Emulsions and adhesives

Microemulsions [276,277] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically 

A paper by Ninham [281] clearly shows that the understanding of emulsions is not trivial. That work on the theoretical forces due to liquid structures at interfaces is well beyond the scope of this book, but it stands as an example of the state of this rather complex field which must deal with the dynamics of liquid-liquid and solid-liquid interfaces. 

The major structural unit of interest in emulsions, microemulsions, colloids and latexes is the particle . It is well known that the particle shape, size and distribution of a latex controls the properties and end use applications. Many latexes are manufactured with a controlled and sometimes monodisperse distribution of particle sizes. Polymer liquids, in the form of emulsions and adhesives, are wet and sticky, and therefore specimen preparation for electron microscopy is very difficult. As a result of the importance of the determination of particle size distribution, microscopy techniques have focused on specimen preparations which do not alter this distribution or which alter it as little as possible. Methods have included special cryotechniques (Section 4.9), staining-fixation methods (Section 4.4), microtomy (Section 4.3) and some simple methods (Section 4.1) such as dropping a solution onto a specimen holder. This section is meant to provide a brief survey of the types of microscopy applications which have been found useful in the evaluation of emulsions and latexes. 

Molau and Keskkula [282] were among the first to study the mechanism of particle formation in rubber containing polymers. They showed that phase separation occurs between the rubber and a vinyl polymer during the polymerization of solutions of rubber in vinyl monomers which is followed by formation of an oil-in-oil emulsion. During phase inversion of the emulsion, rubber solution droplets are formed which change into solid rubber particles in the final polymer. Structural investigations by phase contrast optical 

The formation of micelles, or colloidal particles, by block copolymers in organic solvents has been described and reviewed by Price [2881. The molecular weight of polystyrene was estimated from specimens prepared by spraying and evaporation for TEM. Freeze etching a drop of solution rapidly frozen with liquid nitrogen [289] was described (Section 4.9.4) where the solvent was allowed to evaporate and a replica produced of the fracture surface. Another method [290] 

Microemulsions [345, 346] are special types of emulsions that form spontaneously and have very small particles. Microemulsions are optically clear, thermodynamically stable dispersions of two immiscible liquids obtained by the use of carefully adjusted surface active molecules (surfactants). Both liquids in a microemulsion will be present in regions of the same order of magnitude, with the dispersed phase on the order of 10-100 nm. Aggregates of surface active mole- 

The structure and morphology of multiphase polymers have been discussed (Section 5.3) and the particle size and distribution have been shown to be quite important for both mechanical properties and applications. In many polymers (e.g. ABS), the particle size distribution is 

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Chem. Analysis Sodium benzoate (90-92%), sodium nitrite (8-10%) Uses Corrosion inhibitor for antifreeze, closed cycle heating and cooling systems, domestic and industrial heating systems, summer coolants for internal combustion engines, hydraulic systems, emulsion paints, textile emulsions and adhesives corrosion-inhibiting impregnant for wrapping papers... 

Alpha-protein n. Protein obtained from Soya beans which is chiefly glycinin. The process of extraction of the protein from the beans involves dissolution of the protein and separation from cellulose, removal of carbohydrates, and hydrolysis of the protein to reduce its chemical complexity. Its uses are similar to those of casein, with which it has much in common, being employed in water paints, emulsions, and adhesives of all types. 

The PVOH-protected emulsion and adhesives made with it spray veiy well in airless spray systems, but relatively poorly in air spray systems. 

Acrylates are primarily used to prepare emulsion and solution polymers. The emulsion polymerization process provides high yields of polymers in a form suitable for a variety of appHcations. Acrylate polymer emulsions were first used as coatings for leather in the eady 1930s and have found wide utiHty as coatings, finishes, and binders for leather, textiles, and paper. Acrylate emulsions are used in the preparation of both interior and exterior paints, door poHshes, and adhesives. Solution polymers of acrylates, frequentiy with minor concentrations of other monomers, are employed in the preparation of industrial coatings. Polymers of acryHc acid can be used as superabsorbents in disposable diapers, as well as in formulation of superior, reduced-phosphate-level detergents. 

Higher alkyl acrylates and alkyl-functional esters are important in copolymer products, in conventional emulsion appHcations for coatings and adhesives, and as reactants in radiation-cured coatings and inks. In general, they are produced in direct or transesterification batch processes (17,101,102) because of their relatively low volume. 

Adhesives. Acryhc emulsion and solution polymers form the basis of a variety of adhesive types. The principal use is in pressure-sensitive adhesives, where a film of a very low T (<—20 " C) acrylic polymer or copolymer is used on the adherent side of tapes, decals, and labels. Acrylics provide a good balance of tack and bond strength with exceptional color stabiUty and resistance to aging (201,202). AcryUcs also find use in numerous types of constmction adhesive formulations and as film-to-film laminating adhesives (qv). 

Acryhc modifiers for cement impact strength and adhesion to substrates are discussed in reference 211. Both water-soluble acryhc and acryhc emulsion polymers are used in the ceramic industry as temporary binders, deflocculants, and additive components in ceramic bodies and glazes (212) (see Ceramcs). 

Emulsion Adhesives. The most widely used emulsion-based adhesive is that based upon poly(vinyl acetate)—poly(vinyl alcohol) copolymers formed by free-radical polymerization in an emulsion system. Poly(vinyl alcohol) is typically formed by hydrolysis of the poly(vinyl acetate). The properties of the emulsion are derived from the polymer employed in the polymerization as weU as from the system used to emulsify the polymer in water. The emulsion is stabilized by a combination of a surfactant plus a coUoid protection system. The protective coUoids are similar to those used paint (qv) to stabilize latex. For poly(vinyl acetate), the protective coUoids are isolated from natural gums and ceUulosic resins (carboxymethylceUulose or hydroxyethjdceUulose). The hydroHzed polymer may also be used. The physical properties of the poly(vinyl acetate) polymer can be modified by changing the co-monomer used in the polymerization. Any material which is free-radically active and participates in an emulsion polymerization can be employed. Plasticizers (qv), tackifiers, viscosity modifiers, solvents (added to coalesce the emulsion particles), fillers, humectants, and other materials are often added to the adhesive to meet specifications for the intended appHcation. Because the presence of foam in the bond line could decrease performance of the adhesion joint, agents that control the amount of air entrapped in an adhesive bond must be added. Biocides are also necessary many of the materials that are used to stabilize poly(vinyl acetate) emulsions are natural products. Poly(vinyl acetate) adhesives known as "white glue" or "carpenter s glue" are available under a number of different trade names. AppHcations are found mosdy in the area of adhesion to paper and wood (see Vinyl polymers). 

A number of chemical products are derived from Sasol s synthetic fuel operations based on the Fischer-Tropsch synthesis including paraffin waxes from the Arge process and several polar and nonpolar hydrocarbon mixtures from the Synthol process. Products suitable for use as hot melt adhesives, PVC lubricants, cormgated cardboard coating emulsions, and poHshes have been developed from Arge waxes. Wax blends containing medium and hard wax fractions are useful for making candles, and over 20,000 t/yr of wax are sold for this appHcation. 

Itaconic acid is a specialty monomer that affords performance advantages to certain polymeric coatings (qv) (see Polyesters, unsaturated). Emulsion stabihty, flow properties of the formulated coating, and adhesion to substrates are improved by the acid. Acrylonitrile fibers with low levels of the acid comonomer exhibit improved dye receptivity which allows mote efficient dyeing to deeper shades (see Acrylonitrile polymers Fibers, acrylic) (10,11). Itaconic acid has also been incorporated in PAN precursors of carbon and graphite fibers (qv) and into ethylene ionomers (qv) (12). 

The Hercules viscometer was originally designed for paper and paperboard coatings, but its use has been extended to paints, adhesives, mineral slurries, emulsions, and starch solutions. The iastmment, noted for being robust and rehable, is particularly well suited for quaUty control and product formulation. It is capable of measuting viscosity over a moderate range 1-10 mPa-s) up to high shear rates (115,000 ). A more recent model is the... 

Because of its high cold flow, poly(vinyl acetate) is of little value in the form of mouldings and extrusions. However, because of its good adhesion to a number of substrates, and to some extent because of its cold flow, a large quantity is produced for use in emulsion paints, adhesives and various textile finishing operations. A minor proportion of the material is also converted into poly(vinyl alcohol) and the poly(vinyl acetal)s which, are of some interest to the plastics industry. 

Insulants for pipework and curved pressure vessels can be obtained ready shaped, so that they fit tightly to the surface. All surfaces should be quite dry before the material is applied, even if the adhesive is a water-based emulsion, and the water or other solvent must be given ample time to dry or set before any outer wrapping is applied. 

In the operations of a consulting laboratory, one is constantly confronted with a variety of problems which involve the technology of emulsions and foams. These problems cover a diversity of interests such as foods, cosmetics, adhesives, polymers, and others. 

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