Fillers and HDT / Vicat
HDT / Vicat
This property tells us how much we can heat a material before it softens. HDT is measured by putting a weight on a plastic bar and heating until the bar bends by a defined amount. Vicat softening temperature is measured by pressing a pin against the part surface using a defined force and heating the plastic until the pin penetrates the surface by a specified amount. Both methods tend to give similar numbers and are essentially measuring the stiffness of the material as the temperature is raised. Therefore, HDT and Vicat are affected by fillers in exactly the same way as modulus is (as described on a separate page.
Plastics are not as stiff as other materials like glass or metal, so fillers are often added to increase the stiffness and therefore the HDT. 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 HDT & Vicat?
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
Fillers can be very effective at improving HDT and Vicat but only in semi-crystalline polymers like PE, PP, nylons, PBT and so on. For example, 10 weight% chopped glass fiber or high aspect ratio mica can increase HDT almost to the melting point of the polymer. In amorphous plastics like polystyrene, SAN, PMMA and polycarbonate, fillers have hardly any effect on HDT or Vicat.
Sometimes the goal is to get some improvement in HDT 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 HDT 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 HDT, 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 HDT 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.