Brothers in arms for fine filtration
Nonwovens and membranes are considered to be very separate products which in fact, in many filtration applications, are highly interconnected. Nonwoven media-based filtration processes are not generally considered absolute methods of purification, since particles of the size of one micron or less can pass through them into the filtrate.
Typical pore sizes for microfiltration membranes are in the range of 0.1 microns to 0.45 microns, whereas a meltblown nonwoven media’s pore opening is at least ten times larger.
Membranes, however, are manufactured by solvent casting or spinning, biaxial stretching, sol-gel and other sophisticated processes which make them much more expensive to make than nonwoven filter media.
It’s for this reason that work is ongoing in attempting to develop nonwoven filter media that are capable of achieving the performance of membranes.
Nanofibres must surely be the key to this, and also promise to make single material products for microfiltration possible – as opposed to the laminates which are widely employed at present.
At recent conferences held by EDANA – the owner of the INDEX nonwovens show which is next taking place at Geneva Palexpo in Switzerland from 4th-7th April 2017 – Bryan Haynes of Kimberly-Clark has spoken of the incredible scale that is achievable with nanofibres.
A small cube of polypropylene, the size of a sugar cube, with sides of 1.58cm weighing 3.5 grams, he pointed out, can support the production of 15 micron spunbond fibres that would stretch for 14 miles. The same amount of polypropylene employed to make 3 micron meltblown fibres, meanwhile, would result in 350 miles of fibre.
But that same sugar lump employed to make 300 nanometre nanofibres, would result in enough to stretch 35,000 miles.
“The questions is,” Haynes asks, “how can we produce these fibres cost effectively on a mass scale?”
A product which has attracted a great deal of attention across many areas of the filtration industry is Ahlstrom’s Disruptor.
Disruptor’s filter effect works by both electrical charge and mechanical entrapment. It employs a submicron microglass support fibre to which nanofibres are attached, to result in an average pore size of 2 microns – so a square metre of this material has more than 42,000 square metres of available nanofibre surface.
Another product employing nanofibres is DuPont Hybrid Membrane Technology (HMT), which is billed as filling the performance gap between meltblown nonwovens and microporous films.
HMT nanofibre sheets contain continuous polymeric filaments with diameters between 200 and 600 nanometres and have been available on a commercial scale for a number of years now.
So nonwovens are getting close to the performance of membranes and there will be many examples of attempts to get them closer on show at the next edition of INDEX™17.
However, most membranes lack the mechanical strength that is often required in harsh operating conditions and in many applications – most notably for liquid filtration and separation – need nonwovens to provide it.
Membranes supported by nonwovens exhibit significantly longer lifetimes compared with conventional unsupported membranes and their complexity depends on this required mechanical strength. In addition, the nonwoven can also improve overall particle-retention capacity.
Companies like Freudenberg Filtration Technologies supply nonwovens that are specifically engineered for the membranes employed in micro-filtration, ultra-filtration or reverse osmosis processes to suit a number of filter configurations – flat, tubular or cartridge.
Flat membranes are employed in many filter systems of different configurations and sizes – in spiral windings, plate or cassette modules and in punched blanks. Usually, these membranes are so thin and fragile that they can only be produced by being directly coated onto the carrier nonwoven’s surface.
In order to produce flawless membranes, such carrier nonwovens have to exhibit a high degree of uniformity in terms of thickness, porosity and surface properties. In addition, very good fibre bonding with the membrane is essential in order to reduce defects.
In the continuous production process for tubular membranes, meanwhile, a narrow strip of the nonwoven carrier is wound to form a tube, welded using ultrasonics, and coated with the membrane solution. This process and the application conditions (particularly the maximum operating pressure and temperature) require nonwovens with high longitudinal and transverse strength, rigidity and good weldability. Suitability for welding in turn demands an appropriately uniform density and thickness.
Filter cartridges with pleated membranes will deliver their maximum performance only if the filter’s entire surface area can actually be used. Nonwovens make this possible by acting as ‘spacers’ between the pleats on the face side and as a drainage layer on the clean side.
The nonwoven media’s performance profile can be very specifically modified in terms of weight per unit area, thickness or permeability and in addition, the nonwovens can be easily pleated without damaging the membrane, prior to converting.
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