Kinetics of high conversion polymerization of vinyl acetate. Effects of mixing and reactor type on polymer properties

Abstract

SynopsisIn the present article the kinetics of polymerization of vinyl acetate in suspension up to high conversion was studied. The molecular weight distribution and the side chain branching of polyvinyl acetate produced were examined with respect to micro and macro mixing as well as to reactor type. The following results were achieved: the time-activity curves of the polymerization can be described up to high conversions considering the exponential increase in viscosity of the polymerizing system and combining the viscosity with rate constants of the polymerization. The change of volume of the polymerizing system has no significant influence on kinetics. The narrowest molecular weight distribution of the poly(viny1 acetate) produced was achieved when polymerizing in the homogeneous continuous stirred tank reactor while the broadest molecular weight distribution was observed in the segregated continuous stirred tank reactor. The batch reactor and the flow tube reactor produce polymers with molecular weight distributions lying in between. Considering the side chain branching, another order was found. The batch reactor and the tube reactor show the lowest side chain branching, the homogeneous continuous stirred tank reactor shows a larger one and the segregated continuous stirred tank reactor shows the largest. Possible reasons for the different behavior of the different reactors are discussed. The degree of segregation was determined by experiments. EXPERIMENTALVinyl acetate (Hoechst AG, polymerization grade) used for the polymerization was rectified in a large column and stored at low temperature. Dicyclohexyl peroxidicarbonate (P1652, Peroxid Chemie) was used as initiator and 2,2-diphenyl-l-pikryl-hydrazyl (DPPH) (Fa. Fluka) was used for stopping the reaction after sampling. The dispersion agent utilized was partly saponified poly(viny1 acetate) (Mowioll8-88, Hoechst AG). Tertiary butanol (Bayer AG) was used as solvent for the polymerization in solution because this solvent has the smallest transfer constant of a poly(viny1 acetate) radical to a solvent molecule.Unless mentioned otherwise, the volume fraction of vinyl acetate to water was 0.12 to 0.25, and the initiator concentration was 0.5 wt % related to the monomer. For the polymerization in solution, 15 wt % monomer was solved in tertiary butanol and the same monomer to initiator ratio was chosen.For polymerization, a 1.5 1 thermostatic stirred tank reactor (ratio height/ diameter = 2.5) with a blade stirrer (stirrer diameter to reactor diameter 1:3, centrosymmetrically arranged one-third away from the bottom of the vessel) of 600 rpm for the polymerization in suspension was used. During the continuous process, feeding was put in through an injector at the bottom of the vessel. Outlet and sampling took place in the upper part of the reactor. All experiments were

Uma revisão sobre os processos de polimerização em suspensão

“…As mudanças nas constantes de terminação (k T ) e propagação (k P ), devidas às limitações impostas pelo aumento da viscosidade do meio reacional, geralmente são expressas como funções da temperatura e da composição do meio, embora possam também ser representadas como funções da viscosidade do meio [59,60] ou por outras formas empíricas [17,61,62] . Correlações baseadas na teoria do volume livre são também bastante utilizadas, pois o volume livre pode caracterizar extremamente bem as resistências à difusão em meios complexos [54,63] .…”

Transient Behavior and Gelation of Free Radical Polymerizations in Continuous Stirred Tank Reactors

“…In other works, the polymerization of vinyl acetate was done in different experimental conditions [17] (higher polymerization rates) but on comparing the experimental results obtained with an improved version of previous developed models, no satisfactory agreement was achieved. [18] A significant part of the above mentioned discrepancies between experimental observations and model predictions can be explained by the approximations introduced in the calculations.…”

A New Look at Kinetic Modeling of Nonlinear Free Radical Polymerizations with Terminal Branching and Chain Transfer to Polymer

“…22,23 The results obtained in those simulations demonstrate a fairly good agreement with experimental results of several other previous works. 9,[24][25][26][27] In those cases, we have considered the situation where transfer to polymer can occur in two different sites in the molecules. Then the formation of saponified and nonsaponified branches can also be simulated.…”

“…In more recent works, the need for approximation C was avoided, but the other two simplifications are used. 8,9 Many key features of kinetic modeling of nonlinear free radical polymerizations have been acknowledged by Kuchanov and Pis'men, 10 namely the need of avoiding quasi-steady-state hypothesis for radical concentrations and the usefulness of consideration of multiple radical centers. Nevertheless, in current methods used to describe this kind of systems some of these approximations have been kept: published works using the method of moments, 11 the numerical fractionation technique, 12 and Galerkin finite-elements method 13 have made use of approximations A and B above.…”