Latex dispersion rheology

Rheological Studies of Aqueous Concentrated Polystyrene Latex Dispersions with Adsorbed Poly(vinyl alcohol) Layers... 

Any fundamental study of the rheology of concentrated suspensions necessitates the use of simple systems of well-defined geometry and where the surface characteristics of the particles are well established. For that purpose well-characterized polymer particles of narrow size distribution are used in aqueous or non-aqueous systems. For interpretation of the rheological results, the inter-particle pair-potential must be well-defined and theories must be available for its calculation. The simplest system to consider is that where the pair potential may be represented by a hard sphere model. This, for example, is the case for polystyrene latex dispersions in organic solvents such as benzyl alcohol or cresol, whereby electrostatic interactions are well screened (1). Concentrated dispersions in non-polar media in which the particles are stabilized by a "built-in" stabilizer layer, may also be used, since the pair-potential can be represented by a hard-sphere interaction, where the hard sphere radius is given by the particles radius plus the adsorbed layer thickness. Systems of this type have been recently studied by Croucher and coworkers. (10,11) and Strivens (12). 

In this paper we report some rheological studies of aqueous concentrated polystyrene latex dispersions, in the presence of physically adsorbed poly(vinyl alcohol). This system has been chosen in view of its relevance to many practical systems and since many of the parameters needed for interpretation of the rheological results are available (15-18). The viscoelastic properties of a 20% w/w latex dispersion were investigated as a function of polymer coverage, using creep measurements. 

Interpretation of rheological results The trends in the variation of Xg with are similar to those obtained recently (3) using a model polystyrene latex dispersion. The 1 values obtained in the present system are also close to tSose obtained with the model dispersion(0.017, 0.008 and 0.005 for PEO 20,000, 35,000 and 90,000 respectively). As mentioned before the sharp increase in x above + indicates that at the onset of flocculation the dispersions show marked viscoelasticity. The flocculation obtained at corresponds to the onset of the "semidilute" region, p, i.e., where the polymer coils in solution begin to arremge themselves in some... 

Latex dispersions display a rich range of rheological properties. The exact behavior of dispersions is dependent on both the volume fraction of particles and the interaction potential. 

Differences in dispersion rheology associated with the surface energies (i.e., adsorption coverage) of the different latex types (e.g., vinyl acetate, 18 mN/m methyl methacrylate, 26 mN/m and styrene, 40 mN/m) are not realized (34). 

Chem. Desaip. o-Formal with isothiazolinones Uses Preservative for latex dispersions with residual redox agents, emulsion paints, caulks, sealants, adhesives, pigment pastes, rheology modifiers, stucco, and plasters... 

The influence of particle size distribution in the use of latex dispersions is shown to be of great practical importance. The practical consequences are examined of bimodal particle size distribution with respect to coatings applications. The introduction of polydispersity in acrylic dispersions is examined as a way of obtaining a lower dispersion volume loading at an equivalent viscosity as for monodisperse spheres. Aspects such as film formation, rheology, and drying behaviour are discussed. 39 refs. 

Rheological studies of sterically stabilised concentrated polystyrene latex dispersions under conditions of incipient flocculation... 

Rheological Measurements Three types of rheological measurements have been carried out. In the first type, transient (creep) measurements were performed on a 20% w/w dispersion of latex A, as a function of coverage by PVA. These experiments were carried out using a "Deer" rheometer (PDR 81, Integrated Petronic Systems, London) fitted with a stainless steel concentric cylinder. The procedures used have been described in detail before (21,22). 

Won-Aqueous Dispersions (NADs). These arc the solvcnt-bomc analogues of latexes. They were used as automotive finishes in the 1970s, but their use is now declining. The use of NADs as an auxiliary binder, however, is increasing as it has been found that the addition of a small amount of NAD can improve a coating s drying characteristics and rheology. 

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. 

Direct Emulsification. Polymer colloids called "artificial latexes" can be prepared by dispersion of bulk polymers or polymer solutions into an aqueous medium. Direct emulsification processes are reviewed by ElAasser (23). The preparation procedures involve mechanical dispersion that may be followed by removal of solvent. According to ElAasser "the efficiency of emulsification," and hence the particle size characteristics of the latex, "is determined by the efficiency of formation of fine droplets and the efficiency of stabilization of the formed droplets." Important parameters in the process include the source of energy or agitation, its intensity, and duration type and concentration of emulsifiers mode of addition of emulsifier and the two phases density ratio of the two phases temperature and the rheology of the two phases. 

An important feature of general concern to the vinyl plastisol compounder is the oil absorptivity of the inert and its effect on the rheology of the dispersion. A parallel property known as water absorptivity or water demand is of prime importance to the latex compounder. 

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