Assessment of moisture barrier, mechanical, and thermal property of base/nanophased carbon-epoxy composites in seawater

Abstract

Polymeric composites absorb moisture by quick surface absorption followed by slow diffusion through the matrix in humid environment. Moisture absorption damages the matrix and interface of fiber reinforced polymer (FRP) composites and consequently, weakens their mechanical and thermal properties. Effects of seawater on mechanical and thermal behavior of base and nanophased carbon fiber reinforced epoxy polymer (CFRP) composites were investigated in this study. It was observed that moisture barrier property of carbon/epoxy composites can be improved by adding a small amount (1 to 3% by weight) of nanoclay as filler. Base and nanophased carbon/epoxy composite panels were fabricated by the vacuum assisted resin transfer molding (VARTM) process and samples were prepared from these panels for mechanical and thermal testing according to ASTM standards. Some of these samples were exposed to seawater for 180 days. Moisture intake by the samples was measured every 10 days for 90 days by which time samples reached a saturated state. Mechanical and thermal behavior of base and nanoclay-infused carbon/epoxy composites with exposure to seawater for 180 days were compared with those with no exposure to seawater for 180 days. Mechanical characterization was performed by compression test. Thermal characterizations were carried out by dynamic mechanical analysis, thermo-gravimetric analysis, and thermo-mechanical analysis tests. Results showed that the 2 wt% nanoclay loading is optimum in terms of moisture barrier capability, compression, and thermal properties. After 90-day exposure to seawater, the 2 wt% nanophased composite absorbed 0.39% moisture compared to 0.67% moisture absorption by the base carbon/epoxy composite. Compressive strength and modulus were decreased by 8.20% and 7.11% respectively, for 2 wt% nanophased composites submerged in seawater for 180 days compared with identical samples with no exposure to seawater. Corresponding data from thermal characterization tests indicate storage modulus and glass transition temperature to be 6.59% and 6.1 °C lower respectively, and the coefficient of thermal expansion to be increased by about 6% for the 2 wt% conditioned sample. SEM study revealed that the nanophased composites exhibited a better bonding between the fiber and matrix compared to the base one even after exposure in the seawater. It is concluded that the excellent barrier capacity, higher surface area, and high aspect ratio of nanoclay are responsible for enhanced durability of the nanophased carbon/epoxy composite in seawater compared to the base one in the same condition.