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Ageing behavior of plastics used in automotive fuel systems
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology.
2019 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The increase in service temperature and the use of biobased fuels, such as biodiesel, have raised concerns on the short/long-term performance of plastic components used in automotive fuel systems.

In this work the ageing behavior of unreinforced and glass-fibre reinforced polyamide 12 (PA12), exposed to three different fuels (petroleum diesel, biodiesel, and a mixture of these (80/20)) at high temperature, was investigated. The interactions between the polymer and the fuel, and the associated polymer ageing mechanisms (fuel uptake, extraction of monomer and oligomers, annealing and oxidation), were found to be “generic” in the sense that they occurred, although to various extent, for all fuels. In the glass-fibre reinforced polyamides, the ageing occurred mainly in the polyamide matrix and not in the matrix-fibre interface. The semi-aromatic polyamide showed better performance when exposed to fuels than the aliphatic PA12.  

At a component level, multilayer polyamide-based pipes, with polyamide or fluoropolymer as inner layer, were aged under “in-vehicle” conditions where the pipes were exposed to fuel on the inside and to the air on the outside. All pipes stiffened during ageing but embrittlement occurred only for the pipes with polyamide being the inner layer. Compared to polyamide, the fluoropolymer inner layer showed significantly better barrier properties towards the fuel and no material was extracted into the fuel. The plasticizer loss from the PA12 outer layers into air was diffusion controlled and its diffusivity followed a linear Arrhenius behavior in the high temperature region. Relationships between plasticizer loss and the changes in mechanical properties were established.

The polyamides experienced diffusion-limited oxidation when exposed to air and/or fuel, involving the formation of a thin oxidized surface layer which was responsible for a significant decrease in strain-at-break. 

The fracture behavior of PA 6 in air at high temperature, found to involve three distinct stages, were systematically studied and linked to underlying mechanisms responsible for the reduction in strain-at-break.

Abstract [sv]

En ökning i servicetemperatur och användning av biobaserade drivmedel, t.ex. biodiesel, har väckt frågeställningar om hur detta påverkar egenskaperna på kort och lång sikt hos plastkomponenter som används i fordonsbränslesystem.

I detta arbete har egenskaperna undersökts hos oförstärkt och glasfiberarmerad polyamid 12 (PA12), utsatt för tre olika bränslen (petroleumdiesel, biodiesel och en blandning av dessa (80/20)) vid hög temperatur. Interaktionen mellan polymeren och bränslet och tillhörande åldringsmekanismer hos polymeren (bränsleupptag, extraktion av monomer och oligomer, anlöpning och oxidation) befanns vara "generiska" i den mening att de inträffade, dock i olika grad, för alla bränslen. För glasfiberarmerad polyamid skedde åldrandet huvudsakligen i polyamid-matrisen och inte i gränsskiktet mellan matris och fiber. Semi-aromatisk polyamid visade bättre prestanda än alifatisk PA12 när dessa utsattes för bränslena.

På komponentnivå åldrades flerskikts-polyamid-baserade rör, med polyamid eller fluorpolymer som inre skikt, under ”fordons-lika förhållanden” med bränsle på insidan och luft på utsidan. Alla rör förstyvades under åldrandet men en försprödning inträffade endast för rören med polyamid som inre skikt. Jämfört med polyamiden uppvisade det inre skiktet av fluorpolymer betydligt bättre barriäregenskaper gentemot bränslet och inget material extraherades ut i bränslefasen. Mjukgöraravgången från ytterskikten av PA12 i kontakt med luft var diffusionsstyrd och följde ett linjärt Arrhenius-beteende i högtemperatur-området. Sambanden mellan mjukgöraravgång och förändringar i de mekaniska egenskaperna fastställdes.

Polyamiderna genomgick diffusionsbegränsad oxidation när de utsattes för luft och/eller bränsle, vilket resulterade i bildandet av ett tunt oxiderat ytskikt, vilket i sig bidrog till en avsevärd sänkning i brottöjning.

Brott-beteendet hos PA6 i luft vid hög temperatur studerades systematiskt, vilket ledde till att underliggande mekanismer som var ansvariga för minskningen av brottöjningen i tre distinkta stadier, kunde tas fram.

 

Place, publisher, year, edition, pages
KTH Royal Institute of Technology, 2019. , p. 65
Series
TRITA-CBH-FOU ; 2019:51
Keywords [en]
Automotive, Ageing, Polyamide, Diesel, Biodiesel, Diffusion, Oxidation, Plasticizer migration, Mechanical properties, Fracture behavior.
Keywords [sv]
Fordon, Åldrande, Polyamid, Diesel, Biodiesel, Diffusion, Oxidation, Mjukgöraravgång, Mekaniska egenskaper, Brottegenskaper
National Category
Polymer Technologies
Identifiers
URN: urn:nbn:se:kth:diva-260472ISBN: 978-91-7873-320-0 (print)OAI: oai:DiVA.org:kth-260472DiVA, id: diva2:1355644
Public defence
2019-10-25, F3, Lindstedtsvägen 26, Stockholm, 10:00 (English)
Opponent
Supervisors
Note

QC 2019-10-01

Available from: 2019-10-01 Created: 2019-09-30 Last updated: 2019-10-03Bibliographically approved
List of papers
1. Long-term performance of polyamide-based multilayer (bio)diesel fuel lines aged under “in-vehicle” conditions
Open this publication in new window or tab >>Long-term performance of polyamide-based multilayer (bio)diesel fuel lines aged under “in-vehicle” conditions
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2017 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 144, p. 100-109Article in journal (Refereed) Published
Abstract [en]

The behaviour of a polyamide (PA)-based multilayer fuel pipe was investigated in “close to real” conditions using specially-designed ageing equipment with a program designed according to known customer driving modes and conditions (a key life test). The pipe was exposed to petroleum diesel and a combination of petroleum diesel and biodiesel. The fuel exposure pattern, as well as the temperature profile, followed a specified scheme in the key life test. It allowed for the investigation and understanding of complex ageing mechanisms, often observed in multi-layer systems with a variation in the running conditions. The mechanisms involved included migration of plasticizer from the innermost PA6 layer of the pipe to the fuel, and from the PA12 outer layer to the ambient air. At the same time, fuel was absorbed in the inner layer of the pipe. The oxidation of the innermost PA6 layer was promoted by the oxidation products of biodiesel. The diffusion-limited oxidation of the PA6 layer led to the formation of a 30 μm highly oxidized zone at the inner surface of the pipe, resulting in discoloration and oxidative crosslinking of the polymer. The toughness and extensibility of the pipe decreased significantly after prolonged ageing, and the extensibility was only 7% of that of the unaged pipe after 2230 h.

Place, publisher, year, edition, pages
Elsevier, 2017
Keywords
Ageing, Biodiesel, Degradation, Diesel, Embrittlement, Fuel pipe, Mechanical properties, Polyamide
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-212222 (URN)10.1016/j.polymdegradstab.2017.07.026 (DOI)000414111700012 ()2-s2.0-85026869970 (Scopus ID)
Funder
Swedish Energy Agency, 32519-3
Note

QC 20170817

Available from: 2017-08-17 Created: 2017-08-17 Last updated: 2019-09-30Bibliographically approved
2. Long-term performance of a polyamide-12-based fuel line with a thin poly(ethylene-co-tetrafluoroethylene) (ETFE) inner layer exposed to bio- and petroleum diesel
Open this publication in new window or tab >>Long-term performance of a polyamide-12-based fuel line with a thin poly(ethylene-co-tetrafluoroethylene) (ETFE) inner layer exposed to bio- and petroleum diesel
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2018 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 156, p. 170-179Article in journal (Refereed) Published
Abstract [en]

The long-term performance of a polyamide-12 (PA12)-based (bio)diesel fuel line/pipe with a thin poly(ethylene-co-tetrafluoroethylene) (ETFE) inner layer was investigated in “close to real” and high-temperature isothermal conditions with fuel on the inside and air on the outside of the pipe. The inner carbon-black-containing ETFE layer resisted fuel attack, as revealed by the small fuel uptake, the very low degree of oxidation, and the unchanged electrical conductivity, glass transition and melting behaviour. The properties of the ETFE layer remained the same after exposure to all the fuel types tested (petroleum diesel, biodiesel and a blend of 80% diesel with 20% biodiesel). Because of the presence of the ETFE layer on the inside, the fuel pipe experienced noticeable changes only in the outer PA12 pipe layer through migration of plasticizer, annealing and slight oxidation. The evaporation of plasticizer was found to be diffusion-controlled and it led to an increase in the glass transition temperature of PA12 by 20 °C. This, together with a small annealing-induced increase in crystallinity, resulted in a stiffer and stronger pipe with an increase in the flexural/tensile modulus and strength. The oxidation of PA12 remained at a low level and did not lead to an embrittled pipe during the simulated lifetime of the vehicle. This study reveals that fluoropolymers have a great potential for use as fuel-contacting materials in “demanding” motor vehicle fuel line systems. 

Place, publisher, year, edition, pages
Elsevier Ltd, 2018
Keywords
Ageing, Biodiesel, Diesel, Fuel pipe, Plasticizer loss, Poly(ethylene-co-tetrafluoroethylene), Polyamide, Carbon black, Diesel fuels, Ethylene, Fluorine containing polymers, Gasoline, Glass, Oxidation, Plasticizers, Polyamides, Reinforced plastics, Electrical conductivity, Fuel pipes, High temperature, Isothermal conditions, Long term performance, Tetrafluoroethylene, Glass transition
National Category
Environmental Engineering
Identifiers
urn:nbn:se:kth:diva-236649 (URN)10.1016/j.polymdegradstab.2018.09.003 (DOI)000448091400017 ()2-s2.0-85053044298 (Scopus ID)
Funder
Swedish Energy Agency, 32519-3
Note

Export Date: 22 October 2018; Article; CODEN: PDSTD; Correspondence Address: Wei, X.-F.; KTH Royal Institute of Technology, School of Engineering Sciences in Chemistry, Biotechnology and Health, Fibre and Polymer TechnologySweden; email: xinfengw@kth.se; Funding details: 32519-3; Funding text: The financial support from the Swedish Energy Authority (project no. 32519-3 ), Volvo Cars AB (Göteborg, Sweden) and Scania CV AB (Södertälje, Sweden) is gratefully acknowledged. QC 20181113

Available from: 2018-11-13 Created: 2018-11-13 Last updated: 2019-09-30Bibliographically approved
3. Ageing properties and polymer/fuel interactions of polyamide 12 exposed to (bio) diesel at high temperature
Open this publication in new window or tab >>Ageing properties and polymer/fuel interactions of polyamide 12 exposed to (bio) diesel at high temperature
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2019 (English)In: npj Materials Degradation, ISSN 2397-2106Article in journal (Refereed) Published
National Category
Engineering and Technology Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-260464 (URN)
Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-09-30
4. High-performance glass-fibre reinforced biobased aromatic polyamide in automotive biofuel supply systems
Open this publication in new window or tab >>High-performance glass-fibre reinforced biobased aromatic polyamide in automotive biofuel supply systems
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(English)Manuscript (preprint) (Other academic)
National Category
Polymer Technologies Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-260469 (URN)
Note

QC 20191010

Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-10-10Bibliographically approved
5. Plasticiser loss from plastic or rubber products through diffusion and evaporation
Open this publication in new window or tab >>Plasticiser loss from plastic or rubber products through diffusion and evaporation
2019 (English)In: npj Materials DegradationArticle in journal (Refereed) Published
National Category
Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-260466 (URN)
Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-09-30
6. Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties
Open this publication in new window or tab >>Plasticizer loss in a complex system (polyamide 12): Kinetics, prediction and its effects on mechanical properties
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2019 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321Article in journal (Refereed) Published
National Category
Engineering and Technology
Research subject
Fibre and Polymer Science
Identifiers
urn:nbn:se:kth:diva-260433 (URN)2-s2.0-85072653914 (Scopus ID)
Note

QC 20191011

Available from: 2019-09-30 Created: 2019-09-30 Last updated: 2019-10-11Bibliographically approved
7. Diffusion-limited oxidation of polyamide: Three stages of fracture behavior
Open this publication in new window or tab >>Diffusion-limited oxidation of polyamide: Three stages of fracture behavior
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2018 (English)In: Polymer degradation and stability, ISSN 0141-3910, E-ISSN 1873-2321, Vol. 154, p. 73-83Article in journal (Refereed) Published
Abstract [en]

Polyamides (PAs) frequently experience diffusion-limited oxidation (DLO) under elevated temperatures due to their combination of relatively high oxygen barrier properties and high susceptibility to, and rate of, oxidation; under DLO conditions, oxidation is uneven and limited to a thin surface layer. In this study, the reduced extensibility/embrittlement of unstabilized PA6 under DLO conditions was understood by revealing DLO-induced fracture behavior. The DLO was induced by thermally ageing PA6 samples at 180 degrees C; the built-up of the thin oxidized layer by ageing was revealed by infrared microscopy. Notably, the formation of the thin oxidized layer significantly reduced the strain-at-break. Depending on whether the oxidized layer was brittle, two types of surface behavior (voiding and cracking) occurred during the tensile tests, which in turn lead to three types (stages) of tensile fracture behavior. In particular, in the early stage (Stage I) of ageing, the fracture was caused by a long crack formed by the coalescence of adjacent surface voids, leading to a decrease in the strain-at-break from 300% to 30%. In Stage II, multiple surface cracks, which initiated in the oxidized layer, was arrested by the interface between the oxidized and unoxidized material, leading to an almost constant strain-at-break (at or close to the necking strain). Maximum brittleness occurred in Stage III, where a more extensive oxidation of the oxidized layer initiated cracks with high propagation rate, causing the interface to be unable to arrest the cracks. 

Place, publisher, year, edition, pages
ELSEVIER SCI LTD, 2018
Keywords
Diffusion-limited oxidation, Polyamide, Surface-induced embrittlement, Fracture behavior, Cracks, Mechanical properties
National Category
Other Materials Engineering Textile, Rubber and Polymeric Materials
Identifiers
urn:nbn:se:kth:diva-234202 (URN)10.1016/j.polymdegradstab.2018.05.024 (DOI)000441488100008 ()2-s2.0-85047652570 (Scopus ID)
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2019-09-30Bibliographically approved

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34567896 of 18
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