Latex method

Some peculiarities of the thermal and thermooxidative decomposition of polyvinyl chloride depend on the conditions of its production. Thus, it is known that samples of the polymer produced by initiating the polymerization of vinyl chloride with ultraviolet irradiation possess higher stability in comparison with samples produced in polymerization under the action of chemical agents [26, 27]. Reversibility of the process of dehydrochlorination in the decomposition of samples of polyvinyl chloride produced by the latex method is noted, while in the process of decomposition of suspension polymer, the phenomenon of reversibilily is not observed [21]. It has been shown that the rate of dehydrochlorination of the latex polymer is significantly higher than that of the suspension polymer under the same conditions [21]. It has been established that the polymerization of vinyl chloride in the presence of oxygen leads to the formation of unstable peroxide groups, which can initiate decomposition of the polymer [28, 29]. It is noted that an extremely substantial influence on the stability of polyvinyl chloride is exerted by the purity of the monomer, as well as the presence of impurities of metals of variable valence [28]. 

Copolymersof vinyl chloride with vinylidine chloride, produced by the suspension method, are more stable than the latex copolymers. The rates of thermal decomposition of polyvinyl chloride and the copolymer of vinyl chloride with vinylidine chloride, produced by the latex method, are practically the same for the latex copolymer, just as for the latex homopolymer, reversibility of the process of dehydrochlorination is observed [21]. In an investigation of the stability of copolymers of vinyl chloride with vinyl acetate, vinylidine chloride, and with vinylisobutyl ether in nucleophilic substitution reactions, it was found that the copolymer with vinyl acetate is the least stable to the action of alcoholic alkali the copolymers with vinylidine chloride and vinylisobutyl ether proved more stable [56]. The stability of the copolymer of vinyl chloride witii methyl acrylate is substantially increased when the degree of homogeneity of the copolymer with respect to composition is increased, and when monomers with a smaller content of impurities are used, as well as when the copolymerization is conducted in the presence of chain carriers [57, 58]. 

Latex is a colloidal dispersion of polymer in an aqueous solvent. This method is more suitable for those polymers that can be prepared via emulsion polymerization or those that have the ability to form emulsion. It consists of an aqueous dispersion/ stabilization of filler using a surfactant followed by the addition of the dispersed filler into the polymer latex. Nanocomposites can be obtained after freeze-drying the above mixture followed by melt processing. The latex method has several advantages including no requirement for organic solvent, reliability, ease of processing, and improved dispersion of the filler in the viscous polymer matrix [70]. 

Of course not all particulate matter is homogeneous in terms of its density, for example river sediments, so the method is not universal. In these cases, and also for general convenience, it is usual instead to perform a "secondary" calibration method using a narrow size ranged polymer latex sample, whose mean, mode or some other "size"has been measured by another technique. The latex method is commonly used. Suitable particles are obtained from several sources including, in europe, the Community Bureau of Reference (B.C.R.),44 and, in the U.S.A., the National Institute of Science and Technology (formerly the National Bureau of StandardsFull traceability to those Reference Materials is recommended. 

Synthesis routes (a) and (b) above are idealizations, and many permutations of the general idea exist, particularly in patent literature. " IPN s have been synthesized by bulk, suspension and latex methods. For the latter, two variations are still possible the first method places both networks in a single latex particle, called a latex IPN, and the second method is called interpenetrating elastomeric networks, lEN s, where two different latexes are mixed, coagulated and subsequently crosslinked. ... 

One method (116) of producing cellular polymers from a variety of latexes uses primarily latexes of carboxylated styrene—butadiene copolymers, although other elastomers such as acryUc elastomers, nitrile mbber, and vinyl polymers can be employed. 

Three generations of latices as characterized by the type of surfactant used in manufacture have been defined (53). The first generation includes latices made with conventional (/) anionic surfactants like fatty acid soaps, alkyl carboxylates, alkyl sulfates, and alkyl sulfonates (54) (2) nonionic surfactants like poly(ethylene oxide) or poly(vinyl alcohol) used to improve freeze—thaw and shear stabiUty and (J) cationic surfactants like amines, nitriles, and other nitrogen bases, rarely used because of incompatibiUty problems. Portiand cement latex modifiers are one example where cationic surfactants are used. Anionic surfactants yield smaller particles than nonionic surfactants (55). Often a combination of anionic surfactants or anionic and nonionic surfactants are used to provide improved stabiUty. The stabilizing abiUty of anionic fatty acid soaps diminishes at lower pH as the soaps revert to their acids. First-generation latices also suffer from the presence of soap on the polymer particles at the end of the polymerization. Steam and vacuum stripping methods are often used to remove the soap and unreacted monomer from the final product (56). 

The viscosity of the latex can also be dependent on pH. In the case of some latices, lowering the pH with a weak acid such as glycine is an effective method for raising the viscosity without destabilising the system. Latices made with poly(vinyl alcohol) as the primary emulsifier can be thickened by increasing the pH with a strong alkaU. 

Binders. Latices are used as fiber binders by the paper and textile industries. The two principal methods of appHcation are (/) wet-end addition, wherein the ionic latex is added to a fiber slurry and then coagulated in the slurry prior to sheet formation, and (2) saturation of the latex into a formed fiber web wherein the latex is coagulated by dehydration. Latices are also used as binders for particulate matter such as mbber scrap. 

Latexes of synthetic resins are identified by ir spectrometry. Selective extraction with organic solvents is used to obtain purified fractions of the polymers for spectrometric identification. Polymeric films can be identified by the multiple internal reflectance ir technique, if the film is smooth enough to permit intimate contact with the reflectance plate. TAPPI and ASTM procedures have not been written for these instmmental methods, because the interpretation of spectra is not amenable to standardization. 

Acrylonitrile—Butadiene—Styrene. ABS is an important commercial polymer, with numerous apphcations. In the late 1950s, ABS was produced by emulsion grafting of styrene-acrylonitrile copolymers onto polybutadiene latex particles. This method continues to be the basis for a considerable volume of ABS manufacture. More recently, ABS has also been produced by continuous mass and mass-suspension processes (237). The various products may be mechanically blended for optimizing properties and cost. Brittle SAN, toughened by SAN-grafted ethylene—propylene and acrylate mbbets, is used in outdoor apphcations. Flame retardancy of ABS is improved by chlorinated PE and other flame-retarding additives (237). 

In the second process, a small particle-size latex is prepared and treated so that a limited and controlled degree of particle agglomeration occurs. The agglomerated latex is then concentrated as before but, because of the particle-size distribution obtained, the soHds may be raised to ca 70 wt %. Two methods exist for agglomeration of latices, ie, chemical and freeze agglomeration (45,46). 

Vulcanization. Vulcanization of articles made from latex compounds generally foUows the same principles and methods as in vulcanizing... 

Latex continues to drip after the initial collection and coagulates naturally in the cup to form cup lump. Coagulum which forms as a film of latex on the tapped cut, called tree lace, or from latex that has dripped onto the ground, called earth scrap, is collected the next day along with the cup lump. Some smallholders may not collect the latex at all, but allow it to coagulate in the cup and collect it as cup lump. AH these methods produce mbber known as field coagulum. 

The main distinguishing features of evaporated latex is that 100% mbbet recovery is achieved, but unlike the other methods, the nonmbbets ate also concentrated. The most weU-known evaporated latex is Standard Revertex (trade name from Revertex). 

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