Coating apparatus – Gas or vapor deposition – Crucible or evaporator structure
Reexamination Certificate
1999-09-20
2001-06-12
Crispino, Richard (Department: 1734)
Coating apparatus
Gas or vapor deposition
Crucible or evaporator structure
C118S724000, C118S716000, C427S255600, C427S488000
Reexamination Certificate
active
06245150
ABSTRACT:
This invention relates to an apparatus and method for generating and condensing a vapor onto a substrate to form a coating. More specifically, this invention relates to such an apparatus and method in which the vapor is generated from an atomized mist comprising the materials to be coated.
BACKGROUND OF THE INVENTION
Coatings are applied to a wide variety of substrates for widely divergent purposes. Just a few examples of the many different types of coatings include adhesive coatings, primer coatings, decorative coatings, protective hard coatings, varnish coatings, antireflective coatings, reflective coatings, interference coatings, release coatings, dielectric coatings, photoresist coatings, conductive coatings, nonlinear optic coatings, electrochromic/electroluminescent coatings, barrier coatings, biologically-active coatings, biologically inert coatings, and the like. Such coatings can be applied to substrates that are made from many different materials and have many different shapes. For example, in terms of materials, substrates can be metal, wood, cloth, polymeric, ceramic, paper, mineral, glass, composite, and the like. In terms of shape, substrates can be flat, curved, undulating, twisted, microstructured, smooth, rough, porous, particulate, fibrous, hollow shaped, three-dimensional, regular or irregular surfaced, and the like.
In conventional industrial coating processes, an admixture (which can be an emulsion, solution, slurry, two-phase fluid mixture, and the like) comprising the coating constituents and a suitable solvent is applied to the substrate using a suitable coating technique such as spraying, roll coating, brush coating, spin coating, or the like. The coated composition then is typically dried and cured in order to solidify the coating. During drying, the solvent is removed from the coating and then discarded into the environment or recovered.
The solvent is generally an essential component of the coating composition for a variety of reasons. First, the solvent helps ensure that the coating composition has a suitable coating viscosity. The solvent also helps ensure that the coating composition can be applied to the substrate evenly to form a uniform coating. The solvent may also provide the composition with an acceptable shelf-life.
The presence of the solvent, however, has many drawbacks. If the solvent is to be discarded after use, the solvent becomes waste in the environment. This is particularly problematic if the solvent is hazardous. Indeed, disposal of hazardous solvents tends to involve expensive and elaborate disposal schemes regulated by governmental authorities in an effort to minimize harm to the environment resulting from the disposal. Solvent recovery, therefore, is often preferred to solvent disposal. However, solvent recovery, like solvent disposal, also suffers from several disadvantages. Firstly, solvent recovery tends to require expensive, capital intensive procedures and equipment. Sometimes, the materials used to clean a solvent are hazardous wastes themselves.
In short, the need to handle the solvents from coating operations is a serious burden in industry. Accordingly, it would be desirable to find a way to carry out coating operations with minimal solvent, or more preferably, in solventless fashion, to avoid the burden of having to dispose of, or recover, left-over solvent. It would also be desirable to find a way to accomplish this for a wide variety of different coating compositions.
SUMMARY OF THE INVENTION
The present inventors have now discovered an extremely versatile coating system and method that allows coatings to be formed from a wide variety of coatable compositions that are entirely free of any solvents or have relatively little solvent in amounts effective to help dissolve one or more components of such compositions. This eliminates all of the environmental drawbacks and concerns associated with solvents used in conventional coating processes.
The present invention is based upon the concept of atomizing a fluid coating composition, which preferably is solvent-free, to form a plurality of fine liquid droplets. The droplets are contacted with a carrier gas, which causes the droplets to vaporize even at temperatures well below the boiling point of the droplets. Vaporization occurs quickly and completely, because the partial pressure of the vapor in admixture with the carrier gas is still well below the vapor's saturation pressure. When the gas is heated, the gas provides the thermal/mechanical energy for vaporization.
After vaporization, the vapor flows to the substrate to be coated. The substrate is maintained at a temperature well below the condensation point of the vapor. This causes the vapor to condense as a thin, uniform, substantially defect-free coating that can be subsequently cured, if desired, by various curing mechanisms. The coating may be continuous or discontinuous. The present invention is particularly useful for forming thin films having a thickness in the range from about 0.001 &mgr;m to about 5 &mgr;m. Thicker coatings can be formed by increasing the exposure time of the substrate to the vapor, increasing the flow rate of the fluid composition, increasing the temperature of the carrier gas, and/or increasing the pressure of the carrier gas. For flexible web substrates, increasing the exposure time of the substrate to the vapor can be achieved by adding multiple vapor sources to the system or by decreasing the speed of the web through the system. Layered coatings of different materials can be formed by sequential coating depositions using a different coating material with each deposition.
The principles of the present invention may be practiced in a vacuum. Advantageously, however, atomization, vaporization, and coating can occur at any desired pressure, including ambient pressure. This avoids the need to rely upon costly vacuum chambers commonly used in previously known vapor coating processes. As another advantage, atomization, vaporization, and coating can occur at relatively low temperatures, so that temperature sensitive materials can be coated without degradation that might otherwise occur at higher temperatures. The present invention is also extremely versatile. Virtually any liquid material, or combination of liquid materials, having a measurable vapor pressure can be used to form coatings.
Generally, atomization of the fluid coating composition can be accomplished using any atomization technique known in the art, including ultrasonic atomization, spinning disk atomization, and the like. In particularly preferred embodiments, atomization is achieved by energetically colliding a stream of the carrier gas with a stream of the fluid composition. Preferably, the carrier gas is heated, and the fluid stream flow is laminar at the time of collision. The energy of the collision breaks the preferably laminar flow fluid coating composition into very fine droplets. Using this kind of collision to achieve atomization is particularly advantageous, because it provides smaller atomized droplets with a narrower size distribution and a more uniform number density of droplets per volume than can be achieved using some other atomization techniques. Additionally, the resultant droplets are almost immediately in intimate contact with the carrier gas, resulting in rapid, efficient vaporization. Although the present invention may be used to carry out coating operations in a vacuum, the use of gas collision for atomization is less suitable for use in vacuum chambers because the carrier gas would tend to increase the pressure in the chamber.
In one aspect, the present invention relates to a method of forming a coating on at least a portion of a surface of a substrate. A stream of a carrier gas is caused to collide with a stream of a fluid composition. The collision occurs under conditions such that vaporization of substantially all of the fluid composition occurs so as to form a vapor having a condensation temperature. The vapor is caused to flow to the surface of the substrate due to the velocity and momentum of th
Blette Russell E.
Fleming Robert J.
Lyons Christopher S.
Ruta Constantin I.
3M Innovative Properties Company
Crispino Richard
Little Douglas B.
Tadesse Yewebdar T
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