Plastic and nonmetallic article shaping or treating: processes – Optical article shaping or treating – Utilizing plasma – electric – electromagnetic – particulate – or...
Patent
1997-09-08
1999-03-16
Vargot, Mathieu D.
Plastic and nonmetallic article shaping or treating: processes
Optical article shaping or treating
Utilizing plasma, electric, electromagnetic, particulate, or...
264 17, 264 21, 264 26, 264496, B29D 1100
Patent
active
058825568
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to the manufacture of plastic optical articles such as video discs, ophthalmic lenses and the like. In particular, the present invention relates to the manufacture of photochromic optical articles.
A number of attempts have been made in the prior art to incorporate photochromic compounds into a synthetic polymeric host material.
For example, U.S. Pat. No. 3,212,898 describes preparing a photosensitive composition by suspending a photochromic benzospiropyran in a preformed polyester resin. U.S. Pat. No. 3,666,352 describes dispersing a mercury thiocarbazone compound in a solidified plasticised vinyl chloride-vinyl acetate copolymer, which copolymer is laminated between two plastic or glass layers, thereby to form a photochromic sunglass lens.
Photochromic plastic lenses in which photochromic molecules are dispersed throughout the lens, are often not useful for ophthalmic purposes because the dyes are activated by bands of radiation wavelengths which are quite dissimilar from dye to dye. This radiation interacts through the full section thickness of the lens to a penetration depth defined by the ultraviolet (UV) and near visible (NV) absorption characteristics of the lens/dye system. Both the transmission and colour of portions of different thickness vary according to the form of the lens. Whilst this same effect occurs to a degree with photochromic glass lenses, it is not as noticeable.
Accordingly, conventional wisdom in the prior art indicates that organic photochromic dyes must be provided in regions or zones of constant thickness, independent of the optical geometry of the lens. Surface imbibition is the only commercial process for achieving a satisfactory product. However, surface imbibation is costly and is not freely available to the public.
Particular difficulties which have been encountered in the prior art when attempts have been made to incorporate photochromic material prior to the formation of a photochromic optical article include uneven colouration caused by the variable thickness of the lens which is required to provide the lens with the necessary optical power.
Further, for example as described in U.S. Pat. No. 5,130,353 or U.S. Pat. No. 5,185,390, the prior art describes the inclusion of photochromic dyes into the subsurface regions of a plastic lens by first positioning a polymeric or other carrier of photochromic dye physically against the surface of the lens and then using heat (in the range 100.degree. to 150.degree. C.) to cause the dyes to undergo sorption and to diffuse into the lens. The depleted carrier is removed from the lens after the passage of sufficient time at elevated temperature, typically of order 1/2 to 4 hours. It is found that adequate darkening is achieved with inclusion of 0.5 to 10 .mu.gm/mm.sup.2 of surface diffused to a depth of about 50 .mu.m. The average dye concentration in this region of the lens is in the range of 0.01 to 0.2 mgm/mm.sup.3, or 0.7 to 14% (w/w) of the polymer weight in that region. This corresponds to an Absorbance or Optical Density (OD) in the range of 0.1 to 0.5, dependent on the wavelength dependent absorption characteristics of the individual Photochrome. A typical useful practical range is 0.1 to 0.25 OD units based upon the 390 nm wavelength absorption of a Chromene. Typically 4 hours imbibition at 140.degree. C. produces 0.2 to 0.3 OD units of absorption at 390 nm and produces lenses with an effective working photochromic performance.
For this procedure to operate effectively, the carrier must accommodate a sufficient concentration of dye in sufficient volume to deliver the required level of dopant. Commonly, carrier films are in the range 50 to 80 .mu.m thick. The intensity and duration of the heating step should be sufficient to obtain thermal transfer by permeation of the dyes without decomposing them or causing significant softening of the host polymer (lens). Either decomposition or softening will result in lenses being rejected.
An alternative approach is to include the dyes within an optical coating
REFERENCES:
patent: 5130353 (1992-07-01), Fischer et al.
patent: 5185390 (1993-02-01), Fischer et al.
patent: 5405557 (1995-04-01), Kingsbury
patent: 5523030 (1996-06-01), Kingsbury
patent: 5531940 (1996-07-01), Gupta et al.
Kloubek Helena
Perrott Colin Maurice
Pidgeon Kenneth John
Threlfall Ian Michael
Sola International Holdings Ltd.
Vargot Mathieu D.
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