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08 October 2009

Static Electricity and Petrol Stations

In an earlier post, we looked at how filling a metal container on a vehicle (e.g. a truck) can lead to a build up of static electricity. The static electricity cannot be discharged when it is on the vehicle. As a result, a build up of static of electricity ensues. When another element is nearby, the static electricity build up may become discharged. It is common in the discharge of static electricity that a spark is formed. When a spark is in the vicinity of petrol vapours, the petrol vapours may become ignited.

Static electricity is an everyday occurrence. To understand the phenomena of static electricity, one should understand that atoms, the basic building blocks of the universe, are composed of protons, neutrons, and electrons. Protons are positively charged, and sit in the center of the atom with the neutrons, forming what is known as the "nucleus". Electrons orbit the nucleus. Electrons are negatively charged. When two objects come into contact, electrons may move from one object to another, leading to a charge building up. (Ref: ScienceMadeSimple.com, Static Electricity. Accessed: 8th October 2009)

Triboelectric Effect

Most sparks caused by the discharge of static electricity can be considered a direct outcome of the triboelectric effect. When two objects come into contact, electrons may flow from one object to another. The question is which object will be likely to lose its electrons, and which object will be likely to gain more electrons? The outcome will determine which object will become positively charged, and which object will become negatively charged. Scientists, working empirically by observation and deduction, have compiled the "triboelectric series" which rank, in a hierarchy, objects that will likely lose their electrons to other objects. A simple example of the "triboelectric series" is as below:


TRIBOELECTRIC SERIES
your hand
glass
your hair
nylon
wool
fur
silk
paper
cotton
hard rubber
polyester
polyvinylchloride plastic

(Source: ScienceMadeSimple.com, Static Electricity. Accessed: 8th October 2009)

A more in-depth example is the following table. The following table was reproduced from the website of AlphaLab Inc, which produces electricity meters. (Ref: AlphaLab Inc., TriboElectric Series. Accessed: 8th October 2009.) These include static electricity meters.

Column 1 (this col.): Insulator name. Col.2: Charge affinity in nC/J (nano ampsec/wattsec of friction). Col.3: Charge acquired if rubbed with metal (W=weak, N=normal, or consistent with the affinity). Col.4: Notes.
Affinity
nC/J
Metal
effect
Triboelectric Table
Tests were performed by Bill Lee (Ph.D., physics). ©2009 by AlphaLab, Inc. (TriField.com), which also manufactured the test equipment used. This table may be reproduced only if reproduced in whole.
Polyurethane foam
+60
+N
All materials are good insulators (>1000 T ohm cm) unless noted.
Sorbothane
+58
-W
Slightly conductive. (120 G ohm cm).
Box sealing tape (BOPP)
+55
+W
Non-sticky side. Becomes more negative if sanded down to the BOPP film.
Hair, oily skin
+45
+N
Skin is conductive. Cannot be charged by metal rubbing.
Solid polyurethane, filled
+40
+N
Slightly conductive. (8 T ohm cm).
Magnesium fluoride (MgF2)
+35
+N
Anti-reflective optical coating.
Nylon, dry skin
+30
+N
Skin is conductive. Cannot be charged by metal rubbing.
Machine oil
+29
+N

Nylatron (nylon filled with MoS2)
+28
+N

Glass (soda)
+25
+N
Slightly conductive. (Depends on humidity).
Paper (uncoated copy)
+10
-W
Most papers & cardboard have similar affinity. Slightly conductive.
Wood (pine)
+7
-W

GE brand Silicone II (hardens in air)
+6
+N
More positive than the other silicone chemistry (see below).
Cotton
+5
+N
Slightly conductive. (Depends on humidity).
Nitrile rubber
+3
-W

Wool
0
-W

Polycarbonate
-5
-W

ABS
-5
-N

Acrylic (polymethyl methacrylate) and adhesive side of clear carton-sealing and office tape
-10
-N
Several clear tape adhesives are have an affinity almost identical to acrylic, even though various compositions are listed.
Epoxy (circuit board)
-32
-N

Styrene-butadiene rubber (SBR, Buna S)
-35
-N
Sometimes inaccurately called "neoprene" (see below).
Solvent-based spray paints
-38
-N
May vary.
PET (mylar) cloth
-40
-W

PET (mylar) solid
-40
+W

EVA rubber for gaskets, filled
-55
-N
Slightly conductive. (10 T ohm cm). Filled rubber will usually conduct.
Gum rubber
-60
-N
Barely conductive. (500 T ohm cm).
Hot melt glue
-62
-N

Polystyrene
-70
-N

Silicones (air harden & thermoset, but not GE)
-72
-N

Vinyl: flexible (clear tubing)
-75
-N

Carton-sealing tape (BOPP), sanded down
-85
-N
Raw surface is very + (see above), but close to PP when sanded.
Olefins (alkenes): LDPE, HDPE, PP
-90
-N
UHMWPE is below. Against metals, PP is more neg than PE.
Cellulose nitrate
-93
-N

Office tape backing (vinyl copolymer ?)
-95
-N

UHMWPE
-95
-N

Neoprene (polychloroprene, not SBR)
-98
-N
Slightly conductive if filled (1.5 T ohm cm).
PVC (rigid vinyl)
-100
-N

Latex (natural) rubber
-105
-N

Viton, filled
-117
-N
Slightly conductive. (40 T ohm cm).
Epichlorohydrin rubber, filled
-118
-N
Slightly conductive. (250 G ohm cm).
Santoprene rubber
-120
-N

Hypalon rubber, filled
-130
-N
Slightly conductive. (30 T ohm cm).
Butyl rubber, filled
-135
-N
Conductive. (900 M ohm cm). Test was done fast.
EDPM rubber, filled
-140
-N
Slightly conductive. (40 T ohm cm).
Teflon
-190
-N
Surface is  fluorine atoms-- very electronegative.

Dear Reader,

You may now be wondering: What in the world can this table be used for? From the website of AlphaLab Inc:

This table can be used to select materials that will minimize static charging. For example, if uncoated paper (with a positive charge affinity value of +10 nC/J) is squeezed by a pinch roller made of butyl rubber (@-135 nC/J), there will be about 145 pico coulombs of charge transfer per joule of energy (associated with pinch and friction). This is about 20 times more than 7 nC/J, which is the static charge per joule that results from squeezing paper with a roller made of nitrile rubber (@+3 nC/J). In general, materials with an affinity near zero (e.g. cotton, nitrile rubber, polycarbonate, ABS) will not charge much when rubbed against metals or against each other.

In short, you can use it to design your products, packaging, etc. to prevent (as much as possible) build up of static electricity.

From Wikipedia's entry on the Triboelectric Effect:

The triboelectric effect is related to friction only because they both involve adhesion. However, the effect is greatly enhanced by rubbing the materials together, as they touch and separate many times. ...

Because the surface of the material is now electrically charged, either negatively or positively, any contact with an uncharged conductive object or with an object having substantially different charge may cause an electrical discharge of the built-up static electricity; a spark. ... Low relative humidity in the ambient air increases the voltage at which electrical discharge occurs by increasing the ability of the insulating material to hold charge and by decreasing the conductivity of the air, making it difficult for the charge build-up to dissipate gradually. ... car travel can lead to a build-up of charge on the metal car body (which acts as a Faraday cage). When the driver alights, sparks jump from frame to driver as he makes contact with the ground.

In other words, the very act of travelling in your car can cause the car to become electrically charged! Also, very interestingly, you may also note that humidity does play a role in determining the voltage at which discharge of static electricity occurs.

Countering Static Electricity

Mist Fans!

Those readers who have stepped on a treadmill at their local gym may have experienced a shock from static electricity build-up. In most cases, you would notice that the treadmill is grounded. However, static electricity build-up can still occur because the tread (which which is made of non-conductive material) can act as an insulator and can prevent discharge of static electricity. In one forum discussion, it was suggested that "spraying a mist of water" could help to discharge the static electricity build-up. (Ref: PhysicsForums.com, 25th December 2006. How can static electricity shocks from treadmill be prevented?)

It seems that the above suggestion may be of valid consideration. One manufacturer of mist fans in Shanghai, China, is marketing a "high pressure misting fan" which, among others, claims to "prevent static electricity". (Ref: Shanghai Airequip Engineering Co Ltd, High Pressure Misting Fan. Accessed: 8th October 2009.) Perhaps in the future, petrol stations would be well advised to install mist fans to prevent static electricity. Not only would these mist fans reduce static electricity, customers would also benefit from the added cool during hot afternoons, and the mist also traps dust particles in the air (hence, the air becomes less dusty).

Making Surfaces Slightly Conductive

As build-up of static electricity causes a discharge when the charged object touches another object, the important issue is how much charge is discharged. If the build-up of static electricity carries on for some time, naturally the build-up will be more. The electricity discharged will also be more, and thus more likely to cause a spark. Hence, one strategy would be to render more surfaces conductive, or slightly conductive. Material used to coat surfaces to render them conductive, or slightly conductive, is known as antistatic agent. (Ref: Wikipedia, Antistatic Agent. Accessed: 8th October 2009)

This particular suggestion is not quite useful. Perhaps it would be helpful if petrol stations were to sport a little signboard next to each petrol pump, stating: "TOUCH YOuR HAND HERE TO DISCHARGE YOUR STATIC ELECTRICITY". Such a signboard would, of course, need to be (a) conductive. and (b) grounded. So far, I have yet to see any such signboards. Another possible innovation may even be a "grounding clip" which can be attached to a vehicle, to discharge any static electricity build-up. Such a practice (grounding) would be common when airplanes are being filled with fuel, but not so for the ordinary petrol station which serves motorists. (Ref: Alaska Division of Forestry, Aviation Fueling Handbook. URL: http://www.experimentalaircraft.info/doc/aviation-fueling.pdf. Accessed: 8th October 2009) (Ref also: Experimental Aircraft Info, Aircraft Refueling Safety, Grounding and Test Procedures. URL: http://www.experimentalaircraft.info/homebuilt-aircraft/aircraft-refueling.php. Accessed: 8th October 2009.)

Adding Antistatic Agents To Fuels

From the Wikipedia article on Antistatic Agents:

Antistatic agents are also added to some military jet fuels, to impart electrical conductivity to them and avoid buildup of static charge that could lead to sparking igniting the fuel vapors. Stadis 450, with dinonylnaphthylsulfonic acid (DINNSA) as the active ingredient, is the agent added to some distillate fuels, solvents, commercial jet fuels, and to the military JP-8 Stadis 425 is a similar compound, for use in distillate fuels and solvents.

Naturally, very few civilians use military jet fuels. Some rich tycoons, like a certain Mr Tiong from Sarawak, manage to own private jets. (Ref: The Star Online, 6th October 2009. Police freeze Tiong's jet and properties pending investigations.) But since the majority of Malaysians use cars, and motorcycles, it would be logical to request petrol companies to include antistatic agents in the formulation of petrol which is dispensed at the filling pump. However, in one engineering forum, it was stated that antistatic additives in fuel tend to reduce the effectiveness of the fuel. (Ref: Eng-Tips Forums, 24th Feb 2009. Standard Petrol - Anti Static Additives Or Just Rely On Earthing?)

Grounding And Bonding vs Static Grounding

These are actually two different things. From a blog by Newson Gale, comes this explanation:

1. What is Grounding and Bonding

Grounding & Bonding is a combination of Grounding and Bonding techniques. Bonding is the process of connecting two or more conductive objects together by means of a conductor so that they are at the same electrical potential, but not necessarily at the same potential as the earth. On the other hand, Grounding describes the process of bonding one or more conductive objects to the ground, so that all objects are at zero (0) electrical potential.

2. What is Static Grounding

Static Grounding is the process of bonding one or more conductive objects to the ground, so that all objects are at zero (0) electrical potential, to prevent static build up. In order to secure hazardous areas Static Grounding Systems use explosion protection and intrinsically safe techniques to monitor acceptable resistance to ground and provide verification and interlocks.Static Grounding and Grounding and Bonding.)

Further Reading

Readers are advised to look at AlphaLabs Inc's page on static electricity problems and their fixes. It discusses, among others,
  1. Removing or avoiding charge on materials.
  2. Attraction/repulsion--unintended static forces (e.g., contamination) and intended forces (pinning).
  3. Electrostatic spray painting, powder deposition.
  4. Surface conductivity-- measuring "ohms per square", and making surfaces more conductive.

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