Self healing coatings mohammed alshammasi

Mohammed AlShammasi

Texas A&M University at Qatar

Spring 2012

29 April 2012

Self-Healing Coatings A Case Study: Poly(DCPD)

Self-Healing Coatings

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Table of Contents

Abstract ............................................................................................................................................................... 2

1. Introduction ..................................................................................................................................................... 2

1.1. Background ............................................................................................................................................... 2

1.2. Structural Schemes ................................................................................................................................... 5

3. Synthesis .......................................................................................................................................................... 6

3. Characterization .............................................................................................................................................. 8

3.1. Chemical Properties ................................................................................................................................. 8

3.2. Mechanical Properties .............................................................................................................................. 9

4. Applications ................................................................................................................................................... 10

5. Conclusion ..................................................................................................................................................... 11

References ......................................................................................................................................................... 12


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Abstract

Self-healing coating systems have the unique trait of self-repairing any structural damage

caused by microcracking, crazing, or other types of mechanical failure. Characterized by their

responsiveness, external stimuli, and programmed structures, self-healing coatings can be utilized in

many critical applications to extend a structure’s lifetime and enhance product safety.

This paper presents a brief discussion of the types and characteristics of self-healing coating

systems. Three main structural schemes used in these systems are explained, followed by a review of

the synthesis, characterization, and applications of poly(dicyclopentadiene)/Grubbs catalyst system

as a case study. The ring-opening metathesis polymerization of dicyclopentadiene in this system is

explored, then, methods for chemical and mechanical characterization of the resulting polymer are

identified. Finally, advantages of utilizing such a system as well as several potential applications are

highlighted.

Keywords: coating systems, dicyclopentadiene, microencapsulation, self-healing, smart materials,

responsive polymers.

1. Introduction

1.1. Background

The development of self-healing polymeric coatings has attained great attention over the last

two decades with the application of nanotechnology as the key of progress in the field.1 Self-healing

coatings, a special category of smart materials, are characterized by their ability to repair themselves

upon exposure to structural damage. Other categories of smart coating systems include antimicrobial,

antifouling, conductive, and stimuli responsive. This last category includes a wide variety of

multifunctional coating systems which have the ability to respond to different external stimuli such

as corrosion, pressure and temperature.1


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Polymers form the largest part of smart materials due to the presence of functional groups in

their structure and the nature of polymerization reactions. Functional groups in polymer chains allow

certain types of reactions to occur under certain conditions, and thus, can create a sensing mechanism

for a specific stimulus. For example, the insolubility of PNIPAM in an aqueous solution above its

low critical temperature is attributed to the presence of the hydrophobic isopropyl groups and the

hydrophilic amide groups in its structure.2 On the other hand, the reversibility and controllability of

polymerization reactions provide means to produce instantaneous responses for different stimuli.

Protective coatings of poly(DCPD) in bullet-proof glass, for instance, would undergo a

polymerization reaction that is initiated by Grubbs’ catalyst which would prevent the breakage, or

even the penetration, of a protected window under a blast of bullets.3

In a comprehensive review of the field of self-healing materials, Murphy and Wudl divided

smart healable systems based on their type of stimuli into three main types: thermal, electrical and

mechanical.4 Healable systems with thermal stimuli function based on either utilizing thermally-

reversible cyclic dienes reactions, as demonstrated in the work of Murphy et al. on Diels-Alder

reactions5, or on embedding liquid healing agents in particles that would release their contents upon

exposure to heat, as suggested by Hayes et al. for the healing of polymeric composites by heat

treatment.6

Healable systems with electrical stimuli undergo a built-in healing mechanism at the crack’s

site since electrical resistance of the material would increase where cracked. Both carbon fiber

composites and shape memory alloys (SMAs) show self-healing ability upon resistive heating.4 An

example of such systems, studied by Kirkby et al. in 2008, is a shape memory alloy of Ni/Ti/Cu

composite incorporated into an epoxy composite that contains Grubbs’ catalyst. Upon injection of

DCP in the crack’s location along with resistive heating to 80oC for 30 minutes, an average healing

efficiency of 49% was achieved.7 Other healable systems with electrical stimuli include


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organometallic polymers which have intrinsic reversible interactions in their networks that can be

activated upon resistive heating.4

Self-healing coating systems are

characterized by their responsiveness,

external stimuli, and programmed

structure. Regarding responsiveness, as

shown in Figure 1, self-healing coatings

behave similar to biological healing

systems in human skin except that their

response is usually much faster. The

healing mechanism is similar too to the

biological one. As soon as damage occurs, an actuator triggers the healing mechanism in which some

healing materials are moved to replace the damaged part of the skin or coatings and to finally restore

the matrix through some chemical reaction.

The external stimuli for

which self-healing coatings

respond are mechanical in nature.

They include many types of

mechanical damages to a

material’s structure such as

microcracks and scratches (see

Figure 2).

Since the stimulus in self-healing coatings is mainly a mechanical crack, a detailed discussion

of poly(dicyclopentadiene)/Grubbs’ catalyst coating system, a healable system with a mechanical

Figure 1: Comparison of synthetic healing process and biological one8

Figure 2: Types of mechanical damages that stimulate healing in self-healing coatings8


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stimulus, will be presented later in this paper after a brief discussion of different structural schemes

of self-healing coating systems with mechanical stimuli.

1.2. Structural Schemes

Healable coating systems with mechanical stimuli have three key types of healing methods that

were examined in the available research literature: capsule-based, microvascular networks, and

intrinsic self-healing methods (Figure 3).8 In the capsule-based method, healing agents are stored in

microcapsules which are distributed in an epoxy that contains a catalyst. When damage occurs, the

capsules release their contents which undergo a polymerization reaction as soon as they come into

contact with the catalyst that is embodied in the epoxy. For microvascular networks, the healing

agents are stored in hollow fibers which release their content when ruptured. Finally, intrinsic self-

healing materials have an inherent capability to repair any structural damage through internal

reactions, intermolecular diffusion, or dynamic arrangement.8

Figure 3: Types of self-healing methods for healable systems with mechanical stimuli8

Different techniques of encapsulation are used in the capsule-based healing method including

interfacial, in situ, and meltable dispersion. Thermosetting resins such as urea-formaldehyde,

melamine-formaldehyde, and polyurethane were among the materials used to for interfacial

microencapsulation.8 The work of White et al. at the University of Illinois in 2001 on the

dicyclopentadiene-Grubbs’ catalyst system has brought interfacial microencapsulation of the healing


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agents to light.9 The poly(DCPD)/Grubbs’ catalyst system, is an example of a healable coating

system in which the Grubbs’ catalyst is microencapsulated in discrete capsules of urea-formaldehyde

while the monomers of DCPD are dispersed into a surrounding resin. White et al. incorporated this

self-healing coating system using Ruthenium-based first- and second-generation Grubbs’ catalysts in

different types of materials such as bulk matrices of epoxy9,10

, epoxy vinyl esters11

, and fiber-

reinforced epoxy composites.12,13

This paper reviews the poly(DCPD)/Grubbs’ system in three sections: synthesis,

characterization, and applications. In the first section, the ring-opening metathesis polymerization

(ROMP) of DCPD is explained. In the second section, different methods for characterization of the

mechanical and chemical properties of the system and evaluation of healing recovery are presented.

The last section examined various areas for utilizing this autonomic coating system and screened a

variety of potential uses.

3. Synthesis

As a highly-branched cross-linked polymer, poly(DCPD) is well-known for its high impact and

high chemical corrosion resistance. For the poly(DCPD)/Grubbs’ catalyst coating system, the

polymerization is based on the ring-opening metathesis polymerization (ROMP) of DCPD monomer

via Grubbs’ catalyst as shown in Figure 4 below. ROMP is a well-known method of polymerization

that allows synthesizing long chains of polymers with C=C double bonds.

Figure 4: Ring-opening metathesis polymerization of DCPD14

From a mechanistic point of view, ROMP of DCPD with Grubbs’ catalyst can be explained in

terms of two common steps of polymerization: initiation and propagation. Grubbs’ catalyst, through


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the formation of transition-metal/alkylidene complex, serves as an initiator. The transition-

metal/alkylidene complex is formed via a 2+2 cycloaddition of an alkylidene to C=C double bond

which results in a metallocyclobutane intermediate followed by cycloreversion (Figure 5).15

While

earlier generations of Grubbs’ catalysts were sensitive to functional groups and water, recently

developed generations have higher metathesis activity and greater tolerance of a large number of

functional groups. The relief of ring strain energy in the stained ring makes the reaction very

exothermic, and thus, derives the reaction thermodynamically.14

Figure 5: General mechanism of ROMP15

ROMP of DCPD monomer is used to make numerous commercial products such as Telene®,

Metton®, Prometa

®, and Pentam

®.16

The presence of two C=C double bonds of different reactivities

in DCPD makes it great to use in commercial applications. As the strained norbornene C=C double

bond experiences high angle strain, it can be polymerized rapidly followed by the polymerization of

the other C=C double bond to yield a cross-linked polymer.17

As stated by Grubbs, this second C=C

double bond can bear a variety of functional groups for further synthetic elaboration which provides

an amazing method to produce a variety of consumer products from baseball bats to ballistic

panels.17

As illustrated in Figure 6 below, the DCPD monomer can exist in two isomer forms: endo and

exo. The activation parameters and mechanisms of the polymerizations of both isomers were studied


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by Rule et al. using in situ NMR.18,19

Rule et al. concluded that exo-DCPD is much more reactive

than endo-DCPD due to steric interaction with a bulky Grubbs’ catalyst.10

Figure 6: The two isomer forms of dicyclopentadiene monomer10

3. Characterization

3.1. Chemical Properties

The scarcity of literature on chemical characterization of the poly(DCPD) formed by in situ

ROMP with Grubbs’ catalyst might be due to the research emphasis on studying the mechanical

properties and improving the healing efficiency. However, characterization of similar systems of

cyclic olefins via living polymerization with Grubbs’ catalyst was extensively investigated by

Bielawski and Grubbs in 2006.20

As measured by GPC, narrow polydispersities as low as 1.05 were

reported in excellent yields (up to 95%) for different derivatives of norbornene using Mo- or Ti-

based Grubbs’ catalysts.20

Using solution 1H NMR, Yang et al. studied the ROMP of DCPD in polyester resin systems

presenting the limitations of this characterization method at high metathesis reaction rates.21

In a

groundbreaking work by Constable et al. in 2003, ROMP of DCPD with a ruthenium-based Grubbs’

catalyst was monitored by ultrasonic spectroscopy in a reaction injection modeling cell.22

Reaction

kinetics’ parameters were computed from real-time measurements of density, sound velocity,

acoustic modulus, and attenuation. Consequently, through the analysis of spectra obtained at

different temperature, effects of temperature on reaction kinetics were studied. In the same work,

FTIR spectroscopy was proven to be an ideal method of measuring the extent of polymerization.22


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3.2. Mechanical Properties

In order to quantify the healing performance in self-healing coating systems, a quantity called

healing efficiency, , is defined as follows:

where is a specific mechanical property, such as toughness or strength, measured before the

material is cracked (virgin state) and after it is repaired (healed state).8 Many factors, including

structural scheme, temperature, crack size, loading conditions, and cracking rate, may affect the

mechanical properties of the self-healing system after recovery such as fracture toughness, strength,

ductility, and elastic modulus.8 Table 1 below summarizes the results of mechanical tests on

poly(DCPD)/Grubbs’ system as conducted by White et al. based on fracture toughness.8

Table 1: Healing efficiencies based on fracture toughness for poly(DCPD) self-healing system

Material Structure

scheme

Loading Conditions Maximum healing efficiency

(%)

Epoxy Microvascular Mode I four-point

bend

38-70 23,24

Expoy/E-glass FRC Capsule-based Mode I DCB 60 25

Epoxy Capsule-based Mode I TDCB 75-93 9,10,26,27,28

Epoxy/embedded SMA

wires

Capsule-based Mode I TDCB 77 7,29

Epoxy vinyl ester Capsule-based Mode I TDCB 30 11

Epoxy/carbon FRC Capsule-based Mode I WTDCB 80 12

Scanning electron microscopy (SEM) is commonly used to validate the healing mechanism and

to check the surface before and after healing. SEM images such as Figure 7 are usually in the range


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of 10-100 m depending of the crack size. Capsule-based systems can usually be applied for small to

moderate cracks, while microvascular systems can work for larger cracks. However, the maximum

limit for microvascular systems is virtually when the crack is large enough to intersect the vascular

network.8

Figure 7: SEM image of healed fracture surface of poly(DCPD)/Grubbs' coating system12

4. Applications

Poly(DCPD)/Grubbs’ self-healing coating system has a wide variety of potential applications in

a number of industries including automotive, defense, space, and construction industries. The

elimination of microcracks through self-healing is particularly of great significance in applications

where failure prevention in critical designs is desired. Such potential applications include automotive

coating, airplane external parts, microelectronics packaging, military tanks, and many others.

The benefits of utilizing such self-healing coatings in structural bodies made out of polymers or

composites include enhancing product safety, protecting structural bodies, and reducing relevant

costs. In applications where a mechanical failure could lead to a catastrophic disaster, such as in

aerospace parts and military equipment, a material’s ability to repair its structural damage is of great


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interest. For other applications, where the crack detection is either difficult or costly, the use of self-

healing coating systems prolongs the material’s lifespan and enhances its structural durability.

Consequently, the use of such systems will significantly reduce the costs of replacement,

maintenance, and inspection.

5. Conclusion

Self-healing coatings systems have undergone spectacular development in the last two decades.

Microencapsulation of healing materials in different types of resins was the most extensively

researched structural scheme for these systems. Poly(DCPD)/Grubbs’ catalyst system was one of the

most promising systems researched. Although this system is still under research and development, a

wide range of potential applications will benefit from its commercialization such as its potential use

in automotive coatings and microelectronics packaging.


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Technology

Self healing coatings mohammed alshammasi