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The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres

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Abstract

Developing hemp fibre composites for structural applications requires both reconsideration and optimisation of the transformation processes to obtain stable, high-quality fibre reinforcements. In this context, field retting remains an important issue because it is weather dependent and has not been completely mastered by the hemp industries. Retting can be achieved voluntarily to facilitate fibre separation and extraction from the stalks prior to mechanical decortication. However, retting can also be involuntary and result from climatic misfortune and unforeseeable events at the time of harvest. Therefore, this study aimed to quantify the influence of involuntary and non-controlled field retting on the physicochemical and mechanical properties of industrial hemp bast fibres. A wide spectrum of analytical techniques was applied, including colour spectrophotometry; morphological, microscopic (SEM), surface (EDX, roughness), biochemical (HPLC and pXRD) and thermogravimetric (TGA) analyses; dynamic vapour sorption; and tensile characterisation. The results indicate that retting induced a decrease in the average width of fibre elements after mechanical processing and a loss of pectic substances. We also observed a change in colour from yellow to dark grey, an increase in surface roughness and an increase in the decomposition temperature for the third mass loss region. A decrease in tensile properties at the scales of both single and technical fibres was also observed. Since no significant decrease in cellulose content was measured, this decay in mechanical performance was connected with both the significant degradation of hemicelluloses and a decrease in the fraction of crystalline cellulose that was quantified in this work.

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Abbreviations

NR-SF:

Non-retted single fibres

NR-TF:

Non-retted technical fibres

R-SF:

Retted single fibres

R-TF:

Retted technical fibres

SF:

Single fibres

TF:

Technical fibres

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Acknowledgements

The authors thank Francois Gaudard from FARE for his skilful technical assistance in HPLC, lignin quantification and MEB support. The authors also express theirs thanks to Miguel Pernes for the DVS support. Additionally, Justine Padovani is acknowledged for her critical discussion. Roland Salut, Thomas Jeannin and Paul Tourneroche from FEMTO-ST are thanked for their assistance in energy-dispersive X-ray spectroscopy, surface roughness measurements and ATG, respectively, and Virginie Moutarlier from UTINAM is acknowledged for performing XRD measurements. The authors would also like to thank La Chanvrière Company in France for supplying the hemp fibres. Johnny Beaugrand acknowledges the financial support from the CPER MATRICE state-to-country France-Champagne-Ardenne programme.

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Placet, V., Day, A. & Beaugrand, J. The influence of unintended field retting on the physicochemical and mechanical properties of industrial hemp bast fibres. J Mater Sci 52, 5759–5777 (2017). https://doi.org/10.1007/s10853-017-0811-5

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