Original release | MET Polymer Zhong Ming1. What is static electricity?
When two solids with different physical states come into contact and rub against each other, their respective surfaces will redistribute charges. After re-separation, each solid surface will have an excess of positive (negative) charges than before contact. This phenomenon is called static electricity. Electrostatic phenomena are very common in production, processing and use of polymers. Due to high resistivity of conventional polymers, their volume resistivity is in range of 1010-1020 Ohm·cm. Once static electricity is generated, it is difficult to eliminate it. The accumulation of this charge can lead to serious hazards such as formation of dust and bacteria in medicine, even serious consequences such as an explosion after sparks are generated. As shown in fig. 1-1.
Figure 1-1 Electrostatic physical map of a polymer
Currently, most common and effective method is use of antistatic agents to reduce specific surface resistance of polymers, so development and application of antistatic agents is of great importance. An antistatic agent is a kind of chemical additive added to a resin or applied to surface of a polymer to prevent or eliminate static charge. The antistatic agent itself has no free electrons and belongs to category of surfactants. It forms a charge leakage channel by ionization or transfer of polar group ions or absorption of moisture, and achieves purpose of antistatic agent.2. Classification and characteristics of antistatic agents
Table 1 lists types of antistatic agents and applicable resins. Among them, cationic antistatic agents have excellent antistatic properties, but relatively poor heat resistance and are harmful to skin, so they are mainly used for external use. The anionic type has better heat resistance and antistatic properties, but has poor resin compatibility and affects transparency of product. Nonionic antistatic agents have good compatibility and heat resistance, do not adversely affect physical properties of product, but their dosage is relatively large. The biggest feature of amphoteric antistatic agents is that they can be used in combination with both cationic and anionic antistatic agents. The antistatic characteristics are similar to cationic ones, but heat resistance is inferior to non-ionic ones. The type of polymer is mainly used for permanent antistatic.
Table 1: Main types of antistatic agents3. Practical technology and mechanism of action of antistatic agents
- Using anti-static technology
There are two main types of antistatic technology. (1) The outer coating method is to apply a layer of antistatic agent on surface of resin material so that it can play role of a surface antistatic agent. Specific steps: first prepare antistatic agent in a solution of 0.5% to 2.0% concentration with water, ethanol or ethyl acetate, and then directly spray, dip or clean surface of material, and dry it at room temperature. or hot air to form an antistatic agent. Electrostatic coating. The advantage of this method is that operation is simple, dosage is small, and molding and processing performance of product is not degraded. The disadvantage is that service life is short, and anti-static coating is easy to fall off or disappear after washing or wiping, so this is a temporary anti-static treatment method. In foreign countries, polymeric surfactants are used as antistatic coatings, which to a certain extent increased durability of polymeric antistatic agents. (2) The internal mixing method is to mechanically mix antistatic agent and resin and then shape them. The antistatic agent molecules migrate from inside of resin material to surface and form a uniform antistatic layer on surface. If antistatic agent on surface is washed or wiped from behind, molecules of internal antistatic agent can also move to surface, thereby restoring its antistatic properties, so it becomes a permanent antistatic agent. This technology is widely used.
- Mechanism of action of antistatic agent
According to state of charge, permanent antistatic agents can be divided into cationic, anionic and non-ionic, while antistatic ability, in turn, decreases. Copolymers of ethylene oxide and its derivatives were studied before anyone else and are currently main industrial varieties. They are widely used: polyethylene oxide (PEO), polyetheretheramide and polyetheretherimide (PEEA). Whether it is outer coating method or internal mixing method, action mechanism of polymer material antistatic agent is mainly manifested in two aspects: First, antistatic agent forms a continuous conductive film on surface of material, which can endow surface of product with a certain hygroscopic and ionic film, thereby reducing specific surface resistance, so that generated static charge can quickly flow to achieve antistatic purpose; second is to give surface of material a certain lubricity reduces coefficient of friction, thereby suppressing and reducing formation of static charge. Whether an external antistatic agent or an internal antistaticstatic agent for antistatic material treatment, factors such as relative humidity, temperature and surface concentration of environment will have a certain effect on the antistatic performance. as shown in figure 2.
Figure 2 Schematic diagram of conduction mechanism of antistatic polymer composite materials, interval 1-antistatic, 2-antistatic4. Factors affecting antistatic characteristics of antistatic
Use various hydrophilic polymers as antistatic agents and add them to base resin to make permanent antistatic polymer resins. The key technology is to improve degree of dispersion of permanent antistatic agents in A state of resin, since it forms a core-shell structure in matrix and uses it as a path for static electricity to escape. Permanent antistatic agent is mainly distributed on surface of product in form of thin layers or tendons, while in center it is less and it mainly exists in form of particles. The main factors that determine morphological structure are molding conditions and compatibility with resin of matrix. The most direct factor is difference in melt viscosities or viscosity ratio between matrix and permanent antistatic agent, which is often controlled by shear rate and processing temperature..
- Influence of humidity and temperature
Water has a certain electrical conductivity. The electrical conductivity of pure water is about 3.3×10 - 5 S/cm, and charging half-life is only 10 - 6s. Therefore, if there is still a certain amount of moisture on surface of material, that is, under certain humidity conditions, insulating material will also exhibit a certain surface conductivity. If hydrated ions are formed, conductivity can be improved. Wet electrolyte ions form ionic conduction channels, non-ionic compounds, due to their own moisture, hydrophilic groups form hydrogen bonds with water molecules in air, resulting in ionization tendencies. exchange contact point carries out exchange of charges. Recent studies have shown that with antistatic agents -OH, -NH3, charge transfer can be carried out by proton transfer through formation of hydrogen bonds. Therefore, higher ambient humidity, better antistatic effect.
- Resin compatibility effect
Compatibility between antistatic agent and resin depends on molecular structure of resin material and polarity of antistatic agent. If compatibility between them is too high, migration of antistatic molecules is difficult, and antistatic lost on surface cannot be replenished in time, so it is difficult to play a role; while compatibility is too poor, it will cause processing difficulties, and antistatic will be deposited in large quantities. The appearance is reduced, and deposited antistatic will be lost quickly, and it is also difficult to maintain a long-term antistatic effect. Therefore, choice of an appropriate combination of hydrophilic and lipophilic groups is a top priority in molecularm design of antistatic agents, especially internal antistatic agents.
- Influence of polymer molecular structure
Among parameters related to molecular structure, glass transition temperature (Tg) is first. Above glass transition temperature, antistatic molecules added to polymer continuously migrate to surface due to microscopic Brownian motion of polymer molecule... Below this temperature, polymer molecules are frozen state, and antistatic agent is practically closed between polymer molecules, which makes it difficult to migrate to surface. Therefore, in this case, it is necessary to select an antistatic agent with a large polarity difference from resin, and increase amount of additive accordingly, and transfer it from mold surface to product surface through forming process. On other hand, difference in state of crystallization of polymer also causes a difference in migration rate of antistatic agent. Secondly, this is effect of surface concentration of antistatic agent and other additives.
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