Common Problems and Precautions in Corona Treatment
1 Principle
The corona treatment described in this article is a process in which electrons are accelerated away from an electrode under high voltage and collide with the polymer surface. Since the conduction between the two poles is blocked, the gas in the electric field, upon electron collision, experiences a sharp increase in ionization concentration. The main reaction process is as follows: O2 + high-energy electrons → 2O + low-energy electrons 2O + 2O2 → 2O3 + heat That is: 3O2 + electrical energy → 2O3 + heat The previous equation can also be written as: 3O2 + M → 2O3 + M
In the equation, M represents any other gas molecule in the air, such as nitrogen. They can also be dissociated into nitrogen atoms by high-energy electron impact, triggering a series of reactions, which are omitted here. During ozone generation, a diffuse blue-violet glow corona phenomenon accompanies it, hence the name corona. In other words, corona treatment of film involves placing the film in an electric field as a medium that blocks conduction, obtaining high energy under the electric field, activating other ions or molecules, and distributing this energy to the film surface, forming polar chemical free radicals on the film surface, creating dangling bonds. In this process, high-energy electrons collide with oxygen, nitrogen, and water molecules in the air, accompanied by oxidation-reduction reactions, producing ozone and nitrogen oxides. Since ozone has strong oxidizing properties, when it contacts the polypropylene film surface, complex organic reactions occur at the nanometer level, producing hydroxyl (-OH), carboxyl (-COOH), carbonyl (> C=O), etc. The introduction of these oxygen-containing functional groups is key to increasing the film's surface tension. Therefore, through oxidation, not only can the film's surface tension be improved, but also its metallizability and printability can be enhanced.
Corona treatment equipment generally includes a high-frequency high-voltage generator and a corona treatment station with metal electrodes and a supporting roller. They are parallel to each other, separated by a 1.5mm air gap. When a high voltage of 20-40kHz or several thousand volts is input to the corona treatment station, discharge occurs between the electrodes, forming uniform sparks on the film surface. 2 Discussion
4.1 Equipment configuration and adjustment status of the corona treatment station
4.1.1 Ideal corona treatment requires correct motor operating frequency, moderate output voltage and current, and regular discharge process to achieve good treatment results.
4.1.2 The gap between the corona treatment roller and the electrode must be consistent, meaning they should have a certain distance and be parallel to each other, so that the field strength on the film surface is uniform, producing uniform corona treatment. Generally, the gap is 1.5-2.5mm.
4.1.3 Adjust the parallelism between the corona treatment roller and other pulling rollers, and the uniformity of pressure on the corona treatment roller, so that the film runs smoothly without wrinkling or skewing, maintaining uniform and sufficient corona dosage. 4.2 Effects of film surface temperature and air relative humidity on corona treatment
During corona treatment, film surface temperature and air relative humidity are two significant variables affecting it.
As air relative humidity and film surface temperature increase, the required corona treatment time becomes longer, meaning the film is less easily corona treated. This is because when relative humidity increases, the water molecule content in the air increases, and ozone produced during corona can dissolve in water. At normal temperature and pressure, ozone's solubility in water is about 13 times higher than oxygen and 25 times higher than air. Due to the decrease in ozone concentration, the formation of oxygen-containing functional groups on the film surface and electret opportunities are greatly reduced, thereby reducing the corona treatment effect. As film surface temperature increases, the stability of electret molecules deteriorates, and the proportion of surface molecular migration increases, which is not conducive to the formation of high surface energy regions on the film surface, partially offsetting the effect of increasing film surface tension through corona. On the other hand, based on practical production experience, the film surface temperature is not necessarily lower the better. During production, the corona treatment process generates a large amount of heat. To prevent excessive film surface temperature, we typically use circulating water in the roller for closed-loop circulation and add a heating device to maintain the cooling water at a certain temperature. Too low a temperature can cause insufficient basic energy for molecular polarization and chemical changes on the film surface, leading to insufficient surface tension. Therefore, controlling the film surface temperature at the corona treatment roller within an appropriate range is a key issue in corona treatment, which we have discovered through long-term production experience. 4.3 Control of corona intensity and treatment time in corona treatment
In production, to achieve a certain level of film surface tension treatment, the common method is to increase the corona treatment intensity. Within a certain limit, this method is effective, but beyond this limit, even increasing corona will not improve the film surface energy level. This is because during instantaneous corona treatment, the air volume between the electrode and the corona roller is in a relatively stable state, and the oxygen molecule content in this relatively stable air volume is fixed. Even increasing the electrode voltage and current cannot activate more oxygen molecules to electret more oxygen-containing functional groups onto the film surface to increase surface energy.
Generally, the longer the corona treatment time, the higher the surface energy. However, on one hand, under existing equipment conditions, extending corona treatment time inevitably reduces production efficiency; on the other hand, excessively long corona treatment time can cause excessive surface tension, embrittlement, and even degradation, which is detrimental to primary winding and secondary rewinding. We have successfully conducted experiments on this at a certain unit's equipment.
4.4 Parallelism of corona treatment roller and electrode
Non-parallelism between the roller surface and electrode surface leads to uneven corona treatment due to differences in corona intensity, as shown by the following equation:
Corona intensity = P/W·V
Where: P — discharge power of corona device, W; W — film width, m; V — film speed, m/min;
From the above equation, corona intensity is proportional to corona discharge power, and the power equation for corona discharge is:
P=4CdVsf［V0-(Cd+Cg/Cd)Vs］ Where: Cd — dielectric capacitance, F; Cg — discharge gap capacitance, F; V0 — driving voltage (peak), V; Vs — gap ignition voltage (peak), V; f — driving voltage frequency, Hz.
From the above equation, it is evident that inconsistent gap sizes directly lead to uneven discharge power per unit area, and due to differences in corona at various points, ultimately affect the consistency of wetting tension. 4.5 Stability of film running
If the tension applied by the roller to the film is inconsistent during running, this difference can cause local wrinkles, changing the capacitance at that location during corona treatment, ultimately leading to uneven wetting tension.
In addition to the aforementioned roller synchronization differences, the spreading of the film by the pinch roller on the corona treatment roller and the expansion from the corona treatment roller to the subsequent spreading roller, as well as parallelism between rollers and roller surface levelness, can all cause unstable film running posture.
4.6 Film embrittlement and misalignment issues
In corona treatment, there must be a limit. If the input power is too high, the water temperature may rise excessively, affecting the secondary bond forces between film molecules and causing embrittlement, which affects film usage and makes it impossible to wind capacitor edges neatly, requiring reduced speed to complete winding, adversely affecting capacitor manufacturing processes. 4.7 Waste liquid pollution issue
During corona treatment, due to the high voltage applied between the two poles, electrons collide with and activate molecules in the air between the poles, producing substances such as ozone and nitric oxide. At certain temperatures, due to their hydrophilicity, they combine with water molecules in the air to produce waste liquid containing nitrite and nitrate components, which increases with air humidity. If the generated waste liquid is not cleaned promptly and accumulates in exhaust pipes or electrode housings, it may drip onto the roller surface and, with roller rotation, contaminate the film surface, causing local reduction in wetting tension, poor adhesion, and product quality issues.
2. Factors affecting corona treatment intensity of BOPP film
The corona treatment device consists of an electrode, a high-potential device, and a silicone rubber roller. When voltage passes through a 2.5mm air gap, continuous discharge occurs. Additionally, to exhaust the generated ozone and cool down, an exhaust fan removes air near the corona treatment device, and process water cooling dissipates heat inside the silicone rubber roller. The main factors affecting corona treatment effectiveness include the following: 2.1 Electrode type
The corona treatment effect is significantly related to electrode design. Using single or double electrodes on the equipment results in certain differences in treatment effect. Double electrodes have several advantages over single electrodes: 1. Higher treatment values with lower energy consumption; 2. Reduced surface tension decline during storage; 3. Reduced film heating during corona treatment; 4. Reduced surface-induced static electricity. 2.2 Film temperature
BOPP is a film produced by extruding a thick sheet, quenching, then stretching in both machine and transverse directions. After entering the pulling unit, it undergoes cooling, edge trimming, thickness measurement, preheating, and then corona treatment. At this point, the film temperature directly affects the corona treatment effect, and the film temperature is mainly controlled by the set temperature of the preheating roller,
As the film temperature increases, the surface treatment dyne value also increases. Adjusting the film's surface treatment dyne value by setting the preheating roller temperature is one of the effective methods commonly used in process control. 2.3 Production line speed
Production line speed is another important factor affecting corona treatment effectiveness. BOPP film passes through the high-voltage electrode gap in a very short time, improving the surface dyne value. The residence time in the high-voltage electrode gap affects the corona treatment effect.
It can be seen that the electrode voltage for corona treatment should be adjusted accordingly with changes in production line speed, increasing as the line speed increases. 2.4 Electrode exhaust volume
During corona treatment, as air ionizes, plasma is generated, containing electrons, oxygen ions, ozone, etc. Plasma penetrates the film, breaks other chemical bonds, excites free radicals, and reacts with oxygen ions to form oxidation polarization groups, which affect the film's surface wetting characteristics. On the other hand, the concentration of plasma on the film surface directly affects the corona treatment effect. Generally, the larger the opening of the electrode exhaust valve, the smaller the film's surface treatment dyne value; conversely, the smaller the opening, the larger the dyne value. 2.5 Surface material
BOPP production involves different materials and additive masterbatches. BOPP can be divided into heat-sealable and non-heat-sealable types, with the base materials in the surface layer being copolymers and homopolymers, respectively. Due to differences in the materials themselves, after the same corona treatment, there is a certain difference in surface tension. Generally, for copolymers, such as commonly used SOLVAY KS413, MONTEL PLZ679, BASEAL EP5C37, etc., the plasma penetration efficiency into the film surface is greater than for homopolymers, so heat-sealable films are more easily treated to higher intensities.
Additionally, in the formulation design of heat-sealable films, to meet packaging machine requirements, slip agents are often used to improve the film's friction properties. When selecting slip agents, silicone-based slip agents should be avoided as much as possible because silicone has a low surface tension, about 12 dynes at room temperature, significantly lower than PP's 31 dynes. Using silicone-based slip agents can greatly reduce the surface tension of BOPP.
Antistatic agents also have a certain impact on the corona treatment effect of BOPP film. Most antistatic agents are added to the core layer and have migratory properties. Antistatic agents that migrate to the surface can affect the film's surface corona treatment characteristics, with treatment intensity values decreasing to some extent. 2.6 Surface material
BOPP film also undergoes structural state changes after production. Within a few days, the polymer changes from amorphous to crystalline, affecting the corona treatment effect. Treatment intensity gradually decreases over time and eventually stabilizes. The reduction amplitude of corona treatment is related to storage temperature; the higher the temperature, the faster the reduction.
3. Effects of corona treatment on film physical properties
Corona treatment not only changes the film's surface dyne value but also affects other physical properties, mainly including the following: 3.1 Coefficient of friction
Since the principle of corona treatment is that the film passes through two high-voltage...
