Formaldehyde is made by oxidising methanol: methanol vapour mixed with air is passed over a heated copper or silver gauze at about 500-600 degrees Celsius, and the gas is absorbed in water to give formalin, a solution of about 37% formaldehyde. Both the methanol feed and the product are hazardous, because methanol is toxic and flammable, hot methanol-air mixtures can explode, and formaldehyde irritates the airway and is a carcinogen.
Part of Plastics and Polymers
Formaldehyde is the one-carbon aldehyde used as the cross-linking partner of phenol in Bakelite and as the hardener of casein plastic.
Methanol Comes From Wood Distillation
Plastics from Scratch sources the methanol from destructive distillation of wood, which gives “wood alcohol” in the first distillate fraction. See Wood Tar Production for the wood distillation itself, which yields tar and wood vinegar; it does not describe collecting the methanol. Methanol is poisonous, which is the central point of Methanol Hazards: it cannot be treated as drink or handled casually. Do not use it near flame.
The Reaction And What It Needs
The carrier’s steps:
- Obtain methanol.
- Pass methanol vapour mixed with air over a heated copper or silver gauze at 500-600 °C.
- The overall reaction is CH₃OH + ½O₂ → HCHO + H₂O.
- Collect the formaldehyde in water as a roughly 37% solution, formalin.
The catalyst gauze takes part in the reaction at the surface; the hot metal passes the oxygen to the methanol and releases formaldehyde. A second reaction can run alongside it, in which methanol simply loses hydrogen to give formaldehyde and hydrogen gas (general knowledge). That means flammable hydrogen can be in the product stream.
Worked Example: Methanol Needed For A Litre-Scale Batch
Formalin is about 37% formaldehyde. One kilogram of formalin contains 0.37 × 1,000 g = 370 g of formaldehyde. Formaldehyde has a molar mass of about 30.0 g/mol, so 370 / 30.0 = 12.3 mol. The reaction uses one mole of methanol per mole of formaldehyde. Methanol has a molar mass of about 32.0 g/mol, so the minimum methanol is 12.3 × 32.0 = 395 g. Real yields are below 100%, so more is needed. Molar masses are general knowledge, not corpus figures, and the corpus gives no yield.
Why The Hot Gas Stream Is An Explosion Hazard
A mixture of methanol vapour and air burns across a range of proportions, and a hot catalyst is an ignition source. A reactor fed with such a mixture can flash back or explode. Industrial plants control the mixture to stay outside the flammable range (general knowledge), which needs measurement and flow control. The corpus gives no safe feed ratio, and this article gives none: a settlement-scale build must have a way to relieve pressure, to keep the feed from burning back into the supply, and to run outdoors with everyone clear of it.
Handling The Product
- Formalin irritates eyes, nose, throat and skin, and is a recognised carcinogen on long exposure. It also has a sharp smell that is a useful warning at the start but not a measure of safety.
- Keep it covered and store it in a closed container, labelled, away from food and children.
- Hardening casein uses it at much lower strength: see Formaldehyde Hardening.
- Bakelite uses it as the second reactant: see Synthesis and Molding.
Common Mistakes
- Feeding the catalyst a rich-in-air mix without flow control.
- Closing the apparatus completely — pressure builds from heat and gas.
- Assuming formalin is dilute and safe to splash.
- Using the same vessels for food.
Toxic methanol, explosive vapour, carcinogenic product
Methanol is toxic by swallowing, skin contact and breathing. Methanol vapour and air burn or explode, and a catalyst at 500-600 °C ignites them. Formaldehyde irritates the lungs and is a carcinogen. Work outdoors, with a vented apparatus, no flames, eye and skin protection, and a volume of water at hand. Move anyone who has breathed fume into fresh air at once.
What’s Next
- Learn next: Synthesis and Molding — combining formaldehyde with phenol
- Combine with: Phenol Sources — the other half of the resin