Why hockey pucks are frozen - Kansas City Star

"Rubber is one of the most elastic materials on earth, and even vulcanization can’t stop hockey pucks from bouncing," Hache wrote. "Smashed against a hard surface like concrete or ice, a puck rebounds with between 45 and 55 percent of its original velocity (less so on a softer surface like a board). This percentage is the so-called coefficient of restitution. In an ideal world, the puck wouldn’t bounce off the ice at all. To minimize this unruly behavior, someone discovered a long time ago that freezing the puck before a game would make it slide better and bounce less, owing to its increased stiffness."

Hache even proposed a test. Take two pucks of the same weight and size and put one in a freezer for an hour. Then drop both pucks on a concrete surface.

"You will find that the cold puck bounces less than half the height of the warm puck," Hache wrote. "In fact, they will bounce to about 12 percent and 27 percent of their original height, respectively."

Basically if you warm a puck, the added heat means it has more energy. That’s why it will bounce higher.

A hockey puck is made of vulcanized rubber which means sulfur has been added to it and then heated.

This generates more bonds between the sulfur atoms (disulfide bonds) which makes the chains tighten more quickly. This increases elastic force constant and makes the rubber less extendable.

When a puck is cooled, chain segments freeze and the rubber loses its elastic properties. Thus is will not be as bouncy.

via Why hockey pucks are frozen - Kansas City Star.