Polymers emulsified systems

Latexes are usually copolymer systems of two or more monomers, and their total solids content, including polymers, emulsifiers, stabilizers etc. is 40-50% by mass. Most commercially available polymer latexes are based on elastomeric and thermoplastic polymers which form continuous polymer films when dried [88]. The major types of latexes include styrene-butadiene rubber (SBR), ethylene vinyl acetate (EVA), polyacrylic ester (PAE) and epoxy resin (EP) which are available both as emulsions and redispersible powders. They are widely used for bridge deck overlays and patching, as adhesives, and integral waterproofers. A brief description of the main types in current use is as follows [87]. 

The ozonization method has been extended to the most varied polymer/monomer systems, such as polybutadiene-03 with acrylamide, methyl methacrylate or styrene, cellulose-03 with styrene or acrylonitrile (127), starch-03 with styrene (126). In this last case the formation of some homopolystyrene as side-product has been mentionned by the authors. The starch-styrene graft copolymers are claimed to be good emulsifiers for water-oil suspensions. 

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

In order to aoab se the colloidal furocesses occurring in colloidal polym erization systems containing monomer, polymer monomer and polymer panicles it is important to know about adsorption characteristics of the emulsifier on the monomer-aqueous phase and polymer-aqueous phase interfaces. 

Polymerization starts in the micelles of the emulsifier, because a considerable part of the monomer is dissolved in its hydrocarbon moiety. At 13-20% conversion of the monomer, emulsifier micelles are completely destroyed, and the emulsifier passes into the adsorption layer on the surface of polymer particles. Polymerization continues in the polymer-monomer system, i.e. in a latex into which the monomer penetrates by diffusion from drops. 

Probably the most widely used industrial emulsion or dispersion adhesives are those based on poly(vinyl acetate), commonly referred to as PVA. These product are normally manufactured by emulsion polymerization whereby, basically, vinyl acetate monomer is emulsified in water with a suitable colloid-emulsifier system, such as poly(vinyl alcohol) and sodium lauryl sulfate, and, with the use of water soluble initiator such as potassium persulfate, is polymerized. The polymerization takes place over a period of four hours at 70°C. Because the reaction is exothermic, provisions must be made for cooling when the batch size exceeds a few liters. The presence of surfactants (emulsifiers) and water-soluble protective colloids facilitates the process resulting in a stable dispersion of discrete polymer particles in the aqueous phase. 

The formation of stable droplets of the polymer solution with drug incorporated in as an emulsified system... 

Although some brief comments on the actions of emulsifiers and stabilizers have been presented, it is useful to have a Uttle more detailed concept of just how these materials complete their function in the emulsified system. The actions of the most important systems—polymers, sols, and surfactants—will therefore be explained in a bit more detail. 

The last major class of emulsifiers and stabilizers is that of the monomeric surfactants which adsorb at interfaces, lower the interfacial tension, and, hopefully, impose a stabilizing barrier between emulsion drops. Surfactants are the most widely studied and perhaps best understood class of emulsifiers and stabilizers. Perhaps because they are more amenable to both experimental and theoretical analysis, they have been used to probe the finer points of emulsified systems. They will therefore be discussed in more detail than polymers and sols. 

The compositional chemistry of the resins and elastomers has not changed, so the same compatibility should be observed. However, there are additional factors which complicate the situation. Factors such as compatibility of the emulsifier systems must now be taken into consideration. The molecular weight of the polymer which is generally much higher than that found in solvent-based systems, plays a major role. The particulate configurations of the components must also be considered. However, even with all of these complicating factors, in general we can still say that the same types of resins used to tackify specific elastomers in solvent can also be used successfully to tackify those same polymers in water. 

Polymers that themselves have substantial interfacial activity, such as synthetic hydro-phobically modified water-soluble polymers (HM-P) or natural proteins, acting alone (see Chapter 3) or, more especially, in concert with conventional surfactants, constitute a much more complicated, if very interesting, case. (See Chapter 4.) Although instances of such combined use may be found in the literature, this branch of emulsion science must still be regarded as relatively new and often empirical in the case of HM-P. On the other hand, as proteins, in combination with selected surfactants, have been a traditional emulsifier system for edible emulsions, their behavior at the oil/water interface with selected surfactants has been extensively studied. This area is rather specialized and the interested reader is referred to published treatises on the subject (62,63), and also to general technical literature in the field of latex paints. 

In general, polymer latexes are copolymer systems of two or more different monomers, and their total solid content including polymers, emulsifiers, stabilizers, etc. are 40 to 50 % by weight As seen in Fig. 3.2, most commercially available polymer latexes for cement modifiers are based on elastomeric and thermc lastic polymers which form continuous polymer films when dried. The polymer latexes that are underl ined in Fig. 3.2 are the main ones that are in general use today in the world. Table 3.3 gives the... 

The preparation of polymer colloids is both a science and an art. It is a science in which the kinetic principles of free radical-initiated vinyl addition polymerization are superimposed on the heterogeneous polymer latex system. It is an art in that the preparer uses a recipe which comprises monomer, water, emulsifier, initiator, and other ingredients, and the quality of the latex obtained depends upon small variations in the polymerization parameters as well as the skill of the preparer. The purpose of this paper is to review the different types of polymer latexes and the mechanisms proposed for their preparation, and to give examples of the preparation of different types. 

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