Revitalization of Industrial Hemp Cannabis sativa L. var. sativa in Slovenia: a Study of Green Hemp Fibres Oživljanje navadne konoplje Cannabis sativa L. var. sativa v Sloveniji: raziskava zelene konoplje

The importance of industrial hemp as a source of highly valuable textile fi bres is briefl y presented through its use for textiles and composites and its increasing cultivated areas in the 21st century. On the territory of present Slovenia, about 160 ha of agricultural area was cultivated with hemp before WWII, then it quickly began to decline and at the end of the 1970s, it was no longer cultivated. Revitalization of industrial hemp in Slovenia with fi eld experiments started already in the years 2000/2001 for producing seeds, whereas hemp fi bres were used only as an insulation for buildings. The textile technological properties of hemp fi bres from diff erent varieties grown in Slovenia have not been examined till now. They are important for using hemp fi bres in highly valuable textile products. The properties of green hemp fi bres extracted mechanically from non-retted hemp stems of Cannabis sativa L. var. sativa (varieties: Novosadska, Juso-11, Bialobrzeskie, Unico-B and Beniko) were determined. All the analysed varieties except Beniko had stem height over 200 cm. The highest yield of green fi bres was 33.1% (Novosadska). The analysed green fi bres’ content was 1.24–3.26% of ash, 7.77–8.50% of moisture regain, 10.69–13.92% of water-soluble substances and 8.45–10.83% of pectin. Through a biodegradation process of retting green hemp fi bres in tap water at temperature 35°C, 9.01– 18.89% of dry mass was removed after ten days. Average linear density of green hemp fi bres was very high, around 200 tex. Tenacity of fi bres’ bundles was in the range of 167–272 MPa, but tenacity of elementary fi bres was 548–672 MPa. From the curves of specifi c stress-strain, it is seen that green hemp fi bres from all fi ve varieties had similar superstructure. All analysed green hemp fi bres had high linear density and low mechanical properties. For textile application, they should be further processed into fi ner fi bres in order to increase their tensile stress and become also more fl exible and soft.


Introduction
Hemp (Cannabis sativa L. var. sativa) is an increasingly attractive industrial plant for food, textile and constructional industries because of its valuable seeds, fi bres and shives. Aft er removal of seeds, hemp stems (also called straw) remain as a cheap by-product, which can be technologically converted into highly valuable products. Dried hemp straw consists of about 28% of extremely long, strong bast fi bres (textile fi bres) and about 55% of short, woody fi bres (called also hurds or shives) [1]. Bast fi bres present a sclerenchyma supporting tissue of stems. A hemp stem cross-section with primary (main) and secondary fi bres' layers is presented in  [2]. Fibres' bundles, running from the root to the top of a stem, are around 1500-2000 mm long and have 10-40 elementary fi bres in cross section [3]. Dry non-retted hemp stems (green hemp) can be mechanically separated into bark and shives. Since bast fi bres in bark contain a preserved whole middle lamella, they are very rough and stiff and should be soft ened for further uses. On the contrary, fi bres from retted hemp stems (dew, water, tank, enzyme retting) are soft er and more fl exible, because pectin in middle lamella degrades through retting [6].

Use of hemp bast fi bres in textiles
Hemp is a plant of genus Cannabis, which originates from India and Central Asia [7]. In ancient times,  Figure 1: Cross-section of hemp stem and fi bres: a) cross-section of a hemp stem with a bark tissue (A), a woody part (B) and a pith (C) [4]; b) fragment of cross-section of hemp stem with a primary (main) fi bre layer (F), a discontinued secondary fi bre layer (f) and a woody tissue (B) [5]; c) cross-section of hemp fi bres' bundle (a) and a longitudinal view of elementary fi bres (b) [5].
hemp was regarded more as a source of drug than as a textile fi bre plant. In a Neolithic site Yuan-Shanu (today Taiwan), the remains of hemp ribbons dated from 10,000 BC were found [8]. In 2,800 BC, the Chinese emperor Sei Nung ordered the cultivation of cannabis for the purpose of producing fabrics [5,9]. In 500 BC, the Scythians used hemp fibres for cordages [5]. Th e Scythians also brought hemp to Europe [7]. In the 15 th century, hemp was transferred from Europe to America [10], where later, in the 19 th century, one of the most popular trousers was made from hemp fabric (sail cloth). Th ose trousers were patented by Jacob Davis and Levi Strauss in 1873 [11]. Despite economic success, a prohibition of industrial hemp cultivation was enforced in the USA in 1937. It coincided with the emergence of the fi rst synthetic fi bre, nylon 6.6. Th e prohibition of cultivating industrial hemp was later enforced in other parts of the world. Hemp bast fi bres can be exploited for textiles, papers, composites and biomass materials. Diff erent fi ne woven and knitted fabrics for apparel, denim, decoration fabrics for interior, cordages (strings, twines, cords, ropes), nets, canvas, carpets, insulation etc. can be produced from hemp bast fi bres [3]. New uses of hemp fi bres include geotextiles [12], composites (NFC, natural fi bres composites) [13,14] and biocomposites [15,16] with biodegradable matrices, like polylactide resins [17], wheat gluten plastics [18] and biobased linear low density polyethylene [19,10].

Industrial hemp in the 21 st century
Only industrial hemp (Cannabis sativa L. var. sativa), which contains very low percentage of psychoactive drug delta-9-tetrahydrocannabinol (delta9-THC) and is not suitable for medicinal or recreational uses, is today allowed for cultivation [20,21]. In the European Union, the cultivation of industrial hemp varieties with up to 0.2% THC content in dry mass is allowed [22], in Canada, the limit is up to 0.3% [23], but in Switzerland, it is even up to 1% of THC [24]. Th e cultivation of industrial hemp is today still illegal in most parts of the USA [23]. Only about thirty countries in the world legally cultivate industrial hemp [1,25]. Th e world's largest producers of industrial hemp are China, Canada and France. Th e global harvested area for industrial hemp was about 85.000 hectares in the year 2011, approximately 60,000 hectares (70.6%) for fi bres and 25,000 hectares for seeds [26]. Th e production of hemp fi bres in the year 2011 was around 77,000 tons [25]. In the European Union, 25,224 hectares of cultivated areas were sown with industrial hemp in the year 2015, with almost half of the planted areas located in France (Fig. 2) [27].

Industrial hemp in Slovenia
Th e beginnings of cultivation of industrial hemp on the territory of present Slovenia go back at least 400 years [28] when hemp fi bres were processed mainly into cordages and woven fabrics. In the year 1939, industrial hemp was planted on over 160 hectares of land, aft er the WWII, the cultivation was gradually abandoned to be completely terminated in the 1970s (Fig. 3).  (Fig. 4). From the average values of yield of stem of all three years, it is seen that the variety Novosadska gave the highest yield with 7,164 kg/ha, followed by Unico-B with 6,599 kg/ha, Juso-11 with 6,094 kg/ha and Bialobrzeskie with 5,608 kg/ha and Beniko, which gave only 4,823 kg/ha. Current legislation in Slovenia enables anyone who has a minimum cultivated area of 0.1 hectare to be engaged into the cultivation of industrial hemp [22]. In 2016, four hundred forty-fi ve registered farmers in Slovenia cultivated industrial hemp on 360 hectares (Fig. 5), which represented about 3% of all production area in Slovenia. Industrial hemp in Slovenia is mainly cultivated for seeds, which brings farmers the main income, but straw remains mostly unused [34].  [29] In  [37].
Tekstilec, 2017, 60(1), [36][37][38][39][40][41][42][43][44][45][46][47][48] By using genetic engineering, the diversity of varieties has increased signifi cantly which infl uences the authorized varieties that change all the time. Th is puts the farmers and processors in front of a number of issues regarding the suitability of cultivation in a given territory and yield of plants. Hemp varieties have diff erent ability to adapt to growing conditions (weather conditions, soil, harvest time etc.), which infl uences stem and fi bre yields and their quality. Th e cultivation of industrial hemp in Slovenia is mainly intended for the production of seeds and therefore, non-retted hemp stems are available as a by-product. Th e aim of our study was to determine the properties of green hemp bast fi bres obtained from non-retted stems, which are not available in professional literature. sown in a row at distance of 20 cm, a row distance from each other in a series was 10 cm, which quite restricts plants spatially for growing branches only at the top of stems. Precipitation in the year 2011 was lower in spring and considerably higher in autumn than the average of the last thirty years; the temperatures were slightly higher than the average of the last thirty years (Fig. 6).
Growing of hemp crop was initially poor due to drought in the spring months. It improved in May to July due to higher rainfall. In the late summer, the weather was dry. At the beginning of autumn, the growth stopped and the seeds developed.

Figure 6: Precipitation and temperatures in Ljubljana in the year 2011
From each hemp variety, ten whole stems were selected for analyses. Dried non-retted stems were, after harvesting seeds and removing leaves, decortificated by beating with a plastic hammer. Green hemp fi bres (Fig. 7a) were manually separated from broken wooden core (shives) (Fig. 7b).
Green hemp fi bres Hemp shives Figure 7: Mechanically treated dry stems were separated into green hemp fi bres and broken wooden core (hemp shives)

Methods
Volume density of a stem (ρ s ) was calculated according to Eq. 1, where a stem volume (V s ) according to Eq. 2 was simplifi ed as a truncated cone with height (H s ), thickness in the bottom (2R = d 1 ) and thickness at the top (2r = d 2 ): .
Yield of green hemp fi bres per stem (η f ) was calculated from the ratio of dry mass of extracted green hemp fi bres (m f ) to dry mass of stem (m s ) (Eq. 3):  (Beniko). Th e stems with lower volume density were more hollow and as such more suitable as a potential thermal insulation material than the stems with higher volume densities.

Longitudinal view of green hemp fi bres
Non-retted green hemp fi bres (Fig. 9a) are rough and of diff erent thickness, which formed at peeling of stalks when the stem bark was mechanically disintegrated. On the darker fi bres in Fig. 9a, a stem cuticle is seen. In Fig. 9a-9c, some remains are seen of non-fi brous cortex material 3 , which is fi rmly connected with fi bres and has not been removed by the treatment in boiling water or in a solution of ammonium oxalate, but was destroyed through microbiological retting as it is seen in Fig. 9e and 9f. Th ick fi bres were mechanically easily split longitudinally into fi ner fi bres' bundles aft er treatment in ammonium oxalate (Fig. 9d) and aft er ten days retting in tap water of 35 °C (Fig. 9e and 9f) and also aft er treatment. Hemp fi bres' bundles are composed of elementary fi bres, which are bound to each other with pectin and lignin. Pectin can be removed through microbiological retting or by boiling in a 1% solution of ammonium oxalate, whereas lignin is hardly destroyed in these procedures.

Chemical properties of green hemp fi bres
Hemp fi bres consist of cellulose (74-78%), hemicellulose (4-20%), lignin (2-11%), water and solvent soluble substances, wax, ash, pectin, protein and water [42]. Th e content of water-soluble substances of absolutely dry green hemp fi bres (Tab. 3) was in the range of 13.10-13.92% for the varieties Juso-11, Bia lobrzeskie and Beniko; for the varieties Novosadska and Unico-B, it was lower, 10.69% and 11.57%. Th e content of the removed pectin aft er the treatment for one hour in a 1% solution of ammonium oxalate was in the range of 8.49−10.83%. In biodegradation process of ten days of retting in tap water at temperature 35°C 9.01-18.89% of dry mass, mainly pectin, was removed (Tab. 3). Th e reasons for great diff erences should be specifi cally examined. Th e moisture regain was in the range of 7.77-8.50%. Th e content of ash, which represents a quantity of inorganic substances in fi bres, was in the range of 1.24−3.26%.
In absolutely dry green hemp fi bres, 21.5% non-fibrous material (water-soluble substances and pectin) and 78.5% fi bres were determined on average.

Physical-mechanical properties of green hemp fi bres
Linear density (Fig. 10) of each fi bre was measured before measuring its tensile properties (Tab. 4). Average linear density of measured green hemp fi bres was in the range from 149.8 tex (Juso-11) to 220.5 tex (Beniko) with a high coeffi cient of variation, between 27% and 52%.  Tensile properties of hemp fi bres' bundles depend on initial length of a fi bre, clamped between two grips of a dynamometer before testing. Smaller initial lengths result in higher tenacities [44]. Tensile properties of green hemp fi bres were studied at two diff erent initial length, 100 mm and 5 mm. Breaking force measured at initial length of 100 mm was in the range of 17. 4-29.8 N, but at initial length of 5 mm, it was 2.7-4.8-times higher, in the range of 80.5-96.2 N. Specifi c breaking stress of green hemp fi bres measured at initial length of 100 mm was 167-272 MPa, but at initial length of 5 mm, it was 548-672 MPa, i.e. 2.5-4-times higher than at initial length of 100 mm (Tab. 4). Th e diff erences in average breaking stress measured at initial length of 100 mm varied a lot between the varieties: the strongest variety was Juso-11, the weakest Bialobrzeskie green hemp fi bres. Tensile stress measured at initial length of 5 mm was for all varieties very similar except for Beniko the tensile stress of which was the lowest. Because the length of elementary hemp fi bres is only 50-55 mm [45] or less, tensile properties measured at initial length of 100 mm demonstrate primarily the tenacity of links between hemp fibres and pectin lamella which glues elementary hemp fi bres together into bundles. Much higher tenacity measured at initial length of 5 mm demonstrates the tenacity of elementary fi bres, and depends on the strength of cellulose fi brils that form elementary fi bres. Breaking elongation (Tab. 4) measured at initial length of 100 mm was in the range 0.9-1.0% and confi rms a brittle nature of green hemp fi bres. Breaking elongation measured at initial length of 5 mm was in the range of 5.8-7.2%. It is typical for elementary hemp fi bres, which are much tougher fibres than green hemp fi bres.  Figure 11: Th e correlation between (a) breaking force and linear density and (b) between breaking stress and linear density of green hemp fi bres (all measured at initial length of 100 mm) Duval et al. [46] confi rmed that a diameter of a fi bre near its rupture point infl uenced the hemp fi bre tensile strength: a general observation was that tensile strength decreased with increasing fi bre diameter. Th is interdependence was confi rmed also by Shahzad [15] who attributed such behaviour to the amount of fl aws in the fi bres (kinks, dislocations [2]) and to the number of elementary fi bres which decreased with lower diameters of fi bres and resulted in the increase of tensile properties of fi bres. In our experiment, a correlation between tensile force and linear density (R 2 = 0.330251) and also between tensile stress and linear density (R 2 = -0.17197) (Fig. 11) of green hemp fi bres was low and showed only a weak tendency to the interdependence between the studied properties. Th e curves of specifi c stress/strain of green hemp fibres from the tensile test at initial length of 100 mm (Fig. 12a) are almost linear, which means that the tensile deformation of fi bres was near elastic in the range of less than 1%. Th e variety Juso 11with the steepest curve resists to strain the most and has the highest modulus of elasticity, the variety Bialobrzeskie with the least steep curve has the lowest modulus of elasticity. On the contrary, the curves made on fi bres at initial length of 5 mm (Fig. 12b) show linear behaviour up to strain of about 2% where helically packed fi brils in S2 layer at an angle of 5-30° [41] to the fi bre axis align along the fi bres' axis. Aft er the alignment of fi brils is fi nished, the fi bres' resistance to tensile force increases suddenly and the stress/strain curve bends towards the ordinate, which causes a defl ection of the curves. With increasing tensile force, the fi bres show again an elastic deformation till they break at the end.

Conclusion
Th e quality and yield of hemp fi bres depend on many factors, which include hemp variety, growing conditions (growing plan, temperature, raining/watering, plant nutrients in soil, pests etc.), the way of retting of stems and the way of extracting fi bres from stems. Retting facilitates the extraction of bast fi bres from stems, but it usually causes the deterioration of textile fi bres' mechanical properties [47]. In our study, the following properties of green hemp fi bres extracted mechanically from non-retted stems were determined: analysed varieties except Bialobrzeskie had a comparable stem height and quantity of green fibres, but they diff ered in volume density, which was for Novosadska only 27.5 kg/m 3 , but for Beniko, it amounted to 218 kg/m 3 ; analysed green hemp fi bres diff ered in content of ash, but Unico B and Novosadska deviated in lower content of pectin and water soluble substances; average linear density of green hemp fi bres was very high, around 200 tex; tenacity of fi bres' bundles was comparable with literature [42], i.e. between 167 MPa (Bialobrzeskie) and 272 MPa (Juso-11); tenacity of elementary fi bres was between 548 -MPa (Beniko) and 672 MPa (Bialobrzeskie), which is typical for hemp elementary fi bres; curves of specifi c stress-strain of fi bres from all fi ve varieties are similar, which means that fibres from diff erent varieties have similar superstructure. a b Figure 12: Average stress-strain curves of green hemp fi bres bundles, measured at initial length of (a) 100 mm and (b) 5 mm Tekstilec, 2017, 60(1), [36][37][38][39][40][41][42][43][44][45][46][47][48] Green hemp fi bres from all analysed varieties have high linear density and low mechanical properties. Th ey can be used for technical applications, like ropes and geotextiles. For textile application, they should be further processed into fi ner fi bres (e.g. retted, treated in autoclave with alkalis -cotonisation) to increase their specifi c tensile stress, fl exibility and soft ness.