Advances in self-healing intelligent coatings: concepts, working methods and applications

1 Self-healing coatings the production of
coatings as a polymer-based composite materials due to its light and strong, excellent electrical properties, good corrosion resistance, good electrical properties and other advantages in the construction, aerospace, transportation, electronics, sports and military supplies and other fields widely used. However, one of the main shortcomings of polymer-based composites is that they are susceptible to shock and damage during processing and use. In addition to material damage caused by strong shocks, it is more common for materials to have micro-damage (micro-cracks), which are often visually difficult to detect
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. At this point, the surface of the material may not see anything unusual, but the strength of the material has been greatly reduced. Micro-cracks cause the mechanical strength of composite materials to decrease, polymer materials once the production of micro-cracks, the integrity of the material is seriously damaged, and even lead to the overall destruction of the material.
on the other hand, organisms are damaged and have self-healing functions, and this self-healing function of biological materials will be of high application value if they can be re-used in composite materials. Inspired by this, the scientists built a self-healing model of the material, so that damage to the material, especially those that cannot be detected internally, can be repaired to a certain extent without the use of an added repair material, which is important for maintaining the mechanical strength of the material, eliminating hidden dangers, and extending service life. The concept of self-diagnosis, self-healing intelligent materials was proposed by the U.S. military in the mid-1980s and quickly became the focus of national research.
application of self-healing technology to the field of coatings, i.e. the production of self-healing coatings. The so-called self-healing coating, that is, the coating is damaged after the self-healing function, or under certain conditions with self-healing function of the coating. In recent years, coating technology and the development of materials science are closely linked, all kinds of functional coatings with the continuous progress of materials science continue to emerge, in this context, self-healing coatings in theoretical research and practical applications have been rapid development, this paper mainly on the basic principles of self-healing coatings and some applications to do a review.
2 Theory of self-healing coatings
Foundations
Self-healing coatings can be broadly divided into three types according to the repair process: (1) repair agent release type self-healing coatings; Among them, the self-healing coating of the refrigerant release type mainly includes: (1) the self-healing coating of microcapsules, (2) the self-healing coating of the core/hollow fiber, and (3) the self-healing coating of the redox reaction.
and reversible chemical/physical reaction self-healing coating mainly includes: (1) reversible reaction self-healing coating (Diels-Alde reversible reaction);(2) shape memory self-healing coating (mainly for polyurethane). At present, the rapid development of self-healing coatings is mainly based on one or several of the above principles, this paper respectively on the above-mentioned coating self-healing principles, and their application to do some introduction.
2.1 microcapsules self-healing coating
self-healing system can be divided into buried repair and in-place repair according to the different action process. In-place self-healing system has strict requirements for substation structure and only for special substations. Buried restoration is a relatively early development, research is more in-depth. This method is to bury the repair agent in the coating substation, receive changes from the outside world, release the repair agent, and by the capillary phenomenon causes the repair agent to spread in the substation, and finally bond the crack according to some mechanism.
most classic buried self-healing coating in the world, the most mature self-healing coating based on microcapsules technology. Microcapsules technology is a composite technology with nuclear shell structure formed by coating a stable polymer membrane with a layer of solid or liquid particle surface bread. Microcapsules technology will self-healing microcapsules buried in the substrate, after the substrate produces micro-cracks, the microcapsules buried in the inside of the substrate by external force rupture, release the core material, in the siphon effect core material filled with cracks in the reaction to complete the self-healing process. White, for example, has designed classic microcapsules for self-healing composites. As shown in Figure 1, the gibbus catalyst that makes the monosome polymerization can be repaired by dispersing in the substation by using a deformed resin as a coating in a microcapsules to repair monoceptodylene (DCPD). When the crack produced by the composite material under the external force develops to the surface of the microcapsules and makes the microcapsules rupture, due to the siphoning effect of the capillary tube, the repair monolith is filled with the micro-crack inside, and when the developing cracks touch the catalyst distributed in the substitut, the repair agent has an open-loop polymerization reaction, thus repairing the cracks and restoring the material properties to a certain extent.
2.2 Core/Hollow Fiber Self-Healing Technology
The advantages of self-healing coating of microcapsules make the self-healing system develop rapidly, but the self-healing coating of microcapsules also has some disadvantages: due to the micro-fixing agent covered with repair agents Capsules and the main resin or coating for the non-erjoic phase system, when the addition of microcapsules less, can not achieve repair effect, so it is necessary to increase the use of microcapsules, but when increasing the use of microcapsules, microcapsules added to the coating body performance is actually reduced. In this context, another repair agent release type self-healing coating has been came, which can effectively increase the use of repair agents without reducing the performance of the main coating. This self-healing system is a core/hollow fiber self-healing coating. Its principle is to compound in the polymer substate with adhesive core or hollow fibers, in the resin substation to form intelligent bionic self-healing neural network system, when the material cracks, part of the core or hollow fiber, adhesive liquid flow into the crack, so that the damaged area rehealsed.
Bond and his collaborators used bionics to prepare a hollow fiber containing un cured epoxy resins and curing agents and planted them in the main epoxy resin (figure 2). The test results show that when the coating is damaged, the coating can maintain 97% of the material's initial bending strength after repair of hollow fibers covered with repair agents. In the experiment, the UV fluorescent material is mixed in the repair agent, and the release and penetration of the repair agent can be clearly detected.
2.3 Redox reaction self-healing coatings
steel plate, galvanized steel plate, galvanized plate or aluminum and other metal plate materials are widely used in automotive, building materials and home appliances and other fields. In order to prevent corrosion of steel, it is generally necessary to passivate the steel.
, the passivation layer of galvanized steel plate is also a self-healing coating, in general, the passivation layer is an inorganic composite coating containing chromium. After the damage of the coating, the 6-price chromium in the passivation layer has a redox reaction, generating 3-price chromium, forming a new dense coating, which plays a self-healing role. The self-healing process is shown in Figure 3.
, however, the 6-price chromate contained in chromate passivar is a highly toxic and carcinogenic substance with mutageng effect. In recent years, regulations such as RoHS, promulgated by the European Union, clearly restrict or prohibit the use of six substances, including Cr6 plus, in products. In order to meet the needs of environmentally friendly galvanized surface treatment steel plates, countries are actively exploring and studying new surface treatment methods to replace the 6-price chromate passivation process, and the passivation of galvanized layers is developing in the direction of environmental protection and chromium-free. The development of new environmentally friendly passivation treatment fluids with excellent performance has become an important development direction for the galvanized steel plate pre-treatment industry. Environmentally friendly passivation process is studied more in foreign countries, and some processes are applied. According to the composition of environmentally friendly passivation liquid, the current galvanized surface chromium-free passivation technology is divided into two categories: (1) inorganic passivation;
organic-inorganic composite passivation is a passivation method developed gradually in recent years, and it is also the development direction of chromium-free passivation technology in the future. This method is mainly specific organic matter and chromium-free metal salt compounding use, more than 3 parts: the first part is silane-type coupled agent, can increase the resistance and tightness of the coating; After the destruction of environmentally friendly passivation coating, the alternative chromium metal salt in the passivation layer as a repair agent of chromium-free metal salts has a redox reaction, forming a new oxidation layer and completing the self-healing effect. The principle is shown in Figure 4.
2.4 Reversible Self-Healing Coatings
Recent studies have found that coatings can use some reversible reactions for self-healing. In this type of reaction, diels-Alde reversible reactions (Dils-Alder reversible reactions, referred to simply as DA reactions, as shown in Figures 5 and 6) can be used as classic reversible reaction coating reactions due to their excellent thermodynamic recovery properties. In general, the monosome is added to a group of functions (e.g. a structure of adjacent double bonds such as furan or malayamide) and then a polymer is formed by a DA reaction. Such polymers break down into their monosome forms under heating conditions. Under cooling or other conditions similar to previous synthesis, the anti-DA reaction restores the polymer to its original form.
self-healing of coatings using reversible reactions is a new technology developed in recent years, which is simple to operate and can be repaired repeatedly many times. When a crack occurs locally in the material, the reversible reaction of the DA can be achieved by heating to a certain temperature, thus realizing the automatic repair of the material. The method is easy to operate, without the need for catalyst, repair agent or monolith, can be directly through heating to achieve the recovery of the material co-price key, and because the reaction is reversible, can be repeated repair, and repair efficiency is high, after 2 fracture repair, the fracture strength can still be restored to the initial 78%. However, the method is still in the initial stage of research, the theory of repair is not perfect, and further research is needed. This method provides a new way of thinking for self-healing technology.
2.5 Shape Memory Self-Healing Coating
Polyurethane-based materials are strong and flexible by the presence of two types of chemical bonds. Rigid co-priced bonds are responsible for hardness, and weak hydrogen bonds are reversible, providing elasticity. This is due to the polyurethane material having a property called "shape memory". What happens when a groove is formed on the surface of the polyurethane material? It causes physical deformation of the polymer chain. The co-price bond remains stationary while the hydrogen bond weakens. The material breaks the balance in this way in a tense state. If the deformed material is provided with energy, the material is heated so that the whole system is re-molecularly available and returned to its original configuration. The polymer relaxes again and the chain begins to move, just like muscle, returning to the state it was in before the material was destroyed. At present, the temperature suitable for self-healing coatings is 50 to 60 degrees Celsius. In recent years, Scientists at Bayer Materials Technology have been working on self-healing automotive coatings, acrylic polyurethanes for special molecular structures, and have applied some self-healing products to automotive OEM coatings, including metal and plastic substrates (as shown in Figure 7).
3 Self-Healing Coatings
Currently, self-healing coatings are mainly used in the fields of concrete coatings, anti-corrosion coatings, automotive coatings and 3C products. Here's a brief introduction.
3.1 Concrete coating
Cement concrete has gradually developed towards high strength, high performance, versatility and intelligence, and the concrete structure built with it tends to be large and complex. However, concrete will inevitably produce micro-cracking and local damage, such as micro-cracking of the substation and other micro-range damage, repair is also very difficult. If these damaged areas are not repaired in time, they will cause irreparable damage. In the past, the maintenance of concrete was mainly through after-the-fact maintenance and timed maintenance, with the advent of intelligent self-healing coating, research and development of new cracks self-healing concrete coating, so that it can independently repair the damaged parts of cement concrete has become one of the development trends of concrete coating.
researchers in South Korea are working on a self-healing protective coating that can be used as a solution to the problem of concrete cracking, which is not expensive and is environmentally friendly. Concrete is still an irreplaceable building material worldwide, but cracking and corrosion of concrete, especially reinforced concrete, caused by cracking, have been hot issues around the world. The concrete's self-healing protective coating encapsulates the material that heals the cracks in microcapsules, which break when the concrete surface cracks, and the material inside the capsule flows into the cracks, which are stimulated and cured whenever sunlight shines on the concrete surface (figure 8). The researchers call this process the photo-induced self-healing system because it does not require catalysts, is environmentally friendly and inexpensive, and is a practical way to repair concrete cracks.
Tian Fenglan and others proposed a self-healing polymer cement waterproof coating, including liquid part A and powder part B, in terms of quality ratio of 1: (1.0 to 1.2). Among them, the liquid fraction A by mass score includes: ethylene-ethylene acetate co-polylactic emulsion 15% to 50%, acrylic emulsion 35% to 65%, acrylic resin 0.2% to 0.8%, water 14.20% to 19.80%. Powder part B in mass fractions includes: low alkali fast hard sulfur aluminate cement 19.50% to 37.50%, quartz sand 62.10% to 80.35%, hydroxypropyl methyl cellulose ether 0.05% to 0.5% 15%, water-reducing agent 0.05% to 0.10%, desiccant 0.05% to 0.15%, the waterproof coating provided by the present invention in addition to having strong bonding, high stretch strength, good impermeability, not easy to crack characteristics, for small cracks have self-healing function.
3.2 Metal anti-corrosion coating
In order to inhibit white rust on galvanized steel plate, chromate passivation treatment on the surface of galvanized steel plate is widely used in steel enterprises. The treatment is actually a self-healing coating. At the same time, in order to replace the harmful to the environment and human health chromium passivation, after recent years of research, chrome-free surface treatment products came out. Chromium-free passivants can form covalent bonds through chemical reactions and metal surfaces, and form a dense layer of corrosion protection, while increasing the adhesion of metal and organic coatings. When the coating is damaged, some of the chromium-free metal salts in the chromium-free passivation layer will be resolubleed in water, migrated to the coating damage, and the oxidizing chromium-free metal salts will react with the substrate at the damage, forming a new chemical transformation layer, which will play a role in easing and preventing chemical corrosion. Domestic has a number of large-scale coil companies push