J Polym Res (2015) 22:177 DOI 10.1007/s10965-015-0811-6
ORIGINAL PAPER
Preparation of biodegradable poly(butylene succinate)/halloysite nanotube nanocomposite foams using supercritical CO2 as blowing agent Wei Wu 1 & Xianwu Cao 1 & Hong Lin 1 & Guangjian He 1 & Mengmeng Wang 1
Received: 15 January 2015 / Accepted: 21 July 2015 # Springer Science+Business Media Dordrecht 2015
Abstract Biodegradable poly(butylene succinate) (PBS) was melt compounded with halloysite nanotube (HNT) to prepare PBS/HNT nanocomposites, and both pure PBS and PBS/ HNT nanocomposites were foamed successfully using supercritical carbon dioxide as a physical blowing agent. The cell morphologiesshowed that the cell size decreased, and both cell density and volume expansion ratio increased with the addition of HNT. Within the HNT content used in this work (1, 3, 5, and 7 wt.%), the content of 5 wt.% was found to be the one that lead to the smallest cell size and highest cell density and volume expansion ratio. In addition to the HNT content, both saturation temperature and saturation pressure were found to significantly influence the cell morphology. Higher saturation pressure led to smaller cell size and higher volume expansion ratio. Interestingly, a close-celled to interconnect open-celled morphology transition occurred for PBS/HNT nanocomposites at a saturation temperature of 120 °C. The formation of interconnect open-celled morphology was mainly attributed to the stress induced by the HNT in the cell solidification process. With the increase of HNT content, saturation temperature and saturation pressure, the enthalpy of fusion of the foamed samples increased.
Keywords Poly(butylene succinate) . Halloysite nanotube . Nanocomposites . Foam . Supercritical carbon dioxide * Xianwu Cao
[email protected] 1
National Engineering Research Center of Novel Equipment for Polymer Processing, Key Laboratory of Polymer Processing Engineering of Ministry of Education, School of Mechanical and Automative Engineering, South China University of Technology, Guangzhou 510640, People’s Republic of China
Introduction Biodegradable polymers have attracted great attention from both industrial and academic areas, since they are environmentally benign and able to be degraded into carbon dioxide, water, biomass and humic matter by microorganisms [1]. In particular, biodegradable polymers in the form of foams have drawn interest from both engineers and researchers, because these biodegradable polymer foams can be widely used in many applications, such as acoustic and thermal insulation, impact resistance and scaffolds for tissue engineering [2–4]. A large number of reports have been published on the preparation of biodegradable polymer foams including poly(epsilon-caprolactone) (PCL) [5, 6], polylactide (PLA) [7, 8], poly(lactide-co-glycolide) (PLGA) [9, 10] and poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV) [11] by using supercritical carbon dioxide (sc-CO2) as the blowing agents. As compared to these traditional physical blowing agents, the sac-CO2, which is chemically inert, non-toxic and environmentally friendly, has been gradually widely used. Poly(butylene succinate) (PBS) is a biodegradable and biocompatible aliphatic polyester with excellent mechanical properties, good thermal resistance and melt processability [12]. It can be synthesized through condensation polymerization of 1,4-butanediol and succinic acid [13]. In light of its biodegradability and harmless degradation products, PBS has been used as a novel biomaterial for tissue engineering [14–17] and drug delivery [18–20]. However, there are few reports focusing on the preparation of PBS foams. Bahari et al. [21] formed PBS foams with cross-linked structures by irradiating the PBS with electron-beam irradiation, and then blends with different chemical blowing agents. Lim et al. [22–24] mixed PBS with carbon nanofiber, organoclay and multi-walled carbon nanotube, respectively, and then studied the effects of the processing methods, nanofiller content and
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foaming conditions on the cell morphology of the expanded PBS foams. The cell morphology observations exhibited that a small amount of nanofillers could increase the cell density. Li et al. [25] improved the viscosity of PBS with dicumyl peroxide (DCP) and trimethylolpropane trimethacrylate (TMPTA), and finally foamed PBS with a chemical blowing agent named azodicarbonamide (AC). The result showed that a well-closed cell structure of PBS foam could be produced with appropriate content of DCP and AC. Zhang et al. [26] prepared PBS foams by using ammonium bicarbonate as the chemical blowing agent and obtained the smallest cell size of pristine PBS foam with the addition of 5 wt.% ammonium bicarbonate. Xu et al. [27] and Son et al. [28] prepared PBS microcellular foams with sc-CO2, and studied the effects of processing parameters, including foaming temperature, saturation pressure and depressurization step and rate on the morphology of PBS foams. The results revealed that higher saturation temperature and lower depressurization rate contributed to an increment in cell size. It is well known that the nanofillers loaded into a polymer matrix can act as a homogenous nucleating agent to facilitate the cell nucleation process. Compared with the neat polymer foams, polymer nanocomposite foams show higher cell density, smaller cell size and narrower cell size distribution [29, 30]. However, few work about preparing PBS nanocomposite foams using sc-CO2 as a physical blowing agent have been reported. Halloysite nanotube (HNT) is a kind of natural kaolinite mineral with hollow nanotubular structure [31]. The typical size of HNT varies from 1 to 30 nm in the inner diameter, 30– 50 nm in outer diameter and 100–2000 nm in length [32]. Due to a high length-to-diameter ratio and a low hydroxyl functional group density on the surface, HNT can disperse uniformly in a polymer matrix [33, 34]. It has been emerged as the additive in polymers to improve their mechanical properties and thermal properties [35–38]. As reported in our previous study [39], the addition of HNT can improve the strength
Fig. 1 Schematic diagram of a custom-designed high pressure supercritical CO2 foaming instrument: (1) CO2 gas cylinder; (2) valve; (3) syringe pumps; (4) temperature controller; (5) pressure gauge; (6) mechanical agitator; (7) tubular vessel; (8) sample; (9) oil bath tub (melting); (10) oil bath tub (saturation) and (11) ice-water bath
and the modulus of PBS simultaneously without much loss of ductility. In this study, PBS and PBS/HNT nanocomposite were foamed with a batch foaming apparatus by using sc-CO2 as the physical blowing agent. The effects of HNT content, saturation temperature and saturation pressure on the foam morphology and melting behavior were investigated.
Experimental Materials A commercial poly(butylene succinate) (PBS, Bionolle 1903MD) was purchased from Showa Highpolymer, Japan, with melt flow index of 4.5 g/10 min (190 °C, 2.16 kg). It was dried at 80 °C for 6 h before melt compounding. Halloysite nanotube (HNT, grade ultrafine) with a specific surface area of 25.6 m2/g, was supplied by Imerys Tableware, New Zealand. Prior to mixing, it was heated in a furnace at 500 °C for 1 h to remove the absorbed and crystal water. The silane coupling agent (KH-560) was an industrial grade product and used as received. Industrial carbon dioxide (with the purity of 99.5 %) obtained from Guangzhou Golden Zhujiang Chemical Co. Ltd., China, was directly used as a physical blowing agent. Nanocomposites preparation The PBS/HNT nanocomposites were prepared by melt compounding as mentioned in our previous work [39]. The series of nanocomposites with HNT content of 0, 1, 3, 5, and 7 wt.% were abbreviated as PBS0, PBS1, PBS3, PBS5, and PBS7, respectively. Foaming apparatus and procedures The neat PBS and PBS/HNT nanocomposites were foamed by using a pressure quenching method [40]. In a typical batch
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of the high-pressure vessel was about 10 cm3. The vessel was placed in a custom-designed electronically controlled temperature oil bath tub which was equipped with a mechanical agitator. In a typical foaming process, the PBS/ HNT nanocomposite samples were placed on a sample stage in the tubular vessel. Then, low pressure CO2 gas was used to flush the vessel slowly. Subsequently, the vessel was immersed in an oil bath tub which was preheated to the melting temperature of 150 °C meanwhile the CO 2 loading was achieved to a desired saturation pressure with positive-displacement syringe pumps (model 260D; Teledyne Isco, USA). After the sample immersing in CO2 for 30 min, the vessel was quickly moved to another oil bath tub at desired saturation temperature for another 120 min. The pressure was then rapidly released to atmospheric pressure in 3 s and the foam morphology was fixed by cooling in an ice-water bath. The vessel pressure and temperature in the experiment are shown in Fig. 2.
Temperature Pressure Tmelting saturation
foaming Pressure
Temperature
melting
Tsaturation
Time (min) Fig. 2 Schematic diagram of the pressure and temperature evolution versus the time
foaming process [3], samples were saturated with sc-CO2 under certain temperatures and pressures. Subsequently, the release of pressure resulted in supersaturation and cell nucleation and growth. The schematic diagram of the batch foaming apparatus used in this study is shown in Fig. 1. The internal volume
Rheological measurements The dynamic rheological properties of various samples were determined on a MCR302 Rheometer (Anton Paar, Austria)
5
(a)
4
Storage modulus (Pa)
10
5
10 PBS0 PBS1 PBS3 PBS5 PBS7
Loss modulus (Pa)
10
3
10
2
10
1
10
(b)
4
10
PBS0 PBS1 PBS3 PBS5 PBS7
3
10
2
-1
10
0
1
10 10 Frequency (rad/s)
2
10
10
-1
0
10
(c)
PBS0 PBS1 PBS3 PBS5 PBS7
4
10 Complex viscosity (Pa• s)
1
10 10 Frequency (rad/s)
3
10
-1
10
0
1
10 10 Frequency (rad/s)
2
10
Fig. 3 Plots of a storage modulus (G′), b loss modulus (G″), and c complex viscosity (|η*|) versus frequency
2
10
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)
PBS0 PBS1 PBS3 PBS5 PBS7
Heat Flow (endo
which was equipped with a parallel-plate fixture (25 mm diameter). The samples about 1.0 mm in thickness were melted at 150 °C for 5 min in the parallel plate fixture to eliminate residual thermal history, and carry out measurements subsequently. Frequency sweep tests ranging from 0.1 to 100 rad/s were carried out at a fixed temperature of 150 °C under nitrogen atmosphere. A fixed strain of 1 % was used to ensure that the measurements were performed within a linear viscoelastic region for each sample.
Foam characterization The cell morphologies of cryo-fractured foam samples were observed by a scanning electron microscope (SEM, Quata 200, FEI, USA). The fractured surfaces were goldsputtered prior to SEM observation. The cell diameter was the average size of more than 100 cells in the SEM images. The cell density (N0) could be calculated using Eq. (1): N0 ¼
nM 2 A
ð1Þ
ρ ρf
ð2Þ
where ρ and ρf are the apparent density of unfoamed and foamed samples, respectively. The crystallization behaviors of foamed samples were measured using a differential scanning calorimetry (DSC, DSC-204F1, Netzch, Germany). Prior to the DSC recording, approximately 3 to 5 mg of the samples were compressed and sealed in alumina pans. The prepared materials were heated from 30 to 140 °C at a heating rate of 10 °C / min in a nitrogen flow.
Table 1
90
100
110
120
130
140
o
Temperature ( C)
Fig. 4 Melting curves of the foamed PBS/HNT nanocomposites with various HNT content
Results and discussion Dynamic rheological properties
3=2
Where A is the area of the SEM micrograph, n is the number of cells in the area A, and M is the magnification factor of the SEM micrograph. The density of unfoamed and foamed samples was evaluated by Archimedes’ principle with the help of a density kit mounted on a balance (BP211D, Sartorius, Germany). It was assumed that no water penetrated in the sample as the measurement time was very short [41]. The volume expansion ratio (ϕ) of the foam could be calculated using Eq. (2): ϕ¼
80
Zero shear viscosity of PBS/HNT nanocomposites
Samples
PBS0
PBS1
PBS3
PBS5
PBS7
η0 (Pa*s)
5842
6040
6127
6179
7897
To understand the effect of HNT incorporation on the structural modification of PBS, dynamic frequency sweep tests were carried out. It is generally believed that the addition of HNT to a polymer matrix will improve the melt rheological properties [33, 42]. The storage modulus (G′), loss modulus (G″), and complex viscosity (η*) of PBS and PBS/HNT nanocomposites as a function of frequency for a typical response at 150 °C are shown in Fig. 3. As shown in Fig. 3, the PBS/HNT nanocomposites show higher values of G′ and G″ than those of pure PBS over the entire frequency range. Moreover, the increase of G′ and G″ is the highest when the content of HNT is 7 wt.%, which is caused by the strong interaction of HNT with PBS, and the homogeneous dispersion of HNT within the PBS matrix [39]. In addition, the increment in the moduli doesd not appear to be sensitive to HNT content in the highfrequency region. This result is ascribed to the fact that polymer behavior plays sa prominent role in the rheological behavior of the PBS/HNT nanocomposites at higher
Table 2 Thermal properties data of the foamed PBS/HNT nanocomposites obtained from DSC Samples
HNT content (wt%)
Tp (°C)
ΔHm (J/g)
PBS0 PBS1 PBS3 PBS5 PBS7
0 1 3 5 7
116.5 115.8 114.2 114.4 114.6
64.3 67.6 70.9 74.3 77.9
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Fig. 5 SEM micrographs of PBS/HNT nanocomposite foams with various HNT content at 115 °C and 20 MPa a PBS0, b PBS1, c PBS3, d PBS5, e PBS7
frequencies. When a polymer sample is deformed at a high frequency, the entanglements do not have sufficient time to relax, leading to the increment in the moduli [43]. The zero shear viscosity (η0) of PBS/HNT nanocomposites is calculated from the dynamic viscosity data with the Carreau model. As illustrated in Table 1, the value of η0 increases monotonically with the increase of HNT content. The η0 increases from 5842 Pa*s for pure PBS to 7897 Pa*s for PBS7. The increase of the complex viscosity implies that the HNT restricts the chain mobility of PBS. This type of rheological behavior was also observed in other polymer/ HNT nanocomposites [44, 45].
Figure 5 presents the SEM micrographs of the foamed PBS/HNT nanocomposite with various HNT content. The samples were saturated at 20 MPa and 115 °C. The distribution of cell diameter was calculated from SEM micrographs, and the results were showed in Fig. 6. As illustrated, the cell size distribution of the foams nicely obeys to a Gaussian distribution. The cell morphology observations suggest that the addition of HNT improves the foam morphology, significantly. It is also noticed that the incorporation of 5 wt.% HNT in PBS matrix reduces the mean cell size from 20.45 to 13.04 μm. Moreover, the PBS/HNT nanocomposite foams showe more uniform cell
Effect of HNT content on the cell morphology of foamed sample
PBS0 PBS1 PBS3 PBS5 PBS7
The melting curves of the foamed samples with different content of HNT are shown in Fig. 4, and the results are summarized in Table 2. It reveals that the enthalpy of fusion (ΔHm) of PBS increases gradually with an increase of HNT content. It indicates that the nanosized HNT is able to act as an effective nucleating agent. The increased nucleating sites are likely to facilitate the PBS crystallization process in the nanocomposites. Moreover, the melting endotherm peak (Tp) and the onset melting point of PBS/HNT nanocomposite foams tend to shift toward lower temperatures as compared with neat PBS foam. These might be explained that the increased nucleating sites restricts the growth of the crystals, leading to smaller and/or imperfect crystal [46].
Distribution Ratio
0.6
0.4
0.2
0.0 0
5
10
15 Cell size (µm)
20
25
30
Fig. 6 Cell-size distribution of PBS/HNT nanocomposite foams with various HNT content at saturation temperature of 115 °C
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10
2.5x10
8
Cell density Volume expansion ratio
8
2.0x10
6 8
1.5x10
4 8
1.0x10
2
7
5.0x10
diameter distribution as compared with that of neat PBS foam. This is because the HNT serves as a heterogeneous nucleation agent to greatly increase the formation of bubble nuclei during the foaming process. According to the classical nucleation theory [47, 48], the HNT particles provide a large interfacial area and reduce the activation energy for cell nucleation at the interface. Figure 7 shows the corresponding cell density and volume expansion ratio of the PBS/HNT nanocomposite foams. As expected, the
Fig. 8 SEM micrographs of the PBS3 nanocomposite foam at various saturation temperature and fixed at saturation pressure of 20 MPa
Volume expansion ratio
3
Fig. 7 The effect of HNT content on the cell density and volume expansion ratio of PBS/HNT nanocomposite foams at saturation temperature of 115 °C
Cell density (Number of cells/cm )
177
0
PBS0
PBS1
PBS3
PBS5
PBS7
cell density and volume expansion ratio of foams increase with the increment of HNT content. By increasing the HNT content to 5 wt.%, the cell density increases to 2.17 ×108 cells/cm3 and the volume expansion ratio increases to 5.70. But with a further increase of HNT content, the cell density and volume expansion ratio of PBS7 decrease to 1.49 × 108 cells/cm3 and 5.25, respectively. From Fig. 5(e), it can be deduced that the coalescence of the cell occurs and the cells show complicated shape.
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Fig. 9 SEM micrographs of the PBS5 nanocomposite foam at various saturation temperature and fixed at saturation pressure of 20 MPa
Effect of saturation temperature on the cell morphology of foamed sample It has been reported that the saturation temperature in a batch foaming process is also crucial to control the foams cell morphology [51]. The cell morphologies of PBS3 and PBS5 nanocomposite foams prepared at different saturation temperatures are shown in Figs. 8 and 9, respectively, and the saturation pressure is fixed at 20 MPa. At the saturation temperature of 105 °C (see Figs. 8(a) and 9(a)), it is noted that only a few smaller cells is observed on the fracture surface as compared with those prepared at higher saturation temperatures. According to VogelFulcher-Tammann equation [52–54], the relationship between viscosity (η) and temperature (T) can be expressed as follows: ηðT Þ ¼ η0 exp
B T −T 0
where η0 and B are material constants, T0 represents the temperature. It can be seen from the above equation (see Eq. 3) that the viscosity increases as the saturation temperature decreases, which results in the resistance of cell growth. Furthermore, the PBS5 nanocomposite foamed at 115 °C shows more cells as compared with that of PBS3. This is probably because the HNT can promote cells to nucleate and grow. The volume expansion ratios of PBS3
4
Volume expansion ratio
These results can be explained that PBS7 has a high viscosity which retards the cell growth [49]. Although the increased content of HNT could improve the nucleation site during the foaming process, it is also expected to decrease the distance between particles and hence facilitates the cells coalescence [50].
PBS3 PBS5
3
2
1
0
105
110
115 o
Saturation temperature ( C)
ð3Þ
Fig. 10 The effects of saturation temperature on volume expansion ratio of PBS3 and PBS5 nanocomposite foams
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Fig. 11 SEM micrographs of PBS/HNT nanocomposite foams with various HNT content at 120 °C and 20 MPa
and PBS5 nanocomposite foams as a function of the saturation temperature are shown in Fig. 10. Within these composites, the volume expansion ratio at 120 °C is not included in Fig. 10, because it cannot be calculated by the Archimedes’ principle. It is shown that the sample saturated at the 115 °C has the highest expansion ratio. An increment of saturation temperature decreases the viscosity of the polymer matrix and promotes the CO2 diffusion, which subsequently contributes to larger cells and higher volume expansion ratio [55]. Interestingly, a close-celled to interconnect open-celled morphology transition occurs when the saturation temperature increases to 120 °C (see Figs. 8(d) and 9(d)). Figure 11 presents the PBS/HNT
o
nanocomposite foams with various HNT content. It can be observed that only the PBS/HNT nanocomposite foams have the open-celled morphology. The interconnect opencell structure may account for the stress induced by the HNT during the cell growth process [56]. This is because the HNT, as a rigid additive, has different stretching capability with the polymer. The shared cell wall becomes thinner as the cells grow. When the cell wall thickness reaches a critical value, the cell wall will be damaged by the stress, which results in interconnect open-cell structure. Figure 12 depicts the melting behavior of the PBS3 and PBS5 nanocomposite foams, and the values of the enthalpy
(a)
(b)
o
PBS5-105 C o PBS5-110 C o PBS5-115 C o PBS5-120 C Heat Flow (endo
Heat Flow (endo
)
)
PBS3-105 C o PBS3-110 C o PBS3-115 C o PBS3-120 C
80
90
100
110
120
130
140
80
90
o
Temperature ( C)
Fig. 12 Melting curves of the foamed PBS/HNT nanocomposites at various saturation temperatures
100
110
120 o
Temperature ( C)
130
140
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Table 3 Thermal properties data of the PBS/HNT nanocomposite foams obtained from DSC Samples
Saturation temperature (°C)
Saturation pressure (MPa)
Tp (°C)
ΔHm (J/g)
PBS3 PBS3 PBS3 PBS3 PBS5 PBS5 PBS5 PBS5 PBS5
105 110 115 120 105 110 115 120 115
20 20 20 20 20 20 20 20 16
114.6 114.7 114.2 113.9 114.5 114.8 114.4 114.6 114.9
59.8 63.8 70.9 71.3 70.1 71.5 73.8 77.0 64.2
PBS5
115
18
114.5
67.4
of fusion and the melting endotherm peak are listed in Table 3. It can be observed that the enthalpy of fusion continually increases when the saturation temperature increases. This phenomenon may be attributed to the increased polymer chain mobility associated with the plasticization effect of dissolved CO2 [57, 58]. However, in the PBS5 case, the increased content of HNT restricts the mobility of PBS chain. Therefore, the melting enthalpy of PBS5 does not increase significantly as compared with that of PBS3.
Fig. 14 that the foams with small cell diameter are achieved when the saturation pressure increases. Meanwhile, more uniform cell size distribution is also observed in Fig. 14(c). The mean cell diameter decreases from 19.70 to 13.04 μm and volume expansion ratio increases from 2.79 to 5.70 (see Fig. 15) as the saturation pressure increases. These results indicate that more CO2 are absorbed in the PBS at higher saturation pressure, because the diffusion of CO2 obeys the Fick’s law to some extent. The higher content of CO2 in PBS matrix would cause more supersaturation when the pressure is released, which would promote the formation of nucleation sites. Consequently, the cell diameter decreases and the cell density increases with the increment of saturation pressure. On the other hand, the more CO2 penetrated in the polymer matrix would lead to a decrease of melt viscosity [59]. Thus, the sample foamed at higher saturation pressure has a higher volume expansion ratio. The effect of saturation pressure on the melting behavior of PBS5 nanocomposite foam is calculated in Table 3. As shown, the melting peak shifts toward to lower temperatures as the Sc-CO2 saturation pressure enhances. This is ascribed to the plasticization effect caused by the dissolved CO2 [60]. In addition, the enthalpy of fusion increases with the increase of the saturation pressure. Similar results have been reported by other researchers [61, 62]. It is believed that the dissolved CO2 induces the crystallization of PBS.
Effect of saturation pressure on the cell morphology of foamed sample
Conclusions Saturation pressure is also an important parameter for batch foaming. Figure 13 shows the effect of the saturation pressure on the cell morphology of PBS5 nanocomposite foams. The corresponding distribution of cell diameter at different saturation pressure is presented in Fig. 14. The saturation temperature was fixed at 115 °C and saturation pressure ranged from 16 to 20 MPa. It can be seen from
In this work, PBS/HNT nanocomposites were prepared by melt compounding. The dynamical rheology measurements show that the storage modulus, loss modulus and complex viscosity of PBS/HNT nanocomposites increase with the content increase of HNT. Then biodegradable PBS/HNT nanocomposite foams were successfully produced by batch
Fig. 13 SEM micrographs of the PBS5 nanocomposite foam at various saturation pressure
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0.5
16.16 µm
(a) 16 MPa 19.70 µm Distribution Ratio
Distribution Ratio
0.4
(b) 18 MPa
0.4
0.3
0.2
0.3
0.2
0.1
0.1
0.0
0.0 0
10
20
30
40
0
50
10
20
30
40
50
Cell size (µm)
Cell size (µm)
0.5
13.04µm
(c) 20 MPa
Distribution Ratio
0.4
0.3
0.2
0.1
0.0 0
10
20
30
40
50
Cell size (µm) Fig. 14 The effect of saturation pressure on cell-size distribution of the PBS5 nanocomposite foams at saturation temperature of 115 °C
8
Volume expansion ratio
6
4
2
0 16
18 Saturation pressure (MPa)
20
Fig. 15 The effect of saturation pressure on volume expansion ratio of the PBS5 nanocomposite foams at saturation temperature of 115 °C
foaming with supercritical carbon dioxide (sc-CO2) as physical blowing agent. The effects of HNT, saturation temperature and saturation pressure on cell structures and melting behaviors of PBS/HNT nanocomposite foams were investigated. As expected, by introducing a small amount of HNT, the foam cell diameter, cell density and volume expansion ratio were improved as compared with that of pure PBS foam, indicating that HNT acted as heterogeneous nucleating agent. With the addition of 5 wt.% HNT, the PBS/HNT nanocomposite foam achieved the smallest mean cell diameter of 13.04 μm and the largest cell density of 2.17×108 cells/cm3. Furthermore, a lower saturation temperature and a higher saturation pressure were more favorable for obtaining uniform foams with lower cell diameter. When the saturation temperature increased up to 120 °C, the interconnected cell structure were achieved. The addition of HNT, higher saturation temperature and saturation pressure would enhance the crystallinity of PBS/HNT nanocomposite foams.
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