Note: Descriptions are shown in the official language in which they were submitted.
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k~-l~ A~D COMPOSITION FOR ~E MU~n~FACllnRE
OF ~H~ MTC T-
FIELD OF l~HE lNV~NLlON:
This invention relates to improved methods by which
plastic resins can be cured to form ophth~lmic lenses,
semifinished blanks and optical preforms. Ophthalmic
lenses o~ten have complex geometries, with certain
prescriptions having variations in thicknesses across
the optic area of greater than an order of magnitude.
Since the curing process is accompanied by shrinkage, a
key objective of curing process development efforts is
to be able to accommodate shrinkage without unduly
increasing the cure time. I have developed a curing
method for ophth~1 mi c lenses that uses visible light to
initiate cure, while nevertheless crea~ing a colorless
product.
~A~r-RO~ND:
Curing of organic polymerizable resins to form
ophthalmic lenses and semifinished blanks has
traditionally involved the use of thermal polymerization
initiators as described, for example, in US Paten~
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3,038,210, issued to Hungerford, et al., and US Patent
3,222,432, issued to Gr~n~p~ret~ More recently,
photocuring processes have been disclosed involving the
use of ultraviolet initiators, for example, US Patent
4,166,088, issued to Neefe, and US Patent Nos. 5,364,256
and 4,879,318 to Lipscomb. Photocuring processes have
allowed the development of cure cycles that are
considerably shorter than standard thermal curing
cycles.
In all cases, it is necessary to ensure that the
cure profile, which determines the rate of shrinkage,
allows the cure of the bulk resin to take place in a
controlled fashion while the surface still retains
substantial adhesion to the mold. In this way, the lens
does not undergo a prerelease, does not develop optical
aberratlons caused by the formation of local
heterogeneities in the resin mass due to uneven flow,
and does not develop surface defects or cracks due to
resin shrinkage.
In U.S. Patent No. 4,919,850, issued to me, I
disclose a two stage cure process involving the use of
ultraviolet polymerization initiators that allow the
resin to gel under a low level of ultraviolet
illumination. In this way, the initial cure rate is
25 maintained at a low level, until the resin mass becomes
a gel and mass flow ceases within the curing lens. This
is important, because the risk of developing optical
aberrations is highest at the initial stages of the
curing process when local exotherms can induce optical
30 aberrations through resin flow. After the material has
undergone gelation, the cure rate is accelerated by
increasing ultraviolet light intensity. Increasing Y
light intensity also serves to maintain the pace o~
curing as the initiator becomes depleted.
35 Alternatively, the cure rate is also accelerated in the
second stage by using W light of a shorter wavelength.
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A commercial version of this process has now been
introduced by the Rapidcast Corporation.
A disadvantage of the two stage process, as
disclosed in U.S. Patent No. 4,919,850 is that it
typically uses two curing chambers for efficient
implementation.
SU~$~RY OF I~E lNV~-~-lON:
An advantage of the present invention is that a
process is provided which can be efficiently implemented
using a single chamber.
Another advantage of the invention is that the
scope of the two stage curing process is expanded,
rendering it applicable to resin formulations covering a
wide range o~ chemical reactivities, f~nctionalities,
shrinkage properties, and thermal expa~sion
characteristics.
According to an embodiment of the present
invention, a curing method for ophthalmic lenses or
semi-finished lens blanks is described wherein a curable
resin is ~irst exposed to radiation in the wavelength
range o~ 400-800 NM. Then, the curable resin is
subjected to heat or radiation of di~erent wavelength
or intensity than that used in the ~irst step. The
polymerizable resin preferably comprises: (1) a first
photoinitiator that is activated by radiation in the
wavelength range 400-800 NM and (2) a thermal initiator
which is activated by heat, or a second photoinitiator
which is activated by radiation of different wavelength
or intensity than that used to activate the first
photoinitiator.
According to another embodiment of the invention,
an ophthalmic lens, semi~inished blank or optical
pre~orm is provided according to any of the methods
described or claimed herein.
The above and other objects, advantages and
embodiments will become readily apparent to those of
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skill in the art upon reading the description and claims
set forth below.
Unless indicated to the contrary, each reference
cited herein is incorporated by reference in its
entirity.
DETATT-~n DESCRIPTION OF TRE lNV~N-LlON:
I have developed a cure process that utilizes room
light in a first stage to initiate cure and reach the
gel state. A second stage of the cure process can
subsequently be completed, by application of thermal
energy, by application of W light, or both. The
initial curing stage may take place either directly
under room light, or in chambers employing visible light
bulbs.
Since polymerization initiators which are activated
by visible light are generally highly colored, it may
appear at first sight that their use would be
incompatible with the proposed application (i.e., to
make an ophthalmic lens which is preferably colorless,
or water white). Recently, a new class of
photopolymerization initiators has been commercialized
which begin as a colored species and are activated by
visible light, but upon activation form colorless
photodissociation products. I discovered that such
photoinitiators can be used to develop cure processes
for ophthalmic lenses and semi-finished blanks.
A preferred photoinitiator is BAPO, available from
Ciba Geigy Corp. This photopolymerization initiator is
actually a mixture of two photoinitiators,
Bisdimethoxybenzoyl Trimethylpentyl Phosphine Oxide
(25~, by weight) and 2-Hydroxy 2-Methyl 1-Phenyl
Propanone (75~ by weight). The phosphine oxide
derivative absorbs visible light in the wavelength range
400-450 NM range, and initiates polymerization of resins
incorporating acrylic, methacrylic, vinylic or allylic
derivatives.
=
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Under normal room illumination, the cure rate is
slow. Therefore, the mold assemblies do nct require
cooling or other temperature control to undergo
gelation. Nevertheless, precise temperature control
does produce a more uniform product and improves product
consistency and yield. If temperature control
mechanisms are provided, they should be employed to
control the temperature at or near room temperature,
i.e., at about 15~C to 35~C.
While not wishing to be bound by any particular
theory, it is believed that this photoinitiator works in
the following fashion. The phosphine oxide derivative
is activated and undergoes photodissociation under room
light, leaving the acetophenone derivative unaffected.
15 The phosphine oxide derivative undergoes bleaching on
photodissociation, so that the polymerizing resin mass
becomes less colored as polymerization progresses. Once
the resin has undergone gelation, the mold assembly is
placed in a chamber equipped with ultraviolet light
20 bulbs emitting radiation in the wavelength range of 300-
380 NM. The near ultraviolet radiation activates the
acetophenone derivative, causing the curing process to
become accelerated. At the same time, the dissociation
of the phosphine oxide derivative is completed,
25 completing the bleaching process. A residual faint
yellow hue can be corrected by an addition of a small
amount of a bluing additive, such as TINOPAL (available
from Ciba Geigy Corp.) to the resin formulation.
When W photoinitiators (such as the acetophenone
30 derivative) are used during the second stage of cure,
the mold assembly may be heated along a preestablished
r temperature profile, ultimately reaching a final
temperature in the range of about 90~-150~C, to complete
1 the cure process and to obtain a final lens product with
35 a glass transition temperature in the range of about
100~-175~C. If no heat is applied and the temperature
maintained at or near room temperature, the final
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product has a lower glass transition temperature (e.g.,
in the range of about 30~-50~C). In all cases, the cure t
process should be completed. The extent of the cure
process can be monitored, for example, by a differential
5 sc~nn-ng calorimetric analysis of the material after
cure. Whether heating is necessary to complete the cure
generally depends on the monomers used in the resin
formulation. Thus, if monomers used in the formulation
can form homopolymers which have glass transition
10 temperatures considerably above the room temperature
(15~-30~C), then an elevated temperature is desirable to
complete the cure process.
Alternatively, a phosphine oxide derivative may be
used which initiates cure under visible light as before.
15 However, a thermal polymerization initiator, such as a
peroxide, a peracetate, a percarbonate or an azo
derivative may be used to complete the second stage
(post-gel cure) by placing the mold assembly in a
thermal curing oven, typically a convection oven.
The two stage polymerization process described
above may be carried out in glass molds, in metal molds
or in a combination thereof. Metal molds with
reflective inner surfaces may be especially useful in
reflecting radiation back into the resin mass and
conducting excess heat away from the resin mass. Metal
molds may also be made thinner, and thus can have a
lower thermal mass than glass molds. Alternatively,
glass molds with metallized sur~aces may be employed for
resin formulations which require a metal mold for
adhesion and thus prevent prerelease during cure.
The two stage polymerization method can be employed
to produce lenses from resin formulations covering a
wide range of chemical reactivities, functionalities,
shrinkage properties, and thermal expansion
characteristics. Both monomers and oligomers may be
employed, and polymeric or small molecular weight
additives can be included to alter physical properties
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of the resin formulation, such as viscosity and sur~ace
energy, as well as chemical proper~ies of the
formulation, such as oxidative and photothermal or
hydrolytic stability.
