In the present work, chalcone benzoxazine (Chal-Bz) was co-polymerized with two bio-based benzoxazines namely cardanol (CrAb) and eugenol (EuAb) benzoxazine and coated over cotton fabric. The Chal-Bz/CrAb and Chal-Bz/EuAb blends coated cotton fabrics show water contact angle values of 163° and 156° respectively and their oil–water separation efficiency were observed found to be 97 and 96% respectively. In addition, the glycidoxypropyltrimethoxysilane (GPTMS) functionalized bio-silica in varying weight percentages (1, 3, 5, 7 and 10 wt%) was also reinforced with two co-polymerized blended matrices to obtain respective composites. The developed composites were studied for their dielectric, thermal and morphological behaviours. The values of dielectric constant of 10 wt% bio-silica reinforced Chal-Bz/CrAb and Chal-Bz/EuAb composites were found to be 2.1 and 2.3, respectively. Similarly the values of contact angle of 10 wt% bio-silica reinforced chal-Bz/CrAb and Chal-Bz/EuAb composites were 137° and 116°, respectively. Data obtained from different studies suggest that these bio-silica reinforced benzoxazine composites can be used in the form of coatings, adhesives, sealants and encapsulants for high performance micro-electronics insulation applications under adverse environmental conditions. Graphic Abstract: Thermal assisted polymerization of chalcone based benzoxazine (Chal-Bz) with bio based benzoxazines cardanol-aniline based benzoxazine (CrAb) and eugenol-aniline based benzoxazine (EuAb)) for low k and oil–water separation application.