Gee, I spoil you guys: a blog about a broken screwdriver.

Not just any old screwdriver though, because the handle of this one is made from nitrocellulose, and they don’t do that anymore – not since the 1940s.  I found the remains in a garage I’ve been clearing out over the past couple of days.

Nitrocellulose is an interesting material on many levels; its tendency for spontaneous disintegration is only one of the reasons you’ll no longer find it in tool handles, movie film, guitar pick guards, billiard balls, and dice.  Its flammability made early nitrocellulose film stock a safety liability; even today the UK Health & Safety Executive publish guidance on its handling (downloadable pdf file).

My first encounter with nitrocellulose came as a 12 year old schoolboy, when in the school library I learnt from a popular science book, ‘The Oddities of Heat’, how to apply nitrocellulose as gun-cotton to the blowing up of bridges.  There was even a diagram showing how to position the charge.  Ah, the innocent diversions of less troubled times.

More recently, on a visit to the Museum of Jurassic Technology in Los Angeles (don’t ask, I’m going to write that visit up in due course), I saw an exhibition featuring magician Ricky Jay’s collection of disintegrating nitrocellulose dice (you’re already gauging the character of this museum – right?).

So what’s the science behind this fun stuff. And why the spontaneous disintegration?

Nitrocellulose

Nitrocellulose is made by treating cellulose, a natural organic compound found in the cell walls of plants such as cotton, with chemicals containing nitrogen – normally nitric acid.  Some hydrogen atoms in the cellulose polymer [C₆H₇O(OH)₃]_n are replaced with nitrogen in the form of the nitryl group NO2.  The exact properties of the resulting nitrocellulose depend on how much of the hydrogen is replaced with nitrogen.

The fully nitrated and highly explosive gun-cotton version of nitrocellulose thus has the formula [C₆H₇O(ONO₂)₃]_n.  Nitrocellulose with less nitrogen in the chemical structure, known as pyroxylin or (with camphor added to reduce brittleness) celluloid.  This is the variety used in old film stock, and is probably what my screwdriver handle is made of.  It certainly burns well; I tested it.

Spontaneous Disintegration

Googling this topic yielded a few examples akin to my screwdriver scenario: knife handles, film restoration sites and such like.   But a convincing explanation of the spontaneous failure mechanism was more elusive.  There are several academic papers in the literature dealing with the reaction chemistry and kinetics (speed) of nitrocellulose breakdown in the laboratory, with more practical discussions focusing on movie film conservation.

In all cases, the disintegration appears to happen in two stages: an initial phase where NO2 groups in gaseous form come free from the nitrocellulose structure, to combine with any water present to form nitric acid.  The acid then auto-catalyses the same reaction but at a much higher rate.

It seems fair to hypothesize from this that the release of gases in the surface layers of the screwdriver handle create micro-cracks that transmit the decomposition reaction into the body of the material, the increasing pressure driving the reaction harder.  It certainly looks like that’s what has happened.

Yet I still don’t really understand what triggers the timing of the initial decomposition.

Ideas?