Powder formation chemical

In chemical crosslinking, one must either extend the polymerization process, or add suitable reactants and additives to induce the formation of chemical bonds between adjacent polymeric chains. One example of chemical crosslinking is rubber, which may be chemically crosslinked by addition of sulfur. With UHMWPE, peroxides such as dicumyl peroxide may be mixed with the resin powder to chemically crosslink the polymer during conversion to bulk rod or sheet (Shen, McKellop, and Salovey 1996). However, chemical crosslinking is not used to process UHMWPE for medical applications, and for this reason we will focus on radiation crosslinking for the remainder of this chapter. 

Chemical modeling of gas-phase and surface reactions, nucleation, and growth is based on concepts developed for surface reactions on heterogeneous catalysts. In order to understand the reaction rates and the microstructure of the products observed during CVD, the driving forces for the reaction, the kinetics in the pyrolytic regime, powder formation at high supersaturation, and the uses of thermodynamic calculations have to be discussed briefly. 

GI materials, the second component is a powder produced from an ion-leachable aluminosilicate glass (9), whereas in ZP cements, the powder is essentially pulverized zinc oxide, containing, in some cases, small amounts of magnesium oxide (10). Both powders are chemically basic, and thus react with the aqueous solution of the pol3mieric acid. The acid/base reaction that takes place when powder and liquid components are mixed, transforms the paste to a rigid mass within ten to twenty minutes. The mechanistic details of this reaction, as well as the structure/property relations obeyed by the solid product obtained are not well known at this time. Supposedly, the reaction involves the formation of ionic crosslinks between... 

Just as, in Group VB, niobium, so, in this Group, molybdenum provides most of the examples of the chalcogenide halides. The occurrence and preparation of such compounds are described in numerous publications. In most cases, they have been obtained as powders, with the composition based on chemical analyses only. The presence of defined, homogeneous phases is, therefore, in many cases doubtful. In addition, some published results are contradictory. A decision is possible where a complete structure analysis has been made. As will be shown later, the formation of metal-metal bonds (so-called clusters), as in the case of niobium, is the most characteristic building-principle. Such clusters... 

The physical and chemical characteristics of zinc oxide powders are known to affect cement formation (Smith, 1958 Norman et al., 1964 Crisp, Ambersley Wilson, 1980 Prosser Wilson, 1982). The rate of reaction depends on the source, preparation, particle size and surface moisture of the powder. Crystallinity and lattice strain have also been suggested as factors that may change the reactivity of zinc oxide powders towards eugenol (Smith, 1958). 

The white crystals change to yellow powder upon grinding, presumably with loss of THF and possibly some AuCN formation. Thermal gravimetric analysis of [Au2(2,6-Me2Ph-form)2] 2Hg(CN)2 2THF showed the release of THF gradually at >120 °C followed by decomposition at >200 °C. The powder diffraction pattern of the yellow residue after heating above 265 °C showed a pattern typical of AuCN (IR 2236 cm ) as confirmed by comparison with the powder diffraction pattern of a sample of AuCN obtained from the Aldrich Chemical Co. 

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