Adhesive (Pressure Sensitive) - What is a pressure sensitive adhesive?
Definition of PRESSURE SENSITIVE ADHESIVE:
A type of adhesive that creates an adhesive bond through the application of light pressure only; while other adhesives (sometimes collectively known as “structural adhesives”) form an adhesive bond once they have hardened (via a chemical or physical process), pressure sensitive adhesives or PSAs are tacky (sticky) in their dry form at room temperature and form a bond through a combination of flow and resistance to flow.
PSAs create bonds through interfacial forces between two surfaces (e.g. the adhesive and the adherend or substrate), which means that they need to be soft enough to be able to flow (creating the maximum area of contact between the two surfaces) and hard enough to resist flow when stress is applied to the bond. The three key properties of all PSAs, therefore, are the initial tack, ultimate adhesion, and shear resistance. Initial tack describes the strength of the bond that is created immediately upon contact between an adhesive and an adherend, while ultimate adhesion describes the maximum strength of the bond once the adhesive has fully set. Shear is a type of stress, whereby the external force acting on an object or surface is parallel to the surface; while resistance to shear stress involves both the adhesive and cohesive strength of an adhesive, shear resistance is most closely associated with cohesive strength (shear resistance is also known as cohesive strength, as well as shear strength, shear adhesion, and holding power). PSAs with low shear resistance (soft) have more tendency to flow (creating higher initial tack) and are more likely to split apart under stress, while adhesives with high shear resistance (firm) have less tendency to flow (which may lower their initial tack) and are less likely to split under stress because they have better cohesion.
Generally speaking, PSAs are made up of an elastomer (the base) and a tackifier (to improve the initial tack of the adhesive), although additives may be used to introduce or enhance key properties that are required for a specific label application. Elastomers are polymers that have weak inter-molecular forces and have the property of viscoelasticity, which means that they have both viscosity (the ability to resist flow) and elasticity (the ability to return to their original size and shape after being deformed). Common examples of elastomers used to make PSAs include acrylics, rubbers, and silicone rubbers.
PSAs are made as emulsions, solutions, hot melts, ultraviolets, or 100% solids. In emulsions and solutions, the adhesive components are suspended in a liquid (emulsion) or dissolved in a solvent (solution) so that they can be coated in liquid form before the liquid or solvent evaporates to leave behind a layer of pressure sensitive adhesive. In hot melts and ultraviolets, the adhesive is also coated in a liquid form (molten form) before hardening into their final form when they cool (hot melts) or are cured with ultraviolet light (ultraviolets). 100% solids are sometimes known as “warm melts” and may react with radiation or may be heated and then cooled to produce their final form. Emulsion and solvent based adhesives are the most common types of PSAs; while solvent adhesives offer greater strength and higher moisture resistance, emulsion adhesives are the most popular type of PSA used to create adhesives for labels, packaging, and tapes.
Most PSAs are manufactured from rubbers (rubber-based adhesives) or acrylics (acrylic-based adhesives), although some adhesives may be made with a different base material, such as silicone adhesives (which offer high resistance to moisture, chemicals, and other environmental conditions, and are often used for highly specialised applications such as those that involve skin contact, including medical applications (e.g. wound dressings, EKG electrodes, athletic tape, analgesic and transdermal drug patches) or drama production applications (e.g. make up and prosthetics)). Rubber-based adhesives can be made using natural rubber or synthetic rubber and tend to be softer adhesives, which produce good initial tack, although they are more prone to adhesive bleed and can be affected by oxidation and UV light. Acrylic adhesives are made from acrylic polymers and are more expensive than rubber-based adhesives, although they offer greater resistance to heat, oxidation, UV light, and solvents.
While adhesives can be grouped according to manufacturing method or material base, they are most commonly classified as either “permanent adhesives” or “removable adhesives”; permanent adhesives are designed to create a strong adhesive bond that will result in the label and/or the substrate being damaged when a label is removed, while removable adhesives are designed to create a good adhesive bond that will hold the label in place but that will also allow the label to be removed cleanly and easily without damaging the label and/or the substrate when the label is no longer needed (labels with a removable adhesive are therefore also known as temporary labels). Some removable adhesives may also be repositionable adhesives, which allow a label to be removed (without damaging the label and/or substrate) but also retain enough tack to allow the label to be reapplied onto the same surface or a different surface. PSAs may be further classified according to the intended label application or key properties of the adhesive; common examples include marine adhesives, VOID adhesives, deep freeze adhesives, glass removable adhesives, and opaque adhesives.
The efficiency of PSAs can be influenced during and after the application of a label by a number of factors, including the strength of the adhesive itself (both adhesive and cohesive), the length of time a label has been stuck onto a surface, the surface type, and environmental conditions. Obviously, an adhesive must have enough adhesive and cohesive strength to allow an efficient bond to be formed and sustained, while the passage of time is vital to allow a PSA to develop into its ultimate adhesion. The surface can influence the efficacy of an adhesive bond according to its smoothness (if the surface is completely smooth (no troughs) or very rough (too many troughs OR very deep troughs) the surface area that the adhesive can reach will be too small for a successful bond – the best surface type has some shallow troughs), its surface energy (high energy surfaces have greater interfacial interactions with the adhesive, creating a stronger adhesive bond), and the presence of any contaminants (particles of dust, oils, or moisture on the surface reduce the amount of surface area available and the adhesive may actually bond with these particles instead of the surface, resulting in a weaker bond). If the surface changes over time, this can also result in the failure of an adhesive bond.
The main environmental factor that can influence a PSA is temperature; exposure to extreme temperatures or fluctuating temperatures can result in the failure of an adhesive bond. At higher temperatures, PSAs will become more fluid and mobile, which can result in adhesive bleed or reduced adhesion. If the temperature then suddenly drops, the adhesive may experience additional stress because it has stretched more than it could initially and so loses some of its contact area as it cools, which reduces its adhesion. Likewise, if a PSA is exposed to lower temperatures, the adhesive may become harder and stiffer, which reduces its elasticity and makes it harder for the adhesive to maintain surface contact, resulting in the adhesive becoming less adhesive and brittle like glass. Other environmental factors that can cause issues are humidity, UV light, and exposure to other substances, such as chemicals.
Pressure sensitive adhesives are used for a wide variety of applications because of their ability to create a bond between a wide range of different materials (e.g. paper, wood, metals, plastics, and ceramics) and surface types. Common applications include pressure sensitive labels, packaging, tapes, glue dots, note pads, post-it notes, automobile trim, protective films, sound/vibration damping films, masking tapes, price marking labels, promotional materials, and skin contact applications.
At Label Planet, all of our labels are made with pressure sensitive adhesives; we have a range of permanent adhesives and removable adhesives, including a number of “special adhesives” that have been designed for a specific type of label application.
Here is the Harvard-style citation to use if you would like to reference this definition of the term pressure sensitive adhesive:
Label Planet (2017) What is a pressure sensitive adhesive? | Definition. Available at: https://www.labelplanet.co.uk/glossary/adhesive-pressure-sensitive/ (Accessed: January 1, 2023).