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.
The long-term structural viability of a six-ply watercontaminated quartz fiber-reinforced bismaleimide laminate is investigated via dynamic mechanical analysis and flexural strength assessment over a four-year experimental time frame. Water contamination is achieved via immersion in water at 258C for short-term (2 weeks and 1 month), long-term (6 months), and very long-term (4 years) duration. Long-term moisture uptake exhibits distinctly non-Fickian behavior. Maximum moisture content exceeds 1.5% by weight after four years of immersion. Laminates exhibit a remarkable resistance to degradation for all exposure durations. Flexural strength decreased by roughly 5% in the worst case. Further, no significant change in glass transition temperature was observed. Scanning electron microscopy revealed no micro-crack formation and a relatively low incidence of fiber-matrix debond. Experimental evidence suggests that quartz fiberreinforced bismaleimide is a viable option for longterm moisture-prone structural applications at moderate temperatures. POLYM. COMPOS., 00:000-000, 2016. POLYMER COMPOSITES-2018 FIG. 2. Experimental flexure stress vs. strain for dry, and watercontaminated (immersed two weeks and one month) six-ply BMI/quartz laminates. FIG. 7. Storage modulus vs. temperature of long-term (over four years) water-immersed BMI/quartz laminates.
The potential of nanoclay reinforcement to improve radome performance and longevity is quantified via a resonant technique. Epoxies used for radome applications are susceptible to environmental degradation through moisture absorption. Moisture in composite systems can degrade mechanical and dielectric properties, which is of particular concern in radome applications where low dielectric properties are crucial for maintaining radar transparency. The addition of nanoclay may prove a viable method for dielectric and structural performance improvement through moisture absorption minimization. The dielectric properties of an epoxy/montmorillonite nanocomposite are evaluated as a function of nanoclay weight percentage and moisture content using a split-post dielectric resonator operating at 10 GHz. An increase of 25% in relative permittivity and 480% in loss tangent is observed for nanocomposites contaminated with 8.4% water by weight in the most extreme case. The addition of 2% nanoclay by weight effectively delayed a 16% degradation in relative permittivity by 760 hours.
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