U.S.D.A. FOREST SERVICE RESEARCH PAPER

FPL 126 MARCH 1970

TENSILE STRESS-STRAIN BEHAVIOR OF

FLEXIBILIZED EPOXY ADHESIVE FILMS

U. S. Department of Agriculture Forest Service

Forest Products Laboratory Mad son, Wis.

ABSTRACT

Films of several flexibilized epoxy adhesives were cast and the tensile stress- strain behavior was determined. The modulus of elasticity, maximum tensile s t ress , strain at failure and work to failure were measured. The maximum s t ress and the modulus of elasticity decreased in an approximately linear man­ner with flezibilizer content. A polysulfide flexibilizer definitely increased values for both strain at failure and work to failure. Polyamide, flexible epoxy and m e r c a p t a n - terminated polybutadiene flexibilizers showed very little effect on strain at failure and work to failure. These results demonstrate that the me­chanical properties of epoxy adhesives can be manipu­lated. If the required mechanical performance for a specific end use can be defined, it should be possible to engineer a suitable epoxy formulation for a specific use.

TENSILE STRESS-STRAIN BEHAVIOR OF FLEXIBILIZED EPOXY ADHESIVE FILMS

By

W. T. SIMPSON, Forest Products Technologist and V. R . SOPER, Physical Science Technician

Forest Products Laboratory,1

Forest Service U.S. Department of Agriculture

INTRODUCTION to prevent swelling, or use a deformable adhesive that would move with the wood. The disadvantage

Epoxy resin adhesives are quite versatile be- of the first method is that if the adhesive is cause they possess a combination of such desirable strong and rigid enough to prevent swelling, the characteristics as ease of cure, low shrinkage adherend may not be able to withstand the re-during cure, high adhesive strength, good me- sulting stress. chanical properties, and good chemical resistance Krueger and Blomquist (3) found that a deform-when cured. A wide variety of modifications are able adhesive performed much better than a possible through the use of curing agents, fillers, rigid adhesive, with a very low incidence of rup­plasticizers, and flexibilizers. One of the purposes ture in either the adhesive or adherend. If the of this study was to demonstrate that certain deformable adhesive is superior in this applica­mechanical properties can be manipulated. If this tion, then it would be beneficial to know how can be done and the required mechanical per- much strain the adhesive can withstand before formance for a specific use can be defined, then rupture. Although Krueger (2) did not attempt to it should be possible to engineer a suitable epoxy engineer the adhesive on the basis of the strain formulation. limits he had, he did observe that the adhesive

Examples of this type of reasoning canbefound formulation was able to deform 0.625 inch per in the work of Krueger and Blomquist (3)2 and inch in shear under a sustained load for 6 days Krueger (2). They studied the weakening or without failure. The important point here is that, destruction of plywood-to-lumber joints caused considering the mechanical properties of both by the shrinkage and swelling of the wood. At the the adhesive and adherend, there is a critical outset there appeared to be two ways of over- strain in the adhesive above which the adhesive coming this problem: use a strong, rigid adhesive may rupture and below which the adherend may

1 Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2 Underlined numbers in parentheses refer to literature cited at end of this report.

rupture. AS Krueger (2) points out, this lies between the theoretical extremes of no stress in the adherend (maximum adhesive strain) and maximum stress in the adherend (zero strain in the adhesive).

Another purpose of this study was to determine the tensile strengths of various epoxy adhesive formulations for comparison with the tensile strength of white pine butt joints. Butt joints are considered inefficient in attempting to attain the full structural strength of an adherend, and tests on epoxy-aluminum butt joints have shown that the tensile strength of butt joints is considerably less than the tensile strength of the epoxy alone (5). Recent work at the Forest Products Labora­tory was concerned with developing techniques for end-to-end grain bonding of white pine wit the same epoxy formulations used in this study.3

Since the tensile strength of the epoxy adhesive in the joint is the highest strength that could possibly be attained in a butt joint (considering only adhesive failure), these data provided an upper limit that might possibly be realized. The results of the butt joint study showed that the strength of these joints could approach that of the epoxy adhesive formulation.

In this study the tensile stress-strain behavior of thin films of flexibilized epoxy adhesive formulations was measured.

MATERIALS AND FLEXIBILIZERS

Adhesives and Flexibilizers

The basic material studied was a commercial epoxy resin. This resin was flexibilized to five different levels with four commercial flexibi­lizers: a polyamide, a polysulfide, a mercaptan­terminated polybutadiene, and an epoxy resin with more inherent flexibility than the basic epoxy resin. The adhesives were catalyzed with DETA (diethylenetriamine). The various formu­lations are listed in table 1.

The amount of DETA used in each formulation was based on the number of epoxy equivalent weights. The formulation with the flexible epoxy had more epoxy equivalents than the others and therefore required more DETA. The polyamide flexibilizer has a catalytic action of its own and

therefore the formulation required less DETA than the others.

Mold Preparation

In earlier work (7) on the stress-strain be­havior of adhesives, the films were cast on plate glass and specimens cut from the cured film. Preliminary work on epoxies showed that specimens could not be cut from a cured film because the films were too brittle. Since the volumetric shrinkage of epoxy resins in the curing process is relatively small (4), the test specimens were cast in closed molds. The molds (fig. 1) were made with silicone rubber.

Figure 1.--Right: Male mold metal shim material mounted on plate glass and inserted into frame. Left: Finished rubber mold with specimen impression for casting thin specimens.

M 135 439

3Schaeffer, R . E., and Gillespie, R. H. Improving end-to-end grain butt joint gluing of white pine. Forest. Prod. J . in press. 1970.

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Table 1.--Summary of epoxy adhesive formulations

1Diethylenetriamine.

To prepare the rubber molds it was first necessary to make a male mold to leave the specimen impression in the rubber. A specimen 3/4 inch wide and 8 inches long, with a necked-down section 2-3/4 inches long and 1/2 inchwide, was made from a piece of shim steel 0.004 inch thick. This specimen was fastened to a 2-1/2- by 10-inch glass plate by applying pressure-sensi­tive tape to one side and centering it on the glass. The tape was 0.004 inch thick, giving a total thickness of 0.008 inch for the mounted specimen above the surface of the glass. The mounted specimen and plate glass were then encased in a plywood frame extending 1/8 inch above the glass plate to form the male mold.

A coat of mold release agent was sprayed on the male mold and the catalyzed rubber was poured. A piece of plate glass was lowered onto the male mold by scissor action (see fig. 2) to

remove excess rubber and to bring the surface of the rubber even with the plywood frame of the male mold. After conditioning for 24 hours at 80° F., the rubber was cured and could be re­moved from the male mold.

The rubber molds are quite durable and can be used to prepare many specimens if they are properly cleaned after each use. The mold must be coated with a release agent before casting adhesive specimens, and the agent must be re­moved after casting. The molds were washedwith warm water and soap, dried, and then cleaned further with methyl alcohol.

After coating the mold and top glass plate with the release agent, the mold and plate wereplaced in a circulating oven at 145° F. for 15 minutes to drive off the solvents of the release agent. After cooling to room temperature the mold was ready for casting.

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Figure 2.--Lowering the glass plate with a scissor action to completely fill the impression and force any existing bubbles to the edge. M 135 403

Specimen Preparation

The method of casting the adhesive specimens is illustrated in figures 2 to 6. After filling the specimen impression with adhesive, a glass plate was lowered onto the mold to completely and evenly fill the impression, and to force out any air bubbles (fig. 2). A weight was then placed on the glass plate (fig. 3). After the specimen had cured for 16 hours at 80" F. and 30 percent relative humidity, the mold and glass plate were heated at 145" F. for about 2 minutes. The rubber

Figure 4.--Rubber mold being lifted from the cured adhesive film. The film adheres to the glass but separates easily from the rubber mold. M 135 406

mold could then be peeled from the glass plate quite easily (fig. 4). The squeezeout (0.0005 in. thick or less) of an adhesive could be removed from the specimen with a razor blade (figs. 5 and 6).

The specific details for mixing each of the formulations are given in the Appendix. All mixing was done at 80° F. and 30 percent relative humidity. Most of the mixtures were heated at 145° F. for several minutes. The pur­pose of the heating was to reduce the viscosity of the mixture so that air bubbles would rise to the surface and disperse.

Figure 3.--The adhesive-filled mold with a glass plate on top and a steel plate used as a weight to give equal distribution of pressure until the adhesive cures. M 135 404

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Figure 5.--Free film specimen being removed from glass, after scoring around edge with razor blade. The squeezed-out edges beyond the speci­men are very thin (0.0005 in. or less) and easily break away from the molded specimen. M 135 408

Figure 6.--Cured specimen after removal from mold. Specimen is wiped lightly with methyl alcohol to remove release agent from surface. M 135 407

Stress-Strain Measurements

The stress-strain behavior of the adhesive films was measured up to the point of tensile failure with a universal \ testing machine. The strain gage (1) (fig. 7) was a lever arm device that could be balanced so that no force would be exerted on the specimen. A differential trans­former indicated the strain in a 1-inch gage length. A continuous stress-strain curve was recorded up to failure for each specimen.

All tests were performed at 72° F., 50 percent relative humidity, and at a rate of 0.05 inch of crosshead motion per minute. Ten replicates were tested for each type and level of flexibilizer.

Figure 7.--Specimen with the strain gage attached clamped between the grips of the universal testing machine. M 135 409

RESULTS AND DISCUSSION

Most of the test specimens exhibited brittle failure, which is characterized by a low strain at failure (less than approximately 5 pct. (6)) and a positive slope of the stress-strain curve at failure. Table 2 shows, however, that a yield point occurred more often in the tests as flexi­bilizer content increased. The yield point is defined as that point on the stress-strain curve where stress no longer increases with strain. The specimens that showed a yield point exhibited a brittle-ductile behavior, that is, failure occurred shortly after the yield point.

The maximum tensile s t r e s s , modulus of elasticity, strain at failure, and work to failure are listed in table 2, and shown in figures 8 to 11 as a function of the amount of flexibilizer. The modulus was taken as the initial slope of the stress-strain curve, and the work to failure as the area under the stress-strain curve.

To show the general trend of the stress-strain p a r a m e t e r s with flexibilizer content, the parameters were fit by least squares regression

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Table 2.--Tensile stress-strain parameter for films of flexibilized epoxy adhesive formulations. (PHR = parts per hundred of resin)

1The yield point is that point on the stress-strain curve where stress no longer increases with strain.

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to flexibilizer content. The curves indicating these limits in figures 8 to 11. Because work to failure general trends are shown in figures 8 to 11. reflects strain at failure, data for both show the

The maximum tensile stress and modulus of scatter. With the exception of the polyamide flexi­elasticity decreased with flexibilizer content for bilizer, strain at failure appeared to increase each of the four flexibilizers involved. The maxi- with flexibilizer content. The increase was rela­mum stress ranged from a high of a little less tively small for the mercaptan-terminated poly­than 10,000 pounds per square inch (p.s.i.) for butadiene and epoxy flexibilizers, where the zero flexibilizer content down to a low of about increase was from approximately 3.00 to 3.75 per­4,500 p.s.i. for 40 parts per hundred of resin for cent. The strain at failure for the polysulfide the mercaptan-terminated polybutadiene and poly- formulation increased to about 5 percent at amide flexibilizers. The modulus ranged from a 40 PHR, and appeared to be increasing quite high of about 450,000 p.s.i. for zero flexibilizer rapidly with flexibilizer content at that point. content to a low of about 200,000 p.s.i. for the With the exception of the polysulfide flexibilizer, mercaptan-terminated polybutadiene flexibilizer. work to failure decreased with flexibilizer content.

The effect of flexibilizer content on strain at This was quite pronounced with the polyamide failure and work to failure is not as clear as it flexibilizer, where the decrease was from 170 to is on the maximum stress and modulus. There 67 inch-pounds per cubic inch. As with strain at was considerably more scatter in the data on failure, the work to failure for the polysulfide strain at failure and work to failure than in the formulation appeared to be increasing quite data on maximum stress or modulus of elasticity. rapidly with flexibilizer content at the higher This is apparent from the 95 percent confidence content levels.

Figure 8.--Tensile stress-strain parameters of an epoxy adhesive with several levels of polysulfide flexibilizer. Solid lines are based on least squares and vertical bars are 95 percent confidence Iimits around the mean of 10 specimens. PHR = parts per hundred of resin. M 137 206

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Figure 10.--Tensile stress-strain parameters of an epoxy adhesive with several levels of a more flexible epoxy added to impart flexibility. Solid lines are based on least squares and vertical bars are 95 percent confidence limits around the mean of 10 specimens. PHR = parts per hundred of resin. M 137 209

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Figure 11.--Tensile stress-strain parameters of an epoxy adhesive with several levels of a polyamide flexibilizer. Solid lines are based on least squares and vertical bars are 95 percent confidence limits around the mean of 10 specimens. PHR = parts per hundred of resin. M 137 208

CONCLUSIONS

The results show that it is possible to manipu­late the maximum tensile stress and the modulus of elasticity of epoxy adhesive formulations by adding various amounts of certain flexibilizers. By using a polysulfide flexibilizer, it is possible to increase both the strain at failure and the work to failure of the epoxy formulation. The other flexibilizers had an inconclusive effect on strain at failure and a slightly negative effect on work to failure.

The a d d i t i o n of polysulfide, mercaptan­terminated polybutadiene, flexible epoxy, and

polyamide flexibilizers to a commercial epoxy adhesive decreases the maximum stress and modulus of elasticity of the formulation in an approximately linear manner with flexibilizer content (parts per hundred of epoxy resin). The flexibilizers appear to slightly increase the strain at failure, with the exception of the polyamide which showed a slight decrease. Considering the scatter in the data on the strain at failure, however, these trends cannot be considered con­clusive, except in the case of the polysulfide, which showed a definite effect of increasing both the strain at failure and work to failure at the higher levels of flexibilizer content. The work to failure of the other three formulations decreased slightly with flexibilizer content.

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LITERATURE CITED

1. Jewett, D. M. 1963. An electrical strain gage for the

tensile testing of paper. U.S. Forest Serv. Res. Note FPL-03, Forest Prod. Lab., Madison, Wis.

2. Krueger, G. P. 1965. Behavior of an epoxy-polysulfide

adhesive in wood joints exposed to moisture content changes. U.S. Forest Serv. Res. Pap. FPL 24, Forest Prod. Lab., Madison, Wis.

3. , and Blomquist, R. F. 1965. Performance of a rigid and a

flexible adhesive in lumber joints subjected to moisture content changes. U.S. Forest Serv. Res. Note FPL-076, Forest Prod. Lab., Madison, Wis.

4. Lee, H., and Neville, K. 1967. Handbook of epoxy resins. McGraw-

Hill Book Co., New York. 5. Lewis, A. F., and Ramsey, W. B.

1966. Mechanical behavior of polymers and adhesive joint strengths with amine cured epoxy resins. Ad­hesives Age 9(2): 20-27.

6. Marin, J. 1965. Mechanical relationships in testing

for mechanical properties of poly­mers. In Testing of Polymers Vol. 1, U. V. Schmitz, ed., Inter­sci. Publ., New York.

7. Simpson, W. T., and Soper, V. R. 1968. Stress-strain behavior of films of

four adhesives used with wood. U.S. Forest Serv. Res. Note FPL­0198, Forest Prod. Lab., Madison, Wis.

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Polyamide-- all mixtures. --The DETA wasAPPENDIX mixed with the polyamide and warmed for 3 min­

utes at 145° F. The basic epoxy resin was added Basic epoxy resin.--The resin and DETA were to the warm mixture and allowed to stand 10

mixed and then heated in an oven at 145°F. for minutes before casting.3 minutes to disperse air bubbles. The mixture was then cooled to 80° F. in an ice bath before casting.

Polysulfide--5 and 10 PHR (parts per hundred of resin).--The epoxy resin and the polysulfide flexibilizer were mixed and warmed at 145° F. for 5 minutes. The mixture was cooled in an ice bath, the DETA was added, andthe mixture allowed to stand for 10 minutes at 80° F.

Polysulfide--20, 30, and 40 PHR.--The DETA was mixed with the polysulfide flexibilizer and then the basic epoxy resin was added. The mixture was heated for 3 to 4 minutes at 145° F. and then cooled to 80° F. before casting.

Polybutadiene--5 PHR.--The epoxy resin and flexibilizer were mixed and warmed at 145° F. for 3 minutes. After cooling to 80° F. in an ice bath, the DETA was added. The mixture was allowed to stand for 5 minutes to allow additional bubbles to surface before casting.

Polybutadiene--10 PHR.--The epoxy resin and flexibilizer were mixed and warmed at 145° F. for 3 minutes. The DETA was added while the mixture was still slightly above 80° F. The mix­ture was then cooled to 80° F. and cast immedi­ately.

Polybutadiene--20, 30, and40 PHR.--The DETA and flexibilizer were mixed together, and then the basic epoxy resin was added. The mixture was warmed for 3 to 4 minutes at 145°F. and cooled to 80° F. in an ice bath before casting,

Flexible epoxy--5 PHR.--The basic epoxy and the flexible epoxy were mixed and warmed at 145" F. for 10 minutes. The DETA was added while the mixture was still slightly warm, and the mixture was cast immediately.

Flexible epoxy--10 PHR.--The procedure was the same as with 5 PHR, except that the warming period was 15 minutes.

Flexible epoxy--20, 30, and 40 PHR.--The DETA was mixed with the flexible epoxy and the basic epoxy was added. After warming at 145° F. for 4 minutes, the mixture was cooled to 80° F. in an ice bath and allowed to stand 10 minutes before casting.

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