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Master of Static
Basic Course

Mechanism of Static Electricity Generation

Mechanism of Static Electricity Generation

Master of Static
Basic Course

Do you remember rubbing a balloon on your hair in your childhood? Do you experience pain when you touch a door knob in the winter? How is static electricity generated? This material clearly describes that the underlying cause of static generation is deeper than friction or separation.

1. Invisible static electricity
It is commonly known that static electricity is generated by friction or separation of objects. However, static electricity is generated when objects make contact, before friction or separation occurs. This is called “contact charging.” An object that is not charged with static electricity is electrically neutral. Being charged with static electricity is a state where electric charge was exchanged between materials and the balance between the protons and electrons in a molecule has been lost. Of course, electric charge is not exchanged suddenly through space under normal conditions. When materials make contact, electric charge is exchanged between their surfaces, and static electricity is generated. For example, assume that there are materials A and B as shown in Figure 1. When materials A and B get closer and make contact, electrons are transferred from material A to material B. This is called “contact charging.” In this state, however, it appears that the charge is 0 V and static electricity is not obvious. Consequently, there is no trouble such as “adhesion of foreign objects” and “damage by electrostatic discharge,” which are common problems in manufacturing settings. In fact, this state is the root of static electricity generation and the cause of static electricityrelated problems. When materials A and B then separate, the unbalance of electric charge becomes perceptible, which allows recognition of static electricity. This is the reason why people generally think that static electricity is generated by friction and separation.

Electron transfer Get closer

? ?

? ? ? ? ?

Material A

Material B

? ? ?

Positively charged

Negatively charged

Apparently 0 V

? ? ? ? ?

? ? ? ? ?

Friction/Separation

Contact charging
[Figure 1: Mechanism of contact charging]

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Mechanism of Static Electricity Generation

Master of Static
Basic Course

“Contact charging” is a universal phenomenon. Contact between different materials creates static electricity regardless of their types. This is true for not only solids but also liquids. For example, tap water is considered to be a conductor because it contains many impurities. If water contains few impurities and has no carriers allowing the flow of electrons, such as pure water, it may charge electrically by several kV. Therefore, static electricity is always generated by contact between solid and solid, solid and liquid, and liquid and liquid. It is not an exaggeration to say that perfect elimination of static electricity is impossible unless there is no contact between materials.

2. Method of contact charging
To know the method of contact charging, you need to know the structure of an atom, which is the smallest particle that constitutes a material. As Figure 2 shows, an atom consists of an atomic nucleus containing protons and neutrons, and electrons that orbit around the nucleus.

Atomic nucleus

Electron

[Figure 2: Structure of an atom]

Individual atoms have different levels of energy to attract electrons. The state where two different materials make contact can be considered as the state where two different types of atoms make contact. When two different types of atoms make contact, the atom with a stronger force to attract electrons takes electrons from the atom with a weaker attractive force. The atom that took electrons now has excess negative charge so it is negatively charged; and the atom that lost electrons lacks negative charge so it is positively charged. This is the method of contact charging.

Get closer

Negatively charged

Positively charged

[Figure 3: Exchange of electrons between atoms]
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Mechanism of Static Electricity Generation

Master of Static
Basic Course

3. Characteristics of contact charging
In manufacturing sites, contact cannot be avoided. Static electricity is continuously generated during manufacturing, invisibly causing problems. This section describes the characteristics of contact charging. (1) Static electricity strengthens between materials of different properties. (2) The polarity and amount of generated charge are determined by the combination of materials. (3) Charging occurs at the instant of contact.

(1) and (2) can be explained using the concept of the triboelectric series (See Figure 4). The triboelectric series is a “guide for the phenomenon of electric charging” to know the polarity and amount of charge by experimentally rubbing two materials together.

Positive (+)

Negative (-)

Air

Cotton

Wood

Nylon

Fur

Silk

Amber

Aluminum

Sulfur

Glass

Lead

Steel

Vinyl

Human body

Acetate

Urethane

Tin/Silver

Gold/Platinum

Polyester

Nickel/Copper

Celluloid

[Figure 4: Triboelectric series]

For example, when a glass plate and cotton make contact, the glass plate, which is on the left of the table, is charged positively, and cotton, which is on the right of the table, is charged negatively. When cotton and a Teflon plate make contact, cotton, which is now on the left of the table, is charged positively, and the Teflon plate is charged negatively. Consequently, the polarity of static electricity is determined by the relationship between two materials. Next, when you need to prevent the generation of electric charge and there is a possibility of contact/friction between materials, select materials that are close in the triboelectric series as much as possible. On the contrary, the friction between materials that are far apart in the triboelectric series tends to create a greater charge. Note, however, that the order in the triboelectric series may not always be reproducible. The result varies depending on humidity, temperature, shape, surface condition, and structure of the materials in contact. In reality, it is impossible to eliminate the generation of static electricity from the root cause. (3) can be explained by the difference in the amount of electric charge due to the contact condition. By experimentally comparing the change in electric charge as a result of contact time, it was found that shorter contact time generated a greater amount of electric charge. This means that making contact for a longer time does not cause the transfer of more charge, and that almost all electric charge transfers at the instant of contact.

4

Polyethylene

Silicone

Teflon

Mechanism of Static Electricity Generation

Master of Static
Basic Course

4. Maximum electric charge
If charging occurs when different materials come into contact, then what will the maximum charge be when contact or friction/separation is repeated many times? In fact, electric charge does not increase endlessly, even for a material that is charged easily. When electric charge exists, an electric field is generated around it. An electric field is represented as E = V/d, and when the amount of charge increases, the electric field strengthens. When the strength of the electric field around a charged object increases to 3 kV/m or more, an electrostatic discharge (aerial discharge) occurs and part of the electric charge will be lost to the surrounding air. As a result, it can be said that the maximum electric charge is the upper limit of the charge before an electrostatic discharge occurs.

Electrostatic discharge

No charge

Small charge

Electric field strength: 3 kV/m or more

Maximum charge

[Figure 5: Maximum electric charge]

5. Common measures against static electricity
When objects come into contact, they create static electricity. To prevent the generation of static electricity, you need to reduce the amount of contact between objects or to use materials which are close in the triboelectric series. These measures are effective only in theory, and in principle it is impossible to stop the generation of static electricity in a precise sense. If this is true, then what can be done as a countermeasure to static electricity? In conclusion, the important thing is to eliminate generated static electricity efficiently. This section describes common measures of static elimination. The countermeasures for static elimination can be broadly divided into cases where the target is a conductor and an insulator. [When the target is a conductor] (1) Grounding Connecting a ground wire to a conductor can release generated static electricity to the ground. Advantage: Low cost and easy installation Disadvantage: Not effective for insulators [When the target is an insulator] (1) Using conductive materials Mixing carbon or metal particles into the composition of an insulator allows the use of grounding. Advantage: Grounding can be used. Disadvantage:  The measure cannot be used for finished products without conductive materials. Changing the composition cannot be used for all products.

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Mechanism of Static Electricity Generation

Master of Static
Basic Course

(2) Increasing humidity Increase the relative humidity in the environment to promote aerial discharge of generated static electricity and remove the static charge. Advantage: Static elimination is possible throughout the entire environment. Disadvantage:  The use of moisture may damage the facility. It is difficult to keep the humidity level constant throughout the entire environment, and consequently static elimination cannot be controlled precisely.

(3) Using a static eliminator A static eliminator applies high voltage to its sharp tip to induce corona discharge and generate ions from airborne molecules. When a negatively charged object comes close, the positive ions are attracted to the charged object and electrically neutralize it. After the neutralization, the remaining ions return to the air. Advantage:  Precise static elimination of ±30 V or less is possible. Applicable to all objects regardless of whether they are insulators or conductors. Countermeasure can be used for the processing of finished products. Disadvantage: A static eliminator is costly.
Static elimination in a cell production process

In production sites, there are various materials, both conductors and insulators. In an environment with only conductors, static elimination is quite easy by just grounding. In reality, insulators are usually present and grounding is not a sufficient countermeasure. To completely eliminate problems caused by static electricity, you need to consider the type of static elimination according to your site.

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KEYENCE STATIC ELIMINATOR LINEUP

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The information in this publication is based on KEYENCE’s internal research/evaluation at the time of release and is subject to change without notice. Copyright (c) 2013 KEYENCE CORPORATION. All rights reserved. MasterofStaticB-KA-EN-US 1103-1 E 611797 Printed in Japan
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