Paper-based flexible laminates tendency to curl

Sammanfattning: The main purpose of this study is to assess the impact of environmental conditions on paper-based flexible laminates tendency to curl. Any deviation in flatness may indeed affect materials processability. Two different paper-based flexible laminates are considered: a widely processed heat-sealable yogurt lid (paper / metallized polyethylene terephthalate / heat-sealing lacquer) and a prototype laminate (aluminium / paper / polyethylene). Packaging materials sheets (54'36 cm) are namely sampled from packaging materials rolls. Their tendency to curl is assessed by implementing the cross-cut method described in the German DIN 55403 standard (Deutsches Institut für Normung (DIN), 2014). Fifteen climates are tested i.e. five different relative humidity values (30%, 40%, 50%, 60% and 70%) and three different temperature values (20°C, 25°C and 30°C). The effect of a polyethylene wrapping is also evaluated as a potential solution to prevent or reduce curl. For the yogurt lid which is shown to be sensitive to climate conditions, deformation occurs in both machine and cross-machine directions: average curl values respectively reach 13 millimetres and 37 millimetres. An equation to predict curl parameters as a function of climate and room conditions is furthermore suggested (r² = 72%). For the prototype laminate which appears not to be sensitive to climate conditions, deformation happens to be much lower than for the first material and only occurs in the machine direction. Average curl values reach 3 millimetres in the machine direction and zero millimetres in the cross-machine direction. Finally, considering the material-specific results obtained for the two laminates, no general conclusion can be drawn as regard to the polyethylene wrapping effect.