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Comparative evaluation of dental resin compositesbased on micron- and submicron-sizedmonomodal glass filler particles

Lisia L. Valentea, Sonia L. Peraltaa, Fabrício A. Ogliarib,Larissa M. Cavalcantec, Rafael R. Moraesa,∗

a School of Dentistry, Federal University of Pelotas, Rua Goncalves Chaves 457, 96015-560 Pelotas, RS, Brazilb Materials Engineering School, Federal University of Pelotas, Rua Félix da Cunha 809, 96010-000 Pelotas, RS, Brazilc School of Dentistry, Federal Fluminense University, LABiom-R, Rua Mário Santos Braga 30, 24020-140 Niterói, RJ,Brazil

a r t i c l e i n f o

Article history:

Received 21 January 2013

Received in revised form 2 May 2013

Accepted 9 September 2013

Keywords:

Dental materials

Filler particles

Physical and chemical properties

Particle size

SEM

a b s t r a c t

Objectives. A model resin composite containing a novel monomodal inorganic filler system

based on submicron-sized Ba–Si–Al glass particles (NanoFine NF180; Schott) was formulated

and compared with an experimental composite containing micron-sized particles (UltraFine

UF1.0; Schott).

Methods. The filler particles were characterized using X-ray microanalysis and granulome-

try, while the composites were characterized in terms of filler–resin morphology, radiopacity,

degree of C C conversion, hardness, flexural strength/modulus, work-of-fracture, surface

roughness and gloss (before and after simulated toothbrushing abrasion), and bulk compres-

sive creep. The composites were formulated from the same photoactivated dimethacrylate

co-monomer, incorporating mass fractions of 75% micron- and 78% submicron-sized parti-

cles. Quantitative data were analyzed at a significance level of p < 0.05.

Results. Both filler systems exhibited a narrow grain size range (175 ± 30 and 1000 ± 200 nm),

with differences restricted to the size and specific area of the particles. The composites

were similar in radiopacity, flexural strength, work-of-fracture, and creep. The submicron

composite was harder but had lower flexural modulus and C C conversion. No significant

differences in roughness were observed before brushing, although the submicron composite

had higher gloss. Brushing increased roughness and decreased gloss on both materials, but

the submicron composite retained higher gloss after brushing.

Significance. The monomodal submicron glass filler system demonstrated potential for use

comp

emy

silane) to bond the two phases [1,2]. Recent efforts to improve

in restorative dental

© 2013 Acad

1. Introduction

Dental resin composites consist of a polymeric matrix basedon dimethacrylate monomers, inorganic fillers for polymer

∗ Corresponding author. Tel.: +55 53 3225 6741x135; fax: +55 53 3225 674E-mail address: moraesrr@gmail.com (R.R. Moraes).

0109-5641/$ – see front matter © 2013 Academy of Dental Materials. Puhttp://dx.doi.org/10.1016/j.dental.2013.09.006

osites, particularly due to improved esthetic properties.

of Dental Materials. Published by Elsevier Ltd. All rights reserved.

reinforcement, and a coupling agent (usually an organo-

1x135.

the resin phase have focused on reducing polymerizationshrinkage [3,4], while improvements in the inorganic phasehave involved optimization of particle shape and size [5].

blished by Elsevier Ltd. All rights reserved.

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The inorganic phase is usually composed of glass and/orxide ceramic particles. The mechanical properties, handling,nd esthetic characteristics of the composites are stronglynfluenced by their filler components [6,7]. It is generallyelieved that smaller particles provide enhanced resistance tourface wear [8–10], which generally occurs through a combi-ation of polymer degradation, loss of resin–filler bonding, and

oss of filler particles. Studies have demonstrated that smallerarticles may generate materials with increased wear resis-ance [11–14], with potentially increased retention of surfaceloss and smoothness.

Dental composites containing nano- and submicron-sizedarticles were introduced relatively recent in an effort to pro-uce materials with longer-lasting esthetic qualities [13,15].anocomposites contain fillers with particle sizes below00 nm, while submicron composites contain fillers with par-icle sizes below 1 �m and typically near 300 nm [1,15]. Theoss of smaller particles from a surface results in less rough-ning than the loss of larger particles, producing less variationn the reflection of light following abrasive processes such asrushing.

Several studies have investigated commercial resin com-osites containing various inorganic filler types and sizes.owever, proprietary materials possess differences beyond

he characteristics of the fillers that may affect comparisons.he aim of this study was to evaluate an experimental den-

al composite based on a novel submicron-sized monomodalller system by comparing it to an otherwise similar micron-ized filler system. The study hypothesis was that compositesormulated using submicron-sized particles would exhibitomparable physical–chemical properties in conjunction withmproved esthetic qualities.

. Materials and methods

he characteristics of the silanized micron (UltraFine UF1.0;chott, Landshut, Germany) and submicron (NanoFine NF180;chott) monomodal glass particles are provided in Table 1. The

ilanization procedure was carried out by the manufacturerccording to the specific surface area of the particles using-methacryloxypropyl trimethoxysilane.

Table 1 – Properties of the filler particles tested.

Filler system

Micron-sized Submicron-sized

d50, nm 1000 ± 200 175 ± 30d99, nm ≤4000 387Specific area, m2/ga 8 40Refractive index, nD

a 1.53 1.53Density, g/cm3 a 2.75 2.75Silane %a 3.2 13

Elemental composition% Ba 57.1 54.8% Si 32.7 35.5% Al 9.4 9.1% Sr 0.8 0.6

a Data obtained from the supplier.

0 1 3 ) 1182–1187 1183

2.1. Particle characterization

The particles were washed in ethanol to remove impurities.Granulometric (1064; Cilas, Orleans, France) and energy-dispersive X-ray spectroscopic (SSX-550; Shimadzu, Tokyo,Japan) analyses were carried out to evaluate the grain sizedistribution and elemental composition of the glass particles.

2.2. Preparation of the composites

A dimethacrylate comonomer blend was prepared by mixing35 wt% of 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]-propane, 35 wt% of ethoxylated bisphenol-A gly-cidyl dimethacrylate with 8 ethylene oxide units, 25 wt%of urethane dimethacrylate, and 5 wt% of triethylenogly-col dimethacrylate (Esstech Inc., Essington, PA, USA). Themixture was rendered photosensitive by adding 0.4 wt% cam-phorquinone and 0.8 wt% ethyl 4-(dimethylamino)benzoate(Sigma–Aldrich, St. Louis, MO, USA). The experimentalcomposites were prepared by adding either 75 mass% micron-sized or 78 mass% submicron-sized fillers. The filler contentin each composite corresponded to the maximum filler loadfor which the mixture maintained good consistency andhandling characteristics similar to commercial composites[4]. The materials were heated to 60 ◦C, vacuum treatedto remove entrained air bubbles, and homogenized usinga mechanical mixer. All photoactivation procedures werecarried out using a light-emitting diode curing unit (Radii;SDI, Bayswater, Victoria, Australia) with an irradiance of600 mW/cm2.

2.3. Filler–resin morphology

Disk-shaped specimens (diameter 8 mm, thickness 2 mm) ofeach composite (n = 3) were photopolymerized using 40 s expo-sures at the top and bottom surfaces. The specimens wereembedded in epoxy resin and their surfaces were polishedunder refrigeration with 600, 1200, 1500, 2000, and 2500-grit SiC abrasive papers followed by 3, 1, 0.25, and 0.1-�mdiamond suspensions. The specimens were ultrasonicallycleaned, dried and gold-coated prior to scanning electronmicroscopy (SEM) analysis (SSX-550; Shimadzu).

2.4. Radiopacity

Radiographic images of disk-shaped specimens of each com-posite (n = 5) were obtained using a digital phosphor platesystem (VistaScan; Dürr, Bietigheim-Bissingen, Germany)operating at a voltage of 70 kV and a current of 8 mA. The expo-sure time was 0.2 s and the focus–film distance was 400 mm.An aluminum step-wedge was included in the radiograph fora density reference [16]. The gray levels (pixel density) of eachimage were analyzed and the equivalence of aluminum (inmm) was recorded [17].

2.5. Degree of C C conversion

The degree of C C conversion in each composite (n = 9) wasevaluated using Fourier transform mid-infrared spectroscopy(Prestige-21; Shimadzu). A uniform volume of material was

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1184 d e n t a l m a t e r i a l

used for each sample. A spectrum of the unpolymerizedcomposite (monomer) was acquired using 24 co-added scansat 4 cm−1 resolution. The composite was photoactivated for40 s and another spectrum was obtained (polymer). The C Cconversion (%) was calculated as previously described [18].

2.6. Flexural strength, flexural modulus andwork-of-fracture

A total of 10 bar-shaped specimens (25 mm × 2 mm × 2 mm)were photopolymerized using exposures of 120 s at the topand bottom surfaces. The cured specimens were stored in dis-tilled water at 37 ◦C for 24 h before being subjected to a 3-pointbending flexural test on a mechanical testing machine (DL-500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speedof 0.5 mm/min. The flexural strength, flexural modulus, andwork-of-fracture were obtained from the stress–strain curves[19].

2.7. Hardness

Specimens fractured in the flexural test (n = 7) were embeddedin epoxy resin and polished under refrigeration using 600- and1200-grit SiC abrasive papers. Five Knoop indentations weremade in each specimen using a microhardness tester (FM-700;Future-Tech Corp., Kawasaki, Japan) in which a 50 g load wasapplied for 15 s. The Knoop hardness number (kgf/mm2) foreach specimen was determined from the average of the fivereadings and calculated as previously described [20].

2.8. Bulk compressive creep

A total of 5 disk-shaped specimens (diameter 8 mm, thick-ness 4 mm) of each composite were prepared and subjectedto bulk compressive testing on a dynamic mechanical test-

ing machine (EletroPuls E3000; Instron, Norwood, MA, USA) inwhich a constant 36 MPa load [21] was applied for 30 min. Thecreep was calculated from the axial deformation (%) of eachspecimen as previously described [21].

Fig. 1 – SEM images depicting the filler–resin morphology of the

were homogeneously dispersed into the resin matrix in both sysamount of composite surface area. Both particle types were irreg

( 2 0 1 3 ) 1182–1187

2.9. Surface roughness and gloss after brushingabrasion

A total of 5 disk-shaped specimens (diameter 10 mm, thick-ness 2 mm) of each composite were prepared and polishedusing medium, fine, and superfine alumina abrasive discs(Sof-Lex system; 3M ESPE, St. Paul, MN, USA). The baselinesurface roughness was measured using a profilometer (SJ-201; Mitutoyo, Tokyo, Japan) and the baseline surface glosswas measured with a glossmeter (ZGM1110; Zehntner, Sis-sach, Switzerland) at a 60◦ angle. The averages of 5 readingson each specimen were recorded as the surface roughness (Ra,�m) or surface gloss (gloss units, GU). The simulated tooth-brushing abrasion test was carried out under a 150-gf load andconsisted of 20,000 brushing cycles (4 Hz) using soft-bristledtoothbrushes (Dr. Veit Soft; Dr. Veit Produtos Oral Care, Rio deJaneiro, RJ, Brazil) and a 1:2 distilled water/dentifrice mixture.The Ra and GU measurements were repeated following thebrushing procedure.

2.10. Statistical analysis

Data from the tests described in Sections 2.4–2.8 were sepa-rately analyzed using Student’s t-test. Surface roughness andgloss data before and after abrasion were separately ana-lyzed using two-way repeated measures ANOVA (one factorrepetition) followed by the Student–Newman–Keuls’ post hoccomparison test. A 5% significance level was used for all anal-yses.

3. Results

The characteristics of the inorganic filler systems are listedin Table 1. Both systems exhibited a narrow grain size dis-tribution and the main differences between them were fillerparticle size and specific surface area. Fig. 1 contains SEM

images depicting the filler–resin morphology of each compos-ite. The particles were homogeneously dispersed into the resinmatrix in both systems, although the difference in filler sizewas clearly evident. The submicron-sized particles occupied a

micron (A) and submicron (B) composites. The particlestems. The submicron-sized particles occupied a greaterular in shape.

d e n t a l m a t e r i a l s 2 9 ( 2

Fig. 2 – Results for surface roughness and gloss. Nosignificant differences in roughness were observed beforebrushing, although the submicron material exhibitedhigher gloss. After brushing, an increase in surfaceroughness was observed in both composites, as well as adecrease in surface gloss. While the composites hadss

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imilar post-brushing roughness, the gloss of theubmicron material remained significantly higher.

reater amount of composite surface area. Both particle typesere irregular in shape.

Table 2 is a compilation of the physical–chemical propertiesf the composites and the associated statistical significancealues. There was no significant difference in radiopacity. Theubmicron composite underwent significantly less C C con-ersion but had significantly higher hardness. The flexuralehavior was similar in both materials except for a signifi-antly lower flexural modulus in the submicron material. Bothomposites experienced similar levels of bulk creep deforma-ion.

Fig. 2 presents the results for surface roughness andloss analyses. No significant differences in roughness werebserved before brushing (p = 0.573), although the submicronaterial exhibited a significantly higher gloss (p < 0.001). After

rushing, a significant increase in surface roughness was

bserved in both composites (p < 0.004), as well as a significantecrease in surface gloss (p < 0.003). While the composites hadimilar post-brushing roughness (p = 0.896), the gloss of theubmicron material remained significantly higher (p = 0.02).

Table 2 – Means (standard deviations) for the physical–chemica

F

Micron-sized

Mass filler fraction 75%

Radiopacity, Al mm 1.4 (0.1)

Degree of C C conversion, % 66 (6)

Flexural strength, MPa 92 (29)

Flexural modulus, GPa 7.0 (1.2)

Work-of-fracture, kJ/m2 2.0 (0.5)

Knoop hardness, kgf/mm2 46 (1)

Bulk creep deformation, % 0.56 (0.1)

a Student’s t-test, = 5%.

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4. Discussion

Physical tests including elemental composition indicated thatthe micron-sized particles served as an adequate control forthe submicron filler system, with the only major differencebeing the particle size. However, the increased specific sur-face area of the smaller particles increased the amount ofsilane required to coat the glass particles, and the amountof silane used for coating the submicron-sized particles wasapproximately four times greater than the amount used forthe micron-sized particles. Although silanization is impor-tant for promoting chemical coupling between the fillers andthe resin phase, addition of excess silane may result in athicker and less resistant filler–resin interphase. Excess silanemight also interfere with the orientation of the coupling agentmolecules on the particle surface [2,14,22], potentially affect-ing the physical properties of the composite. The additionalsilane required to coat the particles may be considered a short-coming of the submicron filler system, although it is difficultto predict the effect that increased amounts of silane wouldhave on the material properties.

Despite the slightly higher filler concentration used in thesubmicron composite, no significant differences in radiopacitywere observed. This may be explained by the similar elementalcomposition of both filler systems. Taking into considerationthat 1 mm of aluminum has a radiopacity similar to that of1 mm of dentin (data not shown here), both experimental com-posites were more radiopaque than dentin.

The SEM images indicated that the submicron-sized par-ticles occupied a greater amount of surface area within thecomposite than the softer resin phase, resulting in higherhardness for the submicron composite [23]. The increasedcontent of submicron particles and the difference in particlesize could also have increased the amount of light reflectionand scattering inside the bulk of the material, decreasing themonomer conversion. The lower C C conversion of the morehighly filled submicron material could also be due to reducedmobility of the monomers around the filler particles. TheSEM images revealed an irregular particle morphology in both

filler systems, most likely a result of the manufacturing pro-cess used to produce the filler particles involving a top-downapproach [8].

l properties of the composites.

iller system p-Valuea

Submicron-sized

78%

1.8 (0.5) 0.11860 (3) 0.03282 (11) 0.3626.1 (0.5) 0.0331.7 (0.3) 0.31259 (7) 0.0030.55 (0.1) 0.906

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Mechanical tests of flexural properties and work-of-fracture produced similar results for both composites exceptfor flexural modulus. In spite of the greater filler loadingthe submicron composite exhibited a lower modulus, pos-sibly because of the larger amount of silane used to treatthe particles. Previous studies have demonstrated that thedynamic elastic modulus of composites decreases as theamount of silane increases [2,14], most likely due to changesin the viscoelastic behavior of the composites as a result ofpoorer particle–resin interfacial interaction. However, whensubjected to a constant compressive bulk loading both com-posites displayed similar creep deformation, indicating thatthe differences in flexural modulus had a minor effect on themechanical behavior of the materials. For reference, both com-posites met the flexural strength criterion outlined in the ISO4049 [16] standard for dental restorative polymeric materials.

Surface gloss is a visual attribute describing the geometricdistribution of light reflected at the surface of a material. Thedecrease in gloss after brushing is due to increased roughnessand change in surface topography resulting from abrasion ofthe resin matrix and loss of surface filler particles. Althoughthe surface roughness of both composites was similar, the sur-face gloss of the submicron composite remained higher evenafter brushing, most likely because the submicron materialmay experience a more subtle alteration in light reflection dueto the approximately five-fold smaller particle size. This resultis similar to that of a previous study [24] in which a lowergloss reduction following wear testing was observed in a com-mercial composite containing a submicron filler system whencompared to traditional hybrid materials.

Most commercially available dental composites use a com-bination of large and small particles (hybrids) and particlesof dissimilar composition (bimodals) [1]. These formulationsare mainly intended to increase the amount of filler thatmay be incorporated into the resin, consequently reduc-ing polymerization shrinkage and improving the mechanicalproperties. Hybrid composites are considered to be the ‘goldstandard’ materials for the restoration of posterior teeth [25].The incorporation of different filler types differs from theapproach used in the present study, in which a monomodalfiller system with a narrow particle size distribution wasused. The present results indicate that submicron compos-ites may possess physical and mechanical properties similarto composites containing micron-sized particles. Moreover,the higher surface gloss of the submicron composite bothbefore and after abrasion suggests that smaller inorganic fillerparticles may be advantageous in retaining superior estheticproperties following exposure to the oral environment. Theseresults should motivate further studies evaluating the use ofsubmicron-sized glass particles in dental composites. How-ever, the study hypothesis was only partially accepted, as thesubmicron composite exhibited a slightly lower flexural mod-ulus. The mechanical strength of composites based on thesefillers should therefore be addressed in future studies.

5. Conclusion

The monomodal submicron-sized glass filler system demon-strated potential for use in the development of dental

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restorative composites with enhanced esthetic properties rel-ative to composites containing micron-sized particles.

Acknowledgements

This study was supported by CNPq/Brazil (protocol476446/2009-0 and 308404/2011-4). L.L.V. is grateful toCapes/Brazil for a scholarship. Authors thank Esstech Inc. fordonation of the monomers used in the study.

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