Gelation and precipitation

In conclusion, the self-condensation of 2-furaldehyde promoted by heat occurs with the formation of di- and trifurylic intermediates. The functionality of the growing chain increases after each oligomerization step until gelation and precipitation of the resin occurs. Thus, the process is non-linear from the onset since the condensation product 4 possesses three sites for further attack, namely the free C-5 position and the two formyl groups. It is interestering to note that while the polycondensation of 2-furfuryl alcohol is essentially linear and cross-linking is due to side reactions, the thermal resinification of 2-furaldehyde is intrinsically non-linear and gel formation occurs at earlier conversions. 

In a first part of this paper, we will discuss results of a 27A1 NMR study of the binding of Al ions on acrylamide-acrylic acid copolymers as a function of pH, at the light of a simple electrostatic model. The second part deals with the phase diagrams, physical gelation and precipitation phenomenon, for different copolymer compositions and under various conditions of concentrations, pH and salinity. 

Propyleneglycol alginates do not precipitate from acidic media and their acid stability increases in proportion to the propyleneglycol content. Additionally, those esters with DS > 60% are not precipitated by Ca2 +, because the bulky propyleneglycol substituent hinders the prerequisite alignment for gelation and precipitation. These esters are more tolerant of alcohol than the unesterified alginate. 

Gelation and precipitation occurs with multivalent cations and with some polycations. However, solubilization was observed with proteins (soy protein, caseinate) in the isoelectric range. At low pH, CMC may form cross-links through lactonization between carboxylic acid and free hydroxyl groups. ... 

Retrogradation - The ordered reassociation of starch molecular strands by hydrogen bonding that occurs after paste formation. The hydrogen bonds form junction zones which can lead to higher viscosity, gelation, and precipitation. 

Cyclamate [IV] is generally used in the form of a sodium salt [V] because it is more soluble in water than the free acid. The calcium salt is also used as a sweetener, but, for some applications, it is not suitable as it can cause gelation and precipitation. Sodium cyclamate exhibits good stability in the solid form and is also stable in soft drink formulations within the pH range 2-10. Cyclamate is permitted in several countries (EU, Australia, Canada, New Zealand, etc.). However, it has been banned in the USA after controversial toxicity studies. 

Drying of ink n. The conversion of an ink film to a solid state. This can be accom-phshed by any of the following means, either singly or in combination oxidation, evaporation, polymerization, penetration, gelation, and precipitation. 

V.2.8. Dilution effects competition between gelation and precipitation... 

The second effect (dilution) is much more important. As we shall see, when the monomers become attached together by chemical links, they tend to set up clusters in the solvent, that is, to precipitate. Thus there is a competition between gelation and precipitation, which is essential for many practical applications. 

SAXS study of gelation and precipitation in titanium-based systems. / Non-Cryst Solids, 147 148, 189-193. 

Besides pH, other preparative variables that can affect the microstructure of a gel, and consequendy, the properties of the dried and heat-treated product iaclude water content, solvent, precursor type and concentration, and temperature (9). Of these, water content has been studied most extensively because of its large effect on gelation and its relative ease of use as a preparative variable. In general, too Httie water (less than one mole per mole of metal alkoxide) prevents gelation and too much (more than the stoichiometric amount) leads to precipitation (3,9). Other than the amount of water used, the rate at which it is added offers another level of control over gel characteristics. 

An ink is considered dry when a print does not stick or transfer to another surface pressed into contact with it. Drying is accompHshed by one or more of the following physical or chemical mechanisms absorption, evaporation, precipitation, oxidation, polymerization, cold setting, gelation, and radiation curing. 

Most ultrafiltration membranes are porous, asymmetric, polymeric structures produced by phase inversion, i.e., the gelation or precipitation of a species from a soluble phase. See also Membrane Separations Technology. Membrane structure is a function of the materials used (polymer composition, molecular weight distribution, solvent system, etc) and the mode of preparation (solution viscosity, evaporation time, humidity, etc.). Commonly used polymers include cellulose acetates, polyamides, polysulfoncs, dyncls (vinyl chlondc-acrylonitrile copolymers) and puly(vinylidene fluoride). 

A schematic diagram of a commercial-scale thermal gelation polymer precipitation process is shown in Figure 3.16. The hot polymer solution is cast onto a water-cooled chill roll, which cools the solution, causing the polymer to precipitate. The precipitated film is passed through an extraction tank containing methanol, ethanol or isopropanol to remove the solvent. Finally, the membrane is dried, sent to a laser inspection station, trimmed and rolled up. 

Preparations took place in a stirred glass reactor equipped with sensors (pH electrode, thermocouples and torquemeter on stirring shaft) which allow to control silica gelation and alumina precipitation. 

Owens, H.S., McReady, R.M. and Maclay, W.D. 1949. Gelation and characteristics of acid-precipitated pectinates, Food Technol., 3 77-82. 

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