Abstract
Polyamide 6 (PA6) is able to absorb water from the surrounding air and bond to it by forming hydrogen bonds between the carbonamide groups of its molecular chains. Diffusion processes cause locally different water concentrations in the (component) cross-section during the sorption process, resulting in locally different mechanical properties due to the water-induced plasticisation effect. However, the water content of PA6 is usually specified as an integral value, so no information about a local water distribution within a component is provided. This paper shows a method to characterise moisture distributions within PA6 samples using low-energy computer tomography (CT) techniques and comparing the reconstructed results with a developed finite elements (FE) modelling method based on Fick’s diffusion laws with concentration-dependent diffusion coefficients. For this purpose, the ageing of the samples at two different water bath temperatures as well as at different integral water contents are considered. The results obtained by CT reconstruction and FE modelling are in very good agreement, so that the concentration distributions by water sorption of PA6 calculated by FEM can be regarded as validated.
Highlights
Published: 17 September 2021Polyamides (PAs), in particular the material Polyamide 6 (PA6), have been indispensable in a multitude of technical applications for several decades: PA6 is used in particular when components are exposed to high mechanical and thermal loads in contact with different media
The variation in the storage periods in water results in different integral water contents for the samples determined by mass increase, which are given as integer values
A characteristic grey value distribution over the sample cross-section can be recognised, which is evaluated subsequently along the dotted lines inserted in the samples shown in Figure 3, and which correspond to the reconstruction method of the water concentration distribution described in Equation (3)
Summary
Published: 17 September 2021Polyamides (PAs), in particular the material PA6, have been indispensable in a multitude of technical applications for several decades: PA6 is used in particular when components are exposed to high mechanical and thermal loads in contact with different media. PA6 undergoes different climatic conditions which the material can interact with [1]: Due to its hygroscopic properties, PA6 can absorb water from its environment at higher humidities and release it again at lower humidities until an equilibrium is reached in the material with the surrounding atmosphere. The diffusion rate of the water into the material is significantly influenced by various parameters such as temperature, load and time [2]. This leads to mainly transient conditions prevailing in the material and locally different moisture distributions [3]. The water absorption results in a swelling of the material, which strongly reduces the dimensional stability of the PA components [5,6], and on the other hand, it causes the so-called water-induced
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