Dental Materials Journal 19 (2): 186-195, 2000Original paper

Development of Metal-resin Composite Restorative Material

Part 3 Flexural Properties and Condensability of Metal-resin

Composite using Ag-Sn Irregular Particles

Somchai URAPEPON, Kiyoshi KAKUTA1, Hideo OGURA1

Chatcharee SUCHATLAMPONG and Apiwat RITTAPAIProsthodontics Department,Faculty of Dentistry, Mahidol University6, Yothi Road, Rachathevi, Bangkok, 10400, Thailand1Department of Dental Materials Science,School of Dentistry at Niigata, The Nippon Dental University1-8 Hamaura-cho, Niigata City, Niigata, 951-8580, Japan

Received January 15, 2000/Accepted March 16, 2000

Powder-liquid type metal-resin composites, using Ag-Sn irregular particles as the filler, 4-

META as coupling agent and UDMA+TEGDMA as resin matrix, were experimentally prepared

under 9 different conditions (three different particle sizes and three different filler contents).

The flexural strength and flexural modulus were measured. Three different irregular particle

size MRCs without redox-initiator at 94% filler content, as well as amalgam, conventional hy-

brid composite and Ag-Sn spherical particle MRC were evaluated for condensability.

The flexural strength of the Ag-Sn irregular particle MRC was significantly influenced by

both the filler particle size and filler contents (p<0.01). It increased when either the filler con-

tent increased or the particles size decreased. The highest flexural strength (97.6MPa) was ob-

tained from the condition of particles size <20ƒÊm and 94% filler content. The flexural modulus

was significantly influenced by filler content and it increased with increasing filler content.

The condensability of the Ag-Sn irregular particle MRC was lower than that of amalgam but

much higher than presently available conventional composites and spherical particle MRC.

Key words: Silver alloy, Condensation, Resin composite

INTRODUCTION

Recently, the metal-resin composites (MRC) for direct restorative materials have been developed1-3). Acceptable mechanical and physical properties of MRC using Ag-

Sn spherical particles as filler, as a posterior restorative material were reported. However, one of the disadvantages of the present posterior composite is the lack of resistance to condensation, which is associated with amalgam, and which is of some benefit for manipulative ease and may be related to good adaptation to cavity walls. In our previous studies1-3), however, the prepared MRC was able to be syringed, which indicated the similar consistency to the conventional composites and is insufficient re-sistance to condensation.

It has been reported by Ogura et al.4) that the resistance to condensation of an amalgam mix is influenced by the shape of the alloy particles, and that irregular par-ticles show higher resistance to condensation than spherical particles. Therefore, the

URAPEPON et al. 187

resistance to condensation of the MRC may be improved by using irregular particles for the filler. The strength of MRC, however, may be influenced by such irregular

particles.The resistance to condensation is different from the consistency of the composite,

which has been studied by several researchers5,6). Currently, there is no standard test for the resistance to condensation of composite materials. Ogura et al.4) developed a

quantitative test method for testing the resistance to condensation of amalgam. However, the test method may not be applicable to the testing of composite materials

due to the great difference in resistance between these materials. Opdam et al.5) uti-lized a modification of the consistency test of elastomeric impression materials (ISO 4823, 1992) to test the consistency of the composites. Since the load is static, the re-sults of the measurement of the area of the material as the consistency may not be clinically relevant. Tyas et al.6) developed a more clinically relevant penetration test

method in order to evaluate the consistency of resin composites. However, the mold and plunger size were very big and also the rate of the plunger when driven into the material was high, which are all different from clinical conditions.

The purpose of this study was to evaluate the flexural properties of the MRC

using irregular Ag-Sn particles as filler, and to evaluate the resistance to condensa-tion of MRC using a modified penetration test method

MATERIALS AND METHODS

Composite specimen preparation, flexural strength and flexural modulus test

Ag-Sn spherical alloy particles (73.2 mass% Ag, 26.8 mass% Sn, Lot no.9804AS750)

were produced by the research unit, Faculty of Dentistry, Mahidol University, Thai-

land, using an atomization method. The particle, which size was bigger than 53ƒÊm,

was crushed by a ball-milling machine (Pulverisette, Fritsch, Germany) at rate 8 (595

rpm) for 5h to produce irregular particles. After crushing, the particles were then

sieved with 53 and 20ƒÊm mesh size. Three different particle sizes; under 20ƒÊm, be-

tween 20-53ƒÊm, and a combination of these two with 1:1 ratio by mass were utilized

for the filler of MRC. MRC were prepared in powder-liquid form following the pro-

cedure reported in the previous study3). The filler was treated with 4-META and BPO

at 0.1 concentration ratio of 4-META for 30 seconds and then mixed with resin mono-

mer consisting of 75 mass% of 1, 6-Bis (methacryloyloxy-2-ethoxy carbonyl-amino)-

2, 4, 4 (2, 2, 4)-trimethylhexane (UDMA) and 25 mass% of triethylene glycol

dimethacrylate (TEGDMA) with additive 0.25 mass% of N, N-dimethyl-p-toluidine

(DMPT) to prepare the MRC specimens. The flexural test was carried out following

ISO 4049: 1998 for resin based filling materials. The filler content was 92, 93 or 94%.

Three replications were prepared for each of nine conditions (three different particles

and three filler contents) for the flexural test. The flexural strength and flexural

modules data were subjected to a two-way ANOVA and then the mean values were

compared using Tukey's multiple range test.

188 DEVELOPMENT OF METAL-RESIN COMPOSITE, PART 3

The resistance to condensation (condensability) test

The condensability of MRC was evaluated using a modified penetration test. Three

different irregular Ag-Sn particle fillers (under 20ƒÊm, between 20-53ƒÊm, and the com-

bination of these two with 1:1 ratio by mass) were treated with 4-META as described

above. According to the penetration test method, which is meticulous and takes time,

MRC specimens were prepared without an accelerator (DMPT) to avoid quick setting,

which interferes with accurate measurement. The penetration test was carried out as

follows, the treated Ag-Sn particles were mixed with the resin monomer at 94% filler

content using a plastic spatula for 30sec, and then packed into a polyethylene tube

(4mm in diameter, 8mm long), which was placed on an acrylic block (4mm in di-

ameter and 2mm high). They were assembled and the specimen size was 6mm high

and 4mm in diameter (Fig. 1). After packing and making a leveled flat surface of

the material, the mold was placed on a universal testing machine. A stainless steel

rod (2mm in diameter) with a flat end was attached under the crosshead of the uni-

versal testing machine and lowered slowly until it just made contact with the surface

of the center of the composite. After this procedure the rod was driven into the com-

posite at a crosshead speed of 5mm/min. The resistance force was continuously re-

corded on the chart. From the chart, the resistance force at 2mm penetration depth

was measured. In addition to the irregular particle MRC, amalgam (Dispersalloy,

Lot no 24003, Johnson & Johnson, Japan), conventional hybrid composite (Z-100, Lot

no.5906, 3M, Japan) and spherical particle MRC at the highest flexural strength con-

dition in the previous study3) (<38-53ƒÊm filler size, 0.1 4-META concentration ratio,

30 second immersion time, 94% filler content) were evaluated for comparison. Six

replications were made for each of these materials. As a big difference in variation

among these groups was found, the logarithms of the data were subjected to one-way

ANOVA and then the mean values were compared using Tukey's multiple range test.

Fig. 1 Schematic of the penetration test apparatus.

URAPEPON et al. 189

Scanning electron microscope (SEM) observationThe polished and fractured surfaces after the flexural test of the prepared compos-ites, and cross sectional surface at 3mm depth from the top of the mold of the com-

posite before and after condensation were observed under a scanning electron microscope (S-800, Hitachi Ltd, Tokyo, Japan) at 15kV acceleration voltage.

In addition, for comparison, the fractured surface of the spherical particle MRC from the highest strength specimen of the previous study3) was also observed, with the permission of the authors.

RESULTS

Table 1 shows the summaries of the flexural strength and flexural modulus of the ir-

regular particle MRC. The flexural strength ranged from 70.9 to 97.6MPa and the

flexural modulus from 8.6 to 11.0GPa. Table 2 shows the results of ANOVA for the

flexural strength, flexural modulus and the resistance force (condensability). The

flexural strength was significantly influenced by all both factors (p<0.01). The flex-

ural modulus was significantly influenced only by filler content (p<0.01). Figs. 2 and

3 show the flexural strength and the flexural modulus of the experimental composite

by the respective influential factors. As shown in Fig. 2, the flexural strength in-

creased either as the filler content increased or as the particle size decreased. The

highest flexural strength of the composite in this study (97.6MPa) was slightly

higher than the previous MRC using spherical particles (91.8MPa3)). The flexural

modulus increased as the filler content increased (Fig. 3).

As shown in Fig. 4, the polished surface of the bigger particle (20-53ƒÊm) MRC

showed larger spacing between the particles than that of the smaller particle (<20

ƒÊ m) MRC. In Fig. 5, the fractured surfaces of the irregular particle MRC in differ-

ent conditions did not show any difference from each other but they were different

from those of spherical particle MRC. All of the irregular particles were almost cov-

Table 1 Summaries of the flexural strength and flexural modulus of

the irregular particle MRC

190 DEVELOPMENT OF METAL-RESIN COMPOSITE, PART 3

Table 2 Results of ANOVA for the flexural strength, flexural modulus and condensability

Fig. 2 Flexural strength of the irregular particle MRC.

Fig. 3 Flexural modulus of the irregular

particle MRC.

URAPEPON et al. 191

Fig. 4 SEM photographs of polished surface of MRC;

20-53ƒÊm MRC (A) and <20ƒÊm MRC (B).

Fig. 5 SEM photographs of fractured surface of MRC; irregular particle MRC (A) and spherical particle MRC (B). Note: Photograph B is taken from a previous study with the

permission of the authors of the previous study3).

192 DEVELOPMENT OF METAL-RESIN COMPOSITE, PART 3

Fig. 6 Resistance to condensation of the materials.

Table 3 Resistance force at 2mm penetration

depth

The values at the same vertical line are not sig-nificantey different at p<0.05

erect by the resin matrix while only some of the spherical particles were covered by the resin.

Fig. 6 and Table 3 show the change of the resistance force to condensation with the depth and the resistance force at 2mm depth of the materials, respectively. As shown in Fig. 6 and Table 3, the condensability of the irregular particle MRC was lower than that of amalgam, but higher than that of spherical particle MRC and the

present conventional hybrid composite. The particle fillers after condensation of the irregular particle MRC showed more closely packed textures compared with before

condensation (Fig. 7A), while those of spherical particle MRC did not show any dif-ference (Fig. 7B).

URAPEPON et al. 193

Fig. 7 SEM photographs of cross-sectional surface of MRC before and after condensation; irregular particle MRC (A) and spherical particle MRC (B).

DISCUSSION

In the previous study1-3), a metal-resin composite was experimentally prepared using

Ag-Sn spherical particles as filler, and could be syringed, which indicated a similar

consistency to conventional composites and is insufficient resistance to condensation,

which is an important property for the modern composite substituting for amalgam.

Based on the study of amalgam by Ogura et al.4), the metal-resin composite was

194 DEVELOPMENT OF METAL-RESIN COMPOSITE, PART 3

prepared using irregular Ag-Sn particles in order to obtain a condensable property.

It should be noted that in this study, we chose filler contents of 92-94%, instead of

the 93-95% used in the previous study3), due to results of the pilot study. At 95%

filler content, the irregular particles, especially<20ƒÊm particles were very difficult

to mix and the mixed paste could not be well molded.

The flexural strength of the irregular particle MRC significantly increased as the

filler particle size decreased. From the polished surface of MRC in Fig. 4, we found

that the smaller particles were more closely packed together than the bigger particles.

It has been reported that the failure of composites occurs mainly by slippage along

the fracture plane inside the materials7). The increase in filler content, decrease in

particle size, change of particle shape to irregular, or higher packing of the particles

increased the internal friction in the slide plane, leading to an increase in composite

strength7).

Although it was expected that the irregular particle MRC would be stronger than

spherical particle MRC from the previous study3), the result was contrary to the ex-

pectation. At the same filler content of 94% with almost the same particle size (ir-

regular particle: 20-53ƒÊm, spherical particle: <38-53ƒÊm3)), the irregular particle MRC

showed both lower flexural strength and lower flexural modulus (81.0MPa and 10.7

GPa respectively) than spherical particle MRC (91.8MPa and 16.6GPa respectively3)).

As shown in Fig. 5, the fractured surfaces of the irregular particle MRC showed cohe-

sive failure of the resin matrix while the majority of the spherical particle MRC

showed adhesive fracture. Therefore, the one possibility seemed to be the difference

in the rate of conversion of the resin matrix, resulting in the higher strength of

spherical particle MRC. A further study should be carried out to investigate this

matter.

The filler content influenced both the flexural strength and flexural modulus.

They increased as the filler content increased. These results agreed with the several

previous studies8-11).

The condensability of the irregular particle MRC was significantly different from

those of spherical particle MRC, amalgam and conventional composite (p<0.01). It

was lower than that of amalgam, but higher than that of spherical particle MRC and

the present conventional hybrid composite. Fig. 7 shows that the particles of the ir-

regular particle MRC were condensed together after condensation (Fig. 7A) while that

of the spherical particle MRC did not show any difference (Fig. 7B). This result sug-

gests that the shape of the particles is one of the greatest factor influencing the

condensability of the materials, as was reported for amalgam4). However, we first

expected that the particle size should also influence this property, but the results in

Fig. 6 and Table 3 were unclear with respect to this factor. This result was also

agreed with Tyas et al.6). As MRC is a self-curing composite, the setting reaction oc-

curs continuously while the material is used. It should therefore be noted that the

condensability of the irregular particle MRC might be higher than indicated by the

present results if the initiator is involved, since the setting reaction may promote an

increase in the viscosity of MRC. In addition, only one type of resin monomer

URAPEPON et al. 195

(UDMA+TEGDMA) was evaluated in this study. Other types of resin monomer such as Bis-GMA, etc. should also be examined; this may result in different resistances to condensation.

A further factor affecting the condensability is the stickiness of composites. If the material sticks to the condenser, it may pull back when the condenser is removed resulting in marginal opening and non-condensation. A further study for the im-

provement for this issue is required.

CONCLUSION

The flexural strength of the irregular particle MRC was slightly improved compared

with the spherical particle MRC. The highest flexural strength (97.6MPa) was ob-

tained from<20ƒÊm particle size at 94% filler content.

The condensability of the irregular particle MRC was lower than that of amal-

gam but much higher than the presently available conventional composite and spheri-

cal particle MRC.

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