Comparative Study of Raw and HMDS-Treated Pigment-Rich Agro-Industrial By-Products as Functional Fillers in PDMS Composites

Agro-industrial by-products from beetroot, raspberry, sea buckthorn, and shadbush are valuable sources of natural pigments and renewable filler materials, but their hydrophilic surfaces limit compatibility with hydrophobic polydimethylsiloxane (PDMS) matrices. This study evaluated the effect of catalyst-free vapor-phase hexamethyldisilazane (HMDS) treatment of pigment-rich powders and their incorporation into PDMS composites at 5 and 20 wt.% filler loadings. Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy, and contact angle measurements were used to characterize surface modifications. Mechanical behavior under puncture loading was evaluated using texture analysis. FTIR confirmed successful silylation through the reduction in hydroxyl groups and the emergence of silicon-containing functionalities. Treated fillers exhibited rougher, more irregular surfaces, which suggested improved filler dispersion and interfacial compatibility within the PDMS matrix. Modified composites showed enhanced hydrophobicity, with contact angles up to 102.84°. Composites containing 5 wt.% HMDS-treated raspberry filler demonstrated the highest puncture resistance and elasticity, indicating improved interfacial interactions. In contrast, 20 wt.% filler loading generally reduced puncture resistance and elasticity, possibly due to increased particle agglomeration and reduced matrix continuity, as suggested by the SEM observations, whereas shadbush-filled composites showed decreased performance after treatment. Overall, HMDS surface modification effectively improves the compatibility of pigment-rich agro-waste fillers with PDMS and supports their use as functional fillers and natural colorant sources in silicone-based composites, providing a value-added route for the utilization of agro-industrial by-products.