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Why do plastics get brittle when they get cold?

When your recycling bin becomes a recyclable after a wintertime crack, you’ve experienced a loss of molecular mobility…

By Peter Dunn

When a dropped soda bottle hits the floor and bounces instead of shattering, or a car bumper flexes instead of denting, the credit goes to modern plastics, and their remarkable ability to resist breaking when placed under stress. And in the process, they can remove some stress from a visit to the supermarket or subsequent drive through the parking lot.

Many plastics owe their resilience to ductility—the ability of the plastic’s long, chain-like molecules to stretch, sometimes to several times their original length, explains Greg Rutledge, professor in MIT’s Department of Chemical Engineering. Individually, the stretching molecules absorb energy; collectively they dissipate stress from the point of impact, preventing breakage.

This communitarian approach, however, only works when molecules are free to slip past, around, or through one another (imagine a bowl of just-cooked spaghetti coated with olive oil). If the motion is restricted in some way, the molecules can’t stretch and the stress remains concentrated in a small area. And if the concentration gets too great, the material will fail, creating a crack that can propagate into a fracture. “This ability to slip without letting go is the key to ductility, and to avoiding brittle fracture in plastics,” notes Rutledge.

A key factor in the molecules’ ability to slip and slide is temperature. Specifically, there is something called the “glass transition temperature” (Tg), which is the point below which an amorphous solid (such as glass, polymers, tire rubber, or cotton candy) goes from being ductile to brittle. For most common materials, says Rutledge, this temperature is so high or so low that it is not easily observed – the Tg of window glass is 564 degrees C, and that of tire rubber is -72 degrees C.

But many plastics exhibit their transition at everyday temperatures, and can be “frozen” into brittleness. One example: polypropylene, an inexpensive material often used in containers, toys, outdoor furniture, and recycling bins has a Tg of between -20 and 0 degrees C, so it can easily lose its molecular mobility and become shatter-prone on a winter day.

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