Plastic microspheres: Although with less compressive strength, they offer many of the same advantages as rigid glass microspheres and are among the lightest fillers available. Unlike glass microspheres, they are much less susceptible to breakage. Excessive pressure will not cause the plastic sphere to burst, but to flatten. When the pressure is released, the microspheres tend to recover. Standard specific gravities are as low as 0.025, providing large volume displacement at a very low weight. They are primarily used in spray-up fiber-reinforced thermosetting composites and extrusion applications. Heat limitations (damage can occur at temperatures above 191�C/375�F) make injection molding challenging but possible for a small part with low pressures and low heat.

Expancel has added an ultralightweight microsphere with a density of 0.015 g/cc to its line, consisting of a very thin thermoplastic shell (a copolymer, such as vinylidene chloride, acrylonitrile or methyl methacrylate) that encapsulates a hydrocarbon blowing agent (typically isobutene or isopentane). When heated, the polymeric shell gradually softens, and the liquid hydrocarbon begins to gasify and expand. When the heat is removed, the shell stiffens and the microsphere remains in its expanded form. Expansion temperatures range from 80�C to 190�C (176�F to 374�F), depending on the grade. The particle size for expanded microspheres ranges from 20-150 �m, depending on the grade. When fully expanded, the volume of the microspheres increases more than 40 times. Microspheres deform when the resin is pressurized prior to spraying, but 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 as well. 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. A part that is subjected to thermal stress, such as a windmill blade that gets hot in the summer and cold in the winter, is aided by the microsphere that will help absorb some of the expansion/contraction force.

Expancel microspheres can be supplied in either expanded or unexpanded form. 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% to more than 30% 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 decreases the board�s resistance to cutting and drilling.

Density reductions of 38% can be achieved with the optimal concentration of 3% thermoplastic microspheres (by weight) and between 20% and 30% wood content. While plastic microspheres do not burst, and are 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. The most obvious benefit of the hollow microsphere is its potential to reduce part weight, which is a function of density. Compared to traditional mineral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow microspheres have much lower densities. The density of the sphere will have a huge impact on the formulation of the part. Typical loadings are 1-5% by weight, which can equate to 25% or more by volume. On an equal weight basis, Q-Cell spheres occupy about five times more volume than the resin, which can reduce compound weight, VOC content and cost.

 The density and crush strength of microspheres made from a particular material will depend, in part, on two structural variables, wall thickness and particle size.
Wall thickness: This variable is primarily, but not exclusively, responsible for the sphere is density and its crush strength. Generally, the thicker the wall, the stronger the material. But there are many factors that affect strength and density, including glass chemistries and manufacturing processes.

Particle size: Particle size also plays a critical role in the microsphere is relative density and its survival rate, because smaller microspheres are better able to withstand the processing conditions of higher shear rates and faster screws.
One of the microsphere is greatest assets is the contribution it makes to part processability, which, in a filler, is a direct function of particle shape. Arguably, the microsphere is small, spherical structure is the perfect shape for a filler. Without exception, the mineral fillers available to composites manufacturers are irregularly shaped. That irregularity results in a relatively large surface area, which increases the viscosity of the resin into which the filler is added.

By contrast, the microsphere is regularity minimizes its surface area. The low surface area allows for higher solids loading with less of an impact on the viscosity and flow characteristics of the composite. Additionally, the microsphere has a nominal 1:1 aspect ratio, giving it inherently isotropic properties that composites manufacturers can use to great advantage. For example, in parts fabricated by a resin injection process, chopped glass fiber, with a high aspect ratio, results in ~60% less stiffness in the cross-flow direction than in the flow direction because the fibers become oriented in the direction of flow. This alignment of the fibers can contribute to warpage, especially when introduced to crystalline matrices, such as nylon or polypropylene, which have molecular chains that also tend to align along flow lines. Microspheres, on the other hand, do not orient and, in fact, tend to obstruct directional orientation of reinforcing fibers and matrix. The result is that stresses are more evenly distributed, enhancing both reinforcement and dimensional stability. The �ball bearing effect� of microspheres enables the resin to more easily infiltrate complex mold geometries, resulting in faster cycle times. Further, successful infiltration can occur at lower mold temperatures and injection pressures than are possible when mineral fillers are used. The microsphere�s regular shape can contribute to product surface quality as well. Unlike chopped fiber, which tends to migrate to the part surface during processing, microspheres tend to remain more evenly dispersed throughout the part. They also help shorten mold heating and cooling cycles- because the spheres are hollow, there is less mass to heat or cool, which leads to faster overall throughput. The cost of microspheres varies considerably depending on a variety of factors, including material, density, strength and volume. When comparing the cost of microspheres to resins and competing mineral fillers, it�s critical to think in terms of cost per unit of volume rather than cost per pound because microspheres can displace a large volume of higher-density material at a very low weight.

Specialized surface treatments also can drive up cost; however, such coatings add properties beyond those inherent in the microsphere�s materials and construction, allowing manufacturers to tailor their products for specific applications. Surface treatments can be added to make microspheres magnetic, fluorescent and/or conductive or simply to improve bonding between the microsphere and the matrix.