Adhesives and sealants are often considered together because they both adhere and seal and share several common characteristics. However, the main purpose of a sealant is not to withstand substantial stress but to seal and to fill gaps between joints and to provide a barrier or protective coating.
Sealants have normally lower strength and higher elongation than adhesives and are often used to join and/or seal materials with different thermal coefficients of expansion or modulus, and, therefore, need to have sufficient flexibility and elasticity. In some cases, the substrates may even move, requiring the sealant to expand and shrink significantly without losing adhesion to the substrates.
Depending on the chemistry, the adhesive properties of a sealant will depend on water exposure, temperature, and surface cleanliness. In some cases, a priming step might be necessary to improve wetting and adhesive strength of the sealant.
Sufficient flexibility and adhesive strength are not the only important properties. Depending on the application, a sealant has to have adequate resistance to heat, tear, ultraviolet light, moisture, and oxidation. Important application properties are adequate open and working time, good sag resistance, paint-over-ability, color, self-leveling properties, and hardening time.
Adhesives and sealants are often made of similar compounds, that is, both can be formulated with similar monomers/polymers, fillers, plasticizers, and elastomers to achieve the needed properties such as cure speed, cost, flexibility, and elongation.
Sealants have usually the consistency of a paste to allow filling of (large) gaps between substrates and have low shrinkage to prevent formation of cracks after cure or physical solidification.
The most important sealant types are asphalt, butyl, polysulfide, urethane, acrylic, silicone, polyester, and epoxy
Sealants and adhesives have similar application requirements and failure mechanisms. However, the specifications and test methods for adhesives and sealants are often quite different, because they are designed to perform different functions, and therefore, have to meet different performance requirements.