Mechanical Properties of Hydrocolloid and Rubber Impression Materials
Hampson' has studied the elasticity of alginates under compression and has observed that materials with smaller amounts of permanent deformation were more suitable for use in the presence of undercut areas. Schoonover and Dickson2 report the breaking stress, percent strain, and percent set of alginates. Cresson3 suggested the revision of the testing procedure for compressive strengths of dental elastic impression materials that would apply the load at the rate of 18,000 Gm. per minute. Skinner et al.4 and Phillips5 point out the importance of the rate of deformation on the technic of handling alginate hydrocolloids. Warner" also shows that the breaking strength and percent deformation of alginates are functions of the loading rate where the higher the rate, the greater is the strength qualities. Anderson7 shows the recovery from deformation of alginate to be a direct function of time from the start of mixing. Donnison and Docking8 have measured the permanent deformation, stiffness, and toughness of alginates, the latter being determined as a combination of compressive strength and the strain at rupture. The ADA Specification 189 requires the compressive strength to be determined at a rate of 10,000 -+ 2 000 Gm. per minute. Based on an average strength of 7,000 Gm./cm.2 and a specimen '2 inch in diameter, the time required for rupture would be approximately half a minute. Recently, Braden'0 has determined the energy of tearing of one each of mercaptan, silicone, and alginate impression materials as a function of the rate of tearing and the temperature of testing. As the rate of tearing increased and the temperature decreased,
Physical properties and manipulation of reversible and irreversible hydrocolloid Physical
The organisms did penetrate the margins of some of the fillings which had under­ gone thermal changes, especially if the exposure o f the test organisms had been for periods o f 30 days or more. Hardly any of the thermally treated amalgam fillings permitted penetration o f the or­ ganisms (one out of 2 0 ), but many of the acrylic fillings permitted penetration (17 out o f 80 ). A comparison of acrylic fill­ ings inserted by the brush-on technic with those inserted by the pressure technic showed that the brush-on fillings per­ mitted fewer o f the B. globigii organisms to penetrate. With respect to S. marcescens, there was no difference between brush-on fillings and pressure fillings. The difference may be explained by the differ­ ent sizes o f the two microorganisms. 2035 Spruce Street
SOME FACTORS WHICH INFLUENCE THE SURFACE OF STONE DIES POURED IN ALGINATE IMPRESSIONS
Abstract Critical examination of stone surfaces which have been poured into alginate impressions reveals: 1. Fixing solutions must be used where they are specified by the manufacturer. 2. Plastic bowls and spatulas for mixing alginate do not produce superior stone surfaces on casts. 3. With the particular alginates tested, placing the poured cast in tap water during the set of the stone produced an inferior surface. 4. The stone cast must not be separated before one hour. 5. There is a difference in the texture of the mix attained from various commercial brands of alginate.