THE ROLE OF THE
THE ROLE OF THE
THE ROLE OF THE
Create successful ePaper yourself
Turn your PDF publications into a flip-book with our unique Google optimized e-Paper software.
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