Monomer removal of residual

Taylor has described a method for the removal of residual monomer by a combination of chemical and steam stripping [12]. An added amount of fresh 

This method appears to be twice as efficient in reducing the residual monomer as conventional steam stripping. Kelly [13] has reported a similar method using steam stripping under vacuum instead of at atmospheric pressure. The chemical depletion of monomer with initiator is done prior to the vacuum steam stripping. 

A promising technique for residual monomer removal is pervaporation, as no additional chemicals are needed for this membrane process and the energy costs are typically low. It has been shown that pervaporation can remove a considerable amount of acryhc monomer from polymethylmethacrylate (PMMA) latexes [15]. Apparently, the Hmiting factor for mass transfer does not occur in the polymer particles, mainly because of the high specific area of the polymer-water interface as compared to the membrane area. Although the high initial costs, as well as fouling of the membrane surface with the polymer particles, are potential drawbacks, pervaporation may thus be expected to provide a viable alternative. 

Temperature increase Simple to operate No toxic solvents required High energy costs Time consuming 

Addition of an inert diluent Simple to operate Removal of inert diluent required Large reactor volume 

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MPa, corresponding to about 90% conversion, excess monomer is vented off to be recycled. Removal of residual monomer typically involves passing the reaction mixture through a countercurrent of steam. The reaction mixture is then cooled, and the polymer separated, dried in hot air at about 100°C, sieved to remove any oversized particles, and stored. Typical number-average molecular weights for commercial PVC are in the range 30,000-80,000. 

Comonomer l-vinyl-2-pyrrolidone (VP) comonomer can be purified by distillation at reduced pressure prior to use. Potassium persulfate (KPS) can be purified in a mixture of water and methanol. NIPAM-co-VP copolymers with different amounts of VP can be prepared at temperatures lower or higher than the LCST of PNIPAM by free radical polymerization in water with an initiator of KPS/N,N,N/,N/-tetramethylethylenediamine (TEMED) redox. The resultant copolymer can be harvested by precipitation, i.e., pouring the reaction mixture into an equal volume of methanol. Each resultant copolymer can be further purified by several cycles of re-dissolution in water and precipitation in methanol to ensure a complete removal of residual monomers. The final product can be dried under reduced pressure at 40 °C. 

In this process uncrosslinked rubber is dissolved in a mixture of the monomers and solvent(s). This solution is pumped into the first reactor which is connected to a series of reactors. The polymerization is started by increasing the temperature, eventually in the presence of an initiator. Most of the rubber grafting and particle sizing happen early in the process. Chain transfer agent level, initiator (type/amount) and shear have a great influence in this stage. Crosslinking of the rubber particles occurs later in the process. The final step is the removal of residual monomer and solvent. 

Removal of Residual Monomer from Polymeric Materials.108... 

Devolatilization. Following polymerization of many polymers, removal of residual monomer or solvents is necessary. Ultimately this removal becomes controlled by diffusion from the polymer no matter what type of process is employed (27). Many processes use a steam-stripping operation for this purpose, and others employ devolatilization in a vented extruder. Removal of residual vinyl chloride monomer from poly(vinyl chloride) was an issue of much concern following the discovery that this monomer is a carcinogen. 

Other cleaning techniques are repeated centrifugation with replacement of the serum, contacting the latex with an activated carbon cloth, and gel filtration (see ref. 18 and references therein). For the removal of residual monomer in lattices, steam stripping is sometimes used [24]. 

Advantages of venting are (1) little risk of contamination (2) operation independent of moisture content (3) reliability (4) consistency of quality and (5) removal of residual monomers under favorable conditions. 

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. 

In contrast to most conventional techniques for removal of residual monomer, such as temperature rise, there is considerable flexibility in the C02-based process. For example, doubling the processing capacity would only require twice the extractor volume [48]. Fig. 14.9 shows that if the permitted amount of residual monomer in the product latex is limited to 10 ppm, a less than twofold increase in the extractor volume will be needed. In addition, an increase in the partition coefEdent, m2, by a factor of 10 results in a decrease in the extractor volume of approximately 20%. 

M.H.W. Qeven, Removal of residual monomer from latex products using supercritical carbon dioxide. Graduate Report... 

The mechanisms involved in monomer removal by post polymerisation were studied. Three redox initiator systans which generate radicals with different hydrophobidties were investigated tert-butyl hydroperoxide, hydrogen peroxide and potassium persulphate. Ascorbic acid was used as a reductant in all cases. The efficiency of these initiator systems for the removal of residual monomers from commercial latexes was studied. The examples exauuued were removal of unreacted vinyl acetate from a vinyl acetate/butyl aerylate/acrylie acid latex, methyl methacrylate from a methyl aaylate, butyl aaylate/acryhc acid latex and butyl acrylate from a butyl aaylate/styrene/ acrylic acid latex. Efficieucy of monomer ranoval by post polymerisation increased with the hydrophobidty of the radical formed from the initiator system and this was independent of the water solubihty of the residual monomer. Reasons for the observations were discussed. 35 refs. 

Antec 97. Volume III. Conference proceedings. Toronto,27th April-2nd May 1997,p.3524-32. 012 CHEMICAL REMOVAL OF RESIDUAL MONOMER FROM POLYVINYL CHLORIDE LATICES AND SLURRIES Marshall R A Parker D K Goodyear Tire Rnbber Co. 

The plasticization of polymers by CO2, which causes a decrease in the glass-transition temperature (Tg) of the polymer, is another important feature that must be taken into accoimt. This plasticization facilitates the occurrence of important effects that are essential to polymer synthesis. Plasticization of the polymer allows enhanced diffusion of monomer and initiator into the polymer phase which often results in increased polymerization rates in heterogeneous polymerizations, removal of residual monomer, solvent, or catalyst from the polymer, and the formation of blends by polymerization within a CO2-swollen host polymer. An important emerging issue is the role that CO2 plays in manipulating the loci of reactants such as monomer/comonomer/initiator partition coefficients. Each of these topics is discussed in this chapter. 

In the case of moderate interactions of carbon dioxide with the polymer, substantial levels of sorption and swelling can occur. The swelling of the polymer significantly enhances diffusivities of solutes in the polymer matrix, allowing for rapid extraction, impregnation, shaping, and blending processes. Extraction applications include removal of residual monomers, solvents, and catalysts [84, 85]. An impor-... 

Fig. 21.8. Process flow sheet for the removal of residual monomer from latex-products using SCCO2 [84].

The swelling method can be applied to all four of the methods listed. However, the direct method can be used only for methods (3) and (4). When the swelling method is used, removal of residual monomer is easily accomplished. The lens can be rim polished while it is a hard solid. 

Removal of residual monomer and by-products from polymers. 

Poly(4-acetoxystyrene) with = 10000, PDI =1.12 was prepared by bulk RAFT polymerization of 4-acetoxystyrene at 90 °C using AIBN as initiator and a-acetic acid dithiobenzoate as chain transfer agent, and used as a macrotransfer agent in the block copolymerization of St with AIBN initiator after reprecipitation and the removal of residual monomer. The block copolymer, RB-2 was obtained [46]. The block copolymer can be hydrolyzed under mild basic conditions to give poly(4-hydroxy styrene)- )-PSt [46]. 

May be high due to incomplete removal of residual monomer in first H2O extraction. 

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