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

Abstract

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.

Effect of filler on the compression set, compression stress–strain behavior, and mechanical properties of polysulfide sealants

“…As a result, sample PSF06 has the lowest swelling value because of high‐carbon black loading. Addition of filler loading to rubbers has long been known to reduce swell, and this is attributed to the lower volume fraction of polymer, which is available to the swelling agent 24, 25. Moreover, the addition of silicon dioxide filler also helps reduce swelling ratio due to the same reason.…”

Early stages of degradation of polysulphide sealants in an aqueous environment

“…It is clear that there are many contributions to this peak envelope. It is possible to assign some of the peaks by using standard reference spectra, 3 but this does not cover all of the species present. To assign binding energies to some of the carbon-sulphur species, ab initio calculations were employed using Cerius II software.…”

“…For MC 770-B2 these are a mixture of phenolic-resin hollow spheres of ¾20 µm diameter and solid organic particles. This reduces the density to 1.1 g cm 3 . The work in this paper describes the physical behaviour and the surface chemistry of MC 770-B2 after immersion in deionized water for periods of up to 3 months.…”

“…This sealant is based on a currently available material, MC238-B2: a manganese dioxide cured polysulphide with a density of 1.5 g cm 3 . In order to reduce weight, lower density fillers are used.…”