GB2155940A

GB2155940A - Method for the polymerization of monomer mixtures in the production of contact lenses by centrifugal casting

Info

Publication number: GB2155940A
Application number: GB08506648A
Authority: GB
Inventors: Otto Wichterle
Original assignee: Czech Academy of Sciences CAS
Current assignee: Czech Academy of Sciences CAS
Priority date: 1984-03-16
Filing date: 1985-03-14
Publication date: 1985-10-02
Also published as: AU3984085A; FR2561401A1; GB8506648D0; JPS61201A; CS253754B1

Abstract

The invention pertains to a method for polymerization of monomer mixtures in the production of contact lenses by centrifugal casting consisting in addition of a polymerization photocatalyst, which effect is induced by irradiation at normal temperature, and a catalyst, which effect is induced by temperature increase, to a monomer mixture, which is then exposed, in a constantly rotating mold, to the effect of light at normal temperature as long as a gel state is attained and the mold is then heated without rotation to temperature 50 - 100 DEG C as long as the complete polymerization conversion is attained.

Description

SPECIFICATION Method for the polymerization of monomer mixtures in the production of contact lenses by centrifugal casting The invention pertains to a method for the polymerization of monomer mixtures in the production of contact lenses by centrifugal casting.

The production of contact lenses by the method of centrifugal casting, for example, according to the US Patents No. 3 699 089, 3 408 429, 3 822 089, 4 153 349, proved highly efficient above all in such cases, when the period of time required for the almost complete polymerization of a monomer mixture is very short, i.e. not longer than few minutes.

This is the case, above all, in the polymerization of monomer mixtures containing methacrylic esters of glycols as a basic component. However, development of new materials with an extraordinarily high permeability, in particular for oxygen, led to some types of polymers, which are formed from the particular monomers much slower and where the monomers are almost completely converted to the gel even after many hours. In such case, the productivity of continuous casting machines decreases, because it is approximately inversely proportional to the time of polymerization. The main advantage of the method of centrifugal casting is thus lost.

The method according to the present invention enables a highly economical utilization of the method of centrifugal casting in the production of contact lenses also for very slowly polymerizing monomer mixtures by using the equipment for the continuous casting of a carrousel or column type, which has been employed so far, for the production of precastings consisting of the prepolymerized monomer mixtures which still contain a quite substantial amount of nonpolymerized monomeric components, but having the stabilized shape of the final contact lens fixed in a casting mold after a preliminary crosslinking. The precastings are exposed, in the following operation, without undergoing rotation, to the polymerization conditions for the period of time required for attaining the complete polymerization conversion.In the first stage, characterized by rotation of molds, photopolymerization is used according to the invention, i.e. the polymerization by means of photocatalysts which become effective by irradiation, for example, by means of 2,2-diethoxyacetophenone or alkyl ethers of benzoin wherein alkyl is methyl, ethyl, isopropyl, n-butyl, sec-butyl etc., whereas, in the second stage, when molds with the photo-chemically prepolymerized mixture remain at rest, thermal polymerization is used, which is induced, for example, by the presence of a catalyst of peroxide type (diisopropyl peroxocarbonate, ammonium persulfate, dibenzol peroxide, and the like) or instable azo compounds (azobisisobutyronitrile, and the like). Both types of catalysts, for the first as well as for the final stage of polymerization, are added simultaneously already into the starting monomer mixture.The said monomer mixture is the exposed to light in a constantly rotating casting mold at normal temperature as long as a gel state is attained and then the mold containing the gel is heated without rotation to temperature 50 to 1000C as long as the complete polymerization conversion is attained. The amount of the photocatalyst is advantageously between 0,05% and 5% by weight.

The amount of the catalyst for thermal polymerization is preferably in the range of from 0,05% to 5% by weight. The photocatalyst is decomposed and consumed in the main part during photopolymerization, while the catalyst for the thermal polymerization (second stage) remains intact during photopolymerization which proceeds at lower temperature. In special cases catalyst can be used having the capacity of both, photochemical and thermal initiation of radical polymerization, e.g.

photochemically unstable azo compounds.

Equipment adapted in various ways may serve in the performance of this method. The simplest equipment consists in an arrangement where the molds with a prepolymerized mixture, leaving continuous production apparatuses of the carrousel or column type, are allowed, after the respective inspection, to fall against a stream of protective gas (e.g. pure nitrogen or carbon dioxide) into a vessel rinsed with the protective gas. This vessel, with a large number (up to several thousands) of molds, is then air-tightly sealed and placed in a box heated to a constant temperature, where it is kept for the period of time necessary for the almost completed polymerization. The lenses are eventually removed from molds and processed in the usual way.This simple procedure proved suitable especially in the production of large series of lenses in the uniform type, which have been cast in uniform molds, are collected to undergo the second stage in large vessels, and do not require sorting.

Another equipment, where the sequence of molds leaving the apparatus for centrifugal casting is preserved during completion of polymerization, consists in the arrangement of mold discharge from this apparatus, where the molds, just passing through an irradiation zone and containing the already gelated prepolymer, are periodically slided out from the rotating polymerization column by a sliding gate above an opening and fall through this opening into a tubular magazine placed under the opening, through which a moderate stream of protective gas is led ascendently. The charged magazines are then air-tightly sealed and kept in heated thermostats to carry out the final polymerization.

The arrangement of molds in the magazines, which better suits to a partially automated handling in the following checking and measurement steps, is the advantage of the above method.

Further advantage of this method is that the prepolymerized lens remains closed in a relatively small space between closely fitting molds and evaporation of more volatile components of the polymerizing mixture is reduced at elevated temperature.

Another arrangement of the equipment consists in the possibility to carry out also the second stage continuously by forcing the molds with prepolymerized lenses along a heated tunnel filled with protective gas. Advance the molds is achieved either by a stepwise shifting in a heated track or by smooth motion of a conveyor. This method can be employed only when the second polymerization stage does not require extremely long heating periods. Otherwise the necessary length of the advancing row of molds grows too much with high demands to attendance and maintenance.

As an example of slowly polymerizing monomer mixtures, they can serve such mixtures, which contain vinylpyrolidone as a main monomer component in addition to a minor amount of hydrophobic monomers, for example, butyl methacrylate, allyl methacrylate, allyl methacrylate oligomers, and the like. If mixtures of these monomers, in particular with addition of a nonpolymerizing solvent, should be carried to the almost complete polymerization in the presence of the most efficient photocatalysts, their exposure to effective radiation takes a whole hour or longer. Consequently, the capacity of the expensive apparatus for the continuous centrifugal casting would be very low, because only 5 to 20 cast lenses in hour, or even less, could be discharged from the apparatus at the usual number of five to twenty rotating molds in the irradiation zone.On the other hand, only three to five minutes in enough under the same polymerization conditions to achieve the conversion of three to ten percent, when the mixture is already transformed into a non-fluid gel, so that as much as several hundreds of molds containing prepolymerized lenses pass through the photopolymerization apparatus during one hour and are brought to the final stage of completely polymerized lenses in the following above described operation as a whole and with minimum production expense.

The invention is further illustrated in an example.

Example The conventional apparatus for continuous spin casting of contact lenses using photoiniciated polymerization was charged by molds having sagittal depth 3.5 mm, diameter of their sharpe edge 13 mm and central radius 8.33 mm containing 25 microliters of monomer mixture consisting of 100 volume parts hydroxyethylmethacrylate (HEMA), 0.3 volume parts ethylene dimethacrylate, 0.2 parts 10%- water solution of ammonium persulfate, 0.4 parts benzoin ethyl ether and 15 parts of glycerol.

The passage of molds at room temperature through the spincasting apparatus was maintained constant at such a rate that the exposure of rotating molds (385 R.P.M.) in the zone illuminated by ultraviolet light took 30 to 50 seconds. The molds leaving the rotating sites of the spin casting machine contained a gelly substance of partly polymerized but already sufficiently crosslinked monomer, unable to change its contact lens shape by gravitational forces. The molds collected under pure nitrogen atmosphere in boxes containing each about one thousand of them were stored during 10 to 20 hours at a temperature 65 to 70 C. Under these conditions the polymerization was brought to full conversion. Lenses were separated from molds as usual by swelling by water and stored after inspection in physoiogical solution.

Using this arrangement the expensive and sophisticated spin casting equipment is exploited 5 to 10 times more efficiently as compared with well known photopolymerization only up to full conversion which requires many times longer exposure in rotating sites.

Claims

1. A method for the polymerization of monomer mixtures in the production of contact lenses by centrifugal casting, wherein a polymerization photocatalyst, which effect is induced by irradiation at normal temperature, and a catalyst for thermal polymerization, which effect is induced by temperature increase, are added to the monomer mixture and the said mixture is then exposed to light in a constantly rotating casting mold at normal temperature as long as a gel state is attained and then the mold containing the gel is heated without rotation to temperature 50 to 100 C as long as the complete polymerization conversion is attained.

2. A method as set forth in claim 1, wherein said photo-catalyst is alkyl ether of benzoin where alkyl is methyl, ethyl isopropyl, n-butyl, sec-butyl.

3. A method as set forth in claim 1, wherein said photo-catalyst is 2,2-diethoxyacetophenone.

4. A method as set forth in claim 1, wherein said catalyst for thermal polymerization is a catalyst of peroxide type.

5. A method as set forth in claim 4, wherein said catalyst of peroxide type is selected from the group consisting diisopropyl peroxocarbonate, ammonium persulfate, dibenzoyl peroxide.

6. A method as set forth in claim 1, wherein said catalyst for thermal polymerization is selected from instable azo compounds.

7. A method as set forth in claim 6, wherein said catalyst for thermal polymerization is azobisisobutyronitrile.

8. A method as set forth in claim 1, wherein said photocatalyst is added in the amount from 0,05% to 5% by weight.

9. A method as set forth in claim 1, wherein said catalyst for thermal polymerization is added in the amount from 0,05% to 5% by weight.

10. A method for the polymerization of monomer mixtures in the production of contact lenses by centrifugal casting substantially as described in the example disclosed herein.