Mechanical degradation of elastomers in the presence of silica and inhibitors using a new design of mechano reactor

A newly designed mechanochemical reactor has been home built and used to investigate mechanical degradation of SBR rubber and SBR/SiO 2 blends under N 2 and O 2 atmosphere and in the presence of aromatic amines (DTPD) and tetramethyl piperidine (HALS) radical inhibitors. Continuous on line determination of torque, temperature and O 2 uptake have been made together with solution viscosity and mechano-radical yields at definite time intervals. The fraction of mechanical energy resulting in chemical effects is determined to be less than 0.01%. Chain scission and crosslinking are found to be major mechanochemical reactions with crosslinking acquiring importance with increasing the processing time. According to the results of COGEF type DFT UB3LYP 6-311++G (d,p) relaxed scan computations, the favoured chain scissions sites are predicted to be the ones leading to allylic (prominent) and benzylic radicals. The kinetics of mechano-radicals formation is coherent with torque and viscosity decay kinetics being expressed by the equation [R •] (mol / kg) = - 0.0775 × 10 - 2 t 2 + 0.6755 × 10 - 2 t. The mechano-radical yield is greater than expected from the chain scission yield; furthermore, oxygen and radical inhibitors have minor effects on torque and viscosity decay kinetics in the first part of the process where chain scissions are dominant, whilst they do inhibit crosslinking. It is inferred from these results that a significant part of permanent chain scissions takes place via caged-radical disproportionation and that only scavengeable mechano-radicals escaping cage reactions are responsible for the initiation of crosslinking. From free radical chemistry analysis supported by DFT calculations it is inferred that H abstraction and 1-3 double bond addition by chain scission allyl radicals can be major paths for crosslinking initiation, the addition mechanism acquiring importance with increasing reaction time. The oxygen uptake is approximately linear with process time and consequently does not fit the formation and decay kinetics of scavengeable mechano-radicals; it is suggested that the contribution by other mechanisms involving mechanical excitation coupled with thermal activation may be important. The addition of SiO 2 filler causes an increase of total absorbed mechanical energy and a decrease of the equilibrium molecular weight limit (enhanced chain scissions). In contrast, the scavengeable mechano-radicals yield is found to be less than 50% of that determined in the absence of SiO 2. These observations are related to the mechanical degradation of bound rubber in conditions of enhanced cage effect and build up of frictional energy by silica-rubber Van der Waals interaction