Pressure-sensitive adhesive (PSA, self-adhesive, self-stick adhesive) is a type of nonreactive

adhesive Adhesive, also known as glue, cement, mucilage, or paste, is any non-metallic substance applied to one or both surfaces of two separate items that binds them together and resists their separation. The use of adhesives offers certain advantage ...

which forms a bond when

pressure Pressure (symbol: ''p'' or ''P'') is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure (also spelled ''gage'' pressure)The preferred spelling varies by country and eve ...

is applied to bond the adhesive with a surface. No solvent, water, or heat is needed to activate the adhesive. It is used in

pressure-sensitive tape Pressure-sensitive tape or pressure-sensitive adhesive tape (PSA tape) is an adhesive tape that sticks when pressure is applied without the need for a solvent (such as water) or heat for activation. It is also known in various countries as self ...

label A label (as distinct from signage) is a piece of paper, plastic film, cloth, metal, or other material affixed to a container or product. Labels are most often affixed to packaging and containers using an adhesive, or sewing when affix ...

s, glue dots,

sticker A sticker is a type of label: a piece of printed paper, plastic, vinyl, or other material with temporary or permanent pressure sensitive adhesive on one side. It can be used for decoration or for functional purposes, depending on the situation. ...

sticky note A Post-it note (or sticky note) is a small piece of paper with a re-adherable strip of glue on its back, made for temporarily attaching notes to documents and other surfaces. A low-tack pressure-sensitive adhesive allows the notes to be easily ...

pads, automobile trim, and a wide variety of other products. As the name "pressure-sensitive" indicates, the degree of bond is influenced by the amount of pressure which is used to apply the adhesive to the surface. Surface factors such as smoothness,

surface energy In surface science, surface energy (also interfacial free energy or surface free energy) quantifies the disruption of intermolecular bonds that occurs when a surface is created. In solid-state physics, surfaces must be intrinsically less energe ...

, removal of contaminants, etc. are also important to proper bonding. PSAs are usually designed to form a bond and hold properly at room temperatures. PSAs typically reduce or lose their tack at low temperatures and reduce their shear holding ability at high temperatures; special adhesives are made to function at high or low temperatures.

Structural and pressure-sensitive adhesives

Adhesives may be broadly divided in two classes: structural and pressure-sensitive. To form a permanent bond, structural adhesives harden via processes such as

evaporation Evaporation is a type of vaporization that occurs on the Interface (chemistry), surface of a liquid as it changes into the gas phase. A high concentration of the evaporating substance in the surrounding gas significantly slows down evapora ...

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

(for example, white glue), reaction with UV radiation (as in dental adhesives),

chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...

(such as two part

epoxy Epoxy is the family of basic components or Curing (chemistry), cured end products of epoxy Resin, resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide fun ...

), or cooling (as in hot melt). In contrast, pressure-sensitive adhesives (PSAs) form a bond simply by the application of light pressure to marry the adhesive with the adherend. Pressure-sensitive adhesives are designed with a balance between flow and resistance to flow. The bond forms because the adhesive is soft enough to flow, or wet, the adherend. The bond has strength because the adhesive is hard enough to resist flow when stress is applied to the bond. Once the adhesive and the adherend are in proximity, there are also molecular interactions such as

van der Waals forces In molecular physics and chemistry, the van der Waals force (sometimes van der Waals' force) is a distance-dependent interaction between atoms or molecules. Unlike ionic or covalent bonds, these attractions do not result from a chemical ele ...

involved in the bond, which contribute significantly to the ultimate bond strength. PSAs exhibit

viscoelastic In materials science and continuum mechanics, viscoelasticity is the property of materials that exhibit both Viscosity, viscous and Elasticity (physics), elastic characteristics when undergoing deformation (engineering), deformation. Viscous mate ...

(

viscous Viscosity is a measure of a fluid's rate-dependent resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for example, syrup h ...

and

elastic Elastic is a word often used to describe or identify certain types of elastomer, Elastic (notion), elastic used in garments or stretch fabric, stretchable fabrics. Elastic may also refer to: Alternative name * Rubber band, ring-shaped band of rub ...

) properties, both of which are used for proper bonding. In contrast with structural adhesives, whose strength is evaluated as lap shear strength, pressure-sensitive adhesives are characterized by their shear and peel resistance as well as their initial tack. These properties are dependent, among other things, on the formulation, coating thickness, rub-down and temperature. "''Permanent''" pressure-sensitive adhesives are initially pressure-sensitive and removable (for example to recover mislabeled goods) but after hours or days change their properties, by becoming less or not viscous, or by increasing the bond strength, so that the bond becomes permanent.

Effects of shape

The adhesive bonding of a tape or label can be affected by its shape. Tapes with pointed corners start to detach at those corners; adhesive strength can be improved by rounding the corners.

Applications

Pressure-sensitive adhesives are designed for either permanent or removable applications. Examples of permanent applications include safety labels for power equipment, foil tape for

HVAC Heating, ventilation, and air conditioning (HVAC ) is the use of various technologies to control the temperature, humidity, and purity of the air in an enclosed space. Its goal is to provide thermal comfort and acceptable indoor air quality. ...

duct work, automotive interior trim assembly, and sound/vibration damping films. Some high performance permanent PSAs exhibit high adhesion values and can support kilograms of weight per square centimeter of contact area, even at elevated temperature. These build adhesion to a permanent bond after several hours or days.

Removal

Removable adhesives are designed to form a temporary bond, and ideally can be removed after months or years without leaving residue on the adherend. Removable adhesives are used in applications such as surface protection films,

masking tape Masking tape, also known as painter's tape, is a type of pressure-sensitive tape made of a thin and easy-to-tear paper, and an easily released pressure-sensitive adhesive. It is available in a variety of widths. It is used mainly in painting, ...

s, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, transdermal drug patches, etc.). Some removable adhesives are designed to repeatedly stick and unstick. They have low adhesion and generally cannot support much weight. Sometimes clean removal of pressure sensitive tape can be difficult without damaging the substrate that it is adhered to. Pulling at a slow rate and with a low angle of peel helps reduce surface damage. PSA residue can be softened with certain organic

solvents A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

or heat. Extreme cold (

dry ice Dry ice is the solid form of carbon dioxide. It is commonly used for temporary refrigeration as CO2 does not have a liquid state at normal atmospheric pressure and Sublimation (phase transition), sublimes directly from the solid state to the gas ...

freeze spray Freeze spray (cold spray or vapocoolant) is a type of aerosol spray product containing a liquified gas used for rapidly cooling surfaces, in medical and industrial applications. It is usually sold in hand-held spray cans. It may consist of var ...

, etc.) can cause

materials to change to a

glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...

phase; thus it is useful for removing many types of PSAs.

Manufacture

Pressure-sensitive adhesives are manufactured with either a liquid carrier or in 100% solid form. Articles such as tapes and labels are made from liquid PSAs by coating the adhesive on a support and evaporating the

organic solvent A solvent (from the Latin '' solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a solution. A solvent is usually a liquid but can also be a solid, a gas, or a supercritical fluid. Water is a solvent for p ...

or water carrier, usually in a hot air dryer. The dry adhesive may be further heated to initiate a

cross-link In chemistry and biology, a cross-link is a bond or a short sequence of bonds that links one polymer chain to another. These links may take the form of covalent bonds or ionic bonds and the polymers can be either synthetic polymers or natural ...

ing reaction and increase molecular weight. 100% solid PSAs may be low

viscosity Viscosity is a measure of a fluid's rate-dependent drag (physics), resistance to a change in shape or to movement of its neighboring portions relative to one another. For liquids, it corresponds to the informal concept of ''thickness''; for e ...

polymers that are coated and then reacted with radiation to increase molecular weight and form the adhesive (radiation cured PSA); or they may be high-viscosity materials that are heated to reduce viscosity enough to allow coating, and then cooled to their final form ( hot melt PSA, HMPSA).

Composition

PSAs are usually based on an

elastomer An elastomer is a polymer with viscoelasticity (i.e. both viscosity and elasticity) and with weak intermolecular forces, generally low Young's modulus (E) and high failure strain compared with other materials. The term, a portmanteau of ''ela ...

compounded with a suitable

tackifier Tackifiers are chemical compounds used in formulating adhesives to increase tack, the stickiness of the surface of the adhesive. They are usually low-molecular weight compounds with high glass transition temperature. At low strain rate, they prov ...

(e.g., a

rosin Rosin (), also known as colophony or Greek pitch (), is a resinous material obtained from pine trees and other plants, mostly conifers. The primary components of rosin are diterpenoids, i.e., C20 carboxylic acids. Rosin consists mainly of r ...

ester In chemistry, an ester is a compound derived from an acid (either organic or inorganic) in which the hydrogen atom (H) of at least one acidic hydroxyl group () of that acid is replaced by an organyl group (R). These compounds contain a distin ...

). The elastomers can be based on acrylics, which can have sufficient tack on their own and do not require a tackifier.

Styrene Styrene is an organic compound with the chemical formula C6H5CH=CH2. Its structure consists of a vinyl group as substituent on benzene. Styrene is a colorless, oily liquid, although aged samples can appear yellowish. The compound evaporates easi ...

block copolymer In polymer chemistry, a copolymer is a polymer derived from more than one species of monomer. The polymerization of monomers into copolymers is called copolymerization. Copolymers obtained from the copolymerization of two monomer species are som ...

s (SBC), also called styrene copolymer adhesives and rubber-based adhesives, have good low-temperature flexibility, high elongation, and high heat resistance. They are frequently used in hot melt adhesive applications, where the composition retains tack even when solidified; however non-pressure-sensitive formulations are also used. They usually have A-B-A structure, with an elastic rubber segment between two rigid plastic endblocks. High-strength film formers as standalone, increase cohesion and viscosity as an additive. Water-resistant, soluble in some organic solvents; cross-linking improves solvent resistance. Resins associating with endblocks (cumarone-indene, α-methyl styrene, vinyl toluene, aromatic hydrocarbons, etc.) improve adhesion and alter viscosity. Resins associating to the midblocks (aliphatic olefins, rosin esters, polyterpenes, terpene phenolics) improve adhesion, processing and pressure-sensitive properties. Addition of plasticizers reduces cost, improves pressure-sensitive tack, decrease melt viscosity, decrease hardness, and improves low-temperature flexibility. The A-B-A structure promotes a phase separation of the polymer, binding together the endblocks, with the central elastic parts acting as cross-links; SBCs do not require additional cross-linking. Styrene-

butadiene 1,3-Butadiene () is the organic compound with the formula CH2=CH-CH=CH2. It is a colorless gas that is easily condensed to a liquid. It is important industrially as a precursor to synthetic rubber. The molecule can be viewed as the union of two ...

-styrene (SBS) is used in high-strength PSA applications, styrene-ethylene/ butylene-styrene (SEBS) in low self-adhering non-woven applications, and styrene-ethylene/propylene (SEP) and styrene-

isoprene Isoprene, or 2-methyl-1,3-butadiene, is a common volatile organic compound with the formula CH2=C(CH3)−CH=CH2. In its pure form it is a colorless volatile liquid. It is produced by many plants and animals (including humans) and its polymers ar ...

-styrene (SIS) are used in low-viscosity high-tack PSA applications.

Temperature considerations

The properties of pressure sensitive adhesives can be strongly affected by temperature. The tack or ‘’quick stick’’ characteristics are critical for the initial bonding to the intended substrate; cool temperatures can make a PSA too firm, losing its tack. Once applied, temperature affects the performance on its intended use: Heat can soften an adhesive, reducing its shear holding ability. Cold temperatures can also contribute to premature release. Most PSA and tape test methods are conducted at 23 (°C) and 50% relative humidity but it is common to also conduct testing at other temperatures (and lighter rub-down pressures) to better match PSA performance with requirements of end users. Adhesive formulators often use the more fundamental temperature characteristics using the advanced methods such as

dynamic mechanical analysis Dynamic mechanical analysis (abbreviated DMA) is a technique used to study and characterize materials. It is most useful for studying the viscoelastic behavior of polymers. A sinusoidal stress is applied and the strain in the material is measured, ...

and

differential scanning calorimetry Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and re ...

. Of particular importance is identifying the glass-transition temperature’'', ‘’T’'_g.