The main difference between adhesives and sealants lies in that sealants typically have lower strength and higher elongation than adhesives. Because the main objective of sealants is to seal assemblies and joints, sealants need to have sufficient adhesion to the substrates and resistance to environmental conditions to remain bonded over the required life of the assembly. When sealants are used between substrates with different thermal coefficients of expansion or different elongation under stress, they need to have adequate flexibility and elongation. Sealants generally contain inert filler material and are usually formulated with an elastomer to provide the required flexibility and elongation. They usually have a pasty consistency to allow filling of gaps between substrates. Low shrinkage after application is often required.
Sealants fall between higher-strength adhesives at one end and extremely low-strength putties and caulks at the other. Putties and caulks serve only one function, namely, to take up space and fill voids. Sealants do convey a number of properties despite not having great strength. They seal the substrate at the glue line, and are particularly effective in keeping moisture in or out of the components in which they are used. They provide thermal and acoustical insulation and may serve as fire barriers, and sometimes they contain electrical properties. They may also be used for smoothing or filleting. In short, sealants are often called upon to perform several of these functions at once.
No matter what the application, a sealant has three basic functions, including filling a gap between two or more substrates; forming a barrier through the physical properties of the sealant itself and by adhesion to the substrate; maintaining sealing properties for the expected lifetime, service conditions and environments.
The sealant performs these functions by correct formulation to achieve specific application and performance properties. But unlike adhesives, there are not many functional alternatives to the sealing process. Soldering or welding can perhaps be used as a sealant in certain instances, depending on the substrates and the relative movement that the substrates will see in service. However, the simplicity and reliability offered by organic elastomers usually make them the clear choice for performing these functions.
Sealants are generally classified into one-component sealants, two-component sealants, and sealant tapes, in terms of physical form.
• One-component sealants: Packaged in a cartridge. No special equipment is required to apply one-component sealants. Chemical technologies include acrylic solvent-based, butyl solvent-based, latex water-based, silicone, and urethane.
• Two-component sealants: Composed of two parts, a base component and an activator component. The activator is typically added to the base component and mixed for a set period of time before application. Two-component sealants require bulk guns and mixing equipment to prepare and apply, and are typically packaged in separate buckets. Chemical technologies include epoxy-penetrating solvent-based (supplied as two-component high-solid compounds), silicone, and urethane.
• Sealant tapes: Supplied as sealant on a flexible backing. Types include butyl and silicone tapes (both preformed shape) and urethane tape (supplied in a compressed state).
This article is reproduced from Adhesives.org/Sealants.org.