Dent Mater 8274-277, July, 1992
A comparison of monomer conversion and inorganic filler content in visible light-cured denture resins D. J. Barron ~, F. A. Rueggeberg 2, G. S. Schuster ~ Department of Oral Biology, ~Department of Restorative Dentistry, Section of Dental Materials, Medical Collegeof Georgia, School of Dentistry, Augusta, Georgia, USA
Abstract. Visible light-cured (VLC) denture resins are relatively new products used for the reline and repair of dentures. The conversion of monomer into polymer in 3 brands of visible lightcured denture resins was investigated. The relationship of the inorganic filler content to this conversion was also studied. It was determined that these reline materials vary in monomer conversion and weight percentage of filler, and this variation is brand dependent. The monomer conversion ranged from 77% to 97%. Significant differences in these values were found when duration of light exposure was increased. In addition, resin nearest to the light source polymerized to a greater extent when compared to resin that was 1 mm deep to this surface, hence furthest from the light source. The inorganic filler content ranged from 0% to 15%. For the resin systems studied, the relationship between monomer conversion and inorganic filler loading was inversely proportional, Results indicated that monomer conversion of VLC repair resins was affected by the duration of light exposure as well as the amount of inorganic filler present in the material,
INTRODUCTION Visible light-cured (VLC) denture resin systems were introduced in 1983. These materials are marketed with the purported advantages of more efficient handling, fewer patient appointments, elimination of the potential toxicity of methyl methacrylate, and less dimensional change upon curing than occurs in conventional heat-cured denture resins, The chemical compositions of various VLC brands differ widely. Smith and Powers (1991) have shown these products to contain constituents such as urethane dimethacrylate, poly(ethyl-methacrylate), and ethoxylated bisphenol A dimethacrylate. Since 1983, use of these materials has increased greatly, They are currently being used to fabricate rigid connections between implant transfer copings (Ivanhoe et al., 1991), transfer copings used on stone dies (Passon and Goldfogel, 1991), for orthodontic appliances, oral obturators (DaBreo, 1991), custom impression trays, provisional restorations (Prestipino, 1989), denture baseplates, as well as for vertebral implants (Alsawafet al., 1991). Monomer conversion of resin materials is the degree to which carbon double bonds (C=C) are converted into carbon single bonds (C-C). This conversion occurs during polymerization, and the degree to which this occurs greatlyinfluences not only the physical properties of the resulting polymer (Asmussen, 1982a; 1982b), but also affects its biocompatibility. 274 Barron et aL/Monomer conversion~filler in light-cured resins
The degree of cure of restorative composites has been shown tobedirectlyrelatedtothelevelofcellularbiocompatibilityin vitro (Caughman et al., 1991). As the degree of monomer conversion of composites decreases, protein synthesis by gingiva] fibroblasts also decreases. Similarly, Lefebvre et al. (1991) found differences in the biocompatibility of various VLC denture materials in vitro. However, until now there have been no studies examining the differences in monomer conversion or filler concentrations among the various VLC denture resin products. Inorganic fillers are added to restorative resins for the purpose of increasing wear resistance and adding bulk. Such fillers also are found in VLC denture resin materials. The proportion of filler has yet to be determined, but its presence could affect the degree to which light can penetrate the resin and cause polymerization. Therefore, the relationship between inorganic filler content and monomer conversion also may be an important factor affecting these resin products. The purpose of this investigation was to determine the monomer conversion and inorganic filler content of various brands of VLC denture resins. A possible relationship between these two parameters was also studied. MATERIALS AND METHODS Three VLC systems were tested, as listed in Table 1. Astron and Extoral are both pour-type materials, and utilize a cornbination of chemical and light polymerization (Smith and Powers, 1991, Lefebvre et al., 1991). Triad is polymerized solelyby exposure to visible light andis available in preformed ropes or in sheets. Monomer conversion. Disc-shaped specimens (10 mm diam x 1 mm thick) were fabricated for each material using an aluminum mold. Where applicable, the resin specimens were mixedaccordingtothemanufacturer'sdirections. Mylar(0.06 mm thick, Du Pont Corporation, Wilmington, DE, USA) was placed on the bottom of the mold. The cylindrical cavity was then packed with resin, and another Mylar sheet was placed over the top. The assembly was then pressed so that the resin took the shape and dimensions of the mold. The aluminum mold with uncured resin and Mylar strips was then placed in a Triad II curing unit (Caulk/Dentsply, Milford, DE, USA) for the Triad specimens. The other two resins were polymerized in a Pro-Lite 3 fluorescent unit (Pro-Den Systems, Portland, OR, USA). The duration of light exposure was 10 min. Following polymerization, the Mylar sheets were removed
TABLE 1: CHARACTERISTICSOF VISIBLE LIGHT-CUREDDENTURE
DIRECTION OF INCIDENT LIGHT
MATERIALS USED Material
Powder/Liquid Batch Ratio No. Astron LCH 2.5/1 P-181 L-329
Chemical Manufacturer Composition* Astron Corp. PEMA Wheeling,IL EBDMA
Extoral
2.0/1
P-N/A L-7
Pro-DenSyst. Portland, OR
Triad
single paste
UDUEAA Dentsplylnter. UDMA
PEMA EBDMA
York, PA
PEMA: Polyethyl methacrylate EBDMA: Ethoxylated bis phenol A methacrylate UDMA: Urethane Dimethacrylate *Smith and Powers, 1991.
and the cured discs were allowed to remain in the aluminum mold. A method was developed to determine the monomer conversion of the resin surfaces that were nearest to (superficial resin) and farthest from (lmm deep) the light sources. A diagram of the assembly used is shown in Fig. 1. A small amount of freshly mixed (or freshly dispensed) resin was placed between two sheets of Mylar and pressed into a thin wafer (approximately 30 ~m thick). Two ofthese"sandwiches" (uncured resin between Mylar sheets) were made. The first "sandwich" of resin was placed on the counter top and a small amount of immersion oil (Type B, Cargille Laboratories, Inc., Cedar Grove, NJ, USA) was spread on the upper Mylar surface. The resin in one "sandwich" was labeled as the uncured deep resin wafer (Fig. 1). The 1 mm thick polymerized resin disc was then placed directly over this sandwich and pressed down, spreading the immersion oil into a thin layer, A small amount ofimmersion oil was then placed on the upper surface of the polymerized resin disc and the second sandwich of uncured resin was pressed into place on top of this disc (Fig. 1). This resin was labeled as the superficial resin wafer, Care was taken to ensure that immersion oil did not come in contact with the uncured resin of the superficial or deep wafers. The oil was used to maximize light transmission throughout the specimen by eliminating an air interface between the cured disc and the Mylar sheets of the uncured specimens, The assembly was then placed in the light-curing unit as described previously and exposed for either the manufacturer-recommended 5 rain or for 10 rain. Following polymerization, the apparatus was disassembled and the upper and lower Mylar sandwiches were recovered. The immersion oil was wiped from the outer surfaces and the cured resin wafers were recovered by peeling away the Mylar sheets. In this manner, two thin cured resin specimens were obtained, one from the upper surface of the assembly and the other from the lower. The superficial wafer from the upper upper surface of the assembly was comparable to the tissue bearing surface of a denture. The cure of the lower wafer was affected by the attenuation of light by the superficial resin wafer and the polymerized resin disc (Fig. 1). In this manner, the lower wafer reflected the cure of the resin surface that was further from the light in the curing unit. The upper and lower cured wafers were then placed in the microtransmission holder of a beam condensing unit (Model 4XV-COO, Harrick Scientific Corporation, Ossining, NY, USA).
k
MYLAR STRIPS POLYMERIZED RESIN
DISC
~k~ ~ MYLAR STRIPS Fig. 1. Waler assembly usedfor monomerconversion determination.
The infrared spectrum of each wafer was then obtained with a Fourier transform infrared spectrometer (FTS-40, Bio-Rad, Digilab Division, Cambridge, MA, USA) using 8 scans at 2 cm -1 resolution. An infrared spectrum of a portion of the uncured resin derived from the mixture from which the wafers were made was also obtained using transmission spectroscopy. The uncured mass was pressed into a thinlayer between two ZnSe discs. The infrared spectrum was obtained using similar conditions, as previously mentioned. Five replicate specimens were made for each of the cured and uncured conditions. The monomer conversion of each specimen was determined by comparison of the ratio of the aliphatic carbon-carbon double bond (C=C) to that of the aromatic component for the cured and uncured states (Ruyter and Gy6r6si, 1976). Triad did not have an aromatic C=C absorption, so the aliphatic C=C was rationed to the urethane absorption peak which is absorbed at 3350 cm 1. The mean resin monomer conversion for each testing variable was then determined. The testing variables were upper and lower surfaces, polymerized for 5- or 10-rain exposures. A three-way analysis of variance (ANOVA), with independent variables being resin brand (3 levels), curing duration (2 levels), and surface measured (2 levels), was performed to test for the presence of a significant difference among all the mean conversion values. Tukey's hsd post-hoc procedure was then used to compare specific pairs of mean conversion values for significant difference. All statistical testing was performed at the 95% level of confidence. Inorganic filler content. The inorganic filler content for each of the three resin systems tested was determined using standardized methods (ISO, 1978). This method involves the determination of weight loss of a resin specimen after heating to a temperature that volatilizes all organic components. Four replications were made for each of the three resin systems tested and the mean weight percent of inorganic filler content was determined. A one-way ANOVA (the independent variable being resin brand) was performed to test for the presence of a significant difference in mean inorganic filler content between specific pairs of brands. All statistical testing was performed at the 95% level of significance.
RESULTS Monomer conversion. Table 2 and Fig. 2 display the mean monomer conversion values of the superficial and deep wafer layers for each of the brands tested. When comparing the cure between the superficial and deep wafers within each brand Dental Materials~July 1992 275
TABLE 2: PERCENT MONOMER CONVERSION COMPARED TO
Resin
POLYMERIZATION TIME 5 MIN. 10 MIN.
POLYMERIZATIONTIME, AND SURFACE EXPOSURE Percent Monomer Conversion (+SD) Surface 5 Min Polymerization 10 Min Polymerization
z_0 ~ l OO-
superficial deep
~z cO
9°i 8o-
"--ii RON
rr
70-
F-~XT("}~-RAL
95 (1.3) , 92(1.9)
LU
:~ O ~ :~ LL
eo-
TRAD
86 (1.1)
w
A comparison of monomer conversion and inorganic filler content in visible light-cured denture resins D. J. Barron ~, F. A. Rueggeberg 2, G. S. Schuster ~ Department of Oral Biology, ~Department of Restorative Dentistry, Section of Dental Materials, Medical Collegeof Georgia, School of Dentistry, Augusta, Georgia, USA
Abstract. Visible light-cured (VLC) denture resins are relatively new products used for the reline and repair of dentures. The conversion of monomer into polymer in 3 brands of visible lightcured denture resins was investigated. The relationship of the inorganic filler content to this conversion was also studied. It was determined that these reline materials vary in monomer conversion and weight percentage of filler, and this variation is brand dependent. The monomer conversion ranged from 77% to 97%. Significant differences in these values were found when duration of light exposure was increased. In addition, resin nearest to the light source polymerized to a greater extent when compared to resin that was 1 mm deep to this surface, hence furthest from the light source. The inorganic filler content ranged from 0% to 15%. For the resin systems studied, the relationship between monomer conversion and inorganic filler loading was inversely proportional, Results indicated that monomer conversion of VLC repair resins was affected by the duration of light exposure as well as the amount of inorganic filler present in the material,
INTRODUCTION Visible light-cured (VLC) denture resin systems were introduced in 1983. These materials are marketed with the purported advantages of more efficient handling, fewer patient appointments, elimination of the potential toxicity of methyl methacrylate, and less dimensional change upon curing than occurs in conventional heat-cured denture resins, The chemical compositions of various VLC brands differ widely. Smith and Powers (1991) have shown these products to contain constituents such as urethane dimethacrylate, poly(ethyl-methacrylate), and ethoxylated bisphenol A dimethacrylate. Since 1983, use of these materials has increased greatly, They are currently being used to fabricate rigid connections between implant transfer copings (Ivanhoe et al., 1991), transfer copings used on stone dies (Passon and Goldfogel, 1991), for orthodontic appliances, oral obturators (DaBreo, 1991), custom impression trays, provisional restorations (Prestipino, 1989), denture baseplates, as well as for vertebral implants (Alsawafet al., 1991). Monomer conversion of resin materials is the degree to which carbon double bonds (C=C) are converted into carbon single bonds (C-C). This conversion occurs during polymerization, and the degree to which this occurs greatlyinfluences not only the physical properties of the resulting polymer (Asmussen, 1982a; 1982b), but also affects its biocompatibility. 274 Barron et aL/Monomer conversion~filler in light-cured resins
The degree of cure of restorative composites has been shown tobedirectlyrelatedtothelevelofcellularbiocompatibilityin vitro (Caughman et al., 1991). As the degree of monomer conversion of composites decreases, protein synthesis by gingiva] fibroblasts also decreases. Similarly, Lefebvre et al. (1991) found differences in the biocompatibility of various VLC denture materials in vitro. However, until now there have been no studies examining the differences in monomer conversion or filler concentrations among the various VLC denture resin products. Inorganic fillers are added to restorative resins for the purpose of increasing wear resistance and adding bulk. Such fillers also are found in VLC denture resin materials. The proportion of filler has yet to be determined, but its presence could affect the degree to which light can penetrate the resin and cause polymerization. Therefore, the relationship between inorganic filler content and monomer conversion also may be an important factor affecting these resin products. The purpose of this investigation was to determine the monomer conversion and inorganic filler content of various brands of VLC denture resins. A possible relationship between these two parameters was also studied. MATERIALS AND METHODS Three VLC systems were tested, as listed in Table 1. Astron and Extoral are both pour-type materials, and utilize a cornbination of chemical and light polymerization (Smith and Powers, 1991, Lefebvre et al., 1991). Triad is polymerized solelyby exposure to visible light andis available in preformed ropes or in sheets. Monomer conversion. Disc-shaped specimens (10 mm diam x 1 mm thick) were fabricated for each material using an aluminum mold. Where applicable, the resin specimens were mixedaccordingtothemanufacturer'sdirections. Mylar(0.06 mm thick, Du Pont Corporation, Wilmington, DE, USA) was placed on the bottom of the mold. The cylindrical cavity was then packed with resin, and another Mylar sheet was placed over the top. The assembly was then pressed so that the resin took the shape and dimensions of the mold. The aluminum mold with uncured resin and Mylar strips was then placed in a Triad II curing unit (Caulk/Dentsply, Milford, DE, USA) for the Triad specimens. The other two resins were polymerized in a Pro-Lite 3 fluorescent unit (Pro-Den Systems, Portland, OR, USA). The duration of light exposure was 10 min. Following polymerization, the Mylar sheets were removed
TABLE 1: CHARACTERISTICSOF VISIBLE LIGHT-CUREDDENTURE
DIRECTION OF INCIDENT LIGHT
MATERIALS USED Material
Powder/Liquid Batch Ratio No. Astron LCH 2.5/1 P-181 L-329
Chemical Manufacturer Composition* Astron Corp. PEMA Wheeling,IL EBDMA
Extoral
2.0/1
P-N/A L-7
Pro-DenSyst. Portland, OR
Triad
single paste
UDUEAA Dentsplylnter. UDMA
PEMA EBDMA
York, PA
PEMA: Polyethyl methacrylate EBDMA: Ethoxylated bis phenol A methacrylate UDMA: Urethane Dimethacrylate *Smith and Powers, 1991.
and the cured discs were allowed to remain in the aluminum mold. A method was developed to determine the monomer conversion of the resin surfaces that were nearest to (superficial resin) and farthest from (lmm deep) the light sources. A diagram of the assembly used is shown in Fig. 1. A small amount of freshly mixed (or freshly dispensed) resin was placed between two sheets of Mylar and pressed into a thin wafer (approximately 30 ~m thick). Two ofthese"sandwiches" (uncured resin between Mylar sheets) were made. The first "sandwich" of resin was placed on the counter top and a small amount of immersion oil (Type B, Cargille Laboratories, Inc., Cedar Grove, NJ, USA) was spread on the upper Mylar surface. The resin in one "sandwich" was labeled as the uncured deep resin wafer (Fig. 1). The 1 mm thick polymerized resin disc was then placed directly over this sandwich and pressed down, spreading the immersion oil into a thin layer, A small amount ofimmersion oil was then placed on the upper surface of the polymerized resin disc and the second sandwich of uncured resin was pressed into place on top of this disc (Fig. 1). This resin was labeled as the superficial resin wafer, Care was taken to ensure that immersion oil did not come in contact with the uncured resin of the superficial or deep wafers. The oil was used to maximize light transmission throughout the specimen by eliminating an air interface between the cured disc and the Mylar sheets of the uncured specimens, The assembly was then placed in the light-curing unit as described previously and exposed for either the manufacturer-recommended 5 rain or for 10 rain. Following polymerization, the apparatus was disassembled and the upper and lower Mylar sandwiches were recovered. The immersion oil was wiped from the outer surfaces and the cured resin wafers were recovered by peeling away the Mylar sheets. In this manner, two thin cured resin specimens were obtained, one from the upper surface of the assembly and the other from the lower. The superficial wafer from the upper upper surface of the assembly was comparable to the tissue bearing surface of a denture. The cure of the lower wafer was affected by the attenuation of light by the superficial resin wafer and the polymerized resin disc (Fig. 1). In this manner, the lower wafer reflected the cure of the resin surface that was further from the light in the curing unit. The upper and lower cured wafers were then placed in the microtransmission holder of a beam condensing unit (Model 4XV-COO, Harrick Scientific Corporation, Ossining, NY, USA).
k
MYLAR STRIPS POLYMERIZED RESIN
DISC
~k~ ~ MYLAR STRIPS Fig. 1. Waler assembly usedfor monomerconversion determination.
The infrared spectrum of each wafer was then obtained with a Fourier transform infrared spectrometer (FTS-40, Bio-Rad, Digilab Division, Cambridge, MA, USA) using 8 scans at 2 cm -1 resolution. An infrared spectrum of a portion of the uncured resin derived from the mixture from which the wafers were made was also obtained using transmission spectroscopy. The uncured mass was pressed into a thinlayer between two ZnSe discs. The infrared spectrum was obtained using similar conditions, as previously mentioned. Five replicate specimens were made for each of the cured and uncured conditions. The monomer conversion of each specimen was determined by comparison of the ratio of the aliphatic carbon-carbon double bond (C=C) to that of the aromatic component for the cured and uncured states (Ruyter and Gy6r6si, 1976). Triad did not have an aromatic C=C absorption, so the aliphatic C=C was rationed to the urethane absorption peak which is absorbed at 3350 cm 1. The mean resin monomer conversion for each testing variable was then determined. The testing variables were upper and lower surfaces, polymerized for 5- or 10-rain exposures. A three-way analysis of variance (ANOVA), with independent variables being resin brand (3 levels), curing duration (2 levels), and surface measured (2 levels), was performed to test for the presence of a significant difference among all the mean conversion values. Tukey's hsd post-hoc procedure was then used to compare specific pairs of mean conversion values for significant difference. All statistical testing was performed at the 95% level of confidence. Inorganic filler content. The inorganic filler content for each of the three resin systems tested was determined using standardized methods (ISO, 1978). This method involves the determination of weight loss of a resin specimen after heating to a temperature that volatilizes all organic components. Four replications were made for each of the three resin systems tested and the mean weight percent of inorganic filler content was determined. A one-way ANOVA (the independent variable being resin brand) was performed to test for the presence of a significant difference in mean inorganic filler content between specific pairs of brands. All statistical testing was performed at the 95% level of significance.
RESULTS Monomer conversion. Table 2 and Fig. 2 display the mean monomer conversion values of the superficial and deep wafer layers for each of the brands tested. When comparing the cure between the superficial and deep wafers within each brand Dental Materials~July 1992 275
TABLE 2: PERCENT MONOMER CONVERSION COMPARED TO
Resin
POLYMERIZATION TIME 5 MIN. 10 MIN.
POLYMERIZATIONTIME, AND SURFACE EXPOSURE Percent Monomer Conversion (+SD) Surface 5 Min Polymerization 10 Min Polymerization
z_0 ~ l OO-
superficial deep
~z cO
9°i 8o-
"--ii RON
rr
70-
F-~XT("}~-RAL
95 (1.3) , 92(1.9)
LU
:~ O ~ :~ LL
eo-
TRAD
86 (1.1)
w
