Curtain Coating Processing Technique

1 Principles and Features

High-frequency, high-voltage electricity is applied to the treatment device, causing corona discharge and producing fine, dense purple-blue sparks. After air is ionized, the resulting various plasmas, under the influence of a strong electric field, accelerate and impact the plastic materials within the treatment device. These plasma particles generally possess energies of several to several tens of electron volts, which are close to the energy levels of the chemical bonds in the plastic molecules. This enables the triggering of chemical bond断裂 on the plastic surface, leading to degradation and an increase in surface roughness. During corona discharge, a significant amount of ozone is also produced. Ozone is a powerful oxidizing agent that oxidizes the surface molecules of the plastic, resulting in the formation of carbonyl compounds, peroxides, and more. Additionally, corona treatment serves to remove oil stains, moisture, and dust. After undergoing the aforementioned physical and chemical modifications, the plastic surface's wettability and adhesion are significantly improved.

The corona treatment method offers the following advantages: 1) Wide range of materials processed, suitable for polyethylene, polypropylene, polyvinyl chloride, polyamide, polyester, polycarbonate, fluoro plastics, and various corresponding copolymers; 2) Short processing time and fast speed, capable of being processed on the production line; 3) Simple operation and easy control; 4) The corona treatment only involves a very shallow surface layer of the plastic, generally only in the nanometer range, which basically does not affect the mechanical properties of the product; 5) No effluent discharge, resulting in minimal environmental pollution.

Due to the aforementioned advantages of corona treatment, it is now widely used in surface treatment prior to film printing, coating, and lamination processes, as well as for materials with a thickness less than 0.6mm.

2 Process Route Format

There are generally three methods for corona treatment processes. The first method is carried out on the film production line, commonly referred to as heat treatment, which is a common form of processing. Since the treatment is done before the high polymer molecules have completed crystallization, it results in strong treatment effects, but it is generally only suitable for immediate post-processing applications (such as printing, coating, or lamination) for the film. The treatment equipment is located after the cooling and shaping section of the film and before the winding equipment.

The second form of processing is conducted on the reprocessing line for the film, commonly referred to as cold film treatment. The quality of this processing is also related to the film's history. If the film is stored for a long time after production, a large amount of additives precipitate and adhere to the film surface, resulting in reduced processing quality. When opting for this method, attention should be given to the possibility of electrical sparks igniting the solvent used. The processing unit is set up before the printing (or coating, lamination) unit.

The third form is a combination of the aforementioned two, involving initial processing on the film production line followed by a second processing on the film reprocessing line. It is suitable for films that have poor initial processing quality or have been stored for an extended period post-initial processing, resulting in a significant decline in the effectiveness of the treatment.

3. Key Points of Process Control

3.1 Degree of Processing

The degree of treatment for the film will directly affect the quality of subsequent processing and must be strictly controlled. If the degree of treatment is insufficient, the film's wettability will not be significantly improved, leading to poor adhesion of inks, weak adhesion of adhesive tapes, and low剥离 strength of composite films. Conversely, if the degree of treatment is excessive, the film's surface will age, adhesion will decrease, and luster will worsen; there will be excessive cross-linking of surface molecules, resulting in poor heat-sealability; the film is prone to sticking together (especially during hot summer days), making it difficult to process, such as cutting, and hard to peel off when in use. In summary, the principle of controlling the degree of treatment is to minimize it while meeting the requirements of subsequent processing, avoiding unnecessary over-treatment. The critical surface tension is commonly used to detect the degree of treatment.

Due to variations in film types, post-processing forms, and post-processing times, different degrees of treatment are required. Table 1 lists the critical surface tension values needed for various film printing and lamination processes. For immediate post-processing, a lower value is advisable; conversely, a higher value should be chosen.

3.2 Electrode Gap

It refers to the spatial distance between two electrodes, i.e., the distance from the discharge head to the edge of the ground electrode. It affects the treatment process in two aspects: on one hand, as the gap increases, the corona treatment range widens, and the relative residence time of the film in corona treatment lengthens, which is beneficial for improving the treatment effect; however, on the other hand, it disperses energy over a larger space, reducing the treatment intensity and worsening the treatment effect. The combined effect of both aspects is that the optimal electrode gap is typically maintained between 1mm to 2mm. Additionally, replacing a single-head electrode with a multi-head (such as 6-head) electrode can also enhance the treatment level under the same processing conditions. Before operation, the electrode gap must be carefully calibrated to ensure uniformity, otherwise, there may be uneven film critical surface tension due to varying treatment intensities.

The blade edge of the electrode should be straight and free of notches. Otherwise, the treatment intensity at the notch may be uneven, leading to longitudinal lines with low critical surface tension on the film. After some use, an oxide layer may form on the electrode blade, affecting the quality of the treatment. At this point, the layer can be removed with sandpaper.

3.3 Voltage Handling

Voltage applied to the processing device increases, enhancing the processing effect, but this enhancement is not proportional. Once the processing voltage reaches a certain level, it basically no longer changes. The processing voltage is mainly determined by the thickness of the product; the thicker the product, the higher the processing voltage. Typically, the processing voltage for thin films is controlled between 10,000 and 20,000 volts.

The alternating current (AC) power supply for the corona treatment machine must be stable; otherwise, it will cause fluctuations in the output high-frequency voltage, thereby affecting the film treatment quality. For manufacturers experiencing significant power fluctuations, it is recommended to use an AC voltage stabilizer to ensure voltage stability.

3.4 Processing Power (Current)

Processing power is the primary factor determining the degree of treatment. To achieve the critical surface tension value shown in Table 1, a certain level of processing power must be applied. Due to variations in processing speed, film type, experience, and width, the applied processing power differs. Table 2 indicates that when the processing speed is 25 m/min and the film width is 0.4 m, if the processing speed is doubled, the processing power should also be doubled; similarly, if the film width is doubled, the processing power should also be doubled; the power for cold film treatment should be at least twice that of hot film treatment. These considerations can also serve as a basis for selecting the size of a corona treatment machine.

The treatment device can be regarded as a capacitor with a lossy medium. As the input power frequency increases, the current and power also rise; conversely, they decrease. The corona treatment machine typically adjusts the power size by modifying the power frequency.

Temperature also affects the degree of treatment; as the temperature rises, the degree of treatment increases. In actual production, if the corona treatment machine has low power and insufficient treatment, a red radiation lamp (referring to cold film treatment) can be set before the treatment device to enhance the degree of treatment. Since corona discharge occurs in the air, the humidity of the air will also impact the degree of treatment. High air humidity (such as the plum rain season in the south) often results in less than ideal treatment effects.

4 Processed Timeliness Impact

The degree of processing for the film diminishes exponentially over time of storage, with the rate of diminishment being influenced by various factors such as storage environment temperature, raw material grade, and film thickness. Higher storage temperatures lead to faster and more thorough diminishment. For instance, under storage temperatures below 20°C, polyethylene film typically maintains a critical surface tension of 38mN/m to 42mN/m for about a month, or even half a year. However, if the storage temperature is raised above 36°C, regardless of the initial degree of film processing, it generally drops below 38mN/m after one month.

Films made from raw materials containing additives degrade faster than those without, due to the easy crystallization of these low molecular weight substances, which cover the surface of the film, reducing its adhesion.

Generally, thicker films dissipate more quickly than thinner ones, so it is advisable to print, coat, or laminate the film immediately after processing to achieve better wettability and adhesion. For films that cannot be processed immediately, consider the following three points: 1) Choose film grades with slower dissipation rates as the raw material, which requires accumulating experience over time; 2) Increase the processing power and intensity moderately; 3) Store the film at lower temperatures to slow down the dissipation rate as much as possible.