Rubber recycling refers to the circular economy process of processing waste rubber products into reclaim or rubber crumb/granulates through methods such as chemical treatment (e.g., devulcanization) and grinding. Especially the devulcanization has the goal of restoring its plasticity for reuse and revulcanization. However, the recycling process degrades rubber quality and incurs high energy consumption and costs, limiting its application. This study investigates the effects of varying concentrations of devulcanized EPDM on the rheology, mechanical properties, and thermo-oxidative aging behavior of EPDM compounds. Devulcanized rubber obtained through selective cracking of sulfur crosslinks was incorporated into an EPDM compound at different ratios. The addition of devulcanized EPDM accelerates the curing, but also reduces the crosslink density and the network uniformity. Mechanical testing indicates that higher devulcanized EPDM content increases shore A hardness but decreases tensile strength and elongation, attributed to insufficient interaction to the new rubber matrix, residual crosslinks and filler content in the recycled rubber. Thermo-oxidative aging evaluations indicate that devulcanized EPDM does not accelerate degradation or embrittlement, generally matching or even lagging behind the reference material. Furthermore, recipe adjustment can moderately improve properties of devulcanized EPDM, such as crosslink density and flexibility. These findings provide valuable insights for optimizing recycled rubber formulations, balancing sustainability with performance requirements.

The characterization and modeling of the out-of-plane behavior of fiber-based materials is essential for understanding their mechanical properties and improving their performance in various applications, especially in the forming process. Despite this, research on paper and paperboard has mainly focused on its in-plane behavior rather than its out-of-plane behavior. However, for accurate material characterization and modeling, it is critical to consider the out-of-plane behavior. In particular, delamination occurs during forming processes such as creasing, folding, and deep drawing. In this study, three material models for paperboard are presented: a single all-material continuum model and two composite models using different cohesion methods. The two composite models decouple in-plane and out-of-plane behavior and consist of continuum models describing the behavior of individual layers and cohesive interface models connecting the layers. Material characterization experiments are performed to derive the model parameters and verify the models. The models are validated using three-point bending and bulge tests and show good agreement. A case study is also conducted on the application of the three models in the simulation of a deep drawing process with respect to wrinkle formation. By comparing the simulation results of wrinkle formation in the deep drawing process, the composite models, especially the cohesive interface composite model, show greater accuracy in replicating the experimental results, indicating that a single continuum model can also be used to represent wrinkles.

The evaluation of wrinkles in deep-drawn fiber-based materials is crucial for the assessment of product quality and the optimization of manufacturing processes. Wrinkling is a common phenomenon in the deep-drawing process and is caused by tangential compressive stresses on the flanges of the blank. This phenomenon is particularly prevalent for fiber-based materials with high tensile depths and can seriously affect the appearance and mechanical properties of the final product. The objective of this study is to identify the key process parameters affecting wrinkling and to deepen the understanding of their roles and interactions using wrinkle data for deep-drawn paper products. Image analysis techniques are employed, supported by a specially constructed darkroom platform to ensure uniform light intensity for capturing photographs. An automated program is developed for the detection and evaluation of wrinkle characteristics and distribution, which allows the free choice of the region to be detected and the representation of the wrinkle geometry not limited by the number. To enhance the precision of this program, the ellipticity is initially rectified for products without flanges, specifically cup-shaped deep-drawn products. The ellipticity is caused by the pronounced springback effect of the paperboard. The approach is employed to investigate the impact of material properties, blank holder force, drawing depth, drawing clearance, and punch speed on wrinkling formation after the deep-drawing process. The findings reveal that the blank holder force and drawing clearance are critical factors in wrinkle formation, with higher blank holder force generally leading to increased wrinkle numbers.

This study presents a novel transmission-based method for characterizing local structural features, including the grammage, thickness, and fiber orientation, of paper materials. Some non-destructive techniques, such as micro-computed tomography ($\mu$-CT), microscopy, and radiation-based methods, are costly, time-consuming, and lack the ability to provide comprehensive local structural information within a single measurement. The proposed method utilizes a single light transmission measurement to assess local grammage and thickness through histogram matching with reference data obtained via $\beta$-radiography and profilometry. The same light transmission images are also used to determine local fiber orientation, employing image analysis techniques. The structure tensor method, which analyzes gradients of light transmission images, provides detailed insight into the local fiber orientation. The results show that thickness and grammage measurements are independent of which side of the paper is evaluated, while the fiber orientation distribution varies between the front and back sides, reflecting differences in fiber arrangement due to manufacturing processes. Various distribution functions are compared, and the Pearson Type 3, log-normal, and gamma distributions are found to most accurately describe the grammage, thickness, and fiber orientation distributions. The study includes a variety of paper types, ensuring a robust and comprehensive analysis of material behavior, and confirms that the method can effectively infer the inhomogeneous features from a single light transmission measurement.

For pure metals, typically a homogeneous distribution of material properties is assumed. This assumption reduces the complexity of the models significantly. For inhomogeneous materials like paperboard, however, this assumption is questionable. Experimental findings indicate that the structural inhomogeneity can lead to variations in mechanical properties, which in turn reduce the robustness of processes and require actions to control the product quality. In this work, we introduce an approach to modeling the local material structure in numerical simulations and investigate the material response to an uniaxial tensile test. The effect of various inhomogeneities, e.g., distribution of mass, density, and fiber orientation, on material properties was investigated, and it was found that fiber orientation has the greatest effect in most cases, while the effect of density is usually the least.