Silicate melt process

Fraser et al. (1983, 1985) deduced that, in several instances, the structure of the silicate melt should mimic the structural arrangement of the solid phase at liq-uidus. In incongruent melting processes, the structure of the liquid differs substantially from that of the solid phase of identical stoichiometry. 

The age of meteorites tells us that the solar system - and therefore the Earth - was born roughly 4.6 billion years ago. The oldest terrestrial rocks are zircone crystals (zirconium silicates) which are 4.2 billion years old, but these stones do not tell us much apart from their age, because they are igneous, or magmatic, rocks whose melting processes have erased any trace of history. Much more interesting are the sedimentary rocks, because these were formed by materials that sank to the bottom of ancient seas, and may still contain remnants of the past. The oldest sediments have been found at Isua, in Greenland, and are 3.8 billion years old, which means that there were immense streches of water on our planet at that time, and that the first oceans had originated many millions of years earlier. 

High-Temperature Processes. Silicate Melt Process. One of the high-temperature processes we are studying involves simple melting of a mixture of salt cake with either crushed basalt rock and B2O3 or sand... 

Figure 3. Conceptual flowsheet for application of the silicate melt process to...

The optical micrographs in Figure 5 show the effect of oxygen plasma exposure to pure nylon 6 and a nylon 6/7.5wt% layered silicate nanocomposite. Both are melt-processed samples recast from the 1,1,1,3,3,3-hexa-fluoro-2-propanol solution. The nylon 6 sample experiences almost complete deterioration after 8 hours (480 minutes) of continuous exposure. In contrast, deterioration of the nanocomposite is minimal, with no significant decrease in thickness. Buckling of the nanocomposite sample after exposure arises from differences in thermal expansivity of the self-generating ceramic surface and the bulk polymer nanocomposite. 

Several techniques such as intercalation of polymer from solution, in-situ intercalative polymerization, melt intercalation, direct mixture of polymer and particulates, template synthesis, in-situ polymerization and solgel process, are being employed for the preparation of polmer-layered silicate nanocomposites. Among them the most common and important approaches are in-situ polymerization, solution-induced intercalation method, and melt processing method, which are briefly discussed below. 

Nanocomposite technology using small amounts of silicate layers can lead to improved properties of thermoplastic elastomers with or without conventional fillers such as carbon black, talc, etc. Mallick et al. [305] investigated the effect of EPR-g-M A, nanoclay and a combination of the two on phase morphology and the properties of (70/30w/w) nylon 6/EPR blends prepared by the melt-processing technique. They found that the number average domain diameter (Dn) of the dispersed EPR phase in the blend decreased in the presence of EPR-g-MA and clay. This observation indicated that nanoclay could be used as an effective compatibilizer in nylon 6/EPR blend. X-ray diffraction study and TEM analysis of the blend/clay nanocomposites revealed the delaminated clay morphology and preferential location of the exfoliated clay platelets in nylon 6 phase. 

A more extensive account of the formation, structural characterisation and physical properties of silicate layer nanocompsites can be found in Chapter 10. The discussion below focuses on nanocomposite preparation by melt processing, since this provides the most promising route for more widespread acceptance and subseqnent commercial exploitation of these materials. 

To assist in the production of silicate layer nanocomposites SCCO2 has also been used. Intercalcated poly(methyl methacrylate) clay nanocomposites, containing 25-50% by weight of clay, have been made by polymerisation of methyl methacrylate in a SCCO2 pressure vessel [112]. This overcame previous melt processing difficulties caused by the... 

In conclusion, in this study the sol-gel synthesis of ZnO nanoclusters embedded in silica has been faced by the gel-derived binary system ZnO-Si02. As compared to oxide mixtures prepared by conventional solid-state reactions, in gel-derived systems the formation of ceramic silicate compounds is observed at lower temperatures than expected for traditional melting processing [173]. Through XPS it was possible to evidenee the importance of the choice of proper precursor compounds and proper annealing conditions to produce stable ZnO guest clusters dispersed into the host silica glassy matrix. 

Quaternary alkylammonium salts or alkylamines are the cationic surfactants most commonly used in the modification of layered silicates. They are synthesized by complete alkylation of ammonia or amines. Many efforts have been made to produce ammonium surfactants to improve the affinity between the clay mineral and the polymer [3]. Ammonium organoclays undergo thermal degradation at temperatures below or comparable to the melt-processing temperatures of many polymers. 

Because both melt-processing and polymerization of PET necessitate high temperatures (250-300 °C), it becomes obvious at the outset that any organically modified layered silicates that are intended as reinforcing fillers for PET should employ surfactants with appropriately high thermal stability. The typical alkylammoniums, for example, decompose below these temperatures. Two examples of higher-temperature surfactants that have been employed as modifiers for layered silicates in PET nanocomposites are pyridinium and phosphonium specifically, cetylpyridinium, via solution dispersion [22], and dode-cyltriphenylphosphonium, via in situ polymerization [23], In these two cases, both the... 

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