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Diffusion-limited oxidation of polyamide: Three stages of fracture behavior
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology.
Volvo Car Corp, Polymer Ctr, SE-40531 Gothenburg, Sweden..
Scania CV AB, Mat Technol, SE-15187 Sodertalje, Sweden..
Scania CV AB, Mat Technol, SE-15187 Sodertalje, Sweden..
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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. Vol. 154, p. 73-83
Keywords [en]
Diffusion-limited oxidation, Polyamide, Surface-induced embrittlement, Fracture behavior, Cracks, Mechanical properties
National Category
Other Materials Engineering Textile, Rubber and Polymeric Materials
Identifiers
URN: urn:nbn:se:kth:diva-234202DOI: 10.1016/j.polymdegradstab.2018.05.024ISI: 000441488100008Scopus ID: 2-s2.0-85047652570OAI: oai:DiVA.org:kth-234202DiVA, id: diva2:1246365
Note

QC 20180907

Available from: 2018-09-07 Created: 2018-09-07 Last updated: 2019-09-30Bibliographically approved
In thesis
1. Ageing behavior of plastics used in automotive fuel systems
Open this publication in new window or tab >>Ageing behavior of plastics used in automotive fuel systems
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
Automotive, Ageing, Polyamide, Diesel, Biodiesel, Diffusion, Oxidation, Plasticizer migration, Mechanical properties, Fracture behavior., Fordon, Åldrande, Polyamid, Diesel, Biodiesel, Diffusion, Oxidation, Mjukgöraravgång, Mekaniska egenskaper, Brottegenskaper
National Category
Polymer Technologies
Identifiers
urn:nbn:se:kth:diva-260472 (URN)978-91-7873-320-0 (ISBN)
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

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Wei, Xin-FengGedde, Ulf WHedenqvist, Mikael S.

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