Modified materials

A number of innovations made in the 1920s and 1930s may be noted. Several attempts were made to reduce the dissolution of these cements in oral fluids and their adverse effect on the pulp by inclusion of oils and greases (Simon, 1929, 1932 Eberly, 1934). None have been considered beneficial (Palfenbarger, Schoonover Souder, 1938), a not surprising result because the inclusion of hydrophobic substances is bound to interfere in the setting of an aqueous cement. 

Poetschke (1925) patented a dental silicate powder prepared by fusing zinc silicate with calcium fluoride. This is a kind of silicophosphate cement (Section 6.6). Thomsen (1931) attempted to formulate a water-setting dental cement. Heynemann (1931) included lithium salts in the flux and Brill (1935) included them in the liquid. 

During this period a number of attempts at reinforcing these cements were made. Fillers described include carborundum (Salzmann, 1930), cellulose fibres (Schonbeck Czapp, 1936) and even diamonds (Salzmann, 1930). None of these innovations found their way into commercial materials (Palfenbarger, Schoonover Souder, 1938). 

More recently, Stanicioiu, Chinta Hartner (1959) attempted to reinforce the cement with glass fibres, but this was not successful. The most serious study on the reinforcement of dental silicate cement was made by J. Aveston (in Wilson et al., 1972). Silicon carbide whiskers, carbon fibres and alumina powder were introduced into the cement mix. Unfortunately, the glass powder/liquid ratio had to be reduced, and the strength gained by reinforcement was thereby lost. It is clear that dental silicate cement cannot be strengthened by fibre or particulate reinforcement. 

Systematic attempts to formulate improved materials have met with no success (Manly et al., 1951 Rockett, 1968). The last and, in some ways, most promising attempt at improving the dental silicate cement was made by Pendry (Pendry Cook, 1972 Pendry, 1973) who improved its resistance to acid by adding indium to both powder (5-8 %) and liquid (5-65 %). The cement, however, lacked suflScient translucency, and by this time the glass-ionomer cement had arrived with its advantages of translucency and resistance to staining and acid attack. 

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Another sol—gel abrasive, produced by seeding with a-ferric oxide or its precursors, has been patented (30). A magnesium-modified version of this abrasive, also called Cubitron, is being produced as a replacement for the earlier type. Yttria [1314-36-91-vnc>A V eA sol—gel abrasives have also been patented (31), as well as rare earth oxide modified materials (32). These abrasives are all produced by 3M Corporation they have performed very well ia various applications such as ia coated abrasives for grinding stainless steel and exotic alloys. 

Flexographic Inks. Fluorescent toners such as the Radiant GF, Lawter HVT, and Day-Glo HM and HMS Series toners are used in flexographic ink formulations. These products are soluble in blends of alcohol (80%) and ester solvents (20%) and are compatible with modifying materials such as nitroceUulose resins and acryHc solution polymers. Flexographic inks of this type are used most commonly to print products such as ceUophane and polyethylene film for packaging, and also to print paper products such as gift wrap and price labels. 

In this section discussion will be confined to the true polyimides whilst the modified materials will be considered in Section 18.14. 

Fig. 7.10. The solid state reactivity of shock-modified zirconia with lead oxide as studied with differential thermal analysis (DTA) shows both a reduction in onset temperature and apparent increase in reaction rate. The shock-modified material has a behavior much like the much higher specific surface powder shown in B (after Hankey et al. [82H01]).

Fig. 7.13. The conversion of theta- to alpha-phase alumina was found to be strongly affected by shock modification in work of Beauchamp and co-workers [90B01]. Whereas the unshocked powder showed evidence for an incubation period of 60 min, the shock-modified materials show immediate conversion typical of the presence of shock-formed nuclei.

Currently, graft post-polymerization of monomers in the gaseous phase (2) is the more widely used process because it has at least two basic advantages. First, side processes of homopolymerization are minimized which reduces the consumption of monomers and makes unnecessary additional treatment of the modified materials with solvents. Second, this method is universal and allows the grafting to the surfaces (such as silica) to be carried out with low radiation yields of active sites as compared to the monomers. 

Modify materials, or process, if production materials properties not adequate... 

Protect or modify materials if service environment causes excessive degradation of properties... 

The data provided by Toyota Research Group of Japan on polyamide-MMT nanocomposites indicate tensile strength improvements of approximately 40%-50% at 23°C and modulus improvement of about 70% at the same temperature. Heat distortion temperature has been shown to increase from 65°C for the unmodified polyamide to 152°C for the nanoclay-modified material, all the above having been achieved with just a 5% loading of MMT clay. Similar mechanical property improvements were presented for polymethyl methacrylate-clay hybrids [27]. 

The investigation of the chemical modification of dextran to determine the importance of various reaction parameters that may eventually allow the controlled synthesis of dextran-modified materials has began. The initial parameter chosen was reactant molar ratio, since this reaction variable has previously been found to greatly influence other interfacial condensations. Phase transfer catalysts, PTC s, have been successfully employed in the synthesis of various metal-containing polyethers and polyamines (for instance 26). Thus, the effect of various PTC s was also studied as a function of reactant molar ratio. 

Modify material or product. If a material is not capable of meeting established material or product criteria, it is useful to consider whether additional or alternative material processing or product modification could achieve the desired results. 

The specific capacity obtained in such Ams, actually corresponds to near theoretical limit of 372 mA-h/g, as calculated on a basis of classical LiC6 stoichiometry). Further increase of capacity is possible only via switching to new or modified materials, composites or alloys, which are capable for reversible and stable intercalation of lithium. 

As it is seen from the data of Figure 8, all modified materials have poor cycling performance their reversible capacities fade faster than the one of initial non-modified material, and become lower after the first 8-10 charge-discharge cycles. Thus, we can conclude that no positive effect is achieved by means of modification of the Carbon-Type material with bimetal tri-nuclear complex of Co(III)-Ni(II). 

The mesoporous molecular sieve SBA-15 has been functionalized with aminopropyl moieties via grafting. Further treatment of the 3-aminopropyl-modified material with glutardialdehyde (GA) results in GA-ATS-SBA-15. The modified silica materials were characterized by NMR and IR spectroscopy as well elemental analysis confirming the successful modification. Furthermore, the elemental analysis suggests that two of three amino moieties of the 3-aminopropyl modified material react further with... 

Various attempts have been made to improve the stability of amorphous sulphur. Addition of small amounts of halogens, i.e., chlorine, bromine, iodine, treatment with sulphur monochloride or the addition of a terpene have some effect on the rate of conversion. As far as is known, none of the modified materials are commercially available. 

Prediction of the behavior of osmotically modified materials during further processing and storage. 

In 1990, Choudary [139] reported that titanium-pillared montmorillonites modified with tartrates are very selective solid catalysts for the Sharpless epoxidation, as well as for the oxidation of aromatic sulfides [140], Unfortunately, this research has not been reproduced by other authors. Therefore, a more classical strategy to modify different metal oxides with histidine was used by Moriguchi et al. [141], The catalyst showed a modest e.s. for the solvolysis of activated amino acid esters. Starting from these discoveries, Morihara et al. [142] created in 1993 the so-called molecular footprints on the surface of an Al-doped silica gel using an amino acid derivative as chiral template molecule. After removal of the template, the catalyst showed low but significant e.s. for the hydrolysis of a structurally related anhydride. On the same fines, Cativiela and coworkers [143] treated silica or alumina with diethylaluminum chloride and menthol. The resulting modified material catalyzed Diels-Alder reaction between cyclopentadiene and methacrolein with modest e.s. (30% e.e.). As mentioned in the Introduction, all these catalysts are not yet practically important but rather they demonstrate that amorphous metal oxides can be modified successfully. 

Lewis, L. N. From Sand to Silicones An Overview of the Chemistry of Silicones. In Silicones and Silicone-Modified Materials Clarson, S. J., Fitzgerald, J. J., Owen, M. J., Smith, S. D., Eds. ACS Symposium Series 729 American Chemical Society Washington, DC, 2000 pp 11-19. 

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