Why Study Static Electricity in Textiles?
Static Electricity Phenomenon and Generation Principles in Textiles
There are multiple explanations for the mechanism of static electricity generation, and static electricity in textiles is mainly generated by the friction between surfaces. Textile materials are poor conductors of electricity and have a high resistivity. Fibers and their products are prone to static electricity generation during production, processing, and use due to factors such as friction, stretching, compression, peeling, electric field induction, and hot air drying. Especially with the increasing production and application of synthetic fibers in textiles, the inherent high insulation and hydrophobicity of these polymer materials make them highly susceptible to static electricity generation and accumulation.
Hazards of Static Electricity in Textiles
In civilian applications, static electricity can cause dust accumulation on textiles during use, leading to discomfort and adhesion of clothing to the body. Research has also shown that static electricity stimulation can have adverse effects on human health. In industrial applications, static electricity is one of the main factors causing accidents such as fires and explosions in industries such as pyrotechnics, chemicals, and petroleum processing. It is also a hidden danger for quality and safety accidents in the textile industry, including chemical fiber processing. With the development of high technology, the consequences of static electricity hazards have exceeded the boundaries of safety issues. Spectrum interference caused by static discharge can result in equipment malfunctions and signal loss in electronic, communication, aviation, aerospace, and all other modern electronic device and instrument applications. Therefore, the demand for anti-static textiles is increasing.
Mechanisms of Anti-static Textiles
Static electricity on the surface of insulators can be dissipated through three pathways:
- Dissipation through the air (mist)
- Dissipation along the surface
- Dissipation through the interior of the insulator.
Dissipation through the air relies on oppositely charged particles in the air coming into contact with the surface of the insulator, neutralizing the static charge or causing the charged particles to disperse due to kinetic energy. High-voltage corona-type static eliminators, based on the principle of sharp discharge, have been applied in the production of chemical fibers.
The speed at which static electricity dissipates along the surface of an insulator depends on the surface resistivity of the insulator. Increasing the humidity of the air can form a continuous water film on the hydrophilic insulator surface, enhancing surface conductivity due to the dissolution of CO2 and other impurities in the air. Another method is the use of anti-static agents, mainly ion or non-ion surfactants.
The rate at which static electricity dissipates through the interior of an insulator primarily depends on the resistivity of the insulator. Generally, when the resistivity of a polymer is less than 107 Ω·m, the generated static charge will dissipate quickly. To improve the volume conductivity of polymers, the most convenient method is to add carbon black, metal powders, or conductive fibers.
Fiber polymer materials are theoretically insulators, but the actual conductivity of fibers is higher than the estimated values due to the presence of moisture, impurities, and other low molecular substances in the fibers. Therefore, the conductivity of fibers mainly depends on the impurities present in the fibers, followed by the intrinsic conductivity of the fiber molecules and the influence of external conditions. When the surface ionizable substances have higher conductivity and the water vapor pressure is higher, the conductivity of the fibers will be significantly enhanced.
Approaches for Anti-static Textiles
Anti-static textiles can be divided into two main categories: civilian anti-static fabrics and industrial anti-static protective clothing. The latter can be further classified based on their specific applications, such as dust-free and sterile workwear, fire-resistant and explosion-proof workwear, surgical gowns, and safety uniforms (e.g., anti-static clothing worn by electrical workers, conductive clothing, etc.).
Anti-static Treatment of Fibers
Surface Treatment with Surfactants to Enhance
The principle of this treatment is that the hydrophobic end of the surfactant molecule adsorbs onto the fiber surface, while the hydrophilic polar groups face the surrounding space, creating a polar surface. This polar surface adsorbs water molecules from the air, reducing the surface resistivity of the fiber and accelerating charge dissipation. Surfactants used for this purpose include cationic, anionic, and non-ionic types. Among them, cationic surfactants have the best anti-static effect, while high molecular weight non-ionic surfactants provide better durability. The advantage of this method is its simplicity and suitability for eliminating static interference during textile processing. However, its drawback is that the anti-static effect is not long-lasting, as surfactants can easily volatilize and are not resistant to washing. Additionally, they do not exhibit anti-static properties in low-humidity environments.
Blending, Copolymerization, or Graft Modification of Synthetic Polymers
Similar to the previous method, this approach involves adding hydrophilic monomers or polymers to synthetic polymers to improve their moisture absorption and achieve anti-static properties. In addition to the typical blending of hydrophilic polymers with conventional synthetic polymers, there is also a method of incorporating hydrophilic polymers during the polymerization process to form a micro-phase dispersed system. For example, adding polyethylene glycol to the caprolactam reaction mixture, where polyethylene glycol is dispersed in the PA6 matrix. At the same time, polyethylene glycol reacts with the hydroxyl groups in the aminocaproic acid generated after the ring-opening of caprolactam, thereby improving the durability of the anti-static performance.
Furthermore, hydrophilic polar monomers can be copolymerized onto the hydrophobic synthetic fiber backbone. For example, polyethylene glycol can be embedded in PET macromolecules to enhance the fiber’s moisture absorption and anti-static properties.
Grafting onto the fiber surface through chemical initiation, thermal initiation, high-energy radiation, or UV radiation can effectively improve the moisture absorption of synthetic fibers and exhibit good durability. This method requires a smaller amount of hydrophilic monomers compared to other methods and provides long-lasting anti-static properties. However, the anti-static performance of these fibers, which rely on improved hydrophilicity to accelerate charge dissipation, may be compromised in dry environments with relative humidity below 40%.
Anti-Static Yarn Production
By incorporating a small amount of conductive short fibers during the spinning process, it is possible to produce anti-static yarn and reduce or even eliminate static electricity issues that may arise during spinning. Ordinary textile fibers are used as the main fibers in the spinning process, with a small amount of conductive fibers mixed in. The amount of conductive fibers added depends on the final product’s intended use and cost considerations. Extensive experiments have shown that adding a small amount (a few percentage points) of organic conductive fibers to the yarn significantly reduces its electrical resistivity and greatly improves its conductivity.
Conductive Fibers Conductive fibers include metal fibers, plated metal fibers, and organic conductive fibers. The most widely used metal fiber is stainless steel fiber, which is manufactured through methods such as wire drawing, melt spinning, and cutting. Stainless steel fibers exhibit good conductivity and mechanical properties. However, in textile processing, metal fibers have poor cohesion and spinning performance and are expensive when produced with high fineness. Therefore, except for specific requirements, the use of metal fibers in the development of anti-static products is not yet widespread. Plated metal fibers involve coating the surface of ordinary fibers with a metal layer to enhance the anti-static effect. They have a significantly lower cost compared to metal fibers but are not durable during washing and have a poor hand feel. Currently, organic conductive fibers are commonly used in the development of anti-static blended yarns.
Organic conductive fibers are based on ordinary synthetic polymers and have conductive substances added through coating or composite methods. The commonly used organic conductive fibers include nylon-based, polyester-based, and acrylic-based fibers, with carbon and metal compounds as the conductive substances. Fibers made with carbon conductive substances are dark in color (black, gray), while fibers with metal compounds as conductive substances are white. The latter has slightly lower conductivity but is more suitable for subsequent finishing processes such as dyeing.
Spinning Process Due to the higher cost and smaller proportion of conductive fibers, they are generally manually opened and blended. To ensure uniform mixing, the predetermined weight of conductive fibers and main fibers are simultaneously fed into the carding machine and then go through multiple carding processes. Additionally, the selected conductive fibers should ideally be consistent with the material of the main fibers. The process of blending includes cotton carding (first pass) → cotton carding (second pass) → head doubling → second doubling → third doubling → coarse yarn → fine yarn → winding onto a bobbin.
Incorporating Conductive Filaments or Anti-Static Yarn during Weaving
In the development of anti-static textiles, in addition to improving the raw materials, conductive filaments (or composite yarns with conductive fibers) can be embedded into the fabric at regular intervals during the weaving process. These filaments can be embedded along the warp or weft direction, or both, to form a grid pattern. Extensive experiments have shown that regardless of the embedding method, the anti-static effect of the fabric is significantly improved. Still, the best results are achieved when the conductive filaments are embedded in a grid pattern. Additionally, the anti-static performance of the fabric decreases as the spacing between the conductive filaments increases. The spacing of the conductive filaments (or the content of conductive fibers in the fabric) should be determined based on the intended use of the anti-static product and the required level of conductivity.
Due to the higher cost of conductive fibers and the resulting higher fabric production cost, it is important to consider using the minimum amount of conductive fibers to achieve optimal anti-static performance. By conducting an optimization analysis of various influencing factors such as filament spacing and fabric density, the optimal embedding spacing (conductive fiber content) that meets the product requirements can be determined. Furthermore, since most conductive filaments are black in color when designing the fabric structure, efforts should be made to hide the warp organization points of the conductive filaments beneath the base fabric structure to ensure that the front-facing fabric structure remains intact. On the reverse side of the fabric, the conductive filaments should be exposed as much as possible to facilitate discharge.
Finishing the Fabric with an Antistatic Agent
The method originated in the 1950s and is suitable for various fiber materials. The anti-static agents are mostly high molecular weight polymers with structures similar to the fibers. They are applied to synthetic fibers or fabrics through immersion, rolling, and baking processes. These polymers are hydrophilic, so coating them on the surface increases the fiber’s conductivity through moisture absorption, preventing the accumulation of excessive static charges that could cause harm. In addition to providing anti-static effects, fabrics treated with this method also exhibit moisture absorption, stain resistance, and dust repellency. Due to the simplicity of the anti-static method, the finished products are relatively inexpensive.
The process flow is as follows: fabric – immersion and rolling with anti-static resin (two immersions and two rollings) – drying (100-110℃) – baking (150-160℃for 2 minutes) – stretching – finished product.
There are several methods for anti-static finishing:
- Auxiliary adsorption and fixation method
- Surface grafting polymerization method
- Low-temperature plasma surface treatment method
The latter two methods require special initiators, high-energy radiation, or plasma treatment, making the process complex and operationally challenging. Therefore, the first method is generally preferred. It can be performed during post-finishing for anti-static processing or simultaneously with dyeing in the same bath, both of which can achieve desirable results.