The purpose of this work was the detailed study of sorption characteristics of water or artificial saliva, the determination of flexural strength and the flexural modulus, and the study of the thermal stability of some current commercial dental light-cured nanocomposites containing nano-sized filler particles.
Three nanohydrid dental composites (Tetric EvoCeram (TEC), Grandio (GR) and Protofill-nano (PR)) and two nanofill composites (Filtek Supreme Body (FSB) and the Filtek Supreme Translucent (FST)) were used in this work. The volumetric shrinkage due to polymerization was first determined. Also the sorption, solubility and volumetric increase were measured after storage of composites in water or artificial saliva for 30 days. The flexural strength and flexural modulus were measured using a three-point bending set-up according to the ISO-4049 specification, after immersion of samples in water or artificial saliva for 1 day or 30 days. Thermal analysis technique TGA method was used to investigate the thermal stability of composites.
GR and TEC composites showed statistically no difference in volumetric shrinkage (%) which is lower than the other composites, which follow the order PR < FSB < FST. The amount of sorbed water and solubility is not statistically different than those in artificial saliva. In all the composites studied the amount of water, which is sorbed (% on composite) is not statistically different than the amount of water, which is desorbed and follows the order: GR < TEC < PR < FSB < FST. After immersion in water for 1 day the highest flexural strength showed the FSB and the lowest TEC. GR, PR and FST showed no statistically different flexural strength. The flexural modulus of composites after immersion for one day follows the order TEC < PR≤FST < FSB < GR.
Among the composites studied, Grandio had the lowest polymer matrix content, consisting mainly of Bis-GMA. It showed the lowest polymerization shrinkage and water sorption and the highest flexural strength and flexural modulus after immersion in water or artificial saliva for 30 days. The water and artificial saliva generally showed the same effect on physical properties of the studied composites. Thermogravimetric analysis gave good information about the structure and the amount of organic polymer matrix of composites.
The term “nanotechnology” has evolved over the years via terminology drift to mean “anything smaller than microtechnology” such as nano powders and other things that are nanoscale in size, but not referring to mechanisms that have been purposefully built from nanoscale components. This evolved version of the term is more properly labeled “nanoscale bulk technology” while the original meaning is more properly labeled “molecular nanotechnology .
The most traditional dental composites for restorative purposes are hybrid and microfill types. Hybrids offer intermediate esthetic properties but excellent mechanical properties by the incorporation of fillers with different average particle sizes (15–20 μm and 0.01–0.05 μm). Microfill composites were launched in the market to overcome the problems of poor esthetic properties. These materials are usually formulated with colloidal silica (around 50% in volume) with an average particle size of 0.02 μm and a range of 0.01–0.05 μm. Unfortunately the mechanical properties are considered low for application in regions of high occlusal force .
Based on the definition “nanoscale bulk technology” new classes of dental composites, so-called nanocomposites, have been developed and marketed during recent years. Nanocomposites are claimed to combine the good mechanical strength of the hybrids and the superior polish of the microfills . Other positive features reported are high wear resistance improved optical characteristics and reduced polymerization shrinkage .
Nanocomposites are available as nanohybrid types containing milled glass fillers and discrete nanoparticles (40–50 nm) and as nanofill types, containing both nano-sized filler particles, called nanomers and agglomerations of these particles described as “nanoclusters” . The nanoclusters provide a distinct reinforcing mechanism compared with the microfill or nanohybrid systems resulting in significant improvements to the strength and reliability .
This work is concerned first with the determination of the volumetric shrinkage after polymerization of three current commercial nanohybrids – Tetric EvoCeram (TEC), Grandio (GR) and Protofill-nano (PR) – and two nanofill composites – Filtek Supreme Body (FSB) and Filtek Supreme Translucent (FST). The composites were then immersed in water or artificial saliva 37 °C for 30 days for the determination of the sorbed liquid and volume increase. After that the composites were put into a desiccator at 37 °C for 30-days and desorption of the absorbed water or artificial saliva was determined. Based on this experimental data the amount of unreacted monomers extracted by water or artificial saliva during immersion for 30 days, known as “solubility” of the composites in these liquids, was calculated.
In an aqueous oral environment, polymer composites absorb water and release unreacted monomers. The release of unpolymerized monomers from polymer composites may stimulate the growth of bacteria around the restoration and promote allergic reactions in some patients. Also the water ingress into dental composites in the oral cavity can, over time, lead to deterioration of the physical/mechanical properties due to hydrolytic breakdown of the bond between the silane-filler particles, filler matrix debonding or even hydrolytic degradation of the fillers. However, some water ingress may have a positive side effect, such as the expansion of the composite compensating for polymerization shrinkage leading to improved marginal sealing. Thus the solvent uptake by dental composite is generally a very important property which must be investigated. In the authors’ previous work the sorption of water or ethanol/water solution or ethanol by light-cured dental resins and commercial polymer composites, was studied. Also the sorption kinetics of ethanol/water solution by dental resins and composites, was investigated .
In the present study the flexural strength and modulus were also determined, using a three-point bending set-up according to the ISO-4049 specification, after immersion of samples in water or artificial saliva for 1 day or 30 days. The weight changes of the above composites were also measured as a function of temperature by Thermogravimetric Analysis (TGA). TGA is a technique in which the mass of the sample is monitored as a function of temperature, while the sample is subjected to a controlled program. TGA has been used for the study of the thermal stability of dental composites .
Materials and methods
Five commercially available dental light-cured composites were studied; Tetric EvoCeram (TEC; Ivoclar-Vivadent, Schaan, Liechtenstein), Grandio (GR; VOCO, Cuxhaven, Germany), Protofill-nano (PR; Germany), Filtek Supreme Body (FSB; 3M-ESPE, St. Paul, MN, USA) and Filtek Supreme Translucent (FST; 3M-ESPE, St. Paul, MN, USA). Their specifications are listed in Table 1 .
|Composite||Classification||Lot no.||Shade||Matrix||Filler||Total filler content|
|Tetric EvoCeram ( TEC )||Nanohybrid||K34042||A1||Bis-GMA, UDMA||Barium glass, ytterbium trifluoride, mixed oxide and prepolymer; 40–3000 nm, 550 nm||82–83 wt%||82.5 vol%|
|Grandio ( GR )||Nanohybrid||780610||A2||Bis-GMA, TEGDMA||Silica: 20–60 nm; barium-aluminaborosilicate: 0.1–2.5 μm||87.0 wt%||71.4 vol%|
|Protofill-nano ( PR )||Nanohybrid||8W603A||A1||Bis-GMA, TEGDMA, UDMA||Strontium aluminum borosilicate: 0.6 μm; nanoparticles 20 nm||81.9%|
|Filtek Supreme Body ( FSB )||Nanofill||5AM||A2D||Bis-GMA, TEGDMA, UDMA, Bis-EMA||Silica: 5–20 nm nanoparticle (8 wt%); zirconia/silica: 0.6–1.4 μm nanocluster (71 wt%)||79 wt%||59.5 vol%|
|Filtek Supreme Translucent ( FST )||Nanofill||7BN||GT||Bis-GMA, TEGDMA, UDMA, Bis-EMA||Silica: 75 nm nanoparticle (40 wt%); silica: 0.6–1.4 μm nanocluster (30 wt%)||70 wt%||57.5 vol%|
The SAGF medium used as artificial saliva in this work and its composition is given in Table 2 . The pH of the SAGF medium was adjusted to 6.8, because this value was closer to that of saliva in the mouth after its emission from the canals. The artificial saliva and the samples were sterilized together (0.5 atm/120 °C/20 min) to avoid colonization of microorganisms. The use of the SAGF medium required special care, because the solution was supersaturated in carbon dioxide with regard to the air. As a result it tended to lose CO 2 gas, which led to an increase in pH, so before each experiment, the pH of solution was controlled.
|Components||Concentration (mg l −1 )|
|KH 2 PO 4||654.5|
|NaSO 4 .10H 2 O||763.2|
|NH 4 Cl||178.0|
|CaCl 2 .2H 2 O||227.8|
The volumetric shrinkage was measured based on Archimedes’ Principle, as described in Refs. . The densities of uncured composites were first measured, using a Mettler-Toledo AG64 balance. From each test material, uncured sphere-shaped specimens were carefully formed in such a way that trapped air bubbles were avoided. Since the uncured materials were rather sticky, a thin polyester film (thickness 0.05 mm) was fixed on the special holder of the balance and its mass measured in air and in water. Next, the respective material samples were carefully placed on the polyester film and the mass of the whole assembly was measured again in air and in water. Slight deformations of the materials during the test were of no importance since they do not influence density. The mass of each material was calculated by subtracting the mass of the polyester film from the mass of the whole assembly. Now the density of the uncured material ( ρ uncured ) was computed. The volumetric shrinkage (Δ V ) was calculated using the following equation:
where ρ uncured is the density of the uncured composite and ρ cured ( ρ d ) is the density of the cured composite.
Sorption and desorption of water or artificial saliva – solubility – volumetric change
Sorption and solubility tests were determined according to the method described in ANSI/ADA Specification No. 27-1993 (ISO 4049) regarding filling materials. Specimen discs were prepared by filling a Teflon mold (15 mm in diameter and 1 mm in thickness) with the unpolymerized material. Samples were irradiated for 60 s on each side, using the XL3000 (3M-ESPE, St. Paul, MN, USA) dental photocuring source. The unit was used without the light guide in contact with the sample. Four specimen discs were prepared for each composite material.
The percentage amount of water or artificial saliva sorbed ( WS (%) or ASS (%)) and desorbed ( WD (%) or ASD (%)), the solubility ( SL (%)) of these liquids, the volumetric change (VI (%)) were determined according to the method described in detail in our previous works .
All the specimens were placed in a desiccator and transferred to a pre-conditioning oven at 37 °C. After 24 h they were removed, stored in the desiccator for 1 h and weighed to an accuracy of ±0.00001 g using a Mettler H54AR balance. This cycle was repeated until a constant mass ( m i ) was obtained.
The densities of all samples were measured in dry ( ρ d ) or saturated conditions ( ρ s ) using a Mettler-Toledo AG64 balance and they were calculated based on Archimedes’ principle.
Subsequently, the discs were immersed in water or artificial saliva at 37 ± 1 °C. At fixed time intervals they were removed, blotted dry to remove excess liquid, weighed and returned to the liquid. The uptake of the liquid was recorded for 30 days. The percentage weight increase in specimens, WI (%), was calculated using the following formula:
where m s represents the weight of the saturated specimen after 30 days of immersion. This is an apparent value for the liquid sorbed, because unreacted monomer is simultaneously extracted resulting in a decrease of specimen weight.
For the determination of monomer extracted, the samples were transferred to a drying oven maintained at 37 °C and a similar process to that of sorption repeated during desorption. The percentage amount of water or artificial saliva desorbed from specimens, WD (%) or ASD (%), was calculated using the following formula: