Engineering Synthetics: Phenol-Formaldehyde, Epoxy Resins, Silicones
1. Phenol-formaldehyde (bakelite family)
Condensation of phenol + formaldehyde.
- Acid catalyst + excess phenol → novolac (linear, thermoplastic); heating with hexamine cross-links it to bakelite (thermoset 3D network).
- Base catalyst + excess formaldehyde → resol/resite directly to cross-linked bakelite.
- Properties: hard, heat/chemical resistant, electrical insulator, dark colour.
- Uses: electrical switches, plugs, handles, PCB laminates, adhesives, brake linings.
2. Epoxy resins
Made from bisphenol-A + epichlorohydrin → prepolymer with terminal epoxide rings; cured with hardener (amine/anhydride) which opens rings and builds 3D network:
- Properties: outstanding adhesion (polar –OH + ether groups bond to almost anything), low shrinkage on cure, chemical/corrosion resistance, good electrical insulation.
- Uses: structural adhesives (Araldite), protective coatings (tanks, ships), composites (carbon-fibre aircraft parts, wind blades), potting/encapsulation of electronics, flooring.
- Limitation: UV yellowing outdoors unless top-coated; hardener ratio must be exact.
3. Silicones (polysiloxanes)
Backbone of alternating Si–O instead of C–C: repeat unit
Made by hydrolysis of dimethyldichlorosilane followed by condensation.
- Why special: Si–O bond is long, flexible, partly ionic → very low (~−125 °C), water-repellent (methyl groups outward), stable from −60 to 250 °C, biocompatible, good dielectric.
- Forms tuned by cross-linking: fluids/oils (lubricants, antifoam, hydraulic fluid), greases, rubbers (seals, tubing, baking moulds, implants), resins (heat-resistant paints, electrical insulation).
| Resin | Monomers | Type | Star property | Typical use |
|---|---|---|---|---|
| Phenol-formaldehyde | phenol + formaldehyde | thermoset | hard insulator | switches, handles |
| Epoxy | bisphenol-A + epichlorohydrin | thermoset (needs hardener) | strongest adhesion | adhesives, composites |
| Silicone | chlorosilanes → Si–O–Si | fluid/rubber/resin | heat-stable, water-repellent | sealants, implants, oils |
Exam Points
- Monomers + novolac vs resol conditions for bakelite (3 marks).
- Epoxy cure reaction + two properties + two uses (3 marks).
- Silicone structure (Si–O backbone), why heat-stable/hydrophobic, three forms with uses (5 marks favourite — draw repeat unit).