Axial Notch Tear Strength of Alginate Impression Materials

Objective: With their relatively high water content and associated hydrophilicity, alginate impression materials are susceptible to tearing when removed(�snapped�) from the mouth. Marginal tearing of an impression material can reduce the accuracy of the final cast.� Boghosian and Lautenschlager(IADRAbstract2003) have reported on the tear strengths of low-viscosity elastomeric impression materials using notched specimens meant to represent the average marginal thickness of clinical impressions. The objective of this study was to use an axial-notched mold to compare the tear strengths of various alginate impression materials.�

Methods: A v-notched tear strength mold with the following dimensions was used in this study 101.6mm longX19.05mm wide with v- notched region approximately 0.230mm-thick(SeeFig.1a-1b). Hand-spatulated specimens were placed in the mold (conditioned to 35�C) exactly one minute after start of mix.� The specimens were then stored in a water bath at (35�1�C) for the manufacturer's-recommended time to leave in the mouth(minimum time to remove from mouth).� Within 90sec of removal from the water bath (specimens were transported in a damp paper towel), the top and bottom 1-inch of the specimen was gripped in the Instron5582 equipped with pneumatic grips (17-20psi) and 240-grit abrasive paper.� A tensile force was applied at a cross-head speed of 500mm/min until failure.� A razor was used to slice three sections 1-mm wide from the notched region of the torn specimen.� The thickness of each section was measured using a Nikon profile projector.� The average thickness (h_a) of the notch cross-section was used to calculate tear strength, Ts(MPa) using the following equation:

Ts=F/(h_a x w),T_s=tear strength(N/mm),F=maximum force(Newtons),h_a=average thickness(mm) w=specimen width(mm).

Data was analyzed using One-WayANOVA(p<0.05).�

Results: See Table 1.

Conclusion: A test method using an axial-notched mold was successfully employed to determine tear strength values of various alginate impression materials and report statistically significant differences.