Several years ago, if you played a game of ping pong or table tennis, there was a real chance the ball could explode when hit or burst into flame from friction as it struck the table. Do you know why ping pong balls burn? Contrary to a popular myth, they are filled with air and not a flammable gas. The reason they burn is because they are made of celluloid. Celluloid is a flammable polymer, much like nitrocellulose or guncotton.
Flammable Celluloid in Ping Pong Balls
Celluloid is a thermoplastic made from nitrocellulose and camphor. Sometimes dyes and stabilizers are added. A typical ping pong ball might consist of 70% to 80% nitrocellulose, with up to 30% camphor. Small amounts of dye, ethanol, and stabilizers make the ball colorful, more stable, and less flammable. A fresh ping pong ball isn’t going to burst into flame, although it can be ignited using a match. However, as celluloid ages, camphor gets squeezed out of the plastic. Exposure to ultraviolet light from the Sun or fluorescent bulbs and exposure to humidity break the chains in the polymer and weaken it. Eventually, the ball becomes brittle and can ignite from impact or friction.
If celluloid is flammable, why is it used for ping pong balls? There are two reasons:
The first is that celluloid is easy to form into a smooth, light ball. To make the ball, nitrocellulose and camphor sheets are soaked in hot ethanol until they are soft. The sheet is pressed into hemisphere molds. After the plastic hardens, the two halves of the ball are glued together using an alcohol-based adhesive and the balls are agitated to smooth their seams. Ping pong balls only contain air when they are made, but people often think they are filled with a flammable gas because the plastic and glue off-gas into the center of the ball, leaving a nasty chemical odor. The resulting vapor is noxious and possibly flammable.
The main reason celluloid is used for the balls is because of its coefficient of restitution. In other words, they bounce better than other materials. Since 2015, the International Table Tennis Federation has encouraged ball manufacturers to seek alternatives to celluloid. Modern balls may be made of a mixture of celluloid and another plastic or another polymer altogether. However, all balls must have a matte finish, be 40 millimeters in diameter, have a mass of 2.7 grams, and have a coefficient of restitution of 0.89 to 0.92.
How to Light a Ping Pong Ball on Fire
If you want to watch a ping pong ball burn, you can try it yourself. You only need basic materials:
- Ping pong ball
- Long-handled lighter (for safety)
- Fire-safe surface (for safety)
- Set the ball on a non-flammable surface. Good examples include a brick, a metal bowl, or pavement. You’ll see videos of people holding ping balls in their hands and lighting them. Don’t hold the ball!
- Touch the lighter to the ball and back away. Ping pong balls are small, but they produce a serious flame. You’ll get different effects, depending whether you ignite the ball from the top or the bottom. Another option is to cut a small hole in the ball and ignite it from the inside!
References
- Painter, Paul C.; Coleman, Michael M. (2008). “The Early History of Polymers”. Essentials of Polymer Science and Engineering. DEStech. pp. 7–9. ISBN 9781932078756.
- The International Table Tennis Federation (December 2009). The Ball (version for 40mm balls). Technical Leaflet T3. (archived from the original)\