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Selected curing characteristics of light-activated composite resins
Selected curing characteristics of I.ight-actvated composite resins
H. Onose, H. Sano, H. Kanto, S. Ando, T. Hasuike Nihon University School of Dentistry, Department of Operative Dentistry, Tokyo, Japan.
Onose H, Sano H, Kanto H, Ando S, Hasuike T. Selected curing characteristics of light-activated composite resins. Dent Mater 1985: 1: 48-54. Abstract. - The purpose of this study was to clarify some curing characteristics of light-activated composite resins. Three visible light-activated and 1 UV lightactivated resin were cured with each generator recommended by the manufacturer. The relative degree of cure was determined using Knoop hardness measurements on the irradiated and longitudinally sectioned surfaces of cylindrical specimens. The maximum surface hardness numbers were obtained from the area just under the top of the specimen (0.5 to 1.0 mm below the irradiated surface) and the hardness gradually decreased with the depth. In the specimens cured in 100% N 2 gas environment, the degree of cure on the irradiated surface was improved. Coloring agents in composites have an effect on decreasing the degree of cure. Re-irradiation on the longitudinally sectioned surface of the specimen increased the degree of deep cure. The degree of cure at each depth was improved by prolonged irradiation time.
Composite resins are used in dentistry as esthetic restorative materials. For the chemically-activated systems, it is usually necessary to mix 2 pastes and during the mixing procedures air can be entrapped in the material. This porosity of the material influences the mechanical properties and esthetics for the restoration (1-5). Recently, the light-activated composite systems have been developed. It is not necessary to mix pastes because a photosensitive initiator is used in the single paste, which is activated by the irradiation of light. This system has advantages compared to the paste-paste system, such as adequate working time and less porosity (6-8), and is now used widely in dentistry. The purpose of this study was to determine some curing characteristics of light-activated composite resins. Material and methods In this comprehensive study, 4 light-activated composite systems; (a) Superlux Daylight with Daylight lamp, (b) Plurafil Super with Pluraflex HL150, (c) Fulfil with Prisma-Lite, (d) Estilux with Duralux UV-300, plus (e) a chemically-cured control, Silar, (Table 1) were investigated in 5 phases.
Preparation of specimens A cylindrical vinyl mold (inside diameter 4 mm, height 6 mm) was placed on a glass plate and each resin material was slightly overfilled in the mold. After removal of excess material, the glass plate and matrix strip were eliminated. The tip of the activator light was placed at the top of the resin material and the material was irradiated by the light for the time recommended by each of the manufacturers. The specimens were immediately sectioned longitudinally by means of a thin sectioning machine under water cooling. For the chemically-cured material, Silar, both universal and catalyst pastes were mixed in accordance with the manufacturer's instructions. The mixed paste was placed in the mold as previously described. The Silar specimen was kept for 24 h from the start of mixing at room temperature, in order to achieve a properly cured specimen. The specimen removed from the mold was sectioned longitudinally by means of a thin sectioning machine under water cooling. Then the longitudinally sectioned surfaces of the specimen were polished with emery polishing papers (#800, 1000, 2000 and 3000) under water to
Key words: compositeresins, hardness, exposure, irradiation, color.
Professor H. Onose, Nihon University School of Dentistry, Tokyo, Japan. Accepted for publication 11 December 1984.
achieve the same smooth surface. Under infrared light, the 5 specimens for each phase were prepared in air at room temperature (23 + I~ relative humidity 50 + 5%, except for the study on the effect of environment on the degree of cure. Firstly, Knoop hardness measurements were made on the sectioned surface of each specimen (within 1 h after the irradiation). The indenter point was kept on the surface for 30 sec with a 25 g load. Fig. 1 shows the locations indented by the (Knoop hardness) diamond point on the longitudinally sectioned surface, from 0.1 mm below the top surface of the specimen to 5.5 mm below the top. At each distance from the top surface of the specimen, 3 measurements were carried out. The second part of the study was conducted in order to compare the curing behavior of a visible light-activated composite under a different atmosphere. Visible light cured PS was selected and cured in air with and without matrix strips on the top of the specimen. Also PS was cured in an anaerobic chamber filled with 100% Nz gas. For the 3 groups, Knoop hardness values were obtained from the longitudinally sectioned surface of the specimens (which will be called the sec-
Curing character&tics of light-activated composite resins
49
Table 1. Materials. Composite Resin
Code
Shade
Manufacturer
Batch No.
Activator Light
Wavelength* Irradiation (peak) Time* (sec.)
Typeof polymerization
Plurafil Super
PS
U Y B G
LITEMA
30470 30365 30600 30473
Pluraflex HL150 380-750 nm (505)
30
Visible Light Cure
Superlux Daylight
SD
U
SHOFU
0483
Daylight Lamp
380-750nm (513)
30
Visible Light Cure
Ful-fil
FF
U
CAULK
081782
Prisma Lite
280-540nm (500)
10
Visible Light Cure
Estilux Microfil
EM
U
KULZER
ch-B
Duralux UV-300 350-450nm (435)
20
Ultra-Violet Light Cure
Silar
S
U
3M
-
30**
Chemical Cure
Paste A 2B2 Paste B 2B3
-
* According to manufacturers' instruction. ** Mixing time (sec.). tioned specimen) and the top surface of the cylindric~il specimen (which will be called the irradiated surface specimen), respectively. For the irradiated surface specimen, 10 surface hardness numbers were obtained at the central portion of the irradiated surface within 1 h after the irradiation. The third phase of this study was to clarify the phenomenon of variation in the curing with different shades of resin. Four different shades of PS resin (universal, gray, yellow and brown) were used for this part of the study. Five specimens for each shade were d i r e c t i o n of irradiation mm surface 0.7 0.5 1.0 1.5
m
2.0 2.5 3.0" 3.5
m
4.0 4.5 5.0 5.5
4
/
m
bottom
Fig. 1. Location of measuring points (KHN) on longitudinaUy sectioned surface. O: point of indentation.
cured for 30 sec. Immediately after the irradiation, longitudinally sectioned specimens were prepared and the Knoop hardness numbers were measured according to the method described previously. The fourth phase of the study was to clarify the effect of re-irradiation on the curing of the longitudinally sectioned surface of PS (universal shade). After the 1 irradiation, for the manufacturer's recommended time, the tip of the light unit was placed directly on the center of the longitudinally sectioned surface and a reirradiation for 30 sec was accomplished and the surface hardness was measured. The fifth part of this study was to determine the effect of prolonged irradiation time. PS universal shade materials were cured for 5 different irradiation times (30, 60, 90, 120 and 240 sec). After each irradiation, the degree of cure was measured by using Knoop hardness numbers. Furthermore, increasing the irradiation time meant that the activator light generated more heat in the resin restorations. The temperature rise was due to the exothermic polymerization reaction and heat energy from the activator light itself during the irradiation. Therefore, the temperature change under the resin restoration was measured by a thermocouple during the 240 sec of irradiation. A vinyl mold (inside diameter 6 mm and height 2 mm) was filled with PS material and, in order to prevent heat escape during the exposure of visible light, the mold was embedded in a formed polystyrene plastic as heat insul-
ation. A copper/constantan thermocoupie was placed on the bottom of specimen. To monitor the resin temperature change during the irradiation, the thermocouple was connected to a digital display and X-Y recorder and the temperature versus time was plotted on a chart. Results The surface hardness for the resin materials tested
In this study, Knoop hardness measurements were used as the indicators of the relative degree of cure in the composite resins. The Knoop hardness numbers on the longitudinally sectioned surface of various composite resins at 0.1 mm to 5.5 mm under the top surface of the specimen are shown in Table 2 and Fig. 2. Generally speaking, for all of the light-activated composite resin products cured in air without matrix strips on the top of specimens, a relatively lower hardness was obtained from the area just under (0.1 mm) the top surface of the specimen irradiated by the light, compared to the value obtained 0.5 m m - l . 0 mm under the top surface which was the maximum hardness. Below that maximum value, the hardness gradually decreased with increasing depth. For the UV-cured microfilled composite resin (EM) this tendency was very obvious. However, for the chemically-cured resin (Silar) different behavior was observed. The minimum hardness number was detected at the surface layer
50
Onose et al. Table 3. Surface hardness (KHN) on longitudinally sectioned surface of PS in different environments
Table 2. Surface hardness (KHN) on longitudinally sectioned surface of various composite resins Materials
SD
PS
FF
EM
S
Daylight Lamp
Pluraflex HL 150
Prisma Lite
Duralux UV-300
30
30
10
20
Light Light Irradiation time (sec.) Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Material Environment
6.8 7.2 6.9 5.8 4.9
(1.2) (1.1) (0.8) (0.9) (0.7) . . .
13.0 (3.0) 13.4 (1.9) 12.9 (2.4) 11.5 (1.4) 10.0 (1.8) 8.7 (1.5) 7.6 (1.4) 6.3 (1.4) 4.8 (0.8) . . . . . .
12.8 (0.3) 15.4 (2.6) 16.9 (3.1) 16.0 (4.3) 13.9 (0.5) 12.8 (0.9) 9.2 (1.7) 5.5 (1.4) . . .
43.9 (2.5) 45.7 (9.2) 28.2 (3.2) 14.7 (2.1) 8.8 (0.6) -
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
27.8 (2.7) 28.6 (1.7) 32.2 (1.9) 30.8 (1.9) 31.0 (1.5) 29.4 (1.5) 32.6 (2.6) 32.3 (2.7) 30.4 (3.4) 31.0 (4.0) 31.1 (2.9) 30.7 (1.9) -
-
Number = 5. (): Standard Deviation. : immeasurable. KHN
(0.1 to 0.5 mm below the top surface) of the specimen and the surface hardness gradually increased with the depth, up to approximately 1 m m depth and then r e m a i n e d essentially constant, W h e n comparing the hardness numbers among the 4 light-activated resin systems at the same depth from the top
~3
ir ~
~ o
~ air
o----i
o
,
,
o
Nigas
100+/,
s 7 5
0
40
ii 0
45-
,,'----
SD PS FF ENI S
i
i I
i
i 2
i
Pluraflex HL150
i 3
I
r
i
i
t,
Fig. 3. Surface hardness (KHN) on longitudinally sectioned surface of PS in different environments.
PS Air
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
Nz gas 100%
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4) (1.4) (0.8)
14.1 12.4 11.0 10.5 9.4 8.1 7.4 6.7 4.8
-
(1.0) (0.5) (1.1) (1.1) (0.6) (0.6) (0.5) (0.3) (0.2) -
Number = 5. ( ): Standard Deviation. : immeasurable. Irradiation time: 30 sec.
100% N2 gas and air. The m a x i m u m hardness value was obtained at the area 0.1 m m below the top of the specimen for the longitudinally sectioned surface of the specimen cured in a 100% N2 gas environment. H o w e v e r , for the specimens cured in air without matrix strips on the irradiated surface, the m a x i m u m value was not detected at the nearest point to the light tip (0.1 m m below the top). For the 3 groups of the irradiated surface specimens, the hardest top surface was the specimen cured in 100% N 2 gas, followed by the specimen cured in air with a matrix strip, while the specimen cured in air without a matrix strip showed the lowest (Table 4 and Fig. 4).
35
surface, E M was significantly harder than the others from 0.1 mm to 1.0 m m below the top surface, but the hardness dramatically decreased with depth until 2.5 m m below the top when the hardness was no longer measurable. For the visible light-activated composite systems; F F showed the highest K n o o p hardness n u m b e r and was followed by PS and then SD. For these 3 visble light-curing systems, the K n o o p hardness measurements were obtained from the d e e p e r area of specimens c o m p a r e d to the UV-cured system, E M .
30
25
20
15
10.
Effect of resin shade on the cure of lightactivated composites
The degree of cure for 4 different shades of PS resin are shown in Table 5 Table 4. Surface hardness (KHN) on irradiation surface of PS in different environments Light Material Environments (1) (2)
(3) Effect of environment on the cure of lightactivated resins (mm)
Fig. 2. Surface hardness (KHN) on longitudinally sectioned surface of various composite resins.
Table 3 and Fig. 3 show the K n o o p hardness numbers of the PS specimens cured in different atmospheres such as
PluraflexHL150 PS
10.2(0.3) 9.4 (0.1)
7.5 (0.3)
(1) Surface without matrix in Nz gas 100%. (2) Surface with matrix in air. (3) Surface without matrix in air. Number = 5. ( ): Standard Deviation. Irradiation time: 30 sec.
Curing characteristics of light-activated composite resins Table 5. Surface hardness (KHN) on longitudinally sectioned surface of PS with different colors
K~
~--
15
Light
Pluraflex HL150
Material Universal
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
~a
Gray
Yellow
Brown
Reirradiatior
Reirradiation
%
9.5 (0.1) 10.6 (0.5) 9.3 (0.4) 5.7 (0.1)
9.4 (0.5) 10.1 (0.3) 8.8 (0.3) 6.3 (0.2) -
12.4 12.6 9.6 7.5 6.0
(0.6) (0.8) (0.1) (1.1) (0.1) -
-
-
Light
Pluraflex HL150
11
T
Material Re-irradiation
, 9
(2)
(3)
Fig. 4. Surface hardness (KHN) on irradiation surface of PS in different environments. (1) Surface without matrix in N2 gas 100%. (2) Surface with matrix in air. (3) Surface without matrix in air.
9
13-
Unlversol
I
Groy
o
Yellow
o
Brown
11
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
PS without
Xo 5
O"F
i
in
i
0
I
I
2
I
~
3
I
I
4
I
I
5 mm
5 ,, 4
(mm)
Fig. 5. Surface hardness (KHN) on longitudinally sectioned surface of PS With different colors.
h a r d n e s s v a l u e was always higher t h a n the control g r o u p at every d e p t h tested, a n d especially large differences of h a r d n e s s b e t w e e n the 2 groups were o b t a i n e d f r o m 2.5 m m to 5.0 m m below the t o p of t h e specimen.
The influence of irradiation time on the cure of light-activated composite resin
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4)
13.3 13.6 13.7 13.5 13.0 13.6 13.7
(1.4)
12.5 (0.5)
(0.8)
11.9 (1.7) 10.4 (1.2) 8.9 (1.6)
-
(1.2)
(1.1) (0.8) (0.9) (0.7) (0.7) (0.5)
Reirradiation time: 30 sec. Number = 5. ( ): Standard Deviation. - : immeasurable.
a n d Fig. 5. F o r s p e c i m e n s of all shades the surface h a r d n e s s at the longitudinally s e c t i o n e d surface d e c r e a s e d with increasing d e p t h . H o w e v e r , at each d e p t h of s p e c i m e n , t h e m a x i m u m hardness v a l u e was always o b t a i n e d from the universal s h a d e s p e c i m e n followed by gray, yellow a n d brown.
Effect of re-irradiation on the cure of longitudinally sectioned surface
7
3
x
with
9
2
\
hardness of longitudinally sectioned PS specimen.
KHN
(1
x 7
Fig. 6. Effect of re-irradiation on Knoop
Table 6. Effect of re-irradiation on Knoop hardness of longitudinally sectioned PS specimen
1
it
9
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4) (1.4) (0.8)
Number = 5. ( ): Standard Deviation. - : immeasurable.
0
without with
PS
Shade
0 T
-o
o-------0
51
T h e longitudinally sectioned surface of PS s p e c i m e n s was r e - i r r a d i a t e d by the same light unit for 30 sec a n d the hardness of t h e r e - i r r a d i a t e d s p e c i m e n was c o m p a r e d to t h e s p e c i m e n w i t h o u t reirradiation. Table 6 a n d Fig. 6 d e m o n strate t h e results of re-irradiation. For the r e i r r a d i a t e d group, the K n o o p
Table 7 a n d Fig. 7 show the respective d e g r e e of cure for PS s p e c i m e n s cured for 5 different i r r a d i a t i o n times. T h e surface h a r d n e s s for PS was i m p r o v e d by p r o l o n g i n g the i r r a d i a t i o n time, a n d also the d e p t h of cure was i m p r o v e d . H o w e v e r , m o r e t h a n 120 sec irradiation t i m e did n o t i m p r o v e the surface h a r d n e s s for the PS s p e c i m e n a n d the m a x i m u m surface h a r d n e s s at 0.5 m m to 1.0 m m b e l o w the t o p of the specim e n was n o t i m p r o v e d by i r r a d i a t i o n time o v e r 60 sec.
Temperature change during the irradiation by visible light
T h e t e m p e r a t u r e at the b o t t o m of the PS resin s p e c i m e n during t h e irradiation to visible light gradually rose with increasing the i r r a d i a t i o n time. However, e v e n w h e n t h e s p e c i m e n was irradiated for 240 sec, the t e m p e r a t u r e did n o t r e a c h m o r e t h a n 45~ D u r i n g the 120 sec i r r a d i a t i o n time t h e t e m p e r a t u r e rose to a p p r o x i m a t e l y 43~ (Table 8 a n d Fig. 8). Discussion
T h e d e g r e e of cure for c o m p o s i t e resins influences t h e physical p r o p e r t i e s of set materials a n d t h e longevity of restorations in the e x t r e m e s of t h e oral envi-
O n o s e et al.
52
Table 7. Surface hardness (KHN) on longitudinally sectioned surface of PS cured with various irradiation times Irradiation time (sec.) Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
30
60
90
120
240
13.0 (3.0) 13.4 (1.9) 12.9 (2.4) 11.5 (1.4) 10.0 (1.8) 8.7 (1.5) 7.6 (1.4) 6.3 (1.4) 4.8 (0.8) -
17.6 (2.1) 18.1 (1.3) 16.9 (1.8) 15.3 (1.9) 15.6 (2.3) 15.4 (3. I) 13.2 (1.9) 11.6 (2.2) 8.9 (0.9) 8.1 (0.8) 6.2 (0.6) -
16.4 (3.8) 18.5 (3.0) 17.6 (0.5) 16.1 (1.5) 15.2 (0.4) 14.6 (0.8) 13.6 (0.8) 13.1 (0.5) 12.4 (0.6) 11.2 (0.3) 10.7 (0.6) 9.7 (0.2)
17.1 (0.3) 18.1 (0.2) 17.3 (0.6) 17.2 (0.4) 17..1 (0.6) 17.0 (0.8) 17.0 (0.6) 16.5 (0.3) 15.9 (0.3) 14.7 (0.4) 13.7 (0.6) 12.6 (0.7)
17.3 (0.7) 18.8 (0.3) 17.6 (0.2) 17.5 (0.5) 16.9 (0.2) 16.6 (0.1) 16.7 (0.5) 16.9 (0.4) 16.2 (0.1) 14.8 (0.1) 13.8 (0.2) 12.4 (0.1)
UV-activated material, it is advisable to place and cure material incrementally (9) because of the lower penetrability of U V light (10, 11). However, not all the characteristics of light-activated resins have been clarified yet. Table 2 and Fig. 2 show the results of the surface harness for the specimen cured in air without a matrix strip on the top surface. The hardest value was obtained at 0.5 m m to 1.0 m m below the irradiated surface, even though the top surface was the nearest point to the light source. After the maximum, the hardness gradually decreased with increasing specimen. K n o o p hardness was not measurable b e y o n d more than 2.5 m m in depth for some materials (SD and EM). This tendency was especially obvious for the UV-cured system. Polymerization is d e p e n d e n t u p o n the formation of free radicals. Since there was no matrix strip in place, the presence of oxygen could have caused retardation of polymerization if the oxygen reacted with the free radicals. Kanto et al. (12) m e n t i o n e d in their studies that the mixing e n v i r o n m e n t clearly influenced the curing of chemically-cured resin. Finger & Jorgensen (13) also said that the polymerization of composites exposed in air was inhibited by oxygen. Ruyter (14) measured the inhibited layer on the U V light- and chemicallycured sealant materials exposed in air. Therefore, the visible light-cured material (PS) was cured in a 100% N2 gas environment without influence of oxygen on the curing. The m a x i m u m hardness value was obtained at the closest area to the top surface of the longitudinally sectioned specimen, and the re-
Number = 5. ( ): Standard Deviation. - : immeasurable. Table 8. Temperature changes of PS through irradiation at room temperature (23 + I~ Light Material
Irradiation time (sec.) 0
Pluraflex HL150
PS
10
20
30
60
90
120
150
180
210
240
23.0 32.1 36.3 37.9 39.5 40.4 41.3 41.8 42.1 42.5 43.0 (0.00) (0.62) (0.14) (0.33) (0.41) (0.45) (0.47) (0.47) (0.38) (0.36) (0.33)
Number = 3. ( ): Standard Deviation. Relative humidity: 50 + 10%. ronment. Therefore, clinicians who want to use a material should know the characteristics of that material. According to this study the chemically-cured resin showed similar K n o o p hardness values throughout the speci-
:
"
30 sec. 60 ,. ~ 90 ,'
J
i
120
%
9
,,
men, except for the inhibited surface layer. However, light-activated composite resins developed recently have different polymerization characteristics from chemically-cured resins. O n the polymerization of light-activated material, the Knoop hardness n u m b e r of the resin decreased with the depth, especially for the resin cured by U V light which has a relatively shorter wavelength compared to visible light. Therefore, when restoring a deep cavity with
17
I 50 I
15
11' 30
9" i-
23
7' 5'
0: 0
I
2
3
4
5 mm
Fig. 7. Surface hardness (KHN) on longitu-
dinally sectioned surface of PS cured with various irradiation times.
i
i
i
i
i
10
20
30
80
90
i
120
i
IS0
i
180
I
210
i
240
Irrediation Time ( ~ e e . )
Fig. 8. Temperature changes of PS through irradiation at room temperature (23 _+ I~
Curing characteristics of light-activated composite resins sult obtained from the specimens cured in air with matrix strips was comparable to the specimen cured in N 2 gas. So the cure of visible light-activated composites at the surface exposed to air was also influenced by oxygen. The photo initiator system in visible light-activated composite resins requires a sufficient intensity of light in a certain range of wavelengths (15). If enough intensity of light cannot be supplied from the generator, the polymerization will not begin properly (16). The degree of cure for tight-activated resins decreased with increasing depth of the materials. The reason for this phenomenon is probably to be explained in that the energy absorption of the surrounding media decreased the amount of penetrating light (11), and consequently the light intensity decreased with increased depth of resin, until at the deeper part of the specimen the polymerization was not complete. In order to determine the influence of light absorbancy of the surrounding media on the curing of visible light-activated composite resins, 4 different shades of composites were cured. The universal shade resin always showed a higher Knoop hardness number than the others. This result was in agreement with the previous study on the effect of shade on the depth of cure for light-cured resin by Swartz et al. (9). The degree of cure for light-activated composite resins was influenced by the penetrability of light. Therefore, there is a possibility that unpolymerized substances such as residual monomer and a catalyst still remained in the depths of the resin specimens. In order to clarify this possibility, the longitudinally sectioned surface of the specimen was reirradiated and the Knoop hardness was compared. For the re-irradiation group, the Knoop hardness number was dramatically increased by the re-irradiation. This result clearly indicated that unpolymerized material still remained on the sectioned surface. However, at the area near the bottom surface of the bottom surface of the specimen (more than 4 mm depth from the top surface), in spite of re-irradiation to the light for 30 sec, the surface hardness was not very much improved compared with the upper half of the specimen. This was presumably because the procedures of sectioning and polishing under water cooling washed away some unpolymerized material. From a clini5 DentalMaterials1:2, I985
cal point of view, residual unpolymerized material on the cavity floor could be a cause of chemical pulpal irritation (17). In order to improve the physical properties of resin restorations and to reduce the possibility of irritation on pulp tissue, prolonged irradiation time is one of the ways for curing unpolymerized ,substance at the bottom part of the restoration. The prolonged irradiation (120 sec) improved the Knoop hardness numbers of the PS resin, especially in the deeper part of the specimen, compared to the specimens cured for the manufacturer's recommended irradiation time (30 sec). Furthermore, when the irradiation time was prolonged to achieve a properly cured restoration, the activator light supplied more energy to the resin during the irradiation and consequently the temperature of the restoration increased. A higher peak temperature was obtained at the bottom of the PS specimen irradiated by the light for the manufacturer's recommended time (60 sec) than the chemically-cured composite resin cured at room temperature
(18). With regard to the peak temperature, from the specimen irradiated for 60 sec, 40~ was obtained at the bottom and the temperature gradually increased with increasing irradiation time. For the specimen irradiated for 120 sec, approximately 43~ was reached. This peak temperature at the bottom of this composite material hopefully should not irreversibly damage the pulp (19); however, dentists must take a prudent attitude in using visible light-activated composite systems in their practice, from a biological point of view, as another brand of resin may show peak temperatures beyond the physiologic tolerance of the pulp (2O).
53
Conclusions In order to clarify the characteristics of cure in light-activated composite resins, hardness values on the longitudinally sectioned and irradiated surface were determined as a measure of the degree of cure. The following conclusions were reached. 1) For all of the light-activated resin products cured in air without matrix strips, the surface hardness number at the area just below the top surface irradiated by light sources (0.1 mm) was relatively lower than the area 0.5 m m 1.0 mm under the top surface, which showed the maximum hardness values, and the hardness then gradually decreased with depth. 2) For the specimen cured in the 100% N 2 gas environment, the maximum hardness number was always obtained from the top surface nearest the light source. 3) The yellow, grey, and brown shades always showed lower Knoop hardness numbers at any point tested, compared to the universal shade. 4) Re-irradiation by the light source improved the surface hardness on the longitudinally sectioned surface of specimen, this tendency being especially obvious in the deeper part of specimens. 5) Prolonged irradiation time was effective in improving the hardness, especially at the deeper levels of the specimen. However, after more than 120 sec irradiation, the hardness no longer improved significantly. 6) The temperature at the bottom of the resin gradually increased during the irradiation by light. During the effective prolonged irradiation time of 120 sec, the peak resin temperature was approximately 43~
References 1. JORGENSENKD. Restorative resins: abrasion vs. mechanical properties. Scand J Dent Res 1980: 88: 557-68. 2. BASSIOUNYM A , GRANT A A . The surface finish of a visible light-cured composite resin. J Prosth Dent 1980: 44: 175-82. 3. DEGEE AJ. Some aspects of vaccum mixing of composite resins and its effect on porosity. Quint Int 1979: 7: 69-74. 4. TANI Y, NAMBU T, TOGAYAT, IDA K. A comparison of the physical properties of four UV-cured composite restorative materials. JDMA 1979: 36: 242-5. 5. FORSTENL. Working time and strength in relation to consistency of composite resins. Scand J Dent Res 1977: 85: 480-4. 6. RE1NHARm JW, DENEHY GE, JORDAN RD, RlrrMAN BRJ. Porosity in composite resin restorations. Operative Dentistry 1982: 7.' 82-5.
54
Onose et al.
7. PmLLIPSRW. Skinners science o f dental materials, 8th ed. Philadelphia: W. B. Saunders, 1982. 226. 8. LuTz F, SETCOSJC, PHILLIPSR W ROULETJF. Dental restorative resins types and characteristics. Dent Clin North A m 1983: 4: 697-712. 9. SWARTZML, PHILLIPSRW, RHODES B. Visible light-activated resins-depth of cure. J A D A 1983: 106: 634-7. 10. VON REINHARDTKJ, MUNSTERJV. Ein Vergleich lichthartender and UV-polymerisierbarer Vr Versiegler und Komposite. Dtsch Zahnarztl Z 1979: 34: 24550. 11. RUYTERIE, OYSAEDH. Conversion in different depths of ultraviolet and visible light activated composite materials. Acta Odontol Seand 1982: 40: 179-92. 12. KANTOH, TAKAHASHIM, TAMURAT ET AL. Effect of mixing environment on the cure of chemically cured composites. Japan J Conserv Dent 1983: 26: 2989. 13. FINGER W JORGENSEN KD. Polymerizationsinhibition durch Sauerstoff bei Komposit fullings materialien and Schmelzversieglern. Schweiz Monatsschrift far Zahnheikunde 1976: 86: 812-24. 14. RUYTERIE. Unpolymerized surface layer on sealants. Acta Odontol Scand 1981: 39: 27-32. 15. KILIAN RJ. Visible light cured composite: dependence of cure on light intensity. JDR 1979:58 (A): 243 Abst 603. 16. BLANKENAURJ, CAVELWT, KELSEYWT, BLANKENAUP. Wavelength and intensity of seven systems for visible light-curing composite resins: a comparison study. J A D A 1983: 106: 471-74. 17. STANLEYHR, MYERSCL, HEYDE JB, CHAMBERLAINJ. Primate pulp response to an ultraviolet light cured restorative material. J Oral Pathol 1972: 1: 10814. 18. MASUTANIS. Effect of the clinical variation of the mixing ratios to set of the two paste type composite resins. Japan J Conserv Dent 1981: 24: 46-57. 19. ZACHL, COHEN G. Pulp response to externally applied heat. OS O M and OP 1965: 19: 515-30. 20. MASUTANIS, SETCOSJC, SCHNELLRJ, PHILLIPSRW. Temperature rise during polymerization of visible light activated composite resins. JDR 1984: 63: Abst 1096.
H. Onose, H. Sano, H. Kanto, S. Ando, T. Hasuike Nihon University School of Dentistry, Department of Operative Dentistry, Tokyo, Japan.
Onose H, Sano H, Kanto H, Ando S, Hasuike T. Selected curing characteristics of light-activated composite resins. Dent Mater 1985: 1: 48-54. Abstract. - The purpose of this study was to clarify some curing characteristics of light-activated composite resins. Three visible light-activated and 1 UV lightactivated resin were cured with each generator recommended by the manufacturer. The relative degree of cure was determined using Knoop hardness measurements on the irradiated and longitudinally sectioned surfaces of cylindrical specimens. The maximum surface hardness numbers were obtained from the area just under the top of the specimen (0.5 to 1.0 mm below the irradiated surface) and the hardness gradually decreased with the depth. In the specimens cured in 100% N 2 gas environment, the degree of cure on the irradiated surface was improved. Coloring agents in composites have an effect on decreasing the degree of cure. Re-irradiation on the longitudinally sectioned surface of the specimen increased the degree of deep cure. The degree of cure at each depth was improved by prolonged irradiation time.
Composite resins are used in dentistry as esthetic restorative materials. For the chemically-activated systems, it is usually necessary to mix 2 pastes and during the mixing procedures air can be entrapped in the material. This porosity of the material influences the mechanical properties and esthetics for the restoration (1-5). Recently, the light-activated composite systems have been developed. It is not necessary to mix pastes because a photosensitive initiator is used in the single paste, which is activated by the irradiation of light. This system has advantages compared to the paste-paste system, such as adequate working time and less porosity (6-8), and is now used widely in dentistry. The purpose of this study was to determine some curing characteristics of light-activated composite resins. Material and methods In this comprehensive study, 4 light-activated composite systems; (a) Superlux Daylight with Daylight lamp, (b) Plurafil Super with Pluraflex HL150, (c) Fulfil with Prisma-Lite, (d) Estilux with Duralux UV-300, plus (e) a chemically-cured control, Silar, (Table 1) were investigated in 5 phases.
Preparation of specimens A cylindrical vinyl mold (inside diameter 4 mm, height 6 mm) was placed on a glass plate and each resin material was slightly overfilled in the mold. After removal of excess material, the glass plate and matrix strip were eliminated. The tip of the activator light was placed at the top of the resin material and the material was irradiated by the light for the time recommended by each of the manufacturers. The specimens were immediately sectioned longitudinally by means of a thin sectioning machine under water cooling. For the chemically-cured material, Silar, both universal and catalyst pastes were mixed in accordance with the manufacturer's instructions. The mixed paste was placed in the mold as previously described. The Silar specimen was kept for 24 h from the start of mixing at room temperature, in order to achieve a properly cured specimen. The specimen removed from the mold was sectioned longitudinally by means of a thin sectioning machine under water cooling. Then the longitudinally sectioned surfaces of the specimen were polished with emery polishing papers (#800, 1000, 2000 and 3000) under water to
Key words: compositeresins, hardness, exposure, irradiation, color.
Professor H. Onose, Nihon University School of Dentistry, Tokyo, Japan. Accepted for publication 11 December 1984.
achieve the same smooth surface. Under infrared light, the 5 specimens for each phase were prepared in air at room temperature (23 + I~ relative humidity 50 + 5%, except for the study on the effect of environment on the degree of cure. Firstly, Knoop hardness measurements were made on the sectioned surface of each specimen (within 1 h after the irradiation). The indenter point was kept on the surface for 30 sec with a 25 g load. Fig. 1 shows the locations indented by the (Knoop hardness) diamond point on the longitudinally sectioned surface, from 0.1 mm below the top surface of the specimen to 5.5 mm below the top. At each distance from the top surface of the specimen, 3 measurements were carried out. The second part of the study was conducted in order to compare the curing behavior of a visible light-activated composite under a different atmosphere. Visible light cured PS was selected and cured in air with and without matrix strips on the top of the specimen. Also PS was cured in an anaerobic chamber filled with 100% Nz gas. For the 3 groups, Knoop hardness values were obtained from the longitudinally sectioned surface of the specimens (which will be called the sec-
Curing character&tics of light-activated composite resins
49
Table 1. Materials. Composite Resin
Code
Shade
Manufacturer
Batch No.
Activator Light
Wavelength* Irradiation (peak) Time* (sec.)
Typeof polymerization
Plurafil Super
PS
U Y B G
LITEMA
30470 30365 30600 30473
Pluraflex HL150 380-750 nm (505)
30
Visible Light Cure
Superlux Daylight
SD
U
SHOFU
0483
Daylight Lamp
380-750nm (513)
30
Visible Light Cure
Ful-fil
FF
U
CAULK
081782
Prisma Lite
280-540nm (500)
10
Visible Light Cure
Estilux Microfil
EM
U
KULZER
ch-B
Duralux UV-300 350-450nm (435)
20
Ultra-Violet Light Cure
Silar
S
U
3M
-
30**
Chemical Cure
Paste A 2B2 Paste B 2B3
-
* According to manufacturers' instruction. ** Mixing time (sec.). tioned specimen) and the top surface of the cylindric~il specimen (which will be called the irradiated surface specimen), respectively. For the irradiated surface specimen, 10 surface hardness numbers were obtained at the central portion of the irradiated surface within 1 h after the irradiation. The third phase of this study was to clarify the phenomenon of variation in the curing with different shades of resin. Four different shades of PS resin (universal, gray, yellow and brown) were used for this part of the study. Five specimens for each shade were d i r e c t i o n of irradiation mm surface 0.7 0.5 1.0 1.5
m
2.0 2.5 3.0" 3.5
m
4.0 4.5 5.0 5.5
4
/
m
bottom
Fig. 1. Location of measuring points (KHN) on longitudinaUy sectioned surface. O: point of indentation.
cured for 30 sec. Immediately after the irradiation, longitudinally sectioned specimens were prepared and the Knoop hardness numbers were measured according to the method described previously. The fourth phase of the study was to clarify the effect of re-irradiation on the curing of the longitudinally sectioned surface of PS (universal shade). After the 1 irradiation, for the manufacturer's recommended time, the tip of the light unit was placed directly on the center of the longitudinally sectioned surface and a reirradiation for 30 sec was accomplished and the surface hardness was measured. The fifth part of this study was to determine the effect of prolonged irradiation time. PS universal shade materials were cured for 5 different irradiation times (30, 60, 90, 120 and 240 sec). After each irradiation, the degree of cure was measured by using Knoop hardness numbers. Furthermore, increasing the irradiation time meant that the activator light generated more heat in the resin restorations. The temperature rise was due to the exothermic polymerization reaction and heat energy from the activator light itself during the irradiation. Therefore, the temperature change under the resin restoration was measured by a thermocouple during the 240 sec of irradiation. A vinyl mold (inside diameter 6 mm and height 2 mm) was filled with PS material and, in order to prevent heat escape during the exposure of visible light, the mold was embedded in a formed polystyrene plastic as heat insul-
ation. A copper/constantan thermocoupie was placed on the bottom of specimen. To monitor the resin temperature change during the irradiation, the thermocouple was connected to a digital display and X-Y recorder and the temperature versus time was plotted on a chart. Results The surface hardness for the resin materials tested
In this study, Knoop hardness measurements were used as the indicators of the relative degree of cure in the composite resins. The Knoop hardness numbers on the longitudinally sectioned surface of various composite resins at 0.1 mm to 5.5 mm under the top surface of the specimen are shown in Table 2 and Fig. 2. Generally speaking, for all of the light-activated composite resin products cured in air without matrix strips on the top of specimens, a relatively lower hardness was obtained from the area just under (0.1 mm) the top surface of the specimen irradiated by the light, compared to the value obtained 0.5 m m - l . 0 mm under the top surface which was the maximum hardness. Below that maximum value, the hardness gradually decreased with increasing depth. For the UV-cured microfilled composite resin (EM) this tendency was very obvious. However, for the chemically-cured resin (Silar) different behavior was observed. The minimum hardness number was detected at the surface layer
50
Onose et al. Table 3. Surface hardness (KHN) on longitudinally sectioned surface of PS in different environments
Table 2. Surface hardness (KHN) on longitudinally sectioned surface of various composite resins Materials
SD
PS
FF
EM
S
Daylight Lamp
Pluraflex HL 150
Prisma Lite
Duralux UV-300
30
30
10
20
Light Light Irradiation time (sec.) Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
Material Environment
6.8 7.2 6.9 5.8 4.9
(1.2) (1.1) (0.8) (0.9) (0.7) . . .
13.0 (3.0) 13.4 (1.9) 12.9 (2.4) 11.5 (1.4) 10.0 (1.8) 8.7 (1.5) 7.6 (1.4) 6.3 (1.4) 4.8 (0.8) . . . . . .
12.8 (0.3) 15.4 (2.6) 16.9 (3.1) 16.0 (4.3) 13.9 (0.5) 12.8 (0.9) 9.2 (1.7) 5.5 (1.4) . . .
43.9 (2.5) 45.7 (9.2) 28.2 (3.2) 14.7 (2.1) 8.8 (0.6) -
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5
27.8 (2.7) 28.6 (1.7) 32.2 (1.9) 30.8 (1.9) 31.0 (1.5) 29.4 (1.5) 32.6 (2.6) 32.3 (2.7) 30.4 (3.4) 31.0 (4.0) 31.1 (2.9) 30.7 (1.9) -
-
Number = 5. (): Standard Deviation. : immeasurable. KHN
(0.1 to 0.5 mm below the top surface) of the specimen and the surface hardness gradually increased with the depth, up to approximately 1 m m depth and then r e m a i n e d essentially constant, W h e n comparing the hardness numbers among the 4 light-activated resin systems at the same depth from the top
~3
ir ~
~ o
~ air
o----i
o
,
,
o
Nigas
100+/,
s 7 5
0
40
ii 0
45-
,,'----
SD PS FF ENI S
i
i I
i
i 2
i
Pluraflex HL150
i 3
I
r
i
i
t,
Fig. 3. Surface hardness (KHN) on longitudinally sectioned surface of PS in different environments.
PS Air
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
Nz gas 100%
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4) (1.4) (0.8)
14.1 12.4 11.0 10.5 9.4 8.1 7.4 6.7 4.8
-
(1.0) (0.5) (1.1) (1.1) (0.6) (0.6) (0.5) (0.3) (0.2) -
Number = 5. ( ): Standard Deviation. : immeasurable. Irradiation time: 30 sec.
100% N2 gas and air. The m a x i m u m hardness value was obtained at the area 0.1 m m below the top of the specimen for the longitudinally sectioned surface of the specimen cured in a 100% N2 gas environment. H o w e v e r , for the specimens cured in air without matrix strips on the irradiated surface, the m a x i m u m value was not detected at the nearest point to the light tip (0.1 m m below the top). For the 3 groups of the irradiated surface specimens, the hardest top surface was the specimen cured in 100% N 2 gas, followed by the specimen cured in air with a matrix strip, while the specimen cured in air without a matrix strip showed the lowest (Table 4 and Fig. 4).
35
surface, E M was significantly harder than the others from 0.1 mm to 1.0 m m below the top surface, but the hardness dramatically decreased with depth until 2.5 m m below the top when the hardness was no longer measurable. For the visible light-activated composite systems; F F showed the highest K n o o p hardness n u m b e r and was followed by PS and then SD. For these 3 visble light-curing systems, the K n o o p hardness measurements were obtained from the d e e p e r area of specimens c o m p a r e d to the UV-cured system, E M .
30
25
20
15
10.
Effect of resin shade on the cure of lightactivated composites
The degree of cure for 4 different shades of PS resin are shown in Table 5 Table 4. Surface hardness (KHN) on irradiation surface of PS in different environments Light Material Environments (1) (2)
(3) Effect of environment on the cure of lightactivated resins (mm)
Fig. 2. Surface hardness (KHN) on longitudinally sectioned surface of various composite resins.
Table 3 and Fig. 3 show the K n o o p hardness numbers of the PS specimens cured in different atmospheres such as
PluraflexHL150 PS
10.2(0.3) 9.4 (0.1)
7.5 (0.3)
(1) Surface without matrix in Nz gas 100%. (2) Surface with matrix in air. (3) Surface without matrix in air. Number = 5. ( ): Standard Deviation. Irradiation time: 30 sec.
Curing characteristics of light-activated composite resins Table 5. Surface hardness (KHN) on longitudinally sectioned surface of PS with different colors
K~
~--
15
Light
Pluraflex HL150
Material Universal
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
~a
Gray
Yellow
Brown
Reirradiatior
Reirradiation
%
9.5 (0.1) 10.6 (0.5) 9.3 (0.4) 5.7 (0.1)
9.4 (0.5) 10.1 (0.3) 8.8 (0.3) 6.3 (0.2) -
12.4 12.6 9.6 7.5 6.0
(0.6) (0.8) (0.1) (1.1) (0.1) -
-
-
Light
Pluraflex HL150
11
T
Material Re-irradiation
, 9
(2)
(3)
Fig. 4. Surface hardness (KHN) on irradiation surface of PS in different environments. (1) Surface without matrix in N2 gas 100%. (2) Surface with matrix in air. (3) Surface without matrix in air.
9
13-
Unlversol
I
Groy
o
Yellow
o
Brown
11
Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0
PS without
Xo 5
O"F
i
in
i
0
I
I
2
I
~
3
I
I
4
I
I
5 mm
5 ,, 4
(mm)
Fig. 5. Surface hardness (KHN) on longitudinally sectioned surface of PS With different colors.
h a r d n e s s v a l u e was always higher t h a n the control g r o u p at every d e p t h tested, a n d especially large differences of h a r d n e s s b e t w e e n the 2 groups were o b t a i n e d f r o m 2.5 m m to 5.0 m m below the t o p of t h e specimen.
The influence of irradiation time on the cure of light-activated composite resin
13.0 13.4 12.9 11.5 10.0 8.7 7.6 6.3 4.8
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4)
13.3 13.6 13.7 13.5 13.0 13.6 13.7
(1.4)
12.5 (0.5)
(0.8)
11.9 (1.7) 10.4 (1.2) 8.9 (1.6)
-
(1.2)
(1.1) (0.8) (0.9) (0.7) (0.7) (0.5)
Reirradiation time: 30 sec. Number = 5. ( ): Standard Deviation. - : immeasurable.
a n d Fig. 5. F o r s p e c i m e n s of all shades the surface h a r d n e s s at the longitudinally s e c t i o n e d surface d e c r e a s e d with increasing d e p t h . H o w e v e r , at each d e p t h of s p e c i m e n , t h e m a x i m u m hardness v a l u e was always o b t a i n e d from the universal s h a d e s p e c i m e n followed by gray, yellow a n d brown.
Effect of re-irradiation on the cure of longitudinally sectioned surface
7
3
x
with
9
2
\
hardness of longitudinally sectioned PS specimen.
KHN
(1
x 7
Fig. 6. Effect of re-irradiation on Knoop
Table 6. Effect of re-irradiation on Knoop hardness of longitudinally sectioned PS specimen
1
it
9
(3.0) (1.9) (2.4) (1.4) (1.8) (1.5) (1.4) (1.4) (0.8)
Number = 5. ( ): Standard Deviation. - : immeasurable.
0
without with
PS
Shade
0 T
-o
o-------0
51
T h e longitudinally sectioned surface of PS s p e c i m e n s was r e - i r r a d i a t e d by the same light unit for 30 sec a n d the hardness of t h e r e - i r r a d i a t e d s p e c i m e n was c o m p a r e d to t h e s p e c i m e n w i t h o u t reirradiation. Table 6 a n d Fig. 6 d e m o n strate t h e results of re-irradiation. For the r e i r r a d i a t e d group, the K n o o p
Table 7 a n d Fig. 7 show the respective d e g r e e of cure for PS s p e c i m e n s cured for 5 different i r r a d i a t i o n times. T h e surface h a r d n e s s for PS was i m p r o v e d by p r o l o n g i n g the i r r a d i a t i o n time, a n d also the d e p t h of cure was i m p r o v e d . H o w e v e r , m o r e t h a n 120 sec irradiation t i m e did n o t i m p r o v e the surface h a r d n e s s for the PS s p e c i m e n a n d the m a x i m u m surface h a r d n e s s at 0.5 m m to 1.0 m m b e l o w the t o p of the specim e n was n o t i m p r o v e d by i r r a d i a t i o n time o v e r 60 sec.
Temperature change during the irradiation by visible light
T h e t e m p e r a t u r e at the b o t t o m of the PS resin s p e c i m e n during t h e irradiation to visible light gradually rose with increasing the i r r a d i a t i o n time. However, e v e n w h e n t h e s p e c i m e n was irradiated for 240 sec, the t e m p e r a t u r e did n o t r e a c h m o r e t h a n 45~ D u r i n g the 120 sec i r r a d i a t i o n time t h e t e m p e r a t u r e rose to a p p r o x i m a t e l y 43~ (Table 8 a n d Fig. 8). Discussion
T h e d e g r e e of cure for c o m p o s i t e resins influences t h e physical p r o p e r t i e s of set materials a n d t h e longevity of restorations in the e x t r e m e s of t h e oral envi-
O n o s e et al.
52
Table 7. Surface hardness (KHN) on longitudinally sectioned surface of PS cured with various irradiation times Irradiation time (sec.) Depth (mm) 0.1 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5
30
60
90
120
240
13.0 (3.0) 13.4 (1.9) 12.9 (2.4) 11.5 (1.4) 10.0 (1.8) 8.7 (1.5) 7.6 (1.4) 6.3 (1.4) 4.8 (0.8) -
17.6 (2.1) 18.1 (1.3) 16.9 (1.8) 15.3 (1.9) 15.6 (2.3) 15.4 (3. I) 13.2 (1.9) 11.6 (2.2) 8.9 (0.9) 8.1 (0.8) 6.2 (0.6) -
16.4 (3.8) 18.5 (3.0) 17.6 (0.5) 16.1 (1.5) 15.2 (0.4) 14.6 (0.8) 13.6 (0.8) 13.1 (0.5) 12.4 (0.6) 11.2 (0.3) 10.7 (0.6) 9.7 (0.2)
17.1 (0.3) 18.1 (0.2) 17.3 (0.6) 17.2 (0.4) 17..1 (0.6) 17.0 (0.8) 17.0 (0.6) 16.5 (0.3) 15.9 (0.3) 14.7 (0.4) 13.7 (0.6) 12.6 (0.7)
17.3 (0.7) 18.8 (0.3) 17.6 (0.2) 17.5 (0.5) 16.9 (0.2) 16.6 (0.1) 16.7 (0.5) 16.9 (0.4) 16.2 (0.1) 14.8 (0.1) 13.8 (0.2) 12.4 (0.1)
UV-activated material, it is advisable to place and cure material incrementally (9) because of the lower penetrability of U V light (10, 11). However, not all the characteristics of light-activated resins have been clarified yet. Table 2 and Fig. 2 show the results of the surface harness for the specimen cured in air without a matrix strip on the top surface. The hardest value was obtained at 0.5 m m to 1.0 m m below the irradiated surface, even though the top surface was the nearest point to the light source. After the maximum, the hardness gradually decreased with increasing specimen. K n o o p hardness was not measurable b e y o n d more than 2.5 m m in depth for some materials (SD and EM). This tendency was especially obvious for the UV-cured system. Polymerization is d e p e n d e n t u p o n the formation of free radicals. Since there was no matrix strip in place, the presence of oxygen could have caused retardation of polymerization if the oxygen reacted with the free radicals. Kanto et al. (12) m e n t i o n e d in their studies that the mixing e n v i r o n m e n t clearly influenced the curing of chemically-cured resin. Finger & Jorgensen (13) also said that the polymerization of composites exposed in air was inhibited by oxygen. Ruyter (14) measured the inhibited layer on the U V light- and chemicallycured sealant materials exposed in air. Therefore, the visible light-cured material (PS) was cured in a 100% N2 gas environment without influence of oxygen on the curing. The m a x i m u m hardness value was obtained at the closest area to the top surface of the longitudinally sectioned specimen, and the re-
Number = 5. ( ): Standard Deviation. - : immeasurable. Table 8. Temperature changes of PS through irradiation at room temperature (23 + I~ Light Material
Irradiation time (sec.) 0
Pluraflex HL150
PS
10
20
30
60
90
120
150
180
210
240
23.0 32.1 36.3 37.9 39.5 40.4 41.3 41.8 42.1 42.5 43.0 (0.00) (0.62) (0.14) (0.33) (0.41) (0.45) (0.47) (0.47) (0.38) (0.36) (0.33)
Number = 3. ( ): Standard Deviation. Relative humidity: 50 + 10%. ronment. Therefore, clinicians who want to use a material should know the characteristics of that material. According to this study the chemically-cured resin showed similar K n o o p hardness values throughout the speci-
:
"
30 sec. 60 ,. ~ 90 ,'
J
i
120
%
9
,,
men, except for the inhibited surface layer. However, light-activated composite resins developed recently have different polymerization characteristics from chemically-cured resins. O n the polymerization of light-activated material, the Knoop hardness n u m b e r of the resin decreased with the depth, especially for the resin cured by U V light which has a relatively shorter wavelength compared to visible light. Therefore, when restoring a deep cavity with
17
I 50 I
15
11' 30
9" i-
23
7' 5'
0: 0
I
2
3
4
5 mm
Fig. 7. Surface hardness (KHN) on longitu-
dinally sectioned surface of PS cured with various irradiation times.
i
i
i
i
i
10
20
30
80
90
i
120
i
IS0
i
180
I
210
i
240
Irrediation Time ( ~ e e . )
Fig. 8. Temperature changes of PS through irradiation at room temperature (23 _+ I~
Curing characteristics of light-activated composite resins sult obtained from the specimens cured in air with matrix strips was comparable to the specimen cured in N 2 gas. So the cure of visible light-activated composites at the surface exposed to air was also influenced by oxygen. The photo initiator system in visible light-activated composite resins requires a sufficient intensity of light in a certain range of wavelengths (15). If enough intensity of light cannot be supplied from the generator, the polymerization will not begin properly (16). The degree of cure for tight-activated resins decreased with increasing depth of the materials. The reason for this phenomenon is probably to be explained in that the energy absorption of the surrounding media decreased the amount of penetrating light (11), and consequently the light intensity decreased with increased depth of resin, until at the deeper part of the specimen the polymerization was not complete. In order to determine the influence of light absorbancy of the surrounding media on the curing of visible light-activated composite resins, 4 different shades of composites were cured. The universal shade resin always showed a higher Knoop hardness number than the others. This result was in agreement with the previous study on the effect of shade on the depth of cure for light-cured resin by Swartz et al. (9). The degree of cure for light-activated composite resins was influenced by the penetrability of light. Therefore, there is a possibility that unpolymerized substances such as residual monomer and a catalyst still remained in the depths of the resin specimens. In order to clarify this possibility, the longitudinally sectioned surface of the specimen was reirradiated and the Knoop hardness was compared. For the re-irradiation group, the Knoop hardness number was dramatically increased by the re-irradiation. This result clearly indicated that unpolymerized material still remained on the sectioned surface. However, at the area near the bottom surface of the bottom surface of the specimen (more than 4 mm depth from the top surface), in spite of re-irradiation to the light for 30 sec, the surface hardness was not very much improved compared with the upper half of the specimen. This was presumably because the procedures of sectioning and polishing under water cooling washed away some unpolymerized material. From a clini5 DentalMaterials1:2, I985
cal point of view, residual unpolymerized material on the cavity floor could be a cause of chemical pulpal irritation (17). In order to improve the physical properties of resin restorations and to reduce the possibility of irritation on pulp tissue, prolonged irradiation time is one of the ways for curing unpolymerized ,substance at the bottom part of the restoration. The prolonged irradiation (120 sec) improved the Knoop hardness numbers of the PS resin, especially in the deeper part of the specimen, compared to the specimens cured for the manufacturer's recommended irradiation time (30 sec). Furthermore, when the irradiation time was prolonged to achieve a properly cured restoration, the activator light supplied more energy to the resin during the irradiation and consequently the temperature of the restoration increased. A higher peak temperature was obtained at the bottom of the PS specimen irradiated by the light for the manufacturer's recommended time (60 sec) than the chemically-cured composite resin cured at room temperature
(18). With regard to the peak temperature, from the specimen irradiated for 60 sec, 40~ was obtained at the bottom and the temperature gradually increased with increasing irradiation time. For the specimen irradiated for 120 sec, approximately 43~ was reached. This peak temperature at the bottom of this composite material hopefully should not irreversibly damage the pulp (19); however, dentists must take a prudent attitude in using visible light-activated composite systems in their practice, from a biological point of view, as another brand of resin may show peak temperatures beyond the physiologic tolerance of the pulp (2O).
53
Conclusions In order to clarify the characteristics of cure in light-activated composite resins, hardness values on the longitudinally sectioned and irradiated surface were determined as a measure of the degree of cure. The following conclusions were reached. 1) For all of the light-activated resin products cured in air without matrix strips, the surface hardness number at the area just below the top surface irradiated by light sources (0.1 mm) was relatively lower than the area 0.5 m m 1.0 mm under the top surface, which showed the maximum hardness values, and the hardness then gradually decreased with depth. 2) For the specimen cured in the 100% N 2 gas environment, the maximum hardness number was always obtained from the top surface nearest the light source. 3) The yellow, grey, and brown shades always showed lower Knoop hardness numbers at any point tested, compared to the universal shade. 4) Re-irradiation by the light source improved the surface hardness on the longitudinally sectioned surface of specimen, this tendency being especially obvious in the deeper part of specimens. 5) Prolonged irradiation time was effective in improving the hardness, especially at the deeper levels of the specimen. However, after more than 120 sec irradiation, the hardness no longer improved significantly. 6) The temperature at the bottom of the resin gradually increased during the irradiation by light. During the effective prolonged irradiation time of 120 sec, the peak resin temperature was approximately 43~
References 1. JORGENSENKD. Restorative resins: abrasion vs. mechanical properties. Scand J Dent Res 1980: 88: 557-68. 2. BASSIOUNYM A , GRANT A A . The surface finish of a visible light-cured composite resin. J Prosth Dent 1980: 44: 175-82. 3. DEGEE AJ. Some aspects of vaccum mixing of composite resins and its effect on porosity. Quint Int 1979: 7: 69-74. 4. TANI Y, NAMBU T, TOGAYAT, IDA K. A comparison of the physical properties of four UV-cured composite restorative materials. JDMA 1979: 36: 242-5. 5. FORSTENL. Working time and strength in relation to consistency of composite resins. Scand J Dent Res 1977: 85: 480-4. 6. RE1NHARm JW, DENEHY GE, JORDAN RD, RlrrMAN BRJ. Porosity in composite resin restorations. Operative Dentistry 1982: 7.' 82-5.
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7. PmLLIPSRW. Skinners science o f dental materials, 8th ed. Philadelphia: W. B. Saunders, 1982. 226. 8. LuTz F, SETCOSJC, PHILLIPSR W ROULETJF. Dental restorative resins types and characteristics. Dent Clin North A m 1983: 4: 697-712. 9. SWARTZML, PHILLIPSRW, RHODES B. Visible light-activated resins-depth of cure. J A D A 1983: 106: 634-7. 10. VON REINHARDTKJ, MUNSTERJV. Ein Vergleich lichthartender and UV-polymerisierbarer Vr Versiegler und Komposite. Dtsch Zahnarztl Z 1979: 34: 24550. 11. RUYTERIE, OYSAEDH. Conversion in different depths of ultraviolet and visible light activated composite materials. Acta Odontol Seand 1982: 40: 179-92. 12. KANTOH, TAKAHASHIM, TAMURAT ET AL. Effect of mixing environment on the cure of chemically cured composites. Japan J Conserv Dent 1983: 26: 2989. 13. FINGER W JORGENSEN KD. Polymerizationsinhibition durch Sauerstoff bei Komposit fullings materialien and Schmelzversieglern. Schweiz Monatsschrift far Zahnheikunde 1976: 86: 812-24. 14. RUYTERIE. Unpolymerized surface layer on sealants. Acta Odontol Scand 1981: 39: 27-32. 15. KILIAN RJ. Visible light cured composite: dependence of cure on light intensity. JDR 1979:58 (A): 243 Abst 603. 16. BLANKENAURJ, CAVELWT, KELSEYWT, BLANKENAUP. Wavelength and intensity of seven systems for visible light-curing composite resins: a comparison study. J A D A 1983: 106: 471-74. 17. STANLEYHR, MYERSCL, HEYDE JB, CHAMBERLAINJ. Primate pulp response to an ultraviolet light cured restorative material. J Oral Pathol 1972: 1: 10814. 18. MASUTANIS. Effect of the clinical variation of the mixing ratios to set of the two paste type composite resins. Japan J Conserv Dent 1981: 24: 46-57. 19. ZACHL, COHEN G. Pulp response to externally applied heat. OS O M and OP 1965: 19: 515-30. 20. MASUTANIS, SETCOSJC, SCHNELLRJ, PHILLIPSRW. Temperature rise during polymerization of visible light activated composite resins. JDR 1984: 63: Abst 1096.