Page 375 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

PLASTICS ADDITIVES 5.55

the ratio of the two polymers in the blend. Miscibility depends on equal polarity or mutual
attraction such as hydrogen bonding or cocrystallization. This is not very common, but
there are several important examples of such completely miscible blends. It gives the com-
pounder simple straightforward control over balance of properties.

5.11.2 Practical Compatibility
Most polymer pairs are too dissimilar for complete miscibility. They reject each other and
separate into two or more phases. Generally, the major polymer forms a continuous matrix
phase and retains most of its original properties. The minor polymer separates into dis-
persed “domains” and may affect certain specific properties. When the domains are ex-
tremely fine, sensitive properties may detect the phase separation, but many practical
properties may resemble homogeneous single-phase blends. When the domains are larger
in size, they will have distinct effects on certain specific properties; when these effects are
beneficial, the blend is described as theoretically immiscible but practically compatible.
When the domains are too coarse, most properties will suffer, and the blend is described as
incompatible.

5.11.3 Interface/Interphase
In multiphase polyblends, a critical factor is the interface between the phases. If the two
polymers reject each other and separate into phases, they are likely to reject each other at
the interface as well. Such a weak interface will fail under stress, and most properties will
suffer. Thus, most polymer blends are practically incompatible. Yet, most successful com-
mercial polyblends are multiphase systems. This means that there must be a mechanism to
strengthen the interface.
In some cases, the two polymers have some partial miscibility, so the interface is not a
sharp separation of one polymer from the other but, rather, a modulating solution of the
two polymers in each other, offering a gradual interphase rather than a sharp interface.
Such an interphase can modulate properties gradually from one phase to the other and thus
reduce the stress.

5.11.4 Compatibilizers
In most cases, it is necessary to add a compatibilizing agent to strengthen the interface. In
basic research, the preferred compatibilizing agent is a diblock copolymer, with one block
soluble in one phase and the other block soluble in the other phase. The block copolymer
tends to locate at the interface. This creates primary covalent bonds across the interface
and thus strengthens it. In commercial practice, the compatibilizing agent is usually a graft
copolymer, with a backbone soluble in one phase and side-chains soluble in the other
phase; this is not as theoretically satisfying, but it is usually easier and more economical to
make and appears to work perfectly well in practice. In some cases, the graft copolymer is
made separately and then added to the polyblend during compounding; in other cases, it
may be formed directly during compounding by reactive processing.

5.11.5 Effect of Polyblend Ratio on Polyblend Properties
When two polymers are blended in ratios from 100/0 to 0/100, and the effect on properties
is measured, we may observe one of four types of behavior (Fig. 5.14).

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