Pressure-sensitive adhesive

Source: Wikipedia, the free encyclopedia.
Stickers on a laptop, applied with pressure-sensitive adhesive

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

sticky note
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, 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

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.[citation needed]

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.[citation needed
] 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,

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

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

hot melt PSA
, HMPSA).

Composition

Main article:
Chemistry of adhesive tapes

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.

hot melt adhesive applications, where the composition retains tack even when solidified; however non-pressure-sensitive formulations are also used.[4] 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.[5]

Styrene-

butylene-styrene (SEBS) in low self-adhering non-woven applications, and styrene-ethylene/propylene (SEP) and styrene-isoprene
-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 and differential scanning calorimetry.[6] Of particular importance is identifying the 'glass-transition temperature’, ‘’T’'g.

See also

References

  1. ISSN 2223-7690
    .
  2. ^ Friction Physics (2017-12-06), Science friction: Adhesion of complex shapes, archived from the original on 2021-12-14, retrieved 2018-01-02
  3. ^ US5,798,169, Smith, "SELF. CONTAINING TAMPER EVIDENT SEAL", published 1998 
  4. ISBN 978-0-8155-1851-8
    .
  5. ISBN 9780387690025
    – via Google Books.
  6. ^ Lim, Dong-Hyuk (2006), "PSA performances and viscoelastic properties of SIS-based PSA blends with H-DCPD tackifiers", Journal of Applied Polymer Science, 103 (3): 2839–2846, retrieved 27 June 2023

Further reading

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