Tear strength of elastomeric impression materials Thomas W. Herfort, B.S.E., M.Sc.,* William W. Gerberich, Ph.D., M.S.,** Christopher W. Macosko, Ph.D., M.S.,*** and Richard J. Goodkind, D.M.D., M.S.**** University of Minnesota, Minneapolis, Minn. Impressions are not always successful due to frac- ture of the impression rubber at the margins (Fig. 1). This difficulty is especially encountered in multiple preparation impressions. Tensile strength and tear resistance of the elastic impression materials are important factors in determining whether an impres- sion is a Success or failure. The use of stronger impression rubbers would presumably lead to higher success rates, resulting in better impressions, reduced patient discomfort, and reduced injury to the gingival tissues. Rivlin and Thomas ~ developed the criteria that are currently used to study tear strength. They introduced the simple extension tear test piece (Fig. 2) which was later adapted to the investigation of dental impression materials by Webber and Ryge. 2 The simple extension tear test piece was employed by Braden 3 to evaluate the tear strengths of a silicone, a polysulfide, and an irreversible hydrocol- loid impression material. His results showed that the polysulfide rubber was twice as strong as the silicone system, which in turn was twice as strong as the irreversible hydrocolloid. He also found that increased tear rates resulted in greater tear strengths. Crosslinking kinetics for the polysulfide and sili- cone systems have been studied by Elliot and Braden?' 5 Their results, coupled with laboratory determinations, showed that these materials reach a Read before the Midwest Academy of Prosthodontics, Chicago, Ill. *Dental student, University of Pennsylvania; formerly graduate student, Department of Chemical Engineering and Materials Science. **Professor, Department of Chemical Engineering and Materials Science. ***Associate Professor, Department of Chemical Engineering and Materials Science. ****Professor and Director of Graduate Prosthodontics, School of Dentistry. maximum elastic modulus 10 to 15 minutes after ~ mixing. It was not clear whether tear strength reached a similar maximum within such a period. This investigation was proposed to evaluate tear resistance of commercial elastomeric impression materials and to introduce guidelines for their use so that maximum tear strengths will be obtained in practice. MATERIALS AND METHODS Three synthetic types of impression materials are currently marketed for use in dentistry: polysulfide, silicone, and polyether rubbers. The compositions of these products were characterized by Skinner 6 and Braden?. 5 The filler materials were determined by Herfort and associates. 7 In all cases commercial samples were mixed according to the manufacturers' recommendations. Modulus measurements during crosslinking. All base and catalyst materials were weighed and spatu- lated before testing procedures commenced. Mixing was done at 25 ~ C. A two-plate assembly was used with a Rheometrics mechanical spectrometer*; the upper spindle was driven and the lower spindle rotated freely. To simulate actual curing conditions the test was carried out in an oven set at 37 ~ C. Spatulation for 45 seconds was followed immediately by placing of the mixture in the test fixture. Data were recorded on a chart recorder from which shear modulus vs. time could be deduced. Tear testing. Commercial samples were mixed by weight according to the manufacturers' recom- mendations. A 1/3z-inch-thick copper sheet with a 3/4 • 6 inch cutout was placed between two glass sheets to serve as a mold for each specimen. A silicone spray was applied to the glass slab to facilitate release of the test specimens upon their polymerization. Samples were spatulated for 45 seconds and trans- *Rheometries, Inc., Union, N. J. THE JOURNAL OF PROSTHETIC DENTISTRY 59