--- title: "Thermosetting vs thermoplastics; Bakelite" subject: "Engineering Chemistry" chapter: 4 key: "semester_2/engineering_chemistry:thermosetting-bakelite" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/thermosetting-bakelite site: BCT Engineering --- # Thermosetting vs thermoplastics; Bakelite Engineering Chemistry · Chapter 4: Engineering Polymers - Asked in: 2068 Magh, 2+3 marks - Video: [Thermoplastic & Thermosetting Plastic #Types_of_Plastic #Engineering_Plastics, #PVC,PVA, #Bakelite](https://www.youtube.com/watch?v=qiv5vq5tWww) — Goswami Classes - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/thermosetting-bakelite - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4 ## Question Distinguish between thermosetting and thermoplastics polymers. Mention the preparation and uses of bakelite. ## Short answer Thermosetting polymers have cross-linked 3-D networks, set permanently on heating and cannot be remoulded, whereas thermoplastics are linear, soften and can be remoulded. Bakelite, a phenol–formaldehyde resin, is made by condensation of phenol and formaldehyde; used for electrical insulators, switches and handles. ## Answer #### Thermosetting vs thermoplastic polymers | Point | Thermoplastic | Thermosetting | |---|---|---| | Structure | Linear or slightly branched chains | Cross-linked 3-D network | | Effect of heat | Softens on heating, hardens on cooling (reversible) | Sets permanently on heating; does not soften again | | Moulding | Can be remoulded and recycled | Cannot be remoulded | | Solubility | Soluble in some organic solvents | Insoluble and infusible | | Mechanical nature | Soft, flexible, less strong | Hard, rigid, strong, brittle | | Examples | Polyethylene, PVC, nylon, polystyrene | Bakelite, epoxy resin, melamine–formaldehyde | #### Bakelite: preparation and uses Bakelite is a phenol–formaldehyde resin, the first fully synthetic thermosetting plastic. It is made by **condensation polymerisation** of phenol and formaldehyde (methanal). 1. Phenol reacts with formaldehyde (acid or base catalyst) to give o- and p-hydroxymethyl phenols: C₆H₅OH + HCHO → HOCH₂–C₆H₄–OH 2. These condense with more phenol with loss of water, forming a linear, low molecular mass prepolymer (novolac with an acid catalyst and excess phenol; resole with a base catalyst and excess formaldehyde). 3. On heating with a hardener (hexamethylenetetramine for novolac), or simply on heating for resole, –CH₂– bridges form between the rings at the o- and p-positions, giving a hard, **cross-linked three-dimensional network**. ```text OH OH OH | | | (C₆H₃)–CH₂–(C₆H₃)–CH₂–(C₆H₃)– (rings joined by –CH₂–; | | more CH₂ bridges cross-link CH₂ CH₂ the chains in 3-D) ``` n C₆H₅OH + n HCHO → [C₆H₃(OH)–CH₂]ₙ + n H₂O (simplified) **Uses:** Bakelite is hard, rigid, heat resistant, a good electrical insulator, and does not soften on reheating. It is used for: - Electrical switches, plugs, sockets, switchboards and insulating parts of computers and appliances. - Handles of cookers, pans and irons. - Laminates (e.g. phenolic printed circuit boards), brake linings, and adhesive for plywood. - Telephone cases, radio cabinets, billiard balls, gears and bearings. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which describes a thermosetting polymer? A. A linear chain that softens on heating B. A cross-linked 3-D network that sets permanently on heating C. A solution of monomers D. A branched elastomer that melts Answer: B. Thermosets are cross-linked, infusible and cannot be remoulded. (Key point: Structure) 2. Which is true of solubility? A. Thermosets dissolve in all solvents B. Both are insoluble C. Thermoplastics dissolve in some organic solvents; thermosets are insoluble D. Thermoplastics are insoluble in all solvents Answer: C. Cross-linking makes thermosets insoluble and infusible. (Key point: Solubility) 3. Bakelite is prepared by A. Addition of styrene B. Hydrolysis of silanes C. Heating PCl5 with NH4Cl D. Condensation of phenol and formaldehyde forming CH2 cross-links Answer: D. Phenol–formaldehyde condensation, with water loss, gives a cross-linked resin. (Key point: Bakelite preparation) 4. Which is a use of Bakelite? A. Electrical switches, plugs and pan handles B. Elastic fibres C. Non-stick coatings D. Drug capsules that dissolve Answer: A. Bakelite is hard, heat resistant and insulating. 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