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Magalhães, F. D.
|Title:||Xanthan gum and chitosan as natural adhesives for cork|
|Abstract:||Composites based on cork (bark of Quercus suber L.) have been receiving increasing interest for different applications. Cork combines a unique set of properties like elasticity, resilience, impermeability, low density, and very low conductivity of heat, sound and vibration. Moreover, its use is supported by sustainable forest practices. Cork composites provide added design flexibility and environmental advantages, like ease of incorporation of pre and post-consumer wastes. But one increasing requirement has been the replacement of synthetic adhesives, like reactive polyurethane (PUR) or formaldehyde-based resins, by natural binders from renewable resources. This presents particular challenges, especially when mechanical performance and water resistance are needed.The viability of using two widely available polysaccharides, xanthan gum (XG) and chitosan (CS) as cork binders was studied. The adhesives were used as water solutions with concentrations of 6 wt.% or lower. For XG, chemical oxidation was investigated as a strategy to provide water resistance and reactivity. Periodate oxidation allowed C-C cleavage between adjacent -CHOH groups, forming dialdehydes. NH2OH·HCl titration of the oxidized gum (XGox) indicated aldehyde contents of about 60 %. 1H-NMR confirmed formation of aldehyde groups.Bond strength (BS) tests were performed on cork samples after drying the glued joints for 2 h at 120 °C. Results showed that CS (6 wt.%) yielded similar BS as a reference PUR adhesive, being 50 % higher than XG. BS decreased as CS or XG concentrations were lowered, due to depletion of binder in the glued joint. When performing the tests after immersion of the glued samples in water for 24 h at 20 °C, XG showed null BS, while CS displayed 40 % lower strength. This was expected considering that XG is soluble in water while CS in insoluble, as long as nonacidic conditions are used.Interestingly, XGox showed dry BS 76 % higher than XG and 15 % higher than CS. Wet BS was similar for XGox and CS. Oxidation of XG reduces its water solubility, as confirmed by total soluble matter assays, due to decrease in hydroxyl content and possible self-crosslinking promoted by aldehyde groups, which explains higher water resistance. On the other hand, aldehydes also allow for reaction with groups present in the cork structure, providing a covalent mechanism for increasing bond strength and water resistance. Mixtures of XGox and CS did not yield better performances than XGox alone.|
|Source:||Polychar 23 - 23rd World Forum on Advanced Materials|
|Document Type:||Resumo de Comunicação em Conferência Internacional|
|Appears in Collections:||FEUP - Resumo de Comunicação em Conferência Internacional|
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