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Fang, C.-H., Cloutier, A., Jiang, Z-H.., He, J.-Z., and Fei, B.-H. (2019). "Improvement of wood densification process via enhancing steam diffusion, distribution, and evaporation," BioRes. 14(2), 3278-3288.

Abstract

Mechanical densification treatments make it possible to increase the density of low- or moderate-density woods, and thus a high mechanical strength of densified wood and high-value products can be obtained. The authors’ previous treatments showed that the diffusion and distribution of steam and the release of vapor inside densified wood were prevented to some extent during thermo-hydro-mechanical (THM) densification, causing the occurrence of protrusions, carbonization, blisters, and blows. This study aimed to overcome these problems. Based on the authors’ previous THM densification, different materials, such as fabric, metal mesh, metal foam, and sintered metal mesh laminate (SMML), were used to improve the process. Densification was tested on different wood species. The results showed that SMML was the preferable material for THM densification through enhancing diffusion and distribution of steam, and evaporation of moisture inside wood. No protrusion, carbonization, blisters, or blows were found after densification with SMMLs. The densified wood specimens showed uniform color and a neat surface.


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Improvement of Wood Densification Process via Enhancing Steam Diffusion, Distribution, and Evaporation

Chang-Hua Fang,a,* Alain Cloutier,b Ze-Hui Jiang,a Jia-Zhong He,c and Ben-Hua Fei a,*

Mechanical densification treatments make it possible to increase the density of low- or moderate-density woods, and thus a high mechanical strength of densified wood and high-value products can be obtained. The authors’ previous treatments showed that the diffusion and distribution of steam and the release of vapor inside densified wood were prevented to some extent during thermo-hydro-mechanical (THM) densification, causing the occurrence of protrusions, carbonization, blisters, and blows. This study aimed to overcome these problems. Based on the authors’ previous THM densification, different materials, such as fabric, metal mesh, metal foam, and sintered metal mesh laminate (SMML), were used to improve the process. Densification was tested on different wood species. The results showed that SMML was the preferable material for THM densification through enhancing diffusion and distribution of steam, and evaporation of moisture inside wood. No protrusion, carbonization, blisters, or blows were found after densification with SMMLs. The densified wood specimens showed uniform color and a neat surface.

Keywords: Wood densification; Heat treatment; Steam treatment; Thermo-hydro-mechanical densification; Wood compression

Contact information: a: State Forestry Administration (SFA) and Beijing Co-built Key Laboratory of Bamboo and Rattan Science & Technology, SFA, Department of Biomaterials, International Centre for Bamboo and Rattan, 8 Futong East Street, Chaoyang District 100102, Beijing, China; b: Renewable Materials Research Centre (CRMR), Université Laval, Quebec City, Canada; c: Zhejiang Jiazhong Wood Industry Co., Ltd., Huzhou City, Zhejiang, China;

* Corresponding authors: cfang@icbr.ac.cn; feibenhua@icbr.ac.cn

INTRODUCTION

Generally, high-density wood species are preferable for many engineering structures and applications due to their high mechanical strength. However, high-density wood resources are limited and usually expensive. Densification treatments make it possible to increase the density of low- or moderate-density woods, as well as to obtain a specific strength of densified wood higher than that of most structural metals and alloys. These characteristics make it a low-cost, high performance, and lightweight alternative (Song et al. 2018). Many densification processes, including mechanical compression and chemical impregnation, have been attempted and developed (Inoue et al. 1993a,b, 2008; Higashihara et al. 2000; Kamke 2006; Boonstra and Blomberg 2007; Fukuta et al. 2008; Gabrielli and Kamke 2008; Rautkari et al. 2011; Belt et al. 2013; Laine et al. 2013; Li et al. 2013; Fu et al. 2017; Kariz et al. 2017). Compared to mechanical compression, chemical treatments affect the nature and sustainable character of wood and are usually more expensive (Navi and Heger 2004).

Different mechanical densification treatments have been reported for over a century (Kollmann et al. 1975; Fang et al. 2012c; Sandberg et al. 2013; Song et al. 2018). Many densification treatments have been performed through compression combined with heat. Such methods are also called thermo-mechanical (TM) densification. However, this type of densified wood is unstable with a high percentage of compression set recovery (Navi and Heger 2004; Laine et al. 2013, 2016; Kariz et al. 2017). Recently, Song et al. (2018) reported a two-step densification process involving the partial removal of lignin and hemicellulose from natural wood by boiling in a chemical solution followed by hot-pressing. They obtained densified wood with extremely high density (1300 kg/m3). However, for a small-size sample (120 mm × 44 mm × 44 mm), their two-step process took a long time (more than 31 h). Compared to TM densification, compression combined with heat and steam treatment, also called thermo-hydro-mechanical (THM) densification, is a more efficient way to achieve more dimensionally stable densified wood. Many THM, or similar densification processes, in an open or sealed press system have been reported (Navi and Girardet 2000; Kamke 2006; Li et al. 2013; Popescu et al. 2014; Fu et al. 2016, 2017). Some of them still cannot solve the problem of dimensional stability. Other processes take a long time or are complex. Furthermore, most of them have dealt with small-size wood samples. All these problems and limitations could be due to the difficulty of distributing steam on large wood surfaces and vapor release from core-densified wood due to the high pressure of the hot press platens. Recently, the authors’ team developed a relatively simple THM densification process (Fang et al. 2011, 2012a,b,c; Fu et al. 2016, 2017; Cruz et al. 2018) that can deal with large-size wood samples with high efficiency. Steam treatment showed a positive effect on wood softening, time saving, and dimensional stability for wood densification (Inoue et al. 1993a; Navi and Girardet 2000; Navi and Heger 2004; Gabrielli and Kamke 2008; Fang et al. 2012c). However, diffusion and distribution of steam, and the release of vapor inside densified wood were still prevented to some extent. The objective of this study is to improve the densification process by enhancing steam diffusion and distribution on the samples’ surface and its release from the core of the densified wood samples.

EXPERIMENTAL

Materials

The THM densification treatment was tested on trembling aspen (Populus tremuloides), hybrid poplar clone 15303 (Populus maximowiczii × Populus balsamifera), sugar maple (Acer saccharum), red oak (Quercus rubra), black cherry (Prunus serotina), and yellow birch (Betula alleghaniensis) obtained from Quebec, Canada. For aspen and hybrid poplar, veneers of 700 mm × 700 mm were used for densification. The nominal thicknesses of aspen and hybrid poplar veneers were 3.2 mm and 4.3 mm, respectively. For the other species, thin-sawn strips were used for the tests. The nominal thickness of the strips varied from 3.4 mm to 6.0 mm. The length and width were 700 mm and 90 mm, respectively. They were conditioned at 20 °C and 60% relative humidity before treatment.

Methods

Densification treatments- Previous THM densification treatments

The general densification process was described in previous work (Fang et al. 2011, 2012a,b,c). A Dieffenbacher steam injection press (Dieffenbacher North America Inc., Windsor, Canada) with dimensions of 862 mm × 862 mm was used for THM densification (Fig. 1). Holes were distributed on both the upper and lower platens at 32-mm intervals. Holes with a diameter of 1.5 mm were used for steam injection and venting. The specimens were compressed, using the procedure shown in Fig. 2, from initial to target thickness at a maximum hydraulic pressure of 4.5 MPa to 9.0 MPa. Platens were then maintained at the same position during post-treatment. Steam was continuously injected at a line pressure of 550 kPa during the whole compression and maintaining process. At the end, steam injection was stopped and steam was vented through the holes in the platens. Tests were performed at 180 °C, 200 °C, and 220 °C, respectively. Four different total treatment durations were tested: 10 min, 15 min, 20 min, and 30 min.

Fig. 1. Steam injection hot press used for the densification treatment: (a) 862 mm × 862 mm hot press, (b) steam injection and venting holes on both the upper and lower platens

Fig. 2. Schematic diagram of the densification process: pre-treatment (T0 to T1), compression (Tto T2), and post-treatment (T2 to T3)

THM densification treatment with fabric

The THM densification process was performed as described above in the previous treatment. Specimens were covered with a special fabric, on both the upper and lower sides (Fig. 3a) which is steam-permeable and heat-resistant. It is made of Nomex® III A and manufactured by Dupont™ (Chatham, Canada) with a fabric thickness of 0.5 mm. Densification was tested at 180 °C, 200 °C, and 220 °C, respectively.

Fig. 3. Both sides of the sugar maple specimens were covered with steam-permeable and heat-resistant fabric before densification (a), and protrusions surrounded with dark color were found after densification (b)

Fig. 4. Both sides of red oak specimens were covered with copper wire mesh before densification (a), and the specimens’ surface became rough after densification (b)

THM densification treatment with metal mesh

Instead of fabric, specimens were covered with a metal mesh on both sides and then densified using the same densification process. The mesh was made of copper wire as shown in Fig. 4a. The thickness of the mesh was approximately 2 mm. The tests were performed at 200 °C.

THM densification treatment with metal foam

The same THM densification process was used. Both sides of the specimens were covered with 2-mm-thick metal foam (MTI Corporation, Hefei, China) (Fig. 5a). A metal foam is a cellular structure consisting of a solid metal with pores comprising a large portion of the volume. The pores can be sealed (closed-cell foam) or interconnected (open-cell foam). Open-cell metal foam, also called metal sponge, can be used in heat exchangers, energy absorption, flow diffusion, and lightweight optics. In this study, open-cell metal foam was used due to its high porosity to ensure steam permeability. Two different metal foams were tested. One was made of nickel and the other one was made of steel. The tests were performed at 200 °C.

Fig. 5. Both sides of specimens were covered with open-celled metal foams with interconnected pores (a), specimens’ surface became very rough and the metal foams stuck to wood after densification (b and c)

Fig. 6. Five-layer SMML used to cover wood specimens

THM densification treatment with sintered metal mesh laminate

Two pieces of special sintered metal mesh laminate (SMML; Beijia Filter Equipment Co., Ltd., Anping, China) were used to cover both sides of the specimens and the same densification process was performed. The SMML is a porous metal composite made from multilayer stainless steel wire mesh and sintered into one metal panel (Fig. 6). It is heat-resistant and anti-corrosive with high mechanical strength and wide filter rating ranges.

The SMML is a new, fine material usually used for purification and filtration of liquid and gas, separation and recovery of solid particles, control of airflow distribution, and enhancement of heat and mass transfer. It is easy to clean and reusable. In this study, SMMLs consisting of five layers of metal weave mesh, as shown in Fig. 6, were used and the thickness was 1.5 mm. The SMMLs were placed on the press with the rough reinforcing layer towards the platens and the fine smooth protective layer towards the wood specimens. The tests were performed at 200 °C and 220 °C. To test the reusability of SMML, hundreds of tests were performed.

RESULTS AND DISCUSSION

Previous THM Densification Treatments

After densification treatment, protrusions were found on all of the densified specimens at all the corresponding spots of steam injection holes (Fig. 7). The holes on the platens were also found plugged with wood particles. These phenomena were caused by the high mechanical pressure of the platens during the THM densification process. At a high temperature with steam treatment, the specimens were softened, which facilitated the protruding into the holes in the platens. The plugging of the holes might have blocked the injection and venting of steam to a great extent during densification. Furthermore, the protrusions on the densified specimens needed a further sanding or planing process to remove them. In addition, the spots with protrusions were not densified as elsewhere on the same specimens. Thus, the spots with protrusions could be weak points of densified wood.