NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

31 1.0 5-11 -31 10 56 1 January 20, 1 97 1 1 0 521

P rogress Report

on INFLUENCE OF ALLOY PARTICLE SIZE AND SHAPE

R. M. Waterstrat*

* Research Associate for the American Dental

Association Research Unit at the National

Bureau of Standards, Washington, D. C. 20234

This investigation was supported in part by research

grant D E— 02455 to the American Dental Association from

the National Institute of Dental Research and is part

of the dental research program conducted by the National

Bureau of Standards in cooperation with the Council on Dental Research of the American Dental Association;

the United States Medical Research and Development

Command; the Dental Sciences Division of the School of Aerospace Medicine, USAF; the National Institute of

Dental Research and the Veterans Administration.

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U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

INFLUENCE OF ALLOY PARTICLE SIZE AND SHAPE

R. M. Waterstrat

The influence of alloy particle size and shape in determining the behavior of dental amalgam was apparently not fully

recognized in the earliest work since there is little mention

of this subject in the available literature dating prior to

1918. Between 1918 and 1923, however, there were several

papers published by A. W. Gray in which the author reported

that alloy particle size has a significant effect upon the

dimensional changes which occur during the hardening of dental

(1-3) amalgam . He also showed that it is possible to control

the dimensional change by a suitable blending of particle sizes

Following Gray's paper there was a period of about

twenty years during which the subject of particle size was

virtually absent from the published literature. However, it

appears that during this period some alloy manufacturers

conducted their own proprietary researches which ultimately

led them to adopt a more effective control over the particle

size of their products. Coy and Liebig^^ showed that a finely

divided alloy had a much higher tensile strength after one day

than did samples from a coarse-cut alloy but that after five

days there was only a slight difference in tensile strength.

2

They emphasized the importance of particle size and shape in

obtaining a dense structure with a minimum mercury content

and one which would provide good adaptation to the cavity

walls. They also indicated that the alloy particles are

probably broken up during trituration. Thus the particle

size in a freshly mixed amalgam is somewhat finer than the

particle size of the alloy powder used in making it.

(5) In 1942, Jarabak v presented the results of his studies

on the effects of particle size, trituration and condensation

pressure on the dimensional changes occurring in dental amalgam

during hardening. An interesting review and discussion of

. (< particle size effects in dental amalgam was published by Smith

in 1949.

In all of these studies the particle size separations

were accomplished by passing the alloy powders through sieves

of various sizes. This procedure is not entirely satisfactory

since the particles are usually in the form of elongated flakes

or needles. Such particles may pass through the sieve in the

direction of their longest axis and one thus obtains considerable variations in particle length for a given sieve size.

Furthermore, it is possible that size fractions which are

produced by sieving will not be greatly different in respect

to their specific surface areas. For example, alloys passing

3

a coarse sieve may have significantly larger axial ratios than

those passing a finer sieve. (The axial ratio being here

defined as the length of the longest axis of a particle divided

by the length of its shortest axis) . The coarse particles

with large axial ratios may then have a greater specific

surface area than finer particles with an axial ratio closer

to unity although on the basis of sieve size, one might have

expected the opposite.

( 7 ) This problem was recognized by Crowell and Phillips

who attempted to separate the particles according to their

thicknesses and then, by making certain simplifying assumptions,

estimating the specific surface area corresponding to each

thickness. The results indicated that increasing the specific

surface area produces decreases in setting expansion, setting

time and flow.

It produces an increase in the 1-hour and the 24-hour

compressive strengths. Setting times were shown to be approximately a linear function of the reciprocal of the specific

surface areas in agreement with a Noyes-Nernst equation.

Setting expansion was directly related to surface area by a

linear relationship.

4

With the appearance on the market of the so-called

"fine-cut" alloys there was considerable discussion of the

relative merits of the "fine-cut" versus "regular-cut" alloys.

Mostellar has discussed this subject in relation to the ADA

Specif ication #1 for dental amalgam

.

The distinction between

a fine-cut, regular-cut or coarse-cut alloy was difficult to

establish on a quantitative basis, however, since there was

a considerable variation in particle shape and size for a

given type of alloy. Measurements of specific surface area

for a given alloy were subject to much uncertainty and it was

not feasible to measure the particle dimensions directly in

order to reduce these uncertainties.

(9) In a study of zinc and non-zinc alloys, Jendresen and Ryge

pointed out that although sieving separates large and small

particles it does not produce a great difference in specific

surface. By this time, therefore, it seems to have been

recognized that the specific surface area of the particles was

a parameter more directly related to the properties of the

amalgam than "particle-size" as measured by sieving. It had

become increasingly obvious that a method was needed for

separating alloy particles on the basis of their specific areas

rather than by sieve sizing .

5

The use of spherical alloy powders permits such a separation based on surface areas since the particle diameter of a

sphere is directly related to its surface area. Furthermore,

one may conveniently use the sieving method for separating the

various sphere diameters since there is no longer any problem

of elongated particles passing through the sieve in a preferred

direction

.

In 1961 Demaree and Taylor prepared a silver dental alloy

having a typical composition and then had this alloy converted into spherical powders by a recently patented atomization

process^^^ . After separating the particles by sieving and

by elutriation, they were able to show some remarkable correlations between the average particle size and such properties

(ID

as setting change, flow and compressive strength . There

was also a relationship between average particle size and mercury content. In addition, they suggested that amalgams

prepared from spherical particles would offer advantages from

a clinical standpoint, such as a higher 1-hour compressive

(11, 13)

strength and less sensitivity to manipulative variables.

Subsequent comparisons of amalgams prepared from spherical

particles with those prepared from conventional lathe-cut

6

powders showed that the spherical-particle amalgams were as

(12, 13)

strong or stronger than conventional amalgams , that

they were stronger than conventional amalgams when both are

(12, 13)

subjected to low condensation pressures , and that they

exhibit superior adaptability in conforming to a cylindrical

( 12 ) cavity . In addition, data was obtained which established

a quantitative relationship between average particle size and

tensile strength. This showed that a maximum tensile strength

is obtained at an average particle diameter of 15 to 25 microns

The carvability and polish of the spherical-particle amalgams

(13) was also shown to depend on the particle size . Clinical

studies of spherical particle amalgams yielded favorable

results and these amalgams are now used throughout the world.

In summary: quantitative relationships have been established linking the alloy particle size to tensile strength,

compressive strength, flow, mercury content and dimensional

changes during hardening. Indirectly, these parameters are also

linked quantitatively with the specific surface area of the

particles through the use of spherical particle amalgams.

There are semi-quantitative or qualitative relationships linking

alloy particle size to adaptability, carvability, plasticity,

surface smoothness, polishability , and trituration time. Such

( 12 )

7

parameters as these, however, are difficult to define rigorously

and it is even more difficult to obtain significant measurements

.

One must therefore rely heavily on clinical experience. In

most cases, however, there is a sufficient knowledge of these

variables to permit a reasonable estimate of clinical behavior.

The particle shape seems to be of some significance in

determining the response of the amalgams to variations in

packing pressure. The particle shape may also affect surface

smoothness, carvability, adaptability and plasticity. Other

properties may be affected indirectly due to a relationship

between particle shape and the specific surface area of the

particles

.

The surface condition of the alloy particles is fairly

important since surface films may be so thin as to be undetectable even by means of electron-diffraction techniques. They

may nevertheless produce a significant change in the rate of

hardening or the ultimate strength of the amalgam. The alloy

particles may be given certain chemical treatments to remove

surface films and further research may be needed in this area.

I would conclude, however, that at the present time our

knowledge of the effects of particle size and shape upon the

properties of dental amalgam is satisfactory. Some research

8

may be desirable in this area but it seems probable that this

type of research can be best conducted by the manufacturers of

these alloys who have access to proprietary information and

who would be prompted by commercial motivations to provide

the properties which are desired by the dentist. It seems

unlikely that any major improvements will result from further

study of the effects of particle size and shape. Future research

might be more profitably directed toward understanding the basic

reasons for the brittle fracture of the amalgams, their low

tensile strengths as compared with their fairly high compressive strengths, and more details about the exact nature of the

hardening reaction.

9

References

1. Gray, A. W. Metallographic Phenomena Observed in Amalgams,

Trans AIME Suppl. to Bull. 144 Dec. (1918), also, J.

Natl. Dent. Assoc. VI 513-531 (1919).

2. Gray, A. W. Contraction and Expansion of Amalgams with Time,

Phys. Rev. 18., 108-113 (1921).

3. Gray, A. W. Volume Changes in Amalgams,

J. Inst. Metals 2_9 139-189 (1923).

4. Coy, H. D. and Liebig, E. 0. Dental Amalgam,

Items of Int. Den. _60 135-147 (1938).

5. Jarabak, J. R. The Effect of Particle Size on Dimensional

Change in Dental Amalgams, J.A.D.A. 2_9 593-605 (1942).

6. Smith, E. A. Particle Size of Dental Amalgam Alloys,

British Dental Journal 8j5 34-36 (1949).

7. Crowell, W. S. and Phillips, R. W. Physical Properties of

Amalgam as Influenced by Variation in Surface Area of

the Alloy Particles, J. D. Res., 30. 845-853 (1951).

8. Mosteller, J. H. An Evaluation of the A.D.A. Specification

For Amalgam Alloy in Relation to Particle Size, Annals

of Dentistry XII 19-24 (1953)

.

109.

Jendresen, M. D. and Ryge, G. Effects of Particle Thickness of Zinc and Non-Zinc Alloys, Dental Progress

_1 29/25, 33/29 (1960) .

10. Probst, R. L. , Karp, P. I., Sayre, C. H. and Beebe, A. H. , Jr.

assignors to Federal-Mogul-Bower Bearings Inc. Detroit,

Michigan, Atomizing Nozzle and Pouring Cap Assembly

for the Manufacturer of Metal Powders. U. S. Patent

No. 2,968,062 Jan. 17, 1961.

11. Demaree, N. C. and Taylor, D. F. Properties of Dental

Amalgams Made From Spherical Alloy Particles.

J. D. Res., 41 890-906 (1962).

12. Eden, G. T. and Waterstrat, R. M. Effects of Packing

Pressures on the Properties of Spherical Alloy Amalgams,

J.A.D.A. 74 1024-1029 (1967).

13. Koran, A. and Asgar, K. A Comparison of Dental Amalgams

Made From a Spherical Alloy and From a Comminuted Alloy.

J.A.D.A. 75 912-917 (1967)

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