Stabilization of suspensions, polymeric surfactants for

Steric repulsion results from the presence of adsorbed layers of surfactants and/or polymers. The use of natural and synthetic polymers (referred to as polymeric surfactants) for stabilization of suspension concentrates and emulsions (EW s) plays an important role in agrochemical formulations. Polymers are particularly important for preparation of concentrated dispersions, i.e. at high volume fraction ( j of the disperse phase,... 

This chapter, will begin with a brief description of polymeric surfactants and their solution properties, followed by a description of the fundamental principles of using polymeric surfactants for stabilization of emulsions (as well as suspensions), starting with a section on the adsorption and conformation of these molecules at the interface. This is followed by a section on stabilization of dispersions by polymeric surfactants. Particular... 

Another graft copolymer that could be used for stabilization of suspensions is that based on a polyfructose backbone on which several alkyl groups have been grafted (INUTEC SPl), mentioned earlier for stabilization of emulsions. This polymeric surfactant was used to investigate the stability of polystyrene (PS) and PMMA suspensions in the presence of electrolytes [NaCl, CaClj, and Al2(S04)3l [25]. The polystyrene latex was prepared by emulsion polymerization without surfactant, and it was fairly monodisperse, with a diameter of 210 nm. The PMMA latex was prepared... 

Emulsion polymerization presents similar processing difficulties to those of suspension polymerization. The product has to be recovered, in this case usually by coagulation, and then washed and dried. Again it may be difficult to remove all traces of the surfactant, etc., used to stabilize the emulsion, as with the product from suspension polymerization. However, for some applications, such as for latex (water-based) paints and carpet adhesives, the aqueous product from emulsion polymerization may be used directly. For applications, such as these, it is possible to produce lattices (latexes) containing as high as 50% solids. 

Stable aqueous emulsions of poly(2-ethylhexyl acrylate) (PEHA) were also produced by RESAS from CO2 (68). In this case, a polymer suspension in CO2 was expanded instead of a dissolved solute. A C02-philic surfactant, Monasil PCA (PDMS-g-pyrrolidonecarboxylic acid), was utilized in dispersion polymerization to form a stable polymer suspension at 65°C and 345 bar. A hydrophilic surfactant, (e.g., SAM 185, Pluronic L61, or Pluronic L62), that is soluble in CO2 and CO2/2-EHA monomer mixtures as well as water was added to CO2 to stabilize the suspension after it had been rapidly expanded through a capillary into aqueous solution. The resulting aqueous emulsion with up to 15.6 wt % polymer content was stable for weeks with an average particle size of 2 to 3 pm. Another approach is to introduce the hydrophilic surfactant in the aqueous phase in addition to the surfactant in the CO2 phase. This approach is more general, since many hydrophilic surfactants are not soluble in CO2. During expansion of the suspension into an aqueous solution, the hydrophilic surfactant—for example, triblock Pluronic copolymers—dilfuses to the particle surface to provide stabilization. The resulting aqueous latexes were stable for 100 days for a polymer content reaching 12.7 wt %. 

Surfactants like ammonium salts of fatty acids or hydroxy fatty acids, fatty alcohol sulphates and alkylbenzenesulphonates are also used in micro-suspension polymerization mainly employed for manufacturing of paste or plastisol PVC. Certain surfactants like fatty acid polyglycolesters and glycerylmono- or glycery-Imono/distearate are also used as co-stabilizers in suspension PVC manufacturing. 

An emulsion has been defined above as a thermodynamically unstable heterogeneous system of two immiscible liquids where one is dispersed in the other. There are two principal possibilities for preparing emulsions the destruction of a larger volume into smaller sub-units (comminution method) or the construction of emulsion droplets from smaller units (condensation method). Both methods are of technical importance for the preparation of emulsions for polymerization processes and will be discussed in more detail below. To impart a certain degree of kinetic stability to emulsions, different additives are employed which have to fulfil special demands in the particular applications. The most important class of such additives, which are also called emulsifying agents, are surface-active and hence influence the interfacial properties. In particular, they have to counteract the rapid coalescence of the droplets caused by the van der Waals attraction forces. In the polymerization sense, these additives can be roughly subdivided into surfactants for emulsion polymerization, polymers for suspension and dispersion polymerization, finely dispersed insoluble particles (also for suspension polymerization), and combinations thereof (cf. below). 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to understanding how these molecules act as stabilizers for suspensions and emulsions. Most basic theories on polymer adsorption and conformation have been developed for the solid/liquid interface (9). The same concepts may be applied for the liquid/liquid interface, with some modifications whereby some part of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modifications do not alter the basic concepts, particularly when one deals with the stabilization by these molecules. 

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