Abstract
To effectively utilize sawmill slab residues, self-manufactured combing equipment was used to transform residues into individual bundle sticks. The scrimber was prepared from Chinese fir by hot pressing the resulting bundle sticks. In this study, the combing process and effects of density on the mechanical and physical properties of the scrimber were investigated. The morphology of the scrimber was tested with computed tomography. The results showed that a rotary speed of 120 rpm for the coarse roller and a rotary speed of 360 rpm for the fine roller combed along the grain were the optimum combing parameters to prepare a uniform individual bundle of sticks. The moisture content of the feeding sawmill residues was above 30%. The scrimber prepared via bundle sticks is a new type of structural scrimber, and the density of the scrimber influenced the physical and mechanical properties. As the scrimber density increased, the modulus of rupture, modulus of elasticity, and internal bond gradually increased, while the 24-h thickness swell decreased. The studied method enhanced the density and improved the physical and mechanical properties. The striped structural characteristic of the scrimber was evident, and the density distribution varied for different faults.
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Preparation and Characterization of Combed Sawmill Slab Residues from Chinese Fir and its Scrimber
Chungui Du,a,b,* Xiaoling Yao,a Yating Hua,a and Qiuli Huang a
To effectively utilize sawmill slab residues, self-manufactured combing equipment was used to transform residues into individual bundle sticks. The scrimber was prepared from Chinese fir by hot pressing the resulting bundle sticks. In this study, the combing process and effects of density on the mechanical and physical properties of the scrimber were investigated. The morphology of the scrimber was tested with computed tomography. The results showed that a rotary speed of 120 rpm for the coarse roller and a rotary speed of 360 rpm for the fine roller combed along the grain were the optimum combing parameters to prepare a uniform individual bundle of sticks. The moisture content of the feeding sawmill residues was above 30%. The scrimber prepared via bundle sticks is a new type of structural scrimber, and the density of the scrimber influenced the physical and mechanical properties. As the scrimber density increased, the modulus of rupture, modulus of elasticity, and internal bond gradually increased, while the 24-h thickness swell decreased. The studied method enhanced the density and improved the physical and mechanical properties. The striped structural characteristic of the scrimber was evident, and the density distribution varied for different faults.
Keywords: Sawmill slab residues; Chinese fir; Combing processing; Wood bundle sticks; Scrimber; Properties; Computed tomography (CT)
Contact information: a: School of Engineering, Zhejiang A&F University, Lin’an 311300, China; b: National Engineering and Technology Research Center of Wood-based Resources Comprehensive Utilization, Lin’an, Hangzhou, Zhejiang Province, 311300, China;
* Corresponding author: chunguidu@163.com
INTRODUCTION
Chinese fir is a softwood with the largest cultivated area and softwood yield in China (Sheng 2018). A considerable number of small-diameter logs has been used in the core-board for blockboards and glulams. However, the resulting sawmill slab residues are often wasted, which has consequently decreased Chinese fir utilization, and there is not a current way to efficiently utilize these residues. Few researchers have explored the utilization of sawmill slab residues from Chinese fir (Jin et al. 2003; Liu et al. 2005; Du et al. 2008a,b; Zhou et al.2012). A new method developed through this research used a combing process to enhance the utilization efficiency of the sawmill slab residues. Feedstocks were combed into wood bundle sticks after the residues were compressed into a net feedstock. The wood bundle sticks had a noticeable structural difference when compared with veneers in plywood, fibers in fiberboard, and flakes in particleboard (Jin and Ma 1998; Mei et al. 2012; Zhang et al. 2012; Zhou et al. 2012). This was considered to be a new type of unit in wood-based panels.
Scrimber, which consists of wood bundles with a network structure, provides a new approach to improve wood utilization (Jin and Ma 1998; Shang et al. 1998). However, technological barriers such as the need for specialized equipment, the wood bundle drying process, and orientation matting, have led to the failure of scrimber industrialization (Ma 2011; Yu and Yu 2013). Among them, the irregularities of the network structure and the large size differences of the network wood bundles appear to be the main reasons that interfered with unit operations in industrialized production, such as the drying process, gluing, and forming. Therefore, in the present work, Chinese fir sawmill slab residues were combed into the single wood bundle sticks, and prepared the scrimber by using them as the component units. These steps addressed problems associated with the network-like character of wood bundles. Recently, researchers have specifically focused on scrimber (Zhao and Zhao 2006; Wu et al. 2014; He et al. 2016; Zhang et al. 2016, 2018), while little work has focused on the preparation of scrimber viasawmill slab residues. Therefore, this study aimed to investigate the combing process and properties of the Chinese fir sawmill slab residues scrimber, and the morphology was tested by computed tomography (CT). This method has promising potential for scrimber industrialization.
EXPERIMENTAL
Materials
Sawmill slab residues were harvested from the edges of Chinese fir logs (Zhejiang Shenghua Yunfeng new material Co., Ltd, Deqing, China) with a small diameter, the dimensions were 103 mm × approximately 40 mm to 55 mm × approximately 10 mm to 25 mm (length × width × thickness), and an arch-shaped cross section (Fig. 1). The moisture content (MCs) of the air-dried residues and as-received residues were 15.3% and 36.1%, respectively. Water-based phenol-formaldehyde resin was purchased from Bamboo Scrimber Flooring Co. (Anji, China) and had a pH of 10.50 and a solids content of 23.7% after dilution.
Fig. 1. Appearance of the Chinese fir sawmill slabs
Methods
Combing process
Figure 2 shows the structural schematic diagram and running mechanism of the combing equipment that was manufactured in the laboratory.
Fig. 2. Structural schematic diagram and running mechanism of the combing equipment: (1) feedstock (sawmill slabs residue), (2) compression rollers, (3) and (5) spring controllers; (4) combing rollers, and (6) combing knives
The essential structure of the combing device consisted of three pairs of rollers, including compression rollers with a shallow groove line, coarse rollers with less distributed combing knifes, and fine rollers with densely distributed knives. These three roller pairs corresponded to the preparation of the raw feedstock, preliminary combing process, and further combing process, respectively. The combing direction included combing along the grain of the Chinese fir (feeding direction was parallel to the clockwise cutting direction), combing against the grain (feeding direction was parallel to the counterclockwise cutting direction), and a combination of both combing methods. The optimum combing process was obtained by controlling the rotary speed of the coarse and fine rollers, calculating the strip yields, and observing the morphology.
Morphology of the bundles
The micro-structural morphology of the wood bundle sticks with MCs of 15.3% and 35.7% was imaged via scanning electron microscopy (SEM; SS-550, Shimadzu Co., Kyoto, Japan) with an accelerating voltage of 5 kV and magnification of 1000x.
Preparation and characterization of the scrimber
An optimal combing process was used to prepare bundles with the dimensions 450 mm × 6.9 mm × 3.9 mm (length × width × thickness). The bundles were dried until they reached a MC between 8% and 10%, and were then impregnated with phenol-formaldehyde resin (PF) for 7 min. The resulting bundles were dried until the MC was between 12% and 15% after removing the extra PF. They were then weighed to determine the scrimber densities, which were set to 0.7 g/cm3, 0.8 g/cm3, and 0.9 g/cm3, respectively. The weighed bundles were oriented along the grain in the mold and were hot pressed to prepare the scrimber. After placing the scrimber at room temperature conditions, the thickness swelling (TS; 24 h and 63 °C), modulus of rupture (MOR), modulus of elasticity (MOE), and internal bond (IB) of the scrimber were tested according to the standard GB/T17657-2013(General Administration of Quality Supervision, Inspection and Quarantine 2014). Each set of process conditions was repeated three times, and the properties of six samples were tested, and the average properties were taken.
Test faults structure of the scrimber
The morphology of the scrimber with a density of 0.8 g/cm3 was tested with a 64 Slice Helical CT machine (TSX-101A, Toshiba Co., Kyoto, Japan). The horizontal (surface, core, and bottom layer) and vertical faults (front, middle, and tail section) of the scrimber were scanned at equal intervals.
RESULTS AND DISCUSSION
Combing Results
Table 1 shows the experimental design, stick yields, and visual observations of the geometric shapes.
Table 1. Combing Processing Design and Results
A: cut with the grain, R: cut against the grain
The rotary speed of the coarse and fine rollers had a noticeable influence on both the stick yield and morphology. The stick yield increased as the rotary speed increased. Under the conditions of combing along the grain and a coarse roller speed of 60 rpm, the stick yield at a 360-rpm fine roller speed was 30% higher than that at a 180-rpm fine roller speed. Under the conditions of combing with the grain and a 360-rpm fine roller speed, the stick yield increased from 82% to 91% with an increase in the coarse roller speed from 60 rpm to 120 rpm.
Additionally, the enhanced rotary speed of the coarse and fine rollers led to more uniform geometric shapes. As a result, the optimum rotary speeds were 120 rpm and 360 rpm for the coarse and fine rollers, respectively.
The specific morphology of the combed bundles after being cut with the grain and against the grain are shown in Fig. 3.