Author: Site Editor Publish Time: 2024-03-13 Origin: Site
Silicone sealants are based on polydiorgano siloxanes polymers, which have the following general formula:
For example, PDMS:
Two main types of silicone sealant are following:
One component silicone sealant is formulated by mixing and reacting in anhydrous conditions the silanol-functional polysiloxane with an excess of hydrolyzable trifunctional silane RSiX3 as shown here under
When the sealant is extruded, the atmospheric moisture reacts with the hydrolyzable groups, and the silanol condenses. This reaction continues until a 3-dimensional network is formed. The by-products which result from the cure may be acetic acid (which gives a typical smell), oximes, amides, alcohols.
Two components silicones are used only for architectural glazing because this glazing is made in the factory to obtain preglazed windows and panels.
These sealants are 2 components products with neutral cure, which have:
Very good adhesion to glass and metals,
Tensile strength up to 1 MPa,
Excellent tear resistance,
Moderate elongation at break (100 to 160%),
Shore A hardness ranging from 35 to 45,
Excellent resistance to ozone, UV, aging, heat (service temperature from -40° to +150°C).
The sealing operation can only be made in the factory prior to installation on-site, in order to guarantee excellent bonds for maximum safety.
Silicone sealants are the most successful sealants since the seventies because they display a combination of many excellent and important features, such as:
Excellent resistance to water, chemicals, weathering, aging, heat, temperature cycles (heat and cold), and consequently excellent durability up to 40 years.
Modulus may be low or higher according to the formulations, elongation at break is very high, up to 500%, so that the service elongation may reach 25 to 50% which are the best values achievable for all sealants.
Price is now very moderate because they are produced in very large quantities.
There are 2 types of polyurethane sealants:
The single-component sealants which are terminated by isocyanate groups -NCO and react with the ambient humidity,
The 2 components sealants where part A is a polymer with -NCO terminal groups and Part B a polymer with hydroxyl -OH terminal groups, these 2 groups reacting together in several well-known modes and reactions.
By varying polymer composition, NCO/OH ratio, catalyst, a wide range of products and properties may be obtained.
