Page 1
Transparent Healable Materials A Major Qualifying Project Submitted to the Faculty of
Worcester Polytechnic Institute in partial fulfillment of the requirements for the
Degree in Bachelor of Science in
Chemical Engineering Chemistry
By
______________________________________
Thomas Harless
______________________________________
Paul Hastings
_______________________________________
Andrew Portera
Date: 04/30/15 Project Advisors
________________________________________
Amy Peterson
________________________________________
Arne Gericke
This report represents work of WPI undergraduate students submitted to the faculty as evidence of a degree requirement. WPI routinely publishes these reports on its web site without editorial or peer review. For more
information about the projects program at WPI, see http://www.wpi.edu/Academics/Projects. 1 | Page Abstract Infrastructure maintenance costs billions of dollars every year
Page 2
Abstract
Self-healing polymers are a smart material, capable of autonomic restoration of properties
following damage. This WPI project team was tasked with researching the feasibility of creating
a self-healing material and applying it to vehicles windshield. A healing agent, dicylcopentadiene
and the Grubbs’ catalyst were encapsulated in poly(urea formaldehyde) and paraffin wax,
respectively. Sample mock windshields were prepared using two pieces of borosilicate glass and
a poly(vinyl butyral) interlayer containing the microcapsules and wax protected Grubbs’ catalyst.
These samples were tested for optical properties and then cracked. Following a healing period of
seven days, the samples were tested again for optical properties as well as mechanical properties
Page 3
Table of Contents 1.0 Introduction ............................................................................................................................................. 1
2.0 Background ............................................................................................................................................. 4
2.1 Glass .................................................................................................................................................... 4
2.1.1 Ceramic Materials ........................................................................................................................ 4
2.1.2 Properties of Glass ....................................................................................................................... 4
2.2 Polymers and Polymerization ............................................................................................................. 5
2.2.1 Glass Transition Temperature and Classification of Polymers .. Error! Bookmark not defined.
2.2.2 Applications of Polymers in Glass ............................................................................................... 7
2.3 Damage ............................................................................................................................................... 8
2.4 Healing Materials ................................................................................................................................ 9
2.4.1 Smart Materials ............................................................................................................................ 9
2.4.2 Self-Healing Materials ............................................................................................................... 10
2.4.3 Impacts of Self-healing Glass .................................................................................................... 10
2.5 Methods of Self-healing .................................................................................................................... 11
2.5.1 Self-healing Using the Hollow Tube Technique ........................................................................ 11
2.5.2 Self-healing Using the Microencapsulation Technique ............................................................. 13
2.5.3 The Use of DCPD as a Healing Agent ....................................................................................... 13
2.6 Testing Techniques ........................................................................................................................... 14
2.6.1 UV-Vis Spectroscopy ................................................................................................................ 14
2.6.2 TGA ........................................................................................................................................... 15
2.6.3 Impact Strength Analysis ........................................................................................................... 15
3.0 Methodology ......................................................................................................................................... 16
3.1 Microencapsulation of DCPD ........................................................................................................... 16
3.1.1 Agitation Rates ........................................................................................................................... 16
3.1.2 pH Adjustment ........................................................................................................................... 17
3.2 Wax Protection of Grubbs’ Catalyst ................................................................................................. 17
3.3 Characterization of Microcapsules .................................................................................................... 18
3.3.1 TGA to Determine Weight Percent of DCPD ............................................................................ 18
3.4 Application of Material to Mock Windshield: .................................................................................. 18
3.4.1 Impact Testing of Mock Windshields ........................................................................................ 18
3.4.2 UV-Vis Spectroscopy of Mock Windshields ............................................................................. 19
i
Page 4
3.4.3 Flexural Strength Testing of Mock Windshields ....................................................................... 19
4.0 Results and Discussion ......................................................................................................................... 20
4.1 Microencapsulation of DCPD ........................................................................................................... 20
4.1.1 Effects of Agitation Rate ............................................................................................................ 20
4.1.2 Effects of pH .............................................................................................................................. 22
4.1.3 Final Trial ................................................................................................................................... 23
4.2 TGA Results...................................................................................................................................... 25
4.3 Mock Windshield Assembly ............................................................................................................. 29
4.4 Transparency of Mock Windshield Samples .................................................................................... 30
4.4.1 Transparency of Control Samples .............................................................................................. 30
4.4.2 Transparency of Mock Windshield Trials .................................................................................. 32
4.5 Drop Tower Testing .......................................................................................................................... 35
4.6 Transparency of Healed Mock Windshields ..................................................................................... 37
4.7 Flexural Strength ............................................................................................................................... 38
5.0 Conclusions and Recommendations ..................................................................................................... 42
5.1 Conclusions ....................................................................................................................................... 42
5.1.1 Microcapsule Properties Varied with Agitation Rates and pH Levels ....................................... 42
5.1.2 Optical Transparency was Not Obtained ................................................................................... 42
5.1.3 Autonomic Healing Occurred .................................................................................................... 42
5.1.4 It is Feasible to Create a Self-Healing Laminated Glass ............................................................ 43
5.2 Recommendations ............................................................................................................................. 43
6.0 References ............................................................................................................................................. 45
Appendix A: SEM Results Trial 1 Microcapsules ...................................................................................... 48
Appendix B: SEM Results Trial 2 Microcapsules ...................................................................................... 48
Appendix C: SEM Results Trial 3 Microcapsules ...................................................................................... 48
Appendix D: SEM Results Trial 4 Microcapsules ...................................................................................... 49
Appendix E: SEM Results Trial 5 Microcapsules ...................................................................................... 49
Appendix F: SEM Results Trial 6 Microcapsules....................................................................................... 49
Appendix G: Diameter Distribution Trial 1 Microcapsules ........................................................................ 50
Appendix H: Diameter Distribution Trial 2 Microcapsules ........................................................................ 50
Appendix I: Diameter Distribution Trial 3 Microcapsules ......................................................................... 51
Appendix J: Diameter Distribution Trial 4 Microcapsules ......................................................................... 51
Appendix K: Diameter Distribution Trial 5 Microcapsules ........................................................................ 52
Page 5
Appendix L: Diameter Distribution Trial 6 Microcapsules ........................................................................ 52
Appendix M Thermogravimetric Analysis Trial 1 (550 rpm pH=3.5) ....................................................... 53
Appendix N Thermogravimetric Analysis Trial 2 (1000 rpm pH=3.5) ...................................................... 54
Appendix O Thermogravimetric Analysis Trial 3 (1500 rpm pH=3.5) ...................................................... 55
Appendix P Thermogravimetric Analysis Trial 4 (550 rpm pH=3.0) ......................................................... 56
Appendix Q Thermogravimetric Analysis Trial 5 (550 rpm pH=4.0) ........................................................ 57
Appendix R Thermogravimetric Analysis Trial 6 (1250 rpm pH=3.5) ...................................................... 58
Appendix S Thermogravimetric Analysis Wax Protected Grubbs’ Catalyst .............................................. 59
Appendix T: UV-Vis Data for Mock Windshield Control Samples ........................................................... 60
Appendix U: UV-Vis Data for Mock Windshield Samples, Trial 1 Microcapsules, Undamaged .............. 61
Appendix V: UV-Vis Data for Mock Windshield Samples, Trial 1 Microcapsules, Healed...................... 62
Appendix W: UV-Vis Data for Mock Windshield Samples, Trial 2 Microcapsules, Undamaged ............. 68
Appendix X: UV-Vis Data for Mock Windshield Samples, Trial 2 Microcapsules, Healed...................... 69
Appendix Y: UV-Vis Data for Mock Windshield Samples, Trial 3 Microcapsules, Undamaged .............. 72
Appendix Z: UV-Vis Data for Mock Windshield Samples, Trial 3 Microcapsules, Healed ...................... 73
Appendix AA: UV-Vis Data for Mock Windshield Samples, Trial 4 Microcapsules, Undamaged ........... 76
Appendix BB: UV-Vis Data for Mock Windshield Samples, Trial 4 Microcapsules, Healed ................... 77
Appendix CC: UV-Vis Data for Mock Windshield Samples, Trial 5 Microcapsules, Undamaged ........... 80
Appendix DD: UV-Vis Data for Mock Windshield Samples, Trial 5 Microcapsules, Healed ................... 81
Appendix EE: UV-Vis Data for Mock Windshield Samples, Trial 6 Microcapsules, Undamaged ............ 85
Appendix FF: UV-Vis Data for Mock Windshield Samples, Trial 6 Microcapsules, Healed .................... 86
Appendix GG: Drop Tower Impact Testing Data Mock Windshields Controls ......................................... 90
Appendix HH: Drop Tower Impact Testing Data Mock Windshields Trial 1 Microcapsules.................... 92
Appendix II: Drop Tower Impact Testing Data Mock Windshields Trial 2 Microcapsules ....................... 93
Appendix JJ: Drop Tower Impact Testing Data Mock Windshields Trial 3 Microcapsules ...................... 94
Appendix KK: Drop Tower Impact Testing Data Mock Windshields Trial 4 Microcapsules.................... 95
Appendix LL: Drop Tower Impact Testing Data Mock Windshields Trial 5 Microcapsules .................... 96
Appendix MM: Drop Tower Impact Testing Data Mock Windshields Trial 6 Microcapsules .................. 97
Appendix NN Stress Strain Mock Windshield Controls............................................................................. 98
Appendix OO Stress Strain Curve Mock Windshields Trial 1Healed ...................................................... 101
Appendix PP: Stress Strain Curve Mock Windshields Trial 1 Undamaged ............................................. 104
Appendix QQ: Stress Strain Curve Mock Windshields Trial 2 Healed .................................................... 105
Appendix RR: Stress Strain Curve Mock Windshield Trial 2 Undamaged .............................................. 107
Page 6
Appendix SS: Stress Strain Curve Mock Windshields Trial 3 Healed ..................................................... 108
Appendix TT: Stress Strain Curve Mock Windshield Trial 3 Undamaged .............................................. 110
Appendix UU: Stress Strain Curve Mock Windshields Trial 4 Healed .................................................... 111
Appendix VV: Stress Strain Curve Mock Windshield Trial 4 Undamaged ............................................. 113
Appendix WW: Stress Strain Curve Mock Windshields Trial 5 Healed .................................................. 114
Appendix XX: Stress Strain Curve Mock Windshield Trial 5 Undamaged ............................................. 116
Appendix YY: Stress Strain Curve Mock Windshields Trial 6 Healed .................................................... 117
Appendix ZZ: Stress Strain Curve Mock Windshield Trial 6 Undamaged .............................................. 119
Page 7
Table of Figures
Figure 1: Chain Growth Polymerization Mechanism8 .................................................................................. 6 Figure 2: Step-Growth Polymerization Mechanism9 .................................................................................... 7 Figure 3: Poly(vinyl butyral) Structure ......................................................................................................... 8 Figure 4: Hollow Tubes Stacked in a Matrix System ................................................................................. 12 Figure 5: Diameter Distribution of Trial 1 Microcapsules .......................................................................... 21 Figure 6: SEM of Trial 1 Microcapsules .................................................................................................... 23 Figure 7: SEM of Trial 4 Microcapsules (Left) and Trial 5 Microcapsules (Right) ................................... 23 Figure 8: Diameter Distribution of Trial 6 Microcapsules .......................................................................... 24 Figure 9: SEM Imaging of Trial 6 Microcapsules ...................................................................................... 25 Figure 10: TGA Curve of Trial 1 Microcapsules ........................................................................................ 27 Figure 11: TGA Curve of Trial 4 Microcapsules ........................................................................................ 28 Figure 12: TGA Curve of Wax-protected Grubbs’ Catalyst ....................................................................... 29 Figure 13: Control Sample 1 ....................................................................................................................... 31 Figure 14: Absorption Curve of ControL Mock Windshield Samples ....................................................... 32 Figure 15: Mock Windshield Sample 1D, Trial 1 Microcapsules .............................................................. 33 Figure 16: Absorption Curves of Mock Windshields with Trial 4 Microcapsules ..................................... 35 Figure 17: Impact Force of Control 2 ......................................................................................................... 36 Figure 18: Absorption Curve of Mock Windshield with Trial 1 Microcapsules Undamaged (Red) and After Healing (Black) ................................................................................................................................. 38 Figure 19: Instron Tensile and Flexural Strength Machine ........................................................................ 39
Table of Tables Table 1: Microencapsulation Trial Conditions ........................................................................................... 20 Table 2: Mass Loss Percentage of DCPD in Microcapsules ....................................................................... 26 Table 3: Median Absorption Value of Mock Windshields ......................................................................... 34 Table 4: Flexural Strength Results of Mock Windshields .......................................................................... 41
Table of Equations Equation 1: ASTM D790-10 Flexural Strength Equation......................................................................... 401
Page 8
1.0 Introduction
Glass is used in a wide variety of modern day applications and is often overgeneralized as
a material. Its definition can vary depending on its use and composition. For the purpose of this
project, glass is defined as a transparent material made from silicon dioxide (SiO2), sodium
carbonate (Na2CO3), and calcium carbonate (CaCO3). Glass that fits this definition is used widely in
architecture, household glassware, and in the automotive industry.
One of the problems with glass is its brittleness. Glass has a tendency to break under less
strain than other materials, and electronic devices such as cell phones are very susceptible to cracks
and scratches. As a result, the glass industry is trying to create stronger glasses that can be used in
cellphones, windows, T.V.s, and many other devices. There are approximately 7 billion people in
the world today1 and about as many phones, laptops, T.V.s, and cars.2 Broken cell phones, T.V.s,
and windshields are everyday occurrences for the average person. Producing a glass that could
potentially “heal itself” would improve people’s lives by allowing them to save money and safely
continue to use these items Additionally, self-healing glass could have a large environmental
impact. Glass materials that self-heal would not need to be replaced as frequently, reducing landfill
waste and energy consumed during the recycling process.
Visible and microscopic cracks (ones unobservable to the naked-eye) can have an impact
on the usability and functionality of a device. An example where this can pose a significant
problem is in vehicle windshields. With car windshields, general movement of the car does not
usually result in cracking; however, when other forces such as debris impact the windshield or
uneven heating may result in cracks that affect a driver’s ability to see.3 In addition to these chips,
there are cracks that are classified as micro-cracks and can lead to large cracks when put under
1
Page 9
stress. Generally, the car owner can call a company to apply a resin to fill in the damaged area,
and subsequently fix major cracks. Repair is typically carried out in the following process:
1) Damage occurs (i.e. a rock cracking a windshield)
2) A company is hired to fix the damage
3) The company applies an outside material such as a resin to repair the damage and restores
the glass to its original properties.
In both of these cases, the most important property to be maintained or recovered would be optical
transparency.
In the past 20-30 years, a heavily researched field of material science is smart materials, in
particular self-healing materials. Self-healing materials can restore some functionality to a certain
extent. There are several applications of this technology such as paint on cars, computer chips, and
composites for aerospace.4 In each of these applications, certain characteristics must be maintained
or recovered in the system with healing, such as the physical barrier for paints or electrical
conductivity in computer chips. With glass, a working self-healing material would fill in a crack,
without outside intervention, to an extent that it is almost unnoticeable, and restore an optical
transparency exactly like the glass it would be released into. With the glass industry as large as it
is, creating a self-healing glass would be desirable to extend the life of glass products.
A transparent self-healing material would have a substantial impact on the glass industry.
It could be applied to glass products that are used in everyday life. The technology of transparent
self-healing materials in glass products would minimize cracking, allow for optical transparency
and improve the overall quality of the products.
2
Page 10
This WPI team has two goals for the project in order to produce a material that can be for
consumer use:
1. Synthesize an optically transparent self-healing material
2. Explore the feasibility of applying the self-healing material to car windshields
The completion of these project goals can lead to a material that could extend the lifespan and
overall quality of many glass consumer products.
3
Page 11
2.0 Background
2.1 Glass
Glass is composed of three main oxides: silica (SiO2), sodium oxide (Na2O), and calcium
oxide (CaO). Sodium oxide and calcium oxide are typically derived from the raw materials soda-
ash (NaCO3) and limestone (CaCO3). The materials are placed in a furnace to melt, generally at
temperatures exceeding 1600 °C. Once at the desired temperature, the molten glass is formed into
the product, whether it’s a windshield or a soda bottle. At this point the glass can be heat treated,
chemically treated, or otherwise manipulated depending upon the application.
Glass is found in many applications such as car windshields, food containers, and electronic
devices. The different types of glasses that are found in these applications vary in properties and
depend on the specific application’s purpose. For example, car windshields need to withstand a
substantial amount of force in order to protect passengers in the car, while the Gorilla Glass on a
smartphone does not have the same requirement, but rather needs to be scratch resistant and
lightweight. The way glass is made, and the materials used in the process determine the properties
of the glass.
2.1.1 Ceramic Materials
In addition to devices containing conventional glass, there are many different types of
glass-ceramics. These materials share properties with both glasses and ceramics. They differ from
glasses because they have a polycrystalline structure instead of being amorphous.5 The main
applications for glass-ceramics are dental devices, Corning Ware, and optical elements.
2.1.2 Properties of Glass
Glasses exhibit several properties that are desirable. They can have tremendous strength,
can be manipulated, and are transparent (in most cases). Car windshields must withstand the force
4
Page 12
of an airbag when deployed (approximately 2000 lbs. of force ).6 Windshields are made up of two
panes of glass that have been annealed (heat treated) and adhered together with a polymer
interlayer. The interlayer is generally poly(vinyl butyral) (PVB). This polymer is chosen due to
several properties that make it desirable: outstanding binding efficiency, optical clarity, adhesion
to a large number of surfaces, and toughness combined with flexibility.7
2.2 Polymers and Polymerization
Polymers are very large molecules, often referred to as macromolecules. They are
composed of repeat units of monomers along a chain. Although there are many naturally occurring
polymers, most polymers used commercially are synthetic. The process of creating synthetic
polymers from their building blocks (monomers) is known as polymerization. There are two main
synthesis mechanisms: chain-growth and step-growth.
Chain-growth polymers are formed by the rapid addition of monomers to a reactive
polymer chain. They are most commonly addition polymers, which are formed by reacting the
double bonds of monomers to form repeating units. Chain-growth polymers are most commonly
formed from existing alkene monomers. The mechanism for chain growth polymerization can be
seen in Figure 1.
5
Page 13
Figure 1: Chain Growth Polymerization Mechanism8
The second class of synthetic polymers is step-growth polymers. In contrast to chain-
growth polymerization, step-growth polymerization can occur using two different monomers, or
even a combination of two chains of polymers. Most step-growth polymers are condensation
polymers. This means that the reaction of the monomers results in the loss of a small molecule.
Step-growth polymerization can take place for any two monomers having the correct functionality
for reaction. The mechanism for step-growth polymerization can be seen in Figure 2.
6
Page 14
Figure 2: Step-Growth Polymerization Mechanism9
2.2.1 Applications of Polymers in Glass
Polymers are used for many different applications with glass materials. One specific
polymer application to glass is the use of PVB in car windshields. It is used to make the windshield
safer by increasing its toughness. A glass windshield is composed of three main layers. There are
two sheets of glass surrounding a central layer of PVB. When one of the glass sheets cracks, the
PVB layer prevents the windshield from shattering by absorbing the energy and preventing the
crack from spreading through the second layer. The use of polymers in windshields has reduced
major crack damage.10 The structure of PVB is shown in Figure 3.
7
Page 15
Figure 3: Poly(vinyl butyral) Structure
2.3 Damage
As has been mentioned, there are numerous applications today in which glass is used for
its transparent quality. The greatest problem with glass is its tendency to crack, due to its brittle
character. Cracked glass is detrimental to both the physical appearance and the user interface. For
example, a car windshield with a large enough crack in it can pose problems with the driver’s
vision. To address this, many companies are investigating how glass can be made tougher and
withstand more damage.
Car windshields must be able to flex in different directions as the car moves without
breaking. In most cases these glass materials are susceptible to thermal and mechanical stresses,
which can add to, and even be the cause of, cracking. Uneven heating in the winter is a form of
thermal stress. Due to this uneven heating, windshields are susceptible to thermal fatigue that can
propagate large cracks from small cracks. On occasion, an incident may occur where a rock or
8
Page 16
stone makes contact with some force to the windshield, causing a crack or even a small chip. Over
time, this deformation in the glass is subject to the other stresses that have been mentioned. In its
damaged state, the area of the crack is much weaker than the rest of the glass, and may be the
beginning location for the formation of larger cracks.
2.4 Healing Materials
When a material is damaged or fractured, it loses its original properties. When a glass
material is cracked or shattered it can lose desired properties such as transparency. Depending on
the severity of damage the glass can be healed, or regain its original properties to a useful extent.
For our purposes healing regarding glass materials can be defined as significant restoration of a
damaged glass to near its original flexural strength and transparency.
It is imperative for a glass to maintain these two properties for practical reasons. In car
windshields, it is important for the glass to be optically transparent to allow the driver visibility of
his or her surroundings. It is also important for the windshield to have a flexural strength that can
withstand normal driving conditions. This is to ensure that the windshield does not break or shatter
while the vehicle is being operated. Windshields play a vital role in protecting the occupants of
the vehicle from high wind speeds, outdoor elements, and in the event of a crash. A windshield
capable of healing and restoring these properties after damage would greatly improve the current
state of technology.
2.4.1 Smart Materials
A smart material is characterized as enhancing or controlling one or several properties of
the material to utilize a specific function. Smart materials typically require an outside force or
condition to propagate a change in properties, such as stress, electricity, temperature, or change in
pH.11 Some examples of smart materials are shape-memory alloys, eyeglass lenses that change
9
Page 17
colors when exposed to UV rays, and self-healing materials. Self-healing materials are of particular
interest for their wide range of applications, and unique chemistry. These materials have the ability
to self-repair when damaged and can return their original structure and functionality.
2.4.2 Self-Healing Materials
The difference between healing and self-healing is that self-healing is an autonomic
process, in which internal thermal or mechanical stresses initiate the healing process. Most healing
processes require an addition to the system to restore the material’s properties. Self-healing
materials do not require outside intervention to initiate the healing process, but rather heal
spontaneously. Applying self-healing concepts to glass would enhance glass products in many
ways.
Applying self-healing to glass opens up the potential of having a composite material that
would heal itself when damaged by thermal or mechanical stresses. In order for the glass to be
healed as previously defined, its properties must be significantly restored at the damaged area.
Glass windshields used in cars today have a refractive index between 1.51 and 1.53.12 The
refractive index is the ratio between the speed of light in a vacuum to the speed of light through
whatever medium is being measured. A self-healing material that fills a cracked area of glass,
would have to have a similar refractive index to maintain the glass’s transparency. This is
necessary to ensure there is no noticeable difference between the repaired area and the rest of the
glass.
2.4.3 Impacts of Self-healing Glass
In 2009 alone, there were over 9.5 million vehicles involved in an accident.13 When cars
are in severe accidents, the windshields are either disposed of as landfill waste, or they are
recycled. Landfill waste is not an environmentally friendly waste method as some materials buried
10
Page 18
are not biodegradable. The issue with recycling windshields is that the process requires a
significant amount of energy and is economically inefficient.14 With self-healing windshields, both
of these processes could be mitigated. If the damaged windshield could self-heal and restore its
properties to nearly original levels, the windshield would not need to be replaced, eliminating the
need to recycle or waste the glass. This would both lower energy costs and reduce landfill waste
having great environmental and economic impacts.
2.5 Methods of Self-healing
There have been several different methods developed over the years for producing self-
healing polymers. Two of the most practiced techniques of self-healing are hollow tube, and micro-
encapsulation. Both methods can involve liquid dicyclopentadiene (DCPD) as a healing agent.
DCPD is encapsulated and surrounded by the Grubbs’ catalyst.
2.5.1 Self-healing Using the Hollow Tube Technique
An approach to creating a system that combines the self-healing material, a catalyst, and
the polymer matrix is the use of hollowed tubes. The tubes or capillaries contain a self-healing
liquid surrounded by a polymer matrix. The mechanism takes place when the tubes are cracked or
ruptured; the self-healing liquid is activated and polymerizes to fill the propagated crack.
The hollow tubes in the matrix are typically narrow glass capillaries. Some factors that
need to be considered are what components are being encapsulated in the tubes and their
orientation in the network. The hollow tube can contain either the healing agent, or the catalyst.
This is usually determined by the properties of each component such as shelf-life, or reactivity. If
one component has a poor shelf-life or is too reactive, it will be injected in the hollow tubes. If
reactions occur spontaneously, or components decompose, they will not be effective when the
tubes are ruptured.
11
Page 19
The use of hollow tubes has its benefits over other methods of encapsulation. The hollow
fibers are multifunctional because they store the liquid healing agent and provide structural
reinforcement. The work conducted by Bond and co-workers from the University of Bristol used
tubes that had diameters of 60 µm.15 Although hollow tube encapsulation can be useful due to its
structural reinforcements, when applied to a car windshield system, it may not be the most effective
method.
Since the tubes will be crossed or stacked in the system as shown in Figure 4, visibility
may be a concern. If the tubes have any optical defects or opacity to them, the efficiency of the
system will be lower. The second method of self-healing relies on a process called
microencapsulation of a healing agent. In this method, microencapsulated healing agents are
incorporated in the material.
Figure 4: Hollow Tubes Stacked in a Matrix System16
12
Page 20
2.5.2 Self-healing Using the Microencapsulation Technique
Microencapsulation is a useful process through which chemicals are placed in a system
that then can be released upon external force being applied. For our purposes, we will focus on the
microencapsulation of a monomer. Microcapsules are microscopic spheres containing a healing
agent that, when released, reacts with its surroundings and heals the damaged area. The healing
agent is encapsulated within a protective shell that eliminates direct contact with a catalyst
preventing undesired reactions.
The process in which microcapsules are prepared is a simple oil-in-water emulsion
polymerization. This process generally starts with a solution that will form the shells of the
microcapsules. An emulsifier is necessary because it stabilizes the two immiscible substances. This
prevents separation of the solution. The emulsifier typically has hydrophilic and hydrophobic tails,
so to surround the non-water material, but remain associated within the water.17 One common
emulsifier is ethylene maleic anhydride (EMA).17 It is important to wash the microcapsules after
their synthesis, as they will have residual healing agents on the outside of the shell.
In application, these microcapsules are dispersed in the material that one wishes to heal,
along with a catalyst. When the material is scratched, or damaged, the microcapsules within are
ruptured. This releases the healing agent (monomer) into the damaged area. The healing agent then
comes into contact with a catalyst which increases the rate of polymerization.18
2.5.3 The Use of DCPD as a Healing Agent
DCPD is a commonly used monomer that can be used as a healing agent. It is a monomer
that does not polymerize at room temperature. In order to use DCPD as a self-healing polymer,
there must be a catalyst present to lower the activation energy and allow the polymerization to occur
readily at room temperature. An effective catalyst for DCPD is the Grubbs’ catalyst (benzylidene-
13
Page 21
bis(tricyclohexylphosphine)dichlororuthenium)). The use of DCPD and Grubbs’ catalyst can be
applied to both the hollow tube method, and the microencapsulation method of self-healing.
When a crack in the material ruptures a capsule, DCPD is released. When DCPD comes in
contact with the catalyst, DCPD undergoes a ring-opening metathesis polymerization (ROMP).
This reaction fills the crack in the material and allows for self-healing to take place. The use of
DCPD as a liquid healing agent has resulted in the significant restoration of materials properties.
The low viscosity of the monomer causes an even flow through the crack allowing for healing of
a polymer network to its original fracture toughness.19 Microencapsulation of DCPD and the
Grubbs’ catalyst is a very effective method for self-healing materials.
2.6 Testing Techniques
Several techniques can be used to analyze the system of glass, PVB, and self-healing
material. These techniques include UltraViolet-Visible (UV-Vis) Spectroscopy,
Thermogravimetric Analysis (TGA), and Instron testing (flexural/impact strength). These
techniques measure properties of the material such as flexural strength, absorbance/transmittance,
and weight percent of components in a system. These properties can be used to determine
transparency, strength, and healing efficiency of the system.
2.6.1 UV-Vis Spectroscopy
UV-Vis is a useful spectroscopic technique for determining absorption properties of a
compound. The spectrometer measures light absorption over the electromagnetic spectrum of 200-
750 nm. This range in wavelengths covers the ultra-violet electromagnetic spectrum (100-400 nm)
and the human visible electromagnetic spectrum (400-700 nm). UV-Vis spectroscopy measures
the absorption of light over this range indicating the intensity related to specific wavelengths.20
14
Page 22
2.6.2 TGA
TGA is a useful technique to measure the mass loss of a substance during thermal
degradation. TGA works by heating the sample at certain rate, and simultaneously measuring the
change in mass as this occurs. The mass loss between certain temperatures can be attributed to the
loss of certain substances. For example, weight losses between 70-100 °C are often attributed to
the loss of water. From the end result, the least volatile components weight percent can be
obtained.21
2.6.3 Impact Strength Analysis
Impact strength or energy is a material’s ability to withstand failure under an outside
impact. The Tower Drop Test will be used for this analysis and involves dropping an object, such
as a steel ball, onto a material to do two things:
1) Create a crack
2) Cause failure in the material after initial cracking
This technique will be useful in determining the resistance of the material to failing.22 The ability
to resist failure upon impact is an important characteristic of a windshield.
15
Page 23
3.0 Methodology
3.1 Microencapsulation of DCPD
200 mL of deionized water and 50 mL of a 2.5 wt. % aqueous solution of EMA was mixed
in a beaker at room temperature. The beaker containing the solution was then suspended in a
temperature controlled water bath on a hotplate. The solution was agitated using a mechanical
stirrer. Once the solution was under agitation, 5.00 g of urea, 0.50 g of ammonium chloride and
0.50 g of resorcinol was dissolved in the solution. Then, by drop-wise addition of sodium
hydroxide and hydrochloric acid, the pH of the solution was raised from approximately 3.50. One
or two drops of 1-octanol was added to eliminate surface bubbles. A slow stream of 60 mL of
DCPD was then added to the solution to form an emulsion. The emulsion stabilized under
continuous stirring for 10 minutes. Once the emulsion stabilized, 12.67 g of a 37 wt. % aqueous
solution of formaldehyde was added to obtain a 1:1.9 molar ratio of formaldehyde to urea. The
emulsion was then covered with aluminum foil and paraffin wax, and heated at a rate of
approximately 1 °C per minute until it reached a temperature of 55 °C. This temperature was held
with continuous agitation for 4 hours. After 4 hours, the hotplate and stirrer were turned off and
the emulsion was allowed to cool. Once it reached ambient temperature, the suspension of
microcapsules was separated by coarse-fritted vacuum filtration. The microcapsules were then
rinsed with deionized water and allowed to air dry for 24 hours.18
3.1.1 Agitation Rates
After completion of our base case at an agitation rate of 550 rpm, the effects of agitation
rates during the emulsion process was examined. The second and third agitation rate were 1000
1500 rpm respectively. Following this the microcapsules were examined using SEM.
16
Page 24
3.1.2 pH Adjustment
In the base case of microencapsulation, the reaction was carried out at a starting pH of 3.5.
The formation of poly(urea-formaldehyde) occurs at acidic conditions. To understand how pH
levels will affect the formation of the microcapsules, the pH was adjusted to a more acidic level of
3.0 and another at a more alkaline environment with a pH of 4.0. This sample was then analyzed
for characteristics of how well it performs when combined with the epoxy and Grubbs’ catalyst.
With these results, a comparison was made to the base case to determine which method would give
the most beneficial microcapsule shells.
3.2 Wax Protection of Grubbs’ Catalyst
In an N2 gas-filled glovebox, 10 g of paraffin wax and 525 mg of Grubbs’ Catalyst was
sealed in a vial. Once the vial was sealed, it was removed from the glovebox. An aqueous solution
of 225 mL of water, 63 g of 0.28 wt. % poly(ethylene-co-maleic anhydride) and one drop of octanol
was prepared in a 1000 mL beaker. The beaker was then placed in an 82 °C water bath and stirred
using a mechanical stirrer at 900 rpm. The vial containing the paraffin wax and the catalyst was
then submerged in the same 82 °C water bath. Both the vial and the beaker of solution remained
in the hot water bath for approximately 10 minutes, until the wax had melted. At this point the
stirred solution reached a temperature of approximately 65 °C. The vial containing the wax was
shaken in order to disperse the catalyst. The vial was then opened (in air) and the melted wax was
poured quickly into the aqueous solution still under continuous stirring. After 2 minutes, 600 mL
of cold water (<5 °C) was quickly added to the solution and the stirring was stopped. The wax
microspheres containing the catalyst were then collected by filtration and dried under vacuum.23
17
Page 25
3.3 Characterization of Microcapsules
There were several parameters that needed to be measured to determine the efficiency of
the microencapsulation. One major characteristic was the weight percent of DCPD that was
encapsulated.
3.3.1 TGA to Determine Weight Percent of DCPD
This property was measured by thermal gravimetric analysis (TGA). By heating up
microcapsules as well as the encapsulated Grubbs’, the mass loss over a temperature range was
determined. The boiling point of DCPD is 170 ºC, so the mass loss between the ranges of 150-250
ºC was analyzed for the loss of the encapsulated healing agent.
3.4 Application of Material to Mock Windshield:
The process in which a mock windshield were created included two pieces of glass
(borosilicate glass) and the liquid PVB mixture. Grubbs’ catalyst, and the microcapsules (all
solids) were added in amounts giving an 85 wt. % toluene-ethanol to 15 wt. % solids. Following
this, 2 ml of the mixture were pipetted on each 2”x2” sample to give the desired thickness. A
second piece of glass was placed on top of the mixture, sandwiching it between.
3.4.1 Impact Testing of Mock Windshields
Working in collaboration with the Civil Engineering department, the flexural strength of
the mock windshield was examined. By utilizing the Instron Dynatup drop tower, the mock
windshield, with the healing agent added were cracked, at a determined speed of the weight
dropped. Once this was done, the windshield were allowed to heal. After a 7 day period, the cracks
were assessed for optical and mechanical properties.
18
Page 26
3.4.2 UV-Vis Spectroscopy of Mock Windshields
To measure the transparency of the mock-windshield, UV-Vis spectroscopy was
performed before and after the healing period. A control sample of laminated glass was used as
the base line for the absorption curve. It was placed in the spectrometer and its absorption was
measured over the visible wavelength spectrum. Then, the mock windshields containing the PVB
mixture interlayer was tested over the same light spectrum. The absorption curve for this sample
was used to determine its transparency in visible light. If the healed glass had low absorption for
this range, it will have similar transparency to the control glass.
3.4.3 Flexural Strength Testing of Mock Windshields
Following the allowed healing time, and UV-Vis, each sample was tested for flexural
strength. Using an Instron 3 point bend test, a load rate was applied to each sample until failure.
Once the max load was determined, the equation found in ASTM 790-10 was used to determine
the flexural strength of each sample. These results were compared to those of control samples and
undamaged samples to determine if mechanical properties had returned following the healing
period.
19
Page 27
4.0 Results and Discussion
4.1 Microencapsulation of DCPD
The microencapsulation method described in section 3 produced microcapsules of different
average diameters, shell structure, and weight percentage of the healing agent. These
microcapsules were generally spherical in shape, and had high weight percentages of DCPD. Six
trials were performed at varying agitation rates and pH values to further investigate their impact
on microcapsules properties. The conditions for each trial are shown in Table 1
Table 1: Microencapsulation Trial Conditions
Trial Speed (rpm) pH
1 550 3.5
2 1000 3.5
3 1500 3.5
4 550 3.0
5 550 4.0
6 1250 3.5
4.1.1 Effects of Agitation Rate
By changing the agitation rate of the process by approximately 500 rpm from an
intermediate value for Trials 1-3, it was anticipated that three different extremes of microcapsules
could be produced and analyzed. Following the procedure outlined in Brown et al, the first
agitation rate used was 550 rpm.18 1000 and 1500 rpm were chosen, to understand how drastically
the average diameter would change for the microcapsules. ImageJ, a program developed by the
20
Page 28
National Institutes of Health, was used to determine microcapsule diameters from representative
SEM images of each trial.
The microcapsules formed in Trial 1 had the largest average diameter of approximately
156 ±32.6 μm. The distribution of microcapsule diameters for this trial can be seen in Figure 5
below. The resulting microcapsules in Trial 2 had an average diameter of approximately 132 ±27.8
μm. Trial 3, which was performed under the highest agitation rate, produced an average diameter
of approximately 33 ±34.3 μm. The microcapsules at this rate were embedded within larger pieces
of poly(urea formaldehyde) and were observed to have many more collapsed shells than the other
trials.
Figure 5: Diameter Distribution of Trial 1 Microcapsules
In the interest of optimizing optical transparency of the mock windshields, minimizing
microcapsule size while reducing the amount of ruptured spheres was desired. Because the
microcapsules formed in Trial 3 were both highly agglomerated and damaged, it was concluded
1
7
11 11
27
21
17
4
10
5
10
15
20
25
30
60-80 80-100 100-120 120-140 140-160 160-180 180-200 200-220 220-240
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
21
Page 29
that 1500 rpm was not an ideal condition. Trial 6 was conducted at an agitation rate 1250 rpm in
order to avoid the problems observed in Trial 3 and to reduce the microcapsules size from Trial 2.
4.1.2 Effects of pH
To investigate the effects of pH on the microcapsules, Trials 4 and 5 were conducted at a
constant agitation rate of 550 rpm and pH of 3.0 and 4.0, respectively. The results from these trials
were compared to the results of Trial 1. The average diameters for both trials were approximately
140-160 μm, supporting the hypothesis that the pH of the system does not affect the average
diameter of the microcapsules. However, upon comparison of the SEM results of Trials 1, 4, and
5 it was apparent that the surface morphology of the microcapsules were different.
The SEM results for Trials 1, 4, and 5 are shown in Figures 6 and 7. The surface of the
microcapsules in Trial 4 exhibit a higher roughness than those in Trial 1. The imaging also
indicates a smaller quantity of microcapsule as well as excess poly(-urea formaldehyde). This
implies that the formation of poly(-urea formaldehyde) microcapsules is inhibited by acidic
conditions. Trial 5 experienced similar complications. The microcapsules formed in Trial 5
appear to be less rough than the microcapsules formed in Trial 1. They exhibit more protrusions
stemming from the shell walls and, similarly to Trial 4, display excess poly(urea formaldehyde).
These results indicate that the basic conditions of Trial 5 are not favorable to synthesizing poly
urea formaldehyde microcapsules. To further investigate the effects of pH on microcapsules, TGA
was performed on Trials 1, 4, and 5. TGA results are discussed in section 4.2.
22
Page 30
Figure 6: SEM of Trial 1 Microcapsules
Figure 7: SEM of Trial 4 Microcapsules (Left) and Trial 5 Microcapsules (Right)
4.1.3 Final Trial
Using the results from the first 5 trials and analyzing the effects of agitation rates and pH
level conditions, Trial 6 was designed to optimize the ideal properties of microcapsules for the
mock windshield system. The Trial 6 experiment was conducted with an agitation rate of 1250
23
Page 31
rpm and at a pH value of 3.5. SEM of the first two attempts at Trial 6 indicated that many
microcapsules were ruptured and the trial was inadequate for the mock windshield system. This
was determined to be caused by an incorrectly calibrated pH meter. The third attempt of Trial 6
resulted in microcapsules, which had an average diameter of 44 ±12.4 μm, and smooth surface
morphology. The distribution of microcapsule diameters for this trial can be found in Figure 8 and
the SEM results can be found in Figure 9.
Figure 8: Diameter Distribution of Trial 6 Microcapsules
2
8
24
34
20
74
0
5
10
15
20
25
30
35
40
20 30 40 50 60 70 80
Num
ber o
f Mic
roca
psul
s
Diameter μm
24
Page 32
Figure 9: SEM Imaging of Trial 6 Microcapsules
4.2 TGA Results
TGA was performed on each trial of microcapsules as well as the wax-protected Grubbs’
catalyst. TGA allows users to analyze several aspects of sample thermal stability. We focused on
the mass loss percentage to establish that DCPD and Grubbs’ catalyst were encapsulated. Table 2
details the mass loss results of each trial, where the mass losses ranged from approximately 58-
84%.
25
Page 33
Table 2: Mass Loss Percentage of DCPD in Microcapsules
Trial Mass loss (%)
1 72.5
2 74.7
3 83.9
4 58.6
5 67.1
6 69.5
Trial 1 represents the control parameters of a mechanical stirrer speed of 550 rpm and pH
of 3.5. Figure 10 is the mass loss for Trial 1. There was a mass loss of 72.48% from approximately
150-300 ºC. DCPD has a boiling point of 170 ºC, which corresponds to the mass loss temperature
range.22 In addition to this large mass loss, there was a smaller mass loss from the start of the first
trial (20 ºC, room temperature) to approximately 150 ºC. Trials 2, 3, and 6 show similar TGA
curves with a minimal mass loss until the 150-300 ºC temperature range.
26
Page 34
Figure 10: TGA Curve of Trial 1 Microcapsules
Trials 1-3, and 6 show that encapsulation took place and that the poly(urea formaldehyde)
(PUF) shell wall protected the DCPD. Trials 4 and 5 also support this hypothesis, but show how
the pH effects the efficiency of the PUF in protecting the DCPD from degradation.
In Trials 4 and 5, the pH was adjusted to determine what could potentially change the
encapsulation process. Trial 4 was encapsulated using 550 rpm and a pH of 3.0, which is 0.5 lower
than the standard pH of Trials 1, 2, 3, and 6. Figure 11 shows a steady mass loss until
approximately 250 ºC, where a steep slope presents itself. Trial 5, which was run at 550 rpm and
a pH of 4.0, had a similar curve to Trial 4. This result shows that the integrity of the shell wall may
have been compromised with pH adjustment. Under stress, the shell walls of the microcapsules
may rupture with more ease, or be more porous then those produced at a pH of 3.5.
27
Page 35
Figure 11: TGA Curve of Trial 4 Microcapsules
The Grubbs’ catalyst was encapsulated in paraffin wax. Based on two trials of TGA for the
wax encapsulated Grubbs’ catalyst, an average weight percent of 5.49 ±0.25% Grubbs’ catalyst
was obtained. Figure 12 shows that the first trial of Grubbs’ encapsulated contains approximately
5.24 wt. % Grubbs’ catalyst. This is consistent with the literature value.23 Trial 2 resulted in
approximately 5.74 wt. % of Grubbs’ encapsulated in the wax. The graph of this data can be found
in appendix S. These values represent the first region on the TGA curve for mass loss. Both trials
for the Grubbs’ encapsulation had another region where there was a significant mass loss. This
occurred around 275-350 ºC. This area of the graph corresponds to the boiling point of paraffin
wax. 24
28
Page 36
Figure 12: TGA Curve of Wax-protected Grubbs’ Catalyst
4.3 Mock Windshield Assembly
To create the mock windshields, a 33”x40” piece of borosilicate glass was purchased and
cut into 2”x2” pieces. Due to what was available, the glass purchased had a thickness that is
0.25mm less than what is generally used in car windshields. Knowing the dimensions of each mock
windshield, and the desired thickness of 0.76 mm for the PVB interlayer, a required PVB-
microcapsule mixture volume for each trial was found to be approximately 1.97 mL. To account
for possible overflow and loss of PVB mixture, it was decided to apply 2.0 ml for each sample.
When applying the mixture to each sample, it was very difficult to not have the mixture pour out
the sides of the glass. As a result, it was extremely difficult to maintain the desired thickness in
29
Page 37
each sample. A sample size of at least three mock windshields per trial was used for most accurate
results in mechanical testing.
Following the methodology provided for creating the liquid PVB resin, an 85:15 wt. %
solvent-Butvar (solid PVB resin) ratio, the microcapsules and Grubbs’ catalyst were added in their
respective amounts stated in the methodology. It was found that at this weight ratio, the solution
was too viscous, and could not be pipetted onto the glass sample. To account for this, it was decided
to use a mixture of 85:15 wt. % solvent-solid ratio. The solids consisted of Butvar, the
microcapsules, and the Grubbs’ catalyst.
4.4 Transparency of Mock Windshield Samples
4.4.1 Transparency of Control Samples
The first six control samples of mock windshields were tested to provide the expected
absorption curves for transparent samples. Because the control samples appear to be optically
transparent, it was expected that the samples would absorb little to no light across the visible light
spectrum. A control sample, composed of two pieces of glass and a PVB interlayer can be seen
below in Figure 13.
30
Page 38
Figure 13: Control Sample 1
All six control samples displayed nearly linear absorption curves at approximately zero
across the visible light spectrum. The results of the UV-Vis data for the six control mock
windshields is below in Figure 14. The results support the hypothesis that the control mock
windshields are optically transparent.
31
Page 39
Figure 14: Absorption Curve of Control Mock Windshield Samples
4.4.2 Transparency of Mock Windshield Trials
The mock windshields containing the microencapsulated DCPD and wax protected
Grubbs’ catalyst, did not appear to be optically transparent. All six sample sets of mock
windshields appeared to be relatively opaque. A sample mock windshield is shown below in
Figure 15.
32
Page 40
Figure 15: Mock Windshield Sample 1D, Trial 1 Microcapsules
UV-Vis testing was performed on each mock windshield to assess their optical properties.
The results of the UV-Vis testing for all of the mock windshield samples can be found in
Appendices T-FF. All of the mock windshields for each trial of microcapsules, absorbed light
across the visible light spectrum. To quantify and compare the optical properties of the mock
windshields, the median absorbance was recorded for each trial of mock windshields at a
wavelength of 550 nm. The median absorption values for each trial are shown below in Table 3.
33
Page 41
Table 3: Median Absorption Value of Mock Windshields
Microcapsule Trial Median Absorption Value (λ=550nm)
1 0.50
2 0.87
3 0.56
4 1.50
5 0.75
6 0.50
The absorption data in Table 3 indicates that the mock windshields are not fully transparent.
The mock windshield samples containing the microcapsules from Trial 1 and Trial 6, have the
lowest absorption value of 0.50. The mock windshields containing the microcapsules from Trial
4 have the highest median absorption value of 1.50. The absorption curves of the trial 6 mock
windshield samples is shown below in Figure 16.
34
Page 42
Figure 16: Absorption Curves of Mock Windshields with Trial 4 Microcapsules
4.5 Drop Tower Testing
After assembling the mock windshields, and allowing the resin to cure, the samples were
impact tested in Kaven Hall on the WPI campus. Using the drop tower available, cracks were made
in the glass, in a manner similar to how an object hitting a car windshield would do so. The lightest
weight that could be used in the drop tower was 5.26 lbs. Due to this, the weight had to be dropped
from very small distances to avoid shattering. By placing each sample on a 1” washer and
contacting the samples in the center, a better representation of a windshield without support from
behind was achieved.
To determine the height from which to drop the weight, tests with several control samples
were performed. An initial distance of 20 mm was used, and shattered the control. Following this,
35
Page 43
a distance of 10 mm was chosen. At this height, a max force of 789.8 lbf shattered the control. The
impact result for control 2 can be seen below in Figure 17. In one instance however, at a height of
11 mm, a control (9) cracked only the first layer of glass and reached a maximum force of 851.3
lbf.
Figure 17: Impact Force of Control 2
From these control results it was determined that each sample would break at different
impact forces. Due to this, a standard height from which to drop the weight could not be
determined. This is attributed again to the difficulty in maintaining an equal resin thickness for
each sample. In order to avoid shattering, an initial drop height of 6 mm from the sample was
chosen. In cases where this force did not result in a crack, the height was adjusted in 1 mm
increments until a crack was formed. In some cases, samples shattered instead of cracking.
Samples were tested in the drop tower until a sample size of at least three per condition was
attained. Following the induced damage, the samples were allowed to heal for a seven day period
0
100
200
300
400
500
600
700
800
900
0:00
0:57
1:55
2:52
3:50
4:48
5:45
6:43
7:40
8:38
9:36
10:3
311
:31
12:2
813
:26
14:2
415
:21
16:1
917
:16
18:1
419
:12
20:0
921
:07
Forc
e (L
bf)
Time (ms)
36
Page 44
at room temperature, after which further testing was performed. The literature value for healing
time in an epoxy matrix is 48 hours.25 Because microcapsules in this application were in a PVB
interlayer, which has not been previously investigated, a seven day healing period was chosen to
ensure polymerization was complete when the healed samples were tested.
4.6 Transparency of Healed Mock Windshields
After allowing the cracked mock windshields to heal, UV-Vis testing was performed to
measure the transparency of the samples. The absorption curves of all of the healed samples,
before and after healing, can be found in Appendices U-GG. All of the healed samples absorbed
light across the visible light spectrum. This indicates that after cracking, the mock windshields
were still not optically transparent. Although there were some variations, almost all of the healed
mock windshields experienced slightly higher absorption curves. For example, Trial 1 sample I
originally had an absorption value of 0.20 at 550 nm. When tested after the healing period, the
sample had an absorption value of 0.43 at the same wavelength. The absorbance curves of mock
windshield sample 1I undamaged and after healing, can be seen below in Figure 18. The trend of
increased absorption values after healing shows that the samples became less optically transparent
after healing.
37
Page 45
Figure 18: Absorption Curve of Mock Windshield with Trial 1 Microcapsules Undamaged (Red) and After Healing (Black)
4.7 Flexural Strength
Following the predetermined healing time of seven days, each sample was tested in an
Instron Tension and Flexural machine. Figure 19 shows the setup of the flexural strength testing.
The machine is hooked up to a computer with BlueHill software to graph different parameters.
This testing was performed in accordance with the ASTM D790-10. For the purpose of this project,
the y-axis was chosen as extension in inches and the x-axis was chosen as load in pound-force
(lbf).
38
Page 46
Figure 19: Instron Tensile and Flexural Strength Machine
The measured load at failure for each trial varied. This can be explained by the extent of
the crack in each sample as well as the inconsistent thickness of the samples. The already cracked
controls ranged from 106.2-159.8 lbf while the undamaged controls ranged from 132.8-155.1 lbf.
The flexural strength of each sample was calculated using Equation 1.26
39
Page 47
Equation 1: ASTM D790-10 Flexural Strength Equation
(1) Sf= 3𝑃𝑃𝑃𝑃2𝑏𝑏𝑏𝑏2
Where:
Sf = stress in the outer fibers at midpoint (Psi)
P = load at a given point on the load-deflection curve (lbf)
L = support span (in.)
b = width of beam tested (in.)
d = depth of beam tested (in.)
The maximum value for flexural strength of all the trials was for Trial 5 sample B
(Damaged) which had a load of 212.6 lbf and a flexural strength of 8418.9 Psi. It is believed that
due to the poor quality of the microcapsules in Trials 3, 4, and 5 that DCPD was released into the
interlayer before damage occurred. This likely caused formation of poly(DCPD), which has a very
high flexural strength.27 This explains the high flexural strength values for the undamaged mock
windshields containing microcapsules from Trials 3, 4 and 5. Table 4 shows the average flexural
strength data of all 6 trials of mock windshields and controls. Due to complications initiating
sample cracks, only one undamaged sample mock windshield was tested for flexural strength for
each trial of microcapsules. All healed samples were tested for flexural strength; average values
and standard deviations were calculated.
40
Page 48
Table 4: Flexural Strength Results of Mock Windshields
Trial T1 T2 T3 T4 T5 T6 Control
Sf Healed(Psi) 2515 2797 4213 3991 4401 3543 5176
Standard Deviation (Sf Healed) (±) 1050 876 770 1040 2344 979 964
Sf Undamaged(Psi) 1261 4110 3102 1825 3430 3247 5815
ΔSf (Psi) 1254 -1313 1111 2166 971 297 -639
In almost every trial, the healed samples exhibited higher flexural strength than compared
to the undamaged samples. Though the standard deviation for every trial was large, it seems Trial
6 was more consistent in results relative to other trials such as 4 and 5. Trial 6 when healed returned
to a flexural strength most similar to it undamaged state comparison. The control samples showed
a greater flexural strength both before and after a crack was propagated. However, in trials with
microcapsules and Grubbs’ catalyst loaded into the PVB interlayer, increased flexural strength was
achieved following the healing period. The polymerization of DCPD within the samples created a
need for a greater load force to be applied onto the windshields before failure occurred.
41
Page 49
5.0 Conclusions and Recommendations
After analyzing the data and comparing results, the project group was able to come to the
following conclusions and offer recommendations for future research in self-healing windshields.
5.1 Conclusions
5.1.1 Microcapsule Properties Varied with Agitation Rates and pH Levels
Following the first five trials it was confirmed that microencapsulation would occur at
varying parameters such as agitation rates and pH. These results showed that the agitation rate
affects the size of the microcapsules while pH affects the shell quality and formation of poly(Urea
formaldehyde). The sixth trial (1250 rpm, pH=3.5), produced the best microcapsules for optical
transparency.
5.1.2 Optical Transparency was Not Obtained
Despite the small diameter size produced in the sixth trial of microencapsulation, the mock
windshield samples were not optically transparent. In all cases the samples were visibly opaque,
which was confirmed by the UV-Vis data. Following the healing period, the samples remained
non-transparent, which was also confirmed by the UV-Vis data.
5.1.3 Autonomic Healing Occurred
The flexural strength of the control samples made were greater than that of the mock
windshields assembled with both the microcapsules and Grubbs’ catalyst. After the controls were
damaged, there was a reduction in flexural strength. After the mock windshields with
microcapsules were allowed to heal, flexural strength testing showed there was an increase in
flexural strength when compared to the undamaged trials. This shows that not only did
polymerization occur, but mechanical properties were improved after the healing period. Although
42
Page 50
the optical properties were not consistent with that of windshields used in vehicles, the improved
mechanical property indicates that healing did take place upon cracking.
5.1.4 It is Feasible to Create a Self-Healing Laminated Glass
Although optical transparency like that of a vehicle windshield was not obtained, and
overall flexural strength did not exceed that of the control samples, improvement of mechanical
properties was observed. The polymerization of DCPD which was initiated by damage occurring
to the first layer of glass resulted in a higher flexural strength in most samples compared to
undamaged samples. With these results it is believed that the creation of a self-healing laminated
glass is feasible.
5.2 Recommendations
The following recommendations have been made based off of the results and experience
gained while working on this project. For future work on this project, it is recommended to
investigate the use of a different catalyst. The Grubbs’ catalyst has a red hue to it, and therefore
believed to affect the optical transparency of the PVB mixture.
Working with the Instron drop tower, every sample cracked at different forces. While some
would crack from a height of 11 mm away, others would shatter from a shorter distance such as 8
mm. This is believed to be caused by an ineffective loading method of the mixture. There was an
inability to prevent the mixture from seeping out the sides of the sample once sandwiched between
the two pieces of glass. Inconsistent thicknesses of mock windshields were then obtained.
Finally, the distribution of the microcapsules and catalyst in the interlayer mixture seemed
to play a large role in the resulting opacity of the samples. Further research into the necessary
43
Page 51
amount of both microcapsules and catalyst, as well as a method for even distribution of both could
result in a greater optical transparency of samples.
44
Page 52
6.0 References
1. Population, Total. http://data.worldbank.org/indicator/SP.POP.TOTL (accessed
September 2014).
2. Central Intelligence Agency, Telephones-Mobile Cellular.
https://www.cia.gov/library/publications/the-world-factbook/rankorder/2151rank.html
(accessed September).
3. Mackey, R., The Physics of Windshield Repair. Glass Magazine 2007.
4. Blaiszik, B. J.; Kramer, S. L. B.; Olugebefola, S. C.; Moore, J. S.; Sottos, N. R.; White, S.
R., Self Healing Polymers and Composites. Annual Review of Material Research
2010, 40, 179-211.
5. Scientific Principles. http://matse1.matse.illinois.edu/ceramics/prin.html (accessed October).
6. Bloch, B., The Coming Revolution in Airbag Technology. NHTSA, Ed. pp 1176-1190.
7. X Q Xu et al A numerical model on PVB laminated windshield subjected to headform low
speed impact,2013 J. Phys.: Conf. Ser. 451 012016
8. Chemwiki.ucdavis.edu,. 7.11: Polymers and Plastics – Chemwiki (accessed Apr 26, 2015).
9. Bishop, M. Addition Polymers
http://preparatorychemistry.com/Bishop_Addition_Polymers.htm
(accessed Apr 26, 2015).
10. Rugh, J.; Chaney, L.; Ramroth, L.; Venson, T.; Rose, M. Impact Of Solar Control PVB Glass
On Vehicle Interior Temperatures, Air-Conditioning Capacity, Fuel Consumption,
And Vehicle Range. SAE Technical Paper Series 2013.
45
Page 53
11. Leng, J.; Haibao, L.; Yanju, L.; Huag, W. M.; Du, S., Shape Memory Polymers- A Class of
Smart Material. MRS Bulliten 2009, 34.
12. Munger, C., Gates, K. M. and Hamburg, C. (2014), Determining the Refractive Index
Variation within Panes of Vehicle Windshield Glass. Journal of Forensic Sciences,
59: 1351–1357. doi: 10.1111/1556-4029.124552
13. Bureau, U. S. C., Vehicles Involved in Crashes by Vehicle Type, Rollover Occurance, and
Crash Severity since 2009. Washington, D.C., 2012.
14. Federation, W. W. Recycling Glass.
http://wwf.panda.org/about_our_earth/teacher_resources/project_ideas/recycling_glass/
(accessed October).
15. Trask, R.; Bond, I. Biomimetic Self-Healing Of Advanced Composite Structures Using
Hollow Glass Fibres. Smart Mater. Struct. 2006, 15, 704-710.
16. M. Hucker, I. Bond, A. Foreman, and J. Hudd, Optimisation of hollow glass fibres and their
composites, Advanced Composites Letters, vol. 8, no. 4, pp. 181–189, 1999.
17. Fleet, E., Self-Healing Polymers. Chemistry Review 2011, 20 (3), 2.
18. Brown, E. N.; Kessler, M. R.; Sottos, N. R.; White, S. R., In situ poly(urea formaldehyde)
Microencapsulation of Dicyclopentadiene. Journal of Microencapsulation 2003, 20
(6), 719-730.
19. Ghosh, S. K., Self-healing materials: fundamentals, design strategies, and applications. John
Wiley & Son 2009
20. Stanfield, C. F., UV-Vis spectroscopy. Biopharm 2001, The BioPharm Guide to
Bioanalytical Methods, 54-55.
46
Page 54
21. Kalista, S. J.; Ward, T. C., Thermal characteristics of the self-healing response in
poly(ethylene-co-methacrylic acid) copolymers. J R Soc Interface 2007, 4 (13), 405
411.
22. Sherman, L. M., IMPACT: Which Test To Use, Which Equipment to Buy. Plastics
Technology 2001.
23. Rule, J. D.; Brown, E. N.; Sottos, N. R.; White, S. R.; Moore, J. S., Wax-Protected Catalyst
Microspheres for Efficient Self Healing Materials. Advanced Materials 2005, 17 (2),
205-208.
24. Paraffin Wax; MSDS No. 76243 [Online]; Sigma Aldrich: St. Louis, MO. Mar 02, 2015.
http://www.sigmaaldrich.com/MSDS/MSDS/DisplayMSDSPage.do (accessed Apr 26,
2015).
25. S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N.
Brown & S. Viswanathan Nature 409, 794-797(15 February 2001)
26. ASTM D790-10, Standard Test Methods for Flexural Properties of Unreinforced and
Reinforced Plastics and Electrical Insulating Materials, ASTM International, West
Conshohocken, PA, 2010, www.astm.org
27. Matweb.com,. Overview of materials for Polydicyclopentadiene (PDCPD)
http://www.matweb.com/search/datasheettext.aspx?matid=78164 (accessed Apr 29,
2015).
47
Page 55
Appendix A: SEM Results Trial 1 Microcapsules
Appendix B: SEM Results Trial 2 Microcapsules
Appendix C: SEM Results Trial 3 Microcapsules
48
Page 56
Appendix D: SEM Results Trial 4 Microcapsules
Appendix E: SEM Results Trial 5 Microcapsules
Appendix F: SEM Results Trial 6 Microcapsules
49
Page 57
Appendix G: Diameter Distribution Trial 1 Microcapsules
Appendix H: Diameter Distribution Trial 2 Microcapsules
1
7
11 11
27
21
17
4
10
5
10
15
20
25
30
60-80 80-100 100-120 120-140 140-160 160-180 180-200 200-220 220-240
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
0
1412
5
18
1210 10
35
3
02468
101214161820
<90 90-100 100-110 110-120 120-130 130-140 140-150 150-160 160-170 170-180 >190
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
50
Page 58
Appendix I: Diameter Distribution Trial 3 Microcapsules
Appendix J: Diameter Distribution Trial 4 Microcapsules
2
14
11
21
18
1 2 1
0
5
10
15
20
25
<10 10.0-20 20-30 30-40 40-50 50-60 60-70 >70
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
6
16
14
19
14
1211
3
1
0
2
4
6
8
10
12
14
16
18
20
<100 100-120 120-140 140-160 160-180 180-200 200-220 220-240 >240
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
51
Page 59
Appendix K: Diameter Distribution Trial 5 Microcapsules
Appendix L: Diameter Distribution Trial 6 Microcapsules
6
1819
18
13
5 5
0
2
4
6
8
10
12
14
16
18
20
<100 100-120 120-140 140-160 160-180 180-200 200-220
Num
ber o
f Mic
roca
psul
es
Microcapsule Diameter μm
2
8
24
34
20
74
0
5
10
15
20
25
30
35
40
0:00 0:00 0:00 0:00 0:00 0:00 0:00
Num
ber o
f Mic
roca
psul
s
Diameter μm
52
Page 60
Appendix M Thermogravimetric Analysis Trial 1 (550 rpm pH=3.5)
53
Page 61
Appendix N Thermogravimetric Analysis Trial 2 (1000 rpm pH=3.5)
54
Page 62
Appendix O Thermogravimetric Analysis Trial 3 (1500 rpm pH=3.5)
55
Page 63
Appendix P Thermogravimetric Analysis Trial 4 (550 rpm pH=3.0)
56
Page 64
Appendix Q Thermogravimetric Analysis Trial 5 (550 rpm pH=4.0)
57
Page 65
Appendix R Thermogravimetric Analysis Trial 6 (1250 rpm pH=3.5)
58
Page 66
Appendix S Thermogravimetric Analysis Wax Protected Grubbs’ Catalyst
59
Page 67
Appendix T: UV-Vis Data for Mock Windshield Control Samples
Control Samples 1-6
60
Page 68
Appendix U: UV-Vis Data for Mock Windshield Samples, Trial 1 Microcapsules, Undamaged
Mock Windshield Samples 1A-1K
61
Page 69
Appendix V: UV-Vis Data for Mock Windshield Samples, Trial 1 Microcapsules, Healed
Mock Windshield Sample 1A Undamaged (Red) and Healed (Black)
62
Page 70
Mock Windshield Sample 1B Undamaged (Red) and Healed (Black)
63
Page 71
Mock Windshield Sample 1E Undamaged (Red) and Healed (Black)
64
Page 72
Mock Windshield Sample 1H Undamaged (Red) and Healed (Black)
65
Page 73
Mock Windshield Sample 1I Undamaged (Red) and Healed (Black)
66
Page 74
Mock Windshield Sample 1J Undamaged (Red) and Healed (Black)
67
Page 75
Appendix W: UV-Vis Data for Mock Windshield Samples, Trial 2 Microcapsules, Undamaged
Mock Windshield Sample 2A- 2H
68
Page 76
Appendix X: UV-Vis Data for Mock Windshield Samples, Trial 2 Microcapsules, Healed
Mock Windshield Sample 2A Undamaged (Red) and Healed (Black)
69
Page 77
Mock Windshield Sample 2C Undamaged (Red) and Healed (Black)
70
Page 78
Mock Windshield Sample 2G Undamaged (Red) and Healed (Black)
71
Page 79
Appendix Y: UV-Vis Data for Mock Windshield Samples, Trial 3 Microcapsules, Undamaged
Mock Windshield Samples 3A- 3K
72
Page 80
Appendix Z: UV-Vis Data for Mock Windshield Samples, Trial 3 Microcapsules, Healed
Mock Windshield Sample 3D Undamaged (Red) and Healed (Black)
73
Page 81
Mock Windshield Sample 3G Undamaged (Black) and Healed (Red)
74
Page 82
Mock Windshield Sample 3J Undamaged (Red) and Healed (Black)
75
Page 83
Appendix AA: UV-Vis Data for Mock Windshield Samples, Trial 4 Microcapsules, Undamaged
Mock Windshield Samples 4A- 4H
76
Page 84
Appendix BB: UV-Vis Data for Mock Windshield Samples, Trial 4 Microcapsules, Healed
Mock Windshield Sample 4B Undamaged (Red) and Healed (Black)
77
Page 85
Mock Windshield Sample 4C Undamaged (Red) and Healed (Black)
78
Page 86
Mock Windshield Sample 4F Undamaged (Red) and Healed (Black)
79
Page 87
Appendix CC: UV-Vis Data for Mock Windshield Samples, Trial 5 Microcapsules, Undamaged
Mock Windshield Samples 5A- 5H
80
Page 88
Appendix DD: UV-Vis Data for Mock Windshield Samples, Trial 5 Microcapsules, Healed
Mock Windshield Sample 5A Undamaged (Red) and Healed (Black)
81
Page 89
Mock Windshield Sample 5B Undamaged (Red) and Healed (Black)
82
Page 90
Mock Windshield Sample 5E Undamaged (Red) and Healed (Black)
83
Page 91
Mock Windshield Sample 5F Undamaged (Red) and Healed (Black)
84
Page 92
Appendix EE: UV-Vis Data for Mock Windshield Samples, Trial 6 Microcapsules, Undamaged
Mock Windshield Samples 6A- 6H
85
Page 93
Appendix FF: UV-Vis Data for Mock Windshield Samples, Trial 6 Microcapsules, Healed
Mock Windshield Sample 6A Undamaged (Red) and Healed (Black)
86
Page 94
Mock Windshield Sample 6D Undamaged (Red) and Healed (Black)
87
Page 95
Mock Windshield Sample 6F Undamaged (Red) and Healed (Black)
88
Page 96
Mock Windshield Sample 6G Undamaged (Red) and Healed (Black)
89
Page 97
Appendix GG: Drop Tower Impact Testing Data Mock Windshields Controls
Control Sample 1, Shattered
Control Sample 2, Shattered
-200
-100
0
100
200
300
400
500
600
700
800
900
134.
412
6.4
127.
713
3.0
129.
012
7.7
127.
712
9.0
134.
413
5.7
131.
758
3.0
156.
913
3.0
98.5
123.
713
9.7
90.6
119.
813
4.4
123.
712
5.1
126.
412
5.1
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
700
800
900
0.0
1.3
2.5
3.8
5.0
6.3
7.5
8.8
10.0
11.3
12.5
13.8
15.0
16.3
17.5
18.8
20.0
21.3
22.5
23.8
25.0
26.3
27.5
28.8
Forc
e (lb
f)
Time (ms)
90
Page 98
Control Sample 9, Cracked
0
100
200
300
400
500
600
700
800
900
0.0
2.1
4.2
6.3
8.3
10.4
12.5
14.6
16.7
18.8
20.9
22.9
25.0
27.1
29.2
31.3
33.4
35.4
37.5
39.6
41.7
43.8
45.9
48.0
Forc
e (lb
f)
Time (ms)
91
Page 99
Appendix HH: Drop Tower Impact Testing Data Mock Windshields Trial 1 Microcapsules
Mock Windshield Sample 1E
Mock Windshield Sample 1J
0
100
200
300
400
500
600
700
800
0.0
0.4
0.8
1.3
1.7
2.1
2.5
2.9
3.4
3.8
4.2
4.6
5.0
5.5
5.9
6.3
6.7
7.1
7.6
8.0
8.4
8.8
9.2
9.7
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
0.0
1.3
2.5
3.8
5.0
6.3
7.5
8.8
10.0
11.3
12.5
13.8
15.0
16.3
17.5
18.8
20.0
21.3
22.5
23.8
25.0
26.3
27.5
28.8
Forc
e (lb
f)
Time (ms)
92
Page 100
Appendix II: Drop Tower Impact Testing Data Mock Windshields Trial 2 Microcapsules
Mock Windshield Sample 2A
Mock Windshield Sample 2G
0
100
200
300
400
500
600
700
800
900
20.0
20.5
21.0
21.5
22.1
22.6
23.1
23.6
24.2
24.7
25.2
25.7
26.3
26.8
27.3
27.8
28.4
28.9
29.4
29.9
30.5
31.0
31.5
32.0
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
700
0.0
1.3
2.5
3.8
5.0
6.3
7.5
8.8
10.0
11.3
12.5
13.8
15.0
16.3
17.5
18.8
20.0
21.3
22.5
23.8
25.0
26.3
27.5
28.8
Forc
e (lb
f)
Time (ms)
93
Page 101
Appendix JJ: Drop Tower Impact Testing Data Mock Windshields Trial 3 Microcapsules
Mock Windshield Sample 3D
Mock Windshield Sample 3G
0
100
200
300
400
500
600
700
800
30.0
30.4
30.8
31.2
31.6
32.1
32.5
32.9
33.3
33.7
34.2
34.6
35.0
35.4
35.8
36.3
36.7
37.1
37.5
37.9
38.4
38.8
39.2
39.6
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
700
0.0
0.2
0.4
0.6
0.8
1.1
1.3
1.5
1.7
1.9
2.1
2.3
2.5
2.7
2.9
3.2
3.4
3.6
3.8
4.0
4.2
4.4
4.6
4.8
Forc
e (lb
f)
Time (ms)
94
Page 102
Appendix KK: Drop Tower Impact Testing Data Mock Windshields Trial 4 Microcapsules
Mock Windshield Sample 4B
Mock Windshield Sample 4C
0100200300400500600700800900
1000
15.0
15.7
16.5
17.3
18.1
18.9
19.7
20.5
21.2
22.0
22.8
23.6
24.4
25.2
26.0
26.7
27.5
28.3
29.1
29.9
30.7
31.4
32.2
Forc
e (lb
f)
Time (ms)
0100200300400500600700800900
1000
30.0
30.2
30.5
30.8
31.1
31.4
31.7
32.0
32.2
32.5
32.8
33.1
33.4
33.7
34.0
34.2
34.5
34.8
35.1
35.4
35.7
35.9
36.2
Forc
e (lb
f)
Time (ms)
95
Page 103
Appendix LL: Drop Tower Impact Testing Data Mock Windshields Trial 5 Microcapsules
Mock Windshield Sample 5E
Mock Windshield Sample 5F
0
200
400
600
800
1000
1200
20.0
20.8
21.7
22.6
23.4
24.3
25.2
26.1
26.9
27.8
28.7
29.5
30.4
31.3
32.1
33.0
33.9
34.8
35.6
36.5
37.4
38.2
39.1
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
700
5.0
5.5
6.1
6.7
7.2
7.8
8.4
8.9
9.5
10.0
10.6
11.2
11.7
12.3
12.9
13.4
14.0
14.6
15.1
15.7
16.3
16.8
17.4
18.0
Forc
e (lb
f)
Time (ms)
96
Page 104
Appendix MM: Drop Tower Impact Testing Data Mock Windshields Trial 6 Microcapsules
Mock Windshield Sample 6D
Mock Windshield Sample 6F
0100200300400500600700800900
1000
25.0
25.3
25.6
26.0
26.3
26.6
26.9
27.3
27.6
27.9
28.3
28.6
28.9
29.3
29.6
29.9
30.2
30.6
30.9
31.2
31.6
31.9
32.2
Forc
e (lb
f)
Time (ms)
0
100
200
300
400
500
600
700
800
900
10.0
10.2
10.4
10.7
10.9
11.1
11.3
11.6
11.8
12.0
12.3
12.5
12.7
13.0
13.2
13.4
13.6
13.9
14.1
14.3
14.6
14.8
15.0
15.3
Forc
e (lb
f)
Time (ms)
97
Page 105
Appendix NN Stress Strain Mock Windshield Controls
Control Sample 9 Cracked
Control Sample 10 Cracked
98
Page 106
Control Sample 15, Cracked
Control Sample 18 Undamaged
99
Page 107
Control Sample 19 Undamaged
Control Sample 21 Undamaged
100
Page 108
Appendix OO Stress Strain Curve Mock Windshields Trial 1Healed
Mock Windshield Sample 1A
Mock Windshield Sample 1B
101
Page 109
Mock Windshield Sample 1C
Mock Windshield Sample 1H
102
Page 110
Mock Windshield Sample 1I
Mock Windshield Sample 1J
103
Page 111
Appendix PP: Stress Strain Curve Mock Windshields Trial 1 Undamaged
Mock Windshield Sample 1K
104
Page 112
Appendix QQ: Stress Strain Curve Mock Windshields Trial 2 Healed
Mock Windshield Sample 2A
Mock Windshield Sample 2C
105
Page 113
Mock Windshield Sample 2G
106
Page 114
Appendix RR: Stress Strain Curve Mock Windshield Trial 2 Undamaged
Mock Windshield Sample 2H
107
Page 115
Appendix SS: Stress Strain Curve Mock Windshields Trial 3 Healed
Mock Windshield Sample 3D
Mock Windshield Sample 3G
108
Page 116
Mock Windshield Sample 3J
109
Page 117
Appendix TT: Stress Strain Curve Mock Windshield Trial 3 Undamaged
Mock Windshield Sample 3K
110
Page 118
Appendix UU: Stress Strain Curve Mock Windshields Trial 4 Healed
Mock Windshield Sample 4B
Mock Windshield Sample 4C
111
Page 119
Mock Windshield Sample 4F
112
Page 120
Appendix VV: Stress Strain Curve Mock Windshield Trial 4 Undamaged
Mock Windshield Sample 4H
113
Page 121
Appendix WW: Stress Strain Curve Mock Windshields Trial 5 Healed
Mock Windshield Sample 5A
Mock Windshield Sample 5B
114
Page 122
Mock Windshield Sample 5E
Mock Windshield Sample 5F
115
Page 123
Appendix XX: Stress Strain Curve Mock Windshield Trial 5 Undamaged
Mock Windshield Sample 5H
116
Page 124
Appendix YY: Stress Strain Curve Mock Windshields Trial 6 Healed
Mock Windshield Sample 6A
Mock Windshield Sample 6D
117
Page 125
Mock Windshield Sample 6F
Mock Windshield Sample 6G
118
Page 126
Appendix ZZ: Stress Strain Curve Mock Windshield Trial 6 Undamaged
Mock Windshield Sample 6H
119
