How Are Hollow Ceramic Spheres Used?
Spherical fillers are the preferred choice for many applications because they offer an excellent property set including:
- Excellent flow
- Increased stiffness
- Low shrinkage & warpage
- Significant density reduction
Lightweighting is an important topic, for example, transportation consumes over 40% of oil, so there is always a push to lighten cars, trucks, trains and aircraft. In other instances, lighter weight is needed for buoyancy control. The typical path to lightweighting is to use a foam but standard foams have disadvantages such as reduced mechanical properties and difficulties to reproducibly create the proper cell structure. Syntactic foams circumvent those issues by using pre-made bubbles with a hard outer shell. Hollow glass beads or expandable polymer microspheres can be used, but ceramic spheres are available at a more attractive price point and offer many of the same advantages.
Hollow ceramic spheres are made of aluminosilicate and are known generically as “cenospheres” but customers seek out the Extendopheres® brand due to superior supply consistency and quality.
What Are Extendopheres®?
Extendopheres® are used in many different products spanning a wide range of industries. For example, they are added to thermoplastics, thermosets and elastomers, mainly for density reduction, but also for their other attributes such as high flow, smooth surface finish, sound damping, heat insulation and scratch resistance. Whereas conventional foams have reduced strength and stiffness, the hard ceramic shell actually increases stiffness and allows for good retention of strength.
They are also beneficial in concrete, especially self-levelling concrete due to not only excellent flow but buoyancy and ability to chemically bond to the concrete via their pozzolanic activity to give better interfacial adhesion and durability. Even at high loadings approaching 50 volume percent, good flow is retained in part due to the spherical shape and partly due to the low surface area, both of which reduce interparticle friction.
High hardness makes Extendopheres® ideal for abrasion resistance coatings, brakes and grinding wheels. In the same vein, they perform well in anti-skid coatings where they can add surface roughness combined with resistance to wear, extending durability of the coating. In fact, studies show that ceramic spheres are able to improve the hardness and wear resistance of metals, in particular aluminum and alloys thereof.
An Improved Performance
Electrical and thermal properties can be favorably adjusted using hollow ceramic spheres. Electrical dielectric constant is reduced because of the gas inside the spheres and thermal insulation is improved for the same reason. In both instances, the performance is improved in proportion to the volume fraction of spheres used. In addition, less heat is required to heat and cool materials containing hollow ceramic spheres, which allows for energy and time savings during part production.
Sound damping is often achieved using foams and hollow ceramic spheres have proven highly affective. Interestingly, they work best when debonded from the host matrix. The tan (loss factor) is increased when the spheres are surface treated to prevent interfacial adhesion. This creates an air pocket around each particle and that helps absorb vibrations.
Cost reduction is another factor worth mentioning. Because of their relatively low cost per unit volume and their ability to replace space occupied by costly resin, they help formulators optimize the performance : price balance.
Typical Sphere Properties
- Chemistry: aluminosilicate (inert)
- Max temperature: 1300 – 1600°C
- Compressive strength: 2500 – 5000psi
- Bulk density: 0.4g/cc
- Density: 0.55 – 0.75g/cc
- Appearance: Grey
Surface treated grades are available with a wide range of organosilane coupling agents tailored to each customer’s requirements.
Although ceramic spheres are resistant to fracture, it is prudent to use mixing and processing conditions that are gentle enough to prevent breakage. For example, the use of high-flow resin and low shear and always dose the powder into liquid to avoid dry particles from grinding together.
Learn More About Extendopheres®
If you would like to hear more about our Extendopheres® product line, then feel free to contact us today. We have experts to help you select the best grade for your application and to supply samples for evaluations.