Influence of photoactivation method on conversion, mechanical properties, degradation in ethanol and contraction stress of resin-based materials

Journal of Dentistry (2005) 33, 773–779

www.intl.elsevierhealth.com/journals/jden

Influence of photoactivation method on conversion, mechanical properties, degradation in ethanol and contraction stress of resin-based materials ´via Gonc Marcelo F. Witzela, Fernanda C. Calheirosa, Fla ¸alvesa, Yoshio Kawanob, Roberto R. Bragaa,* a

´ria, Department of Dental Materials, School of Dentistry, Av. Prof. Lineu Prestes, 2227, Cidade Universita ˜o Paulo, Sa ˜o Paulo, SP, Brazil CEP 05508-000 Sa b ˜o Paulo, Department of Fundamental Chemistry, Institute of Chemistry, University of Sa ˜o Paulo, SP, Brazil Sa Received 3 December 2004; received in revised form 9 February 2005

KEYWORDS Composite resin; Mechanical properties; Degree of conversion; Contraction stress

Summary Objectives: To investigate the influence of photoactivation method on degree of conversion (DC), flexural strength (FS), flexural modulus (FM) and Knoop hardness (KHN) of a composite and an unfilled resin (Filtek Z250 and Scotchbond multi-purpose plus, 3 M ESPE) after storage in water or ethanol, and on composite contraction stress (CS). Methods: Specimens 1!2!10 mm were prepared for FS test, photoactivated by 600 mW/cm2!40 s (A), 200 mW/cm2!120 s (B), or 600 mW/cm2!1 s C3 min delay C600 mW/cm2!39 s (C), and tested after 24 h in water or ethanol. Load and displacement values were used to calculate FM. Specimen fragments were used to measure KHN. DC was determined by FT-Raman spectroscopy. CS was determined by mechanical testing. Data were submitted to ANOVA/Tukey’s test (aZ0.05). Results: Composite DC was not affected by photoactivation (A: 65G1.8%; B: 66G3.4%; C: 65G2.9%). Unfilled resin DC was statistically higher using method A (79G0.3%) than B (74G1.0%) and C (73G0.9%). Photoactivation did not influence composite properties, regardless of the storage medium (pO0.05). After ethanol storage, FS of the unfilled resin was lower for specimens irradiated by method B (p!0.001). Pulse-delay curing (C) significantly reduced CS (7.7G1.3 MPa), compared to A (10.7G1.2 MPa) and B (10.1G1.3 MPa). Significance: Photoactivation method did not affect composite properties or susceptibility to ethanol degradation. For the unfilled resin, DC was lower with the use of low intensity and pulse-curing, while FS after ethanol storage was reduced by low intensity curing. Pulse-delay curing significantly reduced CS. Q 2005 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel./fax: C55 11 3091 7840. E-mail address: [email protected] (R.R. Braga).

0300-5712/$ - see front matter Q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jdent.2005.02.005

774

M.F. Witzel et al.

Introduction Clinical performance of BisGMA-based materials is to great extent dependent upon their mechanical properties and resistance to chemical degradation by acids and other organic substances found in the oral cavity.1–3 These characteristics are determined by the material’s degree of conversion.4,5 Degree of conversion in photoactivated materials is related to the energy density delivered by the light unit (expressed in J/cm2).6 Considering energy density as the product of the power density (expressed in mW/cm2) by the exposure time (in seconds), it should be possible to obtain similar conversions using different combinations of these two parameters.7 In recent years, the use of low power densities has become widespread in clinical practice, as several studies have shown that the use of continuous low intensity curing routines, as well as those characterized by reduced power density at the initial seconds, may lead to significant reductions in microleakage and gap formation in composite restorations.8–11 Reduced power densities result in lower polymerization rates, allowing more time for the composite to flow before gelation.12 As a consequence, the material can yield to forces resultant from the polymerization contraction being restricted by the adhesion to the dental substrate. By undergoing plastic deformation, stress build-up is delayed and a smaller fraction of the total shrinkage will be responsible for stress development.13 In fact, significant reductions in contraction stress values have been reported with the use of alternative curing routines. A study evaluating a pulse-delay method showed that 19–30% less stress was developed by three restorative composites, compared to a continuous high intensity photoactivation.14 Another study evaluating three alternative photoactivation modes observed that one of them was capable of significantly reduce contraction forces, compared to a continuous, high intensity photoactivation routine, used as control.12 The development of new photoactivation methods raised the concern that low curing rates may alter the final structure of the polymer and negatively affect the material’s chemical and Table 1

physical properties, in spite of reaching degrees of conversion similar to those reached with the use of continuous high intensity photoactivation. For example, a previous study found that unfilled resin specimens photoactivated by different pulsedelay methods showed degree of conversion and hardness values similar to specimens cured by continuous high intensity irradiation. However, after storage in ethanol for 24 h, specimens in the pulse-delay groups presented more severe reductions in hardness than the control specimens.15 In another study, a hybrid composite cured by pulse-delay also generated specimens with lower hardness after ethanol storage and lower glass transition temperature, compared to other photoactivation methods.16 It has been hypothesized that low power densities would generate a small number of free radicals, resulting in a more linear polymeric structure, with lower crosslinking density, as evidenced by reduced glass transition temperature and increased susceptibility to ethanol degradation. Other studies have shown that the use of continuous low-intensity photoactivation did not seem to affect flexural strength,17,18 fracture toughness17 or elastic modulus17,19 of BisGMA-based materials compared to high intensity photoactivation, given that energy density delivered was similar. However, specimens’ susceptibility to ethanol storage was not evaluated. The present study was outlined to verify the hypothesis that different photoactivation methods with equivalent energy densities would not affect the degree of conversion or the mechanical properties of a composite and an unfilled resin. The effect of ethanol storage on mechanical properties was also verified, in order to indirectly access possible differences in polymer structure caused by curing routines. Finally, photoactivation methods were evaluated regarding composite’s polymerization contraction stress development.

Materials and methods The materials used in this study are described in Table 1. A hybrid composite and an unfilled resin were chosen because they represent extreme

Materials used in this study.

Material

Composition

Batch

Manufacturer

Filtek Z250

BisGMA, UDMA, BisEMA, zirconia/silica filler (0.19–3.3 mm, 60% vol) 2-hydroxyethylmethacrylate, BisGMA, amines

3ER, 3AP

3 M ESPE St Paul, MN, USA

Scotchbond multi-purpose plus

3NK

Photoactivation of resin-based materials opposite situations in terms of mechanical properties. For all tests, photoactivation was performed using a halogen bulb light unit (VIP, Bisco Inc., Schaumburg, IL, USA). Three photoactivation methods were tested: 600 mW/cm2 for 40 s (high intensity), 200 mW/cm2 for 120 s (low intensity) or 600 mW/cm2 for 1 s, followed by a 3 min waiting time and a final irradiation of 600 mW/cm2 for 39 s (pulse-delay). Power density was periodically checked using a radiometer (Model 100 Optilux Radiometer, SDS Kerr, Danbury, CT, USA). The three-minute interval used in the pulse-delay method was chosen to reproduce the condition that led to the most severe reduction in hardness after 24 h ethanol storage, according to results of a previous study.15 In all the cases, the total energy density was 24 J/cm2. Degree of conversion (DC) was measured using FT-Raman spectroscopy. Disc-shaped specimens, 5-mm diameter and 1-mm thick, were built using a steel mold. Photoactivation was performed according to the protocols described above using a mylar strip between the composite surface and the light tip. Specimens were stored in air for 24 h at 37 8C (nZ3). Spectra were obtained by co-addition of 128 scans, at a resolution of 4 cmK1. The ratio between aliphatic (1640 cmK1) and aromatic (1610 cmK1) carbon double bonds peaks was used to calculate the degree of conversion, according to the formula:   cured DC Z 1 K !100 uncured For the flexural strength test (FS), specimens with rectangular crossection (1!2!10 mm, nZ20) were prepared using a steel split mold. Half of the samples were stored in 100% ethanol (Labsynt Ltda., Diadema, SP, Brazil) and the other half was stored in distilled water, both for 24 h at 37 8C. Before testing, specimen dimensions were recorded using a digital caliper (Starrett Ind. e Com. Ltda, Itu, SP, Brazil). The specimens were submitted to three-point bending in an universal testing machine (Kratos Equipamentos Industriais Ltda., Cotia, SP, Brazil) at a crosshead speed of 0.5 mm/min. The distance between the supports was 6 mm. Flexural strength was calculated using the following formula:20 Fs Z

3 !L !D 2 !w !h2

where Fs is the flexural strength (in MPa), L is the failure load (in Newtons), D is the distance between the supports (in mm), w is the width and h is the height of the specimen (both in mm). Flexural modulus (FM) was calculated by applying the specimen displacement values registered during

775 the flexural test to the formula:20 Ef Z

L1 !D3 !10K3 4 !w !h3 !d

where Ef is the flexural modulus (in GPa), L1 is the load (in Newtons), D is the distance between the supports (in mm), w is width of the specimen and h is the height of the specimen (both in mm), and d is the specimen displacement at load L1 (in mm). The average of the FM values corresponding to the 1, 3, 5 and 7% displacement was recorded. Knoop hardness number (KHN) was measured on fragments obtained after the flexural test. Immediately after the test, these fragments (nZ3) were embedded in polyvinyl chloride (PVC) cylinders with acrylic resin, polished using 200–1200 grit silicon carbide sandpapers and subjected to ten indentations under a load of 25 g for 30 s (Shimadzu Corp., Tokyo, Japan) on the irradiated surface. Composite resin was also tested for contraction stress (CS). Glass rods (13-mm long, 5-mm diameter) had one of their flat surfaces sandblasted with 250 mm alumina, silanated (Dentsply Ind. e Com., Rio de Janeiro, RJ, Brasil) and coated with one layer of unfilled resin (Scotchbond Multi-Purpose Plus, 3 M ESPE), photoactivated for 30 s. The glass rods were attached to opposite clamps of a testing machine (Instron, Canton, MA, USA) and composite was applied on the treated glass surfaces. The distance between the glass surfaces was set to 1 mm. An extensometer (Instron) was attached to the rods in order to detect any approximation between the glass surfaces resulting from composite shrinkage, with an accuracy of 0.1 mm. The compliance of the testing system was 7.9!10-6 mm/N. The software of the testing machine would command the actuator to move in the opposite direction to the axial shrinkage force, maintaining the initial composite thickness constant. Photoactivation was performed by placing the light guide tip in contact with the opposite end of the lower glass rod, as described elsewhere.21 In order to compensate for light attenuation caused by the glass, power density output for low intensity condition was increased to 300 mW/cm 2 . For experimental conditions requiring 600 mW/cm2, a 7-mm diameter ‘turbo’ tip was used, instead of the standard 10-mm tip. In both cases, light intensity values reaching the composite surface were found to be approximately 10% higher than those used in the other tests. Contraction force was monitored for 10 min. Maximum contraction stress was calculated by dividing the maximum force by the cross-section area of the glass rod. Five specimens were tested in each group.

776 Table 2

M.F. Witzel et al. Results for the composite (in the same column, all results are statistically similar, pO0.05). DC

2

High intensity (600 mW/cm !40 s) Low intensity (200 mW/cm2!120 s) Pulse-delay (600 mW/cm2!1 s C 3 min C600 mW/cm2!39 s)

65 (1.8) 66 (3.4) 65 (2.9)

Flexural strength (MPa)

Flexural modulus (GPa)

KHN

water

water

water

ethanol

42 (1.7) 43 (1.2) 41 (1.0)

47 (4.6) 48 (3.8) 40 (5.7)

ethanol

ethanol

163 (22.7) 172 (16.6) 12 (1.7) 12 (2.3) 159 (20.6) 164 (16.4) 11 (2.0) 12 (3.0) 146 (23.0) 163 (21.2) 11 (2.5) 11 (2.6)

Average composite CS curves for the different photoactivation methods are displayed in Fig. 1. Pulse-delay curing resulted in significantly lower stress values (7.7G1.4 MPa, p!0.01) compared to high intensity (10.7G1.24 MPa) and low intensity curing (10.1G1.32 MPa).

For all the tests performed, data was submitted to one-way ANOVA/Tukey’s test (aZ0.05), separately for each storage medium (except for DC measurements).

Results Discussion

Averages and standard deviations for DC and mechanical properties for the composite are shown in Table 2. DC was not influenced by photoactivation method (pZ0.810). Photoactivation method did not significantly influence mechanical properties, regardless of the storage medium (for water and ethanol, respectively: pZ0.213 and 0.456 for FS, pZ0.571 and 0.358 for FM and pZ0.180 and 0.193 for KHN). Averages and standard deviations for DC and mechanical properties of the unfilled resin are shown in Table 3. DC was statistically higher when specimens were photoactivated using the high intensity method, compared to the other methods (p!0.001). Photoactivation method did not significantly affect flexural modulus and hardness, either after storage in water (pZ0.724 and pZ0.296, respectively) or ethanol (pZ0.371 and pZ0.787, respectively). Flexural strength results revealed no significant effect of photoactivation after water storage (pZ0.708). After ethanol storage, however, low intensity curing resulted in lower values than those attained with the use of high intensity and pulse-delay curing (p!0.01).

Photoactivation methods tested in the present study did not affect the degree of conversion of the composite, in accordance with results of other studies that used different curing routines with similar energy density.7,12,14,19 Low intensity photoactivation should not negatively affect the degree of conversion if irradiation time is prolonged up to the stage when free-radical propagation becomes diffusion limited.7 Also in agreement with previous studies, since the same energy density was delivered by all photoactivation methods, mechanical properties were not influenced by different polymerization rates.17,19 For the unfilled resin, however, high intensity photoactivation resulted in a significantly higher degree of conversion than those obtained with other two methods. A similar observation has been previously reported.22 It is likely that the association of a high intensity irradiation with a high concentration of carbon double bonds in the unfilled resin generated enough reaction heat to significantly increase degree of conversion, in comparison with the other

Table 3 Results for the unfilled resin (in the same column, results followed by same superscript are statistically similar, pO0.05). DC

Flexural strength (MPa) water

2

High intensity (600 mW/cm !40 s) Low intensity (200 mW/cm2!120 s) Pulse-delay (600 mW/cm2!1 s C 3 min C600 mW/cm2!39 s)

a

79 (0.3) 74 (1.0)b 73 (0.9)b

Flexural modulus (GPa)

ethanol A

water a

KHN

ethanol A

water a

ethanol A

103 (7.4) 23 (6.6) 1.9 (0.5) 0.3 (0.1) 10 (0.6) 6.0 (0.0)a A 99 (10.9) 15 (2.9)b 1.8 (0.1)A 0.2 (0.1)a 11 (0.6)A 6.0 (1.0)a 100 (15.8)A 22 (6.3)a 1.9 (0.3)A 0.3 (0.1)a 10 (0.0)A 5.7 (0.6)a

Photoactivation of resin-based materials

777

12

Contraction Stress (MPa)

10 8 6

low intensity 4

high intensity pulse-delay

2 0 0

200

400

600

Time (s)

Fig. 1 Average composite contraction stress curves for the three photoactivation methods tested (high intensity: 600 mW/cm2!40 s; low intensity: 200 mW/cm2!120 s; pulse-delay: 600 mW/cm2!1 s C3 min C600 mW/cm2! 39 s).

photoactivation methods tested.5,19,23 Nevertheless, this difference in conversion did not correspond to an increase in mechanical properties values of water-stored specimens. A possible explanation for such finding is that the material’s properties noticeably improve up to a certain threshold of polymer network formation. Above that point, increases in conversion will not significantly affect its mechanical behavior.24 When ethanol penetrates the polymer network, it causes an expansion of the structure, allowing the release of uncured monomers and causing dissolution of linear polymer chains.25 This expansion is facilitated when crosslink density is low, since the solvent can disrupt secondary inter-chain bonds, but not primary crosslink bonds.16 In the present study, ethanol storage had different effects on the composite and on the unfilled resin. For the composite, the 24 h storage in ethanol did not affect mechanical properties. It is possible that the storage time was not long enough to disturb the polymer network to an extent that could reflect on the mechanical properties. In fact, a clear tendency for increased values after ethanol storage was observed in most experimental groups. It could be speculated that, during this relatively short period of immersion, residual monomers having a plasticizing effect on the resin matrix may have been released, slightly increasing mechanical properties. 26 Some studies have shown significant reductions in mechanical properties of composites only after 7 days of solvent immersion.27,28 However, other authors reported significant

composite deterioration after 24 h storage in ethanol.16 These discrepancies may be related to differences in the energy density applied to the composite and to the components of the resin composite (e.g., type of monomers, concentration of photoinitiators and inhibitors). Another aspect that should be taken into account is that part of mechanical properties of composites is related to the presence of an inorganic phase.29 Therefore, the effect of polymer degradation may be less evident in highly filled composites, like the material used in this study. Specifically for the hardness test, polishing of the sample may have removed part of the composite affected by ethanol, contributing with the lack of differences between storage media. Differently than composites, mechanical properties of the unfilled resin were severely reduced by ethanol storage. Unfilled resins have a high concentration of monomers with low molecular weight, which are more easily removed by ethanol storage.25 In flexural modulus and hardness tests, no significant differences between photoactivation methods were observed after ethanol storage. However, in the flexural strength test, different than what was observed after water storage, specimens photoactivated using continuous lowintensity irradiation showed significantly lower values after ethanol storage compared to the other curing routines. These findings agree only partially with previously mentioned studies reporting that low curing rates resulted in polymers with high susceptibility to ethanol degradation.15,16,30 Again, variations in energy density levels used by other researchers may explain the different results. In general, studies reporting the influence of curing mode in composite properties after ethanol storage used energy densities that were lower than the 24 J/cm2 used in the present study. In one study, a hybrid composite receiving 16 J/cm2 by pulse-delay curing showed a more severe drop in KHN after 24 h ethanol storage and lower glass transition temperature than specimens cured by other non-continuous or a high intensity curing method.16 Other authors using the same commercial composite tested here and energy densities below 17 J/cm2 reported significant differences in KHN and in the amount of leached monomers between photoactivation methods after 7 days immersion in ethanol.30 In another study, energy densities delivered to unfilled resin specimens through a pulse-delay method did not exceed 19 J/cm2 and, again, a significant drop in hardness was observed after 24 h storage in ethanol compared to specimens photoactivated in continuous mode.15 It has been demonstrated that, at early stages of

778 polymerization, pendant carbon double bonds are preferentially consumed by primary cyclization and high degrees of conversion must be reached in order to substantially increase crosslinking.31 Therefore, it may be speculated that the reduced susceptibility to ethanol degradation observed in the present study may be the result of the formation of a densely crosslinked polymer network due to the use of a high energy density, compensating for the potentially negative effects of a low curing rate. Finally, regarding the pulse-delay method, it could be argued that the power density of the initial pulse, rather than the total energy density, could determine the reduction in mechanical properties after ethanol storage. In fact, in two of the studies mentioned above,16,30 the power density of the initial pulse varied between 100 and 180 mW/cm2. However, in the third study,15 the power density of the initial pulse was either 450 or 650 mW/cm2. Moreover, in the studies using a low-intensity pulse, the duration of the pulse was 10 s, resulting in a higher energy density than the pulse used in this study. Several authors have reported the efficacy of low intensity or pulse-delay methods in reducing the polymerization contraction stress of composites.12,14,32 In the present study, a reduction of approximately 28% was obtained with the pulsedelay curing, compared to the high intensity photoactivation. This reduction is equivalent to what was obtained with a pulse-delay method in a previous study using a similar testing apparatus.14 On the other hand, reducing the power density from 600 to 200 mW/cm2 did not lead to a significant reduction in contraction stress values. The difference in the initial slope of stress development curves (Fig. 1) shows that reaction rate was reduced by supplying of a lower power density. However, though the curing rate can be significantly reduced, it does not mean that the decrease in contraction stress will also be significant. Findings of previous studies suggest that polymerization rate must be reduced under a certain threshold in order to significantly reduce contraction stress. 12,21,33 According to some authors, the gelation of multiacrylates happens in very low degrees of conversion.34 Therefore, the period allowed for the material to flow is very restricted, and a substantial decrease in curing rate is needed to significantly affect contraction stress development. Clinically, it is also important to notice that composites from different manufacturers will show different behaviors when submitted to alternative photoactivation methods. Further investigations are necessary to verify if the final energy density employed in the pulsedelay method is, in fact, the factor determining

M.F. Witzel et al. the polymer susceptibility to ethanol degradation. Other composite formulations must be evaluated as well. Regarding the hypothesis of this study, it must be accepted for the composite, considering that photoactivation method did not significantly affect its degree of conversion or mechanical properties. However, for the unfilled resin, the hypothesis must be partially rejected, because high intensity photoactivation led to a significant increase in degree of conversion, compared to other two methods. Within the conditions of this study, ethanol storage did not significantly affect composite’s mechanical properties. For the unfilled resin, differences only in flexural strength were observed after ethanol storage for the low intensity curing, compared to the other methods. Finally, it was possible to conclude that pulse-delay curing was effective in reducing polymerization contraction stress of the composite, in comparison to the other photoactivation methods evaluated.

Acknowledgements This investigation was supported by FAPESP (grants 03/13002-0 and 03/03686-9), CAPES and FUNDECTO.

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