Tensile properties of bamboo fiber (BF)/polypropylene (PP) composites were investigated for different impregnation modification processes. The surface morphology of BF and the fracture morphology of the BF/PP composites were examined by scanning electron microscopy (SEM). Images showed that CaCO3 nanoparticles (nano-CaCO3) adhered to the fiber surfaces. Tension tests of individual bamboo fibers (IBFs) and the BF/PP composites were conducted. The results indicated that the tensile strength (TS), modulus of elasticity (MOE), and elongation at break (EAB) of IBFs increased by 16.0%, 22.2%, and 5.2%, respectively, while those of the BF/PP composites increased by 3.0%, 7.0%, and 15.8%, respectively, compared to the control samples. Various process parameters during the impregnation with nano-CaCO3 had marked effects on the tensile properties of the composites. The optimal conditions for BF-reinforced PP composites by impregnation modification were determined to be a nano-CaCO3 concentration of 1.0×10-2 g/mL, an ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) additive concentration of 8.5×10-4 g/mL, and an impregnation time of 25 min.
Tensile Properties of Bamboo Fiber-reinforced Polypropylene Composites Modified by Impregnation with Calcium Carbonate Nanoparticles
Cuicui Wang,a Yu Xian,b Haitao Cheng,a,b,* Wenyan Li,a and Shuangbao Zhang a*
Tensile properties of bamboo fiber (BF)/polypropylene (PP) composites were investigated for different impregnation modification processes. The surface morphology of BF and the fracture morphology of the BF/PP composites were examined by scanning electron microscopy (SEM). Images showed that CaCO3nanoparticles (nano-CaCO3) adhered to the fiber surfaces. Tension tests of individual bamboo fibers (IBFs) and the BF/PP composites were conducted. The results indicated that the tensile strength (TS), modulus of elasticity (MOE), and elongation at break (EAB) of IBFs increased by 16.0%, 22.2%, and 5.2%, respectively, while those of the BF/PP composites increased by 3.0%, 7.0%, and 15.8%, respectively, compared to the control samples. Various process parameters during the impregnation with nano-CaCO3 had marked effects on the tensile properties of the composites. The optimal conditions for BF-reinforced PP composites by impregnation modification were determined to be a nano-CaCO3 concentration of 1.0×10-2 g/mL, an ethylenediamine tetraacetic acid disodium salt (EDTA-2Na) additive concentration of 8.5×10-4 g/mL, and an impregnation time of 25 min.
Keywords: Calcium carbonate nanoparticles (nano-CaCO3); Polypropylene (PP); Bamboo fiber (BF); Composites; Tensile properties
Contact information: a: Beijing Key Laboratory of Wood Science and Engineering, Beijing Forestry University, Beijing, China, 100083; b: International Centre for Bamboo and Rattan, Beijing, China, 100102; *Corresponding author: firstname.lastname@example.org, email@example.com
The use of natural fibers in place of synthetic fibers to reinforce polymer composites has attracted great attention among researchers in recent years because of the interest in developing environmentally friendly materials (Jindal 1986; Chattopadhyay et al. 2011). Natural fibers have many desirable characteristics, such as abundance, renewability, recyclability, and biodegradability, and they can provide the basis for a high-tech revolution (George et al. 2001). The advantages of natural fiber-reinforced polymer composites arise because of their sustainability, low cost and density, high dimensional stability, good processability, and favorable mechanical properties (Sain et al. 2005; Dominkovics et al. 2007; Banga et al. 2015). Various researchers have studied natural fibers containing polyolefins (Sain et al. 1993; Saxena and Gowri 2003; Ying et al. 2013). Natural fiber-reinforced polymer composites are widely used in the automotive industry, aircraft, interior decoration, and many other fields encountered in daily life (Davoodi et al. 2012).
Natural fiber-reinforced polymer composites may have problems related to fiber porosity and dispersion, affecting the interfacial compatibility between matrix and fiber (Bledzki et al. 2005). At present, there are three main methods to combat these issues: the modification of natural plant fibers or thermoplastic polymers or the addition of coupling agents to develop the interface compatibility. These strategies effectively work to improve the mechanical properties of composites (Albanoa et al. 2002; Lee et al. 2004; Plackett 2004).
Among the modification methods, nano-CaCO3 modification has been developing considerable prospects because of the nano-scale, ultra-fine solid structure of CaCO3, and the high economic value of the process. Nano-CaCO3 plays an important role in the reinforcing and toughening of materials and enhances the processing and operational performance when used in plastic packing (Huang et al. 2011; Qiu and Zhou 2011; Zhang et al. 2011). Meanwhile, research has shown that nano-CaCO3 on the surface of fibers has a strong electrostatic attraction, which can affect the interface performance of fibers and polymer (Lee et al. 2007). Shi et al.(2011b) prepared CaCO3 particles on the surface of kenaf and conducted cell cavity in situ synthesis successfully, finding that nano-CaCO3 particles could adhere to the surface of fibers, which enhanced the interface compatibility. Liang et al. (2014) reported that the mechanical properties of kenaf fibers and their composites were increased by impregnation with CaCO3 particles. Gao (2012) found that the processing parameters of nano-CaCO3 production by ion solution in situ synthesis had a significant impact on the tensile properties of bamboo fiber (BF) and its composites.
It is inconvenient to produce nano-CaCO3 industrially because of its tedious manufacturing process and high price, although the nano-CaCO3 in situ modification of BF induced an obvious improvement in the interface performance. In this study, nano-CaCO3 impregnation modification of BF was carried out to study the feasibility of in situdeposition modification. The influence of the modification process on the tensile properties of the composites was explored, further revealing the functional mechanism of interfacial bonding performance in the inorganic nanoparticle-reinforced composites.
Neosinocalamus affinis, aged 1 year, with a moisture content of 8% to 12% was collected from a bamboo plantation located in Sichuang Province, China. Nano-CaCO3 with a particle size of 15 to 40 nm was supplied by Boyu Gaoke New Materials Co. Ltd. (China). Ethylenediamine tetraacetic acid disodium salt (EDTA-2Na), analytically pure, with a melting point of 25 °C, was purchased from Huabo Stand (Beijing, China). Polypropylene (PP) film with dimensions of 150 mm × 150 mm × 0.5 mm, a density of 0.905 g/cm3, and a melting temperature of 165 to 170 °C was supplied by Beijing Gemstar Commerce Co. Ltd. (China).
Preparation of individual bamboo fiber (IBF)
According to the standard GB/T 15780 (1995), samples were prepared by cutting 1.5-m to 3.0-m samples of bamboo tube root, wiping off the bamboo green and tabasheer, and keeping the bamboo meat, which was then further split into bamboo sticks with an axial length of 1.5 to 2.0 cm, a radial length of 0.2 cm, and a tangential length of 0.2 cm. The sulfate process described by Chen (2011) was adopted to process the BF, resulting in IBFs with lengths of 2 to 4 mm and diameters of 8 to 22 μm.
Modification of BF
(1) Nano-CaCO3 impregnation modification: At 25 °C, a certain content of BF was dissolved in distilled water for 30 min at 500 rpm through a magnetic stirring apparatus (ETS-D5 S25; IKA, Germany), then a varied amount of EDTA-2Na and nano-CaCO3 were added and mixed for differing time periods. The process parameters for the nano-CaCO3 impregnation are shown in Table 1.
(2) Nano-CaCO3 in situ modification: At 25 °C, 0.5 g of BF was blended with 100 mL of a 0.3 mol/L CaCl2 solution for 30 min at 500 rpm, then 0.5 g of EDTA-2Na and 0.03 mol of Na2CO3 solution, at a rate of 25 mL/min, were added and mixed for 25 min.
The suspensions were washed on a 200-mesh nylon net with distilled water and air dried. The modified bamboo fibers (MBFs) were obtained and preserved in a constant temperature and humidity box at 23 °C and 50% relative humidity (RH). An unmodified BF with 0.5 g of EDTA-2Na was used as the control.
Table 1. Process Parameters for Nano-CaCO3 Impregnation
Preparation of BF/PP composites
Circular BF papers with diameters of 100 mm, made from 0.2 g of MBF, were dried for 20 min at 103 °C in an oven until the moisture content was approximately 2%. A hot press (R-3221; Wuhan Qi’en Technology Co., Ltd., China) was used to form a sheet consisting of five BF papers with a mass of 1.0 g and six PP films with a mass of 2.0 g, layered in an alternating pattern; the sheet thickness was 1.0 mm, and three sheets were used for each specimen. The hot-pressing was conducted at a temperature of 180 °C, a pressure of 2 MPa, and a dwell time of 2 min.
Performance testing of BF
A trinocular microscope (LEICA DM LB2; Leica Microsystems, Germany) was used to test the length and diameter of the BF and to calculate the length-diameter (L-D) ratio. Three hundred testing samples were used. The contents of benzene-alcohol extractives, cellulose, holocellulose, and lignin were measured according to the standards GB/T 10741 (2008), GB/T 744 (1989), GB/T 2677.10 (1995), and GB/T 2677.8 (1994), respectively.
The tensile properties of IBF were measured using a high-precision short fiber mechanical properties tester (JSF08; Beijing YiKe Letter Technology Co., Ltd., China) to test the load and displacement of IBF; the tensile speed was set to 0.0008 mm/s, and the ball and socket system was used. The cross-sectional area of the IBF was measured by a laser scanning confocal microscope (CLSM; LSM 510 META; Zeiss, Oberkocken, Germany). The preparation of the sample and the testing process were carried out in accordance with the procedure described by Cao et al. (2010). Thirty effective samples were used for each group.
The tensile properties of the composites were measured according to the standard GB/T1040.3. (2006). Dumbbell-shaped specimens (type V), which were shorten to a half, were used for testing, and the tensile speed was set to 2 mm/min. Five samples were made from one sheet, and fifteen effective samples were used for each group.
The instrument used for observing the surface morphology of BF and the BF/PP composites under various process parameters was an field emission environmental scanning electron microscope (SEM; XL30 ESEM-FEG; FEI Co., USA). A vacuum was pulled to less than 5 × 10-5 Pa in the sample chamber, while the scanning voltage ranged from 7 to 10 kV.
RESULTS AND DISCUSSION
Basic Properties of BF Produced by the Sulfate Process
As can be seen in Fig. 1, the surface of the BF had many small pores with a diameter of 0.7 to 1.5 μm, and folds formed on the surface of air-dry BF. This process may have occurred because the cell walls of the natural fiber have a mesoporous structure, and the sulfate process can remove the lignin and degrade parts of the hemicellulose through oxidation. These mesopores and folds could provide attachment sites for CaCO3. The length, diameter, and L-D ratio of the BF made by the sulfate process were 1992.9 μm, 21.9 μm, and 93.7, respectively.
Fig. 1. Surface morphology of BF
Long-BF-reinforced composites have been shown to have superior mechanical properties compared with short-BF composites (Ochi 2014). Also, it is generally believed that, when the L-D ratio is less than 45, the value of pulping and papermaking is low (Pei et al. 2012). The number of winding times between fibers per unit area when forming the sheet increases, as does the strength of the resulting paper, as the L-D ratio increases. Therefore, the BF after the sulfate process could be used to make paper with a certain strength.
Table 2. Chemical Composition of BF
Table 2 shows that the lignin content of the BF was only 0.72%. The TS and EAB of the fiber cell wall increase with decreasing lignin content (Zhang 2011). The sulfate cooking liquor had a small effect on cellulose, with a content of 74.10%. Cellulose exists in the fiber cell walls in the state of a microfibril beam composed of orderly molecular chains and serves as the main provider of the cell walls’ mechanical properties. However, the hydroxyl groups of the cellulose chain could have caused strong polarity on the surface of the BF, which would cause the interface compatibility of the two phases to weaken when the BF was combined with the thermoplastic polymer.
Tensile Properties of IBF and Its Composites under Various Modification Methods
As shown in Fig. 2, CaCO3 particles were successfully deposited during the impregnation and in situ modification of BF, which was also reported in a previous study by the authors (Cheng et al. 2014a). This success was attributed to the ability of nano-CaCO3 to fill the fissures or pores of the fibers and grow along with them (Bergert et al. 2005). The microporous and fold structures on the surface of the fiber cell walls could provide attachment locations for inorganic particles, whereas nano-CaCO3 adhered to the cell walls could offer nucleation sites for initiating the crystalline formation of the semicrystalline PP matrix (Shi et al. 2011a). Nano-CaCO3 with a particle size of 70 to 120 nm adhered to the surface of BF from the impregnation modification had the shape of an irregular tetrahedron, which dispersed very well without reunion. The amount of nano-CaCO3 on the surface of BF from the in situ modification was noticeably greater, almost covering the whole fiber, but united the larger particles. Most of the nano-CaCO3 particles were rice-shaped, and the particle size of CaCO3, approximately 300 to 1280 nm, allowed it to distribute widely. Cementation of the nano-CaCO3appeared, especially in the concave parts of the fiber. This phenomenon might have occurred because the Ca2+ and CO32- ions in the solution and the solid CaCO3 were not in equilibrium. At the beginning of the reaction, Ca2+ would first combine with CO32-, and CaCO3 molecular clusters would start growing with a crystal nucleus until the concentration of the CaCO3 molecular clusters in the solution reached a supersaturated state (Lin et al. 2001), which would lead to a gradual increase in the particle size of the CaCO3 adhered to the surface of the BF.
Table 3 shows that the impregnation and in situ modifications had little impact on the cross-sectional area of the IBF, but it had a noticeable effect on tensile properties. Compared with the unmodified IBF, the breaking load, TS, MOE, and EAB increased. Some studies indicate that cell wall deficiencies, initially present in the fiber itself and generated by the preparation process, could affect the mechanical properties. The defective parts concentrated stress easily, which caused the deformation and rupture of the fiber (Mott et al. 1996). Modification could have allowed CaCO3 to fill the defective parts such as pores and folds, improving the density of the fibers and also transmitting the load around the pores to the CaCO3 particles. This load transfer would effectively prevent the fibers from fracturing because of defects, thus improving the tensile properties. The TS, MOE, and EAB of the unmodified IBF were 15.98%, 22.15%, and 5.21% lower than those of the IBF after impregnation modification, respectively, and showed a decrease of 18.00%, 26.19%, and 21.88%, respectively, compared to the IBF after in situ modification. The trend of TS correlated with the loading of CaCO3. The specific surface area increased as the loading of inorganic nanoparticles increased, which would have a prominent effect on the properties of the BF (Xia et al. 2015).
Table 3. Mechanical Properties of IBF Produced by Various Modification Processes
Fig. 2. Surface morphology of nano-CaCO3-modified BF: (a) impregnation modification and (b) in situ modification
As shown in Table 4, the breaking load, TS, and MOE of unmodified BF/PP composites were 3.00%, 6.95%, and 15.80% lower than those of nano-CaCO3 impregnation-modified BF/PP composites, respectively, and showed decreases of 5.28%, 18.68%, and 25.41%, respectively, compared to the nano-CaCO3 in situ-modified BF/PP composites. These differences might have arisen because the CaCO3-filled BF could develop compatibility between the BF and PP matrices and thus improve the TS of the composites (Cheng et al.2014b). The fracture surfaces of the BF/PP composites produced by various modification treatments are shown in Fig. 3.
Table 4. Tensile Properties of BF/PP Composites Produced by Various Modification Processes