Cao su EPDM được tổng hợp từ 3 monomer chính là ethylene, propylene và monomer thứ ba diene. Mặc dù monomer thứ ba này được sử dụng với một lượng rất nhỏ, nhưng nó ảnh hưởng đáng kể đến cấu trúc và tính chất của cao su EPDM tạo thành. Tài liệu sau đây sẽ trình bày cụ thể vấn đề này.

As has been mentioned, three termonomers are employed in the manufacture of EPDM's: ethylidene norbornene (ENB), 1,4 hexadiene (1,4 HD), and dicyclopentadiene (DCPD). Even though all three introduce unsaturation into the EPDM backbone as vulcanization sites, they impart varying characteristics to the elastomers because of their different structure.

Ethylidene Norbornene (ENB). ENB is the most widely used termonomer employed even though it is the most expensive, the reasons being that it is the most readily incorporated during copolymerization and the double bond introduced has the greatest activity for sulfur vulcanization. This activity is also of such a nature that EPDM's containing ENB have the greatest tendency to be cocured with diene elastomers.6 Another unique characteristic of this termonomer is that it makes it possible to prepare linear as well as branched polymers by varying the conditions under which the polymers are synthesized.5 Branching has an important role in establishing the rheological properties of a polymer.7 Under proper control, it can introduce properties to the EPDM that are beneficial in certain applications.

1,4 Hexadiene (1,4 HD). Polymers containing 1,4 HD exhibit a slower cure rate than ENB but possess certain properties that are superior. One such property is its excellent heat characteristic, which is closest to EPM. Such polymers exhibit a good balance of chain scission and crosslinking reactions.8 Polymers prepared with 1,4 HD are normally linear in structure and possess excellent processing characteristics.

Dicyclopentadiene (DCPD). The main advantages of DCPD are its low cost and relative ease of incorporation, in which it is similar to ENB. Of the three termonomers, it has the slowest cure rate. All polymers prepared with it are branched as a result of the slight polymerizability of its second double bond. As mentioned previously, this branching can be beneficial, for example, by imparting ozone resistance to diene rubber blends.

Trích đăng từ sách Rubber Technology – Third Edition, Maurice Morton, Springer, 1999, trang 264 - 265


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