Smart Coatings with Carbon Nanoparticles

*Xoan Xosé Fernández Sánchez-Romate, Alberto Jiménez Suárez and Silvia González Prolongo*

## **Abstract**

Smart coatings based on polymer matrix doped with carbon nanoparticles, such as carbon nanotubes or graphene, are being widely studied. The addition of carbon nanofillers into organic coatings usually enhances their performance, increasing their barrier properties, corrosion resistance, hardness, and wear strength. Moreover, the developed composites provide a new generation of protective organic coatings, being able to intelligently respond to damage or external stimuli. Carbon nanoparticles induce new functionalities to polymer coatings, most of them related to the higher electrical conductivity of nanocomposite due to the formation of percolation network. These coatings can be used as strain sensors and gauges, based on the variation of their electrical resistance (structural health monitoring, SHM). In addition, they act as self-heaters by the application of electrical voltage associated to resistive heating by Joule effect. This opens new potential applications, particularly deicing and defogging coatings. Superhydrophobic and self-cleaning coatings are inspired from lotus effect, designing micro- and nanoscaled hierarchical surfaces. Coatings with self-healable polymer matrix are able to repair surface damages. Other relevant smart capabilities of these new coatings are flame retardant, lubricating, stimuli-chromism, and antibacterial activity, among others.

**Keywords:** polymer nanocomposite, structural health monitoring, self-heating, self-healing, anti- and deicing

### **1. Introduction**

Smart coatings are special covering materials which are able to sense and respond to an external stimulus. They are made with programmable materials, which respond to changes in light, chemical, thermal, or other stimuli. This brings them new performances, typically self-healing, self-cleaning, self-sensors, etc. due to their piezoelectric, thermoelectric, piezoresistive, and chemical properties (**Figure 1**). Most of the current smart coatings are based on nanoreinforced polymers. The incorporation of functional organic and inorganic nanofillers usually improves the thermal and mechanical properties of polymers, providing them new functionalities. As it is well-known, one of the main advantages to add nanofillers is their high specific area, which reduces significantly the nanofiller content and enhances the load transfer from the matrix, when the interface is suitable.

In this work, we focused on the addition of carbon nanoparticles, mainly graphene (G), graphene nanoplatelets (GNP), and carbon nanotubes (CNT). They

Carbon nanofillers must be dispersed on polymer or prepolymers depending on the polymer nature. The dispersion of nanofillers on thermoplastic polymers often carries out during the polymer manufacturing process, as extrusion or calandering. However, the nanofiller dispersion on rubber and thermosetting polymers is usually carried out in a previous step of curing process into

monomers or prepolymers. In this last case, different dispersion techniques can be also applied, based on the application of mechanical forces or an electric or

coating, applying different common **processing techniques of coatings**. Cold spray process is commonly used for processing polymer nanocomposite coatings, avoiding the thermal deterioration of substrate. Dispersion, emulsion, and latex in

situ polymerizations are other applied manufacturing processes.

**2.2 Properties of nanocomposite polymer coatings**

As it was mentioned above, the second step consists on manufacturing the own

Graphitic nanofillers are often used to improve the **mechanical properties** of polymer coatings. The poor tribological performance of polymer coatings can be improved by adequately addition of graphitic nanofillers into the matrix because graphite is a solid lubricant. Polymer coating containing graphene can present excellent tribological properties, with low friction coefficient and reduced wear rate [7]. The increment of graphene content gradually decreases both friction coefficient and wear rate of composite coating. Under high temperature, graphene-reinforced thermosetting coatings show better friction reduction and wear resistance than neat coating. The values of these properties are enhanced by the increase of graphene content. Meanwhile, the friction coefficient and the wear rate of the graphene/ composite coatings do not show a clear tendency with the increase of temperature. This behavior could be explained by the formation of a transfer film on the surface, which suppresses the huge heat and contact pressure [7]. CNT/polymer coatings can induce anti-friction, wear-proof, and self-lubrication performance [8], reducing the friction and improving the wear resistance. However, numerous factors affect their tribological behavior, such as the composition and properties of sliding pairs, such as their surface roughness and main mechanical properties (hardness, stiffness, and fracture toughness) and the sliding parameters, such as load, speed, temperature, and lubrication state, among others. This behavior is explained by the different involved mechanisms: bridge crack of CNT and lock the propagation of cracks, lubricant effect by dislodgement of individual graphene layers, strengthening of reinforced polymer matrix and dissipation of heat, and reducing the temperature induced wear [8]. It is worthy to note that there is an optimum carbon nanoparticle content to achieve the best tribological properties. However, this value depends on many factors such as aspect ratio of nanofiller, the dispersion degree and orientation of nanofillers, and the interactions with polymer matrix at

The incorporation of carbon nanoparticles into polymer composites also increases their hardness. Increasing nanofiller content leads to improvement of

One of the most important applications of polymers reinforced with graphitic nanofillers is as **anticorrosive coatings**. The anticorrosive coatings can be classified

hardness; however, the slope of the curve is reduced as the amount of graphitic nanofiller increases, which is attributed to agglomerations in the

in accordance to the protection mechanism against corrosion [10]: barrier protection, cathodic protection, anodic passivation, electrolytic inhibition, and

magnetic field.

*Smart Coatings with Carbon Nanoparticles DOI: http://dx.doi.org/10.5772/intechopen.92967*

interfaces.

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composite coating [9].

active corrosion inhibition.

**Figure 1.** *Summary of smart coatings with carbon nanoparticles.*

have extraordinary electrical and thermal conductivity and a unique combination of mechanical properties with great stiffness and high toughness [1–6]. They are composed of carbon, exhibiting low toxicity and environmental friendliness. For all these reasons, they are considered as multifunctional fillers of polymer matrix. In fact, polymer nanocomposites reinforced with carbon nanoparticles usually present enhanced mechanical, electrical, and thermal properties together with new performance as smart materials.

They can act as strain sensors due to their piezoresistive behavior, varying the electrical resistance of composite induced by the deformation of the electrical network formed by graphitic nanofillers. On the other hand, the nanofillers can be used as actuators, for example, as self-heater due to Joule's heating or as chemical absorbents. In this case, the matrix is a neat stimulus-responsive polymer, while the carbon nanofillers provide the stimuli to induce the polymer response.
