November/December 2012 | Volume 31, No. 6

Nanofibers and Nonwoven Layers in Focus at Filtrex
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.