self healing of polymeric composites

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SELF HEALING OF POLYMERIC COMPOSITES

ABSTRACT

Inspired by biological system, in which damage triggers an autonomic healing response a

new class of polymeric composite materials was developed which begins to repair itself

as soon as crack forms. Self healing polymeric composite has the potential to defendagainst material failure, it greatly improves product safety and reliability ultimately

extends product life time. Polymeric matrix, microcapsules containing healing agent and

catalyst are the major components of the self healing polymeric composites. These new

classes of materials find applications in the field of biomedical, paint and coatings,sporting equipments, automobiles, aircraft and aerospace.

Key words: self healing polymers, healing agent, microcapsules

INTRODUCTION:

Composite materials are macroscopic combinations of two or more materials. In which

each constituents retain their identity. The basic constituents of Polymeric Matrixcomposites (PMCs) are :( 1) Matrix made up of polymers and (2) Reinforcing agents

(particles, fibers etc.).

Inspired by biological system, in which damage triggers an autonomic healing

response a new class of polymeric composite materials was developed which begins to

repair itself as soon as crack forms. These self healing polymeric composites havepotential to defend against structural failure and greatly improve product safety and

reliability leads to extend product lifetime. Self-healing is accomplished by incorporating

a microencapsulated healing agent and a catalytic chemical trigger within a polymer

matrix. When the material is damaged, the microcapsules rupture and release the healingagent into the damaged region through capillary action. As the healing agent contacts the

catalyst, polymerization is initiated and the damage is repaired.[1]



GENERAL MECHANISM:

Constituents of Self- Healing PMC:The main constituents of a self-healing polymeric matrix composite (SHPMC) are shown

as:

Fig. 1: Constituents of self healing polymeric composite

Matrix: The matrix is made up of a polymers either thermoplasts or thermo sets.

Healing Agent: The healing agent is a fluid in the form of encapsulated tiny bubbles that

are spread throughout the composite material.Microcapsule: This contains the healing agent and separates the catalyst and healing

agent.

Catalyst: The catalyst triggers the polymerization process and usually is Grubbs’

catalyst. The function of catalyst is to enable the healing agent to heal the composite

material. Catalyst and healing agent are separated until they are required to seal a crack.[2]



Concept of Autonomic Healing:

The figure below illustrates the autonomic healing concept. The healing is accomplishedby incorporating a microencapsulated healing and a catalytic chemical trigger within an

epoxy matrix. An approaching crack ruptures embedded microcapsules, releasing healing

agent into the crack plane through capillary action. Polymerization of the healing agent is

triggered by contact with the embedded catalyst, bonding the crack faces. [2]

Fig. 2: The autonomic healing concept

Case Study – Self Healing Epoxy Composite:



Matrix: The matrix is epoxy matrix. The epoxy matrix composite was prepared by

mixing 100 parts epoxide with 12 parts DETA (diethylenetriamine) curing agent. Self-

healing epoxy specimens were prepared by mixing 2.5 wt% Grubbs’ catalyst and 10 wt%microcapsules with the resin mixture described above. The resin was then poured into

silicone rubber moulds and cured for 24 hours at room temperature, followed by post

curing at 400

C for 24 hrs.Healing Agent – (DCPD): The healing agent is dicyclopentadiene (DCPD). It is a low

viscosity (.736cP @ 21oC) hydrocarbon monomer. [2]

Figure 3: Structure of DCPD

Chemistry of Self-Healing:

Completion of self-healing process requires a suitable chemistry to polymerize the

healing agent in the fracture plane. We identified the living ring-opening metathesispolymerization (ROMP) as meeting the diverse set of requirements of the self-healing

system, which includes long shell life, low monomer viscosity and volatility, rapidambient conditions; and low shrinkage upon polymerization. The ROMP reactioninvokes the use of a transition metal catalyst (Grubbs’ catalyst) that shows high

metathesis activity while being tolerant of a wide range of functional groups as well as

oxygen and water. The reaction polymerizes dicyclopentadiene (DCPD) at rtemperature in several minutes to yield a tough and highly cross-linked polymer network.

The characteristics of this reaction are: (1) Fast reaction time, (2) High monomer to

catalyst loading 10000:1 and (3) Minimal volume shrinkage. [2]



Fig. 4: Chemistry of self healing

Preparation of Microcapsules:

Microcapsules are prepared by in-situ polymerization, which is also referred as Microencapsulation, which is described as under:

In a 600ml beaker, urea (7 gm) is dissolved followed by resorcinol (0.5 gm)

and ammonium chloride (0.5 gm) in water (150 ml). A 5 wt% solution of ethylene-Maleicanhydride copolymer (100 ml) is added to the reaction mixture and the pH of the

reaction mixture was adjusted to 3.5 using 10% NaOH solution. The reaction mixture

was agitated at 45 RPM and to this stirred solution 60 ml of dicyclopentadiene was added

to achieve an average droplet size of 200 m. To the agitated emulsion 37

formaldehyde (0.23 ml, 18.91gm) solution was added and then the temperature of the

reaction mixture was raised to 50oC and maintained for 2 hours. After 2 hours, 200 ml of

water was added to the reaction mixture. After 4 hours the reaction mixture was cooled toroom temperature and the microcapsules were separated. The microcapsule slurry was

diluted with additional 200 ml water and washed with water. The capsules were isolated

by vacuum filtration, and dried. The yield was 80%. Their average size was 220 m. [2]

Catalyst:

The catalyst used is Grubbs’ catalyst. It is a stable transition metal catalyst used to initiate

a living polymerization (developed in the mid 1990’s by Robert Grubbs at Caltech). Thecatalyst remains active even after triggering polymerization. Hence when additional



cracking occurs, the catalyst crystals continue to trigger polymerization, allowing

multiple healing to occur. [2]

APPLICATIONS:

1. Paints and Adhesives :A new group of nontoxic , corrosion resistant self healing PMC replaced

chromates which have been used commercially as an anticorrosive constituent of

many paints and adhesives. As soon as the surface is scuffed the coating starts self repairing.[3]

2. Medical Field:

Self healing bandages: Small tissue sample is taken from a patient and a culture

is grown from the cells in laboratory. These cells are then placed on a membrane

made from a medical grade self healing PMC. This membrane is treated with aspecial cell friendly coating which enables skin cells to attach and grow with it. It

is serve best to heal diabetic ulcers and other slow healing wounds.

For healing of bone and cartilage : A new class of biocompatible self healing

PMCS act as scaffolding when it is placed inside a severe fracture or in the small

cavities using laser beams. The main advantage is that it maintains its strength and

eliminate problem of weaker and more porous bones and necessity of futuresurgeries.

Dental applications : Bioactive self healing PMCs containing amorphous calcium

phosphates are used as filler for teeth and applied by root canal therapy. This can

eventually cause teeth to repair their own small cavities. [4]

3. Electronics:

New class of self healing PMCs are used to design electrodes of capacitors, so

whenever short circuit cause damage to the dielectric layer these electrodes willvaporize and heal that areas thus continue functioning of the capacitor. [5]

4. Self Healing Fuel Tanks: Fuel tanks in military planes and helicopters which currently used several layersof heavy rubber for protection. Self healing PMCs developed from surlyn and

ethylene-methacrylic acid copolymer which are self healed against bullet‘s attack.

It is also fuel resisting so long lasting and prevents leakage of fuel by bulletsstrike.

5. Air Crafts and Space Applications :



A clear and crack free air craft windows are developed by coating it with water

repellant self healing PMCs, it is also flexible enough to accommodate changes in

shape that a window undergoes without itself crazing or coming unstuck. [5]

ADVANTAGES AND LIMITATIONS:

ADVANTAGES:

Addition of microcapsules and reinforcing agents strengthens the material.. [4]

Such materials will increase the product safety, reliability.

Self-healing polymers increase the life of thermosetting plastics by as much as 2-3

times.

Requires less maintenance.

These polymers will eliminate the deleterious effects of environmentally assisted

degradation such as moisture swelling and stress corrosion cracking.

Self-healing polymers are used where it is not possible to repair the material once

it has been put into use and in any plastic subject to micro cracking. This polymeric composite recovers 70-90% of its original fracture toughness.

LIMITATIONS:

It is necessary to obtain optimum size of microcapsules according

characteristics of matrix material and different loading conditions.

Microcapsules are not capable of multiple healing.

The catalyst is expensive, but it’s cost will be compensate when a longer lifetime .

less maintenance of product is obtained. [6]

CONCLUSION:

Self healing polymeric composites are efficient and will eliminate deleteriouseffects of mechanical or environmental degradation of materials. This new class

of polymeric composites continues to gaining acceptance. Further research and

technological advancements will extend it’s usefulness in structural andaerospace applications.

REFERENCES:



N.R.Sottos & E.N.Brown, “Microcapsules induced toughening in a Self healingpolymeric

composites”, Journal of Material Science, 39, Pg.1703 -10, 2004.

S.R.White & N.R.Sottos & E.N.Brown, ”Autonomic healing of polym

composites”, Letters to the Nature, 409, Pg.794 -97, Feb.2001.

http://www.uri.edu.

http://www.sciencedaily.com.

http://www.csiro.au.

E.N.Brown, N. R. Scottos & S.R. White, “Fracture Testing of Self healing

polymeric composites”, Experimental Mechanics: An International Journal, 2001.

Document By

SANTOSH BHARADWAJ REDDY

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