Page 338 - Handbook of Plastics Technologies

PLASTICS ADDITIVES

5.18 CHAPTER 5

5.2.1.4 Modulus. Modulus is increased greatly, because, when flexible polymer mole-
cules bump against the hard surface of inorganic particles, they lose much of their inherent
flexibility. The effect is most pronounced for fibers in the axial direction, because, even if
the polymer is willing to respond, the high-modulus fibers absolutely refuse to respond at
all. Creep Resistance correlates with modulus, both theoretically and practically. This can
bring performance of plastics much closer metals and ceramics.
5.2.1.5 Breaking Strength. Breaking strength is increased greatly by continuous fibers;
when the polymer is ready to fail, the high-strength fibers absolutely are not. Short fibers
may or may not increase strength somewhat, depending on stress-transfer across the fiber/
polymer interface; they may actually decrease it, because the fiber ends act as stress con-
centrators, causing premature failure. Particulate fillers usually decrease strength due to
stress concentration at sharp edges and corners of the filler particles.

5.2.1.6 Impact Strength. This is increased tremendously by continuous fibers
(Fig. 5.5); they seem to distribute the shock over the entire length of the fiber so that the
stress at any one point is very small. Short fibers are unpredictable; they may increase im-
pact strength moderately or not at all, or even decrease it, their ends acting as stress con-
centrators. Particulate fillers almost always decrease impact strength, again due to stress
concentration at their sharp edges and corners. Impact strength theory is seriously handi-
capped by the assumption that the same failure mechanisms operate at both low speed and
high speed; it would be much better to recognize that high-speed impact failure is a com-
pletely different phenomenon that deserves its own theoretical analysis.
5.2.1.7 Friction and Abrasion Resistance. These qualities are increased by the sharp
edges of filler particles and the sharp ends of fibers that protrude from the surface of the
polymer matrix.

FIGURE 5.5 Unbreakable plastics.

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