--- title: "Preparation and applications of Bakelite, polyurethanes" subject: "Engineering Chemistry" chapter: 4 key: "semester_2/engineering_chemistry:bakelite-polyurethane-prep" source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/bakelite-polyurethane-prep site: BCT Engineering --- # Preparation and applications of Bakelite, polyurethanes Engineering Chemistry · Chapter 4: Engineering Polymers - Asked in: 2068 Bhadra, 4 marks - Video: [Preparation, Properties & Uses of Polyurethane, Polycarbonates & Phenol-Formaldehyde Resins [PART 4]](https://www.youtube.com/watch?v=RrV-7kwDRfM) — Dr. Vikrant Palekar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/bakelite-polyurethane-prep - Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4 ## Question Give the preparation and applications of bakelite and polyurethanes. ## Short answer Bakelite is a phenol–formaldehyde thermoset, made by condensation then cross-linking; used in electrical switches and handles. Polyurethane is formed from diisocyanate and diol, with water for CO2 foam; used in foams, coatings, and spandex fibres. ## Answer #### Bakelite 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) **Applications:** Bakelite is hard, rigid, heat resistant, a good electrical insulator, and does not soften on reheating. Uses: - 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. #### Polyurethane Polyurethane is formed by **step-growth addition polymerisation** of a diisocyanate with a diol (polyol). For example, toluene-2,4-diisocyanate (TDI) or methylene diphenyl diisocyanate (MDI) reacts with ethylene glycol or a polyether polyol: ```text n OCN–R–NCO + n HO–R'–OH → [–CO–NH–R–NH–CO–O–R'–O–]ₙ (diisocyanate) (diol) polyurethane ``` The urethane link (–NH–CO–O–) joins the units, and no small molecule is lost. Water added to the mixture reacts with –NCO to give CO₂, which blows the polymer into a foam. With a polyol of higher functionality, a cross-linked polymer is obtained. **Applications:** - Flexible foams for mattresses, cushions and car seats. - Rigid foams for thermal insulation of refrigerators and buildings. - Paints, varnishes and coatings, adhesives and sealants. - Elastic fibres (spandex/Lycra), shoe soles, rollers, wheels and synthetic leather. - Medical uses such as catheters and artificial heart parts. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Bakelite is formed by which type of reaction? A. Addition polymerisation of styrene B. Ring-opening polymerisation of caprolactam C. Condensation polymerisation of phenol and formaldehyde with loss of water D. Free-radical polymerisation of CF2=CF2 Answer: C. Phenol and HCHO condense, losing water, to make a thermosetting resin. (Key point: Bakelite type) 2. Which conditions give novolac? A. Base catalyst with excess formaldehyde B. Acid catalyst with excess formaldehyde C. No catalyst and excess water D. Acid catalyst with excess phenol Answer: D. Novolac: acid and excess phenol; resole: base and excess formaldehyde. (Key point: Novolac and resole) 3. What produces the hard 3-D network in Bakelite? A. –CH2– bridges between rings at o- and p-positions on heating B. S–S bridges C. Si–O links D. Urethane links Answer: A. Methylene bridges cross-link the chains. (Key point: Cross-linking) 4. What is the link in polyurethane and how is a foam made? A. Amide –CONH–; SO2 gas blows the foam B. Urethane –NH–CO–O–; water reacts with –NCO to give CO2 that blows the foam C. Ester link; no gas is involved D. Ether link; air is stirred in Answer: B. No small molecule is lost in forming urethane links, and CO2 from water reaction forms the foam. 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