13 resultados para Dental erosion
em Greenwich Academic Literature Archive - UK
Resumo:
In order to find a link between results obtained from a laboratory erosion tester and tests carried out on a pneumatic conveyor, a comparison has been made between weight loss from bends on an industrial-scale pneumatic conveyor and erosion rates obtained in a small centrifugal erosion tester, for the same materials. Identical test conditions have been applied to both experiments so that comparable test results have been obtained. The erosion rate of mild steel commonly used as the wall material of conveyor pipes and pipe bends was determined individually on both test rigs. A relationship between weight loss from the bends and erosion rate determined from the tester has been developed. A discussion based on the results and their applicability to the prediction of wear in pneumatic conveyors concludes the paper.
Resumo:
Particle concentration is known as a main factor that affects erosion rate of pipe bends in pneumatic conveyors. With consideration of different bend radii, the effect of particle concentration on weight loss of mild steel bends has been investigated in an industrial scale test rig. Experimental results show that there was a significant reduction of the specific erosion rate for high particle concentrations. This reduction was considered to be as a result of the shielding effect during the particle impacts. An empirical model is given. Also a theoretical study of scaling on the shielding effect, and comparisons with some existing models, are presented. It is found that the reduction in specific erosion rate (relative to particle concentration) has a stronger relationship in conveying pipelines than has been found in the erosion tester.
Resumo:
An investigation into predicting failure of pneumatic conveyor pipe bends due to hard solid particle impact erosion has been carried out on an industrial scale test rig. The bend puncture point locations may vary with many factors. However, bend orientation was suspected of being a main factor due to the biased particle distribution pattern of a high concentration flow. In this paper, puncture point locations have been studied with different pipe bend orientations and geometry (a solids loading ratio of 10 being used for the high concentration flow). Test results confirmed that the puncture point location is indeed most significantly influenced by the bend orientation (especially for a high concentration flow) due to the biased particle distribution and biased particle flux distribution.
Resumo:
The potential of employing zinc polycarboxylate dental cement as a controlled release material has been studied. Benzalkonium chloride was used as the active ingredient, and incorporated at concentrations of 1, 2 and 3% by mass within the cement. At these levels, there was no observable effect on the speed of setting. Release was followed using an ion-selective electrode to determine changes in chloride ion concentration with time. This technique showed that the additive was released when the cured cement was placed in water, with release occurring by a diffusion mechanism for the first 3 h, but continuing beyond that for up to 1 week. Diffusion coefficients were in the range 5.62 × 10(−6) cm(2) s(−1) (for 1% concentration) to 10.90 × 10(−6) cm(2) s(−1) (for 3% concentration). Up to 3% of the total loading of benzalkonium chloride was released from the zinc polycarboxylate after a week, which is similar to that found in previous studies with glass-ionomer cement. It is concluded that zinc polycarboxylate cement is capable of acting as a useful material for the controlled release of active organic compounds.
Resumo:
The water loss behaviour of a clinical glass-ionomer dental cement has been studied with and without the addition of alkali metal chlorides. Dehydrating conditions were provided by placing specimens in a desiccator over concentrated sulphuric acid. Cements were prepared using either pure water or an aqueous solution of metal chloride (LiCl, NaCl, KCl) at 1.0 mol/dm(3). In addition, NaCl at 0.5 mol/dm(3) was also used to fabricate cements. Disc-shaped specimens of size 6 mm diameter x 2 mm thickness were made, six performulation, and cured at 37 degrees C for 1 hour They were then exposed to desiccating conditions, and the mass measured at regular intervals. All formulations were found to lose water in a diffusion process that equilibrated after approximately 3 weeks. Diffusion coefficients ranged from 2.27 (0.13) x 10(9) with no additive to 1.85 (0.07) x 10(9) m(2)/s with 1.0 mol/dm(3) KCl. For the salts, diffusion coefficients decreased in the order LiCl > NaCl > KCl. There was no statistically significant difference between the diffusion coefficients for 1.0 and 0.5 mol/dm(3) NaCl. For all salts at 1.0 mol/dm(3) and also additive-free cements, equilibrium losses were, with statistical limits, the same, ranging from 6.23 to 6.34%. On the other hand, 0.5 mol/dm(3) NaCl lost significantly more water 7.05%.
Resumo:
The role of aluminum in glass-ionomers and resin-modified glass-ionomers for dentistry is reviewed. Aluminum is included in the glass component of these materials in the form of Al(2)O(3) to confer basicity on the glass and enable the glass to take part in the acid-base setting reactions. Results of studies of these reactions by FTIR and magic-angle spinning (MAS)-NMR spectroscopy are reported and the role of aluminum is discussed in detail. Aluminum has been shown to be present in the glasses in predominantly 4-coordination, as well as 5- and 6-coordination, and during setting a proportion of this is converted to 6-coordinate species within the matrix of the cement. Despite this, mature cements may contain detectable amounts of both 4- and 5-coordinate aluminum. Aluminum has been found to be leached from glass-ionomer cements, with greater amounts being released under acidic conditions. It may be associated with fluoride, with which it is known to complex strongly. Aluminum that enters the body via the gastro-intestinal tract is mainly excreted, and only about 1% ingested aluminum crosses the gut wall. Calculation shows that, if a glass-ionomer filling dissolved completely over 5 years, it would add only an extra 0.5% of the recommended maximum intake of aluminum to an adult patient. This leads to the conclusion that the release of aluminum from either type of glass-ionomer cement in the mouth poses a negligible health hazard.
Resumo:
Objective: The aim of this study was to investigate the adaptation of different types of restorations towards deciduous and young permanent teeth. Materials and Methods: Class V cavities were prepared in deciduous and young permanent teeth and filled with different materials (a conventional glass-ionomer, a resin-modified glass-ionomer, a poly-acid-modified composite resin and a conventional composite resin). Specimens were aged in artificial saliva for 1, 6, 12 and 18 months, then examined by SEM. Results: The composite resin and the polyacid-modified composite had better marginal adaptation than the glass-ionomers,though microcracks developed in the enamel of the tooth. The glass-ionomers showed inferior marginal quality and durability, but no microcracking of the enamel. The margins of the resin-modified glass-ionomer were slightly superior to the conventional glass-ionomer. Conditioning improved the adaptation of the composite resin, but the type of tooth made little or no difference to the performance of the restorative material. All materials were associated with the formation of crystals in the gaps between the filling and the tooth; the quantity and shape of these crystals varied with the material. Conclusions: Resin-based materials are generally better at forming sound, durable margins in deciduous and young permanent teeth than cements, but are associated with microcracks in the enamel. All fluoride-releasing materials give rise to crystalline deposits.
Resumo:
The water desorption behaviour of three different zinc oxide dental cements (two polycarboxylates, one phosphate) has been studied in detail. Disc-shaped specimens of each material were prepared and allowed to lose water by being subjected to a low humidity desiccating atmosphere over concentrated sulfuric acid. In all three cements, water loss was found to follow Fick's second law for at least 6 h (until M(t)/M(infinity) values were around 0.5), with diffusion coefficients ranging from 6.03 x 10(-8 )cm(2 )s(-1) (for the zinc phosphate) to 2.056 x 10(-7 )cm(2 )s(-1) (for one of the zinc polycarboxylates, Poly F Plus). Equilibration times for desorption were of the order of 8 weeks, and equilibrium water losses ranged from 7.1% for zinc phosphate to 16.9% and 17.4% for the two zinc polycarboxylates.
Resumo:
The ability of zinc oxide-based dental cements (zinc phosphate and zinc polycarboxylate) to take up fluoride from aqueous solution has been studied. Only zinc phosphate cement was found to take up any measurable fluoride after 5 h exposure to the solutions. The zinc oxide filler of the zinc phosphate also failed to take up fluoride from solution. The key interaction for this uptake was thus shown to involve the phosphate groups of the set cement. However, whether this took the form of phosphate/fluoride exchange, or the formation of oxyfluoro-phosphate groups was not clear. Fluoride uptake followed radicaltime kinetics for about 2 h in some cases, but was generally better modelled by the Elovich equation, dq(t)/dt = alpha exp(-beta q(t)). Values for alpha varied from 3.80 to 2.48 x 10(4), and for beta from 7.19 x 10(-3) to 0.1946, though only beta showed any sort of trend, becoming smaller with increasing fluoride concentration. Fluoride was released from the zinc phosphate cements in processes that were diffusion based up to M(t)/M(infinity) of about 0.4. No further release occurred when specimens were placed in fresh volumes of deionised water. Only a fraction of the fluoride taken up was re-released, demonstrating that most of the fluoride taken up becomes irreversibly bound within the cement.
Resumo:
Water uptake and water loss have been studied in a commercial resin-modified glass-ionomer cement, Fuji II LC, under a variety of conditions. Uptake was generally non-Fickian, but affected by temperature. At room temperature, the equilibrium water uptake values varied from 2.47 to 2.78% whereas at low temperature (12 degrees C), it varied from 0.85 to 1.18%. Cure time affected uptake values significantly. Water uptake was much lower than in conventional glass-ionomer restorative cements exposed to water vapor. Loss of water under desiccating conditions was found to be Fickian for the first 5 h loss at both 22 and 12 degrees C. Diffusion coefficients were between 0.45 and 0.76 x 10( -7) cm(2)/s, with low temperature diffusion coefficients slightly greater than those at room temperature. Plotting water loss as percentage versus s(-(1/2)) allowed activation energies to be determined from the Arrhenius equation and these were found to be 65.6, 79.8, and 7.7 kJ/mol respectively for 30, 20, and 10 s cure times. The overall conclusion is that the main advantage of incorporating HEMA into resin-modified-glass-ionomers is to alter water loss behavior. Rate of water loss and total amount lost are both reduced. Hence, resin-modified glass-ionomers are less sensitive to water loss than conventional glass-ionomers.
Resumo:
This article reviews the means by which fluoride is supplied to populations. Many public health authorities provide fluoridated drinking water, with typical concentrations of fluoride of between 0.5 and 1.0 ppm. This has been found to be safe and effective, though differences in caries incidence between fluoridated and non-fluoridated regions are less than they were 50 years ago, because of the wider availability of fluoridated products to the whole population. Concerns about the effect of fluoride on bone density and associated conditions are reviewed and the general conclusion from considering the literature on fluoride is that there is almost no cause for concern. Alternatives to water as a means of delivering fluoride to the general public that are being used in a number of countries are salt and milk. These alternatives are also reviewed and have been shown to give satisfactory levels of protection against caries, though milk is shown to be less satisfactory than water as a vehicle for fluoride delivery. Milk is also less effective in providing fluoride to individuals in the population, and is less likely to be consumed by people in lower socio-economic groups, precisely those who suffer most from dental caries. This study concludes that mass water fluoridation remains an important contribution to good oral health throughout the community.
Resumo:
There are two major types of erosion testing devices that are used throughout the world for quantifying particle impact erosion against a solid surface. The first of these uses pressurised air to accelerate abrasive particles through a nozzle so that they impinge upon a target specimen. The second adopts a rotating disc to accelerate abrasive particles using the centripetal effect so that they impinge upon a series of targets arranged around the periphery of the disc. This paper reports the findings of a collaborative project that was designed to compare the performance and results obtained from a rig of each of the two types mentioned above. The sand blast type rig was provided by The Department of Powder Science Technology (POSTEC) at The Telemark Technological Research and Development Centre (TEL-TEK), Porsgrunn, Norway while the centripetal effect accelerator was provided by The Wolfson Centre for Bulk Solids Handling Technology, University of Greenwich, London, UK. The test programme included tests against a wide range of materials that are commonly used in pneumatic handling facilities. (Pneumatic handling is a means of conveying and transporting powders and granular solid materials in bulk in industrial process plant, through pipelines using a gas as the carrier medium.) Olivine sand was used as the abrasive and it was projected against the test specimens at velocities and concentrations commensurate with those seen in pneumatic conveyors. In all instances the materials used in the test programme were taken from the same batch so that scatter of experimental results due to specimen variation was minimised. The paper contains a series of recommendations for erosion testing equipment. A discussion based on the results and their applicability to the prediction of wear in pneumatic conveyors concludes the paper.
Resumo:
The gas-blast and centrifugal-accelerator testers are the two most commonly used erosion testers. An experimental and analytical study was made of the effect of particle characteristics (size, shape and concentration) on particle dynamics in each of these testers. Analysis showed that in the gas-blast tester both particle velocity and the dispersion angle of the particle jet were relatively sensitive to the particle characteristics. Particle characteristics, within the ranges studied, had little influence in the centrifugal accelerator tester. Consequently, during an erosion test, the range of particle velocities and dispersion angles in the gas-blast tester ismuch wider than in the centrifugal-accelerator tester. It was concluded that the centrifugal-accelerator tester gave closer control of the important erosion test parameters and therefore more consistent erosion test measurements. However, one drawback of the centrifugal-accelerator tester is the need to account for erosion effects associated with the impact of rotating particles, an inherent feature of this tester.