Kinetics vinyl acetate monomer process

The industrial process for the vapor-phase manufacture of vinyl acetate monomer is quite common (Daniels, 1989) and utilizes widely available raw materials. Vinyl acetate is used chiefly as a monomer to make polyvinyl acetate and other copolymers. Hoechst-Celanese, Union Carbide, and Quantum Chemical are reported U.S. manufacturers. DuPont also currently operates a vinyl acetate process at its plant in LaPorte, Texas. To protect any proprietary DuPont information, all of the physical property and kinetic data, process flowsheet information, and modeling formulation in the published paper come from sources... 

The process considered in this chapter involved the production of vinyl acetate monomer. It features many unit operations, many components, nonideal phase equilibrium, unusual reaction kinetics, two recycle streams, and three fresh reactant makeup streams. 

The other benefit is that PVC /vinyl acrylate copolymers tend to have lower volatile organic contents (VOCs) than PVC/vinyl acetate copolymers. This is because the reaction kinetics favor consumption of the vinyl acrylate monomer during polymerization, yielding very low levels of residual vinyl acrylate monomer. Most PVC/vinyl acetate copolymers have residual vinyl acetate monomer, which can volatilize during processing or during the use life of the aiticle. The lower VOC content typical of PVC/vinyl acrylate copolymer resins versus their PVC/vinyl acetate counterparts make them an option in automotive applications with stringent fog requirements. 

Most addition polymers are formed from polymerizations exhibiting chain-growth kinetics. Such processes include the typical polymerizations of the vast majority of vinyl monomers such as ethylene, styrene, vinyl chloride, propylene, methyl acrylate, and vinyl acetate. Furthermore, most condensation polymers are formed from systems exhibiting stepwise kinetics. Industrially, such systems include those used for the formation. pa of polyesters and polyamides. Thus, a large overlap exists between the terms... 

In a complex apparatus, Gimesch and Schneider [30, 119] studied the suspension polymerization of vinyl acetate. Their procedure involved equipment which automatically added tempered water to the reacting system as heat was evolved as a result of the polymerization process. Thus they maintained isothermal reaction conditions. The rate of reaction could be followed by recording the water uptake of the equipment with time. The heat of polymerization was also determined (found to be 23 kcal/mole which was considered a satisfactory check of the literature value which is scattered around 21.4 kcal/mole). From this work, a somewhat different mechanism of the suspension polymerization process emerges than the widely accepted concept of the water-cooled bulk polymerization of small particles. It was noted that with an increase in the initiator concentration, there was the expected increase in polymerization rate. With increasing stirring rate, the rate of polymerization decreased. Along with the suspension polymerization, there was always a certain amoimt of imdesirable emulsion polymerization. It was postulated that in the process, free radicals, formed in a monomer drop may be extracted into the aqueous phase where they may act on dissolved vinyl acetate by kinetic processes unique to this system and different from the conventional mechanism of suspension polymerization. 

The emulsion copolymerization of vinyl acetate and butyl acrylate has received considerable attention. The butyl acrylate confers improved film forming characteristics to the polymer. The disparities in their water solubilities and of their individual polymerization rates may help to explain the variations in reactivity ratios that have been reported [170,171]. The variation in reactivity ratios may also by related to the following observations The reaction method has an effect on the morphology of the polymer particles. In a batch emulsion process, a butyl acrylate—rich core is formed which is surrounded by a vinyl acetate-rich shell, in a process in which the monomers are fed into the reactor in a semicontinuous manner, particles form with a more uniform distribution of the monomers [172]. The kinetics for a batch process indicates that the initially formed polymer is indeed high in butyl acrylate. As this monomer is used up, eventually a copolymer high in vinyl acetate develops. It is this latter polymer which forms the final shell around the particles. 

Desorption results in a decrease in the concentration of growing radicals inside the particles, and causes the rate of polymerization to decrease. It is strongly connected to the probability of chain transfer to monomer as smaller radicals cross the interface faster than larger radicals. For monomers with higher chain-transfer rates to monomer (such as ethylene, vinyl acetate, or vinyl chloride), radical exit represents the major process of reducing n to values much less than 0.5. However, for styrene also, the correct kinetic description requires the consideration of radical desorption. Exit is not only literally, but also mechanistically, the opposite of entry the radical must reach the particle surface and must then overcome the barrier for desorption exerted by the interface. 

Solution polymerization n. A polymerization process in which the monomer, or mixture of monomers, and the polymerization initiators are dissolved in a non-monomeric solvent at the beginning of the polymerization reaction. The liquid is usually also a solvent for the resulting polymer or co-polymer. Solution polymerization is most advantageous when the resulting polymeric solutions are to be used for coatings, lacquers, or adhesives. Vinyl acetate, olefins, styrene, and methyl methacrylate are the monomers most often employed. Odian GC (2004) Principles of polymerization. John Wiley and Sons Inc., New York. Elias HG (2003) An introduction to plastics. John Wiley and Sons, New York. Solomon DH (1969) Kinetics and... 

The continuous bulk polymerization of methyl methacrylate was used as an example in Section 5.2. A stirred bulk polymerization like that used for styrene (Section 5.4) could be adapted for methyl methacrylate. A suspension process for poly(methyl methacrylate) was described in Section 5.4. The polymerization of ethyl acrylate most often is carried out in emulsion. A process such as that used for vinyl acetate is suitable (Section 16.4). Like vinyl acetate, the monomer is slightly water soluble, so true emulsion polymerization kinetics are not followed. That is, there is initiation of monomer dissolved in water in addition to that dissolved in growing polymer particles. Ethyl acrylate is distinguished by its rapid rate of propagation. Initiation of a 20% monomer emulsion at room temperature by the redox couple persulfate-metabisulflte can result in over 95% conversion in less than a minute. As with vinyl acetate polymerization, a continuous addition of monomer at a rate commensurate with the heat transfer capacity of the reactor is necessary in order to control the temperature. 

Glusker (37, 38) attempted to prove that these processes are absent by an estimation of active chains by reaction with C14 labelled C02 or H8(T) labelled acetic acid, followed by measurements of the radioactivity of the polymer isolated. Most of the experiments were carried out with fluorenyllithium as initiator in toluene containing 10% diethyl-ether at —60°. At —78° at least 80% of the polymer chains were found to be active at the end of polymerization. The lowest fraction was appreciably less active. Similar results were obtained at —60° although no examination was made of the fractions of lowest molecular weight. Kinetic experiments indicated a first order decay of monomer concentration after an initial rapid consumption of about 3 molecules of monomer per initiator molecule. The mechanism suggested to explain these results involves rapid addition of fluorenyllithium across the vinyl double bond followed by the rapid addition of three monomer units. At this stage it is... 

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