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Conductive

Textile materials, natural or synthetic are predominantly insulators and as such do not allow for the flow of electricity through their structure, many possess dielectric properties, defined by their Dielectric Constant?. This inability to transport electrical current and the ability to generate trioelectricity allows electrical charges to remain static on the surface of materials. This static electricity and risk of Electro Static Discharge ?is problematic in textile processing and end use applications, the way of controlling this is through the introduction of conductive materials to transport and dissipate the charge.

Control of static electricity is not the only application of conductive materials in textiles. The emergence and growth of smart textiles has found applications such as the integration of sensors and electronic devices into textile products across a range of market sectors. Conductive textiles can transport electrical signals as well as electrical current, and as such can be used to transport or store data.

Textile integrated sensors can allow remote monitoring of vital biological signs such as heart rate, having applications in medical, military and sports sectors. This sensing functionality has further use in engineering, automotive, aerospace and geotextile structures to monitor temperatures or pressure loads, with actively smart systems then able to respond intelligently to this electrical signal. The fundamental components in any smart textile are sensors and actuators. Interconnections, power supply and control units are also needed to complete the system, all of which require conductivity and, if to produce a system fit for purpose ease of integration into a textile product. 

Electrical current charged and stored in textiles has the potential to offer an alternative power source for battery operated technology, increasing efficiency and reducing battery weight.  

As technology advances human beings are becoming increasingly exposed to Electro magnetic radiation? from sources such as radios, mobile phones, televisions, microwaves and X-rays. Exposure on a small level has not show to be a risk; however protection is required from prolonged or high level exposure. Electrical equipment is sensitive to Electromagnetic interference (EMI), where exposure may cause temporary malfunction or complete failure. A way of reducing this exposure is through the implementation of EMI shielding materials. Conductive materials weaken electromagnetic waves by Reflectionhttp://missionscience.nasa.gov/ems/03_behaviors.html, conductive materials in the form of textiles or garments offer lightweight and practical shielding solutions.

Perhaps the most controversial application of conductive textiles is that of Taser proof clothing, clothing of this nature acts like a Faraday cage to protect the wearer from the charge. There are concerns this may pose a hazard or become used by criminals. 

The development of Conductive textiles is of importance across a range of applications and products, products which have the potential to revolutionise the way we live and interact with our environment. But two crucial hurdles – unobtrusiveness and reliability – impede widespread adoption of such clever clothes. This is why continual development is required, to meet the increasing needs with smarter, lighter and more efficient solutions. 

The approaches taken to add conductivity to textiles varies greatly dependant upon application which will dictate use factors such as required voltage, strength, durability and ductility, etc.

 Approaches can include:

  • Using an inherently conductive material/polymer
  • Coating with conductive materials
  • Use of conductive materials as fibres, yarns or threads.  

In clothing, comfort is a requirement so conductive yarns have to remain flexible and soft whilst maintaining their conductive function. Wires have been found to not provide a good comfort level and the metallic materials used can be brittle and fail following prolonged wear conditions. Therefore traditional textile materials which already exhibit the required qualities in terms of comfort are being modified in order to obtain conductive qualities. This can be in the form of a coating of conductive materials or, for synthetic fibres, the introduction of these materials at the fibre manufacture stage.

Anti-static treatments introduce conductive materials in order to dissipate the static build up; this is often applied as a finish.  

 

 

There are three main application areas for the functionality of conductivity in textiles;

  • To reduce or dissipate static electricity
  • To carry electrical current or data signals
  • To shield again electromagnetic radiation

Anti Static

As textile materials are largely insulators they do not possess the inherent quality of charge transfer. This coupled with their ability to generate electricity through Triboelectrichttp://en.wikipedia.org/wiki/Triboelectric_effect charging can make the control of static electricity a challenge. This is discussed in more detail in the Anti Static section (LINK to Anti static)

Electronic Textiles and EMI shielding  

The integration of electronic devices and sensors requires electrical current and signals be transferred. Wires or cables are largely unsuitable for textile integration due to their bulk, metal conductors have high stiffness and poor Elasticity? so product failure can result following flexing and bending, such as the movement experienced in wear.  Therefore they require covering in a thick insulative material in order to protect them from this degradation. This bulk and inflexibility provides poor comfort qualities in clothing applications and increases weight, so more suitable solutions have been explored.

Due to these disadvantages of metallic conductors the introduction of conductive polymers or ‘synthetic metals’ has been widely pursued as a solution, the most common of these polymers are outline below, along with their generalised properties.

·         Polyaniline- (PANI) environmental, thermal and chemical stability, displays characteristics similar to traditional textile yarns such as bending, shearing and twisting.

·         Polypyrrole (PPY) ease of preparation, non toxic, high conductivity, good environmental stability.

·         PPy and PANI, little is known about long term stability and unwanted side reactions occur during polymerisation

The potential application methods of these conductive polymers are;

·         Fabric coating with a binder to encourage bonding

·         Dispersions of particles in the fibre spinning solution

·         Coating of the fibre/yarn

The method used is dictated by the level of surface resistivity required, which is generally quite high in anti-static finishes and low in applications to transport current or signal. Novel techniques of application have included introduction at the dyeing stage ) or screen printing

Another material used to impart conductivity to textile materials is carbon nanotubes, which are often used along side conducting polymers. The conductivity of carbon is dependant upon its allotropic form, Carbon Nanotubes display a high level of conductivity, because of their molecular structure, and due to their nano scale can be applied to textiles through conventional finishing techniques with minimal impact on handle or look.  Carbon nanotubes (CNT’s) display a conductivity similar to that of copper and are applied as above, either as a coating of the fabric or yarns or as particles in the fibre.

Innovative and environmentally friendly ways of imparting conductive properties to textiles are continually explored to advance the functionality, bringing increased durability and reducing the cost, which would substantially open up the end use possibilities. These methods include the use of Chemical Vapour Deposition (CVD) as a means to impact nano coatings using plasma processing. More information and links available in the Innovations tab. 

Conductivity is the allowance of electrical current to flow. Resistance refers to a materials resistance to allow this electrical flow, therefore conductive materials have low resistance and insulators have a high resistance. 

Fig 1

Figure 1. Depiction of an atom. Image adapted from; www.sciencepages.co.uk http://www.richardanderson.me.uk/keystage4/GCSEChemistry/m3revisionnotes.php

Electricity is a transfer of electrons that results in positive and negatively charged materials. An atom consists of positively charged protons and negatively charged electrons, generally each atom has an equal amount of protons to electrons so that overall the atom remains neutral. However electrons can become unattached from their atom and flow through a material as electricity, creating negative and positively charged materials. These positive and negatively charged materials will then be attracted to one another, and as everything in nature does, want to again reach an equilibrial, neutral state.

Not all atoms readily give up their electrons, and the mechinsims that bind atoms together can vary greatly therefore the ability to allow a flow of electrons is not equal across all materials.

Metal has a unique metallic bonding system where the electrons of the atoms are shared by the nucleus of all atoms in the material, creating a sea of electrons, as seen in figure 2 allowing for their easy flow. Some materials do not allow for electron flow, and electrons are held more tightly to their nucleus.

Heat inhibits electrical flow thus increasing a materials resistance. This occurs because when atoms are heated they begin to vibrate and move, thus creating moving obstacles that inhibit the free flow of electrons. 

Fig 2

Figure 2. Image from The Hitchhiker’s Guide to Ionic, Covalent and Mettalic Bonding  

(http://apocketmerlin.tumblr.com/post/14579062204/the-hitchhikers-guide-to-ionic-covalent-and-metallic)

Short animated video explains metallic bonding

Video on Conductors and insulators (Click lionk below)

http://www.youtube.com/watch?v=s-Ughfpes0o&feature=channel&list=UL

 

A Dielectric is a material which is classed as an Insulator? as it does not allow for a flow of electricity, however when placed in an electrical field polarisation will occur where electrons are displaced, but remain around their nucleus, creating positive and negative fields. The level to which this polarisation occurs is expressed by a numerical value, the Dielectric Constant?.  

Atoms, as explained in basic principles, consist of a nucleus containing neutral neutrons and positive protons, and an outer cloud of negatively charged electrons which travel around the nucleus, this movement allows for the displacement, as shown in figure 3 which creates positive and negative ends of the atom.

Fig 3

Figure 3. Demonstrate polarization in a atom. From: http://www.technologyuk.net/electronics/electrical_principles/electrostatics.shtml

The Dielectric Constant? of materials is largely spoke about in terms of their use in a capacitor, which is a device used to store energy. The higher the Dielectric Constant? of the insulative material used in the capacitor, the more charge can be stored, which is why Dielectric Constant? is largely associated with capacitors however it is a material property in its own right, determined by its chemical structure.

There has been a great deal of experimentation into the methods of measuring the constant, this is usually undertaken by placing the material to be measured into a capacitor. The presence of the Insulator? in the capacitor will increase its capacitance, and this can be used to calculate the materials Dielectric Constant?.

Measurement of the Dielectric Constant? is useful in gauging information such as; Moisture content?, static generation, fibre structure and dielectrical properties have been demonstrated to be linked to mechanical and thermal properties.

Temperature has been show to greatly affect the result, and the accuracy when measuring fabrics has been question due to the presence of air and moisture within the structure.

In a recent review article published in the Indian Journal of Fibre and Textile Research Bal and Kothari  concluded that due to the lack of homogeneity in textiles fibres such as Moisture content?, amorphous/crystalline regions, etc., exact values of dielectric parameters are still not conclusively found. 

The below video explains with diagrams the Dielectric Constant?:

 

You-tube video, how a capacitor works

 

Carbon, at an atomic level, has the potential to be a good Conductor?, due to its availability of valence electrons, which can transfer to other materials to create an unequal proton to electron ratio and therefore charges. However not all allotropes of carbon are conductive, it depends upon the molecular structure. If all valence electrons are taken up in the bonding, there are none available to be transferred in the creating an electrical current. For example in a diamond structure all 4 of the carbon atoms valence electrons are covalently bonded to other carbon atoms, therefore none are available to transfer. However, as illustrated in figure A, this is not the case in graphite, as each atom is only attached to 3 other carbon atoms, leaving one available for transfer. 

Fig A

Figure A. Display molecular structure of Diamond and Carbon

Fig B

Figure B. Displays the molecular structure of a carbon nanotube. Image from www.topnews.in   http://topnews.in/proteinrecognizing-polymer-coated-carbon-nanotubes-may-identify-proteins-2265676

 

Figure B displays the molecular structure of a carbon nanotube, which demonstrates a structure similar to that of graphite, with each atom bonded to three other atoms, however the cylindrical structure allows for electron transfer throughout the structure, equating a good Conductor?. Carbon nanotubes are the basis of the carbon fibre, a zoom in look at the carbon fibre is shown in the below video. 


Zoom into a Carbon Fiber by Weird_Weird_Science

Video from: www.explainthatstuff.com http://science.wonderhowto.com/how-to/explore-atomic-structure-carbon-fiber-160131/

Where conductivity is applied as an anti-static or to enable safe electrostatic discharge, information can be found in the Anti Static section (LINK TO)

All products placed for sale within the EU must conform under the general product safety directive

http://eur-lex.europa.eu/LexUriServ/LexUriServ.do?uri=OJ:L:2002:011:0004:0017:en:PDF

As conductive properties are usually applied to textiles to create the function of a smart textile, they will fall under the regulation as such. However the regulation on smart textiles is currently being investigated. A working group on smart textiles is currently assessing the need for standards and regulations on smart textiles looking at;

  • Functionality
  • Safety
  • Durability

Including defining smart textiles and the integration with other Directives.

http://www.cen.eu/CEN/Sectors/TechnicalCommitteesWorkshops/CENTechnicalCommittees/Pags/Standards.aspx?param=6229&title=CEN%2FTC+248

If chemical compounds are used these must conform under the REACH regulation.

Test Standards

BS 6524;1984

Determination of the surface resistivity of a textile fabric

A method for determining the surface resistivity of a textile fabric, intended for low or moderate surface resistance. Measures specific resistivity measured along the surface of the textile in ohms