High temperature adhesives durability

For adhesive applications, thermal stability alone is insufficient. Processability of high-temperature polymers is of great importance. These polymers should be processed under moderate conditions with no volatile evolution. The adhesives should be compatible with adherends and should have good, durable properties. For broader uses, the manufacturing cost should be low because cost, so far, has been the major obstacle to developing high-temperature adhesives. 

During the last decade, many high-temperature adhesives have been synthesized for aerospace applications. Supersonic cruise aircraft adhesives must be durable for tens of thousands of cumulative hours at 232 C (500 F), while spacecraft adhesives must withstand... 

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. 

The disadvantages of the urea-formaldehyde adhesives lie in their lack of durability and in their characteristic pungent formaldehyde odor. For particleboard applications subject to high temperature and moisture exposure, phenol-formaldehyde adhesives are required, since the urea-formaldehyde polymer is hydrolyzable and hydrolysis is enhanced with moisture and heat. 

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... 

Oxidative stability depends on the adherend surface as well as on the adhesive itself. Some metal adhesive interfaces are chemically capable of accelerating the rate of oxidation. For example, it has been found that nearly all types of structural adhesives exhibit better thermal stability when bonded to glass or aluminum than when bonded to stainless steel or titanium.12 For any given metal, the method of surface preparation can also determine oxide characteristics, and hence bond durability. Thus, the use of primers is common practice with high-temperature structural adhesives. 

Many applications for adhesives and sealants require high strength and durability at low temperatures. Many of these same applications also require resistance to thermal cycling between high and low operating temperatures. Unfortunately, the properties of adhesives and sealants at low temperatures are not as well studied or documented as they are at high temperatures. 

All of the commercial epoxy adhesives presented in App. B bond well to aluminum and to a wide variety of other materials. Sell22 has ranked a number of aluminum adhesives in order of decreasing durability as follows nitrile-phenolics, high-temperature epoxies, elevated-temperature curing epoxies, elevated-temperature curing rubber-modified epoxies, vinyl epoxies, two-part room temperature curing epoxy paste with amine cure, and two-part urethanes. 

Figure 2 illustrates that the same effect is present for high temperature cured epoxy systems. The DMP-30 composition is more durable than the methylenedianiline cured material because it involves polymerization of the epoxy group, forming a polyether network. Hence, the number of polar groups is reduced and the water durability of the adhesive joint is improved. 

A comparison of the various surface treatments is presented in Table V. The resin used was methylenedianiline cured at high temperature. All of the joints were polished with 600 grit emery cloth and treated with ammonium citrate, with the exception of the control and sand blasted joints. Sandblasting the surface increases the surface area and provides a means of mechanical interlocking of the adhesive with the substrate surface. This does improve the water durability to some extent, but the effect is small when compared to the results achieved with the coupling agents. Ammonium citrate, in addition to activating the surface, has a beneficial effect on durability. 

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