Indian Journal of Dental Research

ORIGINAL RESEARCH
Year
: 2007  |  Volume : 18  |  Issue : 4  |  Page : 173--176

Degree of conversion and residual stress of preheated and room-temperature composites


N Prasanna, Y Pallavi Reddy, S Kavitha, L Lakshmi Narayanan 
 Department of Conservative, Dentistry and Endodontics, College of Dental Surgery, Saveetha University, Chennai, India

Correspondence Address:
N Prasanna
Department of Conservative, Dentistry and Endodontics, College of Dental Surgery, Saveetha University, Chennai
India

Abstract

The aim of this study was to determine the degree of conversion and residual stress of resin composite preheated to different temperatures and to compare it to room-temperature composite. The composite resin was preheated to 40C, 50C, and 60C and packed into brass rings and light-cured. The degree of conversion and residual stress were analysed using Fourier transform infra-red spectroscopy and X-ray diffraction, respectively. The results obtained were tabulated and statistically analyzed using Kruskal-Wallis test and TukeySQs honestly significantly different test. The results showed significant increase in the degree of conversion and residual stress with increase in preheating temperature.



How to cite this article:
Prasanna N, Pallavi Reddy Y, Kavitha S, Lakshmi Narayanan L. Degree of conversion and residual stress of preheated and room-temperature composites.Indian J Dent Res 2007;18:173-176


How to cite this URL:
Prasanna N, Pallavi Reddy Y, Kavitha S, Lakshmi Narayanan L. Degree of conversion and residual stress of preheated and room-temperature composites. Indian J Dent Res [serial online] 2007 [cited 2014 Aug 30 ];18:173-176
Available from: http://www.ijdr.in/text.asp?2007/18/4/173/35827


Full Text

Continued improvements in composite restorative materials have led to the widespread clinical acceptance of these materials. Monomeric resins can be easily converted into cross-linked polymers by exposure to visible light. This complex polymerization process results in the final polymers which, however, are not without intrinsic problems.

The photo-polymerization of composite resins exhibits incomplete conversion of double bonds. This leaves a significant proportion of methacrylate groups unreacted, depending upon the monomer and filler composition, initiator system, and light-curing procedure. [1] This reaction is self-limiting, arising from the increase in system viscosity (also called the gel effect or Trommsdorf-Norrish effect) and the resulting decrease in mobility of the reactive species, which is imposed by the rapid formation of a highly cross-linked polymeric network. [2],[3] The residual unreacted monomer acts as a plasticizer and reduces the mechanical properties of the restorative material. [2] They can also produce allergic reactions. Colour stability is also known to decline due to formaldehyde formation. [4],[5],[6]

The term 'degree of conversion' applied to resin composites, refers to the conversion of monomeric carbon-carbon double bonds into polymeric carbon-carbon single bonds. [7] Increasing the conversion results in higher surface hardness, flexural strength, flexural modulus, fracture toughness, and diametral tensile strength. The wear resistance is also known to be increased. This improvement in its properties may be because of increased cross-linkage. [8],[9]

Composites, when photo-polymerized, exhibit an incomplete degree of conversion. Preheating the composite prior to photo-polymerization can increase the degree of conversion. [10],[11] Preheating has also been shown to increase the flow and enhance the adaptation of the resin to the prepared tooth walls and, thus, potentially reduce microleakage. [10]

The overall shrinkage that occurs during the process of polymerization is proportional to the degree of conversion. The resulting shrinkage stress that develops during the curing of a bonded restoration can induce defects within the composite, the tooth, or at the interface, resulting in compromised clinical performance and aesthetics. [12],[13]

The aim of this study was to evaluate the degree of conversion and residual stress of composites that were photo-polymerized, after heating it to different temperatures, and to compare it to room-temperature composites.

 Materials and Methods



A commercially available photo-activated resin composite (Ceram x, shade M1, Dentsply / Caulk, USA) was used for this study. An incubator was used for heating the composite to temperatures of 40C, 50C, or 60C for 30 s.

The composite was then immediately packed into brass rings (6 mm diameter and 2 mm height), covered with a sheet of clear plastic matrix (0.08 mm, Mylar type D, DuPont, DE, USA) and pressed, to force the composite resin to conform to the ring's dimensions. The composite was then cured using an LED light-curing unit (Bluephase C5, Ivoclar Vivadent, Lichtenstein), for 30 s according to the manufacturer's instructions, with the light guide held 1 mm from the top surface. Spectral irradiance was determined with the help of a radiometer. The irradiance of the light source measured 600 mW/cm 2 between 430-490 nm.

The composite sample was removed from the ring and a section of 1 mm thickness was made from the surface and another at 2 mm depth, using a diamond saw under a water coolant. There were 10 samples in each group. The groups were as follows:

Group I: Room-temperature composite (control)

Group II: Composite preheated to 40C

Group III: Composite preheated to 50C

Group IV: Composite preheated to 60C

Each group had 2 subgroups (5 samples each).

Subgroup A was further divided into division I (surface section of the sample) and division II (section of the sample at 2 mm depth). This subgroup was subjected to Fourier transform infra-red (FTIR) spectroscopy for evaluating the monomer conversion.

Subgroup B was subjected to X-ray diffraction analysis for evaluation of the residual stress. The measurements for all the groups were carried out after the samples had been stored at room temperature for 24 h.

Evaluation of the degree of conversion

Infrared spectra was collected between 1680 and 1550 cm -1 at a rate of one per second, using a FTIR spectrometer (FTS- 40, Digilab, Germany) using 8 scans at 2 cm -1 resolution. The composite pellets were ground into a fine powder and 50 g of the powder was mixed with 5 mg of potassium bromide (KBr) powder; following this, KBr pellets were prepared under a pressure of 10 tonnes. An unpolymerized specimen of each group was smeared on the KBr pellets (using Merck's spectroscopically pure KBr) with a press to detect the degree of conversion.

Monomer conversion was calculated using the changes in ratios of aliphatic to aromatic carbon-carbon double bond absorption peaks in the uncured and cured states [Figure 1]. The degree of conversion percentage was calculated using the following formula: [7]

{1 - (a / b)} 100

Where a = absorption of aliphatic C-C / absorption of aromatic C-C (polymer)

b = absorption of aliphatic C=C / absorption of aromatic C=C (monomer).

Evaluation of residual stress

Residual stress was evaluated using X-ray diffractometer (X'Pert PRO, PANalytical, Netherlands). The stress was calculated from the 2θ peak values that were obtained. Using Bragg's law, 'λ = 2d sinθ, the value of the interplanar spacing 'd' was calculated. The residual stress was calculated using the following formula: [14]

σ = m E / d (1 + ν)

Where, σ = stress

m = slope of d against sin 2 ψ

ψ = angulation of the X-ray beam

d = interplanar spacing

E = modulus of elasticity

ν = density

The values obtained were tabulated and statistical analysis was carried out using Kruskal-Wallis' test to calculate the P value, and comparison of the groups amongst each other was done using Tukey's HSD test.

 Results



The mean values of the degree of conversion percentage of the samples at the surface and at 2 mm depth and the significant groups are given in [Table 1]A and B. Statistical analysis of the data showed higher degrees of conversion with increase in preheating temperature. The results were statistically significant (P [2] The extent of conversion affects both the physical and mechanical properties of the polymer, both of which depend on the polymer network formation. [6],[12] Hence, the degree of conversion of the room-temperature composites and preheated composites was evaluated in this study.

The methods to evaluate the degree of conversion include FTIR spectroscopy, physical determination of surface hardness, Raman spectroscopy, and photo-differentiated scanning calorimetry. [11] FTIR spectroscopy was chosen for this study because of its accuracy.

The degree of conversion of the samples at the surface at room temperature was found to be 52.08%, whereas composites preheated to 40C, 50C, and 60C showed values of 58.6, 64.7, and 68.3%, respectively. The values of the degree of conversion at 2 mm depth was found to be 50.06% for room-temperature samples as compared to 57.62, 63.14, and 66.26% for the samples preheated to 40C, 50C, and 60C, respectively.

The increased degree of conversion can be due to several reasons. The viscosity of the system increases with increase in temperature and this enhances radical mobility. The collision frequency of unreacted active groups and radicals also increases with elevated curing temperature when it is below the glass transition temperature. [2],[11]

In our study, we heated the composite resins for 30 s, as recommended by Daronch et al., [2] who determined that 30 s was sufficient to warm the composite sample homogeneously. In the clinical scenario, when the composite is preheated, there are two factors to be considered: one is the temperature of the composite resin when it is placed in the cavity and the second factor is the time delay between dispensing it from the syringe and placing it into the preparation, contouring, and light-curing it. The heated composite may cool down rapidly, decreasing the benefits of heating. But even if it cools to below 50C, to about 40C, benefits may still be seen in comparison to that of room-temperature composites. [4] However, heating the composite to a temperature higher than 60C may not be advisable in the clinical scenario, due to the potential risk of pulp injury at high temperatures. An intrapulpal temperature increase of 5.5C can produce significant pulp injury. [15]

Residual stress is defined as the stress that exists in the bulk of the material, without the application of an external load. It can result in reduced resistance to wear, which will result in surface cracks. It can also result in adhesive bond failure. [13],[14] Residual stress can be measured by X-ray diffraction, hole drilling, neutron and synchrotron diffraction, laser Raman spectroscopy, photoelastic analysis, and the ring slitting method. [13],[14],[16] The most accurate method is X-ray diffraction and, hence, it was chosen for this study.

The residual stress evaluated in this study was found to be 4.3 MPa for room-temperature composite, as compared to preheated composite, where it was found to be 5.1 MPa (40C), 8.7 MPa (50C), and 10.6 MPa (60C).

Polymerization of resin composites is accompanied by volumetric shrinkage of about 1-1.5%, which results in the development of residual stress. The heat that is generated during polymerization and the thermal expansion-coefficient mismatch between the polymer matrix and the fillers can result in stress development at the filler-matrix interface, which can generate internal 'hoop' stresses around the fillers. [15],[16] Increasing the rate of polymerization increases the contraction stresses by decreasing the viscous flow and molecular mobility in the highly cross-linked polymer, due to the increased degree of conversion. In this in vitro study, the degree of conversion and residual stress seem to be directly proportional to the temperature rise.

 Conclusion



Monomer conversion increases significantly with increases in temperature, when a resin composite is preheated prior to photo-activation. However, residual stress is also greatly increased with increases in temperature. So, within the limitations of this study, preheating the composite may not be a recommended method to increase the conversion.

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