Copolymerization continuous process

In this chapter, the big four thermoplastics are covered polyethylene, polypropylene, polyvinyl chloride, and polystyrene. Like most other thermoplastics, they are long-chain polymers that become soft when heated and can be molded under pressure. They are linear- or branch-chained and, except for some exotic copolymers, have little or no cross-linking. Technological advances continue. Research in copolymerization, catalysts, processing, blending, and fabricating continues even as you read this. 

Except in very special cases (azeotropic copolymerizations), copolymerization via radical mechanism shows a drift in the composition of the copolymers produced through the polymerization process. Emulsion copolymerization obeys this rule too, although the special features of its mechanism can change the drift process. The most common way to obviate that composition drift is to use the semi-continuous process where, after polymerization has been initiated with a small percent of the total charge (say 10 to 20 %) like in the batch process, most of the charge is added continuously at a much smaller rate (Ra) than the rate (Rp) at the end of the batch period, so that the added charge is polymerized quite instantaneously (J, 2). Then,the composition drift is limited to the initial period and most of the product does possess actually a constant composition. 

We will describe its use for controlling the styrene-acrylonitrile emulsion copolymerization system. Results concerning copolymer compositions, molecular characteristics and particle sizes will be compared to the corresponding ones from batch or semi-continuous processes. 

High polymer/surfactant weight ratios (up to about 15 1) of polystyrene microlatexes [73] have been produced in microemulsions stabihzed by polymerizable nonionic surfactant by the semi-continuous process. The copolymerization of styrene with the surfactant ensures the long-term stabihty of the latexes. Nanosized PS microlatexes with polymer content (<25 wt%) were also obtained from an emulsifier-free process [74] by the polymerization of styrene with ionic monomer (sodium styrenesulfonate, NaSS), nonionic comonomer (2-hydroxyethylmethacryalte, HEM A), or both. The surfaces of the latex particles were significantly enriched in NaSS and HEMA, providing better stabilization. 

In addition to the above investigations, free-radical high-pressure polymerizations should also be studied in continuously operated devices for three reasons. (1) Because of the wealth of kinetic information contained in the polymer properties, product characterization is mandatory. Sufficient quantities of polymer, produced under well defined conditions of temperature, pressure, and monomer conversion, are best provided by continuous polymerization, preferably in a continuously stirred tank reactor (CSTR). (2) Copolymerization of monomers that have rather dissimilar reactivity ratios, such as in ethene-acry-late systems, will yield chemically inhomogeneous material if the reaction is carried out in a batch-type reactor up to moderate conversion. To obtain larger quantities of copolymer of analytical value, the copolymerization has to be performed in a CSTR. (3) Technical polymerizations are exclusively run as continuous processes. Thus, in order to stay sufficiently close to the application and to investigate aspects of technical polymerizations, such as testing initiators and initiation strategies, fundamental research into these processes should, at least in part, be carried out in continuously operated devices. 

Vinyl acetate homopolymers are simply-made adhesive bases manufactured by addition polymerization in the presence of water and stabilizers. They are made commercially by the batch reactor process or by the Loop reactor continuous process. External plasticizers such as dibutyl phthalate are often added to confer flexibility and to lower the temperature at which they form a film on drying. Higher-quality products may be made by the copolymerization of ethylene with vinyl acetate to form an EVA. This involves the safe handling of ethylene gas under high pressure, and the plant required is more complex and considerably more costly. The Loop process has considerable attraction in the field of pressure polymerization. 

At first only little styrene is converted, and it is not until the monomer mixture is largely depeleted of 1,3-butadiene that the principal amount of styrene polymerizes forming a block consisting mainly of styrene. In THF, also, the content of vinyl double bonds increases and thus the glass transition temperature is raised [475]. A uniform copolymer composition can be achieved by a continuous process or by programmed addition of butadiene to the reaction mixture [476,477]. Copolymers with a high trans-1,4 content are obtained with barium compounds [478]. In contrast, the coordinative copolymerization yields products with a high cis-1,4 content. 

Emulsion copolymerizations can be carried out using batch, semi-continuous, or continuous processes. The copolymers made by these processes differ according to the process used, the copoly-meriztion reactivity ratios of the monomers, and the monomer solubilities in the aqueous phase. To show the difference between batch and semi-continuous polymerization, the latex particle size, surface characteristics, latex stability, copolymer properties, and latex film morphology were investigated for the vinyl acetate-butyl acrylate system (37). The water solubilities are 290 mM and llmM for vinyl acetate and butyl acrylate, respectively, and the copoly-merization reactivity ratios of = 0-0.04 and r 2 show... 

SBR is produced by copolymerizing butadiene and styrene in a 3 1 weight ratio. In the past, emulsion SBR has been favored due to better processing properties which make consumers able to switch easily between suppliers without any need to reconfigure their processing machines. Emulsion SBR is typically produced in a continuous process making it cost effective, while SSBR can be produced in both continuous and batch processes. 

There are claims that terpolymers of ethylene, propylene, and MA (mol % 47 50 3) can be produced by a continuous process in hexane solution with a modified Ziegler catalyst (ethyl aluminum dichloride-vanadyl trichloride).For the procedure, a carbon tetrachloride solution of the anhydride was added to the reaction mixture after copolymerization of the olefin mixture was well established. The copolymers, obtained by this technique as sticky glasses, have not had adequate study. Also fundamentals of the polymerization reaction have not been explored. 

Butadiene and myrcene have been copolymerized by either batch or continuous processes using finely divided alkali metal (Na or K) as catalyst and ether (preferably diethyl ether or 1,4-dioxane) as solvent at 25-95°C. Conversions of 90% or higher were obtained within 6-24 h. Other terpenes, like a-terpinene, dipentene (racemic limonene), or p-pinene, react little or not at all with 1,3-butadiene, while the copolymerization of alloocimene with 1,3-butadiene gives a low conversion of partially gelled copolymer [37]. 

Batch trials were performed on a kneader reactor where a bulk co-polymerization was carried out. Polymerization conversion, viscosity build, reaction kinetics, and heat transfer calculations were performed using the experimental data from the batch trials. A continuous process was proposed for this bulk copolymerization and the models and results from the batch trials were used in designing the continuous process. Predictions of the continuous process using the batch trial data are compared to the actual continuous process, with a focus on polymer conversion, heat transfer, and torque prediction. 

Emulsion Process. The emulsion polymerization process utilizes water as a continuous phase with the reactants suspended as microscopic particles. This low viscosity system allows facile mixing and heat transfer for control purposes. An emulsifier is generally employed to stabilize the water insoluble monomers and other reactants, and to prevent reactor fouling. With SAN the system is composed of water, monomers, chain-transfer agents for molecular weight control, emulsifiers, and initiators. Both batch and semibatch processes are employed. Copolymerization is normally carried out at 60 to 100°C to conversions of - 97%. Lower temperature polymerization can be achieved with redox-initiator systems (51). 

A schematic of a continuous bulk SAN polymerization process is shown in Figure 4 (90). The monomers are continuously fed into a screw reactor where copolymerization is carried out at 150°C to 73% conversion in 55 min. Heat of polymerization is removed through cooling of both the screw and the barrel walls. The polymeric melt is removed and fed to the devolatilizer to remove unreacted monomers under reduced pressure (4 kPa or 30 mm Hg) and high temperature (220°C). The final product is claimed to contain less than 0.7% volatiles. Two devolatilizers in series are found to yield a better quaUty product as well as better operational control (91,92). 

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