EP0951947A1

EP0951947A1 - Radiation-cured barrier coating and process for manufacturing same

Info

Publication number: EP0951947A1
Application number: EP98400720A
Authority: EP
Inventors: Rapahel Cohen; Eliahu Hakin; Itzhak Kenigsbert
Original assignee: GETRATEX SA
Current assignee: GETRATEX SA
Priority date: 1998-03-26
Filing date: 1998-03-26
Publication date: 1999-10-27

Abstract

The invention relates to a gas barrier coating, formed of radiation-cured, especially EB-cured or UV-cured, repeating units, such as acrylate-based repeating units containing pendant polar group(s) selected from the group consisting of hydroxy, carboxy and amino.

The invention also relates to the substrate thus coated and to a process for manufacturing same.

Description

BACKGROUND OF THE INVENTION

The invention relates to a new barrier coating, that is obtained by radiation curing, especially EB and UV curing, and to a process for manufacturing same.
Films are widely used in numerous industries, for specific end-uses requiring specific properties. It is however very often that one film, although having good mechanical properties, lacks specific barrier properties.
Laminates, formed of a plurality of films, have thus been proposed. One example is a 5-layer film, such as PET/binder/EVOH/binder/PET, where PET is used for its mechanical strength while EVOH is used for its oxygen barrier properties. Other films have been proposed, with less or even more layers. Multilayer films are however costly to manufacture.
Another example of gas barrier film is a metal foil, such as the aluminum foil. However, although widely used, this foil suffers from the drawback of being not transparent, thus preventing customers from seeing the packaged goods.
JP-A-08294989 to Sumitomo Bakelite, discloses a process involving coating a radiation curable acrylate resin for protection purposes onto an alumine/silica gas-barrier layer.
EB curing has been used so far in numerous fields. For example, JP-A-7102089, JP-A-61243 and JP-A-6216047 disclose that EB curing will improve the gas barrier properties of an already polymerized layer, especially PVA.
Numerous prior art documents disclose polymerization of repeating units, where all functionalities are caused to react. EP-A-69635 (to Kureha) discloses polycarboxylic acid and polysacharride (hydroxyl groups) that create the gas-barrier film. EP-A-571074 (to Morton) discloses a polymeric article based on EVOH and carboxylic acid groups containing polyolefin. EP-A-567327, US-A-5567768, US-A-5545689 (to Rohm & Haas) disclose melt processable polymeric blends containing vinyl alcohol and alkyl polymethacrylate copolymer. US-A-5221719 (to Eastman Kodak) discloses a polyester gas barrier obtained by blending dicarboxylic acid and aliphatic glycol. WO-A-9215455 (to Mitsubishi Chem.) and US-A-4959446 (to Eastman Kodak) disclose a polyamide gas barrier obtained by blending dicarboxylic acid and diamine. US-A-5096738 and US-A-5215822 (to Energy Sciences) disclose a process where siloxane and carboxylic monomers in a solvent are solubilized and hydrolized, the solvent is evaporated off and the Si-O-Si bonds are grafted onto the organic polymer by EB curing.
The prior art however is silent on the use of EB- or UV-technique for in situ and solvent-less curing of coating to obtain a gas barrier coating with polar groups.

SUMMARY OF THE INVENTION

The invention thus provides a barrier coating, formed of radiation-cured repeating units, containing pendant polar group(s).
According to one embodiment, the repeating units are monomers.
According to another embodiment, the repeating units are acrylate-based repeating units.
According to yet another embodiment, the polar groups are selected from the group consisting of hydroxy, carboxy and amino.
According to yet another embodiment, the coating comprises straight polymer chains.
According to yet another embodiment, the radiation-cured coating is EB-cured.
According to yet another embodiment, the radiation-cured coating is UV-cured.
According to yet another embodiment, the coating exhibits oxygen barrier and/or moisture barrier and/or aroma barrier and/or methyl bromide barrier.
According to yet another embodiment, the coating is transparent.
The invention also provides a substrate coated with a coating of the invention.
According to one embodiment, the substrate is polymeric.
According to another embodiment, the coating is sandwiched between two substrate layers.
The invention finally provides a process for manufacturing the coating of the invention, comprising the steps of:
(i) applying a composition comprising the repeating units containing pendant polar group(s) onto a substrate, and
(ii) radiation-curing same to obtain the said coating.
According to one embodiment, the radiation-curing is EB-curing.
According to another embodiment, the radiation-curing is UV-curing.
According to yet another embodiment, the process is solvent-less.
According to yet another embodiment, the substrate is polymeric and has been Corona treated prior to step (i).

DETAILED DESCRIPTION OF THE INVENTION

The instant coatings impart improved barrier properties to the substrates onto which they are applied and radiation-cured. The barrier properties are preferably gas-barrier properties, and may be oxygen barrier and/or moisture barrier and/or aroma barrier and/or methyl bromide barrier.
The thickness of the coating can be comprised within broad limits, such as between 2 and 50 µm.

REPEATING UNITS

The instant repeating units are either oligomers or monomers; small units are however preferred. Any repeating unit comprising radiation-curable bonds is appropriate; the units comprising the classical C=C double bond are preferred.

Examples of oligomers are:

epoxy acrylates, urethane acrylates, polyester acrylates, silicone acrylates and silane acrylates.

Examples of monomers are:

Monofunctional, difunctional and multifunctional acrylates, such as phosphoric acid ester acrylates, hydroxy acrylates, carboxy acrylates, amine acrylates, acrylic acid and acrylamide.
Mixtures are also available.
These oligomers and monomers are either commercially available or can be prepared by routine procedures.
The term "acrylate" (or "acrylic"), as used in the invention covers both the methacrylate (or "methacrylic"), form as well as the acrylate ("acrylic") form, stricto sensu.
Preferred repeating units are acrylate-based.
The repeating units will contain pendant polar group(s). In contradistinction with the above-cited prior art, these pendant polar groups do not react during the polymerization reaction and will thus remain on the final barrier polymer. These groups will provide interchain hydrogen bonding and thus will impart further tightness to the polymeric coating. These polar groups can be selected from the group consisting of hydroxy, carboxy and amino. Preferably, where the repeating units are monofunctional (with respect to these polar groups).
Preferably, the cured coating will comprise a minimum of branching, so as to further enhance the tightness of the polymeric coating. Thus, straight polymers are the preferred coatings.
The resulting coating is in most cases transparent, and can be either soft or rigid, depending on the substrate and the final intended use.
Various additives can be added to the coating (at the stage where the repeating units are available as a composition). However, plasticizers should be avoided, while surfactant (or otherwise denominated wetting agent) are welcome. Silicone-based surfactants are preferred additives, especially silicone-acrylate based surfactant.

SUBSTRATE.

The substrate can be any substrate that is compatible with the radiation-cured coating. Preferred substrates are polymeric and include PE, PP, PET, PVC, PA, etc. Corona treatment prior to coating composition applying is preferred. The substrate can also be paper, fabrics, non-woven, and the like. The thickness of the substrate can be comprised within broad limits, such as between 10 and 2000 µm.
The thus coated substrates can be easily laminated on other substrates by using an appropriate adhesive, such as a polyurethane adhesive. According to one embodiment, the radiation-cured coating is sandwiched between two substrates (that can be identical or different).

PROCESS FOR MANUFACTURING THE COATING

Different alternatives can be used in the manufacturing process, such as one step on-line production or more than one step (for each stage in the process, a different step would be present).
The following elementary steps are typically used :

(1) Unwind station:

the film forming substrate (plastic, paper, etc.) roll is mounted and starts to unroll towards the :

(2) Corona station:

the film forming substrate, if necessary, is submitted to a classical Corona discharge. Said treatment improves the adhesion properties of the film. The thus-treated film forming substrate is then processed to:

(3) a coating station:

the composite comprising the functional radiation curable coating is applied on the surface of the film. The film forming substrate is then processed to:

(4) a radiation-curing station:

the coating film is irradiated by EB, UV, etc.. Said curing station may be coupled to a thermal station or it can be a hybrid curing station in the cases of a thermal/EB or thermal/UV or UV/EB hybrid process. The functional coating is thus cured, and the film coated is processed to:

(5) a winding station:

the coated film, optionally with a peeling film, is wound into rolls. The rolls are the stored before final use.
Below are described in more details the steps indicated above; the steps that are not in the following are well-known to the skilled artisan.
In a most preferred embodiment, the process of the invention is a solvent-less process.

Corona treatment

Corona treatment is a well-known treatment and is used to enhance bonding. In fact, plastics in general have chemically inert surfaces with low surface tension, which causes them to be less or non-receptive to bonding with coatings and adhesives. Polyethylene and polypropylene have the lowest surface tensions of the various plastics, and are the two materials most often subjected to corona treatment so as to improve their bonding characteristics. Corona treatment is however not limited to those two materials and can be used on any plastics. The corona treatment is carried out in a classical manner using a classical Corona apparatus. The Corona treatment may not be necessary when the film is for e.g. paper or fabrics.

Coating techniques used in the coating steps

Different types of coating techniques can be used for different applications, depending upon the thickness of the coating, the viscosity of the coating solution, etc. Below are given, by way of illustration, three coating techniques.
The first coating technique is reverse roll coating. Said technique is the most versatile and accurate coating technique for many applications. It can be used in a large range of coating solution viscosities. The main operating feature of reverse roll coaters, is the application of the coating by a roll rotating in the opposite direction to the substrate movement. The coating formulation is premetered on reverse roll coaters, and the deposit thickness is substantially constant regardless of the substrate. The final coating thickness is controlled by the speed ratio between the applicator and backing rolls, the backing roll running substantially at the substrate speed.
The second coating technique is gravure coating. This technique utilizes a driven engraved cylinder, an impression roll, a web transport roll, a doctor blade, a pan and/or applicator to apply a liquid formulation onto a web. This technique operates on the principle of pressing the web onto the engraved cylinder, removing liquid from the engraved cells or lines by capillary action and/or vacuum. This coating method is generally considered as well suited for long run jobs which are repeated often and require a very thin wet film thickness. The accuracy of a gravure coater is generally not speed sensitive (as long as each element is properly adjusted).
The third coating technique is blade coating. The general principle in blade coating is that the applied coating is levelled with a thin steel blade of a 0,2-0,5 mm thickness, for example. By varying the pressure of the blade against the film, the final coat weight is adjusted.
Other coating techniques may of course be used.

Curing process

The radiation-curing techniques utilize an emitting source, which generates the proper actinic radiation. The following describes some specific techniques which may be used.

UV curing process

UV-radiation encompasses radiation of the wavelength 200-400 nm. UV-curable coating formulations consist typically of a blend of reactive monomers or oligomers capable of free radical or cationic initiated polymerization. Photoinitiators (PI) are often used with UV-curable coating and they are the source of free radicals or cations produced upon irradiation. Many different chemical compounds can serve as PI. benzoin and its derivatives, aromatic carbonyls, halogenated chemicals and amines have found application in free radical mechanism, while aryliodonium salts and arylsulphonium salts have found application in cationic mechanism. Different PI may require different wavelengths and the radiation from the source and the PI sensitivity should be matched. The PI should absorb UV radiation in a range which is not absorbed by the monomer, oligomer or other additives. The most commonly used UV source, a medium pressure mercury lamp, emits over a wide range of wavelengths and is thus suitable for all UV applications.
The rate of curing reaction depends upon the number of free radicals or cations produced and thus upon the density of the UV reaction. Since most of the UV radiation is absorbed near the surface coating, a thicker layer usually requires extended irradiation time.
UV irradiation equipment is typically comprised of the following parts: radiation source, lamp housing and reflector, accessories, power supply and controls, shielding and safety equipment.

EB curing process

The electron beam (EB) curing process has been developed recently and is an expanding technology. Electron beams belong directly to ionizing beams having energies greater than 3 eV. Electrons are generated in a vacuum, accelerated over a potential difference, and are then emitted via a thin film, generally made of titanium, into the atmosphere. Then they usually interact with some material which is deliberately put into their path. There are a number of effects which occur while the electrons are slowed down due do their interactions with this material. Besides generating primary electrons, back scattered electrons are also generated. The unit of energy delivered by an electron processor is usually recorded in megarads ; typical values of energy are comprised between 0,5 and 20 Mrad. The dose is used to express the energy required to cure a particular coating and is experimentally determined. For the coating formulations to be used in the instant invention, a typical dose is comprised between 1,5 and 10 Mrad. Also, the voltage applied is usually comprised between 150 et 250 kV. The dose received by a given coating and the voltage applied may vary within the depth of the coating, depending on the accelerating voltage, coating thickness and coating density.
Typical EB-curing equipment is comprised of a vacuum chamber, an electron gun assembly, a window, a processing zone, self-shield, high voltage power supply and controls, shielding and safety equipment.

EB/UV, thermal/EB, thermal/UV, thermal/EB/UV hybrid processes

EB or UV hybrid processing is a combination of either UV curing or thermal drying processes or EB treatment, a manufacturing method that fully cures, among others, coatings applied to paper or film substrates. Such a hybrid process is obtained by combining two or more curing techniques such as EB, UV or thermal curing into a manufacturing process.
During EB/UV hybrid processing, low energy, low heat UV is used to cure just the surface of each application of the coating formulation. Additional coats of wet formulation may then be applied and UV treated on the semi-dry surface before the full and final curing is completed by EB.
Thermal coat formulation may be treated the same way as UV coat formulations to obtain a full cure, scratch resistance and improved adhesion to films. EB curing rapidly accelerates a chemical reaction in the coat formulation to produce a full curing.
Other radiation-curing processes may however be used to perform the curing of the radiation-curable coating.
Classical EB process, UV process and hybrid process are contemplated in the instant invention.
The invention will be further illustrated by the following examples, which should not be construed as limiting the scope of said invention.

EXAMPLES.

Oxygen barrier :

The oxygen permeability through the coated film was tested by the Ox-Tran 100A and 2/20 - Mocon Oxygen Transmission Rate System (ASTM D-3985-81). The samples were tested at dry conditions at room temperature.

Moisture barrier :

The D-1653-85 test method was used for moisture barrier effects of the coated films.

Example 1

The EB curable composition included acrylic acid (AA) (BASF) and the silicone acrylated base wetting agent Ebecryl 350 (U.C.B.) at the ratio of 100/1. The solution was coated on a 100 µm thick polyethylene (PE) film and cured by the EB at 175 kV and 6 Mrad. The coating thickness was 10 µm. The results are summarized in Table 1.

Example 2

The EB curable composition included the hydroxy ethyl acrylate (HEA) (BASF), pentaerythritol triacrylate (PETA) (Cray Valey) and the Ebecryl 350 at the ratio of 90/10/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 3

The EB curable composition included AA, PETA and Ebecryl 350 at the ratio of 90/10/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 4

The EB curable composition included AA, HEA and Ebecryl 350 at the ratio of 80/20/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 5

The EB curable composition included AA, HEA and Ebecryl 350 at the ratio of 50/50/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 6

The EB curable composition included AA, PETA, HEA and Ebecryl 350 at the ratio of 50/40/10/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 7

The EB curable composition included AA, PETA, HEA and Ebecryl 350 at the weight ratio of 20/70/10/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 8

The EB curable composition included β-carboxy ethyl acrylate (β-CEA) (U.C.B.), PETA and Ebecryl 350 at the weight ratio of 90/10/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 9

The EB curable composition included HEA, SiO2 and L-540 silicone wetting agent (U.C.C.) in the weight ratio of 70/30/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 10

The EB curable composition included HEA, Acrylamide (AAm), Triethanolamine (TEA) in order to increase the amount of hydroxy and amino functionalities, PETA and wetting agent DC 193 - U.C.C at a weight ratio of 60/10/10/15/1. It was coated and cured as described in Example 1. The results are summarized in Table 1.

Example 11

The UV curable composition included AA, Ebecryl 350 and Darocure 1173 (Ciba Geigy) as a photoinitiator, at a weight ratio of 100/1/5. The solution was coated on a 100µm thick PE film, cured twice by a UV lamp, 80W/cm at a speed of 20m/min. The coating thickness was 10µm. The results are summarized in Table 1.

Example 12

The UV curable composition included AA, PETA, Ebecryl 350 and Darocure 1173 (Ciba Geigy) as a photoinitiator at a weight ratio of 90/10/1/5. It was coated and cured as described in Example 11. The results are summarized in Table 1.

Example	Oxygen Permeability (ml/m2/24 hrs)	Moisture Permeability (ml/m2/24 hrs)	Flexibility
1	4,0	0,5	Flexible
2	38,0	1,4	Flexible
3	9,0	0,4	Semi Rigid
4	5,0	nd	Flexible
5	6,0	nd	Flexible
6	8,0	0,8	Flexible
7	19,0		Flexible
8	44,0	0,5	Flexible
9	22,0	1,3	Semi Rigid
10	19,0	nd	Flexible
11	3,0	1,1	Flexible
12	32,0	1,5	Flexible

The invention may be varied within broad limits by the skilled man and is not limited to the embodiments disclosed above.

Claims

A barrier coating, formed of radiation-cured repeating units, containing pendant polar group(s).
The coating of claim 1, where the repeating units are monomers.
The coating of claim 1 or 2, where the repeating units are acrylate-based repeating units.
The coating of any one of claim 1 to 3, where the polar groups are selected from the group consisting of hydroxy, carboxy and amino.
The coating of any one of claims 1 to 4, where the coating comprises straight polymer chains.
The coating of any one of claims 1 to 5, where the radiation-cured coating is EB-cured.
The coating of any one of claims 1 to 5, where the radiation-cured coating is UV-cured.
The coating of any one of claims 1 to 7, exhibiting oxygen barrier and/or moisture barrier and/or aroma barrier and/or methyl bromide barrier.
The coating of any one of claims 1 to 8, which is transparent.
A substrate coated with a coating of any one of claims 1 to 9.
The substrate of claim 10, which is polymeric.
The substrate of claim 10 or 11, where the coating is sandwiched between two substrate layers.
A process for manufacturing the coating of any one of claims 1 to 9, comprising the steps of:

(i) applying a composition comprising the repeating units containing pendant polar group(s) onto a substrate, and

(ii) radiation-curing same to obtain the said coating.
The process of claim 13, where the radiation-curing is EB-curing.
The process of claim 13, where the radiation-curing is UV-curing.
The process of any one of claims 13 to 15, which is solvent-less.
The process of any one of claims 13 to 16, where the substrate is polymeric and has been Corona treated prior to step (i).