As we all know, materials used in the packaging and printing industry such as paper, plastics, inks, adhesives, and guide rollers (partially anodized to produce alumina) are insulators; printing, compounding, rewinding, slitting, and Other processes are at high speeds. , contact, separation of the "surface" project. However, in the process of rapid friction, contact, and separation of different types of insulating materials, a material that has a strong ability to adsorb electrons exhibits a negative charge due to electrons, and another material has a positive charge due to the loss of electrons.

The generation of static electricity is mainly related to the chemical composition of insulating materials, molecular structure, mechanical properties, degree of smoothness, electrical properties and environmental temperature and humidity, external mechanical effects such as contact pressure, the speed of friction separation and other factors. The type of the film, the contact type, the contact time, the contact area, and the separation speed are different, and the generated electrostatic discharge time and voltage are different. For example, the better the electrical properties, the greater the static strength of the material; and the same material is also prone to static electricity. For example, when two closely contacting plastic materials such as polyethylene film are quickly separated, the static electricity generated can be more than 10,000 volts instantaneously. In addition, moisture-free liquids and gases are also prone to static electricity.

Papers, plastics, inks, and printing plates (such as flexo plates) in printed materials have different affinity properties for electrons, and the pressure during the printing process makes them in close contact, and the high speed makes them quickly. Separation, fully equipped with static electricity conditions. Therefore, thousands of volts of static electricity are easily accumulated and discharged during the printing process. In particular, during the manufacturing process of paper and plastic materials, static electricity builds up due to friction between pellets, the melt and the screw, the cartridge, the die (lips), the winding, and the friction of the guide rollers. The fugitives are sold to packaging and printing plants and are electrostatically charged before printing, compounding, etc., providing conditions for the continued accumulation and discharge of static electricity. The law of charging plastic materials is: plastic's work function is large, it is easy to negative charge; its work function is small, then it is positively charged. In common plastic materials, the order of work function from big to small is: PP, PE, PS, PVDC, PET, paper, AL (aluminum foil), PA. That is, PP, PE can easily be negatively charged. PET, paper, AL (aluminum foil), and PA are easily positively charged.

The harm of static electricity

Once charged with static electricity, it will be brought to the printing under the electrostatic effect, and the paper and the paper will be sucked firmly and cannot be neat. In the printing process, due to electrostatic attraction, the sheets are firmly stuck together, sometimes two, sometimes several, sometimes a sheet of paper is difficult to separate, resulting in the paper suction nozzle can not absorb the paper, seriously affecting the printing efficiency . During the color printing of plastic materials, due to the electrostatic discharge, burr-like ink overflows and overprint deviations often appear on the edges of the printed crystals; static electricity generated by the ink may cause shallow webs or missed printing; the plastic film and ink absorb dust in the environment. , hair and other foreign objects prone to knife and other quality problems. When rewinding, slitting, bag making and other processes, electrostatic discharge will destroy the device's photoelectric correction system, electromagnetic control system. Up to tens of thousands of volts of electrostatic discharge electrodes can easily ignite organic solvents and cause fire accidents. In particular, the packaging material will cause contamination of the packaged product due to the absorption of foreign substances, and reduce the automatic filling efficiency when the customer uses it. When the powder products such as washing powder, medicament, and milk powder are packaged, the strength of the sealing cannot be achieved because the bag mouth absorbs the powder. Guarantee.

Antistatic technology

It is necessary to adopt different anti-static technologies in combination with actual production, the use of packaging materials (what products are packaged), and the conditions of use of customers. Common antistatic techniques are as follows:

1. Physical elimination

Physical elimination is a method that eliminates the inherent characteristics of static electricity without changing the properties of the material. Such as "grounding" elimination method is to install anti-static brush on the process. Put the brush body in the winding or unwinding position of the paper or plastic coil, and make the grounding end of the antistatic brush reliably grounded, and it cannot be connected to the equipment or the guide roller. Because the device may be poorly grounded; some of the rollers are anodized to produce aluminum oxide on the surface, while alumina is non-conductive. For example, a high-voltage discharge type static elimination method uses a high-voltage discharge type static eliminator to eliminate static electricity. The static eliminator is divided into unipolar and bipolar according to the polarity of the discharge. The unipolar static eliminator neutralizes only one type of charge (positive or negative). A bipolar static eliminator eliminates both positive and negative charges. Environmental humidity control method is also a common one, increasing the relative humidity of the workshop environment (suitable in the range of 60% to 70%), can increase the moisture on the surface of plastics, paper, and form a thin, conductive water film, thereby accelerating the static electricity leakage. The above three combined static elimination methods are common when printing, compounding, slitting, and bag making processes.

2. Chemical elimination

The chemical antistatic method, ie, the antistatic agent treatment technology, is mainly a method for modifying the electrical properties of the resin or the substrate by adding (filling) technology or coating technology to the antistatic agent (surfactant), and is more thorough. Perfect antistatic technology. However, since the addition or coating of an antistatic agent causes a change in the chemical composition of the material, this technique is not suitable for the treatment of paper and is only applicable to the modification of plastic resins.

Especially in packaged foods, medicines, cosmetics, chemical products, etc., attention should be paid to safety, sanitation, and compatibility with matrix resins, etc. Therefore, the technical content is high. The packaging material with antistatic properties not only eliminates all kinds of quality accidents caused by static electricity, but also improves the packaging efficiency for the customer and ensures the strength of the seal, which is recognized by the customer. It is gratifying to note that the plastics industry has strengthened the study of antistatic agents as early as the early 1990s, the performance of antistatic agents has matured, and antistatic agent treatment technology has also been widely promoted and used.

2.1 Additive processing technology

This technology (ie masterbatch technology) is to add additive antistatic agent at a certain concentration (a few percent to a few tens percent) with a thermoplastic resin, and add a variety of additives, through melting, mixing, granulation The antistatic masterbatch was prepared. The selection of antistatic agents should pay attention to the compatibility with the matrix resin. If the compatibility is too poor, the prepared antistatic particles have poor performance; however, if the compatibility is too good, the migration speed of the antistatic agent to the surface is too slow to form an antistatic water film. The same resin as the resin of the product is selected as the matrix resin. In the process of melting, mixing 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 be appropriately adjusted according to the test results, so that the surface resistivity ps can be about 1011n; the increase in the amount of addition will not only increase the product cost, but also the later period. The manufacturing process has an adverse effect. To prepare antistatic composite packaging materials, add the antistatic particles to the functional surface (the hot cover) without adding the composite surface (the corona treatment surface) so as to avoid migration of the antistatic agent and the water film to form a barrier layer, affecting the composite Post-material peel strength. It is also possible to add antistatic masterbatches to coated grade resins such as PE and PP in a certain proportion.

A composite (casting) process is used to avoid the quality problems caused by the adhesion and foreign matter adsorption of the film.

2.2 coating type processing technology

The coating type antistatic agent treatment technology is to make an ionic surfactant into an antistatic coating and apply it on the surface of a plastic film to prevent charge accumulation. The choice of coating type antistatic agent should be determined according to the work function of the coated substrate. If the work function of a plastic material is large, it is easily negatively charged; if its work function is small, it is positively charged. The order of the work function of the common plastic materials from large to small has been previously discussed, PP, PE is extremely easy to negatively charged, it should be coated with cationic surfactant; PET, PA is easy to positively charged, should use anionic surface Active agent coating. The surface tension of the plastic film is required to be greater than 38 dyn/cm; the antistatic coating has good film-forming properties, abrasion resistance, chemical resistance, and durability. The method for preparing the antistatic coating is: dissolving the antistatic agent in an alcohol-soluble or ester-soluble resin or emulsion with good film-forming properties and adhering to the plastic film; the polyurethane adhesive can also be used as a carrier to dissolve with the ester; Can be dissolved in solvent-based plastic coatings and made antistatic coatings. Pay attention to choose the appropriate coating amount when coating, set the drying oven step temperature, control the winding speed and the relative humidity of the environment, to prevent the coating from producing haze, rainbow and so on.

Static electricity measurement and testing

The electrostatic measurement of the packaging material is mainly to measure the voltage of the accumulated charge. The static voltmeter can be used to measure, and the maximum range must be selected first, and then gradually reduce the range. The test of the surface resistivity of packaging materials shall be conducted in accordance with GB/1410-89 "Test Methods for Volume Intrinsic Resistivity and Surface Resistivity of Solid Insulating Materials" under specified conditions of ambient temperature and relative humidity. The measured surface resistivity is in the range of 109n to 1012~, which means that it has antistatic properties.

Static electricity is a common physical phenomenon. Static electricity is easily generated when objects are moved. In addition, almost all manufacturing processes in the packaging and printing industry are performed on the surface of insulating materials, and it is not surprising that static electricity is generated. As long as we deepen our understanding of the mechanism and hazards of static electricity generation and apply antistatic technology scientifically and economically, we can maintain the rapid development of flexible plastic packaging materials and apply them to various fields.


Source: Chutian Graphic Art Network

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