Modem modified composite resins

Table 3.3 Modem modified composite resins used in dentistry...

The essential features of the two basic types of restorative material are given in Table 2.1. From this, it can be seen that each type has its own advantages and disadvantages. In terms of overall properties, modem composite resins appear to be favoured, and there is evidence that these materials are the ones used in the majority of aesthetic repairs in dentistry, particularly in adults. However, as the development of the polyacid-modified composite resins (compomers) shows, these materials are far from perfect, and there is unquestionably scope to enhance their properties. Glass-ionomer cements have properties that would seem to indicate the direction in which improvements could be made, despite the technical difficulties in doing so. 

As mentioned, these materials seem to have found particular application in children s dentistry. The successive reformulations mean that they may have lost their original distinctive characteristic of having a small amount of acid-base reaction following post-cure moisture uptake. There is evidence that modem polyacid-modified composite resins primarily release fluoride as a result of the additional fluoride compound, as with fluoridated conventional composites, and that any acid-base reaction is so slight that it has little, if any, effect on the properties of the material. Overall, these materials do not duphcate the properties of either of the parent materials particularly well, and their current use in clinical dentistry is fairly limited [1]. 

Composites used in Dentistry were developed in 1962 by combining dimethacrylates (epoxy resin and methacrylic acid) with silanized quartz powder [49]. Modem restorative composites are comprised of synthetic monomers, typically dimethacrylates, reinforcing fillers, typically made from radiopaque glass, quartz or silica, chemicals which promote or modify the polymerization reaction, and silane coupling agents which bond the reinforcing fillers to the polymer matrix [26]. 

The forerunner of the modem methods of asphalt fractionation was first described in 1916 (50) and the procedure was later modified by use of fuller s earth (attapulgite [1337-76 4]) to remove the resinous components (51). Further modifications and preferences led to the devdopment of a variety of fractionation methods (52—58). Thus, because of the nature and varieties of fractions possible and the large number of precipitants or adsorbents, a great number of methods can be devised to determine the composition of asphalts (5,6,44,45). Fractions have also been separated by thermal diffusion (59), by dialysis (60), by dectrolytic methods (61), and by repeated solvent fractionations (62,63). 

Having established the essential features of resin-modified glass-ionomers, in the rest of the chapter we consider their composition, properties and clinical applications in modem clinical practice. 

Both anion- and cation-exchange columns are available with a choice of resin matrix (polymeric or sUica) and ionic functional group. Modem IC resins often contain imbedded polar groups in addition to the ion-exchange groups. The purpose is to reduce the hydrophobic nature of the soHd ion exchanger and to modify the nonionic attraction of analyte ions for the soHd phase. The effects of resin composition on anion selectivity will be discussed Section 3.4. 

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