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Six commercially available flowable and one conventional composite resins were selected for this study. Their characteristics including manufacturers, shades, main compositions, lot numbers are listed in Table 1. A photograph of the materials used is shown in Figure 1.

Figure 1. 
Six flowable and one conventional composite resins tested in this study.

According to the ISO 4049 specification, a stainless steel split mold was used to prepare flexural property specimens with a dimension of 2 mm × 2 mm × 25 mm. Ten specimens were made for each materials. For light-cured resin specimens, the paste was syringed into the mold, covered by Mylar, and a microscopic slide (1 mm thick) was used to expel extra resin on top of the mold. The specimen was light-cured as specified by the manufacturer, with the curing tip being as close as possible to the Mylar surface, but not touching it, in 5 overlapping sections to ensure maximum conversion on both the top and bottom surfaces, according to ISO 4049. A light-curing unit (Elipar TriLight, 3M ESPE, Seefeld, Germany; standard mode) with 10-mm optic diameter was used. The output intensity of 750 mW/cm² was constantly monitored during the experiment by a built-in radiometer. Specimens were removed from the mold, trimmed, and stored in distilled water at 37℃ for 24 h before testing.

Flexural properties were then tested using the 3-point bending method with a 20-mm span and a cross-head speed of 0.5 mm/min with a universal testing machine (Model 3343, Instron Inc., Canton, MA, USA). Dimensions of the specimens were determined by a digital caliper. The flexural strength (FS) was calculated using the formula (Lu et al. 2005):

σ = (3Fl)/(2bh²)

where σ is the flexural strength (MPa), F is the load at fracture (N), l is the support span (mm), b is the width (mm), and h is the depth (mm) of the specimen.

The flexural modulus (FM) was calculated using the formula (Lu et al. 2005):

E = [(Fl³)/(δ4bh³)] × 10^-3

where E is the flexural modulus (GPa), and F/δ is the slope of the force-displacement curve (N/mm).

To determine fracture toughness, single-edge notch specimens were fabricated. A razor blade notch was fixed in a slit of a 5 × 2.5 × 30 mm split mold. The resulting notch/width ratio of the specimen was controlled in the range of 0.45 to 0.55. After resin was packed into the mold and excess was removed, the specimen was exposed to light-polymerization in five 30-s steps on each side. Specimens were removed from the mold, trimmed, and stored in distilled water at 37℃ for 24 h before testing.

At a crosshead speed of 0.1 mm/min, a 3-point bending test was performed on five specimens per composite in a universal testing machine. The stress intensity factor, K_Q (MPa√m), was obtained from the peak load and specimen configuration (Kim et al. 2002):

K_Q = [(P_Q × S)⁄(B × W^1.5)] × f(a⁄W)
f(a⁄W) = [3(a⁄W)^0.5[1.99−(a⁄W)(1−a⁄W)(2.15−3.93a⁄ W＋2.7a²⁄W²)]]/[2(1＋2a⁄W)(1−a⁄W)^1.5]

where P_Q is the peak load (N), S is the span (m), B is the specimen thickness (m), a is the crack length (m), and W is the specimen width (m). The span length was 2 × 10^–2 m. As defined by ASTM E-399, if KQ satisfies condition B, a ≥ 2.5 (K_Q/σ_ys)², then K_Q may be defined as K_IC (fracture toughness) (Kim et al. 2002).

For roughness and microhardness tests, five specimens per material were collected from the specimens previously subjected to fracture toughness test. The surface roughness was measured using a previously calibrated profilometer (Surftest SV-400, Mitutoyo Corp., Kawasaki, Japan) at a stylus speed of 0.1 mm/s, a cutoff of 0.8 mm, and a range of 600 μm. The R_a of each specimen was recorded as the average of the three readings.

Using a Knoop hardness tester (HMV-2, Shimadzu Corp., Kyoto, Japan), three indentations were made on the top surface of each specimen along a middle line with each indentation separated by approximately 0.5 mm. To make the indentations, a 10-s dwell time and a 25 g load were chosen. The Knoop hardness (KH) of each specimen was recorded as the average of the three readings.

Each test parameter was evaluated with one-way analysis of variance (ANOVA). Tukey’s multiple range test was used for post-hoc analysis (p < 0.05).

The flexural strengths and flexural moduli of the resin-based composites tested in this study are shown in Table 2. Of the materials, the UL material showed the lowest flexural strength, the difference being statistically significant (p < 0.05). As shown in Table 1, the UF and UL materials had similar chemical compositions (including the filler contents) except for the viscosity. Therefore, it was assumed that the UL materials had a low monomer conversion than the LF materials, thereby produced a significantly lower flexural strength value, although the degrees of conversion were not measured in the present study. Although the other materials exhibited similar flexural strength values, the EQ material, which had a relatively low filler content (Table 1), showed significantly lower values than the rests (p < 0.05).

In ISO 4049:2009(E), Type 1 materials indicate polymerbased restorative materials claimed by the manufacturer as suitable for restorations involving occlusal surfaces. According to the criteria, the use of the UL material was not suitable for restorations involving occlusal surfaces. Instead, the material belonged to Type 2, Class 2 (Group 1), in which the requirement of flexural strength is minimum 50 MPa. Therefore, it can be concluded that all the flowable materials can be used in the posterior regions as well as the anterior regions, except for UL.

The UL material also showed the lowest flexural strength among all the materials tested, the difference being statistically significant (p < 0.05). The PE and FZ materials exhibited significantly higher flexural modulus than the other materials (p < 0.05). Between the two materials, the FZ material showed significantly higher flexural modulus than the PE material. Therefore, in terms of flexural modulus, a conventional composite resin is stiffer than flowable materials (Ryou et al. 2008).

In materials science, fracture toughness is a property which describes the ability of a material containing a crack to resist fracture, and is one of the most important properties of any material for many design applications (Abd Wahab et al. 2011). As shown in Table 2, the FZ material showed a significantly higher fracture toughness values than flowable materials (p < 0.05). This finding indicates that flowable composite resins may have weaker fracture resistance than conventional composites (Baroudi & Rodrigues 2015). Therefore, flowable materials should be highly polished without any surface cracks or defects in order to compensate such a drawback (Vichi et al. 2013). In this study, the UL material, which exhibited a low flexural properties, also showed low fracture toughness.

The surface roughness and microhardness values of the resin-based materials tested are also summarized in Table 2. In this study, there were no significant differences in Ra among the materials tested, indicating that the surfaces prepared against Mylar strip had a similar surface topography.

As shown in Table 2, the conventional composite resin FZ showed a significantly higher microhardness value than the flowable materials (p < 0.05). The hardness of a dental restorative material must be sufficient to resist wear from opposing teeth or restorations and not so hard as to wear enamel and other materials such as porcelain (Wataha & Messer 2004). A restorative material with a low is susceptible to wear, and a hardness greater than enamel may wear existing teeth (Wataha & Messer 2004). The flowable materials appear to have too low microhardness for the use as a restorative material involving occlusal surfaces. Therefore, the formulation of flowable materials should be modified to enhance the microhardness and, as a results, wear resistance (Ryou et al. 2008).

The parameters of the materials can be divided into two categories: bulk and surface properties. The flexural properties and fracture toughness belong to the former, and the correlations between the properties were evaluated, using Spear correlation analyses (Kim et al. 2014). As shown in Figure 2, there was no significant correlation between the flexural strength and the modulus (a). Similarly, no significant correlation was found between the flexural strength and fracture toughness. In contrast, a significant positive correlation was found between the flexural modulus and fracture toughness. This finding indicate the fracture toughness of a resin-based materials should be evaluated together with the stiffness or flexibility of the material (Liu et al. 2010); further investigations should be performed to confirm this.

Figure 2. 
Spearman correlation analysis results.