---
title: "Define thermosetting plastic"
subject: "Civil Engineering Materials"
chapter: 10
key: "semester_3/civil_engineering_materials:thermosetting-plastic-define"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/thermosetting-plastic-define
site: BCT Engineering
---

# Define thermosetting plastic

Civil Engineering Materials · Chapter 10: Asphalt, Bitumen, Tar and Miscellaneous Materials

- Asked in: 2080 Bhadra, 1 marks
- Video: [Thermoset Resins: General Concepts - Session 1](https://www.youtube.com/watch?v=qKFmkEpdsgk) — Epoxy Channel
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/thermosetting-plastic-define
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-10

## Question

Define thermosetting plastic.

## Short answer

A thermosetting plastic hardens permanently upon first heating and moulding due to cross-linking of polymer chains, and cannot be remelted or reshaped; it chars if overheated. Examples include Bakelite, melamine, epoxy, and polyester resin.

## Answer

A **thermosetting plastic** is a plastic that becomes hard and permanently rigid on the first heating and moulding because of cross-linking of its polymer chains. It cannot be softened or reshaped by heating again and burns or chars if overheated. Examples: Bakelite, melamine formaldehyde, urea formaldehyde, epoxy resin and polyester resin.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does a thermosetting plastic stay permanently rigid after its first heating and moulding?
   A. Its polymer chains form cross-links
   B. Its chains stay linear and slide past each other
   C. It absorbs moisture that hardens it
   D. It melts and recrystallises on cooling
   Answer: A. Cross-linking of the polymer chains during the first heating creates a rigid network that cannot be softened again. (Key point: Cause of permanent setting)

2. What happens when a moulded thermosetting plastic is heated again?
   A. It softens and can be remoulded many times
   B. It cannot be reshaped and burns or chars if overheated
   C. It becomes more flexible and elastic
   D. It dissolves into its monomers
   Answer: B. Thermosets cannot be softened by reheating; excessive heat only makes them burn or char. (Key point: Effect of reheating)

3. Which of the following is a thermosetting plastic?
   A. PVC
   B. Polyethylene
   C. Bakelite
   D. Polystyrene
   Answer: C. Bakelite, melamine formaldehyde, urea formaldehyde, epoxy and polyester resin are thermosets; PVC, PE and PS are thermoplastics. (Key point: Examples)