Christoph Hacker of RWTH Aachen University (Germany) observed that the electrospun webs needed to improve the filtration efficiency of drinking water filters are very fragile and therefore hard to laminate to stronger meltblown or spunbond nonwovens. They are easily destroyed by hydroentanglement, needlepunching or thermal bonding, and the use of lasers alone fails because the polymers do not absorb laser energy. The solution is to electro-spray submicron droplets of an energy absorber onto the substrate before electrospinning the fibrous layer, and for the purposes of these experiments the droplets contained carbon black to indicate how uniformly the spot-bonds were distributed.
The beam from a 100watt diode laser was spread using a prism to 35mm wide and as the laminate containing the droplets – held under pressure beneath a glass plate - was illuminated by it, the droplets heated up and spot-bonded the layers together without destroying any of the electrospun filaments.
Microscopy and peel strength tests showed the bonding process was successful and filtration testing showed that the bond points did not affect filtration efficiency.
Work continues to convert this batch demonstration process into a continuous process with wider lasers, simultaneous electrospinning and electrospraying, and a search for improved energy absorbers. Mr Hacker thought the process might ultimately run at 10m/min.