Enhanced mechanical properties of surface modified and silica reinforced nomex sandwich composites

Nomex cored glass fiber sandwich composites find their use in several aerospace, automotive, and military applications due to their high specific mechanical properties. This study reports a novel procedure to modify/enhance the mechanical properties of Nomex sandwich composites through surface modification as well as surface nano-silica reinforcement. Surface modification of Nomex was conducted in dilute phosphoric acid solution with varying soaking times. In another set of experiments, the surface of Nomex was reinforced with different concentrations of silica particles by dipping in nano-silica suspension (NexSil™ 8). A combination of chemical modification and silica reinforcement treatments were investigated. Scanning Electron Microscopy (SEM) was used for morphological characterizations while Energy Dispersive Spectroscopy (EDS), Atomic Force Microscopy (AFM), and Fourier Transform Infrared Spectroscopy (FTIR) were used to analyze the chemical changes at Nomex surfaces. The mechanical properties of the composite samples were investigated using bend tests (i.e., 3-point and 4-point) and flat-wise compression tests. It was observed that the acid treated samples displayed an increase in flexural strength with etch time. Nano-silica reinforced composite samples also led to an enhancement in flexural as well as compressive strengths with each dip. In contrast, it was found that the compression strength values of the composite samples increased only for short acid treatment time (i.e., 30 sec), and decreased thereof for longer duration(s). Enhancement in flexural strength values were attributed mainly to an increase in Nomex-epoxy interfacial physio-chemical changes. This study demonstrates that surface treated Nomex sandwich composites, through the procedures outlined here, possess superior mechanical/structural properties as compared to non-treated ones.