Emulsion controlling styrene-acrylonitrile

In addition to graft copolymer attached to the mbber particle surface, the formation of styrene—acrylonitrile copolymer occluded within the mbber particle may occur. The mechanism and extent of occluded polymer formation depends on the manufacturing process. The factors affecting occlusion formation in bulk (77) and emulsion processes (78) have been described. The use of block copolymers of styrene and butadiene in bulk systems can control particle size and give rise to unusual particle morphologies (eg, coil, rod, capsule, cellular) (77). 

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. 

For rubber-modified, high-impact PS (HIPS), polybutadiene (PB) is dissolved in the styrene monomer (5-10%) and grafting onto the PB takes place. The copolymer styrene/acrylonitrile (SAN) is made in a diluent with controlled addition of S and AN using a free-radical initiator. Copolymers with high AN (70-80%) have low gas permeability and are used for containers. Acrylonitrile/butadiene/styrene (ABS) copolymers are often made in water using a polybutadiene latex together with AN and S monomers and a water-soluble initiator (e.g. persulphate). The random copolymer rubber 75% styrene/25% butadiene (SBR) is also made as an aqueous emulsion using a persulphate initiator. 

Emulsion polymerization is the most important process for production of elastic polymers based on butadiene. Copolymers of butadiene with styrene and acrylonitrile have attained particular significance. Polymerized 2-chlorobutadiene is known as chloroprene rubber. Emulsion polymerization provides the advantage of running a low viscosity during the entire time of polymerization. Hence the temperature can easily be controlled. The polymerizate is formed as a latex similar to natural rubber latex. In this way the production of mixed lattices is relieved. The temperature of polymerization is usually 50°C. Low-temperature polymerization is carried out by the help of redox systems at a temperature of 5°C. This kind of polymerization leads to a higher amount of desired trans-1,4 structures instead of cis-1,4 structures. Chloroprene rubber from poly-2-chlorbutadiene is equally formed by emulsion polymerization. Chloroprene polymerizes considerably more rapidly than butadiene and isoprene. Especially in low-temperature polymerization emulsifiers must show good solubility and... 

The free-radical kinetics described in Chapter 6 hold for homogeneous systems. They will prevail in well-stirred bulk or solution polymerizations or in suspension polymerizations if the polymer is soluble in its monomer. Polystyrene suspension polymerization is an important commercial example of this reaction type. Suspension polymerizations of vinyl ehloride and of acrylonitrile are described by somewhat different kinetic schemes because the polymers precipitate in these cases. Emulsion polymerizations aie controlled by still different reaetion parameters because the growing macroradicals are isolated in small volume elements and because the free radieals which initiate the polymerization process are generated in the aqueous phase. The emulsion process is now used to make large tonnages of styrene-butadiene rubber (SBR), latex paints and adhesives, PVC paste polymers, and other produets. 

Statistical and gradient copolymCTs have bear prepared successfully using NMP and ATRP in bulk, solution, and mini-emulsion, particularly when styrene is used as a monomer with acrylates, methacrylates, acrylonitrile, and substituted styrenes as comonomers. A number of BPO/TEMPO-mediated statistical polymerizations have been reported, but in some cases, control can only be effected if the initial comaiQmer feed is rich in styrene. 

Grafting a second polymer to the NR molecule in the latex stage is one of the many routes to chemically modified NR. An olefinic monomer with unsaturated double bonds such as methyl methacrylate (MMA), styrene and acrylonitrile are important monomers used for such grafting. " For example, MMA monomer is first converted into an emulsion with some suitable emulsifiers and then mixed with NR latex to copolymerize the monomer in a seeded emulsion polymerization process. It is important to ensure the seed latex particles are saturated with the monomer supplied through diffusion from the emulsified monomer droplets. An oil- or water-soluble initiator can be used to start the reaction. With proper control of the system and reaction conditions, the free radical reaction can be made to propagate within the latex particles as far as possible, so that only grafted NR occurs, without the formation of free homopolymer from the monomer. In this way only chemically modified NR... 

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