The present invention relates to the dental field. More particularly, the invention relates to amalgamable alloys for obtaining low-mercury dental amalgams.

BACKGROUND OF THE INVENTION As known, commercial dental materials for filling of cavities are based on an amalgam obtained by trituration of a silver-based amalgamable alloy powder and mercury. The usual alloying element used in the amalgamable alloy is tin, generally being in the range of between 26% to 28% by weight, the main phase constituent being Ag ₃ Sn(σ) . The amalgamation reaction which occurs,may be represented by the following chemical equation:

(1) Ag ₃ Sn(δ) _s + Hgi Ag ₂ Hg ₃ (Sι)s +

Sn? -sHg(δ2 ) ( s ) + Ag ₃ Sn(s> (the unconsumed portion) Generally, the amalgam obtained contained about 60%(volume percent) of δi matrix; 30% of the unreacted o and 10% of the δ ₂ -ρhase. A main disadvantage of this dental amalgam, is the mechanical weakness of the δ ₂ -phase and its high susceptibility to corrosion.

An elegant solution was suggested in the U.S. Patent No. 3,305,356 which eliminates the formation of said weak

phase by producing a new amalgamable alloy known as "Dispersalloy" . This alloy was produced by utilizing a controlled amount of a non-amalgamable alloy powder which was admixed to a conventional amalgamable alloy powder, thus producing a dental amalgam. In this manner, the amalgam obtained consisted of non-amalgamable particles embedded in a continuous matrix (δi) formed by the conventional amalgamable alloy. In order to avoid the formation of the 62- phase, a non-amal amable constituent consisting of Ag-Cu eutectic powder was added, which combines the tin released during the amalgamation. As known, tin combines with copper producing the CueSns phase, located mainly arround the particles of the silver- copper alloy. Therefore, Ag-Cu alloys should be consider¬ ed as substantially non-amalgamable alloys since the silver and copper rich phases present at the surfaces of Ag-Cu particles, dissolve in Hg during the initial stages of the amalgamation. However, when all the surfaces of Ag-Cu particles are covered by the CueSns phase, said dissolution stops. In a later U.S. Patent No. 3,980,472 it was suggested an additional alloying of Ag-Cu eutect by a controlled quan¬ tity of tin in order to decrease its oxidation potential. Ag-Cu-Sn alloys were used as non-amalgamable addition

admixed to conventional lathe cut powder. The known dental amalgams contain between 41% to 54% Hg, which is considered quite a high content. Although, no harmful health effects related to filling of dental cavities were encountered, there is a trend to decrease to a minimum, the amount of mercury used in amalgams. In a very recent paper (Nicholson et al , JADA, Vol. 125, April 1994, pages 392-399) dealing with the environmental factor regarding the use of dental amalgam, there are mentioned restrict- ions which exist in some European countries, due to the fact that a significant portion of the mercury from the amalgam scrap reaches the soil.

The amalgamation reaction in dental amalgam, is quite similar to a liquid sintering process which requires a certain volume ratio between the solid and liquid phases. Therefore, in order to decrease the mercury content in dental amalgams it is required to use liquid alloys which contain mercury instead of pure mercury. Indium was suggested for alloying of mercury, due to the fact that a significant amount of 54% can be dissolved in mercury at room temperature. Also, indium is not toxic and is capable to form intermetallic compounds with the main amalgamable alloy components, such as silver and tin.

According to the U.S. Patent No. 4,039,329 indium powder is admixed to an amalgamable alloy powder in order to decrease the mercury content in the respective amalgam. As mentioned therein, the indium powder reacts with mercury during trituration, thus forming a liquid Hg-In alloy before a significant amalgamation of the amalgama¬ ble alloy powder occurs. In this manner, the initial ratio between the amalgamable powder and mercury may be increased, thus enabling to decrease the mercury content in the respective amalgam to about 33% by weight.

In a more recent paper by (Okabe et al . J.Dent.Res. 73, 1711-16, 1994), it is mentioned that Hg-In liquid alloys, reduce significantly the mercury vaporization from freshly prepared amalgams. However, as mentioned therein, the presence of large amounts of indium,has a significant drawback to the respective amalgam, since it decreases the setting rate of the amalgam. As a result, the early strength of the amalgam, as measured by the one hour compressive strength, is also affected. This is a serious drawback of these amalgams because of the low resistance to the occlusal stresses and due to the fact that curving and removing the matrix band can not be performed during the available span.

In our previous Israeli Patent Application No. 114,248, (not yet published) there were described indium contain¬ ing liquid amalgamating alloys useful for trituration of an amalgamable alloy powder. As mentioned therein, the dental amalgam produced does not affect the initial setting rate and its one hour compressive strength, by using a ternary or a quaternary system. According to said invention, in addition to the mercury-indium liquid alloy, it is incorporated at least one noble metal selected from gold, silver, platinum and palladium or any mixture thereof.

In another recent work of H.Hero et al.(J.Dental Res. 73, 1994, IDAR Abstracts, abstract No. 29, p.105) there are described compositions of dental amalgams prepared with mercury-indium liquid alloys. It is mentioned that when the indium content in the liquid alloy approaches appro¬ ximately 20% by weight, the amalgam prepared does not meet the ISO 1559:1986 (Alloys for Dental Amalgam requirements) since its 24 hours compressive strength, drops below 300 MPa.

The Inventors of the present invention, also carried out a series of experiments with amalgams prepared with a conventional lathe cut powder and Hg-In liquid alloys. When the indium content in the Hg-In liquid alloys

increased up to 20%, an increase in the quantity of the weak 62 phase was found in the resultant amalgam struc¬ ture. However, when the indium content in the liquid alloy reached about 50% by weight, a change in the amalgamation reaction occurred which can be expressed as follows:

(2) Ag ₃ Sn(δ)s + In ₂ Hg(i)

In ₄ Ag9(s) + Sn7-sHg( _S ) + [Hgln] ( I )

It was found that the In Ag9 phase contained about 10% (atomic percent) of mercury and no unconsumed Ag ₃ Sn-phase particles were found. The 24 hour compressive strength of the amalgam obtained with conventional lathe cut powder and Hg-In liquid alloy containing 47.4% (by weight) of In was only 130 MPa, instead of the required 300 MPa. It may be assumed that the very low compressive strength is a result of the presence of a very high content of the 2 phase together with some amount of unconsumed liquid alloy trapped in the amalgam structure.

It is an object of the present invention to provide novel amalgamable alloys compositions to be used for amalgama¬ tion by liquid mercury-indium and mercury-indium metal alloys with a high indium content.

It is another object of the present invention to provide novel amalgamable alloys compositions to be used for preparation of amalgams with liquid alloys containing high content of indium.

BRIEF DESCRIPTION OF THE INVENTION

The invention relates to amalgamable alloys for low mercury dental amalgams compositions, wherein powders comprising silver and copper the amount thereof being in the range of between 5% to 70% by weight, are used as amalgamable alloys for obtaining dental amalgams with mercury-indium liquid alloys. These alloys containing up to 82% by weight indium, optionally in the presence of a noble metal selected from platinum, palladium, silver and gold, or a mixture thereof.

DETAILED DESCRIPTION OF THE INVENTION.

According to the present invention it was found that powders of silver-copper alloys are capable to react with liquid alloys comprising mercury and indium and produce a continuous matrix. The amalgams produced, comprise the unreacted particles of silver-copper embedded in a conti-

nuous matrix of silver-indium and copper-indium inter- metallic compounds with a part of the disolved mercury. The chemical reaction involved nay be presented as follows : (3) α (Cu)s + δ (Ag) _s + [Hgln]ι »►

[AgInHg]s + [CuInHg]s + α (Cu)s + α (Ag) _s The amounts of copper in the amalgamable powders of Ag-Cu are in the range of between 5% to 70% by weight, the most preferred range being between 45% to 55% by weight. Thus, using a powder of 50%:50% Cu:Ag, with an average particle size of 25 μm, the minimum liquid to solid ratio, which provides a workable amalgam when amalgamating with a liquid alloy consisting of 48Hg:52In, was found to be between 0.7 to 1.0. However, in this case, when the condensation is carried out under the standard pressure of 14 MPa (according to ISO 1559:1986), a significant amount of liquid alloy remains unconsumed and found to be trapped in the resultant amalgam structure. As a conse¬ quence, only a minimum required compressive strength could be obtained. On the other hand, when the amount of liquid alloy was increased, the 24 hours compressive strengh of the resultant amalgam drops (as appears in Example 1). By increasing the condensation pressure to 50MPa, the excess of liquid alloy was removed, fact which

increased the amalgam compressive strength (as appears in Example 2) .

Since Ag-Cu powder showing the best strength (when amal¬ gamating by 48Hg-52In liquid alloys) comprises approx. 50% copper, its oxidation potential is quite high. This fact affects the shelf life of the suggested alloys. In order to decrease the oxidation potential of Ag-Cu powders with high copper contents, a controlled quantity of tin and indium in the range from 0.1% up to about 5% tin and from 0.1% up to 10% indium may be added. Quanti¬ ties of tin in excess of about 5% are undesirable due to their negative effect on the resultant amalgam strength. In addition to that, an amount of between 0.1% up to 5% zinc may be added as a deoxidizer. In order to increase the amalgamable alloy reaction potential in respect to mercury containing liquid alloys, a ternary or a higher system may be used by incorporating to the Cu-Ag based powder a noble metal selected from platinum, palladium, and gold or a mixture thereof. As known, the noble metals react readily at room temperature with mercury, producing intermetallic compounds. Accord¬ ing to the present invention an amount of between 0.1% to 10% by weight of a noble metal may be added to a binary Ag-Cu amalgamable alloys, the preferred amount being

between 3% to 6% by weight. Thus, when an amalgamable powder of a ternary system: 50%Cu - 45% Ag and 5% Pd, with an average particles size of 25 μm was amalgamated with a liquid alloy consisting of 48% Hg and 52% In, workable amalgams were obtained in a wide range liquid to solid of between 0.7 : 1 to 1:1. Also, when the amount of the liquid phase in the above range was increased, the compressive strength of the resultant amalgams were found to increase substan ially,as can be noticed in Example 3. This effect is contrary to that obtained with a binary system of Ag-Cu alloy as shown above.

When adding the above mentioned noble metals to Ag-Cu- based powders, a high strength amalgam may be obtained at a lower copper content. In this case, the preferred copper content will be between 10% to 30% by weight. Moreover, high strength may be obtained also under a very low con¬ densation pressure (approx. only 10% of the standard value) .For Ag-Cu-Pd powders with decreased copper content (as presented in Example 4) better results were obtained for one hour compressive strength <Tihour of the discussed amalgams. The importance of this characteristic for a dentist was already mentioned above.

In order to increase the initial hardening speed, a pre- amalga ation of the amalgamable alloy may be performed by

incorporating an amount of between 0.1% to 5% of mercury. In order to improve the workability of the amalgams as produced in the plastic slurry, as resulted after tritu¬ ration of spherical particles in the amalgamable powder, it is suggested to add an amount of between 10% to 50% by volume of non-regular shaped particles to the spherical powder.

The invention will be hereafter illustrated by the fol¬ lowing Examples,being understood that these are presented only for a better understanding of the invention, without limiting its scope. A person skilled in the art after reading the present specification will be in a position to insert slight modifications, without being outside the present invention as covered by the appended Claims. The compressive strength measurements, as given in the Examples, were obtained on standard cylindrical samples according to the ISO specification 1559:1986.

EXAMPLE 1.

Two experiments were carried out which illustrate the influence of the ratio between the liquid alloy to powder on the compressive strength of two amalgams prepared with a binary 50%Ag-50%Cu powder and 48%Hg-52%In liquid alloy. In the two experiments (1,2) the average size of the

powder was 25μm, trituration time 7 seconds and the condensation pressure was 14 MPa.

The results obtained are given in the following Table 1.

TABLE 1. Liquid to powder Compressive strength (in MPa) No. weight ratio after lhr.after 24 hr)

1 0.7 : 1 60 309

2 0.8 : 1 60 255

EXAMPLE 2.

Two experiments were carried out in order to illustrate the influence of condensation pressure on the compressive strength of the amalgams prepared in experiment 1 above. It was found that in experiment (2),when the condensation pressure was 50 MPa - instead of 14 MPa as in experiment 1 - the compressive strength was 373 MPa instead of 309 MPa.

EXAMPLE 3.

Two experiments were carried out which illustrate the influence of the ratio between the liquid alloy to the powder, on the compressive strength of the amalgams pre-

pared with the ternary powder 45%Ag-50%Cu-5%Pd powder and 48%Hg-52%In liquid alloy.

The average powder size of 25μm and condensation pressure of 14 MPa was the same in the two experiments 3 and 4. The results obtained are given in the following Table 2.

TABLE 2: Compression strength as a function of the ratio between the liquid alloy to powder.

Exper. Liquid to powder Trituration Compression strength No. weight ratio time (sec) in MPa, after 1 hour 24 hours

3 0.7:1 5 80 312

4 1 :1 7 100 360

EXAMPLE 4.

Two experiments were carried out to illustrate the influence of the condensation pressure on the compressive strength of the amalgams prepared with a ternary alloy consisting of 77% Ag, 18% Cu, 5% Pd powder and 48% Hg-52% In liquid alloy.

The average powder size of 25 μm, liquid to solid ratio being 1 to 1 and a trituration time of 6 seconds was the

same in the two Examples 5 and 6. The results obtained are given in the following Table 3.

TABLE 3: Compressive strength as a function of the condensation pressure.

Experiment Condensation Compressive strength (MPa) No. pressure after 1 hr after 24 hr

2

14 360 513 1.4 242 415

All of the amalgams presented in the above Examples, comprise from 19.8% to 25% of mercury instead of 41% to 54% in the commercial amalgams.

- 1.