Polymer Coatings Based on Nanocomposites
Uliana Licea-jimenez, Ulises Antonio Mendez-Romero, Abraham Mendez-Resendiz, Arturo Roman Vazquez-Veiazquez, Ricardo Antonio Mendoza-jimenez, Diego Fernando Rodfguez-Dfaz, and Sergio Alfonso Perez-Garcfa
Centro de Investigacion en Materiales Avanzados S.C., CIMAV
Based on the fact that coatings are present all around us, and due to the high demand for new materials and new technologies in our daily life that allow humankind to have comfort and a good quality of life, it is necessary to develop multifunctional materials, considering production processes that are environment-friendly. Such is the case for coatings, which are important for changing the characteristics of surfaces and providing new properties and functionalities. There is an ongoing trend of requests for coatings that cover several different functionalities at once; examples are combinations of extreme wetting behavior, special optical properties, and high mechanical robustness.
In this framework, nanotechnology has opened up new and promising possibilities for the development of coatings with functional features. In particular, the development of polymeric nanocomposites (consist of a reinforcement, mainly NPs and a polymer matrix) has become an opportunity to meet the demands of functional materials.
Polymer nanocomposites offer the possibility of new materials with a unique manifold of structure-property relationships. Current interest in nanocomposites has been generated due to the functionality that can be given by the appropriate combination of nanomaterials, resulting in properties that are not achieved with traditional materials.
Polymer coatings based on nanocomposites provide a real opportunity to make a difference across a wide variety of applications and markets, the application of nanocomposite technologies to further enhance current commercial products or add completely new properties to an existing technology. These coatings can improve product longevity, surface strength, and product performance, while enhancing energy efficiency performance. Thus, properties with incredible practical applications for mechanical, optical and electronic products are expected.
Why are Nanocomposites Attractive for Polymer Coatings?
There is a long list of characteristics that justify the use of nanocomposites for polymer coatings. Some of them are listed below:
- • Because of the outstanding features of nanoparticles (NPs), advanced products and coatings can be produced by their incorporation into a polymer.
- • The large specific surface area of NPs allows them to have enhanced reactivity, superior absorption, higher solubility, lower melting point, and enhanced electronic properties, such as quantum effects found on particles with particle size <10nm (important for electronic and optoelectronic applications).
- • NPs are invisible to the human eye; when they are incorporated in a polymer matrix, they do not affect visible light transmission, making them ideal materials for advanced clear coatings.
- • No sacrifice in light transmission, which is critical in optical applications.
- • In general, the improvement of the mechanical properties of a polymer coating (such as scratch resistance, wear, and adherence)
- • Nanocomposites require very low loading of nanomaterial to yield significant improvement in mechanical and other desired properties.
General Aspects, Considerations, and Challenges
Coatings can be produced by the incorporation of NPs into polymer matrices, due to their characteristics such as particle size, surface area, and properties. NPs and nanomaterials are considered for use in selective coatings, self-cleaning coatings, coatings for energy applications, printed electronics, etc. To fabricate technologically relevant functional coatings, one needs to understand and control the interactions in different materials by manipulating interfaces at the nanoscale. As a consequence, it is very important to create an optimal NP-polymer interface through an appropriate nanometric scale design.
A proper incorporation of nanomaterials into the polymer is a key issue, since many of these properties are closely related to interfaces. Although there are significant advances in the manufacture of nanocomposite coatings, processing remains one of the challenges for the full utilization of the nanoscale properties of the particles. Also, one of the first difficulties is the dispersion of NPs. Consideration of the key issues enable a coating formulation with improved properties, as well as the optimization of the developed materials.
The development of an easy and innovative method to create and apply a functional coating is the main objective of various applications. The search for a high-quality and low-cost coating is a challenge for the application and transfer of technology, so the factors that directly influence the way to obtain the best properties must be understood. This type of coatings can be applied by different easy methods and economical materials such as sol-gel, immersion, spin coating, printing, and others. The selection of the appropriate method to create a functional coating depends on the mechanical and physical-chemical properties of the substrate used. Preparation of the substrate is one of the most important parameters in the procedure for coating application, and its results directly affect the quality of the final product. In this way, some threads represent additional costs to the general process; however, they represent improvements in the quality of the final product.
The performance evaluation of coatings is an important point to consider, together with its formulation. The characterization of materials and the analysis of the properties of the coating play a very important role.
The design of new functional coating materials, the characterization of novel and tailored physicochemical properties, and enhanced processing capability are among the most crucial challenges related to achieving coating products with smarter, stronger, and more durable characteristics.
The formulation strategy of polymer coatings based on nanocomposites depends on the application and different approaches can be followed. However, some general facts are as follows:
- • Often NPs require functionalization (seldom used as prepared) and formulation before their integration into coatings.
- • Both aqueous and nonaqueous solvents are used to disperse NPs.
- • Often the type of coating dictates the chosen solvent.
- • In most cases, nanocomposite coating formulation consist of dispersed particles, dispersion media, and other additives.
In Figure 3.1, a proposed approach to achieve a specific and functional polymer nanocomposite coating is presented.
As it has been mentioned, coatings can be described by their function. In the following sections, functional polymer coatings based on nanocomposites are described.
FIGURE 3.1 A schematic approach for a polymer coating based on nanocomposites.
Self-Cleaning Coatings Based on Polymer Nanocomposites
Nowadays, the development of self-cleaning coatings has gained importance in the field of research, caused by the multiple commercial applications that can be achieved. Automotive, building, aerospace, and energy sectors are some of the fields where selfcleaning coatings have been used; these are mainly deposited on glass materials like windows, mirrors, lenses, and optical and outdoor applications (Cedillo-Gonzalez et al. 2014).
Preparation of a polymer coating based on nanocomposites incorporating NPs (metal oxides, carbon-based, among others) allows improvement and confer new characteristics to the polymer coating. Such nanocomposites have the potential to be used in various applications, including interior and exterior of buildings, houses, textile, glass, ceramic, plastics, wood, as well as different kinds of transportation vehicles and structures (Ganbavle et al. 2011; Guldin et al. 2013; Wang et al. 2013).
These novel coatings are an alternative solution to specific requirements, for example, scratch resistance, self-cleaning, anti-icing, self-healing and antireflective properties, and anticorrosion (Davis et al. 2014; Niu and Wang 2009; Li, Du, and He 2010).
Wettability of coatings is a feature that has received interest for technological applications due to its unique superhydrophobic and superhydrophilic characteristics, such as self-cleaning, antifouling, and fluid drag reduction. Self-cleaning coatings have the ability to be cleaned out through natural processes (Viswanadam and Chase 2012; Decker et al. 1999). A hydrophilic surface possesses a large affinity to water, forming a layer of water that avoid the incrustation and deposition of dust, dirt, organic matter, and other different kinds of impurities. In a hydrophobic surface, there is water repellence forming water droplets on the surface. These waterdrops roll on the surface of the hydrophobic coating, taking dirt with it. The self-cleaning property is related to the contact angle of a water droplet to a solid surface (Figure 3.2). When the water contact angle is less than 90°, the surface can be classified as hydrophilic; when the water contact angle is more than 90°, the solid surface can be considered as a hydrophobic surface; and if the contact angle is above 150°, the solid surface is superhydrophobic. Accordingly, if the contact angle is near to 0°, the solid surface can be considered superhydrophilic (Bhushan and Jung 2011; Zhou et al. 2015; Lee et al. 2016).
FIGURE 3.2 Water contact angle classification.
The nanocomposite materials that can be used for self-cleaning coatings are those containing carbon-based nanomaterials and metal oxide NPs as reinforcements. Some aspects that should be considered are as follows: (1) the dispersion of the nanomaterials in the polymer matrix, that is, the nanoscale homogeneity of the materials, and (2) the functionalization of the nanomaterial to create an affinity between the reinforcement and the polymer matrix. The main carbon-based materials that can be used as reinforcement are graphene-derived materials, such as graphene oxide (GO) and reduced graphene oxide (rGO) (Velasco-Soto et al. 2016). On the other hand, metal oxide NPs such as Si02, Ti02, Ce02, ZnO, and A120, are just some examples of NPs employed for the development of polymer coatings based on nanocomposites. Natural polymers and synthetic polymers like acrylic, polyurethane, rubber, polyethylene, polypropylene, and different kinds of resins are the most commonly used polymers.
Self-Cleaning Coatings Based on Metal Oxide NPs/Polymer Nanocomposites
Usually, superhydrophilic and hydrophilic coatings present features like high surface energy, which means that water is going to spread out along the surface coating, generating low water contact angle. These coatings have a similar surface energy than water, creating a phenomena that keep clean the surface when it is wet (Nakajima, Hashimoto, and Watanabe 2001).
Various metal oxide NPs have been used for self-cleaning coatings, such as Ti02, Si02, ZnO, Ce02, and A120,. This kind of NPs show catalytic and good optical transmission values, among other properties, that can be used and applied for this purpose. In addition, this type of coatings can be applied in various easy methods and economical materials such as sol-gel, immersion, spin coating, printing, and others.
Due to the strong tendency of NPs such as metal oxides to agglomerate, homogeneous dispersion of these materials in a polymer coating is extremely challenging. In order to overcome this problem and to enhance the reinforcement-polymer interaction, a functionalization or chemical modification of NPs is always required. The formulation of novel materials with modified surfaces by organic and inorganic functional groups has gained technological importance.
As previously mentioned, nanocomposite coatings allow designing new functionalities and tailoring physicochemical properties. Thus, it can be considered the combination of NPs. In a study of a binary nanocomposite system based on covalently functionalized Ti02-Si02 (f-Ti02-Si02) with trimethylolpropane triacrylate (TMPTA) and
FIGURE 3.3 Water contact angles of (a) hydrophilic nanocomposite 0.1wt% f-Ti02Si02/ PAA and (b) superhydrophilic nanocomposite 0.5%wt% f-Ti02-SiO,/PAA. Reprinted with permission from Vazquez-Velazquez et al., “Functionalization effect on polymer nanocomposite coatings based on Ti02-Si02 nanoparticles with superhydrophilic properties”. Nanomaterials 8(6):369, 2018.
embedded in a poly acrylic acid (PAA), it was possible to produce a synergy between the constituents (Vazquez-Velazquez et al. 2018). This effect the wettability of the polymer nanocomposite achieving a hydrophilic (13°) and superhydrophilic (5°) coating. Figure 3.3 presents the water contact angles of 0.1wt% f-TiO,-Si02/PAA and 0.5wt% f-Ti02-SiO,/PAA; here is showm the concentration effect of the modified NPs in the acrylic polymer: change from a hydrophilic (Figure 3.3a) to superhydrophilic (Figure 3.3b) behavior, through an increase in the concentration of NPs from 0.1wt% to 5wt%, respectively. Coatings with water contact angles near to 0° are useful for designing self-cleaning coatings.
In self-cleaning coatings, the counterpart of superhydrophilic coatings are the superhydrophobic ones. A superhydrophobic coating can be understood primarily because it is governed by a process similar to the lotus effect; this feature allows the waterdrop to remove and to carry particles of soiling from the surface of the material, keeping the surface clean (Valipour, Birjandi, and Sargolzaei 2014). The growing need to achieve a self-cleaning property through a superhydrophobic coating has generated the development of different strategies. It is known that to obtain these coatings, there are key issues: roughness at a nanometric scale and NPs with hydrophobic features and good polymeric affinity. The proper interaction permits surfaces w'ith high water contact angle and the development of polymeric coatings- based nanocomposites. However, not all the materials present hydrophobic features, so chemical surface modification is required. These modifications can be carried on through a functionalization process, producing a chemical change on the material surface (Ma and Hill 2006).
As a case study, the development of a superhydrophobic coating through SiO, NPs chemically modified with a silane group (f-SiO^ and its incorporation in a polyurethane matrix is described. The incorporation of the f-SiO, improves the wettability properties of a polyurethane coating. The water contact angle analysis showed that modified NPs increase the contact angle of the polyurethane coating (Figure 3.4a) from 79° to 98° for a concentration of 50w't% of NPs and 10wt% of polymeric matrix in the nanocomposite (Figure 3.4b); w'hen the polymer concentration was 5wt%
FIGURE 3.4 Water contact angles of (a) polyurethane coating, (b) 50wt% f-SiO2/10wt%PU, and (c) 50wt% f-SiO,/5wt%PU.
(Figure 3.4c), the water contact angle reached a value of 137°. The superhydrophobic features display important perspectives to be implemented in different industrial applications, for example, in outdoor weather-proof coatings, glass, textile, windows, solar panels, mirrors, and automotive (Paul and Robeson 2008).
Polymer Coatings Using Carbon-Based Nanomaterials
Recently, polymer coatings that incorporate graphene (or derivatives) have gained a commercial attention due to market demand for stronger, more durable, and environment-friendly solutions. Due to its unique properties, for example, chemically inert, impermeability, friction, and good antiwear properties, among others, the use of graphene is growing for application in protective and/or functionalized coatings.
Graphite can be oxidized to obtain graphite oxide, which can be easily exfoliated due to the oxygen atoms that aid to separate its layers, obtaining GO (Paredes et al. 2008). Assisted by reducing agents, rGO can be obtained, being a material with properties similar to graphene, but with remaining functional groups that were gained by oxidation, facilitating the functionalization of this material (Fernandez- Merino et al. 2010) As mentioned before, to improve the dispersion, it is an important functionalization process. The chemical nature of the polymer must be known to find the appropriate functionalization agent.
For the formulation of a nanocomposite system using carbon-based nanomaterials in a polymeric coating, it is important to think about the final application. For instance, to develop a coating for outdoor use without affecting its optical properties, it is essential to have high resistance to abrasion and transparency to visible light. A suitable material for these functions is a polyurethane coating but with certain restrictions. Thus, a multifunctional polymer nanocomposite coating can be tailored. Recalling the critical aspects that have been already discussed, a possible approach could be as follows.
In order to achieve a good dispersion of carbon-based nanomaterials (GO and rGO), functionalization has to be done by choosing the proper coupling agent. Also, it is important to consider the correct solvent for both the functionalized nanomaterial and the polymer. In this case, both the GO and the rGO can be functionalized with a material containing long carbon chains giving a nonpolar character, which can be dispersed in organic solvents as well as in polyurethane.
In this coating, among other desired characteristics is its functionality, that is, being a self-cleaning or photocatalytic material. The use of additional nanomaterials can lead to obtain such characteristics. Ti02 is a widely studied material with interesting properties such as high photocatalytic activity, strong oxidation power, low cost, chemical and thermal stability, and a band gap (3.2 eV) ideal to be photoactive with UV radiation (Momeni, Ahadzadeh, and Rahmati 2016). To add this NPs as a reinforcement to the polymer nanocomposite could render photocatalytic properties, enhancing the self-cleaning.
Based on the latter, a polyurethane-based nanocomposite coating can be formulated, where functionalized rGO and TiO, NPs act as a reinforcement. So, functional coatings can be obtained with either hydrophobic or hydrophilic properties. A polyurethane coating has a contact angle near to 70°, but with the addition of the reinforcement, the contact angle increases close to 90°, starting to become a hydrophobic material. As it can be seen in Figure 3.5, by adding the reinforcement at different concentrations, the contact angle changes. With the addition of just 0.1% of this reinforcement, the contact angle approaches a hydrophobic behavior. A proportional increment of the contact angle with respect to the concentration of the reinforcement could be expected. But at 0.5% of the reinforcement, a slight decrease of the contact angle is reflected. This can be attributed to several factors; for instance, by adding more reinforcement, it becomes difficult to achieve a stable dispersion of the nanomaterial, which can lead to a nonhomogeneous deposit of the coating in the substrate. This will result in differences in the contact angle across the coating. Hence, optimization in the design of the nanocomposite is essential to achieve the desired properties, in this case for self-cleaning.
A self-cleaning coating can also have hydrophilic behavior, but in order to achieve this kind of coating, different approach needs to be used. Utilizing GO as a reinforcement could be a better choice, since it is inherently hydrophilic in character and the only modification would be the addition of TiO, NPs for the photocatalytic
FIGURE 3.5 Contact angle of hydrophobic coatings of polyurethane with reduced graphene oxide (rGO) and TiO, as reinforcement at different concentrations.
FIGURE 3.6 Contact angle of hydrophilic coatings of polyacrylic acid with GO and TiO, as reinforcement at different concentrations.
property (Oribayo et al. 2017). The nature of the polymer also needs to be hydrophilic, while giving optimal mechanical properties and being transparent. A polymer that can fit these parameters is polyacrylic acid. The hydrophilic nature of functional groups attached to the GO's surface permit to be dispersed in solvents such as ethanol (Othman et al. 2019). As previously mentioned, dispersion of the reinforcement affect directly the properties of the composite; also the concentration of the reinforcement affects the dispersion.
As shown in Figure 3.6, different behaviors are distinguished. At a concentration of 0.1% of reinforcement, the contact angle dropped until 11.7°. But, when the concentration increased until 0.5%, the contact angle drastically increased above 30°, which is even higher than the contact angle of the polyacrylic coating (around 22°). Then, at a concentration of 1%, the contact angle drops. This erratic behavior can be attributed to various factors, such as a poor dispersion of the reinforcement, poor cleaning of the substrate, deposition method, and contamination of the formulation. Therefore, it is important to highlight that the design of a polymer coating based on nanocomposites demands to consider the proper nanomaterials, solvents, functionalization agent, dispersion, formulation, and even the application method.