Pressure-sensitive adhesive

Pressure-sensitive adhesive (PSA, self-adhesive, self-stick adhesive) is a type of non reactive adhesive which forms a bond when pressure 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 tapes, labels, glue dots, note pads, automobile trim, and a wide variety of other products.

Stickers on a laptop, applied with pressure-sensitive adhesive

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, 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 of solvent (for example, white glue), reaction with UV radiation (as in dental adhesives), chemical reaction (such as two part epoxy), 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 involved in the bond, which contribute significantly to the ultimate bond strength. PSAs exhibit viscoelastic (viscous and elastic) 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;[1] adhesive strength can be improved by rounding the corners.[2]

Applications
Post-it notes

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 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 tapes, bookmark and note papers, price marking labels, promotional graphics materials, and for skin contact (wound care dressings, EKG electrodes, athletic tape, analgesic and 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 or heat. Extreme cold (dry ice, freeze spray, etc.) can cause viscoelastic materials to change to a glass phase; thus it is useful for removing many types of PSAs. [3]

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 or water carrier, usually in a hot air dryer. The dry adhesive may be further heated to initiate a cross-linking reaction and increase molecular weight. 100% solid PSAs may be low viscosity 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 compounded with a suitable tackifier (e.g., a rosin ester). The elastomers can be based on acrylics, which can have sufficient tack on their own and do not require a tackifier.; bio-based acrylate – recently, a biological-based macromonomer was grafted onto a backbone of acrylate so that the resulting PSA uses 60% bio-based materials,[4] butyl rubber, ethylene-vinyl acetate (EVA) with high vinyl acetate content; can be formulated as a hot-melt PSA, natural rubber, nitriles, silicone rubbers, requiring special tackifiers based on "MQ" silicate resins, composed of a monofunctional trimethyl silane ("M") reacted with quadrafunctional silicon tetrachloride ("Q").

Styrene block copolymers (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. High heat resistance, good low-temperature flexibility.[5] Lower strength than polyesters. 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 improve 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,[6] styrene-butadiene-styrene (SBS), used in high-strength PSA applications styrene-ethylene/butylene-styrene (SEBS), used in low self-adhering non-woven applications, styrene-ethylene/propylene (SEP), styrene-isoprene-styrene (SIS), used in low-viscosity high-tack PSA applications, vinyl ethers.

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See also

References
  1. Popov, Valentin L.; Pohrt, Roman; Li, Qiang (2017-09-01). "Strength of adhesive contacts: Influence of contact geometry and material gradients". Friction. 5 (3): 308–325. doi:10.1007/s40544-017-0177-3. ISSN 2223-7690.
  2. Friction Physics (2017-12-06), Science friction: Adhesion of complex shapes, retrieved 2018-01-02
  3. US5,798,169, Smith, "SELF. CONTAINING TAMPER EVIDENT SEAL", published 1998
  4. "Bio-based PSA".
  5. Liesl K. Massey (1 January 2003). Permeability Properties of Plastics and Elastomers, 2nd Ed.: A Guide to Packaging and Barrier Materials. William Andrew. pp. 582–. ISBN 978-0-8155-1851-8.
  6. Mark, James E. (21 March 2007). Physical Properties of Polymers Handbook. Springer Science & Business Media. ISBN 9780387690025 – via Google Books.

Further reading
  • "Pressure-Sensitive Adhesives and Applications", Istvan Benedek, 2004, ISBN 0-8247-5059-4
  • "Pressure Sensitive Adhesive Tapes", J. Johnston, PSTC, 2003, ISBN 0-9728001-0-7
  • "Pressure Sensitive Formulation", I. Benedek, VSP, 2000, ISBN 90-6764-330-0
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