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Polyethylene – metal oxide particle nanocomposites for future HVDC cable insulation: From interface tailoring to designed performance
KTH, School of Chemical Science and Engineering (CHE), Fibre and Polymer Technology, Polymeric Materials.ORCID iD: 0000-0003-2201-2806
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Low-density polyethylene (LDPE) nanocomposites containing metal oxide nanoparticles are considered as promising candidates for insulating materials in future high-voltage direct-current (HVDC) cables. The significant improvement in dielectric properties compared with unfilled polymer is attributed to the large and active interface between the nanoparticles and the polymer. The nanoparticles may also initiate cavitation under stress and potential accelerated aging risks due to the adsorption and inactivation of the protecting antioxidants. This study is focused on the possibilities of achieving well-balanced performances of the polyethylene nanocomposites for HVDC insulation via tailoring the particle interface in the nanocomposites.

A facile and versatile surface coating method for metal oxide particles was developed based on silane chemistry. The developed method was successfully applied to 8.5 nm Fe3O4, 25 nm ZnO and 50 nm Al2O3 particles, with the aim to develop uniform coatings that universally could be applied on individual particles rather than aggregates of particles. The surface properties of the coatings were further tailored by applying silanes with terminal alkyl groups of different lengths, including methyl (C1-), octyl (C8-) and octadecyl (C18-) units. Transmission electron microscopy, infrared spectroscopy and thermal gravimetric analysis confirmed the presence of uniform coatings on the particle surface and importantly the coatings were found to be highly porous.

The capacity of metal oxide particles to adsorb relevant polar species (e.g. moisture, acetophenone, cumyl alcohol and phenolic antioxidant) was further assessed due to its potential impact on electrical conductivity and long-term stability of the nanocomposites. The oxidative stability of the nanocomposites was affected by the adsorption of phenolic antioxidants on particles and transfer of catalytic impurities (ionic species) from metal oxide particles to polymer matrix. It was found that carefully coated metal oxide particles had much less tendency to adsorb antioxidants. They could, however, adsorb moisture, acetophenone and cumyl alcohol. The coated particles did not emit any destabilizing ionic species into the polymer matrix. 

The inter-particle distance of the nanocomposites based on C8-coated nanoparticles showed only a small deviation from the ideal, theoretical value, indicating a good particle dispersion in the polymer. Scanning electron microscopy of strained nanocomposite samples suggested the cavitation mainly occurred at the polymer/nanoparticles interface. The microstructural changes at polymer/nanoparticle interface were studied by small-angle X-ray scattering coupled with tensile testing. The polymer/nanoparticle interface was fractal before deformation due to the existence of the bound polymers at the nanoparticle surface. Extensive de-bonding of particles and cavitation were observed when the nanocomposites were stretched beyond a critical strain. It was found that the composites based on carefully coated particles showed higher strain at cavitation than the composites based on uncoated particles. The composites based on C8-coated nanoparticles showed the largest decrease in electrical conductivity and the lowest temperature coefficient of the electrical conductivity among the composite samples studied.

Abstract [sv]

Nanokompositer av polyeten anses vara de mest lovande materialen för isolering av framtidens högspänningskablar (HVDC-kablar). De avsevärda förbättringarna i dielektriska egenskaper jämfört med ren polyeten kan härledas till den stora aktiv nanopartikelytan som existerar mellan partiklarna och polymermatrisen. Tillsatsen av nanopartiklar till polymeren kan dock även leda till ojämn spridning av partiklarna, där aggregat kan fungera som startpunkter för bildning av hålrum som skapas vid partikelytorna då materialen utsätts för mekaniska påfrestningar, samt accelererad åldringsbenägenhet på grund av potentiell lokal ansamling polyetens stabilisatorer vid partikelytorna. Detta arbete har fokuserats runt möjligheterna att skapa nanokompositer för HVDC-isolering med optimerad prestanda som relaterad till partikelytorna i nanokompositerna.

En metod för ytbehandling av metalloxidpartiklarna har utvecklats utifrån silankemi med fokus på användarvänlighet. Metoden tillät framgångsrik kondensering av silaner på 8.5 nm Fe3O4, 25 nm ZnO and 50 nm Al2O3 partiklar, och resulterade ytmodifiering av individuella partiklar snarare än aggregat. Ytegenskaperna hos partiklarna kunde därefter skräddarsys till att innefatta olika funktionella alkylgrupper med varierande längder från metyl (C1-), octyl (C8-) till octadecyl (C18-) enheter. Mikroskopi, infraröd spektroskopi samt termogravimetrisk analys bekräftade ytmodifieringarna resulterade i jämntjocka ytbeläggningar av individuella partiklar, medan molekylsimuleringar och densitetsvärden avslöjade att ytbeläggningarna visade hög porositet.

Metallpartiklarnas förmåga att adsorbera polära molekylstrukturer (fukt (H2O), acetofenon, kumylalkohol och fenoliska antioxidanter) utvärderades på grund av dess möjliga inverkan på elektrisk ledningsförmåga och långtidsstabilitet hos nanokompositerna. Stabiliteten gentemot oxidations visade sig var påverkad av adsorptionen av antioxidanter på partikelytorna, samt migration av katalyserande orenheter från partiklar till polymermatris. De modifierade partiklar var dock mindre benägna att adsorbera antioxidanter, men visade fortfarande adsorption av fukt, acetofenon och kumylalkohol. De ytmodifierade partiklarna bedömdes därför mindre benägna att inverka negativt på stabiliteten hos polymermatrisen.

Partikelavstånden mellan partiklarna i kompositerna baserade på C8-funktionaliserade nanopartiklar visade sig endast avvika obetydligt från det beräknade teoretiska värdet, vilket indikerade närmast en ideal dispergeringen av partiklarna. Svepelektronmikroskopi (SEM) av kompositer som utsatts för töjning visade att hålrumsbildningen främst uppstod i gränsytan mellan polymer/nanopartiklar. Förändringar av mikrostrukturen hos kompositerna studerades även med långvinkelröntgen (SAXS - small-angle X-ray) i kombination med dragprovning. Gränskiktet mellan polymer och nanopartikel hade en fraktalstruktur före deformationen på grund av närvaron av bunden polymer på nanopartikelytan. Sprickbildningen och utvecklingen av hålrum runt partiklarna kunde mest tydligt observeras när kompositerna hade sträckts mer än till en viss kritisk töjningsgräns, vilket även bekräftade att hålrumsbildningen uppstod vid högre töjningsvärden då partiklarna var ytmodifierade, i jämförelse med omodifierade partiklar. Kompositerna som framställts med C8-funktionalisering av nanopartiklarna visade den största sänkningen av ledningsförmågan och den lägsta temperatur koefficienten i elektrisk ledningsförmåga bland samtliga kompositer som utvärderats.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. , 48 p.
Series
TRITA-CHE-Report, ISSN 1654-1081 ; 2016:28
Keyword [en]
HVDC, electrical insulation, core-shell nanoparticles, surface modification, silane chemistry, nanocomposites, polyethylene, adsorption, long-term stability, interface, particle dispersion, cavitation, conductivity
Keyword [sv]
HVDC, elektrisk isolation, ytmodifiering, silankemi, nanokompositer, polyeten, adsorption, långtidsstabilitet, ytskikt, partikeldispergering, kavitation, ledningsförmåga
National Category
Textile, Rubber and Polymeric Materials
Research subject
Fibre and Polymer Science
Identifiers
URN: urn:nbn:se:kth:diva-192355ISBN: 978-91-7729-054-4OAI: oai:DiVA.org:kth-192355DiVA: diva2:967845
Public defence
2016-09-30, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20160915

Available from: 2016-09-15 Created: 2016-09-09 Last updated: 2016-09-15Bibliographically approved
List of papers
1. Morphology and properties of silica-based coatings with different functionalities for Fe3O4, ZnO and Al2O3 nanoparticles
Open this publication in new window or tab >>Morphology and properties of silica-based coatings with different functionalities for Fe3O4, ZnO and Al2O3 nanoparticles
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2015 (English)In: RSC Advances, ISSN 2046-2069, Vol. 5, no 59, 48094-48103 p.Article in journal (Refereed) Published
Abstract [en]

A facile single-step method for obtaining 2–3 nm thick silsesquioxane coatings on metal oxide nanoparticles using different carbon-functional silane precursors is presented. Iron oxide nanoparticles 8.5 nm in diameter were used as a model to evaluate the possibilities of forming different uniform carbon-functional coatings, ranging from hydrophobic to hydrophilic in character. Electron microscopy showed that all the coated nanoparticles could be described as core-shell nanoparticles with single Fe3O4 cores and carbon-functional silsesquioxane shells, without any core-free silicone oxide phase. Steric factors strongly influenced the deposited silicon oxide precursors with octyl-, methyl- or aminopropyl functionalities, resulting in coating densities ranging from 260 to 560 kg/m3. The methyl-functional coatings required several layers of silsesquioxane, 3–4, to build up the 2 nm structures, whereas only 1-2 layers were required for silsesquioxane with octyl groups. Pure silica coatings from tetraethoxysilanes were however considerably thicker due to the absence of steric hindrance during deposition, allowing the formation of 5–7 nm coatings of ca. 10 layers. The coating method developed for the iron oxide nanoparticles was generic and successfully transferred and up–scaled 30 and 325 times (by volume) to be applicable to 25 nm ZnO and 45 nm Al2O3 nanoparticles.

National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-168258 (URN)10.1039/C5RA04452A (DOI)000355703700096 ()2-s2.0-84930623622 (ScopusID)
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

Updated from "Submitted" to "Published". QC 20150703

Available from: 2015-05-29 Created: 2015-05-29 Last updated: 2016-09-15Bibliographically approved
2. Influence of nanoparticle surface treatment on particle dispersion and interfacial adhesion in low-density polyethylene/aluminium oxide nanocomposites
Open this publication in new window or tab >>Influence of nanoparticle surface treatment on particle dispersion and interfacial adhesion in low-density polyethylene/aluminium oxide nanocomposites
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2015 (English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945, Vol. 66, 67-77 p.Article in journal (Refereed) Published
Abstract [en]

The effect of silsesquioxane coating of aluminium oxide nanoparticles on their dispersion and on the interfacial strength between nanoparticles and polymer matrix in low-density polyethylene composites was studied. The surface chemistry of the nanoparticles was tailored from hydroxyl groups to alkyl groups with different lengths by reacting methyltrimethoxysilane (C1), octyltriethoxysilane (C8) or octadecyltrimethoxysilane (C18) with aluminium oxide nanoparticles. The core–shell structure of the coated nanoparticles was assessed by transmission electron microscopy, infrared spectroscopy and thermogravimetry. The inter-particle distance of the nanocomposite based on C8-coated nanoparticles showed only a small deviation from the ideal value, indicating a very good particle dispersion in the polymer. The interfacial adhesion between nanoparticles and matrix was determined by stretching nanocomposite specimens in a tensile testing machine to strains well beyond the yield point. A drop in the stress–strain curve indicated the onset of cavitation and necking in the nanocomposites. Samples stretched to different strain levels were studied by scanning electron microscopy and the cavitation was found to be confined to particle interfaces. The composite based on C18-coated nanoparticles showed the highest strain at cavitation/necking suggesting a high interfacial adhesion between nanoparticles and polymer.

National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-168256 (URN)10.1016/j.eurpolymj.2015.01.046 (DOI)000353854000007 ()2-s2.0-84922811618 (ScopusID)
Funder
Swedish Foundation for Strategic Research , EM11-0022
Note

QC 20150529

Available from: 2015-05-29 Created: 2015-05-29 Last updated: 2016-09-15Bibliographically approved
3. Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene
Open this publication in new window or tab >>Interactions between a phenolic antioxidant, moisture, peroxide and crosslinking by-products with metal oxide nanoparticles in branched polyethylene
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2016 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 125, 21-32 p.Article in journal (Refereed) Published
Abstract [en]

Polyethylene composites based on metal oxide nanoparticles are emerging materials for use in the insulation of extruded HVDC cables. The short-term electrical performance of these materials is adequate, but their stability for extended service needs to be assessed. This study is focussed on the capacity of the nanoparticles to adsorb polar species (water, dicumyl peroxide and byproducts from peroxide-vulcanisation, acetophenone and cumyl alcohol) that have an impact on the electrical conductivity of nanocomposites, the oxidative stability by adsorption of phenolic antioxidants on the nanoparticles and the potential transfer of catalytic impurities from the nanoparticles to the polymer. The adsorption of water, dicumyl peroxide, acetophenone, cumyl alcohol and Irganox 1076 (phenolic antioxidant) on pristine and coated (hydrophobic silanes and poly(lauryl methacrylate)) Al2O3, MgO and ZnO particles ranging from 25 nm to 2 gm was assessed. Composites based on low-density polyethylene and the particles mentioned (<= 12 wt.%) were prepared, the degree of adsorption of Irganox 1076 onto the particles was assessed by OIT measurements, and the release of volatile species at elevated temperature was assessed by TG. The concentration of moisture adsorbed on the particles at 25 degrees C increased linearly with both increasing hydroxyl group concentration on the particle surfaces and increasing relative humidity. Dicumyl peroxide showed no adsorption on any of the nanoparticles. Acetophenone and cumyl alcohol showed a linear increase in adsorption with increasing concentration of hydroxyl groups, but the quantities were much smaller than those of water. Irganox 1076 adsorbed only onto the uncoated nanoparticles. Uncoated ZnO nanoparticles that contained ionic species promoted radical formation and a lowering of the OIT. This study showed that carefully coated pure metal oxide nano particles are not likely to adsorb phenolic antioxidants or dicumyl peroxide, but that they have the capacity to adsorb moisture and polar byproducts from peroxide vulcanisation, and that they will not introduce destabilizing ionic species into the polymer matrix. Low contents of dry, equiaxed ZnO and MgO particles strongly retarded the release of volatile species at temperatures above 300 degrees C.

Place, publisher, year, edition, pages
Elsevier, 2016
Keyword
Polyethylene, Metal oxide nanoparticles, Water, Acetophenone, Cumyl alcohol, Phenolic antioxidant, Adsorption
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-184031 (URN)10.1016/j.polymdegradstab.2015.12.014 (DOI)000370894900003 ()2-s2.0-84954186925 (ScopusID)
Note

QC 20160324

Available from: 2016-03-24 Created: 2016-03-22 Last updated: 2016-09-15Bibliographically approved
4. Cavitation in strained polyethylene/aluminium oxide nanocomposites
Open this publication in new window or tab >>Cavitation in strained polyethylene/aluminium oxide nanocomposites
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(English)In: European Polymer Journal, ISSN 0014-3057, E-ISSN 1873-1945Article in journal (Refereed) Submitted
National Category
Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-192569 (URN)
External cooperation:
Funder
Swedish Foundation for Strategic Research , EM11-0022
Available from: 2016-09-15 Created: 2016-09-15 Last updated: 2016-09-15
5. Influence of nanoparticle surface coating on electrical conductivity of polyethylene/aluminium oxide nanocomposites for HVDC cable insulations
Open this publication in new window or tab >>Influence of nanoparticle surface coating on electrical conductivity of polyethylene/aluminium oxide nanocomposites for HVDC cable insulations
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(English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135Article in journal (Refereed) Submitted
National Category
Textile, Rubber and Polymeric Materials Engineering and Technology Composite Science and Engineering
Identifiers
urn:nbn:se:kth:diva-192573 (URN)
External cooperation:
Funder
Swedish Foundation for Strategic Research , EM11-0022
Available from: 2016-09-15 Created: 2016-09-15 Last updated: 2016-09-15

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