Question

Which of the following is a thermosetting plastic?
(A) Melamine
(B) Polyethene
(C) PVC
(D) Nylon

Answer 1

Think about the definition of thermosetting plastics and what kind of structures they form on a molecular level. Define the structures based on whether they are one, two, or three dimensional.

Complete step by step solution:
Thermosetting plastics are polymers that can be melted only once and once they are given shape; they cannot be easily moulded into other shapes and become rigid after they are heated once. Before heating, they can be seen as soft solids or liquids. Polymers that are cross-linked are considered to be thermosetting polymers.
Thermoplastic polymers on the other hand can be heated and moulded repeatedly to form various shapes and have a lower boiling point than that of the thermosetting polymers.
First, let us look at the different types of polymers based on their linkage. This will help us determine which class of polymers is usually considered to be a thermosetting polymer.
- One-dimensional polymers
These polymers are single-chain polymers that do not branch in any way and have the next monomeric units beside them. No units are present above, below, behind, or in front of the monomers in these polymers. They are also known as linear polymers. PVC, high-density polythene, and nylon are considered to be linear polymers. Their structures are:
- PVC

- High-density polythene

- Nylon

- Two-dimensional polymers
These types of polymers form sheets since some monomeric units can be branched and form another linear chain. Chains can be present on either side of the monomeric unit and also the top and the bottom. An example of a branched polymer is low-density polythene. The structure is:

Here we can see that branching is present in the ethylene monomeric units.
- Three-dimensional polymers
In these polymers, sheets are joined to each other by intermolecular bonding of the monomeric units. This is also called cross-linking. These types of polymers are called thermosetting polymers. An example of this type of polymer is melamine. The structure is:

Sheets of this melamine polymer are attached to each other by cross-linking.

Hence, the correct answer to this question is ‘A. Melamine’

Note: Note that in the example for the two-dimensional polymers, the structure given is not a monomeric unit, nor do the branches in low-density polythene occur in the same way. The given image is just a representation as to how the branching will look.