Resin composite blocks via high-pressure high-temperature ...

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532 d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534where is density of the sample, A the mass of the samplein air, B the mass of the sample in deionized water, w thedensity of deionized water determined from the measurementof water temperature, and a the density of air (0.0012 g/cm 3 ).Thirty determinations on five specimens were made for eachmaterial.2.6. Fractured surface morphologyRepresentative fractured NTP specimens of HT/HP polymerized,Paradigm, and control PP composites were sputtercoated(SC500, Bio-Rad, UK) with gold. The surface morphologywas then characterized under a scanning electronmicroscope (SEM) (JSM-6400, JEOL Ltd., Tokyo, Japan) at lowand high magnification.2.7. Statistical analysisSince the main purpose of this study was to assess the effect ofHT/HP polymerization on the physical/mechanical propertiesand not to compare materials among themselves, the resultsof mechanical/physical tests for each material were analyzedby t-tests, performed at a 0.05 level of significance. The resultsobtained for Paradigm were used as a reference only and werenot included in the statistical analysis.Weibull statistics parameters were calculated for the flexuralstrength data. The description of the Weibull distributionis given byP f = 1 − e −(/ 0) mwhere P f is the fracture probability, defined by the relationP f =N + 1where K is the rank in strength from least to greatest, Ndenotes the total number of specimens in the sample, m isthe shape parameter (Weibull modulus), and 0 is the scaleparameter or characteristic strength 63,21% [8,9]. The Weibullmodulus, characteristic strength, and the strength at a probabilityof failure of 5% were obtained using the Weibull statisticsoption in Excel ® (Microsoft, USA).3. ResultsThe results of the physical/mechanical characterizations aresummarized in Table 2, along with the results of the statisticalanalysis. Fig. 1 presents Weibull plots of the flexural strengthresults where, to facilitate observation of the effect of HT/HPpolymerization, open symbols were used for PP composites,corresponding filled symbols for their HT/HP counterparts,and a distinct symbol for Paradigm. The results have shownthat HT/HP polymerization resulted in a significant (p < 0.05)increase in flexural strength, hardness, and density for allcomposites investigated (Table 2) in comparison with their PPcounterparts. Even if the fracture toughness of all materialswas increased by HT/HP polymerization, significant (p < 0.05)increases were detected only for Gradia and EsthetX. TheWeibull modulus of the HT/HP polymerized composites wasalso higher than that of PP counterparts. The results for f , itsassociated Weibull modulus, and K IC obtained for the HT/HPTable 2 – Results (Mean ± SD) of physical/mechanical characterizations and statistical analysis. *PVIL VIP PercentincreaseDEL DEP PercentincreaseVOL VOP PercentincreaseGroup a property GCL GCP PercentincreaseK IC (in MPa·m 1/2 ) 0.85 ± 0.08 1.58 ± 0.18 85.88 * 1.03 ± 0.11 1.17 ± 0.21 13.59 1.26 ± 0.21 1.52 ± 0.10 20.63 * 0.97 ± 0.15 0.99 ± 0.17 2.06 0.78 ± 0.21Hardness (in HVN) 33.01 ± 5.11 53.78 ± 2.26 62.92 * 85.66 ± 5.65 110.02 ± 4.31 28.43 * 38.95 ± 3.48 79.64 ± 2.52 104.46 * 20.01 ± 1.98 40.73 ± 3.16 103.54 * 114.80 ± 4.34Density (in g/cm 3 ) 1.6250 ± 0.0023 1.6337 ± 0.0018 0.53 * 2.1795 ± 0.0017 2.1868 ± 0.0012 0.33 * 2.0754 ± 0.0038 2.1008 ± 0.0068 1.22 * 1.5044 ± 0.0012 1.5087 ± 0.0010 0.29 * 2.1261 ± 0.0042Weibull modulus 7.04 10.34 11.42 22.39 5.62 8.87 4.66 13.88 5.61Identifies significantly different (p < 0.05) results based on pairwise t-test comparisons.a GCL – Gradia light cured; GCP – Gradia HP/HT polymerization; VOL – Grandio light cured; – Vita VM LC light cured; VIP – Vita VM LC HP/HT polymerization; P – Paradigm.
d e n t a l m a t e r i a l s 2 8 ( 2 0 1 2 ) 529–534 533Fig. 1 – Weibull plots of flexural strength results; open symbols were used for photo-polymerized (PP) materials andfilled-symbols were used for high-temperature high-pressure (HT/HP) polymerized materials.Fig. 2 – SEM micrographs of specimens fractured during fracture toughness (K IC ) testing: (a) Grandio photo-polymerized (PP);(b) Grandio polymerized under high-temperature high-pressure (HT/HP).materials were higher than those of the commercially availableCAD/CAM material Paradigm.The most significant observation during the SEM characterizationsof fractured K IC specimens from all the materialstested was the presence of fewer and smaller voids in HT/HPpolymerized composites. As an example, Fig. 2a and b showshigh (1500×) magnification micrographs of PP and HT/HP polymerizedGrandio samples, respectively. No attempt had beenmade to quantify the differences identified.4. DiscussionHigh pressure polymerization is an appealing approach in theattempt to improve the properties of polymers/composites.As early as 1930, Conant and Tongberg [10] reported theirresult on the application of high pressure [(0.9–1.2) GPa] inthe polymerization of isoprene. A high pressure apparatusused to study a series of organic reactions and severalpolymerization reactions under pressure was described byFawcett and Gibson. [11]. Later on, high density polyethyleneand poly(methyl methacrylate) (PMMA) [12,13], among severalother polymers, were obtained under high pressure. Studyingthe polymerization of tetraethylene glycol dimethacrylate(TEGDMA) under pressure (1 GPa) and with no catalyst, Kaminskiet al. [14] reported a degree of crosslinking of 12% after96 h, similar to that obtained in the industrial production ofPMMA.The primary effects of pressure on a monomer mixtureare to decrease intermolecular distances and to reduce thefree volume [15]. Furthermore, as the pressure increases, themobility of the monomers is reduced and limited to relativelysmall translations, which suggests that the polymerizationkinetics are slower as the pressure increases. It is likely thatas the pressure increases above the GPa range, monomerslose their mobility necessary for polymerization and mostlikely transform into solids, forming monomer crystals [16].Low degrees of conversion would result in polymers withpoor mechanical properties while pressures in the GPa rangewould make the process unsuitable for large scale industrial
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