When two solids with different physical states come into contact and rub against each other, their respective surfaces undergo charge redistribution. After re separation, each solid surface will carry an excess of positive (or negative) charge compared to before contact, which is called static electricity. To characterize the conductivity of a material, the concept of resistivity is used. Volume resistivity is the ratio of the potential gradient parallel to the current direction on the material surface to the current per unit width on the surface. Generally, polymers are high insulation materials with high electrical resistivity, so once they are charged, it is difficult to eliminate them. In daily life, when walking on a plastic floor, the friction between the sole of the shoe and the floor can make the human body charged. In severe cases, if the hand comes into contact with a door handle or object, it may also produce discharge, causing a tingling sensation; An electric shock accident occurs during medical surgery. In the electronic industry, human electrification can easily break through circuits, causing damage to integrated circuits; The textile industry causes fiber aggregation and so on.
There are two methods to eliminate static electricity in polymers
(1) Add conductive fillers, such as metal, carbon fiber, and carbon. This method requires a large amount of filler. The sudden decrease in conductivity of the filler must reach a certain percentage in order to achieve anti-static effect. The color and quality of products with a large amount of addition will be greatly limited. For example, the conductivity of carbon black filled PP only changes significantly when the percentage reaches 15%. At this time, the color of the material can no longer meet the requirements of multiple and beautiful materials. Metal fillers increase the quality of materials. Metal fiber fillers have low quality but are prone to breakage and oxidation during processing, making them more expensive.
(2) Adding anti-static agents to activate the surface and improve surface conductivity: 1. Surface coated anti-static agents have low durability and are easily lost due to friction and washing, providing only temporary or short-term anti-static effects. 2. Mixed anti-static agents have high durability, but require high requirements for anti-static agents.
Antistatic agent
The generation of static electricity on plastic surfaces can cause various problems, such as hindering production, sparks causing explosions, and damage to integrated circuits of electronic devices. The general method to remove static electricity is to use surfactants, such as antistatic agents, to reduce the surface resistance of polymers. Due to the hygroscopicity of such additives, they absorb moisture from the atmosphere on the surface of the polymer, forming a thin conductive film that quickly eliminates static electricity. Water plays an important role in this process. With the increase of atmospheric humidity, the surface conductivity of the polymer also improves, causing rapid loss of static charge and producing good anti-static performance.
According to different usage, there are two types of surface active antistatic agents, namely external and internal. External or local antistatic agents are applied to the surface of polymers through spraying, wiping, or impregnation. Although this external anti-static agent is suitable for various polymers, its effectiveness is only temporary, and it is easy to lose it after contact with solvents or friction with other substances. Internal anti-static agents are added during polymer processing. This type of surface active anti-static agent can supplement the anti-static function that has been eroded due to handling. The effect of this internal anti-static agent depends on frost spraying. The meaning of frost spraying here refers to the process in which the internal anti-static agent added to the resin partially migrates to the surface of the polymer. Therefore, internal anti-static agents have a long-term anti-static protection effect.
Surface active antistatic agents can be divided into cationic, anionic, and non-ionic types.
Cationic antistatic agents are usually long-chain alkyl quaternary ammonium, phosphorus, or lead salts, with chlorides as equilibrium ions. They work well in polar matrices such as rigid polyvinyl chloride and styrene based polymers, but have a negative impact on their thermal stability. This type of anti-static agent is usually not allowed to be used in items in contact with food; And the anti-static effect is only 1/5 to 1/10 of that of internal anti-static agents such as ethoxylated amines.
Anionic antistatic agents are usually alkali metal salts of alkyl sulfonic acid, phosphoric acid, or dithiocarbamate, and are mainly used in polyvinyl chloride and styrene resins; Their application effect in polyolefin resins is similar to that of cationic antistatic agents. Sodium alkylsulfonate has been widely used in styrene based resins, polyvinyl chloride, polyethylene terephthalate, and polycarbonate as an anionic antistatic agent.
Non ionic antistatic agents such as ethoxylated aliphatic alkylamines represent the largest class of antistatic agents. They are widely used in polyethylene, polypropylene, ABS, and other styrene based polymers. There are several ethoxylated alkylamines currently produced and sold, with the difference being the length of the alkyl chain and the degree of unsaturation. Ethoxyalkylamine is a highly effective antistatic agent, even in low relative humidity conditions, and is effective for a long time. This type of antistatic agent has been approved by the Federal Food and Drug Administration for use in items that come into indirect contact with food. Other commercially valuable non-ionic antistatic agents include ethoxylated alkylamine, such as ethoxylated lauroylamine, and glycerol monostearate (GMS). Ethoxylauryl amine is suitable for polyethylene and polypropylene used in low humidity environments, and requires quick and long-lasting anti-static functions. GMS anti-static agents are only considered for electrostatic protection during the processing. Although GMS migrates rapidly to the surface of polymers, it cannot exert long-lasting anti-static effects like ethoxylated alkylamine or ethoxylated alkylamine.
Up to 75% liquid or low melting point ethoxylated alkyl groups and polymers can be mixed to form concentrated masterbatch. These masterbatches are free flowing spherical products that are easy to transport, while they are easily dispersed during mixing. The advantages of ethoxylated alkylamine masterbatch can be summarized as follows:
(1) Good dispersibility, with the addition of pre dispersed active materials.
(2) A small ball shaped product with good transportability and free flow, easy to measure and mix.
(3) Good processing performance, with less screw slippage in the extruder.
The selection and dosage of anti-static agents depend on the properties of the polymer, processing methods, processing conditions, types and quantities of other additives, relative humidity, and the ultimate use of the polymer. The time required to obtain sufficient anti-static effect varies, and the rate and duration of anti-static protection can be increased by increasing the concentration of the anti-static agent. However, excessive use of anti-static agents may result in a slippery surface of the final product, which can damage the printing or bonding performance. Untreated inorganic fillers and pigments can adsorb anti-static agent molecules onto their surfaces, thereby reducing the effectiveness of anti-static agents. This phenomenon can be compensated for by increasing the amount of anti-static agent used. However, for products that come into contact with food, the amount of anti-static agents added must comply with the regulations of the Federal Food and Drug Administration (see Federal Regulations Code, 21 (21CFR)). (Codeof Federal Regulations, Title21 (21CFR)).
When using polyethylene, the choice of ethoxylated alkylamine antistatic agent should consider their physical form, such as paste, liquid, small particles, or solid. If ethoxylated tallow amine cannot be treated due to its paste like nature, liquid ethoxylated oleamine can be used. Under high-temperature processing conditions (above 180 ℃), ethoxylated stearphthalamine can be selected. If quick acting anti-static effect is required, ethoxylated lauryl amine can be selected. The issues to consider when using polypropylene are similar to those when using polyethylene. Regardless of the type of resin used, consideration must be given to the regulatory limits of the Federal Food and Drug Administration for various uses. When used for styrene based polymers, it is recommended to choose ethoxylated coconut amine or one of its appropriate masterbatches.
Mixing and processing
In general, anti-static agents are mixed with pigments and other additives in a mixer or extruder. Technically speaking, pure antistatic agents, such as ethoxylated alkylamine, have another advantage, which is that they can melt during liquid injection molding, thereby acting as dispersants for pigment masterbatch. Antistatic agent masterbatch can be directly added to the final processing equipment. The effect of internal anti-static agents is closely related to the production and processing conditions of the final product. For example, the anti-static performance of injection molded products depends on the temperature of the mold. Usually, when the temperature of the mold is low, the anti-static agent migrates quickly, thereby improving the anti-static performance.
There are two testing methods for evaluating the effectiveness of anti-static agents: surface resistance (rate) method and electrostatic decay method; Both methods are widely used.
According to the definition of ASTMD257-78, the surface resistivity of a material is the ratio of the potential gradient to the current passing through the unit width of the material surface, which is generally related to the geometric shape of the specimen. Place two electrodes on the same side of the plastic sample surface and apply direct current to the electrodes; Measure the current passing through the sample and calculate the resistance; Then represent the measurement results of surface resistivity in ohms.
According to the definition of Federal Test Method 4046, electrostatic decay refers to the discharge rate of induced charges. Place the sample (usually a thin plate or film) between two electrodes, with a distance of several millimeters between the electrodes and the surface of the sample. One electrode is connected to the power supply, and the other electrode is connected to the ammeter and recorder. The electric field change caused by the charge induced by one electrode on the sample surface is measured by the other electrode. Antistatic samples will exhibit decay of induced charges. The decay half-life (in seconds) is the time it takes for a charge to decay by half from its initial value.
Another widely used standard testing method in industry is the American standard, which is used for packaging electronic products. The choice of the appropriate method depends on the ultimate use of the plastic that needs to be tested.
The electrical resistivity of plastic itself is 1014 ohms. When adding anti-static agents according to the amount shown in Table A, the electrical resistivity may decrease to 1013 to 109 ohms. If we want to further reduce the resistivity, we can only rely on improving the conductivity, such as using conductive carbon black.
Anti static packaging technology is being developed to emphasize environmental concerns. The widely used ethoxylated alkylamine is now packaged in reusable bulk containers. Suppliers tend to produce anti-static agents with higher concentration, which can be diluted according to processing needs after being delivered to users. The purpose of doing so is to reduce the cost of solid waste treatment. By developing high concentration anti-static agents, manufacturers can ship more anti-static agents at once and reduce the number of packaging containers that need to be handled by users.
Technically speaking, a lot of research and development work still revolves around the packaging market of electronic products. Ethoxylated laurylamide, commonly seen as an amine free antistatic agent, is commonly used in this area. The amount of ethoxylated lauryl amine used in blow molding LDPE and LLDPE films is also increasing, as its anti-static effect is also better under low humidity conditions. The concentrate and masterbatch of this product can also be purchased. Ethoxylated stearophthalamine (containing fully saturated 18 carbon alkyl chains) has been applied in the production of biaxially oriented polypropylene films. In this production process, high processing temperatures require anti-static agents to have high thermal stability.