Continuous emulsion polymerization of vinyl chloride

For batch emulsion polymerizations of vinyl chloride, the reaction rate can be well represented by the empirical expression dx/dt = k (1 + ax), where x is the polymer concentration, and k and a are constants dependent upon initiator concentration and particle number. A simple mathematical model for continuous stirred-reactor polymerizations has been derived from this expression. The model predicts that at short residence times, a steady state of low PVC concentration is reached quickly; with longer residence time, the system more slowly approaches a higher steady-state polymer concentration. Above a critical residence time, equal to 1/ak, the rate of monomer supply becomes the limiting factor. Experimental data on continuous polymerizations are in good qualitative accord with the predictions of the model using constants fitted to batch-charge data. Quantitative agreement is best when the particle number is large; discrepancies may be attributed to a broader particle size distribution in continuous runs. Particle size measurements show evidence of intermittent particle formation in unseeded continuous runs; continuous seeding narrows the particle size distribution, but it is still broader than in batch charges. The model seems sufficiently valid to yield a number of useful implications about practical aspects of continuous emulsion PVC polymerization.     

Kinetics of vinyl chloride and vinyl acetate emulsion polymerization

The kinetics of vinyl chloride and vinyl acetate emulsion polymerization are reexamined. The validity of Ugelstad's model for systems with high desorption rate is confirmed by simulating conversion histories for both systems at different initiator concentrations and particle numbers. On the basis of the model, it is shown that at ordinary initiation rates, termination reactions are unimportant with respect to molecular weight development in both systems, and as a consequence, molecular weight development is independent of number and size distribution of polymer particles and of initiator and emulsifier level. Based on this conclusion, it is shown that in accordance with experimental facts, the molecular weight distribution obtained in vinyl chloride emulsion polymerization is the most probable distribution, and it is concluded that the number of long-chain branch points per repetition unit is less than 2 × 10^?4 at high conversions. In vinyl acetate emulsion polymerization, an almost logarithmic normal distribution is obtained. The distribution is strongly broadened by branching reactions with the number of long-chain branch points increasing rapidly with monomer conversion. The increase of M_n with increasing conversion is due to terminal double-bond polymerization, while the increase in M_w is due mainly to transfer to polymer.     

Dynamic behavior of continuous vinyl chloride suspension polymerization reactors: Effects of segregation

Assumed is that there is no mass transfer among the droplets of reacting organic phase in a continuous vinyl chloride (VCM) suspension polymerization reactor. A mathematical model is thus developed to allow the calculation of steady states and the analysis of their dynamic stability. The results obtained are compared with those obtained with the hypothesis of perfect micromixing. It Is shown that both assumptions lead to similar dynamic structures, which confirms the complex behavior of these reactors. There are major differences, however, that can be exploited in future experimental work.     

A calorimetric investigation of suspension and emulsion polymerization of vinyl chloride

An isothermal calorimeter for studying radical polymerization in heterogeneous systems is presented. The calorimeter was used for investigation of suspension and emulsion polymerization of vinyl chloride at 50°C. The results from the suspension polymerization experiments showed that the termination rate constant in the PVC phase was diffusion controlled and very sensitive to the vinyl chloride activity. During suspension polymerization the pressure started to drop at a much lower conversion than in emulsion polymerization. Mass balance calculations showed that this was due to entrapment of a large amount of liquid vinyl chloride in the fine capillary system of the suspension particles. The results from emulsion polymerization experiments in which the particle number spanned the range from 0.094.10¹⁸ to 1.7.10¹⁸ dm^?3, were analyzed according to the kinetic model developed by Ugelstad et al. Using the concentration-dependent termination rate constant obtained from the suspension experiments and assuming proportionality between the radical adsorption and desorption rate constants, this model gave a very good fit of the experimental rate over the entire interval II and most of interval III. The results strongly indicated that termination in the aqueous phase was of measurable importance during interval II and became the dominating termination in interval III.     

A population balance approach to modelling of continuous emulsion polymerization

A population balance approach is used to mathematically model continuous emulsion polymerization. The Smith-Ewart recursion relation is incorporated directly as the “rate of formation” relation for polymer, particles. As a result of this approach, the mechanism of free radical desorption from polymer particles is easily included, and finite termination rates in particles are allowed. Particle growth is incorporated in the model, and particle size distributions are easily obtained. A brief parameter study is conducted. Model predictions are compared to data from the literature. Practical consequences of changes in system parameters are discussed.     

Molecular weight distribution in continuous stirred tank vinyl acetate emulsion polymerization

Emulsion polymerization typically produces polymers of high molecular weight with broad distributions of molecular size. Many models have been proposed that use either a Flory type distribution, resulting in a polydispersity of two, or a distribution function to represent the breadth of the distribution. In this work, a model for the distribution of molecular weights is derived as a approximation from a detailed molecular weight model that was previously developed. The model is found to approximate the distribution of molecular weights well for several emulsifier concentrations is continuous stirred tank emulsion polymerization of vinyl acetate. An analysis is also conducted of the transformation of gel permeation chromatography data to weight and number fractional distributions of the molecular sizes.     

Dynamic behavior of continuous vinyl chloride bulk and suspension polymerization reactors. A simple model analysis

Based on previous works, a simple model is built to simulate the vinyl chloride monomer (VCM) polymerization in continuous suspension and bulk reactors. The branches of steady-state solutions are computed and their dynamic behavior is analyzed. It is observed that these systems present multiple steady-state solutions for some ranges of parameter values and that non-adiabatic reactors can also present ISOLAs. These reactors can also experience self-sustained oscillations in a very wide region of operation conditions.     

Bulk polymerization of vinyl chloride

The bulk polymerization of vinyl chloride initiated by AIBN at temperature levels of 30°, 50°, and 70°C has been studied. Molecular weight averages and distribution (MWD) were measured by gel permeation chromatography. A model has been proposed which accurately predicts conversion to high levels and MWD. Molecular weight measurements show that transfer to monomer plays the important role in controlling molecular weight averages. Disproportionation is probably the dominant mode of termination.     

Kinetics of the emulsion polymerization of vinyl acetate

Recent investigations of the emulsion polymerization of vinyl acetate report a variable behavior. The reaction has been reported to exhibit a half-order rate dependence and no particle number dependence on the initiator level, or a first-order dependence for both the reaction rate and particle number on the initiator concentration, or some behavior between these two extremes. While two recent models have attempted to account for the changes in the reaction rate dependence by postulating either an aqueous phase or a polymer phase termination, no adequate explanation of the particle number behavior has been given, nor has a single model been suggested that agrees with all of the experimental data. This investigation confirms the results of previous investigators and develops an empirical model for the particle number, taking into account the effects of the initiator level, emulsifier level and alkyl chain length, temperature, and ionic strength on the particle number. This particle number model, used with a polymer phase termination reaction model, yields a polymerization rate expression which agrees with all previous data.     

Seeded emulsion polymerization of vinyl chloride. A new approach to mechanism and kinetics

Some peculiarities concerning the mechanism and the kinetic behaviour of the vinyl chloride emulsion polymerization are described. Seeded emulsion polymerizations in the presence of an adequate mixture of anionic and non-ionic surfactants were carried out in order to avoid the micellar initiation. New kinetic parameters were determined from the experimental data as follows: (i) MSA, the minimum surface area of polymer seed particles necessary to capture all ion-oligoradicals generated in aqueous phase at a given initiator concentration; (ii) MCCI, the maximum critical concentration of initiator per unit surface of polymer particle under which the formation of new polymer particles is avoided; (iii) PVR₁, the polymer volume per active growing radical, necessary to be within the particle for 1 s. It was found that the average number of propagating radicals per particle, n, depends on the size of the polymer particle, as a resultant of entry and exit and then of the initiation and termination reactions. The practical consequences of the accurate control of the polymer particles growing during seeded emulsion polymerization of vinyl chloride were emphasized.     

乙烯基化合物乳液聚合的研究

研究了多种因素对乙烯基化合物乳液聚合的影响,发现十二烷基苯磺酸钠的乳液化学稳定性远不如十二烷基硫酸钠。分析证明,聚甲基丙烯酸甲酯胶乳粒子由于具有极性,粒子粒径及分布除受乳化剂浓度影响外,主要受粒子合并控制,受温度的影响则不明显。用计算机精确控制的量热反应器检测所得的动力学数据表明,易水溶性（极性）单体均相（水相）成核显著,难水溶性单体则甚微。     

Reaction kinetics of vinyl acetate emulsion polymerization

Transport of free radicals out of latex particles into the aqueous phase plays an important role in the emulsion polymerization of vinyl acetate. This so-called “desorption” process involves chain transfer to monomer which generates a mobile and rather stable monomer-unit free radical followed by the diffusion of this free radical out of the latex particle. A kinetic model is developed based on the reaction mechanisms of such an emulsion system. The experimental data available in the literature are used to test the model under various polymerization conditions. Reasonable agreement between the model predictions and experimental data is observed.     

Vapor phase polymerization of vinyl chloride

A study of the polymerization of vinyl chloride was made at conditions under which the monomer exists in the gas phase. In the presence of a bed of poly(vinyl chloride) particles, free-flowing product was obtained. By using free-radical initiators, such as dialkylperoxy dicarbonates or acetylcyclohexane sulfonyl peroxide, high initiator productivities were obtained in reaction periods of 1.5 to 3 hr. Although some modifiers, such as ethyl bromomalonate and bromoform, were demonstrated to lower molecular weight, satisfactory molecular weight modification was accomplished by adjusting polymerization temperature. Several peroxy compounds of different reactivity were studied. Hydrocarbon additives improve the polymerization rate. It is believed that the hydrocarbon and initiator are absorbed on the PVC particle surface and allow the monomer concentration to be increased at the polymerization locus. PVC particles of small size may be prepared in a liquid bulk polymerization for use as seed polymer for the vapor phase process. This provides a seed of high porosity which enables one to prepare a vapor phase product with suitable plasticizer absorption. The physical properties of products prepared by the vapor phase process were evaluated in a rigid formulation.     

On the modelling of continuous emulsion polymerization using a population balance with instantaneous radical termination

An analytical solution of a population balance is used to mathematically describe continuous emulsion polymerization. Steady state performance is examined. The assumption of instantaneous free radical termination within particles is made. The desorption mechanism is included. Particle size distribution information is obtained, and the effect of the desorption mechanism is noted. A desorption rate constant is calculated when the model is fit to data found in the literature.     

Kinetics of continuous addition emulsion polymerization

An analysis of the kinetics of continuous addition emulsion polymerization is presented. For systems that follow the Smith-Ewart mechanism, this analysis predicts that the reaction will approach a steady-state condition if the rate of monomer addition R_a is constant. In the steady state, the rate of polymerization depends on R_a according to a reciprocal relationship, 1/R_p = (1/K) + (1/R_a), where K depends on the number of particles and the propagation rate constant. Above a critical value of R_a, the particles will become saturated with monomer and R_p will be constant and independent of R_a.     

氯乙烯聚合研究进展

对近期氯乙烯聚合研究进展情况进行了综述,在聚合化学方面的研究包括复合引发剂引发氯乙烯聚合动力学、氯乙烯活性自由基聚合、茂金属催化聚合和阴离子聚合,在成粒机理方面的研究包括氯乙烯悬浮聚合和新型非均相聚合机理。     

State estimation for continuous emulsion polymerization

Various aspects of the use of extended Kalman Filters for tracking the states of continuous emulsion polymerization reactors are investigated. The importance of introducing meaningful nonstationary stochastic states to account for unknown impurities, initiator efficiencies, modelling errors, etc. is illustrated. The robustness of these state estimators to unmodelled and unmeasured disturbances, to modelling errors, and to input errors is evaluated. A procedure for selecting an optimal set of on-line sensors is presented. The emulsion polymerization of styrene-butadiene rubber (SBR) is used as the example system.