Okamura

Asian Pac J Dent 2014; 14: 7-12

Effect of light irradiation distance during intermediate
polymerization on depth of cure and hardness of indirectcomposite
Kota Okamura, DDS
Nihon University Graduate School of Dentistry
Purpose: This study evaluated the influence of distance between a halogen lamp and an indirect composite
material surface on depth of cure and Knoop hardness number of the material.
Materials and Methods: An indirect composite material (Signum Ceramis DA2) and single-lamp intermediate
light polymerization unit equipped with a 150 W halogen lamp (Sublite S) was assessed. Relative lightintensity and temperature rise of the halogen lamp, depth of cure and Knoop hardness of the composite material
were assessed with different lamp-material distances.
Results: Relative light intensity of the lamp, depth of cure and hardness of the material were maximal when thelamp-material distance was set as 20 mm. However, temperature rise during light exposure was apparent
around the lamp aperture.
Conclusion: It can be concluded that the distance between the halogen lamp and the material to be polymerizedshould be set around 20 mm, when the composite material is polymerized with an intermediate polymerization
unit equipped with the 150 W halogen lamp.
(Asian Pac J Dent 2014; 14: 7-12.)
Key Words: halogen lamp, indirect composite, intermediate polymerization, light intensity

Introduction
The use of indirect composites has become increasingly common during the last decade.

They are used

for resin-veneered restorations, superstructure of implant prostheses, metal-free restorations, and fixed partial
dentures (FPDs).

According to previous reports,1-5 water absorption and solubility of a new generation ofcomposites has been reduced, and wear resistance has been considerably improved.
It is important for these materials that light polymerization should be performed appropriately in order tofacilitate sufficient physical properties.

Direct composite materials are substantially polymerized by means of aquartz-tungsten-halogen (QTH) or a light-emitting diode (LED) light source, whereas indirect composite
materials are polymerized with both visible and ultraviolet light sources.

During the layering process, anintermediate polymerization unit equipped with a visible light source is frequently applied.

This is due to thefact that the presence of an oxygen-inhibited layer, generated with the use of a visible light source, isindispensable for layering and bonding different shades of indirect composite materials.6

Radiation energy andexposure time period also affect post-polymerization properties of indirect composite materials.1-14

The light

sources employed for polymerization of indirect composites are xenon flash lamps, fluorescent tubes, halogen
lamps, and metal halide lamps.7-14
Light intensity is in general inversely proportional to the square of the distance between the light source andthe material surface to be light-exposed.

This study evaluated the influence of distance between a halogen lamp

and an indirect composite material surface on the depth of cure and Knoop hardness number of the material.

Materials and Methods
Materials
The materials used in this study are summarized in Table 1.

7

An indirect composite material (SignumOkamura

Asian Pac J Dent 2014; 14: 7-12

Ceramis DA2, Heraeus Kulzer Japan Co., Ltd., Tokyo, Japan) was assessed.
inorganic filler, 15% prepolymerized filler, and 30% bifunctional monomers.

The material consists of 55%15A single-lamp intermediate

light polymerization unit equipped with a halogen lamp (Sublite S, Shofu Inc., Kyoto, Japan) was used forpolymerizing the composite material.
Table 1. Materials assessed
Material/Trade name

Manufacturer

Lot

Indirect composite

Composition*
Heraeus Kulzer
Japan Co., Ltd.

Signum Ceramis (DA2)010139Polymerizing unit

70 wt% 0.04-2.0 µm hybrid fillers (SiO2, Ba·Al·Si
glass), 30 wt% urethane and triethylene glycol
dimethacrylatesLight source

Sublite S

Shofu Inc.

0707

*Quoted from Alves et al.15

Halogen lamp
JCR 110V 150 W×1Spectroradiometry
A spectroradiometer (USR-40d-14R/40-d16, Ushio Inc., Tokyo, Japan) was used to determine relative lightintensity emitted from the light source.

The distance between the lamp aperture and the composite specimen

surface was set at 10, 20, 30, and 40 mm.

Measurements were performed per 1 nm wavelength between 200

and 800 nm for 60 s of exposure time period.

The relative light intensity value measured for each distance wasconverted to cumulative light energy values.
Depth of cure
A cylindrical stainless steel split mold 4 mm in inner diameter and 8 mm in height was coated with a thinlayer of separator (Estenia C&B CR Sep III, Kuraray Noritake Dental Inc., Tokyo, Japan) before being filledwith the composite material.

A cover glass (Tempax, Matsunami Glass Ind., Ltd., Osaka, Japan) was used toapply pressure via a resin compression sheet (polyethylene film, GC, Tokyo, Japan) in order to flatten thelight-exposed surface of the material during light exposure.

The distance between the lamp aperture and thecomposite specimen surface was set at 10, 20, 30, and 40 mm.

Six specimens were prepared for each condition.

After 60 s of light exposure, an unexposed period of 8 minutes was ensured to maintain identical temperature of
pre-polymerization material surfaces.
with ISO 4049.

16

The depth of cure of the composite material was measured in accordanceAfter light exposure, the unpolymerized layer of the specimen was removed withalcohol-treated gauze, and the length of the hardened cylinder of the composite material was measured with adigital micrometer (MDC-SB, Mitutoyo Corp., Kawasaki, Japan).12,14
Knoop hardness testing
A piece of filter paper (Whatman Filter Paper 5, GE Healthcare Japan, Tokyo, Japan) and a Teflon mold
(10 mm in inner diameter and 2 mm in height) were placed on a glass plate.
C&B SEP III) was applied to the inner surface of the mold.

A thin layer of a separator (EsteniaThe mold was filled with the composite material.

Pressure was applied to a glass plate and a piece of plastic film during polymerization of the material.

Afterlight exposure, the polymerized specimens were sliced in half using a low-speed rotary diamond saw (IsoMet,Buehler, Lake Bluff, IL, USA).
oxygen-inhibited layer.

The light-exposed surface was ground flat to 200 µm in thickness to remove

Specifically, both the ground and cut surfaces were identically wet-ground with8Okamura

Asian Pac J Dent 2014; 14: 7-12

water-resistant abrasive paper (WetorDry Sheet, 2000 grit, 3M, St. Paul, MN, USA).

The specimens were thenpolished with a series of diamond pastes (metaDi, Buehler; particle diameters 3.0, 1.0, and 0.25 µm) and felts.
A microhardness tester (HMV-1, Shimadzu Corp., Kyoto, Japan) was used to measure the Knoop hardness ofthe specimen.

The measurement of the specimen was performed at one point on the light-exposed surface andat seven points in the central part of the cut surface.

The interval was 0.2 mm from the top to the bottom

direction, and a loading schedule applied to the specimen was set at 490.3 mN (HK0.05) for 30 s.

Six

specimens were prepared for each condition.
Temperature measurements
Temperature of the composite specimen surface during light exposure was determined with a temperaturelogger (Ondotori TR-71UI, T&D Corp. Matsumoto, Japan).

The sensor was placed on the top of the specimen

surface perpendicularly to the light exposing direction with the irradiation distances of 10, 20, 30, and 40 mm.
The temperature after 60-s light exposure was recorded for eight specimens.
Statistical analysis
The Kolmogorov-Smirnov test was applied to the experimental results for the assessment of normality(SPSS Version 16.0 for Windows, SPSS Inc., Chicago, IL, USA).

When normality was not observed, amultiple comparison was performed using the Steel-Dwass test (Kyplot5.0, KyensLab, Tokyo, Japan).

Whennormality was observed, Levene’s test was carried out to confirm equality of variance, after which one-way
analysis of variance and a multiple comparison using Tukey’s HSD test were performed (SPSS).

The

significant levels were set at 0.05 for all testing.

Results
Figure 1 shows the relative light intensity of the halogen lamp determined with different lamp-specimendistances.

Spectral irradiance exhibited a gentle curve with a peak at around 515 nm for all conditions, andshowed the highest value when the distance between the lamp and the specimen surface was 20 mm.

Fig. 1.

Relative light intensity of the 150 W halogen lamp
Lamp-specimen distance; 10, 20, 30, and 40 mm

Fig. 2.

9KHN values of the composite material
Distance from the material surface; 0.2-1.4 mm

Okamura

Asian Pac J Dent 2014; 14: 7-12

Since a wavelength peak was also detected at around 580 nm for all conditions, this study focused on the400-600 nm wavelength ranges, which includes the maximal absorption wavelength of camphorquinone (CQ)
visible-light initiator (470 nm).

It was confirmed that light energy, i.e., integral values of relative light intensity

within the 400-600 nm wavelength range was the highest (41.3 mW/cm2) when the lamp-material distance was
20 mm.
Table 2. Depth of cure (mm) of the composite material
Irradiation distance

Mean (SD)MedianIQR

Category

10 mm

4.0 (0.1)

4.0

0.1b20 mm

4.2 (0.1)

4.2

0.1

30 mm

4.0 (0.1)

4.0

0.1

40 mm

3.6 (0.1)

3.7

0.1

abcn=6; SD, Standard deviation; IQR, Interquartile range;
Identical letters indicate that the values are not statistically different (p>0.05).
Table 3. Knoop hardness number (KHN) of the composite material
Irradiation distance

Mean (SD)MedianIQR

Category

10 mm

46.0 (0.7)

45.9

1.3b20 mm

53.1 (4.1)

53.5

6.130 mm

46.4 (2.0)

46.5

4.1

40 mm

41.3 (2.0)

41.3

3.3

abcn=6; SD, Standard deviation; IQR, Interquartile range;
Identical letters indicate that the values are not statistically different (p>0.05).

Table 2 shows the results depth of cure determination.

They correlated well with relative light intensity,with the greatest mean depth of 4.2 mm when the distance between the lamp and the specimen was 20 mm.
The mean value of other specimens was 4.0 mm for 10- and 30-mm distances, and 3.6 mm for the 40-mm
distance.
Table 3 shows Knoop hardness number (KHN) values for the top surface of the specimens.

The greatest

Knoop hardness number (53.1) was generated when the distance between the lamp and the specimen was 20 mm.
The value was significantly higher than that of other specimens; 46.4 for 30 mm, 46.0 for 10 mm, and 41.3 for40 mm.

Figure 2 shows the KHN values obtained from the longitudinal section of the specimens.

gradually decreased with increasing distance from the material surface.
Table 4. Temperature (˚C) of the composite material surface
Irradiation distance

Mean (SD)MedianIQR

10 mm

51.6 (1.2)

52.1

2.3

20 mm

49.2 (0.3)

49.3

0.5

30 mm

41.1 (0.5)

41.0

1.1

40 mm

38.6 (0.3)

38.6

0.5

n=8; SD, Standard deviation; IQR, Interquartile range;
Different letters indicate that the values are statistically different (p<0.05).

10CategoryabcdHardnessOkamuraAsian Pac J Dent 2014; 14: 7-12

Temperature of light-exposed surfaces is summarized in Table 4.

The temperature after light exposure

was highest (51.6˚C) when the distance between the lamp and the specimen was 10 mm, followed by 49.2˚C for
20 mm, 41.1˚C for 30 mm, and 38.6˚C for 40 mm.

Discussion
Factors affecting properties of indirect composites are divided into two categories; material compositionand polymerization condition.

Sobrinho et al.7 reported that there were differences in hardness between thelight-exposed surface and inner areas when the exposure time period was short.7

However, when the durationof light exposure was gradually increased, polymerization proceeded throughout the entire specimen during thelonger exposure times.

Also, differences in hardness almost disappeared.7

The current study fixed theexposure time period as 60 s, and focused on the relation between the lamp-material distance and thepost-polymerization material properties.
Although the author performed the experiment under the hypothesis that light intensity is inversely
proportional to the square of the distance from the light source, the results were somewhat different.

This may

be derived from the fact that the halogen lamp of the Sublite S unit is equipped with a reflector for theenhancement of light intensity at a specific distance from the aperture.
Experimental results showed that relative light intensity, depth of cure, and Knoop hardness number was
highest when the distance between the lamp and the specimen surface was set at 20 mm.

The results suggestthat the reflector of the halogen lamp is designed for maximizing the light energy at about 20 mm distance fromthe aperture.
Determination of temperature revealed that the heat energy emitted from the lamp was highest at the 10 mmdistance from the aperture.

It has been reported that the conversion of indirect composite materials primarily

irradiated with QTH lamps and LED lamps is temperature dependent.17-21

From the results of the current

experiments and the literature, it is reasonable to consider that the high-temperature around the halogen lightsource is not a negative factor for improvement in properties of indirect composite materials.

Also, the author

speculates that accumulation of heat energy, i.e, temperature rise for the 150 W halogen lamp is dependent on
the distance between the halogen light source and the material surface.

This may be derived from accumulationof the radiant heat from the halogen lamp as well as the reflected heat from the lamp reflector.
Within the limitation of the current experimental settings, it can be concluded that the distance between thehalogen lamp and the composite material to be polymerized should be set at approximately 20 mm, when thematerial is polymerized with an intermediate polymerization unit equipped with a representative 150 W halogenlamp.
Acknowledgements
This work was supported in part by a grant from the Promotion and Mutual Aid Corporation for Private Schools ofJapan (2012 and 2013).

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Correspondence to:Dr. Kota Okamura
c/o Department of Fixed Prosthodontics, Nihon University School of Dentistry
1-8-13 Kanda-Surugadai, Chiyoda-ku, Tokyo 101-8310, Japan
Fax: +81-3-3219-8351 E-mail: deko10010@g.nihon-u.ac.jp
Copyright ©2014 by the Asian Pacific Journal of Dentistry.

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Accepted January 29, 2014.
Online ISSN 2185-3487, Print ISSN 2185-3479