Biomacromolecules 2004, 5, 1200-1205
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Bio-Based Polymeric Composites Comprising Wood Flour as Filler Elizabeth Grillo Fernandes, Matteo Pietrini, and Emo Chiellini* Department of Chemistry & Industrial Chemistry, University of Pisa, v. Risorgimento 35, 56126 Pisa, Italy Received December 6, 2003; Revised Manuscript Received March 26, 2004
The increasing effort on development of bio-based polymeric materials in recent years is motivated by the basic concept of meeting the sustainability criteria for industrial development in the third millennium. Within this framework, our research group is currently involved in assessing the potentiality of some agro-industrial overproduction and byproducts in the formulation of eco-compatible bio-based polymeric materials displaying, among others, the propensity to biodegrade under controlled environment conditions. In the present work, beech wood flour (Bwf) composites were prepared from plasticized poly(3-hydroxybutyrate) (PHB). The type of plasticizer [tri(ethylene glycol) bis(2-ethylhexanoate) (TEGB) and poly(ethylene glycol) (PEG200)] and the amount [5 and 20 wt %] were selected as independent variables in a factorial design. Thermal and mechanical properties of 90 wt % PHB composites were investigated. Incorporation of PEG200 was found to compromise thermal stability of PHB as demonstrated by the higher decrease on the onset decomposition temperature (Td) and the drop in its average molecular weight (Mw). The present study underlines the fact that TEGB/PHB/beech wood flour composites can be optimized to obtain new materials for disposable items. Introduction Most of the academic and technical literature about composites containing natural fillers is based on polyolefin matrices as continuous phase.1-3 At present, several companies are producing durable goods as decks, door and window trim, and automotive interiors with these materials.4 Conversely, the market of disposable food service packaging such as cups, plates, bowls and hinged-lid containers seems to be another fruitful field for biodegradable polymer-woodbased products. Composites where both components are biodegradable have been recently surveyed by Mohanty et al.5 Several reinforcing natural fibers and biodegradable polymers have been analyzed considering the structure-property relationship. The performance of these biomaterials was greatly influenced by properties of the fiber and fiber-polymeric matrix adhesion. Natural fiber composites based on polyesters (Bionelle and Biopol), polysaccharides (Biocell), and blends of thermoplastic starch (Bioplast and Mater-Bi) were investigated by Wollerdorfer and Bader.6 Modest increases in tensile strength were detected in polyesters composites containing up to 25 wt % of fiber. Fiber length distribution (FLD) was evaluated after solvent extraction from the polymeric matrix by optical microscopy. It was observed that FLD was considerably influenced by the kind of polymeric matrix, its rheological characteristics, and the kind of processing. Lou and Netravali7 analyzed physical and tensile properties of laminated composites based on PHBV and 20-30% of * To whom correspondence should be addressed. E-mail: emochie@ dcci.unipi.it. Telephone: +39-50-2219299. Fax: +39-50-28438.
pineapple fibers placed in a 0°/90°/0° arrangement of plies. These composites were compared with three kind of wood in both parallel and perpendicular directions. Although PHBV-pineapple fiber composites presented lower mechanical properties in the parallel direction than woods, it was superior to them in the perpendicular direction. The authors verified that the kind of mechanical failure of composites was adhesive. They proposed that PHBV-based composites can be a promise material after surface treatment of fibers. Isotactic poly[(R)3-hydroxybutyryric acid] (PHB) obtained by microbial fermentation is actually high-priced. Secondary crystallization (post crystallization on aging) is a typical negative event in the course of storage after an initial crystallization. As an outcome, this polyester is brittle with low deformability. In addition, at the process temperature, a significant drop in the molecular weight is observed due to thermal-degradation.8-13 With the aim of developing materials based on PHB, our group is carrying out studies oriented to resolve these drawbacks. The present article intended to evaluate the effect of type and amount of plasticizer on physical-chemical and mechanical properties of PHB/10 wt % beech composites through a full two-levels full factorial design.14 Experimental Sections Materials. PHB, obtained from Burkholderia sacchari, was kindly supplied by PHB Industrial S. A. (Brazil) [average molecular weight (Mw) ) 350 kDa ( 4.0%; polydispersity (Mw/Mn) ) 2.8; melt flow index (MFI) ) 14.7 ( 1.2% g/10 min at 190 °C/2.16 kg]. Beech (hardwood) wood flour was from local sources. Particle size, humidity, and approximate
10.1021/bm034507o CCC: $27.50 © 2004 American Chemical Society Published on Web 05/18/2004
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PHB-Wood Flour Composites Scheme 1
Table 1. Sample Identification Codes PHB beech B BM Bnm BMnm
poly(3-hydroxybutyrate) processed on Brabender mixer beech wood flour as received PHB/beech (90/10) composite processed on Brabender mixer PHB/beech (90/10) composite processed on both Brabender mixer and compression molded. PHB/beech(90/10) plasticized composite processed on Brabender mixer at different composition (n ) 5 and 20) of plastisizers (m ) TEGB or PEG) PHB/beech(90/10) plasticized composite film obtained by compression molding after Brabender mixing process at different composition (n ) 5 and 20) of plastisizers (m ) TEGB or PEG)
cellulose/lignin/hemi-cellulose ratio were 335 µm, 14.4 wt % and 47/27/22.15 Technical grade tri(ethylene glycol)bis(2-ethylhexanoate) (TEGB) (mp: -50 °C and bp: 344 °C) and poly(ethylene glycol) (PEG) (Mn ca. 200 and Tg: -65 °C) were used as plasticizers. Scheme 1 shows the chemical structures of the plasticizers PEG and TEGB and of PHB polymer. Sample Processing. Before the melt compounding process, wood flour and PHB components were mixed physically and dried in a vacuum at ambient temperature for 48 h. Composite processing was performed in a torque rheometer W 50 EHT (with roller blade) connected to a Plastograph Can-Bus Brabender. Composite films of about 312 ( 19 µm (mean for 30 measurements with confidence interval at 95%) were obtained by compression molding in a laboratory mold press. Process temperature and residence time for both torque rheometry and compression molding were 170 °C by 7 min and 180 °C by 4 min, respectively, and were based on data of PHB process optimization obtained by our group.12 Sample identification codes are presented in Table 1. Characterization. A Jasco PU-1580 HPLC liquid chromatograph connected to Jasco 830-RI and Perkin-Elmer LC75 spectrophotometric (λ ) 260 nm) detectors and equipped with two PLgel 5 µ mixed-D columns was used to obtain molecular weights and polydispersities. Chloroform was used as eluent at 1.0 mL min-1 flow rate. Monodisperse polystyrene standards were used for calibration. A Mettler TA 4000 System instrument consisting of a DSC-30 differential scanning calorimeter, a TGA-50 furnace with a M3 microbalance, and TA72 GraphWare software were employed for thermal analyses. DSC samples of 1015 mg were weighed in a 40 µL aluminum pan and an empty pan was used as reference. Measurements were carried out under 80 mL min-1 nitrogen flow rate according to the following protocol: first and second heating from -30 to +210 °C at 10 °C min-1; first cooling (quenching after the first heating) from +210 to -30 °C at 100 °C min-1. TGA evaluations were performed on ca. 20 mg samples at 10 °C min-1 from 25 to 700 °C, under 200 mL min-1 nitrogen flow rate. Dynamic mechanical properties were evaluated in the three-point bending mode using a Rheometrics DMTA V. Thin rectangular strip specimens (22 × 5 × 0.4 mm) were
Table 2. Coded Design of Experiment (DOE) in Standard Order for 23 Full Factoriala expt
processing
plasticizer amount
type of plasticizer
sample
(1) a b ab c ac bc abc
Brabender (-) Brabender-Molding (+) Brabender (-) Brabender-Molding (+) Brabender (-) Brabender-Molding (+) Brabender (-) Brabender-Molding (+)
5 wt % (-) 5 wt % (-) 20 wt % (+) 20 wt % (+) 5 wt % (-) 5 wt % (-) 20 wt % (+) 20 wt % (+)
PEG (-) PEG (-) PEG (-) PEG (-) TEGB (+) TEGB (+) TEGB (+) TEGB (+)
B5P BM5P B20P BM20P B5T BM5T B20T BM20T
a (-) Low and (+) high level variables; (1) The low level of each factor; a, b, and c the main effects; ab, ac, and bc two factors interaction; abc three factors interaction. Bold letters correspond to a 22 factorial design applied with molecular weight response.
investigated from -30 to +130 °C at a heating rate of 4 °C min-1, under nitrogen atmosphere. At a frequency of 1 Hz, linear viscoelasticity was observed for strain at 0.04%. Tensile measurements for tensile strength, elongation, Young’s modulus, and toughness were performed with an Instron testing system (model 5564, Instron Corporation, Canton, MA). Samples were cut into microtensile test specimens (distance between tabs 22.25; width 4.75 mm) and preconditioned for more than one week at 25 °C and 50% RH inside desiccators containing saturated solutions of magnesium nitrate. A micrometer was used to monitor film thickness. Testing protocols were based on ASTM Standard D170893 and D638M-93. Pneumatic grips of the testing machine were set at an initial separation of 22.2 mm. Crosshead speed was set at 1 mm/min. At least 12 specimens for each treatment were tested. Wood flour adhesion to the PHB matrix was evaluated with a JEOL 5600 LV scanning electron microscopy (SEM) operating at 12 kV. Sample films were fractured in liquid nitrogen and then vacuum metallized prior to observations. Table 2 presents the selected settings for a 23 and 22 full factorial design and coded design of experiment (DOE) runs for PHB-based composites. Results and Discussion PHB experiences drops in molecular weight when heated at high temperature (ca. 180 °C) and/or in the presence of
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Table 3. Effect of Beech Wood Flour and Plasticizer on Both PHB Mw and Mw/Mn sample
Mw (kDa)
Mw/Mn
PHB BM BM5P BM5T BM20P BM20T
275 ( 43 137 ( 66 174 ( 33 213 ( 15 133 ( 0.5 183 ( 8
2.1 2.4 2.1 2.2 2.0 2.1
Table 4. Calculated Effects for PHB Mw Response from a 22 Factorial Design effect average main effect type of plasticizer (TP) plasticizer amount (PA) two factors interaction TP × PA
estimated effects and standard errors (kDa) 176 ( 3.7 44.5 ( 7.5 -35.5 ( 7.5 -5.5 ( 7.5
substances containing hydroxyl groups.12,16,17 PHB average molecular weight (Mw) and its polydispersity (Mw/Mn) were evaluated after its chloroform extraction from plasticized PHB-Bwf composites films. GPC data by a refractive index (RI) detector are presented in Table 3. The error was calculated using Student’s t test at a confidence level of 95% for three replications. As can be observed, not only the addition of 10 wt % of beech (BM) can change PHB Mw. Both the type and the amount of plasticizer has a supplementary effect. PHB molecular weights from plasticized composites resulted in general higher than that from PHBBwf composite (BM). Melt viscosity of polymers decreases when plasticized. Accordingly, it is postulated that at least two factors contribute to the changes of the PHB Mws from plasticized composites. Decreasing melt viscosity will consequently decrease the PHB shear in the presence of wood flour. Besides, it is possible that the establishment of a hydrogen bond between plasticizer and wood flour moderate the harmful effect of the hydroxyl groups. Data from Table 3 were analyzed statistically to verify if there is interdependence between both type and amount of plasticizer on PHB Mw. Table 4 shows the results of the factorial analysis with three replications for the samples highlighted in bold in Table 2. Both the type and the amount of the plasticizer main effect are large. This means that higher PHB Mw decreasing was observed for composites plasticized with PEG. The effect difference between both plasticizers was ca. 44 kDa. Besides, increasing the plasticizer amount in the PHB-Bwf composites causes a reduction of the PHB Mw with a estimated value of ca. 35 kDa when going from 5 to 20 wt %. Therefore, the formulation with a less negative effect on PHB Mw is represented by the sample with 5 wt % of TEGB (BM5T). Two factors interaction indicated that there is not any interdependence between these variables in the range studied. Thermo-decomposition of PHB follows one step. Beech wood flour and parent composites thermo-decompose in two steps as indicated by the TGA and DTGA profiles in Figure 1, parts a and b, respectively. Thermo-gravimetric data of
Figure 1. TGA (a) and DTGA (b) traces of PHB, Beech wood flour, 10 wt % composite (sample B) and composite with 5 wt % of PEG. Table 5. Thermogravimetric Data of Plasticized PHB/Beech Wood Flour Compositesa sample
WL25-150 (wt %)
Td (°C)
Tp1 (°C)
PHB B BM B5P BM5P B20P BM20P B5T BM5T B20T BM20T Beech
nd 1.0 0.3 0.8 0.5 2.5 0.3 0.9 0.3 0.8 9.5
281 284 278 263 281 271 274 281 285 281 287 275
301 303 297 281 300 294 294 300 303 303 308
Tp2 (°C)
residueb (wt %)
365 365 342 368 nd 359 359 365 359 371 362
0.25 0.82 1.32 0.66 1.26 0.76 0.90 0.66 0.90 0.49 1.05 17.07
a WL 25-150 is the weight loss between 25 and 150 °C: Tp is the temperature of maximum decomposition rate and Td is the onset decomposition temperature at the crossover of tangents drawn on both side of the decomposition trace with a slope of ca. -1.8% K-1; nd ) not detected. Standard error of mean on Td data: (1°C. b At 690 °C.
PHB-Bwf composites as a function of type and amount of plasticizers and processing (see Table 2) are shown in Table 5. In this table, the WL25-150 means the weight loss probably due to moisture content. Samples processed on a Brabender mixer present lower moisture contents than that re-processed by compression molding. Besides, composites containing 20
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PHB-Wood Flour Composites Table 6. Estimated Effects for Decomposition Temperature (Td) of PHB from a 23 Factorial Design effect average main effect processing (P) plasticizer amount (PA) type of plasticizer (TP) two factor interaction P × PA P × TP PA × TP three factor interaction P × TP × PA
estimated effects and standard errors (°C) 277.9 ( 1.5 7.7 ( 2.9 0.7 ( 2.9 11.2 ( 2.9 -3.2 ( 2.9 -2.7 0.2
Table 7. Effect of Composite Composition on PHB Thermal Parametersa sample
Tg (°C)
PHB B B5P B5T B20P B20T Beech
1.1 0.4 -7.5 -6.6 -20.3 -26.5 162.8
Tcc1 ∆Hcc1 Tcc2 ∆Hcc2 (°C) (J g-1) (°C) (J g-1) 52.0 47.2 37.0 36.6 27.1 21.9
44.7 43.1 34.5 22.5 41.9 31.7
80.3 78.6 77.6 78.6 73.8 69.7
34.6 31.8 34.8 28.2 46.5 28.3
Tm (°C)
∆ Hm (J g-1)
Xc (%)
174.0 174.2 171.4 172.3 162.3 166.0
81.4 84.3 83.6 82.9 83.8 90.4
55.7 57.8 57.2 56.8 57.4 61.9
a T , T , and T are inflection point glass transition, cold crystallization g cc m and melting temperatures, respectively; ∆Hcc and ∆Hm are cold-crystallization and melting enthalpy; Xc is the degree of crystallinity.
4.2
wt % PEG indicated higher propensity to uptake moisture. Most likely these characteristics are concerned not only with the capacity of the sample to absorb moisture but also with the nature of the container used to keep the samples prior to analysis. Samples from the Brabender mixer were left in a closed plastic bag, and the compression molding films were left in a paper envelope. Tp1 and Tp2 are the temperatures of the maximum thermodegradation rate obtained from the first derivative traces (Figure 1b). They are related to the first and the second weight loss step after 150 °C, respectively. The first weight loss corresponds to PHB thermo-degradation. Its maximum thermo-degradation rate is at ca. 300 °C. Tp1 showed values slightly lower for composites plasticized with PEG and viceversa for that with TEGB. The second step on composite thermo-degradation is related to Bwf. Tp2 values apparently do not show any correlation with the studied variables. Table 6 shows the estimated effects of the experiments following the 23 factorial design presented in Table 2, without replicates, on the decomposition temperature (Td) of PHB in the plasticized PHB-Bwf composites. The estimates of standard errors were not direct since there were no replicates for Td measurements. However, it was verified that only the processing × plasticizer amount (P × PA) two factors interaction effect does not lie along the straight line as the data are plotted in a normal probability paper (data not shown). This observation indicates that these two variables are interdependent and that have a significant effect on Td of PHB in the composites. On the other hand, the P × TP, PA × TP, and P × TP × PA interactions effects are negligible. So, an estimation of the error was obtained by combining their mean squares and is reported in Table 6. An increase in processing times of plasticized PHB-Bwf composites from mixing (Brabender) to mixing and molding (Brabender-Molding) increases the decomposition temperature (Td) of PHB by ca. 8 °C (main effect, processing (P)). Thermo-decomposition of PHB follows a cis-elimination reaction producing carboxylic acid and unsaturated end groups.18,19 Increasing processing times will increase the probability of PHB thermo-degradation and consequently the concentration of these end groups. Therefore, it is possible that these new groups will favor new interactions with the other components that will result in a more stable PHB. Different behavior was observed in the nonplasticized
samples (B and BM). This means Td of nonplasticized composites decreased by about 6 °C when processed two times (Brabender-Molding). Changing the type of plasticizer (TP) from PEG to TEGB increases the Td of PHB by ca. 11 °C. This result is in accordance with literature16,17 by considering that the amount of hydroxyl groups in PEG plasticizer is higher than that in TEGB. The plasticizer amount (PA) was not significant as main effect for the levels 5 and 20 wt %. However, its two factors interaction with processing (P × PA) is to be take into account as noticed above. For both levels of plasticizer amount, Td of PHB increases with increasing of processing times (from Brabender to Brabender-Molding, see Table 2). However, this behavior was more significant for the composites plasticized with TEGB. Thermal parameters shown in Table 7 were evaluated from the DSC traces of melt-quenched samples. The glass transition temperature (Tg) of PHB in the presence of 10 wt % of beech wood flour (sample B) decreased slightly. Avella et al.20 observed a slight Tg increases of PHB in steam exploded straw fiber composites. Their explanation, as substantiated by infrared spectroscopy, was based on the interaction between CO groups of PHB and OH groups of cellulose that decreases chain mobility. In the present work, one possible justification for the above result would be the lower molecular weight of PHB in the composite after processing. On the other hand, plasticized PHB composites have Tg values corresponding to those of PHB-plasticizer blends.17 PHB presents two cold crystallization peaks at ca. 52 °C and 80 °C, after a melt-quenching thermal treatment. The first peak temperature (Tcc1) decreases about 5 °C in the composite without plasticizer (sample B). This composite containing 5 or 20 wt % of plasticizers showed additional Tcc1 decreasing of ca. 15 and 30 °C, respectively. Composites based on PHAs with modified pine wood and microcrystalline cellulose presented comparable results. Reinsch and Kelley concluded that wood fiber reinforcements increase the crystallization rate with consequent Tcc depression. Besides, the presence of wood fibers induces changes in the nucleation mechanism and growth geometry but not in the degree of crystallinity.21 Crystallization enthalpy (∆Hcc1+∆Hcc2) was generally lower than that of melting (∆Hm), principally for the PHBBwf composites plasticized with TEGB. The observed difference was about 30 J g-1. In this case, not only the
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Figure 2. SEM photomicrography of (a) BM5P, (b) BM20P, (c) BM5T, and (d) BM20T composites. Table 8. Effect of Beech Wood Flour and Plasticizer on Both PHB Dynamic Elastic Modulus (E ′) and Young Modulus (E) at 25 °C sample
E ′ (GPa)
E (GPa)
PHB BM BM5P BM5T BM20P BM20T
4.5 4.0 3.3 3.1 1.8 1.3
2.0 2.2 1.9 1.8 1.2 0.9
presence of wood flour has a nucleating effect but TEGB too. It is possible that these results have some contributions of not controllable variables (instrumental and sample homogeneity). Therefore, a further investigation of the effect of TEGB on the crystallization of PHB-Bwf composites seems interesting. Melting temperature peak (Tm) depression was observed as a function of plasticizer amount for all plasticized composites. Consequently, the addition of plasticizer into composites produces a less perfect crystalline phase compared with composites formulated in the absence of plasticizer. Table 8 displays the effect of beech wood flour and plasticizers in both dynamic (E′) and static modulus (E) obtained by three point bending DMTA and stress-strain tensile test, respectively at 25 °C. The addition of 10 wt % of Bwf (sample BM) does not increases so much Young modulus (E) of composite. Besides, composites with 5 wt % of plasticizer reveal values that are equivalent to that of the PHB matrix. Both moduli of PHB-Bwf composites decreased with the addition of plasticizers. The lower values were verified for the composites plasticized with TEGB, confirming its superiority plasticization capacity in relation to PEG. In these plasticized composites, the opposite effect of each component (wood and plasticizer) seems to be compensated by maintaining approximately original matrix characteristics.
As can be observed in Figure 2, the adhesive compliance between PHB and beech in the presence of both PEG and TEGB plasticizers does not appear to be strongly effective. Furthermore, the presence in PHB/beech/TEGB composites of a stain at the polymer/wood flour interface independently of plasticizer amount may be due to fraction of TEGB adsorbed by the wood flour when processed in Brabender and then when compression molded migrated to the surface. TEGB is a good plasticizer for PHB17 and as a consequence was absorbed by adjacent matrix. Concluding Remarks Composites based on plasticized PHB 10 wt % Bwf were prepared for an experimental screening based on a two level factorial design. The variables studied were type and amount of plasticizer and processing (times that the composites were worked at temperatures higher than that of melting). The responses analyzed statistically were the PHB weight average molecular weight (Mw) and onset decomposition temperature (Td). Addition of plasticizers decreased PHB Mw. More positive results were found for PHB-Bwf containing 5 wt % of tri(ethylene glycol)bis(2-ethylhexanoate) (TEGB). Interdependence between both variables was not observed. Besides, this formulation with TEGB indicated a possible protection of PHB from the degradative effect exerted by Bwf. Probably this protection is due to the lowering of melt viscosity. The results of thermal degradation were analyzed considering not only kind and type of plasticizer but also the processing times. Plasticized composites processed two times (mixing followed by compression molding) showed higher thermal stability. As indicated above, composites plasticized with TEGB are more stable than PHB alone. Temperature depression at glass transition (Tg), cold crystallization (Tcc), and melting (Tm) was observed for
PHB-Wood Flour Composites
plasticized PHB-Bwf composites. These behaviors can be attributed to the increase of chain mobility associated with plasticization of PHB and the nucleating effect of both wood flour and plasticizer. Once again, composites plasticized with TEGB were the ones with the best results. The same considerations hold true when the composite modulus is considered. Acknowledgment. The work was performed within the framework of the E.C. funded project “Dairy Industry Waste as Source for Sustainable Polymeric Material Production”, WHEYPOL, Growth Project GRD2-2000-30385. The authors thank Dr. Sylvio Ortega Filho of the PHB Industrial S.A. (Brazil) for having kindly supplied the PHB sample and Dr. Piero Narducci of the Department of Chemical Engineering of the University of Pisa for SEM micrographs. References and Notes (1) Abe S. Jpn. Kokai Tokkyo Koho, Jp 2003206367; Tosoh Corp.: Japan, 2003; pp 9. (2) Lai, S.-M.; Yeh, F.-C.; Wang, Y.; Chan, H.-C.; Shen, H.-F. J. Appl. Polym. Sci. 2003, 87, 487-496. (3) Wang, Y.; Yeh, F.-C.; Lai, S.-M.; Chan, H.-C.; Shen, H.-F. Polym. Eng. Sci. 2003, 43, 933-945. (4) Black, S. Extruded wood-filled/thermoplastic composite materials experience explosiVe growth; www.compositesworld.com/ct/issues/ 2003/june/136. (5) Mohanty, A. K.; Misra, M.; Hinrichsen, G. Macromol. Mater. Eng. 2000, 276/277, 1-24.
Biomacromolecules, Vol. 5, No. 4, 2004 1205 (6) Wollerdorfer, M.; Bader, H. Ind. Crops Prod. 1998, 8, 105-112. (7) Luo, S.; Netravali, A. N. J. Mater. Sci. 1999, 34, 3709-3719. (8) Sudesh, K.; Abe, H.; Doi, Y. Prog. Polym. Sci. 2000, 25, 15031555. (9) De Koning, G. Can. J. Microbiol. 1995, 41, 303-309. (10) Hammond, T.; Liggat, J. J. Properties and applications of bacterially derived polyhydroxyalkanoates. In Degradable Polymers; Gilead, D., Ed.; Chapman & Hall: London, 1995; pp 88-111. (11) Melik, D. H.; Schechtman, L. A. Polym. Eng. Sci. 1995, 35, 17951806. (12) Chiellini, E.; Fernandes, E. G.; Pietrini, M.; Solaro, R. Macromol. Symp. 2003, 197, 45-55. (13) Renstad, R.; Karlsson, S.; Albertsson, A.-C. Polym. Degrad. Stab. 1997, 57, 331-338. (14) Montgomery, D. C. Design and Analysis of Experiments, 3rd ed.; John Wiley & Sons: New York, 1991. (15) Fara, S. Caratteristiche Fondamentali del Legno. Materiali Fibrosi e AdesiVi Course; Industrial Chemistry and Chemical Engineering Department of the Politecnico di Milano, http://pcsiwa12.rett.polimi.it/ ∼ciic/esami/mafib/pdf. (16) Lehrle, R.; Williams, R.; French, C.; Hammond, T. Macromolecules 1995, 28, 4408-4414. (17) Fernandes, E. G.; Pietrini, M.; Chiellini, E. Macromol. Symp. In press. (18) Kopinke, F. D.; Mackenzie, K. J. Anal. Appl. Pyr. 1997, 40-41, 43-53. (19) Nguyen, S.; Yu, G.-e.; Marchessault, R. H. Biomacromolecules 2002, 3, 219-224. (20) Avella, M.; Martuscelli, E.; Pascucci, B.; Raimo, M.; Focher, B.; Marzetti, A. J. Appl. Polym. Sci. 1993, 49, 2091-2103. (21) Reinsch, V. E.; Kelley, S. S. J. Appl. Polym. Sci. 1997, 64, 1785-1796.
BM034507O
