Tear strength (ASTM D624, Die B) varied within a range of 28 KN/m to 37 KN/m and passed through an optimum when DBTU was in the order of 2.
Viewing graphical representations of sulfur versus DBTU, a number of similar observations was noted which may be pertinent to offering an interpretation of the mechanism of DBTU in butyl rubber vulcanization (figure 8).
activation energy showed little change with increase in DBTU, but decreased as sulfur level increased from 1 phr to 2.
compound modulus increased with both an increase in sulfur and DBTU.
The profiler also illustrates how these parameters may be adjusted through changes in the concentration of the independent variables MBTS, DBTU and sulfur.
Probably the most effective solution is use of higher molecular weight molecules, such as DMTU, DETU, DBTU and the aromatic derivatives DPDU and DOTTU, which is the basis of this work in butyl rubber.
Such observations would be in agreement with the better reversion resistance found with DBTU compared with DPTU and DOTTU.
Applying their interpretation to the dibutyl substituted thiourea (DBTU) in the systems investigated in this work, DBTU may also first react with MBTS in the formation of the sulfurating complex (figure 14).
However, these data should be viewed with the rheometer cure profile (figure 3), which illustrates that DMTU, DETU or DBTU may be optimized to match that for TMTD.
Figure 15 illustrates cure state and cure system reversion resistance for the five thiourea derivatives, and suggests DBTU would be the preferred molecule for further optimization.
DBTU is more effective at building modulus than MBTS (figure 7), which may also act as a retarder (ref.
DBTU has shown to be particularly effective in this regard.