Development and Mechanical Characterization of Halloysite Nanotubes Reinforced Polymer Syntactic Nanocomposite Foams for Weight-Sensitive Structural Applications

Abstract

Lightweight syntactic foam composites exhibit high specific strength and modulus. Thus, these are popularly used, from electric vehicle construction to space applications. In the present work, syntactic foam composites are fabricated using cenospheres. Cenosphere, a waste by-product of thermal power plants, is chosen as hollow filler in composites for eco-friendly redressal curbing its environmental impact. Also, a halloysite nanotube (HNT) is an abundantly available natural nanofiller that is utilized to uphold the load-bearing and thermal characteristics of reinforced syntactic foam (RSF) composites. The RSF composite fabrication involves the probe sonication of HNTs and homogenizing them with an epoxy matrix. Later the cenospheres are gently mixed in the HNTs/epoxy blend to obtain a uniformly dispersed mixture and is thus solution casted in the aluminum molds. A constant content of 1 vol.% addition of HNTs is maintained to fabricate all the RSF composites with cenospheres content being varied from 20 - 50 vol.%. Furthermore, the cenosphere epoxy syntactic foam (CESF) composites are fabricated without HNTs addition for comparison study. In this work, the influence of HNTs reinforcement in syntactic foam on mechanical, water absorption, viscoelastic, and thermal properties are studied. Furthermore, the impact of post-curing on the mechanical and thermal characteristics is also investigated. The tensile and flexural tests are carried out to evaluate the mechanical performance of CESF and HNTs RSF composites. The enhancement in tensile modulus and flexural modulus was witnessed by up to 42% and 66%, respectively, for the HNTs RSF as compared to CESF composites. The morphology studies prove the existence of hydrogen bonding among the HNTs, cenosphere, and neat epoxy matrix in RSF composite. Field emission-scanning-electron-microscopy (FESEM) affirms the unique crack deflection phenomenon by HNTs, thus elucidating the structure-property correlation. Furthermore, the effect of post-curing on flexural and compressive properties is discussed. The post-cured HNTs RSF containing 40 vol.% cenospheres (NSF40_H) exhibited a compressive modulus of 33.2% higher than room temperature cured neat epoxy due to improved crosslinking. The addition of HNTs in NSF40_H augments the flexural modulus up to 26.9% compared to post-cured neat epoxy. iiiMoreover, the glass transition temperature (Tg) of CESF composites with 40 vol.% cenospheres was increased by 24.3 °C compared to the room temperature cured sample. The positive shift in Tg can be attributed to the beneficial impact of post-curing, as indicated by differential scanning calorimetry study. A water absorption study is carried out to characterize the efficiency of the HNTs RSF composites exposed to the marine environment. The HNTs addition considerably reduces the diffusion coefficient, sorption coefficient, and permeability of the syntactic foam composites. The compressive modulus of wet HNTs RSF composite registered a higher value than the corresponding sample without HNTs. Dynamic mechanical analysis with temperature sweep (30 – 140 °C) reveal that the storage and loss modulus of RSFs is 1 - 36% and 59 - 113% higher than the neat epoxy. Storage modulus increases with an increase in cenospheres content in the epoxy matrix. However, with the incorporation of HNTs, the storage modulus obtained is higher than that of neat epoxy but still lower as compared to CESFs. With the increase in cenospheres content, loss modulus reduces due to increased frictional energy dissipation compared to matrix viscoelasticity. The thermal studies depict that the Tg value ameliorates with HNTs reinforcement. Also, better thermal stability with appreciable char content is reported from gravimetric analysis with HNTs addition. Further, to understand the underlying mechanism of filler interaction with the matrix, structure-property correlations of evaluated properties are presented using exhaustive SEM, FESEM, and TEM images.