I am investigating some biocomposites formed by PBAT and tiny particle fillers derived from natural wastes. I have noticed that with a loading of 10%wt. of these fillers, the elongation at break and the tensile strength values increase meanwhile the Elastic Modulus is scarcely affected. This fact is particularly surprising because PBAT breaks at 500%.

I was expecting the opposite trend, indeed an improved Young Modulus and elongation at break and tensile strength decreased (as in the case of these biocomposites with 20-30%wt. of loading).

What can be the reason of this mechanical behaviour? I was thinking about a possible plasticizing effect induced by some lipid fractions present in the filler but the Tg is practically unchanged and the lipid content is low.

Any suggestion?

Thank you!