EFFECT OF COMPOSITION ON FRACTURE AND CREEP OF COMPOSITE

Although dental composite resin materials are beginning to live up to
expectations as anterior restoratives, they have not been proven to be
durable enough to be used routinely in occlusal, load-bearing
applications. The main problem appears to be an inadequate ability to
resist the degradative forces accompanying the wear process. Recently, the
phenomena of wear in composites has been related to the ability of the
material to resist internal crack formation and propagation, as well as to
its ability to resist subsurface deformation and degradation by solvents
and physical stresses. The restorative material's ability to resist
internal deformation and fracture depends upon its composition and the
physical properties of its components. The goal of this research is to
describe the nature of the subsurface microscopic changes in structure
which lead to failure of dental composite materials by fracture
mechanisms. The fracture toughness, time dependent compressive creep and
compressive yield strength of commercial composites and experimental resins
will be evaluated, and the results will be correlated to the degree of
polymerization (DP) of the resin matrix, as well as to the concentration
and size distribution of the particulate reinforcing fillers. Since the
properties of the polymeric resin and the filler/matrix interface are
suspected to degrade with time, the long-term stability of the materials
will also be evaluated by storing specimens in several aqueous solvents for
one year prior to testing. The fracture surfaces and the microstructure of
the composite subjected to static loading during creep will be examined by
scanning electron microscopy. Evidence of microcrack formation in the
polymer resin matrix, filler/matrix debonding, redistribution of fillers
and other defects will be identified, since these microstructural changes
may initiate the fracture processes which cause composite restorative
materials to fail during service in the oral environment.