In this study, the influence of the open bath and closed-injection pultrusion (CIP) processing methods of epoxy resins on the quality of glass fiber composites was investigated. In addition to the state-of-the-art epoxy resin system with long pot life, new resin systems with short pot life have recently been developed. These systems require processing by closed-injection pultrusion. The epoxies with long pot life allow both processing variants. The experimental work was carried out with two types of injection and impregnation chambers (ii_chamber), namely with a conical and a teardrop design. Fully impregnated composites, which were used for further analyses, could be produced by using the conical ii_chamber. The composite properties of the open bath and the conical ii_chamber impregnation methods were compared. No significant influence on the bending stress could be determined; the interlaminar shear strength was up to 10% better with open bath impregnation than with ii_chamber. For the composites investigated, it was shown that the open bath and ii_chamber impregnation methods can be used to produce parts with partially comparable properties, as demonstrated for the epoxy formulation with long pot life. These results indicate that processing of epoxy systems with a short pot life is also possible by closed-injection pultrusion. Furthermore, the influence on the composite properties of the time interval between the mixing of an epoxy resin and processing in an open bath was investigated. No significant effect on the bending stress and interlaminar shear strength could be determined.
A pseudo‐analytical model for the forces exerted on fibers during flow that lead to fiber damage is proposed and solved. The fundamental derivations for the forces on fibers moving in suspensions developed by Burgers (1938) were used as a comparison. The cases of the motion of a fiber along its axis and perpendicular to its axis and in shear flow at a −45 degree‐angle were investigated for aspect ratios between 10 and 300. The values for the overall forces on the fiber were in good agreement with the results found earlier by Burgers and others. However, the force distribution along the fiber was found to be significantly different from the constant distribution assumed by Burgers. Because of the higher forces on the fiber from the exact solution, the criterion for the onset of buckling in shear flow was revised. The pseudo‐analytical solution was also compared to numerical results done with the boundary element method (BEM); the results were in good agreement.
Pultrusion is an established and efficient process for producing continuous fiber-reinforced composites. The resin systems that are currently most frequently used are unsaturated polyesters and vinylesters. These have a long pot life, are well known, and have good processing properties. Highly reactive resins such as polyurethane (PU) and amine hardening epoxy have been in use for a few years. These resin classes are remarkable for their extended range of properties. This opens up new application fields for pultrusion technology but poses challenges for the processing technology. Short pot lives of just a few minutes require a modified process: closed injection pultrusion (CIP). Various approaches about the design and layout of the internal geometry of the injection and impregnation chambers (ii-chamber) are the subject of ongoing research. Numerous parameters influence the impregnation process in the ii-chamber and the quality of the resulting composite. In this study, two innovative, highly reactive resins for use in the pultrusion process were analyzed, both resins based on aliphatic polyurethanes. In phase 1 of the experiments, a commercial aliphatic polyurethane-system for pultrusion applications was used. In Phase 2, the recently developed bio-based aliphatic polyurethane-system for pultrusion applications was used for the study's main experiments. The aim of the study was to analyze the material and processing properties with various modifications of the impregnation setup. Therefore, a newly developed ii-chamber and die were tested. The ii-chamber was designed to enable easy adjustment of some of the main influencing parameters during the pultrusion process. A test strategy was developed to evaluate the properties of the composites. An assessment of the influence of the process- and die-based parameters should enable an evaluation of the optimal processing settings by analysis of the material characteristics. The most significant effect of variations in the pultrusion process was found in the interlaminar shear strength (ILSS). ILSS was analyzed for all process variations for both resin systems.
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