Thermal Shrinkage
Internal mechanical forces induced by differential thermal contraction during cooling of composite or coated substrates generate dimensional instabilities within paperboard structures. Polymer coating evaluations measure residual thermal contraction stress to assess forces developing when extruded polyethylene coatings solidify onto paperboard webs. As molten polymer cools from extrusion temperatures to ambient room conditions, its thermal contraction coefficient exceeds that of underlying cellulose fibers.
Restrained contraction generates tensile stress within the polymer layer and balanced compressive stress in the paperboard substrate.
Interfacial Tension
Magnitude of internal thermal stress depends on polymer thermal expansion coefficients, elastic modulus, and substrate stiffness. Fast chilling on cooling cylinders locks in high stress levels before polymer molecules relax. Process engineers adjust extrusion temperatures, cooling rates, and web tension to minimize stress buildup along composite material interfaces.
Board Curvature
Unbalanced internal stress across substrate layers causes curl and dimensional distortion in converted paperboard products. High residual thermal contraction stress leads to curl toward the coated side, disrupting sheet feeding in offset printing presses and high speed carton gluers. Multi-ply packaging designs offset this effect by balancing extrusion coatings on both sides or modifying fiber furnish stiffness.
Quantifying thermal stress parameters helps converters predict dimensional stability during storage under variable climate conditions. Controlling residual thermal contraction stress prevents costly warp defects in extrusion coated paperboard and laminated packaging webs.
