This study was performed to investigate the effects of thermal cycling on the strength of light cured restorative composite resins. Five, commerically available light-activated composite resins(Photo Clearfil A : CA, Lite-Fil A : LF, Clearfil Photo posterior : CP, Prisma AP.H.: PA, Z100 : ZH) were used. The specimens of 12 mm in diameter and 0.7 mm in thickness were made, and a thermal cycling test of 1000 cycles at 15 seconds dwell time each in 5 C and 55 C baths was performed. In nrder to investigate the deterioration of composite resins in the thermal cycling test, the bi-axial flexure strengths were measured, and the surface after thermal cycling test were examined by SEM.
Be Tresults obtained were summarized as follows
1. biaxial flexure strength significantly decreased due to the thermal cychng(P<0.01).
2. Biaxial flexure strengths` showed the maximum value of 125.65MPa for ZH group and the minimum value of 64.86M, Pa for ZA group, after thermal cycling test, Biaxial fleure strengths of ZH and CP group were higher then those of PA, CF, and CA groups, after thermal cycling test (p<0.05).
:3. Fracture surfaces showed that the composite resin failure developed along the matrix and the filler/resin interface region, and the cracks propagated in the-´conical shape from the maximum tensile stress zone.
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