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Tensile strengths of selected restorative dental materials
Tensile
strengths
dental
materials
of selected
restorative
Cameron McD. Hannah, B.Ch.D., L.D.S.,* and Dennis C. Smith, MSc., Ph.D.** The Turner Dental School, University of Manchester, of Dentistry, University of Toronto, Canada
England,
and Faculty
1 he strengths of brittle restorative dental materials have usually been determined either by compressionl, 2 or to a lesser extent by transverse or flexural methods3-” Tensile tests have been used for testing ductile materials.‘* ’ The compressive stresses of mastication may induce both shear and tensile stresses within a compound restoration. 6 The tensile properties of a brittle restorative material may be of significance in the durability of a clinical restoration because the tensile strength of a brittle material is usually much lower than the compressive strength. Measurement of tensile strength with the use of conventional dumbbell-shaped specimens of a size comparable with a dental restoration is difficult. Problems are encountered in gripping the specimen, in precise axial loading, and in measuring the load. Consequently, large specimens have been used previously,7-*0 necessitating the manipulation of large quantities of materials which are clinically unrealistic. Relatively large specimens have been used in diametral compression tests by Phillips and associatesll and by Koran and Asgar.l? In this investigation, a modification of the diametral compression testl” was used to determine the tensile strengths of several brittle dental cements and amalgam ai10~s. Short cylinders of material, 3.18 mm. in diameter, were subjected to diametral compression which produces a uniform tensile stress across the loaded diameter directly proportional to the applied load. Tensile failure occurs across this diameter. eliminating the need to grip the specimens or to load axially. In order to assess errors introduced by using larger quantities of material than those used clinically, cylinders of 4.00 and 6.00 mm. in diameter were prepared and tested in a similar manner. MATERIALS The lining and luting ratios are given in Table
cements tested, their general uses, and the powder-liquid I. Cements A, R, C: and 1) are zinc oxide and eugenol
“Lecturer, Department of Conservative Dentistry. **Professor, Department of Dental Materials Science. 314
pm& ”
23”
Table
I. Lining
Tensile
strengths
of restorative
materials
315
and luting cements tested
Cement
Zinc oxide/eugenol B.P.
I
Code letter
A
I
Manufacturing
Co.
-
Liquid/100 Gm. powder”
B
0.27 Gm.
B
0.17 ml.
Sedan01
B
Dental Manufacturing
S. S. White cavity lining
C
S. S. White Dental Mfg. Co., Ltd.
B
0.27 ml.
Kalzinol
D
Amalgamated Dental
B
0.19 ml.
Staline super E.B.A.
E
Staident Products, Ltd.
B 0.14 Gm. C(i) 0.22 Gm. C(ii) 0.16 Gm.
Opotow E.B.A.
F
Opotow Dental Mfg. Corp., N. Y.
C
0.19 Cm.
Durelon
G
E.S.P.E. Fabrik Pharmazeuticher
B C
0.50 Gm. 0.66 Cm.
Co., Ltd.
*B, Ratio for use as dressing/base; C, ratio for use as luting material. formulations. A is slow setting, B and C are accelerated, and D is a resin-reinforced accelerated cement. E and F are ortho-ethoxyben.zoic acid (E.B.A.) cements containing hydrogenated resin and fused quartz in the powder, and G is a polyacrylate cement. The powder-liquid ratios used for cements A, B, C, and D were determined by means of the standard consistency test described in American Dental Association Specification No. 8. I4 This test was also used for cement E(ii) as a luting agent after it had been noted that the manufacturer’s recommended ratio E(i) gave a consistency unsuitable for cementation. The ratios recommended by the respective manufacturers were used for cement E as a base and for cements F and G. Ten currently available silicate cements were selected for testing (Table II). The powder of cement H contained glass fibers incorporated by the manufacturer. Cement I was the same powder without glass fibers. These cements were selected for comparison because it has been suggested l5 that inclusion of glass fibers is unlikely to enhance the mechanical properties of a silicate cement. Four conventional and three spherical-particle dental amalgam alloys were tested with the use of the manufacturers’ recommended alloy-mercury ratios (Table III). The respective manufacturers’ amalgamators were used when mechanical trituration was necessary. Alloys A, D, E, and G were triturated by means of a mortar and pestle. METHODS The short cylindrical specimens were prepared in stainless steel ring molds 3.18 mm. long with 3.18 mm. internal and 6.25 mm. external diameters. The rings were lubricated by a solution of silicone grease in toluene which was allowed to evaporate to a grease film before the specimen material was placed. Larger cylindrical specimens of Velmix” stone, a lining cement D, and three of the silicate cements A, B, and I were prepared in a split steel mold 11.90 mm. long with a 6.00 mm. internal *Kerr Manufacturing
Company, Europe, S.P.A.
316
Hannah
arzd Smith
Table
II. Silicate
cements tested C’ode , letter
Cement S. S. White new filling S. S. White
porcelain
M.Q.
Manufacturing
0.1 ml. liquid. ~ i;,ri. fmi&f (Cl%;
Co.
A
S. S. White Dental
Mfg. Co., Ltd.
1.io
B
S. S. White Dental
Mfg. Co., Ltd.
! .io i.ti
I.63
DeTrey’s
Super Sytnex
c/
.4malgamated Ltd.
Dental
Trade Dist..
DeTrey’s
Bio Trey
D
Amalgamated Ltd.
Dental
Trade
E
Produits
F
Zahn Porzellan K.G.E., & Co.. Hambur,q
G
Dental
re-
H
&tablessement Vivadent, Liechtenstein
Schaan,
1.80. 1.60
fiber-
I
l?tablessement Vivadent, Liechtenstein
Schaan,
I .60
J
fitablessement Vivadent, Liechtenstein
Schaan,
Predet.
“P.D.” lain
translucent
Terralux
Ultra
True
silicate porce-
Universal
Shade
Vivadent Achatit inforced
fiberglass
Vivadent glass
Achatit
without
Vivadent
Silicap
Dentaires
Fillings,
Dist..
S.A., Vevey
I.iO !.,%I
Muhlbauer
Ltd., London
___-_
diameter. The larger amalgam cylindrical specimens were prepared from one of the conventional alloys B. A steel mold with 4.00 mm. internal diameter and 8.25 mm. long was used. Cement powders, amalgam powders, and mercury were weighed to an accuracy of I mg. and liquids were dispensed to the nearest 0.01 ml. to conform to the ratios given in Tables I, II, and III. The materials were manipulated in accordance with American Dental Association Specification Nos. 1. 8. and 9l’ and were allowed to harden in a humidor cabinet at 37’ C. Silicate cement J was mechanically nlixed for 20 seconds to obtain optimmn homogeneity.‘” Amalgam alloys were condensed mechanically into the molds by means of a Bergendal condenser* with a point of 2.5 mm. in diameter and a load of 2.5 Kg. In addition, specimens were prepared from two of the alloys by hand packing, using the same load and point diameter. After removal from the molds, specimens were stored in distilled water at 37’ C. for predetermined time intervals before testing. Cement specimens were tested with the use of a Hounsfield Tensometrr with a precision compression rig and a crosshead speed of I /IS” per minute. Amalgam specimens were tested by means of an Instron Universal testing machine with a crosshead loading speed of 0.1 cm. per minute. Filter paper packing pieces were used between the silicate and amalgam specimens and the loading platens. *A. B. Dentatus,
Stockholm.
Sweden
Volume 26 Number 3
Table
Tensile
III. Dental
amalgam
strengths
of restoratiue
Katalloy
Code letter
Manufacturing
A
Dr. Walter
B
Dental
S. S. White New True Dentalloy
C
S. S. White Dental
Luna Atomic
D
The G. C. Chemical Ltd.
Alloy
spherical-particle
Experimental*
Co.
u. Schmitt
Mfg.
Mfg. Co., Ltd. Mfg. Co.,
-
E F
Vivadent stein
Spheraloy
G
Kerr Mfg. Co., Mich.
by Johnson Matthey
GmbH.
Co., Ltd.
Amalcap
*Supplied
317
Alloy:Hg.
ratio
alloys tested
Alloy
.4lston Tablet
materials
Chemicals,
Corp., Schaan, Liechten-
5 : 4.5 5:5 5:s 5:4 5:4 Preproportioned 55
Ltd.
RESULTS It has been shown that stresses are not significantly affected by variations in strain rate. Williams,” using zinc phosphate cement cylinders, demonstrated that a fourfold increase in strain rate was accompanied by an increase of only 0.6 per cent in failure stress. Consequently, reliable strength comparisons may be made between large and small specimens without modification of crosshead loading speed. Tensile strength of cements. The early and ultimate tensile strengths of the lining and luting cements are given in Table IV. The ultimate strengths of conventional slow-setting zinc oxide and eugenol and the two accelerated zinc oxide and eugenol cements were all in the order of 20 Kg. per square centimeter. It appears that the accelerating agents themselves did not enhance the strength of the material. The resin incorporated in the accelerated cement D gave a 50 per cent increase in strength to 30 Kg. per square centimeter. The E.B.A. and polyacrylate cements were as strong in tension at 50, 70, and 85 Kg. p er square centimeter as zinc phosphate cements, which were usually in the range 50 to 80 Kg. per square centin1eter.l’ The strength of cement G was even greater when a base powder-liquid ratio was used. This strength of 100 Kg. per square centimeter overlapped the 7 day strength range of some silicate cements. The early strength of these cements is of great clinical significance. After 20 minutes cement B reached only 30 per cent of the 7 day strength. Cements C and D attained the 7 day strength in 15 minutes, and cement E reached its ultimate in 20 minutes. Cements F and G gained strength very rapidly during the first 15 minutes but thereafter showed a further increase in strength to the 7 day values. Cements E, F, and G were stronger at 15 and 20 minutes than a zinc phosphate cement (43 Kg. per square centimeter) at the same time interva1s.l’ A limiting compressive strength range for cements which are to be used as bases for amalgam condensation has been suggested. Ifi No minimum tensile strength ranges have been defined for either base or luting cements. However, since the E.B.A. and polyacrylate cements compare favorably with zinc phosphate cement, from tensile
318
Hannah
.J. P~osthet. September,
and Smith
Table IV. Tensile centimeter)
strength
of lining
and luting
cements
(in kilograms
Dent. 1971
per square ----
~
15 minutes
I /
Use
a*
s
A
Dressing/base
-
-.
B
Dressing/base
6.1+
C
Dressing/base
D
Dressing/base
E
Base Cement (i) Cement (ii)
48.8t
F
Cement
G
Base Cement
Cement
I I
/
n
7 days a
I I
5
n
--
18.3
5.8
9
1.8
5
18.1
4.3
8
20.4
3.7
6
21.8
3.6
8
28.4
5.6
6
29.1
2.1
10
5.0
5
53.4 47.7 52.4
7.8 10.7 6.8
7 6 7
56.5
5.2
6
71.3
9.0
10
87.7
9.9
7
101.9 85.5
4.5 16.3
9 9
*u, Mean tensile strength in kilograms
per square centimeter;
s> standard deviation
z;(G)? , n, numbers of specimens tested. n-l tMateria1 not set at 15 min. tested at 20 min. Table
V. Tensile
strength
of silicate
cements in kilograms
per square centimeter
Cement
15 min. 30 min. 45 min. 1 hr. 1% hr. 3 hr. 6 hr. 9 hr. 12 hr. 24 hr. 7 days
57.4 8.2 90.5 18.6 105.0 15.4 102.8 16.8 108.8 22.0 117.5 29.5 137.3 32.0 133.8 21.1 144.7 22.5 143.9 30.9
25 25 25 25 25 25 25 25 25 25
41.8 71.8 64.7 74.8 77.5 87.3 101.1 102.0 _ 109.5 147.4
8.5 13.7 13.2 18.1 19.9 22.1 24.5 31.6 __ 16.6 28.5
12 12 12 12 12 12 12 12 12 12
39.3 9.3 83.1 16.8 81.2 14.8 101.1 19.7 89.8 13.2 91.4 24.4 124.5 25.2 121.3 20.6 .-. ..122.5 35.9 125.8 32.9
12 12 12 12 12 12 12 12 12 12
49.2 4.8 85.7 16.0 83.1 22.2 99.2 13.2 104.7 19.7 123.8 19.9 126.7 18.7 145.1 19.4 144.9 21.5 152.5 24.8
25 25 25 2:-I 25 25 2.5 2:i 21; 25
*Symbols as in Table IV. strength considerations alone they appear well suited for use both as luting agents and as bases. Strength of silicate cements. The strengths of the silicate cements tested are given in Tables V and VI. The curve of increase in strength with time of one of these cements is shown (Fig. 1). Strength was gained rapidly during the first 30 minutes to a point between 50 and 65 per cent of the ultimate strength. After 30 minutes the rate of increase in strength was reduced, but by 6 hours the cements had reached
Volume 26 Number 3
Tensile
strengths
of restorative
materials
319
TIME (hours) 6
14
12
7 DAY
TIME (minutes)
Fig. 1. Rate of increase in tensile strength for a typical silicate cement.
Table VI. Tensile
strength
of silicate
cements in kilograms
per square centimeter
Cement E
15 1 24 7
min. hr. hr. days
F
G
a*
5
n
rr
s
n
a
s
n
43.1 65.9 107.2 114.6
8.7 13.2 23.8 16.2
12 12 12 12
51.3 46.1 78.6 82.8
8.2 12.7 12.4 14.8
12 12 12 12
27.9 62.4 73.2 85.3
5.7 11.4 11.0 16.6
12 12 12 12
H (high
ratio)
H (low
ratio)
a
15 1 24 7
min. hr. hr. days
64.2 11.3 99.6 17.2 115.3 20.0 112.8 18.9
12 69.4 12 95.8 12 109.5 12 97.8
9.0 13.0 13.3 13.6
12 12 12 12
54.1 68.9 93.6 95.7
10.8 15.1 27.3 23.6
12 49.4 8.3 12 83.1 19.7 12 125.9 29.0 12 110.3 35.4
12 12 12 12
*Symbols as in Table IV. between 70 and 100 per cent of their ultimate strength. This rapid rate of increase in strength is clinically desirable because normal mastication may be resumed soon after restorative dental treatment. Glass fibers enhanced the strength of silicate cement in the early stages of setting
320
Hannah
and Smith
Table VII. Tensile centimeter
strength
of dental
amalgam
alloys in kilograms
per square ---.I__-.
Alloy
Thiration
.4 B c
Condensation
Interval
no sec.
Mwhan.
24 hr.
15 sec.
Hand Mechan.
10 sec. (IS
sec.)
0
-?I
I
j61.7
18.0
9
7 da) 7 da)
600.4 632.2
3.5. 1 X7.”
Ill 9
Mechan. Mechan.
7 da) 7 da?
.X3.7
32.6
8
604.1
64.‘;
7
Hand Me&an.
7 da? 7 day
480.2 j46.2
100.6 73.7
8 :i
D
20 sec.
E
20 sec.
Mwhan.
7 da!
.50’,.2 ‘.
:ih.:i
Ii
F
5 sec.
Mcchan.
7 da)
636.1
66.11
!i
G
20 sec.
Mechan.
I da!
713.4
77.0
i
Table VIII. Tensile strength per square centimeter Alloy G. C. Luna Atomic c s II Kerr
/
of spherical-particle
4%
and conventional
hr.
6 hr.
alloys in kilograms
I hr.
3 hr.
7% hr.1 24 hr. I48
hq-
164.8 21.2 8
198.5 18.8 9
208.:1 l12.4 7
247.2 61.6 8
‘9:,.7 56.9 9
352.i! 40.9 8
449.2 h7.‘i 11
i46.2 7.5.7 8
123.9 30.5 6
218.9 33.9 6
242.8 18.4 4
304.0 28.” i
:+5&H 30.4 6
653.11 58.9 3
615.7 24.9 1
71:1.+ 77.0 i
191.8 18.9 6
259..i 26.3 6
305.2 22.1 6
416.8 363 6
618.9 48.6 6
620.1 60.5 i
615.0 IVI..{ h
602 ..!: 116.1; R
115.9 10.1 9
257.i 32.6 7
3’0.5 18.4 6
492.8 48.2 8
634.7 5 3 .7 6
518.6 60.2 9
583.6 7.i.:: 9
5 5 3 ,7 52.6 8
Spheraloy F s Ii
Experimental F b n
alloy
S. S. White t? s II
N.T.D.
(Table VI). The difference in strength became less pronounced with an increase irr the time interval before testing, but the glass fibers in no way impaired the ultimate tensile strength of the set cement. Strength of amalgam alloys. The ultimate strengths of the amalgam alloys tested ranged from 480 to 713 Kg. per square centimeter (Table VII). The rate of increase in strength of one fine-grain conventional alloy and three spherical-particle alloys is given in Table VIII. The experimental spherical-particle alloy gained strength fairly uniformly at a rate similar to that of the conventional alloy. The increase in
Volume 26 Number 3
Tensile
. ..I x... -m-o -
l
NEW TRUE DENTALLDY SPHERALDY LUNA ATOMIC
-a -
strengths
of restorative
t
500= ,
Y 0.
X/ ,’ kg/cm2
#’
Fig. 2
321
655
,
,*
-E T z
materials
0 I
4’
40 .
80 .
120 .
160 I
F 300-
Fig. .‘;
x 2 IF-l .+
IL
P
I
3 S TIME (hours)
7
C D
a =
c D
Fig. 2. A comparison of the rates of increase in strength of a conventional and two sphericalparticle dental amalgam alloys. Fig. 3. Relative tensile strengths of various materials for cylinder (C) and disc (D) specimens.
strength of the two commercial spherical-particle alloys, on the other hand, was not uniform. Initially these alloys gained strength very rapidly, but between 1 and 7 hours the rate of increase in strength was retarded when compared with the fairly uniform rate of increase of the conventional alloy (Fig. 2). This retardation was followed by a recovery and by 48 hours the strengths of the spherical-particle alloys differed little from that of the conventional alloy. The reduced strength of the spherical-particle alloys at an early stage in setting may present problems in their clinical use. DISCUSSION The manufacturers’ recommended ratios for silicate cements and the standard consistency test, A.D.A. Specification No. 9,14 appear to be related to the desired consistency of the large amount of material necessary to prepare relatively large specimens for compressive testing. In order to obtain clinically satisfactory mixes, higher powder-liquid ratios are necessary. For example: 0.53 Gm. of silicate cement H can be incorporated into 0.1 ml. of liquid to give a clinically satisfactory mix. This corresponds to a ratio of 2.1 Gm. to 0.4 ml. The ratio recommended by the manufacturer and determined by the standard consistency test is 1.6 Gm. to 0.4 ml. This means that 25 per cent more powder than that recommended can be incorporated into the liquid to give a clinically satisfactory mix. These ratios should be more appropriately related to the small amount of material used clinically and the specimen size for laboratory testing should be modified accordingly. The effects of specimen size on strength of material can be seen in Fig. 3 (Table
322
Hannah
and
Table
IX. Comparing
J. l’roslhet. Dent. Srprmber, 1971
Smith
cylindrical
and disc test specimens _____I-.
Material Velmix
Specimen
stone
a
1
s
n
Time
Disc Cylinder
(a)
57.6 48.4
8.7 6.6
IO 6
7 days 7 days
Disc Cylinder
(a)
29.1 28.”
2.1 3 :,
10 3
7 days 7 days
Disc Cylinder
(a)
93.6 31.9
27.3 12.3
12 5
Disc Cylinder
(a)
143.9 87.1
30.9 35.1
12 5
7 days 7 thys
Disc Cylinder
(a)
147.4 72.2
28.5 3.3
12 5
7 days 7 days
Alston Tablet Alloy (hand cond.)
Disc Cylinder
(b)
600 515,
35.1 54.9
10 5
7 days 7 days
Alston Tablet Alloy (mech. cond. )
Disc Cylinder
(b)
652 499
37.2 49.9
9 5
7 days 7 days
Kalzinol Achatit
without
fibers
S. S. White porcelain
new filling
S. S. White
M.Q.
The dimensions of the specimens tested were: Disc 3.18 mm. diameter Cylinder (a) 6.00 mm. diameter 4.00 mm. diameter Cylinder (b)
21 hr. 24 hr.
-
3.18 mm. li:ns. 11.9 mm. long. 8.25 mm. ions.
IX). The 6 mm. diameter cylinders of silicate cements tested at 24 hours and ‘i days are considerably weaker than the 3 mm. diameter cylinders of the same material. This difference in strength is possibly a result of difficulty in spatulating the large amount of material thoroughly and also of the unavoidable incorporation of an increased number of voids in preparing the large cylindrical specimens. The difference in diameter of the two sizes of specimens of amalgam was iess than that of the silicate cement specimens. The difference in strength was correspondingly less pronounced. The quantity of material necessary for the preparation of the small cylindrical specimens
is clinically
more
realistic
and is more
easily
manipulated
than
that
which
is necessary to prepare the larger specimens. For this reason the small cylinders considered more suitable for laboratory testing.
are
SUMMARY The tensile strengths of several lining and luting cements, silicate cements, and dental amalgam alloys were determined by diametral compression. Two E.&I. cements and one polyacrylate cement are stronger within 20 minutes of mixing than a zinc phosphate cement. Limiting tensile strengths for base and luting ct:rrlc~nts remain undefined. However, since these cements compare favorably with a zinc phosphate cement for tensile strength considerations, they are well suited to similar clini.. cal uses. Silicate cements reach between 70 and 100 per cent of their ultimate strength within 6 hours. The early tensile strength of these cements is enhanced h\: incorporating glass fibers into them.
Volume 26 Number 3
Tensile
strengths
of restorative
materials
323
Dental amalgam alloys are three times as strong in tension as silicate cements. The rate of increase in strength of two spherical-particle alloys is reduced between 1 and 6 hours when compared with the rate of increase in strength of a conventional alloy. CONCLUSIONS The results obtained using materials in quantities recommended to conform to specification tests differ from those obtained using clinical quantities of material. Both specimen size and bulk of material used for laboratory testing should be more closely related to clinical procedures. References 1. Skinner, E. W., and Phillips, R. W.: The Science of Dental Materials, ed. 6, Philadelphia and London, 1968, W. B. Saunders Company. 2. Peyton, F. A.: Restorative Dental Materials, ed. 3, St. Louis, 1968, The C. V. Mosby Company.
3. Hollenback, 4. 5. 6. 7. 8.
9. 10. 11.
12. 13. 14. 15. 16. 17.
18.
G. M., and Villanyi,
A. A.: The Physical Properties of Dental Amalgam,
J. S. Calif. Dent. Assoc. 12: 455-459, 1964. Mahler, D. B., and Mitchem, J. C.: Transverse Strength of Amalgam, J. Dent. Res. 43: 121-130, 1964. Forsten, L.: Physical Properties of Dental Amalgams, Academic Dissertation, Inst. of Dent., Univ. of Turku, Finland, 1969. Mahler, D. B.: An Analysis of Stresses in a Dental Amalgam Restoration, J. Dent. Res. 37: 516-526, 1958. Ward, M. L.: Modern Tests for the Strength of Amalgam, J. Amer. Dent. Assoc. 11: 487496, 1924. Taylor, N. 0.: A Specification for Dental Amalgam Alloys: A Report to the Scientific Foundation and the Research Commission of the American Dental Association, J. Amer. Dent. Assoc. 17: 112-124, 1930. Bowen, F. L., and Rodriguez, M. S.: Tensile Strength and Modulus of Elasticity of Tooth Structure and Several Restorative Materials, J. Amer. Dent. Assoc. 64: 378-387, 1962. Rodriguez, M. S., and Dickson, G.: Some Tensile Properties of Amalgam, J. Dent. Res. 41: 840-852, 1962. Phillips, R. W., Swartz, M. L., Norman, R. D., Schnell, R. J., and Niblack, B. F.: Zinc Oxide and Eugenol Cements for Permanent Cementation, J. PROSTHET. DENT. 19: 141150, 1968. Koran, A., and Asgar, K.: A Comparison of Dental Amalgams Made From a Spherical Alloy and From a Comminuted Alloy, J. Amer. Dent. Assoc. 75: 912-917, 1967. Williams, P. D., and Smith, D. C.: Determination of the Tensile Strength of Restorative Materials by the Diametral Compression Test, J. Dent. Res. 46: 1297, 1967. American Dental Association: Guide to Dental Materials, ed. 3, Chicago, 1966. McConnell, D., and Brawley, R. G.: Compressive Strength of “Reinforced” Silicate Cement, J. PROSTHET.DENT. 10: 1092-1093, 1960. Phillips, R. W., Swartz, M. L., and Chong, W. F.: Properties of Silicate Cements Mixed by Hand and Mechanical Means, J. S. Calif. Dent. ASSOC. 33: 239-242, 1965. Williams, P. D.: Studies on the Diametral Compression Test as Applied to Dental Restorative Materials and of Their Adhesion to Enamel and Dentine, M.Sc. Thesis, Victoria University of Manchester, p. 28, 1967. of Cement Bases by Chong, W. F., Swartz, M. L., and Phillips, R. W.: Displacement Amalgam Condensation, J. Amer. Dent. ASSOC.74: 97-102, 1967.
THF.TTJRNERDENTAL SCHOOL,UNIVERSITYOF MANCHESTER BRINEFORD STREET MANCHESTER Ml5 6FH, ENGLAND
strengths
dental
materials
of selected
restorative
Cameron McD. Hannah, B.Ch.D., L.D.S.,* and Dennis C. Smith, MSc., Ph.D.** The Turner Dental School, University of Manchester, of Dentistry, University of Toronto, Canada
England,
and Faculty
1 he strengths of brittle restorative dental materials have usually been determined either by compressionl, 2 or to a lesser extent by transverse or flexural methods3-” Tensile tests have been used for testing ductile materials.‘* ’ The compressive stresses of mastication may induce both shear and tensile stresses within a compound restoration. 6 The tensile properties of a brittle restorative material may be of significance in the durability of a clinical restoration because the tensile strength of a brittle material is usually much lower than the compressive strength. Measurement of tensile strength with the use of conventional dumbbell-shaped specimens of a size comparable with a dental restoration is difficult. Problems are encountered in gripping the specimen, in precise axial loading, and in measuring the load. Consequently, large specimens have been used previously,7-*0 necessitating the manipulation of large quantities of materials which are clinically unrealistic. Relatively large specimens have been used in diametral compression tests by Phillips and associatesll and by Koran and Asgar.l? In this investigation, a modification of the diametral compression testl” was used to determine the tensile strengths of several brittle dental cements and amalgam ai10~s. Short cylinders of material, 3.18 mm. in diameter, were subjected to diametral compression which produces a uniform tensile stress across the loaded diameter directly proportional to the applied load. Tensile failure occurs across this diameter. eliminating the need to grip the specimens or to load axially. In order to assess errors introduced by using larger quantities of material than those used clinically, cylinders of 4.00 and 6.00 mm. in diameter were prepared and tested in a similar manner. MATERIALS The lining and luting ratios are given in Table
cements tested, their general uses, and the powder-liquid I. Cements A, R, C: and 1) are zinc oxide and eugenol
“Lecturer, Department of Conservative Dentistry. **Professor, Department of Dental Materials Science. 314
pm& ”
23”
Table
I. Lining
Tensile
strengths
of restorative
materials
315
and luting cements tested
Cement
Zinc oxide/eugenol B.P.
I
Code letter
A
I
Manufacturing
Co.
-
Liquid/100 Gm. powder”
B
0.27 Gm.
B
0.17 ml.
Sedan01
B
Dental Manufacturing
S. S. White cavity lining
C
S. S. White Dental Mfg. Co., Ltd.
B
0.27 ml.
Kalzinol
D
Amalgamated Dental
B
0.19 ml.
Staline super E.B.A.
E
Staident Products, Ltd.
B 0.14 Gm. C(i) 0.22 Gm. C(ii) 0.16 Gm.
Opotow E.B.A.
F
Opotow Dental Mfg. Corp., N. Y.
C
0.19 Cm.
Durelon
G
E.S.P.E. Fabrik Pharmazeuticher
B C
0.50 Gm. 0.66 Cm.
Co., Ltd.
*B, Ratio for use as dressing/base; C, ratio for use as luting material. formulations. A is slow setting, B and C are accelerated, and D is a resin-reinforced accelerated cement. E and F are ortho-ethoxyben.zoic acid (E.B.A.) cements containing hydrogenated resin and fused quartz in the powder, and G is a polyacrylate cement. The powder-liquid ratios used for cements A, B, C, and D were determined by means of the standard consistency test described in American Dental Association Specification No. 8. I4 This test was also used for cement E(ii) as a luting agent after it had been noted that the manufacturer’s recommended ratio E(i) gave a consistency unsuitable for cementation. The ratios recommended by the respective manufacturers were used for cement E as a base and for cements F and G. Ten currently available silicate cements were selected for testing (Table II). The powder of cement H contained glass fibers incorporated by the manufacturer. Cement I was the same powder without glass fibers. These cements were selected for comparison because it has been suggested l5 that inclusion of glass fibers is unlikely to enhance the mechanical properties of a silicate cement. Four conventional and three spherical-particle dental amalgam alloys were tested with the use of the manufacturers’ recommended alloy-mercury ratios (Table III). The respective manufacturers’ amalgamators were used when mechanical trituration was necessary. Alloys A, D, E, and G were triturated by means of a mortar and pestle. METHODS The short cylindrical specimens were prepared in stainless steel ring molds 3.18 mm. long with 3.18 mm. internal and 6.25 mm. external diameters. The rings were lubricated by a solution of silicone grease in toluene which was allowed to evaporate to a grease film before the specimen material was placed. Larger cylindrical specimens of Velmix” stone, a lining cement D, and three of the silicate cements A, B, and I were prepared in a split steel mold 11.90 mm. long with a 6.00 mm. internal *Kerr Manufacturing
Company, Europe, S.P.A.
316
Hannah
arzd Smith
Table
II. Silicate
cements tested C’ode , letter
Cement S. S. White new filling S. S. White
porcelain
M.Q.
Manufacturing
0.1 ml. liquid. ~ i;,ri. fmi&f (Cl%;
Co.
A
S. S. White Dental
Mfg. Co., Ltd.
1.io
B
S. S. White Dental
Mfg. Co., Ltd.
! .io i.ti
I.63
DeTrey’s
Super Sytnex
c/
.4malgamated Ltd.
Dental
Trade Dist..
DeTrey’s
Bio Trey
D
Amalgamated Ltd.
Dental
Trade
E
Produits
F
Zahn Porzellan K.G.E., & Co.. Hambur,q
G
Dental
re-
H
&tablessement Vivadent, Liechtenstein
Schaan,
1.80. 1.60
fiber-
I
l?tablessement Vivadent, Liechtenstein
Schaan,
I .60
J
fitablessement Vivadent, Liechtenstein
Schaan,
Predet.
“P.D.” lain
translucent
Terralux
Ultra
True
silicate porce-
Universal
Shade
Vivadent Achatit inforced
fiberglass
Vivadent glass
Achatit
without
Vivadent
Silicap
Dentaires
Fillings,
Dist..
S.A., Vevey
I.iO !.,%I
Muhlbauer
Ltd., London
___-_
diameter. The larger amalgam cylindrical specimens were prepared from one of the conventional alloys B. A steel mold with 4.00 mm. internal diameter and 8.25 mm. long was used. Cement powders, amalgam powders, and mercury were weighed to an accuracy of I mg. and liquids were dispensed to the nearest 0.01 ml. to conform to the ratios given in Tables I, II, and III. The materials were manipulated in accordance with American Dental Association Specification Nos. 1. 8. and 9l’ and were allowed to harden in a humidor cabinet at 37’ C. Silicate cement J was mechanically nlixed for 20 seconds to obtain optimmn homogeneity.‘” Amalgam alloys were condensed mechanically into the molds by means of a Bergendal condenser* with a point of 2.5 mm. in diameter and a load of 2.5 Kg. In addition, specimens were prepared from two of the alloys by hand packing, using the same load and point diameter. After removal from the molds, specimens were stored in distilled water at 37’ C. for predetermined time intervals before testing. Cement specimens were tested with the use of a Hounsfield Tensometrr with a precision compression rig and a crosshead speed of I /IS” per minute. Amalgam specimens were tested by means of an Instron Universal testing machine with a crosshead loading speed of 0.1 cm. per minute. Filter paper packing pieces were used between the silicate and amalgam specimens and the loading platens. *A. B. Dentatus,
Stockholm.
Sweden
Volume 26 Number 3
Table
Tensile
III. Dental
amalgam
strengths
of restoratiue
Katalloy
Code letter
Manufacturing
A
Dr. Walter
B
Dental
S. S. White New True Dentalloy
C
S. S. White Dental
Luna Atomic
D
The G. C. Chemical Ltd.
Alloy
spherical-particle
Experimental*
Co.
u. Schmitt
Mfg.
Mfg. Co., Ltd. Mfg. Co.,
-
E F
Vivadent stein
Spheraloy
G
Kerr Mfg. Co., Mich.
by Johnson Matthey
GmbH.
Co., Ltd.
Amalcap
*Supplied
317
Alloy:Hg.
ratio
alloys tested
Alloy
.4lston Tablet
materials
Chemicals,
Corp., Schaan, Liechten-
5 : 4.5 5:5 5:s 5:4 5:4 Preproportioned 55
Ltd.
RESULTS It has been shown that stresses are not significantly affected by variations in strain rate. Williams,” using zinc phosphate cement cylinders, demonstrated that a fourfold increase in strain rate was accompanied by an increase of only 0.6 per cent in failure stress. Consequently, reliable strength comparisons may be made between large and small specimens without modification of crosshead loading speed. Tensile strength of cements. The early and ultimate tensile strengths of the lining and luting cements are given in Table IV. The ultimate strengths of conventional slow-setting zinc oxide and eugenol and the two accelerated zinc oxide and eugenol cements were all in the order of 20 Kg. per square centimeter. It appears that the accelerating agents themselves did not enhance the strength of the material. The resin incorporated in the accelerated cement D gave a 50 per cent increase in strength to 30 Kg. per square centimeter. The E.B.A. and polyacrylate cements were as strong in tension at 50, 70, and 85 Kg. p er square centimeter as zinc phosphate cements, which were usually in the range 50 to 80 Kg. per square centin1eter.l’ The strength of cement G was even greater when a base powder-liquid ratio was used. This strength of 100 Kg. per square centimeter overlapped the 7 day strength range of some silicate cements. The early strength of these cements is of great clinical significance. After 20 minutes cement B reached only 30 per cent of the 7 day strength. Cements C and D attained the 7 day strength in 15 minutes, and cement E reached its ultimate in 20 minutes. Cements F and G gained strength very rapidly during the first 15 minutes but thereafter showed a further increase in strength to the 7 day values. Cements E, F, and G were stronger at 15 and 20 minutes than a zinc phosphate cement (43 Kg. per square centimeter) at the same time interva1s.l’ A limiting compressive strength range for cements which are to be used as bases for amalgam condensation has been suggested. Ifi No minimum tensile strength ranges have been defined for either base or luting cements. However, since the E.B.A. and polyacrylate cements compare favorably with zinc phosphate cement, from tensile
318
Hannah
.J. P~osthet. September,
and Smith
Table IV. Tensile centimeter)
strength
of lining
and luting
cements
(in kilograms
Dent. 1971
per square ----
~
15 minutes
I /
Use
a*
s
A
Dressing/base
-
-.
B
Dressing/base
6.1+
C
Dressing/base
D
Dressing/base
E
Base Cement (i) Cement (ii)
48.8t
F
Cement
G
Base Cement
Cement
I I
/
n
7 days a
I I
5
n
--
18.3
5.8
9
1.8
5
18.1
4.3
8
20.4
3.7
6
21.8
3.6
8
28.4
5.6
6
29.1
2.1
10
5.0
5
53.4 47.7 52.4
7.8 10.7 6.8
7 6 7
56.5
5.2
6
71.3
9.0
10
87.7
9.9
7
101.9 85.5
4.5 16.3
9 9
*u, Mean tensile strength in kilograms
per square centimeter;
s> standard deviation
z;(G)? , n, numbers of specimens tested. n-l tMateria1 not set at 15 min. tested at 20 min. Table
V. Tensile
strength
of silicate
cements in kilograms
per square centimeter
Cement
15 min. 30 min. 45 min. 1 hr. 1% hr. 3 hr. 6 hr. 9 hr. 12 hr. 24 hr. 7 days
57.4 8.2 90.5 18.6 105.0 15.4 102.8 16.8 108.8 22.0 117.5 29.5 137.3 32.0 133.8 21.1 144.7 22.5 143.9 30.9
25 25 25 25 25 25 25 25 25 25
41.8 71.8 64.7 74.8 77.5 87.3 101.1 102.0 _ 109.5 147.4
8.5 13.7 13.2 18.1 19.9 22.1 24.5 31.6 __ 16.6 28.5
12 12 12 12 12 12 12 12 12 12
39.3 9.3 83.1 16.8 81.2 14.8 101.1 19.7 89.8 13.2 91.4 24.4 124.5 25.2 121.3 20.6 .-. ..122.5 35.9 125.8 32.9
12 12 12 12 12 12 12 12 12 12
49.2 4.8 85.7 16.0 83.1 22.2 99.2 13.2 104.7 19.7 123.8 19.9 126.7 18.7 145.1 19.4 144.9 21.5 152.5 24.8
25 25 25 2:-I 25 25 2.5 2:i 21; 25
*Symbols as in Table IV. strength considerations alone they appear well suited for use both as luting agents and as bases. Strength of silicate cements. The strengths of the silicate cements tested are given in Tables V and VI. The curve of increase in strength with time of one of these cements is shown (Fig. 1). Strength was gained rapidly during the first 30 minutes to a point between 50 and 65 per cent of the ultimate strength. After 30 minutes the rate of increase in strength was reduced, but by 6 hours the cements had reached
Volume 26 Number 3
Tensile
strengths
of restorative
materials
319
TIME (hours) 6
14
12
7 DAY
TIME (minutes)
Fig. 1. Rate of increase in tensile strength for a typical silicate cement.
Table VI. Tensile
strength
of silicate
cements in kilograms
per square centimeter
Cement E
15 1 24 7
min. hr. hr. days
F
G
a*
5
n
rr
s
n
a
s
n
43.1 65.9 107.2 114.6
8.7 13.2 23.8 16.2
12 12 12 12
51.3 46.1 78.6 82.8
8.2 12.7 12.4 14.8
12 12 12 12
27.9 62.4 73.2 85.3
5.7 11.4 11.0 16.6
12 12 12 12
H (high
ratio)
H (low
ratio)
a
15 1 24 7
min. hr. hr. days
64.2 11.3 99.6 17.2 115.3 20.0 112.8 18.9
12 69.4 12 95.8 12 109.5 12 97.8
9.0 13.0 13.3 13.6
12 12 12 12
54.1 68.9 93.6 95.7
10.8 15.1 27.3 23.6
12 49.4 8.3 12 83.1 19.7 12 125.9 29.0 12 110.3 35.4
12 12 12 12
*Symbols as in Table IV. between 70 and 100 per cent of their ultimate strength. This rapid rate of increase in strength is clinically desirable because normal mastication may be resumed soon after restorative dental treatment. Glass fibers enhanced the strength of silicate cement in the early stages of setting
320
Hannah
and Smith
Table VII. Tensile centimeter
strength
of dental
amalgam
alloys in kilograms
per square ---.I__-.
Alloy
Thiration
.4 B c
Condensation
Interval
no sec.
Mwhan.
24 hr.
15 sec.
Hand Mechan.
10 sec. (IS
sec.)
0
-?I
I
j61.7
18.0
9
7 da) 7 da)
600.4 632.2
3.5. 1 X7.”
Ill 9
Mechan. Mechan.
7 da) 7 da?
.X3.7
32.6
8
604.1
64.‘;
7
Hand Me&an.
7 da? 7 day
480.2 j46.2
100.6 73.7
8 :i
D
20 sec.
E
20 sec.
Mwhan.
7 da!
.50’,.2 ‘.
:ih.:i
Ii
F
5 sec.
Mcchan.
7 da)
636.1
66.11
!i
G
20 sec.
Mechan.
I da!
713.4
77.0
i
Table VIII. Tensile strength per square centimeter Alloy G. C. Luna Atomic c s II Kerr
/
of spherical-particle
4%
and conventional
hr.
6 hr.
alloys in kilograms
I hr.
3 hr.
7% hr.1 24 hr. I48
hq-
164.8 21.2 8
198.5 18.8 9
208.:1 l12.4 7
247.2 61.6 8
‘9:,.7 56.9 9
352.i! 40.9 8
449.2 h7.‘i 11
i46.2 7.5.7 8
123.9 30.5 6
218.9 33.9 6
242.8 18.4 4
304.0 28.” i
:+5&H 30.4 6
653.11 58.9 3
615.7 24.9 1
71:1.+ 77.0 i
191.8 18.9 6
259..i 26.3 6
305.2 22.1 6
416.8 363 6
618.9 48.6 6
620.1 60.5 i
615.0 IVI..{ h
602 ..!: 116.1; R
115.9 10.1 9
257.i 32.6 7
3’0.5 18.4 6
492.8 48.2 8
634.7 5 3 .7 6
518.6 60.2 9
583.6 7.i.:: 9
5 5 3 ,7 52.6 8
Spheraloy F s Ii
Experimental F b n
alloy
S. S. White t? s II
N.T.D.
(Table VI). The difference in strength became less pronounced with an increase irr the time interval before testing, but the glass fibers in no way impaired the ultimate tensile strength of the set cement. Strength of amalgam alloys. The ultimate strengths of the amalgam alloys tested ranged from 480 to 713 Kg. per square centimeter (Table VII). The rate of increase in strength of one fine-grain conventional alloy and three spherical-particle alloys is given in Table VIII. The experimental spherical-particle alloy gained strength fairly uniformly at a rate similar to that of the conventional alloy. The increase in
Volume 26 Number 3
Tensile
. ..I x... -m-o -
l
NEW TRUE DENTALLDY SPHERALDY LUNA ATOMIC
-a -
strengths
of restorative
t
500= ,
Y 0.
X/ ,’ kg/cm2
#’
Fig. 2
321
655
,
,*
-E T z
materials
0 I
4’
40 .
80 .
120 .
160 I
F 300-
Fig. .‘;
x 2 IF-l .+
IL
P
I
3 S TIME (hours)
7
C D
a =
c D
Fig. 2. A comparison of the rates of increase in strength of a conventional and two sphericalparticle dental amalgam alloys. Fig. 3. Relative tensile strengths of various materials for cylinder (C) and disc (D) specimens.
strength of the two commercial spherical-particle alloys, on the other hand, was not uniform. Initially these alloys gained strength very rapidly, but between 1 and 7 hours the rate of increase in strength was retarded when compared with the fairly uniform rate of increase of the conventional alloy (Fig. 2). This retardation was followed by a recovery and by 48 hours the strengths of the spherical-particle alloys differed little from that of the conventional alloy. The reduced strength of the spherical-particle alloys at an early stage in setting may present problems in their clinical use. DISCUSSION The manufacturers’ recommended ratios for silicate cements and the standard consistency test, A.D.A. Specification No. 9,14 appear to be related to the desired consistency of the large amount of material necessary to prepare relatively large specimens for compressive testing. In order to obtain clinically satisfactory mixes, higher powder-liquid ratios are necessary. For example: 0.53 Gm. of silicate cement H can be incorporated into 0.1 ml. of liquid to give a clinically satisfactory mix. This corresponds to a ratio of 2.1 Gm. to 0.4 ml. The ratio recommended by the manufacturer and determined by the standard consistency test is 1.6 Gm. to 0.4 ml. This means that 25 per cent more powder than that recommended can be incorporated into the liquid to give a clinically satisfactory mix. These ratios should be more appropriately related to the small amount of material used clinically and the specimen size for laboratory testing should be modified accordingly. The effects of specimen size on strength of material can be seen in Fig. 3 (Table
322
Hannah
and
Table
IX. Comparing
J. l’roslhet. Dent. Srprmber, 1971
Smith
cylindrical
and disc test specimens _____I-.
Material Velmix
Specimen
stone
a
1
s
n
Time
Disc Cylinder
(a)
57.6 48.4
8.7 6.6
IO 6
7 days 7 days
Disc Cylinder
(a)
29.1 28.”
2.1 3 :,
10 3
7 days 7 days
Disc Cylinder
(a)
93.6 31.9
27.3 12.3
12 5
Disc Cylinder
(a)
143.9 87.1
30.9 35.1
12 5
7 days 7 thys
Disc Cylinder
(a)
147.4 72.2
28.5 3.3
12 5
7 days 7 days
Alston Tablet Alloy (hand cond.)
Disc Cylinder
(b)
600 515,
35.1 54.9
10 5
7 days 7 days
Alston Tablet Alloy (mech. cond. )
Disc Cylinder
(b)
652 499
37.2 49.9
9 5
7 days 7 days
Kalzinol Achatit
without
fibers
S. S. White porcelain
new filling
S. S. White
M.Q.
The dimensions of the specimens tested were: Disc 3.18 mm. diameter Cylinder (a) 6.00 mm. diameter 4.00 mm. diameter Cylinder (b)
21 hr. 24 hr.
-
3.18 mm. li:ns. 11.9 mm. long. 8.25 mm. ions.
IX). The 6 mm. diameter cylinders of silicate cements tested at 24 hours and ‘i days are considerably weaker than the 3 mm. diameter cylinders of the same material. This difference in strength is possibly a result of difficulty in spatulating the large amount of material thoroughly and also of the unavoidable incorporation of an increased number of voids in preparing the large cylindrical specimens. The difference in diameter of the two sizes of specimens of amalgam was iess than that of the silicate cement specimens. The difference in strength was correspondingly less pronounced. The quantity of material necessary for the preparation of the small cylindrical specimens
is clinically
more
realistic
and is more
easily
manipulated
than
that
which
is necessary to prepare the larger specimens. For this reason the small cylinders considered more suitable for laboratory testing.
are
SUMMARY The tensile strengths of several lining and luting cements, silicate cements, and dental amalgam alloys were determined by diametral compression. Two E.&I. cements and one polyacrylate cement are stronger within 20 minutes of mixing than a zinc phosphate cement. Limiting tensile strengths for base and luting ct:rrlc~nts remain undefined. However, since these cements compare favorably with a zinc phosphate cement for tensile strength considerations, they are well suited to similar clini.. cal uses. Silicate cements reach between 70 and 100 per cent of their ultimate strength within 6 hours. The early tensile strength of these cements is enhanced h\: incorporating glass fibers into them.
Volume 26 Number 3
Tensile
strengths
of restorative
materials
323
Dental amalgam alloys are three times as strong in tension as silicate cements. The rate of increase in strength of two spherical-particle alloys is reduced between 1 and 6 hours when compared with the rate of increase in strength of a conventional alloy. CONCLUSIONS The results obtained using materials in quantities recommended to conform to specification tests differ from those obtained using clinical quantities of material. Both specimen size and bulk of material used for laboratory testing should be more closely related to clinical procedures. References 1. Skinner, E. W., and Phillips, R. W.: The Science of Dental Materials, ed. 6, Philadelphia and London, 1968, W. B. Saunders Company. 2. Peyton, F. A.: Restorative Dental Materials, ed. 3, St. Louis, 1968, The C. V. Mosby Company.
3. Hollenback, 4. 5. 6. 7. 8.
9. 10. 11.
12. 13. 14. 15. 16. 17.
18.
G. M., and Villanyi,
A. A.: The Physical Properties of Dental Amalgam,
J. S. Calif. Dent. Assoc. 12: 455-459, 1964. Mahler, D. B., and Mitchem, J. C.: Transverse Strength of Amalgam, J. Dent. Res. 43: 121-130, 1964. Forsten, L.: Physical Properties of Dental Amalgams, Academic Dissertation, Inst. of Dent., Univ. of Turku, Finland, 1969. Mahler, D. B.: An Analysis of Stresses in a Dental Amalgam Restoration, J. Dent. Res. 37: 516-526, 1958. Ward, M. L.: Modern Tests for the Strength of Amalgam, J. Amer. Dent. Assoc. 11: 487496, 1924. Taylor, N. 0.: A Specification for Dental Amalgam Alloys: A Report to the Scientific Foundation and the Research Commission of the American Dental Association, J. Amer. Dent. Assoc. 17: 112-124, 1930. Bowen, F. L., and Rodriguez, M. S.: Tensile Strength and Modulus of Elasticity of Tooth Structure and Several Restorative Materials, J. Amer. Dent. Assoc. 64: 378-387, 1962. Rodriguez, M. S., and Dickson, G.: Some Tensile Properties of Amalgam, J. Dent. Res. 41: 840-852, 1962. Phillips, R. W., Swartz, M. L., Norman, R. D., Schnell, R. J., and Niblack, B. F.: Zinc Oxide and Eugenol Cements for Permanent Cementation, J. PROSTHET. DENT. 19: 141150, 1968. Koran, A., and Asgar, K.: A Comparison of Dental Amalgams Made From a Spherical Alloy and From a Comminuted Alloy, J. Amer. Dent. Assoc. 75: 912-917, 1967. Williams, P. D., and Smith, D. C.: Determination of the Tensile Strength of Restorative Materials by the Diametral Compression Test, J. Dent. Res. 46: 1297, 1967. American Dental Association: Guide to Dental Materials, ed. 3, Chicago, 1966. McConnell, D., and Brawley, R. G.: Compressive Strength of “Reinforced” Silicate Cement, J. PROSTHET.DENT. 10: 1092-1093, 1960. Phillips, R. W., Swartz, M. L., and Chong, W. F.: Properties of Silicate Cements Mixed by Hand and Mechanical Means, J. S. Calif. Dent. ASSOC. 33: 239-242, 1965. Williams, P. D.: Studies on the Diametral Compression Test as Applied to Dental Restorative Materials and of Their Adhesion to Enamel and Dentine, M.Sc. Thesis, Victoria University of Manchester, p. 28, 1967. of Cement Bases by Chong, W. F., Swartz, M. L., and Phillips, R. W.: Displacement Amalgam Condensation, J. Amer. Dent. ASSOC.74: 97-102, 1967.
THF.TTJRNERDENTAL SCHOOL,UNIVERSITYOF MANCHESTER BRINEFORD STREET MANCHESTER Ml5 6FH, ENGLAND