Fillers and Modulus / Stiffness
Modulus / Stiffness
Modulus is simply a technical term for stiffness of a material. Tensile modulus measures how stiff a material is when you pull on it and flexural modulus is how much a material bends when you apply a force to it. Plastics are not as stiff as other materials like glass or metal, so fillers are often added to increase the stiffness. Mineral fillers are basically made from finely ground rocks, so they are much stiffer than the polymer. The more stiff mineral you add, the stiffer the mixture of plastic and filler becomes.
What filler property affects modulus?
Round and cubic fillers like calcium carbonate, silica or glass beads increase modulus by the least amount. These fillers have very low aspect ratio meaning the longest dimension of the particle divided by the shortest dimension. For example, spheres have an aspect ratio of 1 and are therefore not so effective at increasing modulus.
Fillers with medium aspect ratio include talc, kaolin and wollastonite. These are good at increasing stiffness due to their aspect ratio, which is typically in the range 10:1 to 40:1
High aspect ratio fillers are typified by glass fiber, wet ground mica, nanoclay and graphene. These are highly effective at increasing stiffness of the plastic.
Recommendation
Sometimes the goal is to get some improvement in stiffness at the lowest possible cost. In that case, ground calcium carbonate is the most common choice. Dolomite is similar and may also be used.
If the goal is a moderate improvement in modulus without too much added cost, then talc is typically chosen. Dry ground mica, wollastonite and kaolin clay may also be considered depending on the other demands on the material.
When the goal is maximum stiffness, then chopped glass fiber is the first choice in most instances. Long glass fiber, carbon fiber or even Kevlar fiber can be used when cost is no object.
Conclusions
This gives an introduction to selecting the right mineral filler when stiffness is your primary concern. However, there are many other factors to consider. For example, do you care what colour the filler is? Are there impact resistance requirements? The list goes on.
Furthermore, all grades are not created equal. For example, particle size has a significant effect on many properties of the composite material. Similarly, the aspect ratio of different grades of any given mineral has a strong effect on properties. It is not enough to select mica, one needs to choose the correct size and aspect ratio too.
Contact us to describe your exact needs, so we can make a recommendation based on your particular circumstances.