Plasma enhanced polymer deposition onto fixtures

Details Plasma enhanced polymer deposition onto fixtures Plasma enhanced polymer deposition onto fixtures

1998-12-16

2001-04-17

Beck, Shrive (Department: 1762)

Coating processes

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

Polymerization of coating utilizing direct application of...

C427S398100, C427S255230, C427S569000

Reexamination Certificate

active

06217947

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to a method of making plasma polymerized films on a fixture.
As used herein, a fixture is a discrete item. Examples include but are not limited to plumbing fixtures, cabinetry fixtures, tools, optical fixtures including reflectors, light covers, solar collectors and combinatiions thereof which are clearly distinct from a continuous item for example a sheet, wire, or rope.
As used herein, the term “(meth)acrylic” is defined as “acrylic or methacrylic”. Also, “(meth)acrylate” is defined as “acrylate or methacrylate”.
As used herein, the term “cryocondense” and forms thereof refers to the physical phenomenon of a phase change from a gas phase to a liquid phase upon the gas contacting a surface having a temperature lower than a dew point of the gas.
BACKGROUND OF THE INVENTION
The basic process of flash evaporation is described in U.S. Pat. No. 4,954,371 herein incorporated by reference. This basic process may also be referred to as polymer multi-layer (PML) flash evaporation. Briefly, a radiation polymerizable and/or cross linkable material is supplied at a temperature below a decomposition temperature and polymerization temperature of the material. The material is atomized to droplets having a droplet size ranging from about 1 to about 50 microns. An ultrasonic atomizer is generally used. The droplets are then flash vaporized, under vacuum, by contact with a heated surface above the boiling point of the material, but below the temperature which would cause pyrolysis. The vapor is cryocondensed on a substrate then radiation polymerized or cross linked as a very thin polymer layer. The material may include a base monomer or mixture thereof, cross-linking agents and/or initiating agents. A disadvantage of the flash evaporation method with radiation cross linking is that it requires two sequential steps, cryocondensation followed by curing or cross linking, that are both spatially and temporally separate. A disadvantage of this radiation crosslinking method is the time between cryocondensation and curing permitting the cryocondensed monomer to flow or run, especially on fixtures having irregular non-flat geometry, leading to non-uniformity of coating (
FIG. 1
a
) so that the coating surface
150
is geometrically different from the substrate surface
160
. Reducing surface temperature can reduce the flow somewhat, but should the monomer freeze, then cross linking is adversely affected. Using higher viscosity monomers is unattractive because of the increased difficulty of degassing, stirring, and dispensing of the monomer
The basic process of plasma enhanced chemical vapor deposition (PECVD) is described in THIN FILM PROCESSES, J. L. Vossen, W. Kern, editors, Academic Press, 1978, Part IV, Chapter IV-1 Plasma Deposition of Inorganic Compounds, Chapter IV-2 Glow Discharge Polymerization, herein incorporated by reference. Briefly, a glow discharge plasma is generated on an electrode that may be smooth or have pointed projections. Traditionally, a gas inlet introduces high vapor pressure monomeric gases into the plasma region wherein radicals are formed so that upon subsequent collisions with the substrate, some of the radicals in the monomers chemically bond or cross link (cure) on the substrate. The high vapor pressure monomeric gases include gases of CH
4
, SiH
4
, C
2
H
2
, C
2
H
2
, or gases generated from high vapor pressure liquid, for example styrene (10 torr at 87.4° F. (30.8° C.)), hexane (100 torr at 60.4° F. (15.8° C.)), tetramethyldisiloxane (10 torr at 82.9° F. (28.3° C.) 1,3,-dichlorotetra-methyldisiloxane) and combinations thereof that may be evaporated with mild controlled heating. Because these high vapor pressure monomeric gases do not readily cryocondense at ambient or elevated temperatures, deposition rates are low (a few tenths of micrometer/min maximum) relying on radicals chemically bonding to the surface of interest instead of cryocondensation. The low deposition rate is not useable in a high rate industrial application. Remission due to etching of the surface of interest by the plasma competes with deposition of the radicals. Lower vapor pressure species have not been used in PECVD because heating the higher molecular weight monomers to a temperature sufficient to vaporize them generally causes a reaction prior to vaporization, or metering of the gas becomes difficult to control, either of which is inoperative.
According to the state of the art of making plasma polymerized films, PECVD and flash evaporation or glow discharge plasma deposition and flash evaporation have not been used in combination. However, plasma treatment of a substrate using glow discharge plasma generator with inorganic compounds has been used in combination with flash evaporation under a low pressure (vacuum) atmosphere as reported in J. D. Affinito, M. E. Gross, C. A. Coronado, and P. M. Martin, A Vacuum Deposition Of Polymer Electrolytes On Flexible Substrates. “Paper for Plenary talk in A Proceedings of the Ninth International Conference on Vacuum Web Coating”, November 1995 ed R. Bakish, Bakish Press 1995, pg 20-36., and as shown in
FIG. 1
b
. In that system, the plasma generator
100
is used to etch the surface
102
of a moving substrate
104
in preparation to receive the monomeric gaseous output from the flash evaporation
106
that cryocondenses on the etched surface
102
and is then passed by a first curing station (not shown), for example electron beam or ultra-violet radiation, to initiate cross linking and curing. The plasma generator
100
has a housing
108
with a gas inlet
110
. The gas may be oxygen, nitrogen, water or an inert gas, for example argon, or combinations thereof. Internally, an electrode
112
that is smooth or having one or more pointed projections
114
produces a glow discharge and makes a plasma with the gas which etches the surface
102
. The flash evaporator
106
has a housing
116
, with a monomer inlet
118
and an atomizing nozzle
120
, for example an ultrasonic atomizer. Flow through the nozzle
120
is atomized into particles or droplets
122
which strike the heated surface
124
whereupon the particles or droplets
122
are flash evaporated into a gas that flows past a series of baffles
126
(optional) to an outlet
128
and cryocondenses on the surface
102
. Although other gas flow distribution arrangements have been used, it has been found that the baffles
126
provide adequate gas flow distribution or uniformity while permitting ease of scaling up to large surfaces
102
. In the method of radiation curing, a curing station (not shown) is located downstream of the flash evaporator
106
. The monomer may be an acrylate (
FIG. 1
b
). This system was for planar layer coatings. With radiation curing, the time between deposition and curing permits flow of thicker coating layers leading to non-uniformity of coating on non-uniform surfaces or tilted planar surfaces.
Therefore, there is a need for an apparatus and method for coating fixtures with polymerized layers at a fast rate while avoiding flow of the coating.
SUMMARY OF THE INVENTION
The present invention is a method of making a plasma polymerized film on a fixture. More specifically, the method is for making a self-curing polymer layer, especially self-curing PML polymer layer on a fixture. The method relies upon a combination of flash evaporation with plasma enhanced chemical vapor deposition (PECVD) that provides the unexpected improvements of permitting use of low vapor pressure monomer materials in a PECVD process and provides a self curing from a flash evaporation process at a rate surprisingly faster (2 orders of magnitude or more) than standard PECVD deposition rates. Another advantage of the present invention is the ability to make a conformal coating on a fixture. Because of rapid self curing, the monomer has less time to flow and is therefore more uniformly thick.
The method of the present invention has the steps of (a) flash evaporating a liquid monomer forming an evaporate; (b) passing the evaporate to a glow di

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