Expandable microspheres are white microscopic spheres (average particle size is 10~30 μm and average density is approximately 1100 kg/m3) that comprise a thermoplastic shell encapsulating a low boiling point liquid hydrocarbon. When heated to a temperature high enough to soften the shell, the increasing pressure of the hydrocarbon will cause the microsphere to expand by 40 to 100 times. The microsphere ceases to expand when its inner pressure, shell tension and external pressure reach a balance. Its density after expansion can be lower than 30 kg/m3（0.03 g/cm3), making it the most lightweight application material in the world.