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Degree of polymerization between adjacent visible light-cured composite areas
Degree of polymerization between adjacent visible light-
M. A. Abdalla, S. H. Ashrafi, M. S. Bapna, I. Punwani College of Dentistry, Health Science Center University of Illinois at Chicago, USA
cured composite areas Abdalla MA, Ashrafi SH, Bapna MS, Punwani I. Degree of polymerization between adjacent visible light-cured composite areas. Dent Mater 1985: 1: 188190 Abstract - The degree of polymerization between 2 adjacent visible light-cured composite areas was characterized by hardness measurements and scanning electron microscopy. It was observed that minimum of 20 s exposure for 1 mm overlap and 40 s for 0.5 mm overlap, between 2 light-exposed areas, produced optimal hardness. The interface between the areas was uniformly cured. The 1 mm overlap would produce a homogeneous structure, optimal hardness and eliminate any possibility of partially cured area in the filling.
Visible light-cured composite resins are currently in widespread use for restorative, preventive, orthodontic and cosmetic applications. T h e popularity of these materials is largely due to their ease of manipulation, extended working time, greater depth of cure and the ability to be cured through the enamel layer (1-4). The depth of cure of these photo-sensitive composite materials has been extensively studied using hardness data as a function of depth to indicate this parameter (4-5). Occasionally, the degree of polymerization at different depth is also investigated (6). It has been shown that the depth of cure depends on the composition of resin, exposure time, intensity and wave length of light (1-6). The characterization of the area of composite between 2 light-irradiated positions has not been investigated. There are situations where more than one light application from different positions is required. Whenever the surface area of composite is greater than the exit window of light source, the material must be irradiated from more than one position to achieve uniform polymerization of the resin through the entire mass. Further, the in vitro test specimens are often of large size (4, 7) as compared to those routinely used in clinical practice. Ruyter and Oysaed (6) have polymerized large-size specimens by multiple exposure to activated light from different positions so that each time an area corresponding to the
area of exit window of light source was irradiated. Apparently no overlap in exposure was deemed essential. It is suggested that in the absence of a prescribed regimen for multi-position exposure, a set resin mass of non-uniform structure and heterogeneous properties may result. The purpose of this investigation was to characterize the area between 2 adjacent visible tight-cured composite areas and to establish an optimum overlap needed between 2 exposed areas to attain a uniform cure in the entire section. The characterization was carried out using scanning electron microscope (SEM) and microhardness measurements.
Dr. Shahid H. Ashrafi, Department of Histology, College of Dentistry, University of Illinois at Chicago, P.O. Box 6998, Chicago, Illinois 60680, USA. Received May 3, 1985; accepted for publication June 24, 1985.
[~
]
6 mm
X
Y
L
I0 i
j
J
I
I
R
B
B
Material and methods
Fig. l. Geometry of the specimen molds. For molds 1A S = 6 mm for 1B S = 5.5 mm and 1C S = 5.0 mm. Figure 1A shows the plan of mold whilo the Fig. 1B is longitudinal section. Hardness was measured in L, O and R regions.
The three visible light-cured composite materials investigated in this study were Denmat Shade-65 (Denmat Co., Placentia, CA.), Durafill Shade-8 (Degussa, Inc., Placentia, CA.) and SiluxUniversal (3-M Co., St. Paul, MN.). A durafill visible light source (Kulzer & Co., Hamburg, F R G ) with a light-exit window of 6 mm diameter was used to cure all 3 materials. The geometry of the 3 kinds of molds utilized for specimen preparation is shown in Fig. 1. All 3 molds were constructed from a 3 mm thick plexiglass Plate. Mold 1A was fabricated by drilling 2 contacting cylindrical holes of 6 mm in diameter. Similarly, molds 1B and 1C were made by drilling holes of
same diameter, but this time they had an overlap of 0.5 mm and 1.0 mm respectively. The mold with contacting cylindrical holes was considered of zero overlap (O-overlap). Four samples of each material in each mold were prepared against plastic matrix sheet. Samples were exposed by placing the light directly on the matrix. The light source was so positioned each time that the light-exit window covered one 6 mm circular area of the mold. Thus, 2 light exposures were required for each mold for the setting of the entire specimen and the material in the overlap area was covered twice by the light-exit window. Two sets of sam-
Polymerization of light-cured adjacent composite areas
189
Table 1. Mean of Knopp hardness number in three different areas of sample for three different overlaps. Samples were exposed to light for 20 seconds. Type of Overlap
Materials Durafill Right area, (R)
O overlap 0.5 mm overlap 1.0 mrn overlap
Junction area, (0)
Denmat Left area, (L)
54.1+2.9 43.9-+3.6 54.0_+3.2 53.8_+3.7 48.6_+3.4 53.9+3.5 54.2_+3.1 54.0_+4.2 54.0_+2.8
pies were prepared. Samples of the first set were irradiated to light for 20 s, whereas the samples of the second set were exposed for 40 s. Exposed sampies were kept in an incubator at 37 _+ 0.5 ~ and 95 + 5% relative humidity for at least 24 h. All the samples were then longitudinally sectioned along xy (Fig. 1B). Two samples of each type were coated by gold and examined by SEM while other specimens were used for hardness measurements. Microhardness was measured in 3 areas of cut surface, 2 areas (L in left cylinder, and R in right cylinder) were in the middle of the cut surface of the cylinders and the third area, i.e., the junction area was in the overlap portion of the light (0, Fig. 1B). Five hardness readings were obtained in each area with 10 gm load using Tukon hardness tester (Wilson Instrument Division, Bridgeport, CT.).
Right area
Junction area
Silux Left area
Right area
52.3+2.6 41.7_+3.4 52.1+3.0 51.9_+3.1 46.9_+3.3 52.0_+3.4 51.1_+3.3 51.1_+4.3 51.1_+4.l
Junction area
Left area
64.2-+3.8 52.0+4.3 64.0+_4.9 63.9_+3.5 58.1_+4.0 63.8_+2.9 64.0_+3.4 63.8_.+4.3 63.9-+4.7
Fig. 3. SEM micrograph of Denmat with Ooverlap and 40 seconds exposure time. Showing incomplete polymerization at the union. (orig. mag. 20x).
Fig. 5. SEM micrograph of Durafill with 0.5 mm overlap and 40 seconds exposure time. Showing complete polymerization. (orig. mag. 50x).
Fig. 4. SEM micrograph of Durafill with 0.5 mm overlap and 20 seconds exposure time. The bottom junction area (B) showing less polymerization. (orig. mag. 50x).
Fig. 6. SEM micrograph of Silux with 1.0
Results The hardness values at 3 positions for 3 overlap conditions are given in Table 1. When the overlap was less than 0.5 mm, there was a statistically significant difference in the hardness (p < 0.05) values between the junction area (0)
Fig. 2. SEM micrograph of Silux with Ooverlap and 20 seconds exposure time. T = top and B = bottom. Showing incomplete polymerization. (orig. mag. 20x). 15 Dental Materials 1:5, 1985
and the other 2 areas (L and R). The hardness value was almost 20% less in the junction area for O-overlap, and about 6% less for 0.5 mm overlap, but this difference in hardness disappeared when the overlap of 2 light positions was about 1 mm. All 3 materials behaved similarly. Figs. 2 and 3 are SEM micrographs of the sample with O-overlap. The junction area along T and B between the light positions (L and R) was structurally different to the other 2 areas.
mm overlap and 20 seconds exposure time. The convex curvature at the top (T) appears due to the rotation of speciment in SEM. Showing good polymerization in the entire junction area. (orig. mag. 20x). Further, low hardness values in this region compared to area L and R seems to indicate insufficient polymerization in the junction area. In other words, there is incomplete union between 2 light polymerized areas. Even a n increase in light exposure time to 40 s did not form homogeneously cured material in the entire section (Fig. 3). However, if an overlap of 0.5 mm in 2 light positions was made (Figs. 4, 5) then at 40 s exposure time, the SEM micrograph (Fig. 5) appeared to show uni-
190
Abdalla et al.
form polymerization in the entire section of the sample. Fig. 6 is a SEM micrograph of the specimen which was exposed for 20 s at each light position with 1 mm overlap. Uniform microstructure and same magnitude of hardness in each area (Table 1), clearly indicated a homogeneous curing in the entire matrix of the specimen. Table 2 presents the analysis of SEM micrographs for all 3 materials at various levels of overlap of light source and exposure time.
Discussion The hardness measurements and SEM micrographs of visible light-cured composite areas indicate that unless there was an overlap of about 1 mm between 2 adjacent exposed areas, the junction area would be insufficiently polymerized and would be weaker than the rest of the matrix. A n increase in light exposure time from 20 s-40 s with Ooverlap did not improve the curing of the material, and resulted in a weak union with softer matrix at the junction area. However, an increase in exposure time from 20 s--40 s for 0.5 mm overlap produced uniform curing in the entire mass. There could be 2 possible explanations for these observations in O-overlap composite areas. Light intensity at the exit window of the light source is less at the peripheral region than the central zone of the light beam. This may have been caused by the loss of light energy from the periphery of the beam to the surroundings. This could have limited the amount of radiation energy in this region to activate the photo-initiators for polymerization. The other possibility could be that, light at exit was uniform, but the inten-
Table 2. Analyses of SEM micrographs for three materials at various levels of overlap (IU = incomplete union, PU = partial union and CU = complete union). Materials
Type of overlap and Exposure time (sees.) O-overlap 20 Sec. 40 Sec.
Durafill Denmat Silux
IU IU IU
IU IU IU
sity of the scattered light from the filler particles (6) was not uniform in the entire section of the resin. A t the periphery, the magnitude of the scattered light was less because the outer surroundings of the peripheral zones could not contribute enough scattered light to it due to the lack of incident light in this region. This could have resulted in only some localized areas attaining the critical intensity required to initiate polymerization reaction.
0.5 mm overlap
1.0 mm overlap
20 Sec.
40 Sec.
20 Sec.
40 Sec.
PU CU PU
CU CU CU
CU CU CU
CU CU CU
Conclusion This study showed that about 1 mm or greater overlap between 2 adjacent light positions was required to achieve uniform degree of polymerization throughout the resin. With 1 mm overlap dentists could be assured of having a homogeneous structure and optimal hardness; and this would eliminate the possibility of the presence of partially cured areas in the resin.
References 1. POLLACKBF, BLITZER MH. The advantages of visible light curing resin. N Y S Dent J 1982: 4: 228-30. 2. SWARTZML, PHILLIPS RW, RHODES BF. Visible light activated resins: depth of cure. J Dent Res 1982: 61: 270. 3. NEWMANWM, MURRAYG A , YATESJL. Visible lights and visible light-activated composite resins. J Prosthet Dent 1983: 50: 31-35. 4. TIRTHAR, FAN PL, DENNISON JB, POWERSJM. In vitro depth of cure of photoactivated composites. J Dent Res 1982: 61: 1184-87. 5. SWARTZML, PHILLIPS RW, RHODES B. Visible light-activated resins: depth of cures. J A D A 1983: 108: 534-36. 6. RtJYrER IE, OVSAIEH. Conversion in different depths of ultra violet and visible light activated composite materials. Scand J Dent Res 1981: 6: 179-92. 7. BASSIOUNYMA, GIgANT A A . Physical properties of visible light-cured composite resin. J Prosthet Dent 1980: 43: 536--41.
M. A. Abdalla, S. H. Ashrafi, M. S. Bapna, I. Punwani College of Dentistry, Health Science Center University of Illinois at Chicago, USA
cured composite areas Abdalla MA, Ashrafi SH, Bapna MS, Punwani I. Degree of polymerization between adjacent visible light-cured composite areas. Dent Mater 1985: 1: 188190 Abstract - The degree of polymerization between 2 adjacent visible light-cured composite areas was characterized by hardness measurements and scanning electron microscopy. It was observed that minimum of 20 s exposure for 1 mm overlap and 40 s for 0.5 mm overlap, between 2 light-exposed areas, produced optimal hardness. The interface between the areas was uniformly cured. The 1 mm overlap would produce a homogeneous structure, optimal hardness and eliminate any possibility of partially cured area in the filling.
Visible light-cured composite resins are currently in widespread use for restorative, preventive, orthodontic and cosmetic applications. T h e popularity of these materials is largely due to their ease of manipulation, extended working time, greater depth of cure and the ability to be cured through the enamel layer (1-4). The depth of cure of these photo-sensitive composite materials has been extensively studied using hardness data as a function of depth to indicate this parameter (4-5). Occasionally, the degree of polymerization at different depth is also investigated (6). It has been shown that the depth of cure depends on the composition of resin, exposure time, intensity and wave length of light (1-6). The characterization of the area of composite between 2 light-irradiated positions has not been investigated. There are situations where more than one light application from different positions is required. Whenever the surface area of composite is greater than the exit window of light source, the material must be irradiated from more than one position to achieve uniform polymerization of the resin through the entire mass. Further, the in vitro test specimens are often of large size (4, 7) as compared to those routinely used in clinical practice. Ruyter and Oysaed (6) have polymerized large-size specimens by multiple exposure to activated light from different positions so that each time an area corresponding to the
area of exit window of light source was irradiated. Apparently no overlap in exposure was deemed essential. It is suggested that in the absence of a prescribed regimen for multi-position exposure, a set resin mass of non-uniform structure and heterogeneous properties may result. The purpose of this investigation was to characterize the area between 2 adjacent visible tight-cured composite areas and to establish an optimum overlap needed between 2 exposed areas to attain a uniform cure in the entire section. The characterization was carried out using scanning electron microscope (SEM) and microhardness measurements.
Dr. Shahid H. Ashrafi, Department of Histology, College of Dentistry, University of Illinois at Chicago, P.O. Box 6998, Chicago, Illinois 60680, USA. Received May 3, 1985; accepted for publication June 24, 1985.
[~
]
6 mm
X
Y
L
I0 i
j
J
I
I
R
B
B
Material and methods
Fig. l. Geometry of the specimen molds. For molds 1A S = 6 mm for 1B S = 5.5 mm and 1C S = 5.0 mm. Figure 1A shows the plan of mold whilo the Fig. 1B is longitudinal section. Hardness was measured in L, O and R regions.
The three visible light-cured composite materials investigated in this study were Denmat Shade-65 (Denmat Co., Placentia, CA.), Durafill Shade-8 (Degussa, Inc., Placentia, CA.) and SiluxUniversal (3-M Co., St. Paul, MN.). A durafill visible light source (Kulzer & Co., Hamburg, F R G ) with a light-exit window of 6 mm diameter was used to cure all 3 materials. The geometry of the 3 kinds of molds utilized for specimen preparation is shown in Fig. 1. All 3 molds were constructed from a 3 mm thick plexiglass Plate. Mold 1A was fabricated by drilling 2 contacting cylindrical holes of 6 mm in diameter. Similarly, molds 1B and 1C were made by drilling holes of
same diameter, but this time they had an overlap of 0.5 mm and 1.0 mm respectively. The mold with contacting cylindrical holes was considered of zero overlap (O-overlap). Four samples of each material in each mold were prepared against plastic matrix sheet. Samples were exposed by placing the light directly on the matrix. The light source was so positioned each time that the light-exit window covered one 6 mm circular area of the mold. Thus, 2 light exposures were required for each mold for the setting of the entire specimen and the material in the overlap area was covered twice by the light-exit window. Two sets of sam-
Polymerization of light-cured adjacent composite areas
189
Table 1. Mean of Knopp hardness number in three different areas of sample for three different overlaps. Samples were exposed to light for 20 seconds. Type of Overlap
Materials Durafill Right area, (R)
O overlap 0.5 mm overlap 1.0 mrn overlap
Junction area, (0)
Denmat Left area, (L)
54.1+2.9 43.9-+3.6 54.0_+3.2 53.8_+3.7 48.6_+3.4 53.9+3.5 54.2_+3.1 54.0_+4.2 54.0_+2.8
pies were prepared. Samples of the first set were irradiated to light for 20 s, whereas the samples of the second set were exposed for 40 s. Exposed sampies were kept in an incubator at 37 _+ 0.5 ~ and 95 + 5% relative humidity for at least 24 h. All the samples were then longitudinally sectioned along xy (Fig. 1B). Two samples of each type were coated by gold and examined by SEM while other specimens were used for hardness measurements. Microhardness was measured in 3 areas of cut surface, 2 areas (L in left cylinder, and R in right cylinder) were in the middle of the cut surface of the cylinders and the third area, i.e., the junction area was in the overlap portion of the light (0, Fig. 1B). Five hardness readings were obtained in each area with 10 gm load using Tukon hardness tester (Wilson Instrument Division, Bridgeport, CT.).
Right area
Junction area
Silux Left area
Right area
52.3+2.6 41.7_+3.4 52.1+3.0 51.9_+3.1 46.9_+3.3 52.0_+3.4 51.1_+3.3 51.1_+4.3 51.1_+4.l
Junction area
Left area
64.2-+3.8 52.0+4.3 64.0+_4.9 63.9_+3.5 58.1_+4.0 63.8_+2.9 64.0_+3.4 63.8_.+4.3 63.9-+4.7
Fig. 3. SEM micrograph of Denmat with Ooverlap and 40 seconds exposure time. Showing incomplete polymerization at the union. (orig. mag. 20x).
Fig. 5. SEM micrograph of Durafill with 0.5 mm overlap and 40 seconds exposure time. Showing complete polymerization. (orig. mag. 50x).
Fig. 4. SEM micrograph of Durafill with 0.5 mm overlap and 20 seconds exposure time. The bottom junction area (B) showing less polymerization. (orig. mag. 50x).
Fig. 6. SEM micrograph of Silux with 1.0
Results The hardness values at 3 positions for 3 overlap conditions are given in Table 1. When the overlap was less than 0.5 mm, there was a statistically significant difference in the hardness (p < 0.05) values between the junction area (0)
Fig. 2. SEM micrograph of Silux with Ooverlap and 20 seconds exposure time. T = top and B = bottom. Showing incomplete polymerization. (orig. mag. 20x). 15 Dental Materials 1:5, 1985
and the other 2 areas (L and R). The hardness value was almost 20% less in the junction area for O-overlap, and about 6% less for 0.5 mm overlap, but this difference in hardness disappeared when the overlap of 2 light positions was about 1 mm. All 3 materials behaved similarly. Figs. 2 and 3 are SEM micrographs of the sample with O-overlap. The junction area along T and B between the light positions (L and R) was structurally different to the other 2 areas.
mm overlap and 20 seconds exposure time. The convex curvature at the top (T) appears due to the rotation of speciment in SEM. Showing good polymerization in the entire junction area. (orig. mag. 20x). Further, low hardness values in this region compared to area L and R seems to indicate insufficient polymerization in the junction area. In other words, there is incomplete union between 2 light polymerized areas. Even a n increase in light exposure time to 40 s did not form homogeneously cured material in the entire section (Fig. 3). However, if an overlap of 0.5 mm in 2 light positions was made (Figs. 4, 5) then at 40 s exposure time, the SEM micrograph (Fig. 5) appeared to show uni-
190
Abdalla et al.
form polymerization in the entire section of the sample. Fig. 6 is a SEM micrograph of the specimen which was exposed for 20 s at each light position with 1 mm overlap. Uniform microstructure and same magnitude of hardness in each area (Table 1), clearly indicated a homogeneous curing in the entire matrix of the specimen. Table 2 presents the analysis of SEM micrographs for all 3 materials at various levels of overlap of light source and exposure time.
Discussion The hardness measurements and SEM micrographs of visible light-cured composite areas indicate that unless there was an overlap of about 1 mm between 2 adjacent exposed areas, the junction area would be insufficiently polymerized and would be weaker than the rest of the matrix. A n increase in light exposure time from 20 s-40 s with Ooverlap did not improve the curing of the material, and resulted in a weak union with softer matrix at the junction area. However, an increase in exposure time from 20 s--40 s for 0.5 mm overlap produced uniform curing in the entire mass. There could be 2 possible explanations for these observations in O-overlap composite areas. Light intensity at the exit window of the light source is less at the peripheral region than the central zone of the light beam. This may have been caused by the loss of light energy from the periphery of the beam to the surroundings. This could have limited the amount of radiation energy in this region to activate the photo-initiators for polymerization. The other possibility could be that, light at exit was uniform, but the inten-
Table 2. Analyses of SEM micrographs for three materials at various levels of overlap (IU = incomplete union, PU = partial union and CU = complete union). Materials
Type of overlap and Exposure time (sees.) O-overlap 20 Sec. 40 Sec.
Durafill Denmat Silux
IU IU IU
IU IU IU
sity of the scattered light from the filler particles (6) was not uniform in the entire section of the resin. A t the periphery, the magnitude of the scattered light was less because the outer surroundings of the peripheral zones could not contribute enough scattered light to it due to the lack of incident light in this region. This could have resulted in only some localized areas attaining the critical intensity required to initiate polymerization reaction.
0.5 mm overlap
1.0 mm overlap
20 Sec.
40 Sec.
20 Sec.
40 Sec.
PU CU PU
CU CU CU
CU CU CU
CU CU CU
Conclusion This study showed that about 1 mm or greater overlap between 2 adjacent light positions was required to achieve uniform degree of polymerization throughout the resin. With 1 mm overlap dentists could be assured of having a homogeneous structure and optimal hardness; and this would eliminate the possibility of the presence of partially cured areas in the resin.
References 1. POLLACKBF, BLITZER MH. The advantages of visible light curing resin. N Y S Dent J 1982: 4: 228-30. 2. SWARTZML, PHILLIPS RW, RHODES BF. Visible light activated resins: depth of cure. J Dent Res 1982: 61: 270. 3. NEWMANWM, MURRAYG A , YATESJL. Visible lights and visible light-activated composite resins. J Prosthet Dent 1983: 50: 31-35. 4. TIRTHAR, FAN PL, DENNISON JB, POWERSJM. In vitro depth of cure of photoactivated composites. J Dent Res 1982: 61: 1184-87. 5. SWARTZML, PHILLIPS RW, RHODES B. Visible light-activated resins: depth of cures. J A D A 1983: 108: 534-36. 6. RtJYrER IE, OVSAIEH. Conversion in different depths of ultra violet and visible light activated composite materials. Scand J Dent Res 1981: 6: 179-92. 7. BASSIOUNYMA, GIgANT A A . Physical properties of visible light-cured composite resin. J Prosthet Dent 1980: 43: 536--41.