Page 316 - Handbook of Plastics Technologies

ELASTOMERS

4.108 CHAPTER 4

Just as with thermoset rubbers, nonpolar fluids such as oils or fuels cause varying de-
grees of swelling, fluid absorption, and loss of properties by the TPV. Resistance to such
fluids is similar to the fluid resistance of the elastomeric component of the TPV. Thus,
NBR/PP TPVs were developed as TPEs of improved oil resistance.

4.6.3.7 Other TPEs. An emerging group of TPOs are produced by the copolymeriza-
tion of ethylene and alpha olefins, in the presence of metallocene catalysts, to give a poly-
mers with a narrow molecular-weight distributions, controlled levels of long-chain
branching, and homogeneous comonomer distribution. The polymer structure is similar to
that of a block copolymer. Produced in a single reactor, these TPEs have a major cost ad-
vantage over the older TPOs and TPVs. They likely will compete most directly with the
TPOs. It is highly unlikely that their performance can be brought up to the level of the
TPVs.
Blends of PVC and NBR (possibly the oldest of commercial polymer blends) have
found some use in areas where the service temperature is at or near ambient and oil resis-
tance is needed, such as in the hose and electrical wire and cable markets. They are nor-
mally custom compounded for a specific use.
Comparison of Properties. Properties of examples of the above TPEs are given in
Table 4.16.

TABLE 4.16 Properties of Selected Thermoplastic Elastomers *
PU Styr. TPO Copolyester TPVs Polyamides

Hardness, Shore A or D 70A 72A 71A 40D 72A 75A
UTS, MPa 24 26 6.4 26 8.3 29
Stress at 100% strain, MPa 3.5 2 2.2 6.4 3.2 4.3
Ultimate elongation, % 500 560 800 450 375 350
Dry heat performance F P P G G VG
Hot oil resistance E VP VP E G E
Elevated-temp. set resistance P VP VP G E F–G
*.P = poor, F = fair, G = good, E = excellent.

4.6.4 Processing of TPEs
4.6.4.1 Need for Drying TPEs. Many TPEs can absorb sufficient moisture in a short
time to cause difficulties such as poor surface appearance (or splay) on injection molded
parts and rough surfaces and porosity in extrusions. A moisture pickup of 0.10 percent or
greater can give highly significant problems, even in the case of nonpolar hydrocarbon
TPEs. An exposure of less than one day can be unacceptable.
While some TPEs have been developed to specifically avoid the need for drying, most
should be dried immediately before use at a temperature of 70 to 100°C for two to six
hours, depending on the specific TPE. Because of high surface areas, regrind material (for
recycling to the process) should be dried an extra one to two hours.

4.6.4.2 Extrusion. Simple extrusion can be used to manufacture a variety of TPE
shapes such as tubing, hose, sheet, and complex profiles. A thermoplastic extruder with a

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.