In recent years there has been a proliferation of new and low cost plastic portable containers. Containers ranging in size from 1/4 gal. bottles, to 55 gal. drums and 260 gal. IBCs have provided the supply chains of the hazardous process industries with a diverse range of material packaging options. While some packaging options will require plastics that demonstrate specific levels of material compatibly with different products, one of the major drivers of plastic packaging is their relative low cost in comparison to metal containers including metal drums and metal IBCs. The increasing use of plastic containers within the hazardous process industries is coming under increasing scrutiny due to the hazards associated with static electricity. This brief article will address the issues associated with static electricity on plastic packaging, draw on reports and expertise of industry and safety bodies and provide solutions to grounding non-metallic containers, with a particular focus on composite drums and IBCs.
Defining the meaning of the terms “static dissipative”, “conductive” and “insulating”.
It is important to define the terms “conductive”, “insulating” and “static dissipative” (anti-static) in order to fully appreciate the capability of materials to safely dissipate electrostatic charges from objects that are correctly earthed (grounded). Conductive materials permit the transfer of electrostatic charges instantaneously. In static dissipative materials, electrostatic charges are adequately dissipated, albeit at a slower rate than conductive materials. In insulating materials, or to be more precise, poorly conducting materials, electrostatic charges tend to be retained on the material and not readily transferred, even when the material is connected to earth.
Understanding the difference between volume resistance and surface resistance is also important. Resistivity is determined by the intrinsic properties of a material that resist the flow of electrical currents. Volume resistivity, ρ, represents the total resistivity value of a section of material through its entire volume. The overall resistance to charge transfer is calculated by multiplying the resistivity value for the material by its length and dividing by the cross sectional area through which the charge is flowing:
R = ρl/A
For example, the resistance through a large volume of 1 m length by 1m2 cross sectional area of PTFE with a resistivity (ρ) value of 1019 Ω.m, is equal to 1 x 1019 ohms(1). For a similar volume of copper with a resistivity value of 1 x 10-8 Ω.m, the resistance through the copper will be 1 x 10-8 ohms. So even if the PTFE is correctly earthed, charges will experience a very high degree of resistance to their movement to earth, whereas as for metals, charges will experience little or no resistance and be transferred to earth immediately.
Resistance experienced by current flowing through material is influenced by the
resistivity value, ρ, for the material and the length and cross-sectional area of the material.
Surface resistivity, λ, represents the total resistivity across the surface of a material. In essence, a material with a high volume resistivity could be engineered to have a low surface resistivity value, meaning charges that would otherwise not transfer easily through the material, are allowed to transfer across its surface.
Overall surface resistance is calculated in a similar way, where the resistance is calculated from R = λ L1/L2.
Resistance experienced by current flowing across surface is influenced by the
surface resistivity value, λ, of the material and the length and breadth of the material.
In general materials can be segmented into three categories, depending on their volume and surface resistivity values.
Table 1: range of resistivity values for conductive, static dissipative and insulating materials(1).
In regard to electrostatic ignition hazards within hazardous areas the correct use and specification of containers made from conductive, static dissipative and insulating materials is critical to the safety of workers and the processes in which these containers are used.
Testing of composite IBCs and industry guidance.
A report prepared for the Health & Safety Executive in the UK highlights key selection criteria hazardous area operators should take into consideration when using portable containers within hazardous areas(2). The report tested and quantified the levels of electrostatic discharge on containers ranging in size from small 1 litre plastic bottles to 1000 litre rigid IBCs. Rigid IBCs are supplied in a wide range of different materials of construction and can be made of insulating plastic, static dissipative plastic, and insulating plastics surrounded by metal sheet cladding or steel frames. 55 gal. plastic drums were not included in these tests.
The generation and measurement of electrostatic discharge was conducted in accordance with EN 13463-1:2001, “Non-electrical equipment for use in potentially explosive atmospheres. Basic method and requirements”.
Controlled laboratory testing highlighted that levels of electrostatic discharge capable of igniting commonly used gases and vapours is possible from all container types. A plastic composite IBC, manufactured with a static dissipative outer layer was tested and this demonstrated safe electrostatic discharge levels, however, the report does indicate that a representative sample would need to be tested to determine if these characteristics are consistent.
Just some, of a number of the report’s conclusions and recommendations, are listed below:
- “It is very important with all designs that the frame and any other conducting parts are electrically bonded to earth during any operation where electrostatic charging may occur and that they should not be stored on a highly insulating surface unless separately earthed”.
- The earth connection between the frame and conducting parts of taps should be checked at regular intervals.
- Exposed plastic components (e.g. taps and filling caps) should be made of static dissipative materials.
- Metal frames and conducting objects located on IBCs should be “electrically bonded to earth” with a sufficient charge relaxation time permitted.
- A thorough risk assessment should be carried out to determine the most appropriate type of container with a particular focus on electrostatic charging potentials and the presence of flammable gases and vapours in the IIA, IIB and IIC categories.