Thermal characterization, degree of conversion, flexural strength and Knoop hardness of heat treated commercial composites

Visando otimizar as propriedades químicas e mecânicas de compósitos de uso direto aplicados na forma indireta, associados a tratamentos térmicos experimentais (TT), torna-se necessária uma caracterização térmica que permita a determinação de uma temperatura de TT segura (abaixo da temperatura de início de perda de massa). Assim, a primeira etapa deste estudo consistiu em caracterizar termicamente dez compósitos comerciais (Z100, Filtek Z250, Z350 e Supreme XT- 3M ESPE, Esthet-X e TPH Spectrum Dentsply, Charisma Heraeus Kulzer, Tetric Ceram Ivoclar Vivadent, Herculite XRV e Point 4 - Kerr), fotoativados pelo método contínuo (C) e pulse-delay (PD) (20 J.cm-2), por termogravimetria (TG) (n=1) e calorimetria exploratória diferencial (DSC) (n=3). Foram escolhidas duas temperaturas de TT: 100 e 170°C, aplicadas por 10 min, 24h após a fotoativação. A partir disto, avaliaram-se: 1) o grau de conversão (GC), por espectroscopia FT-Raman (n=3); 2) a resistência à flexão em três pontos (RF) (n=10); e 3) a dureza Knoop (KHN) para as faces topo e base (n=3) de quatro destes compósitos (Filtek Z250 e Supreme XT, Esthet-X e Point 4), após diferentes condições: imediatamente, 1h, 6h e 24h após a fotoativação e 24h após fotoativação seguida de TT a 100 ou 170°C. Os dados obtidos foram submetidos à análise de variância e ao teste de Tukey (nível de significância de 5%). Quanto ao GC (%), houve significância estatística para o fator material (p=0,000): Point 4 (68,42) > Z250 (63,05) = Esthet-X (61,69) > Supreme (54,27); condição (p=0,000): TT170 (73,20) = TT100 (73,58) > 24h (62,60) = 6h (60,18) > 1h (55,10) = imediato (53,66); e ativação (p=0,006): C (62,97) > PD (60,75). Também foram significantes as interações material × condição (p=0,007) e material × condição × ativação (p=0,013). Para a RF (MPa), foram encontradas significâncias estatísticas para os fatores material (p=0,000): Z250 (165,48) > Supreme (153,96) > Point 4 (131,30) = Esthet-X (128,06); e condição (p=0,000): TT170 (194,56) > TT100 (182,91) > 24h (150,88) > 6h (131,79) > 1h (111,77) > imediato (96,30); assim como para as interações: ativação × condição (p=0,000), material × condição (p=0,000) e material × ativação × condição (p=0,000). Para a dureza, foram encontradas significâncias estatísticas para os fatores: materialativação (p=0,000): Supreme C (82,46) > Supreme PD (80,29) = Z250 C (77,46) > Z250 PD (74,30) > Esthet-X C (69,17) > Esthet-X PD (65,67) > Point 4 C (57,21) = Point 4 PD (56,71); condição (p=0,000): TT170 (81,35) > TT100 (73,72) > 24h (70,36) > 6h (67,92) > 1h (64,99) > imediato (64,12) e face (p=0,000): topo (71,40) > base (69,40); assim como para as interações: material-ativação × face (p=0,011), material-ativação × condição (p=0,000) e face × condição (p=0,000). A partir dos resultados obtidos, pôde-se concluir que os TTs experimentais propostos foram capazes de aumentar as propriedades estudadas, sendo, de maneira geral, os resultados obtidos com 170°C melhores do que os com 100°C, e ambos melhores do que as demais condições.Aiming to optimize chemical and mechanical properties of direct composites applied in an indirect way, with an association to experimental heat treatments (TT), it is necessary a thermal characterization, in a way to determine a safe temperature for the TT (below the significant mass loss temperature). Thus, the first step of this study was to perform the thermal characterization of ten commercial composites (Z100, Filtek Z250, Z350 and Supreme XT 3M-ESPE, Esthet-X and TPH Spectrum Dentsply, Charisma Heraeus Kulzer, Tetric Ceram Ivoclar Vivadent, Herculite XRV and Point 4 - Kerr) photoactivated by continuous (C) and pulse-delay (PD) methods (20 J.cm-2), by thermogravimetry (TG) (n=1) and differential scanning calorimetry (DSC) (n=3). After that, two temperatures were chosen for the TT: 100 and 170°C, which were applied for 10 min, 24h after photoactivation. Thus, some properties were evaluated for four composites (Filtek Z250 and Supreme XT, Esthet- X and Point 4): 1) degree of conversion (GC) by FT-Raman spectroscopy (n=3); 2) three-point bending test (RF) (n=10); and 3) Knoop hardness (KHN) for top and bottom surfaces (n=3). The conditions were: immediately, 1h, 6h and 24h after photoactivation and 24h after photoactivation followed by TT at 100 or 170°C. Data were analyzed by ANOVA and Tukeys test (level of significance of 5%). In relation to GC (%), there were statistical significance for the factors material (p=0.000): Point 4 (68.42) > Z250 (63.05) = Esthet-X (61.69) > Supreme (54.27); condition (p=0.000): TT170 (73.20) = TT100 (73.58) > 24h (62.60) = 6h (60.18) > 1h (55.10) = immediately (53.66); and activation (p=0.006): C (62.97) > PD (60.75); and for the interactions: material × condition (p=0.007) and material × condition × activation (p=0.013). For RF (MPa), statistical significance were found for the factors material (p=0.000): Z250 (165.48) > Supreme (153.96) > Point 4 (131.30) = Esthet-X (128.06); and condition (p=0.000): TT170 (194.56) > TT100 (182.91) > 24h (150.88) > 6h (131.79) > 1h (111.77) > immediately (96.30); and also for the interactions: activation × condition (p=0.000), material × condition (p=0.000) and material × activation × condition (p=0.000). For KHN, statistical significance were found for the factors material-activation (p=0.000): Supreme C (82.46) > Supreme PD (80.29) = Z250 C (77.46) > Z250 PD (74.30) > Esthet-X C (69.17) > Esthet-X PD (65.67) > Point 4 C (57.21) = Point 4 PD (56.71); condition (p=0.000): TT170 (81.35) > TT100 (73.72) > 24h (70.36) > 6h (67.92) > 1h (64.99) > immediately (64.12), and face (p=0.000): top (71.40) > bottom (69.40); as for the interactions: material-activation × face (p=0.011), material-activation × condition (p=0.000) and face × condition (p=0.000). By the results, it could be concluded that both proposed experimental TTs were capable of increasing the studied properties, with, in a general way, 170°C generating better mechanical properties than 100°C, and both better than the other conditions

Influence of polymeric matrix on the physical and chemical properties of experimental composites

Nowadays, the main reasons for replacement of resin-based composite restorations are fracture or problems with the integrity of their interface, such as marginal staining, microleakage, or secondary caries. The aim of the present study was to evaluate the influence of the organic matrix on polymerization stress (PS), degree of conversion (DC), elastic modulus (E), flexural strength (FS), Knoop hardness (KHN), sorption (SP), and solubility (SL). In order to obtain a material which combines better mechanical properties with lower PS, seven experimental composites were prepared using BisGMA to TEGDMA molar ratios of 2:8, 3:7, 4:6, 5:5, 6:4, 7:3 and 8:2 and 40% of silica. PS was obtained in a universal testing machine, using acrylic as bonding substrate. DC was determined using Fourier Transform Raman spectroscopy. E and FS were obtained by the three-point bending test. KHN was measured by a microindentation test using a load of 25 g for 30 s. SP and SL were assessed according to ISO 4049. The data were submitted to one-way ANOVA. The increase in BisGMA concentration resulted in the decrease of PS, DC, E, FS and KHN. However, it did not change the SP and SL values. FS, E and KHN showed a strong and direct relationship with the DC of the materials. The composite material with a BisGMA to TEGDMA molar ratio of 1:1 was the one with better mechanical properties and lower PS

Heat treatment of a direct composite resin: influence on flexural strength

The purpose of this study was to evaluate the flexural strength of a direct composite, for indirect application, that received heat treatment, with or without investment. One indirect composite was used for comparison. For determination of the heat treatment temperature, thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were performed, considering the initial weight loss temperature and glass transition temperature (Tg). Then, after photoactivation (600 mW/cm² - 40 s), the specimens (10 x 2 x 2 mm) were heat-treated following these conditions: 170ºC for 5, 10 or 15 min, embedded or not embedded in investment. Flexural strength was assessed as a means to evaluate the influence of different heat treatment periods and investment embedding on mechanical properties. The data were analyzed by ANOVA and Tukey's test (α = 0.05). TGA showed an initial weight loss temperature of 180ºC and DSC showed a Tg value of 157°C. Heat treatment was conducted in an oven (Flli Manfredi, Italy), after 37°C storage for 48 h. Flexural strength was evaluated after 120 h at 37°C storage. The results showed that different periods and investment embedding presented similar statistical values. Nevertheless, the direct composite resin with treatments presented higher values (178.7 MPa) compared to the indirect composite resin (146.0 MPa) and the same direct composite submitted to photoactivation only (151.7 MPa). Within the limitations of this study, it could be concluded that the heat treatment increased the flexural strength of the direct composite studied, leading to higher mechanical strength compared to the indirect composite