Page 118 - Handbook of Plastics Technologies

THERMOPLASTICS

2.58 CHAPTER 2

2.4 POLYMER BLENDS

There is considerable interest in polymer blends. This is driven by consideration of the dif-
ficulty in developing new polymeric materials from monomers. In many cases, it can be
more cost effective to tailor the properties of a material through the blending of existing
materials. One of the most basic questions in blends is whether the two polymers are mis-
cible or exist as a single phase. In many cases, the polymers will exist as two separate
phases. In this case, the morphology of the phases is of great importance. In the case of a
miscible single phase blend, there is a single T , which is dependent on the composition of
g
the blend. 408 Where two phases exist, the blend will exhibit two separate T s—one for
g
each of the phases present. In the case where the polymers can crystallize, the crystalline
portions will exhibit a melting point (T ), even in the case where the two polymers are a
m
miscible blend.
Although miscible blends of polymers exist, most blends of high-molecular-weight
polymers exist as two-phase materials. Control of the morphology of these two-phase sys-
tems is critical to achieve the desired properties. A variety of morphologies exist, such as
dispersed spheres of one polymer in another, lamellar structures, and co-continuous
phases. As a result, the properties depend in a complex manner on the types of polymers in
the blend, the morphology of the blend, and the effects of processing, which may orient
the phases by shear.
Miscible blends of commercial importance include PPO-PS, PVC-nitrile rubber, and
PBT-PET. Miscible blends show a single T that is dependent on the ratios of the two com-
g
ponents in the blend and their respective T s. In immiscible blends, the major component
g
has a large effect on the final properties of the blend. Immiscible blends include toughened
polymers in which an elastomer is added, existing as a second phase. The addition of the
elastomer phase dramatically improves the toughness of the resulting blend as a result of
the crazing and shear yielding caused by the rubber phase. Examples of toughed polymers
include high-impact polystyrene (HIPS), modified polypropylene, ABS, PVC, nylon, and
others. In addition to toughened polymers, a variety of other two-phase blends are com-
mercially available. Examples include PC-PBT, PVC-ABS, PC-PE, PP-EPDM, and PC-
ABS.

2.5 REFERENCES

1. Carraher, C.E., Polymer Chemistry, An Introduction, 4th ed., Marcel Dekker, New York, 1996, p.
238.
2. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 516.
3. Kroschwitz, J.I., Concise Encyclopedia of Polymer Science and Engineering, John Wiley and
Sons, New York, 1990, p. 4.
4. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 517.
5. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 518.
6. Billmeyer, F.W., Jr., Textbook of Polymer Science, 2nd ed., John Wiley & Sons, New York, 1962,
p. 439.
7. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 519.
8. Berins, M.L., Plastics Engineering Handbook of the Society of the Plastics Industry, 5th ed.,
Chapman and Hall, New York, 1991, p. 61.
9. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 521.
10. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 523.
11. Brydson, J.A., Plastics Materials, 6th ed., Butterworth-Heinemann, Oxford, 1995, p. 524.
12. Berins, M.L., Plastics Engineering Handbook of the Society of the Plastics Industry, 5th ed.,
Chapman and Hall, New York, 1991, p. 62.

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