Chemistry Reference
In-Depth Information
Microsuspension polymerization is an alternative technique that can yield parti-
cles in the same size range as emulsion processes. This method uses a monomer-
soluble initiator and anionic emulsifiers similar in nature and concentration to
those used in emulsion polymerizations. A microdispersion of the mixture of the
reaction ingredients is first produced mechanically and is then polymerized to
provide polymer with essentially the initial fine particle size distribution.
Emulsion polymerization reactors are made of stainless steel and are normally
equipped with top-entry stirrers and ports for addition of reactants. Control of the
reaction exotherm and particle size distribution of the polymer latex is achieved
most readily by semibatch (also called semicontinuous) processes, in which some
or all of the reactants are fed into the reactor during the course of the polymeriza-
tion. Examples are given in Chapter 10. In vinyl acetate copolymerizations, a con-
venient monomer addition rate is such that
it keeps the vinyl acetate/water
azeotrope refluxing, at about 70
C.
Vinyl acetate and acrylic emulsion copolymers usually contain significant pro-
portions of insoluble “gel” material. This fraction results from chain transfer to poly-
mer (Section 8.8.4). It is not deleterious in products like surface coatings and
adhesives, and may even confer some advantages, like faster drying after application
to substrates. The molecular weight distributions of such polymers are not of practi-
cal interest, since insoluble material has infinite molecular weight, on the scale of
the methods summarized in Chapter 3. However, the properties of these latexes are
affected by their particle size distributions. Industrial-scale emulsion polymeriza-
tions are characterized by variable initial induction periods, as the inhibiting effects
of dissolved oxygen in the water feed (Section 8.9) are overcome by decomposing
initiator. (It is more economical to waste initiator for this purpose than to eliminate
dissolved oxygen in large volumes of water by sparging with an inert gas like nitro-
gen.) As a consequence, however, it is very difficult to produce polymers with con-
sistent particle size distributions, by starting emulsion polymerizations with a
charge of water, monomer, surfactants, and the other ingredients listed in Chapter 10.
Particle sizes of latex polymers are neatly controlled, however, by including a small
quantity of “seed latex” in the initial charge to the reactor. The seed latex has an
appropriate small particle size that has been measured before hand. The polymer
emulsion is grown on the seed latex, controlling the feed rate of other reactants as
outlined in Chapter 10, in connection with the production of “core-shell” particles.
The important factors here are the particle size distribution of the seed latex and its
availability for a large number of seeded polymerizations. The seed polymer need
not even have the same chemical composition as the final polymer.
12.4.2.5
Gas Phase Polymerizations
Transition metal catalysts that produce high yields of olefin polymers per unit
weight of catalyst metal were mentioned in Section 11.5.5. In the gas phase poly-
ethylene processes, ethylene is polymerized or copolymerized in a solvent-free
fluidized bed reactor. (Fluidized beds are suspensions of solid particles in fast-
moving gas streams. Major applications are in hydrocarbon cracking and other
