U.S. patent application number 09/783926 was filed with the patent office on 2002-10-24 for vacuum deposition of cationic polymer systems.
Invention is credited to Mikhael, Michael G., Yializis, Angelo.
Application Number | 20020156142 09/783926 |
Document ID | / |
Family ID | 25130836 |
Filed Date | 2002-10-24 |
United States Patent
Application |
20020156142 |
Kind Code |
A1 |
Mikhael, Michael G. ; et
al. |
October 24, 2002 |
Vacuum deposition of cationic polymer systems
Abstract
A thermally-stable cationic photoinitiator capable of flash
vaporization under vacuum and temperature conditions of an
available flash-evaporation chamber is selected. The photoinitiator
is mixed with a cation-polymerizable monomer and/or oligomer of
interest and the mixture is flash evaporated and condensed in
conventional manner as a film on a cold substrate. The resulting
vacuum-deposited, homogeneous layer is cured with a high-energy
radiation source that causes the cationic photoinitiator to
liberate acidic species that catalyze the crosslinking of the
monomer/oligomer compounds in its deposited film form. As a result
of the homogeneous, pinhole-free nature of the vacuum deposition
process, the thin-film polymer product does not suffer from the
disadvantages attendant to prior-art atmospheric processes for
cationically-cured polymers. In addition, because of the
versatility afforded by vacuum deposition, hybrid films of such
polymers with inorganic materials are also easily manufactured
in-line during the same process.
Inventors: |
Mikhael, Michael G.;
(Tucson, AZ) ; Yializis, Angelo; (Tucson,
AZ) |
Correspondence
Address: |
Antonio R. Durando
Durando Birdwell & Janke, P.L.C.
2929 E. Broadway Blvd.
Tucson
AZ
85716
US
|
Family ID: |
25130836 |
Appl. No.: |
09/783926 |
Filed: |
February 13, 2001 |
Current U.S.
Class: |
522/15 ; 427/497;
427/509; 522/25; 522/29; 522/66 |
Current CPC
Class: |
B05D 3/144 20130101;
B05D 3/068 20130101; B05D 7/04 20130101; C23C 14/12 20130101 |
Class at
Publication: |
522/15 ; 522/25;
522/29; 522/66; 427/497; 427/509 |
International
Class: |
C08J 003/28; C08G
002/00 |
Claims
We claim:
1. A process for forming a solid polymeric structure from
flash-evaporated vacuum-deposited cationically-curable monomeric
material, the process comprising the following steps: (a) preparing
a mixture of a cationically-curable monomer with a thermally
stable, chemically inactive at room temperature, cationic
photoinitiator; (b) flash-evaporating said mixture in vacuum to
produce a vapor; (c) condensing the vapor to produce a film; and
(d) exposing said film to a radiation source to produce a polymeric
solid film.
2. The process of claim 1, further comprising the step of: (e)
vacuum depositing an inorganic material over said polymeric solid
film.
3. The process of claim 2, wherein steps (b)-(e) are repeated to
produce a multilayer structure.
4. The process of claim 1, further comprising the step of vacuum
depositing an inorganic material over a substrate, and then
condensing the vapor over the inorganic material in step (c).
5. The process of claim 4, wherein the steps of vacuum depositing
an inorganic material and condensing the vapor over the inorganic
material are repeated to produce a multilayer structure.
6. The process of claim 1, wherein said monomer is selected from
the group consisting of epoxies, cycloaliphatic epoxies, vinyl
ethers, styrene, styrene derivatives, glycidyl derivatives, vinyl
pyridines, vinyl carbazole, vinyl-carbazole derivatives, vinyl
imidazole, vinyl-imidazole derivatives, oxetanes, dienes, and
mixtures thereof.
7. The process of claim 1, wherein said cationic photoinitiator is
selected from the group consisting of diaryl iodonium salts,
triaryl sulfonium salts, and mixtures thereof.
8. The process of claim 1, wherein said monomer is a cycloalyphatic
epoxy and said photoinitiator is a diaryl iodonium salt, and the
monomer and photoinitiator are used to produce a low-shrinking film
adapted to adhere to metal, ceramic, and plastic substrates.
9. The process of claim 1, wherein said monomer is selected from
the group consisting of cycloalyphatic epoxies, hydrocarbon styrene
derivatives, or mixtures thereof, said photoinitiator is a diaryl
iodonium salt, and the monomer and photoinitiator are used to
produce a gas- and vapor-barrier coating.
10. The process of claim 1, wherein said monomer is selected from
the group consisting of perfluorodivinyl ether, a perfluoro epoxy,
or mixtures thereof, said photoinitiator is a diaryl iodonium salt,
and the monomer and photoinitiator are used to produce an
anti-smudge, contaminant-resistant coating.
11. The process of claim 1, wherein said monomer is selected from
the group consisting of divinylbenzenes, dienes, or mixtures
thereof, said photoinitiator is a diaryl iodonium salt, and the
monomer and photoinitiator are used to produce an
infrared-transmitting, chemically-inert coating.
12. The process of claim 1, wherein said monomer is selected from
the group consisting of divinylbenzenes, dienes, or mixtures
thereof, said photoinitiator is a diaryl iodonium salt, and the
monomer and photoinitiator are used to produce a low dielectric
constant, high dielectric strength, and low dissipation factor
coating.
13. The process of claim 1, wherein said monomer is selected from
the group consisting of N-vinylcarbazole, vinyl pyridines,
vinylimidazole, divinylbenzene, or mixtures thereof, said
photoinitiator is a diaryl iodonium salt, and the monomer and
photoinitiator are used to produce a photonic coating with
photoconductivity and luminescence properties.
14. The process of claim 4, wherein said inorganic material is
selected from the group consisting of aluminum, aluminum oxide,
silicon oxide, and mixtures thereof.
15. The process of claim 5, wherein said inorganic material is
selected from the group consisting of aluminum, aluminum oxide,
silicon oxide, and mixtures thereof.
16. The process of claim 4, wherein said substrate is selected from
the group consisting of polypropylene, polyethylene,
polytetrafluoroethylene, polyethyleneglycol terphthlate, polyimide,
polyamide, polyphenylsulfides, polycarbonate, and cellulose
derivatives.
17. The process of claim 5, wherein said substrate is selected from
the group consisting of polypropylene, polyethylene,
polytetrafluoroethylene, polyethyleneglycol terphthlate, polyimide,
polyamide, polyphenylsulfides, polycarbonate, and cellulose
derivatives.
18. The process of claim 1, wherein said radiation source is an
electron beam.
19. The process of claim 1, wherein said radiation source is a
source of ultra-violet radiation.
20. The process of claim 1, wherein said mixture of a
cationically-curable monomer with a thermally stable, chemically
inactive at ambient conditions, cationic photoinitiator also
includes a free-radical polymerizable monomer.
21. A process for forming a solid polymeric structure from
cationically-curable monomeric material in a vacuum vapor
deposition unit operating at a predetermined pressure, a
predetermined evaporator temperature, and a predetermined condenser
temperature, the process comprising the following steps: (a)
preparing a mixture of a cationically-curable monomer with a
thermally stable, chemically inactive at ambient conditions,
cationic photoinitiator, said monomer and photoinitiator having
respective vaporization temperatures at said predetermined pressure
that do not exceed said predetermined evaporator temperature, and
having respective decomposition temperatures at said predetermined
pressure that exceed said predetermined evaporator temperature; (b)
evaporating said mixture in said vapor deposition unit to produce a
vapor at said predetermined pressure and evaporator temperature;
(c) condensing the vapor to produce a film; and (d) exposing said
film to a radiation source to produce a polymeric solid film.
22. The process of claim 21, wherein said predetermined pressure is
about 10.sup.-1 to 10.sup.-6 torr.
23. The process of claim 21, wherein said predetermined evaporator
temperature is about 70.degree. C.-350.degree. C.
24. The process of claim 21, wherein said predetermined condenser
temperature is about -20.degree. C. to 30.degree. C.
25. The process of claim 21, wherein said predetermined pressure is
about 10.sup.-1 to 10.sup.-6 torr, said predetermined evaporator
temperature is about 70.degree. C.-350.degree. C., and said
predetermined condenser temperature is about -20.degree. C. to
30.degree. C.
26. The process of claim 21, further comprising the step of vacuum
depositing an inorganic material over said polymeric solid
film.
27. The process of claim 21, further comprising the step of vacuum
depositing an inorganic material over a substrate, and then
condensing the vapor over the inorganic material in step (c).
28. A polymeric structure prepared according to the process of
claim 1.
29. A polymeric structure prepared according to the process of
claim 2.
30. A polymeric structure prepared according to the process of
claim 3.
31. A polymeric structure prepared according to the process of
claim 4.
32. A polymeric structure prepared according to the process of
claim 5.
33. A polymeric structure prepared according to the process of
claim 20.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention is related in general to the process of
manufacture of cationic polymer thin films by vacuum vapor
deposition. In particular, it pertains to a process of flash
evaporation, vapor deposition, and radiation curing of
cation-polymerizable monomers and oligomers.
[0003] 2. Description of the Related Art
[0004] Inorganic and polymeric coatings are used on various
substrates to add or promote desirable properties for particular
applications. For example, foils used to preserve food need to have
very low permeability to oxygen. Therefore, it is desirable and
sometimes necessary to modify the physical properties of polymeric
films to improve their suitability for the intended purpose.
Preferably, the films are directly formed with a composition and
molecular structure characterized by the desired properties.
[0005] Thin films of metals, ceramic and polymers are created by
deposition onto appropriate substrates by a variety of known
processes, most notably through film formation by wet chemistry or
vapor deposition. Chemical processes produce soluble thermoplastic
as well as insoluble thermoset polymers and involve the use of
solvents; thus, thin film formation is achieved through solvent
diffusion and evaporation. As a result, these processes require
relatively long residence times and the undesirable step of
handling solvents.
[0006] Vacuum deposition processes involve the flash evaporation of
a liquid monomer in a vacuum chamber, its deposition at room
temperature or on a cold substrate (referred to in the art as
"crycondensation"), and the subsequent polymerization by exposure
to a high-energy source of radiation, such as electron beam or
ultraviolet radiation. As illustrated schematically in FIG. 1, the
liquid monomer from a supply reservoir 12 is fed through a
capillary tube 14 and an atomizer 16 into the heated evaporator
section of a vacuum deposition chamber 10, where it flash vaporizes
under vacuum. The resulting monomer vapor is then passed into a
condensation section of the unit where it condenses and forms a
thin liquid film upon contact with the cold surface of an
appropriate substrate, such as a film 18, which in turn is in
contact with a cold rotating drum 20 as it progresses from a feed
roll 22 to a take-up roll 24. A metal vaporization unit 26 may also
be used to deposit in line a thin metal layer on the film 18 for
multilayer deposition. The liquid deposited film is then cured by
exposure to an electron-beam or ultraviolet radiation source 28. A
duplicate polymer coating system with a corresponding liquid
monomer supply reservoir 12', capillary tube 14', atomizer 16', and
radiation source 28' may be utilized to apply multiple monomer
coats over the film substrate 18. Since the ultimate objective is
the formation of solid films, the initial liquid monomer must be
capable of polymerization and contain enough reactive groups to
ensure that a sufficiently large polymeric molecule results and
yields a solid product. A conventional plasma-gas treating system
30 is also used to clean and prepare the film 18, if desired.
[0007] This conventional approach of utilizing a polymerizable
monomer as the raw material for thin-film forming processes has
been followed over the years because it is not possible to vaporize
the final polymeric product under the range of operating conditions
of a commercially viable vapor deposition chamber (typically,
10.sup.-3 to 10.sup.-6 torr and 70.degree. C.-350.degree. C.). The
higher temperatures required to effect the vaporization of polymers
having molecular weight greater than about 5,000 would destroy the
polymer. Thus, the practice in the industry has been to identify or
develop polymers having specific characteristics deemed
advantageous for a particular film application. A solid thin film
of the polymer is then formed on a target substrate by evaporating
the corresponding monomer or oligomer, cryocondensing it as a
monomer or olygomer in liquid form and polymerizing or
cross-linking it to reach the required molecular weight to ensure
its solidification. Many variations of this basic approach have
been developed for particular applications, but conventional
prior-art vacuum deposition processes involve the formation of a
solid film by polymerization of a liquid monomer evaporated under
vacuum or atmospheric conditions and recondensed on a cold surface
to obtain the desired film characteristics.
[0008] The high rate of deposition and the better quality of the
coatings produced make the vacuum film-forming process a
commercially preferred technique. Therefore, considerable research
has been conducted to develop processes for improving the
properties of thin films obtained by polymerization of vacuum
deposited monomers and oligomers. See, for example, U.S. Pat. Nos.
5,681,615, 5,440,446, 5,725,909, 5,902,641 and 6,010,751. A new
approach to overcome some process limitations, involving the flash
evaporation of oligomers, has been disclosed in copending U.S. Ser.
No. 09/346,877, hereby incorporated by reference.
[0009] In the prior art, vacuum deposition and radiation curing
have been used only with monomers and oligomers that polymerize via
the free-radical polymerization mechanism. As such, mostly
acrylates and methacrylates have been utilized to produce a variety
of useful film products. When electron-beam radiation is applied,
no initiator is needed because electrons are capable of creating
the free radicals that initiate the polymerization. If ultraviolet
or other high-energy photoradiation is used, free-radical
photoinitiators such as aromatic ketone derivatives are used. These
are non-ionic, easy-to-evaporate organic molecules.
[0010] By contrast, vacuum deposition of cation-polymerizable
monomers or oligomers has not been available because conventional
Lewis-acids and Bronstead-acids cationic initiators cause the
polymerization reaction to start at room temperature before flash
evaporation can be carried out. Even photoactive aryldiazonium
cationic initiator salts are not sufficiently thermally stable to
survive the flash evaporation process. In addition, earlier
generations of vacuum equipment (atomizers, evaporators and
nozzles) were not efficient for flash evaporation of mostly
heterogeneous cationic systems that contain thermally stable,
low-vapor-pressure cationic photoinitiator salts. Moreover, the
commercial availability of cationic polymerization systems
(monomers, oligomers and initiators) was limited.
[0011] These limitations thus made it impractical to attempt to use
flash-evaporation vacuum-deposition technology for cationic
polymerization systems, especially for commercial applications that
require high rates of coating, superior properties, and low costs.
As a result, the advantages afforded by the combination of
flash-evaporation vacuum-deposition coating techniques
(solventless, defect-free, ultra-thin, and in-line metallization)
and cationically polymerized coatings (excellent adhesion, high
barrier, electrochemical stability, high dielectric strength and
low infrared absorption) have not been obtained in a single
product.
[0012] Because of the poor solubility of cationic-photoinitiator
salts in monomer or oligomer blends, most cationic polimerization
reactions require solvents to produce the formation of homogeneous
polymeric products. Solvent-based formulations are inherently
incompatible with vacuum-deposition techniques. However, the
process of flash evaporation followed by vacuum deposition
overcomes the problem of photoinitiator solubility and produces a
clear homogeneous coating because of the very short time between
deposition and curing, which does not allow phase separation, even
starting with a heterogeneous monomer/initiator blend.
[0013] During the last few years, progress has been made in
different areas which set the stage for attempting the processing
of cationically polymerizable monomers and oligomers as commercial
coating materials via flash evaporation, vacuum deposition and
radiation crosslinking techniques. Thermally stable, photoactive,
cationic initiators (e.g., diaryliodonium, triarylsulfonium and
ferrocenium salts) have been developed and have become commercially
available. These cationic initiators are completely inactive at
ambient conditions. They are themselves activated by either
electron beam or ultraviolet radiation. Therefore, the possibility
of combining them with cationically polymerizable monomers and
oligomers in a flash-evaporation process became more realistic.
[0014] Thermally stable, photoactive, cationic initiators have been
used extensively for conventional cationic polymerization
processes, but mostly in solvent systems and only at atmospheric
conditions. See, for example, U.S. Pat. No. 6,020,508. In related
research work, non-ionic, metal free, organic soluble cationic
photoinitiators were also synthesized and investigated for
atmospheric systems (Mikhael et al, Macromolecules, 28, 5951,
1995). These initiators were not initially thought to be good
candidates for flash evaporation because of their very low
volatility.
[0015] Besides the recent developments that led to the commercial
availability of thermally stable cationic photoinitiator salts, a
larger variety of cationically polymerizable monomers and oligomers
(e.g., vinyl ethers, cycloaliphatic epoxy, glycidyl derivatives,
styrene derivatives, divinyl styrene, oxetanes, vinyl pyridine,
vinyl carbazole, vinyl imidazole) have also become commercially
available. In addition, a new generation of monomer-evaporating
units has become available with larger surface areas, multiple
injectors, and shorter path to the nozzle, which justified
attempting the flash evaporation of cationic photoinitiator salts
(which have a much lower vapor pressure than the free radical
photoinitiators used for acrylates) together with cationically
polymerizable monomers and oligomers, hoping to avoid the premature
thermal polymerization or salt separation. The present invention
consists of a viable process for the production of polymer coatings
from cation-polymerizable monomers and oligomers through flash
evaporation, vacuum deposition, and radiation curing.
BRIEF SUMMARY OF THE INVENTION
[0016] The main objective of this invention is the flash
evaporation, vacuum deposition and radiation curing of
cation-polymerizable, non-acrylate, monomers and oligomers.
[0017] Another objective is a process for producing
cation-polymerized coatings at a high production rate.
[0018] Another object is the production of cation-polymerized
coatings in a solvent-free environment.
[0019] Yet another object is the production of cation-polymerized
coatings in high-barrier pinhole-free films.
[0020] Still another objective is a process that can be implemented
to coat large surface areas.
[0021] Another goal is a process that produces cation-polymerized
coatings in highly uniform, defect-free, ultra-thin films (0.01-10
microns).
[0022] Specifically, a goal of the invention is a process for
manufacturing thin films of cation-polymerized polymer composites
with superior barrier properties, electrochemical stability,
dielectric strength and infrared transmittance than exhibited by
prior-art (acrylate-based) coatings.
[0023] Another specific goal is a process that produces hybrid
films of cation-polymerized polymers and inorganic materials, such
as metals and/or ceramics.
[0024] Another objective is a procedure that can be implemented
utilizing prior-art flash-evaporation vapor-deposition
technology.
[0025] A final objective is a procedure that can be implemented
easily and economically according to the above stated criteria.
[0026] Therefore, according to these and other objectives, the
present invention consists of selecting a thermally stable
photoactive (chemically inactive at ambient conditions) cationic
initiator suitable for polymerizing non-acrylate
cationically-polymerizable monomers or oligomers of interest and
capable of vaporization under vacuum and temperature conditions of
available flash-evaporation chambers. Such photoinitiator is mixed
with monomers and/or oligomers similarly suitable for vaporization
under the same vacuum and temperature conditions; and the mixture
is flash evaporated and cryocondensed in conventional manner as a
film on a cold substrate. The resulting vacuum-deposited,
homogeneous, highly uniform layer is then cured with a high-energy
radiation source that causes the cationic photoinitiator to
dissociate into acidic species that initiate the cationic
crosslinking reaction of the monomer/oligomer compounds in its
deposited film form. As a result of the homogeneous, solventless
pinhole-free nature of the vacuum deposition process, the thin-film
polymer product does not suffer from the disadvantages attendant to
prior-art atmospheric processes for cationically-cured polymers. In
addition, because of the versatility afforded by vacuum deposition,
hybrid films of such polymers with inorganic materials are also
easily manufactured in-line during the same process at higher rate
than that of conventional atmospheric coating processes.
[0027] Various other purposes and advantages of the invention will
become clear from its description in the specification that follows
and from the novel features particularly pointed out in the
appended claims. Therefore, to the accomplishment of the objectives
described above, this invention consists of the features
hereinafter illustrated in the drawings, fully described in the
detailed description of the preferred embodiment and particularly
pointed out in the claims. However, such drawings and description
disclose but one of the various ways in which the invention may be
practiced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1 is a schematic representation of a prior-art vacuum
vapor deposition and radiation curing unit.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0029] One aspect of the present invention lies in the recognition
that some thermally stable photoactive cationic initiator salts can
be flash evaporated and vacuum deposited under selected operating
conditions. Based on this realization, it became likely that the
manufacture of polymer-coating thin films by vacuum vapor
deposition of cationically-crosslinkable monomers and oligomers
could also be possible by judiciously selecting vaporizable,
completely inactive at ambient conditions, thermally stable
cationic photoinitiators. Accordingly, the invention consists of
flash evaporating, condensing, and then irradiating a particular
cationically-curable, non-acrylate, monomer or oligomer of interest
mixed with such a thermally stable photoinitiator, so that no
curing occurs prior to exposure of the deposited film to a
high-energy radiation source.
[0030] As used herein, the term oligomer is intended to refer not
only to molecular chains normally designated as such in the art
(typically containing between two and ten monomer molecules) but
also to low-molecular weight polymers; specifically, oligomer is
meant to encompass any polymerized molecule having a molecular
weight sufficiently low to permit its vaporization under vacuum at
a temperature lower than its temperature of thermal decomposition.
With current vacuum technology, such maximum molecular weight is
approximately 5,000, the precise molecular weight depending on the
specific monomer used, but it is understood that greater molecular
weights would become available for inclusion in the practice of the
invention if greater vacuum conditions were obtained. Therefore,
the invention is not to be limited to polymeric chains with
molecular weight less than about 5,000, but is intended to include
any polymeric molecule, herein defined as oligomeric, that is
capable of vaporization at a temperature lower than the temperature
at which it decomposes or otherwise degrades. For convenience of
description, such oligomers are included for the purposes of this
disclosure within the definition of term "monomer," which is mostly
used alone to describe the invention.
[0031] As is well understood in the art, the term cryocondensation
refers to a phase-change process from vapor to liquid obtained upon
contact with a surface having a temperature lower than the dew
point of the gas at a given operating pressure. Since vapor
deposition can also occur at room temperature, the terms
condensation and cryocondensation are used interchangeably for the
purposes of this disclosure to describe the vapor condensation step
of the invention. The term thin film is intended to encompass any
layer of material with a thickness in the order of microns or
sub-microns, the thickness typically consisting of polymeric films
produced either by vapor deposition or by wet-chemistry processes.
The term degradation is used interchangeably with the term
decomposition which includes the premature uncontrolled thermal
polymerization. Finally, any cationically-curable monomer and any
cationic photoinitiator used to practice the invention must have a
vaporization temperature that is below its decomposition
temperature at the operating pressures of conventional vapor
deposition chambers (10.sup.-1 to 10.sup.-6 torr), and that is also
within the temperature operating range of such equipment
(70.degree. C.-350.degree. C.).
[0032] A cationically-curable monomer is defined as any organic
molecule functionalized with one or more electron rich .pi.-bond
and/or hetrocyclic ring structures. These are non-acrylate
molecules that are capable of high-rate cationic
photopolymerization and crosslinking. Table 1 shows examples of
various cationically polymerizable molecules.
1TABLE 1 Examples for Cationically Polymerizable Molecules. Vinyl
ethers 1 Styrene and its derivatives 2 Glycidyl ether derivatives 3
Epoxy derivatives 4 Cycloaliphatic epoxy 5 Vinyl pyridine and its
derivatives 6 Vinyl carbazole and its derivatives 7 Dienes 8
Oxetane derivatives 9 R, is hydrogen or any other organic
moiety
[0033] The main requirement for a cationically polymerizable
monomer is electron-donating atoms or groups attached to the .pi.
bond or the heterocyclic ring. This makes the monomer's
polymerizable functional group electron rich and more reactive
towards the positively charged acidic species produced by the photo
dissociation of the cationic initiator. Epoxies, cycloaliphatic
epoxies, vinyl ethers, styrene, styrene derivatives, glycidyl
derivatives, vinyl pyridines, vinyl carbazoles, oxetanes, and
dienes have all been found to be suitable monomers for the process
of the invention.
[0034] Thermally stable, chemically inactive at room temperature,
cationic photoinitiators are defined as organic salts that
dissociate into acidic species when exposed to high-energy
radiation. These cationic photoinitiators should be thermally
stable enough to survive the flash evaporation process without heat
dissociation which may cause premature polymerization inside the
evaporating unit. (Chemically-stable cationic initiators that
release acidic species when exposed to heat are expressly excluded
from the definition of cationic photoinitiators for the purposes of
this invention, even if they are also activated by exposure to
radiation.) Diaryl iodonium salts and triaryl sulfonium salts have
been found to be particularly suitable initiators to practice the
invention.
[0035] Moreover, for the purposes of the present disclosure,
high-energy radiation is defined as either electron-beam radiation
or naturally occurring or artificial radiation having wavelength
less than about 0.7 micron and capable of producing chemical
changes also referred to in the art as actinic radiation.
[0036] The crux of the invention lies in the realization that some
thermally stable cationic photoinitiators consist of very large
size cations and anions (salt molecules) which, notwithstanding
their ionic nature, are capable of vaporization because of their
relatively weak ionic character. These salts have highly
non-nucleophilic anions, such as PF.sub.6.sup.-, AsF.sub.6.sup.-,
and SbF.sub.6.sup.-. The low nucleophilicity of these anions
reduces the termination and chain transfer processes and allows a
longer propagation step of cationic polymerization reactions.
Accordingly, the inventors successfully tested diaryl iodonium
hexafluoroantimonate and triaryl sulfonium hexafluoroantimonate
salts, under conventional vacuum evaporation and deposition
conditions. The discovery prompted the successful implementation of
flash evaporation, vacuum deposition, and subsequent curing by
irradiation of cationically-curable monomers previously processed
only under atmospheric conditions.
[0037] The following examples illustrate the invention.
EXAMPLE 1
[0038] A cycloaliphatic epoxy monomer
(3,4-epoxycyclohexylmethyl-3,4-epoxy- cyclohexane carboxylate), 99%
w/w, was mixed with a thermally stable cationic photoinitiator
(diaryl iodonium hexafluoroantimonate), 1% w/w. The mixture was
heated to 60.degree. C. and stirred for 10-15 minutes. The blend
was transferred to the monomer delivery unit of a vacuum chamber
(FIG. 1) and injected into a pre-heated evaporator at 200.degree.
C. The material was evaporated under vacuum (about
5.times.10.sup.-4 Torr) and recondensed on a plasma-treated (for
cleaning and preparation) plastic film running at 300 ft/min and in
good contact with a cold drum. Immediately after recondensation,
the condensed liquid layer was irradiated with electron beam
radiation (10 KV, 100 mA) and crosslinked. The resulting plastic
substrate coated with a clear, 0.1-micron, polycycloaliphatic epoxy
film was tested for its adhesion, surface energy, chemical
stability, and mechanical properties.
[0039] The film showed 100% adhesion to the substrate, which was
achieved without adding any adhesion promoter material to the
monomer. This may be attributed to the cationic polymerization
mechanism of the epoxy monomers which creates hydroxyl groups
(--OH) or hydroxide ions during the crosslinking process. These
groups or ions dramatically enhance the adhesion to either plastic
or inorganic (metal or ceramic) substrates.
[0040] The process also produced a high surface-energy (50-54
dyne/cm) coating, which was obtained without oxygenated additives
(e.g., acids, alcohols, ethers). This makes the coating a perfect
material for printing, laminating and adhering. The coating also
exhibited very high chemical stability and mechanical
properties.
EXAMPLE 2
[0041] This example was conducted as Example 1, but increasing the
monomer feed rate to deposit a one-micron crosslinked coating. The
resulting coated film showed properties similar to those reported
for Example 1, with a higher hardness (4H-5H on the pencil test)
for the thicker coating.
EXAMPLE 3
[0042] Same as Example 1, but ultraviolet radiation (300 W/inch
mercury lamp) was used for curing instead of the electron beam. The
same properties were found in the resulting coating.
EXAMPLE 4
[0043] Same as Example 1, where the substrate was a metalized
plastic film. Two different line speeds, 200 and 800 ft/min, were
applied to obtain 1.0 and 0.25 micron coatings, respectively.
Moisture vapor transmittance rate (MVTR) and oxygen transmittance
rate (OTR) were tested for the metalized film coated with
0.25-micron cycloaliphatic epoxy film. Table 2 shows the barrier
data (MVTR, OTR) for metalized uncoated film, metalized
acrylate-coated film and metalized epoxy-coated film of the same
thicknesses.
2TABLE 2 MVTR, 38.degree. C., 90% RH OTR, 23.degree. C., 0% RH Film
Type (g/m.sup.2/day) (cc/m.sup.2/day) Metalized uncoated 2.3 26
polyester film Metalized acrylate 0.23 3.3 coated polyester film
Metalized epoxy coat- 0.09 0.38 ed polyester film
[0044] The same materials and conditions were used to build
multilayer gas-and vapor-barrier structures over a
polyethyleneglycol terphthlate (PET) base film consisting of the
following layers: PET-base-film/Al/epoxy/Al/epoxy/Al/epoxy; and
PET-base-film/epoxy/Al/epox- y/Al/epoxy/Al/epoxy. The second
structure, with an epoxy layer between the base film and the first
metal layer, helps to improve the base-film surface properties, so
that a lower grade PET material can be used to achieve the
performance of higher grade, more expensive substrate materials.
This multilayer coating showed ultra-high barrier for both oxygen
and moisture (WVTR<0.005 cc/m.sup.2/day and OTR<0.005
cc/m.sup.2/day). Similar results are obtainable when other
inorganic barrier materials, such as aluminum oxide, silicon oxide,
and indium tin oxide (ITO), are used in the multilayer structure
instead of aluminum metal. Plasma treatment of any layer could be
used if needed to enhance adhesion between layers.
EXAMPLE 5
[0045] Same as Example 4, where the substrate was a ceramic-coated
(SiO.sub.2 or Al.sub.2O.sub.3) plastic film. Two different line
speeds, 200 and 800 ft/min, were applied to obtain 1.0 and 0.25
micron coatings, respectively.
EXAMPLE 6
[0046] Same as Example 1, where tri(ethylene glycol) divinyl ether
was used as a monomer instead of the cycloaliphatic epoxy. The same
cationic photoinitiator and processing conditions of Example 1 were
used in this experiment. A higher crosslinking rate was observed
for this system, which made it possible to run at higher
speeds.
EXAMPLE 7
[0047] Same as Example 1, where a 1:1 mixture of tri(ethylene
glycol) divinyl ether and
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane carboxylate was used
as a monomer blend.
EXAMPLE 8
[0048] A hybrid cationic/free-radical polymerization system was
tested by mixing 3,4-epoxycyclohexylmethyl-3,4-epoxycyclohexane
carboxylate with hexanediol diacrylate (1:1 ratio) and evaporating
the blend at 200.degree. C. and 5.times.10.sup.-4 torr, then curing
the condensed film with electron beam radiation. Both monomers
copolymerized and produced a clear, hard coating. The hybrid film
coating showed much better adhesion, higher surface energy and more
abrasion resistance than a crosslinked hexanediol diacrylate
coating alone.
EXAMPLE 9
[0049] Same as Example 1, where
3,4-epoxycyclohexylmethyl-3,4-epoxycyclohe- xane carboxylate was
blended with various ratios of a fluorinated epoxy monomer (such as
heptadecafluorononyl oxirane) or a perfluorinated divynil ether.
The blend was used to coat fabrics and non-woven substrates. The
coated substrate showed hydrophobic and oleophobic surface
properties without change in the material's porosity and
breathability. This application (coating fabric and non-woven
fibers) benefits the most from combining flash evaporation vapor
deposition with the cationic polymerization mechanism. Atmospheric
coating is not applicable for these kinds of materials because of
the resulting blocking of pores. Similarly, acrylate coating in
vacuum is not efficient because the liquid monomer deposited on the
backside of the fibers does not receive enough radiation (EB or UV)
for curing. In the case of cationically polymerizable monomers, the
continuing post-radiation reaction (dark curing) helps to fully
cure all monomer deposited on the three-dimensional fabric or
fiber. Surface coating and treatment of fabric and non-woven fibers
have great application in the biomedical and filtration
industries.
EXAMPLE 10
[0050] Same as Example 9, where the substrate was a special-grade
polyester film intended for display applications. As little as
0.015 microns of that fluorinated coating turned the base film into
an anti-smudge, contaminant-resistant surface. The contact angle
with water was >105 degrees and the surface energy was <25
dyne/cm.
EXAMPLE 11
[0051] Same as Example 1, where a hydrocarbon monomer
(divinylbenzene, a diene, or a cycloaliphatic diene) was used. The
crosslinked thermoset hydrocarbon coating showed a very high
chemical resistance towards acids, bases and organic solvents. A
one-micron thick coating of this material is characterized by very
low infrared absorption (<3%), especially at wavelengths between
4 and 12 microns. It is impossible to achieve this level of
infrared transmittance with acrylate-based (free-radical cured)
films. The carbon-oxygen bond in acrylate polymers strongly absorbs
infrared light at 4-12 micron range. Infrared transmitting
materials are very important for heat mirror and signature control
applications. The same chemically-inert coating was characterized
with a low dielectric constant (<2.5), a low dissipation factor
(<1%) and a high dielectric strength, which make the coating
perfect for battery and capacitor applications.
EXAMPLE 11
[0052] Same as Example 1, where electron donor monomers, such as
N-vinylcarbazole, vinyl pyridine, or vinylimidazole, were blended
with various ratios of an electron-rich crosslinking agent such as
divinyl benzene and doped with hole-transporting and
electron-transporting components, such as triphenyl diamine
derivatives and aluminum quinolinolate respectively. The blend was
used to coat electrically conductive ITO-coated glass or plastic
substrates. The coated substrate shows photoconductive and
luminescence properties. This kind of photonic coating has
advantageous applications in xerography and light-emitting
industries. Acrylate based coatings produced less efficient
photoconductive and luminescence materials because of the
reactivity of the acrylate ester moiety, which acts as a charge
trap.
[0053] These examples demonstrate the feasibility of manufacturing
thin polymeric coatings by vapor deposition of cation-polymerizable
monomers. The process possesses the advantages of being
solvent-free, environmentally safe, and high rate. Large areas of
coating (with web speeds higher than 1,500 ft/min) can be applied
at low cost. The resulting products are in the form of highly
uniform, defect-free, excellent adhesion coatings, which provide a
better function as moisture and vapor barrier materials than
heretofore achieved with prior-art acrylate-based films. Also the
vacuum deposited cationic coatings exhibit very low dissipation
(<1%) and high dielectric strength which make the coatings very
promising materials for capacitor applications. Moreover, with this
invention, entirely hydrocarbon (polyethylene-like or
polystyrene-like materials) crosslinked thin coatings can be
produced at high rate for various applications that require low
infrared absorption, high chemical stability, low surface energy
and low dielectric constant. In addition, the process of the
invention is suitable for in-line vapor deposition of metal and
ceramic layers to produce hybrid films.
[0054] With respect to prior-art processes of flash vaporization
and cryocondensation of acrylate and metacrylate monomers, which
polymerize through free-radical mechanisms, the polymerization
process, which involves only materials that polymerize via cationic
mechanisms, has the advantages of no oxygen inhibition;
post-radiation curing (dark curing); low volume shrinkage;
excellent adhesion to plastics and metals; low level of extractable
components; absence of odor in the cured film; and low skin
irritation.
[0055] Moreover, the cationically-curable monomers of the invention
produce polymeric films that have better properties than attainable
with prior-art materials. For example, the process enables the
manufacture of films that provide higher moisture and oxygen
barriers; low shrinking and better adhesion to metals, ceramics and
plastics; high chemical resistance to acids, bases, and solvents;
and low infrared absorption for low-emissivity coatings. Finally,
the reactants' low viscosity and unsusceptibility to thermal
polymerization make the process of evaporation easier to carry
out.
[0056] Various changes in the details, steps and components that
have been described may be made by those skilled in the art within
the principles and scope of the invention herein illustrated and
defined in the appended claims. For example, polypropylene,
polyethylene, polytetrafluoroethylene (TEFLON.RTM.), polyimide
(KAPTON.RTM.), polyamide (NYLON.RTM.), polyphenylsulfides,
polycarbonate, cellulose derivatives, are equivalent materials that
may be used instead of PET or other substrates to practice the
invention. Therefore, while the present invention has been shown
and described herein in what is believed to be the most practical
and preferred embodiments, it is recognized that departures can be
made therefrom within the scope of the invention, which is not to
be limited to the details disclosed herein but is to be accorded
the full scope of the claims so as to embrace any and all
equivalent processes and products.
* * * * *