Introduction

In the last decades, resin-matrix composites have become the most suitable materials in restorative dentistry due to their technological development on the chemical composition and processing. Nowadays, resin-matrix composites are used for indirect and direct restorations in restorative dentistry [1,2,3]. The chemical composition involving the inorganic fillers and polymeric matrix of the resin-matrix composites is a major factor that determines their optical and mechanical properties [2, 3]. However, the chemical composition and physical properties of the currently available resin-matrix composites widely vary from manufacturers and types of materials [4,5,6]. Inorganic fillers are added in the chemical composition of the resin-matrix composites to increase their mechanical properties and to mimicking the optical properties of enamel and dentin [7,8,9]. On light curing procedures, the inorganic fillers must allow the transmission of visible light required for the activation of the polymerization reaction of the polymeric matrix. Nevertheless, the polymerization can vary depending on the content, type, and size of inorganic fillers considering optimal conditions on the procedure and equipment of light curing procedures [9,10,11]. A lack of polymerization decreases the strength, elastic modulus, hardness, and wear resistance of the resin-matrix composites leading to the degradation and release of monomers to the surrounding tissues [12,13,14,15].

A resin-matrix composite comprises inorganic vitreous fillers dispersed in an organic matrix. The organic matrix involves a cross-linking of dimethacrylate monomers such as bisphenol A-glycidyl dimethacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), urethane dimethacrylate (UDMA), and ethoxylated bisphenol A dimethacrylate (Bis-EMA) [16,17,18]. Such combination of molecules results in an organic matrix that depends on the structure of the monomers and the polymerization reaction. On visible-light polymerization, camphorquinone (CQ) combined with a tertiary amine are incorporated in resin-matrix composites as a photoinitiator system [19,20,21,22]. Also, acyl and bisacyl phosphine oxide initiators can be utilized as single-component visible-light alpha-cleavage initiators at wavelengths usually below 450 nm [20, 23, 24]. In the organic matrix, CQ is stimulated by visible light irradiation in the range between 420 and 490 nm [15, 25, 26]. Commercially available resin-matrix composites show a weight percentage (wt. %) of inorganic filler content ranging from 40 up to 90 wt. % [7, 27]. A mixture of different inorganic fillers (i.e., glass ceramics and silica) at different sizes can be found in the chemical composition of current resin-matrix composites [6, 8, 28]. Nano- and micro-scale particles are combined in the resin-matrix composite microstructure to promote a mechanical reinforcement under chewing loading [6, 9, 28]. In fact, a high content of nano- and micro-scale particles results in a low organic matrix volume under polymerization. Indeed, inorganic fillers affect the polymerization shrinkage, wear, surface roughness, translucency, opalescence, and fluorescence of resin-matrix composites [18, 29, 30]. Resin-matrix composites with volume fraction up to 60 wt. % revealed increased values of flexural strength and elastic modulus [31]. Despite the influence of the inorganic filler content on the mechanical properties of resin-matrix composites, the nature and shape of the inorganic particles must also be considered [33, 34]. Spherical-shaped inorganic particles allow higher amount of fillers within the resin-matrix composites, and the enhancement of materials' strength comparatively to the materials with irregular filler particles, once the stresses tend to accumulate in the protuberances of the irregular-shaped particles [32]. Nevertheless, the effects of the content, size, and shape of inorganic fillers on the light transmission through resin-matrix composites are not entirely elucidated in literature.

The polymerization of resin-matrix composites for direct restorations can be achieved by demand using light-curing units (LCU) as a source of light ranging from 360 up to 500 nm [25, 35, 36]. Currently, light-emitting diodes (LED) are the most typical light source within wavelength ranging from 360 up to 500 nm, light irradiance between 400 and 1765 mW/cm2, and light exposure from 20 to 60 s [37,38,39]. The time of light exposure depends on the light irradiance as well as type and thickness of the restorative materials  to reach the energy required for the polymerization of the resin-matrix composite. The incident light can be reflected, refracted, absorved, scattered, and/or transmitted towards the material. On the resin-matrix composites’ surfaces, part of the light is reflected, due to the differences in the material refractive indexes [9]. Scattering also occurs when light passes through the material due to the existence of fillers and defects such pores or cracks. Scattering changes with the wavelength of incident light and is mostly determined by the particle size and by the relationship between their refractive indexes [9, 40, 41]. The accomplishment of the polymerization of the organic matrix leads to densely crosslinked, glassy polymer networks that provide high values of physical properties such as strength, elastic modulus, hardness, toughness, and wear resistance [42,43,44]. The optical and mechanical behavior of resin-matrix composites is strongly influenced by the degree of conversion (DC) of the organic matrix [14, 17]. DC percentage of resin-matrix composites is determined by a comparison of the peak height absorbance intensity of aliphatic carbon–carbon double bond (C = C) with aromatic C = C, before and after irradiation [18, 45]. Manufacturers recommend resin-matrix composite increments with 2-mm thickness [46, 47], to achieve adequate DC proportion. The clinical success of direct restorations depends on the light transmission, polymerization, and material properties [4, 5].

The relationship among light, polymerization, DC, and fillers raised the question on the influence of inorganic fillers content on light transmission through resin-matrix composites. Thus, the purpose in this study was to evaluate the light transmission through five different resin-matrix composites regarding the inorganic filler content. The following hypotheses were established: (i) the percentage of inorganic fillers has a significant influence on the light-curing transmission through the resin-matrix composites; (ii) the increase in the inorganic fillers’ amount induces a high light-curing transmission through the material.

Materials and methods

Preparation of specimens

Disc-shaped specimens (diameter at 10 mm × thickness of 2 mm) were prepared from five different commercially available resin-matrix composites. Traditional and flowable resin-matrix composites (shade A1) were selected considering a variation proportion of the inorganic fillers’ content ranging from 60 up to 89 wt. %. The chemical composition of the resin-matrix composites was provided by the manufacturers, as seen in Table 1. Data of the resin-matrix composites were given by the manufacturers. Seventy-five specimens were prepared in a custom-made glass holder following the standard incremental technique, as seen in Fig. 1. Five specimens were prepared for each set of polymerization time point.

Table 1 The chemical composition of the tested resin-matrix composites provided by the manufacturers. 
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Fig. 1
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A Preparation of specimens and light irradiance time points. B Optical transmittance measurements

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The polymerization of the resin-matrix composites was carried out using a LED light curing unit (Cordless LED light B™, Woodpecker Co., Poland). Light-curing unit (LCU) emitted light with a wavelength ranging from 430 up to 480 nm. Prior to the light-curing procedure, the irradiance of the device was measured by a calibrated radiometer (ProclinicExpert™, Montellano, Lisbon), showing systematically a steady irradiance at 350 mW/cm2. The LCU tip was mechanically positioned perpendicularly to the specimen surface plane and at 7 mm away from the surface, as seen in Fig. 1. The light irradiance was carried out following the guidelines from a previous study [44]. It should be emphasized that the light-curing procedure was performed in a controlled environment, with no expected light losses or dissipation through the surrounding environment.

Optical transmittance tests

Optical transmittance measurements were performed for each specimen before and after light curing for 10, 20, or 40 s (Fig. 1A). Thus, the setup comprised the optical transmittance measurement of non-polymerized specimens, followed by the polymerization of the specimens, and then the optical transmittance measurement of light-cured specimens (Fig. 1B). The measurements were immediately performed after polymerization. For calibration purposes, optical reference measurements were acquired between each set of measurements. All the transmittance spectra were measured using a top-bench spectrophotometer with an integrated monochromator (AvaSpec-ULS2048XL EVO, Avantes, NS Apeldoorn, The Netherlands). The apparatus comprised a 200 W Quartz Tungsten Halogen light source (model 66,881, Oriel Newport, USA), optical-fiber probes, and a metallic holder for specimens (Fig. 1B).

The assays were performed for a wavelength range between 350 and 850 nm, with a measurement integration time of 10 ms. The integrated monochromator was coupled to a computer for data acquisition and smoothing, based on 100 measurements’ average (1 s total analysis time). The spectrophotometer has an optical bench with 37.5-, 50-, 75-, or 100-mm focal length, developed in a symmetrical Czerny-Turner design. Light enters the optical bench through a standard SMA-905 connector and is collimated by a spherical mirror. A plain grating diffracts the collimated light and a second spherical mirror focuses the resulting diffracted light. An image of the spectrum is projected onto a 1-dimensional linear detector array (as manufacturer information). The experiments were run in triplicate and carried out in five independent assays.

Nano-indentation and micro-hardness tests

Resin-matrix composite specimens were embedded in autopolymerizing polyether-modified resin (Technovit 400™; Kulzer GmbH, Germany) in polyvinyl chloride mold [49]. Then, assemblies were cross-sectioned at 90° relative to the surface plane. Specimens were cross-sectioned by wet grinding on low speed using a standard laboratory metallographic machine (Struers, USA) and SiC papers ranging from 120 down to 400 mesh [49]. Cross-sectioned specimens were wet ground by using SiC paper from 300 down to 2400 mesh followed by polishing with 3000 and 6000 mesh (3-μm particle size) SiC paper (Trizact ™, 3M ESPE, USA). Then, specimens were ultrasonically cleaned in prophyl alcohol for 5 min and in distilled water for 10 min [50]. An optical micrography of a cross-sectioned specimen at × 200, is shown in Fig. 3F.

Nanoindentation tests were carried out with the loading axis perpendicularly to the specimen’s surfaces (Fig. 1). A nano-hardness tester (Nano Instruments, Inc. Knoxville; TN, USA) operated with a Berkovich diamond pyramid tip (apex angle of 143°) was used to make nanoscopic indentations in triplicate over the surfaces (n = 9). Load of 5 mN was applied at 0.04 mN/s onto the specimens for 15 s. Five points at different regions  were determined along the specimen. The shape function was determined by the Oliver and Pharr method (1992). Nanoindentation tests performed at 90° relative to the surface allow evaluating stress/strain fields induced during indentation across the specimen with respect to the displacement axis of the indenter. Nano-hardness and elastic modulus were acquired as a function of the position of the indenter axis relative to the specimens’ surface. Additionally, Vickers micro-hardness of the specimens was measured using a micro-indenter apparatus (Leica VMHT30™, Leica Microsystems, Germany) with a diamond pyramid-shape indenter. An indentation load at 200 g was applied onto the specimens for 20 s [44, 51, 52] at 5 different points on the surface of each specimen (n = 15). The plastic deformation of the indentation area was inspected using an optical microscope (Leica DM 2500 ™; Leica Microsystems, Germany) coupled to a computer for image processing by Leica Application Suite™ software.

Chemical and microstructural analyses

At first, cross-sectioned specimens were inspected by optical microscopy at magnification ranging from × 30 up to × 1000. Microstructural analyses were performed using an optical microscope (Leica DM 2500 ™; Leica Microsystems, Germany) coupled to a computer for image processing, using Leica Application Suite™ software (Leica Microsystems, Germany). A number of five micrographs were acquired at different magnifications, for each specimen (n = 15). The software Adobe PhotoshopTM (Adobe Systems Software, Ireland) was used to analyze black and white images, considering the black region represented the organic matrix and the white regions represented the inorganic fillers. Image JTM software (National Institutes of Health, USA) was used to quantify the content of inorganic fillers. Then, surfaces were sputter coated with a AgPd thin layer for scanning electron microscopy (SEM) analyses by using a SEM unit JSM-6010 LV™ (JEOL, Japan) coupled to energy dispersive spectroscopy (EDS). The inorganic fillers and microstructure of the specimens were evaluated at high magnification ranging from × 1000 up to × 20,000 under (SE) secondary and (BSE) backscattered electrons, as shown in Fig. 4 [34,35,36]. Even though the chemical composition of the resin-matrix composites was provided by the manufacturers (in Table 1), elemental analyses were performed by energy dispersive spectroscopy (EDS) coupled to the SEM apparatus.

Statistical analysis

Results were statistically analyzed by normality test Shapiro–Wilk and two-way ANOVA to determine statistical differences in the micro-hardness, nano-hardness, elastic modulus, and transmittance values between groups. The Student t-test was used to compare the results considering the content of inorganic fillers and polymerization time. A probability value  below 0.05 was considered significant. The power analysis performed by Student t-test or ANOVA to determine the number of specimens for each group (n) revealed a test power of 100% in the present study. Statistical analyses were carried out using Origin Lab statistical software (Origin Lab, Northampton, MA, USA).

Results

Chemical and microstructural analyses results

The microstructure of the materials acquired by optical microscopy can be noticed in Fig. 2A–C. At low magnification, the white spots in the micrographs represent the micro-scale inorganic fillers with an irregular morphological aspect. The morphological aspects were evaluated at higher magnification by SEM at secondary or backscattered electrons mode, as seen in Fig. 2D–I.

Fig. 2
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A-C Optical microscopy recorded at × 500 magnification for resin-matrix composites reinforced with A 60, B 78–80, and C 89 wt. % inorganic fillers. D-I SEM images recorded at × 5000 magnification for resin-matrix composites reinforced with D 60, E 62.5, F 74, G 78–80, and H 89 wt. % inorganic fillers. I SEM images recorded at × 10,000 magnification for resin-matrix composites reinforced with 89 wt. % inorganic fillers

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On SEM analyses, the micro-scale inorganic fillers were clearly inspected and the sizes of the particles were measured for each material. The chemical analyses by EDS detected the presence of zinc oxide in the flowable resin-matrix composites reinforced with 60 wt. % inorganic fillers and ytterbium fluoride and silica for flowable resin-matrix composites reinforced with 62.5 wt. %. Traditional resin-matrix composites reinforced with 74 wt. % inorganic fillers revealed the presence of silica (spherical particles) and glass ceramic (with irregular shape). Traditional resin-matrix composites reinforced with 78–80 wt. % inorganic fillers showed the presence of spherical silica particles and glass ceramic with irregular shape.

The resin-matrix composite with 78–80 wt. % inorganic fillers revealed the presence of silica, ytterbium fluoride, and barium glass, while composites reinforced with 89 wt. % inorganic fillers revealed silica and zirconia in their microstructure. Chemical composition of the inorganic fillers corroborated with the technical information provided by the manufacturer (Table 1). As seen in Fig. 2I, the size of the zirconia particles ranged from 0.74 up to 5.01 μm.

Optical transmittance analysis

As seen in Fig. 3, the transmittance spectra of non-polymerized specimens (0 s) were quite similar with a high transmittance within the ultraviolet (UV) region, below 400-nm wavelength.

Fig. 3
figure 3

Transmittance (a.u.) of the resin-matrix composites in the wavelength range from 350 up to 850 nm at light irradiance for 0 s (non-polymerization), 10 s, 20 s, and 40 s. Resin-matrix composites containing A 89, B 78–80, C 74, D 62.5, and E 60 wt. % inorganic fillers. F Schematics of the light transmission through resin-matrix composites

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The non-polymerized (np) specimens showed very low optical transmittance from 400- up 850-nm wavelength once the light was completely absorbed. Thus, non-polymerized resin-matrix composites showed no significant differences in optical transmittance for wavelengths above 450 nm (Fig. 3).

After the polymerization, there was an increase in optical transmittance of all specimens above 650-nm wavelength, i.e., when the optical spectrum approaches the near-infrared (near-IR) region. On higher polymerization time, the increase in the light transmittance was also higher, showing that the polymerization of the resin-matrix composites increases with the irradiance time. For instance, similar optical transmittance behavior was noticed for resin-matrix composites containing 62.5 and 78–80 wt. % inorganic fillers since an increase in transmittance occurred for polymerization time from 0 up to 40 s. Both resin-matrix composites contained the same types and quite similar dimensions of inorganic particles, as seen in Fig. 2.

Also, resin-matrix composites containing 60 and 74 wt. % inorganic fillers revealed similar light transmittance at irradiance for 20 or 40 s. The findings also suggested the light irradiance time can be performed for 20 s and therefore the clinical procedure validate such findings. Resin-matrix composites containing 60 and 74 wt. % inorganic fillers were composed of identical types of inorganic particles. Nevertheless, different light transmittances were recorded for resin-matrix composites reinforced with 89 wt. % inorganic fillers (Fig. 3A). A lower increase of light transmittance was detected for such composite for 20 or 40 s when compared to the other resin-matrix composites (Fig. 3B–E). Light transmittance was higher at irradiance time for 20 s compared to values recorded for 40 s. Statistical analyses of transmittance measurements at 850-nm wavelength are shown in Table 2.

Table 2 Analysis of variance of the transmittance results at 850-nm wavelength. Each measurement corresponds to an individual sample
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In Fig. 4, the average ratio of the light transmittance between the polymerized (p) and non- polymerized (np) specimens (n/np ratio) is shown regarding the light wavelength range from 600 up to 850 nm for different polymerization time points: 10, 20, and 40 s. The results showed 1.6 times higher transmittance ratio for resin-matrix composites reinforced with 89 wt. % inorganic fillers under light irradiance at 850-nm wavelength in comparison with light irradiance at 650-nm wavelength (p = 0.0038). Also, p/np ratio was higher for 40 s compared to the ratio for 10 or 20s (Fig. 4A). The p/np ratio for resin-matrix composites reinforced with 78–80 wt. % inorganic fillers was higher under light irradiance for 20 s (p = 0.018) and 40 s (p = 0.0009) and lower for 10 s (p = 0.001), being 2.5 times higher at the 850-nm wavelength than at 600-nm wavelength (Fig. 4B). The p/np ratio for resin-matrix composites reinforced with 74 wt. % inorganic fillers was similar under light irradiance for 10 or 20 s (p < 0.0001), although p/np ratio was higher for 40 s (p = 0.0002) and increased × 2.5 from 600-nm up to 850-nm wavelength (Fig. 4C). Resin-matrix composites reinforced with 62.5 wt. % inorganic fillers showed a higher p/np ratio for 40 s (p < 0.0001) and lower p/np ratio for 10 or 20 s (p < 0.0005), but it was 2.5 times under light irradiance at 850-nm wavelength (Fig. 4D). At last, the resin-matrix composites reinforced with 60 wt. % inorganic fillers showed the highest p/np ratio since it was recorded at 3.5 times higher under light irradiance at 850-nm wavelength than that at 600-nm wavelength (p < 0.0001) (Figs. 4E and 5).

Fig. 4
figure 4

Transmittance ratio (a.u.) between the polymerized and non-polymerized resin-matrix composites in the wavelength range from 600 up to 850 nm at light irradiance. Resin-matrix composites containing A 89, B 78–80, C 74, D 62.5, and E 60 wt. % inorganic fillers. F Schematics of the light transmission through resin-matrix composites

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Fig. 5
figure 5

Ratio of the transmittance of the polymerized and non-polymerized (p/np ratio) resin-matrix composites for different light irradiance exposures. *Statistically significant (p < 0.05); **highly significant (p < 0.001)

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An overall comparison of the average ratio of the light transmittance between the polymerized and non-polymerized specimens is shown in Fig. 5. Statistical data on the p/np ratio at 850-nm wavelength are shown in Table 3.

Table 3 Analysis of variance of the p/np ratio at 850-nm wavelength for 10, 20, and 40 s compared with p/np ratio at 0 s
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Mechanical behavior

In Fig. 6, the micro-hardness mean values are reported for the resin-matrix composites reinforced with different contents of inorganic fillers and polymerized under light irradiance for 20 or 40 s.

Fig. 6
figure 6

A SEM image of the Vickers indentation on the resin-matrix matrix specimen. Mean values of B micro-hardness, C elastic modulus, and D nano-hardness recorded for resin-matrix composites after polymerization for A 20 s or B 40 s. *Statistically significant (p < 0.05); **highly significant (p < 0.001)

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As expected, the highest microhardness mean values were recorded for resin-matrix composites reinforced with 89 wt. % inorganic fillers after polymerization for 20 or 40 s (p < 0.005). In the same way, resin-matrix composites reinforced with 78–80 wt. % inorganic fillers showed higher microhardness mean values than the other resin-matrix composites after polymerization for 20 s (p < 0.05). Flowable resin-matrix composites showed low micro-hardness values and the composites reinforced with 60 wt. % inorganic fillers revealed the lowest mean values.

Mean values of elastic modulus and nano-hardness of the materials recorded by nano-indentation assays are shown in Fig. 6C and D. After light irradiance for 20 or 40 s, the highest mean values of elastic modulus and nano-hardness were measured for resin-matrix composites reinforced with 89 wt. % (p < 0.001), followed by the composites reinforced with 78–80 wt. % (p < 0.05), while the lowest values were recorded for composites reinforced with 62.5 wt. %.

Discussion

The present study focuses on the effect of inorganic particles on the light transmittance through resin-matrix composites by using visible and near-IR spectrophotometry. Thus, the findings acquired in this study revealed the percentage of inorganic fillers influences the light transmittance through the resin-matrix composites. Resin-matrix composites reinforced with 89 wt. % inorganic fillers showed a low light transmittance after polymerization for 10, 20, or 40s. Also, the highest content of fillers resulted in a low ratio of light transmittance on the resin-matrix composites regarding light absence before and light irradiance. On the other hand, flowable resin-matrix composites reinforced with 60 wt. % inorganic fillers revealed the highest light transmittance and consequently light transmittance ratio regarding light absence before and after light irradiance. In this way, the results validate the first hypothesis of the present study. The second hypothesis was rejected on the increase in the fillers’ amount inducing a high light-curing transmission through the material.

In this study, five types of resin-matrix composites were assessed considering the content of inorganic fillers, as seen in Table 1. One group of resin-matrix composites showed a high content of inorganic fillers at 89 wt. %, while another group revealed a low content of inorganic fillers at 60 wt. %. Inorganic fillers were properly identified by scanning electron microscopy coupled to energy dispersive spectroscopy. Resin-matrix composite reinforced with 89 wt. % inorganic fillers was composed of irregular shape glass fillers and spherical amorphous silica (Fig. 2). The dimensions of glass fillers were identified at micro-scale, while silica was detected at submicron- and nano-scale. Considering the type of inorganic fillers, a similar microstructure was noticeable on the resin-matrix composites reinforced with 62.5 or 80 wt. % inorganic fillers composed of ytterbium fluoride or amorphous silica. The mean size of most inorganic fillers was measured at submicron- and micro-scales although nano-scale silica was also enclosed. Furthermore, a similar microstructure was noticeable on the resin-matrix composites reinforced with 60 or 74 wt. % inorganic fillers composed of ytterbium fluoride or amorphous silica at micro-, submicron-, or nano-scale dimensions. The resin-matrix composites were selected considering a similarity of organic components as follows: Bis-GMA, Bis-EMA, TEGDMA, UDMA, HEMA (Table 1). The photoinitiator system, namely, camphorquinone, was analogous for each resin-matrix composite. Thus, the refractive index of monomers and inorganic fillers can affect the light transmittance through resin-matrix composites [1, 53, 54] that implies the light transmittance is highly dependent on the type of materials [4, 54, 55].

Low-light transmittance through resin-matrix composites reinforced with a high proportion of inorganic fillers (i.e., 89 wt. %) corroborates with the inorganic particles influencing the light distribution through the resin-matrix composites. Light transmittance increased with the content of inorganic fillers decreased down to 60 wt. %. Such findings are consistent with the literature data which indicates that increased amount of filler promotes lower transmittance values [1, 56, 57]. That was also noticed by the evaluation of the ratio between the transmittance of the polymerized and non-polymerized specimens since specimens with a high inorganic fillers’ content showed lower ratio values. Nevertheless, a lower ratio was detected for composites containing 62.5 wt. %, that can be explained by a significant proportion of opaque and irregular inorganic fillers, such as ytterbium fluoride [58]. After polymerization, the optical transmittance values were lower at UV region than the values recorded for the spectra infrared region. Light transmittance in the UV region for non-polymerized and polymerized specimens for 10, 20, or 40 s indicates that light is mainly absorbed in the visible light spectra, while UV light is mainly transmitted through the resin composites. It can be explained by the arrangement of the polymer networks after polymerization, which allows a proper and efficient light pathway [4, 56]. Camphorquinone consumption, which reduces light absorption, can also be a factor to increase the light transmittance [53]. Additionally, the exothermic nature of the polymerization chain reaction causes a transient decrease in the refractive index of resin-matrix composites due to a decrease in their density leading to an increase in the light transmittance [4, 56]. Thus, the decrease of the light transmittance in function of a high filler content can be expected although the nature of the inorganic fillers also plays a key role on the light transmission pathways. For instance, glass ceramics such as silica can provide a high translucency, while other glass ceramics provide a reflection of visible light. The size of inorganic particles also affected the light transmittance ratio since the light transmittance increased when the inorganic particles’ size decreased [4, 59]. A mixture of different inorganic fillers (i.e., glass ceramics and silica) at different sizes can be found in the chemical composition of current resin-matrix composites. In this way, a balance in the type and size of glass ceramics should be investigated in further studies.

The indirect evaluation of polymerization efficiency and relative DC percentage [29] can also be predicted by measuring the mechanical properties of the materials including hardness, strength, fracture toughness, or elastic modulus [60, 61]. Most of resin-matrix composite groups revealed higher light transmittance values after light irradiation for 40 s when compared to the values for 20 s. The results of ratio and light transmittance were consistent with the micro-hardness measurements, indicating a high polymerization of the materials after light curing for 40 s. However, commercially available resin-matrix composites can reveal varying optical behavior under light irradiation depending on several factors including exposure time. As seen in Figs. 4 and 5, the inorganic particles lead to light scattering that interferes in the light transmittance concerning a puzzling function of the filler size, distribution, and the mismatch of refractive indexes between the inorganic fillers and organic matrix [62, 63]. For instance, resin-matrix composites reinforced with 60 wt. % inorganic fillers were significantly dependent on the light exposure time in the present study. The transmittance ratio was lower under light exposure for 10 s when compared to the light irradiation for 20 or 40 s. Specimens containing 62.5 or 78–80 wt. % fillers showed a similar behavior regarding the ratio, although resin-matrix composites with 62.5 wt. % fillers were more dependent on light exposure time. On the other hand, the resin-matrix composite reinforced with 89 wt. % inorganic fillers was not dependent on the light exposure for 20 or 40 s. Vickers micro-hardness values showed slightly higher values for 20 s when compared to those recorded after polymerization for 40 s, which indicates the resin-matrix composite was properly polymerized for 20 s. The increased filler content of resin-matrix composites tends to improve mechanical properties of resin-matrix composites [58]. Micro-hardness values recorded for resin-matrix composites containing 89 wt. % and 78–80 wt. % inorganic fillers after polymerization for 20 and 40 s were similar to the results from previous studies [64]. Also, the elastic modulus and nano-hardness results were consistent with the values provided by the manufacturers. In this study, mechanical behavior of resin-matrix composites was highly dependent on the light exposure time, except on the Vicker’s microhardness for the resin-matrix composites containing 89 wt. % fillers. The highest values of Vicker’s microhardness recorded for the resin-matrix composite containing 89 wt. % fillers were resultant from the high inorganic fillers’ content. The elastic modulus values for resin composites with 89 wt. % fillers were in concordance with the values recorded in previous studies [31,32,33]. Also, the results showed differences in values recorded after light curing exposure over light curing for 20 or 40 s. The other groups of resin-matrix composites reinforced with 60, 62.5, and 78 wt.% fillers possess a higher portion of organic matrix, which requests a longer exposure of visible light for adequate polymerization. Then, the DC of monomers in the organic matrix of those resin-matrix composites is dependent on the light exposure time to achieve the required mechanical properties. The findings were validated considering the differences in light transmittance through the tested groups of resin-matrix composites with inorganic fillers ranging from 60 up 89 wt.%. Such findings are clinically relevant since an adequate polymerization via chair side light curing units must be accomplished by the clinicians to guarantee proper mechanical properties in the oral cavity environment [65]. Thus, the relationship among light irradiance, polymerization, DC, and inorganic fillers plays an important role on the physicochemical behavior of the resin-matrix composites. Commercially resin-matrix composites are progressively improved by manufacturers and a wide range of materials are available for restorative dentistry. Then, novel materials must be analyzed by traditional and alternative physicochemical approaches. The present data show that visible and near-IR spectrophotometry could be considered a potential alternative methodology to estimate the DC of resin-matrix composites through the evaluation of the light transmittance behavior. Considering the findings shown in the present study, further studies can be carried out involving a comparison of data with other methods such as Fourier transform infrared (FTIR) or near-infrared (IR).

Conclusions

Within the limitations of the present in vitro study, the results indicate a decrease in the light transmittance through resin-matrix composites when the content of inorganic fillers increased in their chemical composition. However, the chemical composition, microstructure, and light irradiance exposure also varied concerning different commercially available resin-matrix composites. The concluding remarks can be drawn as follows:

  • Glass ceramics, ytterbium fluoride, and amorphous silica were the inorganic fillers found within the tested resin-matrix materials. The size of inorganic fillers ranged from nano- up to micro-scale dimensions and the high content of micro-scale inorganic particles can change the light pathway and decrease the light transmittance through the materials. Also, inorganic fillers with a high light refractive index interfere in the light irradiance, and therefore a balance in the light refractive index between inorganic fillers and organic matrix plays a key role in the light transmittance through the materials.

  • The lowest light transmittance values were recorded for a resin-matrix composite reinforced with 89 wt. % inorganic fillers, while the highest light transmittance was recorded on the resin-matrix composite reinforced with 60 wt. % inorganic fillers. However, a low light transmittance was noticed for the resin-matrix composite reinforced with 62.5 wt. % fillers due the presence of opaque inorganic fillers.

  • The elastic modulus and hardness values of resin-matrix composites reinforced with 89 wt.% fillers (11 wt.% organic matrix) were not significantly affected by the variation in light exposure from 10 up 40 s. On the other side, resin composites reinforced with 60 wt. % fillers (40 wt.% organic matrix) showed low mechanical properties only after light irradiance for 10 s, and therefore their light transmittance ratio increased with the light exposure time, resulting in enhanced mechanical properties. That reveals a dependence of those composites to the light irradiance regarding the kinetics of degree of conversion of monomers in function of the proportion of the organic matrix.

  • The present study data showed that visible and near-IR spectrophotometry could be considered a potential alternative methodology to estimate the DC of resin-matrix composites through the evaluation of the light transmittance behavior.