Chemistry Reference
In-Depth Information
second-stage polymer was produced by batch polymerization in the presence of
the first.
Similarly, when intermingling of the core and shell polymers is required, in
order to provide mechanical interlocking of one polymer with the other, then it is
advantageous to let some of the second monomer soak into the first-stage parti-
cles before starting polymerization. This may be followed by semibatch polymeri-
zation of more second-stage monomer in order to produce the desired outer shell
[27]
.
It is possible to lock the polymer phases into a particular core-shell configura-
tion by introducing a sufficient level of cross-linking. This is not always feasible,
of course, if this results in other undesired changes in polymer properties.
Conversely, the inclusion of solvents or plasticizers will facilitate inversion of
second-stage polymer into the first-stage material, if this produces a lower energy
state. Not all such phase inversions are clear-cut. In many cases this phenomenon
will be observed as penetration of shell polymers into the core polymer.
Unreacted monomer from stage one polymerization will act as a plasticizer to
lower the kinetic barrier to such rearrangements. Use of a chain transfer agent
that lowers polymer molecular weight and viscosity also facilitates phase
rearrangements.
An approach to raising the kinetic barrier to stage 2 inversion involves lower-
ing the polymerization temperature of the second stage. This will reduce the
mobility of the first-stage polymer and retard phase mixing.
10.3
Other Ingredients in Emulsion Recipes
The essential ingredients in an emulsion polymerization are the water, monomer,
surfactant, and free-radical source. Other ingredients are frequently added for a
variety of reasons. Stabilizers, which are usually water-soluble high polymers or
carbohydrate gums, are employed to control latex viscosity and freeze
thaw sta-
bility of products that are used in the latex form.
Chain transfer agents are used to control the molecular weights of polymers
like butadiene copolymers, which are subsequently isolated and processed.
Molecular weight control is not normally needed for latexes which are used
directly as surface coatings or adhesives. The most important class of transfer
agents are aliphatic mercaptans, and these are used particularly with hydrocarbon
monomers that do not transfer readily with the corresponding macroradicals. The
mercaptans used are C
8
C
16
species which are soluble in the monomer. The rate
of consumption of the transfer agent depends partly on its reactivity with the par-
ticular polymeric free radical and partly on the rate of diffusion of the mercaptan
from the monomer droplets to the polymer particles where polymerization is tak-
ing place. The hydrocarbon chain length of the mercaptan transfer agent is
selected to control this diffusion rate at a desirable level. If this is not done most
of the mercaptan would be consumed in early portions of the polymerization.
