Browsing by Author "Darvell, B.W."

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    Aspects of water sorption from the air, water and artificial saliva in resin composite restorative materials
    (Dental Materials, 2003) Musanje, L.; Darvell, B.W.
    Primarily to establish whether artificial saliva (AS) at 37 °C is essential as a clinically relevant environment for testing filled, resin composite restorative materials. The effect of other storage conditions was also investigated for comparison and controls: desiccation, exposure to the laboratory atmosphere, high humidity cabinet, saturated water vapor, and deionized water. Two visible light-cured products were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein). Bar specimens (26×1.5×1.0 mm3) were cured at five overlapping spots for 60 s per spot and randomly distributed into groups of six. Trial 1: one group of each material was exposed first to atmospheric air at 24 °C, ∼50% RH (24WV50), then to water vapor at 37 °C, ∼97% RH (37WV97), and then immersed in deionized water at 37 °C (37DW). Trial 2 used three groups of each material, one first exposed to 37WV97 followed by 37DW, the other two were immediately immersed in 37DW or artificial saliva (37AS). Trial 3: two groups of each material were vacuum desiccated at 37 °C, then exposed to 37 °C, ∼100% RH (37WV100), then immersed in 37DW or 37AS. Trial 4: four groups of HR were treated similarly to Trial 3; one was left under desiccation, and another in 37WV100 for the remaining period. Three-point bend tests for flexural strength (F), flexural modulus (E), and total energy to failure (W) were performed at the end of Trials 2–4. Environmental moisture absorption was substantial at 24WV50(c. 0.2%), at least 40% of that in 37DW (HR: c. 0.7%, TC: c. 0.5%). Saturation was achievable in 37WV100. Mass loss on desiccation (HR: c. 0.4–0.5%, TC: c. 0.25%) was reversible in 37WV100. There were some significant effects of exposure conditions on mechanical properties (e.g. F for HR: after desiccation, 85.7±1.4 MPa; after 37WV100, 73.2±3.6 MPa; difference: p<0.0002), but overall the results were unclear. After a rapid gain in mass, there was a gradual loss in both 37DW and 37AS for both materials, slightly more in 37AS than 37DW. Water vapor absorption is substantial, hence attention must be paid to the laboratory working environment and conditions of storage and testing, i.e. temperature and RH must be stated to assist interpretation of data and comparisons between studies. Test conditions need to be standardized and with reference to normal oral conditions, immediate immersion in artificial saliva at 37 °C is the preferred treatment for these materials, whatever time of testing is chosen.
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    Curing-light attenuation in filled-resin restorative materials
    (Dental Materials, 2006) Musanje, L.; Darvell, B.W.
    To characterize the attenuation of the curing light in filled resin restorative materials (FRRMs) to aid understanding of curing depth. One hundred and eighty materials of various shades from several manufacturers were tested in various ways. One set (66 materials) was used to determine the applicability of Lambert's Law using a quartz-tungsten-halogen curing light (Optilux 400, Demetron Research) by measuring the transmitted light with a dental radiometer (Cure Rite, EFOS) for successive thicknesses of ground 10mm diameter specimens from 3 to 0.5mm in 0.5mm steps. A second set (17 materials) were similarly tested with separate specimens from 1 to 5mm in thickness using a transmission densitometer (DT1405, RY Parry) fitted with a curing-light dichroic filter. For a third (overlapping) set (165 materials), the 1mm pure (reflectance-free) optical density (D1 value) was determined from two specimens, ∼1 and ∼2mm thick using the densitometer as above. From D1 the critical thickness (xCRIT), identified as depth of cure (DoC) for an excess surface exposure factor of 2, was calculated. Lambert's Law was found to hold with no evidence of appreciable differential absorption effects. Attenuation coefficient and D1 were significantly correlated (P<1×10−13). D1 varied between about 0.23 and 0.72, for corresponding xCRIT values of 1.3 and 0.4mm. There was no correlation between D1 and reflectance (P>0.09), and no systematic effect due to shade letter, but a highly significant (P<7.5×10−8), but weak (−0.066mm/unit), correlation between shade number and D1.
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    Effects of strain rate and temperature on the mechanical properties of resin composites
    (Dental Materials, 2004) Musanje, L.; Darvell, B.W.
    To evaluate the effects of strain rate and temperature on the mechanical properties of resin composite restorative materials (RCs) and to investigate the construction of temperature—strain rate equivalence ‘master curves’. Four visible light-cured resin composite RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy Condensable (PR) (Kerr, Orange, CA, USA). Bar specimens were cured for 50 s at an irradiance of 500 mW cm−2 and were randomly distributed into groups of six for each type of material. All specimens were stored in artificial saliva at pH 6, for 7 d. The specimens tested at 12, 24 and 37 °C were stored at the corresponding temperature but those tested at 0 °C were stored at 24 °C. Three-point bend tests for flexural strength (F), flexural modulus of elasticity (E) and total energy to failure (W) were performed at cross-head speeds (XHS) of 0.1, 1.0, 10, 50 and 100 mm min−1 for all materials as well as at 0.01, 0.03, 0.2 and 0.5 mm min−1 for some materials. There was a common pattern of behavior across materials. At constant temperature, F showed a slight variation with cross-head speed, with a broad peak in the region of 1–10 mm min−1. E, on the other hand, showed a more marked and steady increase with XHS at all temperatures except at 0 °C, where it tended to level off above about 10 mm min−1. In contrast, the values of W showed a decline with increasing XHS, except at 37 °C where an initial rise followed by a decline was observed. At constant XHS, increase in temperature caused a small, but highly significant (P<10−3) decline in F but a marked decline in E. W, again in contrast to F and E, showed a general increase with temperature. A master curve model for the temperature–strain-rate equivalence was fitted to the E and W data (all P<10−5) and the fitted parameters interpreted in terms of strain rate and temperature sensitivity.
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    Polymerization of resin composite restorative materials: exposure reciprocity
    (Dental Materials, 2003) Musanje, L.; Darvell, B.W.
    To examine whether there is reciprocity between irradiation time and irradiance with regard to the mechanical properties of filled, resin composite restorative materials (RCs). Four visible light-cured RCs, all of shade A3, were used: Heliomolar Radiopaque (HR) and Tetric Ceram (TC) (Ivoclar, Schaan, Liechtenstein), Filtek Z250 (FZ) (3M, St Paul, MN, USA) and Prodigy condensable (PR) (Kerr, Orange, CA, USA). Bar specimens (1.0×1.5×16.0 mm3) were cured at irradiances (I) ranging from 25–1500 mW/cm2 and irradiation times (t) of 1–3000 s. Six specimens at 250 combinations of t and I were prepared and stored in artificial saliva of pH 6, at 37 °C for 7d before performing three-point bend tests for flexural strength (F), flexural modulus (E) and total energy to failure (W). Contour plots of property value vs. t and I on log scales were prepared. Results. The contour plots showed three regions: unset at low I·t, a plateau corresponding to more or less full property development, and connecting ramp. The boundary between the plateau and the ramp suggests the minimum acceptable exposure. No practical lower limit to irradiance was detected, but there may be no benefit from increasing I beyond about 1000 mW/cm2. The slopes of the contours in the log–log plots provided a test of the hypothesis of reciprocity. These slopes were ∼−1.5 for HR, TC and PR; and ∼−1 for FZ, compared with the expected value of −1. The general hypothesis therefore fails. The existence of localized maxima in property values is further evidence of that failure, even for FZ. Significance. Dentists may use any lamp, including LED sources, and attain satisfactory results providing irradiation time is long enough. Manufacturers ought to supply a graph indicating the minimum acceptable exposure for each product for specified curing lamps. Calculations based on total energy delivered to guide irradiation protocols are invalid and do not recognize product behavior.
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    Water sorption and mechanical behaviour of cosmetic direct restorative materials in artificial saliva
    (Dental Materials, 2001) Musanje, L.; Shu, M.; Darvell, B.W.
    To evaluate the water sorption and mechanical behaviour of a compomer in comparison with those of its nominal forerunners, a filled resin restorative material and a conventional glass ionomer cement. Compomer (Dyract AP) (D-AP), filled resin (SureFil) (SF), and glass ionomer (ChemFlex) (CF) (all Dentsply, Addlestone, UK) restorative materials were tested. Forty bar specimens (26×1.5×1.0mm3) of each material were prepared according to the manufacturer's instructions and randomly distributed into eight groups: dry air (22% RH), saturated water vapour (WV) (100% RH), and five in artificial saliva (AS) at pH6, all at 37°C, as well as untreated control (UC) (23°C, 50% RH). Water sorption was assessed gravimetrically; flexural strength and elastic modulus were determined in three-point bend. The control group was tested at 24h; AS groups were separately tested after 0.5, 1, 3, 6 and 9 months; the other two at 9 months. Mass gain for SF, D-AP and CF in AS was up to 0.17%, 1.2% and 7.0%, respectively. CF showed a marked decrease of strength in AS compared with other groups, followed by a gradual slight rise to a peak at 3 months. Unlike SF and CF, whose flexural strength remained relatively stable, that of D-AP showed a sharp decline from the 1 month peak (P=6×10−7) after 6 months in AS. D-AP also showed a slight decline in flexural modulus from a peak, that of SF was quite stable, while CF showed no peak. The values of flexural strength for both CF and D-AP at 9 months were significantly lower in AS than WV, but SF showed no such difference. Materials intended for service in the mouth must be stored in a realistic medium if the results of testing are to be interpretable. Dyract AP, a compomer, does not seem suitable for application in stress bearing areas as is currently recommended by its manufacturer. The rapid decline in flexural strength after 1 month of exposure to AS and its progressive fall in flexural modulus suggest a progressive deterioration of the material and this necessitates re-examination of the chemistry of compomers, if the behaviour is typical of the class.