Single and double-layers of charge

When electrostatic charge builds up on an insulating material, it typically does so on one surface as a result of contact and rubbing against other materials, a process called triboelectric charging or tribocharging. In FIBC operations, tribocharging usually occurs when the packaged product moves against the bulk bag during filling and emptying.

There is an electric field associated with electrostatic charge, and this field projects into the air next to the charged surface. As charge builds up, the field strength increases and will continue to increase until the field strength reaches the breakdown value (dielectric strength) for air, which is approximately 3 MV/m (76 kV/in). Any additional charging after this point can cause electrical breakdown and ionization of the air. Ions released in the air will move to opposite polarity ions on the surface and will partially neutralise the charge. This mechanism acts as a charge limiter so that the charge density on a single surface cannot increase any more. In the example in Figure 1, the surface is shown as being positively charged, but it could also be negatively charged, depending on the materials contacting it.

Tribocharging and charge limitation

Figure 1. Tribocharging and charge limitation on single charged surfaces.

In some special cases, it is possible that charge can exist on both surfaces of a material, one surface being positively charged and one surface being negatively charged. In this situation, the electric field is directed through the material between the two charged surfaces and very little field is projected into the air. Insulating materials have a much higher dielectric strength than air. For example, polypropylene has a dielectric strength of 30 to 40 MV/m. Therefore, when a double-layer of charge occurs on a material, the charge density can increase above the value limited by air breakdown (Figure 2).

Single and double-layers of charge

Figure 2. Field configuration for a single and double-layer of charge.

Double-layers of charge on FIBC

Type A and Type B FIBC are made from polypropylene without any charge dissipation mechanisms. It is possible for double-layers of charge to build up on the sides of such bulk bags. Initially, charge can build up on the inside surface during filling or emptying. The field from this single layer of charge will project outwards into the air next to the bag (Figure 3(a)). Any sharp or small radius metal structures near the bag (e.g. screws or rivets in support frames, etc.) will cause the field to concentrate and increase in strength. As charge builds up on the inside of the bag, the field strength will also increase. The field strength near the metal structures can eventually reach the breakdown value for air. At this point air ionization can occur and air ions of opposite polarity to the charge on the inside of the bag will be attracted towards its outside surface (Figure 3(b)). As the bulk bag is made from insulating material, the air ions cannot combine with charge inside the bag. Instead, they will build up on the outside of the bag forming a double-layer of charge (Figure 3(c)).

Double-layer of charge on Type A & B FIBC

Figure 3. Example of how a double-layer of charge can occur on Type A and Type B FIBC.

Electrostatic discharges from single and double-layer charged surfaces

The type of electrostatic discharge that occurs from insulating materials with a single layer of charge is called a brush discharge because of its brush-like appearance (Figure 4). When a brush discharge occurs, a limited area of charge is discharged from the surface of the insulating material. Although the area discharged is relatively limited, the energy released can still be sufficient to ignite flammable gases (e.g. pentane, butane, etc.) and solvent vapours (e.g. toluene, xylene, etc.). For this reason, Type A and Type B FIBC must never be used in hazardous areas containing flammable gases and vapours.

Brush discharge

Figure 4. Brush discharge.

Brush discharges can, in theory, ignite very sensitive flammable powders such as bisphenol-A, but most combustible powders (e.g. sugar, flour, etc.) require more energy to ignite than is released in a brush discharge.

When there is a double-layer of charge on the sides of a FIBC, another form of brush discharge can occur, which is extremely dangerous because it releases a large amount of energy. This type of discharge is called a propagating brush discharge or PBD. It starts as a brush discharge, but then propagates across the entire charged surface area. The energy released in a PBD from one side of a FIBC can be several joules. Not only is this orders of magnitude greater than the minimum ignition energy (MIE) required to ignite gases and solvent vapours (MIE ≈ 0.1 mJ) and sensitive flammable powders (MIE ≈ 1 mJ), it is also sufficient to ignite combustible powders (MIE = 3 mJ to 1J).

The PBD mechanism is explained in Figure 5. When a grounded conductor (e.g. a person’s hand, forks of a forklift truck, etc.) closely approaches or touches FIBC with a double-layer of charge on the sides, a brush discharge occurs (Figure 5(a)). This discharges a limited area of charge from one surface. The charge on the opposite surface remains, but now the field from this charge passes through the air next to the discharged surface (Figure 5(b)). As the field strength is very high, this causes the air to ionise and the discharge starts to propagate out across the surface. This process continues until the entire charged surface area is discharged (Figure 5(c). The whole process takes a tiny fraction second and releases most of the electrical energy stored on the surface.

PBD mechanism

Figure 5. PBD mechanism (cross-section on top, plan view below).

The short animation in Figure 6 shows the PBD mechanism at a very much reduced speed. In reality, it is all over in less than the blink of an eye.

Figure 6. Animated PBD mechanism.

Preventing PBD in FIBC

Propagating brush discharges cannot occur from FIBC with charge dissipation mechanisms, i.e. Type D FIBC and grounded Type C FIBC. The charge dissipation mechanisms in these types of bulk bag prevent charge accumulating and double-layers of charge forming.

When packaging low-sensitivity combustible powders, the full static protection provided by Type D FIBC and grounded Type C FIBC is not strictly necessary. However, protection against propagating brush discharges is necessary. Type B FIBC were developed to meet this requirement.

Essentially, Type B FIBC are the same as Type A FIBC, in that they are made from polypropylene without charge dissipating properties. They prevent propagating brush discharges by allowing breakdown between double-layers of charge before the charge density can reach the level where PBD can occur. This can be achieved by using uncoated fabric in which the air gaps in the woven structure breakdown before PBD occur. Alternatively, fabric with very thin coating can be used, typically less than 24 g/m2. Research has established that if the breakdown voltage of FIBC fabric is less than 6 kV, PBD do not occur. This requirement is specified in IEC 61340-4-4 and other safety standards for static protective FIBC.

It must be remembered that whilst Type B FIBC do provide some static protection, it is limited to preventing PBD. As there is no mechanism for dissipating charge, brush discharges with sufficient energy to ignite flammable gases and solvent vapours can occur from Type B FIBC.

Type B FIBC are intended only for packaging combustible powders, or for packaging non-combustible powders that might be emptied in an environment containing combustible powders or dusts.

In environments where flammable gases or solvents are present, the full static protection provided by Type D FIBC or grounded Type C FIBC is an absolute requirement, as specified in national and international safety standards, including IEC 61340-4-4, IEC TS 60079 32 1 and NFPA 77. And, for applications where reliable grounding cannot be ensured, Type D FIBC such as CROHMIQ® is the only safe option.