Little holes with a big impact
A stainless steel disc around 4 mm thick, is illuminated by a bright green beam of laser light. With high energy, the ultra-short-pulsed laser beam bores into the surface of the metal repeatedly, until a large number of holes have been made. What they are producing here at the Institute of Laser Technologies (IFSW) at the University of Stuttgart is the result of a joint research project together with the Institute of Textile Chemistry and Chemical Fibers (ITCF) in Denkendorf: spinnerets with a diameter of 30 – 40 µm, much finer than a human hair, which are utilized in the production of ultra-fine cellulose fibers, or so-called micro- or supermicrofibers.
Microfibers which are made of different polymers finer than 1 dtex. This is a measure of the fineness of a fiber, which indicates the length-related mass of a single thread. 1 dtex corresponds to one gram per 10,000 m of fiber length. Supermicrofibers are fibers whose fineness is even less than 0.3 dtex. The diameter of such fibers is approximately 3 μm. Due to their large surface area, such fibers are particularly suitable for use in the fields of hygiene and medicine, due to the high moisture absorption capacity and the soft handle of textile products made from them. They are also effectively utilized within the industrial sector as technical textiles, for example in filters, as they are both fine and extremely strong.
New ways of producing microfibers
Typically, microfibers are produced in a two-stage process: the fiber-forming polymer is spun together with a matrix polymer, which provides mechanical stability during the thread formation. From these so-called bicomponent fibers, the matrix fraction is chemically detached during the second step. What remains thereafter is the supermicrofiber.
This process is not applicable in the production of cellulose fibers. Cellulose is not fusible, but it is rather spun in a wet spinning process, in which the cellulose is dissolved in ionic liquid and pressed through a spinneret, behind which it is precipitated into the solid fiber in a coagulation bath. The fineness of the fibers is therefore determined by the size of the holes in the spinneret. And this is where the advantages of a laser-drilled spinneret nozzle come into play. The well-established drilling techniques, such as micro-stamping, mechanical drilling or spark erosion, do not allow for the production of such fine holes.
The laser drilling process is highly technically demanding. The laser beam is moved along a circular path by means of a helical drilling robot, which was specially developed at IFSW. The laser beam rotates at 30,000 revolutions per minute during the drilling process and can also be adjusted at an angle to the drill bit. This allows for high precision drilling, reproducibility and a variably adjustable drill hole geometry. Drilling deep holes with a small diameter presents a major challenge for IFSW scientists, as the laser has to penetrate stainless steel plates up to 4 mm thick without creating irregularities on the hole walls or ridges on the hole edges, which would immediately reduce the quality of the fibers.
In the conventional wet spinning process, the spinning dope is forced through the spinneret nozzles under high pressure. The cellulose then coagulates into a fiber in a coagulation bath, directly behind the spinneret. A more complex process, so-called dry-wet spinning, offers even more possibilities: the spinning solution is first pressed into an air gap directly behind the nozzle. This is followed by a drawing process, and thereby the alignment of the chain molecules in the spinning solution. Immediately thereafter, the spinning mass passes into the coagulation bath and coagulates into a fiber. The molecular orientation is retained and the cellulose fibers therefore gain an even greater strength than is the case in the conventional wet spinning process. The thick-walled, laser-drilled stainless steel nozzles also allow for particularly high working pressures of up to 200 bars. This allows for the use of higher concentrations of cellulose in the spinning solution, which makes the production process more efficient.
Defined nozzle geometry determines fiber properties
The shape of the spinning channel determines the handling of the spinning process: "Our goal is to create funnel-shaped nozzles, a drilling channel with a defined geometry that tapers to the desired final diameter," explains Thomas Arnold, a team member at IFSW. Now at the latest it is clear which improvement is made possible thanks to scientific exchange between the two research institutes: "Working with the ITCF gives us a direct input on the required geometries and accuracies." From the results of the fiber spinning, it can then be directly ascertained whether there is still any need for optimization of the spinnerets.
Another promising aspect is the testing of new materials. Recently, the first ceramic spinneret was drilled from silicon nitride. This material is more rigid than stainless steel, and can withstand higher pressures without bending, despite a reduced wall-thickness. The drilling process is also easier to control, as ceramics have no melting phase, which can negatively impact the drilling process. In addition, the partial transparency of the material allows for more accurate observation of the drilling process.
It is not only an advanced optimization of spinnerets that was realized at the ITCF Denkendorf. It are as well cellulose fibers with completely new properties that have been produced by using the dry-wet spinning process: "We have been able to significantly improve the textile-mechanical properties of our fibers compared to those produced by wet spinning," explains Dr. Johanna Spörl, chemist at the ITCF. "Our next step will be to realize further nozzle geometries in collaboration with the IFSW. That will provide our fibers with additional property profiles."
This development project is a successful example of a fruitful interdisciplinary exchange. The fact that this collaboration has generated products which are equally beneficial to both industry and consumers allows the success of this cooperation to speak for itself.