Monomer batch process, effect

The presence of a polyinitiator has a small effect only in batch mode, whereas the polymer obtained in a semi-batch process is more strongly branched (DB=2/3 as compared to DB=0.465 without initiator) [40]. A similar result was found by Hanselmann et al. for ABj monomers in the presence of a core-forming molecule [79]. 

Like the monomers, the co-monomers are diols or diacids, and according to their functional groups, their reactions with TPA and EG follow the principal mechanisms outlined above. Very few data have been published on reactions with co-monomers, and it may be assumed that the same mechanisms and catalysis concepts should hold. Nevertheless, it has been observed that co-monomers influence the overall reaction rates significantly. In a typical batch process, the polycondensation time needed to prepare a polymer with an IV of 0.64 dL/g increases by about one third with co-monomer IPA and by about two thirds with co-monomer CHDM, in comparison to homo-PET. This may in part be due to the differing correlations between Pn and IV, but additionally a reduced reactivity due to steric and electronic effects or the influence of co-monomers on the mobility of functional groups seems probable. 

Some reagents react with the initiating radical to give unreactive substances, a process known as inhibition. A common inhibitor for vinyl polymerisations is hydroquinone, which reacts by the transfer of two hydrogen radicals to the initiator radicals (Fig. 2.4). This gives quinone and unreactive initiator and has the net effect of causing a lag time in the polymerisation and a decrease in the initiator concentration. Monomers are often stored in the presence of inhibitor in order to prevent polymerisation. The amount and type of inhibitor may vary depending on the monomer batch and the manufacturer. For inter-laboratory comparisons of materials to be possible, it is therefore important to remove the inhibitor and purify the monomers prior to use [13]. 

Winyl polymerization as a rule is sensitive to a number of reaction variables, notably temperature, initiator concentration, monomer concentration, and concentration of additives or impurities of high activity in chain transfer or inhibition. In detailed studies of a vinyl polymerization reaction, especially in the case of development of a practical process suitable for production, it is often desirable to isolate the several variables involved and ascertain the effect of each. This is difficult with the conventional batch polymerization technique, because the temperature variations due to the highly exothermic nature of vinyl polymerization frequently overshadow the effect of other variables. In a continuous polymerization process, on the other hand, the reaction can be carried out under very closely controlled conditions. The effect of an individual variable can be established accurately. In addition, compared to a batch process, a continuous process normally gives a much greater throughput per unit volume of reactor capacity and usually requires less labor. 

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. 

Let us first ignore contributions from monomer partitioning and examine the effects of conversion upon copolymer composition. In most cases, thCTe will be a difference in monomer reactivities (Le. reactivity ratios 1) and a consequent drift in copolymer composition with conversion as the more reactive monomer is consumed preferentially (see Section 1.6.3). Since the total quantity of each monomer is added at the beginning of a batch emulsion polymerization, there is no control over this drift in copolymer composition. Hence, copolymors formed using batch processes can have quite broad composition distributions, the breadth of the distribution for each particular system depending upon the monomer reactivity ratios, the initial comonom composition and monomer partitioning (which is dealt with in Section 7.3.2.2). 

Another type of controlled radical polymerization employs a reversible termination with a nitroxide compound [21]. Rosenfeld et al. [22] reported details of the nitroxide-mediated radical polymerization of styrene and butyl acrylate at 140 °C in a 2.9 m tubular micro-reactor with an inner diameter of 900 gm. Whereas, for the low-heat-producing monomer, styrene, the differences between a batch process and the microtubular reaction were small, in the case of butyl acrylate the difference was high. This situation, which may have been due to the Trommsdorff effect in the batch reaction (Figure 14.11), indicated that the polymerization was no longer under control. By contrast, no such effect was observed in the tubular micro-reactor, and the degree of conversion remained quite low under the applied conditions. 

The batch-suspension process does not compensate for composition drift, whereas constant-composition processes have been designed for emulsion or suspension reactions. It is more difficult to design controUed-composition processes by suspension methods. In one approach (155), the less reactive component is removed continuously from the reaction to keep the unreacted monomer composition constant. This method has been used effectively in VT)C-VC copolymerization, where the slower reacting component is a volatile and can be released during the reaction to maintain constant pressure. In many other cases, no practical way is known for removing the slower reacting component. 

There are three important processes for preparing PVAc latexes in the presence of PVA as a protective colloid batch, semi-continuous, and delayed addition of monomer [10]. In this Chapter, the effects of the addition of VAc and initiators on the properties of PVAc latexes are discussed using the three methods under the same charge of ingredients for polymerization as shown in Fig. 1 [1,11]. 

Polymer production technology involves a diversity of products produced from even a single monomer. Polymerizations are carried out in a variety of reactor types batch, semi-batch and continuous flow stirred tank or tubular reactors. However, very few commercial or fundamental polymer or latex properties can be measured on-line. Therefore, if one aims to develop and apply control strategies to achieve desired polymer (or latex) property trajectories under such a variety of conditions, it is important to have a valid mechanistic model capable of predicting at least the major effects of the process variables. 

Three forms of the reactor operator, R(Y), are shown in Figure 3. These are generally differential operators which operate on each monomer and polymer species to describe the effects of accumulation and the physical processes which move material in and out of the reactor or reactor element. The concentration of a specific species is given by the variable Y. In a simple batch reactor, the reactor operator, RB, is merely defined as the rate of accumulation of a certain species with time per unit volume of reactor—i.e., the rate of change of concentration of the species. 

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