Abstract:
A method for laminating an optical assembly and the optical assembly formed thereby. An ultra thin lens is injection molded and may include a bifocal feature. A support lens, e.g. of the single vision type, is provided with a photochromic coating. A flexible two stage compound application process prepares the two lenses for lamination.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of co-pending application bearing U.S. Ser. No. 11/147,614, filed on Jun. 8, 2005 entitled Method of Injection Molding Thin Thermoplastic Lenses. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for producing photochromic lenses by laminating an ultra thin lens to a support lens with a photochromic layer therebetween. 
     2. Description of the Related Art 
     Lenses and other articles manufactured at optical quality have exacting demands for mold replication, high optical transmission and impact resistance. In the 2002  Polycarbonates  publication, Brunelle and Kailasam describe how polycarbonate is prepared by the transesterification of a bisphenol-A with a carbonate. U.S. Pat. No. 5,212,280 describes diphenols which are useful in the condensation method of producing polycarbonate in the presence of phosgene. 
     Injection molding of lenses requires edge gating so that the runner ends up remote from the lens surfaces. The paths from the gate to the edge points of the mold cavity are not symmetrical and therefore make it difficult to control the thermodynamics of the cooling melt flow. As lens cavities become thinner, straight injection molding techniques are unable to fill the mold without premature freeze-off. Accordingly, injection molding machines have been modified to enlarge the cavity during some phase of the injection cycle, in a so-called injection/compression process. Recent improvements in injection molding techniques by the applicant have resulted in ultra thin lenses that can be effectively incorporated into laminated lenses. 
     Previously, relatively thick lenses were utilized in laminated optics, as can be seen for example in U.S. Pat. No. 6,256,152. The process as described in all of the examples, uses a pair of 2.5 mm center thickness Diallyl glycol carbonate lenses. Dially glycol carbonate is a thermoset polymer sold under the trade name CR-39, that is formed into lenses by casting. CR-39 is more brittle, and therefore less flexible, than polycarbonate. As a result the prior art requires perfectly matching base curves on the mating surfaces of the two lenses. An inner film or foil is cast, molded or blown into a solid form having the same base curve as the interfacial surfaces of the two lenses. Despite the uniform base curves, the prior art requires an additional step of treating the interfacial surfaces with plasma or corona discharge inter alia, in order to modify or improve bondability. 
     U.S. Pat. No. 4,867,553 also relates to cast CR-39 lenses having a center thickness at least 1.0 mm and an edge thickness of approximately 1.7 mm. The patent describes a two component assembly. The cover lens can include coatings, filters or tints. However, within the specified ratio of 1.5 to 2 times more edge thickness than center thickness, a photochromic dye will appear much darker in the thicker, peripheral portions than it will in the center. 
     Accordingly, it would be desirable to provide a lens assembly having a uniformly thick photochromic layer, along with a streamlined process to laminate a thin and flexible front lens thereon, without having to supply both of the lenses and the photochromic film in the same base curve. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a method for incorporating injection molded ultra thin (less than 1 mm thickness) thermoplastic lenses into an optical assembly. 
     It is a further object of the present invention to provide photochromic functionality to the laminated optical assembly. 
     It is another object to provide a flexible two stage compound application step for accommodating a variety of lens and coating conditions. 
     These and other related objects according to the invention are attained by a method for producing laminated photochromic lenses starting with injection molding an ultra thin front lens having a thickness less than 0.8 mm. A liquid photochromic solution is solidified in situ onto a single vision (SV) support lens without pressure to form a photochromic layer directly on a SV support lens surface in the absence of an intermediate adhesive layer. Next, at least two compounds are applied between the photochromic layer and the ultra thin front lens. The compounds may include a protective layer, a primer layer, an acrylic-based layer, a polyurethane latex layer, an adhesive layer, and combinations thereof. The ultra thin front lens is then laminated onto the photochromic layer with compressive pressure, whereby the shape of the ultra thin front lens can be deflected up to 0.5 base curves to completely conform to the shape of the SV support lens surface. 
     The ultra thin lens is made from polycarbonate having a viscosity of less than 400 Pa for shear rates below 1,000/s. The injection molding step includes coining an ultra thin lens having a thickness between 0.8 mm and 0.3 mm. The ultra thin front lens is a straight top bifocal lens having an add power between +1.00 and +3.00 diopters. For example, the distance portion is between about 0.7 mm and about 0.5 mm thick. 
     The step of applying at least two compounds includes first coating a polyurethane latex primer onto the photochromic layer of the support lens. The primer layer is spin-coated onto the convex surface at room temperature then dried at a temperature between 50 degrees and 100 degrees C. There is a second applying step of an optical adhesive between the primer and the ultra thin front lens. A UV curable optical adhesive may be used. For example, a UV curable optical acrylate adhesive that is dispensed from a syringe at room temperature. Alternatively, a pressure sensitive adhesive film may be employed. 
     In another embodiment, there is a first application of a protective coating onto the photochromic layer of the support lens. Then apply an optical adhesive between the protective coating and the ultra thin front lens. A UV curable optical adhesive may be used. For example, a UV curable optical acrylate adhesive that is dispensed from a syringe at room temperature. Alternatively, a pressure sensitive adhesive film may be employed. 
     The laminating step occurs at room temperature with a pressure between 5 psi and 60 psi. For example, inflating a silicon rubber bladder to apply pressure onto the ultra thin front lens between about 10 psi and about 25 psi. 
     The ultra thin lens is a bifocal or multi-focal lens. For example, a straight top bi-focal lens having an add power of between +1.00 and +3.00 diopters. The front surface may include a hard coat or an antireflective coating or both. 
     The SV support lens has a back surface that is adapted to be ground so that the SV support lens prescription can be customized, thereby providing a straight top bi-focal lens with photochromic properties contributed by a uniformly thick, internally laminated photochromically active layer. The invention also includes the laminated optical assembly made according to the methods described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages, nature, and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments now to be described in detail in connection with accompanying drawings. In the drawings wherein like reference numerals denote similar components throughout the views: 
         FIG. 1  is a flowchart illustrating a lamination process according to the prior art. 
         FIG. 2  is a flowchart illustrating a streamlined lamination process according to an embodiment of the invention. 
         FIG. 3  is a flowchart detailing the available coating options according to the streamlined lamination process according to the invention. 
         FIG. 4  is a diagram designating the various surfaces and coatings in the laminated assembly. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring now in detail to invention, there is provided a method for producing laminated photochromic lenses. The method presents a specific use for ultra thin lenses, made according to applicants injection molding process and described in U.S. patent application Ser. No. 11/147,614, the entire contents of which is incorporated herein by reference thereto. 
     In summary, the related invention provides methods for producing thin lenses made from a thermoplastic material. The invention overcomes difficulties typically associated with injection molding lenses less than 1 mm thick, for example lenses of about 70 mm in diameter. Applications for such lenses include their use as carriers in the backside transfer (BST) process or the front side transfer (FST) process. Further applications include the production of powered lenses having center or edge sections up to about 1 mm thick. The present specification addresses methods for incorporating these thin powered lenses into laminated optical assemblies. 
     As used herein, the term lens means an optical grade article. The term includes plano lenses as well as powered lenses. Thermoplastic means an optically clear thermoplastic of optical grade. Thermoplastics that may be used include, by way of example only, polycarbonates, polycarbonate/copolyester blends, acrylics like PMMA, cyclic olefin copolymers, amorphous polyamides, polyesters, copolyesters, polyurethanes, and the like. 
     Laminated optical assemblies have certain advantages over one piece lenses. They allow different lens elements to be combined in a way that offers a large variety of lens powers and functions. For example, thin front lenses can provide bi-focal or multi-focal optics. The support lens can provide basic single vision optics. During the assembly operation, functional filters, like photochromic filters can be sandwiched between the two lenses. This offers a distinct advantage since it is difficult to incorporate photochromic dyes into the lens material. Even if photochromic dyes were compounded into the resin, it would provide different degrees of darkening as a function of lens thickness. As will be described in greater detail below, the present invention provides a more streamlined process for the selection and assembly of laminated lenses, due in large part to the material processing that imparts photochromic functionality as well as the properties of the thin front lenses. 
     Referring now to  FIG. 1 , there is shown a series of steps to fabricate laminate lenses according to the prior art. In step  10 , the front lens may be made of glass, although cast lenses made from CR-39 seems to be the preferred material. Once this lens is selected, the interfacial base curve is set, and the other components of the assembly must be exactly matching. In step  11  a second cast lens is selected. To impart a photochromic or polarizing film to the assembly, a film mold is needed with the matching base curve and an appropriate film thickness, per step  12 . The inner layer is then mold or cast into a shaped film in step  13 . The film may be trimmed in various ways in step  14 . 
     When adhering the shaped film to the two lenses, there is an apparent problem with subsequent delamination. As a result the prior art introduces a burdensome step of treating the surfaces to be adhered. Step  15  indicates the surface treatment step as being one of plasma discharge, corona discharge, radiation treatment, laser treatment, etc. In step  16  there is the first application of adhesive to bond the shaped film to one of the lenses. If a flange is present on the film, it may be trimmed in this step, or in a later step. In step  17  there is the second application of adhesive and to bond the second lens to the first lens/film assembly. 
     Referring now to  FIG. 2  there is shown several steps pursuant to a streamlined method according to the invention. For consistency and clarity, we refer to  FIG. 4  for lens side labels. Reading from left to right, front lens  20  has an outward side  20   a , and an opposed inward side  20   b . Support lens  30  has an interior side  30   c  and an exterior side  30   d . The ultra thin front lens less than 1.0 mm thick is injection molded in step  20 . Step  22  indicates an optional coating on the outward surface  20   a  of the front lens. The photochromic solution is delivered in liquid form directly on to the interior side  30   c  of the support lens, which may be a single vision (SV) lens. Once solidified the photochromically enabled surface is referred to as  30   p . Step  32  indicates a grinding process on the exterior side  30   d  of the support lens to impart a custom prescription thereto. 
     Step  50  includes the application of two compounds, one after the other, as can be seen in  FIG. 3 . The first compound is applied to surface  30   p , and is referred to as  50   a . The second compound is applied to either surface  20   b  or  50   a . Generally, the second compound  50   b  is an adhesive. The lenses are laminated together in step  60  with the three layers sandwiched therebetween to form the completed optical assembly. 
     Certain aspects of the method will now be described in greater detail. 
     Summary of Method for Injection Molding Ultra Thin Lenses 
     The ultra thin lens is one component of the optical assembly described herein. It is the result of recognizing the fact that carriers or lenses with a center thickness about 0.5 mm achieve an excellent balance between flexibility for laminating and rigidity for coating. Severe challenges arise in attempting to injection mold a meniscus-shaped article, at optical quality, via edge-gated injection molding. However, our related application describes injection molding such lenses from optically clear thermoplastics having a viscosity below 400 Pa at a shear rate of 1,000/s measured at 300 degrees C. (Celsius). By adjusting the clamp force it was possible to consistently produce PC lenses through a coining process with a center thickness of about 0.56 mm. High yields were characterized by precision mold replication and uniformity in critical physical lens parameters. Similarly, these factors are intrinsic to the successful production of bifocal and multi-focal lenses. 
     The related application describes how the mold halves are closed with a predetermined clamp force F c  that is less than the net integrated force exerted on the mold inserts during injection. Molding material is injected into the cavity with a force F 1  greater than F c  to force the mold open thereby reducing the flow resistance and allowing the material to reach portions of the insert periphery. A lens having a section less than 1 mm thick is coined as the net integrated force on the inserts subsides. 
     The flow resistance is reduced by venting the cavity through the open parting line of the mold or reducing the flow resistance, or a combination of both. During the injecting step, the molding material is in intimate contact with the inserts at a thickness greater than the final coined lens thickness. The coining step reduces the thickness of the lens to between 0.5 and 0.6 mm thick as the mold closes. Following the injecting step, the method further includes the step of shifting the injector to packing pressure mode so that molding material stops flowing into the cavity. For clamp-end coining, the inserts are secured in a fixed position within their respective sleeves, so that force exerted on the inserts by the molding material is transferred to the mold clamp. 
     The method involves closing the mold with a clamp force less than the force exerted on the insert during injection. During injection the mold is allowed to breathe thereby overcoming many of the obstacles associated with injection molding very thin lenses. As the injection pressure subsides, the mold begins to close to perform a coining operation, resulting in a strong, high yield carrier or lens. 
     For the lamination method, we provide injection molded PC coined lenses having a plano distance portion with a thickness between 0.8 mm to 0.3 mm. For example, ultra thin lenses  20  would have a distance portion with a thickness of between about 0.7 mm to about 0.3 mm. These lenses may be configured as any type of bifocal or multi-focal lens, for example, curved top, round seg, no-line or executive bifocals. The lamination method is well suited for use with straight top or flat top bifocal having an add power of about +1.00 to about +3.00 diopters. 
     Plastic lenses, and in particular polycarbonate (PC), may be subject to scratching. Typically the outward surface  20   a  of such PC lenses are provided with a hard coat solution. Such lenses may also include an antireflective (AR) coating, or both AR and hard coat. These coatings may be applied to front lens  20  at any time that does not interfere with the lamination process. In addition, when a straight top bifocal is utilized, the coating(s) must be applied in a manner that avoids pooling or other unacceptable conditions at the straight top ledge. 
     SV Support Lens 
     Semi Finished (SF) lenses are commonly produced with one optically finished side. This is represented by interior side  30   c . SF lenses are manufactured in the first instance to be thicker than the final lens. The invention may use thermoplastic lenses, for example, polycarbonate. The exterior side  30   d  is ground at the lab into a predetermined shape that provides a varying thickness at points emanating radially from the lens center out to the lens periphery. When ground into this predetermined shape, the resulting lens is a Single Vision (SV) lens. The method according to the invention preserves this customization of SF lenses. This is referred to as the custom Rx for a particular customer. Within the context of the inventive method, the grinding process can occur at any time that does not interfere with the lamination. 
     The optically finished interior side  30   c  is coated with a photochromic solution that dries and solidifies to form a photochromic coating  30   p . The application of the solution in a liquid form allows photochromic coating  30   p  to be applied without pressure and without an intermediate adhesive. As will be appreciated by those skilled in the art, this liquid application replaces several steps that are part of the prior art process. Those are the steps of: molding the photochromic layer into a shaped film; any trimming of the film; plasma/corona discharge treatment prior to adhering film; dispensing adhesive; and adhering the film onto one of the lenses. 
     Flexible Two-Stage Compound Application 
     Some SV lenses with photochromic layers  30   p  are designed to receive a protective coating. Accordingly, we consider the application of such protective coating as the first compound  50   a  to be applied in our applying step. 
     The protective layer  50   a  is suitable for adhering directly to the inward surface  20   b  of the ultra thin front lens  20 . The adhesive layer  50   b  can be applied on top of first compound  50   a  or on inward surface  20   b . For example, apply at least one drop of an optical adhesive on either  50   a  or  20   b . The optical adhesive may be applied at room temperature with a syringe. Suitable adhesives are a UV curable adhesive, an acrylate based adhesive, and a UV curable acrylate based adhesive. 
     Alternatively, a film based adhesive can serve as second compound  50   b . For example, a film based pressure sensitive adhesive (PSA) may be employed. One commercially available product is PSA tapes from Nitto Denko Europe. 
     Using a pressure-sensitive adhesive (PSA) is particularly advantageous since the layered structure (thin lens) is permanently retained on the lens (SFSV) in a simple and inexpensive manner, without impairing the optical properties of both the lens and the structure. In particular, no irradiation, such as ultraviolet irradiation, nor intensive heating is required for obtaining a permanent bonding with a pressure-sensitive adhesive. All pressure-sensitive adhesives exhibit permanent tack and have a low elastic modulus at room temperature, typically between 10 3  and 10 7  Pa (pascals). It is pointed out that the adhesion mechanism involved with pressure sensitive adhesives does not involve chemical bonding, but it is based on special viscoelastic properties of pressure-sensitive adhesives. These properties intrinsic to each pressure-sensitive adhesive formulation make it possible to create electrostatic van der Waals interactions at the bonding interface. This occurs when a pressure-sensitive adhesive is brought into contact with a solid material with pressure. The pressure and the low modulus of the pressure-sensitive adhesive create intimate contact of this latter at a molecular scale with the topology of the solid material. Moreover, bulk viscoelastic properties of the pressure-sensitive adhesive lead to dissipation, within the thickness of the adhesive layer, of the energy resulting from mechanical stressing of the bonding interface. Therefore the interface can withstand pull-strengths and debonding mechanisms. 
     In addition, pressure-sensitive adhesives can be deposited in the form of a thin layer with uniform thickness. Such thickness may be comprised between 0.5 and 300 μm. Then, image formation through the lens is not impaired by the layer of pressure-sensitive adhesive and the optical power of the lens is not altered. 
     Several pressure-sensitive adhesives may be used in a process according to the invention. Advantageously, the pressure-sensitive adhesive is selected from a compound based on a polyacrylate, a styrene-based block copolymer and a blend incorporating a natural rubber. Non-limiting examples of pressure-sensitive adhesives have general compositions based on polyacrylates, in particular polymethacrylates, or based on ethylene copolymers, such as ethylene vinyl acetate, ethylene ethyl acrylate and ethylene ethyl methacrylate copolymers, or on synthetic rubber and elastomers, including silicones, polyurethanes, styrene-butadienes, polybutadienes, polyisoprenes, polypropylenes, polyisobutylenes, or based on polymers containing nitriles or acrylonitriles, or on polychloroprene, or on block copolymers that include polystyrene, polyethylene, polypropylene, polyisoprene, polybutadiene, on polyvinylpyrrolidone or vinylpyrrolidone copolymers, or are blends (with continuous or discontinuous phases) of the above polymers, and also may comprise block copolymers obtained from the above-listed compounds. These pressure-sensitive adhesives may also include one or more additives selected from tackifiers, plasticizers, binders, antioxidants, stabilizers, pigments, dyes, dispersing agents and diffusing agents. 
     Some SV lenses simply have a photochromic layer  30   p . The laboratory would have the option of applying a protective layer or a primer layer. In the case of the latter, we consider the primer layer as the first compound layer  50   a . For example, a polyurethane latex primer may be used. The primer can be spin-coated on to photochromic layer  30   p , at room temperature, and then dried at a temperature between 50 and 100 degrees C. 
     The second compound layer  50   b  would then be an adhesive. The adhesive layer  50   b  can be applied on top of first compound  50   a  or on inward surface  20   b . For example, apply at least one drop of an optical adhesive on either  50   a  or  20   b . The optical adhesive may be applied at room temperature with a syringe. Suitable adhesives are a UV curable adhesive, an acrylate based adhesive, and a UV curable acrylate based adhesive. 
     Alternatively, a film based adhesive can serve as second compound  50   b . For example, a film based pressure sensitive adhesive (PSA) may be employed. One commercially available product is the PSA tapes from Nitto Denko Europe. 
     EXAMPLE 1 
     A 0.58 mm thick 6.50 base+2.00 add PC front lens was laminated to a 6.5 base semi-finished single vision (SFSV) Transitions® photochromic polycarbonate lens using a UV curable adhesive. The resulting SFSV bifocal lens laminate exhibited a very uniform darkening when exposed to sunlight. 
     EXAMPLE 2 
     A 0.58 mm thick 6.50 base+2.00 add PC front lens was laminated to a 6.5 base SFSV Transitions® photochromic polycarbonate lens using a PSA. The resulting SFSV bifocal lens laminate exhibited a uniform darkening when exposed to sunlight. 
     Lamination 
     After the intermediate layer  50   a  and second compound layer  50   b  have been prepared, the ultra thin front lens is ready for lamination. With SV lens  30  properly supported, front lens  20  is placed down with inward surface  20   b  facing the compound layers  50 . Compressive pressure is applied to the outward surface  20   a  at room temperature. Suitable pressure is between 5 and 60 psi. For example, an inflatable bladder may apply between about 10 and about 25 psi. The bladder may be made from silicon, which is durable and avoids scratching the outward surface  20   a . This compressive pressure is sufficient to press front lens  20  flat onto the stack, even if the interfacial surfaces  20   b  and coated surface  30   a  are of different base curves. Once front lens  20  is set, the assembly may be exposed to UV radiation to cure the adhesive, in the event that a UV curable adhesive has been employed. 
     The method according to the invention has numerous advantages and benefits over the prior art. The photochromic layer is easily solidified in situ without requiring the use of an intermediate adhesive. The photochromic layer is uniformly thick and near the front of the optical assembly. When a hard coat or AR coating is present on outward surface  20   a , the photochromic layer  30   p  resides within about 0.7 to about 0.8 mm from the hard coat or AR coating. 
     In addition to the uniform darkening and preservation of the optical integrity of the original SV, there are several advantages of the present invention over other known approaches. Unlike in-mold decoration (IMD) or film insert molding (FIM) which requires mass production, the present lamination method can be performed on an individual bases, as needed, in the laboratory. 
     The method effectively utilizes our proprietary ultra thin coined lens in an optical assembly to deliver a straight top bifocal lens with photochromic functionality. The ultra thin lens can be flexed 0.5 diopters in base curve to conform to the base curve of surface  30   c . This is particularly significant when providing a laminated lens series, that incorporates SV lenses having a range of base curves (on surface  30   c ). For example, consider a series that includes SV lenses with base curves from 6 to 8 diopters, in 0.25 diopter increments. That would result in 9 different SV lenses. One would only need a 6.5 base front lens and a 7.5 base front lens to create a lens assembly, where the 6.5 base front lens could be employed on the 6.00, the 6.25, the 6.50, the 6.75 and the 7.00 base SV lens. Accordingly, in a laminated lens series, one front lens, can accommodate a full 1.00 diopter range of support lenses. 
     Having described preferred embodiments for laminating lens assemblies, materials used therein and coatings for same (which are intended to be illustrative and not limiting), it is noted that modifications and variations can be made by persons skilled in the art in light of the above teachings. It is therefore to be understood that changes may be made in the particular embodiments of the invention disclosed which are within the scope and spirit of the invention as outlined by the appended claims. Having thus described the invention with the details and particularity required by the patent laws, what is claimed and desired protected by Letters Patent is set forth in the appended claims.