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By Adrian Wilson,
European Correspondent
The contribution of nanofibers and
nonwoven layers based on them to enhanced filter media performance
was high on the agenda at EDANA's recent Filtrex conference
in Cologne, Germany. |
The invention of the automatic washing machine
should not be underestimated.
This observation - first made by the famous Swedish
doctor and academic Hans Rosling - was underlined by Jeanette Huber,
Director of Germany's future-gazing Zukunfts Institute, in her keynote
address at EDANA's Filtrex 2012 conference held in Cologne, Germany,
from September 17-18.
"In freeing up the time of countless generations
of women to do better things with their time, the washing machine
was a true innovation," she said. "Because true innovation
always needs to be about making people happier and making their
lives better - allowing for self improvement. Companies are always
chasing growth while nations measure success by higher GDP, but
above a certain level, GDP does not equate to increased happiness."
Wealth, she added, has taken millions out of poverty,
but there are, for example, still 900 million people without access
to clean water around the world.
It is in directly addressing such problems, she
implied, that companies in fields like filtration need to take a
leading role in innovation.
Nanofiltration
One key area in which there continues to be much development work
is in nanofibers and the addition of them in nonwoven layers to
filter media. In a second keynote Filtrex address, Kent Hofacre
provided details of his work with colleague Aaron Richardson at
the Battelle Memorial Institute - the world's largest non-profit
R&D organization headquartered in Columbus, Ohio - in identifying
what is known as 'the thermal rebound'.
In terms of filtration efficiency, the effectiveness
of nanofibers in filters is expected to improve as their size gets
smaller, he explained, and when they go below 100 nm their increased
efficiency in capture becomes in part due to Brownian diffusion.
But at some point, the particles begin to behave more like vapor
and collection efficiency declines - the thermal rebound.
Determining at what size this occurs has already
been the subject of a number of research papers, which were cited
by Mr. Hofacre. In aerosol filtration, which has been the focus
of his work with Richardson, the parameters that influence filter
collection efficiency include:
- Fiber diameter
- Electrostatics
- Filter thickness
- Filter solidity
- Aerosol size distribution
- Filtration velocity (surface area)
"In addition to identifying at what particle
size this thermal rebound occurs, we have tried to determine the
relative contributions of electrostatic and diffusion capture mechanisms,
and what level of protection current filtration media used in respirators/HVAC
systems provide against nanoparticles," he said.
Technology
In reviewing the available technology for the commercial manufacture
of nanofibers falling into three key approaches - electrospinning,
electroblowing and centrifugal spinning - Bengt Hagström of
the Swedish research institute Swerea IVF said he believed there
were key markets yet to be substantially exploited, not only in
respect of filtration, but also in medical drapes and gowns, battery
separators and potentially even wipes.
Since around 1995, he said, there have been thousands
of research projects undertaken on nanofibers.
"This is mainly because it doesn't cost much
to set up a lab-sized electrospinning system, since all you need
is a syringe device, an electrical voltage system and a collector.
As a result every possible combination of polymer and solution has
been explored. But the problem, of course, is that production output
is very low with such systems, and attempts to scale them up have
not always proved successful."
One problem with nozzle or needle-based solutions
has been their tendency to clog. Mr. Hagström cited a system
that proposed the use of 10,000 such nozzles and was always going
to be highly impractical.
"At the end of the day, a polymer together
with a solvent becomes basically a glue," he said.
Donaldson, meanwhile, has patents on its rotating
nozzle-based processes for the production of its UltraWeb and SpiderWeb
product ranges, while Korean company TopTec has a system employing
upwards extrusion.
DuPont holds a patent on the electroblowing technique,
while another technology cited by Mr. Hagström was the centrifugal
system of Germany's Dienes, which is similar to that developed by
his own team at Swerea and installed at a cost of around ?16,000
at the plant of Swedish filtration company Dinair.
Elmarco's nozzle-free technology was also said
to be based on a rotating system. However, fellow speaker Ales Gardiàn,
who just happens to be Elmarco's R&D director, pointed out that
this is no longer the case and a stable wire coated with spinning
solution, rather than rotating cylinders, is now at the heart of
the company's technology.
"As a result, our system is much more robust
and convenient," he added.
Accurate costing
Mr. Gardiàn's message was that in the selection of nanofiber
manufacturing technology for a specific application, it is not enough
to base a cost comparison on basis weight (in terms of processing
time, the machine width and number of electrodes) or on materials
cost, since polymers have varying densities and produce different
fiber diameters and some polymers are more prone to specific defects
than others.
"A true cost comparison needs to be application
oriented, since it's the performance of the final product that matters,
and not the polymer/nanofiber, provided it is suitable in the first
place," he said.
"What really matter are product homogeneity,
uniformity and repeatable results based on all parameters. Reliability
must be verified and proven in industrial production."
Elmarco provides such detailed analyses and Mr.
Gardiàn provided scenarios for production with four polymers
- PA6, PVDF, PUR, PAN - based on their properties and performance.
"Simplification, such as straight-forward
scale-up calculations must be avoided," he said, "as must
producing a cost model for other materials based on an existing
cost model, even if there seems to be only a small difference."
A customer's variable costs will include:
- Material - polymers, solvents, additives and
other chemicals used for operation
- Energy - including electricity and pressurized
air
- Labor
- Waste disposal - including not only the disposal
of materials not converted into nanofibers, but those employed
for cleaning the equipment or chemicals produced in waste treatment
Fixed costs, of course, relate to the investment
in the line and its peripherals, as well as the facility costs -
including heat, light, water, maintenance, cleaning, lease cost
or building depreciation, etc.
"Elmarco has produced millions of square meters
of material for different applications and application-oriented
values measured on hundreds of samples from production, running
at a broad range of production speeds and at optimized conditions,"
said Mr. Gardiàn.
"As a result, we can provide very reliable
estimates based on the full-package set of correlations."
There was further disagreement, however, when he
asserted that the quoted Nanospider output of 100 meters per hour
was on the low side.
Dr. Benham Pourdeyhimi, executive director of the
Nonwovens Cooperative Research Centre (NCRC) at North Carolina State
University, which uses the system, said this was actually on the
high side.
Electret efficiency
Dr. Pourdeyhimi's paper detailed work at the NCRC in improving electret
filter efficiency by modifying fibrous webs with melt additives,
specifically barium titanate (BaTiO3).
A repeatable and practical surface potential decay
measurement for fibrous samples has been established and it has
been proven that BaTiO3 addition enhanced the initial charge density
and charge stability, particularly after charging at Curie point,
with the BaTiO3 particles well distributed within PP filaments without
the need for compatibilizers.
Thermal charging also produces high charge retention
and stability for nucleating agent/PP samples due to dipole polarization,
while antioxidants, by contrast, have been shown not to effectively
enhance filtration performance.
Nanoweb
Also over from the U.S. at the Filtrex event was Jack Manns, director
of marketing at Hollingsworth & Vose, who after outlining a
scale of suitability for various materials to accommodate increasingly
smaller particles as shown on page 12, introduced his company's
Nanoweb.
In surface filtration media, Nanoweb was shown
to have performance characteristics similar to membranes via the
result of comparative tests including fractional efficiency in water,
flux, and dirt holding capacity and pore structure.
The key difference is that its cost is significantly
lower for suitable applications than either membranes or alternative
nanofiber technologies.
"A lot of membranes are used as pre-filters
for other membranes and this is where we see beneficial substitution
with our new product," Mr. Manns said.
Optimization is now underway at Hollingsworth
& Vose to develop a 0.1-0.2 µm nanofibrous media with
the efficiency, resistance and dirt loading performance similar
to even higher-performing membranes.
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