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Reducing Contamination Using Ionized-Air Blow-Offs


Posted in Cleanroom by bmichaels on October 17, 2014

Ceiling-mounted and overhead ionization systems remove vastly more particles than regular-air blowers do.


Mark Hogsett and Roger J. Peirce

When using air blow-off equipment to clean medical plastics used to manufacture such devices as catheters, syringes, and vials, etc., it is critical to include ionization in the process. Ionized air blow-off techniques are more effective than other methods at removing unwanted particle contamination on such highly charged materials because they can hold much of the contamination to the surface electrostatically.

This article provides data from experiments conducted using both regular air and ionized air blow-off techniques and details the increased effectiveness of the ionized approach when blowing off surfaces. In addition, particle-count data in typical gowning rooms are provided to document the increased particle burden that personnel carry into the cleanroom if ionization techniques are not employed to remove particles and limit them to the gowning room.

Medical Device Cleaning Operations

Two cleaning techniques—alcohol wipes and regular-air blow-offs—are widely used in medical device manufacturing operations, both of which seek to eliminate unwanted particles on device surfaces.

Alcohol Wipes. During manufacturing, alcohol wipes are commonly used to clean plastic devices. This method removes the static charge on the surface temporarily. Once the alcohol wipe-down has been performed, the product is allowed to ‘dry.’ However, at this stage, evaporation and the final rubbing action of the cloth wipe can regenerate the charge, causing the device to rapidly attracts particles again and regenerate particle contamination. To eliminate this recharging effect, alcohol wipe-downs should be performed using ionized air.

Regular-Air Blow-Offs. Medical device products are also commonly cleaned at various stages of the manufacturing process with guns that dispense regular air. Although the sheer force of moving air along the device surface removes some particles, many particles remain statically attached. Moreover, because particles moving across the surface induce triboelectric charging, the product can become even more statically charged than before the air blow-off. As a result, even greater particle attraction forces can be created, increasing contamination. In contrast, ionized air blow-offs are far more effective at removing particles from the device surface than regular-air methods, preventing recharging and subsequent particle attraction.

Ionization

In the presence of a static field, electrostatic attraction can cause small fibers to cling to the surface of a device and stand up perpendicularly. This phenomenon is a telltale sign that static charge is present on the surface.

A 1-mm fiber standing vertically on a charged surface (left) and an FEM showing that fiber on the charged surface.

Figure 1 (left) shows a 1-mm-long clothing fiber standing vertically on a charged plastic surface. This effect is caused by charge migration from the charged surface to the fiber. As the oppositely charged fiber takes on additional electrostatic charge from the surface, a like-charge repulsion effect lifts the fiber away from the surface. The longer a fiber remains on the charged surface, the greater the charge migration and the more likely this phenomenon will be observed. A finite element model (FEM) can be used as a first-order approximation to describe this phenomenon. As shown in Figure 1 (right), both the surface and the fiber are positively charged. The electrostatic field and potential structure show the repulsion dynamic that makes the fiber lift up from the surface.

Air ionization is the most effective method of eliminating static charges on nonconductive materials and isolated conductors. Air ionizers generate large quantities of positive and negative ions in the surrounding atmosphere, which serve as mobile carriers of charge into the air. As ions flow through the air, they are attracted to oppositely charged particles and surfaces. Electrostatically charged surfaces can be neutralized quickly using this technique.

Insulative materials such as plastics, glass, rubber, and ceramics do not dissipate their charge when they are connected to ground. Only bringing air ions close to their surface using ionization equipment removes the charge. Ionization ‘loosens’ the particles on the surface of the device by eliminating the attraction force, enabling airflow to remove most of the adhered particles.

Experiments

Before air blow-off, many charged particles are electrostatically attracted to a plastic surface.

Regular- vs. Ionized-Air Blow-Off. Experiments were performed in a laboratory environment to investigate how many particles were removed using regular- versus ionized-air blow-off techniques. First, a clean 6 x 6–in. piece of regular plastic material was rubbed and charged to more than 20 Kv. Then, it was placed in close proximity to a dusty surface. Oppositely charged particles from the dusty surface were electrostatically attracted to the plastic, as illustrated in Figure 2.

Next, using regular air from a standard air gun set to 22 psi and positioned approximately 18 in. from the target, the surface was blown off for 4 sec into the collection bowl of a particle counter. This instrument recorded the number of all particles collected above 1 µm in size. It was visually obvious at the end of the 4-sec blow-off that approximately only half of the particles were removed from the surface, as recorded in Figure 3). Electrostatic attraction was responsible for particles remaining on the surface. Even much longer regular-air flows of 20 sec and longer could not dislodge the remaining particles from the target. At 4 sec, the point of diminishing returns had been reached.

After regular-air blow-off, approximately half of the particles were removed from the plastic surface.

An ionized air gun was then used to blow off the plastic surface again using the same air pressure, distance, and time parameters as those used during regular-air experiment. As a result, the electrostatic bonds were broken, and it was visually clear that the surface was now virtually spotless, as highlighted in Figure 4.

To confirm the visual observations, a Met One laser instrument was used to gather particle-count data during two separate runs. In the first run, 6304 particles were removed using regular air, and an additional 8927 particles were removed using ionized air. In the second run, 6912 particles were removed using regular air, and an addition 7054 particles were removed using ionized air.

Extrapolating these results to personnel walking into a gowning area, the roughly 14,000 particles measuring 1 µm and larger that were eliminated from a 6 x 6–in. plastic surface using ionization correlate to between 500,000 and 1 million particles on the surface of a person’s body and clothing. Thus, ionization should be used in the gowning area to remove these particles and prevent them from being transported into the cleanroom.

After ionized-air blow-off, the plastic surface was virtually spotless.

Particle Counts in a Gowning Room. Particle counts were measured in a typical medical device facility gowning room before personnel entered a Class 1000 cleanroom, providing a typical example of the particle-count burden associated with street clothes. The measurements were performed without the use of ionization. Because the particle counter was positioned 6 ft above the floor just inside the entrance to the gowning room and inside the entrance to the cleanroom, the number of particles measured varied greatly from the actual number of particles introduced by the staff. The particles were counted while personnel removed cleanroom clothing, but it can be assumed that similar values would be recorded for changing into cleanroom attire. Nine particle count readings were taken as personnel came through the gowning room during a break.

As summarized in Figure 5, light activity in the gowning room resulted in 1523 particles measuring >0.3 µm and 28 particles measuring >10.0 µm. Moderate activity resulted in 2337 particles measuring >0.3 µm and 68 particles measuring >10.0 µm. Heavy activity resulted in 2976 particles measuring >0.3 µm and 165 particles measuring >10.0 µm. In the Class 100,000 cleanrooms used by many medical device manufacturers, particle counts can number into the tens of thousands in the gowning room and decrease dramatically in the cleanroom.

Particle counts in a gowning room without the use of ionization.

Although many particles are produced when personnel gown up and ungown, ionization eliminates the electrostatic attraction that bonds particles to clothing and personnel, leaving only particles that are deposited through standard diffusion or gravity. By removing the electrostatic forces, a strong ionized air shower following gowning can remove particles from cleanroom clothing before personnel enter the cleanroom. It is also critically important for personnel to take an ionized air shower in their street clothes as in order to eliminate as many particles as possible before they gown up. In such environments, ceiling ionization can be used to discharge clothing, personnel, and airborne particles.

Effective contamination control requires that the gowning room as well as the cleanroom workstations themselves be ionized.1,2 Ionization employed only in the production areas can result in increased particle contamination, since local ionization at the workbench can cause personnel to shed particles onto exposed product.

Conclusion

Ionization is a crucial and effective air blow-off technique for minimizing particle contamination. In the experiments described in this article, the use of a regular-air blow-off gun could remove only about 50% of the particles from a statically charged surface. By the same token, both ceiling-mounted and overhead ionization systems at the entrance and exit of a gowning room removed vastly more particles than regular-air blowers did, preventing personnel from transporting particles into the production cleanroom.

References

1. RJ Peirce, “Confronting Static Attraction in Medical Plastics Manufacturing,” Medical Device and Diagnostic Industry 33, no. 8 (2011): 48–52.
2. RJ Peirce, “Ionization in Gown-Up and Product Transfer Rooms,” Medical Device and Diagnostic Industry 35, no. 11 (2011): 36–39.

Mark Hogsett is the application engineering manager at Alameda, CA–based Simco-Ion Technology Group, an ITW company. With more than 17 years of experience in applied electrostatics, he regularly gives technical presentations and seminars on particle contamination for various industries. He is also an active member of the SEMI standards organization, the SEMI ESD Task Force, the Electrostatic Discharge Association, the Electrostatic Society of America, IEEE, and the International Society for Bayesian Analysis. Hogsett graduated from the University of California at Berkeley. Reach him at mhogsett@simco-ion.com.

Roger J. Peirce has been manager of technical services at Simco-Ion Technology Group since 2006. After starting a 13-year career at Bell Labs in 1970, he cofounded Voyager Technologies in 1983 to design innovative ESD test equipment. Then, in 1986, he founded ESD Technical Services, at which he provided electrostatic discharge/electrostatic attraction consulting services to the semiconductor, medical device, and electronics manufacturing communities. Peirce holds 10 U.S. patents and has authored and published more than 20 technical papers on ESD/ESA. He graduated from Fairleigh Dickinson University. Reach him at rpeirce@simco-ion.com.
 

 

 


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