Fillers and CTE / CLTE

CTE / CLTE

The coefficient of expansion is simply a measurement of material expansion when it is heated. CTE is measured by measuring the length of a test specimen, heating it and measuring the length again. It is often expressed as ppm/°C or μm/m°C. So a value of 17 μm/m°C means that a 1 meter long sample increases in length by 17×10^-6 meters if it is heated by 1°C. Most materials expand when heated and have a positive CTE value.

What filler property affects CTE?

To understand CTE is is best to think of the material as a spring. Weak springs are easily stretched and stiff springs require much more force. As a polymer sample is heated, the polymer wants to expand and that creates a force. How much that force can expand the polymer depends on how stiff the polymer is. Now let us consider mineral-filled plastics. If I add high aspect ratio filler like mica or glass fiber, then the stiffness of the polymer increases dramatically. So, now when I heat that composite, it wants to expand but it can’t expand much so it has a low CTE. If we added soft rubber particles to a polymer, then the CTE would actually increase. Overall, we can see that CTE depends on fillers in the same way modulus does.

Recommendation

Fillers are highly effective for lowering CTE.

Round and cubic fillers like calcium carbonate, silica or glass beads decrease CTE 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 lowering CTE. Low aspect ratio (isotropic) fillers reduce CTE equally in all three directions (x, y and z), so they do not cause warpage as a part heats up and cools down.

Fillers with medium aspect ratio include talc, kaolin and wollastonite. These are good at increasing stiffness due to their aspect ratio so they are fairly good at lowering CTE. Medium aspect ratio fillers can cause some warpage because they reduce CTE differently in different directions.

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 to reduce CTE. However, this type of filler creates the most warpage. Often chopped glass fiber is combined with another filler with low aspect ratio in order to control warpage. Of the two types of mica, phlogopite is most effective at lowering CTE due to the exceptionally low CTE of that mineral compared to other mineral fillers.

Conclusions

This gives an introduction to selecting the right mineral filler when CTE 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, type and aspect ratio too.

Contact us to describe your exact needs, so we can make a recommendation based on your particular circumstances.