Polymers, Thermosoftening & Thermosetting

Instructor: Laura Foist

Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

In this lesson, we will learn about the difference between thermosoftening and thermosetting polymers. We will also explore one kind of thermosoftening polymer, polyethylene.

Polyethylene Properties

When you are grocery shopping, you probably get the flimsy plastic bags that seem to easily tear. On the other hand, when you go clothes shopping, you may often get a nice sturdy plastic bag. These are both plastic bags - in fact, they are both typically made from a polymer of ethene. So what makes the difference between the two types of bags? The grocery shopping bag is a high-density polyethylene (HDPE), while the sturdy bag is a low-density polyethylene (LDPE).

HDPE has greater strength per unit area than LDPE. This may not make sense when we first think about the kind of bags made by each. The HDPE bags seem a lot weaker than the LDPE bags. But, think about how thin the plastic is for the HDPE grocery bags. Despite being a lot thinner than the department store bags, the grocery bags are still very strong. If we made an HDPE plastic bag as thick as the department store LDPE bags, then it would be very strong!

HDPE has straight chains, which allows them to stack, forming a strong structure
HDPE structure

A big difference between HDPE and LDPE is the molecular structure of each. It's true that both are made from a long chain of ethene molecules, but they differ based on how many side chains they have. HDPE is a long, mostly straight chain, and it has very few side chains. On the other hand, LDPE has lots of side chains. The straight chains allow HDPE to stack in layers, which is what makes it stronger. On the other hand, LDPE cannot stack because the side chains get in the way.

LDPE has branches in the chain, which makes it difficult to stack, making a weaker polymer
LDPE structure

Since both HDPE and LDPE are polyethylenes, they are both thermosoftening polymers. Thermosoftening polymers are polymers that can be manipulated into different shapes when they are hot, and the shape sets when it cools. If we were to reheat the polymer again, we could re-shape it once again. So both the department store bag and the grocery bag can be heated and manipulated into a new shape.

Thermosetting

In contrast to thermosoftening polymers, some polymers are thermosetting. Thermosetting polymers cannot be reshaped even when heated. The rubber to make car tires is made out of a thermosetting polymer. Let's look at the structure of HDPE again:


HDPE and all thermosoftening polymers have no cross-links between the chains
HDPE structure


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