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Self-healing of high temperature cured epoxy and composites

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Title: Self-healing of high temperature cured epoxy and composites
Author(s): Jin, Henghua
Director of Research: White, Scott R.
Doctoral Committee Chair(s): White, Scott R.
Doctoral Committee Member(s): Sottos, Nancy R.; Geubelle, Philippe H.; Chasiotis, Ioannis
Department / Program: Aerospace Engineering
Discipline: Aerospace Engineering
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: Ph.D.
Genre: Dissertation
Subject(s): Self-healing adhesive Self-healing epoxy Amine microcapsule Hollow microcapsule Fracture toughness High temperature cured epoxy
Abstract: The use of structural fiber-reinforced polymeric composites and epoxy adhesives in aircraft, ships, vehicles, and wind turbine blades has increased markedly in the past decades. However, the polymeric matrix in these composites and adhesives typically consists of a brittle thermoset, which results in poor resistance to crack initiation and growth. Inspired by living systems, self-healing polymers are designed to autonomically repair damage whenever and wherever it occurs, thus providing a means to significantly extend the service life and reliability of polymeric structural elements. A variety of applications of the capsule-based self-healing motif have been demonstrated over the past decade. However, the limitations of available capsule-based self-healing systems, including catalyst stability and availability, cost, environmental toxicity, and difficulties in materials processing, have limited applications in low-temperature cured polymer matrices. Initially, self-healing epoxy adhesives were developed using a proven healing system - encapsulated dicyclopentadiene (DCPD) monomer and Grubbs’ first generation catalyst particles. Both quasi-static fracture and fatigue performance of a self-healing non-toughened epoxy adhesive were evaluated using the width-tapered-double-cantilever-beam (WTDCB) specimen geometry. The same healing system was then incorporated into a commercial high temperature cured rubber toughened epoxy adhesive, FM®73M. The addition of 117 μm average diameter microcapsules into ca. 750 μm thick rubber-toughened adhesives was shown to reduce the virgin fracture toughness, with this reduction increasing with microcapsule concentration. This reduction in virgin toughness was traced to localization of the fracture plane to the center of the adhesive where the capsules were relegated and a reduction in damage zone size. Improved methods of integration led to more uniform dispersion of microcapsules and retention of virgin toughness. Due to limitations of chemical and thermal compatibility of the DCPD/Grubbs’ catalyst healing system, a dual-capsule self-healing system using amine-epoxy chemistry was then developed. One capsule contains the epoxy monomer while the other contains a reactive amine curing agent. Amine microcapsules were prepared by vacuum infiltration of the target amine into hollow polymeric microcapsules. Epoxy microcapsules were prepared by an in situ polymerization method. Both types of capsules were incorporated into an epoxy matrix and recovery of mode-I fracture toughness was measured using tapered-double-cantilever-beam (TDCB) specimens. Good healing performance was obtained in room temperature cured epoxy using EPON 815C epoxy monomer and EPIKURE 3274, an aliphatic polyamine. However, this system proved unstable at high temperature (121 °C) and healing performance was dramatically reduced under high temperature curing conditions. Improved thermal stability was obtained by encapsulating EPON 813 epoxy monomer using a double shell wall encapsulation method and EPIKURE 3233 (polyoxypropylenetriamine). For this system, high healing efficiencies were retained for 121 150, and 177 °C curing conditions. Finally, a two part PDMS-based healing chemistry with high temperature stability was incorporated in a woven glass/epoxy fiber-reinforced composite and cured at 121 ºC. The healing system is comprised of one set of microcapsules containing silanol end-functionalized poly(dimethyl siloxane) and a crosslinking agent, poly(diethoxysilane), and a second set containing dibutyltin dilaurate catalyst in the solvent hexyl acetate. The effects of microcapsules on self-healing and mechanical properties including short beam strength, tension and compression strength/modulus were investigated. Self-healing of mechanical damage was assessed through the use of a pressure cell apparatus to detect nitrogen flow through a damaged composite. Complete self-healing was achieved when 35 μm diameter microcapsules at a loading of 7.5 wt% were added to the matrix.
Issue Date: 2012-09-18
URI: http://hdl.handle.net/2142/34437
Rights Information: Copyright 2012 Henghua Jin
Date Available in IDEALS: 2012-09-18
Date Deposited: 2012-08
 

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