Abstract

Materials

Industrial wood particles

We used industrial wood particles (composed of 80% Pinus kesiya wood and 20% other wood) with a moisture content (MC) of 3 to 5 percent, provided by the Xinfeilin Wood-Based Panel Co. Ltd., Kunming, Yunnan, China, and with the particle size shown in Table 1.

Table 1. The particle size.

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Hemp shiv particle preparation

We cut the whole hemp shiv or hemp shiv from different stem parts (0 to 20, 100 to 120, 200 to 220 cm above the base of the hemp stem) to 3 to 5 cm in length, 0.5 to 1.0 cm in width, and 3 to 12 mm in thickness using a chipping machine at the Wood-Based Panel Laboratory. We then rechipped at the same location with a hogging machine to create smaller sized particles of 5 to 8 mm in length, 3 to 5 mm in width, and 2 to 3 mm in thickness, and then oven-dried the particles until they had a MC of 3 to 5 percent at a constant 100°C ± 2°C temperature; the particle sizes are shown in Table 1.

Particleboard manufacturing

We followed the project details of particleboard manufacturing methods shown in Table 2.

Table 2. The project of particleboard manufacturing.

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Particleboard testing

The panels were tested for their physical and mechanical properties according to the Chinese local standard (GB/T 17657‐1999; Chinese National Standardization Technical Committee for Wood-Based Panel 1999) for wood-based panels. Target properties included modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), and thickness swelling (TS). After the panels were stored at 20°C and 65 percent relative humidity for 48 hours, they were cut into samples. Four samples with dimensions of 250 by 250 by 10 mm were used for the MOR and MOE tests, and six samples with dimensions of 50 by 50 by 10 mm were used for the IB test and were tested with a Universal Testing Machine (AG-50KNI, made in Japan). In addition, six samples of every panel for the TS test, with dimensions of 50 by 50 by 10 mm, were placed in conditioned water (with temperatures at 20°C ± 2°C and pH at about 7.0) for 2 and 24 hours.

Mixture of hemp shiv/wood–based particleboard panels

The IB of the panels, from 1.48 to 1.88 MPa, is shown in Table 3. The addition of hemp-stalk particles resulted in an increase of IB under all conditions, especially for the 3:2 ratio of hemp stalk to wood; the IB values were significantly higher than those of other particleboard, and all the IB values met the IB requirements (0.4 MPa) of the Chinese local standard for interior decoration and furniture using particleboard (GB/T 4897.3‐2003; Chinese National Standardization Technical Committee for Wood-Based Panel 2003). The bending properties of the panels (MOR and MOE) also improved after the hemp stalk was added into the particleboard (see Table 3) and were higher than those required by the Chinese standard for interior decoration and furniture using particleboard. The MOR and MOE reached the highest values when the ratio of industrial hemp stalk to wood particles was 1:1. In addition, the mechanical properties of the particleboard panels made with industrial hemp shiv particles were better than those of the panels made with castor stalks under the same manufacturing parameters (Table 4). Also, the TS of the particleboard panel could not satisfy the requirements of the Chinese standards for particleboard panels, since there was no hydrophobing agent added to the panels.

Table 3. Properties of industrial hemp shiv/wood particle–based particleboard panels.^a

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Table 4. Properties of particleboards made with different raw materials.^a

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First, the chemical composition of raw materials can greatly influence the properties of panels; changes in the chemical composition of wood or nonwood materials can alter the distribution of the resin on the wood surface (Hosseinaei et al. 2011, Nikvash et al. 2013). There were some differences between the chemical composites of hemp stalk, wood particles, and castor stalk. In addition, there was a definite difference between the physical and mechanical properties of the hemp stalks, wood particles, and castor stalks.

The hemp shiv is a light nonwood material with lower mechanical properties (including elastic modulus and hardness) in fiber cell walls, as tested by nanoindentation, compared with those of fiber cell walls in castor stalks (Li et al. 2013, 2014; see Table 4) and possess a density of only 0.25 g/cm³, which is 64 percent of that of castor stalk (see Table 4). Therefore, the fiber cell wall is easy to compress due to its low mechanical properties (Hosseinaei et al. 2011), just as when the wood-based panel was made, the mat needed some moisture content (Cai et al. 2006). Moisture content can improve the plasticity properties and can also reduce the mechanical properties of fiber cell walls (Yu et al. 2010). In addition, the mechanical properties of the hemp shiv–based particleboard panels with different parts of the stem were negatively correlated with the mechanical properties of the hemp shiv fiber cell walls (Table 5) when different hemp stem parts with similar chemical characters were used in making the particleboard panel.

Table 5. Properties of hemp shiv particleboards made with different stem parts.^a

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Thus the low mechanical properties of fiber cell walls and the low density of hemp shiv both allowed the hemp shiv pieces to form a good interface in the particleboard panels. The microstructures of the panels and stalk fibers in the castor stalk–based particleboard, tested by scanning electron microscope, are shown in Figures 1A and 1B (Li et al. 2011), and the microstructure and hemp shiv fibers in the hemp shiv–based particleboard are shown in Figures 1C and 1D. Based on the Figure 1 images, the interface was poor, and the fiber cell wall was broken in the castor stalk–based particleboard panels, reducing the mechanical properties of the fiber itself. In contrast, in the particleboard with hemp shiv particles, the interface was good, and the fiber cell wall was whole, preventing damage to the mechanical properties of the hemp fiber itself. These results confirmed that the hemp shiv was easy to compress and was capable of forming a good glue interface with the resin, keeping the mechanical properties of hemp shiv fiber intact because of the lower mechanical properties of the fiber cell wall. The fiber cells with lower mechanical properties are soft and were therefore easy to compress, whereas fiber cells with higher mechanical properties are hard and when compressed at high pressure, some cells were broken and could not retain their strength.

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Hemp shiv–based particleboard panels of different density

The chemical, mechanical, and physical properties of hemp shiv in fiber cell walls were all advantages in forming a good interface between the fibers in the particleboard panels discussed above. The mechanical properties of hemp shiv–based particleboard panels with different densities are shown in Table 6. As the density of the panels increased from 0.54 to 0.75 g/cm³, the properties of hemp shiv–based particleboard panels also increased. When the density of the panels is 0.54 g/cm³, the properties of particleboard panels can satisfy the local Chinese standards for common particleboard panels, and when the density is 0.65 g/cm³, the properties of panels, including IB, MOR, and MOE, can satisfy all of the requirements of the local Chinese standards for particleboard used for interior decoration and furniture. In a word, the hemp shiv is an excellent nonwood material, and it can be used to make the low-density particleboard panels of 0.54 g/cm³.

Table 6. Mechanical properties of hemp shiv–based particleboards with different densities.^a

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