Particles, primary, hydrolysis product

From the size distribution of hydrolysis product particles obtained after fixed time intervals (Figure 2), it follows that 1 min after adding the reagents to the simulated water, an appearance of a large number of primary aggregates with radius of about 1-2 pm is registered. 

Crystallization of TiO occurs during solvothermal treatment at high pressure, and the crystals grow to primary particle size through homo-coagulation. At this point, excess solvent partially suppresses further crystal growth as a result, the particle size becomes smaller than that in the sol-gel method. However, hydrolysis and condensation reactions occur very rapidly in sol-gel synthesis of transition metal oxides, therefore uniform and ultrafme products are difficult to obtain. 

Diisocyanates are an important class of chemicals of commercial interest, which are frequently used in the manufacture of indoor materials. such as adhesives, coatings, foams and rubbers (Ulrich, 1989). In some types of particle board, the diisocyanates have replaced formaldehyde. Isocyanates are characterized by the electrophilic -N=C=0 group, which can easily react with molecules containing hydroxy groups, such as water or alcohols. On hydrolysis with water, primary amines are formed, while a reaction with alcohols leads to carbamates (urethanes). Polyurethane (PUR) products are then obtained from a polyaddition of diisocyanate and diol components. Compounds commonly used in industrial surface technology are 4,4 -diphenylmethane diisocyanate (MDI) and hexamethylene diisocyanate (HDI). The diisocyanate monomers are known as respiratory sensitizers and cause irritation of eyes, skin and mucous membrane. Therefore, polyisocyanates such as HDI-biuret and HDI-isocyanurate with a monomer content <0.5 % are used for industrial applications, and isocyanate monomers will not achieve high concentrations in ambient air. Nevertheless, it is desirable to measure even trace emissions from materials in private dwellings. 

It was found in this way that in the presence of an excess of silica sol, no free aluminum hydroxide is precipitated by the addition of a base to aluminum salt solutions. Instead, at an acidity level higher (ca. 1 pH unit lower) than corresponds to the precipitation of aluminum hydroxide, the primary products of hydrolysis of aluminum ions are consumed by the interaction with silica sol particles. One equivalent of acid is produced for each atom of aluminum consumed in this reaction, indicating the formation of an aluminosilicic acid, so that ultimately four equivalents of a base are required to reach neutrality for each aluminum atom present. AH aluminum ions are consumed by this reaction if an excess of silica sol is present (see Fig. 14). No visible precipitate is formed and the insoluble alkali salt remains in the sol form it can be converted to gel or coagulated... 

Fumed silicas used in rubber are made by flame hydrolysis of silicon tetrachloride. The products that result have a primary particle size in the range of 5-50 nm. The product is found in the form of agglomerates. The surface is highly active and contains silanol groups, which make the unmodified product hydrophilic. Modified hydrophobic forms are available. As a product, fumed silicas have limited use in rubbers but they are used as a reinforcing filler for silicone rubbers. 

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