Epoxy thermal aging

Chelating agents are sometimes used as scavengers to capture undesirable metal ions. These compounds react directly with the metallic substrate, thereby inhibiting its catalytic effects on oxidation. The effect of several different chelating agents on the resistance of epoxy-phenolic bonded aluminum joints to thermal aging is shown in Table 15.5. 

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. 

In order to study the effect of physical aging on the carbon-fiber reinforced epoxy, the freshly quenched materials were then sub-Tg annealed at 140 °C. After annealing for only 10 minutes at that temperature, the toughness of the composite was restored to a level comparable to that of the postcured material (see Fig. 7). It is likely that residual thermal stresses resulted from the quenching were annealed away during this 10 minutes thermal aging at 140 °C. 

Finally, a few articles have appeared on chemiluminescence of polymers. This technique has been used to detect hydroxy radicals in wood oxidation,y-irradiation effects on polyethylene, oxidation of nitrile-butadiene rubber, rubber under stress,antioxidant efficiencies in polyethylene, reactions of peroxy radicals, stereoregularity in poly(propylene), colour development in epoxy resins and structural changes in thermally aged poly(phenylene sulfide). ... 

Fig. 28.1 Initial spectroscopic degree of oxirane conversion U p for epoxy films on Au, Al, and Cu prepared for hygro-thermo-oxidative aging (preparation 1) and for thermal aging (preparation 2).

Fig. 29.3 PALS analysis of the epoxy thin films on an Au substrate. Both ortho-positro-nium formation probability Ps and average lifetime To-ps are depicted as functions of the average positron implantation depth. The data are from an unaged, a thermally aged, and a hygro-thermally aged epoxy film prepared in an identical way. Solid lines are included in the graphs to stress the implantation depth dependence of To-ps and /g.ps for every data set.

Additionally, both aging mechanisms have less effect on in the volume of the epoxy films on a Cu substrate than on Au and A1 substrates for hygro-thermal aging there is an increase of 15 ps instead of approximately 30 ps, and for thermal aging the decrease is 20 ps instead of approximately 25 ps. 

Fig. 29.7 Depth-resolved XPS results of the analysis of the atomic nitrogen concentrations of a hygro-thermally aged thin epoxy film versus the sputter depth of the adhesive.

At selected aging times t, samples are taken for experimental characterization. After three months of immersion, some samples are re-dried under thermal aging conditions at 40 °C and 60 °C, respectively. With the dry samples, the effect of aging in the epoxy network is studied without artifacts from evaporating water during the experiments. 

Fig. 30.13 IR-ATR intensity of the normalized epoxy band (915 cm ) for EPl-PC40 in the bulk during thermal and hydro-thermal aging.

Water induces a similar phase separation at least in the bulk. It breaks the imino ether-like crosslinks specific for the DDA-cured epoxy systems in the bulk as well as in the layers on the two types of stainless steel. The proposed mechanism describes the aging behavior of DDA-cured epoxy systems well, since it explains all the observed effects of chemical modification, irreversible plasticization, and irreversible water uptake. Additionally, it is understood why there is no complete disintegration of the network the hydrolysis cleaves only the imino ether-like crosslinks but not the amine-like or the ether-like crosslinks that are also formed during curing. Hence, after hydro-thermal aging the macromolecu-lar mobility in EP2 is similar in the layers and in the bulk because the content of amine-like and ether-like crosslinks is similar. 

Table 2.52 Summary of thermal aging of epoxy and polyimide system (7)...

Ordinarily solvent cementing or thermal welding is used with PMMA. These methods provide stronger joints than adhesive bonding. Adhesives used are cyanoacrylates, second-generation acrylics, and epoxies, each of which provides good adhesion but poor resistance to thermal aging. "... 

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