PROCESS FOR PREPARATION OF OXYGEN BARRIER FILM

Abstract

The invention relates to a process for the preparation of an oxygen barrier film having an oxygen barrier layer and a treated substrate, comprising the steps of: a) subjecting a polyolefin substrate to a surface treatment to obtain the treated substrate wherein the surface under the parts of the curves corresponding to the C—O, C—N and C═O groups is at least 10% based on the total surface under the curve as obtained by X-ray photoelectron spectroscopy (XPS), b) applying an aqueous or hydroalcoholic coating composition comprising 0.1 to 25 wt % of polyvinyl alcohol, 0.1 to 30 wt % of a metal alkoxide and optional silicate minerals on the treated substrate obtained by step a) and c) drying the coating composition to obtain the oxygen barrier layer resulting in the oxygen barrier film, wherein the amount of the optional silicate minerals in the coating composition is less than 10 parts by weight per 100 parts by weight of the polyvinyl alcohol in the coating composition.

Claims

1. A process for the preparation of an oxygen barrier film having an oxygen barrier layer and a treated substrate, the process comprising the steps of: a) subjecting a polyolefin substrate to a surface treatment to obtain a treated substrate wherein the surface under the parts of the curves corresponding to the C—O, C—N and C═O groups is at least 10% based on the total surface under the curve as obtained by X-ray photoelectron spectroscopy (XPS), b) applying an aqueous or hydroalcoholic coating composition comprising 0.1 to 25 wt % of polyvinyl alcohol, 0.1 to 30 wt % of a metal alkoxide and optional silicate minerals on the treated substrate obtained by step a) and c) drying the coating composition to obtain the oxygen barrier layer resulting in the oxygen barrier film, wherein the amount of the optional silicate minerals in the coating composition is less than 10 parts by weight per 100 parts by weight of the polyvinyl alcohol in the coating composition.

2. The process according to claim 1, wherein the polyolefin substrate is a polypropylene substrate or a polyethylene substrate.

3. The process according to claim 1, wherein step a) is a plasma treatment, a corona treatment or a flame treatment.

4. The process according to claim 1, wherein step a) is a plasma treatment performed at a pressure of 10 to 2000 Pa, in oxygen, nitrogen, hydrogen, ammonia, a mixture of oxygen and nitrogen, or air.

5. The process according to claim 1, wherein step a) is a plasma treatment performed at a power output of 10 to 80 W, for a period of 5 to 500 seconds.

6. The process according to claim 1, wherein the surface under the parts of the curves corresponding to the C—O, C—N and C═O groups is at least 15% based on the total surface under the curve as obtained by X-ray photoelectron spectroscopy (XPS) of the treated surface obtained by step a).

7. The process according to claim 1, wherein the metal alkoxide is a compound of the formula MRn, in which M is a metal atom, n is the valency of M and the groups R taken n times each independently represent an alkyl or alkyloxy radical having from 1 to 4 carbon atoms, provided that at least two of the groups R are alkyloxy radicals.

8. The process according to claim 1, wherein the polyolefin substrate has a thickness of 7 to 500 μm.

9. The process according to claim 1, wherein the oxygen barrier layer has a thickness of 0.2 to 10.0 μm.

10. The oxygen barrier film obtained or obtainable by the process according to claim 1.

11. The oxygen barrier film according to claim 10, having an oxygen transmission rate of at most 100 cc/(m.sup.2.Math.24 h), at a relative humidity of 0% and a pressure of 1.013×10.sup.5 Pa.

12. The oxygen barrier film according to claim 10, having an oxygen transmission rate of at most 250 cc/(m.sup.2.Math.24 h), at a relative humidity of 0%% and a pressure of 1.013×10.sup.5 Pa, after being stretched for 20% in one direction at a speed of 10 mm/min at 150 to 170° C.

13. An oxygen barrier film comprising a polyolefin substrate and an oxygen barrier layer provided on the polyolefin substrate, wherein the oxygen barrier film has an oxygen transmission rate OTR1 measured at a relative humidity of 0% and a pressure of 1.013×10.sup.5 Pa, the oxygen barrier film has an oxygen transmission rate OTR2 measured at a relative humidity of 0%% and a pressure of 1.013×10.sup.5 Pa after being stretched for 20% in one direction at a speed of 10 mm/min at 150° C., wherein OTR1 is at most 100 cc/(m.sup.2.Math.24 h), and OTR2-OTR1 is at most 200 cc/(m.sup.2.Math.24 h).

14. The oxygen barrier film according to claim 13, wherein OTR2 is at most 250 cc/(m.sup.2.Math.24 h).

15. The oxygen barrier film according to claim 13, wherein the oxygen barrier layer is made from an aqueous or hydroalcoholic coating composition comprising 0.1 to 25 wt % of polyvinyl alcohol, 0.1 to 30 wt % of a metal alkoxide and optional silicate minerals.

16. An article comprising the oxygen barrier film according to claim 10.

17. The article according to claim 16, wherein the article is a packaging of food or a pharmaceutical products.

18. A process for the preparation of a treated substrate, comprising the step of: a) subjecting a polyolefin substrate to a surface treatment to obtain the treated substrate wherein the surface under the parts of the curves corresponding to the C—O, C—N and C═O groups is at least 10% based on the total surface under the curve as obtained by X-ray photoelectron spectroscopy (XPS).

19. The process according to claim 7, wherein M is silicon (Si), aluminium (Al), zirconium (Zr) or titanium (Ti).

20. The process according to claim 7, wherein the metal alkoxide is tetraethoxysilane (TEOS).

Description

EXPERIMENTS

Ex 2-Ex 5

[0083] Preparation of Oxygen Barrier Film

[0084] Polypropylene substrates having a thickness of 25 μm made from polypropylene homopolymer 525P having MFR of 3.0 dg/min (ISO1133-1:2011, 230° C./2.16 kg) were subjected to vacuum plasma treatment at a pressure of 30 Pa using air at conditions indicated in table 1. The treated substrates were stored in aluminum foil just after the treatments to avoid oxygen. XPS was performed on the treated substrates.

[0085] Within 24 hours from the plasma treatment, a coating composition Oxyflav commercially available from High Materials Innovation srl (Parma, Italy) was poured on the surface of the treated substrate and coated over the total surface by means of a roll. The coating composition was subsequently dried. The thickness of the coating was 5 μm. Oxygen transmission rates were measured.

[0086] Stretching

[0087] The coated films were subjected to stretching tests. The coated films were stretched in one direction for 22% (±2%) at 10 mm/min speed while heating at 150-170° C. and the oxygen transmission rates were measured. Subsequently, these films were stretched in the other direction under the same conditions and the oxygen transmission rates were measured.

[0088] XPS

[0089] X-ray Photoelectron Spectroscopy (XPS) was performed in an UHV (Ultra High Vacuum) system (base pressure 2×10.sup.−10 mbar). The apparatus is equipped with a manipulator with five freedom degrees, receiving a XL25 type sample holder (from ThermoFisher Scientific, VG). Samples with typical dimensions of 20 mm×15 mm were extracted for each material type by cutting the original PP foils in two distant regions (far from the external borders), the first one being almost central, the second one in a peripheral region. Each sample was fixed by carbon tape on a silicon wafer, so that the surface planarity was granted getting rid of the high flexibility typical of polymers. Sample surface was grounded by a metal clip. No surface treatments have been performed before insertion in vacuum, which occurs by a fast entry load lock system (base pressure 7×10.sup.−5 mbar): due to the typical high outgassing rate of polymers, each sample remained one night in fast entry.

[0090] XPS was performed by using a Mg photon source (1253.6 eV), a XR3E2 non monochromated Mg/Mg double anode source (from VG, now ThermoFisher Scientific), at a typical power of 300 W (20 mA and 15 kV). The electron energy analyzer is a hemispherical VSW HA100 with PSP electronic control unit, working in constant Pass Energy (PE) mode with a maximum energy resolution of 0.86 eV. The electron acceptance is normal to the surface, leading to a maximum sampled depth of about 8 nm, while the angle between photons and emitted electrons is 54.7°. No charge compensation was applied. Survey spectra were acquired using a PE of 50 eV, while all core levels spectra were at a PE of 20 eV. Quantitative analysis (leading to the evaluation of the atomic percentage) is based on the elemental sensitivity factor standard approach, properly taking into account the analyzer electron transmission coefficient. Binding energy is typically referred to the Au4f7/2 core level at 84.00 eV, as well as in our case to the C1s core level at 285 eV (C—C species). The peak lineshape was deconvoluted using a Voigt profile, after subtracting the background by means of a Shirley function. The typical uncertainty for the peak energy position is ±0.05 eV, whereas for the full width at half maximum (FWHM) is about ±5%. The maximum error for the atomic percentage evaluation is about ±0.1.

[0091] The surface under the parts of the curves corresponding to the C—O and C═O groups based on the total surface under the curve (sum of the C—O and C═O functional groups) is indicated as % O in Table 1. No C—N functional group was detected in these examples. The results are the average of 2 results from 2 areas.

[0092] Oxygen Transmission Rate

[0093] Oxygen transmission rate (OTR) were measured at relative humidity (R.H.) of 0% and 50% at 23° C. and 1.013×10.sup.5 Pa. At least two films were measured for each sample, and obtained transmissions are reported as the average of both measurements.

[0094] The results of the XPS measurements and the OTR measurements are shown in Table 1.

TABLE-US-00001 TABLE 1 OTR (cc/m.sup.2*24 h) % C % O Before 1.sup.st stretch 2.sup.nd stretch Name Power Time (±0.5) (±0.5) 0% RH 50% RH 0% RH 50% RH 0% RH 50% RH CEx 1 — — 100% 2200 3500 2210 2379 2210 2379 Ex 2 78 W 60 s 83.15 16.85 14 14 76 174 216 219 Ex 3 50 W 20 s 83.91 16.09 68 70 134 152 314 308 Ex 4 50 W 10 s 86.80 13.20 40 57 Not measured Ex 5 32 W 10 s 87.80 12.2 40 38 229 225 1781 1848

[0095] The difference between the OTR at 0% RH after first stretching (OTR2) and the OTR at 0% RH before stretching (OTR1) is:

[0096] Ex 2: 62 cc/(m.sup.2.Math.24 h)

[0097] Ex 3: 66 cc/(m.sup.2.Math.24 h)

[0098] Ex 5: 189 cc/(m.sup.2.Math.24 h)

[0099] CEx 1 is an experiment on the same PP substrate but on which no plasma treatment was performed and no coating composition was applied. The PP substrate has a very high oxygen transmission rate (low oxygen barrier property).

[0100] It can be seen that the oxygen transmission rate has dramatically decreased by preparing the oxygen barrier film by the process according to the invention. Further, even after stretching, the good oxygen permeability performance is largely maintained. This good oxygen permeability performance is still maintained after being subjected to a relatively high temperature. Same trend can be observed at a relative humidity of 0% and 50%.

[0101] With the increase in power and time of the plasma treatment, the amount of the functional groups increases. This in turn retains a higher oxygen barrier property being retained after stretching.

CEx 6-Ex 9

[0102] LLDPE

[0103] In experiments 7 and 9, experiment 2 was repeated using an LLDPE substrate having a thickness of 25 μm or 70 μm. The LLDPE was low linear density polyethylene 6118NE having MFR of 0.9 dg/min (ISO1133-1:2011, 190° C./2.16 kg). CEx 6 and CEx 8 correspond to the LLDPE substrates on which no plasma treatment was performed and no coating composition was applied.

[0104] OTR measurements were performed on the samples which have not been stretched. The measurements were performed at relative humidity (R.N.) of 0%, 25% and 50% at 23° C. and 1.013×10.sup.5 Pa. Results are shown in Table 2.

TABLE-US-00002 TABLE 2 OTR (cm.sup.3/m.sup.2*24 h) Name Thickness Power Time 0% RH 25% RH 50% RH CEx 6 25 μm — — 7700 — — Ex 7 25 μm 78 W 60 s 216 181 216 CEx 8 70 μm — — 2779 — — Ex 9 70 μm 78 W 60 s 63  77  75

[0105] It can be seen that the oxygen transmission rate has dramatically decreased by preparing the oxygen barrier film by the process according to the invention.