HAROLD H. LEVINE
HIGH TEMPERATURE
STRUCTURAL
ADHESIVES FOR THE FUTURE
T&zs military hardware --tomosrow's civilian produst
t is true that present needs for high temperature adhesives are primarily military and potential volume is relatively small. Consequently industry has investigated less exotic but more profitable systems for nonmilitary use.~. But jet aircraft and the Manhattan F'roject originated as a result of military need. What are missiles now may evolve into passenger-carrying space vehicles of the future. For exploring environments where temperatures are rapidly approaching pyrolytic intensity, research in high temperature adhesives must be accelerated. It lags behind design requirements by a considerable margin. For example, a 2000 mile-an-hour plane is on the drawing board, but it cannot be built until suitable adhesives are developed. The lag has resulted because temperatures at which adhesives must function have increased spectacularly. Twelve years ago, the first structural adhesive useful at 300' F. was considered a major accomplishment. In a relatively short time, this was followed by a 500' F. system. Today, we are attempting to find a material useful at 1000° F. and higher, yet problems for materials
I
WHAT IS A niGn TEMPERATURE ADHESIVE?
To be delined precisely, exposure conditions must be describad.
The term can be
applied to a material capable of performing under any of several time-temperature combinations-e.g., 30,000 hours at 350", 1000 hours at 500°, 1 hour at 1000°, or 5 minutes ot lsoOoF.
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INDUSTRIAL AND E N G I N E E d I N G CHEMISTRY
usable at lower temperatures remain unsolved. Adhesives to withstand these extreme temperatures are not available. About 1952, various defense agencies realized that design requirements were rapidly outpacing progress with purely organic adhesives. Consequently, they initiated intensive efforts, in both industrial and government laboratories, to develop new high temperature adhesives. New improved materials were found, such as silicones and fluoro polymers, hut these were ineffective as structural adhesives. Emphasis was then shifted to semi-inorganic and inorganic polymers, chelates, ceramics, and cermets. Much original and interesting chemistry resulted from these efforts. Various study cycles emerged, which concentrated on ceramic adhesives, s-triazine polymers, phosphonitrilic halides, phosphinoboranes, ferrocene derivatives, and others. But each material had shortcomings. Generally, the new polymers failed to reach useful molecular weights because their insolubility caused premature separation from the reaction phase. Hydrolytic resistance of some was poor and caused depoly-
merization or other adverse effects. Thermostability of others was high in inert atmospheres hut poor in air at much lower temperatures. Many were either so highly cross-linked or inert that they were useless powders. The situation was further complicated because thermal, hydrolytic, and oxidation stability, along with the ability to adhere to steel, were only part of the over-all requirements. Also, structural adhesives must have toughness, some peel strength, processability, a reasonable modulus, and curing conditions that are compatible with line as well as laboratory processes. These failures provided valuable lessons and a much stronger foundation from which adhesive chemists could launch more sophisticated probes. The following conclusions are illustrative. GUIDEPOSTS FOR FUTURE RESEARCH
Thermal stability without oxidation stability is virtually useless. In too many instances workers
conducted weight-lossstudies and thermogravimetric and differential thermal analyses in only inert atmospheres. VOL. 5 4
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The reverse procedure is more realistic: I t is safer to =me that if an adhesive is stable in air, it is at least equally stable in an inert atmosphere. Invariably, stability is higher in argon or nitrogen. Heat- and oxidation~esistanceof compounds used to lpnthesisc polymers i not necessarily imparted to the product. Melamine is extremely &ant, but melamine-formaldehyde polymers are d e s s for exposures as low as 500" F. Similarly, very heatstable metal chelates of ketones have been known for many years, but invariably the c o r m p o n d i g polymers are less stable. Senritive functional groups or bonds a r e not the d y cause of hydrolytic instability. Low molecular weight, low cross-linking density, or spatial geometry of the molecule are important facton. For example, an epoxy resin, if insu5ciently cured, will fail to prevent hod deterioration, but when properly cured, its stability under identical conditions may be outstanding. AB expoawe temperature increases, high crossIiukbg density and nonporolity b-e less sign%oxidation and thermal degradation are rate piuccsm which are temperature- as well as time*dent. At higher temperatures, decomposition of a dense of highly crowliied resin can become so rapid as to swamp any benefits observed at lower temperatures. Also, reaction kinetics can change. For example, at a certain temperature, a system may degrade by a chemical oxidation mechanism, but at bigher temperatures it may degrade by free radical nsctions which result from a new set of h e t i c conditions.
Even though they oxidize more readily, highly uow-linked phenolics are better high temperature adhesives than epoxies with relatively lower crossliuking density. Organic polymen can be surprisingly heat-stable and many are oxidation-resistant as well. Aromatic ether polymers typify both heat and oxidation resistance. Also, unusual stability is found in polynudear hydrocarbons which can be considered as benzene polymers. For example, 1,Z-, 3,4-, 5,6-, and 7,8-tetra(l,8-naphthylene)-anthacene (I) and its quinone are very heat stable-the hydrocarbon remains unmelted at 500' C. (72) and the quinone melts at 481' to 483'.
Linear polymenzimidazole polymers have been described recently (17), which have high melting points (400' C.) and excellent heat and oxidation resistance. These polynudear polymers become more exciting when it is realized that they have molecular weights up to 54,000, are hydrolytically stable, and can be converted into strong polymer films. Unlcss new polymers are developed, formulation research cannot provide an adhesive that t stable at high temperatures. The term, high temperature adhesive cannot be defined precisely because exposure conditions must be described. However, the remainder of this article deals with adhesives capable of satisfactory performance for an hour at 1000° F.on stainless steel exposed to air.
wny
ORGANIC POLYMERS SHOULD BE INVESTIGATED
Ceramic adhesives have serious shortcomings. Their heat and oxidation-resistance is better than other compounds, but attempts to &cumvent brittle-
ness have failed. Knowledge of organic polymeriaation and polymers is more advanced than that of the newer semiinorganic or inorganic polymers. Recent developmcnta in organic systems are
promising. Organic polymers generally are easier to modify by fivfher chemical re+s. Thus, properties can be wried and mol& can be tailored to meet
speci6c rcqnirements. T h e shift from organic polymer research, caused by increasing pressure of requirements and Russian progress (Sputnik I) was too great. Research in semi-inorganic and inorganic areasmust be continued, b u t a readjustment in emphasis may be necessary. Research should increase on polynuclear organic polymers, particularly those exhibiting high resonance stabilization energies. These monomer units must be connected with L i g e s that are also heat- and oxidationstable. The Macallum polymers [poly(phenylene sulfides)] typify this approach (8) and are stable to about 460' C. (5). Sulfur compounds other than sulfoxides, sulfones, or sulfonic acids were previously considered too oxidation-sensitive and therefore sulfur-containing polymers were practically ignored. Another example is the polybenzimidazole system (7 7) which can be regarded as aromatic nuclei Liked by the very heat- and oxidationstable imidazole rings: NH
NH
Another example illushates polymers containing s-triazine rings connected by stable urea moieties (70).
Harold H . Lduinc is with t b Research and Developmenl Dinision, Nmmco Industries, Inc., Tclccomputing Gorp., San Diego 17, Calif.
AUTHOR
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INDUSTRIAL A N D ENGINEERING C H E M I S T R Y
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!
These examples have one important common denominator-namely, a fresh approach which resulted from applying new thinking to an old area. I n an area as difficult as high temperature structural adhesives, conditioned thinking can be detrimental. New thinking is needed also in antioxidant research. Value of these materials in elastomers requires no further comment, but for use under drastic temperatures, much new work is necessary. Antioxidants need not operate by reaction with oxygen. In fact this can be a shortcoming because it involves consumption of the stabilizer. At high temperatures free radicals can form which render an oxygen scavenger inoperative. Heat-stable free radical inhibitors or scavengers require investigation. Condensed, polynuclear hydrocarbons are promising and show a remarkable ability to retard gellation of silicone fluids at high temperatures ( 7 ) . Also they inhibit autoxidation of benzaldehyde (7). Recent work has shown that perylene inhibits benzaldehyde formation almost completely when diphenylmethane is oxidized by air (6). Benzaldehyde can only result by bone rupture (hence degradation) between the aliphatic methylene carbon-aromatic carbon linkage ; probably by a hydroperoxide-induced free radical reaction. Antioxidants could also be regarded in terms of negative catalysis. I t is believed that ferric ions can migrate into the adhesive from the stainless steel adherend. Iron promotes more rapid oxygen degradation of an adhesive (2). Arsenic compounds caused a marked increase in oxidation resistance ( 4 ) . Arsenic pentoxide may react with ferric ions to produce insoluble ferric arsenate and thus block oxidation. Because its effects are well known, oxygen has been considered a number one problem in degradation of high temperature adhesives. Yet by using proper techniques,perhaps it can be converted to a beneficial reactant. Infrared spectra show that carbonyl absorptions appear when phenolics are heated in air. Indeed, the model compound, diphenylmethane, gives fair yields of benzophenone as well as benzaldehyde, benzylbenzoate, and other compounds (6). Benzophenone involves no rupture of bonds and is completely unaffected when air is blown through the refluxing compound. One would suspect that a more stable polymer would result if bond-rupturing oxidations (degradation), such as benzaldehyde and benzylbenzoate formation, could be prevented, while more heat- and oxidation-stable keto groups are permitted to form from the methylene group. This is the tantalizing thought about condensed, polynuclear aromatic hydrocarbons which permit benzophenone formation from diphenylmethane while inhibiting formation of benzaldehyde. Applying this concept to practice would convert oxygen into a useful role. To say a t this stage that a high temperature adhesive can be flexible is contradictory. All such adhesives depend on rigidity, imparted by high cross-linking density, and as a result are brittle. Some flexibility is necessary to show good tensile shear strength. When substantial loads are applied to an ordinary tensile shear overlap
specimen, a peeling moment is introduced along the edges of the bond. If flexibility is present, peel can be overcome to a certain amount, and results in high tensile shear values. Chemists have been unable to solve this problem for high temperature applications. A general approach may be to increase oxidation resistance of a rubber-based adhesive such as nitrile rubber-phenolic system. This type of adhesive has good heat stability for long periods-13,000 hours at 350" F. ( 9 ) . Some sacrifice in flexibility can be tolerated if oxidation resistance is improved. Organic polymers can be used to synthesize semiinorganic adhesives. The problems of low molecular weight and insolubility can be avoided by allowing an organic polymer which already has a useful molecular weight to react with an inorganic reagent to give a new polymer. An adhesive containing arsenic pentoxide as the sole curing agent was found useful for 10 minutes at 975" F. or 1000 hours at 500" F. ( 4 ) . The curingmechanism was investigated (7) and it was shown that the polymeric alkoxysiloxane Bisphenol A-condensation product in the adhesive system reacted with the arsenic pentoxide. This resulted in the formation of a new polymer containing repeating units of Si-0-As (V) in the chain with heat-resistance organic polymer and phenyl side groups. This new type of polymer reaction should be capable of extension to other organic polymers and inorganic reactants. The cohesive strength of high temperature adhesives needs to be increased. So far, all attempts to produce a true adhesive failure have been unsuccessful. Even where there appeared to be bare metal, a t least a thin layer of adhesive remained. The layer may have been only a monolayer thick, but failure did occur in the adhesive. Thus, adhesion is always stronger than cohesion. If cohesive strength could be increased, more advantages could be realized from adhesive strength. Such research would benefit all types of adhesives, and when problems such as oxidation stability have been solved, the results would be applicable to high temperature adhesives. SUGGESTED READING
(1) Acton, Moran, Silverstein, J . Chem. Eng. Data 6, 64 (1961). (2) Black, Blomquist, WADC Tech. Rept. 55-330, June 1955. (3) Dum, Waters, Roitt, J . Chem. SOC.1954, 580. (4) Janis, Boram, Susman, Riel, WADC Tech. Rept. 59-11, February 1959. (5) Lenz, Handlovits, J . Polymer Science 43, 167 (1960). (6) Levine, U. S. Navy NOW-61-0254-c, Quart. Rept. 2, March 1961. (7) Levine, Boram, U. S. Navy NORD-19075, Quart. Rept. 2, 3. (8) Macallum, J . Org. Chem. 13, 154 (1948).
(9) Narmco, R&D Division, Telecomputing Corp., unpublished. (10) Reimschuessel, Lovelace, Hagerman, J . Polymer Science 40, 270 (1959). (11) Vogel, Marvel, Zbid., 50, 521 (1961). (12) Zander, Chem. Ber. 92, 2740 (1959). VOL. 5 4
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