Preparation and characterization of novel biodegradable composites based on acylated cellulose fibers and poly(ethylene sebacate)

Composites Science and Technology 71 (2011) 1908–1913

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Preparation and characterization of novel biodegradable composites based on acylated cellulose fibers and poly(ethylene sebacate) Tânia F. Fernandes a, Eliane Trovatti a, Carmen S.R. Freire a,⇑, Armando J.D. Silvestre a, Carlos Pascoal Neto a, Alessandro Gandini a, Patrizia Sadocco b a b

Department of Chemistry, and CICECO, Campus de Santiago, University of Aveiro, 3810-193 Aveiro, Portugal Stazione Sperimentale Carta, Cartoni e Paste per Carta, Piazza Leonardo Da Vinci, 16 20133 Milano, Italy

a r t i c l e

i n f o

Article history: Received 1 July 2011 Received in revised form 6 September 2011 Accepted 9 September 2011 Available online 16 September 2011 Keywords: A. Fibers A. Polymers B. Surface treatments B. Mechanical properties B. Thermal properties Biodegradability

a b s t r a c t The preparation and characterization of biodegradable composite materials with improved properties based on poly(ethylene sebacate) (PES) and acylated cellulose fibers is reported. These biocomposites showed improved mechanical properties, as evidenced by the increase in both elastic and Young moduli and in the tensile strength, and also showed low water sensitivity and a high biodegradability rate. These novel biocomposites were prepared essentially from renewable resources and therefore constitute an important contribution to the development of the area of sustainable composite materials. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Biocomposites are a class of composite materials obtained by blending natural fibers, or other natural reinforcement materials, with biodegradable polymers [1,2]. The interest on biocomposites results on the one hand in the fact that they are at least partially based on renewable resources, and, on the other hand on their full biodegradability. Consequently, after their use they can be simply disposed with the organic waste fraction and composted without negative effect on the environment. In these perspectives, the development of new biocomposites within a perspective of sustainability and reduced environmental impact is a strategy increasingly searched in the development of new materials and applications. Therefore, composite components such as natural fibers and other biopolymers are gaining considerable and growing interest in detriment of petroleum –derived products [3]. A high diversity of natural fibers (e.g. jute, ramie and sisal) and a considerable number of biodegradable matrices (polysaccharide derivatives, proteins and polyesters, among others) are available for use in such green composites [3]. However, apart from the well recognized advantages of natural fibers comparatively with their synthetic counterparts (e.g. glass fibers), their use on composite materials presents some drawbacks. In particular, the high polar character of natural fibers, mainly com-

⇑ Corresponding author. Tel.: +351 234370695. E-mail address: [email protected] (C.S.R. Freire). 0266-3538/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2011.09.005

posed of cellulose, are responsible for (i) a very low interfacial compatibility with common thermoplastic matrices (ii) moisture uptake and (iii) inter-fiber aggregation by hydrogen bonding, resulting in composite materials with low mechanical performance and dimensional stability. Several strategies have been explored to overcome these problems, generally involving specific surface treatments of the fibers, which include both physical and heterogeneous chemical modifications (in order to limit the functionalization to the outmost layers of the fibers, thus preserving their bulk mechanical properties) [3,4]. The acylation of cellulose fibers with fatty acids under heterogeneous reaction conditions [5,6] has been recently reported as an interesting and efficient way to prepare reinforcing elements for composites with non-polar matrices such as low-density polyethylene (LDPE) [7,6]. However, to the best of our knowledge, these modified fibers have never been applied for the preparation of biocomposite materials. Within this context, they may be particularly relevant because of the renewable and biodegradable nature of acylation agents such as fatty acids. In the present study we describe the preparation and characterization of biocomposite materials based on hexanoylated cellulose fibers and a biodegradable polyester matrix, namely poly(ethylene sebacate). This polyester is also largely derived from renewable resources since it is produced from sebacic acid, a derivative of castor oil [8], and ethylene glycol, nowadays mainly derived from petrochemistry, but that can also be obtained from renewable resources.

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Table 2 Melting temperature (Tm), melting enthalpy (DHm) and degree of cristallinity (Xc) of PES-based cellulose composites obtained from the DSC curves.

2. Experimental section 2.1. Materials The cellulose fibers used in this study were in the form of industrial Eucalyptus globulus ECF (DEDED) bleached kraft pulp kindly provided by a Portuguese pulp mill. Hexanoic acid 99%, thionyl chloride, pyridine and toluene were supplied by Sigma–Aldrich. Toluene was dried with sodium wire. Pyridine was purified by distillation over sodium hydroxide. 2.2. Synthesis of poly(ethylene sebacate) Poly(ethylene sebacate) (PES) was synthesized by the bulk condensation of sebacic acid and ethylene glycol in two steps, following a published protocol [8]. 2.3. Fibers modification The details related to the preparation of hexanoyl chloride by the standard reaction of hexanoic acid with thionyl chloride (4 h at 80 °C), the heterogeneous esterification reactions with hexanoyl chloride in toluene (1 h at 115 °C) and the determination (by elemental analysis) of the degree of cellulose substitution (DS) have been described elsewhere [5,9]. 2.4. Compounding and processing Composites were prepared by compounding the polymeric matrix with the unmodified and hexanoylate cellulose fibers in a melting mixer (Haake Rheomix 600P) working at 90 °C. Firstly, PES pellets were charged and, after melting, dried fibers were added. The systems were mixed during 10 and 15 min, at 50 rpm, respectively for hexanoylated and unmodified cellulose fibers. A 15% and 30% loading of unmodified and modified fibers was tested. The identification of the composites prepared is summarized in Table 1. Subsequently, the composites were molded, in an injection molding machine (Thermo-Haake Minijet II) for tensile (bar according to ISO 527-2-5A) and DMA analysis. 2.5. Materials characterization Molecular weight of poly(ethylene sebacate) (and its distribution) were obtained using a home-made Size Exclusion Chromatographer (SEC) equipped with a PL-EMD 960 light scattering detector. The column set consisted of a PL HFIP gel guard followed by two PL HFIP columns (300  7.5 mm), kept at 40 °C. The equipment was set with a flow rate of 1.0 mL/min and a mixture of dichloromethane/ chloroform/1,1,1,3,3,3-hexafluoro-2-propanol (CH2Cl2/CHCl3/HFIP) (70/20/10) in%(v/v/v), was used as the eluent. The sample solutions with a concentration of about 3 mg/mL were filtered through a PTFE membrane before injection. A molecular weight calibration curve

Table 1 Identification of the composite materials prepared and characterized. Sample

Modification

Fibers (wt.%)

PES

–

–

PESCel15 PESCelC615

– Hexanoyl chloride

15 15

PESCel30 PESCelC630

– Hexanoyl chloride

30 30

Sample

Tm (°C)

DHm (J g1)

Xc (%)

PES PESCel15 PESCelC615 PESCel30 PESCelC630

76.7 76.8 75.9 76.4 76.3

66.1 53.5 54.5 42.6 43.9

51.6 49.2 50.1 47.6 49.0

was obtained with 8 polystyrene standards in narrow-range of molecular weights comprised between 790 and 96,000. 1 H and 13C NMR spectra of CDCl3 PES solution were recorded using a Brücker AMX 300 spectrometer operating at 300.13 and 75.47 MHz, respectively. FTIR spectra were acquired using a Brucker IFS 55 FTIR spectrometer equipped with a single horizontal Golden Gate ATR cell. The resolution was 8 cm1 after 128 scans. Spectra were collected from 4000 to 500 cm1. The degree of substitution (DS) of the esterified fibers was determined by elemental analyses [9], using a Leco CNHS 932 elemental analyzer. Each sample was analyzed in triplicate. XRD patterns were measured using a Phillips X’pert MPD diffractometer using Cu-Ka radiation. Contact angles (h) with water, formamide, ethylene glycol and diiodomethane were measured 5 s after deposition of the droplet (to ensure the attainment of equilibrium) using a Surface Energy Evaluation System commercialized by Brno University (Czech Republic). Each reported value is the average of five determinations. Surface energies (including the polar and dispersive components) of the fibers before and after acetylation were calculated by applying the Owens–Wendt approach. TGA essays were carried out with a Shimadzu TGA 50 analyzer equipped with a platinum cell. Samples were heated at a constant rate of 10 °C/min from room temperature to 800 °C, under a nitrogen flow of 20 mL/min. The thermal decomposition temperature was taken as the onset of significant (P0.5%) weight loss, after the initial moisture loss. DSC thermograms were obtained in a Shimadzu DSC-50 apparatus calibrated with indium. The samples (5–10 mg) were analyzed by heating–cooling cycles between 25 and 120 °C, under nitrogen at a scan rate of 10 °C/min using 40 lL aluminum standard pans. DMA measurements were carried out with a Tritec 2000 DMA Triton equipment, in the bending (single cantilever) mode. Tests were performed at 1 Hz and the temperature was varied from 60 to 100 °C by 4 °C steps. Mechanical analyses were performed at room temperature in a Shimadzu Machine Tester, equipped with an Extensometer ME46, using a load cell of 1 kN. The data acquisition rate was 10.0 Hz and the deformation rate 1.00 mm/min. SEM micrographs of the composite fractured surfaces were obtained with a SU-70 equipment operating at 4 kV. The samples were coated with carbon. 2.6. Water uptake Composite samples (dimensions 60  10  1 mm) were immersed in distilled water at room temperature to study their water uptake. A minimum of three samples were tested for each composite material. The weight increase due to water absorption was periodically assessed for 1 month. When samples were taken out of the water, the wet surfaces were immediately wiped dry, weighted and then reimmersed in water. The water uptake at time t, Wuptake, was calculated as

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1

462ºC

m/mi

0,8

PES

0,6 0,4 0,2 0 0

1

100

200

300

400

500

600

700

800

Temperature (ºC)

372ºC

1

462ºC

0,8

Cellulose

0,6

m/mi

m/mi

0,8

0,4

PESCel30 381ºC

0,4

0,2 0

0,6

0,2

0

100

200

300

400

500

600

700

0

800

0

200

Temperature (ºC) 1

400

600

800

Temperature (ºC)

340ºC 1

0,8

456ºC

m/mi

m/mi

0,8 0,6

256ºC

0,4

CelC6

0,6

0,2

PESCelC630

395ºC

0,4 0,2

0 0

100

200

300

400

500

600

700

800

Temperature (ºC)

0 0

200

400

600

800

Temperature (ºC)

Fig. 1. Thermograms of unmodified and C6 esterified cellulose-based PES composites.

W uptake ¼

  Wt  W0  100 W0

ð1Þ

where W0 is the specimen initial weight and Wt the specimen weight after an immersion time t.

2.7. Biodegradation essays The ultimate aerobic biodegradability was determined under controlled composting conditions according to standard methods such as ISO 14855-1999, EN 14046-2003. The test measures the complete transformation (mineralization) of the organic carbon contained in the sample to CO2 and water by the action of the compost microorganisms. The ultimate biodegradation test was conducted by mixing the sample with mature compost. To allow the testing of small quantities of sample the composting mixture volume was enhanced by the addition of an inert material (vermiculite).

 Reactors employed: 500 ml glass vessels: 2 reactors without the sample (blank), 2 reactors with the biodegradable reference (Avicel cellulose) and 2 reactors for each sample.  Reactor composting mixture: 30 g vermiculite as inert support, 50 g mature compost, the water content of the vermiculite/ compost mixture was adjusted at 40%.  Sample quantity: ranging from about 1 g to 2 g for each reactor.  Reactor aeration: the reactors were equipped with inlet and outlet tubes for the aeration with compressed humidified air. The CO2 was previously removed from the inlet air (by adsorption on soda lime).  Temperature: the test was conducted in an incubator at 58 ± 2 °C.  Inoculum: mature compost from a composting plant treating 70% municipal garden green waste and 30% municipal organic waste.  CO2 production measurement: the % of CO2 content in the outlet reactors air was measured using an infrared gas analyzer. The measurements were performed once a day for the first 20 days and afterwards once every 2–3 days.

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3

A

2

Young Modulus (GPa)

1,8

e Elastic modulus E' (GPa)

d

a-PES b-PESCel15 c-PESCelC615 d-PESCel30 e-PESCelC630

2

c b

1,6 1,4 1,2 1 0,8 0,6 0,4 0,2 0 PES

1

PESCel15

a

20

60

Temperature (ºC) Fig. 2. Logaritm of the storage modulus E0 vs temperature (at 4 Hz) of the cellulose based PES composites studied.

Tensile strength (MPa)

-20

PESCelC615 PeSCelC630

B

45

0 -60

PESCel30

40 35 30 25 20 15 10 5

3. Results and discussion The heterogeneous esterification of the cellulose fibers with hexanoyl chloride was performed according to the literature [5], using reaction conditions (temperature, reaction medium and time) that limited the modification to the amorphous or outmost layers of the fibers, aiming to preserve their mechanical properties. The degree of substitution DS attained was around 0.5 while the surface energy was 37 mJ/m2. The thermogravimetric, FTIR, X-ray diffraction results of the modified fibers were also in perfect agreement with the previously reported data [5]. The FTIR and 1H NMR spectra of poly(ethylene sebacate) were in tune with the expected structure, with a clear indication that it had reached a molecular weight higher than that of the previously reported material [8], since no absorption was detected in the IR spectrum in the typical OH-stretching region at 3400– 3500 cm1 (very low concentration of terminal groups), contrary to the published spectrum. SEC gave a Mw of about 40,000 and a polydispersity of 1.8. In addition, the surface energy of PES is very similar to that of the hexanoylated cellulose fibers, specifically 40 mJ/m2. The composites prepared in this study with modified cellulose fibers were in general very homogenous, whereas those with the unmodified fibers showed some fiber agglomeration. All composites were then characterized in terms of thermal and mechanical properties, morphology, water uptake capacity and biodegradability profile. 3.1. Thermal properties The DSC thermograms of all the cellulose-based PES composites showed an endothermic peak ascribed to the melting of the crys-

0

PES

PESCel15

PESCel30

PESCelC615 PeSCelC630

C

100

Elongation at break (%)

 The percentage of biodegradation was expressed as % of CO2 production with respect the theoretical CO2 content of the sample (% ThCO2). For each reactor, the theoretical amount of CO2 (ThCO2) that can be produced by the sample was calculated from the sample organic carbon content value and from the amount of sample added to each reactor.

90 80 70 60 50 40 30 20 10 0 PES

PESCel15

PESCel30

PESCelC615 PeSCelC630

Fig. 3. Young’s modulus (A), tensile strength (B) and elongation at break (C) of the cellulose based PES composites studied.

talline domains of the matrix, with a maximum at about 76 °C (Table 2). The melting temperature (Tm) and the enthalpy of fusion (DHf) were determined by the maximum and the area of the fusion endothermic peak, respectively. The incorporation of both unmodified and hexanoylated cellulose fibers did not significantly affect the melting temperature, regardless of the different fiber loadings. This behavior has already been observed with fatty acids acylated cellulose fibers LDPEbased composite materials [7] as well as with other natural fiberbased composites [10]. Conversely, as expected, the enthalpy of fusion (DHm) decreased with the fiber loading due to the corresponding decrease in the amount of matrix. Moreover, the degree of crystallinity of the composite materials was not significantly affected by the fiber content and modification. Thermogravimetric analysis of cellulose PES-based composites was carried out to evaluate their thermal stability and degradation profiles (Fig. 1). All cellulose PES based-composites were considerably less stable than the unfilled polyester matrix because of the presence of the natural fibers, and their TGA profiles were in gen-

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PESCel30

Fig. 6. Biodegradation profile of PESCelC615, PES, cellulose fibers (pulp fibers) and hexanoylated cellulose fibers. The reported data are the average values of two replicates.

PESCelC630 Fig. 4. Scanning electron micrographs of the fractured surfaces of PESCel30 and PESCel630 composites, resulting from the tensile tests.

5

water uptake (%)

4,5 4 3,5 3 2,5 2 1,5 1 0,5 0

0

5

10

15

20

25

30

Time (days) PES

PESCel15

PESCel30

PESCelC615

PESCelC630

Fig. 5. Water uptake as a function of time for the cellulose based PES composites when immersed in water at room temperature.

eral a combination of those of the corresponding cellulose fibers and matrix (Fig. 1). The composites with unmodified fibers gave a thermal degradation profile that was a almost a sum of those of the separated components, indicating that they degraded separately (Fig. 1). These results were a clear evidence of the poor compatibility; i.e. lack of interfacial adhesion, between the unmodified cellulose fibers and the PES matrix. On the contrary, the composites with hexanoylated cellulose fibers showed different weight losses associated with the two elements of the composite, and, more significant increase (55 °C) in the thermal stability of the cellulose fibers, accompanied by a slightly (5 °C) decrease in the maximum decomposition temperature of the PES matrix (Fig. 1). In this case the two components did not degrade independently. These results were an indication of the excellent interfacial adhesion between the modified cellulose fibers and the PES matrix, as confirmed below based on mechanical essays and SEM.

3.2. Mechanical properties Fig. 2 shows the curves of (E0 /Pa) (storage tensile modulus) of PES based composites prepared as a function of temperature. Spe-

cifically, the effect of the cellulose fibers surface hexanoylation and load on the viscoelastic properties of the neat PES was assessed. The unfilled PES showed a typical behavior with three different regions. In the glassy state (80 to 50 °C) the tensile storage modulus E0 only slightly decreased with temperature, and then dropped considerably in the range 50/30 °C, with a maximum decrease at around 40 °C. This relaxation phenomenon is associated with the glass-rubber transition of the polymer. All the composites showed higher storage moduli than that of neat PES, but the increments increased with the fiber loading, and were in general slightly higher for composites with modified fiber than for those with the same percentage of the unmodified fiber, particularly in the glassy state. This behavior is certainly a result of the improvement of the cellulose-PES matrix interfacial adhesion promoted by the surface chemical modification of the cellulose fibers and has already been reported for acylated cellulose fibers-PE based composites [7]. Fig. 3 shows the tensile mechanical properties, including Young modulus (E), tensile strength and elongation at break, determined from the typical stress–strain curves, of the composite materials. Young moduli of all composites were higher than those of the unfilled PES, increased with the filler contents, and for the same filler percentage were also in general higher for those with modified fiber. However, this difference was not very significant probably because of the heterogeneity of the composite samples with unmodified fibers that present large agglomerates of fibers that could have influenced the results in a wrong way. The tensile strength also increased with increasing fiber load. Additionally, the incorporation of both unmodified and hexanoylated cellulose fibers caused a considerable decrease in the elongation at break.

3.3. Morphology The SEM analysis of the fractured surface of the composites resulting from tensile tests (Fig. 4) was used to study the fiber dispersion within the matrix and the interfacial adhesion between the two components of the composites. The micrographs that correspond to the composites with unmodified fibers clearly show that the interfacial adhesion between the unmodified cellulose fibers and the polyester matrix was very poor, as the fibers were pulled out from the matrix practically intact and therefore, fracturing the composite sample did not lead to the fibers fracture. Moreover, the presence of cellulose fiber aggregates, particularly for the composite with a 30% fiber loading, provided strong evidence of the poor dispersion of the

T.F. Fernandes et al. / Composites Science and Technology 71 (2011) 1908–1913

filler within this polymeric matrix and consequently of the nonhomogeneity of the final materials. Conversely, the SEM images of the hexanoylated fiber-based composites (Fig. 4) provided evidence of the strong interfacial adhesion between the two components, as shown by the cellulose fibers breaking during fracture and also by their good dispersion within the matrix, without aggregate formation. This behavior is the result of the surface acylation with hexanoyl chloride that conferred a non-polar character to the cellulose fiber surface [5]. The SEM analysis clearly corroborated the DMA and tensile tests results. 3.4. Water uptake Another important property that can be achieved through the surface modification of cellulose fibers is the reduction of their water absorption, since the moisture pickup, is known to be closely related to significant variations in mechanical strength, dimensional stability and appearance of cellulose-based composites [11]. Fig. 5 shows the evolution of water uptake for unmodified and hexanoylated cellulose-based composites during 1 month. All the composites absorbed water during the experiment, however in different extents and following distinct profiles. Since PES only showed a very small amount of water uptake after this period (0.2%) owing to its hydrophobic character and high crystallinity, the water uptake of the composites was largely due to the presence of the natural or modified fibers. As expected, water uptake was considerably higher for the unmodified cellulose-based composites, than for their hexanoylated-cellulose counterparts, and increased considerably with the fibers loading, particularly for composites with unmodified fibers. The lower water uptake of composites with modified fibers clearly reflected the increase in hydrophobic character of modified fibers surface, due to the presence of aliphatic chain moieties on the fibers surface [5] and were in perfect agreement with the results obtained with LDPE basedcomposites reinforced with acylated cellulose fibers [7]. Moreover, after this aging period, only the composites with modified fibers and the sample PESCel15 had reached an equilibrium moisture level. These samples displayed a water uptake profile with two well-separated regions. Initially, the absorption was fast and linear Fickian behavior before reaching an equilibrium plateau. 3.5. Biodegradability Aiming to evaluate the biodegradability profile of the biocomposites prepared in this study, the PESCelC615 sample was submitted to a laboratory biodegradation test, together with the hexanoylated cellulose fiber sample (CelC6) and PES for comparative purposes, unmodified cellulose fibers was also tested as positive reference (Fig. 6). PES and cellulose biodegradability profiles are well known [12,13]. In the testing conditions used in this study PES was completely degraded within 15 days whereas the unmodified cellulose fibers required about 25 days. The PES based composites presented also a high degree of biodegradability. The PESCelC615 composite sample was completely degraded within 28 days of testing, while

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for the hexanoylated cellulose fiber alone a slower degradation rate was observed; 60% biodegradation value was reached within 30 days of testing. 4. Conclusions Novel biodegradable composites with improved properties were prepared from PES and hexanoylated cellulose fibers. The surface modification of the cellulose fibers increased substantially their hydrophobicity and therefore the compatibility with the PES matrix, as evidenced by SEM analysis. As a result, these composite materials showed enhanced homogeneity, fibers dispersion and mechanical properties as well as lower water sensitivity, when compared with those with unmodified fillers. These composites could be labeled as sustainable composite materials since they were prepared mainly from renewable resources and therefore represent an important contribution to this field. Acknowledgment Thanks are due to CICECO for financially supporting this work. References [1] Mohanty AK, Misra M, Hinrichsen G. Biofibres, biodegradable polymers and biocomposites: an overview. Macromol Mater Eng 2000;276(3–4):1–24. [2] Mohanty AK, Misra M, Drzal LT. Sustainable bio-composites from renewable resources: opportunities and challenges in the green materials world. Abstr Pap Am Chem Soc 2002;223:D70. [3] Belgacem MN, Gandini A. Monomers, polymers and composites from renewable resources. Amsterdam: Elsevier; 2008. [4] Belgacem MN, Gandini A. The surface modification of cellulose fibres for use as reinforcing elements in composite materials. Compos Interface 2005;12(1– 2):41–75. [5] Freire CSR, Silvestre AJD, Neto CP, Belgacem MN, Gandini A. Controlled heterogeneous modification of cellulose fibers with fatty acids: effect of reaction conditions on the extent of esterification and fiber properties. J Appl Polym Sci 2006;100(2):1093–102. [6] Pasquini D, Teixeira EDM, Curvelo AADS, Belgacem MN, Dufresne A. Surface esterification of cellulose fibres: processing and characterisation of lowdensity polyethylene/cellulose fibres composites. Compos Sci Technol 2008;68(1):193–201. [7] Freire CSR, Silvestre AJD, Neto CP, Gandini A, Martin L, Mondragon I. Composites based on acylated cellulose fibers and low-density polyethylene: Effect of the fiber content, degree of substitution and fatty acid chain length on final properties. Compos Sci Technol 2008;68(15–16):3358–64. [8] More AB, Chilgunde SN, Kamble JC, Patil PS, Malshe VC, Vanage GR, et al. Polyethylene sebacate: genotoxicity, mutagenicity evaluation and application in periodontal drug delivery system. J Pharm Sci-Us 2009;98(12):4781–95. [9] Freire CSR, Silvestre AJD, Neto CP, Rocha RMA. An efficient method for determination of the degree of substitution of cellulose esters of long chain aliphatic acids. Cellulose 2005;12(5):449–58. [10] Tome LC, Pinto RJB, Trovatti E, Freire CSR, Silvestre AJD, Neto CP, et al. Transparent bionanocomposites with improved properties prepared from acetylated bacterial cellulose and poly(lactic acid) through a simple approach. Green Chem 2011;13(2):419–27. [11] Thirmizir MA, Ishak ZM, Taib RM, Sudin R, Leong YW. Mechanical, water absorption and dimensional stability studies of kenaf bast fibre-filled poly(butylene succinate) composites. Polym Plast Technol 2011;50(4):339–48. [12] Doi Y, Kasuya K, Abe H, Koyama N, Ishiwatari S, Takagi K, et al. Evaluation of biodegradabilities of biosynthetic and chemosynthetic polyesters in river water. Polym Degrad Stab 1996;51(3):281–6. [13] Drimal P, Hoffmann J, Druzbik M. Evaluating the aerobic biodegradability of plastics in soil environments through GC and IR analysis of gaseous phase. Polym Test 2007;26(6):729–41.