SynopsisDichromate 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 Mn0,-cured sealants after prolonged immersion at 7O-9O0C, suggests the formation of a water-soluble acid which then reacts with the CaC03 filler, 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 must also be present.

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The suitability of polysulfide sealants for applications involving contact with hot water has been assessed through changes in swell, permeability, peel strength, and lap shear strength that result from immersion at 25-90 °C. Peel and lap shear strengths decline with prolonged contact times, but this is ascribed to swelling rather than thermal effects. A dichromate-cured sealant (PR-1422) gave a standard saturation time-swell curve, whereas with manganese dioxide cured materials (PR-1750, Pro-Seal 899) a linear relationship existed over wide time (>100 days) and swell (160%) ranges. The generality of this behavior was established by examination of laboratory sealants prepared from various combinations of curing agents and polysulfide prepolymers. Permeability studies confirmed that continual swelling of manganese dioxide cured sealants leads to a volume swell level, between 80 and 120%, where water transmission occurs freely and sealing fails. Dichromate-cured materials were consequently more effective sealants in hot water than those cured with manganese dioxide.

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By modifications to the dies and rotor of an oscillating-disk curemeter, continuous monitoring of features of the cure of two-part polysulfide sealants has been achieved. The principal innovations included use of a larger rotor and relocation of the parting line of the dies to the top of an enlarged cavity, which itself was altered in shape to a square design. By these means and through the introduction of an O-ring on the lower die, the fluid mixtures produced after introducing the sealant components were retained in the cavity. Reproducible torque-time curves were obtained using an arc of ±3°. The diverse applications of the procedure for studying curing variables have been demonstrated through examination of the influence of different curatives, curing ratios, liquid polymers, accelerators, fillers, dispersants, and the thermal resistance of cured sealants in confined spaces. The procedure has also been shown to provide a meaningful discrimination between sealants of different crosslink density. The value of the technique may be further enhanced through introduction of new instruments capable of providing loss factor (tan δ) measurements.