Epoxy water uptake

Another type of thermoset polymers is based on epoxy monomers. These thermosets are cured by use of a two-component system or by photo initiators. Disadvantages of epoxies are high water uptake in service and polymerization shrinkage (22). 

Figure 5.25 shows that the glass transition of composites containing carbon fibers may be affected by water uptake. The glass transition of carbon fiber/PEEK composite remains the same under diy and wet conditions. But carbon fiber/epoxy composites may experience a decrease in Tg as high as 77°C depending on the properties of the matrix resin." "... 

Treatment of glass beads with epoxy silane improved the adhesion of glass beads/epoxy eomposite which resulted in a substantially lower water uptake and... 

Corresponding with the main part of water uptake T gY , decreases from 177.4°C initially to about 105 °C within 35 days (Fig. 30.20) due to the plasticizing effect of water. Then the principal glass transition stays at this temperature. The plasticization shifts by -72 K, which is very high in comparison with shifts of -20 K to -30 K reported for other hot-cured epoxies [18, 19]. 

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. 

Applications of FTIR on Epoxy Resins -Identification, Monitoring the Curing Process, Phase Separation and Water Uptake... 

Fig. 2.22. Water uptake plots for a range of epoxy adhesives (Ref. 26).

Fig. 4.21. Theoretical water concentrations within a lap joint bondline quadrant, after immersion in water at 20 °C. Adhesive aliphatic amine cured epoxy at 20 °C. M, = mass o.f water absorbed at time t. = equilibrium mass water uptake at time t = . Coefficient of diffusion D = 6.6 X 10- m /s (H2O, 20 °C).

Effect of water on the adhesive. The influence of water on the adhesive is generally reversible, so that any deterioration in, say, mechanical properties is recovered upon drying. The extent of this influence depends upon the adhesive s composition. All polymers absorb greater quantities of water when above their Tg, so that rubbery materials tend to show greater water absorption than rigid adhesives. Interestingly, the key position in structural metal-to-metal adhesives for airframe construction is occupied by epoxy-nylon adhesives, some of which display water uptakes of the order of 14%. 

The water absorption behavior of the clay-epoxy nanocomposites in terms of maximum water uptake and diffusion coefficient are given in Table 9.18 [47]. The presence of nanoparticle as reinforcement reduces the water absorption of the composite system. The maximum water uptake of epoxy decreases gradually with the increase in clay content. The maximum water absorption decreases by 14.1,17.9 and 24.8% after the incorporation of 1, 3 and 5 wt% nano-clay, respectively, compared with neat epoxy. The presence of high aspect ratio nanofillers can create a tortuous pathway for water molecules to diffuse and enhances the resistance to water absorption. The diffusivity also decreases in the same manner and a significant reduction in diffusivity is obtained for the composites containing 5 wt% clay [112]. 

Table 9.18 Maximum water uptake and diffusion coefficient (D) of clay-epoxy nanocomposites. Reprinted from [47] with permission from Elsevier.

Liu, W, Hoa, S. and Pugh, M. (2005), Fracture toughness and water uptake of high performance epoxy nanocomposites , Composites Science and Technology, Yol. 66, pp. 2116-2125. 

In addition to decreasing water uptake and increasing temperature resistance, interest has also focused on materials for use in severe environments. For instance, in one such system, cycloaliphatic epoxy resins such... 

In a similar study, Brewis et al prepared sandwiches of a nylon-epoxy adhesive between aluminum adherends and exposed them to water vapor labeled with tritium for up to 6 weeks at 50°C. The bonds were then cut into small rectangular sections and the fractional water uptake measured by liquid scintillation counting. The values were compared with those calculated for diffusion and were found to agree well for the outer pieces. For the inner pieces the measured uptake was less than the predicted value. The adhesive contained a carrier and the results seem to imply that this hinders the passage of water into the adhesive bond. These results show... 

By measuring water uptake, the diffusion coefficient and equilibrium concentration of water for the bulk adhesive were obtained at different temperatures. A value of 37 kJ/mol was also calculated for the activation energy of diffusion. A value for the plane-strain stress intensity factor, Kic, for the bulk adhesive was obtained using compact tension specimens. Tensile butt joints were prepared from mild steel blocks bonded with the epoxy adhesive and the fracture stress determined as a function of time of exposure to water at the different temperatures. An activation energy of 32 kJ/mol was calculated for joint failure, in close agreement with that obtained for the diffusion of water. This supports the view that the processes responsible for loss of joint strength are controlled by water diffusion. It was found that joints exposed to 20°C/55% RH showed no reduction in strength, even... 

Seawater aging of epoxy and vinyl ester resins reinforced with different fibers (e g. glass and carbon) has also been investigated (Narasimha Murthy et al., 2010). The experimental results have evidenced that the vinyl ester composites retain better their mechanical properties than epoxy ones in particular, the flexural strength and ultimate tensile strength (UTS) dropped by about 35% and 27% for glass/epoxy, by 22% and 15% for glass/vinyl ester, by 48% and 34% for carbon/epoxy, and by 28% and 21% for carbon/ vinyl ester composites, respectively. In contrast, the authors have shown that the water uptake of the epoxy-based composites is lower than that of the vinyl ester ones. 

Becker O, Vailey RJ, Simon GP (2004) Thermal stabiUly and water uptake of high ptaformance epoxy layered silicate nanocomposites. Eur Polym J 40(1) 187—195 Bergaya F, Jaber M, Lambert IF (2012) Qays and clay minerals as layered nanofillers for (bio) polymers. Green En Tech 50 41—75... 

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