Improved covulcanization and dynamic properties of millable polyurethane elastomer/natural rubber blends: Thermal pretreatment

Abstract Millable polyurethane elastomer (MPU) with polytetramethylene etherglycol as the soft segment, 4,4′‐diphenylmethane diisocyanate extended with 3‐Allyloxy‐1,2‐propanediol as the hard segment, was synthesized. The vulcanization characteristic showed that MPU compounds and natural rubber (NR) compounds presented a mismatch in curing rates under the accelerator‐sulfur vulcanization system, with the former showing slower curing rates than the latter regardless of the curing temperature. A feasible thermal pretreatment process was thus used to modify the MPU compounds. The IR spectra showed that the peak intensity ratios of 1460–1600 cm −1 increased with increasing pretreated time, indicating the enhanced reactivity of the pretreated compounds (PMPU). Vulcanization kinetic results further confirmed that the pretreatment process facilitated the curing reaction, as the activation energies of the PMPU compounds were all reduced during the induction period, with the lowest value obtained for the sample pretreated for 2 min (PMPU2). PMPU2 was subsequently mixed with NR in various ratios and compared with the corresponding MPU/NR blend. Here improved covulcanization characteristics were observed for the PMPU/NR blends, which exhibited faster cure rates and higher crosslink density. In addition, higher mechanical properties were presented for these pretreated blends, with the tensile strength of PMPU2/NR vulcanizates increasing by 57.3%, 153.7%, and 49.1% when the blend ratios were 75/25, 50/50, and 25/75, respectively. In particular, the 75/25 PMPU/NR vulcanizate had excellent abrasion resistance comparable to MPU. The dynamic compression performance of the pretreated blends was also improved, with fatigue life and temperature rise increasing by 119.1% and decreasing by 40.5%, respectively, for the 75/25 PMPU2/NR vulcanizate relative to the reference MPU/NR. The DMA data showed that the blend vulcanizates exhibited lower tan δ at 60°C compared with the MPU vulcanizate, and this value was further reduced for the pretreated blend, which has obvious environmental significance in terms of reduced energy consumption.