Hollow Glass Microspheres (K1, Density 0.125 g/cc)

Hollow glass microspheres (HGM) grade K1 are ultra-lightweight borosilicate glass spheres with a true density of approximately 0.125 g/cc, making them among the lightest commercially available mineral fillers. Each microsphere is a thin-walled hollow sphere filled with inert gas (typically nitrogen or CO2), with outer diameters ranging from 10 to 100 µm.

At 0.125 g/cc, K1 microspheres can dramatically reduce the density of filled composites. A 10% volume fraction of K1 in a UPR system can reduce laminate density by 8–12%, enabling significant weight savings in marine, aerospace, and transportation applications without major mechanical penalty at appropriate loadings. The smooth, spherical morphology acts as a 'ball bearing' effect, reducing resin viscosity at equal volume fraction compared to irregular fillers, which can ease processing in casting and infusion.

K1 microspheres are used in syntactic foam cores, deep-sea buoyancy modules, lightweight casting resins, fairing compounds, and low-density putties. Their closed-cell glass structure provides excellent resistance to water absorption and chemical attack. True density is significantly lower than cenospheres (0.4–0.8 g/cc) or expanded perlite (0.05–0.15 g/cc bulk), with the advantage of controlled, uniform particle size and chemistry.