Typical Coalescing Solvents

Hydrophobic solvents decrease the MFFT most efficiently and increase the hardness of the coating due to improvements in film formation. They also penetrate into the shell and swell the polymer, increasing the viscosity dramatically. Coalescing solvents distribute throughout the three phases, water, polymer shell and polymer core, in differing volumes. Optimum film formation takes place when the coalescent is present in the shell of the polymer. 

The most stable systems are obtained with the minimum volume of hydrophobic coalescent, but this volume should be sufficient to retain good film formation under adverse conditions. The hydrophobic coalescent should be coupled with the water soluble type and pre-blended with water to prevent problems such as polymer shock during addition. An extremely efficient hydrophobic cotdescent is the ester alcohol, Texanol from Eastman, best utilised where stoving facilities are available since there is a tendency for it to be retained in the film. Being insoluble, Texanol is preferentially absorbed by the polymer particles. This not only provides better coalescence at a lower level, but also prevents rapid release from the surface, thereby aiding more complete film formation. Texanol s ability to produce a continuous, fully coalesced, film often provides an improved barrier when resistance to the elements is required. 

Clear coatings coalesced with diethylene glycol monobutyl ether demonstrate improved flow and wetting characteristics, especially on plastics. Hte use of coalescents such as butyl diglycol acetate should be avoided, as they are susceptible to alkaline hydrolysis. 

Recent developments have seen the release of a coalescing solvent that is classified as a non-VOC for paint applications (B.Pt. 250 ). This could well be the answer to a formulator s dream, since many acrylic polymer latices with high Tg require levels of coalescent beyond the compUant Umits. A Usting of some coalescing solvents is included in Table 7-8. 

Type Trade Name Volatility BuAc = 1000 Solubility (20°C)WT%  

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Apart from coalescence solvents to improve the freeze/thaw stability of emulsion paints are used. Typical... 

The largest commercial category for latex paint, the architectural market, used 618 million gallons in the United States in 2001. Coalescing solvents typically constitute 2—3% of the finished paint by volume, corresponding to an estimated 120 million pounds of coalescing solvents in the United States, and perhaps three times that amount globally. Currently, nearly all of these solvents are lost into the atmosphere each year. 

A typical coalescing agent is an organic solvent that exhibits most of the following characteristics ... 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

Typically, spinning is carried out by melt-spinning extrusion through a spinneret to obtain polymer fibers with a diameter of 10-20 (xm. However, solvent spinning has been reported (19). The fragile polymer fibers need to be cured (cross-linked) below their melting point to prevent coalescence... 

A latex paint is an aqueous emulsion. After the paint dries, the surfactant is still present and may interfere with the resistance of the paint to washing to remove fingerprints or other marks. A destructible surfactant would be valuable in this application. It would have to destruct by the action of light or air. Latex paints are not entirely free of solvent. A few percent of a higher-boiling solvent, a coalescing agent, is usually, but not always, necessary to form a pinhole-free film from the particles of polymer after the water evaporates. A typical one (8.45) is derived from isobutyraldehyde. 

The step after impregnation is the elimination of the solvent. Typically, the impregnated system, spread in fine layers, is heated in an oven up to the boiling point of the solvent or at a lower temperature for gentle drying, in static conditions or under a flow of gas. The elimination of water from the pores leads to an increase of the precursor concentration up to saturation and crystallization, preferably on seeds resulting from the interactions with the support, if one wants to obtain a highly dispersed phase. However, hydrated salts like nitrates melt at moderate temperature [35, 36], which may cause the coalescence of their initially dispersed particles, or even their exclusion from the pores. 

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