Fracture Toughness and Total Fracture Work of Dental Thermoplastic Resins for Flexible Denture Base

Objectives: Although thermoplastic resins are widely used to fabricate removable denture with/without metal framework, there has been little information on the mechanical properties of those materials. The purpose of this study was to investigate the fracture toughness (FT, maximum stress intensity factor) and total fracture work (TFW) of several kinds of thermoplastic resins for flexible denture.
Methods: A total of eleven flexible denture materials are used in the experiment; six polyamides Flexident Regular (FR), Flexstar-V (FV), Valplast (VP), Rezen NF (RN), Luciton flexible (LF), Smiletone (ST); three polyacrylics Flexident AC (FA), Snow Resin (SR), Acrysthot (AS); one polyester Estheshot Brght (ES) and one polyprophylen Unigum C1 (UG). Six specimens from each group were fabricated by injection molding methods under the respective manufacturer’s recommendation. Specimens divested were ground parallel and were finished by wet grinding using SiC paper (P1200) to the final dimensions of 4.0 x 8.8 x 40 mm. After preparing pre-crack and sharp notch, specimen strips were stored in distilled water at 37°C for 48 h before test. Fracture toughness and TWF of specimens were measured by using a universal testing machine (Instron and bluehill2) with a 32-mm span according to ISO 20795-1 at a loading speed of 10 mm/min. Data were analyzed ANOVA and post hoc test at P<0.05.
Results: Mean fracture toughness values (MPam^0.5) were ranged 2.90~4.04 for polyamides, 2.25~2.80 for polyacrylics, and 3.18 for one polyester and 2.40 for one polypropylene. Mean TFW values (J/m²) were ranged 4057~8445 for polyamides, 972~1315 for polyacrylics, and 3646 for one polyester and 2664 for one polypropylene. FT value of polyamide materials, except for FV, was significantly higher than polyacrylics or polyprophylene (P<0.05). Thermoplastic resin materials investigated satisfied the minimum requirement (fracture toughness 1.9 MPam^0.5 and total fracture work 900 J/m²) indicated by ISO 20795-1 for impact resistant denture base material. Mean FT value of specimen groups showed a poor correlation with their TFW and flexural strength values. TFW value of polyacrylics was significantly lower than other thermoplastic resins.
Conclusions: Both fracture toughness properties of dental thermoplastic resins are vary depending on the type of material.