Which one of the following is an example of melamine used for making floor tiles ?

Question

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Correct — C, Thermo setting plastic. Plastics divide into two families according to what heat does to them once they have been moulded. A THERMOPLASTIC has long polymer chains that lie alongside one another with only weak forces between them; heat lets the chains slide, so the material softens, can be reshaped, and hardens again on cooling — and it can be put through that cycle repeatedly. A THERMOSETTING plastic is different in structure: during moulding the chains cross-link into a single three-dimensional network held by covalent bonds, and once that network has formed heat cannot undo it. The article can only char or burn; it can never be remelted and remoulded. Melamine belongs to the second family. What is sold as 'melamine' is melamine-formaldehyde resin, made by condensing melamine (C3H6N6, a triazine ring carrying three amino groups) with formaldehyde, and it cures into exactly such a cross-linked network. That structure gives it the properties the question alludes to: it is hard, scratch-resistant, resists fire and tolerates heat better than most plastics, which is why the NCERT science textbook lists its uses as floor tiles, kitchenware and fabrics that resist fire. Note the wording of the option carefully. The examiner is not asking what melamine is chemically; he is asking which CLASS of plastic it exemplifies, and 'thermo setting plastic' is that class. The three other options are all on the wrong side of the divide — one is the opposite class, and the other two are named members of it.

Why the other options are wrong — (a) The exact opposite class, and the option that will collect most of the wrong marks because the two words look almost alike on a printed page under time pressure. A thermoplastic softens on heating and can be remoulded again and again — polythene, PVC, polystyrene, nylon. Melamine cannot be remoulded at all once cured.  (b) Polythene is a specific thermoplastic — the polymer of ethene, used for carry bags, bottles, pipes and packaging film. It is soft, easily melted and reshaped, and would deform under a hot pan, which is exactly why it is not the material of a floor tile or a dinner plate.  (d) Polyvinyl chloride is another named thermoplastic, used in pipes, cable insulation, flooring sheet and window frames. It appears here because PVC really is used in flooring, so a candidate reading only the words 'floor tiles' may reach for it — but the question is about the class melamine belongs to, and PVC softens on heating while melamine does not.

Concept: Polymers are long molecules built by repeating a small unit, and their bulk behaviour follows from how those long molecules are arranged relative to each other. Linear or lightly branched chains held together by weak intermolecular forces give a thermoplastic: heat overcomes those weak forces, the material flows, and it can be extruded, injection-moulded and recycled. Polythene, PVC, polystyrene, PET and nylon are all in this family, and it is the family that dominates packaging precisely because it can be melted and reformed. Cross-linked chains give a thermoset: the moulding step itself is an irreversible chemical reaction, so the finished article is a single giant molecule that cannot be melted. Bakelite — phenol-formaldehyde — was the first commercial plastic of this kind, and is used for electrical switches and the handles of cookware because it is a poor conductor of heat and electricity; melamine-formaldehyde is the other standard school example, chosen where hardness, heat resistance and a wipeable surface matter. The practical consequences run both ways: thermosets are more durable and heat-resistant but are far harder to recycle, since they cannot be melted down and reformed.

State PCS 2024 Q82 — Science & Technology PYQ with Answer

State PCS 2024 Q82 — Science & Technology PYQ with Answer