Microspheres: Fillers filled with possibilities

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.

“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

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.).

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

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.

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’s density and its crush strength (also see particle size, below). “Generally speaking, the thicker the wall, the stronger the material,” says Gladysz, whose company readily tailors its trademarked sodium borosilicate glass Eccospheres for clients in the aerospace, military, electronics and oil and gas industries. “However,” he adds, ”there are many factors that affect strength and density, including glass chemistries and manufacturing processes.” Both factors are important in the company’s range of aerospace-grade, high-purity glass and ceramic microballoons, for which strength and density (the latter ranging from 0.16 g/cc to 0.380 g/cc) are dependent on the company’s ability to produce spheres with uniform wall thicknesses and consistent size distribution.

Particle size. Along with wall thickness, particle size plays a critical role in the microsphere’s relative density and its survival rate, because smaller microspheres are better able to withstand the processing conditions of higher shear rates and faster screws.

In pursuit of increased strength in its hollow glass microspheres, 3M Energy and Advanced Materials Div. (St. Paul, Minn.) recently introduced iM30K, which it touts as “the first 30,000 psi (~200 MPa) isostatic compressive strength hollow glass microsphere with a density of 0.6 g/cc.” At 16 µm in diameter, iM30K microspheres are half the size of 3M’s S60HS grade — rated at 18,000 psi (124 MPa). According to ETS Inc.’s Tres, loadings of 3M’s iM30K can be as high as 20 percent with very little change in the molded part’s impact strength, which is due, in part, to its small particle size.

Automotive parts manufacturer Hyundai Mobis (Seoul, South Korea) tested iM30K for use in an automotive instrument panel (IP) core, with positive results (see photo, p. 31). Glass-filled polypropylene that contained iM30K reportedly achieved a 16.8 percent weight reduction and a 50 percent cost reduction compared to a similar polycarbonate/acrylonitrile butylene styrene (PC/ABS) IP core.

“Improved dimensional stability was very important where the IP core met the windshield,” adds 3M Energy’s business manager, Lou Lundberg, who notes that parts produced using iM30K reportedly had better dimensional stability than the current talc-filled polypropylene (PP) part and improved material flow compared to the PC/ABS core.

In the same way, small balloons perform better during molding processes as well. Potters’ Sphericel-brand glass microspheres, for example, range in size from 11 µm to 18 µm and have crush strengths ranging from 10,000 psi to 8,000 psi (69 MPa to 55 MPa) depending on the grade. They have been compounded successfully with a 25.4-mm/1-inch diameter Killion 2-stage single screw ex-truder and injection molded on a 75-ton Newbury machine without significant sphere breakage, reports the company.

PART PROCESSABILITY

As important as these previously discussed properties are, one of the microsphere’s greatest assets is the contribution it makes to part processability, which, in a filler, is a direct function of particle shape.

Arguably, the microsphere’s 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’s 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,” explains Rosenbusch.

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 percent 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.

Microspheres can act as “mini ball bearings,” adds 3M’s Lundberg. The “ball bearing effect” 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. To illustrate, ETS Inc.’s Tres notes the example of a major automotive parts supplier that was having difficulty achieving the required stiffness and Class A finish on an exterior automotive panel. “The company was using a formula with 20 to 25 percent, by weight, chopped fiber, but was unable to achieve a Class A surface,” says Tres. Unlike chopped fiber, which tends to migrate to the part surface during processing, microspheres tend to remain more evenly dispersed throughout the part. “By adding 10 to 15 percent, by weight, of high-strength glass microspheres and reducing the glass fiber to 5 percent, they were able to achieve the desired mirror finish without sacrificing the stiffness required by the part.”

No less important is the fact that the spheres 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,” says Lundberg.

COMPONENT COST

The cost of microspheres varies considerably depending on a variety of factors, including material, density, strength and volume. Expancel microspheres, for example, range in price from $5/lb to $30/lb, depending on grade and volume. The manufacturing process employed in the production of the microsphere also affects cost. “In general, high-strength, glass microspheres cost two to three times more than chopped glass fiber,” says Tres. However, according to Tres, the cost of 3M’s iM30K is significantly reduced because of the proprietary manufacturing process the company developed.

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. “Although most microspheres are sold by weight, users need to be aware of volume costs,” stresses Rosenbusch. “By comparing the cost of a gallon of resin (or other material) with a gallon of microspheres, you can see what the cost effect will be.”

For example, even if the specified hollow microspheres cost twice as much per pound as the filler or resin that it will displace, significant density reduction (for example, a microsphere with a density of 0.6 g/cc displacing nylon 6/6 with a density of 1.15 g/cc) means that for a given volume, the price is nearly the same. Volume cost can be calculated by multiplying true (not bulk) specific gravity times 8.34 to give pounds per gallon, then multiplying that by the cost per pound to give cost per gallon.

OPTIMIZED FUNCTIONALITY

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. “Coating the microsphere adds new levels of functionality, such as dielectric or thermal imaging properties,” says Trelleborg’s Gladysz. For example, coatings such as titanium dioxide (TiO₂) or silver can provide signature management — that is, control over the way in which objects are viewed when imaged using technologies such as radar or infrared (thermal) imaging. One obvious application is to help reduce the radar detectability of aircraft.

Surface treatments can be added to make microspheres magnetic, fluorescent and/or conductive or simply to improve bonding between the microsphere and the matrix. Microsphere Technology Ltd. (MTL; Edinburgh, Scotland, U.K.) specializes in coating hollow glass microspheres with a variety of pigments and metals for specific applications. Typically, to coat a microsphere with a metal, such as aluminum, silver, copper, stainless steel, platinum, gold or zinc, microspheres are mixed with an adhesive until coated, after which metal flakes are slowly added to coat the spheres. Curing permanently bonds the metal flakes onto the microsphere. Coating thicknesses can range from a few nanometers to several microns to manipulate final part characteristics.

“Microspheres are another tool in the composites manufacturer’s toolbox,” Rosenbusch sums up. “They’re like icing on the cake — a way to fine-tune the end product.”

{See two additional short articles on next page.)

Microspheres: Material Alternatives

Although glass and plastic microspheres are the most widely used in composite manufacture, they are not the only materials used to manufacture microspheres. Wall materials can be chosen to suit varying application requirements. Here are some of the other options, many of which are heavily used in paints and coatings:

Cenospheres. These low-density, hollow, free-flowing alumino-silicate microspheres are extracted from pulverized fuel ash that is produced by coal-fired electric power plants. Cenospheres are available from a number of suppliers, including Trelleborg Emerson & Cuming (Mansfield, Mass.), which supplies trademarked Fillite cenospheres in sizes ranging from 5 to 500 µm. E-Spheres, produced by Envirospheres Ltd. (Lindfield, Australia), are used in spray-up, hand lay-up, resin transfer molding and syntactic foam applications.

Ceramic. Engineered ceramic microspheres are available from a wide range of manufacturers, including 3M Energy and Advanced Materials Div. (St. Paul, Minn.). Although they are used in syntactic foams, paints and coatings remain the primary market for ceramic microspheres.

Carbon. Phenolic microspheres can be carbonized or pitch can be treated and carbonized to produce carbon spheres, which can be used in composites and syntactic foam for a variety of applications. Due to their smooth surface, good mobility and thin walls, which permit deformation in response to sound pressure, carbon microspheres have been effectively used in the production of carbon microphones. Also, specially processed pitch carbon microsphere composites are suitable for use as honeycomb fillers for high-temperature or ionizing radiation fields.

Composite and Metal. Aluminum and copper/silver microspheres are currently available. Companies are continuously manipulating materials to create a seemingly endless array of composite microspheres to suit very specific applications. Present developments include polymer-metal spheres that combine a polymer core with a metal shell as well as a product in which multiwalled carbon nanotubes are bonded to the surfaces of polystyrene microspheres.

Solid. Solid glass microspheres, commonly called glass beads, are widely used as resin extenders. Though they do not offer low density like hollow spheres, they can enhance physical properties. Potters Industries (Valley Forge, Pa.) produces Spheriglass solid microspheres, which it says can increase the strength of plastic. Shrinkage of glass-filled nylon 6/6 reportedly can be cut by 70 to 80 percent when 30 percent solid glass spheres are used, and warpage is said to be reduced by 95 to 97 percent.

CORE, ACOUSTIC & ANTIBLAST APPLICATIONS

Microspheres of all materials (see “Microshperes: Material Alternatives,” above) are well suited for foaming processes. In fact, the makers of low-density composite syntactic foam systems, such as those used in deepwater buoyancy applications, require the controlled, closed cells that only microspheres can deliver.

“Syntactic foams are highly functional materials that can be optimized for very specific applications,” says Gary Gladysz, VP of technology at Trelleborg Emerson & Cuming (Mansfield, Mass.). “If you want strength plus thermal insulation plus acoustic properties plus energy absorption, then you’re in the realm of syntactic foam,” he explains. “And the basis for all of that functionality is the wide selection and targeted properties that can be achieved through microballoon selection.”

By definition, syntactic foams are a combination of microspheres and a polymeric resin. A variety of different wall materials and resins can be used to create optimized foam. For applications ranging from acoustic panels to aerospace structural cores, Cornerstone Research Group (CRG; Dayton, Ohio) builds its engineered foams (trade named Advantic) using glass, polymer or ceramic microspheres embedded in a resin matrix such as cyanate ester, silicone or epoxy. Using a proprietary low-stress resin removal system, excess resin and fractured microspheres are extracted from the syntactic material prior to curing, reportedly resulting in a low density, void-free content. Densities for Advantic, which can be fabricated in a variety of shapes — blocks, cylinders or sheets — can range from 0.30 g/cc to 0.55 g/cc.

Trelleborg Emerson & Cuming typically relies on its Eccosphere microspheres to build its syntactic foam. “If you want functionality beyond the basic glass microsphere,” explains Gladysz, “You can tailor a foam by using a different wall material, such as carbon for higher conductivity, add a coating, combine different densities of microballoons, or even add interstitial void.” Syntactic foam offers a high compressive-strength-to-weight ratio. Compressive properties are driven by the properties of the microsphere, while tensile properties depend on the matrix material.

Trelleborg supplies syntactic foams for a variety of end uses, including lightweight panels for deepsea submarines and marine barriers, acoustic damping and insulation panels as well as a variety of aerospace applications. Notably, the company also is developing a syntactic foam product for blast mitigation. “There are millions of microballoons in one square inch of syntactic foam, and at each interface there is an event that dissipates the energy of a blast,” explains Gladysz. He believes this makes syntactic foams ideal for production of moldable protective panels that absorb and dissipate blast energy rather than just transmit and reflect blast effects.

Key applications for antiblast products include lightweight armor plating for military vehicles, lightweight helmet covers, flak jacket inserts and protective composite walls, doors and other structures.