The uniform degradation rate of a plastic item in the environment can be defined as the differential mass loss per unit time ( 𝑑𝑚 𝑑𝑡 ), equal to the product of the surface area (𝐴 𝑠 ) of the item and the density (𝜌) and specific surface degradation rate (𝑘 𝑑 ) of the item's material (Equation ).