Densification of fast-growing paulownia wood for tough composites with stab resistance property

doi:10.21203/rs.3.rs-3407388/v1

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Densification of fast-growing paulownia wood for tough composites with stab resistance property

https://doi.org/10.21203/rs.3.rs-3407388/v1

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published 19 Mar, 2024

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Paulownia wood, a fast-growing wood native to China, has limited application due to its loose texture. However, the mechanical properties of materials can be improved by densification after delignification. In this study, paulownia wood was treated with sodium hydroxide and hydrogen peroxide to remove most of the lignin, then densified by hot pressing. The influence of the densification process of paulownia wood/epoxy composite on the mechanical properties, especially stab resistance, was discussed. The results show that densification after delignification significantly improves the mechanical properties of the materials. The densification improves the fracture strength and modulus of the materials, reching 171.98 MPa and 14.55 GPa. Hybrid composites made of densified wood/carbon fiber/epoxy resin have good stab resistance. The punch strength per unit area density and puncture worker unit area density of these hybrid composites are superior to carbon fiber reinforced composites.

densification

paulownia wood

mechanical property

stab resistance

composites

Recent demands for sustainable development goals (SDGs) have led to increased interest in the use of composites made from lignocellulose and polymers as a new material for buildings and furniture [1]. Lignocellulosic material (LCMs) is a renewable, recyclable, and abundant resource that is widely available globally [2]. The LCMs can be obtain from many natural materials such as wood, bamboo, flax, and silk. Lignocellulose reinforced composite as one kind of biocomposite is a type of material that has minimal impact on the environment and is beneficial for environmental protection [3]. The biocomposite can be used as a substitute for synthetic fibers reinforced composite like aramid, glass, basalt, and carbon fiber standard composites. Biocomposites have several advantages, including being lightweight, low cost, high fracture toughness renewability and biodegradable [4]. However, they also have some disadvantages, such as lower strength, modulus, and shorter fatigue life compared to synthetic materials [5].

The densification of partially delignified lignocellulose material to improve mechanical strength [6] is a comman used method. This process involves removing the lignin and hemicelluloses before densifying the material through hot-pressing. By optimizing the delignification conditions, the delignified bamboo showed a tensile strength of 301% and a tensile modulus of 233% compared to natural bamboo [7]. The delignified generated bioplastics from coconut husk fragments achieve a tensile Young’s modulus of 2.1 ± 0.4 GPa and a tensile strength of 22.8 ± 4.4 MPa [8]. The research on densification of various natural materials, such as lignocellulose and bamboo, has been extensively studied. However, the study of paulownia trees for densification has not received much attention.

Paulownia trees are a fast-growing species that originated in China and can be used in the industry after just 5 years of growth on the plantation [9]. Due to the increasing shortage of wild forest resources worldwide, there is a need for using fast-growing wood species to replace traditional tropic hardwood species [10]. Fast-growing wood generally has loose texture, which benefits it with high sound absorption properties [11]. However, despite being abundantly available and acoustically beneficial, its wood properties such as inferior dimensional stability and mechanical properties limit its end-use potential [10, 12]. Densification can increase the mechanical properties of materials by reducing their porosity. One potential application of desilicated paulownia wood is to use as a stab resistant material.

Currently, the most commonly used materials for stab-resistant applications are carbon fiber[13], aramid fiber[14], basalt, and high molecular weight polyethylene from petroleum, which are expensive [15]. Additionally, the shear thickening fluid (STF) have been used to improve stab resistance[16]. However, STF only reacts toinstantaneous high-speed impact and has little effect on low-speed piercing or cutting [17]. The natural cellulose materials cotton and paper are reported with stab resistant properties [18]. The application of paulownia trees in stab-resistant fields will reduce the need for high-performance fibers.

In this paper, the structure and mechanical properties of densified paulownia composites are explored by hot pressing delignified paulownia wood and epoxy resin. The hybrid composites are prepared by compounding the paulownia composites with carbon fiber plates, and their stab-resistant properties are tested and analyzed.

Preparation of densified paulownia wood/epoxy composites

The raw paulownia wood (1mm) used in this paper is purchased from Xuancheng Anhui province, China. The wood had been washed with running water to remove the impurities such as dust. The hydrogen peroxide (NaOH & H₂O₂) treatment had been used to remove the lignin and some of the hemicellulose [19, 20]. The paulownia wood treated with 4% sodium hydroxide and 2% hydrogen peroxide at a bath ratio of 1: 80 under 80 ℃ for 2 h for 2 times to get the delignification paulownia wood (DPW). The specific preparation details are shown in Fig. 1. The overlapping angle between different layers is 45^o.

Different layers of densified wood with 100% water content are hot pressed with a temperature of 110°C, pressure of 20 MPa, and time of 12 hours. The densified wood is put into epoxy resin and then put into vacuum oven to defoam. Then the mixture of wood and epoxy resin are used to hot pressing molding. The final hot-pressing process involves the following steps: 1) applying heat at 85°C for 0.5 hours under a pressure of 5 MPa. 2) Repeating step 1 at 90°C for 14 hours under a pressure of 15MPa. The DPW2c, DPW3c, DPW6c, and DPW6 samples are referred to the number of DPW layers in the final hot-pressing process. For instance, the combined hot-pressed material of three pieces of DPW can be referred to as DPW3c.

Preparation of densified DPW/C hybrid composites

The carbon fiber reinforced composites are prepared by hot-pressing lamination. The carbon fibric with specified layers is laminated with resin. Then it is compressed for 4 hours at 80 ℃ under the pressure of 1 MPa. The hybrid composites are made of three layers of composite materials bonded by resin. The top layer is made of four layers of carbon fibric with density of 200 g/m^− 2. The lowest layer is also made of 6 layers of carbon fibric with the density of 280 g/m^− 2. The DPW/epoxy composites are placed in the hybrid material as the middle layer.

Table 1

Relevant parameters of the samples
Samples	Preparation	Thickness /mm	Density /g·cm^− 3
DPW2c	Tow pieces of three-layers compressed wood are impregnated with resin, then laminated and hot pressed	1.15	1.53
DPW3c	Three pieces of tow-layers compressed wood are impregnated with resin, then laminated and hot pressed	1.17	1.59
DPW6c	Six pieces of single-layer compressed wood are impregnated with resin, then laminated and hot pressed	1.15	1.52
DPW6	Six pieces of single-layer wood are impregnated with resin, then laminated and hot pressed	1.24	1.37
C-DPW2c-C	Inner layer is DPW2c	3.20	1.54
C-DPW3c-C	Inner layer is DPW3c	3.20	1.54
C-DPW6c-C	Inner layer is DPW6c	3.30	1.57
C-DPW6-C	Inner layer is DPW6	3.25	1.50

Characterization of different types of windmill palm fiber

Scanning Electron Microscopy (SEM)

The sample is observed by scanning electron microscope (DXS-10ACKT, Hitachi, Tokyo, Japan). The surface morphology of the material before and after treatment is analyzed. Before the test, the sample is sprayed with gold for 70 s (E-1045; Hitachi, Tokyo, Japan). The test voltage and the magnification are 15 kV and 200-10000 times.

Tensile property

The tensile test of composites is carried out by ETM series electronic universal testing machine at room temperature with the tensile rate of 2 mm/min. The test sample size is 70 mm× 6 mm ×1 mm [21].

Digital image correlation full-court strain test

The three-dimensional strain measurement and analysis system had been used to test the full-court strain. The Digital image correlation (DIC) technology needs to identify the changes of speckle on the surface of materials to characterize the strain on the surface of materials. To begin, the surface of the sample is evenly coated with white primer, and then black speckles are added to the white primer using black spray paint. The tensile experiment is carried out on a universal testing machine, with the tensile speed of 2mm/min. The software GOM correlate 2016 is used to analyze the speckle change characteristics of the material surface and reflect the strain characteristics of the material surface in the form of cloud images.

Quasi-static puncture test

The tensile fixture of ETM series electronic universal testing machines is replaced with a puncture tool to test the quasi-static puncture performance. The lower fixture is secured with a composite sample, while the upper fixture containing the test tool is connected to the sensor. In accordance with the domestic test standard GA 68-2019 "Stab-proof Clothing", a specialized test blade is utilized for this purpose. The test blade will puncture the center of the composite sample at a speed of 10 mm/min, and the displacement-load curve is recorded by the sensor that is connected to the tool. The puncture strength and initial modulus under quasi-static puncture are then compared for each sample.

Morphonology of paulownia wood and its composites

Figure 2 presents the surface morphology of paulownia wood samples. In Fig. 2b, the wood ray and axial parenchyma cells can be observed [11, 22]. The enlarged view of the wood ray is shown in Fig. 2a, revealing an obvious cavity structure on the anatomical surface. Figure 2c displays the interpenetrating pits. The surface of the paulownia wood becomes smooth and flat after densification, as illustrated in Fig. 2d. SEM images of delignified paulownia wood/epoxy composite and compressed paulownia wood/epoxy composite are presented in Figs. 2e and 2f, respectively. The surfaces of these two composite materials are smoother than that of the material without resin. After impregnation in vacuum, the low-viscosity epoxy resin glue fills the cell cavity of the wood, functioning to connect the cellulose skeleton instead of lignin.

Tensile mechanical property of paulownia wood composites

Figure 3 presents the tensile fracture curve of densified wood spirally laid at an angle of 45. The stress-strain curve of the material is almost straight, indicating a strong brittleness with less than 2% elongation at break. When multi-layer laminated materials are subjected to tension, delamination can occur. In the case of paulownia wood reinforced composites, the parts bearing the load are the fiber, matrix, and interface, with the interface being typically more vulnerable to destruction than the matrix and fiber [23, 24]. This is due to the composite's weak joint nature. Some of the fibers in the material are arranged in a tensile direction, leading to their fragmentation. This highlights the role of bridging fibers in preventing the rapid propagation of cracks to some extent.

The tensile mechanical properties of the material are shown in Fig. 4. Among them, the tensile strength and modulus of DPW6c are the largest, reaching 171.98 MPa and 14.55 GPa, respectively. However, the breaking strength and modulus of DPW6 are only 134.20 MPa and 11.94 GPa, respectively, showing a 22% and 18% increase in strength. While the tensile strength of raw paulownia is about 18 MPa [25, 26]. This indicates that impregnating compressed wood with resin can enhance the rigidity of the material, effectively reducing the potential damage to human bodies during stab protection. The specific work of fracture of DPW reinforced epoxy composite is greater than that of the epoxy resin/modified wood composite with the same thickness, demonstrating an improvement in impact resistance when compressing wood. In summary, when a composite material fails, it requires more force to do so and can absorb more energy simultaneously.

DIC of paulownia wood composites

The full-field stress analysis of composite materials is shown in Fig. 5. The stress distribution of the material before fracture is shown in the cloud picture. The strain distributions of two materials, DPW2c and DPW6, are relatively uniform. However, the stress of DPW3c and DPW6c is noticeably concentrated, indicating that having too many overlapping layers in the final lamination process can lead to more defects, resulting in stress concentration and weakened mechanical strength.

Quasi-static puncture of paulownia wood composites

The modified paulownia wood was prepared into composite materials through hot-press methods. A quasi-static puncture test was conducted on these composites’ samples are shown in Fig. 6. The tip of the test tool contacts the surface of the composites to start the test. The displacement of the test tool was 4 mm as the end point. The influence of different processing methods on the stab-resistant performance of the composite materials was explored. The puncture force increases with increasing puncture depth, and then the growth curvature slows down. DPW6, DPW3c, DPW2c, and DPW6c reach a turning point in turn. The minimum puncture force of DPW6 composite is 42.77 N, and the maximum puncture force of DPW6c is 87.91 N, which has increased by 206%.

The quasi-static puncture performance of various samples was further investigated, and the results are presented in Fig. 7. The lowest puncture work per unit area density of DPW6 is 0.38 J/gm^− 2, while the composite material laminated after single-layer compression reaches 0.51 J/gm^− 2, which is 34% higher. The change in quasi-static initial modulus and puncture strength per unit area density is similar for all the samples. The initial puncture modulus of DPW6c is 38.48 MPa, whereas the initial puncture modulus of DPW6 is only 14.45 MPa. Compression significantly enhances the quasi-static initial modulus of the material, increasing it by approximately 166%.

Quasi-static puncture of paulownia wood/carbon fiber hybrid composites

Figure 8 demonstrates that the hybrid composites prepared through various processes have similar puncture strength when they are of similar thickness. The puncture strength of the densified materials is 1200 N, and DPW6 has a slightly higher puncture strength compared to the other hybrid composites, reaching 1450 N.

Hybrid composites were prepared by incorporating densified paulownia reinforced with epoxy resin composite as the intermediate layer. The hybrid composite material has carbon fiber reinforcing layers in both the upper and lower regions. The quasi-static puncture test was conducted on the samples. The displacement-load curve is presented in Fig. 9. The initial modulus of the composites during the test is consistent with the results of densified paulownia reinforced composites. The puncture work per unit area density of hybrid compliant materials prepared by compressed wood/epoxy resin composites are equivalent, with the latter having a 33% higher puncture strength per unit area density. This indicates that the compressed wood/epoxy resin hybrid composites have better puncture resistance. However, the quasi-static initial modulus accounts for DPW6c /carbon hybrid composite more than 170% of the DPW6 /carbon hybrid composites, indicating good energy absorption effect.

The stab resistance of hybrid composites is demonstrated in Table 2. During piercing of carbon fiber composites, the formation of fine cracks due to bayonet cutting gradually accumulates. When the bearing capacity of the materials reaches its limit, these cracks will rapidly spread along the direction of warp and weft arrangement. If the warp and weft of the fabric in the composites are arranged in a fixed direction, the full potential of the stab resistance cannot be achieved. The spirally arranged fiber composites can effectively extend the crack propagation path and provide better energy absorption performance when placed in the middle layer of the composites. Compared to pure carbon fiber board, C-DPW6c-C has similar puncture modulus. However, the density per unit area punch work is even 20% higher than that of carbon fiber board. By using wood and epoxy to prepare hybrid composite materials with stab resistance like that of carbon fiber composite materials, environmental pollution can be reduced while costs can also be minimized.

Table 2

Comparison of stab resistance of different samples
Samples	Quasi-static initial modulus/(N/mm)	Puncture strength per unit area density/(N/(g/m²)	Puncture work per unit area density /(10^− 4J/(g/m²))	Thickness /mm
C-DPW2c-C	88.59	0.23	5.36	3.21
C-DPW3c-C	73.95	0.24	5.68	3.20
C-DPW6c-C	110.68	0.23	6.71	3.28
C-DPW6-C	61.16	0.30	7.10	3.25
C-C-C	124.30	0.24	5.68	3.18

In this chapter, paulownia wood was modified by alkaline solution to produce delignified paulownia wood. Densified paulownia wood/epoxy composites, densified delignified paulownia wood/epoxy composites, as well as the hybrid composites (carbon fiber composites were bonded with these wood reinforced composites as an intermediate layer) were prepared to study its mechanical property. The mechanical properties of the densified paulownia wood/epoxy composites are significantly higher than those of the paulownia wood/epoxy composites. This is because the wood structure is more compact after the densified and delignified. The resin fills the pores, resulting in a strong binding between the wood and resin. The minimum puncture modulus of DPW6 composite is 15 MPa, while the maximum puncture modulus of DPW6c is 40 MPa, which has been increased by 300%. The density per unit area punch work of C-DPW6c-C is 20% higher than that of carbon fiber board, indicating that the hybrid composites have better energy absorption properties. The use densified delignified paulownia wood/carbon fiber to prepare hybrid composites provides an effective way to improve the mechanical properties of composite materials and reduce costs.

Author contributions Changjie Chen: Writing—original draft, Methodology, Data analysis, and Supervision. Leiming Shen: Experiment, and Data analysis. Ye Sun: Data analysis. Limin Bao: Investigation and review. Xinhou Wang: Supervision.

Funding This research is funded by the special fund support for basic scientific research business expenses of central universities (2232023G-01); the basalt fiber and composite key laboratory of Sichuan province (XXFC-2201)；the modern silk National Engineering Laboratory (SDGC2244). key laboratory of high performance fibers & products (23D110632/011).

Conflict of interest The authors declare that they have no conflict of interest.

Ethics approval This article does not contain any studies with human participants or animals performed by any of the authors.

Consent for publication All authors gave their consent for publication.

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No competing interests reported.

Download PDF

Journal Publication

published 19 Mar, 2024

Read the published version in Cellulose →

Editorial decision: Major revision
08 Oct, 2023
Editor assigned by journal
07 Oct, 2023
Submission checks completed at journal
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You are reading this latest preprint version

Densification of fast-growing paulownia wood for tough composites with stab resistance property

Status:

Journal Publication

Version 1

Abstract

Figures

Introduction

Experimental methods

Preparation of densified paulownia wood/epoxy composites

Preparation of densified DPW/C hybrid composites

Characterization of different types of windmill palm fiber

Scanning Electron Microscopy (SEM)

Tensile property

Digital image correlation full-court strain test

Quasi-static puncture test

Results

Morphonology of paulownia wood and its composites

Tensile mechanical property of paulownia wood composites

DIC of paulownia wood composites

Quasi-static puncture of paulownia wood composites

Quasi-static puncture of paulownia wood/carbon fiber hybrid composites

Conclusion

Declarations

References

Additional Declarations

Status:

Journal Publication

Version 1