Review
Recent progress on the metal-organic frameworks decorated graphene oxide (MOFs-GO) nano-building application for epoxy coating mechanical-thermal/flame-retardant and anti-corrosion features improvement

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Highlights

  • The GO and MOF-filled composites are widely applied because of their fantastic mechanical/thermal, and barrier properties

  • Mixture of MOFs/GO reinforces the polymers' physicochemical properties that enable them to be used in a wide application ranges.

  • The impacts of GO decoration by MOFs on the epoxy mechanical, anti-corrosion, and anti-flame properties were reviewed

Abstract

Epoxy resins are the most prominent thermoset material, which has been extensively applied in a diversity of industries including aerospace, coatings, electronics, and biomedical systems. Due to the shortcomings of epoxy, the desire to use nanocomposites has been increased and become a hot research topic. The GO and MOF-filled composites, due to their fantastic mechanical, thermal, and barrier characteristics, have provided crucial insights for decades. Additionally, although the inherent defects of GO and MOF particles have vastly bounded their constructive applications, presently, the GO-modified MOF particles have been prospered. The mixture of the porous MOFs and GO nanosheets reinforces the physicochemical properties that enable them to be used in a wide range of fundamental technologies. We will entirely summarize the recent advances of MOFs decorated GO nanohybrids in the epoxy resin, and their effects on the constructed composite coating's mechanical & thermal, anti-corrosion, and flame-diminishing properties. Reports evidenced that the introduction of GO and MOF-based nanohybrids into Epoxy resin has provided central insights for decades because of their outstanding mechanical, thermal, and barrier characteristics. The plus and minus of GO and MOF nanofillers can be avoided and boosted by modification of GO via MOFs, respectively. A high reduction of peak heat release rate (p-HRR) up to 41% and the total smoke production decrement up to 30% were reported by the addition of GO-MOF nanohybrids to the EP in comparing with GO-EP nanocomposite, proving the improvement of the flame-retardant properties of epoxy resin. Through screening the literature it is obvious that through the introduction of GO/Ln-MOF nanoparticles into Epoxy resin the anti-corrosion stability of the epoxy resin was greatly improved and reached the 500% level. The adhesion strengths were enhanced up to 100%. In addition, the storage modulus ratio improvement percentile would be about 20% and the energy at the breakpoint was improved by 275%.

Introduction

Epoxy resin is regularly used as a thermosetting resin in the vast majority of fields such as adhesives, electronic materials, aerospace industry, biomedical systems, and coatings because of their exceptional adhesion to many substrates, high electrical resistance, good mechanical properties, easy processing, and good heat resistance [1], [2], [3], [4], [5]. Epoxy resin is capable of reacting with functional amine-based curing agents that provide coatings with superior properties, i.e., high strength, and good chemical resistance. Numerous nanofillers have been used to promote the mechanical, anti-corrosion, and thermal properties of the epoxy matrix, notably, to address the big weaknesses such as poor impact resistance, poor toughness, and high brittleness [6], [7]. In other words, the mentioned properties are relevant to the undesirable brittleness of epoxy resin, due to their highly cross-linked structure, poor impact strength, and delamination bring about the minimal application of epoxy [8].

Though there can be found a diversity of nanoparticles, the recent investigations have been put the spotlights on the nanofillers as their high specific surface area, functionality, and small size can make some extreme advancements in the properties at a lower amount of the particles along with good dispersion. The common nanofillers include graphene, graphene oxide, α-alumina, nanosilica, carbon nanotube, and layered double hydroxide [9], [10], [11], [12], [13]. In this review, the improvement of the anti-corrosion, thermal-mechanical, and fire-resistance properties of the epoxy coatings will be discussed.

It has been generally acknowledged that carbon-based nanomaterials like carbon nanotubes, active carbon, carbon nanofibers, graphene, and so forth play a more and more promising role to deal with the major challenges and make breakthroughs in practical application. Graphene as a two-dimensional atomic nanomaterial comprising of the sp2 carbon atoms has been known with a hexagonal honeycomb structure [14]. GO also, as a vital derivative of graphene, is rich in functional oxygen-based groups. A graphene sheet is bonded to the oxygen atoms including hydroxyl (single bondOH), carboxyl (Cdouble bondO), the carboxylic acid (single bondCOOH), and alkoxy (single bondCsingle bondOsingle bondC)-based groups relying on both the basal GO plane and the edges of the sheets. These oxygenated groups play a key role in achieving high water stability and the probability for surface functionalization, which have provided many opportunities to be used in nanocomposite materials. This highly oxygenated nanosheet consists of the hydrophobic and hydrophilic sectors including a huge basal plane and many edges [15]. In addition, the C atoms (being sp3 hybridized) of GO enhance the interlayer space, improving its ability to hold functional groups and functionalization of the surfaces. GO has drawn great interest in most of the applications including biotechnology and biomedicine [16], [17], [18], membranes and water treatment [19], [20], coatings [21], batteries [22], [23] Field-Effect Transistors [24], sensors [14], [25], solar cells [26], [27] and etc. GO shows good dispersion in a majority of solvents such as organic solvents and water due to the presence of many surface-bound functional groups and high surface area, [28], [29], [30], [31]; therefore, GOs illustrate chemical/thermal stability [32], electrical conductivity [33], molecular barrier abilities [34], high elasticity and flexibility [35], and corrosion resistance [36], as shown in Fig. 1. For many years, graphene oxide nanoparticles have been employed in the new polymer-based nanocomposites to achieve effective improvement of the properties. For similar reasons, the introduction of the filler into the nanocomposites can cause superior properties for the polymers, especially epoxy coating. GO is an ideal nano-filler for epoxy matrices owing to the fact that they have some similar oxygen-containing functional groups like epoxy and hydrogen groups to increase their compatibility perfectly. For instance, Jiang [37] fabricated GO/epoxy composite coatings and indicated that due to the high aspect ratio of GO sheets, the more tortuous path of diffusion for eroding medium and finally better anti-corrosion properties were displayed. In other word, the cracks and micropores of the pure EP allowed corrosive medium reaching the metal substrate in a short time, however, the GO sheets improve the barrier performance of EP coatings by obstructing their defects and pores, and hampering the penetration of corrosive media (see Fig. 2). Similarly, Zongxue [38] added GO-based nanofillers to the epoxy resins to remedy their shortcomings and enhance their resistance at a low content (2% nanofiller). Furthermore, GO had a great influence on the epoxy resin thermal resistance properties. The addition of GO (3 wt%) into epoxy resin accounted for considerable suppression on the fire risk of the epoxy matrix, decreasing the smoke production rate by half from 0.23 m2/s, and the heat release rate (HRR) from 727 kWm−2 to 367 kWm−2 [39]. Cone calorimetry is roundly used for checking out the combustion behavior of the materials to analyze the elements including the releasing rate of heat, smoke and toxic gas production. In another, GO as a flame-retardant additive, was introduced into an epoxy blanket to enhance the flame/smoke depression properties [40]. The char layers can prevent from decomposition of the materials and suppress liberating of the flame by acting as a barrier. The properties such as strength, viscosity and oxidation resistance, are the key factors to make a continuous and dense protective barrier layer. This study increased the dimensional stability of polymer via high melt viscosity of GO nanoparticle without melt spreading, leading to the reduction of contact area between fire and epoxy coating, which effectively covered and conserved the inner matrix without decomposition (see Fig. 3). Wang [41] evaluated the impact of GO on the mechanical-thermal properties of the neat epoxy coating. The outcome indicated the improvement in the tensile strength by 28% compared to the pure EP (1 wt% of nanofiller) because the nanofiller could restrict the molecular chain mobility of the polymer matrix. There is no denying that the generation of oxygen-containing groups result in making GO functionalize with most of the organic-inorganic molecules. On the other hand, GO sheets reveal a structure of close-packed layers arising from their intrinsic van der Waals' interaction [42], thus leading to poor dispersion and exfoliation in the polymer matrix. Hence, the insufficient hydrophobicity of graphene oxide nanosheets and therefore improper dispersion in the resin matrix is the most challenging issue in further application of this material. Ultimately, to overcome this challenge, much effort has been done and remarkable advancement has been accomplished in recent past years.

The universality and prominence of porous functional materials in the world are indisputable. Such materials have drawn widespread attention owing to their potential applications. Therefore, over the last two decades, the field of organic-inorganic hybrid materials for instance Metal-organic Frameworks (MOF) is undergoing continuous and rising growth. Their structures stem from the self-assembly of metal (as a connector) and ligand (as a link) via a metal-ligand bond. One of the most important features of the MOFs is that they enable to be used in a wide range of applications due to the possibility of designing the variety of pores with determined sizes (selecting ligands with determined shapes) with high surface area (commonly ranging from 1000 to 10,000 m2/g) [43]. Additionally, the determined Metal-organic Frameworks structures enable to study of the relationships between the property and structure of MOFs that are pressing for the workable design of new MOFs in the diversity of applications and processes such as sensing and gas storage [44], [45], selective adsorption and separation [46], [47], drug delivery, imaging and therapy [48], Magnetism [49], catalytic reactions [50], interlayer dielectrics [51] and etc. Meanwhile, MOFs have been introduced in all areas of chemical sciences that allow tailoring according to the specific requirements, as can be seen in Table 1.

More recently, the metal-organic-frameworks have become well-known among other nanoporous materials because of their immense surface areas and unique topology. Inducing MOFs into polymers such as epoxy resins brings to make most of both materials advantages: the crystallinity, porosity, and regularity of MOFs, along with the process-ability, chemical stability, and controllable structure of polymers. Diversity of MOFs, such as MIL-53 [52], ZIF 8 [53], [54], layered double hydroxide (LDH) [55], [56], and UiO-66 [57], [58] can be considered as potential fillers for the breakthrough of the thermal-mechanical, flame-retardant and anti-corrosion properties of the EP coatings. A usual way to attain new properties and develop new applications is by applying inorganic materials into polymers. Recent studies have shown that MOFs are given prominence in inorganic chemistry and also material science; in addition, they have the potential for prevailing over the flaw and limitations of polymers to improve their performance and get highly regular polymeric structural arrangements. To put it another way, MOF particles release some types of gases like NH3, which is likely to weaken the concentration of flammable gases. The performance of char can be demonstrated in Fig. 4. Firstly, the epoxy decomposition generates the final materials during combustion that are absorbed and reacted by MOF and its residue forms the skeletons of char. Meanwhile, not only does the porosity of MOF absorb the decomposed EP products, but also it performs catalytic oxidation reactions that lower the release of smoke and toxic gas. Thereby, improves the flame-retardant and smoke-suppression performance of the epoxy resin [59]. Employment of Sn-MOF in the epoxy matrix can reduce the peak heat release rate (p-HRR) and total heat release (THR) by 42% and 32%, respectively, and delay the production of smoke and carbon monoxide gases up to 66.7% during the combustion process [60]. Yang [61] investigated a smart dynamic polymer coating to protect the metal surfaces against the acidic environment by ZIF 7 MOF. Fig. 5 represents the self-healing procedure via MOF as described as follows; first and foremost, when the matrix of coating is damaged and the contact of the substrate is exposed in front of corrosive species like H+, then the pH located in defect areas decreases. Secondly, the BI molecules released from the ZIF 7 nanoparticles adsorb on the metal surface, hence, the uniform dissemination of the ZIF 7 can defer the corrosion operation, so cover the scratched areas thoroughly and effectively. Eventually, this system has the ability to interact and adapt to the acid environment through forming a novel barrier on the metal substrates with anticorrosive performance. In the other study, Manju [62] reported the increase of 230% in the fracture energy, as well as, the increase of 68% in impact strength by the loading of MOF-5 (0.3% w/w) into epoxy resin. MOF-5/EP composite demonstrated a strong interface among the MOF-5 particles and epoxy matrix, which drops the chain's mobility around the microporous powder with high surface area and perfect load transfer. Some impressive improvements of the epoxy resins properties by introduction of GO and MOF nanofillers are summarized as the Table 2.

However, the poor stability is considered as a weakness of MOFs such as low stability against water, moisture, acid/based materials, and weak chemical-thermal-mechanical stability. The stability of MOFs plays apart role in diverse practical applications. In other words, MOFs are vulnerable and prone against the shear loading, and so the most critical feature over evaluating the mechanical stabilities of MOFs is shear modulus that should be considered [69]. In spite of the uniform and ordered configuration of the framework-based materials, the MOFs incorporation into the polymer composites needs the fabrication of the close-packed dense structure by adjusting the pore-free volume. This structure is the key limitation for the formation of a predominant inhibitive, barrier, and mechanical-thermal impact [70]. To overcome the limitations of MOFs effectively and reach the great potential of MOFs in a large number of applications, the modification and functionalization can be considered as a perfect procedure.

Additionally, π–π stacking, the high surface area of nanosheets, and van der Waals forces cause the agglomeration of GO in the polymer matrix. This agglomeration causes a sharp loss in their desired properties and performance [71]. Generating the composites has been getting prominent to overcome the problems. In particular, for graphene oxide (GO), many endeavors have been made to fabricate GO sheets functionalized by metal-organic oxide (MOF) with a uniform MOF distribution to be used in many applications [72], [73], [74], [75].

Consequently, on account of the importance of GO and MOFs, in this review, the history of the inserting MOFs-modified GO nanosheets into epoxy coatings from the first advances up to the latest ones on the thermal-mechanical, flame-retardant, and anti-corrosion properties of EP have been summarized.

Section snippets

MOF/graphene oxide-based epoxy coatings

These composite-based materials are probable to receive the benefits of both parent components, along with getting rid of their respective weaknesses, which is vital to progress the overall stability of polymers.

Meanwhile, the compatibility of MOFs with various kinds of nanoparticles, metal complexes, organic dyes, polymer matrices, polyoxometalates, and small enzymes [76], [77], [78] is a result of having a wide size of porosity from microporous to macroporous. What's more, the organic ligands

Design and preparation of MOF/GO nanohybrids

The existence of different types of methods for the synthesis of MOF/GO composites demonstrates that different strategies are likely to affect the natural instinct of the interactions between the GO and the MOFs and thus the composite properties. Three synthetic routes are categorized as follows:

Applications

Recently, desirable aspects of graphene oxide and MOF hybrids mentioned above have whipped up great interest between researchers due to the augmentation of the flame-retardant, anti-corrosion, and thermal-mechanical properties of polymers, especially epoxy coatings. However, the resistance of MOF/GO-based materials is just in their infancy and they are in high demand for more attempts in the future. In the next parts, we primarily discuss the applications of MOF/GO-derived nanocomposites as

Future trends in this field

There is no denying that not only do most types of MOFs provide compatible and strong interactions with both organic and inorganic compounds, but also lots of MOFs have ideally intrinsic hydrophobic and water-stable properties. Therefore, the number of anticorrosive MOF-polymer protective coatings has been increasing and their strong performance has become an important focus of attention. The loading of GO sheets modified by MOF has shown a bright future with their superior performance as

Conclusion

In summary, GO/MOF nanohybrids could be considered as a new generation for epoxy resins owing to their distinctive properties. These properties can stem from boosting the unique advantages of the neat GO (numerous functional groups and high surface area) and MOF (versatile structures, low density, and high ability for selective interactions) separately, as well as alleviating some of their disadvantages (the poor dispersion of GO and the low stability of MOF). All of these reported procedures

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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