Author: Site Editor Publish Time: 2024-03-12 Origin: Site
The 4 chemical types of sealants which display elastomeric properties are the following:
These sealants have been developed in the sixties in the USA by THIOKOL Corporation, and they were the first elastomeric sealants. They are based on polymers with -SH end groups with an average molecular weight of 4000.
One such example is THIOKOL LP® 32 which has the following formula:
Curing - Curing proceeds by converting the -SH termination into disulfide bonds. This is achieved by oxidizing agents like peroxides, PbO2 and MnO2. It is accelerated by an alkaline environment.
One component polysulfide has limited package stability. A dry to the touch skin will form after 30 minutes to 1 hour at 20°C and 50 to 60% RH, and then the cure will progress into the depth of the sealant at a speed which depends on the thickness of the joint, the temperature and the humidity of ambient air. The cure of polysulfide is slow: it takes one week to reach 50% of ultimate strength. Shrinkage after cure is negligible.
Hardness – Depending on the formulation, hardness may vary from Shore A 20, equal to soft rubber, for vertical joints such as curtain walls, to 50, (hard rubber hardness) with heavily filled formulations, for floor and concrete joints or aircraft runways, where the joints must resist penetration and traffic.
Solvent, fuel and oil resistance – They have excellent resistance, this is the reason why polysulfides have been used widely and are still used for airport runways joints.
Water resistance and weathering - Polysulfide sealants have excellent resistance to water, oxidation, sunlight and weathering. They maintain excellent adhesion after UV and water exposure. A durability of 20 years outside in normal conditions may be expected. Polysulfides are waterproof to water vapor so that they are used for double insulated windows for the exterior seal.
Modulus, Ultimate elongation, Service elongation - Most Polysulfide have high modulus and fairly high elongation at break (100 to 200%). Because modulus is high, these sealants will develop high stresses when elongated, so the recommendation is to use polysulfide only at 15 to 25% service elongation. They have poor puncture resistance.
Creep and stress relaxation - Creep test is a recording of elongation versus time at a constant load. Figure 1 shows a typical creep curve for polysulfide sealants. We can see that the behavior of polysulfides is partly elastic and partly viscous or plastic, and after unloading there is an irreversible deformation resulting from the plastic creep. Elastic recovery is only 60 to 80%.
Application of Polysulfide Sealants: Because they are not 100% elastic and their prices are fairly high, polysulfide sealants are less and less used, and have been replaced by silicones and polyurethanes. However, some jobs still use it:
In Construction: floor joints between concrete and/or metal elements, expansion joints, curtain walls joints, joints between prefabricated panels (concrete panels…), double insulated windows.
In Civil engineering: joints between concrete slabs in airport runways, joints in concrete bridges.
