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Polymer Degradation and Stability 97 (2012) 1979e1987

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Polymer Degradation and Stability

journal homepage: www.elsevier .com/locate /polydegstab

Effect of ligno-derivatives on thermal properties and degradation behavior ofpoly(3-hydroxybutyrate)-based biocomposites

Fabio Bertini a,*, Maurizio Canetti a, Adriana Cacciamani a, Graziano Elegir b, Marco Orlandi c, Luca Zoia c

a Istituto per lo Studio delle Macromolecole - C.N.R., Via E. Bassini 15, 20133 Milano, Italyb Stazione Sperimentale Carta Cartoni e Paste per Carta, Piazza L. da Vinci 16, 20133 Milano, ItalycDipartimento di Scienze dell’Ambiente e del Territorio, Università Milano-Bicocca, Piazza della Scienza 1, 20126 Milano, Italy

a r t i c l e i n f o

Article history:Received 2 November 2011Received in revised form7 March 2012Accepted 9 March 2012Available online 17 March 2012

Keywords:Thermal degradationLigninPoly(3-hydroxybutyrate)Thermogravimetric analysisBiocomposites

* Corresponding author. Tel.: þ39 2 23699356; faxE-mail address: bertini@ismac.cnr.it (F. Bertini).

0141-3910/$ e see front matter � 2012 Elsevier Ltd.doi:10.1016/j.polymdegradstab.2012.03.009

a b s t r a c t

Rice husk lignin was isolated by means of acidolytic (AL) or alkaline enzymatic extraction method.Biocomposites of poly(3-hydroxybutyrate) (PHB) and acetylated lignin were prepared by casting fromchloroform solution. The morphological, structural and thermal characteristics of the biocompositeswere extensively studied. The ligno-derivatives features, i.e. purity, chemical structure and molecularweight, influenced the thermal properties of the PHB-based biocomposites. The AL sample evidenceda marked interference on the crystallization behavior and thermo-oxidative degradation of the PHB. Thedecrease of PHB crystallization rate and the increase of thermal stability were observed as a function ofthe lignin amount in PHB-AL biocomposite series.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

The steady increase of social environmental awareness and theforecast of oil shortage that will endanger the production ofconventional plastics in the future have prompted the attention onbiopolymers. Nowadays, the biopolymer sector still represents onlya very limited share of the global market, however it is expected tohave a huge potential for the future, due to the countless applica-tions that polymers have gained in our society [1e3].

Poly (3-hydroxybutyrate) (PHB) is accumulated by a widevariety of micro-organisms as an intracellular storage source oforganic carbon and chemical energy. PHB has attracted muchattention as a biocompatible and biodegradable thermoplasticpolymer but its application has often been limited by its brittleness.PHB was blended with several synthetic polymers to improve itsthermal andmechanical properties [4e7]. Additionally, the thermalstability of PHB was improved by chemical modification [8e10].

Biocomposites are novel materials obtained by compoundinga biodegradable polymer with biodegradable fillers [11]. In recentyears, fillers from renewable source have been increasingly used inthe preparation of PHB-based biocomposites [12e14]. Rice is one of

: þ39 2 70636400.

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the most cultivated crops in the world with a global production ofabout 680 million tons/year (www.fao.org). Rice husk, the outercover of rice grain, is among the principal processing side-productsof the rice milling industry and accounts for about 20% by weight ofrice. The main organic components of the rice husk are lignin,cellulose and hemicellulose.

Lignin is an amorphous polyphenolic macromolecule used asa filler for the production of polymer matrix composites [15]. Thepresence of lignin gives particular properties to the composite.Lignin can act as a stabilizer preventing polymer aging due to itsantioxidant activity [16e19]. Lignin is able to produce a largeamount of char residue upon heating at elevated temperature in aninert atmosphere; this feature is a basic aspect of flame retardantadditives, since char reduces the combustion heat and heat releaserate of polymeric materials [20e22]. Lignin can also behave asa nucleating agent during the crystallization of different thermo-plastic polymers and interfere on their supermolecular structure[23,24].

Recent papers reported about the influence of lignin on theproperties of PHB-based composites prepared by melt mixing[25,26].

In the present paper, PHB and acetylated ligninbiocomposites were prepared by casting from chloroform solutionto enable interactions at molecular level between lignin andbiopolymer matrix. The study aims at establishing the relationship

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e19871980

between the biocomposite properties and the ligno-derivativescharacteristics.

2. Experimental

2.1. Materials

Rice husk kindly provided by a local factory (Gariboldi S.p.A.,Italy) was ground in a laboratory blender, passed through a 1 mmscreen and stored at �20 �C.

The poly(3-hydroxybutirate) investigated was a commercialBiopol� sample (Mw¼ 134,500 g/mol,Mw/Mn ¼ 2.9) provided by ICIas a fine white powder.

2.1.1. Alkali enzymatic lignin (AEL) preparationDry rice husk was treated 4 h in 0.3 M NaOH at 90 �C under

mechanical stirring. After cooling the solid residue was separatedfrom the black liquor andwashedwith a 0.3MNaOH solution. Blackliquor and washing solution were combined and lignin wasprecipitated by adding 5 M HCl to reach pH 3. The precipitate wasrecovered by centrifugation, washed and freeze-dried. In order toclean lignin from carbohydrates contamination, the ligninwas thensubjected to two hydrolysis steps with a crude cellulase fromT. reesei ATCC 26921 (50 U/g per step).

2.1.2. Acidolysis lignin (AL) preparationDry rice husk was milled in a ball mill for 20 h at 300 rpm. The

milled rice husk sample was refluxed under nitrogen for 2 h in0.1 M HCl dioxane/water solution (9:1, v/v). The insoluble materialremained after lignin solubilization was collected by centrifugation(3000 rpm, 15 min). The supernatant was added dropwise into0.01 M HCl aqueous solutionwhich was then kept at 4 �C overnightto allow for a complete lignin precipitation. The precipitate wascollected by centrifugation (3000 rpm, 15 min), washed withacidified water (pH 2) and freeze-dried.

2.1.3. Biocomposites preparationThe extracted lignins (AEL and AL) were acetylated in an acetic

anhydride: pyridine solution (1:1, v/v) kept overnight at 40 �C. Afterstripping with ethanol, toluene and chloroform (3 times, eachsolvent), the samples were dried in vacuum.

Different amounts of acetylated lignin samples were solubilizedin chloroform. PHB was also solubilized in chloroform. Then thetwo solutions were mixed and dried in rotavapor and vacuumpump, in order to obtain PHB-AL and PHB-AEL biocomposites withweight ratios of 97.5/2.5, 95/5, 90/10 and 85/15.

Pure PHB processed under identical conditions was prepared asreference material. The casting procedure does not modify thepolymer molar mass (Mw ¼ 133,100 g/mol, Mw/Mn ¼ 3.1).

2.2. Methods

2.2.1. Lignin analysisThe amount of total ligninwas calculated as the sum of the acid-

insoluble and acid-soluble lignin content, measured according tothe method reported by Yeh et al. [27].

31P NMR spectra were recorded at 25 �C on a Bruker Avance500 MHz instrument. Samples were dissolved in a pyridine-deuterated chloroform solution (1.6:1, v/v) containing chromiu-m(III) acetylacetonate, alongwith an e-HNDI solution as the internalstandard. AnhydrousDMFwas added to the alkaline lignin specimento improve their scarce solubility. 2-chloro-4,4,5,5-tetramethyl-1,3,2-dioxaphospholane was used as the derivatizing agent.

The evaluation of the molecular weight distribution was per-formed by gel permeation chromatography (GPC) analysis according

to the methodology developed by Himmel et al. [28]. The analyseswere carried out on aWaters 600 E chromatograph connected to anHP1040 ultraviolet diode array detector set at 280 nm. The GPCcolumn system was composed by a sequence of an Agilent PL gel5 mm, 500 Å and an Agilent PL gel 5 mm,104 Å. The acetylated ligninsampleswere dissolved inTHF (1mg/ml) and analyzed at a flow rateof 1 ml/min. Polystyrene standards were used for calibration.

Thermogravimetric analyses (TGA) were performed on a PerkinElmer TGA-7 instrument with platinum pan using about 1.5 mg oflignin as probe. The samples were heated at 20 �C/min in air ornitrogen atmosphere under a flow rate of 35 ml/min. TGA andderivate thermogravimetry (DTG) curves were recorded from 50 to750 �C.

2.2.2. Biocomposites analysisGPC measurements were carried out on a Waters GPCV2000

system equipped with a Waters 2414 refractive index detector, 2 PLgel Mix C columns, chloroform as solvent and polystyrene asreference.

Differential scanning calorimetry (DSC) measurements werecarried out on a Perkin Elmer Pyris 1 instrument equipped witha liquid subambient device and calibrated with indium standard.The sample was placed in a sealed aluminum pan and melted at190 �C for 3 min. The sample was cooled to 10 �C at 10 �C/min rateand then heated to 185 �C with a scan rate of 10 �C/min. Theisothermal crystallization kinetics was investigated by DSC usingthe following standard procedure: the sample was heated up to187 �C and held at this temperature for 3min. Then, the samplewascooled at a nominal rate of 500 �C/min to the selected crystalliza-tion temperature (Tc). The heat flow evolved during the isothermalcrystallization was recorded as a function of time.

TGA measurements were carried out using a Perkin Elmer TGA-7 under a 35 ml/min flowing air atmosphere at a scan rate of 20 �C/min from 50 to 650 �C. The weight of the samples was kept within4e5 mg.

Wide angle X-ray diffraction (WAXD) data were obtained at20 �C using a Siemens D-500 diffractometer equipped witha Siemens FK 60-10 2000 W tube (Cu Ka radiation, l ¼ 0.154 nm).The operating voltage and current were 40 kV and 40 mA,respectively. The data were collected from 5 to 35 2q� at 0.02 2q�

intervals.Small-angle X-ray scattering (SAXS) measurements were con-

ducted with a Kratky Compact Camera. Monochromatized Cu Ka

radiation (l ¼ 0.154 nm) was supplied by a stabilized SiemensKrystalloflex 710 generator and a Siemens fine focus KLF, 2200WCutarget, ceramic tube operated at 40 kV and 45 mA. The scatteredintensity was counted in different ranges of 2q�, by using a stepscanningproportional counterwith pulse height discrimination. Forall the SAXSmeasurements the abscissa variablewas h¼ sin(q) 4p/l.

The morphology and the growth rate of PHB spherulites weredetermined by polarized optical microscopy (POM) using a NikonEclipse TE 2000-U microscope equipped with a Mettler FP82 hotstage. Thin sample films were placed between two microscopecover glasses and inserted into the hot stage. In a typical spherulitegrowth rate determination, the specimenwas maintained at 190 �Cfor 3 min and then cooled to room temperature at 2 �C/min.Nitrogen gas was purged through the hot stage.

3. Results and discussion

3.1. Lignin characterization

Two different extraction methods were used to isolate ligninfrom rice husk: the acidolytic and the alkaline enzymatic. Theacidolytic isolation method was taken into account as a simple and

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e1987 1981

well-defined procedure for the isolation of a pure lignin sample;whereas, the alkaline enzymatic method was considered as aneconomic and industrial applicable extraction procedure. Theoptimization of lignin isolation procedures from rice husk wasalready reported [29].

3.1.1. Molecular and chemical characterizationIn order to rationalize the role and the interferences of the

different lignin preparation (AL and AEL) on the properties of bio-composites, the lignins have been characterized by gravimetric,GPC and 31P NMR analyses. Table 1 displays an overview of theobtained characterization data.

The best result with regard to the purity was identified in the ALsample, which showed an appreciable high purity lignin recovery(86%) respect the AEL sample (78%). In this kind of analysis, theimpurities are usually composed by residual carbohydrates and, inminor amount, by ashes.

The molecular weight indexes provide the evidence of an ALsample characterized by a higher molecular weight distribution ifcompared to the AEL one. The molar mass values and the poly-dispersity are higher for AL sample than AEL showing a differentability of the extracting method to determine the final molecularweight of the product.

The lignin samples were also characterized by means of quan-titative 31P NMR spectroscopy. Table 1 shows that rice husk lignin ismainly formed by guaiacyl and p-coumaryl units, not depending onthe applied extraction procedure. AEL sample contains a largeamount of acidic functionalities originated either by carbohydratesdegradation or lignin side chains oxidation (or both), along witha modest amount of alcohols and phenols. The OH-functionalgroups were implicated on the acetylation reaction performedbefore the casting, while the carboxylic groups remainedunreacted.

Comprehensively, gravimetric and spectroscopic analyses areconsistent with a significant difference between the chemicalfeatures of AL and AEL that could lead to a different affinitybetween polymer matrix and lignin component, and as a conse-quence to dissimilar final properties of the biocomposites.

3.1.2. Thermal stabilityThe thermal decomposition of acetylated lignins was deter-

mined by thermogravimetric analysis under inert atmosphere andoxidative atmosphere. Before TGA run, the samples were main-tained at 110 �C for 60 min to eliminate the physically adsorbedwater [17,30]. Fig. 1 shows the TGA and DTG curves of AL and AELsamples under air atmosphere.

In general, AL sample was found to be more thermally stablethan AEL. The initial degradation temperature corresponding to 5%weight loss (T5%) of AL sample is marked higher than that of AELsample, 261 and 201 �C, respectively. The main degradation stepoccurs in the temperature range between 150 and 420 �C and isassociated with the fragmentation of inter-unit linkages [31,32].The weight loss involved is about 40 and 50% of the total weightloss for AL and AEL, respectively. Differently from AL, whichexhibits a single well-defined peak of degradation centered at

Table 1Compositional evaluation, molecular and chemical features of rice husk lignins.

Lignin sample Purity (Klason, %) Molecular weighta (GPC, g/mol)

Mp Mn Mw

AEL 78 4800 5500 1350AL 86 6100 11900 3630

a Mp: peak molecular weight; Mn: number-average molecular weight; Mw: weight-aveb Cond.: condensed phenols; S-OH: syringyl phenols; G-OH: guaiacyl phenols; P-OH:

332 �C with a maximum rate of 0.07 mg/min, the DTG trace of AELsample is characterized by overlapping decomposition events. Themulti-stage decompositionwith amain peak centered at 351 �C anda pronounced shoulder at about 250 �C, indicates that the organicsubstances are released in steps, which reflect the differentinvolved mechanisms. The presence of high carbohydrates contentin AEL sample leads to the differences in the DTG pattern, being theresidual carbohydrate moieties more prone to thermal decompo-sition [33,34]. Beyond 420 �C both lignins continue to degrade ata much slower rate. The weight loss registered in this region isattributed to the decomposition of some condensed aromaticstructures [35]. The oxidation of the char residue takes place in thetemperature range 480e700 �C with a DTG maximum at about580 �C, accounting for ca. 50 and 43% of the total weight loss for ALand AEL, respectively. The larger mass loss ascribed to the charoxidation for AL sample is consistent with its higher purity andtherefore higher carbon content. The total weight loss for thethermogravimetric run on AL sample is nearly 100%, whereas theAEL sample shows an ultimate residue at 750 �C of about 1.5% dueto inorganic components.

TGA experiments performed under inert atmosphere evidencedthe high thermal stability of AL sample (Fig. 2). The thermogramsare characterized by a noticeable non-volatile residue at 750 �C, 36and 31% for AL and AEL, respectively. The residue is almost exclu-sively due to the formation of highly condensed aromaticstructures.

Therefore, the TGA experimental data point out significantdifferences in the thermal degradation behavior of the investigatedlignins. These results are attributed to the differences in theirpreparation as well as in detailed chemical structure.

3.2. Comparison between the thermal properties of PHB-basedcomposites

Thin films of PHB-AL and PHB-AEL biocomposites with differentcomposition were prepared by casting from chloroform. The ligninfeatures, i.e. the lignin purity, the molecular weight distributionand the amount of chemical functional groups present, could playan important role in the final biocomposite properties and in thisrespect we compared the thermal stability and the crystallizationbehavior of PHB-based composites.

The effect of the lignin presence on the stability of the bio-composites was studied by means of TGA experiments carried outunder oxidative conditions. The thermograms of pure PHB and thebiocomposites containing 15% of lignin are reported in Fig. 3.

On heating at 20 �C/min, pure PHB volatilizes completely ina single narrow step from 220 to 320 �C with a maximum rate of1.5 mg/min at 293 �C. Compared to the polymer matrix, both PHB-based composites show differences in the values of onset degra-dation, taken as the temperature at which 5% degradation occurs,and maximum rate degradation temperature (Tmax). PHB-AEL15composite shows an increase of 8 �C in onset temperature (T5%)compared to pure PHB, while PHB-AL15 presents a higher increaseof 12 �C. PHB and PHB-AEL15 composite show similar Tmax values,whereas the Tmax of PHB-AL is markedly higher, around 17 �C.

Chemical propertiesb (31P NMR, mmol/g)

Alcohol Cond. þ S-OH G-OH P-OH COOH

0 2.70 0.21 0.38 0.24 0.600 3.03 0.23 0.65 0.65 0.27

rage molecular weight.p-coumaryl phenols; COOH: carboxylic acid.

0

20

40

60

80

100

150 250 350 450 550

wei

ght

(%)

Temperature (°C)

Fig. 3. TGA curves under air atmosphere of: PHB ( ), PHB-AEL15 ( ) and PHB-AL15( ).

0

20

40

60

80

100

-10

-8

-6

-4

-2

0

100 200 300 400 500 600 700

Wei

ght

(%)

dw/dt (%

/min)

Temperature (°C)

Fig. 1. TGA (straight line) and DTG (dashed line) thermograms under air atmosphere ofAL ( ) and AEL ( ).

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e19871982

PHB-AL15 is different from PHB-AEL15 because of its higherresidual amount obtained at the end of the first decompositionstep, i.e. before the char oxidation, 12 and 9% respectively. This isconsistent with aforementioned TGA data on plain acetylatedlignins.

Summarizing, the weight loss is slowed down in both bio-composites and the PHB-AL composite presents the higher stabi-lization effect on the thermo-oxidative degradation of the polymermatrix. The remarkable improvement in the thermal stability is dueto a physical barrier effect of the char yield. Indeed, the char acts asa barrier to heat and mass transfer, hindering the diffusion of theoxygen from the gas phase to the polymer matrix and, at the sametime, the out-diffusion of the volatile decomposition products. Thestabilization effect exercised by the charring lignin was previouslyreported for polypropylene- and poly(ethyleneterephthalate)-based composites [21,24]. The barrier effect was particularly effi-cient for the polymers in which the oxidative atmosphere acceler-ates the volatilization by chain scission, as for polypropylene.

The DSC analysis of PHB and biocomposites containing 5% oflignin is reported in Fig. 4, where successive scans are displayed.The choice of appropriate melting conditions is a key point for theanalysis of crystallization kinetics of PHB because of its low resis-tance to thermal degradation [36,37]. With the aim to cancel

0

20

40

60

80

100

-8

-6

-4

-2

0

100 200 300 400 500 600 700

Wei

ght (

%) dw

/dt (%/m

in)

Temperature (°C)

Fig. 2. TGA (straight line) and DTG (dashed line) curves under inert atmosphere of AL( ) and AEL ( ).

previous thermal history and to minimize the degradation of themacromolecole chains, the melting was carried out at 190 �C for3 min.

During the cooling step, the PHB sample shows an exothermicpeak of crystallization with a maximum at about 74 �C (Fig. 4a). Asimilar trend is observed for the PHB-AEL5 cooling scan where thecrystallization peak is shifted to 68 �C (Fig. 4c). Differently, the PHB-AL5 biocomposite does not show any exothermic peak when cooledfrom the melt, while an exothermal event crystallization peaked at52 �C occurs during the heating step, corresponding to the coldcrystallization from the amorphous state (Fig. 4eef). Thus, the non-isothermal crystallization kinetics of PHB is influenced by thepresence of the lignin and the biocomposite containing AL showa more marked interference on the crystallization behavior of thePHB.

Pure PHB displays characteristic double melting peaks at 163and 171 �C (Fig. 4b). The first peak is due to the melting of thecrystals formed during the primary crystallization and the secondtransition is assigned to the melting of the crystals formed asa result of recrystallization on heating [38,39]. Likewise, thesefeatures are evidenced in both PHB-based composites, as reportedin Fig. 4d and f. The melting curves present a broad endothermic

20 70 120 170

endo

>

Temperature (°C)

a

b

c

d

e

f

Fig. 4. DSC cooling and heating scans of PHB (aeb), PHB-AEL5 (ced) and PHB-AL5(eef).

4

6

8

10

12

14

-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0

ln h (nm-1)

ln I

(h)

(a.u

.)

Fig. 6. Double logarithmic scale of SAXS intensity as a function of the scattering vectorh for PHB-AL15 biocomposite. The straight line is the linear fit to the experimentaldata.

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e1987 1983

transition centered at about 170 �C with a shoulder that emergedon the lower temperature side.

As far as the glass transition (Tg) is concerned, DSC heating scanswere carried out upon quenching of the samplemelted at 190 �C for3 min. The Tg of the composites is not affected by the lignin pres-ence for all investigated compositions (2.5e15% of AL or AEL)remaining at about 3 �C, almost the same value obtained for purePHB.

In summary, the above reported results showed that the inter-ference of the AL on PHB thermal stability and crystallizationbehavior is stronger than that of the AEL. Therefore, a second part ofthe study is dedicated to the structural and morphological char-acterization of PHB-AL composites and to the evaluation of theinfluence of lignin content on the thermal properties of PHB-ALcomposites.

3.3. Morphological and supermolecular characterization of PHB-ALbiocomposites

The dispersion degree of AL in the biocomposites was visualizedby means of POM analysis. The well distribution of AL microparti-cles or aggregate of particles is displayed as an uniform texture forthe PHB-AL15 sample (Fig. 5).

The morphological analysis of PHB-AL15 composite was alsoperformed by SAXS to evaluate the dispersion of AL particles atnanoscale level. For complex and random systems, the concept offractal geometry has been applied to interpret some of themeasured scattering profiles [40,41]. Fractal objects are made fromself-similar motif over different ranges of length scale. The intensityof radiation scattered on a fractal object is proportional to a nega-tive power of the wave vector h:

IðhÞzh�a (1)

where the fractal nature of the system can be determined from thevalue of a.

Fig. 6 shows the scattered intensity as a function of the scat-tering vector h measured at 180 �C for the molten PHB-AL15composite. The data are presented in double logarithmic scale inorder to emphasize the power law followed by the scatteredintensity. For systems that exhibit mass fractal characteristics theexponent a varies between 1 and 3, and can be directly determinedfrom the slope of the straight line. The a value of 2.2 observed forPHB-AL15 sample can be interpreted as due to the scattering ofa mass fractal consisting on AL particles or aggregate of particles.

Fig. 5. Micrograph of PHB-AL15 biocomposite at 190 �C.

The fractal geometry in the length scale between 25 and 120 nmwas deducted by the h values of the extreme points of the straightline. The value of 120 nm represents the maximum dimensiondetectable in our SAXS experimental conditions.

In conclusion, the morphological characterization pointed outthe presence of AL particles having dimension ranging from sometens of nm to some mm.

The biodegradability of poly(3-hydroxybutyrate) is influencedby its structure [42]. Pure PHB and PHB-AL15 biocomposite werecharacterized by X-ray techniques for evaluating the influence oflignin presence on the crystal features. TheWAXD profiles reportedin Fig. 7a are very similar and characteristic of the PHB crystallizedin the orthorhombic crystal lattice structure. The crystallinity valueof about 46%, referred to the PHB component, was calculated forboth samples.

The SAXS profiles show the presence of a maximum, which isassociated with the periodicity resulting from the presence ofmacrolattice formed by centers of adjacent lamellae (Fig. 7b). Forpure PHB and PHB-AL15 composite a long period of 5.9 nm wascalculated from the maximum of the Lorentz-corrected intensityprofile.

Thus, the WAXD and SAXS investigations revealed that thepresence of the AL does not influence the crystal and super-molecular characteristic of the PHB.

3.4. Effect of lignin content on the thermal properties of PHB-ALbiocomposites

3.4.1. Thermal stabilityThe thermal degradation behavior of the PHB-AL samples, with

lignin content varying from 2.5 to 15%, was investigated by TGAmeasurements carried out under air flow, and compared to that ofreference PHB (Fig. 8).

A first general observation is that the TGA and DTG curvesprogressively shift toward the higher temperature with theincrease in the amount of AL in the sample. Table 2 summarizes thecharacteristic temperatures of active degradation, i.e. T5%,a measure of the decomposition onset, T50% the mid-point of thedegradation process, Tmax, and the fraction of residual material atdifferent temperatures.

It is observed that in the biocomposites the initial weight losstakes place at higher temperature than that for pure PHB and theT5% value progressively increases with enhancing the AL content inthe composite. Analogously, as the amount of AL increases, T50% and

0

20

40

60

80

100

150 250 350 450 550

wei

ght

(%)

Temperature (°C)

150 250 350 450 550Temperature (°C)

a

b

-100

-80

-60

-40

-20

0

dw/d

t (%

/min

)

Fig. 8. TGA (a) and DTG (b) curves in air atmosphere of PHB (▬▬▬), PHB-AL2.5 ( ),PHB-AL5 ( ), PHB-AL10 ( ) and PHB-AL15 ( ).

Table 2TGA data under air atmosphere for PHB-AL biocomposites.

Sample T5% (�C) T50% (�C) Tmax (�C) R350 (%) R400 (%) R450 (%) R500 (%)

PHB 261 288 293 0.3 0 0 0PHB-AL2.5 271 296 300 2 1 0 0PHB-AL5 279 303 306 4 3 2 0PHB-AL10 283 307 310 7 5 3 0.5PHB-AL15 283 308 310 10 8 6 3

5 10 15 20 25 30 35

I (a

.u.) PHB-AL15

PHB

a

0.0 0.5 1.0 1.5 2.0 2.5 3.0

I(h)

h2

(a.u

.)

h (nm-1)

PHB-AL15

PHB

b

Fig. 7. X-ray patterns of pure PHB and PHB-AL15 biocomposite: (a) WAXD profiles, (b)Lorentz-corrected SAXS curves.

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e19871984

Tmax values increase. The biocomposite containing 10% of ligninshows degradation temperatures markedly higher than that ofPHB: the increase results to be 22 �C, 19 �C and 17 �C for T5%, T50%and Tmax, respectively.

In general, the enhancement of the thermal stability of thecomposites depends on the obtainment of an effective dispersion ofthe filler into the polymer matrix. TGA results indicate that at ALcontent up to 10%, the presence of charring lignin strongly inter-feres increasing the thermal stability of the composite. Indeed,a progressive accumulation of carbonaceous residue efficient in airshielding, takes place on the polymer surface. When the ligninamount reaches 15%, the degradation temperatures result to besimilar to those of PHB-AL10 sample, thus suggesting that thethermal stability of the PHB-AL composites is not proportional tothe AL fraction.

In contrast, the amount of non-volatile material is strictlyrelated to the AL amount in the biocomposites. At 350 �C, pure PHBresults to be almost completely decomposed, while the bio-composites present a residue due to the lignin component. The charresidue oxidizes in a second degradation step, which Tmax increaseswith enhancing the AL content, going from 434 �C for PHB-AL2.5 to507 �C for PHB-AL15.

For the studied PHB-AL biocomposite series, the AL content of10% seems to represent the optimal concentration in delaying thethermo-oxidative degradation of the PHB matrix.

3.4.2. Kinetics of crystallizationIsothermal crystallizations were performed at 117 �C by DSC.

The weight fraction of the material crystallized at time t, Xt, wascalculated by the relation:

Xt ¼Zt

0

ðdH=dtÞdt=ZN

0

ðdH=dtÞ=dt (2)

where the first integral is the heat generated between the begin-ning of crystallization and time t, and the second is the total heatgenerated at complete crystallization.

Fig. 9 reports the Xt values as a function of crystallization timefor pure PHB and PHB-AL biocomposites.

It can be seen that characteristic sigmoid curves shift to the rightwith increasing the AL content. From these crystallinity curves, thehalf-time of crystallization t1/2, defined as the elapsed time fromthe onset of crystallization until the crystallization reaches 50% of

0

0.2

0.4

0.6

0.8

1

0 10 20 30 40

Xt

time (min)

Fig. 9. Development of relative crystallinity with time for isothermal melt crystalli-zation of PHB (-), PHB-AL2.5 (>), PHB-AL5 (C), PHB-AL10 (B) and PHB-AL15 (:).

Fig. 10. Polarizing optical photomicrograph of: (a) pure PHB, (b) PHB-AL15 biocomposite.

40

50

60

70

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e1987 1985

the whole crystallization event, was obtained (Table 3). The pres-ence of AL causes an increase of the time of crystallization and therate of crystallization decreases with enhancing the AL content inPHB-AL biocomposite series.

The isothermal crystallization kinetics of pure PHB and PHB-ALcomposites was described by the well-known Avrami equation[43]:

1� Xt ¼ expð�KntnÞ (3)

where Kn and n, i.e. the overall crystallization rate constant and theAvrami exponent, respectively, are parameters depending on thetype of nucleation and on the geometry of the growing crystals.

Table 3 collects the values of n and Kn determined from the slopeand the intercept, respectively, of the straight lines obtained byplotting log[�ln(1�Xt)] versus log t. For all samples, a straight linewith a good correlation was observed over a wide range ofconversion (Xt values included between 0.05 and 0.95), and Avramiindex close to 2 was obtained. The n-value around 2 indicates a bi-dimensional growth of crystalline units, developed by heteroge-neous nucleation [5]. The crystallization rate parameter Kn

decreases by increasing the AL content in the composite.From the Avrami analysis, it is clear that the crystallization of

PHB, in the pure state as well as in the biocomposites, is charac-terized by the same n-value. Thus, although the overall crystalli-zation rate decreases, the nucleation mechanism and geometry ofcrystal growth of PHB phase are not affected by the presence of AL.

Fig.10 shows the optical micrographs of pure PHB and PHB-AL15composite taken during the crystallization from the melt. The PHBcrystallizes in a typical spherulitic morphology.

According to Chen and Chung [44], the spherulite growth rate Gcan be estimated by taking the first derivative of the plot of thespherulite radius (r) vs. temperature (T), at each experimentalpoint, when the crystallization is performed at constant coolingrate:

Table 3Isothermal crystallization kinetics parameters.

Sample t1/2 (min) n Kn (min�n)

PHB 6.1 2.2 0.0127PHB-AL2.5 7.3 2.2 0.0078PHB-AL5 8.1 2.1 0.0077PHB-AL10 10.4 2.0 0.0066PHB-AL15 14.1 2.0 0.0033

dr=dT ¼ ðdr=dtÞðdt=dTÞ (4)

where dr/dt is the radial growth rate and dt/dT is the reciprocal ofthe cooling rate.

Fig. 11 reports the G value for PHB-AL15 composite and refer-ence PHB as a function of temperature. A decrease of G value was

0

10

20

30

110 115 120 125 130 135Temperature (°C)

Fig. 11. Spherulite radial growth rate of pure PHB (A) and PHB-AL15 biocomposite (�).

F. Bertini et al. / Polymer Degradation and Stability 97 (2012) 1979e19871986

observed for the PHB-AL15 composite into the whole temperaturerange.

The kinetics data showed that the addition of AL causesa decrease of the overall crystallization rate and the spheruliteradial growth of PHB. Taking into account that the PHB-AL15composite is constituted by two separated phases, the interfer-ence on the crystallization kinetics is not attributable to a diluenteffect of AL component. The depression of the crystallization rate isascribed to the increase of energy related to the transport of thePHB macromolecules in the melt, caused by the presence of lignindomains [45].

4. Conclusions

In this work the rice husk lignin was isolated by means ofacidolytic or alkaline enzymatic extraction procedure. The isolationmethods led to lignin samples characterized by significant differ-ences among their molecular, thermal and chemical features. TheAEL sample showed a lower molecular weight and a higher contentof carbohydrates, even after the cellulolytic treatment; whereas theAL sample presented a higher purity. A different behavior was alsoevidenced in the thermal stability as well as in the char formation.

The lignin characteristics gave rise to a different affinitybetween the polymer matrix and the lignin component, thusdetermining dissimilar final properties of the PHB-basedbiocomposites.

The AL evidenced a marked interference on the thermo-oxidative degradation and the crystallization behavior of the PHB.The PHB-AL biocomposite showed an enhancement of the thermalresistance, being the thermal degradation process shifted to highertemperatures. The increase of thermal stability was observed asa function of the lignin amount in PHB-AL biocomposite series.

The addition of AL caused a decrease of the overall crystalliza-tion rate and the spherulite radial growth of the PHB. The inter-ference of the separated non-crystallizable AL domains on the PHBkinetics properties was ascribed to the enhancement of energyrelated to the motion of the macromolecules in the melt.

Acknowledgments

The authors thank Fondazione Cariplo for financial support ofthe project (Lignoplast-2008-2292) in the framework of theprogram “Scientific and technological research on advancedmaterials”.

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