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Chemically expanded graphite-based ultra-high molecular weight polyethylene nanocomposites with enhanced mechanical properties
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0002-4940-8176
Centre for Mechanical Technology and Automation (TEMA), University of Aveiro, Portugal.
Institute of Material Science of Barcelona, Spain.
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Machine Elements.ORCID iD: 0000-0001-8676-8819
2022 (English)In: Materials & design, ISSN 0264-1275, E-ISSN 1873-4197, Vol. 224, article id 111304Article in journal (Refereed) Published
Abstract [en]

Chemically expanded graphite (CEG) has recently been identified as promising reinforcement for polymer composites with the ability for commercial up-scaling. In this work, silane and polydopamine functionalized CEG were successfully synthesized and employed to prepare ultra-high molecular weight polyethylene (UHMWPE) nanocomposites with an enhanced interfacial compatibility. Characterisation of the functionalized CEG indicated a significant oxygen reduction, which gave rise to a restoration of the graphitic structure. The polydopamine functionalized CEG showed an enhanced exfoliation and dispersion in organic solvents and the polymer matrix with respect to the non-modified CEG. The silane functionalized CEG provided a higher affinity towards the matrix with polymer chains covering the CEG sheets on the fracture surfaces. The addition of functionalized CEG enhanced the mechanical properties of the UHMWPE matrix with an increase in micro-hardness of up to 25% and storage modulus of up to 58%. Furthermore, the hydrophobicity of the composites was significantly enhanced with an increase in water contact angle from 98.6° for the pure polymer to 119° for 5 wt% silane functionalized CEG. Preliminary wear experiments indicated the potential of the composites for tribological applications with a decrease in wear rate of up to 99% under water lubricated conditions.

Place, publisher, year, edition, pages
Elsevier Ltd , 2022. Vol. 224, article id 111304
Keywords [en]
Chemically expanded graphite, Dynamic Mechanical Analysis, Hydrophobicity, UHMWPE, Wear
National Category
Textile, Rubber and Polymeric Materials
Research subject
Machine Elements
Identifiers
URN: urn:nbn:se:ltu:diva-93838DOI: 10.1016/j.matdes.2022.111304ISI: 000882476000005Scopus ID: 2-s2.0-85140887818OAI: oai:DiVA.org:ltu-93838DiVA, id: diva2:1709013
Funder
Vinnova, 2017–03609
Note

Validerad;2022;Nivå 2;2022-11-07 (joosat);

Funder: TEMA (UIDB/00481/2020, UIDP/00481/2020); Portuguese Science Foundation (FCT) (CEECIND/01913/201); European Regional Development Fund; 3D - PRINTGRAPH (705875)

Available from: 2022-11-07 Created: 2022-11-07 Last updated: 2024-03-14Bibliographically approved
In thesis
1. Graphene as Solid Lubricant in Polymer Composites: With Application in Hydropower Bearings
Open this publication in new window or tab >>Graphene as Solid Lubricant in Polymer Composites: With Application in Hydropower Bearings
2024 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Grafen som fasta smörjmedel i polymerakompositer : med tillämpning i vattenkraftslager
Abstract [en]

With a rising global demand for green energy production, hydropower plays a key role in securing a reliable and sustainable energy supply. Hydropower is the largest and most efficient renewable energy source, with an ever-increasing capacity. To enable this high efficiency and moderate the turbine output, regulating surfaces including guide vanes and runner blades, can be adjusted. The frequent start-and-stop cycles, as well as the large variation in turbine output levels introduces harsh sliding conditions in the turbine bearing systems. Premature failure of the self-lubricating polymeric bearings is currently a major limiting factor for the reliability of hydropower systems. Consequently, there is an urgent need for new high performance bearing materials with a significantly enhanced service life. The currently used commercial bearing materials primarily use polytetrafluorethylene (PTFE) as solid lubricant. However, due to environmental concerns related to the production and use of PTFE, alternative solid lubricants are required. Graphene has been identified as potential solid lubricant with a great friction and wear performance at the nanoscale. At macro-scale, the introduction of graphene and its derivatives has not yet led to similar low friction properties as PTFE when used as solid lubricant in non-polar polymers, such as, ultrahigh molecular weight polyethylene (UHMWPE). 

In this thesis, graphene and its derivatives is evaluated as solid lubricant in polymer composites. Different graphene derivates are characterised compared and evaluated with respect to their tribological performance under dry and lubricated sliding. Furthermore, with a poor interface often observed between graphene derivatives and thermoplastic polymer matrices such as and UHMWPE, different methods of surface functionalization were explored to enhance the adhesion and stress transfer between the graphene and matrix. The tribological properties of the resulting composites were analysed in detail. Additionally, two multiscale reinforced composites based on UHMWPE and polyphenylene sulfide (PPS) were processed and evaluated with respect to commercial grade bearing materials. The friction and wear performance of these composites were characterised under varying sliding conditions including contact pressures of up to 40 MPa, simulating conditions as found in hydropower turbines.

The results show a surprising increase in sliding friction when introducing the different graphene derivates, in comparison to the neat polymer matrix. Characterisation of the pin surface highlighted the presence of stick-slip which can be correlated to a reduction in degree of crystallinity and plastic deformation of the polymer at the sliding surface. By using surface functionalization with hydrophobic silanes, a well-defined interface between the chemically expanded graphite (CEG) and polymer matrix was successfully created. This effect was confirmed by fracture surface analysis and an increase in storage modulus with respect to the non-functionalized CEG. Sliding tests furthermore indicated significant reduction in sliding friction at a low CEG content, lower than both the non-functionalized CEG composites and the neat polymer. 

Evaluation of the two in-house processed multiscale reinforced composites and commercial bearing materials revealed a low dry sliding friction and wear for the commercial materials. However, when introducing water as lubricant, friction and wear increased dramatically in the absence of a well-developed transfer film. The PPS based composites performed exceptionally well under water lubrication, with a low coefficient of friction of 0.04. Furthermore, the specific wear rate was a factor of 3 lower than the best performing commercial material, confirming the potential of this novel multiscale reinforced thermoplastic composite for highly loaded hydropower bearings.

Place, publisher, year, edition, pages
Luleå: Luleå University of Technology, 2024
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Keywords
Graphene, Solid Lubricant, Polymer Composite, Friction, Hydropower
National Category
Tribology (Interacting Surfaces including Friction, Lubrication and Wear)
Research subject
Machine Elements
Identifiers
urn:nbn:se:ltu:diva-104606 (URN)978-91-8048-501-2 (ISBN)978-91-8048-502-9 (ISBN)
Public defence
2024-04-19, E231, Luleå University of Technology, Luleå, 13:00 (English)
Opponent
Supervisors
Available from: 2024-03-15 Created: 2024-03-14 Last updated: 2024-03-27Bibliographically approved

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