Emulsion-polymerisation

The final polymerised product is formed in particles much smaller (50-500 nm) than produced with suspension polymerisation. Emulsion polymerisation can lead to rapid production of high molecular weight polymers but the unavoidable occlusion of large quantities of soap adversely affects the electrical insulation properties and the clarity of the polymer. 

It is also possible to polymerise emulsion PVC. While as an emulsion in water, the spherical particles have diameters of 0.1-1 pm. On drying, these agglomerate to grains of mean size 30-60 pm (Fig. 2.17), smaller than the 100-160 pm grain size of suspension PVC. 

Polymerisations may be categorised by both the polymerisation mechanism (e.g. radical polymerisation, anionic polymerisation etc.) and by the polymerisation technique (e.g. solution polymerisation, emulsion polymerisation etc.). A third factor is how the reactor is operated in batch mode, or by adding monomers during the process (semi-continuous) or by continuous operation. Mechanism, technique and process strategies (mode of operation) all have an influence on rates of polymerisation and characteristics of the formed polymer. In this chapter, we will focus on the special characteristics that can be distinguished in an emulsion polymerisation related to rate, development of molar mass and chemical composition. In Chapter 4 the effects of the process strategy will be discussed. 

An emulsion polymerisation system comprises water, an initiator (usually water soluble), a water-insoluble monomer and a colloidal stabiliser, which may be added or maybe formed in situ. The main locus of polymerisation is within the monomer-swollen latex particles, which are either formed at the start of polymerisation or may be added initially (in which case one has a seeded emulsion polymerisation). The term emulsion polymerisation is a misnomer (arising for historical reasons the process was originally developed with the aim of polymerising emulsion droplets, although, in fact, this does not occur). The starting emulsion is not thermodynamically stable. An inverse emulsion polymerisation is one where the continuous phase is organic in combination with an aqueous discrete phase containing a water-soluble monomer (e.g. acrylamide). Two variants of emulsion polymerisation are... 

Emulsion polymerisation kinetics have important differences from solution and bulk polymerisations. These differences can lead to many advantages for example, an increase in molar mass can be achieved without reducing the rate of polymerisation. Emulsion polymerisation is known for its relatively high rates of polymerisation and high molar masses as compared to other polymerisation techniques. A disadvantage of emulsion polymerisation is the presence of surfactant and other additives, which may result in deleterious properties under some circumstances. 

Tetrafluoroethylene. Emulsion polymerisation of tetrafluoroethylene, catalysed by oxygen, yields polytetrafluoroethylene (Tejlon) as a very tough horn-hke material of high melting point. It possesses excellent electrical insulation properties and a remarkable inertness towards all chemical reagents, including aqua regia. 

Emulsion polymerisation of a mixture of butadiene and styrene gives a synthetic rubber (Buna S GBS rubber), which is used either alone or blended with natural rubber for automobile tyres and a variety of other articles. 

Fig. 2. Emulsion polymerisation plant. A, Emulsion feed tank B, polymerisation reactor C, dmmming tank E, filter M, meter P, pressure gauge T,...

Monomer emulsions ate prepared in separate stainless steel emulsification tanks that are usually equipped with a turbine agitator, manometer level gage, cooling cods, a sprayer inert gas, temperature recorder, mpture disk, flame arrester, and various nossles for charging the ingredients. Monomer emulsions are commonly fed continuously to the reactor throughout the polymerisation. 

Emulsion Polymerisation of Acylie Monomers, CM-104, Rohm and Haas Co., Philadelphia, Pa. 

The synthesis of the high molecular weight polymer from chlorotrifluoroethylene [79-38-9] has been carried out in bulk (2 >—21 solution (28—30), suspension (31—36), and emulsion (37—41) polymerisation systems using free-radical initiators, uv, and gamma radiation. Emulsion and suspension polymers are more thermally stable than bulk-produced polymers. Polymerisations can be carried out in glass or stainless steel agitated reactors under conditions (pressure 0.34—1.03 MPa (50—150 psi) and temperature 21—53°C) that require no unique equipment. 

D. C. Blackley, Emulsion Polymerisation—Theory and Practice, John Wiley Sons, Inc., New York, 1975, Chapt. 6. 

A third source of initiator for emulsion polymerisation is hydroxyl radicals created by y-radiation of water. A review of radiation-induced emulsion polymerisation detailed efforts to use y-radiation to produce styrene, acrylonitrile, methyl methacrylate, and other similar polymers (60). The economics of y-radiation processes are claimed to compare favorably with conventional techniques although worldwide iadustrial appHcation of y-radiation processes has yet to occur. Use of y-radiation has been made for laboratory study because radical generation can be turned on and off quickly and at various rates (61). 

The ionic nature of the radicals generated, by whatever technique, can contribute to the stabilisation of latex particles. Soapless emulsion polymerisations can be carried out usiag potassium persulfate as initiator (62). It is often important to control pH with buffets dutiag soapless emulsion p olymerisation. 

Cha.in-Tra.nsferAgents. The most commonly employed chain-transfer agents ia emulsion polymerisation are mercaptans, disulfides, carbon tetrabromide, and carbon tetrachloride. They are added to control the molecular weight of a polymer, by transferring a propagating radical to the chain transfer agent AX (63) ... 

The newly formed short-chain radical A then quickly reacts with a monomer molecule to create a primary radical. If subsequent initiation is not fast, AX is considered an inhibitor. Many have studied the influence of chain-transfer reactions on emulsion polymerisation because of the interesting complexities arising from enhanced radical desorption rates from the growing polymer particles (64,65). Chain-transfer reactions are not limited to chain-transfer agents. Chain-transfer to monomer is ia many cases the main chain termination event ia emulsion polymerisation. Chain transfer to polymer leads to branching which can greatiy impact final product properties (66). 

M. S. El-Aasser and co-workers, ia M. S. El-Aasser and. W. Vanderhoff, eds.. Emulsion Polymerisation of VinylA.cetate AppHed Science PubHshers, London, 1981, p. 215. 

The principal use of the peroxodisulfate salts is as initiators (qv) for olefin polymerisation in aqueous systems, particularly for the manufacture of polyacrylonitrile and its copolymers (see Acrylonitrile polymers). These salts are used in the emulsion polymerisation of vinyl chloride, styrene—butadiene, vinyl acetate, neoprene, and acryhc esters (see Acrylic ester polymers Styrene Vinyl polymers). 

D. R. Basset and A. E. Hamielec, eds.. Emulsion Polymers and Emulsion Polymerisation, ACS Symposium Series 165, American Chemical Society, Washington, D.C., 1981. 

F. Bovey and co-workers. Emulsion Polymerisation, Vol. 10, High Polymers, Interscience PubHshers, New York, 1955. 

In the late 1920s Bayer Company began reevaluating the emulsion polymerisation process of polybutadiene as an improvement over their Buna technology, which was based on sodium as a catalyst. Incorporation of styrene (qv) as a comonomer produced a superior polymer compared to polybutadiene. The product Buna S was the precursor of the single largest-volume polymer produced in the 1990s, emulsion styrene—butadiene mbber... 

Eree-radical initiation of emulsion copolymers produces a random polymerisation in which the trans/cis ratio caimot be controlled. The nature of ESBR free-radical polymerisation results in the polymer being heterogeneous, with a broad molecular weight distribution and random copolymer composition. The microstmcture is not amenable to manipulation, although the temperature of the polymerisation affects the ratio of trans to cis somewhat. 

Emulsion Polymerization. Emulsion SBR was commercialised and produced in quantity while the theory of the mechanism was being debated. Harkins was among the earliest researchers to describe the mechanism (16) others were Mark (17) and Elory (18). The theory of emulsion polymerisation kinetics by Smith and Ewart is still vaUd, for the most part, within the framework of monomers of limited solubiUty (19). There is general agreement in the modem theory of emulsion polymerisation that the process proceeds in three distinct phases, as elucidated by Harkins (20) nucleation (initiation), growth (propagation), and completion (termination). 

D. C. Blackley, Emulsion Polymerisation Theory and Practice, Applied Science Publishers Ltd., London, 1975, pp. 58—72. 

Chain transfer to monomer and to other small molecules leads to lower molecular weight products, but when polymerisation occurs ia the relative absence of monomer and other transfer agents, such as solvents, chain transfer to polymer becomes more important. As a result, toward the end of batch-suspension or batch-emulsion polymerisation reactions, branched polymer chains tend to form. In suspension and emulsion processes where monomer is fed continuously, the products tend to be more branched than when polymerisations are carried out ia the presence of a plentiful supply of monomer. 

The low vinyl acetate ethylene—vinyl acetate copolymers, ie, those containing 10—40 wt % vinyl acetate, are made by processes similar to those used to make low density polyethylene for which pressures are usually > 103 MPa (15,000 psi). A medium, ie, 45 wt % vinyl acetate copolymer with mbber-like properties is made by solution polymerisation in /-butyl alcohol at 34.5 MPa (5000 psi). The 70—95 wt % vinyl acetate emulsion copolymers are made in emulsion processes under ethylene pressures of 2.07—10.4 MPa (300—1500 psi). 

I. Piirma, ed.. Emulsion Polymerisation, Academic Press, New York, 1982. 

D. C. BEddey, Emulsion Polymerisation Theoy andPractice,JohnWHey 8cSons,Inc.,NewYoik,1975. 

R. K. Greene, Continuous Emulsion Polymerisation of Vinyl Acetate, Ph.D. dissertation, 1976. 

R. G. Gilbert, Emulsion Polymerisation. A Mechanistic Approach, Academic Press, Inc., New York, 1995. 

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