Psychrometric Boundary Layer Analysis in Wrapped Paperboard Pallets during Transit
Polyethylene film wraps trap moisture desorbed by paperboard during thermal swings, elevating internal relative humidity and triggering mechanosorptive failure.
Partial Vapor Pressure Gradient
Moisture Transport
Thermodynamic forces that drive water vapor through a sheet of paperboard are determined by the difference in temperature and relative humidity between its two sides. The partial vapor pressure gradient is the difference in moisture pressure that causes water molecules to diffuse through the porous cellulose network from the high-pressure side to the low-pressure side. This phenomenon occurs when a package is filled with a wet or cold product and stored in a dry or warm environment.
The transport process continues until the vapor pressure on both sides of the board reaches equilibrium.
Barrier Resistance
The rate of this vapor movement is controlled by the thickness and porosity of the paperboard, as well as the presence of synthetic polymer coatings. Under a high partial vapor pressure gradient, a standard uncoated board will absorb moisture rapidly, losing its structural strength and stiffness. To counter this, converters apply barrier layers such as polyethylene or wax, which increase the vapor resistance of the material.
This treatment reduces the flow rate of the water vapor, protecting the package contents and maintaining the box’s integrity.
Package Design
Designing packages for high-humidity routes requires calculating the vapor pressure gradient across the board under expected transport temperatures. This calculation helps packaging engineers select the appropriate coating thickness for the board. An incorrect calculation can lead to premature packaging failure and spoiled goods.