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Advisory Committee Chair

Uday K Vaidya

Advisory Committee Members

Derrick Dean

Selvum Pillay

Nasim Uddin

Garry W Warren

Document Type

Dissertation

Date of Award

2010

Degree Name by School

Doctor of Philosophy (PhD) School of Engineering

Abstract

Hybrid materials featuring thermoplastic polymer composites in conjunction with metals can be used as structural materials in military and commercial transport vehicles and for protection of buildings and infrastructure. Constituent thermoplastics and metals have distinct advantages as protective materials; however, metals on their own are heavy; hence, hybrid materials offer an option as lighter materials. Since the optimal performance of a composite strongly depends on the behavior of the interface, the present study focuses on understanding the mechanisms of mechanical and chemical bonding as well as the effect of thermal stresses at the interface of metal-thermoplastic composites. The mechanical interactions between a thermoplastic polymer and steel cord have been studied with a goal to improve interfacial shear strength and cohesive strength. Steel cord was combined with nylon 6, polypropylene, soft thermoplastic polyurethane and hard thermo-plastic polyurethane to evaluate the parameters that affect mechanical bonding via pull-out tests and friction coefficient tests. The test parameters have been correlated to inter-facial shear strength using an experimental and modeling approach. This work also establishes the basis that polar groups and free radicals improve adhesion between polymeric (thermoplastics) and metallic surfaces. Chemical adhesion between the steel cord and thermoplastic polymer was investigated. Surfaces treatments of surfaces are analyzed to improve adhesion. The primary approaches for maximizing interfacial adhesion are (a) modification of the surface to create greater surface area, and (b) imparting free radical surface to providing additional surface bonds with chemical functionalities. Plasma-activated chemical vapor deposition (PACVD) is used to impart silicon, carbon and hydrogen radicals to the metal surface. Thermoplastic polymers with chemical modification were used to investigate the effect of polar groups (-NH, -CO= and -OH) and its influence on the surface energy and adhesion properties. The surface energy was found to have a direct correlation with the amount of polar groups on the surface of a modified polymer and free radicals on the metal surface. Higher surface energy correlates with superior interface adhesion. The influence of thermal stresses at the interface is investigated by finite element model (FEM). The analysis takes into consideration the difference in coefficients of thermal expansion between the fiber and the matrix in a steel cord thermoplastic composite and its effect during the cool down from processing temperature to room temperature. As temperature changes during cooling down, thermal stresses are induced at the matrix-fiber interface. Since both the fiber and matrix are dissimilar materials, the fiber-matrix interface constitutes the weak link in the progression of the composite failure. The study investigated the correlation of interfacial strength to the residual thermal stresses. The overall study provides a basis to determine the limiting bond strength that establishes continuity of the interface between the two phases, i.e. the steel cord and thermoplastic polymer composites.

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Engineering Commons

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