Water Absorption, Mechanical, and Crystallization Properties of High-density Polyethylene filled with Corncob Powder
Shiliu Zhu,a Yong Guo,a,* Daowu Tu,a Yuxia Chen,a Shengquan Liu,a,* Wei Li,b and Li Wang a
Corncob powder filled high-density polyethylene (HDPE) composites were prepared by extrusion. The microstructure, water absorption, mechanical properties, and crystallinity of composite at different corncob powder content were investigated. Results demonstrated that when the corncob powder levels were moderate and uniformly dispersed within the HDPE matrix, the powder acted as a reinforcing agent. As the corncob content increased, the water absorption of the resulting composite gradually increased, which adversely affected the composite’s water resistance. Flexural strengths and moduli initially increased with increasing corncob powder levels, and then consequently decreased at higher powder levels; maximum values for flexural properties were achieved at 40% corncob powder content. The composite’s impact strength and toughness weakened with corncob powder addition. The X-ray diffraction and differential scanning calorimetry analyses indicated that when the corncob content increased, the peak crystallization and melting temperatures of the matrix increased and decreased, respectively. Meanwhile, the presence of the corncob restricted the movement and arrangement of the HDPE polymer chains, which affected HDPE crystal growth and causing a decrease in crystallinity.
Keywords: Corncob powder; High-density polyethylene (HDPE); Composites; Crystallinity; Mechanical properties
Contact information: a: Department of Forest Products Industry, Anhui Agricultural University, Hefei 230036, China; b: Department of Suzhou Dongda Wood Industry Co., Ltd, Suzhou 234101, Anhui, China;
Corresponding authors: firstname.lastname@example.org (Yong Guo) and email@example.com (Shengquan Liu)
Research of renewable and recyclable resources derived from biomasses has received increased attention to address the depletion of non-renewable petrochemical resources. China is a large agricultural country that has abundant biomass resources that are available at low costs (Tingyun et al. 2007). These biomass resources can be used to prepare wood-plastic composites (WPCs).
WPCs are a more environmentally sustainable material, which utilize biomass resources. At present, the reinforcing agents utilized for the preparation of wood-plastic composites include: wood and bamboo fibers (Zakikhani et al. 2014; Ansari et al. 2017); jute, hemp, and flax fibers (Bledzki et al. 2015; Haag et al. 2017; Sullins et al. 2017); agriculture straw and forestry waste fibers (Leão et al. 2015; Cisneros-López et al. 2017); rice husks (Aridi et al. 2016); and walnut and hazelnut shells (Ayrilmis et al. 2013; Tufan and Ayrilmis 2016). Ashori and Nourbakhsh (2010) extensively studied the effects of hot-water treatment and particle sizes of fibers, which were obtained from various wood species, on select physical and mechanical properties of WPCs. Panthapulakkal et al. (2006) investigated the mechanical properties of polypropylene composites that contained 30% wheat straw fibers, as well as the effects of chemical treatments of these fibers. Wheat straw fiber reinforced polypropylene composites exhibited significantly enhanced properties compared to virgin polypropylene. However, the strength properties of the composites were less for chemically prepared fiber filled composites. The mechanical, thermal, thermo-mechanical, and dynamic properties of PLA/HSF (poly(lactic acid)/hazelnut shell flour) composites with various compositions have been studied by Balart et al. (2016). Faruk et al. (2012) examined several variables of biomass-plastic composites, which included the fiber types, the environmental conditions, the processing methods, and the fiber modifications; the authors reported that these variables have complex interactions relative to the impact on the performance of the resulting composite.
Corncobs are a by-product waste of corn processing. It is a promising biomass resource in China due to its high abundance and stable availability. The National Bureau of Statistics of the People’s Republic of China indicated that corn production is approximately 200 million tons a year, which affords corncob annual yields around 40 to 50 million tons. In the past, corncobs were regarded as agricultural waste by farmers, and they were used as fuel or for composting. These disposal methods greatly underutilize this biomass resource. But as technology develops, corncobs are starting to be used in biological feed, industrial raw materials, edible fungi, and energy. Much research has been conducted on alternative usages for corncobs. At present, most of this work has focused on its use for the production of xylose (Deng et al. 2016; Yan et al. 2017), furfural (Liu et al. 2017; Zhang et al. 2017), activated carbon (Qu et al. 2015), nanocrystalline cellulose (Silvério et al. 2013; Ditzel et al. 2017), and various nanocomposites (Guo et al. 2015; Mao et al. 2015). However, a high amount of residue and waste water produced in the preparation process of xylose, furfural, and other chemical products, cause environmental pollution. In addition, the post-treatment consumes a high amount of energy and manpower. However, the use of corncob resources to fabricate WPCs is the realization of green production, and contributes to finding sustainable ways to utilize this waste.
Theoretically, corncob is a typical non-wood fiber material with an average mass composition of 35% to 40% hemicelluloses, 17% to 20% lignin, and 32% to 36% cellulose, in addition to a low amount of inorganic ash (Xu 2011). This indicates that the main components of corncob are similar to wood fiber, which makes it is entirely possible to produce wood-plastic composite. For instance, Luo et al. (2017) found that corncob was the more suitable corn fiber for the preparation of corn fiber plastic composites; and the mechanical properties of the composites were affected by the composition in terms of the cellulose, hemicellulose, and lignin fractions. A high cellulose and lignin content improved the mechanical properties, whereas a high hemicellulose would decrease the water resistance and mechanical properties. Panthapulakkal and Sain (2007) studied the surface chemistry and thermal degradation characteristics of corncob fiber to demonstrate the effect of surface chemistry on the end use properties of the composites and the practicability of processing it with thermoplastics. Furthermore, the viability of corncob as reinforcements in HDPE was evaluated as compared to wheat straw and cornstalk. In addition, composites composed of LDPE, PVC plastisol, and different amounts of untreated and pre-treated ground corncob were prepared, and the physical and mechanical properties was studied as compared to the composites filled with eucalyptus wood and brewery’s spent grain (Georgopoulos et al. 2005). In these studies, the difference between the corncob and other plant fibers was dominated, and most of them remained in the laboratory stage and the standard splines by injection molding were mainly used, and there was no upper pilot line or production line. In addition, the mechanical properties of the composites in these studies need further improvement to meet practical application requirements and a comprehensive feasibility analysis is required, including the materials, formula, preparation method, compounding mechanism and product performance. In order to save costs and efficient use of the waste corncob, a high-filled corncob composite is required. Therefore, the authors selected the corncob powder (CP) to prepare corncob powder filled HDPE (CP-HDPE) composites and studied the water absorption, mechanical properties, and crystallization properties of the composites. Furthermore, this experiment was carried out on a pilot-scale production line, which was based entirely on the production mode of existing factories. The results can be directly extended to the actual production of the factory. It will provide important research and production data on the utilization of this non-wood biomass for the production of plastic composites.
Corncob powder (CP) with an average particle size of 180 m was purchased from Yanggu Ruikang Technology Co., Ltd. (Yanggu, China). The particle size distribution of CP is shown in Table 1, which is based on the mass fraction. High-density polyethylene (HDPE; grade: 5000S; the melt flow index (MFI) is 0.9 g/10 min) was obtained from Dongguan Baoyu Plastic Materials Co., Ltd. (Dongguan, China). Lubricant (Westrchem STR-530, recommended for wood flour / fiber-filled polyethylene (PE) systems) and compatibilizer (maleic anhydride grafted polyethylene (MA-g-PE)) were obtained from Shanghai Guangyu Chemical Co., Ltd. (Shanghai, China). Figure 1 shows the corn and its byproducts.
Fig. 1. Corn and its byproducts
Table 1. Corncob Powder Particle Size Distribution
Manufacturing of corncob powder filled HDPE composites
The formulated composites and their corresponding designations are presented in Table 2. The corncob powder was dried prior to its use (dried at 103 °C for 24 h), and had a moisture content of less than 2% (by mass). The required amounts of CP, HDPE, and other additives were blended together in accordance to the formulations of Table 2 in a high-speed mixer (model: SHR-25A; Zhangjiagang Jiahua Plastic Machinery Co., Ltd. (Zhangjiagang, China)) for 20 min at 80 °C; the speed of the rotor is 1200 r/min. Among them, the amount of lubricant (Westrchem STR-530) and compatibilizer (MA-g-PE) is 2% of the total amount of corncob powder and HDPE, respectively. The blended mixture was pelletized with a granulator (model: SWMSZ-S1; the L/D (length to diameter) ratios is 40; Nanjing Sky Win Technology Co., Ltd. (Nanjing, China)). The pelletizer had temperature profiles of 150, 180, 180, 175, and 160 °C for the five temperature zones, and operated with a screw rotation speed of 400 rpm. The resulting pellets were added to a second single screw extruder (model: SWMSD-S1; L/D =30; Nanjing Nanjing Sky Win Technology Co., Ltd. (Nanjing, China)), which operated with the temperature profiles 135, 145, 150, 155, and 155 °C for the five temperature zones and a screw rotation speed of 6 rpm. The extruded material was injected into a mold at a temperature of 140 °C. Figure 2 illustrates the process schematic for fabricating the composites.
Table 2. Formulated CP-HDPE Composites and Their Designations
Fig. 2. Schematic of the fabrication process of CP-HDPE composites
Scanning electron microscopy (SEM) was performed using a Hitachi S-4800 microscope (Tokyo, Japan). The SEM operated with an accelerating voltage of 1.0 kV. The impact fractured surfaces of the composite samples were coated with a thin layer of gold prior to the SEM analysis (Kaewtatip and Thongmee 2012; Dikobe and Luyt 2017).
X-ray diffraction (XRD) analyses of the composites were obtained utilizing a MSAL XD-3 X-ray diffractometer (Beijing Purkinje General Instrument Co., Ltd. (Beijing, China)). The instrument used CuK radiation ( = 1.5406 Å; 36 kV; and 20 mA) to scan the samples at a rate of 1°/min; samples were scanned over the range of 2θ = 5° to 80°. The data was processed by MDI Jade 6.0 software and the crystallite size was calculated from the Scherrer formula (Babiker et al. 2010),
D = K/βcosθ (1)
where D is the crystallite size, K is Scherrer constant (K=0.89), β is the full width at half maximum, and θ is the diffraction angle.
Differential scanning calorimetry (DSC) analyses of the CP-HDPE composites were performed using a DSC 200F3 differential calorimeter (Netzsch, Germany), which was fitted with a liquid nitrogen cooling system. The heating and cooling steps were conducted under a N2 atmosphere. Samples were heated from –25 °C to 200 °C using a heating rate of 10 °C/min, then held at 200 °C for 5 min (to eliminate the thermal history), then cooled to 25 °C using a cooling rate of 5 °C/min, and finally heated again to 200 °C using a heating rate of 10 °C/min (Hristov and Vasileva 2003). The amount of sample used during the DSC scans was between 5 and 7 mg. Samples were hermetically sealed in a DCS device prior to being placed into the instrument. The crystallization (Tc) and melting (Tm) temperatures were determined during crystallization and endothermic melting, respectively, whereas the heat of fusion (∆H) was calculated from the peak area of the DCS thermogram. The degree of crystallinity (Xc) was calculated from the DSC thermogram using the following expression (Hristov and Vasileva 2003),
Xc = ∆H/∆H0 × 100% (2)
where ∆H is the heat of fusion of the CP-HDPE composite (J/g), and ∆H0 is the heat of fusion of 100% crystalline HDPE, which is taken to be 293 J/g according to Na et al. (2002).
Physical and mechanical properties
Water absorption tests were conducted with 50 mm×50 mm×5 mm sized samples and determined in accordance to Chinese standard GB/T 1462-2005 “Test Methods for Water Absorption of Fiber Reinforced Plastics.” Adsorption tests were replicated five times; reported values for the samples were the averages from the replicates. The specimens were oven-dried at 50 ± 2 °C for 24 h, which was then cooled to room temperature in a desiccator; the conditioned mass of the specimen (W1) was measured with an accuracy of ±0.001 g. The test specimen was then immersed in a container of distilled water at room temperature (23 ± 1 °C) for 24 h. Afterwards, the test specimen was carefully dried with a dry cloth and the wet mass (W2) was determined immediately to the nearest 0.001 g. The soaked sample was oven-dried at 50 ± 2 °C for 24 h, and then cooled to room temperature in a desiccator; the mass of the soaked sample after drying and conditioning (W3) was measured. The absolute water absorption of the specimen (Wa) was determined by the following equations:
The percentage increase in specimen mass (W) during water immersion was calculated as:
W = Wa/W1 × 100% (4)
Three point flexural tests (sample size: 120 mm×10 mm×5 mm) and Charpy un-notched impact tests (sample size: 80 mm×10 mm×5mm) were measured using a universal testing machine (model: AG-Xplus; Shimadzu, Tokyo, Japan) and a simple beam impact testing machine (model: XJJ-50; Chengde Juyuan Testing Equipment Manufacture Co., Ltd., Chengde, China), respectively, in accordance to Chinese standards GB/T 9341-2008 (“Plastics – Determination of Flexural Properties”) and GB/T 1043.1-2008 (“GB/T 1043.1-2008 Plastics – Determination of Charpy Impact Properties – Part 1: Non-Instrumented Impact Test”). Mechanical test were repeated ten times and the reported values are the averages of the replicates average.
RESULTS AND DISCUSSION
Figure 3 depicts the typical fracture surface images from the SEM analyses of the CP-HDPE composites, where: (a) and (b) correspond to 30% CP-HDPE; (c) and (d) correspond to 40% CP-HDPE; and (e) and (f) correspond to 70% CP-HDPE. As shown in Fig. 3(a), the corncob powder was heterogeneously distributed within the HDPE matrix as small pockets (i.e., “island” shapes) when the corncob powder substitution levels were low (i.e., 30%). The image shows apparent pores caused by poor compatibility, and it was easy to distinguish the interphase between the corncob powder and the HDPE. By contrast, the distribution area of the corncob powder was apparently increased, and the powder was well coated by the HDPE matrix (Fig. 3(c) and 3(d)). The interface interaction enhanced between the corncob powder and the HDPE, and the interface diffusion and mechanical interlocking effects improved. As the corncob powder substitution level increased above 70% in the composites, its distribution throughout the HDPE matrix became more heterogeneous (Fig. 3(e), (f)). The corncob powder exhibited an agglomeration effect and accumulated on the HDPE matrix due to the intermolecular effect of hydroxyl groups. The interfacial interaction between corncob powder and HDPE matrix is weakened and the compatibility is poor. From these observations, it was apparent that an optimum substitution level of corncob powder content existed where the fibers and HDPE matrix had intimate contact and mechanical interlocking. In these circumstances, the interfaces between the two phases were strong.