Endeavour to balance mechanical properties and self-healing of nature rubber by increasing covalent crosslinks via a controlled vulcanization

Introducing covalent crosslinks into reversible supramolecular networks would effectively improve their mechanical strengths, but sacrificed the desired self-healing effect. So, it is a practical significance to explore the influence of irreversible covalent crosslinks on self-healing behavior to prepare self-healing rubbers with enhanced mechanical properties for further application. In this study, the trade-off of covalent crosslinks between the balanced mechanical properties and self-healing effect of a natural rubber/methacrylic acid/zinc oxide (NR/MAA/ZnO) supramolecular network system was studied. Zinc dimethacrylate (ZDMA) is in situ formed from MAA and ZnO during the mastication of NR, which introduce mass of ionic crosslinks to generate a reversible ionic supramolecular network at the initial stage of peroxide-induced vulcanization at 140℃. Then, free radicals continue to initiate covalent crosslinks to strengthen the NR matrix. The tensile strength of NR with slight covalent crosslinks could achieve 1.63 ∼ 3.86 MPa when vulcanization time keeps in 240 ∼ 360 s. At this time, primary continuous covalent crosslink network has little binding effect on rubber chains, and ionic interactions can be smoothly reorganized to ensure excellent self-healing behavior. The considerable self-healing efficiency of 67%∼95% indicates that the vulcanization time at 240 ∼ 360 s is an appropriate choice to achieve a win–win situation for the mechanical properties and self-healing efficiency of NR/MAA/ZnO composites.