Abstract
To enhance the resistance of eucalyptus (EU) reinforced high density polyethylene (HDPE) composites to exposure in simulated seawater, the EU fibers were modified by alkali treatment with NaOH to prepare wood-plastic composites (WPCs). The materials were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), moisture absorption, mechanical properties, and color change tests. After exposure to the sea water, all composites experienced deterioration of water repellency and mechanical properties, and color change. The NaOH concentration greatly influenced the properties of the EU/HDPE composites both before and after exposure. The alkali-treated EU fibers presented low polarities, which resulted in better interfacial bonding, improved mechanical properties, lower moisture absorption, and lower color change relative to the untreated samples after immersion in simulated sea water. The results showed that the HDPE composite prepared with 3% NaOH treated EU fiber had better degradation resistance compared with the untreated composite. The tensile strength, flexural strength, flexural modulus, and impact strength increased 29.9%, 19.8%, 35.4%, and 39.3%, respectively, in comparison with the untreated composite after 21 d exposure. The improved degradation resistance of the alkali-treated EU/HDPE composites could ensure the expected service life of their products and widen their practical applications.
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Degradation Resistance of Alkali-Treated Eucalyptus Fiber Reinforced High Density Polyethylene Composites as Function of Simulated Sea Water Exposure
Jingjing Fu,a,b Chunxia He,b,* Caiyun Jiang,b and Yongsheng Chen a,*
To enhance the resistance of eucalyptus (EU) reinforced high density polyethylene (HDPE) composites to exposure in simulated seawater, the EU fibers were modified by alkali treatment with NaOH to prepare wood-plastic composites (WPCs). The materials were characterized by scanning electron microscope (SEM), Fourier transform infrared spectroscopy (FTIR), moisture absorption, mechanical properties, and color change tests. After exposure to the sea water, all composites experienced deterioration of water repellency and mechanical properties, and color change. The NaOH concentration greatly influenced the properties of the EU/HDPE composites both before and after exposure. The alkali-treated EU fibers presented low polarities, which resulted in better interfacial bonding, improved mechanical properties, lower moisture absorption, and lower color change relative to the untreated samples after immersion in simulated sea water. The results showed that the HDPE composite prepared with 3% NaOH treated EU fiber had better degradation resistance compared with the untreated composite. The tensile strength, flexural strength, flexural modulus, and impact strength increased 29.9%, 19.8%, 35.4%, and 39.3%, respectively, in comparison with the untreated composite after 21 d exposure. The improved degradation resistance of the alkali-treated EU/HDPE composites could ensure the expected service life of their products and widen their practical applications.
Keywords: Alkali treatment; Eucalyptus; High density polyethylene; Sea water; Mechanical properties; Color change
Contact information: a: Nanjing Research Institute for Agricultural Mechanization, Ministry of Agriculture and Rural Affairs, Nanjing 210014, China; b: College of Engineering, Nanjing Agricultural University, Nanjing 210031, China; *Corresponding authors: chunxiahe@hotmail.com and cys003@sina.com
INTRODUCTION
Wood-plastic composites (WPCs) have low cost, low density, good mechanical properties, potential sustainability, and biodegradability. They are widely used in outdoor and indoor fabrication applications, such as decking for fencing, landscaping timbers, furniture, boardwalk, timber bridges, and so on. (Leu et al. 2012; Olakanmi and Strydom 2016). There also have been numerous cases of WPCs failures in service; with the exceptions of design and construction errors, and accidental or unexpected usage, many failures are due to the service environment, such as exposure to the light, humidity, freezing, thawing, or biodeterioration. These cases stress the need to ensure suitable and safe performance during its expected working life, and its durability and aesthetic aspects play an important role. The serviceability of WPCs applied in UV radiation (Hou et al. 2013), natural weathering (Zukowski et al. 2018), aging (Wang et al. 2017; Wang et al. 2018), mildew corrosion (Shao et al. 2019), and some outdoor conditions have already been studied. While it is not surprising that sea water diminishes the mechanical properties of WPCs, there has been little research on the serviceability of WPCs in sea water conditions.
Eucalyptus is one of the most common wood species used for furniture, decking, and other wood products. The hemicellulose and lignin in wood fibers are sensitive to UV radiation and moisture, and the wood fibers have weak interfacial bonding with the matrix of the composites (Cai et al. 2015). It may be sensible to proceed with surface modification before the WPCs have been prepared. Alkali treatment with NaOH is commonly used to remove nanocelluloses from natural fibers. It exposes hydroxyl groups and roughens fiber surfaces, leading to improved interfacial bonding (Islam et al. 2011). Among the different surface modifications of wood fibers, alkali treatment is one of the most popular and lowest cost methods, and it improves the mechanical properties of WPCs (Liu et al. 2016; Li et al. 2017).
This study explored the degradation resistance of eucalyptus (EU)/high density polyethylene (HDPE) composites under simulated seawater conditions. An alkali treatment was applied to modify the eucalyptus fibers, and hot compression molding was chosen to promote material shaping. The effects of the different NaOH concentrations on the degradation resistance of EU/HDPE composite in simulated sea water were examined. The morphology, moisture absorption, mechanical properties, Fourier transform infrared spectroscopy (FTIR), and color change analysis were examined. It was expected that improved performance of the EU/HDPE composite will broaden its potential application and facilitate large-scale production.
EXPERIMENTAL
Materials
Eucalyptus (EU) powder was purchased from Weihua Perfumery Plant (Jiangmen, China). High density polyethylene (HDPE) and maleic anhydride polyethylene (MAPE) was purchased from Baoxinnuo Plastic and Chemical Co., Ltd. (Suzhou, China). The size of the EU and HDPE particles was 149 μm (100 mesh). The NaOH (1 M), NaCl, MgCl2, Na2SO4, CaCl2, KCl, NaHCO3, KBr, H3BO3, SrCl2, and NaF (technical pure) were purchased from Zhiyuan Chemical Additives Co., Ltd. (Tianjin, China).
Preparation of EU/HDPE Composites
The EU powder was soaked in 1%, 3%, and 5% NaOH at a temperature of 100 °C for 1 h (solid: liquid = 1:2, v:v). The alkali-treated fibers were separated by filtration and washed with deionized water until the rinsed solution became neutral. The rinsed fibers were dried at a temperature of 90 °C for 24 h, and the HDPE powder and MAPE were dried at a temperature of 90 °C for 12 h in a DHG-9140A electro-thermostatic drum-wind drying oven (Dongmai Scientific Instrument Co., Ltd., Nanjing, China) before the composite preparation step.
The EU fiber, HDPE, and MAPE were mixed with the mass ratio 100:100:3 in an SBH-5L 3D linkage mixer (Xinbao Mechanical and Electrical Industry Co., Ltd., Nanjing, China) followed by hot compression molding at 155 °C, 3.7 MPa for 9 min. All samples were 120 mm in length, 100 mm in width, and between 5 mm and 8 mm thick. They were processed to obtain the required dimensions for characterization after demolding and cooling. The untreated EU fiber reinforced HDPE composite was the control group and labeled as “untreated”; the different concentrations of alkali treated EU/HDPE composites were labeled as “1% NaOH”, “3% NaOH”, and “5% NaOH”, respectively.
Degradation Testing of EU/HDPE Composites in Simulated Seawater Exposure
The simulated sea water was prepared according to the standard ASTM D1141-1998 (2013), and its chemical components are listed in Table 1. NaOH was used to adjust the solution to pH 8. The degradation test of the samples was carried out in a constant temperature water-bath (35 °C) in the HH-600 thermostatic water tank (Bongxi Instrument Equipment Co., Ltd., Shanghai, China) by total immersion in a stagnant solution without agitation. All samples were subjected to 7 d, 14 d, and 21 d exposure followed by air-drying. Then, the composites were sealed and stored for further characterization analysis.
Table 1. Chemical Components of the Simulated Sea Water
Characterization
The morphology of the samples was studied with a HITACHI S-4800 field emission scanning electron microscope (SEM; Tokyo, Japan). The samples were sputter-coated with gold-palladium using an E-1010 ion sputter and operated at 3 kV.
The composites were subjected to 90% relative humidity at a temperature of 23 ± 0.5 °C for 6, 12, 24, 48, 72, or 96 h in an HPX-160BSH-III temperature humidity incubator (CIMO Medical Instrument Manufacturing Co., Ltd., Shanghai, China). The moisture absorption value was calculated from the weight gains after removing excess water on the exterior surface.
The tensile strength, flexural strength, and flexural modulus of samples were measured using a CMT6104 electronic universal testing machine (MST Systems (China) Co., Ltd., Shanghai, China) at a loading speed of 2 mm·min-1. The impact strength of the composites was evaluated using an XJJ-5 beam impact testing machine (Jinjian Testing Instrument Co., Ltd., Chengde, China) at an impact energy of 2 J.
The Fourier transform infrared spectroscopy (FTIR) spectra of the samples were recorded by a Nicolet iS10 FTIR spectrometer (Thermo Fisher Scientific, Shanghai, China) at the range of 4000 cm-1 to 400 cm-1 and resolutions of 4 cm-1 for 16 scans.
The color change of the composites was evaluated using a HP-200 colorimeter according to the CIE 1976 (L*a*b*) color system. Lightness (L*) and chromaticity coordinates (a* and b*) were measured for six replicate samples. L* represents the lightness coordinate and varies from 100 (white) to 0 (dark); a* represents the red (+a*) to green (-a*) coordinate; b* represents the yellow (+b*) to blue (-b*) coordinate. The discoloration (∆E*) was calculated according to the following equation:
∆E*=[(∆L*)2+(∆a*)2+(∆b*)2]1/2 (1)
where +∆L* and -∆L* represent whitening and darkening, respectively; +∆a* and -∆a* represent the color shift toward red and green, respectively; +∆b* and -∆b* represent the color shift toward yellow and blue, respectively. It should be noted that the ∆E* represents the magnitude of the color difference but does not indicate the direction of this difference.
RESULTS AND DISCUSSION
Morphology of EU/HDPE Composites
The SEM images of the untreated and alkali-treated EU fiber-reinforced HDPE composites before and after 21 d of simulated seawater exposure are shown in Fig. 1. Before the exposure, the interface between the EU fiber and HDPE was well jointed and revealed good compatibility, except in the case of the 1% NaOH-treated composite (Fig.1a-1d). With the increase of the NaOH concentration, fewer fibers were exposed and the composites presented smooth and integrated surfaces (Fig. 1c, 1d). Fiber delamination and several fiber fines appeared in the micro-surface of the 1% NaOH-treated sample (Fig. 1b). After the alkali treatment, most of the hemicellulose, lignin, and other soluble materials were removed, and the alkali-treated wood fiber resulted in a rough surface, which allowed the polymer to adhere onto it through mechanical interlocking and improved the interaction between the two phases (Mngomezulu et al.2011). In the lower alkali concentration (≤1%), only a small part of the EU fibers could be bonded with HDPE, and some fibers remained uncovered outside the matrix. After 21 d of simulated sea water exposure, some fibers were pulled out, and fiber breakage (large voids formed by the fibers pulling out of the matrix) was apparent on the fractured surface of the untreated composite (Fig. 1e). Interfacial delamination and small voids emerged on the 1% NaOH sample (Fig. 1f). The surfaces of the 3% NaOH and 5% NaOH composites exhibited well-defined interfaces after 21 d exposure, especially the 3% NaOH sample (Fig. 1g, 1h). The alkali pretreatment could induce dispersion of the fiber bundles into fine fibers, which caused an increase in aspect ratio of the fiber and the effective contact area between the fiber and matrix. Therefore, the alkali treatment of the EU fibers would enhance the adhesive strength between the fiber and HDPE matrix. During the treatment at a high NaOH concentration (≥ 5%), the EU fiber was dispersed into many small pieces, indicating more contact points, but each contact area between the fiber and matrix became smaller. The smaller contact area would be disrupted after 21 d exposure. The fiber pull-outs, fiber breakage, and voids that appeared in the microstructure all indicated the presence of micro-cracks and weak interfacial interaction, which would directly correspond to the degradation of the mechanical properties of the composites.
Moisture Absorption of EU/HDPE Composites
The moisture absorption of the untreated and alkali-treated EU fiber-reinforced HDPE composites before and after 7 d, 14 d, and 21 d simulated sea water exposure are presented in Fig. 2. The moisture absorption in each exposure period, including the untreated sample was similar: with the increased absorbing-time, the moisture content of all composites grew rapidly at first (in 24 h) and then reached saturation gradually. The moisture content before exposure and after 21 d exposure of the untreated and 1%, 3%, and 5% NaOH treated composites increased from 1.41% to 3.23%, 0.99% to 2.4%, 0.83% to 2.24%, and 0.63% to 1.89% when saturated, respectively. The untreated EU/HDPE composite exhibited higher moisture absorption relative to the three alkali-treated composites before exposure and after 7d, 14d, and 21d exposure. And as expected, the order was as follows: untreated > 1% NaOH > 3% NaOH > 5% NaOH. This result demonstrated that the reduction of hemicellulose, lignin, and other hydrophilic components endowed the composite with hydrophobicity, and this behavior increased with NaOH concentration. Under the alkali treatment, the individual fibers were detached from the fiber bundles, and the effective contact area of fibers and matrix was expanded. Hence, the moisture absorption could also indicate the changes in interfacial bonding.
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