Written by Dr. Jody Muelaner
Pressure adhesive, also known as pressure-sensitive adhesive (PSA) or self-adhesive, flows between the surfaces being bonded. If the surfaces are smooth enough to get close together, the viscosity of the thin film combined with molecular interactions, form a strong bond.
This is the type of adhesive used on self-adhesive tapes and stickers. Putties such as Blu Tack are also pressure adhesives.
Most other types of adhesives harden to form a bond. This hardening may be due to evaporation of a solvent, a chemical reaction causing curing, or cooling in the case of a hot-melt adhesive.
Pressure adhesives are different, however, in zero chemical or compositional change occurs at the time of bonding. Pressure-sensitive adhesives are typically low-viscosity polymers that do not need to harden to form a bond. The adhesive remains in the same state but it flows between the surfaces to form a thin film. When the film is thin enough, covering both surfaces, the low viscosity of the adhesive binds the surfaces together.
Molecular interactions, such as van der Waals forces, may also add to the bond strength. Some types of pressure adhesive undergo secondary hardening over time, causing the bond strength to increase, sometimes over long periods of many days.
The strength of the bond formed by pressure-sensitive adhesives depends on the pressure applied, the closeness of the interface between the surfaces, and how clean and dry the surfaces remain. PSA’s typically only perform well at room temperature. At low temperatures, they’re unable to flow between the surfaces properly to form the initial bond. At high temperatures, they become too soft and established bonds will fail.
The bond strength often increases gradually over a few days. Tapes and stickers can, therefore, typically be easily repositioned a few minutes after attachment. They will be much more difficult to move some days after fitting. This property varies, with some types of PSA remaining reusable after long periods of use.
Some common pressure-sensitive adhesives include:
- Duct tape is a heavy-duty self-adhesive fabric tape widely used for general repairs as well as for sealing insulation within construction.
- Insulation or electrical tape is a tape used to insulate electrical wires and fittings. It is typically a stretchy vinyl tape with pressure-sensitive adhesive on one side.
- Sticky tape is also known as sticky backed plastic or by the brand names Scotch Tape and Sellotape . This is typically a translucent polypropylene tape for general household and office use, especially for sticking paper and card.
- Packing tape is also known as parcel tape or box-sealing tape, is similar to household sticky tape but the roll is wider. It is used for sealing cardboard packaging boxes.
- Mounting putty such as Blu Tack or Poster Putty is simply a mass of pressure-sensitive adhesive. It can be broken into pieces and is often used to stick posters to the wall.