Moisture Transport In Polyamide (PA6) and Its Effect On the Mechanical Behaviour

Abstract

Polyamide exhibits hygroscopic nature and can absorb up to 10 % of moisture relative to its weight. The absorbed moisture increases the mobility of the molecular chains and causes a reduction in the glass transition temperature. Thus, depending on the moisture distribution, a polyamide component can show different stiffness and relaxation times. The moisture distribution also depends on the mechanical loading of the material. However, it was noticed that the diffusion process remains unaffected when the process is compared for a loaded and an unloaded material. It is postulated that the moisture redistribution is due to an external force that takes place as a result of the pressure acting on the moisture. The diffusion process is unaffected by preloading of the dry material as the pressure is applied purely on the material and not on the absorbed moisture. However, when a saturated specimen is loaded, the pressure is exerted on the moisture too which causes its redistribution. In this work, the distribution of the absorbed moisture is simulated by a non-linear diffusion model. It is coupled with the viscoelastic behaviour of PA6. The stiffness of the viscoelastic model changes and the relaxation time reduces with increasing moisture concentration. The coupling of diffusion to mechanical loading is achieved with the recalculation of the moisture concentration caused due to the redistribution of volume. It is assumed that there is no transport of moisture, but the transport of volume and the change in volume creates a change in concentration in the specimen. This strongly coupled model has been implemented using the finite element method. The model results are compared to experiments for validation. A strongly coupled model was thus created which could reproduce the experimental results with reasonable accuracy.

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