Emulsion polymerization stages

Core/shell latexes refer to systems with a submicroscopic particle morphology of one polymer forming the center part (the core) and the other polymer covering the core (the shell layer). Core/shell latexes are made via two consecutive emulsion polymerization stages, usually forming a particle structure with the initially polymerized material at the center and the later-formed polymer as the outer layer. If more than two stages are employed in the emulsion polymerization process, latex particles with multilayered morphology can be obtained. 

Emulsion polymerization is commonly used to produce film-forming polymers, and hence it is generally carried out at temperatures above the Tg of the polymers. Therefore, propagation does not become diffusion-controlled [64], and the kinetics of post-polymerization is not influenced by the reaction temperature differently than the emulsion polymerization stage [65]. 

Although there are other nudeation mechanisms available such as homogeneous nudeation, most commerdal emulsion polymerization uses micdlar nudeation. The poor reprodud-bility of stage I in the commerdal practice is responsible for poor batch-to-batch product quality control. To overcome this variability, polymer seed partides are often used. These small polymer partides at 30-50 nm are prepared by emulsion polymerization. Stage 1 is eliminated in the seeded emulsion polymerization. 

Figure 7.3 The overall polymerization process in emulsion polymerization. Stage [...

Kinetics and Mechanisms. Early researchers misunderstood the fast reaction rates and high molecular weights of emulsion polymerization (11). In 1945 the first recognized quaHtative theory of emulsion polymerization was presented (12). This mechanism for classic emulsion preparation was quantified (13) and the polymerization separated into three stages. 

The quahty of the water used in emulsion polymerization has long been known to affect the manufacture of ESBR. Water hardness and other ionic content can direcdy affect the chemical and mechanical stabiUty of the polymer emulsion (latex). Poor latex stabiUty results in the formation of coagulum in the polymerization stage as well as other parts of the latex handling system. 

A kinetic model for the particle growth stage for continuous-addition emulsion polymerization has been proposed (35). Below the monomer... 

The progression of an ideal emulsion polymerization is considered in three different intervals after forming primary radicals and low-molecular weight oligomers within the water phase. In the first stage (Interval I), the polymerization progresses within the micelle structure. The oligomeric radicals react with the individual monomer molecules within the micelles to form short polymer chains with an ion radical on one end. This leads to the formation of a new phase (i.e., polymer latex particles swollen with the monomer) in the polymerization medium. 

With some limitations. Single-stage soapless emulsion polymerization can be used for the direct copolymerization of a relatively apolar monomer with a polar... 

DSEP direct soapless emulsion polymerization, SSEC seeded soapless emulsion copolymerization, DDC direct dispersion copolymerization, TDSC two-stage dispersion copolymerization, ATES Allyl trietoxysilane, VTES vinyl trietoxysilane, DMAEM dimethylaminoethyl-methacrylate, CMS chloromethylstyrene, GA glutaraldehyde, AAc Acrylic acid Aam Acrylamide HEMA 2-hydroxyethylmethacrylate. 

PS/PHEM A particles in micron-size range were also obtained by applying the single-stage soapless emulsion copolymerization method [124]. But, this method provided copolymer particles with an anomalous shape with an uneven surface. PS or PHEMA particles prepared by emulsifier-free emulsion polymerization were also used as seed particles with the respective comonomer to achieve uniform PS/PHEMA or PHEMA/PS composite particles. PS/PHEMA and PHEMA/PS particles in the form of excellent spheres were successfully produced 1 iLitm in size in the same study. 

One of the longest known synthetically prepared surfactants are the fatty alcohol sulfates, which were prepared on technical scale before 1940. Along with their ethoxylated counterparts, the fatty alcohol ether sulfates, which appeared on the stage shortly after, their use in toiletries is very popular but they can also be found in products for textile industry and auxiliaries in emulsion polymerization. With the exception of soaps, the mentioned anionic surfactants all have a sulfur-containing functional group. Denying the differences between these, their skin irritancy potential is remarkably high. 

Artificial control of the monomer concentrations is possible by changing the monomer feed methods, which includes multishot, stage feed (19), and continuous feed. A multishot emulsion polymerization is expected to form multilayered particles if the monomers are chosen properly. When the layers are sufficiently thin, the particles exhibit unique thermal and mechanical properties. The stage feed system is shown in Figure 11.1.6. It makes it possible to produce particles having gradient composition of different monomer units. 

First stage of polymerization emulsion polymerization of styrene, hydroxye-thyl methacrylate, and acrylic acid in the presence of isooctane. 

ASA structural latexes have been synthesized in a two stage seeded emulsion polymerization. In the first stage, partially crosslinked poly(n-butyl acrylate) and poly( -butyl acrylate-sfaf-2-ethylhexyl acrylate) rubber cores are synthesized. In the second stage, a hard styrene acrylonitrile copolymer (SAN) shell is grafted onto the rubber seeds (16). 

Recent microscopical studies by Thomas, Thomas and Deichert (130) lend support to view that these systems share some features of emulsion polymerization. During the early stages of polymerization the particles grow as rather uniform aggregates (Fig. 1). The number of these particles is fairly constant and is in the range of 1011 to 1012 per cc. This is approaching the number found in typical emulsion systems. 

Core-shell polymers were commercially introduced as impact modifiers for poly(vinyl chloride) PVC, in the 1960s. They are produced by a two-stage latex emulsion polymerization technique (Cruz-Ramos, 2000). The core is a graftable elastomeric material, usually crosslinked, that is insoluble in the thermoset precursors. Typical elastomers used for these purposes are crosslinked poly(butadiene), random copolymers of styrene and butadiene,... 

Thus monomers such as styrene, isoprene and octyl acrylate, which have low solubilities in water, nucleate and grow very slowly during the early stages of an emulsion polymerization, whereas more soluble monomers such as methyl acrylate, vinyl acetate and acrylonitrile give rapid rates. In the presence of surfactant micelles the less soluble monomers are solubilized (11), so that they nucleate and grow more rapidly. 

The preparation of a latex by emulsion polymerization comprises two stages (i) particle nucleation (ii) particle growth. For the latex to be monodisperse, the particle nucleation stage must be short relative to the particle growth stage. Despite many investigations, there is disagreement as to the locus of particle nucleation (i) monomer-swollen emulsifier micelles (ii) ad-... 

In continuous emulsion polymerization of styrene in a series of CSTR s, it was clarified that almost all the particles formed in the first reactor (.2/2) Since the rate of polymerization is, under normal reaction conditions, proportional to the number of polymer particles present, the number of succeeding reactors after the first can be decreased if the number of polymer particles produced in the first stage reactor is increased. This can be realized by increasing emulsifier and initiator concentrations in the feed stream and by lowering the temperature of the first reactor where particle formation is taking place (2) The former choice is not desirable because production cost and impurities which may be involved in the polymers will increase. The latter practice could be employed in parallel with the technique given in this paper. 

Our final goal in the present paper is to devise an optimal type of the first stage reactor and its operation method which will maximize the number of polymer particles produced in continuous emulsion polymerization. For this purpose, we need a mathematical reaction model which explains particle formation and other kinetic behavior of continuous emulsion polymerization of styrene. 

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