How to eliminate static electricity in packaging and printing. It is well known that materials used in the packaging and printing industry, such as paper, plastics, inks, adhesives, and guide rollers (some of which are anodized to produce alumina) are insulators; printing, compounding, rewinding, and slitting The other processes are high-speed "surface" engineering of friction, contact and separation. The generation of static electricity in packaging and printing factories is mainly related to the chemical composition, molecular structure, mechanical properties, smoothness, electrical properties, environmental temperature and humidity, external mechanical effects such as contact pressure and friction separation speed of insulating materials. The type, contact type, contact time, contact area, and separation speed of the film are different, and the generated electrostatic discharge time and voltage are different.
1. The harm of static electricity
Once the paper is charged with static electricity, it will bring a lot of trouble to printing. The first is that under the effect of static electricity, the paper is firmly held between the paper and cannot be neatly arranged. In the printing process, due to the electrostatic attraction, the single sheets of paper are firmly pasted together, sometimes two, sometimes several, sometimes a stack of paper is difficult to separate, resulting in the separation nozzle can not absorb the paper, which seriously affects the printing efficiency . When plastic materials are used for color printing, due to electrostatic discharge, burr-like ink overflows and increased overprinting deviations are often found on the edges of the printed products; the static electricity generated by the ink will produce shallow screens or leakage; the plastic film and ink absorb dust in the environment , Hair and other foreign objects are prone to quality problems such as knife wire. In the process of rewinding, slitting, bag making, etc., electrostatic discharge will destroy the equipment's photoelectric correction system, electromagnetic control system, etc.
Second, the method of eliminating static electricity
It is necessary to adopt different antistatic technologies to eliminate static electricity according to the actual production, the use of packaging materials (what products are packaged), and the conditions of use of customers. Common antistatic technologies are as follows:
1. Physical elimination method
The physical elimination method is a method to eliminate the inherent characteristics of static electricity without changing the properties of the material. For example, the "grounding" elimination method is to install a static elimination brush on the process. Place the brush body at the rewinding or unwinding position of paper or plastic coiled material, and make the grounding end of the static elimination brush reliable grounding, and can not be connected to the equipment or guide roller. Because the equipment may be poorly grounded; some guide rollers are anodized to produce alumina on the surface, which is non-conductive.
2. Chemical elimination method
The chemical static elimination method, that is, the antistatic agent treatment technology, is mainly to modify the electrical properties of the resin or substrate by adding (filling) technology or coating technology to the antistatic agent (surfactant), which is more thorough and Perfect antistatic technology. However, due to the addition or coating of antistatic agents, the chemical composition of the material is changed, so this technology is not suitable for the treatment of paper, only for the modification of plastic resins.
Especially in packaging food, medicine, cosmetics, chemical products, etc., we must pay attention to safety, hygiene and compatibility with the matrix resin, etc., so the technical content is higher. Packaging materials with antistatic properties not only prevent various quality accidents caused by static electricity, but also improve the packaging efficiency for customers and ensure the sealing strength, so they are recognized by customers.
2.1 Additive processing technology
This technology (that is, masterbatch technology) is to mix the added antistatic agent with the thermoplastic resin at a certain concentration (several percent to several tens of percent), and add a variety of additives, after melting, mixing, granulation , Made antistatic masterbatch. The selection of antistatic agent should pay attention to the compatibility with the matrix resin. If the compatibility is too poor, the performance of the produced antistatic particles is poor; but if the compatibility is too good, the rate of migration of the antistatic agent to the surface is too slow to form an antistatic water film. Choose the same resin as the product resin as the matrix resin. During the process of melting, kneading and granulating, the processing temperature should be kept as low as possible to prevent the antistatic agent from decomposing or even deteriorating due to poor thermal stability. The use of antistatic particles to prepare antistatic plastic films often uses a three-layer (ABC) co-extrusion blow molding process. Note that the proportion of antistatic masterbatch added should be determined according to the concentration of its effective substance, and adjusted appropriately according to the test results, so that the surface resistivity ps can be about 1011Ω; the increase in the amount of addition not only increases the product cost, but also affects the later stage. Adversely affects the manufacturing process.
2.2 Coating type processing technology
The coating type antistatic agent treatment technology is to make an ionic surfactant into an antistatic coating, which is applied to the surface of a plastic film to prevent the accumulation of electric charge. The selection of coated antistatic agent should be determined according to the work function of the coated substrate. A plastic material with a large work function tends to be negatively charged; a small work function with a positive charge. The order of work functions in common plastic materials from large to small has been mentioned above. PP and PE are easily charged negatively and should be coated with cationic surfactants; PET and PA are easily charged positively and anionic surfaces should be used Active agent coating. The wetting tension of the surface of the plastic film is required to be greater than 38dyn / cm; the antistatic coating has good film formation, friction resistance, chemical corrosion resistance and long-lasting effect.
3. Measurement and test of static electricity
The static electricity measurement of packaging materials by packaging and printing factories is mainly to measure the voltage of the accumulated charge. An electrostatic voltmeter can be used for measurement, and the maximum range must be selected first, and then the range is gradually reduced. The test of the surface resistivity of the packaging material shall be carried out under the specified ambient temperature and relative humidity according to GB / 1410-89 "Test Methods for Volume Resistivity and Surface Resistivity of Solid Insulating Materials". The surface resistivity measured by the packaging and printing factory is in the range of 109Ω-1012Ω, that is, it has antistatic properties.
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