Heat-resistant adhesives stability

It has been previously outlined that the thermal stability of organic materials is a time-temperature dependent concept. Thus, heat-resistant adhesives are those that can be safely used for thousands of hours between 200 and 300°C. This definition excludes the epoxy and epoxy-phenolic adhesives which are, however, the predominant classes of polymers used in aeronautics. Epoxy adhesives based on... 

A preaccelerated, medium reactive, thixotropic, terephthalic/NPG resin offering good chemical and heat resistance, hydrolytic stability and good adhesion to PVC in the manufacture of chemical tanks, ducts, pipes. 

Resistance to weathering. Zinc oxide and magnesium oxide stabilize poly-chloroprene against dehydrochlorination. Further, zinc oxide helps vulcanize the rubber, and magnesium oxide reacts with /-butyl phenolic resin to produce a resinate which improves heat resistance of solvent-borne polychloroprene adhesives. 

New copolymers based on a copolymerization of isobutylene and p-methyl-styrene with improved heat resistance have been reported [64]. Once copolymerization was accomplished, the polymer was selectively brominated in the p-methyl position to yield a terpolymer called EXXPO. In contrast to butyl and halobutyl, the new terpolymer has no unsaturation in the backbone and therefore shows enhanced thermal stability and resistance to oxidation. Useful solvent-based adhesives can be formulated using the new terpolymer in combination with block copolymers [65]. The hydrocarbon nature of the new terpolymer results in excellent compatibility with hydrocarbon resins and oils. 

Cured phenolic resins have outstanding heat resistance, resistance to cold flow, good electric (insulation) properties, and good dimensional stability. Phenolic resins have good adhesive properties and are employed in the production of sandpaper, abrasive wheels, and brake linings. These resins are also used as casting resins. 

Addition copolymers of cycloolefin compounds with a polar substituents in the side chain exhibit excellent heat resistance and transparency. They are also capable of crosslinking to improve the adhesion properties, the dimensional stability and the chemical resistance (35). 

Crosslinked products exhibit superior optical transparency, heat resistance, and adhesion, as well as improved dimensional stability, solvent resistance, and chemical resistance. In addition, crosslinking prevents the occurrence of cracks in films. 

The adhesives are suitable for use in flip chip bonding of semiconductor parts. They exhibit good shelf stability, productivity, strength properties and heat resistance. The electrical properties, such as dielectric constant and dielectric loss tangent are highly satisfactory. 

Epoxy Coreactants. One of the most successful epoxy coreactant systems developed thus far is an epoxy-phenolic alloy. The excellent thermal stability of the phenolic resins is coupled with the valuable adhesion properties of epoxies to provide an adhesive capable of 371°C short-term operation and continuous use at 175°C. The heat resistance and thermal-aging properties of an epoxy phenolic adhesive are compared with those of other high-temperature adhesives in Fig. 15.5. Epoxy-phenolic adhesives are generally preferred over other high-temperature adhesives, such as the polyimides and polybenzimidazoles, because of their lower cost and ease of processing. 

Contact adhesives can also be processed in the form of aqueous dispersions, whereby the longer time required for evaporation of the water must be taken into account. These contact adhesives are also more sensitive to moisture. Typical examples of contact adhesives are solutions of polychloroprene (CR) (e.g., Baypren firom Bayer AG, Neoprene from DuPont). The solvents are xylene, benzine, cyclohexanone, ethyl acetate, or methylene chloride. CR is characterized by a pro-notmced crystallization behavior with a crystallite melting temperature of 53 C. The crystallinity is controlled in accordance with the specific formula in each case (adhesive contains Zn and Mg oxides that result in partial crosslinkage of chlorine to carbon with double bonds). The adhesive strength and heat resistance increase with the degree of crystallinity, UV stability and flexibility are very high. 

Blends of PPE and PA-66 are suited for exterior automotive applications, where they produce class A surface on body panels, because they combine the processing and solvent resistance of polyamides with the moisture resistance and dimensional stability of PPE. Therefore, they have good heat resistance, impact resistance, moldabUity, paint adhesion and chemical resistance. 

Polypropylene Thermoplastic polymer of propylene. Has low density and good flexibility and resistance to chemicals, abrasion, moisture, and stress cracking, but decreased dimensional stability, mechanical strength, and light, fire, and heat resistance. Processed by injection molding, spinning, and extrusion. Used in fibers and films for adhesive tapes and packaging. Also called PP. 

---