CA1114115A

CA1114115A - Direct casting method for producing low-stress glass/plastic composites

Info

Publication number: CA1114115A
Application number: CA311,087A
Authority: CA
Inventors: Anton A. Spycher
Original assignee: Corning Glass Works
Current assignee: Corning Glass Works
Priority date: 1977-11-04
Filing date: 1978-09-12
Publication date: 1981-12-15
Also published as: GB2008480B; GB2008480A; JPS6259649B2; FR2407898A1; FR2407898B1; JPS5473859A; DE2846233C2; DE2846233A1

Abstract

DIRECT CASTING METHOD FOR PRODUCING
LOW-STRESS GLASS/PLASTIC COMPOSITES

Abstract of the Disclosure A glass-plastic composite combining a glass element with a high-shrinkage thermosetting plastic is provided in accordance with a direct casting process comprising a con-solidation heating step, subsequent to plastic curing and shrinkage, during which a thermoplastic coating between the glass element and the cured plastic bonds the assembly into a unitary low-stress composite.

Description

sackground of the Invention . .." ~
;i~ ~ The present invention generally relates to the pro-duction of composite glass-plastic articles, and is parti-~
_ cularly concerned with a process for the manufacture of ~ optically clear composites of laminated structure for a :'~
~ 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 ' ~J"I ~`r 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 -~ 30 provide a direct casting method for producing composite , .. ........ . .,.. ,.~.~ . , i .~
~ 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 tn~
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.
=51 ~ It is desirable to select for the coating a thermo-,~.:, ., 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 ~ ~.
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 -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 -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 -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;
~` "; 1 , 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.
i Detailed Description _ . _ ~$~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 .. i ~.~
-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 , ~. .
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 ;'l ~ . , 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 , ,, 1 - - -temperature of 95C. to complete the curing process and to _ provide a hard, strong plastic element.
~ The casting of the thermosetting plastic against a .... ..
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 ... . ~
carbonate) resin is injected into the cavity, the filled , ~ ~,~,Aj) mold assembly is placed in a curing oven and heated accord-~7 - ~ 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 ; .~. '~ .
~ ~ be substantial, as exemplified by the 14% curing shrinkage -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 .; ~, 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 . .- 2., ~-~ ou~ of the differences in thermal expansion between ~he __ .
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
-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 -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 *Trade Mark 9 ~ .
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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 *Trade Mark -10-' .

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`i to the initial curvature of the glass element used to form the front surface of the lens.
-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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Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A direct casting method for producing a composite article comprising a glass element bonded to a high-shrinkage thermosetting plastic element, with low residual stress between the glass and plastic elements, which comprises the steps of:
coating selected surface portions of the glass element with a thermoplastic adhesive having a heat sealing tempera-ture above the minimum curing temperature of the thermosetting plastic;
casting the thermosetting plastic in liquid form against the surface portions of the glass element which have been costed with the thermoplastic adhesive;
curing the thermosetting plastic by heating the plastic, the glass element and the thermoplastic adhesive to a tempera-ture below the heat sealing temperature of the thermoplastic adhesive but above the minimum curing temperature of the thermosetting plastic;
consolidating the cured thermosetting plastic, thermo-plastic adhesive, and glass element into a unitary glass-plastic composite by heating them to a temperature above the heat sealing temperature of the thermoplastic adhesive.

2. A direct casting method for producing an optical element composite article, comprising a glass element bonded to a high-shrinkage thermosetting plastic element, with low residual stress between the glass and plastic elements, which comprises the steps of:
coating selected surface portions of the glass element with an optically clear thermoplastic adhesive having a heat sealing temperature above the minimum curing temperature of the thermosetting plastic;
casting the thermosetting plastic in liquid form against the surface portions of the glass element which have been coated with the thermoplastic adhesive;
curing the thermosetting plastic by heating the plastic, the glass element and the thermoplastic adhesive to a tempera-ture below the heat sealing temperature of the thermoplastic adhesive but above the minimum curing temperature of the thermosetting plastic;
consolidating the cured thermosetting plastic, thermo-plastic adhesive, and glass element into a unitary glass-plastic composite by heating them to a temperature above the heat sealing temperature of the thermoplastic adhesive.

3. A method in accordance with claim 1 or 2 wherein the glass element ranges up to about 0.020 inches in thickness.

4. A method in accordance with claim 1 or 2 wherein the thermoplastic adhesive consists of polyvinyl butyral.

5. A method in accordance with claim 1 or 2 wherein the high-shrinkage thermosetting plastic is an allyl diglycol carbonate plastic.