Abstract
In this research, the influence of water absorption on the dielectric properties of epoxy resin and epoxy micro-composites and nano-composites filled with silica has been studied. Nanocomposites were found to absorb significantly more water than unfilled epoxy. However, the microcomposite absorbed less water than unfilled epoxy: corresponding to the reduced proportion of the epoxy in this composite. The glass transition temperatures (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> ) of all the samples were measured by both differential scanning calorimetry and dielectric spectroscopy. The T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">g</sub> decreased as the water absorption increased and, in all cases, corresponded to a drop of approximately 20 K as the humidity was increased from 0% to 100%. This implied that for all the samples, the amount of water in the resin component of the composites was almost identical. It was concluded that the extra water found in the nanocomposites was located around the surface of the nanoparticles. This was confirmed by measuring the water uptake, and the swelling and density change, as a function of humidity as water was absorbed. The water shell model, originally proposed by Lewis and developed by Tanaka, has been further developed to explain low frequency dielectric spectroscopy results in which percolation of charge carriers through overlapping water shells was shown to occur. This has been discussed in terms of a percolation model. At 100% relative humidity, water is believed to surround the nanoparticles with a thickness of approximately 5 monolayers. A second layer of water is proposed that is dispersed but sufficiently concentrated to be conductive; this may extend for approximately 25 nm. If all the water had existed in a single layer surrounding a nanoparticle, this layer would have been approximately 3 to 4 nm thick at 100%. This "characteristic thickness" of water surrounding a given size of nanoparticle appeared to be independent of the concentration of nanoparticles but approximately proportional to water uptake. Filler particles that have surfaces that are functionalized to be hydrophobic considerably reduce the amount of water absorbed in nanocomposites under the same conditions of humidity. Comments are made on the possible effect on electrical aging.
Highlights
EPOXY resins are highly crosslinked amorphous polymers used for the insulation of power transformers, switchgear, rotating machines, etc
We present measurements of the dielectric spectrum, water uptake, swelling and density change, depression of glass transition temperature, and low-frequency charge transport processes to support the hypothesis that water collects around silica nano-particles whose surfaces have not been made hydrophobic
Water diffusion in epoxy materials was measured at room temperature and nominally 75% RH, which was controlled by NaCl saturated solution (75.29 %RH at 298K, according to literature [19])
Summary
EPOXY resins are highly crosslinked amorphous polymers used for the insulation of power transformers, switchgear, rotating machines, etc. Epoxy resins can absorb up to a few weight percent of water in a humid environment, leading to an overall degradation of the dielectric properties [3]. Such a situation might be further exacerbated by inorganic fillers, which are often used to improve their mechanical and thermal properties and to reduce their cost. The synthesis of nanometer-sized particles is becoming routine Because their length scale is comparable to that of polymer molecules and the high specific area of the particle surfaces within a composite, nanoparticles exhibit novel properties as fillers. It is possible that a weakness caused by water will have a detrimental effect on the otherwise improved mechanical [10][11] and electrical behaviors experienced by incorporating nano-fillers
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