Effects of Moisture Absorption on the Dielectric Properties of Nanoclay-Reinforced Epoxy for Radome Applications

Abstract

The relative permittivity and loss tangent at 10 GHz of a nanoclay-reinforced epoxy is investigated as a function of nanoclay loading percentage and moisture content. The energy dissipation associated with frictional and inertial losses during the reorientation of absorbed dipole water molecules exposed to an oscillating electromagnetic field has a significant impact on the relative permittivity and loss tangent of moisture-contaminated polymer materials. This can damage the performance of polymer-based radar-protecting structures (radomes) designed to protect sensitive radar equipment. Thus, prevention or minimization of water absorption in these materials is critical to mitigating this effect. The moisture barrier properties of nanoclay reinforcement are well known, and are targeted in this study as a potential method to reduce the moisture absorption rate and therefore improve the performance of polymer-based radomes exposed to precipitation and humid air. The ability of a water molecule to rotate freely in the presence of an EM field is dependent on its physical and chemical state; whether it be bound and unable to rotate, or unbound and able to dissipate energy through unrestricted rotation. Therefore, any potential dielectric property changes associated with the physical and chemical interaction of water and nanoclay must be quantified prior to exploiting prospective moisture-barrier benefits. In this study, the relative permittivity and loss tangent of an epoxy system reinforced with nanoclay up to 5% content by weight are assessed using a resonant cavity technique at 10 GHz during moisture uptake due to immersion in distilled water at 25°C. Variations in moisture diffusion behavior are observed due to the nanoclay loading percentage. Although deviations in the dielectric properties due solely to nanoclay loading percentage are minimal, effects due to moisture absorption are much more prominent. In the most extreme case, a nearly 15% increase in relative permittivity is observed at 5% moisture content by weight, with a direct correlation between diffusion behavior and degradation of relative permittivity observed for all samples. Likewise, an increase in the loss tangent of approximately 220% is observed at 5% moisture content by weight.

Equilibrium moisture content of a crosslinked epoxy network via molecular dynamics simulations

“…The presence of water in a polymeric system leads to a host of damaging consequences such as degradation of mechanical strength and modulus [14,15] hygroscopic swelling [16][17][18] and the accompanying mismatched swelling stresses, reduction of dielectric strength [19,20], and increase in relative permittivity [21][22][23][24]. The latter is detrimental to the performance of advanced semiconductors that use porous polymers as dielectric materials.…”

Nanoclay-based Polymer Nanocomposites for Electromagnetic Interference Shielding

“…This energy dissipation is associated with frictional and inertial losses during the reorientation of absorbed dipole water molecules when exposed to an EM field. This reorientation or rotation of water molecules has significant impact on the relative permittivity and loss tangent of the polymer nanocomposite materials [30].…”

“…Optimal loading of 2 wt.% showed great extent of dispersion and increased moisture-barrier properties. Several nanoclay loadings revealed a higher moisture uptake rate, which is owing to insufficient clay dispersal in the polymer structure caused by the natural moisture content in the organoclays ahead of usage [30]. The diffusion route of penetrating molecules can be obstructed and altered by polymer composites by including nanofillers with a high aspect ratio.…”

“…The penetrating molecules can be either oxygen (water vapor) or moisture absorbed from the environment. This approach to prevent moisture uptake and water vapor penetration is primarily used in Radome structures in defence applications [30,31]. A tortuous pathway for water diffusion through the polymer-matrix nanocomposite is created by well dispersed nanofillers, thereby lowering the water absorption rate and transverse diffusivity [30].…”

“…This approach to prevent moisture uptake and water vapor penetration is primarily used in Radome structures in defence applications [30,31]. A tortuous pathway for water diffusion through the polymer-matrix nanocomposite is created by well dispersed nanofillers, thereby lowering the water absorption rate and transverse diffusivity [30]. In addition, regardless of the kind of gas molecules, a polymer nanocomposite reinforced with clay reported a decrease in gas permeability.…”