Chemistry of Silly Putty

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Silly putty (or science putty, as it’s sometimes referred to) is an odd material. Stretch it slowly and it happily deforms, and can be molded very easily. However, pull it apart with enough force and it’ll snap clean in two. It flows imperceptibly slowly, as if it were a thick liquid, but when rolled up into a ball will bounce if thrown at a hard surface. A closer look at the chemicals that make up silly putty can help us explain this strange behaviour.

A number of ingredients go into making silly putty: by weight, 65% of it is a compound called polydimethylsiloxane, but colouring agents are also included to give the wide range of colours that the putty comes in. It’s the polydimethylsiloxane (PDMS for short) that’s a large contributor to the strange properties of silly putty, however.

PDMS is a type of silicone, a group of polymers defined by the fact that they all contain Si-O-Si units as the basis of their polymeric structure. Silicone polymers are, of course, more commonly known for their use in breast implants – and aren’t to be mistaken for the element silicon on its own, which would be a lot more uncomfortable!

The properties of PDMS are partly responsible for silly putty’s properties. It’s what’s known as a viscoelastic solid. This basically means that it is capable of flowing like a liquid in some cases, but behaves like an elastic solid in others. The polymer chains are quite flexible, and when they are particularly long, as in the case of silly putty, they can become loosely entangled around one-another. This is what causes PDMS’s viscoelasticity.

However, the viscoelasticity of PDMS alone isn’t enough to account for silly putty’s oddities. Another ingredient in the mix, boric acid, also makes a decisive contribution. The PDMS chains in silly putty end in OH groups. The boric acid can react with these to form transient boron-mediated linkages between different polymer chains. These ‘crosslinks’ help hold the putty together, and also contribute to its properties.

When the putty is slowly molded, the crosslinks have time to break and reform at different points in the polymer chains. This means we are able to see the viscous flow of the putty. However, when the putty is pulled with suitable force, the crosslinks do not have time to break and reform, so elastic behaviour is seen. With suitable force exerted, the putty can even be shattered.

As well as it’s obvious appeal for entertainment value alone, silly putty has actually found some serious uses. It was used to help secure tools during some of the Apollo space missions, due to its mild adhesive characteristics, and it’s also found use in some therapy techniques for patients recovering from injury to their hands.

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