Self-healing superhydrophobic shape memory epoxy resin/polydimethylsiloxane@ZnO@SiO2 coating and its anticorrosion performance
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摘要: 本文针对环氧树脂基类超疏水防腐涂层的物理损伤修复速度较慢,并结合不锈钢金属长期处于高湿高盐等环境下高效防腐与保护的实际应用需求,以微晶蜡强化的形状记忆环氧树脂涂层(SMEP)为底层,以超疏水材料聚二甲基硅氧烷(PDMS)@ZnO@SiO2(PZS)为表层,基于双层设计制备了一种较快修复机械损伤、不锈钢持久防腐的自修复超疏水涂层SMEP/PDMS@ZnO@SiO2(SMEP/PZS),并着重对其修复前后SMEP/PZS涂层的润湿性、耐蚀性及其自修复与防腐机制等进行了深入探讨。结果表明,SMEP/PZS涂层具有良好的超疏水性及自清洁等相关性能,其超疏水性随着低表面能物质全氟癸基三甲氧基硅烷 (PFDTMS)含量增加而增强,水接触角、滚动角最佳可达157.6°和2.6°。其次,SMEP/PZS涂层有较快的自修复能力,将该机械模拟的受损涂层于85℃下在20 min较短时间内进行修复,其最佳机械划痕由45 μm缩小至1.0 μm,修复率达97.8%。此外,SMEP/PZS涂层表现良好的耐蚀性,且将修复后的该涂层置于3.5wt% NaCl溶液中浸泡14天后,其耐腐蚀性接近于原始涂层。将SMEP/PZS涂层涂覆在不锈钢基底上,在3.5wt% NaCl溶液中所测点蚀电位Eb与裸不锈钢的相比正移近10倍,维钝电流密度Ip下降2个数量级,对304不锈钢基底具有相对更为持久的防腐与保护。最后,进一步探讨了SMEP/PZS涂层的自修复与防腐蚀机制。Abstract: In this paper, based on the two-layer design, a self-healing superhydrophobic coating self-healing shape memory epoxy resin (SMEP)/polydimethylsiloxane (PDMS)@ZnO@SiO2 (SMEP/PZS) that could quickly repair physical damages and durably corrosion resistance of stainless steel was prepared. Aiming to solve the slow physical damage repair of epoxy-based superhydrophobic anti-corrosion coatings, and inegrate with the practical application requirements of high efficiency anticorrosion and protection of stainless steel in high humidity and high salt environment for a long time. The double-layer coating was designed by combination of a SMEP and a superhydrophobic material PDMS@ZnO@SiO2 (PZS). Furthermore, the wettability, corrosion resistance, self-healing and corrosion resistance mechanism of SMEP/PZS coating before and after repairing were discussed in detail. The results show that SMEP/PZS coating has excellent superhydrophobicity and self-cleaning properties, and its superhydrophobicity increases with the increase of perfluorodecyltrimethoxysilane (PFDTMS) content, its optimal water contact angle and rolling angle are 157.6° and 2.6°. Secondly, SMEP/PZS coating has a faster self-healing ability, the best mechanical scratches of SMEP/PZS coating are reduced from 45 μm to 1.0 μm in a shorter time of 20 min at 85℃, and the repair rate reaches 97.8%. In addition, the SMEP/PZS coating shows good corrosion resistance, and after the repaired coating is immersed in 3.5wt% NaCl solution for 14 days, its corrosion resistance is closed to the original coating. When the SMEP/PZS coating is coated on the stainless steel substrate, the pitting corrosion potential Eb measured in 3.5wt% NaCl solution has increased by nearly 10 times compared with that of the bare stainless steel, and the passive current density Ip has decreased by 2 orders of magnitude, showing relatively longer corrosion resistance and protection for 304 stainless steel substrate. Finally, the self-healing and anti-corrosion mechanism of SMEP/PZS coating is further discussed.
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全球每年因腐蚀造成的金属损失量占全年金属总产量的20%~40%,约为自然灾害损失的4倍左右[1-2]。与常用的阳极保护、缓蚀剂、有机涂层等[3]防腐方法不同,超疏水防腐涂层因其表面独特的润湿性,能够在固-液界面处形成空气夹层,腐蚀性介质难以浸入涂层内部[4],使其具有一定的耐腐蚀、防污等性能,为高盐高湿等特殊条件下金属防腐与保护提供了一种新思路与方法。然而,超疏水防腐涂层在实际应用过程中不可避免地会遭受化学或物理损伤,若不能得到及时修复,容易导致涂层丧失其超疏水及相关的耐腐蚀等性能。
最近研究表明,通过超疏水防腐涂层的自修复功能化,在光、热等刺激下能够恢复其受损表面及功能,实现金属防腐与保护并延长其使用寿命[5-7]。目前关于超疏水涂层的自修复研究主要基于低表面能组分迁移、自愈合剂释放等,然而这种自修复仅针对由O2等离子体造成的化学损伤[8-10],对于在应用过程中造成的物理损伤修复通常无效。超疏水防腐涂层的物理损伤通常是由于机械划伤、表面磨损及相互碰撞等造成的,在实际应用中更常见,但其相关报道与化学损伤修复研究相比较少[11-12]。目前超疏水涂层的物理损伤修复主要通过分子间氢键、动态共价键作用和引入形状记忆聚合物等实现。其中,通过分子间作用来修复超疏水涂层的物理受损区域,存在原料需具有特定官能团、种类有限且制备过程复杂的问题。而在超疏水涂层中引入环氧树脂、聚氨酯等形状记忆性聚合物(SMPs)[13-17],无需引入化学可逆键或交联点,可以在热刺激下诱导其形状记忆效应使涂层恢复至原始形状,同时制备过程相对简单且成本较低。Zhao等[18]以镁合金为基底,利用环氧树脂和全氟改性的纳米SiO2制备了自修复超疏水涂层,该涂层受到机械划伤后,需经两步加热60 min才可修复其受损表面。Zhang等[19]以环氧树脂和凹凸棒土(ATP)、缓蚀剂苯丙三氮唑(BTA)为主要原料,制备了用于镁合金防腐的自修复超疏水涂层,该受损涂层需加热60 min后才可恢复其超疏水性、防腐性能等。然而,目前基于形状记忆环氧树脂型自修复超疏水涂层研究主要集中于镁合金基底,且存在修复时间较长等难题。
本文针对目前环氧树脂基超疏水防腐涂层的物理损伤修复时间较长,结合不锈钢这种常用金属长期处于高湿高盐等环境下极易发生腐蚀等实际问题,制备一种用于不锈钢基底防腐的自修复超疏水双层涂层微晶蜡强化的形状记忆环氧树脂涂层(SMEP)/聚二甲基硅氧烷(PDMS)@ZnO@SiO2(SMEP/PZS)。通过在环氧树脂基底层SMEP中引入低熔点物质微晶蜡,基于环氧树脂的形状记忆效应与微晶蜡熔融流动,双重加快底层自修复并快速带动PDMS@ZnO@SiO2 (PZS)表层愈合。基于PZS超疏水表层与SMEP自修复底层的特点及其各自耐蚀性的优势,提升SMEP/PZS涂层对不锈钢防腐与保护。在此基础上,探讨该涂层在不锈钢基底上的自修复与防腐机制。
1. 实验材料及方法
1.1 原材料
无水乙醇、乙酸乙酯、NaCl购自天津市富宇精细化工有限公司;SiO2((30±10) nm)、ZnO((30±10) nm)、微晶蜡、双酚A二缩水甘油醚(DGEBA)、新戊二醇二缩水甘油醚(NGDE)、聚醚胺(D-230)、全氟癸基三甲氧基硅烷(PFDTMS)、胺固化剂(脂肪族,胺值600~700 mg KOH/g,黏度(25℃:80~150 mPa·s))、十二烷基苯磺酸钠(纯度90%)购自上海麦克林生化科技有限公司;PDMS (PDMS-SYLGARD 184)及其固化剂购自道康宁;304不锈钢(40 mm×20 mm×1 mm)购自深圳市入君怀贸易有限公司。所有试剂均为分析纯,无需进一步纯化直接使用,实验中所使用到的水均为超纯水。
1.2 SMEP自修复涂层的制备
将0.85 g DGEBA于65℃预热30 min后,加入0.54 g NGDE、0.56 g D-230、0.5 g胺固化剂及3 mL乙酸乙酯,磁力搅拌至体系均一透明后加入0.05 g SiO2纳米填料和2 mL乙酸乙酯,继续磁力搅拌0.5 h得到淡黄色浊液A。同时,在85℃下使用5 mL正己烷完全热熔0.5 g微晶蜡,再加入0.025 g十二烷基苯磺酸钠,磁力搅拌0.5 h后得到白色浊液B。将A、B混合搅拌至一定黏度即得到形状记忆环氧树脂混合液SMEP,将其涂覆于304不锈钢基底上,在70℃下固化5 h即得到SMEP涂层。
1.3 PDMS@ZnO@SiO2 (PZS)超疏水涂层的制备
0.5 g ZnO((30±10) nm)和0.5 g SiO2((30±10) nm)与10 mL乙酸乙酯超声混合20 min,向其中加入0.1 g·mL−1 PDMS/乙酸乙酯溶液,在温度为35℃下磁力搅拌混合体系(r=400 r/min) 20 min,再加入0.1 g PDMS固化剂(PDMS和固化剂质量比为10∶1)和不同量的PFDTMS,继续搅拌并反应1 h后得到白色的超疏水分散液PZS,将其用HD-470喷枪喷涂于不锈钢基底上即可得到PZS涂层。
1.4 SMEP/PDMS@ZnO@SiO2 (SMEP/PZS)自修复超疏水涂层的制备
SMEP/PZS自修复超疏水涂层的制备流程如图1所示,以304不锈钢为基底(40 mm×20 mm×1 mm),涂覆一层厚度约为(450±10) μm的自修复SMEP作为底层,并在70℃下固化5 h。在此基础上,采用HD-470喷枪在15 cm的距离和0.2 MPa的压力下,将2 mL超疏水悬浮液PZS喷涂于SMEP自修复涂层表面,在70℃下固化3 h后,最终形成厚度为(500±10) μm的SMEP/PZS自修复超疏水涂层。
图 1 形状记忆环氧树脂(SMEP)/聚二甲基硅氧烷(PDMS)@ZnO@SiO2 (SMEP/PZS)涂层的制备过程示意图Figure 1. Preparation process of shape memory epoxy resin (SMEP)/polydimethylsiloxane (PDMS)@ZnO@SiO2 (SMEP/PZS) coatingPFDTMS—Perfluorodecyltrimethoxysilane; NGDE—Neopentyl glycol diglycidyl ether; DGEBA—Bisphenol A diglycidyl ether; D-230—Polyetheramine D-230; EA—Ethyl acetate1.5 表征与分析
采用美国热电公司的Nicolet 5700傅立叶变换红外光谱仪测定SMEP、PZS的特征官能团,用干燥的KBr压片法进行测试,波长测试范围为500~4000 cm−1。采用日本日立FlexSEM1000扫描电镜对所制备的SMEP/PZS涂层的表面形貌和修复情况进行表征,加速电压3 kV,测试前对样品进行喷金处理。采用德国Kruss的DSA100接触角测量仪对涂层的润湿性进行表征,液滴体积为5 μL,测量5次求平均值。采用法国Bio-logic的VSP-300电化学工作站,通过交流阻抗谱(EIS)和极化曲线(TP)分别评价涂层耐蚀性、不锈钢防腐性能。用传统的三电极系统,以3.5wt% NaCl溶液为电解液,铂电极(1.0 mm×1.0 mm×0.1 mm)为对电极,饱和甘汞电极为参比电极,不锈钢涂层为工作电极,暴露表面积为1 cm2,测试频率为10−2~105 Hz。在−250至50 mV范围内以1 mV·s−1的速率记录极化曲线,并通过EIS对修复后的SMEP/PZS涂层进一步分析表征。通过SEM标尺和PS标尺工具对涂层的划痕宽度进行5次测量,求其平均值。
2. 结果与讨论
2.1 SMEP/PDMS@ZnO@SiO2 (SMEP/PZS)涂层的润湿性与表面形貌
接触角与滚动角是评价涂层表面润湿性的重要指标,若涂层表面润湿性低,液滴在其表面不易扩散且易滚落,所测表面的接触角大、滚动角小,具有较强疏水性。表1为4种超疏水涂层PZS (质量比PDMS∶ZnO∶SiO2 =2∶1∶1)、4种自修复超疏水涂层SMEP/PZS的水接触角及滚动角。可知8种涂层均具有超疏水性。与单独喷涂PZS涂层相比,在不锈钢基底上依次涂覆自修复涂层SMEP与超疏水涂层PZS,所得到的SMEP/PZS涂层超疏水性略有降低。将其浸泡于3.5wt% NaCl溶液12 h后,未添加PFDTMS的SMEP/PZS和PZS涂层丧失了其超疏水性,随着其中低表面能物质PFDTMS的增加,PZS、SMEP/PZS涂层的润湿性得到提升。这可能是当涂层中PFDTMS含量较多时,纳米粒子表面的羟基与全氟硅烷水解的羟基之间缩合作用更完全,使其表面的亲水性基团被大量的—CF2CF3基团所取代,导致涂层疏水性较好,因此可以起到对金属的防腐保护作用。此外,由图2的PZS超疏水涂层FTIR图谱看出,伴随3400 cm−1处纳米粒子—OH特征峰的消失,由于ZnO和SiO2纳米颗粒上的羟基被—CF2CF3疏水性基团取代,在1470 cm−1处出现了PFDTMS中C—F键的特征峰,这是PZS超疏水涂层和SMEP/PZS涂层具有超疏水性的关键因素之一。
表 1 PZS和SMEP/PZS涂层表面的水接触角(CA)及滚动角(SA)Table 1. Contact angle (CA) and rolling angle (SA) of PZS and SMEP/PZS coatingsPZS SMEP/PZS 0wt% 7wt% 21wt% 42wt% 0wt% 7wt% 21wt% 42wt% Unsoaked CA/(°) 151.8 154.1 156.7 159.4 150.9 153.6 155.9 157.6 SA/(°) 6.9 4.6 3.6 1.8 7.8 5.3 3.9 2.6 Soaked 12 h CA/(°) 149.1 152.1 154.5 155.8 148.9 151.9 153.8 155.4 SA/(°) 12.6 6.5 5.0 4.1 13 6.4 5.2 4.4 图3为500倍的SMEP/PZS、SMEP、PZS涂层表面形貌,其中左上角为其1000倍的表面形貌。图3(a)中SMEP自修复涂层的表面较光滑,且含有丰富的氮/氧亲水性基团[20-21],水接触角CA去离子水仅为86.6°,表现出亲水性。由图3(b)~3(g)可知,涂层表面形貌均为典型Cassie-Baxter模型[18]的超疏水微纳米粗糙结构,且随着涂层中低表面能物质的增多,表层变得相对致密与颗粒分布更均匀。然而,将涂层表面形貌放大1000倍后观察,与在光滑的不锈钢硬基底上单独喷涂PZS涂层相比,若在不锈钢上先涂覆SMEP自修复涂层再喷涂PZS涂层(图3(e)~3(g)),受SMEP涂层中环氧树脂具有高黏性和喷枪喷涂时压力作用,两层涂层之间的黏附增强且出现部分颗粒嵌入与聚集现象,导致SMEP/PZS涂层表面的致密度有所降低,这与超疏水性略有下降的研究结果一致。此外,使用石墨粉模拟人造粉尘对自修复超疏水涂层SMEP/PZS进行自清洁性评价见图4,不同含量PFDTMS的原始涂层均具有自清洁能力,当PFDTMS含量为42wt%时涂层的自清洁性最好,这与该涂层的表面形貌及润湿性研究结果一致。
图 3 SMEP涂层 (a);7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d)的PZS涂层;7wt% PFDTMS (e)、21wt% PFDTMS (f) 和42wt% PFDTMS (g) 的SMEP/PZS涂层的SEM图像Figure 3. SEM images of SMEP coating (a); PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d); SMEP/PZS coating with7wt% PFDTMS (e), 21wt% PFDTMS (f), 42wt% PFDTMS (g)2.2 SMEP/PZS涂层的防腐性能
EIS图通常用来分析涂层的耐腐蚀性能[22]。图5为裸不锈钢基底和3种涂层在3.5wt% NaCl溶液中的Nyquist图和Bode图。Nyquist图中容抗弧半径越大,电荷转移电阻越大,对电解质中腐蚀离子的屏蔽作用越强[17]。可知,与裸不锈钢、SMEP、PZS涂层相比,其耐蚀性SMEP/PZS>SMEP>PZS>304不锈钢。图5(b)中SMEP/PZS涂层的低频阻抗模量(|Z|0.01 Hz = 3.6×105 Ω·cm2)最大,其|Z|0.01 Hz值代表反应电阻和溶液电阻之和[23-24],反映对腐蚀性介质良好的阻隔性。图5(c)中SMEP/PZS涂层的相位角更接近90°,可近似等效为纯电容,其耐蚀性最佳。此外,从图5(d)看出,随着低表面能物质PFDTMS含量的增加,SMEP/PZS涂层的耐蚀性趋于上升,这与润湿性变化趋势一致。
图 5 裸不锈钢、PZS、SMEP及SMEP/PZS涂层的交流阻抗谱:(a) Nyquist图;(b) 阻抗模量曲线;(c) 相位角曲线;(d) 不同PFDTMS含量SMEP/PZS涂层的阻抗模量曲线Figure 5. Alternating current impedance spectroscopy of bare stainless steel, PZS, SMEP and SMEP/PZS coatings: (a) Nyquist diagram; (b) Impedance modulus curves; (c) Phase angle curves; (d) Impedance modulus curves of SMEP/PZS with different content of PFDTMSZ′—Real impedance; Z"—Imaginary impedance; Z—Polarization impedance在304不锈钢基底上分别涂覆SMEP/PZS涂层、SMEP底层、PZS表层,在3.5wt% NaCl溶液中进行动电位极化曲线测试,其动电位极化曲线及相关参数见图6和表2。
表 2 由裸不锈钢和分别涂覆PZS、SMEP及SMEP/PZS涂层的不锈钢基底的极化曲线所得相关评价参数Table 2. Related evaluation parameters obtained from the polarization curves of bare stainless steel and stainless steel substrates coated with PZS, SMEP and SMEP/PZSCoating Ecorr/V Icorr/A Eb/V Ip/A 304 stainless steel −0.752 9.728×10−5 0.0562 1.020×10−4 PZS −0.406 1.458×10−6 −0.04 7.476×10−6 SMEP −0.269 6.042×10−7 0.0455 6.783×10−6 SMEP/PZS −0.160 5.283×10−8 0.312 6.672×10−6 Notes: Ecorr—Self-corrosion potential; Icorr—Self-corrosion current density; Eb—Pitting potential; Ip—Passive current density. 由图6的极化曲线分析可知,在活化区域内不锈钢受活化极化控制,金属发生活性溶解腐蚀。相比于裸不锈钢,在其表面分别涂覆3种涂层后,通过Tafel外推法获得的自腐蚀电位(Ecorr)出现正移,自腐蚀电流密度(Icorr)降低1~3个数量级,其中,涂覆SMEP/PZS涂层的不锈钢在3.5wt% NaCl溶液中的活性溶解腐蚀速率相对较小。
作为评价钝态金属防腐性能的重要指标,点蚀电位为钝态金属表面引起点状腐蚀的最低电位值,维钝电流密度反映阳极保护时钝态金属的腐蚀速率。与裸不锈钢相比,在其表面涂覆涂层后所测的点蚀电位均正向移动,维钝电流密度下降了2个数量级,这表明涂覆涂层后对不锈钢基底具有明显的防腐与保护作用。其中,在不锈钢表面单独涂覆SMEP底层或PZS表层时点蚀电位相对较小,而在不锈钢表面依次涂覆SMEP底层和PZS表层形成SMEP/PZS双层涂层时,其点蚀电位提升了近10倍。相对而言,对于涂覆SMEP/PZS涂层的不锈钢,其点蚀电位达到最高(Eb=0.312 V),维钝电流密度最小(Ip=6.672×10−6 A/cm2),且钝化区的电位范围较宽,不锈钢基底的钝化与防腐效果较好。以上结果表明SMEP自修复涂层与PZS超疏水涂层在不锈钢防腐方面具有一定的协同增效作用,这与涂层耐蚀性的变化趋势相一致。
2.3 SMEP/PZS涂层的自修复性能
用刀片划伤涂层表面以模拟其机械受损,在SEM下观察受损涂层的愈合程度,图7为修复前后涂层的表面形貌。由图7(a1)~7(d1)可知(横向看),用刀片划伤不同含量PFDTMS的SMEP/PZS涂层,所形成的机械受损宽度大约为38~45 μm,经85℃加热20 min后,划痕修复至1~10 μm,见图7(a2)~7(d2),其中7wt% PFDTMS的SMEP/PZS涂层修复后受损宽度由45 μm缩小至1 μm,修复率达到97.8%,划痕愈合效果最佳。与图7(a1)、图7(a2)的SMEP涂层相比,在SMEP表面喷涂超疏水PZS材料后得到的SMEP/PZS双层涂层,其自修复性能仍然较好(图7(b1)~7(d2))。其中,环氧树脂受到机械破损后,当加热温度高于其玻璃化转变温度Tg时,由玻璃态变为高弹态,形状记忆效应被激活,使受损涂层恢复原始形状。同时,当加热温度高于微晶蜡的熔点时,熔融微晶蜡的流动进一步提升底层SMEP的自修复性能。基于以上两者的协同作用,带动PZS表层的流动,促进了整个SMEP/PZS涂层的自主修复,进而恢复其防腐、防污等性能。
图 7 SMEP涂层 ((a1)、(a2))和7wt% PFDTMS((b1)、(b2))、21wt% PFDTMS ((c1)、(c2))、42wt% PFDTMS ((d1)、(d2)) 的SMEP/PZS涂层修复前后涂层的SEM图像(脚标1为修复前,脚标2为修复后)Figure 7. SEM images of SMEP coating ((a1), (a2)) and SMEP/PZS coating with 7wt% PFDTMS ((b1), (b2)),21wt% PFDTMS ((c1), (c2)), 42wt% PFDTMS ((d1), (d2)) before and after healing (Pin 1 is before repair, pin 2 is after repair)表3展示了涂层修复前后的润湿性。研究表明,不同含量PFDTMS的SMEP/PZS涂层机械划伤后,涂层均丧失了超疏水性,其划伤处水接触角下降了6.6°~11.5°,滚动角增加了11.3°~13.5°。将受损涂层SMEP/PZS置于85℃条件下自修复20 min后,其CA值和SA值接近原始涂层,重新恢复了其超疏水性。其中PFDTMS含量为7wt%的SMEP/PZS涂层超疏水性修复相对最好,其CA值由受损涂层的147.0°恢复至152.4°,与SMEP/PZS原始涂层的相比仅差1.2°。
表 3 修复前后不同含量PFDTMS的SMEP/PZS涂层的接触角及滚动角Table 3. Contact angle (CA) and rolling angle (SA) of SMEP/PZS coating with different contents of PFDTMS before and after healingCA/(°) SA/(°) 7wt% 21wt% 42wt% 7wt% 21wt% 42wt% Original 153.6 155.9 157.6 5.3 4.9 2.6 Scratched 147.0 145.4 146.4 16.6 17.8 16.1 Healed 152.4 153.8 155.3 5.9 5.8 4.9 通过在3.5wt% NaCl溶液中进行EIS测试,根据修复后涂层耐蚀性的恢复情况进一步评价SMEP/PZS涂层的自修复性能。图8为涂层修复前后的低频阻抗模量曲线,对于涂覆SMEP涂层的不锈钢(图8(a)),其低频阻抗模量(|Z|0.01 Hz)为2.8×105 Ω·cm2,划伤后,降至1.83×105 Ω·cm2,而经过热处理后,恢复至2.74×105 Ω·cm2,这表明SMEP涂层具有较好的自修复性能。而在SMEP表面喷涂不同PFDTMS含量的PZS超疏水材料后,所得SMEP/PZS双层涂层的低频阻抗模量曲线见图8(b)~8(d)。当PFDTMS含量分别为7wt%、21wt%、42wt%时,SMEP/PZS原始涂层的|Z|0.01 Hz分别为2.9×105 、3.01×105 、3.6×105 Ω·cm2,划伤后下降至1.96×105 、1.62×105 、1.73×105 Ω·cm2,经加热后其|Z|0.01 Hz值修复至接近于原始涂层,分别为2.88×105 、2.9×105 、3.14×105 Ω·cm2,这表明SMEP/PZS涂层具有优异的机械损伤修复能力。相对而言,SMEP/PZS双层涂层中PFDTMS低表面能物质含量为7wt%时,修复前后SMEP/PZS涂层的低频阻抗模量曲线的重合度最高,证明7wt% PFDTMS涂层的自修复性能最好,这与表面形貌和润湿性研究结果一致。
2.4 自修复后SMEP/PZS涂层的耐腐蚀性
通过3.5wt% NaCl溶液浸泡试验研究修复后涂层对腐蚀性介质的阻隔性能,经过1天、7天、14天浸泡后其低频阻抗模量值变化情况,如图9所示。由图9(a)可知,对于修复后SMEP涂层,浸泡1天后,该涂层|Z|0.01 Hz由2.28×105 Ω·cm2降至1.46×105 Ω·cm2,随着浸泡时间的延长,该值持续下降,浸泡14天后下降至9.0×104 Ω·cm2。而由图9(b)~9(d)看出,在SMEP表面喷涂PZS后,用刀片划伤涂层模拟机械损伤,对于修复后不同PFDTMS含量SMEP/PZS涂层,其|Z|0.01 Hz值分别为2.88 ×105、2.9×105 、3.14×105 Ω·cm2,经过14天浸泡后其|Z|0.01 Hz值分别减小至1.79×105、1.23×105、1.12×105 Ω·cm2,所测|Z|0.01 Hz值在数量级上未发生下降。由此可知,与SMEP涂层的相比,SMEP/PZS涂层的|Z|0.01 Hz值下降较缓慢,其耐蚀性更好。这表明PZS超疏水表层与SMEP自修复底层对SMEP/PZS涂层的耐蚀性具有一定的协同增效作用。
图 9 SMEP涂层 (a)、7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d) 的SMEP-ZPS涂层修复后于3.5wt% NaCl溶液浸泡1天、7天、14天后的阻抗模量曲线Figure 9. Impedance modulus curves after healing and immersion in 3.5wt% NaCl solution for 1 day, 7 days, 14 days of SMEP coating (a) and SMEP/PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d)图10为修复后各涂层浸泡于3.5wt% NaCl溶液14天后的光学照片。可以看出,若不喷涂PZS超疏水表层,修复后的SMEP涂层表面经3.5wt% NaCl溶液浸泡14天后,出现了明显的腐蚀现象,见图10(a)。然而,若将PZS超疏水表层喷涂在SMEP底层,所制备的SMEP/PZS涂层经过自修复后其表面在3.5wt% NaCl溶液浸泡14天后,并未出现的腐蚀痕迹。关于PFDTMS含量不同对修复后涂层的耐腐蚀性影响见图10(b)~10(d),研究表明,如果仅从宏观光学图片进行观察,修复后涂层的耐蚀性均表现良好且未发现明显差异。然而,通过低频阻抗模量值进行评价,相对而言,当SMEP/PZS涂层中PFDTMS含量为7wt%时,随着浸泡时间的延长,其修复后的|Z|0.01 Hz值变化最小,浸泡14天后仍然高达1.79×105 Ω·cm2,涂层修复效果及耐蚀性最佳。
图 10 SMEP涂层 (a)、7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d) 的SMEP-ZPS涂层修复后浸泡于3.5wt% NaCl溶液14天后的光学照片Figure 10. Optical photos of the healed coating after being immersed in 3.5wt% NaCl solution for 14 days of SMEP coating (a), SMEP/PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d)2.5 SMEP/PZS涂层的自修复与防腐机制
所制备SMEP/PZS涂层兼具SMEP底层的热刺激自修复和PZS表层超疏水防腐等特点,该涂层自修复与防腐机制示意图见图11。当SMEP/PZS涂层受到相对严重的机械划伤时,导致其PZS表层的多级粗糙结构、SMEP底层的结构缺陷或者化学物质损失,表面润湿性由Cassie态易变为Wenzel态而丧失了超疏水性,腐蚀性介质会沿着局部受损处渗入从而对金属造成腐蚀。
在85℃的热刺激下,由于该温度超过了SMEP/PZS涂层中SMEP底层的环氧树脂玻璃化转变温度和微晶蜡的熔点,有效激发环氧树脂的形状记忆效应与微晶蜡熔融流动,双重引发了SMEP底层划痕两侧趋于相向移动而发生自愈合。另外,SMEP底层中环氧树脂具有高黏附性,不仅增强了与不锈钢基底、PZS表层的黏附力,而且SMEP底层划痕相向移动修复的同时,进一步快速带动PZS表层受损处的划痕缩小,从而缩短了整个SMEP/PZS双层涂层受损区域的修复时间。若对涂层受损区域持续热辐射20 min,随着受损涂层中底层划伤处的逐渐愈合及其对表层受损处的持续带动,完成了对整个SMEP/PZS涂层的受损区域快速修复,其修复后涂层表面润湿情况重新变为稳定的Cassie-Baxter状态,并恢复其超疏水性、耐蚀性等。
此外,在不锈钢基底上涂覆SMEP/PZS涂层,将其浸入含Cl−、O2等腐蚀性介质中,首先接触的是该涂层中具有独特润湿性的PZS超疏水表层,在接触界面处能够形成一层稳定的空气夹层,很大程度上阻碍了腐蚀性介质渗入SMEP自修复底层。同时,该涂层中SMEP底层含有耐腐蚀性优异的环氧树脂,且位于表层与不锈钢基底之间,成为对不锈钢基底防腐与保护的第二道屏障。因此,SMEP/PZS涂层中的表层与底层共同阻碍了Cl−、O2等离子的扩散,使金属表面长期处于缺氧状态,阻碍了不锈钢表面氧化还原反应的发生,使体系在钝化区的维钝电流密度大幅降低且点蚀电位较大正移,从而达到了对不锈钢基底相对更为持久地防腐与保护。
3. 结 论
(1) 不锈钢基底上通过简单的涂覆与喷涂法制备了具有较快修复、耐腐蚀等性能的微晶蜡强化的形状记忆环氧树脂涂层(SMEP)/聚二甲基硅氧烷 (PDMS)@ZnO@SiO2 (SMEP/PZS)自修复超疏水涂层。该涂层的超疏水性随着PDMS@ZnO@SiO2 (PZS)表层中低表面能物质全氟癸基三甲氧基硅烷 (PFDTMS) 的增加而增强,当其含量为42wt%时,SMEP/PZS涂层的超疏水性、自清洁性最佳。
(2) 在3.5wt% NaCl溶液中,涂层耐腐蚀性遵循SMEP/PZS>SMEP>PZS的规律,与裸不锈钢相比,在其表面涂覆SMEP/PZS涂层后,维钝电流密度下降了2个数量级,点蚀电位正移近10倍,金属钝化与防腐效果明显增强。
(3) SMEP/PZS涂层具有较快的自修复能力,将SMEP/PZS受损涂层在85℃下自修复20 min,SMEP/PZS受损涂层(7wt% PFDTMS)的自修复能力最强,其修复率高达97.8%,且经14天浸泡后其表面仍未出现明显腐蚀。
(4) 基于自主修复时底层较快带动表层愈合、对不锈钢防腐时底层与表层的协同增效,探讨了SMEP/PZS涂层的自修复与防腐机制。
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图 1 形状记忆环氧树脂(SMEP)/聚二甲基硅氧烷(PDMS)@ZnO@SiO2 (SMEP/PZS)涂层的制备过程示意图
Figure 1. Preparation process of shape memory epoxy resin (SMEP)/polydimethylsiloxane (PDMS)@ZnO@SiO2 (SMEP/PZS) coating
PFDTMS—Perfluorodecyltrimethoxysilane; NGDE—Neopentyl glycol diglycidyl ether; DGEBA—Bisphenol A diglycidyl ether; D-230—Polyetheramine D-230; EA—Ethyl acetate
图 3 SMEP涂层 (a);7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d)的PZS涂层;7wt% PFDTMS (e)、21wt% PFDTMS (f) 和42wt% PFDTMS (g) 的SMEP/PZS涂层的SEM图像
Figure 3. SEM images of SMEP coating (a); PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d); SMEP/PZS coating with7wt% PFDTMS (e), 21wt% PFDTMS (f), 42wt% PFDTMS (g)
图 5 裸不锈钢、PZS、SMEP及SMEP/PZS涂层的交流阻抗谱:(a) Nyquist图;(b) 阻抗模量曲线;(c) 相位角曲线;(d) 不同PFDTMS含量SMEP/PZS涂层的阻抗模量曲线
Figure 5. Alternating current impedance spectroscopy of bare stainless steel, PZS, SMEP and SMEP/PZS coatings: (a) Nyquist diagram; (b) Impedance modulus curves; (c) Phase angle curves; (d) Impedance modulus curves of SMEP/PZS with different content of PFDTMS
Z′—Real impedance; Z"—Imaginary impedance; Z—Polarization impedance
图 7 SMEP涂层 ((a1)、(a2))和7wt% PFDTMS((b1)、(b2))、21wt% PFDTMS ((c1)、(c2))、42wt% PFDTMS ((d1)、(d2)) 的SMEP/PZS涂层修复前后涂层的SEM图像(脚标1为修复前,脚标2为修复后)
Figure 7. SEM images of SMEP coating ((a1), (a2)) and SMEP/PZS coating with 7wt% PFDTMS ((b1), (b2)),21wt% PFDTMS ((c1), (c2)), 42wt% PFDTMS ((d1), (d2)) before and after healing (Pin 1 is before repair, pin 2 is after repair)
图 9 SMEP涂层 (a)、7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d) 的SMEP-ZPS涂层修复后于3.5wt% NaCl溶液浸泡1天、7天、14天后的阻抗模量曲线
Figure 9. Impedance modulus curves after healing and immersion in 3.5wt% NaCl solution for 1 day, 7 days, 14 days of SMEP coating (a) and SMEP/PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d)
图 10 SMEP涂层 (a)、7wt% PFDTMS (b)、21wt% PFDTMS (c)、42wt% PFDTMS (d) 的SMEP-ZPS涂层修复后浸泡于3.5wt% NaCl溶液14天后的光学照片
Figure 10. Optical photos of the healed coating after being immersed in 3.5wt% NaCl solution for 14 days of SMEP coating (a), SMEP/PZS coating with 7wt% PFDTMS (b), 21wt% PFDTMS (c), 42wt% PFDTMS (d)
表 1 PZS和SMEP/PZS涂层表面的水接触角(CA)及滚动角(SA)
Table 1 Contact angle (CA) and rolling angle (SA) of PZS and SMEP/PZS coatings
PZS SMEP/PZS 0wt% 7wt% 21wt% 42wt% 0wt% 7wt% 21wt% 42wt% Unsoaked CA/(°) 151.8 154.1 156.7 159.4 150.9 153.6 155.9 157.6 SA/(°) 6.9 4.6 3.6 1.8 7.8 5.3 3.9 2.6 Soaked 12 h CA/(°) 149.1 152.1 154.5 155.8 148.9 151.9 153.8 155.4 SA/(°) 12.6 6.5 5.0 4.1 13 6.4 5.2 4.4 表 2 由裸不锈钢和分别涂覆PZS、SMEP及SMEP/PZS涂层的不锈钢基底的极化曲线所得相关评价参数
Table 2 Related evaluation parameters obtained from the polarization curves of bare stainless steel and stainless steel substrates coated with PZS, SMEP and SMEP/PZS
Coating Ecorr/V Icorr/A Eb/V Ip/A 304 stainless steel −0.752 9.728×10−5 0.0562 1.020×10−4 PZS −0.406 1.458×10−6 −0.04 7.476×10−6 SMEP −0.269 6.042×10−7 0.0455 6.783×10−6 SMEP/PZS −0.160 5.283×10−8 0.312 6.672×10−6 Notes: Ecorr—Self-corrosion potential; Icorr—Self-corrosion current density; Eb—Pitting potential; Ip—Passive current density. 表 3 修复前后不同含量PFDTMS的SMEP/PZS涂层的接触角及滚动角
Table 3 Contact angle (CA) and rolling angle (SA) of SMEP/PZS coating with different contents of PFDTMS before and after healing
CA/(°) SA/(°) 7wt% 21wt% 42wt% 7wt% 21wt% 42wt% Original 153.6 155.9 157.6 5.3 4.9 2.6 Scratched 147.0 145.4 146.4 16.6 17.8 16.1 Healed 152.4 153.8 155.3 5.9 5.8 4.9 -
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