Microspheres: Fillers Filled With Possibilities
These hollow microstructures not only displace a lot of volume at very low weight but also add an abundance of processing and product enhancements.
By Karen Wood, Contributing Writer | April 2008
Of the many fillers now available to composites manufacturers, microspheres, also called microballoons, are the most versatile. To the naked eye, the small, hollow spheres appear like fine powder. Ranging from 12 to 300 µm in diameter (by comparison, a human hair is approximately 75 µm in diameter), microspheres pack a lot of functionality into a very small package. Integrated into composite parts, they provide a variety of product enhancements and process improvements — including low density, improved dimensional stability, increased impact strength, smoother surface finish, greater thermal insulation, easier machinability, faster cycle times, and cost savings. Composite manufacturers, already adept at making the most of their materials, regularly exploit these benefits — sometimes all at once.
Source: 3M Energy and Advanced Materials Div.
A wide range of potential benefits stem from the isotropic properties of hollow spherical reinforcement — microspheres — including enhanced strength and stiffness.
“The composites industry is unique when it comes to hollow microspheres,” says Chris Rosenbusch, marketing manager for microsphere manufacturer Expancel Inc. (Duluth, Ga.), part of Sweden-based Akzo-Nobel. “Most users focus on one or two attributes of the spheres, but in the composites industry, manufacturers are taking advantage of six or seven attributes of the spheres.”
“Both the matrix and the microsphere can be tailored to achieve multiple objectives in one part,” adds Gary Gladysz, VP of technology at Trelleborg Emerson & Cuming (Mansfield, Mass.).
Moreover, microspheres can be used in all standard processing methods for thermoset and thermoplastic composites, including extrusion and injection molding, and have found a variety of applications across all industries. Microspheres find end uses in applications as diverse as simulated wood furniture and lumber, fiberglass-reinforced core materials, automotive brake components and engineered syntactic foams.
There are a number of producers of hollow microspheres, and the performance of their products varies greatly across product lines. “Microspheres are not all interchangeable,” Rosenbusch warns, explaining that each manufacturer has developed proprietary processes to control a wide range of microsphere variables that include chemical composition, wall thickness, and particle size and shape. Each of these variables makes a contribution to one or, more typically, several desirable properties that have made microspheres an effective delivery system for a number of notable benefits.
CHEMICAL COMPOSITION & CONSTRUCTION
Source: 3M Energy and Advanced Materials Div.
Though they appear as fine powder to the naked eye, hollow microspheres offer a filler with extremely low densities, able to displace large volumes of resin, fiber or traditional mineral-based fillers at a very low weight.
Microspheres are produced for a variety of applications using a fairly broad range of materials (see “Microspheres: Material Alternatives” on p. 3). However, most of the microspheres commonly used in composites manufacturing are hollow and are made of either glass or plastic.
Glass microspheres. In general, a multistep process is used to produce high-temperature glass microspheres. Glass is initially produced at high temperatures from soda-lime-borosilicate, after which it is milled to a fine particle size. Trace amounts of a sulfur-containing compound, such as sodium sulfate, are then mixed with the glass powder. The particles are run through a high-temperature heat transfer process, during which the viscosity of the glass drops and surface tension causes the particles to form perfect spheres. Continued heating activates the blowing agent, which releases minute amounts of sulfur gas that form bubbles within the molten glass droplets. The result is a rigid, hollow sphere manufactured with an eye to increasing crush resistance (that is, the ability to withstand external pressure and avoid fracture of the bubbles) without sacrificing low density.
Glass microspheres also can be produced by processing perlite — common volcanic glass. Noble International SA (La Pin, France) produces its trademarked Noblite microspheres by chemically processing perlites. Typically, the process involves an acid-leaching treatment, using hydrochloric or sulfuric acid at temperatures from 150°C to 200°C (302°F to 392°F), which is followed by a heat treatment process for finishing. Unlike engineered glass microspheres, which consist of a single closed cell, those produced from perlites are multicellular.
Plastic microspheres. Although they have less compressive strength, plastic microspheres offer many of the same advantages as rigid glass microspheres and are among the lightest fillers available. Standard specific gravities are as low as 0.025, providing large volume displacement at a very low weight. Plastic microspheres are primarily used in spray-up fiber-reinforced thermosetting composites and extrusion applications, according to Rosenbusch. Heat limitations (damage can occur at temperatures above 191°C/375°F) make injection molding challenging but not impossible. “You would be limited to a small part with low pressures and low heat,” says plastics consultant Paul A. Tres, president of ETS Inc. (Bloomfield Hills, Mich.).
Source: Expancel Inc.
Shown here at 100x magnification is a cross-sectional view of a fiber-reinforced composite structure filled with Expancel’s expandable 551 DU 40 plastic microspheres. Delivered as dry unexpanded microspheres, they expand to their maximum size (between 10 µm and 16 µm) when processing temperatures reach 139°C to 147°C (282°F to 297°F).
Two of the main producers of hollow plastic microspheres are Asia Pacific Microspheres Sdn Bhd (APM; Selangor Darul Ehsan, Malaysia) and Expancel Inc.
Originally founded as a joint venture with Union Carbide Chemicals and Plastics of USA (Danbury, Conn.), APM produces phenolic and amino-based spheres. Phenolic microsphere production is based on a process originally developed by Sohio (Cleveland, Ohio), an American oil company that was acquired by British Petroleum (BP, Chicago, Ill.). The technology was eventually sold to Emerson & Cuming Inc. (now Trelleborg Emerson & Cuming) and then licensed to Union Carbide. In the process, water-miscible phenolic resole resins are dissolved in water, after which a blowing agent, ammonium carbonate, is added. Spray drying produces discrete, uniform hollow spheres in sizes ranging from 5 µm to 50 µm in diameter. Significantly, the use of phenolic resin, which is naturally fire resistant, provides fabricators a nonhalogenated flame-resistant filler with far less mass than other flame-retardant fillers, such as alumina trihydrate (ATH).
Expancel recently added an ultralightweight microsphere with a density of 0.015 g/cc to its line of expandable thermoplastic microspheres. Expancel-brand microspheres consist of a very thin thermoplastic shell (a copolymer, such as vinylidene chloride, acrylo-nitrile 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 µm to 150 µm, depending on the grade. When fully expanded, the volume of the microspheres increases more than 40 times.
Unlike glass microspheres, plastic micro-spheres are much less susceptible to breakage. “Excessive pressure will cause the plastic sphere to flatten but not burst,” says Rosenbusch. When the pressure is released, the microspheres tend to recover. “In a spray-up application, for instance, the microspheres will deform when the resin is pressurized prior to spraying,” explains Rosen-busch. “However, 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, says Rosenbusch. “The heat of exotherm during cure can be problematic in composite manufacture,” he explains. “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. “Therefore, if you have a part that is subjected to thermal stress, such as a windmill blade that gets hot in the summer and cold in the winter, the microsphere will help absorb some of the expansion/contraction force,” says Rosenbusch.
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 percent to more than 30 percent 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 decrease the board’s resistance to cutting and drilling. According to Maf Ahmad, technical and business manager at Expancel, density reductions of 38 percent can be achieved with the optimal concentration of 3 percent thermoplastic microspheres (by weight) and between 20 and 30 percent wood content (see last photo, this page).
Rosenbusch points out, however, that while plastic microspheres do not burst, and are, therefore, 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.
DENSITY & CRUSH STRENGTH
Source: Expancel Inc.
Expandable microspheres work as foaming agents in the production of wood plastic composites (WPCs) to reduce density and improve physical properties. Shown here is a cross-section of a WPC composed of sawdust, ABS, and Expancel’s 092 MB 120, a masterbatch of microspheres and carrier ethylene vinyl acetate.
The most obvious benefit of the hollow microsphere is its potential to reduce part weight, which is a function of density. Compared to traditional min-eral-based additives, such as calcium carbonate, gypsum, mica, silica and talc, hollow microspheres have much lower densities. For example, at a density of 0.6 g/cc, Sphericel hollow glass microspheres from Potters Industries (Valley Forge, Pa.), an affiliate of PQ Corp., can displace the same volume as talc at one-quarter the weight. Densities and crush ratings, however, vary dramatically across product lines.
“The density of the sphere will have a huge impact on the formulation of the part,” says Rosenbusch. Typical loadings are 1 to 5 percent by weight, which can equate to 25 percent or more by volume. For example, Potters’ lightweight Q-Cell hollow glass microspheres have a density (from 0.14 to 0.20 g/cc) approximately one-fifth that of most thermosetting resins. Therefore, 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.
Historically, crush strength for hollow glass microspheres has been directly linked to density — i.e., a glass sphere with a density of 0.125 g/cc would be rated at 250 psi (1.8 MPa), while one with a density of 0.60 g/cc would be rated at 18,000 psi (124 MPa). To some degree, there remains a correlation.
