Page 130 - Engineering Plastics Handbook

104 Engineering Plastics

Figure 6.2 TEM of crazes in ABS.

microscopy (TEM). (See Fig. 6.2.) The major fracture mechanism of HIPS
is craze with the optimum particle size in the range of 2 to 6 µm, whereas
in the case of ABS resins the mechanism is shear yielding that is maximized
with very small particles (0.1 µm).
The procedures for manufacturing ABS resins can be largely classified
into the mass polymerization process and the emulsion polymerization
process. In the case of the mass polymerization process, the size and struc-
ture of particles can be controlled through change in viscosity of continu-
ous and discontinuous phases, change in stirring speed, change in
molecular weight, and change in added amount of polybutadiene rubber.
The typical particle size is about 1 to 10 µm.
The high impact resistance of ABS resins is generally obtained with
the particle size below 1 µm, which can be readily obtained by the emul-
sion polymerization process. Therefore, currently ABS resins are mostly
produced by the emulsion polymerization process.
The emulsion polymerization produces ABS resins having rubber par-
ticles below 1 µm, from which characteristic features of high gloss and
surface distinction result, whereas by mass polymerization the resins
having large particles are prepared, and, therefore, low-gloss products
are obtained and rubber efficiency is increased.

Emulsion polymerization process
In the emulsion process, ABS resins are obtained by addition of the initia-
tor, the molecular weight regulator, the emulsifier, etc., to polybutadiene