Page 133 - Engineering Plastics Handbook

Acrylonitrile-Butadiene-Styrene (ABS) Resin 107

the reaction are added. Then the reaction progresses at elevated tem-
perature. The emulsifier that can be used in this reaction is identical
to that used in preparation of polybutadiene latex, and the oil-soluble
initiator using the redox system is utilized as the initiator in this
reaction.
The ratio between polybutadiene latex and styrene and acrylonitrile
monomers is controlled according to the purpose of use, and is generally
in the ratio of 1:1 by weight. The ratio between styrene and acrylonitrile
monomers is controlled according to the purpose of use, and is generally
in the ratio of 7:3 by weight. The used amounts of molecular weight reg-
ulator, initiator, and activator determine the molecular weight of the final
ABS resin. Thus, when they are used in a small amount, the phenomenon
of lowering the polymerization conversion rate may occur.
The amount of styrene/acrylonitrile copolymer to be grafted onto polybu-
tadiene latex is increased as the particle size of the polybutadiene latex is
increased, but is decreased as the cross-linking density is increased.
According to such change in grafting rate the properties of the final
ABS resins are also changed. The kinds of initiator also greatly influ-
ence the grafting rate. The water-soluble initiator such as KPS allows
grafting of styrene/acrylonitrile copolymer on the outside, rather than
the inside, of polybutadiene latex, whereas the oil-soluble initiator allows
grafting of styrene/acrylonitrile copolymer on the inside, rather than the
outside, of polybutadiene latex. The reaction time is usually 4 to 6 h, and
upon completion of the reaction the resulting product is transported to the
coagulation and drying steps.

Coagulation and recovery
After the reaction is completed, the solution containing a certain amount
of antioxidant is added to the resulting ABS latex, to prevent the oxida-
tion of ABS during the coagulation and drying steps. A phenolic-based
antioxidant can be mainly used, but a phosphate-based antioxidant may
also be used. The amount of antioxidant generally used is about 1 part by
weight with respect to 100 parts of ABS resins by weight. The ABS latex
to which the antioxidant is added is transported to the coagulation cham-
ber at high temperature.
As a coagulator for the coagulation step, the dilution of acids or the
dilution of CaCl , etc., may be used. The particle size distribution of coag-
2
ulated ABS powder is very important, and therefore, it is kept at the
desired level by the added amount of coagulator, coagulating tempera-
ture, residence time in the coagulation chamber, stirring condition, etc. The
particle size is decreased as the coagulating temperature is lowered. In
addition, as the stirring speed (rpm) is increased, the particle size is
decreased and the particle distribution is also narrowed. Long residence