Because a wide range of coagents are available, the selection of coagents for achieving
optimum properties is paramount.48 The reactivity of coagents also depends on the elastomer,49 and
the nature of the cross-links formed determines the properties of the resulting network. Cross-link
density of the NR–DCP system with different coagents was measured using equilibrium swelling
studies in toluene. Comparison of cross-link densities and tensile strengths of various coagents with
a control (gum vulcanizate) is shown in Figure 9.
The swelling studies showed coagent TMPTMA produced the maximum cross-link density.
Because it is highly reactive, when coagent TMPTMA is added to NR, a very fast reaction occurs
between the methacrylic groups of the coagent and the macroradical of the polymer. The resulting
polymer macroradical undergoes addition reaction with the coagent (further confirmed by FTIR
studies), and new radicals are produced. It may undergo further reaction using the second and third
methacrylic groups of the same coagent molecule, which results in a coagent bridge or starlike
network between the polymer chains.50 At the same time, the coagent can undergo
homopolymerization and subsequent grafting to polymer chains. Both reactions increase crosslink
density of the vulcanizate. Because C–C bonds are formed, coagent TMPTMA is applicable
where high modulus and ageing resistance are required.
The effect of TAC as a coagent in the peroxide vulcanization of NR is somewhat different.
Cross-link density was higher for samples containing 5 phr coagent, and as the concentration of
TACincreased (10 phr), cross-link density decreased. For highly unsaturated rubbers, usually Type
II coagents provide no increase in cross-link density because of the relative ease of allylic hydrogen
abstraction from the polymer chain compared with the vinylic group of coagent. The polymer
macroradical formed by allylic hydrogen abstraction added to the coagent, generating a new
radical; combined with another polymer radical to form a C–C cross-link; or abstracted hydrogen
from the coagent. Some peroxide radicals also initiate isomerization of TAC to triallylisocyanurate
(TAIC; Scheme 2). The former two reactions will increase cross-link density. This might be the
reason for increased cross-link density at lower concentrations of TAC.
The share of ionic cross-links with coagent ZDA was calculated (Eq. 4) and was found to be
20%. The characteristics of the ionic bonds have been discussed in the literature.51 Ionic bonds
exhibit both good heat stability and the ability to slip along the hydrocarbon chain. It reforms after a
break, similar to polysulfidic cross-links in sulfur vulcanization, which is reflected in the higher
tensile strength of vulcanizate with coagent ZDA (Figure 9). Therefore, coagent ZDA would
provide good dynamic properties in peroxide-cured rubber. A suitable cross-linking mechanism
was proposed for NR with coagent ZDA and is given in Figure 10.