Latent solvent

In terms of general solvency, solvents may be described as active solvents, latent solvents, or diluents. This differentiation is particularly popular in coatings applications, but the designations are useful for almost any solvent appHcation. Active solvents are strong solvents for the particular solute in the apphcation, and are most commonly ketones or esters. Latent solvents function as active solvents in the presence of a strong active solvent. Alcohols exhibit this effect in nitrocellulose and acryUc resin solutions. Diluents, most often hydrocarbons, are nonsolvents for the solute in the apphcation. 

Butanol is used as a direct solvent ia paints and other surface coatings. It acts synergisticaHy with butyl acetate as a latent solvent system for nitrocellulose lacquers and thinners to give a solvent system stronger than either solvent alone. Other direct solvent appHcations for -butyl alcohol are ia the formulation of pharmaceuticals, waxes, and resias. 

When the solubility of the thermoplastic is poor at room temperature it may be possible to produce a dispersion in a mixture of diluents and latent solvents. This dispersion may be applied by conventional paint spray equipment. The coated item is placed in an oven where the diluents evaporate off. The latent solvents then dissolve the thermoplastic and evaporate from this solution at a controlled rate, thus producing a continuous him. P.V.F., and p.v.d.f. and p.t.f.c.e. coatings are produced from dispersions of this type. 

Structurally modified PVF has been extruded.66 Thin films are manufactured by extrusion of a dispersion of PVF in a latent solvent.67 Such dispersion contains usually pigments, stabilizers, plasticizers, and flame retardants as well as deglossing agents if needed. The solvent is removed by evaporation. The extruded film can be biaxially oriented and the solvent is removed by evaporation only after the orientation is completed. 

PVF Latent solvent >200 (392) Weather resistant paints and coatings,... 

Use Organic synthesis, latent solvent in paints and lacquers, intermediate for amino coating resins, substitute for n-butanol. Paint removers, fluorometric... 

Use Manufacture of acetone and its derivatives, manufacture of glycerol and isopropyl acetate, solvent for essential and other oils, alkaloids, gums, resins, etc. latent solvent for cellulose derivatives, coatings solvent, deicing agent for liquid fuels, lacquers, extraction processes, dehydrating agent, preservative, lotions, denaturant. 

Metal powder processing techniques in which a preform is molded and sintered are used to process PTFE. Compression molding may also be used to fabricate PTFE parts. Its dispersions are applied by similar techniques to other coatings. Paste extrusion in which PTFE is blended with a hydrocarbon, prior to molding a preform, is used to continuously fabricate PTFE into tubes, tapes, and wire insulation. The hydrocarbon is vaporized before the parts are sintered. PVF is dispersed in a polar latent solvent such as dimethyl acetamide and is melt-extruded as a plastisol, followed by solvent removal by drying. 

The thermal process is perhaps the most universally applicable of all the phase inversion processes because it can be utilized over the widest range of both polar and nonpolar polymers. However, its commercial use for membrane applications will probably be restricted to polyolefins, particularly polypropylene. A large number of the substances can function as latent solvents (Table X). They usually consist of one or two hydrocarbon chains terminated by a polar hydrophilic end group. Therefore, they exhibit surface activity which may explain their ability to form the emulsion-like Sol 2 micelles at elevated temperatures. One latent solvent which is worthy of special mention because of its broad applicability is N-Tallowdiethanolamlne (TDEA). 

Table X. Thermal Process Polymers and Latent Solvents...

The latent solvent is removed from the membrane gel after the latter has attained sufficient strength to allow further processing. Typical leaching agents include volatile liquids such as... 

The cellulose acetate most suitable for coatings have acetyl contents ranging from about 38 to 40% and a hydroxyl range between 3.0 and 4.0%. The lacquer-type cellulose acetates possess certain fundamental characteristics that have made them difficult to formulate as coatings, yet it is exactly these characteristics that make them valuable in certain areas of the protective coating field. The cellulose acetates are soluble only in very strong solvents (Table IV). They respond well to latent solvents, but they have very low tolerance for hydrocarbons. Compatibility with resins is poor, only a dozen or so commercially available resins are compatible. Only very active plasticizers like dimethyl phthalate, triacetin, or dimethoxyethy1 phthalate will remain in the film without exudation. 

Polyvinyl fluoride (PVF) -CHF-CH2- 190 - - Melt extrusion of dispersion in latent solvent... 

Most of the membranes listed in Table 5.20 are formed through phase separation processes, i.e., melt extrusion or coagulation of a polymer solution by a nonsolvent. In melt extrusion, a polymer melt is extruded into a cooler atmosphere which induces phase transition. The melt extrusion of a single polymer usually gives a dense, isotropic membrane. However, the presence of a compound (latent solvent) that is miscible with the polymer at the extrusion temperature but not at the ambient temperature, may lead to a secondary phase separation upon cooling. Removal of the solvent then yields a porous isotropic membrane. Anisotropic membranes may result from melt extrusion of a dope mixture of polymers containing plasticizers. 

Latent solvents, which become effective only in the presence of active or true solvents, include alcohols (e.g., methanol, ethanol, and propanol). Like the nonsolvents these are used to reduce costs. The lower alcohols (e.g., methanol or ethanol) are, of course, true solvents for alcohol-soluble nitrocellulose. 

Coatings with Thermoplastic Fluoropolymers. Poly(vinylidene fluoride), PVDF, is the only conventional thermoplastic fluoropolymer that is used as a commercial product for weather-resistant paints. This crystalline polymer is composed of -CHjCFj- repeating units it is soluble in highly polar solvents such as dimethyl-formamide or dimethylacetamide. Poly(vinylidene fluoride) is usually blended with 20 30 wt% of an acrylic resin such as poly(methyl methacrylate) to improve melt flow behavior at the baking temperature and substrate adhesion. The blended polymer is dispersed in a latent solvent (e.g., isophorone, propylene carbonate, dimethyl phthalate). The dispersion is applied to a substrate and baked at ca. 300 °C for ca. 40-70 s. The weather resistance of the paints exceeds 20 years [2.16]-[2.18]. 

The predictions that can be made for solvent mixtures on the basis of the solubility diagram are not strictly valid because they involve thermodynamic simplifications and empirical parameters, and disregard temperature effects [14.29], [14.33]. A strong warning must therefore be given against the uncritical use of solubility parameters. Nevertheless, description of the solvents with the aid of the solubility parameter concept often provides useful information about their solvency and reveals similarities which can otherwise only be characterized empirically (e.g., latent solvents or dilutability). 

The expressions solvent, latent solvent, non-solvent only apply to a specific substance to be dissolved. A solvent can be a non-solvent for another substance and vice versa. 

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