To fabricate indirect restorative composites for CAD/CAM applications and evaluate the mechanical properties.
Polymer-infiltrated-ceramic composites were prepared through infiltrating polymer into partially sintered sodium aluminum silicate ceramic blocks and curing. The corresponding samples were fabricated according to standard ISO-4049 using for mechanical properties measurement. The flexural strength and fracture toughness were measured using a mechanical property testing machine. The Vickers hardness and elastic modulus were calculated from the results of nano-indentation. The microstructures were investigated using secondary electron detector. The density of the porous ceramic blocks was obtained through TG-DTA. The conversion degrees were calculated from the results of mid-infrared spectroscopy.
The obtained polymer infiltrated composites have a maximum flexural strength value of 214 ± 6.5 MPa, Vickers hardness of 1.76–2.30 GPa, elastic modulus of 22.63–27.31 GPa, fracture toughness of 1.76–2.35 MPa m 1/2 and brittleness index of 0.75–1.32 μm −1/2 . These results were compared with those of commercial CAD/CAM blocks. Our results suggest that these materials with good mechanical properties are comparable to two commercial CAD/CAM blocks.
The sintering temperature could dramatically influence the mechanical properties.
Restorative composites with superior mechanical properties were produced. These materials mimic the properties of natural dentin and could be a promising candidate for CAD/CAM applications.
Because of the polymerization shrinkage and low mechanical properties of direct dental restorative composites, indirect restorative materials emerged recently as initials for patients and doctors. Especially, over the last few years, indirect restorative materials underwent great revolutions. As the significant developments of indirect restorative materials were made under the backgrounds of computer-aided design and computer-aided manufacturing, these restorative materials were also called CAD/CAM blocks. Indirect restorative materials can be classified into two groups, ceramics and ceramic-polymer composites. The ultimate aim of restorative composites would be to replace the lost part of the tooth. And therefore the restorative materials must mimic the natural tooth in structure and properties. Mechanical properties, aesthetic properties, biocompatibilities and anti-bacterial properties are among the most important performances.
CAD/CAM composites can be obtained simply by polymerizing direct restorative composites filled with inorganic particles. However, the mechanical properties of this kind of composites are often low because of the low inorganic filler loading ensuring the fluidity during restoration. High filler loadings often bring about high mechanical properties. And polymer infiltrated ceramic network (PICN) is an ideal method for high inorganic loadings. PICNs are categorized into phase interpenetrating composites with two continuous phases. Ceramic-metal interpenetrating composites and ceramic-glass interpenetrating composites have been reported before PICN. Different from ceramic-metal and ceramic-glass, interpenetrating composites, PICNs possess dual networks. The dual networks possess the properties of both ceramics and polymers. The ceramic parts provide hardness, strength and wear resistance, while the polymer parts provide flexibility and machinability. Lava Ultimate and Vita Enamic are two representative commercial composite CAD/CAM blocks. While Vita Enamic could be classified as PICN, Lava Ultimate is essentially filled composite with ultrahigh filler loading.
The main aim of this paper was to fabricate a PICN that could best imitate the natural tooth in mechanical properties. However, as mechanical properties of dentin and enamel are of gradient characteristics and are different from samples to samples, the true values are difficult to be determined. A series of literatures are summarized, and some mechanical properties of two parts (dentin and enamel) of human tooth are listed in Table 1 .
|Flexural strength||Elastic modulus||Hardness||Fracture toughness|
|Enamel||60–90 MPa||48–115 GPa||2.7–6.4 GPa||0.52–1.5 MPa m 1/2|
|Dentin||213–280 MPa||8.7-25 GPa||0.12–0.67 GPa||1.8–3.1 MPa m 1/2|
In this paper, the flexural strength, flexural modulus, elastic modulus, hardness, fracture toughness and brittleness index were compared with respect to the sintered temperature of the green body.
Materials and methods
Fabrication of the samples
Sodium aluminum silicate (SIPERNAT 820A, Degussa AG, Germany) was used to fabricate ceramic networks. The partially sintered sodium aluminum silicate blocks were infiltrated with monomer mixtures containing bisphenol-A-dimethacrylate, BisGMA, and triethylene glycol dimethacrylate, TEGDMA (Aladdin Reagents Company, Shanghai, China). TEGDMA was used as a diluent because the viscosity of BisGMA was rather high. The mass ratio of BisGMA and TEGDMA was 50:50. Dibenzoylperoxide, BPO (J & K Scientific LTD.), was used as thermo-initiator. The process involved three steps.
In the first step, partially sintered sodium aluminum silicate blocks with porous structure were prepared. Sodium aluminum silicate powder was mixed with PVA aqueous solution adhesive thoroughly. The concentration was 3 wt%, and the dosage was 1 drop per gram of powder. After pushing the treated powder through a 60-mesh sieve, sodium aluminum silicate block green bodies were fabricated through mold pressing with the pressure of 3 MPa. The pressure holding time was 3 min. The green bodies were treated with isostatic cool pressing at the pressure of 220 MPa for 1.5 min. Then the blocks were sintered at different temperature (from 550 °C to 850 °C). A porous structure of sodium aluminum silicate was obtained through heating rate of 5 °C/min and heat preservation of 1 min in case of forming dense ceramics. The inorganic content of the composites, which reflects the density of the porous ceramic blocks in contrast to dense ceramics was listed in Table 3 .
|550 °C||600 °C||650 °C||700 °C||750 °C||800 °C||850 °C|
|brittleness indices (μm −1/2 )||0.76 ± 0.07||0.84 ± 0.11||0.81 ± 0.05||1.02 ± 0.11||1.14 ± 0.06||0.91 ± 0.10||1.16 ± 0.04|
|p-value between groups||p = 0.07||p = 0.44||p < 0.05||p < 0.05||p < 0.05||p < 0.05|
In the second step, the partially sintered porous blocks were infiltrated with monomers and cured. The polymerizable monomers along with dibenzoylperoxide (1 wt%) were infiltrated into the porous block by vacuum capillary action at a vacuum degree of 0.1 MPa in vacuum drying oven. The staying time in vacuum drying oven was more than 24 h, in order that the porous blocks were infiltrated with monomers thoroughly. Then the samples were cured at 70 °C for 8 h and 110 °C for 8 h respectively. As a control group, pure resin with the same monomer components was cured at the same curing condition. Cured pieces of polymer-infiltrated-ceramic composites and pure resin are illustrated in Fig. 1 . The composites exhibited an opalescent characteristic, similar to the natural color of most teeth. Opalescent restorative materials look natural in any light and are preferred by most patients. Because of the opalescent property, these composites could also be tuned to match the color of patients’ teeth through addition of pigments.
In the third step, testing bars were cut out of the cured blocks (7 groups of composites and a group of cured pure resin) and prepared according to corresponding standards. The cured blocks were sanded to 2.2 mm thick using 2000# water proof abrasive paper. Thereafter, beams with dimensions of 2.2 mm × 2.2 mm × 20 mm (n = 10), 4.2 mm × 2.2 mm × 20 mm (n = 10) and 4.2 mm × 2.2 mm × 10 mm (n = 1) of each material (seven groups of composites with the sintering temperatures of the green bodies from 550 °C to 850 °C and one group of cured pure resin) were cut from the above cured blocks using a diamond saw with rotating speed of 3000 rpm and feeding speed of 6 mm/min.
Flexural property tests
Beams with dimensions of 2.2 mm × 2.2 mm × 20 mm (n = 10) were sanded with 2000# water proof abrasive paper to the final dimension of 2 mm × 2 mm × 20 mm (n = 10) for flexural property testing. The bending bars were chamfered slightly using 2000# water proof abrasive paper to remove the micro-cracks at the edges. Then bending bars were polished with a diamond suspension polishing agent until no visible scratches could be observed on the surface.
The flexural strength was tested in a three-point bending format with a universal test machine (Shimadzu, EZ-100, Japan) according to ISO-4049 and calculated by Eq. (1) :
σ f = 3 F l 2 b d 2
Where F is the maximum load at the point of fracture; l is the distance of the roller span; b is the width and h is the height of the specimen. The support span was 10 mm and the loading speed was 0.5 mm/min.
The elastic modulus was calculated from the results of three-point bending results by Eq. (2) :
E f = ( F m ) l 3 4b d 3
Where L is the distance of the roller span; b is the width and h is the height of the specimen; (F/m) is the slope of the force-displacement curve.
Fracture toughness tests
Beams with dimensions of 4.2 mm × 2.2 mm × 20 mm (n = 10) were sanded with 2000# water proof abrasive paper to the final dimension of 4 mm × 2 mm × 20 mm (n = 10) for fracture toughness testing. The same as flexural strength testing bars, these bars were chamfered and polished. Thereafter, a notch with a length of 2 mm through the width of 4 mm was sawed at the middle of each bar with a high speed cutting machine. The exact length of the notch was measured using an optical microscope with an accuracy of 1 μm. The length of every notch was double measured from two sides and averaged.
Standard single-edge-notched beam (SENB) method was used to measure the fracture toughness (K 1c ). The prepared samples were loaded in a three-point bending format with the support span of 15 mm at a cross-head speed of 0.05 mm/min till failure in a universal test machine (Shimadzu, EZ-100, Japan). The following equation was used to calculate the K 1c ,
K 1c = FL BW 3/2 f ( a W )
In which F is the maximum load at fracture, L is the span of supporting rollers, B is the breadth of the beam, W is the width of the beam, and a is the length of the notch. Geometrical factor f(a/W) is a function of (a/W) which can be calculated by Eq. (4) :
f ( a W ) = 2.9 ( a W ) 1 / 2 − 4.6 ( a W ) 3 / 2 + 21.8 ( a W ) 5 / 2 − 37.6 ( a W ) 7 / 2 + 38.7 ( a W ) 9 / 2
One specimen (4.2 mm × 2.2 mm × 10 mm) of each material were sanded and polished. A nano-indentation tester (MTS, XP, America) was used to characterize elastic modulus (E) and Vickers hardness. The load was exerted on the surface of each sample with a triangular pyramid Berkovich tip. 10 indentation points were chosen randomly. For each point, a series of loads were measured in response to corresponding displacements when exerting force. The depth of indentation was 1000 nm. Elastic modulus (E) and Vickers hardness (HV) were calculated from the results of nanoindentation tests using the Oliver–Pharr method.
Brittleness index calculation
The machinability of dental restorative composites is quantified by the brittleness index, the average values and the standard deviations (SD) of which were calculated according to Eq. (5) and Eq. (6) respectively:
B I = H V K 1 C
σ B I = H V K 1 C ( ∂ ln B I ∂ H V ) 2 ( σ H V ) 2 + ( ∂ ln B I ∂ K 1 C ) 2 ( σ K 1 C ) 2
Where HV is the Vickers hardness, K 1C is the fracture toughness. σ BI , σ HV and σ K1C are standard deviations (SD) of brittleness index, Vickers hardness and fracture toughness.
The fractured and polished samples were platinum coated for scanning electron microscope (SEM) observation. The fractured and polished surfaces were investigated using both secondary electron detector and backscattered electron detector.
The pure resin samples cured at 70 °C and 110 °C for 8 h were tested with uncured resin as control using mid-infrared spectroscopy. The aromatic C-C absorption at 1608 cm −1 in the mid infrared spectroscopy remains constant before and after polymerization and serves as internal standards. The unconverted C C bonds were calculated by comparing intensity of the aliphatic C C absorption at 1637 cm −1 and the aromatic C-C absorption at 1608 cm −1 of polymer and monomer. Degrees of the functional group (double bond) conversion were calculated from the results of mid-infrared spectroscopy using Eq. 7:
% c o n v e r s i o n = 1 − [ a l i p h a t i c ( C = C ) / a r o m a t i c ( C − C ) ] p o l y m e r [ a l i p h a t i c ( C = C ) / a r o m a t i c ( C − C ) ] m o n o m e r