In this article we will discuss the applications and technical properties of PMMA microspheres.

What are PMMA Microspheres?

PMMA micropsheres are also known as Poly(methyl methacrylate) or acrylic microspheres. PMMA, an ester of methacrylic acid (CH₂=C[CH₃]CO₂H), is a synthetic resin produced from the polymerization of methyl methacrylate.

Ever since PMMA resin was discovered and first commercialized in 1930s, it became widely used due to a combination of highly desirable technical properties. Currently the acrylic polymer is often sold under the trademarks Plexiglas, Lucite, and Perspex.

Due its optical properties and durability that are similar to glass, PMMA polymer became a go-to substitute for glass in products such as shatterproof windows, skylights, illuminated signs, and aircraft canopies. PMMA resin is also an economical alternative to polycarbonate (PC) when extreme strength is not necessary.

PMMA plastic is a fully recyclable material, with some grades approved for food and medical contact. The material does not contain any toxic materials or heavy metals, which may cause environmental damage or health risks. When acrylic burns, it does not produce toxic or corrosive gases which is compliant with international standard.

PMMA microspheres, which are also often referred to as PMMA beads, microbeads, spheres, or acrylic balls, are spherical engineered particles manufactured out of acrylic resin with precise technical specifications of controlled shape and size of each particle. Precision PMMA microspheres are the material choice when the application requires the exceptional optical clarity similar to glass in combination with high durability and robustness of the plastic.

Typically acrylic microspheres are manufactured by suspension polymerization and range in diameter from one micron to five hundred micron. Spherical shape of particles offers advantages of large surface area for quicker dissolving in solvents, light rendering properties including diffusion and scattering, and processing advantages due to ball-bearing properties of microspheres.

What are Technical Properties of PMMA Microspheres?((https://www.pmma-online.eu/pmma-science/faq/))¹

Mechanical and Physical Properties of PMMA Microspheres:

PMMA polymer that is used in formulation of acrylic microspheres is an amorphous, transparent, and colorless thermoplastic that is hard and stiff, which makes PMMA spherical particles suitable to withstand a wide range of applications.

Poly(Methyl Methacrylate) has good abrasion and UV resistance which makes PMMA microspheres highly durable.

Poly(Methyl Methacrylate) or PMMA is less hydrophobic than polystyrene and reported to show reduced nonspecific protein and peptide binding. The density of PMMA beads, at ~1.2 g/cc, is considerably heavier than polystyrene particles, allowing more rapid separation and making them easier to concentrate by centrifugation. On the other hand, the density of PMMA is significant lighter than glass beads, minimizing settling when the preference is to keep the spheres suspended in a fluid.

Mechanical Properties

(source: Goodfellow)

Elongation at break (%)	2.5-4
Hardness – Rockwell	M92-100
Izod impact strength (J m–1)	16-32
Poisson’s ratio	0.35 – 0.4
Tensile modulus (GPa)	2.4-3.3
Tensile strength (MPa)	80

Physical Properties

Abbe number	57.2
Density (g cm–3)	1.19
Flammability	HB
Limiting oxygen index (%)	17-20
Radiation resistance	Fair
Refractive index	1.49
Resistance to Ultra-violet	Good
Water absorption – over 24 hours (%)	0.2

Optical Properties of PMMA Microspheres:

Typical PMMA grades allow at least 92% of light to pass through it, which is more than glass or other plastics. This outstanding clarity enables the use of PMMA in many different optical and related applications.

The particles typically have a hydrophilic anionic surface with refractive index of 1.48.

Electrical Properties of PMMA Microspheres:

Dielectric constant @1MHz	2.6
Dielectric strength (kV mm–1)	15
Dissipation factor @ 1MHz	0.014
Surface resistivity (Ohm/sq)	1014
Volume resistivity (Ohmcm)	2-14 x 1015

Thermal and Combustion Properties of PMMA Microspheres:

Though flammable, PMMA polymer material has low smoke emission.

Compared with many other plastics, and especially with several types of wood and other natural materials, PMMA develops almost no smoke. Typically PMMA products will comply with the Euro Class E, which confirms the low smoke levels of acrylic materials.

The smoke gases emitted by PMMA were examined in detail by a specialized institute. The combustion gases generated by PMMAs are typically toxicologically inoffensive according to EN 50267-2-2 and do not impede escape from fire.

PMMA acrylic polymer burns with a bright flame, virtually without smoke. Under normal circumstances, combustion only gives rise to carbon dioxide and water. Due to the material’s chemical composition (carbon, hydrogen and oxygen), no acutely toxic substances like phosgene, acid vapors and sulfur dioxide can form, even in a real fire. Since the material does not contain any halogens, no dioxins can form either.

Coefficient of thermal expansion (x10–6 K–1 )	70-77
Heat-deflection temperature – 0.45MPa (C)	105
Heat-deflection temperature – 1.8MPa (C)	95
Lower working temperature (C)	-40
Specific heat (J K–1 kg–1)	1400 – 1500
Thermal conductivity @23C (W m–1 K–1)	0.17-0.19
Upper working temperature (C)	50 to 90

Chemical Resistance Properties of PMMA Microspheres:

PMMA is generally characterized by good chemical resistance. Parts made from PMMA are resistant to most inorganic chemicals, aliphatic hydrocarbons, cycloaliphatic compounds, fats and oils at room temperature, and also to diluted acids and concentrated solutions of most alkalis at temperatures up to 60 degrees Celsius.

By contrast, PMMA is attacked by chlorinated hydrocarbons, acetone, benzene, ketones, esters, ethers, alcohols and aromatic compounds.

The fact that acrylic polymer resin has somewhat poor solvent resistance is seen as an advantage in applications where it is desired to cleanly “burn-off” the microspheres leaving behind a porous structure of precise diameter, such as in formation of porous ceramics (e.g. artificial bone).

Acids – concentrated	Good-Poor
Acids – dilute	Good-Poor
Alcohols	Good-Poor
Alkalis	Good
Aromatic hydrocarbons	Poor
Greases and Oils	Good
Halogenated Hydrocarbons	Poor
Halogens	Poor
Ketones	Poor

Full chemical compatibility chart for PMMA.

Crosslinking Effects of PMMA Microspheres:

Various grades of PMMA microspheres are commercially available, which offer different levels of polymeric cross-linking.((Albeladi HK, Al-Romaizan AN, Hussein MA. Role of cross-linking process on the performance of PMMA. Int J Biosen Bioelectron. 2017;3(3):279–284. DOI: 10.15406/ijbsbe.2017.03.00065))

Cross-linking is the process of interlinking the polymer chains through ionic or covalent bonds, in which it confines the polymer chains to slide past each other by impeding their free movement and produces elasticity in amorphous polymers at the same time, and this process can be carried out by using (i) chemical cross-linking or (ii) physical procedures.

The degree of cross-linking of PMMA affects thermal stability, tensile strength, surface hardness, mechanical stability, flame retardant qualities, elasticity, solvent and chemical resistance of the finished product.

Biocompatibility Properties of PMMA Microspheres:

This biocompatibility can be attributed to PMMA’s resistance to:

• Temperatures stress
• Chemical reactions
• Human tissue
• Bioprocesses

PMMA is biocompatible with human tissue. Because it is BPA free, it is also a useful BPA-free alternative to polycarbonate frequently used for making component parts for medical, biological and biopharmaceutical applications.

What Applications are PMMA Microspheres Used In?

Poly(methyl methacrylate) or PMMA microspheres and spheres are often used in a wide variety of applications including porous ceramics, self assembled microfluidic devices, biomedical research and life sciences.

The first major application of the new plastic took place during World War II, when PMMA was made into aircraft windows and bubble canopies for gun turrets. Now, PMMA micropsheres are used in such a wide variety of industries as investigations in the colloidal crystal field, toners for copying machines, biomedical devices and injectable dermal fillers.

PMMA Microspheres in Porous Ceramics:

Polymer microspheres, such as Poly(Methyl Methacrylate) (PMMA) or Polyethylene (PE) beads, offer an excellent solution to creating precise pore sizes in ceramics. Microspheres are mixed in with the ceramic material and then “burned off,” leaving a precise porous structure.

Using polymer microspheres, such as PMMA or PE, offers the added benefit of minimal residue after firing and the availability in many diameters. Highly spherical microspheres have the added benefit of creating strong pores without any stress risers that might cause fracturing of the parent ceramic.

PMMA Microspheres as Bondline Spacers:

PMMA microspheres are widely used as flexible (deformable) spacer particles, where the polymeric spacer material is more compliant to deformation compared to glass spheres and, therefore, ensures uniformity of contact across a specific area.

Additional advantage of PMMA spheres as a bondline spacer is the low density of ~1.2g/cc, which is typically close to the density of the bonding material. Density match between the adhesive and the spacer material helps to minimize settling of spheres, compared to using much denser glass spheres. In addition, since many adhesive formulations are acrylic-based, better bonding to the spacer particles is achieved.

For bondline spacer applications, having a narrow particle diameter distribution is not as critical as ensuring the sharp cut-off at the maximum particle diameter. The largest particles will be the element that holds and ensures the bondline thickness.  PMMA microspheres are commercially available in many different grades, most of them offering a standard bell-shaped (normal) particle size distribution, which is not suitable for bondline applications. Unlike other microsphere manufacturers, Cospheric grades of PMMA microspheres have very strict specifications on the number of particles above maximum diameter requirement, making them a great fit for spacer applications.

When sourcing PMMA microspheres for a bondline spacer project, it is critical to inquire about % of particles that may be outside the maximum specified diameter, in addition to other important considerations of selecting microspheres for bondline spacer applications.

PMMA Microspheres in Medicine and Dentistry:

Due to superior biocompatibility of PMMA microspheres, they are frequently used in cosmetics, dental, and medical applications.((Lin, Zhi & Shah, Vishva & Dhinakar, Arvind & Yildirimer, Lara & Cui, Wenguo & Zhao, Xin. (2016). Intradermal fillers for minimally invasive treatment of facial aging. Plastic and Aesthetic Research. 3. 72. 10.20517/2347-9264.2015.121. ))

The first use of polymethyl methacrylate (PMMA) as a dental device was for the fabrication of complete denture bases. Its qualities of biocompatibility, reliability, relative ease of manipulation, and low toxicity were soon seized upon and incorporated by many different medical specialties.

PMMA has been used for (a) bone cements; (b) contact and intraocular lens; (c) screw fixation in bone; (d) filler for bone cavities and skull defects; and (e) vertebrae stabilization in osteoporotic patients.((Frazer, Robert Q et al. “PMMA: an essential material in medicine and dentistry.” Journal of long-term effects of medical implants 15 6 (2005): 629-39.))

For dermal filler applications, the technical properties of PMMA microspheres that relate to particle diameter and narrow size distribution are especially critical. This is due to the fact  that the spheres must be small enough to pass through a tiny needle and large enough to escape phagocytosis by macrophages, the cells which clean the inner vertebrate tissues from all foreign materials. Cospheric offers a variety of narrow particle diameter distributions of PMMA spheres.

PMMA in Optical Applications:

PMMA is a tough and rigid plastic. In addition, it has almost perfect transmission of visible light, and, because it retains these properties over years of exposure to ultraviolet radiation and weather, it is an ideal substitute for glass. A most successful application is in internally lighted signs for advertising and directions.

PMMA is also employed in domed skylights, swimming pool enclosures, aircraft canopies, instrument panels, and luminous ceilings. For these applications the plastic is drawn into sheets that are machined or thermoformed, but it is also injection-molded into automobile lenses and lighting-fixture covers. Because PMMA displays the unusual property of keeping a beam of light reflected within its surfaces, it is frequently made into optical fibres for telecommunication or endoscopy.

Precision PMMA spherical particles are often in optical instruments or medical devices as a lightweight alternative to glass microspheres.

Conclusion:

PMMA microspheres or beads are spherical polymer microparticles that offer superior functionality suitable for many applications due to desirable properties of PMMA polymer as a strong yet lightweight material, which is also optically clear and biocompatible.

1. http://www.goodfellow.com/A/Polymethylmethacrylate.html [↩]