Sustainable smart anti-corrosion coating materials derived from vegetable oil derivatives: a review

Sustainable development is a critical concern in this fast-paced technological world. Therefore, it is essential to employ renewable resources to move towards sustainable development goals (SDGs). The polyols attained from renewable resources, including lignin, chitosan, vegetable oils, cellulose, etc. and the polymers derived from them have attracted the attention of the majority of researchers, both in academia and industry. The development of bio-based polymers from vegetable oils start emerging with different properties to generate a value-added system. This review will give an impression to readers about how coatings generated from vegetable oils can find a way towards better protective properties against corrosion either by using fillers or by using molecular structure modifications in the system, thus covering a range of vegetable oil-based self-healing polymers and their application in anti-corrosion coatings.


Introduction
Since mid of the 1900s, petroleum-based polymers have been widely used in the coating industry; however, from the environmental point of view, it is high time we shi towards renewable resources. [1][2][3] The major challenge in today's emerging world is the replacement of conventional petroleumbased polymers with renewable polymers, where vegetable oils (VOs) are potential starting materials among other bio-based Poonam Singh is presently pursuing her PhD degree from University of Petroleum and Energy Studies (UPES), Dehradun, India. She has received her bachelor degree in Chemistry from Zakir Husain Delhi college (University of Delhi), New Delhi, India followed by Post Graduation diploma in Nanotechnology from Maitreyi College (University of Delhi), New Delhi, India. She obtained her Master degree in Organic Chemistry from RCCV girls college (CCS University), Ghaziabad, India. Her research interests include nano-materials, bio based green materials, self-healable vitrimeric coatings.  19 amidation and thiol-ene reaction is performed. [20][21][22] Among different vegetable oil-based polymers, polyurethanes are widely used due to their excellent physiochemical properties. 23 These properties vary with sources of polyols, preparation methods and functional groups present in resultant polymers. 24,25 To enhance the durability of polymeric materials, the addition of a self-healing function is highly advantageous. [25][26][27] Selfhealing can be generally classied into two main groups: intrinsic and extrinsic self-healing materials. These materials primarily differ in their mechanisms as well as their chemistries. While intrinsic self-healing materials are based on supramolecular hydrogen bonding interactions and reversible reactions. [28][29][30][31][32] Extrinsic self-healing materials require the Table 1 Density, iodine value, saponification value and molecular structure of some commonly used vegetable oils mbedding of microcapsules lled with healing agents in a matrix system or vascular networks. [33][34][35][36] The capsule-based self-healing system (an approach that releases the conned healing agent through the rupture of the microcapsule), and microvascular networks are the two pathways frequently used in the preparation of extrinsic self-healing materials. 22,[37][38][39][40][41][42][43][44][45] For the addition of self-healing properties, the presence of phenolic groups is advantageous in order to support the cross-linking, which results in a fast self-damage curation 9 therefore, some phenolic biomolecules known for their affinity towards the modication of vegetable oil-based polymers for coating applications. The -OH (hydroxyl group) containing molecules like avones, tannin, and avonoids also act as a suitable corrosion inhibitors. Taking Tannin biomolecules for this purpose provides adequate corrosion protection and self-healing abilities. Tannins are one of the components found in natural extracts and organic products. 46,47 However, polymeric coatings derived from vegetable oils perform poor chemical and thermal resistance as compared to petroleum-based polymeric coating. 48 To enhance the thermal and mechanical properties including preventing the moisture/water penetration, a wide range of nanollers have been incorporated into the coating matrix. 5,13,[49][50][51] Carbonized materials such as carbon nanotubes (CNTs), graphene, and mesoporous carbon structures have been attracted to researchers due to their chemical, thermal, electrical, and mechanical properties, thus, leading towards the enhancement of mechanical, anti-corrosion, antimicrobial properties. 10,46,[52][53][54][55][56] Carbonaceous hybrid nanostructure materials implanted with different functionalities enhance the selfhealing properties. Hydrogen bond donor-acceptor moieties bonded to ller walls show repetitive dynamic repair on damages due to hydrogen bond exchange in the polymeric network via click chemistry 30,35,57,58 vegetable oils derived metals containing polymeric coatings are considered to enhance anticorrosion and anti-fungal properties, whereas aliphatic triester moiety of oils provides hydrophobic nature and inherent exibility to the coating. [59][60][61] This review will give an impression to readers about how coatings generated from vegetable oils can nd a way towards better protective properties against corrosion.

Signicance and background
Although so many polymeric protective coatings are present in the market, approx. 3.6% of total GDP (total gross domestic product) is being spent on the maintenance of structural substrates and instruments caused by distruction due to corrosion. 62,63 Therefore, an intelligent coating able to repair itself and protect the assets from corrosion and improve durability is highly desirable. The whole idea of self-healing coating was inspired by nature, given how natural healing of wounds and cuts occur in living species. The era of self-healing materials is gradually settling in where the macro/micro-cracks in structural materials can close on their own without any external inuence, and the scratches on the designed materials can seal up with a complete restoration. In particular, self-healing coatings are helpful to reduce costs and labor associated with coating maintenance for offshore application, where inspection and maintenance are complex due to the location of the asset. However, due to geopolitical challenges, coating industry need to shi its focus from conventional petroleum-based polymers to bio-chemicals derived polymers. Lined up from the biomaterial to smart protective coatings, vegetable oil is worth to utilize for the synthesis of high-performance material, given that oils provide a wide range of fatty acids like oleic acid, linoleic acid, palmitic acid and stearic acid. Leaving an impression of interest, sustainable polymeric protective coatings have been in prime focus for corrosion inhibition of metallic surfaces and thus explored in this report. 6

Castor oil-based polymers
Castor oil, a readily available organic chemical can be extracted via solvent extraction or mechanical pressing. In the case of solvent extraction, the main constituent obtained is ricinoleic acid, which is present at around 90% of the whole oil and comes under the euphorbiaceous group. Other components, including linoleic acid, oleic acid, stearic acid, palmitic acid, etc., are present in trace amounts. 9,67 The presence of -COOH groups in castor oil facilitates the esterication and amidation, whereas double bonds proceed to hydrogenation and epoxidation. In addition, -OH groups promote acetylation and alkoxylation or dehydration, leading to the formation of a different range of polymers, including polyether, polyurethanes, polyesters, polyamides, etc. which is being discussed in detail in Table 2 along with their possible applications. To enhance coating properties, ller incorporation is highly advantageous. 68 Deewan and coworkers studied the effects of incorporating boron-based nanollers in polyester and polyurethane matrix during synthesis. 60 The reaction involves esterication of hydroxyl group of castor oil (BCPE) followed by polyaddition between BCPE and Toluene,2,4-diisocyanate (TDI). The coating results in highly effective anti-corrosion properties with no weight loss in the basic medium for 50 hours, whereas a loss in weight was observed in the acidic medium. Good thermal stability (∼220°C) and physio-chemical properties were observed for boron-polyurethane-based coatings. To understand the signicance of castor oil-based shape memory PUs, Karak and coworkers discussed the advantages of using sulfur containing reduced graphene oxide based coatings (SRGO). 69 The presence of functional groups on the graphene surface gives better dispersion properties in polymeric matrix than to pristine graphene nanosheet. In addition, poly(3caprolactone)-diol (PCL) and SRGO were mixed to get the nal hyperbranched PU/SRGO (HPU/SRGO) hybrid. Due to the strong interaction between pre-polymer and functionalised GO, improvement in thermal stability and mechanical properties    was observed. The presence of PCL helped to achieve excellent shape recovery and shape-healing properties (due to crystalline character) under sunlight and microwave. In addition, authors have studied microwave radiation-promoted SH composites. 70 The microwave radiation oscillates dipoles, thus helpful in transferring the energy into the polymer matrix. This absorbed energy is benecial to raise the matrix temperature above T g and consequently facilitates the repairing of reported coatings. The synthesis mechanism and exclaimed properties of nanocomposite are substantiated in schematic diagram in [ Fig. 1B and C].
Hyperbranched polyurethane iron oxide reduced graphene oxide (HPU-IO-RUO) nano-composites were prepared using different compositions of nanohybrids [ Fig. 1A]. The healing efficiency of coating was judged by cutting scratching the surface and calculating the recovery in tensile strength.
Urethane linkages can be obtained via reacting isocyanates and alcohols catalysed by tertiary amines (with low steric hindrance to facilitate the reaction), or Pb, Sn based catalysts, e.g. N,N-dimethyl cyclohexylamine or stannous octoate. 71 Reacting pre-urethanes along with another polymeric matrix results in the formation of interpenetrating polymer networks (IPN) possessing impressive thermal stability. By controlling the composition of isocyanates and polyols, the matrix demonstrated adhesive properties and is benecial for vast applications including coatings. Shahidul and coworkers have reported the TiO 2 dispersed hyperbranched poly-(ester amide)(HBPEA) nanocomposites derived from castor oil using A 2 + B 3 approach. 72 In this process, rstly, N,N-bis(2-hydroxy ethyl) castor amide (HECA) was synthesized by reacting diethanol amine and sodium methoxide at 110°C and the addition of castor oil was performed dropwise, which was further used for the synthesis of hyperbranched poly (ester-amide). The reported composite was found to have hydrophobic properties (due to the hydrophobic surface of TiO 2 increasing cross-linking density) with contact angle in the range of 89°to 107°, and thermal stability of 452°C.
Moving further, eucalyptus leaves were used to synthesize the carbonized nanoparticles as bio-resourced nano-llers. 10 These carbonised nanoparticles were obtained by burning the leaves in atmospheric oxygen and treating them with piranha solution, which leads to delignied carbon nanoparticles. Further, biobased hybrid polyurethane was synthesized by reacting polymeric 4,4 ′ -methylene diphenyl diisocyanate, carbonized nano particles (CNM-COOH) and 4-methyle pentene-2-one along with castor oil. The study found that with increasing the percentage of CNM doping, cross-linking power of urethane and thermal stability (399°C) was enhanced. The corrosion resistance of the composite coating was improved with E corr = 12.4 mV, I corr = 4.4 × 10 −4 A cm −2 , R p = 7.02 × 10 4 K ohm cm 2 and R corr = 5.1 × 10 −6 ohm on mild steel.
Kashif and coworkers investigated corrosion inhibition behaviour of castor oil-derived polyurethane, synthesised by reaction of the castor oil with diethanol-amine and sodium methoxide followed by the addition of toluene2,4-diisocyanate(TDI). 73 The reported polymer matrix possesses high hardness due to TDI (because of aromatic moiety), impact resistance 200 lb per inch, and corrosion inhibition efficiency of 94% & 90% in acidic and alkaline environments, respectively. In another study, it was observed that castor oil-derived alkyd resin has a good affinity towards corrosion inhibition. 74 In order to strengthen the anti-corrosion and hydrophobic properties; lotus leaves inspired self-cleaning coatings is highly advantageous. Wei and coworkers has developed anti-smudge bio-based polymeric coatings ncorporated with poly(dimethylsiloxane) (PDMS)). 75 Hyperbranched polyol was synthesised by a transesterication reaction between CO, 2,2-bis (hydroxymethyl)propionic acid (DMPA) and p-toluene-sulfonic acid (P-TSA) under nitrogen environment [ Fig. 2A(a)]. PDMS-OH was applied to the coating surface to provide the low surface tension and change in contact angle [ Fig. 2B]. A study found that it inhibits the deposition of dirt on the surface, and no trace of liquid was le behind. The reported coating was found to repel water, inks, and other organic solvents; thus, it can be used for anti-graffiti and anti-ngerprint purposes [ Fig. 2A(b)]. Mechanical and excellent adhesive properties were observed with considerable exibility and transparent, robust qualities in the coating.
Recently, there has been a high demand for electromagnetic interference (EMI) shielding materials. Jingu Lu and coworkers worked on castor oil-based polyurethanes coating lms with properties like self-healing and EMI shielding. 76 Castor oil was treated with Iso-phorone diisocyanate (IPDI) to synthesize the water-borne polyurethane network. Aerwards, titanium carbide was graer on the polymer moiety in different proportions along with 2-aminophenyl disulde (AD). The lm prepared was mechanically robust due to presence of Hbonding. Though ADWPU polymeric lm synthesised is very effective against electromagnetic waves, aer adding Ti 3 AlC 2 about 99.9% of waves were found to be blocked either by reecting, scattering or absorbing the radiations.

Soybean oil-based polymers
Soybean oil has majorly been used in the polymeric coating industry for last decade. In spite of high T g and strength-based challenges epoxidized soybean oil has remain magnicent starting point for the formation of polymers with diverse properties. 47 The properties exhibited and possible applications for a different set of polymers derived from soybean oil has been listed in Table 3. Bio-based acrylic monomers show strong hydrophobicity, which is benecial for the anti-corrosion coating application. 7 In the wake of exploring soybean oil for synthesising self-healing coating materials, Altuna and coworkers tailored the epoxy resin with gold nanoparticles. 24 In this study, citric acid was used to synthesise epoxy to synthesise an epoxy-based polymer network followed by the incorporation of gold nanoparticles, where gold NPs were prepared in situ through the Turkevich poly(vinyl-pyrrolidone) (PVP) medium. The prepared network is a non-toxic and biologically compatible polymer. The presence of PVP facilitates the linear alignment of the NPs in the polymeric matrix, which promote stress relaxation and self-healing properties of the nanocomposite. The tensile strength of the coating was quite good, and stress relaxation at 60°C and self-healing properties were well facilitated through localized healing with green laser radiation to ease the reach of thermal effect for the crack healing process. Thiol-ene click reactions are currently in vogue in the selfhealing coating industry as it is very economical to carry out at very high-speed kinetics. Prakash and coworkers 83 used a photo-induced thio-ene click reaction approach to synthesize the castor oil polyol(MCO) and soybean oil(MSO) polyol in the presence of DMPA and ME followed by the formation of thermoplastic PU from MCO and MSO using chain extender 1,4butanediol (BD) and dibutyltin dilaurate (DBTDL). 24 The prepared thermoplastic PU was found to have excellent transparency, elasticity and high mechanical strength. Altogether, Kocaman suggested the use of coconut-based bio-llers to enhance the coating properties. 82 Acrylated and epoxidized soybean oil (AESO) based epoxy resins can be fused with bio-llers generated from grinded coconut waste (CW) particles. Coconut waste particles were functionalized with poly(hexa-uoro butyl acrylate) (PHFBA) which is hydrophobic in nature. This modication in polymeric matrix benets the material with several properties like water repellence, high thermal stability, ammability, good acid and alkali resistance which exhibit anti-corrosion properties. The results show that with high cellulosic content, the ammability of the material increases as lignin content is high. Prakash and coworkers suggested that soybean polyols can be prepared through a thioene reaction between soybean oil and 2-mercaptoethanol (ME) in the presence of DMPA and THF. 83 The soybean-based polyol was further used for the formation of polyurethane coating by reacting it with toluene 2,4-diisocyanate (TDI) and DBTDL at room temperature. The resultant coating was found to have good anti-corrosion properties on mild steel panels (tests conducted in 3.5% NaCl and 0.5N HCl aqueous solution). The coating provides excellent tensile strength and barrier properties against corrosion, due to notable high H-bonding in methane linkage between PU coating where no damage or fracture was observed on the coating surface. According to thermal studies, rst degradation was observed at 250°C and second degradation observed at 390°C due to breakage of ester linkage.
Generally, imine-based polymers show good mechanical properties and irretrievability, where due to lower in bond energies and easy dissociation, H-bonds provide efficient selfhealing properties. Initially, epoxidized soybean oil was treated with trivanillinyl-phosphate (TVP), which was further used to prepare the multilayer-CNT (MWCNT) composites. With the increase in TVP concentration H-bonding abundancy increases while stable imine bonds go down, which results an increase in cross-linking, thus enhances the rigidity of polymer. With the elevation in temperature multi-exchange of network come in existence to achieve self-healing and re-processibility. With a low TVP concentration polymer show rubbery character above T g , however, a further increase in TVP concentration T g increase from 40°C to 75°C. 84 Vitrimeric polymers can be synthesised from the epoxidized oil in the absence of the catalyst. Liu and coworkers developed catalyst-free epoxy based vitrimeric network using soybean oil and 4,4'dithiodiphenylamine(APD) as curing agents [Fig. 3A]. The polymer composite shows excellent tensile strength, reprocesibility bonding strength and super high stretch-ability [ Fig. 3B]. 85 Kamal and coworkers produce three different types of novel polyester resins by reacting mono-glyceride (from soybean oil) with phthalic and maleic anhydride. The monoglyceride was obtained by glycerolysis of oil via reversible ester interchange reaction. 86 Yuan and coworkers have reported the synthesis of lignin and soybean oil-based azide-functionalized sustainable elastomers via controlled thermal azide−alkyne cycloaddition (TAAC) [Fig. 4a and c] along with the exibility and appreciable mechanical strength prepared elastomer exhibit excellent elasticity with T g less than 5°C [ Fig. 4b and d] 50 . Alongside this, a catalyst-free bio-based vitrimer from soybean oil was done by Guillermina and coworkers. The thermal and mechanical behaviour of the material was found to be controlled by the content of the precursor used. 87 Despite limited application due to water resistance, methacrylate gelatine can be used in bio-coating to improve moisture resistance. Being able to perform photo-crosslinking reactions and be prepared from renewable resources, methacrylate gelatine is found to enhance hydrophobicity and mechanical properties. Single layer application is reported to be very useful and spontaneously stratifying into layers, whereas multilayer application is quite complex, expensive and energy consuming. Self-stratication is blending incompatible polymers to form a collective multilayer structure. Sahar and coworkers reported  that to enhance the mechanical properties and breathability, this self-stratifying light curable AESO-based coating material could be formulated. 88

Linseed oil-based polymer
Due to its rich inherent properties, linseed oil is one of the highly utilised vegetable oils in the coating industry. Linseed oil has been widely used among researchers because it is ecofriendly, economical, available, and suitable to form the polymeric network. 79 Drying oil Linseed oil-derived polymers are known for their self-healing abilities, and the recent advance in properties and applications for the same has been discussed in Table 4. Due to the drying ability of linseed oil has been used as a healing component within the microcapsules in self-healable polymeric coatings. However, temperature curing is one of the major challenges with the vegetable oil-based polymeric coating. Manawwer and coworkers has synthesized a linseed oilbased polyester-amide for anti-corrosion applications. 95 The reported coating prepared was cured at the ambient environment, which possess appreciable anti-corrosion and physiomechanical properties. Initially, linseed oil was used to form N, N-bis 2-hydroxy ethyl linseed oil fatty amide diol (HELA), followed by the formation of hydroxyl-terminated polyester-amide (Ed-PEA) by treating with ethylenediaminetetraacetic acid (EDTA). The resultant material was cured while reacting with poly (styrene co-maleic anhydride) (SMA) at ambient temperature. Polyester-amide coating was thermally stable upto 150°C and possesses signicant corrosion inhibiting properties. Subsequently, polyester amines have also been known for their uses in anti-bacterial and anticorrosion coatings. To improve the anti-bacterial properties, durability and anti-corrosion properties, H. Abd El-Wahab and coworkers modied polyester amine with N-phthaloyl-glutamic acid. 96 In this study, linseed oil and diethanolamine (DEA) reacted to get N,N-bis-(2-hydroxyethyl)linseed oil fatty acid amide (HELA) which was made to treat with phthalic anhydride (PA) and N-phthaloyl-glutamic acid (NPGA) (synthesised in situ) to obtain modied resin for coating. The reported coating exhibits notable chemical resistance and anti-corrosion properties along with excellent mechanical properties. The coating was tested to have less permeability for water, oxygen, and other ions to pass, owing to enriching the adhesion of the hydroxyl group connected to metal surfaces. Although bio oil-based nanocomposites are potential alternatives for replacing petroleum coatings, advancing the properties of composite polymers can do wonders to address the issues like corrosion, optical transparency, and exibility. Shabnam Pathan and team attempted to address these noted issues by tailoring the linseed oil-based water-borne alkyd resin with 3-iso-cynato-propyl-triethoxy silane (IPTES) and di-methyl-tin-di-neo-decanoate. 97 In nanohybrid composite, exibility is due to the synergistic reinforcing effect and merging of organic and inorganic phases of the system. Shabnam Pathan and coworkers used waterborne linseed oil-based alkyd composites to improve the mechanical properties, 98 where silane-based coupling agents have been used to address the limitations and escalate the interaction between two phases. Cross-linked polymer coatings derived from linseed oil using melamine formaldehyde (MF) and 3 iso-cynato-propyl tri-ethoxy silane (IPTES) exhibits great mechanical and anticorrosive performance [ Fig. 5A]. It was found that the prepared coating exhibits remarkable adhesive and barrier properties against chemical environment and water along with high thermal stability which comprises to corrosion inhibition [ Fig. 5B and C].
In another approach, Mohamed and coworkers introduced Cu 2 O nano-cube based ller into alkyd resin to achieve high thermal stability (∼324°C) along with better mechanical and corrosion inhibition properties. 79 The hyper-branched polyalkyd resin was synthesized by adopting A 2 + B 3 approach, where glycerol and adipic acid were reacted in the presence of dibutyltin oxide (catalyst). The prepared materials were and further treated with linseed oil fatty acid and P-TSA as catalyst under poly-condensation reaction. The resultant resin incorporated with Cu 2 O nanoparticles was used for coatings, where coating possess excellent corrosion inhibition abilities, signicant mechanical features as well as stability against chemicals. For further advancement of the study in vegetable oil-based alkyd resin Deepak M. Patil and coworkers synthesised epoxidized alkyd resin from linseed oil using itaconic acid for the application as anti-corrosion coating. 99 It was a two-step process, where linseed oil was converted into mono-glyceride via an esterication process. Furthermore, the unsaturated binds present in the alkyd moity were converted into oxirane and later to epoxidized alkyd resin. The resultant polymer was modied by using 3-amino propyl-trimethoxy-silane (APTMS) to obtain a coating with good mechanical and adhesive properties. It is due to its ability to form H-bonding with the metal surface. The coating was able to create a good barrier against acid alkaline medium and corrosion and showed increased thermal stability up to 247°C and high cross-linking density.
Regarding microcapsule-based matrix, incompatibility of microcapsules with polymeric metric results in poor mechanical properties of the coating. 100 To address these issues, Mirabedini and coworkers have modied the microcapsules with 3-aminopropyltrimethoxy silane coupling agent to improve the surface interaction. 101 Microcapsules generated from polyurea-formaldehyde lled with linseed oil were found to exhibit self-healing properties. The reported polymeric matrix shows improved tensile strength and anticorrosion properties. To create a coating with enhanced anti-corrosion and self-healing properties, Sehrish and coworkers made an attempt to incorporate of talc nanoparticles (TNP) and urea-formaldehyde micro (UFM) capsules [ Fig. 6A]. 102 Talc falls into the category of mineral clay having corrosion inhibitor properties, composed of magnesium silicate [ Fig. 6B]. The authors observed that composite exhibit excellent thermal stability and self-healing properties. When any damage occurs on the coating surface, it makes microcapsule breaks due to mechanical stress and linseed oil releases[ Fig. 6C]. Linseed oil works as a healing agent as it starts to cross-link when it interacts with air and forms a protective lm with 99% healing efficiency. In combination with NaNO 3 , this lm inhibits corrosion from spreading on the substrate surface with a corrosion inhibition efficiency 99%. Likewise, to extend the lifetime of the oil-based polymeric coatings. David and coworkers used acrylated epoxidized linseed oil as the base matrix. 103 Linseed oil was converted into acrylated epoxidized linseed oil and copolymerised with dihemiacetal ester (1,10-dibutoxyethyl sebacate (DBES)) and 3,4-dihydroxyphenetyl acrylamide(DHPA) for adding desired cross-linking property to the polymeric matrix. The corrosion protection by the coating on the surface was found to be appreciable, as DBES addition to the matrix results in an increase in hydrophobic property and high thermal stability, however, it does not lead the matrix towards barrier properties. Though interesting barrier properties were observed in absence of DB.

Other vegetable oils
Due to the difference in their functionality and degree of unsaturation present in their molecular structures, vegetable oils exhibit their unique features and qualities to deliver specic properties, as discussed in Table 5. In the coating network, drying time and durability of the matrix have quite an important roles, which varies oil to oil. Characteristics of neem oil are full of surprises as it delivers an enormous number of properties to the polymeric matrix. Neem oils great potential in the coating industry as it contains anti-corrosion and antimicrobial and is highly available (in India). Counting on it, Chaudhari and coworkers studied self-healing poly(esteramide) synthesised from neem oil and their properties. 105 Neem oil underwent amidation reaction with a diethanolamine and PbO as catalyst to result into Azadirachta indica juss Fattyamide (AIJFA), which was used to synthesis polyester amide.
Microcapsules were prepared using poly-amido-amine (PAMAM) and linseed oil as core material in it and further used for the development of polyurethane coatings. The prepared coating shows thermal stability up to 221°C, resistance against water and chemical interaction including good adhesive and anti-corrosion properties on the metal substrate.
To add on to green composite materials with improved properties, Manawwer and coworkers studied corn oil-based organic coating using isosorbide. 106 Corn fatty amide reacted with isosorbide in the presence of sulfuric acid as catalyst to get poly(isosorbide-ether-amide) (PIEtA), which was further converted into PUIEtA/fused silica nanocomposite. Prepared silica based nanocomposite manifests excellent anticorrosion properties and barrier against chemicals with the notable thermal stability of 275°C and increased adhesion up to 98%, impact resistance with corrosion inhibition efficiency 99%. In addition,  authors have investigated corn oil-based poly(urethanemalonic-ester-amide) network to improve the properties of the matrix by incorporating multi-walled carbon nanotubes (MWCNTs) as ller. 107 N,N-bis(2-hydroxyethyl) corn oil fatty amide (HECA) was reacted with malonic acid in toluene to obtain malonic polyester-amide(MPEA) followed by the preparation of poly(urethane-malonic-ester-amide) using iso-phorone diisocyanate (IPDI) which nally get incorporated with MWCNTs. The integration of CNTs in polymeric network increases contact angle (110°) and provides hydrophobic nature to it, making it suitable for anti-corrosion coatings with excellent thermal stability and mechanical performance. Aung and coworkers suggested that ZnO can be used as a nano-ller for corrosion inhibition in polymeric coating. 108 To synthesize the epoxy acrylate of jatropha oil (AEJO), epoxidized jatropha oil was made to react with triethylamine, 4-methoxyphenol and acrylic acid, which is further processed to prepare the composite by reacting AEJO with reactive diluent tetra-methyl-ol-propane triacrylate (TMPTA) and photo-initiator 2-hydroxy-2methylpropiphenone, followed by the addition of ZnO as nano-ller in different compositions. These nano-hybrid resins possess properties to protect the metal surface from corrosion owing to the hydrophobic nature of ZnO. Incorporating a 5% weight% ZnO loading showed signicant enhancement in corrosion resistance and coating performance. Tung oil is being used in polymeric coating industries for its positive impact as ller inside microcapsules. A study by Gonçalves and coresearchers reported that composites prepared from poly(urea-formaldehyde) positively impact corrosion inhibition and self-healing properties. 109 Zinc phosphate is being used as a corrosion inhibitor as the hydroxyl group in the base network reacts with zinc phosphate and results in an even more stable zin oxide lm, helping to protect the substrate from corrosion.
Along with all the properties of the coating, the thickness of the coating is also essential to study in polymeric coatings. Usually, in self-healing polymeric coatings, large microcapsules facilitate the release of a good amount of healing agents, which results in a thick coating. In comparison, a thin application of the coating is enough to protect the metal surface. Aiming toward the characteristic of diameter, Li and coworkers studied the tung oil microcapsules subsumed in epoxy matrix [ Fig. 7A(a)]. 110 Tung oil-based microcapsules were prepared in situ using formaldehyde for the formation of shell and urea, resorcinol and ammonium chloride [ Fig. 7A(c) and (d)]. Selfhealing coating embedded with tung oil microcapsules was investigated, and found that a decrease in adhesion of coating was found, but the barrier ability of the coating was good along with self-healing and anti-corrosion properties [ Fig. 7B and C].

Summary and future prospectives
In view of the governing procedures and pressing need to reduce CO 2 -emissions, the perspective of using biobased materials in the coating applications is attractive. The use of bio-chemical derived molecules mainly aims to reduce the number of synthetic molecules, however, achieving material performance similar to the commercial coating product represents a signicant challenge. Certainly, in spite of great economic and ecological values, vegetable oil based polymeric coatings are facing numerous challenges in making place in industrial market, 114 e.g. polymeric network oen possess low crosslinking density which lead towards lower in mechanical properties and solvent resistivity, due to the triglyceride long chain structure of vegetable oil based polymeric moiety, though it also offers exibility to the network compared to the petroleum based polymeric networks. 5,20 These drawbacks can be overcome through the addition of suitable crosslinker as well as by addition of llers. In addition, to avoid the environmental oxidation and physical stability, vegetable oil based self-healing materials can be used in encapsulated form for developing selfhealable polymeric coatings. But with encapsulated networks there are limitation in terms of reusability, re-processibility and low mechanical properties. 115,116 As the low thermal stability of the VO based coating network ultimately lead towards the lower shelf life and hence the early degradation of the network, which can be answered by using the various llers in the form of graphene based nanosheets, metal nanoparticles or metal-legend composites. 117,118 On the other hand, vegetable oil based self-healing networks require particular conditions to reprocess and to reect the characteristic behavior (like high temperature and high pressure), which is not feasible to provide at industrial level. 100,119 To address such issues various techniques can be applied in case of advanced materials (e.g. vitrimers), where instead of conventional thermal heating electromagnetic radiation or induced heating can be used to attain the energy, required for the dynamic bond exchange reaction in order to facilitate the self-healing property at different working environment. 100,118 To acquire the radiation heating of the system, the chemistry can be obtained with the presence of carbonaceous materials or the molecules with dielectric constant. In consideration of bio-integrated electronic devices, longer healing time of the cracks could result into failure of the device, apparently rapid healing of the network at ambient temperature would be required to avoid the damage. 120 Overall, bio-molecules derived coatings can be advantageous when factors like the availability of the biobased additives and their true sustainability are considered. In addition, sometimes designing desirable molecules from petrochemicals is difficult, thus, availability of new bio-molecules/monomers opens the door for researchers to construct novel molecules difficult to be obtained from petrol-chemicals. Along with this, a bio-material design is made from scratch, allowing them to plan material properties in line with their future life cycle from the very beginning, thus, these functional materials could nd versatile application by collaboration of chemist and industrial researchers.

Conflicts of interest
There are no conicts to declare.