Phenolic resins are thermosets made by reacting phenol with formaldehyde, which cure under heat into a hard, rigid, heat-resistant network that does not conduct electricity, and they are the plastics known as Bakelite. They are moulded with a filler such as wood flour in a heated press, and the result is the standard early electrical insulator. No batching recipe is given, because phenol and formaldehyde are toxic and the reaction can run away without temperature control.
Part of Thermosets
Phenolic resins are phenol-formaldehyde thermosets used as heat-resistant moulding compounds, laminates and binders.
A Dense Network Makes It Hard, Heat-Proof And Brittle
Phenol has three positions where formaldehyde can attach, so each molecule becomes a junction in a three-dimensional network. The result is rigid, does not soften with heat, resists most solvents, and does not carry electric current. Plastics from Scratch explains why this matters: Bakelite is the plastic to prioritise for electrical insulation because it is heat-resistant and insulating, while casein absorbs moisture and celluloid burns violently. A cured phenolic part will char before it melts. The same network makes it brittle, so it is nearly always filled.
Fillers Do The Toughening
Pure cured phenolic is glassy and cracks. Mixing in fibres or flour toughens it and cuts cost:
- Wood flour. The usual general-purpose moulding powder.
- Paper or cloth layers. A laminated sheet or tube with strong insulating properties, as in Laminated Composites.
- Mineral fillers. Improve heat and arc resistance.
Moulding is done by pressing the partly reacted powder into a heated mould, where it flows, then cures into shape. Pressure removes gas, and the part is ejected hot. Nothing can be remoulded.
Two Resin Types, Two Routes
In standard industrial practice, a resol is made with excess formaldehyde under alkaline conditions and cures with heat alone, while a novolac is made with excess phenol under acid conditions and needs an added cross-linker to cure. The corpus’s Plastics from Scratch Advanced Materials article covers the route for a settlement. This article states no ratios, times or temperatures.
Where The Inputs Come From, And Why Care Is Needed
Coal Tar is the traditional source of phenol, among its hundreds of compounds. Formaldehyde comes from oxidising methanol. Both are hazardous: phenol burns skin and is absorbed through it, and formaldehyde is a respiratory sensitiser and carcinogen. The reaction between them releases heat, and heated in a closed vessel it can foam and rupture. For these reasons the sensible first step is to salvage existing phenolic parts (old switch bodies, handles, laminated boards) and understand their properties before attempting to make any.
Using Salvaged Phenolics
- Identify by hardness, dark colour, a faint phenolic smell when warmed, and failure to soften in heat.
- Do not heat or burn it in the open: it gives off phenol and formaldehyde.
- Reuse whole parts for insulating bases, handles and switch bodies.
- Grind scrap for filler, but not for remoulding.
Phenol and formaldehyde are toxic and phenolic dust is harmful
Phenol penetrates skin quickly and can cause serious poisoning from a small area of contact, and formaldehyde vapour harms eyes and lungs and is a recognised carcinogen. Do not attempt synthesis without gloves, eye and face protection, a respirator rated for organic vapour, outdoor or strongly ventilated space and water for flushing. Sanding or machining cured phenolic makes fine dust to avoid breathing.
Common Mistakes
- Treating the recipe as the hard part — control of heat is.
- Burning scrap — toxic fumes.
- Using it as a load-bearing part unfilled — it is brittle.
What’s Next
- Learn next: Bioplastics — polymers from renewable sources
- Combine with: Plastics from Scratch — the full route for phenolic plastics