Static electricity has been known since ancient times, but its scientific understanding emerged with the discovery of electrons. Modern polymer materials in shoes and carpets have exacerbated static electricity issues. Shocks resulting from this phenomenon when touching metal objects due to accumulated voltage (sometimes up to tens of thousands of volts) are common. Despite extensive efforts to reduce static electricity accumulation, these shocks still occur regularly, with the level of discomfort varying among individuals. Different materials, including polymers and natural fibers like wool, play a role in static electricity generation.

1. Basics of Static Electricity in Carpeted Environments

1.1. Basic Principles and Terminology

Static electricity arises from an imbalance between positive and negative electrical charges within materials, often generated by direct friction between two bodies in a phenomenon known as “triboelectric charging.” When materials come into contact, electrons may shift, resulting in one material acquiring an excess positive charge and the other an excess negative charge.
The creation of static electricity is akin to filling a basin with water. If electric charges can exit as quickly as they are generated, static electricity remains minimal. However, if charge generation exceeds dissipation, static electricity accumulates. Insulating materials impede charge dissipation, allowing static electricity to accumulate rapidly.
Materials vary in their ability to conduct electrical charges. Metals are excellent conductors, while insulators hinder charge movement. Materials with moderate electrical resistance allow charges to move slowly, determining the rate of charge dissipation and static electricity voltage. Higher conductivity leads to rapid charge dissipation, while lower conductivity results in slower dissipation and higher static electricity voltage.
In summary, static electricity accumulates when charge generation exceeds dissipation. Effective dissipation prevents the accumulation of static electricity.

1.2. The Role of Materials in Charge Generation

Charge generation is influenced by the types of materials in contact, as depicted in the provided chart. This chart ranks materials based on their empirical tendency to generate electrical charges when rubbed together. When a material from the upper part of the chart (such as wool or nylon) is rubbed against a material lower in the chart (such as rubber), the higher-ranked material typically acquires a positive charge. In contrast, the lower-ranked material (rubber) acquires a negative charge. The further apart materials are in the chart, the stronger the charging effect is expected to be. Therefore, polyurethane (a common shoe sole material) in contact with wool or nylon used as a floor covering will likely generate significant static electricity.

1.3. The Impact of Air Humidity

Static electricity shocks vary seasonally and with different humidity levels. Heating systems in colder months dry the air, enhancing static charge accumulation, while rainy weather reduces static electricity. Air humidity, measured by relative humidity (RH), significantly affects static charge accumulation. Low humidity levels (<30%) intensify electrostatic effects. A 10-degree temperature rise can halve relative humidity, exacerbating static charge accumulation. For instance, indoor air at 5°C and 50% RH heated to over 15°C may drop to less than 25% RH.

1.4. Electrostatic Discharge (ESD) Risk Thresholds

Undesirable effects of static electricity, like shocks or electronic component damage, can be controlled if they remain below certain thresholds. Personnel feel shocks when body voltage exceeds about 2kV. European electronic equipment must pass ESD tests at specified stress levels. The flammable material ignition risk threshold is typically converted to body voltage, with a maximum accepted body voltage of around 100V.

1.5. Static Charge Build-Up on Walking Personnel

Static charge accumulation on people during daily activities can lead to discomforting shocks. Factors affecting charge accumulation include floor and footwear resistance, air humidity, walking style, and interaction with furniture. The repeated contact and separation between feet and floor generate static electricity, which cannot discharge if insulated. Lower floor surface resistance facilitates charge neutralization, reducing the risk of shocks and ESD damage.

1.6. Measurements Made on Floor Materials

Floor resistance-to-ground (Rg) is commonly measured, indicating charge dissipation to the ground. Point-to-point resistance (Rp) measures how easily static charge can move across the floor surface. Resistance values above 1010Ω promote static charge accumulation on people or moving items. Lower resistance levels are preferred to control static electricity-related risks.

2. Methods of Reducing Static Electricity in Carpet Materials

Controlling body static voltage can be achieved through two main methods. Firstly, the electric charge generation can be managed by selecting appropriate materials or applying surface coatings. Secondly, voltage buildup can be controlled by using materials with lower resistance.

2.1. Choice of Materials

Material selection for carpets influences charge generation based on the earlier presented chart. However, materials in contact with carpets vary in electrostatic properties, making control challenging. Footwear, typically insulating unless designed for static control, complicates static electricity prediction. Nonetheless, materials in the middle of the chart may generate less charge. Two main technologies for static control are conductive fibers and topical finishes.

2.2. Conductive Fibers

Conductive fibers, often carbon-based, provide a long-term solution for controlling static charge accumulation. Fibers with stainless steel or silver coatings offer antimicrobial benefits. Incorporating conductive fibers into carpets reduces static electricity via conduction, lowered resistance, and discharge.

2.3. Topical Finishes

Topical finishes contain ionic or cationic surfactants or quaternary ammonium compounds applied during manufacture or post-installation to control static electricity’s nuisance effects. While effective for general use, critical environments like computer rooms or data centers require conductive fibers for comprehensive static control.

3. Standards

Several international, national, or industry organizations provide standards in this field. Some of the most important international standards are developed by the International Electrotechnical Commission (IEC), the International Organization for Standardization (ISO), and the European Committee for Standardization (CEN). IEC and ISO, endorsed by the World Trade Organization (WTO), cover areas such as electrotechnology, electromagnetic compatibility, safety, and nearly all other technical fields. CEN develops standards for use in Europe, often following European directives.

4. Future Trends

With the continued use of polymer materials in carpets, flooring, and furniture, static electricity will remain a source of discomfort and danger to individuals and equipment. Controlling static electricity in processes where damage to parts or ignition of flammable materials is a concern will remain vital. Incorporating technologies to reduce static electricity in the design and specifications of future materials is crucial. While existing techniques are available to carpet manufacturers, further innovation in materials and payment methods to reduce material resistance and production costs of static electricity is necessary.

5. Polypropylene Carpets

Wool and synthetic fibers such as nylon, acrylic, and polyester generate static electricity due to friction and use. As a result, these fibers attract more dust particles and can even create sparks, causing mild but unpleasant shocks to consumers.
In comparison to natural fibers, it must be said that polypropylene fibers have another advantage. Carpets woven from natural fibers tend to conduct static electricity. However, static electricity generation is likely present in polypropylene but much lower.
Another noteworthy advantage is that polypropylene carpets retain their color density for a longer period. They do not lose their color and are leading among all synthetic counterparts!
Sahand’s polypropylene carpets are especially durable to resist spills, stains, scratches, water, and dirt. That’s why people choose polypropylene carpets for both outdoor and indoor spaces. Cleaning these carpets is easy, and if you use the services of a professional carpet cleaner, your carpet will look as good as new!

References

[1] Smallwood, J. (2017). Reducing static electricity in carpets’, in Goswami, K. K. (ed.) Advances in Carpet Manufacture. UK: Woodhead Publishing, pp.135-161.

[2]  J.  Cross, Electrostatics: Principles, Problems and Applications, Adam Hilger, IoP Publishing, Bristol, ISBN: 0-85274-589-3, 1987.

[3]  J.M. Smallwood, Static electricity in the modern human environment, in: D. Clements Croome (Ed.), Electromagnetic Environments and Health in Buildings, Taylor & Francis, ISBN: 0-415-31656-1, 2004. Ch. 19.

[4] N. Wilson, The static behaviour of carpets, Textile Inst. Industry 10 (8) (1972) 235.

[5]  J.M.  Smallwood, D.E.  Swenson, Evaluation of performance of footwear and flooring systems in combination with personnel using voltage probability analysis, J. Phys. Conf. Ser. 301 (2011) 012064.