Diluent processes

Heat removal and viscosity increases during polymerization are facilitated by using a diluent solvent. However recovery and repurification of the solvent, together with flammability hazards, have limited this technique. Heat removal can be conveniently carried out using the latent heat of vaporization, e.g. the cationic polymerization of isobutene to make polyisobutylene (butyl rubber) is maintained at — 100°C by the refluxing ethylene solvent. 

Water as diluent has obvious advantages and has been developed for many free-radical polymerizations. Thus suspension polymerization involves the dispersion of the non-miscible monomer in water as droplets (0.1-5 mm diameter) by means of agitation and protective colloids or dispersing agents (e.g. polyvinyl alcohol, PVAL), and adding a monomer-soluble initiator. The polymer ends up approximately the same size as the original droplets and the system can be viewed as many small bulk polymerizations. As water is the continuous phase the viscosity remains constant and good heat transfer occurs. This process is used for PVC. 

Emulsion polymerization is similar to suspension polymerization in that water is the continuous phase, but the main difference is that a water-soluble initiator is used. The water-insoluble monomer is dispersed in the water using emulsifying agents, such that the latex is made up of droplets (1.5 pm diameter) and micelles (0.01 pm). Because of the enormous surface area of the micelles, initiation and polymerization takes place at this interface and the monomer is replenished by diffusion from the larger droplets to the micelle/growing polymer particle. A latex is obtained which is often used directly as such for paints, adhesives, etc. 

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In order to faciUtate heat transfer of the exothermic polymerization reaction, and to control polymerizate viscosity, percent reactives are adjusted through the use of inert aromatic or aUphatic diluents, such as toluene or heptane, or higher boiling mixed aromatic or mixed aUphatic diluents. Process feed streams are typically adjusted to 30—50% polymerizable monomers. 

The unusual sensitivity of some composite-modified double-phase propellants before curing has justified intensive effort to exploit a nonmechanical mixing process. First introduced in about 1959 as the quick-mix process by Rocketdyne Division of North American Aviation (5, 10), the inert diluent process has been developed at the Naval Ordnance Station, Indian Head, Md. for application to a variety of propellant compositions. Separate streams of solids, slurried in heptane, and an emulsion of plasticizers in heptane, are combined in a non-mechanical mixing chamber. The complete propellant slurry is allowed to settle, and the heptane is separated and recycled in a continuous operation. Figure 1... 

Figure I. Inert diluent process flow diagram Adhesives for Case Bonding...

The slurry bulk process is slightly more expensive than the slurry diluent process, the claimed advantage being low catalyst residues in the polypropylene produced [43,51]. The bulk process may be simplified by not removing the atactic polymer fraction (Figure 3.56) [51]. 

Since each bead of a given external diameter that is made by the inert diluent process will contain some void volume, there is actually less polymer available per unit volume for the introduction of functional groups. Therefore, these macroporous resins are inherently of lower total exchange capacity than gel-type resins of the same composition. 

Major Developments since 1980. During the 1980s, two fundamental changes in the polypropylene process led to a breakthrough in terms of improved process economics, energy efficiency, and reduced environmental impact. The first of these changes was the conversion from a diluent process to a process in which liquid propene acts as both reactant and diluent for the reactor slurry. This mode of operation, which is called LIPP (Liquid Propylene Process) leads to a considerable increase in the pressure at which such a process is operated. 

The second breakthrough was the use of a high-activily catalyst. This proprietary catalyst called Shell High Activity Catalyst (SHAC) was developed by Shell Research and achieved a hundredfold improvement in product yield with respect to the qi n ity of titanium catalyst employed, leading to a titanium residue in the polymer on the order of 1 to 2 pg/g [222]. As a result, removal of the catalyst residues could be omitted, leading to further process simplification. An impression of the simplification in terms of unit operations between the diluent process and new LIPP process can be gained from Figures 93 and 94. 

Alcohol In the diluent process, an alcohol is used to deactivate the catalysts. Although the alcohol is recovered from the aqueous stream, a small residual... 

The polymerization of olefins with coordination catalysts is performed in a large variety of polymerization processes and reactor configurations that can be classified broadly into solution, gas-phase, or slurry processes. In solution processes, both the catalyst and the polymer are soluble in the reaction medium. These processes are used to produce most of the commercial EPDM rubbers and some polyethylene resins. Solution processes are performed in autoclave, tubular, and loop reactors. In slurry and gas-phase processes, the polymer is formed around heterogeneous catalyst particles in the way described by the multigrain model. Slurry processes can be subdivided into slurry-diluent and slurry-bulk. In slurry-diluent processes, an inert diluent is used to suspend the polymer particles while gaseous (ethylene and propylene) and liquid (higher a-olefins) monomers are fed into the reactor. On the other hand, only liquid monomer is used in the slurry-bulk pro-... 

Slurry-diluent processes use an inert diluent to suspend the polymer particles. Although the diluent does not directly affect the polymerization, it has been shown that different diluents might change catalyst behavior, probably due to electronic interaction with the active sites. Gaseous monomers and hydrogen are continuously bubbled through the diluent. Liquid a-olefin comonomers, diluent, catalysts, and cocatalyst are continuously fed into the reactor. Alternatively, liquefied propylene can be fed into the reactor (slurry-bulk process). Except from this difference, all other conditions are similar to the slurry-diluent process. 

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