Plastics don't forget

by Alfredo Mateus

In this experiment we will observe what happens with a sample of PET polymer, obtained from a plastic bottle, when it is heated. We will also discover that plastics have memory too. Watch the video below to see the experiment.

This is one of the three experiments described in my article in the Journal of Chemical Education in 2019, I hope to post about the others here soon. “Polymer Processing Demonstrations Using PET Bottles”.

What is happening

Plastic bottles made with PET polymer are produced in a two-step process. The polymer is processed into small pellets, which can be injection molded into preforms for bottle production. The preforms are then transported to the beverage industry where the bottles are made. The preform is heated in an oven, placed in a mold and blown with pressurized air (Stretch Blow Molding). The mold is chilled with cold water so the plastic can quickly harden, and the bottle is released, ready for use. A single blowing machine can produce 10,000 bottles per hour. 

Quando aquecemos o polímero, existe uma faixa de temperaturas em que ele se torna mais flexível, mesmo bem antes de atingirmos a sua temperatura de fusão. Muitos polímeros como o PET das garrafas não são sólidos cristalinos, com suas moléculas organizadas. Quando resfriamos rapidamente o polímero líquido, as moléculas não conseguem se organizar completamente antes de travarem em uma posição no sólido. Com isso, o polímero fica num estado amorfo ou semi-cristalino. 

PET bottles are transparent, a desirable property for a packaging material. The transparency is associated with the degree of crystallinity of the polymer. Highly transparent polymers are amorphous or semi-crystalline.

Amorphous solids undergo a glass transition between a solid state and a more flexible and rubber-like state. This change occurs over a range of temperatures that is characteristic of each polymer and that depends on the level of cristallinity. PET has a glass transition temperature (Tg) that is close to the boiling temperature of water, around 75 oC. Above this temperature, the plastic is soft and can be stretched. Cooling the plastic below its Tg in this elongated state will “lock” the molecules in this new position. When the plastic is reheated above the Tg, we obtain a surprising result: the molecules “relax” and the plastic returns to its original shape. 

Last thoughts

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