Inflating milk bottles to make plastic bags

When school students visit our Materials department, they are usually given a tour of the polymer processing facilities. They normally get to see processes like injection moulding and extrusion blow moulding. However, for the most recent visit the processing lab was busy, so I was given the challenge of demonstrating polymer processing in a lecture room!

First, I showed the students the fundamental stages of polymer processing (melting, shaping and cooling) using polycaprolactone, which can be melted with boiling water.

To demonstrate processing using a common polymer at real-world polymer processing temperatures, usually much higher than 100 C, I simply used a hot air gun (the type used for paint stripping, like a super-hot hair dryer) on the side of a 1-litre HDPE milk bottle. Within a few seconds the polymer melts, and changes from cloudy to clear.

Milk bottle before heating

After heating with a heat gun

If you blow into the neck of the bottle, you can inflate the melted portion into a bubble, which eventually bursts. It's fun! Here's a video of me inflating a bottle:

 The polymer in the skin of the bubble cools and freezes very quickly. Take care though, there will still may be some very hot melted polymer around the bottom of the bubble. Here's the finished 'product':

I get the students to feel the bubble - it feels exactly like a supermarket plastic bag. This is because these bags are also made out of HDPE, and also made by inflating molten polymer (in the blown film process). Inflation is also how the milk bottle was made in the first place - by extrusion blow moulding.

I've been doing this demo at the end of a lecture on glass transition temperature. To tie the two ideas together, I dunk the neck of the bottle in liquid nitrogen and smash it with a hammer:

The students have then seen the bottle in three states: solid/glassy, solid/rubbery and melted.

Using polycaprolactone for introducing polymer processing

Polycaprolactone is a polymer with a low glass transition temperature (-57 C [1]) which melts at 60 C [2], and so can be melted simply by dunking the polymer in hot water. You can see the polymer become clear as the crystalline regions melt, and you can feel that melted polymer still has strength and high viscosity (unlike a small molecule melting, like water) since it is made from entangled chains.

A great deal of polymer processing relies on having decent 'melt strength' and high melt viscosity. For example, in extrusion blow moudling a molten polymer tube is extruded, clamped into a mould and inflated with air. This process would not be possible if the viscosity and melt strength of the polymer were low - the polymer would either flow to the bottom of the mould, or burst on inflation!

Here's what the process of melting polycaprolactone looks like:

Solid polymer

Melting in boiled water

Melted polymer - the material becomes transparent

I bought mine under the trade name 'Polymorph' from Maplin in the UK, although it's available from other places and under other names (e.g. ShapeLock or Friendly Plastic).

I think the easily accessible melting temperature makes it ideal for teaching the fundamentals of thermoplastic polymer processing: plastication (melting), shaping, shape stabilisation (cooling).

Last week, I tried this out with some A-Level school students (16-18 years old). Before the session, I had a play around to come up with some ideas for things to make.

Roll the polymer between your hands into a tube, then shape round a big pen to make a spring. On cooling, the spring is nice and springy.

Again, after rolling into a tube, you can pull the ends apart to draw fibres – we managed to get it many metres long.

Once the fibre has cooled, you can stretch them further to demonstrate strengthening by fibre drawing, since the polymer is well above the glass transition temperature at room temperature.

By stretching a sheet you can form a film, although we found difficult to get large areas without holes.

Making linked rings is something that is quite difficult to do with 'normal' polymer processing techniques!

The students had fun playing with the material and seemed to grasp the relevance to real-world polymer processing. Apparently some of them were soon to be introduced to it as a 'smart material' as part of their A-Level studies

References

[1] Sivalingam et al., Polymer Degradation and Stability 2004, 84, 345-351
[2] Goldberg, Journal of Environmental Polymer Degradation 1995, 3, 61-67