Water-Based Aqueous Processing

Swan dive, belly flop, or mile swim  

Oswald Chambers is generally the person attributed with the slippery slope picture. When you re on the pinnacle (where you should be), one step in any wrong direction leads to a drop. One wrong step is easy to make, but getting back to where you should be is a hard and frustrating task. What s a famous preacher got to do with it Tape casting started as a water-based production method. It fell way, way down that slippery slope. It s a long way back up. 

One of the hardest parts of water-based slip development is that everyone in the capitalist industry wants it. So when manufacturers find something really great, they hide it instead of telling others. Don t worry...we don t think capitalism is such a horrible thing, or that the companies keeping secrets are evil, but trade secrets don t aid technology development in this case, now do they  

One of the most troublesome aspects of water-based slip development is the high dielectric constant of the universal solvent, which, after a few steps of logic, which we ll exclude to give you a puzzle to ponder, leads to the point that water-based slip formulations tend to be aggressively powder dependent. Where a PVB slip might be useful for alumina, zirconia, titania, ferrite, and nickel metal, a water-based slip will tend to work only for one certain powder chemistry and fail miserably for other similar powders. 

Two basic approaches have been used in water-based tape casting. These approaches differ in the types of binder selected. In organic systems, all binders used were soluble in at least one of the vehicles chosen. This same approach in the water-based area leads to the use of water-soluble film formers, including certain celluloses, vinyls, and acids. The second approach is to stick with the polymer families we know well (acrylates and vinyls) and to form water-based emulsions of these resins. A caution must be issued at this point due to the fact that many in the field consider emulsions to be water soluble. 

---

CHAPTER 7—WATER-BASED (AQUEOUS) PROCESSING Introduction... 

Recovery and Purification. AH processes for the recovery and refining of maleic anhydride must deal with the efficient separation of maleic anhydride from the large amount of water produced in the reaction process. Recovery systems can be separated into two general categories aqueous- and nonaqueous-based absorption systems. Solvent-based systems have a higher recovery of maleic anhydride and are more energy efficient than water-based systems. 

The major advantage of the use of two-phase catalysis is the easy separation of the catalyst and product phases. FFowever, the co-miscibility of the product and catalyst phases can be problematic. An example is given by the biphasic aqueous hydro-formylation of ethene to propanal. Firstly, the propanal formed contains water, which has to be removed by distillation. This is difficult, due to formation of azeotropic mixtures. Secondly, a significant proportion of the rhodium catalyst is extracted from the reactor with the products, which prevents its efficient recovery. Nevertheless, the reaction of ethene itself in the water-based Rh-TPPTS system is fast. It is the high solubility of water in the propanal that prevents the application of the aqueous biphasic process [5]. 

The Arrhenius concept was of basic importance because it permitted quantitative treatment of a number of acid-base processes in aqueous solutions, i.e. the behaviour of acids, bases, their salts and mixtures of these substances in aqueous solutions. Nonetheless, when more experimental material was collected, particularly on reaction rates of acid-base catalysed processes, an increasing number of facts was found that was not clearly interpretable on the basis of the Arrhenius theory (e.g. in anhydrous acetone NH3 reacts with acids in the absence of OH- and without the formation of water). It gradually became clear that a more general theory was needed. Such a theory was developed in 1923 by J. N. Br0nsted and, independently, by T. M. Lowry. 

While minimization possibilities are being investigated, substitutions should also be considered as an alternative or companion concept that is, safer materials should be used in place of hazardous ones. This can be accomplished by using alternative chemistry that allows the use of less hazardous materials or less severe processing conditions. When possible, toxic or flammable solvents should be replaced with less hazardous solvents (for example, water-based paints and adhesives and aqueous or dry flowable formulations for agricultural chemicals). 

The kinetics of the alkaline hydrolysis of 2-methylpentyl salicylate (24) have been studied in various aqueous propanol and r-butanol mixtures and in mixtures of water and ethane-l,2-diol. ° Further smdies of the aminolysis of ionized phenyl salicylate (25) have been reported, in which it was observed that, in mixed acetonitrile-water solvents, glycine, 1,2-diaminoethane and 3-aminopropanol all reacted as did simple amines, via an intramolecular general-base-catalysed process. ... 

The final product of emulsion polymerization is an emulsion —a stable, heterogeneous mixture of fine polymer beads in an aqueous solution, sometime called a latex emulsion. Water-based paints, for example, can be formed from the emulsion polymerization of vinyl acetate. In this process, I m of water containing 3% poly(vinyl alcohol) and 1% surfactant are heated to 60°C in a reaction vessel (see Figure 3.27) The temperature rises to around 80°C over a 4 to 5 hour period as monomer and an aqueous persulfate solution are added. The rate at which heat can be removed limits the rate at which monomer can be added. 

In the discussion of the general base catalyzed addition step above (p. 120) the objection was raised that it was difficult to believe that general base catalysis would be necessary for the addition of water to so reactive a species as a protonated ester. An answer to this objection is implicit in the discussion above of the mechanism of hydrolysis of orthoesters. It appears that the protonated orthoester, which would be the initial product of the simple addition of a molecule of water to a protonated ester, is too reactive a species to exist in aqueous solution, and that carbon-oxygen bond-cleavage is concerted with the transfer of the proton to the orthoester. The formation of a protortated orthoester by the addition of a molecule of water to the conjugate acid of an ester will be even less likely, and it seems entirely reasonable, therefore, that the formation of the neutral orthoester, by a general base catalyzed process, should be the favoured mechanism. 

The gauge glass will normally be somewhat colder than the process vessel as a result of ambient-heat losses (an exception to this would be a refrigerated process). For every 100°F decrease in the gauge-glass temperature or level-trol temperature, the specific gravity of the liquid in the glass increases by 5%. This rule of thumb is typical for hydrocarbons only. Aqueous (water-based) fluids are totally different. 

Acid-base neutralization reactions are processes in which an acid reacts with a base to yield water plus an ionic compound called a salt. You might recall from Section 2.9 that we defined acids as compounds that produce H+ ions when dissolved in water and bases as compounds that produce OH- ions when dissolved in water. Thus, the driving force behind a neutralization reaction is the production of the stable covalent water molecule by removal of H + and OH- ions from solution. The reaction between hydrochloric acid and aqueous sodium hydroxide to yield water plus aqueous sodium chloride is a typical example ... 

---