Permeation Process
Mass transfer of water molecules through a solid sheet occurs along a concentration gradient until equilibrium is reached between both sides. In barrier paperboards and flexible packaging, moisture transport diffusion dictates the rate at which water vapor passes through the cellulose fibers and coating layers to affect the packaged product. This physical movement establishes the shelf life of dry foods or pharmaceutical goods packaged in paper-based containers.
Mechanism Description
The flow begins when water molecules adsorb onto the warm or high-humidity surface of the packaging material. These molecules then dissolve into the polymer or migrate along the internal surfaces of the cellulose pores. This moisture transport diffusion follows a concentration gradient, moving from areas of high relative humidity to dry zones.
In uncoated paper, this process is incredibly fast because of the open, porous nature of the fiber network. Coating the board with a continuous synthetic or bio-based polymer layer blocks these pathways, forcing the water molecules to navigate a highly tortuous path through the material.
Barrier Evaluation
Measuring the rate of this transmission helps packaging designers select the appropriate thickness of protective coatings. High-speed testing instruments measure the weight change of sealed cups or monitor the humidity rise in a dry chamber to quantify moisture transport diffusion under controlled conditions. These tests are conducted at specific temperatures and humidity levels to simulate tropical or ambient storage conditions.
