Why do thermosetting plastics decompose instead of melting when heated?

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major industrial polymers Figure 1: The linear form of polyethylene, known as high-density polyethylene (HDPE).

Thermosetting plastics do not melt when heated because their molecular structure consists of a permanent, three-dimensional network created through cross-linking during their initial processing.123

Unlike thermoplastics, which are composed of separate, individual molecules that can flow past one another when heated, thermosets undergo a chemical reaction that binds the entire finished article into one large, infusible molecule. Because these molecules are covalently tied together, they cannot disentangle or flow to reach a liquid state. Consequently, when heat is applied to a thermoset, the material remains dimensionally stable until it reaches a temperature high enough to cause chemical degradation, effectively breaking down the material rather than melting it.134