Anionic emulsion polymerisation

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. 

Dodecyl acrylate microspheres, crosslinked with hexanediol diacrylate, were prepared by emulsion polymerisation and characterised by scanning electron microscopy, particle size measurement, and differential scanning calorimetry. The microspheres were doped with 9-(diethylamino)-5-(octadecanoylimino)-5H-benzo(alpha)phenoxazine (ETH 5294), a chromoionophore whose fluorescence emission spectrum is sensitive to pH. These microspheres adsorb anions selectively according to their lipophilicity, and therefore can be used as anion-selective optical sensors. The responses of the microspheres to chloride, nitrate and perchlorate ions were reported. 27 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 preparation of SAN latexes by micro-emulsion polymerisation using sodium dodecyl sulphate as an anionic surfactant. Kinetics of copolymerisation was studied at different temperatures using different concentrations of potassium persulphate and hydrogen peroxide/ascorbic acid. The latexes were characterised for particle size and number of particles by dynamic light scattering and TEM. Products were examined using NMR and thermal analysis. 20 refs. INDIA... 

PS latexes were prepared by emulsion polymerisation and characterised according to concentration, density, pH, ionic strength, particle size, particle size distribntion and surface charge. For latexes prepared using anionic surfactants the effects of temperature, initiator concentration, surfactant concentration and amonnt of monomer on the latex size were investigated. 46 refs. IRAN... 

It is well-known that viscosity, self-diffusion coefficient and glass transition temperature are a strong function of molecular weight (MW). The film formation characteristics of an acrylic (BM A/MM A/M AA) latex as a function of its average MW is reported. The latex with the high MW is compared to an identical formulation with a low MW. The latex dispersions are prepared by standard techniques of emulsion polymerisation using an anionic surfactant, ammonium dodecyl benzene sulphonic add, and ammonium persulphate... 

EMULSION POLYMERISATION OF STYRENE STABILISED BY MIXED ANIONIC AND NONIONIC SURFACTANTS Chem C S Lin S Y Chen L J Wn S C Taiwan,National Institnte of Technology Taiwan,National University... 

It is always convenient to control the heat evolved during polymerisation and this is achieved when the reaction takes place in a solvent. It is possible to control the heat transfer by solution, suspension and emulsion polymerisations [5]. A short discussion of emulsion polymerisation will be given next. In this polymerisation technique, the initiator must be soluble in water. A common initiator for this purpose is persulfate-ferrous initiator which yields a radical sulfate anion through the reaction shown in Reaction 6.9. 

Emulsion polymerisation produces latex particles in the size range from 0.03 to 0.1 pm. The process comprises monomer + initiator + solvent (usually water) + surfactant (usually anionic, e.g. sodium dodecyl sulphate). 

The monomer has only a very limited (but finite) solubility in the solvent (e.g. styrene in water). Most of it is present initially in dispersed droplets (hence the term emulsion polymerisation) one role of the (anionic) surfactant is to help stabilise these droplets, by adsorbing at the droplet/water interface. However, some of the monomer is present in the water phase. 

Low degrees of ethoxylation tend to render the surfactant oil soluble, whilst higher levels confer water solubility. The use of non-ionic surfactants may sometimes lead to the formation of small aggregates or grainy emulsions. This tendency is due to the weaker surface activity and the relative difficulty with which non-ionic surfactants form micelles. As a result of these deficiencies non-ionic/anionic surfactant mixtures are normally employed in emulsion polymerisation. The ionic component allows easy solubilising of the monomer whilst the non-ionic component confers emulsion polymer stability. 

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. 

S-SBR made from initiators other than anionic has a broader MW distribution, which depends upon the polymerisation conditions. The branching probability is much less than that in the emulsion polymerisation primarily because of the lower local concentration of polymer. The mechanisms of the long branch-formation are different for different initiator (catalyst) systems. This aspect will be discussed with butadiene (BR) polymerisation. 

The stability improvement of Ti02 suspensions is important not only for water-based paints, but also for paints based on non-polar or low-polar solvents. It is shown in [208] that Ti02 powders modified with an anionic surfactant, e.g. sodium dodecyl sulphate, are dispersed to smaller sizes, and their sedimentation stability increases. The production of water-alkyd emulsions is inhibited due to low mechanical stability. These emulsions can easily break when exposed to shear forces such as those produced by pumps, and when intensively agitated during dispersion. [209] demonstrates that most stable emulsions can be obtained with alkyds showing high acid numbers, as well as with highly polymerised alkyds of low viscosities. 

Styrene-butadiene latices were prepared by emulsion-free polymerisation in the presence of N,N-diethylaminoethyl methacrylate to supply a positive charge and their colloidal behaviour and interaction with anionic pulp fibres investigated. It was found that the latices were positively charged and stabilised by electrostatic repulsion and deposited readily on anionic fibres suspended in water, forming a monolayer on the fibre surface. Dewatering and drying resulted in coalescence of the particles and fibres covered with a polymeric film, which improves the bonding between the fibres. 9 refs. CANADA... 

The polymerisation and the properties of the latexes depended sensitively on the emulsifier and on the charge of the initiator. There was no visible correlation between the properties of the final latexes and the properties of the emulsifiers such as surface activity, solubilisation capacity, or the ability to stabilise the initial monomer emulsion. When a cationic (2,2 -dimethyl-2,2 -azo-N-benzylpropionamidine hydrochloride, VA-552 from Wako Chemicals) initiator was employed, all emulsifiers lead to stable monodisperse latexes, except for polysoaps with low hydrophobe content. The polymerisable, as well as the polymeric emulsifiers, yielded latex solutions with very high surface tensions, different from the use of the standard surfactant. In contrast, the use of an anionic initiator (potassium persulphate) can pose difficulties. The... 

The particle size of the monomer is reduced to colloidal dimensions by more vigorous stirring and use of synthetic surfactants (anionic, cationic or non-ionic) in place of protective colloids used in sus )ension polymerisation. Usually water soluble catalysts such as persulphate or hydrogen peroxide are u.sed. Thermal dissipation and viscosity problems are absent. Both the rate of polymerisation and the molecular weight of the product formed arc very high. The product, which is in the fonn of fine particles dispersed in water (called latex), can used directly as adhesive or an emulsion paint, or it can be isolated by coagulating with an electrolyte. The technique is employed for the industrial production of PVC,... 

The anionically initiated solution polymerisation methods also have the virtue of considerable flexibility with much greater scope than with the emulsion polymers for ringing the changes in their molecular architecture. This scope was demonstrated by Cooper and Nash who described the various techniques for making polymers of six different types of molecular architecture, namely ... 

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