Self-healing Materials – The Future of Repairs

Topics Covered

Introduction
How Self-healing Materials Work
Different Methods of Manufacturing
Uses of Self-healing Materials
References

Introduction

The ability to self-heal is one of the true wonders of nature, and now material scientists are closer than ever to reproducing this ability in everyday materials, which would hail in a new era in civil engineering and construction. Cracks and damages in materials even at a microscopic level alter their acoustical, electrical and thermal properties resulting in failure of structures.

Self-healing materials are those that have a potential to repair mechanical damages and cracks caused by over-usage, without the need for human interference. The key potential benefits of self-healing materials are as follows:

  • Minimizing the production cost of various industrial processes required for repairing damage.
  • Prolonging the service life of the materials.
  • Reducing the inefficiency of the materials due to degradation.

How Self-healing Materials Work

Although self-healing materials facilitate good mechanical performance, they are restricted to autonomic healing of damage in a particular location. These polymers function based on a three-step process, which is similar to that of a biological response. The first step involves triggering action immediately after the damage occurs. The second step is the transportation of materials to an affected region. The third step involves the chemical repair process. Self-healing materials are divided into 3 general sub-categories: capsule-based materials, vascular materials and intrinsic materials.

In the capsule-based materials, the healing agent is contained within the small capsules that release the agent upon rupture. The healing agent is present inside the capillary type hollow channels in case of vascular materials, and the channels are capable of being interconnected one-dimensionally, two-dimensionally or three-dimensionally. The capillary network can be refilled by an external source or another channel that remains undamaged. Intrinsic self-healing materials function on latent self-healing property rather than using healing agent.

However, with the recent advancements of various new lithographic techniques, self-healing materials with complex embedded microvascular networks for autonomously repairing repeated damage events are being developed. This microvascular coating–substrate architecture of the materials is similar to that of a human skin. The self-healing material structure is composed of a microvascular substrate containing microvascular network filled with a healing agent and a brittle epoxy coating containing catalyst particles. When a coating-substrate beam is loaded in a four-point bending configuration, the cracks are developed at a coating surface and propagated towards a coating–substrate interface. The healing agent is released into the cracks when the coating is damaged, through capillary action. Excess healing agent is secreted on the surface of the coating. The cracks are repaired after a sufficient time period, which leads to the restoration of structural integrity of the coating. The healing cycle is repeated when the cracks reopen.

Different Methods of Manufacturing

Smart self-healing materials are commonly produced using two methods:

  • Host-guest interactions sensitive to water
  • Covalent bonds that are resistant to water and other functional groups.

Hideyuki Otsuka and his team at the Kyushu University found a new solution for producing self-healing materials using a cross-linking unit, diarylbibenzofuranone (DABBF), which can be divided to form a radical pair. This cross-linking system is capable of achieving autonomous repair without the need for stimulation. The DABBF units can break into arylbenzofuranone radicals and reform. This ability enabled the material to repair itself when the damaged parts are made to contact with each other.

Prof. Nancy Sottos along with her team at the University of Illinois Urbana-Champaign developed a new self-healing technology that involves incorporation of plastics into a channel network. Researchers at the University of Southern Mississippi have developed a coating that has an ability to heal upon exposing to sunlight when damaged. The coating is made from the molecules of chitosan obtained from the shells of crab or any crustaceans. These molecules are combined with oxetane rings and the combination was added to a mixture of polyurethane. When the polyurethane coating is damaged, the oxetane rings are split to yield loose ends, which are highly reactive. Upon exposure to UV light in the sun, the chitosan molecules combine with the free ends to repair the damage.

Uses of Self-healing Materials

Self-healing systems can be developed from a variety of metallic materials and polymers. Carbon nanotubes (CNTs) are now considered as an ideal material for mechanical reinforcement purpose and molecular storage devices as well. This is mainly due to the fact that CNTs have a large interfacial area, good mechanical and chemical properties and a hollow tubular structure despite their small structure. Polymer/CNTs composites have provided favorable results for self-healing applications in storage devices. Currently, innovative self-healing nanosystems are being developed with the help of computer simulations for repairing damages.

Potential future uses for self-healing plastics include longer lasting cell-phones, cars and laptops, whilst larger scale structures will also benefit, from aeroplanes to spacecraft. The future possibilities for these smart materials look to be almost limitless. 

References


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