Thermo-oxidative aging of butadiene rubber and its mechanism

Butadiene rubber (also known as polybutadiene rubber) has the advantages of high elasticity, cold resistance, wear resistance, flexural resistance and good dynamic performance. It is currently mainly used in tires, shoes, high impact polystyrene and ABS resin. modification, etc. There are unsaturated bonds in the structure of butadiene rubber. Under the condition of thermal oxygen or ultraviolet light, it is easily attacked by oxygen free radicals, which leads to the destruction of its composition and structure, thus greatly reducing its performance. At present, most of the aging research on cis-butadiene rubber focuses on the change of physical and mechanical properties [5-7], but there is almost no research on the change of chemical structure in the aging process. In this paper, cis-butadiene rubber is used as the raw material, and the thermo-oxidative aging study is carried out. -Vis) absorption spectroscopy focuses on exploring its chemical structure change and aging mechanism. So far, the method and results of using nuclear magnetic resonance spectroscopy to study the aging of cis-butadiene rubber have not been reported. This is of great significance for developing new formulations and proposing protective measures to improve the storage and service life of butadiene rubber. 1 Experimental part 1.1 Raw material butadiene rubber (BR≥98%): commercially available; deuterated chloroform (CDCl3): D>99.8%, TMS is 0.03%, Sigma-Aldrich; cyclohexane: analytical grade, Xilong Science Co., Ltd. 1.2 Experimental Instruments Nuclear Magnetic Resonance Spectrometer: Bruker Avance 400 MHz, Bruker, Germany; Infrared Spectrometer: Nicolet iS10, Nicolet Instruments, USA; UV-Vis Spectrophotometer: UV-2550, Shimadzu, Japan. 1.3 Sample preparation and aging test Sample preparation: Dissolve 3 g of cis-butadiene rubber in a three-necked flask containing 30 g of cyclohexane, heat and stir at 80 °C for 2 h to make it fully dissolved, cool to room temperature, and drop quantitatively. Films are made on glass slides. Put it in a fume hood and wait until all the reagents are evaporated. The film thickness is about 0.02 mm. Thermo-oxidative aging: According to the national standard GB/T3512-2001 [8] (hot air accelerated aging and heat resistance test of vulcanized rubber or thermoplastic rubber), the prepared butadiene rubber film is placed in an electric heating constant temperature blast drying oven, and the temperature is set The temperature was 120 °C, aged for 0.5 d, 1 d, 2 d, 3 d, 5 d, 7 d, 10 d, and 15 d, respectively, and then taken out for testing. 1.4 Test and Characterization 1.4.1 ATR-FT-IR Test: Using Attenuated Total Refraction (ATR) technology, the number of scans is 16 times, the scanning range is 500-4000 cm-1, and the cis-butadiene rubber film is directly tested by infrared [9]. 1.4.2 1H-NMR test: Cut about 10 mg of cis-butadiene rubber film, dissolve it in 0.5 mL of deuterated chloroform, and conduct hydrogen nuclear magnetic resonance spectrum test. 1.4.3 UV-vis test: Cut about 2 mg of cis-butadiene rubber film, dissolve it in 10 mL of cyclohexane, and carry out UV spectrum test, scanning range 500-190 nm, medium speed, scanning gap 0.5 mm. 1.5 Calculation of reaction rate 2 Results and discussion 2.1 Infrared spectrum of cis-butadiene rubber during thermo-oxidative aging Fig.1 is the ATR-FT-IR spectrum of cis-butadiene rubber before and after aging for 15 d. Among them, in the infrared spectrum of unaged cis-butadiene rubber, 3068 cm-1 is =CH2 antisymmetric stretching vibration, 3004 cm-1 and 1654 cm-1 are cis-1,4-butadiene structural units- The stretching vibration peak of CH2=CH2, 2939 cm-1 is the antisymmetric stretching vibration peak of C-H on -CH2 on methylene, 1448 cm-1 is the rocking vibration peak of C-H on methylene-CH2-, 993 cm-1 And 911 cm-1 is the out-of-plane rocking vibration peak of -CH2, and 736 cm-1 is the in-plane rocking vibration peak of the repeating unit -CH2-CH2-, which is also a characteristic band of the cis-1,4-cis-butadiene rubber structure [10]. When the butadiene rubber was thermally aged at 120 ℃ for 0.5 d, it was observed that the infrared spectrum began to change, and an obvious new material peak appeared. Among them, the spectral peak of hydroxyl-OH appears near 3418 cm-1, which is attributed to the stretching vibration of -OH group of the alcohol compounds generated by the oxidation reaction; the spectral peak of carbonyl-C=O appears at 1710 cm-1 [11 ], the broad absorption bands in the range of 1176 cm-1 and 1030 cm-1 are attributed to the C-O and C-O-C bonds, respectively, which indicate that the cis-butadiene rubber undergoes an oxidation reaction during the thermal oxidation aging process; while the spectral peak at 736 cm-1 The height decreases sharply, indicating that the thermo-oxidative aging reaction occurs in the molecular chain.α-H site. The absorption peaks at 3004 cm-1 and 993 cm-1 suddenly weakened after 0.5 d of aging, indicating that the hydrogen content of double-bonded alkenes decreased sharply and continued to decrease, but never completely disappeared within 15 d of aging. From the infrared spectrum, it can be concluded that the structure of cis-butadiene rubber changes significantly within 0.5 d of thermal-oxidative aging, indicating that its resistance to heat and oxygen is poor, which is attributed to the existence of a large number of unsaturated bonds in its molecular structure.

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