Antistatic Materials - ESD - Definition and Benefits (12 4962)

Electrostatic discharge can be a problem in the manufacturing environment: the hazards range from damage to sensitive components to fire and explosion hazards. There are a variety of options for minimizing its occurrence. Antistatic or ESD materials are one of them. The following article covers causes for electrostatic charge, introduces antistatic materials (ESD materials), and describes general principles for avoiding electrostatic discharge.

How does an electrostatic charge develop?

Electrical charge is a physical conservation parameter. To understand the latter, one must consider matter at the atomic level. Each individual atom consists of an atomic shell and an atomic nucleus. While the main mass of the atomic nucleus consists of positively charged particles (protons) and neutral elementary particles (neutrons), there are negatively charged particles (electrons) in the atomic shell. Equal charges repel each other and opposing charges attract each other. If protons and electrons are present in the same quantity, the external effects of the individual charges present, but separated, in the atom cancel each other out in the charge summation. The atom is electrically neutral. However, for example, if more protons are present than electrons, this is called a positive charge.

The electrostatic charge is closely linked to the electrical charge. If two materials rub against one another, electrons are transferred from one material to the other. Although while grounded, conductive materials simultaneously compensate the electron transfer with free electrons, this is not readily possible for insulated or insulating materials. In these cases, the material that can hold the electrons less well in the atomic shell loses these electrons to the friction partner. The electron-releasing friction partner has a positive charge summation due to electron deficiency, and the partner to which the electrons are transferred has a negative charge summation due to excess electrons. There is a charge difference between the two friction partners. Electrostatic discharge occurs when two objects with different electrical charges come into contact with each other. Electrons are exchanged on contact and electrical breakdowns can occur. The statically charged components discharge.

When discharging a built-up electrostatic charge, high electrical currents can flow, which can even cause flammable substances to burn or explode.

How the electrostatic charge manifests itself depends on the voltage range:

• > 3,000 V: Electrostatic discharge is noticeable, e.g. when touching a metal blade.
• > 5,000 V: Electrostatic discharge is audible, e.g. crackling, when removing clothing.
• > 10,000 V: Electrostatic discharge is visible, e.g. in the form of a spark or flash.

Lightning and sparks occur, for example, when a high field strength is created between objects that carry a different charge. If the voltage is large enough, a sudden charge equalization occurs and an electrical current flows briefly (flash).

The large difference between insulating and conductive materials is caused by the stability or strength of electrons embedded in the atomic shell. While all electrons in an insulator are bonded to their atom, the electrons in metals can also move freely in the de-energized state and jump from one atomic shell to the atomic shell of another atom.

Protection against electrostatic charge

Protection from electrostatic charge is an important aspect of occupational safety, as also explained in our article on safety standards in mechanical engineering. But why is protection so important? Everyone has certainly gotten an electric shock when making contact with metallic objects. However, the higher the electrical charge, the more dangerous these discharges are.

In industrial environments, voltages of up to 10,000 V can easily occur, which can lead to damage or destruction, especially on electronic components. Standard plastics have the risk of being electrostatically charged, as they have a high surface resistance. Problems can occur quickly if electronic components are transported in them. Ongoing miniaturization, especially in the electrical and semiconductor sector, has increased the susceptibility to electrostatic discharge. Static charging of plastics is therefore particularly important. Components that are particularly sensitive to electrostatic discharge are marked with the ESDS symbol for "Electrostatic Sensitive Device":

There is also a risk of fire and explosion in the production of films, semiconductors, or paper. Protection is therefore essential.

Definition: ESD materials and antistatic materials

Fundamentally, antistatic and ESD materials differ in their manner of handling electrostatic charging. For antistatic material, the focus is to prevent electrostatic charging from affecting the electrostatic properties of the material. For ESD materials, electrical conductivity is in the foreground so that electrostatic charges can be discharged particularly quickly. This is achieved by adding carbon. However, this does not mean that antistatic materials cannot also discharge electrostatic charges at the same time; their conductivity is only lower than that of ESD materials.

There are different standards for both material groups. Antistatic materials are generally used in personal protection, in standards such as EN 1149 for antistatic clothing or EN ISO 20345 for antistatic safety shoes. The electrical resistance must be so low that no charging can occur and, for example, sparks can also be prevented. For ESD materials, the focus in on protecting components. EN 61340-4-1 specifies requirements for the conductivity of surfaces and materials. However, there are ESD materials that are used for personal protection, e.g. ESD shoes. For ESD shoes, stricter specifications apply with regard to the approved electrical resistance: They have an electrical resistance between 0.1 megaohms and 100 megaohms, while the range for antistatic materials is 0.1 to 1000 megaohms. ESD and antistatic materials are also used in conductive floor coverings and packaging materials and special coatings, for example.

Depending on the contact resistance, materials can be divided into different categories, which in turn is also crucial in antistatic and ESD technology:

Operating principles

The functionality of the various material categories differs according to the following principles:

• Dissipation: ESD materials guide the created charge in a controlled manner through the material and distribute it to avoid dangerous voltage differences.
• Insulation: Insulating materials prevent the movement of charges, which can be useful in certain areas to protect sensitive components from uncontrolled discharge.
• Rejection: By means of special surface structures and additives, antistatic materials prevent charges from accumulating on the surface.
• Grounding: Conductive materials are connected to ground potential with defined grounding points in order to discharge excess charges in a controlled manner.

Test methods

The electrical insulation behavior of a material can best be determined by determining the surface resistance and contact resistance. The contact resistance, also called electrical resistance, is the resistance to the current flow through a material and is specified in ohms. The contact resistance provides information about how well or poorly a material can discharge charges in the direction of a ground. Readings are taken by attaching one electrode each to the top and bottom of the surface and passing a measurement current through these. The surface resistance in turn describes the electrical resistance on the material surface. The electrical resistance is determined by applying an electrical voltage by means of two parallel electrodes on the surface and by the current flowing through both electrodes.

Design-based approaches for preventing static charges

Static charging can already be effectively minimized or prevented by purposeful design measures on systems, components and critical work areas. Relevant control parameters include:

• Choice of material
• Grounding
• Humidity control
• Ionizers
• Avoidance of friction

Choice of material

In general, the choice between antistatic or ESD materials depends on the application. ESD materials are recommended if an electrostatically sensitive product is to be transported, mounted or processed. If static charges must be prevented from the outset, for example to protect personnel against sparks, antistatic materials are better suited, as they minimize charge buildup. MISUMI has various antistatic materials. Specialty plastics with specific properties can also be suitable materials. Find out more in our article Specialty Plastics for Practitioners.

Electrostatic grounding

If grounded correctly, excess electrical charges can be discharged directly against ground potential. This can be achieved, for example, by special electrically conductive workstation mats, earthing straps, or special grounding points on machines. All conductive components should have a permanent connection to ground.

Humidity control

The higher the humidity, the higher the conductivity of the air and the better excess charges can be released and distributed to the airborne water molecules. If permitted by the manufacturing environment, a higher humidity can therefore be used and monitored. A relative humidity of 50-60% is recommended.

Ionizers

If grounding alone is insufficient, ionizers can be used for electrostatic discharge. They generate positively and negatively charged ion pairs that bind to and compensate for existing static charges in the effective range of the ionizer. Ionizers are often used when working parts made of insulating material are machined to eliminate unwanted static charges.

Avoidance of friction

Friction between insulators or high-impedance surface materials is one of the main causes of static charging. Friction can be avoided by using aids, such as antistatic lubricants or sliding additives. A deeper understanding of the frictional properties of a material can also help to better classify the operational processes. For more information, see friction and dry friction coefficients.

Another option is a special surface treatment, which ensures that a smoother structure is produced. This can be achieved, for example, by polished surfaces or also by special materials such as antistatic silicone or PTFE (Teflon) with conductive fillers.