Hollow glass polymeric or ceramic microspheres are used in coatings or composites and exhibit unusual mechanical and heat-insulation properties. In many countries, heat insulation by means of coatings is achieved by adding hollow or solid microspheres in acrylic-based coatings. Many coatings are developed with high absorbtivity in the visible regime and very low emissivity in the IR regime for architectural as well as industrial coatings.
Issues like stringent environmental regulations, customer requirements and competitive markets can be tackled with innovative raw material usage and developing new processes. There are many raw materials available that can be called ‘answers awaiting questions’; a good example being microspheres.
These specialty materials are used for many purposes in many diverse industries. Microspheres are innovative raw materials in the coatings industry, and are diverse because both solid and hollow formats are readily available. Solid spheres are most commonly used in reflective traffic paints where the microspheres are used as light reflectors. Hollow glass microspheres are small, spherical particles ranging in size from 12-300 microns in diameter, and wall thickness up to 0.1 micron. As these microspheres are hollow, the true density is very low, ranging from 0.60 g/cc to as low as 0.025 g/cc. Organic hollow glass microspheres are mostly composed of polystyrene, polyacrylonitrile or phenolic materials, while inorganic microspheres are glass, ceramic or made from fly ash from thermal power plants.
Glass Microspheres
Glass microspheres give high heat and chemical resistance with density ranges of 0.125 - 0.60 g/cc. The collapse strength depends directly on the wall thickness and density, thus the higher the density, the higher the strength.
Solid or hollow glass microspheres are used in various applications in the coatings industry. They are commonly used to improve the performance of epoxy primers, powder coatings, floor applications, aircraft paints and industrial coatings. Hollow spheres are used in thermal insulating coatings for construction and transportation applications, and also for acoustic insulation coatings. Hollow glass microspheres with a density of 0.6 g/cc with a fine particle size distribution can be used in flat wall paint.
Plastic Microspheres
Thermoplastic microspheres are compressible hollow particles with thin shell walls having densities as low as 0.025 g/cc. As resilient plastic materials, these microspheres can deform under stress (during high shear mixing or pumping), and there is very low to no breakage. The compressible nature of plastic can absorb impact, reducing damage caused by stone chips, foot traffic or freeze-thaw cycles.
Use in Coatings
Uniform spherical-shaped microspheres have lower surface area then irregular fillers and extender pigments, which means a lower resin demand. Another benefit to the spherical shape is the ability to roll past one another, hence there is minimal impact on viscosity when they are added to a liquid. As coatings are manufactured on weight basis and sold on volume basis, microspheres are used to increase the solid content of a coating, maintaining application and flow properties. Higher-volume solids reduce VOCs, shrinkage and drying time.
Since hollow spheres lower the density of materials, they are added to coatings. If added in coatings, it will atomize better while spraying and it will give less spatter while rolling also sag less once applied. [4]
One of the most important applications microspheres have been developed for is the Space Shuttle program. When the space shuttle re-enters the earth's atmosphere, incredible heat is generated due to increasing air friction. In order to prevent the space shuttle from burning up during re-entry, NASA scientists developed a superior insulating material using Ceramic technology. This technology can now be applied to roofs and sidewalls of buildings, piping, ducts, tanks, various storage devices, refrigerated containers, cold rooms, etc. in order to insulate them from the radiant heat of the sun and the atmosphere by using hollow ceramic spheres.
Stagnant air is a bad conductor of heat because heat is transferred by convection currents. Stagnant air inside hollow spheres acts as an insulator for heat and hence can be used as heat insulating material in coatings. This characteristic of hollow glass microspheres allows improved thermal and acoustic insulation properties of coatings or composites. Currently, markets taking advantage of this property include fire retardant materials, sensitive acoustic equipment, and roof coatings.
This article comes from pcimag edit released