Addition emulsion polymerisation

In the early days of the commercial development of PVC, emulsion polymers were preferred for general purpose applications. This was because these materials exist in the form of the fine primary particles of diameter of the order of 0.1-1.0 p,m, which in the case of some commercial grades aggregate into hollow secondary particles or cenospheres with diameters of 30-100 p,m. These emulsion polymer particles have a high surface/volume ratio and fluxing and gelation with plasticisers is rapid. The use of such polymers was, however, restricted because of the presence of large quantities of soaps and other additives necessary to emulsion polymerisation which adversely affect clarity and electrical insulation properties. 

Polychloroprene is produced by emulsion polymerisation, during which the following forms of addition are possible ... 

Residues of fatty acids from emulsion polymerisation and from cure activation provide sites for bacterial attack when the rubber product is exposed to warm moist conditions. The addition of a biocide/fimgicide will give excellent fungal growth protection. 

Emulsion polymerisation is a special case of heterogeneous addition polymerisation in which the reaction kinetics are modified because the A are compartmentalised in small polymer particles [48, 49]. These particles are usually dispersed in water and reaction (78) occurs in the aqueous phase. Initiating radicals diffuse to the particles which are stabilised by surfactant material. Chain termination becomes retarded physically and a relatively high polymerisation rate is obtained. If chain transfer is not prominent, a high molecular weight polymer is produced. The polymerisation rate is given by the expression... 

CR is a polymer with relatively large monomer repeating units that consequently exhibit little vibrational coupling between the chemical units along the chain [85], Infrared analysis [86] showed the following bands cis 1652 cm 1, trans 1660 cm"1 1,2, 925 cm 1 3,4, 883 cm"1. For emulsion polymerisation addition is almost entirely 1,4 with no more than about 2% 1,2- and 3,4-. Cis 1,4 - CR exhibits characteristic infrared absorption bands at 847, 1652, 3025 and 3282 cm 1 (C = C overtone). The corresponding trans bands are at 822, 1660, 3018 and 3295 cm 1. Typical CR has 78%-96% trans, 1,4 18% cis 1,4,... 

Recently, a water-based polymer synthesised from N-chloro-2,2,6,6-tetramethyl-4-pipridinyl methacrylate (Cl-TMPM) was prepared via emulsion polymerisation (Figure 10.3). Furthermore, the addition of this antimicrobial material to commercial water-based latex paints led to antimicrobial activity against methicillin-resistant Staphylococcus aureus, vancomycin-resistant enterococcus, Escherichia coli, Candida tropicalis, bacteriophage MS2 virus and Staphylococcus chartarum [53]. 

In this chapto we have reviewed recent developments in the technology dealing with the emulsion polymerisation of VAc and its copolymers. In particular, we have examined problems peculiar to VAc, such as reactivity and water solubility, and the resulting consequences regarding particle formation, growth and stabilization options. In addition, the development of particle structure and its effect on the film formation ttnd ultimate film properties were discussed. Finally, a family of branched vinyl esters for the production of higher performance vinyl copolymers was described as one possible path of future development in this area. 

The choice of initiating systems was made by considering their possible effect on particle morphology (see section 6.3.2). The other additives (emnlsifiers, protective colloids) were standard materials commonly used in emulsion polymerisation of acrylic monomers. 

Flexibility is the key word in emulsion polymerisation. Latex properties can be tailored to the application (65,384). Various types of monomers, processing methods, and additives can be used during emulsion polymerisation, making the process flexible (276). A wide variety of products with specialised properties can be manufactured. Emulsion polymerisation allows for the production of particles with specially-tailored properties, including size, composition, morphology, and molecular weight. Functional groups can also be incorporated (160). Blends of different types of latexes have been formulated to provide the desired properties without copolymerisation (139,156, 213, 386). 

Although free radical polymerisation is most common, other types of polymerisations have been carried out in emulsion polymerisation, including reversible addition-fragmentation transfer (RAFT) (131), atom transfer radical polymerisation (ATRP) (76, 222), and stable free radical polymerisation (SFRP) (77). 

Hybrid (or composite) latexes (169) are essentially a combination of the artificial latex and emulsion polymerisation methods (68, 167). A water-insoluble species (such as polymer) may be dissolved in monomer and dispersed in water in the same marmer as the artificial latexes. However, rather than removing the monomeric solvent, it is polymerised in the droplets by the addition of initiator. The monomer-swollen polymer particles capture radicals and polymerise to form a polymeric blend or structured domains. In this maimer, polystyrene particles with styrene-butadiene mbber (SBR) inclusions have been prepared for impact modification applications. 

Latex with hydroxyl functionalised cores of a methyl methacrylate/butyl acrylate/2-hydroxyethyl methacrylate copolymer, and carboxyl functionalised shells of a methyl methacrylate/butyl acrylate/methacrylic acid copolymer was prepared by free radical polymerisation. The latex was crosslinked using a cycloaliphatic diepoxide added by three alternative modes with the monomers during synthesis dissolved in the solvent and added after latex preparation and emulsified separately, then added. The latex film properties, including viscoelasticity, hardness, tensile properties, and water adsorption were evaluated as functions of crosslinker addition mode. Latex morphology was studied by transmission electron and atomic force microscopy. Optimum results were achieved by introducing half the epoxide by two-step emulsion polymerisation, the balance being added to the latex either in solution or as an emulsion. 8 refs. 

Carboxylic monomers, such as acrylic or methacrylic acid, are included in emulsion polymerisation formulations for several reasons to increase the stability of latex particles, to improve the adhesion of resultant films to various substrates, to provide functional groups for interparticle crosslinking reactions and to control the viscosity of latex via neutralisation. Acrylic latices with and without incorporation of carboxylic groups, together with addition of various amounts of anionic surfactant, are used to investigate rheological and drying behaviours of the latices. 9 refs. 

Details are given of the control of latex particle size and particle size distribution in semibatch emulsion polymerisation of polybutyl methacrylate and polymethacryhc acid. A seeded technique was used to examine secondary nucleation during monomer addition. 12 refs. 

A rapid and low cost method was developed for direct analysis of residual monomer concentration of acrylamide from inverse-emulsion reactions. Inverse-emulsion polymerisations involve the dispersion of a water-soluble monomer in aqueous solution in a continuous organic phase. The addition of a low-medium hydrophilic-lyophilic balance steric stabiliser and continuous agitation is required to maintain emulsification. 19 refs. 

The process of emulsion polymerisation begins when the free radicals derived from the, usually water-soluble, polymerisation initiator enter the monomer-saturated micelles where they find a sufficient number of solubilised molecules to start a rapid chain reaction (Elgood and Gilbekian, 1973). Each polymer radical first exhausts the monomer contained in the micelle and then captures additional supplies from 50 or more other micelles before the chain reaction is terminated. Some of the depleted micelles then break up and the released emulsifier molecules are adsorbed at the surface of the newly formed primary polymer particles (Dunn, 1971). The remainder are replenished by diffusion from the emulsified monomer droplets, which act essentially as reservoirs. 

A superabsorbent nanocomposite based on partially neutralised acrylic acid, recycled PS foam and Na-MMT was prepared via emulsion polymerisation [65]. The results indicated that the acrylic acid monomer had successfully grafted onto the PS chains and the layers of Na-MMT were exfoliated after copolymerisation. Moreover, the addition of Na-MMT not only improved the thermal stability of the samples, but also increased the content and rate of water absorbency. 

The packing efficiency of a monodisperse latex may be increased by the addition of one or more latices of smaller particle size. Hence certain blends of PVC latices can be used to produce plastisols with low viscosity over a wide range of shear rates (5). However it is more usual to produce multi-disperse particle size distributions at the polymerisation stage in a seeded emulsion polymerisation process. 

Clearly, an objective of commercial production is to obtain a stable latex which has as high a solids content as remains consistent with producing the required balance of pol3naer properties after drying. The colloidal stability of latices is enhanced by the addition of surfactants. Latices prepared by emulsion pol3mierisation techniques usually have solids content of 40-45%. The kinetics and mechanism of emulsion polymerisation of VCM have been extensively reviewed by Ugelstad et al (9). 

Water is the ideal solvent from the cost and pollution viewpoints, but it is a non-solvent for many surface coating polymers. It will ssolve a small number of homopolymers, notably those derived from acrylamide, acrylic acid, itaconic acid, vinyl methyl ether, vinyl pyrrolidone and vinyl sulphonic acid, but none of these homopolymers forms flexible films of use in the coatings industry. While copolymers of acrylic or methacrylic acids with acrylate esters are generally insoluble in water, their salts are soluble when the acid content is over 5% (for hydrophilic monomers) and 12% (for hydrophobic monomers). Such polymers can be prepared in solution, or in emulsion, but not in aqueous solution. This is because the acrylate esters are insoluble in water. The acid is copolymerised in the un-ionised form because the ion is unreactive to free radicals. In emulsion polymerisation, care has to be taken to avoid homopolymerisation of the acrylic or methacrylic acid in the water phase. Suppression of homopolymerisation requires a low concentration of acid throughout the polymerisation process. This can be achieved by using a long reaction period and slow addition of monomer mixture, or by careful pH buffer selection. 

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