Relationship between impact strength and test method of denture resins

Objectives: Charpy impact method according to ISO1567:1999/Amd.1:2003 has been adopted for impact test of denture base resins with 1.2mm notch depth. However, the discrepancy between the impact strength and the static properties (e.g. facture toughness) was often found. The aim of this study was to investigate the effect of impact strength on various notch conditions using Charpy impact method.

Methods: Two high impact resins (sample A includes fiber = low fracture toughness, sample B NOT includes fiber = high fracture toughness) were used in this study. Both specimens were produced using normal dental laboratory practice, polymerized in heated water at 70ºC during 30minutes, and then in boiled water for 40minutes. The cured specimens were polished using abrasive papers, and finished the all surfaces using 5micron alumina solution and cut to give specimens of 50x6x4 mm. In each specimen, an A type notch was then cut edgewise to a depth of 0, 0.5, 1.2 and 2.0mm. For each depth of notch, six specimens were tested according to Charpy impact test from ISO1567. All data were analyzed by two-way ANOVA, and Tukey's test. In addition the stress distribution was analyzed by 3D-FEM.

Results: Mean values and standard deviation of Charpy impact strength were as follows. Same letter indicates no significant difference (p>0.05). In 0mm notch depth, sample A had significantly lower Charpy impact strength than sample B (p<0.01). On the other hand, in each of 0.5, 1.2, and 2.0mm notch depth, no significantly difference was found between sample A and sample B (p>0.05), respectively.These results were also demonstrated by 3D-FEM.

Conclusion: It was suggested that impact strength is depend on the presence of notch, not the depth. Therefore the impact strength of denture base resins should be determined by Charpy impact method without notch.

Table: Charpy impact strength (kJ/m²)