Coating processes – Spraying – Moving the base
Reexamination Certificate
2000-02-29
2002-03-05
Beck, Shrive P. (Department: 1762)
Coating processes
Spraying
Moving the base
C427S162000, C427S163100, C427S164000, C427S240000, C118S052000, C118S320000
Reexamination Certificate
active
06352747
ABSTRACT:
DESCRIPTION OF THE INVENTION
The present invention relates to a process for producing uniform coatings on circular substrates and the coated substrates produced by the process. In particular, the present invention is directed to an improved spin and spray coating process that optimizes the use of coating materials. More particularly, the present invention relates to efficiently producing optical elements having uniform coatings or imbibed areas containing photochromic compounds. Further, the coated optical elements meet commercially acceptable “cosmetic” standards for optical coatings applied to optical elements, e.g., lenses.
Photochromic compounds exhibit a reversible change in color when exposed to light radiation involving ultraviolet rays, such as the ultraviolet radiation in sunlight or the light of a mercury lamp. Various classes of photochromic compounds have been synthesized and suggested for use in applications in which a sunlight-induced reversible color change or darkening is desired.
In recent years, photochromic articles, particularly photochromic plastic materials for optical applications, have been the subject of considerable attention. In particular, photochromic ophthalmic plastic lenses have been investigated because of the weight advantage they offer, vis-à-vis, glass lenses. Moreover, photochromic transparencies for vehicles, such as cars and airplanes, have been of interest because of the potential safety features that such transparencies offer.
Methods for producing uniform coatings on substrates have been disclosed in numerous patents. U.S. Pat. No. 3,494,326 describes a spin-coating machine that includes a sealed compartment and means for providing a pressure differential in the area of the spinning lens. The controlled withdrawal and flow of air around the spinning lens due to the pressure differential reportedly enables providing a uniform coating on certain elongated lens shapes.
U.S. Pat. No. 5,094,884 describes an apparatus for applying a uniform layer of a fluid material on a substrate by using a dispensing nozzle having a rectangular or oblong shaped opening. The fluid material is dispensed while the nozzle moves along a radial path inwardly from the peripheral edge of the substrate.
U.S. Pat. No. 5,246,499 describes a coating system for applying a scratch-resistant coating to plastic ophthalmic lenses. The coating station of this system includes a coating arm assembly having nozzles which can be moved radially inwardly and outwardly over spinning lenses to apply the coating solution.
U.S. Pat. No. 5,514,214 describes an apparatus for and method of applying a UV curable monomer to the surface of an ophthalmic lens or mold. The substrate is sprayed with a UV curable solution while spinning at a high rate of speed to achieve a uniform coating. The coated substrate is then moved to a curing chamber to polymerize the monomer on the lens or mold.
U.S. Pat. No. 5,571,560 describes a method of coating a substrate that minimizes waste using a proximity dispenser that dispenses a liquid coating in the form of a stream from nozzles placed from 5 to 10 mm above the substrate.
U.S. Pat. No. 5,685,908 describes an apparatus for coating multifocal lens elements. The coating solution is applied using a nozzle that may be tilted while the lens is spun about an axis offset from the geometric center.
U.S. Pat. No. 5,766,354 describes a spin-coating device having the means to position the substrate at a predetermined angle while the fluid coating is dropped onto the rotating substrate.
A process has now been discovered for coating a substantially circular substrate with a liquid coating material dispensed onto the surface of a spinning substrate through a dispenser positioned above the substrate surface in a pattern which forms a uniform coating and reduces the amount of coating required. The improved process comprises the steps of applying the liquid coating material through a single opening in the dispenser onto the substantial center of the spinning substrate, and then moving radially the dispenser opening to near to the edge of the substrate while maintaining the dispenser opening at a predetermined distance from the substrate surface for substantially the entire radius of the substrate.
DETAILED DESCRIPTION
The dispenser used in the process of the present invention is one that produces a pattern having greater than 90%, preferably greater than 95%, and more preferably, greater than 99% of the dispensed material within the boundaries of the pattern. The term “pattern” as used herein describes a cross-section of the droplets and/or fluidized stream dispensed from the nozzle in a plane perpendicular to the direction of the discharge from the nozzle. Typically, the pattern may have an outside diameter of 1.2 inches (3 cm) when the coating material is dispensed through a single opening at a distance of from 0.3 to 0.6 inches (0.76 to 1.52 cm) from the substrate. Preferably, the pattern has an outside diameter of up to 1 inch (2.54 cm) when the distance is from 0.37 to 0.5 in (0.94 to 1.27 cm) and more preferably, the pattern appears as a ring having an uncoated inside circular region of about 0.4 inches (1.0 cm). Any type of disperser that produces the desired pattern may be used.
Preferably, the dispenser used in the process of the present invention is a Select Coat® Slim Swirl Applicator fitted with a swirl nozzle. This dispenser is described in U.S. Pat. No. 5,336,320 and Customer Product Manual, Part 303776A from the Nordson Corporation, both of which are incorporated herein by reference. The diameter of the aforementioned swirl nozzle may range from at least 0.008 inches (0.02 cm) preferably, at least 0.016 inches (0.04 cm) and more preferably, at least 0.020 inches (0.05 cm) to not more than 0.050 inches (0.13 cm), preferably, not more than 0.040 inches (0.1 cm) and most preferably, not more than 0.030 inches (0.076 cm). The diameter of the swirl nozzle used may range between any combination of these values, inclusive of the recited values.
In addition to the single opening in the swirl nozzle for dispensing fluid, there are 12 air holes angled toward the fluid discharged from the nozzle. The amount of air supplied to the dispenser has an effect on the pattern formed. For example, a fluid stream forming amount of air under pressure, i.e., an amount of air that does not cause the atomization or formation of individual droplets, e.g., from 1 to 3 pounds per square inch (psi) (6.89×10
3
to 2.07×10
4
Newtons per meter
2
(N/m
2
)) produces a coating pattern having a donut or ring-like pattern, whereas an atomizing amount of air pressure, e.g., greater than 10 psi (6.89×10
4
N/m
2
), produces a filled-in circular pattern.
In the process of the present invention, the dispenser is attached to a means for moving it vertically and horizontally, such as a robotic arm. The radial horizontal movement of the robotic arm may be accomplished using a telescopic movement or by using a fixed arm that swings in an arc over the spinning substrate. The angle at which the dispenser is maintained during application of the coating material ranges from perpendicular to the substrate to 45 degrees from perpendicular to the substrate.
A programmable logic controller (PLC) may be used to coordinate the movement of the dispenser, the supply of the coating material and the rotation of the substrate. In one contemplated embodiment, a controller is used to move the dispenser over the substantial center of the substrate, spin the substrate at a first speed to wet the surface, e.g., from 100 to 500 rpm, initiate the dispensing of coating material, move the dispenser radially from the center to the edge of the substrate while maintaining a distance of from 0.3 to 0.6 inches (0.76 to 1.52 cm) to the substrate surface, and spin the coated substrate at a second speed to form a uniform coating, e.g., from 500 to 3000 rpm. The controller may also direct the placement and removal of substrates from a substrate holder attached to means for rotating the substrate at variable spe
Blackburn William P.
Bowles, III Robert J.
Levesque Michael B.
Maldonado Ernesto
Beck Shrive P.
Calcagni Jennifer
Mallak Frank P.
PPG Industries Ohio Inc.
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