Effect of the Silanization of Aerosil OX50 in the Properties of Light-Cured Dental Composites

Citation: Díaz-Rodríguez, S.d.l.C.; Tarano-Artigas, O.; Herrera-Kao, W.; Cauich-Rodríguez, J.V.; Cervantes-Uc, J.M.; Rosa-Sainz, A.; La Serna, A.A.; Veranes-Pantoja, Y. Effect of the Silanization of Aerosil OX50 in the Properties of Light-Cured Dental Composites. Appl. Sci. 2024,14, 2453. https://doi.org/10.3390/ app14062453 Academic Editor: Vittorio Checchi Received: 28 January 2024 Revised: 7 March 2024 Accepted: 12 March 2024 Published: 14 March 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). applied sciences Article Effect of the Silanization of Aerosil OX50 in the Properties of Light-Cured Dental Composites Selena de la Caridad Díaz-Rodríguez 1, Oridayma Tarano-Artigas 1, Wilberth Herrera-Kao 2, Juan Valerio Cauich-Rodríguez 2, JoséManuel Cervantes-Uc 2, Ana Rosa-Sainz 3,* , Amisel Almirall La Serna 1 and Yaymarilis Veranes-Pantoja 1,* 1 Centro de Biomateriales, Universidad de La Habana, Ave. Universidad, e/Ronda y G, Plaza de la Revolución, La Habana C.P. 10400, Cuba; [email protected] (S.d.l.C.D.-R.); [email protected] (O.T.-A.); [email protected] (A.A.L.S.) 2 Centro de Investigación Científica de Yucatán (CICY), Calle 43, No. 130 x 32 y 34, Col. Chuburnáde Hidalgo, Mérida C.P. 97205, Yucatán, Mexico; [email protected] (W.H.-K.); [email protected] (J.V.C.-R.); [email protected] (J.M.C.-U.) 3Escuela Politécnica Superior, Departamento de Ingeniería y Ciencia de los Materiales y del Transporte, Universidad de Sevilla, Calle Virgen de África, 7, 41011 Sevilla, Spain *Correspondence: [email protected] (A.R.-S.); [email protected] (Y.V.-P.); Tel.: +53-52866259 (Y.V.-P.) Abstract: In this study, the surface of aerosil OX50 was treated with γ -methacryloxypropyltrimethoxy- silane in order to evaluate its effect on light-cured dental composites. Four composites were prepared with Bis-GMA/tetraethylene glycol dimethacrylate/ γ -methacryloxypropyltrimethoxysilane, Bis- GMA/tetraethylene glycol dimethacrylate, and a 45% treated or untreated aerosil OX50, using the camphorquinone/N,N-dimethylaminoethyl methacrylate pair as initiator. Evidence of filler silanization was provided by FTIR as a low-intensity absorption at 1707 cm −1 (carbonyl functional group) while thermogravimetric analysis showed a mass loss of approximately 2% associated with the decomposition of γ -methacryloxypropyltrimethoxysilane. The experimental composites studied meet the requirements of the ISO 4049:2019 standard for depth of cure, water sorption, and solubility. The composites are shown to be thermally stable and presented a degree of conversion higher than 70%, being higher than that reported for many commercial composites. Based on the observed properties, the best formulations were those in which the silane is incorporated into the matrix and the filler was previously treated. Keywords: aerosil OX50; light-cured dental composites; silanization 1. Introduction Dental caries is one of the most widespread diseases in humanity, and it is defined as an infectious, multifactorial, chronic, and localized disease that affects the hard tissues of the tooth. It is produced by the action of organic acids from microbial fermentation of carbohydrates in the diet, the characteristics of the dental tissue, and the hygiene of each individual [ 1 ]. For a long time, the most widely used restorative material in the repair of dental tissue has been silver amalgam. Its adequate physical–mechanical properties and ease of handling have made it the ideal material for the restoration of posterior teeth. However, it presents drawbacks such as its unnatural appearance, the lack of adhesion to dental tissue that generates more sacrifice of healthy dental tissue, and the contamination produced by mercury [ 2 ]. For this reason, some countries prohibit the use of amalgam, and work is underway worldwide on the gradual reduction of the use of compounds that contain mercury, which is declared in the Minamata Convention, and its total elimination is proposed for the year 2030 [3,4]. The appearance in restorative dentistry of light-cured composite resins has been one of the most significant contributions as it considers the preservation of healthy tissue, Appl. Sci. 2024,14, 2453. https://doi.org/10.3390/app14062453 https://www.mdpi.com/journal/applsci Appl. Sci. 2024,14, 2453 2 of 14 reduction of microleakage, and prevention of postoperative sensitivity. In general, composite resins or dental composites are made up of a mixture of acrylic monomers made up of an organic matrix, filler particles, a coupling agent, whose function is to achieve an optimal bond between both components, and an initiator system [ 5 ]. The organic matrix of dental composite resins is basically made up of two types of monomers that are divided according to their viscosity into base monomers and diluents. 2,2 [p-(2-hydroxy- 3-methacryloxypropoxy) phenyl] propane (Bis-GMA) is the most commonly used base monomer for this purpose due to its properties such as low shrinkage upon polymerization, rigidity, and high resistance [ 6 ]. However, it has a high viscosity that negatively affects the final properties of the composite, requiring the use of diluent monomers such as tri- and tetraethylene glycol dimethacrylates (TEGDMA and TEEGDMA, respectively) [ 7 ]. Quartz, glass, titanium oxide, and silicon oxide are generally used as inorganic fillers, providing an increase in the resistance to traction, compression, and abrasion of composites. Hydroxyapatite could be a promising bioactive filler for dental resin composites, since it is the main biological component of dentin and enamel [ 8 , 9 ]. In addition to reducing shrinkage in composites it is desirable that mechanical properties are like that of the dental structure in order to guarantee a restoration that meets physical, chemical, and mechanical needs, i.e., an aesthetic and biocompatible dental composite. Currently, there is a trend to use nanofillers to obtain composite resins with dental applications, since these improve the wear properties of the composites, such is the case of aerosil OX50. This is a fumed silica made up of spherical particles of 40 nm in diameter. This silica is obtained by flame hydrolysis of silicon tetrachloride, SiCl4[10,11]. Among the most important properties of dental resins for clinical use are the degree of conversion, water sorption and solubility, and the mechanical properties. The values required for these properties are well known and have been established in various international standards [ 12 ]. The degree of conversion of the composites expresses the number of double bonds of the methacrylate monomeric groups that react. It can be determined by infrared spectroscopy, considering the decrease in the intensity of the absorption band of the double bonds corresponding to the aliphatic carbons [ 9 ]. The depth of cure is also a property that indicates the effectiveness of the polymerization process and is carried out according to the stablished parameters in the ISO 4049 standard [ 12 , 13 ]. Other properties that are evaluated and regulated (ISO 4049) are water sorption and solubility. The latter is of great importance since these materials will be placed in a humid environment where unreacted monomers may dissolve or the filler particles can be leached into the oral environment, affecting its clinical behavior and its biocompatibility [14]. Obtaining good properties in composite resins depends largely on the bond between the matrix and the filler. With a good bond, the detachment of inorganic particles is avoided, as well as the penetration of water in the interface, and then the deterioration of the restoration. To ensure the formation of a proper interphase, the filler is treated with coupling agents, generally silanes. The most widely used silane is γ - methacryloxypropyltrimethoxysilane (MPS), capable of chemically interacting with both the matrix and the surface of the filler [15,16]. In the 1970s, “two-step” silanization methods were developed, either by dry contact or in solution, which involve a pretreatment of the filler with the silane before its incorporation into the monomeric mixture for the formulation of composite resins. Other methods that have been used include the “one-step” treatment, which consists of incorporating the silane directly into the organic matrix [ 17 ]. Several types of aerosil have been used as previously reported and MPS silanized aerosil with smaller particle size rendered better adhesion with a Bis-GMA matrix but the amount of sorbed water increased. However, the use of larger particle sizes (40 nm) like aerosil OX50 did not severely affect their mechanical performance as their bending strength is slightly lowered without sacrificing flexural modulus [17]. Recently, changes have been introduced in the composition of dental resins with the aim of improving their properties and in order to reduce the use of bisphenol A, where the main concern is the possible release of monomers [ 5 ]. In addition to these alternative Appl. Sci. 2024,14, 2453 3 of 14 monomers, the use of monomers with antibacterial activity has also been pursued but the most significant enhancements and changes have been in fillers such as size reduction and shape enhancement (plate-like, rod-like, nanoparticles) [18]. In this research, MPS is introduced into organic matrices not only as a partial diluent of Bis-GMA (Bis-GMA/TEEGDMA/MPS) but also as a coupling agent. Therefore, the aim of this study was to evaluate the influence of the surface treatment of aerosil OX50, used as an inorganic filler, on various properties of dental composites including degree of conversion, depth of cure, water sorption, and solubility. 2. Materials and Methods In this section, the materials and methods used are presented. The section is subdivided into: silanization of aerosil OX50, preparation of dental composite formulations, and characterization of the filler and experimental composites. 2.1. Silanization of Aerosil OX50 To improve the surface adhesion between the organic matrix and inorganic particles, the filler was treated with MPS (Sigma Aldrich, Burlington, MA, USA). The silanizing solution was prepared with 80 mL of acetone, 0.5 mL of H 2 O, 1 mL of MPS, stirred for 20 min, and then 1 g of aerosil OX50 (DEGUSSA, Tokyo, Japan) was added. Silanization in acetone/water mixtures was conducted as in previous studies that demonstrated a better incorporation of silicon on hydroxipatatite surfaces in comparison to the use of methanol/water mixtures [ 19 ]. Magnetic stirring was continued for 2 h. Subsequently, it was placed in an ultrasonic bath for 20 min and stirred again for 2 h with a magnetic stirrer. It was decanted and washed with sufficient acetone. The treated aerosil OX50 was dried for 12 h in an oven at 60 ◦ C. Figure 1shows the schematic diagram of the procedure performed to obtain the silanized aerosil OX50. Appl. Sci. 2024, 14, x FOR PEER REVIEW 3 of 14 Recently, changes have been introduced in the composition of dental resins with the aim of improving their properties and in order to reduce the use of bisphenol A, where the main concern is the possible release of monomers [5]. In addition to these alternative monomers, the use of monomers with antibacterial activity has also been pursued but the most significant enhancements and changes have been in fillers such as size reduction and shape enhancement (plate-like, rod-like, nanoparticles) [18]. In this research, MPS is introduced into organic matrices not only as a partial diluent of Bis-GMA (Bis-GMA/TEEGDMA/MPS) but also as a coupling agent. Therefore, the aim of this study was to evaluate the influence of the surface treatment of aerosil OX50, used as an inorganic filler, on various properties of dental composites including degree of conversion, depth of cure, water sorption, and solubility. 2. Materials and Methods In this section, the materials and methods used are presented. The section is subdivided into: silanization of aerosil OX50, preparation of dental composite formulations, and characterization of the filler and experimental composites. 2.1. Silanization of Aerosil OX50 To improve the surface adhesion between the organic matrix and inorganic particles, the filler was treated with MPS (Sigma Aldrich, Burlington, MA, USA). The silanizing solution was prepared with 80 mL of acetone, 0.5 mL of H 2 O, 1 mL of MPS, stirred for 20 min, and then 1 g of aerosil OX50 (DEGUSSA, Tokyo, Japan) was added. Silanization in acetone/water mixtures was conducted as in previous studies that demonstrated a better incorporation of silicon on hydroxipatatite surfaces in comparison to the use of methanol/water mixtures [19]. Magnetic stirring was continued for 2 h. Subsequently, it was placed in an ultrasonic bath for 20 min and stirred again for 2 h with a magnetic stirrer. It was decanted and washed with sufficient acetone. The treated aerosil OX50 was dried for 12 h in an oven at 60 °C. Figure 1 shows the schematic diagram of the procedure performed to obtain the silanized aerosil OX50. Figure 1. Scheme of the silanization process. 2.2. Preparation of Dental Composite Formulations Four experimental light-cured dental composites were prepared with two different monomeric matrices and 45% aerosil OX50 as an inorganic filler (Table 1). The M1 matrix is made up of Bis-GMA/TEEGDMA/MPS (60:30:10) and the M4 matrix is made up of Bis- Figure 1. Scheme of the silanization process. 2.2. Preparation of Dental Composite Formulations Four experimental light-cured dental composites were prepared with two different monomeric matrices and 45% aerosil OX50 as an inorganic filler (Table 1). The M1 matrix is made up of Bis-GMA/TEEGDMA/MPS (60:30:10) and the M4 matrix is made up of Bis- GMA/TEEGDMA (60:40). All composites contain Bis-GMA from Sigma Aldrich, USA, in their organic matrix while tetraethylene glycol dimethacrylate (TEEGDMA) was purchased from Fluka. The C145 composite has MPS incorporated into its organic matrix as part of the diluent, while C445 does not. In addition, the C145s and C445s composites contain Appl. Sci. 2024,14, 2453 4 of 14 filler previously silanized with MPS. The camphorquinone (CQ, Aldrich, USA)/N,N- dimethylaminoethyl methacrylate (MDMAE, Riedel de Haen, Seelze, Germany) pair was used as the initiator system. As reference material, the commercial Tetric Evo Ceram ® (TT) was used. All the reagents used were 98% pure. Table 1. Composition of experimental light-cured dental composites. Composites Organic Matrix Inorganic Filler C145 Bis-GMA/TEEGDMA/MPS (M1) Aerosil OX50 (45%) C445 Bis-GMA/TEEGDMA (M4) C145s Bis-GMA/TEEGDMA/MPS (M1) Aerosil OX50 silanized (45%) C445s Bis-GMA/TEEGDMA (M4) TT Bis-GMA/TEGDMA Mixtures of oxides and copolymers (82%) The light source employed to cure the materials was a light-emitting diode (LED) from Woodpecker (Guilin, China) with a wavelength range from 420 to 480 nm and a light intensity of 1000 mW/cm2. 2.3. Characterization of the Filler and Experimental Composites To verify the occurrence of filler silanization and to analyze the thermal behavior of dental composites, a thermogravimetric analysis (TGA) of 50 to 800 ◦ C was performed in a Perkin Elmer TGA-7 (Norwalk, CT, USA), with a heating rate of 10 ◦ C/min and nitrogen flow of 50 mL/min. For the study, 3 mm diameter and 1 mm thickness samples were prepared after polymerization for 40 s. The morphology of silanized and unsilanized aerosil OX50 was observed on a TESCAN VEGA3 scanning electron microscope, and it was coated using an ion sputter coater SBC-12 (KYKY TECHNOLOGY CO., Beijing, China). The study was carried out at an acceleration voltage of 25 kV with a magnification of 50,000 times. From the micrographs obtained, 100 measurements of aerosil OX50 particles were made using the ImageJ program, version 1.54d. These measurements allowed us to obtain particle size distribution of the filler and thus estimate their average size. Fourier transform infrared spectroscopy (FTIR) was used for the characterization of the filler and determination of the degree of conversion in the composites. The aerosil OX50 was analyzed in a Thermo Nicolet 8700 spectrometer (Madison, WI, USA), with a resolution of 4 cm −1 , 100 scans, and in the spectral interval of 4000–500 cm −1 . The samples were dispersed in KBr and the degree of conversion (DC) determined by recording transmission spectra in the 4000–600 cm −1 range, with the same resolution and 50 scans. Three replicates of each sample were run at 0, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, and 80 s after irradiation. The DC of every dental resin composite was calculated from the absorbance intensities (R) of the aliphatic C=C bond (1638 cm −1 ) against the internal standard of the aromatic C=C bond (1608 cm −1 ), determined before and after polymerization. The degree of conversion (%) was calculated according to Equation (1) [20–22]. DC (%) = 100 ×[1 −(Rcured/Rnot cured)] (1) The depth of cure, water sorption (A), and solubility (S) tests were evaluated according to the specifications described in the ISO 4049:2019 standard [ 12 ]. For the depth of cure, three cylindrical specimens (6 mm in height and 4 mm in diameter) were irradiated for 40 s on one side. After the polymerization process, the specimen was removed from the mold and any soft material was scraped using a plastic spatula. The height was measured with a micrometer (0–25 mm, 0.01 mm precision), the obtained value was divided by two, and the measured magnitude was reported as the depth of cure of the material. Water sorption and solubility tests were performed using five disc-shaped specimens (1 mm in Appl. Sci. 2024,14, 2453 5 of 14 height and 15 mm in diameter). The samples were cured with nine overlapped irradiations on one side (40 s for each irradiation; 360 s in total). Diameter and thickness are measured with a micrometer (0–25 mm) to calculate the volume (V). The samples are weighed and stored in a desiccator until constant weight, the final mass is taken as m 1 . Then, they are immersed in distilled water (m 2 ) for seven days at 37 ◦ C and placed again in a desiccator until a constant mass (m3). For the calculations, Equations (2) and (3) were used. A= (m2−m3)/V(2) S= (m1−m3)/V(3) The Origin8 computer program was used for data processing. Statistical analysis was performed by comparing means and variances with the Statgraphics Plus 5.5 program, using Student’s “t” statistic and Fisher’s “F” statistic with 95% confidence. The mean and standard deviation (Sd), as an interval (±), were reported for all cases. 3. Results This section presents the obtained results, which are divided into: characterization of the filler, physical appearance of the experimental composites, and characterization of the experimental composites. 3.1. Characterization of the Filler FTIR spectra, mass loss (TGA), and the first derivative (DTGA) of the silanized and untreated aerosil OX50 are shown in Figure 2a,b. The FTIR spectrum of non-silanized aerosil OX50 showed two typical bands of silicon oxide at 1100 and 801 cm −1 . In the case of silanized aerosil OX50, an almost imperceptible third band can be observed around 1707 cm−1 . In the TGA of the untreated aerosil OX50 no mass loss was observed, which can be explained by the high stability of SiO 2 . In contrast, silanized aerosil OX50 showed a decomposition temperature at 437 ◦C with a mass loss of approximately 2%. Appl. Sci. 2024, 14, x FOR PEER REVIEW 5 of 14 two, and the measured magnitude was reported as the depth of cure of the material. Water sorption and solubility tests were performed using five disc-shaped specimens (1 mm in height and 15 mm in diameter). The samples were cured with nine overlapped irradiations on one side (40 s for each irradiation; 360 s in total). Diameter and thickness are measured with a micrometer (0–25 mm) to calculate the volume (V). The samples are weighed and stored in a desiccator until constant weight, the final mass is taken as m1. Then, they are immersed in distilled water (m2) for seven days at 37 °C and placed again in a desiccator until a constant mass (m3). For the calculations, Equations (2) and (3) were used. A = (m2 − m3)/V (2) S = (m1 − m3)/V (3) The Origin8 computer program was used for data processing. Statistical analysis was performed by comparing means and variances with the Statgraphics Plus 5.5 program, using Student’s “t” statistic and Fisher’s “F” statistic with 95% confidence. The mean and standard deviation (Sd), as an interval (±), were reported for all cases. 3. Results This section presents the obtained results, which are divided into: characterization of the filler, physical appearance of the experimental composites, and characterization of the experimental composites. 3.1. Characterization of the Filler FTIR spectra, mass loss (TGA), and the first derivative (DTGA) of the silanized and untreated aerosil OX50 are shown in Figure 2a,b. The FTIR spectrum of non-silanized aerosil OX50 showed two typical bands of silicon oxide at 1100 and 801 cm−1. In the case of silanized aerosil OX50, an almost imperceptible third band can be observed around 1707 cm−1. In the TGA of the untreated aerosil OX50 no mass loss was observed, which can be explained by the high stability of SiO2. In contrast, silanized aerosil OX50 showed a decomposition temperature at 437 °C with a mass loss of approximately 2%. (a) (b) Figure 2. Characterization of the filler: (a) FTIR spectrum; (b) TGA thermograms of untreated and silanized aerosil OX50. Several micrographs of aerosil OX50 were taken before and after silanization and representative ones are shown in Figure 3. In general, aerosil OX50 particles are spherical in shape and easily agglomerate, which is typical of nanoparticles. The agglomerates in the untreated aerosil OX50 (Figure 3a) are round and 150 nm in size. In the case of the Figure 2. Characterization of the filler: (a) FTIR spectrum; (b) TGA thermograms of untreated and silanized aerosil OX50. Several micrographs of aerosil OX50 were taken before and after silanization and representative ones are shown in Figure 3. In general, aerosil OX50 particles are spherical in shape and easily agglomerate, which is typical of nanoparticles. The agglomerates in the untreated aerosil OX50 (Figure 3a) are round and 150 nm in size. In the case of the silanized aerosil OX50 (Figure 3b), there is an increase in the cohesion between the agglomerated particles. Appl. Sci. 2024,14, 2453 6 of 14 Appl. Sci. 2024, 14, x FOR PEER REVIEW 6 of 14 silanized aerosil OX50 (Figure 3b), there is an increase in the cohesion between the agglomerated particles. (a) (b) Figure 3. Characterization of the filler: (a) micrograph of unsilanized aerosil OX50; (b) micrograph of silanized aerosil OX50. The particle size distribution of aerosil OX50 (Figure 4), obtained from micrograph measurements, shows a normal distribution. Most of the particles are in the 40–50 nm range in agreement with the manufacturer’s description. Furthermore, no polydispersity is observed, and the average particle size was 44 ± 1 nm. Figure 4. Particle size distribution of aerosil OX50. 3.2. Physical Appearance of the Experimental Composites The four experimental composites, prepared with the monomeric matrices M1 (Bis- GMA/TEEGDMA/MPS) and M4 (Bis-GMA/TEEGDMA) and 45% aerosil OX50, are homogeneous pastes, easy to handle, and of similar consistency and color to commercial resins. The most viscous composites, as visually appraised and the most difficult to manipulate, are those that contain the M4 matrix, because the diluent monomer (TEEGDMA) is more viscous than the TEEGDMA/MPS mixture. In addition, in composites prepared with the Bis-GMA/TEEGDMA/MPS matrix, the filler is better incorporated. The commercial resin is a homogeneous, cream-colored paste, slightly more viscous than that prepared with the Bis-GMA/TEEGDMA matrix. The monomeric mixtures have a similar color (yellow) due to the presence of camphorquinone, with a color change after polymerization. Figure 3. Characterization of the filler: (a) micrograph of unsilanized aerosil OX50; (b) micrograph of silanized aerosil OX50. The particle size distribution of aerosil OX50 (Figure 4), obtained from micrograph measurements, shows a normal distribution. Most of the particles are in the 40–50 nm range in agreement with the manufacturer’s description. Furthermore, no polydispersity is observed, and the average particle size was 44 ±1 nm. Appl. Sci. 2024, 14, x FOR PEER REVIEW 6 of 14 silanized aerosil OX50 (Figure 3b), there is an increase in the cohesion between the agglomerated particles. (a) (b) Figure 3. Characterization of the filler: (a) micrograph of unsilanized aerosil OX50; (b) micrograph of silanized aerosil OX50. The particle size distribution of aerosil OX50 (Figure 4), obtained from micrograph measurements, shows a normal distribution. Most of the particles are in the 40–50 nm range in agreement with the manufacturer’s description. Furthermore, no polydispersity is observed, and the average particle size was 44 ± 1 nm. Figure 4. Particle size distribution of aerosil OX50. 3.2. Physical Appearance of the Experimental Composites The four experimental composites, prepared with the monomeric matrices M1 (Bis- GMA/TEEGDMA/MPS) and M4 (Bis-GMA/TEEGDMA) and 45% aerosil OX50, are homogeneous pastes, easy to handle, and of similar consistency and color to commercial resins. The most viscous composites, as visually appraised and the most difficult to manipulate, are those that contain the M4 matrix, because the diluent monomer (TEEGDMA) is more viscous than the TEEGDMA/MPS mixture. In addition, in composites prepared with the Bis-GMA/TEEGDMA/MPS matrix, the filler is better incorporated. The commercial resin is a homogeneous, cream-colored paste, slightly more viscous than that prepared with the Bis-GMA/TEEGDMA matrix. The monomeric mixtures have a similar color (yellow) due to the presence of camphorquinone, with a color change after polymerization. Figure 4. Particle size distribution of aerosil OX50. 3.2. Physical Appearance of the Experimental Composites The four experimental composites, prepared with the monomeric matrices M1 (Bis- GMA/TEEGDMA/MPS) and M4 (Bis-GMA/TEEGDMA) and 45% aerosil OX50, are homogeneous pastes, easy to handle, and of similar consistency and color to commercial resins. The most viscous composites, as visually appraised and the most difficult to manipulate, are those that contain the M4 matrix, because the diluent monomer (TEEGDMA) is more viscous than the TEEGDMA/MPS mixture. In addition, in composites prepared with the Bis-GMA/TEEGDMA/MPS matrix, the filler is better incorporated. The commercial resin is a homogeneous, cream-colored paste, slightly more viscous than that prepared with the Bis-GMA/TEEGDMA matrix. The monomeric mixtures have a similar color (yellow) due to the presence of camphorquinone, with a color change after polymerization. 3.3. Characterization of the Experimental Composites The measurement of the degree of conversion of the unsilanized and silanized experimental composites and the monomeric mixtures was carried out at 0, 1, 2, 3, 4, 5, 10, 15, Appl. Sci. 2024,14, 2453 7 of 14 20, 25, 30, 35, 40, and 80 s of irradiation. Figure 5a shows the FTIR spectrum of the C145 composite at the different irradiation times mentioned above. The curves corresponding to each time are indicated in different colors from blue for time 0 to red for 80 s. The two bands under study in this research can be observed in the spectrum. The band corresponding to the aliphatic C=C bond appears around 1637 cm −1 , while the one associated with the vibrations of the C=C aromatic bond can be seen around 1612 cm −1 . Figure 5b shows the results obtained from the determination of the degree of conversion with time. Generally, only the degree of conversion at the final irradiation time, which in normally 40 s, is reported. In this study, in addition, the results of the degree of conversion at several times up to 80 s are reported and analyzed. Appl. Sci. 2024, 14, x FOR PEER REVIEW 7 of 14 3.3. Characterization of the Experimental Composites The measurement of the degree of conversion of the unsilanized and silanized experimental composites and the monomeric mixtures was carried out at 0, 1, 2, 3, 4, 5, 10, 15, 20, 25, 30, 35, 40, and 80 s of irradiation. Figure 5a shows the FTIR spectrum of the C145 composite at the different irradiation times mentioned above. The curves corresponding to each time are indicated in different colors from blue for time 0 to red for 80 s. The two bands under study in this research can be observed in the spectrum. The band corresponding to the aliphatic C=C bond appears around 1637 cm −1 , while the one associated with the vibrations of the C=C aromatic bond can be seen around 1612 cm −1 . Figure 5b shows the results obtained from the determination of the degree of conversion with time. Generally, only the degree of conversion at the final irradiation time, which in normally 40 s, is reported. In this study, in addition, the results of the degree of conversion at several times up to 80 s are reported and analyzed. (a) (b) Figure 5. (a) FTIR spectrum of the C145 composite in the region from 1680–1580 cm −1 at different irradiation times; (b) Degree of conversion vs. irradiation time of the matrices and composites. The results of the depth of cure and degree of conversion at 40 s for the matrices and composites are shown in Table 2. Composite materials meet the requirements of the ISO 4049:2019 standard [12] for cure depth, showing values greater than 2 mm. The highest values are exhibited by the monomeric matrices M1 and M4, (2.95 ± 0.01) mm and (2.94 ± 0.01) mm, respectively. In addition, there are no statistically significant differences between the monomeric matrices M1 and M4, nor between the composites prepared with them, so no effect of the silanization process was observed for this property. The experimental composites showed a greater value of curing depth than the commercial TT composite used as a reference. In the case of the degree of conversion for the matrices and composites at 40 s of irradiation, values higher than 67% and, in several cases, more than 75% (C145s and C445) were obtained. As shown in Table 2, all experimental composites present DC values higher than TT, except for composite C145, which does not have significant differences from it. Furthermore, the highest values of this property correspond to matrices M1 and M4, (86.3 ± 0.1)% and (85 ± 2)%, respectively. Figure 5. (a) FTIR spectrum of the C145 composite in the region from 1680–1580 cm −1 at different irradiation times; (b) Degree of conversion vs. irradiation time of the matrices and composites. The results of the depth of cure and degree of conversion at 40 s for the matrices and composites are shown in Table 2. Composite materials meet the requirements of the ISO 4049:2019 standard [ 12 ] for cure depth, showing values greater than 2 mm. The highest values are exhibited by the monomeric matrices M1 and M4, (2.95 ± 0.01) mm and ( 2.94 ±0.01 ) mm, respectively. In addition, there are no statistically significant differences between the monomeric matrices M1 and M4, nor between the composites prepared with them, so no effect of the silanization process was observed for this property. The experimental composites showed a greater value of curing depth than the commercial TT composite used as a reference. Table 2. Degree of conversion at 40 s and depth of cure. Samples DC (%) Depth of Cure (mm) Property Value 55–80 ≥2 M1 86.3 (0.1) a2.95 (0.01) a M4 85 (2) a,b 2.94 (0.01) a,b C145 71 (5) c2.93 (0.02) b C145s 79 (2) d2.93 (0.01) b C445 80 (3) b,d 2.93 (0.01) a,b C445s 76 (6) c,d 2.93 (0.02) a,b TT 63 (4) e2.67 (0.09) c a,b,c,d,e Statistically homogeneous groups (analysis by properties). Appl. Sci. 2024,14, 2453 8 of 14 In the case of the degree of conversion for the matrices and composites at 40 s of irradiation, values higher than 67% and, in several cases, more than 75% (C145s and C445) were obtained. As shown in Table 2, all experimental composites present DC values higher than TT, except for composite C145, which does not have significant differences from it. Furthermore, the highest values of this property correspond to matrices M1 and M4, (86.3 ±0.1)% and (85 ±2)%, respectively. The results of the water sorption and solubility test are shown in Figure 6. The highest values of this property evaluated for the different formulations correspond to the pristine monomeric matrices M1 and M4, which slightly exceed the limit value in the ISO 4049:2019 standard [ 12 ]. In the case of absorption, the best values correspond to the C145s, C445 and C445s composites, and no significant differences between them are observed. Furthermore, the commercial TT composite showed the lowest water sorption values and significant differences with respect to the experimental composites. Regarding water solubility, no significant differences were observed between C145 and C145s, nor between C445 and C445s. Appl. Sci. 2024, 14, x FOR PEER REVIEW 8 of 14 Table 2. Degree of conversion at 40 s and depth of cure. Samples DC (%) Depth of Cure (mm) Property Value 55–80 ≥2 M1 86.3 (0.1) a 2.95 (0.01) a M4 85 (2) a,b 2.94 (0.01) a,b C145 71 (5) c 2.93 (0.02) b C145s 79 (2) d 2.93 (0.01) b C445 80 (3) b,d 2.93 (0.01) a,b C445s 76 (6) c,d 2.93 (0.02) a,b TT 63 (4) e 2.67 (0.09) c a, b, c, d, e Statistically homogeneous groups (analysis by properties). The results of the water sorption and solubility test are shown in Figure 6. The highest values of this property evaluated for the different formulations correspond to the pristine monomeric matrices M1 and M4, which slightly exceed the limit value in the ISO 4049:2019 standard [12]. In the case of absorption, the best values correspond to the C145s, C445 and C445s composites, and no significant differences between them are observed. Furthermore, the commercial TT composite showed the lowest water sorption values and significant differences with respect to the experimental composites. Regarding water solubility, no significant differences were observed between C145 and C145s, nor between C445 and C445s. Figure 6. Water sorption and solubility for the matrices and composites. A, B, C, D and a, b, c are the statistically homogeneous groups for absorption and solubility, respectively. The thermal degradation of dental composites and matrices can be visualized from the thermograms shown in Figure 7. As observed in Figure 7a, M1 and M4 have highest mass loss (90% at 700 °C and 89% at 500 °C, respectively) with respect to the other samples. In the case of composites, the greatest mass loss corresponds to the commercial TT (24.40%), followed by C145 (47.08%), C145s (49.00%), C445 (49.36%), and C445s (55.68%). The analysis of the first derivative (Figure 7b) indicates that the degradation of the tested materials occurs in an interval of 250–650 °C. In addition, the matrices present two decomposition temperatures (M1: 400, 424 °C; M4: 408, 432 °C), and the C145, C145s, and C445 Figure 6. Water sorption and solubility for the matrices and composites. A, B, C, D and a, b, c are the statistically homogeneous groups for absorption and solubility, respectively. The thermal degradation of dental composites and matrices can be visualized from the thermograms shown in Figure 7. As observed in Figure 7a, M1 and M4 have highest mass loss (90% at 700 ◦ C and 89% at 500 ◦ C, respectively) with respect to the other samples. In the case of composites, the greatest mass loss corresponds to the commercial TT (24.40%), followed by C145 (47.08%), C145s (49.00%), C445 (49.36%), and C445s (55.68%). The analysis of the first derivative (Figure 7b) indicates that the degradation of the tested materials occurs in an interval of 250–650 ◦ C. In addition, the matrices present two decomposition temperatures (M1: 400, 424 ◦ C; M4: 408, 432 ◦ C), and the C145, C145s, and C445 composites experience a single decomposition at 430, 424, and 420 ◦ C, respectively. The commercial composite experiences two endothermic effects, one at 368 ◦ C and the other at 451 ◦ C. On the other hand, the C445s composite experiences three endothermic effects (372, 420, and 600 ◦C). Appl. Sci. 2024,14, 2453 9 of 14 Appl. Sci. 2024, 14, x FOR PEER REVIEW 9 of 14 composites experience a single decomposition at 430, 424, and 420 °C, respectively. The commercial composite experiences two endothermic effects, one at 368 °C and the other at 451 °C. On the other hand, the C445s composite experiences three endothermic effects (372, 420, and 600 °C). (a) (b) Figure 7. (a) Thermogravimetric curves; (b) First derivative of the thermogravimetric curves of matrices and composites. 4. Discussion Silicon dioxide presents three characteristic absorption bands at 1080, 800, and 450 cm−1, which correspond to the vibration modes of stretching, bending, and rocking of the Si-O-Si bonds, respectively. In the IR spectra of the untreated and silanized aerosil OX50 (Figure 2a) only two of the characteristic bands of silicon dioxide are observed, since the spectrum was recorded between 4500 and 650 cm−1. The highest-intensity band appears around 1100 cm−1 and the least intense one at approximately 801 cm−1, corresponding to the stretching and bending vibrations of the Si-O-Si bonds, respectively. The authors have verified the occurrence of silanization by the presence of a band of low intensity around 1707 cm−1 in the spectrum of silanized aerosil OX50 (Figure 2a), corresponding to the vibrations of the carbonyl group present in the MPS coupling agent [23]. The low intensity of this band suggests that a very small percentage of silane is incorporated, making it almost imperceptible for this characterization technique. When the TGA curves of the unsilanized aerosil OX50 (Figure 2b) are analyzed, it is observed that the mass of the sample remains practically constant, which is due to the thermal stability of silicon dioxide. In the case of the treated aerosil OX50, the loss of 2% mass is associated with the decomposition of the MPS. In this sense, it has been reported that the physical decomposition of silane physically absorbed on the surface occurs between 50 and 150 °C, while the silane chemically bonded to the filler decomposes between 200 and 600 °C [24]. These results demonstrate the appearance of the silanization process of the filler. The authors report that nanoparticle resins are highly aesthetic and their physical and mechanical properties are superior to others, presenting less contraction during polymerization and high resistance to fracture, compression, and traction [25]. With a smaller particle size, a better finish of the restoration is achieved, which is observed in its surface texture, reducing the possibilities of biodegradation of the material. In addition, this technology has allowed the mechanical properties of the resin to be suitable for both the anterior and posterior sectors [26]. In the micrographs of aerosil OX50 (Figure 3) it can be seen that it is formed by spherical particles in agreement with Aminoroaya et al. [8], suggesting that dental composites containing smaller spherical silica particles result in Figure 7. (a) Thermogravimetric curves; (b) First derivative of the thermogravimetric curves of matrices and composites. 4. Discussion Silicon dioxide presents three characteristic absorption bands at 1080, 800, and 450 cm−1 , which correspond to the vibration modes of stretching, bending, and rocking of the Si-O-Si bonds, respectively. In the IR spectra of the untreated and silanized aerosil OX50 (Figure 2a) only two of the characteristic bands of silicon dioxide are observed, since the spectrum was recorded between 4500 and 650 cm −1 . The highest-intensity band appears around 1100 cm−1 and the least intense one at approximately 801 cm −1 , corresponding to the stretching and bending vibrations of the Si-O-Si bonds, respectively. The authors have verified the occurrence of silanization by the presence of a band of low intensity around 1707 cm −1 in the spectrum of silanized aerosil OX50 (Figure 2a), corresponding to the vibrations of the carbonyl group present in the MPS coupling agent [ 23 ]. The low intensity of this band suggests that a very small percentage of silane is incorporated, making it almost imperceptible for this characterization technique. When the TGA curves of the unsilanized aerosil OX50 (Figure 2b) are analyzed, it is observed that the mass of the sample remains practically constant, which is due to the thermal stability of silicon dioxide. In the case of the treated aerosil OX50, the loss of 2% mass is associated with the decomposition of the MPS. In this sense, it has been reported that the physical decomposition of silane physically absorbed on the surface occurs between 50 and 150 ◦ C, while the silane chemically bonded to the filler decomposes between 200 and 600 ◦ C [ 24 ]. These results demonstrate the appearance of the silanization process of the filler. The authors report that nanoparticle resins are highly aesthetic and their physical and mechanical properties are superior to others, presenting less contraction during polymerization and high resistance to fracture, compression, and traction [ 25 ]. With a smaller particle size, a better finish of the restoration is achieved, which is observed in its surface texture, reducing the possibilities of biodegradation of the material. In addition, this technology has allowed the mechanical properties of the resin to be suitable for both the anterior and posterior sectors [ 26 ]. In the micrographs of aerosil OX50 (Figure 3) it can be seen that it is formed by spherical particles in agreement with Aminoroaya et al. [ 8 ], suggesting that dental composites containing smaller spherical silica particles result in more uniform stress distribution and a reduction of stress at the filler–matrix interface. An increase in filler size is expected to increase the stress concentration at the filler–matrix interface, causing more susceptibility to crack nucleation in this area due to the different rigidities of organic matrix and inorganic filler, leading to lower mechanical performance, such as lower strength of the dental composites.