Latex form

Poly etrafluoroethylene is manufactured and sold in three forms granular, fine powder, and aqueous dispersion each requires a different fabrication technique. Granular resins are manufactured in a wide variety of grades to obtain a different balance between powder flows and end use properties (Pig. 1). Pine powders that are made by coagulating aqueous dispersions also are available in various grades. Differences in fine powder grades correspond to their usefulness in specific appHcations and to the ease of fabrication. Aqueous dispersions are sold in latex form and are available in different grades. A variety of formulation techniques are used to tailor these dispersions for specific appHcations. 

Waterborne contact adhesives contain an elastomer in latex form, usually an acryflc or neoprene-based latex, and a heat-reactive, cross-linkable phenohc resin in the form of an aqueous dispersion. The phenoHc resin improves metal adhesion, green strength, and peel strength at elevated temperature. A typical formulation contains three parts latex and one part phenohc dispersion (dry weight bases). Although metal oxides may be added, reaction of the oxide with the phenohc resin does not occur readily. 

Rubber processed in latex form accounts for about 10% of new mbber consumption. Rubber latex is a Hquid, oil-in-water emulsion which is used to make foam or thin-walled mbber articles. The same accelerators and antidegradants used in dry mbber are used in latex, with longer-chain versions preferred for greater oil solubiHty. To prepare these and other additives for addition to latex, they must be predispersed in water and the surface of the powder or oil droplet coated with a surface-active agent to prevent destabilization (coagulation) of the latex. 

In addition to the soHd form of natural mbber it is available as a soHd suspended in water, known as latex. Synthetic mbbers are also available in latex form. Latex has become an important commodity used in the manufacture of dipped goods for health and disease protection. The principal uses of natural mbber are as follows tires and retreading, 70% latex (gloves, balloons), 12% mechanical goods, 9% load-bearing components, 4% and other, 5%. 

Synthetic. The main types of elastomeric polymers commercially available in latex form from emulsion polymerization are butadiene—styrene, butadiene—acrylonitrile, and chloroprene (neoprene). There are also a number of specialty latices that contain polymers that are basically variations of the above polymers, eg, those to which a third monomer has been added to provide a polymer that performs a specific function. The most important of these are products that contain either a basic, eg, vinylpyridine, or an acidic monomer, eg, methacrylic acid. These latices are specifically designed for tire cord solutioning, papercoating, and carpet back-sizing. 

In contrast to other polymers the resistance to water permeation is low due to the hydrolysis of the poly(vinyl acetate) (163,164). Ethylene copolymers have been developed which have improved water resistance and waterproofness. The polymer can be used in the latex form or in a spray-dried form which can be preblended in with the cement (qv) in the proper proportion. The compressive and tensile strength of concrete is improved by addition of PVAc emulsions to the water before mixing. A polymer-soHds-to-total-soHds ratio of ca 10 90 is best. The emulsions also aid adhesion between new and old concrete when patching or resurfacing. 

The molecular weight of the polymers is controlled by temperature (for the homopolymer), or by the addition of organic acid anhydrides and acid hahdes (37). Although most of the product is made in the first reactor, the background monomer continues to react in a second reactor which is placed in series with the first. When the reaction is complete, a hindered phenoHc or metal antioxidant is added to improve shelf life and processibiUty. The catalyst is deactivated during steam coagulation, which also removes solvent and unreacted monomer. The cmmbs of water-swoUen product are dried and pressed into bale form. This is the only form in which the mbber is commercially available. The mbber may be converted into a latex form, but this has not found commercial appHcation (38). 

In suspension processes the fate of the continuous liquid phase and the associated control of the stabilisation and destabilisation of the system are the most important considerations. Many polymers occur in latex form, i.e. as polymer particles of diameter of the order of 1 p.m suspended in a liquid, usually aqueous, medium. Such latices are widely used to produce latex foams, elastic thread, dipped latex rubber goods, emulsion paints and paper additives. In the manufacture and use of such products it is important that premature destabilisation of the latex does not occur but that such destabilisation occurs in a controlled and appropriate manner at the relevant stage in processing. Such control of stability is based on the general precepts of colloid science. As with products from solvent processes diffusion distances for the liquid phase must be kept short furthermore, care has to be taken that the drying rates are not such that a skin of very low permeability is formed whilst there remains undesirable liquid in the mass of the polymer. For most applications it is desirable that destabilisation leads to a coherent film (or spongy mass in the case of foams) of polymers. To achieve this the of the latex compound should not be above ambient temperature so that at such temperatures intermolecular diffusion of the polymer molecules can occur. 

An early approach was to use butadiene as the comonomer but the resultant copolymers have largely been used only in latex form in paper and board finishes and are no longer believed to be important. 

From the electron micrographs, assuming that PVAc particles in the latex are the same size, the formation model of the porous film from the latex film can be illustrated as in Fig. 3 [19]. When the latex forms a dried film over minimum film-forming temperature, it is concluded that PVA coexisted in the latex and is not excluded to the outside of the film during filming, but is kept in spaces produced by the close-packed structure of PVAc particles. 

The research in this area has a great potential from scientific and technological aspects and requires further exploration. However, the reported attempts are a welcoming endeavor to hydrogenate nitrile rubber in latex form. 

Naturally occurring fibers such as cotton, cellulose, etc., have short whiskers protruding from the surface, which help to give a physical bond when mixed with rubber. Glass, nylon, polyester, and rayon have smooth surfaces and adhesion of these fibers to the rubber matrix is comparatively poor. In addition, these synthetic fibers have chemically unreactive surfaces, which must be treated to enable a bond to form with the mbber. In general, the fibers are dipped in adhesives in the latex form and this technology is the most common one used for continuous fibers. The adhesion between elastomers and fibers was discussed by Kubo [128]. Hisaki et al. [129] and Kubo [130] proposed a... 

One component of a terpolymer of butadiene, styrene and vinyl pyridine used in latex form to promote good adhesion between rubber and textiles, particularly rayon and nylon. Viscoelasticity... 

Before World War II, hevea rubber accounted for over 99% of all elastomers used, but synthetic elastomers account for more than 70% of all rubber used today. NR and many synthetic elastomers are available in latex form. The latex may be used, as such, for adhering carpet fibers or for dipped articles, such as gloves, but most of the latex is coagulated and the dried coagulant used for the production of tires and mechanical goods. 

Latex materials are classified into three types according to the type of surfactant used in the production of the latexes cationic (positively charged), anionic (negatively charged) and nonionic (uncharged). The action of the emulsifier (surfactant) is due to its molecules having both hydrophilic and hydrophobic parts and the properties of the latex formed are very dependent on how the various constituents are put together. 

Polystyrene 10.0 kg polymer in solution 10.0 kg polymer dissolved in the monomer 10.0 kg polymer in stable latex form... 

The prevulcanization of natural rubber in latex form has also been a subject of much investigation. The cross-linking mechanism is not yet fully understood, but the water apparently plays a major role in it. Irradiation results in the cross-linking of the rubber molecules and in coarsening of the latex particles. A process of cross-linking of natural rubber latex has been developed to the point that it can be used for an industrial-scale application. The irradiation is performed in aqueous media by electron beam without a prorad (sensitizer) at a dose of 200 kGy (20 Mrad) or in the presence of n-butyl acrylate at considerably lower doses, typically 15 kGy. The cross-linked film exhibits physical properties comparable to those obtained from sulfur cured (vulcanized) film. As an alternative, the addition of a variety of chloroal-kanes makes it possible to achieve a maximum tensile strength with radiation doses of less than 5 Mrad (50 kGy). ... 

GR-S employs styrene as the minor ingredient with butadiene, and it will be discussed under the heading of the latter compound. Copolymers in which styrene is the major component—i.e., about 70 to 85 parts of styrene with 30 to 15 parts of butadiene— are also produced (23), and they have found markets both as a solid in applications such as shoe soling and floor covering and in latex form as an emulsion paint for inside use. Latex paints (19) of this type have been introduced to the consumer market only within the last few years, but they are receiving widespread acceptance as a result of their excellent film properties, good covering power, and ease of application. 

The final product in latex form may be used for water-type paints or coatings or the water may be removed from the finely divided high-molecular weight polymer. Separation may be brought about by the addition of electrolytes, freezing or spray drying. 

Because of its excellent high- and low-temperature properties, many products used in the arctic and tropical areas of the world are made from natural rubber. However, it is not suitable for applications where there is contact with naphtha, e.g, gasoline hoses, because the solvent swells the material. Almost all clastic bands arc made from natural rubber. Because of its excellent tack properties, the material is used in solvent and latex form as the base for adhesives. 

The properties of these elastomers are widely different. All require vulcanization. In general, sulfur is used only for unsaturated polymers, peroxides, quinones, metallic oxides, or cliisocyauates effect vulcanization with saturated types. Many are special-puipose rubbers, some can be used in tires when loaded with carbon black, others have high resistance to attack by heat and hydrocarbon oils and thus are superior to natural rubber for steam hose, gasoline and oil-loading hose. Most are available in latex form. See also Elastomers. 

Continuous emulsion polymerization processes are presently employed for large scale production of synthetic rubber latexes. Owing to the recent growth of the market for polymers in latex form, this process is becoming more and more important also in the production of a number of other synthetic latexes, and hence, the necessity of the knowledge of continuous emulsion polymerization kinetics has recently increased. Nevertheless/ the study of continuous emulsion polymerization kinetics hasf to datef received comparatively scant attention in contrast to batch kinetics/ and very little published work is available at present/ especially as to the reactor optimization of continuous emulsion polymerization processes. For the theoretical optimization of continuous emulsion polymerization reactors/ it is desirable to understand the kinetics of emulsion polymerization as deeply and quantitatively as possible. 

Fluorocarbon Elastomers in Latex Form, Processing and Applications... 

FLUOROCARBON ELASTOMERS IN LATEX FORM, PROCESSING AND APPLICATIONS... 

A certain proportion of fluoroelastomers is used in latex form. The compounding techniques used are similar to those used for standard latexes i.e., solid ingredients are first dispersed in water with the use of surface active agents and liquid ingredients are prepared as emulsions prior to their addition to the latex. The dispersions of solids are prepared in ball mills or high-speed mills (e.g., Kady). 

Copolymerization. Vinyl chloride can be copolymerized with a variety of monomers. Vinyl acetate [9003-22-9], the most important commercial comonomer, is used to reduce crystallinity, which aids fusion and allows lower processing temperatures. Copolymers are used in flooring and coatings. This copolymer sometimes contains maleic acid or vinyl alcohol (hydrolyzed from the poly(vinyl acetate)) to improve the coating s adhesion to other materials, including metals. Copolymers with vinylidene chloride are used as barrier films and coatings. Copolymers of vinyl chloride with maleates or fumerates are used to raise heat deflection temperature. Copolymers of vinyl chloride with acrylic esters in latex form are used as film formers in paint, nonwoven fabric binders, adhesives, and coatings. Copolymers with olefins improve thermal stability and melt flow, but at some loss of heat-deflection temperature (100). Copolymerization parameters are listed in Table 5. 

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