Environmental Engineering Reference
In-Depth Information
6.2 PolymerizationTechniques
Acrylic resins have been widely used in the organic coatings/adhesive
industry. Depending on the method of polymerization, the copolymers
i nd dif erent applications. Bulk polymerized methacrylates are widely
used as materials in dentistry [34], but when polymerized in solution, they
can be used as varnishes [35], and suspension polymerization results in
acrylic beads [36]. Emulsion polymerization has been an important and
widely used process for the manufacture of polymer products, e.g., paints,
adhesives, coatings, and binders [37-54]. One of the advantages of this
process is the possibility of obtaining polymers of high molecular weight
at a reasonable rate of reaction. h e kinetics and mechanism of conven-
tional emulsion polymerization have been investigated extensively. From
an industrial perspective, one of the major objectives in the operation of
emulsion polymerization processes is that of a faster and safer operation
with consistent quality. h e polymerization reaction proceeds as a classi-
cal double bond addition reaction initiated via a free-radical mechanism
[37-39]. In order to obtain well-dei ned latex, it is important to optimize
the polymerization process with regard to emulsii ers, initiator, and mono-
mers. However, particle size, Tg, surface charge density of latex particles,
and type of monomers change the properties of polymers synthesized by
emulsion polymerization.
h e advantages of the emulsion polymerization process are numerous
and mainly arise from the absence of organic solvents and the compart-
mentalization of the reaction. Water constitutes an inert and harmless con-
tinuous phase, which acts to maintain a relatively low viscosity of the end
product and provide good heat transfer. An additional dif erence between
emulsion polymerization [55], bulk and solution polymerization is that,
in emulsion polymerization, the size of the gel is limited by the size of the
polymer particle. Monomer-swollen micelles may also exist in the reac-
tion system when the concentration of surfactant in the aqueous phase is
above its critical micelle concentration (CMC). Only a small fraction of the
relatively hydrophobic monomer is present in the micelles (if present) or
dissolved in the aqueous phase. h e polymerization proceeds by decompo-
sition of a water-soluble initiator producing the free radicals which propa-
gate in the aqueous phase to form oligomeric radicals by adding a small
fraction of the monomers present in the aqueous phase.
Emulsion polymerization is applicable to a number of hydrophobic
monomers such as butadiene, styrene, MMA, BA, etc. h e emulsion tech-
nique was adopted by several scientists [56-59] to synthesize the crosslinked
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