Interfacial bond strengths between layers of visible light-activated composites.

The bond strengths between composite layers either cured to themselves or to other types (classes) of composites of similar or different brands were measured by using a direct tensile test (true tension). The diametral compression test for tension for each material tested was also conducted according to ADA specification No. 27. The values were used for comparison. 1. The interfacial bond strengths were generally found to be higher than the cohesive strengths of the weaker materials when cured to different types of composites or of the weak region in the specimen when composites were cured to themselves. 2. The cohesive tensile failure of the materials occurred at much lower stress levels than their corresponding diametral tensile strength (ranging from 1/4 to 1/3 of the diametral strength). 3. When two types of composites were bonded together the cohesive failure occurred consistently within the materials with lower diametral strength. Correlation was observed between the values of cohesive strength of material measured with true tension and the diametral test. 4. A urethane dimethacrylate microfilled composite bonded weakly to BIS-GMA composite, therefore, their combined use should be avoided. 5. Incremental placement produced a clinically acceptable bond strength because it exceeded or was at least comparable to the cohesive strength of the material.