Influence of foaming-induced crystallization behavior on the structure of poly(butylene adipate-co-terephthalate) supercritical foamed beads

Abstract

The crystallization behavior during foaming directly affects the foaming properties. For crystalline polymers, there is no consensus on the influence of the crystallization behavior during foaming process on the stabilization of the cell structure. In this work, PBAT foamed bead and unfoamed pellets were prepared by controlling the saturated temperatures in supercritical CO₂, soaking step (one or two) and the depressurization rate, respectively. Double melting peaks were observed in the DSC curve of supercritical CO₂ foamed PBAT beads. By comparing the outgassing rates we find that the stretching-induced crystallization caused by the rapid expansion of the gas during foaming plays an important role in the stabilization of the cells. Although the crystalline perfection or crystal size at this time is much smaller than that of the crystalline grains formed during static cooling, the rapid crystallization is effective in stabilizing the cell structure of the foamed pores. Compared to normal supercritical foaming processes, the two-step foaming process of soaking CO₂ at high temperatures followed by foaming at low temperatures results in an increase in cell size and expansion ratio. At high temperatures, more CO₂ diffuses into the PBAT pellets, increasing the instantaneous gas concentration in the pellets for foaming, and the rapid stretching produces stretching-induced crystallization that raises the average size of the cells, further increasing the expansion multiplicity of individual cells. The average cell size of foam beads rises from 37.8 to 48.8 µm and the expansion ratio also increases to 8.6 with saturated temperature increasing form 95 to 105°C. The two-step soaking foaming method is a more efficient way of manufacturing industrial foamed beads, allowing for the preparation of better foam beads at low temperatures.

Keywords

Poly (butylene adipate-co-terephthalate)cell structure crystallization foaming process

Introduction

With the extensive use of disposable plastic bags and tableware, these consumer products made of low-density polyethylene (LDPE), polystyrene (PS) and other plastics have caused serious white pollution problems, and a large part of the pollutants are foam lunch boxes made of foamed PS. In order to alleviate the white pollution problem, polybutylene adipate terephthalate (PBAT) foam has attracted increasing attention. This material, which is similar to LDPE foam, has high flexibility, strong resilience, and excellent biodegradability, and its promotion and use in the field of disposable consumer goods is of great significance to sustainable green development.^1–3

As a semi-crystalline material, many factors affect PBAT foam structure. The crystalline properties and melt strength are the two key factors affecting not only the diffusion and permeability of foaming agent, but also the cell size and the foam expansion ratios.⁴ In general, the melt strength of polymers directly affects the formation of cells, but for semi-crystalline polymers, the contribution of polymer crystallization during cooling cannot be ignored.^5,6 The foaming of semi-crystalline polymers faces a number of problems, including (1) a narrow foaming temperature window due to changes in melt strength before and after crystallization, (2) a significant inhibition of cell growth by crystallization, and (3) the diffusion of foaming agent in the resin becomes difficult due to crystallization.^4,7,8 The effect of crystallization on cell growth and stability during foaming is complex. An important research content in the preparation of semi-crystalline polymer foams is to control the crystallization behavior during foam cooling.

Due to its non-toxicity and environmental friendliness, supercritical CO₂ (scCO₂) foaming is considered as a green way to manufacture polymer foams.⁹ In the autoclave, CO₂ will simply diffuse into the amorphous zone in the PBAT spherical crystal. During pressure relief, the rapid expansion of the gas brings about both initial wafer destruction and stretching-induced crystallization of the amorphous zone. This behavior leads to a complex crystallization during foaming.^10–12 In the present study of supercritical CO₂ foams of semi-crystalline polymers such as polylactic and polypropylene (PP), the phenomenon of double melting peaks in the DSC curves of foamed beads was found. Nofar et al.¹³ suggests that the low temperature melting peak is related to the formation of small crystalline grains during foaming, while the high temperature melting peak is related to the crystallization of the polymer during CO₂ immersion. At present, the causes and mechanisms of high melting temperature crystallization formation during the CO₂ isothermal saturation stage have been explained in detail. However, there is no consensus among researchers on the effect of rapid stretching-induced crystallization produced by foaming on the cell structure.¹⁴

Recently, we found that large and small crystals coexistence in supercritical carbon dioxide foamed PBAT beads. The expansion ratio and the average cell size of the beads increase as the size difference between the large and small crystalline components decreases. Apparently, the small crystals induced by melt stretching during foaming have a significant effect on the cell structure. In this work, we investigate the effects of depressurization rate and CO2 immersion temperature on the formation and growth of small crystals. The contribution of small crystals formed during rapid stretching to the stability of the cell structure is elucidated. The relationship between foaming process conditions-crystallization performance-cell structure is further established, which can better guide industrial production.

Experimental

Materials

PBAT (A400) was supplied by Zhuhai Vantone Co., Ltd with a mass density of 1.186 g/cm³; Carbon dioxide (CO₂) was supplied by Guangzhou Shengying Chemical Co., Ltd with a purity of 99.99%.

Sample preparation

PBAT bead foams were prepared under different saturated temperatures in supercritical CO₂ and one soaking step. First, the pure PBAT resin was placed in the autoclave chamber. Then, the foaming system was heated to different temperatures (80°C, 85°C, 90°C, 95°C and 100°C, respectively) and soaked in scCO₂ for 1 h to ensure all the PBAT resins were thoroughly saturated with CO₂. The choice of foaming temperature is related to the stability of the bead shape. When the foaming temperature is lower than 80°C, the expanded PBAT beads cannot be formed. When the temperature is higher than 100°C, the melt strength is extremely low, and the foamed beads Visible collapse occurred during cooling. Finally, the pressure of the foaming system was released promptly from 15 MPa to 0.1 MPa, which offered a driving force for cell nucleation and cell growth to get various PBAT bead foams.¹⁵ After the foaming system was soaked in scCO₂ for 1 h, the pressure of the foaming system was released slowly from 15 MPa to 0.1 MPa (Unloading rate below 0.1 MPa/min) to suppress the foaming of PBAT and obtain the unfoamed pellet.

Two soaking steps were carried out as follows. First, the pure PBAT pellets was placed in the autoclave chamber. Then, the foaming system was heated to different temperatures (95°C, 100°C, 105°C and 110°C, respectively) and soaked in scCO₂ for 1 h to ensure all the PBAT resin were thoroughly saturated with CO₂ (the saturated temperature was represented by T₁). Thirdly, the foaming system was cooled to 90°C and soaked in scCO₂ for 30min (this temperature was represented by T₂). Finally, the pressure of the foaming system was released promptly from 15 MPa to 0.1 MPa, which offered a driving force for cell nucleation and cell growth to get various PBAT bead foams.

Characterization

Scanning electron microscope

An electron scanning microscope (S-3400N) from Hitachi, Japan, was used to analyse the cell morphology of PBAT samples. Various PBAT foams were placed in liquid nitrogen for 1 h and removed for fracture. A thin layer of platinum was sprayed on the fractured surfaces of various PBAT foams before putting into S-3400N for observation. The cell size of various PBAT bead foams was measured by Image-J. Based on equation (1), (2), (3) and no less than 10 figures of the SEM experiment, the average cell size, cell density and the expansion ratio were calculated, respectively:¹⁵

\bar{D} = \frac{\sum_{i = 1}^{n} D_{i}}{n}

(1)

N_{f} = {[\frac{n M^{2}}{A}]}^{\frac{3}{2}} φ

(2)

φ = \frac{ρ_{p}}{ρ_{f}}

(3)

where n was the number of cells counted in the SEM photographs, M was the magnification factor, A was the selected statistical area (cm²) in the electron microscopy photographs, and

ρ_{p}

is density of the pre-foamed sample and

ρ_{f}

is density of the foamed sample.

ρ_{f}

is measured according to ASTM D792-00 by weighing a sample of the polymer foam in water using a sinker.

Differential scanning calorimetry

A differential scanning calorimeter (DSC3) from Mettler Group, USA was used for the thermal property analysis of PBAT samples. Firstly, 5-10 mg of samples were weighed and placed under the protection of high purity nitrogen. Then, various PBAT bead foams and unfoamed pellets were quickly heated from room temperature to 190°C at a rate of 10°C/min under a 50 mL/min nitrogen gas flow. Finally, the crystallization temperature (T_c) and melting temperature (T_m) of various PBAT bead foams and unfoamed pellets were determined. The relative crystallinity ( $χ_{c}$ ) of various PBAT bead foams and unfoamed pellets was calculated using the following equation (3):

χ_{c} = \frac{∆ H_{m}}{∆ H_{m}^{0}} \times 100 %

(4)

where

∆ H_{m}

(J/g)was the experimental melting enthalpy, and

∆ H_{m}^{0}

was the heat fusion of 100% crystalline PBAT, 114 J/g.¹

Extensional viscosity test

Extensional viscosity test sample preparation: PBAT resin were evenly placed in the mold and pressed into 0.5 mm, 20 mm (L) × 10 mm (W) samples, pressing temperature 190°C, pressure 10 MPa, holding pressure 5min and then cooling.

Extensional viscosity test: Anton Paar rotational rheometer MCR302 was used, and the sample was clamped vertically in the fixture with the SER fixture, the Hencky deformation was set to 3%, and the thermal equilibrium time was 60s. The transient viscosity test was performed at 80°C, 85°C, 90°C, 95°C and 100°C, respectively, with a strain rate of 0.1/s.

Foaming properties

A precision balance (AUW20D) of Shimadzu Manufacturing Co., Ltd, Kyoto, Japan, was used to analyse the CO₂ absorption of PBAT samples. First, a certain amount of sample was weighed and its mass was measured with a precision balance. The sample was then spread on the bottom of the autoclave and foamed. Finally, the samples were removed after cooling and their masses were measured. The CO₂ absorption was calculated using the following equation (5):

M = \frac{m_{1} - m_{2}}{m_{2}}

(5)

where

m_{2}

was the mass of the pre-foamed sample and

m_{1}

was the mass of the foamed sample.

Calculation of cell nucleation rate

High melting temperature crystal was used as the heterogeneous nucleation point to trigger the cell nucleation. The nucleation rate was calculated using the following equation (6):¹⁶

N_{h e t} = C_{1} f_{1} \exp (- \frac{∆ G_{h e t}^{*}}{k T})

(6)

where

∆ G_{h e t}^{*}

was the critical Gibbs free energy for heterogeneous nucleation (Equation (7)),

f_{1}

was the frequency factor (Equation (9)),

C_{1}

was the concentration of the heterogeneous nucleating agent.

∆ G_{h e t}^{*} = \frac{2 ∆ P R^{3}}{3} f (θ)

(7)

f (θ) = {(2 + \cos θ) (1 - \cos θ)}^{2} / 4

(8)

f_{1} = {(∆ PR / π m)}^{1 / 2}

(9)

C_{1} = \frac{w_{P} \cdot ρ_{b l e n d}}{m_{P}}

(10)

where

f (θ)

was the energy decay factor,

∆ P

was the gas saturation pressure (15 MPa),

θ

was the contact angle (20°),¹³

ρ_{b l e n d}

was the density of the polymer/nucleating agent blend (1.186 g/cm³),¹⁶ m was the mass of the CO₂ gas and

m_{P}

was the mass of CO₂ (foaming nucleating agent) absorbed by unit mass of PBAT in the high-pressure reactor (measured by an accurate balance), R was the average cell size (measured by SEM), and

w_{P}

was the percentage of the nucleating agent in polymer/nucleating agent blend measured by DSC.

Results and discussion

Cell morphology of scCO₂ foamed PBAT beads by one step soaking under different temperatures

Figure 1 shows the morphologies of scCO₂ formed PBAT resin by one step soaking at different temperatures, and specific size parameters are listed in Table 1. It can be seen that the average cell size increases from 16.8 to 46.1 µm and the foaming ratio is from 2.9 to 8.5 by increasing the CO₂ soaking temperature. According to the statistical distribution of cell size, we find that the size distribution is compound Gaussian with distribution width from 5-35 µm to 20-70 µm.¹⁷ When the CO₂ soaking temperature is lower than 85°C, the foaming effect of PBAT beads is very poor. At this time, the cell size is small and a large number of unfoamed areas appear. Increasing the CO₂ soaking temperature will help improve the final cell structure. When the CO2 soaking temperature near the PBAT melting onset temperature, the average cell diameter rises, the unfoamed area disappears, the cell size distribution becomes wider, the cells appear to merge, and the cell density decreases continuously. Since the foaming process is a common process of gas diffusion expansion and crystallization as well as melt strength maintaining morphology, it is obvious from the SEM results that the increase in temperature increases the gas diffusion capacity and the average cell diameter. Next, we will continue to study the effects of melt strength and crystallization properties on the structure of the cells.

Figure 1.

SEM and cell diameter distribution of foamed beads by one-step soaking under different temperatures.

Table 1.

The cell density, the expansion ratio and the average cell size of foamed beads by one step soaking under CO₂ saturation and foaming temperature of 80°C–100°C.

CO₂ saturation and foaming temperature	80°C	85°C	90°C	95°C	100°C
The average cell size (μm)	16.8 ± 0.1	25.0 ± 0.2	29.9 ± 0.2	41.9 ± 0.1	46.1 ± 0.2
The expansion ratio	2.9 ± 0.2	3.8 ± 0.2	5.2 ± 0.1	7.5 ± 0.2	8.5 ± 0.1
The cell density (cell/cm³)	1.9 × 10⁸	2.1 × 10⁸	1.6 × 10⁸	1.2 × 10⁸	1.1 × 10⁸

The effects of melt strength on the structure of the cells

Figure 2 shows the DSC curve of PBAT, where the square on the right is the CO₂ saturated temperature we set, and the square on the left is the interval after the pressure is unloaded resulting in a lower temperature, which happens to be the crystallization region of PBAT. So at the set saturated temperature, CO₂ diffuses into the PBAT resin as supercritical liquid. During pressure relief foaming, with the rapid escape of gas, the CO₂ in the PBAT resin is gasified and expanded, and the near-melt polymer is stretched to form cells.¹⁵ At the same time, the escape of gas leads to rapid heat exchange, which makes the temperature in the autoclaves drop rapidly, reaching about 20°C. At this time, the melt strength and crystallization ability play a crucial role in the stability of the cell structure.¹⁸ Therefore, we further tracked the extensional viscosity of PBAT at different foaming temperatures, as shown in Figure 3. Since the temperature change at the moment of gas expansion is almost negligible, we set the test temperature of extensional viscosity at the foaming temperature.

Figure 2.

DSC curve of PBAT resin.

Figure 3.

Extensional viscosity of PBAT resin.

The extensional viscosity can relatively characterize the melt strength, the larger the value is, the higher the melt strength is.¹⁹ At 80°C, there is obvious tensile hardening phenomenon, hence the foaming is difficult at this time, resulting in the overall low foaming ratio. At 85°C–95°C, the extensional viscosity is approximate, there is a certain amount of tensile hardening, the cell structure is stable at this time, the cell merging phenomenon is not obvious, so the size distribution is uniform. At 100°C, the phenomenon of tensile hardening almost disappears. At this time, it is difficult to maintain the cell structure after gas expansion, and the phenomenon of cell merging intensifies, resulting in a wide size distribution and a large average cell size.

Effect of crystallization behavior on cell structure during foaming process

Above we know that the melt strength affects the cell structure mainly in the foaming and cooling stages, then the influence of crystallization on the cell structure throughout the whole stage of foam forming, including the carbon dioxide saturation stage to limit the absorption of gas, in the foaming and cooling stages to maintain the stability of the cell structure, etc.¹⁹ Here, we first discuss the origin of the crystals inside the PBAT foam beads. The crystalline properties of foamed beads by one step soaking and unfoamed pellets at different foaming temperatures were tested and shown in Figure 4. All curves show a thermal relaxation peak among 40°C-50°C.²⁰ For foamed beads, double melting peaks are observed, whereas only one melting peak appears for unfoamed pellets. The corresponding crystalline data are shown in Tables 2 and 3. It can be seen from the DSC melting curve that compared with PBAT beads (Figure 2, black line), the crystallization properties of PBAT are greatly improved after high temperature and high pressure. The melting peak changes from broad and diffuse to narrow and sharp, and the melting onset temperature increases significantly. At the same time, the high pressure forces the orientation of the PBAT molecular chains, and an obvious thermal relaxation signal appears on the heating curve.

Figure 4.

DSC curves of foamed beads by one-step soaking (a) and unfoamed pellets (b) at different CO₂ saturation and foaming temperatures.

Table 2.

The melting points of bead foams by one-step soaking and unfoamed pellets with CO₂ saturation and foaming temperature of 80°C–100°C (a: thermal relaxation peak, b: low melting temperature crystal, c: high melting temperature crystal).

Melting point %	80°C			85°C			90°C			95°C			100°C
Melting point %	a	b	c	a	b	c	a	b	c	a	b	c	a	b	c
Beads	50.0	80.6	118.1	50.0	86.2	121.5	50.0	90.0	123.6	50.0	93.4	125.5	50.0	101.1	128.6
Pellets	42.3	/	115.8	42.5	/	117.8	44	/	121.5	44.3	/	125.2	46.3	/	128.6

Table 3.

The crystallinity (DSC) of bead foams by one step-soaking and unfoamed pellets with CO₂ saturation and foaming temperature of 80°C–100°C (a: low melting temperature crystal, b: high melting temperature crystal).

Crystallinity %		80°C		85°C		90°C		95°C		100°C
Crystallinity %		a	b	a	b	a	b	a	b	a	b
Beads	Each peak	0.5	10.3	0.9	8.9	1.5	8.2	2.1	6.1	2.4	4.9
Beads	Total	10.8		9.8		9.7		8.2		7.3
Pellets	Each peak	/	13.2	/	10.9	/	9.7	/	6.9	/	5.6

The thermal relaxation peak

First, from the DSC curve, the melting points of the two samples are close (Table 2), but an obvious endothermic peak appears in the range of 40°C–50°C far below the melting point and foaming temperature. We believe that this peak is caused by the relaxation of the forced orientation of the PBAT molecular chain under high pressure. This relaxation temperature reaches 50°C for the rapidly depressurized sample due to the plasticization of PBAT by CO₂ increasing the free volume size. During rapid decompression, carbon dioxide is vaporized and expanded, foam beads are formed, and molecular chains are stretched and cooled, which weakens the restriction of pressure on the chain conformational balance, resulting in an increase in relaxation temperature.²¹

The high melting temperature peak

Next, we discuss the formation of the high melting temperature peak. First, as it is closely related to the formation of the low melting temperature peak, it is known from Figure 4 that the melting points of PBAT foam and unfoamed pellets increase and the crystallinity decreases as the saturated temperature rises. Under the high temperature and pressure field, the fine and imperfect crystals with melting temperature lower than the foaming temperature will melt completely, resulting in a decrease in crystallinity. In contrast, the crystals with melting temperature higher than the foaming temperature will not melt and become the “nucleus” for new crystal growth. At the same time, the compressed CO₂ gas has a strong plasticizing effect. This plasticizing effect will cause the polymer to swell and lower the energy barriers for molecular contraction and folding, accelerating the crystallization process of PBAT foams,²² which leads to the close alignment of the molecular chain segments in the “nucleus”, resulting in more perfect crystals and contributing to the increase of melting point and the decrease of the melting range. As to why the melting point of PBAT foam will be mostly higher than that of unfoamed pellets, we believe that the stretching and cooling of molecular chains during rapid decompression will also promote the formation of more perfect crystals, but this effect is limited. As the CO₂ soaking temperature increases, the original fine crystal grains gradually become perfect, and the damage to the crystals caused by rapid expansion during pressure relief foaming can be ignored.

The low melting temperature peak

Compared with unexpanded beads, there is an additional melting peak on the melting curve of PBAT foam, and its temperature is close to the CO₂ immersion temperature, as shown in Figure 4(a). This new melting peak is related to the stretch-induced crystallization of the foaming process.²¹ When the pressure was released, the temperature in the autoclave dropped by about 20°C to the PBAT crystallization starting temperature. At the same time, the gas expansion caused the PBAT molecular chains were stretched on the cell walls, resulting in a typical stretch-induced crystallization phenomenon. At this time, cell wall crystallization and cell expansion proceed simultaneously, and stretch-induced crystallization stabilizes the cell structure. As the CO₂ soaking temperature increases, the contribution of this part to the stabilization of the cell structure becomes more and more significant. Therefore, although the foaming temperature increases, the melt strength decreases, and the foaming ratio becomes larger, the stretch-induced crystallization promotes the uniformity of the cell structure.

The relationship between double melting peaks and the foam structure by one step soaking

It can be seen in Figure 5 and Figure 6, as the CO₂ saturated temperature rises, part of the initial crystal melts and the absorption of CO₂ in the polymer then increases. The amount of CO₂ uptake reaches equilibrium when the crystallinity of the high melting parts of the crystal is less than 5% (CO₂ saturated temperature reaches 95°C). Although the nucleation rate of the cells increased significantly with increasing CO₂ absorption (Figure 5(c)), the low melt strength did not guarantee a stable gas envelope for the PBAT melt. During foaming, the merging of adjacent cells leads to an increase in cell size. It can be seen that raising the CO₂ saturated temperature contributes to gas diffusion, while foaming at a relatively low relief temperature contributes to the production of fine crystals induced by low stretching, which promotes the structural stability of the cells.¹⁵ Therefore, we further investigated the effect of the two-step CO₂ saturated foaming process on the structure and properties of PBAT supercritical CO₂ foamed beads.

Figure 5.

Relationship between high melting temperature crystallization (a), gas uptake (b), nucleation rate (c) and CO₂ saturation and foaming temperatures.

Figure 6.

The relationship between double melting peaks and the foam structure by one step soaking.

SEM and DSC curves of scCO₂ foamed PBAT beads by two step soaking under different temperatures

In the two-step soaking experiment, we set the CO₂ saturated temperature to 95°C–105°C, then lowered the autoclave temperature to 90°C and released the pressure to foam when the temperature stabilized. Figure 7 and Table 4 show SEM and expansion ratio of foamed beads by two soaking steps experiment. The average cell size of foam beads rises from 37.8 to 48.8 µm and the expansion ratio also increases to 8.6 with T₁ increasing from 95 to 105°C. In contact with the data in Table 1, the cell size and expansion ratio can be effectively increased by CO₂ saturation at high temperature, keep soaking in supercritical CO₂, and then foam at low temperature. This phenomenon is related to the increase of carbon dioxide content diffused in PBAT pellets. Due to the relative stability of melt strength at uniform foaming temperature, the increase of carbon dioxide content per unit volume leads to the increase of gas expansion rate during pressure relief, and this change causes the increase of melt tensile strain, which further increases the tensile-induced crystallization of PBAT, thus increasing the crystallinity of low melting temperature crystallization, which in turn stabilizes the increase of cell size and expansion rate and prevents the rupture of cell.

Figure 7.

SEM of foamed beads by. at different temperatures.

Table 4.

The cell density, the average cell size and the expansion ratio of foamed beads by two soaking steps at temperature of 95°C–110°C.

Saturated temperature (T₁)/Foaming temperature (T₂)	95/90°C	100/90°C	105/90°C	110/90°C
The average cell size (µm)	37.8 ± 0.1	40.1 ± 0.2	41.9 ± 0.2	48.8 ± 0.2
The cell density (cell/cm³)	1.62 × 10⁸	1.65 × 10⁸	1.67 × 10⁸	1.58 × 10⁸
The expansion ratio	6.6 ± 0.1	7.5 ± 0.1	8.6 ± 0.2	9.1 ± 0.1

Figure 8 and Table 5 gives the DSC curves of two steps foamed beads by saturated temperature at 95°C–110°C. As the CO₂ saturated temperature increases, the initial crystals in the PBAT pellets begin to melt and the crystallinity of the high temperature melting point crystal decreases from 6.4% to 1.5%. However, the crystalline perfection of the unmelted crystals increases and the melting point is raised from 127.3°C to 137.8°C. This is due to the increased activity of chain segments in the dissolved CO₂ region along with the increase in crystal melting and CO₂ uptake, which contributed to the increase in crystalline perfection of the unmelted crystals. The grain refinement caused by gas expansion and stretching is consistent during foaming due to the uniformity of the unloading and foaming temperatures, which is also shown by the fact that the melting points of the low melting temperature crystals are all around 90°C, but the degree of crystallinity increases significantly from 3.9% to 8% with increasing saturation temperature. This increase in crystallinity represents an increase in the number of grains, which also contributes to the structural stability of large cells during foaming and prevents rupture due to violent merging of cells, which is also reflected in the relative stability of cell density.

Figure 8.

DSC curves of foamed beads by two step soaking at different temperatures.

Table 5.

The crystallinity and the melting point by two soaking steps at temperature of 95°C–110°C.

Saturated temperature (T₁)/Foaming temperature (T₂)	95/90°C	100/90°C	105/90°C	110/90°C
The crystallinity of high melting temperature peak (%)	6.4	5.7	3.9	1.5
The crystallinity of low melting temperature peak (%)	3.9	4.5	6.2	8.0
The melting point of high melting temperature peak (°C)	127.3	130.2	120.3/133.3	121.7/137.8

Conclusion

In this paper, the effect of foaming process on the structure of supercritical CO₂ foamed PBAT beads was investigated. By increasing the CO₂ saturated temperature and foaming temperature from 80°C to 100°C, the foaming ratio of the beads was increased from 2.9 to 8.5, and the average size of the cells was increased from 16.8 to 46.1 μm. As the initial crystals in the PBAT particles melted with increasing temperature, the CO₂ gas uptake increased and the cell nucleation rate increased, while the melt stretching viscosity decreased with increasing temperature. At this time during pressure relief, the multi-point rapid expansion of CO₂ causes PBAT to stretch rapidly, accompanied by a decrease in temperature to form small crystals. As a result, a typical double melting peaks phenomenon appears in the DSC curves. Based on this phenomenon, we further investigated the two-step soaking method by raising the saturated temperature of CO₂ from 90°C to 105°C and foaming at 90°C to obtain beads with higher foaming ratio and cell size. The average cell size of foam beads rises from 37.8 to 48.8 µm and the expansion ratio also increases to 8.6 with saturated temperature increasing form 95 to 105°C. The two-step soaking method can promote the diffusion of CO₂ in PBAT and prepare foam beads with higher expansion ratio in low-temperature foaming.

Footnotes

Acknowledgements

The authors also would like to thank the Project of National Science Foundation of China under Grant (52173033, 51773044, 51603047), Guangzhou Science and Technology Plan Project (202102020952), Research and Development Plan for Key Areas in Guangdong Province (No. 2019B090914002), the Project of Science Foundation of Guangdong Province (2021A1515011914), Foshan Science and Technology Innovation Project (No. FS0AA-KJ919-4402-0145).

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51603047, 51773044, 52173033), Guangzhou Science and Technology Plan Project (202102020952), Research and Development Plan for Key Areas in Guangdong Province (No. 2019B090914002), the Project of Science Foundation of Guangdong Province (2021A1515011914), Foshan Science and Technology Innovation Project (No. FS0AA-KJ919-4402-0145).

ORCID iDs

Ruijie Xu

Caihong Lei

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Influence of foaming-induced crystallization behavior on the structure of poly(butylene adipate-co-terephthalate) supercritical foamed beads

Abstract

Keywords

Introduction

Experimental

Materials

Sample preparation

Characterization

Scanning electron microscope

Differential scanning calorimetry

Extensional viscosity test

Foaming properties

Calculation of cell nucleation rate

Results and discussion

Cell morphology of scCO2 foamed PBAT beads by one step soaking under different temperatures

The effects of melt strength on the structure of the cells

Effect of crystallization behavior on cell structure during foaming process

The thermal relaxation peak

The high melting temperature peak

The low melting temperature peak

The relationship between double melting peaks and the foam structure by one step soaking

SEM and DSC curves of scCO2 foamed PBAT beads by two step soaking under different temperatures

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

ORCID iDs

References

Cell morphology of scCO₂ foamed PBAT beads by one step soaking under different temperatures

SEM and DSC curves of scCO₂ foamed PBAT beads by two step soaking under different temperatures