Blends with Other Latex Systems

Many epoxy dispersions are compatible with most types of latex emulsions including acrylic, urethane, styrene butadiene, vinyl chloride, and polyvinyl acetate. The epoxy dispersion can be used as a modifier for these emulsions to alter handling and application characteristics such as emulsion rheology, foaming tendencies, pH sensitivity, wetting properties, and coating coalescence. They can also be reacted into the latex resin either by reacting the epoxy with a functionalized latex or by use of an epoxy with a coreactant. In this way adhesive systems can be formulated that are cured at room or elevated temperatures. 

Adhesives formulated with epoxy-modified latex retain the tack and conformability of the original latex but show improvements in green bond strength and fully cured bond strength. Cured epoxy latex epoxy resin systems also exhibit improved water and chemical resistance over unmodified latex systems. 

In choosing an epoxy and polymeric latex, it is important that they have compatibility. Incompatibility usually occurs when the pH of the epoxy resin dispersion alters the pH of the latex into a range where the ionically stabilized latex is broken, causing agglomeration of the latex polymer. The pH of the epoxy resin s emulsion may need to be adjusted before blending with the polymeric latex. 

It has been shown that tensile shear and peel strength for several latex polymers (ethylene vinyl acetate, polyvinyl alcohol, ethylene vinyl chloride, polyvinyl chloride, and acrylic) can be significantly increased by the addition of 10 percent by weight of an epoxy emulsion cured with a tertiary amine curing agent.17 The epoxy modification improves the bond strength in all cases. The degree of improvement is dependent on the selection of the latex type and the chemistry of the latex polymer. 

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Epoxy dispersions also can easily be blended with other waterborne polymers to make modified latex adhesives. The resulting hybrid adhesive produces performance properties and application characteristics that are superior to those of the originating latex system. 

A second generation of phenolic dispersions, patented by J. S. Fry (33). involved the post dispersion of phenolic resins in a mixture of water and water-miscible solvents. To conform with air pollution regulations, the solvent was held to 20 volume %, or less, of the volatiles. A heat-reactive phenolic resin dispersion (34) and a phenolic-epoxy codispersion have become commercially available based on the above technology. Supplied at 40-45% solids, these products, which have a small particle size (0.75-1.0 ym), are better film formers than the earlier dispersions. Used alone or in blends with other waterborne materials, corrosion-resistant baking coatings may be formulated for coil coating primers, dip primers, spray primer-surfacers, and chemically resistant one-coat systems. Products of this type are also tackifiers for acrylic latexes, and such systems have been employed as contact, heat seal, and laminating adhesives for diverse substrates. 

A similar technique was used for the preparation of polystyrene (PS)-Si02 nanohybrids, where colloidal silica solutions were mixed with PS solutions by means of ultrasonic homogenization [60]. Also, latex-silica nanohybrid films were synthesized upon mixing aqueous colloidal suspensions of silica and nanolatex polymer beads [61-63], Other silica-based nanohybrid systems with poly(ethylene oxide) (PEO) [64, 65], polyfvinyl alcohol) (PVA) [66], PS [67], polybutylacrylate [68], or PMMA [69] can be prepared by using the same suspension blending method. 

Instead of a hydrocarbon diluent, a ketone or ester may be used instead, in which case it is usually referred to as a dispersant rather than a diluent, reflecting its vinyl compatibility. Typical additives in this class are diisobutyl ketone and hexyl acetate. They have a greater effect on viscosity than hydrocarbons, but require greater investment to achieve solvent recovery. Blends of hydrocarbons with ketones or esters provide cost savings, but not only are they difficult to recover efficiently but it is also difficult to maintain constant composition during recirculation. If suitable solvent recovery systems are not available, the fabricator would be well advised to consider other approaches, such as compoimding vinyl latex. 

From the above, we can see that the mechanism of morphology formation is quite different from that of traditional polymer solution organic-inorganic nanocomposites. For traditional polymer solution systems, the phase separation mechanism is similar to that of polymer blends, i.e., nucleation and growth mechanism and spinodal decomposition mechanism.Generally, the component with higher content is apt to form the continuous phase and the other the dispersed phase. However, for the PEA/bentonite emulsion system, whether PEA can be continuous or not depends mainly on whether the PEA latex particles are in close contact before the complete volatilization of water. Instead, it depends on whether the content of PEA is larger than that of bentonite. 

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