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Industrial Crops and Products 34 (2011) 1572– 1582

Contents lists available at ScienceDirect

Industrial Crops and Products

jo ur nal homep age: www.elsev ier .com/ locate / indcrop

ulping and papermaking properties of Tunisian Alfa stems (Stipaenacissima)—Effects of refining process

. Marrakchia,b, R. Khiari a,b, H. Oueslati c, E. Maureta,∗, F. Mhennib

Laboratory of Pulp and Paper Science (LGP2), UMR CNRS 5518, Grenoble INP-Pagora - 461, rue de la papeterie, 38402 Saint-Martin-d’Hères, FranceResearch Unity of Applied Chemistry and Environment, FSM-Monastir, TunisiaResearch Unity of Materials Environment and Energy, FSG-Gafsa, Tunisia

r t i c l e i n f o

rticle history:eceived 12 May 2011eceived in revised form 21 May 2011ccepted 25 May 2011vailable online 25 June 2011

eywords:lfatipa tenacissimahemical composition

a b s t r a c t

The objective of this work is to study characteristics of chemical pulps (soda cooking process) and ofpaper obtained from Alfa, also known as Stipa tenacissima. For this purpose, Tunisian Alfa stems, andboth unbleached and bleached pulps were characterized by determining their chemical compositionas well as their morphological and physical properties. Through a detailed comparison with the otherpulps obtained from various species, we show that the properties of Alfa stem fibres are intermediatebetween those of non-wood and wood plants, and most often close to those of Eucalyptus fibres. Refiningprocess (PFI mill device) was then applied to the unbleached and bleached Alfa pulps. The modificationsof the morphological properties of the fibres and the drainability and water retention values of the pulpswere studied as a function of the refining degree. Here again, Alfa fibres exhibit a behaviour similar

efiningulp propertiesaper properties

to that of Eucalyptus fibres, as the fibre shortening is very limited during the refining process. Finally,conventional handsheets with a basis weight of 65 g/m2 were prepared from the unrefined and refinedpulps. Their characterization showed that Alfa based papers present low density values and quite goodmechanical properties, which are significantly enhanced by the refining treatment, particularly for theunbleached pulp. This study demonstrates the high potentiality of this non-wood species for papermakingapplications.

. Introduction

The increasing consumption of fibre-based products reliesainly on the forest exploitation for the production of wood

ulps as well as on the recycling of fibres. In European coun-ries, for instance, more than 50% of the lignocellulosic fibressed are secondary fibres. Nevertheless, and although the use ofon-wood fibres remains relatively limited in the world (around0%), annual or perennial plants and agricultural residues consti-ute the major local source of cellulosic fibres in certain countriesr regions like North-Africa, where forestry resources are notvailable or are limited. Increasing the use of these non-woodbres by rationalizing their valorization could allow preserving the

orestry resources, limiting the dependence of such countries inerms of pulps and fibres and bringing new properties to fibre-

ased products considering the specific properties of non-woodbres.

∗ Corresponding author. Tel.: +33 4 76 82 69 17; fax: +33 4 76 82 69 33.E-mail addresses: evelyne.mauret@pagora.grenoble-inp.fr,

velyne.mauret@grenoble-inp.fr (E. Mauret).

926-6690/$ – see front matter © 2011 Elsevier B.V. All rights reserved.oi:10.1016/j.indcrop.2011.05.022

© 2011 Elsevier B.V. All rights reserved.

Alfa, also known as Stipa tenacissima, is a common grass of NorthAfrica, which covers approximately a surface of 3 million hectaresin Algeria (Akchiche and Messaoud-Boureghda, 2007) and morethan 400 thousand hectares in Tunisia, mainly located in Kasser-ine, Sidi Bouzid, Gafsa and Kairouan region. Alfa is considered as oneof the most interesting annual plants for the production of fibresfor papermaking and it has already been used in such applicationsfor four decades (Nadji et al., 2006). For instance, in Tunisia, theproduction of Alfa pulp exceeds 30,000 tons per year (Nadji et al.,2006). However, and to the best of our knowledge, very few dataabout the properties of pulps and papers prepared from this rawmaterial are available in the literature. In particular, no exhaustivestudy provides a detailed description of the effects of refining on theproperties of Alfa pulps and papers. Only Bouiri and Amrani (2010),who evaluated the effect of the bleaching process on the proper-ties of Algerian Alfa based papers, studied partially the impact ofrefining. Nevertheless, few mechanical properties are reported anddiscussed (breaking length, tear and burst indexes). That is the rea-son why we have decided to undertake a systematic work dealing

with the effects of refining on the properties of pulps and papersobtained from Tunisian Alfa. For this purpose, the chemical com-position of Tunisian Alfa stems and that of the obtained soda pulpswere assessed according to standard methods. Then, the impact

ps and

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Z. Marrakchi et al. / Industrial Cro

f refining on the morphology of the Alfa fibres (unbleached andleached) and on their swelling behaviour was studied. Finally, thehysical properties of conventional handsheets with a basis weightf 65 g/cm2 prepared from pulps refined at different levels wereetermined. All these results are discussed and compared in thensuing sections with data available in the literature for non-woodnd wood plants.

. Materials and methods

.1. Raw material

Alfa stems used in this work were provided by a Tunisian pulp-ng factory located in the region of Kasserine – Tunisia. They wereollected in October 2008, dried, and after removing dust, cut into–8 cm long pieces before pulping.

.2. Alfa pulping and bleaching

Unbleached (UBAP) and bleached (BAP) Alfa pulps used inhis work were manufactured by the same Tunisian mill. Thenbleached pulp was obtained by cooking the raw material at

temperature of 160 ◦C and a pressure of 0.75 MPa, in a NaOHqueous solution at 72 g L−1. The total cooking time was about3 min. The UBAP was then washed several times until attaining aeutral pH. For the production of BAP, a three stage bleaching pro-ess, namely: (i) chlorination, (ii) alkali extraction, and (iii) calciumypochlorite treatment, was applied to the UBAP. For confiden-ial reasons, the experimental conditions of pulping and bleachingere not detailed in this work.

.3. Refining process

The refining kinetics of UBAP and BAP were studied. Thus,ifferent samples were disintegrated in a laboratory pulper accord-

ng to ISO 5263-2:2002 standard protocol, before being refinedn a PFI mill (ISO 5264-2:2002). The selected revolution numbers

ere: 0 (unrefined pulp), 1500, 2500, 3500, 5000 and 7500 for thenbleached pulp and 0, 500, 1000, 1500 and 2500 for the bleachedne. All refining treatments employed here were repeated threeimes at least.

.4. Characterization of Alfa stems and pulps

In order to ensure reliable determination of the chemical com-osition of the Alfa stems and those of the unbleached and bleachedulps, Alfa stem pieces were washed first in order to eliminatehe residual sand and dust, rinsed with distilled water and thenir-dried. The obtained materials were ground and sieved to aranulometry ranging from 0.2 to 0.4 mm. The relative amounts ofxtractives were then evaluated in different liquids: cold and hotater (T207 cm-08), 1% sodium hydroxide solution (T212 om-07)

nd ethanol–toluene (T204 cm-07). Ash content and the amounts ofignin, �-cellulose and holocellulose were assessed by using the fol-owing methods: T211 om-07, T222 om-06, T203 cm-99 and Wiset al.’s method (1946), respectively. As recommended by the vari-us pertinent standards, all these experiments were duplicated andhe difference between the two values was within an experimentalrror of 5%.

The viscosity � (mPa s) of the pulps (UBAP and BAP) dissolvedn a cupriethylene-diamine solution was determined according toappi standard (T230 om-99). These values were then converted

nto the degrees of polymerization (DP) using the following relationroposed by Sihtola et al. (1963):

P = [0.75(954 Log10� − 325)]1.105 (1)

Products 34 (2011) 1572– 1582 1573

The pulps were also characterized in terms of their brightness,which was measured by using a spectrophotometer Color Touchaccording to the standard ISO 5351/1. Infrared spectra of UBAPand BAP were also obtained by using a bio-Rad spectrophotome-ter. 1 mg of each pulp was mixed with 100 mg of anhydrous KBrand was formed into a small pellet using a laboratory press. Eachsample was scanned 64 times in transmittance mode, at 4 cm−1

resolutions and the wave number ranged from 500 to 4000 cm−1.The X-ray photoelectron spectroscopy (XPS) experiments were per-formed with a XR3E2 apparatus (Vacuum Generators, UK) equippedwith monochromatic MgK� X-ray source (1253.6 eV) and operat-ing at 15 kV under a current of 20 mA. Samples were placed in anultrahigh vacuum chamber (10−8 mbar) with electron collectionby a hemispherical analyzer at an angle of 90◦. Signal decomposi-tion was done using Spectrum NT, and the C1s signal was shiftedto ensure that the C–H signal of the decomposition occurred at285.0 eV. These measurements were carried out at least in tripli-cate and the difference between the resulting values was within anexperimental error of 5%.

Alfa stems were observed with a Quanta 200 environmentalscanning electron microscope (FEI Company, USA) under vacuum,at an operating voltage of 10 kV. The dried samples were previouslygold coated by sputtering for 15 s. UBAP and BAP were then qual-itatively characterized in terms of their morphological propertiesby using an optical microscope (ZEISS Image M1) equipped with aCCD camera and an image analysis software. A MorFi (LB-01) ana-lyzer developed by Techpap – France (Passas et al., 2004) was alsoused. It allows the determination of the main fibre parameters byimage analysis of a diluted pulp suspension: the average weightedlength, the average width and the proportion of fine elements (% inlength) were measured.

Water retention values (WRV) were assessed by centrifugingwet pulp samples for 15 min at 3000 × g (Silvy et al., 1968). Beforeand after drying, the samples were weighed and WRV were calcu-lated by using the following equation:

WRV(%) = 100 ×[

M1 − M2

M2

](2)

where M1 and M2 are the weights of the sample after centrifuga-tion and after drying, respectively. The drying step was performedat 105 ◦C for, at least, 24 h. The pulp drainability was determinedby measuring the Schopper Riegler degree (◦SR) according to thestandard method ISO 5267-1. All these measurements were repli-cated 3 times, except for the WRV, which was measured six timesfor each pulp. The difference between the values did not exceed anexperimental error of 5%.

2.5. Preparation and characterization of Alfa based papers

The previously refined pulps were then used to prepare paperhandsheets with a basis weight of 65 g/m2, using a standard sheetformer (Frank apparatus according to the Rapid-Köthen method –ISO Standard 5269). Prior to testing, the handsheets were condi-tioned for 48 h, at 23 ◦C and 50% of relative humidity (ISO 187).The effect of refining on the paper structure was observed by usingscanning electron microscopy. Then, the main physical characteris-tics of the obtained handsheets were determined. The basis weight(NF Q 03-019), the thickness (NF Q 03-016) and the air permeabil-ity (NF Q 03-075) were measured as well as the tensile properties(breaking length, Young modulus and elongation (NF Q 03-002)),the burst (NF Q 03-053) and tear indexes (NF Q 03-011) and the

zero-span breaking length (dry and wet). As recommended by thevarious standards used, all the measurements were replicated 10times for each series, apart from thickness which was repeated 30times.

1574 Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582

Table 1Chemical composition of Alfa stems – comparison with data collected from previously published studies (%, w/w, o.d. materials).

Reference CW HW 1% NaOH Ash Ext Holo K lignin �-cell

Tunisian Alfa stems

This work 9.1 11.1 19.4 3.7 7.9 68.2 22.3 46.1Ben Brahim and Ben Cheikh (2007) n/a n/a n/a 2 n/a 69 24.0 45.0b

Paiva et al. (2007) n/a n/a n/a 2 n/a n/a 23.0 45.0b

Nadji et al. (2006) n/a 11.0 n/a 2.4 7.7 72.3 19.2 44.2b

Algerian Alfa stems

Houacine (1983) n/a 5.0 n/a 4.0 4.1a n/a 18.5 43.4b

Nadji et al. (2006) n/a 4.9 n/a 2.5 6.0 n/a 24.3 44.1b

Bouiri and Amrani (2010) n/a n/a n/a 5.1 n/a 69.8 17.7 47.6b

Akchiche and Messaoud-Boureghda (2007) n/a 5.1 n/a 4.6 n/a n/a 18.7 43.8b

CW: cold water extractives, HW: hot water extractives, 1% NaOH: 1% NaOH extractives, Ext: ethanol–toluene extractives, Holo: holocellulose, K lignin: Klason lignin, and�-cell: �-cellulose.n

3

3

3

stihiharbdc

lrl

T

TC

H

/a: non available.a In alcohol–benzene.b Cellulose.

. Results and discussion

.1. Characterization of Alfa stems

.1.1. Chemical characterizationThe chemical composition of the ground Tunisian Alfa stems

tudied in this work is summarized in Table 1, which also includeshe corresponding data from previous studies, for sake of compar-son. From our results, it appears that Tunisian Alfa stems containigh amounts of extractives (more than 10% in hot water, for

nstance) and lignin (the Klason lignin is about 22%). The contents inolocellulose and �-cellulose (68% and 46%, respectively) are quitecceptable for papermaking applications. Moreover, the presentesults on chemical composition of Tunisian Alfa stems seem toe in line with the other studies available. There is no significantifference between Algerian and Tunisian Alfa stems in terms ofhemical composition.

Table 2 presents the chemical compositions for various cel-ulosic biomasses such as hardwood and softwood, agricultural

esidues and other various non-wood sources collected from theiterature.

As it can be seen from Table 2, the extractive amounts of theunisian Alfa stems studied in this work, in cold water, hot water

able 2hemical composition of Alfa stems – comparison with some other lignocellulosic plants

Reference

Wood

Pinus pinasterd Jimenez et al. (2008)

Acer rubruma Chow et al. (2008)

Eucalyptus globulusd Jimenez et al. (2008)

Eucalyptus grandisd Baeza et al. (1991)

Non-wood

Date palm rachis Khiari et al. (2010a, 2011)

Hesperaloe funiferad Sanchez et al. (2011)

Luecaena diversifoliad Jimenez et al. (2008)Leucaena colinsid Jimenez et al. (2008)

Arundo donaxd Jimenez et al. (2008)

Prosopis albad Jimenez et al. (2008)

Annual and perennialplants

Parthenium argentatuma,b Chow et al. (2008)

Asclepias syriaca L.a Chow et al. (2008)

Parthenium tomentosuma Chow et al. (2008)

Kenaf (Hibiscus cannabinus)c Jonoobi et al. (2009)

Wheat strawd Jimenez et al. (2008)

Canola straw Hosseinpour et al. (2010)

Cotton stalksd Jimenez et al. (2008)

Luffa cylindrica Siqueira et al. (2010)Posidonia oceanica Khiari et al. (2010a)

Alfa stems (Stipa tenacissima) This work

em: hemicellulose.a Results related to the woody parts of the stems.b Average values for three germplasm lines of mature guayule.c In alcohol–acetone.d In alcohol–benzene.e Cellulose.

and in ethanol–toluene are similar to those encountered in the caseof non-wood and annual or perennial plants. As expected, thesecontents are higher than those found in wood (hardwood and soft-wood). The same tendency is also observed for their ash contents.Considering now the extractives in 1% NaOH, the holocellulose,the �-cellulose and hemicelluloses as well as the Klason lignin, itappears that Alfa stems show a chemical composition close to thatof hardwood or intermediate between hardwood and non-woodspecies.

3.1.2. Morphological characterizationAlfa stems were observed with a scanning electron microscope.

Longitudinal views of one stem (outside and inside: 1(a) and 1(b))as well as cross sections (1(c) and 1(d)) at different magnificationsare shown in Fig. 1.

From the SEM images, it may be observed that Alfa stems havea circular section shape with small pores (Fig. 1(c)). Fibres arealso characterized by a circular section and are bound togetherwith lignin (Fig. 1(d)). The inside longitudinal view of the stem

(Fig. 1(b)) shows a lot of small elements placed in a regular wayand surrounding the fibre bundles. These elements correspond tothe “comma-shaped” cells found abundantly in the Alfa pulps, as itwill be discussed hereinafter.

(%, w/w, o.d. materials).

CW HW 1% NaOH Ash Ext Holo K lignin �-cell Hem

n/a 2.0 8.0 0.5 2.6 69.6 26.2 55.9 13.7n/a 7.5 17.1 1.0 4.6 67.4 26.0 37.7 29.7n/a 2.8 12.4 0.6 1.2 80.5 20.0 52.8 27.7n/a n/a n/a n/a 5.8 75.2 24.8 53.1e n/a

5.0 8.1 20.8 5.0 6.3 74.8 27.2 45.0 29.8n/a 13.5 29.5 5.9 4.0 76.5 7.3 40.9 35.6n/a 3.2 17.4 n/a 4.4 77.9 19.1 40.1 37.8n/a 4.3 20.0 n/a 4.6 80.8 17.0 43.8 37n/a 4.7 26.8 n/a 7.3 70.2 22.3 40.5 29.7n/a 4.7 20.9 n/a 4.6 63.6 19.3 41.6 22

n/a 9.6 30.6 2.0 5.9 80.2 20.8 46.6 33.6n/a 10.3 31.3 4.6 2.7 76.5 21.8 72.5 4n/a 6.5 19.3 1.7 3.2 82.9 18.6 50.6 32.3n/a n/a n/a 2.2 4.0 81.1 12.7 63.5e n/an/a 12.3 43.6 6.5 4.0 76.2 17.3 39.7 36.5n/a n/a n/a 6.6 n/a 77.5 20.0 36.6 40.9n/a 3.3 20.3 2.2 1.4 72.9 21.4 58.5 14.4n/a n/a n/a 0.7 3.1 83.0 15.2 65.5e n/a7.3 12.2 16.5 12.0 10.7 61.8 29.8 40.0 21.89.1 11.1 19.4 3.7 7.9 68.2 22.3 46.1 22.1

Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582 1575

nd in

3

3

bT

rsaNliatfibns

Fig. 1. SEM images of one Alfa stem: longitudinal views (outside (a) a

.2. Characterization of unrefined Alfa pulps (UBAP and BAP)

.2.1. Chemical and spectroscopic analysesThe chemical compositions of UBAP and BAP as well as their

rightness and degrees of polymerization are summarized inable 3, which also includes the prior literature data.

For the UBAP, it is readily seen that the content in extractivesemains relatively high when compared with the other pulps pre-ented in Table 3. Moreover, this pulp still contains a significantmount of lignin (as revealed by the Klason lignin determination).ow regarding the BAP, it clearly appears that the bleaching steps

ead to the removal of significant amount of extractives therebymproving its brightness. Nevertheless, the resulting pulp exhibits

low value of the degree of polymerization thus denoting a nega-ive effect of the bleaching process on the intrinsic strength of the

bres (Chirat et al., 2008). Finally, it is worth noting here that, foroth pulps, but especially for BAP, the sum of the constituents doesot reach 100%; this feature, frequently observed for non-woodpecies, reveals that the standard methods used are not perfectly

side (b)) and cross section views ((c)–(d)) at different magnifications.

designed for the characterization of non-wood or annual/perennialplants.

The infrared spectra for unbleached and bleached Alfa pulps aredisplayed in Fig. 2. Both spectra, which are quite close, exhibit trans-mittance profiles similar to that of cellulose. This observation is inperfect agreement with the chemical composition of the two pulps,thus confirming that they are rich in holocellulose and cellulose. Afurther analysis of the spectra shows some other common peaks.In fact, the wide strip at 3340 cm−1 and peaks between 1150 and950 cm−1 are attributed to OH functional group’s vibration (Khiariet al., 2010b). The band at 2920 cm−1 is characteristic of the elonga-tion of the methylene (–CH2–) groups. Finally, the band displayed ataround 900 cm−1 indicates the presence of glucosidic units (Khiariet al., 2010b) and the peak in wave number regions from 3000to 2842 cm−1 is assigned to C–H stretch in methyl and methylene

groups (Nadji et al., 2006).

The main noticeable difference between the two spectra isrelated to the carbonyl group bands at 1659 cm−1 (conjugatedgroups) and at 1737 cm−1 (non-conjugated groups). These peaks

1576 Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582

Tab

le

3C

hem

ical

com

pos

itio

n

of

un

blea

ched

and

blea

ched

Alf

a

pu

lps

(%, w

/w, o

.d. m

ater

ials

),

DP

and

brig

htn

ess

–

com

par

ison

wit

h

dat

a

coll

ecte

d

from

the

lite

ratu

re.

Pulp

ing

pro

cess

Ble

ach

ing

pro

cess

Ref

eren

ce

Ash

Ext

Hol

o

K

lign

in

�-c

ell

DP

Bri

ghtn

ess

Un

blea

ched

pu

lp

Tun

isia

n

Alf

a

(Sti

pate

naci

ssim

a)So

da

pro

cess

–

This

wor

k

2.8

5.0

75.3

8.1

67.5

850

47.3

Alf

a

(Sti

pate

naci

ssim

a)K

raft

pu

lpin

g–

Bou

iri a

nd

Am

ran

i (20

10)

1.0

n/a

73.8

2.7

68.4

n/a

23.5

Ken

af

(Hib

iscu

sca

nnab

inus

)So

da-

AQ

pro

cess

–

Jon

oobi

et

al. (

2009

)

n/a

0.9a

94.2

2.5

81.5

dn

/a

n/a

Dat

e

pal

m

rach

is

Sod

a-A

Q

pro

cess

b–

Kh

iari

et

al. (

2010

a)

4.0

1.8

69.3

5.2

n/a

1203

n/a

Ric

e

stra

wSo

da

pro

cess

c–

Gon

zale

z

et

al. (

2008

)n

/a3.

285

.017

.075

.013

62

n/a

Ble

ach

ed

pu

lpA

lfa

(Sti

pate

naci

ssim

a)So

da

pro

cess

Ch

lori

nat

ion

-alk

ali

extr

acti

on-c

alci

um

hyp

och

lori

te

This

wor

k2.

02.

2

80.8

n/a

76.9

694

83

Ken

af

(Hib

iscu

sca

nnab

inus

)So

da-

AQ

pro

cess

Sod

ium

chlo

rite

-alk

ali

extr

acti

on-S

odiu

mch

lori

te

Jon

oobi

et

al. (

2009

)

n/a

0.5a

97.2

0.5

92.0

dn

/a

n/a

aIn

alco

hol

–ace

ton

e.b

Coo

kin

g

tem

per

atu

re

160

◦ C.

cIn

alco

hol

–ben

zen

e.d

Cel

lulo

se.

Fig. 2. FTIR spectra of unbleached and bleached Alfa pulps (transmittance versuswave number).

Table 4Atomic composition of unbleached and bleached Alfa pulp surfaces.

Samples C (%) O (%) O/C

UBAP 74.16 ± 0.73 23.64 ± 0.46 0.32BAP 67.38 ± 0.48 32.15 ± 0.27 0.48

appear clearly in the case of BAP, and they can be attributed toa cellulose side chain oxidation and opening ring reactions (Nadjiet al., 2006) resulting from the bleaching process. This result con-firms the strong effect of the bleaching as denoted by the significantdecrease of the degree of polymerization. FTIR technique does notallow detecting peaks associated with hemicelluloses, lignin andextractives, since their associated bands are mostly overlapped bythose of cellulose and/or because these components are present invery low amounts.

In order to investigate the chemical composition of the surfaceof UBAP and BAP, XPS analysis was carried out. As expected, theXPS low-resolution spectra of the pulps (not shown here) presenttwo typical peaks: oxygen peak at around 532 eV and carbon peakat 285 eV, respectively. The details concerning the atomic compo-sition of the surface of the fibres are summarized in Table 4 for thetwo pulps.

From these results, it appears that the O/C ratios are 0.32 and0.48 for UBAP and BAP, respectively. These values are much lowerthan that of pure cellulose, which is close to 0.8 (Gaiolas et al.,2009) and than those of other lignocellulosic substrates (Belgacemand Gandini, 2005; Gandini and Belgacem, 2011). The significantchange in the O/C ratio between UBAP and BAP clearly indicatesthat the bleaching treatment introduces an important modificationof the atomic composition of the surface. The C1s deconvolutionXPS spectra of the UPAB and BAP pulps, shown in Fig. 3(a and b)and summarized in Table 5, display similar profiles.

The main difference between them is the decrease of the C1peak from 55 to 32% and the increase of O/C atomic ratio after the

bleaching process. These intensity evolutions are strongly linkedwith the removal of impurities and lignin. This result corroboratesthe previous data already discussed in our analysis of the chemical

Table 5C1s deconvolution XPS data of unbleached and bleached Alfa pulp surfaces.

Samples C–C (C1) C–O (C2) O–C–O (C3)

UBAP 54.85 ± 0.55 34.66 ± 0.35 10.50 ± 0.21BAP 32.44 ± 0.25 53.40 ± 0.36 14.17 ± 0.13Binding energy (eV) 285 286.8 288.3

Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582 1577

F

cmaasna2

3

fiamip

lbtatpitFfieosclfi

ig. 3. C1s deconvolution XPS spectra of unbleached and bleached Alfa pulps.

omposition of the UBAP and BAP. In fact, the bleaching treat-ent reduces the amounts of extractives in several liquids (cold

nd hot water, 1% sodium hydroxide solution and ethanol–toluene)nd the amount of lignin which implies that many surfactant typetructures (resinic acids, for instance) and phenolic fragments (tan-ins and lignin) were removed during this treatment (Belgacemnd Gandini, 2005; Gandini and Belgacem, 2011; Gustafsson et al.,003).

.2.2. Morphological characterization, WRV and drainabilityTable 6 collects the main morphological parameters of Alfa

bres from UBAP and BAP namely: (i) average weighted length,verage width, and content in fine elements (% in length). Sucheasurements are essential (Passas et al., 2004), since they directly

mpact the properties of the pulps and the quality of the producedapers.

From this table, it can be deduced that fibres from UBAP areonger and thicker than those from BAP. This result can be explainedy the damaging effect of the bleaching steps. Table 6 also reportshe characteristics of some other plants such as Algerian Alfa, woodnd non-wood plants. The comparison of the different data showshat Tunisian and Algerian bleached Alfa fibres exhibit similar mor-hological properties. Moreover, when compared to wood species,

t appears that the length and the width of Alfa fibres are similaro those of common hardwood like Eucalyptus and Aspen fibres.inally, it is worth noting here that Alfa pulps contain high levels ofnes. This high content may be related to the presence of additionallements (Aitken et al., 1988), as illustrated by Fig. 4 where we canbserve epidermal cells taking the shape of comb and hairs (Cb),

clerous cells (S), parenchymatous cells (unusually present) andomma-shaped cells (C). On these micrographs, Alfa fibres appearike regular cylinders with thin ends, which resemble to phloembres of hardwood.

Fig. 4. Optical images of the Alfa pulps (a: unbleached; b: bleached).

UBAP and BAP were also characterized in terms of their drain-ability (Schopper Riegler degree) and water retention values(WRV). Schopper Riegler degrees (◦SR) are 16 and 23 for UBAPand BAP, respectively. These values are similar to those obtainedfor hardwood pulps and lower than those generally associated tonon-wood sources and plants like Kenaf (Fuwape, 1993), Posidoniaoceanica and date palm rachis (Khiari et al., 2010a), for instance.

It is worth noting that the drainability of unrefined UBAP is thusquite good especially when considering the high content in fineelements. The bleaching steps induce a decrease of the drainability(that is an increase of the Schopper Riegler degree), which can berelated to the modification of the morphological properties of thepulp.

The water retention values (WRV) of the two pulps were foundto be 91% for UBAP and 95% for BAP. Here again, these valuesare similar to those of unrefined pulps arising from softwood andhardwood (90–100%) and lower than those obtained for non-woodpulps. This trend can be probably explained by the relative lowcontent of hemicelluloses.

3.3. Effect of refining on properties of Alfa based pulps and papers

3.3.1. Effect of refining on pulp propertiesFig. 5 shows the evolution of the drainability (◦SR) and water

retention value (WRV) as a function of the PFI revolution number.

As expected, the drainage rate decreases with the increasing

degree of refining which also promotes the swelling of fibres. Thebleaching steps highly modify the refining kinetics. Thus, in orderto reach the same values of the Schopper Riegler degree and WRV,

1578 Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582

Table 6Main morphological properties of UBAP and BAP (average of three measurements).

Reference Fibre lengthc (mm) – l̄A Fibre lengthd (mm) – l̄W Fibre width (�m) Fine elements (% in length)

UBAPa This work 0.63 0.95 19.2 25.1BAPa This work 0.59 0.87 18.7 26.2BAPa Nadji et al. (2006) 0.48 0.84 20.1 n/aEucalyptus kraft pulpa Abrantes et al. (2007) 0.76 n/a 15.4 n/aOlive tree (organosolv pulp)a Mutjé et al. (2005) n/a 0.45 n/a 24.4Aspenb Ai and Tschirner (2010) 0.69 0.92 n/a 2.94Scandinavian Spruce Shackford (2003) 3.5 n/a 27 n/aDouglas Fir Shackford (2003) 4.0 n/a 44 n/aSlash Pine Shackford (2003) 2.3 n/a 36 n/aPosidonia oceanicaa Khiari et al. (2010a) n/a 0.55 21.3 7.5Date palm rachisa Khiari et al. (2010a) 0.69 0.89 22.3 30.8Switchgrassb Ai and Tschirner (2010) 0.42 0.78 n/a 15.3Alfalfab Ai and Tschirner (2010) 0.35 0.78 n/a 14.3

a Morfi apparatus.b FQA.c Arithmetic mean length.d Weighted mean length.

ned A

tc

Uaw

r

Fig. 5. Evolution of drainability and WRV for refi

he number of PFI revolutions for BAP is divided by about 2.5 whenompared to that of the UBAP.

Refining kinetics, illustrated in Fig. 6, were compared betweenBAP and some other pulps obtained from wood, non-wood and

nnual plants. Evidently, UBAP is easier to refine than some soft-ood pulps and also it has a similar behaviour as Eucalyptus pulp.

When examining the water retention values, it appears thatefining increases the ability of fibres to absorb water (swelling).

Fig. 6. Refining kinetics of pulps made from various lignocellulo

lfa pulps as a function of PFI revolution number.

The WRV of the unrefined pulp is relatively low (92%) for a non-wood species and, even if it regularly increases with refining, itsfinal value at 65◦SR reaches only 158%. Again, this behaviour issimilar to that of wood pulps and can be related to the content

in hemicelluloses of Alfa pulps.

Fibres’ morphology of unrefined and refined UBAP andBAP were also assessed and these results are listed inTable 7.

sic materials (drainability versus PFI revolution number).

Z. Marrakchi et al. / Industrial Crops and Products 34 (2011) 1572– 1582 1579

Table 7Effect of refining on the main morphological properties of the unbleached and bleached Alfa pulps (average of three experimental data).

PFI revolutions UBAP BAP

0 1500 2500 3500 5000 7500 0 500 1000 1500 2500

Fibre lengtha (mm) 0.95 0.94 0.93 0.94 0.92 0.9 0.87 0.83 0.79 0.77 0.75Fibre width (�m) 19.2 18.9 19.1 19.0 19.2 19.5 18.7 18.7 18.8 18.9 19.2Macrofibrils (% in length) 0.54 0.49 0.50 0.58 0.55 0.63 0.60 0.63 0.67 0.64 0.70Fine elements (% in length) 25.1 21.5 22 21.2 22.6 25.3 26.2 24 25.5 26.1 28.8

rmIbrf

a Weighted mean length.

It is interesting to note from the results shown in Table 7 thatefining process did not induce significant variations of the fibres’orphology, despite the relatively high final refining degrees.

n fact, the variation of the main parameters is negligible foroth pulps, especially for UBAP. The content in fine elementsemains roughly constant. As expected, the fibre width of the dif-erent pulps slightly increases with the refining level. This classical

Fig. 7. 1200× SEM images of unrefined (a–c) and refined (65◦SR) (

behaviour can be related to the swelling and/or fibrillation of thefibres.

3.3.2. Effect of refining on paper propertiesTable 8 reports the main physical properties of the handsheets

obtained from UBAP and BAP. From these data, it appears that theproduced papers exhibit quite good properties. As expected, the

b–d) Alfa handsheets (cross section (a–b) and surface (c–d)).

1580 Z. Marrakchi et al. / Industrial Crops an

Tab

le

8Ef

fect

of

refi

nin

g

on

the

mai

n

stru

ctu

ral a

nd

mec

han

ical

pro

per

ties

of

pap

ers

mad

e

from

UB

AP

and

BA

P

(ave

rage

valu

es

and

stan

dar

d

dev

iati

ons)

.

PFI r

evol

uti

ons

UB

AP

BA

P

015

0035

0050

0075

000

500

1000

1500

2500

Sch

opp

er

deg

ree

1622

3244

6523

3035

4258

Stru

ctu

ral p

rop

erti

es

Bas

is

wei

ght

(g/m

2)

64.7

9

±

0.03

63.0

5

±

0.03

62.9

8

±

0.06

64.1

3

±

0.02

64.2

2

±

0.03

63.3

5

±

0.04

64.8

8

±

0.03

62.1

7

±

0.06

65.0

2

±

0.03

64.3

7

±

0.02

Thic

knes

s

(�m

)

153

±

2.98

133

±

1.69

124

±

3.63

120

±

0.87

113

±

1.50

137

±

2.54

130

±

2.39

118

±

2.67

123

±

1.69

113

±

0.81

Bu

lk

(cm

3g−1

)2.

36

2.11

1.96

1.87

1.76

2.16

2.00

1.90

1.89

1.75

Perm

eabi

lty

(cm

3/s

Pa

m2)

859

±

11.4

6

541

±

16.8

4

181

±

7.16

64

±

0.89

28

±

1.83

395

±

4.50

247

±

8.14

182

± 10

.57

120

±

7.72

36

±

1.45

Mec

han

ical

pro

per

ties

Bre

akin

g

len

gth

(km

)3.

75

±

0.20

4.39

±

0.15

5.57

±

0.14

6.02

±

0.46

6.64

±

0.68

2.16

±

0.12

3.29

±

0.23

3.95

± 0.

15

4.35

±

0.17

5.14

±

0.26

Elon

gati

on

(%)

0.51

±

0.07

1.89

±

0.23

2.51

±

0.34

2.71

±

0.39

3.07

±

0.63

0.62

±

0.07

1.05

±

0.17

1.42

±

0.17

1.69

±

0.22

2.15

±

0.35

You

ng

mod

ulu

s

(GPa

)1.

82

±

0.12

2.83

±

0.07

3.42

±

0.15

3.70

±

0.17

4.11

±

0.20

2.28

±

0.13

2.94

±

0.07

3.23

±

0.16

3.36

±

0.13

3.93

±

0.19

Zero

-sp

an

brea

kin

gle

ngt

h

(dry

)

(km

)10

.72

12.1

0

11.6

2

11.4

7

12.3

1

6.99

8.84

8.86

8.89

8.61

Zero

-sp

an

brea

kin

gle

ngt

h

(wet

)

(km

)8.

87

9.71

10.5

6

10.3

7

10.7

9

5.92

5.87

6.75

6.91

7.13

d Products 34 (2011) 1572– 1582

bulk is a decreasing function of the refining degree. This indicatesa densification of the fibrous network resulting from an increase ofthe fibre–fibre bonding. This behaviour is also confirmed by SEMimages of the cross sections of two samples (Fig. 7(a) and (b)): thefirst one shows paper obtained from the unrefined pulp whereasthe other one shows paper prepared from the pulp refined at 65◦SR.

In the same way, the paper permeability values greatly decreasewhen the Schopper Riegler increases. Indeed, the permeabilitydepends on both the porosity and the specific surface area of thefibrous network. These properties are related to the fibrillation andthe swelling of the fibres as well as the content in fine elements andthe fibre shortening. As discussed above, swelling of fibres occurredduring the PFI refining even if WRV increased in a moderate wayand the fibrillation phenomenon is well demonstrated by the SEMimages (see Fig. 7(c) and (d)). We also showed that refining of UBAPand BAP did not induce significant fibre shortening and/or increaseof fine element content. Thus, one can postulate that the increasein the paper density mainly results from the fibrillation and theswelling of the fibres whose conformability is improved. By con-sidering the high value of the Schopper Riegler degree for the mostrefined pulps (UBAP and BAP), it appears that Alfa based papers arecharacterized by a relatively high value of the bulk, which is closeto 1.75 cm3 g−1 at about 60◦SR. Alfa fibres then allow maintaining acertain level of porosity of the fibrous network, even at high inten-sity of refining. Finally, it is worth mentioning here that the bulkand permeability of the handsheets are unaffected by the bleachingprocess. Indeed, for a given Shopper Riegler degree, the bulk and, ina less extent, the permeability exhibit the same values irrespectiveof the nature of the pulp (UBAP or BAP).

Considering now the mechanical properties, they are signifi-cantly improved by the refining process. It is interesting to notethat the unbleached Alfa based papers exhibit very good proper-ties in terms of tensile, bursting and tearing strengths, even forthe low levels of refining, which correspond to a high drainabil-ity of the pulps. Moreover, the refining also has a positive impacton the tear resistance regardless the number of PFI revolutions. Inthis sense, the behaviour of the Alfa pulps is close to that of Euca-lyptus which can be linked to the very limited shortening of thefibres during refining. The effect of refining is particularly strikingfor papers obtained from pulps refined at a relatively low revolu-tion number (between 0 and 1500 PFI revolutions). The breakinglength was seen to increase by a factor of 2 when we considerthe unrefined BAP and the BAP refined at 1500 revolutions. Nev-ertheless, this gain must be somewhat balanced by the fact thatthe mechanical properties of the BAP based papers are far lowerthan those of the UBAP. The bleaching process greatly damages theAlfa fibres. Even if the refining kinetics of the BAP is higher thanthat of the UBAP, it is necessary to refine at the same level (1500revolutions) to reach similar levels of properties for the two papers,apart from the tearing strength which is lower for the BAP. Underthese conditions, the drainability of the BAP is quite lower (42◦SR)than that of the UPAB (22◦SR). This must be related to the signifi-cant decrease of the intrinsic strength of the fibres as revealed bythe sharp drop of the wet zero-span breaking length for the BAPbased papers and the DP values (as discussed above), although thevalues of these properties are quite adequate for the UBAP, at 1500revolutions.

In order to complete this analysis, we compare our results tothose of Bouiri and Amrani (2010) who studied the effect of bleach-ing processes. Even if their values for both the burst and tear indexesare somewhat confusing (presumably due to inadvertent inter-change of the results for bursting and tearing), their data in termsof breaking length are quite comparable to the present values (seeFig. 8) and show again the damaging effect of the bleaching pro-

cess for the experimental conditions used, perhaps indicating thesensitivity of the Alfa fibres to this process.

Z. Marrakchi et al. / Industrial Crops and

Ft

4

pttbpcipat

of the effect of refining on organosolv pulp from olive trimmings and kraft pulp

ig. 8. Breaking length and tear and burst indexes of UBAP and BAP as a function ofhe refining degree.

. Conclusion

In this work, a detailed characterization of stems, pulps andapers obtained from Tunisian Alfa plant was carried out in terms ofheir physical, morphological and chemical properties. Moreover,he effects of refining process on the properties of unbleached andleached pulps and papers were investigated. Results confirm theotentiality of the fibres obtained from S. tenacissima and justify theooking of this vegetal species, in view of using the prepared fibresn papermaking applications. The physical properties of the papers

roduced from Alfa pulps and particularly from unbleached pulp,re quite good and they are significantly enhanced by the refiningreatment, although the morphological characteristics of the fibres

Products 34 (2011) 1572– 1582 1581

are not greatly modified. Consequently, tear index remains quitehigh irrespective of the degree of refining.

Acknowledgments

The authors would like to thank the program of “Mobilité Inter-nationale de la région Rhône-Alpes” (MIRA), for its financial supportand to express their sincere gratitude to Bertine Khelifi (Agefpi,Saint-Martin-d’Hères, France) for the SEM characterization andWim Thielemans from the DICE (Driving Innovation in Chemistryand Chemical Engineering, Nottingham, UK) for his expertise in XPStechnique.

References

Abrantes, S., Amaral, M.E., Costa, A.P., Duarte, A.P., 2007. Cynara cardunculus L. alka-line pulps: alternative fibres for paper and paperboard production. Bioresour.Technol. 98, 2873–2878.

Ai, J., Tschirner, U., 2010. Fiber length and pulping characteristics of switchgrass,Alfalfa stems, hybrid poplar and willow biomasses. Bioresour. Technol. 101,215–221.

Aitken, Y., Cadel, F., Voillot, C., 1988. Constituants fibreux des pâtes - Papiers et car-tons. Pratique de l’analyse, Edition: EFPG et CTP, ISBN: 2.906579.01.7, Grenoble.

Akchiche, O., Messaoud Boureghda, K., 2007. Esparto grass (Stipa tenacissima L.), rawmaterial of papermaking. First part. ХИМИЯ РACTИTEЛЬНO�O CІРЬЯ.4,25–30.

Baeza, J., Urizar, S., Erismann, N.M., Freer, J., Schmidt, E., Duran, N., 1991. Organo-solv pulping. V. Formic acid delignification of Eucalyptus globulus and Eucalyptusgrandis. Bioresour. Technol. 37, 1–6.

Belgacem, M.N., Gandini, A., 2005. Physical, chemical and physico-chemical modifi-cation of cellulose fibers. Compos. Interfaces 12 (1–2), 41–75.

Ben Brahim, S., Ben Cheikh, R., 2007. Influence of fibre orientation and volumefraction on the tensile properties of unidirectional Alfa-polyester composite.Compos. Sci. Technol. 67, 140–147.

Bouiri, B., Amrani, M., 2010. Elemental chlorine-free bleaching hAlfa pulp. J. Ind. Eng.Chem. 16, 587–592.

Chirat, C., Lachenal, D., Mishra, S.P., Pipon, G., 2008. Including an ozone stage inbleaching sequence: effect on pulp properties. In: The International Pulp andPaper Congress and Exhibition , October 2008, Brazil.

Chow, P., Nakayama, F.S., Blahnik, B., Youngquist, J.A., Coffelt, T.A., 2008. Chemicalconstituents and physical properties of guayule wood and bark. Ind. Crops Prod.28, 303–308.

Fuwape, J.A., 1993. Paper from Kenaf fibre. Bioresour. Technol. 43, 113–115.Gaiolas, C., Belgacem, M.N., Silva, L., Thielemans, W., Costa, A.P., Nunes, M., Silva,

M.J.S., 2009. Green chemicals and process to graft cellulose fibers. J. ColloidInterface Sci. 330, 298–302.

Gandini, A., Belgacem, M.N., 2011. Physical and chemical methods of fiber surfacemodification. In: Zafeiropoulos, E. (Ed.), Interface Engineering in Natural FibreComposites for Maximum Performance. Woodhead Publishing Ltd, Cambridge,UK, pp. 3–42.

Gonzalez, M., Canton, L., Rodrıguez, A., Labidi, J., 2008. Effect of organosolv andsoda pulping processes on the metals content of non-woody pulps. Bioresour.Technol. 99, 6621–6625.

Gustafsson, J., Ciovica, L., Peltonen, J., 2003. The ultrastructure of spruce kraftpulps studied by atomic force microscopy (AFM) and X-ray photoelectron spec-troscopy (XPS). Polymer 44, 661–670.

Hosseinpour, R., Fatehi, P., Latibari, A.J., Ni, Y., Sepiddehdam, S.J., 2010. Canola strawchemimechanical pulping for pulp and paper production. Bioresour. Technol.101, 4193–4197.

Houacine, A., 1983. Ph.D. thesis, National Polytechnic Institute of Grenoble France.Jimenez, 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.

Jonoobi, M., Harun, J., Shakeri, A., Misra, M., Oksman, K., 2009. Chemical composition,crystallinity, and thermal degradation of bleached and unbleached kenaf bast(Hibiscus cannabinus) pulp and nanofibers. BioRes. 4 (2), 626–639.

Khiari, R., Mhenni, M.F., Belgacem, M.N., Mauret, E., 2010a. Chemical compositionand pulping of date palm rachis and Posidonia oceanica—a comparison with otherwood and non-wood fibre sources. Bioresour. Technol. 101, 775–780.

Khiari, R., Mhenni, M.F., Belgacem, M.N., Mauret, E., 2010b. Valorisation of VegetalWastes as a Source of Cellulose and Cellulose Derivatives. J. Polym. Environ. 19(1), 80–89.

Khiari, R., Mauret, E., Belgacem, M.N., Mhenni, F., 2011. Tunisian date palm rachisused as an alternative source of fibres for papermaking applications. BioRes. 6(1), 265–281.

Mutjé, P., Pèlach, M.A., Vilaseca, F., Garcia, J.C., Jimenez, L., 2005. A comparative study

from eucalyptus wood. Bioresour. Technol. 96, 1125–1129.Nadji, H., Brochier Salon, M.C., Bruzzèse, C., Benaboura, A., Belgacem, M.N., 2006.

Chemical composition and pulp properties of Alfa (Stipa tenacissima). Cellul.Chem. Technol. 40 (1–2), 45–52.

1 ps an

P

P

S

S

582 Z. Marrakchi et al. / Industrial Cro

aiva, M.C., Ammar, I., Campos, A.R., Ben Cheikh, R., Cunha, A.M., 2007. Alfa fibres:mechanical, morphological and interfacial characterization. Compos. Sci. Tech-nol. 67, 1132–1138.

assas, R., Lecourt, M., Nougier, P., Minko, W., Khelifi, B., 2004. Effet de la remise ensuspension des pâtes sur leur caractérisation mophologique. ATIP 58 (4), 6–13.

anchez, R., Rodríguez, A., García, J.C., Rosal, A., Jiménez, L., 2011. Exploitation of

hemicellulose, cellulose and lignin from Hesperaloe funifera. Bioresour. Technol.102, 1308–1315.

hackford, L., 2003. A comparison of pulping and bleaching of kraft softwood andEucalyptus pulps. In: 36th International Pulp and Paper Congress and Exhibition, October 2003, Brazil.

d Products 34 (2011) 1572– 1582

Sihtola, H., Kyrklund, B., Laamanen, L., Palenius, I., 1963. Comparison and conversionof viscosity and DP-values determined by different methods, special number 4a.Paperi ja Puu, 225–232.

Silvy, J., Romatier, G., Chiodi, R., 1968. Méthodes pratiques de contrôle du raffinage.ATIP 22 (1), 31–53.

Siqueira, G., Bras, J., Dufresne, A., 2010. Luffa cylindrica as a lignocellulosic source

of fiber, microfibrillated cellulose and cellulose nanocrystals. BioRes. 5 (2),727–740.

Wise, L.E., Murphy, M., D’Addieco, A.A., 1946. Chlorite holocellulose: its fractionationand bearing on summative wood analysis and on studies on the hemicellulose.Paper Trade J. 122 (2), 35–43.