It was known that fillers play an important role in restorative composite filling. In the present work new composite resins were synthesized including M1-M10 and N1-N4 with various constituents’ ratios of glass fillers. Nano CaF₂ particles were replaced the normal sized powders studied in our previous work. Physical characterization of glass fillers the Nano compo glass filling including shrinkage strain, depth of care, degree of conversion and thermal expansion coefficient were studied and showed an increasing of the above properties, thermal expansion was found to varies between 2.28-36.4 .Water absorption and water solubility decreased in comparison with standard values. Mechanical properties of the compo glass studied which include flexural strength was found around 95-135 MPa while the diametric strength (DTS) showed a value of 46-60 MPa especially on using 10% Nano CaF₂

Keywords: compo glass, dental filling, Nano Calcium Fluoride

The influence of Nano particle and surface area are important on the properties of the Nano composites. The interphase development and the resulting Nano composite properties have been shown to be greatly affected by Nano particle such as size, shape, aspect ratio, concentration and surface chemistry. Many experimental studies have been performed to understand the employment of particle morphology (size, shape, and aspect ratio) on Nano composite properties. Mechanical properties, such as strength and modulus, have generally increased with decreasing particle size in polymer Nano composites,¹ and direct comparisons between micron-scale and Nano scale particles showed improved mechanical properties in Nano composites when compared with the same filler loading.² Much work has focused upon varying the glass components to reduce shrinkage and improve the mechanical properties and also to prevent bacterial microleakage.³ Shahin et al.,⁴ have used Nano silver within filler to increase the physical property of composite as well as anti bacterial action. Nano gold and Nanosilver composites exhibited higher micro hardness in comparison to non-modified resin composites higher Diametric Tensile Strength.⁵ Many researchers have studied composite compositions on their properties .These effects includes Filler effects,⁶ effects of coupling agent used⁷ and monomer effects⁸ TiO₂ has been used as filler to improve mechanical properties of composites⁹ which showed improvements of micro hardness and flexural strength. Monika et al.,¹⁰ have investigate the mechanical properties on addition 0.5-5%wt CaF₂ of composite and found that addition of 0.5-1.0% will increase the tensile strength. In our study we used Nano CaF₂ to improve the mechanical properties of the composite resin and the depth of curve while the micro leakage of fluoride ion as Caries-prevention Fillers will be our next work.

All glass components were Annular, CaF2 preparation and characterization was presented in our previously published work.^11,12

Compoglass synthesis

The glass containing six component (SiO₂, Al₂O₃, CaF₂, AlPO₄, AlF₃, and NaF). The weight ratio between Al₂O₃/SiO₂ change between (0.516 to 0.86). Two method of preparation were used in glass synthesis, the first group A: slowly cooling (N1, N2, N3, and N4). The second group B: melt–quench route (M1 … M10). Each batch (50g) of different compositions belonging to two types was prepared by weighing out the components required according to the pre-planned formula (Table 1). The powders were mixed by subjected to vibratory milling and homogenization for 30min, the resulting mixture as-milled powders were sieved and particles within size <75μm were isolated. The selected particles were placed into a recrystallized alumina crucible and placed in an electric furnace. The glass melting operation consisted of several stages which include preparation of batch, melting, conditioning, quenching and followed by powder production by ball milling.

Glass synthesized by slowly cooling route (Group A)

The homogenized batch was transferred into a recrystallized alumina crucible and placed at the centre of a raising furnace. Using ivoclar bivalent program at P500 the range of temperature 1200ºC Germany. The melting process was initiated at a heating rate of 10°C/min from room temperature to temperature of furnace reached to around 1200°C. When the furnace temperature reached the fusion temperature (1150-1200°C), batch was subjected to melt homogenization for 60min. slowly cooled (the temperature was decreased to room temperature) at a rate of 25°C/min let cool down to room temperature.

Glass synthesized by melt quench route (Group B)

The second method to synthesized glass, the homogenized batch was transferred into a recrystallized alumina crucible and placed at the centre of a raising furnace using MIHM- VOGT P6/B GERMANY to 1200ºC. The furnace temperature was set according to the fusion temperature of batch mix. The melting process was initiated at a heating rate of 5°C/min from room temperature to temperature of furnace reached to around 500°C after which a heating rate was increased to 10°C/min. When the furnace temperature reached the fusion temperature (1150–1200°C), batch was subjected to melt homogenization for 60min. The raising heat was quickly moved down. The raising hearth was quickly moved down and the resulting glass melt was poured directly into a water container to produce granular glass frit. Resultant glass frits were collected and dried, and then it was ground manually by using agate mortar and pestle Subsequently dry ball milling using (RETSCH. PM 100 Germany) was performed using zirconium balls with diameter cm for 90min .The resulted powder was passed through a sieve to retrieve a powder with mean particle size of <25µm Sieve shakers SCL-SD8-12 Iran and sieve mesh 500 for sieving powder to get particles <25µm. Table 1 shows the privileges of the results of the studied samples physical and mechanical properties. The order of the samples is in the following order: M2.... N4, M1. In our analysis of the Nano, we have chosen the first three samples of the composites, because they proved to be the best with regard to the physical and mechanical properties. In this paper materials used and following the same vein composites and method of preparation as well as the method of measurement used.¹⁰

The discussion is divided into two parts. The first part, Related to the treatment of the CaF₂NPs. The second part addresses the Nano composites physical and mechanical properties due to filler incorporation

Synthesis of Nano composites dental material

Surface modification of CaF₂NPs

As mentioned earlier in our previous paper¹⁰ CaF₂NPs having specific surface area about 25m²/g. The theoretical loading of silane was calculated to be ~1.5wt.% for CaF₂NPs grafted with γMPT assuming that all silane provided during the reaction formed covalent bonds with silanol groups on the particle surface. After treatment of the CaF₂NPs surface by γMPT, the surface was investigated by FTIR spectroscopy and TGA. Figure 1 shows the FTIR spectra of the chemical structure of CaF₂NPs. The CaF₂NPs exhibited strong absorption peaks at around 1460 and 1650cm^-1 assigned to the =CH stretching vibrations and C=O bending vibrations, respectively. At 1121cm^-1 absorption peak is assigned to Si–O, week absorption peaks at around 1720cm^-1 due to the stretching vibration of ester carbonyl group, a medium band at 2928cm^-1 related to the CH₂– of the coupling agent and weak but visible absorption peaks at 3130cm^-1 are assigned to the –OH. Also the theoretical value for silane density on the surface was calculated from TGA data Figure 2. Table 2 shows the mass loss for CaF₂NPs about 1.4%.This value is in good correlation with TGA results for CaF₂NP. Results obtained for silane loading of CaF₂NPs are relatively low.

Figure 1 FTIR spectra of the untreated and treated CaF₂NPS.

Figure 2 TGA of the untreated and treated CaF₂NPS.

Sample preparation

To determine the weight percent of filler (Wt%= weight fraction of filler (100%). Volume percent of filler could then be calculated based on filler weight percent, filler density and composite density, according to following equation

Where Vol% volume fraction of filler *100%, C and F are density of the composite and filler, respectively. The respective series of composites, namely M1, M10, and N4, The total filler included the CaF₂NPs 76% wg% (~58% vol), for each composites three ratio of Nano filler loading 5%, 10% and 15% Vol, The CaF₂NPs mass ratio was determined by weight.

Characterization of Nano composite

Physical properties

Shrinkage strain and rate of shrinkage

Figure 3 shows the typical shrinkage strain curves for the M10 tested Nano composites 5%, 10% and 15% %Vol, respectively. It clearly shows the ratio 10%vol has low shrinkage. The result of shrinkage strain for Nano composites were shown in Figure 4 & Table 3.

Figure 3 Shrinkage curve and rate of polymerization for M10 Nano composite tested.

Figure 4 Shrinkage polymerization for Nano composite tested.

Depth of curve

Figure 5 & Table 4 show the depth of curve for tested Nano composites according to ISO 4049:2000, it varies between 3.2mm for M1, 5% Nano and 3.4mm for M10, 10% Nano.

Figure 5 Depth cure for Nano composites tested.

Degree of conversion (DC)

Conversion values obtained from the FTIR results as shown in Table 5 & Figure 6. The degree of conversion for all Nano composites varies between 57.8 and 81.2 for M1, 5% Nano and M1, 15% Nano respectively. All values of Nano composites are nearly lower than M1 composites.

Figure 6 Degree of conversion for Nano composite tested.

Thermal expansion coefficient

Linear coefficients of thermal expansion CTE were calculated for temperature ranges 0 to 60ºC as show in Table 6. Less CTE obtained in the ratio 15% for all nanocomposites tested 42.28, 36.47 and 36.40 for M1, M10 and N4 respectively (Figure 7) (Figure 8).

Figure 7 Shrinkage curves and rate of polymerization for M2 composite tested.

Figure 8 Mean of CTE for Nano composite tested.

Water sorption and water solubility

Table 7 shows the mean water sorption and water solubility for each three Nano composites, illustrated in Figure 9 for water sorption and Figure 10 for water solubility.

Figure 9 Water sorption for Nano composite tested.

Figure 10 Water solubility for Nano composite tested.

Density

(Table 8) (Figure 11)

Figure 11 Water solubility for Nano composite tested.

Mechanical properties

Flexural strength and flexural modulus

Table 9 shows the means and standard deviations of flexural strength and elastic modulus. Under the conditions of 40 sec light irradiation at a power density of 500mW/cm², results of flexural strength and elastic modulus ranged from 90.5 to 135.1MPa and from 4.7 to 7.6GPa, respectively (Figure 12) (Figure 13).

Figure 12 Mean and stand flexural strength (MPa) of Nano composites.

Figure 13 Mean and stand flexural modulus (GPa) of Nano composites.

Diametrical tensile strength (DTS)

Mean values and standard deviation of the DTS of the tested materials are shown in Figure 14 & Table 10. The values varied from 46MPa to 60MPa.

Figure 14 Mean and stand DTS (MPa) of Nano composites.

Dental restorative materials should mimic as much as possible the physical behavior of the tooth and approximate the mechanical properties of enamel and dentin. Thus, any differences in properties may be attributed to the different Nano composite formulations evaluated. Besides irradiation temperature, host temperature and environment, the irradiation regime, light intensity and exposure duration, intrinsic factors such as monomer system, concentration of the catalyst, amount of filler, filler type, size and coupling agent used have been shown to affect the curve of Nano composites reins. Light intensity of 500mW/cm² for 40seconds has been chosen based on the recommendation by manufacturers for most commercial materials to ensure maximum curve. This study was specifically designed to compare and to evaluate the recent Nano composites, using Nano technology in composites fabrication, which is claimed to have superior mechanical properties than the commonly used hybrid and the mono-filled composites. The improvement of Nano filled composite used comes from its fabrication property where its filler particle size ranges from (0.005-0.01 microns). These very small particles will not interact with the visible light and as a result will not cause scattering which leads in modulus of elasticity, depth of curve and esthetics Shrinkage.

The results of depth of curve showed that, a significant difference was found between the three tested composites ratio, where the 10%vol of Nano composite recorded the best depth of curve followed by Nano filled composite then 5%. Utilization of Nanotechnology is very clear in terms of reducing the CTE decreased from 50×10^-6ºC to 36×10^-6ºC in the ratio of 15% and this is a good indicator and a new relationship with the proportion of Nano and CTE for Nano composite. International Organization for Standardization (ISO) specification 4049 for polymer-based restoratives classifies dental polymer classified restorative materials into two different types. The first type is type I which is the material claimed by the manufactures to be appropriate for restoration involving occlusal surfaces. The second type is type II which includes all other polymer-based filling materials. The lowest value of flexural strength required for type I is 80MPa and 50MPa for type II {ISO, 2000}. In this study all materials investigated showed higher mean flexural strength values than ones recommended by the ISO suggesting that these materials can be used as direct restorative materials.

The diametric tensile strength is a property which is suited only for testing brittle material. For this reason, the tensile strength of these materials may be considered to have more clinical relevance than the compressive strength. So Compressive strengths of various composites are about the same, but DTS of some Nano filled composites may be lower than the other Nano filled or hybrid composites. The DTS mean values of the composites tested in the present study are in the DTS range of dental composites, 30‑55MPa.¹³ Results of the present study showed that, some Nano filled composites may have lower DTS than the other composite resins (Table 5). The results (in MPa) obtained in this study is similar to the average previously recorded as 38.69 for Palfique Estelite, 44.6MPa for Esthet X and 44.42MPa for Supreme. Mota et al,¹⁴ determined the diametric tensile strength results ranged from 49.24MPa (±7.10, Grandio enamel) to 36.08 (±8.71, 4 Seasons dentin),¹⁴ and ERL4221 Epoxy resin 28MPa M50 42MPa G60 47MPa Commercial methacrylate Composite 50MPa.¹⁵ However, in our study results are different because large particle size showed lower values of DTS. This could be due to a very high filler contents or large filler particle size that may interfere with light penetration during polymerization and influences the DTS of the material. The Overall results obtained from the present study were in accordance with the previous studies mentioned before^{1,11,12,17,18} and ADA No27 Resinbased Filling Material1993.

Adding Nano technology caused to a decrease in the liner thermal coefficient of expansion to drop from 50×10^-6ºC to 36×10^-6ºC while using the ratio of 15%, it is gradually decreased. The employment of Nano particles is also cause to increase in the flexural strength and achieved a 10% higher strength while it decreased in using 15% ratio. As a conclusion, Using 10% of Nano filler of Nano composites showed potential for use as dental restoratives and present a promising approach to achieve novel low shrinkage and water sorption, high flexural strength for all Nano composites tested and also higher depth of curve (more than 3mm) which is novel result compared with the conventional value of just 2mm.

The Authors would like to appreciate Iraqi Ministry of Higher Education and Research for offering Rapid Scholarship to do this work which is part of his PhD Thesis.

The author declares that there is no conflict of interest.

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