FLASH OF FIRE, NO SMOKE
My favourite chemical reaction? Combustion of cellulose nitrate tops the list
“What is your favourite chemical reaction?” That unexpected question from a student was prompted by my dropping a piece of sodium into water and watching the hydrogen produced burst into flame. Certainly, I had considered favourite athletes, movies, Broadway shows and meals before, but I had never thought about chemical reactions in that context.
I didn’t have to think long, though. Combustion of cellulose nitrate tops the list. Countless times in a lecture or on television I have ignited a piece of “flash paper” or a wad of “gun cotton” that to the astonishment of the audience disappears in a flash without a hint of smoke. The reaction is impressive, especially when its importance in shaping history is considered.
It all starts with the classic story of Swiss chemist Christian Schonbein’s clumsiness. As the story goes, back in 1845 he was distilling a mixture of nitric and sulphuric acids in his kitchen when the flask broke and the contents spilled all over the floor. Quickly grabbing his wife’s cotton apron hanging nearby, he proceeded to wipe up the mess. After washing the apron he hung it in front of a hot stove to dry and was stunned when it just vanished in a flash!
Actually some seven years earlier, Théophile-Jules Pelouze had reacted cellulose, the major component of cotton, with nitric acid to produce a flammable material but never capitalized on his observation. Schonbein, however, did. He recognized that his discovery could have a practical application as a propellant in firearms, especially given that it produced no smoke. Battlefields at the time were commonly filled with smoke, obscuring targets. Schonbein’s “gun cotton” was far more flammable than Pelouze’s nitrocellulose because sulphuric acid catalyzed the reaction of cellulose with nitric acid and resulted in a higher degree of nitration.
Cellulose is composed of glucose units linked together, with each glucose having three sites where nitro groups can attach, forming highly unstable oxygen-nitrogen bonds. It is the breaking of these bonds, coupled with the release of oxygen by the nitro groups, that leads to rapid combustion. Pelouze’s method attached two nitro groups per glucose on average, while Schonbein’s reaction resulted in cellulose trinitrate, the basis of modern smokeless gunpowder, three times more powerful than classic black powder made of sulphur, charcoal and saltpetre.
Not only did smokeless gunpowder change the course of warfare, it also launched the plastic industry. Around the same time that the disappearing apron startled Schonbein, one of Pelouze’s students, Louis Menard, discovered that dissolving nitrocellulose in a mixture of alcohol and ether led to a gelatinous liquid that dried to a hard, transparent film. Named “collodion,” it eventually found a use as a dressing for minor wounds. Applied as a liquid, it dried to provide a waterproof protective layer. Menard, it seems, did not have the imagination to mould substances out of collodion, but Alexander Parkes in England, likely independently, recognized the possibilities afforded by dissolving nitrocellulose in a solvent.
In 1862, at the International Exhibition in London, Parkes displayed a number of “Parkesine” items, made by dissolving nitrocellulose in methanol, known at the time as “wood naphtha.” Allowing the solvent to evaporate left behind a material able to be moulded into various shapes when heated and subjected to pressure. In other words, Parkes had formulated a plastic! His co-worker Daniel Spill improved Parkesine by substituting castor oil for methanol and mixing in camphor to produce “Xylonite.”
However, it was American inventor John Wesley Hyatt who perfected the nitrocellulose camphor mixture and introduced “celluloid,” the first commercially successful plastic, triggering a legal battle with Spill about patent rights. Hyatt had been motivated by a $10,000 prize offered by the Phelan & Collender Company to anyone who could come up with a substitute for ivory in billiard balls. Celluloid was not quite up to that task, but it found widespread use in combs, brushes, toys, dentures, jewelry, gentlemen’s detachable collars and later as the basis of photographic and movie film.
There was, however, one problem that plagued celluloid. It was highly flammable! That was dramatically brought home to the public when the Celluloid Manufacturing Company’s four-story building in Newark was levelled in just two hours by a fire, fuelling fear about consumer items made of celluloid. Scientific American fanned the flames with a story about a young lady whose celluloid buttons burst into flame when she parked herself in front of a fire. While that account is questionable, a story in the Dec. 18, 1910 issue of the Pittsburgh Gazette Times was factual. It recounted the story of a gentleman who lost his life while caring for his long grey beard with a comb.
The unfortunate man had the habit of combing his beard and then holding the comb over a small gas stove to burn away the hairs that had become stuck in the comb’s teeth. This time he seems to have held the comb just a bit too long in the flame and the comb caught fire, setting his clothing ablaze, causing lethal burns. Celluloid combs were responsible for other disasters as well. In 1909 in Brooklyn nine workers in a comb factory were killed and scores injured when the celluloid being used to make combs ignited. Employees stampeded to the fire escapes and the roof with some being forced to leap from windows to escape the suffocating smoke. There were also a number of devastating fires in movie theatres cause by the celluloid film being ignited by the hot projector lamp.
Celluloid film was eventually replaced by cellulose acetate “safety film,” but we were reminded of celluloid’s flammability by 2012’s Oscar winning film The Artist in which the hero, shaken by the invention of talkies, sets fire to a spool of celluloid film, almost causing his demise. Luckily he is saved when his loyal dog draws the attention of a police officer who saves him from the burning house. That’s why I’m careful to keep my celluloid collectables, including a vintage comb, away from heat. I don’t want them to engage in my favourite chemical reaction.
The reaction is impressive, especially when its importance in shaping history is considered.