Chemistry Reference
In-Depth Information
The reactions take place at all temperatures, but industrially they are carried out from 50 to 75
C
and above. At lower temperatures, however, the process may take days to complete. At temperatures
of 135-155
C, approximately 8% of sulfur (by weight of rubber) reacts [
272
]. Also, sulfur dissolves
in unvulcanized rubber even at room temperature. The overall mechanism of the reaction is still being
studied. Most evidence points to an ionic mechanism and a sulfonium ion intermediate [
272
]. It was
shown [
273
] that a straightforward reaction of sulfur with rubber is insufficient. Somehow, between
40 and 100 atoms of sulfur must be combined in order to obtain one cross-link. Out of 40-100 atoms,
only 6-10 are actually engaged in the formation of the cross-links. The rest of the atoms form cyclic
sulfide units that become spread along the main chain [
273
].
To improve the efficiency of the vulcanization reaction, various accelerators were developed.
Among them are zinc oxide combined with fatty acids, and/or amines. Zinc oxide forms zinc
mercaptides like (XS)
2
ZnL
2
where X is an electron withdrawing substituent and L is a ligand from
a carboxyl or an amine group. The function of the ligand is to render the complex soluble. The
mercaptide complexes are believed to react with additional sulfur to form zinc perthiomercaptides.
The accelerators that are most commonly used are derivatives of 2-mercaptobenzothiazole. They are
very effective when used in combinations of metal oxides with fatty acids (referred to as
). The
favorite activators are zinc oxide combined with stearic acid. The combinations permit rapid
vulcanizations that take minutes compared to hours when sulfur is used alone. In the process of vulcaniza-
tion, 2,2
0
-dithiobisbenzthiazole forms initially and then reacts with sulfur to form polysulfides [
273
]:
activators
N
N
S
x
N
N
C
S
S
C
C
S
S
x
S
C
S
S
S
S
The products from reactions with sulfur in turn react with natural or synthetic rubber at any allylic
hydrogen. This is a concerted reaction that results in formation of sulfur containing adducts of the
polymers:
N
N
+
SH
S
N
N
S
S
S
S
S
S
S
S
crosslinking
S
x
Once the cross-links are formed, further transformations take place. Some of them consist of
reactions that shorten the polysulfide links:
S
x
S
2
S
