Feeding starved conditions

The power exponent, x determines what the monomer feed profile will look like as a function of of. Some examples of more common feed profiles are shown in Figure 1. If the polymerization is carried out under monomer-starved conditions, the composition of the polymer being formed at any instant is the same as the feed composition, C-. Therefore, the cumulative polymer composition at any timeef may be obtained from ... 

The process usually starts with the polymerization of a small proportion of the reagents at a very low monomer to water ratio (the seed stage), followed by the feeding of the remaining monomer (which may take several hours) and of other materials (if needed) once the conversion in the reactor has reached 70% or more. The in-reactor conversion will then depend upon the rate of polymerization compared to the rate of feed. If the reaction is continued under the so-called monomer-starved conditions, the in-reactor conversion is kept at a high 80-90%, which reduces the polymerization rate. To compensate, temperature is raised however, then the initiator depletes faster and more has to be added during the reaction. 

Pietsch noted that the peripheral roll speed and particulate powder speed are not equivalent in the entire compaction zone. Throughput does not increase proportionally with roll speed. There are two effects that hinder throughput starved conditions in the feed zone, and secondly, too much squeezed air from the particle mass flows upward and against the powder flow, reducing fhe supply of maferial fo fhe nip... 

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 previous discussion leads to the definition of the fourth classical control problem, which is the control of the copolymer composition along the reaction batch (or at the end of the batch). This objective is normally attained through manipulation of monomer feed flow rates [44, 45]. The feed stream usually contains the most reactive monomer species, so that composition control is obtained by keeping the concentration of the most reactive monomer concentration at the desired low levels throughout the batch time. It is important to emphasize that implementation of monomer feed strategies may lead to runaway conditions in the presence of heat transfer limitation [ 46 ], which partially explains why control of copolymer composition in emulsion reactors is normally attained by working under starved conditions. 

The change in the monomer feed rates during reactions allowed simultaneous influence and verification of the evolution of these parameters during the synthesis. The adjustable feed rate confers great power for controlUng during the reaction. Thus, the hybrid reaction shown in Figure 12.10 [50] starts in semicontinuous mode, under starved conditions, achieved by slow monomer feed in the reactor, and becomes essentially a batch reaction upon the one-shot addition of MM A at 15% conversion. 

Chern [42] developed a mechanistic model based on diffusion-controlled reaction mechanisms to predict the kinetics of the semibatch emulsion polymerization of styrene. Reasonable agreement between the model predictions and experimental data available in the literature was achieved. Computer simulation results showed that the polymerization system approaches Smith-Ewart Case 2 kinetics (n = 0.5) when the concentration of monomer in the latex particles is close to the saturation value. By contrast, the polymerization system under the monomer-starved condition is characterized by the diffusion-con-trolled reaction mechanisms (n > 0.5). The author also developed a model to predict the effect of desorption of free radicals out of the latex particles on the kinetics of the semibatch emulsion polymerization of methyl acrylate [43]. The validity of the kinetic model was confirmed by the experimental data for a wide range of monomer feed rates. The desorption rate constant for methyl acrylate at 50°C was determined to be 4 x 10 cm s ... 

Suitable conditions for AM-type polymerization may be created, however, by applying the overall ratio of [M]/[I] high enough to allow the formation of polymer chain with desired length, but at the same time keeping instantaneous concentration of monomer low to ensure low instantaneous ratio of [M]/[I]. Thus, polymerization should be conducted by slow feeding of monomer to reaction mixture, that is, at monomer-starved conditions. 

There have been few basic studies of flow in Buss Kokneters. Only in the 1990s do we have pubUcations of Elemans and Meijer [89] and Lyu and White [90 to 95] with which to seek to understand and model the flow mechanisms and the flow fields in the various machine elements. Certain things are clear, such as the fluid mechanics indicates the axially oscillating screw inducing an oscillatory output. The machine operates under starved conditions with alternating fully filled and starved sections. The secondary extruder into which compounds exit from the Kokneter is starved and the length of fill oscillates but the output is uniform. This would feed a palletizing die. 

The power feed addition strategy is also conducted under monomer starved conditions but aiming at producing a copolymer with varying composition, see Basset and Hoy (1981). In this case a heterogeneous copolymer composition distribution, that is, a copolymer product with a predefined composition distribution, is sought. This is obviously achieved by varying the ratio of the monomer flow rates into the reactor continuously with time. 

As a first approach a cylindrical barrel rather than the complicated barrel of Transfermix may be chosen. With a single screw extruder a filled channel is required in order to create pressure necessary for feeding material to a mixing zone. This leads to excessive temperatnre rise. With a twin-screw a starved condition may be used to move the material forward with a minimum of temperature rise. A successful example of the continuous mixing of EPDM with a Werner Pfleiderer twin screw extruder was described by Tyler [6]. 

Primary radical termination is also of demonstrable significance when very high rates of initiation or very low monomer concentrations are employed. It should be noted that these conditions pertain in all polymerizations at high conversion and in starved feed processes. Some syntheses of telechelics are based on this process (Section 7.5.1). Reversible primary radical termination by combination with a persistent radical is the desired pathway in many forms of living radical polymerization (Section 9.3). 

Table 9.9 Block Copolymers Prepared by Macromonomer RAFT Polymerization under Starved-Feed Conditions.380"595...

Transfer constants of the macromonomers arc typically low (-0.5, Section 6.2.3.4) and it is necessary to use starved feed conditions to achieve low dispersities and to make block copolymers. Best results have been achieved using emulsion polymerization380 395 where rates of termination are lowered by compartmentalization effects. A one-pot process where macromonomers were made by catalytic chain transfer was developed.380" 95 Molecular weights up to 28000 that increase linearly with conversion as predicted by eq. 16, dispersities that decrease with conversion down to MJM< 1.3 and block purities >90% can be achieved.311 1 395 Surfactant-frcc emulsion polymerizations were made possible by use of a MAA macromonomer as the initial RAFT agent to create self-stabilizing lattices . 

In the case of network formation controlled by (irreversible) kinetics programmed polymerization regime (starved feed conditions, etc.). 

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