- Basic Principles
- Anti Static Finishes
- Conductive F&Y
Static electricity is electricity at rest and is built up when materials contact and then separate allowing for a transfer of electrons resulting in an unequal electron balance creating negative or positive charged items, this electron transfer is activated by frictional forces. Electricity generated in this manner is known as triboelectricity. It is static because it remains on the surface of insulative materials as they do not possess the property to conduct the electrical charge throughout the structure, resulting in the charge remaining static on the surface.
The way different materials behave is due to their properties, the Triboelectric series is a prediction of how materials will react based on these properties, ie which are more reactive and whether they become positive or negatively charged, the further apart in the series materials are listed illustrates that a greater charge will be created on their interaction.
Figure 1. Triboelectric Series
The Triboelectric series is useful in predicting results from material interactions, however this is a generalisation and a range of variable factors will ultimately influence the charge generated, such as;
· Area of contact
· Speed of separation
· Materials chemistry
From the Triboelectric series it is demonstrated that human skin is most likely to loose electrons, becoming positively charged, whereas materials such as polyester, polyurethane, polypropylene, PVC and Teflon, tend to gain electrons and become negatively charged. Therefore when synthetic fibres are worn in close to skin applications static electricity occurs, the amount of charge generated depends on variables such as the materials properties, mechanics of contact, surface contact and Relative humidity?.
Whereas static electricity in clothing applications can be troublesome for the wearer, the consequences are generally within the guidelines for safe working, in such applications anti-static finishes can be implemented to minimise the problem.
In Textile processing static electricity becomes more problematic, as materials are often processed at high speeds and contact with materials that create triboelectricity, which in insulating materials can result in a static charge remaining on the surface, which can result in an electrostatic discharge. As well as a possible safety risks for workers, charged surfaces can also attract and hold foreign matter that causes a contamination risk during process affecting the quality of the end product.
The Human body is the biggest generator of triboelectricity, when this electrostatic is then discharged a small shock can be experienced, but this mostly only causes mild discomfort, however in certain environments this electrostatic discharge can pose health and safety risks and the malfunction of electronic equipment. In areas where flammable of volatile liquids or gases are processed an electrostatic discharge can trigger fire or explosion. In areas where sensitive electronic equipment is manufactured or stored an electrostatic discharge can cause malfunction, which is costly, because of these risks controlling Electro Static Discharge ?is of high priority to many industries. Therefore means of controlling electrostatic discharge are implemented, and due to the human body being a main factor in its generation, clothing and accessories such as wrist straps are implemented in controlling it. Many environments requires workers to wear personal protective clothing with anti-static properties and these usually work by dissipating the charge, ie, not allowing it to sit static on the surface, this is done through the introduction of conductive materials which allow for flow through of the charge, often allowing it to flow into the ground.
Terms and Definitions
On a floor (from BS 1815:1997) The ESD journal defines Static electricity as an electrical charge caused by an imbalance of electrons on the surface of a material.
Electrostatic discharge is defined as the transfer of charge between bodies at different electrical potentials.
Staic electrical propensity- the static electrical charge generated by a person walking
During the contact of materials a form of adhesion will occur, this is a complex interaction but generalised in the case of Triboelectric generation, it is an atomic interaction occurring between electrons and protons. As the materials then separate, not all electrons remain around their nucleus and can transfer between materials, leading to an unequal amount of electrons, which results in a positive, if electrons are lost or negative if electrons are gained, charge.
The chemical structure of the material dictates how it will behave in relation to triboelectricity, as the fundamental elements and molecular structure determine electron activity, determining the circumstances under which electron transfer will occur and to what extend, leading to whether a positive or negative charge will be incurred. This chemical structure will also determine if/how the occurring charge is dissipated.
As all materials want to exist in equilibrium, they strive to maintain balance and again reach their neutral state, therefore they want to regain or discharge the electrons. The rate at which this occurs is again due to chemical structure of the materials and 2 main groups exist;
- Insulators (http://en.wikipedia.org/wiki/Insulator_(electricity) )
- Conductors (http://en.wikipedia.org/wiki/Electrical_conductor )
Insulators do not allow movement of electrons within the structure, therefore electricity cannot flow through them, and so acquired charge stays on the surface of the Insulator?, remaining static. Resulting in all the charge being focused in one area, which can lead to a build up that may be quite strong when released. Insulators are therefore highly resistive as they restrict the flow of electricity.
Conductors have low resistance as they allow for movement of electrons within their structure meaning electricity can flow freely.
Static Dissipative materials lay in between insulators and conductors, they can allow static to build up on them, but they can conduct, doing so at a slower rate then a Conductor?. Charges transfer faster than they would from an Insulator?, but slower than from a Conductor?.
Selecting materials that will minimise the generation of triboelectiricty will reduce the resultant charge, however this approach will be unlikely to eliminate the build up completely, especially in applications where frictional forces are frequently experienced. To minimise the risk of electrostatic discharge the basic principle is that the charge cannot remain static, it has to be moved, and therefore it needs to encounter materials within which it can do so. Therefore a way of dissipating, or distributing the charge is required, and this is done through the implementation of conductive materials to enable the charge to be moved safely.
The size of the charge being moved and the rate at which this occurs is dependant upon the specific application. In textiles there are generally two approaches taken to control static electricity;
- Anti-static finishes- provides a surface treatment to traditional textile materials, altering their inherent electrostatic behaviour, allowing them to dissipate the charge by allowing for the current to move along the fabric surface.
- Introduction of conductive fibres and yarns- works in the same principle, ie, allowing the generated charge to dissipate through conduction, however this property is introduced in the form of conductive yarns or fibres instead of as a surface finish.
Influence of Moisture in Textile materials
From Figure 1, the Triboelectric series it can be generalised that synthetic fibres are more prone to static generation that natural. This is related to the low Moisture regain? of synthetic fibres. Pure water, as in H2O is unable to conduct electricity, however water is not very often experienced in its pure state, most likely to contain contaminants, such as salts, which make the water conductive.
The polar groups within the fibre dictate to what extent water molecules will be attracted, for example the groups OH and –COOH. The fibre structure also dictates the extent to which water molecules can be held, for example in highly ordered regions (crystalline) there is no place for the molecules, whereas more disordered regions (amorphous) are able to absorb water molecules. Therefore the fibres ability to hold moisture affects its ability to generate a charge and to conduct it, which is why Relative humidity? affects textiles electrostatic properties.
Static Electricity is measured in Coulombs. The charge of an object (Q) is determined by the product of the capacitance of the object (C) and the voltage potential on the object (V)
However we speak of electrostatic potential, which is expressed as voltage.
Water is implemented as an anti static agent because of the conductivity of the electrolytes it holds, it is cheap, non-toxic, and non-flammable. In natural fibres there inherent Hydrophilic? nature allows them to hold water molecules, which gives them some conductivity, reducing the problem of static charge. Therefore in synthetic fibres that do not allow for a high water regain, anti-static agents are used to encourage the presence of water in the textile.
Surfactants have a hydrophobic head and a Hydrophilic? tail, the phobic head is embedded within the synthetic material, and the Hydrophilic? head attracts water and salts to create a dissipative layer, as displayed in figure 2. However this relies upon atmospheric conditions and as they are intended to attract a surface layer they may not be suitable for cleanroom applications. (REF: http://www.ce-mag.com/archive/01/Spring/Rosner.html )
Surfactants are applied at fabric finishing stage, and are often found in fabric softeners, however they do not last for the life of the product and are often lost during the laundering process. There have been wide concerns over their safety and the environmental impact of using them on such a wide scale.
Figure 2. Antistatic agent. Image from; http://www.ce-mag.com/archive/01/Spring/Rosner.html
Conductive Fibres and Yarns
More on Conductive Fibres and Yarns can be found in Conductivity.
Standards & Regulation
Static electricity is a naturally occurring phenomena and as such it cannot be regulated, however controlling its generation can be managed as can steps be taken to ensure electrostatic discharge occurs safely.
EN 14041:2004 is a mandatory conformity marking for floor coverings, it covers these areas;
• Reaction to fire (EN 13501-1)
• Content of Pentachlorophenol – PCP (EN 14041 Annex B)
• Formaldehyde emissions (EN 717-1 and/or EN 717-2)
• Water tightness (EN 13553)
• Slip resistance (EN 13893)
• Electrical behaviour (Antistatic EN 1815 or ISO 6356 – Static dissipative/Conductive floors EN 1081 or ISO 10965 dependent on product)
• Thermal conductivity (EN 12667)
Conformity of this directive in order to obtain a CE marking for floor coverings has been mandatory since 2007.
Those test methods are;
Method of evaluating electrostatic propensity of textile floor coverings. It cannot give accurate in use readings due to the large amount of variables, such as shoe material, humidity and usage, but it will provide a comparison of the performance. It measures the difference in electrical potential, in relation to the earths potential (zero), produced by a person walking;
- On the textile flooring under test
- With standardised footwear
- In a prescribed manner
- Under controlled atmospheric conditions
It measurements are used to evaluate the risk of a person experiencing discomfort of static electric shock.
Textile floor coverings- determination of electrical resistance
Laboratory method to determine electrical resistance of textile floor coverings, horizontal or vertical.
Another relevant test method, but not mark of the EN directive is;
BS 1815:1998 - Resilient and Textile floor coverings- Assessment of static electrical propensity
(Similar to ISO but can be used in-situ as well as under laboratory conditions)
Anti static textiles/garments used in Personal Protective Equipment fall under the legislation for Personal Protective equipment, of which there is information in the Market Sectors area. (link to PPE? section)
The EN1149 - series of standards outlines a series of test methods and requirements for electrostatic properties of protective clothing.
Different parts are necessary because of the different fields of application and materials, the different parts are as follows;
For materials intended to be used in the manufacturing of electrostatic dissipaitive protective clothing, it determines resistance over short periods, not specifically appropriate for evaluating full garments.
EN1149-2; 1997 - Test method for measurement of the electrical resistance through a material (vertical resistance)
EN1149-3; 2004 - Test method for measurement of charge decay
EN1149-4:2008 - Material Performance and design requirements
Specifies material and design requirements for electrostatic dissipative protection clothing, used as part of a total earthed system to avoid incendiary discharges.
Are a set of standards addressing electrostatic generation and prevention of electrostatic discharges in relation to specific applications, such as wrist straps, footwear and electronic shielding. There are also Methods for simulation of electrostatic effects using various models.
Electrostatic properties have long been exploited in industries such as automotive and electronics, using the attraction between negative and positively charged materials to aid assembly. A commonly know example of this is in the Spraying? of automotives, where as the spray particles pass out of the nozzle of the spray gun they become positively charged, and become attracted to the negatively charged surface, aiding in the achievement of a evenly coated surface, limiting the number of air borne particles reducing human risk and waste.
This self assembly technique is used to add nano coatings in the electronics industry and in recent years has been explored as a means of adding nano coatings to textiles
Layer by Layer deposition of antimicrobial silver particles on textile fibres
Layer by Layer deposition of polyelectrolyte nanolayers on natural fibres; cotton