Method for treating plastic mold pieces

Surface characteristics of cast molded contact lenses are improved by removing substantially all the oxygen from the plastic mold pieces prior to casting the lenses.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of treating plastic mold pieces used in 
cast molding contact lenses. More specifically, in one aspect, the present 
invention is directed to a method for removing oxygen from plastic mold 
pieces and, in another aspect, to a method of improving the surface 
characteristics of cast molded contact lenses polymerized by a free 
radical polymerization process. 
2. Description of Art 
The cast molding of contact lenses is well known. Typically, the monomer 
mix containing monomers capable of forming suitable polymers, 
crosslinkers, catalysts, polymerization initiators, and the like are mixed 
neat or mixed in suitable diluents or solvents and are placed into a 
female mold half. The male mold half is then pressed into the female mold 
half and the monomer mix is polymerized. The lenses obtained from this 
process conform to the shape of the cavity formed between the two mold 
halves and exhibit surface characteristics which correspond to the mold 
surfaces. After the lenses are cast, they are ready for further processing 
such as cleaning, polishing and/or edging and hydration, as necessary. 
The mold pieces used in casting contact lenses are generally made of 
plastic materials which are substantially inert to the monomers employed 
and to the polymerization process employed. Typically, these plastic mold 
pieces are discarded after a single use. 
Certain contact lenses manufactured by the cast molding process, 
particularly those known as hydrogels, have a noticeable frequency of 
cosmetic defects on their surfaces. As used herein, cosmetic defects is 
meant to describe sites or areas found on the surface of the lenses which 
can scatter light and which indicate the occurrence of an irregular 
surface area as compared to the rest of the lens surface. While these 
cosmetic defects cannot usually be seen by the naked eye, they do appear 
when examined under a slit lamp or magnification. Although these cosmetic 
defects are generally not associated with any medical or health concerns, 
they can lead to slight deficiencies in optimal visual performance of the 
lenses. Moreover, these cosmetic defects can be associated with 
processing, shipping and handling problems as lenses which contain these 
defects tend to more easily stick to themselves and to the packaging 
materials. Finally, improving the polymerization at the lens surfaces 
results in improved yields of acceptable lenses obtained from the cast 
molding process. 
It has long been known that the presence of oxygen inhibits complete free 
radical polymerization. Accordingly, conventional cast molding processes 
are conducted in inert environments in order to eliminate the effects of 
oxygen on the polymerization process. These cast molding processes have 
been found adequate for overall polymerization resulting in a degree of 
polymerization of greater than about 99% of the bulk lens. Such cast 
molding techniques are widely used commercially. 
However, it has now been found that oxygen may still have an effect on the 
free radical polymerization of the contact lens material even when the 
polymerization is conducted under inert atmospheric conditions. It has 
surprisingly been found that certain plastic mold pieces contain 
sufficient oxygen within the structural matrix of the plastic to adversely 
affect polymerization at the interface between the mold surface and the 
surface of the lens. It is believed that the oxygen migrates to the 
surface of the plastic mold piece during free radical polymerization and 
inhibits complete polymerization at the lens surface. The presence of 
oxygen is also believed to cause reduced crosslinking density at the lens 
surface. It is this incomplete polymerization or reduced crosslinking 
density at the lens surface which is believed to cause the cosmetic 
defects described above. 
SUMMARY OF THE INVENTION 
In accordance with this invention, it has now been found that removing 
substantially all of the oxygen from the thermoplastic resin prior to 
molding the plastic mold piece, removing substantially all the oxygen from 
the plastic mold piece after molding, or both, results in cast molded 
contact lenses with less cosmetic defects and more complete polymerization 
at the lens surface. Further, removing substantially all of the oxygen as 
described above increases the yield of acceptable contact lenses made from 
cast molding processes. 
Accordingly, the present invention is a method for removing oxygen from 
plastic mold pieces comprising contacting the plastic resin with an inert 
gas for a period of time sufficient to remove substantially all of the 
oxygen prior to molding the plastic resin into plastic molded pieces 
having predetermined shapes. Alternatively, the oxygen can be removed by 
contacting the molded plastic mold piece with an inert gas for a period of 
time sufficient to remove substantially all of the oxygen. Moreover, the 
oxygen can be removed by the use of a vacuum in lieu of or in combination 
with contacting the thermoplastic resin or plastic mold piece with an 
inert gas. Preferably, the plastic resin and the plastic mold pieces will 
be contacted with inert gases and maintained in an inert environment prior 
to casting 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. Generally, the composition of the contact 
lenses, the molding process, and polymerization processes are well known 
and this invention is concerned primarily with treating the plastic mold 
pieces to achieve contact lenses with improved surface characteristics and 
decreased frequency of cosmetic defects. Of course, the invention can also 
be used to improve surface quality with any free radical polymerization 
process using plastic mold pieces to provide a predetermined shape to the 
final polymerized product. 
The present invention can be used with all contact lenses such as 
conventional hard, soft and rigid gas permeable lenses and the composition 
of the monomer mix 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 useful in making contact lenses may be used. 
Hydrophobic lens forming monomers may also be included such as those 
containing silicone moities. The degree of polymerization and/or the 
crosslinking density at the surface of the lens is believed to be improved 
in all contact lenses, even those which do not typically 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, 
lens blanks lathed into finished contact lenses, and other optical 
implants. 
The monomer mix used in forming the contact lenses useful with this 
invention typically includes crosslinking agents, strengthening agents, 
free radical initiators and/or catalysts and the like as is well known in 
the art. Further, suitable solvents or dilueuts can be employed in the 
monomer mix, provided such solvents or diluents do not adversely affect or 
interfere with the polymerization process. 
The method of polymerization or cure is not critical to the practice of 
this invention, except that this invention is limited to free radical 
polymerization systems as are well known in the contact lens art. Thus, 
the polymerization can occur by a variety of mechanisms depending on the 
specific composition employed. For example, thermal, photo, X-ray, 
microwave, and combinations thereof which are free radical polymerization 
techniques can be employed herein. Preferably, thermal and photo 
polymerizations are used in this invention with UV polymerization being 
most preferred. 
Cast molding techniques are also well known. Generally, conventional cast 
molding techniques employ thermoplastic male and female mold halves of 
predetermined configuration which imparts the desired shape and surface 
configurations to the lenses formed therebetween. Examples of cast molding 
processes are taught in U.S. Pat. Nos. 4,113,224; 4,121,896; 4,208,364; 
and 4,208,365 which are fully incorporated herein by reference. Of course, 
many other cast molding teachings are available which can be used with the 
present invention providing the molds are made from thermoplastic 
materials. 
As described above, the mold pieces used to cast contact lenses are 
generally made of plastic materials which provide the specific physical 
characteristics to the lenses. The plastic materials which can be employed 
with the present invention are thermoplastic which generally have high 
oxygen permeabilities. As mentioned above, it is believed that dissolved 
or free oxygen migrates through the plastic material and to the interface 
between the surface of the mold piece and the surface of the lens. It is 
the dissolved or free oxygen which is removed by this invention. The 
oxygen permeability of a polymeric material (a thermoplastic material) is 
an intrinsic property of that material and is defined as the amount of 
oxygen which transports across a film of the polymeric material having 
unit thickness subjected to unit driving force which is measured by the 
difference of partial pressure of the oxygen on the two sides of the film. 
Actual permeability values depend on the units used to express the mount 
of gas, thickness of the film and the driving force. The oxygen 
permeabilities of most common thermoplastic materials or resins are 
readily available and can be found in J. Brandrup and E. H. Immergud's The 
Polymer Handbook, 3rd ed., J. Wilely & Sons, 1989, which is incorporated 
herein by reference. It has been found that plastic materials having an 
oxygen permeability of greater than 0.035.times.10.sup.-13 
##EQU1## 
are useful with this invention. Preferably, the oxygen permeability of the 
plastic materials used herein will be greater than 0.035.times.10.sup.-13 
and most preferably greater than 1.0.times.10.sup.-13 
##EQU2## 
The preferred plastic materials are those polymers and copolymers which 
contain predominantly polyolefins such as polyethylene and polypropylene, 
and polystyrene. Polypropylene is the most preferred plastic mold 
material. 
The plastic mold pieces are generally injection molded from thermoplastic 
resins which are often in the form of pellets in finished metal master 
molds or dyes. However, the method of manufacturing the plastic mold 
pieces can vary according to any of the known techniques. Preferably, the 
resin will be injection molded under a substantially oxygen free 
atmosphere. Conventionally, injection molding techniques do not require 
removal of oxygen from the thermoplastic resin prior to molding. However, 
in accordance with one embodiment of this invention, the oxygen 
concentration is substantially removed by contacting the resin with an 
inert gas prior to molding. 
Thus, one method of employing the present invention is to remove 
substantially all the oxygen from the resin by contacting the resin with 
an inert gas. While any inert gas can be employed to remove the 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. Moreover, the oxygen can be removed by the use of a 
vacuum. 
It should be understood that it is preferred to remove substantially all of 
the oxygen from within the resin. While it is difficult to quantify the 
exact concentration of oxygen required for optimal polymerization at the 
lens surface, it is believed that about 51% to about 99% of the oxygen 
normally contained in the plastic material when at equilibrium in normal 
oxygen containing environments should be removed prior to casting the 
contact lenses. However, it may not be desirable to remove all of the 
oxygen. It is believed that excessive removal of oxygen may lead to 
difficulties in removing the lenses from the plastic mold pieces for 
certain thermoplastic materials. Thus, the presence of a slight amount of 
oxygen in the plastic mold piece may be desirable depending upon the 
composition of the plastic mold piece, the polymerization techniques and 
the overall cast molding process to obtain optimal release of the contact 
lenses from the molds during manufacture. 
Accordingly, the amount of oxygen to be removed for any particular plastic 
mold piece depends on several variables such as plastic mold composition, 
lens composition, method of polymerization and the like. The mount of 
oxygen to be removed to achieve the desired degree of polymerization at 
the lens surface and optimal removal from the molds can be determined for 
any specific mold material by simple trial and error testing as will be 
apparent to and well within the skills of one skilled in the art. 
Preferably, the thermoplastic resin is placed in an inert environment or 
otherwise contacted with an inert gas for a period of about 2 to about 96 
hours at ambient temperature and pressure. Of course, the duration of 
exposure to the inert gas can vary depending on the conditions of 
temperature and pressure selected. The use of elevated temperatures has 
been found to reduce the duration of exposure for polypropylene mold 
process. 
It has also been found that the removal of oxygen from the plastic mold 
piece after the mold piece has been formed can be effectively employed 
either in addition to the removal of the oxygen from the resin as 
discussed above or in lieu of this earlier step. Generally, the removal of 
oxygen from the molded plastic mold piece when used alone requires placing 
the plastic mold piece in an inert environment for between about 10 and 
about 48 hours preferably for about 20 to 40 hours at ambient temperature 
and pressure depending on the thermoplastic materials employed. When the 
removal of oxygen from the molded plastic mold piece is used in 
conjunction with the first oxygen removal step, the plastic mold piece is 
contacted with the inert gas for a period of about 0.5 to about 6 hours, 
preferably 1 to 4 hours at ambient temperature and pressure. As mentioned 
above, a vacuum can be also used to remove the oxygen. 
It is most preferred to employ both steps described above in preparing 
plastic mold pieces for use with this invention. In the most preferred 
embodiment of this invention, polypropylene resins are placed in an inert 
environment for a period of between 8 and 72 hours. The treated resins are 
then molded into the plastic mold pieces which are maintained in an inert 
environment for a period of about 0.5 and 4 hours. The plastic mold pieces 
are then moved directly into a cast molding process which is also 
conducted in an inert environment. 
The following examples serve to illustrate certain embodiments of the 
invention.

EXAMPLES 
Formation of Molds 
Several batches of polypropylene resin used for making the plastic mold 
pieces of this invention were contacted with nitrogen for 12, 48 and 72 
hours at ambient temperature and pressure as shown in Table I. A separate 
batch of resin was not treated as a control. Female and male polypropylene 
mold halves were prepared by injection molding in steel master molds. The 
nitrogen treated resin was used for the mold halves representing this 
invention and some of the untreated resin was used for the mold halves 
used as controls and the other untreated resin was used below. Mold halves 
made from the nitrogen treated resins were contacted with nitrogen to 
remove additional oxygen for between 0.5 to 6 hours at ambient temperature 
and pressure as shown in Table I. Some of the mold halves made from 
untreated resin were contacted with nitrogen to remove the oxygen for 
between 16 and 72 hours at ambient temperature and pressure. 
Formation of Lenses 
Lenses were polymerized in the treated and untreated polypropylene molds 
prepared above. Monomer mix consisting of 85% by weight 2-hydroxyethyl 
methacrylate, 15% by weight glycerin, 0.3% by weight ethylene glycol 
dimethacrylate, and 0.2% by weight benzoin methyl ether was placed in the 
cavity of the female mold halves and the male mold halves were inserted to 
displace excess monomer mix. Pressure was applied to the molds to ensure 
proper seating and the monomer mix was polymerized using UV energy. All 
female mold halves and all male mold halves had identical configurations 
and all 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 occurrence of cosmetic defect. The percent 
of cosmetic defects seen by optical comparator inspection are shown in 
Table I. The controls (lenses molded from untreated resin and plastic mold 
pieces) are designated as letters A-L. Each example below represents a lot 
of contact lenses containing from about 50 to about 200 lenses. 
TABLE I 
______________________________________ 
Cosmetic Defects in Cast Molded Contact Lenses 
Treatment (hrs.) 
*Example Resin Molds % Defects 
______________________________________ 
A -- -- 24 
1 48 2 0 
2 48 2 0 
3 -- 72 0 
4 -- 72 0 
5 -- 72 0 
B -- -- 53 
6 12 2 6 
7 12 2 10 
8 -- 24 0 
9 -- 24 4 
C -- -- 11 
10 12 3 0 
11 12 3 0 
12 -- 24 0 
13 -- 24 0 
D -- -- 52 
14 12 3 4 
15 12 3 0 
16 -- 16 0 
17 -- 16 0 
E -- -- 28 
18 12 3 0 
19 -- 24 0 
F -- -- 41 
20 48 2 0 
21 -- 48 0 
G -- -- 39 
22 12 3 0 
23 -- 24 0 
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 
______________________________________ 
*One control lot was run for each scrics of test lots on a given day. 
As shown in Table I, the contact lenses molded from plastic mold pieces in 
which the oxygen was removed have significantly less cosmetic defects than 
those molded from the untreated plastic mold pieces. 
The present invention is not to be limited by the embodiments specifically 
disclosed herein. It should be understood that the scope of this invention 
includes all modifications, variations and equivalents which fall within 
the scope of the attached claims.