Surfactants emulsion polymerisation

Emulsion polymerisation represents the next stage in development from the suspension technique and is a versatile and widely used method of polymerisation. In this technique droplets of monomer are dispersed in water with the aid of an emulsifying agent, usually a synthetic detergent. The detergent forms small micelles 10-100 /im in size, which is much smaller than the droplets that can be formed by mechanical agitation in suspension polymerisation. These micelles contain a small quantity of monomer, the rest of the monomer being suspended in the water without the aid of any surfactant. 

For the thermally initiated (potassium persulphate) emulsion polymerisation of styrene, we have observed [73] a twofold increase in the initial polymerisation rate in the presence of ultrasound (20 kHz), the increase being dependent upon the level of surfactant employed. Several workers have suggested that possible explanations for the observed increase in rate are ... 

Fig. 5.35. Emulsion polymerisation in the presence and absence of inhibitor and in the presence and absence of ultrasound (Conditions 8% styrene (w/v), 0.8% surfactant (w/v), potassium persulphate, T= 70°C).

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... 

Emulsion polymerisation used for the production of various elastomers is carried out in a number of processes. Generally surfactants are used in EP to foam the material consistently to give it certain properties. Without surfactants many products would cease to exist. Can one imagine a world without rubber products, or one reduced to hand-made, weak-performing... 

Applications. At present there are very few known applications, although the surfactants have significant potential due to their unique properties. Sulphonated fatty acids are used in some hard-surface cleaning formulations where their low foam is a benefit and in emulsion polymerisation, where they perform similarly to LAS but with greatly reduced tendency to foam. Future applications for these products may include machine dishwash, extended use in detergent products and industrial applications such as pigment dispersants. For these to be realised, further process development will be required to give a more consistent and better defined product. 

Vidal, F., Guillot, J. and Guyot, A. (1995) Surfactants with transfer agent properties (transurfs) in styrene emulsion polymerisation. Colloid Polym. Sci., 273, 999-1007. 

Guyot, A. and Vidal, F. (1995) Inifer surfactants in emulsion polymerisation. Polym. Bull, 34, 569-76. 

Tauer, K. (2003) The role of emulsifiers in the kinetics andmechanism of emulsion polymerisation. In D.R. Karsa (ed.), Surfactants in Polymers, Coatings, Inks and Adhesives. Blackwell Publishing, CRC Press, Boca Raton, FL. 

VCM and an emulsifier. These components are circulated through a mixing pump (homogeniser) which causes the mixture to disperse into very fine droplets, before being placed in the autoclave. The droplets are coated with surfactant which stabilises them during the reaction. Initiation and polymerisation occur within the droplets. After polymerisation, the autoclave contains a stable dispersion of fine particles of PVC in water. Thereafter the subsequent operations for obtaining the final product are similar to the emulsion polymerisation process. 

Effect of surfactant level. Much of the earlier work on this reaction system was carried out using sodium dodecylbenzene-sulphonate as the surfactant. In the course of experiments which were Intended to investigate the effect upon polymerisation rate of varying the surfactant level, it was discovered that purified sodium dodecylbenzenesulphonates are apparently able to act as initiators of free-radical emulsion polymerisation in the absence of other added Initiating substances. Furthermore, as the results summarised in Figure 5 show, the rate of polymerisation in the absence of added initiator is directly proportional to the concentration of sodium dodecylbenzenesulphonate In the aqueous phase. 

The reason for this behaviour is obscure it is not apparently due to the presence of peroxides in the surfactant. However, it is perhaps significant that an emulsion polymerisation reaction in which the rate of polymerisation is first-order with respect to surfactant level is consistent with Smlth-Ewart "Case 2" kinetics for a system in which the surfactant functions as an initiator as well as a micelle generator. 

Other workers claim a crystal violet and ascorbic acid mixture is similarly very effective in inducing the photopolymerisation of vinyl monomers. The photoinduced emulsion polymerisation of styrene with sodium dodecyl sulphate as a surfactant and dibenzyl ketone as the initiator has been... 

Emulsion polymerisation in this case the monomer is emulsified in a nonsolvent (commonly water), usually in the presence of a surfactant. A water-soluble initiator is then added, whereupon particles of polymer form and grow in the aqueous medium as the monomer reservoir in the emulsified droplets is gradually used up. 

A typical emulsion polymerisation formulation contains water, 50% monomer blended for the required glass transition temperature (T ), surfactant (and often coUoid), initiator, pH buffer and fungicide. Hard monomers with a high Tg used in emulsion polymerisation may be vinyl acetate, MMA, and styrene, while soft monomers with a low include butyl acrylate, 2-ethylhexyl acrylate, vinyl versatate,... 

Several types of surfactant can be used in emulsion polymerisation, and a summary of the various classes is provided in Table 17.1. 

Most reports on emulsion polymerisation have been limited to commercially available surfactants which, in many cases, are relatively simple molecules such as sodium dodecyl sulphate and simple nonionic surfactants. However, studies on the effects of surfactant structure on latex formation have revealed the importance of the structure of the molecule. Block and graft copolymers (polymeric surfactants) are expected to be better stabilisers when compared to simple surfactants. The use of these polymeric surfactants in emulsion polymerisation and the stabilisation of the resulting polymer particles is discussed below. 

An important technique is that in which it is the precursor of the final colloidal particle that is reduced to a colloidal size. Thus a liquid reactant may be emulsified and then caused to react to form a colloidal dispersion of solid particles whose particle size distribution is related to that of the emulsion precursor. The commonest application of this method is in suspension polymerisation, in which an emulsion of monomer droplets, stabilised by a surfactant, is polymerised by adding an initiator which is soluble in the monomer. Polymerisation occurs within the monomer droplet, leading to the formation of a polymer latex. 

The core of a micelle has been shown to have properties akin to a liquid hydrocarbon. One piece of evidence for this is that a surfactant solution above the c.m.c. is capable of taking up substantial quantities of non-polar organic (lipophilic) substances. These enter the core of the micelle, which can now swell because the added material has no hydrophilic moiety which needs to he on the surface (Figure 11.8). Solubilisation plays an important role in detergency and in the detailed mechanism of emulsion polymerisation. 

An excess of maleic anhydride (up to 5 % wt.) is generally used. Unreacted ROH, diester, and maleic acid/anhydride are formed as by-products. Note, monoalkylethoxymaleates are of interest as polymerisable surfactants in the emulsion polymerisation [77]. 

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