Semi-Batch Emulsion Copolymerization of Vinyl Acetate and Butyl ...

Semi-Batch Emulsion Copolymerization of Vinyl Acetate ... 2615
VAc, BuA is depleted very rapidly due to its high reactivity
ratio, while the polymerization rate of VAc prominently
increases after the consumption of BuA. [1] Generally,
this leads to heterogeneous particle structures with a
soft core, rich in the more hydrophobic butyl acrylate,
and a harder hydrophilic shell, rich in the softer vinyl
acetate. On the other hand, a semi-continuous emulsion
copolymerization process can produce latex particles
[1, 2, 4]
which are nearly homogeneous. Thus, copolymer
composition control in terms of optimal monomer
addition policy has widely been studied. [2, 5, 6] It has been
shown that homogeneous copolymers having a desired
composition can be obtained by feeding the more reactive
comonomer into the reactor according to an optimal
[3, 7]
control law. Semi-batch polymerization can prolong
the nucleation period and result in a decrease of particle
growth rate under monomer starved conditions, leading
to the production of small sized, high solid-content
latexes. These latex products exhibit increased emulsion
viscosity which is frequently desirable in paint
formulations. Moreover, based on the structural differences
of the latex particles, the semi-continuous polymerization
of VAc/BuA has been found to lead to the production
of latexes having improved film forming properties.
[1]
Another important feature in the emulsion copolymerization
of VAc/BuA is the significant difference in the
water solubilities of the two monomers which strongly
affects the respective monomer partition coefficients in
the polymer and water phases. The high water solubility
of VAc indicates that water phase polymerization will be
dominant at low conversions. Thus, particle nucleation
will predominately occur by the homogeneous nucleation
mechanism. [2] At higher monomer conversions the particle
phase volume increases and gradually becomes the
main locus of polymerization. [1, 8] Kinetic studies on the
semi-continuous emulsion polymerization of VAc/BuA
have shown that the number of radicals per particle is
large and, thus, the polymerization does not follow the
classical Smith-Ewart kinetics. [9] The number of radicals
per particle was determined to be between 1 and 7 for
BuA compositions between 20 and 100%. [1] On the other
hand, in batch copolymerizations the number of radicals
per particle was initially large but decreased below 0.5 at
higher monomer conversions after the consumption of
BuA. [1]
In semi-batch emulsion polymerizations the initiator
can initially be charged or/and continuously fed during
polymerization. Increased initiator concentrations affect
not only the polymerization kinetics but also the stability
of the particles by increasing the ionic strength of the
continuous aqueous phase. Vandezande and Rudin [4]
observed a nearly linear relationship between the ionic
strength of the aqueous phase and the latex particle surface
area. Moreover, they reported that the final monomer
conversion increased when a continuous initiator addition
policy was employed.
In general, particle stabilization is achieved by the use
of anionic surfactants, which are extensively employed in
many emulsion polymerization systems. However, the
limited stabilizing effectiveness of anionic surfactants at
high solids (e.g., A40%) and electrolyte concentrations
has led to the development of alternative stabilization
techniques, including the combination of anionic and
nonionic surfactants. [10] VAc/BuA latex particles produced
in batch reactors can have a core-shell morphology.
Such core-shell latex particles are difficult to stabilize
due to poor surfactant adsorption characteristics of
the hydrophilic VAc surface. [11] This problem appears
even with the production of homogeneous particles in
semi-batch reactors and has led to the development of
alternative stabilization methods. These include the use
of functional comonomers containing weak or strong acid
groups (e.g., acrylic acid), the use of steric stabilizers [12]
(e.g., PVOH, HEC, PEG) and the use of polymerizable
surfactants. [13, 14] Unfortunately, the use of alternative stabilization
methods has often led to the appearance of
other problems (e.g., deterioration of film formation
properties). As a result, there is a persisting need for the
development of new surfactants for stabilization of high
solids-content latexes.
A limited number of publications, dealing with the use
of nonionic surfactants for latex stabilization, have
appeared in the open literature. [12] In general, nonionic
surfactants provide stabilization that is lower than that
obtained by anionic surfactant systems, often leading to
the formation of multi-modal and broad particle size distributions.
[10] Vandezande and Rudin [4] described a seeded
semi-batch emulsion polymerization process for the production
of high solid VAc/BuA latexes, using the anionic
surfactant sodium dodecyl sulfate, (SDS). The production
of high solid-content latexes (e.g., up to 55–60%) could
only be achieved by employing a nonionic surfactant in
addition to SDS. However, Vandezande and Rudin
observed a reduction in the polymerization rate in the
presence of a nonionic surfactant, which was attributed to
the inhibition of oligomer radical entry rate by the viscous
surfactant layer. In a similar study, Bataille et al. [9]
employed polyoxyethylene-b-polypropylene nonionic
surfactants with limited success.
In the present work, a new generation of nonionic surfactants
(e.g., alkyl polyglucosides) was employed for
the production of high quality environmentally benign
water-borne VAc/BuA latexes. All emulsion polymerization
experiments were carried out in the presence of alkyl
polyglucosides. The main goal of this work was to investigate
the effect of the surfactant molecular structure and
concentration as well as the initiator addition policy on
the polymerization kinetics, latex stability and particle
size distribution.