STOMATOLOGY EDU JOURNAL 2017, Volume 4, Issue 3 SEJ_3-2017_Online - Page 16

Polymerisation kinetics in a fibre reinforced resin-based composite 2 x 16 mm. Irradiation occurred with the above described LCU on top and bottom of the specimens, with three light exposures of 20 seconds per side, overlapping one irradiated section no more than 1 mm of the diameter of the light guide, in order to prevent multiple polymerization. After removal from the mould the specimens were grinded with silicon carbide sand paper (grit size P 1200/4000 (Leco)) in order to remove excess material. All specimens were then stored in distilled water at 37°C for 24 h prior to testing. The specimens were loaded until failure using a universal testing machine (Z 2.5, Zwick/ Roell, Ulm, Germany) in a three-point bending test device, constructed according to the guidelines of NIST No. 4877 with 12 mm distance between the supports. Specimens were immersed in distilled water at room temperature during testing. The crosshead speed was 0.5 mm/min. The universal testing machine measured the force during bending as function of deflection of the beam. The bending modulus was calculated from the slope of the linear part of the force-deflection diagram. 2.3. Micro-mechanical properties Fragments (n = 10) of the three-point-bending test specimens were used to determine the micro-mechanical properties – Vickers hardness (HV), indentation modulus (Y HU ) and Creep (Cr) - according to DIN 50359-1:1997-10 by means of a universal hardness device (Fischerscope H100C, Fischer, Sindelfingen, Germany). Prior to testing, the specimens were polished with a diamond suspension (mean grain size: 1 µm). Measurements were done on the first polymerised side of the specimens (6 measurements per specimens, 60 measurements in total). The test procedure was carried out force controlled; the test load increased and decreased with constant speed between 0.4 mN and 500 mN. The load and the penetration depth of the indenter were continuously measured during the load-unload-hysteresis. The Universal hardness is defined as the test force divided by the apparent area of the indentation under the applied test force. From a multiplicity of measurements, a conversion factor between Universal hardness (Martens Hardness) and Vickers hardness was calculated and implemented in the software, so that the measurement results were indicated in the more familiar Vickers hardness (HV) units. The indentation modulus (Y HU ) was calculated from the slope of the tangent of indentation depth-curve at maximum force. By measuring the change in indentation depth with constant test force, a relative change in the indentation depth can be calculated. This is a value for the creep of the materials. 2.4. Curing unit characteristics The analysis of the variation in irradiance delivered at distances up to 10 mm away from the light tip was performed in 1 mm steps (n = 3) on a laboratory- grade NIST-referenced USB4000 Spectrometer (MARC (Managing Accurate Resin Curing) System, Bluelight Analytics Inc., Halifax, Canada). The miniature fiber optic USB4000 Spectrometer uses a 3648-element Toshiba linear CCD array detector and high-speed electronics. The spectrometer has been spectro-radiometrically calibrated using Ocean Optics’ NIST-traceable light source (300- 1050 nm). The system uses a CC3-UV Cosine Corrector to collect radiation over 180º field of view thus mitigating the effects of optical interference associated with light collection sampling geometry. 2.5. Statistical analysis Statistical analysis were performed using statistical software (SPSS Inc. 23.0, Chicago, IL, USA). A Shapiro-Wilk test verified the normal distribution of the data. Descriptive statistics and 95% confidence interval are indicated. DC and irradiance results were statistically compared using one-way ANOVA followed up with Tukey’s post hoc test. In all statistical tests, p-values < 0.05 were considered statistically significant. 3. Results 3.1. Degree of conversion (DC) and polymerisation kinetic The degree of conversion as function of increment thickness measured 300 s after initiating the polymerisation as well as the kinetic parameters are summarised in Table 1. One way ANOVA revealed no significant difference in DC measured 300 s post irradiation in a depth of 100-µm and 2-mm (p = 0.281). Similarly, no significant difference was identified between DC measured at 2-mm and 4-mm (p = 0.724), while the DC measured at 6-mm depth was significantly lowest (p < 0.001). The polymerisation kinetic (Fig 1, Table 1) was well described (R² > 0.95) by an exponential sum function. Parameters “a” and “b” are describing the gel phase of the polymerisation process. Table 1. DC (%) measured 300 s post-irradiation (mean and standard deviation in parenthesis; superscript letters indicate statistically homogeneous subgroups as function of specimen’s thickness, Tukey’s HSD test, α = 0.05), maximal rate of polymerisation, Rate max , and parameters of the exponential sum function (95% c