Section 6 Printing of Plastic Products - -2

(d) The electrostatic charge on the surface of the antistatic treatment plastic can cause dust dirt to be absorbed on the surface, resulting in defects in the coating and decoration applied to the plastic surface. The ink covering the dust in the screen printing pattern of the plastic product dries off after drying and leaves the needle eye. In addition, the static charge on the plastic will cause certain patterns to appear after the ink film has dried. This is because the charge on the surface will polarize the solids in the lacquer or the ink, causing the solids to swirl or light in appearance depending on the texture pattern of the surface charge. Related to this other problem is that many filaments are drawn from the coated area along the edge of the surface of the decor and the whisker-like or fluffy edges are attached to the image of the decor. This is called the whisker effect. Of course, a solution to this problem is to remove the static charge from the plastic surface cloth before printing the ink and painting, by treating the surface with ionized air from a blower, air gun or static eliminator.
The cause of static electricity is frictional electrification or contact electrification. When two materials are contacted and separated, one material produces a greater electrophilicity, and electrons are transferred to the surface of this material. The surface of the obtained electron can be considered as It is negatively charged, and the surface of another material is considered to be positively charged due to the loss of electrons.
This phenomenon occurs in all kinds of materials, and its degree is related to various factors such as the degree of contact, the speed of separation, the size of friction, and the conductivity of the material. If two smooth surfaces are in contact, the total number of contact points is greater than the total number of points contacted by the two rough surfaces, which means that there are more charges due to the closer contact. The faster the separation, the more charge it produces, because the electrons have the opportunity to leak back to the original surface when the separation speed is slow.
Conductivity has two effects on the generation of static electricity, since the charge can move freely on a conductive surface and is easily distributed over the entire surface rather than concentrated at the point where the charge is generated. The distribution of the charge on the entire surface is low due to the low charge density. The second effect of conductivity is that once any part of the material is in contact with the ground, all the charge on the material is immediately introduced into the earth.
Whether a material accumulates and retains charge depends on the conductivity of the material. The conductive material quickly loses its charge when it comes in contact with the ground. Even if the non-conductive material is in contact with the ground, the charge cannot move the contact point through the surface and thus can retain the charge.
Causes of non-conductive material dust collection: For example, plastics and molded parts are ejected from the mold and fall into the container made of cardboard. When the plastic surface and the mold surface are separated, a large amount of charge can be generated on the plastic surface. From the air Dust and dirt in and around the environment.
The method of eliminating static electricity is as follows:
1. Ionized air The so-called ionized air is the cracking of air in a typical room into positive and negative ions. Since positive and negative charges attract each other, these positive and negative ions can be restructured. Therefore, the life of ionized air is very short, only a few seconds. Therefore, ionized air must be continuously generated and supplied. Certain ions are attracted by the surface charge and will attract its charge to neutralize. The opposite-phase charge attracts, so positive ions neutralize negative charges and negative ions neutralize positive charges. In order to generate ionized air, it is necessary to consume energy. For example, ionized air can be obtained by supplying enough energy to the space in the form of flame, high voltage, and radiation.
(1) Ionization Air Generators Ionized air nozzles and ionized air blowers are a good way to remove dust particles from plastic surfaces. Handheld compressed air guns with ionization elements are used to blow off 25nm or larger particles. This type of device uses nuclear bombardment with high-voltage electricity or alpha particles to ionize the compressed air.
(2) Energy ion generator The electric energy air ionization device consists of two basic parts, namely the ion generation part and the high voltage supply part. The ionizer consists of one or a row of sharp metal pins. The ionizer is: a sharp metal needle, ie an air ionization gun, with a metal needle mounted on a grounded metal cover at the outlet of the air gun. The device is powered by high voltages of 5000-8000V. The ions in the air ionization gun are created in the space around the highly charged tip. Compressed air from the air gun nozzle passes through the ionized air zone and transmits the ions to the charged surface. The ionizer of the static eliminator on the printing press is composed of a row of sharp metal needles, and the mechanism of high-voltage power supply and its elimination of static electricity is the same as that of the air-electricity anti-gun.
(3) Nuclear Energy Ion Generator There are two types of nuclear ion generators. One is to generate ions by irradiating air molecules in air nozzles. The ion generating element is a nuclear source fixed in the metal foil, and the metal tube is installed at the outlet of the air gun. The air is ionized by bombardment of the compressed air through the metal tube by alpha rays. There is also a nuclear ion generator called a printed static eliminator. The ion generating element is a nuclear source enclosed in a rectangular metal mesh. The air on the metal mesh shell is bombarded by the rays to ionize the air. The ionized ions can be ionized. Neutralizes the charge on the paper and film that passes through the Internet, eliminating static electricity in the print.
The α-ray emitting isotope is usually used as a nuclear source. The high-speed α-particles interact with the compressed air in the air barrel and have enough energy to strip away an outer layer of air to form a positive ion. The free electrons are adsorbed onto another molecule, producing a corresponding negative ion.
There are three isotopes that have been used in industrial static removal equipment, Strontium-241, Radium-226, and Epi-210. The half-life of ytterbium-241 is quite long, 458 years. When it emits alpha-rays, it also emits a considerable amount of gamma-rays that are harmful to human tissue. Radium-226 has a very long half-life of almost 1700 years. Like 镅-241, it emits α-rays and emits gamma-rays that are harmful to the human body. The outer-210 is actually 100% emitting alpha-rays, completely harmless to the human body, its radioactive half-life of 138 days, and directly change to non-radioactive stable elemental lead. The static electricity-removing equipment containing Y-210 can be considered as having a service life of one year. At that time, the intensity of the isotope decays to 16% of the original strength. Such equipment can be updated once a year in accordance with the rental agreement with the manufacturer.
At present, there are two kinds of nuclear source processing methods for electrostatic neutralizer on the market: roll foil type and microsphere type. In foil wound nuclear sources, isotopes are enclosed between a thin layer of gold foil and a thicker layer of silver foil. The microsphere-type nuclear source isotopes are chemically bonded and then physically enclosed in a ceramic ball, and then plated with nickel, and the prepared microspheres are firmly bonded to the aluminum substrate.
2. Antistatic Agents There are two types of general antistatic agents used in plastics. The most commonly used are surfactants that are sprayed or coated on the surface. The coating tends to absorb moisture from the air, which increases the conductivity of the plastic surface and causes static charge to diffuse. The second type is the incorporation of antistatic agents into plastic resins prior to molding. Such antistatic agents can migrate to the surface to increase the electrical conductivity of the surface. If these two types of chemicals appear on the surface of plastic products, the adhesion of paint and ink can be reduced. Therefore, the surface to be decorated should be avoided. Commonly used antistatic types are: a. general surfactants; b. functional group polymers; c. organic or inorganic salt compounds. This kind of material has a weak ionization, can give the plastic film antistatic effect, because it does not adsorb powder, prevent pollution, and can improve the welding ability.
(1) Selection of antistatic agents Antistatic agents are mainly surfactants. Their molecules contain polar hydrophilic groups and non-polar lipophilic groups. The lipophilic groups have certain compatibility with plastics, and hydrophilic groups. It can ionize or absorb water in the air, forming a thin layer of conductive layer that can leak charge, thus playing an antistatic role. There are many types of antistatic agents, including liquid and white powders. Different types of plastics should be selected corresponding to the antistatic agent (see Table 3-32). Powdery amine glycidyl ether adducts and higher aliphatic amine ethylene oxide adducts are generally used. Mix with resin during use.
(2) Examples of Antistatic Agents for Polyethylene Films Currently used antistatic agents are water and powder. Since the liquid agent needs to be dried after being stopped, it is generally not used. There are many types of powders, such as the "antistatic agent HZ-1 (complex)" produced in Hangzhou, which is a white powder. The main technical performances are: hydroxyl value 220-240mgKOH/g, amine value 65-75mgKOH/g, melting point range 42-44°C, ash content 8.0%-10%. Mix the powder directly into the resin, mix it by high-speed mixer or evenly mix it with other effective methods and resin (you can use the waste vertical small drilling machine to insert the drill rod into the large iron drum for use), mix it and then directly pour it Into the extruder feed hopper extrusion. The film product is generally used in an amount of 0.24%-0.5% (per hundred parts of resin), depending on the degree of static electricity of the resin, and does not affect the transparency of the film after molding. The trial results showed that the surface static electricity of the film can be reduced from the original 1016 ohms to 109-10 ohms. After the printing, the film no longer has adhesion due to static electricity, which facilitates slitting and finishing. The surface of the finished product is less likely to absorb dust, the internal powder is clearly structured, and it is easy to heat-seal, achieving the purpose of improving product quality, improving work efficiency, and reducing costs. One year later, the sample test still plays an antistatic role. HZ-1 antistatic agent, identified by the health department, acute oral LD5000mL/kg or more. Is a non-toxic product, so the treated plastic film can be used for food packaging. Use range: LDPE, HDPE, LLDPE, PP and other plastics.
3. Elimination of static electricity on the plastic sheet The special equipment allows the plastic sheet to pass through two specially placed rubber rollers, eliminating static electricity on the plastic sheet. The reason is that the rubber of the special rubber roller contains enough antistatic agent.
4. Conductive fiber or conductive rubber static eliminator Static electricity is often generated during the printing of paper or film when the climate is dry. By installing a conductive fiber or conductive rubber static eliminator at an appropriate location and grounding it with a wire, it can also eliminate static electricity in printing.
Table 3-32 Suitable antistatic agents for various plastics
Anionic (phosphates) 18-carbon alkyl trimethyl ammonium chloride
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