A PAPER ON

“SELF-HEALING POLYMER TECHNOLOGY”

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SUBMITTED BY:
Sreeja Gadhiraju,
Naga vaishnavi.A,
III/IV Mech,
Sri Sivani Institute Of Technology. sreeja.btech@gmail.com INDEX

|Topic |Page No. |
| | |
|ABSTRACT |1 |
|introduction |2 |
|INTRINSIC SELF-HEALING |2-3 |
|EXTRINSIC SELF-HEALING |4-5 |
|HEALING MECHANISM | 5-7 |
|APPLICATION |7-8 |
|PROBLEM AND CHALLENGES |9 |
|CONCLUSION |9 |
|REFERENCES |9-10 |
| | |

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.

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, and (ii) chemical
Interactions.
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 [1]. Then, the molecules at the cracking surfaces would interdiffuse and the residual functional 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[2] and poly-phenylene ether (PPE) in which the repairing agent was regenerated by oxygen [3]. 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.

[pic] Figure 1. Hydrolysis and recombination reaction of PCs with the catalyst of NaCO3

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 [4]. 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 (corresponding to retro-DA reaction) but then reconnect upon cooling (i.e. DA reaction). This process is fully reversible and can be used to restore fractured parts of the 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 [5]. 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 [6]

2.1.2. Three-dimensional microvascular networks
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 [7]. 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 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.

2.2. Self-healing in terms of healant loaded microcapsules
The principle of this approach resembles the aforesaid pipelines but the containers for storing 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. [pic] Figure 3. Schematic drawing of the principle of self-healing epoxy based microcapsules[8]

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 develop small 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 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 [9].
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 [10].
2.Paint
Apart from cosmetic reason, paint is typically serves to protect surfaces. Self-healing protection coating for cars from Nissan [11] 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.
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[12] . 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.
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. • 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. • 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. • 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 already exists. Dupont’s Surlyn® show good properties to heal after ballistic damage [13]. • 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. • Electronics: Polymer composite circuit boards and electronic components can suffer from mechanical and electrical failures if microcracks progress unabated. This self-healing technology would help to prevent such failures. • 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. • 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 longer lasting seal.

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.
Problems and Challenging:
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.

Conclusion
Achievements in the field of self-healing polymers and polymer composites are far from satisfactory, 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.