SYNERGETIC EFFECTS OF SILANE-GRAFTING AND EVA ON WATER TREE RESISTANCE OF LDPE

Chinese Journal of Polymer Science Vol. 28, No. I, (2010), 1-11Chinese Journal of Polymer Science◎Chinese Chemical SocietyInstitute of Chemistry, CASSpringer-Verlag 2010SYNERGETIC EFFECTS OF SILANE-GRAFTING AND EVA ON WATER TREERESISTANCE OF LDPEZhi-shen Ma, Xing-yi Huang, Jun-sheng Yang and Ping kai JiangSchool of Chemistry and Chemical Engineering, Shanghai Key Laboratory of Electrical Insulation and Thermal Aging,Shanghai Jiao Tong University, Shanghai 200240, ChinaAbstract The synergetic infuence of silane grafting and polar additives (EVA) on the water tree resistance of the lowdensity polyethylene has been investigated. A series of samples obtained before and after hydration have been characterizedby measuring gel content, infrared spectroscopy (FTIR), differential scanning calorimetry (DSC) and dieletricmeasurements. The results obtained clearly show that the siane condensation occurred and that the silane-grafting and polaradditives have synergetic efects on the water tree resistance of LDPE with lttle influence on its dielectric properties, e.g. thedielectric breakdown strength, dielectric permittivity and loss tangent.Keywords: Water teeing; Vinyl trimethoxysilane; Silane-grafting Synergetic influences; Cable insulation.INTRODUCTIONWater treeing is one of the major degradation phenomena of polyethylene (LDPE) cable insulation under thecombined action of water and electric stress"l. Due to the importance of the water treeing deterioration, manyspeculations have been put forward to explain the processes and mechanisms of the initiation and growth ofwater trees, but controversies still exist at present2-].Besides mechanism, a number of studies have been carried out for improving the water-tree resistance, e.g.chemical and physical modification, including the use of additives, blending, copolymerization and othertechniquesl",8-18. Our research group has investigated the water tree resistance of polyethylene by graftmodification with sodium acrylate and ethylene ionomersMany authors have attempted to use ethylenevinyl silane copolymer as additives for development of cable insulation performances.According to the previous researches, it can be found that water tree resistance may be improved by vinyltrimethoxysilane (VTMS) being grafted onto polyethylene chains, and the reactive groups of VTMS(Si- (OCH)3) can absorb permeated water to form crosslinked structure. However, the water tree resistantmechanism of silane grafting is not so clear.On the other hand, EVA used as a polar additive also can greatly improve the water tree resistance ofpolyethylene or crosslinked polyethylene 10 ), and EVA also can improve the general performance of silanecrosslinked polyethylene, such as dieletric properties, crystallinity and mechanical properties23.The purpose of this work is to explain the water tree resistant mechanism of silane grafting on polyethyleneand to observe the synergetic effect of silane-grafing and EVA on the water tree resistance of LDPE.中国煤化工Corresponding author: Ping-kai Jiang (江平开), E-mail: pkjiang@sjtuReceived October 20, 2008; Revised November 17, 2008; Accepted DeMYHCNMHGdoi: 10.1007/s10118-010-8191-1Z.S. Ma etal.EXPERIMENTALMaterialsThe polyethylene used in this study is LDPE (100BW) from ExxonMobil in Saudi Arabia, which has a meltflow index (MFI) of 2.0 g/10min and a density of 0.9225 g/cm'. Vinyl trimethoxysilane (VTMS Z-6300) issupplied by Dow Corming Corp. in Midland, MI, USA. Ethylene vinyl acetate (EVA VA600) is supplied byHyundai Petrochemical Corp. in South Korea, which has a melt flow index (MFI) of 6.0 g/10min and a densityof 0.95 g/cm'. The dicumyl peroxide (DCP) (Chemical Reagent) is supplied by Shanghai Gaoqiao PetroleumCo., Ltd, China, which has purity higher than 99.5%. Catalyst used here was dibutytin diaurate (DBTL),purchased from Atofina Corp. The NaOH (AR) and sodium chloride (NaCI) (AR) are from Shanghai MedicineChemical Co, Ltd, China.Sample PreparationLDPE or the definite ratio of LDPE and EVA was mixed in a HAAKE rheometer RC90 at a temperature of170°C and a rotor speed of 30 r/min. After the mixture was melted, VTMS and DCP were added into therheometer for 2 min, then DBTL was added and blended for 2 min, and the residence time was 5 min. Allsamples were pressed at 120°C for 10 min.Infrared Transmission Spectra AnalysisThe samples obtained were pressed into flms of 100 pum in thickness. Prior to the FTIR measurement, the filmswere extracted in a Soxhlet extractor for 36 h by using acetone as extractant to remove non-reacted silane andprobable silane oligomers, and then the films were dried in an oven under vacuum at 70°C for 8 h before testing.All the hydration samples were obtained after being immersed in distilled water or sodium chloride solution for30 days, and then FTIR measurements were carried out.On the other hand, in order to consider the effect of electric field on the hydration crosslinking reaction ofgrafted silane, after the accelerated aging for 30 days, the samples that contained water tree were also used forFTIR measurements.Gel ContentThe gel content was determined on thin films with thickness of about 0.2 mm. Like FTIR analysis, we alsoconsider the effect of water-immersion on the gel content of samples. About 0.2 g of the hot-pressed films andthe films after water-immersion for 30 days were exactly weighed and then placed in a 120-mesh copper net,respectively. Then, they were extracted in a Soxhlet extractor for 36 h by using p-xylene as extractant. Aferextraction, the copper nets containing samples were dried in a vacuum oven at 80°C. Finally, the remaining gelwas weighed and calculated as follows:Gel content (%)= W/Wox 100where, Wo and w are the mass values of samples before and afer extraction, respectively.Differential Scanning CalorimetryA Perkin Elmer Pyris 1 DSC calorimeter was used to perform differential scanning calorimtry analysis in aflowing nitrogen atmosphere. Before DSC tests, flms were dried in an oven under vacuum at 80°C for 8 h.Samples of 3.0 mg were initially heated fom 20°C to 150°C ata heat rate of 20 K:min~', held at 150°C for3 min to eliminate thermal history effects, and then cooled down to 209C at the rate of 20 K min '. The value of290 Jg' was taken as the melting enthalpy of 100% crytalline polyethylene in order to calculate crstalinity ofsamples from the melting enthalpies(41.Dielectric PropertiesThe dielectric constant and the dissipation factor were meas中国煤化工h-voltage bridge(Shanghai Huguang Corp, Shanghai, China) according to A:YHCNMHGof specimens is1.0 mm), the measuring frequency is 50 Hz.Synergetic Effects of Silane grafting and EVA on Water Tree Resistance of LDPEThe dielectric strength was measured using an AC dielectric strength tester AHDZ-10/100 (ShanghaiLanpotronics Corp., Shanghai, China) according to ASTM D 149-2004. During testing, the applied voltage wasraised step by step, and the voltage at each step was kept for 1 min.Water Treeing TestThe circular specimens for water treeing test were prepared based on the method and molding apparatusaccording to ASTM D6097-97. After ten specimens of the same kind were fixed in a PMMA specimen holder,the holder was flld with 1.0 mol/L NaCl solution, and then the test specimen bolder was also placed in the bathcontaining 0.01 molL NaCl solution. Water treeing test was carried out at room temperature under the action ofan applied voltage of5 kV (1500 Hz). After the accelerated water tree test was performed for 30 days, allsamples were stained with methylene blue, and the water tree length was determined by measuring the length ofthe longest branch of the tree under a polarizing microscope. The value averaged from the values of10 measurements was taken as the water tree length (WTL).RESULTS AND DISCUSSIONFTIR CharacterizationFigures 1-3 shows FTIR spectra of LDPE-g-TMS and LDPE-g-TMS/EVA at different conditions, respectively.From the comparison between the FTIR spectra of LDPE and LDPE-g-TMS in Fig. 1, the absorption peaks at1082 cm~l assigned to the functional groups of VTMS can be easily found. There is an absorption peak around1100 cm-' corresponding to the vibration of C- c single bonds in the spectrum of LDPE, but the peak shape ofwhich is different from those observed in the other spectra of LDPE-g-TMS and the transmittance at 1082 cm-'tends to increase with the increase of content of VTMS, indicating the occurrence of VTMS grafting onto LDPEchains. On the other hand, EVA has a characteristic absorption peak, which is assigned to the C= 0 strethingvibration of EVA at 1740 cm-' and can be found in Fig. 1.pd30001000Wavenumber (cm^')Fig. 1 FTIR spectra of LDPE-g-TMS and LDPE.g-TMS/EVAa) VTMS 0 phr; b) VTMS 0.4 phr; c) VTMS 2.0 phr; d) VTMS 0.4 phr/EVA 1.0 phr,e) VTMS 2.0 phr/EVA 1.0 phrIn order to characterize the silane condensation reaction at room temperature and the effect of appliedelectric field, FTIR spectra measurements were performed on LDPE-g-TMS and LDPE-g-TMS/EVA series afterbeing immersed into distilled water for 30 days at room temperature and after 30 days accelerated aging processrespectively, and FTIR spectra are shown in Figs. 2 and中国煤化工m-' related withSi-OCH; vibration of Si- (OCH3)3 groups decreased, howevCNMH- Si vibration ofcrosslinks at 1025, 1050 and 1150 cm~' became strong. Ththe shoulder atZ.S. Ma et al.1120 cm^ ' may be ascribed to Si- 0 vibration in a strictly hindered configuration resulting ftom an increasedcrosslink density.All these results of Fig.2 confrm the occurrence of the silane condensation reaction when silane contactswith water molecules.舍300020001000Wavenumber (cm )Wavenumber (cm~ )Fig.2 FTIR spectra of LDPE-g-TMS and LDPEg-Fig. 3 FTIR spectra of LDPE-g-TMS and LDPE-g-TMS/EVA after silane condensation reactionTMS/EVA under electric fielda) VTMS 0.4 phr; b) VTMS 2.0 phr; c) VTMS 0.4 phr/EVAa) VTMS 0 phr; b) VTMS 0.4 phr; c) VTMS 2.0 phr;1.0 phr; d) VTMS 2.0 phr/EVA 1.0 phrd) VTMS 0.4 phr/EVA 1.0 phr; e) VTMS 2.0 phr/EVA 1.0 phrOn the other hand, comparing with the FTIR spectra of LDPE-g TMS and LDPE-g-TMS/EVA series underthe action of electric field, besides the absorption peak of Si- 0- Si groups, it can be also found that the peakaround 2980 cm^". becomes wider and the transmittance becomes quickly weaker, and the absorption peakaround 2340 cm^' that may correspond to CO2 and -CH=C=CH- - is quickly enhanced, which may beattributed to that carbon hydrogen chains have been broken to form degraded structures. Comparing the FTIRspectra of LDPE-g-TMS and LDPE-g-TMS/EVA series before and after the action of electric field, there arehardly any changes of the IR absorption peak and area corresponding to C=0 stretching vibration of EVA,indicating that EVA has a physically-resistant performance on water tree growth, the IR spectrum changes ofsilane groups (TMS) indicate that the silane condensation occurs, it may form crosslinked structure and result inwater tree resistance improvement, indicating that VTMS grafting has a chemically-resistant efect, therefore,water tree resistant mechanisms of silane grafting and EVA are different from each other in nature.Gel ContentIn order to investigate the occurence of silane condensation cos-linking during the wate-teeing test, the gelcontent of samples containing different contents of VTMS before and after the hydrolyzed condensation wasmeasured (Fig. 4).It can be seen from Fig. 4 that there is an obvious difference of gel content between LDPE-g -TMS beforeand after the hydrolyzed condensation. Although the silane cross-linking rate is very low at room temperature!24,the gel content greatly increases after a long time period of water immersion. The reason may be that thediffusion rate of water molecules at room temperature is so low that silane cross-linking condensation needsmore time to finish. On the other hand, the comparison of the gel content ofLDPE_g-TMS with that of LDPE-g-TMS/EVA before and after the hydrolyzed condensation sho |中国煤化工increases the gelcontent of LDPE-g-TMS, regardless of whether hydrolyzed corYC N M H Gor not. Howeverthe difference of the gel content between LDPE-g-TMS and LDF cg-1 MO/LV A is ivi su sguricant.Synergetic Efects of Silane grafing and EVA on Water Tree Resistance of LDPE580-60-2001.21.2.0VTMS content (phr)Fig. 4 Plots of the gel content ofLDPE.g TMS and LDPE-g-TMS/EVAa) LDPE-g-TMS before the hydrolysed crosslinking; b) LDPE-8-TMS/EVA before hydrolysedcrosinking; c) LDPE-g-TMS after the hydrolysed crosslinking for 30 days; d) LDPE-g-TMS/EVA after the hydrolysed crosslinking for 30 daysDifferential Scanning CalorimetryBecause the DSC melting and crystallization curves for LDPE-g-TMS and LDPE-g-TMS/EVA blends beforeand after the accelerated aging for 30 days is similar, and juts the DSC melting and crystallization curves ofLDPE and samples containing the largest VTMS content are exhibited in Figs. 5 and 6, respectively, whichshow that silane content has litle effects on the melting and crystallization temperatures of LDPE-g-TMS, andthe addition of EVA causes the melting temperature to further shift towards the low temperature ranges. On theother hand, the accelerated aging also hardly has any effect on the melting and crystallization temperatures,however, it can be easily found from the melting curves that the apparent second peaks occur in the meltingDSC curves after the action of eletric fields, which may be due to that the growth of water tree destroys thepolyethylene chains, some sub-crystals forms by the formation of silane cross-linked structure and rudimentalwater. However, such second peaks cannot be found in the crystallization curves, indicating that some degradedstructure may be formed during water tree growth and also can forms sub-crystals, but the structure of degradedregion is similar with polyethylene. When melts rerystallize, these sub-crystals will rearranged and form perfectcrystals, and the second peaks will not appear in crystallization curves.6800060100120Temperatur (CC)Temperature (C)Fig. 5 DSC melt curves of LDPE-g-TMS and LDPEgFig.6 DSC crystallization curves of LDPE_g-TMS andTMS/EVA under nitrogenLDPE--JTMS/EV A lunder nitrncena) VTMS 0 phr; b) VTMS 2.0 phr; c) VTMS 2.0 phr/ .a)VI中国煤化工rMS 2.0 pht/EVAEVA 1.0 phr; d) VTMS 0 phr after eletric field aging;1.0pld aging; e) VTMSe) VTMS 2.0 phr after electric field aging; f) VTMS 2.02.0 p| YHCNMHGMS2.0phr/EVAphr/EVA 1.0 phr after electric field aging1.0 phr after electric field aging6Z.. Ma et al.Tables 1 and 2 show the thermal properties of LDPE.g-TMS and LDPE-g-TMS/EVA blends before andafter accelerated aging for 30 days. It can be found that crytallinity tends to decrease with the increase ofVTMS content, which may be ascribed to that the large volume of TMS groups may inhibit the thickening of thelamellae of Si-grafted polyethylene.Table 1. Effect of VTMS on the thermal properties ofLDPE-g- TMS and LDPE-g-TMS/EVAbefore acclcrated agingSamplesTm (C)T(°C)_ AH.(J/g)_ X (%)_LDPE108.9493.0276.3726.33LDPE-g-0.4 phr TMS108.2292.2572.2224.90LDPE-g-0.8 phr TMS106.8693.8574.0725.54LDPE-g-1.2 phr TMS108.6393.0174.6625.74LDPE-g-1.6 phr TMS107.7494.3273.9825.51LDPE-g-2.0 phr TMS107.9492.9273.3425.29LDPE-g-0.4 phr TMS/1.0 phr EVA108.0792.6271.5224.66LDPE-g-0.8 phr TMS/1.0 phr EVA108.0993.6172.2624.92LDPE-g-.2 phr TMS/1.0 phr EVA107.6993.0672.1324.87LDPE-g-1.6 phr TMS/1.0 phr EVA107.3092.44472.6925.07LDPE-g-2.0 phr TMS/L.0 phr EVA107.2993.63 _71.9024.79Table 2. Effect of VTMS on the thermal properties of LDPE-.g-TMS and LDPE-g-TMS/EVAafter accelcrated agingTm(°C) _T.(°CAH (0/g)X (%)108.1493.4470.2024.21108.2992.94109.3071.1624.54LDPEg-1.2 phr TMS108.4591.8569.8124.07107.7792.8468.4223.59LDPE-g 2.0 phr TMS107.8292.5867.4523.26108.0292.6869.7524.05108.5592.8767.6223.32LDPE-g-1.2 phr TMS/1.0 phr EVA108.5291.5366.7723.02LDPE-g-1.6 phr TMS/.0 phr EVA108.8691.3865.7522.67LDPE-g-2.0 phr TMS/1.0 phr EVA108.2492.2964.5722.27Comparing the crystallinity degree of samples before and after accelerated aging process, it can be foundthat the crystallinity degree values of samples after accelerated aging process are obviously lower than those ofsamples before accelerated aging process.Dielectric ProperiesIt is well known that the addition of additives can cause the modification of the dielectric properties of matrix,including the dielectric permittivity, the dissipation factor, dielectric breakdown strength etc.The dielectric properties for LDPE-g-TMS and LDPE-g-TMS/EVA specimens containing differentconcentrations of VTMS before and after hydrolyzed condensation are shown in Figs. 7 and 8, respectively. Itcan be easily expected that the functional group of TMS (Si - -OCH3)3 is a polar and reactive group and EVA isalso a polar copolymer, the silane grafting and the addition of EVA can cause the increase of electrical polarityof the LDPE-g-TMS and LDPE-g-TMS/EVA. However, as clearly seen from Fig. 7, the groups of TMS(Si-OCH3) show a lttle influence. The dielectric constant of the LDPE-g-TMS tends to slightly decrease withthe VTMS concentration increasing, and the addition of EVA leads to an increase of dielectric constant. Thecrosslinked polyethylene after silane-crosslinking might be expected to bave the higher dielectric permitivitythan the non: crosslinked one, but the dielectric constant is obse中国煤化工: of the existenceof rudimental water.TYHCNMHGSynergetic Efets of Silane grafing and EVA on Water Tree Resistance ofLDPE2.After condensation0I Before condensationLDPE-g-TMS/EVA.2旨2.2.DPE-g-TMS/EVA732.1 LDPE-g-IMS’| 1.DPE-g-TMS2.0f LDPE3g-TMSEVA F二45量2.111‘.9+2告LDPE-g-TMS1.8L0040.8亡0.00.4 0.8i .2.6VTMS content (phr)Fig. 7 The effect of VTMS content on dissipationFig.8 The efect of VTMS content on dissipationfactor and dielectric constant of LDPE-g-TMS andLDPE-g-TMS/EVA before immersionLDPE-g-TMS/EVA after immersionThe dielectric loss tangent (tan8) has similar trends before and after silane cross-linking: with increase ofVTMS content, the dielectric loss increases, and after the addition of EVA the dielecric loss further increases.After 30-day-hydrolyzed condensation, the reactive TMS groups decrease because of silane cross-linking, thedielectric loss should also decrease. However, the existence of rudimental water, the dieletric loss of which ismuch higher than polyethylene, leads to the increase of dieletric loss of samples after silane cross-linking.The dielectric strength of LDPE-g-TMS and LDPE.g-TMS/EVA before and after hydrolyzed condensationis shown in Figs. 9 and 10. It should be noted that the dielecric breakdown behavior of the LDPE-g-TMS beforehydration is quite silmilar with its dependence of gel content on VTMS concentration and strongly supports theabove- mentioned considerations. It is well known that non-crosslinked polyethylene shows higher dielectricstrength than the cosinked one at 30°C[251.100Before immersionAfler immersionAter inmcrsion80H80广50H时40H心20H20 HoL.40.121.0.40.81.21.6 2.0Fig. 9 Characteristic dielectric strength of LDPE-g-Fig.10 Characteristic dielectric strength of LDPE-g-TMS samples with different VTMS contentsTMS/EVA samples with different VTMS contentsAlso very interesting is that the values of dielectric breakdown strength before hydration apparentlyincrease corresponding to the increase of VTMS concentrations, while after hydration, the dielectric strengthwill greatly decrease. And also, the difference between dielectpic stength behavinre hefore and after hydrationalso corresponds to the water tree resistant effect of VTMS中国煤化工ix, the dielecricstrength values both before and after hydration become lower,YHC N M H Gicletic strengthvalues of LDPE-g-TMS/EVA before and after hydration also snows une smar uenus as une behavior of water8Z.S. Ma et al.tree length.Water Tree ResistanceThe water tree resistance of the LDPE-g-TMS and LDPE-g-TMS/EVA samples studied is evaluated by thewater tree length and is shown in Fig.11.口LDPE-g-TMS。LDPE-g-TMS/EVA600福500g 400302000.0.40.82 1.6 2.0VTMS content (phrFig. 11 Effect of the VTMS concentration on the water tree length ofLDPE-g-TMS and LDPE-g-TMS/EVAIt can be seen from Fig. 11 that silane-grating can considerably improve the water tree resistance ofpolyethylene: the water tree length decreases from 709 μm to 386 μm when the VTMS content increases from0 phr to 2.0 phr. After adding 1.0 phr EVA, the water tree length further decreases from 709 um to 261 umwhen the VTMS content increase from 0 phr to 1.2 phr, while beyond 1.2 phr VTMS, it increases again up to439 um at 2.0 phr VTMS, and according to our experiment, the number of water trees is also the smallest whenVTMS content is 1.2 phr and EVA content is 1.0 phr.The results obtained are likely due to both the grafing of hydrophic and reactive TMS onto the basic chainsof polyethylene and the silane condensation reactions occur during water treeing test. The addition of EVAresults in that the water tree length further decreases because EVA itself can resist the growth of water tree andmay be possible to take part in the silane grafting to improve the mechanical properties of silane cross linkedpolyethylenel231.According to the propagation mechanisms of water trees, water and electrical field are the most crucialfactors for water tree formation!”9，20. The grafted reactive and hydrophilic Si- (OCH3)3 groups of polyethylenecan react with water and then form silane crosslinked structure under the action of catalyst (DBTL). The totalreaction process of silane in polyethylene matrix can be easily understood according to Scheme I. Silanecrosslinking reaction could lead to the production of water molecules, but more water molecules would beconsumed during the change of Si - (OCH3)3 groups into Si - (OH)3 groups, that is, the total pumber ofpenetrated water molecules decreased and the effect of penetrated water on water tree growth would bedepressed during the water tree growing process. Besides cross-linked silane groups, there existed some silanolgroups which did not react with the polymer matrix and had a strong interaction with water molecules. Thesesilane groups were dispersed uniformly in the matrix and could prevent the aggregation of water molecules,resulting in the decrease of water tree length. This is the mechanism of the water treeing resistance from theviewpoint of chemical resistance. On the other hand, the introduction of hydrophilic EVA can also absorbpenetrated water molecules and water molecules produced in silane crosslinking reactions to prevent the watersediment at high electric stress point, which is the mechanism of water tree resistance from the viewpoint ofphysical aspect. Thus, silane-grafting and EVA have different中国煤化工ns, and they cansynergistically improve the water tree resistance of polyethylenEn from the FtIRspectra of LDPE-g-TMS and LDPE-g-TMS/EVA before and af;Y片CNMHGsmaybegraftedSynergetic Efecs of Silane grafting and EVA on Water Tree Resistance of LDPE9onto EVA chain, the silane grafted EVA will also take part in the silane condensation of LDPE-g-TMS and theprocess can be shown in Scheme 2, although VTMS that be grafted onto EVA chain may also have effect on thewater tree resistance, the added content of EVA is only one percent of LDPE amount, and thus the effect ofEVA-g- TMS is too small to be discussed in the paper.Silane gratingQCH;OCH;+ CH2=CIH- Si-OCH; .IntationCH2- -CH2- -Si-OCHzOCH3+ H2OOHQHr CH2-CH2-Si-0-0- -Si-CH2-CH2-DBTL-CH2- -CH2-Si-OHOtSilanol condensationScheme 1 Water tree resistant mechanism of LDPE-g-TMSQCH3_DCP_+ CH2=CH-si- -OCH3IntuationO-CH2- -CH2-si-OCH30CH;+ 3H2OH-H20_, 0-CH2-CH2-Si-0-o-Si-CH2- -CH2-- DBrL-O>-CH-CH-Si-OHScheme 2 Water tree resistant mechanism of LDPE-g-TMS/EVAFigure 12 shows the water tree photos of several samples, and it is more intuitionistic to find the effect ofsilane-grafting and the synergetic effect of silane-grafting and EVA on water tree resistance of LDPE. On theother hand, you will also found the saturation of modification of polar additives, after adding 1.0 phr EVA, thewater tree length will increase again when VTMS content exceed 1.2 phr.中国煤化工MYHCNMHG1Z.S. Ma et al.d岗Fig. 12 Water tree photos of samplesa) LDPE; b) LDPE-g-2.0 phr TMS; c) LDPE.g-1.2 phr TMS/1.0 phr EVA;d) LDPE-g-2.0 phr TMS/1.0 phr EVACONCLUSIONSThe effect of silan-grafting on the water tree resistance of LDPE has been experimentally investigated togetherwith the analyses of physico-chemical and dielectric characterization.It can be concluded from the resuts that the VTMS grating shows a signifcant efet on improvement ofthe water tree resistance of LDPE, and after introducing EVA, the water tree resistance will be further improved.It has been found that there exist optimum concentrations of VTMS (about 1.2 phr) and EVA (1.0 phr)corresponding to the minimum water tree length.The water teeing mechanism of the LDPE-g TMS can be explained by the elimination of the absorbedwater molecules by silane cross-linking. The grafted Si- OCH3 is uniformly dispersed in matrix, and its strongcontact with water molecules prevents the aggregation of water at high electric field points. 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