Chemistry Reference
In-Depth Information
CHAPTER
10
Dispersion and Emulsion
Polymerizations
“Science” means simply the aggregate of all the recipes that are always
successful. All the rest is literature.
—Paul Val ´ ry, Moralit ´ s (1932)
The discussion of free-radical polymerizations in Chapters 8 and 9 focused pri-
marily on homogeneous reaction systems, in which monomer, polymer, and any
solvent were all miscible. This conventional presentation makes it much easier to
grasp the fundamentals of free-radical polymerizations. In fact, however, many
large-scale processes are carried out in heterogeneous systems, because these
offer advantages over alternative procedures. Their overall importance is such as
to justify this chapter describing the effects of process conditions on polymer
properties.
There are two basic types of heterogeneous polymerizations:
1.
The initial reaction medium comprises several phases and polymerization and
occurs in a heterogeneous system, as in emulsion and suspension reactions.
2.
The reaction medium is initially homogeneous and the polymer forms a
separate phase as the polymerization proceeds. Examples are dispersion
reactions, described below, and polymerization of acrylonitrile. Various
polyolefin processes discussed in Chapter 11 are other examples.
Description of suspension polymerizations fits most appropriately in Chapter 12,
where polymer reaction engineering is the topic. This chapter focuses on dispersion
and emulsion polymerizations.
10.1
Dispersion Polymerization
In this process, the monomer and initiator are soluble in the continuous phase and
the polymer particles, which precipitate as they are produced, are stabilized
against coagulation by dispersants that comprise different segments that are
respectively soluble and insoluble in the continuous phase. Dispersion polymeri-
zations have been used successfully as an alternative to solution polymerization
of vinyl polymers for application as surface coatings. In that case the diluents are
usually aliphatic hydrocarbons, and the process acronym is NAD (for nonaqueous
dispersion).
