Effect of temperature, curing time, and filler composition on surface microhardness of composite resins.

AIM: The aim of this study was to evaluate the microhardness of two composite resins when subjected to three different temperatures and three different light-curing times.
MATERIALS AND METHODS: Two composites were used; Filtek Z250 and Grandio. Three different temperatures (23, 37, and 55(o)C) were used, utilizing a composite warmer. The heated samples were immediately injected into cylindrical molds (6 mm × 2 mm) and the top surface of the specimens was polymerized for 10, 20, and 40 sec, using a Quartz-Tungsten-Halogen light-curing unit (QTH LCU). Vickers microhardness measurements were performed from both the top and bottom surface of the specimens, following dry storage for 24 hours in the dark. Statistical analysis were performed using one-way analysis of variance (ANOVA) and Tukey post-hoc test at a level of significance of a = 0.05.
RESULTS: The results indicated that there was an increase in microhardness as the temperature of the composite was increased for either the top or the bottom surface (P < 0.05). Furthermore, there was a general increase in microhardness for both composites as curing time increased (P < 0.05). The type of composites did not influence the surface microhardness (P > 0.05).
CONCLUSIONS: Temperature of composites affects their surface microhardness. Also, light-curing time influence microhardness values of the composites tested.

INTRODUCTION

The success of dental composites in restorative dentistry stems from their good esthetic properties and adequate durability. The clinical performance of composite resins is directly related to the degree of monomer conversion after photopolymerization.[1] There are many parameters that influence degree of polymerization of composite resins such as their composition,[2] shade and translucency,[3] characteristics of the light-curing unit (LCU) used,[45] distance between light-curing tip and restoration surface,[6] duration of photopolymerization and composite temperature.[7]

Measurement of surface microhardness of composite resins is a useful method to indirectly evaluate the degree of polymerization.[8] According to ISO 4049:2000, to achieve the acceptable degree of polymerization composite resins have to meet the requirement of ≥80% bottom/top percentage microhardness at 2 mm depth. Surface microhardness is also evaluated for the estimation of composite wear after tooth-bleaching procedures.[9]

Placing composites at an elevated temperature reduce their viscosity[10] and increase the efficiency of polymerization.[11] Heating the composite prior to placement in the cavity and polymerizing increases monomer conversion rate and therefore the duration of the irradiation period may be reduced.[12] With increased paste temperature, free radicals, and developing polymer chains become more fluid as a consequence of decreased paste viscosity and they react to a greater extent, resulting in a more complete polymerization reaction and greater cross-linking.[13] The increase in the degree of polymerization of composites may lead to better internal adaptation to cavity walls,[14] improved mechanical properties, and increased wear resistance.[12]

The purpose of this in vitro study was to evaluate the surface microhardness of a microhybrid and a nanohybrid composite when subjected to three different temperatures (23, 37, and 55ºC) and three light-curing times (10, 20, and 40 sec). The first null hypothesis (Ho1) of the study was that the filler composition of the composite resins tested does not influence their surface microhardness. The second null hypothesis (Ho2) was that there are no significant differences in surface microhardness between experimental groups subjected to different temperatures. Finally, the third null hypothesis (Ho3) was that different curing times do not affect the surface microhardness of the composite resins investigated.

MATERIALS AND METHODS

Two different types of composites were used to determine if filler characteristics affect the outcome variables of time and temperature. The composite resins used in this study were a microhybrid Filtek Z250 (3M ESPE) and a nanohybrid Grandio (Voco) with shade A2.

The composites were tested at three different temperatures; room temperature (23ºC), body temperature (37oC), and preheating temperature (55ºC). Preheating of the composites was achieved utilizing a commercially available composite warmer (ENA Heat, Micerium SpA, Avegno GE, Italy). The heated samples were immediately injected into cylindrical Teflon molds (6 mm diameter and 2 mm depth). Prior to packing the mold, a Mylar strip was placed on a glass slab, the mold was then placed, and the composite resin packed. After placing the composite, a second Mylar strip was placed on top of the mold and a glass microscope slide was placed over the composite, in order to achieve a standardized surface finishing and to remove the excess of the material.

Subsequently, the top of each specimen irradiated for the designated time. Three different polymerization times (10, 20, and 40 sec) were used, which are commonly cited in the literature as acceptable polymerization times, using a Quartz-Tungsten-Halogen (QTH) LCU (Elipar 2500, 3M ESPE, St. Paul, MN, USA) with a light output of 1300 mW/cm2. Five specimens were prepared for each combination of the parameters (composite-polymerization time-temperature) resulting in 18 groups and a total of 90 specimens.

Vickers microhardness measurements were performed from both the top and bottom surface of the specimens with a microhardness tester (HMV-2000, Shimadzu, Tokyo, Japan), following dry storage for 24 hours in the dark. Four readings for each specimen surface were carried out and were independently averaged and reported in Vickers Hardness Number (VHN).

The statistical analysis of the data was done by Statistical Package for the Social Sciences (SPSS) 19.0 and performed using one-way analysis of variance (ANOVA) and Tukey post-hoc test at a level of significance of a = 0.05. For the surface microhardness measurements the ISO standard for composites requires that when a 2 mm-layer of composite is polymerized from the top, the bottom surface hardness should be 80% of the top surface hardness. Therefore, in the present study the percent differences between the top and bottom surfaces of the specimens were also calculated.

RESULTS

The mean values and standard deviations of surface microhardness measurements for each experimental group are shown in Table 1. The highest VHN presented the composite specimens which were preheated at 55ºC and light-cured for 40 sec (Filtek Z250: 81.1 ± 4.0 VHN and Grandio: 79.2 ± 3.7 VHN), while the lowest microhardness values presented the room temperature composite specimens which were light-cured for 10 sec (Filtek Z250: 24.7 ± 2.9 VHN and Grandio: 30.4 ± 1.7 VHN).

Table 1

Mean values and standard deviations (VHN) of each experimental group for composite specimens

The results indicated that there was a statistically significant increase in surface microhardness as the temperature of the composite resin was increased from 23ºC to 55ºC for both composites for either the top or the bottom surface (P < 0.05) [Table 1]. However, the VHN between experimental groups preheated at 37ºC and 55ºC did not exhibit any significant differences (P > 0.05). The increase in VHN with temperature in Filtek Z250 ranged between 8.1 and 21.7% (top) and between 16.3 and 44.8% (bottom). Likewise, in Grandio the increase in microhardness values ranged between 9.6 and 16.2% (top) and between 13.4 and 35.4% (bottom). At the bottom surface of the composite specimens the increase of microhardness values as the temperature increase was higher than the top surface of the specimens. Moreover, this increase in surface microhardness was higher as the duration of light-curing decreases [Table 2].

Table 2

The increase in surface microhardnes (% VHN) of the experimental groups with temperature and curing time

There was a statistically significant increase in surface microhardness for both composites as light-curing time increased (P < 0.05), regardless composite temperature. Nevertheless, there were no significant differences in surface microhardness between the experimental groups of the same temperature photopolymerized for 20 sec and 40 sec (P > 0.05), except for room temperature specimens (P < 0.05). The increase in VHN with light-curing time in Filtek Z250 ranged between 17.8 and 29.9% (top) and between 39.5 and 60.2% (bottom). Similarly, in Grandio the increase in microhardness values ranged between 24.2 and 29.6% (top) and between 37.4 and 53.3% (bottom). At the bottom surface of the composite specimens, the increase of microhardness values as the duration of light curing increases was higher than the top surface of the specimens, and this increase in surface microhardness was higher as the composite temperature decreases [Table 2].

Additionally, different filler composition of the composite resins did not influence surface microhardness (P > 0.05), independently on curing time or composite temperature. The bottom surface microhardness of specimens which were photopolymerized for 10 sec did not meet the ISO requirement of ≥80% bottom/top percentage microhardness at 2 mm depth, in all experimental groups [Table 1].

DISCUSSION

On the basis of the results reported in the current study, Ho1 should be accepted. However, previous studies have reported that filler composition of the composite resins may affect their surface microhardness.[1516] Inorganic fillers of composite resins are able to scatter light, and both filler size and content affect light dispersion through the material. The smaller the size of filler particles, the higher the light scattering occurs. Microhybrid and nanohybrid composite resins contain a combination ofdifferent filler particle sizes. Degree of composite polymerization is reduced due to light attenuation as the light beam is scattered and reflected within the composite material, leading to lower microhardness values.[17] In the present study, Filtek Z250 and Grandio did not exhibit significant differences in VHN in the same polymerization conditions, which may be explained by their similar filler content (82% wt and 87% wt, respectively), shade (A2) and photoinitiator (camphoroquinone). This is in agreement with Sabatini,[18] who found that some nanohybrid and microhybrid composite resins presented no significant differences in microhardness values.

The results of the present study also demonstrate that preheated composite specimens at 37ºC or 55ºC exhibited significant higher microhardness values than room temperature composite specimens. As a result, Ho2 is rejected. This is in agreement with previous reports.[111219] The VHN data of this study indicate that preheating of composite resins improves their degree of monomer conversion, as evidenced by the increase in surface microhardness. The explanation of this phenomenon is that preheated composite resins exhibit increased monomer mobility, as a result of higher thermal energy, which leads to less viscosity and enhanced mobilities of growing chain moieties in the composite material.[20]

Previous studies have reported that the preheating of composites may reduce microleakage of their restorations,[21] enhance marginal adaptation[14] and improve their flow characteristics.[9] Furthermore, an interesting issue is that in clinical conditions, some temperature drop of the composite material may be expected during its removal from the composite warmer to the tooth cavity. Daronch et al.,[22] found a 50% temperature decrease within 60 sec of removing a composite from a preheating device. In the present study, this temperature decrease of the composites may be occurred during their removal from the composite warmer to the Teflon mold simulating clinical situation.

Daronch et al.,[12] found that by preheating composites to 60ºC the degree of monomer conversion increased between 31.6 to 67.3%. As a consequence, they suggested that duration of polymerization may be reduced by up to 75% with preheating of composites. The same researchers reported that light curing of a preheated composite for 5 sec resulted in a greater degree of polymerization than light curing at room temperature for 40 sec. However, in the present study light-curing of preheated composites at 55ºC for 10 sec exhibited significant lower VHN than room temperature composites light curing for 20 sec.

According to the results of this study, Ho3 is also rejected. However, there were no significant differences in VHN between the experimental groups of the same temperature light-cured for 20 sec and 40 sec, except for room temperature specimens. Marchan et al.,[23] reported that among five nanocomposites investigated, polymerization for 10 sec was insufficient for two of them, while for the other three it was adequate. Moreover, Akram et al.,[24] found that microhardness of composite specimens light-cured for 20 sec was significantly lower than those light-cured for 30 sec, 40 sec, and 60 sec. In another study,[25] the extended polymerization time (40 sec) did not influence the degree of conversion of the composite material tested compared with light-curing for 20 sec.

The manufacturers of the composite materials tested recommend 20 sec light-curing with a QTH LCU. This reflects on the energy value required for adequate polymerization of the composite materials. It has been reported that depending on the brand and shade of a composite, the energy density required for optimal light-curing ranged between 6and 36 J/cm2.[26] In a previous study, it has been reported that there is correlation between the amount of energy delivered from a LCU to a composite material and the resulting degree of polymarization and physical properties.[27]

The VHNs achieved in the present study at the top surface of composite specimens were higher compared to the bottom surface in all experimental groups. This is due to the attenuation of light as it travels through the composite material. This was coincident with previous studies, which have demonstrate that at a depth of 2 mm, the light attenuation may decrease irradiance to approximately 75% of that reaching the top surface.[28] In addition in this research, all the experimental groups meet the ISO requirement of ≥80% bottom/top percentage microhardness at 2 mm depth, except the specimens polymerized for 10 sec, regardless the composite temperature. These results coincide with the results of other studies.[1123] Moreover, in the same studies the researchers concluded that the type of LCU and the composition of the material are crucial parameters for the efficiency of polymerization at 10 sec.

CONCLUSIONS

Within the limitations of this in vitro study the following statements can be concluded:

Preheating composites at 55oC with a commercially available composite warmer increases the surface microhardness of the tested materials.

Increasing the duration of photopolymerization of the composites investigated increases the surface microhardness of the materials.

The different filler content of the two composites does not affect their surface microhardness.

Photopolymerization time of 10 sec is not recommended for the composites tested according to ISO standards.