分子结构与交联度对聚乙烯耐水树枝老化特性影响的研究

发布时间：2018-11-15 12:38

【摘要】：在潮湿的环境下,水分子会在电场的作用下进入电力电缆交联聚乙烯绝缘中并逐渐形成水树枝结构,水树枝的存在不仅会劣化绝缘的机械和介电性能,而且长期发展会形成电树枝造成永久性破坏。因此,研究聚乙烯水树枝老化的内在机理对电力电缆的安全运行十分重要。本文采用水刀电极法对试样进行加速水树枝老化实验,主要分为两部分:(1)分子结构对聚乙烯耐水树枝老化能力的影响。试样为主链带有长支链的LDPE、主链带有大量短支链LLDPE和主链具有极少短支链的HDPE;(2)交联度对聚乙烯耐水树枝老化能力的影响。通过向聚乙烯基础树脂中加入不同含量的交联剂得到不同交联度的XLPE试样。其中LDPE采用过氧化物交联法,而LLDPE则采用新型的紫外光辐照交联法。利用DSC对试样的熔融-结晶曲线进行测量、利用PLM和SEM对其球晶和片晶进行观测、并利用DMA和电子拉力机对试样的动态热机械曲线和应力-应变曲线进行测量。结合水树枝的生长机理,分别从聚乙烯的结晶形貌和片晶之间分子链的缠结两种角度,解释分子结构和交联度对聚乙烯耐水树枝老化性能的影响机理。研究发现:与LDPE和HDPE相比,LLDPE的耐水树枝老化能力强。原因在于LLDPE密集的短支链分子结构,大量的短支链促进了片晶之间的缠结,水分子在LLDPE非晶相中需要更多的能量与时间来破坏这些缠结分子链,并且这些缠结分子链在一定程度上也会抑制片晶在水分子挤压力下产生的滑移,这就相当于削弱了电致应力的破坏作用,抑制了水树枝的生长。从熔融-结晶曲线和PLM、SEM观测结果可知,随着试样交联度的升高,XLPE三维网状结构增强,抑制了球晶和片晶的生长,使其结晶度下降,无定形区域面积增大,理论上会促进水树枝的生长,但交联使其形成的三维网状结构限制了材料内部水分沿电场方向发生形变挤压材料形成水树枝的能力。从实验结果可知,交联形成的三维网状结构在提高材料耐水树枝能力上起主导作用。从动态热机械曲线和应力-应变曲线可知,交联度越高的XLPE,α松弛强度越弱,应变硬化现象也越明显。交联键能够有效加强非晶区分子链的缠结行为,在电致应力应力作用下的片晶之间的滑移与破坏变得更为困难,从而提高了抑制水树枝生长的能力。
[Abstract]:In humid environment, water molecules will enter the XLPE insulation of power cable under the action of electric field and form water tree structure gradually. The existence of water tree will not only deteriorate the mechanical and dielectric properties of insulation. And long-term development will form electric branches and cause permanent damage. Therefore, it is very important to study the inherent mechanism of polyethylene water tree aging for the safe operation of power cable. In this paper, the water knife electrode method is used to test the accelerated water tree aging of the sample, which is divided into two parts: (1) the effect of molecular structure on the aging resistance of polyethylene to water tree. The effects of crosslinking degree of HDPE; (2) on the aging resistance of LDPE, with long branched chain and HDPE; (2) with a large number of short branched chains on the aging resistance of polyethylene were investigated. XLPE samples with different crosslinking degree were obtained by adding different amount of crosslinking agent to PE base resin. The peroxide crosslinking method was used in LDPE and the new ultraviolet irradiation crosslinking method was used in LLDPE. The melt-crystallization curves of the samples were measured by DSC, the spherulites and tablets were observed by PLM and SEM, and the dynamic thermo-mechanical curves and stress-strain curves were measured by DMA and electronic tension machine. Based on the growth mechanism of water tree, the effects of molecular structure and crosslinking degree on the aging resistance of polyethylene were explained from the point of view of the crystalline morphology of polyethylene and the entanglement of molecular chains between sheets. It was found that compared with LDPE and HDPE, LLDPE had stronger water resistance to aging. The reason lies in the dense molecular structure of short branched chains of LLDPE. A large number of short branched chains promote the entanglement between wafers. Water molecules need more energy and time to destroy these entangled molecular chains in LLDPE amorphous phase. To some extent, these entangled molecular chains can also inhibit the slippage of lamellar crystals under water molecular extrusion pressure, which is equivalent to weakening the damage of electroinduced stress and inhibiting the growth of water branches. From the melting crystallization curve and PLM,SEM observation, it can be seen that with the increase of the cross-linking degree of the sample, the three-dimensional network structure of XLPE is enhanced, the growth of spherulites and tablets is inhibited, the crystallinity decreases, and the amorphous area increases. In theory it can promote the growth of water branches but the three-dimensional network structure formed by crosslinking limits the ability of the internal water to deform and extrude the materials along the direction of electric field to form water branches. From the experimental results, it can be seen that the three-dimensional network structure formed by cross-linking plays a leading role in improving the water-resistance of the materials. From the dynamic thermo-mechanical curves and stress-strain curves, it can be seen that the higher the crosslinking degree is, the weaker the relaxation strength of XLPE, 伪 is, the more obvious the strain hardening is. The cross-linking bond can effectively enhance the entanglement behavior of the molecular chains in the amorphous region, and the slip and destruction of the lamellae under the electrically induced stress becomes more difficult, thus improving the ability to inhibit the growth of water dendrites.
【学位授予单位】：哈尔滨理工大学
【学位级别】：硕士
【学位授予年份】：2017
【分类号】：TM247

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