Static electricity in solid dosage manufacturing

February 1, 2007
Fred A. Rowley
Pharmaceutical Technology Europe
Volume 19, Issue 2

The same phenomena that create lightning and thunderstorms are around us every day, producing incredibly high voltages, which cause sparks and shocks. Static electricity is a mighty force. Each year excessive electrical charge build cause explosions in the grain industry.1 Look around any flammable storage area and you will see both grounding bars on the wall and cables, from the grounding bars connected to the drums of solvents. Take any material safety data sheet (MSDS) for a powder and look in section V; it highlights that any dry powder has the potential to attract and store a charge.

The same phenomena that create lightning and thunderstorms are around us every day, producing incredibly high voltages, which cause sparks and shocks. Static electricity is a mighty force. Each year excessive electrical charge build cause explosions in the grain industry.1 Look around any flammable storage area and you will see both grounding bars on the wall and cables, from the grounding bars connected to the drums of solvents. Take any material safety data sheet (MSDS) for a powder and look in section V; it highlights that any dry powder has the potential to attract and store a charge.

Yet our understanding of the role of a static electricity build, even in simple blending and handling,2 and its resulting aberrant potency and content uniformity data is under-appreciated on the production floor in both the pharmaceutical and nutritional supplement industries.

A review of the problem

Static electricity is everywhere. Consider the hazards of static electricity in the hospital operating room, the local gas pump. Static electricity can be generated simply when opening a door in our carpeted living rooms or walking around a nonconductive floor. Other industries, such as the electronics industry, use an electrostatic discharge (ESD) shield bag when storing components. Further, employees must wear heel grounders and wrist straps, and the safety checks, procedures and audits that determine how effective they are as a system are continually challenged. The audits may be conducted by an National Association of Radio and Telecommunications Engineers (NARTE)-certified EDS engineer.3

Figure 1 Thieves must be verified for proper installation and grounding prior to use.

On the production floor, consider asking any process operator if he or she has ever been 'zapped' by static electricity when their gloves come in contact with a double-lined plastic bag that lines drums of granulation and tablets and you will discover that operators deal with static electricity every day. The bag manufacturers themselves try to minimize (but cannot totally prevent) static charge build up on the bag rolls.4 Picking up a common plastic bag from a bench generates 1200–20000 volts of static build. Static electric charges can build up on trolleys (tote bins) as you push them around. The charge is usually generated by the combined movement of the trolley wheels and your foot action as you walk — thousands of volts may build up on your body and on the trolley. The charge remains unnoticed until you touch something, which closes the loop, giving the characteristic shock.5 These events are common and drive our point for consideration.

Electric static charge build and blended powders

Static electricity exists whenever there are unequal amounts of positive and negative charged particles present. It does not matter whether the region of imbalance is flowing or still. Only the imbalance is important, not the 'staticness'.6

Tips for grounding

All solid objects contain vast quantities of positive and negative charges whether the objects are electrified or not. When these quantities are unequal, we say that the object is 'charged' or 'electrified'. Because static electricity is actually an imbalance in the quantities of positive and negative, it is wrong to believe that the phenomena has anything to do with lack of motion, with being 'static'. In fact, static electricity can easily be made to move along conductive surfaces. When this happens, it continues to display all its expected characteristics as it flows, though it does not stop being 'static electricity'. As negative particles are pulled away from the positive particles, equal and opposite areas of imbalance are created (ibid.).

Triboelectric charges (a transfer of electrical charges) occurs when two or more materials are in contact and then separated.7 One material acquires an excess of negative ions while the second, third or fourth material acquires an excess of positive ions; the force, pressure and separation of these materials are the major causes of industrial static electricity.

Figure 2 Tote bins must be properly grounded.

The interparticle forces of these powders are extremely large and cause clumping, sticking and channelling when attempts are made to fluidize them. Although flow problems can be experienced in the transport of free-flowing solids, they pale in comparison to the problems experienced with the flow of cohesive powders. Interparticle forces accounted for 70–80% of cohesive behaviour while mechanical interactions accounted for 20–30%.8 Thus, most nonpharmaceutical process designers have shied away from using cohesive powders. However, these powders are often produced in processes by attrition, and their presence must be acknowledged and addressed for successful processing. The majority of the active pharmaceutical ingredients (APIs) used in pharmaceutical tablets and capsules would be classified as cohesive powders.

The nature of APIs

Most APIs are fine powders. Many are, in fact, micronized powders that provide for rapid dissolution profiles. The advantages these powders provide for the end-user can be very problematic when a firm attempts to validate the content uniformity of blender powders and does not consider the potential electrical aspects of fine powder. This becomes even more challenging when the powders are both micronized and cohesive.

Figure 3 Connect and ground the bin then open the valve and move the powder into the transition piping.

What exactly are cohesive powders?

Cohesive powders are composed of fine particles that tend to attach to one another. Besides particle size, powders may also be classified as 'free-flowing' or 'cohesive'. Colijn defines a cohesive powder as particles generally less than 140 mesh or 100 μm in size.9 One might very well use this definition for many APIs on the market as tablets and capsules. It is, however, important to note that not all cohesive powders will attract and hold an electrical charge; but most cohesive powders do exactly that.

How does one know that the API or the final blend is cohesive? Confirmation of the particle size distribution may be achieved by applying the Hausner ratio test. The Hausner ratio test is particularly suitable for assessing both the flow and cohesivity of both the active ingredient and the final blend. According to the Hausner criterion, an API or a final blend that has a value greater than 1.40 is deemed cohesive and likely to hold an electrical charge.10,16 Muzzio et al. took the position that the pharmaceutical industry as a whole was relatively unattentive to how difficult precision sampling of cohesive powder can be.16

The ability of a solid to transmit electric charges is characterized by its volume resistivity.11 Powders may be conveniently divided into the following three groups:

  • Low-resistivity powders having volume resistivities in bulk up to 108 ohm-m. Examples include metals, coal dust, and carbon black.

  • Medium-resistivity powders having volume resistivities of 108–1010 ohm-m. Examples include many organic powders and agricultural products.

  • High-resistivity powders having volume resistivities above 1010 ohm-m. Examples include organic powders, synthetic polymers, and quartz [NFPA 77 8.4.2.1].

Powders that are free-flowing and are noncohesive have low resistivity. An individual powder, whether present in small or large quantities, but having medium or high resistivity will be prone to effects of static charges and can become cohesive or charged during flow. The charge rapidly dissipates when the powder is conveyed into a storage device or container that is grounded. However, if conveyed into a nonconductive container, the accumulated charge can result in a small spark as the charge in the dust and powder attempts to equalize potential differences during this process (ibid.). The powder blend may also adhere to the container wall causing a slight to high increase in tablet or capsule content as the batch progresses and the active increasingly attaches to the vessel.

Powder blends may collectively take the profile of an individual active ingredient, regardless of the overall API content and exhibit medium to high resistivity. The charge rapidly dissipates when the powder is conveyed into a storage device or container that is grounded.

It is important to recognize that mixing cohesive actives with free-flowing excipients will produce a different set of expected difficulties than mixing cohesive actives with cohesive excipients. When a small amount of cohesive active is blended into a much larger amount of a free-flowing excipient, it is imperative to ensure that the agglomerates of the active are effectively comminuted and dispersed into the mixture. When the active and excipients are both cohesive, micromixing (i.e., agglomerate breakup by application of shear force) will be the major performance-limiting phenomenon and must be accounted for throughout the mixing process.12

If conveyed into a nonconductive container, such as a blender, drum, tote bin or tablet press, an accumulated charge can result in microsparks as the charge in the dust and powder attempts to equalize potential differences during this process by drawing together. We commonly refer to these pockets as 'hot spots' and they may in part account for the 'erroneous numbers' we sometimes see and cannot understand in our content uniformity results from the laboratory.

Powder then transiting through a closed feeding system to the die table may attract and retain an electrical charge through contact with container and piping walls. This may take the form of a film inside the tote bin, the transition pipe, the plastic bag in a drum or in the powder hopper on a tablet press. It is easily dissipated when the container becomes grounded.

Electrostatic charge insulation, grounding and discharge

Conductors can be insulated (e.g., a jacketed or plastic-coated cable) or uninsulated (i.e., bare conductors). Uninsulated electrical conductors (wires) are recommended, because it is easier to detect defects in them. Charged insulators may attract and retain a charge for multiple hours or even days. Further, opposite charges may exist on an insulator simultaneously. Charges of either polarity will not migrate on an insulator. Grounding an insulator will neither remove nor prevent the accumulation of surface charges. The charge of one of either polarity can remain on a conductor as long as the conductor remains isolated from the ground. But a charged conductor will discharge completely when grounded.13

How do we know when a bin, mixer, drum or thief is in fact grounded? Where the bonding/grounding system is all metal, resistance in continuous ground paths will typically be less than 10 ohms (ibid.).

For all practical purposes, when the terms 'discharge' or 'grounding' are used, they should be thought of as including a connection or path to the earth to put electrically conductive materials at the same potential as the earth.

Pulling it all together

Early in product development, a determination of potential cohesivity and static charge attraction must be made. If there is an issue, the firm must begin planning for the day validation and commercial manufacturing commences. Maintaining a high level of attention throughout the scale-up and commercialization process helps prevent procedural mistakes that commonly result in erroneous data. While on the production floor, there are certain procedures that will help the overall effort:

  • A basic fact is to maintain the relative humidity (RH) in the room and in the containers at or above 35% RH and static charges are frequently automatically drained off to a ground. There is usually no need for grounding straps or ionizers.14 However, do not assume that the conditions inside the blender, the tote bin or the drums conform to this guideline or that these conditions were met during both blending and handling.15

  • Most powders meant for tablet compression are, in fact, dry powder with moisture content at or below 5% and they may potentially attract and retain an electrical charge. This is especially true if there is an interaction between a cohesive powder, poorly grounded equipment, bag-lined totes or drums and a sampling thief that is ungrounded.

  • Our recommended procedure involves the following key steps, usually contained in a standard SOP in the granulating, blending and tablet compression/powder encapsulation departments:

  • Use a static meter to determine the polarity and severity of the static charge on a surface. The basic goal here is to create a path of dissipation that will not be subject to the negative effects of pressure piling of electrons. A Static Charge Meter is a noncontact, low cost, static monitoring and locating instrument. It utilizes a centre-zero meter display to indicate the magnitude and polarity of the electrostatic charge.

  • Treat grounding cables and powder thieves with care. Grounding cable are to be used for drums, totes, transition tubes and tablet presses/encapsulators.

Conclusion and summary

Static electricity is real. Cohesive powders can and do attract and retain small electrical charges. Proper grounding and sampling procedures are necessary to prevent problems. Understanding the nature of static electricity and cohesive powders, and their potential role in solid dosage and hard-shell capsule manufacturing will enable a company to deal with the problem long before commercial manufacturing begins.

Fred A. Rowley is a guest lecturer at Solid Dosage Training Inc. (USA).

References

1. A. Castellanos, "Open Problems in Powder Mechanics," Thesis abstract, University of Sevilla, Avenida Reina Mercedes, Sevilla, Spain.

2. R.E. Freeman, "The Flowability of Powders — an Empirical Approach," International Conference on Powder and Bulk Solids Handling (London, UK, 2000).

3. National Association of Radio and Telecommunications Engineers standardized tests ANSI/ESD 20.20. www.narte.org

4. "Static Electricity Stays on the Bags and Rolls Down to the End User," Ion Systems Industrial. www.ion.com

5. J. Smallwood, "Static Electricity", white paper. www.electricstatics.com

6. "Misconceptions of Static Electricity", Science Education Center. www.sec.org.za

7. L.G. Britten, Avoiding Static Ignition Hazards in Chemical Operations, (John Wiley & Sons Ltd, UK, 1999).

8. V.G. Reiling, "The Effects of Ultrafine Particles on Powder Cohesion and Fluidization," Thesis, Case Western Reserve University, Chemical Engineering, Cleveland, OH, USA (1992).

9. M.J. Johnson, "Static Protection Through Bonding and Grounding", International Association of Electrical Inspectors, Newsletter, May/June 2004 www.iaei.org

10. H.H. Hausner, Int. J. Powder Metall., 3, 7–13 (1967).

11. H. Colijn, Bulk and Powder Magazine, June 2004. www.powderandbulk.com

12. A. Alexander, Pharm. Technol., 28(9), 54–74 (2004).

13. A. Hashish, "Is Static Electricity Static?" Thesis, Department of Physics, Faculty of Science, United Arab Emirates University, Al-Ain, United Arab Emirates.

14. "The Use of Himidifiers in Industrial Situations," Carel USA. www.carelusa.com.

15. S. Kamath and V.M. Puri, Powder Tech., 102(2), 184–193 (1999).

16. F.J. Muzzio et al., Int. J Pharmaceutics, 250, 51–64 (2003). www.sciencedirect.com