Investigators: Gery R. Stafford, Christian E. Johnson, David R. Kelley, Yoshi Oshida* and Moshe P. Dariel**

* Guest Scientist, Indiana University School of Dentistry
** Guest Scientist, Ben Gurion University, Beer-Sheva, Israel

Technical Description:

The occupational and environmental hazards associated with the use of mercury-containing dental alloys are a recurring source of public concern. Since dental amalgams have performed exceedingly well over more than one hundred years, the development of a direct filling material still based on the common constituents of dental amalgams, other than mercury, is the objective of this program. The approach chosen by the Electrochemical Processing Group is based on three main premises: (1) the cold-welding of oxide-free silver; (2) the in-situ formation of Ag_xSn_y intermetallics by the room temperature fast diffusion of silver and tin; and (3) the homogeneous precipitation of silver by Sn(II) in solution.

Technical Objectives:

• Determine, develop and quantitatively characterize the basic processes involved in the room temperature in-situ consolidation of suitable amalgam-free alternative restorative materials.

• Develop a chemical or electrochemical process that reproducibility generates a restorative material with consistent properties.

• Develop test methods to obtain mechanical properties for comparison of alternative restorative materials to dental mercury amalgams.

• Optimize the composition of the alternative restorative material by studying the effects of various strengthening mechanisms on both the properties and processing parameters.

Anticipated Outcome:

• The silver-based compacts have the advantage of being mercury-free and, thereby, provide an alternative in an anticipated situation where the use of mercury-containing restoratives will be curtailed.

• The development of a successful technology that meets the requirements and displays advantages with respect to the various alternatives will be transferred to industrial and marketing partners.

Accomplishments for FY1995:

• In-situ silver-tin intermetallic compound formation was achieved by immersion deposition of pure silver onto very high surface area tin particles.

• The presence of intermetallic compounds increased the hardness of the silver based restorative composite, but adversely affected the propensity to undergo consolidation and consequently yielded low transverse rupture strengths.

• An in-depth reassessment was made to shift the main thrust of the program toward optimizing unalloyed silver metal, condensed at low but realistic consolidation pressure, before addressing the issue of strengthening the restorative material.

• Acid-assisted consolidation continues to be a critical factor in the development of the metallic substitute for amalgams.

• Developed a process to precipitate silver powder (0.3µm-3.0 µm) via two solution technique as a result of Sn²⁺ to Sn⁴⁺ oxidation.

• Determined that the annealing (450 ^oC - 2 hours) of silver powders prior to consolidation resulted in doubling the transverse rupture strength of consolidated samples compared to unannealed powders.

• 75-80% of the theoretical density for silver was obtained on precipitated silver compacts using acid-assisted hand consolidation with normal dental tools.

• Current state of the art for precipitated silver compacts, prepared by acid-assisted hand consolidation with normal dental tools, have transverse rupture strengths of 110MPa- 140 MPa compared to 115 MPa for a dental amalgam.

• The range of rupture strengths indicate sensitivity to the consolidation technique which includes different personnel, consolidation pressure, and tip sizes and geometries of hand condensers.

Impacts and Technical Highlights:

• With the contention that feasibility has been demonstrated for the development of a silver-based, mercury-free restorative material, NIST has transferred the current state of the art to our CRADA partner, Dentsply/Caulk, for in-house evaluation.

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Last modified: Mon Jan 06 09:46:15 1997 Metallurgy Webmeister