Thermoplastic Microspheres - Ultra Lightweight

Print

Although they have less compressive strength, plastic microspheres offer many of the same advantages as rigid glass microspheres and are among the lightest fillers available.

Standard specific gravities are as low as 0.025, providing large volume displacement at a very low weight. Plastic microspheres are primarily used in spray-up fiber-reinforced thermosetting composites and extrusion applications.

Unlike glass microspheres, plastic micro-spheres are much less susceptible to breakage. Excessive pressure will cause the plastic sphere to flatten but not burst. When the pressure is released, the microspheres tend to recover.

In a spray-up application, for instance, the microspheres will deform when the resin is pressurized prior to spraying. However, once the material hits the mold and returns to ambient pressure, the microspheres will rebound to their spherical shape.

This compressive capability can provide some control over thermal expansion. The heat of exotherm during cure can be problematic in composite manufacture. By incorporating plastic microspheres as the part heats up, the resin is able to expand inward causing the microspheres to compress. Once the heat dissipates, the spheres rebound.

The microspheres retain this flexibility even after cure. Therefore, if there is a part that is subjected to thermal stress, such as a windmill blade that gets hot in the summer and cold in the winter, the microsphere will help absorb some of the expansion/contraction force.

Unexpanded microspheres, which can be expanded in-situ, have been effectively used as foaming agents in wood plastic composites (WPCs).

Foaming can remove from 5 percent to 30 percent+ of a WPC board’s weight and the internal pressures generated during the foaming process reportedly result in a texture and appearance that is more like wood. The presence of the thin-walled, hollow spheres in the finished board also decrease the board’s resistance to cutting and drilling.

While plastic microspheres do not burst and are, therefore, well suited for high shear mixing and spray-up applications, they are more susceptible to heat damage and chemical interaction than glass spheres. Therefore, the choice of material could be dictated, to some extent, by the molding process and the product end use.

 

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