Nonwovens as lightweight construction material

The industry partners come from all relevant process stages. The expertise in textile processing is covered by the glass fiber and nonwovens producer Johns Manville, Bobingen/Germany, the machine builder Dilo Machines GmbH, Eberbach/Germany, the nonwovens manufacturers BWF Group, Offingen/Germany, and Filzfabrik Fulda, Fulda/Grmany. In addition, there are 2 plastics processing companies, the injection molding company Dittrich + Co. GmbH & Co. KG, Schwabmünchen/Germany, and the specialist in polyamide casting, Ensinger Tecarim, Linz/Austria. Under the leadership of the Institut für Textiltechnik Augsburg gGmbH (ITA Augsburg), Augsburg/Germany, research focuses on the processing parameters, mechanical performance and economic efficiency of web-based composites.

The members of the IRG set the research priorities themselves in an agile and demand-oriented manner. So far, the cooperation has focused on 5 major topics:

• Design of stiffness-related lightweight construction applications with WBC
• Acquisition of end users with the powerful IRG consortium
• Investigation of the potential of single-layer structures from WBC
• Development of a tool for the calculation of lightweight construction potentials
• Enabling the processes of the partners to process nonwovens

The results of the investigation of the potential of single-layer structures – components made of individual nonwoven layers – showed a particular potential of WBC for saving process steps. At the same time characteristic mechanical values were improved. The component production from a single nonwoven layer has the enormous advantage that a whole process step, the stacking process, is completely eliminated. The aim of the study is therefore to evaluate the influence of the number of nonwoven layers on the mechanical properties. Fig. 1 shows an example of 2 of the manufactured components.

For this purpose, nonwovens were produced at the ITA Augsburg consisting of recycled carbon fibers (rCF) from SGL Carbon GmbH, Meitingen/Germany, and polyamide 6 (PA 6) fibers from Ems-Chemie, Dormat/Switzerland. For the production, a compact nonwoven line from Dilo was used. Further processing of the nonwovens is carried out on the thermoforming process of ITA Augsburg using the Engel injection molding machine insert 1050H/200 single, from Engel Spritzguss GmbH, Schwertberg/Austria. The nonwovens are infrared heated and then consolidated in the press unit to form a generic component. The thermoforming and injection molding process also offers the possibility of functional integration, which reduces the total cost of ownership of the finished product. The components are then used to produce test specimens for 3-point bending tests according to DIN EN ISO 14125.

The average fiber length of the carbon fibers is 60 mm and the mechanical properties are in the range of the classic high tenacity types with an average tensile modulus of 240 GPa and a tensile strength of 3,500 GPa. The PA 6 fibers of type P300 have a fineness of 1.7 dtex and an average fiber length of 40 mm. The blend ratio of the nonwovens are 40 % rCF by weight to 60 % PA 6 by weight, leading to a fiber volume content of the consolidated board of 29.7 %. 4 different basis weights were produced for the tests, whereby the nonwoven line parameters remained constant except for the number of layers in the cross lapping process of the nonwovens production. The basis weights are 250 g/m2, 500 g/m2, 900 g/m2 and 1250 g/m2. From the different basis weights, panels of the same thickness of 2 mm are produced during consolidation by means of a multi-layer structure.

Fig. 2: Bending stiffness for different surface weights

The bending stiffness increases with increasing basis weight, as shown in Fig. 2. On average, the highest mechanical properties are achieved with a grammage of 900 g/m2 at 14.3 GPa. The maximum mechanical values reach 22 GPa.

The values are always lower in the machine direction than in the cross direction, because the nonwovens are rotated by 90° by the cross lapper of the line.
The strong scattering is due to the thermoforming process used. The heat input by IR emitter is suboptimal for nonwovens. Despite a pre-consolidation, the nonwovens spring open during heating due to the restoring forces of the fibers and form an insulating layer. This means that the core of the panels is not melted throughout. In further steps, contacting heating will be employed and is recommended for the processing of nonwovens.

The improvement of the bending stiffness is favored by 2 effects. On the one hand, the delamination resistance of nonwovens consisting of fewer layers is improved by better mixing of the fibers within the nonwovens. On the other hand, the carbon fibers experience less fiber damage at high weights due to needling. More material is available as a buffer for the same needle density.
The investigation shows that the use of tailor-made surface weights offers the potential for both cost reduction and improvement of mechanical properties. Conclusion: Win-Win.
In November 2020 the Industry Research Group Web Based Composites (IRG WBC) will enter its second year. Interested parties can join the IRG at this time or a future time depending only on compatibility with the current consortium.