Bakelite is made in three stages: phenol is reacted with a deficiency of formaldehyde under acid to give a fusible novolac resin, the ground resin is mixed with hexamethylenetetramine and a filler such as wood flour to give a moulding powder, and the powder is cured in a hot steel mould under pressure. The scratch carrier uses a phenol to formaldehyde ratio of 1 to 0.8, which keeps the resin fusible in the kettle, and that is the safer of the two ratios the plastics articles give.
Part of Plastics and Polymers
Bakelite synthesis is the staged reaction of phenol and formaldehyde into a resin, its formulation into moulding powder, and its final cure in a heated press.
Three Stages, Each With A Different Job
The steps below follow Plastics from Scratch. Read them with the hazards at the end of this article.
Stage A: novolac resin (soluble, fusible).
- Mix phenol and formaldehyde at a molar ratio of 1 to 0.8.
- Add 1-2% acid catalyst, hydrochloric or oxalic.
- Heat to 100 °C with stirring for 2-4 hours. The mixture thickens to a viscous resin.
- Pour into trays, cool, and grind to powder.
Stage B: moulding powder.
- Mix the novolac powder with 8-10% hexamethylenetetramine (hexa), the extra cross-linker.
- Add filler: wood flour at 50-60% by weight, or cotton flock. Wood flour gives a machinable, impact-resistant material.
- Mix thoroughly by ball mill or kneading.
Stage C: moulding and final cure.
- Load the powder into a preheated steel mould at 160-180 °C.
- Apply 10-30 MPa in a press.
- Hold 1-5 minutes per millimetre of thickness.
- The resin cross-links permanently. Open the mould, extract the part and trim flash.
The press cycle in detail is in Compression Mold.
Why A Novolac Is The Controlled Route
With less than one formaldehyde per phenol and an acid catalyst, the chains stay linear enough to remain fusible: they cannot cross-link until hexa supplies the extra cross-linking carbon at the press. So the resin stays a processable solid in the kettle and sets only in the mould. Phenolic Resins describes the novolac and the resol, the other route, made with excess formaldehyde under alkaline conditions, which cures with heat alone and so can gel in the vessel.
Where The Two Plastics Articles Disagree
Plastics Manufacturing gives a different recipe: phenol and formaldehyde at about 1 to 1.2 molar, a few drops of hydrochloric acid per 100 ml, heating at 70-80 °C, curing at 150-180 °C for 2-10 minutes. Acid catalysis with more formaldehyde than phenol is the combination that can gel in the kettle, and the heat of the reaction in a closed vessel can foam it out. The 1 to 0.8 route above is the one to follow. This article does not alter either set of figures; it points at the safer one. The scratch carrier’s mention of “novolac with alkaline catalyst” for casting resin also mixes terms: an alkali-catalysed, formaldehyde-rich resin is a resol.
The same carrier lists asbestos fibre as a filler. Do not use it: asbestos fibre is a lung hazard, and wood flour or cotton flock does the job.
Worked Example: Reagents For A Small Batch
- Take 1 mole of phenol: about 94 g (molar mass, general knowledge).
- At a 1 to 0.8 ratio, formaldehyde = 0.8 mol × 30.0 g/mol = 24 g of pure formaldehyde.
- As 37% formalin this is 24 / 0.37 = 65 g of solution.
- So the batch is 94 g of phenol to 65 g of formalin, a mass ratio of 94 / 65 = 1.45 to 1. The carrier’s “roughly equal weights” is only loosely right.
- Acid at 1-2% of the batch is about 1.6-3.2 g of catalyst if taken on the 159 g charge (94 + 65), an assumed basis.
- Hexa at 8-10% of 100 g of novolac powder is 8-10 g.
Water in the formalin comes out during the cook as steam. That is part of why the vessel must not be sealed.
Check The Cure By Test Pieces
Mould a small test piece with every batch and adjust the cure time on test pieces, not on real parts. A well-cured piece is hard, dark and glossy; a soft, tacky or blistered one is undercured (general knowledge, not a corpus test). The carriers’ hold times differ widely, 1-5 minutes per millimetre in one and 2-10 minutes in the other, so begin at the longer end for a thick part and shorten from there.
Common Mistakes
- Using the formaldehyde-rich recipe with acid — it can gel in the kettle.
- Sealing the vessel — steam and gas pressure.
- Skipping the hexa — the powder will not cure.
- Heating a thick part too fast — gas inside, blisters.
- Using asbestos filler.
Phenol, formaldehyde, steam, ammonia and heat
Phenol is corrosive, numbs skin and is absorbed through it; formaldehyde irritates the lungs and is a carcinogen; the cook releases steam and a mixing-in of hexa gives off ammonia and formaldehyde as it cures (general knowledge). Work outdoors with a vented vessel, chemical-resistant gloves, eye and face protection and a respirator rated for organic vapour, and keep water at hand. Hot moulds burn. Do not heat salvaged phenolic in the open.
What’s Next
- Learn next: Compression Mold — the press cycle
- Combine with: Machining Plastics — finishing the cured part