Fig 4 shows a polymer identification chart used in a workshop. (a) Using the flow chart in Fig 4, identify the polymer that self-extinguishes when removed f...

(a) The polymer that self-extinguishes when removed from a flame and produces an acrid smell is PVC (polyvinyl chloride). [1]

Following the flow chart: the sample burns initially, but when removed from the flame it self-extinguishes rather than continuing to burn. PVC is well known for producing a sharp, acrid smell (from hydrogen chloride gas released during combustion) and for self-extinguishing due to its high chlorine content, which inhibits sustained combustion.

(b) One other observation (apart from flame behaviour and smell) that can help identify a polymer during a burn test:

Whether the material drips when it melts. [1] Some thermoplastics (such as polyethylene and nylon) produce molten drips during burning, while others (such as polystyrene) tend to produce sooty smoke without dripping. The colour and density of the smoke can also vary between polymers.

(c) Why thermosets generally do not burn in the same way as thermoplastics:

1. Thermosetting plastics have strong covalent cross-linked bonds between their polymer chains, forming a rigid three-dimensional network structure. [1]
2. When heated, these permanent cross-links do not break down to allow the material to melt and flow. Instead, the material chars and decomposes at high temperatures, rather than melting into a liquid that could sustain a flame as thermoplastics do. [1]