Rice straw was obtained from Jin He Agricultural Technology Co., Ltd. (Shanxi, China). Bis-(γ-triethoxysilylpropyl)-tetrasulfide and anhydrous ethanol were purchased from Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Lipase was obtained from Shandong Longkete Enzyme Preparation Co., Ltd. (Shandong, China). The phenolic resin was obtained Guangdong Yitong Plastic Chemical Raw Materials Co., Ltd. (Guangdong, China). All chemicals were used as received without further purification. All solutions were prepared with deionized water.

1,000 g rice straw was washed with deionized water and dried completely in the oven at 105°C. Then rice straw was cut into small sections with a length of 1–2 cm and put into a centrifugal mill for crushing. The unmodified rice straw fibers (SF) were obtained after the crushed rice straw passed the 0.5 mm sieve.

Silane-modified rice straw fibers: 9 g double-[γ-(triethoxysilicon) propyl] tetrasulfide was dissolved in 285 g ethanol/water solution with the mass ratio at 95:5 and stirred for 1 h at room temperature (25°C). Then 90 g rice straw fibers were added into the prepared mixed solution and stirred for another 1 h. After the reaction was finished, the rice straw fibers were washed with ethanol three times to remove the unreacted silane coupling agent. Finally, the silane-modified rice straw fibers (S-SF) were obtained by drying at 80°C ± 2°C for 24 h and stored in a sealed bag for further use.

Hydrothermal treatment of rice straw fibers: 90 g rice straw fibers and 300 ml deionized water were added into a three-necked flask and stirred at 90°C with a water bath for 1 h. Then the hydrothermally treated rice straw fibers (H-SF) were obtained by drying at 80°C for 24 h and stored in a sealed bag.

Lipase treatment of rice straw fibers: 9 g lipase was dissolved into 300 ml deionized water, and 90 g rice straw fibers were added into the solution with stirring for 4 h under a 40°C water bath. After the treatment, the obtained rice straw fibers were washed with ethanol three times to remove the unreacted lipase. Then the L-SF were obtained by drying at 80°C for 24 h and stored in a sealed bag for further use.

30 g SF, S-SF, H-SF, and L-SF were mixed with phenolic resin with the weight ratio at 1:1 respectively. 1% curing agent of the weight ratio of phenolic resin was added to the mixture and stirred for 1 h. Then the uniformly mixed fibers and phenolic resin were added into the mold and cured by hot-pressing under different conditions with a pressure of 5 MPa.

Hot-pressing temperature is an important factor to influence the curing process of phenolic resin, which affects the mechanical properties and thickness swelling of the rice straw/phenolic resin composites. As is shown in Figure 1, the region of temperature from 150°C to 170°C was selected to carry out the three-point bending test. The results presented that the static bending strength and elastic modulus at 150°C were 14.95 MPa and 1,606.53 MPa, and then static bending strength and elastic modulus increased to 15.25 MPa and 1783.61 MPa when the temperature increased to 160°C. The thickness swelling decreased from 15.3% to 10.2%. However, when the temperature increased from 160°C to 170°C, the static bending strength and elastic modulus decreased to 14.76 MPa and 1,695.68 MPa. At the same time, the thickness swelling increased from 10.2% to 12.8%. Therefore, the temperature of 160°C is the optimal hot-processing condition to fabricate the rice straw fiber/phenolic resin composites.

Hot-pressing time is also an important factor for phenolic resin curing. The influence of hot-pressing time on the mechanical performance and thickness swelling of the rice straw/phenolic resin composites was also investigated in Figure 2. We can see that the static bending strength and elastic modulus were 14.68 MPa and 1,668.27 MPa with 7 min hot-pressing treatment. Then the static bending strength and elastic modulus increased to 17.32 MPa and 1981.98 MPa with the hot-pressing time increased to 11 min, and they became stable with a further extension of hot-pressing time. What’s more, the rice straw/phenolic resin composite showed the lowest thickness swelling at 11 min. Thus, 11 min was the optimal hot-pressing time for the preparation of the rice straw/phenolic resin composites.

The weight ratio of the rice straw fibers:phenolic resin has a significant influence on the mechanical properties and thickness swelling of the composites. As is shown in Figure 3, the static bending strength of the composites with the fiber content at 35% was 14.52 MPa. Then the static bending strength increased with increasing the fiber content and reach 17.33 MPa. However, when the fiber content increased to 55%, the static bending strength decreased to 15.56 MPa, mainly due to the aggregation of the fibers in the matrix of phenolic resin (Shah, 2013). The elastic modulus of the composites with the fiber content at 35% was 1724.93 MPa and increased to 1950.34 MPa with the fiber content increased to 55%. The thickness swelling of the composite increased from 7.2% to 12.5% with the fiber content increasing from 35% to 55%, indicating the water resistance decreased with decreasing the phenolic resin content.

Rice straw fiber (SF) is mainly composed of three main natural polymers (cellulose, hemicellulose, and lignin). In order to study the influence of hydrothermal treatment, silane coupling agent modification, and enzyme treatment on the chemical structure of the SF surface, the samples were characterized by FT-IR. As shown in Figure 4, the FT-IR spectra of raw rice straw fiber show characteristic bands within the selected region, including a band at 3,450 cm⁻¹ attributed to the stretching vibration absorption peak of the -OH group (Tan et al., 2022), a peak at 2970 cm⁻¹for -CH₂- stretching vibrations (Mo et al., 2019), the peaks at 1,600 cm⁻¹ and 1,486 cm⁻¹ corresponding to the skeleton vibration absorption of the aromatic ring of lignin (Cheng et al., 2016), and a peak at 1,055 cm⁻¹ belonging to the characteristic absorption peak of the hemicellulose ether bond (Liang and Wang, 2017).

Compared with SF, the absorption peaks at 3,450 cm⁻¹ and the absorption peak at 2970 cm⁻¹ of lipase-treated fiber (L-SF) increased, indicating that the lipophilic substances on the surface of the SF were removed (Jiang et al., 2009). Therefore, the functional groups in cellulose, hemicellulose, and lignin could be fully exposed, which is beneficial to the formation of good interfacial bonding with the resin adhesive. For the S-SF, the enhancement of the -CH₂-stretching vibration absorption peak at 2940 cm⁻¹ and the characteristic absorption peak of the -O-Si- group at 1,019 cm⁻¹ (Herrera-Franco and Valadez-Gonzalez, 2005), which proved the silane coupling agent was successfully introduced onto the SF. In the H-SF, the absorption peak intensities at 1,055 cm⁻¹ and 1,019 cm⁻¹ became weaker, presenting that the hydrothermal reaction result in the reduction of the hemicellulose and lignin, which may be beneficial to improve the interfacial adhesion between the rice straw fibers and the polymer matrix.

The interface compatibility between natural polymers and synthetic polymers is usually poor, which results in a relatively low mechanical strength. In order to improve the mechanical performance of the composite, many efforts have been reported to treat the surface of natural polymers with various approaches. In this work, three different approaches (silane modification, hydrothermal treatment and enzyme treatment) were used to treat the SF, and the static bending strength and elastic modulus of the rice straw/phenolic resin composites were also tested in Figure 5. For the PF/SF, the static bending strength and elastic modulus were 17.32 MPa and 1981.98 MPa. After hydrothermal treatment, the static bending strength and elastic modulus of the PF/H-SF were 18.94 MPa and 2096.85 MPa. The silane modification and enzyme treatment can also increase the mechanical performance of the PF/S-SF and PF/L-SF, and the static bending strength and elastic modulus were 20.44 MPa and 2362.33 MPa, and 23.03 MPa and 2588.39 MPa. What’s more, the thickness swelling of PF/H-SF (9.2%), PF/S-SF (8.6%), and PF/L-SF (7.9%) reduced significantly compared to the PF/SF (9.8%). It can be seen from the above results that the mechanical properties and thickness swelling of the rice straw/phenolic resin composites can be improved by the three different treatments of the SF. This is because the pretreatment can benefit their dispersion in the phenolic resin matrix by improving the interface compatibility.

DMA is an effective tool to investigate the interactions between the polymer matrix and fiber at different temperatures (Gupta and Singh, 2018). The storage modulus of PF/SF, PF/H-SF, PF/S-SF, and PF/L-SF was investigated in Figure 6. As is shown, the storage modulus of PF/SF, PF/H-SF, PF/S-SF, and PF/L-SF at room temperature (25°C) were 903.5 MPa, 1,583 MPa, 1,325 MPa, and 1,630 MPa, indicating the treatment of the SF can improve the interface compatibility between the fibers and the polymer matrix. Therefore, the treated fibers present better inhibition for the mobility of the phenolic resin molecule chain and resulted in the increased storage modulus. The storage modulus of PF/SF, PF/H-SF, PF/S-SF, and PF/L-SF decreased with increasing temperature. When the temperature is over 225°C, the storage modulus of the composites decreased dramatically because the composite start to degrade (Stevulova et al., 2016).