WO2004061148A2 - Surface treatments for increasing transmission and durability of a transparent articles - Google Patents

Abstract

The reflectivity of a hardcoated optical element is reduced via modification Of the surface topography. Selective etching of the hardcoat surface in a alcoholic solution containing dissolved alkali or basic reagent results in increased transmission; yet without significantly affecting the haze or light scattering, or other desirable optical property. The durability and abrasion resistance of nano-structured optical surfaces, particularly those formed of or on polymeric substrate, are enhanced by at least one of adhering to it a fluorinated carbon molecule or other reactive lube composition as a monolayer or a thin multi-layer coating, and treatment with a surface active cross linking agent.

Description

Surface Treatments for Increasing Transmission

and Durability of a Transparent Articles

Cross Reference to Related Applications

This application claims priority to the provisional patent application having serial # 60/437,208 and filed on December 30, 2002.This application claims priority to the provisional patent application having serial # 60/437,412 and filed on December 30, 2002

Background of Invention

This invention relates to a method of imparting a lower reflectance to a transparent article by surface modification, to create nano-structured surfaces. The present invention also relates to a method of imparting improved durability and abrasion resistance nano- structured optical surfaces, and more particularly to those formed from or on polymeric substrates or other optical elements or articles.

When light travels through a transparent object, part of the light "bounces back" or reflects causing unwanted glare or reflected images. In many applications this imparts a significant loss to the performance of the article. As examples Ophthalmic lenses, displays of various sorts (e.g. monitors, televisions, picture frames), and telescopes and the like suffer a loss of as much as 8% of the light that travels through them. This phenomenon has been well known to us for a long time and various methods throughout the past have been proposed and utilized to reduce that unwanted reflection.

The principle of these proposed methods involve adding coatings onto the surface of the article that significantly reduce the back reflection. They are generally known as "anti- reflective coatings". The method generally employs a plurality of thin films differing in their refractive index on a substrate by multi-coating procedures. US patents 3,781,090; 3,799,653; 3,854,796; 3,914,023; 3,984,581 and 4,196,246, among others, all speak to various multilayers of inorganic oxide layers onto polymeric ophthalmic lenses. To deposit the aforementioned coatings, many techniques have been used such as vacuum evaporation, sputtering (for improving adhesion) and electron beam evaporation method. However, it is problematic to apply such coatings onto plastic or polymeric materials by these methods. In the last few years this has become more of an issue as polymeric materials are finding more and more uses as optical elements such as spectacle lenses, TN screens and various plastic films and sheets that are subsequently applied to windows, and display screens. Most polymeric materials when used in ύ. "mechanically harsh" environment (i.e. abrasion, or recurring impact) quickly wear away their usefulness. A method of coating these articles with "hardcoats" to impart robustness has been shown (e.g. hardcoated ophthalmic lenses, polymeric windows and laminates) to greatly improve useful life of the polymeric article (among others, US patents no. 3,700,487; 4,049,868; 4,137,365; 4,611,892; 5,619,288 and 5,958,514.) In most applications were the polymeric material has some optical function, an anti-reflection (AR) coating is also required. Numerous problems arise when these coating methods are applied, especially to the plastic materials having a hardcoat formed on them for improving scratch and impact resistance.

More specifically, polymeric materials have relatively poor heat resistance and cannot withstand the thermal stress imparted by the above-mentioned coating processes. This causes deformation, pitting, crazing and even melting of the substrates during the deposition process. Furthermore, the adhesion is ordinarily poor on plastic materials. These disadvantages are mainly due to differences in the expansion coefficient and surface energy between a plastic material and the inorganic substance to be coated thereon. If the adhesion is extremely reduced when the plastic material is exposed to an elevated temperature or a high humidity, cracks and other defects are often formed in the inorganic coating layer.

A more serious problem is that the impact resistance and flexibility of a hardcoated plastic material are drastically reduced by formation of such brittle inorganic substances onto it. Namely, the superiority of plastic materials to glass materials is lost by the presence of such coatings. A less well known alternative to multilayer anti-reflection are optical devices in which surface reflections are reduced by altering the surface topography to provide a relief pattern that somehow diminishes reflection losses, and increases transmission. A moths' eye has such a relief structure, comprising a regular array of conical protuberances. This is believed to suppress reflections by providing a graded refractive index between the air and the cornea and thereby contribute to reduce reflection (Bernard, C.G., Endeavor 26, pp. 79-84 (1967)). This recognition has led to the suggestion that a glass lens having such a surface would exhibit similar reductions in reflectivity. One method for providing such an altered surface is disclosed by Nicoll et.al. in U.S. Pat. No. 2,445,238 which proposes a method for reducing reflections from glass surfaces by exposing the glass to a vapor of hydrofluoric acid. This was thought to form a microscopically roughened glass surface which is similar to a structure of a moth's eye. Difficulties in reproducing these "skeletonized" structures and in maintaining a uniform structure over the entire surface area of optical devices has led others to develop alternative structures. Moulton (U.S. Pat. No. 2,432,484) developed a technique for forming on glass surfaces a nonuniformly dispersed layer of colloidal particles containing a random arrangement of peaks to provide the antireflection characteristics. Lange et al. U.S. Pat. No. 4,816,333 discloses anti-reflective coatings of silica particles. The coating solution contains colloidal silica particles and optionally a surfactant ("Trition.TX.X-100" and "Tergitol TMN-6") to improve the wettability of the coating solution. U.S. Pat. No. 4,374,158 (Tanigucki et al.) discloses an anti-reflective coating using a gas phase treatment technique. Clapham in U.S. Pat. No.4,013,465 teaches that a clear article may have reduced reflection to a particular wavelength band provided that the surface of such articles have specific protuberances on its surface; "having a height not less than 1/3 the longest wavelength and a spacing that is shorter than the shortest wavelength divided by the index of refraction of the material". Numerous patents have been granted which have demonstrated that imparting a nanostructure to a surface dramatically reduced back reflection of visible light from it. However, none of these anti-reflective techniques produced a durable coating (Cathro et al. in "Silica Low-Reflection Coatings for Collector Covers by a Dye-Coating Process," Solar Energy, Vol. 32, No. 5, pp. 573-579 (1984); and by J.D. Masso in "Evaluation of Scratch Resistant and Anti-reflective Coatings for Plastic Lenses," Proceedings of the 32^nd Annual Technical Conference of the Society of Vacuum Coaters, Vol. 32 p. 237-240 (1989)).

It is therefore, the object of this invention to impart antireflection properties to a durable surface, thereby, providing both antireflection and mechanical durability (such as scratch resistance, flexibility, impact resistance) to an optical element.

It is another object of the invention to provide a process that can impart antireflective properties to an optical element without difficulty and complication of process. It is another object of the present invention to provide a method of imparting an antireflection property to ophthalmic lenses as they are provided from the manufacturer without any additional layers

It is therefore, an object of this invention to impart abrasion durability to a nano- structured surface. Among others, this invention can furnish mechanical durability (such as scratch, wear, and impact resistance) to an optical element that has a Moths' Eye type antireflection treatment on its surface. '

It is another object of the invention to provide a process that can impart low surface energy properties to an optical element without difficulty and complication of process.

It is another object of the present invention to provide a method of imparting lubricity to an article that uses a nano-structure for its functionality (e.g. microdisplays, sensors, LEDs, memory devices, and other nano-technology devices).

Summary of Invention

In accordance with a first aspect of present invention, there is provided a process for producing a hardcoated transparent shaped article having an enhanced anti-reflective effect, via the surface modification of the hardcoat itself. The present invention provides a method for modifying the surface of a hardcoat disposed on an optical element to provide the anti reflection property. The surface modification is achieved by controlled etching of the hardcoat itself. This affords the article both mechanical durability and antireflection in a single layer.

In accordance with another present invention, there is provided a process for producing an abrasion resistant surface by, in a preferred embodiment, chemically attaching fluorocarbon molecules to it. This is accomplished by dipping the articles in a solution of reactive fluorocarbons and removing the excess unreacted molecules from its surface via the vapor degreasing method.

In yet another aspect the present invention provides a method for modifying the surface properties of a nano-structured hardcoat disposed on an optical element. In particular it is aimed to provide it with a low surface energy property. It is surmised that the lowering of surface energy reduces the friction coefficient and thus abrasion is reduced. The modification is achieved by reacting the surface with a reactive lubricant. This affords the article the capability to have both a microscopically rough surface, as well as, abrasion resistance in a single layer. In a further aspect, the present invention provides a method for modifying the mechanical and surface properties of a nano-structured hardcoat disposed on an optical element. In particular it is aimed to cross-link the nano-structured surface. It is surmised that surface cross-linking of the nano-structure enhances the mechanical durability of the optical element. The modification is achieved by reacting surface functionality present on the nano-structure surface with a surface active cross-linking agent. This affords the article the capability of having enhanced abrasion resistance.

More specifically, the present invention seeks to furnish durability to moth eye type, anti- reflection treatments that are imparted to optical articles such as ophthalmic lenses, displays, solar cells etc. In most cases, these articles require both properties for their proper functionality (i.e. durability and anti-reflection as well a transmission increase for transparent articles).

The above and other objects, effects, features, and advantages of the present invention will become more apparent from the following description of the embodiments thereof taken in conjunction with the accompanying drawings.

Brief Description of Drawings

Figure 1 is a graph showing the wavelength dependence of the transmittance of each thick lens in example 2 (for treatment times of 35, 40, and 45 minutes) in comparison to a blank (untreated) lens and a commercial antireflective lens (AR).

Figure 2 is a graph showing the wavelength dependence of the transmittance of each thin lens in Example 2 (for 40, 50 and 60 minutes) in comparison to a blank (untreated) lens and a commercial antireflective lens (AR).

Figure 3 is a schematic illustration of a nano-structured surface coated with a mono layer of reactive lube

Figure 4 is a schematic illustration of an apparatus used in one embodiment of a method of applying a mono layer of reactive lube to an article.

Detailed Description

The term "etchant" in this embodiment is generally referred to any alkali or basic material such as sodium hydroxide, potassium hydroxide and related material that can partially dissolve and or hydrolytically react with inorganic and organic surfaces and mixed inorganic-organic surfaces. Optical elements of the present invention, such as Ophthalmic lenses, protective eyewear, picture frames, display screens, solar cells, windows and the like, are preferably already supplied with an organic or polymeric hardcoat that provides scratch resistance to the element.

A hard-coated optical article is dipped in an aqueous or alcoholic solution of an alkali such as Sodium Hydroxide. The concentration of the etchant is preferably from 2%-30% by weight and more preferably between 10% and 20%. The treatment time is from 2 minutes to one hour at 10°C to 40°C, but more preferably at room temperature. The resultant article, after washing and drying, embody superior anti reflective properties, along with other performance benefits attributable to the remaining hardcoat. If the optical article does not have a hardcoat, one must be coated onto the article by any method known to one skilled in the art prior to the etching step. The hardcoat has been established as a means of providing mechanical durability to polymeric optical articles. As hardcoats can be applied to organic and inorganic surfaces, the invention is not limited to polymeric optical articles. Suitable transparent hardcoats are well known in the art, and comprises at least a cross-linked polymer matrix and generally some portion of inorganic filler that is well dispersed therein being formed of colloidal particles of silica, alumina, Titania, tin oxide and the like. Many hardcoats are formed from polymeric resins having at least in part a siloxane (Rl, R2-Si-0) repeat unit, while other resins may be formed or cross-linked via pendent or terminal acrylate groups. If treated correctly, the polymeric optical devices of the present invention exhibit reflectance as low as 0.1%, and a relatively low uniform reflectivity throughout the visible region (380-700 nm). In contrast, untreated polymeric devices with or without a hardcoat typically exhibit reflectance on the order of about 4% from each surface.

The present invention is thus a significant improvement over prior art polymeric optical elements in both anti-reflection and mechanical durability. To better elucidate the process and resulting anti-reflection properties a brief description of the process is outlined below:

Scratch resistant coated plastic lenses available from Silor Corporation (St. Petersburg FL,) under the tradename "TRUETINT"consist of a siloxane type hardcoat material coated on "CR-39" type resin. These lenses were treated in the base solution system, as follows; a. Sodium Hydroxide was dissolved in methanol and the concentration varied from 10% to 20% w/v. b. The lenses were soaked in base solution for 5, 10, 20, 30, 35, 40, 45, 50 and 60 minutes. c. Samples were washed in methanol using ultrasonic cleaning, and then dried at room temperature.

The percentage transmittance of each sample was measured and compared to both an untreated lenses and a commercial available lens having a conventional antireflective multiple layer coating. The results have shown the significant increase in percentage transmittance of the base treated material over the untreated material. Indeed, the base treatment method at the appropriate concentration and soaking times increases the circa 92 % transmittance up to about 99.6%, which is higher than commercially available antireflective-coated ophthalmic lenses.

It appears that the conditions for controlled etching of the hard cover layer to produce a near ideal nano-structure for anti-reflection optical phenomenon is a function of the composition and reactivity of the etching solution, but also at least somewhat dependent on the chemical nature of the hard coat. Silor "TRUETINT" lenses are the currently preferred substrate for this process. Other commerical lens substrates include "SOLA" lenses. We have come to appreciate that the solutions of methanol and sodium hydroxide may also comprise from 0% to perhaps as much as 30 percent water depending on the ambient humidity and atmospheric exposure time. It appears that the actual amount of water will vary the strength or aggressiveness of the etching, as a perfectly dry for anhydrous solution has been found not to attack the siloxane hardcoats in any reasonable amount of time. However, deliberately adding up to 30 percent water actually produces an etching solution so aggressive that it completely removes the hard coat in a relatively short period of time, leaving no practical treatment time range for process control and optimization. Lower water content and increased temperatures will affect etching in a reasonable amount of time depending upon the water concentration and temperature. It should be noted that while these etching procedures were originally developed for promoting adhesion of hard coats to bare ophthalmic lenses, that is "CR-39" resin, the criticality of the concentration of water, time and temperature of action has not been previously appreciated, as a broader range of chemical roughening of the bare lens surface will yield adequate adhesion without interfering with the desired optical performance of the subsequently hard coat lens.

Therefore, in the preferred embodiment of the inventive process the etching solution is made up from commercially available anhydrous methanol and sodium hydroxide such that the actual water concentration can be controlled by the addition of known amounts of water and conducting the etching process in a relatively low humidity environment (less than approx. 50% R.H.). Controlling the water content may also be achieved by maintaining the etching environment of an anhydrously prepared methanolic alkali solution at about 20 percent relative humidity, which is believed to produce a solution having between about 0.2% and 9% water, depending on the atmosphere exposure time. However, if it is not commercially practical to control the relative humidity within this range then the etching time, or base concentration, or etching temperatures should be lowered or altered accordingly from the conditions provided herein. Alternatively, one may also control the actual amount of water in the solution, preferably between about 2% to about 10%. In either case, the same sequence of time and temperature dependent tests can be used to determine the preferred treatment time for the given lens and/or hardcoat type.

By way of examples, the present invention will be further clarified as to what it means to "correctly" treat the surface.

EXAMPLE 1 : Sodium hydroxide was dissolved in methanol at various concentrations (5%, 10%, 15%, 20% w/w). The samples were soaked in the base solution for 10, 20, 30, 40, 50 and 60 minutes. The samples were then washed in methanol and dried at room temperature. Table 1: Treatment in various concentrations and soaking time

EXAMPLE 2: Two different thickness lenses were investigated by the base treatment system. Sodium hydroxide was dissolved in methanol at the concentrations of 10 w/v (12.7% w/w). The samples were soaked in the base solution for 30, 35, 40, 45, 50 and 60 minutes. Then the samples were washed in methanol and dried at room temperature. Transmittance of each lens was measured and compared with a blank lens and a commercial antireflective lens. Figure 1 is a graph showing the wavelength dependence of the transmittance of each thick lens in example 2 (for treatment times of 35, 45 and 55 minutes) in comparison to a blank (untreated) lens and a commercial antireflective lens (AR). The modulation of transmission between about 90 and 92% of the blank lens (hardcoat without treatment) arises from the slight difference in refractive index of the hardcoat and resin that forms the bulk of the lens, the periodicity being related to the hardcoat thickness. However, as the average transmission increases with etching time this modulation weakens, having essentially disappeared after 45 minutes, when the transmission reaches a maximum value at about 600 nm. Figure 2 is a graph showing the wavelength dependence of the transmittance of each thin lens in Example 2 (for 40, 50 and 60 minutes) in comparison to a blank (untreated) lens and a commercial antireflective lens (AR). The variation between Figures 1 and 2 may be due to control of water in the methanol -sodium hydroxide solution, the lens thickness or batch-to-batch variations in the hardcoat chemical composition or structure. However, a peak in transmission of about 99.5% is still obtained at about 600 nm. Additionally, it appears that the process is self-stabilizing, as the transmission profiles after 50 and 60 minutes are essentially identical, exhibiting about the same average transmission (over 400 to 750 nm) as the conventional multi-layer (AR) treatment, that is an average transmission of greater than 95%.

EXAMPLE 3: A 10% NaOH etch solution (w/w) is prepared by dissolving 50.0g of NaOH (Reagent Grade Sodium Hydroxide) in 450.0 grams of anhydrous methanol (MeOH, 568.5 mL) plus 20.0 mL of deionized water (d. H₂O). A hardcoated optical lens (Silor TruTint^® Lens; Silor, Division of Essilor of America, Inc., St. Petersburg, Florida) is suspended in the NaOH etch solution for a period of 10 minutes at 21 °C, after which time the lens is removed, rinsed with methanol and dried with a heat gun. Optical reflectance (450-750 nm) of the lens after etch treatment was <0.5% (optical reflectance of the untreated lens was 5.5%). EXAMPLE 4: A 10% NaOH etch solution (w/w) is prepared by dissolving 50.0g of NaOH (Reagent Grade Sodium Hydroxide) in 450.0g of anhydrous methanol (MeOH, 568.5 mL) plus 20.0 mL of deionized water (d. H₂O). A hardcoated optical lens (SOLA Lens; SOLA International Holdings Ltd, Lonsdale, South Australia) is suspended in the NaOH etch solution for a period of 20 minutes at 21 °C, after which time the lens is removed, rinsed with methanol and dried with a heat gun. Optical reflectance (450-750 nm) of the lens after etch treatment was <0.5% (optical reflectance of the untreated lens was 5.0%).

It should be appreciated that alternative chemical etching agents or etchantsinclude solutions comprised of the following: A. Metal Hydroxide, where said metal includes any one or combination of the following: 1) Alkali metals (Group I Elements), such lithium, sodium, potassium and the like, 2) Alkaline earth metals (Group II Elements) such as magnesium, calcium and the like; B. Metal Alkoxide or Alcoholate, where said metal includes any one or combination of the following: 1) Alkali metals (Group I Elements), such lithium, sodium, potassium and the like, 2) Alkaline earth metals (Group II Elements) such as magnesium, calcium and the like; alternatively, the Alkoxide or

Alcoholate is derived from any one or combination of methanol, ethanol, propanol and related homologs and isomers; and, ethylene glycol, propylene glycol, glycerol and related homologs and isomers; C. Ammonia, Ammonia Solution and Ammonium hydroxides; D. Quaternary Amine Hydroxides, where said quaternized amine may be ligated with alkyl, aryl, aralkyl, and related substituents; E. Fluoride Salts and Related Complexes, where the fluoride counterion includes any one or combination of: 1) Alkali metal (Group I Elements), such lithium, sodium, potassium and the like, 2) Alkaline earth metal (Group II Elements) such as magnesium, calcium and the like. 3.) Ammonium and the like. 4.) Quaternized an ine, which said quaternized amine may be ligated with alkyl, aryl, aralkyl, and related substituents; and F. Dissolved or partially solubilized alkali metal and alkaline earth metal. It should be appreciated that alternative solvents for the preparation and use of etchant solutions include any one or combination of the following: A. Alcohols such as methanol, ethanol, propanol and related homologs and isomers, B. Glycols and Polyols such as ethylene glycol, propylene glycol, glycerol and related homologs and isomers, C. Amines such as ammonia, methylamine, ethylamine, propylamine and related homologs and isomers; and, polyamines such as ethylenediamine, diethylenetrkmine and related homologs and isomers, D. Strongly polar aprotic solvents such as dimethylsulfoxide, dimethylformamide, hexamethylphosphoramide and related solvents, and E. Water.

Further, it should be appreciate the preparation and use in etchant solutions include any one or combination of the following additives: A. Surface Active or Wetting Agents for the lowering of the etchant solution surface tension; B. Cationic Surfactants as counterions or surface transport agents for the hydroxide, alkoxide, fluoride and related reactive anionic species; C. Metal Binding Agents and Chelators; and D. pH Modifying Agents.

Although it has been discovered that chemical hardcoats used on ophthalmic lens can be etched in alkali solutions to provide a nano-structure having superior anti reflective properties, this treatment reduces the durability with respect to the hardness and scratch resistance afforded by the hardcoat itself. Significantly, in yet another aspect of the instant invention it has been discovered that the application of a surface-active cross- linking agent and or monolayer of reactive lube has been found to restore the hardness and scratch resistance otherwise lost in an alkali etching process.

The present invention is thus a significant improvement over prior art polymeric optical elements in both anti-reflection and mechanical durability. Furthermore, it is a cost effective as well as simple method to implement in providing durable AR coatings for optical articles. The term "lube" in this application is generally referred to any substance that is used as a lubricant to lower the occurrence of abrasion during motion between intimately contacting surfaces. There are many lubes used for a myriad of applications, the properties and performance of different types being generally known within the applicable technology field. The term "surface cross-linking reagent" in this apphcation is generally referred to any substance that can react with a plurality of surface functionality present on the nano- structured surface of the optical element. The resulting interaction between the surface functionality and the surface cross-linking reagent may be covalent bonds, ionic bonds or hydrogen bonded interactions that confer additional mechanical durability to the optical element.

A sub set of lubricants called reactive lubes utilizes a long chain molecule (that can, in some instances, be considered either an oligomer or polymer) that has a reactive group (e.g. carbonyl, amine etc.) at one end of the molecule. The reactive end is chosen such as to chemically bond to a specific surface it is designed for. This in turn ensures that the attached polymer is secured to the surface and cannot be easily removed. Example of such polymers are "Krytox 157FS" from Dupont and "ZDOL-7007" from Ausimont. These types of reactive lube polymers are employed in this invention to impart durability to a nano-structured surface. A preferred subset of such reactive lubes is that in which the long chain molecule is a fluorinated molecule, in which is fluorine otherwise replaces a substantial amount of hydrogen in the equivalent long chain hydrocarbon.

The present invention can be best elucidated by way of an example of how it can be used to enhance the property of devices that rely on optical properties for their functionality. Optical articles (such as ophthalmic lenses, displays and solar cells) typically require a means of reducing back reflection for optimal performance. Creating a nano-structure on the surface of the article has been shown to provide adequate AR properties (e.g. US patents #, 2,445,238; 2,432,484; 4,816,333; 4,374,158 and 4,013,465). However, the inherently weak mechanical structure affords the article insufficient durability.

Thus, Figure 3 is a schematic illustration showing successive higher magnification cross sections from optical element or article 100 at the bottom, Figure 3 (a), having a hardcoat layer 110 in Figure 3 (b). Not wishing to be bound by theory, Figure 3(c) illustrates conceptually the nano-structured hardcoat layer 110a coated with a monolayer of polymeric lube molecules 120 attached thereto.

Thus in one embodiment of the invention hardcoat layer 110 can be applied to the surface of an optical article 100. Hardcoat layer 110 is subsequently modified to form a nano- structured layer 110a, which has potentially a graded refractive index or a periodic variation in height with sufficient pitch to reduce reflection, and enhance transmission. However, the application of the invention also embraces such variations in nano- structured surfaces to include combinations of these structures and diffractive optical surfaces. Accordingly, in either variant the nano-structure at surface 110a is either at least partly porous or not as smooth as the original surface and thus more susceptible to erosion or abrasion, through either cleaning or casual contact in the environment. However, any further treatment to strengthen or protect the nano-structures surface must not interfere with the topographic profile. As the polymeric lube molecules have predicable dimension based on there molecular weight they can be selected in accordance with the nano- structure height, thickness, porosity, refractive index or and pitch variation so that the monolayer substantially conforms with the nano-structure relief, thus avoiding detracting from the optical properties provided by the nano-structure. As will be further described, the reactive lube molecules 120 are limited to a monolayer, each molecule preferably anchored at one end by a chemical reaction with the surface 110a, having the opposite end extending away from the surface to provide lubrication that increase the wear resistance and durability of the otherwise fragile nano-structured surface.

The choice of specific reactive lube is made in made in consideration of the reactivity and functional groups that may be present on the nano-structured surface of the optical article or device, as well as the necessary times and conditions of reaction. The optical article is preferably coated with the monolayer by dip coating in a solution of solvent with this reactive lube followed by slow withdrawn through a vapor cloud of that solvent. This process ensures that the article is densely covered with a single molecular layer of the reactive lube. All the excess unreacted lube flows back to the solution during passage through the vapor cloud. The remaining lube is essentially a monolayer, having become anchored to the surface by a chemical reaction of on end of the molecule with the surface. Thus, the remaining end of the molecule is free to at least partially extend from the surface. The free end provides the lubrication of tribological modification while being of limited length and constrained avoiding a significant alteration of the surface topography of the nano-structure. In other words the anti-reflection property is unaffected by the presence of a monolayer uniformly covering the surface.

In an alternative embodiment of the invention excess reactive lube is removed after the lens or optical element is removed from the reactive lubricant solution (or other solution coating methods, such as spray coating, curtain coating, spin coating and the like) by placing it in pure solvent solution such that excess, un-reacted lube is removed by dissolution in the solvent used as a rinse bath. As the solvent rinsing solution will eventually accumulate the excess reactive lube, a further preference is to use a series or so-called "cascade" of rinse baths, each in a progressively purer solvent. The following is a more specific example of how this invention may be used in conjunction with a nano-structured surface to afford articles both AR and durability. To produce the AR effect, a hard-coated optical article is dipped in an aqueous or alcoholic solution of an alkali such as Sodium Hydroxide. The concentration of the etchant is preferably from 2%-30% by weight and more preferably between 5% and 15%. The treatment time is from 2 minutes to one hour at 10 °C to 40 °C, and more preferably at room temperature.

If the optical article does not have a hardcoat, one must be coated onto the article by any method known to one skilled in the art prior to the etching step.

If treated correctly, the polymeric or inorganic optical devices exhibit reflectance as low as 0.1 %, and a relatively low uniform reflectivity throughout the visible region (380-700 nm). In contrast, untreated polymeric devices with or without a hardcoat typically exhibit reflectance about 4% from each surface.

The article is then dipped into a reactive lube solution comprising 1% (w/w) of "ZDOL- 7007" (Ausimont USA, Inc., 10 Leonards Lane, Thorofare, NJ 08086) dissolved in "HFE-7100" a (3M Performance Materials, 223 Portsmouth, NH 03801.), a fluorinated hydrocarbon solvent. As illustrated in Figure 2 the vessel 200 is equipped with a chiller ring 210 above the vessel opening and has a means of being heated, schematically illustrated as element 220 beneath vessel 200. The solution 230 is heated to a temperature of about 40 ° C. The chilling coil 210 above the fluid containing portion 200a of vessel 200 condenses the solvent, vapors back into the vessel thus creating a uniform vapor cloud above the solution surface in the region surrounding lens 252. The AR or nano- structured article, for example a lens 251, is then immersed into the heated solution, such as by lowering it along arrow 270. Next lens 251 is slowly withdrawn, as indicated by arrow 280, at a constant rate (circa ~l"/min.). This sequence results in a monolayer of the lube deposited on the nano-structured surface, as the condensation of solvent vapor on the lens after withdrawal re-dissolves excess reactive lubricant, which is before the reaction with the surface or other lube molecules is completed.

While the present invention is applicable to any nano-structured surface that suffers from poor mechanical durability it is particularly useful in the treatment of nano-structured polymeric surfaces, as these structures can be quite delicate depending on the process used to form the surface. For example, although it is possible to form nano-structure polymer surfaces by molding and embossing, etching via mechanical or chemical attach is a commercially attractive method of modifying optical surfaces, as it is independent of the method used to produce the primary, which is image forming, optical surface.

An alternative embodiment of the present invention entails surface cross-linking of the nano-structured optical element. Moreover, if additional mechanical durability is desired or warranted, the resulting cross-linked nano-structure surface may be further treated with a lube agent.

Surface cross-linking of the nano-structure may be achieved by treatment with polyvalent ions such as Ca+2, Mg+2, Fe+3, Al+3, and related ions, which may form ionic or hydrogen-bonded bridges that are capable of spanning several silanol or surface active functional groups.Cross-linking may also be achieved by the judicial selection and employment of polyfunctional reactive molecules.A monolayer or relatively thin multilayer of polyfunctional organic molecules or organometallic molecules capable of chemically reacting with a plurality of appropriate surface functionality on the nano- structure surface such as, surface silanols or other surface active functional groups that may be present depending upon the nano-structure optical element.

EXAMPLE 5:A fluorolube coating solution is prepared as follows: 4.01g of Fomblin^® Z- DOL (a fluorocarbon lubricant from Solvay Solexis, Inc., formerly Ausimont USA, Inc.) is dissolved in 396.21g of HFE-7100 Solvent (3M Novec™ Engineered Fluid, C₄F₉OCH₃). An etched Silor lens prepared in accordance to Example 1 is suspended in the fluorolube solution for a period of 10 minutes at 21 °C, after which time the lens is removed, rinsed with HFE-7100 Solvent and air-dried. A minimal change in lens reflectance (<0.3%) was observed after the fluorolube treatment. Scratch or rub resistance (using a heavy gauge white cloth) of the fluorolube treated lens improved as compared to the etched only lens (with heavy gauge white cloth wrapped around the index finger, and rubbed across the surface of the lens three times in three different areas: Fluorolube treated lens: no visual rub marks, Etched-Only lens: visual rub marks after three rubs).

EXAMPLE 6: A fluorolube treatment solution is prepared as follows: l.Og of Fluorolink™ 7007 (a fluoropolyether derivative from Solvay Solexis, Inc. , formerly Ausimont USA, Inc.) is dissolved in 94.0g isopropanol (i-PrOH), 4.0g deionized water (d. H₂O) and 1.0 g Glacial Acetic Acid (AcOH). An etched Silor lens prepared in accordance to Example 1 is suspended in the fluorolube solution for a period of 5 minutes at 21 °C, after which time the lens is removed, rinsed with efhanol and then dried with a heat gun. The lens was then oven cured at 100 °C for 30 minutes. A minimal change in lens reflectance (<0.5%) was observed after the fluorolube treatment. Scratch or rub resistance (using a heavy gauge white cloth) of the fluorolube treated lens improved as compared to the etched only lens (with heavy gauge white cloth wrapped around the index finger, and rubbed across the surface of the lens three times in three different areas: Fluorolube treated lens: no visual rub marks, Etched-Only lens: visual rub marks after three rubs). Example 7: A fluorosilane treatment solution is prepared as follows: 2.0g of

(tridecafluoro-l,l,2,2,-tetrahydrooctyl)-triethoxysilane (a reactive organofluorosilane from Gelest, Inc.; CF₃(CF₂)₅CH₂CH₂Si(OEt)₃) is dissolved in 98.0g of ethanol (EtOH). The resulting solution is then adjusted to pH 4.5 using glacial acetic acid (approx. 0.3-0.5 mL). An etched Silor lens prepared in accordance to Example 1 is suspended in the fluorosilane treatment solution for a period of 5 minutes at 21 °C, after which time the lens is removed, rinsed with ethanol and then dried with a heat gun. The lens is then oven cured at 110 °C for 15 minutes. A minimal change in lens reflectance (<0.5%) was observed after the fluorosilane treatment. Scratch or rub resistance (using a heavy gauge white cloth ) of the fluorosilane treated lens improved as compared to the etched only lens (with heavy gauge white cloth wrapped around the index finger, and rubbed across the surface of the lens three times in three different areas: Fluorosilane treated lens: no visual rub marks, Etched-Only lens: visual rub marks after three rubs).

EXAMPLE 8:A 20% (w/w) aqueous calcium chloride solution is prepared by dissolving 20.0g of CaCl₂ in 80.0 mL of deionized water (d. H₂O). An etched Silor lens prepared in accordance to Example 1 is rinsed with 10% HC1 (aq.) followed by rinsing with d. H₂O and then suspending in the 20% CaCl₂ (aq.) for a period of 15 minutes at 21 °C, after which time the lens is removed, rinsed with d. H₂O and then dried with a heat gun. The lens is then oven cured at 110 °C for 15 minutes. A minimal change in lens reflectance (<0.2%) was observed after the calcium treatment. Scratch or rub resistance (using a heavy gauge white cloth ) of the calcium treated lens improved as compared to the etched only lens (with heavy gauge white cloth wrapped around the index finger, and rubbed across the surface of the lens three times in three different areas: Calcium treated lens- no visual rub marks, Etched-Only lens: visual rub marks after three rubs).

EXAMPLE 9:A fluoropolymer treatment solution is prepared as follows: l.Og of FluoroPel™ PFC802A (a 2.0% fluoroaliphatic polymer dissolved in a fluorosolvent, from Cytonix Corp.) is dissolved in 99.0g of HFE-7100 Solvent (3M Novec™ Engineered Fluid, C₄F OCH₃). An etched SOLA lens prepared in accordance to Example 2 is suspended in the fluoropolymer treatment solution for a period of 5 minutes at 21 °C, after which time the lens is removed, rinsed with HFE-7100 Solvent and then air-dried. The lens is then oven cured at 50 °C for 15 minutes followed by an additional cure at 90 °C for 15 minutes. An increase in lens reflectance (approx.1-2%) was observed after the fluoropolymer treatment. Scratch or rub resistance (using a heavy gauge white cloth ) of the fluoropolymer treated lens improved as compared to the etched only lens (with heavy gauge white cloth wrapped around the index finger, and rubbed across the surface of the lens three times in three different areas: Fluoropolymer treated lens: no visual rub marks, Etched-Only lens: visual rub marks after three rubs).

Accordingly, one of ordinary skill in the art will select a surface cross-linking agent and or reactive lube based in part on the reactivity to the surface group on the optical article, taking into account the compatibility of the size or long chain portion of the molecule with the substrate chemistry, preferably trying to maximize the durability of the nano- structure surface and minimize the difference in refractive index between the "fluid" like monolayer and the substrate it is disposed on or reacted with. While the invention has been described in connection with a preferred embodiment, it is not intended to limit the scope of the invention to the particular form set forth, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents as may be within the spirit and scope of the invention as defined by the appended claims.

Claims

Claims

[cl] A process for increasing the transmission of an article having a siloxane type hardcoat as an outer surface, the process comprising: a) immersing the hardcoated article in solution of an alkali having a basic pH for a predetermined time and temperature to alter the hardcoat topography, b) removing the hardcoated article from the alkali solution, c) rinsing the hardcoated article in an alcohol or suitable solvent to remove excess base, d) drying the hardcoated article to remove excess alcohol or solvent, e) wherein the predeteπnined time and temperature is selected so as to be sufficient to increase the transmission by at least about 2%.

[c2] The method of claim 1 wherein the basic pH solution etches at least a portion of the hardcoat.

[c3] The method of claim 1 wherein the basic pH solution has a solvent selected from the group consisting of alcohols, glycols, amines, strongly polar aprotic solvent and water.

[c4] The method of claim 1 wherein the alkali reagant used to form the alkali solution is selected from the group consisting of a metal hydroxide, metal alkoxide or alcoholate, ammonia, ammonia solutions, ammonium hydroxides and quaternary amine hydroxides and dissolved alkali metal , dissolved alkaline earth metal, partially solubilized alkali metal and partially solubilized alkaline earth metal.

[c5] The method of claim 4 wherein the metal used to form the hydroxide or alkoxide is selected from the group consisting of alkali metals and alkaline earth metals.

[c6] The method of claim 4 wherein the alkoxide or alcoholate is derived from the group consisting of methanol, ethanol, propanol and related homologs and isomers; and, ethylene glycol, propylene glycol, glycerol and related homologs and isomers.

[c7] The method of claim 1 wherein the alkali reagent used to form the alkali solution is a fluoride salts and related complexes, where the fluoride counter ion is selected from the group consisting of alkali metal, alkaline earth metal, ammonium and quaternized amine.

[c8] The method of claim 1 wherein the etchant comprises at least one additive selected from the group consisting of surface active, wetting agents, cationic surfactants, metal binding agents, chelators and pH modifying agents. [c9] A transparent optical element consisting essentially of: a) a transparent body having a front surface, b) an transparent organic hardcoat disposed on the front surface of said transparent body, wherein the average overall transmission through the hardcoat to the interior of the optical element, in the wavelength range of 450 to 650 nm, is greater than about 95%.

[c 10] A transparent optical element comprising: a) a transparent body having a front surface, b) a transparent organic hardcoat disposed on the front surface of said transparent body as the external surface of the optical element, c) wherein the internal transmission through the front surface is greater than 97%.

[el l] A transparent optical element according to claim 10 further comprising an transparent organic hardcoat disposed on the rear surface of said transparent body, wherein the average overall transmission at through the hardcoat to the interior of the optical element a wavelength range of 450 to 650 nm is greater than about 95% [cl2] A transparent optical element according to claim 9 wherein the transparent body is a lens.

[cl3] A transparent optical element according to claim 9 wherein the transparent body is a plastic resin.

[cl4] A transparent optical element according to claim 10 wherein the transparent body is a plastic resin.

[cl 5] A transparent optical element according to claim 10 wherein the hardcoat is a silicone polymer. [cl6] A transparent optical element comprising: a) a transparent body having a front surface, b) a transparent organic hardcoat deposited on the front surface of said transparent body, and etched to increase the overall transmission of the optical element by at least 2%. [c 17] A transparent optical element according to claim 16 wherein said organic hardcoat is etched to increase the overall transmission of the optical element by at least 3%

[cl 8] A transparent optical element according to claim 16 further comprising a) a transparent organic hardcoat deposited on the back surface of said transparent body, and etched to increase the overall transmission of the optical element by at least 4%.

[cl9] A transparent optical element according to claim 18 wherein the optical element is a lens.

[c20] A transparent optical element according to claim 18 wherein the optical element is a plastic [c21 ] A transparent optical element according to claim 18 wherein the optical element is a plastic ophthalmic lens.

[c22] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection, the process comprising: a) providing an article having a nano-structured optical surface, b) applying a solution formed of a solvent and a reactive lube, c) withdrawing the article from the solution source, d) removing excess reactive lube from the article such that a substantially uniform layer of reactive lube remains on the surface, e) wherein the layer of reactive lube has the opportunity to chemically react with the nano-structured surface.

[c23] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 22 wherein the uniform layer is a monolayer. [c24] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 22 wherein the reactive lube is a fluoroaliphatie polymer. [c25] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 22 wherein the reactive lube is a fluorosilane polymer.

[c26] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 22 wherein the solvent is a fluorosolvent. [c27] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection, the process comprising: a) providing an article having a nano-structured optical surface, b) applying a solution formed of a solvent with a surface cross-linking reagent, c) withdrawing the article from the solution source, d) removing excess surface cross-hr-king reagent from the article, e) wherein the surface-active cross-lmking agent has the opportunity to chemically react with the nano-structured surface. [c28] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 27 wherein the cross-Jinking agent is a polyvalent ion. [c29] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 28 wherein the polyvalent ion is selected from the group consisting of Ca⁺², Mg⁺², Fe⁺³ and Al⁺³' [c30] A process for enhancing the durability of an article having a nano-structured surface to improve transmission or reduce reflection according to claim 24, the process further comprising: a) applying a solution formed of a solvent and a reactive lube, withdrawing the article from the solution source, b) removing excess reactive lube from the article such that a substantially uniform layer of reactive lube remains on the surface, c) wherein the layer of reactive lube has the opportunity to chemically react with the nano-structured surface. [c31 ] A transparent optical element consisting essentially of: a) a transparent body having a front surface, the front surface of the transparent body having a nano-structure to reduce reflection there from and increase overall transmission through the optical element, b) a substantially uniform layer of a reactive lube is disposed on the front surface of the transparent body. [c32] A transparent optical element according to claim 31 wherein the reactive lube is a monolayer. [c33] A transparent optical element according to claim 31 wherein the average optical reflection from 450-750 nm is less than about 4%.

[c34] A transparent optical element according to claim 31 wherein the average optical reflection from 450-750 nm is less than about <0.2%.

[c35] A transparent optical element according to claim 31 wherein the average optical reflection from 450-750 nm is less than about <0.5%.

[c36] A transparent optical element according to claim 31 wherein the average optical reflection from 450-750 nm is less than about < 2.0 %. [c37] A transparent optical element according to claim 31 wherein the average optical reflection from 450-750 nm is less than about <0.5%.

[c38] A transparent optical element according to claim 36 wherein the transparent body is a lens.

[c39] transparent optical element according to claim 36 wherein the transparent body is a plastic resin.

[c40] A transparent optical element according to claim 37 wherein the transparent body is a plastic ophthalmic lens.