Materials: What are Microspheres Made of?

What materials can microspheres even be made from? How can we choose correct materials for microspheres with our specific needs in mind?

The possibilities of raw materials to make microspheres out of are almost endless but they can be grouped into four basic material categories of glass, polymer, ceramics, and metal. Each of these core categories of material types often have an option of further being dyed, coated, functionalized, or impregnated with a wide variety of additives to achieve desired functionality and performance of the particle for the specific research project or a unique application.

Materials for Microspheres – Glass

Glass particles have numerous benefits for those high temperature and harsh environment applications. They are inert, stable, durable, and able to withstand high temperatures. Typically, the broad category of glass microspheres is separated into solid glass microspheres, also known as glass beads, and hollow glass microspheres, which are also often referred as microballoons or microbubbles.¹

Solid Glass Microspheres / Glass Beads

Solid glass materials for microspheres offer the advantages of durability and high density. Optically clear grades of solid glass have numerous benefits in optical instruments and in retroreflective coatings. High precision glass spheres and microspheres can be used as calibration particles in a wide variety of applications, such as tooling and diagnostic devices. Due to their high crush strength and spherical shape, precision glass spheres make superior bondline spacer beads.

There are several formulations of glass including sodalime glass, borosilicate glass, and barium titanate glass. These formulations are most frequently used as glass raw materials for microspheres manufacturing. Each formulation of glass gives microspheres unique properties.

For example, SodaLime glass beads are the most economical type and offer the basic benefits of high temperature and chemical resistance, while Borosilicate glass beads bring lower coefficient of expansion, lower density, higher softening point, higher resistance to acids and thermal shock, which is especially useful for laboratory applications that require high heat. Barium Titanate glass beads offer additional benefit of higher Refractive Index (n) and higher density than most other glass formulations.

Depending on the grade of material, refractive index of Barium Titanate glass is between 1.9 to 2.2 compared to 1.5 of Soda Lime and Borosilicate glasses. Barium Titanate glass beads 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. Barium Titanate glass core material for microspheres also offers significant higher density at 4.2g/ml compared to 2.2-2.5g/ml for Borosilicate and Soda Lime glass beads respectively.

Hollow Glass Microspheres / Microballoons / Microbubbles

Hollow glass microspheres preserve the benefits of the spherical particle shape as well as high temperature functionality and inertness of glass. In addition, glass microbubbles bring super low density that has benefits of floating to the surface in most fluids or significantly reducing the weight of the material while preserving the volume when used as a low-density filler. Microbubbles can be used to improve thermal insulation, increase buoyancy and reduce permeability.  They are also non-combustible and non-porous.

Typically the glass formulation for hollow microspheres is a proprietary sodalime-borosilicate glass blend. The thickness of the wall is most often around 10% of the diameter and the exact density varies dramatically as a function of the particle diameter of the individual spheres. Because the shell is so thin, microballoons are highly fragile and should not be used in high shear processes. Crush strength of each size and grade of material has to be considered when choosing microballoons for a specific application.

The applications of microbubbles range from uses in light-weight vehicles, furniture, and sporting goods to biomedicine, optics and photonics.²

Materials for Microspheres – Polymer

Polymer is a very broad term that encompassed a large amount of materials. The world “polymer” refers to materials that are composed of repeating identical molecules. The type of molecule, the length of the chain, the amount of cross-linking, the presence of various chemical groups – all affect the behavior as well as chemical and physical properties of the material.

Many different polymers can be used as raw materials for microspheres. The most common polymer microspheres are made from polyethylene, polystyrene, and poly(methyl methacrylate).

Solid Polymer Microspheres / Polymer Microbeads

The advantages of solid polymer microspheres include the ability of these materials to be surface functionalized, dyed, or embedded with colorants and other additives. Polymer microbeads can be clear, opaque, colored, fluorescent, phosphorescent, or paramagnetic, among many other possible options. Polymers offer significantly lower density compared to glass formulations which creates opportunities to using polymers for dispersion into a fluid, flow visualization studies, and biomedical diagnostics.

For example, poly(methyl methacrylate) spheres (often also called PMMA or acrylic spheres)  have good biocompatibility and are often used as  deformable spacers or medical/dermal fillers. Polystyrene spheres can be easily functionalized and serve as a template of choice for biological applications. Polyethylene microspheres can be manufactured in sizes up to 1mm in variety of densities, colors, and fluorescent properties. Precision polyethylene spheres are often used a highly visible tracer or seed particle when performing simulations such as dispersion of pollen in a forest, spread of fish eggs in a stream, or studies of an animal’s digestive tract. Polyethylene microbeads also frequently used in studying environmental effects of microplastics.

Hollow Polymer Microspheres / Microballoons

Even though not used as widely as hollow glass microspheres, hollow polymer particles also offer the characteristics of low density, optical scattering, high specific surface area, and good heat-insulation. Hollow polymer microspheres are used as a light-weight filler and high quality insulating material, as well as in pharmaceutical, cosmetics, and biotechnology applications.

Materials for Microspheres – Ceramics

In addition to the benefits of spherical particle shape, using ceramic materials for microspheres offers additional advantages of being highly durable and chemically unreactive with superior thermal, mechanical, and electrical properties.

Solid Ceramic Microspheres

Solid silica microspheres are frequently used in cosmetics due to their inertness, high opacity, and ball-bearing effect (gliding smooth feel). Uncoated silica spheres exhibit excellent dispersibility, good oil absorbancy, and good fluidity.

Silica spherical particles are often used in emulsion and make-up products, as anticaking agents, and as additive and perfume carriers. Optional silicone coating makes silica spheres hydrophobic, transparent, easy to disperse, resistant to acids and alkalis, as well as resistant to UV and moisture.

Silica microspheres are used in biomedical applications due to their biocompatibility, low toxicity, and scalable synthetic availability. It is possible to precisely control silica particle size, porosity, crystallinity, and shape to tune the structure for diverse applications. Furthermore, the many possible surface modifications of silica particles allow precise control of surface chemistry to modulate drug or chemical loading, blood circulation, and site specific targeting. The ability to combine these properties makes silica particles a desirable platform for biomedical imaging, assaying, therapeutic delivery, monitoring, and ablative therapies.((Liberman A, Mendez N, Trogler WC, Kummel AC. Synthesis and surface functionalization of silica nanoparticles for nanomedicine. Surf Sci Rep. 2014;69(2-3):132-158. doi:10.1016/j.surfrep.2014.07.001))

Zirconia microspheres can survive operating temperatures up to 1000 degrees C, when most other materials for microspheres would not survive. Due to the Zirconia’s toughness and high wear resistance these spherical beads can be used in high energy and high temperature processes and applications in extreme environments. Zirconia particles also offer high density at over 6 g/ml and unmatched durability.

Chemically-stable, inert, and safe ceramic materials for microspheres are used in fillers and spacers for biotechnology, sintering, microfluidics, electronics, optical coatings, medical devices and other high tech applications. Due to their high refractive index, as well as their chemical and mechanical robustness Zirconia microspheres are interesting building blocks for various photonic applications considered for energy systems and heat management.³

Hollow Ceramic Microspheres

Similar to hollow glass microspheres, hollow ceramics preserve the benefits of spherical particle shape and bring additional advantages of high temperature stability, low density, chemical stability, and ability to be functionalized with a range of molecules. These characteristics make hollow ceramic spheres a great additive to reduce the weight of plastics, rubbers, resins, and cement.

Materials for Microspheres – Metal

Metals materials for microspheres (metal powders) are characterized by their high density, opacity, and high thermal and electrical conductivity.

Solid Metal Microspheres

Solid metal microspheres are highly spherical and round metal spheres which are often used as conductive spacers in bond line applications or as high density test projectiles, offering high density, corrosion resistance and narrow size ranges.

Titanium microspheres are made from a bio-compatible metal commonly used for implants, and medical & dental devices. Combination of high density at 4.5g/cc and small particle sizes make these particles suitable for use in semiconductor, Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD) processes, including Thermal and Electron Beam (E-Beam) Evaporation, Low Temperature Organic Evaporation, Atomic Layer Deposition (ALD), Metallic-Organic and Chemical Vapor Deposition (MOCVD).

Applications for metal microspheres, also referred to as metal balls, include bearings, ball
screws, linear motion guides, automotive parts, food processing, dispenser valve balls,
stainless-steel bearings, chemical-related applications, casters, bicycle parts, light-duty
bearings, hardware and toys.

Metal-coated Microspheres

Metal-coated microspheres have gained acceptance due to the combined benefits of precision dimensions and a unique combination of electrically-conductive properties, especially in the area of advanced electronics. Silver, nickel and gold are the most common coated microspheres.

Conductive properties of materials for microspheres are usually achieved by applying a layer of silver, nickel, or gold coating on hollow or solid glass, polymer, or ceramic microspheres.

Electrically conductive microspheres are often used as a conductive filler that is lighter than solid silver in paints, adhesives and composites to provide electrical conductivity. They are also used as catalysts, laser fusion targets, and electrically conducting bond line spacers in advanced electronics and other applications.

The lower particle density compared to solid metal and large surface area of the spherical particles facilitates slow phase separation in paints and adhesives compared to heavy metallic and inorganic fillers.  Metal-coated microspheres are used as catalysts, laser fusion targets and materials for preparing the following:

• Electrically conductive adhesives and polymers.
• Composites and reinforced polymers.
• Copper and silver pastes for electronics.
• Thermoelectrical elements.
• Ablative coatings.
• Electrically conductive bond-line spacers.

Metal coatings can provide the following properties to the substrate:

• High electrical conductivity.
• High thermal conductivity.
• Chemical resistance.
• Catalytic and electrocatalytic activity.
• Ferromagnetism.
• High specific weight.
• Mechanical strength.
• Electromagnetic absorption.

 

Silver-coated hollow glass microspheres are specifically optimized for use in EMI shielding and designed for use in paints, polymers, composites and resins. Silver coating with optimized thickness provides the material with good conductivity and shielding properties
while maintaining the weight-saving benefit associated with hollow-core materials. Silver-coated hollow glass spheres have been demonstrated to be a more efficient conductive filler than materials such as nickel and graphite powders and flakes. By incorporating silver-coated microspheres into their products, composites manufacturers can realize significant weight savings over solid silver particles and other conductive fillers. Conductive surface materials for microspheres are also suitable for use in military applications, electronics, paints and other industries.

A unique benefit of metal coating on glass spheres is that it can offer retroreflective properties. Retroreflective Microspheres are made by applying a half-shell aluminum coating on solid Barium Titanate glass microspheres. These high index of refraction glass spheres are hemispherically coated with a thin aluminum shell produce a bright retroreflective response directed back to the light source.

Summary:

In conclusion, many options exist for the selecting the right materials for microspheres suitable for a specific application. Materials differ in size, density, compression, crush strength, durability, temperature and chemical resistance, biocompatibility, temperature tolerance and much more. Some materials for microspheres are easier to surface-functionalize for biomedical applications, other materials may be better suited for conductive coatings or added colorants and fluorophores. When choosing materials for microspheres we should always start by investigating and truly understanding the technical requirements and the desired functionality that the microsphere component will bring to the success of the project.

1. Righini G.C. Glass micro- and nanospheres. Physics and applications. Pan Stanford Publishing; Singapore: 2018. [↩]
2. Righini GC. Glassy Microspheres for Energy Applications. Micromachines (Basel). 2018;9(8):379. Published 2018 Jul 30. doi:10.3390/mi9080379 [↩]
3. E. Leib, R. Pasquarelli, J. Do Rosario, P. Dyachenko, S. Döring, A. Puchert, A. Petrov, M. Eich,  G. Schneider, R. Janssen, H. Weller, T. Vossmeyer. (2015). Yttria-stabilized zirconia microspheres: Novel building blocks for high-temperature photonics. J. Mater. Chem. C. 4. 10.1039/C5TC03260A. [↩]