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Basic Principles

 

During the contact of materials a form of adhesion will occur, this is a complex interaction but generalised in the case of Triboelectrichttp://en.wikipedia.org/wiki/Triboelectric_effect 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 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?.

(REF http://www.tandfonline.com/doi/pdf/10.1080/01445340802442096 )

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 Triboelectrichttp://en.wikipedia.org/wiki/Triboelectric_effect 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.

 (http://trj.sagepub.com/content/38/1/28.abstract )

Measurement

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) 

Q=CV

However we speak of electrostatic potential, which is expressed as voltage.

Ref; http://www.esda.org/fundamentalsP1.html