The automotive industry is the third most important consuming sector of polymers after packaging and building & construction. Therefore, changes in material usage can have major implications on polymer demand and the financial performance of polymer producers.

More than 70% of the plastic used in automobiles comes from four polymers: polypropylene, polyurethane, polyamides and PVC. Plastic has become one of the key materials required for the structure, performance, and safety of automobiles in recent years, with growth in plastic consumption being driven by light-weighting trends for fuel efficiency and consequently lower greenhouse gas emissions.

The glass transition temperature (Tg) of polymers is defined as the temperature at which they transition from a hard, glassy substance to a soft rubbery substance. After this transition area, physical material parameters such as size, volume, and coefficient of thermal expansion (CTE) rise. Beyond this point, other qualities like modulus and hardness deteriorate.

This temperature (in degrees Celsius or degrees Fahrenheit) is determined by the chemical structure of the polymer and can thus be used to identify polymers.

Tg is the only property that amorphous polymers have. Because there is frequently an amorphous part ("semi"), crystalline polymers have a Tm (melt temperature) and a Tg (glass transition temperature). Tg is determined by the mobility of the polymer chain and ranges from 170 to 500 °C for most synthetic polymers.

The transition from a glass to a rubber-like state is a key aspect of polymer behavior, indicating a period of significant changes in physical qualities such as hardness and elasticity. Hardness, volume, percent elongation to break, and Young's modulus of solids are the most noticeable changes at Tg.

Polystyrene, poly (methyl methacrylate), and other stiff and brittle polymers are utilized below their Tg (in a glassy state). Their Tg is higher than those of ambient air.

Rubber elastomers such as polyisoprene and polyisobutylene, for example, are utilized above their Tg (in a rubbery condition). Their Tg is lower than room temperature, making them soft and flexible.

Melting Temperature vs. Glass Transition Temperature

At Tg, the chains in the amorphous (i.e., disordered) parts of the polymer gather enough thermal energy to start sliding past each other at a significant rate. The melting point (Tm), which is higher than the Tg, is the temperature at which the complete chain moves.

Melting is a property of the crystalline zone, whereas glass transition is a property of the amorphous region. A disordered amorphous solid exists below Tg, where chain motion is frozen and molecules begin to wiggle around above Tg. The higher the value of Tg, the more immobile the chain is. While it is an ordered crystalline solid below Tm, it transforms into a disordered melt above Tm.

Glass Transition Temperature Values of Commonly Used Plastics in the Automobile Industry

It's a complex topic, to be sure. We hope this fairly technical explanation helped reduce confusion on the topic. Have an injection molding project in the works? Reach out at info@procurabl.com. We’re always happy to geek out on polymers.