Process Optimization: Innovative Combination of Adapted Resin System and Winding Process

The European Union has identified climate change and environmental damage as the existential threats to Europe and the world. For this reason, EU presented the European "Green Deal" in December 2019, which is intended to present a new growth strategy to facilitate the transition to a modern, resource-efficient and competitive economy. The core objectives of the Green Deal are:

• No release of net greenhouse gas emissions until 2050
• Decoupling economic growth from resource use
• No person or region is abandoned [1]

To achieve the targeted greenhouse gas aims, the change from today’s combustion engines to progressive motors is essential. By 2015, the transport sector already accounted for 30% of end energy consumption and 20% of total greenhouse gas emissions in Germany. The largest share is attributed to road traffic. A further increase in car and especially in truck traffic is expected by 2030 [2]. Nevertheless, to achieve the required reduction targets for greenhouse gas emissions in the transport sector, an increase in battery electric e-mobility is currently planned, especially in the passenger car sector. However, the commercial vehicle sector (trucks and busses), as well as rail and shipping, will also have to switch to alternative drive systems due to the impending emission restrictions. Due to its relatively low gravimetric energy storage density and the resulting poor weight/range ratio, battery technology is a disadvantage for heavy commercial vehicles. For this reason, various private companies are developing alternative energy storage systems such as hydrogen to power electric motors in mobile applications (e.g. Nikola One, Wright-bus (UK) Pulsar Hydrogen, Alstrom Coradia iLint, to name a few).

Gaseous hydrogen as a mobile energy carrier in individual or freight transport has not yet achieved a high market penetration due to its comparatively low volumetric storage density. To be able to store sufficient quantities of hydrogen for economically efficient operation in mobile applications, the current state of the art requires storage pressures of 700 bar. As an alternative to high-pressure storage, hydrogen can also be stored in liquid form. However, this storage method leads to major technical challenges in practical handling (evaporation of warming hydrogen and high costs for vacuum insulation of the storage system components). In practical comparison, hydrogen storage at 700 bar currently provides the best storage density in relation to system costs [3].

Pressure vessels for liquid and gaseous media are divided into five different types according to their design (see: Figure 1). For the pressure storage of hydrogen in mobile applications, pressure vessels of type IV are usually used today. The vessels of this type consist of a plastic liner to ensure sufficient gas impermeability and a coating of fiber material to provide the required strength of the composite system. For the sake of completeness, it is worth mentioning the pressure vessels of type V, which are pressure vessels whose reinforcing fiber-plastic coating additionally takes over the function of the liner. As these vessels are currently state of research, they do not take a relevant part in the vessel production and are only mentioned in passing. Type IV vessels are usually manufactured using the wet-wound or Towpreg winding process. In wet winding, dry fiber strings, so-called rovings, are fed through an impregnation device and impregnated with resin. The impregnation of fiber rovings is mainly carried out by the roller impregnation process, whereby the fibers are guided over one or more rotating rollers rotating in a resin bath and thereby absorb the resin. The fibers are then usually guided several times over stripping and deflection rods or rollers to improve the penetration of the resin into the fiber strands and to strip off the surplus resin. The penetration of the resin into the fiber strands depends on the rheology of the resin, depending on the temperature, as well as on the residence time of the fiber on the impregnating roller and the deflection rods. Operation at high winding speeds and the associated fast-rotating rolls lead to unwanted resin spinning-off. As a result of this spray, increased contamination and the corresponding cleaning effort is caused in the affected area of the winding system. The moist rovings also lead to a limitation of the deposit angle on the winding core: if an attempt is made to deposit the roving on the surface of the winding core in radii that are too small, the fibers can slip due to the poor adhesion of the rovings. The calculated winding pattern is not deposited and in the worst case, the mechanical properties of the component are reduced [4]. Another possibility for producing pressure vessels is the use of so-called Towpregs. Towpregs are pre-impregnated fiber strings. The fibers in these strings are already fully impregnated and perfectly parallel. Since this process separates the impregnation process from the actual shaping of the component, significantly higher winding speeds and high fiber volume contents of up to 70 % can be achieved in the winding process, with high placement quality compared to conventional wet winding. Due to the dry and tacky surface of the Towpregs, it is possible to achieve tighter deposition radii compared to wet winding technology. However, the processing of the Towpregs has been more complex up to now due to the necessary machine concepts and considerably more expensive due to the high material costs of the Towpregs. Therefore, the use of pre-impregnated fiber tapes for the industrial mass production of pressure vessels is hardly economically possible so far.

IVW, in cooperation with JWS Maschinenfabrik GmbH and the Department of Polymer Materials at University of Bayreuth, developed a novel combination of adapted resin system and winding process within the Public Funded ZIM project "SpeedPreg". Here, the advantages of the wet winding process and the Towpreg winding process are combined to be able to lay down fibers at high speeds with high quality. For this purpose, a resin system and an adapted winding process were developed in an iterative process. To avoid the higher costs caused by separate processes, the conventional wet winding process was chosen as the basis for the development. Here, the fibers are impregnated in the process with the help of a classic roller impregnation unit and then treated in a way that allows processing similar to the Towpreg process. To impregnate the fibers, the resin should have low viscosity to allow flow into the fiber string. During subsequent processing, the viscosity should increase to such an extent that the problems of spinning-off and slipping, which occur during classic wet winding, do not arise. For this purpose, a resin system has been developed which, through clever temperature control, makes it possible to guarantee both, the low viscosity during impregnation as well as the required high viscosity during processing. For implementation, the classic wet winding design, as shown schematically in Figure 2, must be extended by a suitable cooling section. The process sequence is described in detail below:

The resin is heated in a conventional resin bath to achieve a reduction in viscosity. With the sufficiently low viscosity resin, the roving can be impregnated and penetrated and stripped at the deflection rollers and scrapers. The wet roving is then passed through a cooling section to cool it down to room temperature. By reducing the roving temperature, the viscosity of the resin increases significantly, resulting in a dry roving with excellent tack for depositing on the winding core.

Promising tests that have already shown the potential of the process have been carried out on the IVW pilot plant. Figure 3 shows test specimens that were wound during a test series on the prototype line set up at IVW. In the further progress of the project, this prototype line will be extended and improved to reach the status of a pre-series line. Finally, a demonstrator in type IV pressure vessel design will be wound.

The SpeedPreg production technology presented here offers the potential to further optimize the winding process of thermoset reinforced plastic composites. By combining the advantages of wet and Towpreg winding, it is possible to minimize the cleaning effort and improve the depositing process. Furthermore, it is possible to save fiber material due to the increased adhesion of the roving surface and the resulting adjustment of the winding pattern. The achievable savings and optimizations lead to a price reduction for hydrogen storage and make the energy carrier economically more attractive.

The ZIM project "SpeedPreg - Developments of a winding process based on the new fiber impregnation system and resin formulation for high winding speeds" was funded by the German Federal Ministry of Economics and Energy within the framework of the Central Innovation Program for SMEs based on a resolution of the German Bundestag. We would like to thank the institutions mentioned above for providing the financial means.

References:

(1) E. Kommision, "The European Green Deal," Brüssel, 2019.

(2) F. Dünnebeil, C. Reinhard, U. Lambrecht, A. Kies, S. Hausberger und M. Rexeis, "Zukünfigte Maßnahmen zur Kraftstoffeinsparung und Treibhausgasminderung bei schweren Nutzfahrzeugen," Umweltbundesamt, Dessau-Roßlau, 2015

(3) M. Kell, H. Eichlseder und A. Trattner, Wasserstoff in der Fahrzeugtechnik, Wiesbaden: Springer Fachmedien, 2018

(4) J. Tölper und J. Lehmann, Wasserstoff und Brennstoffzelle, Heidelberg: Springer, 2014

(5) F. Henning, Handbuch Leichtbau Methoden, Werkstoffe, Fertigung, München, Wien: Hanser2011

Contact:
Dipl.-Ing. Benedikt Bergmann
Phone: +49 631 2017 304
E-Mail: benedikt.bergmann@ivw.uni-kl.de