Evaluating Surface Energy Decay in Thermal Polyurethane Film Lamination
Evaluating surface energy decay in thermal TPU lamination requires measuring polar force components to ensure bond strength before heat and pressure passes.
Erucamide Blooming
Surface Migration
Organic molecules added to polymer coatings to reduce the coefficient of friction migrate to the surface of the film over time. This process, known as erucamide blooming, is common in polyolefin-coated paperboards used for high-speed packaging. The slip agent crystallizes at the surface, which reduces friction during forming and filling but can interfere with the bonding of inks and adhesives.
Slip Mechanism
Erucamide is partially soluble in molten polyethylene but becomes incompatible as the polymer cools and crystallizes. The molecules are squeezed out of the polymer matrix and move toward the surface, a process that takes between twenty-four hours and several days to reach equilibrium. This layer allows the coated board to slide easily over metal forming collars on packaging machines.
Without this slip effect, the film would stick to the metal, causing tension peaks, tearing and line stoppages.
Adhesion Interference
Excessive accumulation of the slip agent on the surface creates a barrier that prevents inks and adhesives from making direct contact with the polymer. This layer can be removed by corona treatment, but the effect is temporary because more erucamide will migrate from the bulk to replace it. In high-speed packaging, this can cause poor seal strength or ink rub-off.
To avoid these issues, mills must control the concentration of slip additives in the resin, keeping the dosage below one thousand parts per million, while converters must use specialized inks formulated to penetrate the amide layer.