[
    {
        "id": 1,
        "chunk": "# Review",
        "category": " Introduction"
    },
    {
        "id": 2,
        "chunk": "# Research progress of UV-curable polyurethane acrylate-based hardening coatings  \n\nJunchao $\\mathtt{F u}^{\\mathrm{a}}$ , Li Wanga,⁎, Haojie $\\mathrm{Yu^{a,*}}$ , Muhammad Haroona, Fazal Haqa, Wenlei $s\\mathrm{{hi}^{\\mathrm{{b}}}}$ , Bin Wub, Libo Wangc  \n\na State Key Laboratory of Chemical Engineering, College of Chemical and Biochemical Engineering, Zhejiang University, Hangzhou 310027, China b Suzhou Taihu Electric Advanced Material Ltd., Fenhu New & Hi-Tech Industrial Development Zone, Wujiang 215200, China c Ningbo Haoxin YURON New Material Co., Ltd., NO. 7, Dajiang North Road, Jiangkou Sub District, Fenghua 315514, China",
        "category": " Abstract"
    },
    {
        "id": 3,
        "chunk": "# A R T I C L E I N F O",
        "category": " Abstract"
    },
    {
        "id": 4,
        "chunk": "# A B S T R A C T  \n\nKeywords:   \nUV-curable   \nPolyurethane acrylate Hardening coatings  \n\nWith the development of society, plastics play a significant role in daily supplies owing to their advantages. Whereas, insufficient scratch resistance and vulnerable plastic surfaces result in the constraint of their range of application fields, for instance, electronic products. Hence, it is a highly desirable objective of researchers to investigate hardening coatings for protecting plastic surfaces, by selecting polyurethane acrylate (PUA) as filmforming materials attributing to their adjustable features. This article reveals components of PUA and principles for its hardening modification, and summaries various methods of hardening modification of PUA-based coatings, such as improving the crosslinking density, strengthening hydrogen bonding, incorporating rigid groups into molecular structure, introducing inorganic nanoparticles into resin matrix and transferring linear PUA into hyperbranched analogs. Moreover, optimal strategies for the preparation of PUA-based hardening films from above five tactics are discussed.",
        "category": " Abstract"
    },
    {
        "id": 5,
        "chunk": "# 1. Introduction  \n\nPlastic is an indispensable material in the industry to produce daily supplies and high-tech products for specific applications owing to its lightweight, easy process and low cost, such as optical glasses, electronic product shells or protective films of precision instruments and so on. However, as optical resins and instrument housings, the insufficient mechanical strength of plastic results in the constraint of their range of application fields, because poor scratch resistance causes soft plastic surfaces which can be easily damaged [1–3].  \n\nUp to now, there exist many methods to improve the hardness of plastic which can be divided into three groups. One is an additive modification, in which hardening additives are added to plastics. The commonly used hardening additives are rigid inorganic fillers (kaolin or silica hydrated etc.) and fibers. However, these additives have significantly increased the surface roughness of plastic products, which brings bad influence to plastic. Second is blocking or grafting at the molecular level, which can be operated by incorporating polar groups or rigid groups into molecular chains of plastic to increase the crystallinity or rigidity of plastic, respectively. The hardness of plastic can be enhanced, possibly while the other mechanical properties will be affected, such as the considerable decrease in toughness. Third is hardening modification of plastic surface, which means that only the hardness of surface is promoted, and the internal hardness of products does not change. The examples are coating, plating and surface treatment. The coating has lower cost, easier process and slighter influences on the other performances of plastic than the above hardening modifications. So, it is an urgent need to extend the application domain of hardening coatings with high mechanical properties in plastic products.  \n\nCurrently, considerable efforts have been made to investigate highperformance UV-curable coatings through changing materials or operational parameters [4–6]. Compared with thermal curing, UV-curing technology is noted as 5E, which stands for Efficiency, Energy saving, Enabling, Economical, and Environmental friendly [7–12]. Generally, UV-curing systems mainly consist of three basic components: a monoor multifunctional acrylate monomer, an acrylate prepolymer, and a photo-initiator. Until now, diverse sorts of additives are constantly employed in such systems [13,14].  \n\nAs the one of most popular resin, PUA has attracted much attention in UV-curable coatings attributing to its excellent flexibility, prominent adhesion on substrates and a variety of adjustable features. [15] Significant results can be acquired when it is applied in coatings for metals, mobile phones, and other electronic products. However, the density and content of photosensitive groups of the existing photo-curable resins are not abundant, and have influence on the film performance. For example, the hardness of coating films is poor and UV-curing speed is slow which limits its practical applications in some fields [16,17]. Therefore, improving these performances of PUA is priority.  \n\nIn this review paper, the components of PUA and principles for hardening modification of PUA-based coatings is revealed in the first part. Afterward, the second part will be devoted to review diverse methods of its hardening modification. Eventually, a discussion about the optimal approaches for the preparation of PUA hardening coatings will be drawn.",
        "category": " Introduction"
    },
    {
        "id": 6,
        "chunk": "# 2. Compositions of PUA and principles for hardening modification of PUA-based coatings  \n\nPUA is an important category of photo-curable crosslinking resins, and is also widely employed in protective coatings. It is based on polyurethane, and then the double bond of acrylates is introduced into the molecular chain terminal of polyurethane, eventually, oligomers are used to initiate double-crosslinking reaction under the action of photoinitiators [18].",
        "category": " Introduction"
    },
    {
        "id": 7,
        "chunk": "# 2.1. Compositions of PUA  \n\nPUA comprises a significant category of polymeric materials whose properties can be tailored by regulating its compositions, the ratio of polyurethane/polyacrylate or -NCO/−OH, and the structure of raw materials [19–22]. The molecular structure of PUA mainly consists of urethane segments, the main chain of polyols or polyamines, and acrylate hydroxyalkyl ester segments (Fig. 1). The curing characteristics are determined by the acrylates located in the segments, and the structure and composition of the resin backbone mostly affect the properties of products.  \n\nGenerally, researchers frequently employ diisocyanates containing toluene diisocyanate (TDI), isophorone diisocyanate (IPDI), diphenylmethane diisocyanate (MDI), dicyclohexylmethane diisocyanate (HMDI), hexamethylene diisocyanate (HDI) as the urethane segments, including ethylene diamine (EDA) or ethane glycol (EG) etc. as chain extenders (Table 1) [18]. Attributing to the fact that diisocyanates possess different molecular structures, bringing diverse features for segments, we can acquire the properties of coatings what we want through varying the category or content of diisocyanates. For example, selecting HMDI and MDI for improving mechanical properties of resins owing to the cyclic structure or benzene rings they have, or choosing HDI to receive flexible coatings because of the exiting long-chain alkane, or using IPDI to control the reaction process which can design the chemical structure of compounds ascribing it to the different reactivity of two isocyanate groups of IPDI at low temperature. For the parts of main chain of polyols or polyamines, investigators usually employ polyethylene glycols (PEG), polytetrahydrofuran (PTMEG), poly (caprolactone glycol) (PCL), or polycarbonate diols (PCDL) etc. as flexible chain extenders whose terminals contain many hydroxyl or amino groups (Table 1) which can react with diissocynates by semi-adduct reaction. For the acrylate segments, we constantly choose hydroxyethyl acrylate (HEA) as end-capper to obtain unsaturated bonds. Nevertheless, in order to get compounds containing high functionality, raise the hardness or mechanical properties of coatings after curing, sometimes, we are more willing to introduce trimethylolpropane diallyl ether(TMPDE) or pentaerythritol triacrylate (PETA) containing lots of unsaturated bonds into system which are used as end-capping reagent (Table 1).  \n\nWhereas, in order to follow new policies of sustainable chemistry development, academic and industrial researchers have to seek for some greener resources or processes to replace hazardous chemicals and rigorous reaction conditions [23]. The greener raw materials can be divided into two groups. One is the preparation of non-isocyanate polyurethane (NIPU) by transurethanization polycondensation (Fig. 2), such as the reaction between cyclic carbonate and diamine, representing one of the most promising surrogates to the traditional route for synthesizing polyurethanes [24–28]. Another is discovery of renewable resources, for instance, vegetable oils, which are the most promising sustainable building blocks that can efficiently substitute for fossil-feedstock-derived polyester and polyether polyols [29]. Vegetable oils are triglycerides mainly consisting of saturated and unsaturated fatty acids, such as soybean oil [30], castor oil [31] or jatropha oil [32] and so on. The relationships between structure-property and resulting polyurethanes dramatically depend on the kind of triglyceride used [33], the category of diisocyanates and the degree of cross-linking28].  \n\nIn another aspect, some researchers add several reactive diluents into UV-curing systems (Table 1), such as tripropylene glycol diacrylate (TPGDA), trimethylolpropane triacrylate (TMPTA) or pentaerythritol tetraacrylate (PETTA), not only to decrease the viscosity of curing system, but also to increase the crosslinking density of coating films after cured owning to its plentiful double carbon bonds. For the reason that PUA occupies so many adjustable characteristics which combines the advantages of both acrylic and polyurethane resins, it has high reactivity, excellent flexibility, adhesion, low temperature resistance, abrasion resistance, chemical resistance and elasticity.",
        "category": " Materials and methods"
    },
    {
        "id": 8,
        "chunk": "# 2.2. Principles for hardening modification  \n\nFor the past few years, a large number of researchers have made much effort to modify the hardness of PUA coatings, some of which show excellent results. [11,12] We can receive the conclusion, whether from strategies they have used or the PUA molecular structure (Fig. 1), existing four techniques to elaborate hardening modification.  \n\nFrom Fig. 1, we can see that PUA can be divided into three sections, section a contains double bond functional groups, which are mainly used for photo-curing crosslinking, section b includes urethane bonds, which forms the hard segments, and section c comprises with weak or no other intermolecular forces, forming the soft segments. Sequentially, some tactics will be utilized. We can select section a to improve the hardness of PUA, promoting its functionality (unsaturated bonds), thereby increasing the crosslinking density to achieve more compact network structures [34], or incorporating some chain extenders that can form more hydrogen bonds into section b or section c reinforcing hydrogen bonding, accordingly strengthening the micro-phase separation or producing mixed phases between hard and soft segments, respectively, and then increasing the hardness [35], or introducing rigid groups (containing cyclic groups) for section c which can promote the hardness of soft segment owing to its rigidity [36]. Certainly, adding some inorganic fillers into the resin, such as nano- $s\\mathrm{iO}_{2}$ or $z_{\\mathrm{{nO}}}$ and so on, can also endow the considerable result of hardness for composite films. [10] The inorganic nanoparticles have capabilities to remarkably improve physical properties of polymers due to the strong interaction between particles and polymer interface caused by nanometer effect and its high specific area. The above strategies can not only be employed alone but also be united, to achieve the best outcomes.  \n\n![](images/fde373c0c2e43d1d2215f732ad74ba994287e5fbc5ffaafb8a14c107031fdea3.jpg)  \nFig. 1. General chemical structure of PUA.  \n\n![](images/69bc1fcc67aee22316961b19f266dbdeeb050647c625e68d78bfeae95e7c121c.jpg)  \nFig. 2. Overview of synthetic routes to polyurethanes. [23] Copyright 2015. Reproduced with permission from American Chemical Society [23]",
        "category": " Results and discussion"
    },
    {
        "id": 9,
        "chunk": "# 3. Strategies of hardening modification of PUA  \n\nFrom the mechanisms of hardened modification for PUA, we can get that there are five methods to enhance the hardness of coating films, which include improving the crosslinking density, strengthening the effect of hydrogen bonding, introducing rigid groups or adding inorganic nanoparticles into matrix to augment the rigidity of films and transferring linear PUA into hyperbranched PUA.",
        "category": " Results and discussion"
    },
    {
        "id": 10,
        "chunk": "# 3.1. Improving the crosslinking density  \n\nTheoretically speaking, there are two ways to reinforce the crosslinking density. One is forming multi-functional groups (carbon double bonds) and another is the introduction of silicone coupling agent. Silanol groups produced by the silicone coupling agent will react with each other to form siloxane bonds (Si-O-Si), consequently forming stable Si-O-Si siloxane networks [37]. The crosslinking result of silane coupling agents enhances the hardness and abrasion resistance of coatings.",
        "category": " Results and discussion"
    },
    {
        "id": 11,
        "chunk": "# 3.1.1. Polyfunctionality modification  \n\nAs a rule, the modification of double bonds content exists two strategies. One is adding reactive diluents [38] and another is promoting functionality of oligomers [39].  \n\nFor the reactive diluent, Xia et al. [40] synthesized a cluster of UVcurable polyurethane mixed components including (Nethylperfluorooctylsulfonamido) methyl acrylate (EFCSA) and pentaerythritol tetra (3-mercaptopropionate) (PETMP, a reactive oligomer). Subsequently, EFCSA and PETMP reacted with (2-hydroxyethyl acrylate)-terminated polyurethane resin to fabricate coatings (Fig. 3a). With increasing contents of PETMP, the pencil hardness was raised from 2B to HB due to the promotional crosslinking density of coatings caused by the increasing of thiol groups. Meanwhile, one point should be emphasized that this study introduced thiol-ene click reactions into crosslinking system to form polyurethane coatings, offering some benefits like high curing rate, low energy consumption [41] and enhanced mechanical properties due to its high reactivity for emerging highly dense networks. Li et al. [42] used PETA as end-capper and PETTA as reactive diluents to form PETA/PETTA composite system, including many unsaturated double bonds, subsequently enhancing the crosslinking density of films (Fig. 3c). The results revealed that dense network structures were formed by introducing PETTA with higher reactivity into the polyurethane molecule after cured and pencil hardness was improved from 2H to 3H. And the best consequence of mechanical properties was acquired when nearly $35\\mathrm{wt.\\%}$ PETTA was added (Fig. 4d). It was found that if the weight ratio of PETTA was tremendous, it might bring something worse effects to films owing to its high reactivity, which can take the shape of compact networks sharply, and then restrict the diffusion and motion of PETTA or radicals out of networks. In consequence, it brought a reduction of the crosslinking density because of premature termination of polymerization that was caused by partially unreacted double bonds trapped in the polymeric networks [15]. On the same method, Xu et al. [43] introduced tripropylene glycol diacrylate (TPGDA) as a multifunctional acrylate molecule reactive diluents into resins (Fig. 3b) that can form the preferable crosslinking structure, and then raise the coating hardness due to two unsaturated double bond $\\mathbf{\\tilde{\\Sigma}}.\\mathbf{C}=\\mathbf{CH}_{2})$ ) and shorter soft chains of TPGDA. As a whole, we can conclude that reactive diluents play an important role in the film hardness attributing to their high functionality.  \n\n![](images/b4f305dd9a76f73621e253bdd0dc16cd6cbbccedbdd4edebb16b37e03a8e3a56.jpg)  \nFig. 3. a) Preparation process of HFTPU. [40] b) Reaction mechanism of UV-WPUA based on PETA/PETTA [42]. c) The formation of UV-WPUA coating film [43].Copyright 2013. Copyright 2014. Reproduced with permission from John Wiley and Sons [42]. Reproduced with permission from John Wiley and Sons [43].  \n\n![](images/99d4c5deca0cf58a83f8e79afbe2ede692ad07edc47acb7cc962610373d3d1c8.jpg)  \nFig. 4. a) Synthesis process of UV-WPUA emulsion and b) Tensile properties of UV-WPUA. [45] c) Synthesis of fluorinated/methacrylated soybean oil [48]. d) Tensile properties of UV-WPUA [42]. Copyright 2014. Reproduced with permission from Elsevier [45]. Copyright 2009. Reproduced with permission from Elsevier [48]. Copyright 2014. Reproduced with permission from John Wiley and Sons [42].  \n\nFor the functionality of oligomers, Yuan et al. [44] developed a series of PUA oligomers terminated with multiple unsaturated bonds by using PETA as an end-capping reagent. Poly (propylene oxide) and PETA have significant effects to furnish more double bonds, which can increase the content of multi-functional groups. The results demonstrated that the functionality and content of oligomers (PETA content) of PU prepolymer have a huge impact on the film hardness, raising from 4H to 6H when PETA was added. Li et al. [45] introduced both castor oil (CO) and end-capper of PETA into waterborne polyurethane (WPU) molecules to obtain UV-WPUA oligomers containing multiple unsaturated double bonds and polar groups attributing to the high functionality and existing ester groups of CO (Fig. 4a), and the excellent mechanical properties was obtained. Simultaneously, researchers found that when the additive amount of CO was more than $6.86\\%$ , tensile strength received a sharp reduction (Fig. 4b). The excessive crosslinking results in some negative effects in the molecular level, such as molecular chains hardly moved freely and the mechanical strength decreased. Nevertheless, owing to its good advantages, castor oil and its derivatives have been considerably employed in polyurethane coatings field. [39,46,47] In another article, Kahraman et al. [48] used epoxidized soybean oil and methacrylic acid to synthesize methacrylated soybean oil terminated with multiple double bond, and then introduced it into PUA resins to get the harden coating (Fig. 4c). The result exhibited that the modification of film hardness was great, which can be supposed to the modified soybean oil acting as a cross-linking agent, because its polyfunctionality will raise the crosslinking behavior, afterward developing a tight network structure. Coincidentally, Li et al. [30] disclosed the similar consequence that acrylated epoxidized soybean oil (AESO) can increase the crosslinking density and form network structures during the curing process. The reason can be attributed to the fact that AESO contains two types of functional groups: one is hydroxyl groups which can covalently bond by reacting with other reactive groups or form hydrogen bonds to strengthen the effect of intermolecular chains, and the another is double carbon bonds that can be UV-cured.  \n\nObviously, the increment of double bonds content of curing systems or oligomers can make the chemical crosslinking density raise as well, so that the crosslinking network structures of components are more compact, reducing the free space for chain motion. Subsequently, the overall hardness and abrasion resistance of film is enhanced. With aggrandizing functionalities of compounds, however, the viscosity of system remarkably ascend which is not conducive to the leveling of coatings, resulting in insignificant increase in hardness. As a result, the lower viscosity of the system could promote a full crossing-linking reaction to form a denser crossing-linking structure by its leveling of films and motion of free chains [44].  \n\n![](images/b6ceeea9a83e74413a1237f639e8fafbcd6b42f8bfdf96e4c3bc595f21042385.jpg)  \nFig. 5. Schematic diagram of PDMS-based polyurethane acrylate oligomers. [52] Copyright 2011. Reproduced with permission from Elsevier [52]",
        "category": " Results and discussion"
    },
    {
        "id": 12,
        "chunk": "# 3.1.2. Siloxanes and silane coupling agents modification  \n\nNumerous researchers have certified that preparing harder films only with siloxanes and silane coupling agents through sol-gel technique can be done successfully [49], which can even raise the pencil hardness of coatings from 2B to 5H [50]. It can be attributed to the formation of stable Si-O-Si siloxane networks [51]. The most studied and applied silicon-containing acrylate monomers are silane coupling agents containing only one silicon atom, such as $\\upgamma$ -methacryloxypropyltrimethoxysilane (MPTMS), trimethylsilyl methacrylate (TMSM) and so on. $\\upgamma$ -Methacryloxypropyltris (trimethylsiloxy) silane containing several silicon atoms is also a commonly used monomer.  \n\nFor the modification of PUA coatings, hydroxy-terminated polydimethylsiloxane (PDMS) was introduced into the soft segments of PUA dispersions by Hwang et al. to reinforce the thermal and surface property (Fig. 5) [52]. From the results, they revealed that the curing rate and conversion of unsaturated bonds were diverse when end-cappers containing different functionality were added. For the conversion of unsaturated bonds, PDMS with mono-functional methacrylate obtained a little increment, but PDMS with high functionality reduced slightly. The reason could be supposed to that the former can be attributed to chain flexibility of PDMS, and the latter may be influenced more by the steric hindrance caused by PDMS. Furthermore, coatings with PDMS, especially including tri-acrylate end-capping, showed high initial modulus and excellent tensile strength and got a sharp reduction of elongation at break due to the polyfunctionality of tri-acrylate endcapping. Park et al. [37] investigated the effect of silane coupling agents in coatings, and introduced acrylic monomer and vinyltrimethoxysilane (VTMS) to acquire the UV-curable polyurethane acrylates (Fig. 6a). The consequences exhibited that as the amount of VTMS augmented, the storage modulus/hardness of the UV-cured coating enhanced significantly and the tensile strength/glass transition temperature raised slightly (Fig. 6b and c), whereas, the elongation at break decreased sharply owing to the occurrence of rigidity by the stable and dense Si-O-Si network structures. Wang et al. [36] adopted  \n\nMPTMS (KH-570) as the silane coupling agent and then found that the introduction of KH-570 can form a more compact spatial structure by increasing crosslinking density, afterwards improving the tensile strength of the latex films properly while keeping other performances well.  \n\nFor silane coupling agents, they can condense by themselves to form the structure of polyhedral oligomeric silsesquioxane (POSS), which can obtain the nanometer effect and excellent mechanical properties. Octavinyl-POSS was incorporated into UV-curing PUA matrixes by Kim et al. [53] to prepare hybrid nanocomposite films with distinctive thermal and mechanical properties. The PUA was consisted of poly (tetramethylene glycol), IPDI and HEA. Researchers found that the Shore A hardness (Hardness value measured by Shore hardness tester) of coating raised from 70 to 85 by adding and increasing POSS content in hybrid coatings. Addition of silicones to the matrix made films harder which can be ascribed to the increment of the cross-linking density as well as the reinforcing effect produced by the multi-functionality and rigidity of octavinyl-POSS. In another research, the silane coupling agent was utilized for the modification of interfacial compatibility. Kim et al. [54] gained the UV-curable PUA based hybrid materials, and MPTMS as a silane coupling agent was incorporated into the matrix to promote interfacial attraction between main organic part and inorganic silicate in the curing system, to receive a high degree of cross-linking and compact organic-inorganic network structure. By adjusting the adding quantity of MPTMS, morphological variation was obtained in Fig. 7. From the figure, we can see that with increasing the additive amount of the silane coupling agent MPTMS, the dispersion of silica particles was remarkably improved, eventually the stable and homogeneous morphology can be observed. Furthermore, this phenomenon delivered a significant information that raising interaction between organic and inorganic phases can substantially suppress the tendency of agglomeration among nano-inorganic particles and bring the silane coupling agent into full play.  \n\nFrom the above discussions, some conclusions can be made that the addition of siloxanes and silane coupling agents reinforce the crosslinking density of the system to form a dense network structure owning to its compact and stable siloxane networks generated from silanol groups, to acquire the effect of improving coatings hardness. Whereas, siloxanes are easily crosslinked together and then make clusters, which brings inferior effects to coatings, such as the low increase in hardness and high reduction of elongation at break after cured [55]. So, in order to receive excellent results of modification, we should control the number of siloxanes in a moderated range.  \n\n![](images/646cd109986830ce5c5445e4efd164e07fa4cfdd4fe5739c16471413a45acb4d.jpg)  \nFig. 6. a) Synthetic route of UV-curable PUAs. b) The storage modulus and c) Stress-strain curves of UV-cured coatings (FPUA $6/0$ , FPUA $_{6/3}$ , FPUA $6/6$ and FPUA 6/ 9). [37] Copyright 2015. Reproduced with permission from Springer Nature [37].  \n\n![](images/66aee6f7485de02b919463fd0b94329d16cdf435304977a48af2e686880a94e3.jpg)  \nFig. 7. SEM images of acrylate $\\mathrm{\\Delta}^{\\prime}\\mathrm{SiO}_{2}$ hybrids without MPTMS, a) $\\mathrm{TEOS}=0.01\\mathrm{mol}$ , b) $0.03\\mathrm{mol}$ , and hybrids with addition of MPTMS, c) $\\mathrm{TEOS}=0.01\\mathrm{mol}$ , d) $0.03\\mathrm{mol}$ . [54] Copyright 2010. Reproduced with permission from Springer Nature [54].  \n\n![](images/cea78c79ee44d5e6902d995339e84f8b797920de3a239572485add264c8a9b2f.jpg)  \nFig. 8. a) Preparation of soybean-oil-based WPU dispersions. b) The relation between glass-transition temperature $(T_{g})$ of the SPU coatings and the hydroxyl number of the MSOL. c) Stress-strain curves for MSOLs-based SPU films with different hydroxyl numbers. [29] Copyright 2008. Reproduced with permission from American Chemical Society [29].",
        "category": " Results and discussion"
    },
    {
        "id": 13,
        "chunk": "# 3.2. Strengthening hydrogen bonding  \n\nPUA contains thermodynamically incompatible hard segment and soft segment units. The hydrogen bonding generated between hard segments form physical crosslinking among polymer molecular chains which have a significant effect on physical properties of polymers, and are easy to produce mixed phases, giving the material excellent overall performance [29,56].  \n\nLu et al. [29] incorporated a derivative of soybean oil (MSLO) into prepolymers to prepare a cluster of vegetable-oil-based WPU dispersions, among them, hydroxyl groups of MSLO ranged from $2.4\\mathrm{up}$ to 4.0 (Fig. 8a). Certainly, MSLO can play the role of UV-cured functional groups owing to its unsaturated double bonds as well. In addition, the experimental results disclosed that hydroxyl functionalities of the MSOLs had significant effects in controlling mechanical properties of coatings. With increasing the $-\\mathrm{OH}$ number in MSOL, the $T_{g}$ value and mechanical properties were strengthened due to the higher physical cross-linking in the soft segment provided by hydrogen bonding (Fig. 8b and c). Jofre-Reche et al. [57] modified the abrasion resistance and hardness of PU with polycarbonate diol (PCD) and polytetramethylene glycol diol (PTMEG). The results revealed that PUPTMEG possessed poor abrasion resistance, while the hardness and wear resistance of PU-PCD and PU- $50\\%$ $\\mathrm{PCD}+50\\%$ PTMEG were significantly improved, almost up to 60 Shore A hardness, which attributed to stronger interactions of the carbonate groups in soft segments that were able to create hydrogen bonds with urethane groups of hard segments, producing a higher miscibility of the hard and soft domains and then reinforcing mechanical properties. Definitely, we should note that the role played by hydrogen bonds is mainly augmenting the abrasion resistance of coatings, which makes resins tough owing to its role of buffers, conversely, restricting the improvement of the hardness of films.  \n\nAnother approach is not the modification of oligomers but blending. Zhang et al. [35] successfully acquired a new type of waterborne PUPA ester emulsion through a physical blend between polyurethane emulsion (PU) and polyacrylic ester emulsion (PA). The film properties of  \n\n![](images/500a8f3ab9e167f8ae48d9f86505a1d28bbd23bb578fb1d1b41c3cfa652ba660.jpg)  \nFig. 9. a) Formation of the PUPA polymer particles. [35] b) The different patterns of $\\scriptstyle{\\mathsf{C}}=0$ in polyurethane: (a) free carbonyls, (b) disordered H-bonded carbonyls, (c) ordered H-bonded carbonyls. c) XRD graphs of the WPU and WFPU coatings [59]. Copyright 2013. Reproduced with permission from Springer Nature [35]. Copyright 2017. Reproduced with permission from Elsevier [59].  \n\nPUPA coating was characterized and exhibited reasonable hardness, improving the stability of the PUPA coating by the hydrogen bonding between $\\boldsymbol{\\mathrm{N-H}}$ of PU and $\\scriptstyle0=\\mathbf{C}$ of PA [58]. The forming mechanism of hydrogen bonding between PU and PA was represented and shown in Fig. 9a. From the above researches, it is worth noting that the introduction of more hydrogen bonds into soft segments can increase the interaction between hard and soft segments to enhance mixed phases, and then endow coatings with favorable properties. The method strengthens hydrogen bonding between hard and hard segments to raise micro-phase separation, whereas, also can work pretty well to increase the hardness of films. As an instance, Yang et al. [59] developed a series of novel waterborne fluorinated polyurethane and acquired the conclusion that the increment of H-bonded carbonyl groups in hard domains has a significant effect on crystallization, bringing the increase of crystalline in hard domains (Fig. 9b), which was contributed to raising the hardness of films. All samples showed a strong peak at $2\\Theta=19^{\\circ}$ (Fig. 9c), demonstrating that micro-phase separation between the soft and hard segment was generated to endow coatings with excellent properties because of the crystallinity in the hard domains. This enhancement can be related to the restricted movement of polymer chains caused by the larger degree of hydrogen bonding between the hard and hard segments [29,60]. However, the large degree of micro-phases separation will lead to uneven film surface because of its crystallization on the surface.  \n\nAlthough the hydrogen bonding improves the mechanical properties of materials, its enhancement in hardness is limited, because it mainly improves the abrasion resistance of coatings. The reason can be attributed to taking the shape of buffers by hydrogen bonding, which will absorb impact energy when subjected to force. At the same time, the hydrogen bonds in coatings break at relatively high temperatures, whose thermal stability is relatively poor.",
        "category": " Results and discussion"
    },
    {
        "id": 14,
        "chunk": "# 3.3. Incorporation of rigid groups  \n\nResearches have shown that rigid groups give the PUA a very high hardness improvement, for example, bisphenol A epoxy resin itself contains benzene rings, whose hardness after curing is very considerable [36]. In contrast, it was demonstrated that the addition of compounds containing tough chains into epoxy resins or PUA coatings will decrease coating hardness and increase damping properties [61]. Rigid groups that can be introduced into PUA include benzene rings or sixmembered heterocyclic rings, especially six-membered heterocyclic rings that can form large $\\uppi$ bonds, such as triazine groups, which not only provide considerable rigidity, but also endow coatings with antiyellowing.  \n\nShi et al. [62] introduced melamine into PUA matrix to get phasechange heat-storage UV-PUA coatings by microencapsulated technology, where melamine-formaldehyde shell and paraffin core were synthesized to form phase change materials. Certainly, the melamine possesses triazine group that can take the shape of large $\\uppi$ bonds. Attributing to dense crosslinking density and rigidity of melamine-formaldehyde, the mechanical properties of films were enhanced [63]. Besides, Pathak et al. [64] selected hexamethoxymethylmelamine (HMMM) as a crosslinking agent, which was incorporated into resin system to prepare films, getting the conclusion that triazine groups of HMMM play a significant role in strengthening mechanical properties of coatings owing to its rigidity. In another study, Mishra et al. [65] synthesized a new intermediate through the reaction between epoxy resin and dimer fatty acid, which called dimer acid modified epoxy (DME) polyol containing both hydroxyl and epoxy groups (Fig. 10a), and then prepared UV-curable polyurethane by adding trimethylolpropane tris(3-mercaptopropionate) as a cross-linking agent. Evaluation of cured samples showed that with incresing the amount of thiol ratio, the significant improvement in storage modulus (Fig. 10b) and hardness can be observed. Furthermore, the high hardness value of coatings can be attributed to the rigidity of DME, which contains rigid phenyl groups.  \n\n![](images/4a73499aced67259f34b0b643e47f9808912903c97eeef98673f9a0d9cd64c90.jpg)  \nFig. 10. a) Synthetic route of DME. b) Storage modulus of cured films. [65] c) Chemical structures of hard/soft monomers containing acrylic groups [66]. Copyright 2017. Reproduced with permission from Springer Nature [65]. Copyright 2017. Reproduced with permission from Elsevier [66].  \n\nIn addition, Yong et al. [66] selected different ratios of hard/soft monomers containing acrylic groups as an adjusting mean to prepare a series of WPUA hybrid emulsions (Fig. 10c). The research disclosed the relationship between mechanical properties and amounts of acrylic monomers. Comparing to the WPU film, the hardness of WPUA coatings increased remarkably owing to the introduction of acrylic monomers, which is due to the reason that the increment of the weight ratio of hard monomers can endow films with rigidity and excellent mechanical properties by considerable phenyl skeleton structures of acrylic monomers. Certainly, they also discovered that each film possessed two $T_{g}$ values, indicating that the phase separation phenomenon existed due to the appearance of hydrogen bonding between hard and hard segments. Hence, they can form buffers by breaking hydrogen bonds when subjected to force, giving coatings toughness. Moreover, Beniah et al. [56] used 1,4-diaminobutane, isophorone diamine, methylene bis(cyclohexyl amine), and bis(aminomethyl) norbornane as chain extenders to investigate the influence between polyhydroxyurethane (PHU) structure and properties of PHUs (Fig. 11a), eventually demonstrating that structure and content of chain extenders played an important role in the properties of PHUs (Fig. 11b and c). The most remarkable improvement in mechanical properties of the resulting PHUs can be obtained when the norbornane-based chain extender was applied owing to the norbornane ring acting as an effective physical cross-linking point since no crystalline structure or hydrogen bonding was observed in their elastomers. This inference was the same as what Jiao et al. [67] got, who synthesized UV-curable PUA oligomers modified with cycloaliphatic epoxide resin.  \n\nOne conclusion we should note is that it is a capital idea to introduce rigid groups into PUA which will significantly improve the hardness of coatings. Whereas, the introduction of benzene rings may result in yellowing of coatings, as benzene ring can be easily oxidized into quinones. In consequence, we should avoid employing materials contained benzene rings to synthesize PUA resins when the appearance of films is a priority.",
        "category": " Results and discussion"
    },
    {
        "id": 15,
        "chunk": "# 3.4. Introduction of inorganic nanoparticles  \n\nInorganic nanoparticles can provide excellent mechanical properties for organic/inorganic composites because nano-inorganic fillers are not only small in size but also large in specific surface area. As a result, there exists a strong interaction between particles and polymer interface, which significantly improves physical properties of polymers [68,69].  \n\nGenerally, nano-inorganic fillers that can be introduced into the PUA matrix include silica [70], carbon nitride [17], calcium carbonate [71], alumina [72] and zinc oxide [73] and so on. Lv et al. [16] acquired some waterborne UV-curable $\\mathrm{PUA}/\\mathrm{SiO}_{2}$ nanocomposites via traditional sol-gel method, in which KH-570 was used as the coupling agent of inorganic phases and organic phases, making sure $\\mathrm{{siO}}_{2}$ had good dispersion in the PUA matrix and then incorporated modified- $s\\mathrm{i}0_{2}$ into the ends of the PUA main chains by radical polymerization. Comparing to the physical blending method, from results, it was easier to obtain a uniform emulsion by the sol-gel technique owning to $\\mathrm{{siO}}_{2}$ nanoparticles showing a tendency to aggregate together without any KH-570 added (Fig. 12), which can bring some good effects in practical production applications. At that, comparing with neat PUA, the pencil hardness of $\\mathrm{{PUA}}/{\\mathrm{{SiO}}_{2}}$ coatings enhanced from the HB up to 4H when 6 wt. $\\%$ of silica was added. Certainly, Kim et al. [73] and Xu et al. [74] both also obtained similar consequences by introducing ZnO (Fig. 13a and b) and $\\mathsf{C a C O}_{3}$ (Fig. 13c) into PUA matrix, respectively, in which KH-570 was used as the coupling agent to modify inorganic fillers. Afterward, Liu et al. [75] revealed the relationship between pencil hardness and modified inorganic fillers. As expected, increasing modified fillers loading resulted in apparent improvement of pencil hardness, raising from HB to 2H with $2\\mathrm{wt.\\%}$ filler content. Nevertheless, the pencil hardness occurred a reduction from 2H to H when high filler content was added, which may be caused by the occurrence of inorganic particles aggregation (Fig. 14). As a result, appropriate dispersion of the modified fillers is crucial to take advantage of nanoscale reinforcement and to acquire desired physical and mechanical properties of composites films.  \n\nIn another research, Liao et al. [17] prepared a suite of UV-curable waterborne Wsi-PUA- $\\mathrm{.C_{3}N_{4}}$ composites including vinyl hydroxyl silicone oil and different contents of $\\mathrm{C}_{3}\\mathrm{N}_{4}$ without any couple agents. The results showed that the dispersion of $\\mathrm{C}_{3}\\mathrm{N}_{4}$ particles in composite films were homogeneous when the additive contents of $\\mathrm{C}_{3}\\mathrm{N}_{4}$ were low, endowing composite films with the excellent mechanical property. Nevertheless, agglomerates could be found at higher $\\mathrm{C}_{3}\\mathrm{N}_{4}$ content, which can be supposed to the fact that the high concentration induced phase separation (Fig. 15). Certainly, the pencil hardness of films could increase from 2H to 4H when the content of $\\mathrm{C}_{3}\\mathrm{N}_{4}$ was low. Nam et al. [71] investigated the effect of inorganic nanoparticles $\\mathsf{C a C O}_{3}$ in the UVcurable PUA coating and revealed that the performance of organic/ inorganic nanocomposite film was intensively linked with organicallymodified colloidal $\\mathsf{C a C O}_{3}$ nanoparticles. This was because the weak interfacial interaction between organic phases and inorganic interfaces could be disconnected when the amount of additive $\\mathsf{C a C O}_{3}$ was high, resulting in discontinuity of bond matrix, which gave rise to the disastrous fault of the nanocomposite films. Hence, in order to get highperformance UV-curable PUA nanocomposites coatings, inorganic nanoparticles homogeneously dispersed in organic matrix is crucial.  \n\n![](images/2bc4facc701bc220b8107d411dc5859b0a3a7242226b8aa633b4ea7250b0b555.jpg)  \nFig. 11. a) Synthetic route of PHUs. b) Stress-strain curves of PHUs chain extended with $50\\mathrm{wt.\\%}$ hard-segment content and c) Norbornane diamine at several har segment contents. [56] Copyright 2017. Reproduced with permission from John Wiley and Sons [56].  \n\nAbove of all, something we can discover is that, although PUAs modified with inorganic fillers can raise the hardness of coatings significantly, inorganic nanoparticles may be poorly dispersed due to miserable dispersibility of the high inorganic fillers content in the organic phase, resulting in unfortunate performance enhancement [76]. Therefore, in order to avoid this negative effect, the amount of inorganic filler should be controlled in the appropriate range, because the appropriate dispersion of the nanofillers is crucial to take advantage of nanoscale reinforcement and to obtain desired physical and mechanical properties of nanocomposites [75]. Of course, it must be mentioned that introducing inorganic fillers into the resin matrix will roughen the surface of coatings.",
        "category": " Results and discussion"
    },
    {
        "id": 16,
        "chunk": "# 3.5. Hyperbranched modification  \n\nHyperbranched polymers have advantages of high solubility and reactivity, low solution viscosity and melt viscosity, which are widely used in coatings [77–79]. Modifying PUA with hyperbranched structure not only improve the functionality but also can reduce the viscosity of system, contributing to the dispersion of materials within matrix [80–83]. Some researchers have demonstrated that introducing dendritic hyperbranched PUA into curing system will augment the crosslinking density [84,85] and will form the highly compact structure of films owing to its high functionality.  \n\nJana et al. [86] acquired the hyperbranched core (Fig. 16a) through esterification of pentaerythritol (PE), 2, 2-bis (methylol) propionic acid (DMPA) and trimethylolpropane (TMP), whose branching can be controlled with a varying amount of DMPA as chain extender, and then can form the alkyd polyurethane resin by employing phthalic anhydride and benzoic acid as end-cappers, both containing phenyl groups. The research revealed that with increasing the branching in the hyperbranched core, the pendulum hardness of coatings was reinforced by the introduction of rigid groups of end-cappers. However, the increment of the extent of branching in polyurethane structure lost the orientation or structural regularity of alkyd polyurethane chain, resulting in decreasing the glass transition temperature. In another research, a cluster of hyperbranched polyurethane acrylate (F-HBPUA) with diverse hydroxyl numbers and flexible chains was successfully developed by Xiang et al. [87] (Fig. 16b), and then the effect of generation number and flexible chain on the performances of resin and film was investigated. The results indicated that with increasing generation numbers and chain lengths, the increment of the viscosity was occurred, whereas hardness decreases from F to HB owing to its flexibility which increases with longer soft chains. Certainly, the number of polar groups (nitrogen and oxygen) was raised with the increment of the degree of branching and soft chain length, which strengthened the intermolecular interaction and then endowed good mechanical properties to films. [88,89] Apart from that, Jeong et al. [90] also investigated the effect of degree of HBPUA’s branching on performance, obtaining the result that with increasing the branching from 8 to 16, the hardness of HBPUA coatings enhanced slightly, exhibiting that the effect was not significant. The reason for this phenomenon will be discussed below. Absolutely, one thing for synthesizing oligomers we should be aware is that HBPUA can provoke rapid gelation of the mixed solution in a brief period during preparation owing to its high reactivity. In order to prevent this issue from occurring, sometimes, the reaction was carried out with excess polymerization inhibitors or solvent [31].  \n\n![](images/caa9fd18331bc8a935915710d583716f886ed2ebe8e13cca61a24bd4f5670c9e.jpg)  \nFig. 12. TEM micrographs of PUA and PUA $\\mathrm{\\SiO}_{2}$ hybrid particles: (a) pure PUA, (b) PUA with $4\\mathrm{wt.\\%}$ unmodified $\\mathrm{{SiO}}_{2},$ (c) PUA with $4\\mathrm{wt.\\%}$ modified $\\mathrm{SiO}_{2}$ and (d) PUA with $6\\mathrm{wt.\\%}$ modified $\\mathrm{{SiO}}_{2}$ [16]. Copyright 2015. Reproduced with permission from Royal Society of Chemistry [16].  \n\n![](images/e4288ff7a01ae4da684f8873ef046bd3f1667ea280232bede47f3918bff4fd29.jpg)  \nFig. 13. a) Reaction mechanism of KH-570 with $z_{\\mathrm{{nO}}}$ surface hydroxyl groups. b) The relationship between hardness and modulus values of PUA/ZnO nanocomposite films and ZnO content. [73] c) Brief mechanism of hydrolysis of KH-570 and surface modification of inorganic carbonate [74]. Copyright 2012. Reproduced with permission from Elsevier [73]. Copyright 2018. Reproduced with permission from Springer Nature [74].  \n\n![](images/1ba46e6d4c180fe3a05b5a2841af8a8d1df3a6154f5d8c2d1dfdebf2efc3db04.jpg)  \nFig. 14. SEM micrographs of cross-section of the UV-curable nanocomposite coatings (coated PC) containing a, b $2\\mathrm{wt.\\%}$ and d, e 5 wt. $\\%$ $\\mathrm{TiO}_{2}$ $\\scriptstyle\\phantom{+}_{2}-S\\mathrm{iO}_{2}/\\mathbf{P}$ (MMA-coPMPM), and surface of the UV-curable nanocomposite coatings (coated PC) c 2 wt.% and f 5 wt. $\\%$ $\\mathrm{TiO}_{2}$ - $\\mathrm{SiO}_{2}$ /P(MMA-co-PMPM). [75] Copyright 2018. Reproduced with permission from Springer Nature [75].  \n\nFrom the above research, what we can conclude is that hyperbranched modification both improves functionality and reduces system viscosity, resulting to coatings leveling [91–93]. Nevertheless, singlecomponent hyperbranched polymers have low crosslink density and acquire miserable results after curing. The reason is that although functionalities of polymers are considerable, each of functional groups can be cross-linked together after curing is impossible due to the spherical shape of polymers, which results in a low crosslink density [94–97]. Whereas, if researchers employ it as an additive, especially as an additive of low-functionality PUA, its advantages can be fully exerted, and properties of PUA can be improved well [98].  \n\nZhang et al. [99] chose toluene diisocyanate (TDI) as the main part of PUA matrix. Bifunctional PUA was first prepared by the reaction between polyethylene glycol, hydroxyethyl acrylate and TDI. Subsequently, hyperbranched HBPUA was synthesized via trimethylolpropane as the core of the dendritic polymer, which was used as additives and then introduced into PUA matrix to acquire coatings (Fig. 17a). The results showed that with an increase in HBPUA content, the hardness of coatings improved from 6H to 9H, simultaneously the abrasion resistance and storage modulus also raised markedly (Fig. 17c). Definitely, the cured film with about $10\\mathrm{wt.}\\%$ HBPUA displayed strongly raised tensile strength while the elongation at break received a little reduction. However, the elongation at break was reduced by about $30\\%$ when $20\\mathrm{wt.}\\%$ HBPUA was added, indicating a significant decline in toughness (Fig. 17b). Some researches also disclosed the similar issue that although the rigidity of coatings was reinforced with the increase of crosslink density, the remarkable decrease in elongation at break values resulted in the restrained toughness [31]. And so, it’s a crucial issue that how to keep the toughness constant or decrease slightly while increase the hardness of coatings. One is selecting chain extenders or long soft segments that can generate more hydrogen bonds to promote micro-phases separation or mixed phases [100], which can play a role of the buffer when coatings subjected to force, obtaining the effect of toughness [101]. But for the former, it will lead to uneven coating surface owning to its crystallinity, which is bad for films as the smooth appearance of them is a priority. Another is increasing the content of flexible chain extenders in soft segments, so that PUA chains can easily move, and thus improve the toughness of films. But beyond that, Xiang et al. [87] also certified that as the soft chains increased, the dendritic arms became more flexible, the cured films were more flexible, accordingly. Although the toughening effect can be acquired, the film hardness descended [102]. So, we can receive that only moderate addition of soft chains can keep hardness and toughness both well.  \n\n![](images/b359c529ab1effc66a59036c860f480aab00c5e60d4874bc4f18aeb1b19f6e00.jpg)  \nFig. 15. TEM micrographs of ultra-thin sections taken from the coating samples filled with: a) $0.25\\mathrm{wt.\\%}$ , b) $0.5\\mathrm{wt.}\\%,$ , c) 1.0 wt. $\\%$ and d) $2.0\\mathrm{wt.\\%}$ $\\mathsf{g}\\mathrm{-}\\mathsf{C}_{3}\\mathsf{N}_{4}$ particle [17] Copyright 2015. Reproduced with permission from Elsevier [17].  \n\nGiving a similar example, Zou et al. [103] designed hyperbranched polyurethane (HBPU) by reacting IPDI and poly (tetrahydrofuran), which brought flexible segments for HBPU resin, sequentially generating more hydrogen bonds among molecular chains. HBPU and the linear analog polyurethane (LPU) were used as tougheners in the diglycidyl ether of bisphenol A (DGEBA)/amine system, respectively. This research revealed that the average crosslinking density, comparing with DGEBA/LPU films, DGEBA/HBPU samples were higher attributing to high functionality of HBPU. Furthermore, though adding HBPU raised the crosslink density, the HBPU introduced into matrix enhanced the flexibility of the network structure as well (Fig. 18). It should be noted that the enhancement in toughness is associated with micro-phase separation structures which prevent the crack to freely develop and absorb the impact energy. Apart from that, the stronger interface interaction in the DGEBA/HBPU films promotes the stress transfer when films subjected to force, which is caused by the generation of hydrogen bonding that formed the buffer upon loading.  \n\n![](images/eb8e19a04b94a1fe50d2fc5c2b167d3d5065d1b06e0cb0a9616338ac15c265a6.jpg)  \nFig. 16. a) Scheme of the synthesis of the hyperbranched core. [86] b) Schematic representation for the preparation of F-HBPUA [87]. Copyright 2017. Reproduced with permission from John Wiley and Sons [86]. Copyright 2017. Reproduced with permission from Elsevier [87].  \n\n![](images/79fc80b8b7dfd8b3256ee7493e343c1385d2fc7d40d0466b21bba88e1ceaf933.jpg)  \nFig. 17. a) Synthetic process of PUA/HBPUA UV-curable coatings. b) Stress-strain curves of the cured films. c) Storage modulus (E′) graphs of UV-curable coatings as a function of temperature. [99] Copyright 2016. Reproduced with permission from Royal Society of Chemistry [99].  \n\n![](images/27900aa5eb45069edb84782a067391aeaaf8912ca7992517092ffad1c2f52c05.jpg)  \nFig. 18. The influence of modifier content on the a) impact strength and b) flexural strength. c) Schematic illustration of separate particles in the DGEBA/HBPU films. [103] Copyright 2016. Reproduced with permission from Royal Society of Chemistry [103].",
        "category": " Results and discussion"
    },
    {
        "id": 17,
        "chunk": "# 4. Conclusion  \n\nIn summary, five types of strategies are used for hardening modification of PUA, such as the improvement of the crosslinking density of system, the enhancement in the effect of hydrogen bonding, the incorporation of rigid groups at molecular level, the introduction of inorganic fillers and hyperbranched modification, which have their own advantages and disadvantages. Of these, hyperbranched polyurethane acrylate is optimal because it not only provides polyfunctionality (increasing the cross-linking density), but also reduces system viscosity and contributes to the dispersion of compositions. Although the effect of single hyperbranched polymer on film formation is not very good, its advantages can be maximized when it employs as an additive to coatings. Next one is followed by the introduction of rigid groups owning to their pronounced effect on the hardness of coatings. The material containing rigid groups should be a benzene-free substance which can prevent coatings from yellowing. Furthermore, in order to maximize the increment of hardness, this material will be introduced into the hyperbranched polymer by the formation of hyperbranched nuclei. For the polyfunctional modification, it is a distinct method that using vegetable oils as hyperbranched polyols which can increase the functionality of PUA by epoxidation and ring-opening reaction. In addition, it can be a sustainable route to solve the problem of environment and depletion of the world crude oil stock due to their green and abundant resources, such as palm oil [104] and olive oil [105]. Certainly, siloxanes and silane coupling agents, or inorganic fillers can be introduced into PUA coatings to enhance the hardness of films significantly when the contents of those materials are moderate, which is economical for industrial manufacture. In another expect, we can endow hardening coatings with excellent toughness by the introduction of flexible aliphatic chain or others that can form remarkable hydrogen bonds, accordingly generating physical cross-linking buffers, eventually absorbing impact energy or strengthening stress transfer in the intermolecular chains.",
        "category": " Conclusions"
    },
    {
        "id": 18,
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        "category": " References"
    }
]