nal

J.

Env

nos

tra

ed fine

over 10 dierent types of linkages (Abreu et al., 1999; Bru-now et al., 1999; Tsujino et al., 2003; Ralph et al., 1998).

as dispersants (Gosselink et al., 2005; Northey, 1992)(about 90% of commercialized lignin is used as concreteadditive due to this property), emulsiers (Garca et al.,1984) and ion-exchange resins (Dizhbite et al., 1999;Zoumpoulakis and Simitzis, 2001). On the other handkraft lignin, which accounts near the 85% of total lignin

* Corresponding author. Tel.: +34 943 017271; fax: +34 943 017130.E-mail address: inaki.mondragon@ehu.es (I. Mondragon).

Bioresource Technology 98 (21. Introduction

With the exception of cellulose there is not a more abun-dant renewable natural resource than lignin, a polyphenolicmacromolecule present in the cell wall of plants. Lignin iscreated by enzymatic polymerisation of three monomers,called coniferyl alcohol, synapyl alcohol and p-coumarylalcohol that lead, respectively, to guaiacyl (G), syringyl(S) and p-hydroxyphenyl propane (p-H)-type units(Fig. 1a). The resulting structure is a complex macromole-cule (Fig. 1b) with a great variety of functional groups and

Depending on the original source and extraction methodused the physico-chemical characteristics of the dierentlignins, and thus their suitability to be used in polymer for-mulations, can vary noticeably.

Traditionally, lignosulfonates (lignins resulting from thesulte pulping process) have been the only type of ligninextensively used in the industry. The polymeric characterand the high content in sulfonic acid groups give them sur-face-active and binding properties (Rodrguez et al., 1990;Van der Klashorst, 1989) These properties have been usedin several applications developed for lignosulfonates, suchAbstract

During the last decades lignin has been investigated as a promising natural alternative to petrochemicals in phenolformaldehyde (PF)resin production, due to their structural similarity. Physico-chemical characterization of three types of lignin, namely kraft pine lignin(L1), sodaanthraquinone ax lignin (L2), and ethanolwater wild tamarind lignin (L3) has been evaluated to determine which one is themost suitable chemical structure for above purpose. Characterization has been performed using Fourier transform infrared spectroscopy(FT-IR) and proton nuclear magnetic resonance spectrometry (1H NMR) to analyse the chemical structure, gel permeation chromatog-raphy (GPC) for determining molecular weight (MW) and molecular weight distribution (MWD), dierential scanning calorimetry(DSC) to measure the glass transition temperature and thermogravimetric analysis (TGA) to follow the thermal degradation. Both struc-tural and thermal characteristics suggest that kraft pine lignin (L1) would be a better phenol (P) substitute in the synthesis of ligninphe-nolformaldehyde (LPF) resins, as it presents higher amounts of activated free ring positions, higher MW and higher thermaldecomposition temperature. 2006 Elsevier Ltd. All rights reserved.

Keywords: Lignin; Characterization; Organosolv; Kraft; Soda; LPF resinsPhysico-chemical characterizatiofor use in phenolform

A. Tejado a, C. Pena a, J. Labidi a,a Materials + Technologies Group, Department of Chemical and

Pza. Europa, 1, 20018 Dob Bakelite Iberica S.A., Epele, 39 C

Received 27 March 2006; received in revisAvailable onl0960-8524/$ - see front matter 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.biortech.2006.05.042of lignins from dierent sourcesdehyde resin synthesis

M. Echeverria b, I. Mondragon a,*

ironmental Engineering, University of Basque Country (EHU),

tia-San Sebastian, Spain

a Navarra, 20120 Hernani, Spain

orm 23 May 2006; accepted 25 May 200614 July 2006

007) 16551663

OCC

C

yrin

H3

(a)

ecOH

CH2

CHOH2C

OCH3

H

HOH2

H

H3CO

guaiacyl (G) s

CH

CH

CH2OH

OCH3O OCH3

CH

CH

CH

OCH3O OC

3

O

C

C

H

H2

OHC

HOCH2

1656 A. Tejado et al. / Bioresource Tproduction in the world, is mainly used as fuel to recoverpart of the energy of the pulping process. Apart from that,lignin is the raw material to obtain low molecular weightcompounds like vanillin (Northey, 1992), widely used incosmetics, simple and hydroxylated aromatics, quinones,aldehydes, aliphatic acids, etc. for which the economicfeasibility is being studied (Johnson et al., 2005).

The use of lignin for the synthesis of new polymericmaterials is the most promising alternative for its revalori-zation. In the last 20 years huge research activity has beendone though industrial applications are at the momentrather scarce. Blends of lignin with thermoplastic polymerslike polypropylene have shown good thermal and mechan-ical properties (Cazacu et al., 2004; Gosselink et al., 2004;Kadla et al., 2002). It is also being studied as compatibiliz-ing agent between thermoplastics and natural bers (Roz-man et al., 2000, 2001). The most research activities arefocused on the use of lignin as substitute of phenol in thesynthesis of lignin-modied phenolformaldehyde (LPF)resins (Alonso et al., 2004; Benar et al., 1999; Danielsonand Simonson, 1998; Kazayawoko et al., 1992; Khanet al., 2004; Kharade and Kale, 1998; Peng and Riedl,1994; Sarkar and Adhikari, 2001; Simitzis et al., 1995; Tur-unen et al., 2003; Vazquez et al., 1995; Ysbrandy et al.,

OOC

OCH3HOCH

CH3O OCH3O

HC

(b)Fig. 1. (a) Schematic representation of the structural units ofH

H2

OCH3

gyl (S)OH

CH2C

HOH2C

H

p-hydroxyphenyl propane (p-H)

C

CH2OH

CH2

CH3

O

H

C

C

CH2OH

H

OOCH3

O

CH CHOH2C

HC

HC

hnology 98 (2007) 165516631997), due to the structural similarity between them. Thenal resin behaviour is very dependent on the chemicaland physical properties of the lignin, and thus a completecharacterization of it prior to the synthesis becomes ofgreat interest.

In this work three types of lignin, namely kraft pine(pinus radiata) lignin, soda/anthraquinone ax (linum usita-tissimum) lignin and ethanol/water wild tamarind (leucaenaleucocephala) lignin have been characterized for further usein novolac-type PF resin synthesis. These lignins wereextracted from black liquors kindly supplied by SmurtNervion (L1), Celulosa de Levante (L2) and the Universityof Cordoba (Spain) (L3). Pine and ax are two well-knownspecies belonging to pinaceae (softwood) and linaceae(dicotyl crop) families, respectively. Wild tamarind belongsto the leguminosae family (hardwood) and is one of thefastest-growing leguminous trees. It is an important speciesencouraged under the social forestry schemes in drought-prone areas and semi-arid tracts.

2. Experimental

Lignins from kraft and soda/anthraquinone liquors (L1and L2), commonly named alkali lignins, were extracted by

H3

OOCH3

O

lignin, and (b) lignin structure proposed by Adler (1977).

Tecacid precipitation. According to Sharma and Goldstein(1990), HCl-isolated lignin samples show higher reactivitytowards phenol than those isolated using sulfuric acid,for this reason we have used a solution of 1 M hydrochloricacid. After lowering the pH to 2 the precipitated ligninswere ltered on a Buchner funnel and successively washedwith water twice, to remove unreacted compounds anddegraded sugars, and then dried in an oven at 60 C undervacuum.

Isolation of organosolv (ethanol/water) lignin (L3) wasperformed under a liquor-to-water ratio of 1:2 and acidula-tion with HCl to pH 2. Such conditions are reported to beadequate for extraction (Botello et al., 1999; Vila et al.,2003). The lignin sample obtained after centrifugation at3500 rpm for 10 min was washed twice and dried in anoven at 60 C under vacuum until constant weight wasobtained.

Lignin samples were subjected to acetylation in order toenhance their solubility in organic solvents, used in bothGPC and NMR techniques. This reaction implies thesubstitution of all the hydroxylic functions by new acetylgroups. With that purpose, the three lignins were placedin an acetyl anhydride/acetyl acid mixture (1:1 by weight)containing sodium acetate as a catalyst (0.5 equivalentsper mole of acetyl anhydride), being the nal lignin concen-tration 20 wt% (Glasser and Jain, 1993). Reactions werecarried out at room temperature for 48 h and then reuxedfor 1 h. Products obtained by precipitation in ice-coldwater with 1% HCl were ltered, washed with distilledwater and dried in the same way as the isolated ligninsamples.

FT-IR measurements were performed in a PerkinElmer16PC instrument by direct transmittance using KBr pellettechnique. Each spectrum was recorded over 20 scans, inthe range from 4000 to 400 cm1 with a resolution of4 cm1. Background spectra were collected before everysampling. KBr was previously oven-dried to avoid interfer-ences due to the presence of water. The chemical structureof samples has also been studied through 1H NMR spec-trometry using a Bruker 500 MHz spectrometer at a fre-quency of 250 MHz with an acquisition time of 0.011 s.DMSO has been used as solvent.

Gel permeation chromatography provides a rapid wayto obtain information on the molecular weight of poly-mers. Samples have been examined through THF-elutedGPC using a PerkinElmer instrument equipped with aninterface (PE Series 900). Three Waters Styragel columns(HR 1, HR 2 and HR 3) ranging from 100 to 5 105 anda refractive index detector (Series 200) were employed, witha ow rate of 1 mL/min. The calibration was made usingpolystyrene standards.

Glass transition temperatures (Tg) were determinedusing a Mettler DSC20 dierential scanning calorimeterlinked to a TC 15 TA processor by using medium pressurepans. The scans were run at 10 C/min under a nitrogen

A. Tejado et al. / Bioresourceow rate of 10 mL/min. Before being tested, the sampleswere extensively dried for 24 h in an oven at 60 C undervacuum to eliminate the presence of water. The Tg wasdened as the mid point of the temperature range at whichthe change in heat capacity occurs. The thermal stability oflignin was measured using a TGA-92 thermobalance fromSetaram under dynamic scans from 30 to 800 C at 10 C/min with helium atmosphere.

3. Results and discussion

3.1. Chemical structure

3.1.1. FT-IR spectroscopy

The FT-IR spectrum (Fig. 2a) of all lignin samples showbands at 1600, 1515 and 1425 cm1 corresponding toaromatic ring vibrations of the phenylpropane skeleton.A wide absorption band focused at 3400 cm1 is assignedto aromatic and aliphatic OH groups while bands at2960, 2925, 2850 and 1460 cm1are related to the CHvibration of CH2 and CH3 groups. Bands assignment isshown in Table 1.

Some remarkable singularities due to their dierent ori-gin can be observed. It is well known that in softwoods thelignin network is mainly composed by G moieties withsmall contents of S and only traces of p-H type units(Boeriu et al., 2004) while in hardwoods and dicotyl cropsdierent amounts of G/S have been reported (Dence,1992). These characteristics become visible in the spectraof the three studied lignins. L1 presents mainly G bands(1270, 1125, 855 and 810 cm1), while L2 shows typical Sbands (1326, 1115 and 825 cm1) as it was expected. L3,coming from the fast-growing hardwood specie, clearlyshows a mixture of both types of units, as remarked inFig. 2b.

Bands in the range of 17051715 cm1 shown by L1 andL3 can be attributed to non-conjugated carbonyl groupsand the wideness of the 1600 cm1 band in L2 spectrumcan be related to the presence of conjugated carbonylgroups (focused at 1650 cm1) in its structure.

The delignication method used for the isolation of lig-nin plays an important role in the nal structure (Fig. 3).Kraft method cleaves b-O-4 and a-O-4 linkages leaving alot of non-etheried phenolic OH groups in lignin, visiblein L1 spectrum at 1365 cm1. On the other hand, sodapulping of non-woody materials produces the cleavage ofthose aryl-ether linkages through the formation of smallquantities of phenolic hydroxyls (Gellerstedt and Lindfors,1984) and the loss of primary aliphatic OH. This can alsobe seen in L2 spectrum, where there is practically no vibra-tion at 1365 cm1 and the signal of 1030 cm1, attributedto primary OH (among others), is less intense than in theother samples. In the organosolv method (L3) that usesan ethanol/water mixture as pulping solvent the acetic acidreleased from the hemicelluloses of wood is the chemicalagent that leads to lignin dissolution. This method pro-motes the acid hydrolysis of lignin generating both phe-

hnology 98 (2007) 16551663 1657nolic hydroxyl groups (visible at 1365 cm1) and newcarbonyl groups (1715 cm1) in its structure.

ec% T

rans

mitt

ance

L1L2L3

1658 A. Tejado et al. / Bioresource TPolymerisation reactions during the synthesis of PF res-ins take place through electrophillic substitution of F at afree position of the aromatic ring. In lignin, G-type unitshave a free C5 position (ortho to the phenolic hydroxyl)in the ring, susceptible of reacting with F, while in S-typeunits both C3 and C5 positions are linked to a methoxygroup, resulting in low reactivity with F. From this pointof view, lignins with G groups as the principal structuralunits must be a priori more suitable for PF formulations.Another factor that must be considered is the existence ofbig quantities of phenolic hydroxyls in the lignin structure:they activate the free ring positions making them reactivewith F, but they can also promote higher non-covalentinteractions between lignin moieties making lignin behaveas a crowded and sti macromolecule (Feldman et al.,

4000 3500 3000 24000 3500 3000 2500Wave numb

1400 1300 1200 1

% T

rans

mitt

ance

Wave number

112

1365

1330

1270

1

(a)

(b)

Fig. 2. (a) FT-IR spectra of L1 (), L2 (- -) and L3 ( )hnology 98 (2007) 165516632001; Thring et al., 2004). In ligninphenolformaldehyde(LPF) formulations this fact can cause an importantdecrease of nal properties (Kazayawoko et al., 1992;Kharade and Kale, 1998; Peng and Riedl, 1994; Sarkarand Adhikari, 2001; Vazquez et al., 1995; Ysbrandyet al., 1997).

3.1.2. 1H NMR spectrometry

The chemical structure of acetylated lignin samples hasbeen studied through 1H NMR spectrometry. Fig. 4 showsthe 1H NMR spectra of acetylated lignin samples (AL1,AL2 and AL3). As pointed out in Table 2, the integral ofall signals between 6.0 and 8.0 ppm can be attributed toaromatic protons in S and G units, while those between0.8 and 1.5 ppm are related to the aliphatic moiety in the

2 1 1000 5002000 1500 1000 500er (cm )-1

100 1000 900 800 (cm )-1

L1

5 825

810855

L2

115

L3

1030

samples, and (b) detail of the 1480770 cm1 range.

Table 3 shows the results derived from these analyses.By integration of the dierent signals, the following param-eters can be calculated: the ratio between G and S structures(G:S), the content in aromatic protons (aromatic H), meth-oxyl groups (OCH3) and total hydroxyl groups (OHtotal)per average phenylpropane unit (C9), and the ratiobetween phenolic and aliphatic hydroxyls (OHph:OHal).All calculations have been made supposing that lignin sam-ples are only composed by G and S-type units. As expected,acetylated L1 exhibits higher content of aromatic protonsthan L2 and L3, as the former is mainly composed by Gunits. Therefore, it will show a priori higher reactivitytowards electrophilic substitution reactions. L2, withmajority of S units, presents high quantities of methoxylgroups as shown by FT-IR results, while L3 shows anintermediate behaviour. Total OH content is much biggerfor kraft pine lignin (L1) than for alkali ax lignin (L2)as shown by FT-IR results, being L3 in the middle of them.On the other hand, the relative quantity of aromatic andaliphatic hydroxyls is similar for all samples and can be

Table 1FT-IR bands assignment

Band (cm1) Vibration Assignment

34203405 st OH Phenolic OH + aliphatic OH29602925 st CH CH3 + CH228502840 st CH OCH317051715 st C@O Unconjugated C@O1650 st C@O Conjugated C@O1600 st CC Aromatic skeleton1515 st CC Aromatic skeleton1460 dasymmetric CH CH3 + CH21425 st CC Aromatic skeleton1365 dip OH Phenolic OH1326 st CO S1270 st CO G1220 st CO(H) + CO(Ar) Phenolic OH + ether1125 dip Ar CH G1115 dip Ar CH S1030 st CO(H) + CO(C) 1st order aliphatic OH + ether855 dop Ar CH G825 dop Ar CH S810 dop Ar CH G

st: Stretching vibration.

A. Tejado et al. / Bioresource Technology 98 (2007) 16551663 1659lignin. Methoxyl protons (OCH3), closely related to G:Sproportion, give an intense signal centred at 3.8 ppm.

Acetylation produces derivatives that display broad pro-ton signals in regions with small interference with otherhydrogen signals. In addition, aromatic and aliphaticacetyl groups are shifted dierently (2.3 and 2.1 ppm, res-pectively) giving rise to separate peaks. Thus, 1H NMRspectrometry of acetylated lignin samples reveals both thetotal content of acetyl groups (proportional to OH content)

dip: In-plane deformation vibration.dop: Out-of-plane deformation vibration.and the ratio between aromatic and aliphatic substituents(Glasser and Jain, 1993).

CH3OO

CC

HOH2C

OR1O R2

- OR1

CH3OO

CHC

HOH2CHH

H O R2

+ HS

- H

CH3OO

CC

HOH2C

OR1O R2

- OR1

CH3OO

CHC

HOH2CHH

H O R2

CH3O

- HCHO

- H

CH3OOH

CC

HOH2C

OR1O R2

- HOR1

HH

+ H

CH3OOH

HCC

HOH2CO R2H

- H

CH3O

HOH2

(a)

(b)

(c)

Fig. 3. Schematic representation of the main changes occurring in the ligninexplained by taking into account the eect of the pulpingmethod suggested above: while kraft and organosolv pulp-ing generate phenolic OH in L1 and L3 samples respec-tively, soda process eliminates aliphatic OH from L2.

3.2. Molecular weights

3.2.1. Gel permeation chromatography

The molecular weights of lignin samples have been ana-lysed through THF-eluted GPC. As shown before ligninsamples have been acetylated to enhance their solubilityin such solvent. Data and chromatograms obtained for lig-nin preparations, shown in Table 4 and Fig. 5 respectively,are in overall agreement with published reports (Angle`s

CH3OO

CHCH

HOH2C

CH3OO

CHCH

HOH2C

CH3OO

CHC

HOH2CH

S

O R2

- OArR2

S

-S0

O

CHCH O R2

HCC

CO R2

- HOArR2+ H2O

CH3O

CH2C

HOH2COOH OH

structure during the (a) kraft, (b) soda and (c) ethanol/water processes.

ec1660 A. Tejado et al. / Bioresource Tet al., 2003; Gellerstedt and Lindfors, 1984; Glasser andJain, 1993).

8 7 6 5 4 ppm

AL1

Aromatic Me

AL2 AL3

Fig. 4. 1H NMR spectra of acetylated L

Table 2Signal assignment for 1H NMR spectrometry of acetylated lignin samples

Signal (ppm) Assignment

8.06.0 Aromatic H in S and G units6.9 Aromatic H in G6.6 Aromatic H in S3.14.2 Methoxyl H2.52.2 H in aromatic acetates2.21.9 H in aliphatic acetates1.50.8 Aliphatic H

Table 3Chemical structure of lignin samples studied by 1H NMR

Sample G:Sa Aromatic H OCH3 OHtotal OHph:OHal

/C9b /C9 wt% /C9 mmol/g

L1 73:27 2.73 1.2 20.9 1.23 6.52 0.54L2 19:81 2.19 1.8 27.5 0.82 4.01 0.54L3 28:72 2.29 1.5 22.6 1.17 5.81 0.49

a All calculations have been done considering MW of 180 and 210 g/molfor G-type and S-type units, respectively.b C9 represents one phenylpropane structural unit.

Table 4Weight average MW Mw, number average MW Mn and polydispersityMw=Mn of acetylated lignin samples analysed by GPCSample Mw=10

3 Mn=103 Mw=Mn

AL1 8.7 2.4 3.6AL2 2.6 1.5 1.8AL3 3.1 1.7 1.8AliphaticAcetylthoxyAromatic Aliphatic

hnology 98 (2007) 16551663AL2 and AL3 samples show weight average MW of2.6 103 and 3.1 103 while AL1 presents a much highervalue, close to 104. These results can be explained accord-ing to the dierent lignin composition. Although b-O-4 isthe most common linkage in all lignin types, there are alsoimportant quantities of CC bonds between the structuralunits; among them, those involving C5 of the aromatic ringare the most abundant (Brunow et al., 1999; Sjoholm,1999; Sjostrom, 1981). G-type units are able to form thiskind of bonds, but this is not possible in S-type units asthey have C5 position substituted by a methoxy group.This is a very important feature when studying the MWof lignins because these CC bonds are not cleaved duringthe pulping of wood due to their higher stability. As a con-

3 2 1 0

1 (), L2 (- -) and L3 ( ) samples.

16 18 20 22 24 26 28 30

1103

2.210 33.7103

1.310 5

1.210 4

Det

ecto

r res

pons

e

Elution time (min)

AL1 AL2 AL3

Fig. 5. GPC chromatograms of acetylated L1 (-d -), L2 (-s -) and L3(-m -) samples.

sequence, lignins strictly composed by G units are expectedto show higher MW than those presenting high contents ofS units. This seems to be valid for the lignin samples stud-ied in this work, as their MW is correlated with their Gcontent.

Repolymerisation reactions may be promoted duringalkaline pulping aecting the MW of lignins. Under highly

of thermal degradation, while the rst derivative of thatcurve (DTG) shows the corresponding rate of weight loss.The peak of this curve (DTGmax) may be expressed as asingle thermal decomposition temperature and can be usedto compare thermal stability characteristics of dierentmaterials. The DTGmax appears between 350 and 425 Cfor all lignin samples analysed, as can be seen in Fig. 6and Table 5. Pyrolytic degradation in this region involvesfragmentation of inter-unit linkages, releasing monomericphenols into the vapour phase, while above 500 C the pro-cess is related to the decomposition of some aromatic rings(El-Saied and Nada, 1993; Sun et al., 2000). The DTGmaxfor non-wood lignin (L2) is clearly lower than for L1 andL3, what can be related to the above mentioned dierentcontent in CC linkages. These data are again in agreementwith those found in the literature (El-Saied and Nada,1993; Glasser and Jain, 1993; Sun et al., 2000).

After heating to 800 C the 4050 wt% of all lignin sam-ples still remains unvolatilized due to the formation ofhighly condensed aromatic structures. This thermal charac-teristic has also been reported for PF resins (Gabilondo,2004) and remarks the similarities between both types of

eriv

wei

ght (

%/mi

n)

L1 L2

A. Tejado et al. / Bioresource Technology 98 (2007) 16551663 1661alkaline conditions some a-hydroxyl groups form quinonemethide intermediates that react easily with other ligninfragments giving alkali-stable methylene linkages (Ishizuet al., 1958; Van der Klashorst, 1989). This reaction, occur-ring especially during kraft method due to the more severeconditions used, causes the appearance of high MW speciesthat lead to higher average MW and MWD values.

Most of the research works found in the literature sup-port the utilization of high MW fractions of lignin for thesynthesis of modied phenolic resins (Forss and Fuhr-mann, 1979; Lange et al., 1983; Olivares et al., 1988; Vander Klashorst, 1989). Due to the larger number of phenyl-propane units per fragment, each fragment has a much bet-ter chance to contribute to polymerisation, compared withthe monomeric and dimeric fractions (Van der Klashorst,1989). For this reason and due to its higher content of aro-matic protons, L1 would be more appropriate for this aimthan the other lignin samples.

3.3. Thermal behaviour

3.3.1. Dierential scanning calorimetry

Glass transition temperature (Tg) of lignins are often tooindiscernible to be determined with reliability and they arealso very inuenced by the water content. For this reason,Tg have been determined by DSC scans after extensivelydrying the samples (60 C, 24 h, vacuum-oven dried) inorder to release all the moisture content.

Dierent underivatized lignin preparations are reportedto have Tg values between 90 and 180 C (Feldman et al.,2001; Glasser and Jain, 1993), corresponding the highervalues usually to softwood kraft lignins and the lower onesto organosolv lignins (Glasser and Jain, 1993). Theseranges show good agreement with our data, as shown inTable 5.

3.3.2. Thermogravimetric analysis

The thermal decomposition of organic polymers is com-monly determined by thermogravimetric (TG) analysisunder helium or nitrogen atmosphere. TG curves revealthe weight loss of substances in relation to the temperature

Table 5Values of glass transition temperature (Tg), maximum of thermaldecomposition temperature (DTGmax) and unvolatized weight fractionat 800 C (residue) for all lignin preparations

Sample Tg (C) DTGmax (C) Residue (%)

L1 144 421 48

L2 138 356 53L3 100 413 38100 200 300 400 500 600 700 800

D

Temperature ( C)

L3

100 200 300 400 500 600 700 800-70

-60

-50

-40

-30

-20

-10

0

L3

Wei

ght l

oss (

%)

Temperature ( C)

L1 L2

(a)

(b)Fig. 6. (a) DTG and (b) TG curves of L1 (-d -), L2 (-s -) and L3 (-m -)obtained from TGA analysis.

that kraft pine lignin (L1) is mainly composed by G units

than the other samples. This behaviour is related to its

of curing kinetic parameters of ligninphenolformaldehyde resol by

ecseveral dynamic dierential scanning calorimetry methods. Thermo-chim. Acta 419, 161167.

Angle`s, M.N., Reguant, J., Garca-Valls, R., Salvado, J., 2003. Charac-teristics of lignin obtained from steam exploded softwood with soda/AQ pulping. Wood Sci. Technol. 37, 309320.

Benar, P., Goncalves, A.R., Mandelli, D., Schuchardt, U., 1999. Euca-lyptus organosolv lignins: study of the hydroxymethylation and use inresols. Bioresour. Technol. 68, 1116.

Boeriu, C.G., Bravo, D., Gosselink, R.J.A., van Dam, J.E.G., 2004.higher content in CC linkages and to repolymerisationreactions occurring during kraft pulping. Finally, L1 andL3 show degradation temperatures up to 80 C above thatof unmodied novolac PF resin. Both structural and ther-mal characteristics analysed suggest that L1 will be a prioria better P substitute in the synthesis of LPF resins, as itpresents higher amount of activated free ring positions,higher MW and higher thermal decomposition tem-perature.

Acknowledgements

The authors wish to thank Bakelite Iberica, GobiernoVasco/Eusko Jaurlaritza (OD02UN46) and Ministerio deEducacion y Ciencia (CTQ2004-06564-C04-03/PPQ) fortheir nancial support of this work.

References

Abreu, H.S., do Nascimento, A.M., Maria, M.A., 1999. Lignin structureand wood properties. Wood Fiber Sci. 31, 426433.

Adler, E., 1977. Lignin chemistry: past, present and futures. Wood Sci.Technol. 11, 169218.

Alonso, M.V., Oliet, M., Perez, J.M., Rodrguez, F., 2004. Determinationwith high quantities of non-etheried hydroxyl groups,while soda/AQ ax lignin (L2) shows mainly S units andless free hydroxyls; organosolv hardwood lignin (L3) pre-sents both G and S structural units. Molecular weight oflignin samples has been studied through THF-elutedGPC showing that L1 has much higher MW and MWDsubstances. The maximum rate loss in non-modied novo-lac resins occurs at 345 C. Therefore the introduction oflignins in PF formulations, especially those with higherthermal stability, will lead to higher thermal decompositiontemperatures. This enhanced thermal behaviour may sup-pose a wider temperature range of application for lignin-modied PF resins.

4. Conclusions

Physico-chemical characterization of three lignin sam-ples has been performed, and the results have been relatedto their dierent origin and dierent extraction method.FT-IR spectroscopy and 1H NMR spectrometry reveal

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Brunow, G., Lundquist, K., Gellerstedt, G., 1999. Lignin. In: Sjostrom,E., Alen, R. (Eds.), Analytical Methods in Wood Chemistry Pulpingand Papermaking. Springer-Verlag, Berlin, Heidelberg, New York,pp. 77124.

Cazacu, G., Pascu, C., Prore, L., Kowarskik, A.L., Mihaies, M., Vasile,C., 2004. Lignin role in a complex polyolen blend. Ind. Crops Prod.20, 261273.

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A. Tejado et al. / Bioresource Technology 98 (2007) 16551663 1663

Physico-chemical characterization of lignins from different sources for use in phenol-formaldehyde resin synthesisIntroductionExperimentalResults and discussionChemical structureFT-IR spectroscopy1H NMR spectrometry

Molecular weightsGel permeation chromatography

Thermal behaviourDifferential scanning calorimetryThermogravimetric analysis

ConclusionsAcknowledgementsReferences