Coupling agent between filler particle and resin is:

Coupling agent : A bonding agent applied to the surfaces of reinforcing particles (filler) to ensure that they are chemically bonded to the resin matrix. Organo-silane compounds are the more common class of dental composite coupling agents. Although titanates and zirconates can be used as coupling agents, organosilanes—such as γ-methacryloxypropyl tri-methoxysilane are used most commonly. Phillips 12 ed page no. 275,286