Currently, commercial photochromic plastic lenses with photochromic dyes dis- persed throughout the lens substrate are made by thermal processes. These processes yield product with good mechanical, optical and photochromic proper- ties, but they require normally around 20 hours before the curing is complete. Ex- amples of such thermally cured lenses are disclosed in U. S. Patent No. 5,763, 511, to Chan et al. ; U. S. Patent No. 5,973, 039, to Florent et al. ; and U. S. Patent No.

6,034, 193, to Henry et al.

Attempts have also been made to produce synthetic resin lenses by radiation cur- ing. Examples of such attempts to produce radiation cured lenses are described in U. S. Patent No. 5,621, 017, to Kobayakawa et al., and U. S. Patent No. 5,910, 516, to Imura et al. Although the use of radiation curing makes it possible to reduce processing time, the resulting lenses have not found successful commercial appli- cation due to one or more of the following reasons: (1) inferior thermal/mechanical properties, (2) poor optical and/or photochromic properties, and (3) lack of commercial capability for mass production.

Thus, despite the efforts of the prior art to make synthetic resin lenses through radiation curing, there are still no commercial photochromic lenses produced via radiation cure in the market. There has remained a need to solve these problems by finding a proper radiation curable photochromic composition, along with the process to produce superior synthetic resin lenses.

The technical problem underlying the present invention is thus to develop a cur- able core resin composition which can be cured rapidly by radiation curing meth- ods and/or thermally to produce synthetic resin lenses with desirable thermal and mechanical properties, good optical characteristics and, in the case of photochro- mic lenses, photochromic characteristics, and which are capable of mass produc- tion on a commercial scale.

The solution to the above technical problem is achieved by the embodiments characterized in the claims.

In particular, the present invention relates to a new curable core resin composition suitable for photochromic dyes, along with a new initiation system containing both photo and thermal initiators. The unique cure process includes combination of both step radiation cure and thermal annealing, leading to lenses with superior physical and/or photochromic properties in as little as one hour.

In a first aspect, the present invention relates to a high refractive index curable synthetic resin composition comprising a core resin composition of ethylenically unsaturated compounds and a photo initiator and a thermal initiator.

In accordance with a further aspect of the present invention the synthetic resin composition further comprises at least one photochromic dye and is suitable for the production of photochromic lenses. The photochromic composition according to the present invention provides superior photochromic properties and physical properties, short processing time through both step radiation and thermal curing, and easy commercial capability for mass production.

The present invention also relates to a radiation curing process of the composi- tions of the present invention via a single stage cure or via a multiple stage cure with an appropriate cooling period at the end of each cure stage, followed by thermal annealing.

The present invention may be understood more readily by reference to the follow- ing detailed description of particular embodiments of the invention and the specific examples included therein. Before the present compositions and their process are disclosed and described, it is to be understood that this invention is not limited to a particular formulation and a process, as such may, of course, vary. It is also to be understood that the terminology used herein is for the purpose of describing par- ticular embodiments only and is not intended to be limiting. As used herein, the term" (meth) acrylate" is intended to refer generally to both acrylate and methacry- late functional groups.

The present invention relates to a high refractive index curable core resin compo- sition suitable for photochromic application. When proper photochromic dyes, ini- tiation system containing both photo and thermal initiators and additives are incor- porated into the composition according to the present invention, lenses with supe- rior physical and photochromic properties can be made through the curing process according to the present invention.

In one aspect, the core resin composition suitable for photochromic application comprises per 100 parts by weight: (i) from 2 to 70 parts by weight of at least one first compound corresponding to formula (I) : wherein n and n'independently are 0-30 R1-R4 independently represent H or Ci-Ce alkyl, X is O, S, SO2, C02, CH2, CH=CH, C (CH3) 2 or a single bond, and yi-ys independently represent H, OH, halogen, mercaptan or Ci-C4 alkyl ; (ii) from 2 to 80 parts by weight of at least one second compound corresponding to formula (ii) : wherein m is at least 1, and R5-R7 independently represent H or C1-C6 alkyl ; (iii) from 2 to 60 parts by weight of a reactive diluent selected from the group con- sisting of 1, 6-hexanediol di (meth) acrylate, isobornyl (meth) acrylate, 2-hydroxyethyl (meth) acrylate, 2-phenoxyethyl (meth) acrylate, vinyl benzoate, vinyl 4-t-butyl ben- zoate, styrene, divinyl benzene, polyethylene glycol di (meth) acrylate, polypropyl- ene glycol di (meth) acrylate and mixtures thereof; and (iv) from 2 to 60 parts by weight of a multi-functional (meth) acrylate or (meth) acrylate derivative with three or more acrylate functional groups selected from the group consisting of trimethylolpropane tri (meth) acrylate, pentaerythritol tri (meth) acrylate, glyceryl tri (meth) acrylate, pentaerythritol tetra (meth) acrylate, di- trimethylolpropane tetra (meth) acrylate, dipentaerythritol penta (meth) acrylate, penta (meth) acrylate ester and mixtures thereof.

In preferred compounds of formula 1, Ri and R4 are CH3 ; R2, R3, and Y1-Y8 are hy- drogen; and X is C (CH3) 2. n and n'are preferably 3 to 5. In preferred compounds of formula 11, R5and R7are CH3, and R6 is hydrogen.

To make a photochromic article, the core resin composition of the present inven- tion may further comprise an effective photochromic amount of at least one photo- chromic dye incorporated therein to form photochromic lenses which darken when exposed to bright light and fade when under less intense light exposure. Suitable photochromic dyes may, for example, comprise chromenes, fulgides, fulgimides, spirooxazines, naphthopyrans, and/or mixtures thereof. Examples of useful photo- chromic dye compounds and/or mixtures are disclosed in U. S. Patent Nos.

5,399, 687; 5,498, 686; 5,623, 005; 5,645, 768; 5, 707, 557; 5,801, 243; 5,932, 725; 5,952, 515; 5,990, 305; 6,022, 496; 6,036, 890; 6,102, 543; 6,146, 554; 6,171, 525; 6,190, 580 and 6,225, 466, the entire disclosures of which are incorporated herein by reference. The amount of the photochromic dye or dye mixture may vary de- pending on the desired photochromic effect, but typically will amount to from 0.0001 to 1, preferably 0.0001 to 0.1, and particularly preferably 0.001 to 0.1, part by weight per 100 parts by weight of the core resin. The selection of specific dyes or dye mixtures to be used in any given case will depend on the desired coloration, as well as the desired performance characteristics of the resulting lens, such as darkening rate and/or fade rate, and is considered to be within the skill of the art. A suitable neutral gray photochromic dye mixture is described in U. S. Patent No.

6,373, 615, at, for example, column 10, line 50+, the entire disclosure of which is incorporated herein by reference. In one preferred aspect, a resin composition in- cludes 0.05 to 0.06%, more preferably 0.056%, photochromic dye mixture by weight of the core resin composition, including 380 to 460 ppm, more preferably 423 ppm, spiro-9-floureno-13'- (6-methoxy-3- (4-N-morpholinyl) phenyl-3-phenyl- indeno [2, 1-flnaphtho (1,2-b) pyrane (gray-blue), 40 to 60 ppm, more preferably 46 ppm, of 3-phenyl-3'- (4-N-piperidinyl)-phenyl-6-N-morpholinyl-3H-naphtho [2,1- b] pyrane (ruby), 45 to 65 ppm, more preferably 54 ppm, of 3-phenyl-3'- (4-N- morpholinyl)-phenyl)-6-N-morpholinyl-3H-naphtho [2,1-b] pyrane (orange), and 30 to 50 ppm, more preferably 38 ppm, of 3-phenyl-3'- (4-methoxy)-phenyl-6-N- morpholinyl-3H-naphtho [2,1-b] pyrane (yellow).

To form a radiation curable synthetic resin composition according to the present invention, especially a short wavelength, visible light, radiation curable composi- tion, a suitable photo initiator may be incorporated in the core resin composition.

Typically the resin composition will include from 0.01 to 3, and preferably 0.02 to 1, part by weight of photo initiator per 100 parts by weight of the core resin. Exam- ples of suitable photo initiators include benzophenone, 2, 2-dimethoxy-2-phenyl acetophenone, 1-hydroxycyclohexyl phenyl ketone, 2-methyl-1- [4- (methylthio) phenyl]-2-morpholino propan-1-one, 2-hydroxy-2-methyl-1-phenyl- propan-1-one, bis (2, 6-dimethoxybenzoyl)-2, 4, 4-trimethylpentyl phosphine oxide, 2,4, 6-trimethylbenzoyl diphenylphosphine oxide, bis (2,4, 6-trimethylbenzoyl)- phenylphosphine oxide and mixtures thereof. Preferred photo initiators include 2,4, 6-trimethylbenzoyl diphenylphosphine oxide, bis (2,4, 6-trimethylbenzoyl)- phenylphosphine oxide, or mixtures thereof.

If desired, a thermal polymerization initiator may also be incorporated in the syn- thetic resin composition to facilitate use of thermal curing or combination radia- tion/thermal curing process techniques. For example, the resin composition may advantageously contain from 0.01 to 3, and preferably 0.02 to 1, part by weight of a thermal initiator per 100 parts by weight of the core resin. Examples of suitable thermal initiators include t-amyl peroxy-2-ethylhexanoate, t-butyl peroxy-2- ethylhexanoate, t-butyl peroxyl-2-methylbenzoate, 1, 1-di- (t-butylperoxy)-3, 3,5- trimethylcyclohexane, t-butyl peroxy-3,5, 5-trimethylhexanoate, t-butyl peroxy 2- ethylhexyl carbonate, dibenzoyl peroxide, t-amyl peroxy benzoate, 2,2'-azobis (2,4- dimethylpentanenitrile), 2,2'azobis (2-methylpropanenitrile), 2, 2'azobis (2- methylbutanenitrile), 1,1'azobis (cyclohexane-carbonitrile), and mixtures thereof.

Particularly preferred thermal initiators include 2,2'-azobis (2,4- dimethylpentanenitrile), 2,2'azobis (2-methylpropanenitrile), 2,2'azobis (2- methylbutanenitrile), 1,1'azobis (cyclohexanecarbonitrile), or mixtures thereof.

In one preferred aspect, the resin composition comprises an initiator mixture com- prising both a photo initiator and a thermal initiator. Castable resin compositions according to the invention desirably will have a viscosity of less than 400 cps at room temperature.

The curable synthetic resin composition of the present invention may also com- prise up to 5 parts by weight of other additives, such as light stabilizers, mold re- lease agents and/or other processing agents, per 100 parts by weight of the core resin composition. For example a benzophenone UV absorber, such as 2-hydroxy- 4-methoxybenzophenone, may be included in the resin composition in an amount from about 50 ppm to about 1,000 ppm, preferably from about 100 ppm to about 500 ppm. In one preferred aspect, a resin composition includes 0.019% 2-hydroxy- 4-methoxybenzophenone and 0.056% photochromic dye mixture by weight of the core resin composition. One effective processing agent is isopropylxanthic disul- fide, which extends pot life and minimizes yellowing of the resin. Pot life is the time from when a resin batch is prepared until the last acceptable lens can be produced from that particular batch. In manufacturing photochromic resins, the possibility of yellowness increases after initiators are mixed with the resins, thus the ability to extend pot life is a useful benefit.

The present invention also relates to a radiation cure process in which a mold as- sembly is filled with a curable synthetic resin composition of the present invention as described above and the filled mold assembly is subjected to a source of actinic radiation. Desirably the radiation source is a filtered actinic radiation source having a primary output at or above the cutoff wavelength of the filter. Where the synthetic resin composition is intended to produce a photochromic article and contains a photochromic dye which is activated at certain visible wavelengths, e. g. wave- lengths above 400 nm, it is advantageous to use a filter to cut off substantially all (e. g. , 99% or more) of the radiation up to the wavelength where the photochromic dye is active, so that the photochromic dye will not be activated or darkened during the curing process.

The radiation curing process may comprise either a single stage cure up to 30 minutes in length or a multiple stage cure comprising a plurality of stages from 10 seconds to 20 minutes in duration. The use of the multiple stage procedure is ad- vantageous in order to facilitate control of the rate of reaction and the temperature of the resin during curing depending on the type of lenses being processed. It is also desirable to have adequate cooling time and cooling media at the end of each radiation stage to further control the rate of cure and the temperature of the resin.

Suitable cooling media include ambient air, chilled air, ambient water and/or chilled water. The multiple stage radiation and cooling process permits rapid effective cur- ing while keeping the temperature of the lens material below the temperature where thermal polymerization of the resin becomes uncontrollable and the reaction autoaccelerates. This maximizes the yield of the process, optimizing optical, physical, and photochromic properties of the cured article.

In one preferred aspect, after the radiation cure it is desirable to have a thermal anneal at a temperature of 50 to 150°C for up to 2 hours. While various thermal curing processes are suitable subsequent to the radiation exposure process de- scribed above, the following temperature profile has provided acceptable results. After a radiated lens is placed in the oven, the oven temperature is maintained at 60°C for 15 minutes; the temperature is then raised at a rate of 3. 3°C/minute to 76°C and is maintained at 76°C for 10 minutes. The temperature is then raised at a rate of 3. 3°C/minute to 100°C and is maintained at 100°C for 35 minutes. There- after, the temperature of the oven is allowed to cool to room temperature. The thermal annealing may be carried out prior to and/or after removing the plastic lens from the mold assembly in which it is formed.

The present invention also relates to an article, especially a photochromic article, made by molding and curing a synthetic resin composition according to the pre- sent invention as described above.

The following table describes a number of methods for producing lenses with ac- ceptable optical, physical, and photochromic performance when cast using the formulation (s) as described above.

Table 1 Methods for producing lenses Cure Description Example 1 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter, expose both sides of the lens or article for 20 minutes. Following the radiation exposure, the articles were put into an oven at 120°C for 25 to 30 minutes. 2 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter, expose both sides of the lens or article for 2.75 to 3 minutes. After this initial exposure, the gasket was removed from the article and exposed to the actinic light for an additional 9 minutes. Following the radia- tion exposure, the articles were put into an oven at 80°C for 25 to 30 minutes. 3 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter, both sides of the lens or article were exposed for 12 minutes continuously. Follow- ing the radiation exposure, the articles were put into an oven at 70 to 90°C for 30 to 60 minutes. Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. Lenses were placed on a conveyor belt with 3 curing zones and 2 cooling zones. The exposure consisted of an initial 2.75-minute actinic light exposure followed by a 1.25 minutes period out of the light with an ambient temperature of 55°F to 75°F. This was re- peated and then followed by another 2. 75 minute exposure and quenching into 20°C water for 2 minutes. After the final portion of the radiation exposure process, lenses were placed in an oven 45 to 90 minutes at temperatures between 60 and 100°C. 5 Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. The exposure consisted of an initial 2.75-minute actinic light ex- posure followed by a 1.25 minutes period out of the light. This was repeated and then followed by another 2.75 minute exposure and quenching into 20°C water for 2 minutes. After the final por- tion of the radiation exposure process, lenses were placed in an oven 45 to 90 minutes at temperatures between 60 and 100°C. 6 Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. The exposure consisted of an initial 3 minutes actinic light expo- sure after which the gasket was removed. The article was then exposed to actinic light two more times for 3.5 minutes each time. Following the actinic light exposure, the lenses were placed in an oven for 1 hour at 60°C, followed by 1 hour at 70°C. 7 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter,-expose both sides of the lens or article for 3 minutes continuously. After this initial exposure, lenses were exposed using 420 nm peak fluo- rescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article be- ing cured by continuous exposure from both sides for 7 minutes. Following the radiation exposure, the articles were put into an oven at 70°C for 30 to 60 minutes. 8 Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. Lenses were placed on a conveyor belt that carries the lenses between the lamps. The method consisted of exposing the lenses for 4.25 minutes twice. After the final portion of the radiation ex- posure process, lenses were placed in an oven 45 to 90 minutes at temperatures between 60 and 100°C. 9 Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. Lenses were placed on a conveyor belt and exposed to the ac- tinic light source described for 9 minutes. After the final portion of the radiation exposure process, lenses were placed in an oven 30 to 90 minutes at temperatures between 60 and 100°C. 10 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter, expose both sides of the lens or article for 10 minutes continuously. Following the radiation exposure, the articles were put into an oven at 120°C for 25 to 30 minutes. 11 Using 420 nm peak output low pressure Hg lamps spaced three to six inches apart and GG420 glass as a filter, lenses were ex- posed from each side separately using shutters to control the ex- posure time from each side. The reaction was monitored using a thermocouple embedded in the lens so the temperature could be controlled as low as possible. Exposure times were from 30 sec- onds to 4 minutes per side. Following the radiation exposure, the articles were put into an oven at 60 to 120°C for 20 to 90 mi- nutes. 12 Using 420 nm peak fluorescent tubes spaced three to seven inches apart as well as GG420 glass placed between the lamps and the lens/article being cured, lenses were cured as follows. Lenses were placed on a conveyor belt with 3 curing zones and 2 cooling zones. The exposure consisted of an initial 2.75-minute actinic light exposure followed by a 1.25 minutes period out of the light with an ambient temperature of 55°F to 75°F. This was re- peated and then followed by another 2.75 minutes exposure. This whole exposure was repeated and then followed by placing the lens into cool water. After the final portion of the radiation expo- sure process, lenses were placed in an oven 45 to 90 minutes at temperatures between 60 and 100°C.

The following tables exhibit examples of photochromic compositions, process and physical properties of the resulting cured articles. Examples 1-28 below, employ formula (la) where n = n'= 3, R1-R4 = CH3, X = C (CH3) 2, and Y1-Y8 = H; formula (lb) where n = n'= 5, Ri-R4 = CH3, X = C (CH3) 2, and yi-ys = H; Formula (Ila) where mavg = 9, R5 and R7 = CH3, and R6 = H (polyethylene glycol 400 dimethacry- late) ; and Formula (Ilb) where maV9 = 4 to 5, R5 and R7 = CH3, and R6 = H (poly- ethylene glycol 200 dimethacrylate) in the amounts indicated in the table.

Bis (2,4, 6-trimethylbenzoyl)-phenylphosphine oxide at 0.04% and 2,2'-azobis (2,4- dimethylpentanenitrile) at 0.05% were used as photo and thermal initiators, re- spectively, based on 100 parts by weight of core resin. A neutral gray photochro- mic dye mixture, at 0.073%, and a UV-absorber, 2-hydroxy-4- methoxybenzophenone, at 0.025%, were also employed. Isopropylxanthic disul- fide was used at 0.03%, based on 100 parts by weight of core resin, in examples . 1-5. During one hour mixing at room temperature with the photochromic dye mix- ture, nitrogen bubbling was employed, followed by degassing under vacuum for 15 minutes. The dye mixture employed in these examples includes a combination of 550 ppm spiro-9-floureno-13'- (6-methoxy-3- (4-N-morpholinyl) phenyl-3-phenyl- indeno [2, 1-fanaphtho (1,2-b) pyrane (gray-blue), 60 ppm of 3-phenyl-3'- (4-N- piperidinyl)-phenyl-6-N-morpholinyl-3H-naphtho [2,1-b] pyrane (ruby), 70 ppm of 3- phenyl-3'- (4-N-morpholinyl)-phenyl)-6-N-morpholinyl-3H-naphtho [2,1-b] pyrane (or- ange), and 50 ppm of 3-phenyl-3'- (4-methoxy)-phenyl-6-N-morpholinyl-3H- naphtho [2,1-b] pyrane (yellow), which provides a gray photochromic dye blocking visible light. All samples were cured under low pressure Hg lamp with 420 nm peak output for 15 to 20 minutes at an intensity of 2.5 mw/cm2 with GG420 filter glass (from Schott), followed by annealing at 120°C for 25 to 30 minutes.

Table : Formulation (by wt) of core resin Example Formula Formula Formula Formula Styrene Divinyl Dipenta- No. (la) (lb) (I la) (I lb) Benzene erythritol Pentaacrylate 1 0 29.3 41.5 0.0 18.0 0.0 11.3 2 10 19.3 41.5 0.0 18.0 0.0 11.3 3 20 9.3 41.5 0.0 18.0 0.0 11.3 4 0 29. 3 31. 5 10. 0 18. 0 0. 0 11. 3 5 0 29. 3 21. 5 20. 0 18. 0 0. 0 11. 3 6 0 39. 3 24. 0 0. 0 18. 0 0. 0 18. 8 7 0 45. 3 18. 0 0. 0 18. 0 0. 0 18. 8 8 0 39.3 18.0 0.0 18.0 0.0 24.8 9 0 39. 3 24. 0 0. 0 0. 0 18. 0 18. 8 10 0 45. 3 18. 0 0. 0 0. 0 18. 0 18. 8 11 0 39. 3 18. 0 0. 0 0. 0 18. 0 24. 8 12 0 39.8 17.0 0.0 15.8 10.8 16.5 13 0 34. 5 17. 0 0. 0 15. 8 10. 8 21. 8 14 0 39. 8 17. 0 0. 0 10. 8 15. 8 16. 5 15 0 34.5 17.0 0.0 10.8 15.8 21.8 16 0 36. 3 14. 0 0. 0 24. 4 4. 0 21. 3 17 0 41. 0 14. 0 0. 0 21. 0 7. 4 16. 6 18 0 30. 0 15. 6 0. 0 24. 4 4. 0 26. 0 19 0 41. 0 17. 1 0. 0 21. 4 4. 0 16. 5 20 0 32. 6 17. 1 0. 0 20. 9 5. 9 23. 5 21 0 36. 3 14. 0 0. 0 17. 6 10. 8 21. 3 22 0 41.0 14.0 0.0 13.2 10.8 21.0 23 0 31. 1 24. 0 0. 0 24. 4 4. 0 16. 5 24 0 31. 6 14. 0 0. 0 17. 6 10. 8 26. 0 25 0 41. 0 14. 0 0. 0 21. 0 7. 4 16. 6 26 0 30. 0 24. 0 0. 0 17. 6 10. 8 17. 6 27 0 36. 0 19. 0 0. 0 17. 6 10. 8 16. 5 28 0 36. 0 19. 0 0. 0 24. 4 4. 0 16. 5 Table 3 Properties of cured photochromic samples Example Activation Darkness Deactivation Refractive Abbe Flex modulus No. Index Value (kpsi) Compared to ColorMatic extra gray* 1 Slower similar Faster 1. 5413 41. 8 123 2 Slower similar Faster 1. 5398 40. 8 143 3 Slower lighter Faster 1. 5423 40. 5 152 4 Slower lighter Faster 1. 5436 41. 2 167 5 Slower lighter Faster 1. 5413 41. 1 179 6 Similar similar Faster 1. 5396 41. 9 115 7 Similar similar Faster 1. 5396 42. 2 120 8 Similar darker Faster 1. 5403 41. 6 136 9 Faster darker Much faster 1. 5426 41. 1 80 10 Faster darker Much faster 1. 5461 38. 4 79 11 Faster darker Much faster 1. 5451 39. 1 81 12 Faster darker Faster 1. 5542 38. 8 168 13 Faster darker Faster 1. 5516 38. 4 159 14 Faster darker Faster 1. 5533 38. 7 149 15 Faster darker Faster 1. 5510 38. 8 162 16 Slower lighter Faster 1.5529 39 195 17 Slower similar Faster 1. 5522 39 178 18 Similar similar Faster 1. 5450 41. 8 197 19 Similar similar Faster 1. 5496 38. 8 168 20 Faster darker Faster 1. 5493 39. 6 174 21 Faster similar Faster 1. 5525 39. 7 173 22 Faster darker Faster 1. 5493 39. 3 132 23 Similar similar Faster 1. 5484 40. 2 174 24 Faster Darker Faster 1. 5506 39. 9 175 25 Faster Darker Faster 1. 5532 39. 1 195 26 Faster Darker Faster 1. 5494 39. 3 174 27 Faster Darker Faster 1. 5514 39. 4 174 28 Slower Similar Faster 1. 5493 39. 6 191 * ColorMatic extra gray is the best photochromic lenses in over-all proper- ties on the market and is produced by Optische Werke G. Rodenstock located in Munich, Germany. Photochromic properties were obtained af- ter exposure under the natural sun light.

Thicker lenses may also be produced according to the processes of the present invention. Because slower heat transfer rates are associated with thicker lenses, the processing temperatures of the material employed in thicker lens tend to be higher and the selection of thermal initiators may be affected. For example, a use- ful monomer charge for a typical lens may be 24.37% styrene, 3.99% divinylben- zene, 0.05% 2,2'-azobis (2, 4-dimethylpentanenitrile), 0. 04% bis (2,4, 6- trimethylbenzoyl)-phenylphosphine oxide, 0.20% triphenyl phosphine, 6.29% poly- ethylene glycol (PEG 400), 36. 18% bisphenol A, 21.22% dipentaerythritol pen- taacrylate, and 7.67% photochromic dyes. The monomer charge for a similar, but thicker, lens may substitute thermal initiator 1,1'azobis (cyclohexane-carbonitrile) for 2, 2'-azobis (2, 4-dimethylpentanenitrile).

The foregoing description and examples have been set forth merely to illustrate the invention and are not intended to be limiting. Since modifications of the de- scribed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed broadly to include all variations falling within the scope of the appended claims and equiva- lents thereof.