Improvement of Mechanical Properties of Polymeric Composites

Abstract

Thesis (Ph.D.)--University of Washington, 2014The use of reinforced polymer-matrix composite materials is growing rapidly. Their low weight, high strength and stiffness and resistance to corrosion make them particularly well suited for aerospace applications. The next generation of aircraft, including the Boeing 787 and the Airbus A350, are made largely out of continuous fiber reinforced plastics, reducing fuel consumption and improving range. The continued growth of the market is dependent not only on improving manufacturability and reducing cost, but also on the continuous improvement of material properties, necessitating the development of new materials and an improved understanding of the chemistry and mechanics of composites. Composite properties are dependent on many factors including the properties of the constituent phases, their relative volume fraction, their spatial orientation and arrangement, and the adhesion between them. It is recognized that good composite mechanical properties are dependent on a strong and tough adhesive bond between the polymer and the reinforcement, necessitating the development of novel techniques to improve the adhesion. Additionally, with the weight sensitive applications of polymeric composites, a reduction in the material weight, without adversely affecting mechanical properties, is desired. In the current study, the adhesion in both continuous fiber and particulate reinforced plastics is investigated, as is the synthesis of a nano-void lightened resin. More specifically, the applied stress necessary to cause adhesive failure in a particulate composite, as a function of interparticle separation and surface functionalization is determined with in situ transmitted light optical microscopy. A simple model is developed for the system, based on the superposition of stress concentrations, determined with an analytical solution, and a critical local tensile stress failure criterion. For continuous fiber reinforced plastics, the use of electrostatically deposited poly(ethyleneimine) functionalized silica nanoparticles as a means to improve the interfacial shear strength is investigated. The use of these topological surface modifiers, in contrast to the typical chemical modification of the interface, provides an increase in surface area and mechanical interlock, improving the adhesion, as measured by the single fiber fragmentation test. The particle size is optimized, as very small particles, 71 nm, are too large, and do not improve the adhesion. A method for reducing the density of a thermosetting resin is discussed. From a literature review, it is established that the most promising means for the incorporation of nano-voids into a thermoset is with blowing agent wells, self-assembled block copolymer micelles that solubilize blowing agent. A number of potential block copolymers are studied, and blowing agent wells are successfully made with the commercially available block copolymer epoxy-toughener Dow Fortegra®, which are used to solubilize heptane. The method for controlled vaporization of the blowing agent to make a low density material is still under development