Which material chars and burns but does not flow when exposed to heat?

The correct choice is thermoset materials, such as polyurethane. When exposed to heat, thermosets undergo a chemical change during curing that makes them hard and inflexible. This means that, unlike thermoplastics, they do not melt or flow when heated. Instead, they char and burn while maintaining their structural integrity up to a certain temperature, providing a degree of resistance to deformation. This characteristic is crucial in fire service contexts, as it can affect the behavior of building materials during a fire.

In contrast, thermoplastics can melt and flow when heated, which can lead to a loss of shape and structural support during a fire. Polyurethane foam, while often classified under thermosets when cured, can behave differently under certain conditions, as it may still break down, expand, or spread flames rather than holding its shape. PVC also has its own unique behavior; it can soften and deform under heat, making it unsuitable for situations requiring materials that do not flow under fire conditions. Hence, the choice of thermoset materials aligns with the requirement for charring without flow when exposed to heat.