GB1587538A - Coating of plastics optical elements - Google Patents

Description

(54) COATING OF PLASTICS OPTICAL ELEMENTS (71) We, AMERICAN OPTICAL CORPORATION, a corporation organised under the laws of the State of Delaware, 14 Mechanic Street, Southbridge, State of Massachusetts, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed to be particularly described in and by the following statement: This invention relates to a method of coating plastics optical elements, such as ophthalmic lenses made of synthetic polymers.

Synthetic polymers are used extensively for the manufacture of lenses, since such lenses can be manufactured inexpensively by casting or injection moulding, which methods enable lenses of complicated surface configurations to be obtained directly without an expensive grinding operation.

Such lenses often have inadequate abrasion and scratch resistance, and it is therefore conventional to provide them with abrasion and scratch-resistant coatings. Such coatings are generally applied by vacuum deposition while the lenses are supported in a vacuum chamber.

The disadvantage of this method is that a coating tends to be vacuum. deposited on all the surfaces present in the vacuum chamber, such as the internal walls of the vacuum chamber, the lens support structure, thickness monitoring systems and heaters.

This undesirable coating of parts of the apparatus necessitates frequent cleaning, in order to prevent outgassing, dust contamination, and malfunctioning of various mechanical parts. Several methods of facilitating cleaning have been proposed, including the following: (1) All surfaces exposed during vacuum deposition are constructed of very hard material such as stainless steel, or plated to form hard surfaces with a material such as Kanigen (an electroless nickel coating material which is very hard) so that undesired coatings can be removed by abrasion, for example, by steel wool scrubbing or other mechanical means; (2) The exposed surfaces are coated with a protective layer of a non-stick polymer material such as Teflon (Teflon is a Trade Mark) which is a waxy, opaque polytetrafluoroethylene.

Undesired coatings can be removed by a mechanical operation, e.g. scrubbing or a chemical operation, e.g. solvent washing; and (3) The covering of all exposed surfaces with disposable metal foils, such as aluminium foil.

None of the above-mentioned methods has proven fully satisfactory. The first method is labour-intensive and the cleaning of the surfaces by scrubbing, or sandblasting, can often damage the close tolerances necessary in the coating manufacturing procedures carried out on ophthalmic substrates. The second method involving the use of Teflon is costly and, in addition, the expensive coating is easily damaged by the hand tools which may be necessary for mechanical cleaning; furthermore Teflon may not function as a non-stick material for all coating materials. The use of disposable foils is only effective in covering large, substantially flat areas such as chamber walls but is not satisfactory on other fixtures such as the fixtures used to support the lenses, which fixtures often have surfaces with complex geometry. In some instances, the surfaces may have moving parts.

According to the present invention, there is provided a method of coating a plastics optical element by vacuum deposition while said element is mounted on a re-usable support structure, in which at least those surfaces of the support structure which are exposed to coating material during vacuum deposition have thereon a protective layer comprising a polar polymer, said protective layer being applied in the form of a solution of the polar polymer.

Preferably a protective layer as just described is also present on other surfaces in a vacuum chamber used for the vacuum deposition.

The polar polymer used in the protective layer must have polar groups, preferably hydroxyl groups. The polymer may be, for example, polyethylene oxide, polyvinyl pyrrolidone, polyacrylamide or polymethacrylic acid. Another possible polar polymer is polyvinyl alcohol (PVA). Of these polymers, polyethylene oxide, such as the material avaiiable commercially under the Trade Mark Carbopol, is particularly preferred. Carbopol is relatively impure, and purer grades of poly ethylene oxide can also be used.

The solvent for the polar polymer solution is preferably a mixture of an organic com pound (which should be relatively easily, rapidly evaporable) and water, the mixture preferably comprising 5-95 percent of the organic compound with the remainder being water. Suitable such organic compounds in clude, for example, acetone, methanol, ethanol, propanol and iso-propanol. Methanol is preferred. The solution preferably contains about 5% by weight of the polar polymer.

Higher concentrations of polymer will build up a coating faster but there are attendant handling problems. Too much polymer in the solvent may give a viscous, or syrupy, material which is difficult to spray. The concentration of polymer is therefore preferably less than 10% by weight, more preferably not more than 5% by weight.

The protective layer preferably contains a dye which is chemically inert with respect to the coating material to be applied by vacuum deposition and also with respect to the plastics material of which the optical element is made.

The use of such a dye gives an indication of whether or not the protective layer is com plete and also of whether or not the latter layer has been completely removed during cleaning.

For example, where the protective layer comprises Carbopol as the polar polymer, the solution thereof may contain approximately 0.2 grams of a dye, such as Rodimens Dye, per litre, an example of such a solution being 500 millilitres of water (demineralized), 500 millilitres of methanol, 5.5 grams of Carbopol and 0.2 grams of Rodimens dye. This solution may be prepared by mixing in an ordinary elec tric blender at blend speed adding the Carbopol and dye to the circulating mixture of water and methanol. Mixing is typically continued for two minutes the resulting solution being allowed to stand for ten minutes to dissipate air bubbles.

Dyes other than Rodimens may be used. We prefer red or violet dyes on grey metal surfaces, since they contrast satisfactorily with the latter surface. Those skilled in the art can easily select their preference from "Color Index", 3rd Edition, Vol. 5, by The Society of Dyes and Colorants, published by the American Association of Textile Chemists and Colorists, published by Chorley & Pickersgille Ltd., Leeds and London, pgs. 5077-5086. We prefer dyes which are water soluble, such as Rodimens Basic Red 1-71, or other types such as Basic Violet 1-41.

As mentioned above, the protective layer is applied in the form of a solution of the polar polymer. The solution may be applied by any convenient conventional means such as, for example, spraying, dipping or brushing, of which spraying is preferred, particularly for coating inside surfaces of large commercial vacuum chambers. Relatively small support structures, for example as shown in the accompanying drawings, may be coated by dipping.

Of the above-mentioned polar polymers, PVA is unsuitable for application by spraying, but it can be applied by brushing or dipping. The remainder of the polar polymers can be applied by any of these methods, spraying being preferred.

The coated surfaces may be allowed to dry at room temperature, or inside an oven, and the resulting protective layers should be dry before evacuation of the vacuum chamber. The protective layer has been found to resist outgassing in vacuo, particularly when the layer is very thin, i. e. on the order of 1 micrometre, and when properly dried.

It has been found that most metal and dielectric evaporants adhere well to the protective layer during and after vacuum deposition. When the vacuum-deposited coating has accumulated, the support structure can be taken from the vacuum chamber and placed in a solvent bath, an ultrasonic cleaning tank, or even a commercial dishwasher to clean the unwanted deposit together with the protective layer. The support structure can then be dried and provided with a fresh protective layer before a further vacuum deposition cycle.

The removal of unwanted deposits is thus cheap, convenient, safe, complete and repeatable without deleterious effects on the surface quality or mechanical alignment of the optical elements being coated. The removal does not produce dust and is, therefore, compatible with coating of the optical elements in clean-room conditions. The preferred polar polymers for use in the protective layer are non-toxic and are water-soluble (they can therefore be removed by water washing).

The preferred material for application by vacuum deposition is quartz.

In order that the present invention be more clearly understood, an embodiment thereof will now be described by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side elevation of a support structure for use in the method according to the invention; and Figure 2 is a sectional side elevation of the support structure of Figure 1 along the line 2-2.

Referring to Figure 1, there is shown a support structure for ophthalmic-quality plastics optical elements, the support structure comprising a panel 10 having formed therethrough a plurality of apertures 11. Each aperture has formed thereabout a circular raised ledge or ring 12. A plurality of wing nuts 13 are shown threaded over complementary bolts extending through apertures formed in the panel sections 10.

Referring to Figure 2, in preferred practice a pair of panels 10, 1 0A are joined together by the wing nuts 13 and complementary mating bolts. A rotatable, spring-loaded clip 20 goes over slots 10C to mount the support structure in a coating chamber (not shown). The panels 10, 10A are identical and when mounted together in pairs adjacent raised rings 12 and 12A, together with aligned apertures 11, are arranged to receive and hold in place lenses 15 to be coated.

According to the invention, at least those surfaces of the support structure which are exposed to coating material during vacuum deposition have thereon a protective layer of a polar polymer, the protective layer being applied in the form of a solution of the polar polymer.

All surfaces to be provided with a protective layer should be first thoroughly cleaned, for example, in a commercial dishwasher with an ordinary dishwashing detergent. A preferred dishwashing detergent is "Calgonite" (Trade Mark) in which the surfaces are preferably first soaked for at least 15 minutes and then placed in a dishwasher with additional detergent. Preferably, the support structure is placed on end or in such a manner as to allow gravity drainage and evaporation of all residual moisture.

When the support structure has been dried and is at a temperature where it can be easily handled with ordinary rubber gloves (this temperature should be as hot as convenient, for example, about 200 F) it is preferably placed on a conventional spray rack and the protective layer applied by spraying. A conventional aspirating spray gun is preferably used, this being adjusted to very fine spray and the solution of the polar polymer applied in a sweeping motion over the support structure to be coated. The presence of a dye in the solution is a good indicator of complete coating, as indicated above.

In the aspirating spray gun, we have used both compressed air and bottled dry nitrogen as the propellant charge (using a pressure of, for example, 35 pounds per square inch).

Nitrogen is preferred as it deposits the solution quicker and more evenly.

After removal of the coated structures from the spray rack, they are preferably placed on drying racks and dried. Two to ten minutes drying per rack is generally sufficient, although the drying time will depend on the air flow around the support structure.

After drying of the protective layer, each support structure has a plurality of lenses 15 placed in the apertures 11 with the edges thereof supported within the annular pockets formed by opposed pairs of rings 12 and 12A.

Assembled support structures and lenses are placed in a vacuum chamber and the lenses 15 are provided with a coating by vacuum deposition, such a coating preferably being a durable abrasion-resistant vitreous composition by, for example, as described and claimed in our U.K. Specification No. 1,459,813. That specification discloses the use of synthetic polymers, for example, diallyl glycol carbonate polymers and polycarbonates which are used in the manufacture of ophthalmic lenses and other lenses for use in the optical industry.

After coating by vacuum deposition the support structure is removed from the vacuum chamber, the lenses removed, and the support structures are examined. When the latter indicate an undesirable build-up of vitreous coating material (which can be in one coating cycle), they are disassembled and moved to a cleaning station. Preferably, the support structures firstly have loose coating material removed, e.g. by an air blast or by being vacuumed, and are then moved to a protective layer removal step. A preferred method of removing a water-soluble protective layer is to immerse in boiling water until all of the pro tective layer is removed, as indicated by absence of dye, followed by rinsing in a hot water bath, blow-drying, visual inspection for cleanliness, defects and warpage, followed by application of a fresh protective layer as above described.

In application of the coating by vacuum deposition in the method according to the invention, the vacuum deposited material should tend to adhere to the protective layer so as to prevent undesirable flaking (that is pieces of the vacuum deposited material undesirably breaking off and attaching to the surface of the optical elements, causing undesirable surface roughness). The mechanism of quartz adhesion (quartz being the preferred vacuumdeposited material) is such that a water impervious layer is not formed; this facilitates washing off polymer and adhered deposited quartz from the support structure. Of course, repeated usage of the support structure could cause an impervious structure to build up and thus the structure must be washed before this occurs.

WHAT WE CLAIM IS: 1. A method of coating a plastics optical element by vacuum deposition while said element is mounted on a re-usable support structure, in which at least those surfaces of the support structure which are exposed to coating material during vacuum deposition have thereon a protective layer comprising a polar polymer, said protective layer being applied in the form of a solution of the polar polymer.

2. A method according to Claim 1, in which said protective layer is also present on other surfaces in a vacuum chamber used for said vacuum deposition.

3. A method according to Claim 1 or 2, in which said protective layer contains a dye which is chemically inert with respect to the coating material and to the optical element.

4. A method according to any of Claims 1 to 3, in which the polar polymer is polyethylene oxide, polyvinyl pyrrolidone, polyacrylamide or polymethacrylic acid.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (9)

**WARNING** start of CLMS field may overlap end of DESC **. the wing nuts 13 and complementary mating bolts. A rotatable, spring-loaded clip 20 goes over slots 10C to mount the support structure in a coating chamber (not shown). The panels 10, 10A are identical and when mounted together in pairs adjacent raised rings 12 and 12A, together with aligned apertures 11, are arranged to receive and hold in place lenses 15 to be coated. According to the invention, at least those surfaces of the support structure which are exposed to coating material during vacuum deposition have thereon a protective layer of a polar polymer, the protective layer being applied in the form of a solution of the polar polymer. All surfaces to be provided with a protective layer should be first thoroughly cleaned, for example, in a commercial dishwasher with an ordinary dishwashing detergent. A preferred dishwashing detergent is "Calgonite" (Trade Mark) in which the surfaces are preferably first soaked for at least 15 minutes and then placed in a dishwasher with additional detergent. Preferably, the support structure is placed on end or in such a manner as to allow gravity drainage and evaporation of all residual moisture. When the support structure has been dried and is at a temperature where it can be easily handled with ordinary rubber gloves (this temperature should be as hot as convenient, for example, about 200 F) it is preferably placed on a conventional spray rack and the protective layer applied by spraying. A conventional aspirating spray gun is preferably used, this being adjusted to very fine spray and the solution of the polar polymer applied in a sweeping motion over the support structure to be coated. The presence of a dye in the solution is a good indicator of complete coating, as indicated above. In the aspirating spray gun, we have used both compressed air and bottled dry nitrogen as the propellant charge (using a pressure of, for example, 35 pounds per square inch). Nitrogen is preferred as it deposits the solution quicker and more evenly. After removal of the coated structures from the spray rack, they are preferably placed on drying racks and dried. Two to ten minutes drying per rack is generally sufficient, although the drying time will depend on the air flow around the support structure. After drying of the protective layer, each support structure has a plurality of lenses 15 placed in the apertures 11 with the edges thereof supported within the annular pockets formed by opposed pairs of rings 12 and 12A. Assembled support structures and lenses are placed in a vacuum chamber and the lenses 15 are provided with a coating by vacuum deposition, such a coating preferably being a durable abrasion-resistant vitreous composition by, for example, as described and claimed in our U.K. Specification No. 1,459,813. That specification discloses the use of synthetic polymers, for example, diallyl glycol carbonate polymers and polycarbonates which are used in the manufacture of ophthalmic lenses and other lenses for use in the optical industry. After coating by vacuum deposition the support structure is removed from the vacuum chamber, the lenses removed, and the support structures are examined. When the latter indicate an undesirable build-up of vitreous coating material (which can be in one coating cycle), they are disassembled and moved to a cleaning station. Preferably, the support structures firstly have loose coating material removed, e.g. by an air blast or by being vacuumed, and are then moved to a protective layer removal step. A preferred method of removing a water-soluble protective layer is to immerse in boiling water until all of the pro tective layer is removed, as indicated by absence of dye, followed by rinsing in a hot water bath, blow-drying, visual inspection for cleanliness, defects and warpage, followed by application of a fresh protective layer as above described. In application of the coating by vacuum deposition in the method according to the invention, the vacuum deposited material should tend to adhere to the protective layer so as to prevent undesirable flaking (that is pieces of the vacuum deposited material undesirably breaking off and attaching to the surface of the optical elements, causing undesirable surface roughness). The mechanism of quartz adhesion (quartz being the preferred vacuumdeposited material) is such that a water impervious layer is not formed; this facilitates washing off polymer and adhered deposited quartz from the support structure. Of course, repeated usage of the support structure could cause an impervious structure to build up and thus the structure must be washed before this occurs. WHAT WE CLAIM IS:

1. A method of coating a plastics optical element by vacuum deposition while said element is mounted on a re-usable support structure, in which at least those surfaces of the support structure which are exposed to coating material during vacuum deposition have thereon a protective layer comprising a polar polymer, said protective layer being applied in the form of a solution of the polar polymer.

2. A method according to Claim 1, in which said protective layer is also present on other surfaces in a vacuum chamber used for said vacuum deposition.

3. A method according to Claim 1 or 2, in which said protective layer contains a dye which is chemically inert with respect to the coating material and to the optical element.

4. A method according to any of Claims 1 to 3, in which the polar polymer is polyethylene oxide, polyvinyl pyrrolidone, polyacrylamide or polymethacrylic acid.

5. A method according to Claim 4, in

which the solution is an aqueous solution of polyethylene oxide containing sufficient dye to give a visual indication of the uniformity of the protective layer.

6. A method according to Claim 5, in which the aqueous solution comprises water, methanol, polyethylene oxide and the dye, the water, methanol and polyethylene oxide being present substantially in the proportions 500 millilitres of water:500 millilitres of methanol:5.5 grams of polyethylene oxide.

7. A method according to any of Claims 1 to 6, in which the solution is applied by spraying.

8. A method according to any of Claims 1 to 7, further comprising the step of dissolving out the protective layer after vacuum deposition.

9. A method according to Claim I, substantially as described herein with reference to the accompanying drawing.