The demand for lightweight construction materials is steadily increasing, particularly in the transport sector, which contributes greatly to the global CO2 emission levels. If vehicles are built with lighter materials, they are able to travel greater distances with a smaller energy consumption, regardless of whether that energy comes from fossil fuels or electricity. Thereby, resources and CO2 emissions are reduced and the distances travelled by electric cars can be increased. Therefore, constant further development of lightweight materials is essential. In the research of lighter materials, the development of glass or carbon fiber reinforced composites, which are prized for their low specific weight and great stiffness and stability, has made great strides. Such composites are comprised of the aforementioned glass or carbon fibers and a so-called matrix that is a synthetic material wrapped around these fibers. However, there remains room for optimization of the connection between fiber and matrix. This bond is usually purely physical which often leads to a ripping or tearing thereof during high stress or other strains. Researchers from the Department of Microsystems Engineering (IMTEK) and the Fraunhofer Institute for Mechanics of Materials (IWM) therefore came up with an idea to tackle this problem. Their goal was to develop a molecular adhesive which could create a chemical bond between fibers and the matrix. In order to achieve this, the researchers came together in the pilot project of the Sustainability Center Freiburg “Chemical modification of fiber-matrix interfaces for enhancing the strength and durability of lightweight construction materials”.
“We divided the work among us in order to capitulate on the expertise of our colleagues. The researchers from the University of Freiburg focused on the chemical synthesis and characterization of the new material. The Fraunhofer IWM researchers were responsible for the mechanical analyses and simulations of the composite materials,” explains Prof. Dr. Jürgen Rühe from IMTEK. Furthermore, the team hoped to enable a recycling process of the new material as per the cradle-to-cradle concept. This meant that after the material was used for its primary purpose, it needed to be able to be broken back down into its primary components, which in turn were primed for other structures. Thereby, a complete closed life cycle could be established. As a final goal, the material was meant to be applicable in a variety of fields, not only in the auto industry. “What’s great about this material is that we can adapt it to any specific needs that need to be met,” reports Prof. Rühe.
The adhesion connection is established by two simultaneous processes. The first is the reaction of surface groups of the fibers with the matrix which become interlinked within the layer of adhesive. The second process is the chemical bonding of the matrix polymer via C,H-insertion crosslinking reactions. This chemical bond between substrate (with its polymer molecules) and the matrix (with is reactive groups) is made possible by the activation of the chemical groups within the adhesive via heat or light. The form and internal structure of the material is thereby stabilized.
The coating of the fibers is, as Prof. Rühe describes, the key factor. Among other things, the researchers needed to thoroughly analyze the influence of the process parameters on the thickness of the coating and the process of the activation process. The researchers from Fraunhofer IWM and the University examined thereunto various combinations of fibers, synthetic material and coating. They thereby analyzed characteristics of the individually coated fibers, how evenly the coating needed to be applied, and how well the layers adhered to one another. Furthermore, the researchers were able to examine the structure of the materials during a burden on its interface in a simulation. They were able to visualize a possible break and precisely determine where the weakness of the material lay. The IWM-team was also able to successfully produce and examine its first samples.
“The possible application of our material in the bio-medical field is particularly interesting. Possible areas were in hydrogels or in a blood-compatible design of medical interfaces,” says Prof. Rühe. Hydrogels, or water swellable polymer networks, are often used in medicine in order to create bio-analytical surfaces or to coat implants. This allows for a good adaptation of the synthetic material to the biological environment. However, a common problem with the currently used hydrogels is that the contact between the coating of the hydrogel and the underlying substrate fails and the layers separate. This separation can sometimes lead to significant damages. Thereby, the implementation of covalently bonded structures (as is the case when using the new adhesive) would be beneficial.
The main focus of this project within the Sustainability Center Freiburg was however on classical lightweight components. Amongst others, cars and airplanes could entail this new composite material in the future. Their lower weights would result in an increased fuel efficiency. Plus, the implementation of the material allows for an increase in sustainability via recycling. If the matrix is comprised of a fusible or dissolvable composite, it is possible to simply remove it from the fibers at the end of the life cycle of the product. Both components are then ready for a new implementation. An interesting feature thereof is that the coating can be applied to the fibers time and time again. The (re)-coated fibers can then be placed back into a new, perhaps even different matrix, and complete the same bonding process. This process can also be repeated for multiple life cycles.
An important first step towards a more powerful yet recyclable adheisve has thus been completed. The researchers still need to delve further into this area of research in order to develop this system into a scalable process for large-scale practical applications. “We were able to achieve many of our goals in the last three years,” reports Prof. Rühe proudly. The successful collaboration also strengthened the pre-existing relationship between Fraunhofer and the University of Freiburg. The researchers are looking forward to the next possibility to work once more again on a common topic.