Curl is the tendency of a coated film to curve towards the coated side of the film instead of lying flat can be created in several different parts of the web coating process, substrate manufacture, drying in coating line, lamination, storage and normal usage. Curl is caused in the dryer by residual stresses in the coating applying a force to the substrate, which causes the substrate to bend. These stresses are affected by the shrinkage of the coating during drying and are increased by rapid drying, and low solvent concentration in the dryer air. The magnitude of these stresses will depend on the drying conditions, amount of shrinkage from the initial wet thickness, the properties of the binder system, the transition temperatures and stress relieving in the dryer
Depending on its severity, curl can have a detrimental effect during the drying process and if not reduced will effect the functionality of the product when used by the customer. In the dryer the drying generated forces substrate edge to curl and can result in the wet coating contacting the dryer nozzles and causing defect in the coated layer. Figure 1 shows the curl and the initiating stresses. Curl can be a major problem in a floater dryer because of the close distance, 0 " to 1" between nozzle and web. This is dependent on web type, web tension, and nozzle design, between substrate and floater dryer nozzles. Gelatin containing coatings are particularly susceptible to curl and will exhibit severe curl at low relative humidity in the dryer. Also, finished product can curl if stresses are not relieved after exiting the dryer
There are several basic actions that can be taken to reduce dryer curl; Insure substrate has no inherent curl from manufacturing process, reduce stresses induced by the drying conditions and modify the product structure to generate less force and make the substrate more resistant to deformation.
The shrinkage forces in the dryer can be reduced by several actions. Decreasing the drying rate by lowering temperature and increasing drying air solvent content. This will give more time for the coating to reduce stresses. Increasing the solvent concentration the falling rate zone drying zone will prevent the surface layer from prematurely drying. It will reduce shrinkage stress in the drying coating, thereby reducing curl. Increasing the relative humidity in space between the end of the dryer and the rewind can plasticize binders that are moisture sensitive reducing stress. Keep the dry point close to the end of the dryer and do not over dry. Reduce the solids concentration to slow down drying and generate less forces.