How to Eliminate Static Electricity in Packaging Printing
It is well known that materials used in the packaging and printing industry, such as paper, plastic, ink, adhesives, and guide rollers (some of which are anodized to form alumina), are insulators. The processes of printing, laminating, rewinding, and slitting are high-speed operations that involve friction, contact, and separation—essentially "surface" engineering. The generation of static electricity in packaging and printing plants is mainly related to the chemical composition, molecular structure, mechanical properties, smoothness, and electrical properties of the insulating materials, as well as environmental factors like temperature, humidity, and external mechanical actions such as contact pressure and the speed of frictional separation. The type of film, contact method, contact time, contact area, and separation speed all influence the time of static discharge and the voltage produced.
1. The Hazards of Static Electricity
When paper becomes static-charged, it brings numerous challenges to printing. First, static causes sheets of paper to stick tightly together, making it difficult to neatly separate them. During the printing process, static attraction causes individual sheets to adhere together firmly, sometimes in twos, threes, or even stacks, making it hard for the paper feeder to pick up the sheets, which severely impacts printing efficiency. In color printing on plastic materials, static discharge can lead to issues such as jagged ink overflow around the edges of prints, increased misalignment of overprinting, and other printing defects. Static on ink can also result in light-screen or missing-print issues. Plastic films and inks attract dust, hairs, and other contaminants from the environment, leading to problems like blade streaks. During processes like rewinding, slitting, and bag-making, static discharge can damage equipment such as the optical correction system and electromagnetic control systems.
2. Methods for Eliminating Static Electricity
Different anti-static techniques should be applied based on production conditions, the packaging materials' intended use (what product is being packaged), and customer usage requirements. Common anti-static techniques include:
2.1 Physical Elimination Method
The physical method eliminates static without altering the material's properties by utilizing static's inherent characteristics. One such method is the "grounding" elimination method, which involves installing a static elimination brush. The brush body is placed at the roll or unrolling area of paper or plastic materials, and the grounding end of the static brush must be reliably grounded—not connected to equipment or rollers, which may not be properly grounded. Some rollers are anodized, producing alumina, which is non-conductive.
2.2 Chemical Elimination Method
Chemical static elimination, also known as antistatic agent treatment, involves adding or coating antistatic agents (surfactants) to modify the electrical properties of resins or substrates. This method is more thorough and effective but can change the chemical composition of materials. Therefore, this technique is unsuitable for treating paper but is commonly used to modify plastic resins.
Special care must be taken when using this technique for packaging food, pharmaceuticals, cosmetics, and chemical products, considering factors like safety, hygiene, and compatibility with the base resin. Packaging materials with antistatic properties not only prevent various quality issues caused by static but also improve packaging efficiency and ensure sealing strength, earning customer approval.
2.2.1 Additive Treatment Technique
This method, also known as the masterbatch technique, involves mixing antistatic agents at specific concentrations (ranging from a few percent to several tens of percent) with thermoplastic resins along with various additives. The mixture is then melted, compounded, and granulated to produce antistatic masterbatches. The selection of antistatic agents should consider their compatibility with the base resin. Poor compatibility results in poor performance of the antistatic particles, while excessive compatibility slows the migration of the antistatic agent to the surface, making it difficult to form an antistatic water film. Selecting a resin identical to the product's base resin is important during the melting, compounding, and granulation process. Care should be taken to maintain low processing temperatures to prevent decomposition or deterioration of the antistatic agent due to poor thermal stability. The production of antistatic plastic films often employs a three-layer (ABC) co-extrusion blown film process. The proportion of antistatic masterbatch should be adjusted according to the concentration of the effective ingredient and based on test results, ensuring that the surface resistivity (ps) is around 10^11 Ω. Excessive addition not only increases production costs but may negatively impact subsequent processing.
2.2.2 Coating Treatment Technique
The coating antistatic agent technique involves applying ionic surfactants as antistatic coatings on the surface of plastic films to prevent charge accumulation. The selection of coating antistatic agents depends on the work function of the substrate being coated. If the work function of the plastic material is large, it tends to carry a negative charge; if it is small, it tends to carry a positive charge. Among common plastics, the order of work function from large to small has been discussed earlier, with PP and PE tending to carry negative charges, making cationic surfactants more suitable for coating. PET and PA tend to carry positive charges, making anionic surfactants more appropriate. The plastic film surface should have a wetting tension greater than 38 dyn/cm. The antistatic coating should have good film-forming properties, be resistant to friction and chemical corrosion, and offer long-lasting effects.
3. Static Electricity Measurement and Testing
In packaging printing plants, static measurement mainly involves determining the voltage of the accumulated charge on packaging materials using an electrostatic voltmeter. It is recommended to select the maximum range first and then gradually reduce it. For surface resistivity testing of packaging materials, follow the GB/1410-89 standard "Test Methods for Volume and Surface Resistivity of Solid Insulating Materials." Testing should be conducted under specified environmental conditions of temperature and relative humidity. If the surface resistivity of packaging materials falls within the range of 10^9 Ω to 10^12 Ω, the material has antistatic properties.
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