Molecular weight high-solvency

Other than fuel, the largest volume appHcation for hexane is in extraction of oil from seeds, eg, soybeans, cottonseed, safflower seed, peanuts, rapeseed, etc. Hexane has been found ideal for these appHcations because of its high solvency for oil, low boiling point, and low cost. Its narrow boiling range minimises losses, and its low benzene content minimises toxicity. These same properties also make hexane a desirable solvent and reaction medium in the manufacture of polyolefins, synthetic mbbers, and some pharmaceuticals. The solvent serves as catalyst carrier and, in some systems, assists in molecular weight regulation by precipitation of the polymer as it reaches a certain molecular size. However, most solution polymerization processes are fairly old it is likely that those processes will be replaced by more efficient nonsolvent processes in time. 

Supercritical fluid chromatography is a form of chromatography in which the temperature is held near the critical temperature of the mobile phase and pressure utilized to effect solvency and hence migration. The advantages of this technique have been shown to be Increased r nsfer and the migration of high molecular weight... 

The adsorption of polymeric surfactants is more complex, since in this case the process is irreversible and produces a high-affinity isotherm with a steep rise in the adsorption value at low polymer concentrations (in this region most of the molecules are completely adsorbed). Subsequently, the adsorbed amount remains virtually constant, giving a plateau value that depends on the molecular weight, temperature and solvency of the medium for the chains (this topic was discussed in detail in Chapter 6). 

For high molecular-weight polymer-solvent systems, the polymer critical concentration is close to zero and the interaction parameter has a value close to 0.5. Thus, a good solvent (polymer soluble in the solvent at all proportions) is obtained if < 0.5, whereas values greater than 0.5 indicate poor solvency. Since we mentioned that the model is only an approximate representation of the physical picture and that the FH parameter is often not a constant at all, this empirical rule is certainly subject to some uncertainty. Nevertheless, it has found widespread use and its conclusions are often in good agreement with experiment. 

In addition to these major market segments, plasticizers are used in a variety of smaller segments. Adhesives and sealants generally use lower molecular weight phthalate esters, benzoate esters and diesters, and dibutyl terephthalate because of improved solvency and lower fusion temperatures. One major use of plasticized PVC is automotive underbody coatings these products are highly filled plastisols that are mostly commonly based upon GP plasticizers often with a fast fusing plasticizer added to improve processability however, plasticizer volatility can be a major concern. 

The industry defines this group with a minimum 55% paraffinic side chains (psc). Because of its high level of saturates, this group has excellent resistance to color development and the oxidation resistance increases with molecular weight. However, low aromatic content gives poor solvency and this group of process oils is not recommended for polar elastomer compounding. 

Water-miscible semi-aqueous cleaners include low molecular weight alcohols, ketones, esters, and organic amines. Table 13.4 gives some properties of water-miscible cleaners. N-methyl-2-pyrrolidone-based solvents have a high solvency for a number of contaminants and are completely water-soluble. Acetone (CH3COCH3) removes heavy oils quite effectively hut tends to leave a residue and it is also quite flammable. Acetone cleaning or wipeclean should be followed by a methanol rinse or wipeclean to remove the residue. 

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