Preparation of Carbon Fiber and Bamboo Fiber Reinforced Poly (butylene Adipate-co-terephthalate) Foams by Supercritical Carbon Dioxide Foaming

Summary

This study describes a supercritical carbon dioxide foaming method for the preparation of carbon fiber and bamboo fiber reinforced poly (butylene adipate-co-terephthalate) foams.

Abstract

Fiber-reinforced polymer foams have found extensive applications across numerous fields. Particularly, poly (butylene adipate-co-terephthalate) (PBAT) has garnered significant attention in the field of biodegradable materials due to its exceptional processing properties, mechanical performance, and biocompatibility. However, the high shrinkage rate of pure PBAT foam limits its application. Among the various materials capable of enhancing foam performance, carbon fiber (CF) and bamboo fiber (BF) exhibit unique advantages. This study employed supercritical carbon dioxide (scCO₂) foaming technology to successfully prepare PBAT/CF/BF foams. The results demonstrate that the synergistic effect of CF and BF significantly improves the anti-shrinkage properties of PBAT foam. Compared to pure PBAT foam, the shrinkage rate of PBAT/15CF/5BF foam decreased from 55.63% to 6.92%, a reduction of 87.56%. PBAT/10CF/10BF foam achieved the lowest density of 0.13 g/cm³, representing a 31.58% decrease. PBAT/10CF/10BF foam exhibited the highest expansion ratio of 9.43, an increase of 51.36%. Compression resilience tests revealed that all PBAT/CF/BF foams displayed favorable compression resilience, with PBAT/15CF/5BF foam demonstrating the highest compressive strength of 0.26 MPa. This research provides an effective solution for fabricating fiber-reinforced biodegradable polyester foams. Those foams have anti-shrinkage properties, lightweight characteristics, and excellent resilience.

Introduction

Fiber-reinforced polymer foams have been widely applied in various fields, including aerospace¹, transportation², construction³, and environmental protection⁴, due to their high strength-to-weight ratio, excellent thermal stability, and versatile composability. Driven by global "dual-carbon" goals and plastic pollution control initiatives, biodegradable polymer foams are becoming a research hotspot^5,6,7. PBAT, as a typical bio-based biodegradable polyester, is considered an ideal alternative to traditional petroleum-based foam materials due to its outstanding processing performance and environmental friendliness^8,9,10,11. However, the significant shrinkage exhibited by pure PBAT foams limits their application¹².

Among various modification techniques, research has found that employing fiber reinforcement technology can significantly enhance the performance of PBAT foam¹³. CF, renowned for its high strength, high modulus, and low density, is frequently utilized to reinforce polymers, thereby enhancing their mechanical properties¹⁴. The inclusion of CF not only offers stronger mechanical support for foams but may also modify their microstructure, thus improving their overall performance^15,16. Yimu Qiao et al.¹⁷ prepared a sandwich structure composed of PBAT/CF foam and continuous CF reinforced polypropylene (PP) prepregs. The study found that the foam cells with prepregs were more uniform and dense. In contrast, BF presents unique advantages in reinforcement materials due to its exceptional biocompatibility and renewability¹⁸. Its unique internal microstructure bestows certain toughness and cushioning properties on the material¹⁹. Wei Guo et al.²⁰ prepared alkali-modified BF reinforced polypropylene BF/PP foams. The research shows that the addition of BF improves the cellular structure and significantly enhances the mechanical properties of BF/PP foams. Consequently, the judicious combination of CF and BF in the fabrication of PBAT foam is anticipated to maximize the benefits of both materials, mitigate the significant shrinkage issue of pure PBAT foam, and further broaden its application potential across various fields.

Although scCO₂ foaming technology has made the aforementioned progress, there are still significant shortcomings in the field of fiber-reinforced biodegradable polyester foam. First, existing research mostly uses single fiber reinforcement (such as pure CF or pure BF), failing to fully leverage the synergistic effects of different fibers, such as the combination of CF's rigidity enhancement and BF's interface control advantages. Second, avoiding foaming defects caused by decreased melt flowability at high fiber content remains a challenge. The innovation of this paper lies in addressing the above issues. On the one hand, it constructs a scCO₂ foaming system for CF/BF dual-fiber synergistic reinforcement of PBAT, which compensates for the shortcomings of single-fiber systems in nucleation efficiency and interface bonding. At the same time, optimising the CF/BF ratio improves the mechanical properties of the foam while ensuring foaming, providing a new design concept for the high-performance of biodegradable foam.

ScCO₂ foaming technology has opened new avenues for producing high-performance microcellular foam materials, leveraging its environmental benefits, tunable micron-scale pore size, and flexible process parameter adjustability²¹. Through scCO₂ foaming, CO₂ dissolves uniformly within the composite material system. Upon sudden pressure release, the carbon dioxide rapidly escapes, forming numerous uniformly distributed bubble nuclei that subsequently grow into pores²². This process not only enables precise control over pore size and density but also enhances the material's thermal insulation, soundproofing, and cushioning properties while maintaining its original performance²³.

This study investigates a scCO₂ foaming system for PBAT composites reinforced with varying ratios of CF and BF, both at a total content of 20 %. To prepare PBAT/CF/BF foam, the CF/BF ratio was selected as 3:1, 1:1, and 1:3. The foaming temperature was 100 °C, the foaming pressure was 10 MPa, and the foaming time was 60 min. Initially, foams were produced, and their density, expansion ratio, and shrinkage rate were measured to visually illustrate the differences in the fundamental physical properties of the foams with different fiber ratios. Subsequently, scanning electron microscopy (SEM) was utilized to characterize the cellular structure, with cell diameters measured to disclose, at the microscopic level, the impact of varying fiber ratios on cell morphology, size, and uniformity. Lastly, compression cycle tests were performed to assess the compression performance, indicating the mechanical load-bearing capacity conferred to the foams by different fiber reinforcement combinations. The results of this study will offer theoretical bases and technical insights for the development of bio-based, biodegradable foam materials with superior physical properties, microstructure, and mechanical strength, thus promoting their sustainable use in areas such as packaging and construction.

Protocol

The reagents and the equipment used in this study are listed in the Table of Materials.

1. Raw material pretreatment

1. Put the PBAT, CF, and BF in an oven at 80 °C for 8 h (Figure 1A).

2. Weighing and pre-mixing

1. Taking a total mass of 200 g as an example, use an electronic balance to proportionally weigh PBAT and CF (for instance, PBAT/20CF: 180 g PBAT and 20 g CF).
2. Pour the weighed PBAT and CF into a polyethylene bag, mix for 5 min to ensure preliminary uniform dispersion of CF.

3. Melt extrusion

1. Turn on the power switch of the twin-screw extruder (Figure 1B).
2. Set the temperature of each zone to 135 °C, 145 °C, 155 °C, 165 °C, and 160 °C respectively.
3. Initiate the heating program and maintain for 30 min after the temperature in each zone reaches the set value.
4. Set the host speed to 5 rpm.
5. Set the feeder rotation speed to 3 rpm.
6. Add pure PBAT pellets into the extruder and run for 10 min to clean the screw.
7. Switch to PBAT/20CF premix and extrude continuously.
8. Prepare samples with PBAT/15CF/5BF, PBAT/10CF/10BF, PBAT/5CF/15BF, PBAT/20BF, content respectively according to the above steps. Clean the screw with pure PBAT before each raw material change.
9. Collect the extruded mass and crush it using a powder granulator (Figure 1C).
10. Seal and store the crushed samples in polyethylene bags, labeling them with the formulation ratio and date.

4. Compression molding

1. Turn on the power switch of the flat vulcanizing machine (Figure 1D).
2. Set the temperature of the upper and lower plates to 165 °C.
3. Initiate the heating program and maintain for 10 min after reaching the target temperature.
4. Based on the mold dimensions of 100 mm × 100 mm × 4 mm, the calculated required material mass is 60 g.
5. Lay a polytetrafluoroethylene film over the flat gasket, followed immediately by placing the mold.
6. Evenly distribute the material into the mold.
7. Lay a polytetrafluoroethylene film over the mold for material placement, then cover with a flat gasket.
8. Place the laid material in the center of the heating plate of the flat vulcanizing machine.
9. Initiate the mold closing procedure, apply 1 MPa pressure, maintain pressure for 3 min, and expel air from the material to avoid bubble formation.
10. Continue the mold closing procedure, increase the pressure to 10 MPa, and maintain for 10 min to ensure the material is melted and uniformly fills the mold.
11. Turn off the heating, activate the cooling system, and wait until the temperature drops below 40 °C before depressurizing and removing the mold.

5. ScCO₂ foaming process

1. Use a blade to cut the molded sheet into dimensions of 10 mm × 10 mm × 4 mm (Figure 1E).
2. Place the cut panels into the reaction vessel.
3. Tighten the bolts of the reactor using a wrench tool and close the reactor.
4. Close the exhaust valve of the reaction kettle.
5. Open the valve of the CO₂ cylinder vent.
6. Open the intake valve of the booster pump.
7. Open the inlet valve of the reactor and inject CO₂ gas into the reactor.
8. Close the inlet valve of the reactor and maintain for 3 min.
9. Open the exhaust valve of the reactor to discharge the air inside the reactor.
10. Turn on the power switches of the two oil baths, and set the temperatures to 165 °C and 100 °C, respectively.
11. Place the reaction vessel containing the sample in an oil bath at 165 °C for 10 min.
12. Open the inlet valve of the reactor to pressurize the reactor and introduce CO₂, with the pressure controlled at 10 MPa, and maintain the temperature for 10 min.
13. Close the ventilation valve of the CO₂ cylinder.
14. Close the intake valve of the booster pump.
15. Close the inlet valve of the reaction kettle.
16. Transfer the reaction vessel containing the sample to an oil bath at 100 °C, maintaining the temperature and pressure for 60 min.
17. Open the exhaust valve of the reactor and rapidly release pressure within 3 s to obtain the foam material.

6. Foam characterization

NOTE: The density of the specimen is directly measured using a densitometer. Taking pure PBAT as an example.

1. Switch the power on. The display will change from 8888888 to 0.00 g.
2. Press the 0 key to reset it to zero.
3. Place the sample on the measurement platform.
4. Press the ENTER key after stabilization.
5. Remove the sample and wipe it dry.
6. Place the sample on the submerged hanging platform in water.
7. After stabilization, press the ENTER key. The reading value is 1.223 g/cm³.
8. Press the SET button to exit and return to the weighing mode, then proceed to measure other products.
   NOTE: The density of the specimen is directly measured using a densitometer.
9. Calculate the initial foaming ratio of the foam (φ₁) using the following formula²⁴.
        (1)
   NOTE: ρ₀ is the solid density of the specimen, and ρ₁ is the initial foaming density of the foam.
10. Calculate the foaming expansion ratio after foam stabilization (φ₂) using the following formula²⁴.
         (2)
    NOTE: ρ₂ is the foaming density after foam stabilization.
11. Calculate the shrinkage ratio (S) using the following equation²⁴.
         (3)

7. SEM characterization

1. Place the foam cutting sample on the SEM specimen stage.
2. Apply gold spray treatment to the foam.
3. Obtain SEM images by setting the acceleration voltage to 5 kV.
4. Open the SEM image using Image J and use the Straight Line tool to match the scale bar of the image.
5. Click on Analyze > Set Scale, set the Known Distance to the scale bar of the SEM image, then click on Global > OK.
6. Click on Analyze > Tools > ROI Management to measure the cell size of the foam, then click on Add it. Measure the required number of samples as specified.
7. Click on Measure to obtain quantitative data of the figure for further calculations.
8. Calculate the average cell diameter of the foam using the following formula²⁴.
        (4)
   NOTE: n_i refers to the number of cells with a diameter of D_i in the SEM image.
9. Calculate the nucleation density (N₀) of foam using the following formula²⁴.
        (5)
   NOTE : n is the total number of cells in SEM, and A is the area of SEM.

8. Detemination of compressive properties

1. According to the GB/T 1041-2008 testing standard, cut 6 sets of specimens measuring 10 mm × 10 mm × 8 mm from each type of foam.
   NOTE: Conduct a visual inspection of the specimen to ensure the surface is flat and free from obvious defects such as cracks, material shortages, etc.
2. Measure the specimen dimensions.
3. Open the software.
4. Turn on the testing machine and connect it to the software.
5. Install the compression fixture, ensuring its parallelism and perpendicularity meet the requirements.
6. Place the foam specimen at the center of the compression fixture, ensuring good contact between the specimen and the fixture.
7. Set the test parameters of the testing machine. The loading speed is 5 mm/min, with 10 cycles and a compressive strain of 50%.
8. Conduct the operational test, commence applying compressive force to the foam specimen while simultaneously recording data.
9. After the test is completed, remove the foam sample.
   NOTE: During the experiment, observe the deformation of the foam specimen, such as whether local deformation, cracking, or collapse occurs, and keep detailed records.
10. Repeat the above steps for all foam samples to verify and ensure the reproducibility of the results.
11. After the experiment, calculate the compressive strength of the foam sample.

Results

The physical image and physical properties of PBAT foams reinforced with varying ratios of CF and BF prepared according to the aforementioned protocol are shown in Figure 2. It can be observed that increasing the proportions of CF and BF effectively resists the shrinkage of PBAT foams. Through measurements, the density of pure PBAT foam was found to be 0.19 g/cm³ (Figure 2B), with a foaming ratio of 6.23 (Figure 2C) and a...

Discussion

Drying treatment is a critical step preceding melt extrusion. Its primary objective is to remove moisture from the material to prevent hydrolysis, vaporization, and interfacial defects under high-temperature conditions. This ensures the mechanical properties, processing stability, and appearance quality of PBAT composites⁸. In this study, the specific procedure involved placing PBAT, CF, and BF in an oven for 8 h of drying treatment at a temperature of 80 °C.

By me...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Bamboo fiber	Sichuan Lvzhu Plant Fiber Co.,Ltd., China	1000 mesh	Raw material supplied for the experimental work
Booster pump	Dongguan Suncenter Fuild Control Equipment Co.,Ltd., China	DGA15C	Used to Controlle Pressure
Carbon dioxide	Harbin Qinghua Industrial gases Co. Ltd., China	99.9% purity	Used as the foaming agent
Carbon fiber	Toray Industries Incorporated,  Japan	1000 mesh	Raw material supplied for the experimental work
Ceactor	Zhengzhou Zechen Instrument Equipment Co., Ltd., China	0.5 L	Used as a foaming container
Compactor	Zhejiang Kaishan Compressor Co., Ltd., China	XK06-010-00487	pressurized
Digital oil bath	Boho Temptech Co.,Ltd., China	HH-5-12L	Used to heat
Drying oven	Shanghai Yiheng Scientific Instrument Co., Ltd., China	DGG-9070	Used to dry raw materials
Electronic balance	Mettler Toledo (Changzhou) Weighing Equipment System Co., Ltd	SPN501F	Used to weigh of raw materials
Flat plate sulphurisation instrument	Bolon Precision Testing Machines Co.,Ltd., China	BL-6170-A	For hot compression moulding of composites
High-precision solid density meter	Yuemang Intelligent Equipment Co., Ltd., China	JHY-120Y	Device used to determine  density
Image analysis softwareNational Institutes of Health	National Institutes of Health	ImageJ	Software used for determining the morphological characteristics of foam
Poly(butylene adipate-co-terephthalate)	Xinjiang Blue Ridge Tunhe Sci.& Tech. Co.,Ltd., China	TH801T	Raw material supplied for the experimental work
Polytetrafluoroethylene film	Luo Huixing Plastic Materials Trading Company, China	0.5 mm	Used to demould
Pulveriser	Shenzhen Chengzhongsheng Electromechanical Technology Co., Ltd.,China	380 w, 50 Hz	Used to Crush and granulate
Scanner	FEI Inc., USA	QUANTA 220	Scanner used for taking an image of foams
Twin-screw extruder	Nanjing Jiente Machine Point Co. Ltd., China	SHJ-20	Melt Mixing
Universal mechanical testing machine	Wance Technologies Ltd., China	TestPilot X10A	Used to smeasure compression performance