Hybrid syntactic foam core cased natural-glass fibre sandwich composite
Authors:   , Afolabi Olusegun Adigun 
Published: July 2023

Composite materials comprised of two separates with different properties to form a single material that reflect the properties of the combined materials. Syntactic foam composites (SFC) are made from the combination of hollow glass microspheres and epoxy resin. They are lightweight and used as a core in the hybrid sandwich composite. Hollow glass microspheres (HGM) are high strength microballoons that provide closed cell porosity and help to reduce material weight. SFCs made of HGM, and resin matrix are used as the core in sandwich composite material and reinforced with natural or synthetic fiber materials. The sandwich syntactic foam composite (SSFC) has a wide range of applications in the marine, aerospace, structural, and automobile industry. Therefore, it is important to investigate their physical, mechanical, thermal, and morphological properties to achieve high strength and low density. Most of the previous work in literature employed the use of different fillers and core materials in sandwich composite but are limited in strength because of their high density. In this study, a single HGM filler was employed as heterogeneous and homogenous by varying into four different particle sizes to investigate the effect of these particle sizes on the mechanical and physio-mechanical properties of the SFC used as the core in the SSFC. The effect of wall thickness and radius ratio of the HGM on the microstructural properties of SFC was also determined. The heterogeneous and homogeneous SFC was fabricated by degassing method mixing the epoxy matrix with HGM filler, the filler was varied into five-volume fractions of 5, 10, 15, 20, and 25%. The functional group of the HGM filler and the neat epoxy was determined and compared with that of the SFCs fabricated using Fourier Transform Infrared Spectroscopy (FTIR). The results showed that the filler contain various functional groups such as hydroxyl group, phenol-OH, aldehyde C-H group, aromatic proton, epoxy group, which enhanced the bonding process. It was determined that the intensity of the SFCs for all the volume fractions increased more than the neat epoxy due to the shifts in the peaks representing the filler and the matrix groups. The physical (density, water absorption, buoyancy) properties and the mechanical (hardness, tensile, flexural, and impact) properties of the SFCs improved significantly compared with the neat epoxy. The Scanning Electron Microscopy (SEM), Dynamic Mechanical Analysis (DMA), and Thermo-gravimetric Analysis (TGA) were also used to determine the morphological structure, the viscoelastic properties, and degradation temperature of the HGM and the neat epoxy and compared with the fabricated SFCs. The surface of the HGM showed the microballoons in their different sizes before separation. The surface of the SFCs showed the epoxy matrix, matrix porosity, microballoons porosity, and microballoons structure in their mixed state. It was an indication of good interaction between the epoxy matrix and the HGM filler using degassing processing method. The DMA showed improved storage and loss modulus values by 9% and above 100% respectively compared to the neat epoxy and the TGA showed better glass transition Tg values of 4.5% and 2.7% at 20% and 55% weight loss respectively compared to the neat epoxy. This indicated that good interaction and interfacial bonding existed between HGM and the epoxy matrix and because of lower density and void content. The SFC was used as the core to fabricate a lightweight sandwich syntactic foam composite (SSFC). The SSFC was made into four different orientations (kenaf-SFC-kenaf, as KK; glass –SFCglass, as GG; glass/kenaf – SFC – kenaf/glass, as GK; and kenaf/glass –SFC- glass/kenaf, as KG) using kenaf and glass fibers as reinforcement. The physical properties (density, water absorption capacity, and buoyancy), mechanical properties (hardness, tensile, compression, and flexural), morphological properties (SEM), and acoustic properties were determined. The porosity of KK increased by 21.6% because the kenaf fiber is less dense and more porous in terms of water absorption which makes it require higher buoyancy force to stay afloat. The mechanical properties results showed that GK and KG have the highest hardness, flexural and compressive strength of 70.2%, 74.4%, and 42.7% respectively, while GG has improved tensile strength of 210.96% increase than KK. The acoustic properties results showed that GG improved in sound level (P) dB by 24.1% compared to KK, while the sound pressure (Lp) dB does not show a significant difference in the SSFC. In conclusion, the degassing processing method of SFCs improved its physical and mechanical properties by reducing the density using particle distribution analysis (PSA) and particle variation analysis (PVA) with the aid of a gas pycnometer, and porosity values thereby making it a suitable core material for the sandwich composite. A novel sandwich syntactic foam composite (SSFC) material was fabricated by hybridizing the face-sheets in different layering pattens. The SSFC physical and mechanical properties improved significantly with the use of hybrid fibers. Hence, this study has demonstrated that for structural and marine purposes, hybrid fibers can perform better as reinforcement in the sandwich composite than using a single fiber.