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. 2025 Sep 29;25:1489. doi: 10.1186/s12903-025-06886-3

Degree of conversion and microhardness of different composite resins polymerized with an advanced LED-curing unit

Gozde Ozciftci 1, Hayal Boyacioglu 2, Lezize Sebnem Turkun 1,✉
PMCID: PMC12482808  PMID: 41024048

Abstract

Background

The aim was to evaluate the effects of different curing times (3s/10s/20s) of an advanced Light-Emitting Diode (LED) curing unit on the degree of conversion (DC%), microhardness, and hardness ratio of three different composite resin materials.

Methods

180 cylindrical specimens (3 × 4 mm; n = 60) were prepared from PowerFill (Ivoclar, Liechtenstein), PowerFlow (Ivoclar), and Omnichroma Flow Bulk (Tokuyama, Japan). Half of the samples were used for the degree of conversion analysis (nDC = 30), and the other half for the microhardness test (n VHN = 30). All specimens were irradiated using a Light-Emitting Diode (LED) curing unit (Bluephase PowerCure, Ivoclar). Based on polymerization times: 3-second rapid, 10-second conventional, and 20-second conventional; three sub-groups were formed for each material (n = 10). The specimens’ degree of conversion (DC%) was analyzed using Fourier Transform Infrared Spectroscopy (FTIR). Microhardness values were determined using the Vickers microhardness test, and the bottom-to-upper surface hardness ratios were calculated. The degree of conversion and microhardness were analyzed with a paired t-test, ANOVA, and Tukey tests (p < 0.05).

Results

Comparisons among all groups revealed significant differences in upper and bottom DCs (p < 0.05). PowerFill exhibited higher values on both surfaces after 3s polymerization, while PowerFlow required 10s and Omnichroma Flow Bulk 20s to achieve similar results (p < 0.05). When the upper surface microhardness values were compared, no significant difference was found between PowerFlow and Omnichroma Flow Bulk in the 3s group (p = 0.623) and after 20s polymerization, between PowerFill and Omnichroma Flow Bulk (p = 0.082). Material type and polymerization duration significantly affected bottom surface microhardness values (p = 0.000). As the polymerization time increased, the bottom microhardness values of all materials increased. The highest microhardness ratio (HR) was obtained in the PowerFill group at 20 s (p = 0.029). In the PowerFlow group, no difference was found between HR values of 10s and 20s (p = 0.238). For Omnichroma Flow Bulk group, all polymerization durations showed significant differences (p = 0.000).

Conclusion

The polymerization performed with an advanced LED curing unit in a shorter time (3s) resulted in varying polymerization efficiencies among different types of composite resins. For more homogeneous polymerization, it is advised to extend the duration to 10 s or longer.

Keywords: Polymerization, Composite resin, LED curing unit, Degree of conversion, Microhardness, Bulk-fill

Background

It has been reported that a significant portion of a dentist’s clinical workload involves direct restorations polymerized using light-curing technologies [1]. In this context, efforts have been directed toward developing rapidly cured materials, techniques involving fewer clinical steps, and applications that reduce the overall procedure time and cost. The trend toward shortening the duration of restorative procedures has intensified competition within the dental industry, driving innovation in faster polymerization techniques [2].

Light-emitting diode (LED) polymerization units have been shown to achieve greater polymerization depth compared to quartz-tungsten-halogen (QTH) curing units [3]. Previous studies have demonstrated that methacrylate-based composite resins often exhibit incomplete cross-linking, leaving unreacted carbon-carbon double bonds (C = C) within the matrix and potentially containing up to 10% residual monomer, which may adversely affect the physical stability of the composite resin materials [4].

Since 2011, to mitigate the intense yellow hue of the camphorquinone (CQ) and to develop composite resins more compatible with lighter and bleached tooth shades, manufacturers have introduced alternative photoinitiators. These include phenylpropanedione (PPD), monoacylphosphine oxide (MAPO or Lucirin TPO), and bisacylphosphine oxide (BAPO or Irgacure 819), which have been incorporated into the resin formulations to enhance photoinitiation efficiency. However, these photoinitiators possess different absorption spectra compared to the classical camphorquinone. To address this discrepancy, third-generation LED curing devices have been designed to combine two emission peaks in the blue and violet spectra. These multi-wave devices, which emit radiation exceeding 1100 mW/cm² and total light output surpassing 8 W, have garnered attention in the field [5].

The latest generation of LED polymerization units, referred to as fourth-generation devices, exhibits numerous technological advancements, most notably, wavelength scanning technology. This innovation allows clinicians to select light output modes tailored to the specific material and clinical scenario [6]. One such device, the Bluephase PowerCure, utilizes Poly-Wave LED technology to emit across a broad spectrum (385–515 nm), rendering it compatible with all currently used photoinitiator systems. This high-performance unit offers four distinct curing modes: high power, turbo, a 3-second PowerCure mode, and a pre-curing mode. The 3-second PowerCure mode facilitates rapid and effective polymerization for pediatric patients, fissure sealants, and individuals with reflex sensitivity [7].

To enhance the structural stability of the polymer network, controlled radical polymerization mechanisms have been proposed. A step-by-step polymerization mechanism may lead to more homogeneous and durable polymer networks. Previous studies have suggested that incorporating the β-allyl sulfone (AFCT) reactive component allows the reversible addition–fragmentation chain transfer (RAFT) polymerization mechanism to be used in dental polymers [8, 9]. Studies have shown that including AFCT in dimethacrylate-based polymer networks improves its architecture during polymerization, increases network homogeneity, raises the glass transition temperature, and enhances mechanical properties [9, 10]. This innovative modification has been implemented in bulk-fill composite resins as part of the PowerCure restorative system (PowerFill and PowerFlow; Ivoclar), which is designed to polymerize under high light intensity (3050 ± 10% mW/cm²) using a Polywave light-curing unit (Bluephase PowerCure) in 3 s [8, 10, 11].

Although the polymerization process is fundamentally chemical, several key parameters are still clinician controlled. Polymerization time and light intensity determine the amount of energy available to activate photoinitiators, directly impacting the speed and quality of the curing process [12]. Inadequate light exposure may lead to reduced monomer conversion and decreased mechanical performance, which can subsequently result in volumetric and marginal breakdown, and secondary caries formation that contribute to the failure of the restorations [13]. Nonetheless, the effects of high-intensity, rapid-curing protocols remain inadequately explored and analyzed.

The degree of conversion (DC) refers to the proportion of carbon-carbon double bonds (C = C) transformed into single bonds during polymerization, representing the extent of monomer-to-polymer conversion [14]. Generally, increased light energy correlates with higher conversion rates [15]. For Bis-GMA–based composite resins, a DC range of 50–60% reflects a high cross-linking density, indicating that approximately 50–60% of methacrylate groups have been polymerized. However, this does not imply that 40–50% of the monomer remains unreacted. When identical monomers and adequate light exposure are employed, chemically and light-cured composite resins exhibit comparable overall conversion degrees, typically between 50 and 70% at room temperature [16].

Microhardness, a critical mechanical property of polymerized restorative materials, is one of the indicators of resistance to scratching and wear and plays a vital role in enhancing the clinical performance of restorative materials. Increased hardness enhances resistance to deformation under functional loading, thereby promoting the material’s long-term durability [17].

In the evaluation of polymerization depth, the hardness ratio (HR), calculated as the ratio of bottom surface microhardness to upper surface microhardness, is frequently used [18]. This parameter serves as a critical indicator of whether composite resins have been uniformly cured. Some previous researches have proposed that, to assess the adequacy of bottom polymerization in composite resins, the HR value should be no less than 80% [19, 20]. Attaining this threshold suggests that the composite resin has reached a sufficient depth of cure and possesses clinically acceptable mechanical strength [21].

Considering the aforementioned literature, the present study aims to evaluate the effects of various modes (3 s/10 s/20 s) of an advanced LED curing unit—capable of effective polymerization up to 4 mm in depth—on the degree of conversion, microhardness, and hardness ratio of three different categories of composite resin materials.

The hypothesis tested was as follow:

  • H₀: The time of curing will not significantly affect the degree of conversion, microhardness, and hardness ratio of the different composite resin groups.

Methods

This in vitro study was conducted at the Research Laboratory of Ege University and the FTIR Laboratory of the Institute of Nuclear Sciences, Ege University. The composite resins used in this study were PowerFill (Ivoclar, Liechtenstein), PowerFlow (Ivoclar), and Omnichroma Flow Bulk (Tokuyama, Japan). Table 1 presents the compositions, manufacturers, and lot numbers of these materials. A third-generation advanced LED light-curing device, Bluephase PowerCure (Ivoclar), was used for polymerization, with three different light intensities and durations: 20 s at 1000 mW/cm², 10 s at 1000 mW/cm², and 3 s at 3050 mW/cm².

Table 1.

Composition, manufacturers, and lot numbers of composite resins used in the study (BisGMA: bisphenol-A Diglycidil ether dimethacrylate; UDMA: urethane dimethacrylate; TEGDMA: triethylene glycol dimethacrylate; Bis-EMA: ethoxylated bisphenol-A-dimethacrylate; DCP: tricyclodecane-dimethanol dimethacrylate; vol%: volume %; wt%: weight%)

Composite Resins Resin Matrix Composition Filler Composition Filler Content (wt%/vol%) Photoinitiator Manufacturer Lot
Number
PowerFill Bis-GMA, UDMA, Bis-EMA, aromatic dimethacrylate, DCP Barium aluminum silicate glass, ytterbium trifluoride, spherical mixed oxide, isofillers

79%/

53–54%

 Ivocerin-dibenzoyl germanium Ivoclar; Schaan, Liechtenstein Z009GW
PowerFlow Bis-GMA, Bis-EMA, UDMA Barium glass, ytterbium trifluoride, copolymer 68.2%/46.4% Ivocerin-dibenzoyl germanium Ivoclar; Schaan, Liechtenstein Z00V4H
Omnichroma Flow Bulk UDMA, TEGDMA Supranano spherical filler (SiO₂-ZrO₂) 69%/55% Camphoroquinone/amine Tokuyama; Japan 011E03
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Power analysis was conducted using the G*Power 3.1 software. Based on a one-way analysis of variance (ANOVA) with a significance level of 0.05, statistical power of 0.80, and an effect size of 0.50, the minimum total sample size was calculated to be 45 (n = 15 for each composite resin group). However, to accommodate potential data loss, 180 samples were prepared: nDC = 90 and n VHN= 90 for microhardness testing (n = 30 for each composite resin group and n = 10 for the sub-groups).

Preparation of composite resin samples

To ensure procedural standardization, all specimens were prepared by a single operator. Composite resin samples were fabricated using transparent cylindrical plastic molds measuring 3 mm in diameter and 4 mm in height. The flowable composite resins were injected into the molds positioned on glass slides and covered with Mylar strips to prevent air entrapment (Fig. 1). A second Mylar strip and a glass slab were placed over the mold with gentle pressure to achieve a smooth, uniform surface and to eliminate excess materials.

Fig. 1.

Fig. 1

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Injection of composite resins into molds for specimen preparation

The polymerization of the samples was performed through the upper surface of the mold using the Bluephase PowerCure LED device. Different polymerization protocols were applied across the groups: (1) standard power mode (1100 mW/cm² for 20 s), (2) standard power mode for a shorter duration (1100 mW/cm² for 10 s), and (3) high power mode (3050 mW/cm² for 3 s). Light irradiance was monitored after curing of every five samples using a radiometer (Bluephase Meter, Schaan, Liechtenstein).

The prepared 180 samples were stored dry in amber-colored containers at 37 °C for 24 h in an incubator. Subsequently, they were divided into two groups for further evaluation: FTIR-ATR spectroscopy and microhardness testing.

Degree of conversion analysis: FTIR-ATR spectroscopy

To determine the degree of monomer conversion in the composite resin specimens, the Fourier-transform infrared (FTIR) spectrometer (Perkin-Elmer Spectrum Two, Waltham, MA, USA) suitable for both solid and liquid sample analysis of the Institute of Nuclear Sciences of Ege University was used.

Initial FTIR measurements were obtained from uncured samples to establish baseline spectra. The FTIR spectra were recorded within the 400–4000 cm⁻¹ range, at a resolution of 4 cm⁻¹, with 25 scans per sample. After 24 h of dry storage in incubator, to avoid any monomer elution prior to DC evaluations, each cured sample was carefully positioned onto the ATR crystal plate and secured with a clamp to ensure optimal contact. Spectral data were collected from both the upper and bottom surfaces.

The degree of conversion (DC) was calculated based on the change in the absorption ratio of the aliphatic C = C (1638 cm⁻¹) and aromatic C–C (1608 cm⁻¹) bands using the formula below [14]:

graphic file with name d33e515.gif

Microhardness testing of composite resin specimens

For the microhardness analysis, the 90 composite resin samples were first stored in amber containers at 37 °C in an incubator for 24 h and then polished. The upper surfaces of all samples were polished using the OptiDisc system (Kerr Corporation, Orange, CA, USA). All the discs were subsequently applied dry at 9000 rpm for 30 s. No polishing was performed on the bottom surfaces.

The Vickers microhardness values were obtained using a microhardness tester (HMV-2, Shimadzu, Japan). This test uses a 136° diamond pyramid indenter to create impressions under a specific load, with the diagonals of the resulting indentations measured via an optical system. Specimens were positioned on the tester’s platform, and both upper and bottom surfaces were indented subsequently using a constant load of 200 g for 15 s. To limit the surface variations, three indentations per surface were made, spaced 1 mm apart.

After indentation, samples were examined under a ×40 magnification lens. The diagonals of each pyramidal impressions were manually measured using the device’s x-y-z stage controls. The Vickers hardness values (VHN) were then automatically calculated by the device. For each surface, the mean of three measurements was used as the representative microhardness value.

Microhardness ratio of composite resins

To evaluate microhardness ratios, the bottom surface hardness value of each specimen was divided by its corresponding upper surface hardness value and multiplied by 100, in accordance with the method described by Lombardini et al. [18].

graphic file with name d33e535.gif

Statistical analysis

All statistical analyses were conducted using SPSS 25.0 (SPSS Inc., Chicago, IL, USA). Descriptive statistics (mean ± standard deviation) were initially calculated for all data sets. The normality of the data distribution was assessed using the Kolmogorov Smirnov test. Since all variables followed a normal distribution, inferential analyses were conducted using paired t-tests, three-way ANOVA, and Tukey’s post-hoc tests for intergroup comparisons of degree of conversion and microhardness values. Partial eta square was calculated in three-way ANOVA analyses. Partial eta squared is telling us how large the effect of the independent variable(s) on the dependent variable is. A significance level of p < 0.05 was considered statistically significant throughout the study.

Results

The FTIR-ATR spectra of PowerFill, PowerFlow, and Omnichroma Flow Bulk composite resins following polymerization with the Bluephase PowerCure LED light device for 3, 10, and 20 s are presented in Figures 2, 3, 4, respectively.

Fig. 2.

Fig. 2

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FTIR-ATR spectra of PowerFill composite resin before polymerization and after polymerization with three different curing durations using the Bluephase PowerCure LED device

Fig. 3.

Fig. 3

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FTIR-ATR spectra of PowerFlow composite resin before polymerization and after polymerization at three different durations using the Bluephase PowerCure LED device

Fig. 4.

Fig. 4

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FTIR-ATR spectra of Omnichroma Flow Bulk composite resin before polymerization and after polymerization at three different durations using the Bluephase PowerCure LED device

The degree of conversion (DC) values measured from the upper surface of all groups are summarized in Figure 5. Paired t-test comparisons revealed that all pairwise comparisons among the groups were statistically significant (p < 0.05). Among the tested materials, PowerFlow exhibited the highest DC at all curing durations (p < 0.05), while Omnichroma Flow Bulk consistently showed the lowest DC (p < 0.05), especially demonstrating a significantly low conversion at 3 s (p < 0.05). The optimal polymerization duration varied by material: 3 s for PowerFill, 10 s for PowerFlow, and 20 s for Omnichroma Flow Bulk.

Fig. 5.

Fig. 5

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Graphical representation of upper surface DC values of composite resins. Different letters above the bars indicate statistically significant differences (p < 0.05)

For PowerFill, DC values numerically decreased with increasing polymerization time (p < 0.05), though all values remained within the clinically acceptable range of 52–75% [26–28]. PowerFlow consistently exceeded the desired range across all durations. Omnichroma Flow Bulk achieved acceptable DC levels at 20 s, with 3 s resulting in significantly insufficient polymerization compared to the other groups (p < 0.05).

ANOVA and Tukey post-hoc analyses confirmed significant differences between polymerization durations and among materials across different durations (p = 0.000).

Degree of conversion from the bottom surface

Bottom-surface DC values by group are presented in Figure 6. Statistically, bottom surface DCs were significantly lower than their corresponding upper surface values (p < 0.000). All group comparisons were statistically significant (p < 0.05). PowerFlow achieved the highest bottom DC at all durations (p < 0.05), while Omnichroma Flow Bulk showed the lowest, especially at 3 s, where the values were critically low (p < 0.05). The optimal bottom DC was observed at 3 s for PowerFill, 10 s for PowerFlow, And 20 s for Omnichroma Flow Bulk.

Fig. 6.

Fig. 6

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Graphical representation of bottom surface DC values of composite resins. Different letters above the bars indicate statistically significant differences (p < 0.05)

While 3 seconds was sufficient for acceptable bottom DC in PowerFill and PowerFlow, it was inadequate for Omnichroma Flow Bulk. Notably, a 20-second curing duration led to a decrease in DC for PowerFill and PowerFlow but increased DC for Omnichroma Flow Bulk (p < 0.05). As a results, Three-way ANOVA and Tukey post-hoc tests confirmed that material type (p = 0.000, partial eta-squared = 0.911), curing time (p = 0.000, partial eta-squared = 0.175), and the surface measured (upper vs. bottom) (p = 0.000, partial eta-squared = 0.225) all had significant effects on the degree of conversion. According to the calculated partial eta squares, it was observed that the material had the upper most effect.

Microhardness measurements of composite resins

Figure 7 is presenting the upper and bottom images of the indentations of the composite resins tested at various polymerization times.

Fig. 7.

Fig. 7

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Images of the upper and bottom indentations of the composite resins tested at various polymerization times (3s-10s-20s). PowerFill on the left, PowerFlow in the middle and Omnichroma Flow Bulk on the right

Upper surface findings

Upper surface microhardness values for PowerFill, PowerFlow, and Omnichroma Flow Bulk across different polymerization times are shown in Figure 8. Both composite resin type and curing duration significantly affected microhardness values (p = 0.000). Upper values increased significantly with longer polymerization durations (p < 0.05).

PowerFill demonstrated the highest upper surface microhardness at 3 and 10 s (59.17 VHN And 66.34 VHN respectively) (p < 0.05). PowerFlow and Omnichroma Flow Bulk had very low hardness at 3 s (p < 0.05) but showed considerable improvement with longer curing times. PowerFlow reached its highest value at 20 s polymerization (70.36 VHN). No significant difference was found between PowerFlow and Omnichroma Flow Bulk at 3 s (p = 0.623), nor between PowerFill and Omnichroma Flow Bulk at 20 s (p = 0.082).

Fig. 8.

Fig. 8

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Graphical representation of upper surface microhardness values. Different letters above bars of the same color indicate statistically significant differences for that material at different durations (p < 0.05)

Bottom surface findings

Bottom-surface microhardness values are depicted in Figure 9. Both material type and curing time had a significant impact (p = 0.000), and bottom-surface hardness increased with time for all materials (p < 0.05).

Fig. 9.

Fig. 9

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Graphical representation of bottom surface microhardness values. Different letters above bars of the same color indicate significant differences (p < 0.05)

A comparative visualization of both upper and bottom microhardness values is shown in Figure 10, where all bottom values were consistently lower than their upper counterparts. At 3 s polymerization, PowerFlow and Omnichroma Flow Bulk upper microhardness values were not different (p = 0.623). Similarly at 20 s, upper values of PowerFill and Omnichroma Flow Bulk (p = 0.082) and bottom values for PowerFill and PowerFlow were also not statistically different (p = 0.568).

Fig. 10.

Fig. 10

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Combined graphical representation of upper and bottom microhardness values. Statistically significant differences are marked with p-values on the graphic

Three-way ANOVA for microhardness value confirmed that material type (p = 0.000, partial eta-squared = 0.357), curing time (p = 0.000, partial eta-squared = 0.524), and the surface measured (upper vs. bottom) (p = 0.000, partial eta-squared = 0.433) all had significant effects on the degree of conversion. According to the calculated partial eta squares, it was observed that the curing time had the upper most effect.

Microhardness ratio (HR) findings

The microhardness ratio (HR), calculated as the percentage of bottom-to-upper surface hardness is displayed in Table 2. The Tukey HSD statistical results about the HR values of the tested materials at different curing times are presented below in Table 3.

Table 2.

Microhardness ratios (HR) in % and standard deviations (SD) for each group at different time periods

PowerFill PowerFlow Omnichroma Flow Bulk
Time HR % (SD) HR % (SD) HR% (SD)
3 s 78,30 (4,89162) 76,75 (4,31397) 62,93 (4,02876)
10 s 80,31 (5,02016) 80,96 (4,57665) 75,09 (4,51632)
20 s 81,05 (4,16272) 79,87 (3,50727) 86,97 (4,57977)
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Table 3.

The Tukey HSD statistical results about the HR values of the tested materials for different curing times (p < 0.05)

Curing times (I) material (J) material Mean Difference (I-J) Std. Erro Sig.
3s PowerFill PowerFlow 1.54866 1.14280 0.369
Omnichroma 15.37023* 1.14280 0.000
PowerFlow PowerFill −1.54866 1.14280 0.369
Omnichroma 13.82157* 1.14280 0.000
Omnichroma PowerFill −15.37023* 1.14280 0.000
PowerFlow −13.82157* 1.14280 0.000
10s PowerFill PowerFlow − 0.65090 1.21605 0.854
Omnichroma 5.21397* 1.21605 0.000
PowerFlow PowerFill 0.65090 1.21605 0.854
Omnichroma 5.86487* 1.21605 0.000
Omnichroma PowerFill −5.21397* 1.21605 0.000
PowerFlow −5.86487* 1.21605 0.000
20s PowerFill PowerFlow 1.17858 1.06043 0.510
Omnichroma −5.92213* 1.06043 0.000
PowerFlow PowerFill −1.17858 1.06043 0.510
Omnichroma −7.10071* 1.06043 0.000
Omnichroma PowerFill 5.92213* 1.06043 0.000
PowerFlow 7.10071* 1.06043 0.000
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There was no statistically significant difference between PowerFill and PowerFlow groups (p > 0.05). The highest HR values for these materials were observed at 10 s. Three seconds was generally inadequate, while 20 s caused a reduction in HR in some groups.

For Omnichroma Flow Bulk, the lowest HR was recorded at 3 s (62.93%), confirming insufficient polymerization. Although 10 s improved HR considerably, the highest value (86.97%) was achieved at 20 s.

Discussion

Advancements in the mechanical and esthetic properties of materials used in posterior restorations have necessitated increased practicality in clinical applications [22]. On the other hand, during the polymerization process of these materials, the light energy transmitted decreases as it penetrates deeper layers, leading to reduced monomer conversion in the bottom parts of the restorations. Inadequate polymerization negatively affects both the mechanical and biological properties of the restoration and reducing its longevity [23]. Over time, this can result in clinical issues such as discoloration, biocompatibility problems, and toxic effects on the pulp tissue [24]. Therefore, achieving optimal polymerization conditions in composite resins is considered a critical requirement for the long-term clinical success of esthetic restorations [25].

To improve the long-term success of restorations, the development of next-generation composite resin materials and light-curing units with enhanced transmission and optimized polymerization properties is of great importance [26]. Various techniques have been developed to achieve deeper polymerization in bulk-fill composite resins. One of these, involves the inclusion of Ivocerin, a germanium-based photoinitiator, in addition to the conventional camphorquinone. Due to its ability to absorb light in the 370–460 nm range, Ivocerin allows for faster and deeper polymerization [27, 28]. However, considering the current options for light intensity and exposure time, there is still no definitive consensus on which light-curing protocols are required for different composite resins to achieve clinically acceptable levels of polymerization [29]. On the other hand, extending the polymerization duration beyond the manufacturer’s recommendations must be carefully considered, as it may cause excessive heat accumulation in adjacent tissues and the pulp [30].

The degree of conversion (DC) achieved during the polymerization process of composite resin restorations largely depends on external factors and polymerization conditions. The chemical structure of dimethacrylate monomers, photoinitiator concentration, and polymerization mechanism are directly related to the material’s composition and translucency [31]. Moreover, studies have shown that the degree of conversion is affected by both the formulation of the composite resin and the light source’s wavelength, power density, exposure duration, tip size, and photoactivation technique [32]. Additionally, the filler content and its distribution within the resin matrix have also been reported to influence the degree of conversion by affecting light transmission and polymer network formation [33]. Tetric PowerFill and Tetric PowerFlow differ in filler content, which directly affects their polymerization characteristics. PowerFill contains 79 wt% barium glass and pre-polymerized particles, providing high strength but increasing viscosity and light scattering, which may slightly reduce the degree of conversion (DC). Conversely, PowerFlow’s lower filler content (68.2 wt%) and smaller particles improve its translucency and light penetration, often resulting in higher DC [34].

The use of low-intensity light devices may lead to insufficient polymerization, resulting in increased wear, discoloration, pulp irritation, and recurrent caries risk [35, 36]. To mitigate these risks, high-intensity light-curing units such as Quartz Tungsten Halogen, dental Plasma curing light, and LED have been developed, with LED systems now widely replacing other methods due to their effectiveness [36].

Numerous studies in the literature have evaluated the monomer conversion degree and microhardness properties of composite resins independently [37–39]. These two test methods assess the polymerization efficacy of composite resins from different perspectives and provide important insights into the material’s clinical performance. While the degree of conversion indicates the amount of unreacted monomers and is significant for biocompatibility; microhardness analysis is a critical parameter for evaluating the mechanical durability of the material [40].

Bolaños-Carmona et al. [41] used three different spectroscopic techniques (FTIR-ATR, FTIR, and FT-Raman) to evaluate the degree of conversion of five different bulk-fill composite resins at 0 mm and 4 mm thicknesses. The study found FTIR-ATR spectroscopy to be the most reliable and accurate technique for calculating conversion degrees. Based on this evidence, we also selected the FTIR-ATR method for our study as it allows for measurements without grinding the specimens.

Analyses conducted on different composite resins have shown that the desired monomer conversion degree typically ranges from 52 to 75% [42]. However, there is still no clearly defined threshold in the literature for the clinically acceptable minimum conversion degree [43]. Nevertheless, it has been suggested that conversion degrees below 55% are inadequate and not recommended for occlusal surfaces exposed to high chewing loads [44]. In our study, PowerFill’s degree of conversion remained within the acceptable 52–75% range [42, 44] at all time intervals. PowerFlow consistently showed conversion rates above this range, while Omnichroma Flow Bulk only reached acceptable levels after 20 s of polymerization.

In a study by Hayashi et al. [11], PowerFlow demonstrated a higher degree of conversion than PowerFill at both 3 and 10 s. Our study supports these results. PowerFill and PowerFlow contain a germanium-based photoinitiator ‘Ivocerin’. Literature indicates that Ivocerin absorbs visible light more effectively than camphorquinone and enables deeper light transmission due to its broad absorption spectrum between 370 and 460 nm. This feature allows composite resins to achieve adequate polymerization at depths of up to 4 mm [7]. The findings of this study reveal that Ivocerin containing composite resins, have superior light transmission and more uniform polymerization efficiency compared to the camphorquinone-based material highlighting the advantage of their advanced photoinitiator systems [45].

When comparing the degree of conversion (DC) on the bottom surfaces of PowerFill samples and another composite resin group, PowerFill exhibited higher DC values [39]. Similarly, Randolph et al. [46] reported that composite resins containing 2,4,6-trimethylbenzoyl phosphine oxide (TPO) achieved significantly higher monomer conversion rates than camphorquinone-based materials when cured for 3 s or longer. Our study found that the degree of conversion decreased as depth increased, which is also consistent with previous studies [33, 34]. However, apart from Omnichroma Flow Bulk’s bottom surface values of 30.29% at 3 s and 47.34% at 10 s; all materials demonstrated at least 50% conversion rates. We believe that achieving higher monomer conversion at the restoration’s top surface may enhance resistance to wear and hydrolytic degradation. The results from this study suggest that the lower degree of conversion on the bottom surfaces of 4 mm samples is related to limited light penetration, indicating the need to determine minimum polymerization durations specific to each material. Specifically, Omnichroma Flow Bulk, which contains camphorquinone, required longer light exposure (20 s), while PowerFlow achieved high conversion rates in shorter durations (10 s).

It is known that polymerization continues even after light exposure ends, and microhardness values increase over time. One study reported that microhardness increased during the first week and reached 92% of its maximum value within 24 h [47]. For this reason, microhardness measurements taken immediately after light exposure may not fully reflect the material’s final microhardness, and it has been recommended that measurements be taken at least 24 h after polymerization [14]. In accordance with the literature, we conducted microhardness testing 24 h after sample preparation.

According to the literature, when polishing is not performed on the upper surfaces of the light-cured restorations, microhardness values obtained from that surface may not accurately reflect the overall hardness profile of the material [48]. Even though the use of a clear matrix strip produces a smoother surface, it is emphasized that the oxygen-inhibited layer should be removed through finishing and polishing procedures [49]. In our study, to minimize the potential adverse effects of the oxygen-inhibited layer, clear matrix strips were placed on both top and bottom surfaces of the samples before polymerization, and finishing/polishing was applied to the upper surfaces to improve the accuracy of microhardness measurements.

Studies comparing the post-polymerization surface hardness of bulk-fill and conventional composite resins have shown that upper surface hardness is generally higher than bottom surface hardness [37, 50, 51]. Our results align with this data, as the upper surfaces of 4 mm thick samples showed higher microhardness values than the bottom surfaces. However, all surface microhardness values increased with longer polymerization durations for all materials. A recent study similarly reported higher microhardness values for PowerFill compared to PowerFlow at 3 s [34], which we believe is due to PowerFill’s higher inorganic content, resulting in superior mechanical properties.

Some researchers have suggested that a Vickers microhardness (VHN) value above 50 indicates adequate and ideal polymerization [52], while others argue that the upper/bottom hardness ratio is a more reliable indicator [53]. In our study, PowerFlow and Omnichroma Flow Bulk, polymerized for 3 s, had upper surface microhardness values below 50 VHN (42.13 VHN And 42.61 VHN respectively). The ideal polymerization time for these materials was found to be 10 s, whereas PowerFill reached ideal hardness at 3 s. For bottom surfaces, at 3 s, none of the composite resins reached the ideal 50 VHN threshold (46.24 VHN for PoweFill; 32.3 VHN for PowerFlow; 26.77 VHN for Omnichroma Flow Bulk). At 10 s, PowerFlow still fell short of this value (43.77 VHN) while the others achieve it (PoweFill; 53.12 VHN; Omnichroma Flow Bulk 51.41 VHN). At 20 s, all materials reached the threshold level.

An early study showed a strong correlation between increased microhardness values and higher monomer conversion degree [54]. More recently, Bouschlicher et al. [55] reported that the upper/bottom microhardness ratio of a composite resin accurately reflects its corresponding conversion degree and that these two parameters can be evaluated independently of the resin’s composition.

Abed et al. [43] found no statistically significant correlation between conversion degree and microhardness for the bulk-fill materials that they have tested. Although the degree of conversion and surface microhardness are related properties in composite resins, the material’s mechanical properties do not directly correlate with the conversion rates alone [56]. The degree of conversion alone does not fully characterize the polymer network of composite resins, and a high conversion rate does not always guarantee superior mechanical performance.

In our study, both PowerFill and PowerFlow materials demonstrated optimal degrees of monomer conversion across all curing durations. However, when examining microhardness values, these materials achieved the ideal threshold only at 10-second and 20-second. These findings suggest a consistency between the degree of conversion and microhardness at these exposure times for PowerFill and PowerFlow, indicating that both 10 and 20 s are clinically appropriate curing durations for these materials. In contrast, Omnichroma Flow Bulk exhibited both an optimal degree of conversion and an acceptable microhardness only after 20 s of light exposure. When considered in tandem, the degree of conversion and microhardness results indicate that a 20-second polymerization period is most suitable for Omnichroma Flow Bulk.

Clinically, a hardness ratio of ≥ 80% is considered acceptable [19, 20]. In our investigation, both PowerFill and PowerFlow groups exceeded this 80% ratio following 10 s of light curing. However, Omnichroma Flow Bulk failed to meet this threshold at 10 s, achieving a bottom-to-upper hardness ratio above 80% only after 20 s of exposure. Interestingly, none of the materials tested in this study achieved the desired microhardness ratio under the 3-second polymerization protocol. These results suggest that, while the high-intensity advanced LED light device facilitates faster surface polymerization, additional curing time is necessary to ensure adequate polymerization through the depth of the material. Based on all the findings of our study, the null hypothesis (H₀) stating that the time of curing will not significantly affect the degree of conversion, microhardness, and hardness ratio of the different composite resin groups was rejected.

While the findings obtained in our in vitro study provide noteworthy contributions to the existing literature, certain limitations must nonetheless be acknowledged. In the oral cavity, variables such as moisture, temperature fluctuations, and anatomical complexities could affect the efficiency of polymerization thus our study, being an in vitro one, is not fully replicating the clinical situation. Additionally, although FTIR-ATR and microhardness analyses provide valuable data on polymerization performance, they only offer indirect insights into clinical behavior. In a recent study conducted by Price et al. [57], it was demonstrated that a 300-gram load during microhardness testing successfully penetrated and passed the oxygen-inhibited layer, yielding adequate indentation depth while also offering a suitable balance to prevent deformation of composite resin specimens. In contrast, a fixed load of 200 g was employed in the present study, which may have influenced the depth and accuracy of the microhardness readings. Additionally, Yilmaz Atali et al. [58] reported that Omnichroma exhibited greater hardness values on the 15th day compared to the 24-hour marks. In our investigation, however, microhardness data were exclusively evaluated at the 24-hour interval, potentially overlooking variations in material behavior. Furthermore, only a single high-intensity advanced light-curing unit was utilized, albeit under three different operational modes. Future investigations may benefit from evaluating various curing devices with distinct mode configurations to determine whether the observed outcomes are generalizable across different light-curing units/technologies.

The use of Ivocerin and TPO photoinitiators in combination with CQ/amine systems has been shown to enhance polymerization kinetics. These photoinitiators exhibit higher reactivity when exposed to high-intensity light, facilitating faster and more efficient polymerization when compared to traditional photoinitiators CQ. Several studies have demonstrated the superior efficiency of Ivocerin and TPO in curing composite resins, as they exhibit a higher degree of conversion and better mechanical properties when subjected to high-intensity light [59]. In the present study, this was evident in the polymerization behavior of PowerFill and PowerFlow, where the inclusion of these photoinitiators allowed for rapid polymerization even with a short curing time. The inclusion of the AFCT component in the resin formulations, alongside the advanced photoinitiators, appears to be a key factor in achieving high polymerization efficiency and improved mechanical properties in these composite resins. As noted, AFCT encourages more linear polymer chain growth, reduces crosslinking density, and mitigates polymerization shrinkage stress, which collectively improve the overall material properties [60]. The monomer conversion rates (DC%) and Vickers microhardness values obtained from the composite resins containing AFCT tested in our study were consistent with previous research, highlighting the positive impact of AFCT on polymerization efficiency and mechanical performance [10, 61, 62]. In contrast, Omnichroma Flow Bulk composite resin, which contains only a CQ/amine system, showed lower DC% and Vickers microhardness values when polymerized using the 3-second light-curing protocol.

In conclusion, the findings from this study underline the importance of optimizing both the resin formulation and the curing protocol to achieve the best possible clinical outcomes. Based on the findings of this study, clinical protocols involving 10 s of curing for PowerFill and PowerFlow and 20 s for Omnichroma Flow Bulk are recommended for optimal results. However, careful attention to curing times is essential for achieving optimal outcomes in clinical practice, particularly in cases involving bulk filling or deeper restorations where additional curing time is necessary to ensure adequate polymerization through the depth of the material.

Acknowledgements

The authors acknowledge Dr. Emine Nostar Aslan and Dr. Sevde Gül Batmaz for performing the FTIR-ATR analysis of our study.

Abbreviations

LED

Light-emitting diode

DC

degree of conversion

FTIR

Fourier Transform Infrared

QTH

quartz tungsten halogen

CQ

camphoroquinone

PPD

phenyl propanedione

MAPO

monocylphosphine

BAPO

bisacylphosphine

AFCT

β-allyl sulfone

RAFT

reversible addition-fragmentation chain transfer

ATR

attenuated total reflection

HR

microhardness ratio

ANOVA

analysis of variance

VHN

Vickers hardness

mW

Miliwatt

Authors’ contributions

GO performed material preparation, test analysis, and wrote the first draft. LST wrote/corrected the paper, commented on data statistics. HB performed statistical analysis, wrote the first draft of the result section. All authors contributed to the study’s conception and design. All authors commented on previous versions. All authors read and approved the final manuscript before submission and agreed to be accountable for all aspects of the study. All authors reviewed the manuscript.

Funding

The research does not have financial support.

Data availability

The datasets used and/or analyzed during the current study are available from the corresponding author on request.

Declarations

Ethics approval and consent to participate

Not applicable.

Consent for publication

Not applicable.

Competing interests

The authors declare no competing interests.

Footnotes

Publisher’s Note

Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

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Data Availability Statement

The datasets used and/or analyzed during the current study are available from the corresponding author on request.

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