Of the many fillers now available to composites manufacturers, microspheres, also called microballoons, are the most versatile. To the naked eye, the small, hollow glass microspheres appear like fine powder. Ranging from 12 to 300 µm in diameter (by comparison, a human hair is approximately 75 µm in diameter), microspheres pack a lot of functionality into a very small package. Integrated into composite parts, they provide a variety of product enhancements and process improvements — including low density, improved dimensional stability, increased impact strength, smoother surface finish, greater thermal insulation, easier machinability, faster cycle times, and cost savings. Composite manufacturers, already adept at making the most of their materials, regularly exploit these benefits — sometimes all at once.
CHEMICAL COMPOSITION & CONSTRUCTION
Glass microspheres. In general, a multistep process is used to produce high-temperature hollow glass microspheres. Glass is initially produced at high temperatures from soda-lime-borosilicate, after which it is milled to a fine particle size. Trace amounts of a sulfur-containing compound, such as sodium sulfate, are then mixed with the glass powder. The particles are run through a high-temperature heat transfer process, during which the viscosity of the glass drops and surface tension causes the particles to form perfect spheres. Continued heating activates the blowing agent, which releases minute amounts of sulfur gas that form bubbles within the molten glass droplets. The result is a rigid, hollow glass microsphere manufactured with an eye to increasing crush resistance (that is, the ability to withstand external pressure and avoid fracture of the bubbles) without sacrificing low density.
DENSITY & CRUSH STRENGTH
The most obvious benefit of the hollow glass microsphere is its potential to reduce part weight, which is a function of density. Compared to traditional min-eral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow glass microspheres have much lower densities. For example, at a density of 0.6 g/cc, Sphericel hollow glass microspheres from Potters Industries (Valley Forge, Pa.), an affiliate of PQ Corp., can displace the same volume as talc at one-quarter the weight. Densities and crush ratings, however, vary dramatically across product lines.
Typical loadings are 1 to 5 percent by weight, which can equate to 25 percent or more by volume. For example, Potters’ lightweight Q-Cell hollow glass microspheres have a density (from 0.14 to 0.20 g/cc) approximately one-fifth that of most thermosetting resins. Therefore, on an equal weight basis, Q-Cell spheres occupy about five times more volume than the resin, which can reduce compound weight, VOC content and cost.
Historically, crush strength for hollow glass microspheres has been directly linked to density — i.e., a glass sphere with a density of 0.125 g/cc would be rated at 250 psi (1.8 MPa), while one with a density of 0.60 g/cc would be rated at 18,000 psi (124 MPa). To some degree, there remains a correlation.
The density and crush strength of microspheres made from a particular material will depend, in part, on two structural variables, wall thickness and particle size.
This article comes from compositesworld edit released