Page 220 - Handbook of Plastics Technologies

ELASTOMERS

4.12 CHAPTER 4

• Silicone elastomers, whose molecular main chains contain silicon and oxygen atom
backbones, have excellent high-temperature performance (up to 315°C) and are also
flexible at very low temperatures. However, their physical properties are modest.
• Acrylic elastomer (ACM) has heat resistance that is almost as good as that of fluorinated
compounds and silicones. It also ages well and is oil resistant, but it is sensitive to water.
• Polysulfides (T) have good weather resistance and resistance to a variety of fluids and
other chemical environments; however, their physical properties are modest.
• Millable polyurethanes (U) are either ester- or ether-based. Ester-based elastomers are
poor in water resistance. They have good ultimate mechanical properties and abrasion
resistance.
The above conventional elastomers and others are listed in Table 4.3 with their ASTM
D1418 designated abbreviations.

4.3.2 Thermoplastic Elastomers
These materials have the functional requirements of elastomers (extensibility and rapid re-
traction) but processability similar to that of rigid thermoplastics. The principal advan-
tages of the TPEs compared to vulcanized rubber are (1) reduction in compounding
requirements, (2) easier and more efficient processing cycles, (3) scrap recycling, and (4)
availability of thermoplastic processing methods. Generic classes of TPEs include
• Styrenic block copolymers
• Polyolefin blends (TPO)
• Thermoplastic vulcanizates (TPV)
• Thermoplastic polyurethanes (TPU)
• Thermoplastic copolyesters
• Thermoplastic polyamides
TPEs are processed by extrusion, injection molding, blow molding, and thermoform-
ing, and they can be heat welded. None of these methods are applicable to thermoset-type
elastomers.

4.4 PROPERTIES OF ELASTOMERS

The most frequently measured rubber properties are the mechanical properties. These in-
clude hardness, ultimate tensile strength (UTS), ultimate elongation (UE), and stress at
100, 200, and 300 percent tensile strain. Though not really moduli, these stresses are re-
ferred to, in the rubber technologists’ jargon, as100, 200, and 300 percent modulus (M100,
M200, and M300). Additional properties are measured to be more or less predictive of ser-
vice performance.
Hardness. Hardness is probably the least difficult and most often measured property
of vulcanized elastomers. It is in almost every list of rubber specifications. As measured,
hardness is the relative resistance of the rubber surface to deformation by an indentor. The
hardness of rubber is measured by using a small spring-loaded gauge known as a durome-
ter (ASTM D 2240). One measures the hardness by pressing the indentor against the rub-

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