© 2011 by Jericho Lynn Moll. All rights reserved

© 2011 by Jericho Lynn Moll. All rights reserved.

SELF-SEALING OF THERMAL FATIGUE AND MECHANICAL DAMAGE IN FIBER-REINFORCED

COMPOSITE MATERIALS

BY

JERICHO L. MOLL

DISSERTATION

Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Materials Science and Engineering

in the Graduate College of the University of Illinois at Urbana-Champaign, 2011

Urbana, Illinois

Doctoral Committee:

Professor Nancy R. Sottos, Chair and Director of Research Professor Paul V. Braun Professor Jianjun Cheng Professor Scott R. White

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ABSTRACT

Fiber reinforced composite tanks provide a promising method of storage

for liquid oxygen and hydrogen for aerospace applications. The inherent thermal

fatigue of these vessels leads to the formation of microcracks, which allow gas

phase leakage across the tank walls. In this dissertation, self-healing

functionality is imparted to a structural composite to effectively seal microcracks

induced by both mechanical and thermal loading cycles. Two different

microencapsulated healing chemistries are investigated in woven glass

fiber/epoxy and uni-weave carbon fiber/epoxy composites.

Self-healing of mechanically induced damage was first studied in a room

temperature cured plain weave E-glass/epoxy composite with encapsulated

dicyclopentadiene (DCPD) monomer and wax protected Grubbs’ catalyst healing

components. A controlled amount of microcracking was introduced through

cyclic indentation of opposing surfaces of the composite. The resulting damage

zone was proportional to the indentation load. Healing was assessed through

the use of a pressure cell apparatus to detect nitrogen flow through the

thickness direction of the damaged composite. Successful healing resulted in a

perfect seal, with no measurable gas flow. The effect of DCPD microcapsule size

(51 µm and 18 µm) and concentration (0 - 12.2 wt%) on the self-sealing ability

was investigated. Composite specimens with 6.5 wt% 51 µm capsules sealed

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67% of the time, compared to 13% for the control panels without healing

components.

A thermally stable, dual microcapsule healing chemistry comprised of

silanol terminated poly(dimethyl siloxane) plus a crosslinking agent and a tin

catalyst was employed to allow higher composite processing temperatures. The

microcapsules were incorporated into a satin weave E-glass fiber/epoxy

composite processed at 120⁰C to yield a glass transition temperature of 127⁰C.

Self-sealing ability after mechanical damage was assessed for different

microcapsule sizees (25 µm and 42 µm) and concentrations (0 – 11 vol%).

Incorporating 9 vol% 42 μm capsules or 11 vol% 25 μm capsules into the

composite matrix leads to 100% of the samples sealing. The effect of

microcapsule concentration on the short beam strength, storage modulus, and

glass transition temperature of the composite specimens was also investigated.

The thermally stable tin catalyzed poly(dimethyl siloxane) healing

chemistry was then integrated into a [0/90]s uniweave carbon fiber/epoxy

composite. Thermal cycling (-196⁰C to 35⁰C) of these specimens lead to the

formation of microcracks, over time, formed a percolating crack network from

one side of the composite to the other, resulting in a gas permeable specimen.

Crack damage accumulation and sample permeability was monitored with

number of cycles for both self-healing and traditional non-healing composites.

Crack accumulation occurred at a similar rate for all sample types tested. A 63%

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increase in lifetime extension was achieved for the self-healing specimens over

traditional non-healing composites.

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ACKNOWLEDGEMENTS

I would like to acknowledge many people who have contributed their

time and efforts toward the work presented in this dissertation. Many thanks to

my advisor, Prof. Nancy Sottos for her guidance, advice and compassion as I

struggled through the last five years, and to my co-adviser, Prof. Scott White, for

his encouragement and optimism as well as advice especially in the area of

composites manufacturing. I’d also like to thank the additional members of my

committee, Prof. Paul Braun and Prof. Jianjun Cheng for their time, input and

suggestions. To both past and present AMS members, in particular Dr. Amit

Patel, Dr. Chris Mangun, Dr. Ben Blaiszik, Dr. Mike Keller, Amanda Jones,

Cassandra Kingsbury, Brett Beiermann, Sen Kang and Kevin Hart, I thank you for

your help, support and suggestions in lab. You all are my friends and co-workers

and my experience here at U of I would not have been the same without you.

Finally, I would also like to acknowledge the Air Force Office of Scientific

Research and NASA for funding.

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TABLE OF CONTENTS

LIST OF TABLES ...................................................................................................... viii LIST OF FIGURES ...................................................................................................... ix CHAPTER 1: INTRODUCTION ................................................................................... 1

1.1 Motivation ............................................................................................. 1 1.2 Self-healing Polymers ............................................................................ 3 1.3 Self-healing Composites ........................................................................ 5 1.4 Dissertation Overview ........................................................................... 8 References .................................................................................................. 9

CHAPTER 2: SELF-HEALING OF MECHANICAL DAMAGE IN A LOW TG FIBER-REINFORCED COMPOSITE.................................................................. 12

2.1 Introduction ........................................................................................ 12 2.2 Manufacturing .................................................................................... 12

2.2.1 Preparation of Sample Components ................................... 12 2.2.2 Sample Types ....................................................................... 16 2.2.3 Sample Preparation ............................................................. 17

2.3 Sample Testing .................................................................................... 18 2.4 Damage Area Calculation .................................................................... 21 2.5 Results ................................................................................................. 22

2.5.1 Sealing Performance ............................................................ 22 2.5.2 Damage Characterization .................................................... 25

2.6 Summary and Conclusions .................................................................. 30 References ................................................................................................ 31

CHAPTER 3: SELF-HEALING OF MECHANICAL DAMAGE IN A HIGH TEMPERATURE CURED STRUCTURAL COMPOSITE ..................................................... 32

3.1 Abstract ............................................................................................... 32 3.2 Introduction ........................................................................................ 32 3.3 Materials and Methods ....................................................................... 35

3.3.1 Microencapsulated Healing Agent Preparation .................. 35 3.3.2 Sample Types ....................................................................... 40 3.3.3 Sample Preparation ............................................................. 41 3.3.4 Mechanical Testing .............................................................. 43

3.4 Results ................................................................................................. 45 3.4.1 Sealing Performance ............................................................ 45 3.4.2 Effect of Microcapsule Concentration on Mechanical

Properties ............................................................................. 46

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3.5 Discussion ............................................................................................ 50 3.6 Conclusions ......................................................................................... 50 References ................................................................................................ 51

CHAPTER 4: SELF-HEALING OF THERMAL FATIGUE DAMAGE IN A CARBON FIBER-

REINFORCED COMPOSITE.................................................................. 53 4.1 Introduction ........................................................................................ 53 4.2 Experimental Methods and Procedure ............................................... 55

4.2.1. Microcapsule Preparation ................................................... 55 4.2.2 Layup and Sample Types ...................................................... 57 4.2.3 Coefficient of Thermal Expansion Measurement ................ 58 4.2.4 Cryogenic Cycling and Permeability Testing ........................ 59

4.3 Results and Discussion ........................................................................ 63 4.3.1 Microcrack Accumulation .................................................... 63 4.3.2 Coefficient of Thermal Expansion ........................................ 66 4.3.3 Sealing Performance ............................................................ 68

4.4 Conclusions ......................................................................................... 70 References ................................................................................................ 71

CHAPTER 5: SUMMARY AND FUTURE WORK ....................................................... 74 5.1 Summary ............................................................................................. 74 5.2 Future Work ........................................................................................ 75 References ................................................................................................ 78

AUTHOR’S BIOGRAPHY ......................................................................................... 79

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LIST OF TABLES

2.1 Summary of resin components for various sample types .............................. 16 2.2 Sealing summary for self-healing and control specimens .............................. 25 3.1 Summary of sample types and components .................................................. 41 4.1 Summary of sample types and components .................................................. 57

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LIST OF FIGURES

1.1 Carbon fiber/epoxy composite cryogenic tank ................................................ 2 1.2 Carbon fiber/epoxy composite with a crack initiated by thermal cycling ........ 2 1.3 Composite face sheet due to core delamination in a helicopter rotor blade .. 3 1.4 Schematic of microcapsule based self-healing ................................................. 4 1.5 Schematic of microcapsule based healing in a fiber-reinforced composite .... 6 1.6 Image of hollow glass fibers embedded in a carbon fiber composite .............. 7 2.1 Optical image of 51 µm diameter DCPD microcapsules ................................. 13 2.2 Flow chart of UF/DCPD microencapsulation method..................................... 14 2.3 Histograms of UF/DCPD microcapsules for stirring rates of a) 1000 rpm and b)

1800 rpm ......................................................................................................... 14 2.4 Histograms of a) Grubbs catalyst in wax spheres manufactured at a stir rate

of 2500 rpm and b) plain wax spheres also manufactured with a stir rate of 2500 rpm ......................................................................................................... 15

2.5 Optical image of wax protected Grubbs’ catalyst microspheres .................... 16 2.6 Schematic of side view of sample layup ......................................................... 17 2.7 Schematic of indention apparatus to induce controlled damage in composite

laminates ......................................................................................................... 18 2.8 Schematic of pressure cell setup .................................................................... 19 2.9 Pressure evolution for representative sealed and control samples ............... 20 2.10 Digital image of a type 2 control sample with manual tracing around the

damaged region ............................................................................................. 21 2.11 Comparison of type 1 control samples with self-healing samples containing

varying capsule sizes and concentrations: a) percentage of samples sealing completely, b) initial leakage rate in nonsealed samples, and c) sealing efficiency. All samples were damaged with a peak force of 410 N. Error bars represent one positive standard deviation. .................................................. 23

2.12 Composite damage area as a function of damage load, capsule size and capsule concentration. Error bars represent one standard deviation ......... 26

2.13 Comparison of crack damage in 6.5 wt%, 51 µm diameter capsule type 2 control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c). ............................. 27

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2.14 Comparison of crack damage in 12.2 wt%, 18 µm diameter capsule type 2 control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c). ............................. 29

3.1 Dual microcapsule self-healing concept. a) Both types of microcapsules are dispersed into epoxy resin. b) A crack propagates in the resin, rupturing both types of microcapsules causing the encapsulated components to wick into the crack plane. c) Healing components mix and polymerize in the crack. The new polymerized material bonds the crack faces together, effectively healing the matrix. ....................................................................................................... 35

3.2 Flow chart of HOPDMS/PDES encapsulation procedure ................................ 36 3.3 Histograms of microcapsules used in this study: a) HOPDMS/PDES

microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 25 µm, b) HOPDMS/PDES microcapsules prepared at an agitation rate of 1500 rpm with an average diameter of 42 µm, c) hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 31 µm and d) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 41 µm.. .................. 37

3.4 Scanning electron microscope (SEM) images of a) 25 µm HOPDMS microcapsules prepared at an agitation rate of 2000 rpm and b) 41 µm DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm ………………………………………………………………………………………………….………………..38

3.5 Flow chart of DBTL/hexylacetate or hexylacetate only microencapsulation procedure. ....................................................................................................... 39

3.6 Flow chart of the rinsing procedure for DBTL/hexylacetate or hexylacetate only microcapsules.......................................................................................... 40

3.7 Schematic of composite layup components ................................................... 42 3.8 A polished cross-sectional image of an undamaged region of an 8H satin

weave composite sample. .............................................................................. 42 3.9 A polished tiled cross-sectional image of a control 2 sample with 11 vol% 25

μm capsules showing the percolating crack damage running from one side of the sample to the other. ................................................................................. 43

3.10 Summary of self-sealing results for composite samples with multiple sizes and concentrations of HOPDMS/PDES microcapsules dispersed in the matrix.. .......................................................................................................... 46

3.11 Composite short beam strength as a function of microcapsule concentration. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation. ....................................................................................... 47

3.12 Representative short beam shear loading curves for composite samples with varying microcapsule concentrations. Vc is capsule volume fraction in the composite ................................................................................................ 47

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3.13 Comparison of storage modulus values with expected model predictions. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation ............ 48

3.14 Changing glass transition temperature with microcapsule concentration in fiber reinforced composite and epoxy samples. Microcapsule volume fractions are reported in the resin only, not accounting for the inclusion of fiber reinforcement. Horizontal line represents the Tg of epoxy resin combined with HOPMDS monomer. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation ................................................................. 49

4.1 Histogram of microcapsules used in this study: a) HOPDMS/PDES microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 29 ± 15 µm, b) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 44 ± 16 µm .......... 56

4.2 Representative plot of the change in sample thickness with temperature curve................................................................................................................ 59

4.3 Schematic of thermal fatigue apparatus ........................................................ 60 4.4 Approximate temperature profiles through the thickness of composite

samples for a) heating to 35⁰C and b) cooling to -196⁰C ............................... 61 4.5 Illustration showing how nitrogen gas can escape through a thermally

damaged composite sample in an unmodified pressure cell ......................... 62 4.6 Schematic of the pressure cell apparatus. a) Sealed sample chamber with

sample and rubber o-rings. b) Complete test apparatus with the sealed sample chamber inserted in the pressure cell................................................ 63

4.7 Optical image of a polished cross-section of a control 1 sample with the plies and fiber orientations labeled ........................................................................ 64

4.8 Microcrack density with number of thermal cycles in a) plies 1 and 4 and b) plies 2 and 3 .................................................................................................... 64

4.9 Representative optical micrographs of thermally induced cracks. a) Microcrack in ply 4 in a control 1 sample after 110 thermal cycles. b) Microcrack in ply 2 and 3 in a control 2 sample after 310 thermal cycles. c) Delamination running between plies 3 and 4 in a control 2 sample after 310 thermal cycles ................................................................................................. 65

4.10 Comparison of coefficient of thermal expansion of epoxy resin with expected rule of mixtures values. Error bars represent one standard deviation ...................................................................................................... 67

4.11 Optical images of delamination running between plies after 2500 thermal cycles in a) control 1 and b) control 2 samples .............................................. 68

4.12 Representative output pressure, after 10 minutes of testing, of a control 1 sample with number of thermal cycles .......................................................... 69

4.13 Percentage of samples sealed with number of thermal cycles .................... 70

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CHAPTER 1

INTRODUCTION

1.1 Motivation

Fiber reinforced polymer matrix composites are used in a wide variety of

applications due to their excellent strength to weight ratio and the ability of the

engineer to tailor specific mechanical properties into the composite. Unfortunately,

composite materials are susceptible to matrix cracks, as well as large-scale

delaminations that run between composite layers.

Matrix cracking is particularly problematic in composite cryogenic tanks. Figure

1.1 shows an image of a carbon fiber composite tank which is underdevelopment as a

fuel storage container in liquid oxygen and nitrogen for aerospace applications [1]. The

coefficient of thermal expansion mismatch between the carbon/epoxy layer leads to the

generation of high stresses during thermal cycling. These stresses initiate transverse ply

cracks, which can percolate, rendering the composite walls permeable.

Figure 1.2 shows a typical transverse ply crack due to thermal cycling of a

carbon/epoxy composite. Previous studies have investigated the leakage rate of

gaseous fuel through damaged composite tank walls. Peddiraju et al. predicted

numerically how the leakage rate of cryogens varies with composite ply number,

thickness and damage opening in the material [2]. Other studies measured the effect of

2

mechanical and thermal damage on the leakage rate of hydrogen gas through a

reinforced composite material [3]. Bechel et al. quantified how the density of thermally

induced cracks and resulting permeability increase with the number of thermal cycles

[4-6].

Figure 1.1 Carbon fiber/epoxy composite cryogenic tank.

Figure 1.2 Carbon fiber/epoxy composite with a crack initiated by thermal cycling.

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Composite sandwich structures are used widely for aerospace applications such

as airplane rudders, ailerons and flaps. The face sheets of these sandwich structures are

susceptible to cracking due to mechanical fatigue or low velocity impact such as a tool

drop which can induce microcracking and increase water absorption into the core

material. Moisture absorption not only increases the overall weight of the composite

but can cause the face sheet/core bond to weaken leading to delamination (Figure 1.3)

and ultimately, premature composite failure [7].

Figure 1.3 Composite face sheet due to core delamination in a helicopter rotor blade [8].

1.2 Self-healing Polymers

The introduction of self-healing functionality provides a method of reversing the

damage in polymeric materials and improving the mechanical properties of the part [9].

Self-healing has been successfully demonstrated in thermosetting [10], fiber reinforced

composites [11-13], elastomers [14], coatings [15, 16], as well as self-healing adhesives

4

[17]. A comprehensive review of self-healing was recently published by Blaiszik et al.

[18]. The basic concept for autonomic healing as originally proposed by White et al. is

shown schematically in Figure 1.4 [9]. A solid catalyst and an encapsulated monomer

are dispersed in a polymeric matrix material. Healing is triggered by crack damage

which causes the capsules to rupture. Monomer is wicked into the crack plane due to

capillary forces and contacts the embedded catalyst, initiating a polymerization

reaction. The new polymerized material bonds the crack faces together, effectively

healing the matrix.

Figure 1.4 Schematic of microcapsule based self-healing [9].

While most of the literature has focused on healing of mode I cracks, a few

recent investigation have demonstrated the ability to heal puncture damage and

prevent gas from flowing through the material. Beiermann et al. introduced

Microcapsule

Catalyst

Crack

Healing agent

Polymerized healing agent

5

encapsulated poly(dimethyl siloxane) (PDMS) based healing components into the PDMS

layer of a polyurethane/nylon/PDMS laminate [19]. Damage induced a

polycondensation reaction between the encapsulated healing components effectively

filling the puncture and regaining the ability to seal. Kalista et al. reported a self-healing

poly(ethylene-co-methacrylic acid) copolymer capable of autonomic molecular

rearrangement which was able to maintain pressure up to 3 MPa after projectile

puncture damage [20]. Self-healing is inherent in ionomers, which have a two-step melt

elastic recovery and inter chain diffusion self-healing mechanism.

1.3 Self-healing Composites

In fiber reinforced composites, the healing components (microcapsules and

catalyst) must be sequestered in the matrix rich regions of the material, shown

schematically in Figure 1.5 for woven fiber architecture. Kessler et al. incorporated

dicyclopentadiene (DCPD) microcapsules and solid particles of Grubbs’ catalyst into

graphite epoxy composites [11]. Using a width tapered double cantilevered beam

specimen geometry, they demonstrated a 38% recovery of interlaminar fracture

toughness after healing at room temperature, and a 66% recovery after healing at 80⁰C.

Patel et al. used a similar healing chemistry composed of encapsulated DCPD and wax

protected Grubbs’ catalyst. Wax protection was used to prevent deactivation of the

catalyst by amines present in the epoxy matrix [21, 22]. Patel reported a reduction in

crack length in self-healing samples and the ability to withstand greater impact energies

6

before a reduction in residual compressive strength was observed [23]. Moll et al.

demonstrated the ability to self-seal and prevent gas phase leakage across a composite

containing microencapsulated DCPD and wax protected Grubbs’ catalyst [24]. Using an

encapsulated epoxy resin and latent imidazole curing agent in the matrix, Yin et al.

demonstrated a recovery of interlaminar fracture toughness after the application of

heat (130 °C for 1 hour) [25, 26]. Although this was not autonomic self-healing, the

method did heal the cracks formed in the matrix material.

Figure 1.5 Schematic of microcapsule based healing in a fiber-reinforced composite [23].

Bond and co-workers developed an alternative healing strategy to recover

mechanical properties and visualize the damaged region of a composite [13, 27-37].

Hollow glass fiber reinforcement with an outer diameter of 60 µm was embedded into

glass and carbon fiber reinforced composites (Figure 1.6). These hollow glass fibers

7

were filled with either uncured epoxy resin or curing agent, which upon fracture, were

delivered to the damaged region of the sample [29, 30, 38]. Residual strengths of 87%

were reported after self-healing of impact damage. In an alternate application,

fluorescent dye was incorporated into the composite material with the healing

components. The dye wicked into the cracks in the material enabling visual assessment

of damage in the sample [13, 28].

Figure 1.6 Image of hollow glass fibers embedded in a carbon fiber composite [29].

Employing a vascularized approach, Bond et al. incorporated 1.5 mm PVC tubing

into the core of a composite sandwich structure [33, 36]. These tubes were used to

deliver healing agent to the damaged foam core and shown to recover the flexural and

compressive strength after impact of the sandwich structure composite. Bond and

Trask also developed a method to embed solder wires into advanced composites which

were later removed by the application of heat to create a network of channels for

healing agent delivery [31].

0.10 mm

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1.4 Dissertation Overview

The work described in this dissertation focuses on the manufacture, testing and

sealing performance of self-healing fiber reinforced composite materials with low and

high glass transition temperatures. Self-healing of mechanical and thermal fatigue

damage is investigated along with the influence of microcapsules on the mechanical

properties of the composite material.

Chapter 2 describes the sealing ability of a self-healing low temperature cured (and

low Tg) glass fiber reinforced composite. In this dissertation, self-sealing is defined as

the ability of a material to heal a percolating crack network in such a way that gas

cannot pass through the sample.

This includes the manufacturing of the healing components and the composite

specimens along with the testing procedure and results. The influence of microcapsule

concentration on sealing results and damage area are also discussed.

In chapter 3, a fully cured high Tg self-healing composite is described. Included is

a description of the thermally stable healing chemistry, manufacturing of the

encapsulated components and composite samples along with the influence of

microcapsule concentration on the sealing results, Young’s modulus and interlaminar

properties of the specimens.

Chapter 4 describes the thermal fatigue damage of carbon fiber/epoxy

composites with microcapsules embedding in the matrix. The accumulation of crack

damage with number of thermal cycles along with the ability of these composites to

self-healing thermal damage is discussed.

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Finally, in chapter 5 overall conclusions and results of this dissertation are

summarized and compiled. Suggestions for future work and directions are also

included.

References [1] R. Heydenreich, “Cryotanks in future vehicles,” Cryogenics, vol. 38, no. 1, pp. 125-130,

Jan, 1998. [2] P. Peddiraju, J. Noh, J. Whitcomb et al., “Prediction of cryogen leak rate through

damaged composite laminates,” Journal of Composite Materials, vol. 41, no. 1, pp. 41-71, Jan, 2007.

[3] R. Grenoble, and T. Gates, “Hydrogen Gas Leak Rate Testing and Post-Testing Assessment of Microcrack Damaged Composite Laminates,” in AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, Newport, Rhode Island, 2006.

[4] V. T. Bechel, M. B. Fredin, S. L. Donaldson et al., “Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 7, pp. 663-672, 2003.

[5] V. T. Bechel, J. D. Camping, and R. Y. Kim, “Cryogenic/elevated temperature cycling induced leakage paths in PMCs,” Composites Part B-Engineering, vol. 36, no. 2, pp. 171-182, 2005.

[6] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.

[7] H. S. Choi, and Y. H. Jang, “Bondline strength evaluation of cocure/precured honeycomb sandwich structures under aircraft hygro and repair environments,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 9, pp. 1138-1147, Sep, 2010.

[8] Airwolf_Aerospace. March 3, 2011, 2011; www.airwolfaerospace.com. [9] S. R. White, M. M. Caruso, and J. S. Moore, “Autonomic healing of polymers,” Mrs

Bulletin, vol. 33, no. 8, pp. 766-769, Aug, 2008. [10] E. N. Brown, N. R. Sottos, and S. R. White, “Fracture testing of a self-healing polymer

composite,” Experimental Mechanics, vol. 42, no. 4, pp. 372-379, Dec, 2002. [11] M. R. Kessler, and S. R. White, “Self-activated healing of delamination damage in woven

composites,” Composites Part a-Applied Science and Manufacturing, vol. 32, no. 5, pp. 683-699, 2001.

[12] M. R. Kessler, N. R. Sottos, and S. R. White, “Self-healing structural composite materials,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 8, pp. 743-753, 2003.

[13] J. W. C. Pang, and I. P. Bond, “'Bleeding composites' - damage detection and self-repair using a biomimetic approach,” Composites Part a-Applied Science and Manufacturing, vol. 36, no. 2, pp. 183-188, 2005.

[14] M. W. Keller, S. R. White, and N. R. Sottos, “A self-healing poly(dimethyl siloxane) elastomer,” Advanced Functional Materials, vol. 17, no. 14, pp. 2399-2404, Sep, 2007.

[15] S. H. Cho, S. R. White, and P. V. Braun, “Self-Healing Polymer Coatings,” Advanced Materials, vol. 21, no. 6, pp. 645-+, Feb, 2009.

http://www.airwolfaerospace.com/

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[16] K. S. Toohey, N. R. Sottos, and S. R. White, “Characterization of Microvascular-Based Self-healing Coatings,” Experimental Mechanics, vol. 49, no. 5, pp. 707-717, Oct, 2009.

[17] G. Miller, “Self-Healing Adhesive Film for Composite Laminate Repairs on Metallic Structures,” Aerospace Engineering, University of Illinois, Urbana-Champaign, Illinois, 2007.

[18] B. J. Blaiszik, S. L. B. Kramer, S. C. Olugebefola et al., "Self-Healing Polymers and Composites," Annual Review of Materials Research, Vol 40, Annual Review of Materials Research, pp. 179-211, 2010.

[19] B. A. Beiermann, M. W. Keller, and N. R. Sottos, “Self-healing flexible laminates for resealing of puncture damage,” Smart Materials & Structures, vol. 18, no. 8, Aug, 2009.

[20] S. J. Kalista, “Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture,” Mechanics of Advanced Materials and Structures, vol. 14, no. 5, pp. 391-397, 2007.

[21] J. D. Rule, E. N. Brown, N. R. Sottos et al., “Wax-protected catalyst microspheres for efficient self-healing materials,” Advanced Materials, vol. 17, no. 2, pp. 205-+, Jan, 2005.

[22] G. O. Wilson, J. S. Moore, S. R. White et al., “Autonomic healing of epoxy vinyl esters via ring opening metathesis polymerization,” Advanced Functional Materials, vol. 18, no. 1, pp. 44-52, Jan, 2008.

[23] A. J. Patel, N. R. Sottos, E. D. Wetzel et al., “Autonomic healing of low-velocity impact damage in fiber-reinforced composites,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 3, pp. 360-368, Mar, 2010.

[24] J. L. Moll, S. R. White, and N. R. Sottos, “A Self-sealing Fiber-reinforced Composite,” Journal of Composite Materials, vol. 44, no. 22, pp. 2573-2585, Oct, 2010.

[25] T. Yin, M. Z. Rong, J. S. Wu et al., “Healing of impact damage in woven glass fabric reinforced epoxy composites,” Composites Part a-Applied Science and Manufacturing, vol. 39, no. 9, pp. 1479-1487, Sep, 2008.

[26] T. Yin, L. Zhou, M. Z. Rong et al., “Self-healing woven glass fabric/epoxy composites with the healant consisting of micro-encapsulated epoxy and latent curing agent,” Smart Materials & Structures, vol. 17, no. 1, Feb, 2008.

[27] C. Y. Huang, R. S. Trask, and I. P. Bond, “Characterization and analysis of carbon fibre-reinforced polymer composite laminates with embedded circular vasculature,” Journal of the Royal Society Interface, vol. 7, no. 49, pp. 1229-1241, Aug, 2010.

[28] J. W. C. Pang, and I. P. Bond, “A hollow fibre reinforced polymer composite encompassing self-healing and enhanced damage visibility,” Composites Science and Technology, vol. 65, no. 11-12, pp. 1791-1799, Sep, 2005.

[29] R. S. Trask, G. J. Williams, and I. P. Bond, “Bioinspired self-healing of advanced composite structures using hollow glass fibres,” Journal of the Royal Society Interface, vol. 4, no. 13, pp. 363-371, Apr, 2007.

[30] R. S. Trask, H. Rwilliams, and I. P. Bond, “Self-healing polymer composites: mimicking nature to enhance performance,” Bioinspiration & Biomimetics, vol. 2, no. 1, pp. P1-P9, Mar, 2007.

[31] R. S. Trask, and I. P. Bond, “Bioinspired engineering study of Plantae vascules for self-healing composite structures,” Journal of the Royal Society Interface, vol. 7, no. 47, pp. 921-931, Jun, 2010.

[32] G. Williams, R. Trask, and I. Bond, “A self-healing carbon fibre reinforced polymer for aerospace applications,” Composites Part a-Applied Science and Manufacturing, vol. 38, no. 6, pp. 1525-1532, 2007.

11

[33] H. R. Williams, R. S. Trask, and I. P. Bond, “Self-healing composite sandwich structures,” Smart Materials & Structures, vol. 16, no. 4, pp. 1198-1207, Aug, 2007.

[34] H. R. Williams, R. S. Trask, A. C. Knights et al., “Biomimetic reliability strategies for self-healing vascular networks in engineering materials,” Journal of the Royal Society Interface, vol. 5, no. 24, pp. 735-747, Jul, 2008.

[35] H. R. Williams, R. S. Trask, P. M. Weaver et al., “Minimum mass vascular networks in multifunctional materials,” Journal of the Royal Society Interface, vol. 5, no. 18, pp. 55-65, Jan, 2008.

[36] H. R. Williams, R. S. Trask, and I. P. Bond, “Self-healing sandwich panels: Restoration of compressive strength after impact,” Composites Science and Technology, vol. 68, no. 15-16, pp. 3171-3177, Dec, 2008.

[37] G. J. Williams, I. P. Bond, and R. S. Trask, “Compression after impact assessment of self-healing CFRP,” Composites Part a-Applied Science and Manufacturing, vol. 40, no. 9, pp. 1399-1406, Sep, 2009.

[38] R. S. Trask, and I. P. Bond, “Biomimetic self-healing of advanced composite structures using hollow glass fibres,” Smart Materials & Structures, vol. 15, no. 3, pp. 704-710, Jun, 2006.

12

CHAPTER 2

SELF-HEALING OF MECHANICAL DAMAGE IN A LOW TG FIBER-REINFORCED COMPOSITE

2.1 Introduction

In this chapter, we describe a self-healing plain weave E-glass epoxy composite with the

healing components, microencapsulated dicyclopentadiene (DCPD), and paraffin wax coated

Grubbs’ catalyst dispersed throughout the matrix. Healing is assessed through use of a pressure

cell apparatus to detect nitrogen flow through the thickness direction of a damaged composite.

Successful healing resulted in a perfect seal, with no measureable gas flow. A controlled

amount of microcracking is introduced through cyclic indentation of opposing surfaces of the

sample. The resulting damage zone is proportional to the indentation load. We investigate the

effect of DCPD microcapsule size and concentration on the self-sealing ability of plain weave E-

glass epoxy composites.

2.2 Manufacturing

2.2.1 Preparation of Sample Components

The healing agent, DCPD (Alpha Aesar) monomer was distilled, stabilized with

150 ppm p-t-butylcatechol and encapsulated in a urea-formaldehyde (UF) shell (Figure

13

2.1). Figure 2.2 contains a diagram of the encapsulation procedure. Sealing

performance was investigated for two microcapsule sizes which were varied by

controlling the stir rate. The resulting histograms of microcapsule size distribution are

shown in Figure 2.3. The larger capsules, 51 µm in diameter, were manufactured by the

oil in water emulsion in situ polymerization technique described by Brown et al. [1]. The

smaller capsules, 18 µm in diameter, were manufactured in a similar fashion with a few

procedural modifications. The amount of ethylene-maleic anhydride (EMA) was

increased by 50% from the amount specified by Brown et al. [1]. The additional amount

of stabilizing agent was needed because the surface area of the DCPD/water interface in

the emulsion increases with decreasing droplet size. After completion of the

encapsulation procedure, the excess EMA was eliminated by centrifuging the capsule

solution in deionized water three times, decanting off the water after each cycle. The

capsules were spray dried into a dry, free flowing white powder.

Figure 2.1 Optical image of 51 µm diameter DCPD microcapsules.

14

Figure 2.2 Flow chart of UF/DCPD microencapsulation method.

Figure 2.3 Histograms of UF/DCPD microcapsules for stirring rates of a) 1000 rpm and b)

1800 rpm.

Grubbs’ catalyst in wax microspheres and plain wax microspheres were

manufactured in a method previously outlined by Rule et al. [2]. A histogram showing

15

the size distribution of both of these components is in Figure 2.4. As received first

generation Grubbs’ catalyst (Aldrich) was dissolved in benzene in a method previously

outlined by Jones et al. [3] and freeze-dried and protected from deactivation by primary

amines [4] in the epoxy matrix curing agent by paraffin wax. The 11 µm diameter

Grubbs’ catalyst, shown in Figure 2.5, and wax spheres were centrifuged three times in

water to remove EMA, the emulsion stabilizing agent. The resulting solution was frozen

in liquid nitrogen and freeze dried for approximately 48 h, resulting in a dry powder.

Catalyst-free wax microspheres with an average diameter of 26 µm, were also produced

by this protocol for control samples.

Figure 2.4 Histograms of a) Grubbs catalyst in wax spheres manufactured at a stir rate of

2500 rpm and b) plain wax spheres also manufactured with a stir rate of 2500 rpm.

16

Figure 2.5 Optical image of wax protected Grubbs’ catalyst microspheres.

2.2.2 Sample Types

Self-healing composite panels were fabricated along with two types of

corresponding control panels. The components in each of these sample types are

summarized in Table 2.1. Self-healing samples consisted of epoxy resin, fiber

reinforcement, microcapsules, and wax protected catalyst microspheres. The first type

of control specimens were made of epoxy resin and fiber reinforcement only and

contained no self-healing components (microcapsules or wax microspheres). The second

type of control specimens were comprised of epoxy resin, fiber reinforcement,

microcapsules, and wax microspheres without catalyst.

Table 2.1 Summary of resin components for various sample types.

Sample type Epoxy Resin

E-glass reinforcement

Wax protected catalyst

Wax micro-

spheres

UF/DCPD micro-

capsules

Self-healing X X X - X Control 1 X X - - - Control 2 X X - X X

17

2.2.3 Sample Preparation

Composite panels were manufactured using a hand lay-up and compression

molding technique similar to the method outlined by Kessler et al. [5]. A side view of

the layup schematic can be seen in Figure 2.6. The composite reinforcement consisted

of four 150 mm by 200 mm plain weave E-glass (340 g/m2) plies. We selected Epon 862

epoxy resin with Epikure 3274 curing agent mixed at a ratio of 100:40 as the matrix. The

dry capsules and catalyst spheres were gently mixed into the resin followed by

degassing under vacuum for at least twenty minutes. The layup was compacted under

170 kPa at 35 ⁰C for 72 h, creating samples with an average fiber volume fraction of

0.35. Self-healing and control (type 2) samples were prepared with varying capsule

concentrations.

Figure 2.6 Schematic of side view of sample layup.

18

2.3 Sample Testing

Each composite panel was sectioned into twelve 45 mm by 45 mm samples. We

developed an indentation protocol to induce consistent amounts of damage in the

composite samples. A schematic of the indentation apparatus is shown in Figure 2.7.

The square composite specimens were simply supported on a ring with an inner

diameter of 24 mm and a height of 4 mm. The samples were damaged by driving a

Vicker’s diamond indenter tip into both sides of the laminate with a predetermined

velocity and load. The indenter tip was positioned above the middle of the sample and

was cyclically driven into the surface of the specimen ten times on each side. All

samples, unless otherwise noted, were damaged with a maximum load of 410 N at a

velocity of 150 µm/s. After the damage cycle, the samples were allowed to heal

overnight at 30 ⁰C before testing continued.

Figure 2.7 Schematic of indention apparatus to induce controlled damage in composite

laminates.

19

Self-sealing ability was evaluated using a pressure cell apparatus to quantify

nitrogen flow through the damaged composites. The pressure cell (Figure 2.8) was

designed for prior experiments by Beiermann et al. to study healing of puncture damage

in PDMS laminates [6]. In the cell, the sample served as a barrier between a pressurized

chamber and the atmosphere. Pressure on both sides of the sample was monitored

with time during the experiment.

Figure 2.8 Schematic of pressure cell setup [7].

Laminates were placed in the sample chamber and securely clamped between

two O-rings to eliminate leakage from the cell. A computer controlled regulator (EP211-

X60-5V, Omega) was used to ramp the applied pressure to the cell to 276 kPa. Pressure

transducers were placed on either side of the sample. The first transducer measured

the input pressure applied to the sample by the regulator. The second transducer

measured the output pressure on the opposite side due to nitrogen flow through the

sample. If no output pressure was detected then the specimen was fully healed or there

20

was insufficient damage to create an interpenetrating crack network that allowed

nitrogen to flow through the specimen. Hence, healing of damage was assessed by the

change in pressure recorded by the second transducer. The input pressure and the time

evolution of the output pressure measured for representative control type 1 and type 2

samples along with a fully sealed self-healing sample are plotted in Figure 2.9.

Figure 2.9 Pressure evolution for representative sealed and control samples.

Three different criteria were used to evaluate the self-sealing ability of

composite specimens, the percentage of fully healed samples, the initial slope, and the

sealing efficiency (ηs). A sample was considered fully healed if the output pressure did

not increase by more than 0.07 kPa after thirty minutes of testing. In addition, for

samples that leaked we characterized the leakage rate by calculating the initial slope of

the pressure evolution curve. A large slope corresponded to a high gas flow rate, while

a small slope corresponded to a sample with low gas flow. A sealing efficiency was also

calculated for all samples after thirty minutes of testing,

21

30min

( )1

( )

outS

in t

P t

P t

(2.1)

where pressure readings are taken after thirty minutes. A sealing efficiency of zero

corresponded to a final output pressure at thirty minutes equal to the input pressure

(no healing), while a fully healed sample yielded an efficiency of unity.

2.4 Damage Area Calculation

In both types of composite control samples, the opaque damage area was easily

identified. As shown in Figure 2.10, the damaged region was manually traced using

photo editing software. The damage area was then averaged over all samples of a given

type. In self-healing samples it was not possible to accurately identify the damage area

using this method due to the dark purple color of the Grubbs’ catalyst embedded in the

specimens.

Figure 2.10 Digital image of a type 2 control sample with manual tracing around the

damaged region [7].

22

2.5 Results

2.5.1 Sealing Performance

The effects of capsule size and concentration on self-sealing results were

investigated for 51 µm and 18 µm diameter capsules at concentrations of 12.2 wt%, 6.5

wt% and 2.7 wt% in resin with wax protected catalyst at concentrations of 2.4 wt%, 2.6

wt% and 2.7 wt% respectively. The total amount of wax protected catalyst was kept

constant, but the overall concentration changed due to differences in the mass of

microcapsules added. Figure 2.11 compares the sealing performance of self-healing

specimens with varying microcapsule concentrations and diameters to that of the type 1

control samples (no microcapsules or catalyst). All samples in this data set (see Table

2.2) were damaged at a maximum load of 410 N. The 12.2 wt% and the 6.5 wt%, 51 µm

diameter capsule self-healing samples performed better than the type 1 control samples

in all three of the sealing evaluation categories. The 6.5 wt% specimens with 51 µm

diameter capsules had the highest percentage sealed, the lowest leakage rate and the

highest sealing efficiency. In contrast, none of the 6.5 wt% specimens with 18 µm

diameter capsules fully sealed, although their leakage rate was significantly lower and

sealing efficiency significantly higher than type 1 control samples.

23

Figure 2.11 Comparison of type 1 control samples with self-healing samples containing

varying capsule sizes and concentrations: a) percentage of samples sealing completely, b) initial leakage rate in nonsealed samples, and c) sealing efficiency. All samples were damaged with a peak force of 410 N. Error bars represent one positive standard deviation [7].

24

Specimens with 2.7 wt% 51 µm diameter capsules and both concentrations of 18

µm diameter capsules have a low healing percentage, but a lower leakage rate and

higher sealing efficiency than type 1 controls. The sealing data for these specimens

suggests that insufficient amounts of healing agent are delivered to the damaged region

for full sealing of crack networks.

The sealing performance of type 2 control specimens was also investigated and

compared to corresponding self-healing specimens with various capsule concentrations

for the 51 µm diameter specimens. For all three capsule concentrations investigated,

the self-healing samples performed better than the respective type 2 control samples.

The effect of peak damage load on the sealing results was also investigated by

damaging 6.5 wt% 51 µm capsule type 2 control and self-healing samples at a lower

peak load of 380 N (instead of the standard 410 N used for all other samples). A

summary of the sealing results for these samples is provided within Table 2.2. All of the

self-healing samples sealed fully in these tests (100%), compared to 25% for the control

samples. This result indicates that there is a maximum peak load below which 100%

sealing can be achieved for a given capsule size and concentration.

25

Table 2.2 Sealing summary for self-healing and control specimens.

Sample Type

Capsule Diameter

(m)

Capsule Concentration (wt% in resin)

Peak Load (N)

Percent Healed

(%)

Leakage Rate

(kPa/min)

Sealing Efficiency,

s

Self-healing 51 12.2 410 58 9.1 0.81 Self-healing 51 6.5 410 67 9.2 0.47 Self-healing 51 2.7 410 17 50.9 0.52

Control-2 51 12.2 410 8 17.6 0.43 Control-2 51 6.5 410 0 83.1 0.14 Control-2 51 2.7 410 8 87.6 0.31

Self-healing 18 12.2 410 8 21.4 0.40 Self-healing 18 6.5 410 0 15.1 0.42

Control-1 N/A 0 410 13 111.7 0.21 Self-healing 51 6.5 380 100 N/A 1.00

Control-2 51 6.5 380 25 12.2 0.77

2.5.2 Damage Characterization

To further understand the relationship between capsule size, concentration and

sealing performance, we characterized the area of the damage zone in both types of

control samples and compared the crack density in cross-sections of both healed and

type 2 control specimens. The damage zone area of the control samples was analyzed

from digital specimen images acquired after indentation (Figure 2.10). The damage

area, plotted in Figure 2.12, increased with increasing capsule concentration and size.

Although increasing the capsule concentration increases the amount of DCPD delivered

to the crack, it also increases the amount of crack damage, suggesting that an optimum

capsule concentration exists for a given amount of damage. For the self-healing

samples with 51 µm diameter capsules, the best sealing performance was achieved with

6.5 wt% capsule specimens. The higher capsule concentration (12.5 wt%) specimens led

to a larger amount of damage which could not be filled as effectively with capsules of

26

this size. The lower capsule concentration (2.7 wt%) specimens have less damage, but

insufficient amounts of healing agent for complete sealing. Similarly, samples with 18

µm capsules had less damage area than the 51 µm capsule samples, but the percentage

of samples which sealed was substantially lower. Thus, smaller capsules do not deliver a

sufficient amount of DCPD to seal the damage.

Figure 2.12 Composite damage area as a function of damage load, capsule size and

capsule concentration. Error bars represent one standard deviation [7].

The crack network was evaluated by comparing polished cross-sections of

damaged regions in the composite specimens. Tiled scanning electron microscope

images are shown in Figures 2.13 and 2.14 for type 2 control and self-healing samples

with 6.5 wt% 51 µm capsules and 12.2 wt% 18 µm capsules, respectively. The cracks

have been manually highlighted in white within the image of the type 2 control sample

in a) and the self-healing sample in c). Only the highlighted cracks are shown for the

control sample in b) and the self-healing sample in d). Although cracks are still present

27

in the self-healing sample, enough of the cracks have healed to disrupt the percolation

of crack damage, thus successfully sealing the sample.

Figure 2.13 Comparison of crack damage in 6.5 wt%, 51 µm diameter capsule type 2

control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c) [7].

28

As revealed in Figures 2.13 and 2.14, cracks in the composite samples run

predominately between tows through resin rich regions in the composite, rupturing

microcapsules in this area. The region directly under the indenter tip has the largest

crack separation, which decreases radially. The self-healing cross-sections (Figure 2.13

c, d and Figure 2.14 c, d) reveal a noticeable reduction in the number of cracks and a

smaller crack separation when compared to the type 2 control samples (Figure 2.13 a, b

and Figure 2.14 a, b). Cracks with smaller separation are more easily healed due to the

smaller volume of DCPD needed to fully seal the damage.

29

Figure 2.14 Comparison of crack damage in 12.2 wt%, 18 µm diameter capsule type 2

control, and self-healing specimens: a) tiled SEM images of a polished cross-section of damaged area in a type 2 control, b) highlighted crack damage in black corresponding to the image in a), c) tiled SEM images of a polished cross-section of damaged area in a self-healing specimen, d) highlighted crack damage in black corresponding to the image in c) [7].

30

2.6 Summary and Conclusions

A self-healing fiber reinforced polymer matrix composite was achieved by

incorporating urea-formaldehyde encapsulated dicyclopentadiene monomer and wax

protected Grubbs’ catalyst into the matrix of a glass fiber reinforced epoxy composite

using a wet lay-up technique. The specimens were damaged by repeated indentation at

prescribed loads. The connectivity of cracks in the damaged composite was evaluated

by applying pressurized nitrogen gas to one side of the sample and monitoring the

output pressure on the opposite side of the specimen.

Protocols were established to evaluate the self-sealing capacity of these

composite specimens by monitoring the percentage of samples that fully healed, initial

leakage rate, and the sealing efficiency. The effects of capsule size and concentration on

the self-sealing performance of the samples were evaluated. Specimens with 6.5 wt%

51 µm capsules performed the best in all three sealing assessment categories.

Although the addition of microcapsules increases the fracture toughness of the

matrix [8], increasing capsule concentration and size leads to an increase in damage

area in composites with both microcapsules and paraffin wax microspheres. The

optimum capsule concentration for a given capsule size depends on the size of the

damage volume compared to the volume of healing agent delivered.

Normal composite samples (type 1 controls) were tested and compared to self-

healing samples. Specimens with 12.2 wt% and 6.5 wt% 51 µm capsules outperformed

the normal composites in all three sealing categories. All self-healing samples,

regardless of capsule size or concentration, show lower leakage rate and higher sealing

31

efficiency compared to normal composites. Hence, optimized self-healing composites

show great potential to seal non-catastrophic crack damage in fiber reinforced

composite materials.

References [1] E. N. Brown, M. R. Kessler, N. R. Sottos et al., “In situ poly(urea-formaldehyde)

microencapsulation of dicyclopentadiene,” Journal of Microencapsulation, vol. 20, no. 6, pp. 719-730, Nov-Dec, 2003.

[2] J. D. Rule, E. N. Brown, N. R. Sottos et al., “Wax-protected catalyst microspheres for efficient self-healing materials,” Advanced Materials, vol. 17, no. 2, pp. 205-+, Jan, 2005.

[3] A. S. Jones, J. D. Rule, J. S. Moore et al., “Catalyst morphology and dissolution kinetics of self-healing polymers,” Chemistry of Materials, vol. 18, no. 5, pp. 1312-1317, Mar, 2006.

[4] G. O. Wilson, J. S. Moore, S. R. White et al., “Autonomic healing of epoxy vinyl esters via ring opening metathesis polymerization,” Advanced Functional Materials, vol. 18, no. 1, pp. 44-52, Jan, 2008.

[5] M. R. Kessler, and S. R. White, “Self-activated healing of delamination damage in woven composites,” Composites Part a-Applied Science and Manufacturing, vol. 32, no. 5, pp. 683-699, 2001.



[8] E. N. Brown, N. R. Sottos, and S. R. White, “Fracture testing of a self-healing polymer composite,” Experimental Mechanics, vol. 42, no. 4, pp. 372-379, Dec, 2002.

32

CHAPTER 3

SELF-HEALING OF MECHANICAL DAMAGE IN A HIGH TEMPERATURE CURED STRUCTURAL COMPOSITE

3.1 Abstract

A two capsule healing chemistry, shown to be stable to 150⁰C is used to self-heal

crack damage in a fiber-reinforced composite with a Tg of 127⁰C. The healing system is

comprised of silanol terminated poly(dimethyl siloxane) plus a crosslinking agent and a

tin catalyst. Sealing of mechanical damage is assessed through the use of a pressure cell

apparatus to detect nitrogen flow through a damaged composite. Successful healing

results in a perfect seal, with no measurable gas flow. The effect of microcapsule size

and concentration on self-sealing ability and the effect of microcapsule concentration

on the short beam strength, storage modulus and Tg were investigated.

3.2 Introduction

Microcracking in advanced composites leads to a reduction in stiffness and an

increase in permeability. In cryogenic tanks, microcracks form during thermal cycling

due to the coefficient of thermal expansion mismatch between the fibers and the

33

matrix. When a sufficient density of microcracks form a percolating network of cracks

through the thickness of the composite, cryogens begin to leak through the tank wall

[1]. In sandwich structures mechanical fatigue or low velocity impact can also induce

microcracking and an increase in water absorption into the core material, which not

only increases the overall weight of the composite, but can also cause delaminations

between the face sheet and core [2].

Stacking sequence and fiber architecture affect the permeability of composites

during and after thermal cycling due to the development of thermal stresses in the plies

[3-5]. In an effort to minimize leakage caused by cryogenic cycling and low velocity

impacts, McVay et al. embedded flexible barrier layers within a composite and

demonstrated a 290% improvement over composites without the barrier layers [6].

Beiermann et al. introduced encapsulated poly(dimethyl siloxane) (PDMS) based healing

components into the PDMS layer of a polyurethane/nylon/PDMS laminate [7]. Damage

induced a polycondensation reaction between the encapsulated healing components

effectively filling the puncture and regaining the ability to seal. Kalista et al. reported a

self-healing poly(ethylene-co-methacrylic acid) copolymer capable of autonomic

molecular rearrangement which was able to maintain pressure up to 3 MPa after

projectile puncture damage [8]. Self-healing is inherent in ionomers, which have a two-

step melt elastic recovery and inter chain diffusion self-healing mechanism. Moll et al.

demonstrated the ability to self-heal 67% of samples and prevent gas phase leakage

across a composite containing microencapsulated DCPD and wax protected Grubbs’

catalyst [9]. Due to limitations of thermal stability of the healing chemistry, these

34

composites could not be processed at elevated temperature and their final glass

transition temperature was approximately 60⁰C.

Here we present a thermally stable dual microencapsulated self-healing

chemistry which has previously been used as a healing agent in epoxy and bulk PDMS [7,

10, 11]. The dual microcapsule self-healing concept is shown in Figure 3.1. Both types

of microcapsules are dispersed in the epoxy matrix (a) and are ruptured upon crack

damage (b) triggering the release of healing agents into the crack plane by capillary

forces. Once the healing agents mix by diffusion, polymerization in the crack plane (c)

bonds the crack faces together, effectively healing the matrix. Capsule A contains a

poly(dimethyl siloxane) monomer and cross-linker while capsule B contains a tin

catalyst. Since this healing chemistry is stable until 150⁰C, the composite is cured at

121⁰C for 8 hours, resulting in a composite glass transition temperature (Tg) of

approximately 127⁰C.

35

Figure 3.1 Dual microcapsule self-healing concept. a) Both types of microcapsules are dispersed into epoxy resin. b) A crack propagates in the resin, rupturing both types of microcapsules causing the encapsulated components to wick into the crack plane. c) Healing components mix and polymerize in the crack. The new polymerized material bonds the crack faces together, effectively healing the matrix.

3.3 Materials and Methods

3.3.1 Microencapsulated Healing Agent Preparation

A thermally stable healing chemistry was achieved by the encapsulation of a

PDMS monomer and a catalyst in two separate types of microcapsules. Type A

microcapsules were comprised of a mixture of 53 wt% hydroxyl end-functionalized

polydimethylsiloxane (HOPDMS) with the balance polydiethoxysiloxane (PDES)

encapsulated in a urea-formaldehyde shell. Type B microcapsules contained a 50:50

36

mixture of dibutyltin dilaurate (DBTL) catalyst from Gelest and hexylacetate from Sigma

Aldrich, which acts as a carrier solvent, encapsulated in a polyurethane shell.

Urea-formaldehyde microcapsules containing HOPDMS and PDES were

manufactured using an emulsion in situ polymerization method identical to the one

described by Mangun et al. [10] for S21 HOPDMS. A flow chart outlining the

manufacturing procedure is shown in Figure 3.2. Microcapsules with average diameters

of 25 ± 10 μm (2000 rpm) and 42 ± 20 μm (1500 rpm), were produced by varying the

agitation rate. Representative histograms for each agitation rate are provided in Figure

3.3. Figure 3.4 a) shows a scanning electron microscope image of HOPDMS/PDES

microcapsules manufactured at 2000 rpm.

Figure 3.2 Flow chart of HOPDMS/PDES encapsulation procedure.

37

Figure 3.3 Histograms of microcapsules used in this study: a) HOPDMS/PDES

microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 25 µm, b) HOPDMS/PDES microcapsules prepared at an agitation rate of 1500 rpm with an average diameter of 42 µm, c) hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 31 µm and d) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 41 µm.

38

Figure 3.4 Scanning electron microscope (SEM) images of a) 25 µm HOPDMS

microcapsules prepared at an agitation rate of 2000 rpm and b) 41 µm DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm.

Polyurethane microcapsules containing only hexylacetate (Figure 3.4 b) or a

50:50 mixture of hexylacetate and DBTL were manufactured using an oil in water

emulsion interfacial polymerization similar to the one described by Mangun et al. [10]

with a few notable exceptions. A flow chart outlining the encapsulation procedure is

shown in Figure 3.5. In a 600 mL beaker 2.5 g gum arabic (Sigma-Aldrich) was dissolved

in 100 mL of deionized water under agitation at 1100 rpm. In a separate container 17 g

hexylacetate or 10 g hexylacetate and 10 g DBTL were combined along with 2.5 g

Desmodur L75 polyurethane prepolymer (Bayer) and added to the gum arabic solution

at 85⁰C. This solution was mixed for 1 hour before the agitation rate was decreased to

200 rpm and the temperature was adjusted to room temperature for 1 hour. Next, 50

mL of deionized water and 25 mL of 2.5wt% ethylene maleic anhydride solution

(Zeeland Chemical) were added to the mixture while the agitation rate was kept

constant and the temperature was adjusted to 55⁰C for 4 hours. The procedure for

rinsing the microcapsules is outlined in Figure 3.6. Excess surfactant was eliminated

39

from the aqueous capsule solution by centrifuging four times for 30 minutes at 1000

rpm, discarding the supernatant and replacing with deionized water after each

centrifugation cycle. Finally, the solution was frozen in liquid nitrogen and placed on a

freeze drier to sublime off the water resulting in discrete, free flowing microcapsules.

Figure 3.5 Flow chart of DBTL/hexylacetate or hexylacetate only microencapsulation procedure.

40

Figure 3.6 Flow chart of the rinsing procedure for DBTL/hexylacetate or hexylacetate only microcapsules.

3.3.2 Sample Types

Three types of samples were fabricated for this study, the components of each

are summarized in Table 3.1. Control 1 samples were comprised of epoxy resin and E-

glass reinforcement (no microcapsules). Control 2 samples contained not only the

epoxy resin and E-glass reinforcement, but also HOPDMS/PDES microcapsules and

hexylacetate microcapsules. Finally self-healing samples were composed of epoxy resin,

E-glass reinforcement, HOPDMS/PDES microcapsules and DBTL/hexylacetate

microcapsules. Control 2 samples were designed to mimic the mechanical behavior of

self-healing samples, but without the ability to polymerize the healing agents.

41

Table 3.1 Summary of sample types and components.

Control 1 Control 2 Self-healing

Epoxy resin Epon 862/Epikure W Epon 862/Epikure W Epon 862/Epikure W

Reinforcement 8H satin weave E-

glass 8H satin weave E-

glass 8H satin weave E-

glass

Type A capsules - HOPDMS/PDES HOPDMS/PDES

Type B capsules - hexylacetate DBTL/hexylacetate

3.3.3 Sample Preparation

All composite samples were manufactured with Epon 862/Epikure W (Miller-

Stephenson), at a mix ratio of 100:26.4, epoxy resin and an 8 harness satin weave E-

glass (Style 7781, Fibre Glast) reinforcement. The microcapsules were mixed (10:1 by

weight, HOPDMS/PDES:DBTL/hexylacetate or hexylacetate) into the unreacted epoxy

resin followed by 30 minutes of degassing at 100⁰C. A wet layup procedure was used

with the plies sequenced (0,90,0)s, where 0 represents the warp direction. A schematic

of the layup procedure is shown in Figure 3.7. A Teflon plate was placed under the

sample to create a smooth surface finish from which the sample could be easily

removed. The composite sample was laid up in a layer by layer format with liquid epoxy

resin wetting out each ply. The composite thickness (1.92 mm – 2.03 mm) was

controlled by steel spacers placed around the periphery and a porous steel plate was

placed on top allowing resin to bleed though the plate and into bleeder cloths above. As

pressure was applied, the sample compacted until the porous plate came into contact

with the steel spacers, thus dictating the sample thickness and fiber volume fraction.

The entire layup was cured under 1.2 kPa at 121⁰C for 8 hours.

42

Figure 3.7 Schematic of composite sample layup components.

Figure 3.8 shows a polished cross section of a control 2 sample. The composite is

free of voids and the microcapsules are evenly distributed in the matrix rich regions. The

microcapsule concentrations were calculated based on polished cross-sectional images

such as Figure 3.8. Fiber tows running both parallel and perpendicular to the plane of

the image are visible.

Figure 3.8 A polished cross-sectional image of an undamaged region of an 8H satin

weave composite sample.

43

3.3.4 Mechanical Testing

After the composite panels were manufactured, they were sectioned into

smaller 45 mm x 45 mm samples. Damage was introduced into these specimens by

cyclically (10 times per side) driving an indenter tip into both sides of the sample to a

maximum load of 690 N. The complete details of this damage protocol are contained

both in chapter 2 and in Moll et al. [9]. Figure 3.9 shows a tiled cross-sectional optical

image of a control 2 sample containing 11 vol% 25 µm capsules after the damage

protocol with a large crack spanning the entire thickness of the sample.

Figure 3.9 A polished tiled cross-sectional image of a control 2 sample with 11 vol% 25

μm capsules showing the percolating crack damage running from one side of the sample to the other.

After damaging the samples, they were allowed to heal for 12 hours before

testing them for leaking in a pressure cell apparatus [7, 9] to assess sealing capability.

During this test, the composite permeability was measured by applying a pressurized

gas to one side and monitoring the pressure on the opposite side. A sample was

considered fully healed if the output pressure did not increase by more than 70 Pa over

the 30 minute test.

44

Short beam shear experiments were performed on samples of approximately 4

mm x 25 mm. Prior to testing, the samples were conditioned for at least 48 hours at

23⁰C and 50% relative humidity. Short beam shear tests were performed according to

ASTM standard D2344 [12]. Samples were loaded in 3-point bending at a rate of 1

mm/min with a 12 mm span between bottom supports. The load and displacement

data were recorded. Short beam strength is calculated from,

0.75sbs mPF

b h

(3.1)

where Pm is the peak load, b is the sample width, and h is the thickness.

Dynamic mechanical analysis (DMA) was also performed on samples to obtain

the storage modulus and Tg following ASTM standard D5023-07. After conditioning at

23⁰C and 50% relative humidity for a minimum of 48 hours, the samples were loaded

into a TA Instrument RSA III DMA in 3-point bending with a span of 25 mm. The

temperature of the sample was ramped from 20⁰C to 160⁰C at a rate of 3⁰C/min. The

storage and loss moduli were recorded as a function of temperature. The glass

transition temperature (Tg) is reported based on the peak in the tangent of the phase

angle.

45

3.4 Results


The effect of microcapsule size and concentration on the self-sealing

performance of the composite samples was investigated for microcapsule diameters of

25 μm and 42 μm. Figure 3.10 compares the sealing performance of various

microcapsule concentrations and sizes to that of the control samples. Although the

control 1 samples did not have the ability to heal, 14% of the samples did not leak due

to insufficient damage. Self-healing samples with 8 vol% 25 μm microcapsules sealed

75% of the time, compared to control 2 samples which sealed only 29% of the time. The

small percentage of control 2 samples that seal is likely attributed to the presence of the

viscous HOPDMS liquid in the crack network. Increasing both size and concentration of

microcapsules (9 vol% 42 µm and 11 vol% 25 µm microcapsules), resulted in 100% of the

self-healing samples sealing compared to 0% for the representative control 2 samples.

The delivery of more healing agents by increasing capsule size and/or concentration

improves healing performance. The decrease in sealing performance for control 2

samples is due to the increased crack damage associated with these samples, as was

previously shown by Moll et al. [9].

46

Figure 3.10 Summary of self-sealing results for composite samples with multiple sizes

and concentrations of HOPDMS/PDES microcapsules dispersed in the matrix.

3.4.2 Effect of Microcapsule Concentration on Mechanical Properties

The effect of microcapsule concentration on the short beam strength (SBS),

storage modulus and Tg were investigated for composite samples with 0, 4, 7 and 15

vol% 25 µm capsules. We found that the short beam strength decreased as the

concentration of capsules in the composite increased, as summarized in Figure 3.11.

Samples without any microcapsules present (control 1) have a short beam strength of

45 MPa, while samples with 15 vol% microcapsules have a short beam strength of 33

MPa, corresponding to a 27% decrease. Representative loading curves for all

microcapsule concentrations tested are plotted in Figure 3.12. Samples without

microcapsules failed mostly in bending with very little interlaminar cracking. As the

microcapsule concentration increased, the failure mode changed to interlaminar shear

with more interlaminar cracking. As seen in Figure 3.12, the loading curves have a larger

47

plateau region as the microcapsule concentration increases. The initial slopes of the

loading curves also decreased with increased capsule concentration.

Figure 3.11 Composite short beam strength as a function of microcapsule concentration.

Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.

Figure 3.12 Representative short beam shear loading curves for composite samples with

varying microcapsule concentrations. Vc is capsule volume fraction in the composite.

48

The storage modulus at 25⁰C was measured for microcapsule concentrations

ranging from 0 to 15 vol% (Figure 3.13). The storage modulus decreased by 15% with

the addition of 15 vol% microcapsules. Experimental values are compared with the

model developed by Naik et al. [13-17] for satin weave composites in Figure 3.13. The

matrix properties were modified by assuming the microcapsules did not contribute to

the matrix stiffness. The model over predicts the composite modulus indicating that the

microcapsules may also interrupt the woven fiber architecture, reducing the overall

stiffness of the composite.

Figure 3.13 Comparison of storage modulus values with expected model predictions.

Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.

The Tg of fiber-reinforced composites and non-reinforced epoxy resin samples

was also measured for varying microcapsule concentrations (Figure 3.14). As the

microcapsule concentration increased, the Tg decreased slightly for both the neat resin

49

and the composite. To determine if the decrease in Tg was caused by the capsule

components leaching out of the microcapsules and plasticizing the matrix, we prepared

additional samples of epoxy resin and 0.55 wt% HOPDMS mixed in directly into the

epoxy matrix. No drop in Tg was observed with the addition of HOPDMS. Similar trends

have been reported for the addition of particulates in composites by Crowson et al. [18],

who showed a decrease in the Tg of a glass bead reinforced epoxy resin with increasing

bead concentration. Also, Yuan et al. [19] found that in a glass fiber reinforced epoxy

composite with epoxy resin filled microcapsules the Tg of the composite decreased with

increasing microcapsule concentration.

Figure 3.14 Changing glass transition temperature with microcapsule concentration in

fiber reinforced composite and epoxy samples. Microcapsule volume fractions are reported in the resin only, not accounting for the inclusion of fiber reinforcement. Horizontal line represents the Tg of epoxy resin combined with HOPMDS monomer. Vertical line represents the minimum microcapsule concentration used for healing in this study. Error bars represent one standard deviation.

50

3.5 Discussion

A high Tg structural composite capable of healing crack damage and sealing off

nitrogen gas flow up to at least 275 kPa was achieved with a dual microcapsule system.

This is the first demonstration of microcapsule based autonomic self-healing in a fully

cured, high Tg material. Previously, only 67% of composite samples were capable of

withstanding a pressure of 275 kPa with a low Tg matrix, cured at room temperature [9].

The current PDMS based healing chemistry has achieved sealing in 100% of high Tg

composite samples when 9 vol% 42 µm capsules or 11 vol% 25 µm capsules are

dispersed in the matrix. This healing system has shown excellent sealing ability,

however the addition of microcapsules led to some decreases in mechanical

properties. The presence of some larger microcapsules (ca. 100 um) that were not

sieved or improper rinsing of the capsules may have contributed to change in

mechanical properties. In the future, it will be necessary to optimize composite

processing as well as the size and amount of the microcapsules to achieve healing, but

minimize the reduction in mechanical properties.

3.6 Conclusions

A fully cured, high glass transition temperature (127⁰C) E-glass reinforced self-

healing composite with self-sealing functionality was achieved by the incorporation of a

thermally stable healing chemistry comprised of a hydroxyl end-functionalized

polydimethylsiloxane (HOPDMS) and cross-linking agent polydiethoxysiloxane (PDES)

catalyzed by dibutyltin dilaurate (DBTL). Encapsulated healing components (25 – 42 µm

51

average diameter) were incorporated into the matrix rich regions of the sample during

fabrication.

The effect of microcapsule concentration on short beam strength, and storage

modulus were investigated for composite samples with capsule concentrations from 0 –

15 vol%. At a concentration of 15 vol%, a 27% decrease in the short beam strength was

noted while the storage modulus was reduced by 15%. The glass transition temperature

(Tg) of the fiber-reinforced composite and plain epoxy resin was also reduced slightly,

ca. 3°C, with the addition of microcapsules.

Composite specimens sealed 100% of the time when 9 vol% 42 µm or 11 vol% 25

µm capsules were added. Hence, self-sealing composites have shown the potential to

prolong the lifetime of cryogenic tanks and composite face sheets by eliminating

leakage of cryogenic fuel and reducing water uptake into the sandwich structure core.

References [1] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for

cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.

[2] H. S. Choi, and Y. H. Jang, “Bondline strength evaluation of cocure/precured honeycomb sandwich structures under aircraft hygro and repair environments,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 9, pp. 1138-1147, Sep, 2010.

[3] T. Yokozeki, T. Ogasawara, T. Aoki et al., “Experimental evaluation of gas permeability through damaged composite laminates for cryogenic tank,” Composites Science and Technology, vol. 69, no. 9, pp. 1334-1340, Jul, 2009.

[4] S. Choi, and B. V. Sankar, “Gas permeability of various graphite/epoxy composite laminates for cryogenic storage systems,” Composites Part B-Engineering, vol. 39, no. 5, pp. 782-791, 2008.

52

[5] H. Kumazawa, T. Aoki, and I. Susuki, “Influence of stacking sequence on leakage characteristics through CFRP composite laminates,” Composites Science and Technology, vol. 66, no. 13, pp. 2107-2115, Oct, 2006.

[6] A. C. McVay, and W. S. Johnson, “Permeability of Various Hybrid Composites Subjected to Extreme Thermal Cycling and Low-velocity Impacts,” Journal of Composite Materials, vol. 44, no. 12, pp. 1517-1531, Jun, 2010.


[8] S. J. Kalista, “Self-healing of poly(ethylene-co-methacrylic acid) copolymers following projectile puncture,” Mechanics of Advanced Materials and Structures, vol. 14, no. 5, pp. 391-397, 2007.


[10] C. L. Mangun, A. C. Mader, N. R. Sottos et al., “Self-healing of a high temperature cured epoxy using poly(dimethylsiloxane) chemistry,” Polymer, vol. 51, no. 18, pp. 4063-4068, Aug, 2010.

[11] S. H. Cho, H. M. Andersson, S. R. White et al., “Polydimethylsiloxane-based self-healing materials,” Advanced Materials, vol. 18, no. 8, pp. 997-+, Apr, 2006.

[12] ASTM-D2344, "Standard Test Method for Short-Beam Strength of Polymer Matrix Composite Materials and Their Laminates," 2006.

[13] N. K. Naik, and P. S. Shembekar, “ELASTIC BEHAVIOR OF WOVEN FABRIC COMPOSITES .3. LAMINATE DESIGN,” Journal of Composite Materials, vol. 26, no. 17, pp. 2522-2541, 1992.

[14] N. K. Naik, and V. K. Ganesh, “PREDICTION OF ON-AXES ELASTIC PROPERTIES OF PLAIN WEAVE FABRIC COMPOSITES,” Composites Science and Technology, vol. 45, no. 2, pp. 135-152, 1992.

[15] N. K. Naik, and P. S. Shembekar, “ELASTIC ANALYSIS OF WOVEN FABRIC LAMINATES .2. MIXED COMPOSITES,” Journal of Composites Technology & Research, vol. 15, no. 1, pp. 34-37, Spr, 1993.

[16] N. Naik, Woven Fabric Composites, p.^pp. 1-23, 73-93, Lancaster, Pennsylvania: Technomic Publishing Company, Inc., 1994.

[17] P. S. Shembekar, and N. K. Naik, “ELASTIC ANALYSIS OF WOVEN FABRIC LAMINATES .1. OFF-AXIS LOADING,” Journal of Composites Technology & Research, vol. 15, no. 1, pp. 23-33, Spr, 1993.

[18] R. J. Crowson, and R. G. C. Arridge, “ELASTIC PROPERTIES IN BULK AND SHEAR OF A GLASS BEAD REINFORCED EPOXY-RESIN COMPOSITE,” Journal of Materials Science, vol. 12, no. 11, pp. 2154-2164, 1977.

[19] L. Yuan, A. Gu, G. Lian et al., “Novel Glass Fiber-reinforced Cyanate Ester/Microcapsule Composites,” Journal of Composite Materials, vol. 43, no. 16, pp. 1679-1694, 2009.

53

CHAPTER 4

SELF-HEALING OF THERMAL FATIGUE DAMAGE IN A CARBON FIBER-REINFORCED COMPOSITE

4.1 Introduction

Autonomous self-healing of thermal fatigue damage is achieved in a [0/90]s

carbon fiber/epoxy composite. The healing chemistry is the thermally stable dual

microcapsule system comprised of a tin catalyst and a poly(dimethyl siloxane) monomer

with a cross-linker described in chapter 3. Sealing ability is evaluated with the pressure

cell apparatus to detect nitrogen flow through the damaged composite.

Under thermal fatigue, fiber-reinforced composite materials suffer from the

formation of small microcracks due to the coefficient of thermal expansion (CTE)

mismatch between the fiber reinforcement and the polymer matrix [1, 2].

Microcracking is especially relevant in carbon fiber/epoxy composites where the CTE of

carbon fiber in the longitudinal direction is negative and the CTE of epoxy is positive.

Thermally induced microcracks can lead to composite permeability, hindering their use

in cryotanks [3]. Bechel et al. quantified the density of thermally induced cracks and the

resulting permeability increase with the number of thermal cycles in various carbon

fiber/polymer composites constituents and layup geometries [4-6]. Kumazawa et al.

54

investigated the influence of stacking sequence on the permeability of a bi-axially

loaded carbon fiber composite [7].

Microcrack damage in fiber-reinforced composites is particularly difficult to

detect using non-destructive methods. Only modest success at damage detection has

been achieved using approaches such as line scanning thermography [8], magnetic

resonance detection [9, 10], acoustic emission [11-13] dielectric variations [14, 15], and

electric resistance changes in carbon fiber [16, 17] and nanotube reinforced composites

[13, 18, 19]. Each of these methods has shortcomings associated with them such as

sensitivity of detection, depth of detection, and practicality of use [20].

Self-healing provides a promising method to reduce crack propagation due to

mechanical and thermal fatigue in fiber-reinforced composite materials and extend the

life expectancy of the part. These materials have the ability to heal crack damage due to

mechanical fatigue in bulk polymers through the incorporation of healing agent filled

microcapsules into epoxy [21-24] and poly(dimethyl siloxane) (PDMS) [25] resin. Brown

et al. [21] reported a 213% life extension at a moderate ∆K1 and Jin et al. [24] described

complete crack arrest in an epoxy adhesive using dicyclopentadiene microcapsules and

Grubbs’ catalyst healing chemistry. Keller et al. saw a 24% reduction in torsion fatigue

crack growth in a PDMS resin with an encapsulated PDMS healing agent [26].

Here we investigate the microencapsulated thermally stable tin catalyzed

poly(dimethyl siloxane) healing chemistry, described in chapter 3, to self-heal thermally

induced cracks in a carbon fiber/epoxy composite. This healing chemistry has been used

previously to heal crack damage in epoxy [27, 28], polymer coatings [29] and in bulk

55

PDMS [30]. The effect of microcapsule incorporation on the accumulation of thermally

induced transverse microcracks in the composite and the effect of microcapsule

concentration on CTE of the epoxy resin was also investigated.

4.2 Experimental Methods and Procedure

4.2.1 Microcapsule Preparation

The healing chemistry in this study is composed of hydroxyl end-functionalized

poly(dimethyl siloxane) (HOPDMS) with polydiethoxysiloxane (PDES) acting as a cross-

linker in a reaction catalyzed by dibutyltin dilaurate (DBTL). The HOPDMS, PDES and

DBTL were purchased from Gelest. The HOPDMS and PDES were encapsulated together

(53wt% HOPDMS) in a urea-formaldehyde shell. The encapsulation procedure (Figure

3.2) is identical to the one outlined by Mangun et al. [28]. The microencapsulation

mixture was agitated at a rate of 2000 rpm, producing microcapsules with an average

diameter of 29 ± 15 µm (Figure 4.1 a).

The DBTL catalyst microcapsules, with an average diameter of 44 ± 16 µm

(Figure 4.1 b), were prepared by an interfacial polymerization method (Figure 3.5)

similar to the technique described in Mangun et al. [28] with a few notable

modifications. A polyurethane prepolymer, Desmodure L75, (Bayer) was dissolved in 10

g DBTL along with 10 g hexylacetate, used as a carrier solvent, and poured into a

mixture of 100 mL deionized water and 2.5 g gum arabic (Sigma-Aldrich) mixing at 1100

rpm. This solution was mixed for 1 hour at 85⁰C before the stir rate was reduced to 200

56

rpm and the temperature control was turned off for an additional hour. Next, 50 mL of

deionized water and 25 mL of 2.5 wt% ethylene maleic anhydride (Zeeland Chemical)

solution were added to the aqueous microcapsule mixture and the temperature was

adjusted to 55⁰C for 4 hours. After the reaction was completed, the microcapsules were

cleaned by centrifuging four times for 30 minutes at 1000 rpm, as described in chapter

3. After each centrifuging cycle the aqueous portion was removed and replaced with

deionized water. Finally, the microcapsule solution was frozen in liquid nitrogen and

placed on a freeze drier to remove the water. Control microcapsules were

manufactured in an identical method using only 17 g hexylacetate, instead of the 50:50

mixture of DBTL and hexylacetate.

Figure 4.1 Histogram of microcapsules used in this study: a) HOPDMS/PDES microcapsules prepared at an agitation rate of 2000 rpm with an average diameter of 29 ± 15 µm, b) DBTL/hexylacetate microcapsules prepared at an agitation rate of 1100 rpm with an average diameter of 44 ± 16 µm.

57

4.2.2 Layup and Sample Types

Three types of samples were investigated. The components of each are listed in

Table 4.1. The matrix material for all sample types was Epon 862/Epikure W (Miller-

Stephenson), with a unidirectional carbon fiber fabric (2583-C, Fibre Glast)

reinforcement. All composites were laid up in a [0/90]s geometry with a total sample

thickness of 1.75 ±0.03 mm. Control 1 samples consisted of only the epoxy resin and

carbon fiber reinforcement (no microcapsules). In addition to the epoxy resin and

carbon reinforcement, control 2 samples also contained HOPDMS/PDES and

hexylacetate microcapsules. Finally, self-healing samples were comprised of epoxy

resin, carbon reinforcement, HOPDMS/PDES microcapsules and DBTL/hexylacetate

microcapsules. Control 2 specimens were not able to heal because of the absence of

the DBTL catalyst.

Table 4.1 Summary of sample types and components. Control 1 Control 2 Self-healing

Epoxy resin Epon 862/Epikure W Epon 862/Epikure W Epon 862/Epikure W

Reinforcement Uni Carbon Uni Carbon Uni Carbon

Ply Sequence [0/90]s [0/90]s [0/90]s

Type A Capsules (vol%)

- HOPDMS/PDES (9 vol%)

HOPDMS/PDES (9 vol%)

Type B Capsules (vol%)

- Hexylacetate (1 vol%)

DBTL/Hexylacetate (1 vol%)

All composite samples were manufactured with Epon 862/Epikure W (Miller-

Stephenson), at a mix ratio of 100:26.4, epoxy resin and a unidirectional carbon fiber

(Style 2583-C, Fibre Glast) reinforcement. The microcapsules were mixed (10:1 by

weight, HOPDMS/PDES:DBTL or hexylacetate) into the unreacted epoxy, which was

58

degassed for 45 minutes at 100⁰C to eliminate trapped air which could lead to voids in

the final composite sample. A wet layup procedure was used with the plies sequenced

[0/90]s, where 0 represents the warp direction. A schematic of the layup procedure is

shown in Figure 3.7 in chapter 3. A Teflon plate was placed under the sample to create

a smooth surface finish from which the sample could be easily removed. The initial

overall microcapsule concentration in the resin was 4 wt%, but the final volume fraction

of microcapsules in the composite was 0.09. The composite sample was laid up in a

layer by layer format with liquid epoxy resin wetting out each ply. The composite

thickness was controlled by steel spacers placed around the periphery and a porous

steel plate was placed on top allowing resin to bleed though the plate and into bleeder

cloths above. As pressure was applied, the sample compacted until the porous plate

came into contact with the steel spacers, thus dictating the sample thickness (1.85 ±

0.03 mm) and fiber volume fraction (0.4). The entire layup was cured under 1.2 kPa at

121⁰C for 8 hours. During the application of pressure, the top plies compacted first,

resulting in these plies having a lower thickness than the bottom plies.

4.2.3 Coefficient of Thermal Expansion Measurement

The coefficient of thermal expansion (CTE) was measured for plain epoxy

samples with varying concentrations of microcapsules. The samples were cured in 8

mm diameter cylinders and were sectioned in 4 mm increments with parallel sides. The

specimens were placed between a parallel plate fixture in a rheometer with a constant

normal force of 0.5 N held on the sample. The temperature was increased at a rate of 5

59

⁰C/min from 25⁰C to 120⁰C. The change in displacement with increasing temperature

was recorded. A representative change in displacement with increasing temperature

plot for epoxy resin with 3.5 vol% HOPDMS/PDES microcapsules is shown in Figure 4.2.

The CTE of the entire composite was not measured because the concentration of

microcapsules was easier to control in epoxy only samples.

Figure 4.2 Representative plot of the change in sample thickness with temperature curve.

4.2.4 Cryogenic Cycling and Permeability Testing

After the composite panels were manufactured, they were sectioned into square

samples approximately 45 mm X 45 mm. Each was polished on two, perpendicular sides

and examined under an optical microscope to verify that no cracks were present prior to

thermal cycling.

A schematic of the cryogenic cycler is shown in Figure 4.3. The samples were

placed in a wire basket and were repeatedly cycled between 35⁰C for 8 minutes and

60

liquid nitrogen (-196⁰C) for 8.5 minutes. Approximate sample through thickness

temperature profiles with time for heating and cooling are plotted in Figure 4.4 a) and b)

respectively. The laminates heat up to the ambient oven temperature (35⁰C) in

approximately 8 minutes, and then cool to liquid nitrogen temperature approximately 5

minutes after being submerged. These temperature profiles were calculated by solving

the one dimensional heat equation with transient boundary conditions approximated by

monitoring the surface temperature of the samples with time. After several cycles, the

samples were examined using optical microscopy to determine the number of

transverse microcracks in each ply. Next, the samples were placed in a desiccator for 4

hours to ensure all condensation was eliminated before testing for leaking.

Figure 4.3 Schematic of thermal fatigue apparatus.

61

Figure 4.4 Approximate temperature profiles through the thickness of composite samples for a) heating to 35⁰C and b) cooling to -196⁰C.

Sample permeability was evaluated in a pressure cell apparatus designed for

previous experiments [30, 31]. In this setup, nitrogen was flowed into the cell with a

controlled pressure (275 kPa) recorded by a pressure transducer. If a percolating

network of cracks was present in the sample, nitrogen gas would flow completely

through the sample and result in an increased output pressure on the opposite side of

the sample. This output pressure was measured and recorded by a second pressure

62

transducer. All tests were run for 10 minutes, which was sufficient time to see the

development of the output pressure evolution with time curve, but short enough to be

practical for laboratory experiments.

Prior experiments tested sample permeability after a localized damage event.

The crack networks were confined to the center of the sample, and were completely

encompassed by o-rings on both sides of the sample. Thermal fatigue damage is not

localized, and therefore the o-rings would not completely contain the nitrogen leaking

through the sample as shown in Figure 4.5. To eliminate this problem, a new sample

chamber was fabricated for the pressure cell to contain the nitrogen escaping through

the sides of the sample. A diagram of the removable sealed sample chamber and the

pressure cell is shown in Figure 4.6. The sample chamber consisted of two aluminum

plates with an o-ring sealed against each side of the sample, and a third larger o-ring

completely surrounding the sample. After the samples were tested for leaking, the

process of cycling and testing was repeated until all samples leaked.

Figure 4.5 Illustration showing how nitrogen gas can escape through a thermally

damaged composite sample in an unmodified pressure cell.

63

Figure 4.6 Schematic of the pressure cell apparatus. a) Sealed sample chamber with

sample and rubber o-rings. b) Complete test apparatus with the sealed sample chamber inserted in the pressure cell.

4.3 Results and Discussion

4.3.1 Microcrack Accumulation

Periodically throughout the cryogenic cycling process, the samples were

examined to record the number of transverse microcracks in each ply with number of

thermal cycles. Figure 4.7 shows an optical micrograph of a cross-section of a control 1

sample with each of the plies and fiber orientations labeled. The crack density with

number of thermal cycles for each ply is plotted in Figure 4.8. Representative images in

Figure 4.9 illustrate the types of cracks in the composite samples. Thermally induced

microcracks span the entire thickness of the ply as shown in Figure 4.9 a) and b).

64

Figure 4.7 Optical image of a polished cross-section of a control 1 sample with the plies and fiber orientations labeled.

Figure 4.8 Microcrack density with number of thermal cycles in a) plies 1 and 4 and b)

plies 2 and 3.

65

Figure 4.9 Representative optical micrographs of thermally induced cracks. a)

Microcrack in ply 4 in a control 1 sample after 110 thermal cycles. b) Microcrack in ply 2 and 3 in a control 2 sample after 310 thermal cycles. c) Delamination running between plies 3 and 4 in a control 2 sample after 310 thermal cycles.

Transverse cracking appeared first in the top ply due to a combination of a more

rapid temperature change experienced by the outer plies and the higher fiber volume

fraction in the top ply due to uneven sample compaction during cure. A representative

crack in ply 4 in a control 1 sample is shown in the optical micrograph in Figure 4.9 a).

As displayed in Figure 4.8 a), the cracking in ply 4 begins quickly, after as few as 5 cycles.

After 1500 cycles, the crack initiation rate decreases, and the number of transverse

microcracks remains essentially constant for the remainder of testing. Cracks began to

66

appear in ply 1 after 600 cycles, for all specimen types. The crack growth rate was much

slower than ply 4, but continued to increase modestly throughout the test. Figure 4.8 b)

summarizes the accumulation of transverse cracks for the inner plies. A representative

optical image of a transverse crack crossing plies 2 and 3 and intersecting embedded

microcapsules after 310 cycles is displayed in Figure 4.9 b). Typically, a single transverse

crack will simultaneously go through plies 2 and 3. Cracks begin to appear in these plies

after approximately 100 thermal cycles and continue to increase in number through the

testing period. Delaminations are also present in the interlaminar regions of the

composite samples. An optical image of a delamination in a control 2 sample running

between plies 1 and 2 after 310 cycles is shown in Figure 4.9 c).

4.3.2 Coefficient of Thermal Expansion

Thermally induced microcracks in composite materials are caused by the

coefficient of thermal expansion (CTE) mismatch between the fiber reinforcement and

the epoxy resin. Since changes in CTE can alter the cracking behavior, the effect of

microcapsules in resin on the CTE was investigated. Epoxy samples were made with

HOPDMS/PDES microcapsule concentrations varying from 0 vol% to 15 vol%. The

resulting experimental and predicted CTE values based on the rule of mixtures are

plotted in Figure 4.10. The CTE at 15 vol% capsules was 88 x 10-6 /⁰C, 91% higher than

that of the neat resin (46 x 10-6 /⁰C). Although this is a relatively large increase, these

results follow the rule of mixtures for microcapsules with CTE values the same as that of

bulk PDMS (300 x 10-6 /⁰C). The CTE of carbon fiber in the longitudinal direction is -0.5 x

67

10-6 [32]. The larger difference between the CTE of the microcapsule filled epoxy and

the CTE of the carbon fibers, may have lead to an increase in the localized thermal

stresses in the composite.

Ply level thermal stresses can lead to delaminations between fiber plies as seen

in Figure 4.11 for control 1 and control 2 samples after 2500 cycles. Delaminations

appeared after fewer cycles in samples with microcapsules than in samples without

microcapsules. This is likely caused by a number of factors associated with the addition

of microcapsules into a composite including a reduction in interlaminar strength

(chapter 3) and an increase in local thermal stresses. Possible additional sources could

be poor bonding of the epoxy matrix to the HOPDMS/PDES microcapsules or improper

composite sample manufacturing.

Figure 4.10 Comparison of coefficient of thermal expansion of epoxy resin with

expected rule of mixtures values. Error bars represent one standard deviation.

68

Figure 4.11 Optical images of delamination running between plies after 2500 thermal

cycles in a) control 1 and b) control 2 samples.


Composite sample permeability and crack density (Figure 4.8) increased with

thermal cycles. Figure 4.12 plots the output pressure, measured at 10 minutes of testing

after various amounts of thermal cycles, for a representative control 1 sample. This

particular specimen began to leak after approximately 1225 thermal cycles. As the

number of cycles continued to increase, so did the permeability. After about 1600

cycles the rate of increase in output pressure decreased as the output pressure

approached the value of the input pressure (275 kPa). There was a small decrease in

the output pressure after 1850 cycles, which is likely due to the sample not being

69

perfectly centered in the sealed sample test chamber. A trend similar to Figure 4.12 was

also observed for both control 2 and self-healing samples.

Figure 4.12 Representative output pressure, after 10 minutes of testing, of a control 1

sample with number of thermal cycles.

All samples were thermally cycled and tested in the pressure cell to determine

the number of cycles each sample was able to withstand before leaking. In all samples,

no leaking occurred until a crack network had propagated through all four plies. As

revealed in Figure 4.8 a) transverse microcracks occurred last in ply 1, making it the

critical ply to trigger leaking. Control 1 and control 2 samples were not capable of self-

sealing and therefore leaked as soon as a percolating crack network extended through

all plies. Self-healing samples accumulated transverse microcracks at a similar rate to

both the control 1 and control 2 samples (Figure 4.8), but leaked after a higher number

of cycles as seen in Figure 4.13. Control 1 samples survived an average of 1027 thermal

cycles before they started to leak, while control 2 samples survived only 717 cycles.

Self-healing samples remained leak-free for an average of 1675 cycles, prolonging the

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lifetime of these samples by 63% (649 cycles) over traditional composite samples

(control 1) and 134% (959 cycles) over samples with microcapsules but no catalyst

(control 2). The reduction in sample lifetime seen in control 2 samples is likely due to

the larger CTE mismatch between the resin and the carbon fiber reinforcement as

discussed previously.

Figure 4.13 Percentage of samples sealed with number of thermal cycles.

4.4 Conclusions

Self-healing samples have shown the ability to prolong the lifetime of composite

materials subjected to thermal cycling. Crack accumulation in each ply was tracked as a

function of number of thermal cycles. Composite samples with and without

microcapsules accumulated cracks at a similar rate. Leaking was not observed in any

samples until all four plies had cracked. Transverse microcracks appeared first in the

top ply and last in the bottom ply, which was likely caused by a combination of varying

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ply thicknesses as a result of the manufacturing method and thermal shock experienced

by the outer plies. The thinnest surface ply (ply 4), developed cracks first.

The effect of microcapsule concentration on the coefficient of thermal expansion

(CTE) of the epoxy matrix was investigated for samples with capsule concentrations

ranging from 0 to 15 vol%. The CTE of epoxy increased with increasing microcapsule

concentration by approximately 90% over this concentration range. An increase in CTE

resulted in a larger mismatch between the carbon fibers and the epoxy matrix, possibly

contributing to delaminations between plies which form after a lower number of cycles

and causing the control 2 samples to leak before the other sample types.

Overall, self-healing samples accumulated cracks at a similar rate as both types

of control samples, but survived an average of 959 more thermal cycles than controls

with microcapsules (control 2) and 650 more cycles than controls without microcapsules

(control 1). Hence, self-healing composites have the ability to seal thermally induced

microcracks formed in fiber reinforced composite materials. This technology shows

great promise toward improving thermal fatigue performance of composite storage

vessels.

References [1] N. L. Hancox, “Thermal effects on polymer matrix composites: Part 1. Thermal cycling,”

Materials & Design, vol. 19, no. 3, pp. 85-91, Jun, 1998. [2] M. C. Lafarie-Frenot, “Damage mechanisms induced by cyclic ply-stresses in carbon-

epoxy laminates: Environmental effects,” International Journal of Fatigue, vol. 28, no. 10, pp. 1202-1216, Oct, 2006.

[3] R. Heydenreich, “Cryotanks in future vehicles,” Cryogenics, vol. 38, no. 1, pp. 125-130, Jan, 1998.

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[4] V. T. Bechel, M. B. Fredin, S. L. Donaldson et al., “Effect of stacking sequence on micro-cracking in a cryogenically cycled carbon/bismaleimide composite,” Composites Part a-Applied Science and Manufacturing, vol. 34, no. 7, pp. 663-672, 2003.

[5] V. T. Bechel, J. D. Camping, and R. Y. Kim, “Cryogenic/elevated temperature cycling induced leakage paths in PMCs,” Composites Part B-Engineering, vol. 36, no. 2, pp. 171-182, 2005.

[6] V. T. Bechel, M. Negilski, and J. James, “Limiting the permeability of composites for cryogenic applications,” Composites Science and Technology, vol. 66, no. 13, pp. 2284-2295, Oct, 2006.

[7] H. Kumazawa, T. Aoki, and I. Susuki, “Influence of stacking sequence on leakage characteristics through CFRP composite laminates,” Composites Science and Technology, vol. 66, no. 13, pp. 2107-2115, Oct, 2006.

[8] O. Ley, S. Chung, J. Schutte et al., "Application of line scanning thermography for detection of interlaminar disbonds in sandwich composite structures."

[9] G. LaPlante, A. E. Marble, B. MacMillan et al., “Detection of water ingress in composite sandwich structures: a magnetic resonance approach,” Ndt & E International, vol. 38, no. 6, pp. 501-507, Sep, 2005.

[10] A. E. Marble, G. LaPlante, I. V. Mastikhin et al., “Magnetic resonance detection of water in composite sandwich structures,” Ndt & E International, vol. 42, no. 5, pp. 404-409, Jul, 2009.

[11] R. M. Measures, “SMART COMPOSITE STRUCTURES WITH EMBEDDED SENSORS,” Composites Engineering, vol. 2, no. 5-7, pp. 597-618, 1992.

[12] G. Caprino, and R. Teti, “RESIDUAL STRENGTH EVALUATION OF IMPACTED GRP LAMINATES WITH ACOUSTIC-EMISSION MONITORING,” Composites Science and Technology, vol. 53, no. 1, pp. 13-19, 1995.

[13] L. M. Gao, E. T. Thostenson, Z. G. Zhang et al., “Coupled carbon nanotube network and acoustic emission monitoring for sensing of damage development in composites,” Carbon, vol. 47, no. 5, pp. 1381-1388, Apr, 2009.

[14] A. A. Nassr, and W. W. El-Dakhakhni, “Damage Detection of FRP-Strengthened Concrete Structures Using Capacitance Measurements,” Journal of Composites for Construction, vol. 13, no. 6, pp. 486-497, Nov-Dec, 2009.

[15] A. A. Nassr, and W. W. El-Dakhakhni, “Non-destructive evaluation of laminated composite plates using dielectrometry sensors,” Smart Materials & Structures, vol. 18, no. 5, May, 2009.

[16] Z. H. Xia, and W. A. Curtin, “Damage detection via electrical resistance in CFRP composites under cyclic loading,” Composites Science and Technology, vol. 68, no. 12, pp. 2526-2534, Sep, 2008.

[17] A. E. Zantout, and O. I. Zhupanska, “On the electrical resistance of carbon fiber polymer matrix composites,” Composites Part a-Applied Science and Manufacturing, vol. 41, no. 11, pp. 1719-1727, Nov, 2010.

[18] B. R. Loyola, V. La Saponara, and K. J. Loh, “Characterizing the Self-Sensing Performance of Carbon Nanotube Enhanced Fiber-Reinforced Polymers,” in The International Society for Optical Engineering, 2010.

[19] K. J. Kim, W.-R. Yu, J. S. Lee et al., “Damage characterization of 3D braided composites using carbon nanotube-based in situ sensing,” Composites: Part A, vol. 41, no. 10, pp. 1531-1537, 2010.

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[20] K. Diamanti, and C. Soutis, “Structural health monitoring techniques for aircraft composite structures,” Progress in Aerospace Sciences, vol. 46, no. 8, pp. 342-352, Nov, 2010.

[21] E. N. Brown, S. R. White, and N. R. Sottos, “Retardation and repair of fatigue cracks in a microcapsule toughened epoxy composite - Part II: In situ self-healing,” Composites Science and Technology, vol. 65, no. 15-16, pp. 2474-2480, Dec, 2005.

[22] E. N. Brown, S. R. White, and N. R. Sottos, “Fatigue crack propagation in microcapsule-toughened epoxy,” Journal of Materials Science, vol. 41, no. 19, pp. 6266-6273, Oct, 2006.

[23] A. S. Jones, J. D. Rule, J. S. Moore et al., “Life extension of self-healing polymers with rapidly growing fatigue cracks,” Journal of the Royal Society Interface, vol. 4, no. 13, pp. 395-403, Apr, 2007.

[24] H. Jin, G. M. Miller, N. R. Sottos et al., “Fracture and fatigue response of a self-healing epoxy adhesive,” Polymer, vol. 52, no. 7, pp. 1628-1634, Mar, 2011.

[25] M. W. Keller, S. R. White, and N. R. Sottos, “A self-healing poly(dimethyl siloxane) elastomer,” Advanced Functional Materials, vol. 17, no. 14, pp. 2399-2404, Sep, 2007.

[26] M. W. Keller, S. R. White, and N. R. Sottos, “Torsion fatigue response of self-healing poly(dimethylsiloxane) elastomers,” Polymer, vol. 49, no. 13-14, pp. 3136-3145, Jun, 2008.

[27] S. H. Cho, H. M. Andersson, S. R. White et al., “Polydimethylsiloxane-based self-healing materials,” Advanced Materials, vol. 18, no. 8, pp. 997-+, Apr, 2006.

[28] C. L. Mangun, A. C. Mader, N. R. Sottos et al., “Self-healing of a high temperature cured epoxy using poly(dimethylsiloxane) chemistry,” Polymer, vol. 51, no. 18, pp. 4063-4068, Aug, 2010.

[29] S. H. Cho, S. R. White, and P. V. Braun, “Self-Healing Polymer Coatings,” Advanced Materials, vol. 21, no. 6, pp. 645-+, Feb, 2009.



[32] I. M. Daniel, and O. Ishai, Engineering Mechanics of Composite Materials, 2nd ed., p.^pp. 375: Oxford University Press, 2006.

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CHAPTER 5

SUMMARY AND FUTURE WORK

5.1 Summary

Self-healing composites provide a promising method to solve the problem of

leaking in composite storage vessels and moisture ingression in composite sandwich

structures caused by impact or fatigue. Self-healing ability interrupts the percolating

crack network that causes these problems and prolongs the lifetime of the composite.

A test protocol was established using a pressure cell apparatus, to monitor and

evaluate the ability of a sample to hold pressure up to 275 kPa. Composite samples

cured at room temperature with imbedded encapsulated dicyclopentadiene and

Grubbs’ catalyst were capable of sealing mechanically induced crack damage in 67% of

samples with 6.5 wt% 51 µm capsules compared to 13% of traditional composites

without capsules. The effect of microcapsule concentration and size on the damage

area of composite control samples without catalyst was investigated. It was found that

damage area increased with both increasing capsule size and concentration.

A high temperature cured self-healing structural composite with a glass

transition temperature of 127⁰C was manufactured. A thermally stable healing

chemistry was used to ensure the self-healing ability would remain active after the

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composite was cured at 121⁰C for 8 hours. These composite samples were capable of

sealing mechanically induced crack damage in 100% of samples with 9 vol% 42 µm and

11 vol% 25 µm microcapsules. The effect of microcapsule concentration on short beam

strength, storage modulus and glass transition temperature were investigated for

composite samples. Short beam strength was reduced by 27% and storage modulus

was reduced by 15% in samples with 15 vol% microcapsules when compared to

specimens without microcapsules. The glass transition temperature of the fiber

reinforced samples was reduced by approximately 3 ⁰C when the concentration of

microcapsules in the resin was 32 vol%.

A cross-ply carbon fiber/epoxy self-healing composite was manufactured and

cryogenically cycled a total of 2500 times. Transverse microcrack damage in each of the

four plies was monitored with cycle number. The effect of microcapsule concentration

on the coefficient of thermal expansion (CTE) of epoxy was investigated. The CTE of

epoxy samples with 15 vol% microcapsules increased by 90% over plain epoxy samples.

Self-healing specimens were able to withstand an average of 63% more thermal cycles

before leaking than traditional composite samples without microcapsules.

5.2 Future Work

There are several challenges that need to be addressed before self-healing

composites can be commercially viable. Most of these challenges center on the need

for new and improved processing methods. The wet hand layup technique employed in

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this study can only be used to make flat plates, while actual composite parts for aircraft

or storage vessels have much more complex geometries. There is also a need to

improve processing such that the thicknesses of all plies are equal. Variations in ply

thickness correspond to differences in fiber volume fraction, which dictate many

mechanical properties of the composite. Manufacturing self-healing pregreg would

allow us to take great steps toward solving these problems. Prepreg is composed of

fiber reinforcement coated by partially cured resin which is sold in thin, flat, flexible

sheets. These sheets can be stacked together to form layups with desired fiber

orientations. Incorporating microcapsules into prepreg would enable us to make

samples with plies of consistent thicknesses, dictated by the thickness of the prepreg

and slightly more complex composite shapes.

Self-healing prepreg can be made by adding an additional component to

traditional wet prepregging setups. In this process, fibers are run into a resin bath and

collected on a mandrel where the epoxy is allowed to partially react before it is chilled

to slow the cure process. To add microcapsules to this prepreg, fiber would be run

through an aqueous microcapsule solution and dried prior to the resin bath. It has been

shown previously that microcapsules will adhere to fibers after dipping in an aqueous

capsule bath [1].

The use of smaller (1-10 µm diameter) microcapsules would be useful in

optimizing composites capable of healing thermal fatigue damage. Unlike mechanical

damage, thermal fatigue damage has a relatively small crack separation (<5 µm), which

can be healed using smaller scale capsules. These microcapsules will be easier to

77

incorporate into prepreg using the process outlined above. Finally, the use of smaller

microcapsules will allow better dispersed of capsules in each ply, instead of being

sequestered primarily to the interlaminar regions of the composite making it possible to

heal transverse cracks as well as delaminations.

Additional healing chemistries should be evaluated for self-healing composites.

As mentioned in Chapter 4, the current poly(dimethyl siloxane) system significantly

increases the coefficient of thermal expansion (CTE) of epoxy. Microcapsules containing

other monomers may not cause such a drastic increase in CTE, reducing the thermal

strains in the composite. One potential chemistry to evaluate is an epoxy healing

system. This two capsule system would be comprised of amine capsules and epoxy

capsules. While the microcapsules themselves may still increase the CTE of the epoxy,

the healed material would have the same CTE as the bulk matrix.

Finally, before self-healing composites become mainstream, they need to be

evaluated for long term environmental stability. Testing should be done to determine

how the composites and encapsulated components are affected by prolonged exposure

to non-ambient temperatures and humidities. The composite healing and mechanical

properties should be investigated prior to and post exposure. Analysis of microcapsules

can be done using thermal gravimetric analysis (TGA) and differential scanning

calorimetry (DSC) to determine if components have leached out of the microcapsules or

become inactive over time.

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References [1] B. J. Blaiszik, M. Baginska, S. R. White et al., “Autonomic Recovery of Fiber/Matrix

Interfacial Bond Strength in a Model Composite,” Advanced Functional Materials, vol. 20, no. 20, pp. 3547-3554, Oct, 2010.

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AUTHOR’S BIOGRAPHY

Jericho Lynn Moll was born in Browerville, Minnesota on December 12, 1982. She was

raised in Anchorage, Alaska where she attended Grace Christian School. After

graduating in May, 2001 she enrolled at Hope College in Holland, Michigan. In May,

2005 she graduated with a B.A. in Chemistry and a B.S in Engineering with an emphasis

in Chemical Engineering. After graduation, Jericho moved to Vladivostok, Russia for

nine months to volunteer at Vladivostok Homeless Children’s Rehabilitation Center, a

Russian non-profit organization. In the fall of 2006 she enrolled in the Materials Science

and Engineering Department at the University of Illinois at Urbana-Champaign, where

she worked for Prof. Nancy Sottos on self-sealing composites. In May, 2008 she was

awarded her Masters of Science degree.

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© 2011 by Jericho Lynn Moll. All rights reserved