Page 312 - Handbook of Plastics Technologies

ELASTOMERS

4.104 CHAPTER 4

ulus of the COP, which permits thinner, lighter parts and markedly lower part weights. The
efficiency of thermoplastics processing can combine with the lower part weight to give
pronounced cost savings relative to a thermoset rubber.
The tensile strengths of these TPEs are high, and they have hardnesses ranging from 40
to 63 Shore D (generally harder than conventional thermoset rubber compounds). As hard-
ness increases, these copolymers become more like hard plastics and less like rubber vul-
canizates.
COPs have a useful service temperature range from –40 to 150°C. The lower limit is
set by the soft segment T . Retention of physical properties measured at elevated tempera-
g
tures is quite good. They have very good resistance to a wide range of fluids. However, the
ester and ether linkages in the polymer backbone render them susceptible to hydrolysis in
the presence of both acids and bases.
COPs are resilient, with low hysteresis and heat buildup for uses requiring rapid, re-
peated flexing. In their elastic, low-strain region, COPs have very good resistance to flex
fatigue and to tensile and compressive creep.
4.6.3.3 Thermoplastic Polyurethanes (TPUs). The first commercial TPEs were the
TPUs, which have the same block copolymer morphology as do the COPs. Their general
structure is -A-B-A-B-, where A represents a hard crystalline block derived by chain ex-
tension of a diisocyanate with a glycol. The soft block, represented by B, can be derived
from either a polyester or a polyether. Typical TPU structures, both polyester and poly-
ether types, are represented here:

As with other block copolymers, the nature of the soft segments determines the elastic
behavior and low-temperature performance. TPUs based on polyester soft blocks have ex-
cellent resistance to nonpolar fluids and high tear strength and abrasion resistance. Those
based on polyether soft blocks have excellent resilience (low heat buildup, or hysteresis),
thermal stability, and hydrolytic stability.
TPUs can be made much softer than can the copolyester TPEs—down to a Shore A
hardness of 50. The properties of TPU TPEs are largely determined by the ratio of the
amounts of hard to soft phases, the length and length distribution of the segments, and the
crystallinity of the hard segments.
TPUs are noted for their outstanding abrasion resistance and low coefficient of friction
on other surfaces. However, TPUs deteriorate slowly but noticeably between 130 and

Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2006 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.