Polymerisation latexes

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). 

Styrene emulsions were prepared using a water-soluble, low molecular weight blue dye as the cosurfactant. Their shelf stability was evaluated by monitoring the monomer size, a eamiiig rate and phase separation of the monomer as functions of time. The dye molecule is a potential probe for determining the lod of particle nucleation during emulsion polymerisation, becattse its concentration in toluene can be determined by UV absorbance at678 nm. In the subsequent polymerisation, latex particles were produced by both... 

A novel process for the preparation of latex with high solid content, but maintaining the characteristics of microemulsion polymerisation latex, small particle size (less than 50 nm) and polymer with high molecular weight (more than 10 6) is presented. With the PS latex obtained by microemulsion polymerisation as seed, core shell, styrene-butyl acrylate polymers functionalised with itaconic acid are prepared. Materials were characterised by differential scanning calorimetry, dynamic mechanical thermal analysis and transmission electron microscopy. These polymers have better mechanical properties than the non functionalised or those prepared by emulsion polymerisation. 11 refs. 

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. 

Vmulsifier Type. The manufacturers of NBR use a variety of emulsifiers (most commonly anionic) for the emulsion polymerisation of nitrile mbber. When the latex is coagulated and dried, some of the emulsifier and coagulant remains with the mbber and affects the properties attained with the mbber compound. Water resistance is one property ia particular that is dependent on the type and amount of residual emulsifier. Residual emulsifer also affects the cure properties and mold fouling characteristics of the mbber. 

Poly(vinyl chloride) is commercially available in the form of aqueous colloidal dispersions (latices). They are the uncoagulated products of emulsion polymerisation process and are used to coat or impregnate textiles and paper. The individual particles are somewhat less than 1 p,m in diameter. The latex may be coagulated by concentrated acids, polyvalent cations and by dehydration with water-miscible liquids. 

To produce the Type 2 polymers, styrene and acrylonitrile are added to polybutadiene latex and the mixture warmed to about 50°C to allow absorption of the monomers. A water-soluble initiator such as potassium persulphate is then added to polymerise the styrene and acrylonitrile. The resultant materials will be a mixture of polybutadiene, polybutadiene grafted with acrylonitrile and styrene, and styrene-acrylonitrile copolymer. The presence of graft polymer is essential since straightforwsird mixtures of polybutadiene and styrene-acrylonitrile copolymers are weak. In addition to emulsion processes such as those described above, mass and mass/suspension processes are also of importance. 

Chain reactions are used to prepare a variety of high molar mass polymers of commericial importance and in practice may take one of four forms, namely bulk, solution, suspension, and emulsion methods. These four methods are described in the sections that follow, together with the loop modification which has become of commercial importance recently in producing latexes by emulsion polymerisation for the paint industry. 

As emulsion polymerisation proceeds, like the suspension technique but unlike either the bulk or the solution techniques, there is almost no increase in viscosity. The resulting dispersed polymer is not a true emulsion any more, but instead has become a latex. The particles of the latex do not interact with the water hence viscosity is not found to change significantly up to about 60% solids content. 

Emulsion polymerisation is used in the commercial production of synthetic diene elastomers and also to produce commercial latexes of the type used in paints these paints are known incorrectly as emulsion paints and... 

This is a modification of emulsion polymerisation which has recently been developed for the manufacture of commercially important latexes for emulsion paints. In this process instead of producing the polymer in batches in a tank polymer is produced continuously in a reactor that consists of a continuous tube coiled to a convenient shape. 

Amine-containing accelerators and stabilizers used in the polymerisation of rubber latex... 

Latex or emulsion polymers are prepared by emulsification of monomers in water by adding a surfactant. A water-soluble initiator is added, e.g., persulfate or hydrogen peroxide (with a metallic ion as catalyst), that polymerises the monomer yielding polymer particles, which have diameters of about 0.1 pm. The higher the concentration of surfactant added, the smaller the polymer particles. 

The rubber may be natural, in which case the latex is produced by the rubber tree. Latex of the main synthetic rubbers is produced by the technique of emulsion polymerisation. The term latex has been broadened in recent years and a general definition is now a stable dispersion of a polymeric substance in an aqueous medium . Latices may be classified as natural (from trees and plants), synthetic (by emulsion polymerisation) and artificial (by dispersion of the solid polymer in an aqueous medium). They may also be classified according to the chemical nature of the polymer, e.g., SBR, nitrile, polychloroprene, etc. 

In latex technology, a submicro scopic aggregation of oriented molecules in polymer technology it is synonymous with crystallite. The term is also applied to the aggregates of soap molecules formed in emulsion polymerisation. Micro... 

Emulsion Polymerisation and Applications of Latex, Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers Institute. 

This is superficially similar to suspension polymerisation. But in this process a monomer dispersed in water, in presence of a surface active agent is polymerised to give a stable polymer latex. 

Emulsion Polymerisation It is a very good process which is used for the preparation of polystyrene. Emulsion polymerisation which is mainly used in the production of polystyrene latex used in water-based surface coating. 

By this method, the ABS copolymers are obtained by polymerising acrylonitrile and styrene in the presence of polybutadiene latex at 50°C in the presence of initiator and transfer agent. 

The binders vary quite widely—the most common being starch, soy protein and latexes in conjunction with other soluble polymers. Styrene-butadiene latexes have been the most popular but ethylene-vinyl acetate binders are also used. The method of polymer synthesis provides a way of modifying the properties of the latex. For example, adjustment of the ratio of styrene butadiene in the co-polymer gives rise to different degrees of softness or hardness. This property has a profound influence on the quality of the coating. It is also possible to co-polymerise monomers so as to introduce, for example, carboxy groups on to the surface of the latex particle which in turn assist in... 

Recently Biggs [74] has investigated the emulsion polymerisation of styrene using ultrasonic irradiation as the initiation source (i. e. in the absence of a chemical initiator). Similar to Lorimer and Mason using a thermally initiated system, Biggs found both a marked increase in monomer conversion rate as a function of time as the ultrasonic intensity was increased but remarkable constancy in the resultant latex particle... 

Report 158 Geosynthetics, David I. Cook Report 159 Biopolymers, R.M. Johnson, L.Y. Mwaikambo and N. Tucker, Warwick Manufacturing Group Report 160 Emulsion Polymerisation and Applications of Latex, Christopher D. Anderson and Eric S. Daniels, Emulsion Polymers Institute... 

Polystyrene latexes were similarly prepared by Ruckenstein and Kim [157]. Highly concentrated emulsions of styrene in aqueous solutions of sodium dodecylsulphate, on polymerisation, yielded uncrosslinked polystyrene particles, polyhedral in shape and of relative size monodispersity. Interestingly, Ruckenstein and coworker found that both conversions and molecular weights were higher compared to bulk polymerisation. This was attributed to a gel effect, where the mobility of the growing polymer chains inside the droplets is reduced, due to increased viscosity. Therefore, the termination rate decreases. 

Other latexes which have been produced by this method include poly(butyl methacrylate), poly(butyl acrylate) and poly(styrene/DVB) [161]. Additionally, polymer blends were produced by mixing, under high shear, HIPEs of partially polymerised monomer, followed by completion of polymerisation. The conversion prior to blending had to be less than 5%, to allow efficient mixing of the highly viscous emulsions. The materials thus produced resembled agglomerates of latex particles, due to copolymerisation at the points of contact of partially polymerised droplets. 

Water-in-oil concentrated emulsions have also been utilised in the preparation of polymer latexes, from hydrophilic, water-soluble monomers. Kim and Ruckenstein [178] reported the preparation of polyacrylamide particles from a HIPE of aqueous acrylamide solution in a non-polar organic solvent, such as decane, stabilised by sorbitan monooleate (Span 80). The stability of the emulsion decreased when the weight fraction of acrylamide in the aqueous phase exceeded 0.2, since acrylamide is more hydrophobic than water. Another point of note is that the molecular weights obtained were lower compared to solution polymerisation of acrylamide. This was probably due to a degree of termination by chain transfer from the tertiary hydroxyl groups on the surfactant head group. 

Crosslinked polyacrylamide latexes encapsulating microparticles of silica and alumina have also been prepared by this method [179], Three steps are involved a) formation of a stable colloidal dispersion of the inorganic particles in an aqueous solution containing acrylamide, crosslinker, dispersant, and initiator b) HIPE preparation with this aqueous solution as the dispersed phase and c) polymerisation. The latex particles are polyhedral in shape, shown clearly by excellent scanning electron micrographs, and have sizes of between 1 and 5 pm. 

Other polymer materials which can be prepared include latexes, or particle agglomerates, by dispersed phase polymerisation. These can be either hydrophilic or hydrophobic in nature, or may have core-shell morphologies. They can be employed as support materials for a number of catalyst systems. Polymerisation of both phases of the emulsions produces composite materials, which have found use as selective membranes for the separation of mixtures of liquids with similar physical properties. 

A similar seeding technique can be used to prepare monodispersed polymer latex dispersions by emulsion polymerisation (see page 17). 

Note. The results presented in Table I were obtained using solutions of 5g PAA of molecular weight 1 x 10 in 500 cm ethanol and 280 cm water containing lg BzP. All of the polymerisations were performed at 78 C. Conversions to PST in the 8 hours allowed for the experiments was 100% in all cases except that using the highest styrene concentration. In this case a sticky precipitate was produced instead of a latex and the experiment was terminated after 1 hour). 

Fig. 6. This particle aggregate was formed in an octopole field cage. It has six tails of which four are visible here. The cage was 200 pm in diameter and energised by 25 V at 1 MHz. The aggregate is formed of 3.9 pm diameter latex beads levitated and held by the forces in the cage. By alterations of driving regime, different shapes are possible and suitably prepared particles can be crosslinked to form a permanent structure by photo-polymerisation or by chemical means. The bar is 200 pm...

The absence of dimethylallyl-group in NR indicates that the initiating species for rubber formation in Hevea tree is not FDP, but FDP modified at the dimethylallyl-group, which is abbreviated here as (o [103,109,110]. This was confirmed by 13C-NMR analysis of in vitro polymerised rubber by incubation of the bottom fraction of fresh latex and isopentenyl diphosphate (IDP) [111]. The newly synthesised in vitro rubber formed in the presence of FDP and IDP showed the dimethylallyl group derived from FDP. On the other hand, no dimethylallyl group was detected in the in vivo rubber prepared without the addition of FDP [112]. 

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