U.S. DEPARTMENT OF AGRICULTURE • FOREST SERVICE FOREST PRODUCTS LABORATORY • MADISON, WIS. In Cooperation with the University of Wisconsin

U.S.D.A. FOREST SERVICE R E S E A R C H N O T E F P L - 0 1 9 8 J U L Y 1968

STRESS-STRAIN BEHAVIOR OF FILMS OF FOUR ADHESIVES USED WITH WOOD

Abstract

Mechanical properties of an adesive joint depend largely on the mechanical properties of the adhesive. A logical approach to joint design requires knowledge of these properties and the factors influencing them. This report describes methods of casting thin adhe-sive films and the methods and results of stress-strain tests in tension. The adhesive formulations tested varied from an easily deformable rubber-base adhesive to a stiff resorcinol resin. A comparison of certain mechanical properties of different adhesives can result in a wiser choice of adhesive for a particular application.

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STRESS-STRAIN BEHAVIOR OF FILMS

OF FOUR ADHESIVES USED WITH WOOD

By

WILLIAM T. SIMPSON, Forest Products Technologist and

VERNON R. SOPER Physical Science Technician

Forest Products Laboratory,1 Forest Service U.S. Department of Agriculture

Introduction

Certain mechanical properties of an adhesive in a bonded joint determine joint performance in many application. When failure occurs within the adhesive these mechanical properties become important. Therefore a knowledge of these properties is desirable and could lead to wiser uses of the adhesives.

Two general approaches to the study of the stress-strain behavior of adhesives 6)2are: Measuring the stress-strain behavior of the adhesive in a joint (2, 3, ,

and measuring the stress-strain behavior of the builk adhesive as a film free from the adhesive joint (4). The first approach has the advantage of being a direct measurement on the adhesive as it behaves in a joint; thus it includes the complex stress patterns in the actual joint, The second approach is concerned more with the bulk adhesive as a material possessing certain mechanical prop-erties. This report deals with the stress-strain behavior of bulk adhesive in the form of thin films.

Useful quantities can be determined from a stress-strain curve: Modulus of elasticity, breaking strength, strain at failure, and work to failure. The modulus of elasticity is usually taken as the intitial slope of the stress-strain curve, and

1Maintained at Madison, Wis., in cooperation with the University of Wisconsin. 2Underlined numbers in parentheses refer to the Literature Cited at the end of this note.

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for small deformations and short times is a measure of the resistance to deformation. The breaking strength and the strain at failure are useful because they define the limits of the material. The work to failure is the energy the material can absorb before failure and is an indication of toughness. Whereas uniaxial tension does not necessarily simulate actual end use stress conditions, the four quantities mentioned can be useful for comparing materials and for determining how the variation of certain parameters can affect the mechanical properties of the material.

The purpose of this study was to gather stress-strain data on a few selected adhesives used with wood and to determine if cure time has a measurable effect on the stress-strain behavior. Special techniques had to be developed for pre-paring thin adhesive films to secure the needed results.

Materials and Experimental Procedures

Adhesives

All adhesives evaluated were commercial formulations of the following six types: (1) Thermoplastic polyvinyl acetate emulsion; (2) thermoplastic polyvinyl acetate emulsion with 10 percent walnut shell flour; (3) thermosetting polyvinyl acetate emulsion; (4) thermosetting polyvinyl acetate emulsion with 10 percent Walnut shell flour; (5) resorcinol resin; and (6) rubber-base adhesive.

The thermoplastic polyvinyl acetate emulsion had a solids content of 50 percent; the thermosetting polyvinyl acetate emulsion, 48 percent. The thermosetting polyvinyl acetate required an acidic catalyst for cure; the catalyst was added in the proportion of 5 percent of the weight of the emulsion. The 10 percent walnut shell flow was based on the weight of the wet adhesive. An amount of water equal to that of the walnut shell flour was also added to provide for a spreadable consistency.

The resorinol was a resorcinol-formaldehyde resin containing a cellulosic filler and a formalin hardener. The rubber-base adhesive was a solvent system with methyl ethyl ketone and 20 percent cyclized rubber.

Film Casting

Spreading the adhesive on plate glass coated with a solvent-type wax release agent was the general method for preparing the adhesive films, Before complete

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cure, while the adhesive was still somewhat flexible, the film was removed from the glass, and the specimens were cut. If the cure was allowed to progress for too long, the films became brittle and made it impossible to cut specimens without gross cracks. The cured-film thicknesses were between 0.005 and 0.008 inch.

The polyvinyl-acetate formulations were stirred to an even consistency and spread on wax-coated plate glass (fig. 1). The films were spread with an adust-able doctor blade set to leave a wet film 0.020 inch thick. The films were allowed to dry or cure at 80° F. and 65 percent relative humidity for about 2 hours before the specimens were cut from the film.

The method of spreading the resorcinal film is illustrated in figure 2. The mixed adhesive containing hardener and resin was poured into a glass plate. Shims 0.012 inch thick were placed around the edge of the plate, and a second glass plate was lowered onto the shims of the bottom plate so that the mixture was squeezed into a thin film. The top plate was needed to prevent the warping and the cracking that resulted from unequal rates of drying and cure on the exposed and unexposed sides of the film. The film was allowed to stand for about 16 hours at 80° F. and 80 percent relative humidity before cutting the specimens.

The rubber-base adhesive was spread in a manner similar to that used for the polyvinyl acetates except that the wet film was 0.050 inch thick. The film was cured for about 16 hours at 80° F. and 65 percent relative humidity. To remove the extremely flexible film from the glass with a minimum of stretching, it was necessary to soak the film on the plate in warm water for about 30 minutes.

Specimen Preparation

The test specimens were 7-3/4 inches long, 3/4 inch wide at the ends, and necked down over a 2-inch radius to a 1/2 inch width in the central 1-1/2 inches of the specimen (fig. 3). A cutting die was used to cut the specimens from the adhesive film.

An optimum physical condition for each adhesive film made it possible to cut sound, well-formed specimens. This condition had to be determined in each case by trial-and-error. The polyvinyl-acetate films became brittle rapidly after removing from the plate glass. Therefore, only a small section of film was removed at a time, and specimens were cut as quickly as possible. Speci-mens were cut from the resorcinol-resin films in much the same manner. While each small section of film was being cut, the remaining film was kept

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Figure 1.--Method of spreading polyvinyl-acetate emulsion adhesive

films.

M 129 457

Figure 3.--Specimen for adhesive film tension test.

between the glass plates used in the casting operation. The film from the rubber-base adhesive remained flexible on extended storage; therefore timing was not critical for cutting the specimens from the film.

After cutting, the specimens were kept from curling and warping while they continued to dry and cure by placing them between sheets of hardboard under sufficient weight to hold them flat. All test specimens were stored at 80° F. and 30 percent relative humidity until the time they were tested.

Stress-Strain Measurement

The stress-strain relationships for the adhesive films were measured on an Instron universal testing machine. Stress was applied in the same direction the adhesive film was spread; strain was measured with a strain gage attached to the specimen. The strain gage (1) was a lever arm device that could be balanced so that no force would be exerted on teh specimen (fig. 4). A differential trans-former indicated the strain. A continuous stress-strain curve was recorded up to failure from the strain input of the differential transformer and the stress input of the machine load cells.

The design of the strain gage was essentially that of Jewette (1) as adapted to the measurement of the tensile properties of paper according to Setterholm and Kuenzi (7).

All tests were conducted at 72°F., 50 percent relative humidity; the specimens were exposed to these conditions for at least 12 hours before they were tested. All specimens were strained at a rate of 0.05-inch crosshead motion per minute with 4-1/2 inches the initial distance between specimen clamps. A 1-inch gage length was used with the strain gage.

The stress-strain behavior of the polyfinyl=acetate formulations and of the resorcinol formulation was evaluated periodically for film ages up to about 100 days, or more in some cases, to determine if film age had a measurable effect on the stress-strain behavior. During this time the specimens were stored in an environment maintained at 80° F. and 30 percent relative humidity.

The effect of curing the rubber base adhesive at 200° F. for cure times up to 24 hours was evaluated in terms of a change in the modulus of elasticity.

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Figure 4.--Directreading strain gage for testing adhesive films:

Spring-loaded knife-edged clamps (A); balance weights (B); bearings

(C, D, and E); transformer (F); and transformer core (G).

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Treatment of Data

The modulus of elasticity was taken directly from the stress-strain curves recorded on the testing machine. A dashed line was drawn tangent to the initial portion of the curve, and the slope of this line was determined as the modulus (fig. 5).

The work to failure is the area under the stress-strain curve and was evaluated by a method of numerical integration. The method involves dividing the area under the curve into a number of equal increments (approximately 0.001 to 0.002 inch) along the strain axis, calculating the area of each small increment, and then summing all the small areas (fig. 5).

The portion of the stress-strain curve between each increment was assumed to be parabolic for calculation purposes according to Simpson’s rule of numer-ical integration (5, p. 172).

Results

Typical stress-strain curves to failure for the thermoplastic polyvinyl-acetate, the thermosetting polyvinyl-acetate, and the resorcinol formulations are shown in figure 6. The initial portion of a typical stress-strain curve for the rubber-base adhesive cured at room temperature is shown in figure 7. The averages of modulus of elasticity, stress at failure, strain at failure, and work to failure are shown for the polyvinyl acetates and resorcinol for mulations in the tables 1 to 3. The film age, the number of specimens, and the 95 percent confidence limits are shown in each ease. The number of specimens varied because some speci-mens broke in the test machine jaws or at a place other than the necked-down portion.

The stress-strain curves in figure 6 typify the behavior of the adhesive films. The material was characterized by brittle fracture at the particular test condi-tions used. Strain at failure was usually in the range of 1 to 2 percent, and the stress-strain curves were almost linear all the way to failure. The values of the modulus of elasticity were in the order of 400,00 to 600,000 pounds per square inch.

For the thermoplastic polyvinyl-acetate formulation, the modulus of elasticity was about 400,000 pounds per square inch; the breaking strength about 4,000 pounds per square inch; the strain at failure about 1 to 1.5 percent; and the work to failure about 30 inch-pounds per cubic inch. There did not appear to be any

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Figure 5.--Methods for determining modulus of elasticity and work to failure from stress-strain curves.

Figure 6.--Relationship of stress to strain for three adhesive films.

M 134 906

F igu re 7 . - -Typ i ca l r e l a t i o n s h i p o f s t r e s s t o s t r a i n f o r a room temperature cured rubber-base adhesive.

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trend in the stress-strain behavior with cure time. This was generally found for the other adhesives as well. Test results varied greatly; if any changes in the stress-strain behavior did occur with increasing cure time, they apparently were not great enough to be detected by the methods used here. The 95 percent confidence limits gave an indication of the variation in the results. The average stress-strain parameters and the values of the coefficient of variation are given in table 4; the values are the averages for all of the cure times evaluated because the effect of cure time was not detectable.

The addition of 10 percent walnut shell flour to the thermoplastic polyvinyl acetate does not appear to have changed any of the four stress-strain properties evaluated. Again, an effect on these stress-strain properties may be present but too small to be detected by the methods of this study.

For the thermosetting polyvinyl-acetate formulation, the modulus of elasticity was about 500,000 pounds per square inch; the breaking strength about 5,500 pounds per square inch; the strain at failure a little over 1 percent; and the work to failure about 35 inch-pounds per cubic inch. No trend could be observed in these properties with time.

The addition of 10 percent walnut shell flour to the thermosetting polyvinyl acetate definitely lowered both the modulus of elasticity and the breaking strength. The modulus of elasticity was reduced approximately 100,000 pounds per square inch and the breaking strength close to 1,000 pounds per square inch. The strain at failure and work to failure did not appear to have changed greatly.

The resorcinol resin had a modulus of elasticity about 600,000 pounds per square inch; a breaking strength of about 10,000 pounds per square inch; a strain at failure of about 2 percent; and a work to failure of about 100 inch-pounds per cubic inch.

Due to the highly extensible nature of the rubber-base adhesive, only the modulus of elasticity could be determined. The crosshead of the testing machine reached its limit of extension before specimen failure. The modulus of elasticity of the rubber-base adhesive was plotted against cure time at 200° F. in figure 8. The modulus increased from about 700 pounds per square inch for a room tem-perature cure to about 3,000 pounds per square inch for a 24-hour cure at 200° F.

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Table 4.--Average stress-strain properties of five adhesive films1 .

Figure 8.--Relationship of modulus of elasticity to cure time at 200° F. for a rubber-base adhesive.

Summary

Satisfacory methods were developed for casting thin films of adhesives and for measuring their stress-strain behavior when stressed in uniaxial tension.

Rescorinol-resin, t h e r m o s e t t i n g polyvinyl-acetate, and thermoplastic polyvinyl-acetate adhesives were characterized by brittle fracture when stressed under the teat conditions. The resorcinol-resin adhesive had the highest modulus of elasticity and breaking strength, followed by the thermosetting polyvinyl acetate and the thermoplastic polyvinyl acetate. The addition of 10 percent walnut shell flour reduced the modulus of elasticity and breaking strength of the thermosetting polyvinyl acetate but had little or no effect on the thermoplastic polyvinyl acetate. The resorcinol-resin adhesive exhibited a higher strain at failure than did any of the polyvinyl-acetate formulations. The stress-strain behavior of these three adhesives did not change enough with time of aging to be detected by the method used. The modulus of elasticity of the rubber-base adhesive was increased by curing the film at 200° F. for various periods.

Literature Cited

1. Jewett, D. M. 1963. An electrical strain gage for the tensile testing of paper. U.S.D.A.,

Forest Service Res. Note FPL-03, Forest Products Lab., Madison, Wis,

2. Krueger, G. P., and Blomquist, R F. 1965. Experimental techniques for determining mechanical behavior of

flexible structural adhesives in timber joints. U.S.D.A., Forest Service Res. Paper FPL 21, Forest Products Lab., Madison, Wis.

3. Kuenzi, E, W, and Stevens, G. H. 1963. Determination of mechanical properties of adhesives for use in the

design of bonded joints. U.S.D.A., Forest Service Res. Note FPL-011, Forest Products Lab., Madison, Wis.

4. McBain, J. W., and Lee, W. B. 1972 Adhesives and adhesion: Relation of joint strength to tensile strength

of films. Jour., Soc. Chem. Ind., 46, No. 30, 321T-324T. 5. McCracken, D. D. and Dorn, W. S.

1964, Numerical methods and Fortran programming with applications in engineering and science. New York: John Wiley and Sons, Inc.

6. Norris, C. B., James, W. L., and Drow, J. T. 1956. A strain gage for the measurement of strains in adhesive bonds.

ASTM Bull. No, 218, pp. 40-49. 7. Setterholm V. C., and Kuenzi, E. W.

1956. Method for determining tensile properties of paper. U.S.D.A., Forest Service, Forest Products Laboratory, Madison, Wis., Rep. No. 2066.

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