中国腐蚀与防护学报, 2017, 37(3): 254-260
doi: 10.11902/1005.4537.2016.019
碳纳米管含量对环氧树脂涂层性能的影响研究

Effect of Carbon Nanotube on Properties of Epoxy Coating
张娟, 刘自强, 冯涛, 温世峰, 陈瑞卿

摘要:

以环氧树脂为基料,氨基硅烷为固化剂,气相SiO2为分散助剂,碳化二亚胺为改性助剂,制备了不同碳纳米管含量的环氧树脂涂层。采用拉拔法测附着力,球盘磨损测耐磨性,电化学和丝状腐蚀测耐蚀性,全面评价了碳纳米管含量对环氧树脂涂层性能的影响。结果表明:碳纳米管含量为2% (质量分数) 时就能显著提高环氧树脂涂层的附着力、耐磨性和耐蚀性,同时增强涂层的导电性。当碳纳米管含量为5%和7%时,涂层的附着力和耐磨性进一步提高;当碳纳米管含量为10%时,涂层的附着力和耐磨性开始略微下降,但耐蚀性和导电性达到最佳状态。

关键词: 碳纳米管 ; 环氧树脂涂层 ; 附着力 ; 摩擦 ; 腐蚀 ; 电化学阻抗谱

Abstract:

Epoxy coatings with different mass fraction of carbon nanotube (CNT) is prepared with epoxy as matrix, amine silane as curing agent, fumed silica as disperse dispersant, and carbodiimide as modifying agent. The effect of CNT amount on properties of epoxy coating is overall evaluated by means of pull-off adhesion test, ball-on-disk ear test, electrochemical impedance spectroscopy (EIS) and filiform corrosion test. The results show that, the adhesion strength, abrasion resistance and corrosion resistance are improved remarkable for the epoxy coating with 2% (mass fraction) of CNT in comparison to that without CNT addition. All the above mentioned properties are further improved for the epoxy coating with 5% and 7% of CNT. Furthermore, when the epoxy coating with the addition of CNT is up to 10%, of which the adhesion strength and abrasion resistance decreased, while the corrosion resistance and conductivity reach the optimum.

Key words: carbon nanotube ; epoxy coating ; adhesion ; friction ; corrosion ; EIS

碳纳米管由于其优异的力学性能、化学稳定性、纳米级尺度等特征[1],作为填料可以改善涂层的性能,其含量会对涂层的附着力、摩擦磨损和耐腐蚀等性能产生重要影响[2,3]。碳纳米管作为导电导热材料添加到涂层中,高长径比的碳纳米管在达到一定值时会相互搭接形成不规则的三维空间网络结构,从而通过碳纳米管网络导电导热,该特性符合渗流理论[4-6],通常用电学信号进行检测。根据渗流理论,当导电粒子在复合材料中达到一定含量时就能够相互接触形成导电通道,使复合材料的阻抗模值迅速降低几个数量级[7,8],该值称为渗流域值。环氧树脂涂层在固化过程中会因收缩形成微孔和内应力,从而降低涂层的致密性、力学和耐腐蚀性能[9]。Khun等[10]研究了含0%,0.1%,0.5% (质量分数) 的碳纳米管环氧树脂涂层的附着力、摩擦磨损和耐腐蚀性能,发现随着碳纳米管含量的增加,环氧树脂涂层的各项性能均有提高,但对更高含量碳纳米管的涂层未做进一步研究。Deyab[11]研究了碳纳米管含量不同的聚苯胺涂层在酸性环境下的电化学性能,发现碳纳米管含量为0.8% (质量分数) 的聚苯胺涂层具有最好的耐腐蚀性能,但对更高碳纳米管含量的涂层也未做进一步研究。碳纳米管含量达到渗流阈值之后会显著提高涂层的导电性能,改善涂层的致密性,继续提高碳纳米管含量对涂层性能的影响有待进一步研究。

本文通过使用碳化二亚胺对氧化碳纳米管进行接枝改性[12],添加气相SiO2促进碳纳米管的分散性[13],以低碳钢为基底材料,制备了碳纳米管含量为0% (质量分数),2%,5%,7%和10%的5组环氧树脂涂层,系统测试了碳纳米管含量对环氧树脂涂层附着力、耐磨性、耐蚀性以及电化学等性能的影响。

1 实验方法
1.1 不同碳纳米管含量环氧树脂涂层的制备

实验用基料为环氧硅氧烷杂化树脂,固化剂为3-氨基丙基三乙氧基硅烷,碳纳米管由深圳纳米港有限公司提供,主要参数为直径15~25 mm,长度5~15 μm,纯度>95% (质量分数) ,比表面积150~210 m2/g。

为研究碳纳米管含量对环氧树脂涂层性能的影响,配制碳纳米管含量分别为0%,2%,5%,7%和10%的5种碳纳米管环氧树脂涂料。为使氧化碳纳米管与氨基硅烷充分反应,先将碳纳米管加入到氨基硅烷中,加入一定量的碳化二亚胺促进碳纳米管和氨基硅烷反应,加入少量的二氯甲烷可以减少副产物的产生,且挥发后不影响各组分比例和性质。将碳纳米管、氨基硅烷、碳化二亚胺、二氯甲烷的混合物高速搅拌15 min,超声处理5 min,然后加入适量环氧树脂和等量的气相SiO2,再高速搅拌15 min配制成最终涂料。在配制过程中使用水浴控制涂料的温度约保持在40~60 ℃,以保证N,N-二环己基碳二亚胺为溶液状态,各组分充分反应。

氧化碳纳米管的羧基与环氧树脂的环氧基均与氨基硅烷的氨基反应,为使各组分反应完全,本文采用Boehm滴定法测定氧化碳纳米管的羧基含量。根据滴定计算,1 g碳纳米管大约会消耗0.02 g的氨基硅烷,各组涂料配比如表1所示。

各组涂料进行配制、喷涂、固化,制备出相应比例的碳纳米管环氧树脂涂层试件,各组涂层厚度为160~170 µm。

1.2 涂层力学性能与耐蚀性测试

采用PosiTest AT-A全自动数字显示拉拔式附着力测试仪测试涂层的附着力大小,测试锭子直径20 mm,拉开有效面积大于70%。采用HT-1000型高温摩擦磨损试验机测试涂层的耐磨性能,球盘磨损实验在常温下进行,实验条件为:对磨材料钢,电机频率10 Hz,载荷200 g,摩擦半径7 mm,对磨时间10 min。采用CS350型电化学工作站测试涂层的电化学阻抗谱特性。采用HTP201E型交变湿热试验箱测试涂层的耐丝状腐蚀性能,测试环境:湿度80%,温度40 ℃。

2 结果与讨论
2.1 拉拔法测附着力

图1为碳纳米管含量对环氧树脂涂层附着力的影响。可知,碳纳米管的加入显著提高了环氧树脂涂层的附着力,附着力由碳纳米管含量为0%时的1.87 MPa到碳纳米管含量为5%或7%时的3.3 MPa,提高了约一倍。与未添加碳纳米管的环氧树脂涂层相比,碳纳米管含量为2%时,环氧树脂涂层的附着力升高到3.1 MPa,这与碳纳米管的纳米级尺度和超轻的单位质量相关,即一定体积占比的碳纳米管在单位体积内的数量是一般材料颗粒数量的多倍[4]

图1 碳纳米管含量对环氧树脂涂层附着力的影响

Fig.1 Effect of CNT content on adhesion of the epoxy coating

表1 不同含量碳纳米管的涂料配比
Table 1 Constitutes of the coatings with different contents of CNT
Specimen CNT contentmass fraction / % Epoxyg Amine silaneg
A 0 80.0 20.0
B 2 78.4 19.6
C 5 75.9 19.1
D 7 74.3 18.7
E 10 71.8 18.2

表1 不同含量碳纳米管的涂料配比

Table 1 Constitutes of the coatings with different contents of CNT

由于碳纳米管分散在单位空间内的数量巨大,所以低含量的碳纳米管足以充满单位体积内的整个空间。碳纳米管含量为2%时,单位体积的环氧树脂涂层内有数量庞大的碳纳米管,碳纳米管作为环氧树脂涂层的增强体,能提高涂层的附着力。涂层附着力按机理分为机械附着力和化学结合力两种,碳纳米管在涂层中能与基底材料细微的凹槽与突起等不平整的部分形成铆接等机械附着力[14],同时碳纳米管也可以与基底材料表面形成氢键,与环氧树脂和氨基硅烷形成共价键和氢键。所以碳纳米管的加入可以从机械附着力和化学结合力两方面提高涂层的附着力。但由于少量的碳纳米管能够形成足够多的机械铆接与化学键,所以继续提高碳纳米管的含量并不能显著提高涂层的附着力。当碳纳米管含量过高时,单位体积内的碳纳米管数量过多,会引起分散不均匀,碳纳米管团聚体与环氧树脂结合不牢固,碳纳米管之间会产生滑移,同时碳纳米管占据了太多的空间,使得环氧树脂与基底的接触面积变小,从而导致涂层与基底的附着力减小。所以当碳纳米管含量为10%时,碳纳米管环氧树脂涂层的附着力反而减小。

2.2 球盘磨损测试耐磨性能

常温下钢珠与涂层对磨10 min后,各组涂层材料均未穿透,均出现磨损凹槽。随着碳纳米管含量的提高,涂层的平均磨损质量呈减小趋势;碳纳米管含量为7%和10%时,涂层磨损质量明显减小。由图2可知,在相同条件下,碳纳米管的含量为2%和10%时,涂层的平均摩擦系数显著降低;但当碳纳米管含量为5%和7%时,涂层的平均摩擦系数比未添加碳纳米管的涂层还大。

图2 碳纳米管含量对环氧树脂涂层耐磨性的影响

Fig.2 Effects of CNT content on wear resistance of the epoxy coating

碳纳米管由于其纳米级尺度及特殊的碳结构,与石墨的作用类似,具有润滑效果。碳纳米管具有较强的力学性能,与环氧树脂涂层通过化学键牢固结合时能很好地提高涂层的力学性能,对摩擦磨损的破坏作用能起到较强的阻挡作用[15]。当碳纳米管含量为2%时,涂层摩擦系数显著减小;当碳纳米管含量为5%和7%时,涂层摩擦系数又显著增大。碳纳米管在单位空间内能以巨大的数量存在,当其含量提高时,其分散难度迅速增大,会在局部形成团聚体颗粒。所以当碳纳米管含量提高时,涂层的摩擦系数会因碳纳米管分散不均匀而增大,但并未明显影响涂层的耐磨性能。当碳纳米管含量继续提高到10%时,涂层的摩擦系数又显著下降。这是由于碳纳米管分散不均匀的团聚体过多而在涂层中形成了较均匀的团聚体分布形态,且大量的碳纳米管在环氧树脂包覆层磨损时迅速释放出来,会对磨损部分形成较好的润滑效果,从而降低了涂层的磨损量。

2.3 电化学阻抗谱

将涂层试件浸泡在3.5%NaCl溶液中测得各组涂层的阻抗谱见图3。由阻抗谱测试结果可见,各组涂层的阻抗模值随着浸泡时间的增加,在短时间内反复地增大减小,而在长时间内则是缓慢地减小,且电容性减弱,电阻性增强。涂层在3.5%NaCl溶液中浸泡时,短时间内溶液会从涂层表面的微孔缺陷处渗透到深处,使得涂层的阻抗模值迅速减小,而环氧树脂吸水膨胀又会减小微孔,使得涂层的阻抗模值增大;长时间内水和导电离子则会缓慢渗透到涂层深处使得涂层的阻抗模值减小。在浸泡初期,溶液较少地渗透到涂层中,涂层对NaCl溶液具有良好的阻挡作用,所以电容性较强;随着浸泡时间的延长,溶液渗透到涂层深处,涂层对导电介质的阻挡作用减弱,电容性随之减弱。

图3 不同碳纳米管含量环氧树脂涂层阻抗模值比较

Fig.3 Impendence modules of the epoxy coatings with different CNT contents

此外,阻抗模值减小最快的是含2%碳纳米管的涂层,未添加碳纳米管的涂层次之,减小最慢的是含10%碳纳米管的涂层。阻抗模值反映涂层的导电性能,也能在一定程度上反映涂层的耐蚀性。水和导电离子在涂层中的渗透会降低涂层的阻抗模值,碳纳米管作为导电材料添加到涂层中形成导电回路也会降低涂层的阻抗模值。在环氧树脂涂层中添加碳纳米管,一方面会降低涂层固化过程中的收缩率,减少涂层微孔的形成和封堵微孔,从而减缓水和导电离子在涂层中的渗透;另一方面均匀分散的碳纳米管形成导电回路会减小涂层的阻抗。2%碳纳米管含量的涂层由于碳纳米管含量不足,阻挡作用较弱使得水和导电离子渗透到涂层深处,又由于碳纳米管形成的导电回路作用,所以阻抗模值下降的最多,未添加碳纳米管的环氧树脂涂层没有碳纳米管的作用,其阻抗模值也下降较多。碳纳米管含量为10%的涂层由于碳纳米管形成了良好的导电回路,其初始阻抗模值就很小,由于碳纳米管含量最高,涂层的固化收缩率最小,微孔缺陷最少,对水和导电离子的阻挡作用最强,故阻抗模值下降的最慢,阻抗模值保持较小值,但并不说明其耐蚀性差。

图4为添加不同含量碳纳米管的环氧树脂涂层在3.5%NaCl溶液中浸泡1 h和120 d后的电化学测试结果和Bode图的等效电路以及拟合曲线。其中,Rl为溶液电阻,Rc为有机层电阻,Cc为有机层电容,Rct为电荷转移电阻,Cdl为双电层电容,RCNT为碳纳米管电阻,CCNT为碳纳米管电容,Wcorr为Warburg阻抗原件。比较浸泡1 h后的Bode图可见,随着碳纳米管含量的增加,低频区的相位角增大,高频区的相位角减小,表明涂层整体的电容性减弱,这是由于碳纳米管形成的导电回路增强了涂层的导电性能。各组涂层浸泡120 d与浸泡1 h相比,相位角显著降低的位置由中低频区转移到高频区,且低频区相位角在尾端有增大趋势,表明随着浸泡时间的延长,水和腐蚀性离子渗透到涂层深部,且基底金属发生了一定程度的腐蚀,在涂层与基底的接触界面形成了阻滞作用。

图4e~j可见,含5%,7%和10%碳纳米管环氧树脂涂层对应浸泡1 h和浸泡120 d的等效电路完全一致,含0%和2%碳纳米管的环氧树脂涂层对应的等效电路不同。浸泡1 h条件下,添加了碳纳米管涂层的等效电路包含Rl,Cc,Rc,RCNT,和CCNT,与典型的富锌涂层的等效电路一致,表明添加到涂层中的碳纳米管形成的导电回路导电作用显著,与富锌涂层的锌粉作用一样形成了阻抗与双电层电容。浸泡120 d条件下,含2%碳纳米管的涂层在浸泡1 h等效电路基础上增加了涂层/基底界面的Rct,CdlWcorr,表明水和腐蚀性离子渗透涂层到达基底,并在基底形成了一定程度的腐蚀,涂层正在失去保护作用。浸泡120 d时,环氧树脂涂层和含5%,7%和10%碳纳米管的涂层均在浸泡1 h等效电路基础上增加了RctCdl,表明涂层在后期均发生了一定程度的腐蚀。

所有涂层在浸泡120 d的等效电路均出现RctCdl,含2%碳纳米管涂层的等效电路中还出现了Wcorr,但Bode图中低频区表现的Warburg阻抗段并非与横轴平行、位于45°的直线,与典型的Warburg阻抗特征不符,表明腐蚀层并非半无限扩散层。综上所述,电化学阻抗谱和等效电路表明,添加较高含量碳纳米管能提高环氧树脂涂层的耐蚀性。

2.4 丝状腐蚀

实验选取碳纳米管含量依次为0%,2%,5%,7%和10%的环氧树脂涂层试件进行封边处理,用钢刀在各试件上同一位置划 “X”形状的刻痕,要求刻痕深度刚好划透涂层见到基底,且刻痕距离试件边缘大于10 mm。不同腐蚀时间的丝状腐蚀记录见图5和6。

可见,未添加碳纳米管的环氧树脂涂层低碳钢试件在湿热环境下60 d时就发生了明显的丝状腐蚀,而添加了碳纳米管的环氧树脂涂层试件均未发生明显腐蚀,只在刻痕处生成少量腐蚀产物。到90 d时,无碳纳米管的环氧树脂涂层试件表面的丝状腐蚀显著增多增长,而添加了碳纳米管的环氧树脂涂层试件表面无明显腐蚀发生,但涂层均有不同程度的翘起脱离情况发生。使用3M600型测试胶带将湿热环境下处理90 d的各试件表面黏附剥离,结果显示,添加碳纳米管的涂层均有不同程度的剥离发生。随着碳纳米管含量的提高,涂层剥离的面积有增大的趋势,而低碳钢基底的腐蚀状态反而趋于良好。

图4 不同CNT含量环氧树脂涂层在3.5%NaCl溶液中浸泡不同时间后的Bode图拟合曲线

Fig.4 Fitting results of Bode plots of epoxy coating with 0% (a, b), 2% (c, d), 5% (e, f), 7% (g, h) and 10% (i, j) CNT after immersed in 3.5%NaCl solution for 1 h (a, c, e, g, i) and 120 d (b, d, f, h, j)

金属发生丝状腐蚀的条件要求涂层与基底间形成楔形空间,且楔形空间内凝聚电解液。实验过程中所有试件都处于80%湿度和40 ℃温度条件下,所以造成各试件发生不同程度丝状腐蚀的原因是涂层与基底间形成的空间形貌不同。在使用3M600胶带对涂层进行剥离的过程中显示,碳纳米管含量越高的环氧树脂涂层韧性越低,在发生小变形程度的弯曲时就容易断开脱落。图6显示碳纳米管含量高的环氧树脂涂层和基底结合与分离两种状态形成的界线更加明显,表明涂层剥离处与基底形成的夹角较大,不利于凝聚电解液,从而不利于丝状腐蚀的发生。实验结果表明,在湿热环境下提高碳纳米管的含量会导致环氧树脂涂层与基底在破损处易剥离,但并不会降低其耐蚀性。

图5 碳纳米管环氧树脂涂层丝状腐蚀不同时间的表面形貌

Fig.5 Surface morphologies of the epoxy coatings with 0% (a1, a2), 2% (b1, b2), 5% (c1, c2), 7% (d1, d2) and 10% (e1, e2) CNT after filiform corrosion for 60 d (a1~e1) and 90 d (a2~e2)

图6 碳纳米管环氧树脂涂层丝状腐蚀90 d后胶带剥离状态

Fig.6 Exfoliated morphologies of the epoxy coatings with 0% (a), 2% (b), 5% (c), 7% (d) and 10% (e) CNT after filiform corrosion for 90 d

3 结论

(1) 添加碳纳米管能显著提高环氧树脂涂层与基底的机械结合力与化学结合力。当碳纳米管含量为2%时,涂层的附着力能提高约一倍。

(2) 添加碳纳米管能显著提高环氧树脂涂层的耐磨性和与钢材料的摩擦系数。在一定范围内,碳纳米管含量过高会降低其分散性,导致涂层与钢材料的摩擦系数增大;碳纳米管含量太高则会降低环氧树脂有机材料的连续性,使得涂层变得疏松而降低涂层的耐磨性能。

(3) 碳纳米管会封堵水和腐蚀性离子的传输路径,且降低环氧树脂涂层的固化收缩率,减少微孔缺陷,提高环氧树脂涂层的耐腐蚀性能。在保证碳纳米管分散性良好的前提下,提高碳纳米管含量,环氧树脂涂层耐腐蚀性能改善更明显。

(4) 碳纳米管能显著提高环氧树脂涂层的耐丝状腐蚀能力。在湿热环境下腐蚀会造成涂层与基底间形成楔形空间,碳纳米管含量的提高会降低环氧树脂涂层的韧性,从而使得该楔形夹角增大,阻碍丝状腐蚀的发生,但使得涂层容易脱离基底材料,且容易发生脆性断裂。

The authors have declared that no competing interests exist.

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关键词(key words)
碳纳米管
环氧树脂涂层
附着力
摩擦
腐蚀
电化学阻抗谱

carbon nanotube
epoxy coating
adhesion
friction
corrosion
EIS

作者
张娟
刘自强
冯涛
温世峰
陈瑞卿

ZHANG Juan
LIU Ziqiang
FENG Tao
WEN Shifeng
CHEN Ruiqing