Vinyl acetate emulsion polymerization

Buffers are frequently added to emulsion recipes and serve two main purposes. The rate of hydrolysis of vinyl acetate and some comonomers is pH-sensitive. Hydrolysis of monomer produces acetic acid, which can affect the initiator, and acetaldehyde which as a chain-transfer agent may lower the molecular weight of the polymer undesirably. The rates of decomposition of some initiators are affected by pH and the buffer is added to stabilize those rates, since decomposition of the initiator frequently changes the pH in an unbuffered system. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4—5, eg, with phosphate or acetate, but buffering at neutral pH with bicarbonate also gives excellent results. The pH of most commercially available emulsions is 4—6. 

Erbil, H.Y., 2000, Vinyl Acetate Emulsion Polymerization and Copolymerization with Acrylic Monomers, CRC, Boca-Raton, FL. 

The initiators used m vinyl acetate polymerizations are the familiar free-radical types, Buffers are frequently added to emulsion recipes. Vinyl acetate emulsion polymerization recipes are usually buffered to pH 4-5. The pH of most commercially available emulsions is 4-6. 

A Reinvestigation of Vinyl Acetate Emulsion Polymerization Isotope Effect... 

LiTT and chang Vinyl Acetate Emulsion Polymerization... 

The results definitely prove our hypotheses in the kinetic model for vinyl acetate emulsion polymerization (10), that vinyl radical, CH2=C-0Ac, is the major monomer radical formed and is a stable radical which reinitiates relatively slowly compared to the propagation step. 

Chang, K., Litt, M., and Nomura, M., "A Reinvestigation of Vinyl Acetate Emulsion Polymerization (I) - Polymerization Rate". 

Figure 1. Conversion histories of the batch vinyl acetate emulsion polymerizations (similar recipes only the emulsifier concentration is different).

The kinetics of vinyl acetate emulsion polymerization in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly (vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stability, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Breltenbach et al. (12) established that in the case of vinyl acetate emulsion polymerization the dls-pertion rnedlim of the resultant latex contains the amoimt of emulsifier exceeding by a factor of two the critical micelle concentration (CMC), i.e. the reaction system is not depleted of the emulsifier. We obtained similar results in the case of methylacrylate polymerizing with alkyl- and al larylsulphonates. 

In vinyl acetate emulsion polymerization radical desorption is important, i.e. with this monomer 0 and therefore m 0. Typical values of m and a lie in the intervals 10"1 - 10 3 and 10 3 - 10 6 respectively in the early stages of polymerization. 

The purpose of this paper is first to clarify the detailed characteristics of the emulsion polymerization of vinyl acetate using sodium laiiryl sulfate as emulsifier and potassium persulfate as initiator, and second to propose a new reaction model, based on our theory relating to the role of polymer particles, which enedsle vis to predict the number of polymer pcurticles produced and consequently, the progress of vinyl acetate emulsion polymerization. 

The following are suninerized for kinetic behavior of vinyl acetate emulsion polymerization from the data presented here. 

From recent investigations()( 5 ), it is generally concluded that polymerization tcdces place exclusively in the polymer particles in vinyl acetate emulsion polymerization. As a detailed description of the role of polymer particles was presented in the previous paper(4 ), a brief explanation will therefore be reviewed here. 

A quantitative discussion is given in the authors literatures ( )( ) for the theoretical derivation and the application of kf. In vinyl acetate emulsion polymerization, the value of JCf may be calculated by ... 

Table 1 Elementary reactions of vinyl acetate emulsion polymerization and definition of their rates...

Recently, Ugelstad et al. l969i proposed a semiempirtcal rate coefficient for radical desorption in vinyl chloride emulsion polymerization. On the other hand, Nomura et al. (1971, 1976) have derived a rate coefficient for radical desorption theoretically with both stochastic and deterministic approaches and have successfully applied it to vinyl acetate emulsion polymerization. They also pointed out that radical desorption from the particles and micelles played an important role in micellar particle formation, Fiiis et al. 1973 also derived the rate coefficient for radical desorption in a different way. Lift et al. (1981) discussed in more detail the chemical reactions incorporated in the physical process of radical desorption in the emulsion polymerization of vinyl acetate. 

On the other hand, in vinyl acetate emulsion polymerization the value of p. was 1.2 X 10 (Nomura et al., 1976), This value is also about 10 times greater than that predicted hy the diffusion theory. The reason for this may be that radicals have greater difficulty m entering micelles than polymer panicles, or it may be that radicals, having entered a micelle, may escape from the micelle too rapidly to cause initiation, because the micelle has too small a volume. Both factors will decrease the apparjsnt value of k, and hence increase the value of c. Therefore, e can be regarded as a factor that represents the radical capture efficiency of a micelle relative to a particle. ... 

The prinaples of latex reactor design, operation, and control will he illustrated by a consideration of the homopolymerization of styrene and vinyl acetate. Emulsion polymerization of vinyl acetate follows Case 1... 

The function of the chelator is to complex the ferrous ion and thus limit the concentration of free iron. Redox systems appear very versatile, permitting polymerization at ambient temperatures and the possibility of control of the rate of radical initiation versus polymerization time. This would thus permit control of heal generation and the minimization of reaction time. The use of the redox system ammonium persulfate (2 mmol) together with sodium pyrosulfite (Na S Oj 2.5 mmol) together with copper sulfate (0.002 mmol) buffered with sodium bicarbonate in I liter of water form an effective redox system for vinyl acetate emulsion polymerization. The reaction was started at 25 C and run nonisothermally to 70 C. The time to almost complete conversion was 30 min (Warson, 1976 and Edelhauser, 1975). 

Continuous Poly(vinyl Acetate) Emulsion Polymerization Reactors... 

Emulsion Copolymerizations. Due to the good copolymerizability of VEC with vinyl ester monomers, it seemed likely that VEC could be incorporated into a vinyl acetate/butyl acrylate latex. First, it was important to determine if VEC is prone to hydrolysis in the acidic medium used for vinyl acetate emulsion polymerization. As a check, a single experiment was carried out using an acetic acid-sodium acetate buffer at pH=4 and heating for 4 hours at 80°C. In this experiment, 6.1% of the VEC was hydrolyzed to the 3-butene-1,2-diol. Since VEC is only soluble in water up to 3.3 %, it is expected that most of the VEC will be in the oil phase during the emulsion polymerization and that only a small amount will be hydrolyzed. 

By the way, it is interesting to note that the type of poly(vinyl alcohol) most conveniently dissolvable in cold water contains 10-30% residual poly(vinyl acetate). The compatibility of the poly(vinyl acetate) blocks with latex particles may explain the suitability of these grades of poly(vinyl alcohol) in vinyl acetate emulsion polymerizations [145]. 

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