**6.3 Water diffusion in nanocomposites**

The barrier property is a function of cross-link density of the thermoset. The free volume for such densely cross-linked network is small, and the chain segments are very stiff. These two factors resist the penetrant transport in the matrix. Since the glass transition temperature is far higher than ambient, the thermoset can be described as an amorphous material trapped as a frozen mass below its glass transition. The dependence of diffusion on cross-linking is reflected in nonlinear behavior and also wide range of diffusivities reported in literature for rigid thermosets and FRPs based on these.

Thermoset nanocomposites were studied by many researchers to observe the effect of inclusion of nanoparticles in highly cross-linked networks, such as epoxy, USP and VE.

**Figure 9.**

*A typical process for polymer-nanocomposite using ultrasonication.*

**Figure 10.** *A typical arrangement for pre-bound process.*

Epoxy-amino functionalized carbon nano fiber (CNF) composite was studied by Prolongo et al. [51] and concluded that the CNF effectively controlled the extent of unbound free water, which fills the nano-voids, without swelling (type-I water). However, the final water uptake was not much different than FRP without CNF. Balgis et al. [52] studied MWCNT and milled carbon (spherical graphite and chopped micron scaled carbon fiber) in epoxy resin and found that the dynamic modulus improved by addition of these reinforcements by about 8%. The uptake of water was slow, and the ultimate water was reduced by 12% compared with the neat epoxy resin, and after hydrothermal aging, the dynamic modulus was marginally changed from 2900 MPa to 2700 MPa, but the glass transition temperature changed by about 10–11°C.

Maheshwari et al. [53] studied the effect of nano silica on sea water diffusion of unsaturated polyester resin nanocomposites at varying temperatures (40–60°C) and salinity (0–25%). The effect of inclusion of 3% nano silica in distilled water reduced the saturation water uptake from 0.65 to 0.52% approximately, at ambient temperature, while for a 4% saline water, which is slightly more saline than sea water, the saturation with 3% nano silica is about 0.46%, compared with neat USP at 0.6%. Their study also showed a gradual decrease in saturation of water in nano silica content. See et al. [54] used a organically modified montmorillonite (OMMT) clay treated with a modification agent known as X-treatment using an organically reactive dispersion agent (commercially restricted) in unsaturated polyester resin to make a gel coat with improved barrier property against water immersion/moisture diffusion. It is seen that the moisture uptake increased upon addition of 1% OMMT from 1.74% for without clay to about 2.17%, and with a X-treated OMMT, the figure is about 1.9%. The study clearly shows that the inclusion of the OMMT nanoclay did not reduce the ultimate water uptake, nor it could improve upon glass transition compared with the neat resin after moisture saturation, but the diffusivity was reduced by about 25–30%, and the saturation time is same as the neat resin coating. The life extension by formation of the clay-nano composite cannot be expected to be significant. Shah [55] used two different types of surface-treated OMMT with vinyl resin and studied water diffusion and its effect on properties of the resin-clay nanocomposite. The study indicated similar result as reduction of diffusivity but not the ultimate water uptake and no significant difference in glass transition.

Burla [42] studied the absorption-desorption cycles of water in Cloisite 10A nanocomposites of epoxy, polyester, and vinyl ester thermosets at various values of relative humidity and immersion in water at 25°C under a tensile stress to the extent of 17% of their ultimate tensile strength. Although the diffusivities were reduced upon in addition of the clay, but the ultimate extent of water uptake did not reduce and the time to reach saturation was not improved.

The nanocomposites in all above cases were seen to be non-Fickian in diffusion and in most cases had slightly higher moisture uptake at saturation. This is possibly due to the bound water molecule at the surface of the clay, which is more hydrophilic due to more dOH groups present in the clay compared with that in the resin. A study on fractional free volume and nano hole size distribution was done by Patil et al. [56] with epoxy-Cloisite 10A clay nanocomposites. The authors used Positron Annihilation Lifetime Spectroscopy (PALS) to determine the subnanoscopic free volume in the nanocomposite. The fractional free volume decreased with clay incorporation, but at higher loading, the decrease did not follow a simple linear mixing rule with respect to the volume fraction of the clay, but the reduction was more. This is possibly due to more interaction of the clay with the resin. PALS results showed strong (repulsive) interactions between the clay and the epoxy matrix at lower clay concentrations, which decrease at higher clay concentrations due to the clay-intercalated structure. However, the nanohole size distribution showed an interesting feature. The nanoholes became smaller in size, but the size distribution broadened with respect to nanohole volume beyond the original maximum nanohole volume. There was a net increase in total void volume, although average nanohole volume reduced from 0.075 nm<sup>3</sup> to about 0.05 nm<sup>3</sup> on incorporation of 7.5% cloisite 10A. The reduction of size and increase in overall hole volume are, of course, a function of the clay-resin interaction, for example, in this case it was repulsive.

A study was done by Rath et al. [57] with USP-Cloisite 15A nanocomposites on the reason for reduction of mechanical properties with increase in clay loading. PALS technique was used to find the free volume change on incorporation of the clay. It was seen that clay loading caused an increase in fractional free volume, suggesting a lower chain packing efficiency in these intercalated USP/clay nanocomposites. This could be a reason for higher or at the most similar water uptake on saturation by the clay nanocomposites.
