Shear viscosity and ion viscosity of uncured visible light-curing (VLC) resins and resin based composites (RBC) are correlated with respect to the resin composition, temperature and filler content to check where Dielectric Analysis (DEA) investigations of VLC RBC generate similar results as viscosity measurements.
Mixtures of bisphenol A glycidyl methacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) as well as the pure resins were investigated and compared with two commercial VLC dental resins and RBCs (VOCO, Arabesk Top and Grandio). Shear viscosity data was obtained using a Haake Mars III, Thermo Scientific. Ion viscosity measurements performed by a dielectric cure analyzer (DEA 231/1 Epsilon with Mini IDEX-Sensor, Netzsch-Gerätebau).
Shear viscosity depends reciprocally on the mobility of molecules, whereas the ion viscosity also depends on the ion concentration as it is affected by both ion concentration and mixture viscosity. Except of pure TEGDMA, shear and ion viscosities depend on the resin composition qualitatively in a similar manner. Furthermore, shear and ion viscosities of the commercial VLC dental resins and composites exhibited the same temperature dependency regardless of filler content. Application of typical rheological models (Kitano and Quemada) revealed that ion viscosity measurements can be described with respect to filler contents of up to 30 vol.%.
Rheological behavior of a VLC RBC can be characterized by DEA under the condition that the ion concentration is kept constant. Both methods address the same physical phenomenon – motion of molecules. The proposed relations allows for calculating the viscosity of any Bis-GMA-TEGDMA mixture on the base of the viscosities of the pure components. This study demonstrated the applicability of DEA investigations of VLC RBCs with respect to quality assurance purposes.
Visible light curing resin based composites (VLC RBC) for esthetic tooth-colored dental restorations have successfully replaced amalgam fillings or gold inlays . They consist of an organic light-curable resin matrix, inorganic solid filler powder having micro and/or nano size and functional additives . The flow properties of pure VLC resins, semi-filled so called flowables and highly filled composites play an important role in clinical handling as well as manufacturing .
First, the differences in a monomer mobility, and thus viscosity, affect strongly the curing reaction of the pure resin. Viscosity of visible light curing (VLC) monomer resins varies from 0.01 up to 800 Pa s at room temperature, and is typically Newtonian .
Second, besides resin composition the filler content, its particle size, size distribution, shape and tendency to agglomeration affect the flow characteristics of composites. VLC dental composites are reported to behave pseudo-plastic and/or thixotropic . Furthermore, close to the maximum filler content the viscosity increases dramatically and the filler particles will not be covered completely with resin leading to unfavorable kneading properties, and thus undesired gap formation at the interface of the tooth restoration causing secondary caries.
Despite of a certain viscosity required by clinicians in order to have specific shaping and manipulation properties of the composite , quality, biocompatibility and longevity of composite restorations are also governed by the curing behavior, e.g. curing kinetics, degree of cure (DC), depth of cure (DoC), curing shrinkage and relaxation properties . While DoC, shrinkage and mechanical properties of composites are strongly affected by the filler shape, size, size distribution and content, the curing kinetics, DC, relaxation and post-curing properties are governed mainly by the VLC resin system itself (base and diluent monomers, initiators, accelerators and daylight stabilizers) .
Rheometry is a widely used testing method to investigate both the flow properties during processing of the uncured composites as well as their curing properties . The curing process is mainly investigated in an oscillatory mode . However, it is difficult to design the experiments similar to the dental application process concerning sample mass, layer thickness and illumination intensity. Furthermore, after gelation the dynamic (oscillatory) tests soon reach their resolution limit .
A lot of effort has been spent to fit rheological data of particle filled composites with models intercepting various parameters (especially filler content) . Nevertheless, examination of validity of these models on experimental data for highly filled composites revealed surprisingly ambiguous results. Contrary to the fact that several models fitted relatively well the flow data for lower filler contents, for higher filler contents irregular filler particle shape, agglomeration and size distribution are highly affecting the flow behavior . The predicted values of the maximum volume fraction for the same powder depend on the model employed .
Dielectric Analysis (DEA) is another method to gain information about the flow and curing properties of resins and composites. It is widely used in composites manufacturing in automotive and aircraft industry , but a DEA setup allowing for analyzing the light-curing process of dental composites at high time resolution has been introduced quite recently . Further, the DEA method is advantageous in monitoring the polymerization of VLC composites even after the glass transition .
This study correlates and compares DEA and rheological data of various uncured VCL dental resins and composites differing in their filler content. The aim is to investigate the influence of temperature, resin composition and filler content, and thus only the measurements of initial ion viscosities and shear viscosities prior to curing are presented and discussed.
Materials and methods
Experimental mixtures of bisphenol A glycidyl methacrylate (Bis-GMA) and triethylene glycol dimethacrylate (TEGDMA) in ratios 0:100, 20:80, 40:60, 60:40, 80:20 in wt.% were supplied by Voco GmbH, Cuxhaven, Germany, and investigated prior to curing using both DEA and rheometry at clinically relevant temperatures of 23 °C (extraoral material preparation) and 36 °C (body temperature). The experimental mixtures contained small amounts of storage additives (Bis-GMA contained <1 wt.% Hydochinonmonomethylether and <0.1 wt.% Hydrochinone, TEGDMA contained 0.008–0.012 wt.% Hydochinonmonomethylether), light sensitive initiator system consisting of camphorquinone (CQ, 0.3 wt.%) and ethyl 4-(dimethylamino)benzoate (DABE, 0.5 wt.%) and stabilizer butylated hydroxytoluene (BHT, 0.1 wt.%).
Two commercial VLC dental resins (supplied by Voco GmbH, Cuxhaven, Germany) Arabesk Top (shade A1, Batch no. 1307385) and Grandio (shade I, Batch no. 1316541) were also investigated prior to curing during DEA and rheometry at clinically relevant temperatures of 23 and 36 °C. To account for the effects of the filler content on the ion viscosity, various filler contents were realized by diluting the commercial composites in their corresponding resins (also supplied by VOCO), Table 1 . The two components were stirred until a homogeneous paste was achieved. The stirring process was repeated prior to every measurement to compensate for sedimentation processes. The Arabesk Top resin consists of a monomer mixture of Bis-GMA, UDMA and TEGDMA and is polymerized by a CQ/DABE system. The Grandio resin consists of a monomer mixture of Bis-GMA and TEGDMA and is also polymerized by a CQ/DABE system.
|Material||Filler volume fraction v F|
|Arabesk Top (Bis-GMA + UDMA + TEGDMA)||0.60|
|Grandio (Bis-GMA + TEGDMA)||0.71|
To account for the effect of temperature on the ion viscosity all commercial VLC dental resins and composites were investigated at 20, 30, 40 and 50 °C. Rheometer measurements were performed in continuous rotation in the temperature range from 20 to 50 °C. The filler weight contents were determined by ashing (5 h at 600 °C in air), Table 1 . The volume fraction v F of the diluted commercial composites was calculated using the density of the resin mixture ρ resin of 1.1 g/cm 3 and the density of the filler particles ρ filler of 2.7 g/cm 3 . Due to the fact that the fillers of commercial VLC RBCs are compatibilized with silanes (information provided by Voco) the measured filler content was corrected for the organic components lost during ashing.
The ion viscosity of VLC RBC depends strongly on the DC, and changes several orders of magnitude during the curing process, Fig. 1 . Uncured VLC RBC has a characteristic initial ion viscosity ηion0
η 0 i o n
. After irradiation one observes a small decrease of the ion viscosity due to the introduction of heat by the curing light as well as due to the generation of ion molecule radicals . This is very quickly antagonized by the polymerization reaction leading to a significantly higher final ion viscosity ηion∞
η ∞ i o n
Ion viscosity measurements were performed using a dielectric cure analyzer (DEA 231/1 Epsilon, Netzsch-Gerätebau, Selb, Germany) at a frequency of 1 kHz. The DEA-sensor (Mini IDEX, Netzsch-Gerätebau) consisting of two comb-shaped electrodes (0.1 mm distance, (5 × 7) mm 2 sensing area) on an insulating polyimide film was kept at the desired testing temperature by the Peltier element of a rotational rheometer (AR1000, TA Instruments). The experimental setup is shown in Fig. 2 . For details concerning the measurement principle see Zahouily et al. .
The VLC RBC is placed on the sensor surface and pressed with a microscope glass slide to a thickness of 1 mm. After 20 s the initial ion viscosity has stabilized and is determined as a mean of 10 data points over 3 s. Each measurement was repeated 5 times ( n = 5).
The rheological measurements were performed using a rotational rheometer (Haake MARS III, Thermo Fisher Scientific, Waltham, Massachusetts, USA) with a cone-plate geometry (cone: C35/2°) in the steady-shear mode. The experimental resins were investigated at 23 and 36 °C in a shear rate range from 1 to 100 s −1 . Each viscosity data point is an average of 3 measurements ( n = 3).
Furthermore, the temperature dependency of the initial shear viscosity η 0 of Arabesk Top and Grandio pure resins and their composites with 15 vol.% filler content were investigated in the continuous shear mode between 20 and 50 °C until a constant viscosity was reached to affirm overcoming thixotropic effects. This method was not applicable for filler contents exceeding 15 vol.% due to melt fracture effects. The experiments were performed for the shear rates (1, 10, 50 and 100 s −1 ).
Evaluation of initial ion viscosity with respect to filler content
According to Zahouily the ion viscosity η ion depends reciprocally on ion mobility μ ion , ion charge q ion and ion concentration c ion :
The initial ion viscosity of the resin ηion0,resin
η 0 , resin i o n
is orders of magnitude lower than that of the glass filler ηion0,F
η 0 , F i o n
. According to Lee the ion mobility in solid glass fillers is close to zero. Thus, the initial ion viscosity of a resin composite ηion0,composite
η 0 , composite i o n
is mainly changed by the decrease of ion concentration with reduced pure resin content. Assuming that an increase of the filler content reduces only the overall ion concentration, the effect of resin and filler content on the DEA measurement can be modeled as a parallel capacitor arrangement of the pure resin having low ion viscosity ηion0,resin
η 0 , resin i o n
and the filler containing almost no mobile ions, and thus high ion viscosity ηion0,F
η 0 , F i o n
, Fig. 3 . Then, the ion viscosity of a composite can be calculated with respect to the volume filler fraction v F using the inverse mixing rule:
1 η 0 , composite i o n = 1 η 0 i o n ( v F ) = 1 − v F η 0 , resin i o n + v F η 0 , F i o n