Chemistry of pressure-sensitive adhesives
These rely on adhesive material affixed to a backing such as paper or plastic film.
[1] Because of the inherent tackiness of the adhesive material and low surface energy, these tapes can be placed onto a variety of substrates when light pressure is applied, including paper, wood, metals, and ceramics.
The design of tapes requires a balance of the need for long service life and adaptation to a variety of environmental and human effects, including temperature, UV exposure, mechanical wear, contamination of the substrate surface, and adhesive degradation.
Pressure sensitive adhesives are viscoelastic polymers with their rheology tuned to the desired bonding and de-bonding characteristics needed.
[5] Typical materials used to make the adhesive include: These materials often are blended with a tackifier to produce permanent tack (“grabbing power”) at room temperature,[6][8][9] are somewhat deformable, have low surface energy,[6] and are moisture resistant.
[10] To meet these requirements, these materials are typically low cross-linking density, low viscosity (η < 10,000 cP),[6] and broad molecular weight distribution[6] to enable deformation of the adhesive material to the rough surface of the substrate under various temperatures and peel conditions.
of a binary adhesive mixture of acrylate monomers can be estimated using the Gordon-Taylor equation, where
[Gordon-Taylor Equation] Polyacrylates used in adhesive tapes are readily synthesized by free-radical polymerization.
[6] Such polymerizations are typically carried out in solvent to produce a water-resistant, homogenous coating.
The adhesive is coated onto a flexible material (the backing) such as paper, foil, fabric, or plastic film (such as biaxially oriented polypropylene or polyvinyl chloride[6][9]) to provide strength and protect the adhesive from degradation by environmental factors including humidity, temperature, and ultraviolet light.
Backing tensile strength, elongation, stiffness, and tear resistance can be matched to the intended use of the tape.
The adhesive can be bound to the backing through surface treatments, primers, heating, or UV curing.
This is accomplished by using a material that enables the easy removal of favorable interactions at the adhesive-backing or adhesive-adhesive interface, or by making both surfaces immiscible in one another.
Two common materials used in polyacrylate-based adhesive tapes are fluorosilicones[9] and vinyl carbamates.
[6] Plastic films can have the surface modified by corona treatment or plasma processing to allow increased bonding of the adhesive.
[13] PSAs are able to maintain their tackiness at room temperature and do not require the use of additives such as water, solvents, or heat activation to exert strong adhesive forces on surfaces.
Due to this PSAs are capable of being applied to a variety of surfaces such as paper, plastics, wood, cement, and metal.
The adhesives have a cohesive holding and are also elastic allowing PSAs to be manipulated by hand and also be removed from a surface without leaving behind any residue.
Within this temperature range typical adhesives maintain their balance in viscous and elastic behavior where optimal surface wetting can be achieved.
This could cause problems after application to the surface because if the temperature drops the tape may experience additional stress.
This may lead to the tape losing some of its contact area, lowering its shear adhesion or holding power.
The backing of the adhesives may also be plasticized in order to lower its glass transition temperature and retain its flexibility.
[15] Surfaces with high energy have greater interactions with the adhesive, allowing it to spread out and increase its contact area.
If contaminants are present it may be necessary to clean the surface with a suitable solvent such as benzene, alcohols, esters, or ketones.
However, silicon based removal of moisture will also cause lowering of adhesion and thus failure.
[17] Even the prospect of rapidly changing environmental conditions can be enough to cause a failure in the substrate.
For instance, rapid cooling can cause the substrate to shrink dramatically while the adhesive remains stationary.
[18] Mechanical wear is largely dependent on the amplitude and direction of the forces exerted on the system.
The ideal temperature range is largely dependent on the adhesive identity,[19] which comes down to polymer structure.
Effects on recyclability are particularly important when tape is applied to paper surfaces, such as corrugated fiberboard and other packaging.
