GB2051819A - Epoxy resin adhesive composition - Google Patents

Abstract

An adhesive useful in bonding a foil-type strain gage comprises a cross-linked resin layer resulting from the admixture of a selected epoxy resin and a predetermined amount of trimetallic anhydride as a hardener, enough tetrahydrofuran solvent as needed to facilitate proper application of the two-component resin composition. Upon elevation to a moderate curing temperature for a relatively brief time, sufficient cross-linking occurs to permit the installation to remain set in place without clamping or the like, and somewhat higher-temperature post cure then assures that the installed gage will be highly stable and, in particular, will exhibit very low creep.

Description

SPECIFICATION Strain gage bonding The present invention relates to improvements in the bonding of electrical-resistance strain gages, such as foil gages having non-porous carriers, into stable low-creep relationships with substrates, and, in one particular aspect, to adhesive-coating compositions and practices involving long-pot-life epoxy mixtures of resin, hardener and solvent materials which may be readily applied and cured to transmit accurately to and through a foil-gage carrier the surface strains exhibited by a loaded specimen.

Precision measurements characterizing such phenomena as force, torque, weight and pressure are commonly performed with the aid of strain gages having grids of elongated fine conductive strands which vary in resistance as the underlying specimen surfaces to which they are affixed undergo distortions related to such phenomena. Both wire and foil type strain gages have found widespread applications forthose purposes, and lend themselves well to manufacture in highly miniaturized composite forms wherein the delicate gage strands and their associated end and lead tabs are first permanently united with a thin insulating backing sheet or so-called "carrier" which electrically isolates and supports them and allows them to be handled conveniently.In some instances, wire-type gages have been fashioned atop impregnated-paper carriers, which could in turn be affixed to a specimen surface by way of a simple cellulose cement, and, in others, the gage wire has been molded in a thermosetting phenol resin; the latter type of "bakelite" gages have required the use of heat-hardening phenol-resin cement to secure them to test specimens properly.

Foil gages are modernly produced atop carriers of such materials as polyimide, glass-reinforced phenolic, nitro-cellulose impregnated paper, and epoxy phenolic. Whatever the carrier may be, its bonding, uniformly and closely and stably to the specimen undergoing loading is critical to accuracy, because the gage performance simply cannot be better than the qualities of that bonding will allow.

In addition to the aforementioned cellulose and phenol-resin cements, it has been known to employ a two-component adhesive comprising a separately-packaged resin and activator which will yield a self-polymerizing mixture when combined at room temperature and which will more quickly develop its best strength when further cured by heating. The Strain Gage Primer, by C. C. Perry and H. R.

Lissner, McGraw-Hill Book Company, Inc., N.Y., 1955, pages 29-38. Epoxy resin has also been described for use in a nontacky room-temperature adhesive layer which may be made flowable for bonding purposes upon heating. U.S. Patent No.

3,609,624.

Among the serious problems which attend the cementing or bonding of foil-type gages into place are those concerned with making the "glue line" as thin as possible, so that the special advantages of the minute-thinkness foil layer and the thin carrier will not be lost in their unwanted large separation or spacing from the test-piece surfaces by the underlying adhesive. Further, that thin adhesive should not contain either lumps or voids which would tend to vary spacing or distort the gage or leave any part of the carrier unsecured, and it should both spread uniformly and readily "wet" the carrier and specimen surfaces with which it is intended to cooperate.

Very importantly, such an adhesive ought not to be responsible for any significant gage "creep"; that term identifies slip of a strain gage relative to the test surface with which it should be united, and possible softening and/or other unintended yielding involving the adhesive material which could result in such slip and cause related measurement errors must therefore be minimized insofar as possible. Creep is best suppressed by an adhesive which cures essentially fully, rather than retaining any significant residual thermoplastic characteristics, and which does not involve such high cross-linking temperatures as to risk damage to foil, carrier, lead connections, and the like. Bonding materials which must be freshly mixed and used quickly before their hardening can interfere may of course be quite troublesome to the user.

By way of a summary account of practice of this invention in one of its aspects, there is provided a two-co m po nent type bonding agency, for foil-type strain gages and the like, involving a separate epoxy resin part and a proportioned trimellitic anhydride hardener part which amounts to about 35-45% by weight, relative to the resin part; tetrahydrofuran is added when needed as a solvent, in equal quantities to both parts, sufficientto enable their resulting mixture to have flow and wetting qualities appropriate to its being evenly coated thinly upon a test surface and underside of a carrier by a brush or other applicator.Upon mixture of its parts, the bonding material has a significant pot life, which may be as long afourweeks, within which it may be used if kept uncontaminated and not allowed to evaporate its solvent excessively and not exposed to elevated temperatures. Once the bonding mixture has been applied to specimen and carrier, the two are clamped, preferably with the aid of suitable protective pressure-distributing padding, and a first cure is effected at about 250"F for about two hours; thereafterthe gage will remain well set in place of its own accord, and a final thermosetting-type cure and substantially full cross-linking, which insures that the thin adhesive layer is both hard and of a fixed low modulus of elasticity appropriate to avoidance of creep, is assured by a post cure for about four hours at a temperature at least about 50"F higher than before.

Accordingly, it is one of the objects of this invention to provide unique and improved bonding for foil strain gages backed by non-porous carriers, and the like, which can be achieved readily and which exhibits low creep.

Another object is to provide novel and advantage ous two-component strain gage adhesives which lend themselves to substantially full cross-linking at relatively low curing temperatures and to related improvements in restraint of creep, and which pos sess sustained pot life and low viscosity favoring their simple and effective preparation and application.

Still further, it is an object to provide epoxy strain gage adhesives wherein advantages associated with soundness of cure and ease and convenience of handling are promoted bytrimellitic anhydride hardener proportioned in a special weight relation to resin, and wherein tetrahydrofuran acts as a solvent to adjust viscosity for application purposes orto condition the hardener into a convenient liquid for.

Although the features and aspects of thins invention which are considered to be novel are set forth in the appended claims, further details as to preferred practices and as to further objects and advantages thereof may be most readily comprehended with reference to the following description taken in connection with the accompanying drawings, wherein: FIGURE lisa top plan view of a foil strain gage supported by its carrier and bonded thereby to an underlying surface of a deformable test piece; FIGURE 2 is a transverse cross-sectional view taken along section line 2-2 in FIG. 1; and FIGURE 3 represents in a block-diagrammed convention certain preferred bonding practices and related materials.

Referring to the drawing, wherein like reference characters designate corresponding parts in the differentviews, and more particularlyto FIGS. 1 and 2 thereof, there is shown a foil-type electricalresistance strain gage having a common grid array 4 of parallel conductive metal strands or filaments 5 serially joined by end tabs 4' and cooperating with enlarged terminals with which external leads may be soldered. The carrier 6, upon which the strands are photo-etched utilizing procedures and substances now commonly known in the trade, is preferably an exceedingly thin film of a known non-porous material such as polyimide, phenolic, or epoxy, and the delicate strands are intimately united in permanent abutments with the top of the carrier in their asformed state.The gage strands are of course intended to be varied in effective resistance as their cross-sections change with strand elongations and contractions as they follow elastic distortions of a surface 7 caused by loading-induced tension and compression effects exhibited by the test specimen 8; by way of example, the latter may be in the form of a steel column, beam, shaft or diaphragm.

Optimum measurements are promoted when the gage is as uniformly close and thoroughly bonded to the test surface 7 as possible, and it is well established that the an adhesive layer, such as 9, by which a gage carrier is secured to a test surface is a critical link in the chain of factors which can seriously affect measurement accuracy and long-term reliability and stability. In particular, errors attributable to creep will develop if layer 9 should allow relative slip to develop between the specimen surface and gage, due to lack of hardness of that bonding layer.At the same time, it should be possible for such an adhe siveto be applied readily to metal specimens and non-porous carriers, with good wetting and lack of either voids or lumps, and for that adhesive to be prepared simply and without undue cost and with a sustained pot life promoting ease, convenience and economy of use in affixing strain gages. Further, the resulting gage installations ought to involve relatively short times and reasonable temperatures for curing which will permanently fix the bonding and assure substantial freedom from later creep, while the installations should neverthelessthereafter be capable of withstanding very high temperatures (examples: 500-550"F) without involving thermoplastic-type softening or other damage.

Adhesives which will satisfy such requirements and which are in accord with the present teachings have in part had to be determined empirically, because technical certainties and reliable predictions about what would and would not work properly tend to be elusive in this field. However, such improved adhesives involve, as one component, a selected epoxy resin or a thoroughly-mixed combination of such an epoxy resin and a specific solvent, tetrahydrofuran. A separate second-part component is a specific hardening or curing agent, trimellitic anhydride, in a predetermined weight relation to the resin, with or without the same solvent, tetrahydrofuran.

Both components are pre-prepared and packaged separately, and in FIG. 3 the preparation ofthe epoxy resin component, at 10, preferably involves the separation of a predetermined quantity of a liquid resin such as the Shell Chemical Company's EPON-828 or of a melted and subsequently-dissolved (in tetrahydrofuran) resin such as the Cresol Novolac Resin ECN-l 235 of the Ciba Geigy organization. Preparation of the second component, the trimellitic anhydride, at 11, involves either the pulverizing of trimellitic an hydride flakes into a dry powder (37 micron size or under) or the dissolving of the flakes into the tetrahydrofuran solvent.When the adhesive is to be applied, the two components are thoroughly mixed, at 12, with the weight oftrimellitic an hydride in the mixture being kept to at least about 35% and not in excess of about 45% of that of the resin. Thereafter, during the pot life of the mixture, which may be from about 24 hours to four weeks, depending upon the resin preparation and the tetrahydrofuran solvent usage, the mixture is applied to a strain-gage carrier and clean test-specimen surface, as designated at 13, by spatula, brush or other suitable applicator. At 14, the gage is seen to be clamped against the specimen and an initial curing is effected for about two hours at about 250"F. Subsequent cooling, removal of clamping pressure, and post-curing for about four hours at a temperature at least about 50 F higher than before, as at 15, results in a uniformlythin, hard and thoroughly-bonded layer, such as 9, which will reliably and stably maintain the gage in precision measurement relationship with the test specimen.

Example A A two-component adhesive for strain-gage bonding, which has excellent moisture and chemical resistance and which will function satisfactorily over a temperature range of -320" to 500 F, is made using a viscous clear liquid epoxy resin and a trimellitic an hydride white powder as the two components. The liquid epoxy resin is selected to have a viscosity in the range 100-160 Poises and an epoxide equivalent of 185-192, such as an EPON-828 resin which is available commercially from Shell Chemical Company or Miller Stephenson Co., Inc., and is preferably seal-packed in small containers fiiled with a predetermined weight of the resin.The hardener is formed as a dry powder, by pulverizing trimellitic an hydride flakes to particles of about 37 micron size or under, and that powder is separately dry-packed in small containers each holding a weight of trimellitic an hydride which is about 35-45% of that of the liquid resin with which it is to be mixed. If significantly less than a 35% weight relationship exists, the resulting mixture tends to have an undesirably long hardening time, and, in addition, there is likely to be incomplete cross-linking or only partial curing, such that the resulting bonding layer may exhibit thermo- plasticity or softening which would allow creep. On the other hand, if more than about a 45% weight relationship exists, the bonding layer can exhibit reduced strength in shear and undesirable susceptibilityto fracture.

Typically, small plastic pouches ofthe liquid resin component contain about 7 grams of the epoxy and the cooperating pouches of powder about 3 grams of trimellitic anhydride. Larger pouches with about 35 grams of resin cooperate effectively with powder pouches holding about 15 grams. During their packagings, neither the liquid or powder should be exposed to an environment with relative humidity exceeding 50%, and, even with humidity below that, exposure times before dry sealing should not exceed about an hour; moisture absorption is detrimental to this epoxy system and must not exceed about 0.01%.

The two components are thoroughly mixed, as by kneading, when the adhesive is about to be used to bond a gage to a test surface, and within 24 hours thereafter (which is the recommended pot life, somewhat extendable if it is kept at 40 F) the adhesive is applied in a light coat, smoothed to a uniform thickness, on the back of the gage carrier and on the surface area being gaged. The gage is then applied to the surface, both coats being in intimate contact, and a silicone rubber padding orthe like is then placed over the gage, with a thin Teflon sheet intervening to prevent sticking of the padding and adhesive coating. Clamping pressure of 20-30 psi is then applied, and the installation is cured at about 250 F for about two hours.Upon being cooled down to room temperature, the padding and Teflon parting film are removed, and it will be found that the gage is physically well set in place. A post cure follows, for about four hours at a temperature about 50 F higher than either the maximum operating temperature intended for the installation or the preceding curing temperature (whichever is higher), or for about four hours at about 450 F. Thereafter, the installed gage can be considered ready for use for stable and reliable measurements as referred to hereinabove.

Example B A second two-component adhesive, which will withstand even highertemperatures (to 550 F) and will offer a highly desirable lengthy pot life (to about four weeks after mixing) and which involves only liquid-form components lending themselves to their brushed-on applications or the like, is prepared using for one component a dissolved transparent semi-solid resin which has been melted and dissolved into tetrahydrofuran. A semi-solid resin material suitable for such purposes is a Cresol Novolac Resin having a molecular weight of 540, weight per epoxide of 200, an Epoxide Type Functionality of 1.7, and a melting point of about 95 F.ECN-1235 resin having satisfactory qualities is available commercially from Ciba-Geigy Corp., N.Y., and is first melted at about 225 F until completely liquified and then cooled to room temperature. Thereafter, tetrahydrofuran solvent (Fisher Certified) is added in a weight relationship to the resin of 1.4, and stirring is effected until the resin is completely dissolved. The hardener is prepared from white trimellitic anhydride flakes, such as those which are available commercially from Amoco Chemicals Corporation, Ill., by adding the tetrahydrofuran solvent in a 3.5 weight ratio to the flakes and stirring the mixture until the flakes are completely dissolved.Both components are separately packaged, for example in small bottles containing 12 grams by weight of the resin component, and, in the case of the hardener, 9 grams by weight of that component. When such pre-packaged items are mixed in anticipation of use to bond foil strain gages onto specimens, the weight ratio of trimellitic anhydride to resin is 40%, which is within the 35-45% range referred to in Example A, above, and for the same reasons.

The two components as freshly mixed together offer a useful pot life of about four weeks, when kept closed and free of contaminants. Gage carrier and specimen surfaces are lightly coated, as by a brush, and these coatings are allowed to dry to a tacky condition, for between about 5-10 minutes, before being joined. Thereafter, the clamping, curing, cooling and unclamping, and post-curing, are as set forth in connection with the preceding Example A, except that a higher clamping pressure of about 50-70 psi is preferred.

Claims (22)

1. The method of producing a creep-resisting bonding to a specimen surface of a non-porous member such as the carrier for a foil strain gage which comprises preparing a first separate quantity of epoxy resin in liquid form, preparing a second separate quantity of trimellitic an hydride, mixing the epoxy resin and trimellitic anhydride together to form a flowable mixture in which the trimellitic anhydride is in about a 35-45% weight relation to the epoxy resin, applying the flowable mixture in a thin layer between the specimen surface and the back of the gage carrier, and curing the mixture while ma intaining the carrier and specimen in a predetermined adjacent relationship.

2. The method as set forth in claim 1 wherein the preparation of a first quantity of epoxy resin includes packaging a liquid epoxy resin having a viscosity of about 100-160 poises and wherein preparing the second quantity includes pulverizing trimellitic an hy dride flakes into a fine powder and packaging same.

3. The method as set forth in claim 2 wherein the liquid epoxy resin has the characteristics of EPON-828 resin, and in which the flowable mixture is applied within about 24 hours after mixing ofthe first and second quantities.

4. The method as set forth in claim 1 wherein the preparation of a first quantity of epoxy resin includes completely melting a semi-solid cresol novolac resin, and wherein preparing the second quantity includes dissolving drytrimellitic anhydride in tetrahydrofuran.

5. The method as set forth in claim 4 wherein the preparation of the first quantity further includes dissolving the melted cresol novolac resin in tetrahydrofuran after it has been cooled to substantially room temperature.

6. The method as set forth in claim 5 wherein the weight ratio of tetrahydrofu ran to the melted resin is about 1.4, and the weight ratio of tetrahydrofuran to the trimellitic anydride is about 3.5, and wherein the curing is attemperatureswhich drive offthetet- rahydrofuran solvent.

7. The method as set forth in claim 5 wherein the cresol novolac resin has the characteristics of ECN-1235 resin, and in which the liquid mixture is applied within about four weeks after mixing of the first and second quantities.

8. The method as set forth in claim 1 wherein applying the flowable mixture includes spreading a light coatthereof on each of the specimen surface and the back of the foil gage carrier, and wherein the curing includes clamping the carrier-backed gage to the specimen under pressure and holding the com bination at an elevated temperature.

9. The method as set forth in claim 3 wherein the curing includes clamping the carrier-backed gage to the specimen under a pressure of about 20-30 psi for about two hours at a temperature of about 250 F and then cooling and unclamping the gage and then post-curing for about four hours at a temperature about 50 F higher than the intended operating temperature orthan the curing temperature, whichever is greater, or at about 450" F.

10. The method as set forth in claim 7 wherein the curing includes clamping the carrier-backed gage to the specimen under a clamping pressure of about 50-70 psi for about two hours at a temperature of about 250"F and then cooling to room temperature and unclamping the gage and then post-curing for about four hours at a temperature about 50 F higher than the intended operating temperature or than the curing temperature, whichever is greater, or at about 4500 F.

11. A two-component adhesive formulation for substantially non-creep bonding of a non-porous strain-gage unit to a specimen surface, comprising a pair of cooperating separate proportioned quantities of epoxy resin in liquid form and a trimellitic anhydride hardener, the quantity of said trimellitic anhydride hardener being in a weight relationship to the epoxy resin of about 35-45%, whereupon mechanical mixture of said quantities will yield a flowable mixture which may be applied as a thin layer between the gage unit and specimen surface within a relatively long period after mixing and which will bond them stably upon being cured at elevated temperatures.

12. A two-component adhesive formulation as set forth in claim 11 wherein said resin in said liquid form has the characteristics of EPON-828 resin, and wherein said hardener comprises trimellitic anhydride pulverized into powder form with particles not in excess of about 37 micron size.

13. Atwo-component adhesive formulation as set forth in claim 11 wherein the said resin in said liquid form includes epoxy resin which is the cooled product of a melting of semi-solid cresol novolac resin dissolved in tetrahydrofuran, and wherein said hardener is a liquid product of a dissolving of dry trimellitic anhydride in tetrahydrofuran.

14. Atwo-component adhesive formulation as set forth in claim 13 wherein said resin in said liquid form has the characteristics of ECN-1235 resin, and wherein substantially the same amount by weight of said tetrahydrofuran is present in each of said proportioned quantities.

15. Acreep-resisting strain gage installation comprising a foil gage supporated by a non-porous carrier and having a hard thin layer of fully crosslinked epoxy resin bonding the back of the carrier in adjacent relation to a metal surface of an elastically deformable sensing element, said layer being formed by a mixture of liquid epoxy resin with trimellitic anhydride in a 35-45% weight relation thereto, said mixture in said layer having been initially cured at an elevated temperature with pressure being exerted between the carrier and specimen and then having been post-cured at a further-elevated temperature without such pressure to complete cross-linking therein.

16. Acreep-resisting strain gage installation as set forth in claim 15 wherein said layer is formed by a mixture of liquid epoxy resin having the characteristics of EPON-828 resin with trimellitic anhydride in dry powdered form.

17. A creep-resisting strain gage installation as set forth in claim 15 wherein said layer is formed by a mixture of epoxy resin in liquid form which includes the cooled product of a melting of semisolid cresol novolac resin dissolved in tetrahydrofuran, the said semi-solid resin having the characteristics of ECN-1235 resin, and the tetrahydrofuran having been driven off in the curing.

18. Acreep-resisting strain gage installation as set forth in claim 15 wherein the initial curing under pressure of at least 20 psi is effected at about 250 F for about two hours and the post-curing is effected in about four hours at a temperature of about 50 F greater than the intended operating temperature or than the curing temperature, whichever is greater, or at about 4500 F.

19. The method as set forth in claim 1, substantially as hereinbefore described in Example A or B or with reference to the accompanying drawings.

20. A carrier and specimen when bonded together by the method as set forth in any one of claims 1 to 10 and claim 19.

21. Atwo-component adhesive formulations as set forth in claim 11, substantially as hereinbefore described in Example A or B.

22. A creep-resisting strain gage installation as set forth in claim 15, substantially as hereinbefore described with reference to the accompanying drawing.