New Green Route for Acrylonitrile Butadiene Rubber Latex Hydrogenation Using Water-Soluble Catalyst

The chemical modification of diene-based polymers via hydrogenating the residual C=C in polymers has been extensively examined in the last two decades. It has exhibited special importance in extending the ranges of application for unsaturated polymers. Hydrogenated acrylonitrile butadiene rubber (NBR) is an excellent example. Due to its superior thermal and oxidative stability after hydrogenation, hydrogenated NBR (HNBR) has already been commercialized by different companies.   

The commercial process for manufacturing this HNBR needs to use a large amount of organic solvent and involves complicated pre/post-treatments. With the increasing attention to sustainable development, it is very desirable to have a green process in which high performance elastomers could be produced in a simple process. Direct NBR latex hydrogenation attracted much attention for many years. Recently, our group has developed NBR latex hydrogenation using Wilkinson’s catalyst [RhCl(PPh₃)₃] with co-catalyst ligand triphenylphosphine (PPh₃) and high quality HNBR latex was obtained. However, the hydrophobicity of the catalyst hinders its dispersion in an aqueous system, which greatly slows down the hydrogenation reaction rate (TOF < 20 h^-1).

In order to achieve hydrogenation of the NBR latex, the key to success is to find such a system that catalysts can be easily dispersed in water (hydrophilic) and quickly diffused into the NBR micelles (hydrophobic). Here we reported on our new findings for NBR latex hydrogenation. Water-soluble catalyst RhCl(TPPMS)₃ is active catalyst for the hydrogenation of the NBR in latex. The reaction could proceed under mild conditions (T = 100 ^oC, P_H2 = 34.5 bar) at a molar ratio of C=C units/Rh = 1000 within 8 hours. The industrial required level of hydrogenation (>95 %) could be achieved. Besides, C=C bonds in NBR is selectively hydrogenated (no reduction of CN) and the resultant HNBR latex is under high quality.

Extensive experimental investigation has been carried out with respect to the effects of reaction temperature, pressure, the ratio of catalyst/NBR, co-catalyst ligand, the particle size and the surfactant type of the NBR latex. Meanwhile, the catalyst diffusion process was investigated by monitoring the catalyst concentration change within the NBR particles and the result showed that a high degree of hydrogenation can only be achieved when the catalyst is transported into the micelles.

It is confirmed that such a designed technical route greatly increases the catalysis efficiency. Comparing with the RhCl(PPh₃)₃/PPh₃, faster hydrogenation reaction rate (TOF > 100 h^-1) was observed by RhCl(TPPMS)₃ under the identical reaction condition and no co-catalyst ligand TPPMS is necessary. More importantly, no organic solvent was used in the reaction which makes this process very promising, economical and simple.

Keywords: Acrylonitrile-butadiene rubber latex; Hydrogenation; Organic solvent free; Water soluble catalyst