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The cell-wall composition has been analyzed for 13 batches of flax fibers grown over 3 years under 3 different weather conditions including a ‘normal one, a harsh drought and a rainy weather. It was found that both stresses, drought and excess of rain induced a decrease of uronic acid in the matrix and an increase of the structuring pectins. Besides, a drought led to an increase of hemicellulose polysaccharides (+24%) whereas an excess of rainfall caused a rise in the amount of so-called structuring pectins (+67%). As the fiber’s mechanical properties remained the same over the years, it was assumed that the cell-wall composition was modified to preserve the mechanical role of the fiber in the stem. [ABSTRACT FROM AUTHOR]

The development of flax (Linum usitatissimum L.) fibers was studied to obtain better insight on the progression of their high mechanical performances during plant growth. Fibers at two steps of plant development were studied, namely the end of the fast growth period and at plant maturity, each time at three plant heights. The indentation modulus of the fiber cell wall was characterized by atomic force microscopy (AFM) using peak-force quantitative nano-mechanical property mapping (PF-QNM). Changes in the cell wall modulus with the cell wall thickening were highlighted. For growing plants, fibers from top and middle heights show a loose inner Gn layer with a lower indentation modulus than mature fibers, which exhibit thickened homogeneous cell walls made only of a G layer. The influence of these changes in the fiber cell wall on the mechanical performances of extracted elementary fibers was also emphasized by tensile tests. In addition, Raman spectra were recorded on samples from both growing and mature plants. The results suggest that, for the fiber cell wall, the cellulose contribution increases with fiber maturity, leading to a greater cell wall modulus of flax fibers. [ABSTRACT FROM AUTHOR]

Abstract: Nanoindentation tests were performed on flax fibers or stems sections in order to study the influence of the maturity of the fibers and the one of the location of the fibers within a bundle on their mechanical properties. The first results evidence the interest of this technique to approach differences into mechanical properties of plant fibers and highlight some questions linked to the specific structure of fibers. [Copyright &y& Elsevier]

Abstract: In a composite ecodesign approach, flax fibers could be good candidates for glass fiber substitution. However, their use in industrial applications could be restricted due to their scattered mechanical properties. In this study, we compared tensile properties of 50 batches of flax (Linum usitatissimum) fibers, cultivated in France between 1993 and 2011. Thus, 2954 fibers were tested in the same conditions according to the XP T 25-501-2 standard. Reliable specific data are suggested to be a basis for design calculation. [Copyright &y& Elsevier]

Highlights • The influence of lodging on flax fiber performances is explored. • Flax gravitropic response is investigated at key stages of plant development. • Fibers from the pulling side of tilted stems show changes in mechanical properties. • Only slight dissimilarities in fiber performances remain at plant maturity. • Fiber properties stay adequate for composite reinforcement in case of lodging. Abstract Flax fibers (Linum Usitatissimum L.) are currently used for textile applications and composite reinforcement. Due to its industrial importance, flax is the subject of a varietal selection work in view of obtaining varieties with higher fiber yields, but also exhibiting a greater lodging resistance. Indeed, lodging sometimes happens within flax fields, complicating plant harvest and compromising yields. Interestingly, it sometimes occurs that flax stems restore from lodging through a gravitropic reaction. Depending on the time of lodging, variations in elementary fiber mechanical performances, monitored by tensile tests appeared to be more or less pronounced, being greater in the earliest stage of the experiment, and also depend on the studied side of the stem curvature. Namely, the pulling of the stems provides fibers with the most emphasized changes, in terms of strength at break, filling rate (presence of a fiber lumen) as well as cell wall tangent modulus. Finally, differences between tilted and control fibers diminish as the plant maturity progresses, with only slight remaining dissimilarities at plant maturity. Thus, flax fibers are involved in the plant gravitropic reaction and maintain their efficient mechanical characteristic despite lodging, through the adjustability of their cell wall performances over fiber thickening, which is a major result for fiber suppliers and composite manufacturers. [ABSTRACT FROM AUTHOR]

Highlights • Conventional and greenhouse cultivation of Bolchoï textile flax were compared. • Changes of flax stem architecture attributed to seismomorphogenesis were explored. • Much taller plants were obtained under greenhouse conditions. • The two types of cultivations led to similar fiber number and morphology. • Interestingly, stem and fiber moduli are unaffected despite architectural changes. Abstract Flax fibers (Linum usitatissimum L.) are of great interest for textile and composite applications. Thus, their use for industrial purposes requires increasing quantities and constant quality. In general, plants can change their morphology and mechanical properties when submitted to stress, particularly in the case of the reaction of plants to wind, a phenomenon known as seismomorphogenesis. In this paper, the influence of greenhouse or field cultivation on plant architecture and anatomy as well as the fiber yield and mechanical performances of flax fibers are investigated. The results highlight the development of much taller plants under greenhouse conditions, but similar fiber length and number of fibers per plant with both types of cultivation. Finally, the bending stiffness of stems is estimated by three-point bending tests and fiber performances are measured by tensile tests; in terms of mechanical properties at the stem level but also at the fiber scale, there are no statistically significant differences between greenhouse and field cultivated plants. In conclusion, despite the increased plant height under greenhouse conditions (44% increase in total height), fiber yield and properties are unchanged compared to field cultivation. Hence, the greenhouse cultivation of flax does not appear to favor higher fiber yields or quality, but nevertheless maintains compliance with these essential criteria. [ABSTRACT FROM AUTHOR]

The varietal selection of flax ( Linum Usitatissimum L.) has always focused on specific criteria fulfilling requirements of farmers and textile workers. Thus, the current development of composites using flax as reinforcement presents new challenges for flax breeders in terms of fiber quantities and quality. However, the impact of the varietal selection on the mechanical properties of resulting fibers is yet to be determined. In the present study, several architectural characteristics of flax stems are defined. Stem transverse sections from four varieties selected from the 1940s to 2011 are compared. Anatomical changes over time are highlighted. The most important ones involve the gap between fiber bundles and the amount of fibers which can be improved with the selection (from 7.8% to 13.4% of the tissue area per section). This trend coincides with the increase in biomass production over time expected from the selection work. Moreover, this study demonstrates that flax fibers preserve their good mechanical performances in spite of the anatomical differences. Thus, through varietal selection, it is possible to increase the biomass yield while preserving the excellent specific mechanical properties of flax fibers. Finally, flax fibers can compete with glass fibers to reinforce composite materials. [ABSTRACT FROM AUTHOR]

This paper presents results from an experimental study of three types of non-woven preforms (needlepunched, spunlaced and mat manufactured using a paper-making process) intended as composite reinforcement. These are potentially very attractive for transport applications. First, the influence of processing on elementary fiber tensile properties is shown to be limited. Then the preforms are evaluated in polypropylene matrix composites and mechanical properties are determined. The structure of non-woven reinforcements is strongly dependent on the manufacturing route. By varying the fiber content it is shown that the most efficient reinforcement for flax fibers is the mat produced by paper processing. The new spunlaced reinforced composites are shown to have slightly lower tensile properties (15% lower strength, and 25% lower stiffness) compared to mat composites at equivalent volume fraction, but further optimization is possible for these materials. Based on the measured constituent properties micromechanics models have been used to estimate composite stiffness. A good correlation is obtained between test results and model predictions. [ABSTRACT FROM AUTHOR]

The industrial production of flax fibers yields two fibrous products: scutched fibers are valuable for their length, fineness and cleanness, while tows are entangled, impure and are low-priced. There is a need for technical data to compare these two kinds of fibers beyond the difference of appearance, and to assess their potential as composite material reinforcements. In this work, we have compared the properties of scutched flax and flax tows from the same batch. The morphology was first assessed by measuring the length, cleanness and fineness of fiber bundles. The tensile properties of single fibers and of unidirectionally reinforced epoxy composites were then measured. The results show that the morphology of flax tows is different from but close to the morphology of scutched flax. Tow bundles were 15% shorter than scutched flax bundles and tow bundles were 25% thicker than scutched flax bundles. The tensile properties of single fibers were in the same range. Tensile properties of the unidirectional composites show a similar evolution of properties versus fiber volume fraction. The use of scutched flax allowed volume fractions of 0.7 to be achieved while tow volume fraction was limited to 0.6. Despite the visual and morphological difference between tows and scutched flax both can be good candidates for composite reinforcement in high performance applications. [ABSTRACT FROM AUTHOR]

Abstract: Plant fibers have several aspects, such as low energetic production costs, biodegradability and great mechanical properties, which make them very attractive in the field of composite reinforcement. The aim of this study was to compare the impact of two eco friendly water washings on the thermal behavior, surface morphology and mechanical properties of flax fibers (Electra variety). As a first step, we have carried out scanning electronic microscopy (SEM) to evaluate surface properties and performed thermogravimetric analysis (TGA) to estimate the effects of the treatments on the fiber composition. Secondly, tensile tests were performed on elementary fibers. The results show no negative impact of water treatment on the mechanical properties of the elementary fibers. Moreover, after water washing for 72h at 23°C, the Young''s modulus values appeared to increase slightly, possibly due to better surface state and an easier extraction of flax fibers. Finally, an analysis of eluted sugars suggests some negative effect of the boiling treatment on the fiber structure. [Copyright &y& Elsevier]

Non-woven composites reinforced with plant fibers are widely used in the automotive and construction sectors. The vast majority is composed of petroleum-based, non-compostable polyolefins, which are no longer a viable solution in an environmental context where the end-of-life management of industrial products is becoming a major societal issue. Here, fully green composites are produced by reinforcing three bio-based and biodegradable matrices – poly-(hydroxyalkanoate) (PHA), poly-(butylene-succinate) (PBS) and poly-(lactide) (PLA) – with non-woven flax fiber preforms. Notably, their mechanical performance was observed to be at least equivalent to the industry reference – poly-(propylene) (PP) reinforced non-woven flax. These composites were then buried in an instrumented garden compost, and the evolution in microstructure and mechanical properties was studied over a period of six months. Microtomography studies revealed that evolution in composite microstructure principally depended on the polymer matrix: surface degradation was predominant for PBS and PHA biocomposites, whereas rapid fiber-matrix interface degradation in the core was observed for PLA biocomposites. Interestingly, even after six months in the compost, all composites exhibit tensile strengths of at least 50% of their initial value. Moreover, the strength reduction in biodegradable composites was of the same magnitude as the industry reference, flax/PP composite. These results demonstrate the potential of biocomposites in resolving the 'biodegradation paradox': flax composites with biopolymers like PLA, PHA and PBS can be designed to have adequate mechanical performance for industrial products, even after ageing in harsh conditions, and yet offer an alternative end-of-life route to the typical incineration (with or without energy recovery). • Mechanically speaking, the studied biocomposites are, at least, as good as flax/PP. • Degradation appears in surface and/or at flax fibers/matrix interface. • Interface degradation occurs at the first stage of degradation, impacting strength. • After six months, biocomposites and flax/PP lose 50% of their initial strength. [ABSTRACT FROM AUTHOR]

Highlights: [•] The reproducibility of flax fibers tensile properties was checked. [•] Standard deviation on tensile properties was found to be reducible. [•] Retting degree and cell-wall composition were approached. [•] The best retted flax samples had the best tensile properties. [•] The importance of the percentage of cellulose and uronic acids was confirmed. [Copyright &y& Elsevier]

Highlights: [•] The degree of retting of gradually retted flax fibers was assessed. [•] This parameter influences the tensile properties of single fibers. [•] This also influences the tensile properties of short fiber composite materials. [•] These differences rely on the fiber properties and the microstructure of the material. [ABSTRACT FROM AUTHOR]