A first set of trials are performed at the industrial scale first,using flax dedicated machines and their associated settings to establish a reference.Then,a laboratory scale scutching and hackling equipment was used to investigate/optimise the scutching and hackling process parameters to improve the quantities of fibres and maximise their performances.Projections of long fibre yields in conjunction to dew-retted dry hemp stem yields are also given and permit to discuss the future of a complementary value chain in flax territories.A lab scale scutching/hackling extraction device was used.It,was used to separate the different plant fractions contained in the hemp stems with the main objective to obtain the long line fibres analysed in this study.The hemp stems are stabilised before testing at 65 % relative humidity and 23 ◦C.This lab-scale scutching/hackling device is composed of three distinct modules.The first one has the function of breaking the wooden part of the stems and allowing a first extraction of the shives and the dust.It is composed of a set of three pairs of corrugated rollers with adjustable distance between centres and speed of rotation.The material obtained is then automatically transported to a scutching system.It consists of two rotating turbines,which rotate in opposite direction to each other.Their role is to beat the fibres and to remove the shives still tied to the fibres.The residence time in the scutching module and the turbine speed are adjustable.Finally,the fibres are subjected to a progressive hackling stage to align the fibres and reduce technical fibre diameter.The hackles or combs are mounted on two rotating belts that can be adjusted in speed.The translation speed of the fibre is also adjustable.Stems from the retted and un-retted batches are subjected to extraction by scutching and hackling using the lab scale device.An additional part of the fibres from the dew retted stems were extracted using an industrial scale Depoortere scutching device and a Linimpianti hackling machine located at the “Terre de Lin” company.
Fig.1 shows the different steps of the industrial scutching and hackling of hemp stems.This scutching machine is composed of two distinct devices: a breaking system composed of a succession of horizontal fluted rollers and a beating stage which consists of successive pairs of rotating turbines,grow cannabis with each turbine rotating in opposite direction.The scutching machines are designed to process more than one ton of flax straw per h and globally deliver 250 kg of long line fibres.The hackling machine is a Linimpianti type equipment that was designed to process about 80 kg/h of scutched fibres.It is a fast and high production rate if one compares to traditional Mackie type machines which process globally up to 40 kg/h.For hemp,different settings were specifically applied to the scutching breaking and beating steps,but still with a high extraction speed,close to the one used for flax straw.The extraction parameters chosen for our device were optimised so that to obtain large quantities in mass of hackled long line fibres.Gentle extraction conditions have been applied by the lab-scale device with a low transfer speed during breaking and a low turbine rotation speed during scutching.Globally,a reduction of the transfer speed by 300 % and the turbine rotation speed by 400 % is applied in comparison to what is classically used in industrial scutching facilities for flax.As a result,some of the shives remain after the scutching stage,on the contrary to what is observed during industrial scutching.The type of combs and the hackling machine design offers the possibility to remove this wood without difficulty and to obtain clean long line fibres at the end of extraction.Following the stage of hackling and the realisation of a continuous sliver at the end of this process,the large count sliver is submitted to six drawing stages where the linear mass of the sliver decreases in our case up to a linear mass of about 150 tex depending on the settings.The used apparatus is a lab scale drawing system.This device mimics,at a reduced scale,the six drawing/doubling stages used in the flax spinning industry to prepare the slivers into rovings that will be used at the spinning stage.During the different stages of this process,six parallel flax dedicated systems using pin drive devices to bring the slivers and a wooden wheel to perform the drawing were used.This type of drawing system is also called “gill drawing system”.perform the different drawing operations.
During this stage,the sliver mass is reduced but it is also homogenised as between each drawing stage,six drawn slivers are each time grouped together before the following drawing.During these operations,the technical fibre diameter is also reduced when the technical fibres are pulled from the Gill system pins.The fibre diameter obtained at the end of the extraction and preparation processes was therefore investigated and compared for different batches at this stage.After processing the hemp stems through the various modules of the lab scale scutching hackling extraction device,several plant fractions are obtained.The shives,which are the woody part of the stems,can be separated from the dust generated during the extraction process and from the long line fibres and shorter fibres,also called tows.Thus,the by-products obtained at the output of each module are manually separated and weighed in order to determine the impact of the different extraction steps on each of the plant fractions as well as the yields.The study of the impact of the modules on the losses of the plant fractions is important in order to know which elements should be improved to increase the quantity and quality of the fibres.Fibre yield is computed at the end of the industrial scutching and hackling equipment.In order to investigate the impact of the different extraction steps on the mechanical and physical properties of the long line fibre,single elementary fibres are extracted after each module.Fibres are tested in tension and the evolution of fibre surface defect is investigated.The results obtained are compared to the initial potential of the material prior to any mechanical extraction.In the study of the industrial extraction of hemp fibres,fibres could only be collected after the scutching and hackling modules.To determine the initial mechanical potential of the elementary hemp fibres,prior to any mechanical extraction,fibres are manually extracted.To reach this objective,sections of stems are randomly taken and the bast peeled by hand.The elementary fibres are then carefully separated from the bast after soaking them in distilled water for about 10 s as specified in the NF 25-501-2 standard.Fibre samples were also taken after each extraction module of the lab scale scutching/hackling device.Thirty elementary fibres were then extracted from each batch to determine the impact of the various stages of the process on the mechanical properties of the fibres.The number of defects,as well as the morphological and mechanical properties of the fibres were evaluated.The main defects that can be observed on the surface of the fibres are kink bands which can be examined under polarized light as shown,for example,by Baley or Thygesen.
Kink bands are among the defects that can be visible on the surface of the fibres and are expected to be zones of weakness for the fibres as cracks were shown to preferably initiate from these zones.They can come from a disorientation of the cellulose fibrils due to some compression or bending loads.The number of kink bands on the surface of thirty elementary fibres for each batch is counted after observation with an optical microscope under polarized light and over a distance of 330 μm.In addition,the area of each of these kink bands is also determined,using ImageJ software following a manual identification of each kink band.The surface tool permits computing the area of the identified defects.2.7.2.Determination of the cross-sectional areas of the elementary fibres prior to tensile test The elementary fibres extracted from each batch are glued at each end to plastic tabs with a light-sensitive glue to prevent the fibre slipping during the tensile test.A gauge length of 12 mm is taken for tensile tests.The measurement of fibre cross-sections is carried out using a device manufactured by the company Dia-Stron called the Fibre Dimensional Analysis System and controlled by the UV Win software also developed by the company.This type of device permits to accurately determine the diameters of the fibres using an “automated laser scanning” method based on the light shadow technique performed using a high-precision laser source and photodetector.The fibres mounted on plastic tabs are positioned in the rotating jaws of the FDAS module and held in position by a pneumatic system.By 360 ◦ rotation of the jaws,the diameter of the fibre is measured over its entire circumference locally.The fibre is then translated and another part of the fibre is scanned over its whole circumference again.This operation can be repeated over the entire length of the sample.In this study,ten measurements are distributed over the 12 mm length of the gauge.As the fibre is rotating,the projected diameters are recorded and the maximum and minimum diameters are extracted to determine the fibre cross section using an elliptical model as recommended for technical fibres by who observed that this approach permits obtaining cross section measurements with a higher accuracy than with other models such as the circular model recommended by the NF 25-501-2 standard.This is due to the fact that hemp fibres are not circular.The measurements are carried out with an accuracy of 0.01 μm.A more detailed description of the device is available in Gr´egoire et al..Tensile tests are carried out on thirty elementary fibres from each batch.The device to apply the tension on the individual fibre was developed by the Dia-Stron,company.This is an automated high-precision extensometer which is equipped with a ±20 N load cell.Displacement is achieved using a step by step motor which permits to control the displacement with an accuracy of 1 μm.This makes the device suitable for fibre breaks with low levels of deformation.
The tests are carried out using a displacement speed of 0.0167 mm/s and a break threshold value of 5 gmf as recommended by the NF T25-501-2 standard.The deformation selected for Modulus of elasticity corresponds to the one of the Young’s modulus,indoor cannabis grow system at the beginning of the stress-strain curve.The “fineness” of the fibre bundles is determined using a lab scale type device based on a laser scan technology proposed by Itecinnovation company.It consists in cutting the fibres in short length with a guillotine and dispersing them in an alcoholic liquid to prevent their swelling.Fibres are placed in a fluid flow and they are scanned by a laser: when a laser beam illuminates a fibre,a shadow appears on the photodetector.This area is directly proportional to the fibre diameter if one assumes that the fibres are cylindrical.This device,originally developed for wool fibres was modified by the manufacturer and adapted to bast fibres.This device was available at the Terre de Lin company premises.The tests were carried out on batches of 1000 fibres and a distribution can be obtained.The extraction of dew-retted hemp stems on industrial facilities,with process parameters not optimised for hemp,resulted in fibre yields of 9.15 % after scutching and 5.11 % after hackling.After hackling,the long line fibre mass represents in this case only 17 % of the initial fibre mass in the stem.The feed and beating speeds in the industrial scutching module are the ones generally used on flax.These process parameters appear to be un-adapted as very large quantities of fibres fall in the tows.After analysis,one can observe that a very large part of the scutching tow fibres are long line fibres.Scutching tows were taken randomly during the industrial scutching process right below the machines.One can observe in Fig.2a that a large amount of fibre is present.It contributes to about 50 % of the mixture mass.In Fig.2b,the tows,collected at a different moment contain in a vast majority long line fibre.A more complete analysis of the tows,with a large number of collected samples would be necessary to characterise the amounts of long line,short fibres and shives contained in the scutching tows.One can,however,observe that the mass of fibre is very large and can be represented by large quantities of long line fibres that should not fall in the tows.Different hypothesis can be formulated to explain this unwanted phenomenon.The first one is the use of too aggressive process parameters as,on the contrary to what was expected,hemp requires lower compression and beating loads than an equivalent mass of flax.