Camphor is the plasticiser that makes cellulose nitrate flexible and moldable: at about 25-30% of the weight of the cellulose nitrate it turns a hard, brittle solid into celluloid that softens in hot water or steam and can be rolled, pressed and molded. Camphor comes from the wood of the camphor tree or can be synthesised from turpentine, and it slowly evaporates from finished celluloid, which is one reason old celluloid shrinks and cracks.
Part of Plastics and Polymers
A plasticiser is an additive that sits between polymer chains and lets them slide, making a hard plastic softer and more workable.
A Plasticiser Works By Keeping Chains Apart
Cellulose nitrate on its own is hard and brittle. Camphor dissolves into it, wedges between the chains and lets them move, so the mass becomes pliable instead of snapping. The same idea applies to any plasticiser: a small molecule that spaces the chains. The proportion matters. The scratch carrier gives 25-30% camphor by weight of the cellulose nitrate, and the manufacturing carrier says about 30%.
Too little camphor leaves the material stiff and crack-prone. Too much leaves it soft, greasy and slow to harden. Camphor is a solvent for cellulose nitrate as well as a plasticiser, so a small amount of a volatile solvent such as ethanol is often used to help it mix, per the scratch carrier.
Where Camphor Comes From
- From the tree. Distilled from the wood of the camphor tree (the manufacturing carrier says bark, the scratch carrier says wood; use whichever you have). For a century camphor was a commodity of a few producing regions, which is part of why synthetic routes were developed (recalled history).
- From turpentine. The scratch carrier gives the route as pinene to camphene to isoborneol to camphor. It is a multi-step chemistry job, and the corpus gives no procedure. Turpentine itself is the light volatile oil from resinous wood: see Wood Tar Production.
Worked Example: Camphor For A Batch
At 25-30% of the nitrate weight, 1 kg of cellulose nitrate takes 0.25 × 1,000 g = 250 g up to 0.30 × 1,000 g = 300 g of camphor. For 400 g of nitrate the range is 0.25 × 400 = 100 g to 0.30 × 400 = 120 g. Weigh to the gram, because the property is set by the proportion.
What Camphor Does Over The Years
Camphor does not stay put. It evaporates slowly from celluloid, so an old celluloid object loses plasticiser, shrinks, warps and may crack (general knowledge). The loss is also why salvaged celluloid often smells faintly of camphor and why old film curls. This is a different process from the acid-driven decomposition in Celluloid Safety, and the two can happen together.
Camphor Is Toxic If Swallowed
Camphor is poisonous by ingestion and can cause seizures, particularly in children (general knowledge). The corpus gives no dose and neither does this page. Keep it out of reach of children and away from food, and do not treat it as a home remedy.
Where The Rest Of The Process Sits
Camphor is added to cellulose nitrate that already exists, so the dangerous step has been taken before the plasticiser enters. The nitration is covered, and deliberately not given as a procedure, in Cellulose Nitration. Compounding is hot, solvent-wet work on a flammable mass: this page gives no temperatures or kneading procedure, for the reasons in Celluloid Safety.
Common Mistakes
- Measuring camphor by eye — the proportion sets the properties.
- Using hot dry heat to speed mixing — hot spots near a flammable mass.
- Storing camphorated celluloid in a sealed warm place — shrinkage and decomposition.
- Leaving camphor where children can reach it.
Flammable mass, flammable solvent, toxic additive
Camphor-plasticised cellulose nitrate burns violently, the solvents used to mix it are flammable and their vapour sinks into low places, and camphor itself is poisonous if swallowed. Work outdoors with no flame, spark or electrical arcing, and use hot water or steam, not a flame, if you must soften a piece.
What’s Next
- Learn next: Celluloid Safety — why the finished plastic is still a fire hazard
- Combine with: Lacquers and Coatings — celluloid dissolved as a coating