Corona treatment is an electrical surface treatment applied to prepare the surface of polyethylene film for printing. Polyethylene has low surface energy; on an untreated surface, ink beads up or lifts off easily once dry.
As the film passes between an electrode and an earthed roller, the electrical discharge that forms modifies the surface and helps the ink adhere. The treatment is usually carried out during film extrusion, before winding. Because its effect can fade over time, a roll that has been stored for a long time is checked again before printing. Some additives in the film can also migrate to the surface and affect the result.