Synthesis And Characterization Of Macromolecular Hindered Phenol Antioxidant And Its Thermo-oxidative Aging Resistance For Natural Rubber

At present, the commonly used antioxidants are the compounds with low molecular weight, which have some disadvantages, such as poor thermal stability, easy migration and poor extraction resistance. During polymer processing and long-term service, there is large physical loss because antioxidants easily volatilize by heat, migrate or be extracted out from polymer matrix to the surrounding environment, resulting in the decrease of antioxidative efficiency and the pollution of surrounding environment. How to effectively decrease the physical loss of antioxidant due to volatilize, migrate or be extracted out, and improve efficiency of the antioxidant, is one of the key issues for the development and application of high-performance polymer materials, especially the rubber and its products. In this study, polyhydroxylated polybutadiene containing thioether(PHPBT) was first synthesized by hydroxyl-terminated polybutadiene(HTPB) and 2-mercaptoethanol(ME) via thiol-ene click reaction. Then, using isophorone diisocyanate(IPDI) as bridging agent, the 2,2’-thiobis(4-methyl-6-tert-butylphenol)(TPH) was grafted on the PHPBT, and a macromolecular hindered phenol antioxidant, polyhydroxylated polybutadiene containing thioether bound 2,2’-thiobis(4-methyl-6-tertbutylphenol)(PHPBT-b-TPH) was obtained. The thermo-oxidative aging resistance of the PHPBT-b-TPH for natural rubber(NR) vulcanizate and its extraction resistance in NR vulcanizate were investigated. In addition, the mechanisms of thermo-oxidative aging resistance and extraction resistance of PHPBT-b-TPH were also proposed. Besides, two novel macromolecular hindered phenol antioxidants containing thioether and urethane groups(GMME-IPDI and GM-ME-TDI) were synthesized by thiol-acrylate Michael addition and hydroxylisocyanate nucleophilic addition. Thermal stability of the GM-ME-IPDI and GM-ME-TDI and their thermo-oxidative aging resistance for NR vulcanizate were studied. The main contents and results are as follows.(1) PHPBT was synthesized via a thiol-ene click reaction between HTPB and ME with 2,2’-azoisobutyronitrile(AIBN) as initiator, and characterized by Fourier transform infrared(FTIR) spectroscopy, nuclear magnetic resonance(1H-NMR, 13C-NMR) spectroscopy and gel permeation chromatography(GPC). Effects of reaction conditions on hydroxyl value of the PHPBT were investigated. Relative reactivity of three types of C=C double bonds(1,2-vinyl, 1,4-cis and 1,4-trans units) in the HTPB reacting with the ME was analyzed. The results showed that the hydroxyl value of the PHPBT increased at first and then almost remained unchanged with the reaction time increasing. As AIBN content or reaction temperature increased, the hydroxyl value increased first and then decreased. The optimal temperature, reaction time and AIBN content were 70 oC, 180 min and 2.0wt% based on the HTPB, respectively. All the three types of C=C double bonds in the HTPB could react with the ME and the reactivity order was: 1,2-vinyl unit>1,4-cis unit>1,4-trans unit.(2) Using IPDI as bridging agent, PHPBT-NCO was first synthesized by a nucleophilic addition between the hydroxyl group of PHPBT and higher active secondary NCO group of IPDI. Then a macromolecular hindered phenol antioxidant(PHPBT-b-TPH) was obtained by one phenolic OH of TPH reacting with the NCO in PHPBT-NCO. The structure and thermal stability of PHPBT-b-TPH were characterized by FTIR, 1H-NMR, 13C-NMR, GPC, thermogravimetry(TG) and TG-FTIR. Effects of reaction conditions on the synthesis of PHPBT-NCO and PHPBT-b-TPH were investigated in detail. The results showed that the reaction conditions, such as reaction temperature, catalyst and molar ratio of reactant, had important effect on the synthesis of PHPBT-NCO and PHPBT-b-TPH. The suitable reaction conditions for the synthesis of PHPBT-NCO by nucleophilic addition between PHPBT and IPDI as follow: reaction temperature was 35 oC, catalyst was dibutyltin dilaurate(DBTDL), molar ratio of IPDI to the OH group in PHPBT was 1.1:1, feed way was that the PHPBT added dropwise into IPDI, reaction time was 6~7 h. The suitable reaction conditions for the synthesis of PHPBT-b-TPH by nucleophilic addition between PHPBT-NCO and TPH were that the molar ratio of TPH to the NCO group in PHPBT-NCO was 2:1, reaction temperature was 75 oC, and the catalyst was triethylamine(TEA). In addition, it was found that the PHPBT-b-TPH had execllent thermal stability, whose initial temperature of weight loss reached to 257 oC.(3) PHPBT-b-TPH as antioxidant, NR vulcanizates were prepared by mixing NR with the other ingredients. Thermo-oxidative aging resistance of the PHPBT-b-TPH for NR vulcanizate was studied by oxidation induction time(OIT) and accelerated thermal aging tests. The reasons for antioxidative performance differences of different antioxidants were investigated, and the possible antioxidative mechanism of PHPBT-b-TPH for NR vulcanizate was also proposed. The results showed that when the amount of PHPBT-b-TPH was 1 phr, the NR vulcanizate had excellent thermo-oxidative aging resistance. In addition, it was found that the thioether group and urethane group in PHPBT-b-TPH had antioxidative ability, and had synergism effect of thermo-oxidative aging resistance with hindered phenol.(4) The extraction resistances of the PHPBT-b-TPH and TPH in NR vulcanizate were evaluated by mass loss of antioxidant and oxidation resistance of vulcanizates after NR vulcanizates were soaked in 70 oC water or extracted with a Soxhlet apparatus using methanol as solvent. The results showed that the extraction resistance of PHPBT-b-TPH in NR vulcanizate was excellent, and was much higher than that of TPH. When the NR vulcanizate with PHPBT-b-TPH was extracted with methanol for 24 h, the mass loss of PHPBT-b-TPH was only 56.8wt%, OIT value of NR vulcanizate decreased to 25 min form 51.2 min before extraction. But for NR vulcanizate with TPH, almost all the TPH lost, and the OIT value of NR vulcanizates decreased to 12.6 min form 64.3 min before extraction. In addition, by toluene swelling experiment of NR vulcanizate with PHPBT-b-TPH, it was found that one of the main reasons for excellent extraction resistance of PHPBT-b-TPH in NR vulcanizate was that the covulcanization took place between PHPBT-b-TPH and NR.(5) A hindered phenol with alcoholic hydroxyl and thioether groups(GM-ME) was first synthesized by thiol-acrylate Michael addition between 2-tert-butyl-6-(3-tert-butyl-2-hydroxy-5-methylphenyl) methyl-4-methylphenyl acrylate(GM) and ME. Then two novel macromolecular hindered phenol antioxidans containing thioether and urethane groups, GMME-IPDI and GM-ME-TDI, were obtained successfully after the hydroxyl of GM-ME reacted with IPDI and toluene diisocynate(TDI), respectively. Structures of the GM-ME-IPDI and GMME-TDI were confirmed by FTIR, 1H-NMR and matrix-assisted laser desorption/ionization time of flight mass spectrometry(MALDI-TOF-MS). The OIT and accelerated thermal aging tests of NR vulcanizates showed that the antioxidative efficiency of the GM-ME-IPDI and GMME-TDI were superior to the GM, especially the GM-ME-TDI. In addition, it was found that the antioxidative ability of urethane group was affected by the groups which linked with urethane group. When the urethane group connected with benzene ring, it had better antioxidative ability than the urethane group connected with alicyclic ring.