Monomer feed rate

Experimental Procedure. For the initial start-up of the continuous tirred tank reactor, the mixing speed and bath temperature were adjusted with the reactor full of solvent. The polymer seed and monomer feed rates were then adjusted simultaneously. 

TEMPERATURE FOR SEMI-BATCH AND FINISHING MONOMER FEED RATE DURING SEMI-BATCH STEP INITIATOR FEED RATE DURING SEMI-BATCH STEP TOTAL INITIAL LOADING TIME FOR SEMI-BATCH STEP MONOMER WT. % AT START OF FINISHING INITIATOR WT. PERCENT AT START OF FINISHING TIME FROM START OF... 

Figure 2. Rate representation for MMA semicontinuous emulsion polymerization ((-----) the monomer feed rate (right or-...

The lag between density cell response and reactor events were considerably less for this example and the figures ignore any correction. After establishing a "steady state" response to the monomer feed (about 160 minutes into the reaction), the incremental increase of the feed rate is seen not to alter the overall fractional conversion since the rate of polymerization increases to parallel the monomer feed rate. At the end of this set of data the rate is 2-3 times that observed earlier before the feed. 

The latexes were prepared using a conventional semi-batch emulsion polymerization system modified for power-feed by the addition of a second monomer tank. Polymerization temperatures ranged from 30-85°C using either redox or thermal initiators. Samples were taken periodically during the polymerization and analyzed to determine residual monomer in order to assure a "starved-feed" condition. As used in this study this is a condition in which monomer feed rate and polymerization rate are identical and residual monomer levels are less than 5%. 

Recently Uniqema has introduced commercially a Surfmer under the trade name of Maxemul 5011. Maxemul is produced by esterification of an unsaturated fatty anhydride with a methoxy PEG such that the reactive group is close to the hydrophilic moiety [ 34 ]. Stable latexes with a solid content of 52% were produced in the seeded emulsion polymerization of film-forming methyl methacrylate/butyl acrylate/acrylic acid (3% Surfmer on monomers, constant monomer feeding rate over 4 h, potassium persulfate/sodium metabisulfate redox initiator). The latexes were stable to electrolytes but not to freeze-thaw. 

Compositional control for other than azeotropic compositions can be achieved with both batch and semibatch emulsion processes. Continuous addition of the faster reacting monomer, styrene, can be practiced for batch systems, with the feed rate adjusted by computer through gas chromatographic monitoring during the course of the reaction (54). A calorimetric method to control the monomer feed rate has also been described (8). For semibatch processes, adding the monomers at a rate that is slower than copolymerization can achieve equilibrium. It has been found that constant composition in the emulsion can be achieved after ca 20% of the monomers have been charged (55). 

Figure 11.6c shows the monomer feed rate dependence of internal stress in VpMDSO plasma polymer films at different argon flow rates. The overall values of internal stress in plasma films obtained with argon flow rate at 1500 seem are much higher than those obtained at 750 seem. 

From Figure 11.6c it can also be noted that the internal stress in CAT polymers deceased with increasing VpMDSO monomer feed rate. In plasma deposition process, when the other plasma parameters are kept the same, the increase of monomer feed rate indicates that the same amount of energy input is consumed by a larger number of monomer molecules. In other words, when the other plasma parameters are kept constant, the increase of monomer feed rate will actually reduce the relative energy input in plasma polymerization process. 

Figure 11.7 Monomer feed rate dependence of internal stress in plasma polymers 750 seem Ar, 4.0 A arc current.

P = polymer product rate and monomer feed rate R — monomer recycle rate G = monomer-polymer recycle rate Ti side arm neat exchange temperature T2 = reactor temperature... 

Here, F is the monomer feed rate. At high feed rates, monomer droplets accumulate inside the reactor and the monomer concentration inside the particles reaches its saturation value. Therefore, Cp may be estimated as follows ... 

Since both P and are constant, (pi is also constant. Given that the total polymer radical concentration ([P ]) varies during the reaction, one can readily solve for the time-varying monomer feed rates, Pj and p2,m> using Equations 6.70-6.76. 

The practical implementation of the above policies is not necessarily as straightforward as solving the above equations. As can be deduced from Equations 6.70-6.76, Pjjjj is a function of the propagation rate coefficients, the monomer concentrations, and most importantly, the total radical concentration. Hence, to precalculate the optimal monomer feed rates, the radical concentration must be specified in advance and kept constant via an initiator feed policy and/or a heat production policy. This is especially important considering that a constant radical concentration is not a typical polymer production reality. This raises the notion that one could increase the reactor temperature or the initiator concentration over time to manipulate the radical concentration rather than manipulate the monomer feed flowrates, that is, keep P j constant for simpler pump operation. Furthermore, these semibatch policies provide the open-loop or off-line optimal feed rates required to produce a constant composition product. The online or closed-loop implementation of these policies necessitates a consideration of online sensors for monomer... 

Care also has to be taken not to overcool the reaction. If so, a slowdown in the polymerization rate may occur, with excess free monomer, leading to an exotherm followed by foaming or an overloading of the condenser. A reduction in the monomer feed rate at this time is essential, but again care has to be exercised, as a sudden loss of cooling from the incoming monomer stream coupled with a drop-off in the reflux rate can give an uncontrollable exotherm. 

Other strategies for controlling copolymer composition Although the use of monomer-starved conditions for control of copolymer composition is widespread, the low monomer feed rates which need to be used give rise to low rates of copolymerization and have significant effects upon the molar mass and molar mass distribution of the copolymers formed (see Section 7.4.4.4). Hence, alternative procedures have been developed which facilitate higher feed rates, but nevertheless allow for control of copolymer composition. These procedures are briefly described in this section. 

The simplest strategy involves feeding the individual monomers separately while monitoring the concentration of each monomer in the reactor on-line (e.g. by gas or liquid chromatography). In response to detected changes in the concentrations of unreacted monomers, the individual monomer feed rates are adjusted in such a way as to maintain the comonomer composition in the reactor constant. However, since the time required for analysis is significant, feedback cannot... 

Commercial production of PVA from PVAc is carried out via a continuous process. PVAc is polymerized with a free radical initiator in methanol, usually between 55 and 85°C. Molecular weight is controlled by the residence time in the reactors, monomer feed rate, solvent concentration, initiator concentration, and polymerization temperature. Direct hydrolysis or catalyzed alcoholysis converts the PVAc into the corresponding PVA, a water-soluble polymer (1) The degree of hydrolysis can be controlled to yield various grades of PVA super-hydrolyzed (>99 mol%), fully hydrolyzed (98 mol%), and partially hydrolyzed (88 mol%). There are also specialty grades less than 80 mol%. The annual worldwide capacity of PVA is about 750 million pounds. 

One should note that this ratio should be controlled at the desired level regardless of the magnitude of the total radical concentration, and hence impurities which affect the radical concentration will have no effect on copolymer composition control. The on-line measurement of heat generation rate can thus be used to set the appropriate monomer feed rate. If polymerization is too slow because of radical scavengers, one can increase the radical generation rate to compensate, and, in parallel, increase the monomer inflow rate to maintain the ratio constant (or, vice versa to compensate for a possible auto-acceleration operating point). Additional discussion is provided in Dub6 etal. [56]. 

Reduction of residual monomer (increase of monomer conversion) and increase of polymer productivity can be obtained through manipulation of reaction times [29], of reactor temperatures (which may be allowed to increase at the end of the polymerization to promote the conversion of the residual monomer) [30], of monomer feed rates [31] and of mixtures of initiators (or of bifunctional initiators) and catalysts with different decay characteristics [32]. 

Multi-objective optimization procedures were used for the simultaneous maximization of monomer conversion and minimization of side products during low-density polyethylene polymerizations performed in tubular reactors under steady-state conditions [170]. Genetic algorithms were used to compute the Pareto sets. Multi-objective optimization procedures were also used for the simultaneous maximization of molecular weight averages and minimization of batch times in epoxy semibatch polymerizations [171]. In this case, monomer feed rates were used as the manipulated variable. 

An optimal predictive controller was developed and implemented to allow for maximization of monomer conversion and minimization of batch times in a styrene emulsion polymerization reactor, using calorimetric measiuements for observation and manipulation of monomer feed rates for attainment of control objectives [31]. Increase of 13% in monomer conversion and reduction of 28% in batch time were reported. On-line reoptimization of the reference temperature trajectories was performed to allow for removal of heater disturbances in batch bulk MMA polymerizations [64]. Temperature trajectories were manipulated to minimize the batch time, while keeping the final conversion and molecular weight averages at desired levels. A reoptimization procediue was implemented to remove disturbances caused by the presence of unknown amounts of inhibitors in the feed charge [196]. In this case, temperatiue trajectories were manipulated to allow for attainment of specified monomer conversion and molecular weight averages in minimum time. 

The polymers described in Sect. 2.3 can be considered to be copolymers, and in many cases they are actually called copolymers. However, those polymers have been synthesized from monomers with polymerizable groups (e.g., thiophene), and the monomer already contains the redox functionality. The copolymers that will now be discussed have been prepared from two or more difierent monomers, which can also be electropolymerized separately, and the usual strategy is to mix the monomers and execute the electropolymerization of this mixed system. It should be mentioned that the structures of the copolymers have not been clarified unambiguously in many cases. Usually the cyclic voltammetric responses detected show the characteristics of both polymers, and so it is difficult to establish whether the surface layer consists of a copolymer or whether it is a composite material of the two polymers. However, several copolymers exhibit electrochemical behaviors that differ from the polymers prepared from the respective monomers. The properties of the copolymer depends on the molar ratio of the monomers (feed rate), and can be altered by other experimental conditions such as scan rate, pH, etc., since generally the electrooxidation of one of the comonomers is much faster than that of the other one (a typical example is the comonomer aniline, whose rate of electropolymerization is high even at relatively low positive potentials). In many cases the new materials have new and advantageous properties, and it is the aim of these studies to discover and explore these properties. We present a few examples below. 

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