Self Healing Plastic

Self Healing Plastic

INTRODUCTION The scientists has developed the first material that repairs itself. The material is a form of plastic that has been engineered to fill breaks in its surface. Plastics are used today in everything from airplane wings to hundreds of objects found in the home. Scientists wanted to find a way to make made of plastic last longer. The researches hope their new discovery can be used to make objects that are difficult or impossible to replace. The surface of the plastic objects breaks over time. Very small breaks or cracks are develop every time when a plastic object is used. The researchers wanted to find out how to stop plastic from developing small cracks that grow, weaken and destroy it. The research team at the University of Illinois at Champaign-Urbana found an answer to the problem, they found the answer in the chemical structure of the plastic itself.

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Self Healing Plastic

COMPOSITE Composite is defined as a multiphase material from a combination of material differing in composition, which remain bonded together, but retain their identities and properties, without going to any chemical change Ex. Bone Also polymer composites are advanced materials that consists of two components (Advanced – high modulus fibres such as graphite, boron, low modulus fibres such as carbon, glass, jute etc.) 1) Reinforcing fibres Such as carbon, glass, kevlar flax, hemp or jute etc. 2) Liquid moulding resin Such as epoxy resin, unsaturated polyster resin, vinyl ether or urethane resin. Typically, the fibres are first placed in a mould. The viscous resin is poured or injected into mould. The resin hardens and outcomes the finished item. Composites are generally processed or manufactured by following techniques.  Hand lay-up Techniques.  Bag moulding process.  Thermoset Matched Die Moulding.  Filament winding  Continuous manufacturing process. ( Pultrusion)

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Self Healing Plastic

We use these composite materials in all manner of products, for all manner of applications. The market is huge, some 20 million tones of composites are used annually in civil engineering, aerospace and defense related projects, offshore oil exploration, electronic and biomedicine.

ADVANTAGES OF COMPOSITES : (1) The major advantage that has been driven the widespread use of composite is that of specific properties, when compared to traditional materials, composites are have found to have higher – (a) moduli per unit weight ( specific moduli ) (b) strength per unit weight ( specific strength ) (2) These higher specific moduli and specific strength can be translated directly into weight saving. (3) The reduction in weight, in turn result in more efficient structures, reduced energy cost and reduced material costs.

ADDITIONAL ADVANTAGES Composite also offers many advantages such as 1. Resistance to fatigue and corrosion. 2. Low cost fabrication. 3. Tailored thermal expansion characteristics and thermal conductivity, damping. 4. Design flexibility by tailoring the reinforcement location to design requirements. C.O.E & T. Akola

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Self Healing Plastic

MECHANISM OF POLYMER FRACTURE WHAT IS FRACTURE ? Fracture is a rupture of the bonds between the elements of a body (atoms, molecules or ions ) resulting in a breakage or cleavage of the material (specimen) into parts. Many researchers have tried to calculate the strength of polymer on the basis of forces of interaction between particles of a body ( ie. Atoms, molecules or ions ). Calculated strength is known as theoretical strength which differs from the experimentally found (technical strength ). For real solids, the calculated strength is greater than technical strength by about two orders. To explain this, Griffiths proposed the hypothesis of the surface crack on the strength of real bodies that the ideas of the hypothesis are sound was shown by Loffle and his pupils. They ruptured rock salt specimens simultaneously dissolving their surface layer which contained cracks, and found that this increased the fracture stress from 0.4 to 150 kgf/mm2. It is known that glass filaments picked preliminarily with fluoric acid to remove their surface layer also become much stronger. Following are the hypothesis of Grifiths theory (1) When the polymeric articles are subjected to mechanical action, cracking or crazing occurs, usually starting at the most stressed point.

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Self Healing Plastic

Ex. At the holes by which the parts are fastened. (2) Crazing is especially harmful in the case of polymers used in fields where optical properties are important, as these are deteriorated by crazing. (3) Crack reflect and scatter light, making the surface turbid or “silvering” it. If such a parts are examined it a certain angle, the surface shows a bright luster. (4) Cracking may be caused by external or internal stresses. Internal stresses are stresses arising during fabrication of the material (sample) and balancing each other inside it. Sometimes cracking occurs in a polymer only under the action of such internal stresses. For examples, a polystyrene sample with internal stresses will crack if immersed in a solvent or upon subsequent evaporation of that solvent. (5) By observing the formation and growth of surface cracks in polymers under a microscope , it was established that the cracks do not arise instantaneously after the load is applied, but after some time. (6) Then new crack form and those that appeared earlier gradually grow larger. (7) Therefore a sample contains cracks of a great variety of sizes. The rate of their appearance ( the number of cracks appearing per 1 cm2 of surface area per 1 S) and their (increase of the length of a crack visible under the microscope per 1 S) depend on the stress and temperature.

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Self Healing Plastic

(8) The rate of both processes increase with increasing temperature and increasing stress. After a certain limit is reached the number of cracks ceases to grow but their size continuous to increase. (9) Crazing may occur not only on the surface but throughout the bulk, and it is an irreversible process. (10)Prolonged rest of the sample after removal of the load decreases the size of the crack but does not heal it entirely. The crack remains and begins to grow again as soon as any tensile force is applied. (11)Within a certain temperature interval, deformation of transparent glassy polymers causes whitening at the point of neck formation. This is related to the formation of the microcracks. The higher the temperature of deformation, the weaker will be the whitening and near glass transition temperature it vanishes entirely. (12)Microcracks are also arise in samples which do not form necks when extended, but in this case crack formation is less intensive. If extension of a glass polymer is stopped long before fracture occurs, and the specimen is heated to a temperature above and cooled again to the initial temperature and again strained, the polymer will be found to possess the same mechanical properties as in the first case (before heating). Evidently, small strains do not cause microcracks to form or cause very small ones.

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Self Healing Plastic

When polymer samples are extended to fracture, or almost to fracture, microscopic cracks appear, which do not heal when heated above glass transition temperature. Even if after annealing these cracks become so small as to be invisible under the microscope, the material will crack anyway afterwards during the experiment. Hence, these changes in mechanical properties are irreversible. (13)Cracking in polymers depends largely on the method of their processing. It is caused mostly by the tensile stresses remaining in material after processing , extrusion or other technology operations. Thus, if a polystyrene sample, in the surface layer of which tensile stresses are acting, is immersed in a solvent, it will crack, but if compressive stresses are operating in its surface layer, it will not crack under the same conditions. Preliminary annealing always increases the resistance of a polymer to cracking. (14)The initial act of fracture is the strain of the chemical bond. The strained bond bursts under the action of thermal fluctuations. There are comparatively few such overstressed bonds but they are determinative in fracture process.

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Self Healing Plastic

CHEMICALS USED The following are the chemicals used during the preparation of the microcapsules 1) Dicyclo pentadiene ( DCPD )

2) Urea and formaldehyde

3) Cobalt naphtenate

4) Epoxy resin

5) Ethylene maleic anhydride resin

6) Tricyclic diolefin

7) Dimethylaniline

8) Resorcinol acid

9) Ruthenium carbene

HC

CH

CH

H2 C

CH

CH

NH2

C

NH2

H

C

H

O

CH CH Dicyclopentadiene

Urea

Formaldehyde

COOCo COOH N

CH3 Cobalt naphtenate

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CH3

Dimethylaniline

OH

Resorcinol acid

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Self Healing Plastic

MANUFACTURING Plastic is made of small molecules called monomers.

These monomers link

together to form very long molecules called polymers. Polymers give plastic its ability to be shaped and its strength. The research team found a way to make plastic that contains very small balloons (microcapsules) filled with liquid. The liquid certain monomer molecules, the building material of plastic. The team then created solid plastic that contains a special chemical. This chemical is called as catalyst. A catalyst is a substance that starts a chemical reaction. The new plastic still cracks like common plastic. But when it does, the monomer liquid is released and flows into the crack. The catalyst in the solid plastic then reacts with liquid monomer. The chemical reaction between the liquid monomer and the catalyst creates polymer molecules that repair the break. The repaired plastic has seventy five percent of the undamaged plastic. Structural composites are typically made up of high strength fibers embedded within a polymer matrix. Although lighter than metal, these high-tech materials can fall victim to fatigue. The gradual loss of strength and eventual failure of a material caused by stress cracking.

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Self Healing Plastic

Following are the some of the methods by which self healing plastic can be made. [1] In this method the liquid healing agent uses capillary action to seep into the voids created by the cracks. The best result to date have been achieved using 1.3 % (wt) cobalt naphtenate and 1.3% (wt.) dimethylaniline as a catalyst. To assess the repair agents effectiveness, tensile coupons were made and then fractured and repaired manually using the crack filling agent. However, this agent does not have a suitable shelf life for encapsulation. Following are the manufacturing methods : 1. DCPD + 1.3 % (wt) cobalt

Cap. action

naphtenate and 1.3 % (wt)

Polymer that bonds surfaces of crack together

dimethylaniline 2. DCPD + Catalyst

Room temp

(100 µm)

3. 10 % resin microcapsules + Ruthenium carbene

Polymer that bonds surfaces of crack together

24 hrs bake

Polymer that bonds

at 40° C

surfaces of crack together

(Grubb’s catalyst)

4. DCPD + Mixture of urea & formaldehyde + Resorcinol acid

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Polymer that bonds surfaces of crack together

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Self Healing Plastic

+ Ethylene maleic anhydride [2] The fatigue damage is controlled by embedding microcapsules of di-cyclopentadiene (DCPD), a polymer precursor, into a composite matrix material. When a growing crack ruptures one of 100 µm capsules, the DCPD flows into the fissure and comes into contact with a catalyst in the matrix. Within a minutes at room temperature, the DCPD form a new polymer that bonds the surfaces of the crack together.

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Self Healing Plastic

[3] For this method, the 10 % by weight of resin microcapsules, 100 mm in diameter is stir into the epoxy formulation, the molded epoxy is cure for 24 hours at room temperature followed by a 24-hour bake at 400C.

The

polymerization catalyst dispersed throughout is a ruthenium carbene complex invented by chemistry professor Robert H. Grubbs of California Institute of Technology. The Grubbs catalyst remains active on exposure to air, moisture, or most organic functional groups.

Manipulated image from a scanning electron microscope. The polymerized healing agent emerges from the red ruptured capsule. The fracture plane is light blue. Grubb’s catalyst is the chemical structure with the red, green, and gold. Photo taken from http:/www.news.uiuc.edu C.O.E & T. Akola

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Self Healing Plastic

[4] In this method, the microcapsules are made by high speed stirring of an aqueous mixture of urea and formaldehyde, dicyclopentadiene, resorcinol-acidcatalyst and ethylene-maleic-anhydride resin emulsifying agent. The product is microcapsules of urea-formaldehyde resin containing dicyclopentaline liquid.

This   plastic   is   full  of  tiny,   liquid­filled   spheres,   dyed   red,   and   dark   specks   of   catalyst,  which let the plastic heal itself. A crack, located where the red and clear sections meet,  breaks some of the spheres, whose contents congeal into a polymer and fill in the gap.

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Self Healing Plastic A red­dyed capsule (left), next to which a microscopic crack develops (middle). The crack  then breaks the sphere and a healing agent fills in the crack (right).

When the material cracks, the micro-capsules rupture and release the healing agent into the damaged region by means of capillary action. As the healing agent comes into contact with the embedded catalyst, polymerization is initiated, which then bonds the crack face closed. On the microscopic scale, the small capsules of DCPD provide weak spots towards which crack grow. However this can be advantageous if crack locate and burst these bubbles of healing fluid.

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Self Healing Plastic

A microsphere whose thin wall has collapsed as a result of a nearby fracture.

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Self Healing Plastic The two sides of a crack have been knit together by the healing agent released from the  microspheres. 

REQUIREMENTS FOR SELF HEALING SYSTEM

There are basically four types of requirement for self healing system. (1) It must not degrade physical properties or performance of the plastic. (2) It must sense the damage. (3) It must initiate healing. (4) It must restore the original strength and stiffness i.e. Inclusion of the capsules does not degrade the strength or stiffness of the epoxy.

DESIGN POINTERS

 By preventing the rapid spread of the microcracks material life expectancy can be extended.  The size of the spheres does not have a detrimental effects on the mechanics of the composite.

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Self Healing Plastic

 75% of the initial strength is achieved on repair.

CRITICAL PARAMETERS (1) One of the biggest challenge to this research was developing microcapsules that were weak enough to be ruptured by growing crack but strong enough to withstand the curing step of the composites manufacturing processes. (2) Creation of the correctly sized capsules . Large spheres would have a detrimental effect on the composite structure so the researchers are currently using spheres with a diameter of about 100 microns. (3) Determination of the best wall thickness Thick capsule walls may not rupture when the crack approaches, while thin capsule wall might break during processing. (4) Micro-cracks that are too stiff cause distributions of stress in the plastic that force the crack to grow away from them. (5) One micro-sphere often is not enough to stop crack.

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Self Healing Plastic

(6) Specific chemistry used to create the polymer also limits the maximum temperature it can be explored to, before the autonomic healing process is inactivated ( about 800c). (7) Polymerization of healing monomer should not cause empty space or voids due to contraction or fluid flow.

APPLICATIONS It would be particularly useful for applications for which repair is either impossible or impractical. It should also increase the life of the thermosetting polymers in a massive variety of different guises. The following are the important applications – (1) Electronic circuit board / microelectronics because even the smallest crack can lead to electrical failure which can then be hard to locate on a complex, highly populated board. (2) Air force because of the many polymer based composites are used in aircraft. (3) Rocket motors. (4) Components of deep spare probes-space-vehicles where part can not be repaired. (5) Space stations.

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Self Healing Plastic

(6) Bridge supports ( large bridges ). (7) Implanted medical devices. (8) Such as in prosthetics, artificial organs. (9) Plastic dental fillings (In this silica particles of 50 nanometer are used ). (10)Integrity of adhesive joints and sealants.

CONCLUSION Everyday we observe plastics in endless fields and to be near to this material technology-wise as a polymer technologist is a wonderful experience. Plastics today can be made much more stronger than metal & can be made much more softer than any other material by possessing adequate knowledge of this material. Plastics have found applications not only in day to day purposes but also in special fields such as aerospace, microelectronics etc. However, there are certain hurdles for this wonder material in its applications much more is expected from plastics. Though self healing plastics are in their early stages, the day when they will be fully developed and applied will be the one after which the hurdles will be gone and polymers would be applied inadvertently in any and every field.

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Self Healing Plastic

REFERENCE [1] Mark Bikales, Overber berger, Menges, Encyclopedia of polymer Science & Engineering , Vol . 3, 2nd Edition , A wiley – Interscience publication( 1985 ), Page No. 776 – 779

[2] George Lubin , Hand Book of composites , Vol.1 , 1st Edition ,VAN Norstrand Reinhold New York (1982) , Page No 19, 38, 50, 57,89,321,368, 390, 448,478,490,513.

[3] James M Margolis , Advance thermoset composites

( Industrial &

commercial Application) , Vol. 1, 1st Edition , VAN Norstrand Reinhold New York (1986 ) , Page No.1- 5.

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Self Healing Plastic

[4] A. Tager , Physical Chemistry of Polymer , Vol. 1 , 2nd Edition (1978 ) , Mir publishers Page No. 232-265.

[5] www. google.com [6] www.rediff.com

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