Abstract Biopolymer based composites are designed using glass–silica reinforcement. Surface etching of spherical glass–silica particles is performed using chemical and physical treatments. In particular, treatment with hydrofluoric acid proved to be efficient to achieve acceptable anchoring effect. Experimental testing of thermomoulded composites confirms that samples with chemically modified microbeads have improved mechanical properties, irrespective of phase content. A quantitative evaluation of the improvement of the starch/glass–silica interphase properties is achieved using a finite element model. Generation of typical microstructures is used to simulate phase arrangement and interphase properties. Microstructures are meshed taking into account the interphase region. Finite element results indicate that for all samples, interphase Young’s modulus is lower than those of the intrinsic materials. The thickness weighted interface modulus increases for composites where the mechanical adhesion is improved using HF chemical treatment.