self healing polymer technology

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ACKNOWLEDGEMENT

To put an effort like this requires the determination and help of many people around me

and I would not be doing justice to their efforts by not mentioning each helping hand in

person.

I express my heartful gratitude to Prof. H G. Phakatkar, Head of Department and other

staff members of the Mechanical Engineering Department for their kind co-operation.

I feel privileged to acknowledge with deep sense of gratitude to my guide PROF. M.V.

Walame for his valuable suggestion and guidance throughout my course of studies and

help render to me for the completion of the report.

I would like to give sincere thanks to the Central Library Cell and Reference Library Cell

and Information Access Centre for their kind co-operation throughout my work.

Last but not the least I would like to thank my parents and my friends. It would have not

been possible to complete the report without their moral support, valuable comments and

suggestions which motivated me towards work.

Shinde Atul K.

TE-V_01



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List of Figures

Figure 1 Multiple versus one-time self-healing. (a) Capsule-

based, (b) vascular, and (c) intrinsic self-healing

principles.

8

Figure 2 Schematic diagram of repair concept for polymer

matrix composites using pre-embedded hollow

tubes

11

Figure 3 Microencapsulated Healing Agent and Ruptured

Microcapsule

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Figure 4 Schematic diagram of repair concept using 3D

network

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Figure 5 Self-Healing Coating 12

Figure 6 Schematic drawing of the principle of self-healing

epoxy based microcapsules

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Table of Contents

Sr. No. Name of Topic Page No.

Abstract 4

1 Introduction 5

2 Fracture Mechanics 6

3 Classification of Self-Healing Processes and

Methodology

8

4 Types of self-healing materials and the healing

mechanisms

14

5 Research in Self-Healing Materials 17

6 Applications 18

7 Benefits 20

8 Challenges 21

Conclusion 22

References 23



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SELF-HEALING POLYMER TECHNOLOGY

ABSTRACT

Initiation of cracks and other types of damage on microscopic level is a critical problem

in polymers during their service in structural applications and has been shown to change

thermal, electrical, and acoustical properties, and eventually leading to whole scale

failure of the material. Therefore, early sensing, diagnosis and repair of microcracks

become necessary for removing the latent perils. In this context, the materials possessing

self-healing function are ideal for long-term operation. Self-healing polymers are based

on the concept of human body's natural response to damage and its ability to recover with

minimal external help.

The advances in this field show that selection and optimization of proper repair

mechanisms are prerequisites for high healing efficiency. It is a challenging job to either

invent new polymers with inherent crack repair capability (intrinsic self-healing) or

integrate existing materials with novel healing system (extrinsic self-healing).

Comparatively, extrinsic self-healing techniques might be easier for large-scale usage for

the moment. The works and outcomes in this aspect have broadened the application

possibility of polymeric materials. Also, the extended service life of components made

from these intelligent materials would contribute to reduce waste disposal. It is

undoubtedly important for building up a sustainable society.



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INTRODUCTION

Polymers and polymer composites have been widely used in tremendous engineering

fields because of their advantages including light weight, good processibility, chemical

stability in any atmospheric conditions, etc. However, long-term durability and reliability

of polymeric materials are still problematic when they serve for structural application.

Exposure to harsh environment would easily lead to degradations of polymeric

components. Comparatively, micro cracking is one of the fatal deteriorations generated in

service, which would bring about catastrophic failure of the materials and hence

significantly shorten lifetimes of the structures. Since the damages deep inside materials

are difficult to be perceived and to repair in particular, the materials had better to have the

ability of self-healing.

In fact, many naturally occurring portions in animals and plants are provided with such

function. For healing of a broken bone, similar processes are conducted, including

internal bleeding forming a fibrin clot, development of unorganized fiber mesh,

calcification of fibrous cartilage, conversion of calcification into fibrous bone and

lamellar bone. Clearly, the natural healing in living bodies depends on rapid

transportation of repair substance to the injured part and reconstruction of the tissues.

Having been inspired by these findings, continuous efforts are now being made to mimic

natural materials and to integrate self-healing capability into polymers and polymer

composites. The progress has opened an era of new intelligent materials. On the whole,

researches in this field are still in the infancy. Innovative measures and new knowledge

of the related mechanisms are constantly emerging.



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FRACTURE MECHANICS

The failure of engineering materials is almost always an undesirable event for several

reasons; these include human lives that are put in jeopardy, economic losses, and

interference with the availability of products and services. Even though the causes of

failure and the behavior of materials may be known, prevention of failures is difficult to

guarantee. The usual causes are improper materials selection and processing and

inadequate design of the component or its misuse. Also, damage can occur to structural

parts during service, and regular inspection and repair or replacement are critical to safe

design. It is the responsibility of the engineer to anticipate and plan for possible failure

and, in the event that failure does occur, to assess its cause and then take appropriate

preventive measures against future incidents.

Simple fracture is the separation of a body into two or more pieces in response to an

imposed stress that is static (i.e., constant or slowly changing with time) and at

temperatures that are low relative to the melting temperature of the material. Fracture can

also occur from fatigue (when cyclic stresses are imposed) and creep (time dependent

deformation, normally at elevated temperatures).

For uniaxial tensile loads acting on metals two fracture modes are possible:

(i) Ductile and (ii) Brittle

Any fracture process involves two steps - crack formation and propagation - in response

to an imposed stress. The mode of fracture is highly dependent on the mechanism of

crack propagation. Ductile fracture is characterized by extensive plastic deformation in

the vicinity of an advancing crack. Furthermore, the process proceeds relatively slowly as

the crack length is extended. Such a crack is often said to be stable. That is, it resists any

further extension unless there is an increase in the applied stress. In addition, there will

ordinarily be evidence of appreciable gross deformation at the fracture surfaces (e.g.,

twisting and tearing). On the other hand, for brittle fracture, cracks may spread extremely

rapidly, with very little accompanying plastic deformation. Such cracks may be said to be



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unstable, and crack propagation, once started, will continue spontaneously without an

increase in magnitude of the applied stress.

Ductile fracture is almost always preferred to brittle for two reasons. First, brittle fracture

occurs suddenly and catastrophically without any warning; this is a consequence of the

spontaneous and rapid crack propagation. On the other hand, for ductile fracture, the

presence of plastic deformation gives warning that failure is imminent, allowing

preventive measures to be taken. Second, more strain energy is required to induce ductile

fracture inasmuch as these materials are generally tougher. Under the action of an applied

tensile stress, many metal alloys are ductile, whereas ceramics are typically brittle, and

polymers may exhibit a range of behaviors.

Brittle fracture in crystalline metals can be classified into two broad groups, intergranular

and transgranular. The crack of intergranular failure moves along grain boundaries.

Transgranular fracture occurs through fracture within grains. Within a grain, cleavage

failure occurs along a weak crystallographic plane. In fact cleavage fracture is the most

brittle form of fracture and it hardly damages the fractured surfaces. Once the cleavage

crack reaches the grain boundary, it finds another favorable orientation in the next grain.

Ductile fracture growth occurs due to substantial plastic deformation and creation of

microvoids. The material deforms plastically due to the micromechanisms such as

nucleation and motion of dislocations, formation of twins, etc.



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CLASSIFICATION OF SELF-HEALING PROCESSES AND

METHODOLOGY

Classification based on way of healing

(i) Intrinsic ones that are able to heal cracks by the polymers themselves

(ii) Extrinsic in which healing agent has to be pre-embedded.

1. Intrinsic self-healing

The so-called intrinsic self-healing polymers and polymer composites are based on

specific performance of the polymers and polymeric matrices that enables crack healing

under certain stimulation (mostly heating). Autonomic healing without external

Intervention is not available in these materials for the time being. As viewed from the

predominant molecular mechanisms involved in the healing processes, the reported

achievements consist of two modes:

(i) Physical Interactions

(ii) Chemical Interactions

Figure 1. Multiple versus one-time self-healing. (a) Capsule-based, (b) vascular, and (c)

intrinsic self-healing principles.



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1.1. Self-healing based on Physical Interactions

Heating induced healing of polymers depends on inter diffusion of chains and formation

of entanglements. Crack healing happens only at or above the glass transition

temperature. In order to reduce the effective glass transition temperature polymer is

treated external agent for e.g. PMMA is treated with methanol and ethanol reducing the

glass transition temperature to a range of 40~60°C, and found that there were two

distinctive stages for crack healing: the first one corresponding to the progressive healing

due to wetting, while the second related to diffusion enhancement of the quality of

healing behavior.

Healing of epoxy, for instance, has to proceed above the glass transition temperature.

Then, the molecules at the cracking surfaces would interdiffuse and the residualfunctional groups react with each other. A 50% recovery of impact strength can thus be

obtained.

1.2. Self-healing based on Chemical Interactions

Cracks and strength decay might be caused by structural changes of atoms or molecules,

like chain scission. Therefore, inverse reaction, i.e. recombination of the broken

molecules, should be one of the repairing strategies. Such method does not focus on

cracks healing but on „nanoscopic‟ deterioration. Examples are polycarbonate (PC)

synthesized by ester exchange method and poly-phenylene ether (PPE) in which the

repairing agent was regenerated by oxygen. The above example shows that PPE might be

probably designed as a self-repairing material by means of the reversible reaction. The

deterioration is expected to be minimized if the recovery rate is the same as the

deterioration rate.

Another method is using thermally reversible crosslinking behavior has been known for

quite a while. Wudl et al . combined this with the concept of „self -healing‟ in making

healable polymers. They synthesized highly cross-linked polymeric materials with

multifuran and multi-maleimide via Diels-Alder (DA) reaction. At temperatures above

120°C, the „intermonomer‟ linkages disconnect but then reconnect upon cooling (i.e. DA

reaction). This process is fully reversible and can be used to restore fractured parts of the



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polymers. In principle, an infinite number of crack healing is available without the aid of

additional catalysts, monomers and special surface treatment.

2. Extrinsic self-healing

In the case of extrinsic self-healing, the matrix resin itself is not a healable one. Healing

agent has to be encapsulated and embedded into the materials in advance. As soon as the

cracks destroy the fragile capsules, the healing agent would be released into the crack

planes due to capillary effect and heals the cracks. Taking the advantages of crack

triggered delivery of healing agent, manual intervention (e.g. heating that used to be

applied for intrinsic self-healing) might be no longer necessary. In accordance with types

of the containers, there are two modes of the repair activity:

(i) Self-healing in terms of healant loaded pipelines

(ii) Self-healing in terms of healant loaded microcapsules

2.1. Self-healing in terms of healant loaded pipelines

2.1.1. Hollow glass tubes and glass fibers

The core issue of this technique lies in filling the brittle-walled vessels with

polymerizable medium, which should be fluid at least at the healing temperature.

Subsequent polymerization of the chemicals flowing to the damage area plays the role of

crack elimination. Property matching is important for hollow glass fibers/matrix polymer

pairs, which decides breakage of the hollow fibers and release of healing agent. Zhao et

al . showed that for the epoxy/polyamide compounds with healing agent loaded hollow

plastic fiber, the plastic tubes did not fracture even when the matrix was completely

broken. No healing effect could be observed as a result. One of the possible solutions of

this problem lies in covering the hollow repair fiber with a thin polymeric layer.

Flowability of the released healing agent inside materials to be healed is another problem

that might be encountered in practice.



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Figure 2. Schematic diagram of repair concept for polymer matrix composites using pre-

embedded hollow tubes

2.1.2. Three-dimensional microvascular networks

Figure 3. Microencapsulated Healing Agent and Ruptured Microcapsule

In conventional extrinsic self-healing composites it is hard to perform repeated healing,

because rupture of the embedded healant-loaded containers would lead to depletion of the

healing agent after the first damage. To overcome this difficulty, Toohey et al . proposed

a self-healing system consisting of a three-dimensional microvascular network capable of

autonomously repairing repeated damage events. Their work mimicked architecture of

human skin. When a cut in the skin triggers blood flow from the capillary network in the

dermal layer to the wound site, a clot would rapidly form, which serves as a matrix



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through which cells and growth factors migrate as healing ensues. Owing to the vascular

nature of this supply system, minor damage to the same area can be healed repeatedly.

Figure 4. Schematic diagram of repair concept using 3D network

2.2. Self-healing in terms of healant loaded microcapsules

Figure 5. Self-Healing Coating

The principle of this approach resembles the aforesaid pipelines but the containers forstoring healing agent are replaced by fragile microcapsules. As soon as cracks destroys

the capsules, the healing agent would be released into the crack planes due to capillary

effect and cure crack under initiation of the latent hardener.



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Figure 6. Schematic drawing of the principle of self-healing epoxy based

microcapsules



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TYPES OF SELF-HEALING MATERIALS AND THE HEALING

MECHANISMS:

Although all types of these materials have their own self-healing mechanism, we start

from describing some common features. Virtually all materials with long degradation

time deteriorate through development of microcracks (fatigue). A sharp apex of each

crack works as a knife cutting the materials with ease. This results in larger cracks, and

consequently, mechanical degradation. Example of such material would be plastics used

for construction, artificial bones, dental cement, etc. To heal such materials, one needs to

seal those microcracks before their further growing. The other type of degradation and

the healing mechanism is important for materials that can degrade sufficiently fast.

Example of such materials can be various coatings, armor, all surfaces that can suffer

sudden impact or collision with a projectile. In such a case, not only cracks, but even

holes should be sealed and healed. Definitely there are materials of dual purposes, which

would degrade through both of the above mechanisms.

To classify self-healing materials, one can consider four different classes:

plastics/polymers, paints/coatings, metals, and ceramics/concrete. We will discuss each

of these classes below.

1. Plastics/polymers

Polymers/plastics are attractive from mechanical and chemical points of view. Many

plastic materials are strong and resistant to breaking. However, once fractured, the

material deteriorates irreversibly. Even under normal wearing, plastics used to developsmall cracks that also grow irreversibly. This leads to degradation of their mechanical

properties and decreasing life time of such materials. This is where self-healing is needed

the most. The working principle of self-healing mechanism is based on having small

capsules filled with healing glue. These capsules are mixed within the polymer body. The



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glue activator (needed to rigidify the glue inside the cracks) is also added to the polymer

body.

When microcracks are developed in the polymer body, these also rapture the capsules.

The glue leaks in the cracks and heals them before cracks can get any bigger.

Another approach is based on using hollow fibers instead of microcapsules.

Hollow fibers based on glass tubes are filled with either resin or hardener, which are

released into the damaged area when the fibers are fractured. When the resin and the

hardener are mixed in the crack plane, the resin hardens, repairing the crack.

It is worth noting that thermoplastic materials demonstrate interesting natural healing

property. Being heated, they can recover their mechanical integrity and properties. This

can be used to fix some impact damage even autonomically. For example, after collision

with such a plastic, there can be a dent/hole/scratch. However, as a part of the collision

energy transfers into heat. So the area of the damage can be melted and heal itself. By

manipulating thermally reversible Diels-Alder reactions, a transparent polymer material

with self-repairing functionality at ~120°C is developed.

2.Paint

Apart from cosmetic reason, paint is typically serves to protect surfaces. Self-healing

protection coating for cars from Nissan is one of such examples. In principle, the

mechanism of healing here can be similar to the described previously. However, main

cause of wearing of paint coating is due to scratches, abrasion, and mechanical damage

(collisions). It implies a specific restriction to a possible healing mechanism. Specifically,

recover of mechanical recovery is not as important as recovery of protective property.

This means, for example, that the healing agent can seal or inhibit corrosion of the

surface underneath the crack rather than seal the crack itself. To fix scratches

cosmetically, and up to some extend protect coated surface, a rather viscous polymer can

be used instead of glue.



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3. Metals

Metals being superior materials in many respects, suffer from cracks, dents and

corrosion. Presently, the issue of corrosion is addressed by various coating. Self-healing

of metals is not as developed as that for plastics. Electroconductivity of metals can be

used in self-healing of both metals and ceramics. New methods involving electric-field

induced colloidal aggregation are being explored. When a defect occurs in the insulating

coating, metal is exposed and creates high current density at the damaged site. This leads

to fluid flow though the crack, causing colloidal particles to coagulate around the defect,

and consequently, seal it.

4. Ceramics/concrete

There are different directions in autonomic healing of structural materials. The

first one is the “classical” use of healing capsules. The second one is inhibiting

corrosion of inner reinforcement frame (like the frame in concrete). Studies have

demonstrated these materials to have the potential for increasing the life of reinforced

concrete structures.

The other interesting approach suggest to use chalk as a part of concrete materials that

have direct contact with water. If a crack appears the water the material is standing in

gets inside. While for modern concrete that leads to irreversible deterioration, in the chalk

concrete, the water dissolves the chalk in the mortar. That suspension of chalk penetrates

into the cracks and settles there calcifying, sealing the crack. This approach is rather

promising because chalk is relatively cheap.



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RESEARCH IN SELF-HEALING MATERIALS

Efforts to create autonomic, or self-healing, materials have become a fast-growing line of

research, in large part due to advancements made by Beckman Institute researchers.

In 2001, Beckman faculty members Nancy Sottos, Jeff Moore, and ScottWhite published

a paper in Nature magazine detailing their breakthrough work that demonstrated for the

first time self-healing in an engineered materials system. The paper drew worldwide

attention in newspapers, journals, and websites and earned a front page story in the

Washington Post.

Inspired by biological processes in which damage triggers an autonomic healing

response, their work has used encapsulated microcapsules and microvascular networks as

methods for generating self-healing in a polymer material and, in a recent research line,

in electrical energy storage systems, including batteries. Since the Nature paper first

appeared in 2001, numerous advances have been made by Beckman researchers,

including developing methods that are more practical and cost-effective than the original

approach, developing systems that are able to repair multiple cracks, and the introduction

of mechanochemical approaches to self-healing. Potential applications could includematerials that self-repair damage on coatings such as those applied to airplane fuselages

or bridges, and batteries for electrical vehicles.

Nancy Sottos, JeffMoore, and ScottWhite have developed microvascular composites that

improved the microcapsule concept (in which the healing agent was consumed) through

an interconnected delivery network of microchannels that provide for multiple self-

healing reactions.



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APPLICATIONS

The uses for these self-healing polymer composites are virtually endless. This technology

can be used in nearly any plastic or composite part that is subject to microcracking.

Below are just a few examples.

I. Transportation

Cracks in the structure or components of automobiles, airplanes, and spacecraft

shorten vehicle life and can compromise passenger safety. This self-healing

technology would repair these cracks before they grow to dangerous levels.

II.

Sporting Goods

Many consumers are willing to pay top dollar for high-quality fishing equipment,

tennis rackets, helmets and other protective gear, boats and surfboards, skis, and

other sports equipment. This self-healing technology would improve the quality

of these products.

III. Military

Having armor, body protection that could heal itself even during the battle will be

beneficial for the Army. Air force and Navy can additionally benefit from fast self

disappearing holes in the skin of a jet or ship. A prototype of such material

alr eady exists. Dupont‟s Surlyn® show good properties to heal after ballistic

damage.

IV. Medicine

Once implanted in the body, prosthetics and other medical devices are difficult to

monitor and access for repair. This self-healing technology could prevent

problems caused by damaged pacemakers, hip and knee replacements, dental

materials, and other medical devices.



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V. Electronics

Polymer composite circuit boards and electronic components can suffer frommechanical and electrical failures if microcracks progress unabated. This self-

healing technology would help to prevent such failures.

VI. Civil construction

Calcium for self-healing concrete is cheap. Self-healing coatings on structural

steel components in, for example, bridges can be very popular. Again, here the

healing mechanism is not in recovery mechanics of the coating but rather in protection against rust. This helps sustaining mechanical integrity of the coated

steel constructions.

VII. Paints, Coatings, and Adhesives

Used in a wide variety of products, paints, coatings, and adhesives are subject to

scratches, cracks, and deterioration. This self-healing technology would repair this

damage, maintaining protection from environmental conditions and/or a longerlasting seal.



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BENEFITS

Self-healing

Polymeric and composite materials are subject to weakening due to fatigue cracking. A

self-healing composite has the potential to defend against material failure due to fatigue

and to greatly improve product safety and reliability and to extend product lifetimes.

Improved toughness

Adding the microcapsules to the resin and later initiating the self-healing process

increases the toughness of the resin over what it would have been without the

microcapsules. Improving the toughness of a previously brittle material makes it more

durable and less likely to suffer brittle fracture.

Reduced waste disposal

Also, the extended service life of components made from these intelligent materials

would contribute to reduce waste disposal

Sustainable society

It is undoubtedly important for building up a sustainable society.



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CHALLENGES

Apart from problems and challenges related to high-cost, there are many technological

problems. It would be far beyond the scope of the present overview to discuss these

problems in detail. We will outline just main issues that are common. Virtually any self-

healing mechanism has the following steps. The healing agent has to be delivered to the

damaged region, after that the healing should be initiated, and finally, the result of

healing should be compatible with the surrounding materials. Therefore, technical

challenges can be ordered as follows:

1. Storage of healing agent inside the material for a long period of time. This is especially

difficult inside of polymeric materials, which intrinsically permeable on molecular level.

2. Initiation of healing. The healing agent should start react either with the surrounding

material or with a special initiator. Such an initiator can be impregnated in the

surrounding material or should be mixed with the healing agent. All these create

additional problems of storage of the initiator, and mixing the initiator and the healing

agent.

3. Finally, the healing agent should be strongly bound to the material, and be stable with

respect to the surrounding environment. This indeed is typically the simplest problem,

which is however, restrictive to the type of the healing agent. The main challenge of

course is to find the solution of the above problems in the way that can be scaled up to

the mass production.



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CONCLUSION

Achievements in the field of self-healing polymers and polymer composites are far fromsatisfactory, but the new opportunities that were found during research and development

have demonstrated it is a challenging job to either invent new polymers with inherent

crack repair capability or integrate existing materials with novel healing system. But this

provide aspect for future development and application possibility of polymeric materials.

Also, the extended service life of components made from these intelligent materials

would contribute to reduce waste disposal. It is undoubtedly important for building up a

sustainable society.

Comparatively extrinsic self-healing techniques might be easier for large-scale usage for

the moment but from a long-term point of view, synthesis of brand new polymers

accompanied by intrinsic self-healing function through molecular design and automatic

trigger would be a reasonable solution.



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References

Books

[1] David Broek , “Elementary Engineering Fracture Mechanics”, Martinus Nijhoff

Publishers, Bosten, 1984, pp. 3-62

[2] Prashant Kumar, “ Elements of Fracture Mechanics ”, Wheeler Publishing, 1999,

pp. 1-9

[3] Victor E. Saouma, “Lecture Notes in: Fracture Mechanics”, University of

Colorado, Boulder, 2000, pp. II1-II8

[4] William D. Callister, Jr., “Materials Science And Engineering” 8E, John Wiley &

Sons, Inc., pp. 235-271

Research Papers

[1] B. Aissa, D. Therriault, E. Haddad and W. Jamroz, “Self -HealingMaterials

Systems: Overview of Major Approaches and Recent Developed Technologies ”,

November 2011

[2]

Jay A. Syrett, C. Remzi Becer and David M. Haddleton , “Self -healing and self-

mendable polymers”, The Royal Society of Chemistry, 2010

[3] Klaas van Breugel, “Self -Healing Material Concepts As Solution For Aging

Infrastructure”, 37th Conference on Our World in Concrete & Structures 29-31

August 2012, Singapore

[4] K. Gordon, R. Penner, P. Bogert, W.T. Yost and E. Siochi, “Puncture Self-healing

Polymers for Aerospace Applications”, NASA Langley Research Center

Hampton,2012

[5] R S Trask, H R Williams and I P Bond, “ Self-healing polymer composites:

mimicking nature to enhance performance”, 2014

[6] Y. C. Yuan, T. Yin, M. Z. Rong, M. Q. Zhang, “Self healing in polymers and

polymer composites. Concepts, realization and outlook: A review”,Express

Polymer Letters, 2010

self healing polymer technology

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