Microspheres - Density and Crush Strength

Print

The most obvious benefit of the hollow microsphere is its potential to reduce part weight, which is a function of density.

Compared to traditional mineral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow microspheres have much lower densities.

The density of the sphere will have a huge impact on the formulation of the part. Typical loadings are 1 to 5 percent by weight, which can equate to 25 percent or more by volume.

As important as these properties are, one of the microsphere’s greatest assets is the contribution it makes to part processability, which, in a filler, is a direct function of particle shape.

The microsphere’s small, spherical structure is the perfect shape for a filler. Without exception, the mineral fillers available to composites manufacturers are irregularly shaped. That irregularity results in a relatively large surface area which increases the viscosity of the resin into which the filler is added.

By contrast, the microsphere’s regularity minimises its surface area. The low surface area allows for higher solids loading with less of an impact on the viscosity and flow characteristics of the composite.

Additionally, the microsphere has a nominal 1:1 aspect ratio giving it inherently isotropic properties that composites manufacturers can use to great advantage.

For example, in parts fabricated by a resin injection process, chopped glass fiber, with a high aspect ratio, results in ~60 percent less stiffness in the crossflow direction than in the flow direction because the fibers become oriented in the direction of flow. This alignment of the fibers can contribute to warpage especially when introduced to crystalline matrices, such as nylon or polypropylene, which have molecular chains that also tend to align along flow lines.

Microspheres do not orient and tend to obstruct directional orientation of reinforcing fibers and matrix. The result is that stresses are more evenly distributed, enhancing both reinforcement and dimensional stability.

Microspheres also act as ‘mini ball bearings’, which enable the resin to more easily infiltrate complex mold geometries resulting in faster cycle times. Further, successful infiltration can occur at lower mold temperatures and injection pressures than are possible when mineral fillers are used.

(Originally published 4/1/2008 in Composites Technology)