Article pubs.acs.org/IECR
Thermoforming of Polylactic Acid Foam Sheets: Crystallization Behaviors and Thermal Stability Richard Eungkee Lee,† Yanting Guo,‡ Harinder Tamber,† Mirek Planeta,† and Siu Ning Sunny Leung*,‡ †
Macro Engineering and Technology Inc., Mississauga, Ontario L4Z 2E5, Canada Department of Mechanical Engineering, York University, Toronto, Ontario M3J 1P3, Canada
‡
ABSTRACT: Biobased polylactic acid (PLA) foam packaging has been commercialized as an alternative to conventional polystyrene (PS) foam. However, PLA’s low glass transition temperature results in its inherently poor heat resistance and in turn limits the application to cold-fill packaging. In this study, low-density PLA foam sheet was extruded using an industry-scale tandem extrusion line and subsequently laminated with a PLA film casted from a high heat deflection temperature (HDT) PLA resin with a very low D-lactide monomer content (i.e., ∼0.4 mol %). Experimental results reveal that the crystallization behaviors of PLA foam sheets were sensitive to both thermoforming temperature and process-induced strain. Furthermore, this work has demonstrated that heat-resistant biodegradable PLA foam sheets could be realized by laminating them with solid films made of high-HDT PLA to serve as environmentally sustainable alternatives to PS foams in hot-fill packaging applications.
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INTRODUCTION Polylactic acid (PLA), a biobased and biodegradable thermoplastic polyester, is being considered as an environmentally benign substitute for polystyrene (PS). PLA originates from natural resources such as corn and sugar cane. Generally, it is produced through condensation polymerization of lactic acid monomers or ring-opening polymerization of lactides.1−8 Unlike the majority of expensive biodegradable polymers, the large-scale production of PLA by companies such as NatureWorks LLC9 has led to a steady decrease of material cost to levels approaching that of commodity polymers. This has helped PLA draw more attention from various industrial sectors.10 The wide spectrum of everyday applications include fast food service tableware, grocery and composting bags, wrap films, thin-wall containers, and other packaging products. Furthermore, as the ban on disposable PS foam packaging has become a massive trend in North America and other countries globally, PLA foams are being considered as one of the most feasible alternatives to traditional PS foams.11,12 In fact, PLA-based low-density foam has been commercialized as compostable packaging trays for meat, fish, fruits, and vegetables by manufacturers. Although PLA foams are viewed as an eco-friendly candidate to replace PS foams, its limited foamability has made its use in various plastic foam applications challenging. On the one hand, the relatively low melt strength of linear PLA has limited its expansion to form low-density foams. In light of this, various researchers have used chain extender to improve PLA’s rheological property and foamability.13−15 As a result, lowdensity PLA foams based on physical foaming could be manufactured by continuous reactive extrusion foaming processes. However, the applications of low-density PLA foams are still significantly limited by its intrinsic susceptibility to heat.16−20 First, the glass transition temperature of PLA is as low as about 55 °C,21 which is significantly lower than the boiling point of water. Second, because of the presence of rigid segments in its main chain,22−24 PLA has crystallization kinetics © 2015 American Chemical Society
that are significantly slower than that of other semicrystalline polymers. These two shortcomings have made it challenging to manufacture low-density PLA foam products with high service temperature, which is essential for hot-fill food packaging applications. Aside from the cost issue, this drawback has also been an impediment to using PLA foam packaging to replace conventional PS foam packaging. In this context, many researchers have investigated stereocomplex crystallization of PLA (e.g., poly(D-lactic acid) (PDLA) as a solution to improve PLA’s thermal resistance (i.e., increased glass transition temperature and melting temperature).25−29 However, the high cost of commercialized PDLA resin would likely delay the deployment of its full potential to replace conventional PS foam packaging until the stereocomplex PLA foam becomes sufficiently cost-competitive. Another approach that researchers have proposed to improve PLA’s heat deflection temperature (HDT) is through promoting PLA’s crystallization.6,11,30−35 In particular, studies reported that physical foaming19,36−39 and annealing treatment6,31,34,40 are effective strategies to enhance the crystallization kinetics of PLA. For instance, the plasticization effect of a physical blowing agent and the biaxial stress generated from foam expansion could dramatically increase the crystallization rate of PLA. In this study, a new approach was proposed and demonstrated to produce biodegradable PLA foam packaging that overcomes the aforementioned disadvantages. PLA foam sheets with two-layered structures were manufactured by laminating a solid high-HDT PLA film onto a low-density PLA foam sheet. It is expected that the solid PLA skin layer, which has high HDT attributed to its high crystallinity, would be able to increase the thermoformed foam products’ service temperatures and satisfy the requirements in hot-fill food Received: Revised: Accepted: Published: 560
September 22, 2015 November 25, 2015 December 24, 2015 December 24, 2015 DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
Article
Industrial & Engineering Chemistry Research
length-to-diameter ratios of the two extruders in the tandem system were 32:1 and 30:1, respectively. The primary screw was designed to efficiently melt the PLA resins, whereas the secondary screw was designed to effectively cool the melt in order to maximize the melt strength. A liquid butane metering system was connected to the barrel of the primary extruder to inject i-butane at a designated constant flow rate. A gravimetric feeding system was used to mix and feed different material components into the hopper of the primary extruder. A spider type annular die was attached to the exit of the secondary extruder to manufacture tubular foam sheet, which was then pulled over a cooling mandrel to govern the tube’s diameter. The tube was subsequently slit into two flat foam sheets. The pressure of the annular die was maintained at 1500 to 2000 psi, and the melt temperature in the die was 145 °C. A customized lab-scale vacuum thermoforming system was designed, implemented, and used to thermoform monolayered and two-layered PLA foam sheets into desired shapes. The labscale vacuum thermoforming system is capable of manufacturing PLA thermoformed products of dimensions up to 8 × 8 × 3 in3. An overview of the thermoforming process is illustrated in Figure 2. First, the monolayered PLA8052D foam sheet was
packaging applications. Low D-lactide content PLA with high crystallization rate possesses a relatively narrow foam processing window. Therefore, producing two-layered PLA foam composed of a high D-lactide content PLA foam layer and a low D-lactide content PLA solid skin layer is a feasible strategy to improve the HDT of conventional PLA thermoformed foam products. Furthermore, the effects of thermoforming temperature and process-induced strain on the crystallization behaviors of PLA foam sheets were investigated in this work.
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EXPERIMENTAL DETAILS Materials. Two commercially available PLA resins, Ingeo 2500HP and Ingeo 8052D, with 0.4 mol % and 4.7 mol %, respectively, of D-lactide content were supplied by NatureWorks LLC. Ingeo 2500HP is the high-HDT-grade PLA. The melt flow indexes of Ingeo 2500HP and Ingeo 8052D were 8 g/ 10 min and 14 g/10 min, respectively, at 210 °C. Talc powders (Mistrocell grade) supplied by Imerys were used as the cell nucleating agent. A masterbatch grade of chain extender (CESA Extend OMAN 698493, Clariant, 30% active) was fed along with the PLA8052D resin. The concentrations of talc powder and chain extender were 0.5 and 2.1 wt %, respectively. Iso-butane with a purity of 99.5%, supplied by Praxair Canada, was used as the physical blowing agent in the foam extrusion of PLA foam sheets at a content of 5 wt %. To promote the crystallization of PLA, 1 wt % of crystallization enhancer (CN-L03, Polyvel) was blended with PLA2500HP. Sample Preparation. As shown in Figure 1, a productionscale tandem foaming extrusion line (Macro Engineering and Technology Inc.) was used to manufacture low-density PLA foam sheets. The screw diameter of the primary extruder was 150 mm, and that of the secondary extruder was 180 mm. The
Figure 2. An overview of the vacuum thermoforming process.
clamped between two aluminum frames and heated at preset temperatures ranging from 170 to 185 °C in a vacuum oven for 50 s. In the case of the two-layered PLA foam sheets, because of the presence of the high-HDT PLA2500HP layer, the preset oven temperature and time were set to 180 °C and up to 125 s, respectively. Simultaneously, the mold for thermoforming was preheated to 40 °C to avoid a sudden temperature drop of the PLA foam sheet upon its contact with the mold during the shaping process. The softened PLA foam sheet was subsequently transferred from the oven to the vacuum thermoforming setup. Under vacuumization, the softened PLA foam sheet would conform into the contour of a bowlshaped mold. A perforated mold was used to enhance uniformity in strain throughout the foam sheet during the thermoforming process. Finally, the thermoformed PLA foam bowl was removed from the mold after cooling. Sample Characterization. The thermoformed PLA foam samples were characterized in terms of their foam morpholo-
Figure 1. PLA foam sheet extrusion line (Macro Engineering and Technology Inc.): (a) tandem foam extrusion line and (b) downstream mandrel and slitter. 561
DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
Article
Industrial & Engineering Chemistry Research gies, crystallinity, and glass transition temperature as well as dimensional stability under hot-fill conditions. These analyses were conducted by scanning electron microscopy (SEM), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (XRD), and thermal resistance test against hot water. The PLA foams’ morphologies, before and after thermoforming, were observed by SEM (FEI Company, Quanta 3D FEG). Samples were cryo-fractured under liquid nitrogen to expose their cross sections. The fractured surface was sputter-coated with gold (Denton Vacuum, Desk V Sputter Coater). The crystallinity and crystal structures of the PLA8052D foam and solid film of PLA2500HP were analyzed using an X-ray diffractometer (Rigaku MiniFlex 600). The specimens were exposed to a Cu X-ray with a generator running at 40 kV and 15 mA. The scanning was carried out at a rate of 0.6°/s in an angular region (2θ) of 5−40°. The degree of crystallinity in the specimen was determined based on the ratio of intensity from the crystalline peaks to the sum of the crystalline and amorphous intensities. The glass transition temperatures of the PLA8052D thermoformed at different temperatures and PLA2500HP thermoformed at 180 °C were examined by DSC (TA Instruments, Q2000). Dimensional stability of the thermoformed bowl-shaped PLA foams at elevated temperature was analyzed by pouring hot water at different temperatures into the bowls. It was characterized by observing the time at which thermoformed foam bowls started to deform after they were filled with hot water at different temperatures.
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Figure 4. SEM micrographs of thermoformed PLA8052D foam sheets at (a) 170 °C, (b) 175 °C, (c) 180 °C, and (d) 185 °C.
using heating temperatures ranging from 170 to 185 °C. The thickness and density of the as-extruded foam sheets were about 6 mm and 0.078 g/cm3, respectively. The volume expansion ratio was 16 fold. Figure 3 shows that the extruded PLA8052D foam sheets, before thermoforming, consisted of both closed cells and open cells. Such foam morphology revealed that the melt strength of PLA8052D was not perfectly uniform in the melt despite the long-chain branching reaction induced by the chain extender. In addition, it is believed that the melt temperature during foam extrusion had not been completely optimized, leading to the rupture of cell walls. By analyzing the SEM micrograph, it can be observed that the average cell size and the cell population density with respect to the unfoamed volume were 590 μm and 4.48 × 105 cells/cm3, respectively. Figure 4a−d shows the SEM micrographs of PLA8052D foam sheets after thermoforming at different temperatures. The SEM samples were obtained at a location 60−70 mm from the center of the thermoformed bowls, along the radial direction. As expected, the cells were elongated in the straining direction after the thermoforming process. To understand the influences of the thermoforming temperature on the macroscopic structures of PLA8052D foam sheets, the strain and the thickness of the PLA8052D foam sheets at different distances from the center of the thermoformed bowls, along the radial direction, were measured. Before thermoforming, 1 × 1 cm2 grids were drawn on the PLA foam sheets. The strain was determined by measuring the deformation of the grids after thermoforming. Figures 5 and 6 plot the experimental results. As shown in Figure 5, it can be observed that the level of strain increased with thermoforming temperature. Because higher heating temperature would suppress the stiffness of the PLA8052D foam, such conditions would enhance the foam sheet to deform and elongate during the shaping process. On the other hand, Figure 6 reveals that there existed an optimal thermoforming temperature to promote the uniformity of PLA8052D foam’s thickness along the radial direction of the thermoformed bowl. The standard deviations of the thickness measurements along the radial direction of the thermoformed bowls fabricated at different
RESULTS AND DISCUSSION
The effects of thermoforming temperatures on foam morphologies and crystallization behaviors of PLA foam sheets were investigated. Moreover, the high-temperature dimensional stability of thermoformed PLA8052D foam bowls, with and without the lamination of high-HDT PLA2500HP films, was evaluated. The influences of thermoforming temperatures and the PLA2500HP film’s thickness on the thermoformed PLA foam bowl’s dimensional stability at high temperature were studied. Effects of Thermoforming on the Morphology of PLA Foams. Figures 3 and 4 illustrate the SEM micrographs of PLA8052D foam sheets as-extruded and after thermoforming
Figure 3. An SEM micrograph of extruded PLA8052D foam sheet before thermoforming. 562
DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
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Figure 5. Effect of thermoforming temperature on strains of the thermoformed PLA foam sheets.
Figure 7. XRD spectra of amorphous PLA8052D and PLA8052D foam sheet.
Such enhancement of PLA crystallization was caused by two factors. First, the dissolution of i-butane in the PLA melt would result in the plasticizing effect,45,46 promoting the mobility of polymer chains. Second, local strain fields being generated around the expanding cells would promote the ordering of polymer chains, thereby enhancing their crystallization.47,48 An example of a monolayered thermoformed PLA8052D foam bowl is illustrated in Figure 8. To elucidate the effects of
Figure 6. Effect of thermoforming temperature on thicknesses of the thermoformed PLA foam sheets.
heating temperatures are summarized in Table 1. Experimental data shows that the thermoformed PLA8052D foam bowl prepared by a heating temperature of 175 °C had the smallest thickness deviation. It is believed that if the heating temperature was too low, the high stiffness of PLA8052D foam made it challenging to achieve uniform thickness.41 In contrast, if the heating temperature was too high, sheet sagging and thinning might occur at local regions, leading to nonuniformity of wall thickness.42−44
Figure 8. An example of a thermoformed PLA bowl.
thermoforming on the crystallization behaviors of PLA foams, XRD spectra of thermoformed PLA8052D foams, prepared at different heating temperatures, were obtained. Figure 9 plots the XRD spectra of thermoformed foam bowls prepared at different heating temperatures. The degrees of crystallinity of the thermoformed low-density PLA8052D foams prepared at different heating temperatures were determined, and the data is summarized in Table 2. It can be observed that lower heating temperature for thermoforming resulted in a high degree of crystallinity. When the sample was heated at temperatures close to the melting temperature of the PLA, crystals were partially melted. At higher thermoforming temperatures, the seeds used for crystal perfection during heating of the thermoforming process (i.e., annealing-induced crystallization) became less, leading to decrease in the degree of crystallinity. Similar discussion was reported from annealing experiments of PLA in the literature.49,50 Moreover, when the temperatures were too high, the extremely high chain mobility hindered the regular arrangement of the polymer chain by the strain-induced
Table 1. Thickness Distribution of Thermoformed PLA Foam Sheets thermoforming temperature (°C)
minimum thickness (mm)
maximum thickness (mm)
standard deviation (mm)
170 175 180 185
0.975 0.890 0.995 0.875
2.57 2.28 2.53 2.90
0.574 0.302 0.510 0.512
Effects of Thermoforming on the Crystallization Behaviors of PLA Foam Sheets. The XRD spectra of amorphous PLA and PLA foam sheet are shown in Figure 7. The degree of crystallinity of the as-extruded PLA foam was 26.84%, which indicated that a considerable amount of crystals had already been formed during the extrusion foaming process. 563
DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
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Figure 9. XRD spectra of the thermoformed PLA8052D foams prepared at different heating temperatures.
Figure 10. XRD spectra of the high-HDT PLA (i.e., 2500HP) film before and after the thermoforming process.
Table 2. Degree of Crystallinity of Thermoformed PLA8052D Foam Sheets thermoforming temperature (°C)
degree of crystallinity
170 175 180 185
54.71% 49.18% 47.80% 42.87%
PLA2500HP film). It was found that prior to the thermoforming process, the high-HDT PLA2500HP film could almost be considered as amorphous as evidenced by its “hump-shaped” XRD spectrum. The amorphous nature of the thin film could be attributed to the high cooling rate of PLA2500HP during the cast film process. In contrast, after the heating and thermoforming process, a sharp diffraction peak of α-crystal at 2θ = 16.7° could be identified. Moreover, the degree of crystallinity of annealed PLA2500HP film and thermoformed PLA2500HP film were 42.48% and 47.84%, respectively. The increase in degree of crystallinity of annealed PLA2500HP film solely originated from the annealing-induced crystallization during the heating process in the oven. With the strain-induced crystallization by stretching the high-HDT film during the thermoforming process, the degree of crystallinity of high-HDT PLA2500HP film was further enhanced. The glass transition temperature of PLA2500HP thermoformed at 180 °C was evaluated to be 62.78 °C by DSC. Dimensional Stability of Thermoformed PLA Foam Bowls in Hot-Fill Applications. The dimensional stability of monolayered PLA8052D foam bowls in hot-fill food container applications was evaluated by pouring hot water at different temperatures into the bowls. The time it took the bowl to start deforming at water temperatures ranging from 60 to 99 °C was observed, and the measurements are depicted in Figure 11. It can be observed that the thermoformed monolayered PLA8052D foam bowls prepared at lower heating temperature started deforming at higher water temperature. In other words, lower heating temperature in the thermoforming process was favorable to the enhancement of the PLA8052D foam’s dimensional stability. It is believed that the higher degree of crystallinity contributed to the promoted high-temperature dimensional stability of the PLA8052D foam. The dimensional stability of the thermoformed two-layered PLA foam bowls laminated with solid PLA2500HP films with different thicknesses were also compared by pouring hot water at 99 °C into the bowls. The bowl made of a two-layered foam sheet laminated with 30 μm thick solid skin started to deform after 7 min and showed considerable deformation, which was similar to that made of monolayered PLA8052D foam sheet without lamination. When the thickness of the solid skin increased to 70 μm, the thermoformed bowl also deformed after 7 min and still showed very limited improvement on dimensional stability at high temperature. Therefore, it would
crystallinity.51 This reduced the positive impact of the thermoforming process on PLA’s degree of crystallinity. Nevertheless, the degree of crystallinity of PLA8052D foams, regardless of heating temperature, was dramatically increased after the thermoforming process. In particular, the intensity of α-crystal at 2θ = 16.7° in the XRD spectra increased significantly after the thermoforming process. The higher peak intensity and therefore the higher degree of crystallinity were attributed to the annealing-induced crystallization52,53 during the heating of the PLA foam sheet in the oven, as well as the strain-induced crystallization54,55 by the stretching of the foam sheets during the thermoforming process. The glass transition temperatures of PLA8052D thermoformed at different temperatures were evaluated by DSC and are summarized in Table 3. Table 3. Glass Transition Temperatures of PLA8052D Thermoformed at Different Temperatures thermoforming temperature (°C)
glass transition temperature of thermoformed PLA8052D (°C)
170 175 180 185
64.46 64.41 64.56 64.34
The XRD patterns of the film made of PLA2500HP with a thickness of 110 μm before and after the thermoforming process were also examined, and the results are shown in Figure 10. To decouple the effect of process-induced strain from that of the heating process before the thermoforming, two sets of high-HDT PLA2500HP were prepared. The first set was preheated in the oven and subsequently cooled in air (i.e., annealed PLA2500HP film) without undergoing thermoforming. The second set was preheated in the oven and subsequently underwent the thermoforming process (i.e., thermoformed 564
DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
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CONCLUSION Thermoformed containers made of low-density PLA foam sheets for hot-fill disposable packaging applications were successfully manufactured using a production scale tandem foam extrusion line. The heat endurance of the laminated PLA foams was found to be comparable with that of conventional PS foams. Monolayered foam sheets, being thermoformed into bowl-shaped products using lab-scale thermoforming equipment, had limited resistance to heat. Experimental results show that lower heating temperature, prior to the thermoforming process, would increase the degree of crystallinity and enhance the thermoformed PLA foam bowl’s heat endurance. Regardless, the low glass transition temperature of PLA as well as the residual stress during thermoforming process resulted in unsatisfactory dimensional stability of the PLA foam bowls when hot water was poured into them. In contrast, the high-temperature dimensional stability of thermoformed PLA sheet foams was significantly enhanced by laminating a solid high-HDT PLA film with sufficient thickness onto the PLA foam sheet. Based on experimental findings reported in this work, it is expected that either coextrusion foaming or extrusion coatings can be an effective process for manufacturing twolayered or multilayered heat-resistant PLA foam sheets for hotfill packaging applications. In this context, the thickness of solid high-HDT PLA film layer should be optimized to minimize the material cost while satisfying the required dimensional stability at an elevated temperature. The minimum thickness of solid high-HDT PLA film should be determined based on actual applications. To decrease the total material cost, the thickness of the foamed layer could potentially be reduced because the solid layer could contribute to the required mechanical strength of the two-layered foam sheet.
Figure 11. Effect of heating temperature for thermoforming on the dimensional stability of the thermoformed bowls containing hot water at different temperatures.
be unsatisfactory for various hot-fill applications. However, for the PLA foam bowl laminated with 110 μm thick solid PLA2500HP film, the bowl was able to endure the heat from the hot water without any deformation for 13 min and showed only very minor deformation thereafter. Figure 12a,b reveals that the
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Funding
The research presented here was funded by the Natural Science and Engineering Research Council of Canada (NSERC) Engage Grant (Grant 468707). Notes
The authors declare no competing financial interest.
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Figure 12. Thermal resistance test of thermoformed PLA foam bowls made of PLA foam sheets laminated with high-HDT PLA solid films of different thicknesses: (a) 30 μm and (b) 110 μm.
ACKNOWLEDGMENTS The authors of this study gratefully acknowledge the financial support by the Natural Science and Engineering Research Council (NSERC) of Canada.
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PLA foam bowl laminated with a 110 μm solid film had thermal resistance significantly higher than that of the one laminated with a 30 μm solid film. The PLA thermoformed bowl laminated with a 30 μm solid film exhibited more severe surface wrinkles and geometrical deformation than that laminated with a 110 μm solid film. It can be deduced that lamination of thicker high-HDT PLA2500HP films would increase the dimensional stability at high temperature of the thermoformed twolayered foam sheet. In contrast, monolayered PLA8052D foams showed susceptibility to heat despite the high crystallinity, which might be attributed to the residual stress during the elongation process of thermoforming.56
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ABBREVIATIONS DSC = differential scanning calorimetry HDT = heat deflection temperature PDLA = poly(D-lactic acid) PLA = polylactic acid PS = polystyrene SEM = scanning electron microscopy XRD = X-ray diffraction REFERENCES
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DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
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DOI: 10.1021/acs.iecr.5b03473 Ind. Eng. Chem. Res. 2016, 55, 560−567
