Properties of pressure sensitive adhesives
Pressure sensitive adhesives (PSAs) bond upon the application of pressure, rather than through chemical hardening or evaporation. These adhesives are usually applied to some type of backing, which allows them to be adhered by hand or machine at a later time. A release agent is also applied, which keeps the PSA from bonding before its final application. PSAs can be formulated with different chemistries that fit an application precisely. PSAs are available for permanent, semi-permanent or temporary adhesion. They bond a wide variety of surfaces and can withstand varying environments.
Selecting the appropriate PSA for an application depends on both the chemistry of the adhesive itself, as well as the backing the adhesive is on.
Pressure-sensitives have a few different qualities that determine their usefulness in varying situations.
- Tack - tack is the ability of a PSA to "wet" a particular surface and saturate it with adhesive to form bonds. Tack is different from long-term adhesion, but higher tack PSAs may be necessary in high-speed and other instant-adhesion applications.
- Peel Adhesion - Adhesion is the actual strength of the bond formed between the PSA and the substrate it is being adhered to. This will vary depending on application method, temperature and surfaces being adhered.
- Cohesion/Shear - This is the "internal strength" of the adhesive and its ability to stick together under stresses like high heat and shear (parallel) forces. This can come into play when the surface a PSA is bonded to expands and contracts more than the adhesive itself.
Pressure-sensitive adhesives can be formulated with a number of base-chemistries that determines, in part, their properties and what surfaces they will bond to.
- Rubber-based pressure sensitives offer good adhesion, initial tack and bonding to both high- and low-energy surfaces, though do not provide the best resistance to chemical and environmental factors.
- Acrylic-based PSAs offer excellent environmental, chemical solvent and UV resistance, though they can be less adhesive and have lower initial tack than other PSAs. Adhesion and tack can be improved with the addition of tackifiers, but this comes at the cost of reduced resistance to environmental factors.
- Silicone-based pressure sensitives offer even more robust protection from the environment, chemicals and UV degradation. They are also the adhesive of choice for high-temperature (275C range) applications. Their adhesion, particularly to low-energy surfaces can be lower than other types of PSA, however.
The properties of each of these chemistries can be modified by changing the precise formulation and the addition of other compounds like tackifiers.
In addition to the chemistry and formulation of PSAs, their usefulness in different applications is determined by the backing, or carrier, that is coated with adhesive. These carriers can be any number of materials like plastics (duct tape), fabric (medical tape) paper (labels), foil (HVAC wrap) and others. In specialty applications like transfer-tapes in the graphics industry, the pressure sensitive is applied directly to a release liner.
"Application by hand or machine is the only necessary component for adhesion."
One of the main advantages of using pressure sensitive adhesives is that no specialty equipment needs to be used for curing or dispensing. Because the adhesive bonds under pressure, application by hand or machine is the only necessary component for adhesion. This makes PSAs an extremely quick and efficient adherent in industries where they are applicable.
The uses of PSAs are near-universal. The packaging industry makes up a large section of the PSA industry, as they can be used in box sealing, bottle or other package labeling and temporary holding. In the medical field they are used both in tape that adheres to skin, as well as patches for drug delivery. The food and beverage industry uses them extensively for labeling on a wide variety of surfaces. Another major section of the PSA industry is adhesives tabs on disposable hygiene products like diapers. Because PSAs can be formulated for temporary use, they can also find application in assembly or packaging when pieces need to be held in place until a more permanent bond can be formed. Many types of product assembly use PSAs, particularly paper products like notebooks. They of course also have wide use in the consumer industry as all forms of single and double-sided tapes.