COATING METHOD AND PRODUCT THEREOF

Abstract

A process for the preparation of a film is described, in which a substrate is coated with a coating mixture containing a water-soluble polymer and a layered double hydroxide. The process of the invention is markedly simpler that conventional techniques for affording films having reduced permeability to degradative gases. The films obtainable by the process are particularly useful in packaging applications, notably in the food industry.

Claims

1. A process for the preparation of a film, the process comprising the steps of: a) providing a first substrate; b) providing an aqueous mixture comprising a water-soluble polymer and a water-dispersible layered-double hydroxide; c) coating the first substrate with a layer of the aqueous mixture; and d) drying the coated first substrate.

2. The process of claim 1, wherein the first substrate is selected from polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyamide, nylon, polylactic acid (PLA) and polyvinyl dichloride (PVDC).

3. The process of claim 1 or 2, wherein the water-soluble polymer is selected from one or more of poly(vinyl alcohol) (PVOH), copolymers comprising vinyl alcohol (e.g. polyethylene vinyl alcohol (EVOH)) and polyacrylic acid (PAA).

4. The process of any one of claim 1, 2 or 3, wherein the layered double hydroxide has a structure according to formula (I) shown below:
[M.sup.z+.sub.1-xM.sup.y+.sub.x(OH).sub.2].sup.a+(X.sup.n).sub.m.bH.sub.2O (I) wherein M is at least one charged metal cation; M is at least one charged metal cation different from M; z is 1 or 2; y is 3 or 4; 0<x<0.9; 0<b10; X is at least one anion; n is the charge on anion X; a is equal to z(1-x)+xy-2; and ma/n.

5. The process of claim 4, wherein when z is 2, M is Mg, Zn, Fe, Ca, Sn, Ni, Cu, Co, or a mixture of two or more of these, or when z is 1, M is Li.

6. The process of claim 4 or 5, wherein when y is 3, M is Al, Ga, In, Fe, Ti, or a mixture thereof, or when y is 4, M is Sn, Ti or Zr or a mixture thereof.

7. The process of any one of claims 4, 5 and 6, wherein M is Al.

8. The process of any one of claims 4 to 7, wherein the layered double hydroxide of formula (I) is a Zn/Al, Mg/Al, ZnMg/Al, Ca/Al, Ni/Al or Cu/Al layered double hydroxide.

9. The process of any preceding claim, wherein X is selected from carbonate, bicarbonate, hydrogenphosphate, dihydrogenphosphate, nitrite, borate, nitrate, phosphate and sulphate.

10. The process of any preceding claim, wherein X is carbonate.

11. The process of any preceding claim, wherein the layered double hydroxide is a MgAlCO.sub.3 layered double hydroxide.

12. The process of any preceding claim, wherein the layered double hydroxide has a platelet morphology, wherein the largest dimension of the platelet is 0.01-10 m.

13. The process of claim 12, wherein the largest dimension of the platelet is 0.01-1 m.

14. The process of any one of claims 1 to 11, wherein the layered double hydroxide has a platelet morphology, wherein the average particle size of the platelet is 2.5-10 m.

15. The process of claim 14, wherein the layered double hydroxide has a platelet morphology, wherein the average particle size of the platelet is 3.5-9 m.

16. The process of any preceding claim, wherein the aqueous mixture comprises 1-15 wt % of layered double hydroxide.

17. The process of any preceding claim, wherein the aqueous mixture comprises 1-10 wt % of layered double hydroxide.

18. The process of any preceding claim, wherein the aspect ratio of the layered double hydroxide is at least 10, wherein aspect ratio is the average diameter of the layered double hydroxide platelet divided by the average thickness of the layered double hydroxide platelet.

19. The process of any preceding claim, wherein the aqueous mixture comprises 1-20 wt % of water soluble polymer.

20. The process of any preceding claim, wherein the aqueous mixture comprises 1-10 wt % of water soluble polymer.

21. The process of any preceding claim, wherein the aqueous mixture has a total solids content of 5-15 wt %, wherein the weight ratio of water-soluble polymer (e.g. PVA) to LDH within the aqueous mixture is 0.5:1 to 5:1.

22. The process of any preceding claim, wherein the aqueous mixture has a total solids content of 8-12 wt %, wherein the weight ratio of water-soluble polymer (e.g. PVA) to LDH within the aqueous mixture is 0.5:1 to 5:1.

23. The process of any preceding claim, wherein the aqueous mixture has a total solids content of 8-12 wt %, wherein the weight ratio of water-soluble polymer (e.g. PVA) to LDH within the aqueous mixture is 0.75:1 to 4.5:1.

24. The process of any preceding claim, wherein the aqueous mixture has a viscosity of 1-5000 cP.

25. The process of any preceding claim, wherein after step c) and prior to step d), the coated first substrate is contacted with a second substrate, such that the layer of aqueous mixture is provided between the first and second substrates.

26. The process of any one of claims 1 to 24, further comprising the steps of: e) of applying a layer of adhesive to the dried coated first substrate resulting from step d), such that the layer of adhesive is provided on top of the layer applied during step c); and f) contacting the layer of adhesive applied in step e) with a second substrate.

27. The process of claim 25 or 26, wherein the second substrate is selected from polyethylene terephthalate (PET), polyethylene (PE), polypropylene (PP), polyamide, nylon, polylactic acid (PLA) and polyvinyl dichloride (PVDC).

28. The process of claim 26 or 27, wherein the adhesive is selected from cellulose acetate, poly(vinyl alchohol) (PVOH), polyvinyl acetate, polyvinyl dichloride (PVDC), polyurethane, an acrylic-based adhesive, an epoxy resin, and mixtures thereof.

29. A film obtainable by the process of any preceding claim.

30. A film comprising: a) a substrate; and b) a coating layer provided on a least one surface of the substrate, wherein the coating layer comprises 5-70 wt % of layered double hydroxide dispersed throughout a water-soluble polymeric matrix.

31. The film of claim 30, wherein the layered double hydroxide is randomly dispersed throughout the water-soluble polymeric matrix.

32. The film of claim 30 or 31, wherein the coating layer comprises 10-60 wt % of layered double hydroxide.

33. The film of any one of claim 30, 31 or 32, wherein the weight ratio of water-soluble polymeric matrix (e.g. PVA) to LDH within the coating layer is 0.5:1 to 5:1.

34. The film of claim 33, wherein the weight ratio of water-soluble polymeric matrix (e.g. PVA) to LDH within the coating layer is 0.75:1 to 4.5:1.

35. The film of any one of claims 30 to 34, wherein the layered double hydroxide is as defined in any of claims 4 to 18.

36. The film of any one of claims 30 to 35, wherein the water-soluble polymeric matrix comprises a water-soluble polymer as defined in claim 3.

37. The film of any one of claims 30 to 36, wherein the substrate is as defined in claim 2.

38. The film of any one of claims 30 to 37, wherein the coating layer comprises: a) 10-60 wt % of layered double hydroxide; b) 40-90 wt % of water-soluble polymeric matrix; and c) 0-2 wt % of water.

39. The film of any one of claims 30 to 38, wherein the coating layer has a thickness of 0.1-10 m (e.g. 1-10 m).

40. The film of any one of claims 30 to 39, wherein the substrate is a first substrate, and the film comprises a second substrate disposed on top of the coating layer, such that the coating layer is located between the first and second substrates.

41. The film of claim 40, wherein the film comprises a layer of adhesive provided between the coating layer and the second substrate.

42. The film of claim 41, wherein the adhesive is as defined in claim 28.

43. Use of a film as claimed in any one of claims 30 to 42 in packaging.

44. The use of claim 43, wherein the packaging is food packaging.

Description

EXAMPLES

[0117] The present invention will now be described, for the purpose of illustration only, with reference to the accompanying figures, in which:

[0118] FIG. 1 shows X-ray powder crystallography for the LDHs of Example 1.

[0119] FIG. 2 shows an SEM image of the LDHs of Example 1.

[0120] FIG. 3 shows a schematic flow diagram outlining the coating process.

[0121] FIG. 4 shows SEM images of coated films having (a) 2%, (b) 3.3% and (c) 5% loading of LDHs in the coating formulation (Table 1 coating mixtures).

[0122] FIG. 5 shows cross-sectional SEM images of coated films having (a) 2%, (b) 3.3% and (c) 5% loading of LDHs in the coating formulation (Table 1 coating mixtures).

[0123] FIG. 6 shows thickness measurements of various coated and uncoated films (Table 1 coating mixtures).

[0124] FIG. 7 shows X-ray powder crystallography of various coated and uncoated films, as well as that of the LDHs themselves (Table 1 coating mixtures).

[0125] FIG. 8 shows oxygen transmission rate values (OTR) of various coated and uncoated films (Table 1 coating mixtures).

[0126] FIG. 9 shows water vapour transmission rate values (WVTR) of various coated and uncoated films (Table 1 coating mixtures).

[0127] FIG. 10 shows the total transmittance values of various coated and uncoated films (Table 1 coating mixtures).

[0128] FIG. 11 shows the haze values of various coated and uncoated films (Table 1 coating mixtures).

[0129] FIG. 12 shows the clarity values of various coated and uncoated films (Table 1 coating mixtures).

[0130] FIG. 13 shows SEM images of various coated films (Table 2 coating mixtures).

[0131] FIG. 14 shows cross-sectional SEM images of coated films at 5% LDH loading (Table 2 coating mixtures).

[0132] FIG. 15 shows oxygen transmission rate values (OTR) of various coated and uncoated films (Table 2 coating mixtures).

[0133] FIG. 16 shows TEM image (for 100 nm LDH) and SEM images of the LDHs and commercial clays used in Example 5.

[0134] FIG. 17 shows oxygen transmission rate values (OTR) of the various coated and uncoated films of Example 5.

[0135] FIG. 18 shows the effect of flex testing on the oxygen transmission rate values (OTR) of the various coated and uncoated films of Example 5.

[0136] FIG. 19 shows the thickness of the various coated and uncoated films of Example 5.

[0137] FIG. 20 shows the total transmittance values of the various coated and uncoated films of Example 5.

[0138] FIG. 21 shows the haze values of the various coated and uncoated films of Example 5.

[0139] FIG. 22 shows the clarity values of the various coated and uncoated films of Example 5.

MATERIALS AND METHODS

Powder X-Ray Diffraction (PXRD)

[0140] X-ray diffraction (XRD) patterns were recorded on a PANalytical XPert Pro instrument in reflection mode with Cu Ka radiation. The accelerating voltage was set at 40 kV with 40 mA current (=1.542) at 0.01s.sup.1 from 1 to 70 with a slit size of degree.

Scanning Electron Microscopy (SEM)

[0141] Scanning electron microscopy (SEM) analyses were performed on a JEOL JSM 6100 scanning microscope with an accelerating voltage of 20 kV. Powder samples are spread and film samples are mounted on carbon tape adhered to an SEM stage. For cross-sectional SEM, film samples are cut by a sharp blade and mounted on carbon tape adhered to 90o sample holder. Before observation, the samples are sputter coated with a thick Platinum layer to prevent charging and to improve the image quality.

Transmission Electron Microscopy (TEM)

[0142] Transmission electron microscopy (TEM) was conducted at the Research Complex at Harwell, Oxfordshire on Jeol JEM-2100 TEM equipped with LaB6 filament at an accelerating voltage of 200 kV. Prior to analysis, samples were diluted with deionised water and sonicated in deionised water for 15 minutes. A few droplets of the resulting suspension were left to dry on a copper grid covered with a carbon film (300 mesh, Agar scientific).

Oxygen Transmission Rate Testing

[0143] Films and coated substrates are tested for oxygen transmission rate using an oxygen permeation analyser (Systech Illinois Inc., Oxygen Permeation Analyser 8001) at 23 C. and 0% RH. The oxygen transmission rate (OTR) is recorded after a steady state permeation is reached and reported in units of cc/m.sup.2.Math.day.Math.atm.

Water Vapour Transmission Rate Testing

[0144] Films and coated substrates are tested for water vapour transmission rate using a water vapour permeation analyser (Systech Illinois Inc., Water Vapour Permeation Analyser 7000) at 38 C. and 90% RH. The water vapour transmission rate (OTR) is recorded after a steady state permeation is reached and reported in units of cc/m.sup.2.Math.day.Math.atm.

Thickness Measurements

[0145] All thickness measurements are tested by using a thickness tester (Thwing-Albert Instrument Company, ProGage Thickness Tester). Average of ten measurements is reported in units of micron.

Optical Properties Measurements

[0146] Total transmittance, haze, and clarity of films are measured by using a haze meter (The haze-gard I, BYK-Gardner GmbH Inc). Average of ten measurements is reported in units of percent.

Flex Durability Measurements

[0147] Flex durability measurement of flexible films was adapted from ASTM F392-93, using Gelbo flex tester, IDM Instruments, at SCG Packaging, Thailand. Film samples were cut to a size of 200 mm (width)280 mm (long). The sample was then clamped tightly to the stationary mandrel and the moving mandrel of the instrument. Flexing was done at room temperature with a twisting motion, repeatedly twisting and crushing the film for a certain cycle. After flexing, OTR was performed to observed the change of OTR values.

Example 1

Synthesis of LDHs

[0148] An aqueous solution (100 mL) of 0.40 M Mg(NO.sub.3).sub.2.6H.sub.2O, 0.10 M of Al(NO.sub.3).sub.3.9H.sub.2O, and 0.80 M urea was prepared. The mixed solution were transferred to a Teflon-lined autoclave and heated in an oven at the 100 C. for 24 hours. After the reactions were cooled to room temperature, the precipitate products were washed several times with deionised water by filtration and finally placed in a vacuum oven overnight. The LDHs product shows typical XRD patterns (FIG. 1) of well-crystallised LDHs with an average size of 3-4 m and aspect ratio at about 80. FIG. 2 shows an SEM image of the prepared LDH.

Example 2

Preparation of Coating Mixture

[0149] An aqueous barrier coating solution is prepared as follows. An aqueous polyvinyl alcohol (PVA) solution of defined solid content is freshly prepared; the required amount of polymer is weighed, added to the required amount of pre-heated deionised water under vigorous stirring. The mixture is stirred and heated at 90 C. After complete dissolution of polymer, the PVA solution is kept at 60 C. under stirring. PVA can be chosen from different molecular weights and degree of hydrolysis (POVAL 28-99, MW 145,000 g/mol, 99-99.8% hydrolysis, Kuraray; Mowiol 4-88, MW 31,000, 88% hydrolysis, Sigma-Aldrich) and used as received.

[0150] LDH is firstly added to the deionised water to prepare a 10% of filler suspension. The suspension is stirred for 10 minutes and sonicated for 20-30 minutes before usage. The LDH and PVA solutions of different proportions are vigorously mixed to obtain the coatings with weight ratios of PVA/LDHs, and the obtained coatings are stirred at 60 C. for 1 hour. The solid part of the coating formulations are controlled at 10%.

[0151] Several solutions (Table 1) are formulated using 3-4 m size LDHs. Alternatively, the coating mixtures can be prepared with LDHs having size of 0.5 or 7 m and formulated similarly to above procedure (Table 2).

TABLE-US-00001 TABLE 1 Coating formulation of 3-4 m size LDHs with high molecular weight PVA Part (%) % LDHs PVA/LDHs PVA* LDHs.sup. loading ratio 80 20 2 4 67 33 3.3 2 50 50 5 1 *M.sub.w 145,000, 99-99.8% hydrolysis .sup.3-4 m in size, MgAlCO.sub.3-LDHs (Mg/Al = 4)

TABLE-US-00002 TABLE 2 Coating formulation of low molecular weight PVA with various sizes of LDHs (0.5, 3 and 7 m) Part (%) % LDHs PVA/LDHs PVA* LDHs.sup. loading ratio 80 20 2 4 50 50 5 1 *M.sub.w 31,000, 88% hydrolysis .sup.0.5, 3 and 7 m in size, MgAlCO.sub.3-LDHs (Mg/Al = 4)

Example 3

Preparation of Coated Films

[0152] FIG. 3 provides a schematic flow diagram of the coating process.

[0153] The coating solution is applied to corona-treated polyethylene terephthalate substrate (SARAFIL Transparent TF101, Polyplex Thailand), which is supplied by SCG Packaging PLC, by a Mayer rod coater and an automatic coater (K101, RK Print Coat Instruments Ltd.). The substrate film is secured in the middle of the coating area and the rod is placed on the upper top of the film. Approximately 1-2 mL of the prepared coating solution is applied in the gap between the rod and the substrate along the width of the substrate. A Mayer rod moves down the substrate with a controlled speed and the coated film is obtained. All coated samples are dried naturally at room temperature. Coating thickness is controlled by selecting of the rod number and speed of the coated.

Example 4

Characterisation of Coated and Uncoated Films

[0154] Films Coated with Table 1 Coating Mixtures

[0155] FIG. 4 shows SEM images of coated films having (a) 2%, (b) 3.3% and (c) 5% loading of LDHs in the coating formulation.

[0156] FIG. 5 shows cross-sectional SEM images of coated films having (a) 2%, (b) 3.3% and (c) 5% loading of LDHs in the coating formulation.

[0157] FIG. 6 shows thickness measurements of various coated and uncoated films.

[0158] FIG. 7 shows X-ray powder crystallography of various coated and uncoated films, as well as that of the LDHs themselves.

[0159] FIG. 8 shows oxygen transmission rate values (OTR) of various coated and uncoated films.

[0160] FIG. 9 shows water vapour transmission rate values (WVTR) of various coated and uncoated films.

[0161] FIG. 10 shows the total transmittance values of various coated and uncoated films.

[0162] FIG. 11 shows the haze values of various coated and uncoated films.

[0163] FIG. 12 shows the clarity values of various coated and uncoated films.

Films Coated with Table 2 Coating Mixtures

[0164] FIG. 13 shows SEM images of various coated films.

[0165] FIG. 14 shows cross-sectional SEM images of coated films at 5% LDH loading.

[0166] FIG. 15 shows oxygen transmission rate values (OTR) of various coated and uncoated films.

Example 5

Comparison of LDH-Containing Coatings and Commercial Clay-Containing Coatings

Preparation of Coated Substrates

[0167] LDH were prepared according to the procedure outlined in Example 1.

[0168] Aqueous barrier coating solutions were prepared as follows: an aqueous polyviny alcohol (PVA, Mowiol 4-88, M.sub.w 31,000, 88% hydrolysis, Sigma-Aldrich) solution of defined solid content is freshly prepared; the required amount of polymer is weighed, added to the required amount of pre-heated deionised water under vigorous stirring. The mixture then is stirred and heated at 90 C. After complete dissolution of the polymer, the PVA solution is cooled down and kept at room temperature. Suspensions of LDHs and clays were prepared at 10 wt %. in water for 10 minutes and then sonicated for 20-30 minutes before being used. FIG. 16 provides SEM images of the different LDHs and clays used in this study. The LDH/clay and PVA solutions of were vigorously mixed in different proportions to obtain coating solutions with weight ratios of PVA/LDHs or PVA/clay at 80/20 and 50/50. The total solids content of the coating solution was controlled at 10%.

[0169] The coating solution was then applied to corona-treated polyethylene terephthalate substrate (SARAFIL Transparent TF101, Polyplex Thailand), which is supplied by SCG Packaging PLC, by a Mayer rod coater and an automatic coater (K101, RK Print Coat Instruments Ltd.). The substrate film is secured in the middle of the coating area and the rod is placed on the upper top of the film. Approximately 1-2 mL of the prepared coating solution is applied in the gap between the rod and the substrate along the width of the substrate. A Mayer rod moves down the substrate with a controlled speed and the coated film is obtained. All coated samples are dried naturally at room temperature. Coating thickness is controlled by using a yellow rod and fixing speed of the coater at #7 for all coatings.

Oxygen Transmission Rate (OTR) Studies

[0170] FIG. 17 presents the OTR properties of the various LDH-containing and clay-containing coated films.

[0171] Clay particles are strongly aggregated. In general, a dispersing agent is required to obtain full dispersion of clay in water. To avoid possible unwanted side-effects, such an additive was not included in this study. FIG. 17 shows that the clay-containing samples have poor OTR properties, indicating that using such commercial clays can destroy the barrier performance of the coating layer (possibly due to the poor dispersion of the clay in the PVA solution). On the other hand, FIG. 17 shows that the LDH-containing samples generally gave better OTR results than the clay-containing samples. Particularly good OTR properties were observed for LDHs having a larger platelet size. Without wishing to be bound by theory, it is believed that LDHs of smaller platelet size tend to be aggregated and might therefore be insufficiently covered by PVA, thereby giving rise to a more open coating structure through which oxygen can pass.

[0172] Generally, inorganic-coated films (i.e. oxide-coated, clay-coated) have poor flex resistance. OTR measurement was employed to observe the change in barrier property of the coated films before and after 50 and 200 flex cycles. FIG. 18 shows the effect of flex testing on the OTR properties of the various LDH-containing and clay-containing coated films.

[0173] The results presented in FIG. 18 show that LDH-containing samples of larger platelet size (e.g. 7 m) exhibited excellent flex durability, demonstrating good barrier properties even after 200 flexes.

Optical Studies

[0174] FIG. 19 shows a comparison of the thickness of the various LDH-containing and clay-containing coated films. FIGS. 20, 21 and 22 provide a comparison of the light transmittance, haze and clarity of the various films.

[0175] The results presented in FIGS. 19-22 show that all of the coated films exhibit similar transparency and thickness. Haze and clarity properties tend to worsen when more LDH/clay is incorporated into the coating layer.

[0176] While specific embodiments of the invention have been described herein for the purpose of reference and illustration, various modifications will be apparent to a person skilled in the art without departing from the scope of the invention as defined by the appended claims.