I
W. E. ST. CLAIR and R. H. MOULT Verona Research Center, Koppers Co., Inc., Pittsburgh, Pa.
These adhesives can be used for bonding structural parts of aircraft where heat and pressure are not easily applied, where heat-curing distorts metal structures, and in field repair of damaged airplanes
Room Temperature-Cured Resorcinol Epoxide Adhesives for Metals
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resins of commerce are glycidyl ethers (9): derived from bisphenol A and epichlorohydrin, and well known as adhesives for metals, particularly for aluminum, copper, and steel. Such substrates present oxidized surfaces which may be converted to a more polar character by external forces during bonding. The dependence of bond strength in the epoxy adhesive systems on the nature of the oxide film was demonstrated by Murphy and Page (8). Working with aluminum, they found that the type of oxide film after degreasing, acid etching, and rinsing in water and the consequent bond strength were dependent on rinse water temperature and pH. Similar results were obtained in this study. The strength of the bond, whether tested at room temperature or at
180' F., was directly proportional to the temperature of the rinse water up to 160' F.; a discontinuous rise occurred above 160' F. (Figure 1). These results extended the findings of Murphy and Page to room-temperature-cured epoxy adhesives of different functionality, and showed that adhesion was dependent upon surface polarity. Various authors have described the wide range of properties of the bisphenol epoxide adhesives (6). The bond strengths are dependent largely on the curing agent and curing conditions. The curing agents contain two (usually more) active hydrogens, each capable of adding t o the epoxy linkage in the resin. Because these resins are linear chains containing terminal epoxy groups, they have a relatively low functionality. The usual product possesses
only about 1.85 epoxy groups per mole cule. High strength structural adhesives for metals using these epoxides require heat curing. Only by the use of aliphatic polyamines can thermoset, three-dimensional polymers be obtained at room temperature or ambient conditions ( 4 ) . The epoxides react with all materials containing active hydrogens (5). Even with materials of highest
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How the polyglycidyl resins are formed
908
INDUSTRIAL AND ENGINEERING CHEMISTRY
Figure 1. Influence of surface condition of aluminum on bond strength using adhesive formula 76 1 Bond strength i s proportional t o temperature of rinse water-up t o 160' F.
Table 1. Adhesives Prepared by Simultaneous Etherification of Resorcinol and Penacolite Resin B-1A Heat resistance and room temperature activity were improved Resin Resorcinol Ratio, Oxirane Viscosity, Adhesive Oxygen, Cps. at Moles/ No. % 30' C. Mole 176 70/30 12.3 >100,000 186 50/50 11.2 10,500 192 30/70 12.2 1,475
Table II. Evaluation of Solid Amines as Curing Agents" Solid amines can increase pot life but also make the adhesive too brittle
Amineb Triethylenetetramine (liquid control) 2,4-Toluenediamine m-Phenylenediamine p,p'-Methylenedianiline 1,6-€Iexanediamine p-Phenylenediamine Piperazine (I
possible functionality, room temperature cure is very slow or negligible with materials containing active hydrogens, except the polyamines. Epoxide resins from Novolaks (Epok resin, British Products, Inc.) (7-3) and the commercial bisphenol A epoxides, were unsatisfactory room-temperaturecured adhesives. The resorcinol epoxide resins (7), however, were successfully formulated into high strength, roomtemperature-cured, structural adhesives for metals. These materials, resorcinolformaldehyde resins reacted with epichlorohydrin, with two glycidyl ether groups attached to each aromatic nucleus, have even higher functionality than the phenolic epoxide resins. T h e resorcinol epoxide resins were generally resinous solids a t room temperature. However, a thin liquid diepoxide (resorcinol diglycidyl ether) can react with resorcinol resin epoxides to give a material of 100% reactivity and low viscosity. When resorcinol diglycidyl ether and resorcinol resin polyglycidyl ether were prepared simultaneously, heat resistance and roomtemperature reactivity were improved (Table I). When the resorcinol epoxides were cured with liquid aliphatic polyamines, these mixtures gave acceptable strengths, but the adhesive was brittle and the pot life short. I t was postulated that if solid polyamines were used, the pot life would be dependent on the rate of solution in the resorcinol epoxide. A number of solid polyamines were ground intimately with a plasticizer, Thiokol LP-3, and this paste was used in the evaluation (Table 11). I n gen-
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Active Hydrogens
Gel Time, Hr.
6 4 4 4 4 4 2
0.2 2.5 1.3 1.6 0.2 0.5 0.2
Tensile Shear Strength, P.S.I.c Cured 24 Hr. Cured 168 Hr. at R.T. at R.T. 180'F. R.T. R.T. 180'F. 1305 495 1305 1200 2745 1835 280
345 1120 705 250 855 540 50
1095 940 1025 1925 2590 2625 715
1275 1430 1955 455
995 350 130
Adhesive 186 used in all tests. Amines used equivalent to one active hydrogen reacting with one oxirane oxygen. Average of three specimens each, 2024 T3 clad aluminum.
eral, the pot lives were much longer, as predicted. Although the strengths could be increased to a n acceptable value when the adhesive was tested on aluminum a t room temperature, hot strength was generally higher except when excessive amounts of plasticizer were used. This indicated that the adhesive was too brittle, because the high amount of cross linkages interfered with chain alignment and uniform load distribution. The relatively higher strengths at the higher temperatures were apparently due to relaxed internal stresses and a more even distribution of load over the entire bonded cross section. This increase was not due to postcuring, as heated and cooled specimens were also low in strength. Therefore resorcinol epoxides were prepared by simultaneous epoxidation of resorcinol resins and resorcinol; only a part of the hydroxyl groups available reacted (Table 111). This in effect moved the functional centers further apart and increased the flexibility of the bonded adhesive by reducing the number of cross links. After satisfactory strengths in preliminary tests based on partial resorcinol epoxides were developed, adhesive formulations were thoroughly evaluated for qualification against MIL-A-5090B, a general-purpose specification for aircraft adhesives of high strength. The adhesives that are a t present qualified are hot-pressed or heat-cured for at least 1 hour at temperatures in excess of 300' F. (Table IV).
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When the effect of resin-resorcinol ratio in the adhesive formulation was investigated as a function of bond strength, highest strengths a t 180" F.
Table 111. Effect of Degree of Etherification on Pot Life and Tensile Strength Using Toluenediamine as Curing Agent The more complete etherification is, the longer the pot life Adhesive No. Property 192 186 218 Resinresorcinol ratio, mole/mole 30/70 50/50 50/50 Degree of etherification Complete Complete Partial Gel time, hours 3.3 2.1 1.0 Shear strength, p.~.i.~ R.T. 3655 2640 2700 180' F. 1875 1265 2255 a Average value of three specimens, 2024 T3 clad aluminum.
Table IV.
Target Requirements Curing Condi-. tions, Hours, R.T. ~
Type of Test
Room temperature shear strength Shear 180' F. - 67' F. Bend Creep
24
120 24 120 120 120 120
i Fluid exposure Pot life, hr. Viscosity
120
... ...
Minimum Strength, P.S.I. 1875 2500
950 1250 2500 150-lb.load R.T., 1600 p.s.i.,
