Chemical Composition , Fiber Morphology , and Kraft Pulping of Bracken Stalks ( Pteridium aquilinum ( L . )

In this study, kraft, kraft-NaBH4, and kraft-KBH4 pulp and paper properties of the bracken stalks (Pteridium aquilinum (L.) Kuhn) were determined. Also, the chemical composition and fi ber properties of bracken stalks were evaluated. NaBH4 and KBH4 were separately added to cooking liquor by 0.5 %, 1 %, 1.5 %, and 2 % (oven dried wood). The boron compound-free kraft pulp were also made as control pulp. Fiber length and fi ber width of bracken stalks were determined as 1.25 mm and 24 μm, respectively. Bracken stalks are composed of 73.34 % holocellulose, 32.55 % α-cellulose, and 30.79 % lignin. In addition, the pulp yield was increased with additions of both boron compounds, while kappa number was decreased. Also, highest strength increases determined in 0.5 % NaBH4 added pulp. These results showed that bracken stalks can be used as a raw material for kraft pulp production.


INTRODUCTION
1. UVOD Bracken (Pteridium aquilinum (L.) Kuhn), a weed with a height of 30-200 cm, is the fi fth most distributed common weed species of the world.Bracken is widely distributed from the Equator to the northern parts of Europe, in Central Asia, China and Japan, from central South America to subarctic Canada.It can easily grow and spread on many types of soil (Vetter, 2009).
Lignocellulosic biomass are included in wood and non-wood biomass.Wood-based lignocellulosic biomass, main raw material of pulp production, consists of hardwood and softwood species.Non-wood lignocellulosic biomass, such as wheat straw, rice straw, cotton stalks, canola stalks, sugarcane bagasse, switchgrass and reed, has been an important fi ber resource for pulp production in countries with a shortage of wood raw material.The main differences between woody and non-wood lignocellulosic biomasses are their chemical compositions and physical properties (Zhu and Pan, 2010).The usability of a raw material in pulp production initially depends on these properties.
Global paper and paperboard production in 2000 and 2015 was 324.6 million ton and 406.3 million ton, respectively (FAOSTAT, 2016).Increasing paper production causes the decrease in forest resources.Non-wood lignocellulosic biomass is abundantly available, low cost, and easy to process.Also, it has a short growth and harvest period.For this reason, it is introduced as a potential raw material for pulp and paper production (Tye et al., 2016).However, in 2015, pulp production from non-wood lignocellulosic biomass was only 12.3 million ton (FAO-STAT, 2016).Many studies have examined the utilization of alternative raw materials in the pulp and paper industry.The utilization as an alternative raw material in paper industry of fruit trees, such as orange tree pruning (Gonzáléz et al., 2011), olive tree pruning (Requejo et al., 2012), pomegranate tree pruning (Gülsoy et al., 2015), white mulberry (Gençer et al., 2013), common hazelnut (Gençer and Özgül, 2016), wild cherry (Gençer and Gül Türkmen, 2016), was previously evaluated.On the other hand, pulp and paper properties of many non-wood plant species were determined by several authors (Deniz et al., 2004;Shatalov and Pereira, 2006;Çöpür et al., 2007;Akgül and Tozluoğlu, 2009;Shakhes et al., 2011;Gençer, 2015;Gençer and Şahin, 2015).However, there is no published report on using bracken stalks in pulp and paper production.The aim of this study was to evaluate the suitability of bracken stalks for papermaking.For this purpose, the kraft, kraft-NaBH 4 , and kraft-KBH 4 pulp properties of bracken stalks were evaluated.The chemical composition and fi ber morphology of bracken stalks (Pteridium aquilinum (L.) Kuhn) were also determined.

Plant material and sample preparation 2.1. Biljni materijal i priprema uzoraka
The bracken (Pteridium aquilinum (L.) Kuhn) were collected from the Bartin province of Turkey.The roots and leaves of bracken were removed and only the stalks were used.The stalks were chopped to 3-5 cm.Bracken stalks were air-dried and stored in dry conditions.

Pulping and handsheet properties 2.4. Proizvodnja celuloze i svojstva papira
The kraft pulps made from bracken stalks were prepared under the following conditions: active alkali as Na 2 O 25 %, sulfi dity 30 %, liquor/wood ratio 5:1, pulping temperature 170 °C, heating time to 170 °C 90 min., and time at temperature 75 min.The same pulping conditions were applied to 0.5 %, 1 %, 1.5 %, and 2 % NaBH 4 and KBH 4 added samples.In pulping, laboratory-type 15-L electrically-heated rotary digester was used.In order to remove the black liquor, the pulps were washed and disintegrated.The rejects were retained by a Somerville-type pulp screen with a 0.15mm slotted plate (TAPPI T 275).All pulps were beaten according to TAPPI T 200 to 25 °SR in a Valley Beater for comparison in the same conditions.The kappa number, screened yield and freeness levels of all pulps were determined according to TAPPI T 236, TAPPI T 210, and ISO 5267-1, respectively.Ten handsheets (75 g/m 2 ) were formed with a Rapid-Kothen Sheet Former (ISO 5269-2).The handsheets were conditioned according to TAPPI T 402.The tensile index, tensile energy absorption (TEA), and stretch (ISO 1924), burst index (TAPPI T 403), tear index (TAPPI T 414), and brightness (TAPPI T 525) of the handsheets were measured using the relevant standard methods.

Statistical analysis 2.5. Statistička analiza
All data was performed using SPSS software.The data belonging to the kraft and kraft-NaBH 4 , and kraft-KBH 4 pulp properties of the bracken stalks were analyzed with analysis of variance (ANOVA).The effects of boron compound addition on paper properties were evaluated statistically.All pair wise multiple comparison procedures were performed using Duncan's test (p<0.05).The same letter in fi gures denotes that there were no statistically signifi cant differences between the groups.

Chemical composition 3.1. Kemijski sastav
The chemical composition of a lignocellulosic raw material has been an important factor in evaluating its suitability for pulp production.The high holocellulose content and low lignin content have been desired for pulping to obtain higher pulp yield and lower kappa number.Also, pulp yield substantially depends on α-cellulose content of raw material used in pulp production.Table 1 shows the chemical composition of bracken stalks and its comparison with other lignocellulosic materials.As can be seen in Table 1, holocellu-lose content of bracken stalks was found to be 73.34%, which is comparable with other annual plants, lower than eucalyptus (80.47 %) and aspen (82.68 %), and higher than maritime pine (70.21 %).
The α-cellulose content of raw material is an indicator of pulp yield.Fibers having higher α-cellulose content have been preferred for chemical pulp production.The α-cellulose content of bracken stalks (32.55 %) was lower than that of other annual plants.Also, it was lower than that of eucalyptus (52.79 %), aspen (49.03 %), and maritime pine (47.11 %), higher than that of cotton stalks (29.74 %).
The klason lignin content of bracken stalks (30.79 %) was higher than that of most annual plants.Also, it was higher than that of eucalyptus (19.96 %), aspen (16.69 %), and maritime pine (28.23 %).The high lignin content in raw material causes longer pulping time and increases the requirement of chemicals for delignifi cation.Therefore, low lignin content in lignocellulosic raw material has been seen as an advantage for chemical pulping.
The solubility in water provides no useful information concerning the pulping value of the raw material, but it indicates the nature of certain constituents.In the cold water procedure, inorganic compounds, tannins, gums, sugars, and coloring matters are removed from raw material.The hot water procedure additionally removes starch.Treatment of raw material with 1 % NaOH causes the extraction of low-molecularweight carbohydrates consisting mainly of hemicelluloses and degraded cellulose.Also, this treatment is an indication of decay degree in the raw material.The high content of 1 % NaOH soluble of the rapeseed stalks and depithed stalks may probably affect the pulp yields and also the chemical consumption (Tofanica et al., 2011).Ethanol, hot water, cold water, and 1 % NaOH solubility of bracken stalks were found to be 7.19 %, 15.45 %, 14.60 %, and 28.08, respectively.These values were similar to those of other nonwood lignocellulosic raw materials, higher than those of hardwood and softwood species.According to chemical composition results, bracken stalks can be evaluated as an alternative raw material for pulp production, although they have low α-cellulose content and high lignin content.
Like most nonwood fi bers, the ash content of bracken stalks (7.45 %) was higher than that of wood species.Also, it was similar to that of corn stalks (7.5 %), lower than that of wheat straw (11.63 %), barley straw (10.97 %), and rice straw (16.6 %).High quantity of ash content in the lignocellulosic raw material can cause problems in pulping and papermaking processes.It can have a negative effect on chemical consumption during pulping, refi ning, and recovery of the cooking liquor.

Fiber morphology 3.2. Morfologija vlakana
One of the important factors in evaluating the suitability of pulp production of a raw material is its fi ber morphological properties.The fi ber properties directly affect the runnability on paper machine, strength and optical properties of sheet, response to refi ning, and fi ber-water interactions, such as swelling and water retention of fi bers.Long fi bers are predisposed to form a porous and less uniform paper structure, coarse paper surface.Also, sheets of long fi bers have higher strength properties than sheets of short fi bers.On the other hand, fi ber fl exibility depends on lumen width and cell wall thickness of fi bers.Thick-walled fi bers have a negative effect on the folding endurance, burst and tensile index of paper, and a positive effect on tear index.Also, the paper obtained from thick walled fibers will be bulky, with a coarse surface, and will contain a large amount of void volume.However, thinwalled fi bers provide uniform and denser paper structure.
Table 2 shows fi ber properties of bracken stalks and comparison of these fi ber properties with some lignocellulosic materials.As seen in Table 2, average fiber length of bracken stalks was 1.25 mm.Fiber length of bracken is similar than that of tobacco stalks (1.23 mm), kenaf (1.29 mm).Fiber length of bracken stalks was shorter than that of maritime pine (2.4 mm), bamboo (1.98 mm), sorghum stalks (1.77 mm), and sugarcane bagasse (1.59 mm), and longer than that of European aspen (1.1 mm), river red gum (0.80 mm), cotton stalks (0.83 mm), sunfl ower stalks (0.76 mm), rice straw (0.99 mm).
Slenderness ratio, felting ratio, and Runkel ratio, derived from fi ber dimensions, have been used to determine the suitability of lignocellulosic raw materials for pulp production.High length to width ratio (slenderness ratio) of fi bers results in well bonded paper.High slenderness ratio (>33) means that lignocellulosic raw material is suitable for pulp and paper production (Xu et al., 2006).Slenderness ratio of bracken stalks (52.8) was comparable with that of other raw materials as shown in Table 2.It was very close to slenderness ratio of river red gum (53.33), lower than that of sorghum stalks (90.37) and rice straw (86.04), and higher than that of sunfl ower stalks (30.06) and paulownia (22.58).
Flexibility ratio (lumen to fi ber width) of fi bers is classifi ed as highly elastic fi bers (>75), elastic fi bers (50-70), rigid fi bers , and very rigid fi bers (<30).Flexibility ratio of bracken stalks (42.92) was comparable with that of other raw materials, as shown in Table 2.It was very close to fl exibility ratio of corn stalks (44.03), lower than that of sunfl ower stalks (64.96) and maritime pine (67.5), and higher than that of sorghum stalks (33.79).According to fl exibility ratio, the fi bers of bracken stalks can be classifi ed into rigid fi bers group.
The fl exible fi bers, having low Runkel ratio (wall to lumen ratio, <1) are easily collapsible, and give large surface area for interfi ber bonding.The rigid fi bers, having high Runkel ratio (>1), have low bonded area.These fi bers are least suitable for paper production due to their stiff fi bers.Runkel ratio of bracken stalks (1.33) was comparable with that of other raw materials, as shown in Table 2.It was very close to Runkel ratio of rice straw (1.31), lower that that of switchgrass (1.5), and higher than that of maritime pine (0.5) and European aspen (1.1).Bracken stalks fi bers are classifi ed into rigid fi bers category in terms of their Runkel ratio value.According to fi ber morphology results, bracken stalks can be used for pulp production as FL: Fiber length / duljina vlakna, FW: Fiber width / širina vlakna, LW: Lumen width / širina lumena, CWT: Cell wall thickness / debljina stanične stijenke, SR: Slenderness ratio / omjer vitkosti, FR: Flexibility ratio / omjer fl eksibilnosti, RR: Runkel ratio / Runkelov omjer an alternative raw material, although they have relatively short and rigid fi bers.

Pulp and paper properties 3.3. Svojstva celuloze i papira
Some properties of kraft, kraft-NaBH 4 , and kraft-KBH 4 pulps are given in Table 3.The screened and total yields of bracken stalks control kraft pulp were relatively lower compared to conventional kraft pulp yield.This result can be explained by low α-cellulose content of bracken stalks (Table 2).On the other hand, pulp yields increased with the addition of NaBH 4 and KBH 4 .The highest total pulp yield was found to be 36.9% in 2 % KBH 4 added pulp.Similar total yield increases with the addition of NaBH 4 (Akgül et al., 2007 (Gülsoy et al., 2016) have been reported by several authors.This result could be attributed to carbohydrate retention increases with the addition of boron compounds.
Residual lignin content of pulp was calculated by multiplying the kappa number by 0.13.Bleachable-grade chemical pulp (kraft or sulfi te) usually contains about 1.5-4.5 % residual lignin (Gellerstedt, 2010).The residual lignin content of bracken stalks control pulp (45.1 x 0.13= 5.86 %) was slightly high-er than that of bleachable-grade.This fi nding could be explained by high lignin content of bracken stalks (Table 2).The kappa numbers of pulps decreased with increasing boron compound addition ratios.This fi ndng can be attributed to the acceleration of delignifi cation rate with NaBH 4 and KBH 4 additions.The lowest kappa number was found to be 30.9 in 2 % KBH 4  The comparison of kraft pulp properties of bracken stalks with other lignocellulosic raw materials is presented in Table 4. Bracken stalks had lower screened yield and brightness, and higher kappa number than other nonwood species.This result can be attributed to low α-cellulose and high lignin content of bracken stalks.
At the similar pulp freeness levels, handsheet strength properties of bracken stalks kraft pulp were determined to be comparable with hardwood and nonwood papermaking raw materials.Tensile index (68.69N•m/g) of handsheets of bracken stalks kraft pulp at 25 ºSR freeness level was higher than that of European aspen kraft pulp at 30 ºSR freeness level (61.13 N•m/g), kenaf kraft pulp at 30 ºSR freeness (50.23 N•m/g), sweet bamboo kraft pulp at 25 ºSR freeness (63.01 N•m/g), and lower than switchgrass kraft pulp at 30 ºSR freeness level (74.48 N•m/g).
The effect of NaBH 4 and KBH 4 additions on tensile index of handsheets is given in Figure 1.Tensile index increased with NaBH 4 and KBH 4 additions except for 1.5 % NaBH 4 and 1 % KBH 4 added pulps (p<0.05).The highest tensile index was determined in 0.5 % NaBH 4 added pulp with 74.01 N•m/g.Increased tensile index can be attributed to increasing hemicellulose retention in boron compounds added pulps.Higher hemicellulose content results in an increase of pulp strength.Gulsoy and Eroglu (2011) noted that tensile index increased in European black pine kraft pulp, and Gümüşkaya et al., (2011) noted that it increased in stone pine AS-AQ pulp with NaBH 4 addition.On the contrary, some authors noted that NaBH 4 had a negative effect on tensile index of handsheets (Akgül et al., 2007;Çöpür ve Tozluoğlu, 2008;Istek and Özkan, 2008).The effect of NaBH 4 and KBH 4 additions on burst index of handsheets is given in Figure 5. Burst index increased with NaBH 4 and KBH 4 additions except for 1 % KBH 4 added pulp (p<0.05).The highest burst index was determined in 0.5 % KBH 4 added pulp with 3.27 kPa•m 2 /g.Burst index of European black pine kraft pulp (Gulsoy and Eroglu, 2011) and stone pine AS-AQ pulp (Gümüşkaya et al., 2011) increased with 1 % NaBH 4 addition.On the contrary, Çöpür and Tozluoğlu (2008) noted in Brutia pine kraft pulp and Istek and Gonteki (2009) in maritime pine that NaBH 4 had a negative effect on burst index of handsheets.
The effect of NaBH 4 and KBH 4 additions on brightness of handsheets is given in Figure 6.Brightness increased with NaBH 4 and KBH 4 additions (p<0.05).This result can be attributed to lower kappa number of NaBH 4 and KBH 4 added pulps than that of control pulp (Table 3).The highest brightness was determined in 2 % KBH 4 added pulp with 20.16 %.Similar brightness increases were reported in earlier studies (Akgül et al., 2007;Çöpür and Tozluoğlu, 2008;Istek and Gonteki, 2009;Gulsoy and Eroglu, 2011).

CONCLUSIONS 4. ZAKLJUČAK
Chemical composition analysis showed that bracken stalks had higher holocellulose and lignin, and lower α-cellulose content than those of other lignocel-  lulosic nonwood resources.The fi ber properties of bracken stalks were similar to the fi bers of other nonwood resources and aspen.Kraft pulp properties of bracken stalks were determined to be comparable with those of hardwoods (aspen and eucalyptus) and common nonwood papermaking raw materials.NaBH 4 and KBH 4 additions caused the increase of pulp yield and decrease of kappa number.Also, strength properties of bracken stalks kraft pulp increased with NaBH 4 and KBH 4 additions except for tear index.NaBH 4 and KBH 4 had a positive effect on pulp brightness.Consequently, the bracken stalks can be used as a fi ber source for paper production.
Istek and Gonteki (2009)sed with NaBH 4 and KBH 4 additions except for 1 % and 2 % KBH 4 added pulps (p<0.05).The highest TEA value was determined in 0.5 % NaBH 4 added pulp with 76.66 J/m 2 .Similar TEA losses were reported byIstek and Gonteki (2009).The effect of NaBH 4 and KBH 4 additions on tear index of handsheets is given in Figure4.Tear index decreased with NaBH 4 and KBH 4 additions (p<0.05).Tear index losses can be attributed to higher pulp yield of NaBH 4 and KBH 4 added pulps.Increasing in pulp yield causes the decrease of the fi ber per unit weight of oven-dried pulp.The lowest tear index was determined in 2 % NaBH 4 added pulp with 5.82 mN•m2/g.Similar tear index losses were reported in earlier studies (Akgül et al., 2007; Çöpür and Tozluoğlu, 2008; Istek and Gonteki, 2009; Gulsoy and Eroglu, 2011; Gümüşkaya et al., 2011).