Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
sackground of the Invention
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;i~ ~ The present invention generally relates to the pro-
duction of composite glass-plastic articles, and is parti-
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_ cularly concerned with a process for the manufacture of
~ optically clear composites of laminated structure for a
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~ variety of applications.
The advantages of combining the light weight of clear
i plastics with the scratch resistance and chemical durability
~ of glass have long been recognized. U.S Patent No. 2,361>589
. 20 to Bennett et al., for example, describes durable, lightweightlenses formed by laminating thin glass sheets to the exterior
of plastic lenses.
_ Although composite products exhibiting the desired
~ phy~ical properties can be produced by lamination methods,
- the direct molding or casting of such composites i8 more
efficient and thus more attrac~.ive fr~m the commercial point
of view. One procedure for adapting such direct methods to
~ the manufacture of optical composites is described by Coenen
~ : in German Patentschrift 1,529,861.
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Among the optically clear plastics suitable for use in
_ composites of this type are certain thermosetting plastics
P~ exhibiting high shrinkage on curing. Examples of such
plastics are epoxy and allyl carbonate plastics. As sug-
gested in U.S. Patent No~ 3,382,137 to Schreiber et al.,
curing shrinkage such as is e~hibited by plastics of this
type can generate very high stresses in direct-cast glass-
` /: plastic composite article
_
The minimization of such stresses can be important for
optical applications. For example, in the case of the
plastic-plastic polarizing lenses described in U.S. Patent
No. 3,970l367 to Laliberte, excess stress was found to
introduce birefringence into the lenses. Careful position-
ing of the polarizing plastic e.lement within the optical
plastic lens body was therefore specified in order to
minimize the optical affects of any residual stress.
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~L-`~ Stress is also a problem in the case of cast glass-
plastic lenses having, or example, only a front surface
protected by a thin glass element. In this configuration,
_ 20 shrinkage of the plastic during curing can cause deformation
~ of the lens and cracking of the protective glass.
- Some thermosetting plastics generate only moderate
stress during shrinkage, and then fail due to plastic crack-
ing prior to full curing. In cast glass-plastic composites
produced from these plastics, locally adhering plastic areas
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bounded by a network of cracks in the plastic are usually
produced. Of course such composites are useless for optical
applications.
It is a principal object of the present invention to
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~ 30 provide a direct casting method for producing composite
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~ glass-plastic articles wherein residual stresses attribut-
_ able to shrinkage of the cast plastic are eliminated.
It is a further object of the inven~ion to provide a
direct casting method which can be used for the manufacture
of glass-surfaced plastic lenses exhibiting low residual
stress.
Other objects and advantages of the invention will
; ~ become apparent from the following description.
~''4~ Summary of the Invention
~!~Cs~
In accordance with the invention, a ca~t glass-plastic
article comprising at least one glass element indirectly
bonded to a cast-in-place high-shrinkage thermosetting
;`~f~`3 plastic element is provided by a method which includes a
consolidation heating step subsequent to the step of curing
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the thermosetting plastic element of the article. A thermo-
` ~ plastic adhesive coating provided between the glass and_~ .
plastic elements acts to control stress in the composite
arising from curing shrinkage and to finally bond the glass
and cured plastic elements together during consolidation.
, ~ 20 Through the use of this method, glass-plastic composites
- comprising low curing strength plastics and/or thin glass
elements surrounding or encased in plastic may be provided.
The method of the invention specifically comprises the
initial step of coatlng selected portions of the glass
element to be incorporated into the composite with a thermo-
plastic coating which can act as an adhesive. The surface
portions coated are those against which the thermosetting
plastic selected for the plastic element of the composite is
c~ ~ to be cast.
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~ It is desirable to select for the coating a thermo-
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plastic adhesive having a heat sealing temperature above the
minimum curing temperature of the thermosetting plastic
which is selected. Such an adhesive remains in a relatively
solid, nonreactive state during the curing of the ~hermosetting
plastic, preventing undesirable chemical interactions and
minimizing bonding and stress buildup between the plastic
and the adhesive.
1 After selected surace portions of the glass element
have been coated, the selected thermosetting plastic in
liquid form is cast against the coated surface portions of
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the glass, and the liquid plastic is then cured by heating
the glass, plastic and coating to a temperature below the
heat sealing temperature of the thermoplastic adhesive but
above the minimum curing temperature o~ the thermosetting
~ plastic. During this step, the plas~ic may be held in place
~- against the coated glass by gravity or using conventional
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j mold elements, gaskets and the like.
A~ter the thermosetting plastic has been cured, the
glass element, thermoplastic adhesive, and cured thermo-
~ setting plastic element are further heated to a temperature
_ above the heat sealing temperature of the thermoplastic
adhesive to finally bond the cured plastic element to the
adhesive-coated glass element. During this consolidation
~ heating step, the softened thermoplastic adhesive can also
release any stress generated between the glass and plastic
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due to curing shrinkage, thus providing an essentially
stress-free glass-plastic composite article.
A~ter consolidation has bPen completed, the composite
~'~ 30 article is cooled to ambient temperature. Diferences in
thermal expansion between the glass and plastic elements
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give rise to some stress in the composite as it is cooled;
however, these stresses are low and thus do not ordinarily
-?~ affect the physical or optical properties of the composite.
Description of the Drawings
~ The invention may be further understood by reference to
_ the drawings, wherein:
~ Fig. 1 is a schematic elevational view in cross-section
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of one type of cast glass-plastic composite article provided
in accordance with the invention, together with mold and
gasket elements utilized during the casting of the article;
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O~Fig. 2 is a graph of treating temperature versus time
as a composite glass-plastic article is cycled through
illustrative plastic curing and consolidation heating steps
of the method of the invention; and
Fig. 3 is a graph of plasti.c shrinkage versus time
_during the plastic curing and ~tress-release-heating steps
shown in Fig. 2.
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Detailed Description
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~$~j The method of the present invention is particularly
- 20 useful for the production of optical elements such as light-
weight glass-plastic lenses. Although the configuration of
glass elements for such lenses may vary, the elements are
~ typically quite thin ~e.g., of a thickness up to about 0.020
inches). They may also be spherically and/or cylindrically
curved, with the degree of curvature having a direct effect
upon the refracting characteristics of the lens. Neverthe-
_ less, through the proper selection of a thermoplastic coat-
ing material, the curvature characteristics of the glass
element may be fully reproduced in the composite even when
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-t plastics exhibiting very high shrinkage upon curing are ;~
_ employed.
~ For applications such as lenses, it is important to use
G~ an optically clear thermoplastic adhesive to provlde the
thermoplastic coating on the surface of the glass element.
Although the thickness of the coating is not critical, the
, thermoplastic must be one which can be conveniently applied
in uniform thickness in order to avoid optical distortion in
the product.
An example of a thermoplastic adhesive with desirable
optical properties and good stress-release and heat sealing
characteristics is polyvinyl butryral. Coatings of this
thermoplastic may be conveniently provided on glass using
plastic sheets, or by coating the glass with a solution of
polyvinyl butryral in a suitable solvent by dipping, spray-
ing, brushing, spinning or the like. Polyvinyl butryal also
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has a heat sealing temperature sufficiently high to remain
essentially non-adhesive at the curing temperatures of
¦ severaI optical-quality, high-shrinkage thermosetting plastics. ! 20 For the purpose of the present description, the ~eat sealing
temperature is that temperature at which the thermoplastic
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becomes soft andlor reactive enough to bond to the thermo~
setting plastic.
Among the thermosetting plastics with very desirable
= optical properties for lens applications are the allyl
~ ,
.~.~ diglycol carbonate plastics, e.g., a plastic produced from
diethylene glycol bis(allyl carbonate) resin. This resin,
commercially available and commonly known as CR~39~ resin,
~a typicall~ exhibits a shrinkage on curing of about 14% by
volume. It has a minimum curing temperature of about 70C.
(for sot cure), and is ordinarily heated to a peak ~uring
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temperature of 95C. to complete the curing process and to
_ provide a hard, strong plastic element.
~ The casting of the thermosetting plastic against a
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coated glass element is satisfactorily accomplished using
the coated glass together with supplemental mold and gasket
members to form a cavity into which the plastic in liguid
form is poured or injected. One assembly suitable for this
purpose is illustrated in Fig. 1 of the drawing, which shows
a thermosetting-plastic-filled cavity defined by a mold, a
ring gasket, and a curved sheet glass element having an
interior thermoplastic coating. The curved sheet glass
~.`'~ element is in turn supported by a second mold.
No special heating procedures are required for curing
the thermosetting plastic while in contact with the coated
glass. Thus in a case where a diethylene glycol bis(allyl
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carbonate) resin is injected into the cavity, the filled
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mold assembly is placed in a curing oven and heated accord-
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- ~ ing to a time/temperature schedule conventional or the
curing of such a resin.
_ 20 One example of a suitable curing schedule, shown in
Fig. 2 of the drawing, ~omprises slow heating to a curing
. temperature of 95C. over a time interval of 14 hours, after
which the resin is essentially completely polymerized ~o a
hard clear plastic element. The schedule shown is for a 4
mm-thick CR-39~ plastic element combined with a polyvinyl
butyral thermoplastic coating having a heat sealing tempera-
ture above the maximum plastic curing temperature of 95C.
During the consolidation step which immediately follows
_ the curing step, the cured plastic element, glass element
and thermoplastic coating are consolidated by briefly
heating them above the heat saaling temperature of the
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_~ thermoplastic coating. This step is illustrated in Fig. 2
_ of the drawing for the case of a polyvinyl butyral coating
~ ,J having a heat sealing temperature of about 120C.
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~2~ At temperatures below its heat sealing point, the
polyvinyl butyral thermoplastic coating does not bond well
to allyl cliglycol carbonate plastic, so that stresses due to
plastic shrinkage are largely avoided. Such shrinkage can
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~ ~ be substantial, as exemplified by the 14% curing shrinkage
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illustrated in Fig. 3 of the drawing for a thermosetting
~t;~ 10 allyl diglycol carbonate resin cured in accordance with the
curing schedule of Fig. 2.
At heat sealing temperatures, the soft polyvinyl butyral
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layer can flow to relieve any residual stress and permanently
bond the assembly into an integr,al glass-plastic co~posite
article. The only stress then present in the composite as
i~ is finally cooled to ambient temperature is that arising
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~-~ ou~ of the differences in thermal expansion between ~he
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glass and plastic elements at temperatures below the softening
temperature of the thermoplastic layer.
The invention may be further understood by reference to
the following detailed examples illustrating some preferred
~ procedures for the production of composite lenses in accord-
ance therewith.
Example A
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A glass element for a glass-plastic composlte lens,
consisting of a round, spherically-curved glass disc about
0.010 inches in thickness, having a diameter of 70 mm and a
surface curvature of 6.25 diopters, is thoroughly cleaned
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with acetone, dionized water and alcohol. A solution of
polyvinyl butyral is prepared consisting of 10 parts plastic
solids and 90 parts solvent by weight. The solids component
of the solution consists of 60% by weight of Butvar B-98
polyvinyl butyral powder, available from ~he Monsanto
Company, and 40% by weight of 3GH plasticizer, available
from the Union Carbide Corporation. The solvent component
of the solution consists of 22.5% diacetone alcohol, 22.5%
n-butyl alcohol, 10% ethyl alcohol and 45% xylene by volume.
The polyvinyl butyral solution thus prepared is sprayed
onto the concave surface of the curved glass lens element
until a solid thermoplastic layer about 0.005 inches thick
is formed. The convex (uncoated surface) of the glass is
then placed directly against a curved glass-ceramic support-
ing mold, a thermoplastic rubber rin~ gasket about 4 mm in
thickness and having an inside diameter of about 60 mm is
placed on the coated surface of the glass, and the open
cavity thus provided is filled with prepolymerized CR-39~
thermosetting resin. The convex surface of a second curved
glass-ceramic mold is then placed over the resin and plastic
gasket to form a closed cavity, substantially as shown in
Fig. 1 of the drawing.
This filled mold assembly is clamped, placed in a
curing oven, and heated in accordance wi~h the heating ~-
schedule shown in Fig. 2 of the drawing, that heating schedule
comprising controlled heating to 95C. over a 14 hour interval
to cure the CR-39~ resin to a hard plastic, followed by
heating to 120C. for 10 minutes to heat-seal the cured
plastic to the polyvinyl butyral coating. Finally the mold
assembly is cooled to room temperature.
The mold assembly is opened and the consolidated com-
posite lens removed and examined. The lens is of opticalquallty, free of glass and plastic defects, and the curvature
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of ~he fron~ (glass-clad) surface of the lens substantially
conforms to the 6.25 diopter curvature of the glass lens
element used in fabrication.
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Example B
A glass lens element having a size and configuration
essentially id~ntical to the lens element described in
Example A above is cleaned and positioned in a concave
glass-ceramic supporting mold as therein described. A clean
sheet of thermoplastic polyvinyl bu~yral, consisting of a
section of 10 mil-thick Saflex SR 10 polyvinyl butyral film
from the Monsanto Co., is placed over the glass element,
covered with a thin sheet of polyethylene terepthalate
acting as a release layer, and finally covered with a 10-lb. -
weight having a curvature matching that of the supporting
mold and glass lens element.
This assembly is placed in a vacuum o~en and heated at ~ -
120C. under a partial vacuum (28 inches of mercury) to ; `
remove trapped air from the assembly and to preliminarily
bond the polyvinyl butyral sheet to the glass lens element.
It is then taken out of the vacuum oven and the weight and
polyethylene terepthalate sheet are removed from the glass-
thermoplastic sub-~ssembly.
This sub assembly is then gasketed, filled with thermo-
setting ~R-39~ resin, covered with a curved glass-ceramic
mold, clamped, and the~mally processed to achieve plastic
curing and consolidation of the glass and plastic lens
elements in accordance with the procedure described in
Example A above. The composite lens produced by this process
is again of optical quality, free of glass and plastic
defects and having a front surface curvature corresponding
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`i to the initial curvature of the glass element used to form
the front surface of the lens.
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Of course, the foregoing examples are merely illus-
trative of procedures-by which glass-plastic composite
articles may be provi~ed in accordance with the invention.
Obviously, various other processing techniques may be
~ resorted to in fabricating lenses and other composite glass-
plastic articles within the scope of the appended claims.
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