Journal of Oral Rehabilitation, 1991, Volume 18, pages 353-362
Residual monomers (TEGDMA and Bis-GMA) of a set visible-light-cured dental composite resin when immersed in water K. TANAKA, M. TAIRA*, H. SHINTANI, K. WAKASA* M . Y A M A K I * Department of Operative Dentistry and * Department of Dental Materials, Hiroshima University School of Dentistry, Hiroshima, Japan
Summary
Rods of a visible-light-cured dental composite resin were photo-polymerized and immersed in water at 37 °C for 7 days. The residual monomers (TEGDMA and BisGMA) trapped in the set composite and those eluted into water were analysed by gas-liquid chromatography. It became evident that minor amounts of the residual monomers dissolved in water, but that most residual monomers remained in the set composite. Extension of the irradiation period contributed to the significant reduction in the residual monomer level and its elution into water. Introduction
Visible-light (VL)-cured dental composite resin has been widely accepted as a restorative material due to its ease of handling and its aesthetic merits. Concern about its clinical reliability, however, still remains. Unreacted monomers that leach from the polymerized materials may irritate the soft tissue (Bloch et al, 1977; Stanley, Bowen & Folio, 1979; Hanks et al, 1988). Furthermore, monomers trapped in the set composite may reduce the clinical serviceability of the composite through oxidation and hydrolytic degradation, which may be manifested in forms such as discoloration of the fillings (Asmussen, 1983; Ferracane, Moser & Greener, 1985) and accelerated wear (Roulet, 1987). Thus it appears important to measure the residual monomers of the hardened VL-cured composite held in the aqueous environment. For chemical analysis of the resin matrix in the dental composite consisting of multi-functional methacrylate monomers such as triethyleneglycol dimethacrylate (TEGDMA) and Bisphenol A glycidyl dimethacrylate (Bis-GMA), high-performance liquid chromatography (HPLC) has usually been employed (Hirabayashi et al, 1984; Ruyter & 0ysa;d, 1987). Using HPLC techniques, Inoue and Hayashi (1982) have chemically analysed not only residual Bis-GMA in the set chemically cured dental composites, but also its elution into water. However, it is generally not easy to identify accurately the trace monomers, such as those dissolved from the set composite into water, by HPLC. Another possible technique is gas-liquid chromatography (GC), although the latter has more often been utilized to detect lower-molecular-weight chemical substances such as the monomeric methyl methacrylate released from acrylic appliances (Douglas & Bates, 1978; Fletcher et al, 1983; Stafford & Brooks, 1985; Correspondence: Dr K. Tanaka, Department of Operative Dentistry, Hiroshima University School of Dentistry, 1 - 2 - 3 Kasumi Minami-ku, Hiroshima-shi 734, Japan.
353
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K. Tanaka et al.
Sadamori et al, 1987; Baker, Brooks & Walker, 1988) and photo-initiators in the VL-cured composites (Shintani, Inoue & Yamaki, 1985; Taira et al, 1988). Therefore, the aims of the present study were as follows: (i) by means of GC, to develop a method of measuring tiny amounts of the monomers derived from the set VL-cured dental composite resin; (ii) to measure the quantity of residual monomers left in the composite at photocuring; (iii) to examine the manner in which the residual monomers in the photo-cured composite stored in water elute into water as a function of time. To confirm the GC peaks of the monomers, mass-spectroscopy (MS) was used. In addition, the effects of the irradiation time (30s and 50s) on the residual monomers of the set composite soaked in water were examined. Materials and methods
The VL-cured dental composite resin to be examined was universal shade Photoclearfil A*. Table 1 lists its composition by weight. The inorganic filler content (88% by weight) and the organic fraction (12% by weight) were initially checked by thermogravimetric thermal analysis and HPLC, respectively. The resin matrix of the composite studied basically consisted of TEGDMA and Bis-GMA. For the chemical analyses, two monomer standards of TEGDMAt and Bis-GMA+ were used without further purification. Figure 1 illustrates the procedures for preparation of three kinds of sample. (a) A small portion of the uncured composite resin paste was dissolved in acetone, the solution was then centrifuged, and an aliquot was injected into a gas-liquid chromatographic column for qualitative confirmation of the GC peaks of monomers in the composite. (b) Specimen rods, 6mm in diameter and 3 mm high, were prepared by use of a stainless steel mould. The mould was first filled with the material, the surface of the material open to the air was covered with a polyester film and the material was photo-cured by means of a VL source§. The irradiation period was varied for 30s and 50 s. The hardened rods were then ground by use of a pestle and a mortar, and the resultant powders were immediately dipped in sealed pure
Table I. The composition b) weight (%) of the commercial visiblc-light-cured composite resin that was examined Comptxicnt Inorganic filler* BisGMA TEGDMA Others
Composition by weight {^Vn 88-0 9() 24 0-6
* Determined at 575°C. * t t §
Lot number 1111, Kuraray Co.. Osaka, Japan. Lot number DOOl. 99 9% pure, Tokyo Kasci Co.. Tokyo, Japan. Lot number 1209B, industrially pure grade. Shin-Nakamura Co., Wakayama, Japan. Quick-light model VL-1, Morita Co., Kyoto, Japan.
Residual monomers of VL-cured composite resin (b)
(c)
Uncured composite
Uncured composite
Uncured composite
Dissolved in ocetone
Rod VL cured tor 30s, 50s
Rod VL cured for 30 s, 50 s
Centrifuge
Ground to powders
Immersed in water tar up ta 7 days
Aliquot taken
Immersed in methonol for 7 days
(a)
GC-MS anolysis
GC analysis
355
Elutian taken out at I, 3, 5, 7 days later Chlorafarm added and shaken Acetone added and shaken Oentrifuge and separating funnel Solvent phase treezedried in vacuum JL. Residue dissolved in acetone
i GC analysis
Fig. 1. Procedures for the chemical analysis of (a) the monomers in the uncured composite, (b) the residual monomers remaining in the composite at photo-curing, and (c) the residual monomers that eluted from the set composite into water.
methanol for 7 days for the measurement of the residual monomers left in the composite at photo-curing. (c) VL-cured rods were also immersed in the distilled water for up to 7 days. The water, including the elution of the monomers, was removed 1, 3, 5 and 7 days later. To extract the monomers from water, two solvents, chloroform and acetone, were added stepwise, the mixed solution was shaken well and the two phases of solvent and water were separated using a centrifuge and a separating funnel. The solvent phase fully absorbing the monomers from water was then freeze-dried in vacuo to evaporate the solvents. Acetone was added to the residue to provide the solution for chemical analysis of the monomers eluted from the composite into water. GC was performed using a gas-liquid chromatograph* under the following experimental conditions: detector, F.I.D. (hydrogen flame); capillary column, CBP-1 W25-300* (25 m x 0-53 mm, 0-30 jxm film thickness); injection port temperature, 310°C; column oven temperature, programmed from 100 to 230°C at 30°Cmin~', and from 230 to 310°C at 5°Cmin~'; carrier gas, 25ml min ' He; sensitivity, 10 MQ. MS was performed on a gas-liquid chromatograph-quadrupole mass spectrometer-computer data
* Shimadzu Co., Kyoto, Japan.
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K. Tanaka et al.
system*, under the GC conditions outlined above, and the following MS conditions: electron energy, 70 eV; ion source temperature, 250°C; current, 60 ^lA. The procedure for chemical analysis was as follows. When part of the GC chromatogram for the specimen prepared from the uncured composite correlated closely with regard to characteristic retention time with the GC chromatogram of a monomer standard, it was assumed that the former contained the latter. This was confirmed by the fragmentation patterns of MS coupled with GC. Ouantitation of the residual monomers was then achieved solely by GC, by measuring the peak area of the target monomer extracted from the cured composite and relating this to the calibration line between peak area and concentration constructed previously for the pure monomer standard. All quantitative measurements were repeated three times. Results
Figure 2 shows the mass chromatogram together with the total ion chromatogram (TIC) of the acetone extract obtained from the uncured composite. The TIC is equivalent to the GC chromatogram. Two peaks of TEGDMA and Bis-GMA could
TEGDMA
TIC
BIS-GMA
i
i
Moss
- 69 - 113 143
A-A10
15
20 25 Time (mm)
30
35
Fig. 2. Mass chromatogram and total ion chromatogram (TIC) of the acetone extract obtained from the uncured composite specimen. Shimadzu GCMS-2(XK)A. Shimadzu Co., Kyoto. Japan.
Residtial monomers of VE-cured composite resin
357
be identified because the 'fingerprint' m/e tracing in the mass chromatogram (such as 69 and 113 m/e for TEGDMA and 69 and 143 m/e for Bis-GMA) corresponded closely with the electron-impact mass spectra of their pure standards, as shown in Fig. 3. Two monomers had different GC properties. Although TEGDMA appeared as a single strong peak, Bis-GMA tended to emerge as a few weaker peaks. Figure 4 shows GC chromatograms of (a) the methanol extract containing the residual monomers that remained in the composite at photo-curing, and (b) the acetone extract containing the residual monomers that eluted from the set composite into water. Table 2 shows the proportions of the residual monomers left in the composite specimen at VL curing. It is clear that significant quantities of residual TEGDMA monomer and small amounts of residual Bis-GMA monomer existed in the once photo-cured composite, and that increasing the duration of VL irradiation led to a substantial decrease in residual monomer levels.
( a ) TEGDMA = 286 M+ CH3
CH3
41
1001- 41
69
in
0 80
30
130
:b) Bis-GMA =: 512 M+ CH3
100 r
230
260
CH3
CH3
I OH
69
180 ;nn/e)
OH
CH3
143
69 41
c 50 o to
0
30
ii.i...
130
,..
li,.
230
330
430
530
(m/e)
Fig. 3. Mass spectra of two monomer standards: (a) TEGDMA and (b) Bis-GMA.
358 (0)
K. Tanaka et al. Residuol monomer 0-47 Meihanol
577 TEGDMA 5-99
a> 20-01
(b)
BIS-GMA
Residual monomer into water • 0'24 Acetone
5-85 TEGDMA
cc
BIS-GMA
Fig. 4. GC chromatograms of (a) the methanol extract containing the residual monomers remaining in the composite specimen at VL curing, and (b) the acetone extract containing the residua) monomers that eluted from the photo-cured composite resin into water.
Table 2. Proportions of the residual monomers remaining in the composite specimen at photo-curing, relative to the (original concentration of each monomer in the uncurcd composite Proportion by weight (%) 3()-s irradiation
5()-s irradiation
Rcsiduul TEGDMA
23-5
12-8
Residual Bis-GMA
41
13
Residual monomers of VE-cured composite resin
359
Figure 5 shows the trend for two residual monomers, TEGDMA and Bis-GMA, to elute from the photo-cured composite into water over a period of 7 days. There was extensive elution of residual TEGDMA into water during the initial 1—2 days, but its dissolution subsequently levelled off. Residual Bis-GMA continued to elute into water over a longer time period. For the specimen that had been photo-cured for 30 s more residual monomers eluted into water, compared to the specimen that had been VL-activated for 50 s. Table 3 shows the ratio of the quantity of residual monomers eluted into water over a 7-day period to the amount of residual monomers left at VL-curing. It became apparent that minor amounts of the residual monomers dissolved into water, but that the majority of the residual monomers remained in the set composite stored in water. Discussion
By using GC we were able to measure not only the residual monomers remaining in the composite at photo-curing, but also the trace residual monomers that eluted from the set composite into water. This may be attributed to the use of the capillary
TEGDMA
•0
0-5
50-s irradiation
0
o
3
0-10 BIS-GMA
30-s irradiation
0-05
0
Time (days)
Fig. 5. Concentration profiles of the residual monomers, (a) TEGDMA and (b) Bis-GMA. that eluted from the photo-cured composite into water, plotted against time. Note that the quantity of each residual monomer is expressed relative to the original amount of monomer present in the uncured composite.
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Table 3. Ratio of the quantity of each residual monomer elutcd into water over a 7-day period to the amount of that monomer remaining at VL curing Percentage ratio 3()-s irradiation
50-s irradiation
Eluted TEGDMA/ residual TEGDMA
6 21
1-88
Eluted Bis-GMA/ residual Bis-GMA
2-17
1-69
column, which is suitable for microanalysis of the organic materials with high boiling points (Bailey et al, 1987). The rationale for the use of three solvents is as follows. With regard to the solvent extraction method, for rapid extraction of the monomers dissolved in water, acetone and chloroform were effective solvents, but methanol was a weaker solvent. However, the former solvents tended to degrade two monomers, particularly Bis-GMA, rapidly, so that it was necessary to analyse them chemically immediately following sample preparation. For extraction of the residual monomers from the set composite, we found methanol to be more satisfactory than the other two strong solvents. Methanol disintegrates the monomer to a lesser extent during chemical analysis, while it requires a long time period (e.g. 7 days) to absorb the monomers fully from the set composite (Hirasawa et al, 1984). Therefore solvents should be used selectively according to the specific requirements, an approach that was adopted in this study. The amounts of residual monomers left in the composite specimen at photo-curing were comparable to those reported by other workers using different experimental techniques (e.g. Fourier transform infra-red (FTIR) spectroscopy, Rueggeberg & Craig, 1988). The reason why more TEGDMA remained uncured at VL curing, compared to Bis-GMA, is not well understood and is still under investigation (Table 2). As demonstrated by Inouc & Hayashi (1982) for chemically cured composites, limited amounts of the residual monomers eluted from the photo-cured composite into water over a 7-day period, but most residual monomers remained in the set composite (Table 3). It would therefore be of great interest to study the effect of the latter on the long-term serviceability of the VL-cured composite. The trend for residual TEGDMA to dissolve from the set composite into water differed from that for residual Bis-GMA (Fig. 5), possibly due to the different mobility of each monomer inside the set composite stored in water. Lower-molecular-weight TEGDMA appears to flow more easily into water than higher-molecular-weight Bis-GMA (Ban et al, 1986). When exposed to water for more than 7 days, it is possible to speculate from the curves obtained (Fig. 5) that trapped residual TEGDMA may remain inside the set composite, and that residual Bis-GMA may elute fractionally from the set composite into water, the dissolution rate diminishing with time and eventually becoming negligible. It should be noted here that a small proportion of the residual monomers trapped in the set composite may be consumed by post-irradiation curing. The reaction mechanism also requires investigation.
Residual monomers of VL-cured composite resin
361
It is recommended that dental clinicians apply VL irradiation to the composite examined for 30 s. However, extending the photo-curing time to 50s significantly decreases the residual monomer level and its elution into water, which may enhance the clinical performance of the VL-cured composite. Conclusions
(i)
Gas chromatography identified high concentrations of residual TEGDMA monomer and small amounts of residual Bis-GMA monomer in a set VL-cured dental composite resin that contained two monomers as the resin matrix. (ii) When stored in water at 37 °C for 7 days, minor quantities of the residual monomers eluted from the photo-cured composite into water. The dissolution rate was initially rapid, and decreased with time, but the majority of the residual monomers remained in the hardened composite. (iii) Increasing the period of VL irradiation from 30 s to 50 s resulted in a significant decrease in residual monomer levels and the rate at which they were eluted into water.
References (1983) Factors affecting the color stability of restorative resins. Acta Odontologica Scandinavica, 41. 11. BAILEY, E . . FARMER. P.B.. PEAE. J.A.. HOTCHKISS, S.A. & CAEDWELE. J. (1987) Analytical methodology to determine stable isotopically labelled and unlabelled theophylhne in human plasma using capillary gas chromatography-mass spectrometry. Journal of Chromatography Biomedical Applications, 416. 81. BAKER, S.. BROOKS, S.C. & WAEKER. D . M , (1988) The release of residual monomerie methyl methacrylate from acryhc appliances in the human mouth: an assay for monomer in saliva. Journal of Dental Research, 67, 129.^^. BAN, S., KATO, H . , IINO, S. & HASEGAWA, J. (1986) Release of dimethacrylate monomers into water. Aichigakuin Journal of Dental Science, 24, 383. BEOCH, W.W., AUSTIN, J.C., CEEATON-.1ONES. P.E., WIETON-COX, H . & FAITI, L.P. (1977) Pulpal response to a new visible light-cured composite restorative material: Fotofil . Journal of Oral Pathology, 6, 278. DouGEAS, W.H. & BATES, J.F. (1978) The determination of residual monomer in polymethylmethacrylate denture-base resins. Journal of Materials Science, 13, 26(KK FERRACANE, J.L., MOSER. J.B. & GREENER, E.H. (1985) Ultraviolet light-induced yellowing of dental restorative resins. Journal of Prosthetic Dentistry, 54. 483. ASMUSSEN. E .
FEETCHER, A . M . ,
PURNAVEJA,
S.,
AMIN. W.M.,
RITCHIE, G.M..
MORADEANS. S. & WODD.
A.W.
(1983) The level of residual monomer in self-curing denture-base materials. Journal of Dental Research, 62. 118. HANKS, C.T., CRAIG, R.G , DIEHE. M . L . & PASHEEY, D.H. (1988) Cytotoxicity of dental composites and other materials in a new in vitro device. Journal of Oral Pathology, 17. 396. HIRABAYASHI, S., HIRASAWA, T . . NASU. I. & NAKANISHI, S. (1984) Compositions of various visible light-cured composite resins and elutions of residual monomers from these cured resins (in Japanese). Journal of Japanese Society for Denial Materials and Devices, 3, 655. HIRASAWA, T . . NASU, I., HIRABAYASHI. S. & HARASHIMA. I. (1984) The method of quantitative analysis
of residual monomer in dental resin using thin film samples (in Japanese). Journal of Japanese
Society for Dental Materials and Devices, 3. 243. INOUE. K. & HAYASHI. 1. (1982) Residual monomer (Bis-GMA) of composite resins. Journal of Oral Rehabililation, 9, 493. RouEET. J.-F. (1987) Degradation of Dental Polymers, p. 6(1. Karger. Basel. RuECGEBERG. F.A. & CRAIG, R.G. (1988) Correlation of parameters used to estimate monomer conversion in a light-cured composite. Journal of Dental Research, 67. 932. RUYTER, I.E. & 0YS/ED, H . (1987) Composites for use in posterior teeth: composition and conversion. Journal of Biomedical Materials Research, 21. 11.
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SADAMORI, S., YAMAGAMI, T . , NIKAWA, H . , FURUEBISU, M . , SHIGETO, N . & HAMADA, T . (1987) Studies
on residual monomer in dental acrylic resins. Part 3. Curing methods and residual monomer (in
Japanese). Journal of Hiroshima University Dental Society, 19, 471. SHINTANI, H . , INOUE, T . & YAMAKI, M . (1985) Analysis of camphorquinone in visible light-cured composite resins. Dental Materials. 1, 124. STAFFORD, G . D . & BROOKS, S.C. (1985) The loss of residual monomer from acrylic orthodontic resins. Dental Materials, 1, 1.35. STANLEY, H.R., BOWEN, R.L. & FOLIO, J. (1979) Compatibility of various materials with oral tissues. II. Pulp responses to composite ingredients. Journal of Dental Research, 58, 1507. TAIRA, M . , URABE, H . , HIROSE, T., WAKASA, K. & YAMAKI, M . (1988) Analysis of photo-initiators in visible-light-cured dental composite resins. Journal of Dental Research, 67, 24.
Residual monomers (TEGDMA and Bis-GMA) of a set visible-light-cured dental composite resin when immersed in water K. TANAKA, M. TAIRA*, H. SHINTANI, K. WAKASA* M . Y A M A K I * Department of Operative Dentistry and * Department of Dental Materials, Hiroshima University School of Dentistry, Hiroshima, Japan
Summary
Rods of a visible-light-cured dental composite resin were photo-polymerized and immersed in water at 37 °C for 7 days. The residual monomers (TEGDMA and BisGMA) trapped in the set composite and those eluted into water were analysed by gas-liquid chromatography. It became evident that minor amounts of the residual monomers dissolved in water, but that most residual monomers remained in the set composite. Extension of the irradiation period contributed to the significant reduction in the residual monomer level and its elution into water. Introduction
Visible-light (VL)-cured dental composite resin has been widely accepted as a restorative material due to its ease of handling and its aesthetic merits. Concern about its clinical reliability, however, still remains. Unreacted monomers that leach from the polymerized materials may irritate the soft tissue (Bloch et al, 1977; Stanley, Bowen & Folio, 1979; Hanks et al, 1988). Furthermore, monomers trapped in the set composite may reduce the clinical serviceability of the composite through oxidation and hydrolytic degradation, which may be manifested in forms such as discoloration of the fillings (Asmussen, 1983; Ferracane, Moser & Greener, 1985) and accelerated wear (Roulet, 1987). Thus it appears important to measure the residual monomers of the hardened VL-cured composite held in the aqueous environment. For chemical analysis of the resin matrix in the dental composite consisting of multi-functional methacrylate monomers such as triethyleneglycol dimethacrylate (TEGDMA) and Bisphenol A glycidyl dimethacrylate (Bis-GMA), high-performance liquid chromatography (HPLC) has usually been employed (Hirabayashi et al, 1984; Ruyter & 0ysa;d, 1987). Using HPLC techniques, Inoue and Hayashi (1982) have chemically analysed not only residual Bis-GMA in the set chemically cured dental composites, but also its elution into water. However, it is generally not easy to identify accurately the trace monomers, such as those dissolved from the set composite into water, by HPLC. Another possible technique is gas-liquid chromatography (GC), although the latter has more often been utilized to detect lower-molecular-weight chemical substances such as the monomeric methyl methacrylate released from acrylic appliances (Douglas & Bates, 1978; Fletcher et al, 1983; Stafford & Brooks, 1985; Correspondence: Dr K. Tanaka, Department of Operative Dentistry, Hiroshima University School of Dentistry, 1 - 2 - 3 Kasumi Minami-ku, Hiroshima-shi 734, Japan.
353
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Sadamori et al, 1987; Baker, Brooks & Walker, 1988) and photo-initiators in the VL-cured composites (Shintani, Inoue & Yamaki, 1985; Taira et al, 1988). Therefore, the aims of the present study were as follows: (i) by means of GC, to develop a method of measuring tiny amounts of the monomers derived from the set VL-cured dental composite resin; (ii) to measure the quantity of residual monomers left in the composite at photocuring; (iii) to examine the manner in which the residual monomers in the photo-cured composite stored in water elute into water as a function of time. To confirm the GC peaks of the monomers, mass-spectroscopy (MS) was used. In addition, the effects of the irradiation time (30s and 50s) on the residual monomers of the set composite soaked in water were examined. Materials and methods
The VL-cured dental composite resin to be examined was universal shade Photoclearfil A*. Table 1 lists its composition by weight. The inorganic filler content (88% by weight) and the organic fraction (12% by weight) were initially checked by thermogravimetric thermal analysis and HPLC, respectively. The resin matrix of the composite studied basically consisted of TEGDMA and Bis-GMA. For the chemical analyses, two monomer standards of TEGDMAt and Bis-GMA+ were used without further purification. Figure 1 illustrates the procedures for preparation of three kinds of sample. (a) A small portion of the uncured composite resin paste was dissolved in acetone, the solution was then centrifuged, and an aliquot was injected into a gas-liquid chromatographic column for qualitative confirmation of the GC peaks of monomers in the composite. (b) Specimen rods, 6mm in diameter and 3 mm high, were prepared by use of a stainless steel mould. The mould was first filled with the material, the surface of the material open to the air was covered with a polyester film and the material was photo-cured by means of a VL source§. The irradiation period was varied for 30s and 50 s. The hardened rods were then ground by use of a pestle and a mortar, and the resultant powders were immediately dipped in sealed pure
Table I. The composition b) weight (%) of the commercial visiblc-light-cured composite resin that was examined Comptxicnt Inorganic filler* BisGMA TEGDMA Others
Composition by weight {^Vn 88-0 9() 24 0-6
* Determined at 575°C. * t t §
Lot number 1111, Kuraray Co.. Osaka, Japan. Lot number DOOl. 99 9% pure, Tokyo Kasci Co.. Tokyo, Japan. Lot number 1209B, industrially pure grade. Shin-Nakamura Co., Wakayama, Japan. Quick-light model VL-1, Morita Co., Kyoto, Japan.
Residual monomers of VL-cured composite resin (b)
(c)
Uncured composite
Uncured composite
Uncured composite
Dissolved in ocetone
Rod VL cured tor 30s, 50s
Rod VL cured for 30 s, 50 s
Centrifuge
Ground to powders
Immersed in water tar up ta 7 days
Aliquot taken
Immersed in methonol for 7 days
(a)
GC-MS anolysis
GC analysis
355
Elutian taken out at I, 3, 5, 7 days later Chlorafarm added and shaken Acetone added and shaken Oentrifuge and separating funnel Solvent phase treezedried in vacuum JL. Residue dissolved in acetone
i GC analysis
Fig. 1. Procedures for the chemical analysis of (a) the monomers in the uncured composite, (b) the residual monomers remaining in the composite at photo-curing, and (c) the residual monomers that eluted from the set composite into water.
methanol for 7 days for the measurement of the residual monomers left in the composite at photo-curing. (c) VL-cured rods were also immersed in the distilled water for up to 7 days. The water, including the elution of the monomers, was removed 1, 3, 5 and 7 days later. To extract the monomers from water, two solvents, chloroform and acetone, were added stepwise, the mixed solution was shaken well and the two phases of solvent and water were separated using a centrifuge and a separating funnel. The solvent phase fully absorbing the monomers from water was then freeze-dried in vacuo to evaporate the solvents. Acetone was added to the residue to provide the solution for chemical analysis of the monomers eluted from the composite into water. GC was performed using a gas-liquid chromatograph* under the following experimental conditions: detector, F.I.D. (hydrogen flame); capillary column, CBP-1 W25-300* (25 m x 0-53 mm, 0-30 jxm film thickness); injection port temperature, 310°C; column oven temperature, programmed from 100 to 230°C at 30°Cmin~', and from 230 to 310°C at 5°Cmin~'; carrier gas, 25ml min ' He; sensitivity, 10 MQ. MS was performed on a gas-liquid chromatograph-quadrupole mass spectrometer-computer data
* Shimadzu Co., Kyoto, Japan.
356
K. Tanaka et al.
system*, under the GC conditions outlined above, and the following MS conditions: electron energy, 70 eV; ion source temperature, 250°C; current, 60 ^lA. The procedure for chemical analysis was as follows. When part of the GC chromatogram for the specimen prepared from the uncured composite correlated closely with regard to characteristic retention time with the GC chromatogram of a monomer standard, it was assumed that the former contained the latter. This was confirmed by the fragmentation patterns of MS coupled with GC. Ouantitation of the residual monomers was then achieved solely by GC, by measuring the peak area of the target monomer extracted from the cured composite and relating this to the calibration line between peak area and concentration constructed previously for the pure monomer standard. All quantitative measurements were repeated three times. Results
Figure 2 shows the mass chromatogram together with the total ion chromatogram (TIC) of the acetone extract obtained from the uncured composite. The TIC is equivalent to the GC chromatogram. Two peaks of TEGDMA and Bis-GMA could
TEGDMA
TIC
BIS-GMA
i
i
Moss
- 69 - 113 143
A-A10
15
20 25 Time (mm)
30
35
Fig. 2. Mass chromatogram and total ion chromatogram (TIC) of the acetone extract obtained from the uncured composite specimen. Shimadzu GCMS-2(XK)A. Shimadzu Co., Kyoto. Japan.
Residtial monomers of VE-cured composite resin
357
be identified because the 'fingerprint' m/e tracing in the mass chromatogram (such as 69 and 113 m/e for TEGDMA and 69 and 143 m/e for Bis-GMA) corresponded closely with the electron-impact mass spectra of their pure standards, as shown in Fig. 3. Two monomers had different GC properties. Although TEGDMA appeared as a single strong peak, Bis-GMA tended to emerge as a few weaker peaks. Figure 4 shows GC chromatograms of (a) the methanol extract containing the residual monomers that remained in the composite at photo-curing, and (b) the acetone extract containing the residual monomers that eluted from the set composite into water. Table 2 shows the proportions of the residual monomers left in the composite specimen at VL curing. It is clear that significant quantities of residual TEGDMA monomer and small amounts of residual Bis-GMA monomer existed in the once photo-cured composite, and that increasing the duration of VL irradiation led to a substantial decrease in residual monomer levels.
( a ) TEGDMA = 286 M+ CH3
CH3
41
1001- 41
69
in
0 80
30
130
:b) Bis-GMA =: 512 M+ CH3
100 r
230
260
CH3
CH3
I OH
69
180 ;nn/e)
OH
CH3
143
69 41
c 50 o to
0
30
ii.i...
130
,..
li,.
230
330
430
530
(m/e)
Fig. 3. Mass spectra of two monomer standards: (a) TEGDMA and (b) Bis-GMA.
358 (0)
K. Tanaka et al. Residuol monomer 0-47 Meihanol
577 TEGDMA 5-99
a> 20-01
(b)
BIS-GMA
Residual monomer into water • 0'24 Acetone
5-85 TEGDMA
cc
BIS-GMA
Fig. 4. GC chromatograms of (a) the methanol extract containing the residual monomers remaining in the composite specimen at VL curing, and (b) the acetone extract containing the residua) monomers that eluted from the photo-cured composite resin into water.
Table 2. Proportions of the residual monomers remaining in the composite specimen at photo-curing, relative to the (original concentration of each monomer in the uncurcd composite Proportion by weight (%) 3()-s irradiation
5()-s irradiation
Rcsiduul TEGDMA
23-5
12-8
Residual Bis-GMA
41
13
Residual monomers of VE-cured composite resin
359
Figure 5 shows the trend for two residual monomers, TEGDMA and Bis-GMA, to elute from the photo-cured composite into water over a period of 7 days. There was extensive elution of residual TEGDMA into water during the initial 1—2 days, but its dissolution subsequently levelled off. Residual Bis-GMA continued to elute into water over a longer time period. For the specimen that had been photo-cured for 30 s more residual monomers eluted into water, compared to the specimen that had been VL-activated for 50 s. Table 3 shows the ratio of the quantity of residual monomers eluted into water over a 7-day period to the amount of residual monomers left at VL-curing. It became apparent that minor amounts of the residual monomers dissolved into water, but that the majority of the residual monomers remained in the set composite stored in water. Discussion
By using GC we were able to measure not only the residual monomers remaining in the composite at photo-curing, but also the trace residual monomers that eluted from the set composite into water. This may be attributed to the use of the capillary
TEGDMA
•0
0-5
50-s irradiation
0
o
3
0-10 BIS-GMA
30-s irradiation
0-05
0
Time (days)
Fig. 5. Concentration profiles of the residual monomers, (a) TEGDMA and (b) Bis-GMA. that eluted from the photo-cured composite into water, plotted against time. Note that the quantity of each residual monomer is expressed relative to the original amount of monomer present in the uncured composite.
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Table 3. Ratio of the quantity of each residual monomer elutcd into water over a 7-day period to the amount of that monomer remaining at VL curing Percentage ratio 3()-s irradiation
50-s irradiation
Eluted TEGDMA/ residual TEGDMA
6 21
1-88
Eluted Bis-GMA/ residual Bis-GMA
2-17
1-69
column, which is suitable for microanalysis of the organic materials with high boiling points (Bailey et al, 1987). The rationale for the use of three solvents is as follows. With regard to the solvent extraction method, for rapid extraction of the monomers dissolved in water, acetone and chloroform were effective solvents, but methanol was a weaker solvent. However, the former solvents tended to degrade two monomers, particularly Bis-GMA, rapidly, so that it was necessary to analyse them chemically immediately following sample preparation. For extraction of the residual monomers from the set composite, we found methanol to be more satisfactory than the other two strong solvents. Methanol disintegrates the monomer to a lesser extent during chemical analysis, while it requires a long time period (e.g. 7 days) to absorb the monomers fully from the set composite (Hirasawa et al, 1984). Therefore solvents should be used selectively according to the specific requirements, an approach that was adopted in this study. The amounts of residual monomers left in the composite specimen at photo-curing were comparable to those reported by other workers using different experimental techniques (e.g. Fourier transform infra-red (FTIR) spectroscopy, Rueggeberg & Craig, 1988). The reason why more TEGDMA remained uncured at VL curing, compared to Bis-GMA, is not well understood and is still under investigation (Table 2). As demonstrated by Inouc & Hayashi (1982) for chemically cured composites, limited amounts of the residual monomers eluted from the photo-cured composite into water over a 7-day period, but most residual monomers remained in the set composite (Table 3). It would therefore be of great interest to study the effect of the latter on the long-term serviceability of the VL-cured composite. The trend for residual TEGDMA to dissolve from the set composite into water differed from that for residual Bis-GMA (Fig. 5), possibly due to the different mobility of each monomer inside the set composite stored in water. Lower-molecular-weight TEGDMA appears to flow more easily into water than higher-molecular-weight Bis-GMA (Ban et al, 1986). When exposed to water for more than 7 days, it is possible to speculate from the curves obtained (Fig. 5) that trapped residual TEGDMA may remain inside the set composite, and that residual Bis-GMA may elute fractionally from the set composite into water, the dissolution rate diminishing with time and eventually becoming negligible. It should be noted here that a small proportion of the residual monomers trapped in the set composite may be consumed by post-irradiation curing. The reaction mechanism also requires investigation.
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It is recommended that dental clinicians apply VL irradiation to the composite examined for 30 s. However, extending the photo-curing time to 50s significantly decreases the residual monomer level and its elution into water, which may enhance the clinical performance of the VL-cured composite. Conclusions
(i)
Gas chromatography identified high concentrations of residual TEGDMA monomer and small amounts of residual Bis-GMA monomer in a set VL-cured dental composite resin that contained two monomers as the resin matrix. (ii) When stored in water at 37 °C for 7 days, minor quantities of the residual monomers eluted from the photo-cured composite into water. The dissolution rate was initially rapid, and decreased with time, but the majority of the residual monomers remained in the hardened composite. (iii) Increasing the period of VL irradiation from 30 s to 50 s resulted in a significant decrease in residual monomer levels and the rate at which they were eluted into water.
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