Plastics — which of the following is considered non-inflammable or self-extinguishing under normal conditions? Select the best option.

Introduction / Context:
Fire behavior is a critical selection criterion for plastics in building products, cable insulation, and interior finishes. Materials differ widely in ease of ignition, flame spread, smoke, and tendency to self-extinguish once the external flame is removed. This question probes basic fire-performance knowledge for common plastics.

Given Data / Assumptions:

• Ambient pressure and normal oxygen concentration are assumed.
• “Non-inflammable” here means relatively difficult to ignite and often self-extinguishing without sustained external flame.
• Representative commodity and thermoset plastics are listed.

Concept / Approach:
PVC contains chlorine in its polymer backbone. When heated, it can release hydrogen chloride and forms a char that inhibits further combustion. As a result, PVC is commonly classified as self-extinguishing and is widely used where flame retardance is needed. In contrast, cellulose acetate burns relatively readily; urea and phenol formaldehyde are thermosets with good heat resistance but can still sustain combustion and produce smoke once ignited, and they are not universally regarded as non-inflammable compared with PVC's self-extinguishing behavior.

Step-by-Step Solution:

Verification / Alternative check:
Fire safety data and material handbooks often list PVC among self-extinguishing polymers, widely used in cable insulation and conduit for this reason.

Why Other Options Are Wrong:

• Cellulose acetate: Readily combustible; lacks self-extinguishing behavior.
• Phenol formaldehyde: Heat resistant but can burn; not classed as non-inflammable in general use.
• Urea formaldehyde: Similar caveat; combustible under sufficient heat.

Common Pitfalls:
Equating thermal stability with non-inflammability; a material may resist heat but still propagate flame once ignited.

Final Answer:
Polyvinyl chloride (PVC) plastics