The quest for a safer accelerator for polychloroprene rubber

Abstract

This thesis describes the investigation into finding a safer alternative to the polychloroprene cross-linking agent ethylene thiourea (ETU). The novel approach of utilising spectroscopic analysis in combination with a study of mechanical and physical properties of polychloroprene was employed. Currently the most effective cross-linking system for polychloroprene is ETU in combination with zinc oxide (ZnO). However, due to the health risks associated with it, ETU may in the future be banned or heavily regulated, thus a replacement is sought. An investigation of the cross-linking mechanisms of ETU alone, and ETU in conjunction with ZnO was completed. This was necessary as polychloroprene cross-links differently to many other polymers. The study was carried out using model compounds, such as amines and thiols, in addition to work on more widely used accelerators, including thiurams and dithiocarbamates; this revealed that at least three disparate mechanisms are in operation. These mechanisms, comprise one of a cationic nature employed by ZnO, a bis-alkylation mechanism of ETU and a new proposed mechanism seen when ETU and ZnO work together synergistically. This new mechanism shows ZnO activating the polymer chain, followed by the sulfur of ETU creating a cross-link. Using this new mechanism, several new accelerators were proposed. These accelerators contain a dithiocarbamate group and are complexed with diamines. The diamine present acts as an activator and the dithiocarbamate group a cross-linker. These new accelerators were tested in pure polychloroprene rubber on their own, and in combination with tetrabenzyl thiuramdisulfide (TBzTD) or ZnO. All the proposed accelerators were able to cross-link. The replacement which most closely matched ETU, in terms of physical and mechanical properties of the vulcanisate was a complex of piperazine-1-carbodithioic acid and 1,3-diaminopropane (named PNA5). This accelerator was taken forward and tested in a filled commercial master batch. As a successful conclusion to the project a complete PNA5-based system was shown to closely match the performance of an ETU system in a master batch. Concomitantly the use of a multi-functional additive (a complex formed between stearic acid and 1,4-diaminobutane) allowed a reduction in ZnO needed, due to environmental concerns.