All adhesives in widespread use today can be considered “wet” adhesives. These materials are typically applied to a surface in a liquid state and achieve intimate contact by flowing along the surface and wetting surface asperities. To be an effective adhesive, the liquid must be transformed into a solid with high cohesive strength through either a chemical reaction (crosslinking), or in the case of hot melts, by cooling below the melting temperature.
Structural adhesives transition to the solid state through a chemical reaction. This class of adhesives is extremely strong (shear strength in excess of 1,000 psi), and widely used to join load-bearing parts of a product. Common types of structural adhesives include epoxies (highest strength and temperature resistance), two-part acrylics (bond to the widest variety of substrates and are useful on oily surfaces or low surface-energy plastics), two-part polyurethanes (lower cost solution used in applications requiring flexibility and impact resistance), and anaerobic adhesives (curing proceeds in absence of oxygen, which is useful for threadlocking, gasketing, and sealing applications). The main limitation of structural adhesives is that once cured, they are very challenging to remove. If used in the proper way, structural adhesives will achieve an extremely intimate contact with the target surface and will fail cohesively, which results in a rough fracture surface and is the most irreversible form of adhesion.
Pressure-sensitive adhesives (PSAs) are among the most rapidly growing classes of adhesives because they do not require post-treatment, such as curing or drying, to work – they can be applied and used in the same state. PSAs are predominantly made from acrylics, styrenic block copolymers, or natural rubber, although silicone rubber is used for niche low or high-temperature applications. These are extremely soft materials that are able to conform to, or wet, surfaces under application of light pressure. This ease of compliance is known as “tack” and is an important design parameter. PSAs do not undergo any physical transformation or chemical reaction during bonding, the adhesive strength is determined only through weak van der Waals (vdW) forces between the surfaces. When peeled from a surface, PSAs tend to form cavities that do not coalesce and instead grow as fibrils between the surfaces. This fibrillation is the reason for the high peel strength of PSAs, and is also the reason why PSAs tend to leave behind significant residue after use.
One important characteristic of PSAs is that they are highly dissipative materials. The resistance of any material to deformation can be described as a combination of solid-like (elastic) and liquid-like (dissipative) responses. Materials that we regard as “solids” are characteristically elastic, meaning that after a small deformation, they recover their initial shape. They have very low dissipation, which occurs when the molecules are easily re-arranged when deformed, turning the mechanical energy into heat. To put another way, elastic materials have an excellent “memory” of their shape, whereas dissipative materials do not and will tend to adopt the shape of their container over time.
A major advantage of dry adhesives is that their performance is predominantly elastic and independent of irreversible behaviors such as fibrillation. This allows for vastly improved reusability, in addition to potentially higher strength for a smaller amount of material, clean removal, and other benefits not possible with conventional adhesives.
In contrast, pressure-sensitive adhesives are carefully formulated to be liquid-like enough to conform to a surface and establish a high surface area interface, while remaining elastic enough to resist external forces and not flow down a wall or fail when moved. PSAs are generally developed to have their relative dissipative response (ratio between dissipation and elastic components of response) at its maximum for a given set of use conditions. However, the magnitude of dissipation is sensitive to temperature, so the behavior will be significantly altered by even minor heating or cooling, and may change over time due to various aging phenomena. The dominant liquid-like character also means that PSAs are very susceptible to dimensional changes over time (creep), and that some crosslinking is generally required.