This investigation evaluated the flexural properties of two composite resins, and the influence of unidirectional glass fiber reinforcements, with and without pre-tensioning.
Two composite resins (Q: Quixfil and A: Adoro) were used to fabricate 2 mm × 2 mm × 25 mm beams ( N = 10), reinforced with two fiber bundles along the long axis of the beam and pre-tensioned under a load equivalent to 73.5% of its tensile strength (groups QPF and APF). In two other experimental groups, the bundles were similarly positioned but without pre-tension (groups QF and AF). Two more groups were included without fiber reinforcement (control groups Q and A). After 24 h storage, specimens were subjected to a three-point flexural bending test to establish the flexural module, the deflection at initial failure and the flexural strength. Data were analyzed using a two-way analysis of variance (composite resin system and fiber reinforcement type) and the Tukey HSD post hoc tests ( α = .05).
The results showed that prestressing increased the flexural module of Adoro specimens ( p < .001) but not Quixfil ( p = .17). Prestressed beams reached greater deflection at initial failure than those conventionally reinforced ( p < .001), namely .85–1.35 mm for Adoro and .66–.90 mm for Quixfil. Prestressing also significantly increased the flexural strength of beams ( p < .001) in both Adoro and Quixfil groups, from 443.46 to 569.15 MPa and from 425.47 to 568.00 MPa, respectively.
Pre-tensioning of unidirectional glass fibers increased both deflection until initial failure and flexural strength of Quixfil and Adoro composite resins, however, with limited effects on the flexural modulus.
Nowadays, most advanced applications require materials with unusual blends of properties that are not reachable by a single material alone. Aeronautical engineering for instance, looks for structural materials with a unique combination of properties including low density, strength, rigidity and wear/impact resistance. Composites are distinctive and complex materials able to fulfill the aforementioned requirements. They are defined as multiphase materials presenting a significant proportion of the properties of its various phases in order to achieve a better combination of properties, which is also called “Principle of Combined Action” . Technically, the composites that better fit this model are those reinforced with fibers (fiber-reinforced composite or FRC).
“ And Pharaoh commanded the same day the taskmasters of the people, and their officers, saying, Ye shall no more give the people straw to make brick, as heretofore: let them go and gather straw for themselves ”. One of the most ancient use of fiber as a reinforcement is reported in the book of Exodus of the Holy Bible, when Israelites used straw to reinforce their bricks made of clay almost 4000 years ago. In dentistry, FRCs have been used increasingly since the nineties . Some brands of FRC posts have produced promising results in laboratory and clinical studies . The elastic modulus of FRC endodontic posts is similar to that of dentin unlike existing metal posts. In a similar way, FPDs substructures made of FRCs present a distinctive combination of resilience and resistance, unlike other conventional isotropic materials. This blend of properties enables load transfer to the dental structure in a more physiological way, thus preventing the development of harmful stresses on the cement or adhesive interface .
In many aspects, use of FRCs in clinical practice compares to concrete in civil engineering. Concrete in its simplest form, resists compressive stresses successfully, but not tensile ones (approximate compressive/tensile strength ratio of 10:1). In order to solve this problem, steel frameworks are aligned in the direction of the internal tensile forces, resulting in armed concrete . Alike concrete, composite resins resist compressive forces to a much greater degree than tensile ones and fibers strategically positioned into the resin lead to appropriate resistance to tensile stress (principle of armed concrete).
When subjected to three- or four-point bending, any beam will show compressive stresses on the loaded side and harmful tensile stresses at the opposite side ( Fig. 1 A ) . The problematic compressive/tensile strength ratio of concrete called for the development of the prestressing concept in which a prior stress stage is applied to the structure to optimize its strength and performance. Pre-tensioning the steel framework induces a compressive stress of the matrix phase that opposes the development of tensile forces at the opposite side of load and makes the material more efficient ( Fig. 1 B–D).
In concrete, application of a precompressed state is obtained by pre-tensioning high-strength steel cables placed inside the molds and pulled with very high tensile force before casting the matrix phase. Once the concrete has reached the desired compressive strength (which should always exceed the pre-tensioning stress), the tension is released and the cables tend to recover their original length. Because of the adhesion between cables and concrete, the structure is subjected to compression . The same process can be applied with glass fibers and composite resin to produce reinforced composite resins ( Fig. 2 ).
The purpose of this study is to evaluate whether the established advantages of prestressing in civil engineering and superstructures can be translated to the fabrication small parts to be used in operative dentistry and prosthodontics. Therefore, this study assessed the influence of prestressing on the flexural properties of direct and indirect light-polymerized composite resins. The hypothesis was that inclusion of unidirectional glass fibers (whether pre-tensioned or not) would not influence the flexural properties of the composites resins.
Materials and methods
This study was conducted in two steps. First, basic physical properties of the glass fibers and composite resins were obtained to be used for optimal specimen designing and fabrication ( Table 1 ), including determination of the optimal fiber pre-tension load. The second step consisted of specimen production and load-to-failure in three-point flexure.
|Fiber bundle (S glass)|
|Filament diameter (μm)||9.6 a|
|Number of filaments ( n )||3468 a|
|Cross-section area (mm 2 )||.25 a|
|Linear density (TEX = g/km)||684 b|
|Ultimate tensile strength (MPa)||1344 c|
|Ultimate tensile load (N)||337.5 c|
|Longitudinal Young’s modulus (GPa)||38.5 c|
|Compressive strength (MPa)||266 d||353 d|
|Modulus of elasticity (GPa)||2.4 d||1.7 d|
|Shear bond strength between fiber and resin (MPa)||112.6 e||72.8 e|
Physical properties of the glass fiber and composite resins
First, the detailed mechanical properties of the glass fibers were determined. It is critical that the compressive strength of the composite resin be sufficient to resist the compressive forces generated by the pre-tensioned fibers . Hence, the direct composite (Quixfil U; Dentsply, Germany) and the indirect one (Adoro; Ivoclar, Liechtenstein) were submitted to compressive testing in order to determine their compressive strength and elastic modulus. Finally, the interfacial adhesion between fiber and resin was obtained by a pull-out test. This property must exceed the shear stress generated by the pre-tensioning in order to guarantee the load transfer from the fiber to the resin without loss. All predetermined values are presented in Table 1 along with the test method.
Specimen fabrication and testing
The specimens (2 mm × 2 mm × 25 mm) were fabricated and tested in conformity to ISO 10477 standards. The beams were distributed in six groups according to the composite material and type of reinforcement ( Table 2 ).
|Group||Composite||Pre-tensioning force (N per cable)||Impregnation resin||Reinforcement type||N|
|AF||Adoro||–||Vectris Glue||Glass fiber||10|
|APF||Adoro||248||Vectris Glue||Pre-tensioned glass fiber||10|
|QF||Quixfil||–||Filtek Z 350 flow||Glass fibers||10|
|QPF||Quixfil||248||Filtek Z 350 flow||Pre-tensioned glass fiber||10|
First, the glass fiber bundles were cut to a length of 105 cm. A segment of approximately 6 cm at the middle of the bundle was used for the bonding procedures. The fibers were then treated with a preactivated silane coupling agent (Angelus, Brazil) by dripping. They were hot dried at 78 °C for 30 min and dropped in a low viscosity resin (Vectris Glue; Ivoclar Vivadent, Liechtenstein or Filtek Z 350 flow; 3M, USA) for 24 h to ensure adequate impregnation . Excesses of impregnation resin were removed from the bundles using a spatula before assembling the specimens.
Second, a special device (prestressing bed) was developed to fabricate the prestressed specimens through the pre-tensioning procedure ( Fig. 3 A ). Two fiber bundles were gripped by winding them around capstans at each end of the prestressing bed ( Fig. 3 B). Weights were used to apply the desirable tensile force ( Fig. 3 C).
The fiber bundles were laid on a glass slide and separated by two 1 mm-diameter pins with the purpose of standardizing their position. The composite resin was preheated at 50 °C to decrease viscosity and facilitate placement ( Fig. 3 D). An initial tension of about 215 N was applied to the fibers. The shape of the composite resin was maintained by a stainless steel plate on each side of the specimen and a piece of polyester film (Mylar strip) compressed by another glass slide ( Fig. 3 E).
Before polymerizing the resin, the pre-tension of the two fiber bundles was increased to a total load of 496 N corresponding to 73.5% of the ultimate tensile strength of both bundles . At this load, the elongation of the bundles was approximately 1.2 mm per 100 mm (1.2%). The resin was then light polymerized at 800 mW/cm 2 (Optilux 501, Kerr, USA) at both the top and bottom of the beams. For Quixfil (QPF), 10 light curing cycles of 60 s each were necessary to cover the entire length of the sample (5 cycles on each side). With Adoro (APF), the specimens were additionally heated at 78 °C using a hair dryer while light polymerizing for a total of 25 min (25 cycles) to simulate heat curing oven. Following a delay of 10 min to allow the resin to reach a minimum compressive strength of about 266 MPa for QPF and 353 MPa for APF (see Table 1 ) , the weights were carefully removed from the device, generating an estimated compression prestressing of 145 MPa via the resin–fiber bond.
The AF and QF groups were constructed using the same prestressing device, however, without pre-tensioning. The bundles were initially tensioned with a load of approximately 215 N to maintain the position of the fiber bundles as in the APF and QPF groups. Once the fiber bundles were aligned and the composite resin molded by the steel plates and glass slides, the load was completely released and the composite was light polymerized as for the PF groups. In addition, two control groups (A and Q) were fabricated using the basic procedures but without any fiber reinforcement.
The final shape of each specimen was obtained by cutting the extremities with a diamond disc (Komet, Lemgo, Germany) and finishing with SofLex discs (3M ESPE, St Paul, MN, USA). Exact dimensions were obtained by careful polishing of all surfaces with sandpaper (#1200, Carborundum, Brazil) ( Fig. 4 ). The dimensions of each specimen were measured with a caliper (accuracy of .01 mm) and used to calculate the flexural properties. The specimens were also weighed in an analytical balance (accuracy of .01 mg) (FA-2104N, Bioprecisa, Brazil).
The specimens were stored at 100% humidity for 24 h before load-to-failure with a three-point flexural test in accordance to the ISO 10477:1992(E). The load was applied at a cross-head speed of .7 mm/min by a universal testing machine (Model 4444, Instron, Canton, USA). Load/displacement curves were generated for each specimen. The flexural test allowed determining the flexural modulus, the deflection at the initial failure of the composite resin phase and the flexural strength. The flexural modulus E , in MPa, was determined at the non-destructive interval of the load/deflection curve using the following equation: