Comparative evaluation of dental resin composites based on micron- and submicron-sized monomodal glass filler particles

Abstract

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 granulometry, 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 compressive creep. The composites were formulated from the same photoactivated dimethacrylate co-monomer, incorporating mass fractions of 75% micron- and 78% submicron-sized particles. 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 in restorative dental composites, particularly due to improved esthetic properties.

Introduction

Dental resin composites consist of a polymeric matrix based on dimethacrylate monomers, inorganic fillers for polymer reinforcement, and a coupling agent (usually an organo-silane) to bond the two phases . Recent efforts to improve the resin phase have focused on reducing polymerization shrinkage , while improvements in the inorganic phase have involved optimization of particle shape and size .

The inorganic phase is usually composed of glass and/or oxide ceramic particles. The mechanical properties, handling, and esthetic characteristics of the composites are strongly influenced by their filler components . It is generally believed that smaller particles provide enhanced resistance to surface wear , which generally occurs through a combination of polymer degradation, loss of resin–filler bonding, and loss of filler particles. Studies have demonstrated that smaller particles may generate materials with increased wear resistance , with potentially increased retention of surface gloss and smoothness.

Dental composites containing nano- and submicron-sized particles were introduced relatively recent in an effort to produce materials with longer-lasting esthetic qualities . Nanocomposites contain fillers with particle sizes below 100 nm, while submicron composites contain fillers with particle sizes below 1 μm and typically near 300 nm . The loss of smaller particles from a surface results in less roughening than the loss of larger particles, producing less variation in the reflection of light following abrasive processes such as brushing.

Several studies have investigated commercial resin composites containing various inorganic filler types and sizes. However, proprietary materials possess differences beyond the characteristics of the fillers that may affect comparisons. The aim of this study was to evaluate an experimental dental composite based on a novel submicron-sized monomodal filler system by comparing it to an otherwise similar micron-sized filler system. The study hypothesis was that composites formulated using submicron-sized particles would exhibit comparable physical–chemical properties in conjunction with improved esthetic qualities.

Materials and methods

The characteristics of the silanized micron (UltraFine UF1.0; Schott, Landshut, Germany) and submicron (NanoFine NF180; Schott) monomodal glass particles are provided in Table 1 . The silanization procedure was carried out by the manufacturer according to the specific surface area of the particles using γ-methacryloxypropyl trimethoxysilane.

Table 1
Properties of the filler particles tested.
Filler system
Micron-sized Submicron-sized
d 50 , nm 1000 ± 200 175 ± 30
d 99 , nm ≤4000 387
Specific area, m 2 /g a 8 40
Refractive index, n D a 1.53 1.53
Density, g/cm 3 a 2.75 2.75
Silane % 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.

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 size distribution and elemental composition of the glass particles.

Preparation of the composites

A dimethacrylate comonomer blend was prepared by mixing 35 wt% of 2,2-bis[4-(2-hydroxy-3-methacryloyloxypropyl)phenyl]-propane, 35 wt% of ethoxylated bisphenol-A glycidyl dimethacrylate with 8 ethylene oxide units, 25 wt% of urethane dimethacrylate, and 5 wt% of triethylenoglycol dimethacrylate (Esstech Inc., Essington, PA, USA). The mixture was rendered photosensitive by adding 0.4 wt% camphorquinone and 0.8 wt% ethyl 4-(dimethylamino)benzoate (Sigma–Aldrich, St. Louis, MO, USA). The experimental composites were prepared by adding either 75 mass% micron-sized or 78 mass% submicron-sized fillers. The filler content in each composite corresponded to the maximum filler load for which the mixture maintained good consistency and handling characteristics similar to commercial composites . The materials were heated to 60 °C, vacuum treated to remove entrained air bubbles, and homogenized using a mechanical mixer. All photoactivation procedures were carried out using a light-emitting diode curing unit (Radii; SDI, Bayswater, Victoria, Australia) with an irradiance of 600 mW/cm 2 .

Filler–resin morphology

Disk-shaped specimens (diameter 8 mm, thickness 2 mm) of each composite ( n = 3) were photopolymerized using 40 s exposures at the top and bottom surfaces. The specimens were embedded in epoxy resin and their surfaces were polished under refrigeration with 600, 1200, 1500, 2000, and 2500-grit SiC abrasive papers followed by 3, 1, 0.25, and 0.1-μm diamond suspensions. The specimens were ultrasonically cleaned, dried and gold-coated prior to scanning electron microscopy (SEM) analysis (SSX-550; Shimadzu).

Radiopacity

Radiographic images of disk-shaped specimens of each composite ( n = 5) were obtained using a digital phosphor plate system (VistaScan; Dürr, Bietigheim-Bissingen, Germany) operating at a voltage of 70 kV and a current of 8 mA. The exposure time was 0.2 s and the focus–film distance was 400 mm. An aluminum step-wedge was included in the radiograph for a density reference . The gray levels (pixel density) of each image were analyzed and the equivalence of aluminum (in mm) was recorded .

Degree of C C conversion

The degree of C C conversion in each composite ( n = 9) was evaluated using Fourier transform mid-infrared spectroscopy (Prestige-21; Shimadzu). A uniform volume of material was used for each sample. A spectrum of the unpolymerized composite (monomer) was acquired using 24 co-added scans at 4 cm −1 resolution. The composite was photoactivated for 40 s and another spectrum was obtained (polymer). The C C conversion (%) was calculated as previously described .

Flexural strength, flexural modulus and work-of-fracture

A total of 10 bar-shaped specimens (25 mm × 2 mm × 2 mm) were photopolymerized using exposures of 120 s at the top and bottom surfaces. The cured specimens were stored in distilled water at 37 °C for 24 h before being subjected to a 3-point bending flexural test on a mechanical testing machine (DL-500; EMIC, São José dos Pinhais, PR, Brazil) at a crosshead speed of 0.5 mm/min. The flexural strength, flexural modulus, and work-of-fracture were obtained from the stress–strain curves .

Hardness

Specimens fractured in the flexural test ( n = 7) were embedded in epoxy resin and polished under refrigeration using 600- and 1200-grit SiC abrasive papers. Five Knoop indentations were made in each specimen using a microhardness tester (FM-700; Future-Tech Corp., Kawasaki, Japan) in which a 50 g load was applied for 15 s. The Knoop hardness number (kgf/mm 2 ) for each specimen was determined from the average of the five readings and calculated as previously described .

Bulk compressive creep

A total of 5 disk-shaped specimens (diameter 8 mm, thickness 4 mm) of each composite were prepared and subjected to bulk compressive testing on a dynamic mechanical testing machine (EletroPuls E3000; Instron, Norwood, MA, USA) in which a constant 36 MPa load was applied for 30 min. The creep was calculated from the axial deformation (%) of each specimen as previously described .

Surface roughness and gloss after brushing abrasion

A total of 5 disk-shaped specimens (diameter 10 mm, thickness 2 mm) of each composite were prepared and polished using medium, fine, and superfine alumina abrasive discs (Sof-Lex system; 3M ESPE, St. Paul, MN, USA). The baseline surface roughness was measured using a profilometer (SJ-201; Mitutoyo, Tokyo, Japan) and the baseline surface gloss was measured with a glossmeter (ZGM1110; Zehntner, Sissach, Switzerland) at a 60° angle. The averages of 5 readings on each specimen were recorded as the surface roughness (Ra, μm) or surface gloss (gloss units, GU). The simulated toothbrushing abrasion test was carried out under a 150-gf load and consisted of 20,000 brushing cycles (4 Hz) using soft-bristled toothbrushes (Dr. Veit Soft; Dr. Veit Produtos Oral Care, Rio de Janeiro, RJ, Brazil) and a 1:2 distilled water/dentifrice mixture. The Ra and GU measurements were repeated following the brushing procedure.

Statistical analysis

Data from the tests described in Sections 2.4–2.8 were separately analyzed using Student’s t -test. Surface roughness and gloss data before and after abrasion were separately analyzed using two-way repeated measures ANOVA (one factor repetition) followed by the Student–Newman–Keuls’ post hoc comparison test. A 5% significance level was used for all analyses.

Materials and methods

The characteristics of the silanized micron (UltraFine UF1.0; Schott, Landshut, Germany) and submicron (NanoFine NF180; Schott) monomodal glass particles are provided in Table 1 . The silanization procedure was carried out by the manufacturer according to the specific surface area of the particles using γ-methacryloxypropyl trimethoxysilane.

Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Comparative evaluation of dental resin composites based on micron- and submicron-sized monomodal glass filler particles
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