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. One notable characteristic of PSAs is that when peeled from a surface, they 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 separated into an elastic and a dissipative part. The elastic part is the solid-like response – it is the ability of a material to recover its original shape after being deformed. The archetypical “perfectly elastic” material is a perfect spring (called a “Hookean” spring). Since this is fully recoverable, it is also fully reversible. The opposite of elastic response is dissipation, which is also known as a viscous or liquid-like response. A material is said to exhibit a strong liquid-like response if it dissipates energy as heat, which essentially means it is unable to “remember” its original shape. PSAs operate by dissipation – they are carefully formulated and tailored so that at their use temperature, their relative dissipative response (often described as a “damping factor” or tangent delta, where delta is a physical lag in time response between an applied strain and measured stress) is at its maximum. This gives an “optimal” performance, but has a few inherent drawbacks: the magnitude of dissipation is extremely sensitive to temperature, so the behavior will be significantly altered by even minor heating or cooling. Dissipation is by definition non-recoverable, so it is impossible to have a reusable PSA. In other words, the liquid-like character means PSAs are very susceptible to dimensional changes over time (creep), which means that some crosslinking is generally required and necessitates a very careful balancing of components with inherent trade-offs.