Amalgam - Dental amalgam
Posted by John Doe at Dental Assistant on February 4, 2012.
Categories: Dental Materials
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PROPERTIES OF AMALGAM
Important properties for dental amalgam include dimensional changes, compressive strength, creep, and corrosion resistance. These properties may be explained in part by the composition, mi-crostructure, and manipulation of the amalgam.
ANSI/ADA SPECIFICATION NO. 1 FOR AMALGAM ALLOY
ANSI/ADA Specification No. 1 for amalgam alloy contains requirements that help significantly control the qualities of dental amalgam. The specification lists three physical properties as a measure of amalgam quality: creep, compressive strength, and dimensional change. When a cylindrical specimen is 7 days old, a 36-MPa stress is applied in a 37° C environment. Creep is measured between 1 and 4 hours of stressing. The maximum allowable creep is 3%. The minimum allowable compressive strength 1 hour after setting, when a cylindrical specimen is compressed at a rate of 0.25 mm/minute, is 80 MPa. The dimensional change between 5 minutes and 24 hours must fall within the range of ±20 um/cm.
PHYSICAL AND MECHANICAL PROPERTIES
Compressive Strength Resistance to compression forces is the most favorable strength characteristic of amalgam. Because amalgam is strongest in compression and much weaker in tension and shear, the prepared cavity design should maximize the compression forces in service and minimize tension or shear forces. The early-compressive strengths (after 1 hour of setting) for several low- and high-copper alloys are listed in Table 11-2. The percent mercury used in preparing the samples is also listed; the lathe-cut alloy requires the greatest amount of mercury, and the unicompositional alloy the least. Notice that amalgams are viscoelastic and the compressive strength is a function of the rate of loading. In general, the higher the rate of loading, the higher the compressive strength, although some studies have shown that compressive strength may decrease at very high strain rates. As a result, when comparing the compressive strength of amalgam samples, it is imperative that they be tested at the same rate of loading.
When subjected to a rapid application of stress either in tension or in compression, a dental amalgam does not exhibit significant deformation or elongation and, as a result, functions as a brittle material. Therefore a sudden application of excessive forces to amalgam tends to fracture the amalgam restoration.
The high-copper unicompositional materials have the highest early-compressive strengths of more than 250 MPa at 1 hour. The compressive strength at 1 hour was lowest for lathe-cut alloy (45 MPa), followed by one of the low-copper spherical alloys (88 MPa), and then two low-copper spherical alloys and the high-copper admixed alloy (118 to 141 MPa). These data indicate that only some of the older lathe-cut alloys would not meet the requirement for compressive strength at 1 hour of ANSI/ADA Specification No. 1. High values for early-compressive strength are an advantage for an amalgam, because they reduce the possibility of fracture by prematurely high contact stresses from the patient before the final strength is reached. The compressive strengths at 7 days and the final strengths are again highest for the high-copper unicompositional alloys, with only modest differences in the other alloys.
Tensile Strength The tensile strengths of various amalgams after 15 minutes and 7 days are listed in Table 11-3. The tensile strengths at 7 days for both non-y2 and y2-containing alloys are about the same. The tensile strengths are only a fraction of their compressive strengths; therefore cavity designs should be constructed to reduce tensile stresses resulting from biting forces. The tensile strengths at 15 minutes for the high-copper unicompositional alloys are 75% to 175% higher than for the other alloys. However, no correlation exists between the tensile strengths at 15 minutes and 7 days. The high early tensile strengths of the high-copper unicompositional alloys are important, because they resist fracture by premature biting stresses better than other amalgams.
| Product | Mercury in Mix % | 1-hr Compressive Strength (MPa) (0.5 1dmin) | 7-Day Compressive Strensth (MPa) | Creep (%) | |
|---|---|---|---|---|---|
| 0.2 mm/min | 0.05 mm/min | ||||
| LOW-COPPER ALLOYS | |||||
| Fine-cut | |||||
| Caulk 20th Century Micro Cut | 53.7 | 45 | 302 |
227 |
6.3 |
| Spherical | |||||
| Caulk Spherical | 46.2 | 141 |
366 |
289 | 1.5 |
| Kerr Spheraloy | 48.5 | 88 | 380 | 299 | 1.3 |
| Shofu Spherical | 48.0 |
132 |
364 | 305 | 0.50 |
| HIGH-COPPER ALLOYS | |||||
| Admixed | |||||
| Dispersalloy | 50.0 | 118 | 387 | 340 | 0.45 |
| Unicornpositional | |||||
| Sybraloy | 46.0 | 252 |
455 |
452 |
0.05 |
| Tytin | 43.0 | 292 | 516 | 443 | 0.09 |
| Product | Tensile Strength at 0.5 mm/min (MPa) | Dimensional Change (um/cm) | |
|---|---|---|---|
| 15 min | 7 days | ||
| LOW-COPPER ALLOYS | |||
| Fine-cut | |||
| Caulk 20th Century Micro Cut |
3.2 |
51 |
-19.7 |
| Spherical | |||
|
Caulk Spherical |
4.7 |
55 |
-10.6 |
|
Kerr Spheraloy |
3.2 |
55 |
-14.8 |
|
Shofu Spherical |
4.6 |
58 |
-9.6 |
| HIGH-COPPER ALLOYS | |||
| Admixed | |||
|
Dispersalloy |
3.0 |
43 |
-1.9 |
|
Unicompositional |
|||
|
Sybraloy |
8.5 |
49 |
-8.8 |
|
Tytin |
8.1 |
56 |
-8.1 |
Transverse Strength These values are sometimes referred to as the modulus of rupture. Because amalgams are brittle materials, they can withstand little deformation during transverse strength testing. The main factors related to the high values of deformation are (1) the slow rates of load application, (2) high creep of the specific amalgam, and (3) higher temperature of testing. Thus, high copper amalgams with low creep should be supported by bases with high moduli to minimize deformation and transverse failure.
Strength of Various Phases The relative strengths of the different amalgam phases are important. By studying the initiation and propagation of a crack in a set amalgam, the relative strength of the different phases can be observed. Fig. 11-5 shows the propagation of a crack in a dental amalgam specimen. It is possible to view the crack initiation and propagation of an amalgam specimen under a conventional metallo-graphical microscope with a strain viewer. The propagation of the crack can be halted and the specimen etched to identify the various phases. Results of such studies have led to the following ranking, from strongest to weakest, of the different phases of a set low-copper amalgam: Ag3Sn (y), the silver-mercury phase (y^, the tin-mercury phase (y2), and the voids.
Silver-mercury and tin-mercury act as a matrix to hold the unreacted amalgam alloy together. When relatively smaller amounts of silver-mercury and tin-mercury phases form, up to a certain minimum required for bonding the unreacted particles, a set amalgam is stronger. When a higher percentage of mercury is left in the final mass, it reacts with more of the amalgam alloy, producing larger amounts of silver-mercury and tin-mercury phases and leaving relatively smaller amounts of unreacted particles. The result is a weaker mass. Therefore the effect of various manipulative conditions can be explained in this manner. In high- copper amalgams, there is preferential crack propagation through the Yi phase and around copper-containing particles.
Fig. 11-5 Propagation of a crack in a dental amalgam. A, Unetched. B, After etching. (From Asgar K, Sutf~nL : J Dent Res 44:985, 1965.)
Elastic Modulus When the elastic modulus is determined at low rates of loading, such as 0.025 to 0.125 mm/min, values in the range of 11 to 20 GPa are obtained. High-copper alloys tend to be stiffer than low-copper alloys. If the rate of loading is increased so the viscoelastic property does not significantly influence the elastic modulus, values of approximately 62 GPa have been obtained.
Creep The viscoelastic properties of amalgam are also reflected by the creep or permanent deformation under static loads. Under a continued application of force in compression, an amalgam shows a continued deformation, even after the mass has completely set. Amalgam has no tendency for work hardening or for resisting deformation more effectively after the mass has been deformed, as may be experienced with the cast gold alloys.
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