Adhesives and sealants are often lumped together, as if they were versions of the same product with the same function. But they’re not. Adhesives and sealants are different.
Adhesives: Functions and FormsAn adhesive is a material that joins two surfaces together by bonding them. It is usually applied as a thin layer between two surfaces or substrates.
To meet the performance needs of the particular application, two factors are important:
Adhesion—the adhesive must bond with each substrate (sticking to them) so they can be held together. If the product will not adhere to one of the surfaces, then the materials cannot be joined.
Cohesion—the adhesive must have enough internal strength so that it holds together and does not break within the film of adhesive when force is applied to pull the two surfaces apart.
The level of required adhesion built into a formation depends on the application. The designed adhesive strength-the amount of force needed to pull the two substrates apart-depends on the end use. Less adhesive strength is necessary for temporary labels that are formulated for easy removal than for high-strength products that must hold parts of airplanes together. The level of cohesion depends on the application. In some applications, if enough force is applied to separate the two surfaces, it is better that the adhesive film itself, breaks so that the surface of the bonded material is not damaged.
Adhesives can be simple, made up of only one or two ingredients, or they can be highly complex, using the most advanced chemical components. They can be water-thin and runny, or very thick. They can air dry by evaporation of water or they can cure by undergoing chemical reaction to form a new, stronger composition.
Sealants: Functions and FormsA sealant is a material designed only to fill up a space. The spaces can be joints, gaps or cavities that occur between two substrates.
The space between a bathtub and wall.
The space between two precast concrete panels forming the wall of a warehouse.
The space between the body and fender of a car.
The sealant is not there to hold the two materials together but is used only to form a seal against the entry of:
Liquids—like rain on a window or water on a bathroom wall.
Gases—like air and wind that could come in through gaps between panes of glass and a window frame. For example, air coming into an automobile.
Solids—like dust or blowing dirt.
Like adhesives, sealants can be very thin or very thick, depending on where and how applied. They will harden or cure in place. To form a successful seal, the sealant must adhere to the surfaces that are forming the gap. However, the primary purpose is filling, not bonding. Included in the family of sealants are putties, caulks, mastics, and high performance sealants.
Factors for successful performance of a sealant include:
Impermeability —air and water must not go through the sealant.
Flexibility —while forming a solid, the cured sealant must be elastic enough to maintain the tight seal even when there is movement of the substrates. An example is a glass curtain wall building, where the spaces between the panes of glass and the underlying steel structure are filled with sealant. As the sun hits the wall in the morning, the steel will expand more than the glass, and there will be a shift in relative positions of the steel and the glass. The sealant must be able to move when they move, adhering firmly to the glass and to the steel, and not have cracks or holes formed within the sealant that would let in air.
Stability—the sealant should not substantially change between the time it is manufactured and the time it is applied. It should not pre-cure in the can or cartridge.
Rheology—if applied to a vertical surface, as between two wall panels, it must stay in place and not sag. If applied in a horizontal gap, as between the sections of an airport runway, it must level or even out.
The decision of how much of these properties to formulate into a particular sealant depends upon the application and its economics. At home, a lower performance window-pane putty or bathtub caulk will be fine. It is inexpensive and relatively easy to replace after a few years. But on a glass-curtain-walled building, the sealant must last a long time because it is very expensive to repair broken seals.
Specialty Minerals Products for Adhesives and SealantsSpecialty Minerals Inc. (SMI) manufactures four types of minerals used in adhesives and sealants. They range from inexpensive ground limestone (calcium carbonate) fillers that reduce costs to talcs that contribute to impermeability and chemical resistance and ultrafine or nano-particled precipitated calcium carbonates that add rheology and strength to high performance sealants used in automobile manufacture and commercial and industrial construction.
Each mineral contributes to the successful application and performance of sealants and adhesives.