Rheology of Composite Filling Material Pastes MICHAEL BRADEN Dental School of the London Hospital Medical College, University of London, London, El 2AD, England

The rheology of composite pastes in the unset state has been studied using a cone and plate viscometer. All materials behaved as Bingham bodies, exhibiting linear shear stress-shear rate plots, with a positive shear-stress intercept (the yield stress of the material). The shape of the plot gave the coefficient of viscosity of the material; at sufficiently high shear rates, shear failure occurred, marked by a breakdown of the linear relationship. The materials studied gave a very wide range of the various rheological factors. J Dent Res 56(6): 627-630 June 1977.

Composite filling materials are well established, and much has been written about their physical and biological properties. (A good review has been given by Paffenbarger and Rupp.") Received for publication April 7, 1976. Accepted for publication July 27, 1976.

Many, if not most, materials are supplied as a two-paste system although occasionally the material is supplied as a single paste to which a few drops of catalyst liquid are added. However, it seems that the many materials available have similar properties in the set condition, albeit with quantitative differences. The practitioner's choice of a material may well be governed by other characteristics not hitherto considered. Rheological properties seem, a priori, to be worth considering in this light for the following reasons: (1) The ease of mixing and handling is basically a rheological problem. (2) The acid etch technique depends upon the ability of the material to flow into the interstices of the etched enamel. (3) If repair to an existing abraded composite restoration is required, greater success is achieved with more fluid pastes.2 There are two aspects that could be studied, namely the rheological properties of










DeTreys Cosmic Paste/Paste











Johnson and Johnson, East Windsor, NJ 3M Co. St. Paul, Minnesota Amalgamated Dental Co., London, UK Davis, Schottlander, and Davis,

London, UK S. S. White Dental Manufacturing Co., Philadelphia, Pa. Paste/Catalyst Kerr, Sybron Corp., Liquid Romulus, Mi Paste/Catalyst Allied Laboratories Ltd. Liquid (Member of Glaxo Group), London, England Paste/Paste

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J Dent Res June 1977


a, 3.0 z



2-0 .


0 10

Shear Rate-sec'

FIG 1.-Plot of shear stress versus shear rate for Material A, catalyst paste. Open circles, 7.6 cm diameter, 1037' angle cone (0.0029-inch gap); closed circles, 2.5 cm diameter, 3059' angle cone (0.0066-inch gap).

rate where failure occurred. This was manifest both by material breaking away from the plates and by departures from linearity of the shear stress-shear rate plates. Shear rate (ay) was calculated from [1] o- = 27r n Cor(, where n - number of revolutions per second and ( = cone angle. Shear stress (r) was calculated from the instrument torque (G) and plate diameter (D) from [2] r122GfrD3. At each shear rate, the instrument was run from three to five minutes to ascertain whether the material was thixotropic, although in fact no such effects were observed to any major extent.

Results All graphs plotted show shear stress as a function of shear rate. Figure 1 shows data plotted for the catalyst paste of material A, using both cone and plate systems. Figure 2 shows typical data plotted for a two-paste system (material C), and Figure 3 for a single

the pastes per se and the change of properties during setting. This report is concerned with the first. Materials and Methods Five two-paste and two paste/liquid systems (A to G) were studied; the details of each are given in Table 1. Rheological properties were examined at 25 C with a cone and plate viscometer,* the use of which for dental materials has been described previously.3 This instrument has two alternative cone and plate systems. Preliminary experiments showed that with the 7.5-cm diameter, 10 32' angle plate, which has a minimum gap of 0.0029 inch, considerable sticking of the force transducer occurred, indicating the presence of coarse particles blocking the gap. This was confirmed by the observation of scratches on the plates. Satisfactory measurement were, however, attainable with the 2.5cm diameter 3° 59' angle plate with 0.0066inch gap. In the case of material A, both cone and plate systems could be used satisfactorily and were used to check conformity of data. All materials were studied at a range of shear rates from 0.0139 sec-1 up to the shear * Weissenberg Rheogoniometer, Sangamo Controls Ltd., Bognor Regis, UK.





0.9 "I

a) 0


E z zen 0.7




'5 06 a)

en O

0-4 0*3 0-2

0-i 1.0-0 05 Shear Rate- sec

FIG 2. Plot of shear stress versus shear rate for Material C. Open circles, universal paste; closed circles, catalyst paste.

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Vol. 56 No. 6


where shear failure occurs. q, T0) Of, and rf valfor the materials studied are given in Table 2. Also the rate of energy dissipation (Wf) to cause failure is given; this was calculated from












a) C

T1-0_a) U)


I 0




010 Shear Rate- sec

FIG 3.-Plot of shear stress for material F.




paste system (material F). All graphs have the same basic pattem: namely, (1) A yield stress ( r0), below which flow does not occur; this is of

where the linear graph intercepts the ordinate. (2) A linear plot, over a limited range of r and a, the slope of which is the coefficient of viscosity (n); and ((3) the linear rrelationship breaks down at reasonably well-defined values of r and a (rf and af),




Figure 1 shows that to a first approximation, both cone and plate systems give consistent data. All of the materials studied conform to the remarkably simple pattern of a Bingham body:4 namely, a linear shear stress-shear rate plot with a yield stress. Additionally, these materials exhibit shear failure at sufficiently high shear rates. Hence characterization is surprisingly simple. Considering how similar these seven materials are in the set condition, in terms of strength and modulus parameters, there is a remarkably wide range of rheological properties (Table 2). It would seem that with the paste/liquid systems (F and G) where the paste has a viscosity of -104 N.sec/m2 and the liquid -1e3 N.sec/m2 conditions are not favorable to good mixing and blending of the catalyst in the paste as has been found5 for material F on long-term immersion in water. However, there is no direct evidence as yet of clinical problems. Of these two materials, material G with i.s lower J,, v values seems to be the better.


A Universal

Catalyst B Universal

Catalyst C Universal

Catalyst D Universal

Catalyst E* Universal F G

ro(kN/m2) 0.760 1.16 0.164 0.20 0.020 0.050 0.275 0.200 0.033 0.400 0.115



sec/m2) Tf(kN/m2) af(SeC-1) (Watts/m3) 78.40 0.09 194 2.5 82.70 0.09 170 3.0 5.35 0.03 0.52 124 4.40 0.02 0.56 220 76.5 0.360 0.45 10.8 310.0 0.565 1.15 20.0 165.0 0.40 1.10 17.85 17.50 0.10 40.10 0.55 18.35 0.10 0.40 32.4 77.0 0.07 2.6 328 32.4 0.08 0.93 100

* Catalyst paste of this material caused sticking and plate, indicating presence of coarse particles.


even with the small cone

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The lower viscosity materials (C and D) and possibly the low-yield stress material B would seem to be more favorably placed for the acid etch technique; B and C have been shown to give better bond strengths when used to add to existing set material. The existence of a yield stress (Ir) below which flow will not occur is probably desirable from a clinical viewpoint. It means that the material will not flow under its own weight. Shear failure will facilitate the formation of voids in the material during mixing, hence af should be as high as possible; again materials C and D are best in this respect. Alternatively, stirring rates should not be too fast. There is a wide range of values of the rate of energy dissipation necessary to cause failure; the reasons for this are not clear at present. Extensions of this work would seem to be directed toward its relation to behavior during setting, and toward the more fluid resin-based composites. Conclusions Unset composite filling material pastes

I Dent Res June 1977 behave as Bingham bodies, and so are characterized by a yield stress, and a coefficient of viscosity; in addition, the material breaks down on shearing at well-defined stress/shear rate coordinates. The materials studied showed a very wide range of these parameters. The Weissenberg Rheogoniometer was purchased with grant funds from the Central Research Fund Committee of the University of London, to which the author is indebted.

References 1. PAFFENBARGER, G.C., and RupP, N.W.: Composite Restorative Materials in Dental Practice: A Review, Int Dent J 24:1-11, 1974. 2. CAUSTON, B.E.: Repair of Abraded Composite Fillings: An In Vitro Study, Brit Dent J 139:286-288, 1975. 3. BRADEN, M.: Viscosity of Impression Rubbers, J Dent Res 46:429-433, 1967. 4. REINER, M.: Phenomenological Macrorheology in Rheology, EIRICH, F.R. (ed): Vol. 1:44, 1962. 5. BRADEN, M.: Selection and Properties of Some New Dental Materials, Dental Update, 1:489-501, 1974.

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Rheology of composite filling material pastes.

Rheology of Composite Filling Material Pastes MICHAEL BRADEN Dental School of the London Hospital Medical College, University of London, London, El 2A...
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