Chemistry Reference
In-Depth Information
10.4
Emulsion Polymerization Processes
Continuous processes will generally not be used for acrylic surface coating
latexes, adhesives, and other low tonnage products. Semibatch processes are fre-
quently used in which not all the ingredients are added initially. Continuous sys-
tems are favored for large-volume polymers like SBR in order to increase the
reactor output and reduce fluctuations in product properties. Such continuous
reactor trains usually consist of a series of batch reactors each connected with bot-
tom inlets and top outlets and operated with continuous overflow.
Mixtures of anionic and nonionic surfactants are usually employed. The
anionic emulsifiers are the less water soluble and control the number and size of
the particles. The nonionic surfactants are often ethylene oxide condensates of
alkyl phenols; their water solubility is proportional to the degree of polymeriza-
tion of the poly(ethylene oxide) component. Their function is primarily to provide
colloidal stability against electrolytes, mechanical shearing, and freezing.
The target polymerization temperature will usually be chosen to optimize pro-
duction rates or product quality. “Cold” SBR, which is made near 5
C, is an
interesting case in this regard. The cold product is superior as a rubber to hot
(60
C emulsion polymerization) SBR, because it contains less low-molecular-
weight polymer which cannot be reinforced with carbon black. There is also less
branching and more
-1,4 units in the cold SBR. Hot SBR is easier to mill
and extrude because of its low-molecular-weight fraction and is used mostly for
adhesive applications while cold SBR, which is made mainly for tires, accounts
for about 90% of all production of this polymer.
The completion of the polymerization can be slow when the monomer concen-
tration in the aqueous phase is low and the radicals that reach the polymer parti-
cles are mainly fragments of the primary initiator and not oligomers mentioned
earlier in this chapter. Since water-soluble initiators yield hydrophilic radicals
these may be reluctant to enter the polymer particles. This difficulty can be cir-
cumvented by using an initiator in the late stages of the reaction with a more
favorable partition coefficient toward the polymer phase. Alternatively, a como-
nomer that has some water solubility may be added in the last intervals to gener-
ate oligomers that will enter the polymer particles. Vinyl acetate is sometimes
added in the last stages of vinyl chloride emulsion polymerization, for example.
trans
PROBLEMS
10-1
(a) What happens to the rate of emulsion polymerization if more mono-
mer is added to the reaction mixture during interval II polymerization?
(b) What happens to the number average degree of polymerization?
10-2
A particular emulsion polymerization yields polymer with
M
n
5
500000
:
Show quantitatively how you would adjust t
he
operation of a semibatch
emulsion process to produce polymer with
M
n
5
250000 in interval II
