Page 132 - Engineering Plastics Handbook

106 Engineering Plastics

Preparation of polybutadiene latex
The reaction is conducted by introducing the butadiene monomer, emul-
sifier, initiator, and chain transfer agent into the batch reactor and then
elevating the temperature while stirring the reaction mixture. The ratio
of butadiene to water is generally about 1:1. The emulsifier is added in the
ratio of about 1 to 5 parts with respect to 100 parts of butadiene monomer,
and it is used in the form of the salt of rosin or fatty soap. After the graft-
ing reaction, the emulsifier, which can have the agglomerating property
for acid or salt, is used.
As the reaction initiator, water-soluble initiators such as KPS or oil-
soluble initiators using a redox system can be used. The use of oil-soluble
initiator can provide high reaction rate at relatively low temperature com-
pared to the use of water-soluble initiator.
The particle size can be controlled by the ratio between monomer and
water as used, reaction temperature, etc. The chain transfer agent is used
to control the cross-linking density of polybutadiene rubber, which greatly
influences the properties of the final ABS polymer. In general, the cross-
linking density is controlled to be at least 60%. After completion of the reac-
tion, unreacted butadiene monomer is recovered and reused in the
polymerization reaction.
Although in preparing polybutadiene latex the desired particle size can
be achieved through polymerization, particles having the desired size
may be prepared by polymerization of small-size particles followed by
latex agglomeration. The chemical agglomeration uses an acid or an acid
oligomer, and the physical agglomeration utilizes a homogenizer or a
freezer. For most cases, the chemical agglomeration is utilized. Such a
method is more advantageous than direct polymerization with respect
to production efficiency, and it is used by many manufacturing compa-
nies. It can produce various kinds of polybutadiene latex, even in small
equipment.

Preparation of ABS latex
Generally, ABS resins are produced in the form of a bimodal type comprised
of two kinds of particle sizes for better surface and impact resistance prop-
erties. In this case, either (1) the graft polymerization is also accomplished
in the concurrent presence of two kinds of polybutadiene latex having dif-
ferent particle sizes, or (2) respective polybutadiene latexes, after the graft
polymerization is accomplished, may be mixed in a certain ratio to prepare
the product. The particle size is generally in the range of 0.1 to 0.4 µm.
Polybutadiene latex comprising large and small particles is intro-
duced in a graft polymerization reactor. Styrene and acrylonitrile
monomers are added in a certain quantity to the reactor, and the molec-
ular weight regulator, initiator, activator, and emulsifier as required for