Sellotape: How it deteriorates and changes with time

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What is sellotape?  sellotape3

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.

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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.”

Author: namitajaspal
Namita Jaspal, with inherited skills of art and aesthetics and passion for science, opted to pursue post- graduation in ‘Conservation of Cultural Property’ after her graduation in Science from Delhi University in 1992. Four year intense training at National Museum Institute provided the right foundation to start a career in Conservation. She is currently practicing conservation consultancy for Heritage property including monuments and collections. She has been doing independent research in conservation techniques and procedures in Indian context. She is currently working on the conservation of wall-paintings of Sri Harmandir Sahib Ji (The Golden temple at Amritsar. The project is nearing completion and getting a lot of appreciation for the organized and ethical treatment it is providing. It is for the first time in the history of Sri Harmandir Sahib Ji (The Golden Temple) that wall painting conservation and preservation is being done in a scientific manner, keeping the codes of Ethics into consideration while decision making. Another project just completed is of the Conservation of Krishna Temple at Kishankot, Gurdaspur, Punjab. In her private Conservation Laboratory, she is not only providing onservation services, but also mentoring young aspiring conservators and archaeologists by the way of training and paid internships. She has been a guest lecturer at DIHRM (Delhi Institute of Heritage Research and Management). With all the updated knowledge database and innovative practical approaches relevant to the Indian conditions, she could successfully do curative and preservative treatment of very old textiles like a nine feet long flag from nineteenth century that is regarded as priceless memorabilia of second Dogra regiment at Chandi Mandir, and four hundred years old (seventeenth century) Chola Sahib Ji of Sixth Guru of Sikhs, Sri Hargobind Sahib Ji. Apart from this she has done conservation of Photograph collections, archival records, numerous paintings, manuscripts and other cultural objects. She is also involved in preparation of up-gradation proposals for museums and such organizations. Her expertise includes Conservation and preservation technologies and procedures for conservation of varied material like wall paintings, paper, photographs, textile, ceramics, stone, metal and archaeological objects.

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