Thermal Stability of the Adhesive

This increase in polymer thermal stability translates to improved thermal stability of the adhesive, as shown in Fig. 10 for the steel lapshear adhesive strength after thermal aging at 121 °C for 48 h. 

Fillers can also be used to promote or enhance the thermal stability of the silicone adhesive. Normal silicone systems can withstand exposure to temperatures of 200 C for long hours without degradation. However, in some applications the silicone must withstand exposure to temperatures of 280 C. This can be achieved by adding thermal stabilizers to the adhesive formulations. These are mainly composed of metal oxides such as iron oxide and cerium oxide, copper organic complexes, or carbon black. The mechanisms by which the thermal stabilization occurs are discussed in terms of radical chemistry. 

Reducing the temperature by 75-100°F dramatieally improves the thermal stability of packaging adhesives, resulting in significant cost savings for equipment maintenance, as well as greater worker safety. Such adhesives became possible with the availability of low MW EVA base polymers (MI of 800 and above). They rely on low MW refined paraffin wax and a blend of resins chosen for the specific application [67,68]. 

The position and thermal stability of the 1670 cm band together with the fact that its intensity increases with increased milling time suggests that it may be attributed to water in a very strongly hydrogen bonded environment. It is hypothesised that this band indicates the presence of a type of adhesive water which holds together the aggregates of milled kaolin particles. 

The inherent thermal stability of the phenol formaldehyde chemistry is preserved but with the crosslinking characteristics of the epoxy groups. However, epoxy novolacs also form very rigid and brittle polymers when fully cured because of their high crosslink density. For this reason, they are often used as modifiers in epoxy adhesive systems rather than as the base polymer. 

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. 

Oxane bonds, M—O—Si, are hydrolyzed during prolonged exposure to water but reform when dried. Adhesion in composites is maintained by controlling conditions favorable for equilibrium oxane formation, ie, maximum initial oxane bonding, minimum penetration of water to the interface, and optimum morphology for retention of silanols at the interface. The inclusion of a hydrophobic silane, such as phenyltrimethoxysilane [2996-92-1], with the organofunctional silane increases thermal stability of the silane and make the bond more water resistant (42). 

Compatibilization seems to be of industrial interest in several ways besides the improvement of the phase dispersion and adhesion, leading to superior mechanical properties, it also often can prevent the minor crystallizable dispersed phase from fractionated or retarded crystallization, which make faster production times and higher thermal stability of the products possible. 

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