Key Engineering Materials Vol. 742

Abstract: In order to reduce fuel consumption due to environmental aspects, weight of automotive components has to be reduced. Fibre reinforced polymers have high potential to contribute to this aim as they feature a high ratio of stiffness to weight. The direct processing route for long fibre reinforced polymers is a potential process for the net shape series production of automotive parts. To retain safety and comfort, the material properties of polymers processed in such a way have to be investi-gated thoroughly implementing a deeper understanding of elastic response and damage mechanisms. This work deals with glass fibre reinforced polypropylene manufactured by a direct LFT processing route (D-LFT). After introducing basic properties, studies to determine damage evolution are presented. In this regard, the decrease of stiffness with increasing strain was analyzed using tensile tests featuring loading-unloading cycles. The materials properties have been correlated to fibre orientation measurements from X-ray computed tomography. The stiffness decrease is compared to stiffness measurements carried out by ultrasonic phase spectroscopy (UPS) tests, carried out on juvenile, undamaged specimens. This method is used in this study for the first time to describe the elastic properties of long fibre reinforced thermoplastics.

Abstract: In this work newly developed phosphate-free polyurea/polysilicate resins (denoted as 2P) were explored and compared to market established 3P Resins® regarding their fiber/matrix adhesion with coupling agents introduced for further property improvement. Fiber/matrix adhesion was determined by macro mechanical test, i.e. interlaminar shear strength (ILSS), double-cantilever beam (DCB), Charpy impact, and tensile tests.It was demonstrated that 2P resins containing epoxidized linseed oil (ELO) replacing unfavorable organic phosphate esters in 3P Resins® as phase transfer agents nearly reach similar adhesion properties between fiber and matrix compared to established 3P Resins®. Additionally, some coupling agents were found to increase the ILSS by up to 26 %, the energy release rate by up to 7 %, the Charpy impact strength by up to 18 %, and the tensile modulus by up to 11 %.

Abstract: Fiber-reinforced thermoplastics have a high potential for big scale light weight process applications due to low processing times and recyclability. Further advantages are the low emissions during the manufacturing process and beneficial handling and storing properties of the semi finished materials. Thermoplastic composites are made of reinforcement fibers and a thermoplastic polymer matrix by applying two essential sub processes: (1) melting of the matrix-material and (2) impregnating the textile component with molten matrix-material. At present state of art both sub-processes are applied by using double-belt-presses, characterized by high processing temperatures and high processing forces. Therefore, a large amount of energy is needed to create the necessarily high compaction forces and temperatures with hydraulic cylinders and electric heating. Convection, infrared-radiation and the cooling (dynamic) of tempered machine parts leads to a significant dissipation of energy. Especially the process for generating the hydraulic pressure has a low level of efficiency. Therefore, in respect to economic and ecologic reasons, novel energy-efficient impregnation processes need to be investigated and developed. The represented novel impregnation process is based on ultrasonic technology. A stack of polymer film (outer layers) and a textile ply (inner layer) is formed and the energy is applied with an ultrasonic sonotrode. The efficient, fast and strongly concentrated energy application into the thermoplastic films allows the development of novel and highly flexible machine concepts. These can be used for development of small scale up to large scale production processes. The ultrasonic-technology allows a continuous impregnation of the textile component with molten matrix-material. A custom-designed prototype was developed. First material samples were produced and the technological parameters studied. A characterization of the experimental results, material samples, prototype machine and process is the focus of this paper.

Abstract: During recycling of waste paper from private households different fractions containing plastics and polluted paper fibers are received. Those polluted fibers cannot be recycled in the common paper manufacturing process or for energy recovery like in waste incinerating plants due t o economic reasons. Current research at the Institute of Polymer Materials and Plastics Engineering at Clausthal University of Technology evaluated the use of this waste paper recyclate as a substitution for natural fibers as fillers and reinforcements in polypropylene. Special attention was given to the mechanical properties of the composites. Additionally the influence of maleic anhydride grafted polypropylene (MAPP) as an adhesive was investigated.

Abstract: Carbon fiber reinforced polymers (CFRPs) are promising composite materials for high-performance and lightweight applications, gaining increasing interest in aerospace and automotive industries. Epoxy thermosets are frequently used as polymer matrices of CFRPs, which are usually responsible for failure of the composite. In this work different types of carbon nanotubes (CNTs) and carbon nanofibers (CNF) are added to the epoxy resin to improve mechanical properties of the whole CFRP composite. The dispersion of the fillers on a three-roll mill (TRM) is shown comparing their dispersion behavior in the resin. Results of increased modulus and strength of the hierarchical composite in four-point bending tests are presented.

Abstract: The automotive and aerospace industries ask for lightweight and cost effective materials, material combinations and structures for their products. To achieve these goals certain composite material groups have to be optimized with respect to their lightweight potential and production methods.Thermoplastic sandwich composites - which consist of a core structure (transferring the load) and two face-layers (absorbing tensile and compression loads occurring at bending) - suite the need of minimizing weight per area under bending loads. Reduction of process steps can be achieved by connecting the face layers and core in-situ via in mold assembly process using foam injection molding (FIM). FIM uses a single material system and – as within this work – physical blowing agents (PBA) for foaming. To increase the strength and stiffness of FIM parts, (long glass) fibers are in cooperated to create long fiber reinforced thermoplastic (LFT) materials.A commercially available version of the LFT-FIM process is the MuCell^® process (Trexel, Inc.). LFT granulate (~ 11 mm length) is fed into the injection molding machine, melted and combined with nitrogen as PBA. To skip the needed compounding process step of the rod granules Fraunhofer ICT developed a Direct LFT-FIM process where polymer and continuous fibers are fed into a twin-screw extruder, melted and mixed with nitrogen. This single phase solution then is transferred to an injection unit.Within this work, these two foam injection molding processes will be compared concerning their achievable fiber length. For that purpose, (foamed) long fiber reinforced polypropylene (PP) blanks were manufactured using identical raw materials such as polymer, additives and glass fibers. The semi-finished product, starting material for the MuCell^® process, were manufactured by EASICOMP GmbH using the same raw materials as with the D-LFT process. The different blanks – foamed and fiber reinforced PP (30 wt% and 40 wt %) – were manufactured using one injection unit for the MuCell^® process and one for the D-LFT-FIM process. To compare the fiber length the same mold optimized to reduce fiber breakage was used in both processes.

Abstract: Polybenzoxazines offer excellent properties regarding mechanical performance, chemical stability, good flame resistance and low water absorption. High curing temperatures of benzoxazine monomers, however, impede the processing of composites. Therefore, this contribution presents a formulation of polybenzoxazines with the aim to reduce the curing temperature and facilitate Out of Autoclave (OoA) oven processes. This is accomplished by copolymerizing benzoxazine monomers with a poly(ε-caprolactone) based plasticizer yielding a resin exhibiting viscosities and reduced curing temperatures suitable for a vacuum infusion as well as compression process. Air void free composites were manufactured successfully featuring temperature dependent mechanical behavior.

Abstract: Continuous fibre reinforcements in thermoplastic composites require an enhanced adhesion to the matrix component, in order to effectively divert external forces from the matrix. Therefore different silanes as adhesion promoters are used as a part of the sizing. They operate as a connector to the matrix component. These silanes affect the sliding properties of the sizing during the production of the glass filaments in a negative way, in which case the proportion of the adhesion promoter in the sizing must be kept at a low level in order to maintain the processing speed in the textile production process. With the immersion bath method, it was examined whether the treatment of the surface of textile fabric after the textile production process with a silane-containing aqueous solution could solve these problems. Different silane concentrations and solvents were considered. After drying the textiles were processed during a two-step pressing process directly into a multi-layer organic sheet with a textile-based polypropylene matrix. The successful layering of the adhesion promoter on the glass fibre surface was verified by Fourier transform infrared (FTIR) spectroscopy. With thermogravimetric analysis (TGA), the thermal resistance of the adhesion-promoting layer for the subsequent pressing process could be shown. In order to examine the influence of the layer on the fibre/matrix adhesion within the composites, the Young’s modulus and flexural modulus of the composite panels were determined. Impact experiments were made to measure the required penetration energy and the energy absorption capacity of the composite panels. An optimum for the amount of adhesion promoter could be found. Exceeding the optimum amount of adhesion promoter in the solution led to a decrease in the mechanical properties of the composite.

Abstract: In order to reduce CO2 emissions, for the automotive industry, the most promising area of research is lightweight construction. Next to weight reduction, lightweight materials like fiber reinforced thermoplastic composites (FRTC) may also improve mechanical properties of vehicle body parts. FRTCs, so-called organic sheets, have the potential for large scale series production and they can be back moulded due to the thermoplastic matrix. On the other hand high production cycle times and a poor surface quality are limiting their potential. Therefore, ITA’s current research approaches these problems in two ways. Nanomodified materials and a new tool concept for heat pressing are going hand in hand and may lead to the technology’s breakthrough.To reduce the cycle times of the production of FRTCs innovative and modified matrix systems are investigated. The goal of the public founded project “VarioOrgano” is to analyze the potential of these modified yarns and the tool system during the FRTC production. Moreover, the capability of these composites in visible parts in automotive applications is investigated. Therefore, the whole process chain from compounding, to melt spinning, commingling and consolidation with a heat press is investigated.This paper shows the production steps along the process chain to produce these FRTCs with focus on hybrid yarn development and production.