Glass bubbles can provide new and unique material and design solutions for innovative users. They render polymers with lower density which is directly related to thermal conductivity and insulation properties. The three polymer systems are common in appliance applications for either housings, walls or, in the case of thermoset polyurethane (PU), already used as a foam material for insulation. Polyurethane foam for appliance insulation is usually made with a chemical blowing agent and can achieve a very low density (0.20 – 0.40 g/cc). Typical polyurethane composite density with glass bubbles is in the 0.76 – 0.95 g/cc range so they are not competitive with urethane for achieving the highest insulation properties. But the unique property of the glass bubble foam is that it is rigid and structural and can be applied to the walls and housings themselves for additional insulation value. This may help appliances that are on the edge of an ENERGY STAR rating to move to that level.
Weight or mass reduction can be helpful in other ways such as helping to reduce shipping costs and ease installation issues. Glass bubbles can provide weight reduction for thermoplastics, thermosets and elastomeric polymer substrates.
The addition of glass bubbles to a polymer will result in physical property changes (density being the obvious one). Typically glass bubble addition will cause the composite to become stiffer than the original unfilled base resin. This can be useful for making stronger yet lighter housings and parts but impact strength is usually inversely related to stiffness. Impact usually becomes the property of focus for material specifiers trying to balance the benefits of mass reduction with other physical properties.
The choice of a specific bubble for a given application is important to maximize density reduction and to minimize cost-in-use. Not all glass bubbles can survive all polymer processing methods. As shown in Figure 2, the relationship of strength to density is important in selecting the lowest density glass bubble that will survive the process. With thermoset materials like polyurethanes and epoxies the predictive step in the process is the type of mixing system used. For high shear thermoset mixers such as Cowles mixers, a 3000 PSI bubble or higher strength material is generally required. For thermoplastics and rubber where there is only an extrusion process involved (e.g. sheet extrusion for thermoforming), then typically at least a 5000 PSI bubble is required. Injection molded thermoplastics require the highest compressive strength bubble – generally 16,000 PSI or greater. Finding the lowest density bubble that survives the process will insure the lowest cost in use since the least amount by weight will be required to achieve the targeted composite density.
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