Polymers, liquid dispersion

Resin cements or polymer cements have been reviewed by Chandra, Justnes, and Ohama [343]. Polymer cements are materials made by replacing the cement at least partly with polymers. Cements can be modified by latex, dispersions, polymer powders, water-soluble polymers, liquid resins, and monomers. 

Latex originally meant the sap of the rubber plant and is a dispersion of particulate rubber. Emulsion polymerization produces a similar dispersion of synthetic rubber or polymers and was rapidly developed to obtain a substitute for natural rubber during World War II. Therefore the product of emulsion polymerization was first called polymer latex, but is now known simply as latex. Sometimes the product of emulsion polymerization is called polymer emulsion. But this terminology is incorrect for latices of solid polymer particles, because emulsion indicates liquid-in-liquid dispersion (1). 

The most convenient of these methods is viscosity measurement of a liquid in which particles coated with a polymer are dispersed, or measurement of the flow rate of a liquid through a capillary coated with a polymer. Measurement of diffusion coefficients by photon correlation spectroscopy as well as measurement of sedimentation velocity have also been used. Hydrodynamically estimated thicknesses are usually considered to represent the correct thicknesses of the adsorbed polymer layers, but it is worth noting that recent theoretical calculations52, have shown that the hydrodynamic thickness is much greater than the average thickness of loops. 

When a liquid dispersion contains non-adsorbing polymers there will be a layer of liquid surrounding each dispersed species that is depleted in polymer, compared with the concentration in bulk, solution. This causes an increase in osmotic pressure in the system compared with what it would be were the dispersed species absent. If the dispersed species move dose to each other then the volume of solvent depleted is reduced, reducing the overall osmotic pressure, which provides a driving force for flocculation. Xanthan gum, added in low concentrations, can cause depletion flocculation [291]. 

Vincent, B. The Stability of Solid/Liquid Dispersions in the Presence of Polymers in Solid/Liquid Dispersions, Tadros, Th. F. (Ed.), Academic Press New York, 1987, pp. 147— 162. 

Emulsions and suspensions are colloidal dispersions of two or more immiscible phases in which one phase (disperse or internal phase) is dispersed as droplets or particles into another phase (continuous or dispersant phase). Therefore, various types of colloidal systems can be obtained. For example, oil/water and water /oil single emulsions can be prepared, as well as so-called multiple emulsions, which involve the preliminary emulsification of two phases (e.g., w/o or o/w), followed by secondary emulsification into a third phase leading to a three-phase mixture, such as w/o/w or o/w/o. Suspensions where a solid phase is dispersed into a liquid phase can also be obtained. In this case, solid particles can be (i) microspheres, for example, spherical particles composed of various natural and synthetic materials with diameters in the micrometer range solid lipid microspheres, albumin microspheres, polymer microspheres and (ii) capsules, for example, small, coated particles loaded with a solid, a liquid, a solid-liquid dispersion or solid-gas dispersion. Aerosols, where the internal phase is constituted by a solid or a liquid phase dispersed in air as a continuous phase, represent another type of colloidal system. 

In the slurry process, the reaction is carried out in a liquid dispersant (paraffinic in nature), in which catalyst and polymer remain in suspension. Reaction temperature is held below 110°C. to prevent dissolution of the polymer. Catalyst does not necessarily remain in the middle of a polymer particle but spalls and is scattered throughout the polymer. The slurry of polymer and hydrocarbon is withdrawn from the reactor and flashed to remove diluent and unreacted olefin for recycle. Because of the high productivities obtained in this process, it is unnecessary to remove catalyst for many polymer applications. 

Dispersions of gas in solids are also called foams but the foam cells (bubbles) formed are isolated from one another. An example of such foams are the natural porous materials, cellular concrete, cellular glass and polymer foams. However, if in such disperse systems both phases are continuous (such as in many foamed polymers), they are called sponges. Many porous materials are partially sponge and partially solid foam. The properties of solid foams differ drastically from those of foams with liquid dispersion medium. At the same time the strength and other physical and mechanical characteristics of solid foams depend significantly... 

At least two good ways out exist. One consists of the dispersion at the molecular level of rigid polymer molecules between flexible chains - the concept of molecular composites of Helminiak, Hwang e.a.(2, 4j. The other involves the use of polymer liquid crystals (PLCs). As discussed by Witt (5), compared to widely used engineering thermoplastics, PLCs show clear... 

Polymer capsules Core-shell Flavour composition entrapped in capsule consisting of solvent core covered by protein shell Dry product or liquid dispersion Limited (oxidative) stability of encapsulated flavour Flavour can be loaded in empty particles Flexible particle size Relatively slow release/burst-like release... 

Latex is a dispersion of polymer particles in a liquid medium, where the particles will remain suspended indefinitely. This property means that latices are colloidal dispersions. By nature of its origin, latex is classified into natural latex for dispersions obtained from plants, and synthetic latex for dispersions that are man made, typically by a process called emulsion polymerization. Blackley discusses a number of further classifications including artificial latex for dispersions in which the polymer is dispersed after synthesis, and modified latex where a chemical modification of existing latex is made. 

Sanes J, Carridn FJ, Bermddez MD et al (2006) Ionic hquids as lubricants of polystyrene and polyamide 6-steel contacts. Preparation and properties of new polymer-ionic liquid dispersions. Tribol Lett 21 121-133... 

Polymers are also essential for the stabilisation of nonaqueous dispersions, since in this case electrostatic stabilisation is not possible (due to the low dielectric constant of the medium). In order to understand the role of nonionic surfactants and polymers in dispersion stability, it is essential to consider the adsorption and conformation of the surfactant and macromolecule at the solid/liquid interface (this point was discussed in detail in Chapters 5 and 6). With nonionic surfactants of the alcohol ethoxylate-type (which may be represented as A-B stmctures), the hydrophobic chain B (the alkyl group) becomes adsorbed onto the hydrophobic particle or droplet surface so as to leave the strongly hydrated poly(ethylene oxide) (PEO) chain A dangling in solution The latter provides not only the steric repulsion but also a hydrodynamic thickness 5 that is determined by the number of ethylene oxide (EO) units present. The polymeric surfactants used for steric stabilisation are mostly of the A-B-A type, with the hydrophobic B chain [e.g., poly (propylene oxide)] forming the anchor as a result of its being strongly adsorbed onto the hydrophobic particle or oil droplet The A chains consist of hydrophilic components (e.g., EO groups), and these provide the effective steric repulsion. 

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... 

Polymers that are dispersible in water or organic liquids (dispersion adhesives)... 

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