Polymer foams are frequently used as core materials in sandwich structures for applications such as aerospace, naval, and wind industry. It is known that the core material contributes to the overall mechanical properties of these sandwich structures up to a remarkable extent. In addition, due to the curvature and geometrical complexities of several applications, these cores are available with special cuts/grooves (finishing options) to provide bendability and better resin infusion during processing. The goal of this study is to investigate the mechanical performance of synthetic polymer foams as core materials for sandwich structures. Balsa wood, which is the most common traditional structural core, was used as reference material. Glass fiber reinforced epoxy was employed as face sheets. End-grain Balsa wood, commercially available polyvinyl chloride and polyethylene terephthalate, and experimental grades of polyurethane foams were chosen as alternative core materials. Quasi-static flexural tests were carried out using a four-point bending setup according to ASTM C 393. Sandwich properties such as stiffness, core shear strength, and energy absorption were determined and compared. The contour cuts filled with resin show a reinforcing effect against transverse shear stresses leading to higher shear strengths. Digital image correlation was used to study the yielding and permanent strain of the foam core sandwich beams.
Injection molding of long fiber-reinforced thermoplastics is a well-established method in automotive industry to produce high quality structural parts in mass production without the need of further finishing, where the performance is strongly dependent on the fiber length. It is known that the processing parameters have to be carefully chosen as they directly influence the final fiber length and can therefore have a negative effect on the resulting mechanical properties of the part. Particularly in terms of impact behavior, the fiber length is seen as a key factor. The aim of the present work is to quantify the effects of the injection molding parameters on the impact behavior of long fiber reinforced compounds. In a first step the resulting fiber length of injection molded glass fiber reinforced polypropylene is analyzed as a function of the injection velocity, holding pressure, revolution speed and back pressure. The single steps of fiber length analysis are carefully investigated in terms of feasibility and accuracy, in order to assure that a reproducible and reliable test method is employed. Subsequently, the impact behavior of the PP-GF compounds is quantified by falling dart experiments, and the results are then correlated to the fiber length and the processing parameters to establish a fundamental processing-structure-property relationship. The investigation of the fiber length analysis shows that a high pyrolysis temperature leads to embrittlement of the glass fibers and a decrease of tensile strength of the single fibers. This may consequently lead to incorrect characterization of the fiber length due to fiber breakage. The results of the statistical investigation of the processing parameters indicate a significant influence of the back pressure on both impact energy and fiber length.
Flame retardancy for thermoplastics is a challenging task where chemists and engineers work together to find solutions to improve the burning behavior without strongly influencing other key properties of the material. In this work, the halogen-free additives aluminum diethylphosphinate (AlPi-Et) and a mixture of aluminum phosphinate (AlPi) and resorcinol-bis(di-2,6-xylyl phosphate) (AlPi-H þ RXP) are employed in neat and reinforced poly(butylene terephthalate) (PBT), and the morphology, mechanical performance, rheological behavior, and flammability of these materials are compared. Both additives show submicron dimensions but differ in terms of particle and agglomerate sizes und shapes. The overall mechanical performance of the PBT flame-retarded with AlPi-Et is lower than that with AlPi-H-RXP, due to the presence of larger agglomerates. Moreover, the flow behavior of the AlPi-Et/PBT materials is dramatically changed as the larger rod-like primary particles build a percolation threshold. In terms of flammability, both additives perform similar in the UL 94 test and under forcedflaming combustion. Nevertheless, AlPi-Et performs better than AlPi-H þ RXP in the LOI test. The concentration required to achieve acceptable flame retardancy ranges above 15 wt %.
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