WO1997017181A1

WO1997017181A1 - Method for producing an optical article in a mold sealed under vacuum

Info

Publication number: WO1997017181A1
Application number: PCT/BE1996/000118
Authority: WO
Inventors: Peter Weissman
Original assignee: Ucb, S.A.
Priority date: 1995-11-09
Filing date: 1996-11-07
Publication date: 1997-05-15
Also published as: AU7557596A; WO1997017182A1; AU7557696A

Abstract

The invention provides a method for curing of an optical article, preferably an ophtalmic lens, in which a mold containing a composition to be cured is vacuum sealed in a plastic bag and curing is carried out while maintaining the mold under vacuum sealed conditions.

Description

METHOD FOR PRODUCING AN OPTICAL ARTICLE IN A MOLD SEALED UNDER VACUUM The present invention relates generally to methods for curing optical articles and more particularly to methods for producing ophthalmic lenses It is conventional in the art to produce optical lenses by thermal curing techniques from either allyl, methacrylate, acrylate or vinyl functional monomers and/or oligomers The thermal curing techniques for polymerizing these materials to produce optical lenses have several disadvantages The most significant drawback is that it may take between 6 and 20 hours to produce a lens. Classically, a lens forming mold is capable of producing two lenses per day

Curing of a lens by ultraviolet light (UV) has advantages, such as greatly increased cure speed. However, disadvantages such as degradation of the material during the cure cycle, and concern for the safety of the workers (due to high energy radiation) have also been documented In addition, it is difficult to completely cure polymers with high glass transition temperatures (Tg) (greater than 90°C) using only UV light. As the material crosslinks, the Tg increases from a temperature corresponding to the Tg of the monomer to a temperature corresponding to the Tg of the ultimate polymer. It is well known that polymers formed via radical reactions are capable of achieving Tgs greater than the temperature at which they were cured. However, it has also been demonstrated that these materials are frequently quite yellow at the end of their cure cycle This is due to the trapping of radicals that are produced after the material passes through the vitrification temperature but before exposure is terminated. This lack of mobility during the final stages of cure, limits the extent of cure

Currently, many UV cured lenses are actually cured using two different initiators. One is UV activated and the other is thermally activated. The lenses are partially cured under high intensity UV light and are then placed into an oven and thermally cured to completion.

A final difficulty with UV cured lenses is keeping air from entering the mold during the cure cycle Because the cure and the shrinkage associated with the cure process occurs so rapidly, it is difficult to keep the molds sealed and eliminate air from entering the mold cavity during the cure cycle In conventional thermal curing, a metal clamp is placed on the mold in order to keep the mold sealed during cure, but UV cured lenses must use a transparent clamping mechanism, if they can use one at all As can be seen from the summary of the prior art discussed above, disadvantages of methods of curing optical articles involving the use of ultra-violet light are the need for specific equipment, namely expensive light sources, dangerous working conditions for the personal handling such apparatus, as well as the overall length of such methods. Therefore it is an objective of the present invention to solve the various defects of the hitherto known methods of curing optical articles.

In a first aspect, the present invention provides a process for curing optical articles, in particular ophthalmic lenses, which involves the use of a specific wavelength range of visible light. This light causes no degradation of the polymer material during the cure cycle. Since UV light is not employed, the environment is much safer for the workers involved in the process. In addition to the visible light exposure, the articles, particularly lenses, may also be heated during the cure cycle. However, no thermal initiator is used and the visible light exposure, optionally in conjunction with heat, is critical to developing the appropriate

mechanical and physical properties of the article. It is also critical that a photoinitiator be employed which has an absorption peak in the specific wavelength range of visible light used for curing the article As used in the present invention the term "optical article" is defined as an object that is designed to transmit, focus, defocus, block or otherwise act to interact or prevent interaction with electromagnetic radiation that ranges from a wavelength of about 250 nm to about 1200 nm, preferably from 300 nm to 900 nm, and more preferably from 380 nm to 750 nm.

By "cure" and "curing", Applicant means to cause the chemical reaction of lower molecular weight species such that after they react their molecular weight has significantly increased.

To "actively heat" means to cause an increase in temperature by either electromagnetic radiation, specifically infrared radiation or microwave radiation, radiant energy or convective energy

The method is applicable to the manufacture of optical articles, in particular ophthalmic lenses, from a wide variety of materials, including virtually any double-bond containing materials which are radiation polymerisable. Particular non-limiting examples of suitable materials include various acrylates, methacrylates, vinyl ethers, oxethanes and allyls, although allyls tend to react more slowly More specifically, they generally consist of a mixture of the following reactive components - at least one radiation polymerizable oligomer or reactive prepolymer, with a molecular weight of generally less than about 10,000 and having unsaturated groups such as acrylic, methacrylic, vinyl or allyl chain terminating or side groups. Such oligomers or prepolymers are well known, available commercially and can provide a great variety of structures such as polyesters, polyacrylics, polyepoxides, polyurethanes, etc.. Examples include epoxy acrylates or methacrylates, of the type described in U.S. Patent Nos. 3,676,398, 3,770,602 and 4,511,732, urethane acrylates or methacrylates, such as those described in U.S. Patent Nos. 3,700,643, 4,133,723 and 4,188,455, polyester acrylates or methacrylates, such as those described in U.S. Patent Nos. 4,206,025 and 5,002,976, and acrylic acrylates or methacrylates, etc.

- at least one polyethylemcally unsaturated reactive monomer, preferably a di(meth)acrylate or a poly(meth)acrylate of a low molecular weight polyol, such as a diacrylate of 1,4-butanediol, 1,6-hexanediol, neopentyl glycol, di-, tri- and/or tetraethylene glycol, tripropylene glycol, the

triacrylate of trimethylolpropane or of pentaerythritol, the hexa-acrylate of dipentaerythritol, and

- optionally at least one monoethylenically unsaturated reactive monomer which contains a single ethylenically unsaturated group per molecule.

Examples of such monomers include monoacrylates and monomethacrylates of monohydroxylated and polyhydroxylated aliphatic alcohols, styrene, vinyl toluene, vinyl acetate, N-vinyl-2-pyrrolidone, N-vinylpyridine,

N-vinylcarbazole, and (2-oxo-1-pyrrolidinyl) alkyl (meth)acrylates

This monomer may be added to the composition as a reactive diluent and the amount and nature thereof can be varied to allow the viscosity of the composition to be adjusted at will.

Oligomer formulations which can be mentioned as examples oligomer are as follows: U. S. Patent No. 5,442,022 discloses a composition based on ethoxylated bis-phenol A dimethacrylate; U. S. Patent No. 5,502,139 disclose a composition based on fluorene di(meth)acrylate monomer; U. S. Patent No. 5,147,959 discloses the reaction product of a polyisocyanate with a halobisphenol A epichlorhydrin polycondensate-(meth)acrylic acid adduct, U. S. Patent No. 5,132,384 discloses resins based on

polyalkyeneglycol di(meth)acrylates); U. S. Patent No. 5,373,033 disclose mixtures of bisphenol monomers and polyoxyalkylene-glycol diacrylates; U. S. Patent No. 5,384,380 discloses a reactive mixture of a

polyisocyanate, aromatic multifunctional (meth)acrylates and styrene, U.S. Patent No. 5,133,370 discloses mixtures of (a) polybutylene glycol diacrylate, (b) a urethane polymethacrylate and (c) an aromatic or alicyclic mono (meth) acrylate; U. S. Patent No. 4,912,185 discloses a composition of (a) a polyoxyalkylene glycol di(meth)acrylate, (b) a tπfunctional (meth) acrylic monomer and (c) a polyacrylic urethane monomer. International Patent Publication No. WO96/26184 discloses polymeric compositions for the preparation of optical grade materials specifically formulated from

(a) 60 - 75% by weight of a di(meth)acrylate;

(b) 5 - 20 % by weight of a monothiol, and

(c) 15 - 30% by weight of an aromatic compound with conjugated

unsaturation.

Reference can also be made to lens forming compositions disclosed in U.S. Patent No. 5,410,006, comprising a urethane acrylate prepolymer based on tolylenednsocyanate.

In the process according to the present invention, there may also be used an active energy ray-curable resin composition containing a

urethane (meth)acrylate which comprises the components (a), (b), and (c):

(a) an organic isocyanate having at least two lsocyanate groups,

(b) a polyol having a molecular weight of at least 300 which has at least two hydroxyl groups in the molecule;

(c) at least one compound selected from the group consisting of

compounds represented by general formulae (1) to (5) described below.

wherein R¹ is a hydrogen atom or a methyl group, R² is independently a hydrogen or an alkyl group having a carbon number ranging from 1 to 10, R³, R⁴ and R⁵ are independently a hydrogen, an alkyl group having a carbon number ranging from 1 to 10, a phenyl group or bromine, nl is an integer ranging from 1 to 7, n2 is an integer ranging from 0 to 20, R_a and R_b are independently a hydrogen or a methyl group, n3 is independently an integer ranging from 0 to 10, n4 is 0 or 1, and n5 is an integer ranging from 0 to 5 .

It is to be noted that the compositions employed in this aspect of the invention may also contain UV stabilizer materials since the curing is carried out by employing visible light. Th s constitutes an additional advantage of the present invention since the UV stabilizer can be

incorporated into the final cured article. The wavelength of the visible light used in the method of the invention can vary between approximately 385 and 475 nm, with a preferred range of approximately 400 to 450 nm and a particularly preferred wavelength of approximately 420 nm. The advantage of using such actinic light is that at a wavelength of 420 nm they cause virtually no degradation of the optical article during the process of curing The examples set forth below clearly demonstrate that by heating the article for a sufficient period of time to a temperature at or slightly above the polymer's ultimate glass transition temperature while continuing its exposure to visible light, according to a preferred embodiment of the invention complete cure can be obtained By slightly above the glass transition temperature is meant to a level of up to about 30°C above such temperature (which usually ranges from about 60° to about 160°C) A thermal initiator consequently is not needed to obtain complete cure of the article while using the process of the invention.

As a preferred light source there can be mentioned Fluorescent Super Actinic light from Philips or its equivalent from another manufacturer As an alternative light source, there can be mentioned 420 nm Metal Halide lamps from Electrolite Corporation. This should be used in conjunction with Uvilex 3902 UV Barrier Filter Glass from Schott Corporation. This filter glass will eliminate wavelengths below about 385 nm.

The process of the invention is carried out using a photoinitiator suitable for use with the particular optical article material which is intended The photoinitiator must be one which possesses an absorption peak at or about the same wavelength as that of the visible light which is employed in the curing process. Preferred photoinitiators are Lucirin^® TPO (2,4,6-trimethyl-benzoyldiphenylphosphine oxide) from BASF, Irgacure 184 (1-hydroxycyclohexyl phenyl ketone), Darocur 1173 (2-hydroxy-2-methyl-1-phenyl-propan-1-one), bis(2,6-dimethoxybenzoyl)-2,4,4- trimethylpentylphosphine oxide from Ciba-Geigy and mixtures thereof The molds which are employed in carrying out the curing method of the invention are well known in the art For example, there can be mentioned a glass mold held together by a polyvinyl chloride (PVC) gasket Other molds will be apparent to the art-skilled. The mold must, of course, be such that it is penetrable by the visible light which is employed in curing.

To illustrate equipment configurations which can be employed in carrying out the method of the invention schematic drawings are provided. Fig. 1 is a schematic diagram of a lens curing apparatus. Fig. 1A is a front view showing lens molds (1) sitting in vertical position on the lens support (2) of the lens curing apparatus. Fig. 1B is an end view of the lens support mechanism (2) along with the actinic light assembly and halogen lights for providing heat to the lens mold (1). In this figure, (3) denotes a halogen track lighting which is holded by a movable bracket (4), whereas (5) denotes a fluorescent light fixture with actinic lights mounted on a L bracket (6).

Fig. 2 is a schematic of an alternative apparatus for carrying out the inventive method employing convection heating. The upper chamber thereof comprises a cooling fan (7) and an exhaust port (8) and is provided with a series of fluorescent light fixtures (5) with actinic lamps and with a reflector (9) for actinic lights. The upper chamber is separated from the lower chamber by a heat resistant glass plate (10). The lower chamber is provided with a heating element (11), with a heat resistant glass plate (12) on which the lens molds (1) are set, and with a reflector (9') for actinic lights.

Fig. 3 is a schematic of another alternative apparatus for carrying out the inventive method employing infrared heating. This alternative apparatus comprises elements similar to those of the apparatus of figure 2, except that the lower chamber is not provided with a heating element but instead comprises a series of quartz infrared lamps (13) near the separating heat resistant glass plate (10).

In referring to Fig. 1A and Fig. 1B, four fluorescent lamps are held approximately 20 cm from the lens molds The halogen lamps are on a track

(3) so that they can be easily moved to match two lamps (one on each side) per lens mold. The halogen lamps are moved up and out of the way (as shown) during Phase I of the cure cycle. During Phase I the fluorescent lamps are turned on and curing begins. During Phase II, the halogen lamps are moved vertically down so that the centerline of the halogen lights corresponds with the center of the lens. The lamps are turned on and heating begins while the visible light curing also continues.

In referring to Fig. 2 and Fig. 3, the visible light curing is begun by turning on the fluorescent lights. After a desired period of time, the heating element is turned on and visible light continues with heating until the desired cure is obtained.

In a second aspect of the invention a method of curing is provided in which an optical article mold, preferably an ophthalmic lens mold, containing the composition to be cured is sealed in a clear plastic container, preferably a bag, under vacuum, prior to curing and curing takes place with the mold thus sealed.

"Vacuum" as used herein means a vacuum greater than about 400 mm Hg (i.e. a pressure below 360 mm Hg) as measured by standard vacuum gauge, more preferably greater than 600 mm Hg.

Sealing of the mold in this manner has the beneficial effect of exerting pressure on the mold to hold it together while, at the same time, preventing air from entering the mold cavity during the cure cycle.

According to this second method, the mold containing the composition to be cured is first placed into a bag made of a suitable clear plastic material. A tube is inserted in the bag and the open end of the bag is sealed, for example, with a commercially available heat sealer, up to the inserted tube. A vacuum is then pulled on the bag, causing it to collapse around the mold. The heat sealer is then used to seal the bag in front of the inserted tube.

This technique can be automated using equipment specifically designed to vacuum bag or seal materials on an industrial/commercial scale The plastic film that the mold is sealed into is not necessarily provided as a bag. Frequently these materials are sold on rolls in what is termed a "C Fold" configuration. In this case, a length of film is unrolled and the edge coming off the roll is sealed using a heat sealer. The mold is then placed between the two layers of film, i.e., in the C Fold. The film is then typically sealed and cut simultaneously on the other side of the mold. Thus, the mold is now "in a bag" with one end open. That end is then sealed and a vacuum is pulled as described above By using the proper equipment, it is also possible to pull the vacuum and seal the bag simultaneously. The particular mechanics of the operation are not critical but rather that the end result is the mold encased in a clear film under vacuum. Any method to achieve this end is acceptable.

Also it is not a necessity that the sealing be effected by heat. Adhesive sealing is acceptable so long as the vacuum conditions are achieved and maintained. Heat sealing is, however, preferred.

This aspect of the invention can be carried out in conjunction with the first aspect of the invention, i.e., curing with visible light However, it is also applicable to other conventional curing processes, e.g. UV curing and thermal curing, briefly described hereinabove In such case, the composition to be cured will comprise the photoinitiator and optionally photosensitizer or, respectively, the thermal activator which are suitable for such curing technology.

Non-limiting examples of suitable materials for the bag are low and high density polyethylene, polybutylene, polyvinyl chloride, polypropylene copolymers of ethylene and higher α-olefins such as propylene or 1-butene, copolymers of ethylene and alkyl acrylates or methacrylates and optionally maleic anhydride, etc. The films may also be coextruded films such that the inner film is designed to be easily sealed while the outer film provides the majority of the mechanical properties. Such coextruded films are well known in the packaging art. The bag or film must be flexible, clear, and - if it is to be used in the process of curing with visible light - transparent to light in the range of approximately 385 co 475 nm, preferably approximately 420 nm. It is also preferably heat sealable. The bag or film may also, if desired, be chosen to provide additional protection against UV light if it is made of polymer or contains additives that absorb UV light.

Having described the invention, the following Examples and Comparative Examples are presented to illustrate the invention in its various aspects. These are purely illustrative and not exhaustive of the invention. Unless stated otherwise, all amounts and percentages are expressed by weight. Ther refractive index was measured using an Abbe refractometer

manufactured by Fisher Scientific.

Example 1

A casting composition was prepared from a mixture of

- 67.5% of an oligomer composition,,

- 22.5% of p-methyl styrene from Deltech Industries, and

- 10% of a modified thiol.

The oligomer composition comprises a urethane acrylate which is the reaction product of diphenylmethane dusocyanate and hydroxypropylacrylate in a 1.1 molar ratio, a photoinitiator (Lucirin TPO from BASF), a UV stabilizer and an antioxidant. The modified thiol is a functional product that can be prepared by thermal initiation from 4 equivalents of

pentaerythntol tetra(3-mercaptopropionate) and 3 equivalents of styrene in a manner similar to example 12 of International Publication No.

WO96/26184.

The above components were mixed together in a brown polyethylene bottle The material was allowed to degas overnight The formula was poured into a glass mold held together by a polyvinyl chloride (PVC) gasket The mold was set oetween two sets of fluorescent actinic lights (420 nm) (Phillips Corporation) approximately 60 cm apart. The lights were turned on. After twenty minutes, a pair of 75 watt halogen lamps directed at the mold were turned on. The lens was cured for an additional 30 minutes. At the end of this cycle, the lens temperature was approximately 95°C.

The lens was clear and almost colorless at this point. The mold was then disassembled and the lens removed. The lens was cleaned and then placed in a 100°C oven for 15 minutes. This annealing cycle is performed only to allow the stress induced in the demolding process to be relieved. The lens was then removed from the oven and allowed to cool to room

temperature.

The lens exhibited very low color and very high clarity. The polymerized material was found to have a refractive index of 1.574 at 20°C and an Abbe number of approximately 33. The glass transition temperature was approximately 90°C. The power of the lens was measured to be

approximately -2.0 and was found to be consistent out near the edges.

This lens passed the dress lens impact test.

Example 2

The material and mold were prepared as in Example 1. The initial actinic cure cycle was 20 minutes but the secondary cure with the halogen lamps was modified so that the temperature of the lens at the end of the second cure cycle was only about 75°C. This time the lens was very yellow when it came out of the cure cycle. Thermal post annealing, as described in Example 1, did decrease the color of the lens to a similar color as the lens from Example 1. The polymerized material was found to have a refractive index of 1.574 at 20°C and an Abbe number of approximately 33.

The glass transition temperature was approximately 85°C.

Example 3

The material and mold were prepared as in Example 1. However, this time the lens mold was placed in a flexible, clear, transparent to 420 nm light and heat sealable polyethylene bag. A tube is inserted in the bag and the open end of the bag is sealed with a commercially available heat sealer up to the inserted tube. A vacuum is then pulled on the bag, causing it to collapse around the mold The heat sealer is then used to seal the bag in front of the inserted tube. The lens was then cured as in Example 1. The lens had no bubbles as a result of air entering the mold cavity during the cure cycle

Example 4 A casting composition was prepared from

- 71.29 % of a trifunctional urethane (meth) acrylate oligomer,

- 28.56% of Hexane Diol Diacrylate from UCB Chemicals,

- 0.12% TPO from BASF, and

- 0.03% 2,6-di-tert-butyl-para-cresol (BHT), an antioxidant from PMC

Specialties.

The above components were mixed together in a brown polyethylene bottle. The material was allowed to degas overnight. The formula was poured into a lens mold consisting of a front curve of 200 mm and a back curve of 400 mm. The two halves of the glass mold were held together by a PVC gasket that provided a distance between the edges of the glass molds of 3.5 mm. This resulted in a lens with a center thickness of approximately 1.2 mm. The mold was then vacuum sealed in a polyethylene bag as described in Example 3. It was then set between two sets of fluorescent actinic lights (420 nm) (Phillips Corporation) approximately 60 cm apart. The lights were turned on. After ten minutes, a pair of 75 watt halogen lamps directed at the mold were turned on. The lens was cured for an additional 30 minutes.

At the end of this cycle, the lens temperature was approximately 95°C. The lens was clear and almost colorless at this point. The mold was then disassembled and the lens removed. The lens was cleaned and then placed in a 100°C oven for 20 minutes. This annealing cycle is performed only to allow the stress induced in the demolding process to be relieved. The lens was then removed from the oven and allowed to cool to room

temperature.

The lens exhibited very low color and very high clarity. The polymerized material was found to have a refractive index of 1.50 at 20°C and an Abbe number of approximately 52. The glass transition temperature was approximately 67°C. The power of the lens was measured to be

approximately -2.0 and was found to be consistent out near the edges.

This lens passed the dress lens impact test.

Example 5

A casting composition was prepared from

- 71.40 % of a trifunctional urethane methacrylate oligomer,

- 28.51% of Hexane Diol Dimethacrylate from Hϋls Corporation,

- 0.06% TPO from BASF, and

- 0.03% BHT from PMC Specialties.

This composition was then processed as in example 4. The lens exhibited very low color and very high clarity. The polymerized material was found to have a refractive index of 1.50 at 20°C and an Abbe number of approximately 52. The glass transition temperature was approximately 80°C. The power of the lens was measured to be

approximately -2.0 and was found to be consistent out near the edges. This lens passed the dress lens impact test.

Example 6

Three optical flat lenses were made, using different curing cycles as described in table 1, from an oligomer composition comprising 71.29% of a trifunctional urethane acrylate, 28.56% hexanediol dimethacrylate, 0.12% Lucirin TPO and 0.03% BHT (antioxidant). The color of the lenses were measured before annealing and after annealing (15 minutes at 100 °C). The results indicate that the lowest color lens is produced when heat is applied simultaneously with the actinic lights. This is due to a higher degree of usage of the photoinitiator, TPO being known as a photobleaching initiator (i.e., its fragments absorb less light than the unfragmented molecule). This lens will also be more stable over time since more of the initiator has been consumed. Therefore, there is less likelihood of further reaction occurring in the lens during normal exposure to sunlight.

Comparative Examples

The following formulations were made and cured into lenses. The lenses were cured for approximately 15 minutes using actinic light only. The intensity of the actinic lamps was approximately 3200 micro watts per square centimeter and the exposure occurred from both sides of the molds. A thermocouple was inserted in each lens and the cure response was noted.

As indicated in table 2 below, only samples A and E showed any exothermic which indicates the lenses cured. This data shows that the initiator used must be one which will initiate under visible light conditions as hereinabove defined.

To determine whether the samples can be thermally initiated, all samples were poured into 2 ml ampoules and sealed. The ampoules were then heated for one hour at 100 °C. There was no obvious conversion of any of the samples.

Claims

1. A method for the production of an optical article which comprises

introducing into a mold a radiation polymerizable composition which upon curing forms an optical article, sealing said mold in a plastic bag under vacuum, exposing said sealed mold to electromagnetic radiation or heat to initiate and/or complete the curing of the composition to form an optical article, and removing the optical article from the mold.

2. A method according to claim 1 wherein the plastic bag is composed of a material selected from the group consisting of low density polyethylene, high density polyethylene, polybutylene, polyvinylchloride and

polypropylene.

3. A method according to any of claims 1 and 2 wherein the composition to be cured contains a UV initiator and the electromagnetic radiation is UV light.

4. A method according to any of claims 1 and 2 wherein the composition to be cured contains a photoinitiator which possesses an absorption peak at a visible light wavelength, and following vacuum sealing of the mold in the bag, curing is initiated by application thereto of radiation energy in the form of visible light having a wavelength of 385 to 475 nm.

5. A method according to any of claims 1 to 4 wherein the optical article is an ophthalmic lens.

6. A method according to any of claims 1 to 5 wherein after removal of the optical article from the mold said article is subjected to heat annealing.

7. A method according to any of claims 1 to 6 wherein the composition

comprises a urethane acrylate oligomer.

8. A method according to any of claims 1 to 7 wherein the vacuum is greater than 400 mm Hg.