Development in crosslinking system for natural rubber and epoxidized natural rubber

Abstract

This research was divided into two parts. The first part was to investigate the possibility to use maleic anhydride (MA) as a crosslinking agent for epoxidized natural rubber (ENR). The second one was to investigate the possibility to prepare the thermoreversible crosslinking of natural rubber (NR) when using glycerol (Gly) or diethylene glycol (DGly) as a thermoreversible crosslinking agent. For the first part, it was found that both ENR25 and ENR50 can be crosslinked with MA. From FT-IR spectra, the emerge of two peaks at 1735 cm-1 and 1710 cm-1 indicated that the crosslinking was taken place by the ring opening of oxirane ring of ENR with the hydroxyl groups of diacids derived from the hydrolysis of MA. Therefore, the linkages between ENR chains were the ester linkages. After swelling testing, it was found that the value of swelling index decreases with the increase of MA loading. In addition, the crosslinked ENR with MA showed better thermal resistance and higher mechanical properties than the uncrosslinked ENR. Rheographs showed that the cure characteristics of the ENR crossslinked with MA was marching behavior with a very high induction crosslinking rate. This led ENR50 to scorch during molding. For the second part, the results showed that it was possible to prepare the thermoreversible crosslinking of NR when using Gly or DGly as crosslinking agents. To achieve this, firstly NR was grafted with MA using a reactive processing method in an internal mixer without adding an initiator. This gave a product called maleated NR or MNR. The grafting was occurred by the shear action and the highest % grafting was 1.76%. The formation of MNR was confirmed by the occurrence of two peaks in FT-IR spectra at 1835 cm-1 and 1790 cm-1 attributed to the C=O of the succinic anhydride ring. The results showed after heating the MNR mixed with Gly or DGly, the obtained products had lower swelling index and higher modulus and tensile strength. This was indicated that the thermoreversible crosslinkings of MNR with Gly or DGly were taken place. The FT-IR spectra of the MNR mixed with Gly and with DGly showed that upon heating covalent ester crosslinks were formed. It was proposed here that the thermoreversible crosslinking was formed via a reaction of succinic anhydride ring with hydroxyl groups of Gly or DGly. It was found after remolding the small pieces of rubber at 150 °C the rubber sheet can be newly formed. This indicated that the thermoreversible crosslinking MNR with Gly or DGly can be reprocessed. After remolding, the mechanical properties decreased with increasing recycling round.