Latex yield

A disease of rubber trees affecting the pods and leaves. The resulting pod-rot and leaf-fall seriously reduces the latex yield. It is common in most rubber growing countries with, until recently, the exception of Malaysia, where urgent... 

VEC copolymerizes well with vinyl ester monomers over a range of compositions. To a more limited extent, VEC can also be incorporated into acrylic copolymers, however, we have not achieved quantitative incorporation. In the presence of styrene, essentially no VEC is incorporated into the copolymer. VEC can also be easily incorporated into a vinyl acetate/butyl acrylate latex, yielding a latex polymer containing cyclic carbonate functionality. The Tg of the copolymers can be modeled using extrapolated values for the Tg of a VEC homopolymer. 

When considering a cementitious mortar mix for use in the manufacturing of an ultra-lightweight thin film product, the addition of a suitable polymer modifier can enhance both the fresh and hardened properties of such lightweight material. The addition of styrene butadiene rubber latex (SBR) latex yielded the best overall results in terms of workability, formability, mouldability, flowability, compressive and flexural strengths. The improvements offered by the addition of the acrylic polymer were less impressive and the use of an acrylic modifier is thus not recommended for use in ultra-lightweight thin film products. It is also confirmed that small adjustments to the water cement ratio alter the workability of such mixes and may be used to tweak mix designs to suit specific applications and uses. Thus with the addition of either a SBR latex or acrylic polymer and... 

S.3.3 Tapping, Latex Yield and Carbon Budget of the Rubber Tree... 

A curable epoxy resin containing a crosslinked elastomeric latex " yields a special type of sequential IPN. The elastomers mentioned include SBR, EPR, NBR, silicone rubber, and PEA. Hawkins " described an epoxy/polyurethane semi-SIN, and Mendoyanis " revealed an epoxy/liquid rubber SIN. The liquid rubbers were based on polysulfide rubber or polyethylene. The final products could be extended to 400% at... 

Impranil 4496 particle size is 0.1-0.2 micrometers. Storage life is at least 6 months, but prolonged exposure above 140 °F and cooling to less than 38°F must be avoided. Viscosity is 220 cps at 20°C (Brookfield, No. 1 spindle, 12 rpm). The latex yields continuous film with programmed drying (200°F, 230°F, 265-290°F) but adhesive applications accommodate 265-290°F directly. 

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). 

Thixotropy and Other Time Effects. In addition to the nonideal behavior described, many fluids exhibit time-dependent effects. Some fluids increase in viscosity (rheopexy) or decrease in viscosity (thixotropy) with time when sheared at a constant shear rate. These effects can occur in fluids with or without yield values. Rheopexy is a rare phenomenon, but thixotropic fluids are common. Examples of thixotropic materials are starch pastes, gelatin, mayoimaise, drilling muds, and latex paints. The thixotropic effect is shown in Figure 5, where the curves are for a specimen exposed first to increasing and then to decreasing shear rates. Because of the decrease in viscosity with time as weU as shear rate, the up-and-down flow curves do not superimpose. Instead, they form a hysteresis loop, often called a thixotropic loop. Because flow curves for thixotropic or rheopectic Hquids depend on the shear history of the sample, different curves for the same material can be obtained, depending on the experimental procedure. 

Latex compound viscosity obviously forms an important aspect of dipped product manufacture. Accurate measurement by a Brookfield or similar viscometer is desirable to estabhsh the fundamental viscosity of a compound, but Flow-Cup viscometers (Ford B.3 Cup) are more commonly used for day-to-day control of latex compounds during compounding and product manufacture. It is necessary to ensure that only stainless steel flow cups are used, if the measured latex is allowed to return to the production tanks brass cups yield an unacceptable level of copper contamination, which adversely affects aging properties of products made from copper-contaminated mbber compound. 

Butadiene copolymers are mainly prepared to yield mbbers (see Styrene-butadiene rubber). Many commercially significant latex paints are based on styrene—butadiene copolymers (see Coatings Paint). In latex paint the weight ratio S B is usually 60 40 with high conversion. Most of the block copolymers prepared by anionic catalysts, eg, butyUithium, are also elastomers. However, some of these block copolymers are thermoplastic mbbers, which behave like cross-linked mbbers at room temperature but show regular thermoplastic flow at elevated temperatures (45,46). Diblock (styrene—butadiene (SB)) and triblock (styrene—butadiene—styrene (SBS)) copolymers are commercially available. Typically, they are blended with PS to achieve a desirable property, eg, improved clarity/flexibiHty (see Polymerblends) (46). These block copolymers represent a class of new and interesting polymeric materials (47,48). Of particular interest are their morphologies (49—52), solution properties (53,54), and mechanical behavior (55,56). 

Uses. /-Butyl hypochlorite has been found useful in upgrading vegetable oils (273) and in the preparation of a-substituted acryflc acid esters (274) and esters of isoprene halohydrins (275). Numerous patents describe its use in cross-linking of polymers (qv) (276), in surface treatment of mbber (qv) (277), and in odor control of polymer latexes (278). It is used in the preparation of propylene oxide (qv) in high yield with Httle or no by-products (269,279). Fluoroalkyl hypochlorites are useful as insecticides, initiators for polymerizations, and bleaching and chlorinating agents (280). 

The polymerization iavolves the reaction of sodium polysulftde with ethylene dichloride ia aqueous media at 70°C for 2—6 h, yielding an aqueous dispersion (latex) of the polysulftde mbber. 

The latex of the Sapota achras yields a thermoplastic material, chicle, consisting of about 17.4% hydrocarbon, 40% acetone soluble resin and 35% occluded water. The hydrocarbon appears to contain both trans- and c/s-polyisoprene. Although originally introduced as gutta pereha and natural rubber substitutes, deresinated chicle has become important as the base for chewing gum. Like other polyisoprenes, it is meeting competition from synthetic polymers. 

The emulsion polymerization of acrylamide yields a high-molecular polymer (with the molecular weight reaching 2.5-10 ), which can be easily dispersed in water to obtain water-in-oil type latex (containing 30-60% polymer). On prolonged storage, the emulsion exhibits lamination, but subsequent stirring allows easy redispersal of the product. 

Successful NMP in emulsion requires use of conditions where there is no discrete monomer droplet phase and a mechanism to remove any excess nitroxide formed in the particle phase as a consequence of the persistent radical effect. Szkurhan and Georges"18 precipitated an acetone solution of a low molecular weight TEMPO-tcrminated PS into an aqueous solution of PVA to form emulsion particles. These were swollen with monomer and polymerized at 135 °C to yield very low dispersity PS and a stable latex. Nicolas et at.219 performed emulsion NMP of BA at 90 °C making use of the water-soluble alkoxyamine 110 or the corresponding sodium salt both of which are based on the open-chain nitroxide 89. They obtained PBA with narrow molecular weight distribution as a stable latex at a relatively high solids level (26%). A low dispersity PBA-WocA-PS was also prepared,... 

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