SELF HEALING POLYMER COMPOSITES: MIMICKING NATURE TO ENHANCE

PERFORMANCE

-Seminar By:Abhijith Achuthakumar

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INTRODUCTION

Need for continual improvement in material performance is a common feature of any modern engineering endeavors.

A key focus of current scientific research is the development of bioinspired material systems.

A great many natural materials are themselves self-healing composite materials.

Lightweight, high strength, high stiffness fibre reinforced polymer composite materials are leading contenders as component materials to improve the efficiency and sustainability of many forms of transport.

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The healing potential of living organisms and the repair strategies in natural materials is increasingly of interest to designers seeking lower mass structures with increased service life.

It is simple observation that many natural systems can self-heal.

it is equally simple observation that animals usually achieve this via a ‘bleeding’ mechanism.

A clear distinction is whether the natural mechanism has been simply observed or whether it has been studied and specific functional points mimicked.

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SELF HEALING STRATEGIES IN ENGINEERING STRUCTURES

BIO-INSPIRED SELF HEALING APPROACHES:

One area of interest is the fusion of the failed surfaces.

Polymeric materials possessing selective cross-links between polymer chains that can be broken under load and then reformed by heat.

In certain instances, polymeric material hosts a second solid-state polymer phase that migrates to the damage site under the action of heat .

Possibility of using nanoparticles dispersed in polymer films to deposit at a damage site.

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MICROENCAPSULATION: The third area of interest is based upon a biological ‘bleeding’

approach to repair, i.e. microcapsules and hollow fibres.

Microencapsulation self-healing involves the use of a monomer, dicyclopentadiene (DCPD), stored in urea-formaldehyde microcapsules dispersed within a polymer matrix.

A key advantage of this approach is the ease with which they can be incorporated within a bulk polymer material.

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When the microcapsules are ruptured by a crack, the monomer is comes into contact with a dispersed particulate catalyst, thus initiating polymerization & repair.

Its disadvantage is the necessity for capsule rupture & the need for catalyst.

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HOLLOW FIBRES: Self-healing using hollow fibres embedded within an engineering

structure, is similar to the arteries in a natural system.

Here the self-healing material acts as the structural fibres.

The key advantage is that the fibres can be located to match the orientation of the surrounding reinforcing fibres thereby minimizing Poisson ratio effects.

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The fibres can be placed at any location within the stacking sequence to address specific failure threats .

A few disadvantages are the relatively large diameter of the fibres compared to the reinforcement, the need for fibre fracture etc.

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BIOMIMETIC SELF-HEALING APPROACHES:

The challenge for the future is the evolution of ‘engineering self-healing’ towards a biomimetic solution.

To date, the autonomous healing materials in engineering structures have been distributed randomly throughout the structure or spaced evenly through the composite laminate structure.

In nature the network is tailored for a specific function with the healing medium often being multifunctional.

In the first man-tailored work, the key failure interfaces were identified and then the hollow fibre self-healing network was designed for a specific composite component and operational environment.

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VASCULAR NETWORKS

Biological organisms have a highly developed, multifunctional vascular network.

This network supplies fluid to an area from a point reservoir, giving a branching network.

Over years, the branching & size of vessels have evolved to minimize the power required to distribute and maintain the supporting fluid within many other constraints.

Future of self healing relies on the development of a continuous healing network embedded within a composite laminate.

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This mimiced network has to deliver healing agent from reservoirs to damaged sites to permit repair of all types of composite material modes.

The healing agent needs to be replenished & renewed during the life of the structure.

It must restore matrix material properties & structural efficiency of fractured fibres.

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HEALING AGENT Mammalian blood clotting has evolved around a series of chemical

reactions & their inactive precursors known as Clotting factors, triggered in the form of a 'waterfall' of reactions.

It is initiated by a damage that breaches endothelial cells & culminates in the production of fibrin.

In addition to rapid injury response, system malfunction is extremely rare.

This is achieved by the rapid removal of activated enzymes upon fibrin production & action of endothelial cells.

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Biomimetic hollow fibre self-healing mimics mammalian self-healing in that a liquid healing agent leaks from a region of mechanical damage.

In mammals the immediate response is the need to arrest bleeding; whereas in biomimetic self-healing the rapid response is to restore some degree of structural integrity or prevent crack propagation.

Haemostatic system functions through almost 80 coupled biochemical reactions & this enormous complexity limits its degree to be mimicked.

Synthetic self-healing resin needs to be developed to duplicate the blood clotting approach.

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Self-healing in man-made structures requires intimate contact of 2 part resin systems:

1)A resin & a hardener 2)A resin & a catalyst

In resin & hardener type, the efficiency of healing depends on the extent of their mixing, i.e., molecular transfer across the boundary.

In resin & catalyst type, the chemical reactions spread at the damaged site & beyond it as well, using up the resin supply.

It is desirable to develop a resin system that mimics the clotting of blood to allow multiple ,localized repair events.

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COMPARTMENTALISATION A cut to a tree, triggers the formation of internal impervious

boundary walls that develops over time to protect the tree from damage.

This defense mechanism is termed compartmentalisation & is the main healing mechanism that protects them from pathogen infection through wounds.

It is a 2 part process:

1)A chemical boundary is active in the short term to protect against pathogens present at the time of injury.

2)A long term formation of a barrier zone giving continued protection from pathogens.

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A parallel concern in the use of composite materials, is the effect of environmental attack on damaged structure.

Moisture ingress reduces strength of these structures over time.

A biomimetic system for producing an impervious internal boundary in a damaged structure helps to avoid the secondary risk of moisture ingress.

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RECOVERY AFTER YIELD

Nature offers alternative healing strategies to repair brittle materials, in the form of materials that regain strength after yielding.

An example is: Mussel Byssal thread.(Gains back an amount of its modulus).

A better damage tolerance strategy is to adopt a hybrid composite material that mimics the safety strategies observed in living organisms.

This method, although not self-healing, helps develop an elastic-plastic behavior in composite materials.

This approach can be furthered, if the yielded fibres & the matrix material, could then be healed to regain its original stiffness.

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REINFORCEMENT REPAIR Self healing in fibre reinforced composites has been primarily focused

on polymer matrices as typical impact damage is primarily in the matrix.

However, reinforcing phase provides majority of strength & stiffness within any composite & it is this component that would benefit from a self-healing capability.

The natural process of bone healing is a complicated process, where the initially 'woven' bone is remodeled & replaced by mature 'lamellar' bone.

The process takes around 18 months, that can be accelerated on application of an axial load at fracture site.

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CONCLUSION The problem of damage initiation, propagation & tolerance has

limited the acceptance of composite materials in all engg. disciplines.

Nature has developed materials with healing potential over years.

Self healing approaches applied in composite materials to-date are primarily bio-inspired.

More recent advance is the study of natural healing to allow biomimetic self-healing.

Tailored placement of healing components & adoption of biomimetic vascular networks are very active topics on the same.

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Biomimetic healing agents are a requirement closely linked to the adoption of vascular networks.

Compartmentalisation is a bridge between self-healing & engg damage tolerance philosophy; particularly applicable to the problem of moisture ingress.

Post yield recovery offers self-healing for overloading in primary direction.

Reinforcements extend the concept of healing beyond repair of matrix dominated failure modes.

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REFERENCES R.S Trask, H R Williams & I P Bond 2007 SELF HEALING

POLYMER COMPOSITES: Mimicking nature to enhance performance U.K: University Of Bristol, Bristol. BS8 1TR Department of Aerospace Engineering.

S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown & S. Viswanathan 2008 Autonomic healing of polymer composites University of Illinois Department of Aeronautical and Astronautical Engineering, Department of Theoretical and Applied Mechanics, Department of Chemistry, Urbana-Champaign, Urbana, Illinois 61801, USA

Y. C. Yuan, T. Yin, M. Z. Rong, M. Q. Zhang 2008 Self healing in polymers and polymer composites. Concepts, realization and outlook: A review Zhongshan University, Key Laboratory for Polymeric Composite and Functional Materials of Ministry of Education, OFCM Institute, School of Chemistry and Chemical Engineering, Guangzhou 510275, P. R. China ;Materials Science Institute, Guangzhou 510275, P. R. China

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