Mechanical latices

Natural rubber accounts for about 25% of total rubber consumption. It is produced from the Hevea brasiliensis tree, being formed by isoprene units with cis-1,4 links. Natural rubber is used in tyres and for retreading, latex, mechanical goods, etc. 

Latex mechanical stability The abihty of latex to resist coagulation under influence of mechanical agitation. 

Sperry P R 1984 Morphology and mechanism in latex flocculated by volume restriction J. Colloid Interface Sol. 99 97-108... 

Fig. 1. Stress—strain curves A, hard fiber, eg, nylon B, biconstituent nylon—spandex fiber C, mechanical stretch nylon D, spandex fiber E, extruded latex...

Whatever the nucleation mechanism the final particle size of the latex is determined duting Stage I, provided no additional particle nucleation or coalesence occurs ia the later stages. Monomer added duting Stages II and III only serves to increase the size of the existing particles. 

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 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%. 

The particle size of typical natural mbber latex ranges from slightly higher than 1 )Tm to as small as 20 nm, and can be destabilized by mechanical or chemical action. Machinery used to stir or maintain latex circulation should be designed to minimize fluid shear, particularly where that may be locally intensified, eg, at the junction of blade with shaft. 

Styrene—butadiene latexes generally are quite stable mechanically because of the presence of relatively large amounts of emulsifying and stabilizing agents, and therefore require addition of less stabilizer in compounding. The apphcations of SBR latex are classified in Table 21. This classification indicates the scope of the industry and illustrates the large number of diverse applications in which synthetic latices are employed. The latex types previously found most suitable for particular applications are also listed. 

In dipping generally, but particularly with the anode process, it is desirable to use tanks that circulate the coagulant and latex compound, particularly the latter. Use of circulation keeps the Hquid surface clean and free from lumps, scum, or bubbles. Mechanical circulation can cause mbber particle instabihty, however, and eventually coagulate the compound. Therefore, tanks should be designed to minimize friction or shear action, and the compound stabilized to maintain mechanical stabiUty. 

In the Talalay process, the froth is produced by chemical rather than mechanical means. Hydrogen peroxide and an enzyme decomposition catalyst are mixed iato the latex and the mixture placed ia the mold. Decomposition of the peroxide by the added enzyme results ia the Hberation of oxygen which causes the latex mix to foam and fill the mold. The foam is then rapidly chilled and CO2 is iatroduced to gel the latex. The gelled foam is then handled ia a manner similar to that used ia the Dunlop process. 

P perApplications. In beater additions, the latex is mixed with the beaten paper pulp either by addition at the beater or to the stock chest at the wet end of the paper machine. In either case, the pH of the pulp is reduced to 4.0—4.5, usually by the addition of a solution of alum to the pulp—latex mixture which has been thoroughly agitated. The latex, which for this appHcation must be based on an anionic emulsifier, coagulates as the pH drops. The latex soHds separate ia intimate associatioa with the pulp fibers. The pulp is thea screeaed and the paper web formed ia the coaveatioaal way. A latex for this purpose must possess the proper balance between mechanical and chemical stabiHty. 

Unvulcanized Latex and Latex Compounds. A prime consideration has to be the fluid-state stabihty of the raw latex concentrate and hquid compound made from it. For many years, the mechanical stabihty of latex has been the fundamental test of this aspect. In testing, the raw latex mbber content is adjusted to 55% and an 80 g sample placed in the test vessel. The sample is then mechanically stirred at ultrahigh speed (ca 14,000 rpm) by a rotating disk, causing shear and particle cohision. The time taken to cause creation of mbber particle agglomerates is measured, and expressed as the mechanical stabihty time (MST). 

More recently, a number of tests of chemical stabihty of the latex concentrate have been developed. Chemical stabihty variance in the raw concentrate has considerable effect on the dipping characteristics of latex compounds, and can also affect mechanical stabihty of the compound. A broad rule is that, while latex MST can be increased or decreased without necessarily affecting its chemical stabihty, any change in the latter always is reflected in the MST. A new test, in which chemical stabihty is deterrnined by measurement of the effect of weak 2inc acetate solution added to a second mechanical stabihty sample and the result contrasted with the original MST, is available to numerically quantify chemical stabihty (56). 

The quahty of the water used in emulsion polymerization has long been known to affect the manufacture of ESBR. Water hardness and other ionic content can direcdy affect the chemical and mechanical stabiUty of the polymer emulsion (latex). Poor latex stabiUty results in the formation of coagulum in the polymerization stage as well as other parts of the latex handling system. 

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). 

Hyperbranched polyurethanes are constmcted using phenol-blocked trifunctional monomers in combination with 4-methylbenzyl alcohol for end capping (11). Polyurethane interpenetrating polymer networks (IPNs) are mixtures of two cross-linked polymer networks, prepared by latex blending, sequential polymerization, or simultaneous polymerization. IPNs have improved mechanical properties, as weU as thermal stabiHties, compared to the single cross-linked polymers. In pseudo-IPNs, only one of the involved polymers is cross-linked. Numerous polymers are involved in the formation of polyurethane-derived IPNs (12). 

Emulsion Polymerization. Poly(vinyl acetate) and poly(vinyl acetate) copolymer latexes prepared in the presence of PVA find wide appHcations in adhesives, paints, textile finishes, and coatings. The emulsions show exceUent stabiHty to mechanical shear as weU as to the addition of electrolytes, and possess exceUent machining characteristics. 

Layered Structures. Whenever a barrier polymer lacks the necessary mechanical properties for an appHcation or the barrier would be adequate with only a small amount of the more expensive barrier polymer, a multilayer stmcture via coextmsion or lamination is appropriate. Whenever the barrier polymer is difficult to melt process or a particular traditional substrate such as paper or cellophane [9005-81-6] is necessary, a coating either from latex or a solvent is appropriate. A layered stmcture uses the barrier polymer most efficiently since permeation must occur through the barrier polymer and not around the barrier polymer. No short cuts are allowed for a permeant. The barrier properties of these stmctures are described by the permeance which is described in equation 16 where and L are the permeabiUties and thicknesses of the layers. 

Polymerization Reactions. The polymerization of butadiene with itself and with other monomers represents its largest commercial use. The commercially most important polymers are styrene—butadiene mbber (SBR), polybutadiene (BR), styrene—butadiene latex (SBL), acrylonittile—butadiene—styrene polymer (ABS), and nittile mbber (NR). The reaction mechanisms are free-radical, anionic, cationic, or coordinate, depending on the nature of the initiators or catalysts (194—196). 

---