Cynara cardunculus
L.: chemical composition and
soda-anthraquinone cooking
A. Antunes, E. Amaral, M.N. Belgacem *
Department of Science and Technology of Paper,Uni6ersity of Beira Interior,6200 Co6ilha˜, Portugal
Accepted 14 January 2000
Abstract
This paper presents results about the determination of chemical composition of a new annual plant (Cynara cardunculusL. or Cardoon in English) growing in Portugal at experimental scale. Two raw materials were studied. The first one concerns over mature crops collected in 1996, whereas the second one was collected at the right time in 1997. The Klason lignin content of the first raw material was found to be relatively high (about 30%), whereas that corresponding to mature material was found to be around 20%. This result clearly showed the importance of the time of collection of this annual plant. The polysaccharides content, for both raw materials, was found to be as high as 60% of which 30% was hemicelluloses, the rest, i.e. 70% being cellulose. The quantity of water and ethanol/toluene extractives was also very high, i.e. around 10 and 5%, respectively. The conditions of soda-anthraquinone pulping were varied in order to establish the optimal condition of cooking this material. The criteria used were the obtention of good yield with lowkand an acceptable pulp viscosity. Thus, the following conditions were retained, alkali active
of 20 and anthraquinone of 0.25%, with respect to oven dried material, the time of cooking was 90 min of heating up to 160°C and maintaining this temperature during 120 min. The pulps obtained from both the original materials had a yield of about 35%, aknumber of 17 and a viscosity of about 900 cm3/g. For mature crops it was found that
only 30 min of cooking (after 90 min of heating up) gave pulps with properties as good as those corresponding to pulps from over mature material cooked during 2 h. © 2000 Elsevier Science B.V. All rights reserved.
Keywords:Cynara cardunculusL.; Chemical composition; Soda-anthraquinone pulping;kNumber; Viscosity
www.elsevier.com/locate/indcrop
1. Introduction
Paper consumption is continuously increasing across the world in general and in the West European and North American countries in
par-ticular (Foster, 1998). In fact, in our countries the average consumption of the paper (all grades included) reached 150 kg per capita and per year (Roberts, 1996). This tendency is drastically en-hanced in the US and Canada, since in both these lands, people are consuming the double as that of their European counterparts. Some years ago, the technologies dealing with computer and informa-tion sciences increased very fast, which yielded * Corresponding author. Tel.:+351-75-319792; fax:+
351-75-319740.
E-mail address:[email protected] (M.N. Belgacem)
some prediction about the decrease of paper needs. Today, all the statistical data related to paper consumption since these technologies were introduced, show that the previous predictions were totally wrong (Foster, 1998).
The fact that the need of pulp fibres for pa-permaking is increasing could be enhanced by finding new areas of application of this vegetal fibre, such as composite materials. In fact, dur-ing the last decade a series of studies about the possibility of using cellulose into thermoplastic matrices was published (Felix and Gatenholm, 1991; Felix et al., 1994; Kim et al., 1992; Belgacem et al., 1994; Simonsen et al., 1998). The main idea of these studies resides on the fact that the substitution of classical fibres, e.g. glass fibres, could give materials easy to recycle in term of energy recovering and with a certain economical viability, since cellulose fibres are cheap, renewable, have comparable mechanical properties and do not give any residue when burned.
These tendencies push researchers in the area of pulp and papermaking to find new sources of fibres. In the literature, three alternatives dealing with these aspects are proposed, namely: (i) the modification of existing processes in order to in-crease the yield of pulps keeping the same level of properties; (ii) the using of secondary fibres through recycling and deinking of old papers; and (iii) looking for new fast growing plants or using of agricultural crops.
Portugal has the same preoccupations and Portuguese researchers are concentrating their efforts in the same direction. During the last years, the main topics of our team of re-searchers are devoted to two vegetal species, Eu -calyptus globulus and Pinus pinaster, which are the unique sources of cellulose fibres in our country. Nevertheless, recently, our laboratory has been involved in a new way of getting cellu-lose from a typical Portuguese annual plant.
In fact, the present paper reports results about the characterisation and fibres isolation from Cardoon (Cynara cardunculus L.). The first part of this report will be devoted to the chemi-cal composition of this plant, after which the alkaline soda anthraquinone conditions of
pulp-ing will be optimised and discussed. The choice of this process of cooking is motivated by the fact that it is known to be suitable for annual plants (Blain, 1993; Nezamoleslami et al., 1998). To the best of our knowledge, no report avail-able was devoted to the chemical composition and pulping of this vegetal species.
2. Experimental
The C. cardunculus L. crops were cultivated in an experimental soil at the Instituto Superior de Agronomia in Lisbon. Two kinds of crops were studied. The first was over mature material of the 1996th year, whereas the second one corre-sponded to mature crops of the year after. In order to establish the chemical composition of these materials, crops were ground and sieved and the 60-mesh fraction was selected. The com-ponents quantified were Klason lignin, toluene – ethanol and water extractives, holocelluloses using peracetic and chlorite methods, Kurschner – Hoffner cellulose and ashes. The content of each component of the crops studied was determined following commonly used stan-dards.
Pulping procedures was carried out using three sizes of reactors, namely: (i) mini-digesters with 125 ml of capacity; (ii) medium size reac-tors having a volume of 500 ml; and (iii) larger size reactors with a volume of 10 ml. The quan-tities of O.D. crops were 10, 50 and 600 g for mini-digesters, medium size and large size reac-tors, respectively. The liquor to solid ratio was 6:1 and was kept constant for all cooking exper-iments, except where specified differently. The temperature of cooking was 160°C. The time of cooking as well as the quantities of an-thraquinone were optimised.
The pulps thus obtained were characterised in terms of yield, k number and viscosity using common standards. The degree of polymerisa-tion was calculated from viscosity data using the following equation, according to standard TAPPI 206m-55:
3. Results and discussions
3.1. Chemical composition
As seen from Table 1, which presents the con-tents of lignin, extractives and ashes for both crops, different concluding remarks could be drawn. First, the lignin of over mature material is quite different from that corresponding to the crops collected at the right time. This amount of lignin is considerably high for annual plant and it is close to softwood species. This result clearly indicates the importance of the time of collection of such vegetal material. Indeed, the collection of C.cardunculusL. at the mature age gave a reason-able amount of lignin, i.e. about 20%. Then, the
quantity of toluene – ethanol and water extractives for both materials was also relatively high to compare with annual plants, since it reached about 15 and 18%, for over mature and mature crops, respectively. This result could be consid-ered as a negative finding for our purpose which is devoted to the production of pulps with the highest yield possible. However, we believe that the detailed characterisation and quantification of the constituents of these extractives, could open a rational way of valorising them, e.g. in the area of surfactants, pharmaceutics etc. These aspects are presently under investigation in our laboratory and they will be reported shortly. The other parameter, which seemed to be of high concern was the ash content which reached about 5 and 8% for over mature and mature crops, respec-tively. These data are in the range of ash contents of annual plant which vary from about 3% for alpha (Stipa tenecissima) (Belgacem et al., 1986) and 14% for banana crops (Musa accuminata Colla) (Noronha et al., 1999).
The carbohydrate complex was also quantified using different methods, namely, peracetic and chlorite techniques. The holocelluloses thus ob-tained were submitted to Kurschner – Hoffner ap-proach, in order to determine their cellulose content. On the other hand, the latter approach was applied to the original crops. As shown in Table 2 which summarises these data, the reaction conditions were also optimised, in order to have polysaccharides with the highest purity possible. From these data the main information, which can be deduced is the fact that the holocelluloses content, as determined by both methods and for both original crops was higher than 60% with respect to O.D. material. For our purpose, this finding is very promising, since regardless of the high content of extractives and ashes, the quantity of carbohydrates was high enough to envisage the use of these crops as a source of cellulose fibres and to justify the optimisation of the cooking process. The careful examination of these data, show that the peracetic method of holocellulose determination is more suitable for our material, since it gives carbohydrate residue with lower lignin content. Moreover, it occurred more rapidly and it allowed working with toxic-free chemicals.
Table 1
Ashes, lignin and extractives contents of over mature 1996 and mature 1997 materials fromC.cardunculusL.
Raw material
Ethanol/toluene 4.7 5.0
13.0 10.0
Water
Table 2
Holocellulose content of two raw materials ofC.cardunculus L.
Soluble lignin 4.3
-Peracetic acid method 64.0 63.4 3.6
Klason lignin 3.2
Table 3
Cellulose content of two raw materials ofC.cardunculusL., as obtained directly from original non-woody material and from holocelluloses, as prepared by two techniques (chlorite and peracetic)
Cellulose 1996 1997
From holocellulose as obtained by Chlorite technique
From raw material 43 38
3.8 0.6
Klason lignin
Soluble lignin 0.3 0.3
the chemical composition of 1996 and 1997 raw materials, the sum of all components was 103 and 96% for both materials, respectively, which is within experimental error.
3.2. Pulping
The crops collected in 1996 (over mature) were studied first. They were cooked using different amounts of sodium hydroxide and anthraquinone using medium size reactors. Table 4 summarises the pulping conditions used and some characteris-tics of the pulps obtained. The first observation concerns cooking in the absence of an-thraquinone, which yielded a high amount of uncooked material and consequently a low pulp yield (see Table 4, experiment 1). The second remark which is worth mentioning consists of the fact that the increase of soda concentration did not affect the yield of pulps and uncooked mate-rial, since the increase of the NaOH concentration from 18 to 23 gave the same result. Then, the quantity of added anthraquinone gave a substan-tial decrease of the k number and that of un-cooked material (see Table 4, experiments 3a – c). The k number of the pulps reached a value of about 17, which is quite low and very acceptable (see Table 4, pulps 2b and 3c). Then, the overall yield of pulps is also very promising since it reached a value around 36%. In spite of the fact that this value is lower than those obtained form wood species, it can be considered to be very promising in our context, if one takes into ac-count the fact that original material contains up to 15% of water extractives and ashes. The pulps thus obtained possessed an acceptable viscosity, i.e. in the range 900 – 1000 cm3/g. The mechanical properties of the obtained papers are presently under investigation in our laboratory and will be the topic of the next paper in this area.
From these data the following optimal condi-tions were selected, alkali active, 20%; and an-thraquinone concentrations, 0.25% with respect to O.D. material; cooking temperature and time, 90 min to reach 160°C and 2 h at this temperature. These conditions allowed to have pulps with a yield of about 36%,knumber of about 17, and a viscosity of 850 cm3/g (Table 4).
Fig. 1. Chemical composition of the raw materials.
Table 4
Optimisation of cooking conditions ofC.cardunculusL., collected in 1996, using medium-size reactors
NaOH (%) Anthraquinone Yield Uncooked (%) kNumber Viscosity (cm3/g) Degree of polymerisation
(%)
3c 17.0 940 1517
0.25 35 –
20.0 17.2
4a 910 1465
20.0
5a 0.25 35 – 16.9 930 1500
aExperiments carried out using the large-scale reactor (10 l capacity). The data given are the average of three experiments. Experiment 4 concerns the material 1996, whereas trial 5 deals with 1997 one.
The time of maintaining the isotherm level was also studied using mini-digestors, as shown in Table 5. As expected, increasing the time of cook-ing decreased the k number of the pulps, but at the same time induced polysaccharide degradation as witnessed by the decrease of viscosity and consequently decreasing the degree of polymerisa-tion. This indicates that cooking with shorter time may offer pulps with better quality, if one accepts a slightly higherk number.
Then the mature material collected in 1997 was submitted to cooking, but taking into account the fact that these crops contained much less lignin. Thus, cooking processes were conducted with the same conditions except for the fact that the time of cooking was varied from 15 min to 2 h (see Table 5). To avoid extensive consumption of the raw material, delignification was carried out using mini-digesters, which gave pulps very suitable to investigate in terms of k number and viscosity. Unfortunately, since the initial quantity of raw material was too small (about 10 g O.D. material) and since washing of pulps could induce loss of fibres and fines, the yield of these pulps was not quantified. This parameter is left to be quantified when medium- and large-size reactors are used.
As can be seen from Table 5, which reports the k number and the viscosity of the pulps as a function of delignification time using mini-di-gestors, the increase of the reaction time lowered theknumber of the pulps obtained. The viscosity of the pulps remained in the range 900 – 1000
cm3
/g. It is worth noting that short pulping times gave pulps with an acceptable k numbers, e.g. about 20 for delignification time of 45 min. The viscosity of the pulps thus obtained showed a surprising behaviour since it increased from about 900 cm3/g to reach a relatively constant value of around 1000 cm3/g.
In order to confirm our results and to be able to calculate accurately the yield of pulping, a series of delignification reactions, using medium-size reactors, were carried out in triplicate. The results obtained are presented in Table 6, which shows that, for the raw material 1997, the yield of the pulps obtained by short time cooking (30 min) was found to be about 37% and the pulps
ob-Table 5
Kinetic of delignification of C. cardunculus L. collected in 1997, using mini-digestersa
kNumber Degree of
Time (min) Viscosity
(cm3/g) polymerisation
29.1 905
60 16.4 1018 1657
15.8
120 984 1596
Table 6
Kinetic of delignification of both raw material fromC.cardunculusL., using medium-size reactorsa
Time (min) Yield (%) kNumber Viscosity (cm3/g) Degree of polymerisation
1996
30 33 24.3 1018 1656
34
60 20.2 950 1535
16.7 932 1504
33 120
1997
30 37 17.0 958 1550
16.6 931
35 1504
60
120 33 15.1 833 1328
aReaction conditions are the same as in Table 5.
tained had a k number of 17, which is from our point of view an excellent result (Table 6).
The pulps prepared from mature 1997 crops showed systematically lower k numbers than those obtained from over mature material, as illustrated in Fig. 2. This finding is quite under-standable, since the original materials contained different amounts of lignin, as mentioned in the chemical composition section. If one compares the viscosity of both material, the same trend was observed, except for pulping during 2 h (Fig. 3). Nevertheless, the viscosity of these pulps re-mained within an acceptable range.
The last part of this study deals with cooking of both raw materials using a large-size reactor, i.e. starting from few hundreds of grams. The experi-ments were also conducted in triplicate and the results obtained showed an excellent agreement with those obtained from medium-size digesters. In fact, a pulp yield of about 35% was obtained for both materials. Theknumbers were 17 and 15 for over-mature and mature materials, respec-tively, whereas the viscosity was found about 900 cm3
/g, for both pulps (see Table 4).
4. Conclusions
Before starting this program of research, our main preoccupation was, whether C. cardunculus L. is a suitable source of cellulose fibres or not. Today we are very happy to be able to answer this
Fig. 2. Comparative features of theknumber of pulps, from
both over mature 1996 and mature 1997 materials, as a function of time of cooking using medium-size reactors.
question positively. The suitability of these fibres in the papermaking and boards area is currently ongoing in our laboratory and will be reported very soon.
Acknowledgements
The authors wish to thank PRAXIS XXI, Por-tugal research program (3/3.2/PAPEL/2311/95) who supported this work, as well as, the BIC scholarship (BIC/4650) conceded for Antunes An-abela.
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