Water immersion of polysulfide sealants. II. An interpretation of the influence of curing systems on water resistance

摘要

AbstractDichromate cured polysulfide sealants have superior resistance to swelling in hot water than those cured with manganese dioxide and examinations to identify the reasons for this difference were undertaken. The commercial sealants PR‐1422, PR‐1750, and Pro‐Seal 899 were shown to contain similar types and loading of fillers. Crosslink densities were compared through a procedure involving an oscillating disc cure meter modified to accommodate mobile mixtures. No relationship between stability and crosslinking was observed. Formation of large voids in MnO2‐cured sealants after prolonged immersion at 70–90°C, suggests the formation of a water‐soluble acid which then reacts with the CaCO3filler, and this was supported by pH measurements. The acid was shown to arise through autoxidation of formal groups in the polysulfide, and this process can be suppressed by radical scavengers (including ammonium dichromate). When PR‐1750 was examined under nitrogen or with carbon black as filler, a similar time–swell curve to that of PR‐1422 was produced. It was concluded that abnormal swell does not result simply from autoxidation: a filler capable of reacting with the acid mu