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Chemical composition and pulp characterization of Tunisian vine stems
Industrial Crops and Products 36 (2012) 22–27
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Chemical composition and pulp characterization of Tunisian vine stems Samar Mansouri a , Ramzi Khiari a,b,∗ , Najoua Bendouissa a , Seif Saadallah a , Farouk Mhenni a , Evelyne Mauret b a b
Research Unity of Applied Chemistry & Environment, Department of Chemistry, Faculty of Sciences, University of Monastir, Monastir 5019, Tunisia Laboratoire de Génie des Procédés Papetiers (LGP2), UMR CNRS 5518, Grenoble INP-Pagora – 461, rue de la papeterie – 38402 Saint-Martin-d’Hères, France
a r t i c l e
i n f o
Article history: Received 9 May 2011 Received in revised form 10 July 2011 Accepted 30 July 2011 Available online 24 September 2011 Keywords: Vine stems Chemical composition Pulping Paper properties
a b s t r a c t In this study, the valorisation of Tunisian vine stem wastes was investigated. The chemical composition of the vine stems was studied, and it was found that when compared to non-wood plants, they contain greater amounts of extractives, lignin, and comparable holocellulose content. An elementary analysis of the ashes showed that the major constituents were mineral elements (K and Ca). Soda pulping of vine stems led to a yield of about 35% after the bleaching step. This amount is lower than that obtained for wood plants and similar to that observed for annual plants. The morphological properties, Kappa number, and degree of polymerization of the resulting pulp were determined. Finally, paper handsheets were prepared from the pulp, and their physical properties were investigated. The breaking length, Young’s modulus, and burst index of the produced paper presents quite acceptable values. Further, the silica content of the stems is low, which is advantageous for the pulping process. Experimental results obtained for both the pulp and paper show that this agricultural residue has the potential to be used for papermaking applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cellulosic fibres are widely used for many purposes, for example, in textile industries (to manufacture clothing and furnishing), papermaking and packaging industries (to produce papers, boards, and non-wovens), in pharmaceutical applications (to manufacture compresses, dressings, bandages, drugs, etc.), and preparation of innovative materials such as ‘green’ composites. Consequently, the consumption of cellulosic fibres is increasing, and it is becoming increasingly difficult to satisfy the large demand. In this context, non-wood species can be viewed as alternative sources of cellulosic fibres, especially in regions that are poor in forest resources. Nonwood fibres could potentially be used in applications that require materials with properties similar to these fibres. Finally, non-wood fibres are often obtained from agricultural wastes and can therefore be valorised. There are many studies that have been carried out over many years to investigate the use of annual plants or/and agricultural wastes as alternative sources of fibres. These strategies were already applied in various countries for Helianthus tuberosus L. (Fiserova et al., 2006), Miscanthus sinensis (Barba et al., 2002), Cynara cardunculus L. and banana pseudo-stems (Antunes et al.,
∗ Corresponding author at: Research Unity of Applied Chemistry & Environment, Department of Chemistry, Faculty of Sciences, University of Monastir, Monastir 5019, Tunisia. Tel.: +33 4 76 82 69 59; fax: +33 4 76 82 69 33. E-mail address: khiari [email protected] (R. Khiari). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.07.036
2000; Cordeiro et al., 2004; Abrantes et al., 2007), sorghum stalks (Jiménez et al., 1993), Ipomea carnea and Cannabis sativa (Dutt et al., 2008), kenaf (Chia et al., 2008), holm oak (Eugenio et al., 2006), bamboo (Khristova et al., 2006), Amaranthus caudatus L. (Fiserova et al., 2006), Atriplex hortensis L. (Fiserova et al., 2006), Arundo donax L. (Shatalov et al., 2001), and rachis date palm (Bendahou et al., 2008). In particular, Tunisian agricultural wastes and annual plants (date palm rachis, alfa grass, and Posidonia oceanica) have been studied and the valorisation of extracted fibres has been investigated for the production of paper, green composites, and cellulose derivatives (Belgacem et al., 1986; Khiari et al., 2010, 2011). Nevertheless, numerous other Tunisian cellulosic materials that have so far not been studied in detail are available. An example is the case of vine plants (Fig. 1), of which grapevine is commercially grown (according to a Food and Agriculture Organization report, in Tunisia, grape production reached 1.32 × 108 t in 2008). Owing to grape production, significant quantities of vine fragments have accumulated on Tunisian agricultural lands, and this warrants the cleaning of the vineyards every autumn after grape harvesting. The use of this available and renewable biomass for papermaking can help in the profitable utilization of the vine fragments. To the best of our knowledge, none of the past studies on the chemical composition and pulping of non-wood materials for papermaking have considered vine stems. This study is devoted to the characterization of lignocellulosic fibres from Tunisian vine stems and to the evaluation of their suitability for papermaking applications. The initial part of this paper focuses on the determination of the
S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
Fig. 1. Vine stem.
chemical composition. Soda delignification is adapted to obtain cellulosic fibres from the stems. Characterization of the pulp is carried out in terms of the yield, Kappa number, degree of polymerization (DP), and morphological properties. In the last part of this paper, the physical properties of handsheets prepared from vine stem pulp are presented and discussed. 2. Materials and methods 2.1. Raw material The stems were obtained from Monastir in December 2009 and dried under natural conditions (average relative humidity: 65%; average temperature: around 20 ◦ C). They were then washed in order to eliminate sand and dried again under the same conditions. Before pulping, the stems were cut into small pieces with lengths of about 1–3 cm. 2.2. Chemical composition of vine stems The chemical composition of the Tunisian vine stems was determined by using common procedures, which are described below. Extractives: The evaluation of extractives was carried out in different liquids according to common standards, namely, cold and hot water solubility (T207 cm-08), 1% sodium hydroxide solution solubility (T212 om-07), and ethanol–toluene solubility (T204 cm07). The water solubility test is normally used for wood and annual plants and pulps, which are not extracted with organic solvents. However, if prior extraction with organic solvents is desired, a solvent such as dichloromethane, which will dissolve only a minimum amount of a water-soluble material, should be used. In the present study, the Tunisian vine stems studied was Soxhlet extracted for 6 h, and the amount of soluble products was determined gravimetrically. Ash (T211 om-07): The ash content was determined according the standard method TAPPI T211 om-07. It is defined as the solid residue remaining after ignition at 525 ± 25 ◦ C for at least 4 h. The characterization of this residue was also carried out by elementary analysis at the Service Central d’Analyse, Vernaison (CNRS). Klason lignin (T222 om-06): The acid-insoluble lignin was measured by subjecting the vine stem to acid hydrolysis and filtering the obtained suspension for separating the insoluble lignin. The obtained solid is then dried and weighed. It is worth noting that this procedure is used to determine the acid-insoluble lignin in wood and in all grades of unbleached pulps; however, it is not applicable to bleached pulps containing only small amounts of lignin. Holocellulose (Wise et al., 1946): Stems (5.0 g, oven dried (o.d)) were taken in a flask containing distilled water (160 mL). Sodium chlorite (1.50 g) and glacial acetic acid (0.5 mL) were added to the flask and the mixture was refluxed at 70–80 ◦ C for 1 h. After 1 h, sodium chlorite (1.50 g) and glacial acetic acid (0.5 mL) were again added. The procedure was repeated four times until the
23
material turned white. The obtained residue was washed several times before being dried at 105 ◦ C for 24 h. ˛-Cellulose (T203 cm-99): An holocellulose sample was taken in a beaker containing 35 mL of sodium hydroxide solution (17.5%). After allowing the reaction to proceed for 5 min, 40 mL of sodium hydroxide solution (17.5%) was added to the reaction mixture in four equal portions (4 mL × 10 mL) at intervals of 10 min under constant stirring. Thirty minutes later, 100 mL of distilled water was poured into the beaker, and the beaker was left in the bath for 30 min. Thus, the total extraction time is 60 ± 5 min. At the end of the 60-min period, the pulp suspension was filtered. Subsequently, 25.0 mL of the filtrate and 10.0 mL of 0.5 N potassium dichromate solution were taken in a 250-mL flask. An amount of 50 mL of concentrated H2 SO4 was added cautiously. After heating the solution for 15 min, 50 mL of water was added. Finally, 2–4 drops of Ferroin indicator were added and titration was carried out with 0.1 mol L−1 ferrous ammonium sulphate solution up to the point where the colour of the solution changed to purple. ␣-Cellulose’s quantity was determined after titration. Kappa number of pulp (T 236 om-06): The Kappa number is defined as the amount (in milliliters) of 0.1 mol L−1 potassium permanganate solution consumed by 1 g of moisture-free pulp under controlled conditions at 25 ◦ C. This value provides an evaluation of the residual lignin in pulps, i.e., the degree of delignification. This measurement is applicable to all types of chemical and semichemical pulps and gives Kappa numbers ranging from 1 to 100. As recommended by the various standards used, all the experiments were duplicated and the differences between the two values were within an experimental error of 5%. 2.3. Pulping Several alkaline cookings of the stems were carried out in order to determine the most appropriate procedure. The effects of temperature (120, 140 and 160 ◦ C), sodium hydroxide concentration (10, 12 and 15%) and cooking time at constant temperature (30, 60 and 120 min) were thus studied. Briefly, vine stems were treated with 100 mL of an aqueous soda solution under stirring at fixed temperature by using a set of 15 small reactors of 250 mL each (Ahiba device). Liquor to solid ratio was kept constant at a value of 10 for all the experiments. After the cooking step, the obtained unbleached material was filtered, washed several times with distilled water, and dried. For the most relevant conditions, the obtained pulp was bleached at 60 ◦ C with a sodium hypochlorite solution (12% of active chlorine) for 90 min. Before the bleached pulps were air dried, a neutralization step was performed to eliminate the excess chlorite. Each cooking condition was performed at least twice, and the difference between the various values obtained was within an experimental error of 5%. 2.4. Characterization of pulp and paper obtained from vine stems After delignification, the pulping yield was calculated as the ratio of the weight of the o.d. material after washing to that of the o.d. initial raw material. The amount of residual lignin was evaluated as described previously (Klason lignin). The degree of polymerization (DP) was determined by measuring the viscosity of a pulp sample (in mPa s) dissolved in a cupriethylenediamine solution according to the TAPPI method (T230 om-99). All the experiments were duplicated and the difference between the two values was within an experimental error of 5%. The morphological properties of the obtained vine stem pulps were assessed by using a MORFI (LB-01) analyzer, developed by Techpap (France). The morphological parameters (fibre length, fibre width, and fine element content) were measured by an image analysis procedure. In this procedure, a diluted pulp suspension
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S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
Table 1 Chemical composition of several lignocellulosic materials.
Vine stems (this work) Grape vine stalks (Ping et al., 2011) Vine shoots (Jiménez et al., 2007) Posidonia oceanica (Khiari et al., 2010) Date palm rachis (Khiari et al., 2010) Sorghum stalks (Jiménez et al., 1993) Amaranth (Fiserova et al., 2006) Jerusalem artichoke (Fiserova et al., 2006) Cynara Cardunculus L**. (Antunes al., 2000) Chamaecytisus proliferus (Jiménez et al., 2008) Olive trimmings (Jiménez et al., 1996) Pinus pinaster (Alonso, 1976) Eucalyptus globulus (Alonso, 1976)
CW
HW
1% NaOH
AB
Ash
Lign
Hol
Cell
8.2 n.a. n.a. 7.3 5.0 n.a. 23.5 26.6 n.a. n.a. 15.5 n.a. n.a.
13.9 n.a. 16.0 12.2 8.1 21.7 28.0 31.0 11.5 2.8 12.8 2.0 2.8
37.8 n.a. 39.2 16.5 20.8 41.6 46.8 48.5 n.a. 16.7 30.0 7.9 12.4
11.3 n.a. 4.9* 10.7 6.3 8.0 2.5 2.8 4.8 2.6* 11.5* 2.6* 1.2*
3.9 3.9 3.5 12.0 5.0 4.8 12.0 2.0 6.7 2.3 1.0 0.5 0.6
28.1 39.6 20.3 29.8 27.2 13.4 13.2 14.7 25.4 16.8 18.9 26.2 19.9
65.4 60.0 67.1 61.8 74.8 71.7 58.4 51.6 72.8 79.7 64.7 69.6 80.5
35.0 36.0 41.1 40.0 45.0 42.0 32.0 29.0 40.5 45.4 59.0 56.0 53.0
CW: cold water solubility; HW: hot water solubility; AB: solubility in ethanol–toluene (*or in ethanol–benzene); 1% NaOH: 1% sodium hydroxide solubility; Hol: holocellulose (%); Lign: Klason lignin (%); Cell: ␣-cellulose (%); ** average for two years (1996 & 1997); n.a.: non available.
(0.3 g of pulp in 8 L) flowing in a transparent flat channel was observed by a charge-coupled device video camera. Fine elements are defined as particles with a size less than 200 m and their amount is calculated as the ratio of the total length of fines to the total length of the elements of the suspension. An average of five tests was conducted for each parameter. Several steps were needed for the successful preparation and characterization of handsheets prepared from vine stem pulp. According to the standard method ISO 5263-1, the pulp was disintegrated and passed through a slotted screen with an aperture size of 0.15 mm in order to remove uncooked materials. The pulp drainability was determined by measuring the Schopper Riegler degree (◦ SR–ISO 5267–1). The lower the Schopper Riegler degree, the better is the pulp drainability. Subsequently, after the dilution of the pulp to 2 g L−1 , ten conventional handsheets were prepared on a Rapid Khöten sheet former (ISO 5269-2). As recommended by the standard method ISO 187, the prepared papers were conditioned at 23 ◦ C and 50% relative humidity for 48 h before they were tested and their physical properties were evaluated. The basis weight (ISO 536), thickness (ISO 534), tensile (NF Q 03-002), tear (NF Q 03-011) and burst strengths (NF Q 03-053), as well as brightness (TAPPI method, T 452) were measured.
3. Results and discussion 3.1. Chemical composition of vine stems The chemical composition of Tunisian vine stems was determined, and the results are summarized in Table 1. The obtained data show that the stems are characterized by relatively high amounts of lignin (28.1%), holocellulose (65.4%) and extractives, especially in ethanol–toluene mixture (11.3%). In contrast, the ␣-cellulose content is low (35%).
Table 1 also presents the chemical composition of several other cellulosic materials such as wood, non-wood, and annual plants; these data have been obtained from the literature. When compared with data for grapevine stalks (Ping et al., 2011) or vine shoots (Jiménez et al., 2007), it appears that ␣-cellulose and holocellulose contents are relatively close. The lignin content of the stems is intermediate between those of stalks and shoots and the amount of extractives in the ethanol–toluene mixture is higher in the stems. The comparison with other wood and non-wood species confirms that the amounts of extractives in the vine stems are high. With regard to structural components, the vine stems are characterized by relatively low ␣-cellulose content and high amounts of lignin, whereas the content in holocellulose is quite comparable. Finally, the ash content was found to be around 4%, which is much higher than that of wood and in the same range than that of non-wood plants (Alonso, 1976; Jiménez et al., 1993,1996, 2007, 2008). From a detailed analysis of the chemical composition of these ashes, it appears that they mainly contain potassium (K) and calcium (Ca), which is generally observed for non-wood plants (see Table 2). Moreover, the silicon quantity is around 0.057%, which is a very low quantity. As silica has a negative impact on the chemical recovery in alkaline pulping process, the low amount present in vine stems is then a positive feature, when considering the valorisation of this agricultural waste in papermaking. In conclusion, the Tunisian vine stems are characterized by high amounts of extractives, lignin, and relatively low ␣-cellulose content. Thanks to the acceptable amount of holocellulose, this biomass could be viewed as a potential source of cellulose for the production of cellulose derivatives, and of lignocellulosic fibres for fibre-reinforced composite materials or papermaking applications; the suitability of this biomass for papermaking applications is examined in the present study.
Table 2 Ash composition for vine stems, date palm rachis, banana pseudo-stems and amaranth (in % (w/w) with respect to oven dried raw material; n.a.: non available). %
Vine stems (this work)
Date palm rachis (Khiari et al., 2010)
Si 1.14 2.8 Ca 15.85 21.5 Mg 2.24 3.53 Fe 0.11 240 ppm Cu 243 ppm 360 ppm K 26.22 10.2 P 4.27 0.7 S 1.2 1.69 C 6.07 1.5 Cl 1.39 18.6 Na 2.09 6.79 Absolute silicon contents in raw materials Si 0.057 0.14
Banana pseudo-stems (Cordeiro et al., 2004)
Amaranth (Fiserova et al., 2006)
2.7 7.5 4.3 n.a. n.a. 33.4 2.2 n.a. n.a. n.a. n.a.
0.25 4.17 0.035 n.a. 0.01 36.67 n.a. n.a. n.a. n.a. n.a.
0.38
0.03
S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
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Fig. 4. Effect of sodium hydroxide concentration on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, 10, 12 and 15% of NaOH, liquor/solid ratio = 10, 120 min of heating, temperature: 140 ◦ C. Fig. 2. Effect of temperature on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, sodium hydroxide concentration of 15%, liquor/solid ratio = 10, 120 min of heating, temperature: 120, 140 and 160 ◦ C.
3.2. Effect of cooking conditions on the pulping process Delignification process depends on numerous factors such as temperature, cooking time, concentration of sodium hydroxide solution, etc. In order to reveal the influence of these parameters, pulping yield and Kappa number were determined for different experimental conditions. 3.2.1. Effect of temperature Fig. 1 shows the influence of the temperature on pulp yield and Kappa number. As it can be seen, when temperature increases, both yield and Kappa number decrease (Fig. 2). At 120 ◦ C, for a yield close to 55%, the obtained pulp contains many uncooked materials (see Fig. 3A). At 140 ◦ C, the yield and Kappa number were found to be equal to 42.5% and 21.5, respectively and uncooked materials were no more present (see Fig. 3B). At 160 ◦ C, yield and Kappa number keep decreasing without any observable modification of the pulp homogeneity. Consequently, a temperature of 140 ◦ C was selected for the further experiments. 3.2.2. Effect of sodium hydroxide dosage For the selected temperature, concentration of sodium hydroxide solution was varied between 10 and 15%. Increasing the concentration of sodium hydroxide solution leads to a significant decrease of the yield, as it is shown in Fig. 4. In order to maintain it at an acceptable level, an optimum sodium hydroxide concentration of 12% was chosen. At this concentration, the yield and Kappa number were 45% and 25.5, respectively.
Fig. 5. Effect of cooking time on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, sodium hydroxide concentration of 12%, liquor/solid ratio = 10, 120 min of heating, temperature: 140 ◦ C.
3.2.3. Effect of pulping time Time of cooking also affects the quality of the pulp. Fig. 5 shows the effect of this parameter and, as expected, increasing cooking time has a strong influence on both yield and Kappa number. The pulping yield (Kappa number) varied from 52% (56) to 45% (25.5) when the time was increased from 30 to 120 min. It is worth noting that some uncooked materials were observed for the pulps obtained at 30 and 60 min whereas, at 120 min, uncooked materials were no more present. From these results, it appears that the best cooking conditions correspond to a cooking time of 120 min, at 140 ◦ C, with a sodium hydroxide concentration of 12%. In these conditions, after several washings of the pulp in order to remove the black liquor, the obtained yield was 45%. This pulp was then submitted to a bleaching stage leading to a final yield
Fig. 3. Images (optical microscopy) of Tunisian unbleached vine stem pulps. A – cooking conditions: sodium hydroxide concentration of 15%, 120 min of heating, temperature: 120 ◦ C. B – cooking conditions: sodium hydroxide concentration of 15%, 120 min of heating, temperature: 140 ◦ C.
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S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
Table 3 Main properties of pulps from various Tunisian non-wood species.
Pulping yield, % Kappa number Viscosity (mPa s) DP Schopper Riegler degree (◦ SR) Fibre length – arithmetic mean (mm) Fibre width (m) Fine elements (% in length) a
a
Vine stems (this work)
Date palm rachis (Khiari et al., 2010)
Pinus radiata (Mishra, 2010)
Eucalyptus (Mishra, 2010)
45 25.5 8.5 800 16 0.59 24.6 9.2
44.8 54 15.7 1203 14 0.89 22.3 30.8
55 10.6 20.5 1423 13 1.9 31.6 7.4
50 8.7 18.1 1399 17 0.77 20.9 12.2
Average of three experimental data.
of 35%, which is a typical value for chemical pulps of annual plants (Jiménez and Lopez, 1990; Jiménez et al., 1993, 2008; Fiserova et al., 2006).
observed. In Table 3, the main morphological properties of these fibres are presented. From this table, it appears that the fibres’ width is close to that of fibres obtained from some other annual plants, while their length (0.6 mm) is significantly lower. These properties impact the aspect ratio (ratio of the length to the diameter), whose value is 24. A comparison can be made with the aspect ratios of softwood and hardwood fibres (Mishra, 2010), which are generally around 70 and 50, respectively, and those of annual plants, which are less than 50. High values of the aspect ratio increase the strength of the paper sheet due to fibres’ entanglement, but they also promote the flocculation of the pulp suspension. Moreover, the vine stem pulp is characterized by low fine element content. The drainability of the pulp, also reported in Table 3, which is quite good (16◦ SR), can be partially explained by this low fine element content. This value is similar to that of unrefined softwood pulps and lower than that of other non-wood plants like Cynara cardunlus L. (Antunes et al., 2000; Gominho et al., 2001), hemp or bamboo (Khristova et al., 2006), and date palm rachis (Khiari et al., 2010).
3.3. Characterization of the vine stem pulp The bleached pulp obtained from Tunisian vine stems was then characterized and the results are reported in Table 3. The viscosity of the bleached pulp is equal to 8.5 mPa s. From this value and by using the Mark–Houwink relationship (Khiari et al., 2010), the degree of polymerization (DP) of the pulp was calculated. The resulting DP, around 800, is quite suitable for papermaking applications and corresponds to values commonly encountered for fibres obtained from annual plants (Khiari et al., 2010). Moreover, this DP value indicates that proper pulping conditions have been selected. The morphological characterization of fibres from vine stems was then performed. From optical microscopy analyses, fibres with different lengths as well as some fine elements can be easily
Table 4 Physical properties of paper handsheets made from unrefined bleached chemical pulp of vine stems. Comparison with bleached and unbleached chemical pulps made from other sources of lignocellulosic fibres.
Unbleached pulp Date palm rachis pulp (Khiari et al., 2010) Arundo donax L. reed pulp (Shatalov and Pereira, 2005) Canola Stalks pulp (Enayati et al., 2009) Kenaf pulp (Dutt et al., 2010b) Hibiscus cannabinus pulp (Dutt et al., 2010a, 2010b) Vine shoot pulp (soda) (Jiménez et al., 2006) Eucalyptus citriodora (Khristova et al., 2006) Bleached pulp Kenaf pulp (Dutt et al., 2010b) Hibiscus cannabinus pulp (Dutt et al., 2010a) Canola Stalks pulp (Enayati et al., 2009) Arundo donax L. reed pulp (Shatalov and Pereira, 2005) Softwood (Pinus radiata) pulp (Mishra, 2010) Hardwood (eucalyptus) pulp (Mishra, 2010) This work Bleached vine stem pulp
Basis weight (g/m2 )
◦
63.9
14
2.2
65.0a
n.a.
65.0a
26b
SR
Bulk (cm3 g)
Tear index (mN m2 g)
Burst index (kPa m2 g)
Brightness (%)
4.40
1.32
n.a.
1.8
8.22
0.96
23
n.a.
5.07
1.22
36.5
c
60.0 60.0
17 17
1.6 1.6c
7.60 7.50
1.52 1.1
45 45
67.0
21
n.a.
0.18
1.05
21
60.0
15.5
n.a.
1.9
1.4
22.3
60.0 60.0
18 18
1.6c 1.6c
7.12 7.20
1.18 1.0
80 81.2
65.0a
28b
n.a.
4.76
1.39
78.4
65.0a
n.a.
1.7
9.94
1.37
76.4
66.0
13
2.0
10.30
8.6
n.a.
64.2
15
2.1
1.19
0.53
79.9
69.5
16
2.4
5.74
1.72
70
n.a.: non available. a Basis weight is deduced from the standard methods used and cited by the authors. b Shopper Riegler degree is deduced from results expressed in Canadian Standard Freeness. c Bulk is deduced from apparent density.
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3.4. Characterization of vine-stem-based papers
References
Conventional paper handsheets were prepared from unrefined vine stem pulp and their physical properties were determined. The produced handsheets present quite acceptable values of tensile strength (breaking length: 3220 m and Young modulus: 2.58 MPa), of tear index (5.74 mN m2 g−1 ) and burst index (1.72 kPa m2 g−1 ). It is worth noting that, for the considered basis weight (69.5 g m−2 ), the value of the thickness is high. This may denote the presence of impurities, not well removed by the screening operation. Consequently, the bulk (2.4 cm3 g−1 ) is probably slightly overestimated, whereas the Young modulus is underestimated. In order to compare physical properties of vine stem based papers to those of papers made from various species, Table 4 presents data for handsheets of bleached chemical pulps obtained from hardwood, softwood, and some non-wood plants. Physical properties of papers made from unbleached pulps (like unbleached pulp of vine shoots) are also reported, by assuming that bleaching does not modify the mechanical properties in a great extent (see Table 4 and, for instance, the works of Enayati et al. (2009) for both unbleached and bleached pulps of Canola stalks and of Dutt et al. (2010a) for Hibiscus cannabinus pulps). All these data correspond to papers made from unrefined pulps. The Schopper Riegler degrees are also reported, and they show that most values are lower than 20 apart from the work of Enayati et al. (2009). From this table, it clearly appears that the burst index of vine stem based papers and, in a less extent, the tear index, are comparable or better than those of papers obtained from other non-wood plants or hardwood (Khristova et al., 2006; Mishra, 2010). Thus, properties of papers obtained from vine stem pulp are considerably greater than those of vine shoot based papers (Jiménez et al., 2006). Finally, only the papers made from softwood pulp exhibit higher mechanical properties for both tear and burst indexes. These good results may be related to the DP of the vine stem pulp, as previously discussed. In addition, the acceptable content in holocellulose (65%) of the pulp has certainly a positive impact on the mechanical properties of the sheets, by increasing the number of hydrogen bonds. These properties probably counteract the effect of the low aspect ratio of the vine stem fibres. Considering both these characteristics and the good drainability of the pulp when compared to that of other pulps (Table 4), vine stems can be then viewed as a good candidate for producing papers from agricultural residues.
Abrantes, S., Amaral, M.E., Costa, A.P., Duarte, A.P., 2007. Cynara cardunculus L. alkaline pulps: alternative fibres for paper and paperboard production. Bioresour. Technol. 98, 2873–2878. Alonso, L., 1976. Análisis químico de maderas de diferentes especies forestales. Departamento de Celulosas e Industrias de Extracción, CRIDA 06 (Tajo) INIA, Madrid, p. 5. Antunes, A., Amaral, E., Belgacem, M.N., 2000. Cynara cardunculus L.: chemical composition and soda-anthraquinone cooking. Ind. Crop. Prod. 12, 85–91. Barba, C., De la Rosa, A., Vidal, T., Colom, J.F., Farriol, X., Montane, D., 2002. TCF bleached pulps from Miscanthus sinensis by the impregnation rapid steam pulping (IRSP) process. J. Wood Chem. Technol. 22 (4), 249– 266. Belgacem, M.N., Zid, M., Nicolski, S.N., Obolenskaya, A.V., 1986. Study of the chemical composition of alpha from Tunisia. Chim. Technol. Drev. Mej. Sbor. Trud. 8, 111–114. Bendahou, A., Kaddami, H., Sautereau, H., Raihane, M., Erchiqui, F., Dufresne, A., 2008. Short Palm tree fibres polyolefin composites: effect of filler content and coupling agent on physical properties. Macromol. Mater. Eng. 293 (2), 140–148. Chia, C.H., Zakaria, S., Nguyen, K.L., Abdullah, M., 2008. Utilisation of unbleached kenaf fibers for the preparation of magnetic paper. Ind. Crop. Prod. 28 (3), 333–339. Cordeiro, N., Belgacem, M.N., Torres, I.C., Mourad, J.C.V.P., 2004. Chemical composition and pulping of banana pseudo-stems. Ind. Crop. Prod. 19, 147–154. Dutt, D., Upadhyaya, J.S., Tyagi, C.H., Kumar, A., Lal, M., 2008. Studies on Ipomea carnea and Cannabis sativa as an alternative pulp blend for softwood: an optimization of kraft delignification process. Ind. Crop. Prod. 28, 128–136. Dutt, D., Upadhyaya, J.S., Tyagi, C.H., 2010a. Studies on Hibiscus cannabinus, Hibiscus sabdariffa, and Cannabinus sativa pulp to be a substitute for softwood pulp. Part 1. AS-AQ delignification process. Bioresources 5 (4), 2123–2136. Dutt, D., Upadhyaya, J.S., Tyagi, C.H., 2010b. Studies on Hibiscus cannabinus, Hibiscus sabdariffa, and Cannabinus sativa pulp to be a substitute for softwood pulp. Part 2. SAS-AQ and NSSC-AQ delignification processes. Bioresources 5 (4), 2137–2152. Enayati, A.A., Hamzeh, Y., Mirshokraie, S.A., Molaii, M., 2009. Papermaking potential of canola stalks. Bioresources 4 (1), 245–256. Eugenio, M.E., Alaejos, J., Diaz, M.J., Lopez, F., Vidal, T., 2006. Evaluation of Holm oak (Quercus Ilex) wood as alternative source for cellulose pulp. Cellul. Chem. Technol. 40 (1–2), 53–61. Fiserova, M., Gigac, J., Majtnerova, A., Szeiffova, G., 2006. Evaluation of annual plants (Amaranthus caudatus L., Atriplex hortensis L., Helianthus tuberosus L.) for pulp production. Cellul. Chem. Technol. 40 (6), 405–412. Gominho, J., Fernandez, J., Pereira, H., 2001. Cynara cardunculus L.: a new fibre crop for pulp and paper production. Ind. Crop. Prod. 13, 1–10. Jiménez, L., Angulo, V., Ramos, E., De La Torre, M.J., Ferrer, J.L., 2006. Comparison of various pulping processes for producing pulp from vine shoots. Ind. Crop. Prod. 23, 122–130. Jiménez, L., Lopez, F., 1990. Characterization of Spanish agricultural residues with a view to obtaining cellulose pulp. Tappi J. 73 (8), 173–176. Jiménez, L., Lopez, F., Martınez, C., 1993. Paper from sorghum stalks. Holzforschung 47, 529–533. Jiménez, L., Pérez, A., Jesús, M., Moral, A., Serrano, L., 2007. Characterization of vine shoots, cotton stalks, Leucaena leucocephala and Chamaecytisus proliferus, and of their ethyleneglycol pulps. Bioresour. Technol. 98, 3487–3490. Jiménez, L., Pérez, I., Maestre, F., Ferrer, J.L., 1996. Fabricación de pastas celulósicas a partir de materias primas no madereras utilizando tecnologías menos contaminantes. Inv. Téc. Papel. 129, 606. Jiménez, L., Rodriguez, A., Perez, A., Moral, A., Serrano, L., 2008. Alternative raw materials and pulping process using clean technologies. Ind. Crop. Prod. 28 (1), 11–16. Khiari, R., Mhenni, M.F., Belgacem, M.N., Mauret, E., 2010. Chemical composition and pulping of date palm rachis and Posidonia oceanica – a comparison with other wood and non-wood fibre sources. Bioresour. Technol. 101, 775–780. Khiari, R., Mauret, E., Belgacem, M.N., Mhenni, F., 2011. Tunisian date palm rachis used as an alternative source of fibres for papermaking applications. Bioresources 6 (1), 265–281. Khristova, P., Kordsachia, O., Patt, R., Karar, I., 2006. Comparative alkaline pulping of two bamboo species from Sudan. Cellul. Chem. Technol. 40 (5), 325–334. Mishra, S.P, 2010. Bleaching of cellulosic paper fibres with ozone-effect on the fibre properties. Thesis, Institut Polytechnique de Grenoble, France. Ping, L., Brosse, N., Sannigrahi, P., Ragauskas, A., 2011. Evaluation of grape stalks as a bioresource. Ind. Crop. Prod. 33 (1), 200–204. Shatalov, A.A., Pereira, H., 2005. Arundo donax L. reed: new perspectives for pulping and bleaching. Part 4. Peroxide bleaching of organosolv pulps. Bioresour. Technol. 96, 865–872. Shatalov, A.A., Quilho, T., Pereira, H., 2001. Arundo donax L. reed: new perspectives for pulping and bleaching. 1. Raw material characterization. Tappi J. 84 (1), 96. Wise, L.E., Murphy, M., D’Addieco, A.A., 1946. Chlorite holocellulose: its fractionation and bearing on summative wood analysis and on studies on the hemicellulose. Paper Trade J. 122 (2), 35–43.
4. Conclusion The obtained data show that vine stems could be considered as a promising raw material for papermaking applications. Vine stems contain holocellulose in sufficient amount, which justifies their valorisation for cellulose derivatives, as a source of fibres for cellulose fibre-reinforced composites, or for papers and boards. If the pulping process is performed under proper conditions, the obtained pulp exhibits good properties in terms of degree of polymerization and Kappa number. Nevertheless, the presence of large quantities of lignin and extractives in the stems limits the pulping yield. Finally, morphological characterization shows that the aspect ratio of the vine stem fibres is low. This does not significantly impact the strength properties of the produced papers, which are comparable to those of papers prepared from other non-wood species. Acknowledgment The authors would like to express their sincere thanks to Professor Mohamed Naceur Belgacem, Director of LGP2, for his valuable advice and assistance.
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Industrial Crops and Products journal homepage: www.elsevier.com/locate/indcrop
Chemical composition and pulp characterization of Tunisian vine stems Samar Mansouri a , Ramzi Khiari a,b,∗ , Najoua Bendouissa a , Seif Saadallah a , Farouk Mhenni a , Evelyne Mauret b a b
Research Unity of Applied Chemistry & Environment, Department of Chemistry, Faculty of Sciences, University of Monastir, Monastir 5019, Tunisia Laboratoire de Génie des Procédés Papetiers (LGP2), UMR CNRS 5518, Grenoble INP-Pagora – 461, rue de la papeterie – 38402 Saint-Martin-d’Hères, France
a r t i c l e
i n f o
Article history: Received 9 May 2011 Received in revised form 10 July 2011 Accepted 30 July 2011 Available online 24 September 2011 Keywords: Vine stems Chemical composition Pulping Paper properties
a b s t r a c t In this study, the valorisation of Tunisian vine stem wastes was investigated. The chemical composition of the vine stems was studied, and it was found that when compared to non-wood plants, they contain greater amounts of extractives, lignin, and comparable holocellulose content. An elementary analysis of the ashes showed that the major constituents were mineral elements (K and Ca). Soda pulping of vine stems led to a yield of about 35% after the bleaching step. This amount is lower than that obtained for wood plants and similar to that observed for annual plants. The morphological properties, Kappa number, and degree of polymerization of the resulting pulp were determined. Finally, paper handsheets were prepared from the pulp, and their physical properties were investigated. The breaking length, Young’s modulus, and burst index of the produced paper presents quite acceptable values. Further, the silica content of the stems is low, which is advantageous for the pulping process. Experimental results obtained for both the pulp and paper show that this agricultural residue has the potential to be used for papermaking applications. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Cellulosic fibres are widely used for many purposes, for example, in textile industries (to manufacture clothing and furnishing), papermaking and packaging industries (to produce papers, boards, and non-wovens), in pharmaceutical applications (to manufacture compresses, dressings, bandages, drugs, etc.), and preparation of innovative materials such as ‘green’ composites. Consequently, the consumption of cellulosic fibres is increasing, and it is becoming increasingly difficult to satisfy the large demand. In this context, non-wood species can be viewed as alternative sources of cellulosic fibres, especially in regions that are poor in forest resources. Nonwood fibres could potentially be used in applications that require materials with properties similar to these fibres. Finally, non-wood fibres are often obtained from agricultural wastes and can therefore be valorised. There are many studies that have been carried out over many years to investigate the use of annual plants or/and agricultural wastes as alternative sources of fibres. These strategies were already applied in various countries for Helianthus tuberosus L. (Fiserova et al., 2006), Miscanthus sinensis (Barba et al., 2002), Cynara cardunculus L. and banana pseudo-stems (Antunes et al.,
∗ Corresponding author at: Research Unity of Applied Chemistry & Environment, Department of Chemistry, Faculty of Sciences, University of Monastir, Monastir 5019, Tunisia. Tel.: +33 4 76 82 69 59; fax: +33 4 76 82 69 33. E-mail address: khiari [email protected] (R. Khiari). 0926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.indcrop.2011.07.036
2000; Cordeiro et al., 2004; Abrantes et al., 2007), sorghum stalks (Jiménez et al., 1993), Ipomea carnea and Cannabis sativa (Dutt et al., 2008), kenaf (Chia et al., 2008), holm oak (Eugenio et al., 2006), bamboo (Khristova et al., 2006), Amaranthus caudatus L. (Fiserova et al., 2006), Atriplex hortensis L. (Fiserova et al., 2006), Arundo donax L. (Shatalov et al., 2001), and rachis date palm (Bendahou et al., 2008). In particular, Tunisian agricultural wastes and annual plants (date palm rachis, alfa grass, and Posidonia oceanica) have been studied and the valorisation of extracted fibres has been investigated for the production of paper, green composites, and cellulose derivatives (Belgacem et al., 1986; Khiari et al., 2010, 2011). Nevertheless, numerous other Tunisian cellulosic materials that have so far not been studied in detail are available. An example is the case of vine plants (Fig. 1), of which grapevine is commercially grown (according to a Food and Agriculture Organization report, in Tunisia, grape production reached 1.32 × 108 t in 2008). Owing to grape production, significant quantities of vine fragments have accumulated on Tunisian agricultural lands, and this warrants the cleaning of the vineyards every autumn after grape harvesting. The use of this available and renewable biomass for papermaking can help in the profitable utilization of the vine fragments. To the best of our knowledge, none of the past studies on the chemical composition and pulping of non-wood materials for papermaking have considered vine stems. This study is devoted to the characterization of lignocellulosic fibres from Tunisian vine stems and to the evaluation of their suitability for papermaking applications. The initial part of this paper focuses on the determination of the
S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
Fig. 1. Vine stem.
chemical composition. Soda delignification is adapted to obtain cellulosic fibres from the stems. Characterization of the pulp is carried out in terms of the yield, Kappa number, degree of polymerization (DP), and morphological properties. In the last part of this paper, the physical properties of handsheets prepared from vine stem pulp are presented and discussed. 2. Materials and methods 2.1. Raw material The stems were obtained from Monastir in December 2009 and dried under natural conditions (average relative humidity: 65%; average temperature: around 20 ◦ C). They were then washed in order to eliminate sand and dried again under the same conditions. Before pulping, the stems were cut into small pieces with lengths of about 1–3 cm. 2.2. Chemical composition of vine stems The chemical composition of the Tunisian vine stems was determined by using common procedures, which are described below. Extractives: The evaluation of extractives was carried out in different liquids according to common standards, namely, cold and hot water solubility (T207 cm-08), 1% sodium hydroxide solution solubility (T212 om-07), and ethanol–toluene solubility (T204 cm07). The water solubility test is normally used for wood and annual plants and pulps, which are not extracted with organic solvents. However, if prior extraction with organic solvents is desired, a solvent such as dichloromethane, which will dissolve only a minimum amount of a water-soluble material, should be used. In the present study, the Tunisian vine stems studied was Soxhlet extracted for 6 h, and the amount of soluble products was determined gravimetrically. Ash (T211 om-07): The ash content was determined according the standard method TAPPI T211 om-07. It is defined as the solid residue remaining after ignition at 525 ± 25 ◦ C for at least 4 h. The characterization of this residue was also carried out by elementary analysis at the Service Central d’Analyse, Vernaison (CNRS). Klason lignin (T222 om-06): The acid-insoluble lignin was measured by subjecting the vine stem to acid hydrolysis and filtering the obtained suspension for separating the insoluble lignin. The obtained solid is then dried and weighed. It is worth noting that this procedure is used to determine the acid-insoluble lignin in wood and in all grades of unbleached pulps; however, it is not applicable to bleached pulps containing only small amounts of lignin. Holocellulose (Wise et al., 1946): Stems (5.0 g, oven dried (o.d)) were taken in a flask containing distilled water (160 mL). Sodium chlorite (1.50 g) and glacial acetic acid (0.5 mL) were added to the flask and the mixture was refluxed at 70–80 ◦ C for 1 h. After 1 h, sodium chlorite (1.50 g) and glacial acetic acid (0.5 mL) were again added. The procedure was repeated four times until the
23
material turned white. The obtained residue was washed several times before being dried at 105 ◦ C for 24 h. ˛-Cellulose (T203 cm-99): An holocellulose sample was taken in a beaker containing 35 mL of sodium hydroxide solution (17.5%). After allowing the reaction to proceed for 5 min, 40 mL of sodium hydroxide solution (17.5%) was added to the reaction mixture in four equal portions (4 mL × 10 mL) at intervals of 10 min under constant stirring. Thirty minutes later, 100 mL of distilled water was poured into the beaker, and the beaker was left in the bath for 30 min. Thus, the total extraction time is 60 ± 5 min. At the end of the 60-min period, the pulp suspension was filtered. Subsequently, 25.0 mL of the filtrate and 10.0 mL of 0.5 N potassium dichromate solution were taken in a 250-mL flask. An amount of 50 mL of concentrated H2 SO4 was added cautiously. After heating the solution for 15 min, 50 mL of water was added. Finally, 2–4 drops of Ferroin indicator were added and titration was carried out with 0.1 mol L−1 ferrous ammonium sulphate solution up to the point where the colour of the solution changed to purple. ␣-Cellulose’s quantity was determined after titration. Kappa number of pulp (T 236 om-06): The Kappa number is defined as the amount (in milliliters) of 0.1 mol L−1 potassium permanganate solution consumed by 1 g of moisture-free pulp under controlled conditions at 25 ◦ C. This value provides an evaluation of the residual lignin in pulps, i.e., the degree of delignification. This measurement is applicable to all types of chemical and semichemical pulps and gives Kappa numbers ranging from 1 to 100. As recommended by the various standards used, all the experiments were duplicated and the differences between the two values were within an experimental error of 5%. 2.3. Pulping Several alkaline cookings of the stems were carried out in order to determine the most appropriate procedure. The effects of temperature (120, 140 and 160 ◦ C), sodium hydroxide concentration (10, 12 and 15%) and cooking time at constant temperature (30, 60 and 120 min) were thus studied. Briefly, vine stems were treated with 100 mL of an aqueous soda solution under stirring at fixed temperature by using a set of 15 small reactors of 250 mL each (Ahiba device). Liquor to solid ratio was kept constant at a value of 10 for all the experiments. After the cooking step, the obtained unbleached material was filtered, washed several times with distilled water, and dried. For the most relevant conditions, the obtained pulp was bleached at 60 ◦ C with a sodium hypochlorite solution (12% of active chlorine) for 90 min. Before the bleached pulps were air dried, a neutralization step was performed to eliminate the excess chlorite. Each cooking condition was performed at least twice, and the difference between the various values obtained was within an experimental error of 5%. 2.4. Characterization of pulp and paper obtained from vine stems After delignification, the pulping yield was calculated as the ratio of the weight of the o.d. material after washing to that of the o.d. initial raw material. The amount of residual lignin was evaluated as described previously (Klason lignin). The degree of polymerization (DP) was determined by measuring the viscosity of a pulp sample (in mPa s) dissolved in a cupriethylenediamine solution according to the TAPPI method (T230 om-99). All the experiments were duplicated and the difference between the two values was within an experimental error of 5%. The morphological properties of the obtained vine stem pulps were assessed by using a MORFI (LB-01) analyzer, developed by Techpap (France). The morphological parameters (fibre length, fibre width, and fine element content) were measured by an image analysis procedure. In this procedure, a diluted pulp suspension
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S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
Table 1 Chemical composition of several lignocellulosic materials.
Vine stems (this work) Grape vine stalks (Ping et al., 2011) Vine shoots (Jiménez et al., 2007) Posidonia oceanica (Khiari et al., 2010) Date palm rachis (Khiari et al., 2010) Sorghum stalks (Jiménez et al., 1993) Amaranth (Fiserova et al., 2006) Jerusalem artichoke (Fiserova et al., 2006) Cynara Cardunculus L**. (Antunes al., 2000) Chamaecytisus proliferus (Jiménez et al., 2008) Olive trimmings (Jiménez et al., 1996) Pinus pinaster (Alonso, 1976) Eucalyptus globulus (Alonso, 1976)
CW
HW
1% NaOH
AB
Ash
Lign
Hol
Cell
8.2 n.a. n.a. 7.3 5.0 n.a. 23.5 26.6 n.a. n.a. 15.5 n.a. n.a.
13.9 n.a. 16.0 12.2 8.1 21.7 28.0 31.0 11.5 2.8 12.8 2.0 2.8
37.8 n.a. 39.2 16.5 20.8 41.6 46.8 48.5 n.a. 16.7 30.0 7.9 12.4
11.3 n.a. 4.9* 10.7 6.3 8.0 2.5 2.8 4.8 2.6* 11.5* 2.6* 1.2*
3.9 3.9 3.5 12.0 5.0 4.8 12.0 2.0 6.7 2.3 1.0 0.5 0.6
28.1 39.6 20.3 29.8 27.2 13.4 13.2 14.7 25.4 16.8 18.9 26.2 19.9
65.4 60.0 67.1 61.8 74.8 71.7 58.4 51.6 72.8 79.7 64.7 69.6 80.5
35.0 36.0 41.1 40.0 45.0 42.0 32.0 29.0 40.5 45.4 59.0 56.0 53.0
CW: cold water solubility; HW: hot water solubility; AB: solubility in ethanol–toluene (*or in ethanol–benzene); 1% NaOH: 1% sodium hydroxide solubility; Hol: holocellulose (%); Lign: Klason lignin (%); Cell: ␣-cellulose (%); ** average for two years (1996 & 1997); n.a.: non available.
(0.3 g of pulp in 8 L) flowing in a transparent flat channel was observed by a charge-coupled device video camera. Fine elements are defined as particles with a size less than 200 m and their amount is calculated as the ratio of the total length of fines to the total length of the elements of the suspension. An average of five tests was conducted for each parameter. Several steps were needed for the successful preparation and characterization of handsheets prepared from vine stem pulp. According to the standard method ISO 5263-1, the pulp was disintegrated and passed through a slotted screen with an aperture size of 0.15 mm in order to remove uncooked materials. The pulp drainability was determined by measuring the Schopper Riegler degree (◦ SR–ISO 5267–1). The lower the Schopper Riegler degree, the better is the pulp drainability. Subsequently, after the dilution of the pulp to 2 g L−1 , ten conventional handsheets were prepared on a Rapid Khöten sheet former (ISO 5269-2). As recommended by the standard method ISO 187, the prepared papers were conditioned at 23 ◦ C and 50% relative humidity for 48 h before they were tested and their physical properties were evaluated. The basis weight (ISO 536), thickness (ISO 534), tensile (NF Q 03-002), tear (NF Q 03-011) and burst strengths (NF Q 03-053), as well as brightness (TAPPI method, T 452) were measured.
3. Results and discussion 3.1. Chemical composition of vine stems The chemical composition of Tunisian vine stems was determined, and the results are summarized in Table 1. The obtained data show that the stems are characterized by relatively high amounts of lignin (28.1%), holocellulose (65.4%) and extractives, especially in ethanol–toluene mixture (11.3%). In contrast, the ␣-cellulose content is low (35%).
Table 1 also presents the chemical composition of several other cellulosic materials such as wood, non-wood, and annual plants; these data have been obtained from the literature. When compared with data for grapevine stalks (Ping et al., 2011) or vine shoots (Jiménez et al., 2007), it appears that ␣-cellulose and holocellulose contents are relatively close. The lignin content of the stems is intermediate between those of stalks and shoots and the amount of extractives in the ethanol–toluene mixture is higher in the stems. The comparison with other wood and non-wood species confirms that the amounts of extractives in the vine stems are high. With regard to structural components, the vine stems are characterized by relatively low ␣-cellulose content and high amounts of lignin, whereas the content in holocellulose is quite comparable. Finally, the ash content was found to be around 4%, which is much higher than that of wood and in the same range than that of non-wood plants (Alonso, 1976; Jiménez et al., 1993,1996, 2007, 2008). From a detailed analysis of the chemical composition of these ashes, it appears that they mainly contain potassium (K) and calcium (Ca), which is generally observed for non-wood plants (see Table 2). Moreover, the silicon quantity is around 0.057%, which is a very low quantity. As silica has a negative impact on the chemical recovery in alkaline pulping process, the low amount present in vine stems is then a positive feature, when considering the valorisation of this agricultural waste in papermaking. In conclusion, the Tunisian vine stems are characterized by high amounts of extractives, lignin, and relatively low ␣-cellulose content. Thanks to the acceptable amount of holocellulose, this biomass could be viewed as a potential source of cellulose for the production of cellulose derivatives, and of lignocellulosic fibres for fibre-reinforced composite materials or papermaking applications; the suitability of this biomass for papermaking applications is examined in the present study.
Table 2 Ash composition for vine stems, date palm rachis, banana pseudo-stems and amaranth (in % (w/w) with respect to oven dried raw material; n.a.: non available). %
Vine stems (this work)
Date palm rachis (Khiari et al., 2010)
Si 1.14 2.8 Ca 15.85 21.5 Mg 2.24 3.53 Fe 0.11 240 ppm Cu 243 ppm 360 ppm K 26.22 10.2 P 4.27 0.7 S 1.2 1.69 C 6.07 1.5 Cl 1.39 18.6 Na 2.09 6.79 Absolute silicon contents in raw materials Si 0.057 0.14
Banana pseudo-stems (Cordeiro et al., 2004)
Amaranth (Fiserova et al., 2006)
2.7 7.5 4.3 n.a. n.a. 33.4 2.2 n.a. n.a. n.a. n.a.
0.25 4.17 0.035 n.a. 0.01 36.67 n.a. n.a. n.a. n.a. n.a.
0.38
0.03
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Fig. 4. Effect of sodium hydroxide concentration on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, 10, 12 and 15% of NaOH, liquor/solid ratio = 10, 120 min of heating, temperature: 140 ◦ C. Fig. 2. Effect of temperature on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, sodium hydroxide concentration of 15%, liquor/solid ratio = 10, 120 min of heating, temperature: 120, 140 and 160 ◦ C.
3.2. Effect of cooking conditions on the pulping process Delignification process depends on numerous factors such as temperature, cooking time, concentration of sodium hydroxide solution, etc. In order to reveal the influence of these parameters, pulping yield and Kappa number were determined for different experimental conditions. 3.2.1. Effect of temperature Fig. 1 shows the influence of the temperature on pulp yield and Kappa number. As it can be seen, when temperature increases, both yield and Kappa number decrease (Fig. 2). At 120 ◦ C, for a yield close to 55%, the obtained pulp contains many uncooked materials (see Fig. 3A). At 140 ◦ C, the yield and Kappa number were found to be equal to 42.5% and 21.5, respectively and uncooked materials were no more present (see Fig. 3B). At 160 ◦ C, yield and Kappa number keep decreasing without any observable modification of the pulp homogeneity. Consequently, a temperature of 140 ◦ C was selected for the further experiments. 3.2.2. Effect of sodium hydroxide dosage For the selected temperature, concentration of sodium hydroxide solution was varied between 10 and 15%. Increasing the concentration of sodium hydroxide solution leads to a significant decrease of the yield, as it is shown in Fig. 4. In order to maintain it at an acceptable level, an optimum sodium hydroxide concentration of 12% was chosen. At this concentration, the yield and Kappa number were 45% and 25.5, respectively.
Fig. 5. Effect of cooking time on pulping yield and Kappa number. Cooking conditions: 10 g of oven dried vine stem, sodium hydroxide concentration of 12%, liquor/solid ratio = 10, 120 min of heating, temperature: 140 ◦ C.
3.2.3. Effect of pulping time Time of cooking also affects the quality of the pulp. Fig. 5 shows the effect of this parameter and, as expected, increasing cooking time has a strong influence on both yield and Kappa number. The pulping yield (Kappa number) varied from 52% (56) to 45% (25.5) when the time was increased from 30 to 120 min. It is worth noting that some uncooked materials were observed for the pulps obtained at 30 and 60 min whereas, at 120 min, uncooked materials were no more present. From these results, it appears that the best cooking conditions correspond to a cooking time of 120 min, at 140 ◦ C, with a sodium hydroxide concentration of 12%. In these conditions, after several washings of the pulp in order to remove the black liquor, the obtained yield was 45%. This pulp was then submitted to a bleaching stage leading to a final yield
Fig. 3. Images (optical microscopy) of Tunisian unbleached vine stem pulps. A – cooking conditions: sodium hydroxide concentration of 15%, 120 min of heating, temperature: 120 ◦ C. B – cooking conditions: sodium hydroxide concentration of 15%, 120 min of heating, temperature: 140 ◦ C.
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Table 3 Main properties of pulps from various Tunisian non-wood species.
Pulping yield, % Kappa number Viscosity (mPa s) DP Schopper Riegler degree (◦ SR) Fibre length – arithmetic mean (mm) Fibre width (m) Fine elements (% in length) a
a
Vine stems (this work)
Date palm rachis (Khiari et al., 2010)
Pinus radiata (Mishra, 2010)
Eucalyptus (Mishra, 2010)
45 25.5 8.5 800 16 0.59 24.6 9.2
44.8 54 15.7 1203 14 0.89 22.3 30.8
55 10.6 20.5 1423 13 1.9 31.6 7.4
50 8.7 18.1 1399 17 0.77 20.9 12.2
Average of three experimental data.
of 35%, which is a typical value for chemical pulps of annual plants (Jiménez and Lopez, 1990; Jiménez et al., 1993, 2008; Fiserova et al., 2006).
observed. In Table 3, the main morphological properties of these fibres are presented. From this table, it appears that the fibres’ width is close to that of fibres obtained from some other annual plants, while their length (0.6 mm) is significantly lower. These properties impact the aspect ratio (ratio of the length to the diameter), whose value is 24. A comparison can be made with the aspect ratios of softwood and hardwood fibres (Mishra, 2010), which are generally around 70 and 50, respectively, and those of annual plants, which are less than 50. High values of the aspect ratio increase the strength of the paper sheet due to fibres’ entanglement, but they also promote the flocculation of the pulp suspension. Moreover, the vine stem pulp is characterized by low fine element content. The drainability of the pulp, also reported in Table 3, which is quite good (16◦ SR), can be partially explained by this low fine element content. This value is similar to that of unrefined softwood pulps and lower than that of other non-wood plants like Cynara cardunlus L. (Antunes et al., 2000; Gominho et al., 2001), hemp or bamboo (Khristova et al., 2006), and date palm rachis (Khiari et al., 2010).
3.3. Characterization of the vine stem pulp The bleached pulp obtained from Tunisian vine stems was then characterized and the results are reported in Table 3. The viscosity of the bleached pulp is equal to 8.5 mPa s. From this value and by using the Mark–Houwink relationship (Khiari et al., 2010), the degree of polymerization (DP) of the pulp was calculated. The resulting DP, around 800, is quite suitable for papermaking applications and corresponds to values commonly encountered for fibres obtained from annual plants (Khiari et al., 2010). Moreover, this DP value indicates that proper pulping conditions have been selected. The morphological characterization of fibres from vine stems was then performed. From optical microscopy analyses, fibres with different lengths as well as some fine elements can be easily
Table 4 Physical properties of paper handsheets made from unrefined bleached chemical pulp of vine stems. Comparison with bleached and unbleached chemical pulps made from other sources of lignocellulosic fibres.
Unbleached pulp Date palm rachis pulp (Khiari et al., 2010) Arundo donax L. reed pulp (Shatalov and Pereira, 2005) Canola Stalks pulp (Enayati et al., 2009) Kenaf pulp (Dutt et al., 2010b) Hibiscus cannabinus pulp (Dutt et al., 2010a, 2010b) Vine shoot pulp (soda) (Jiménez et al., 2006) Eucalyptus citriodora (Khristova et al., 2006) Bleached pulp Kenaf pulp (Dutt et al., 2010b) Hibiscus cannabinus pulp (Dutt et al., 2010a) Canola Stalks pulp (Enayati et al., 2009) Arundo donax L. reed pulp (Shatalov and Pereira, 2005) Softwood (Pinus radiata) pulp (Mishra, 2010) Hardwood (eucalyptus) pulp (Mishra, 2010) This work Bleached vine stem pulp
Basis weight (g/m2 )
◦
63.9
14
2.2
65.0a
n.a.
65.0a
26b
SR
Bulk (cm3 g)
Tear index (mN m2 g)
Burst index (kPa m2 g)
Brightness (%)
4.40
1.32
n.a.
1.8
8.22
0.96
23
n.a.
5.07
1.22
36.5
c
60.0 60.0
17 17
1.6 1.6c
7.60 7.50
1.52 1.1
45 45
67.0
21
n.a.
0.18
1.05
21
60.0
15.5
n.a.
1.9
1.4
22.3
60.0 60.0
18 18
1.6c 1.6c
7.12 7.20
1.18 1.0
80 81.2
65.0a
28b
n.a.
4.76
1.39
78.4
65.0a
n.a.
1.7
9.94
1.37
76.4
66.0
13
2.0
10.30
8.6
n.a.
64.2
15
2.1
1.19
0.53
79.9
69.5
16
2.4
5.74
1.72
70
n.a.: non available. a Basis weight is deduced from the standard methods used and cited by the authors. b Shopper Riegler degree is deduced from results expressed in Canadian Standard Freeness. c Bulk is deduced from apparent density.
S. Mansouri et al. / Industrial Crops and Products 36 (2012) 22–27
27
3.4. Characterization of vine-stem-based papers
References
Conventional paper handsheets were prepared from unrefined vine stem pulp and their physical properties were determined. The produced handsheets present quite acceptable values of tensile strength (breaking length: 3220 m and Young modulus: 2.58 MPa), of tear index (5.74 mN m2 g−1 ) and burst index (1.72 kPa m2 g−1 ). It is worth noting that, for the considered basis weight (69.5 g m−2 ), the value of the thickness is high. This may denote the presence of impurities, not well removed by the screening operation. Consequently, the bulk (2.4 cm3 g−1 ) is probably slightly overestimated, whereas the Young modulus is underestimated. In order to compare physical properties of vine stem based papers to those of papers made from various species, Table 4 presents data for handsheets of bleached chemical pulps obtained from hardwood, softwood, and some non-wood plants. Physical properties of papers made from unbleached pulps (like unbleached pulp of vine shoots) are also reported, by assuming that bleaching does not modify the mechanical properties in a great extent (see Table 4 and, for instance, the works of Enayati et al. (2009) for both unbleached and bleached pulps of Canola stalks and of Dutt et al. (2010a) for Hibiscus cannabinus pulps). All these data correspond to papers made from unrefined pulps. The Schopper Riegler degrees are also reported, and they show that most values are lower than 20 apart from the work of Enayati et al. (2009). From this table, it clearly appears that the burst index of vine stem based papers and, in a less extent, the tear index, are comparable or better than those of papers obtained from other non-wood plants or hardwood (Khristova et al., 2006; Mishra, 2010). Thus, properties of papers obtained from vine stem pulp are considerably greater than those of vine shoot based papers (Jiménez et al., 2006). Finally, only the papers made from softwood pulp exhibit higher mechanical properties for both tear and burst indexes. These good results may be related to the DP of the vine stem pulp, as previously discussed. In addition, the acceptable content in holocellulose (65%) of the pulp has certainly a positive impact on the mechanical properties of the sheets, by increasing the number of hydrogen bonds. These properties probably counteract the effect of the low aspect ratio of the vine stem fibres. Considering both these characteristics and the good drainability of the pulp when compared to that of other pulps (Table 4), vine stems can be then viewed as a good candidate for producing papers from agricultural residues.
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4. Conclusion The obtained data show that vine stems could be considered as a promising raw material for papermaking applications. Vine stems contain holocellulose in sufficient amount, which justifies their valorisation for cellulose derivatives, as a source of fibres for cellulose fibre-reinforced composites, or for papers and boards. If the pulping process is performed under proper conditions, the obtained pulp exhibits good properties in terms of degree of polymerization and Kappa number. Nevertheless, the presence of large quantities of lignin and extractives in the stems limits the pulping yield. Finally, morphological characterization shows that the aspect ratio of the vine stem fibres is low. This does not significantly impact the strength properties of the produced papers, which are comparable to those of papers prepared from other non-wood species. Acknowledgment The authors would like to express their sincere thanks to Professor Mohamed Naceur Belgacem, Director of LGP2, for his valuable advice and assistance.