Why does very old clear sticky tape become crumbly? I can understand why the glue on the back might dry out, but really old sticky tape can, when touched, crumble into a pile of shiny dust.
• Clear sticky tape comes in a wide variety of forms and is made using one of many different backing materials. It is intended for short-term fixes, not as a permanent, long-term adhesive, so being degradable is actually beneficial. The UK’s leading brand, Sellotape, uses a cellulose-based backing material that is biodegradable. , like most of the other materials used to make sticky tapes, is a polymer.
Over time, the long polymer chains that make the tape strong and flexible chemically react with their environment and begin to break down. As a result, the tape becomes brittle, begins to flake and eventually crumbles. Depending on the type of tape and the conditions, other changes such as discolouration can occur. How fast this happens depends on the backing material, the adhesive used and the environment. Exposure to sunlight will speed up decay because light, especially ultraviolet, breaks the chemical bonds in the polymer chains.
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Inappropriate use of sticky tape can cause problems for museums and other conservation centres. As well as the tape breaking down, the adhesives on it often react with and damage the documents. Specialist tape is available, using low-acidity adhesives and specially treated or less-reactive backing materials, but that only means the tape lasts longer before decaying and causes less damage.
Alan Clague, Liverpool, UK