Emulsion and Dispersion Polymerisation

Methods based on the above general principles have been developed for the preparation of monodisperse colloidal dispersions of a variety of metal oxides and hydroxides and of polymer latices. The latter have played a major role in the understanding of colloids and their properties. 

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Descriptions of both emulsion and dispersion polymerisation are given below, with particular reference to the control of their particle size and colloid stabiHty, which is greatly influenced by the emulsifier or dispersant used. Particular emphasis will be placed on the effects of polymeric surfactants that have been recently appHed to the preparation of emulsion polymers. 

Most aqueous emulsion and dispersion polymerisation that have been reported are based on a few commercial products with a broad molecular weight distribution and varying block composition. The results obtained from these studies could not establish the effect that the structural features of the block copolymer would have on their stabilising ability and effectiveness in polymerisation. Fortunately, model block copolymers with well-defined structures can be synthesised, and their roles in emulsion polymerisation have been determined using model polymers and model latexes. 

The mechanism of dispersion polymerisation has been discussed in detail in the book edited by Barrett [11]. A distinct difference between emulsion and dispersion polymerisation may be considered in terms of the rate of reaction. As mentioned above, with emulsion polymerisation the rate of reaction depends on the number of particles formed. However, with dispersion polymerisation, the rate is independent of the number of particles formed. This is to be expected, since in the latter case polymerisation initially occurs in the continuous phase, whereby both monomer and initiator are soluble, and the continuation of polymerisation after precipitation is questionable. Although in emulsion polymerisation the initial monomer initiation reaction also occurs in the continuous medium, the particles formed become swollen with the monomer and polymerisation may continue in these particles. A comparison of the rate of reaction for dispersion and solution polymerisation showed a much faster rate for the former process [11]. 

Preparation by Sequential Polymerisation. Two-polymer composite latex particles may be prepared using either emulsion or dispersion polymerisation techniques. A dispersion (latex) of particles of a first polymer may be prepared in the usual manner after complete conversion of monomer to polymer, a different monomer or monomer mixture is added and polymerised to provide the second polymer. 

Sequential polymerisation may be operated by producing particles of the first polymer by either emulsion or dispersion polymerisation and by adding the monomer for the second polymer together with free-radical initiator at a slow and controlled rate such that the rate of addition and the rate of polymerisation are equal. The amount of free monomer remains at a low level throughout the process. This monomer-starved process (5) has been used with an aqueous continuous phase in many studies. Samples may be taken at regular intervals to... 

Dispersion polymerisation may be considered a heterogeneous process which may include emulsion, suspension, precipitation and dispersion polymerisation. In dispersion and precipitation polymerisation, the initiator must be soluble in the continuous phase, whereas in emulsion and suspension polymerisation the initiator is chosen to be soluble in the disperse phase of the monomer. A comparison of the rates of polymerisation of MMA at 80 C for the three systems was given by Barrett and Thomas [11], as illustrated in Figure 17.10. The rate of dispersion polymerisation is much faster than either precipitation or solution polymerisation. TTie enhancement of the rate in precipitation polymerisation over... 

Functional oligomers with a terminal alpha-substituted acrylate group can be synthesised by catalytic free-radical chain transfer polymerisation based on cobalt II or II chelates. The apphcations of such oligomers in the design of low molec.wt., graft and block copolymer emulsions and dispersions for waterborne, two-component PU paints are reviewed. The emulsions and dispersions are shown to have composition and molec.wt. control and to exhibit... 

The book is focusing on emulsion polymerisation in combination with both conventional and controlled radical polymerisation. Except for miniemulsion polymerisation, more exotic techniques, such as inverse emulsion polymerisation, microemulsion polymerisation and dispersion polymerisation are not covered. Chapter 1 gives a historic overview of the understanding of emulsion polymerisation, while also focusing on the solution of the... 

Commercial Polymerisation Methods. Aqueous media, such as emulsion. suspension, and dispersion polymerization, are by far [he most widely used in the acrylic fiber industry. Water acts as a convenient hcai-iranslcr... 

Alkyl sulphates are used industrially as wetting and dispersing agents, and also in emulsion polymerisation. 

However, in a recent publication, Shirinyan, Mnatsalianov, et al. (20) find that differences between the rates of vinyl acetate emulsion polymerisation observed with samples of similar polyvinyl alcohols manufactured by the same process In three different factories could be attributed to a condensation product of acetaldehyde derived from hydrolysis of residual vinyl acetate this gave rise to a conjugated ketone type ultra-violet spectrum and could be extracted from the polyvinyl alcohol under suitable conditions. This could be the uncontrolled factor which appears to have confounded nmuiy of the experiments reported here. Even more recently the same laboratory ( ) has reported that there Is an optimum sequence length of hydroxyl groups in the polyvinyl cdcohol-acetate block copolymer for polymerisation rate and dispersion stability. 

Generally speaking, three methods are available for the preparation of polymer dispersions, namely emulsion, dispersion, and suspension polymerisation ... 

Micro-emulsion is another variant of emulsion polymerisation. Such emulsions are thermodynamically stable systems including swollen monomer micelles dispersed in a continuous phase. In general, they require fairly large concentrations of surfactants to be produced compared with the other dispersed polymerisation systems. Hence, the interfacial tension of the oil/water is generally close to zero. Polymers with ultra-high molecular weight, i.e. above 10 g/mol, can be obtained, as can copolymers with a very well-defined, homogenous composition. Whereas polymerisation can take 24-48 h in the normal emulsion process, it proceeds at a fast rate in micro-emulsion, as total conversion can be obtained in less than 30 min. Polymer particles of very small size (diameter < 100 nm) and narrow distribution can be obtained by this process. 

Polymerisation in disperse media takes pride of place in production of polymer materials. This is because polymers can be obtained as dispersions of particles (latexes) and these are often used in industry. Terminology is adopted from the study of emulsions. A normal latex is a system composed of hydrophobic polymer particles dispersed in water. A reverse latex is a system in which water-soluble polymer particles are swollen with water and dispersed in an organic phase. 

Ruckenstein and Sun [89] have used inverted emulsion polymerisation for the synthesis of PANI rubber composites using an isooctane-toluene mixture and water to form the emulsion and using ammonium persulfate as the oxidant. Inverse emulsion polymerisation consists of an aqueous solution of the monomer, which is emulsified in a non-polar organic solvent and the polymerisation is initiated with an oil-soluble initiator. The reaction is carried out in a heterogeneous system in which the reaction takes place in a large number of reaction loci dispersed in a continuous external phase. 

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. 

The formulation, or the chemical composition, is one major factor affecting the form and properties of the final product. Some components are added to the recipe before or during polymerisation. Some are added after polymerisation (post-additives) to modify the latex properties. There are two phases in an emulsion the dispersed (oil) phase containing the monomers and other monomer-soluble components, and the continuous (aqueous) phase containing water-soluble components. The individual components that comprise the oil and aqueous phases in an emulsion, and their functions, are described below. 

Monomers are of principal interest in emulsion polymerisation, and must be chosen based on the performance requirements of the intended application. Cost is another critical factor in the selection of an appropriate monomer. The monomer cannot be completely miscible with the water phase (otherwise it would be a dispersion polymerisation), nor can it be completely insoluble (or polymerisation by conventional emulsion polymerisation could not proceed). Most monomers are sparingly soluble in water and fall within these guidelines. Typical monomers used in emulsion polymerisation processes are the styrenics (149, 353), acrylics (141), methacrylics (309), vinyl acetate (164), vinyl chloride (363), acrylonitrile (152), butadiene (307), ethylene (114), as well as various speciahty (100) and functional monomers (332). 

This series of articles focuses on the polymerisation processes used to prepare polymers and resins utilised in the coatings industry. This article concerns latexes and emulsion polymerisation. Surfactants and dispersion stabilisation mechanisms, emulsion polymerisation ingredients and processes, particle nucleation, particle... 

Emulsion polymerisation. It makes use of a dispersant medium, which is generally water the monomer is dispersed in the form of droplets in water with the aid of a surfactant. At the end of the reaction, the polymer is present as an emulsion or a latex. This process combines the advantages of the bulk and solution polymerisations, such as high polymerisation rate, control of the reaction temperature, production of polymers with high molecular weights. 

An alternative approach for producing latex with a wide particle size distribution is microsuspension polymerisation. In this process, an initiator such as lauroyl peroxide is used, which is highly soluble in the VCM, but is essentially insoluble in water. Thus, polymerisation takes place within the dispersed VCM droplets. The water insolubility of the initiator also helps to stabilise the VCM droplet, and it may be possible to use lower levels of emulsifier compared with the batch emulsion and continuous emulsion processes. Lower levels of emulsifier can be advantageous, for example for applications coming into contact with food, where water absorption or clarity is important, and also for the environmental impact of the proccess. Such latexes produce polymers which give very low plastisol viscosities, but tend to be dilatant in nature. This can be overcome by modifying the process to have a secondary particle size distribution alongside the primary one. 

Poly (vinyl chloride) (PVC) is one of the highest tonnage polymers produced on a world-wide scale. From the beginning of production over 40 years ago, growth of PVC sales into a wide variety of applications has expanded rapidly, reaching an annual world consumption of 1.2 x 10 tes per annum in 1980 (1). PVC is prepared by four principal polymerisation processes bulk or mass, suspension, microsuspension and emulsion. In addition PVC may also be prepared by solution or dispersion polymerisation, but these remain low tonnage specialised products. 

Acid (or amine) functional emulsion polymerised resin is neutralised and dispersed/solubilised in water. These are sometimes referred to as alkali thickening emulsions (in the case of an acid functional emulsion). 

An ab initio emulsion polymerisation involves the emulsification of one or more monomers in a continuous aqueous phase and stabilisation of the droplets by a surfactant. In a seeded emulsion polymerisation, one starts instead with a preformed seed latex. Usually, a water-soluble initiator is used to start the free-radical polymerisation. The locus of polymerisation is within the submicron polymer particles (either formed during the process or added at the start), which are swollen with monomer during the polymerisation process, and dispersed in the aqueous phase. The final product is a latex comprising a colloidal dispersion of polymer particles in water. Ab initio emulsion polymerisation differs from suspension, mini- and microemulsion polymerisations in that the particles form as a separate phase during the polymerisation process. The particle size is much smaller than those formed in a suspension polymerisation. 

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