Effects of Compressed CO2 and Cotton Fibers on the Crystallization

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Effects of Compressed CO2 and Cotton Fibers on the Crystallization and Foaming Behaviors of PLA Xiaoli Zhang, Weidan Ding, Na Zhao, Jingbo Chen, and Chul B Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04139 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Industrial & Engineering Chemistry Research

Effects of Compressed CO2 and Cotton Fibers on the Crystallization and Foaming Behaviors of PLA Xiaoli Zhang1,2*, Weidan Ding2, Na Zhao1,2, Jingbo Chen1, Chul B. Park2* 1

School of Materials Science and Engineering, Zhengzhou University, Zhengzhou, China 450001

2

Microcellular Plastics Manufacturing Laboratory, Department of Mechanical and

Industrial Engineering, University of Toronto, Toronto, Ontario, Canada M5S 3G8

*Corresponding authors: [email protected] [email protected], Tel: +1 416 978 3053 Abstract The foaming behavior and thermal properties of neat PLA and PLA/cotton-fiber composite foams were investigated in this study. CO2 saturation pressure, temperature, and fibers content significantly affected the PLA’ crystallinity and foaming behaviors. At the same saturation temperature (140 °C), a low CO2 pressure generated non-uniform foam and a large unfoamed area due to too high crystallinity with a close-packed structure. At an intermediate pressure, a fine-cell structure was developed due to the presence of numerous less closely-packed crystals served as cell nucleating agents. A high CO2 pressure also led to a uniform cell structure but with larger cell sizes due to cell deterioration. Similar to the effect of saturation pressure, an intermediate temperature generated the uniform fine-cell structures. The PLA’s cell morphology was improved by the addition of cotton fibers at a low content because of the increased local

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stresses through the fibers and transcrystals surrounding the fibers. Key words: Polylactide, Crystallinity, Foaming, Compressed CO2 1. Introduction Polylactide (PLA) is one of the commercially available and mass-produced biodegradable polymers that have been produced in the last decade.1-2 It is derived from annually renewable resources, such as sugar beets, corn, and other agricultural biomass. PLA has some advantages over traditional petroleum-based polymers. These include its good mechanical properties, biodegradability, and biocompatibility.3-7 Thus, it has been used in the packaging, biomedical, and agricultural fields. But PLA also has some drawbacks including its narrow processing temperature windows, a slow crystallization rate, brittleness, and a low melt strength.8-13 To overcome these limitations and broaden PLA’s industrial applications, various additives have been incorporated, including micro-sized talc,13-14 nanoclay,9, 13-15 carbon nanotube,16 graphene,17 chitin,18 and various types of cellulose fibers.19-24 Among these additives, natural cellulose fibers possess advantages that the others lack, such as their low density, abundance, degradability, and biocompatibility.25 Several kinds of natural cellulose fibers with different sizes have been introduced into biodegradable polymers. These include micro-size cellulose fibers,19-21, 23, 25 cellulose nanofibers,22, 26-27 and cellulose nanocrystals.24, 28 It has been reported that these additives enhanced PLA’s crystallization kinetics by providing heterogeneous crystal nucleation sites.22-23, 26-29 The presence of these particulates lowered PLA’s activation energy barrier for crystal

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nucleation,17, 22, 25 and thereby increased the crystal nucleation density and decreased the crystal size.22, 29 Wang et al. found that a transcrystalline structure was induced by sisal fibers in PLA at isothermal crystallization temperatures of between 123 °C and 130 °C.25 Adding micro- or nano- size cellulose did not generate a new crystalline structure.22, 29 Although the addition of cellulose fibers can lower PLA’s cost, their presence may also deteriorate its ductility and impact strength.10, 15 Microcellular foaming technology, which was introduced at the Massachusetts Institute of Technology in the 1980s,30-33 can help to produce a lighter product with better dimensional stability,34 improved specific flexural modulus and strength, impact strength and toughness,13, 35-36 as well as improved thermal stability.15, 37 For use during the foaming process, carbon dioxide (CO2) and/or nitrogen (N2) have recently been gaining popularity as physical blowing agents due to their low environmental impact.38 With CO2 present, the crystallization kinetics of PLA appears to have changed. It has been reported that, due to its plasticizing effect, supercritical CO2 accelerated PLA’s crystallization kinetics.1, 14, 22, 27, 39-40

The dissolved CO2 depressed both the glass transition temperature (Tg) and the

crystallization temperature remarkably depending on the saturation pressure.1, 14, 22, 39 In addition, the solubility of CO2 in PLA increased with gas pressure, but decreased with temperature.41 In a gas saturated environment, the cellulose fibers also promoted the PLA’s crystallization.22-23, 27 The effect of CO2 saturation on PLA’s crystallography has also been investigated extensively.1, 14, 22, 27, 42-43 However, the conclusions were not in agreement. This was probably due to different experimental conditions and the PLA 3



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materials used. Some researchers reported that CO2 induced a new crystalline structure in PLA,27, 42-43 while others claimed that CO2 did not affect the crystal structure.1, 14, 22 Crystallization kinetics can significantly affect PLA’s foaming behavior. The formed crystals can promote cell nucleation through local stress variations and gas supersaturation.44-46 The desirable tensile stresses can be generated in the amorphous region because the crystals contract.45 During foaming, the bubbles that grow around the crystals can cause polymer chain deformation because the crystals are rigid and resist movement.44-45 The growing crystals lead to a gas supersaturation at the amorphous-crystalline phase interface, thereby reducing the critical radius for cell nucleation.45-46 The presence of a large number of crystals can also increase the PLA’s melt strength through an increased molecular entanglement. The increased melt strength can suppress both cell coalescence and coarsening.47-48 Similar to the crystal effect, the incorporation of cellulose fibers can also promote cell nucleation and improve the foam structure. Improved PLA crystallinities were also observed under various CO2 pressures.1, 14, 22, 39 Natural cellulose Kraft fibers have some specific advantages, such as high cellulose and semi- cellulose contents, less lignin constituent, high strength and aspect ratio, compared to that of the non-treated natural fibers. Moreover, microcellular PLA/Kraft natural- fiber foams have potential industrial applications over some other fossil based polymer materials due to their totally degradable properties, which were seldom studied and reported in the literatures. The presence of lower content cotton fibers we selected in this study can act as the cell nucleation sites to generate more potential bubbles, but 4


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too more loading contents may retard the composites expansion. Transcrystals induced in the fibers surface can also influence the cell nucleation and expansion of the composites. In this study, cotton fibers were mixed with a linear PLA via the melt compounding process. The neat PLA and the PLA/cotton composites were then foamed with compressed CO2 using a batch foaming method. The objective of this study was to investigate the crystallization and foaming behaviors of the neat PLA as well as the PLA/cotton- fiber composites in the presence of compressed CO2 under various pressures. 2. Experimental 2.1 Materials and sample preparation A semi-crystalline PLA (Natureworks LLC, IngeoTM 3052D) with 4% of D-Lactide content, was used as the polymer matrix. It had a specific gravity of 1.24 g/cm3 and MFR 14 (g/10min., 210 °C, 2.16 kg). Bleached Kraft cotton fibers (Mudanjiang Paper Co., Ltd. Mudanjiang, China) had an average length of 2.0–3.0 mm, a width of 15–20 µm, and a statistic aspect ratio of 100-150 (These values were obtained and counted from measuring and counting of at least 100 cotton fibers, using an Olympus BX 51 Polar Microscope). Carbon dioxide (CO2) (99 % pure, Linde Gas LLC, Canada) was used as a foaming agent. PLA pellets and cotton fibers were dried at 50 °C for 12 h. and at 80 °C for 24 h., respectively. Then, the PLA and cotton fibers were compounded using a twin-screw micro-compounder (Xplore® MC 15, Xplore Instruments) at 170 °C and 100 rpm for 10

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minutes. PLA composites with various fibers content, that is, 1, 5, 10, 20, 30 wt. % were fabricated. After that, the compounded PLA/cotton- fiber mixture was compressed into thin samples with a thickness of 1.5 mm using a CARVER Presser at 170 °C. The compressed samples were used for the foaming experiment. Batch foaming experiments were carried out in a high-pressure vessel (shaped in cylinder with an inner diameter of 1.2cm, length of 5.0cm) under the conditions shown in Table 1. At the end of the designated saturation period, the vessel pressure was rapidly released, after a holding time of 10s, the chamber (with sample) was immediately dipped into cold water bath to freeze the PLA samples’ foam structures. The cells were nucleated during the pressure drop and their structure was stabilized during the temperature quenching process. The solubility of CO2 is a key factor to affect the cell nucleating, cell growth and the final cell structure, so the determination of the nucleating agent saturation time is important in this study. For semi-crystallized polymers, pressurized CO2 can easily dissolve into the amorphous regain, but difficult to diffuse into the crystalline domain. 41, 49

In our previous study, Mahmood et al. measured the solubility of compressed CO2 in

three brands of PLAs, 3001D, 8051D, and 4060D, respectively, using a magnetic suspension balance system, at 180, 190 and 200℃. 41 After a 1.5 h thoroughly equilibrium holding time at the tested temperatures, they observed that the PLA types slightly affect the CO2 solubility, for example, at 10.34MPa, 180℃, the solubility is about 0.067g/g (CO2/polymer) for three PLAs. They also found that the D-content has less effect on the CO2 sorption ability. Improved pressure and decreased temperature 6


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could enhance the CO2 solubility. In this study, based on the information of the former testing results, we select a 1h saturation time, which is assumed to reach the equilibrium sorption of CO2 under the conditions illustrated in Table 1. PLA/cotton- fiber composites with 1, 5, 10, 20, 30 wt. % fibers contents were labeled as PLA-1C, PLA-5C, PLA-10C, PLA-20C, PLA-30C, respectively. For contrast, all pure PLA samples were treated with the same procedures as that of whose fibers filled composites counterparts. Table 1 Foaming conditions

Temperature (°C)

Pressure (MPa)

Saturation time (h)

120

6.9

1

140

6.9

1

150

6.9

1

140

5.5

1

140

10.3

1

2.2 Differential Scanning Calorimetry (DSC) Analysis Differential Scanning Calorimetry (DSC, TA Instruments Q2000) was used to investigate the thermal properties of the neat PLA and its composites and their foams. For the foams, the samples were heated at a rate of 10 °C /min to record the heat flow in the first heating process. To identify cotton fibers’ effect on the PLA’s crystallization without the influence of compressed CO2, the neat PLA and its composites with different cotton fibers content were first heated to 200 °C at a rate of 10 °C/min. The

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samples were then equilibrated for 10 min. to eliminate their thermal histories. Following this, the samples were cooled to 25 °C at a rate of 20 °C/min. Finally, the samples were reheated at a rate of 10 °C/min to 200 °C (second heating process). The degree of crystallinity (XcDSC) in the heating cycles were calculated using the following Equation (1): 4, 40 Xc

DSC

=

∆H m − ∆H cc 93.0 × ∆WPLA

(1)

where ∆Hm is the melting enthalpy, and ∆Hcc is the cold-crystallization enthalpy. WPLA is the weight fraction of the PLA in the sample, 93.0 J/g is the theoretical heat of fusion for 100 % crystalline PLA.40

2.3 Rheological testing To investigate the influence of cotton fibers on the rheological properties of PLA, a rheometer (ARES-G2, TA Instruments) was used with a parallel plate fixtures at a gap of 1.5mm, in an oscillatory mode. A fixed strain of 0.1%, angular frequencies ranging from 5 to 628 rad/s were applied. The disc shaped samples with a diameter of 25mm, thickness of 1.0mm were firstly heated to 200℃ and held for 5 min to eliminate any former thermal and mechanical histories, then the samples were cooled to 170℃ at a rate of 10℃/min, finally, the frequency sweeps were subjected.

2.4 Scanning Electron Microscopy (SEM) The cell morphologies of the neat PLA and the PLA/cotton- fiber composite foams were characterized using a scanning electron microscope (SEM) (JSM-6060, JEOL, Japan). The foamed samples were fractured after immersion in liquid nitrogen for 8


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several minutes. The fractured surfaces were sputter-coated with platinum before the SEM. The foams’ porosities were calculated as follows using Equation (2): (2)

where ρunfoamed and ρfoamed are the densities of the composites before and after foaming, respectively. Pure PLA and PLA/cotton- fiber foams were measured using the water displacement method according to the ASTM D792. The cell size was measured from the SEM micrographs, and the cell density N was calculated according to Equation (3) as follows: 50 3

n 2 N =  Φ  A

(3)

(4)

where n is the number of cells counted in the SEM micrograph, A is the analyzed area in cm2 of the counted area, and Φ is the sample expansion ratio.

2.5 Wide-angle X-Ray Diffraction (WAXD) To investigate the crystal structure of specimens, X-ray diffraction profiles were collected by a Bruker D8 Discover apparatus (Bruker, Germany) with CuKα as the radiation source (λ=1.54 Å). It was operated at 35 kV and 30 mA and the scanning range was 10~30 ° in the reflection mode. The test samples were polished to around 1 mm in thickness using metallographic abrasive paper. For comparison with the degree of crystallinity derived from DSC results, the degree of crystallinity using WAXD

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results (XcWAXD) were calculated from the following Equation (5).

X cWAXD =

Ac A c + A a ×100%

(5)

where A c is the crystalline diffraction integral area, A a is the amorphous integral area.

3. Results and Discussion 3.1 Effects of cotton fibers on PLA’s crystallization and rheological properties To exclude compressed CO2’s effect and to better illustrate the influence of cotton fibers on PLA’s crystallization behavior, the second heating curves (as illustrated in 2.2) of the DSC tested results are shown in Figure 1. The DSC curves of the neat PLA and the PLA/cotton-fiber composites exhibited a typical double-melting peak Tm (147°C and 153°C), a Tg of around 62 °C, and a cold-crystallization temperature Tcc of about 111 °C. This double-melting peak was explained by the melt-recrystallization model, wherein imperfect and small PLA crystals were transformed successively into more stable and perfect crystals through the melting and recrystallization process.16 In the non-isothermal crystallization condition, with increased fibers content, the high-melting temperature peak’s XcDSC climbed from 4.55 % for the neat PLA to 9.10 % for PLA-30C composite. The low crystallinity values suggested that the PLA had slow crystallization kinetics in this condition. The enhanced XcDSC value at a high-melting temperature peak in the composites was mainly attributed to the cotton fibers’ heterogeneous crystal nucleating ability to provide more nucleating sites. Figures 2a and 2b are the storage modulus G ′ and complex viscosities η ∗ of pure 10


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PLA and PLA/cotton- fiber composites as a function of frequency, at a sweep temperature of 170℃. At the relatively low frequency range below 100 rad/s in this study, G′ and η ∗ were improved with the increasing of cotton fibers content, compared to that of pure PLA. Especially, a higher fibers content of 20 wt. % enhanced those values of G′ and η ∗ of the composites dramatically, either at low or high frequency ranges. These fibers enhancements could further affect the cell nucleating ability and the microcellular foaming behavior of PLA/cotton- fiber composites.

3.2 Pressure effect on PLA’s crystallization and foaming properties Figure 3 shows the first heating curves of the foamed samples processed at 140°C under various pressures. The melting behavior of the neat PLA and the PLA/cotton-fiber composites was sensitive to the CO2 saturation pressures before foaming. At 5.5 MPa, only one sharp melting peak was detected at around 160 °C (Figure 3a). At 6.9 MPa, a cold crystallization peak and a triple-melting peak (at around 149, 154 and 164 °C) were obtained in the foamed samples (Figure 3b). As the pressure was increased further to 10.3 MPa, the cold crystallization peak became more significant, as shown in Figure 3c. The high temperature melting peak (165 °C) became small, or even disappeared in some samples, and two low temperature melting peaks dominated. We believe that the foamed samples’ melting behavior was related to the crystallinity and the crystal structure. Figure 4 shows the XcDSC results of the PLA materials under CO2 pressures ranging from 5.5 to 10.3 MPa. In general, the XcDSC crystallinity increased when the saturation pressure decreased. For example, the crystallinities of the neat PLA were 27.13, 20.2, and 10.55 % at 5.5, 6.9, and 10.3 MPa, respectively. At a low saturation pressure (e.g., 11



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5.5 MPa), less CO2 was dissolved and less crystals were nucleated. Such crystals could grow into large crystallites with a close-packed structure.1 Therefore, higher crystallinities were obtained. By contrast, at a higher pressure (e.g., 10.3 MPa), more gas was dissolved in the PLA and a larger number of crystals were nucleated. But these crystals could only grow to a small size with a loose-packed structure because of the increased polymer chain entanglement from the crystal-to crystal interactions.1, 22 A close-packed structure can result in a higher melting peak.1, 22 As a result, higher melting peaks (≥ 160°C) are often observed under low saturation pressures and low melting peaks are seen under high-pressure conditions. At the same saturation pressure, the XcDSC value improved slightly with increased cotton content. This was attributed to the crystal nucleating effect of the cellulose fibers, which can provide more heterogeneous nucleating sites.51 With a fixed cotton fibers composition, the lower crystallinity of the foamed samples generated a more significant cold crystallization during the first heating process. This agreed with our observation in Figure 3. Wide angle X-Ray measurements were conducted, as shown in Figure 5, to study the crystalline structure of the neat PLA and the PLA/cotton- fiber composites under various CO2 pressures. All of the PLA foamed samples exhibited diffraction peaks at 2θ=14.6, 16.7, 19.0, and 22.3°. Those corresponded with the α crystal form of PLA.14, 22, 40, 52

This observation is in agreement with the reported diffraction peaks found in PLA

samples by other researchers.14, 22, 40, 52 The major peak position was not influenced by the presence of cotton fibers and the dissolved CO2, and this indicated that a new crystal phase had not been generated in the foamed samples. For the case of fibers, the 12


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diffraction peaks appeared in 15.6° of cellulose IV1 type crystals, and 22.6° of I1 type crystals in Figure 5a. Clearly, the main diffraction peaks at 16.7° of pure PLA and PLA/cotton- fiber composites did not influenced by the fiber diffraction information, while, the diffraction intensities of PLA/cotton- fiber composites at 22.3° might be affected by cotton fibers. The calculated XcWAXD results are 21.55, 44.12, 41.92, 31.69, 47.80, and 41.90%, with the increasing fibers contents from 0 to 30wt. %, after subtracting the composition of fiber proportion. Even the crystallinity values of XcDSC and XcWAXD are inequable due to the different treatments, but the XcWAXD changing trend is similar to that of XcDSC as shown in Figure 4, as a function of fiber content. By comparing the WAXD patterns of the samples foamed at 5.5 MPa and 6.9 MPa (Figure 5a and 5b), it was observed that the differences were in the intensity and broadness of the peaks. This reflected the changes in the degree of crystallinity and in the degree of perfection of the crystal lamellas under different saturation pressures14. Compared to the counterparts foamed at 5.5 MPa, the samples treated at 6.9 MPa (Figure 5b), had decreased intensities at the diffraction peak of 2θ=16.7°. Accordingly, the crystallinities decreased (Figure 4). Even though a triple DSC melting peak appeared in Figure 3b, a new crystal structure was not generated. The peculiar DSC triple melting peak may be attributed to the existence of less close-packed crystals in this foaming condition.14, 53-55 When the samples were exposed to 10.3 MPa CO2 pressure, the relative intensities of the neat PLA, PLA-1C and PLA-5C at 2θ=16.7° almost disappeared, indicating very low crystallinities in these samples. When the fibers content was increased to 10, 20, and 30 wt. %, relative diffraction intensities of 2θ= 13



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16.7, 19.0, and 22.3° were detected. This indicated that a high cotton fibers loading could enhance the PLA’s crystallization at this high pressure, as shown in Figure 5c.

Figures 6 and 7 show the SEM micrographs of the neat PLA and the PLA/cottonfiber composites at 140 °C but under various CO2 pressures of 5.5, 6.9 and 10.3 MPa. At 5.5 MPa, non-uniform foam structures with small cells were observed. Due to the high crystallinities and large crystal sizes with a close-packed structure in the samples, the cells could be nucleated and grow only in the amorphous regions. At 6.9 MPa, small-size bubbles were produced in large quantities. In this condition, a large number of less close-packed crystals were induced1. These crystals, acting as cell nucleating agents, promoted cell nucleation by stress variations44-46. The decreased crystallinity also helped to decrease the polymer matrix’s stiffness so that the cells could be nucleated and grow, unlike the case at 5.5 MPa. At 10.3 MPa, large cell sizes were observed and the cell size distribution became more uniform. Although a large number of cells were nucleated in the beginning due to a higher pressure drop rate and a large number of small crystals, the cell sizes were still bigger than those under low pressure. This was probably due to cell collapse and coalescence. At 10.3 MPa, the CO2 was in a supercritical state, and a large amount of dissolved gas had a significant plasticizing effect. The lower crystallinity at 10.3 MPa enhanced the samples’ porosities due to the large amorphous region, in contrast with the other two pressure conditions. Because CO2 can only be absorbed in the amorphous region, a higher crystallinity can decrease the CO2’s solubility, and thus negatively affect the samples’ expansion. With increased cotton fibers content, an irregular cell structure and smaller cell sizes were observed. 14


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This was attributed to the increased melt strength that came with a high cotton fibers content. The porosities were calculated and are shown in Figure 8. The porosities increased as the pressure increased. The porosities were around 10.0% at 5.5 MPa and gradually increased to about 55.0% at 10.3 MPa. This was because PLA had a higher amount of dissolved CO2 and a lower crystallinity at a higher pressure. If the crystallinity was too high, it would decrease the porosity due to the polymer matrix’s great rigidity. Due to the high melt strength of the samples with cotton fibers, the porosity decreased with the fibers content.

3.3 Temperature effect on PLA’s crystallization and foaming properties To investigate the effect of temperature on the crystallization and the foaming properties of the PLA and its composites, we used 120 °C, 140 °C and 150 °C saturation temperatures under a constant pressure of 6.9 MPa. Figures 9a and 9b show the DSC graphs of the foamed PLA and the PLA/cotton- fiber composites at 120 °C and 150 °C. Figure 10 summarizes the XcDSC as a function of the cotton fibers content at different saturation temperatures (for contrast, XcDSC results of foams at 140°C, 6.9MPa in Figure 4 was also listed in Figure 10). At 120 °C, the melting peaks presented a single peak with a shoulder on its left side, as shown in Figure 9a. This suggested that a higher crystallization had already been achieved for the PLA foams in this condition. The neat PLA’s XcDSC attained a high value of 35.47 %, and PLA/cotton- fiber composites’ XcDSC of were even higher around 40.00 %, as shown in Figure 10. At an elevated temperature of 140 °C in Figure 3b, there were triple melting peak behaviors, which we discussed 15



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earlier in the section on pressure effects. In this case, all of the samples showed a modest crystallinity level of around 27.0 %. We believe that the co-existence of triple melting peaks in a broad temperature range resulted from the co-existence of crystals with a different packing structure, as was noted earlier in the WAXD section in Figure 5b. A high melting peak (Tm3) usually indicates the existence of more close-packed crystal structures. At a high temperature as high as 150 °C, the neat PLA’s XcDSC value decreased to 5.53 %. Because of the high gas-saturation temperature, the polymer chain mobility was too high to crystallize and to grow. Thus, there was a low degree of crystallinity. Consequently, only a low temperature melting peak was seen. Figures 11 and 12 show the SEM images of the cross sections of the fractured neat PLA and the PLA/cotton- fiber composite foams. At 120 °C, large areas were not foamed in the neat PLA and the PLA/cotton- fiber composites. As discussed earlier, too high a degree of crystallinity may have suppressed the foaming. At 140°C, as shown in the middle rows of Figures 6 and 7, a large amount of small-size cells formed. Based on the DSC curves (Figure 3b) and on the WAXD profile (Figure 5b), a large number of imperfect crystals might have been generated. The large number of crystals promoted cell nucleation due to the local stress variations around the crystals and the gas supersaturation at the crystal front during the foaming.44-46 The lower crystallinity also helped the cells to grow. Thus, as shown in Figure 13 (porosity values of foams made at 140°C, 6.9MPa were also listed for the purpose of contrast), there were higher porosities than there were at a lower saturation temperature of 120 °C. At 150 °C, which is close to PLA’s melting temperature, the plasticizing effect of 16


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CO2 further facilitated the molecular movement; therefore, the PLA materials’ melt strength became very low. For this reason, the foam morphologies were extremely poor and a collapsed cell structure was observed (second row in Figure 11). Moreover, the high temperature melted the original crystals, and this dramatically decreased the crystallinity. But it softened the polymer materials, and thus caused a higher expansion ratio. Due to the bubble expansion, the fibers (arrows in Figure 12) were squeezed to the cell wall. In terms of fibers content, the porosity was first promoted to a higher level, and then it gradually decreased after reaching a peak at about 5 wt. % of the fibers loading, as shown in Figure 13. The mountain shape curves may be attributed to the fibers-fibers entanglements at a higher cotton fibers loading level. Such an entanglement might have depressed the bubble expansion.

3.4 Cotton fibers effect on PLA’s foaming properties To understand how the fibers affected PLA/cotton- fiber composites’ foaming properties, we analyzed the SEM micrographs foamed at 140 °C and 10.3MPa as the examples. Figure 14 shows the average cell size and cell density results with different cotton fibers content. With increased cotton fibers loading of up to 5 wt. %, the average cell size was reduced to 35.4 µm from 38.5 µm of the neat PLA. At 30 wt. % cotton fibers content, it was further decreased to 15.8 µm. The cell density increased gradually with increased cotton fibers content, as seen in Figure 14b. The cell density increased by 1 order of magnitude from around 4.8 × 107 cells/cm3 for the neat PLA to 4.4 × 108 cells/cm3 when a 30 wt.% cotton fibers content was added.

Supporting Information Paragraph: 17



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Cotton fibers distribution information at different contents on PLA matrix of the solid composites are presented in Figure S1, and the WAXD fitting schematic graphs of Figure 5s are illustrated in Figure S2. Descriptions of Figures S1 and S2 are listed below, and they were also appended in the attached file, so you can selectively use them. In Figure S1, fibers at lower contents below 10 wt. % are relatively dispersed uniformly in PLA, while, that of higher contents of 20 and 30 wt. % are not very homogeneous, because of the aggregation and uneven distribution of fibers in the matrix. Figure S2 illustrates the fitting procedure of WAXD graph, from which the crystalline diffraction integral area and amorphous integral area can be separately obtained and thus the crystallinity XcWAXD can be calculated.

4. Conclusions In this study, bleached Kraft cotton fibers were selected as the additives for polylactide (PLA). Neat PLA and PLA/cotton-fiber composites were foamed under different conditions using compressed carbon dioxide (CO2). The effects of subcritical and supercritical CO2 and cotton fibers on PLA’s crystallization and foaming behavior at different temperatures and pressures were investigated using DSC, and SEM. Compressed CO2 significantly affected the final crystallinity of the foamed PLA and its composites. At 140 °C, a high crystallinity of 27.13 % for the neat PLA was achieved after one hour of CO2 saturation at 5.5 MPa. The increase in CO2 pressure decreased the crystallinity. This was because the increased crystal-to-crystal interactions of a large

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number of formed crystals at a higher pressure restricted the crystal growth and lead to a loosely packed structure. At a saturation pressure of 6.9 MPa, the crystallinity decreased with increased temperature. This was due to the greater chain mobility that occurred at a higher temperature and which hindered the crystallization. On the other hand, the presence of the cotton fibers increased the final crystallinity of the foams due to its positive impact on crystallization. When the crystallinity was too high, it suppressed the foaming of PLA materials because of the increased stiffness in the polymer matrix. When a moderate crystallinity was achieved from the condition of 140 °C and 6.9 MPa pressure, it resulted in numerous small-size cells, due to the nucleating effect of a large number of formed crystals. A further decrease in the crystallinity generated a uniform cell structure but there were with larger cell sizes owing to the low melt strength. WAXD analysis showed that no new crystal structure was induced in the samples after the foaming process.

Acknowledgements The authors wish to thank Dr. Reza Barzegari for his support during the foaming experiment. This work was supported by the China Scholarship Council and by Zhengzhou University, the Microcellular Plastics Manufacturing Laboratory (MPML) of the University of Toronto, the National Natural Science Foundation of China (11172272, 11372284), and the Henan Province Natural Science Project of China (122300410278, 15A430049).

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Origin 8.5 Figure 1. DSC second heating curves of the PLA and the PLA/cotton- fiber foamed composites 196x135mm (300 x 300 DPI)


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Origin 8.5 Figure 2a. Storage modulus and complex viscosities of PLA and PLA/cotton- fiber composites at a sweep temperature of 170 °C, as a function of frequency (a) storage modulus 203x142mm (300 x 300 DPI)



Origin 8.5 Figure 2. (b) complex viscosity 203x142mm (300 x 300 DPI)


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Origin 8.5 Figure 3. DSC curves of the PLA and the PLA/cotton- fiber composites foamed at 140°C, but different pressures of (a) 5.5MPa 196x135mm (300 x 300 DPI)



Origin 8.5 Figure 3. (b) 6.9MPa 196x135mm (300 x 300 DPI)


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Caption : Origin 8.5 Figure 3. (c) 10.3MPa 196x135mm (300 x 300 DPI)



Origin 8.5 Figure 4. Degree of crystallinity (XcDSC) of composite foams as a function of cotton fibers content foamed at 140°C, but various pressures 215x166mm (300 x 300 DPI)


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Origin 8.5 Figure 5. WAXD profiles of the PLA and the PLA/cotton- fiber composites foamed at 140°C, but different pressures of (a) 5.5MPa 203x142mm (300 x 300 DPI)



Origin 8.5 Figure 5. (b) 6.9MPa 203x142mm (300 x 300 DPI)


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Origin 8.5 Figure 5. (c) 10.3MPa 203x142mm (300 x 300 DPI)



Figure 6. SEM micrographs of the PLA and the PLA/cotton- fiber composites foamed at 140°C, but different pressures 254x129mm (96 x 96 DPI)


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Figure 7. SEM micrographs of the neat PLA and the selected PLA/cotton- fiber composites with higher magnification foamed at 140°C, but different pressures 254x190mm (96 x 96 DPI)



Origin 8.5 Figure 8. Porosities of the PLA and the PLA/cotton- fiber composites foamed at 140°C, but different pressures 215x166mm (300 x 300 DPI)


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Origin 8.5 Figure 9. DSC curves of the neat PLA and the PLA/cotton- fiber composites foamed at (a) 120 °C 196x135mm (300 x 300 DPI)



Origin 8.5 Figure 9. (b) 150 °C 196x135mm (300 x 300 DPI)


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Origin 8.5 Figure 10. Degree of crystallinity (XcDSC) of the PLA and the PLA/cotton- fiber composites foamed at 6.9MPa, but different temperatures 215x166mm (300 x 300 DPI)



Figure 11. SEM micrographs of the neat PLA and the PLA/cotton fiber composites foamed at 6.9MPa, but different temperatures 255x94mm (96 x 96 DPI)


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Figure 12. SEM micrographs of the neat PLA and the selected PLA/cotton fiber composites with higher magnifications foamed at 6.9MPa, but different temperatures 254x131mm (96 x 96 DPI)



Origin 8.5 Figure 13. Porosities of the PLA and the PLA/cotton- fiber composites foamed at 6.9MPa, but different temperatures 215x166mm (300 x 300 DPI)


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Origin 8.5 Figure 14. (a) Average cell size and of the neat PLA and the PLA/cotton- fiber composite foamed at 140°C, 6.9MPa 215x166mm (300 x 300 DPI)



Origin 8.5 Figure 14. (b) cell density of the neat PLA and the PLA/cotton- fiber composite foamed at 140°C, 6.9MPa 215x166mm (300 x 300 DPI)


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Effects of Compressed CO2 and Cotton Fibers on the Crystallization

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