112] PVOH-NPA solutions were prepared by first preparing stock solutions of the different polymers. PVOH (ca 9% (w/w)) was prepared by mixing at 95° C. for one hour, and NPA (1% (w/w)) was prepared through continuous stirring overnight at room temperature. The NPA was added to the PVOH solution at 60° C. under stirring. The system was thereafter removed from the heater and stirred for an additional period of ten minutes.

[0113] In the studies in which nanofiller (CLOISITE-Na+) was employed, the nanofiller was first dispersed in water (during ten minutes), before the addition of PVOH (and subsequent heating).

[0114] LUTENSOL® NO70 (10% based on the weight of the polymer/s) was added to the polymer (nanofiller) blends at 40° C. to improve the performance in curtain coating. Curtain coating was however not used in the testing described below.

2. Coating of the Substrate with PVOH-Based Formulations

[0115] The polymeric solutions were applied on the plastic substrate by a laboratory rod-coater, resulting in a grammage of 2-3 g/m.sup.2. The coating was thereafter dried by IR.

3. Crosslinking of the Barrier Films

[0116] A solution containing glutaraldehyde (0.1% (w/w)) and Zn(NO.sub.3).sub.2 (0.08% (w/w)) was prepared by mixing the components for thirty minutes. Substrates, coated with the different PVOH-based formulations, were immersed in this solution (at 25° C. or 60° C.) for five seconds, and immediately dried by IR, followed by immediate drying for two minutes at 140° C. in an oven. It is noted that the post-treatment (“curing”) improves the effectiveness of the crosslinking.

4. Barrier Properties

[0117] The OTR (measured at 80% RH and 23° C.) of the coated films were measured using an OX-TRAN® instrument from Mocon Inc. according to ASTM F1927-07. The oxygen permeation rates (OP) of the different systems were calculated by normalization of the measured OTR values with the thickness of the coating. Preliminary thickness values (about 1 to 2 μm) were obtained by SEM measurements.

Results

[0118] The results are summarized in Table 1. A non-modified PVOH layer is used as comparison, and the measured OTR compared to that of the non-modified layer. When evaluating the results, it should be kept in mind that the coating thickness was in the order of 1-3 μm. It should also be emphasized that the experimental methods have not been optimized, and the properties of the barrier layers have determined when testing the layers individually, and not in a sandwich construction as they would be used in the packaging industry.

TABLE-US-00001 TABLE 1 Comparative examples. OP means oxygen permeability. OTR means oxygen transmission rate. Unless otherwise indicated, the temperature of the crosslinking solution was 25° C. ≈OTR/ Barrier ≈OP* ≈OTR** OTR.sub.PVOH ≈OTR** ≈OTR.sub.80/ composition 80% RH Thickness 80% RH 80% RH 50% RH OTR.sub.50 PVOH 18 1.6 11 1 6 2 PVOH 8 1.6 6 0.5 2 2 crosslinked with glutaraldehyde PVOH 3 1.6 2 0.2 1 2 crosslinked with glutaraldehyde (60° C.) PVOH 13 1.8 7 0.6 3 2 10%(w/w) nanofiller PVOH 13 1.8 7 0.6 2 3 10%(w/w) nanofiller crosslinked with glutaraldehyde PVOH-PMV.sub.low 9 1.8 5 0.5 N/A N/A PVOH-PMV.sub.low 4 1.8 2 0.2 N/A N/A crosslinked with glutaraldehyde PVOH-PMV.sub.low 2 1.8 1 0.1 N/A N/A crosslinked with glutaraldehyde (60° C.) PVOH-PMV.sub.low 4 1.9 2 0.2 1 2 10%(w/w) nanofiller PVOH-PMV.sub.low 2 1.9 1 0.1 N/A N/A 10%(w/w) nanofiller crosslinked with glutaraldehyde PVOH-PMV.sub.low 2 1.9 1 0.1 N/A N/A 10% nanofiller crosslinked with glutaraldehyde (60° C.) PVOH-NPA 8 1.3 6 0.5 3 2 PVOH-NPA 5 1.3 4 0.4 3 1 crosslinked with glutaraldehyde *Unit: ml μm/m.sup.2 day atm **Unit: ml/m.sup.2 day atm

[0119] It can be seen from Table 1 that the crosslinking of a dried PVOH film resulted in a reduction of the OTR from 11 to 6 ml/m.sup.2 day atm at 80% RH and from 6 to 2 ml/m.sup.2 day atm at 50% RH. The OTR values were further improved, and reduced to 2 ml/m.sup.2 day atm (80% RH) and 1 ml/m.sup.2 day atm (50% RH) when the crosslinking agent was added at a temperature of 60° C. instead of 60° C. Similar effects were obtained when an inter-polymer complexation (IPC) polymer (PMV.sub.low or NPA) was present, but the temperature effect was smaller.

[0120] The lowest OTR values at 80% RH were obtained when the dried PVOH coating (that was subjected to crosslinking) comprised both an IPC polymer (PMV.sub.low, M.sub.w 216 kg/mol) and nanofiller or when it comprised the IPC polymer and was crosslinked at 60° C. That being said, remarkably low OTR values were obtained also without such additives.

[0121] The results indicate that highly desirable barrier properties, in particular barrier properties at high relative humidity, can be achieved by a process which is well suited for industrial application. It may also be possible to produce barrier layers which do not need to be covered with additional protecting layers, which is required in many competing processes.

[0122] Without further elaboration, it is believed that a person skilled in the art can, using the present description, including the examples, utilize the present invention to its fullest extent. Also, although the invention has been described herein with regard to its preferred embodiments, which constitute the best mode presently known to the inventors, it should be understood that various changes and modifications as would be obvious to one having the ordinary skill in this art may be made without departing from the scope of the invention which is set forth in the claims appended hereto.

[0123] Thus, while various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.

REFERENCES

[0124] U.S. Pat. No. 6,444,750—PVOH-based coating solutions, Robert E. Touhsaent WO 2004/089624—Packaging, Arnoldus J. Kruger and Patricia A. Truter [0125] WO 2010/129032—Barrier coatings post-treated with multi-valent metal cations, Carrie A. Feeney et al. [0126] WO 2013/064500—Coating composition, a method for coating a substrate, a coated substrate, a packaging material and a liquid packaging, Johan Larsson and Anders Karlsson [0127] Labuschagne P W, Germishuizen W A, C. Verryn S M, Moolman F S (2008) Improved oxygen barrier performance of poly(vinyl alcohol) films through hydrogen bond complex with poly(methyl vinyl ether-co-maleic acid), European Polymer Journal 44:2146-2152 doi:https://doi.org/10.1016/j.eurpolymj.2008.04.015

[0128] 0Lim M, Kim D, Seo J (2016) Enhanced oxygen-barrier and water-resistance properties of poly(vinyl alcohol) blended with poly(acrylic acid) for packaging applications, Polymer International 65:400-406 doi:10.1002/pi.5068