d e n t a l m a t e r i a l s 3 1 ( 2 0 1 5 ) 391–398
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A study of polymerization shrinkage kinetics using digital image correlation Andrew Lau, Jianying Li, Young Cheul Heo, Alex Fok ∗ Minnesota Dental Research Center for Biomaterials and Biomechanics, School of Dentistry, University of Minnesota, Minneapolis, MN, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the polymerization shrinkage kinetics of dental resin composites
Received 4 June 2013
by measuring in real time the full-field shrinkage strain using a novel technique based on
Received in revised form
digital image correlation (DIC).
26 October 2014
Methods. Polymerization shrinkage in resin composite specimens (Filtek LS and Z100) was
Accepted 7 January 2015
measured as a function of time and position. The main experimental setup included a CCD camera and an external shutter inversely synchronized to that of the camera. The specimens (2 mm × 4 mm × 5 mm) were irradiated for 40 s at 1200 mW/cm2 , while alternating image
Keywords:
acquisition and obstruction of the curing light occurred at 15 fps. The acquired images were
Resin composite
processed using proprietary software to obtain the full-field strain maps as a function of
Polymerization shrinkage kinetics
time.
Digital image correlation (DIC)
Results. Z100 showed a higher final shrinkage value and rate of development than LS. The
Inversely synchronized shutter
final volumetric shrinkage for Z100 and LS were 1.99% and 1.19%, respectively. The shrink-
Real-time measurement
age behavior followed an established shrinkage strain kinetics model. The corresponding characteristic time and reaction order exponent for LS and Z100 were calculated to be approximately 23 s and 0.84, and 14 s and 0.7, respectively, at a distance of 1.0 mm from the irradiated surface, the position where maximum shrinkage strain occurred. Thermal expansion from the exothermic reaction could have affected the accuracy of these parameters. Significance. The new DIC method using an inversely synchronized shutter provided realtime, full-field results that could aid in assessing the shrinkage strain kinetics of dental resin composites as a function of specimen depth. It could also help determine the optimal curing modes for dental resin composites. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The use of light-cured resin composites has become commonplace in restorative dentistry. These dental biomaterials have a high aesthetic value, good mechanical and physical
∗
properties, and are easy to use. A drawback of using resin composites to restore teeth, however, is the polymerization shrinkage that occurs during the curing of these materials. Therefore, resin composites need to be chemically and/or micro-mechanically adhered to the tooth substrate to ensure that no gaps are formed between the tooth substrate and the
Corresponding author. Tel.: +1 612 6255406; fax: +1 612 6261484. E-mail address: [email protected] (A. Fok).
http://dx.doi.org/10.1016/j.dental.2015.01.001 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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restorative material after curing [1,2]. Otherwise, the restored tooth will be structurally compromised and bacteria may invade through the interfacial gaps and cause secondary caries [1]. However, bonding the restoration to the tooth can create problems in itself. As the resin composite shrinks, the tooth cavity walls will oppose this shrinkage, leading to the creation of shrinkage stress. Polymerization shrinkage stress poses three main problems: (1) fracture of the tooth tissues, (2) failure of the composite restoration, and (3) debonding at the tooth-restoration interface [3–5]. Therefore, many studies have been performed to try to measure, understand, and minimize polymerization shrinkage. There are several methods for measuring polymerization shrinkage. Notable examples include dilatometry [6,7], the bonded disc method [7–9], and the use of strain gauges [7,10]. These methods use either a liquid medium or mechanical devices to measure the displacement or volume change of the composite material due to polymerization shrinkage. The mechanical devices require the buildup of a certain level of stiffness in the specimen to deform them, and the liquid medium may exert gravitational or adherent forces on the specimen [11]. In other words, the specimen could be prevented from undergoing free shrinkage when these methods are used for its measurement. Indeed, the strain gage method only measures the so-called post-gel shrinkage of resin composites. An alternative to measuring shrinkage strain mechanically is the use of optical sensors. These have the ability to perform noncontact shrinkage measurement without the aforementioned problems. One such method is the use of digital image correlation (DIC). DIC works by comparing a series of sequential images of the deforming object under observation. Through the tracking and analysis of distinctive features on the object, its displacements and strains can be determined [11–14]. Li et al. [11] pioneered the use of DIC for measuring the polymerization shrinkage strain and depth of cure in dental resin composites. In their work, resin composite bar specimens were prepared with irregular speckle patterns created on their surfaces with spray paints. The specimens were then cured from one side using a curing light, followed by 30 min of continuous imaging of the speckled surface using a CCD camera. Through the analysis of the correlated images, the final shrinkage strains caused by polymerization could be determined over the entire surface of observation. Hence, shrinkage strain could be determined as a function of distance from the curing light, from which the depth of cure could be estimated. The limitation of this approach was the interference of the curing light with the imaging during its operation, which prevented measurement of the specimen’s shrinkage during curing. Note that the majority of shrinkage in resin composites occurs during the first few seconds of the polymerization process. Therefore, the kinetics of polymerization shrinkage during curing, which is probably the most important part of the polymerization process, could not be studied with the regular DIC method. The purpose of this study is to introduce a novel technique for measuring, in real time, the entire process of shrinkage strain development using an enhanced DIC method. The method utilizes an external shutter that is inversely
synchronized with the CCD camera’s internal shutter to temporarily block off the curing light when an image is being taken. This allows the shrinkage behavior of resin composites to be assessed in more detail by studying shrinkage kinetics as a function of depth within the specimen. It could also help to determine the optimal curing modes for dental resin composites.
2.
Theory
Based on Watts’ work [15], the development of polymerization shrinkage strain can be modeled using the following equation:
εi = εmax 1 − e−((t−o )/c )
(1)
where εi is the instantaneous strain at any given position x and time t, and εmax is the maximum strain exhibited by the resin composite material at that position. The characteristic time c controls the rate at which the strain approaches εmax : the larger its value, the slower the rate. is the reaction order exponent which also controls the rate of shrinkage strain development, as well as the overall shape of the shrinkage strain vs. time curve. A large value would produce a curve with a step change in shrinkage around c . The initiation and propagation of polymerization in a resin composite is a diffusive process. Therefore, there is a time delay between turning on the curing light and the onset of conversion, especially for material in a sizable specimen that is some distance away from the light source: the further away a point is from the curing light, the longer the time delay. An additional parameter o has thus been added to the model in [15] to take into consideration this time delay. In general, εmax , o , and c all depend on the distance x from the light source. As seen in Eq. (2) below, by taking double logs on both sides of Eq. (1), the shrinkage strain kinetics equation can be manipulated to obtain the above parameters from experimental data using linear regression. By plotting the left hand side of Eq. (2) against ln(t − o ), the reaction order exponent can be determined from the slope of the straight line fitted to the experimental data. Once is determined, the characteristic time c can be found from the y-intercept.
ln −ln 1 −
3.
εi
εmax
= ln (t − o ) − ln (c )
(2)
Materials and methods
Curing of resin composites in a non-bonding model cavity was performed to characterize shrinkage strain formation under free-shrinkage conditions.
3.1.
Specimen preparation
Experiments were performed using a steel mold with a milled rectangular cavity measuring 2 mm × 4 mm × 50 mm; see Fig. 1a. The steel cavity was lined with PTFE tape to minimize adhesion to the resin composite. The resin composite was packed into the steel cavity using a flat steel bar to create a flat surface for strain measurement using DIC. The compacted
d e n t a l m a t e r i a l s 3 1 ( 2 0 1 5 ) 391–398
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sample had final dimensions of 2 mm × 4 mm × 5 mm (Fig. 1a). The 4 mm × 5 mm exterior surface facing the camera (Fig. 1b) was coated with a thin layer of white spray paint first and then fine black carbon particles. The black carbon particles on the white paint provided irregular speckles for computerized tracking to determine displacements during curing. The dental composites used in this study were Z100TM Restorative and FiltekTM LS Low Shrink Posterior Restorative (both 3 M ESPE, St. Paul, MN, USA); n = 4 specimens for each material. Z100 is known to be a high-shrink material (vol.% = 2.48 [6]) while LS is considered a low-shrink material (vol.% = 0.99 [16]). Manufacturer instructions for curing each material specify a 40 s cure using a LED light with a light intensity of at least 400 mW/cm2 for Z100 [17], and 500 mW/cm2 for LS [18].
3.2.
Apparatus setup
Fig. 1c shows the experimental setup used to induce and measure polymerization shrinkage. A curing light (3 M ESPE Ellipar S10), which emitted blue light of wavelength 460 nm at an irradiation power of 1200 mW/cm2 , was used to cure the specimens. It was positioned less than 5 mm away from the composite specimen. A light shield with a 2 mm × 4 mm throughhole was attached to the side of the specimen mold facing the curing light to ensure that irradiation was applied to that surface of the specimen only. A CCD camera (Point Grey Grasshopper GRAS-20S4C-C) was used to capture images of the speckled surface. An external shutter (Uniblitz VS14) was positioned between the curing light and the specimen. It was inversely synchronized with the camera’s internal shutter by using a shutter control box (Uniblitz VCM-D1) with trigger signal input from the camera. This trigger signal was responsible for alternately opening the external shutter for light-curing and closing it for image-taking. The external shutter had a 50% duty cycle, essentially reducing the light energy provided to the specimen to ∼600 mW/cm2 . Imaging without such an ancillary light-blocking unit would result in overexposure/saturation of the CCD camera. A 550 nm long-pass filter was placed between the camera and the specimen (2 mm from the observation surface) for absorption of extraneous blue light to further improve camera exposure during irradiation. A yellow LED light was used to illuminate the specimen without contributing to the polymerization. Collectively, these components provided a means for continuous imaging during irradiation.
3.3. Fig. 1 – (a) Schematic diagram of experimental setup: (a) curing light, (b) external shutter, (c) light-shield with 2 mm × 4 mm through-hole, (d) specimen mold with 2 mm × 4 mm × 50 mm milled cavity, (e) resin composite sample (2 mm × 4 mm × 5 mm), (f) 550 nm optical light filter. (b) Observation surface of composite specimen painted with speckled pattern (c) Photograph of experimental setup.
Experimental operation
To minimize the effect of slumping, the mold housing the specimen was laid on the 4 mm × 5 mm face until the test was performed. Each of the prepared specimens was irradiated for 40 s. As mentioned, irradiation was delivered only on the 2 mm × 4 mm surface of the specimen facing the curing light. During irradiation, continuous image acquisition occurred at a rate of 15 frames per second (fps). The first image acquired was used as a reference image to which all successive images were compared. Differences in positions of the speckled patterns between any two images indicated displacements and dimensional changes in the specimen caused by polymerization. An additional image was taken at 10 min from the start of
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curing to measure the strain distribution after shrinkage had plateaued. Displacement and strain analysis was carried out using proprietary software (StrainMaster, Davis 7.0 LaVision GmbH, Germany). The magnitude and rate of strain development as a function of time and distance from the curing light were calculated for comparison between the two material groups.
3.4.
Effect of external shutter on polymerization
To evaluate the possible effect of the external shutter on the irradiation and polymerization of the composites, additional strain development tests were performed without the external shutter. Strain measurements were taken at 0 s and 5 min, with irradiation also limited to the first 40 s. Five tests were performed for each setup and the averaged values at each point were compared.
4.
Results
Fig. 2 shows how the polymerization shrinkage strain developed through one of the bar specimens as time increased. Note that shrinkage strains continued to develop after the 40 s of curing. As illustrated in Fig. 2, the strain contour plots revealed heterogeneous strain formation within the specimens. Even at a constant distance from the curing light, there was variation in the shrinkage strain across the specimen height. Therefore, the shrinkage strain averaged across the height was calculated for various distances from the curing light for comparison. Fig. 3 displays the mean and standard deviation for the shrinkage strains in the x (horizontal) and y (vertical) directions, along with the volumetric strain for both material groups at 10 min. There were more fluctuations in the xdirection strain than in the y-direction strain. As expected, LS showed lower average shrinkage strains than Z100. The plots also show that, for both materials, the maximum shrinkage strain was found between 1.0 and 2.0 mm from the surface facing the curing light. The maximum volumetric shrinkage at 10 min for Z100 and LS were 1.99% and 1.19%, respectively. Z100 has a reported volumetric shrinkage of 2.32 at 5 min and 2.48 at 10 min [6]. There was a clear reduction in polymerization shrinkage strain as the distance from the curing light increased. In relation to this, when removing the cured specimens from the mold, it was observed that there was tangible stiffness within approximately the first 4 mm of the bar specimens. Beyond this length, the resin composites remained relatively soft. A small peak in shrinkage strain, mostly in the x-direction, was seen at this transition point. The presence of the external shutter was found to result in a reduction of less than 20% in the average shrinkage strain at all times and positions (Fig. 3c). Representative shrinkage strain vs. time curves for the two composites are shown in Fig. 4. Given their smaller fluctuations, the y-axis shrinkage strain, at distances of 1.0 mm increments are plotted for further analysis. As expected, the shrinkage strain increased with time before plateauing to a maximum. The shrinkage strains found at 10 min were assumed to be the maximum values that the materials attained, i.e. εmax in Eq. (1), at each position. These are also
Fig. 2 – Representative shrinkage strain contours. Irradiation was applied at 0 mm for 40 s.
plotted in Fig. 4. An observation that can be made in these shrinkage strain-time plots, especially in those for LS, was the exhibition of a small amount of expansion (negative shrinkage) in the beginning of curing. This initial expansion was also seen in Z100, albeit small. The time delay to shrinkage strain development for several positions along the specimen length was determined from the times in Fig. 4 at which the shrinkage strain first became positive. Fig. 5a shows that the time delay increased exponentially with the distance from the curing light. LS showed longer time delays than Z100, the values being 5 s and 0.5 s, respectively, at the irradiated surface. Due to the low signal-to-noise ratio when x > 2.5 mm, the results for the remaining portion of the LS specimens were not presented. For Z100 the time delay increased to 7.23 s at x = 5.0 mm, and 12.19 s at x = 2.5 mm for LS. Fig. 5b is a representative plot that was generated using Eq. (2) and the data extracted from Figs. 4b and 5a. The results were fitted with straight lines to derive the reaction order exponent, , from the slopes and the characteristic time, c ,
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Fig. 3 – (a) Filtek LS average shrinkage strain. (b) Z100 average shrinkage strain. (c) Volumetric shrinkage strain of LS measured with and without external shutter present.
Fig. 4 – Representative y-direction shrinkage strain vs. time plots for (a) Filtek LS, (b) Z100. Each line corresponds to a different position along the specimen length. The corresponding maximum strain values measured at 10 minutes are shown in dashed lines.
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Fig. 5 – (a) Time delay ( 0 ) to shrinkage strain development as a function of position along the beam specimen. (b) Representative (Z100) shrinkage strain data plotted according to Eq. (2) to determine the parameters for the shrinkage strain kinetics equation. (c) Reaction order exponents for LS and Z100 found from the slopes of the straight lines fitted to the data at each position x in Fig. 4b. (d) Characteristic times for LS and Z100 derived from the extrapolated y-intercept of the fitted straight lines in Fig. 4b.
from the y-intercept. Fig. 5c shows the reaction order exponent as a function of distance along the specimen length. Z100 had a relatively linear increase in the reaction order exponent from about 0.59 to 1.36 within the first 5 mm of specimen length. LS exhibited similar values for the reaction order exponent in the front portion of the specimens, but showed a steeper increase in values with increasing distance away from the light source. Fig. 5d shows the characteristic time as a function of distance from the curing light. As can be seen, the characteristic time also increased as the distance from the curing light increased. However, there was a reduction in value at a depth of 0.5 to 1 mm. LS had longer characteristic times (>20 s) than Z100 (>10 s), and showed a bigger subsurface reduction in the value before showing an increase with distance from the irradiated surface.
5.
Discussion
For a composite restoration to be durable, the resin composite must be cured adequately. A practical method to assess the extent of cure is to measure the material’s hardness. A resin
composite sample is said to be “cured” when the bottom surface reaches approximately 80–90% the hardness of the top surface in a 2 mm thick sample [4]. A more rigorous method of assessing the extent of cure is to measure the degree of conversion using Fourier transform infrared spectroscopy (FTIR). FTIR operates by measuring the ratio of carbon-to-carbon double bonds to carbon-to-carbon single bonds [19]. A study performed by Silikas et al. [9] found that the polymerization shrinkage strain of the composite they studied was linearly correlated to the degree of conversion. Therefore, the polymerization shrinkage strain can be used as a representative index of the degree of conversion of resin composites, although it is not expected that all composite materials will have the same linear correlation. It has been suggested that shrinkage stress depends mostly on the post-gel shrinkage. In actual fact, the development of shrinkage stress depends on the temporal development of both the shrinkage strain and the Young’s modulus [20]. Therefore, it is important to determine the entire shrinkage strain history when studying polymerization kinetics and the development of shrinkage stress.
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The aim of this study was to use the DIC method, enhanced with an external shutter that was inversely synchronized with that of the CCD camera, to measure the full-field shrinkage strain development in light-cured resin composites in real time. The results allowed the polymerization kinetics of resin composites to be studied as a function of specimen depth. The progression of shrinkage strain from the end closest to the curing light source to the far end shows clearly the diffusive process of polymerization (Fig. 2). The full-field shrinkage strain development with time plotted in Fig. 2 shows a heterogeneous strain development, similar to those found at the end of curing by Li et al. [11]. Similar heterogeneity has also been found using Raman spectroscopy to measure degree of conversion and its relationship with shrinkage strain [21]. These results indicated that the initiation and development of polymerization was not uniform throughout the specimen, even at the same distance from the curing light. As expected, Z100 produced greater shrinkage strain levels than its low-shrink counterpart, LS. The decrease in shrinkage strain with increasing distance from the light source, as seen in Fig. 3, was consistent with those reported by Li et al. [11], who suggested this as a means for determining the depth of cure for composite materials. This is determined by identifying the point where the shrinkage strain falls below a certain percentage of the maximum value. As the distance from the curing light increases, the amount of light and resulting free radicals reaching the material decrease. As a result, the extent of polymerization shrinkage also decreases. If the reduction in shrinkage strain is an indicator of the depth of cure, as proposed in the previous work [11], the results presented would indicate that LS has a shallower depth of cure than Z100 due to the former’s faster reduction in shrinkage strain with distance. The possible complex strain state created at the point (∼4 mm) where the material changed from hard to soft was considered to be the reason for the small peak in shrinkage strain seen in Fig. 3. The x-direction strains showed more fluctuations than the y-direction strains (Fig. 3). This was attributed to the combined effect of friction and the longer dimension that gave rise to more hindrance to movement in the x-direction. In contrast, the shorter dimension allowed the specimens to shrink more freely in the y-direction, thus producing smoother results. As was found by others, Fig. 3 shows that the greatest level of shrinkage strain was located at 1.0 to 2.0 mm away from the irradiated surface. Oxygen inhibition to polymerization has been suggested as being responsible for the lower level of shrinkage measured at the irradiated surface. The observation surface was covered with paint and therefore should not be affected by oxygen inhibition. However, the paint might have reacted chemically with the underlying composite. Shrinkage at the top and bottom edges of the specimen was seen to be lower than that located inward (Fig. 2). This may also be attributed to oxygen inhibition since the PTFE tape lining was permeable to oxygen. Nevertheless, given the similarity between the measured shrinkage strains and those reported by others [6], these effects were considered not to be significant. In addition to determining the final shrinkage strain as a function of specimen depth, the enhanced DIC method can also be used to study the rate at which shrinkage takes
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place. This is useful information when comparing the curing behavior of different resin composites. The new method could also allow the effectiveness of different curing regimes to be studied. Two noteworthy observations can be made of the results presented in Fig. 4. The first is the initial expansion of the resin composites prior to contraction. This is indicated by the initial negative shrinkage strain values. The polymerization reaction can be a highly exothermic process that produces temporary thermal expansion. The silorane-based LS is known to be more exothermic than methacrylate-based resin composites as a result of its bond-breaking mechanism to reduce shrinkage stress/strain. It is therefore not surprising to see more expansion in LS than Z100 at the beginning of curing. Also, as it took time for polymerization to propagate from the irradiated surface, the deeper the location, the later the expansion took place. The implication of this is that the time delay for shrinkage, especially in the case of LS, could have been significantly overestimated, which could have led to further errors in the other parameters. To correct for these errors, the temperature increase during curing and the coefficient of thermal expansion of the materials will need to be established so that the thermal strains could be eliminated from the shrinkage strain measurement. The second observation is that polymerization shrinkage continued well after the end of the 40 s cure, with the shrinkage strains measured at 10 min being significantly greater than those measured at 40 s. The further away a point was from the light source, the greater the difference between the measurements taken at the two different time points. This was partly due to the increasing time delay to the onset and completion of shrinkage with the distance from the curing light (Fig. 4), which again indicates the diffusive nature of the polymerization process. Fig. 5b shows that the shrinkage strain kinetics can be well described by Eq. (1). The values found for the reaction order exponent, Fig. 5c, were similar to those reported in [15]. As shown in Fig. 5c, as the distance from the curing light increased, the reaction order exponent increased. The reaction order exponent is related to the rate of reaction: the higher the exponent, the slower the polymerization shrinkage strain forms. This was consistent with the results seen in Fig. 4, which shows that the initial slope/rate of the polymerization shrinkage strain plots decreased as the distance from the light source increased. Again, this decrease in rate of polymerization with specimen depth can be explained by the decrease in light penetration and hence free radicals present at deeper locations. The slower reaction rate with increased depth can further be seen in Fig. 5d through the increasing characteristic time along the specimen. In the results above, the reaction order exponent and characteristic time could only be plotted from x = 0 mm to x = 2.5 mm for LS. This was because some of the LS specimens did not exhibit high enough strain levels (>0.1% strain) at distances greater than 2.5 mm. A strain value of 0.1% is the detection threshold for DIC, below which it becomes difficult to differentiate noise from meaningful strain values. The use of the external shutter was found to reduce moderately the level of shrinkage strain, when compared to measurements made with an unobstructed light. The external shutter effectively created a high-speed pulsed irradiation
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that reduced the overall energy from the curing light. The inadvertent pulsed curing was different from the slow-start and pulse-curing technique proposed by others as an attempt to reduce shrinkage. These techniques initiate polymerization with a low-power irradiation (100–200 mW/cm2 ), followed by period of rest before switching to a high-power irradiation (450–600 mW/cm2 ) [22,23]. The present work used a continuous high-powered irradiation (1200 mW/cm2 ) that was pulsed for 40 s at a rate of 15 Hz. This constant, high-powered, pulsed light intensity was able to produce shrinkage levels close to those achieved without the external shutter. Other sources of errors in the experiments included inadequate resolution of the camera, variations from the manual preparation of the specimen, non-uniform shrinkage through the specimen thickness, and misalignment between the camera and the specimen. The quality of the images used in the image correlation analysis relies on a camera that provides high enough resolution and close enough proximity to the specimen. Additionally, the quality of the speckled patterns could also have affected the accuracy of the strain maps. It was assumed that the observation surface did not suffer from oxygen inhibition because it was covered with a layer of paint. As a result, the surface strains should be similar to those within the volume. The layer of paint was also assumed to be thin and flexible enough that it would move freely with the bulk of the composite material behind it. However, material heterogeneity and oxygen inhibition at the other surfaces may have resulted in non-uniform shrinkage across the specimen thickness. Misalignment of the camera could impact the shrinkage strain readings by distorting the images. This limitation may be overcome by using two cameras, which also allow the out-of-plane strain to be measured. Concerns have been expressed regarding the effects of slumping on the measurement of shrinkage strain. However, the reference image used was taken at the start of curing, and it only took a few seconds for the material to reach the post-gel state, at which point slumping would no longer be an issue. Therefore, any slumping that might have occurred before the start of curing (when the reference image was taken) would not have affected the measurement of shrinkage.
6.
Conclusions
A new DIC method utilizing inversely synchronized shutters has been successfully developed for measuring the polymerization strain development in light-cured resin composites in real time. Full-field measurements on the resin composite’s surface can now be taken during the irradiation period. This allows polymerization kinetics to be studied readily as a function of time and specimen depth via the shrinkage strain, which has been reported to be directly related to the degree of conversion.
references
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[2] De Munck J, Van Landuyt K, Peumans M, Poitevin A, Lambrechts P, Braem M, et al. A critical review of the durability of adhesion to tooth tissue: methods and results. J Dent Res 2005;84:118–32. [3] Davidson CL, Feilzer AJ. Polymerization shrinkage and polymerization shrinkage stress in polymer-based restoratives. J Dent 1997;25:435–40. [4] Stansbury JW, Trujillo-Lemon M, Lu H, Ding X, Lin Y, Ge J. Conversion-dependent shrinkage stress and strain in dental resins and composites. Dent Mater 2004;21: 56–67. [5] Li H, Li J, Yun X, Liu X, Fok A. Non-destructive examination of interfacial debonding using acoustic emission. Dent Mater 2011;27:964–71. [6] Kleverlaan CJ, Feilzer AJ. Polymerization shrinkage and contraction stress of dental resin composites. Dent Mater 2005;21:1150–7. [7] Sakaguchi R, Wiltbank B, Shah N. Critical configuration analysis of four methods for measuring polymerization shrinkage strain of composites. Dent Mater 2004;20: 388–96. [8] Watts DC, Cash AJ. Determination of polymerization shrinkage kinetics in visible-light-cured materials: methods development. Dent Mater 1991;7:281–7. [9] Silikas N, Eliades G, Watts D. Light intensity effects on resin-composite degree of conversion and shrinkage strain. Dent Mater 2000;16:292–6. [10] Sakaguchi R, Berge H. Reduced light energy density decreases post-gel contraction while maintaining degree of conversion in composites. J Dent 1998;26: 695–700. [11] Li J, Fok A, Satterthwaite J, Watts D. Measurement of the full-field polymerization shrinkage and depth of cure of dental composites using digital image correlation. Dent Mater 2009;25:582–8. [12] Furukawa T, Arakawa K, Morita Y, Uchino M. Polymerization shrinkage behavior of light cured resin composites in cavities. J Biomech Sci Eng 2009;4:356–64. [13] Chuang S, Chen T, Chang C. Application of digital image correlation method to study dental composite shrinkage. Strain 2008;44:231–8. [14] Chuang S, Chang C, Chen T. Spatially resolved assessments of composite shrinkage in MOD restorations using a digital-image-correlation technique. Dent Mater 2011;27:134–43. [15] Watts D. Reaction kinetics and mechanics in photo-polymerised networks. Dent Mater 2005;21: 27–35. [16] Filtek LS. Technical product profile Ins.: 3M ESPE; 2007. [17] Z100 Restorative Instructions for use Ins.: 3M ESPE; 2012. [18] Filtek LS. Technique guide Ins.: 3M ESPE; 2008. [19] Atai M, Watts D. A new kinetic model for the photopolymerization shrinkage-strain of dental composites and resin-monomers. Dent Mater 2006;22:785–879. [20] Fok A. Shrinkage stress development in dental composites—an analytical treatement. Dent Mater 2013;29:1108–15. [21] Huxford I, Holmes B, Fok A. Correlating strain measurements with degree of conversion in commercial composites. In: Proceedings of the 43rd annual meeting of the American Association for Dental Research. 2014 (Abstract nr 883; [abstract]). [22] Lu H, Stansbury JW, Bowman CN. Impact of curing protocol on conversion and shrinkage stress. J Dent Res 2005;84:822–6. [23] Deliperi S, Bardwell D. An alternative method to reduce polymerization shrinkage in direct posterior composite restorations. JADA 2002;133:1387–97.
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ScienceDirect journal homepage: www.intl.elsevierhealth.com/journals/dema
A study of polymerization shrinkage kinetics using digital image correlation Andrew Lau, Jianying Li, Young Cheul Heo, Alex Fok ∗ Minnesota Dental Research Center for Biomaterials and Biomechanics, School of Dentistry, University of Minnesota, Minneapolis, MN, USA
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objective. To investigate the polymerization shrinkage kinetics of dental resin composites
Received 4 June 2013
by measuring in real time the full-field shrinkage strain using a novel technique based on
Received in revised form
digital image correlation (DIC).
26 October 2014
Methods. Polymerization shrinkage in resin composite specimens (Filtek LS and Z100) was
Accepted 7 January 2015
measured as a function of time and position. The main experimental setup included a CCD camera and an external shutter inversely synchronized to that of the camera. The specimens (2 mm × 4 mm × 5 mm) were irradiated for 40 s at 1200 mW/cm2 , while alternating image
Keywords:
acquisition and obstruction of the curing light occurred at 15 fps. The acquired images were
Resin composite
processed using proprietary software to obtain the full-field strain maps as a function of
Polymerization shrinkage kinetics
time.
Digital image correlation (DIC)
Results. Z100 showed a higher final shrinkage value and rate of development than LS. The
Inversely synchronized shutter
final volumetric shrinkage for Z100 and LS were 1.99% and 1.19%, respectively. The shrink-
Real-time measurement
age behavior followed an established shrinkage strain kinetics model. The corresponding characteristic time and reaction order exponent for LS and Z100 were calculated to be approximately 23 s and 0.84, and 14 s and 0.7, respectively, at a distance of 1.0 mm from the irradiated surface, the position where maximum shrinkage strain occurred. Thermal expansion from the exothermic reaction could have affected the accuracy of these parameters. Significance. The new DIC method using an inversely synchronized shutter provided realtime, full-field results that could aid in assessing the shrinkage strain kinetics of dental resin composites as a function of specimen depth. It could also help determine the optimal curing modes for dental resin composites. © 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
The use of light-cured resin composites has become commonplace in restorative dentistry. These dental biomaterials have a high aesthetic value, good mechanical and physical
∗
properties, and are easy to use. A drawback of using resin composites to restore teeth, however, is the polymerization shrinkage that occurs during the curing of these materials. Therefore, resin composites need to be chemically and/or micro-mechanically adhered to the tooth substrate to ensure that no gaps are formed between the tooth substrate and the
Corresponding author. Tel.: +1 612 6255406; fax: +1 612 6261484. E-mail address: [email protected] (A. Fok).
http://dx.doi.org/10.1016/j.dental.2015.01.001 0109-5641/© 2015 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
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restorative material after curing [1,2]. Otherwise, the restored tooth will be structurally compromised and bacteria may invade through the interfacial gaps and cause secondary caries [1]. However, bonding the restoration to the tooth can create problems in itself. As the resin composite shrinks, the tooth cavity walls will oppose this shrinkage, leading to the creation of shrinkage stress. Polymerization shrinkage stress poses three main problems: (1) fracture of the tooth tissues, (2) failure of the composite restoration, and (3) debonding at the tooth-restoration interface [3–5]. Therefore, many studies have been performed to try to measure, understand, and minimize polymerization shrinkage. There are several methods for measuring polymerization shrinkage. Notable examples include dilatometry [6,7], the bonded disc method [7–9], and the use of strain gauges [7,10]. These methods use either a liquid medium or mechanical devices to measure the displacement or volume change of the composite material due to polymerization shrinkage. The mechanical devices require the buildup of a certain level of stiffness in the specimen to deform them, and the liquid medium may exert gravitational or adherent forces on the specimen [11]. In other words, the specimen could be prevented from undergoing free shrinkage when these methods are used for its measurement. Indeed, the strain gage method only measures the so-called post-gel shrinkage of resin composites. An alternative to measuring shrinkage strain mechanically is the use of optical sensors. These have the ability to perform noncontact shrinkage measurement without the aforementioned problems. One such method is the use of digital image correlation (DIC). DIC works by comparing a series of sequential images of the deforming object under observation. Through the tracking and analysis of distinctive features on the object, its displacements and strains can be determined [11–14]. Li et al. [11] pioneered the use of DIC for measuring the polymerization shrinkage strain and depth of cure in dental resin composites. In their work, resin composite bar specimens were prepared with irregular speckle patterns created on their surfaces with spray paints. The specimens were then cured from one side using a curing light, followed by 30 min of continuous imaging of the speckled surface using a CCD camera. Through the analysis of the correlated images, the final shrinkage strains caused by polymerization could be determined over the entire surface of observation. Hence, shrinkage strain could be determined as a function of distance from the curing light, from which the depth of cure could be estimated. The limitation of this approach was the interference of the curing light with the imaging during its operation, which prevented measurement of the specimen’s shrinkage during curing. Note that the majority of shrinkage in resin composites occurs during the first few seconds of the polymerization process. Therefore, the kinetics of polymerization shrinkage during curing, which is probably the most important part of the polymerization process, could not be studied with the regular DIC method. The purpose of this study is to introduce a novel technique for measuring, in real time, the entire process of shrinkage strain development using an enhanced DIC method. The method utilizes an external shutter that is inversely
synchronized with the CCD camera’s internal shutter to temporarily block off the curing light when an image is being taken. This allows the shrinkage behavior of resin composites to be assessed in more detail by studying shrinkage kinetics as a function of depth within the specimen. It could also help to determine the optimal curing modes for dental resin composites.
2.
Theory
Based on Watts’ work [15], the development of polymerization shrinkage strain can be modeled using the following equation:
εi = εmax 1 − e−((t−o )/c )
(1)
where εi is the instantaneous strain at any given position x and time t, and εmax is the maximum strain exhibited by the resin composite material at that position. The characteristic time c controls the rate at which the strain approaches εmax : the larger its value, the slower the rate. is the reaction order exponent which also controls the rate of shrinkage strain development, as well as the overall shape of the shrinkage strain vs. time curve. A large value would produce a curve with a step change in shrinkage around c . The initiation and propagation of polymerization in a resin composite is a diffusive process. Therefore, there is a time delay between turning on the curing light and the onset of conversion, especially for material in a sizable specimen that is some distance away from the light source: the further away a point is from the curing light, the longer the time delay. An additional parameter o has thus been added to the model in [15] to take into consideration this time delay. In general, εmax , o , and c all depend on the distance x from the light source. As seen in Eq. (2) below, by taking double logs on both sides of Eq. (1), the shrinkage strain kinetics equation can be manipulated to obtain the above parameters from experimental data using linear regression. By plotting the left hand side of Eq. (2) against ln(t − o ), the reaction order exponent can be determined from the slope of the straight line fitted to the experimental data. Once is determined, the characteristic time c can be found from the y-intercept.
ln −ln 1 −
3.
εi
εmax
= ln (t − o ) − ln (c )
(2)
Materials and methods
Curing of resin composites in a non-bonding model cavity was performed to characterize shrinkage strain formation under free-shrinkage conditions.
3.1.
Specimen preparation
Experiments were performed using a steel mold with a milled rectangular cavity measuring 2 mm × 4 mm × 50 mm; see Fig. 1a. The steel cavity was lined with PTFE tape to minimize adhesion to the resin composite. The resin composite was packed into the steel cavity using a flat steel bar to create a flat surface for strain measurement using DIC. The compacted
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sample had final dimensions of 2 mm × 4 mm × 5 mm (Fig. 1a). The 4 mm × 5 mm exterior surface facing the camera (Fig. 1b) was coated with a thin layer of white spray paint first and then fine black carbon particles. The black carbon particles on the white paint provided irregular speckles for computerized tracking to determine displacements during curing. The dental composites used in this study were Z100TM Restorative and FiltekTM LS Low Shrink Posterior Restorative (both 3 M ESPE, St. Paul, MN, USA); n = 4 specimens for each material. Z100 is known to be a high-shrink material (vol.% = 2.48 [6]) while LS is considered a low-shrink material (vol.% = 0.99 [16]). Manufacturer instructions for curing each material specify a 40 s cure using a LED light with a light intensity of at least 400 mW/cm2 for Z100 [17], and 500 mW/cm2 for LS [18].
3.2.
Apparatus setup
Fig. 1c shows the experimental setup used to induce and measure polymerization shrinkage. A curing light (3 M ESPE Ellipar S10), which emitted blue light of wavelength 460 nm at an irradiation power of 1200 mW/cm2 , was used to cure the specimens. It was positioned less than 5 mm away from the composite specimen. A light shield with a 2 mm × 4 mm throughhole was attached to the side of the specimen mold facing the curing light to ensure that irradiation was applied to that surface of the specimen only. A CCD camera (Point Grey Grasshopper GRAS-20S4C-C) was used to capture images of the speckled surface. An external shutter (Uniblitz VS14) was positioned between the curing light and the specimen. It was inversely synchronized with the camera’s internal shutter by using a shutter control box (Uniblitz VCM-D1) with trigger signal input from the camera. This trigger signal was responsible for alternately opening the external shutter for light-curing and closing it for image-taking. The external shutter had a 50% duty cycle, essentially reducing the light energy provided to the specimen to ∼600 mW/cm2 . Imaging without such an ancillary light-blocking unit would result in overexposure/saturation of the CCD camera. A 550 nm long-pass filter was placed between the camera and the specimen (2 mm from the observation surface) for absorption of extraneous blue light to further improve camera exposure during irradiation. A yellow LED light was used to illuminate the specimen without contributing to the polymerization. Collectively, these components provided a means for continuous imaging during irradiation.
3.3. Fig. 1 – (a) Schematic diagram of experimental setup: (a) curing light, (b) external shutter, (c) light-shield with 2 mm × 4 mm through-hole, (d) specimen mold with 2 mm × 4 mm × 50 mm milled cavity, (e) resin composite sample (2 mm × 4 mm × 5 mm), (f) 550 nm optical light filter. (b) Observation surface of composite specimen painted with speckled pattern (c) Photograph of experimental setup.
Experimental operation
To minimize the effect of slumping, the mold housing the specimen was laid on the 4 mm × 5 mm face until the test was performed. Each of the prepared specimens was irradiated for 40 s. As mentioned, irradiation was delivered only on the 2 mm × 4 mm surface of the specimen facing the curing light. During irradiation, continuous image acquisition occurred at a rate of 15 frames per second (fps). The first image acquired was used as a reference image to which all successive images were compared. Differences in positions of the speckled patterns between any two images indicated displacements and dimensional changes in the specimen caused by polymerization. An additional image was taken at 10 min from the start of
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curing to measure the strain distribution after shrinkage had plateaued. Displacement and strain analysis was carried out using proprietary software (StrainMaster, Davis 7.0 LaVision GmbH, Germany). The magnitude and rate of strain development as a function of time and distance from the curing light were calculated for comparison between the two material groups.
3.4.
Effect of external shutter on polymerization
To evaluate the possible effect of the external shutter on the irradiation and polymerization of the composites, additional strain development tests were performed without the external shutter. Strain measurements were taken at 0 s and 5 min, with irradiation also limited to the first 40 s. Five tests were performed for each setup and the averaged values at each point were compared.
4.
Results
Fig. 2 shows how the polymerization shrinkage strain developed through one of the bar specimens as time increased. Note that shrinkage strains continued to develop after the 40 s of curing. As illustrated in Fig. 2, the strain contour plots revealed heterogeneous strain formation within the specimens. Even at a constant distance from the curing light, there was variation in the shrinkage strain across the specimen height. Therefore, the shrinkage strain averaged across the height was calculated for various distances from the curing light for comparison. Fig. 3 displays the mean and standard deviation for the shrinkage strains in the x (horizontal) and y (vertical) directions, along with the volumetric strain for both material groups at 10 min. There were more fluctuations in the xdirection strain than in the y-direction strain. As expected, LS showed lower average shrinkage strains than Z100. The plots also show that, for both materials, the maximum shrinkage strain was found between 1.0 and 2.0 mm from the surface facing the curing light. The maximum volumetric shrinkage at 10 min for Z100 and LS were 1.99% and 1.19%, respectively. Z100 has a reported volumetric shrinkage of 2.32 at 5 min and 2.48 at 10 min [6]. There was a clear reduction in polymerization shrinkage strain as the distance from the curing light increased. In relation to this, when removing the cured specimens from the mold, it was observed that there was tangible stiffness within approximately the first 4 mm of the bar specimens. Beyond this length, the resin composites remained relatively soft. A small peak in shrinkage strain, mostly in the x-direction, was seen at this transition point. The presence of the external shutter was found to result in a reduction of less than 20% in the average shrinkage strain at all times and positions (Fig. 3c). Representative shrinkage strain vs. time curves for the two composites are shown in Fig. 4. Given their smaller fluctuations, the y-axis shrinkage strain, at distances of 1.0 mm increments are plotted for further analysis. As expected, the shrinkage strain increased with time before plateauing to a maximum. The shrinkage strains found at 10 min were assumed to be the maximum values that the materials attained, i.e. εmax in Eq. (1), at each position. These are also
Fig. 2 – Representative shrinkage strain contours. Irradiation was applied at 0 mm for 40 s.
plotted in Fig. 4. An observation that can be made in these shrinkage strain-time plots, especially in those for LS, was the exhibition of a small amount of expansion (negative shrinkage) in the beginning of curing. This initial expansion was also seen in Z100, albeit small. The time delay to shrinkage strain development for several positions along the specimen length was determined from the times in Fig. 4 at which the shrinkage strain first became positive. Fig. 5a shows that the time delay increased exponentially with the distance from the curing light. LS showed longer time delays than Z100, the values being 5 s and 0.5 s, respectively, at the irradiated surface. Due to the low signal-to-noise ratio when x > 2.5 mm, the results for the remaining portion of the LS specimens were not presented. For Z100 the time delay increased to 7.23 s at x = 5.0 mm, and 12.19 s at x = 2.5 mm for LS. Fig. 5b is a representative plot that was generated using Eq. (2) and the data extracted from Figs. 4b and 5a. The results were fitted with straight lines to derive the reaction order exponent, , from the slopes and the characteristic time, c ,
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Fig. 3 – (a) Filtek LS average shrinkage strain. (b) Z100 average shrinkage strain. (c) Volumetric shrinkage strain of LS measured with and without external shutter present.
Fig. 4 – Representative y-direction shrinkage strain vs. time plots for (a) Filtek LS, (b) Z100. Each line corresponds to a different position along the specimen length. The corresponding maximum strain values measured at 10 minutes are shown in dashed lines.
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Fig. 5 – (a) Time delay ( 0 ) to shrinkage strain development as a function of position along the beam specimen. (b) Representative (Z100) shrinkage strain data plotted according to Eq. (2) to determine the parameters for the shrinkage strain kinetics equation. (c) Reaction order exponents for LS and Z100 found from the slopes of the straight lines fitted to the data at each position x in Fig. 4b. (d) Characteristic times for LS and Z100 derived from the extrapolated y-intercept of the fitted straight lines in Fig. 4b.
from the y-intercept. Fig. 5c shows the reaction order exponent as a function of distance along the specimen length. Z100 had a relatively linear increase in the reaction order exponent from about 0.59 to 1.36 within the first 5 mm of specimen length. LS exhibited similar values for the reaction order exponent in the front portion of the specimens, but showed a steeper increase in values with increasing distance away from the light source. Fig. 5d shows the characteristic time as a function of distance from the curing light. As can be seen, the characteristic time also increased as the distance from the curing light increased. However, there was a reduction in value at a depth of 0.5 to 1 mm. LS had longer characteristic times (>20 s) than Z100 (>10 s), and showed a bigger subsurface reduction in the value before showing an increase with distance from the irradiated surface.
5.
Discussion
For a composite restoration to be durable, the resin composite must be cured adequately. A practical method to assess the extent of cure is to measure the material’s hardness. A resin
composite sample is said to be “cured” when the bottom surface reaches approximately 80–90% the hardness of the top surface in a 2 mm thick sample [4]. A more rigorous method of assessing the extent of cure is to measure the degree of conversion using Fourier transform infrared spectroscopy (FTIR). FTIR operates by measuring the ratio of carbon-to-carbon double bonds to carbon-to-carbon single bonds [19]. A study performed by Silikas et al. [9] found that the polymerization shrinkage strain of the composite they studied was linearly correlated to the degree of conversion. Therefore, the polymerization shrinkage strain can be used as a representative index of the degree of conversion of resin composites, although it is not expected that all composite materials will have the same linear correlation. It has been suggested that shrinkage stress depends mostly on the post-gel shrinkage. In actual fact, the development of shrinkage stress depends on the temporal development of both the shrinkage strain and the Young’s modulus [20]. Therefore, it is important to determine the entire shrinkage strain history when studying polymerization kinetics and the development of shrinkage stress.
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The aim of this study was to use the DIC method, enhanced with an external shutter that was inversely synchronized with that of the CCD camera, to measure the full-field shrinkage strain development in light-cured resin composites in real time. The results allowed the polymerization kinetics of resin composites to be studied as a function of specimen depth. The progression of shrinkage strain from the end closest to the curing light source to the far end shows clearly the diffusive process of polymerization (Fig. 2). The full-field shrinkage strain development with time plotted in Fig. 2 shows a heterogeneous strain development, similar to those found at the end of curing by Li et al. [11]. Similar heterogeneity has also been found using Raman spectroscopy to measure degree of conversion and its relationship with shrinkage strain [21]. These results indicated that the initiation and development of polymerization was not uniform throughout the specimen, even at the same distance from the curing light. As expected, Z100 produced greater shrinkage strain levels than its low-shrink counterpart, LS. The decrease in shrinkage strain with increasing distance from the light source, as seen in Fig. 3, was consistent with those reported by Li et al. [11], who suggested this as a means for determining the depth of cure for composite materials. This is determined by identifying the point where the shrinkage strain falls below a certain percentage of the maximum value. As the distance from the curing light increases, the amount of light and resulting free radicals reaching the material decrease. As a result, the extent of polymerization shrinkage also decreases. If the reduction in shrinkage strain is an indicator of the depth of cure, as proposed in the previous work [11], the results presented would indicate that LS has a shallower depth of cure than Z100 due to the former’s faster reduction in shrinkage strain with distance. The possible complex strain state created at the point (∼4 mm) where the material changed from hard to soft was considered to be the reason for the small peak in shrinkage strain seen in Fig. 3. The x-direction strains showed more fluctuations than the y-direction strains (Fig. 3). This was attributed to the combined effect of friction and the longer dimension that gave rise to more hindrance to movement in the x-direction. In contrast, the shorter dimension allowed the specimens to shrink more freely in the y-direction, thus producing smoother results. As was found by others, Fig. 3 shows that the greatest level of shrinkage strain was located at 1.0 to 2.0 mm away from the irradiated surface. Oxygen inhibition to polymerization has been suggested as being responsible for the lower level of shrinkage measured at the irradiated surface. The observation surface was covered with paint and therefore should not be affected by oxygen inhibition. However, the paint might have reacted chemically with the underlying composite. Shrinkage at the top and bottom edges of the specimen was seen to be lower than that located inward (Fig. 2). This may also be attributed to oxygen inhibition since the PTFE tape lining was permeable to oxygen. Nevertheless, given the similarity between the measured shrinkage strains and those reported by others [6], these effects were considered not to be significant. In addition to determining the final shrinkage strain as a function of specimen depth, the enhanced DIC method can also be used to study the rate at which shrinkage takes
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place. This is useful information when comparing the curing behavior of different resin composites. The new method could also allow the effectiveness of different curing regimes to be studied. Two noteworthy observations can be made of the results presented in Fig. 4. The first is the initial expansion of the resin composites prior to contraction. This is indicated by the initial negative shrinkage strain values. The polymerization reaction can be a highly exothermic process that produces temporary thermal expansion. The silorane-based LS is known to be more exothermic than methacrylate-based resin composites as a result of its bond-breaking mechanism to reduce shrinkage stress/strain. It is therefore not surprising to see more expansion in LS than Z100 at the beginning of curing. Also, as it took time for polymerization to propagate from the irradiated surface, the deeper the location, the later the expansion took place. The implication of this is that the time delay for shrinkage, especially in the case of LS, could have been significantly overestimated, which could have led to further errors in the other parameters. To correct for these errors, the temperature increase during curing and the coefficient of thermal expansion of the materials will need to be established so that the thermal strains could be eliminated from the shrinkage strain measurement. The second observation is that polymerization shrinkage continued well after the end of the 40 s cure, with the shrinkage strains measured at 10 min being significantly greater than those measured at 40 s. The further away a point was from the light source, the greater the difference between the measurements taken at the two different time points. This was partly due to the increasing time delay to the onset and completion of shrinkage with the distance from the curing light (Fig. 4), which again indicates the diffusive nature of the polymerization process. Fig. 5b shows that the shrinkage strain kinetics can be well described by Eq. (1). The values found for the reaction order exponent, Fig. 5c, were similar to those reported in [15]. As shown in Fig. 5c, as the distance from the curing light increased, the reaction order exponent increased. The reaction order exponent is related to the rate of reaction: the higher the exponent, the slower the polymerization shrinkage strain forms. This was consistent with the results seen in Fig. 4, which shows that the initial slope/rate of the polymerization shrinkage strain plots decreased as the distance from the light source increased. Again, this decrease in rate of polymerization with specimen depth can be explained by the decrease in light penetration and hence free radicals present at deeper locations. The slower reaction rate with increased depth can further be seen in Fig. 5d through the increasing characteristic time along the specimen. In the results above, the reaction order exponent and characteristic time could only be plotted from x = 0 mm to x = 2.5 mm for LS. This was because some of the LS specimens did not exhibit high enough strain levels (>0.1% strain) at distances greater than 2.5 mm. A strain value of 0.1% is the detection threshold for DIC, below which it becomes difficult to differentiate noise from meaningful strain values. The use of the external shutter was found to reduce moderately the level of shrinkage strain, when compared to measurements made with an unobstructed light. The external shutter effectively created a high-speed pulsed irradiation
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that reduced the overall energy from the curing light. The inadvertent pulsed curing was different from the slow-start and pulse-curing technique proposed by others as an attempt to reduce shrinkage. These techniques initiate polymerization with a low-power irradiation (100–200 mW/cm2 ), followed by period of rest before switching to a high-power irradiation (450–600 mW/cm2 ) [22,23]. The present work used a continuous high-powered irradiation (1200 mW/cm2 ) that was pulsed for 40 s at a rate of 15 Hz. This constant, high-powered, pulsed light intensity was able to produce shrinkage levels close to those achieved without the external shutter. Other sources of errors in the experiments included inadequate resolution of the camera, variations from the manual preparation of the specimen, non-uniform shrinkage through the specimen thickness, and misalignment between the camera and the specimen. The quality of the images used in the image correlation analysis relies on a camera that provides high enough resolution and close enough proximity to the specimen. Additionally, the quality of the speckled patterns could also have affected the accuracy of the strain maps. It was assumed that the observation surface did not suffer from oxygen inhibition because it was covered with a layer of paint. As a result, the surface strains should be similar to those within the volume. The layer of paint was also assumed to be thin and flexible enough that it would move freely with the bulk of the composite material behind it. However, material heterogeneity and oxygen inhibition at the other surfaces may have resulted in non-uniform shrinkage across the specimen thickness. Misalignment of the camera could impact the shrinkage strain readings by distorting the images. This limitation may be overcome by using two cameras, which also allow the out-of-plane strain to be measured. Concerns have been expressed regarding the effects of slumping on the measurement of shrinkage strain. However, the reference image used was taken at the start of curing, and it only took a few seconds for the material to reach the post-gel state, at which point slumping would no longer be an issue. Therefore, any slumping that might have occurred before the start of curing (when the reference image was taken) would not have affected the measurement of shrinkage.
6.
Conclusions
A new DIC method utilizing inversely synchronized shutters has been successfully developed for measuring the polymerization strain development in light-cured resin composites in real time. Full-field measurements on the resin composite’s surface can now be taken during the irradiation period. This allows polymerization kinetics to be studied readily as a function of time and specimen depth via the shrinkage strain, which has been reported to be directly related to the degree of conversion.
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