Page 376 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.56 CHAPTER 5

FIGURE 5.14 Properties vs. polymer/polymer ratio in a polyblend.

5.11.5.1 Type I. If the two polymers are completely miscible down to the molecular
level and form a single homogeneous phase, properties are generally proportional to the
ratio of the two polymers in the blend. Even if the two polymers are immiscible and form
fine phase separation, many property tests are relatively insensitive to fine-phase separa-
tion and may still show such “homogeneous behavior.” Practically, this is useful to com-
pounders who want the ability to produce a spectrum of balance of properties at low cost.
5.11.5.2 Type II. When two polymers are immiscible and form two separate phases, the
major polymer will form the continuous matrix phase and retain most of its original prop-
erties, while the minor polymer will form finely dispersed domains and contribute certain
specific properties. Thus, high A/B ratios will have properties similar to poly-A, and high
B/A ratios will have properties similar to poly-B. Obviously, at fairly equal ratios of A and
B, there will be a phase inversion with a rapid change of properties from one plateau to the
other.
This explains the two leading uses of polymer blends. (1) When rigid plastics suffer
from brittleness, dispersion of fine rubbery domains in the rigid matrix can add great im-
pact strength with little sacrifice of rigidity. (2) Rubber molecules must be tied together to
give them strength, creep resistance, and insolubility; while this is usually done by ther-
moset vulcanization, it can also be done by dispersion of fine rigid thermoplastic domains,
either glassy or crystalline, to form thermoplastic elastomers.

5.11.5.3 Type III. When two polymers are immiscible and separate into two phases,
there may be so little attraction between them that the interface between the phases is ex-
tremely weak and will fail under stress. This is most often seen in ultimate tensile strength
and ultimate elongation. In most products, this would be labeled “incompatibility.” How-
ever, there are occasional examples where such behavior is actually beneficial. For exam-
ple, adding an immiscible polymer may decrease melt viscosity and thus improve melt
processing. Or it may decrease breaking strength, producing a package that is easier to
open and therefore more customer friendly. Thus, it is safer to label Type III behavior “U-
shaped” or “trough-shaped,” rather than simply incompatible.
5.11.5.4 Type IV. Once in a while, the polymer blend may exhibit properties greater
than either of the individual polymers, a major synergistic improvement in practical utility.
The leading example of this phenomenon is the use of finely dispersed rubbery domains to
increase the impact strength of a brittle glassy matrix polymer. Commodity examples are

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