Coating Properties of Alkyd Resin, Epoxy Resins and Polyurethane Based Nanocomposites: A Review

The uniqueness of alkyd resin, epoxy resin, and polyurethane nanocomposites has brought prominent recognition to the field of heavy-duty coating materials. This is expected due to the collaborative features of nano-sized materials such as the high surface area to volume ratio, great functionality per-unit space, extremely small sizes with high density, and that of alkyd resin, epoxy resin, and polyurethane (biodegradability, great gloss retention, adaptability, flexibility, durability, good drying properties, and weathering resistance). The objective of this review was to analyze the extent and currency of research and the development of alkyd, epoxy, and polyurethane nanocomposites in coating applications. Some of the several types of modifications discussed in this review are the incorporation of varying types of nanoclay and metal nanoparticles materials into alkyd resins, the incorporation of carbon nanotubes, MGel-graphene oxide (GO)/gelatin (MGel), Ni (II) Complex-Zeolite and starch-modified nano-ZnO into epoxy resin and the incorporation of (Rb 2 Co(H 2 P 2 O 7 ) 2 .2H 2 O), modified nanoparticles of ZnO, diminished graphene oxide (dGO) into polyurethane and their effects on coating applications. The various studied modifications resulted in nanocomposite end-products with much improved properties. However, there are several challenges to the development of nanocomposites that need urgent attention. Some of the challenges discussed are the difficulty involved in transforming fabricated nanocomposites from laboratory to commercialized scale, the capital-intensive nature of synthesizing large nanopowder, etc. increase in the nanoparticle content leads to a corresponding shift in the characteristic temperatures (glass transition temperature-Tg, reaction temperature-Tr) towards a lower value. Mechanical analysis exhibited satisfactory elasticity with an impact resistance of 1 kg/cm2 and bending of f=2cm. The wear resistance changed reasonably as the nanoparticle content increased. This study was able to establish a more satisfactory abrasion resistance for nanocomposite coatings with SiO 2 nanoparticles when correlated to nanocomposites containing the same quantity of TiO 2 . They fabricated remarkable soy alkyd-based nanocomposites (NCs) using TiO 2 NPs surface adjusted with various gallates and imine acquired oleylamine and 3,4-dihydroxybenzaldehyde. This study is different from the work of Grozdanov et al (2019) because of the addition of imine to the nanocomposite. The surface-modified and Unmodified anatase TiO 2 NPs were examined using X-ray diffraction (XRD), electron microscopy (TEM), ultraviolet-visible (UV-Vis) spectroscopy and Fourier transform infrared spectroscopy (FTIR), while the adsorbed ligand amounts were estimated from the thermogravimetric analysis (TGA) outcomes. Surface modification of the TiO 2 NPs was confirmed by FTIR and UV-Vis spectra. The impact of the TiO 2 surface modification on the TiO 2 NPs dispersion in the alkyd resin, barrier, thermal, chemical resistance, and mechanical features of alkyd resin/ TiO 2 nanocomposite coatings were also investigated. The glass transition temperature of all the examined composites was seen to be lower than the unadulterated resin. The introduction of TiO 2 NPs to the surface altered with gallates had no meaningful impact on the thermo-oxidative of coatings were achieved using the basket milling approach with zirconium balls and ten characterized using thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), and contact angle measurements. Enhancement in the anticorrosive features confirmed using accelerated salt spray tests based on 4628 degradation of coatings (EIS). self-cleaning effects Diverse studies have described the modification of graphene oxide with epoxy resin for applications on anticorrosion. Rajitha et al. (2020) reported the anti-corrosion action of epoxy nanocomposites, comprising modified MGel-graphene oxide (GO) and gelatin (MGel), against steel. They showed that the excellent distribution of GO into epoxy resin matrix develops the cross-linking and strength of epoxy resin, leading to a development in the protection efficiency of newly generated nanocomposites by 59%, compared to the unmodified coating. The thermal stability of MGel-GO/epoxy composite was observed to be higher than GO without modification. It was attributed to the conversion of unstable OH groups of GO into a stable structure by interacting with MGel. The exceptional physical and chemical characteristics of nano-ZnO have made its utilisation as a second phase in the composites coating more attractive for numerous researchers (Ng et al., 2017).

Polyurethanes (PUs) are segmented polymers made up of a soft segment (SS) and a hard segment (HS). The SS, generated by a polyol, inflicts flexibility to the polymer, whereas, the HS, formed by a chain extender and an isocyanate, provides the material with rigidity and strength (Santamaria-Echart et al., 2015). Polyurethane can be utilised in diverse applications such as the generation of foam, seals and gaskets, insulation, suspension components, wheels and tires, surface coatings, adhesives, and biomedical devices (Jing et al., 2015). Nonetheless, like other elastomers, alkyd resin, epoxy resin and polyurethanes (PUs) struggle to attain combinations of features such as high strength, high durability and high elasticity concurrently. As a result, the incorporation of nanomaterials into the aforementioned polymers has emerged as a potential reinforcement approach to alleviate these restrictions, resulting in the enhancement of anticorrosive, chemical resistance, drying, electrical, mechanical, thermal properties and so on (Saralegi et al., 2013). The properties of the aforementioned polymers can be deliberately tuned to possess specific desired properties by incorporating nanosised or other materials during or after their synthesis to achieve specific application. The speedy growth of nanotechnology has led to the notable enhancement of the properties of polymeric materials with a concurrent drop in the cost of the final product. It has been established that nanomaterials maintain chemical and physical characteristics that are precisely different from their bulk counterparts (Kamat, 2002).
Several researchers have investigated the impact of several nano-scale fillers incorporated in alkyd resins, epoxy resin and polyurethane on the properties of their respective nanocomposites.
The objective of this review was to analyse the extent and currency of research and development of alkyd, epoxy and polyurethane nanocomposites in coating applications. Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)

ALKYD RESIN CLAY NANOCOMPOSITES COATINGS
Nanoclay materials are optimised nanoparticles of layered mineral silicates, with diverse modification characteristics (Majid et al., 2020). Owing to their high aspect ratio, possibly exfoliation qualities and more reliable mechanical properties, they have become one of the most sought-after reinforcing fillers for composites among other nanoparticles (Rajitha et al., 2020).
Nanoclay is made up of about 1 nm thick alumina surface of silicate layer, stacked with around 10 nm in diameter multilayer stacks. Hence, it has a specific surface area of approximately 657m 2 g -1 and an exceptional aspect ratio (Majid et al., 2020). The fact that nanoclay filler dramatically facilitates the strengthening and alteration of the mechanical characteristics of fibre reinforced polymer, when nanoclay was completely dispersed in epoxy resin composites has fascinated several researchers (Majid et al., 2020). Lin et al. (2008) synthesized biodegradable glycerol-derived alkyd resins from glycerol and maleic anhydride via the polycondensation reactions (Scheme 1). Thereafter, they successfully generated alkyd resin clay nanocomposites by melt blending organo-clays with maleic anhydrideglycerol precursors (Scheme 2). Before usage, the clays were treated with methyl tallow bis-2hydroxyl ethyl ammonium chloride salt (yielding a nanocomposite designated clay30B), and some samples of the clay30B were further treated with the diglycidyl ether of bisphenol A (DGEBA) (clay30BT) (Scheme 3). For comparison, resin mica and resin talc nanocomposites were fabricated similarly. XRD and SEM analysis revealed that the further treatment of clay30B with the DGEBA led to further delamination of the organoclays, which were mainly exfoliated and well dispersed in both series of alkyd resin nanocomposites leading to a reasonable advancement in thermostabilities (Lin et al., 2008). Lin et al. (2008) were able to establish the comparable nature of the mechanical properties, including tensile modulus, toughness and Young's modulus to those of the corresponding neat alkyd resin thermoset even at very low loadings. Some issues like the formation of aggregates in the case of talc or mica in the polymer matrix were recorded, even though there were improvements in the mechanical properties of the synthesized glycerolanhydride alkyd resins. Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)  alkyds were prepared with coconut oil fatty acid (COFA), (PA), glycerine (G), phthalic anhydride, organoclay and dipropylene glycol (DPG). The organoclay (nano clay) used in this study was modified with 15-35wt.% octadecylamine and 0.5-5 wt.% aminopropyltriethoxysilane. However, the chemicals employed in this modification were quite different from pre-treatment chemicals used by Lin et al. (2008). They also went further to subject the modified alkyd resins to a curing process at 140 o C for 2 hr, using different ratios of melamine-formaldehyde and urea-formaldehyde resins.
Experimental analysis recorded an improvement in the physicochemical characteristics of the cured modified alkyd (Bal et al., 2010)). Properties such as hardness, the drying degree, abrasion resistance, adhesion strength, acid, water, solvent resistance, alkaline and resistance to environmental conditions drastically improved. This study was able to establish that the blending of the urea-formaldehyde resin and organoclay can be depended upon to improve the physical and chemical resistance of the alkyd-amino resins. Tahmaz et al. (2015)

ALKYD RESIN METAL NANOCOMPOSITES COATINGS
The most promising strategy for ensuring carbon steel pipelines protection is to inhibit corrosion before it happens, and organic based-coatings have long been employed for that purpose (Vakili et al., 2015). Organic coatings have been used in the protection of carbon steel pipelines and other Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)  materials against corrosion or degradation by initiating a physical obstacle that stops water, oxygen and corrosive ions from getting to the pipelines surface .
The two primary disadvantages of organic coatings are weak adhesion and coating permeability (Deyab, 2015). The all-around performance of the coatings is heightened by the adhesion of the organic coating and decreased resin pore channels. This supplementary shielding is commonly accomplished by integrating nanostructured sized pigments, which leads to diminished enhanced coating adhesion and coating permeability (Zhang et al., 2016).
In modern times, many efforts have been invented to improve the corrosion protection of coatings were achieved using the basket milling approach with zirconium balls and ten characterized using thermogravimetric analysis (TGA), fourier transform infrared spectroscopy (FTIR), and contact angle measurements. Enhancement in the anticorrosive features was confirmed using accelerated salt spray tests based on ISO 4628 degradation of coatings evaluation and electrochemical impedance spectroscopy (EIS). In addition, mechanical and physical investigations were run in accordance with the standard test strategies for coatings. The outcome of the study revealed that cerium oxide nanoparticles inflict UV defensive, anticorrosive, hand self-cleaning effects and outstanding physical resistance to the generated alkyd coatings. The positive influence of cerium oxide nanoparticles was seen to be more pronounced than those of their microparticles counterparts.

COATINGS BASED ON EPOXY RESIN NANOCOMPOSITE
Anti-corrosion coatings have been generated successfully from the epoxy resin in several industries (Deyab et al., 2018;Sumi et al., 2020). In current studies, it was obvious that the exceptional anti-corrosion performance of the epoxy resin was influenced by the functionalisation of carbon nanotubes (CNTs). This assertion was also presented by Deyab and Awadallah (2020).
It was demonstrated that the epoxy/f-MWCNTs nanocomposites can preserve steel tanks from corrosion. The reduction in the agglomeration of CNTs inside the epoxy resin is due to the introduction of functional groups on CNTs surfaces and consequently, this enhances the mechanical and anti-corrosion characteristics of the nanocomposites coating.
The covalent link that is generated between the functional groups of the polymeric matrix and the CNT surface, is the principle that explains the precise description of the mechanism of stabilisation of CNTs dispersion inside the polymeric matrix by utilising functional groups.  coating was observed to be in the order: Mg0.5VOPO4 Zn0.5VOPO4 Ni0.5VOPO4. The net outcome from this investigation is promising for prospect works on the anti-corrosion coating.

COATINGS BASED ON POLYURETHANE NANOCOMPOSITE
Polyurethane coatings have several excellent characteristics, such as not being flammable, high wear resistance, high abrasion resistance and waterproof. The protection of metal from corrosion using nanocomposite coatings based on polyurethane has been attempted by several researchers   . Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)   The obtained nanostructure coatings are simple to design, eco-friendly, and property tunable. The outcome of their study reveals that the vertex group of POSS had a prominent impact on the interaction degree and level of dispersion between POSS and polyurethane that satisfactorily adjusted the discharge routine and duration of coated urea, even when the coating rate was as low as 2 wt.%. The liquid POSS with flexible and long PEG groups had more suitable dispersibility and compatibility in polyurethane matrix than the solid POSS with tough octaphenyl groups, as confirmed by SEM/EDS (Fig 4). The observed outstanding features were ascribed to the various degrees of physical crosslinking. The authors, therefore, concluded that the modification of biobased polyurethane coating with POSS offers an alternative route to controlling and regulating the features of coated fertilizer. Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)

Transforming Fabricated Nanocomposites from Laboratory to Commercialized Scale
At the commercial level, the huge-scale fabrication of polymer nanocomposites is a problem due to the difficulty in their generation (Viswanathan et al., 2006;. In addition to the fabrication difficulty, suitable selection of fabrication techniques and operating conditions should also be put into consideration by research scientist. As a result of this, various parameters must be defined such as interactions between the nanofiller and polymer matrix, the dispersion and stability of nanofiller in the polymer matrix, polymer chain flexibility, surface charge, the crystallisation ability of the nanofiller and surface chemistry.

Generation of Nanopowders
The feedstock materials of most nanocomposite fabrication techniques make use of nanopowders.
Despite the development of nanopowders over the last decade, it is yet to reach the maturity to generate a hefty quantity of nanopowders at an inexpensive price for their successful consolidation into nanocomposite (Rivas et al., 2018). There is an urgent need to create techniques which are cost-effective and also able to produce large amount of nanoparticles.

Handling of Nanopowders
The ultrafine powders have high surface activity due to their high surface area, and thus are vulnerable to contamination (Viswanathan et al., 2006;Ifijen et al., 2018;Ifijen et al., 2019a;Ifijen et al., 2019b;Ifijen et al., 2019c). The physical, chemical, and mechanical properties of the endproducts are easily compromised by the existence of such surface contaminants. As a result, better ways of storing and handling nanopowders should be investigated to prevent any contamination so that the advantages that comes with generating 'nanoscale materials''' can be harnessed to the maximum level.

Fabrication/Consolidation Techniques
The focal point of nanotechnology is dependent on the possibility of creating nanostructured materials that bring into being properties with novel characteristics in the 'macroscale' ( techniques and also to find out some innovative means of nanopowders densification.

Interdisciplinary Effort
The well-known function of nanotechnology for societal gain is stoutly based on the dynamic involvement of all disciplines of science and technology at all levels (Viswanathan et al., 2006;Omorogbe et al., 2019;Omorogbe et al., 2020). To fully comprehend this astonishing, but inadequately explained technology, the expansion of a cross-functional scientific workforce that transcends the predictable limits of a variety of disciplines is necessary. The development and implementation of new paradigms for training students and researchers of a new breed, who are not scared to cross-domains of several scientific disciplines needs to be carried out by academicians and administrators in the universities. .

CONCLUSIONS
This review has divulged current advances in the utilization of alkyd -clay-based nanocomposites, nanocomposite coatings based on epoxy resin and nanocomposite coatings based on polyurethane coatings applications. The alteration made on alkyd resins, epoxy resins and polyurethane over the years has helped to reduce the limitations of the aforementioned resins.
Physical-chemical properties like drying properties, coating performances, anticorrosive properties, resistance to chemicals and thermal stability of the studied resins have experienced drastic developments in current times. This review has confirmed that the potential of nanotechnology in coating applications is boundless and encouraging. Further research on alkyd resin, epoxy resin and polyurethane nanocomposites is recommended to further enhance properties such as adhesion, drying time, gloss retention, resistance to scratch, chemical resistance capability in various corrosive environments, thermal stability, flexibility, roughness, hardness, abrasion resistance and mechanical strength, etc.

ACKNOWLEDGEMENTS
The authors wish to appreciate the Almighty God for the knowledge that made this study possible.
We also wish to acknowledge Mrs. Precious Olohi Ikhazuagbe Ifijen (Wife of the first author), Ikhazuagbe H. I., Nyaknno U. U., Gregory E. O., Eribe M. J and Esther U. I (MEJS)  Mr. and Mrs. Godfrey Ifijen (Parents of the first author) and Prof (Mrs) E.U. Ikhuoria (Mentor of the first author) for their continuous support.

CONFLICT OF INTEREST
Authors declare that there is no conflict of interest.