Self Healing Flashcards
(10 cards)
What are self-healing materials?
Self-healing can be defined as the ability of a material to heal (repair/recover) damages. It can be autonomic or non-autonomic, and extrinsic or intrinsic. Different strategies are used for polymers and composites, ceramics and concrete, and metals.
Non-autonomic are self-healing materials that require a modest external trigger (heat, light…), whereas autonomic self-healing materials do not require one; the damage itself is the stimulus for the healing. The detection of the damage (by a sensor) as well as the repair (by an actuator) proceeds autonomously within the material structure.
Extrinsic self-healing materials have a healing process based on external healing components (intentionally embedded into the matrix), such as capsule based or vascular.
Intrinsic healing materials feature no separate healing agents, but rather physical and chemical interactions (covalent or non-covalent bonds) between the existing material.
Self-healing is based on the same common general principle: the generation of a mobile phase, which can close the crack. As such, in theory, any material can self-heal.
Different types of damage may require different healing strategies, or even combinations of them, to achieve optimal healing.
What is healing efficiency?
Healing efficiency is the ability of a healed material to retain its original optimal properties. healing efficiency [%]=(healed properties)/(original properties)×100%.
Explain microcapsules:
Capsule based healing will have capsule of healing agent which subsequently interact with either a catalyst directly incorporated into the material, or secondary capsules containing the catalyst. These captions are solid emulsions, the preparation therefore requires considering phase separation. Some disadvantages include small amounts of healing agent, single healing event possibility, and brittle phase in the composite reducing mechanical properties post-healing.
Explain vascular systems:
Hollow fibers will contain healing agent/catalyst, activating upon rupture. There are mainly three possible approaches: hollow fibers filled with one part resin, hollow fibers filled with resin coupled with hollow fibers filled with hardener, and hollow fibers filled with resin and hardener/catalyst dispersed around the matrix. All approaches can be used in a 1D, 2D or 3D network, indeed they are inspired from the robust distributed vascular networks in biological systems. Variants such as a pressurized vascular system exist, allowing more efficient filling of cracks, but having the downside of requiring a pumping/pressure system.
Explain self-healing corrosion protection coatings:
An epoxy based vinyl ester matrix contains microencapsulated catalyst and PDMS based capsuled healing agent. Upon corrosion the capsules are ‘opened’, acting much like a normal microcapsule system.
Explain intrinsically self-healing polymers:
The healing ability is incorporated directly into the basic nature of the material. Reversible covalent bonds allow the polymer to revert back to simpler components and rebuild bonds as required. The mobility of the materials at crack faces is utilized to entangle polymer chains that span the damage. Alternatively, in the case of non-covalent bonds, reversible intermolecular interactions such as hydrogen bonding or ionic clustering are utilized.
Reversible covalent bonds include categories such as thermally remendable polymers, where no catalyst or embedded liquid agent is required. They work based simply on reversible covalent bonds such as Diels-Alder or Retro Diels-Alder. Tensile and compressive properties are comparable to epoxy resins. Another category is photo-healable polymers, based on polysulphides, which are photo-responsive compounds with repeatable self-healing capabilities due to the dynamic covalent reshuffling of polysulphides.
Explain supramolecular self-healing:
Supramolecular chemistry focuses on how molecules interact with each other through weaker, reversible, and dynamic interactions. In self-healing materials, supramolecular interactions act like a dynamic “glue” that can break and reform. When damage occurs (like a crack or scratch), these weak bonds break under stress or damage, allowing deformation or rupture, but then reform spontaneously when the damage is removed, restoring the material’s structure and function. This healing can occur autonomously (without any external stimulus) or be triggered by heat, light, moisture, or pH changes.
Advantages include reversibility of bonds that allows multiple healing cycles, healing that often requires mild conditions, tunable properties by molecular design and a compromise between mechanical performance with dynamic behavior. Limitations however are that supramolecular interactions are weaker than covalent bonds, so mechanical strength can be lower, healing might be slow or incomplete depending on the environment, and the fact that stability under long-term or harsh conditions can be challenging.
Explain ionomer self-healing:
An example of a supramolecular interaction would be ionomers, where even f the macro material is non-polar, nano-sized domains may have ionic clusters that serve as physical crosslinks between oppositely charged ends. Through temperature and pressure these segments may dissociate, allowing the polymer chain to move and fill the crack, only to re-form and restore the initial mechanical properties of the material upon cooling/resting.
See figure in doc.
Explain the importance of damage management in traditional vs self-healing materials:
See figures in doc.
Additionally, during the healing process the mechanical properties of the material will change.
What are some common pitfalls in designing the SH material?
If the particle is stiffer, softer, stronger or weaker the crack propagation will be different. Weaker particles attract the approaching crack in both stiff and soft particles, so this is an ideal compromise. One must also take note of existing pre-existing flaws, as interface flaws will have the crack move through them instead of going through the particle.
