---
title: "Polyurethanes and Bakelite"
subject: "Engineering Chemistry"
chapter: 4
key: "semester_2/engineering_chemistry:polyurethane-bakelite"
source: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyurethane-bakelite
site: BCT Engineering
---

# Polyurethanes and Bakelite

Engineering Chemistry · Chapter 4: Engineering Polymers

- Asked in: 2074 Bhadra, 2.5+2.5 marks
- Video: [Preparation,structure,properties applications of Nylon66,Bakelite, Novolac,Polyurethanes,Polycarbona](https://www.youtube.com/watch?v=iaL4HycqjsA) — Chem Chaya
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_2/engineering_chemistry/polyurethane-bakelite
- Whole chapter: https://www.bctengineer.com/md/semester_2/engineering_chemistry/chapter-4

## Question

How your acquaintance to polyurethanes and Bakelite.

## Short answer

Polyurethane is a step-growth addition polymer from a diisocyanate and a diol, giving urethane links; used in foams, coatings, and elastic fibres. Bakelite is a thermosetting phenol–formaldehyde resin formed by condensation polymerisation that cross-links into a 3D network, making it hard, heat-resistant, and used for electrical insulators, handles, and laminates.

## Answer

#### Polyurethanes
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. No small molecule is lost. Water added in the mixture reacts with –NCO to give CO₂, which blows the polymer into a foam. With a higher functionality polyol a cross-linked polymer is obtained.

**Uses:**
- 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.

#### 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 just heat 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)

**Properties and 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, switch-boards 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 monomers form polyurethane?
   A. Diisocyanate and a diol
   B. Phenol and formaldehyde
   C. Diamine and diacid
   D. Styrene and butadiene
   Answer: A. A diisocyanate (TDI or MDI) reacts with a diol or polyol to form urethane links. (Key point: Polyurethane monomers)

2. How does polyurethane become foam?
   A. Water reacts with -NCO to give CO2 which blows the polymer
   B. Heating liberates nitrogen
   C. Air is whipped in
   D. Alcohol evaporates
   Answer: A. Water and -NCO form CO2 gas bubbles that expand the polymer. (Key point: Foam formation)

3. What type of polymerisation produces Bakelite?
   A. Addition
   B. Ring-opening
   C. Condensation of phenol and formaldehyde with loss of water
   D. Free radical chain of styrene
   Answer: C. Bakelite is a phenol-formaldehyde resin formed by condensation. (Key point: Bakelite polymerisation)

4. Why does Bakelite not soften on reheating?
   A. It is linear
   B. -CH2- bridges form a cross-linked 3-D network
   C. It contains metal
   D. It is biodegradable
   Answer: B. Methylene bridges join rings into a rigid network, so it is thermosetting. (Key point: Thermosetting nature)

## Related in Engineering Chemistry

- [Preparation and applications of Bakelite, polyurethanes](https://www.bctengineer.com/question.md?key=semester_2%2Fengineering_chemistry%3Abakelite-polyurethane-prep) — Engineering Chemistry, chapter 4