Abstract
An investigation was conducted of the modification of bamboo fibers (BF) by nano-hydroxyapatite (N-HA) and its impact on the mechanical and thermal properties of BF/PLA composites. The functional groups, and crystallinity of the N-HA modified BF were investigated with Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD), respectively. The effects of different N-HA contents on the properties of the BF/PLA composites were evaluated using mechanical testing (bending, tensile, and impact properties), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results showed that the most suitable N-HA content was 7.5 wt.%. Treating BF with an optimum concentration of N-HA decreased the polarity of the bamboo fiber, while maintaining the crystal structure of the cellulose. Compared with the control group, the mechanical properties and the crystallinity of the modified BF/PLA composites were improved, and the flexural, tensile, and impact strengths increased by 10.2%, 11.6%, and 28.1%, respectively. The thermogravimetric analysis results indicated modified BF/PLA composites has higher thermal stability.
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Effect of Nano-hydroxyapatite Modification of Bamboo Fiber on the Properties of Bamboo Fiber/Polylactic Acid Composites
Kaiqiang Zhang,a,b,c Jianyong Lin,a,b,c Chengyi Hao,a,b,c Gonghua Hong,a,b,c Zhenghao Chen,a,b,c Zhangjing Chen,d Shuangbao Zhang,a,b,c,* and Wei Song a,b,c,*
An investigation was conducted of the modification of bamboo fibers (BF) by nano-hydroxyapatite (N-HA) and its impact on the mechanical and thermal properties of BF/PLA composites. The functional groups, and crystallinity of the N-HA modified BF were investigated with Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD), respectively. The effects of different N-HA contents on the properties of the BF/PLA composites were evaluated using mechanical testing (bending, tensile, and impact properties), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results showed that the most suitable N-HA content was 7.5 wt.%. Treating BF with an optimum concentration of N-HA decreased the polarity of the bamboo fiber, while maintaining the crystal structure of the cellulose. Compared with the control group, the mechanical properties and the crystallinity of the modified BF/PLA composites were improved, and the flexural, tensile, and impact strengths increased by 10.2%, 11.6%, and 28.1%, respectively. The thermogravimetric analysis results indicated modified BF/PLA composites has higher thermal stability.
Keywords: Bamboo fiber; Polylactic acid; Nano-hydroxyapatite
Contact information: a: MOE Key Laboratory of Wooden Material Science and Application, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China; b: Beijing Key Laboratory of Wood Science and Engineering, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China; c: MOE Engineering Research Center of Forestry Biomass Materials and Bioenergy, College of Materials Science and Technology, Beijing Forestry University, Beijing 100083, China; d: Department of Sustainable Biomaterials, Virginia Tech, Blacksburg VA 24061, USA; *Corresponding authors: shuangbaozhangj5@163.com; j5international@163.com
INTRODUCTION
Polylactic acid (PLA) is a polymeric material that exhibits excellent biocompatibility and biodegradation. Thus, it is widely used in the medical field and has received increasing attention (Singh et al. 2004; Tiainen et al. 2006). It is used as a new type of orthopedic internal fixation material for use in medical products, such as bone nails and bone plates. Although PLA has many excellent properties, it cannot meet the fixation requirements of many functional bones because of the low mechanical strength of pure PLA (Wan et al. 2004). At present, the performance of PLA is mainly enhanced by self-reinforcement or by compounding it with other materials, such as natural and carbon fibers. In addition to biodegradable and environmentally friendly nature, bamboo fiber (BF) has a unique structure, good mechanical properties, and natural bacteriostatic properties. Thus, it has attracted attention for enhancing the performance of PLA (Abdul Khalil et al. 2012; Liu et al. 2012; Mishra et al. 2012; Yu et al. 2014). However, BF contains a large number of polar hydrophilic hydroxyl groups, which lead to poor compatibility with non-polar PLA and poor performance of composites; in contrast, the degradation products of PLA are acidic, which easily cause an inflammatory reaction in the human body (Sawpan et al. 2011). These problems plague the application of PLA in composites.
It is necessary to modify the surface of BF to enhance the performance of its composites. Shi et al. (2011) and Wang et al. (2017) reported that the modification of natural fibers by inorganic nanoparticles is an effective method that remarkably improves the interface between the fibers and polymer matrix. The inorganic nanoparticles can adhere to the surface of cell walls. As a nucleation site, it affects the crystallization behavior of the polymer matrix and produces a robust electrostatic attraction to the surface of the non-polar polymer. Therefore, the interfacial compatibility of the composite may be improved (Wang et al. 2016). Nano-hydroxyapatite (N-HA) has good bioactivity and osteoconductivity, and it can form a direct bond with bone, so it is widely used in bone replacement implant materials. Moreover, the acidic degradation products of PLA can be buffered by hydroxyapatite (HA). Additionally, the osteoinductivity of N-HA can provide a suitable environment for bone cell adhesion and growth, which conforms to the requirements of bone tissue engineering (Verheyen et al. 1992; Shikinami and Okuno 2001; Zhao et al. 2002).
In this study, the BF were modified with a series of concentrations of N-HA, and then the treated BF was introduced into the PLA matrix. The purpose of this study was to explore the interfacial enhancement effect of N-HA on composite materials, which may provide new theoretical and practical bases for the practical application of inorganic particle-reinforced bamboo plastic composites.
EXPERIMENTAL
Materials
Bamboo fibers ranging from 60 mesh to 80 mesh in size were purchased from Sentai Wood Plastic Composites Material Co., Ltd. (Huzhou, China). The PLA (grade 4032D, melting point = 160 °C, and density = 1.24 g/cm3) was provided by NatureWorks Ind. Co. Ltd. (Northford, CT, USA). The N-HA was purchased from Nanjing Emperor Nano Material Co., Ltd. (Nanjing, China) in sizes ranging between 60 nm and 80 nm. The deionized water was provided by the laboratory of Beijing Forestry University.
Table 1. Process Parameters for N-HA Impregnation
Modification process of the bamboo fiber
The BF was dried for 8 h at 105 °C in a drying oven. Subsequently, 100 g of BF were dispersed in 1 L of deionized water for 30 min at 300 rpm with a mechanical stirring apparatus at room temperature. Next, different N-HA contents (2.5 %, 5%, 7.5%, 10% and 12.5%) were added to the mixed solution, respectively, and the mixtures were stirred for 30 min. After this, the modified BF was washed with deionized water and placed in an air-dry oven and dried at 105 °C for 8 h. The process parameters are shown in Table 1.
Preparation of the BF/PLA composites
The route of the preparation of BF/PLA composites was similar to our group’s previous research (Lin et al. 2018). The weight ratio of PLA to BF was maintained at 3:2 during preparation of the BF/PLA composites. Firstly, the PLA and BF were mixed uniformly in the blending machine, and then the compound was extruded via a co-rotating twin-screw extruder (KESUN KS-20, Kunshan, China). The twin-screw rotation speed was kept at 100 rpm, and the temperatures of each part of the twin screw were 165 °C, 170 °C, 175 °C, 165 °C, and 160 °C. Then, the mixture was pulverized into small particles with 4-mm diameters. Next, the composites were produced after these particles underwent a hot-pressing process (HAPCO, BY 602 × 2/2 150 T Testing Press, Suzhou, China), which was performed with a pressing time of 6 min, pressing pressure of 4 MPa, and hot-pressing temperature of 175 °C. Finally, the sample was pressed at a pressure of 4 MPa for 10 min at room temperature. The dimensions of the PLA composites were 270 mm × 270 mm × 4 mm, and the density was 1.2 g/cm3.
Characterization of the Bamboo Fiber
Fourier transform infrared spectroscopy
The Fourier transform infrared (FTIR) analysis of the BF was performed using a Vertex 70v (BRUKER, Karlsruhe, Germany), with a resolution of 4 cm-1 and average of 40 scans over a scanning range from 4000 cm-1 to 400 cm-1. The BF sample was mixed with potassium bromide, and then the mixture was compressed into tablets for recording the spectra. Each group was measured three times to minimize error.
X-ray diffraction procedure and analysis
The crystallinities of the BF were measured by X-ray diffraction (XRD) using a D8 Advanced X-ray diffractometer (BRUKER, AXS, Karlsrushe, Germany) with Cu Kα radiation (λ = 0.154060 nm), a radiation tube voltage of 40 kV, and a current of 40 mA. The scanning rate was 2°/min at 2θ angles of 5° to 40°. The cellulose crystallinity index (IXRD) was calculated with Eq. 1 (Le Troedec et al. 2008; Pickering et al. 2011):
(1)
where I002 is the maximum intensity of the (002) lattice diffraction plane at a 2θ angle between 22° and 23° (22° ≤ 2θ ≤ 23°), and Iamp is the diffraction intensity at a 2θ angle close to 18°, which represents amorphous materials in the cellulosic fibers.
Characterization of the Bamboo Fiber/PLA Composites
Mechanical characterization
The tensile property of the composite was measured according to ASTM D638-03 (2003) using an AG-IS (Shimadzu, Kyoto, Japan). The dimensions of the specimens were 165 mm × 20 mm × 4 mm (dumbbell-shape, with a radius of arc of 76 mm, gauge length of 50 mm, and middle part width of 13 mm), and each group had eight specimens that were measured at a stretching speed of 5 mm/min.
The flexural property of the composite was measured according to ASTM D790-03 (2003) using the AG-IS. The dimensions of the specimens were 80 mm × 10 mm × 4 mm, and each group had eight specimens that were measured at a strain speed of 20 mm/min.
The impact property of the composite was measured according to ASTM D256-03 (2003) using a CREE-1002C (XJJ5, KERUI, Dongguan, China). The dimensions of the specimens were 80 mm × 10 mm × 4 mm, and each group had eight specimens that were measured at an impact speed of 2.9 m/s with a 1 J hammer. These tests were performed at room temperature.
Thermogravimetric analysis
The thermal stability of the composites was measured using a Q5000 apparatus (TA Instrument, New Castle, USA) at a heating rate of 10 °C/min from 40 °C to 600 °C under an N2 gas flow rate of 50 mL/min.
Differential scanning calorimetry
Differential scanning calorimetry (DSC) analysis was performed using a Netzsch DSC-204 (Bavaria, Germany). All of the data obtained were taken within a temperature range of 40 °C to 200 °C at a heating rate of 10 °C/min; the samples were then cooled at a rate of 10 °C/min and heated again to 200 °C at the same rate. The crystallinity (XDSC, %) was calculated with Eq. 2 (Fischer et al. 1973; Rana et al. 2000; Joseph et al. 2003),
(2)
where f is the mass fraction of the PLA matrix in the composites, ΔHm is the melting enthalpy of the composites, and ΔH0m is the melting enthalpy of the 100% crystalline PLA (93.7 J/g).
Interfacial morphology analysis
The impact fracture morphology analysis was performed using an FEI Quanta FEG 650 scanning electron microscopy (SEM, Thermo Fisher Scientific Inc., Waltham, MA, USA) with an acceleration voltage of 15 kV. All of the specimens were sputter-coated with gold before observation.
RESULTS AND DISCUSSION
FTIR Analysis of the Bamboo Fiber
The FTIR spectra of the untreated (Control) and treated BF (HA-BF-2 and HA-BF-5) are shown in Fig. 1. As shown, there were characteristic peaks near 3430 cm-1, which according to Fourier transform infrared (FTIR) spectroscopy were designated as stretching vibration of hydroxyl (–OH) groups. The characteristic peaks at 2910 cm-1 were attributed to the symmetric and asymmetric stretching vibration absorption peaks of methyl (–CH3) and methylene (–CH2) groups that belong to cellulose (Sgriccia et al. 2008). The characteristic peaks located at 1740 cm-1, 1425 cm-1, 1370 cm-1, and 1160 cm-1 were assigned to C=O stretching, -CH2– shear, and C-H bending vibrations (Zhang et al. 2012).
Fig. 1. FTIR spectra of the untreated and N-HA-treated BF
It was clear that the intensity of the absorption peaks of the three curves showed a distinct difference at 3430 cm-1. This was explained by the fact that during the BF treatment with N-HA, the -OH groups on the N-HA molecules formed hydrogen bonds with the hydroxyl groups on the surface of the BF. As the N-HA content was increased, the amount of hydrogen bonds formed increased, which resulted in a decrease in the number of -OH groups. Compared with the untreated BF, the curves of the treated BF showed weak peaks at 603 cm-1 and 564 cm-1, which were assigned to PO43- stretching vibration. This indicated that N-HA has potential to improve the interfacial properties of BF and plastic composites.
XRD Analysis of the Bamboo Fiber
Figure 2 shows the results of the analysis of the crystalline performances of the treated and untreated BF by XRD. The crystalline structure of the natural plant fiber is reflected by the two crystalline areas of (101) and (002) in the XRD pattern, whose 2θ angles are 17.5° and 22.3°, respectively. The position of the characteristic peak of the treated BF was barely changed compared with that of the untreated BF, which indicated that N-HA had almost no influence on the crystalline region of the cellulose. The structure of the crystalline areas of the cellulose chain was barely destroyed in the treatment process.
As shown in Fig. 2, the treated BF showed characteristic peaks at 25.7° and 31.8°, which were designated as characteristic bands of HA. When the content of the N-HA was increased from 0% to 7.5%, the crystallinity of cellulose increased gradually. And when the content of N-HA was 7.5%, the crystallinity of cellulose reached the maximum value of 44.0%. Then, with the increase of N-HA content, the crystallinity of the cellulose gradually decreased and reached a minimum of 41% when the N-HA content was 12.5%. This indicates that the low content of N-HA induced the nucleation of the cellulose of the BF and then initiated crystallization. When the content of N-HA exceeds a certain amount, the crystallization of cellulose was hindered when the N-HA content exceeded 7.5 wt.%, agglomeration occurred and the crystal nucleus failed to increase (Qian et al. 2018).
Fig. 2. XRD analysis of the treated and untreated BF
Effect of the BF Treatments on the Mechanical Properties of the BF/PLA Composites
Table 2 shows the effects of the different N-HA contents on the mechanical properties of the BF/PLA composites. Compared with the control group, the flexural strength, flexural modulus, tensile strength, tensile modulus, and impact strength of the treated BF all increased. The main reasons were that the N-HA particles gradually filled in the micropores, grooves, etc. on the surface of the fiber, which reduced the generation of void defects during the compounding process. The N-HA formed strong hydrogen bonds with the fiber molecules. Thus, the hydrophilicity of the fiber surface was reduced and the bonding strength with PLA was enhanced. Thereby, the interface properties of the composites were increased.
Table 2. Mechanical Properties of BF/PLA Composites
Note: The data were analyzed by one-way ANOVA based on a 95% confidence interval. The standard deviations of the test results are shown in parentheses; the letter markers show the statistical differences. The groups do not differ significantly from one another when they have the same letter(s), and vice versa.
With the increase of N-HA content, the mechanical properties of treated BF/PLA composites first increased and then decreased. When the N-HA content was 7.5 wt.%, the mechanical properties of the composites were superior in this study. However, when the N-HA content exceeded this value, the mechanical properties of the composites were adversely affected. This was because the to high concentration N-HA was unevenly distributed on the surface of the fiber, which aggregated into large particles and caused the cross-link density of the modified fiber and PLA matrix to not be uniform. This resulted in poor stress transfer between the interfaces, which reduced the interfacial compatibility between the BF and PLA (Qian et al.2015b; Qian et al. 2016). Therefore, the optimum N-HA content in this study was 7.5 wt.%, which caused the flexural strength, flexural modulus, tensile strength, tensile modulus, and impact strength to increase by 10.2%, 5%, 11.6%, 22.7%, and 28.1%, respectively, in comparison with those of the untreated composites.
Thermogravimetric Analysis
Figure 3 shows the thermogravimetric (TGA) and differential thermogravimetric (DTG) curves based on the PLA matrix composites enhanced with untreated and treated BF. The initial thermal degradation temperature (T1) and maximum rate temperature (T2) of the composite material are listed in Table 3.
Table 3. Thermal Parameters of the BF/PLA Composites
Note:T1 was the 5% mass loss rate of the material. T2 were the first peaks in the DTG curve of the material.
Figure 3 shows that weight loss in the composites can be divided into three stages. In the first stage (from 30 °C to 230 °C), the mass loss was associated with the evaporation of moisture in the BF and depolymerization of macromolecular compounds. In the second stage (230 °C to 400 °C), the decline was obvious within the range of 230 °C to 350 °C, which was mainly because of the thermal decomposition of hemicellulose.
Fig. 3. TGA curves (a) and DTG curves (b)
From 350 °C to 400 °C, which was because of the thermal decomposition of the cellulose . The T1 and T2 of the untreated BF/PLA composites were 291 °C and 312 °C, respectively. With the addition of N-HA, the T1 and T2 showed an increasing trend. It was observed that the thermal stability of the treated BF/PLA was significantly higher than that of the control. This was explained by the following reasons. The first reason was that the characteristics of N-HA prevented heat transfer; and the second reason was that the N-HA improved the cross-linking density of the PLA matrix and strengthened the molecular structure, so that the thermal stability of the PLA matrix-based composites was improved. In the third stage (from 400 °C to 600 °C), the N-HA treated group had a lower mass loss compared with that of the control group. Consequently, the N-HA treatment resulted in a higher thermal stability.
Differential Scanning Calorimetry
Figure 4 and Table 4 show the DSC thermograms and relevant thermal parameters of the composite. Compared with the untreated BF/PLA composites, the glass transition temperature (Tg) of the treated BF-reinforced composites shifted to a higher temperature. The main reason was that the N-HA-modified BF had more hydrogen bonds, which hindered the molecular chain movement of the PLA. Therefore, the molecules required more energy to start moving and needed a higher temperature to convert the PLA from a glassy state to a rubbery state (Qian et al. 2015a). However, the Tg showed a downward trend when the N-HA content was above 7.5 wt.%. The reason was the agglomeration of HA, which led to the uneven cross-linking density of the interfacial region and resulted in a decrease in the total crosslinking density of the PLA matrix (Qian et al. 2018).
Table 4. Typical Thermal Parameters of the DSC Analysis of the Various BF/PLA Composites
Tm – The melting temperature of the PLA matrix; and Xc – the crystallinity of PLA matrix
Fig. 4. DSC curves of the untreated and treated BF/PLA composites
Surface Morphology
The microscopic morphology of the impact cross-section of the composites is shown in Fig. 5.