What is sellotape? 

Sellotape is basically a brand of transparent, polypropylene-based, pressure-sensitive tape. But, this term is also used for any such pressure sensitive tape of any brand. Sellotape is generally used for joining, sealing, attaching and mending.
The older version of sellotape has shown some changes in its physical and chemical properties over a longer period of time due to ageing. Initially such tapes are transparent and with more fluid adhesive, but over many years, it tends to become discoloured, acidic and brittle.
Richard G. Drew (1899-1980) invented masking tape and clear adhesive tape (also called cellophane tape, Sellotape or Scotch tape). Drew was an engineer for the 3M company (the Minnesota Mining and Manufacturing).

Sellotape is broadly made up of two layers.

1. On top there is a clear film, called the carrier, which is traditionally cellophane. Cellophane itself is regenerated cellulose, the stuff that provides structure to plant cells and is also used to make paper and plastic. Nowadays the carrier in clear tapes is generally plastic, but you can find tapes with paper carriers such as masking tape.

2. The bottom layer is the sticky part, the pressure-sensitive adhesive. The adhesive is traditionally rubber-based and is made so that it bonds with what it touches when pressure is applied. When self-adhesive tape was first invented this was a big deal, since before the adhesive would have to be triggered by heat, water, or another solvent.

Deteriorating changes associated with ageing of Sellotape

The (rubber like) adhesive, is a long polymer chain just like the cellulose that makes up paper. Over time as the paper and the adhesive on the tape stay stuck together, the two types of polymers begin to interact and attach to each other. This process is called cross-linking, and this change to the chemical structure causes the sellotape to become insoluble and discolored. The longer this goes on, the more difficult it is to remove the adhesive from the paper and the more discoloured it becomes. However, as it cross-links it also becomes less effective as an adhesive and eventually the tape carrier falls off, leaving solid, discoloured adhesive behind. At this stage there’s not a lot a conservator can do.

How does Sellotape or pressure sensitive tape work?

Pressure-sensitive adhesives (which are polymers) are ‘tacky’ or ‘sticky’ because they are essentially very high viscosity liquids that also have some elastic characteristics–in technical terms, they are ‘viscoelastic.‘

This property means that they exhibit some of the characteristics of liquids, and so they will ‘wet’ a surface to which they are pressed. But then, because of their elasticity, they will resist separation when stressed. Thus, ‘stickiness’ is strictly a physical (viscoelastic) phenomenon, not a chemical one.”

There are two fundamentally different components of tape’s sticky nature; adhesion and cohesion.

Adhesion is the binding force between two different materials, whereas cohesion is the binding force between two similar materials.

When two materials are brought into contact with each other, the surface molecules interact, giving rise to attractive forces that may be physical, chemical or electrostatic (corresponding to adsorption, covalent bonding or van der Waals forces, respectively). When the molecules are similar, as in the case of two ‘glue molecules,’ the cohesive force causes the glue to stick to itself. When the molecules are dissimilar, as in the case of a glue molecule and a molecule of the substrate (the surface the glue is sticking to), the adhesive force holds the glue to the substrate. Hence, the ‘stickiness’ of tape is caused by a combination of the molecular forces of the glue material sticking to itself as well as holding onto the substrate.”