Chemically and thermally stable fibers for environment and membrane architecture
Protective clothing and hot gas filters, high seas sails and construction membranes have something in common: They all require chemically or thermally resilient fibers. Fluoropolymers meet all of these requirements but their mechanical resilience is limited. This property could be provided by a second polymer. Together with partners from the machinery industry, ITV Denkendorf has developed the process technology to spin fluoropolymers into fibers together with a second polymer.
Today, large membrane roofs are constructed from polytetrafluorethylene-coated glass fiber fabrics. They are characterized by a high level of stability, but are they are heavy and an appropriate framework is required to bear the weight. By using polymer-based fibers, more than one third of this weight could be saved. In clothing that protects the wearer from chemicals as well as in hot gas filtration, fluoropolymers have been proven in terms of chemical and thermal resistance. The required strength must be provided by a different polymer, which in turn is to be protected against the aggressive media. In these areas, the solution is a fiber which has a core of a strong polymer, to provide the necessary reinforcement, and a sheath of fluoropolymer.
Bicomponent fibers, particularly in the form of core and sheath fibers, are common today, for example, for heat-activated adhesive applications. However, the combination of fluoroplastics with other high-temperature plastics such as polyether ether ketone (PEEK), polyphenylene sulfide (PPS) or polyphthalamide (PPA) pose challenges to the field of plant engineering which cannot be overcome by conventional systems. Temperature resistance to a temperature of over 400°C was a requirement for which technical solutions already existed. Fluoroplastics or traces of decomposition products which occur during extrusion are however extremely corrosive at temperatures over 350°C, especially when combined with traces of water.
ITV Denkendorf developed the fundamentals of the extrusion process together with REIMOTEC Maschinen- und Anlagenbau GmbH. The extrusion components together with the spinning head, which was designed by ITV, were manufactured from corrosion-resistant Hastelloy. The spinnerets were developed, taking the specific flow behavior of fluoropolymers into account. ITV also developed a gentle cleaning method for removing melted matter from parts such as spinnerets and extruder screws. However, the spinning pumps’ corrosion protection failed due to the high temperatures.
In order to overcome this problem, ITV called upon WITTE Pumps & Technology GmbH. The company is experienced in building highly corrosion resistant chemical pumps but was not in respect of comparatively small spinning pumps. WITTE GmbH succeeded in developing a corrosion-resistant pump able to stand up to our tests of repeated cleaning cycles.
With these new spinning pumps, bicomponent fibers were spun from a molten mass of spinnable PTFE and PFA combined with PEEK. The polymers were provided by ElringKlinger AG and the company Victrex. DIENES Apparatebau GmbH provided a newly developed heating shaft.
Beside the technological prerequisites for the spinning of fluoroplastics, the safety aspects should not be neglected under any circumstances. The hydrofluoric acid that is released during decomposition is not only extremely corrosive; it is also extremely poisonous even in very small quantities when absorbed through the skin or the respiratory tract. For this reason, a hydrofluoric gas detector was installed for constant monitoring and the staff wore supplied air respirators. An emergency plan was developed, in order to manage any possible incidents.
The research work began with this technological setup and the safety measures mentioned. Pre-tests showed that the viscous fluoropolymers do not tolerate high spinning speeds. The best level of stability of the bicomponent fibers was obtained at just one tenth of the typical spinning speeds used in the production of polyesters. At > 30 cN/tex (563 MPa), the stability of the bicomponent fibers attained the desired target.
Analysis of the individual components indicates that with optimized parameters, a further improvement of the stability of around 20 % can be expected. The research results form the basis of further fields of competence for ITV such as environmental and architectural textiles. Thanks to this development, our industrial partners have increased the scope of their portfolios in terms of high-tech applications and products.