Polymer networks: cross-linked polymers, once cured into an infusible three-dimensional network, belong to which broad processing class?

Introduction / Context:
Classifying polymers by their thermal response is fundamental to processing choices. Thermoplastics soften and flow upon heating, allowing reshaping; thermosets form permanent, cross-linked networks that do not melt. The question probes recognition of where cross-linked materials fit in this dichotomy.

Given Data / Assumptions:

• Cross-linking yields a covalently bonded 3-D network.
• Heating a cross-linked network does not lead to viscous flow; instead, it may char or degrade.
• Examples include epoxy, phenolic, melamine, vulcanised rubber (to varying degrees).

Concept / Approach:
Thermosetting polymers are those that cure irreversibly. Their cross-linked topology prevents chain slippage required for melt flow. In contrast, thermoplastics consist of linear or lightly branched chains that can be melted and remolded repeatedly below decomposition temperatures. Cross-linked architecture equates to thermosetting behavior in standard polymer science terminology.

Step-by-Step Solution:

Verification / Alternative check:
Differential scanning calorimetry and rheology confirm absence of melt flow in cured thermosets; gel fraction measurements indicate insolubility consistent with network formation.

Why Other Options Are Wrong:

• Thermoplastic: contradicts the inability to melt/flow.
• Either class: cross-linked state specifically defines thermosets.
• “Fibres only”/“Elastomers only”: describe product forms, not thermal class.

Common Pitfalls:
Confusing partially cross-linked TPV (thermoplastic vulcanizates) with fully cross-linked thermosets; TPVs behave overall as thermoplastics due to continuous thermoplastic phase.

Final Answer:
Thermosetting