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Epoxy resins are frequently used for advanced fiber reinforced composites applications. Even though the fibers are dominating strength and stiffness of the composites, specifically when loaded in fiber direction, the failure usually initiates in the matrix near or directly at the interface. Especially in plies loaded transverse to the fiber direction, the mechanical behaviour is highly influenced by the mechanical properties of the epoxy matrix. Accordingly the mechanical properties of the matrix are of great importance.
Failure of composite materials is initiated by fracture processes on microscale, especially by interfacial debonding. Failure processes taking place on microscale are studied by single fiber experiments. This is, single fibers embedded in tensile specimen are loaded under various off-axis angles. Starting at microdefects interface cracks propagate circumferentially as well as longitudinally, depending on the loading angle. In addition, finite element simulations of interfacial crack propagation around single fibers as well as fibers embedded in a hexagonal composite are shown based on linear elastic fracture mechanics. The course of the energy release rate is given in dependence of the fiber volume fraction.
Epoxy resins are one of the first choices for structural adhesives and are widely used in combination with fibers as fiber reinforced plastics (FRP). The mechanical properties are the result of the complex chemical network structure that is generated by the thermally catalyzed cross linking reaction. Numerical simulations on the atomistic length scale are appropriate tools to understand and improve the mechanical properties and its mechanisms of epoxy resins. This leads to the necessity of a model generation procedure that covers the characteristic cross linking mechanisms of epoxy resins and is able to generate a realistic representation of the network structure. Research in the field of Molecular Dynamic based curing kinematics of polymers has led to cross linking procedures that are based on the main chemical curing reaction and can produce models, whose mechanical properties are in agreement with experimental values. Nevertheless an assessment of the realism of these cross linking procedures is difficult, since various complex aspects, such as the influence of the activator molecules or catalyzing chemical reactions may be important, but are hard to characterize. By using the method of in situ near-infrared spectroscopy (NIR) the time and temperature evolution of the reactive groups, epoxy and either amine or anhydrite curing groups, can be measured. It has been shown that this method is well suited for analyzing the curing process and to characterize the fully hardened epoxy resin. Thus NIR measurements of the cross linking kinetics of epoxy resins give a valuable insight in the curing process that can be used to calibrate and assess numerical approaches of the cross linking procedure. A modeling technique for the curing kinematics of epoxy resins is presented, that is able to realistically represent the cross linking mechanism and generate simulation models with characteristics in good agreement with experimentally analyzed cured epoxy resins. This is achieved by calibrating the cross linking parameters and is shown by a comparison of both, the cross linking procedure and the resulting network structure, with experimental results of NIR measurements. The modeling approach is incorporated in the Molecular Dynamic Finite Element Method (MDFEM) framework and implements a step by step molecular network build-up. This allows to perform MDFEM equilibrium iterations during the curing procedure in order to create realistic and well equilibrated simulation models. Furthermore MDFEM simulations of tensile tests are presented to evaluate the influence of the network structure on the elastic mechanical properties. These numerical tests also illustrate the need for accurate models when deriving material properties from atomistic length scale simulations.
The fire behaviour of carbon fibre (CF) reinforced polymers differs in comparison to polymers. Fibres behave often inert with respect to pyrolysis, they change the melt flow and dripping behaviour, the heat absorption and transfer, the amount and properties of the fire residue and so on. Flame and fire retardancy concepts are needed not only suitable for the different fire protection goals typical for each application, but also tailored for composites. This field is illuminated by examples taken from different projects carried out in the group of the author in the recent years. The examples target on different applications through achieving reduction in reaction to fire controlling the fire risks (flammability, heat release) in the beginning and development of a fire and investigating the fire stability, when a severe flame is directly applied (key property in fully developed fires). Approaches to halogen-free flame retardancy in CF reinforced thermosets are presented as well as building up a bench and an intermediate scale testing of composites in fire applying mechanical load (up to 1 MN compression) and direct flame exposure (180 kW/m2) simultaneously. Indeed, e.g. we have investigated the fire stability of stringer reinforced shell components taken out from the fuselage of an aircraft.
The understanding of fire behaviour, fire resistance, and fire retardant modes of action in composites is a promising basis for target-oriented development. The role of flame inhibition, charring, and protective layer formation is discussed. Successful concepts are presented for fire retardancy tailored for different application as well as general guidelines for future development. Different phosphorus flame retardants are proposed to achieve halogen-free flame retardancy with respect to ignition and developing fires. Different protective approaches are sketched for addressing the fire stability of composites that is the most important fire risk for the fire resistance in structural applications.
Most synthetic polymers have a high fire load, and as a result, they require flame retardants (FRs) to ensure their safe use. Phosphorus plays an important role in flame retardancy and has the potential to replace halogenated variants, which are assumed to be harmful to the environment and health. Among phosphorus-based FRs, there exists a trend towards polymeric, high molar mass molecules with complex molecular architectures. In this project, we synthesized a novel series of so-called phosphorus-based hyperbranched polymeric FRs and investigated their use as multifunctional additives to high-performance polymers, i.e. epoxy resins. By cleverly designing the chemical structure to contain varying amounts of P-O and P-N bonds, new insight into the chemical mechanism of flame retardancy was gained, and by comparing the hyperbranched polymers to their monomeric counterparts, a greater understanding of the role of complex architecture was won. This talk aims at presenting some of these results and proposes chemical mechanisms that illustrate what role these novel hyperbranched flame retardants play in molecular firefighting.
Renewable alternatives for common thermoset resins are demanded to go for sustainability. The objective is to create a flame retarded epoxy resin from a commercial bio epoxy resin with halogen free inorganic and organic flame retardants, respectively. Alumina trihydrate, aluminum diethyl phosphinate, a DOPO-based phosphonamidate and ammonium polyphosphate seem to have promising performances. Properties are enhanced with different bio fillers: pyrolyzed cocoa shells and plant waste (provided by Otto A. Müller Recycling GmbH, thanks!), short fibers and nonwovens of the natural fiber kenaf, hydroxypropyl-ß-cyclodextrin and sulfobutylether-ß-cyclodextrin.
Fire performance is investigated by cone calorimeter examinations, LOI and UL-94 ratings. Thermal analysis is given by TG-FTIR and DSC measurements.
Combinations of flame retardants and bio-fillers lead to reduced PHRR and THR, reach V0 in UL-94 and have a significant increase in LOI of up to 37 vol.-%. 10 % ammonium polyphosphate with 10 % pyrolyzed cocoa shell performs best, builds a magnificent protective layer, and shows good intumescence.