Investigations on Initial Stage of Aging of Tire Rubbers by Chemiluminescence Spectroscopy

Abstract

Abstract The initial stage of the thermal oxidation aging process of polydienes (NR, BR, and SBRs) and polyolefins (EPDM) was investigated using the chemiluminescence (CL) method, which detects the low-intensity light emitted by the polymer oxidation reaction as a function of time. It thus quantitatively characterizes the kinetics of the polymer oxidation reaction and the aspect of the polymer microstructure during the thermal oxidation condition, the influence of the rubber mixture's ingredients or vulcanizates, and the role of the anti-aging agent. The oxidation induction time (OIT) — defined as the time at which the oxidation reaction starts — is introduced as the analytical result of CL analysis, with a standard deviation of 5–6%. The OIT from CL analysis indicates the resistance of the polymer against the thermal oxidation condition. The polydiene microstructure is found to play a significant role in the aging behavior of polymers. In the case of polydienes with higher double-bond concentration in the main chain (1,4 units) — NR or BR, for example — the OIT occurs after a relatively brief exposure to thermal influences and oxygen, depending on the temperature. SBR, with a high vinyl (1,2-butadiene units) or styrene content, is relatively more resistant than NR or BR to radical attack by oxidation reaction. EPDM with no double bonds in the main polymer chain exhibits, moreover, the strongest aging resistance. After extraction of the polymers to remove the stabilizer commonly added to the polymer, OIT values are as much as 400% shorter than for raw polymers. The OIT values of polymers processed (mixed, milled, and pressed) prior to thermal aging analysis are found to be between the OIT values of raw polymers and those of extracted ones. The temperature at which analysis was carried out has an influence on the polymer's OIT. OIT's are shorter at higher temperatures, increasing the maximum intensity of the CL curve and also the CL reaction rate. The activation energy (Ea) of polydienes is calculated on the basis of temperature dependency analysis. NR and BR have slightly similar Ea values of about 55 kJ/mol, while the group of SBRs, with varied vinyl or styrene content, also has similar Ea values of about 80–90 kJ/mol. PPD derivatives are anti-aging substances used to evaluate the role of antioxidants in the polymer. The consumption of antioxidant in the initial stage of the CL reaction is clearly indicated in the CL curve. Antioxidants prolong the durability of the polymer by a factor of about 3.5 to 20. DTPD is the most effective antioxidant, followed by 6PPD, IPPD, and 77PD. Thermal oxidation characterization of vulcanizates by the CL method is more complicated due to various ingredients in the mixture and to the fact that some species are fixed as pendant group in the polymer chain. This results in more than one peak in the CL curves. It is highly likely that this effect depends strongly on the type of accelerator used in the vulcanization system.