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
(vtp-vlab-caosuviet)
