Dent Mater 8:290-295, September,1992
Post-cure heat treatments for composites: properties and fractography J.L. Ferracane, J.R. Condon Department of Dental Materials Science, Oregon Health Sciences University, Portland, Oregon, USA
Abstract. Two commercial and four experimental composites were subjected to post-cure heat treatments of 10 min and 3 h duration immedtatelyafter light-curing. Fracturetoughness, flexural modulus, microhardness and degree of conversion (FTIR) were evaluated 24 h later. The results showed that post-cure heat treatments at 120°C of short or long duration can be used to produce significant improvements in the degree of cure and the mechanical properties of dental composites used as mlays. A 10 min heat treatment was as effective as a 3 h treatment in enhancing properties and degree of cure. In addition, a 3 h heat treatment carned out 7 days after the initial hght-curing was capable of ~mproving properttes and cure to almost the same extent as the immediate heat treatments. The improvement in properties, in conjunctton with the fractography, indicate a toughening of the filled resin matrix and possibly an improved filler/matrtx adhesion in the microfills The changes appear to be predominantly the result of an tncrease tn degree of cure.
INTRODUCTION The difficulties encountered in the placement of successful posterior composite restorations created a market for the composite inlay. The clinical expectations for the inlay restorations are improved marginal adaptation, due to a minimization of contraction stresses, and improved durability, due to an enhanced cure of the polymer matrix. The enhanced cure results from the application of heat to initiate curing or to further the reaction of a light-cured material. The effect of various heat treatments has been evaluated. Lutz et al. (1984) showed some improvement in the wear resistance of a heat-cured formulation of a posterior composite over a chemical-cured formulation. However, another study reported no difference in the wear rates for a light-cured inlay and an inlay that was light-cured and then given an additional heat treatment of 10 min at 120°C (Wendt and Leinfelder, 1990). However, marginal integrity and post-operative sensitivity were improved for the latter. A recent in vitro evaluation of the contact wear of three composites subjected to various post-cure heat treatments reported inconsistent results, showing the wear resistance of only one material, a coarse particle composite, to be improved by heat treatment (Krejci et al., 1991). Numerous studies have reported on the effect of heat treatments on the mechanical properties of dental composites. Wendt (1987a; 1987b) showed 10-20% increases in diametral tensile strength and hardness and 40-50% increases in abrasion resistance for several light-cured composites after postcure heat treatments of 10 min at 100°C to 200°C. Compressive strength and compressive modulus were minimally af-
fected. Reinhardt (1989) reported increases in transverse strength and modulus of elasticity as a result of only 150 s of post-curing at 125°C. At the same time, Wendt (1989) reported that a post-cure heat treatment of 7.5 min at 125°C produced optimal abrasion resistance. Cook and Johannson (1987) showed significant increases, ranging from 10% - 100%, in diametral tensile strength, flexural strength, and fracture toughness for four composites post-cured at up to 100°C for 24 h. In another study, post-cure heating of composites for 1 h at temperatures above 100°C produced only 10% increases in several mechanical properties (Asmussen and Peutzfeldt, 1990). Similarly, McCabe and Kagi (1991) reported minimal increases in mechanical properties for one composite after a 7 rain post-cure at 120°C in a light/heat oven. Fujishima et al. (1991) showed an average increase in fracture toughness of 15% and a 25% increase in tensile strength for several composites post-cure heat treated near 100°C. The greatest increases in these properties were produced in a small-particle hybrid composite. The increase in mechanical properties as a result of heat treating composites has been shown to be related to an increase in degree of cure (Cook and Johannson, 1987). While this is likely to be the main factor, it is also possible that the high temperature treatment provides a relief of internal stresses generated during the initial curing. De Gee et al. (1990) reported 20-60% increases in the initial in vitro abrasion wear resistance for composites post-cured at 125°C for 7 min, and suggested that the improvement was due to stress relief within the composite instead of further polymerization or cross-linking. Haller et al. (1990) reported improved marginal integrity ~n wtro for thermocycled Class V composite inlays as compared with direct-fill restorations, and suggested that an annealing of internal stresses due to the post-curing may have been responsible. The objectives of this study were to quantitate any change in fracture toughness, elastic modulus and surface hardness of composites after three different post-light-curingheat treatments. The degree of cure (DC) for four of the composites was evaluated by micro-FTIR to determine if increases in these properties due to the heat treatments could be attributed to an increase in conversion. Fracture surfaces were evaluated by SEM to identify changes in the crack propagation mode as a result of the post-cure heating.
MATERIALS AND METHODS Four experimental light-cured composites (EXP I, EXP II, EXP III, EXP IV) were made with a 50% Bis-GMA/50% TEGDMA (triethyleneglycol dimethacrylate) resin containing
290 Ferracane & Condon/Post-cure heat treatments for composttes, properties and fractography
camphoroquinone and a tertiary amine reducing agent (Table 1). The resin was the same as that used in Silux Plus (3M Dental Products, St. Paul, MN, USA). The composites were synthesized for the study by 3M using quartz and fumed silica silanated with methacryloxypropyl trimethoxysilane (5 w/o and 20 w/o, respectively). These formulations were chosen to provide the full spectrum of available materials in terms of filler size. In addition, two commercial materials were evaluated: Herculite (Kerr, Romulus, MI, USA), a hybrid containing 58 vol% of barium glass with an average raze of 0.6 pm and Heliomolar (Vivadent, Schaan, Lmchtenstein), a reinforced microfill containing a total of 50 vol% of inorganic particles composed of fumed SiO~ (avg. size of 0.1 pro), YbF3, and pre-polymerized resin fillers (containing fumed SiO 2). Fracture toughness (Krc)was evaluated using single edge notch specimens (25 mm x 5.0 mm x 2.5 ram) prepared in steel molds with a razor blade insert to give an a/w of 0.5, where a = notch length and w = specimen width. The specimens (n = 5; n = 10 for the NORMAL cure) were initially cured in a light-curing unit (Triad II. Caulk/Dentsply, Milford, DE, USA) with 40 s exposures of both the top and bottom surfaces. They were tested on a universal testmg machine (Model TTB, Instron Corp., Canton, MA, USA) in bending (20 mm span) at a cross-head speed of 0.127 mm/min. KI~was calculated as previously described (Ferracane et al., 1987). Hardness was tested on the same bars (n = 5) employed m the Kk tests. Spemmen surfaces were polished to a 5 pm finish with aluminum oxide. A 500 g load was applied through a Knoop diamond indenter over a 20 s loading cycle on a microhardness tester (Kentron, Torsion Balance Co., Chfton, NJ, USA). Elastic modulus (E) was evaluated using bars (25 mm x 2 mm x 2 ram) produced in a spht steel mold. These samples (n = 5) were imtmlly hght-cured in the same manner as the Kic bars and were tested in three-point bending (span = 20 mm) at a cross-head speed of 0.254 mm/mm. The Instron cross-head travel was used in the calculatmn of the modulus because strain was not measured directly. Followmg the initial light-curing, specimens were treated m one of four ways: NORMAL - aged in water at 37°C for 24 h before testing; HEAT 10 MIN- immediate (within 1 min) post-light-curing heat treatment in a dark oven at 120°C for 10 min, followed by 24 h in water at 37°C before testing; HE A T 3 H - immediate post-light-curing heat treatment in a dark oven at 120°C for 3 h followed by 24 h aging in water at 37°C before testing; D E L A Y H E A T - aged m water at 37°C for 7 d before being subjected to a heat treatment in a dark oven at 120°C for 3 h, fi)llowed by 24 h in 37°C water. Degree of conversion (DC) was determined on specimens from each treatment group for Hercuhte (HERC), Heliomolar (HELIO), and experimental compomtes EXP I and EXP II. DC was evaluated using transmission micro-FTIR (XAD MicroFTIR module/RFX 30 FTIR, Laser Precision, Irvine, CA, USA) from chips or flakes removed with a scalpel from the internal areas of the fractured K,~specimens The chips were normally 20-30 gm thick and 100 pm or so m length. In addition, chips were removed from the surface of the specimens and analyzed by micro-FTIR to determine if there was a difference in DC at the surface compared to the bulk. The surfaces were lightly wet ground on #600 SiC paper. The uncured pastes were first examined in transmission and the ratio ofC=C (methacrylate unsaturation) / C C (aromatic ring) before and after curing was determined using techniques previously described
TABLE 1: COMPOSITION OF THE EXPERIMENTAL COMPOSITES Composite
F,Iler Vol(%)
F,Iler Type
F,Iler Avg Size(pm)
F,Iler S,ze Range(pm)
EXP I (m,crof,ll)
38
S,O2 PPRF*
0 04 17
0 01 - 0.2 1 - 60
EXP II (small)
65
quartz
16
0 1-5
EXP III (hybr,d)
65
quartz
1 6 (55 w/o) 8 (45 w/o)
EXP IV (large)
65
quartz
8+
0 1- 5 1 - 15 1 - 15
PPRF* pre-polymenzed resin fillers +(only 75% of the fillers sflanated)
(Ferracane and Marker, 1992). Specimens used in the Kic tests were then mounted and sputter coated (Hummer VII, Anatech, Alexandria, VA, USA) with a 50 nm layer of Au-Pd for analysis by scanning electron microscopy (SEM) using the secondary electron signals (JEOL 6400 SEM, Peabody, MA, USA). Several specimens were coated with a layer of carbon in a vacuum evaporator (VE 10, Varian, Pale Alto, CA, USA Ifor imaging using the backscatter electron signal. Fracture surfaces from at least two specimens for each material were qualitatively compared for the different post-cure treatments. Fractographs were always obtained from an area within 1 mm of the notch tip to avoid bias due to edge effects. ANOVA and Scheff~'s test for multiple comparisons between means were used to compare the results from the different treatments for each composite (p _
Post-cure heat treatments for composites: properties and fractography J.L. Ferracane, J.R. Condon Department of Dental Materials Science, Oregon Health Sciences University, Portland, Oregon, USA
Abstract. Two commercial and four experimental composites were subjected to post-cure heat treatments of 10 min and 3 h duration immedtatelyafter light-curing. Fracturetoughness, flexural modulus, microhardness and degree of conversion (FTIR) were evaluated 24 h later. The results showed that post-cure heat treatments at 120°C of short or long duration can be used to produce significant improvements in the degree of cure and the mechanical properties of dental composites used as mlays. A 10 min heat treatment was as effective as a 3 h treatment in enhancing properties and degree of cure. In addition, a 3 h heat treatment carned out 7 days after the initial hght-curing was capable of ~mproving properttes and cure to almost the same extent as the immediate heat treatments. The improvement in properties, in conjunctton with the fractography, indicate a toughening of the filled resin matrix and possibly an improved filler/matrtx adhesion in the microfills The changes appear to be predominantly the result of an tncrease tn degree of cure.
INTRODUCTION The difficulties encountered in the placement of successful posterior composite restorations created a market for the composite inlay. The clinical expectations for the inlay restorations are improved marginal adaptation, due to a minimization of contraction stresses, and improved durability, due to an enhanced cure of the polymer matrix. The enhanced cure results from the application of heat to initiate curing or to further the reaction of a light-cured material. The effect of various heat treatments has been evaluated. Lutz et al. (1984) showed some improvement in the wear resistance of a heat-cured formulation of a posterior composite over a chemical-cured formulation. However, another study reported no difference in the wear rates for a light-cured inlay and an inlay that was light-cured and then given an additional heat treatment of 10 min at 120°C (Wendt and Leinfelder, 1990). However, marginal integrity and post-operative sensitivity were improved for the latter. A recent in vitro evaluation of the contact wear of three composites subjected to various post-cure heat treatments reported inconsistent results, showing the wear resistance of only one material, a coarse particle composite, to be improved by heat treatment (Krejci et al., 1991). Numerous studies have reported on the effect of heat treatments on the mechanical properties of dental composites. Wendt (1987a; 1987b) showed 10-20% increases in diametral tensile strength and hardness and 40-50% increases in abrasion resistance for several light-cured composites after postcure heat treatments of 10 min at 100°C to 200°C. Compressive strength and compressive modulus were minimally af-
fected. Reinhardt (1989) reported increases in transverse strength and modulus of elasticity as a result of only 150 s of post-curing at 125°C. At the same time, Wendt (1989) reported that a post-cure heat treatment of 7.5 min at 125°C produced optimal abrasion resistance. Cook and Johannson (1987) showed significant increases, ranging from 10% - 100%, in diametral tensile strength, flexural strength, and fracture toughness for four composites post-cured at up to 100°C for 24 h. In another study, post-cure heating of composites for 1 h at temperatures above 100°C produced only 10% increases in several mechanical properties (Asmussen and Peutzfeldt, 1990). Similarly, McCabe and Kagi (1991) reported minimal increases in mechanical properties for one composite after a 7 rain post-cure at 120°C in a light/heat oven. Fujishima et al. (1991) showed an average increase in fracture toughness of 15% and a 25% increase in tensile strength for several composites post-cure heat treated near 100°C. The greatest increases in these properties were produced in a small-particle hybrid composite. The increase in mechanical properties as a result of heat treating composites has been shown to be related to an increase in degree of cure (Cook and Johannson, 1987). While this is likely to be the main factor, it is also possible that the high temperature treatment provides a relief of internal stresses generated during the initial curing. De Gee et al. (1990) reported 20-60% increases in the initial in vitro abrasion wear resistance for composites post-cured at 125°C for 7 min, and suggested that the improvement was due to stress relief within the composite instead of further polymerization or cross-linking. Haller et al. (1990) reported improved marginal integrity ~n wtro for thermocycled Class V composite inlays as compared with direct-fill restorations, and suggested that an annealing of internal stresses due to the post-curing may have been responsible. The objectives of this study were to quantitate any change in fracture toughness, elastic modulus and surface hardness of composites after three different post-light-curingheat treatments. The degree of cure (DC) for four of the composites was evaluated by micro-FTIR to determine if increases in these properties due to the heat treatments could be attributed to an increase in conversion. Fracture surfaces were evaluated by SEM to identify changes in the crack propagation mode as a result of the post-cure heating.
MATERIALS AND METHODS Four experimental light-cured composites (EXP I, EXP II, EXP III, EXP IV) were made with a 50% Bis-GMA/50% TEGDMA (triethyleneglycol dimethacrylate) resin containing
290 Ferracane & Condon/Post-cure heat treatments for composttes, properties and fractography
camphoroquinone and a tertiary amine reducing agent (Table 1). The resin was the same as that used in Silux Plus (3M Dental Products, St. Paul, MN, USA). The composites were synthesized for the study by 3M using quartz and fumed silica silanated with methacryloxypropyl trimethoxysilane (5 w/o and 20 w/o, respectively). These formulations were chosen to provide the full spectrum of available materials in terms of filler size. In addition, two commercial materials were evaluated: Herculite (Kerr, Romulus, MI, USA), a hybrid containing 58 vol% of barium glass with an average raze of 0.6 pm and Heliomolar (Vivadent, Schaan, Lmchtenstein), a reinforced microfill containing a total of 50 vol% of inorganic particles composed of fumed SiO~ (avg. size of 0.1 pro), YbF3, and pre-polymerized resin fillers (containing fumed SiO 2). Fracture toughness (Krc)was evaluated using single edge notch specimens (25 mm x 5.0 mm x 2.5 ram) prepared in steel molds with a razor blade insert to give an a/w of 0.5, where a = notch length and w = specimen width. The specimens (n = 5; n = 10 for the NORMAL cure) were initially cured in a light-curing unit (Triad II. Caulk/Dentsply, Milford, DE, USA) with 40 s exposures of both the top and bottom surfaces. They were tested on a universal testmg machine (Model TTB, Instron Corp., Canton, MA, USA) in bending (20 mm span) at a cross-head speed of 0.127 mm/min. KI~was calculated as previously described (Ferracane et al., 1987). Hardness was tested on the same bars (n = 5) employed m the Kk tests. Spemmen surfaces were polished to a 5 pm finish with aluminum oxide. A 500 g load was applied through a Knoop diamond indenter over a 20 s loading cycle on a microhardness tester (Kentron, Torsion Balance Co., Chfton, NJ, USA). Elastic modulus (E) was evaluated using bars (25 mm x 2 mm x 2 ram) produced in a spht steel mold. These samples (n = 5) were imtmlly hght-cured in the same manner as the Kic bars and were tested in three-point bending (span = 20 mm) at a cross-head speed of 0.254 mm/mm. The Instron cross-head travel was used in the calculatmn of the modulus because strain was not measured directly. Followmg the initial light-curing, specimens were treated m one of four ways: NORMAL - aged in water at 37°C for 24 h before testing; HEAT 10 MIN- immediate (within 1 min) post-light-curing heat treatment in a dark oven at 120°C for 10 min, followed by 24 h in water at 37°C before testing; HE A T 3 H - immediate post-light-curing heat treatment in a dark oven at 120°C for 3 h followed by 24 h aging in water at 37°C before testing; D E L A Y H E A T - aged m water at 37°C for 7 d before being subjected to a heat treatment in a dark oven at 120°C for 3 h, fi)llowed by 24 h in 37°C water. Degree of conversion (DC) was determined on specimens from each treatment group for Hercuhte (HERC), Heliomolar (HELIO), and experimental compomtes EXP I and EXP II. DC was evaluated using transmission micro-FTIR (XAD MicroFTIR module/RFX 30 FTIR, Laser Precision, Irvine, CA, USA) from chips or flakes removed with a scalpel from the internal areas of the fractured K,~specimens The chips were normally 20-30 gm thick and 100 pm or so m length. In addition, chips were removed from the surface of the specimens and analyzed by micro-FTIR to determine if there was a difference in DC at the surface compared to the bulk. The surfaces were lightly wet ground on #600 SiC paper. The uncured pastes were first examined in transmission and the ratio ofC=C (methacrylate unsaturation) / C C (aromatic ring) before and after curing was determined using techniques previously described
TABLE 1: COMPOSITION OF THE EXPERIMENTAL COMPOSITES Composite
F,Iler Vol(%)
F,Iler Type
F,Iler Avg Size(pm)
F,Iler S,ze Range(pm)
EXP I (m,crof,ll)
38
S,O2 PPRF*
0 04 17
0 01 - 0.2 1 - 60
EXP II (small)
65
quartz
16
0 1-5
EXP III (hybr,d)
65
quartz
1 6 (55 w/o) 8 (45 w/o)
EXP IV (large)
65
quartz
8+
0 1- 5 1 - 15 1 - 15
PPRF* pre-polymenzed resin fillers +(only 75% of the fillers sflanated)
(Ferracane and Marker, 1992). Specimens used in the Kic tests were then mounted and sputter coated (Hummer VII, Anatech, Alexandria, VA, USA) with a 50 nm layer of Au-Pd for analysis by scanning electron microscopy (SEM) using the secondary electron signals (JEOL 6400 SEM, Peabody, MA, USA). Several specimens were coated with a layer of carbon in a vacuum evaporator (VE 10, Varian, Pale Alto, CA, USA Ifor imaging using the backscatter electron signal. Fracture surfaces from at least two specimens for each material were qualitatively compared for the different post-cure treatments. Fractographs were always obtained from an area within 1 mm of the notch tip to avoid bias due to edge effects. ANOVA and Scheff~'s test for multiple comparisons between means were used to compare the results from the different treatments for each composite (p _
