Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Utilizing plasma – electric – electromagnetic – particulate – or...
Reexamination Certificate
2001-03-27
2004-12-07
Vargot, Mathieu D. (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Utilizing plasma, electric, electromagnetic, particulate, or...
C264S001380
Reexamination Certificate
active
06827885
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention is directed toward controlled curing of devices requiring optical cure. More specifically, the present invention provides a method for curing optical devices such that the devices undergo a more controlled polymerization, resulting in a reduction in defects such as dimpling and warpage in the cured device. In particular, the optical devices include ophthalmic lenses including contact lenses, intraocular lenses, spectacle lenses, corneal onlays and corneal inlays. More particularly, this method provides for a method to produce contact lenses having a controlled cure profile.
It is often desirable to mold optical devices such as contact lenses and intraocular lenses, rather than form the lenses by machining operations. In general, molded lenses are formed by depositing a curable liquid such as a polymerizable monomer into a mold cavity, curing the liquid into a solid state, opening the mold cavity and removing the lens. In particular, the mold cavity may be formed by a mold assembly comprised of a posterior mold portion and an anterior mold portion, each having a lens-forming surface. When the posterior mold portion and anterior mold portion are mated, the lens-forming surface of the posterior mold portion and the lens-forming surface of the anterior mold portion form the lens-forming cavity. The non-lens-forming surface of both mold portions, herein referred to as non-critical surfaces, are generally molded to have a similar radius (or radii) of curvature as that of the lens-forming surfaces. While the lens-forming surfaces are of optical quality, each having a central optical zone and typically, at least a peripheral carrier zone, the only requirement of the non-critical surface generally is a smooth surface.
Polymerization is typically carried out by thermal means, irradiation or combinations thereof Traditionally, conventional thermo-casting techniques require fairly long curing times and are used when the resultant object is thick. Rods from which rigid gas permeable lenses are lathed from or thicker lenses are often thermally cured. Curing of lenses by irradiation, in particular, ultraviolet (UV) irradiation, frequently offers short curing times. The monomer is poured into a transparent mold having a desired optical surface, and thereafter the UV light is radiated to the monomer through the transparent mold to cure the photosetting monomer.
A common material used as a mold material is polypropylene, which is disclosed in U.S. Pat. No. 5,271,875 (Appleton et al., assigned to Bausch & Lomb Incorporated, the entire contents herein incorporated by reference). The process disclosed in Appleton et al., may be used to produce lenses with predictable and repeatable characteristics.
The use of polypropylene may be desired with certain lens-forming materials. Other lens-forming materials, however, may cast just as well or better in other mold materials. As disclosed in U.S. Ser. No. 09/312105 (Ruscio et al. and assigned to Bausch & Lomb Incorporated, the entire contents herein incorporated by reference), polyvinyl chloride absent any UV stabilizer provides a suitable material for the posterior mold.
While the irradiation of the optical device from the light source may be conducted in a uniform and parallel manner, the material chosen for the mold portions may affect the pathways of the light rays. For instance, some materials, such as thermoplastic crystalline polymers, may diffuse the radiation, causing a scattering of the light rays. Polypropylene is such a material. Other materials such as polyvinyl chloride and polystyrene are thermoplastic amorphous polymers, which permit an unhindered pathway for the light rays during curing.
The radiation may also be reflected off the surface of the glass or plastic mold materials. This may result in non-uniform distribution of light intensity over the lens-forming material.
This invention recognized that the non-critical surface of the posterior mold may act as an optical device, reflecting and/or refracting the irradiation in a non-uniform pathway through the mold portion. In particular, the geometry of the non-critical surface of a posterior mold may affect the cure of the lens article sandwiched between the posterior and anterior mold. For instance, the non-critical surface of a posterior mold may be comprised of a radius of curvature and an outer flat portion. The junction formed at the intersection of the radius and flat portion may produce a molding area in which the radiation does not penetrate well. This would be similar to providing a shadow on the lens article. The resultant lens would then have a circular area corresponding to the junction(s) that may not be as completely cured as the rest of the lens. Any junction formed at the intersection of different geometries may produce “shadowing”. The geometries need not necessarily be spherical.
Additionally, the non-critical surface of the mold may refract the radiation from the optical source. This may lead to non-uniform curing speed of the ultraviolet curable resin. As a result, since the curing is completed faster and more completely in a portion receiving a high radiation intensity (in this instance, the periphery portion of the lens) and slower in a portion receiving a low irradiation intensity (the central portion, respectively), a stress is generated in the cured resin layer. This stress may deteriorate the precision of the optical device face. Additionally, since the faster curable portion receiving higher radiation intensity is cured with absorption of the surrounding uncured resin in order to compensate for the contraction of resin resulting from curing, the slower curable portion (which receives lower radiation intensity) may show defects such as shrinkage. In particular, in the case of contact lenses and spectacle lenses, this can produce lenses with unacceptable optical aberrations caused by uneven curing and stress. “Dimpling” or warpage of the contact lens is a common problem caused by uneven curing. In dimpling, the apex of the lens is flattened or slightly concave in shape. Warpage is generally seen as the inability of the edge of a lens to have continuous contact with the molding surface upon which it contacts. Other drawbacks seen with plastic spectacle lenses include “striations”, which are caused by uneven curing and stress. Thermal gradients form in the gel-state, which produce convection lines (“striations”) that become frozen in place and cannot be dispersed.
U.S. Pat. No. 4,166,088 (Neefe) discloses a method of controlling the polymerization of cast optical (plastic or contact) lenses. The mold section on the bottom is a lens which focuses UV light to the center of the cavity. The bottom mold must have a thickness which corresponds to the focal length of the refractive surface so that the UV light rays converge at the center of the monomer being cured. Neefe also requires an aluminum reflector on the outer surface of the top mold to reflect light back through the monomer.
U.S. Pat. No. 4,534,915 (Neefe) discloses the use of a convex positive refractive power cylinder lens to provide a band of actinic light to a rotating lens monomer. The center of the spin cast lens receives the most radiation, the area adjacent to the center receives less while the periphery receives still less radiation. This allows for the outer portion of the spin cast lens to migrate inward as the lens shrinks during the curing process. A fresnel lens or a Maddox rod may also be used to provide the narrow high-energy line of actinic light.
U.S. Pat. No. 4,879,318 (Lipscomb et al.) discloses the use of mold members formed from any suitable material that will permit UV light rays to pass through. To aid in the even distribution of the UV light, the surfaces of the molds are frosted. In one embodiment, a Pyrex glass plate is used to filter out UV light below a certain wavelength. Lipscomb et al. found that if incident UV light is not uniform throughout the lens, visible distortion pattern may appear in the finished lens. Lipscomb et al. solve
Altmann Griffith E.
Armstrong Lisa A.
Beebe Kevin D.
Cox Ian G.
Huang Horngyih
Bausch & Lomb Incorporated
McGuire Katherine
Vargot Mathieu D.
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