Syntactic foams with high performance impermeable microspheres as filler have drawn much scientific research owing to their robustness, easily-tailored low density, and excellent mechanical strength. The microspheres’ low density is critical when syntactic foams are applied as buoyancy materials in high-pressure aqueous environments. Superior mechanical strength to withstand hydrostatic pressure, low density, and high-water resistance are necessary for syntactic foams to suit applications in deep-sea equipment.
To achieve remarkable low density, the density adjuster’s volume percentage is always high, which inadvertently yields many interfaces between the hollow microspheres and the polymer matrix. Also, reinforcing shells should be appropriately designed to complement mechanical strength and binding strength with the matrix. Only this way can adequate buoyancy and deformation resistance of the syntactic foams be achieved. It’s therefore of great importance to tailor reinforcing shells to improve the syntactic foam instant property and property evolution.
Conventionally, silicate glass materials were used to fabricate high-strength syntactic foams. Silicate materials with alkali and alkaline earth metal contents are easy to shape, possess significantly high strength, and are cost-effective. Unfortunately, the alkali and alkaline earth metal oxides present in the glass phase are responsible for hindering the interface affinity between the glass shell surface zone and the resin matrix. Low interface affinity significantly affects matrix infiltration to the glass shell leading to weak interface bonding. Therefore, bonding status and interface infiltration are essential parameters in defining the durability and mechanical strength of syntactic foams. However, there is less research on the surface alkali and alkaline earth metal tailoring and mechanical strength coupling of syntactic foams crafted for marine applications.
Researchers Professor Zhenguo An, Professor Jingjie Zhang and Dr. Jing Yuan, from the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, China, employed specially designed hollow silicate glass microspheres with high strength to weight ratio for the controlled manufacture of high-strength lightweight syntactic foams. They paid close attention to the surface physicochemical status modification of the hollow silicate glass microspheres on the mechanical performance and strength in several external environments. The research work is currently published in the journal Composites Science and Technology.
In the preparation of hollow silicate glass microspheres, a certain amount of alkaline earth metal oxides: Calcium oxide and Sodium oxide were introduced into the reactor to reduce the melting point of silica, which is a dominant hollow silicate glass microspheres component—the objective for introducing the alkaline metal oxides was to achieve better shaping and high strength. X-ray photoelectron spectroscopy was then applied to study the surface properties of the pristine hollow silicate glass microspheres and after acid leaching and calcination.
It was observed that the surface of the hollow silicate glass microspheres, and by extension, the interface of the syntactic foams, could be tailored by acid quenching and heat treatment of hollow silicate glass microspheres. After acid quenching and heat treatment, alkaline metal oxide content was reduced, thereby increasing hydrophobicity. The synergism of these two factors led to an increased affinity between the hollow silicate microspheres and the polymer matrix and improved stability of the syntactic foams in various conditions. It was also observed that surface modification of the hollow silicate glass microspheres improved hydrolytic resistance, fracture toughness, and compressive strength.
The findings of the study suggest that syntactic foam products based on surface pre-treated hollow silicate glass microspheres show promising application potential in the areas of buoyancy materials for deep sea applications. The products are also good candidates as thermal insulation materials applied in aqueous environments under high pressure. The insights to mechanical strength and aging control of syntactic foams gained from the study can be extended to property enhancement of similar composites with different silicates and glass fillers.
Jing Yuan, Zhenguo An, and Jingjie Zhang. Effects of hollow microsphere surface property on the mechanical performance of high strength syntactic foams. Composites Science and Technology, issue 199 (2020) 108309.