KH560 modified silica xerogel from rice husk ash for epoxy composite coating on steel: effects of post calcination on silane grafting and interfacial bonding

Silica xerogel (SiO2-X) derived from rice husk ash was evaluated as a reinforcement for epoxy coatings. Although γ-glycidyloxypropyl trimethoxysilane (KH560) is widely used to enhance silica–epoxy compatibility, grafting is often hindered by the high silanol density of sol–gel derived SiO2-X. This study examines how the elimination of silanol groups via calcination improves silane coupling, thereby influencing the physico-mechanical properties of epoxy coatings. Calcination at 500 °C for 2 h removed ∼7.5 wt% of water and hydroxyl species, as confirmed by thermogravimetric analysis (TGA), and reduced the BET surface area from 382 to 215 m2/g. Both uncalcined and calcined SiO2-X were silanized with KH560 and incorporated at 5 wt% into a diglycidyl ether bisphenol A (DGEBA) epoxy matrix. The calcined SiO2-X filler enabled more efficient grafting, leading to superior film formation and stronger interfacial bonding. This was demonstrated by a 45% increase in pull-off adhesion strength, a 35% rise in water contact angle (WCA), improved microstructural homogeneity observed via SEM, and significantly enhanced abrasion resistance in a linear reciprocating ball-on-flat wear test. Differential scanning calorimetry (DSC) further showed that uncalcined SiO2-X disrupted epoxy curing, lowering the glass transition temperature, whereas the calcined SiO2-X maintained normal polymerization behavior. The findings indicate that proper control of surface hydroxyl concentration is crucial for optimizing the reinforcing capability of bio-derived SiO2-X in epoxy coatings.