Latex reduction process

Carpet waste generated each year and accumulated in landfills represents a considerable potential resource, as it may be converted into various useful products The rate of carpet disposal is about 2-3 million tons per year in the US A, and about 4-6 million tons per year worldwide. A carpet typically consists of two layers of (i) backing (usually fabrics from polypropylene tape yams), joined by CaCOj filled styrene-butadiene latex rubber (SBR), and (ii) face fibers (the majority being nylon 6 and nylon 66 textured yams) tufted into the primary backing. To use post-consumer carpet (PCC) as concrete or soil reinforcement, the carpet is shredded to recover fibers. It is generally not necessary to disassemble yams in the carpet into individual fibers. The size-reduction process yields the following from PCC ... 

Rubbers. Plasticizers have been used in mbber processing and formulations for many years (8), although phthaHc and adipic esters have found Htde use since cheaper alternatives, eg, heavy petroleum oils, coal tars, and other predominandy hydrocarbon products, are available for many types of mbber. Esters, eg, DOA, DOP, and DOS, can be used with latex mbber to produce large reductions in T. It has been noted (9) that the more polar elastomers such as nitrile mbber and chloroprene are insufficiendy compatible with hydrocarbons and requite a more specialized type of plasticizer, eg, a phthalate or adipate ester. Approximately 50% of nitrile mbber used in Western Europe is plasticized at 10—15 phr (a total of 5000—6000 t/yr), and 25% of chloroprene at ca 10 phr (ca 2000 t/yr) is plasticized. Usage in other elastomers is very low although may increase due to toxicological concerns over polynuclear aromatic compounds (9). 

Continuous polymerization systems offer the possibiUty of several advantages including better heat transfer and cooling capacity, reduction in downtime, more uniform products, and less raw material handling (59,60). In some continuous emulsion homopolymerization processes, materials are added continuously to a first ketde and partially polymerized, then passed into a second reactor where, with additional initiator, the reaction is concluded. Continuous emulsion copolymerizations of vinyl acetate with ethylene have been described (61—64). Recirculating loop reactors which have high heat-transfer rates have found use for the manufacture of latexes for paint appHcations (59). 

Despite these generalizations, the reduction or elimination of coagulum is usually best accomplished by a "systems approach", i.e., a consideration of latex properties to be achieved in the emulsion polymerization, the economics of the polymerization process, and the deliberate design of the reactor system for that particular polymerization system. Each polymerization system must be considered as a separate system and treated as such. The most effective approach to reduce or eliminate the formation of coagulum is to determine the mechanism by which it is formed and... 

The vulcanization accelerators include the thiazole 2-mercaptobenzothiazole (MBT) (169), and its derivative benzothiazole disulfide (dibenzoythiazyl disulfide, 2,2 -dithiobis (benzothiazole), MBTS) (170). Organic accelerators enable reduction in time of vulcanization, more effective use of sulfur in formation of cross-links and use of low processing temperatures. MBTS delays vulcanization, when compared with MBT alone. They are used in production of conveyor belts, footware, etc. The 2-mercaptobenzothiazole zinc salt (MBTZ) (171) is also important, and is used in latex products. Other sulfur donors include 2-morpholinodithiobenzothiazole, 2-(4-morpholinyldithio)benzothiazole (MBSS, MORFAX) (172). 

Garcia et al. [98] conducted coulometric initiation of acrylamide polymerization in oil-continuous AOT-toluene-water microemulsions using platinum/Nafion solid polymer electrodes (SPEs). The SPE served to separate the microemulsion from an aqueous electrolyte phase. Polymerization was initiated at room temperature by constant-potential electrolytic reduction of potassium persulfate initiator solubilized in the microemulsion droplets. Acrylamide monomer behaved as a cosurfactant and was required for the redox process. Latex particles and solid polyacrylamide were obtained. The kinetics of electroinitiated polymerization was slower than observed with UV or thermal initiation. Latex stability results suggest that coalescence is the primary mechanism for particle growth. 

Fig. 14.1 Process concept for the reduction of residual monomer from latex products using high-pressure CO2 in a countercurrent apparatus.

The value of the monomer partition coefBcient between the CO2 and the water phase indirectly determines the ratio between the effect of enhanced polymerization and the effect of extraction on the reduction of residual monomer. Depending on the process conditions, i.e. temperature, pressure, and the phase behavior of the system involved, this ratio between enhanced polymerization and extraction may vary for different latex systems. With respect to the PMMA latex, the high partition coefBcient m2 as shown in Section 14.4, causes extraction to be the predominant effect as compared to conversion of the monomer. Therefore, a preliminary process design has been developed based on C02-extraction. For this purpose, a mass transfer model has been set up to determine the rate-limiting step in the extraction process. In addition, a process flow diagram, including equipment sizing has been developed. Finally, an economic evaluation has been performed to study the viability of this technique for the removal of residual monomer from latex-products. 

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