MXPA97002638A - Method for treating plast mold pieces - Google Patents

Abstract

The present invention relates to the surface characteristics of contact lenses molded by casting are improved by substantially removing all the oxygen from the plastic mold parts before emptying the lent

Description

METHOD FOR TREATING PARTS OF PLASTIC MOLD.

BACKGROUND OF THE INVENTION Field of the Invention This invention relates to a method of treating plastic mold pieces used in casting contact lenses. More specifically, in one aspect, the present invention relates to a method for extracting oxygen from plastic mold parts, and, in another aspect, to a method of improving the surface characteristics of cast-molded contact lenses, polymerized by a free radical polymerization process. Description of the technique Contact casting casting is known. Typically, the combination of monomers containing monomers capable of forming suitable polymers, crosslinking agents, catalysts, polymerization initiators and the like, are mixed alone or in suitable diluents or solvents and placed in the female half of the mold. The male half of the mold is then pressed to the female half of the mold, and the monomer mixture is polymerized. The lenses obtained from this method are adapted to the shape of the cavity formed between the two mold halves, and exhibit surface characteristics corresponding to the surfaces of the mold. After emptying the lenses, they are ready for further processing, such as cleaning, polishing and / or rounding and hydration, as needed. The mold parts used in the emptying of contact lenses are generally made of plastic materials that are substantially inert to the monomers used and to the polymerization process employed. Normally, these pieces of plastic mold are discarded after using them only once. Some contact lenses manufactured by the cast molding process, namely those known as hydrogels, have an appreciable frequency of cosmetic defects on their surfaces. In the sense in which it is used herein, cosmetic defects are understood as the places or areas on the surface of lenses that can scatter light and which indicate the appearance of an irregular surface area in comparison with the rest of the surface of the lens. Although these cosmetic defects can not be seen ordinarily with the naked eye, they appear when examined with a lens drop lamp or amplifier. Although these cosmetic defects are not generally associated with medical or health problems, they may cause slight deficiencies in the optimal visual performance of the lenses. In addition, these cosmetic defects may be associated with problems of processing, transport and handling, because lenses with such defects tend to stick more easily to each other and to the packaging materials. Finally, the improvement of the polymerization on the surfaces of the lenses results in a better performance of acceptable lenses obtained from the cast molding process. It has long been known that the presence of oxygen inhibits the complete polymerization of free radicals. Accordingly, conventional cast molding processes are carried out in inert environments in order to eliminate the effects of oxygen in the polymerization process. These cast molding processes are considered suitable for the general polymerization to result in a degree of polymerization greater than about 99% of the volume of the lens. Such cast molding techniques are widely used in commerce. However, it has been found that oxygen can still affect the free radical polymerization of the contact lens material even when the polymerization is carried out under inert atmospheric conditions. It has unexpectedly been found that some pieces of plastic mold contain, within the structural matrix of the plastic, sufficient oxygen to adversely affect the polymerization at the interface between the surface of the mold and the surface of the lens. It is estimated that oxygen migrates to the surface of the plastic mold part during the polymerization of free radicals and inhibits complete polymerization at the surface of the lens. It is also considered that the presence of oxygen produces a reduced crosslinking density at the surface of the lens. It is believed that this incomplete polymerization or reduced crosslinking density at the surface of the lens produces the cosmetic defects described above. COMPENDIUM OF THE INVENTION According to this invention, it has been found that the substantial extraction of all the oxygen from the thermoplastic resin before molding the plastic mold part, the substantial extraction of all the oxygen from the plastic mold part after the molding, or both, results in Contact lens molded by casting with less cosmetic defects and more complete polymerization on the surface of the lens. In addition, the substantial extraction of all oxygen, as described above, increases the production of acceptable contact lenses made from casting processes. Accordingly, the present invention is a method for extracting oxygen from plastic mold parts which includes putting the plastic resin in contact with an inert gas for a period of time sufficient to extract substantially all of the oxygen before molding the plastic resin into pieces. molded plastic with predetermined shapes. Alternatively, the oxygen can be removed by placing the molded plastic mold part in contact with an inert gas for a sufficient period of time to extract substantially all of the oxygen. In addition, the oxygen can be extracted by using vacuum instead of or in combination with the placing of the thermoplastic resin or the plastic mold part in contact with an inert gas. Preferably, the plastic resin and the plastic mold parts will come into contact with inert gases and will be maintained in an inert environment prior to the emptying of the lenses. DETAILED DESCRIPTION OF THE INVENTION The present invention is directed to improving the surface quality of contact lenses manufactured by cast molding processes using free radical polymerization techniques. In general, the composition of the contact lenses, the molding process and the polymerization processes are known, and this invention relates primarily to the treatment of plastic mold parts to achieve contact lenses with improved surface characteristics and less frequent cosmetic defects. Of course, the invention can also be used to improve the surface quality with any free radical polymerization process using plastic mold parts to give a predetermined shape to the final polymerized product. The present invention can be used with all contact lenses, such as hard, soft and rigid conventional lenses, permeable to gases, and the composition of the monomer mixture and the specific monomers used to form the lenses are not critical. The present invention is preferably employed with soft contact lenses, such as those commonly referred to as hydrogel lenses, prepared from monomers including, but not limited to, hydroxyethyl methacrylate, vinyl pyrrolidone, glycerol methacrylate, methacrylic acid and acid esters. However, any combination of lens forming monomers capable of forming a polymer that can be used in the manufacture of contact lenses can be used. Hydrophobic lens-forming monomers, such as those containing silicone radicals, may also be included. The degree of polymerization and / or the crosslinking density on the surface of the lens is considered improved on all contact lenses, even those that do not normally exhibit cosmetic defects. Thus, the term "contact lenses", as used herein, includes hard, soft and rigid gas-permeable contact lenses, as well as inocular lenses, turned lens parts to finished contact lenses. , and other optical implants. The monomer mixture used in forming the contact lenses that can be used with this invention usually includes crosslinking agents, reinforcing agents, free radical initiators and / or catalysts and the like, as is known in the art. In addition, suitable solvents or diluents can be employed in the monomer mixture, provided that such solvents or diluents do not adversely affect or interfere with the polymerization process. The polymerization or curing method is not critical for the implementation of the invention, except that this invention is limited to free radical polymerization systems, known in the contact lens art. Thus, polymerization can be produced by several mechanisms, depending on the specific composition employed. For example, thermal, photo, X-ray, microwave polymerization and combinations thereof, which are free radical polymerization techniques, can be employed in the present invention. Preferably, thermal and photo polymerizations are used in this invention, with UV polymerization being more preferred. Casting molding techniques are also known. In general, conventional pour molding techniques employ male and female thermoplastic mold halves of predetermined configuration, imparting the desired shape and surface configurations to the lenses formed therebetween. Examples of cast molding processes are described in U.S. Patents 4,113,224, 4,121,896, 4,208,364 and 4,208,365, which are fully incorporated herein by reference. Naturally, there are many other pour molding ideas that can be used in the present invention, provided that the molds are made of thermoplastic materials. As described above, the mold parts used for the emptying of contact lenses are generally made of plastic materials that give the lenses the specific physical characteristics. The plastic materials that can be used in the present invention are thermoplastics that generally have high oxygen permeabilities. As mentioned above, it is considered that dissolved or free oxygen migrates through the plastic material and at the interface between the surface of the mold part and the surface of the lens. What this invention extracts is dissolved or free oxygen. The oxygen permeability of a polymeric material (a thermoplastic material) is an intrinsic property of said material and is defined as the amount of oxygen that it transports through a film of the polymeric material with a unit of thickness subjected to a unit of force of drive that is measured by the partial pressure difference of oxygen on both sides of the film. The actual permeability values depend on the units used to express the amount of gas, the thickness of the film and the driving force. Oxygen permeabilities of most common thermoplastic materials or resins can be easily known and can be found in J. Brandrup and E. H. Mergud, The Polymer Handbook, third edition, J. Widely &; Sons, 1989, which is incorporated herein by reference. It has been found that plastic materials having an oxygen permeability higher than U, UJD X lu 13 CT? 3 gas at Tempera ture - and Pre-sion- Standard • cm cm2 • Seconds • Pascal Preferably, the oxygen permeability of the plastics materials used in the present invention will be greater than 0.035 X 10"13, and most preferably higher than Preferred plastic materials are polymers and copolymers which predominantly contain polyolefins, such as polyethylene and polypropylene, and polystyrene. Polypropylene is the most preferred plastic mold material. The plastic mold parts are generally injection molded from thermoplastic resins which are often in the form of pellets, into finished metal master molds or dies. However, the method of manufacturing the plastic mold parts can vary according to any of the known techniques. Preferably, the resin will be injection molded in an atmosphere substantially free of oxygen. Conventionally, injection molding techniques do not require the extraction of oxygen from the thermoplastic resin prior to molding. However, according to one embodiment of this invention, the oxygen concentration is substantially extracted by placing the resin in contact with an inert gas prior to molding. Thus, one method of employing the present invention is to extract substantially all of the oxygen from the resin by placing the resin in contact with an inert gas. Although any inert gas can be used to extract oxygen from the resin, nitrogen is preferred, because of its availability, safety and cost. However, any inert gas, such as argon or helium, as well as carbon dioxide can be used. In addition, oxygen can be extracted through the use of vacuum. It should be understood that it is preferred to extract substantially all the oxygen from the interior of the resin. Although it is difficult to quantify the exact concentration of oxygen necessary for optimal polymerization at the surface of the lens, it is estimated that, prior to contact lens emptying, approximately 51% to approximately 99% of the oxygen normally contained in the lens should be removed. plastic material when it is in equilibrium in environments that contain oxygen. However, it may not be desirable to extract all the oxygen. It is considered that excessive extraction of oxygen can cause difficulties when removing the lenses from the plastic mold parts for some thermoplastic materials. Thus, the presence of a slight amount of oxygen in the plastic mold part may be desirable, depending on the composition of the plastic mold part, the polymerization techniques and the general cast molding process to release optimally the contact lenses of the molds during manufacture.

Accordingly, the amount of oxygen to be extracted for a particular plastic mold part depends on several variables, such as the composition of the plastic mold, the composition of the lens, the polymerization method and the like. The amount of oxygen to be extracted to achieve the desired degree of polymerization on the surface of the lens and the optimal extraction of the molds can be determined for any specific mold material by a simple trial and error, as will be evident and known by the experts in the field. Preferably, the thermoplastic resin is placed in an inert environment or is otherwise contacted with an inert gas for a period of from about 2 to about 96 hours at room temperature and pressure. Naturally, the duration of exposure to inert gas can vary depending on the selected temperature and pressure conditions. It has been found that the use of elevated temperatures reduces the duration of exposure in the polypropylene mold process. It has also been found that the extraction of oxygen from the plastic mold part, after forming the mold part, can be effectively employed in addition to the extraction of oxygen from the resin, as explained above, or instead of this earlier phase. In general, the extraction of oxygen from the molded plastic mold part, when used alone, requires placing the plastic mold part in an inert environment for between about 10 and about 48 hours, preferably for about 20 to 40 hours, at room temperature and pressure, depending on the thermoplastic materials used. When the oxygen extraction of the molded plastic mold part is used in conjunction with the first oxygen extraction stage, the plastic mold part is brought into contact with the inert gas for a period of about 0.5 to about 6 hours, preferably 1 to 4 hours at room temperature and pressure. As mentioned above, vacuum can also be used to extract oxygen. It is highly preferable to employ both phases described above when preparing plastic mold parts for use with this invention. In the most preferred embodiment of this invention, the polymer resins are placed in an inert environment for a period of between 8 and 72 hours. The treated resins are then molded into plastic mold pieces that are maintained in an inert environment for a period of about 0.5 and 4 hours. The plastic mold parts are then passed directly to a cast molding process which is also carried out in an inert environment. The following examples serve to illustrate some embodiments of the invention. EXAMPLES Formation of molds Various batches of polypropylene resin used to make the plastic mold parts of this invention were contacted with nitrogen for 12, 48 and 72 hours at room temperature and pressure, as set forth in Table I.

A separate batch of resin was not treated, as a control. Male and female polypropylene mold halves were prepared by injection molding in steel master molds. The nitrogen-treated resin was used for the mold halves that represent this invention, and part of the untreated resin was used for the mold halves used as controls, and the other untreated resin was then used. The mold halves made of nitrogen-treated resins were contacted with nitrogen, to extract additional oxygen, for between 0.5 and 6 hours at room temperature and pressure, as set forth in Table I. Some of the mold halves made of untreated resin were placed in contact with nitrogen, to extract oxygen, for between 16 and 72 hours at room temperature and pressure. Lens Formation The lenses were polymerized in the treated and untreated polypropylene molds, prepared above. The monomer mixture consisting of 85% by weight of 2-hydroxyethyl methacrylate, 15% by weight of glycerin, 0.3% by weight of ethylene glycol dimethacrylate, and 0.2% by weight of benzoin methyl ether, was placed in the cavity of the female mold halves, and the male mold halves were introduced to displace the excess monomer mixture. Pressure was applied to the molds to ensure proper settlement, and the monomer mixture was polymerized using UV energy. All the female mold halves and all the male mold halves had identical configurations, and all the polymerization conditions were identical. After cooling, the lenses were removed from the molds, hydrated and packaged in plastic blister containers. After autoclaving, the lenses were removed from the blister packs and inspected for cosmetic defects. The percentage of cosmetic defects appreciated by optical comparative inspection is shown in Table I. The controls (lenses molded from untreated resin and plastic mold pieces) are designated with the letters A-L. Each following example represents a batch of contact lenses that contained from about 50 to about 200 lenses.

LO LO or LO or LO CM CM ro ro TABLE I (Cont.) Cosmetic defects in molded contact lenses by casting Treatment (heiras) * Example Resin Mold% defects H - - 62 24 12 3 0 25 - 24 0 I - - 40 26 12 2,5 0 27 - 24 0 J - - 24 28 72 0,5 0 29 - 72 0 K - - 25 30 12 2,5 0 31 - 24 0 L - - 29 32 12 2 0 33 _ 24 1 * A control batch was used for each series of test batches on a given day. As set forth in Table I, contact lenses molded from plastic mold pieces from which the oxygen was extracted have a considerably lower number of cosmetic defects than those molded from plastic mold pieces without try. The present invention is not limited by the embodiments specifically described herein. It should be understood that the scope of this invention includes all modifications, variations and equivalents that fall within the scope of the appended claims.

Claims (25)

1. Method for preparing plastic mold parts used to cast cast contact lenses including contacting a thermoplastic resin with an oxygen permeability greater than t? cm3gas • TPE • cm 0, 035 X 10 cm 'S • Pa with an inert gas for a time sufficient to substantially remove all of the oxygen from the interior of the resin and subsequently mold the resin into a plastic mold part with a predetermined configuration.

2. The method of Claim 1 wherein the resin is contacted with an inert gas for a period of from about 2 to about 96 hours under pressure and room temperature.

3. The method of Claim 2 further including contacting the plastic mold part with an inert gas for a period of from about 0.5 to about 6 hours at room temperature and pressure before casting the lenses.

4. The method of Claim 3 wherein the plastic mold part has an oxygen permeability of more than

0. 35 * 1 ° - "^ cm ';; ^" "

5. The method of Claim 1 wherein the resin is a polyolefin.

6. The method of Claim 5 wherein the resin is polypropylene.

7. In a method for casting cast contact lenses including emptying the lenses between two pieces of plastic mold with a greater oxygen permeability of 0.35 X 10 -13 cm3gas • TPE • cm cm 'S • Pa using a free radical polymerization process, the improvement consisting in extracting substantially all of the oxygen from the interior of the plastic mold parts before emptying the contact lenses.

The method of Claim 7, wherein the oxygen is removed by placing the thermoplastic resin used to make the plastic mold part in contact with an inert gas before molding the plastic mold part.

9. The method of Claim 8 wherein the oxygen is extracted by contacting the resin with an inert gas for between about 2 and about 96 hours at room temperature and pressure.

The method of Claim 9 wherein the plastic mold part is contacted with an inert gas for between about 0.5 and about 6 hours at room temperature and pressure.

11. The method of Claim 10 wherein the plastic mold part is a polyolefin.

12. The method of Claim 11 wherein the plastic mold part is polypropylene.

The method of claim 7, wherein the oxygen is removed by placing the plastic mold part in contact with an inert gas.

The method of claim 13, wherein the oxygen is removed by placing the plastic mold part in contact with an inert gas for between about 10 and about 48 hours at room temperature and pressure.

15. The process of claim 7, wherein the oxygen is extracted by the use of vacuum.

16. The method of claim 14, wherein the plastic mold part is a polyolefin.

The method of claim 16, wherein the plastic mold part is a polypropylene.

The method of claim 7, wherein the oxygen permeability of the plastic mold part is greater than about 0.035 X 10- "Cm ^ aS 'TPE • c c' S • Pa

19. A method of preparing plastic mold parts used for casting contact lenses including casting a plastic mold part having an oxygen permeability greater than r. , n-i3 cm3gas • TPE • cm 0.035 10"c qm2. s. in contact with an inert gas for a period sufficient to remove substantially all of the oxygen from the interior of the plastic mold parts prior to the emptying of the contact lenses.

The method of claim 19, wherein the plastic mold part is contacted with an inert gas for between about 10 and about 48 hours at room temperature and pressure.

The method of claim 20, wherein the plastic mold part is a polyolefin.

22. The method of claim 21, wherein the polyolefin is polypropylene.

23. The method of claim 22, wherein the oxygen permeability of the plastic mold part is superior to 0.035 X 10 -13 cm3gas • TPE • cm cm 'Pa

24. A contact lens made from a cast molding process using free radical polymerization and casting between two plastic mold parts made of thermoplastic resins having an oxygen permeability greater than 0.035 X 10- "CITt3sraS 'TPE' Cm cm2 • S • Pa and where substantially all of the oxygen present in the plastic mold parts is removed before casting the contact lens.

25. The contact lens of claim 24, wherein the oxygen is extracted from the thermoplastic resin, the plastic mold part or both with vacuum.