Scand. J. Dent. Res. 1976: 84: 426-429 (Key words: dental materials; resins)

Elastic properties of resin-based filling materials NILS ROAR GJERDET NIOM, Scandinavian Institute of Dental Materials, Oslo, Norway ABSTRACT - Some aspects of the viscoelastic behavior of resin-based filling materials have been studied by the use of a bending test. The specimens, designed as cantilever beams, were loaded for 30 sec and then unloaded. The tests were performed at 5/2 h, 1 h, 3 h, 24 h, and 1 week after mixing of the materials. The strains were measured by electric strain gauges cemented to the tensile side of the beam. Six products were tested, five of them composites. Different brands of materials showed considerable variation in degree of viscoelastic behavior. The values for elastic after-effect decreased markedly with time, reaching a fairly constant value at 24 h. Elastic moduli showed a similar increase and the stiffest materials exhibited the least elastic after..effect.

JORGENSEN,

MATONO

&

SHIMOKO3E

(1976) have shown that deformation of cavities during mastication may cause percolation at the tooth/filling interface. Resin-based filling materials are viscoelastic - they show creep when loaded and elastic after-effect when unloaded ( N O WICK 1965). The elastic properties of resin-based filling materials may predispose to percolation. Many investigations have dealt with elastic moduli, both static and dynamic measurements (FINGER 1974, 1975; MGHANNAH & COMBE 1976), but nq data are available on the elastic aftereffect of resin-based filling materials. The purpose of this study was to investigate the viscoelastic behavior of resinbased filling materials by measuring the elastic after-effect and elastic moduli. The elastic after-effect represents a measure of the ability of the material to return to its original shape after a load has been re-

moved, while elastic modulus is a measure of the stiffness of the materials. Material and methods

The investigated products are listed in Table 1. The materials were mixed according to manufacturers' instructions and specimens measuring 2 X 4.5 X 20 mm were prepared by the use of a Teflon® mold. Two specimens of each brand were investigated. Ten minutes after start of mixing an electric strain gauge (Hottinger Baldwin LY 13®) was fixed to the middle of the specimen with fast-setting cement. The specimens were stored dry at 37°G, and tests were performed at 23°G. The specimen was mounted horizontally in a clamp and the strain gauge connected to an amplifier and a recorder with rapid pen movement. A metal weight with a mass of 375 g was suspended on a wire to the free end, 6.5 mm from the center of the strain gauge. The weight rested upon a laboratory jack which permitted rapid but smooth loading and unloading of the specimen (Fig. 1). The specimen made a cantilever beam and deformations on

427

ELASTIGITY OF RESIN FILLINGS Table 1 Investigated products Product

Gode

Manufacturer

Adaptic® Gompocap® Gosmic® Epoxydent® Smile® Sevriton Simplified®

Johnson & Johnson Ivoclar A.G. Gebr. De Trey A.G. Lee Pharmaceuticals Kerr Gebr. De Trey A.G.

the tensile side were recorded by the strain gauge (HOFFMANN 1973). Thirty minutes after commencement of mixing, the specimen was loaded for 30 sec and then unloaded. The tests were repeated on the same specimen after 1 h, 3 h, 24 h, and 1 week. The resulting strains were continuously recorded and typical curves for one product are shown in Fig. 2. To calculate elastic moduli and elastic aftereffect, the strains at 0.5 sec after loading and 0.5 sec after unloading were measured.

AD CP GO EP SM SE

at /2 h. At ^2 h there are considerable differences between products but after 1 week the values are similar except for Sevriton Simplified®, which maintains an elastic after-effect approximately fiv^ times higher than the composites. The elastic moduli of the materials are shown in Fig. 4. The values increase with the age of the specimens, but changes are not as dramatic as for the elastic aftereffect. The differences between products

Results The elastic after-effect is markedly affected by the age of the specimen (Fig. 3). Within 3 h there is a decrease of about 70 % compared with measurements

LOADING

UNLOADING

0.5 h

RECORDER

STRAIN GAUGE ^

\

. - "

1 week



LABORATORY JACK 30 TIME, seconds

Fig. 1. Experimental set-up.

Fig. 2. Gurves obtained at '/i h and 1 week for Gompocap®. Values 0.5 sec after loading and 0.5 sec after unloading were used to calculate elastic moduli and elastic after-effect.

428

GJERDET

ues are therefore about 10 % higher than those reported by FINGER (1974). The present results should be considered when resin-based materials are used. Finishing of fillings includes the application of forces, and in the first hours after insertion the material is easily pulled away from the cavity margins. Particles may be packed into the interspaces and the tendency for marginal discoloration and secondary caries will increase. Finishing of resin fillings should therefore be delayed at least 24 h. Heavy masticatory or incisive forces should not be allowed to act directly on a resin-based filling material, especially not on newly placed fillings. The elastic after-effect may cause heavy percolation which will diminish as a function of time but will probably still be important after 0.5 h

3h

24 h

1 week

log TIME

Fig. 3. Degree of elastic after-effect as a function of specimen age and type of product. Measurements taken 0.5 sec after unloading of the specimens.

AD

are maintained from the J/^-h reading to the 1-week recording. Discussion

The results demonstrate that although the materials appear to be hardened after J/2 h, at that time the elastic properties are different from those at 1 week. The degree of viscoelastic behavior represented by the elastic after-effect is pronounced and the elastic moduli are low. It is important to note that the presented elastic moduli are measured after only 0.5 sec of loading. Compared with testing at low speeds these results will reflect better the effects of rapid forces. The effect of creep is minimized and the val-

SE

lh

3h

24 h

Iweek

log TIME

Fig. 4. Elastic moduli as a function of specimen age and type of product. Measurements taken 0.5 sec after loading of specimens.

ELASTICITY OF RESIN FILLINGS 1 week. The materials with the least degree of elastic after-effect will be more suitable than those with high degree of elastic after-effect if restorations are subjected to heavy forces.

429

K . : Grundlagen der Dehnungsmessstreifen - Technik. Die Messung elementarer Belastungsfdlle mit DMS. Hottinger Baldwin Messtechnik GmbH 1973.

HOFFMANN,

JORGENSEN, K .

D.,

MATONO, R . &

SHIMOKOBE,

H.: Deformation of cavities and resin fillings in loaded teeth. Scand. J. Dent. Res. 1976: 84: 46-50. References MCHANNAH, G. & COMBE, E . C.: Mechanical FINGER, W . : Der Elastizitatsmodul von Gomproperties of composite restorative materials. posite-Fiillungsmaterialen. Schweiz. MonatsBr. Dent. J. 1976: 140: 167-173. schr. Zahnheilkd. 1974: 84: 648-661. No WICK, A. S.: Anelastic phenomena in metals FINGER, W . : Elastisitat von Composite-Fiillungsand non-metallics. In: Internal friction, damping, and cyclic plasticity. ASTM Specmaterialen. Dtsch. Zahnaerztl. Z. 1975: 30: ial Publication No. 378, 1965, p. 21-26. 345-349. Address: NIOM Forskningsveien 1 Oslo 3 Norway

Elastic properties of resin-based filling materials.

Some aspects of the viscoelastic behavior of resin-based filling materials have been studied by the use of a bending test. The specimens, designed as ...
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