Pressure sensitive adhesives (PSAs) require pressure to initiate bonding between the adhesive and the substrate, whereas other adhesives require heat, water or a solvent. Other factors that influence bonding include surface energy, preparation smoothness and temperature; if an environment is too hot or too cold the adhesive may be compromised and lose its tackiness and holding ability. Room temperature is usually preferred for optimal performance, although conditions may vary depending on the adhesive’s chemical composition.
Typically, PSAs are composed of several key elements: a fluid, adhesives micro-web, and some kind of structured backing. An elastomer functions as the primary base material, which can be any one of the following materials: natural rubber, vinyl ethers, acrylics, butyl rubber, styrene block copolymers, silicones and nitriles. A tackifier, a kind of molecular compound, is added to the elastomer to increase adhesion and can include the following common resins: terpenes, aromatic resins, hydrogenated hydrocarbon resins and terpene-phenol resins. Because tackifiers have high viscoelasticity, the adhesive shares several properties with rubber, such as shear flow resistance and strain resistance when under stress.
There are numerous types of pressure sensitive adhesives, which can be divided into categories according to elastomer materials: rubbers, acrylates and silicones.
Rubbers offer good shear strength, flexibility and adhesion, work well in long- and short-term applications, and are low cost. They are also prone to yellowing, do not do well with high temperatures, and require additives to sustain tack and adhesion.
Acrylates offer UV-, solvent- and hydrolosis-resistance, as well as shear strength and an ability to function at temperatures between -45 and 121 degrees Celsius (C). They tend to be more expensive, and some have low creep resistance.
Silicones can function within a wider temperature range, between -73 and 260 degrees C, and feature high chemical and solvent resistance. They are more expensive than acrylates.
Pressure sensitive adhesives, when used as labels, can also be classified based on the strength of adhesion, into the following categories: permanent, peelable, ultra-peelable, freezer and high-tack .
Permanent adhesives cannot be removed once applied, without causing damage to the substrate.
Peelable adhesives can be removed because the adhesive is weaker, without risking much
damage to the substrate. However, even peelable adhesives can sometimes be difficult to remove because adhesion remains fairly strong.
Ultra-Peelable adhesives are the easiest to remove and do not leave any reside. They are typically used on glass and substrates where residue is undesirable.
Freezer adhesives are designed to withstand extremely cold temperatures without compromising the adhesive.
High-Tack adhesives are strong and work well on uneven or rough surfaces.
When considering pressure sensitive adhesives and their applications, it’s useful to keep in mind several key terms and their definitions.
Shear strength refers to an item’s ability to withstand tangential force.
Flexibility refers to an item’s ability to show an increase in a range of motion, as a result of stretching.
Adhesion refers to an item’s ability to firmly attach itself to a substrate or other item.
Creep refers to the slow flow of a material that occurs when a material’s properties change as a result of high temperature or pressure.