Article pubs.acs.org/IECR
Study of Solid and Microcellular Injection-Molded Poly(butylenes adipate-co-terephthalate)/poly(vinyl alcohol) Biodegradable Parts Jun Peng,†,‡ Chunmei Zhang,§ Haoyang Mi,† Xiang-Fang Peng,*,† and Lih-Sheng Turng*,‡ †
South China University of Technology, Guangzhou 510640, China Polymer Engineering Center, University of Wisconsin−Madison, Madison, Wisconsin 53706, United States § Polymer Research Institute, Sichuan University, Chengdu 610065, China ‡
ABSTRACT: Microcellular injection molding using supercritical fluid (SCF) as a physical blowing agent is capable of producing lightweight, dimensionally stable plastic parts while using less material. To improve strength and foamability of the biodegradable poly(butylenes adipate-co-terephthalate) (PBAT), poly(vinyl alcohol) (PVA) was used to compound the biodegradable PBAT/ PVA blends. It was found that the tensile mechanical properties (i.e., the Young’s modulus and ultimate strength) of both the solid and microcellular injection molded PBAT/PVA parts increase with increasing PVA content and the enhancement depends on their blend composition, morphology, and microstructure. As the PVA weight ratio increases, the PVA domains in the solid parts change from tiny, dispersed droplets to elongated filaments, to a cocontinuous structure, and finally to a continuous phase after phase inversion. The evolving microstructure and molecular entanglement result in various rheological melt characteristics and changes in complex viscosity. For foamed parts, the fractured surface of the microcellular injection molded parts present a multilayer structure and the PVA domains help to increase the cell densities as well as the tensile properties. The results provide useful insights into foaming PBAT/PVA blends with tunable microstructures and tensile mechanical properties.
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INTRODUCTION Over the past 2 decades, significant improvements have been made in the development of biodegradable polymeric materials and biomass materials. Degradable polymeric biomaterials have been employed in various applications, such as automotive, medical, and packaging, just to name a few. Poly(butylenes adipate-co-terephthalate) (PBAT) is a biodegradable aliphatic− aromatic copolyester that offers the balance of degradation time and physical properties (i.e., mechanical and thermal properties) by controlling the molar ratio of comonomers in the copolymer.1−3 PBAT possesses a high toughness and good processability4,5 and is an excellent candidate for toughening biodegradable polymers using the melt compounding approach.6 Although PBAT has good toughness and biocompatibility, its relatively low ultimate strength limits its application as a biodegradable material. For foam applications, nucleating agents (e.g., nanoclay and talc) have been employed to improve the foamability of PBAT by promoting heterogeneous nucleation.7,8 The commercially available microcellular injection molding (a.k.a. the MuCell process) process blends supercritical fluids (SCF, usually nitrogen or carbon dioxide) with a polymer melt in the machine barrel to create a single-phase polymer/gas solution that subsequently foams during the injection molding stage to produce lightweight, microcellular injection-molded parts.9−12 This promising technology enables an injectionmolding process with a lower melt viscosity, processing temperature, and pressure, leading to reductions in clamp tonnage, cycle time, and energy consumption.9,13 In addition, the expansion of the gas inside of the polymer melt creates a uniform packing effect and compensates for shrinkage of the material during cooling, leading to reduced residual stresses and excellent dimensional stability of the molded parts. To © 2014 American Chemical Society
overcome the shortcomings of foaming PBAT, an additional polymeric phase is needed. In this study, another biodegradable and high-strength resin, namely, poly(vinyl alcohol) (PVA), was chosen to enhance the PBAT’s foamability and tensile mechanical properties. To study the foaming behavior and tensile mechanical properties of the PBAT/PVA blends, solid and foamed PBAT/ PVA parts were fabricated with various PVA compositions through conventional solid injection molding and microcellular injection molding, respectively. Increasing of the PVA composition stepwise, the melt rheological properties of the PBAT/PVA blends were investigated via rheometry, and the tensile mechanical properties and microstructure of the PBAT/ PVA parts were characterized by tensile tests and scanning electron microscopy (SEM), respectively. The results provide useful insights into foaming PBAT/PVA blends and controlling their microstructures and tensile mechanical properties.
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EXPERIMENTAL PROCEDURE Melt Processing. A commercial grade poly(butylenes adipate-co-terephthalate) (PBAT) with a Tg of −29 °C and a Tm of 110−115 °C was received from BASF (Ecoflex F BX 7011). Figure 1 shows the molecular formula of PBAT. Poly(vinyl alcohol) (PVA, POLYBATCH AquaSol 116) was supplied in pellet form by A. Schulman. Its specific gravity was 1.27 and it had a melt flow index of 10 g/10 min (200 °C/5 kg). The solubility measurements of N2 were facilitated by placing samples in ha igh-pressure vessel under a N2 gas Received: Revised: Accepted: Published: 8493
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using an image analysis tool (UTHSCSA Image Tool). The cell density was calculated using the following formula,15 cell density =
Figure 1. Molecular formula of PBAT.
⎛ N ⎞3/2 ⎜ ⎟ ⎝ L2 ⎠
(1)
where N was the number of cells (bubbles) and L was the linear length of the observed area.
pressure of 2.21 MPa (320 bar) at room temperature for 1 day. The similar procedure for the measurement of CO2 solubility in PVA matrix was described in our group’s previous paper.14 According to the measurement, the solubility of N2 in solid PVA is 0.81 mg of N2/g of PVA. For all experiments, the PBAT and PVA resins were dried at 55 °C for 4 h to remove any excess moisture. Batch materials of PBAT/PVA pellets for injection molding were compounded and granulated using a twin-screw extruder with a 27 mm screw diameter and a L/D ratio of 42. Table 1 summarizes the PBAT/PVA compositions by weight. Injection-molded tensile bars with various amounts of PVA followed the ASTM D63803 Type I standard using an Arburg Allrounder 320S injectionmolding machine (Lossburg, Germany) equipped with MuCell technology (Trexel Inc., Woburn, MA, U.S.). For all molding trials, the injection speed was 20 cm3/s and the barrel temperatures from nozzle to the hopper were 180, 175, 170, 165, 160, and 38 °C, respectively. For the microcellular injection-molding process, no pack/hold pressure was applied to the reciprocal screw as the growing cells (bubbles) in the molded part will compensate for the material shrinkage. For all microcellular injection-molding trials, 0.5 wt % SCF nitrogen was injected into the barrel as the physical blowing agent. The injection volumetric shot size was chosen in such a way to ensure complete filling of the tensile test bars without short shots or visible shrinkage on part surface. Depending on various PBAT/PVA concentrations, the gas solubility in various blends, and their foaming characteristics, the weight reductions of the foamed parts ranged between 10 and 15 wt % less than their solid counterparts. Thirty samples were collected during the course of each molding trial after discarding the first 15 samples. Testing Techniques. Rheological behaviors of PBAT/PVA blends were investigated using an Advanced Rheometric Expansion System (ARES) with 25 mm parallel plates. All samples were dried in a vacuum oven before testing to remove any moisture. The measurements for dynamic frequency sweep tests were performed over a frequency range of 0.01−70 rad/s at 5% strain. The testing gap was set at a range of 0.8−1.0 mm. Tests were performed at 170 °C under a nitrogen atmosphere to avoid thermal degradation. The mechanical tensile properties (modulus, strength, and strain-at-break) were measured at room temperature with a 50 kN load cell on an Instron Model 3369 tensile tester. The crosshead speed was set at 10 cm/min, and all tests followed ASTM D638-08. Five samples from each molding trial were tested and the average results were reported. The morphologies of the solid and foamed PBAT/PVA parts were probed by a scanning electron microscope (SEM; Model LEO 1530, JEOL, Japan) operated at an accelerating voltage of 3 kV. The average void sizes in the foamed parts were analyzed
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RESULTS AND DISCUSSION Rheology of Solid PBAT/PVA Blends. Figure 2 shows the logarithmic dependence of complex viscosity values (η*) on a
Figure 2. Complex viscosity versus sweep frequency for PBAT and blends at 170 °C. The numbers stand for PBAT weight concentrations.
logarithm of sweep frequencies (ω) for pure PBAT and various PBAT/PVA solid blends at 170 °C. PBAT weight compositions are listed in the legend. Across the range of explored frequencies, the complex viscosity of the PBAT and its blends decreased with increasing sweep frequency, which indicates that PBAT and its blends are pseudoplastic liquids. Pure PBAT and its blends exhibited a shear-thinning, power-law type of melt flow behavior. Generally, the complex viscosity of PBAT/PVA blends increased with additional PVA content over the entire frequency range tested. In comparison with its blends, pure PBAT had the longest Newtonian platform and the shear thinning behavior happened at the latest point (around 10 Hz). The addition of the PVA phase in the blends shortened the Newtonian platform and enhanced the shear thinning behavior. Also, the complex viscosity differences among the blends decreased as the frequency increased in the relatively high frequency ranges. At low frequencies, the complex viscosities of the blends increased drastically when the PBAT content decreased, specifically, from 80 to 70 wt %. When the PBAT content was high (100−80 wt %) or low (40−30 wt %), the complex viscosities were close to each other. These changes in
Table 1. Various Weight Compositions of PBAT/PVA Blends samples
PBAT100
PBAT90
PBAT80
PBAT70
PBAT60
PBAT50
PBAT40
PBAT30
PBAT20
PBAT10
PBAT0
PBAT (wt %) PVA (wt %)
100 0
90 10
80 20
70 30
60 40
50 50
40 60
30 70
20 80
10 90
0 100
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Therefore, the G′ of the PBAT/PBA blends increased with the additional PVA phase at the low sweep frequency. Meanwhile, at relatively high frequencies, the addition of PVA had less effect on the storage modulus of the melt due to the shortage of time for the entanglement recoveries. In Figure 4, the loss modulus (G′′) showed a trend similar to the storage modulus (G′). For low-composition PVA blends (i.e., 0−20 wt %), PBAT/PVA blends have closedG′′ values since the dispersed phase of PVA molecules was relatively stiff. As mentioned early, the interactions between the PBAT and PVA molecules intensified as more PVA was compounded with the PBAT matrix. Stiffer PVA molecules indicated a higher loss modulus. In addition, the dynamic rheological properties depended on the changing phase compatibilities and domain microstructures. A Cole−Cole plot is capable of describing the two-phase microstructures of polymer blends. Figure 5 shows a plot of the
complex viscosities corresponded to morphology evolution, discussed later in the SEM images. Figures 3 and 4 show the dependence of the dynamic storage modulus (G′) and the dynamic loss modulus (G′′) on the
Figure 3. Storage modulus versus sweep frequency for PBAT and blends at 170 °C.
Figure 5. Cole−Cole plots of imaginary viscosity versus real viscosity for different compositions.
imaginary viscosity (η′′) versus the real viscosity (η′), where η′′ = G′/ω and η′ = G′′/ω.16−18 As shown in Figure 5, the pure PBAT and PBAT90 have only one circular arc in the curve, suggesting a properly homogeneous phase. SEM images can also visually indicate whether they are miscible or not, which will be shown later in the microstructure section. However, heterogeneous blends indeed display no arc but slopes. High concentrations of PVA domains lead to a coexisting phase like droplets disturbed in a continuous phase or that of cocontinuous morphologies. Tensile Properties of Solid and Microcellular Injection Molded PBAT/PVA Parts. The representative stress−strain curves of solid and microcellular injection-molded samples are featured in Figure 6. Tables 2 and 3 tabulate the average values of the Young’s modulus, ultimate strength, and strain-at-break of tensile test bars according to ASTM D638-03. As shown in Figure 6a and Table 2, solid injection-molded pure PBAT parts had the lowest Young’s modulus and ultimate strength. Recall that PBAT is commonly used for improving the strain-at-break. On the other hand, pure PVA possessed the lowest strain-atbreak. The Young’s modulus and ultimate strength of PBAT/ PVA blends gradually increased with additional PVA content. The high tensile PVA phase was found to enhance the normally flexible PBAT matrix. Therefore, the tensile mechanical
Figure 4. Loss modulus versus sweep frequency for PBAT and blends at 170 °C.
sweep frequency of pure PBAT and PBAT/PVA blends, respectively. The storage modulus corresponded to the elastic behavior of the energy stored and the loss modulus related to the viscid behavior of the energy lost. The G′ and G′′ generally depended on the sweep frequencies and blend compositions. As shown in Figure 3, the storage modulus (G′) of pure PBAT and its blends was augmented as the sweep frequency (ω) increased. For high PBAT concentration blends, G′ increased intensely with sweep frequency changes. When the PBAT concentration was less than 70 wt %, the increase of G′ slowed with the sweep frequency. Taking the entanglement theory into consideration, it is known that the molecular chain of PBAT is more flexible and easier to entangle with itself or other chains as compared to the molecular chains of PVA. 8495
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Table 3. Mechanical Tensile Properties of Microcellular Injection Molded PBAT Parts with Various PVA Concentrationsa Young’s modulus (MPa)
ultimate strength (MPa)
PBAT100 PBAT90
N/A 3.11 × 1001
N/A 1.21 × 1001
PBAT80 PBAT70 PBAT60 PBAT50 PBAT40 PBAT30 PBAT0
3.50 4.86 5.37 5.44 5.53 5.77 7.81
× × × × × × ×
1001 1001 1001 1001 1001 1001 1001
1.14 1.25 1.19 1.23 1.34 1.52 2.28
× × × × × × ×
1001 1001 1001 1001 1001 1001 1001
strain-at-break (mm/mm) N/A (did not break) 6.88 × 1000 2.83 × 1000 2.45 × 1000 1.20 × 1000 1.64 × 1000 1.92 × 1000 3.04 × 1000
weight reduction (%) N/A 3.5 4.2 4.8 8.0 8.2 11.2 13.5 16.6
a
Due to its poor foamability of pure PBAT, no microcellular samples were produced for PBAT100.
various PBAT concentrations. Pure PBAT could not be foamed due to the low solubility of SCF nitrogen in the PBAT matrix, and thus the PVA phase is critical for improving PBAT formability. As a result, PBAT/PVA foamed parts were fabricated at various PVA concentrations. In comparison with solid parts, foamed tensile bars had a relatively lower ultimate strength and lower strain-at-break. Clearly, the foamed structure affected the tensile mechanical properties. The reductions in the ultimate strength and strain-at-break of the foamed parts were attributed to voids in the center of the samples, which were equivalent to reductions in the effective cross-section areas. The ultimate tensile strength of PBAT/ PVA foamed parts improved stepwise with more PVA, but the strain-at-break exhibited differing trends and reached a minimum value at a weight ratio of 50:50, conceivably due to the cell microstructures, mixture compositions, and domain microstructures. Morphology of the Fractured Surface and Microstructures of Solid Injection-Molded Parts. Morphological examinations of the solid injection-molded PBAT/PVA parts revealed the development of the PVA domain as it gradually changed from a dispersed phase in the PBAT matrix to a continuous phase with increasing PVA concentrations. The fractured cross-sectional morphologies are shown in Figure 7. The fractured surface of the specimens with low PVA concentrations (i.e., 10 or 20 wt %), Figure 7a,b, revealed that the tiny PVA domains dispersed well in the PBAT matrix. With increasing PVA concentration (i.e., Figure 7c), the diameters of the PVA droplets (or pores) became larger as a result of the coalescence of the small PVA domains and reduced the total polymer melt surface tension. Moreover, the clear boundaries between the tiny dispersed PVA droplets (10 wt %) and the PBAT matrix indicate that they are immiscible, even though the Cole−Cole plot of PBAT90 has only one arc. Cole−Cole plots depend on rheological storage modulus and loss modulus to indirectly characterize blends miscibility; thus, SEM is a more reliable indicator to directly check miscibility than that of Cole−Cole plot. Figure 7d shows the fracture surface morphologies of solid injection-molded PBAT/PVA (60/40) blends with uniform domains on the cross sections. Furthermore, Figure 8 shows the ribbon-like morphology on the PBAT50 and PBAT60 along the melt flow direction. At these fractions, the round PVA droplets changed into ribbon or filament shapes due to
Figure 6. Tensile mechanical properties of (a) solid and (b) foam injection-molded tensile bars.
Table 2. Mechanical Tensile Properties of Solid InjectionMolded PBAT Parts with Various PVA Concentrations Young’s modulus (MPa) PBAT100 PBAT90 PBAT80 PBAT70 PBAT60 PBAT50 PBAT40 PBAT30 PBAT20 PBAT10 PBAT0
3.13 3.31 3.74 4.51 5.70 6.13 7.14 7.46 7.72 8.99 9.41
× × × × × × × × × × ×
1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001
ultimate strength (MPa)
strain-at-break (mm/mm)
× × × × × × × × × × ×
(did not break) (did not break) (did not break) 6.7 5.48 4.23 3.46 3.53 3.33 3.41 3.25
1.12 1.23 1.33 1.57 1.67 1.73 1.88 1.92 2.12 2.41 2.65
1001 1001 1001 1001 1001 1001 1001 1001 1001 1001 1001
properties could be tailored by compounding flexible PBAT and rigid PVA. In addition, microcellular injection-molded PBAT/PVA parts exhibited similar trends to solid injection-molded blends with 8496
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Figure 7. SEM images of fracture surface structures on the cross sections of solid injection-molded PBAT/PVA blends: (a) PBAT90, (b) PBAT80, (c) PBAT70, (d) PBAT60, (e) PBAT50, (f) PBAT40, (g) PBAT30, (h) PBAT20, and (i) PBAT10.
continuous phase, as shown in Figures 7h,i. Thus, as a function of component concentration, the morphologies of the PBAT/ PVA blends transitioned stepwise from a PVA domain to a dispersed phase, to a cocontinuous phase, and even to phase inversion with the PBAT domain. The domain’s microstructure transition affects the rheological properties of blended hot melts. At low PVA concentrations (i.e., 10 wt %), the Cole−Cole plots showed an arc indicating good compatibility of PBAT and PVA. A PVA content of 10 wt % in the PBAT matrix exhibited tiny (1−2 μm) spherical droplets in the continuous PBAT matrix, as shown in Figure 7a. With an expanded PVA domain, from 20 to 30 wt %, the PVA droplet diameters rose due to the coalescence of small ones (Figure 7b,c). The domain microstructure difference was indicated by the melt complex viscosity and storage modulus (Figures 3 and 4). The complex viscosity difference between PBAT80 and PBAT70 implicated the enhanced PVA effects in the PBAT/PVA blends. The blends displayed mainly flexibility when PVA was less than 30 wt %. On the other hand, the blend melt exhibited more rigid behavior. In addition, the tensile test results supported this transition, as shown in Figure 6a. The samples with low PVA concentrations did not break in the testing range, and the strain-at-break decreased with more PVA domains. Furthermore, the PVA domains positively affected the blend’s
Figure 8. SEM image of the fracture surface structure on the flow direction of solid injection-molded (a) PBAT50 and (b) PBAT60.
coalescence and a more profound shear effect that stretched the droplets. However, the morphology of PBAT60 also consisted of a tightly packed dispersed PVA phase in the PBAT matrix, suggesting that the PVA phase did not completely coalesce to form a continuous phase. The continuity of the PVA domains increased dramatically when the PVA and PBAT had equal weight concentrations and a cocontinuous phase was achieved, as shown as Figure 7e. By further increasing the PVA domain to 70 wt % (Figure 7g), phase inversion occurred and the PVA phase became the primary, continuous phase, which surrounded dispersed and isolated PBAT domains. After the phase inversion, the tiny PBAT droplets dispersed in the PVA 8497
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Figure 9. SEM images of fracture surface structures on the cross sections of microcellular injection-molded PBAT/PVA blends: (a) PBAT90, (b) PBAT80, (c) PBAT70, (d) PBAT60, (e) PBAT50/50, (f) PBAT40, (g) PBAT30, and (h) PBAT0. Inserts show magnified features.
layer thicknesses decreased as PVA content increased in the blends due to the higher nitrogen solubility in the PBAT/PVA blends. Figure 10 shows the cell size and cell density of the foamed samples, indicating the decreased cell size and increased cell density with increased PVA content. The dispersed PVA domains in the PBAT matrix developed from the dispersed drops into elongated, flat strips similar to the transformation process presented in Figure 7a−d. According to the nucleating mechanism, the decreased surface tension at the interface of PBAT and PVA reduced the critical radius of nitrogen cells and increased the nucleating rates. PVA domains also enhanced the nitrogen solubility in the blends. Figure 9a shows the cell structure of PBAT90. Small cells can be found in the skin layer because the low temperature of the mold wall quickly froze the bubbles. Cells in the middle layer had a relatively larger size resulting from the coalescence and a slower cooling rate. The large difference in nitrogen solubility between PBAT and PVA introduced large standard deviations in cell sizes. More PVA domains would be helpful for achieving uniform cell dispersion in PBAT/PVA blends. At 20 wt % PVA, the size of the nitrogen bubbles became more constant at around 110 μm, as shown in Figure 9b. Moreover, the weight reduction of foamed PBAT/PVA blends increased with additional PVA weight ratios; weight reduction data are listed in Table 3. The microstructure in terms of cell density and cell size, as well as the sandwiched multilayer structure, impact the tensile
foamability as well as the ultimate strength of the blends based on the domain microstructures. Fractured Surface and Microstructure Morphology of Microcellular Injection-Molded Parts. To investigate the microstructures of the cells, cross-section morphologies of microcellular injection-molded tensile bars were examined using scanning electron microscopy (SEM). For microcellular foamed samples, the cross-sectional fractural surface (as seen by SEM) located at the middle of the tensile bar along the flow direction, resulting from the tensile tests. The microcells and microstructures were influenced by both blend composition and processing conditions. To investigate the effects of composition on the cell morphology, the processing conditions were kept the same. Different material compositions exhibited various morphologies as shown in Figure 9, and plotted graphically in Figure 10. Figure 9 shows the fractured surface morphologies of the cross sections (the insets show the cells magnified). The microcellular injectionmolded technique did not work for pure PBAT due to low solubility of nitrogen in the PBAT matrix. Under the same processing conditions, pure PVA had good foamability and exhibited uniformly distributed bubbles (Figure 9h), the underlying reason why PVA was compounded with PBAT to improve the foamability. In Figure 9, multiple distinct-layer structures of foamed PBAT/PVA parts were observed: the skin layer possessed a higher density resulting from fewer cells, whereas the middle layer had more bubbles. The average skin 8498
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from the dispersed phase to elongated filaments, to a cocontinuous structure, and to phase inversion as a continuous phase. These microstructure transitions also affected the rheological properties and tensile mechanical properties. The Young’s modulus and ultimate strength of the blends, as well as the complex viscosity and storage modulus, improved with additional PVA. Foamed samples typically had decreased tensile mechanical properties as compared to their solid counterparts, but with a fine cell structure the retention of the mechanical property is possible. The fractured surfaces of the microcellular injection-molded parts presented a multilayer structure.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected];
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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REFERENCES
The authors would like to acknowledge the financial support of the USDA National Institute of Food and Agriculture Award (No. 2011-67009-20056) and the Wisconsin Institute for Discovery at UW−Madison. The first and fourth authors would like to acknowledge the National Nature Science Foundation of China (No. 51073601, No.21174044), the Fundamental Research Funds for the Central Universities (No. 2011ZZ0011), the China Postdoctoral Science Foundation (2012M511791), and the 973 Program (2012CB025902) for their financial support.
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Figure 10. Representative values for (a) the average cell size and the standard deviation, and (b) the cell density of microcellular injectionmolded PBAT/PVA parts with various compositions based on SEM images.
mechanical properties of foamed parts. The cause of the different Young’s moduli of foamed parts was due to the cell structures and the different layered structures; namely, the ratio of the outer skin layer to the core inner layer. Compared to the inner layer that was composed mainly of cells, the outer layer had fewer and smaller cells, resulting in a higher density. Therefore, the skin layer played a more significant role in contributing to the modulus along the flow direction than in the center portion. Furthermore, the cell sizes and cell densities of the foamed parts were similar when the PBAT weight ratios were between 60 and 80 wt %, and hence the Young’s modulus was enhanced due to the addition of stiffer PVA domains. More specifically, the Young’s modulus of the foamed parts depended on the skin layer thickness and density of the various layers rather than the skin layer thickness alone.
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CONCLUSIONS In the present work, solid and microcellular PBAT/PVA biodegradable blends were fabricated and investigated to improve PBAT’s foamability and strength. The rheological behaviors, tensile properties, and cell structures were found to depend on the composition of the blends. With increasing PVA content, the PVA in the solid and foamed blends transitioned 8499
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