Coating composition, a method for coating a substrate, a coated substrate, a packaging material and a liquid package

Abstract

The present invention relates to a curtain coatable gas barrier coating composition comprising polyvinyl alcohol and a surfactant, in which the surfactant is a water-soluble non-ionic ethoxylated alcohol. The present invention also relates to a method for providing a substrate with a gas barrier layer by means of the coating composition, and a coated substrate having at least one gas barrier layer obtained by coating the substrate with the coating composition. Further, the invention relates to a packaging material comprising a coated paperboard coated with the coating composition, and a liquid package comprising such a packaging material.

Claims

1. A curtain coatable gas barrier coating composition comprising polyvinyl alcohol and a surfactant, wherein the surfactant is a water-soluble non-ionic ethoxylated alcohol and the solids content of the composition is 4-20%.

2. The coating composition according to claim 1, wherein the surfactant has the formula RO(CH.sub.2CH.sub.2O).sub.xH, wherein R=iso-C.sub.13H.sub.27 and x is 8 or higher.

3. The coating composition according to claim 1, wherein the surfactant is an alkyl polyethylene glycol ether based on C10-Guerbet alcohol, with a degree of ethoxylation of 7 or higher.

4. The coating composition according to claim 1, wherein the surfactant has the formula RO(CH.sub.2CH.sub.2O).sub.xH, wherein R=iso-C.sub.10 and x is 5 or higher.

5. The coating composition according to claim 1, wherein the composition, when dried to a bone dry composition, comprises a concentration of 0.05-2.0 weight % of non-ionic ethoxylated alcohol surfactant of the total weight of said bone dry coating composition.

6. The coating composition according to claim 1, wherein the solids content is 6-12%.

7. The coating composition according to claim 1, comprising platy particles.

8. The coating composition according to claim 7, comprising Montmorillonite clay in nano particle form.

9. A method for providing a substrate with a gas barrier layer applied in at least one coating step, comprising applying a coating composition as defined in claim 1 by curtain coating the substrate with the coating composition in at least one coating step.

10. The method according to claim 9, wherein a coat weight of the gas barrier layer is 0.4 g/m.sup.2 or more and less than 7.0 g/m.sup.2.

11. The method according to claim 9, wherein a coat weight of the gas barrier layer is 0.8 g/m.sup.2 or more and less than 7.0 g/m.sup.2.

12. The method according to claim 9, wherein the substrate is a fibre substrate or a paper or a paperboard.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention will now be described in more detail, with reference being made to test examples and the enclosed schematic drawings illustrating different aspects and embodiments of the invention, given as examples only, and in which:

(2) FIGS. 1a-1d shows schematically examples of a coated substrate according to the present invention, and

(3) FIGS. 2a-2d shows schematically examples of a packaging material according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

(4) During the search for a suitable surfactant and a suitable method, many different commercially available surfactants were tested that were alleged to be suitable for curtain coating. However, they were all found unsuitable since they all failed either in providing a stable curtain or in providing a sufficiently good barrier.

(5) After having performed many tests it was surprisingly found a group of surfactants that managed to provide the desired result of a stable curtain and a high quality barrier with an acceptable low amount of pinholes. This group of surfactants was non-ionic ethoxylated alcohols, such as sold under the trade names of Lutensol ON and Lutensol TO, by BASF. In particular Lutensol ON50, Lutensol ON70 and Lutensol TO8 were found to provide the required results.

(6) According to the product information supplied by BASF, the Lutensol ON types are non-ionic surfactants. They are alkyl polyethylene glycol ethers made from a saturated synthetic, short-chain fatty alcohol. They conform to the following formula: RO(CH2CH2O)xH, where

(7) R=saturated, synthetic, short-chain fatty alcohol

(8) x=3, 5, 6, 7, 8 or 11

(9) The numeric code in the product name usually indicates the degree of ethoxylation. The Lutensol ON types are manufactured by causing the fatty alcohol to react with ethylene oxide in stoichiometric proportions. The ethoxylation temperature is kept as low as possible. This, combined with the high purity of the feedstocks, ensures that high-performance products with low toxicity are obtained. According to additional information available from BASF, R may be iso-C10.

(10) According to the product information supplied by BASF, the Lutensol TO types are non-ionic surfactants. They are made from a saturated iso-C13 alcohol. They conform to the following structural formula: RO(CH2CH2O)xH, where

(11) R=iso-C13 H27

(12) x=3, 5, 6, 6.5, 7, 8, 10, 12, 15 or 20

(13) The numeric code in the product name indicates the degree of ethoxylation. The Lutensol TO types are manufactured by causing the iso-C13 oxo alcohol to react with ethylene oxide in stoichiometric proportions. The ethoxylation temperature is kept as low as possible. This, combined with the high purity of the feedstocks, ensures that high-performance products with low toxicity are obtained.

(14) In the case of Lutensol ON50, x (the degree of ethoxylation) is 5, and in the case of Lutensol ON70, x is 7. A Lutensol ON having a lower molecular weight, Lutensol ON30 (x=3), was also tested but did not result in the desired effects.

(15) In the case of Lutensol T08, x is 8. Lutensol TO8 has a molecular weight of approximately 600 g/mol. Also for this group, a Lutensol TO having a lower molecular weight, Lutensol TO6 (x=6), was also tested but did not result in the desired effects.

(16) The non-ionic ethoxylated alcohols having the desired effect are water soluble and provide relatively slow wetting. Lutensol ON30 and TO6, on the other hand, are not water soluble. Lutensol ON is water soluble for a degree of ethoxylation of 5 and above, while Lutensol TO is water soluble for a degree of ethoxylation of 8 and above.

(17) Another such water soluble non-ionic ethoxylated alcohol is Lutensol XP.

(18) According to the product information supplied by BASF, the Lutensol XP types are branched non-ionic surfactants. They are alkyl polyethylene glycol ethers based on C.sub.10-Guerbet alcohol and ethylene oxide. The Lutensol? XP types are manufactured by reacting the C.sub.10-alcohol with ethylene oxide in stoichiometric proportions. The numeric portion of the product name indicates the general degree of ethoxylation. The product is water soluble when the degree of ethoxylation is 7 or higher.

(19) Tests with good and satisfying results were performed with a liquid coating composition comprising polyvinyl alcohol, of which a solution was prepared according to the instructions on the particular product, and to which solution the concerned surfactant was added according to the following:

(20) Concentration of surfactant (weight % of added surfactant as from package of the weight of the total bone dry composition): 0.05-2%; preferably 0.1-1.0% or even 0.2-0.5%; Solid content of the composition: 4-20%; preferably 7-15%, or even 8-10%.

(21) The tests were made by depositing the liquid coating composition on a fibre based moving web in a curtain coating process. The web was paperboard.

(22) Test results are listed in Table 1. The results in the table are measured on paperboard coated in a curtain coating process with two layers of the coating composition according to the invention, each layer being equal or not in coat weight to the other layer, totalling 2.0-3.0 g/m.sup.2. In this table are also shown comparative tests performed with a coating comprising polyvinyl alcohol (PvOH) alone and a coating composition comprising polyvinyl alcohol and the by Cytec, USA, commercially available surfactant Aerosol OT75 comprising Sodium dioctyl sulfosuccinate, which did not result in a satisfying gas barrier.

(23) TABLE-US-00001 TABLE 1 Test examples OTR Coat Flow (min) AO2IR ml/m.sup.2* Coating weight Visual Curtain l/min KIT- Pinholes/ ml/dm.sup.2 .Math. 24 h* composition g/m.sup.2 observation stability *m number dm.sup.2 24 h 1 atm PvOH only 3.2 OK at high Holes in 10 1 237 1817 5.8 flow rates the film only PvOH + 0.2% 2.4 OK OK 6 1 252 5152 61.8 Aerosol OT75 PvOH + 0.3% 2.4 OK OK 6 12 22 296 Lutensol ON70 PvOH + 0.3% 3.0 OK OK 6 4 51 7 Lutensol ON70 PvOH + 10% 2.0 OK OK 6 12 4 94 5.2 Cloisite + 0.3% Lutensol ON70 PvOH + 10% 3.0 OK OK 6 12 3 13 2.9 Cloisite + 0.3% Lutensol ON70
KIT=TAPPI T 559 KIT test (repellency of paper and board to grease, oil, and waxes)
AO2IR is short for Ambient Oxygen Ingress Rate which is a method for measuring oxygen gas transmission with a Perm-Mate instrument supplied by FBI Dansensor or Systech Illinois.
OTR is short for Oxygen Transmission Rate, measured at 23? C. and 50% RH by the instrument Mocon Ox Tran 2/21.
A low KIT number indicates resistance only against fat; a high KIT number indicates high resistance against fat and solvents;
A low AO2IR value indicates high resistance against gas transmission.
Flow (min) l/min*m=minimum flow rate in curtain
The OTR is measured on a packaging material comprising the inventive coated paperboard provided with a polyethylene layer of 30 g/m2. A low OTR value indicates good barrier function.

(24) The coating composition may also comprise Montmorillonite clay, as mentioned before, or talc, kaolin clay or other similar solid additives in platy form or nano-particle form, while maintaining good barrier results, which has also been proven in tests. The tests performed with Montmorillonite (Cloisite) are also shown in Table 1. The clay or other suitable platy or nano form additives may be added to the polyvinyl alcohol in an amount of 1-20% dry weight of the total dry weight of the composition, preferably 3-15% and more preferably 5-10%.

(25) The barrier effect represented by a regular pinhole evaluation showed that it was possible to obtain a coating layer with a low amount of pinholes and a low AO2IR-value down to a coating layer of 2.4 g/m.sup.2 and even down to 2.0 g/m.sup.2 with Montmorillonite. The coating layer was obtained in a curtain coating process using the composition and achieving the layer in two coating steps of 1.2 g/m.sup.2 and 1.0 g/m.sup.2 respectively per step.

(26) Particularly good barrier results are the results showing a KIT number of 8 or above (12 being the maximum number) and less than 50 pinholes/dm.sup.2.

(27) Particularly good gas barrier results are the results showing a value of the oxygen transmission rate (OTR) of less than 10 ml/m2*24 h*1 atm.

(28) The tests were performed with a velocity of the web substrate between 420 and 850 m/min with good results. With a curtain flow rate of at least 6 l/min*m and stable curtain conditions with less than 50 pinholes/dm.sup.2, it can be shown that it is theoretically possible to have a velocity of 250-1500 m/min, or even more, with a coat weight of 0.4-4.0 g/m.sup.2 applied in two layers, totalling 0.8-8.0 g/m.sup.2, and achieve satisfactory barrier results.

(29) It should be generally understood that the present invention is not limited to the surfactant products with the above trade names, but there may be similar/corresponding products sold under other trade names by other suppliers or manufacturers which will work accordingly. It should also be mentioned that a possibility is foreseen to mix two or more surfactants, and it can also be foreseen that other components may be added to the coating composition, without departing from the scope of the present invention.

(30) The polyvinyl alcohols used for the tests were supplied from Kuraray Nordic Oy in the form of product range Mowiol 6/98, 15/99 and 28/99, and it was prepared by dissolving in water according to the supplier's instructions. The Montmorillonite clay was supplied by Southern Clay Products, USA in the form of the product Cloisite Na+. Tests were also made with kaolin clay and talc, with similar good results as with Montmorillonite. The kaolin clay used was supplied by Imerys in the form of Product Barrisurf LX. The talc used was supplied by FinnTalc Oy in the form of the product Finntalc C10B. It should be clear that also other similar types of polyvinyl alcohols, and other brands of the mentioned substances may be used without departing from the scope of the present invention. It is concluded that the present invention makes it possible to obtain a barrier coated paperboard that is suitable for liquid packages, in which the polyvinyl alcohol layer provides an oxygen barrier and other additional layers, e.g. polyethylene, provides a liquid barrier and sealing properties.

(31) The concerned liquid coating composition can generally be prepared by adding polyvinyl alcohol and also Montmorillonite (or talc or kaolin clay) when applicable, in powder form to cold water. Thereby a liquid is obtained which is then heated. After preparation, the liquid shall be allowed to cool down before the surfactant is added and then the resulting composition is ready to be used in the curtain coating process. Alternatively, each of the substances may prepared by itself, before mixing the substances with each other, heating and then adding the surfactant in order to obtain the final composition.

(32) In FIGS. 1a-1d is schematically illustrated examples of a coated substrate according to the present invention. In FIG. 1a is shown a coated substrate where the substrate 1 is a paperboard comprising at least one fibre based layer, and the paperboard is coated with a gas barrier coating layer 2 comprising a coating composition according to the present invention. In FIG. 1b, the coated substrate is further provided with a pigment coating layer 3 on the opposite side of the paperboard layer 1, i.e. on that side of the paperboard layer 1 that is not coated with the barrier coating 2. In FIG. 1c, the coated substrate of FIG. 1a is also provided with a pigment coating layer 3, but in this example the pigment coating layer 3 is provided between the paperboard layer 1 and the barrier coating layer 2. Consequently, the barrier coating 2 has been applied on top of the already applied pigment coating layer 3. Finally, in FIG. 1d, the coated paperboard of FIG. 1c has been provided with an additional pigment coating layer 3, resulting in a paperboard layer 1 having a pigment coating layer 3 on both sides and a barrier coating layer 2 applied on one of the pigment coating layers 3.

(33) FIGS. 2a-2d shows schematically examples of a packaging material according to the present invention. FIGS. 2a-2d illustrates a coated substrate corresponding to FIGS. 1a-1d respectively, which has been provided with a polyethylene layer 4 on both sides.

(34) The present invention is not limited to the disclosed examples, but may be modified in many ways that would be apparent to the skilled person, within the scope of the appended claims.