PBT plasticity loss induced by oxidative and hydrolysis ageing
Introduction
Brought to market by Celanese in the late 1960s, poly(butylene terephthalate) (PBT) is a thermoplastic polyester with a good balance between dielectric properties, mechanical strength and dimensional stability [1]. The easy processing and the rapid crystallization [2], [3] makes it ideal to be processed by injection moulding. Its applications cover several fields from design insulator parts, door lockers to connectors for automotive or medical applications [4], [5]. One major disadvantage is its instability regarding hydrolysis and high temperature causing its very narrow processing window (typically 250–260 °C).
In the technical literature, one can find some examples of unexpected PBT brittleness [6] where the skin-core morphology induced by injection molding is reported to be a potential failure, since this latter is well known to impact polymer properties [7], [8], [9], [10]. The influence of morphology, especially spherulites size, on resistance and toughness has mainly been studied on PP [11], [12], [13]. It was shown that these properties are enhanced by increasing the spherulites radius but decrease above a determined value: for example a radius of 46 μm leads to a ductile behaviour whereas a radius about 126 μm leads to brittle fracture [12]. For PP, Huan et al. [13] observed that a 80 µm spherulites radius resulted in a decrease of 15 MPa of tensile strength. The process is also important: the same increase in radius but by an annealing under compression process leads to a rise of 20 MPa in tensile strength [13]. Hence, the mechanical properties are also driven by spherulites size, but these large spherulites are generally not observed in injection molding parts for PP parts [14] as well as for PBT parts.
Another reason is the possible degradation of PBT induced by the process. Polyesters are classified to be both hydrolytically and thermally unstable. Both degradation mechanisms might induce architectural changes in the macromolecular chains (chains scissions in particular) responsible for a loss of toughness. It is well documented that mechanical properties of semi-crystalline polymers depend on both molar mass and crystalline morphology. In the case of polypropylene for example, pioneering works by Fayolle [15], [16] showed the embrittlement occurs when molar mass gets below a critical mass M’c below which there is no more plastic deformation. Later, this end of life criterion was completed in the case of polyethylene [17], [18], polyamide [19], [20] and PLA [21]: it was highlighted that the embrittlement is also dependant on residual amorphous phase content, then crystallinity. Schematically, polymer gets brittle at low molar mass and/or low amorphous phase content and the ductile-brittle transition is represented by a line in La (or Lc, or χc) vs Mw or Mn diagram.
Despite its practical interest, the embrittlement of PBT was, to our best knowledge, scarcely addressed in scientific literature. The aim of the present paper is hence to define failure criteria corresponding to the “critical” macromolecular architecture in relation to the ductile-brittle transition. For that purpose, thin films of one PBT grade will be aged either in thermal or in hydrolytic ageing. The ageing will be systematically monitored by uniaxial tensile tests, molar mass changes (rheometry, gel permeation chromatography) in order to give a fine description of PBT embrittlement.
Section snippets
Material
The poly(butylene terephthalate) used in this study is an injection grade. It was transformed into films with a GIBITRE compression press (200 bar) at 230 °C. This temperature was chosen low enough to limit thermal degradation. Films were about 150 to 200 μm thick and initial average molar mass values Mn = 33.8 kg/ mol and Mw = 74.7 kg/mol.
Ageing conditions
Films previously prepared were exposed to three ageing conditions:
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Thermal ageing at 180 °C and 210 °C under air in ventilated ovens (AP60 by System Climatic
Mechanical changes
Aged samples were first characterized by tensile tests. For unaged PBT, the curve displays a plastic behaviour with an ultimate strain higher than 10% which is consistent with literature [23], [24]. In the case of ageing at 210 °C, the curves of virgin and aged samples are almost similar in the elastic region whereas the main difference of samples having undergone oxidative and hydrolysis ageing is the absence of the plastic region (Fig. 1), which is usually observed after ageing [17], [19],
Discussion
The aim of this section is to understand the nature of architectural changes responsible for PBT embrittlement and later to illustrate commonalities and differences between PBT embrittlement and those of other semi-crystalline thermoplastics.
Conclusions
A PBT grade for injection purpose was thermally aged at 210 °C and 180 °C and hydrolytically aged at 80 °C to determine the evolution of the embrittlement and the associated mechanisms.
First, thermal oxidation and hydrolysis seems to cause mainly chain scission evidenced by rheometry and GPC. There was no evidence of crosslinking here but it remains to be verified for bulkier materials submitted to oxygen gradients. Those macromolecular changes are accompanied by a thickening of the crystalline
CRediT authorship contribution statement
C. Loyer: Data curation, Formal analysis, Investigation, Writing - original draft, Writing - review & editing. G. Régnier: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing - original draft. V. Duval: Conceptualization, Funding acquisition, Methodology, Project administration, Supervision, Validation, Writing - original draft, Writing - review & editing. Y. Ould: Conceptualization, Funding acquisition, Methodology, Project
Declaration of Competing Interest
We hereby confirm that we have no conflict of interest with the paper
Acknowledgments
The authors would like to thank the ANRT for granting this project (N°2019/1467).
The authors would also like to thank V. Michel, RX manager at PIMM laboratory for helping in SAXS measurements and data treatment.
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