Epoxy network water diffusion

The addition of microspheres lowers the glass transition temperature of the epoxy binder (Fig. 13). This seems to be because the filler causes defects in the matrix network. Equal diffusion coefficients of filled and unfilled epoxy binder indicates, therefore, that the diffusion processes are insensitive to binder changes. The sorption of water by epoxy resins is in fact known to depend mainly on their polarity and only slightly on the three-dimensional compactness of the network. 

Apparently, there is less free volume in the aged polymer network. Any water coming into the aged network would tend to swell the polymer because there are simply less vacant sites in the distribution of free volume. In short, there is more polymer-solvent interactions as water diffuses into an aged epoxy network. 

Moisture is a well-known plasticizer for macromolecules (14). Specifically, water penetrates into an epoxy network and can lower the glass temperature of the resin (15). In this report, moisture has for the first time been utilized as a probe to characterize denslfIcatlon process during epoxy aging. Also, using the same rationale, heavy water diffused Into the epoxy resin Is used to study the Interactions of moisture with the aging polymer by hydrogen-2 (deuterium) NMR spectroscopy. 

The structure-diffusivity relationships are not clearly established. A rough correlation between the diffusivity and the reciprocal of hydrophili-city is usually found. For example, for networks absorbing less than 1.5% water, such as unsaturated polyesters and anhydride-cured epoxies, the coefficient of diffusion is often higher than 5 x 10 13 m2 s 1 at 20-50°C. In contrast, for epoxies cured by amines absorbing more than 2% water, the coefficient of diffusion is of the order of 10 13 m2 s 1 at 20-50°C, and it tends to decrease with W% in most of the structural series. 

Numerous studies have been reported on the effect of diffusion of water into epoxy resins 177 178>. It is generally agreed that moisture acts as a plasticizer which lowers the Tg of the resin uo 134). Very little work, however, has been reported on the effect of physical aging on the diffusion behavior of water into network epoxies. This Section of the review summarizes the first attempt to study such an effect on TGDDM-DDS epoxy/water interactions. 

Not only can the water molecule pass through gaps in the network because of its small size, but also because it is compatible with (i.e. effectively soluble in) the polymer. Thus polar adhesives are naturally hydrophilic whereas non-polar plastics, such as PVC and Polythene, are not i.e. like permeates like . The solubility of water in epoxies is of the order of a few mass per cent, and the coefficient of diffusion of water at 20 °C is around 10 m s . 

Figure 7.20. Water vapor permeability, diffusion, and sorption coefficients vs. network composition for a SIN based on a polyurethane and an epoxy. 5)...

Musto, R, et al., Prohing the Molecular Interactions in the Diffusion of Water through Epoxy and Epoxy-Bismaleimide Networks. J. Polym. Sci. Part B-Polym. Phys., 2002. 40 922-938. 

Musto P, Ragosta G, Scarinzi G, Mascia L (2002) Probing the molecular interactions in the diffusion of water through epoxy and epoxy-bismaleimide networks. J Polym Sci B Polym Phys 40(10) 922-938... 

Keywords Interlayer adhesion loss Epoxy coatings Weathering Erosion mechanism UV absorber Humidity Water Crosslinking Network degradation Oxygen diffusion and solubility Photoproducts Oxidation zone Rain Lag time UV radiation degradation... 

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]. 

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