Novel Talc-Filled Biodegradable Bacterial Polyester Composites

Sep 22, 2006 - The tensile and flexural moduli of PHBV-based composites were improved by 70 ... The storage modulus (E′) and heat deflection tempera...
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Ind. Eng. Chem. Res. 2006, 45, 7497-7503

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Novel Talc-Filled Biodegradable Bacterial Polyester Composites Audrey Whaling, Rahul Bhardwaj, and Amar K. Mohanty* Michigan State UniVersity, School of Packaging, 130 Packaging Building, East Lansing, Michigan 48824

Talc-filled composites of a bacterial polyester, i.e., poly(3-hydroxybutyrate-co-3-hydroxyvalerate), PHBV, were prepared by extrusion followed by an injection molding process. The effects of the talc weight content (15-50 wt %) on various mechanical and thermomechanical properties of the PHBV-based composites were studied. The tensile and flexural moduli of PHBV-based composites were improved by 70 and 166%, respectively, when reinforced with a talc content of 50 wt %. Theoretical predictions were used to compare the experimental tensile modulus values of talc-filled PHBV composites. Comparison of the experimental results with theoretical predictions predominantly favored a spherical geometry of the talc particles in the PHBV matrix rather than a platy morphology. The storage modulus (E′) and heat deflection temperature (HDT) of the PHBV-based composites were found to increase when the composites were reinforced with talc particles. The coefficient of linear thermal expansion (CLTE) of PHBV was reduced by 56% for the composite reinforced with a talc content of 50 wt %. The filler orientation, filler dispersion, and filler-matrix adhesion were investigated by scanning electron microscopy (SEM). Introduction The realm of biodegradable plastics is expanding as a result of the depletion of resources of the petroleum-based plastics and the increasing environmental problems associated with plastic wastes. Among biodegradable plastics, poly(hydroxyalkanoate)s (PHAs) have gained a great deal of interest because of their complete biodegradability, biocompatibility, and renewable-resource-based origin.1 Poly(hydroxyalkanoate)s (PHAs) are thermoplastic polyesters synthesized by several types of bacteria as intracellular carbon- and energy-storage compounds that accumulate as granules in the cytoplasm of their cell.2 These bacterial polyesters have properties ranging from stiff and brittle to tough and ductile materials.3 Poly(3-hydroxybutyrate) (PHB) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) are the two most extensively known poly(hydroxyalkanoate)s.4-6 PHB is a homopolymer having a highly stereoregular structure leading to a high crystallinity in this polymer. The high crystallinity leads to a stiffness-toughness imbalance in this polymer, which limits its commercial applications.7,8 PHB also experiences thermal degradation above its melting temperature during processing. PHB has been copolymerized with hydroxyvalerate (HV) in an attempt to reduce its crystallinity and melting point.5 The resulting PHBV copolymer has a higher flexibility and lower processing temperature than PHB. An increase in the hydroxyvalerate (HV) content in PHBV copolymer has a major effect in reducing its crystallinity and melting point.9 However, PHBV has some major drawbacks, which include the development of interlamellar secondary crystallization on storage, a low crystallization rate, and a high production cost.10,11 Fillers such as mica, kaolin, calcium carbonate, and talc are frequently incorporated in thermoplastics to reduce the costs of the molded products. These fillers also improve the properties of the polymers such as strength, rigidity, durability, and hardness.12 Talc is a common filler used for the modification of poly(propylene) (PP).13,14 Common applications of talc-filled poly(olefin)s are found in automotive components, garden furniture, and packaging. Talc has a platelike geometry in which an edge-shared octahedral sheet of Mg(OH)2 is sandwiched * To whom correspondence should be addressed. E-mail: mohantya@ msu.edu. Tel.: +1-517-355-3603. Fax: +1-517-353-8999.

between tetrahedral sheets of silica (SiO2). The properties of talc-filled thermoplastics depend on the dispersion and orientation of the talc particles and the interfacial adhesion between the talc particles and the matrix. Not a great deal of work has been reported on the incorporation of high talc weight contents in biodegradable polymers. Huda et al.15 recently reported talcfilled poly(lactic acid) (PLA) composites with a talc content of 30 wt % that have improved flexural and tensile properties as compared to those of neat PLA. Talc has also been incorporated into PHB and PHBV as a nucleating agent.16 The incorporation of a high talc content in PHBV bioplastic can be beneficial in improving its thermomechanical properties and reducing the amount of expensive PHBV matrix in the molded product. The lower processing temperature and better melt stability of PHBV allow for higher cycle times in the processing of talc-filled PHBV composites that can be beneficial in improving the dispersion of the talc particles. In the present study, different talc weight contents (15-50 wt %) were incorporated into PHBV bioplastic by a melt-mixing technique. The effects of varying talc weight content on the mechanical, thermomechanical, and dynamic mechanical thermal properties of PHBV were studied. The morphology of the talc-filled PHBV bioplastics was evaluated by means of scanning electron microscopy (SEM). Materials Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) [PHBV; trade name, Biopol (Zeneca Bio Products, U.K.); molecular weight, 450 kDa; valerate content, 13%] was procured from Biomer, Germany. The density of the PHBV used was 1.25 g/cm3. The talc (trade name, SILVERLINE 002) was kindly donated by Luzenac America. It is characterized as a flat white microcrystalline mineral with a median particle size of 12.5 µm. It is a soft mineral, with a loose bulk density17 of between 0.38 and 0.43 g/cm3 and a tensile modulus18 of 17.2 GPa. Processing of Talc-Filled PHBV Composites. Poly(3hydroxybutyrate-co-3-hydroxyvalerate) (PHBV) was dried in a vacuum oven at 80 °C for 3 h before processing. A microcompounding instrument (DSM Research, Geleen, The Netherlands) was used for the fabrication of unfilled and talc-filled PHBV composites. The instrument is a co-rotating twin-screw

10.1021/ie060604x CCC: $33.50 © 2006 American Chemical Society Published on Web 09/22/2006

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microcompounder having a screw length of 150 mm, a lengthto-diameter ratio (L/D) of 18, and a barrel volume of 15 cm3. The fabricated PHBV-based composites contained talc contents of 15, 30, 40, and 50 wt %. The processing temperature for PHBV and talc-filled PHBV composites was 160 °C. Different cycle times were used for the filled PHBV composites depending on the talc loading. The processing cycle time was 6 min for composites having talc contents of 15 and 30 wt %, whereas it was 8 min for composites having talc contents of 40 and 50 wt %. The extruder screw rotation is crucial for the dispersion of talc microparticles in the polymer matrix. The screw speed was 50 rpm during the filling of talc and PHBV into the microextruder, whereas during processing, the speed was 100 rpm. The molten material was transferred to a small mini-injection molder for the preparation of the various test specimens. Testing and Characterization. (i) Morphological Studies. Scanning electron microscopy (JEOL model JSM-6400) was used to evaluate the morphology of the talc particles and impactfractured surfaces of the talc-filled PHBV composites. The SEM instrument had lanthanum hexaboride (LaB6) crystal as an electron emitter. (ii) Mechanical Properties. Tensile properties of neat and talc-filled PHBV were measured using a United Calibration Corp SFM 20 testing machine as per ASTM standard D638. The specimens were tested at a crosshead speed of 50.8 mm/ min. Flexural properties were measured according to ASTM standard D 790. The load was applied to the rectangular bar at a crosshead speed of 1.25 mm/min. The notched izod impact strength was measured with a TMI 43-02-01 monitor/impact machine according to ASTM standard D256. The impact energy of the pendulum was 1.35 Nm. (iii) Thermomechanical Properties. The storage moduli (E′) of the neat and talc-filled PHBV composites were evaluated with a TA Instruments DMAQ800 apparatus. The tests were carried out by heating the samples at a rate of 2 °C/min from 30 to 80 °C. The samples were tested in a single canteliverbending mode at an oscillating amplitude of 15 µm and a frequency of 1 Hz. The heat deflection temperatures (HDTs) of the neat polymers and their composites were measured by using the DMAQ800 instrument as per ASTM standard D648. Rectangular bars of nominal size 1.99 mm × 12 mm × 58 mm were used for testing. A three-point bending mode was used to apply a load of 66 psi. The samples were heated at a rate of 2 °C/min from room temperature to the desired temperature. Values of the coefficient of linear thermal expansion (CLTE) of the neat polymers and their composites were determined by an expansion probe using a TMA 2940 thermomechanical analyzer. The samples were heated at a rate of 3 °C/min from 30 to 90 °C. Results and Discussion Morphology of Talc-Filled PHBV Composites. Scanning electron microscopy (SEM) images of the talc particles and talc-filled PHBV composites are shown in Figures 1-4. The talc particles were dispersed in a clear background for SEM observation. The Figure 1A shows that the talc consisted of particles of varying sizes. Both spherical and platy morphologies are evident in Figure 1. Figure 2 shows the morphologies of impact-fractured surfaces of talc-filled PHBV composites having talc contents of 15 and 30 wt %. Figures 3 and 4 show lowand high-magnification images of talc-filled PHBV composites having talc contents of 40 and 50 wt %, respectively. The talc particles were randomly oriented and dispersed in the PHBV matrix with increasing talc content. Talc particles are anisotropic

Figure 1. SEM images of talc particles: (A) low magnification, (B) high magnification.

and tend to orient in the flow direction during processing. The orientation of the particles has a significant effect on the strength and stiffness of a composite. The orientation and orientation distribution of particles in a polymer melt depend on a number of processing factors such as the flow pattern, shear conditions, mold filling rate, and cooling conditions.19 In the present study, a microcompounder was used to mix the talc particles into the PHBV matrix, and a small injection molder was used for the fabrication of test specimens. There was lack of continuous shear stress and flow pattern during the processing. The flow path of the molten material was also very short during the injection molding. Therefore, the large-scale orientation of the talc particles in the PHBV matrix was not expected under these conditions. The talc particles were imbedded in the PHBV matrix at low talc loading (15 wt %) but were visible on the fractured surface for the composites having high talc loadings (30, 40, and 50 wt %). A rheological study of PHBV indicated that it exhibited a shear-thinning regime in a particular range of shear rates.20 It was expected that the low melt viscosity of PHBV could provide a better dispersion of talc particles in the PHBV matrix. However, the talc-filled composites having talc contents of 30, 40, and 50 wt % showed the aggregation of talc particles, the formation of an air cave indicated poor filler dispersion, and filler-matrix adhesion. The dispersion of particles in a polymer matrix also depends on factors such as the shear stress developed to separate the particles during processing and the relative magnitudes of the adhesion and separating forces.21-23

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Figure 2. SEM images of impact-fractured surfaces of the talc-filled PHBV composites: (A) PHBV/talc, 85/15; (B) PHBV/talc, 70/30.

Figure 3. SEM images of impact-fractured surfaces of a PHBV/talc (60/40) composite: (A) low magnification, (B) high magnification.

Mechanical Properties. (i) Tensile Strength and Modulus of Talc-Filled PHBV. The tensile strength and modulus values of PHVB and its talc-filled composites are shown in Figure 5. The tensile modulus of talc-filled PHBV composites improved by 70% upon incorporation of talc contents of 40 and 50 wt %. The morphology of talc (see Figure 1) suggested that it has a platy structure. Therefore, the improvement in the modulus could be attributed to the platy structure of the talc, which allowed better wettability between the filler and polymer matrix, hence leading to better stress transfer.24 However, only a fraction of talc particles had a platy structure, and the rest of the particles were spherical in shape or existed as spherical aggregates in the PHBV matrix. Therefore, the reinforcing ability of talc in the PHBV-based composites was restricted at higher talc weight contents. The observed constancy in the tensile modulus value of talc-filled PHBV composites having talc contents of 40 and 50 wt % might occur because of the insufficient dispersion of the talc particles in the composites containing a talc content of 50 wt %. It was observed that the tensile strength of talc-filled PHBV decreased as the talc content increased from 15 and 30 wt %. However, the tensile strength of the talc-filled PHBV composites improved to the original value of neat PHBV upon incorporation of talc contents of 40 and 50 wt %. This trend could be explained on the basis that, at a higher loading of talc, the particles were able to restrain the PHVB matrix from deformation, thus allowed the composite to withstand a higher stress at relatively lower strains. Our group has previously reported a similar trend in the tensile strength values of PHBV

composites reinforced with recycled cellulose fiber.25 Another possible explanation could be based on the morphology of the talc particles. SEM investigation (Figure 1) of the dispersed talc particles revealed that the talc particles were not of uniform size; rather, they had a graded size. The volume fraction of small particles increased with increasing talc volume fraction. These graded particles can form more compact composite structures in which the smaller particles can help in filling voids and reducing the spaces between larger particles. The densely packed cooperative particles can enhance the stress transfer ability and improve the composite tensile strength at higher talc loadings.26 The experimental values of the tensile moduli of the talcfilled PHBV composites were compared with theoretical predictions to understand the morphology, dispersion, and reinforcing ability of the talc particles in the PHBV matrix. The SEM images of the talc particles suggested that the composition of the talc contained platy as well as spherical geometries. Thus, these two morphological forms of talc particles i.e., platelets and spheres, were considered while choosing theoretical equations. The Einstein equation,27 which was obtained from the analogous viscosity equation, can be used for the straightforward prediction of the composite modulus. The Einstein equation is

Ec ) Em(1 + 2.5φ)

(1)

where Ec and Em are the composite and matrix moduli, respectively, and φ is the filler volume fraction. This equation

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Figure 6. Comparison of experimental data on tensile moduli of talc-filled composites with theoretical predictions.

The Halpin-Tsai equation is as follows

Ec 1 + ABφf ) Em 1 - Bφf

(2)

where B ) (Ef / Em - 1)/(Ef / Em + A), with A ) 2(l/d); Ef is the modulus of the filler; and l/d represents the aspect ratio of the filler. The average aspect ratio of the talc particles was taken as 10 based on the SEM observations. Lewis and Nielsen30 suggested a modification in the Halpin-Tsai equation by including a factor of the maximum packing fraction of the filler, φmax. φmax is defined as the ratio of the true volume of the filler to the apparent volume occupied by the filler.30 The Lewis and Nielsen modification provides the following equation

Ec 1 + ABφf ) Em 1 - BΨφf where Figure 4. SEM images of impact-fractured surfaces of PHBV/talc (50/50) composites: (A) low magnification, (B) high magnification.

Figure 5. Tensile strength and modulus values for PHBV and its talcfilled composites: (A) neat PHBV, (B) PHBV/talc (85/15), (C) PHBV/talc (70/30), (D) PHBV/talc (60/40), (E) PHBV/talc (50/50).

implies that the reinforcement effect of the filler is independent of its size but that the volume of the filler is a crucial variable. The usefulness of this equation is restricted to low filler concentrations. The Halpin-Tsai equation28,29 indicates the significance of the reinforcement geometry on the modulus of a composite material. The modulus of a composite material improves by several orders of magnitude as the reinforcement geometry changes from a sphere to a fiber. The prominent effect comes from the high aspect ratio of the fiber-type geometry.

ψ)1+

(

)

1 - φmax φmax2

(3)

φf

Two conditions were chosen for applying the Lewis and Nielsen equation (corresponds to Lewis and Nielsena and Lewis and Nielsenb in the legend to the Figure 6): (a) The maximum packing fraction31 and aspect ratio were chosen as 0.602 and 2, respectively, considering the spherical morphology of the talc particles and the random loose packing of the filler in the matrix and (b) the maximum packing fraction31 and aspect ratio were taken as 0.52 and 10, respectively, considering the platelet type of morphology of talc particles with random orientations. Figure 6 presents a comparison of the experimental values of the tensile moduli of the talc-filled PHBV composites with the theoretical predictions. Einstein’s equation, which considers a spherical filler geometry and does not take the aspect ratio of the filler into account, showed a reasonable fit to the experimental data. The experimental data points were considerably low compared to the theoretical predictions that take the aspect ratio and maximum filler packing fraction into account. This analysis suggests that the talc particles were not dispersed appropriately in the PHBV matrix and that there was minimal reinforcing effect of the talc particles on the modulus values of the composites from their platy morphology. The theoretical equations that consider a spherical filler geometry fall closer to the experimental values, indicating that the majority of the talc particles have a spherical geometry and that there was also spherical aggregation of the flaky talc particles in the PHBV matrix.

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Figure 7. Variation of flexural strength and modulus values of talc-filled PHBV composites with increasing weight content of talc particles.

Figure 8. Dependency of notched izod impact strength values of talcfilled PHBV composites with increasing weight content of talc particles.

(ii) Flexural Properties of Talc-Filled PHBV. Figure 7 shows the effect of the talc content on the flexural strength and modulus values of the talc-filled PHBV composites. The flexural strength and modulus values of the talc-filled PHBV composites having high talc contents, i.e., 40 and 50 wt %, improved compared to those of the neat PHBV matrix. The flexural strength decreased, and the modulus of the talc-filled composite improved slightly with the addition of a talc content of 30 wt %. The probable reason for this result might be the insufficient talc particle dispersion in the PHBV matrix because of the lower processing cycle of 6 min for this composition. This result could also be interpreted as indicating that insufficient talc dispersion caused a higher amount of PHBV resin to appear at the surface of the flexural test specimens. In a flexural test, the maximum stress is produced at the surface of the specimen,30 so it can be believed that there was improper stress transfer from the PHBV matrix to the rigid talc particles, thus giving an error in the flexural modulus of the talc-filled PHBV composite having a talc content of 30 wt %. However both the flexural strength and modulus showed a significant improvement for the talcfilled composites having talc contents of 40 and 50 wt %. The flexural strength and modulus of the talc-filled PHBV samples improved by around 38 and 166%, respectively, as compared to those of neat PHBV upon incorporation of a talc content of 50 wt %. (iii) Impact Strength. The notched izod impact strength values of talc-filled PHBV composites as a function of talc weight content are shown in Figure 8. The impact strength values of talc-filled composites decreased continuously with increasing talc weight content. There was a drop in impact

Figure 9. Effect of temperature on the storage modulus (E′) values of talc-filled PHBV composites: (A) neat PHBV, (B) PHBV/talc (85/15), (C) PHBV/talc (70/30), (D) PHBV/talc (60/40), (E) PHBV/talc (50/50).

strength of around 64% compared to that of neat PHBV for talc-filled composites having a talc content of 50 wt %. The incorporation of a rigid filler in a rigid polymer matrix usually reduces the impact strength of the polymer.32 The poor adhesion between the talc particles and the PHBV matrix could be the major reason for the decrease in the impact strength of talcfilled PHBV composites. The poor filler-matrix adhesion might have created a concentration of stress at the filler-matrix interface, thus providing a low-energy path for crack propagation and leading to a drop in the impact strength of the talc-filled PHBV composites. Thermomechanical Properties of Talc-Filled PHBV Composites. (i) Storage Modulus (E′). The effect of temperature on the storage modulus (E′) values of talc-filled PHBV composites having different talc weight contents is depicted in Figure 9. The dynamic mechanical thermal analysis (DMTA) curves for the storage modulus were collected above the glass transition temperature (Tg) of PHBV, i.e., 0.3 °C.25 There was a substantial increase in the storage modulus of the talc-filled PHBV composites with increasing talc weight content, as shown by the upward shift in the curves in the Figure 9. The effect of fillers in raising the modulus is usually larger above the glass transition temperature than below it. The higher ratio of between the filler and matrix moduli above Tg, the effect of the larger Poisson ratio in the rubbery state, and the presence of thermal stresses below Tg explain this event.31 The initial storage modulus obtained from the dynamic mechanical analysis (DMA) did not agree well with the tensile modulus. The discrepancy in this result could be attributed to the nature of the testing conditions applied during these two tests. In DMA, a lower stress is applied on the sample as compared to the tensile testing. Because of the lower stress in DMA, the weak interface of the talc-filled PHBV composite is not exposed, and it provides enough stress-transfer ability to the composite. On the other hand, the high stress in tensile testing exposes the interface, and the poor filler-matrix adhesion leads to the formation of microvoids under stress, thus limiting the stress-transfer ability of the talc-filled PHBV composites during tensile testing. The storage modulus decreased with increasing temperature because of the softening of the PHBV matrix. However, the talc-filled composites showed improved storage modulus values at all temperatures as compared to neat PHBV. The presence of the rigid filler in talc-filled PHBV might restrict the deformation of the PHBV matrix, resulting in better storage modulus values for the composite systems. (ii) Heat Deflection Temperature (HDT). The heat deflection temperatures (HDTs) of neat PHBV and its talc-filled composites are presented in Figure 10. The HDT value of neat

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the CLTE values for talc-filled PHBV could be the mechanical restraint of the PHBV matrix by the rigid talc particles, which restricted the molecular motion of the PHBV polymer chains, thus reducing the thermal expansion. Conclusion

Figure 10. Heat deflection temperatures (HDTs) of neat PHBV and its talc-filled composites: (A) neat PHBV, (B) PHBV/talc (85/15), (C) PHBV/ talc (70/30), (D) PHBV/talc (60/40), (E) PHBV/talc (50/50).

Figure 11. Effect of increasing of talc content (wt %) on the coefficient of linear thermal expansion (CLTE) of talc-filled PHBV composites.

PHBV was 106 °C. The HDT values of the talc-filled PHBV composites increased with increasing talc weight contents. The talc-filled PHBV composite having a talc content of 50 wt % showed the maximum HDT value of 121 °C. The incorporation of fillers usually improves the heat deflection temperatures of polymers. The increase of the modulus and reduction of the high-temperature creep for the filled polymers have been found to be the key factors for the improvement of the HDT value.31 The modulus-temperature dependency is also a critical factor in determining the HDT value of a material. In the present case, talc-filled PHBV composites also exhibited higher storage modulus values than did neat PHBV at higher temperatures (see Figure 9), so this result could also be correlated with the improvement in the HDT values for the talc-filled PHBV composites. (iii) Coefficient of Linear Thermal Expansion (CLTE). Figure 11 depicts the effect of the talc weight content on the coefficient of linear thermal expansion (CLTE) of the talc-filled PHBV composites. The high CLTE values of most thermoplastics are a significant negative factor as compared to those of metals and ceramics. The presence of free volume and the ability of the polymer chains to move from their equilibrium state are possible reasons for the high CLTE values of polymers and especially thermoplastics. Rigid fillers are commonly used to lower the CLTE values of the polymers. The incorporation of talc particles in the PHBV matrix decreased the CLTE value of PHBV significantly. There was a continuous decrease in the CLTE value of the talc-filled composites with increasing talc weight content. The CLTE was reduced by 56% as compared to that of neat PHBV for the talc-filled composite having a talc content of 50 wt %. One reason for the low CLTE value of talc-filled PHBV could be that the addition of low-expansion talc particles might have reduced thermal diffusion in the talcfilled PHBV composites. Another reason for the reduction of

PHBV bioplastic-based composites having different talc weight contents (15-50 wt %) were prepared by a melt-mixing technique. The talc-filled PHBV composites showed improved thermal and mechanical properties as compared to those of neat PHBV. There were moderate to significant improvements in the tensile, flexural, and storage moduli of talc-filled PHBV as compared to those of neat PHBV, even at high talc weight content (50 wt %). Interestingly, talc-filled PHBV composites showed recovery in the tensile and flexural strength above a certain talc weight content. The heat deflection temperature (HDT) of talc-filled PHBV was improved, and the coefficient of linear thermal expansion (CLTE) value was decreased as compared to those of unfilled PHBV. Scanning electron microscopy (SEM) revealed poor filler dispersion and filler-matrix adhesion in the talc-filled PHBV composites. A correlation of the experimental results with theoretical models predicted a spherical geometry of the talc particles in the PHBV matrix. The use of a high talc content in the PHBV-based composites was advantageous in reducing the amount of expensive PHBV resin and improving its thermomechanical properties. Acknowledgment The financial support from NSF PREMISE-II, DMI-0400296, is gratefully acknowledged. The authors express their appreciation to Luzenac America and Biomer, Germany, for supplying the talc and PHBV, respectively. The authors are thankful to Sanjeev Singh for his help during the preparation of this manuscript. Literature Cited (1) Lenz, R. W.; Marchessault, R. H. Bacterial Polyester: Biosynthesis, Biodegradable Plastics and Biotechnology. Biomacromolecules 2005, 6, 1-8. (2) Hocking, P. J.; Marchessault, R. H. Chemistry and Technology of Biodegradable Polymers, 1st ed.; Griffin, G. J. L., Ed.; Chapman and Hall: Glasgow, Scotland, 1994. (3) Noda, I.; Green, P. R.; Satkowski, M. M.; Schechtman, L. A. Preparation and Properties of a Novel Class of Polyhydroxyalkanoate Copolymers. Biomacromolecules 2005, 6, 580-586. (4) King, P. P. Biotechnology: An Industrial View. J. Chem. Technol. Biotechnol. 1982, 32, 2-8. (5) Holmes, P. Applications of PHBsA microbially produced biodegradable thermoplastic. Phys. Technol. 1985, 16, 32-36. (6) Bluhm, T. L.; Hamer, G. K.; Machessault, R. H.; Fyfe, C. A.; Veregin, R. P. Isodimorphism in Bacterial Poly(β-hydroxybutyrate-co-βhydroxyvalerate). Macromolecules 1986, 19, 2871-2876. (7) Doi, Y. Microbial Polyesters; VCH Publishers: New York, 1990. (8) Holmes, P. A. DeVelopment in Crystalline Polymers 2; Basset, D. C., Ed.; Elsevier: London, 1988. (9) Kunioka, M.; Tamaki, A.; Doi, Y. Crystalline and Thermal Properties of Bacterial Copolyesters: Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) and Poly(3-hydroxybutyrate-co-4-hydroxybutyrate). Macromolecules 1989, 22, 694-697. (10) Biddlestone, F.; Harris, A.; Hay, J. N.; Hammond, T. Physical Aging of Amorphous Poly(hydroxybutyrate) Polym. Int. 1996, 39, 221-229. (11) Zhang, J.; McCarthy, S.; Whitehouse, R. Reverse Temperature Injection Molding of Biopol and Effect on Its Properties. J. Appl. Polym. Sci. 2004, 94, 483-491. (12) Khunova, V.; Hurst, J.; Janigova, I.; Smatko, V. Plasma treatment of particulate polymer composites for analyses by scanning electron microscopy II. A study of highly filled polypropylene/calcium carbonate composites. Polym. Test. 1999, 18, 501-509.

Ind. Eng. Chem. Res., Vol. 45, No. 22, 2006 7503 (13) Rothon, R N. Particulate-Filled Polymer Composites; Longman Scientific & Technical: Harlow, Essex, U.K., 1995. (14) Wypych, G. Handbook of Fillers; ChemTec Publishing: Toronto, Canada, 1999. (15) Huda, M. S.; Drzal, L. T.; Misra, M.; Mohanty, A. K.; Williams, K.; Mielewiski, D. F. A Study on Biocomposite from Recycled Newspaper Fiber and Poly(lactic acid). Ind. Eng. Chem. Res. 2005, 44, 5593-5601. (16) Kai, W.; He, Y.; Inoue, Y. Fast crystallization of poly(3-hydroxybutyrate) and poly(3-hydroxybutyrate-co-3-hydroxyvalerate) with talc and boron nitride as nucleating agents. Polym. Int. 2005, 54, 780-789. (17) Talc in Plastics; Technical Bulletin; Luzenac America: Nov 2005; available at http://www.luzenac.com. (18) Provided by the manufacturer, Luzenac America, by e-mail communication, 12/06/2005. (19) Pukanszky, B.; Moezo, J. Morphology and Properties of Particulate Filled Polymers. Macromol. Symp. 2004, 214, 115-143. (20) Ramkumar, D. H. S.; Bhatacharya, M. Steady Shear and Dynamic Properties of Biodegradable Polyesters. Polym. Eng. Sci. 1998, 38, 14261435. (21) Pukanszky, B.; Fekete, F. Adhesion and surface modification. AdV. Polym. Sci. 1999, 139, 109-115. (22) Pukanszky, B.; Fekete, F. Aggregation tendency of particulate fillers: determination and consequences Polym. Polym. Compos. 1998, 6, 313-322. (23) Fekete, F.; Molnar, S.; Kim, G. M.; Michler, G. H.; Pukanszky, B. Aggregation, fracture initiation, and strength of PP/CaCO3 composites. J. Macromol. Sci. Phys. 1999, B38, 885-899.

(24) Wake, W. C. Fillers for Plastics; Plastics Institute: London, 1971. (25) Bhardwaj, R.; Mohanty, A. K.; Drzal, L. T.; Pourboghrat, F.; Misra, M. Renewable Resource Based Green Composites from Recycled Cellulose Fiber and Poly(3-hydroxybutyrate-co-3-hydroxyvalerate) Bioplastic. Biomacromolecules 2006, 7, 2044-2051. (26) Li, G.; Helms, J. E.; Pang, S. S.; Schulz, K. Analytical Modeling of Tensile Strength of Particulate-Filled Composites. Polym. Compos. 2001, 22, 593-603. (27) Einstein, A. Motion of suspended particles in stationary liquids required from the molecular kinetic theory of heat. Ann. Phys. 1905, 17, 549-560. (28) Halpin, J. C. Stiffness and expansion estimates for oriented short fiber composites. J. Compos. Mater. 1969, 3, 732. (29) Tsai, S. W. U.S. Government Report AD 834851; U.S. Government Printing Office: Washington, DC, 1968. (30) Lewis, T. B.; Nielsen, E. L. Dynamic Mechanical Properties of Particulate-Filled Composites. J. Appl. Polym. Sci. 1970, 14, 1449-1471. (31) Nielsen, E. L. Mechanical Properties of Polymers and Composites; Marcel Dekker: New York, 1976. (32) Larson, G. P. Kaolinite fractions, their effect upon physical properties of reinforced plastics. Mod. Plast. 1958, 35, 157.

ReceiVed for reView May 16, 2006 ReVised manuscript receiVed July 23, 2006 Accepted August 14, 2006 IE060604X