Application of antistatic technology in packaging and printing industry
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Electrostatic generation mechanism
As we all know, the materials used in the packaging and printing industry, such as paper, plastics, inks, adhesives, and guide rolls (some of which are anodized to form alumina) are all insulators; printing, compounding, rewinding, slitting, etc. are high-speed friction. "surface" engineering of contact, separation. In the process of rapid friction, contact and separation of different kinds of insulating materials, materials with strong electron-withdrawing ability have a negative charge due to the acquisition of electrons, while another material has a positive charge due to the loss of electrons. The generation of static electricity is mainly related to the chemical composition, molecular structure, mechanical properties, smoothness, electrical properties, ambient temperature and humidity, external mechanical action such as contact pressure and speed of friction separation. The type of the film, the type of contact, the contact time, the contact area, and the separation speed are different, and the generated electrostatic discharge time and voltage are different. The better the electrical properties, the higher the electrostatic strength of the material; the static electricity is also easily generated between the same materials. For example, when two closely contacting plastic materials such as polyethylene film are quickly separated, the static electricity generated in an instant can reach more than 10,000 volts. In addition, liquids and gases that do not contain moisture are also susceptible to static electricity. Paper, plastic, ink, printing plates (such as flexo plates) in printed materials have different characteristics of electrons, and the pressure during printing makes them in close contact, and they have a high speed to make them fast. Separated, fully equipped with conditions for static electricity generation. Therefore, thousands of volts of static electricity are easily accumulated and discharged during the printing process. Especially in the manufacturing process of paper and plastic materials, static electricity is accumulated due to the friction between the pellets, the melt and the screw, the barrel, the die (the lip), the winding and the friction of the guide roller, and the static electricity is too late. The escape is sold to the packaging and printing factory, and it is charged with static electricity before printing, compounding, etc., which provides conditions for the static electricity to accumulate and discharge. The law of charging plastic materials is: plastic has a large work function, it is easy to carry negative electricity; its work function is small, it is positively charged. In common plastic materials, the order of work function from large to small is: PP, PE, PS, PVDC, PET, paper, AL (aluminum foil), PA. That is, PP and PE are easily negatively charged, and PET, paper, AL (aluminum foil), and PA are easily positively charged.
Electrostatic hazard
Once it is electrostatically charged, it will bring the printing first, under the action of static electricity, and the paper and the paper will be firmly sucked and cannot be neat. During the printing process, due to electrostatic attraction, the sheets are firmly affixed together, sometimes two, sometimes several, sometimes a piece of paper is difficult to separate, resulting in the paper nozzle not absorbing paper, seriously affecting printing efficiency . When plastic materials are used for color printing, due to electrostatic discharge, it is often found that burr-like ink overflows and the offset deviation increases at the edge of the printed crystal; the static electricity of the ink may cause shallow mesh or missing printing; the plastic film and the ink absorb dust in the environment. Foreign matter such as hair is prone to quality problems such as knife hair. In the process of rewinding, slitting, bag making, etc., electrostatic discharge will destroy the photoelectric correction system and electromagnetic control system of the equipment. Electrostatic discharge electrodes of up to several tens of volts are easy to ignite organic solvents and cause fire accidents. In particular, the packaging materials will cause pollution to the packaged products due to the adsorption of foreign matter; the automatic filling efficiency is reduced when the customer uses it; when the powder products such as washing powder, medicaments, and milk powder are packaged, the sealing strength cannot be obtained because the powder is adsorbed at the mouth of the bag. Guarantee.
Antistatic technology
Different antistatic technologies should be adopted in combination with the 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
The physical elimination method is a method of eliminating the inherent characteristics of static electricity without changing the material properties. For example, the "grounding" elimination method is to install an antistatic brush on the process. Place the brush body on the winding or unwinding of the paper or plastic roll, and make the grounding end of the static eliminating brush reliably grounded, but not on the equipment or the guide roller. Because the equipment may be poorly grounded; some of the guide rolls are anodized to form alumina on the surface, while alumina is not conductive. For example, a high-voltage discharge type static elimination method uses a high-voltage discharge type static eliminator to eliminate static electricity. The 泫 static eliminator is divided into unipolar and bipolar according to the discharge polarity. The unipolar static eliminator neutralizes only one charge (positive or negative), and the bipolar static eliminator eliminates both positive and negative charges. The 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 plastic and paper, forming a thin, electrically conductive water film, thereby accelerating static electricity. leakage. The above three combined static elimination methods are more common in printing, compounding, slitting, and bag making processes.
2. Chemical elimination
The chemical elimination antistatic method, that is, the antistatic agent treatment technology, mainly uses an antistatic agent (surfactant) to modify the electrical properties of the resin or the substrate by adding (filling) technology or coating technology, which is more thorough and Complete antistatic technology. However, since the addition or coating of the antistatic agent causes a change in the chemical composition of the material, the technique is not suitable for the treatment of paper, and is only suitable for the modification of the plastic resin.
Especially in packaged foods, medicines, cosmetics, chemical products, etc., attention should be paid to safety, hygiene and compatibility with matrix resins, etc., so the technical content is high. The packaging material with antistatic properties not only eliminates various quality accidents caused by static electricity, but also improves the packaging efficiency for the customer and ensures the sealing strength, thus obtaining the customer's approval. Fortunately, the plastics industry has strengthened the research on antistatic agents as early as the early 1990s. The performance of antistatic agents has become increasingly mature, and antistatic agent treatment technology has been widely promoted and used.
2.1 Additive processing technology
The technology (ie, masterbatch technology) is to mix the additive antistatic agent with a certain concentration (a few to several tens of percent) with a thermoplastic resin, and add a variety of additives, after melting, mixing, granulation , made of antistatic masterbatch. The choice of antistatic agent should pay attention to the compatibility with the matrix resin. If the compatibility is too poor, the obtained antistatic particles have poor performance; but if the compatibility is too good, the rate of migration of the antistatic agent to the surface is too slow, and it is difficult to form an antistatic water film. The same resin as the resin of the product is selected as the matrix resin, and in the process of melting, kneading, and granulating, the processing temperature is kept as low as possible, and the antistatic agent is prevented from decomposing or deteriorating due to poor thermal stability. The antistatic plastic film is prepared by using antistatic particles, and a three-layer (ABC) co-extrusion blow molding process is often used. Note that the proportion of antistatic masterbatch to be added should be determined according to the concentration of the active substance, and adjusted according to the test results, so that the surface resistivity ps is about 1011n; the increase of the addition amount not only increases the product cost, but also the later stage. The processing steps have an adverse effect. To prepare antistatic composite packaging materials, add antistatic particles to the functional surface (hot cover) without adding a composite surface (corona treatment surface) to prevent the migration of antistatic agent and water film to form a barrier layer, affecting the composite The peel strength of the post material. The antistatic masterbatch can also be added to a coating grade resin such as PE and PP by a certain ratio of an extrusion compounding (casting) process, thereby avoiding quality problems caused by adhesion of the film and adsorption of foreign matter.
2.2 Coating treatment technology
The coating type antistatic agent treatment technology is to form an ionic surfactant into an antistatic coating and apply it on the surface of the plastic film to prevent charge accumulation. The selection of the coating type antistatic agent is determined according to the work function of the coated substrate. Plastic materials have a large work function, which is easy to carry negative electricity; when the work function is small, it is positively charged. The order of work function of common plastic materials is as large as possible. PP and PE are easily negatively charged and should be coated with cationic surfactant. PET and PA are easily positively charged. Anionic surface should be used. The active agent is coated. The surface tension of the plastic film is required to be greater than 38 dyn/cm; the antistatic coating has good film forming property, friction resistance, chemical resistance and long-lasting effect. The antistatic coating is prepared by dissolving the antistatic agent in an alcohol-soluble or ester-soluble resin or emulsion having good film-forming property and adhering to a plastic film; or using a polyurethane binder as a carrier to dissolve with an ester; It can be dissolved in solvent-based plastic coatings to make antistatic coatings. When coating, pay attention to selecting the appropriate coating dry amount, set the drying chamber step temperature, control the coiling speed and the relative humidity of the environment to prevent haze and rainbow after coating.
Static measurement and testing
The electrostatic measurement of the packaging material is mainly to measure the voltage of the accumulated charge. The electrostatic voltmeter can be used for measurement, and the maximum range is selected first, and then the range is gradually reduced. The test of the surface resistivity of the packaging material shall be carried out in accordance with GB/1410-89 "Test method for volume resistivity and surface resistivity of solid insulating materials" under the specified ambient temperature and relative humidity conditions. It is measured that its surface resistivity is in the range of 109n-1012, which has antistatic properties.
Static electricity is a common physical phenomenon, and it is easy to generate static electricity when moving with objects. In addition, almost all processing techniques in the packaging and printing industry are carried out on the surface of insulating materials, and it is not surprising that static electricity is produced. As long as we deepen our understanding of the mechanism and hazards of static electricity, and scientifically and economically use antistatic technology, we can maintain the rapid development of soft plastic packaging materials and apply them to various fields.