What is the purpose of impregnating a material to make it flame retardant?

The purpose of impregnating a material to make it flame retardant is primarily to alter its structure in a way that enhances its resistance to combustion. By incorporating flame-retardant chemicals into the material, the chemical composition is modified to impede the ignition process and slow the spread of flames. This alteration can include changing the thermal stability of the material or creating a char layer that protects the underlying structure from fire.

While other options might appear relevant in different contexts, they do not directly relate to the primary goal of flame retardancy. Freezing a material does not enhance its fire-resistant properties; instead, it could lead to brittleness. Absorbing moisture is unrelated to flame resistance, and reducing density does not inherently provide any benefits concerning a material's flammability or combustion characteristics. Thus, altering the structure through impregnation is the effective method for enhancing flame resistance.