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RSC Advances Review Load–extension plots for different flax and flax carbon hybrid composite samples in tensile testing. Reproduced with permission from ref. 65 [license number: 4992431067492]. showed a large drop in modulus (55%) but a smaller reduction in strength (12%) aer 38 days ageing at 90% RH (Fig. 10). 2.5 Moisture uptake behaviour and mechanisms Moisture ingress in biocomposites takes place in various ways depending on the environments that they are exposed to. For example, if composites are exposed to elevated temperatures, moisture penetration is more aggressive into the bre–matrix interface compared to room temperature exposure. Natural bres include: ax, hemp, jute, sisal, kenaf, date palm, bagasse (brous residue aer sugarcane stalks are crushed) amongst others. Natural bres contain cellulose, and hemicellulose, which contain a large number of available hydroxyl groups, which attract water molecules and create hydrogen bonds. This process allows cell walls to swell as the brils are pushed out.21 This phenomenon causes natural bres to weaken, and results in reduced ability to withstand applied load and an inability to transfer load in the interface region of composite. This disruption causes a weak bre matrix interface. As a result, this Fig. 10 Moisture diffusion characteristics of flax fibre reinforced composites at two different temperatures at 90% humidity. Repro- duced with permission from ref. 66 [license number: 4992431222559]. whole moisture ingress process leads to signicant reduction of strength and stiffness. Understanding moisture ingress mech- anisms especially in a marine environment (seawater exposure) is a complex process.67 The different diffusion mechanisms (matrix, bres and interfaces) and the effects of geometric dimensions and the bre orientation are illustrated in Fig. 1168 and been described in gure caption. The resulting reduction in strength is accentuated by swelling and debonding of the bres.65 The key steps leading to changes and degradation of the bre and composite structure are summarised below: Water molecules diffuse through the microscopic gaps (micropores) between polymer chains in the bre and the composite. Voids, defects and gaps lead to capillary transport between the bres and the matrix. Swelling of the bres, causes expansion of the micro-cracks in the matrix leading to debonding. The overall bre–matrix bond is damaged and this leads to reduction of mechanical properties (strength and stiffness). It has been shown65 that for ax unidirectional (Flax/UD) and ax cross-ply (Flax/CP) samples immersed in de-ionised water at room temperature (23 C), the moisture uptake percentages were approximately 13% and 23%, respectively, aer 648 hours. The moisture absorption values measured for carbon bre hybridised specimens, were much lower. The maximum percentage weight gains for FlaxUD/carbon and for FlaxCP/ carbon immersed at room temperature for 648 h were approx- imately 2% and 8%, respectively. It is worthy to note that moisture ingress is equally inuenced how the bres are laid out in the composite; for example, cross-ply samples show higher moisture uptake compared to unidirectional ax bre composites as highlighted by Dhakal et al.65 2.6 Thermal degradation One important aspect of the durability of biocomposites is their thermal stability. The stability of the composite components, bres and matrix polymer, is rst critical during manufacturing of components, as many thermoplastic matrix polymers require elevated temperatures to ow and impregnate bres. The thermal behaviour of the manufactured composite is also one of the factors which can limit nal applications performance, though high temperature environments are not a feature of most marine applications. Here we will rst consider the base materials, then the resulting composite. First, it should be noted that there a number of factors involved in the thermal stability. A common way to quantify the thermal stability of polymers is through thermo-gravimetric analysis (TGA). This involves heating at a constant rate and measuring weight loss, it can certainly provide useful information on global changes but additional techniques are required to understand the causes of these changes. The polymer microstructure can change during heating (cure, post-cure, crystallinity changes, oxidation) and various other analyses such as differential scanning calorimetry (DSC), Fourier-transform infrared spectroscopy (FTIR), gel permeation chromatography (GPC), can provide valuable Fig. 9 32924 | RSC Adv., 2021, 11, 32917–32941 © 2021 The Author(s). Published by the Royal Society of ChemistryPDF Image | Long-term durability and ecotoxicity of biocomposites in marine environments
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