Calcium and zinc ion release from hydroxyapatite-zinc oxide-poly(acrylic acid) (HAZnO-PAA) composite cements into deionised water was investigated as a function of HA content, PAA concentration, PAA molecular weight and maturation time. At any given maturation time, zinc ion release was constant until the HA content was at the maximum loading (60 wt%) resulting in the cement matrix breaking up, allowing exacerbated ion release. The calcium ion release increased with increased HA content in the composite until the maximum loading where the release drops off. Up to this point, the release of both ionic species was proportional to square root time for the initial 24 hour period, indicating that the release is diffusion controlled. In agreement with related data from conventional Glass Polyalkenoate Cements (GPCs), it is the concentration of the PAA, not the molecular weight, that influences ion release from these materials. However, unlike GPCs, the release of the active ions results in a pH rise in the deionised water, more conventionally seen with Bioglass and related bioactive glasses. It is this pH rise, caused by the ion exchange of Zn(2+) and Ca(2+) for H(+) from the water, leaving an excess of OH(-), that should result in a favourable bioactive response both in vitro and in-vivo.
Glass ionomer cements (GICs) are an important class of biomedical material used extensively for color matched mercury free, dental restorations. GICs can release clinically beneficial amounts of fluoride and have acceptable handling properties which make them suitable as dental restoratives. The fluoride release of model GICs produced from specially synthesized fluoro-alumino-silicate glasses was studied. Nine glasses of varying fluoride content based on 4.5SiO(2)-3Al(2)O(3)-1.5P(2)O(5)-(5-Z)CaO-ZCaF(2) were synthesized and cement disks were prepared from them. The glass transition temperature reduced with increasing fluorine content of the glass. Fluoride ion release was measured into distilled water as a function of time for up to 140 days using a fluoride ion selective electrode. The quantity of fluoride released was found to be proportional to the fluorine content of the glass at all intervals time. The cumulative fluoride release was proportional to square root time. Substituting strontium for calcium in the glass had little influence on the fluoride release behavior of the cements.
In the synthetic rubber production lines for curing and isolation, in order to dewater the moist crumb rubber at a temperature of 140-200~ squeezing and drying screw presses are used which have a single spindle shaft of diameter 250-300 mm [1]. Test operation has shown that the service life of such presses is limited by the intense and nonuniform wear of the crests of the worm spirals, the filter plates (rods) of the filter body of squeezing presses, and the cylinder bushing of drying presses. Wear on the working parts of screw presses reduces the output and quality of the rubber as a result of metal particles entering it. Furthermore, in operation at the eontaetpoints between screw and cylinder there arise considerable loads (up to 6000 kgf and above), leading to fracture of the filter body rods and emergency shutdown of the presses. Analysis of wear on the worm shafts and body components of a type OM-1K-300 squeezing screw press when dewatering type SKS-30 rubber and of a type SM-1K-250 drying screw press when drying type SKD rubber showed that maximum wear at the crests of the worm spirals, filter body rods, and cylinder bushings occurs at the discharge attachment end because the worm shafts are cantilever mounted {Fig. 1). Moreover, the greatest wear is observed in the initial operation period (up to 500 h), and is related to running in rubbing surfaces under different operating conditions. The cause of rod fracture is wear and corrosion cracking accompanied by the development of fatigue microcracks in contact points with the screw. Accordingly, in order to increase the wear resistance and service life of screw presses it is most appropriate to strengthen the crests of the worm spirals, rods, and cylinder bushings on the discharge side and also to select the appropriate materials for all components.To strengthen the worm shafts, the spiral peaks have a welded-on deposit of stellite based on cobalt, while the bushes and rods are made from sand-cast or centrifugally east steilite. Alloys based on cobalt are widely used for this purpose [2, 3], but they are in short supply and costly, while weld overlaying large components is quite complex due to the need for heating up to a temperature of at least 600~ before and during the operation.
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