High temperature adhesives needs

C except for small area bonding. In addition, flexible bagging material and sealants to operate at these temperatures are a problem. Those companies that have a special need for a high temperature adhesive often develop a material for that particular application and do not rely upon a commercial source. These special needs vary from the joining of electronic elements to the bonding of ceramic components in a missile. It is unlikely that a single... 

For durable bonds, adhesives should be tough. However, many high temperature adhesives (addition-type polyimides, bismaleimides (BMI), and acetylene-terminated resins) are brittle and need to be toughened by elastomers. 

There are several other classes of polymers being examined for high-temperature adhesive and composite applications, such as phthalo-cyanines, " aromatic nitriles,polytriazines, ladder polymers such as poly-bis(benzimidazophenanthroline) (BBP polymers), " and polyphenylene." Many of these materials are not commercially available and much more work is needed before successful applications will be found. 

Few applications for high temperature adhesives outside of the industries just discussed are recognized today. However, the current high-level of interest in high-performance plastics and ceramics could generate a need for high-temperature adhesives. 

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. 

The high-temperature glassy or crystalline transitions of the linear aromatic and heterocyclic polymers were an important drawback for die structural aerospace applications, which need a good flow for the adhesive or composite formulations. 

Modem installations often impose furnace conditions so severe that refractories other than lire-clay are needed. High aluminum and silicon carbide refractories are typical of tlie.se. The heal conductivities of the super-refractories are larger than those of lire-clay brick, and such construction should be backed up with high temperature insulation. Silicon carbide blocks are the most refractory and have the quality of resisting clinker adhesion heitcr than ordinary fire-brick. Their fusion temperature is about 4000 F (2204 C)... 

These adhesives require relatively high cure temperatures (above 130°C). In the past this had not been a problem, because the adhesive cured in the paint bake cycle. However, with newer paint systems and efforts to reduce energy consumption, lower-cure-temperature adhesives are needed, and epoxy-urethane adhesives and other flexibilized adhesives are beginning to replace the plastisols. 

Accessory materials and equipment used to process and apply epoxy adhesives can also be hazardous. Solvents that are used either to dilute the epoxy adhesive or to clean equipment after use can be both dangerous and toxic. Mixing equipment can run at very fast speeds, and caution needs to be exercised when one is operating such equipment. Curing equipment often runs at high temperatures and can create a fire or explosion hazard. Electrical hazards are also associated with most equipment. 

Early attempts to use mimosa tannin in particleboard adhesives involved high-temperature alkaline treatment of the extract to reduce viscosity of the 40% solids level needed (43,44) Subsequent improvements followed the same course as with plywood, namely the use of phenol-formaldehyde or phenol-resorcinol-formaldehyde as crosslinking agents (45) and the use of catalysts or mix modifications to reduce press temperature requirements and to extend pot life. Recent work (46) has shown that exterior chipboard adhesives can also be prepared by crosslinking of mimosa tannins with 4,4-diphenylmethane diisocyanate. 

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