High-temperature-resistant adhesives epoxies

The purpose of this book is to provide, in one volume, an overview of structural adhesives. One chapter will be devoted to each of the major classes of structural adhesives, emphasizing the chemistry of the base resin and the main end uses for the adhesives of that class. The choice of systems is restricted to synthetic resins that are of current industrial interest for structural bonding. Some, such as the phenolics and epoxies, have been used successfully for many years and are of considerable industrial importance. Others, notably the structural acrylics and cyanoacrylates, are generating much interest and will probably become more widely used for industrial applications in the future. The newer polymers, for high-temperature-resistant adhesives, are currently of limited use most activity in these systems is at present still in the research and development stage. The desire for higher-temperature-resistant materials is creating much interest in these polymers and adhesives based on them will undoubtedly become important in the future. 

Structural adhesives are formulated from epoxy resins, phenolic resins, acrylic monomers and resins, high temperature-resistant resins (e.g., polyimidcs), and urethanes. Structural adhesive resins arc often modified by elastomers. 

Benzyldimethylamine (BDMA) is another tertiary amine that can be used as either a sole catalyst or an accelerator with other curing agents. It is used with DGEBA epoxy resins at 6 to 10 pph. The pot life is generally 1 to 4 h, and the cure will be complete in about 6 days at room temperature. When used by itself, BDMA can provide epoxy adhesive formulations with high-temperature resistance (Chap. 15). However, BDMA is mostly used as an accelerator for anhydride and dicyandiamide cured epoxy resins. 

Polyacetal polyphenylene oxide are widely used as engineering thermoplastics, and epoxy resins are used in adhesive and casting application. The main uses of poly(ethylene oxide) and poly(propylene oxide) are as macroglycols in the production of polyurethanes. Polysulfone is one of the high-temperature-resistant engineering plastics. 

Epoxy, modified epoxy, and urethane adhesive are commonly used in weldbonding aluminum. Epoxy and polyimide adhesives are used for titanium. Polyimides are particularly suitable for titanium because of their very high temperature resistance (matching the resistance of titanium). Epoxy and modified-epoxy adhesives are available in one-or two-part liquid, paste, capillary, or unsupported-film form. ... 

Polyimide and polyphenylene sulfide resins present a problem in that their high temperature resistance generally requires that the adhesive have similar thermal properties. Thus, hi -temperature epoxies adhesive are most often used with polyimide and PPS parts. Joint strength is superior (greater than 1000 Ib/in ) but thermal resistance is not better than the best epo3 systems (300 to 400°F continuous). 

Polymer blends are often used in adhesive formulations where properties associated with rigid polymers (high temperature resistance, chemical resistance, etc.) must be obtained along with properties associated with tough, elastic polymers (impact strength, high peel strength, etc.). Examples of these adhesive systems are nylon-epoxy, phenolic-nitrile, epoxy-polysulfide, epoxy-nitrile, and epoxyurethane. 

Polysulfides and stability, and compatibility with epoxies high temperature resistance RT rapid cure times Poor performance at Consumer adhesives ... 

The sandwich construction method mentioned above involves joining metals to other materials by bonding. Further examples include the bonding of brake linings (phenolic adhesives) and compound materials in ski manufacture, where aluminum is bonded to plastics, wood, etc. (phenolic and epoxy adhesives). Highly alloyed steels, beryllium and titanium alloys, and other special metals can be bonded with adhesives (e.g., polyi-mides, polybenzimidazoles) that have comparable high-temperature resistance. 

A wide range of chemistries have been evaluated to provide adhesives that have properties that are an improvement over epoxies and phenolics. The primary property that has been sought is an improvement in the high-temperature resistance of these adhesives. Bis-maleimide is one of the chemistries that have been evaluated for this purpose. The stmcture of a his-malermide is shown in Figure 9. Bis-maleimides can be cured into a stractural adhesive by one of three reaction mechanisms. The first one shown is the reaction of a bis-maleimide with an amine via a Michael addition reaction. The second one shown is the Diels-AIder reaction of a diene and the bis-maleimide. A third mechanism (not shown) is radical addition. 

Phenohc resins (qv), once a popular matrix material for composite materials, have in recent years been superseded by polyesters and epoxies. Nevertheless, phenohc resins stiU find considerable use in appHcations where high temperature stabiHty and fire resistance are of paramount importance. Typical examples of the use of phenoHc resins in the marine industry include internal bulkheads, decks, and certain finishings. The curing process involves significant production of water, often resulting in the formation of voids within the volume of the material. Further, the fact that phenoHcs are prone to absorb water in humid or aqueous conditions somewhat limits their widespread appHcation. PhenoHc resins are also used as the adhesive in plywood, and phenohc molding compounds have wide use in household appliances and in the automotive, aerospace, and electrical industries (12). 

As shown in Table 15.5, the epoxy plastics have fair resistance to high temperatures and have good mechanical properties. Cured epoxy resins are resistant to nonoxidizing acids, alkalis, and salts. Because of the presence of polar hydroxyl pendant groups, these polymers have good adhesion to substrates such as wood or metal. 

Much attention has been paid to the synthesis of fluorine-containing condensation polymers because of their unique properties (43) and different classes of polymers including polyethers, polyesters, polycarbonates, polyamides, polyurethanes, polyimides, polybenzimidazoles, and epoxy prepolymers containing pendent or backbone-incorporated bis-trifluoromethyl groups have been developed. These polymers exhibit promise as film formers, gas separation membranes, seals, soluble polymers, coatings, adhesives, and in other high temperature applications (103,104). Such polymers show increased solubility, glass-transition temperature, flame resistance, thermal stability, oxidation and environmental stability, decreased color, crystallinity, dielectric constant, and water absorption. 

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