Substrate Core Thermal Expansion Limits in High Speed Sheetfed Converting Operations
Substrate core thermal expansion limits dictate sheet dimensional stability, requiring precise thermal control to prevent misregister and bimetallic warp.
Polymer Free Volume Expansion
Barrier Performance
Molecular changes in synthetic coatings caused by heat increase the space between polymer chains, allowing moisture or gases to pass through the barrier more easily. Polymer free volume expansion describes the widening of the microscopic gaps within a plastic film as it reaches its glass transition temperature. This expansion reduces the effectiveness of the coating in protecting the food or contents inside the package.
It is a critical factor in the shelf-life testing of laminated cartons.
Permeability Increase
Thermal energy provides the polymer chains with enough movement to push away from each other, creating more empty space at the atomic level. This polymer free volume expansion allows oxygen and water vapor molecules to diffuse through the material more rapidly. The rate of this expansion depends on the specific chemistry of the plastic and the presence of any plasticizers.
Once the material cools, the chains may not return to their original tightly packed state. This permanent change can lead to the premature spoilage of the packaged product.
Storage Conditions
Lower storage temperatures prevent the expansion from compromising the package. This physical property is why certain laminated materials are rated for refrigerated use only. Understanding these molecular limits is necessary for choosing the right barrier for high-temperature applications.