Patent Publication Number: US-2020276778-A1

Title: Photochromic Lens

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 13/741,290 filed Jan. 14, 2013, entitled Photochromic Lens which is a divisional of U.S. patent application Ser. No. 12/959,201 filed Dec. 2, 2010 entitled Photochromic Lens (now U.S. Pat. No. 8,367,211 issued Feb. 5, 2013); which is a divisional of U.S. patent application Ser. No. 11/537,571 filed Sep. 29, 2006 entitled Photochromic Lens (now U.S. Pat. No. 7,858,001 issued Dec. 28, 2010); which claims the benefit of priority from U.S. Provisional Application Ser. No. 60/722,848 filed on Sep. 29, 2005 entitled Photochromic Lens, and which is a Continuation-In-Part of U.S. application Ser. No. 10/938,275 filed on Sep. 9, 2004 entitled Photochromic Polyurethane Laminate (now U.S. Pat. No. 8,298,671 issued Oct. 30, 2012); which claims the benefit of priority from U.S. Provisional Application Ser. No. 60/501,819 filed Sep. 9, 2003 entitled Photochromic Film and Method of Manufacture, and from U.S. Provisional Application Ser. No. 60/501,820 filed on Sep. 9, 2003 entitled Photochromic Laminate; all of which are hereby incorporated herein by reference in their entireties. 
    
    
     BACKGROUND OF THE INVENTION 
     Many methods and devices are known in the art for incorporating photochromic characteristics into an ophthalmic lens. One example of a method known in the art includes imbibing or infusing photochromes into the host material or base lens from a transfer layer (that is subsequently removed) prior to formation of the finished lens product. Another example of a method known in the art includes incorporating photochromes into a lens by imbibing or coating a photochromic composition onto the surface of a base lens. Yet another example of a method known in the art includes incorporating photochromes into a finished lens product by combining a prepared photochromic insert or “laminate” with base lens material, typically via an injection molding process. The following are illustrative examples of such methods known in the art. 
     The imbibing process was one of the first processes to be used to impart photochromicity to plastic lenses. U.S. Pat. No. 4,286,957 to Naour-Sene describes this process. Further refinements were discussed in U.S. Pat. No. 4,880,667 to Welch. Various improvements to this imbibing process have been developed, such as described in U.S. Pat. No. 5,130,353 to Fisher et al., and U.S. Pat. No. 5,185,390 to Fisher et al. These two patents suggest improvements to the transfer process with a unique transfer layer. It was recognized early on that the plastic resins used to make ophthalmic lenses do not provide the best host material for photochromes. In such plastic resin materials, the photochromes do not activate easily and fatigue or wear-out in a short period of time. A strong activation darkness to near sunglass darkness is desired in the marketplace. Another desired characteristic of a photochromic lens is that it should maintain at least 70 percent of its original activation darkness after two years of wear. This is one of the limitations to putting photochromes into polymeric host materials that are used to form the bulk of the lens. 
     A more recent example, U.S. Pat. No. 5,728,758 to Smith describes a photochromic article in which the organic polymeric host material has been impregnated with photochromes prior to formation of the finished lens product. As is described in the &#39;758 patent, one of the drawbacks of incorporating photochromes directly into the polymeric host material is the problem of fatigue or light fatigue. Photochromes are believed to lose their ability to change color specifically resulting from the irreversible decomposition of the photochromic compound, which occurs due to repeated exposure to UV light over time. The &#39;758 patent specifically address this problem by using a unique combination of monomers and surface coating compositions to improve abrasion resistance, chemical attack and improved fatigue resistance. 
     Alternatively, an example that describes coating of a photochromic layer onto the surface of a lens is found in U.S. Pat. No. 4,756,973 to Sakagami et al. The &#39;973 patent specifically describes the use of spirooxazine compound and phenolic compound in the photochromic layer, and describes that such a lens formulation provides successful coloring effects in photochromic lenses that are subjected to environmental conditions ranging from normal to high temperatures. 
     Another example that describes coating photochromes on the surface of a lens substrate is found in U.S. Pat. No. 6,150,430 to Walters et al. Specifically, the &#39;430 patent describes a process that includes the steps of treating the surface of a polymeric substrate to provide reactive groups, applying a polymerizable composition to the surface, exposing the coated substrate to radiation to improve adhesion, and applying and curing a photochromic composition to the coated surface. The &#39;430 patent, at least in part, addresses a method of producing commercially acceptable “cosmetic” standards for photochromic and non-photochromic optical coatings that are applied to lenses. A major limitation of the photochromic coating approach is the poor scratch resistance of such a coating even with another hard coating on top of the photochromic coating. Additionally, if the photochromic coating is scratched, it will result in streaks of areas on the lens that do not activate. 
     The limitations of the performance of the photochromes in the various plastics used to make ophthalmic lenses have resulted in various improvement methods, including making composite or multiple part lenses that combine plastics that are good photochromic hosts with additional plastics to make improved ophthalmic lenses. One example of this approach is described in U.S. Pat. No. 5,531,940 to Gupta et al. Another approach is to put the photochromic dyes in the glue layer between two lens sections as described in U.S. Pat. No. 5,851,328 to Kohan. More recent attempts at making photochromic composites are described in U.S. Pat. Nos. 6,863,844 and 6,863,848 to Engardio et al. and U.S. Publication No. 20050089630 to Schlunt et al. One problem with these approaches is that the mechanical stability of the composite is not very durable in subsequent processing to make the ophthalmic lens and mount it into a frame. Processes such as surfacing the lens to prescription power and edge cutting to fit into a frame result in chipping of the composite due to the different cutting and grinding characteristics of the materials. Drilling of the composite to mount into rimless frames also results in chipping of the composite. 
     Lastly, the following methods known in the art illustrate incorporation of photochromes into ophthalmic lenses via photochromic inserts or laminates, whether by cast-mold-type processes or by injection molding processes. For example, U.S. Pat. No. 4,889,413 to Ormsby et al. describes creation of a finished laminate product that is created by placing two glass or plastic layers into a mold and injecting a photochromically-infused plastic resin between the glass/plastic layers. The resulting photochromic laminate is thereafter cured and processed, producing a finished lens product. 
     Another example that illustrates the use of a photochromic insert or laminate in an injection molding process is described in U.S. Pat. No. 6,328,446 to Bhalakia et al. The photochromic laminate or wafer includes inner and outer resin sheets (or protective layers), which sandwich a photochromic cellulose acetate butyrate layer. The unitary photochromic laminate is thereafter placed inside a mold cavity, after which a molten polycarbonate resin is injected into the cavity and fused to the back of the photochromic laminate. The lens is then cooled to room temperature and the finished product is an injection molded, photochromic polycarbonate lens. 
     While each of the above-referenced patents and published applications describe methods of making photochromic lenses and address particular problems in the art, improvements are still required. For example, problems associated with impregnating photochromes within the host material of a base lens have been described to some degree above. Additionally, if such a lens is a semi-finished product and requires further processing (e.g., grinding, polishing, etc), it is clear that photochromes present in the base lens will be ground and/or polished away, inevitably diminishing the desired coloring effects of the finished lens product. In addition, the prescription lens must be robust enough to maintain its integrity through subsequent processing both to form the prescription and to be edged, cut and possibly drilled for mounting into a frame. 
     Alternatively, the shortcomings of coating photochromic products onto the surface of a lens have to do primarily with coating thickness and the creation of segmented, multi-focal lenses. For example, a coating of about 25 μm or more is needed to incorporate a sufficient amount of photochromic compounds to provide the desired light blocking quality in the lens when the compounds are activated. However, a coating of this thickness is not well suited for application on the surface of a segmented, multi-focal lens because it is too thick. Typically, a coating of this thickness creates such problems as the creation of an unacceptable segment line, as well as coating thickness uniformity issues around the segment line. 
     Problems that have been raised particularly regarding use of photochromic laminates or inserts in injection molding process include, primarily, the bleeding of the functional layer (e.g., photochromic layer) material of the laminate or wafer. By the term “bleeding,” it is meant that the functional layer materials between the transparent resin sheets (e.g., the protective layer of the laminate or wafer) runs out from between the resin sheets in the lateral direction. 
     Often bleeding occurs due to the deformation of the photochromic layer under the high temperature and pressure used during the injection molding process. This is thought to occur due to either an excess amount of functional layer material and/or inadequate softening properties of the functional layer material. Further, this bleeding can interfere with any additional coating layers that are applied to the lens after injection-molding. The Bhalakia patent adequately addresses the issue of making laminates used in injection-molding, through improvement of laminate materials and properties. However, the issues addressed by Bhalakia do not include providing a laminate or insert that may be used in a cast-lens manufacturing process. 
     Therefore, a need exists to create a photochromic lens that addresses the problem of maximizing photochromic properties of a lens produced in a cast-mold manufacturing process. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a photochromic lens that includes a relatively long service life and provides good resistance to photochromic dye fatigue. It is another object of the present invention to provide a photochromic lens that reduces the amount of photochromic dyes used during the manufacturing process. It is yet another embodiment of the present invention to provide a photochromic lens that is not limited from use with surface designs, such as bi-focal lenses. 
     It is yet another object of the present invention to provide a method of manufacturing a photochromic lens that can utilize most commercially available cast resins, such as those made by thermoset or radiation initiated processes. It is yet another embodiment of the present invention to provide a photochromic lens having high impact resistance, using known and commercially available high impact resinous layers and materials, such as polycarbonate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a photochromic lens according to a preferred embodiment of the present invention. 
         FIGS. 2A-2D  illustrate a photochromic film, as embedded in a photochromic lens of  FIG. 1 ), according to a preferred embodiment of the present invention. 
         FIG. 3  illustrates a photochromic lens according to a preferred embodiment of the present invention. 
         FIG. 4  illustrates a cross-sectional view of a cast lens mold known in the art. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As seen in  FIG. 1 , one embodiment according to the present invention relates to a photochromic lens comprising a front cast resin layer  1 , a photochromic film  2 , and a back cast resin layer  3 . Specifically, a preferred embodiment would include a photochromic lens made from a cast resin that is either thermoset or radiation-set. The photochromic lens may be either a finished product or a semi-finished product. In one embodiment (as shown in  FIG. 2 ), a photochromic lens may include a photochromic film  2  including at least one protective layer  4  and a photochromic layer  5  (as shown in  FIG. 2 , examples (A),(B) and (C)). In another embodiment, the photochromic film may include just a photochromic layer  5  (as shown in  FIG. 2 , example (D)). In each of these embodiments, the photochromic film  2  is bonded or adhered strongly to a casting resin layer  1 ,  3  during a casting process (as shown in  FIG. 1 ). The photochromic film  2  in the form of a laminate may be optimized as a host for a photochromic dye to provide for maximum performance of the dye. 
     As seen in  FIG. 2 , additional embodiments according to the present invention would include a photochromic film  2  having at least one protective layer  4  and a photochromic layer  5 , in a variety of orientations to one another (e.g.,  FIG. 2 , examples (A), (B), and (C)). As shown in  FIG. 3 , hard coating, primer, or barrier coatings may be applied onto the front surface of the protective layer  4  (e.g., for enhancing adhesion and other performances), the front surface meaning the surface of the protective layer  4  of film  2  laminate facing away from the photochromic layer  5 . When the photochromic layer  5  is used as a photochromic film  2  without protective layers  4 , the photochromic layer  5  may also have such hard, primer or barrier coatings applied before placement into the cast lens. 
     Additional embodiments according to the present invention may include a cast photochromic lens including a number of layers and/or combinations of layers, comprising, for example, protective layers, photochromic layers or films, and/or additional coating layers (e.g. hard coating, primer layers, and/or other barrier layers) (not shown). Each of the above-described photochromic lenses can be conveniently manufactured through a casting process. 
     A suitable photochromic film (laminate) is disclosed in U.S. patent application Ser. No. 10/938,275 entitled Photochromic Polyurethane Laminate, filed Sep. 9, 2004, which is herein incorporated by reference. A preferred material for the protective layers  4  of the photochromic film  2  should have good compatibility with the lens casting material. By the term “compatibility”, it is meant that the adhesion between the protective layer  4  and the front cast resin layer  1  formed by the lens casting material is sufficient to pass ordinary tests for eyewear lenses without damage or chemical attack to the protective layers  4  of the photochromic film  2  (e.g. FDA drop-ball test, drilling and mounting tests). 
     Examples of preferred materials for the protective layers  4  of the photochromic film  2  include acrylate resins, cellulose esters, and polycarbonate resins. Suitable hard coatings known in the art may further protect such protective layer resins to prevent the casting resin from chemically attacking the protective layer resins during processing. 
     Preferred materials for the photochromic layer  5  of the present invention would include, for example, plastic host resins having a glass transition temperature, T g , of less than 50 degrees Celsius, and more preferably, below 30 degrees Celsius. These plastics tend to be mechanically soft which is ideal for the photochromes to activate and de-activate. When the photochromic dyes absorb UV light, typically a carbon-oxygen bond in the photochromic dye molecule is broken and the dye molecule rotates into a form that absorbs visible light. A soft plastic host for the dye that would facilitate this action preferably would include a class of plastic hosts, such as polyurethanes. However, historically, this mechanical softness of plastic hosts has been shown deficient when such materials are used as ophthalmic lens materials. Therefore, a preferred embodiment of an ophthalmic lens contemplated for use with the present invention would include encapsulation of a host urethane resin, such as a photochromic layer  5 , within an ophthalmic lens. A preferred thickness of the photochromic layer  5 , namely the photochromic urethane layer  5 , would preferably be 10 mil or less, more preferably 4 mil or less and most preferably, to 2 mil or less. 
     Preferred polyurethane photochromic host materials would include, for example, thermoplastic and thermoset polyurethanes. Examples of such host materials are disclosed in U.S. Pat. Nos. 4,889,413, 6,107,395, and 6,166,129, and U.S. patent application Ser. No. 10/938,275, which are herein incorporated by reference. The polyurethane composition would preferably include the following: 0.05% to 6% pbw of photochromic compound(s), and a stabilizer package: 0.5% to 6% pbw of light stabilizer(s), 0.5% to 6% pbw of antioxidant(s), 0.5% to 6% pbw of UV absorber(s). Also, the photochromic film  2  would preferably have a thickness no greater than 40 mil. 
     Any lens casting resins available on the market and known in the art would be suitable to produce the photochromic lens of the present invention. Examples of such casting resins would include the optical monomer CR-39, allyl diglycol carbonate, from PPG (or equivalent from Great Lakes Chemicals denoted by the tradename of RAV-7) and the optical monomer MR series cast resins from Mitsui. In addition to thermoset resins, cast resins that are curable by radiation energy (e.g., UV) are also suitable for use with the present invention. The radiation curing process is advantageous in that it will not interfere or degrade the photochromic dyes due to their protection inside the laminate. Examples of radiation curable cast resins would include, for example, those resins based on acrylate chemistry. 
     It is preferable that the cast resin for the front protective layer  4  of the photochromic film  2  should not include UV absorbers, which would significantly absorb or block the activation wavelength of the photochromic dye. The photochromic layer  5  of the photochromic film  2  would provide adequate UV protection to the eyes, as the photochromic dyes imparted to the photochromic layer  5  are extremely efficient absorbers of UV. 
     Examples of manufacturing the photochromic lens as contemplated in the present invention would include, for instance, a cast molding process, in which a photochromic film  2  is first placed into a cast mold  8 , shown in  FIG. 4 . Thereafter, a cast resin  1 , 3  may also be introduced into the cast mold  8  and the lens would be cured, forming an integrate photochromic cast lens. The photochromic film  2  may be placed in any number of orientations within the mold, depending upon desired results and lens processing applications. 
     One embodiment for manufacturing the lens of the present invention would include the steps of preparing a photochromic film  2 , as earlier described; forming discs of the film into wafers, preferably having a curve matching the front base curve of the lens to be produced; preparing a cast setup comprising a front mold  34 , a formed wafer or film  2 , a back lid (mold)  32 , in a cast gasket; pouring a cast resin into the front cavity  70  formed by the front mold and the wafer  2 , and the back cavity  72  formed by the wafer  2  and the back lid (mold)  32 ; and curing the cast resin to form the photochromic lens. 
     Forming of the photochromic film  2  may be done by a variety of different ways familiar to those in the arts. Examples of lens forming techniques may include, for instance, compression forming and vacuum forming. 
     A cast setup used to produce polarizing lenses from cast resins could also used to cast the photochromic lens of the present invention without any modification. 
     Another embodiment for manufacturing a lens as contemplated by the present invention includes incorporation of photochromes into the polyurethane plastic, and thus the photochromic film  2 , after the polyurethane has been formed (e.g., after urethane monomers and catalyst have reacted to fully form polyurethane). This is contrary to the teaching of U.S. Pat. No. 4,889,413 (herein incorporated by reference), which describes incorporation of photochromes into the urethane monomer/catalyst mixture, prior to formation of polyurethane. Further, the teaching of the &#39;413 patent describes a method of preparing and assembling a lens unlike that of the present invention. The &#39;413 patent describes assembly of pre-cast lenses (either plastic or glass), between which a photochromic host is introduced and thereafter cured. 
     One embodiment of a method contemplated for use with the present invention may include the steps of: dissolving an appropriate amount of photochromic dye into a polyurethane resin with an appropriate solvent; casting the resultant solution on a smooth surface, to allow the solvent to evaporate; placing the resulting sheet or film of photochromic polyurethane into a mold with a thermoset casting monomer liquid and catalyst; and completing a curing or reaction step of the thermoset monomer into a thermoset, fully cured lens with the polyurethane film encased inside the thermoset lens. 
     This particular embodiment would result in a photochromic thermoset, cast lens with improved photochromic properties. Thus, one does not need to mix the photochromes with the polyurethane monomers first. One has the option of putting the polyurethane in a laminate as described above or not in a laminate inside the lens (plastic host). 
     Example 1 
     A photochromic film was prepared according to the examples in U.S. patent application Ser. No. 10/938,275. The polyurethane layer is 40 μm thick, and the protective layers are 76 μm cellulose acetate butyrate (CAB) films (K-Mac). The polyurethane layer and protective layers were bonded together to form a photochromic laminate. The laminate was masked with a polypropylene 3M film (24S56W). A 70-mm disk was die-cut off from the above laminate, and formed into a 6-base laminate wafer through a thermo-vacuum forming process. The temperature was 255° F., and the forming time was 200 seconds. A 70-mm lens cast gasket and two 6-base glass molds (front and back) were used to cast the photochromic lens. The masked film is thereafter removed prior to placement of the laminate wafer into the gasket. The laminate wafer was fixed in the gasket about 1 mm away from the front mold surface with help of a spacer. A clear UV-curable cast resin from OptiCast was injected into the front and back cavities. The front cavity is formed by the front mold and the photochromic film. The cast resin in the above setup was cured under a 12-mW/m 2  exposure for 10 minutes. The result was a cast resin lens having the photochromic film embedded in it. The unactivated transmission of the lens was measured as 70%. The activated transmission after exposure to a Xenon lamp under 20 W/m 2  intensity of UV was measured to be 19%. This demonstrated good photochromic activity. 
     Example 2 
     To a solution of 18% by weight of a polyester urethane (Tecoflex CLC-93A from Thermedics) in THF solvent was added 2% each of Tinuvins 765 (Mixture of Bis(1,2,2,6,6-pentamethyl-4-piperidyl)sebacate and methyl 1,2,2,6,6-pentamethyl-4-piperidyl sebecate), 144 (Bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-[[3,5-bis(1,1-dimethylethyl)-4-hydroxyphenyl] methyl]butylmaloate), and Irganox 1010 (Pentaerythritol tetrakis(3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate)) (all from CIBA Corporation), and 0.8% of naphthopyran photochromic dye VP0762 (Proprietary Vision-Ease Dye). The mixture was then cast on a flat borosilicate glass plate and the solvent allowed to evaporate. A photochromic film of about 0.2 mm thick was obtained. The photochromic film was then placed between two glass molds held together with a standard casting gasket. A mixture of 58.3 grams of casting monomer P11 (NOF Corporation), 0.6 gram of Tinuvin 765, 0.26 gram of Trigonox 7 (tert-Butyl peroxydiethylacet) catalyst and 0.37 gram of Trigonox 21 (tert-Butyl peroxy-2-ethylhexanoat) catalyst was introduced between the glass molds and around the polyurethane film. The mold and gasket assembly was placed in a water bath and cured in a cycle that ramps the lens up to 90 degrees Celsius over a 20 hour period. The resultant thermoset, cast lens is then separated from the glass molds. The lens had a refractive index of 1.55. The lens sample was then fatigued by exposure to a Xenon lamp with an ultraviolet light output of 30 Watts/Square Meter for 144 hours. This simulates actual wear of the lens by someone for a two year wear period. After the 144 hours, the photochromic performance remaining was 97%. This compares to the performance of the commercial photochromic polycarbonate Quantum (Transitions Optical, Inc.) lens product that is believed to be only a 65% remaining performance. Thus, the photochromic film in the cast lens was very fatigue resistant and had longer life than the prior art. 
     Example 3 
     A photochromic film was prepared according to the examples in U.S. patent application Ser. No. 10/938,275 (now U.S. Pat. No. 8,298,671 issued Oct. 30, 2012). The polyurethane layer is 40 μm thick, and the protective layers are 350 μm thick polycarbonate films. The polyurethane layer and protective layers were bonded together to form a photochromic laminate. The laminate was masked with a 3M film (24S56W). A 70-mm disk was die-cut off from the above laminate, and formed into a 6-base laminate wafer through a thermo-vacuum forming process. The temperature was 255 degrees Fahrenheit, and the forming time was 200 seconds. A 70-mm lens cast gasket and two 6-base glass molds (front and back) were used to cast the photochromic lens. The masked film is thereafter removed prior to placement of the laminate wafer into the gasket. The laminate wafer was placed against the front mold surface and placed into the gasket with the back mold. A clear UV-curable cast resin from OptiCast (OPIV-B) was injected into the back cavity. The cast resin in the above setup was cured under a 12-mW/m 2  UV exposure for 7 minutes. The result was a cast resin lens having the photochromic laminate fused to the front of it. The unactivated transmission of the lens was measured as 79%. The activated transmission after exposure to a Xenon lamp under 12 W/m 2  intensity of UV was measured to be 19%. This demonstrated good photochromic activity. 
     Example 4 
     A photochromic film was prepared by laminating 13.5 mil thick CAB film (Kodacel K7896, made by Eastman Kodak) for both top and bottom protective layers and 38 micron photochromic layer (polyurethane). The laminate was masked with 3M protective masking film (24S56W) on both sides, then a 86 mm disk in diameter was cut-out from the above laminate. It was formed into a 6 base laminate wafer through a thermo-vacuum forming process. The forming temperature was 75 degrees Fahrenheit and forming time was 150 seconds. It was further cut to 72.6 mm in diameter to fit the size of casting mold. It was further cut by 3 mm on two locations to allow flow of the cast resin around it. A 73 mm casting mold and two 6 base glass molds were used for front and back to cast a photochromic lens. The masked film is thereafter removed prior to placement of the laminate wafer into the gasket. Said formed photochromic film was placed in the mold about 1 mm away from the front mold surface with help of a spacer. Thermo-set resin RAV-7 (made by Great Lakes Chemical) without UV absorbing agent was injected to fill both front cavity and back cavity which were separated by the photochromic film. The cast resin in the above setup was thermally cured in normal condition. The result was a cast resin lens having the photochromic laminate fused to the front of it. The unactivated transmission of the lens was measured as 82.4%. The activated transmission after exposure to a Xenon lamp under 12 W/m2 intensity of UV was measured to be 16%. This demonstrated good photochromic activity. 
     Example 5 
     A photochromic film was prepared by laminating 12 mil thick polycarbonate film (1151 by Teijin Kasei America) for both top and bottom protective layers and 38 micron photochromic layer (polyurethane). The polycarbonate film was applied with UV curable hardcoat as barrier coating on one side in advance. The hardcoated side was placed outside of lamination that later contacts casting resin. Without this barrier coating, this casting resin monomer has been shown to cause the polycarbonate film to turn white and, therefore, unusable. The laminate was masked with 3M protective masking film (24S56W) on both sides, then a 86 mm disk in diameter was cut-out from the above laminate. It was formed into a 6 base laminate wafer through a thermo-vacuum forming process. The forming temperature was 285 degrees Fahrenheit and forming time was 250 seconds. It was further cut to 72.6 mm in diameter to fit the size of casting mold. It was further cut by 3 mm on two locations to allow flow of the cast resin around it. A 73 mm casting mold and two 6 base glass molds were used for front and back to cast a photochromic lens. The masked film is thereafter removed prior to placement of the laminate wafer into the gasket. Said formed photochromic film was placed in the mold about 1 mm away from the front mold surface with help of a spacer. Thermo-set resin RAV-7 (made by Great Lakes Chemical) without UV absorbing agent was injected to fill both front cavity and back cavity which were separated by the photochromic film. The cast resin in the above setup was thermally cured in normal condition. The result was a cast resin lens having the photochromic laminate fused to the front of it. The unactivated transmission of the lens was measured as 90.0%. The activated transmission after exposure to a Xenon lamp under 12 W/m2 intensity of UV was measured to be 19%. This demonstrated good photochromic activity. 
     In yet another embodiment, the present invention provides a photochromic polyurethane laminate having two transparent resin sheets bonded to a photochromic polyurethane layer formed by curing a mixture of a solid thermoplastic polyurethane, at least one isocyanate prepolymer, at least one photochromic compound, and a stabilizing system. The thermoplastic polyurethane has a theoretical NCO index of from 90 to 105, and a molecular weight (number averaged) of from 20,000 to 100,000. The isocyanate prepolymer has a NCO content of from 1.0% to 10.0%, by weight. The weight ratio of the thermoplastic polyurethane vs. the isocyanate prepolymer in the photochromic polyurethane composition is in the range from 1:9 to 9:1. The photochromic compound(s) counts for 0.1% to 5% of the total polyurethane, by weight. 
     To enhance the fatigue resistance of the photochromic compounds, stabilizers such as antioxidants, light stabilizers, and UV absorbers are added in the polyurethane layer. 
     The photochromic laminate is preferably made through a cast-lamination process. All components described above are dissolved in a suitable solvent, cast on a release liner. After the solvent is evaporated substantially, the thermoplastic polyurethane portion will provide the cast polyurethane film enough rigidity to go through the lamination process without any deformation. After lamination, the polyurethane prepolymer will provide further curability by reacting with active hydrogen atoms such as those of terminal hydroxyl groups, moisture, urethane groups, and urea groups in the system to enhance the dimensional stability of the polyurethane layer under high temperature and high pressure. 
     Transparent Resin Sheets 
     The material used to make the transparent resin sheet is not limited so long as it is a resin with high transparency. In case the photochromic polyurethane laminate of the present invention is incorporated into a thermoplastic article such as a spectacle lens, the transparent resin sheets of the laminate is preferably of a resin material that is thermally fusible to the article base material so that the photochromic laminate is tightly integrated with the article base when produced with the insert injection molding process. Thus, it is more preferred to have same kind of material for both the article base and the transparent resin sheets. 
     Suitable sheet resin materials include polycarbonate, polysulfone, cellulose acetate butyrate (CAB), polyacrylates, polyesters, polystyrene, copolymer of an acrylate and styrene, blends of compatible transparent polymers. Preferred resins are polycarbonate, CAB, polyacrylates, and copolymers of acrylate and styrene. A polycarbonate-based resin is particularly preferred because of high transparency, high tenacity, high thermal resistance, high refractive index, and most importantly, and especially its compatibility with the article base material when polycarbonate photochromic lenses are manufactured with the photochromic polyurethane laminate of the present invention and the insert injection molding process. A typical polycarbonate based resin is polybisphenol-A carbonate. In addition, examples of the polycarbonate based resin include homopolycarbonate such as 1,1′-dihydroxydiphenyl-phenylmethylmethane, 1,1′-dihydroxydiphenyl-diphenylmethane, 1,1′-dihydroxy-3,3′-dimethyldiphe-nyl-2,2-propane, their mutual copolymer polycarbonate and copolymer polycarbonate with bisphenol-A. 
     While the thickness of a transparent resin sheet is not particularly restricted, it is typically 2 mm or less, and preferably 1 mm or less but not less than 0.025 mm. 
     Thermoplastic Polyurethane 
     As the thermoplastic polyurethane, it is preferably made from a diisocyanate, a polyol, and a chain extender. Thermoplastic polyurethanes of this kind are known and may be obtained, for example, in accordance with U.S. Pat. Nos. 3,963,679 and 4,035,213, the disclosures of which are incorporated herein by reference. 
     The thermoplastic polyurethane used in the present invention is particularly prepared from a composition comprising a) an aliphatic isocyanate having a functionality of 2, b) at least one high molecular weight polyol having a nominal functionality of 2 and a molecular weight of from 500 to 6000 g/mole, preferably from 700 to 3000 g/mol, and counting for from about 50% to about 98% by weight, preferably from 70% to 95%, of the total isocyanate reactive species in the composition, and c) at least one low molecular weight diol having a molecular weight of from 62 to 499, and counting for from about 2% to about 50% by weight, preferably from 5% to 30%, of the total isocyanate reactive species in the composition. 
     Polyols 
     The polyols of the present invention are those conventionally employed in the art for the preparation of polyurethane cast elastomers. Naturally, and often times advantageously, mixtures of such polyols are also possible. Examples of the suitable polyols include polyether polyols, polyester polyols, polyurethane polyols, polybutadiene polyol, and polycarbonate polyols, while polyether and polyester types are preferred. 
     Included among suitable polyether polyols are polyoxyethylene glycol, polyoxypropylene glycol, polyoxybutylene glycol, polytetramethylene glycol, block copolymers, for example, combinations of polyoxypropylene and polyoxyethylene glycols, poly-1,2-oxybutylene and polyoxyethylene glycols, poly-1,4-tetramethylene and polyoxyethylene glycols, and copolymer glycols prepared from blends or sequential addition of two or more alkylene oxides. The polyalkylene polyether polyols may be prepared by any known process such as, for example, the process disclosed in Encyclopedia of Chemical Technology, Vol. 7, pp. 257-262, published by Interscience Publishers, Inc. (1951), the disclosure of which is incorporated herein by reference. 
     Polyethers which are preferred include the alkylene oxide addition products of polyhydric alcohols such as ethylene glycol, propylene glycol, dipropylene glycol, trimethylene glycol, 1,2-butanediol, 1,5-pentanediol, 1,6-hexanediol, 1,7-heptanediol, hydroquinone, resorcinol glycerol, glycerine, 1,1,1-trimethylol-propane, 1,1,1-trimethylolethane, pentaerythritol, 1,2,6-hexanetriol, .alpha.-methyl glucoside, sucrose, and sorbitol. Also included within the term “polyhydric alcohol” are compounds derived from phenol such as 2,2-bis (4-hydroxyphenyl)-propane, commonly known as Bisphenol A. 
     The suitable polyester polyols include the ones which are prepared by polymerizing ε-caprolactone using an initiator such as ethylene glycol, ethanolamine and the like. Further suitable examples are those prepared by esterification of polycarboxylic acids. Further suitable polyester polyols include reaction products of polyhydric, preferably dihydric alcohols to which trihydric alcohols may be added and polybasic, preferably dibasic carboxylic acids. Instead of these polycarboxylic acids, the corresponding carboxylic acid anhydrides or polycarboxylic acid esters of lower alcohols or mixtures thereof may be used for preparing the polyesters. The polycarboxylic acids may be aliphatic, cycloaliphatic, aromatic and/or heterocyclic and they may be substituted, e.g., by halogen atoms, and/or unsaturated. The following are mentioned as examples: succinic acid; adipic acid; suberic acid; azelaic acid; sebacic acid; phthalic acid; isophthalic acid; trimellitic acid; phthalic acid anhydride; tetrahydrophthalic acid anhydride; hexahydrophthalic acid anhydride; tetrachlorophthalic acid anhydride, endomethylene tetrahydrophthalic acid anhydride; glutaric acid anhydride; maleic acid; maleic acid anhydride; fumaric acid; dimeric and trimeric fatty acids such as oleic acid, which may be mixed with monomeric fatty acids; dimethyl terephthalates and bis-glycol terephthalate. Suitable polyhydric alcohols include, e.g., ethylene glycol; propylene glycol-(1,2) and -(1,3); butylene glycol-(1,4) and -(1,3); hexanediol-(1,6); octanediol-(1,8); neopentyl glycol; (1,4-bis-hydroxymethylcyclohexane); 2-methyl-1,3-propanediol; 2,2,4-trimethyl-1,3-pentanediol; triethylene glycol; tetraethylene glycol; polyethylene glycol; dipropylene glycol; polypropylene glycol; dibutylene glycol and polybutylene glycol, glycerine and trimethlyolpropane. A preferred polyester polyol is polycaprolactone polyol having an average molecular weight from 500 to 6,000, and preferably from 700 to 3,000. 
     Diols 
     Suitable diols are those polyols listed above having a functionality of 2 and a molecular weight of from 62 to 499. Preferred diols are 1,4-butane-diol and 1,3-propane-diol. 
     Isocyanates 
     The diisocyanate component is preferably an aliphatic diisocyanate. The aliphatic diisocyanate is selected from the group consisting of 1,4-tetramethylene diisocyanate, 1,6-hexamethylene diisocyanate, 2,2,4-trimethyl-1,6-hexamethylene diisocyanate, 1,12-dodecamethylene diisocyanate, cyclohexane-1,3- and -1,4-diisocyanate, 1-isocyanato-2-isocyanatomethyl cyclopentane, 1-isocyanato-3-isocyanatomethyl-3,5,5-trimethyl-cyclohexane (isophorone diisocyanate or IPDI), bis-(4-isocyanatocyclohexyl)-methane, 2,4′-dicyclohexylmethane diisocyanate, 1,3- and 1,4-bis-(isocyanatomethyl)-cyclohexane, bis-(4-isocyanato-3-methylcyclohexyl)-methane, .alpha.,.alpha.,.alpha.′,.alpha.′-tetramethyl-1,3- and/or -1,4-xylylene diisocyanate, 1-isocyanato-1-methyl-4(3)-isocyanatomethyl cyclohexane, 2,4- and/or 2,6-hexahydrotoluylene diisocyanate, and mixtures thereof. Bis-(4-isocyanatocyclohexl)-methane is the preferred diisocyanate in occurrence with the method of the present invention. 
     The polymerization process to make the thermoplastic polyurethane can be carried out in one-pot fashion, that is, all starting materials are initially added into the reaction vessel. The polymerization process can also be carried out with a prepolymer approach. That is, a polyurethane prepolymer terminated with isocyanate groups is first obtained by reacting a stoichiometrically in excess diisocyanate with a polyol. Suitable equivalent ratio of diisocyanate to polyol in the present invention is from 1.2:1.0 to 8.0:1.0. A chain extender of diol is then mixed with the prepolymer to complete the reaction. The NCO index of the thermoplastic polyurethane, formed from the quotient, which is multiplied by 100, of the equivalent ratio of isocyanate groups to the sum of the hydroxyl groups of polyol and chain extender is within a range of 90 to 105, preferably between 92 and 101. 
     Catalysts such as organotin or other metallic soaps may be added in the mixture to make a thermoplastic polyurethane. Example catalysts include dibutyltin dilaurate, stannous octoate, and cobalt naphthenate. 
     Isocyanate Prepolymer 
     The isocyanate prepolymer used in the photochromic polyurethane composition of the present invention is prepared in the same way as the prepolymer used to prepare the thermoplastic polyurethane in a prepolymer method described above. Preferably, the polyol and the isocyanate used to make the isocyanate prepolymer is the same as the polyol to make the thermoplastic polyurethane. More preferably, the isocyanate is an aliphatic diisocyanate described in the previous sections, and the polyol is a polyester polyol having a molecular weight between 700 and 3,000. The molecular weight (number averaged) of the isocyanate prepolymer is preferably between 1,000 and 6,000, and more preferably between 1,500 and 4,000. As an isocyanate group terminated prepolymer, its NCO content is between 1.0% and 10.0%, preferably between 2.0% and 8.0%. 
     When mixing the isocyanate prepolymer and the thermoplastic polyurethane together, the mixing ratio by weight is in the range from 1:9 to 9:1, preferably from 1:3 to 3:1. 
     Photochromic Compounds 
     Suitable photochromic compounds in the context of the invention are organic compounds that, in solution state, are activated (darken) when exposed to a certain light energy (e.g., outdoor sunlight), and bleach to clear when the light energy is removed. They are selected from the group consisting essentially of benzopyrans, naphthopyrans, spirobenzopyrans, spironaphthopyrans, spirobenzoxzines, spironaphthoxazines, fulgides and fulgimides. Such photochromic compounds have been reported which, for example, in U.S. Pat. Nos. 5,658,502, 5,702,645, 5,840,926, 6,096,246, 6,113,812, and 6,296,785; and U.S. patent application Ser. No. 10/038,350, all commonly assigned to the same assignee as the present invention and all incorporated herein by reference. 
     Among the photochromic compounds identified, naphthopyran derivatives are preferred for optical articles such as eyewear lenses. They exhibit good quantum efficiency for coloring, a good sensitivity and saturated optical density, an acceptable bleach or fade rate, and most importantly good fatigue behavior. These compounds are available to cover the visible light spectrum from 400 nm to 700 nm. Thus, it is possible to obtain a desired blended color, such as neutral gray or brown, by mixing two or more photochromic compounds having complementary colors under an activated state. 
     More preferred are naphtho[2,1b]pyrans and naphtho[1,2b]pyrans represented by the following generic formula: 
     
       
         
         
             
             
         
       
     
     Substituents on various positions of the aromatic structure are used to tune the compounds to have desired color and fading rate, and improved fatigue behavior. For example, a photochromic dye may contain a polymerizable group such as a (meth)acryloyloxy group or a (meth)allyl group, so that it can be chemically bonded to the host material through polymerization. 
     The quantity of photochromic compound(s) incorporated into the polyurethane layer of the present invention is determined by the desired light blockage in the activated state and the thickness of the polyurethane layer itself. The preferred outdoor visible light transmission of sunglasses is preferably between 5% and 50%, more preferably between 8% and 30%, most preferably between 10% and 20%. Preferably, the amount of total photochromic substance incorporated into or applied on the polyurethane layer may range from about 0.1 wt. % to about 5 wt. % of the total polyurethane, and more preferably from about 0.5 wt. % to about 3.0 wt. %. If the thickness of the polyurethane layer is 100 μm, between about 0.5 wt. % to about 1 wt. % of photochromic compound(s) is needed to achieve an outdoor light transmission of between 10% and 20%. The amount of photochromic compound(s) needed is inversely proportional to the thickness of the polyurethane layer. In other words, to achieve the same outdoor light transmission the thicker the polyurethane layer, the lower the concentration of photochromic compound(s) needed. The concentration of the photochromic compound(s) also depends on the color intensity of the photochromic compound(s) at the activated state. 
     Stabilizers 
     Additives such as antioxidants and light stabilizers are incorporated into the polyurethane layer in order to improve the fatigue resistance of the photochromic compounds. Hindered amines are usually used as light stabilizers, and hindered phenols are usually used as antioxidants. Preferred hindered amine light stabilizers include, bis(1,2,2,6,6-pentamethyl-4-piperidinyl)-sebacate, or a condensation product of 1,2,2,6,6-pentamethyl-4-piperidinol, tridodecyl alcohol and 1,2,3,4-butanetetra caboxylic acid as tertiary hindered amine compounds. Preferred phenol antioxidants include, 1,1,3-tris(2-methyl-4-hydroxy-5-t-butylphenyl)butane, tetrakis-[methylene-3-(3′,5′-di-t-butyl-4′-hydroxy-phenyl)propionate]methane, and 1,3,5-tris(3,5-di-t-butyl-4-hyroxybenzyl)-1,-3,5-triazine-2,4,6-(1H,3H,5H)-trione. Phenol antioxidants that contain 3 or more hindered phenols are preferable. 
     Process to Make the Laminate 
     A photochromic laminate having a polyurethane layer in between two transparent resin sheets in accordance with the present invention may be produced through a variety of processes. Depending on the nature of the starting material to the polyurethane, processes such as casting-lamination (also referred to in the art as coating-lamination), and extrusion-lamination may be used. 
     To the photochromic polyurethane composition of the present invention, a novel casting-lamination process has been developed by the inventors. The process essentially comprises: a) preparing a solvent casting solution by dissolving a solid thermoplastic polyurethane, at least one isocyanate polyurethane prepolymer, at least one photochromic compound, and optional stabilizers in a proper solvent; b) cast the solution on a release liner film; c) remove the solvent from the cast film to a substantially dry state to form a photochromic polyurethane film; d) transfer-laminate the photochromic polyurethane film between two transparent resin sheets; e) cure the photochromic polyurethane film, thereby forming a photochromic polyurethane laminate. 
     To cast a photochromic polyurethane film, a thermoplastic polyurethane, an isocyanate prepolymer, selected photochromic compounds and other necessary additives are first dissolved in a suitable solvent or in a mix of solvents to form a cast solution. The solid concentration in such a solution is usually 15% to 50%, by weight, and the solution has a viscosity suitable for coating. For example, suitable viscosity of the cast solution for using a slot die method is within the range from 500 cPs to 5000 cPs. Examples of suitable solvents that may be used to dissolve polyurethanes include cyclohexane, toluene, xylene and ethyl benzene, esters such as ethyl acetate, methyl acetate, isopropyl acetate, n-propyl acetate, isobutyl acetate, n-butyl acetate, isoamyl acetate, methyl propionate and isobutyl propionate, ketones such as acetone, methylethyl ketone, diethyl ketone, methylisobutyl ketone, acetyl acetone and cyclohexyl ketone, ether esters such as cellosolve aetate, diethylglycol diaetate, ethyleneglycol mono n-butylether acetate, propylene glycol and monomethylether acetate, tertiary alcohols such as diacetone alcohol and t-amyl alcohol and tetrahydrofuran. Ethyl acetate, methyl ethyl ketone, cyclohexane, tetrahydrofuran, toluene and combinations thereof are preferable. 
     The solution is then cast on a release liner by using a method known to those skilled in the art, such as slot-die, knife-over-roll, reverse-roll, gravure, etc. Slot die and knife-over-film are referred. Slot die method is especially preferred due to its capability to handle wide range of solution viscosity and to cast uniform films. A release liner may consist of a base film and a release coating or simply a film itself. Films with surface energy low enough to provide easy release of the cast film can be used by itself. Examples include low energy polyolefins and fluoropolymers. Most commercially available release liners are based on polyester film coated with a release coating. The release coating has a proper surface energy so that a cast solution or coating forms a uniform film (e.g., without beading) on it. At the same time the release coating does not provide good adhesion to the dried film so that the film can be easily peeled off. Release coatings include silicone (siloxane) based and non-silicone base such as fluoropolymers. A liner based on polyester (PET) with cured siloxane release coating is preferred due to the dimensional stability, flatness, handling, solvent resistance, low cost. Suitable liners should have a thickness of from 25 micrometers to 130 micrometers. 
     The wet photochromic polyurethane film cast on the release liner is sequentially dried in a forced air oven system. The solvent will be substantially evaporated so that the solvent retention in the photochromic polyurethane film is low enough to not cause any defects (e.g., bubbling) in the future laminate. The solvent retention preferably is less than 2 wt. %, more preferably less than 1 wt. %, and most preferably less than 0.5 wt. %. Conventional methods such as hot air dryers may be used to evaporate the solvent before lamination. The drying conditions, such as temperature and air flow rate in the oven, for a desired solvent retention value depends on the nature of the solvent, the thickness of the cast film, the type of the release liner, and the web speed. The drying conditions should not be so aggressive to cause any surface defects in the cast film. Example defects are blisters (bubbles) and orange peel. Preferably, the drying oven system has multi-zones whose drying conditions are controlled separately. 
     The thickness of the dried photochromic polyurethane layer is from about 5 micrometers to about 150 micrometers. For using the photochromic laminate in an insert injection molding process to make plastic photochromic lenses, the thickness of the photochromic polyurethane is preferably between 5 micrometers and 80 micrometers. The thickness variation of the photochromic polyurethane layer should be controlled in order to produce a uniform light blockage at the activated state. A thickness variation of less than 15% over the width of the laminate is required and preferably less than 10% and more preferably less than 5%. 
     The transfer-lamination of the dried photochromic polyurethane film to two transparent resin sheets to form a laminate of the polyurethane film between the two resin sheets, may be done by either a sequential lamination process or an in-line lamination process. In a sequential lamination process, the dried polyurethane film on the release liner is first laminated to the first transparent resin sheet through a first lamination station. The semi-laminate consisting of the release liner, the polyurethane film, and the resin sheet, is then wound up on a core. The wind is then brought to a second lamination station where the release liner is peeled off and the second transparent sheet is laminated to the polyurethane film to form the final photochromic polyurethane laminate. The first and the second lamination stations may be the same one. The lamination may be conducted between two chrome coated steel rolls or between one steel roll and one rubber roll, although the later is preferred. 
     According to the findings of the inventors, an in-line lamination process is more preferred. In such a process, the second transparent resin sheet is immediately laminated to the semi-laminate without first winding the semi-laminate. The in-line lamination may be done with two two-roll lamination stations, or more conveniently be conducted on one three-roll setup in which the first roll and the second roll form a first nip, and the second roll and the third roll form a second nip. The dried polyurethane film on the release liner is first laminated to the first transparent resin sheet through the first nip. Without forming and winding a semi-laminate, the release liner is peeled off, and the second transparent resin sheet is immediately laminated to the exposed side of the polyurethane film on the first transparent resin sheet, through the second nip. This in-line lamination process will significantly increase the productivity. It also eliminates an extra winding step and reduces the possibilities of defects in the polyurethane film associated with the winding step. Example defects are de-lamination between the polyurethane film and the transparent resin sheet, impressions in the polyurethane film caused by possible external particles under winding pressure. 
     The photochromic polyurethane laminate thus formed according to the present invention needs to be cured before application. The curing is preferably carried in two stages: a) ambient curing for 1 day to 1 week, b) post curing at elevated temperature of from 50° C. to 130° C. for 8 hours to 1 week. 
     If the solvent selected to dissolve the photochromic polyurethane composition does not whiten the transparent resin sheet, a direct cast on the resin sheet may be employed. In this case, a simple two-roll lamination setup is acceptable for making a photochromic polyurethane laminate. 
     In an alternative process, the photochromic layer from a thermoplastic polyurethane and isocyanate-terminated polyurethane prepolymer may be co-extruded utilizing a single- or twin-screw extruder. The extruded photochromic polyurethane film will then be immediately hot-laminated between two transparent resin sheets to form the photochromic polyurethane laminate. The photochromic compounds and other additives may be incorporated into the polyurethane during the resin synthesis stage or melt-mixed prior to extrusion. 
     Although the photochromic laminate according to the present invention is especially suitable for making photochromic polycarbonate lenses through the insert injection molding process described in commonly assigned U.S. Pat. No. 6,328,446, it can also be used as-is for other photochromic transparencies such as goggles and face shields. The photochromic laminate may also be incorporated into other types of eyewear lenses such as cast resin lenses with a process described in U.S. Pat. No. 5,286,419. 
     The photochromic polyurethane laminate in accordance with the present invention will now be illustrated with reference to the following examples, which are not to be construed as a limitation upon the scope of the invention in any way. 
     In the examples, all values are expressions of weight %. CR49 and CR59 are tradenames of photochromic dyes available from Corning Corp. Grey-762 is proprietary grey photochromic dye. Irganox-1010 as an antioxidant, Tinuvin-144 and Tinuvin-765 as light stabilizers are available from CIBA (Tarrytown, N.Y., US). 
     To visually evaluate the activation and the photochromic polyurethane layer uniformity, a photochromic laminate or lens was exposed to UV irradiation (12 mw/m2) for 5 minutes. 
     Example 1 
     Preparation of Isocyanate Polyurethane Prepolymer A: In a 3-necked flask equipped with an overhead stirrer, thermocouple, and a vacuum adapter, 393.5 g (3 equivalents) of 4,4′-dicyclohexylmethanediisocyanate (H12MDI, available from Bayer as Desmodur W) was charged into the reactor and stirred at ambient temperature. 1000 g (2 equivalents) of a polycaprolactone diol having an OH number of 112 mg KOH/g and a number average molecular weight of about 1000 g/mole (available from Dow Chemical as Tone™ 2221) was preheated in an oven to 80° C. and added to the reactor. The mixture was allowed to stir for about 15 minutes, before adding 6 g of dibutyltin dilaurate catalyst (available from Air Products as T-12). The reaction flask was evacuated (&lt;0.1 mm HG) and held at 90° C. for 6 hours. An aliquot of the prepolymer was withdrawn and titrated for isocyanate content using standard n-butyl amine titration. The isocyanate content was found to be 2.92% (theory; 3.0%). 
     Example 2 
     Preparation of Isocyanate Polyurethane Prepolymer B: In a 3-necked flask equipped with an overhead stirrer, thermocouple, and a vacuum adapter, 613.0 g (4.67 equivalents) of 4,4′-dicyclohexylmethanediisocyanate (H12MDI, available from Bayer as Desmodur W) was charged into the reactor and stirred at ambient temperature. 1000 g (2 equivalents) of a polycaprolactone diol having an OH number of 112 mg KOH/g and a number average molecular weight of about 1000 g/mole (available from Dow Chemical as Tone™ 2221) was preheated in an oven to 80° C. and added to the reactor. The mixture was allowed to stir for about 15 minutes, before adding 8 g of dibutyltin dilaurate catalyst (available from Air Products as T-12). The reaction flask was evacuated (&lt;0.1 mm HG) and held at 90° C. for 6 hours. An aliquot of the prepolymer was withdrawn and titrated for isocyanate content using standard n-butyl amine titration. The isocyanate content was found to be 6.75% (theory; 7.0%). 
     Example 3 
     Preparation of Thermoplastic Polyurethane: A thermoplastic polyurethane having a theoretical NCO index of 95 was prepared as following. The isocyanate prepolymer B (927.2 g) prepared in Example 2 was heated in vacuo (&lt;0.1 mm HG) with stirring to 80° C. and 1,4-butane-diol (72.8 g) as the chain extender and 3 g of dibutyltin dilaurate catalyst were combined with the prepolymer while keeping stirring. The mixture was stirred for 30 seconds and subsequently poured into a Teflon lined tray. The tray containing the casting was cured in an oven at 85° C. for 24 hours. 
     Example 4 
     A solution was first made by dissolving 4 g of the thermoplastic polyurethane prepared in Example 3 in 16 g of anhydrous tetrahydrofuran. To the solution was further added 4 g of the isocyanate prepolymer prepared in Example 1, 0.14 g of CR49 dye, 0.02 g CR59 dye, 0.17 g each of Irganox-1010, Tinuvin-144, and Tinuvin-765. The mixture was stirred at room temperature for 3 hours before cast on an easy release liner (available from CPFilms as T-50) with draw bar targeting a 38 micrometer dry film thickness. The solvent in the cast film was evaporated at 60° C. for 5 minutes with airflow above the film. The dried film was transfer-laminated between two 0.3 mm thick polycarbonate sheets (available from Teijin as PC-1151) with a bench top roller laminator. After 4 days under ambient, the laminate was cured at 70° C. for 3 days. 
     The laminate was cut into a 76 mm disc and used to make a segmented multi-focal polycarbonate photochromic lens. After the insert injection molding process with common molding parameters, the finished lens had an acceptable thin, crisp segment line. No polyurethane bleeding from the laminate was observed. The lens showed quick and uniform photochromic activation. No any lamination defects were observed. 
     Example 5 
     A solution having 28.2% solid, was first prepared by dissolving 1950 g of the thermoplastic polyurethane prepared as in Example 3 in 7550 g of anhydrous tetrahydrofuran. To the solution was further added 780 g of the isocyanate prepolymer prepared as in Example 1, 59 g each of “762” dye, Irganox-1010, Tinuvin-144, and Tinuvin-765. The mixture was stirred at room temperature for 3 hours then set overnight before cast on an easy release liner (available from Saint-Gobain as 8310) at a web speed of 5.5 feet per minute in a pilot coater equipped with a slot die, a two-zone drying oven, and a three-roll lamination station. The solvent in the cast film was evaporated at 70° C. for 1 minute and 120° C. for another minute with forced airflow above the film. The dried film was 38 micrometer thick and had a solvent retention of 0.1%. It was transfer-laminated between two 0.3 mm thick polycarbonate sheets (available from Teijin as PC-1151) with an in-line process (without winding the semi-laminate of the release liner, polyurethane film, and the first polycarbonate sheet). After 4 days in ambient (22° C. and 35%˜50% RH), the laminate was cured at 70° C. for 3 days. 
     The laminate was cut into 76 mm discs and used to make a segmented multi-focal polycarbonate photochromic lenses. After the insert injection molding process with common molding parameters, the finished lens had an acceptable thin, crisp segment line. No polyurethane bleeding from the laminate was observed. The lens showed quick and uniform photochromic activation. No any lamination defects were observed. 
     Example 6 
     A solution having 35.3% solid, was first prepared by dissolving 1950 g of the thermoplastic polyurethane prepared as in Example 3 in 7742 g of anhydrous tetrahydrofuran. To the solution was further added 1950 g of the isocyanate prepolymer prepared as in Example 1, 68 g of CR49 dye, 10 g CR59 dye, 85 g each of Irganox-1010, Tinuvin-144, and Tinuvin-765. The mixture was stirred at room temperature for 3 hours then set overnight, then cast directly on a first 0.3 mm thick polycarbonate sheet (available from Teijin as PC1151) at a web speed of 5.5 feet per minute in a pilot coater equipped with a slot die, a two-zone drying oven, and a three-roll lamination station. The solvent in the cast film was evaporated at 94° C. for 1 minute and 127° C. for another minute with forced airflow above the film. The dried film was 25 micrometer thick and had a solvent retention of 0.1%. A second 0.3 mm thick polycarbonate sheet was laminated on the exposed side of the dried polyurethane film on the first polycarbonate sheet. After 4 days in ambient (22° C. and 35%˜50% RH), the laminate was cured at 70° C. for 3 days. The laminate obtained was clear. No solvent whitening on the polycarbonate sheets was seen. 
     Comparison Example 1 
     To 10 g of Hysol® (Loctite) U-10FL urethane adhesive resin are dissolved 1.5% of “762” dye, 2.0% of Tinuvin 144, and 2.0% of Tinuvin 765. Then, 9.1 g of Hysol® U-10FL urethane adhesive hardener is mixed in to form a liquid adhesive. 
     The adhesive was coated with a draw bar directly on a polycarbonate sheet (0.3 mm thick, available from Teijin as 1151) to form a 38 micrometer cast film. Another polycarbonate sheet was laminated onto the adhesive through a bench top roller laminator. Some adhesive was squeezed out. The laminate was allowed to cure at room temperature overnight, then is post cured at 65° C. for 10 hours. 
     When the photochromic laminate was activated, thin spots (lightly activated due to thinner spots in the polyurethane layer) and non-uniformity of activation due to thickness gradient across the laminate were observed. 
     Comparison Example 2 
     Example 4 was followed, except the isocyanate prepolymer was neglected. The photochromic polyurethane layer was 38 micrometers thick. The laminate showed uniform photochromic activation. No lamination defects were observed. However, when molded into a polycarbonate lens as in Example 4, severe polyurethane bleeding was observed at the edge of the laminate. 
     The foregoing detailed description of the preferred embodiments of the invention has been provided for the purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed. Many modifications and variations will be apparent to practitioners skilled in the art to which this invention pertains. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, thereby enabling others skilled in the art to understand the invention for various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents. 
     Although the invention has been described in terms of particular embodiments and applications, one of ordinary skill in the art, in light of this teaching, can generate additional embodiments and modifications without departing from the spirit of or exceeding the scope of the claimed invention. Accordingly, it is to be understood that the drawings and descriptions herein are proffered by way of example to facilitate comprehension of the invention and should not be construed to limit the scope thereof.