Durability of epoxies

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... 

De Nicola, A. J., and Bell, J. P., Synthesis and Testing of p-Diketone Coupling Agents for Improved Durability of Epoxy Adhesion to Steel, in Adhesion Aspects of Polymeric Coatings, K. L. Mittal, ed., Plenum Press, New York, 1983, p. 443. 

The outdoor durability of epoxy bonded joints is very dependent on the type of epoxy adhesive, specific formulation, nature of the surface preparation, and specific environmental conditions encountered in service. The data shown in Fig. 15.19, for a two-part room temperature cured polyamide epoxy adhesive with a variety of fillers, illustrates the differences in performance that can occur due to formulation changes. Excellent outdoor durability is provided on aluminum adherends when chromic-sulfuric acid etch or other chemical pretreatments are used. 

The factors that influence the durability of epoxy adhesives in outdoor weathering conditions are... 

Apicella, A., Nicolais, L., Nobile, M. R., Castiglione-Morelli, M. A. Effect of processing variables on the durability of epoxy resins for composite systems, Comp. Sci. Tech., in press... 

Polyfunctional Chelating Agents for Improved Durability of Epoxy Adhesion to Steel... 

The low shrinkage and good durability of epoxies also fits them for capable jointing compounds. Here a measure of flexibility is desirable. The cured article may need to be worked on from time to time, so systems that can be cut or peeled off may be required. Usable lives of 15 min to 1 h are the norm. Components should be selected for good water resistance (Section VI.E). 

Chelating Agents for Improved Durability of Epoxy Adhesion to Steel," Org. Coatings and Appl. Polym. Sci. Proceedings, 489 (1982). 

As a result of their high aspect ratio and electrical conductivity it has been established that carbon nanotubes can form electrically conductive networks in epoxy adhesives and polymer matrix materials and ultimately make them electrically conductive which triggers the opportunity to develop in-situ SUM method. Another important factor is that the addition of carbon nanotubes served to increase the bonding strength and durability of epoxy joints [18]. 

Aluminum powder is often included to modify the thermal conductivity of epoxies. It also reduces undercut corrosion and hence improved adhesion and durability of epoxy adhesives between bare steel substrates, perhaps by a sacrificial electrochemical mechanism. 

Figure 9. Effect of surface treatment on the durability of epoxy/aluminum joints exposed to room temperature water immersion. (1) Anodized, (2) grit-blasted plus vapor degrease, (3) vapor degrease, (4) chromate conversion coating. Reprinted from ref. 3, p. 71, by courtesy of Marcel Dekker Inc.

DeNicola and Bell have reported attempts to improve durability of epoxy-bonded steel joints using coupling agents containing )8-diketone and polyfunctional mercaptoester chelating groups. The success of this approach again depends on the hydrolytic stability of the chemical bonds formed with the oxide surface. 

Narasimha Murthy H N, Sreejith M, Krishna M, Sharma S C and Sheshadri T S (2010), Seawater durability of epoxy/vinyl ester reinforced with glass/carbon composites , J Reinf Plast Compos, 29, 1491-1499. doi 10.1177/0731684409335451. 

Cabral-Fonseca, S., et al. (2009). Durability of epoxy adhesives used to bond CFRP laminates to concrete structures. 17th International Conference of Composite Materials - ICCM17, Edinburgh, UK. 

Vick, C. B., et al. (1998). Reactivity of hydroxymethylated resorcinol coupling agent as it affects durability of epoxy bonds to douglas-fir. Wood and Fiber Science, 30, 312-322. 

The durability of epoxy-aluminium joints that used a homopolymerised epoxy resin was studied by researchers based in Spain [15], and the effects of relative humidity, temperature, and salt concentration analysed. The homopolymerised epoxy resin absorbed little water (1.5 wt%) because of its non-polar network structure. Increasing relative humidity and temperature enhanced water uptake, but the joint strength remained constant because of epoxy plasticisation. A saline environment was damaging to the adhesive joints because of metal corrosion, but was not significantly harmful to the epoxy resin because of the lower diffusion coefficient of salt water. The decrease in glass transition temperature of the epoxy adhesive due to water absorption was dependent upon only the amount of absorbed water and was independent of hydrothermal ageing conditions. The durability of epoxy adhesive joints made underwater has been studied [16]. 

Figure 8.12 Durability of epoxy/mild steel joints employing various silane-based primers [49].

Figure 8.15 Effect on the durability of epoxy/carbon steel joints of a pretreatment based upon preactivation of the substrate with aqueous ammonium citrate followed by application of hexanetriol trithioglycolate as a primer [71].

Ti—Zr-based pretreatment provided improved durability of epoxy-bonded AA6060 aluminum joints relative to an alkaline etch and deoxidation pretreatment but was inferior to a chromate treatment in terms of adhesion and resistance against FFC. The presence of cathodic intermetallic a-Al(Fe,Mn)Si particles in AA6060 is detrimental ... 

The particles cause excessive Ti—Zr oxide deposition in their vicinity during pretreatment, resulting in poor durability of epoxy-bonded joints in humid environments... 

Lunder O, Lapique F, Johnsen B, Nisancioglu K. Effect of pre-treatment on the durability of epoxy-bonded AA6060 aluminum joints. Int J Adhes Adhes. 2004 24 107-117. 

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