In the coating/printing industry, the corona treatment is a commonly adopted step to enhance the surface energy of the coating/printing web, typically made of plastics, paper, fabrics with low surface energy of low 30s mN/m or dyne/cm.
Schematic image about a corona treater. Source: www.3dtllc.com. It includes a power supply, Transformer, converting regular power to high voltage, high frequency AC (10-20KV, 20KHz), and treater station.
Between corona electrode and grounding roll where web is sitting on, electrons are accelerated by high electrical field (about 10-20KV, gap of 2-3 mm, and frequency of 20KHz) and ionize the air in between, generating ozone, nitrogen oxide and emitting light. When the electrons hit into polymer surface, they break C-H or C-C bonds in the polymer. Reactions with the corona take place at these free radicals, mainly towards oxidation. Thus polar functional groups, such C=O, COOH, C-OH, forms consequently (refer to the following figure). As a result the surface energy of the polymer substrate is significantly increased and favors the following coating or printing process.
In addition to plastics, paper, metal foils can also be treated by corona to remove contaminates on the surface (e.g. organic molecules during foil making process).
Depending storage conditions, the treatment effect can last for a couple of days to a couple of weeks. For example, initial treatment level of i.e 48 dynes/cm comes to 44 dynes/cm within 2 days while it takes 15 days from 44 dynes/cm to 40 dynes/cm (ref 2).