Frost-retted hemp fibers were investigated to assess their suitability for composite applications. Chemical analysis of frost-retted hemp fibers highlighted a high amount of solubles (pectins) at the ...fibers surface and a low lignin content in the fibers that was attributed to an unfavorable synthesis of lignin in the cell wall due to the particularly cold temperature during hemp growth in the Nordic countries. The fibers tensile properties were considered at two different scales and the performances of hemp/PLA composites were assessed. Recommendations were provided for the use of frost-retted hemp fibers in the reinforcement of thermoplastic composites.
The quality of fibres obtained from flax, hemp and reed canary grass is dependent on the moisture characteristics of the crops. In this study, the adsorption equilibrium moisture contents of ...un-retted and dew-retted flax straw, un-retted and frost-retted hemp stalks and spring-harvested reed canary grass were determined using the dynamic gravimetric method at different temperatures (5, 15, 25
°C) for relative humidities in the range 35–95%. Non-linear regression was used to fit five commonly used three-parameter isotherm models the modified Henderson model, the modified Chung–Pfost model, the modified Halsey model, the modified Oswin model and the modified Guggenheim–Anderson–de Boer (GAB) model to the data obtained. The goodness-of-fit of the models was compared using the mean relative percentage deviation, the standard error of estimate and residual plots. The modified Halsey model was considered the best for predicting the equilibrium moisture content of un-retted flax and spring-harvested reed canary grass, and the modified Oswin model for predicting the equilibrium moisture content of dew-retted flax and un-retted hemp, while the Chung-Pfost model was the best for predicting the equilibrium moisture content of frost-retted hemp. For flax and hemp, there were statistically significant differences between un-retted and retted plant materials, whereas the differences between varieties were small.
Bast fibre plants are attracting increasing attention in Europe, and the plant straw fractions have already been used as a raw material in many new applications. This study focuses on characterising ...the liquid–fibre interactions of frost-retted and unretted bast fibre plant (
Linum usitatissimum L. and
Cannabis sativa L.) straw fractions. A novel approach was used in order to overcome the problems introduced by the surface heterogeneity in contact angle measurements. First, the wicking rates of water, oil and ethanol in plant straw fractions were measured with a capillary rise method. The results were compared with the results calculated from the Lucas–Washburn equation. Second, the wetting properties of a compressed sample were studied by measuring the contact angle of a liquid. Third, the absorption time of a liquid droplet on a compressed sample was measured. Fibre hemp absorbed water better than flax/linseed. After 100 s the difference was about one order of magnitude. For oil the differences were marginal. Wicking and wetting turned out to be very rapid even when the measured (static) contact angle was almost 90°. Big variations in dynamic contact angle calculated from wicking measurements were observed. The wicking rates and droplet absorption times give suitable information when selecting fibre plant straw fractions as raw materials for new industrial products. On the contrary, a droplet based contact angle measurement does not give useful information of dynamic wetting behaviour.
In this study of the behaviour of the fractions of unretted and frost-retted fibre straws in damp air, a production scale method to separate fibre and shive from fibre plants was introduced and ...tested on bast fibre plants (
Linum usitatissimum L. and
Cannabis sativa L.). The method consists of optional drying of stalks, unloading bales, milling the straws with a hammer mill, separating the fractions from air stream with a cyclone and finally separating fibres from shives with a screening drum. Fractions were characterized focusing on technologically indicative properties such as equilibrium moisture content, ash and microbiological quality. Unretted fractions of the bast fibre plant stem reached higher equilibrium moisture contents than the retted fractions, and hemp fibres absorbed more moisture from air than did the
Linum fibres. In very humid air, all fractions began to lose weight due to moulding. The weight decrease during the first week was lower in frost-retted than in unretted fractions. The frost-retted fractions appeared to be more resistant to humidity in the short term. The total number of microbes and especially the numbers of yeasts and moulds can be used as a criterion of hygienic level. For green fractions, the mould level was similar in fibres and in shives, but frost-retted shives contained more moulds than the unretted shives. The mould content of a fraction had no direct correlation with the moulding tendency of the fraction. The ash contents of fibres were somewhat higher than those of shives, due to a probable soil contamination. Ash content did not have significant correlation with microbiological quality, although ash is a possible risk factor for hygienic quality. According to the results of this study it is highly important to study the quality of the production chain of bast fibre plants to ensure the quality of industrial products. From the producer's point of view, raw material with defined quality can be directed to the most suitable application. The behaviour of fractions in various ambient atmospheres, and other quality aspects such as hygienic level can be used as criteria for defining the most appropriate product applications.
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