高校化学工程学报    2023, Vol. 37 Issue (4): 525-536  DOI: 10.3969/j.issn.1003-9015.2023.04.002
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引用本文 

李通盛, 詹浩成, 蓝慧萍, 王立杰, 林益军, 金日光. 氧化石墨烯改性及其在环氧防腐涂层中的应用进展[J]. 高校化学工程学报, 2023, 37(4): 525-536.   DOI: 10.3969/j.issn.1003-9015.2023.04.002.
LI Tongsheng, ZHAN Haocheng, LAN Huiping, WANG Lijie, LIN Yijun, JIN Riguang. Graphene oxide modification and its application in anticorrosive epoxy coatings[J]. Journal of Chemical Engineering of Chinese Universities, 2023, 37(4): 525-536.   DOI: 10.3969/j.issn.1003-9015.2023.04.002.

通讯联系人

林益军,E-mail:yjl310@163.com

作者简介

李通盛(1973-),男,广西北流人,高级工程师, 学士。

文章历史

收稿日期:2022-06-02;
修订日期:2022-08-29。
氧化石墨烯改性及其在环氧防腐涂层中的应用进展
李通盛 1,2, 詹浩成 1,3, 蓝慧萍 4, 王立杰 1, 林益军 1,5, 金日光 5     
1. 中国电建集团中南勘测设计研究院有限公司, 湖南 长沙 410014;
2. 大唐观音岩水电开发有限公司, 四川 攀枝花 617012;
3. 河海大学 力学与材料学院, 江苏 南京 211100;
4. 中国能建集团湖南化工设计院有限公司, 湖南 长沙 410007;
5. 北京化工大学 化工资源有效利用国家重点实验室, 北京 100029
摘要:石墨烯作为一种新型二维片层碳纳米材料,具有超大比表面积以及优异的疏水、导电、屏蔽、阻隔、自修复和化学惰性,在腐蚀防护领域展现出广阔的应用前景,但分子间的高比表面能和强π-π作用使其难以均匀分散,很大程度上限制了性能的发挥。氧化处理可以扩大石墨烯的层间距,降低团聚倾向,且表面新生的大量功能基团更有利于后续改性。归纳总结了近年来促进氧化石墨烯分散和有序排列的新方法及其在环氧涂层防腐方面取得的新进展,并对现阶段存在的一些问题进行了探讨与展望。
关键词石墨烯    氧化石墨烯    改性    分散与定向    环氧防腐涂层    自修复    
Graphene oxide modification and its application in anticorrosive epoxy coatings
LI Tongsheng 1,2, ZHAN Haocheng 1,3, LAN Huiping 4, WANG Lijie 1, LIN Yijun 1,5, JIN Riguang 5     
1. Power China Zhongnan Engineering Co. Ltd., Changsha 410014, China;
2. Datang Guanyinyan Water and Electricity Development Co. Ltd., Panzhihua 617012, China;
3. College of Mechanics and Materials, Hohai University, Nanjing 211100, China;
4. China Energy Engineering Group Hu′nan Chemical Engineering Design Institute Co. Ltd., Changsha 410007, China;
5. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China
Abstract: Graphene is a novel two-dimensional carbon nanosheet with huge specific surface area, excellent hydrophobicity, conductivity, shielding, barrier, self-repairing and chemical inertia properties, which has great prospects in the application of corrosion protection. However, high specific surface energy and strong π-π interaction between graphene layers make it difficult to be uniformly dispersed, which extremely limits the performance of graphene in anti-corrosive coatings. Oxidation treatment can enlarge layer spacing and reduce agglomeration tendency, and numerous surface functional groups produced can be used for further modification. Recent advances in improving dispersion, orderly alignment and corrosion resistance of graphene oxide in epoxy coating are summarized in this review. Meanwhile, potential problems and challenges in the present stage are discussed.
Key words: graphene    graphene oxide    modification    dispersion and orientation    anticorrosive epoxy coating    self-repairing    
1 前言

据测算,全球每年因腐蚀造成的直接经济损失高达7 000~10 000亿美元,为其他自然灾害总和的4~6倍,年均因锈蚀报废的设备相当于世界钢材年总产量的30%。涂覆防护涂层作为抑制或减缓金属腐蚀最简单、经济、可靠的手段,在生产实践中有着广泛使用[1-2]。为开发高性能重防腐涂层产品,众多科研工作者致力于新型功能材料的研究。石墨烯(G)自2004年由Geim和Novoselov发现以来,在航空航天、工程防腐、海水淡化、离子或分子渗透等领域受到广泛关注,已成为各行业研发焦点之一[3-12]。因突出的力学、阻隔、化学稳定等特性,作为填料分散在涂层中,能够极大增强复合涂层的抗腐蚀能力。然而石墨烯片层间的超疏水性、强范德华力、高比表面能使之在溶剂和树脂基体中很容易发生堆叠和团聚[13],不仅严重影响屏蔽水、氧介质的能力,团聚体与金属基底形成的原电池效应还可能诱发或加速局部腐蚀。氧化石墨烯(GO)作为最常见的石墨烯衍生物,表面有大量含氧基团,在一定程度上扩大了石墨烯的层间距,降低了团聚倾向。本文着重概述了GO改性以促进良好分散和有序排列的新技术,归纳了石墨烯和氧化石墨烯在环氧树脂(EP)涂层中的防腐机制及应用成果,分析了当前存在的主要问题,并就石墨烯防腐涂料未来发展方向提出了展望。

2 氧化石墨烯分散与定向排列技术 2.1 氧化石墨烯分散改性

实现石墨烯及其衍生物的无损均匀分散,是制备防腐涂料的前提和基础。当前传统处理方式主要通过共价键修饰[14-23]、非共价键修饰[24-27]、元素掺杂[28-32]以及原位聚合[33-39]等将其功能化,以增强层间位阻效应,调节表面双亲性,改善其与聚合物间的分散性与相容性。

(1) 共价键修饰。利用石墨烯及其衍生物边缘或缺陷部位较高的反应活性,将羧基、羟基、环氧基、胺基等官能团借助石墨烯晶格中碳原子的sp2→sp3杂化共价键链接到表面,可以降低纳米片层间的共轭耦合,改善分散性、电传导活性、离子迁移率等。有机小分子或聚合物还能够与GO表面的含氧活性基团反应,增强GO在树脂中的相容性。同时,GO表面反应性基团也能参与涂层的固化交联,从而进一步提升复合涂层的整体性能。

Bai等[16]曾以异佛尔酮二异氰酸酯接枝GO,使分子链上的─NCO与水性聚氨酯形成共价键,降低了石墨烯表面的范德华力,进而抑制了GO在基质中团聚倾向。不过该方法需要使用二异氰酸酯对GO进行预处理,步骤较为繁琐。

Xie等[17]通过自由基均聚获得了聚丙烯酸酯(PA)功能化GO。经PA修饰后的GO在EP中的分散性与界面相互作用得以明显提升,这可能与GO改性后表面存在大量可参与环氧固化的羧基有关。此外,通过对修饰物的设计和选择,在实现GO良好分散的同时,还能增强复合涂层的疏水性,进一步提升防腐能力。所制备的PA-GO/EP涂层低频阻抗模量|Z|较纯树脂体系增加了近2个数量级,腐蚀速率也由最初的4.81×10−7 mm⋅a−1下降至1.70×10−8 mm⋅a−1

Ye等[19]采用氨基倍半硅氧烷(POSS-NH2)插层GO,基于倍半硅氧烷的纳米尺寸改善了GO的平整度、层间距与分散性,同时Si─O键的低表面能使涂层表现出优异的超疏水性,接触角最高可达153.8°,大大缩短了腐蚀介质在涂层表面的滞留时间。添加POSS-NH2/GO后,复合涂层的硬度、黏接强度、弹性、润滑性均有一定程度提升。相比于EP、GO/EP体系,当POSS-NH2/GO质量分数为0.5% 时,POSS-NH2/GO/EP涂层显示出最低的摩擦系数(0.32)及磨损率(2.81×l0−6 mm3⋅N−1⋅m−1),该涂层在质量分数为3.5% 的NaCl溶液中长期浸泡150 d后|Z|仍高达2.16×109 Ω⋅cm2,吸水率却仅为2.81% (见图 1)。

图 1 复合涂层防腐蚀机制[19] Fig.1 Anticorrosion mechanism of different composite coatings[19]

Zhu等[20]以聚甲基氢硅氧烷(PMHS)改性GO,利用PMHS的Si─H键与GO表面C═C加成,使GO富含疏水性Si─O键;此外,Si─H键还可结合组分中聚乙烯醇缩丁醛中的─OH,促进GO分散并通过交联固定其位置。与KH560改性剂相比,PMHS-GO涂层接触角增大了26°,在质量分数为3.5% 的NaCl溶液中浸泡1 200 h后依然保持完好。

(2) 非共价键修饰是一种物理吸附,改性方法主要有π-π键、氢键、离子键、静电力、范德华力等相互作用。相对于共价改性,它能够更好地保留石墨烯的原始结构,且反应条件较温和、反应过程可控,但由于作用力偏弱,修饰物的稳定性不及共价改性。譬如,Javidparvar等[24]曾以苯并咪唑-铈离子复合物对GO表面功能化,使得GO在聚合物基体中的分散性明显改观,且断裂应力、杨氏模量、断裂能和断裂伸长率分别提高了99.6%、40.3%、133.4%、446.4%。遗憾的是,此改性表面的结构容易遭到破坏。

Shang等[25]发现石墨烯在二甲基亚砜和水中的良好分散性可能源于边缘群效应和分子间范德华力。Arranz-mascaros等[26]则指出,π电子所携带的负电荷和弥散的电子云之间或许存在某种互相吸引,当2个π体系有非常接近的电荷密度时,表现出的色散作用可促进堆垛石墨烯的分散。Wu等[27]曾按此机理以六方氮化硼为修饰物,改善了GO的均匀分布以及环氧涂层的屏蔽抗腐能力。

综合而言,石墨烯及其衍生物共价与非共价键改性各有利弊,共价键改性虽保留了石墨烯的化学稳定性和机械强度,提高了反应活性,但晶格的本征属性被严重破坏,大大降低了导电、导热能力。非共价键改性对原有的导电、导热特性影响不大,但由于键合作用较弱,石墨烯分散程度也往往偏低。因此,科研人员应根据具体需求选择合适的修饰方式。

(3) 元素掺杂可以改变石墨烯的本征结构,赋予其新的优异性能,其中以引入N、B、P等元素较为常见。含磷基团具有强大的金属螯合能力,磷羟基可在金属表面形成P─O─Fe单分子保护膜,对石墨烯改性后可结合二者的优点,在促进其分散的同时,还能够改善涂层对基材的附着力。例如Wang等[30]曾选取植酸(PhA)为改性剂,利用PhA上的磷酸基促使环氧开环,为GO引入新的含磷五元环(见图 2)。多种检测表明:PhA的存在增加了GO层间距与分散性,PhA-GO/EP涂层极化电阻较纯EP提升3个数量级;在质量分数为3.5% 的NaCl溶液中浸泡1 080 h后涂层损失指数(CDI)不足30%,而同等条件下纯EP层CDI接近90%。

图 2 PhA、GO夹层间的可能反应及其在水中不同存储期的分散性[30] Fig.2 Possible reaction during PhA intercalation of GO and their dispersion in water after various storage periods[30]

李玉峰等[31]同样以PhA为原料,采用热解法制备了高含磷石墨烯。当该含磷石墨烯质量分数为3% 时,有机硅复合涂层表现出较好的疏水性和耐腐蚀能力,接触角达103.5°,吸水率为3.72%,腐蚀电流密度仅为3.53×10−10 A⋅cm−2,电化学阻抗(EIS)达3.82×107 Ω⋅cm2

此外,Huang等[32]也发现,丙烯酸磷酸酯单体可通过γ-巯丙基三乙氧基硅作为分子桥接枝到GO表面(PGO),促进了GO在树脂中的分散,且PGO/EP涂层历经质量分数为3.5% 的NaCl溶液浸泡40 d后|Z降幅仅为纯EP体系的25%。磷元素掺杂的GO,在涂层中不仅发挥了石墨烯自身的阻隔和屏蔽效应,同时起到缓蚀防锈效果,由于此掺杂手段相对简单,实际应用中或具有较大潜力。

(4) 原位聚合作为共价修饰的一种特殊形式,也不失为调节石墨烯分散性的有效途径,它可以将反应单体填充至纳米层状物的层间并聚合。Xue等[33]以聚环氧丙烷原位接枝GO,当质量分数为0.08 %时,聚氨酯涂层在质量分数为3.5% 的NaCl溶液中浸泡168 h后,电阻值仍为纯树脂涂层的50倍。

Wang等[34]借助树枝状双(二辛氧基焦磷酸酯基)乙撑钛酸酯(T2)和异丙基三(二辛基焦磷酸酰氧基)钛酸酯(T3)对GO插层。研究发现,T2、T3增大了石墨烯层间距,改善了GO在水性树脂中的分散性及相容性,同时T2、T3分子携带的─OH可与─NCO基团形成交联网络,使得涂层拉伸强度、断裂伸长率均有近2倍增长。与双支化的T2相比,T3-GO粒径更小,分散效力更佳,因而T3-GO复合涂层呈现更为优异的耐蚀性,|Z|及电阻值较空白涂层各提升1 000倍以上。

近年来,具有电化学防腐功能的聚苯胺分子逐渐应用于对GO的改性中,研究人员将还原氧化石墨烯(rGO)与磺化聚苯胺(SPANI)原位聚合,从而实现了rGO的均匀分散,并进一步增强了涂层的抗腐蚀能力。rGO与SPANI分子间存在氢键和π-π作用,可促进石墨烯分散,延缓腐蚀性介质渗透[35]。Zhu等[36]在rGO-Fe3O4表面连接聚苯胺(PANI),所制备的rGO-Fe3O4/PANI纳米复材中,GO组分高度分散,且Fe3O4与PANI之间独特的p-n键仅允许在某一特定方向上进行电子传输,从而限制了氧的还原;此外,rGO与PANI间π-π作用也有助于扩大PANI对中性介质的氧化还原活性,提高腐蚀防护效果。

Yang等[37]相继报道了一款GO-PANI纳米粒子,0.01 Hz低频下,GO-PANI/EP涂层EIS值为纯EP体系的55.2倍、EP/PANI体系的12.4倍,表明PANI对GO存在团聚抑制,并可通过填充涂层与金属表面间的空隙,进一步阻隔腐蚀介质的侵入。

另外,在石墨烯之间引入亲水性聚吡咯基团,也可实现石墨烯的剥离及其在聚合物中的稳定分散。Zhu等[38]通过原位聚合获得了聚吡咯(PPy)功能化GO纳米片。监测显示,PPy在一定范围内能够补偿GO表面空位等缺陷,扩大层间距,促进其在水和水性环氧中的分散。凭借GO优异的抗渗透性和PPy协同保护,仅质量分数为0.05% 的PPy-GO就可使复合涂层高度致密化,获得最佳防腐能力,腐蚀电流仅纯EP涂层的50%,但GO与PPy质量比m(GO)/m(PPy)不宜过大(< 2:1),若GO过量仍有可能存在堆叠风险。该小组进一步以传统磷酸锌为主填料,PPy-GO为辅助填料,利用磷酸锌与PPy-GO在超声处理过程中的相互作用,更大程度上改善了PPy-GO分散性,使复合涂层可以包含更多的非团聚G2P纳米片,其中m(GO)/m(PPy) =2:1[39]

2.2 石墨烯诱导定向

事实上,石墨烯耐蚀效果不仅取决于其在涂层中能否均匀分散,而且与排列方式密切相关。根据尼尔森修正模型[40],当纳米片平行于基体表面时易形成迷宫效应,有效阻隔电介质进入,但若二者间存在较大倾角,H2O、O2、Cl、SO42−等进入基体的路程大为缩短,腐蚀通道将被打开。一般情形下,石墨烯及其衍生物在涂层中的分布是无序、非定向的,为最大限度发挥屏蔽效能,可采取外界电、磁场诱导或以多层自组装形式促其定向[41-45]。Ding等[43]设计了改性多元醇负载Fe2O3的石墨烯材料,如图 3所示,对磁性石墨烯微纳片施加一定的水平磁转矩,能够使其在环氧涂层中旋转并有序排列。

图 3 磁性石墨烯在均匀磁场中的非定向、定向排列及其在富锌涂层中阻隔机制[43] Fig.3 Non-oriented and oriented arrangement of magnetic graphene in uniform magnetic field and its barrier mechanism in zinc-rich coatings[43]

Lao等[44]也曾利用苝二酰亚胺衍生物(PBI)改性石墨烯,再经旋涂工艺获得了高度取向的PBI-G/EP涂层。测试得知,石墨烯沿水平方向规整排列,当与PBI等比例添加,且质量分数为0.5% 时,PBI-G/EP阻抗值达109 Ω·cm2,较环氧空白涂层增加3个数量级。

Zhu等[45]通过氧化自聚合和电离反应制备了一种阳离子多巴胺还原的新型氧化石墨烯(DRGO+)纳米片。因分子中富含NH3+,它能够稳定分散于水性环氧乳液中(静置45 d无沉降),并在电场作用下牢固地自对准平行排列。从EIS谱图发现,质量分数为0.5% 的DRGO+/EP涂层初始|Z|高达4.79×1010 Ω⋅cm2,10倍于纯EP体系。腐蚀产物分析表明,DRGO+在涂层中形成了屏障网络,进而改善了物理阻隔效能并延长腐蚀剂渗透路径;此外,DRGO+上带正电的NH3+还可以静电吸引电子及Cl等,使金属基板表面形成Fe2O3/Fe3O4致密膜(见图 4)。然而,石墨烯在涂层中的有序分布机理研究还不够深入,加之其分散过程可控性较差,制备高度有序定向石墨烯复合涂层的方法尚未见报道。

图 4 DRGO+合成路线及电泳沉积过程[45] Fig.4 Schematic diagram of DRGO+ synthesis and electrophoretic deposition processes[45]
3 石墨烯在环氧防腐蚀涂层中的应用

环氧树脂因其良好的耐有机溶剂、低收缩率、高黏接强度等特性,常被用作防腐底漆的成膜聚合物,科研人员对石墨烯-环氧的协同增益研究比聚氨酯、丙烯酸体系更为深入[46-57],鉴此,本文针对性综述了石墨烯在该领域的最新进展。

3.1 环氧-石墨烯基防腐蚀涂层

石墨烯及其衍生物的层状纳米片结构,可大幅度增强涂层的阻隔性能。Rajitha等[47]以含萘环结构的2-氨基-4-(1-萘基)噻唑作为修饰物,使得改性后的GO体积更大,能够更好地封堵环氧固化物的孔隙和缺陷,利于生成更致密屏蔽层,涂层的|Z|提高了3个数量级,接触角增大10°。Qiu等[49]以聚(2-氨基噻唑)(PAT)为改性物,增强了石墨烯的分散性和腐蚀抑制性,质量分数为0.5% 的PAT-G/EP复合涂层在质量分数为3.5% 的NaCl溶液中浸泡期长达80 d,磨损率较空白样降低69.48%。

Zheng等[50]利用GO表面的羟基促使环氧基体开环,形成脂肪醚,改善了涂层的抗氯离子渗透性。检测证实,随着GO质量分数的增加,GO/EP复合涂层的疏水性和抗渗能力先增后减,Cl渗透速率则先减后增,质量分数为0.3% 时润湿角最大(96.1°)、饱和吸水率最低(2.18%)、Cl扩散系数最小(1.12×10−12 m2⋅s−1)。

Feng等[51]发现,rGO可提高环氧涂层在高温、高盐环境中的耐腐蚀能力,当质量分数为1.0% 时,rGO/EP涂层拥有最低的腐蚀电流密度(9.52×10−9 A⋅cm−2)、最高的腐蚀电位(−0.15 V)、最大的极化电阻(6.04×109 Ω⋅cm2),在质量分数为10% 的高浓度NaCl溶液中50 ℃下浸泡60 d,仍能维持较好强度与韧性。

由于石墨烯及还原氧化石墨烯结构中的缺陷少,具有良好的导电性,在锌粉涂料中得以大量应用。Ding等[52]尝试在水性环氧富锌漆中加入适量石墨烯以增强锌粉与金属之间电子流的连续性,从而提升有效锌的利用率。测试表明,石墨烯的阻隔作用减缓了腐蚀介质渗透,同时延长了牺牲阳极的持续时间,当质量分数为3% 时,涂层综合性能最佳。作者指出,空白涂层的开路电位最初呈缓降趋势,在−630 mV (铁的腐蚀电位)时维持不变;而G-Zn/EP涂层电位则是先降后升,且最低电位小于锌粉阴极保护的临界值(−860 mV),随着G-Zn质量分数的增加,腐蚀电位负值加大,阴极保护时间延长,其防腐机理可总结为初始屏蔽、阴极保护、屏蔽保护、失效演化4个阶段。类似的,Liu等[53]也确认了石墨烯可促进锌粉与钢材表面间的相互作用,质量分数为0.6% 的石墨烯就可大大改善环氧富锌层的阴极保护和屏蔽功能。

Yang等[54]通过添加锡锑氧化物、GO制得导静电防腐涂层,质量分数为0.3% 的GO复合体系对Q235钢黏接强度达8.92 MPa,远高于纯EP涂层(5.63 MPa),在质量分数为3.5% 的NaCl溶液中浸泡144 d后|Z|值仍可维持在9.8×107 Ω⋅cm2 (纯EP涂层浸泡87 d后仅1.6×106 Ω·cm2)。作者认为这主要归因于GO不仅起到物理隔离效果,还能够互相连接形成导电网络,提高了涂层的导静电和防腐蚀能力。不过在笔者看来,GO分子通常存在大量结构缺陷,其导电性偏弱,锡锑氧化物或许对该导电体系起到了更为明显的促进。

相比于线性聚合物,超支化大分子可对其枝杈结构进行设计,链接特定官能团,使得改性后的GO表面存在更多、更有针对性的功能位点,从而对涂层性能起到更积极的作用。例如Sari等[55]借助羟基封端的超分支聚酰胺(HB)对GO改性,提高其在环氧基体中的分散性,以及复合涂层黏附性和耐盐雾能力,|Z|高于1010 Ω·cm2,且HB分子中大量含氧结构(羟基、羰基)的存在,使附着力提升50% 以上(见图 5)。

图 5 不同环氧涂层对钢材的氢键黏附性及其耐盐雾后的表观形貌[55] Fig.5 Visual morphology of different epoxy coatings after salt spray and their adhesion to iron surface through hydrogen bonding[55]

Ramezanzadehm等[56]采用沸石咪唑酯有机金属骨架(ZIF-8)改性GO,其比表面积较GO提高了79%,GO@ZIF-8/EP涂层的|Z|高于1010 Ω·cm2,湿态附着力、阴极剥离强度、防腐效率较纯EP体系各增加73%、60%、70% (见图 6)。

图 6 GO@ZIF-8合成路线、附着力与阴极剥离强度及其环氧涂层经3.5% NaCl浸泡后的微观形貌[56] Fig.6 Synthesis of GO@ZIF-8 particles, pull-off and cathodic disbonding and micrographs of epoxy samples immersed in 3.5% NaCl[56]

Zhou等[57]以赖氨酸(LY)结合GO,制备出LY-GO纳米片和LY-GO/EP复合涂层。多种表征发现,改性后的LY-GO表面富含胺基,可均匀分散至聚合物基体中,并促进环氧开环,进而提高了涂层的交联密度和阻隔能力,LY-GO/EP腐蚀速率慢于纯EP及GO/EP近2个数量级(10−4 mm⋅a−1对10−2 mm⋅a−1)。在该研究中,赖氨酸的使用为我们提供了一种更为绿色低碳、可持续发展的新策略。遵循此环保理念的还有诸如Wang等[58]以木质素、Zhou等[59]以天然植酸、Mohammadkhani等[60]以骆驼蓬种子提取物、Badr等[61]以壳聚糖等对GO功能化。不过上述天然提取物成本昂贵,现阶段还难以实现其在防腐涂料中的大规模应用。

3.2 石墨烯基自修复涂层

值得一提的是,近年来自修复涂层的防腐能力也日益受到关注,它不需要人为干预,可以在涂层缺陷部位裸露金属表面形成致密的催化钝化膜或缓释吸附膜,通过封闭缺陷,或抑制缺陷处的腐蚀反应来自动恢复涂层的物理屏障功能。石墨烯及其衍生物巨大的比表面积使其能够负载各类缓蚀剂,当涂层出现破损后,缓蚀剂从石墨烯表面释放,并使裸露的金属基底表面钝化,从而实现自修复。

早在2018年,Liu等[62]获得了咪唑离子液体(IL)共价接枝GO纳米材料,SEM、EIS及扫描振动电极表征显示,离子液体的存在可有效促进石墨烯在水及水性环氧聚合物中的稳定分散,并抑制阳极溶解;改性后的IL-GO杂化体呈现典型的片状结构,能在涂层缺陷处抑制腐蚀发生。

Mohammadkhani等[63]选用静电吸附锌离子掺杂导电聚吡咯纳米颗粒改性GO,设计了具有自愈、防腐双重功能的环氧复合体系。

Chen等[64]利用静电力将磷酸盐插层水滑石(PIH)复合至GO表面,PIH提高了GO在环氧涂层中的分散性,而水滑石层间的PO43−又能从GO-PIH释放并沉积在金属表面形成钝化区,赋予涂层主动防御能力。

中科院宁波材料所Ye等[65]多年来致力于石墨烯防腐材料研究,所制备的硅烷化苯胺三聚体(SAT)在一定条件下水解后可与石墨烯形成交联网络,确保了石墨烯在水及环氧涂层中的无损分散,修饰后的石墨烯厚度仅1.4~1.6 nm(4~5层),阻隔作用明显;而SAT仍保持了苯胺低聚物完整的电活性,可作为缓蚀剂抑制微小缺陷处的腐蚀反应。该SAT-G/EP涂层能够在金属基面形成Fe2O3、Fe3O4钝化膜,从而表现出自修复性和长效防护。同时,该课题组通过化学接枝八氨基倍半硅氧烷获得了含GO的多孔骨架(8-PG),随后负载苯并三氮唑(BTA)制得石墨烯基纳米容器(见图 7)。研究发现,随着浸泡时间的推移,纯EP涂层下的腐蚀和扩散非常严重,而8-PG-BTA/EP复合层却相对轻微并伴有逐渐减弱趋势。这归因于BTA分子的释放不仅发挥了缓蚀作用,同时也提高了石墨烯涂层的致密度,抑制了腐蚀介质的纵向扩展[66]

图 7 石墨烯基纳米容器制备及BTA负载过程[66] Fig.7 Preparation of graphene-based nanocontainer and the BTA loading process[66]

Ma等[67]则将GO分散液加入异佛尔酮二异氰酸酯和聚醚胺D2000的混合液中,得到聚脲预聚体,将此预聚体与1, 6-己二胺界面聚合,制备了直径仅0.5 mm的单层或双层微胶囊,该胶囊在低于325 ℃范围内有良好热稳定性,其质量分数为3% 的环氧涂层自修复效率高达80%。

Javidparvar等[68]以Ce3+与苯并咪唑改性GO,成功构建了可智能缓释的纳米容器复合体,促进了GO在环氧基体中的分散,当腐蚀介质侵入时,该复合体能够从GO表面脱附;同时Ce3+还可结合─OH生成氧化铈保护层,在质量分数为3.5% 的NaCl溶液中浸泡42 d后涂层阻隔和主动防腐性能较空白样提升2个数量级(见图 8)。

图 8 GOBICe纳米复合体制备以及铈离子/苯并咪唑分子在阴、阳极的缓蚀性能[68] Fig.8 Preparation of GOBICe nanocomposites and inhibition performance of Ce3+ and benzimidazole molecules at anodic and cathodic areas[68]

Ramezanzadeh等[69]将锌或氧化铈以静电或阳离子-π共轭方式吸附在聚苯胺分子上,彼此间的协同有助于在钢材表面形成高质量防护膜。与此方向相近的还有:Asaldoust等[70]在GO表面负载苯并咪唑和磷酸锌,Akbarzadeh等[71]负载酸角提取物与Zn2+,Hao等[72]在GO表面接枝聚苯胺链段后进一步复合苯并三唑等。

3.3 石墨烯防腐涂层的工程化应用

在基础研究不断深入的同时,石墨烯防腐的商业化也逐渐纳入日程,BASF、PPG、Hempel等知名企业均开展了相关探索,产品迄今已在珠海“万山号”鹰式波浪能发电平台、浙江舟山金塘岛世界第一输电高塔(380 m)、江苏龙源海上风场、泉州湾跨海大桥、柬埔寨“200 MW双燃料电站”、印尼“雅万高铁”等国内外大型工程中得以应用。据石墨烯产业技术创新战略联盟早前预测,2022年仅中国石墨烯防腐涂料市场规模就有望突破48亿元,产业前景极为广阔。

4 总结与展望

被誉为21世纪“新材料之王”的石墨烯,经过十几年的发展,其巨大的商业价值正吸引着全世界的目光,凭借优异的屏蔽、阻隔、自修复和电化学保护特性,在防腐蚀领域具有相当规模的应用空间。大量数据表明,石墨烯及其衍生物在涂层中的均匀分散和有序排列对防腐蚀性能的发挥至关重要,为此包括共价键、非共价键修饰在内的各种改性方法被开发。然而,共价修饰会破坏石墨烯的原始结构,且存在有毒试剂使用量大,容易造成环境污染等问题。相比之下,石墨烯的非共价键修饰,如π-π堆叠、离子键和氢键,是较为环保的途径之一,但当前研究还不充分,需要更系统的探索。此外,石墨烯的所谓有序排列也只是实现了相对均匀的定向,远没有达到理想状态的高度有序,这些都是未来研究的热点。

凭借其片层结构和超大比表面积,石墨烯可以延长腐蚀介质的渗透路径,作为功能填料加入含锌涂层中还能够增强对牺牲阳极的保护,因而比传统涂层具有更令人满意的耐腐蚀性能。但石墨烯的保护机制仍未完全明确,如何规避大阴极小阳极现象可能引发的金属局部腐蚀加速,如何深入剖析耐蚀机理、建立和完善长效考核标准等系列问题尚待进一步厘清和解决。未来通过丝束电极技术与常规电化学方法的联合应用,探究复合涂层保护下的金属电位分布或许是研究石墨烯防腐机理的有效手段。最后,在涂装行业普遍强调降低挥发性有机物含量的大趋势下,石墨烯在水性防腐涂料中的应用或有望成为将来发展方向,需引起重视。

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