Nano-and Micro-Particle Challenges to Effective Containment
Because of comparable behavior of nanoparticles, i.e. particles with at least one dimension less than 100 nm, and particles in the next size range of 100–220 nm median diameter, they are both included in this discussion. The prefix nano- is used when referring to the formal classification of materials and micro- is used for the wider grouping of 1–220 nm.
Microparticles are formed in copious quantities whenever particles are generated under high stress conditions such as heat, pressure, shear force, etc.
In the natural world there are many major contributors – volcanoes, vents, ocean forces, weather conditions, cascading water, etc. Even something as simple as a bubble bursting and liquid splashing through a water film creates significant quantities of microparticles. Without man-made activities, microparticles are both copious and ubiquitous.
In Agriculture, earth moving such as tilling (shear force), pesticide and herbicide spraying (pressure), and grain storage silos generate copious quantities of microparticles.
In Industry, the same forces apply with major emitters being metalworking facilities (steel mills, foundries, refining, machining, welding), plants relying on heating and cooling (sulfuric and phosphoric acid, refrigeration), incinerators and power plants, and of course the fuel engine – even after pollutions controls are instituted.
Many waste treatment (aeration, trickling filters, etc.) and pollution control devices (wet and dry scrubbers, catalytic converters) are themselves responsible for generating masses of microparticles.
Pharmaceutical processing generates microparticles whenever shear forces are applied (milling), air pressurization (liquid filling), and many small-scale development studies.
There are also many microparticle sources in the home such as aerosols, any places where heating and cooling of steam and vapors occurs (cooking, humidification, rapid chilling, freezers). In the garden, spraying and watering are significant sources, which accounts for the taste as well as the smell of herbicides, and casual activities such as pool usage are also culprits.
In spite of the prevalence of microparticle sources, quantitative data is difficult to find. Industry is not prone to publishing seemingly random data unless for patent use. Few Government Agencies fund, or even recognize the need for research, despite the intense interest in environmental factors – possibly because few people recognize the relevance of these materials.
Existing data is scarce and difficult to quantify. In the instances involving major industry and workplace issues in which FabOhio Inc. personnel have participated, the quantitative data confirm a common finding:
Whenever particles are created in the <10 μm diameter range, the number of nanoparticles can exceed the mass of particles >0.2 μm (220 nm) by orders of magnitude.
By merely watching fog formation using a 16 channel optical particle counter, one can see incremental number of particles growing with time, beginning with the 0.2 μm size range and progressively growing in the 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 , 1.0. 2.5 μm and higher ranges hours before the fog actually becomes visible to the naked eye. While the smaller sizes aren’t detectable, it is reasonable to suggest that growth initiates from the molecular water size with progressive development until finally detectable. The driving force – cooling of air saturated with water molecules.
The growth in nanotechnology is one of the fastest seen in many years.
Nanoparticles have come to the fore in a wide range of consumer products, from cosmetics to paints. The biotechnology world is showing increasing success in building molecules based on nanosize backbones to target specific diseases. While development of biologically active products will take years to mature and become of marketable value to patients, improvements in microparticle detection and containment technologies need to meet the current and future demands of the research world. Investment in both medical and industrial nanotechnology research is occurring daily.
Materials handling practices for micro-materials in both industrial and medical environments are dependent on conventional containment practices to minimize contamination and prevent material migration. Sadly, much of the learning in microparticle control, which was generated by major chemical industries in dealing with environmental release issues, has yet to carry over. Industry has learned that controlling microparticle emissions requires an entirely different mind set.
The basic issue is an understanding of the anomalous behavior of microparticles when compared with particles of >0.22 μm median diameter:
- microparticles are smaller than the wavelength of visible light
- optical particle counters only deliver partial counts below 0.2 μm (220 nm) and none below 0.1 μm (Tyndall light scattering)
- no current sampling method can capture and quantify them
- they are not visible using conventional microscopy
- they can be observed using electron microscopy
- when formed, they carry a partial and identical charge
- even when electrically neutralized, the charge is re-established the moment they are disturbed
- settling rates are extremely long
- microaerosols are highly stable
- impaction collection become less effective as size decreases
- membrane filter penetration below 220 nm exceeds 98%
- membrane filters cause liquid droplets to fragment into smaller droplets
- particle size classifiers are ineffective
- Novel, properly designed depth filters are effective in capturing them
- performance of the depth filters (glass and quartz) currently available is hard to quantify
The sum outcome leads to the conclusion that for workers handling microparticle their may be risks which have yet to be accounted for.
a) Reliance on membrane filters for capturing microparticles
The implications are serious if one wishes to capture microparticles, because membrane filters are ineffective as a collection device. This was proven on a quantitative basis when investigating both industrial and worker environments. When challenged with data, the Technology Director of a major membrane filter provider reluctantly admitted that microparticle capture was ineffective using only membrane filters, which is why 0.22 μm was the smallest pore size available. The primary reason that the information was not public was because it had no impact in the food and oil industries where membrane filters were developed for specific applications.
This weakness implies that any worker protection device based on membrane filter technology is ineffective, i.e. respirators and Powered Air Purification Respirators. Which leaves us with air supplied respirators as the only effective protection, but even then the source must be an oil-free system, both for creation and distribution.
Current limitations in conventional filtration systems imply that using either HEPA or UHEPA filtration for both supply and exhaust air is ineffective. They are neither effective at cleaning the air entering a containment enclosure, nor for cleaning the exhaust air. In many cases, when air is exhausted from containment enclosures (flexible and rigid) it is released into the workers’ space. Ventilation recirculation practices merely disperse microparticles throughout a facility. Likewise scrubbers used for conditioning air, and air exhaust baghouses are equally ineffective.
b) Quantifying Microparticle concentrations (Including nanoparticles)
To deal with these issues economically, one has to resort to practices developed in the late 1960’s for pollution control. Dr Joseph Brink researched and published his findings on controlling emissions of liquid aerosols emitted from sulfuric and phosphoric acid manufacturing plants. His finding resulted in development of a patent for Monsanto Chemical Company covering the use of mist eliminators (depth filters) for liquid aerosol control.
With the Brink Mist Eliminator in production, a method of measuring performance was formalized as 999-AP-13 (Air Pollution Control Association). The method segregated large mist droplets, i.e. >3 μm diameter particles using a cyclone separator, and <3 μm diameter using a depth filter to quantify cleaning efficacy, i.e. a scaled down version of a commercial Mist Eliminator. The method involved isokinetic and isothermal sampling of the stack gases to eliminate collection bias. The sample flow rate was established at 1 ACFM (Actual Cubic Foot/min).
When similar testing for <0.2 μm diameter particles was adopted for a workplace evaluation, it repeatedly revealed a particle mass ~100 times higher than measured using a conventional membrane filter cassette. A compound sampling method for segregating vapors from microparticles showed that a properly designed depth filter effectively collects particulates by both impaction and diffusion across the entire gamut – from 1 nm to large airborne sizes. Actual size of the collection devices varies with sample flow rate, but one designed for 400 cc/min airflow works effectively across a range of 100–400 cc/min, while a device for 250cc/min is effective down to 10cc/min.
FabOhio Inc. has developed a family of microparticle sampling devices allowing sampling from 10cc/min through 1 ACFM. While a generic sampler will handle a wide range of flow rates, a specific size can be custom made to match the pseudo-isokinetic concept behind the Institute of Occupational Medicine (IOM) sampler. The samplers will collect the entire range of particulate sizes entering the inlet, but can be modified by pre-filtering using a 0.22, 0.46, or 0.8 μm membrane filter sampling cassette, recognizing that some material will be lost on the backup support pad which behaves as a thin depth sampler.
A selection of microparticulate sampling devices (images below) will be shown for the first time at INTERPHEX 2016.
c) Creating working environments free from microparticles
FabOhio Inc. recognized that all ambient air is potentially contaminated with a high burden of microparticulates, and that current air filtration methods are incapable of eliminating this burden. The challenge was to develop a contained environment as free from microparticles as possible. Based on the learning from industry, adoption of depth filters would solve the problems of preventing airborne microparticle contaminants entering,and leaving the containment zone of an enclosure.,
Microparticulate Filters Canisters (MFCs) are used to isolate the air within the containment zone from all unintentional sources, including ambient air. They are bi-directional, filtering both inlet and exhaust air. With MFC’s installed in all entry and exit locations, Flexible Enclosures afford an ideal way of achieving microparticle control economically. FabOhio Inc. will display a freestanding mobile Microparticle Capable Glovebag (MCG) at INTERPHEX 2016.
Microparticle Capable Glovebags are prepared essentially particle free and ready for use upon delivery.
The ability to compress the size of an enclosure to expel air, then restore to size using inlet air filtered through the MFC allows the pre-cleaned enclosure to be rapidly purged with efficiencies >1,000,000 fold. Upon receipt, the MCG is inflated through the MFC. Once the support frame is either inflated or connected, the enclosure is ready for use.
d) Solving materials transfer challenges
FabOhio Inc.’s development also addressed moving materials into and out of the contained space without allowing contamination. The pass-through sleeve(s) becomes an ideal vehicle once MFC’s are installed. Using the same inflation/deflation cycle technique as used by the supplier, the pass-through can be purged ready for opening to the contained MCG space.
With this advance, the pass-through sleeve becomes a vehicle for maintaining any sealed enclosure free from undesirable contaminants, e.g. moving materials through this novel isolation barrier into the environment within a Gloveboxes by having an appropriate connecting device.
Powder Systems Ltd., in collaboration with FabOhio Inc., are in the process of further developing this technology for its Micro Particle Containment gloveboxes and Isolators. It is important to note that flexible containment and rigid containment systems compliment each other and form a realistic and effective approach to containment at the highest level. This partnership between FabOhio Inc. and PSL offers the best of both worlds to our clients.
The potential user needs to evaluate the value of their research or processes to decide how necessary it is to protect their processes from potential artifacts due to airborne microparticles. The solution is now available at an economical investment.