BACKGROUND OF THE INVENTION
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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.
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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.
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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.
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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.
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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.
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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.
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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.
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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
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The invention thus provides a barrier coating, formed of
radiation-cured repeating units, containing pendant polar
group(s).
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According to one embodiment, the repeating units are
monomers.
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According to another embodiment, the repeating units are
acrylate-based repeating units.
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According to yet another embodiment, the polar groups
are selected from the group consisting of hydroxy, carboxy
and amino.
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According to yet another embodiment, the coating
comprises straight polymer chains.
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According to yet another embodiment, the radiation-cured
coating is EB-cured.
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According to yet another embodiment, the radiation-cured
coating is UV-cured.
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According to yet another embodiment, the coating
exhibits oxygen barrier and/or moisture barrier and/or aroma
barrier and/or methyl bromide barrier.
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According to yet another embodiment, the coating is
transparent.
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The invention also provides a substrate coated with a
coating of the invention.
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According to one embodiment, the substrate is polymeric.
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According to another embodiment, the coating is
sandwiched between two substrate layers.
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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.
-
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According to one embodiment, the radiation-curing is EB-curing.
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According to another embodiment, the radiation-curing is
UV-curing.
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According to yet another embodiment, the process is
solvent-less.
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According to yet another embodiment, the substrate is
polymeric and has been Corona treated prior to step (i).
DETAILED DESCRIPTION OF THE INVENTION
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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.
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The thickness of the coating can be comprised within
broad limits, such as between 2 and 50 µm.
REPEATING UNITS
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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:
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- epoxy acrylates, urethane acrylates, polyester acrylates,
silicone acrylates and silane acrylates.
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Examples of monomers are:
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- Monofunctional, difunctional and multifunctional acrylates,
such as phosphoric acid ester acrylates, hydroxy acrylates,
carboxy acrylates, amine acrylates, acrylic acid and
acrylamide.
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Mixtures are also available.
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These oligomers and monomers are either commercially
available or can be prepared by routine procedures.
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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.
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Preferred repeating units are acrylate-based.
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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).
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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.
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The resulting coating is in most cases transparent, and
can be either soft or rigid, depending on the substrate and
the final intended use.
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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.
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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.
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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
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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).
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The following elementary steps are typically used :
(1) Unwind station:
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- the film forming substrate (plastic, paper, etc.) roll is
mounted and starts to unroll towards the :
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(2) Corona station:
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- 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:
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(3) a coating station:
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- the composite comprising the functional radiation curable
coating is applied on the surface of the film. The film
forming substrate is then processed to:
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(4) a radiation-curing station:
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- 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:
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(5) a winding station:
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- the coated film, optionally with a peeling film, is wound
into rolls. The rolls are the stored before final use.
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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.
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In a most preferred embodiment, the process of the
invention is a solvent-less process.
Corona treatment
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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
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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.
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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.
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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).
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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.
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Other coating techniques may of course be used.
Curing process
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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
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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.
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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.
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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
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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.
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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
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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.
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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.
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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.
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Other radiation-curing processes may however be used to
perform the curing of the radiation-curable coating.
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Classical EB process, UV process and hybrid process are
contemplated in the instant invention.
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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 :
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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 :
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The D-1653-85 test method was used for moisture barrier
effects of the coated films.
Example 1
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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
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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 |
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The invention may be varied within broad limits by the
skilled man and is not limited to the embodiments disclosed
above.