Combating corrosion with self-healing composites

The corrosion of metals is a destructive process that leads to huge economic losses. According to the World Corrosion Organisation, the direct cost of corrosion worldwide is estimated to exceed $1.8 trillion. Karan Thanawala, a research scholar at the IITB-Monash Research Academy, hopes to bring this number down significantly.

Graduate research scholars of IITB-Monash Research Academy study for a dually-badged PhD from both IIT Bombay and Monash University, spending time at both institutions to enrich their research experience. IITB-Monash Research Academy is a collaboration between India and Australia that endeavours to strengthen scientific relationships between the two countries.

Mechanism of self-healing coatings using capsule based approach
[Picture Credit: Adv. Mater. 2009, 21, 645]

Karan explains that corrosion of metallic objects occurs by electrochemical reaction at the surface involving the oxidation of the metal in the presence of electrolyte and oxygen. The efforts taken for the prevention of corrosion are the use of alternative material and design of component, and/or application of a suitable protective coating, depending on the type of environmental conditions and the expected life. Among these, the most efficient and common approaches to control corrosion is the application of organic polymer-based coatings. However, being the outermost layer on structures, these coatings are susceptible to damage and scratches originating at the micro and nano-levels during handling and service. Such damage is hard to detect, allowing the corrosion process to propagate, finally rendering the coating non-protective.

Therefore, says Karan, it is a very attractive concept to design and develop coatings that possess the ability to heal the damage, thereby maintaining the protective properties.

Currently, the three main methods of achieving barrier restoration in self-healing coatings are intrinsic self-healing, capsule based self-healing, and vascular self-healing. The intrinsic and vascular methods require extra energy for triggering the self-healing mechanism, such as high temperature (~80 oC), humidity or UV light. Also, the fabrication and formulation of these methods are expensive due to the costs associated with the preparation of tubes and chemicals necessary for synthesising smart polymers. Microcapsules used in the capsule-based approach have the ability to store the active healing material as core in the outer shell. The shell is a hard cover of polymer over the active liquid state healing agent (core), such as drying oil. These microcapsules are prepared using simple in-situ (emulsion) polymerization technique. Post its preparation, microcapsules are incorporated in regular organic polymeric coatings, wherein the mechanical stresses generated in the area of damage are sufficient to rupture the microcapsules and allow the active healing material to flow through to the damaged region, forming a thin film. This film restores the barrier functionality that might have been damaged due to natural or induced factors.

(a) Optical microscopy and (b) scanning electron microscopy images showing core and shell phenomenon of microcapsules
[Picture Credit: Karan Thanawala ]

So what motivated Karan to take up this project for his research? He says, "Self-healing composites possess great potential for solving many limitations of polymeric coatings and structural materials, viz., micro cracks and hidden damages. Damages in the coating are the precursors to structural failure, and the ability to heal them will enable structures with longer lifetimes and less maintenance. Self-healing coatings mimic the natural healing process, similar to the healing of damaged skin. Therefore, self-healing coatings are very attractive as they can assure durability of the coated components even after damage in the coating due to chemical or mechanical reasons. The synthesis of microcapsules and formulating self-healing coatings pose great challenges. The size of microcapsules, shape and morphologies play an important role in providing an active functionality of rupturing and healing. Optimising the process to prepare tailored microcapsules allows great opportunity to try new methods, often exciting for a researcher."

Karan is confident that this breakthrough in the field of coatings research will go a long way in solving the problems of corrosion and its control using polymeric coatings, which, in turn, will ensure safer workplaces.

Self-healing ability performance of coating embedded with: (a) 1 wt%, (b) 2 wt% and (c) 3 wt% of microcapsule (after zero hours)
[Picture Credit: Karan Thanawala]

Throwing light on work he has done so far, Karan says, "Development of self-healing coatings posed great challenges. The most important factor was to achieve reproducibility of size, shape and morphology of prepared microcapsules, which when added into the coating, delivers the smart healing functionality. The critical parameters of synthesis process such as stirring speed and reaction time were optimized, which dominates formation of size and shape of the microcapsules. Additionally, the components of the microcapsules have been selected on the basis of their biocompatibility and non-hazardous nature, which classifies them as green materials. These prepared microcapsules were dispersed in the organic coating in varying concentrations. These microcapsule-impregnated coatings, after complete curing, were induced with an artificial scribe, before corrosion test in a saline solution (similar to sea water) for evaluation of the performance of the coatings. The effect of addition of the microcapsules on the self-healing functionality was investigated using optical microscopy. The coatings were further analysed for mechanical and adhesion properties for use as commercial industrial coatings. The results of self-healing coatings were comparable with the control coatings (without the microcapsules)."

"The IITB-Monash Research Academy is an exciting chapter in Indian-Australian relations that will see both countries creating binding links. This will enable us to tackle the research challenges that lie ahead and generate some long-lasting high impact outcomes for society," says Prof Murli Sastry, CEO, IITB-Monash Research Academy.

Research scholar: Karan Thanawala, IITB-Monash Research Academy

Project title: Development of Self-Healing Coatings

Supervisors: Prof A S Khanna (IIT Bombay) and Prof R K Singh Raman (Monash University)

Contact details: kktha4@student.monash.edu

The above story was written by Mr Krishna Warrier based on inputs from the research student and IITB-Monash Research Academy. Copyright IITB-Monash Research Academy.