Modeling of the mechanical properties of natural fiber reinforced polymer components

With a neutral CO₂ balance and low energy consumption during processing into composites, natural fibers can make an important contribution to compliance with emission limits. Due to their low density, natural plant fibers provide an ideal reinforcing structure for the application in fiber-reinforced composites.

Compared to glass fibers, the production of technical natural fibers - such as hemp - requires only one seventh of the primary energy. Glass fiber production results in emissions of about 3.5 kg of CO2 per kilogram of fiber. Natural fibers, on the other hand, absorb CO₂ from their environment as they grow so that the balance of production - in terms of greenhouse gas emissions - remains potentially negative. Thus, natural fibers are climate-neutral for the duration of their use until they are recycled [1, 2].

The specific mechanical properties of the two competing reinforcing fibers are at a comparable level [3]. In addition to a good crash behavior with low tendency to splinter, natural fibers have good damping properties. These potentials have already been exploited for several years in automotive applications. Due to the relatively low material price, natural fibers in particular have established themselves in the form of hybrid nonwovens for the production of semi-structural interior components such as roof stiffeners and door panels. Natural fiber-reinforced polymer composites (NFRPC) have also been used for the first time in the visible area of BMW i3.

Despite their good properties and high lightweight potential, NFRPCs play a minor role - in terms of the overall market for fiber-reinforced polymers [4] - with an EU market share of less than 5%. This is mainly caused by variations in the properties of the fibers which in turn make it difficult to meet the required quality standards. As a natural raw material, fibers are subject to  environment influences. Depending on numerous factors - such as the growing region, soil composition and climatic conditions - varying property profiles result, even between directly successive batches. For this reason, the preparation of fiber blends from different material batches is essential for a constant and reproducible property profile.

For the production of nonwoven materials, the natural fiber blend is mixed with thermoplastic fibers (e.g. polypropylene), combined to form a fiber stack and then bonded by needling to form a hybrid nonwoven. The mechanical properties of the nonwovens and the components made from them are currently determined by destructive component tests. An initial blend is created for each batch based on empirical values which is then iteratively adjusted until the desired requirement profile is achieved. This cost-intensive procedure, which is often based on years of experience, is a big hurdle for potential new market participants.

In the NaturePerformance project, Leibniz-Institut für Verbundwerkstoffe GmbH (IVW) and Institut für Textiltechnik of RWTH Aachen (ITA) are developing a model for calculating the achievable properties of NFRPC components, based on single fiber properties that can be determined quickly and inexpensively. For this purpose, the influences of the individual process steps and their process variables (starting from the single fiber via the nonwoven production up to the component manufacturing) are analyzed in detail, and mathematical correlations are derived which allow the characteristic values of the component to be determined in advance. Thus, the currently common iterative adjustment of fiber mixtures as well as the multiple, cost-intensive component testing should be avoided in the future. The goal of this approach is to lower entry barriers and enable greater market development for NFRPC.

Acknowledgements

The project "NaturePerformance – Modellierung der mechanischen Eigenschaften von Bauteilen aus naturfaserverstärkten Kunststoffen (NFK)" is funded within the framework of the program for the promotion of joint industrial research (IGF). Project no.: 21240 N/2.