Latex dispersion film formation

Unfortunately, no values for polymers have been measured in this system. There is no provision for handling dispersion forces which are of great importance in many practical systems. Nevertheless, as this theory is developed it should be of value in handling aqueous solutions, perhaps latex stabilization, film formation, adhesion, or pigment-vehicle interactions. 

The technique of film formation from aqueous polymer dispersions is widely used in the field of paints ( dispersion paints , latex-paints ), coatings, and adhesives. The equipment is simple brush, scraper, rake, or roller. 

The polarity and adsorption data discussed above reveal some interesting aspects of the surface chemistry of vinyl acrylic latex surfaces. It is quite likely that the polarity of the latex films, expecially of the two co-polymers, determined by contact angle measurements may not correspond exactly with their respective latex surfaces in the dispersed state due to reorientation of polymer chains during film formation. But the surfactant adsorption data shows clearly that the three latex surfaces in their dispersed state do exhibit varying polarity paralleling the trend found from contact angle measurements. The result also shows that the surface of the co-polymer latex surface is a mixture of vinyl acetate and acrylate units. This result is somewhat unexpected in a vinyl acrylic latex, prepared by a batch... 

To explain the fact that HSPAN swells in water to form gel sheets or macroparticles rather than disintegrating into a gel dispersion, we initially felt that chemical bonding must take place between individual particles of water-swollen gel as water evaporates. Although we cannot totally eliminate this possibility, the proposal of primary chemical bonding is not necessary to explain the behavior of these films and conglomerates. For example, Voyutskii (19) has reviewed the formation of films from vulcanized rubber latexes and concludes that film formation in these systems is observed because of interdiffusion of ends of individual macromolecules in adjacent latex particles. This diffusion can take place even though individual latex particles are crosslinked, 3-dimensional networks and the continuity of the resulting films, even when... 

Fig. 26 Differences observed in the mechanism of film formation from latex and pseudolatex dispersions and from micronized coating materials.

Fig. 28 Cross-sectional areas of different films. Eudragit L 30 D without pigments, film thickness approximately 25 pm (A), Eudragit L 30 D film containing 50% of magnesium stearate, film thickness approx. 25 pm (B), incomplete film formation at higher magnification showing individual latex particles from an Eudragit L 30 D dispersion in the film (C).

Membrane permeability is a function of thickness, porosity, tortuosity, and composition. Therefore, film formation is a substantial determinant of drug release rate through a sustained release film coat. Drug release through an insoluble polymeric membrane produced from a latex dispersion will decrease with the evolution of film... 

Particle deformation and polymer diffusion can only occur at temperatures above the glass transition temperature of the polymer. Final coatings, however, are required to be at temperatures considerably below the glass transition temperature. To get around this problem, it is common to add plasticizers to water borne latex dispersions to lower the glass transition temperature of the constituent polymer during the film formation process. Subsequent evaporation of the plasticizer results in a hard final coating. A common plasticizer is 2,2,4-trimethyl-l,3-pentanediol monoisobutyrate, usually referred to as Texanol Ester Alcohol. 

When designing a film forming latex dispersion, the properties to consider are the final mechanical properties of the film, as well as the ease of film formation The mechanical properties required, as well as the environment of operation will dictate the polymers suitable for the coating and may well dictate the glass transition temperature of the polymer. The crack points alluded to earlier correspond to the transition from capillary deformation to the receding water front regime. Therefore, a value of X less than 100 will ensure a well-formed film. 

This article has reviewed latex processing. The polymers used, synthesis of particles, major uses, and reasons for loss of dispersion stability have been outlined. The mechanism of latex film formation has been described, and the different properties resulting from different film forming conditions in latex explored. 

After the polymerization, the polymer can be isolated by precipitation, but for many applications latexes are used as such. Film formation upon evaporation of the dispersing medium (Scheme 7.3) is a key step in many apphcations, and the fact that colloids are composed of submicron particles is advantageous in terms of forming continuous films. 

Film formation from a polymer latex upon evaporation of the dispersing medium. 

In this work, the film properties made from mixtures of latex based on hard-chain and elastic copolymers, undergone by vibrowave treatment have been considered. Process of a film formation in this case is known to start with the evaporation of the dispersion media (water) and finishes with transformation of a dispersion into a coating. The period between the change of the film sizes during the coating formation and achievement of an equilibrium condition is governed by relaxation processes [7],... 

The creation of 2D crystals of both micron sized and nanometre sized particles remains a somewhat empirical process due to the ill-defined role of the substrate or surface on which nucleation takes place. Perrin first observed diffusion and ordering of micron sized gamboge 2D crystals in 1909 under an optical microscope [32]. Several techniques have been proposed for the formation of 2D arrays at either solid-liquid surfaces or at the air-water interface. Pieranski [33], Murray and van Winkle [34] and later Micheletto et al. [14] have simply evaporated latex dispersions. Dimitrov and coworkers used a dip-coating procedure, which can produce continuous 2D arrays [35,36]. The method involves the adsorption of particles from the bulk solution at the tricontact phase line. Evaporation of the thin water film leads to an attractive surface capillary force which aids condensation into an ordered structure. By withdrawing the film at the same rate as deposition is occurring, a continuous film of monolayered particles is created. Since the rate of deposition is measured with a CCD camera, it is not possible to use nanometer sized particles with this method, unless a nonoptical monitor for the deposition process can be found. 

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