Manufacture of fibrous reinforcements for biocomposites and hemicellulosic oligomers from bamboo

Chemical Engineering Journal 167 (2011) 278–287

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Manufacture of fibrous reinforcements for biocomposites and hemicellulosic oligomers from bamboo Daniel González a,b , Valentín Santos a,b,∗ , Juan Carlos Parajó a,b a b

Department of Chemical Engineering, Faculty of Science, Campus Ourense, University of Vigo, 32004 Ourense, Spain CITI (Centro de Investigación, Transferencia e Innovación), Avda. Galicia, No. 2, Parque Tecnolóxico de Galicia, San Cibrao das Vi˜ nas, 32900 Ourense, Spain

a r t i c l e

i n f o

Article history: Received 30 June 2010 Received in revised form 4 October 2010 Accepted 21 December 2010 Keywords: Autohydrolysis Biocomposites Bamboo Fibers Polylactic acid

a b s t r a c t Autohydrolysis of bamboo (treatments with hot, compressed water) resulted in liquors containing mainly hemicellulose-derived products (oligo- and mono-saccharides), and in spent solids (enriched in cellulose and lignin) with potential utility as a reinforcement for composites. Autohydrolysis of bamboo was carried out under a variety of operational conditions, and the resulting solid and liquid phases were assayed for composition. Kinetic models (based on pseudo-homogeneous, first-order, and irreversible reactions) were developed for data interpretation. Under selected conditions, 62.6% of the initial hemicelluloses present in the raw material were converted into oligosaccharides. The solid phase from the treatment leading to the maximal concentration of oligomeric compounds derived from hemicelluloses was employed as a fibrous reinforcement for polylactic acid (PLA)-based biocomposites. For comparative purposes, other three types of reinforcements from bamboo were also employed for making PLA-based composites: bamboo flour, short bamboo fibers, and large bamboo fibers. Compared to neat PLA, reinforcement with bamboo-derived materials resulted in increased stiffness and lower strain at break, whereas little effects were caused in tensile strength. Additional information on thermal analysis and surface morphological characteristics of biocomposites is also provided. © 2011 Elsevier B.V. All rights reserved.

1. Introduction In the last decades, the interest in using environmentally friendly materials has grown due to environmental and sustainability concerns. On the other hand, the depletion of petroleum resources and the volatility of oil price foster the research for renewable raw materials. In the case of bio-based plastics, the market is still small compared to the one of petrochemical plastics, but their share is increasing: between 2003 and 2007, the annual growth rate of emerging bio-based plastics was near to 40%, and the market is expected to grow up to 3.5 million tons in 2020. It has been foreseen that up to 90% of petrochemical plastics will be replaced by biobased plastics [1]. Polylactic acid (PLA) is the only thermoplastic polymer based on natural resources produced at a rate over 140,000 tons/year [2]. The company NaturWoks is planning to establish a new PLA plant in China with this latter annual production capacity. PLA can be produced either by ring-opening polymerization of lactide or by

∗ Corresponding author at: Department of Chemical Engineering, Faculty of Science, Campus Ourense, University of Vigo, 32004 Ourense, Spain. Tel.: +34 988387047; fax: +34 988387000. E-mail address: [email protected] (V. Santos). 1385-8947/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2010.12.066

polycondensation of lactic acid monomers, which can be obtained by fermentation from renewable starchy or cellulosic materials. The chiral nature of lactic acid enables the manufacture of different polymers, based on the specific monomers used as starting materials. The most usual PLA is derived from the l-isomer [3,4]. PLA presents satisfactory mechanical properties, with tensile strength and elongation at break comparable to petroleum-based plastics such as PVC or polystyrene, favourable melting point (170–180 ◦ C) to be used with natural fibers, versatility (with many applications in a variety of industrial fields), and processability by extrusion, injection molding, thermoforming, and compression molding [5]. Its production requires less energy, less water and entails less carbon dioxide release than traditional petroleum-based polymers [3,6], keeping compostability and degradability. However, its low heat resistance (resulting in a marked loss of mechanical properties with temperature), slow crystallization, brittleness and poor tenacity limit the wider application of PLA [7]. Reinforcement by a suitable material may result in improved properties [8,9]. Composites reinforced with glass, carbon or aramid fibers are employed in many market sectors. Substitution of these reinforcing agents by natural fibers is an interesting alternative for applications such as building, packaging, automotive components and consumer goods [10,11]. The main advantages of using natural fibers over traditional reinforcers are their low cost, low density, competitive

D. González et al. / Chemical Engineering Journal 167 (2011) 278–287

specific mechanical properties, and low abrasiveness. Additionally, the natural origin of natural fibers implies annual renewability, carbon dioxide sequestration, and biodegradability. Production costs can be reduced by 10–30% when natural fibers are used instead of glass fibers [12]. Many types of natural fibers have been used as reinforcing agents for composites, for example from stems, leaves, stalks, fruits, grass or wood, as well as man-made regenerated cellulose (including cordenka, viscose or lyocell) [10,11]. Bamboo fibers present favourable physical and mechanical properties when compared with other natural fibers [13], and have been proposed for reinforcing different polymer matrices [14–16], including PLA [17–19]. Bamboo species are cheap and fast-growing resources, abundant in Asia, and also appearing in middle and South America. Due to its low density and mechanical resistance, bamboo is used in building structures, scaffolds and furniture. About 1200 bamboo species are spread all over the world, covering a total area of about 18 millions of hectares. With suitable management procedures, bamboo can produce annually an average of 10–15 tons dry biomass/ha. Bamboo has been evaluated in Europe as a biomass resource in the framework of the FAIR Project CT96-1747. The hydrophilic nature of natural fibers, as well as their morphological and chemical characteristics, makes their compatibility with hydrophobic polymeric matrices poor, with tendency to form aggregates, difficult homogenization, poor adhesion and limited stress transfer, showing fiber pull-outs and clean fiber surfaces in fracture surfaces. Influential factors affecting the properties of composites are aspect ratio of particles, fiber loading, and fiber orientation. Autohydrolysis or hydrothermal processing, a technology based on the utilization of liquid water at high temperature and pressure, can be considered as the first step of a biorefinery strategy enabling the following benefits: (i) environmentally friendly character, (ii) ability for causing a selective solubilization of hemicelluloses, (iii) production of spent autohydrolysis solids (mainly composed of cellulose and lignin) of fibrous nature, whose surface may not have strong hydrophilic character owing to the hydrophobic character of lignin, (iv) production of liquors containing oligosaccharides, which can be useful for a variety of applications, and (v) reduced processing costs. As autohydrolysis removes extractives from the raw material while retaining lignin, autohydrolyzed solids are expected to have an improved compatibility with usual polymer matrices [20]. A variety of cellulose-containing reinforcements for biodegradable composites have been proposed in literature, but little attention has been paid to autohydrolyzed substrates. Georgopoulos et al. [21] employed autohydrolyzed samples of wood and brewery spent grains for reinforcing polypropylene-based composites, whereas Vila et al. [20] employed autohydrolyzed Eucalyptus globulus wood and rice husks for reinforcing PLA. In a related study, Joseph et al. [22] used alkali-extracted, bleached fibers from palm oil empty fruit bunches for making natural rubber composites. Steam-explosion was considered by Zhong et al. [23] for obtaining sweet sorghum fibers to be employed as reinforcing agents for PLA-based composites. Steam explosion was also used by Ibrahim et al. [24] to obtain banana microfibrils for PE composites, by Taib et al. [25] for producing polyethylene-based composites containing Acacia mangium fibers, and by Tokoro et al. [18] and Okubo et al. [26] for obtaining bamboo-derived fibers used as reinforcements for PLA-based composites; whereas Yin et al. [27] and Keller [28] employed steam explosion to process pine wood and hemp, respectively, for manufacturing composites with other polymeric matrices. The main objective of this work was to assess the autohydrolysis of bamboo fibers as a method for obtaining soluble hemicellulose-derived products (mainly of oligomeric nature)

279

together with spent solids enriched in fibers (suitable as reinforcements for PLA-based composites). Kinetic models based on sequential pseudo-homogeneous first-order, irreversible reactions were developed to assess the selection of the best autohydrolysis operational conditions. Spent solids obtained under selected conditions were used as reinforcements for PLA-based composites. 2. Experimental 2.1. Materials Bamboo was collected in Galicia (North-West of Spain), airdried, chipped, treated in a Willey mill to pass 1 mm screen, homogenized in a single lot to avoid compositional differences among aliquots and stored in polypropylene bags until use. Four different types of bamboo fibers were studied as reinforcements for PLA composites: bamboo wood flour (BWF, with particle size less than 150 ␮m), short bamboo fiber (SBF, with particle size between 150 and 500 ␮m), large bamboo fiber (LBF, with particle size between 500 and 1000 ␮m), and autohydrolyzed bamboo fiber (ABF, with particle size between 150 and 500 ␮m). Polylactic acid (PLA) Nature Works 3051, supplied by Cargill Dow LLC, USA, was employed as polymeric matrix for composite manufacture. The properties of the matrix were: density, 1.25 g/cm3 ; glass transition temperature (Tg ), 59 ◦ C; melting point (Tm ), 152 ◦ C; melt flow index (MFI), 30.3 g/10 min (210 ◦ C, 21.2 N). 2.2. Analysis of the raw material Aliquots from the homogenized lot cited above were assayed for moisture (oven drying at 105 ◦ C), ash (standard method TAPPI T-244-om-93), ethanol extracts (standard method TAPPI T-264om-88) and quantitative acid hydrolysis with 72% sulfuric acid (standard method TAPPI T-249-em-85). The solid residue after hydrolysis was considered as Klason lignin. Hydrolyzates were neutralized with barium hydroxide, filtered through 0.45 ␮m cellulose acetate membranes, and assayed by HPLC for sugar determination using a 1100 series Hewlett Packard chromatograph equipped with a refractive index detector and an Aminex HPX87P column (Bio-Rad, CA) under the following conditions: mobile phase, deionized water; flow rate, 0.6 mL/min; temperature, 50 ◦ C. Hydrolyzates were also analyzed by HPLC using an Agilent 1200 series chromatograph equipped with a refractive index detector and a Aminex HPX-87H column (Bio-Rad, CA) under the following conditions: mobile phase, 0.003 mol H2 SO4 /L; flow rate, 0.6 mL/min; temperature, 40 ◦ C. Neutralization was not required in this method, which also allowed the determination of acetic acid (coming from acetyl groups bound to hemicelluloses). The latter column was also employed for sugar determination, since glucose, xylose and arabinose were the only monosaccharides detected, and the corresponding peaks were individually resolved in chromatograms. Furfural (F) and hydroxymethylfurfural (HMF), coming from dehydration of pentoses and hexoses, respectively, were also determined using this method. SCAN viscosity of chloritedelignified samples was measured by the SCAN C15:62 method. 2.3. Autohydrolysis of bamboo Bamboo and water were mixed at the desired proportions (8 kg water/kg oven-dry raw material) and reacted in a batch stainless steel reactor (Parr reactor model 4563M, 600 mL volume, Parr Instruments Company, Moline, IL) under nonisothermal conditions. The reaction media were stirred at 150 rpm and heated up to reach the desired temperature (in the range 180–230 ◦ C). The severity factor (R0 ), defined by Overend and Chornet [29], was employed to assess the combined effects of time and temperature caused by

280

D. González et al. / Chemical Engineering Journal 167 (2011) 278–287 Table 1 Injection molding processing parameters.

Temperature (ºC)

240 200 160 120 80

40 0 0

500

1000

1500

2000

Time (s) Fig. 1. Temperature heating profile of the reactor.

the nonisothermal treatments (see the heating–cooling profiles in Fig. 1).



R0 =

t

exp 0

 T (t) − 100  14.75

dt

(1)

where T(t) is the temperature achieved at time t, and the value 14.75 is a parameter reported in the literature [29], related to the activation energy of the reaction. The value 100 corresponds to the reference temperature, at which the effects caused by autohydrolysis are negligible. At the end of treatments, both spent solids and autohydrolysis liquors were collected and analyzed as described below. Aliquots of the spent solids were assayed for moisture and composition using the methods employed for raw material analysis. An aliquot of the liquors was oven-dried to a constant weight to determine the non volatile solids content (NVC). A second aliquot of the liquors was filtered through 0.45 ␮m membranes and analyzed by HPLC for monosaccharides, furfural, hydroxymethylfurfural, and acetic acid. A third aliquot was subjected to quantitative posthydrolysis with 4% H2 SO4 at 121 ◦ C for 45 min, filtered through 0.45 ␮m membranes and analyzed by HPLC. The increase in the concentrations of monosaccharides and acetic acid caused by posthydrolysis provided a measure of the oligomer and acetyl groups present in liquors. Liquors obtained under the conditions leading to maximum oligomer concentration were assayed for molecular weight distribution by GPC using an Agilent instrument with a MCIGEL CK02S column (Mitsubishi Chemical Co.), kept at 85 ◦ C, and a refractive index detector. Water was used as a mobile phase (flow rate, 1.0 mL/min). Xylobiose, xylotriose, xylotetraose, xylopentaose and xylohexaose were used as external standards for quantification. Oligomers with DP > 6 were calculated by difference between the total oligomers and the joint contribution of the oligomers with DP 2–6.

Parameter

Unit

Setting

Extrusion temperature Injection temperature Mold temperature Extrusion speed Injection speed Holding pressure Cooling time

◦

170 170 25 70 10 20 20

C C C % % Bar s

◦ ◦

shows the values preset for the processing parameters. On the basis of previous experiments, the reinforcement:PLA mass ratios employed in this study were 20:80 and 30:70 g/g. Five types of PLA composites were manufactured: PLA with 20% BWF, PLA with 20% SBF, PLA with 20% LBF, PLA with 20% ABF and PLA with 30% ABF. Tensile strength ( t ), stiffness (E) and strain at break (ε) were determined using a Shimadzu Autograph AG-X testing machine with 50 kN load cell, operating at a crosshead speed of 2 mm min−1 . The Izod impact strength (Is ) was measured in a Ceast–Resil Impactor (15 J energy). The values reported for these variables are the average of results obtained in ten replicate experiments. Differential scanning calorimetry (DSC) was used to analyze the thermal behavior of composites, with determination of the glass transition temperature (Tg ), crystallization temperature (Tc ), melting temperature (Tm ), melting enthalpy (Hm ), and cold crystallization enthalpy (Hc ). A Setaram DSC (model Setsys Evolution) instrument was used for DSC determinations, using nitrogen as purge gas. Samples (20 mg) were heated in DSC pans from room temperature up to 200 ◦ C, cooled to room temperature and subjected to a second heating up to 200 ◦ C. Heating and cooling scans were carried out at a rate of 10 ◦ C/min. The percentage of crystallinity (Xc ) was calculated using the following equation: Xc =

Hm − Hc × 100 93W

(2)

where W is the weight fraction of PLA in the composite, and the parameter 93 J/g corresponded to the fusion enthalpy of 100% crystalline PLA. Injection-molded specimens were used to study water absorption. Specimens were immersed in distilled water, withdrawn after the desired time, and wiped with adsorbent paper to remove excess water. Fiber-matrix interface morphologies were examined by scanning electron microscopy (SEM) using a Philips XL 30 instrument. Samples were coated with a thin gold layer using a sputter coater. Selected zones on fracture surface of injection-molded specimens were employed in observations.

2.4. Fitting of data 3. Results and discussion The set of differential equations derived from the kinetic models was solved using the fourth-order Runge–Kutta method. Empirical equations were used to fit the temperature profiles employed in the numerical solving of the differential equations. Pre-exponential factors and activation energies were calculated by minimizing the sum of the squares of deviations between experimental and calculated data using commercial software with a built-in optimization routine based on Newton’s method. 2.5. Production and characterization of composite materials ISO tensile and impact specimens, with 80 mm length and a constant rectangular cross-section of 10 mm × 4 mm, were obtained using an integrated compounding and injection molding machine (twin screw extruder with a screw diameter of 35 mm, L/D ratio of 18 and an injection unit with 2300 kN clamping force). Table 1

3.1. Chemical composition of bamboo Compositional data of bamboo are shown in Table 2. The major fraction was glucan (accounting for 40.1 wt.% of the oven-dried Table 2 Chemical composition of the raw material. Component

Oven-dry basis (wt.%)

Glucan Xylan Arabinosyl groups (as arabinan) Acetyl groups Klason lignin Ash Extractives

40.1 20.8 1.5 3.8 30.9 0.95 2.9

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281

Table 3 Severity factors corresponding to treatments, solid yield, intrinsic viscosity of spent solids, and compositional data of liquors and spent solids from autohydrolysis. T max (◦ C)

R0 × 103 (min)

180 190 200 210 215 217 220 225 230

0.62 1.26 2.37 3.68 4.70 7.06 9.28 14.0 20.6

Mass fraction (g/100 g raw material, o.d.b.)

Viscosity (mL/g)

SY

NVC

OC

93.0 90.7 81.8 74.8 68.8 67.9 67.4 64.1 63.1

4.8 6.6 12.3 22.7 26.5 26.6 28.3 27.5 23.3

2.2 2.7 5.9 2.5 4.7 5.5 4.3 8.4 13.6

434 488 525 586 784 723 717 702 602

Chemical composition of spent solids (g/100 spent solids, o.d.b.) Gn

Xn

Arn

Acn

AIR

40.6 42.9 45.5 52.2 55.0 55.2 57.1 58.2 59.2

23.4 22.7 20.6 14.6 11.0 10.2 7.4 5.7 3.1

1.1 0.9 0.7 0.4 0.2 0.2 0.1 0.0 0.0

4.5 4.3 3.8 2.2 1.6 1.3 1.1 0.6 0.4

30.4 31.0 31.5 32.6 33.0 34.1 35.2 38.6 39.2

Gn, glucan; Xn, xylan; Arn, arabinosyl substituents; Acn, acetyl substituents; AIR, acid-insoluble residue.

3.2. Effects of hydrothermal processing on bamboo fractionation Table 3 shows the values of the severity factor calculated for the various treatments, the solid yields, data concerning the non volatile solids content in liquor (denoted NVC) and other compounds (denoted OC, determined by difference), as well as the chemical composition and intrinsic viscosity determined for the various spent solids. The degree of bamboo solubilization increased with temperature, particularly in the range 190–210 ◦ C (where the solubilization percentage increased from 9.3 to 25.2 wt.%), a behavior ascribed to both extractive removal and hemicellulose solubilization. Non volatile compounds in liquors increased with the maximum temperature of treatments up to 220 ◦ C, and then decreased, with a marked increase in the proportion of OC, indicating the formation of non-saccharide reaction products. The intrinsic viscosity increased with severity up to 220 ◦ C, with a further decrease. As the intrinsic viscosity is related to the molecular weight of polysaccharides, this behavior is in agreement with the expected effects derived from both hemicellulose removal and breakdown of cellulosic chains under harsh conditions. 3.2.1. Composition of spent solids from autohydrolysis of bamboo The partial solubilization of the raw material along autohydrolysis led to spent solids with increased contents of cellulose (measured as glucan) and Klason lignin, which are less susceptible to hydrothermal processing than other polysaccharides [34]. The content of hemicelluloses (xylan, arabinan and acetyl groups) decreased continuously with the severity of treatments, owing to the susceptibility of these components to hydrolytic degradation. The OC content (measuring the amount of nonsaccharide products), was too small for practical purposes under all the conditions assayed.

On the basis of the data determined for solid yield and chemical composition of both raw material and spent solids, it can be calculated that the percentage of the original acid-insoluble material remaining in spent solids (which measures the Klason lignin content) decreased with temperature up to 215 ◦ C, conditions under which its recovery in solid phase reached 73.5%. Harsher conditions resulted in increased recoveries of acid insoluble material, up to reach 80% under the severest conditions assayed. The glucan recovery in solid phase accounted for more than 93% in all the cases considered. Concerning the hemicellulosic fraction, arabinose moieties were removed easily, leading to arabinose-free spent solids under severe conditions. These harsh treatments resulted in samples containing 10–20% of the original xylan. 3.2.2. Composition of liquors from autohydrolysis of bamboo Operating under suitable autohydrolysis conditions, hemicellulosic polysaccharides were principally converted into oligomers and monomers, with negligible generation of sugar-decomposition products. The concentrations of monosaccharides (glucose, xylose, arabinose), acetic acid, and sugar-decomposition products (furfural, denoted F, and hydroxymethylfurfural, denoted HMF), coming from dehydration of pentoses and hexoses, respectively, are shown in Fig. 2. The glucose concentration remained almost constant with temperature, reaching values in the range 0.43–0.50 g equiv. glucan/100 g oven-dried raw material. The arabinose concentration showed a fast increase up to reach 0.47 g equiv. arabinan/100 g raw material at 215 ◦ C, and then decreased slightly, due to the increasing contribution of monosaccharide-consuming reactions. The concentrations of xylose, acetic acid, HMF and F increased

Content (g/100 g raw material)

feedstock). Although some authors indicated the presence of starch in bamboo, with a content dependent on the specie and season [30], no starch was detected in this work using either the iodine or the amyloglucosidase methods. The acid insoluble residue accounted for 30.9 wt.%, and hemicelluloses (including xylan, arabinan, and acetyl groups) accounted jointly by 26.1 wt.% of samples. Other components (including extractives and ashes), which are of minor importance for the purposes of this study, were also determined in order to enable the formulation of more detailed balances. These results were close to the ones reported by Okubo et al. [31], whereas slightly lower contents of lignin and higher contents of polysaccharides were reported in the literature [16,32]. Xylan with low degree of substition by arabinose was the major hemicellulosic polysaccharide, with a xylose:arabinose ratio of 14:1. In a related study, Fengel and Shao [33] reported a xylose:arabinose ratio of 17:1. The SCAN viscosity of the raw material presented a value of 469 mL/g.

7 6

G

X

Ar

AcH

F

HMF

5 4 3 2 1 0 170

180

190

200

210

220

230

TMAX (ºC) Fig. 2. Temperature profiles of glucose (G), xylose (X), arabinose (Ar), acetic acid (AcH), furfural (F), and hydroxymethylfurural (HMF).

D. González et al. / Chemical Engineering Journal 167 (2011) 278–287

Content (g/100 g raw material)

282

Gn s k1

18 16 14 12 10 8 6 4

k2

XOH

k5

k3

G

HMF

GO

Xn s

XO

k4

XOL k 6

X

k7

ArO

F

Ar ns

AcO O

Acns

2 0 170

GO

k9

k12

k10

ArO

AcO

k13

Ar

k8

DecP

k11

AcH

Fig. 4. Kinetic model employed to assess the bamboo autohydrolysis.

190

210

230

TMAX (ºC) Fig. 3. Temperature dependence of glucooligosaccharides (GO), xylooligosaccharides (XO), arabinosyl groups linked to oligosaccharides (ArO), acetyl groups bounded to oligomers (AcO), and total oligomers (O).

continuously with the severity of treatments, reaching values of 5.99 g equiv. xylan/100 g raw material, 2.18 g acetyl groups/100 g raw material, 2.21 g equiv. hexose/100 g raw material, and 0.14 g equiv. pentose/100 g raw material at 230 ◦ C, respectively.

3.2.3. Oligomer in autohydrolysis liquors Fig. 3 shows the temperature dependence of the oligomer concentrations in autohydrolysis liquors. The major components were xylooligosaccharides (XO), whose variation pattern was closely related to the one determined for the acetyl groups bound to oligomers (denoted AcO), with an initial fast increase to reach maximum values (13.7 g equiv. xylan/100 g raw material or 2.09 g acetyl groups/100 g raw material) in the experiment performed up to reach 220 ◦ C. Harsher treatments resulted in decreased concentrations, to achieve values of 6.0 g equiv. xylan/100 g raw material and 1.39 g acetyl groups/100 g raw material, respectively, in the treatment performed under the severest operational conditions. The concentration of glucooligosaccharides (GO) presented a fairly constant profile, with values in the range 0.34–0.43 g equiv. glucan/100 g raw material. Arabinosyl groups linked to oligosaccharides (denoted ArO) showed a behavior similar to the ones described for XO and AcO, reaching their maximum concentration (0.33 g equiv. arabinan/100 g raw material) in the experiment performed at 200 ◦ C. Harsher treatments resulted in decreased concentrations of arabinosyl groups, up to reach 0.12 g equiv. arabinan/100 g raw material at 230 ◦ C. The concentration of total oligomers (denoted O), calculated as the joint contribution of GO, XO, ArO and AcO, reached a maximum value of 16.3 g/100 g oven-dried raw material at 220 ◦ C (corresponding to a liquor concentration of 20.0 g/L, with 19.0 g/L corresponding to XO). Under these conditions, the xylan recovered as XO accounted for 65.7% of the stoichiometric amount. Under the conditions leading to the maximum concentration of oligomers, the relative abundance of compounds with different degrees of polymerization (DP) was as follows: DP2, 10.7%; DP3, 9.8%; DP4, 10.8%; DP5, 16.4%; DP6, 24.7% and DP > 6, 27.6%. The molecular weight distribution of XO plays an important role in applicabitility: for example, comparatively favourable prebiotic properties have been reported for XO of reduced DP [35]. The experimental data confirm that the DP distribution of the products obtained under the optimal conditions is suitable for this type of applications.

3.3. Kinetic modeling of hemicellulose autohydrolysis In order to provide further insight on the process, the kinetics of bamboo autohydrolysis was analyzed using the mechanism shown in Fig. 4, which was based on the following considerations: • The degradation of cellulose upon autohydrolysis is only relevant for harsh processing conditions [36,37]. The small amount of glucooligosaccharides found in this work could come from low molecular weight cellulose fragments and/or hemicellulose heteropolymers. • Glucan (Gn) was made up of two fractions: non-susceptible glucan and susceptible glucan (denoted Gns ). Gns is able to give glucooligomers (GO), which can be decomposed into glucose (G). G can be dehydrated to hydroxymethylfurfural (HMF). • Xylan (Xn) was made up of two fractions: non-susceptible xylan and susceptible xylan (denoted Xns ). Upon autohydrolysis, Xns yields high molecular weight xylooligosaccharides (XOH ), which react to give low molecular weight xylooligosaccharides (XOL ), and these latter are hydrolyzed to xylose (X). X can be dehydrated to yield furfural (F). • Arabinosyl groups are susceptible to hydrolysis (all Arn corresponds to Arns ), as well as a part of the acetyl groups (denoted Acns ). Oligosaccharides contain attached arabinosyl groups (ArO) and acetyl groups (AcO). These compounds can be cleaved to yield arabinose (Ar) and acetic acid (AcH), respectively. Ar can be dehydrated to F. • Material balances calculated for experiments under severe conditions proved the consumption of pentose and F to give decomposition products (DecP). • All the above mentioned reactions present pseudohomogenous, irreversible, first-order kinetics. • The temperature dependence of the various kinetic coefficients follows the Arrhenius equation: Ki = K0i exp

 E  ai RT (t)

(3)

where k0i (h−1 ) is the pre-exponential factor and Eai (kJ/mol) is the activation energy. All saccharides and saccharide substituents are expressed as polymer equivalents (glucan, xylan, arabinan or acetyl groups) per 100 g of dry raw material. The equations derived from the above hypotheses are shown in Appendix B. The set of Eqs. ((B.1)–(B.25)) was solved using the fourth-order Runge–Kutta method. Temperature was estimated from empirical equations fitting the heating and cooling temperature profiles. Table 4 shows the values calculated for the various regression parameters, including susceptible fractions (denoted ˛i , and measured as the mass fraction of the polymer susceptible to hydrolysis), pre-exponential factors, and activation energies, as well as the R2 statistical coefficients from the experimental data. The satisfactory

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283

Table 4 Values determined for the susceptible fractions (˛i ), pre-exponential factors (k0i ), activation energies (Eai ) and R2 from data analysis. Susceptible fractions ˛Gn

˛Xn

˛Arn

˛Acn

0.015

0.890

1.000

0.913

Pre-exponential factors (k0i ), activation energies (Eai ) and R2 Eai (kJ mol−1 )

R2

Gns → GO GO → G G → HMF Xns → XOH XOH → XOL XOL → X X→F F → DecP Arn → ArO ArO → Ar Ar → F Acns → AcO AcO → AcH

k1 k2 k3 k4 k5 k6 k7 k8 k9 k10 k11 k12 k13

11.0 13.4 20.3 42.9 47.3 48.0 36.3 42.5 21.7 18.7 16.3 46.5 36.2

41.6 52.4 83.1 160.1 180.4 183.3 135.6 157.3 74.8 61.8 55.0 174.0 137.9

<0.9 <0.9 <0.9 0.996 0.987 0.939 0.834 0.992 0.993 0.923 0.945 0.994 0.980

GO exp

GO calc

G exp

G calc

HMF exp

HMF calc

Gn exp

Gn calc

3.0

40

2.5

38

2.0 36 1.5 34 1.0 32

0.5 0.0 170

30 190

210

230

TMAX(ºC) Fig. 5. Experimental and calculated values for glucan and glucan-derived products.

Content (g/100g raw material)

agreement between experimental and calculated data confirmed the suitability of this model for giving a quantitative interpretation of bamboo autohydrolysis. The experimental and the calculated values of glucan, xylan, arabinan, acetyl groups and their corresponding hydrolysis products are shown in Figs. 5–8.

Xn exp

XO exp

X exp

F exp

DecP exp

Xn calc

XO calc

X calc

F calc

DecP calc

20

15

10

5

0 170

190

210

230

TMAX(ºC) Fig. 6. Experimental and calculated values for xylan and xylan-derived products.

Fig. 7. Experimental and calculated values for arabinosyl groups and their derived products.

Fig. 5 shows the experimental and calculated behavior of glucan and glucan-derived products. The susceptible glucan fraction accounted for 0.015 g/g glucan, corresponding to 0.6 g glucan/100 g oven-dry raw material. The solubilization of the susceptible glucan was high even in the mildest treatment, presenting limited increases in glucose and HMF with temperature. Fig. 6 shows the experimental and calculated temperature profiles for xylan and their hydrolysis products. Xylan presented an exponential decrease, tending to a limit value in correspondence with the proportion of susceptible xylan (˛XN = 0.890 g/g). XO presented a maximum concentration at 220 ◦ C, closely interpreted by the model. The same Figure presents experimental and calculated values for F and DecP. Furfural is generated simultaneously from xylose and arabinose at temperatures above 210 ◦ C, and increased up to reach 2.2 g equiv. pentose/100 g raw material at 230 ◦ C. The decomposition products presented a marked increase with severity, reaching up to 5.6 g equiv. pentose/100 g raw material. Under the conditions leading to the maximum oligomer concentration, the amounts of furfural and decomposition products accounted for 0.42 and 0.77 g/100 g raw material. The model provided a satisfactory reproduction of the respective concentration profiles. Fig. 7 shows the experimental and calculated results of arabinosyl substituents and their hydrolysis products. Arabinan was easily hydrolyzed and totally consumed at 220 ◦ C, whereas arabinosyl units linked to oligosaccharides were easily transformed into arabinose, which reached a maximum (0.34 g equiv. arabinan/100 g raw material, corresponding to 23% of the initial arabinosyl groups)

Content (g/ 100 g raw material)

Ln k0i (k0i in h−1 )

Gn (g/100 g RM)

Coefficient

GO, G and HMF (g/100 g RM)

Reaction

Acn exp

AcO exp

AcH exp

Acn calc

AcO calc

AcH calc

4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 170

190

210

230

TMAX (ºC) Fig. 8. Experimental and calculated values for acetyl groups and their derived products.

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Water absorption (%)

PLA PLA/20% LBF

PLA/20%BWF PLA/20% ABF

PLA/20% SBF PLA/30% ABF

4 3 2 1 0

0

200

400

600

800

1000

1200

Time (h) Fig. 11. Water absorption in composites made up of PLA and bamboo-reinforced fibers.

Fig. 9. Parameters determined in tensile tests for neat PLA and PLA reinforced with bamboo-derived fibers.

about 205 ◦ C. Again, the model provided a good interpretation of the respective concentration profiles. Fig. 8 shows experimental and calculated data for acetyl groups, which can be present in solid phase, attached to oligomers or converted into acetic. A close correlation can be observed between the variation patterns of xylan hydrolysis and acetyl groups (see Figs. 6 and 8). In the experiment performed to reach up to 220 ◦ C, about 2.1 g acetyl/100 g initial raw material was attached to oligosaccharides, accounting for 55% of the original acetyl groups in the raw material. The experimental trends were well described by the model.

sented comparatively poor strain and break and similar stiffness than the rest of samples. Compared to neat PLA, reinforcement with bamboo-derived materials resulted in increased stiffness, lower strain at break and similar or slightly lower tensile strength, following the general trends reported in literature for composites containing biofibers. In order to improve the properties of composites, a more efficient interaction matrix – reinforcement is necessary. The materials reinforced at 20% loading with SBF and ABF showed similar properties, with a slight increase in stiffness and tensile strength, and a slight decrease in the strain at break. When the proportion of ABF was increased from 20% to 30%, stiffness improved by 12%, whereas the strain at break decreased from 3.0% to 2.5%, and tensile strength showed little variation. Fig. 10 confirms an increase in impact strength with the fiber length, in agreement with the results reported by Serizawa et al. [38] in experiments with kenaf fiber-reinforced PLA. In the case of composites reinforced with ABF, the impact strength was similar to the ones of SBF composites manufactured

3.4. Properties of PLA-bamboo composites Results of tensile and impact tests of composites manufactured with PLA and bamboo-derived reinforcements are shown in Figs. 9 and 10. Reinforcing composites with 20% of BWF, SBF and LBF resulted in similar stiffness, with a slight decrease (from 2.9 to 2.7 GPa) when fiber size was increased (from BWF to LBF). Strain at break and tensile strength increased markedly when SBF was used instead of BWF, and presented similar values for composites manufactured with SBF and LBF. Composites containing BWF pre-

Fig. 10. Parameters determined in impact tests for neat PLA and PLA reinforced with bamboo-derived fibers.

Fig. 12. DSC thermograms obtained first heating (solid line) and second heating (dashed line). (a) PLA; (b) PLA with 20% LBF; (c) PLA with 20% ABW; (d) PLA with 30% ABW.

D. González et al. / Chemical Engineering Journal 167 (2011) 278–287

285

Fig. 13. SEM micrographs showing the tensile-broken composite surfaces. (a) PLA with 20% ABF (200×); (b) PLA with 20% ABF (800×); (c) PLA with 20% ABF (2.0× kx); (d) PLA with 30% ABF (200×); (e) PLA with 30% ABF (800×); (f) PLA with 30% ABF (2.0× kx); (g) PLA with 20% LBF (200×); (h) PLA with 20% ABF (800×); (i) PLA with 20% ABF (2.0× kx).

with the same fiber loading, and increased significantly (up to 23.7 J/m) when the fiber content increased from 20% to 30%. The latter impact strength almost doubled the one determined for neat PLA probes. Water uptake is a key quality parameter of wood-plastic composites. The experimental values determined in this work are presented in Fig. 11. Due to the hygroscopicity of cellulose fibers, their addition as a reinforcement increased water uptake. When the behavior of the various composites was compared in terms of fiber length, longer lengths resulted in increased values of water uptake: as a diffusion controlled process, longer fibers facilitated water penetration into the composite. The water uptakes determined at the maximum immersion time were 2.5%, 3.4% and 4.1% for BWF, SBF and LBF, respectively. The low water uptake determined for BWF (2.5%) is an interesting property of this type of composites. When autohydrolysis spent fibers were used as a reinforcing agent, water uptake was significantly reduced. For example, composites containing 20% SBF or 20% ABF presented 3.4 or 2.1% water uptake (with 38% reduction from the first case to the second). This behavior can be ascribed to an improved interfacial adhesion between polymeric matrix and ABF, leading to a reduction of interfacial voids and micro-cracks formed in the matrix during injection, and/or to differences in the hydrophilic character of the fiber surface [19]. In this field, Wang et al. [39], in a study dealing with PVC reinforcement, found a interrelationship between the hemicellulose content of alkali-pretreated bamboo fibers and water absorption; whereas Kalia et al. [40] found that hemicellulose removal by hydrothermal treatments, jointly with the increase in cellulose crystallinity,

reduced the water uptake of composites reinforced with natural fibers. Interestingly, increasing the ABF content of composites from 20% to 30% resulted less than proportional increases in water uptake (from 2.1% to 2.7%): 50% increment in fiber loading entailed just 29% increase in water absorption. The thermal behavior of representative composites was assessed by DSC (see Fig. 12). Thermograms showed that addition of reinforcing agents to PLA did not result in significant changes of the glass transition temperature (Tg ) and melting temperature (Tm ). Although composites and neat PLA presented a similar degree of crystallinity (around 14%) along the first heating cycle, a different behavior was observed in the crystallization zone before melting: in the case of neat PLA, it occurred mainly at temperatures around 90 ◦ C; whereas composites presented a wider crystallization zone, which started close to the glass transition temperature and reached melting. In the second heating cycle, crystallinity was low in all the cases, and composites presented little premelt crystallization and a small endo melting peak. In the case of neat PLA, melting and premelt crystallization were observed, but at a lower level than in the first heating, and the premelt crystallization occurred at a higher temperature (about 120 ◦ C). According the experimental data, the presence of fibers did not facilitate the crystallization of PLA, oppositely to the behavior described in the literature [8]. Fig. 13 shows SEM images of the fracture surfaces of selected probes from tensile tests. In general, a good impregnation of fibers by the matrix can be observed, even though some hollows from

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pull out fibers and intact fibers reveal that further improvements in adhesion are still possible. 4. Conclusions Autohydrolysis treatments have been applied to bamboo in order to achieve oligosaccharide-containing liquors and spent solids suitable as reinforcing agents for composites. Hemicellulose breakdown was modeled using kinetic equations based on sequential pseudo-homogeneous first-order, irreversible reactions involving coefficients with Arrhenius-type dependence on temperature. The close agreement between experimental and calculated data enabled the utilization of models for a quantitative interpretation of the experimental data. The maximum oligosaccharide concentration was achieved in the experiment performed up to reach 220 ◦ C, conditions under which 62.6% of hemicelluloses were saccharified to give mainly oligomeric compounds having a DP distribution suitable, for example, for prebiotic applications. The spent solids obtained under the optimal conditions for oligosaccharide production were used as fiber-based reinforcements for PLA-based composites. For comparative purposes, three additional types of bamboo-derived fibrous reinforcements were also employed. Reinforcing PLA with these materials led to composites of increased stiffness and resulted in lower strain at break than neat PLA, with slightly decreased tensile strengths. BWF presented the best stiffness, but a reduced tensile strength. No significant differences between the reinforcing ability of SBF and LBF were found. Impact strength of composites varied according to the fiber length. Composites made with spent autohydrolysis solids presented a markedly reduced water uptake. Increased fiber loading (up to 30%) resulted in significant improvements in impact strength. The results of DSC experiments showed closely related glass transition temperatures for both neat PLA and PLA composites, as well as close melting temperatures and similar crystallinities; but the presence o fibers affected the pre-melt crystallization, broadening the temperature range at which this process occurs. SEM of reinforced samples showed a satisfactory compatibility between phases, confirming the potential of composites made up of PLA and bamboo fibers as an environmentally friendly alternative to conventional petrochemical thermoplastics. Acknowledgments

oven-dried spent solids) glucooligosaccharides (g equiv. glucan/100 g oven-dried raw material) XO xylooligosaccharides (g equiv. xylan/100 g oven-dried raw material) ArO arabinooligosaccharides (g equiv. arabinan/100 g ovendried raw material) Ac acetyl groups bound to oligomers (g/100 g oven-dried raw material) G, X, Ar, AcH, F and HMF contents of glucose, xylose, arabinose, acetic acid, furfural and hydroxymethylfurfural (g/100 g oven-dried raw material) RM raw material GO

Appendix B. Kinetic equations The equations describing the kinetic model derived from the mechanism shown in Fig. 3 and from the hypotheses explained in text are as follows (subscripts: S, susceptible fraction; RM, raw material): • Equations describing the behavior of glucan and its reaction products: dGnS = −k1 · GnS dt

(B.1)

Gn = GnS + (1 − ˛Gn ) · GnRM

(B.2)



˛Gn

Appendix A. Nomenclature

BWF SBF LBF ABF DP SY SF

bamboo wood flour short bamboo fiber long bamboo fiber autohydrolysis bamboo fiber degree of polymerization solid yield (g spent solids/100 g oven-dried raw material) solubilized fraction (g solubilized material/100 g ovendried raw material) Gn, Xn, Arn, Acn and AIR contents of glucan, xylan, arabinosyl groups, acetyl groups and acid-insoluble residue (g/100 g

(B.3)

dGO = k1 · GnS − k2 · GO dt

(B.4)

dG = k2 · GO − k3 · G dt

(B.5)

HMF = GnRM − Gn − GO − G

(B.6)

• Equations describing the behavior of xylan, arabinan and their reaction products: dXnS = −k4 · XnS dt

(B.7)

Xn = XnS + (1 − ˛Xn ) · XnRM

(B.8)



˛Xn Authors are grateful to the Spanish Ministry of Science and Innovation for supporting this study, in the framework of the research Project “Properties of new prebiotic food ingredients derived from hemicelluloses” (reference AGL2008-02072, which was partially funded by the FEDER Program of the European Union). Authors thank to A.M Cunha and A.R. Campos (PIEP, Guimaraes, Portugal) for their support in composite formulation and mechanical characterization, as well as to Angel Yanev (PIEP) for his technical assistance.

GnS  =  Gn RM

XnS  =  Xn RM

(B.9)

dXOH = k4 · XnS − k5 · XOH dt

(B.10)

dXOL = k5 · XOH − k6 · XOL dt

(B.11)

XO = XOH + XOL

(B.12)

dX = k6 · XOL − k7 · X dt

(B.13)

dArn = −k9 · Arns dt

(B.14)

Arn = ArnS + (1 − ˛Arn ) · ArnRM

(B.15)



˛Arn

ArnS  =  Arn RM

(B.16)

dArO = k9 · Arns − k10 · ArO dt

(B.17)

dAr = k10 · ArO − k11 · Ar dt

(B.18)

dF = k7 X · +k11 Ar − k8 · F dt

(B.19)

D. González et al. / Chemical Engineering Journal 167 (2011) 278–287 DecP = XnRM + ArnRM − Xn − XO − X − Arn − ArO − Ar − F

(B.20)

• Equations describing the behavior of acetyl groups and acetic acid: dAcnS = −k12 · AcnS dt

(B.21)

Acn = AcnS + (1 − ˛Acn ) · AcnRM

(B.22)



˛Acn

AcnS  =  Acn RM

(B.23)

dAcO = k12 · AcnS − k13 · AcO dt

(B.24)

AcH = AcnRM − Acn − AcO

(B.25)

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