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118 The Role of the Chemist in Automotive Design<br />

R<br />

CH 3<br />

FIgure 8.8 Structure of EPDM.<br />

H<br />

H 3 C<br />

seals, and glass run channels, among other applications. In 2000, production figures<br />

for this synthetic polymer were over 870 metric tons [3]. The structure of EPDM is<br />

shown in Figure 8.8.<br />

EPDMs are utilized in the automobile industry for their heat resistance, ozone<br />

resistance, and overall stability. Several key characteristics that are useful to the<br />

automotive chemist can be derived from the structure. Figure 8.8 shows the straight<br />

chain backbone of the EPDM polymer. EPDM is a terpolymer, or three-monomer<br />

polymer, consisting of propylene, ethylene, and another constituent—in this case,<br />

ENB (ethylidene norbornene).<br />

Ethylene and propylene monomers combine to form the saturated and stable<br />

backbone [4]. This saturated backbone will provide excellent heat, oxidation, ozone,<br />

and weather aging because no reactive double bonds are in the backbone structure<br />

[4]. The third monomer (ENB) is added in a controlled manner and provides a site for<br />

cross-linking via the double bond. The M in EPDM refers to a saturated backbone.<br />

By virtue of the ENB, various amounts of vulcanization can be obtained to acquire<br />

the durometer, tear strength and tensile and other properties needed for the sealing<br />

or isolation needs of the automobile part.<br />

Another of the monomers used in addition to ENB is dicyclopentadiene (DCPD),<br />

which is shown in Figure 8.9. As can be seen from the structure, there are two double<br />

bonds in DCPD. Each of the two dienes will have different tendencies for long chain<br />

branching, which will influence processing rates and cross-linking by sulfur or peroxide<br />

cures [3]. Table 8.5 shows some of the characteristics of the two dienes [5]. In addition,<br />

Table 8.6 shows general features of ethylene–propylene elastomers as related to the ter-<br />

Dicyclopentadiene DCPD<br />

FIgure 8.9 Structure of dicyclopentadiene.<br />

H 3C<br />

R

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