Water Tree Morphology Research Articles

Water-Tree Characteristics and Its Mechanical Mechanism of Crosslinked Polyethylene Grafted with Polar-Group Molecules.

In order to restrain electric-stress impacts of water micro-droplets in insulation defects under alternating current (AC) electric fields in crosslinked polyethylene (XLPE) material, the present study represents chemical graft modifications of introducing chloroacetic acid allyl ester (CAAE) and maleic anhydride (MAH) individually as two specific polar-group molecules into XLPE material with peroxide melting approach. The accelerated water-tree aging experiments are implemented by means of a water-blade electrode to measure the improved water resistance and the affording mechanism of the graft-modified XLPE material in reference to benchmark XLPE. Melting–crystallization process, dynamic viscoelasticity and stress-strain characteristics are tested utilizing differential scanning calorimeter (DSC), dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. Water-tree morphology is observed for various aging times to evaluate dimension characteristics in water-tree developing processes. Monte Carlo molecular simulations are performed to calculate free-energy, thermodynamic phase diagram, interaction parameter and mixing energy of binary mixing systems consisting of CAAE or MAH and water molecules to evaluate their thermodynamic miscibility. Water-tree experiments indicate that water-tree resistance to XLPE can be significantly improved by grafting CAAE or MAH, as indicated by reducing the characteristic length of water-trees from 120 to 80 μm. Heterogeneous nucleation centers of polyethylene crystallization are rendered by the grafted polar-group molecules to ameliorate crystalline microstructures, as manifested by crystallinity increment from 33.5 to 36.2, which favors improving water-tree resistance and mechanical performances. The highly hydrophilic nature of CAAE can evidently inhibit water molecules from aggregating into water micro-droplets in amorphous regions between crystal lamellae, thus acquiring a significant promotion in water-tree resistance of CAAE-modified XLPE. In contrast, the grafted MAH molecules can enhance van der Waals forces between polyethylene molecular chains in amorphous regions much greater than the grafted CAAE and simultaneously act as more efficient crystallization nucleation centers to ameliorate crystalline microstructures of XLPE, resulting in a greater improvement (relaxation peak magnitude increases by >10%) of mechanical toughness in amorphous phase, which primarily accounts for water-tree resistance promotion.

Improved Water-Tree Resistances of SEBS/PP Semi-Crystalline Composites under Crystallization Modifications

Water-tree resistances of styrene block copolymer/polypropylene (SEBS/PP) composites are investigated by characterizing crystallization structures in correlation with the dynamic mechanical properties to elucidate the micro-structure mechanism of improving insulation performances, in which the accelerated aging experiments of water trees are performed with water-knife electrodes. The water-tree morphology in spherulites, melt-crystallization characteristics and lamella structures of the composite materials are observed and analyzed by polarizing microscopy (PLM), differential scanning calorimetry (DSC) and scanning electron microscopy (SEM), respectively. Dynamic relaxation and stress-strain characteristics are specifically studied by means of a dynamic thermomechanical analyzer (DMA) and electronic tension machine, respectively. No water-tree aging occurs in both the highly crystalline PP and the noncrystalline SEBS elastomer, while the water trees arising in SEBS/PP composites still has a significantly lower size than that in low-density polyethylene (LDPE). Compared with LDPE, the PP matrix of the SEBS/PP composite represent a higher crystallinity with a larger crystallization size in consistence with its higher mechanical strength and lower dynamic relaxation loss. SEBS molecules agglomerate as a “island” phase, and PP molecules crystallize into thin and short lamellae in composites, leading to the blurred spherulite boundary and the appreciable slips between lamellae under external force. The high crystallinity of the PP matrix and the strong resistance to slips between lamellae in the SEBS/PP composite essentially account for the remarkable inhibition on water-tree growth.

Research on the water blade electrode method for assessing water tree resistance of cross-linked polyethylene

A new water blade electrode method is presented to accelerate water tree initiation and propagation in cross-linked polyethylene (XLPE) insulation, and the validity was verified by a series of experiments. Water tree morphology and length over time were monitored using a polarized light microscope and compared to the results through the conventional water needle electrode method. The influence of electric field distribution along the water electrodes on water tree initiation and morphology was also investigated on the basis of experimental data and finite element simulation. Furthermore, the effect of voltage frequency on water tree initiation was also investigated. It was found that the water blade electrode method could greatly accelerate water tree initiation and propagation and it was more convenient to slice XLPE for observation. The high frequency voltage could more efficiently facilitate water tree initiation and propagation.

The influence of temperature on water treeing in polyethylene

The influence of temperature on water treeing in polyethylene (PE) is investigated in this paper. Low-density polyethylene (LDPE) and peroxide cross-linked polyethylene (XLPE) are chosen as the test materials. The liquid needle electrode method is used for water tree development at 20, 40, 60 and 80 °C, and a metallographic microscope is used to observe water tree morphology. The sizes and initiation rate of water trees are measured. The results obtained from the water treeing experiments indicate that water treeing is significantly influenced by temperature. The initiation rates of water trees decrease first, and then increase as temperature rises. The sizes of water trees also exhibit an increasing trend with ascending temperature. The experiments conducted on LDPE and XLPE demonstrate similar results. However, XLPE is found to have better anti water treeing performance than LDPE. Mechanical tensile test results show that the mechanical properties of the two materials, such as elastic modulus, tensile strength, tensile breaking stress and tensile yield stress, drop significantly as temperature increases. The results of the crystallinity determination and crystal morphology observation show that the crystal blocks of XLPE are smaller and denser compared with those of LDPE, which are lager and relatively sparser. Based on the analysis, XLPE is found to have better anti water treeing performance than LDPE because of cross-linking limitation to the formation of large crystal blocks and the enhancement of the mechanical properties of the material. Moreover, the failure of the mechanical properties of the materials, pore expansion and the accelerated movement of moisture and ions are the reasons for the acceleration of water treeing at high temperatures.

Non-equilibrium statistical theory of water treeing in polymeric cable insulators

A non-equilibrium statistical theory of water treeing in polymeric cable insulators, which treats water tree growth as a stochastic process, is presented. In this treatment the deterministic equation for the rate of water tree growth is made stochastic by the addition of a fluctuation term. The fluctuations are used to model the effect of the complex topologically connected microstructure of the polymeric insulator on water tree growth. Such considerations lead to a generalized Langevin equation for the water tree's growth rate as well as an equivalent Fokker - Planck equation for the probability density distribution of the water tree length. Many of the major attributes of water tree growth are shown to be a natural consequence of this equation. The self-affine fractal object for water tree morphology is first constructed, based both on the self-affinity of the time-correlating fluctuations and on the scale-invariance of the fundamental dynamic equation dominating water tree growth. The empirical two-parameter Weibull distribution of water tree length in the literature is also derived. Good quantitative agreement between theory and previously reported experimental results is shown.

Influence of polymer morphology on water treeing

The influence of polymer morphology on water tree propagation is studied using water needle laboratory specimens. For six different thermoplastic materials, the growth kinetics of water trees is measured, the water content of individual water trees is evaluated from the water tree opacity and the water tree morphology is examined by optical microscopy. Results are interpreted by considering the mechanical response of the materials to the penetration of water, induced by dynamic forces of electric origin. >

Water tree morphology

Water tree morphology as revealed by oxygen plasma etching of water trees in low-density polyethylene is reported. Water trees sectioned by freeze fracturing are found to be virtually invisible until treated by plasma etching. Etching attack is localized and produces either a pitted or a channeled structure, depending on the water tree growth conditions. The spatial distribution of the pits or channels is reminiscent of water tree void distribution. Secondary electron photomicrographs of trees grown under varied conditions are presented. >

Water treeing in polyethylene cables

Water tree growth in polyethylene cable insulation is discussed. The characteristics of water trees, the effect of aging parameters on water tree growth, and the possible mechanisms of growth are considered, emphasizing vented tree development in polyethylene insulating materials. The morphology of water trees, the characteristics of the tree-infested dielectric cable, and test methods and measures to reduce water treeing are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Water Treeing in Solid Dielectrics

The phenomenon of water-tree degradation of polyolefin insulating materials is reviewed with particular emphasis on the materials aspect of the problem. Specific topics include: methods of producing water trees in the laboratory, water-tree morphology, the nature of tree growth curves, and the effects of temperature, material, electrical variables, chemical factors and physical and mechanical changes on tree growth. Proposed mechanisms for tree initiation and growth, and contradictions in mechanisms and observations, are analyzed. Where possible, suggestions are made for experiments which would help clarify points of dispute. Appendices contain commonly accepted methods for staining treed materials, including filled rubber.