Glass Transition Temperature (Tg)

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Definition - What does Glass Transition Temperature (Tg) mean?

Glass transition temperature (Tg) is the temperature at which the mechanical properties of a material radically change due to the internal movement of the polymer chains that form the material. It is the temperature at which the material physically changes from a solid to a liquid or from a liquid to a solid.

Glass transition temperature is the most important property of a polymer. The value of the glass transition temperature is directly related to the mechanical properties (strength, hardness, brittleness, elongation, etc.) that the polymer presents at the temperature that is exposed. Therefore, knowledge of the Tg value of an adhesive is vital.

Corrosionpedia explains Glass Transition Temperature (Tg)

The glass transition temperature (Tg) is the temperature at which an amorphous solid becomes soft upon heating or brittle upon cooling. The Tg is one of the most important properties of any epoxy and is the temperature region where the polymer transitions from a hard, glassy material to a soft, rubbery material. The glass transition temperature is always lower than the melting temperature (Tm) of the crystalline state of the material, if one exists.

The Tg is a function of chain flexibility. The glass transition occurs when there is enough vibrational (thermal) energy in the system to create sufficient free volume to permit sequences of six to ten main-chain carbons to move together as a unit. At this point, the mechanical behavior of the polymer changes from rigid and brittle to tough and leathery, the behavior defined as plastic behavior.

The glass transition temperature is more important in plastics applications than is the melting point, because it tells a lot about how the polymer behaves under ambient conditions. If a polymer’s glass transition temperature is well above ambient room temperature, the material behaves like a brittle glassy polymer — it is stiff with low impact resistance. Conversely, if the Tg is well below room temperature, the material is what is commonly termed a rubber or elastomer — soft and easily stretched. Those materials whose Tg is reasonably close to the ambient temperature exhibit plastic material behavior — strong and tough with good impact resistance.

There are chemical compounds known as plasticizers that are used to decrease the glass transition temperature during the design and formulation of adhesive. Greater movement also decreases the glass transition temperature and produces more elastic behavior.

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