I
S. S. STlVALAl and W. J. POWERS Ordnance Corps Plastics Laboratory, Picatinny Arsenal, Dover, N. J.
Improved Impact Expoxy Adhesives Greater impact strength and shear resistance point the way to more widespread applications for these modified epoxy adhesives
AN ADHESIVE-bonded joint must withstand temperature variations from -65’ to 600’ F., in such applications as ordnance, aircraft and guided missiles, and electrical equipment. Often the use of heat to cure the adhesive is impractical or prohibited, and roomtemperature-setting adhesivesare needed. Further, for impervious surfaces solventless-type adhesives are generally preferred and sometimes mandatory for rapid setting and high strength. The major purpose of this investigation was to find solventless adhesives which would cure a t room temperature and would have an advantageous balance of important mechanical properties. Among the many existing commercially available room-temperature setting adhesives, those based on the epoxy resins appeared to offer the best over-all properties and utility. In adhesive applications these resins are relatively poor in resisting a sudden blow or impact. Accordingly, effort was directed toward improving the impact-resisting properties of the epoxy resin while simultaneously retaining, or not appreciably altering, its good features. Attempts have been made by both industry and military agencies to improve the impact property of epoxy resin adhesives. Modifiers, such as polysulfide liquid polymers, low molecular weight polyamides, and fillers of various nature, have been successful. However, for more significant improvements it was believed that modification on the molecular structure of the base 1
polymer should be investigsted as a means of improving impact properties.
Chemistry of Epoxy Resins The epoxy resins (5)are low molecular weight resins derived from the condensation of a polyhydric compound with epichlorohydrin, in the presence of an alkaline catalyst-e.g., sodium hydroxide. Most commercial epoxy resins today are made from the reaction of bisphenol A CH3 H
O
O
k
O
-
O
H
CH3 with
epichlorohydrin,
H&-
CH-
\/
0 CH2-Cl. Exceptions to these are Epon 562 (Shell Chemical Corp.) made from glycerol and Cardolite 7019 (Irvington Chemical Division, Minnesota Mining and Manufacturing Co.) made from the bisphenol obtained from cashew nut shell liquid.
The epoxy resins are available as viscous liquids or solids, depending on their molecular weights. These resins, prior to curing, are soluble in ketones, alcohol-ethers, chlorinated hydrocarbons, and some esters. I n many adhesive applications it is desirable to employ nonsolvent liquid system; therefore, the low molecular weight epoxy resin, which is a liquid, is ordinarily employed. The linear thermoplastic resins, as commercially supplied, must be cross-linked or “set” to achieve useful properties. The cross-linking agents include compounds containing active hydrogen, such as organic acids and amines. Resinous substances containing active hydrogen may also be employed as curing systems; these include phenolics, ureas, melamines, polysulfides and low molecular weight polyamides. Reaction or curing can occur a t room or elevated temperatures, depending upon the type of crosslinking agent employed. In its simplest form, the reaction between epichlorohydrin and bisphenol A may be represented as shown below:
Present address, John Harrison Labora-
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./
__ -/-
T-
IZOD HAMMER Typical specimen used to test shear impact of adhesives
PATH
The organic amines, in particular the polyamines such as diethylenetriamine or triethylenetetramine, add to the epoxy ring via the hydrogen attached to the nitrogen of the amine. The reaction occurs a t room temperature within 1.5 to 2 hours with concentration of amine generally between 6 and 1470 based on the weight of resin. The concentration of amine is somewhat critical; any value below or above the calculated amount (approximately one amine hydrogen per epoxy group) can cause incomplete or improper cure. The reaction of epichlorohydrin with the bisphenol derived from cashew nut shell liquid yields a dark brownish liquid epoxy resin that is internally plasticized (6). This resin, Cardolite 7019, is soluble in aromatic hydrocarbons and may be converted to a three-dimensional polymer with the usual types of cross-linking agents for epoxy resins. The major component of the polyphenol derived from cashew nut shell oil is essentially 1,8-bis(hydroxypheny1)pentadecane.
v
OH
This structure is characterized by' a wide distance (seven methylene groups) between the reactive phenolic groups and a long side chain. The 1,8-bis(hydroxypheny1)pentadecane is characterized by a much greater freedom of rotation and bending than is the bisphenol A. The higher impact resistance of the cashew-epoxy resin is thus attributed to the internal plasticization provided by the seven methylene groups separating the phenol groups and by the flexible side chain. Cardolite 6885 is essentially the monoglycidyl ether of cardanol of cashew nut shell liquid. Because of the presence of one terminal epoxy group, it is effective as a coreactive material with epoxy resins, serving as a nonmigrating type of internal plasticizer. Experimental
15 20 % CA?DOLITE 6885
25
Figure 1. Shear strength of Epon 828 modified with Cardolite 0 . -65'F. M. 73.5' F. A. 160' F.
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The compositions studied include Epon 828, an epoxy liquid resin derived from bisphenol A and epichlorohydrin; Cardolite 7019, a diglycidyl ether of 1,8-bis(hydroxyphenyl) pentadecane ;and Cardolite 6885, the monoglycidyl ether of cashew nut shell liquid as plasticizer. The properties of shear and impact strengths were chosen for comparison among the formulations studied, as they represent conditions encountered in many industrial applications. Because of the structural nature of the cashew-epoxy resin (Cardolite 7019) this material was evaluated extensively along with plasticized Epon 828. Cardolic 7019 cures to a hard, tough mass at room temperature with 7 to 10 parts of triethvlenetetramine per 100 parts of resin. ' This resin is compatible, in all proportions, with Epon 828. The mixture sets at room
INDUSTRIAL AND ENGINEERING CHEMISTRY
temperature with 7 to 10 parts of triethylenetetramine per 100 parts of the resin mixture. Properties intermediate to that of Epon 828 andiof Cardolite 7019 result. Cardolite 6885 is used as a nonmigratory reactive type plasticizer with either Epon 828 or Cardolite 7019. Impact. Impact, in adhesive joints, is the minimum force needed to cause the adhesive to fail under a sudden single blow, and impact strength may be determined with the adhesive bond either in shear or in tension. Federal specifications describe impact in shear, and accordingly the shear impact was determined using the Izod impact testing machine. The size of the striking hammer of the Izod machine was so chosen as to cause failure in the sample and give readings that lie reasonably within the two extremes of the scale. All efforts were used to employ the same hammer for all samples. Occasionally samples of very high impact or extremely low impact demanded the use of other hammers. I n comparing the results of Epon 828 with those obtained from Cardolite 7019, the impact strength a t 73.5' F. of the latter showed approximately a threefold increase over the Epon 828 (10.7 ft. lb./sq. inch compared with 3.3). At -65' F. the impact strength of Cardolite 7019 was 5.1 ft. lb./sq. inch compared with 3.3 for Epon 828. However, a t 160' F. Epon 828 had higher impact-resistant properties than the Cardolite 7019 (8.8 ft. Ib./sq. inch compared with 3.6). The somewhat inferior impact property of Cardolite 7019 a t 160' F. may be attributed to its partial softening and poorer adhesion t o the metal. Epoxy resins become less brittle (improved flow) at higher temperatures, and thus have improved impact qualities, but they do not adhere as well to a surface. The impact strength of Epon 828 at 73.5'F. and below (as low as -65' F.) can be improved by incorporating with it the reactive plasticizer Cardolite 6885. Accordingly, Cardolite 6885 was added to the Epon 828 in concentrations of 5, 10, 15, 20, 25, 30, 40, and 50% by weight. Seven parts of triethylenetetramine per 100 parts of mixture were added as curing agent. Impact specimens prepared with the use of these mixtures were tested at -65', 73.5', and 160' F. At -65' and 73.5' F. there was an improvement in impact strength, increasing with increasing concentrations of the plasticizer up to a maximum. lt'ith further addition of plasticizer a drop in impact resulted. The optimum concentration of plasticizer that imparted maximum impactresisting properties to Epon 828, at both -65' and 73.5' F., was approximately 20% by weight. At 25% concentration, the impact strength dropped considerably :
EPOXY A D H E S I V E S Table 1.
Measured Shear and Calculated Impact Values for Epon 8 2 8 and Cardoljte 70 19 Modified with Cardolite 6885" Cardolite 6885 Added, % Temp., F. 0 5 10 15 20 25 30 40 50 Shear, P.S.I. O
Epon 828
,
-65' 73.50 160'
433 408 260
920 597 397
- 65'
3.3 3.3 8.8
9.5 4.1 8.1
523 845 186
1006 496 170
5.1 10.7 3.6
7.2 8.8 4.5
859 587 393
1025 734 597
... ... ...
... ... ...
...
9.1 12.8 4.9
14.9 18.6
10.5 4.5
5.1
...
5.0 3.3 4.9
5.8 2.7 1.7
7.1 4.9 1.1
940 568 107
931 471 103
... ... ...
...
... ...
... ... ...
...
6.1 5.9
3.2 3.6
1.5 1.3
I027 601 385
... ...
... ...
...
Impact, Ft. Lb./Sq. Inch 73.50 160'
10.8 12.8 7.5
Shear, P.S.I.
- 65'
Cardolite 7019
73.50 160'
*
914 485 157
Impact, Ft. Lb./Sq. Inch -65' 73.50 160'
7.4 8.8 1.8
7.7 11.4 3.1
6.2 11.8 2.4
... 4.2
...
...
... ...
...
...
Adhesives cured 2 weeks at room temperature prior to testing. 6
Temp., F.
Cardolite 6885, %
Impact, Ft. Lb./ Sq. In.
73.5 73.5 73.5 -65.0 -65.0
0 20 25 20 25
3.3 18.6 4.5 14.9 10.5
At 73.5' F. impact resistance was, on the average, higher than a t -65' F. with 0 to 20% concentration of Cardolite 6885, but impact values were lower at 73.5' F. for plasticizer concentration from 25 to 50% than a t -65' F. The plasticizer is thus more effective a t 73.5' F. for concentrations up to 20% and more effective a t -65' F. for concentrations beyond 20%. At 160' F. impact strength decreased with addition of plasticizer. The addition of Cardolite 6885 to Cardolite 7019 was not too effective in improving the impact property of the cashew-epoxy adhesive. The best concentration was 20'%, with an- increase in impact from 10.7 to only 11.8 ft. lb./sq. inch. Beyond 20% co.ncentration, the plasticizer proved extremely detrimental-e.g., 4.2 ft. lb./sq. inch with the use of 25Yc Cardolite 6885. In any case, there was no composition of Cardolite 7019 with plasticizer as good on impact or shear as Epon 828 with Cardolite 6885 (90 to 10 for impact resistance and 80 to 20 for shear strength). Shear. Epon 828 and Cardolite 7019 followed a similar pattern in shear as in impact, Cardolite 7019 was stronger a t -65' and a t 73.5' F. than Epon 828, bur a t 160' F. the performance was reversed. The shear strength of Epon 828 was improved by the addition of plasticizer a t all three testing temperatures. However, the effectiveness of the plasticizer varied with the temperature, being greater a t -65' F. than a t 73.5' or
160' F. (Figure 1). There was almost a threefold increase in shear strength on going from 0 to 15% plasticizer concentration a t -65' F. At 73.5' and a t 160' F. the shear values increased at 5% concentration, remained virtually constant between 5 and 15%, and increased again a t 2001,. Generally, high concentrations of plasticizer induce flow; therefore, any further increase in
plasticizer beyond 20% would not appreciably increase the shear strength and, as observed with impact, a drop in strength may result. The shear strength of Cardolite 7019 was not improved with plasticizer a t 73.5' or 160' F. In fact, there was a drop in strength. O n the other hand, a t -65' F. the shear strength increased from 523 to 1006 p.s.i. with 5% plasti-
Table II. Effect of Curing Conditions on Shear Strength of Epon 828 and Cardolite 7019 Modified with Cardolite 6885" Shear, P.S.I. Epon 828 Cardolite 7019 Cardolite Curing Curing Curing Curing condition Ab condition BE condition Ab condition Be 6885, % 0 5 10 15 20 30 40 50
408 597 587 601 734
...
...
...
1534 1 I48 1036 1030 1249 1226 939 890
845 496 485 568 471
... ... *..
1450 1727 2032 1593 1360 416 626 379
Tested at 73.5O F., 50% R.H. Cured 2 weeks at room temperature prior to testing. Cured 2 weeks at+roomtemperature, further cured 3 days at 71O C., and postcured at 90' C. for 1.5 hours.
Table 111.
Cardolite 6885, %
Effect of Curing Conditions on Impact Strength of Epon 8 2 8 and Cardolite 7019 Modified with Cardolite 6885" Energy to Produce Failure, Ft. Lb./Sg. Inch Epon 828 Cardolite 7019 Curing Curing Curing Curing condition Ab condition B" condition Be condition Ab
8.1 15.9 10.7 8.9 8.8 11.9 8.7 8.8 14.8 10 11.1 11.4 17.2 15 11.5 12.2 11.8 20 8.4 4.2 10;9 30 6.3 5.9 5.9 40 5.8 1.3 4.0 50 Tested at 73.5O F., R.H. Cured 2 weeks at room temperature prior t o testing. Cured 2 weeks at room temperature, further cured 3 days at 71O C., and finally postcured 1.5 hours at 90' C. 0
5
3.3 4.1 12.8 12.8 18.6 4.5 3.3 4.9
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cure. However, in many instances, the strength of the room temperaturecured adhesives increased with time and eventually approached the strength attainable at the higher temperatures.
0.25 W
I
I
1060 2000 3000 4000 CALCULATED STRESS, P. S . I.
0
Figure 2. Effect of length of lap joint on shear stress for Epon 828 Higher shear stress values can be expected when length of lap joint is reduced
cizer, and any further increase in concentration decreased the strength slightly. In all cases observed, the shear strength was maximum a t -65’ F. and lowest at 160’ F. (Table I). Some compositions serve best only for shear or only for impact. Further, a specific composition may serve best only a t one given temperature or for a narrow range of temperatures. The best over-all composition for a temperature range of -65’ to 160’ F. consists of 10% Cardolite 6885 and Epon 828. Elevated Temperature Cure. A series of shear and impact specimens was prepared using adhesives formulated from Epon 828 with plasticizer and from Cardolite 7019 with plasticizer. The specimens were cured at room temperature for 2 weeks, heated at 71’ C. for 3 days, and finally postcured at 90’ C. for 90 minutes. Testing was conducted a t 73.5’ F. and 50% R.H. (Tables I1 and 111). Elevated temperature cure produced an appreciable increase in observed shear strengths for both types of epoxy adhesives. O n the other hand, the impact strength of the Epon 828 series showed only a small increase, whereas the Cardolite 7019 series indicated almost no improvement. Elevated temperature cure of these adhesive compositions produced adhesives with higher strength than can be obtained from room temperature
z r i 6000
2000
1000 OVERLAP,
INCH
Figure 3. Effect of length of overlap on load to produce failure A constant value for load required to produce failure will be reached as lap extension is increased
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Preparation of Adhesive Choice of Composition. The choice of epoxy composition depends upon the strength required and the temperature to which the adhesive will be subjected. Table I shows values of measured shear and calculated impact values which can be expected from different formulations when subjected to temperatures of 160”, 73,5’, and -65’ F. Additional intermediate values could be obtained with corresponding intermediate compositions. Preparation a n d Application. Adhesive composition consists of the epoxy resin to which the plasticizer has been added. Both should be weighed with an accuracy of =t5% of the weight involved and the curing or hardening agent with an accuracy of + l % of the weight involved. After thorough mixing, the adhesive can be applied to surfaces by brush or spatula. Both bonding surfaces should be coated with adhesive. The adhesive generally sets within several hours at room temperature. Elevated temperatures may be used to accelerate curing time. Working or Pot Life. The working life of the epoxy mixture after addition of curing agent is generally between 0.5 to 4 hours, depending upon plasticizer-epoxy ratio, type of amine hardener, ambient temperature, and quantity of mixture. After this time, the mixture becomes too viscous for satisfactory spreading, so the curing agent should be added at the time of usage. Epoxy resins without curing or hardening agent have excellent storage qualities and may be stored for almost any length of time at normal conditions. The hardening agent is also stable at room temperature, though it may change color on prolonged storage. Test Procedure. A typical specimen used for impact testing is shown. Bond thickness, on the average, was maintained at approximately 0.002 to 0.004 inch. All adhesives were cured at room temperature (except for the series cured a t elevated temperatures) and conditioned for one week at 73.5‘ F. and 50% R.H. prior to testing. Tests were conducted a t 73.5’, -65’, and 160’ F. Lap-shear joints were prepared from cold rolled steel 1,’s inch thick with the bonding area surface ground. All specimens were vapor degreased prior to application of adhesive, and bond thickness was kept between 0.002 to 0.004 inch. Impact and shear tests were conducted at temperatures of 160°, 73.5’, and -65’ F. (4).
INDUSTRIAL A N D ENGINEERING CHEMISTRY
The length of overlap (width being constant) is an important factor in measuring the shear stress of an adhesive in a lap joint (7-3). The over-all effect is a diminishing shear stress with increasing length of overlap; simultaneously there is a correspondingly slower rate of increase of failure load which tends eventually to approach a constant value. Accordingly, a length of overlap will be reached beyond which any further increase in length will show no increase in strength. Such was the case observed with the use of Epon 828 applied onto two steel strips in a lap joint (Figures 2 and 3). An increase in overlap from 0.25 to 1.25 inches raised the breaking load from 683 to 730 pounds while simultaneously reducing the breaking stress from 2730 to 584 p.s.i. The rate of change of stress with change of overlap described is that obtained from Epon 828 and only under the conditions of preparation and test herein specified. Figure 2 does not represent all adhesives, but for most adhesives the relationship will assume a similar shape. The major significance of this specific test is not the actual values but rather the variance of shear stress that can result. Therefore, studies of epoxy adhesives were based on overlap length of 1 inch, and the values obtained were expected to be much lower than would result if a shorter lap length, say 0.5 inch, were used. Accordingly, the shear values obtained in this investigation are measured stresses resulting from an actual area of 1 square inch having dimensions of 1 X 1 inch. The 1-inch overlap was chosen because a small positive or negative deviation will not cause an appreciable error in stress.
Acknowledgment The authors wish to thank Eileen Kelly and Edward Duda for their efforts in the preparation of test specimens, and M. Chmura for his assistance in testing.
literature Cited (1) Aero Research Tech. Notes, Ciba Ltd., Duxford, England, No. 53 (May 1947). (2) Aircraft Eng. 16, 115, 140 (1944). (3) Bruyne, N. A. de, Houwink, R., “Adhesion and Adhesives,” Elsevier, New York, 1951. (4) Federal Specification MMM-A-175, Government Printing- Office, Washton, D. C. (5) Stivala, S. S., “Polymer Processes,” C. Schildknecht. ed., Cham 10. Interscience, New York, 1956. ( 6 ) Wasserman, D. (to H a r d Gorp.), U. S. Patent 2,665,266 (Jan. 5, 1954).
RECEIVED for review November 19, 1956 ACCEPTED October 30, 1957 Use of trade names does not express endorsement of any product by the U. S. Army.
