Characterisation of Textile and Oleaginous Flax Fibrous and Shives Material as Potential Reinforcement for Polymer Composites

In recent years, the use of fl ax fi bres to replace glass fi bres as reinforcement in polymer composites has gained popularity due to an increasing environmental concern and requirement for developing sustainable materials. Many works deal with the properties of fl ax fi bres cultivated for textile applications, which are today used for polymer reinforcement. As fi brous material from oleaginous fl ax varieties and shives is available in large quantities and not promoted, its use in composites shall be further developed in the forthcoming years. Croatia and Slovenia mainly grow oleaginous fl ax, where after the seed collecting, most of the stems remain unused, the major portion being burned in fi elds, creating environmental pollution, or being disposed by ploughing. Therefore, the aim of this study was to characterise and compare the properties of shives and technical fi bres extracted from fl ax Linum usitatissimum L., a textile variety planted in Croatia and from the Slovenian autochthonous oleaginous variety from Bela Krajina, to be used as potential reinforcement in polymer composites. The fl ax stems of the textile variety were subjected to water retting for 72 hours and the fl ax stems of the oleaginous variety were dew retted for four weeks. Dried retted stems were passed through a mechanical process of breaking and scotching, followed by heckling and combing, where the shives and fi bres were separated into four groups according to their length. The characterisation of the fi brous material of both varieties was studied according to the results of optical and scanning electron microscopy, moisture regain, fi bre length, linear density and tensile strength, and according to the results of Fourier transform infrared spectroscopy and thermogravimetric analysis. Based on the analysis results, it was concluded that the properties of investigated textile and oleaginous fl ax fi brous material were comparable, as were the properties of tested fi bre length groups within the same variety; that fl ax fi bres from textile and oleaginous varieties have adequate morphological and mechanical properties, and thermal stability for reinforcing polymer matrix composites; and that fl ax shives are more appropriate for fi llers in plastics with a lower reinforcing role. As the type of fi bre reinforcement (short fi bres, roving/yarns, nonwoven or woven fabrics) is very important for polymer composite properties, based on the obtained results, fi bres can be selected for specifi c purposes.


Introduction
Flax is a natural bast bre that is widely grown in Europe.Furthermore, it is one of the most widely utilised bio-bres.Two main groups of ax plant Linum usitatissimum L. varieties are cultivated -the rst for bre production (textile ax) and the second for linseed oil (oleaginous ax).e harvested area of ax for seed and oil worldwide production is much larger compared to the cultivation of ax for textile applications [1][2][3][4].Flax bres can be obtained from plants grown primarily for bre or from waste stems generated in the ax seed production.Textile ax varieties are utilised not only for textile application but also in composites and paper production.Plants for textile varieties grow up to 80-120 cm in height with the stem diameter of about 3 mm, while the plants for oleaginous varieties are smaller, i.e. 60-80 cm in height, and they are thicker [5].Commercially important ax bres are historically known by two classes, namely by oriented, long-line bre for valued linen products and tow (short bre by-product) for short staple spinning and composites.However, ax stems that are not grown speci cally for high value linens may be processed to give a "totalbre" in which a single, non-oriented bre product results [1].ese bres can be processed in short staple spinning and nonwoven units.Flax bres obtained from oleaginous varieties are known as tow (mainly Canadian tow is available on the market) which is usually packed in bales of 136 kg for the shipment to pulp mills for subsequent paper formation.e unused short oleaginous ax stems, which are produced in large quantities around the world, represent an abundant, inexpensive and readily available source of lignocellulosic bres.A er the seed collecting, a major portion of these stems is burned in the eld, creating environmental pollution.e exploration of these inexpensive agricultural residues as a bio-source for making industrial products can open new avenues for the utilisation of agricultural residues by reducing the need for disposal and environmental deterioration through pollution, re and pests, and at the same time add value to the creation of rural agricultural-based economy [1,3].Flax bres can be processed into semi-nished products for the reinforcing of polymer composites.As ax bres are suitable for di erent kinds of polymer composite applications, in recent years, the European ax industry has enforced two groups of specially designed ax reinforcement -dry preforms and wet preforms (prepregs), where the full potential of ax can be exploited.Dry preforms consist only of bres and can be classi ed as short bres of speci c length or mixed residues, roving/ lanenih vlaken iz predivnega lana, ki se danes uporabljajo za utrjevanje polimerov.Po drugi strani pa se uporabo vlaknatih materialov in pezdirja iz oljnih sort lanu v polimernih kompozitnih ne spodbuja, čeprav so na voljo v velikih količinah.V prihodnjih letih je zato pričakovati razvoj postopkov njihove uporabe v kompozitih.Na Hrvaškem in v Sloveniji gojijo predvsem oljni lan, kjer po odstranitvi semen ostanejo neuporabljena stebla.Večji del stebel sežgejo na polju, kar povzroča onesnaževanje okolja, ali pa stebla zaorjejo.Zato je bil cilj te študije opredeliti in primerjati lastnosti tehničnih vlaken in pezdirja, pridobljenih iz stebel lana Linum usitatissimum L., ki so bila zasajena na Hrvaškem (predivni lan) in v Sloveniji (oljni lan, slovenske avtohtone sorte iz Bele Krajine), z namenom ugotoviti njihovo potencialno uporabnost za ojačitev polimernih kompozitov.Stebla predivne sorte lana so bila godena z namakanjem v vodi 72 ur, stebla oljnega lana pa so bila štiri tedne godena z rosenjem.Iz posušenih godenih stebel, ki so bila v mehanskem postopku trenja na stopah in trlici ter otepanju in česanju (s čimer je bil pezdir ločen od vlaken) izločena vlakna in razvrščena po dolžini v štiri skupine.Lastnosti vlaknatega materiala obeh sort so bile opredeljene na podlagi rezultatov elektronske in optične mikroskopije, vsebnosti vlage, dolžine, fi noče, nateznih lastnosti, infrardeče spektroskopije (FT-IR) in termogravimetrične analize.yarns, nonwoven (mats) and woven reinforcements (UD -unidirectional, 2D -bidirectional and multiaxial).Using these semi-products, the matrix will be added during the composite production.In prepregs, the bres are already pre-impregnated with the matrix.Pre-impregnated preforms can be classied as compound, thermoplastic and thermoset prepregs (roving/yarn, nonwoven and woven).During the manufacture, the impregnation is completed and the matrix consolidated [3,6,7].
e bre length or its aspect ratio (ratio length-todiameter) has a great impact on the polymer composite processing techniques [2].For structural composites (where bres carry the load), long bre bundles are required [3].For short bre reinforced composites considering the injection and compression moulding techniques, the suggested bre length is approximately 10 mm and 25 mm (for mats), respectively.For palletising (with matrix), cascade mixing and extruder compounding, the bre length should be less than 3 mm [2].
e ax bre is characterised by a very complex structure.When talking about the ax bre, technical ( bre bundles) and elementary bres (single plant cells) should be di erentiated.Technicalbres (length of up to ~ 1 m, apparent diameter 100-200 µm) separated from the ax plant consist of elementary bres (length ~ 50 mm, apparent diameter 10-30 µm).e polyhedron-shape elementary bres overlap one another at a rather large length interval.ey are held together by pectin and hemicellulose.e elementary bres are composed of a very thin (~ 0.2 µm) primary cell wall, a strongly developed secondary cell wall (dominating the cross section) subdivided into three layers, the middle, S2 layer, having the largest dimension, and a lumen, a small, open channel in the centre of the elementary bre [3,5,8].e secondary cell wall contains crystalline cellulose micro brils and amorphous hemicellulose.e micro brils are bundled into meso brils that are highly oriented along the bre axis (at the so-called micro bril angle).eir arrangement in layers is responsible for the mechanical strength and sti ness of the bre [3,8].Flax fibres can be referred to as composites as the cell wall comprises reinforcing oriented semicrystalline cellulose microfibrils which are embedded in a two-phase (lignin-hemicellulose) amorphous matrix [9].e content of three main polymers (i.e.cellulose, hemicellulose and lignin) is known to vary among plant bre types.e presence of pectin and waxes can lead to the formation of an ine ective interface between the bre and polymer matrix with consequent problems such as debonding and In recent years, the use of ax bres to replace glass bres as reinforcement in polymer composites for engineering applications has gained popularity due to an increasing environmental concern and required development of sustainable materials [9].e performance of ax bre reinforced composites depends strongly on the properties of used bres and polymeric matrix, as well as on their ratio, orientation and interface adhesion [13].A general picture of the in uence of axbre properties on a polymer composite properties is given in Table 2.Many works deal with the properties of ax bres cultivated for textile applications, which are today used for polymer reinforcement.Nevertheless, large quantities of oleaginous ax bre are obtained each year and are not promoted.e tensile properties of ax bres are essential when considered as reinforcement in bre reinforced polymer composites.Pillin et al [14] evaluated the tensile deformation of di erent oleaginous ax bres which were cultivated in the same geographic area and lands in a temperate region (Western France).
e varieties of oleaginous ax studied were Oliver, Hivernal, Alaska, Niagara and Everest.
e used test machine, gauge length and cross-head displacement rate were identical.e results show that interesting mechanical properties were obtained with the oleaginous variety and close to those of textile varieties, e.g.Agatha or Electra.Considering the diameters and speci c properties of these elementary oleaginous bres, it was evidenced that they are good candidates for the substitution of glass bres in composite materials.e retting degree has no in uence on the diameters and mechanical properties of bres.
e same conclusion is obtained with agronomic factors such as seeding rate and plant height.In the study by Baley and Bourmaud [15], the bre tensile properties of 50 batches of 14 textile and oleaginous ax (Linum usitatissimum L.) varieties cultivated in France (Normandy) between 1993 and 2011 were compared.eir varietal and geographical origins were known and the tensile test conditions were similar.Contrary to the widespread idea, the stiness of elementary textile ax bres was very close to that of oleaginous ax bres (Young's modulus 52.4 GPa vs. 52.8GPa), whereas their breakage properties were slightly better (tensile strength 976 MPa vs. 855 MPa and elongation at break 2.15% vs. 1.82%).e results show a strong performance of oleaginous ax bres and justify their use as reinforcement for polymers.A detailed analysis of the results does not show any important impact of the variety nor the cultivation year.ere was no very weak batch with poor mechanical properties.It was concluded that by using a blend of batches, it is possible to guarantee speci c mechanical properties that can compete with those of glass bres.Moreover, it should be highlighted that all tensile properties are widely scattered [2,[15][16][17][18]; this phenomenon also occurred at the breakage properties of glass bres in opposition to their Young's modulus, which is more stable [15].Fuqua et al [19] assessed the property variation between polymer matrix composites unidirectionally reinforced with dew retted, nely combed, long-line ax bre versus randomly oriented polymer composite reinforced with combine harvested, minimally retted, short oleaginous ax bre with high percentage of shives.Varieties, farming conditions, harvest and processing of ax impact the manufacturability of ax bre reinforced composites.e ax bre bundle pullout tests proved that with appropriate cleaning, orientation and combing, similar composite properties were obtained from samples.In a study by Mekic et al [20], the oleaginous ax bre was investigated for its composite processability as compared to traditional breglass.e studied liquid ow through ax bre performs was similar to the breglass performs with the same porosity values under identical processing conditions.Despite the use of non-wood and non-cotton plant bres in reinforced plastics having tripled to 45,000 tonnes over the last decade, plant bre reinforced composites make up only around 1.9% of the 2.4 million tonnes of the EU bre reinforced composite market, primarily ax (64% of the market Shives Material as Potential Reinforcement for Polymer Composites Tekstilec, 2016, 59(4), 350-366 share).It is forecasted that about 830,000 tonnes of bio-bres will be consumed by 2020 and that the share will go up to 28% of the total reinforcement materials [2,3,5].It should be noted that the harvested area of oleaginous ax varieties displays a considerable quantity of short oleaginous axbres.As brous materials from oleaginous ax varieties and shives are available in large quantities, their use in composites shall be further developed in the forthcoming years.Moreover, the processing technique for oleaginous ax bres needs to be adapted and optimised to develop alternative bre supply sources for the composite industry.In Croatia and Slovenia, mainly oleaginous ax is grown, where a er the seed collecting, most of the stems remain unused and a major portion is burned in the eld, creating environmental pollution or is disposed by ploughing.
erefore, the aim of this study was to characterise and compare the properties of shives and technical bres extracted from the ax Linum usitatissimum L. textile variety planted in Croatia and from the Slovenian autochthonous oleaginous variety from Bela Krajina, to de ne if they can be used as potential reinforcement in polymer composites.A er the retting and drying, the stems were passed through a mechanical process of breaking and scotching.e next step was heckling and combing of ax to align bres removing neps, dust and extraneous matters whereby bres and shives were separated.With regard to the length, the bres were additionally separated into four length groupsfrom longer to shorter bres (Figures 1 and 2).

Methods
Mechanical properties, chemical composition, physical and morphological properties, thermal stability during processing and use, hygroscopic behaviour and bre/matrix adhesion are important factors in determining the performance properties of a brous material if used as reinforcement in polymer composites.erefore, a characterisation of the brous material of both varieties was studied with: a) Optical and Scanning Electron Microscopy (SEM).e longitudinal and cross-sectional views of ax bres of both varieties were taken by using an Olympus CH20 optical microscope and Dino microscope eye-piece camera.e surface morphology of ax bres and shives was examined with a SEM analysis using JEOL 6060 LV SEM, Japan (at the accelerating voltage of 10 kV).For SEM analyses, the samples were previously coated with a gold/palladium admixture to the extent of 90/10% in sputter coater.b) Moisture regain.e moisture content in bres (for each length group) and shives was determined according to ASTM D 2654-89a [21].e specimens were conditioned in a standard atmosphere (temperature of 20 ± 2 °C and relative humidity of 65 ± 4%) for 24 h, weighed, dried in an oven at the temperature of 105 °C and reweighed.e di erence between the mass of conditioned and the mass of oven-dried samples was calculated as moisture regain and expressed in percentage.c) Length of individual ax bres.For each length group of technical bres, the length was determined according to ISO 6989 [22], method A: on a straightened bre on a graduated rule, under a light tension applied with the aid of forceps and grease.d) Linear density of individual ax bres.For each length group of technical bres, linear density was examined according to EN ISO 1973 [23], using Vibroscop 400, Lenzing.Both, linear density and tensile properties were determined for the same bres.e) Tensile properties of individual ax bres.Using gauge length, shorter than the length of a single bre cell, the properties of the cell wall in technical bres were measured.Breaking force, breaking elongation, tensile strength and Young's modulus for each length group of technical bres were determined according to EN ISO 5079 [24], using Vibrodin 400, Lenzing, with cogged steel clamps at the following conditions -gauge length: 5 mm, elongation rate: 3 mm/min, pretension: 1500 mg.
e bre specimens for testing the length, linear density and tensile properties were conditioned in standard atmosphere.e average values (x -) of 100 measurements and their coe cient of variation (CV) were calculated.e number of measurements was adapted according to the statistical indications of the degree of reliability with 95% con dence interval.As the diameter and tensile properties of a ax bre are not uniform along its length [2], the linear density and tensile properties of bres were determined on a bre section (60 mm in length) that was taken in the middle of each bre bundle of a certain length group.f) Fourier transform infrared (FT-IR) spectroscopy.
e FT-IR spectra of shives and technical ax bres of both varieties were obtained with a Perkin Elmer Spectrum 100 FT-IR spectrometer, using the non-destructive attenuated total-reection (ATR) method.e results are collected from a region between the surface and depth of about 0.5-5.0µm, depending on sample characteristics [25,26].All spectra were recorded over the range of 4000 cm -1 to 380 cm -1 , with the resolution of 4 cm -1 and 8 scans.e spectra were normalised to the absorption band at 1312 cm -1 .All samples for the FT-IR spectroscopy were prepared in the same standard conditions, which enabled the crystallinity index of cellulose to be evaluated using the obtained spectra.e index I c is determined as the ratio of intensities of absorption bands at 1368 and 2918 cm -1 , I 1368 and I 2918 , respectively [27]: g) ermogravimetric analysis (TGA).TGA is an analytical technique used to determine the thermal stability of a material by monitoring the weight change that occurs when a sample is heated at a constant rate (non-isothermal thermogravimetry) in a controlled atmosphere.As the thermal stability of a brous ax material at a higher temperature is one of the most important factors during the processing of polymer composites (especially thermoplastic), a Perkin Elmer Pyris 1 thermogravimetric analyser was used for the thermal degradation of technical axbres and shives as well as for the determination of their thermal stability.e weight of all analysed samples was 7.0 ± 1.0 mg. e analysis was performed in a nitrogen atmosphere with the ow rate 30 ml/min, temperature range from 50 to 700 °C and 10 °C/min heating rate to avoid the unwanted oxidation.

Optical and SEM microscopy
e optical and SEM micrographs of textile and oleaginous ax bres for all tested length groups are very similar.Figure 3 shows the longitudinal and cross-sectional optical views of ax bres of both varieties.It is clearly seen that the surface impurities and non-cellulosic materials are present on the surface of ax bres (Figures 3a and 3c).Both types of bres have kink bands that appear as horizontal bands in elementary bres.It is conrmed that the ax bres of both varieties exist as a bundle of elementary bres of polygonal crosssectional shape, which can vary in their dimensions (Figures 3b and 3d).
e surface morphology of bres and shives was studied using the scanning electron microscopic analysis.e SEM micrographs of the bre surface and bre cross-section for both varieties are given in Figure 4. Figures 4a and 4c con rm that the structure of ax bres includes several elementary bres bonded along the bre axis, as well as the presence of surface pectin material, which is also determined in optical images.e cross-section of bres (Figures 4b and 4d) indicates the presence of a thick secondary wall.
e diameter of elementary bres of both varieties is very similar, as it is shown in Flax bres were extracted from phloem, which surrounded them in ax plants and occurred in bundles under epidermis.Xylem material (woody core or shives) is located in the middle part of the plant [5].e surface morphology of shives is very similar for both varieties and is given in Figure 5.

Moisture regain
e moisture content of ax brous material is one of the most important criteria which has to be considered in choosing the reinforcement material.Moisture content a ects the dimensional stability, electrical resistivity, tensile strength, porosity and swelling behaviour of ax bres in polymer composites.e chemical composition and location of constituents de ne the sorption properties of a ax brous material.Cellulose is a semicrystalline polysaccharide with a large amount of hydroxyl groups, giving ax bres their hydrophilic nature.When they are used to reinforce hydrophobic matrices, the adhesion between them is low and is accompanied by poor resistance to moisture absorption.Hemicelluloses are strongly bonded to cellulose brils presumably by hydrogen bonds.Hemicellulosic polymers are branched, fully amorphous and have a signi cantly lower molecular weight than cellulose.Due to their open structure, containing mainly hydroxyl and acetyl groups, hemicelluloses are hygroscopic and are partially soluble in water.Lignin and pectin act mainly as bonding agents.Lignin is more hydrophobic and composed of amorphous, highly complex, mainly aromatic, polymers of phenyl-propane units.
e waxy substances of ax bres a ect the lower bre wettability and adhesion characteristics [2,26].
e moisture regain of tested technical bres of textile ax was in the range from 9.73% to 9.35% and for oleaginous ax bres in the range from 9.14% to 8.89%, as it can be seen in Figure 6. e moisture regain of textile ax shives (TFS) was 10.38% and for oleaginous ax shives (OFS) 9.54%.e highest valu es were obtained for the shives and the longest bres (TF1 and OF1) of both varieties.e textile ax bre moisture regain is slightly higher than that of oleaginous bres, which indicates a slight di erence in the chemical composition of bres (Table 1).

Fibre length, linear density and tensile properties
e results of average values of bre length and linear density obtained on technical textile (TF) and oleaginous (OF) ax bres, alongside with the corresponding coe cient of variation are presented in Figures 7 and 8.During the bre samples preparation, a smaller proportion of longer bres was determined at oleaginous ax bres than at textile ax bres.By comparing the results of length and linear density of technical ax bres from textile and oleaginous ax, it was established that: Flax bres obtained from textile ax are on average • longer than ax bres obtained from oleaginous ax (Figure 7).Average length of two longer bre groups 1 and 2 is higher in textile ax bres (TF1, 337.5 mm; TF2, 201.6 mm vs. OF1, 260.6 mm; OF2, 132.2 mm).Although the technical bre linear density depends • on the shape and length of elementary bres, their number in the bre bundle to be evaluated, the ax variety and processing method [3], it was found out that the di erences in the bre linear density between the length groups of textile ax variety (TF1-TF4) are minimal (39.47-37.35dtex).In case of oleaginous ax, the longest bres (OF1, 260 mm) are also the nest bres (29.97 dtex).Shorter oleaginous ax bres (OF2-OF4) have higher linear density (39.59-34.75dtex), comparable with textile ax bres (Figure 8).A relatively large coe cient of variation is found • for ax bres within the length groups, even for the same variety, which indicates highly variable properties.e variability is more pronounced in textile ax bres (Figures 7 and 8).e results of average values of bre tensile properties obtained on technical textile (TF) and oleaginous (OF) ax bres, together with the corresponding coe cient of variation are presented in Table 3.Studies by Romhány et al [8] showed that there are three failure mechanisms of technical ax bres: 1) longitudinal splitting of the pectin boundary layer among elementary bres; 2) transverse fracture of elementary bres; and 3) multiple fractures of elementary bres and their micro brils.Generally, a higher tensile strength is observed for bres with shorter test gauge length.e reasons are two-fold: rstly, the longer the bre, the higher the probability of containing weak links or imperfections (e.g.kink bands), and secondly, the failure mechanism of technical bres at shorter clamping length is di erent from that at longer clamping length.At large clamping length, ax bre failure takes place through the relatively weak pectin interphase that bonds the elementary bres together.e pectin interphase is oriented predominantly in the length direction of the bre, it breaks by shear failure.At clamping length below the elementary bre length, usually stronger cellulosic cell wall of elementary bres is loaded [2,28].
e dispersion of tensile properties is due to the variation in the cellulose, lignin and pectin content which is di erent from one bre to another, and also due to the randomness of the location and size of defects in the stressed bre segment.erefore, a relatively high CV for tensile properties was established for technical bres (Table 3), even low gauge length of 5 mm was selected for measurements.By comparing the tensile properties of technical bres from textile and oleaginous ax separated into four length groups, it was established that with the reduction in bre length, the values of breaking force, tensile strength and breaking elongation decrease,  increasing the dispersion of measurement results.e tensile strength values of two ax varieties are comparable.However, it was found out that the longest and the nest oleaginous ax bres (OF1, 260 mm) are also the strongest (tensile strength 66.19 cN/tex).e values of oleaginous ax bre breaking elongation are minimally lower (OF, 3.52-2.92%)than at textile ax (TF, 3.87-3.26%)which is in accordance with the published data [15].With cellulose micro brils spiral angle (MFA) of 6 to 10° in cell bres, ax bres are among the plant bres with higher tenacity [3].e tensile strength values obtained by investigation are high and also within the range of published data (2.6-7.7 g/den) [1].e Young's modulus of bres is governed by the increase of cellulose content and decrease of MFA [3].Although the values of the Young's modulus are higher for the ax bres obtained from oleaginous ax, the relatively high Young's modulus is determined for both varieties -the highest for longer bres (OF1, 204.42 vs. TF1, 173.72).

FT-IR analysis
e lignocellulosic brous material compounds containing cellulose, hemicelluloses and lignin consist of oxygen containing functional groups (ester, ketone and alcohol), alkenes and aromatic groups [1,11,25,26,29,30].e Fourier transform infrared (FT-IR) spectra of ax bres separated into four length groups and shives are shown in Figures 9 and 10. e FT-IR spectra of textile (Figure 9) and oleaginous ax (Figure 10) bres for all tested length groups are very similar, with no signi cant di erences.e spectra of shives are somewhat di erent compared to the spectra of ax bres, namely according to the intensity of identied bands in the spectra.As shown in Figures 9 and 10, for all bre length groups, a broad absorption band was observed at 3333 cm -1 (TF) and 3336 cm -1 (OF) due to the OH stretching in H-bonded hydroxyls groups of cellulose and hemicelluloses (characteristic band position for H-bonded hydroxyl groups is between 3200-3400 cm -1 ).In shives, this band was observed at 3331 cm -1 for TFS and at 3337 cm -1 for OFS, respectively.e absorption bands at 2918 cm -1 and at 2850 cm -1 correspond to the CϪH symmetrical stretching vibration and CϪH symmetrical stretching vibration from CH 2 in cellulose and hemicelluloses.e CϭO stretching vibration of the linkage of carboxylic acid in lignin or ester group in hemicelluloses is centred at 1734 cm -1 (characteristic band position is between 1720-1740 cm -1 , more precisely between 1731-1734 cm -1 ).
e band at 1632 cm -1 (TF) and 1641 cm -1 (OF) is assigned to the OH bending of physically adsorbed water molecules in noncrystalline cellulose (characteristic band position is 1625-1660 cm -1 ).In the spectra of shives, it appears at 1645 cm -1 .e CϭC stretching of the aromatic skeleton ring of lignin appears at around 1515 cm -1 .e same band at 1513 cm -1 , present in the spectra of shives and elsewhere only as a weak shoulder, con rms the lower content of lignin in all ax bres.e band at 1425 cm -1 corresponds to CϪH in plane bending deformation connected with the methoxyl group in lignin and is present in all ax bre spectra (characteristic band position is between 1400-1430 cm -1 ).In the spectra of shives, it appears at 1423 cm -1 , where an additional band at 1463 cm -1 corresponds to the asymmetrical CϪH bending in the CH 2 and OH deformation that is also present in the spectra of OL2-OL4 oleaginous bres (characteristic band position is between 1450-1475 cm -1 ).
e bands around 1370 cm -1 correspond to the inplane CH bending form CH 2 in cellulose and hemicelluloses.In the bre spectra, this band appears at 1368 cm -1 and in shives, at 1373 cm -1 .e band at 1202 cm -1 corresponds to the CϪOϪC symmetrical stretching in cellulose and hemicelluloses.It is present in all ax bre spectra and only as a weak shoulder in the spectra of ax shives.e band at 1247 cm -1 may be attributed to the Guaiacyl ring breathing with the CϪO stretching and is characteristic of lignin (band position is around 1250 cm -1 ).It is clearly present in the spectra of shorter axbres (TF2-4 and OF2-4) and only as a shoulder in the spectra of the long ax bres of both varieties.In the shives spectra, it is present as a very intensive band at 1242 cm -1 that corresponds to the CϪO bond of the acetyl group in xylan and hemicelluloses.
e band detected at 1156 cm -1 all spectra (characteristic band position is between 1150-1160 cm -1 ) corresponds to the asymmetrical stretching deformation of the CϪOϪC band in cellulose.e β-glycosidic linkages between monosaccharides show a band at 896 cm -1 in all spectra (asymmetrical stretching owing to β linkage in cellulose is characteristic of 890-900 cm -1 ).e band at 660 cm -1 corresponds to the CϪOH out-of-plane bending in cellulose and is present in all ax bre spectra.In Shives Material as Potential Reinforcement for Polymer Composites Tekstilec, 2016, 59(4), 350-366 the area around 1600 cm -1 , corresponding to the aromatic skeleton ring vibration and vibrations owing to adsorbed water, no clear band in thebre spectra was observed, which suggests that there is still a certain amount of fats and pectins in the bres.In the shives spectra, an additional band was observed at 1546 cm -1 , corresponding to the presence of lignin.e changes in the magnitude of crystallinity index follow the tendency in the structural changes of cellulose.e crystallinity index of ax cellulose for textile ax bres was calculated as 0.95-0.80,for oleaginous ax bres as 0.94-0.73and for shives of both varieties as 0.72.e index is lower for shives and shorter ax bres in comparison with that of longer bres of both varieties.is can be explained by the fact that shives Table 4: Results of thermogravimetric analysis for all analysed samples: textile ax bres separated into four length groups (TF1-TF4) and shives (TFS); and oleaginous ax bres separated into four length groups (OF1-OF4) and shives (OFS)  and shorter ax bres contain more amorphous hemicelluloses and pectins. is contributes to the increased absorbance at 2918 cm -1 , which is related to the covalent oscillations of CH groups.e deformational oscillations of CH groups at 1368 cm -1 depend on the degree of orientation of macromolecules; therefore, the crystallinity index is lower for the shives and shorter bre samples.A lower content of fat or waxes and pectins leads to the decrease of intensity at 2918 cm -1 , and in turn to the increase in the crystallinity index of longer ax bres.e lower crystallinity index of cellulose and a higher content of amorphous hemicelluloses in shives also in uence the higher moisture content in samples, as shown in Figure 6. e lower crystallinity index of shorter axbres is probably in uenced by a higher amount of pectin and wax in the bre primary wall (this could be connected with determined lower values of moisture regain, Figure 6) and hemicelluloses in the bre secondary wall (this could be connected with determined lower values of bres breaking force, Table 3).

Thermogravimetric analysis
e thermal stability of ax bres and shives was determined using the non-isothermal thermogravimetric analysis (TGA).e thermogravimetric analysis results of the tested textile (TF) and oleaginous (OF) ax brous material are presented in Table 4, and TGA and di erential thermogravimetric (DTG) curves in Figures 11 and 12. Generally, there are three stages of weight loss of the ax brous material.e rst is moisture evaporation, followed by the decomposition of pectin, hemicelluloses and cellulose, and the third is degradation of lignin.e remaining weight a er the third stage represents the percentage of ash.
e thermal decomposition pro les are similar for all analysed textile and oleaginous ax bre samples.e rst weight loss is related to the evaporation of free water in bre samples.In the stage that begins at 50 °C and ends at around 100 °C, the weight loss of tested bres varies in the range of 4.3-6.2%for textile ax bres and for oleaginous ax bres in the range of 4.4-5.4%.e second weight loss corresponds to the degradation of hemicelluloses and cellulose, and the third is related to the degradation of non-cellulosic components.e second degradation step starts at around 240 °C and ends at around 390 °C.e analysis of ax bre samples showed no distinct pectin peak at 256 °C [31], although a shoulder can be observed at DTG curves in that temperature region.Similarly, no hemicellulose peak but a small shoulder was detected at 290 °C [31].ere is a cellulose peak that corresponds to the temperature at the maximum degradation rate, which varies in the range from 354 °C for the longest textile ax bres to the 351 °C for the shortest ones.e temperature at the maximum degradation rate is slightly higher for longer oleaginous axbres and amounts to 367 °C, in comparison with shorter OF3 and OF4 at 357 °C and 363 °C, respectively.e changes in the peak temperature along with weight loss and rate of degradation may reveal the variation in the quality of a bre.e weight changes at the second peak correspond to cellulosic components and show an increase with a high retting degree.e higher peak intensity in bres is associated with the high crystallinity of cellulose [3].In this stage, a signi cant weight loss is observed for both ax bres; the weight of textile ax bres reduced by 65.5-64.4% and of oleaginous ax bres by 68.6-67.0%together with the degradation rate of around 13%/min. e third or lignin peak is signicantly smaller in comparison with the primary or cellulose peak. is degradation step for all samples begins at around 395 °C and ends at around 515 °C.However, no distinct peak at 429 °C [31] attributed to lignin could be observed.e weight of textile ax bres was reduced in this degradation step by 9.1-7.8%due to the degradation of lignin and by 8.9-7.6% in the case of oleaginous ax bres.At around 700 °C, the tested ax brous material possesses a high char residue due to the high cellulose content. is result is consistent with the results reported by other researchers [32].
e residual weight of textile ax bres amounts to around 20% and of oleaginous ax bres it varies within the range of 16.6-19.1%(from longer to shorter bres).A comparison of thermal stabilities between thebres of textile and oleaginous ax shows no signicant di erence.Both samples have a high cellulose content, lower amount of hemicelluloses and pectin, as well as a high degree of bre quality. is implies that the bres are thermally stable to up to 240 °C in a nitrogen atmosphere.Textile and oleaginous ax shives can be considered as thermally stable to up to 220 °C.A er 220 °C, a decline in the thermal stability of shives can be observed trough a signi cant reduction in weight.
e initial weight loss attributed to the evaporation of free water begins at the temperature at around 50 °C with the weight loss of 5.9% for textile and 5.2% for oleaginous ax shives.is is con rmed by a higher moisture release in textile ax shives and a higher amount of free water than in the oleaginous ax shives.e maximum degradation rate occurs at a lower temperature compared to bres: at 349 °C for textile and 352 °C for oleaginous ax shives.e intensity of the cellulose peak for ax shives is signi -cantly lower compared to the bres of both varieties, regardless of weight loss of 64.8% for textile and 65.1% for oleaginous ax shives, which are similar to the corresponding ax bres.e rate of cellulose degradation is lower compared to ax bres and amounts to around 9%/min.e FT-IR analysis determined a lower crystallinity index of cellulose and con rmed a higher amount of hemicelluloses, pectin and amorphous cellulose in shives in comparison with ax bres.During the degradation of lignin, the weight of ax shives reduced by about 8-9% and the remaining percentage of ash amounted to around 20%, these values being similar to those of corresponding ax bres.

Conclusion
According to the performed analysis, it was concluded that the measured properties of the textile and oleaginous ax brous material were comparable, as well as the properties of individual bre length groups within the same variety.e optical and SEM micrographs of textile and oleaginous ax bres for all tested length groups are very similar.e results of the mechanical properties obtained with the oleaginous variety are close to those of the textile variety.e relatively high Young's modulus determined, which con rms a high ax bre cellulose content for both varieties.Contrary to the widespread idea, the FT-IR and TGA analysis show no signi cant di erence in the lignin content in ax bres of textile and oleaginous varieties.e TGA analysis showed for both ax varieties almost the same thermal stability, a high amount of cellulose and a high degree of ax bre quality.e lower crystallinity index of cellulose and a higher amount of hemicelluloses and pectin were obtained for shives and shorter ax bres in comparison with the longer bres of both varieties.e highest values of moisture regain were obtained for shives and the longest bres of both varieties.e hygroscopic behaviour and presence of pectin and waxes on the surface of ax brous material that can lead to the formation of ine ective interface between the bre and polymer matrix could be modi ed by di erent physical and chemical modi cation processes.erefore, it was concluded that ax bres from textile and oleaginous varieties have adequate morphological and mechanical properties, as well as thermal stability for reinforcing polymer composites.As the type of bre reinforcement (short bres, roving/ yarns, nonwoven or woven fabrics) is very important for the polymer composite resulting properties, it is according to the obtained results possible to select bres for speci c purposes.Shorter bres can be used for nonwoven mats or unidirectional prepregs; longer bres can be used for roving yarns and woven reinforcing fabrics -2D waves, UD waves or multiaxial woven reinforcements.Furthermore, short ax bres, cut or crushed up to very short length, can be added to the polymer during the manufacturing in extrusion or injection moulding.It should be noted that crushed up ax shives are more appropriate for llers in plastics, with a lower reinforcing role.As ax reinforced polymer composites already have a very broad range of application in nearly all sectors of everyday life, this study o ers an alternative and eco-friendly brous reinforcement material which may have usability potential in Croatia and Slovenia.is point is signi cant also from the economic perspective, since the whole set of co-product produced by the plants ( bres, seeds, shives) could be valorised.e production of oleaginousbres could be ensured without using new agricultural lands, thus avoiding the competition with food products.Consequently, farmers could secure several remuneration sources.
is work was supported by the research related to the Development of high performance bio-composites reinforced with cellulose bres from domestic sources, coordinator: Assoc Prof DrSc Antoneta Tomljenović, funded by the University of Zagreb, Croatia for 2013/2014, to the Improvement of adhesion between matrix and cellulose reinforcements in bio-composite materials by cold plasma treatment, coordinator: Assist Prof DrSc Sanja Ercegović Ražić, funded by the University of Zagreb, Croatia for 2014, to the Target modi cation of composite reinforcements by sol-gel process and non-thermal plasma, coordinator: Assist Prof DrSc Maja Somogyi Škoc, funded by the University of Zagreb, Croatia for 2014/2015, and to the Optimization of composite materials reinforcement properties by application sol-gel and plasma treatments, coordinator: Assoc Prof DrSc Antoneta Tomljenović, funded by the University of Zagreb, Croatia for 2016.e authors would also like to thank Assoc Prof DrSc Tatjana Rijavec, Assist Prof DrSc Darja Kocjan-Ačko, Assist Prof DSc Ružica Brunšek, Assist Prof DrSc Sandra Flinčec Grgac and Kristina Rusak, mag.ing.techn.text.for their generous assistance.

e
following technical ax bres were used in this study: a) Textile ax (variety Viola, Van de Bilt Zaden, Netherlands) planted in 2009 in Križevci in Croatia.Flax was manually harvested in the phase of early yellow maturity in late June.Flax stems were subjected to water retting for 72 hours in a laboratory tank with tap water heated to the temperature of 32 °C.b) Oleaginous ax (Slovenian autochthonous variety from Bela Krajina) planted in 2010 in Slovenia on an experimental eld of the Biotechnical Faculty, University of Ljubljana, Slovenia.Oleaginous ax was manually harvested in the phase of yellow maturity in early July.Flax stems were laid on the soil for dew retting for four weeks.

Figure 1 :
Figure 1: Cleaning, orientation and combing of textile ax technical bres, and separated shives

Figure 2 :
Figure 2: Cleaning, orientation and combing of oleaginous ax technical bres

Table 1 :
Chemical composition of textile and oleaginous ax bres

Table 3 :
Tensile properties of ax technical bres separated into four length groups -textile ax (TF1-TF4) and oleaginous ax (OF1-OF4) T 0 -onset degradation temperature, T max -temperature at maximum degradation rate, R max -maximum degradation rate, ∆m H2O -evaporation of free water, ∆m 1 -weight loss in the second degradation step (degradation of hemicelluloses and cellulose), ∆m 2 -weight loss in third degradation step (degradation of non-cellulosic components), m f -residual weight