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Microspheres Overview:Microspheres (also known as spheres, balls, beads or micro-balloons) are spherical microparticles typically ranging from 1 micron to 1000 microns in diameter. Microspheres are manufactured from a variety of raw materials. Most common types of microspheres are:
Each type of microspheres comes in different grades depending on the application and quality requirements. Quality of the microspheres ranges from monodispersed microspheres produced under very tight specifications primarily for laboratory applications to microsphere grades with huge variations in particle size and sphere uniformity for applications as fillers in automotive, transportation and other commodity markets.
Applications of Microspheres:Traditional applications for microspheres include traffic paint, lightweight fillers for automotive and marine putties, and density modifiers for explosives. Recently microspheres have become popular in specialty applications such as adhesives, automotive and specialty coatings, cosmetics, EMI shielding, offshore products, paper, specialty sealants, plastics and composites. Microspheres are becoming widely used in specialty applications with high-growth, high-value potential. These include digital displays, medical devices, solar panels, drug delivery, analytical technology, biotechnology, medicine, diagnostics and many other unique applications in latest science and technology development.
Polyethylene Microspheres vs Glass Microspheres:Polyethylene microspheres (also referred to as polyethylene spheres, beads, balls, polymer spheres, polymer microspheres, polymer beads, plastic beads or plastic microspheres) are solid spherical microparticles and are the most common type of solid polymer spheres. Glass microspheres represent a class of additives that offer aesthetic, process control and cost benefits, while providing flexibility in a wide range of potential applications. With advances in microsphere manufacturing processes, polymer spheres and glass microspheres are available in comparable grades, particle sizes and prices.
Which microsphere material is right for your application? There are several major differences to keep in mind when selecting microspheres.
1) Melting Point: Polyethylene Microspheres - The melting point of polyethylene microspheres varies somewhat depending on the grade and molecular weight of the polymer, but is usually between 110C for low molecular weight grades and 130C for higher molecular weight material. The melting point is typically low and sharp, since polyethylene goes through a fast phase transition. This is a very important feature for applications where the spheres are used as a temporary filler but would need to be “melted away” at a later point to create holes or cavities for a sponge effect.Glass Microspheres - The melting point of glass microspheres is from 500C - 800C, depending on the product. High melting point makes glass microspheres attractive for high temperature applications, where the product needs to withstand severe environmental or processing conditions.
2) Density or Specific Gravity of Particles:Polyethylene Microspheres - Typical densities of 0.95 g/cc - 1.3 g/cc as well as ability to color-code spheres by density make polyethylene spheres suitable as density marker beads. These are small colored microspheres of known mass density that are used for calibrating density gradients and determining density in gradient columns. Density gradients are often used for separations and purifucations of cells, viruses and subcellular particles. Generally a set of several density marker beads covering a range of densities is used. Custom density particles are available in polyethylene formulations. Brightly colored and fluorescent polymer microspheres are specifically designed as particles for water flow visualization and particle image velocimetry (PIV) experiments. Highly spherical microbeads with tight particle size distribution and density of 1g/cc, matching to properties of fresh water, are used as tracer or seeding particles clearly visible as they follow the flow of the liquid. Glass Microspheres - Solid glass microspheres have a high density of about 2.2g/cc for borosilicate glass spheres, 2.5g/cc for soda lime glass spheres, and 4.49g/cc for barium titanate glass spheres. Hollow glass microspheres have densities as low as 0.14 g/cc.Depending on the application requirements, solvents used, desired buoyancy, difference in density between polyethylene and glass microspheres might become a critical factor when selecting the right material.
3) Chemical Stability:Polyethylene Microspheres - Most grades of polyethylene have excellent chemical resistance and do not dissolve at room temperature because of their crystallinity. Polyethylene microspheres usually can be dissolved at elevated temperatures in aromatic hydrocarbons such as toluene or xylene, or in chlorinated solvents such as trichloroethane or trichlorobenzene. This feature is benefitial if microspheres need to be dissolved at a precise point in the process.Glass Microspheres - Glass has very high chemical resistance and is the right choice for applications where microspheres need to withstand contact with agressive solvents at elevated temperatures.
4) Crush Strength:Polyethylene Microspheres - Specific crush resistance is strongly dependent on the grade of material and processing techniques. Even though polyethylene microspheres are solid and very robust, they will crush if significant force is applied. They are not suitable for applications were microspheres undergo dramatic mechanical pressing force.Glass Microspheres - Solid glass microspheres have the highest crush strength. Hollow glass microspheres have the lowest crush strength, which varies widely with the grade of material, density, sphere diameter, shell thickness.
If the microspheres are expected to be exposed to strong mechanical pressures such as plastic compounding or injection molding, crush strength numbers need be carefully evaluated and solid glass is preferred. Solid glass spheres have high crush strength which makes them suitable for high stress applications where microspheres are exposed to a lot of stress during processing or application. Solid glass beads do not dust during compounding, have very low oil absorption, and have no effect on the viscosity or thickness of a formulation. Solid glass microspheres will not shatter or shear when used in most high speed spray guns, extruders or powder coating chambers. While they do not thicken a formulation, glass spheres will strengthen the surface of materials into which they are incorporated: for example, allowing a plastic mold to hold its shape better. Because of their spherical shape, glass spheres also promote the flow of paints or coatings, which silica and other minerals do not.
5) Color (Pigments and Additives) and Light Refraction: Polyethylene Microspheres - Pigments, additives, specialty ingredients can be incorporated into polyethylene prior to microsphere manufacturing process. This allows endless possibilities for customization of polyethylene microspheres for specific applications, smaller R&D projects, and unique customer requirements. Colored, fluorescent, phosphorescent, charged, paramagnetic polyethylene microspheres are available.Glass Microspheres - In general, it is very difficult to incorporate additives into glass. Formulating with a small percent of additives is sometimes possible, but typically additives interfere with the formation of glass and hinder its inherent properties (such as clarity, sphericity, strength, etc). Customization of microspheres with pigments and additives is limited.
Solid glass imparts visual and material benefits that cannot be replicated when spheres are made of other materials such as ceramics or polymerics, aluminum oxides, or silicas and mineral fillers. Solid glass refracts, bends and reflects light. Most ceramics do not transmit light or exhibit mirror-like reflection due to their internal crystalline structures and surface irregularities. Instead of being reflected back, the light is “trapped” in the structure and emitted as diffuse or scattered reflectance, which is not as strong or direct as light transmitted through glass, which produces mirror-like reflectance. Hollow glass can also possess numerous surface and interior micro irregularities that also diffuse light. Because the thickness of a hollow bead’s wall is inversely proportional to its diameter, however, the larger hollow spheres that might offer some reflective properties have very low crush strengths, which precludes their incorporation into most formulations.Solid glass microspheres can be made retroreflective by applying a half-shell aluminum coating applied to solid barium titanate glass microspheres. Retroreflective microspheres are hemispherically coated with a thin aluminum shell to produce a bright retroreflective response directed back to the light source and to the observer. The light bounces off the aluminum-coated half of the sphere produces the retro reflective effect that provides the desired high visibility in dark conditions. 6) Particle Size : Glass and Polyethylene Microspheres - The size of a microsphere is an important factor in the advantages spheres offer. There is an inverse relationship between the size of a microsphere and its surface area: the smaller the size of the sphere, the greater the surface area. This property is important because the greater surface area means that the spheres are more compatible with polymers, such as plastics or paints, and other materials, and this compatibility significantly expands the spheres’ potential applications. The greater surface area of a smaller diameter sphere means a larger area for a polymer to adhere to, which means the polymer “wets out” more completely and is better dispersed. Increased “wetting out” results in stronger adhesion to the glass sphere itself and reduces air pockets, which destroy the physical properties of a formulation, such as impact or tear strengths.7) Coatings:Glass Microspheres Proprietary coatings position its coated microspheres at the surface of either water (silane coatings on glass spheres) or solvent-based (fluorochemical coatings on glass spheres) paints or formulations by creating a tension between the microsphere and the resin that repels the sphere to the surface. This placement maximizes the transmission of light through the bead and the light’s “bounce back” to create enhanced retro reflectivity.
Metal coatings can be applied to glass microspheres to create conductivity or retro-reflective properties.
8) Applications:Solid glass microspheres, also called glass beads, provide multiple benefits including enhanced processing, excellent chemical and heat resistance, thermal stability, low oil absorption, and are used in automotive, electrical, household appliance, adhesives, packaging, paint and construction industries. Glass is non-toxic, extremely stable and recyclable. Solid glass microspheres are inert and are not nanoparticles and therefore do not raise the regulatory and other concerns of sub-micron-size materials.
There are many advantages to using solid glass microspheres:
Polyethylene Microspheres are often used in applications requiring low specific gravity, lower temperature, fluorescence, phosphorescence or magnetic properties. These particles are often used in digital displays, medical devices, solar panels, drug delivery, analytical technology, biotechnology, medicine, diagnostics, cosmetics and personal care and many other unique applications in latest science and technology development.
The defining factors become density, melting point, solvent resistance, crush strength, colors, additives and specific customization requirements. The detailed knowledge of the application, the technical judgement of scientists and engineers on the project, and sometimes simple trial and error approach must be relied on to select the right microsphere for the job.
Contact us to discuss your specific application. We will recommend microspheres that fit your needs.