High-temperature-resistant adhesives

Licari, J. J., High Temperature Resistant Adhesives, Product Engineering, December 1964, p. 104. 

PPQ films discolor on heating, but retain their transparency and the major parts of their mechanical properties. To date, PPQs have been tested for application as high-temperature-resistant adhesives and as matrices for bonding materials. In such high-temperature applications, a postcuring must be applied whereby cross-linking occurs as a result of thermal or thermo-oxidative decomposition. 

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. 

During World War II, several new synthetic elastomers were produced and new types of adhesives (mainly styrene-butadiene and acrylonitrile copolymers) were manufactured to produce adequate performance in joints produced with new difficult-to-bond substrates. Furthermore, formulations to work under extreme environmental conditions (high temperature, resistance to chemicals, improved resistance to ageing) were obtained using polychloroprene (Neoprene) adhesives. Most of those adhesives need vulcanization to perform properly. 

Silicones. Silicones are useful where high temperature resistance or compatibility with silicone components such as molded seals are needed. Silicone firewall insulation materials and silicone gaskets and seals are bonded with silicone rubber adhesives. 

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. 

Aluminum powder, in particular, is frequently employed at relatively high concentrations in high-temperature epoxy adhesive formulations. The filler provides improvement in both tensile strength and heat resistance, and it increases the thermal conductivity of the adhesive. Aluminum powder fillers also reduce undercut corrosion and, hence, improve adhesion and durability of epoxy adhesive between bare steel substrates. It is believed that this is accomplished by the aluminum filler providing a sacrificial electrochemical mechanism.27... 

The base polymer, of course, is a key ingredient in a high-temperature epoxy adhesive system. For an adhesive to withstand elevated temperatures, it must have a high melting or softening point and resistance to oxidation. Materials with a low melting point, such as many of the thermoplastic adhesives, may prove excellent adhesives at room temperature ... 

FIGURE 15.5 Comparison of (a) heat resistance and (b) thermal aging of several high-temperature structural adhesives.11... 

Adhesive Polyimide Resin, Film and Paste, High Temperature Resistant, 315°C or 600°F... 

Silicones Adhesives and sealing compounds with the basic framework based on -Si-O- bonds. Available as one and two-component systems (RTV-1 and RTV-2). They are characterized by high-temperature resistance as well as high ageing resistance. 

The use of prefabricated SPS structures and articles has several advantages over monolithic structures, including high quality of manufacturing and repair-ability allowing replacement of individual structural elements. In this regard, the development of acid- and temperature-resistant adhesive compounds and study of... 

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. 

Excellent high temperature resistance When properly cured at elevated temperatures, nitrile adhesives can withstand temperatures above 150°C, with some compounds capable of limited exposure to temperatures above 200°C. 

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