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FLEXO Magazine : March 2013
Technologies & Techniques Surface Energy & Poor Adhesion Corona Treatment optimizes Film’s Performance on Press The problem with writing or printing on plastic films is well known. You have a plastic bag or shrink sleeve label and you would like to write on it with a pen. Unfor- tunately, the ink doesn’t stick to the plastic. Untreated plastic has a structure, which makes it impossible to both write and print on, due to poor adhesion. Corona treatment is the solu- tion to this well know problem. Whether a liquid wets a material effectively or not, depends primarily on the chemical nature of both the liquid and the substrate. Wetting is defined as the ratio between the surface energies of the liquid and substrate. In general the following rule is true: “A material will be wetted, if it’s surface energy (=dyne/cm) is higher than the surface energy of the liquid. If not, there will be an adhesion problem. Figure 1 shows the basic dyne-level of a material and the required dyne-level in the different applications. AVOID ADHESION PROBLEMS Corona treatment is necessary to obtain sufficient wet- ting and adhesion on plastic films or metallic foils before the printing, laminating or coating. A corona discharge unit is the solution to optimize the wetting and adhesion. This corona technique has proved to be highly effective, cost-effective and can take place in-line. What is corona treatment? Plastic is a man-made synthetic material, which contains long homogeneous molecular chains that form a strong and uniform product. The chains of molecules are normally joined end-to-end forming even lon- ger chains, leaving only a few open chain ends, thus provid- ing only a small amount of bonding points at the surface. The small amount of bonding points cause the low adhe- sion and wettability, which is a problem in converting process. When the Danish engineer and founder of Vetaphone, Verner Eisby, was asked about a solution to this problem, he came up with a theory that a high frequency charge would provide both a more efficient and controllable method of increasing the adhesion and wettability of a plastic surface. During corona discharge treatment, electrons are acceler- ated into the surface of the plastic, causing the formation of long chains to rupture and producing a multiplicity of open ends and free valences. Ozone from the electrical discharge creates an oxygen- ation, which in turn forms new carbonyl groups with a higher surface energy. The result is an improvement of the chemical connection (dyne/cm) between the molecules in the plastic and the applied media/liquid. This surface treatment will not reduce or change the strength. Neither will it change the appearance of the material. Corona only changes the top molecule chains, which are 0.00001 micron thick. TREATMENT LEVEL Effectiveness of corona treatment depends on the specific material being used. Different materials have different char- acteristics and different amounts of slip and additives, which will determine the effect of the corona treatment. There are no limits with regard to the materials that can be corona treated. However, the required intensity of the treatment (watt/min/m2) may vary significantly. The treatment level can be calculated by using the following formula: Power(watt)=TxSxWxM P = Total Power (watt) required T = Number of sides to Treat (single/double sided) S = Line Speed (in meters per minute) W = Film Width (in meters) M = Material factor (required Watt per m2 per minute) The exact value is best determined by testing a sample of the actual film that is used for a specific application. AUTOMATIC POWER REGULATION Corona-Plus treaters are equipped with automatic power regulation. When the required treatment level (watt/min/m2) has been determined, Corona-Plus units will automatically regulate the power needed and the film is now influenced by the same energy, regardless of the speed or the width of the film. This ensures even treatment of all the material. Over time, the ob- tained dyne-level will decrease and it can be 46 FLeXO marCh 2013 www.flexography.org CALCULATING TREATMENT LEVEL PowER(wATT)=TxSxwxM • P = Total Power (Watt) required • T = Number of sides to Treat (single/double sided) • S = Line Speed (in meters per minute) • W = Film Width (in meters) • M = Material factor (required Watt per m2 per minute) • Exact value is best determined by testing a sample of actual film used for a specific application