摘要 含有动态受阻硫脲键的有机硅是一种新型智能绝缘材料,可用于电力设备和电子器件。目前关于自修复材料的研究多集中于机械性能的修复效率,较少关注其绝缘性能。为研究自修复材料在电气领域应用的可行性,该文以聚二甲基硅氧烷(PDMS)为基体,通过对苯二异硫氰酸酯和氨丙基双封端聚二甲基硅氧烷合成动态硫脲键,利用无水哌嗪提供空间位阻结构,同时采用三(2-氨乙基)胺调整分子链结构,研制了一种具有修复效果和良好绝缘性能的材料。基于拉伸试验、击穿和电导试验、宽带介电谱测试及热修复试验,对比样品修复前后的机械性能、绝缘性能和介电性能,发现动态键的加入使材料具备自修复能力,且相对介电常数增加。随着交联密度的提高,样品直流击穿强度和体积电阻率上升。该文可为开发同时具有修复性和绝缘性能的材料提供一条有效的途径。
关键词:自修复材料 硫脲 聚二甲基硅氧烷(PDMS) 绝缘性能 空间电荷
聚二甲基硅氧烷(Polydimethylsiloxane, PDMS)材料作为一种常见的硅橡胶绝缘材料,被广泛应用于电力设备中,然而由于其自身模量、机械强度及耐磨性能较差,采用硅橡胶材料制备的电力设备容易在生产、运输、安装的过程中发生刮擦磨损等机械损伤,导致其表面形貌发生改变,进而影响其寿命和可靠性。传统硅橡胶材料的交联结构使其难以通过重塑来恢复损伤[1-3],而自修复材料作为一种新型材料,在受到损伤后,能够在特定条件下修复其表面的微小损伤,从而在一定程度上恢复其原始性能,具有良好的经济效益。
根据自修复材料的修复机理,可将其分为本征型自修复和外援型自修复。其中,外援型自修复材料在制备过程中掺杂含有修复剂的微脉管或微胶囊,当材料被破坏时,损伤处的修复剂会由于脉管或胶囊的破损而释放,对损伤进行修复[4-5]。外援型自修复材料应用范围广,但易引入缺陷,破坏材料本身性能,且修复次数有限,当某个区域的修复剂耗尽后,该区域将不再具备自修复能力。本征型自修复材料通过将动态键引入材料基体中实现修复过程,具有制备简单、可多次修复等优点[6-9]。目前常见的动态键有:二硫键、Diels-Alder(DA)键、动态受阻脲键、氢键、硼酸酯键等[10-12]。Sun Jiawen等将聚脲-硫脲和单宁酸共同引入硅氧烷中,利用单宁酸作为氢键交联位点和抗菌剂,既保证了材料的韧性和拉伸性,又使其具有抗硅藻性,同时,其在人工海水中的修复效率可达95%,为海洋防污涂料提供了一个良好的途径[13]。Li Yanmei等用模块化方式组装聚硫脲基团,详细地研究了不同空间位阻对材料修复温度的影响,所研制的自修复材料在室温到120℃内均具有良好的修复性能[14]。Sun Wenjie等将受阻脲键引入硅氧烷,以材料的介电特性和拉伸特性为表征参数,对材料的修复性能进行了研究,为自修复材料的电气应用开辟了道路[15]。
硫脲键是在脲键的基础上,将氧原子替换为硫原子。相比于只有反/反构象的脲键基团,硫脲基团具有顺/反和反/反两种构象,硫脲单元呈现非线性和无序的结构,不会诱导结晶[9]。本文以氨丙基双封端聚二甲基硅氧烷为基体,通过对苯二异硫氰酸酯和无水哌嗪构建含有动态受阻硫脲键的自修复硅橡胶。哌嗪的空间位阻效应和硫脲的活性使得聚合物网络可以实现解离-缔合过程,从而使受损伤部位的聚合物链可以重新连接,实现修复性能[16-17]。同时,本文通过三(2-氨丙基)胺调整聚合物链的交联程度,以改善其绝缘性能。针对自修复材料的电气应用,本文主要集中讨论材料绝缘性能和机械性能的修复效率,并探究交联度对材料空间电荷分布的影响。本文制备的材料具有良好的绝缘性能和修复能力,可为自修复材料在电气领域的应用提供参考。
对苯二异硫氰酸酯(PDITC,96%)购自北京迈瑞达科技有限公司;氨丙基双封端聚二甲基硅氧烷(APDMS,分子量为3 000 g/mol)和无水哌嗪(PA,98%)购自安徽泽升科技股份有限公司;三(2-氨乙基)胺(TA,98%)购自上海毕得医药科技股份有限公司;四氢呋喃(THF,99.5%)购自上海吉至生化科技有限公司。所有试剂均按照所收到的使用。
含有动态受阻硫脲键的自修复聚二甲基硅氧烷(下简称“自修复PDMS”)的制备原理如图1所示。PDMS-1的制备过程如下:将2 mmol无水哌嗪、6 mmol对苯二异硫氰酸酯以及4 mmol氨丙基双封端聚二甲基硅氧烷的四氢呋喃溶液均匀混合,室温下搅拌4 h,得到线性PDMS-1,将混合溶液70℃烘干12 h后,热压成片状试样。
图1 自修复PDMS制备原理
Fig.1 Synthesis principle of self-healing PDMS
采用类似方法制备PDMS-2和PDMS-3,其中,PDMS-2在室温下搅拌4 h后,加入0.667 mmol三(2-氨乙基)胺的四氢呋喃溶液继续搅拌12 h后烘干;PDMS-3在室温下搅拌4 h后,加入1.333 mmol三(2-氨乙基)胺的四氢呋喃溶液继续搅拌12 h后烘干。自修复PDMS具体合成原料见表1。
表1 自修复PDMS合成原料 (单位:mmol)
Tab.1 Synthesis reagent of self-healing PDMS
样品APDMSPDTICPATA PDMS-14620 PDMS-23620.667 PDMS-32621.333
样品的红外光谱由傅里叶变换红外(Fourier Transform Infrared, FTIR)光谱仪(IN10+IZ10,Nicolet)分析,分辨率设置为8 cm-1,每个光谱在4 000~500 cm-1范围内扫描32次。差示扫描量热分析(Differential Scanning Calorimetry, DSC)由量热仪(DSC822e)测量,温度范围为-140~120℃,在氮气氛围下以10℃/min的速率升高。拉伸试验采用万能试验机(CMT4503-5kN),根据国标GB/T 528—2009《硫化橡胶或热塑性橡胶拉伸应力应变性能的测定》,在室温条件下以200 mm/min的速度拉伸,并测量应力-应变曲线,试样被切割成哑铃状,长度为35 mm,狭窄部分宽度为2 mm,试样厚度为1 mm,每个样本测试8~10次,舍去在狭窄部分以外断裂的试样。相对介电常数和介质损耗角正切由宽带介电谱分析仪(Technologies Concept80)测量得到,频率测量范围为102~106 Hz。直流电导由三电极系统测量,样品厚度为0.3 mm。直流击穿试验采用球-球电极在空气中进行,设置电压以10 kV/s的速度上升直到发生击穿,击穿场强采用威布尔分布进行评估[18],表达式为
式中,P为失效概率;E为击穿场强测量值;为击穿场强特征值;
为形状参数。
样品的交联度由溶胀试验确定,样品被切成小片,在室温下用二甲苯萃取直到样品质量不再发生变化,随后将样品干燥,记录原始样品质量M0、溶胀质量Ms和干燥质量Md。样品的溶胀比和凝胶分数
计算式分别为
样品的修复性能由切割-修复试验验证,用干净的刀片在样品中部进行机械切割,随后将切口拼合,并在90℃下进行自修复,采用偏光显微镜(BX51-P)观察样品修复前后的显微图像变化。通过机械性能(拉伸强度、断裂伸长率)和绝缘性能(击穿场强和直流体积电阻率)的变化,进一步表征材料的修复性能。修复效率计算式[19]为
式中,和
分别为修复前、后的拉伸强度、断裂伸长率、击穿场强或体积电阻率。
样品的化学结构由傅里叶变换红外光谱分析确定,如图2所示。其中,3 320 cm-1和1 630 cm-1处的特征峰属于N—H的弯曲振动和C=O的拉伸振动,1 570 cm-1和1 445 cm-1处的特征峰分别对应N—H的弯曲振动和N—C=S的拉伸振动。以上结果证明,材料是按照本文设计所合成的[20]。
图2 自修复PDMS傅里叶变换红外光谱分析
Fig.2 FTIR analysis of self-healing PDMS
聚合物网络结构由溶胀试验确定,样品的凝胶分数和溶胀比如图3所示。PDMS-1没有引入三胺,是一种线性结构,几乎可以完全溶解在溶剂中,其凝胶分数仅为34.33%;相反地,PDMS-2和PDMS-3的尺寸会随着溶胀而变化,其中PDMS-3长度由原来的3 cm增加到溶胀后的3.5 cm,证明其产生了交联结构。PDMS-3相比于PDMS-2具有更高的凝胶分数和更低的溶胀比,这是因为其含有更多的三胺,形成了更紧密的交联网络和更多的交联点。
图3 自修复PDMS的凝胶分数和溶胀比
Fig.3 Gel fraction and swelling ratio of self-healing PDMS
通过DSC试验得到了样品的玻璃化转变温度Tg,如图4所示。玻璃化转变温度定义为热容阶梯变化的中间点,与受阻硫脲键有关的Tg出现在40~80℃,其中线性结构的PDMS-1具有最低的Tg(48℃)。随着交联度的增加,样品的Tg呈现上升趋势,PDMS-2的Tg约为55℃,PDMS-3的Tg则约为76℃,这可能是因为PDMS-3具有更多的交联点,从而使高分子链的运动受到了限制。
图4 自修复PDMS的DSC分析
Fig.4 DSC analysis of self-healing PDMS
自修复PDMS的机械性能见表2。可见所制备的样品的断裂伸长率接近,为54%~59%。由于PDMS-2具有部分交联结构,其拉伸强度略高于线性结构的PDMS-1。而PDMS-3虽然具有更高的交联度,但拉伸强度较小,这可能是由于交联点属于动态键,在外力的作用下易断开,并不能完全起到增强材料拉伸强度的作用,弹性模量随着交联度的提高呈现下降趋势。
表2 自修复PDMS的机械性能
Tab.2 Mechanical properties of sela-healing PDMS
样品拉伸强度/MPa弹性模量/MPa断裂伸长率(%) PDMS-13.18923.53554.963 PDMS-24.66019.35055.744 PDMS-33.4075.70758.807
样品常温下的相对介电常数如图5所示。测试范围内样品的相对介电常数在2.1~4.8之间,其中PDMS-1和PDMS-3比文献报道的纯PDMS的相对介电常数(2.6~2.7)略高[21],这是由于样品引入了高极性的硫脲基团。电介质的介电常数由可定向的偶极子数量和它们在外加电场下的定向能力决定。PDMS-2的交联结构提高了基团定向的难度,其极化现象弱于线性结构的PDMS-1;而PDMS-3单位体积内引入了更多的高极性硫脲基团,虽然其具有更高的交联度,但相对介电常数仍要高于PDMS-1和PDMS-2。
图5 自修复PDMS常温下的相对介电常数
Fig.5 The relative dielectric constant of self-healing PDMS at room temperature
样品室温下的的介质损耗角正切tanδ如图6所示。随着频率的增加,在高频范围内,tanδ随着频率的增加出现先上升再减小的趋势,这在PDMS-3中更为明显,其中的峰值称为α弛豫。弛豫峰值频率与温度的关系遵循阿伦尼乌斯定律[22-23],即
式中,为弛豫峰值频率;f0为指前因子;
为活化能;k为玻耳兹曼常数;T为热力学温度。由阿伦尼乌斯定律得出,在同等温度下,活化能越高,弛豫峰值频率越低。PDMS-3的弛豫峰值频率出现在105~106 Hz,PDMS-1和PDMS-2的弛豫峰值频率出现在大于106 Hz的位置,因此PDMS-3的活化能高于PDMS-1和PDMS-2。这可能是由于PDMS-3的交联程度较高,链的移动会受到限制,需要更多的能量进行刺激,因此活化能较高;相较而言,PDMS-1和PDMS-2由于交联程度低,活化能较小。在之前的研究中[24],受阻脲键的弛豫峰值频率出现在104~106 Hz,低于本文中的硫脲键,证明脲键的活化能要高于硫脲键。
图6 自修复PDMS室温下的tanδ
Fig.6 The tanδ of self-healing PDMS at room temperature
当外施电场强度为10 kV/mm时,样品的空间电荷分布如图7所示。对于PDMS-1,试样加压后迅速在两电极-界面处建立同极性电荷积聚,体电荷基本为零,空间电荷积聚情况随极化时间延长的变化不明显。对于PDMS-2,阳极处同极性电荷注入和积聚随着极化时间延长出现显著增长,电荷峰向介质内部迁移;阴极附近电荷分布基本稳定,体电荷以正极性电荷为主,正负电荷汇聚于靠近阴极侧。对于PDMS-3,阳极处同极性电荷注入能力进一步增强,大量正极性电荷在阳极附近形成同极性电荷积聚,占据体电荷的大部分;在阴极附近形成异极性的电荷积聚,导致阴极处峰值随极化时间的延长而增长。
图7 自修复PDMS的空间电荷分布
Fig.7 The space charge distribution of self-healing PDMS
样品直流击穿场强的威布尔分布如图8所示。可见三个样品的尺度参数β均大于10,击穿场强分布狭窄,证明合成的材料较为均匀[25]。从图8中还可以观察出PDMS的击穿场强与网络结构密切相关。交联程度最高的PDMS-3具有最高的击穿场强(134.71 kV/mm);交联程度减小,击穿场强降低,PDMS-2的击穿场强仅为107.43 kV/mm,比PDMS-3下降20%;而线性结构的PDMS-1击穿场强仅为76.23 kV/mm。这可能是由于交联结构阻碍了电流的流动,从而提高了材料的绝缘性能。
图8 自修复PDMS直流击穿场强的威布尔分布
Fig.8 The Weibull distribution of DC breakdown strength of self-healing PDMS
这一现象也可以通过直流体积电阻率看出,样品的直流体积电阻率和击穿场强如图9所示。具有交联结构的PDMS-2和PDMS-3的直流体积电阻率远高于线性结构的PDMS-1。较高的三胺含量导致了更致密的交联结构,使得样品的直流体积电阻率更高。本文中的硅橡胶材料具有较好的绝缘性能,这为自修复硅橡胶材料在电力设备上的应用提供了广阔的前景。
图9 自修复PDMS直流体积电阻率和击穿场强
Fig.9 DC volume resistivity and DC breakdown strength of self-healing PDMS
可塑性是高分子链的流动能力的宏观表征,是材料自修复的前提条件。对于常规的交联结构,链的流动受到永久交联网络的限制。当引入动态化学键时,交联网络是活性的,并且能自动地或在外界刺激的作用下发生解离-缔合。本文通过引入动态受阻硫脲键来实现聚合物交联网络的自修复性。当温度较高时,聚合物交联网络由于动态键的解离而分解,链段运动受限制较少,在损伤处发生扩散;当温度降低时,聚合物链段的宏观流动受到抑制,动态键的缔合占主要地位,实现修复过程[24]。修复效率与分子链的结构密切相关,具有链状结构的PDMS-1的修复效果最为显著。本文采用划痕修复试验验证材料具有自修复性:使用洁净美工刀分别在自修复PDMS和普通商用PDMS样品表面划出划痕,伤口拼合后用长尾夹进行固定,如图10a所示;然后在90℃烘箱中进行修复。使用偏光显微镜对样品修复前后的形貌进行观察,图10b为普通PDMS和PDMS-1在90℃条件下修复过程中切口变化的偏光显微镜图像。普通PDMS在90℃下处理48 h后,切口有微弱的修复趋势;而PDMS-1在热处理24 h后,切口显著变小,在热处理48 h后切口几乎完全消失,证明其产生了修复效果。
图10 自修复PDMS热修复方法及修复前后图像
Fig.10 Self-healing PDMS thermal healing method and images before and after repair
用机械性能表征含有动态受阻硫脲键的自修复PDMS的修复性,样品修复后机械性能变化情况如图11所示。PDMS-1和PDMS-2具有较好的修复情况,在90℃热处理48 h后,其上的切口几乎完全消失,拉伸强度和弹性模量恢复60%以上,断裂伸长率恢复90%以上。PDMS-1为线性结构,分子链的热运动更加灵活,导致其修复效率略高于微交联的PDMS-2。交联结构最为紧密的PDMS-3的拉伸强度几乎没有修复效果,这是因为致密的交联网络限制了链段的运动,导致基团碰撞概率较小,从而修复能力较低。
图11 自修复PDMS机械性能修复效率
Fig.11 Mechanical properties repair efficiency of self-healing PDMS
切割损伤修复后样品的绝缘性能,包括直流体积电阻率和击穿场强的恢复情况如图12所示。由于PDMS-3致密的交联结构导致其修复效率较低,在测量的过程中极易发生击穿,因此本文只对比PDMS-1和PDMS-2修复前后绝缘性能的恢复情况。切口修复后,虽然直流体积电阻率的恢复较差,但仍处于绝缘性能较为优良的状态,其中PDMS-1的直流体积电阻率恢复至41.5%,PDMS-2的直流体积电阻率恢复至32.1%,这可能是由于具有网状结构的PDMS-2在修复过程中,其分子链的移动受到阻碍,导致修复效果略差于PDMS-1。与直流体积电阻率类似,PDMS-1修复后的直流击穿场强恢复至84.3%,PDMS-2修复后的直流击穿场强恢复至79.1%,材料的绝缘性能恢复良好。
图12 自修复PDMS绝缘性能修复效率
Fig.12 Insulation properties repair efficiency of self-healing PDMS
动态受阻硫脲键在修复过程中,动态键发生解离-缔合效应,叔胺和C=S键之间的化学强度在高温下很弱,易发生解离,生成的异硫氰酸酯和氨基可以随着链段移动并重新缔合成动态受阻硫脲键。宏观上来说,解离-缔合过程使得交联网络变得动态和可流动,聚合物链可以移动到受损区域,重新进入分离的伤痕表面,使材料在冷却后修复,因此材料保留了原始的分子结构,从而恢复绝缘性能和机械性能[24]。同时研究发现,三(2-氨乙基)胺在提高样品交联度的同时,限制了分子链的移动,基团碰撞概率降低,从而使修复效率降低。在普通PDMS中,如果没有引入动态键,则受损的有机硅只能通过聚合物链缠结来修复。然而,由于普通有机硅是一种交联结构,永久性网络限制了聚合物链的动力,链流动和修复较为困难,修复过程难以进行。
本文将动态受阻硫脲键引入聚二甲基硅氧烷中,得到具有高绝缘性能和修复性能的硅橡胶材料,研究了材料本身机械性能、绝缘性能以及材料修复前后各项性能的变化。同时,以三(2-氨乙基)胺为交联剂,通过调控交联剂含量来调整材料性能。得到以下结论:
1)含有动态受阻硫脲键的自修复PDMS具有良好的修复性能,基于直流击穿场强的修复效率最高可达84.3%,虽然修复后的直流体积电阻率有所下降,但仍具有良好的绝缘效果;线性结构的PDMS-1和交联程度较小的PDMS-2的机械性能修复效率可达到60%以上。
2)交联结构能够通过影响载流子的移动来提高材料的绝缘性能,包括直流体积电阻率和击穿场强等,同时也会通过阻碍分子链的移动而削弱材料的修复效率。
总之,本文研究的含有动态受阻硫脲键的聚二甲基硅氧烷材料在具有良好绝缘性能的同时,还具有修复能力。这表明,将动态化学键引入聚合物材料中可以在不牺牲其绝缘性能的条件下赋予其修复能力,对实际应用十分具有吸引力,可以起到延长材料使用寿命、减少资源浪费等作用。
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Abstract Silicone rubber is a kind of insulating material widely used in power system. However, due to its poor mechanical strength, it is easy to be damaged during use, which affects its service life. Self-healing materials can repair their own minor damages under certain conditions, and then restore certain original properties. Silicone containing dynamic hindered thiourea bonds is a new type of intelligent insulation material, which can be used in power equipment and electronic devices. At present, the research on self-healing materials mainly focuses on the repair efficiency of mechanical properties, and pays less attention to their insulation properties. In order to study the feasibility of self-healing materials in electrical field, dynamic thiourea bond was synthesized by p-phenyldiisothiocyanate and aminopropyl double-ended polydimethicone. Anhydrous piperazine provided steric hindrance structure, and tri(2-aminoethyl)amine was used to adjust the molecular chain structure. A kind of material with healing property and insulating property was developed.
In this study, the crosslinking degree is variable and the cross-linking degree of PDMS-1, PDMS-2 and PDMS-3 increased. Firstly, the structure of the sample was tested, and the successful synthesis of thiourea bond was verified by Fourier transform infrared spectroscopy. The cross-linked structure was verified by swelling experiment, and the glass transition temperature was obtained by differential scanning calorimetry. It was found that the glass transition temperature of the sample increased with the increase of the cross-linking degree. The mechanical tests show that the elongation at break of the sample is close to 55%~59%, and the elastic modulus decreases from 23.535 MPa to 5.707 MPa with the increase of crosslinking degree, and the tensile strength increases first and then decreases. Due to the introduction of dynamic bond, the relative permittivity of the sample increases compared with that of ordinary silicone rubber, ranging from 2.1~4.8. It is found that the activation energy of thiourea bond is lower than that of urea bond through relaxation peak. The results of space charge show that with the increase of cross-linking degree, the homo-charge injection increases. The insulation test results show that the DC resistivity and breakdown field strength increase with the increase of crosslinking degree.
The self-healing properties of the material were verified by scratch repair experiments. Scratches were made on the surface of self-healing PDMS and ordinary commercial PDMS samples with a clean utility knife, and the wounds were fixed with a long tail clip after the wound was split. The morphology of the samples before and after the repair was observed with a microscope. After two days of treatment at 90℃, the incision of ordinary silicone rubber had a weak trend of repair, and PDMS-1 after 24 h of heat treatment, the incision became significantly smaller. The healing efficiency of the sample was evaluated by tensile strength and insulation properties. The repair efficiency of PDMS-1 with linear structure and PDMS-2 with low crosslinking degree can reach more than 60% based on mechanical properties, while the repair efficiency of PDMS-3 is low due to the high cross-linking degree, which limits the movement of chain segments. The breakdown strength of PDMS-1 and the repair efficiency of PDMS-2 can reach 84.3% and 79.1% respectively.
Through the above experiments, the following conclusions are drawn: (1) The self-healing PDMS containing the dynamic hindered thiourea bond has good repair performance, and the repair efficiency based on the breakdown strength can recover up to 84.3%. Although the resistivity decreases before and after repair, it still has good insulation effect; The repair efficiency of PDMS-1 with linear structure and PDMS-2 with low crosslinking degree can reach more than 60%. (2) The cross-linked structure can improve the insulation performance by affecting the movement of carriers, including resistivity and breakdown strength, etc., while also weakening the repair efficiency of the material by hindering the movement of the molecular chain.
Keywords:Self-healing material, thiourea, polydimethylsiloxane (PDMS), insulating property, space charge
中图分类号:TM215.2
DOI: 10.19595/j.cnki.1000-6753.tces.242005
国网北京市电力公司资助项目(ZYKCJS[2023]010)。
收稿日期 2024-11-08
改稿日期 2024-11-26
丁一铭 男,1994年生,助理工程师,研究方向为高压电缆检修和和高分子绝缘材料应用。
E-mail:2386234727@qq.com
吕泽鹏 男,1987年生,教授,博士生导师,研究方向为绝缘材料的电荷输运、电树、局部放电和老化过程。
E-mail:lv.zepeng.insu@xjtu.edu.cn(通信作者)
(编辑 李 冰)