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Everyone at sometime has experienced a sharp shock, from a door handle or had clothing cling to them on cold dry days, or in air conditioned offices. You might have noticed a crackling and even a bright glow when getting undressed in the dark at home, or your hair might “stand on end”. All of these phenomena are due to a build-up of static charge on your clothing which then discharges when it can find a route to earth through, for example, conductors such as a metal door handle.

It is doubtful any of these would be classed as more than irritating and they are hardly dangerous. Of course that depends on where you are and whether or not there is something which can catch fire. If the spark that gave you a shock when you touched the door handle occurred whilst you were filling your fuel tank, then the circumstances could be far more dangerous; a spark under such circumstances could easily result in a fire and with so much flammable vapour around the filling nozzle at the pump, even an explosion.

A number of such fires have been reported at petrol stations all over the world and initially these were put down to incautious use of a mobile phone at or near the pump. Research has since shown this to be unlikely, at least in most instances, and the more likely cause seems to be the discharge of static electricity from clothing which was “charged” as the driver left his seat and/or re-entered the vehicle and returned to the pump without touching the car body (grounding himself). Under such circumstances, the discharge seems to have occurred as the driver then went to grasp the fuel nozzle which of course was surrounded by a flammable atmosphere of fuel vapour and air.

There are a number of YouTube videos on-line showing these explosions and the damage they can cause. There was even a report some years ago about a man in Australia who set fire to a store carpet as he continually discharged static electricity from his clothing - such was the electrostatic build-up!

Power-Dresser Leaves a Trail of Destruction

Reuters

A man in rural Victoria built up a 40,000-volt charge of static electricity in his clothes as he walked, leaving a trail of scorched carpet and molten plastic and forcing a fire brigade to evacuate a building.

Frank Clewer, who was wearing a woollen shirt and a synthetic nylon jacket, was oblivious to the growing electrical charge that was building up as his clothes rubbed together. When he walked into a building in Warrnambool, the electrical charge ignited the carpet. "It sounded almost like a firecracker", Clewer said. "Within about five minutes, the carpet started to erupt."

Employees, unsure of the cause of the mysterious burning smell, telephoned firefighters who evacuated the building. "There were several scorch marks in the carpet, and we could hear a cracking noise – a bit like a whip – both inside and outside the building", said fire official Henry Barton.

Firefighters cut electricity to the building thinking the burns might have been caused by a power surge. Clewer, who after leaving the building discovered he had scorched a piece of plastic on the floor of his car, returned to seek help. "We tested his clothes with a static electricity field meter and measured a potential of 40,000 volts, which is one step shy of spontaneous combustion, where his clothes would have self-ignited," Barton said. "I've been firefighting for over 35 years and I've never come across anything like this," he said. Firefighters took possession of Clewer's jacket and stored it in the courtyard of the fire station, where it continued to give off a strong electrical field.

So how is static electricity generated? The triboelectric effect

The nucleus of an atom contains protons, having a positive charge, and neutrons, having no charge. Clouds of electrons, having a negative charge, orbit this nucleus. This fundamental structure of atoms allows that the surfaces of all materials will possess electrons. When materials with different concentrations of loosely bonded surface electrons are brought into contact with each other, the surface electrons attempt to balance or become electrically neutral. While the two materials are in contact, the surface electrons are freely exchanged. This “bonding” is most prevalent when the materials have a larger number of free electrons in the atom’s outer shell. When the materials are separated, the transfer of electrons stops, and both materials may be left with electrical charge on their surface. When an object has lost (i.e. positively charged) or gained (i.e. negatively charged) electrons, it has developed a static charge.

Left in the static mode, this charge may increase in size until it can dissipate to another material of opposite charge. A common example of this static charging can be found by rubbing a cotton object (such as socks) across polyester (such as carpet). The person in contact (bonded) with the cotton will develop a charge; “the triboelectric effect”. When that person touches another object (such as a door knob) of lesser charge, the charge is transferred (i.e., neutralized). If the charge is of a high potential, the charge may be able to bridge an air space to dissipate in the form of a spark, which is referred to as an electrostatic discharge.

People frequently develop static charges by walking on insulating floors or carpets as we saw in the Australian example above or by removing synthetic clothing. Regardless of the materials involved, the process of charge generation requires that dissimilar surfaces contact each other and transfer free elections. The resulting separation causes the materials to retain a charge. Whilst this Australian example is an extreme case and thankfully rare, discharges can be damaging to sensitive electronics such as computers and technicians go to a lot of trouble to ensure that they are earthed and do not cause damage to precious data.

Cloth finishers often apply auxiliary materials to clothing textiles to reduce the risk of static generation and it is established that the use of fabric conditioners in the final rinse during domestic laundering greatly reduces the likelihood of static build-up in clothing as these conditioners increase the moisture content of the textile material to a level where static build-up does not occur.

Having said all of this, modern technology is trying to harness this triboelectric effect for the purposes of driving small electronic devices worn in SMART clothing such as small flexible screens, key pads and in one example, a vehicle remote control device to unlock the car simply by touching the garment cuff. The fabric has been designed such that even though it can be cut and sewn or even patched, it retains its ability to generate electricity by the triboelectric effect and power the devices.

For more information on this subject contact:

Robert Croskell BTech CText FTI
Softlines Specialist
t: +44 (0)1379 668625
m: +44 (0)7730 544718
e: Rob.Croskell@sgs.com

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