Humidity-Sensing Material Cottonid – Microstructural Tuning for Improved Actuation and Fatigue Performance

Functional Composite Mater

Dörrstein

Hornberger

et al. 2021

Self Cite

The aim of this work is to characterize the moisture-dependent actuation behavior of bioinspired and additively manufactured hygromorphs based by following deductive and inductive design approaches. Fused Filament Fabrication (FFF) is employed to print bilayered structures consisting of swellable active layers and rigid passive layers. The active layer is composed of a polylactic acid (PLA) matrix filled with different hygroscopic cellulosic materials (native and modified) up to a filler content of 50 m%. Acrylonitrile Butadiene Styrene (ABS) is used for the passive layer. The FFF process allows the generation of desired differential swelling properties in the composites upon moisture absorption. The moisture dependent actuation strain of the printed bilayers was determined by video analyses. Some influencing geometrical factors which contribute to the actuation were deduced from x-ray diffraction (XRD) and micro computed tomography (μCT). The investigation of the mean cellulose microfibril orientation on the surface of the active layer suggested a preferential orientation with respect to printing direction. Furthermore, a gradient of cellulosic material within a single printed layer was observed, which indicates fiber sedimentation. Comparison with the thermomechanical model derived from Timoshenko (1925) shows that the computational prediction of the moisture dependent actuation is considerably accurate for most selected cellulosic materials and filler contents.

Section: Theoretical Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Fabrication of 3D-printed hygromorphs based on different cellulosic fillers

Functional Composite Mater

Dörrstein

Hornberger

et al. 2021

Self Cite

“…Cellulose as most abundant biopolymer on earth and therefore a very sustainable resource is an attractive material candidate for substitution of fossil products in specific, i.e. mostly functional, [32][33][34]. Furthermore, the impact of outdoor weathering conditions on the long-term actuation and fatigue behavior is characterized.…”

Section: Cottonid Materials Systemmentioning

confidence: 99%

Impact of Solar Radiation on Chemical Structure and Micromechanical Properties of Cellulose-Based Humidity-Sensing Material Cottonid

Scholz

Hemmerich

et al. 2020

Preprint

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Renewable and environmentally responsive materials are an energy- and resource-efficient approach in terms of civil engineering applications, e.g. as so-called smart building skins. To evaluate the influence of different environmental stimuli, like humidity or solar radiation, on the long-term actuation behavior and mechanical robustness of these materials, it is necessary to precisely characterize the magnitude and range of stimuli that trigger reactions and the resulting kinetics of a material, respectively, with suitable testing equipment and techniques. The overall aim is to correlate actuation potential and mechanical properties with process- or application-oriented parameters in terms of demand-oriented stimuli-responsive element production. In this study, the impact of solar radiation as environmental trigger on the cellulose-based humidity-sensing material Cottonid, which is a promising candidate for adaptive and autonomously moving elements, was investigated. For simulating solar radiation in the lab, specimens were exposed to short-wavelength blue light as well as a standardized artificial solar irradiation (CIE Solar ID65) in long-term aging experiments. Photodegradation behavior was analyzed by Fourier-transform infrared as well as electron paramagnetic resonance spectroscopy measurements to assess changes in Cottonid’s chemical composition. Subsequently, changes in micromechanical properties on the respective specimens’ surface were investigated with roughness measurements and ultra-micro-hardness tests to characterize variations in stiffness distribution in comparison to the initial condition. Also, thermal effects during long-term aging were considered and contrasted to pure radiative effects. In addition, to investigate the influence of process-related parameters on Cottonid’s humidity-driven deformation behavior, actuation tests were performed in an alternating climate chamber using a customized specimen holder, instrumented with digital image correlation (DIC). DIC was used for precise actuation strain measurements to comparatively evaluate different influences on the material’s sorption behavior. The infrared absorbance spectra of different aging states of irradiated Cottonid show decreased absorption at 2,900 cm-1, i.e. the spectral region of C - H stretching vibrations, and increased absorption at 1,800-1,700 cm-1, i.e. the spectral region of C = O stretching vibrations, with respect to unaged samples and therefore indicate oxidative stress on the surface. These findings differ under pure thermal loads. EPR spectra could corroborate these findings as radicals were detected, which were attributed to oxidation processes. Instrumented actuation experiments revealed the influence of processing-related parameters on the sorption behavior of the tested and structurally optimized Cottonid variant. Experimental data supports the definition of an optimal process window for stimuli-responsive element production. Based on these results, tailor-made functional materials shall be generated in the future where stimuli-responsiveness can be adjusted through the manufacturing process.

“…To understand the behavior of the actuators an analytical mechanical model was employed [10], which relates the curvature (κ) of the bilayer actuator to the humidity-induced strain of the material by equation (1). The results of the κ-calculations are shown in table 1.…”

Section: Theoretical Modelmentioning

confidence: 99%

“…They are only driven by environmental humidity gradients [2]. The materials and design approaches inspired by these phenomena are often of non-renewable origin [3]- [5] while several approaches describe the use of wood [6], [7] or cellulose-based materials such as cottonid [8]- [10]. Hygromorphous structural materials respond to the relative humidity (Φ) of the environment by changing their dimensions, and thus their shape by swelling through the uptake of water.…”

Section: Introductionmentioning

confidence: 99%

Fabrication of 3D-Printed Hygromorphs Based on Different Cellulosic Fillers

Dörrstein

Hornberger

et al. 2020

Preprint

Self Cite

The aim of this work is to characterize the moisture-dependent actuation behavior of bioinspired and additively manufactured hygromorphs based by following deductive and inductive design approaches. Fused Filament Fabrication is employed to print bilayered structures consisting of swellable active layers and rigid passive layers. The active layer is composed of a polylactic acid matrix filled with different hygroscopic cellulosic materials up to a filler content of 50 m%. Acrylnitril-Butadien-Styrol-Copolymer is used for the passive layer. The Fused Filament Fabrication process allows the generation of desired differential swelling properties in the composites upon moisture absorption. The moisture dependent actuation strain of the printed bilayers was determined by video analyses. Some influencing geometrical factors that contribute to the actuation were deduced from x-ray diffraction and micro computed tomography. The investigation of the mean cellulose microfibril orientation on the surface of the active layer suggested a preferential orientation with respect to printing direction. Furthermore, a gradient of cellulosic material within a single printed layer was observed, which indicates fiber sedimentation. Comparison with a thermomechanical model shows that the computational prediction of the moisture dependent actuation is considerably accurate for most selected cellulosic materials and filler contents.