Crystallisation and microstructure in stamp formed fibre reinforced polyamide 66

Hamby, William (2022). Crystallisation and microstructure in stamp formed fibre reinforced polyamide 66. University of Birmingham. Ph.D.

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Abstract

Thermoplastic fibre reinforced composites have the potential to reduce the CO\(_2\) emissions of automotive vehicles through processes such as vehicle lightweighting. In comparison to the current structural metallic components, composites offer considerable improvements in the strength-to- weight ratio of automotive parts, ultimately enhancing the fuel efficiency of the vehicle. Polyamide 66 (PA66) is an affordable, tough and durable engineering thermoplastic with intrinsically good tribological and chemical resistance properties. Further, the mechanical properties and dimensional stability of polyamides can be improved by the addition of continuous glass fibre reinforcement. This work aims to characterise through-thickness crystallinity and microstructural morphology in stamp formed glass fibre reinforced polyamide 66 (PA66/GF).

As determined by calorimetry, the through-thickness degree of crystallinity of an 11-ply PA66/GF composite was shown to be inversely proportional to the rate of heat loss throughout the stamp forming process. Within the surface layers of the composite, where cooling rates were observed to reach 1400 °C/min, crystallinity was found to be 3.0% lower than that of the central layers which, by comparison, crystallised under quasi-isothermal conditions. The variation in crystallinity owing to the disparity in cooling rates was further confirmed by x-ray diffraction (XRD). Analysis of one- dimensional (1D) wide angle x-ray scattering (WAXS) patterns demonstrated that despite the distinctly non-isothermal crystallisation conditions of the stamp forming process, the matrix polyamide rapidly crystallises into a triclinic unit cell characteristic of the \alpha-phase of PA66. However, despite this, the intensities of the \(\alpha_1\) and \(\alpha_2\) peaks representative of the (100) and (010)/(110) crystallographic planes, respectively, were found to differ through the thickness of the laminate, indicative of a change in crystal structure.

Isothermal crystallisation kinetics of PA66/GF were analysed over a crystallisation temperature (T\(_c\)) range of 245 to 249 °C where, owing to both models accounting for the contribution of secondary crystallisation, the parallel Velisaris-Seferis and Hay equations were shown to provide the best fit to the experimental data (R\(^2\) > 0.995). Hay’s assumption that both primary and secondary crystallisation occur simultaneously and that total crystallinity at time t, is the sum of the two contributions, was confirmed. Further, having not previously been applied to polyamides or thermoplastic reinforced composites, the findings of this study support the use of the Hay model in determining the isothermal crystallisation kinetics of polymeric materials.

Non-isothermal crystallisation kinetics were also evaluated using models developed by Jeziorny, Ozawa and Mo and over a cooling rate range of 10 to 60 °C/min. Mo’s theory - a combined Avrami/Ozawa approach, was proven to be more successful than the Jeziorny and Ozawa models in describing the non-isothermal crystallisation of PA66/GF.

Post-stamp forming, low temperature annealing (T\(_a\)) of unidirectional and cross-ply PA66/GF laminates over the T\(a\) range 60 to 110 °C was shown to improve the creep performance of the laminates. DSC thermograms obtained post-annealing showed the appearance of a low temperature melting peak (T\(_m’\)) characteristic of the melting of thin crystalline structures. The value of T\(_m’\) was shown to be dependent on T\(_a\) with a 46.7 °C difference in T\(_m’\) observed between annealing temperatures of 60 and 110 °C (t = 4 hrs). With rising T\(_a\) the thickness of the thin lamella structures forming within the interlamella amorphous regions was found to increase which, in accordance with the Thomson-Gibbs equation resulted in T\(_m’\) shifting to higher temperatures. Further, by constraining the amorphous chains, the thin lamella structures developed at T\(_a\) of 110 °C were shown to improve load transfer between the matrix and fibres, increasing creep modulus and reducing creep strain by up to 25.2 GPa and 450%, respectively.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):