Bouncy ball making kit

I've seen a few kits to make bouncy balls for sale, and I recently bought a cheap one to give it a go as they looked like some fun science and I thought my 4 year old might enjoy it.  

The kit I bought

I've seen them marketed as an educational science activity, and it's quite possible that if you pay a bit more, you get something that tells you a bit about what you're doing and why, but there was nothing like this in the cheap kit.  It didn't even tell you what you were making the ball from, which to my mind is a missed opportunity to introduce children to a bit of science whilst they are having fun.  I did a bit of research so I could  explain to my son, hence writing this blog in case anyone else has a similar kit and wants to be able to explain to children how it works!

Contents of the kit - mould and granules

We made the bouncy 'ball' (the mould provided wasn't a ball, and it looks like many kits have non-spherical shapes) by attempting to follow the instructions.  The plastic mould comes in two pieces which slot together and can be separated later in the process.  Then you get some granules to pour into it, ours came in 4 colours.  The kit was totally silent on what these are, but looking at the Materials Safety Data Sheets online for other identical-looking kits, these are almost certainly polyvinyl alcohol.

Pouring the PVA granules into the mould

When putting the granules into the mould, we shook it a few times to make them pack a little more closely.  The instructions didn't say to do this, but there looked to be some fairly large gaps which I feared would mean the ball didn't hold together well.  My son made some coloured layers of his choice.  The instructions said to fill the mould 95% full, which requires a bit of estimating - we didn't quite get this right and should have put a little more in - I think the granules probably need to come up to the 'neck' of the mould, rather than sitting a little below.

Drying after adding water

The next instructions were to add some water, and immerse the mould (which has some holes) in water.  We did this in an empty yoghurt pot.  We left it in the water for 2 minutes, then took it out, leaving the mould intact for a further 2 minutes.  The mould was easy to open, but one half stuck in to the extent that it seemed the ball would break rather than it coming out, and I had to get a knife to assist it.  I suspect a spherical mould would be easier to release the bouncy ball from.  It was a bit sticky to start with, but after 5 minutes of drying it is a pretty good bouncy ball and my son proudly showed it to his friends later in the day.  The shape means it bounces pretty erratically, but it's quite fun chasing after it!

Finished bouncy ball

So how do the granules stick together?  The polyvinyl alcohol (PVA) is a polymer made of lots of repeating units of carbon, hydrogen and oxygen.  The PVA molecules are like long strings that are tangled together.  When you briefly add water, the PVA is soluble, meaning the long polymers are able to move around in the water, and get tangled up with molecules from adjacent granules.  When they dry out, this new tangling holds adjacent granules together.

And why do the balls bounce?  The long tangled polymer chains have gaps between them (imagine taking a ball of string, unwinding it and then letting it tangle together).  When the ball hits something, the ball squashes, with the polymers squashing into the little gaps.  Once the gaps are filled, the polymers start to expand again, releasing the energy they absorbed and making the ball bounce.  We describe things that squash and stretch back to their original shape as elastic.

This was definitely a fun way to introduce polymers to my son, and the bouncy ball will hopefully last a while!