Polymers can get complicated quickly. Thermal properties play a large role in the behavior of polymers, causing them to react in different ways. Let’s look at different properties of polymers in relation to temperature.
There are three types of polymers: amorphous, crystalline, and semicrystalline. Amorphous polymers contain chains that are not ordered in crystalline structures. The thermal properties of an amorphous structure are referred to as the glass transition temperature, denoted as Tg.
Glass transition is exhibited by amorphous polymers and amorphous areas of a semicrystalline structure. Crystalline structures do not have a Tg. Crystalline structures have a melting point where the polymer chains lose their crystal structure and become a disordered liquid. This phenomenon is denoted as Tm. Melting is only seen in crystalline structures and not in amorphous polymers or amorphous regions. Almost all crystalline polymers have an amorphous region, making them semi-crystalline.
Let’s start off with a basic term - kinetic energy. Heat is kinetic energy, defined as the energy of objects in motion. So let’s apply this to polymers. When a polymer is hot, its molecules have a lot of kinetic energy and move around very fast. If the polymer is cold, there is little kinetic energy and the molecules move around very slowly, or not at all.
The temperature of a polymer can greatly affect the way it responds to stress. If the chains are heated and you try to move them, you will find that they are already moving and change their positions very easily to remove stress placed on them. If the chains are cold and you try to move them, the position can’t easily be changed. In this case the chains are either strong enough to resist the stress, or they will break due to the stress.
Thermal expansion is the concept of materials taking up more space as the temperature increases. The reason for this is that as the temperature increases, the kinetic energy increases and the atoms vibrate over a longer distance. This is measured as the expansion of volume.
The thermal expansion can be measured using Thermomechanical Analysis. Thermal expansion is important in packaging to understand the tolerances of polymers. For example, a closure may have a different expansion than the finish of a bottle in a hot fill application or a foam cushion may contract in a shipper when cold and the product will sit loosely in the container.
As you can see, understanding the thermal properties of polymers is critical. Knowing how polymers react to temperature changes gives you further insight into how your plastic packaging is formed and the variances you can have.
To learn more about the thermal properties of polymers enroll in our Polymers in Packaging course at https://www.packagingschool.com/courses/polymers-in-packaging-101