Written by Kerryn Caulfield, Executive Manager of Composites Australia Inc.
In principle, a vacuum has no properties as it contains no air or other gas – it is devoid of matter. Sound can’t be carried in a vacuum as there just aren’t enough molecules for the audio vibrations to move through. There is no air in a vacuum to expand in heated conditions, nor matter to create weight. For hollow spherical shaped ceramic microspheres, the centres of which are a vacuum, the property of “no properties” is what makes them exceptionally functional fillers.
Hollow ceramic microspheres are integrated into composite parts as an additive or reinforcing filler for a variety of product enhancements and process improvements, including to lower the product density, reduce weight and shrinkage, improve flow and wetting of fibreglass with resin, increase impact strength, and thermal insulation and resistance. They can be used in all standard processing methods for thermoset composites including sprayup, hand lay-up, resin transfer moulding, and have found end uses in applications as diverse as fibreglassreinforced materials, automotive brake components and engineered syntactic foams.
E-SPHERES® is the registered name for the hollow ceramic microspheres manufactured by Australia’s own Envirospheres in the South Burnett district of Queensland – just over 200 km north/west of Brisbane. The Envirospheres factory site is a major private employer within the community where it has operated for over 20 years.
The chemical composition of what some call aluminosilicate microspheres is mainly silicon dioxide SiO2 (Silica) 55 – 60%, and Aluminium Oxide Al2O3 (Alumina) 36 – 40%, with a small amount of iron oxide Fe2O3 (Hematite) 0.4 – 0.5%, and titanium dioxide (TiO2) 1.4 – 1.6%. Silica is a combination of silicon and oxygen, two very abundant, naturally occurring materials.
E-SPHERES® work hard for a material that is effectively a chemically unreactive inert substance. Due to the ceramic composition, E-SPHERES® are flame-retardant and non-combustible, which are key properties for many industrial applications including composites. Manny Samano, Business Development Manager for Envirospheres Pty Ltd advises that, “The processing includes drying and sterilising at temperatures above 400⁰ C which produces an absolute pure product, ensuring that the microspheres are suitable not only for composites applications but also in many water based products such as coatings and construction materials.”
The micro-shell structure of E-SPHERES® can survive compressive pressures over 4800 psi, contributed to by its spherical shape, wall thickness and ceramic nature. A sphere is the only shape that has no single weakest point and no single strongest point; anywhere you apply pressure, the stress on the sphere will be the same and its strength is evenly distributed around the entirety of its surface. “Spherical geometry, as well as a smooth surface, improves resin penetration allowing it to penetrate voids in complex moulds, all of which improves cycle times. E-SPHERES® are compatible with all epoxy, polyester, vinyl esther, phenolic and hybrid resin systems,” advises Manny.
Appearing as a fine white powder E-SPHERES® have a unique pale pearl-like colour and have a refractive index of 1.53 which means they can even be used in transparent applications. They range in particle size from 20 to 500 µm in diameter – by comparison, a human hair is approximately 75 µm in diameter. The size of the sphere influences formulation of the part, ultimate finish and end use application. Manny says that as well as density reductions, cost savings can also be achieved thanks to resin extension. “Light weighting composite end products through the use of hollow ceramic microspheres brings both environmental and cost saving benefits, including resin optimisation, lower transport, handling and installation costs.” As non-combustible, non- flammable, nonreactive, non-corrosive and nontoxic, E-SPHERES® are not classified as dangerous goods boosting their environmental credentials.
Applications in composites vary from syntactic foam systems such as deepwater buoyancy and ballistic applications, architectural wall panels, swimming pools and polymarble, to automotive components for trucks and buses.