NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

311.05-11-3110561 April 28, 1971 10 580

Progress Report

on A NEW SERIES OF X-RAY-OPAQUE

REINFORCING FILLERS FOR COMPOSITE MATERIALS

by

R. L. Bowen* and G. W. Cleek**

* Associate Director, American Dental Association Research Unit at the National Bureau

of Standards, Washington, D. C. 20234.

** Research Chemist, Institute for Materials

Research, National Bureau of Standards,

Washington, D. C. 20234.

This investigation was supported in part by Research Grant

D E 02494—04 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 American Dental Association; the

Dental Research Division of the United States Army 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.

IMPORTANT NOTICE NATIONAL BUREAU OF S

for use within the Government,

and review. For this reason, tl whole or in part, is not auth( Bureau of Standards, Washing

the Report has been speciticall

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

3 ss accounting documents intended

; subjected to additional evaluation

3 listing of this Report, either in

le Office of the Director, National by the Government agency for which

copies for its own use. <NBS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

A NEW SERIES OF X-RAY-OPAQUE REINFORCING

FILLERS FOR COMPOSITE MATERIALS

There has been a continued interest in having composite

restorative materials that are radiopaque. This property

would facilitate the detection of associated caries or

underlying decalcified dentin and would call attention to

overhanging margins ,7 voids or other defects, should they

occur in the restoration.

The presence of major proportions of inorganic reinforcing fillers in direct filling, polymeric materials , 1

now called composite materials , 2 suggested the possibility

of making such dental materials opaque to X-rays. This

can be done by the incorporation of an element of relatively

high atomic weight into the glass that constitutes part or

all of the reinforcing filler. Many of the heavier elements,

such as those containing partly-filled d orbitals (Sn2 ~ being

an exception ),

3

are not suitable because of the color they

impart to the glass or, in the case of lead, because of the

possibility of discoloration through the formation of sulfides-
2

Tensile stresses develop in composites upon hardening /

1

and water penetrates such materials . 5 Alkaline elements in

silicate glasses make them more hygroscopic^ and tend to

form a strongly alkaline water film . 7 Assuming that these

alkaline elements are washed7 or otherwise removed from

the surface of the filler before it is treated with an

appropriate organofunct ional silane coupling agent , 8 it

is reasonable to expect that diffusion from the interior

of the glass to the surface (or silane interface region)

would be more probable for smaller g monovalent ions

than for larger polyvalent ions.

Alkaline conditions at the interface generated by alkali

elements, as well as the presence of fluoride ion might

catalyze the hydrolysis (depolymerization) of the interfacial silica, polysiloxane or both . 9 ’ 10 Thus, in the

presence of moisture and internal residual tensile stresses

(and other stresses exerted on surface particles), a form
- 3 -

of stress corrosion7 might increase the rate of surface

material loss, due to normal wear and tear, and decrease

the bulk strength of the composite material . 11

The known commercially-available glasses containing

barium have refractive indexes that are above those desirable for the purposes of this application, contain

alkali elements, such as sodium, potassium, or lithium,

or have such a high barium content so as to warrant

mechanical admixture with other filler materials.'^ Consequently, it was important to determine whether clear, radiopaque glasses having the proper refractive index could be

formulated for use as the sole or major reinforcing filler,

using only polyvalent elements. The purpose of this paper

is to describe barium-containing glasses, some of which

contain no monovalent elements, developed since a previous

report 1 *" of this continuing study.
- 4 -

Mater;ia Is .and.Methods*

The formulations of the glasses are given in the tables.

The glasses were melted in an electric furnace in a platinum

crucible 6.35 cm in diameter by 7.62 cm deep. After the batch

was melted, the melt was stirred with a motor-driven platinum

10%-rhodium double-bladed propeller-type stirrer to obtain

homogeneity. The time of melting and fining depended on the

characteristics of the melts as did the maximum temperature

used. Usually about 1.5 hours was required to fill the

crucible and about the same length of time was required for

stirring the melt. The maximum temperature used was about

1650° C. When possible, a small amount of the molten glass

was poured into a metal mold and the rest was poured into

clean water to quench the glass and break it up into small

pieces so that it could be conveniently ground in a jar

mill (dry gaged)

.

* Certain commercial references are made

in this paper to specify adequately the experimental procedure. In no instance does such identification imply recom

mendation or endorsement by the National Bureau of Standards
5

The refractive indexes were determined with a microscope

by the oil-immersion method. The coefficients of thermal

expansion (between about 40 and 100° C) were measured on an

automatic recording apparatus. 13

The powder of one of the glasses (M17-1445) was passed

through an oxygen-acetylene flame, converting the irregular

shaped particles into spheres. This was done because the

use of rounded particle shapes and an intermittent sizedistribution (gap grading) has been shown 1

4

to give an

increase in the powder-to- liquid ratio, resulting in composites

with lower thermal expansion coefficients, higher moduli

of elasticity, and other desirable properties.

Results

The data given in the tables demonstrate that clear

and colorless glasses can be prepared having refractive

indexes close to those of the polymeric binders of current

2 5 and experimental composite materials with n Q about 1.54 to

1.55. The refractive index of the polymeric binders varies

somewhat depending on its chemical composition, extent of

polymerization, water content, and other factors.
- 6 - Glasses containing about 7 mole percent of barium

(Table 1)* if used as the sole reinforcing filler for a

dental composite material* would yield restorations with

about the same minimal radiopacity as did the mixture of

fillers utilized in a previous clinical study. x

5

Voids in

such restorations might be misdiagnosed as caries. 15 This*

and the need to add other ingredients such as zinc oxide

(Table 1) to obtain the desired refractive index* suggested

the use of an increased barium content to give the desired

refractive index (Tables 1 and 2) and an increased radiopacity .

The coefficients of thermal expansion of four of the

glasses were 3.8* 4.1* 3.3* and 4.9

ppm/°C between 40 and 100°C for F1537, F1541* F1542* and

K-17* respectively.

The portion of glass M17-1445 powder that was passed

through an oxygen-acetylene flame did not change perceptibly

in refractive index during the transformation into spherical

particles. However, comparison of it with F1542, having

the same batch composition (Table 1)* again13 pointed out
7

the sensitivity of refractive index to thermal history

in glasses containing fluoride.

pi. s cu s s i on

The need for a predictable and stable refractive index

in a radiopaque glass, a desire to eliminate monovalent

elements, and other considerations led to the study of the

compositions given in Table 2. The rationale leading to

the study of these particular formulations follows.

After the available information16-2 ^ bearing on the

subject was brought together, the following concept emerged.

At least in silica glasses, in the presence of approximately

stoichiometric concentrations of elements of sufficiently

low electronegat ivity

,

2 ° including barium, 20 aluminum atoms

are converted from a coordination of six (octahedral) to

four (tetrahedral), and residually, boron atoms from a

coordination of three (planar) to four (tetrahedral).

The transformation from three to four coordination

in the case of boron results in a reduced thermal ex-
_ 8 _

o O pansion coefficient. In view of the similarity of the

other "anomalous" characteristics in the transformation

from six to four coordination in the case of aluminum, 1

b

it is reasonable to expect a similar reduction in thermal

expansion coefficient. The lowest possible thermal expansion coefficient is desirable in the reinforcing filler

since this helps lower the thermal expansion of the composite

mater ial

.

1 4

Present evidence 16 ' 22 suggests that in the fourcoordinated configurations, aluminum and boron oxides

can enter into the tetrahedral silicon-oxide framework

of the glass structure. This might be expected to localize

the large, interstitial, eight-coordinated Ba^ + ions to

maintain electrical balance with the [AIO4]" and [BO4]”

segments. It is hoped that this effect will provide

hydrolytic stability to this type of glass and to its

products

.

These considerations led to the stoichiometric formulations

seen in Table 2 and Figure 1. What was left to determine, on

an empirical basis, was the best Ba0*B2 03 to Ba0-Al 2 03 ratio
9

along an isofract of the desired refractive index. A

promising region was -found between the extremes of maximal

Ba0*B2 03 , which is in a region of liquid immiscibility

,

2

4

’ 2

5

and maximal Ba0-Al 2 03 , having a high liquidus temperature212

and viscosity. For example, compositions K-19 and K-17 had

viscosities low enough for dry gaging, 27 apparently had low

liquidus temperatures, and were resistant to devitrification.

Future studies should determine the feasibility of

preparing spherical particles of this kind of glass and

obtaining optimal size distribution. 14 Its resistance,

after preliminary washing and silane treatment, to leaching

of barium constituents should be determined, in view of

the toxicity of soluble barium compounds. 28

Conclusions

X-ray-opaque glasses containing no monovalent elements

can be prepared that are clear and colorless and that have

refractive indexes suitable for use in translucent composite restorative materials. The batch composition in mole

percent for one such glass, for example, is: Si0 2 , 66;

BaO, 17; B2 03 , 11, and Al 2 03 , 6.
- 10

The authors are grateful to Lt . Col- John A. Barton, Jr.,

USAF, DC, for the determinations of the coefficients of

thermal expansion.
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References

1. Bowen, R. L.: Development of an Ahesive Restorative

Material, in Adhesive Restorative Dental Materials II , University of Virginia Workshop, Public Health Service

Publication No. 1494, 1966, pp 225-231.

2. Broutman, L. J., and Krock, R. H.: Modern Composite

Materials , Reading, Mass: Addison-Wesley Publishing

Co., 1967.

3. Martin, R. L., and Stranks, D. R.: The Colour of Inorganic Materials: The Ligand Field Theory and its

Implications, J Oil and Colour Chemists 1 Association

47:141-161, 1964.

4. Bowen, R. L.: Adhesive Bonding of Various Materials

to Hard Tooth Tissues. VI. Forces Developing in

Direct Filling Materials During Hardening, JADA

74:439-445, 1967.

5. Peterson, E. A., II; Phillips, R. W. ; and Swartz, M. L.

A Comparison of the Physical Properties of Four Restora

tive Resins, JADA 7^:1324-1336, 1966.
16

6. Kokorina, V. F.: Influence of Oxides of Alkaline-Earth

Metals on the Chemical Resistance of Glass, The Structure

of Glass y Vol 2, New York: Consultants Bureau, 1960,

pp 385-387.

7. Bascom, W. D. : Water at the Interface, Proceedings of the

25th Anniversary Conference , The Society of Plastics Industry, Inc., New York, 1970, Section 13-C, pp 1-12.

8. Plueddeman, E. P.: Adhesion Through Silane Coupling

Agents, J Adhesion 2i 184-201, 1970.

9. Moulik, S. P., and Mullick, D. K. : Catalysis in the

Polymerization of Silicic Acid, J Polymer Sci: Part A-l

4:811-820, 1966.

10. Her, R. K.: Polymerization of Silicic Acid: Catalytic

Effect of Fluoride, J Phys Chem 56 : 680-683 , 1952.

11. Bowen, R. L.: Properties of a Silica-Reinforced Polymer

for Dental Restorations, JAPA 6j5: 57-64, 1963.

12. Bowen, R. L., and Cleek, G. W. : X-Ray-Opaque Reinforcing

Fillers for Composite Materials, J Dent Res 48:79-82, 1969.
17

13. Dickson, G., and Zelenka, D.: Automatic Recording Thermal

Expansion Apparatus, abstracted, IADR Preprinted Abstracts , 40:90, 1962.

14. Bowen, R. L.: Effect of Particle Shape and Size Distribution in a Reinforced Polymer, JAP

A

64:481, 1964.

15. Chandler, H. H.; Bowen, R. L.; Paf f enbarger , G. C.; and

Mullineaux, A. L.: Clinical Investigation of a Radiopaque

Composite Restorative Material, JADA 8^:935-940, 1970.

16. Biscoe, J., and Warren, B. E.: X-Ray Diffraction Study

of Soda-Boric Oxide Glass, J Am Ceram Soc 21:287-293,

1938.

17. Demkina, L. I.: Additivity of the Properties of Silicate

Glasses in Relation to Their Structure, The Structure of

Glass , Vol 2, New York: Consultants Bureau, 1960, pp 40-45,

472 .

18. Galant, E. I.: Refractive Index and Coordination Transformation in Aluminoborosilicate Glasses, The Structure

of Glass , Vol 2, New York: Consultants Bureau, 1960,

pp 451-453.
18

19. Kozlovskaya, E. I.: Effect of Composition in the Elastic

Properties of Glass, The Structure of Glass , Vol 2, New

York: Consultants Bureau, 1960, pp 299-301.

20. Appen, A. A., and Fu-Si, Kan: Borate and Aluminobor ate

Anomalies of the Properties of Silicate Glasses, The

Structure of Glass , Vol 2, New York: Consultants Bureau,

1960, pp 445-450

21. Douglas, R. W. : Chemistry in Glass Technology, Chemistry

in Britain J5:349-353, 1969.

22. Eitel, W. : Silicate Science , Vol 1, New York: Academic

Press, 1964, p 358.

23. Pauling, L.: General Chemistry , San Francisco: W. H.

Freeman and Co., 1953, p 235.

24. Levin, E. M., and Ugrinic, G. M. : The System Barium

Oxide-Boric Oxide-Silica, J Research Natl Bur Stands

.51:37-56, 1953.

25. Levin, E. M. , and Cleek, G. W. : Shape of Liquid Immiscibility

Volume in the System Barium Oxide-Boric Oxide-Silica, J Am

Ceram Soc 41:175-179, 1958.
19

26. Thomas , R. H.: Phase Equilibrium in a Portion of the

Ternary System BaO-Al2 03 -Si02 , J Am Ceram Soc 3J3:3 5-44,

1950.

27. Hutchins, J. R., Ill, and Harrington, R. V.: Glass,

Kirk-Othmer: Encyclopedia of Chemical Technology (2nd Ed)

Vol 10, 1966, pp 566, 539.

28. Patty, F. A. (ed) : Industrial Hygiene and Toxicology , Vol II, New York: Interscience Publishers, 1962, pp 998-

1002 .
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