What are the differences in thermal conductivity between solid and foamed plastics?

Thermal conductivity is a critical property that distinguishes solid plastics from their foamed counterparts. Solid plastics typically exhibit higher thermal conductivity values, ranging from 0.15 to 0.45 W/m·K, as heat transfers more efficiently through their continuous, dense molecular structure. In contrast, foamed plastics demonstrate significantly reduced thermal conductivity, typically between 0.020 and 0.050 W/m·K, due to their cellular structure containing trapped gas pockets that impede heat transfer. The fundamental difference lies in the presence of gas-filled cells in foamed plastics, which dramatically lowers thermal conductivity by creating barriers to heat flow through conduction, convection, and radiation. This cellular architecture makes foamed plastics exceptionally effective as thermal insulators, while solid plastics serve better applications requiring heat dissipation. The density reduction in foamed plastics directly correlates with decreased thermal conductivity, as the gas within the cells (often air or specialized blowing agents) possesses much lower thermal conductivity than the polymer matrix itself. This principle explains why expanded polystyrene (EPS) and polyurethane foams are widely used in insulation applications, while solid polymers like polyethylene and polypropylene are chosen for applications where moderate heat transfer is acceptable or desired.