The use of sodium birnessite as a curing agent for liquid polysulfide sealant

Abstract

Manganese (IV) oxide phases are widely used as curing agents for polysulfide sealants. The manganese based curing agents, however, do not give a reproducible cure and this results in material waste. As the market for insulating glass applications is increasing, efficient curing becomes more significant both commercially and environmentally. The research described in this portfolio focuses on using a laboratory synthesised manganese (IV) oxide phase, sodium birnessite, as a polysulfide curing agent. The synthesis and full characterisation of sodium birnessite is described and its curing ability measured in terms of the following properties of the cured polysulfide: rate of cure. surface condition, hardness build-up, stress-strain data, low and high temperature properties. and moisture transmission. The results obtained are compared with those of polysulfides cured with a good commercial product. Different concentrations of curing agents, and types and concentrations of accelerators, plasticisers and inert diluents were investigated to optimise the curing process, reduce material use and product wastage. Sodium birnessite is an active and reliable curing agent when used at half of the concentration of the commercial agent in the presence and absence of inert diluents. Diluted sodium birnessite (1: 1 with inert diluent) cures polysulfide to levels comparable in terms of stress-strain properties, moisture transmission rate, hardness, surface condition, low- and high- temperature properties, and degree of polysulfide cross linking to that achieved with commercial agent. The mechanism of the curing of polysulfide with sodium birnessite, as with all curing agents is conversion of mercaptan groups -S-H into disulfide linkages (-S-S-). Sodium birnessite is a mixed oxidation state compound and its synthesis results in the inclusion of Mn 2+ in addition to Mn 4+ ions in the manganese oxide based lattice. A rationale for the improved curing ability of sodium birnessite is presented. It is based on the ability of Mn 2+ to create vacancies in the Mn4+ sub-lattice thus increasing the mobility of the Mn4 in the structure and its transport to the surface of the curing agent where the oxidation reduction reaction responsible for the cure takes place.