Curing characteristics of a composite. Part 2: The effect of curing configuration on depth and distribution of cure

Abstract

Objective

The objective of this study was to examine the effect different configurations of curing would have on the depth and distribution of the cure within each configuration, for a specific resin-based composite (RBC).

Methods

RBC was cured in a variety of configurations, consisting of 6 mm molds of three different colors; large molds that simulated the condition of no mold at all; and 3–6 mm diameter molds to check the effect of size. All specimens were cured for 20 s with a quartz-halogen lamp and were allowed to cure for 24 h in the dark. Transmission measurements were made for these same configurations. Knoop hardness measurements were made across the central plane of some configurations to determine the distribution of curing.

Results

Depths of cure and distribution of curing were significantly affected by changes in configuration. Under the configuration of no mold, the cure extended well beyond the periphery of the light guide due to scattering of the light. When a mold was used, a pronounced effect by the walls resulted in decreased hardness as the mold wall was approached, and the severity of this effect was dependent on the color of the mold. It is believed that this is due to absorption/reflection characteristics of light by the walls, with the white molds showing the least effect. Reducing the diameter of the molds resulted in significant decreases in depth of cure, which are attributed to light absorption by the walls that limits the penetration of light during the curing procedure.

Significance

Configuration of curing has a significant effect on the depth of cure, but also significantly reduces the cure near the mold wall. This can have clinical ramifications for the cure along a stainless steel matrix band for Class II restorations, and for test procedures in general, where there is no standardization regarding configuration or where measurements are made on specimens.

Introduction

There has been a long history of testing light-curable resin-based composite (RBC) materials to establish effective curing conditions for combinations of RBC and related curing lamps. In the conduct of these studies a wide variety of molds have been used in which the RBC is light cured. These have varied in size, depth, shape, color, material and translucency, but for the most part, the mold has been ignored as a factor in the results obtained. However, there has been some evidence that the mold can influence the depth and the distribution of the cure , so it is unlikely that results from these various molds could be directly compared. Therefore, it would seem reasonable that there should be some standardization of how light-curable RBC materials are tested, and recognize that the mold is an important part of the test methodology. There is a standardized test for depth of cure, which is often referred to as the ISO test , which is mandatory for manufacturers to certify their RBC materials and to set their recommendations for cure times relative to RBC increment thickness, which makes it a clinically related test. However, the rationale for this test is unspecified and correlations between the test and clinical applications are lacking.

If a test method is going to be developed it seems reasonable that the configuration for curing the RBC should be understood. The configuration of curing refers to the surroundings of the RBC and the relative positioning or size of the curing lamp, and in its typical form is a cylindrical mold with the lamp directly on top, but it can be more complex than that. Understanding how light is distributed, absorbed, or reflected in the configuration would be an important part of judging the worth of a particular configuration, particularly if the same is done for curing RBC in human teeth. It may be that a test should be designed around a difficult clinical application, such as a deep Class II cavity. For any test configuration that is devised to give clinical guidance on depth of cure, it is important to know what to measure and where to make the measurements and that requires an understanding of how the light and subsequent cure of the RBC is distributed within that configuration.

It might be expected that with configuration changes involving color of opaque molds, that the different amounts of absorption or reflection of the curing light could cause changes in how the material cures, and that was partially confirmed in a study where black molds produced shorter depths of cure than a stainless steel mold when a light shade of composite was cured . However, black molds are still used instead of stainless steel molds, as recommended in the ISO test . White molds have also been used, but they have generally been made of Teflon or some other translucent material that can allow more of the curing light to pass through the mold than through the RBC material . This results in exaggerated depths of cure, and scrape back surfaces that are concave as opposed to the convex scrape-back surfaces found when using opaque molds . Despite these results, translucent plastic and Teflon molds are still used in determining the depth of cure and other properties .

Human tooth molds have also been used , but they have varied in size and have not been compared with a 4 mm diameter, stainless steel mold, as specified in the ISO test to determine depths of cure.

The effect of mold size has been examined for opaque cylindrical molds and the measured or implied depth of cure for RBC materials was found to decrease as the diameter decreased. A change in size would be considered a change in configuration, as would a change in geometry. Most molds used are cylindrical, but other mold geometries have been used: molds with 4 mm square cross section , a slot mold with 2 mm by 4 mm cross section and a hemi-cylindrical mold of 4 mm diameter . These were used to provide a flat surface on which to make measurements such as hardness. In each case, other than one , comparisons with the ISO test were made even though the geometry and/or materials were not what is specified in the ISO test. Also, since the ISO test is relevant to measurements along the central axis, then surface measurements may not be comparable given the evidence above of potential non-uniform curing within specimens.

Given that the walls of a mold may influence the cure distribution within the RBC, then it is of interest to know what the cure distribution is for an unrestricted RBC, which in practical terms means a mold that is large enough that there is no influence on the curing process. Under these conditions, the RBC has been shown to cure laterally beyond the edges of the light guide by a significant amount , and this lateral cure and depth of cure is dependent on the diameter of the light guide, for the same irradiance .

In Part 1 , unique relationships were found between the internal radiant exposure, H, and both DC and KHN, which were called the energy-conversion relationship (ECR) and energy-hardness relationship (EHR) for the RBC. The internal H values were determined from the incident radiant exposure, H 0 , and a measured light transmission relationship as a function of depth, T ( d ), using fully cured RBC. It was postulated that the ECR and EHR were unique to the material and would apply to any curing configuration of that RBC material, however, the T ( d ) relationship was expected to vary with the configuration. Using these relationships for a given configuration, it was postulated that the curing characteristics along the central axis of a configuration could be defined. In addition it was postulated that at the scrape-back depth ( D SB ) the radiant exposure H SB would be nominally the same for all configurations, but the value of D SB would vary as would the T ( d ) relationship. Depth of cure was defined as the scrape back depth, D SB , in Part 1, and is also defined that way in the present work.

It is the intent of this work to examine in a detailed way the effect of various curing configurations on the distribution and depth of cure for an RBC material in a variety of configurations. The specific goals and hypotheses are to show that for a specific RBC: (1) Each configuration is represented by a transmission relationship specific to that configuration. (2) D SB will also be specific to each configuration. (3) H SB will be nominally independent of the configuration. (4) The distribution of curing will be non-uniform along a plane perpendicular to the central axis for each configuration and will be unique to the configuration. (5) The EHR for the RBC material is independent of the configuration.

Materials and methods

As discussed in a previous article, Part 1 , the RBC material that was to be used for all this work was depleted before all of the planned measurements were completed. This was unexpectedly caused by the fact that the material was undergoing an aging process that resulted in decreasing scrape-back depths; which necessitated repeated measurements to first define the problem, and then to redo most of the work in a relatively short time span so that various aspects of the study would be comparable. However, there was not enough material left to make all the measurements intended. Therefore, a more recent lot of the same type material was used to make comparisons with the original material and to complete some of the measurements that were planned. Table 1 provides a description of the two materials, which will be referred to as RBC1 and RBC2.

Table 1
Description of resin-based composite (RBC) materials.
RBC Material Shade Lot Manufacturer
1 Z100 A3.5 3GY 3M ESPE, St. Paul, MN, USA
2 Z100 A3.5 N240183 3M ESPE, St. Paul, MN, USA

Most of the procedures were described in detail in the previous article, Part 1 , therefore, the descriptions here will be somewhat abbreviated.

Depth of cure measurements and specimen molds

The specific RBC materials were cured in a variety of molds, each representing a different configuration for curing the RBC. In all cases the depth of cure was defined as the scrape-back depth ( D SB ). The procedure for forming specimens was the same for all molds. A minimum number of five specimens was used for each group, but in some cases more were used.

Molds were placed onto a glass slide or plate that was covered with a polyester film. RBC material was extruded directly into the mold from syringes produced by the manufacturer until an excess of material was obtained at the top of the mold. This excess was trimmed, leaving a small excess of RBC, then a polyester film was placed on top and a glass slide was used to compress the material flat at the top of the mold. The glass slide was removed and the material was cured for 20 s with a quartz-tungsten-halogen (QTH) lamp (XL 3000, 3M-ESPE, St. Paul, MN, USA). Cured specimens were then stored in the dark at room temperature for 24 h.

After storage, specimens were removed from the molds and uncured material was removed and all crumbly material was scraped away to leave a hard surface, using a stainless steel placement instrument. The length of each specimen was measured with a digital caliper three times and the average length recorded to the nearest hundredth of a millimeter. The average value for a group of specimens was determined and this provided the depth of cure value, D SB . One exception to the above description of procedures was made for a group of specimens cured in 4 mm, cylindrical, stainless steel molds, where the scrape-back procedure was applied immediately after light-curing as specified by the ISO test .

In all stages of handling and making measurements on cured specimens the work was done in reduced lighting or in filtered light to protect against additional curing of the specimens.

Radiant exposure from the light source

All specimens made using RBC1 were light-cured for 20 s with the same quartz–tungsten–halogen curing lamp (XL 3000, 3M-ESPE, St. Paul, MN, USA), with a 7 mm diameter, fiber optic light-guide. The power output was measured with a power meter (Power Max 500D Laser Power Meter, Molectron Detector Inc., Portland, OR, USA) to be 260 mW after a 30 s stabilization period and decreased only 2% over an additional 120 s of operation. This provided an average irradiance of 684 mW/cm 2 from the light guide. When curing RBC with a polyester film, this was reduced by 10% to correct for reflectance from the film so that an irradiance of 616 mW/cm 2 was incident on the RBC material. With a 20 s light exposure time the radiant exposure was 12,320 mJ/cm 2 to the RBC surface. These values were used for all calculations involving RBC1 in the previous article (Part 1) and in this current work.

All measurements with the RBC2 material were made at a later time, after all the RBC1 testing had been completed. The same curing lamp described above was used, but a different power meter was used (351 Power Meter, UDT Instruments, Baltimore, MD, USA) to accommodate the types of measurements planned. This meter had a 20 mm diameter detector head. The power output from the light-guide, at the time the RBC2 measurements were made, as measured with this meter, was 245.9 mW from the 7 mm light-guide. Power measurements were also made for various diameters of the light guide by using appropriate apertures in black tape affixed to the end of the light guide. Table 2 shows these values along with the corresponding irradiance values; this pattern of power distribution is similar to other observations . When curing RBC, using a polyester covering film, these values were corrected for reflectance from the film, as described above.

Table 2
Power and irradiance values of curing for different light-guide apertures.
Light-guide diameter (mm) Power (mW) Area (cm 2 ) Irradiance (mW/cm 2 )
1 8.5 0.00785 1089.2
2 32.6 0.0314 1038.2
4 114.5 0.1256 911.6
6 203.3 0.2826 719.4
7 245.9 0.3846 639.4

Configurations

To test the effect of color differences of molds, cylindrical split-molds made of stainless steel that when assembled provided a 6 mm diameter cylindrical mold 12 mm in height were used. These molds were used as is, to provide one color, or were lined with black or white vinyl tape for the other two colors. The tape was 0.215 mm thick so the diameters of the latter two molds were about 5.6 mm.

Differences in size were examined by comparing the depth of cure for RBC specimens made in cylindrical, stainless steel split-molds of 6, 5, 4 and 3 mm diameter, as described above. To represent a configuration of unrestricted RBC, a 22 mm diameter, cylindrical mold that was 11 mm in height was used to hold the RBC material. This mold was large enough that there was no influence of the mold itself on the distribution of light or the resulting cure within the RBC. This configuration will be referred to as unrestricted composite. The effect of a 2 mm diameter light source in comparison to the 7 mm light guide on the curing characteristics of unrestricted composite was accomplished by placing a 2 mm aperture, made from black tape, over the end of the 7 mm light-guide.

One further mold configuration was examined that was provided by using half of a 4 mm cylindrical, stainless steel mold to form a hemi-cylinder that was capped with a flat stainless steel plate.

Hardness measurements

Specimens of RBC1 that were measured for D SB were mounted on polishing fixtures and polished with water-cooling to expose a longitudinal plane along the central axis having a 4000 grit final polish. Specimens were dried with compressed air and left to dry further, in the dark, for 24 h before measurements were made. The specimens still attached to the polishing fixture were transferred to a specimen stage of a Knoop hardness tester (Buehler Micromet II microhardness tester; Buehler, Lake Bluff, IL, USA). The long axis of the specimen was aligned with the y -axis of the specimen stage and x and y zero points were set at the top center of specimen. Knoop hardness numbers (KHN) were measured using 100 g load and 12 s dwell time.

KHN values were obtained over one half of the specimen from the outer edge to the central axis. Measurements were made along the length in 0.5 mm steps, starting 0.5 mm from the top, and dropping to 0.25 mm steps until KHN values could not be made. At each of these steps, measurements in a radial direction were made, in increments, starting at either 0.05 mm or 0.1 mm from the outer surface of the cylinder and commencing with measurements at 0.2, 0.3, 0.5,1.0, 2.0, mm out to the central axis. The scrape-back surface defined the KHN = 0 contour . Top KHN values were determined by extrapolation of the depth profiles along the central axis to zero depth.

An analysis of the measured KHN data was undertaken to determine isometric contours of various values of %MaxKHN. A series of plots of the %MaxKHN vs. depth for each radial sampling distance was made on graph paper. By choosing a particular percentage on the y -axis, sets of depth and radial coordinate pairs were determined for that percentage, and these defined the isometric profile. Similarly plots of the %MaxKHN vs. radial position for specific depths allowed comparable data pairs to be used in refining the isometric profiles.

Transmission measurements

The curing light transmission relationship for RBC1, and the measurement method used, was described in the previous article (Part 1) . The same basic procedure was used in this work for RBC2. Stainless steel cylinders of various lengths with either 6 mm or 4 mm apertures were filled with RBC2 material and cured extensively to assure complete cure. After 24 h the cylindrical test units were polished on each end to remove excess material and were then measured for thickness. Individual units were placed on the detector head of the power meter and the light guide was centered over the top aperture and transmitted power was measured after 30 s of stabilization of the lamp. Three measurements were made and the average power was calculated. The transmission value for each thickness was determined as the ratio of the transmitted power to the incident power. A transmission relationship with depth T ( d ) was determined by fitting the data with linear regression analysis.

To form unrestricted RBC2 test specimens, the material was placed in various thicknesses in 22 mm diameter cylindrical molds, and light-cured extensively to assure full conversion. Transmission measurements were made as described above with apertures on the detector that matched the light diameter used.

Materials and methods

As discussed in a previous article, Part 1 , the RBC material that was to be used for all this work was depleted before all of the planned measurements were completed. This was unexpectedly caused by the fact that the material was undergoing an aging process that resulted in decreasing scrape-back depths; which necessitated repeated measurements to first define the problem, and then to redo most of the work in a relatively short time span so that various aspects of the study would be comparable. However, there was not enough material left to make all the measurements intended. Therefore, a more recent lot of the same type material was used to make comparisons with the original material and to complete some of the measurements that were planned. Table 1 provides a description of the two materials, which will be referred to as RBC1 and RBC2.

Table 1
Description of resin-based composite (RBC) materials.
RBC Material Shade Lot Manufacturer
1 Z100 A3.5 3GY 3M ESPE, St. Paul, MN, USA
2 Z100 A3.5 N240183 3M ESPE, St. Paul, MN, USA

Most of the procedures were described in detail in the previous article, Part 1 , therefore, the descriptions here will be somewhat abbreviated.

Depth of cure measurements and specimen molds

The specific RBC materials were cured in a variety of molds, each representing a different configuration for curing the RBC. In all cases the depth of cure was defined as the scrape-back depth ( D SB ). The procedure for forming specimens was the same for all molds. A minimum number of five specimens was used for each group, but in some cases more were used.

Molds were placed onto a glass slide or plate that was covered with a polyester film. RBC material was extruded directly into the mold from syringes produced by the manufacturer until an excess of material was obtained at the top of the mold. This excess was trimmed, leaving a small excess of RBC, then a polyester film was placed on top and a glass slide was used to compress the material flat at the top of the mold. The glass slide was removed and the material was cured for 20 s with a quartz-tungsten-halogen (QTH) lamp (XL 3000, 3M-ESPE, St. Paul, MN, USA). Cured specimens were then stored in the dark at room temperature for 24 h.

After storage, specimens were removed from the molds and uncured material was removed and all crumbly material was scraped away to leave a hard surface, using a stainless steel placement instrument. The length of each specimen was measured with a digital caliper three times and the average length recorded to the nearest hundredth of a millimeter. The average value for a group of specimens was determined and this provided the depth of cure value, D SB . One exception to the above description of procedures was made for a group of specimens cured in 4 mm, cylindrical, stainless steel molds, where the scrape-back procedure was applied immediately after light-curing as specified by the ISO test .

In all stages of handling and making measurements on cured specimens the work was done in reduced lighting or in filtered light to protect against additional curing of the specimens.

Radiant exposure from the light source

All specimens made using RBC1 were light-cured for 20 s with the same quartz–tungsten–halogen curing lamp (XL 3000, 3M-ESPE, St. Paul, MN, USA), with a 7 mm diameter, fiber optic light-guide. The power output was measured with a power meter (Power Max 500D Laser Power Meter, Molectron Detector Inc., Portland, OR, USA) to be 260 mW after a 30 s stabilization period and decreased only 2% over an additional 120 s of operation. This provided an average irradiance of 684 mW/cm 2 from the light guide. When curing RBC with a polyester film, this was reduced by 10% to correct for reflectance from the film so that an irradiance of 616 mW/cm 2 was incident on the RBC material. With a 20 s light exposure time the radiant exposure was 12,320 mJ/cm 2 to the RBC surface. These values were used for all calculations involving RBC1 in the previous article (Part 1) and in this current work.

All measurements with the RBC2 material were made at a later time, after all the RBC1 testing had been completed. The same curing lamp described above was used, but a different power meter was used (351 Power Meter, UDT Instruments, Baltimore, MD, USA) to accommodate the types of measurements planned. This meter had a 20 mm diameter detector head. The power output from the light-guide, at the time the RBC2 measurements were made, as measured with this meter, was 245.9 mW from the 7 mm light-guide. Power measurements were also made for various diameters of the light guide by using appropriate apertures in black tape affixed to the end of the light guide. Table 2 shows these values along with the corresponding irradiance values; this pattern of power distribution is similar to other observations . When curing RBC, using a polyester covering film, these values were corrected for reflectance from the film, as described above.

Table 2
Power and irradiance values of curing for different light-guide apertures.
Light-guide diameter (mm) Power (mW) Area (cm 2 ) Irradiance (mW/cm 2 )
1 8.5 0.00785 1089.2
2 32.6 0.0314 1038.2
4 114.5 0.1256 911.6
6 203.3 0.2826 719.4
7 245.9 0.3846 639.4

Configurations

To test the effect of color differences of molds, cylindrical split-molds made of stainless steel that when assembled provided a 6 mm diameter cylindrical mold 12 mm in height were used. These molds were used as is, to provide one color, or were lined with black or white vinyl tape for the other two colors. The tape was 0.215 mm thick so the diameters of the latter two molds were about 5.6 mm.

Differences in size were examined by comparing the depth of cure for RBC specimens made in cylindrical, stainless steel split-molds of 6, 5, 4 and 3 mm diameter, as described above. To represent a configuration of unrestricted RBC, a 22 mm diameter, cylindrical mold that was 11 mm in height was used to hold the RBC material. This mold was large enough that there was no influence of the mold itself on the distribution of light or the resulting cure within the RBC. This configuration will be referred to as unrestricted composite. The effect of a 2 mm diameter light source in comparison to the 7 mm light guide on the curing characteristics of unrestricted composite was accomplished by placing a 2 mm aperture, made from black tape, over the end of the 7 mm light-guide.

One further mold configuration was examined that was provided by using half of a 4 mm cylindrical, stainless steel mold to form a hemi-cylinder that was capped with a flat stainless steel plate.

Hardness measurements

Specimens of RBC1 that were measured for D SB were mounted on polishing fixtures and polished with water-cooling to expose a longitudinal plane along the central axis having a 4000 grit final polish. Specimens were dried with compressed air and left to dry further, in the dark, for 24 h before measurements were made. The specimens still attached to the polishing fixture were transferred to a specimen stage of a Knoop hardness tester (Buehler Micromet II microhardness tester; Buehler, Lake Bluff, IL, USA). The long axis of the specimen was aligned with the y -axis of the specimen stage and x and y zero points were set at the top center of specimen. Knoop hardness numbers (KHN) were measured using 100 g load and 12 s dwell time.

KHN values were obtained over one half of the specimen from the outer edge to the central axis. Measurements were made along the length in 0.5 mm steps, starting 0.5 mm from the top, and dropping to 0.25 mm steps until KHN values could not be made. At each of these steps, measurements in a radial direction were made, in increments, starting at either 0.05 mm or 0.1 mm from the outer surface of the cylinder and commencing with measurements at 0.2, 0.3, 0.5,1.0, 2.0, mm out to the central axis. The scrape-back surface defined the KHN = 0 contour . Top KHN values were determined by extrapolation of the depth profiles along the central axis to zero depth.

An analysis of the measured KHN data was undertaken to determine isometric contours of various values of %MaxKHN. A series of plots of the %MaxKHN vs. depth for each radial sampling distance was made on graph paper. By choosing a particular percentage on the y -axis, sets of depth and radial coordinate pairs were determined for that percentage, and these defined the isometric profile. Similarly plots of the %MaxKHN vs. radial position for specific depths allowed comparable data pairs to be used in refining the isometric profiles.

Transmission measurements

The curing light transmission relationship for RBC1, and the measurement method used, was described in the previous article (Part 1) . The same basic procedure was used in this work for RBC2. Stainless steel cylinders of various lengths with either 6 mm or 4 mm apertures were filled with RBC2 material and cured extensively to assure complete cure. After 24 h the cylindrical test units were polished on each end to remove excess material and were then measured for thickness. Individual units were placed on the detector head of the power meter and the light guide was centered over the top aperture and transmitted power was measured after 30 s of stabilization of the lamp. Three measurements were made and the average power was calculated. The transmission value for each thickness was determined as the ratio of the transmitted power to the incident power. A transmission relationship with depth T ( d ) was determined by fitting the data with linear regression analysis.

To form unrestricted RBC2 test specimens, the material was placed in various thicknesses in 22 mm diameter cylindrical molds, and light-cured extensively to assure full conversion. Transmission measurements were made as described above with apertures on the detector that matched the light diameter used.

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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Curing characteristics of a composite. Part 2: The effect of curing configuration on depth and distribution of cure
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