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Vertical density profiles in thermally compressed balsam fir wood.


Factors influencing the vertical density profiles (VDPs) in thermally compressed balsam fir wood were investigated. The shape of the VDP depends on heat transfer and moisture re-distribution during hot-pressing, which in turn depends on press closing rate, wood initial moisture content, and the proximity to the end grain. Three typical VDP shapes could be obtained, with higher surface density, higher core density, or uniformly high density through the thickness. The VDP should affect the mechanical and physical properties of compressed wood allowing it to be engineered for its intended end use.


High temperature compression of wood aims to improve its strength and surface properties by physical and mechanical densification. Specifically, thermally compressed wood known as "Staypak" (Seborg et al. 1945, Stamm 1964), compressed wood with phenolic resin (PF) pretreatment known as "Compreg" (Stamm and Harris 1953, Stamm 1964), surface densified wood (Tarkow and Seborg 1968) and so forth were studied systematically several decades ago. But the problems such as the low efficiency of hot-pressing and the instability of the wood compression limited the wide development of such wood products. Since the 1980s, wood compression has gained new interest, especially in Asia, in order to utilize some fast-growing and low-density species in a new way. The research has been mainly focused on the formation and the permanent fixation of wood compression set (Iida et al. 1984; Inoue et al. 1993a; Norimoto 1993, 1994; Dwianto et al. 1997, 1999; Wang and Zhao 1999; Navi et al. 2000; Navi and Girardet 2000; Wang et al. 2000). The compression methods include not only lumber compression, lumber surface compression (Inoue et al. 1990, 1991), but also log compression (Ito et al. 1998), sometimes with resin or other chemical impregnation before compression. For these more recent studies, the target wood density was usually lower and the compressed wood could reach wood compression set and even permanent fixation more quickly under certain treatment conditions compared with the earlier research.

The thermal compression of solid wood is usually considered to be analogous to hot-pressing wood-based composites, except that it takes much longer to achieve solid wood compression set without the bonding effect of resins. The density gradient through the thickness of wood-based composites (such as oriented strandboard [OSB] and medium density fiberboard [MDF]) typically shows surface layers with higher density and core layers with lower density. The density distribution is influenced by the combined effects of temperature, moisture, compaction pressure, resin curing, and other factors during hot-pressing and affects the mechanical and physical properties of the products (Strickler 1959, Kamke and Casey 1988, Wang and Winistorfer 2000). Due to the differences in material properties and hot-pressing variables compared to wood composites processing, compressed solid wood could present a different density distribution. This investigation focuses on the effects of hot-press closing rate, wood initial moisture content (MC), sample size, and other factors on the vertical density profiles (VDPs) of compressed fir wood. In a companion study the effects of grain orientation and surface plasticizing methods on the VDPs of compressed balsam fir and spruce were investigated (Wang and Cooper 2004). Future work will evaluate the effect of VDPs on mechanical and physical properties of such compressed wood products.

Materials and methods

Commercial balsam fir boards (Abies balsamea [L.] Mill.) nominal 1 by 6 inches (actual dry dimensions 18 [+ or -] 1 mm thick and 135 [+ or -] 2 mm wide) were used for this investigation. The samples used to study an individual effect were from the same board or from boards with similar density and grain orientation. Since moisture movement and loss from wood under thermal compression is expected to affect the VDPs, it was necessary to confirm whether small samples could be used to approximate the effects on larger wood specimens (sample size effect). In addition, the effects of press closing rate (press closing time included preheat time when the samples contacted the press platens without compression and the real closing time) and wood initial MC were investigated (Table 1). The determination of variables was mainly based on preliminary experiments. A 300-mm by 300-mm laboratory hot-press, equipped with stops to ensure approximately 40 percent thickness reduction, was used to compress the wood. The maximum compression pressure ranged from 5 to 8 MPa depending on the sample and pressing conditions. It was found that the total pressing time had little effect on VDPs once the MC of wood was low enough (close to ovendry state) to lead to the glassy state and the wood compression set (Iida et al. 1984, Norimoto 1993). In this report, samples were held at the stop thickness for 60 minutes at a platen temperature of 180[degrees]C. When the press was opened, the samples did not show obvious recovery, but the thickness of wood varied slightly, especially after the samples were conditioned at ambient condition to gain some moisture from the air (the equilibrium MC of wood was around 6%). Usually, the higher the plasticization during the pressing and the lower the MC at press opening, the lower the wood thickness. The larger specimens (18 by 135 by 220 mm) were cut into small-end and center samples (18 by 30 by 50 mm) with matched grain orientations (Fig. 1). The center samples with grain angles of around 45 degrees were chosen to evaluate the VDPs for this paper, because the VDP curves of the 0 degree grain angle samples were more irregular (Wang and Cooper 2004). The VDP was measured with an x-ray density profiler (QMS Model QDP-01X, Quintek Measurement Systems, Knoxville, Tennessee), with two replicates for each condition.



Results and discussion

The air-dry density of the balsam fir samples used in this investigation varied from 0.32 to 0.40 g/[cm.sup.3]; however, the initial density had little effect on the shape of VDPs.

Effects of sample size and initial wood MC

Samples pressed at a medium press closing rate (5 min. preheat plus 2 min. closing time) had low surface density and high core density (Fig. 2). The VDP of the small sample was similar to that of the end sample from large specimens and the densified core zone for the sample taken from the center of the large specimen was slightly wider. This can be explained by the effects of steam and heat transfer on wood plasticization during hot-pressing, since the steam escapes through the sample end grain, especially in small samples, more quickly, reducing the plasticization of the core wood. However, in general, the proximity to the end grain under these pressing conditions had a relatively minor effect on the density profile and small samples can also represent larger samples quite well. In comparison with the sample size effect, the initial MC of wood at the start of the hot-pressing seemed to have a similar effect (Fig. 3). The higher the initial MC, the wider and the more obvious were the density peaks in the core layers. Both phenomena are related to the wood plasticization degree or the dissipation of moisture from wood during hot-pressing.




Effect of press closing rate

The press closing schedule effectively controlled the basic shapes of the VDPs of compressed fir wood (Fig. 4). At a press closing time of 2 minutes, without preheating the wood, the two surface layers of compressed wood were significantly densified. These VDPs are similar to those in OSB and MDF, suggesting they share a similar formation mechanism. The measurement of temperature inside the wood indicated that it took about 10 minutes for the core of samples to reach the target temperature during such hot-pressing. Hence, during the fast press closing, the surface layers of wood reached a high temperature, while their MC remained relatively high, leading to better plasticization and higher density near the surface. But the gradients of moisture, steam, and also the temperature inside the wood changed drastically with prolonged heating before the final press closure, and the density peaks shifted towards the core layers with increased preheat time or press closing time. This is attributed to the progressive elevation of temperature inside the wood and migration of moisture towards the center, shifting the zone of maximum wood plasticization deeper into the sample. At the longest press closing time of 17 minutes (10 min. of preheat plus 5 min. of closing), except for the small peaks in the surface layers, the wood had uniformly high density, caused by predrying and preheating of the sample, resulting in uniform plasticization through the thickness.


While smaller samples allowed faster moisture loss from the end grain and so had density profiles similar to the ends of larger specimens, the differences between these samples and samples distant from the ends of larger specimens were quite minor. The preheat plus press closing time controlled the basic shape of the VDPs in compressed fir wood according to the results of this investigation. The typical shapes of VDPs of compressed fir could be therefore classified into the three types shown in Figure 5: higher surface density, higher core density, or uniformly high density through the thickness, mainly corresponding to short, medium, and very long press closing times, with some influence from the initial MC of wood. It is anticipated that the different VDPs will affect the final mechanical and physical properties and that densified wood can be engineered to provide optimum properties for its intended end use and future work will investigate this aspect.
Table 1. -- Variables evaluated for the thermal compression of balsam
fir wood, with replicates of two.

Parameters Preheat Closing Pressing
evaluated Dimensions (a) time time time
 (mm) (min.)

Size effect 18 by 30 by 50 5 2 60
 18 by 135 by 220 5 2 60
Press closing rate 18 by 135 by 220 0 2 60
 18 by 135 by 220 1 2 60
 18 by 135 by 220 3 2 60
 18 by 135 by 220 5 2 60
 18 by 135 by 220 10 7 60
Effect of MC 18 by 135 by 220 5 2 60
 18 by 135 by 220 5 2 60
 18 by 135 by 220 5 2 60

Parameters Pressing Wood
evaluated Dimensions (a) temperature initial MC
 (mm) ([degrees]C) (%)

Size effect 18 by 30 by 50 180 11.3
 18 by 135 by 220 180 11.3
Press closing rate 18 by 135 by 220 180 11.3
 18 by 135 by 220 180 11.3
 18 by 135 by 220 180 11.3
 18 by 135 by 220 180 11.3
 18 by 135 by 220 180 11.3
Effect of MC 18 by 135 by 220 180 5.8
 18 by 135 by 220 180 11.3
 18 by 135 by 220 180 14.6

(a) Thickness by width by length.

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Jieying Wang

Paul A. Cooper*

The authors are, respectively, Post-doctoral Research Fellow and Professor. Faculty of Forestry, Univ. of Toronto, 33 Willcocks St., Toronto, ON, Canada M5S 3B3. This paper was received for publication in July 2004. Article No. 9902.

*Forest Products Society Member.
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Title Annotation:TECHNICAL NOTE
Author:Wang, Jieying; Cooper, Paul A.
Publication:Forest Products Journal
Geographic Code:1USA
Date:May 1, 2005
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