Effect of filler content on the adhesion strength between UD fiber reinforced and particulate filled composites

Fiber reinforced composites (FRCs) are increasingly replacing metals in many engineering and medical applications. Recently, splints, post-orthodontic retainers and various load bearing substructures in fixed partial dentures (FPD) are manufactured using special dental grade FRC prepregs. In these applications, FRC substructures are covered with particulate filled dental composites to improve esthetics and patient comfort as well as to maintain good oral hygiene by reducing plaque accumulation. In this paper, effect of the composition of the particulate filled esthetic composite veneer (PFC) on the shear strength of adhesion between the FRC framework and the PFC was investigated. Light-cured, unidirectional S2-glass fiber reinforced composite rods were used to model the FRC dental substructures. A series of PFCs with filler content varying from 0 to 46 vol.% was prepared. Nominal shear adhesion strength, τa, was measured using a modified pull-out test. Resin matrix for both, FRC and PFC was based on the same dimethacrylate monomer mixture. No adhesive was used and the FRC and PFC were joined together during polymerization. Scanning electron microscope (SEM) was used to inspect the loci of failure of the adhesive joint. The stress field within the test specimen was simulated using finite element analysis (FEA) code.Increasing filler content in the PFC resulted in an increase of the measured nominal shear adhesion strength, τa, between FRC and PFC. With increasing embedded length of the FRC rod in the PFC cylinder, the τa became almost independent of PFC composition, and, joint failure was altered from cohesive failure in the PFC to adhesive and/or interlaminar failure in the FRC. The results suggested that the fracture toughness of the PFC composite is of pivotal importance for preventing PFC layer from chipping-off the FRC load bearing framework and, hence, it is critical for long lasting passive dental appliances such as splints, retainers and FPDs. In addition, the interfacial shear strength of the fiber–matrix interface in the FRC controls the crack path in the FRC/PFC joint. Increasing the fiber–matrix adhesion can change the failure from interlaminar in the FRC to adhesive failure running along the FRC/PFC interface or even to cohesive fracture propagating in the PFC.