Cottonid—An Old Material for Autonomous Actuation

Many material systems have already been developed, which are able to respond to external stimuli such as temperature, light, electric fields, such as temperature, light, electric fields, pH-value or humidity by means of dimensional changes. These materials can react in an energy-efficient way to kinetic energy or mechanical force through volume expansion.<br/><br/>One material in such a system could be Cottonid, one of the oldest known (bio-)plastics. In 1859, the Englishman Thomas Taylor described for the first time the production of this material on the basis of rag paper, which he produced by soaking cellulose fibers in a zinc dichloride bath and pressing several layers under heat. Nowadays, unsized specialty paper made from cotton, pulp or a mixture of both serves as a renewable raw material for industrial production. The treatment with zinc dichloride should only last until the fibers have reacted superficially, otherwise the cellulose chains are degraded and the material is damaged. The exact mechanism of this vegetable parchmentization has not yet been identified in detail. During the process, a tremendous swelling can be observed followed by the decomposition of the paper sheet. The result of this conversion process of the cellulose fibers is a considerable change in the material properties, from soft and flexible to horn-like and hard.<br/><br/>The manufacturing process and the material itself is very sustainable. The zinc dichloride, used as a catalyst, can be completely recycled. The washing water is also almost completely reused. The finished material is free from chemical residues and consists only of cellulose and, for marketing purposes in some cases, pigments. In addition, zinc dichloride is a cost effective and a safe “solvent” compared to commonly used solvent systems for dissolving cellulose. The resulting material is quite hygroscopic and can be perfectly used for sustainable actuators due its dimensional changes in response to environmental humidity.<br/><br/>The presented talk will provide an overview of the possibilities of Cottonid in a bilayered material system for autonomous actuations. Inspired by nature we fabricated actuators with Cottonid as an active layer together with different passive layers and investigated the actuation potential of different systems. Different bio-based and non-bio-based materials and adhesives were usedt to build up a two-layered material. The autonomously actuation potential was investigated inside a climate chamber by alternating humidity. The bending motion was captured with a camera and evaluated. The resulting curvature angles reach from 1° up to 70° depending on the change of the relative humidity and the system itself. We generated a portfolio of different material systems for a wide range of possible applications for which a directed autonomous humidity driven actuation is required.<br/><br/>In summary, Cottonid is a promising material in the field of bioderived and sustainable materials. Due to the topicality of the issues of environmental protection and sustainability, these materials are currently experiencing a renaissance. It has great potential to replace conventional petroleum-based materials with a green alternative. Additionally, the reaction route via zinc dichloride can be transferred to the wide area of all cellulose composites and might lead to a new, even more sustainable way for their fabrication. A possible future field of application for Cottonid-based bilayer actuators could be the so-called bio-architecture. It would be conceivable to use Cottonid-based actuators as an intelligent building envelope, which reacts autonomously to the environment. Even with complex architectural geometries shading systems could be applied In addition to this function as a shading element, combination systems with solar modules, for example, would also be conceivable, in which the Cottonid bilayer adjusts the panel to the position of the sun and follows it.