In order to determine which treatment type is best suited for your application, we must first examine the differences between corona, plasma and flame treating. The purpose of all three types of treatment is to improve the wettability and adhesion characteristics between plastic or flexible substrates and inks, coatings and laminations. Unlike the case of an ink penetrating into a porous surface like paper, plastic films will generally need some means of surface treatment to achieve acceptable chemical bonding with the ink or adhesive.
Although there are numerous theories explaining the principles behind the process of corona treating, the most commonly accepted theory appears to be the theory of high-speed oxidation. This theory states that the energy of the high-charged electrical corona breaks the molecular bonds on the surface of the non-polar substrate. The broken bonds then recombine with the free radicals in the corona environment to form additional polar groups on the film surface. These polar groups have a strong chemical affinity to the polar inks and adhesives, which results in improved adhesion. Similarly, the polar surface results in an increased surface energy that correlates with improved wettability.
Plasma treatment or "modified atmospheric plasma" treating is very similar to traditional corona treatments with the exception that gases are injected into the corona discharge to modify the reaction with the substrate. Some materials are less reactive to a traditional corona and require this special atmospheric plasma treatment. Additionally, semi-conductive gases such as helium can be utilized to lower the operating voltage at the corona to meet other particular application requirements.
Flame treating is just that, the exposure of a given substrate to an oxygen rich flame plasma for the purpose of promoting adhesion. It can be beneficial in several ways. Due to the high temperatures achieved with flame treating, it is commonly used on paper or board stock to burn off dust and fibers cleaning the surface for printing, coating or extrusion coating and works very well in the removal of annealing oils from foil to promote coating/laminating. The oxygen rich portion of the flame, known as the secondary zone, promotes oxidation in a very similar way that corona does to plastic substrates. The benefit to flame lies in the intensity of the plasma enabling higher treat levels at faster speeds with no backside treatment. Additionally, flame is not limited by gauge of material.
What type of technology would most likely be used in the printing industry?
Corona treatment is still the most popular means of achieving better adhesion and will continue to be the dominant choice for years to come. Why? Because corona treatment is still the most economical and practical means of promoting adhesion.
While flame and plasma have a place in the printing market, their popularity is limited by the additional cost and complexity of their operation. Only a limited number of applications justify this added cost. Your supplier should help you determine which method is best for you. Keep in mind, the less complex, the better.
What was the impetus for plasma or flame treating technology versus corona? How does it relate?
As mentioned before, certain materials do not react as well to conventional corona treatment. Fluoropolymers and polypropylenes are examples of materials requiring these processes. Converters have long used gases within their corona treating systems to enable surface energy enhancement on fluoropolymers, while BOPP manufactures have long used flame treatment in their process. Today we are starting to see flexography speeds increase to the point where flame treaters are required.
What are some of the more recent developments in corona treatment as it pertains to printing?
As material gauges become thinner and process speeds increase, emphasis has been placed on ways of generating smoother, yet more intense corona discharge within a smaller package. Additionally, emphasis is being placed on providing more complete systems that require less installation coordination for the converter.