Microspheres are engineered materials that are used in a wide variety of industries and applications. Microspheres are not all created equal and functional properties of microspheres need to be carefully considered when choosing the right microsphere for the specific project.
The properties of microspheres that are often considered in product selection are specific gravity (true particle density), particle size (diameter) and particle size distribution, melting point (temperature tolerance), solvent resistance, durability, crush strength (pressure and shear tolerance), color (reflective, fluorescent, or phosphorescent response), optical properties (reflective index or retroreflection), conductivity, bio-compatibility, safety, surface properties (porosity, roughness, hydrophobic vs. hydrophilic surface, ability to coat or functionalize the spheres) among many other properties.
Detailed knowledge of the application, the technical judgment of scientists and engineers working on a project, and sometimes a simple trial-and-error approach, are often used to select the appropriate properties of microspheres for the application.
Properties of Microspheres – Composition
The most common and widely available microsphere materials are polymer, ceramic, and glass. However, each one of these large categories of materials can be further divided into much narrower categories which vary dramatically in their properties. For example, polymer microspheres may be manufactured out of polyethylene, polystyrene, polypropylene, poly(methyl methacrylate). Each of these polymers varies in their melt temperature, resistance to solvents, and ability for the surface to be modified with functional groups.
Category of glass microspheres can also be further broken down. For example, Sodalime microspheres are most available and economical, Borosilicate glass is preferred in higher temperature applications, while Barium Titanate glass formulation offers high density and high index of refraction for retroreflective and optical applications. Most hollow glass microspheres are made out of proprietary Borosilicate-Sodalime glass blend.
Most common ceramic materials used for microspheres are silica and zirconia, which are often used as fillers or grinding media.
Made of these materials are available in solid and hollow forms, clear or colored, and with a variety of coatings that make them suitable for particular applications. The materials from which microspheres are made vary widely in density, operating temperature, crush strength and solvent resistance. Microspheres possess different physical and optical properties, which may present advantages or limitations for different applications, based on their material composition.
Properties of Microspheres – Density/Specific Gravity
True particle density or specific gravity of the particles is one of the critical properties of microspheres which needs to be precisely controlled in many applications. Specifically this property becomes important in systems where the spheres are a component embedded or dispersed into another media, whether suspended in a fluid or used as a filler.
Particles that are heavier than the media in which they are dispersed will settle and collect at the bottom of a container over time. Particles that are lighter than the media will float up and accumulate on the surface. Matching the specific gravity of microspheres to
that of a matrix solution creates a stable suspension of particles, ensures uniform distribution of microspheres in a product, and prevents settling or surface collection over time.
If microspheres need to be suspended in a particular solution without floating to the top or sinking to the bottom, matching the specific gravity of the particle and the solution is a critical factor. Solid glass and ceramic microspheres have specific gravity greater than 2 g/ml. Polyethylene microspheres are available in specific gravities of 0.96 g/ml to 1.3 g/ml, which is close to that of water. A variety of polyethylene microspheres that have been designed with precision density to be neutrally-buoyant in an aqueous solution are commercially available.
The most obvious benefit of hollow microspheres is their 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. For example, at a density of 0.6 g/ml, hollow glass microspheres can displace the same volume as talc at one-quarter of the weight. Densities and crush ratings, however, vary dramatically across product lines.
When microspheres are being used a a bond line spacer to be mixed into epoxy or another adhesive, manufacturer might need the spheres to stay in suspension for a significant period of time. Density of the bondline spacer particles is important to maximize the residence time in the dispensing vessel to increase the amount of time the mix can be used after the particles have been incorporated. To achieve this dispersion the density of microspheres needs to match to the density of the epoxy as closely as possible. Ability to pre-mix offers significant cost savings and efficiency improvements for the manufacturer, as compared to having to disperse particles at the time of use.
Properties of Microspheres – Durability
Durability may be one of the critical properties of microspheres. Depending on the applications microspheres might need to be able to withstand high pressure, temperature, and aggressive solvents, or operate in harsh conditions and aggressive environments.
Crush strength is important when microspheres need to survive high-shear forces and high pressures involved in manufacturing processes such as plastics compounding and injection molding. Typically, solid glass microspheres have better crush strength compared to
hollow glass microspheres. If microspheres have to undergo an aggressive mechanical process, solid glass microspheres will most likely be required.
Historically, crush strength for hollow glass microspheres has been directly linked to density. That is, a glass sphere with a density of 0.125 g/ml is rated at 250 psi (1.8 MPa) while one with a density of 0.60 g/ml is rated at 18,000 psi (124 MPa). To some degree, there is a correlation between density and crush strength. The density and crush strength of microspheres made from a particular material depends, in part, on two structural variables: wall thickness and particle size.
In recent years, significant progress has been made in increasing the strength of hollow glass microspheres. 3M Energy’s Advanced Materials Division introduced iM30K, which are close to 30,000 psi (~200 MPa) isostatic compressive-strength hollow glass microspheres with a density of 0.6 g/ml. Loadings of 3M’s iM30K can be as high as 20% with very little change in the molded part’s impact strength, which is due in part to the small particle size. This product provides an additive with a superior strength-to-density ratio for use in injection molding extrusion and sheet molding compounds. Final part weight can be reduced by 15% or more without compromising mechanical integrity.
Potters’ Sphericel glass microspheres, for example, range in size from 11 to 18 microns and have crush strengths ranging from 8,000 to 10,000 psi (55 to 69 MPa), depending on the grade. They have been compounded successfully with a 25.4 mm/1-inch-diameter Killion two-stage single-screw extruder and injection molded on a 75-ton Newbury machine without significant sphere breakage.
It is critical to know the maximum operating temperature to which microspheres will be exposed during manufacturing processes and in final applications.
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.
Ceramic Microspheres – With operating temperatures up to 1000 degrees C, superior toughness and high resistance to wear ceramic beads can be used in high energy and high temperature processes and applications where other materials would not survive.
Chemical Stability/Solvent Resistance
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 aggressive solvents at elevated temperatures.
Shelf life may be an important property of microspheres, as it might limit the options of how the materials are stored, handled, and used. Most microspheres that are sold as a dry powder have indefinite shelf-life if stored in a sealed container under controlled conditions (low humidity, no temperature extremes). Microspheres that are provided suspended in solution typically have a limited shelf life and need to be either refrigerated or frozen to extend useability.
Safety always needs to be considered when working with materials. Three potential hazards are unique to some microspheres and need to be considered: slip hazard, combustible dust, inhalation hazard.
Ball-bearing effect is one of unique properties of microspheres that applies to spherical particles regardless of what material they are made out of. When spheres are extremely slippery when spilled, so good housekeeping and prompt clean up of any spills is essential.
Like most fine powders (e.g. flour), microspheres made of flammable materials in certain size ranges may be combustible when present in air in certain concentrations. Caution should be taken to read the Safety Data Sheets, prevent creation of dust clouds, and eliminate any potential sources of ignition when working with microspheres. Good housekeeping practices need to be in place to ensure no layers of particles are collecting on surfaces. Only approved vacuums can be used and correct clean-up procedures implemented (e.g. wet wiping as opposed to sweeping to prevent dust clouds).
Lastly, microspheres that are less than 10 micron in diameter may pose an inhalation hazard in some situations. Correct handling of these microspheres requires combination of engineering controls (e.g. exhaust hood, ventilation), personal protective equipment (e.g. dust mask, respirator), and good laboratory practices to minimize potential inhalation of the particles.
As any hazard, the risk depends on the amount of material we are working with and the level of exposure. The risks are low for scientists working with very small quantities of materials in a controlled laboratory environment. The risks are higher when we are continuously working with large volumes of spheres moving through our manufacturing systems.
Properties of Microspheres – Particle Size and Distribution
Microspheres are manufactured in particle sizes as small as 1 micron in diameter and as large as 1,000 microns (1 mm), making them suitable for use in a wide variety of products. Not only particle size but also a particle size distribution (PSD) is important to know when specifying the right microspheres for the application.
Requirements for particle size vary widely. For example, a scientist might be looking for microspheres less than 10 micron to simulate a human cell in a biological application or a 5 micron sphere to simulate bacteria. A microsphere of 50um diameter might be needed to hold a precision epoxy bondline, or a reference particle of 500um might be required to calibrate an optical instrument or a medical device. When used as a filler, smaller microspheres are better able to withstand the processing conditions of higher shear rates and faster screws.and 850 to 1,000 microns (1 mm). When used as a tracer particle in a flow visualization experiments, smaller particles tend to stay in suspension longer, while larger particles are easier to observe.
Microsphere size may be critical to the proper function of an assay, or it may be secondary to other characteristics. Diameter also determines surface area. Small-diameter spheres present more surface area per unit weight while larger spheres present more surface area per bead. Size also affects ease of handling, processing considerations (such as the method used for separations [centrifugation, dialysis, filtration]) and the amount of reagent needed for coating.
Particle size distribution is typically specified in one of two ways:
Median diameter (D50) of microspheres with an associated coefficient of variation (CV%). Sometimes D10 and D90 are also specified. This method of reporting particle size assumes a normal (bell-shaped) distribution of particle diameters.
The lower the coefficient of variation, the tighter (narrower) the size distribution of the spheres in the sample. Please note that no measurement is infinitely precise. Even if very tight tolerances (low CV%) are reported, there is always a level of measurement uncertainty associated with the measurement technique and equipment.
When particle size distribution does not have a bell shape, the property that is specified is size range, as opposed to a median diameter. For example, 1-5um or 53-63um, where the first number is the minimum diameter and the second number is the maximum diameter. In this method, % of particles in this size range are also specified. For example, greater than 95% of particles are between 1 and 5um in diameter.
For either of the above scenarios there is a science to ensuring a statistically significant and representative sample of the powdered material is measured.
For life sciences applications, monodisperse microspheres are typically the most desirable but also the most expensive. For large-volume, low-cost filler and texturizer applications, a wider size distribution might be preferred due to improved packing efficiency. The requirements of each unique formulation must be investigated to find the most economical solution. The wider the particle-size distribution, the more affordable the spheres.
Properties of Microspheres – Electrostatic Charge, Conductivity, Magnetism
Microspheres can be neutrally, positively or negatively charged, allowing formulators to design products that are attracted to or repelled from the surface as needed. Since like charges repel and opposite charges attract, the charge of a coating can be manipulated to be attracted to the surface or repel dust. Charged microparticles also enable formulators to control how the various materials within a formula interact.
A positive or negative charge is internally embedded into each microsphere, and proprietary additives are acquired during the microsphere manufacturing process. The charge is permanent; it does not dissipate over time and cannot be grounded. The whole microsphere is charged and will respond to the electric field. Further, dark-colored or black microspheres can be made magnetic or static-dissipative. Metal or metal-coated microspheres offer electrical conductivity and provide shielding against EMI.
Janus particles, which are bipolar and bichromal can be manufactured with with dipole precisely aligned with two differently colored hemispheres. Due to the dipole the sphere will rotate in electromagnetic field to align more positive hemisphere to the negatively charged stimuli and vise versa.
Properties of Microspheres – Optical Characteristics
Microspheres can be transparent or invisible to the eye, partially translucent or opaque, providing maximum hiding power. Generally, when light strikes an interface between two substances, some of the light is reflected, some is absorbed, some is scattered and the remainder is transmitted.
An opaque substance transmits little if any light and thus light is reflected, scattered or absorbed. One cannot see through an opaque particle since the surface behind it is completely hidden. Since opaque microspheres do not allow light to pass through them, this means that a 50-micron monolayer of opaque spheres will not transmit light, thus resulting in maximum hiding power in one layer of microspheres. Opacity of microspheres is often desired in color products such as paint coatings and in color cosmetics such as powder, concealer, blush, lipstick, eye shadow and nail polish, where uniform color and hiding color underneath the area on which the cosmetic product applied is required.
In general, high levels of opacity become more difficult to achieve in microscopic particles because opacity is proportional to the thickness of the material. Due to the chemistry of glass, it is very difficult to create opaque glass spheres. Most colored glass microspheres are made by attaching dyes to the surface of the particle and do not achieve significant opacity. Ceramic microspheres can be opaque but microspheres in a batch usually do not have the same level of opacity, with some more opaque than others.
Hollow glass spheres can be used to replace some of the titania in a formulation. Hollow glass spheres redirect the angle of light, imparting opacity. Depending on the formulation, equivalent tint strength can be achieved with 2% to 5% replacement of titania.
Retroreflective hemispherically-aluminized high-refractive-index glass beads are used on airport runways, specialized signs and in other applications that require high-intensity retroreflectivity. Reflective pavement marking lines make a significant contribution to highway safety, especially in conditions of poor visibility at night and in wet weather. Applying retroreflective spheres is one of the most cost-effective ways to increase highway safety in these conditions.
Different formulations of glass have different index of refraction. For example, Barium Titanate glass spheres offer a higher Refractive Index (n) than most other glass formulations. Depending on the grade of material, reflecting index of Barium Titanate glass is between 1.9 to 2.2 compared to n=~1.5 of Soda Lime and Borosilicate glasses. Barium Titanate glass microspheres will reflect back more light directly to the viewer, which is beneficial for applications where retroreflectivity is desired, such as motion tracking targets in medical devices, endoscopy, micro-optics, defense and microscopy.
Properties of Microspheres – Color
Colored microspheres add color to a product without the use of other colorants (pigments or dyes) and also offer functional benefits such as lower viscosity and improved flow. Solid polyethylene microspheres can be manufactured in any color, including flesh tone, clear, grey, fluorescent, glow-in-the-dark and multicolor, which provides a multitude of options for color effects in a wide variety of products.
Microspheres can be used to add saturated color, a hint of color or a sparkle to a formula. Colored microspheres enable formulators to achieve saturated colors or desired color effects with much larger particles, which are not respirable and come in a free- flowing dry powder, ensuring a simpler formulation process. In addition, since larger microspheres tend to not agglomerate as easily as submicron pigments, the challenge of pressed-powder agglomeration is greatly reduced.
Since creating an opaque glass microsphere is almost impossible, colored polymer microspheres are often preferred for applications requiring high color, opacity and superior coverage. However, glass microspheres are preferable if maximum clarity of the sphere is desired, for example, for a soft-focus effect.
In science and technology applications color becomes one of the most important properties of microspheres, especially when used as tracer particles that need to have high contrast and visibility in the system, such as in-vivo studies, process diagnostics and troubleshooting, contamination studies, flow visualization, and instrument calibration, to name a few applications.
Colored polyethylene microspheres have colorant embedded into the polyethylene matrix which will not leach out or degrade in a solution.
Properties of Microspheres – Fluorescence and Phosphorescence
Fluorescent microspheres are round spherical particles that emit bright colors when illuminated by UV light. Ability to emit intense color under UV (black light) illumination provides contrast and visibility of microspheres relative to background materials. In addition to the benefits of conventional high quality microspheres, such as sphericity, smoothness and spreadability among others, fluorescent spheres offer extra sensitivity and detectability for analytical methods. For example, fluorescent microbeads are often used as traces to simulate spread of viruses in medical research.
Typical applications of fluorescent spheres include: testing of filtration media and systems, vial and container cleaning studies, flow tracing, flow visualization, and fluid mechanics studies, medical imaging and flow cytometry, fluorescence microscopy and photography, as well as biomedical technology research, qualification and validation of medical devices, biomedical diagnostics, process troubleshooting and process flow among others. Specifically fluorescent microspheres are often used for water- and air-flow testing and bead-based diagnostic applications. New unique applications of fluorescent spheres are being discovered daily.
Phosphorescent particles are used for process study and blind tests especially in medical technology industry, diagnostics, troubleshooting, process flow, medical research and other industries. Particles exhibit a strong long-afterglow phosphorescent response and are stable in solution.
Properties of Microspheres – Shape and Quality
While high-quality microspheres tend to be more expensive, some lower-cost microspheres have limitations such as poor shape and uniformity or lack of consistency between batches, wide particle-size distribution, the presence of dust and debris, poor roundness and sphericity. It is not uncommon for low-cost low-quality microspheres to contain a significant amount of crushed, oblong and non-spherical particles, which could negatively impact the texture, appearance and feel of finished products and/or performance in an application.
It is important for formulators to select a supplier that manufactures particles in a controlled environment and thoroughly screens them to prevent any dust, debris and nonspherical microparticles entering the system. Depending upon the requirements for a formulation and properties of microspheres, specifications for the maximum amount of unacceptable particles must be determined and communicated to the supplier.
For example, with luxury cosmetic applications, most formulators will look for high-quality identical polymer spheres with excellent uniformity. For large-volume low-density concrete-filler applications sold by the truckload, price may be a more critical factor than particle size and color uniformity.
Properties of Microspheres – Summary
To summarize, there are many different microsphere products on the market with properties of microspheres varying dramatically due to the composition of raw materials and additives, particle size and distribution, levels of sphericity and roundness, among other factors. It is essentials to truly understand the technical requirements of the project and the intended functionality of microspheres to select the right product for the application.