U.S. patent application number 09/745083 was filed with the patent office on 2001-05-17 for acrylate composite barrier coating.
This patent application is currently assigned to Presstek, Inc.. Invention is credited to Cline, Daniel, Dawson, Eric, Langlois, Marc, Shaw, David G..
Application Number | 20010001284 09/745083 |
Document ID | / |
Family ID | 26829369 |
Filed Date | 2001-05-17 |
United States Patent
Application |
20010001284 |
Kind Code |
A1 |
Shaw, David G. ; et
al. |
May 17, 2001 |
Acrylate composite barrier coating
Abstract
A thermoplastic container or packaging material is given low
oxygen permeability by coating with a crosslinked acrylate layer
and a layer of oxygen barrier material deposited over the acrylate
layer. Another acrylate layer may be deposited over the oxygen
barrier. The oxygen barrier is selected from the group consisting
of silicon oxide, aluminum oxide and metal. The acrylate layer may
be formed from a photopolymerizable polyfunctional acrylate that is
sufficiently low viscosity to be sprayed on the substrate or
applied by dipping. Alternatively, the acrylate layer is a
polymerization product of an acrylate monomer which is evaporated
in a vacuum, condensed on the substrate and polymerized by
irradiation by ultraviolet or an electron beam. The surface of the
thermoplastic substrate is prepared for deposition of the acrylate
by either flame treating the surface of the substrate to heat it
above its melting point without deforming the substrate to thereby
smooth the surface, or by plasma treating the surface for enhancing
adhesion of the acrylate. Chilling the substrate enhances
deposition efficiency.
Inventors: |
Shaw, David G.; (Tucson,
AZ) ; Dawson, Eric; (Tucson, AZ) ; Cline,
Daniel; (Tucson, AZ) ; Langlois, Marc;
(Tucson, AZ) |
Correspondence
Address: |
CHRISTIE, PARKER & HALE, LLP
350 WEST COLORADO BOULEVARD
SUITE 500
PASADENA
CA
91105
US
|
Assignee: |
Presstek, Inc.
|
Family ID: |
26829369 |
Appl. No.: |
09/745083 |
Filed: |
December 20, 2000 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09745083 |
Dec 20, 2000 |
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08706180 |
Aug 30, 1996 |
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08706180 |
Aug 30, 1996 |
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08228579 |
Apr 15, 1994 |
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08228579 |
Apr 15, 1994 |
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08131328 |
Oct 4, 1993 |
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5440446 |
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Current U.S.
Class: |
428/216 ;
428/35.2; 428/35.7; 428/451; 428/483; 428/520 |
Current CPC
Class: |
Y10T 428/24967 20150115;
Y10T 428/1334 20150115; B05D 1/60 20130101; H01G 4/20 20130101;
Y10T 428/31699 20150401; Y10T 428/3175 20150401; Y10T 428/31935
20150401; Y10T 428/1338 20150115; Y10T 428/24975 20150115; H01G
2/12 20130101; Y10T 428/265 20150115; B65D 1/0215 20130101; Y10T
428/1355 20150115; Y10T 428/2495 20150115; H01G 4/206 20130101;
Y10T 428/1359 20150115; Y10T 428/31743 20150401; H01G 4/32
20130101; Y10T 428/31667 20150401; Y10T 428/31797 20150401; Y10T
428/31928 20150401; Y10T 428/1352 20150115; B05D 3/144
20130101 |
Class at
Publication: |
428/216 ;
428/35.2; 428/35.7; 428/451; 428/483; 428/520 |
International
Class: |
B32B 007/02; B32B
027/30; B32B 001/08 |
Claims
1. A barrier sheet comprising: a thermoplastic substrate; a first
vapor deposited, crosslinked acrylate layer on at least one face of
the substrate, the acrylate layer being a polymerization product of
a blend of acrylate monomers comprising at least one monomer having
a molecular weight in the range of from 150 to 400 and another
acrylate having a molecular weight of more than 600; a
substantially continuous layer of gas barrier material vacuum
deposited over the acrylate layer; and a second vapor deposited
acrylate layer over the gas barrier material which was crosslinked
before the gas barrier material contacted any solid surface.
2. A barrier as recited in claim 1 wherein the substrate is a
polymer.
3. A barrier as recited in claim 1 wherein the gas barrier material
is a metal oxide.
4. A barrier as recited in claim 1 wherein the acrylate layer is
over the gas barrier layer.
5. A barrier as recited in claim 1 comprising at least two acrylate
layers, a first acrylate layer being under the oxygen barrier
material and a second acrylate layer being over the gas barrier
material.
6. A barrier as recited in claim 1 wherein the crosslinked acrylate
layer is a polymerization product of acrylate monomer having a
molecular weight in the range of from 150 to 600.
7. A barrier as recited in claim 1 wherein the crosslinked acrylate
is a polymerization product of a blend of acrylate monomers
comprising at least one monomer having a molecular weight in the
range of from 150 to 400 and an acrylate having a molecular weight
of more than 600.
8. A barrier as recited in claim 1 wherein the crosslinked acrylate
layer is a polymerization product of a blend of acrylate compounds
having a sufficiently low viscosity for spray or dip coating the
substrate.
9. A barrier as recited in claim 1 wherein each crosslinked
acrylate layer has a thickness of more than one tenth
micrometer.
10. A container comprising: a closable hollow container molded from
a thermoplastic; a first vapor deposited, crosslinked acrylate
layer on a surface of the container, the outside surface of the
container being sufficiently smooth that the acrylate layer is
continuous; a substantially continuous layer of gas barrier
material vacuum deposited over the acrylate layer; and a second
vapor deposited acrylate layer over the gas barrier layer which was
crosslinked before the gas barrier material contacted any solid
surface.
11. A container as recited in claim 10 wherein the gas barrier
material is selected from the group consisting of silicon oxide and
aluminum oxide.
12. A container as recited in claim 10 comprising at least two
acrylate layers, a first acrylate layer being under the oxygen
barrier material and a second acrylate layer being over the oxygen
barrier layer.
13. A container as recited in claim 10 wherein the second
crosslinked acrylate layer is a polymerization product of acrylate
monomer having a molecular weight in the range of from 150 to
600.
14. A container as recited in claim 10 wherein at least one of the
crosslinked acrylate layers is a polymerization product of a blend
of acrylate monomers comprising at least one monomer having a
molecular weight in the range of from 150 to 400 and an acrylate
having a molecular weight of more than 600.
15. A container as recited in claim 10 wherein the crosslinked
acrylate layer is a polymerization product of a blend of acrylate
monomers having a sufficiently low viscosity for spray or dip
coating the substrate.
16. A gas barrier sheet comprising: a thermoplastic sheet
substrate; a substantially continuous layer of gas barrier material
vacuum deposited on one face of the sheet substrate; and an
evaporated acrylate layer deposited over the gas barrier layer and
crosslinked before the gas barrier material contacts any solid
surface.
17. A barrier sheet as recited in claim 16 comprising at least two
crosslinked acrylate layers, the second acrylate layer being under
the vapor barrier material.
18. A barrier sheet comprising: a thermoplastic substrate; a first
vapor deposited, crosslinked acrylate layer on at least one face of
the substrate; a substantially continuous layer of gas barrier
material vacuum deposited over the acrylate layer; and a second
vapor deposited acrylate layer over the gas barrier material which
was crosslinked before the gas barrier material contacted any solid
surface.
19. A barrier as recited in claim 18 wherein the gas barrier
material is at least one of metal oxide, metal, and inorganic
material.
20. The barrier sheet of claim 18 further comprising: a second
substantially continuous layer of gas barrier material vacuum
deposited over the second acrylate layer; and a third vapor
deposited acrylate layer over the second layer of gas barrier
material which was crosslinked before the second layer of gas
barrier material contacted any solid surface.
21. The container as recited in claim 10 wherein the gas barrier
material is at least one of metal oxide, metal, and inorganic
material.
22. The container of claim 10 further comprising: a second
substantially continuous layer of gas barrier material vacuum
deposited over the second acrylate layer; and a third vapor
deposited acrylate layer over the second layer of gas barrier
material which was crosslinked before the second layer of gas
barrier material contacted any solid surface.
23. A barrier sheet comprising: a thermoplastic substrate; a first
vapor deposited, crosslinked acrylate layer on one face of the
substrate; a substantially continuous layer of gas barrier material
vacuum deposited over the acrylate layer, wherein the gas barrier
material layer is transparent; and a second vapor deposited
acrylate layer over the gas barrier material which was crosslinked
before the gas barrier material contacted any solid surface.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
1. This application is a continuation of application Ser. No.
08/706,180 filed Aug. 30, 1996, which is a continuation of
application Ser. No. 08/228,579 filed Apr. 15, 1994, which is a
continuation of application Ser. No. 08/131,328 filed Oct. 4, 1993
and now U.S. Pat. No. 5,440,446 issued Aug. 8, 1995. The subject
matter of these applications are incorporated herein by
reference.
BACKGROUND OF THE INVENTION
2. This invention relates to deposition of barrier films for
inhibiting penetration by oxygen or other gases employing a
cross-linked acrylate layer and a layer of oxygen barrier
material.
3. Many products, including many food products, are packaged in
thin plastic sheet bags or the like. The thin films are desirably
resistant to permeation by oxygen, water vapor and odorous gases.
This can, for example, be important for protecting a food from
environmental gases and also for retaining the aroma of food as it
is stored.
4. Such barrier films are commonly made of costly plastics because
less costly films are too permeable to oxygen or water to give a
long shelf life. Reduced cost barrier films are highly
desirable.
5. There are many products that are packaged in plastic bottles,
tubes or vials which also need protection from oxidation or
contamination by environmental gasses or which must be in
impermeable containers so that components of the contents are not
lost by diffusion through the containers. An example comprises
medical products which may presently be enclosed in glass bottles
because plastics are not suitable. It would be desirable to make
such containers of plastic material which is physiologically
acceptable, however, such materials may also be sufficiently
permeable to oxygen that they are unsuitable for medical products.
Food packages are also desirably packaged in plastic, but shelf
life may be compromised by permeability. It is, therefore,
desirable to provide a coating on such vessels for these critical
applications.
SUMMARY OF THE INVENTION
6. There is, therefore, provided in practice of this invention a
barrier with low oxygen and water permeability having a
thermoplastic substrate, a cross-linked acrylate layer on one face
of the substrate and a layer of oxygen barrier material deposited
on the same face of the substrate as the acrylate layer, preferably
over the acrylate layer, or in some embodiments, under the acrylate
layer. In one embodiment, the acrylate layer is a polymerization
product of an acrylate monomer having a molecular weight in the
range of from 150 to 600. Alternatively, the acrylate layer may be
formed from a photopolymerizable acrylate that is sufficiently low
viscosity to be sprayed on the substrate or applied by dipping.
Preferably, there is another cross-linked acrylate layer over the
oxygen barrier layer. It is preferable to deposit a top acrylate
layer over a metallized layer before the metallized layer contacts
any surfaces.
7. The surface of the thermoplastic substrate is prepared for
deposition of the acrylate by either heating the surface of the
substrate above its melting point without deforming the substrate
or by plasma treating the surface for enhancing adhesion of the
acrylate. Chilling the substrate enhances deposition
efficiency.
DESCRIPTION OF THE DRAWINGS
8. These and other features and advantages of the present invention
will be appreciated as the same becomes better understood by
reference to the following detailed description when considered in
connection with the accompanying drawings wherein:
9. FIG. 1 illustrates in transverse cross section a coated
thermoplastic substrate with low oxygen permeability;
10. FIG. 2 illustrates an exemplary container constructed according
to principles of this invention;
11. FIG. 3 is a schematic illustration of apparatus for coating
substrates by spraying; and
12. FIG. 4 is a schematic illustration of apparatus for coating a
sheet substrate.
DESCRIPTION OF THE INVENTION
13. An exemplary barrier material constructed according to
principles of this invention comprises a thermoplastic substrate
having a cross-linked acrylate layer 11 on one face. The acrylate
layer is overlain by a layer 12 of oxygen barrier material. A final
cross-link acrylate layer 13 is optionally formed over the oxygen
barrier layer.
14. It will be recognized that in the drawing, the various layers
are drawn schematically and at a scale suitable for illustration
rather than at the scale of the actual material. For example, such
a composite with low oxygen permeability may be a wall of a four
ounce threaded bottle 14 with a wall thickness of about 1 mm. Thus,
the substrate has a thickness of 1 mm. The thickness of each of the
layers formed on such a substrate may be in the order of 1
micrometer or less. An acrylate layer may be somewhat thicker if
sprayed on and up to about 25 micrometers if applied by dipping. If
one is making thin sheet material for food packaging or the like,
the substrate thickness may be in the order of 5 to 20 micrometers
with the coating layers being in the order of 1 micrometer or
less.
15. It has been discovered that polyethylene, polypropylene,
polyester or nylon substrates with thin surface coatings of
crosslinked acrylate have very low oxygen and water permeability
when combined with a metal or inorganic barrier coating. There is a
great need for low cost packaging materials where the oxygen
permeability of the package is low for preserving the freshness of
the packaged goods. Metallized plastic sheet is used for this
purpose. Typical sheets for packaging foodstuffs include metallized
polypropylene, nylon or polyester sheet. Metallized nylon has an
oxygen permeability of about 0.05 ml/100 in.sup.2/hour (ml/645
cm.sup.2/hour) as measured with a Mocon Oxtran System available
from Modern Controls, Minneapolis, Minn. Metallized polyester has a
typical oxygen permeability of about 0.08. Metallized
polypropylene, on the other hand, has an oxygen permeability of
about 2.5 and is not usually suitable for packaging where low
oxygen permeability is important.
16. It is believed that the high oxygen permeability of metallized
polypropylene is due to the inherent surface roughness of the
polypropylene sheet. Nylon and polyester sheets are considerably
smoother and have a higher temperature capability than
polypropylene. A metal coating of uniform thickness can be readily
applied as a good oxygen barrier. Typically, polypropylene may have
a surface roughness in the order of 1/2to one micrometer, or more
in some sheets. A layer of acrylate in the same order, namely about
1/2to one micrometer thick is adequate for smoothing the surface
for producing a low oxygen permeability.
17. Sheet polypropylene without any coating may have an oxygen
permeability of about 100. However, if a layer of aluminum is
applied to a surface of a polypropylene sheet substrate, the oxygen
permeability decreases to about 2.5. Surprisingly, when an acrylate
layer only about one micrometer thick is formed on the
polypropylene and then covered with a layer of metal, the oxygen
permeability drops to about 0.05, a value lower than metallized
polyester. It is hypothesized that the film of liquid acrylate
monomer deposited on the surface of the polypropylene has a smooth,
high temperature surface and the surface remains smooth when the
acrylate is polymerized. The metallized layer can then form a good
oxygen barrier.
18. Furthermore, a transparent barrier film may be formed on a
polyethylene, polypropylene, polyester or nylon substrate, or other
thermoplastic substrate. First, a layer of acrylate monomer is
deposited on the substrate and crosslinked. The acrylate layer is
then coated with an oxygen barrier layer of SiO.sub.x or
Al.sub.2O.sub.3, both of which have good resistance to oxygen
permeability. The high temperature resistance of the crosslinked
acrylate layer permits the notably higher temperature deposition of
silicon oxide or aluminum oxide on the thermoplastic substrate.
19. A still greater surprise occurs when another polymerized
acrylate layer is formed over the oxygen barrier layer. The
permeability through a polypropylene barrier material drops to
about 0.002 which is appreciably better than the oxygen
permeability of metallized nylon. It is hypothesized that the
second acrylate layer may protect the metallized layer and assure
retention of the oxygen barrier properties of the metal. Oxygen
barriers are further enhanced by multiple layers, such as, for
example, a thermoplastic substrate with layers of acrylate, metal,
acrylate, metal and acrylate. Furthermore, when multiple coating
layers are applied, any pinholes or other local defects in a layer
are likely to be offset from similar pinholes or defects in
underlying layers. Thus, oxygen permeability through pinholes is
effectively eliminated.
20. Thus, a preferred composite material with low oxygen
permeability has a layer of polymerized acrylate, a layer of
barrier material such as SiO.sub.x or Al.sub.2O.sub.3 and another
layer of polymerized acrylate on a thermoplastic substrate. The
layers of acrylate reduce permeability dramatically and the layer
overlying the barrier material protects the barrier material from
mechanical damage and also provides a surface suitable for
printing.
21. The substantial improvement in oxygen permeability is believed
to be attributable to formation of a liquid film of monomer on the
surface of the polypropylene, followed by cross linking of the
polyfunctional acrylate. Applying the layer by condensing or
spraying as a liquid assures smooth and uniform coating of the
substrate, thereby forming an excellent surface for receipt of the
metallization. Cross linking upon curing the acrylate produces a
material having low inherent oxygen permeability. Adding a second
layer of acrylate monomer which is polymerized in situ is believed
to rectify any defects in the underlying layers and provide an
additional thickness of material with inherently low oxygen
permeability. Redundant layers minimize pinhole leakage.
22. The polymerized acrylate layer is believed to be beneficial for
a number of other reasons. As a thermoset material, it has higher
temperature resistance than the thermoplastic substrate. In the
coating process, the product is subjected to elevated temperature
processing such as metallizing, plasma treatment and the like.
Particularly high temperatures may be encountered when depositing
transparent barrier coatings. Various volatile materials, such as
water vapor or plasticizers, may be emitted by thermoplastic
surfaces under these conditions. These may adversely affect the
properties of the coating such as adhesion, nucleation and growth,
and thereby reduce the barrier properties. A cured acrylate coating
would not have such emissions and may seal the surface and inhibit
emission of such materials from a thermoplastic substrate.
23. The acrylate layers in the various embodiments may be deposited
by either of two principal techniques. One may spray a low
viscosity liquid acrylate onto the surface in an open system or a
container may simply be dipped into a liquid acrylate.
Alternatively, one may condense a vaporized acrylate monomer in a
vacuum system.
24. It is particularly preferred to vaporize an acrylate prepolymer
and deposit it on a substrate in a vacuum system since this
technique serves to refine the acrylate. the original
polyfunctional acrylate may contain volatile non-polymerizable
substances that are preferably avoided in the crosslinked coating.
In effect, the vaporization and deposition process is a vacuum
distillation which removes volatiles to the vacuum pumps and
deposits only higher molecular weight acrylates on the substrate.
Removal of volatiles is desirable for subsequent high temperature
processing of the material such as in deposition of metal or other
inorganic barrier films.
25. After any of these deposition techniques, the monomer film is
irradiated with ultraviolet or an electron beam to cause
polymerization of the acrylate to form a monolithic crosslinked
layer. Polymerization by irradiation is a conventional practice and
the electron flux required or wavelength and total flux of
ultraviolet used are commonly known. A photoinitiator may be
included in the acrylate for facilitating polymerization by
ultraviolet radiation.
26. An exemplary process for coating a container can be described
as follows. Such a container may be a food jar, a beverage bottle,
a collapsible tube, a cosmetic container, a medicine bottle, a vial
for blood products, or essentially any other thermoplastic
container. The container is injection molded or blow molded in a
conventional manner from a conventional thermoplastic material.
Preferably the container is then flame treated for activating and
smoothing the surface. It has been found that adhesion of an
acrylate layer on the substrate is enhanced by activating the
surface by plasma or flame treating. High temperature air may also
be used.
27. In a typical production line, a row of containers 14 are moved
successively through a flame treating station, a coating station
and a curing station. In the flame treating station, the containers
are bathed in the flames from a plurality of propane or natural gas
torches 16. The containers may be rotated as they pass through the
flame treating station for uniform heating of the surfaces or they
may be essentially fixed and have a plurality of torches arranged
for completely surrounding the container.
28. In addition to activating the surface for enhanced adhesion,
the flame treating can significantly smooth the surface of the
container to assure that there is complete coverage by subsequent
coatings. The thermoplastic materials employed for such containers
have a relatively low thermal conductivity. The flame is applied to
the surface with sufficient intensity to soften or melt a thin
surface layer on the container. The containers move through the
flame treating station rapidly enough, however, that the container
is not deformed by the heating.
29. The flame treatment melts and rounds off any molding flash and
smooths mold marks on the container so that the coating can bridge
surface irregularities. "Melting" may almost be considered a
misnomer since the thermoplastic materials are effectively
supercooled liquids. Thus, melting is considered to be sufficient
softening of the surface for smoothing irregularities. Furthermore,
there may be embodiments where the quality of the mold in which the
container is made may be good enough that smoothing is not a
significant requirement. In such a case, flame treatment may still
be employed with sufficient intensity for activating the surface
and enhancing adhesion of a subsequent acrylate layer without
noticeable melting.
30. Such an acrylate layer is applied in a coating station where
one or more nozzles 17 sprays a thin coating of acrylate monomer
onto the surface. Such a sprayed coating may be in the order of
from one to twenty micrometers thick, for example. The acrylate
sprayed onto the surface may be a low viscosity monomer or if
desired, a monomer and/or a low molecular weight polymer may be
combined with a solvent for spraying. In the event the acrylate is
to be cross-linked by ultraviolet irradiation, a photoinitiator may
be included in the sprayed composition.
31. An alternative to spraying the surface of the container with
acrylate comprises dipping the container into a liquid acrylate
composition. In one exemplary embodiment, such dipping of a
container yields an acrylate coating thickness of up to about 25
micrometers.
32. Following the coating station, the containers pass a curing
station where a plurality of ultraviolet lamps 18 irradiate the
acrylate layer and cause cross-linking.
33. Following the application of an acrylate layer to the
containers, an oxygen barrier layer is applied. Preferably this is
by deposition in a vacuum chamber. A metal barrier layer, e.g.
aluminum, may be applied by vacuum metallizing or sputtering. A
layer of silicon oxide or aluminum oxide or other oxide material
may be deposited by a plasma assisted chemical vapor deposition
technique. For example, SiO.sub.x may be deposited by a plasma
vapor deposition process using an oxidizing or inert carrier gas.
SiO.sub.x may be evaporated from a crucible by an electron beam and
deposited over the acrylate layer on the thermoplastic container.
Preferably this is conducted in an oxygen rich environment for
obtaining the proper composition of the SiO.sub.x. Aluminum oxide
can be deposited by electron beam evaporation or preferably by
evaporation of aluminum which is converted to an oxide in an oxygen
plasma.
34. Other conventional techniques for depositing silica, alumina or
other oxides may be used. A variety of techniques are used in the
semiconductor industry, but may have deposition temperatures too
high for coating thermoplastics which are not already coated with a
protective layer of crosslinked acrylate.
35. An alternative technique for depositing an acrylate layer is in
a vacuum chamber. Suitable apparatus for coating a sheet substrate
with acrylate and oxygen barrier layers is illustrated
schematically in FIG. 4. All of the coating equipment is positioned
in a conventional vacuum chamber 21. A roll of polypropylene,
polyester or nylon sheet is mounted on a pay-out reel 22. The sheet
23 forming the substrate is wrapped around a rotatable drum 24 and
fed to a take-up reel 26. Idler rolls 27 are employed, as
appropriate, for guiding the sheet material from the payout reel to
the drum and to the take-up reel.
36. A flash evaporator 28 is mounted in proximity to the drum at a
first coating station. The flash evaporator deposits a layer or
film of acrylate monomer on the substrate sheet as it travels
around the drum. After being coated with acrylate monomer the
substrate sheet passes an irradiation station where the acrylate is
irradiated by a source 29 such as an electron gun or source of
ultraviolet radiation. The UV radiation or electron bombardment of
the film induces polymerization of the acrylate monomer.
37. The sheet then passes a deposition station 31 where a coating
of oxygen barrier material is applied by plasma deposition, vacuum
deposition or the like. The sheet then passes another flash
evaporator 32 where another layer of acrylate monomer is deposited
for forming a protective layer over the oxygen barrier. This layer
of monomer is cured by irradiation from an ultraviolet or electron
beam source 33 adjacent the drum. The coated sheet is then wrapped
up on the take-up reel 26.
38. Evaporation of the monomer is preferably from flash evaporation
apparatus 29, 32 as described in U.S. Pat. Nos. 4,722,515,
4,696,719, 4,842,893, 4,954,371 and/or 5,097,800. These patents
also describe polymerization of acrylate by radiation. In such
flash evaporation apparatus, liquid acrylate monomer is injected
into a heated chamber as 1 to 50 micrometer droplets. The elevated
temperature of the chamber vaporizes the droplets to produce a
monomer vapor. The monomer vapor fills a generally cylindrical
chamber with a longitudinal slot forming a nozzle through which the
monomer vapor flows. A typical chamber behind the nozzle is a
cylinder about 10 centimeters diameter with a length corresponding
to the width of the substrate on which the monomer is condensed.
The walls of the chamber may be maintained at a temperature in the
order of 200 to 320.degree. C.
39. Two styles of evaporator are suitable. In one of them, the
orifice for injecting droplets and flash evaporator are connected
to one end of the nozzle cylinder. In the other style, the injector
and flash evaporator section is attached in the center of the
nozzle chamber like a T.
40. It is often found desirable to plasma treat the surface to be
coated immediately before coating. A conventional plasma gun 34 is
positioned in the vacuum chamber upstream from each of the flash
evaporators 28 and 32 for activating the surface of the sheet on a
continuous basis before monomer deposition. Conventional plasma
generators are used. In an exemplary embodiment the plasma
generator is operated at a voltage of about 500 to 1000 volts with
a frequency of about 50 Khz. Power levels are in the order of 500
to 3000 watts. For an exemplary 50 cm wide film traveling at a rate
of 30 to 90 meters per minute, around 500 watts appears
appropriate. Plasma treatment of the surface enhances adhesion of
the deposited materials.
41. An analogous system may be employed for coating containers with
layers of acrylate and barrier material. In the event the
containers are coated with acrylate external to a vacuum system,
the coated containers are placed in or moved through a deposition
station in a vacuum chamber for depositing an oxygen barrier
material by plasma assisted chemical vapor deposition or the like.
In the event both acrylate and oxygen barrier materials are applied
in the vacuum system, the containers are moved successively through
an acrylate evaporation and condensation station and a deposition
station. If two layers of acrylate are used, over and under the
oxygen barrier layer, the second acrylate may be applied by way of
the same flash evaporator or by way of a second similar flash
evaporator. Plasma treatment of the surface of the container is
optional. If the container has been flame treated a short interval
before it is introduced into the vacuum chamber, the surface
probably remains sufficiently activated for good adhesion of the
acrylate layer and plasma treatment may not be of any additional
benefit.
42. The acrylates used for forming the cross-linked coatings on the
thermoplastic substrate differs somewhat depending on the technique
used for depositing the coating. The acrylates used for dipping or
spraying are similar and it is not necessary that the acrylate is a
monomer. Generally, the acrylates used are blends of high and low
molecular weight materials to yield the desired viscosity of the
composition for dipping or spraying. Monomers with molecular
weights in the order of 150 up to partially polymerized materials
have a molecular weights in the order of 20,000 may be blended to
obtain a low viscosity blend. The chemistry of the acrylates is not
known to be significant. There should, however, be polyfunctional
acrylates in the blend so that there is extensive cross-linking.
There should be a minimum of at least about 20% diacrylate or
equivalent.
43. In the event the acrylate layers are applied by the evaporation
and condensation technique, the range of suitable acrylates is more
restricted. These acrylate resins are generally monomers having a
molecular weight in the range of from 150 to 600. Preferably, the
monomers have a molecular weight in the range of from 200 to 400.
Higher molecular weight fluorinated acrylates or methacrylates may
be equivalent to these lower molecular weight materials and also be
used for forming a deposited acrylate layer. For example, a
fluorinated acrylate with a molecular weight of about 2000
evaporates and condenses similar to a non-fluorinated acrylate
having a molecular weight in the order of 300. The acceptable range
of molecular weights for fluorinated acrylates is about 400 to
3000. Fluorinated acrylates include monoacrylates, diacrylates, and
methacrylates. Fluorinated methacrylates are fast curing. Whereas
methacrylates are generally too slow curing to be desirable, the
fluorinated methacrylates cure rapidly. Chlorinated acrylates may
also be useful.
44. If the molecular weight is below about 150, the monomer is too
volatile and does not condense well for forming a monomer film.
Monomer that does not condense on the desired substrate may foul
vacuum pumps and hinder operation of an electron gun used for
polymerizing the resin. If the molecular weight is more than about
600 the monomer does not evaporate readily in the flash evaporator
at temperatures safely below the decomposition temperature of the
monomer.
45. It is desirable that the thickness of the acrylate layer be
sufficient for smoothing any surface roughness of the underlying
substrate. For example, polypropylene which has not been flame
treated may have a surface roughness in the order of 1/2to one
micrometer. A layer of acrylate in the same order, namely about
1/2to one micrometer thick is adequate for smoothing the
surface.
46. In applications where a transparent oxygen barrier is applied
or in some applications with a metallized layer, a thin layer of
acrylate shows a slightly colored or tinted appearance due to
interference patterns. An acrylate layer having a thickness of
about 1.2 to 1.5 micrometers can avoid the interference colors.
47. Suitable acrylates not only have a molecular weight in the
appropriate range, they also have a "chemistry" that does not
hinder adhesion. Generally, more polar acrylates have better
adhesion to metal layers than less polar monomers. Long hydrocarbon
chains may hinder adhesion to metal but may be an advantage for
depositing on non-polar thermoplastic or oxide surfaces. For
example, lauryl acrylate has a long chain that is hypothesized to
be aligned away from the substrate and may hinder adhesion to
subsequent layers. Thus, one acrylate monomer or blend may be used
for condensing acrylate on a thermoplastic substrate, and a
different acrylate may be used for depositing over the oxygen
barrier layer.
48. A typical monomer used for flash evaporation includes an
appreciable amount of diacrylate and/or triacrylate to promote
crosslinking. Blends of acrylates may be employed for obtaining
desired evaporation and condensation characteristics and adhesion,
and for controlled shrinkage of the deposited film during
polymerization.
49. Suitable monomers are those that can be flash evaporated in a
vacuum chamber at a temperature below the thermal decomposition
temperature of the monomer and below a temperature at which
polymerization occurs in less than a few seconds at the evaporation
temperature. The mean time of monomer in the flash evaporation
apparatus is typically less than one second. Thermal decomposition,
or polymerization are to be avoided to minimize fouling of the
evaporation apparatus. The monomers selected should also be readily
capable of crosslinking when exposed to ultraviolet or electron
beam radiation.
50. The monomer composition may comprise a mixture of monoacrylates
and diacrylates. Triacrylates tend to be reactive and may
polymerize at the evaporation temperatures. Generally speaking, the
shrinkage is reduced with higher molecular weight materials.
51. Generally, it is desirable that at least a major portion of the
acrylate monomer evaporated is a polyfunctional acrylate for
crosslinking. Preferably, the acrylate comprises at least 70
percent polyfunctional acrylates such as diacrylate or triacrylate.
If the degree of crosslinking is too low, the polymerized acrylate
layer may not have adequate cure speed.
52. Preferably, the molecular weight of the acrylate monomer is in
the range of from 200 to 400. If the molecular weight is less than
about 200, the monomer evaporates readily, but may not condense
quantitatively on the substrate without chilling of the substrate.
If the molecular weight is more than about 400, the monomers become
increasingly difficult to evaporate and higher evaporation
temperatures are required. As mentioned above, some fluorinated
methacrylates with higher molecular weights are equivalent to lower
molecular weight non-fluorinated acrylates.
53. Preferably, the acrylate monomer has a vapor pressure at
25.degree. C. in the range of from 1 to 20 micrometers of mercury.
If the vapor pressure is less than about one micrometer,
exceptionally high temperatures may be required to evaporate
sufficient material for forming a coating on the substrate in
reasonable coating time. High temperatures may lead to thermal
decomposition or premature curing of the monomers. If the vapor
pressure is higher than about twenty micrometers of mercury,
condensation of the monomer to form a film on the substrate may
have too low an efficiency for practical coating operations.
Adequate efficiency may not be obtained until the surface of the
substrate is cooled below the freezing point of the monomer, in
which case the material may not polymerize properly.
54. There are at least five monoacrylates, ten diacrylates, ten to
fifteen triacrylates and two or three tetraacrylates which may be
included in the evaporated composition. Most preferably the
acrylate comprises hexane diol diacrylate (HDDA) with a molecular
weight of 226 and/or tripropylene glycol diacrylate (TRPGDA) with a
molecular weight of about 300. Other acrylates may be used,
sometimes in combination, such as monoacrylates 2-phenoxy ethyl
acrylate (M.W. 192) , isobornyl acrylate (M.W. 208) and lauryl
acrylate (M.W. 240), epoxy acrylate RDX80095 made by Radcure of
Atlanta, Ga.; diacrylates diethylene glycol diacrylate (M.W. 214),
neopentyl glycol diacrylate (M.W. 212), propoxylated neopentyl
glycol diacrylate (M.W. 328) and polyethylene glycol diacrylate,
tetraethylene glycol diacrylate (M.W. 302), and bisphenol A epoxy
diacrylate; and triacrylates trimethylol propane triacrylate (M.W.
296), ethoxylated trimethylol propane triacrylate (M.W. 428),
propylated trimethylol propane triacrylate (M.W. 470) and
pentaerythritol triacrylate (M.W. 298). Monomethacrylates isobornyl
methacrylate (M.W. 222) and 2-phenoxyethyl methacrylate (M.W. 206)
and dimethacrylates triethylene glycol dimethacrylate (M.W. 286)
and 1,6-hexanediol dimethacrylate (M.W. 254) may also be useful,
but may cure too slowly to be useful for high speed coating
operations.
55. It is known that adhesion may be enhanced between a substrate
and an acrylate coating, by using an acrylate containing high
molecular weight components. In practice very high molecular weight
oligomers are usually mixed with low molecular weight monomers. The
oligomers usually have molecular weights of greater than 1000 and
often as large as 10,000 or even higher. The monomers are used as
diluents to lower the coating viscosity and provide an increased
number of acrylate groups for enhancing cure speed, hardness and
solvent resistance in the resulting coating.
56. It has generally been considered that it is not feasible to
evaporate high molecular weight acrylates because of their very low
vapor pressure and high viscosity. Evaporated acrylate coatings
have been restricted to low molecular weight monomers, generally
below a molecular weight of about 400 and with low viscosity.
Generally the viscosities are below 50 centistoke. For example,
Henkel 4770, which is an amine acrylate, has a sufficiently high
molecular weight that it has a viscosity of about 1000 centistokes
at 25.degree. C. This material cures in the evaporator before
evaporating. Beta carboxy ethyl acrylate (BCEA) which has a
viscosity of over 200 centistokes also cures in the evaporator.
57. It has been found, however, that by mixing a very low and a
very high viscosity material, flash evaporation, condensation and
curing can be obtained. For example, a mixture of 70 percent of
Henkel 4770 and 30 percent diethylene glycol diacrylate has a
viscosity of about 12 centistokes and can be successfully
evaporated, condensed and cured. A mixture of 70 percent
tripropylene glycol diacrylate (TRPGDA) and 30 percent of beta
carboxy ethyl acrylate (BCEA) has a viscosity of about 15
centistokes and can be readily evaporated, condensed and cured. The
low viscosity component lowers the viscosity of the blend, which
improves atomization in the evaporator and assists in the flash
evaporation of the high viscosity acrylate.
58. There is essentially a trade off between the molecular weights
(and hence viscosities) of the high and low molecular weight
acrylates. Generally, the lower the molecular weight and viscosity
of the low molecular weight component, the higher the molecular
weight and viscosity of the higher molecular weight component can
be for satisfactory evaporation and condensation. The reason for
good atomization in the flash evaporator is straightforward. This
is essentially a physical effect based on the viscosity of the
blend. The reason for successful evaporation is not as clear. It is
hypothesized that the low molecular weight acrylate essentially
dilutes the high molecular weight material and energetic
evaporation of the lower molecular weight material effectively
sweeps along the higher molecular weight material.
59. When blends of high and low molecular weight acrylates are
used, it is preferred that the weighted average molecular weight of
the blend be in the range of from 200 to 600 and preferably up to
about 400. This assures that there is good vaporization of the
blend at reasonable temperatures in the evaporator.
60. Some examples of low molecular weight acrylates are hexane diol
diacrylate, diethylene glycol diacrylate, propane diacrylate,
butane diol diacrylate, tripropylene glycol diacrylate, neopentyl
glycol diacrylate, phenoxyethyl acrylate, isobornyl acrylate and
lauryl acrylate. Some examples of high molecular weight acrylates
are bisphenol A diacrylate, BCEA, Radcure 7100 (an amine acrylate
available from Radcure, Atlanta Ga.), Radcure 169, Radcure 170,
acrylated and methacrylated phosphoric acid, Henkel 4770 (an amine
acrylate available from Henkel Corporation, Ambler, Pa.) and
glycerol propoxy triacrylate.
61. Particularly preferred high molecular weight materials include
BCEA which is acid in character and has a shrinkage of only about 4
percent upon curing. Another suitable material is an acrylate or
methacrylate of phosphoric acid. One can also use acrylic acid in
the composition, along with dimers, trimers and tetrameres of
acidic acrylates or methacrylates. For example, Henkel 4770 is
polar and helps increase the cure speed and adhesion. In general,
the higher molecular weight components are used to add flexibility,
reduce shrinkage or provide some particular chemical
characteristics such as acid or caustic resistance.
62. The molecular weight range of the acrylate may be extended by
preheating the prepolymer before it is atomized into the
vaporization chamber. The acrylate is injected into a vaporization
chamber by way of an ultrasonically vibrating tip. Fine droplets of
acrylate are generated, which impinge on the heated walls of the
vaporizer. The preheating lowers the viscosity of the acrylate and
makes it easier to obtain fine droplets that readily flash
evaporate.
63. Preferably the acrylate prepolymer is preheated to a
temperature above ambient temperature and lower than a temperature
where the prepolymer polymerizes. If the acrylate is overheated it
may commence to polymerize in the atomizer. Preheating to about
100.degree. C. is found to significantly enhance the rate of
vaporization. When the acrylate is preheated the substrate may be
moved past the vaporizer more rapidly for a desired thickness of
coating. Thus, preheating the acrylate increases production speed.
As mentioned, it may also extend the molecular weight range of
acrylates suitable for deposition. Even materials that are solid at
room temperature may be vaporized and deposited after preheating
above their melting temperature.
64. It has been found that the temperature of the substrate on
which the monomer film is deposited can have a large influence on
the efficiency of condensation. The effect of temperature depends
on the particular monomer. Because the efficiency of condensation
changes rather steeply in the general vicinity of ambient
temperatures and since the flash evaporation and irradiation tend
to raise the temperature of the substrate, it is desirable to
refrigerate the substrate before it is placed in the vacuum
chamber. Good condensation efficiency can be obtained with monomers
having a molecular weight of at least 200 with the substrate cooled
to temperatures in the range from 0 to 15.degree. C.
65. A rack of containers to be coated can be removed from a
refrigerator and placed in a vacuum chamber, pumped down and coated
before the containers warm to unreasonably high temperatures. If
desired, the containers may be precooled to a temperature well
below the optimum temperature for deposition and the deposition
step timed to occur when the containers have warmed to an optimum
temperature.
66. The surface of a polypropylene or other thermoplastic substrate
can also be activated before any of the coating steps described
above by exposing the substrate to a corona discharge in air or
nitrogen. Oxygen and nitrogen are apparently incorporated onto the
surface and change the surface conductivity and surface tension,
enhancing adhesion and the ability to cure the acrylate. Thus, the
surface can be activated by corona discharge, flame treatment or
plasma bombardment within the vacuum system.
67. There may be embodiments where it is sufficient to deposit an
oxygen barrier layer directly on the substrate and apply an
acrylate layer over the oxygen barrier material. For example, when
the thermoplastic substrate has been flame treated to smooth the
surface sufficiently that a thin oxygen barrier material can bridge
over any surface irregularities, the oxygen barrier material may be
deposited directly on the flame treated substrate. An acrylate
layer may then be applied over the oxygen barrier material to
protect the barrier and further reduce permeability.
68. It is found particularly desirable to provide a protective
crosslinked acrylate coating over a deposited layer of metal such
as aluminum. If an aluminum layer is applied to a sheet substrate
which is rolled for later use or which is passed over a roller
contacting the surface, the aluminum may be abraded off of higher
asperities on the surface. A sheet coated with aluminum and not
protected with an overlying crosslinked acrylate coating may have a
pinhole density in the order of 1000 pinholes/cm.sup.2. If one
deposits an acrylate monomer and polymerizes the acrylate in situ
to form a layer having a thickness of as little as 0.1 micrometer,
the pinhole density through the aluminum layer can be maintained as
low as 10 pinholes per cm.sup.2.
69. It is important to deposit the liquid acrylate on the metal
layer before the metal layer contacts any solid surface, such as
another roll or even the opposite face of a sheet substrate. The
acrylate should, of course, be polymerized for forming a
crosslinked acrylate layer before the metal layer contacts any
solid surface. The crosslinked acrylate has much better abrasion
resistance than the metal and avoids damage during handling.
70. There are various advantages and disadvantages to the
techniques for depositing an acrylate coating inside the vacuum
chamber by evaporation and condensation or outside the vacuum
chamber by spraying or dipping. When the entire process can be
performed in vacuum, it can be essentially continuous by using
loading and unloading airlocks or it can be a batch process. When
the entire process is performed in vacuum, there is essentially no
concern for particulate contamination which may be present when the
process is performed in an open environment. In an embodiment where
multiple layers of both acrylate and an oxygen barrier material may
be desired, the alternating layers can be accumulated in vacuum
without removing the containers or other substrate from the vacuum
chamber.
71. The evaporation and condensation technique may require cooling
of the containers, depending on the acrylate monomers used.
Prechilling of the containers may not be convenient. Cooling of a
sheet substrate on a chilled drum in the vacuum chamber can be less
of a disadvantage.
72. Spraying or dipping may be advantageous since the process can
be readily observed and controlled in an open environment. It is
also inexpensive. There may, however, be a problem with particulate
material unless the process is performed in a suitably clean area.
Oxygen may tend to inhibit curing of the acrylate coating.
Furthermore, unreacted photoinitiator remaining in the cured
coating may be evolved during subsequent vacuum deposition of the
oxygen barrier material. Which of the techniques selected for a
given application will depend on such considerations.
73. Many modifications and variations in the coating of
thermoplastic containers for low oxygen permeability will be
apparent to those skilled in the art. For example, the sequence of
coating operations and the coated substrate may be varied
appreciably. A mix of steps may also be employed. For example, one
may choose to first flame treat the containers, then condense and
cure an acrylate in a vacuum chamber, followed by deposition of an
oxygen barrier. A second layer of acrylate may be applied by
dipping or spraying.
74. Thus, it will be understood that within the scope of the
following claims this invention may be practiced otherwise than as
specifically described.
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