Crosslinking Rubber Via Oxa-michael Reaction: Mechanism, Performance And Applications

Diene-based rubbers such as natural rubber(NR) are the most commonly used types of rubber in a diverse variety of engineering applications. These rubbers exhibit high elasticity and good mechanical properties, including high toughness and abrasion resistance. In most cases, diene-based rubbers are cross-linked by a sulfur-based curing package or by peroxides, in conjunction with various accelerators and antioxidants. Although the cured diene-based rubbers exhibit promising mechanical performance, they generally suffer from insufficient anti-ageing properties, and the curing process involves the use of toxic additives. Accordingly, in the present dissertation, we focused on the studies on crosslinking rubber via oxa-Michael reaction. Epoxidized rubbers were cured by unsaturated metal carboxylates as novel vulcanizer via oxa-Michael reaction. Rubber composites with high ageing resistance and/or high abrasion resistance, and shape memory effects were fabricated accordingly. The vulcanization mechanism via oxa-Michael reaction was revealed.(1) A novel method of fabricating high-performance diene-based rubber was proposed. The curing mechanism via an oxa-Michael reaction was elucidated through characterization of the model compounds. Vulcanization was achieved via the ring-opening reaction of the epoxy group on epoxidized natural rubber(ENR) and subsequent oxa-Michael reaction with the conjugated double bonds of unsaturated metal carboxylates.(2) With the oxa-Michael reaction mechanism, ENR was cured by ZDA under conventional curing conditions. The mechanical properties of ZDA-cured ENR composites are fully competitive with those for ENR compounds cured by the conventional sulfur curing package. Without any additional antioxidant, the antiaging performance of the ZDA-cured ENR composites was significantly improved. ZDA-cured ENR composites still possesses practically useful mechanical performance(tensile strength 17 MPa and strain at break 147%) even after ageing for 21 days. Such observation is revealed to be closely related to the novel crosslinks with higher bond energy, the relatively lower concentrations of epoxide and α-hydrogen, and the absence of sulfides in the ZDA-cured composites.(3) In the presence of Zn O, the renewable conjugated acids are converted into the salts, i.e. zinc disorbate(ZDS) and zinc diferulate(ZDF). The curing capability of in situ formed salts for ENR was been revealed. The vulcanization, resilience and mechanical properties of ZDS-cured and ZDF-cured ENR composites were also competitive with those of the conventional sulfur-cured ENR composites. In the absence of any additional antioxidant, the anti-ageing performance of ZDS-cured and ZDF-cured ENR/silica composites was substantially improved. Most impressively, a super resistance to abrasion was observed in the case of the ZDF-cured ENR/silica composites; this excellent abrasion resistance was attributed to the strong interfacial interaction, uniform dispersion state of the silica, and the much improved anti-ageing properties of the composites.(4) Epoxidized nitrile butadiene rubber(NBR) was prepared by ortho-phthalic monoperoxyacid(MPPA). Epoxidized NBR was demonstrated could be cured by ZDF via oxa-Michael reaction. The obtained NBR vulcanizates exhibited higher mechanical properties, compared to those of sulfur-cured NBR. Furthermore, one-step curing method for NBR by ZDF was proposed. By in situ epoxidation of NBR during conventional processing conditions, NBR could subsequently cured by ZDF and achieved improved mechanical performance. Such one-step method significantly simplified the epoxidation modification and vulcanization of NBR in a facile and effective way.(5) The feasibility of SMPs based on natural rubber via oxa-Michael reaction was revealed. SMPs with varying Tg values(20-46 °C) and recovery times(14-33 s) were developed from a single polymer by adjusting the ZDA concentration. The SMPs possessed excellent shape fixities and shape recoveries. The nanosilica-reinforced SMPs exhibited exceptionally high strength in the rubbery state(over 20 MPa). The ZDA and nanosilica concentration were used to tune the stress and strain of the SMPs. The nanosilica-filled SMPs exhibited high transparency. This makes them suitable for use in visible heat-shrinkable tubes in elevated temperature and oxygen conditions. Besides, SMPs with high content of ZDA(over 40 phr) exhibted wide transition range of Tg. SMPs hence achieved triple- and multi-shape memory performance.(6) Reversible plasticity shape memory polymers(RPSMPs) were fabricated by incorporating compatible, low molecular weight AO-80 into an ENR network cross-linked by ZDA. SMPs with RPSM behavior could simplify the shape fixing process, saving additional time and energy. These SMPs could undergo a large plastic deformation of ～300% strain that was achieved at a Td that was below Tg and maintained when the stress was released. Large energy storage capacities at Td in these RPSM materials were demonstrated compared with those achieved at elevated temperature in traditional SMPs. Upon heating to a temperature above Tg, RPSMPs recovered their original shape almost completely. Therefore, such SMPs could be useful for possessing high recovery stress, such as intelligent and powerful gripper. Self-healing tests showed that nonpermanent damages of these SMPs were well-healed when heated to a temperature above Tg.