Vaccum deposition of cationic polymer systems

Details Vaccum deposition of cationic polymer systems Vaccum deposition of cationic polymer systems

2001-02-13

2002-10-22

Berman, Susan W. (Department: 1711)

Coating processes

Direct application of electrical, magnetic, wave, or...

Polymerization of coating utilizing direct application of...

C427S509000, C427S585000, C427S255110, C427S255600, C427S255700, C522S015000, C522S025000, C522S029000, C522S031000, C522S066000, C522S167000, C522S168000, C522S170000, C522S181000, C522S188000, C522S186000

Reexamination Certificate

active

06468595

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
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.
2. Description of the Related Art
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.
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.
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.
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
−3
to 10
−6
torr and 70° C.-350° 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.
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 U.S. Pat. No. 6,270,841, hereby incorporated by reference.
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.
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.
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.
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.
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.,

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