Tear strength as indicator for the stability of Alginates

Introduction: Tearing or deformation of alginate impressions can result in inaccuracy of gypsum casts that forms the initial base for a prosthetic appliances. Therefore, strength and elasticity of alginate impression materials are important material characteristics determining the performance of these products. In ISO1563, “Dental Alginate Impression materials”, a compressivestrength test is described for evaluation of the strength of the impression material. It was hypothesized that a tear strength test, which is also being used in other standards for hydrocolloid based impression materials, has more clinical significance than a CompressiveStrength test. Aim: The aim was to evaluate the merit of a TearStrength Test for characterising alginate-based impression materials, by comparing results with those obtained by the existing tests for CompressiveStrength and ElasticRecovery. Materials&methods: TearStrength, CompressiveStrength and ElasticRecovery of five different alginate impression materials were measured (CA37;Cavex, Orthotrace;Cavex, Hydrogum;Zhermack, AromaFine;GC, Blueprint; Dentsply) The tests were performed on freshly manufactured products and after accelerated aging during 7 days at 60°C. The CompressiveStrength and Elastic Recovery were determined using the standard testing methods described in the ISO1563:1990. For the TearStrength a new test based upon ASTMD624 was performed. The mould (4x19x102mm with V-notch) was filled with mixed alginate impression material and 90 seconds after setting, the force at failure was measured in a tensilometer at a crosshead speed of 500mm/min. The results were statistically evaluated using 2-way ANOVA. Results: TearStrength and CompressiveStrength for CA37(0,95N/mm-1,06MPa) and Orthotrace(0,83 N/mm–1,06 MPa) were significantly higher than for Hydrogum(0,54N/mm–0,69 MPa), AromaFine(0,51N/mm– 0,57MPa) and Blueprint(0,49N/mm–0,61MPa) Significant decreases in TearStrength and CompressiveStrength were found after accelerated aging. Conclusions: The TearStrength Test is a valuable addition to existing tests of the ISO1563:1990. Correlation was found between TearStrength and CompressiveStrength, both for freshly manufactured materials and for materials after accelerated aging. No correlation was found between TearStrength and ElasticRecovery.