Generation of electrostatic charge

The ability of a solid to carry a charge depends on surface conditions, dielectric constant, surface resistivity, and the relative humidity of the surrounding environment. Its ability to carry a charge is inversely proportional to its dielectric constant and relative humidity, and directly proportional to its surface resistivity. The sign of the charge varies depending on the material; materials with lower dielectric constants are positively charged.

Insulation properties are related to static electricity buildup. The chemical structure of most plastics reveals them to be excellent insulators, making them essential materials for high-frequency equipment such as radar. Because most plastics have low surface conductivity, they cannot quickly dissipate electrical charge, a difference between plastics and metals.

During the use of plastic products, static electricity can cause various problems and lead to serious, even dangerous, consequences. The most common hazards include: severe buildup of dirt on plastic surfaces; static electricity attracting dust that affects the sound quality of records; static electricity causing an unpleasant “electric shock” sensation in people using synthetic fiber carpets or plastic flooring; static adhesion between plastic films and sheets, disrupting normal production; and solid powder clumping together during airflow transport. The discharge sparks generated by large accumulations of static charge can even ignite mixtures of air and dust or organic solvents, becoming a cause of many destructive explosions.

Measures to suppress electrostatic charge

(1) Increasing relative humidity：As the ambient humidity of molded products increases, their surface conductivity also increases, thereby accelerating the dissipation of charge. For example, when the relative humidity of water-absorbing polyamide is higher than 65%, there is practically no static electricity. Conversely, when the relative humidity is much lower than 20%, surface charge balance problems are inevitable. In this case, the only truly effective measure to suppress static electricity is to add a conductive matrix to reduce the volume resistivity.

(2) Increase the conductivity of air:by using an ionizer that works on the principle of electricity or radioactivity to increase the conductivity of air, so that the charge can be quickly dissipated into the ambient air.

(3) Increase surface conductivity by adding chemical additives (antistatic agents) to plastics or applying them to the surface to increase surface conductivity, thereby dissipating static charge.

 Chemical Structure of Antistatic Agents

Antistatic agents are additives that are added to molding compounds or applied to the surface of molded products to reduce static electricity buildup. Generally, based on the application method, antistatic agents can be divided into two main categories: internal and external application.

 2.Internal Antistatic Agents

Internally added antistatic agents are added to polymers as surfactants before or during molding. They all possess surface-active characteristics and can migrate and aggregate on the surface of molded parts. These additives contain both hydrophilic and hydrophobic groups in their molecules. The hydrophobic groups have a certain compatibility with the polymer and can cause its molecules to adhere to the surface of the product, while the hydrophilic groups function by binding and exchanging with water molecules on the product surface. Most antistatic agents with surface-active characteristics can be classified into cationic, anionic, and nonionic types.

1. Cationic Antistatic Agents: In this type of antistatic agent, the active part of the molecule typically contains a large cationic group and often a long alkyl group, such as quaternary ammonium salts, quaternary sulfonium salts, or quaternary sulfonium salts. Anions are generally formed during quaternization reactions, such as chlorides, methyl sulfates, and nitrates. Quaternary ammonium salt antistatic agents dominate this category of commercial products. Cationic antistatic agents are most effective on polar matrices (such as PVC and styrene polymers). However, their use is somewhat limited due to their adverse effects on the thermal stability of certain polymers.

2. Anionic Antistatic Agents: In this type of antistatic agent, the active part of the molecule is anionic. Alkyl sulfonates, sulfates, phosphates, dithiocarbamates, or carboxylates typically carry a large number of anions, while the cations are usually alkali metal ions, and sometimes alkaline earth metal ions. For example, sodium alkyl sulfonate is widely used in industry because it achieves satisfactory antistatic effects in polyvinyl chloride and polystyrene polymers, but its application in polyolefins has certain limitations.

3. Nonionic antistatic agents: These antistatic agents have a surface-active molecular group that is uncharged and has very low polarity (mainly polyethylene glycol esters or ethers, fatty acid esters or ethanolamines, mono- or diglycerides, and ethoxylated fatty amines). They are mostly supplied commercially as liquids or low-softening-point waxes.

The low polarity of these additives makes them ideal internal antistatic agents for polyethylene and polypropylene, and they also exhibit high compatibility. Different types of polyethylene and polypropylene have varying densities, crystallinity, and microscopic molecular structures. Therefore, to obtain the optimal molecular structure for each antistatic agent, the length of the alkyl chain and the number of hydroxyl or ether groups in the compound must be adjusted. Only in this way can the desired application effect be effectively ensured. For example, typical antistatic agents used in polypropylene are less effective when applied to low-density polyethylene, and vice versa.

 External coating type antistatic agent

External antistatic agents are applied to the surface of molded parts in the form of an aqueous or alcoholic solution. Due to the different application methods, the structural requirements mentioned in internal antistatic agents become less important. All surface-active compounds, as well as many non-surface-active hygroscopic substances (such as glycerin, polyols, and polyethylene glycol), possess antistatic properties to varying degrees, and the effectiveness of these compounds is not affected by their compatibility with the polymer or their migration within the polymer.