Journal of Oral Rehabilitation, 1990, Volume 17, pages 487-494
Correlation between degree of conversion, filler concentration and mechanical properties of posterior composite resins K . - H . C H U N G an
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. •••
colour stability and biocompatibility, the degree of conversion may play an important role in the success of the restoration (Lutz et al., 1984; Smith, 1985). Stupp and Weertman (1979) reported the degree of conversion of nine proprietary chemically cured composites, which ranged from 30-70%. Ferracane and Greener (1984) studied the unfilled Bis-GMA based resins and found that the degree of conversion for those experimental resins varied between 55—72%. Recently, Ruyter (1985) reported the degree of conversion of five posterior composites, which ranged from 55-73%. With regard to the development of dental composites, Bowen (1956, 1963, 1964) has shown that an increase in filler fraction increased the modulus and strength, while it reduced the setting contraction and thermal expansion. For dental composites, the value of compressive strength was reported to be correlated approximately with dimensional change and wear (Lee & Orlowski, 1977). Vougiouklakis and Smith (1980) found that some of the Bis-GMA-based resins have strength properties comparable with tooth structure. A negative correlation between wear and tensile strength was found in a chnical evaluation (Goldberg et al., 1983). Positive correlations between elastic modulus, proportional limit, tensile strength and filler concentration were observed during an investigation of the mechanical properties of dental composites (Boyer, Chalkley & Chan, 1982); a small negative correlation was also found between the compressive strength and the filler content in a study of 10 commercial composites, all of which were Bis-GMA- or UEDMA-based resins with filler loadings of 20—65% by volume. Nagata, Lundeen and Turner (1984) revealed that maximum diametral tensile strength (55 MPa) occurred with 75% by weight filler content, and maximum compressive strength (315 MPa) with 80% by weight filler content, in Bis-GMA-based dental composites. Several studies have demonstrated an apparent correlation between increasing diametral tensile strength and hardness and decreasing concentrations of unreacted methacrylate groups in Bis-GMA based resins (Caul, Sweeney & Paffenbarger, 1956; Moser & Greener, 1973; Asmussen, 1982; Ferracane, Newman & Greener, 1982). A positive correlation has also been established between hardness and inorganic filler content (Raptis, Fan & Powers, 1979; Boyer, Chalkley & Chan, 1982; Roberts & Shaw, 1984). In fact, increasing the content of the reinforcing filler and development of bulky and rigid monomer systems are the usual methods of producing dental composites for utilization as posterior restorations. Thus there appears to be ample experimental evidence to support a correlation between the filler concentration, the properties of filled composites and the degree of conversion to properties in the unfilled resin. However, there is very little information in the literature that attempts to correlate degree of conversion, filler concentration and mechanical properties in highly filled dental composites. Materials and methods
Seven commercially available composite resins were used in this study. All these materials are visible Hght activated composites, intended for use as posterior restorations clinically. They are all one-paste systems. A list of composites used, with details of manufacturers and batch numbers, is given in Table 1. The polymerization reaction of the composites was monitored with a Fourier
Posterior composite resins
489
Table 1. Materials used in the study Composite material
Batch number
Manufacturer
Marathon (shade: medium) Ful-fil Compules P-30
248001
DenMat, P.O. Box 1729, Santa Maria, CA 93456, U.S.A. L.D. Caulk Co., Milford, DE 19963, U.S.A. 3M Dental Product, St Paul, MN 55144, U.S.A. Kulzer I Co., GmbW, W. Germany
.
0424841 032RR4 :
Estilux Posterior (shade: YO) Sinterfil (shade: universal) Occlusin Bis-fil I (shade: universal)
201133 40664 March, 84 100584
.
Teledyne Getz, Elk Grove Village, IL 60007, U.S.A. Imperial Chemical Industries, ICI, U.K. ., , Bisco, Lombard, IL 60515, U.S.A.
Transfrom Infrared (FTIR) spectrometer (FX 6200)* in transmission mode, and then converted to absorbance. The thin-film technique (Ferracane & Greener, 1984; Chung, 1985) and standard baseline procedure (Heighl, Bell & White, 1947) were used to calculate the degree of conversion. Each specimen was cured with a visible light cured unit, Visar 2, for 60 s. The degree of conversion was calculated by comparing the absorbance ratio of the aliphatic C=C peak at 1638-6 cm"^ with the unchanging aromatic ring C=C peak at 1609-4 cm~^ for the pre- and post-polymerized resins (Chung & Greener, 1988). The inorganic filler content of the composites was determined by the gravimetric ashing technique (Chung, 1985), in which the difference in weight is compared before and after ashing of specimen discs 6 mm and 3 mm in diameter at 700°C. The density of the filler was measured pycnometrically. The density of the resins and the filler volume (a's a percentage of the composites) were calculated from the measured density, filler density and filler weight (%) of five discs of each material. Cylindrical specimens were prepared for compressive and diametral tensile strength tests by curing the composite resin in glass moulds of length 6-5 mm, internal diameter 3 mm and external diameter 4 mm, with three pieces of white filter paper as the background. A Visar 2 lightt with a 60-s exposure along the length of the tubing was used to cure the composite specimen. The specimens were removed from the tubing and stored in the dark in tap water at 37°C for 24 h. Prior to testing, the ends of the cylindrical specimens were finished on 600 grit silicon carbide paper to remove any air-inhibited layers of resin, and to produce parallel ends. The strength tests were performed using an Instron universal testing machine^ with a cross-head speed of 0-1 inch min~'. Specimens were prepared for Knoop hardness (KHN) tests by curing the composite in 6 mm x 3 mm Teflon moulds, with the upper and lower surfaces covered with glass
* Analect Instruments, Irvine, California, U.S.A. t Denmat Co., Santa Maria, California, U.S.A. X Instron Corp., Canton, Massachusetts, U.S.A.
490
K.-H. Chung and E. H. Greener
sUdes 1-2 mm in thickness*. The curing procedures were similar to the method for preparation of cyhndrical specimens. After light curing, the specimens in the Teflon moulds were stored in the dark at 37°C in tap water for 24 h before testing. Each hardness reading was made at 24±1°C under a 200-g load on a microhardness tester (M12a metallurgical microscope)!, using a Knoop diamond indenter. Five values were recorded for the top surface of five specimens for each composite. The results of the degree of conversion, filler content, strength and hardness tests were analysed by ANOVA and Scheffe's tests at the P
Correlation between degree of conversion, filler concentration and mechanical properties of posterior composite resins K . - H . C H U N G an
: --^
v— -
>
. •••
colour stability and biocompatibility, the degree of conversion may play an important role in the success of the restoration (Lutz et al., 1984; Smith, 1985). Stupp and Weertman (1979) reported the degree of conversion of nine proprietary chemically cured composites, which ranged from 30-70%. Ferracane and Greener (1984) studied the unfilled Bis-GMA based resins and found that the degree of conversion for those experimental resins varied between 55—72%. Recently, Ruyter (1985) reported the degree of conversion of five posterior composites, which ranged from 55-73%. With regard to the development of dental composites, Bowen (1956, 1963, 1964) has shown that an increase in filler fraction increased the modulus and strength, while it reduced the setting contraction and thermal expansion. For dental composites, the value of compressive strength was reported to be correlated approximately with dimensional change and wear (Lee & Orlowski, 1977). Vougiouklakis and Smith (1980) found that some of the Bis-GMA-based resins have strength properties comparable with tooth structure. A negative correlation between wear and tensile strength was found in a chnical evaluation (Goldberg et al., 1983). Positive correlations between elastic modulus, proportional limit, tensile strength and filler concentration were observed during an investigation of the mechanical properties of dental composites (Boyer, Chalkley & Chan, 1982); a small negative correlation was also found between the compressive strength and the filler content in a study of 10 commercial composites, all of which were Bis-GMA- or UEDMA-based resins with filler loadings of 20—65% by volume. Nagata, Lundeen and Turner (1984) revealed that maximum diametral tensile strength (55 MPa) occurred with 75% by weight filler content, and maximum compressive strength (315 MPa) with 80% by weight filler content, in Bis-GMA-based dental composites. Several studies have demonstrated an apparent correlation between increasing diametral tensile strength and hardness and decreasing concentrations of unreacted methacrylate groups in Bis-GMA based resins (Caul, Sweeney & Paffenbarger, 1956; Moser & Greener, 1973; Asmussen, 1982; Ferracane, Newman & Greener, 1982). A positive correlation has also been established between hardness and inorganic filler content (Raptis, Fan & Powers, 1979; Boyer, Chalkley & Chan, 1982; Roberts & Shaw, 1984). In fact, increasing the content of the reinforcing filler and development of bulky and rigid monomer systems are the usual methods of producing dental composites for utilization as posterior restorations. Thus there appears to be ample experimental evidence to support a correlation between the filler concentration, the properties of filled composites and the degree of conversion to properties in the unfilled resin. However, there is very little information in the literature that attempts to correlate degree of conversion, filler concentration and mechanical properties in highly filled dental composites. Materials and methods
Seven commercially available composite resins were used in this study. All these materials are visible Hght activated composites, intended for use as posterior restorations clinically. They are all one-paste systems. A list of composites used, with details of manufacturers and batch numbers, is given in Table 1. The polymerization reaction of the composites was monitored with a Fourier
Posterior composite resins
489
Table 1. Materials used in the study Composite material
Batch number
Manufacturer
Marathon (shade: medium) Ful-fil Compules P-30
248001
DenMat, P.O. Box 1729, Santa Maria, CA 93456, U.S.A. L.D. Caulk Co., Milford, DE 19963, U.S.A. 3M Dental Product, St Paul, MN 55144, U.S.A. Kulzer I Co., GmbW, W. Germany
.
0424841 032RR4 :
Estilux Posterior (shade: YO) Sinterfil (shade: universal) Occlusin Bis-fil I (shade: universal)
201133 40664 March, 84 100584
.
Teledyne Getz, Elk Grove Village, IL 60007, U.S.A. Imperial Chemical Industries, ICI, U.K. ., , Bisco, Lombard, IL 60515, U.S.A.
Transfrom Infrared (FTIR) spectrometer (FX 6200)* in transmission mode, and then converted to absorbance. The thin-film technique (Ferracane & Greener, 1984; Chung, 1985) and standard baseline procedure (Heighl, Bell & White, 1947) were used to calculate the degree of conversion. Each specimen was cured with a visible light cured unit, Visar 2, for 60 s. The degree of conversion was calculated by comparing the absorbance ratio of the aliphatic C=C peak at 1638-6 cm"^ with the unchanging aromatic ring C=C peak at 1609-4 cm~^ for the pre- and post-polymerized resins (Chung & Greener, 1988). The inorganic filler content of the composites was determined by the gravimetric ashing technique (Chung, 1985), in which the difference in weight is compared before and after ashing of specimen discs 6 mm and 3 mm in diameter at 700°C. The density of the filler was measured pycnometrically. The density of the resins and the filler volume (a's a percentage of the composites) were calculated from the measured density, filler density and filler weight (%) of five discs of each material. Cylindrical specimens were prepared for compressive and diametral tensile strength tests by curing the composite resin in glass moulds of length 6-5 mm, internal diameter 3 mm and external diameter 4 mm, with three pieces of white filter paper as the background. A Visar 2 lightt with a 60-s exposure along the length of the tubing was used to cure the composite specimen. The specimens were removed from the tubing and stored in the dark in tap water at 37°C for 24 h. Prior to testing, the ends of the cylindrical specimens were finished on 600 grit silicon carbide paper to remove any air-inhibited layers of resin, and to produce parallel ends. The strength tests were performed using an Instron universal testing machine^ with a cross-head speed of 0-1 inch min~'. Specimens were prepared for Knoop hardness (KHN) tests by curing the composite in 6 mm x 3 mm Teflon moulds, with the upper and lower surfaces covered with glass
* Analect Instruments, Irvine, California, U.S.A. t Denmat Co., Santa Maria, California, U.S.A. X Instron Corp., Canton, Massachusetts, U.S.A.
490
K.-H. Chung and E. H. Greener
sUdes 1-2 mm in thickness*. The curing procedures were similar to the method for preparation of cyhndrical specimens. After light curing, the specimens in the Teflon moulds were stored in the dark at 37°C in tap water for 24 h before testing. Each hardness reading was made at 24±1°C under a 200-g load on a microhardness tester (M12a metallurgical microscope)!, using a Knoop diamond indenter. Five values were recorded for the top surface of five specimens for each composite. The results of the degree of conversion, filler content, strength and hardness tests were analysed by ANOVA and Scheffe's tests at the P
