Latex polymers continuous phase

As another case study a process synthesis of an emulsion polymerization process is given (Hurme and Heikkila, 1998). In emulsion polymerization unsaturated monomers or their solutions are dispersed in a continuous phase with the aid of an emulsifier and polymerized. The product is a dispersion of polymers and called a latex. The raw materials are highly flammable unsaturated hydrocarbons and the reaction is exothermic which both cause a risk. The main phases and systems in an emulsion polymerization plant are listed in Table 31. 

This method was first reported by Vanderhoff [82] for the preparation of artificial latexes. The polymer and drug are dissolved or dispersed in a volatile water-immiscible organic solvent, such as dichloromethane, chloroform, or ethyl acetate. This is emulsified in an aqueous continuous phase containing a surfactant, such as poly(vinylalcohol), to form nanodroplets. The organic solvent diffuses out of the nanodroplets into the aqueous phase and evaporates at the air/water interface, as illustrated in Figure 6. The solvent is removed under reduced pressure. The nanodroplets solidify and can be separated, washed, and dried to form a free-flowing powder. 

An aqueous polybutyl methacrylate latex (average radius == 200 A) has a viscosity of 50,500 cP at 0 = 0.25 and a viscosity of 36.7 cP at the same rate of shear when 1.71 x 10-5 g NaCl is added per gram of polymer, t Assuming that these charged particles must be surrounded by a layer of dissolved ions in solution, what conclusions can you draw about the dependence of the thickness of this layer of ions on the electrolyte content of the continuous phase ... 

In emulsion polymerization the compartmentalization of reaction loci and the location of monomer in polymer particles favor the growth and slow down termination events. The contribution of solution polymerization in the continuous phase is strongly restricted due to the location of monomer in the monomer droplets and/or polymer particles. This gives rise to greatly different characteristics of polymer formation in latex particles from those in bulk or solution polymerization. In emulsion polymerization, where polymer and monomer are mutually soluble, the polymerization locus is the whole particle. If the monomer and polymer are partly mutually soluble, the particle/water interfacial region is the polymerization locus. 

In the methodology developed by us [24], the incompatibility of the two polymers was exploited in a positive way. The composites were obtained using a two-step method. In the first step, hydrophilic (hydrophobic) polymer latex particles were prepared using the concentrated emulsion method. The monomer-precursor of the continuous phase of the composite or water, when that monomer was hydrophilic, was selected as the continuous phase of the emulsion. In the second step, the emulsion whose dispersed phase was polymerized was dispersed in the continuous-phase monomer of the composite or its solution in water when the monomer was hydrophilic, after a suitable initiator was introduced in the continuous phase. The submicrometer size hydrophilic (hydrophobic) latexes were thus dispersed in the hydrophobic (hydrophilic) continuous phase without the addition of a dispersant. The experimental observations indicated that the above colloidal dispersions remained stable. The stability is due to both the dispersant introduced in the first step and the presence of the films of the continuous phase of the concentrated emulsion around the latex particles. These films consist of either the monomer-precursor of the continuous phase of the composite or water when the monomer-precursor is hydrophilic. This ensured the compatibility of the particles with the continuous phase. The preparation of poly(styrenesulfonic acid) salt latexes dispersed in cross-linked polystyrene matrices as well as of polystyrene latexes dispersed in crosslinked polyacrylamide matrices is described below. The two-step method is compared to the single-step ones based on concentrated emulsions or microemulsions. 

Candau and co-workers were the first to address the issue of particle nu-cleation for the polymerization of AM [13, 14] in an inverse microemulsion stabilized by AOT. They found that the particle size of the final microlatex (d 20-40 nm) was much larger than that of the initial monomer-swollen droplets (d 5-10 nm). Moreover, each latex particle formed contained only one polymer chain on average. It is believed that nucleation of the polymer particle occurs for only a small fraction of the final nucleated droplets. The non-nucleated droplets also serve as monomer for the growing particles either by diffusion through the continuous phase and/or by collisions between droplets. But the enormous number of non-nucleated droplets means that some of the primary free radicals continuously generated in the system will still be captured by non-nucleated droplets. This means that polymer particle nucleation is a continuous process [ 14]. Consequently, each latex particle receives only one free radical, resulting in the formation of only one polymer chain. This is in contrast to the large number of polymer chains formed in each latex particle in conventional emulsion polymerization, which needs a much smaller amount of surfactant compared to microemulsion polymerization. 

The submicroscopic emulsion polymerized form of IPN s would be expected to differ in mechanical properties from the counterpart bulk polymerized form in that (1) The latex particles are not crosslinked one to another allowing movement of one latex particle past another. (2) In bulk IPN s (10) it was shown that polymer I forms the continuous or more continuous phase while in latex IPN s polymer II tends to form the more continuous phase (1). 

A latex has basically two parts, a dispersed phase (polymer particles) and the continuous phase (the water the liquid in which the polymer droplets are dispersed), the process is usually referred to as emulsion... 

Lepizzera et al [172] reported in an early publication on the deformation of latex particle films, as detected by AFM. Films made from core/shell latex particles that possess a soft shell and a hard core exhibited a continuous phase composed of the shell polymer in which the hard cores formed long-range hexagonal orderings. Upon elongation of the films (for small draw ratios), linear necklaces of core particles, perpendicular to the elongation direction, were observed at the surface of the films (Fig. 3.81). 

Emulsion polymerization is a heterogenous reaction process in which unsaturated monomers are dispersed in continuous phase with the aid of emulsifiers and polymerized by free-radical initiators. The resulting product is a dispersion of polymer particles, typically smaller than 1 pm in size, in water and is referred to as polymer latex. 

Most paint formulations consist of disperse systems (solid in liquid dispersions) [2]. The disperse phase consists of primary pigment particles (organic or inorganic) which provide the opacity, colour and other optical effects these are usually in the submicron range. Other coarse particles (mostly inorganic) are used in primers and undercoats to seal the substrate and enhance adhesion of the top coat The continuous phase consists of a solution of polymer or resin which provides the basis for a continuous film that seals the surface and protects it from the outside environment Most modem paints contain latexes which are used as film formers. These latexes - which typically have a glass transition temperature (Tg) below... 

Sperry et al. found that the addition of either the anionic surfactant sodium dodecyl sulphate or the nonionic surfactant Triton X-405 completely desorbed any hydroxyethyl cellulose from the surface of the latex particles. This meant that, even in the presence of free hydroxyethyl cellulose in the continuous phase, none of the flocculating polymer was attached to the surface. The latex particles in the presence of the sodium dodecyl sulphate (0-5%) were thus electrostatically stabilized whereas the nonionic Triton surfactant conferred steric stabilization. 

Because a latex polymer is insoluble in water, the factors affecting the viscosity of latexes differ significantly from polymers dissolved in solvents. Since the aqueous continuous phase interacts with the polymer only at the surface of the latex particle, the molecular stracture of the polymer does not have the effect that it would if the polymer were dissolved in the liquid. The molar mass and polymer backbone flexibility, in particular, have no direct effect on the viscosity of the latex. The principal latex parameter that influences latex viscosity is of course... 

The stability of the inverse latex strongly depends on an appropriate formulation. Poor chemical compatibility between oils and emulsifiers produces unstable latices, whereas a good chemical match leads to perfectly transparent and stable latices [17,18]. The latex stability was accounted for by (1) reduced gravity forces ( /, where d is the particle diameter) (2) high entropic contribution from the droplets owing to their large number and (3) low interfacial tension between polymer droplets and the continuous phase [4]. 

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