Improving dimensional stability by acetylation of discrete layers within ...

Improving dimensional stability by acetylation of discrete layers within flakeboard ✳

Jeroen H. van Houts ✳ Paul M. Winistorfer ✳ Siqun Wang

Abstract Dimensional instability of wood composite panels remains a key area of interest for many manufacturers. Cost-effective measures to minimize moisture-induced swelling of these hygroscopic products, while still retaining acceptable levels of mechanical strength, would provide obvious advantages within the highly competitive marketplace. A series of laboratory flakeboard panels were manufactured with the inclusion of discrete layers of furnish that had been treated with acetic anhydride. Acetylation is a treatment known to reduce the swelling and water absorption behavior of wood. Using treated material in discrete layers of the mat targets those layers of known significant thickness swell in a manner that may economically improve overall dimensional stability. A previously developed technique to determine the swelling of discrete layers within a panel was used to identify the contribution of a treated layer relative to the overall thickness swell for the whole panel. Results conclude that the thickness swell of oriented strandboard can be improved significantly by including discrete layers of acetylated strands when these layers are positioned appropriately in the mat. Acceptable internal bond strength was achieved for treated panels. Also, a semi-empirical model was used to verify the effective coverage area of the strands forming the discrete layers. The total uncovered area and the proportion and number of overlapping strands are included in the data output by the model.

Extensive research has been carried

out on the thickness swell (TS) behavior of different types of wood composite panels. Of particular interest, studies investigating layer TS behavior have been carried out by Wang and Winistorfer (2003, 2001, 2000), and Xu and Winistorfer (1995a and 1995b). Results from these studies clearly indicate that the majority of TS in wood composite panels, such as medium density fiberboard and oriented strandboard (OSB), occurs in the outer layers of the panel. By utilizing this knowledge, it was conceived that appreciable gains in dimensional stability could be attained by using treated furnish for specific layers within a panel. OSB manufacture typically involves the forming of a 3-layer mat, with the difference between the layers being the 82

orientation of the strands. The technology associated with this mat-forming step lends itself well to being adapted for including layers of treated strands within the mat structure. Using treated furnish has advantages over post-pressing panel treatments; for example, post-pressing treatments

would necessitate additional handling of large panels that may require significant additional space and may also provide more opportunities for panel damage. Economically, the treatment of furnish used for discrete layers has obvious advantages over treating furnish used for the entire panel.

The authors are, respectively, Post Doctoral Research Associate, Former Professor and Director (currently Professor and Department Head, Dept. of Wood Sci. and Forest Prod., 210 Cheatham Hall, Virginia Tech, Blacksburg, VA 24061-0323), and Assistant Professor, Tennessee Forest Prod. Center, The Univ. of Tennessee, PO Box 1071, Knoxville, TN 37901-1071. This work was supported by USDA Special Grant R11-2218-54. The authors would like to thank colleagues Chris Helton and Tim Young for their helpful assistance. The authors would also like to thank J.M. Huber for providing experimental materials. This paper was received for publication in July 2001. Reprint No. 9337. ✳Forest Products Society Member. Forest Products Society 2003. Forest Prod. J. 53(1):82-88 FOREST PRODUCTS JOURNAL

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Acetylation with acetic anhydride has been used extensively on various products (solid wood, fiberboard, flakeboard etc.) made from different species of wood, resulting in reduced moisture uptake and improved dimensional stability (Ramsden et al. 1997; Rowell et al. 1995, 1988, 1986b; Kumar 1994; Youngquist et al. 1986; Yoshida et al. 1986; Tarkow et al. 1950). Essentially, the mechanism by which acetylation reduces the hydrophilic nature of wood is by chemical reaction with the hydroxyl groups in the cell walls. This reaction bulks the cell wall and reduces available sites for water molecules to bond (Rowell 1982). Acetylation is also a known method for providing resistance to biological attack. The main objectives of this study were to acetylate wood strands with acetic anhydride, form panels with discrete layers of this treated furnish at various locations, test panel TS behavior using standard methods and a previously developed layer swell measurement technique, and measure internal bond (IB) strength. In addition to these objectives, a model to determine the coverage behavior of the strands used to form the discrete layers was also employed. A better understanding of strand coverage would assist in both interpreting the layer TS results, and in future work assist in optimizing the exact weight of furnish required for a treated layer.

Materials and methods Furnish treatment and panel preparation Commercially produced strands, predominantly southern pine but also containing approximately 20 percent mixed hardwood species, which had been manufactured with a target length of 100 mm, thickness of 0.7 mm, and an uncontrolled width dimension, were utilized for this investigation. The furnish moisture content was in the range of 8 to 10

percent when obtained from the OSB manufacturer. A portion of the strands was treated with acetic anhydride using a procedure similar to that presented by Rowell et al. (1986a). In brief, the acetylation procedure involved ovendrying the furnish and then immersing the furnish in acetic anhydride for a 1-minute period to allow chemical uptake. Reaction of the wood material with the absorbed chemical was then carried out in an oven heated to 120°C for a period of 4 hours. Upon removal from the oven, the strands were washed under running water for 48 hours to remove any residual acetic anhydride and by-product acetic acid, after which they were again ovendried. The average weight gain of the furnish, due to the acetylation process, was 18.2 percent. Both the acetylated and the untreated furnish were conditioned to an average moisture content of 3 percent for panel manufacture. Liquid phenol formaldehyde resin was applied to the strands at a 5 percent (solids) loading before manufacturing panels measuring 356 by 254 mm with a target thickness and density of 12.7 mm and 670 kg/m3, respectively. No wax was used in panel preparation in order to further unmask any differences in TS among treatments. The 300-second pressing schedule included 40 seconds of closing time, and a 30-second degassing period. The press platen temperature was set at 200°C. A total of six panels were carefully manufactured with their lay-ups described in Table 1. Due to the expense, time commitments, and the preliminary nature of this study, only one panel of each configuration was prepared. From each panel, two 152- by 152-mm TS specimens and five 50-by 50-mm IB specimens were evaluated. Before testing, the IB specimens were used for vertical density distribution measurement.

Table 1. — A description of the different layer configurations of the panels shown in a weight basis. Panel

Layer configuration of furnish (from top to bottom of the panel)

A

100% untreated

B

100% acetylated

C

25% acetylated, 50% untreated, 25% acetylated

D

25% untreated, 50% acetylated, 25% untreated

E

12.5% acetylated, 75% untreated, 12.5% acetylated

F

12.5% untreated, 12.5% acetylated, 50% untreated, 12.5% acetylated, 12.5% untreated

JANUARY 2003

Vertical density profile Vertical density profile measurement was conducted using a commercially available x-ray technique (Quintek Measurement Systems, Knoxville, Tenn.). A necessary consideration for obtaining accurate data on the density profile of the panels was to determine the mass attenuation coefficients of the 100 percent untreated panel (panel A) and the 100 percent acetylated panel (panel B). Different materials have different mass attenuation coefficients (Moschler and Dougal 1988), and thus it was important to adjust the measured density profiles of the panels containing various layers of untreated and acetylated strands accordingly. This adjustment was made assuming that the various layers were uniform and the transition between untreated and acetylated layers was abrupt. Thickness swell TS tests were carried out based on ASTM 1037-96a (ASTM 1996), with thickness measured 25 mm in from the edge and on the panel edge (referred to as TS and edge TS, respectively). Edge layer TS measurements were also carried out on the same specimens. These edge layer TS measurements were made using an optical technique previously devised by Wang and Winistorfer (2003, 2002). Implementing this method for layer TS allows the contribution of the treated layers to be identified, which is undetectable using the overall measurements prescribed by the ASTM standard. Modeling strand coverage In order to gain an understanding of the degree of coverage by the discrete layers of strands within a panel, a model that incorporates scanned images of a sample of the actual strands used was devised. The images obtained for the model came from a number of predetermined masses (based on ovendry weight) of strands that had been randomly selected from the untreated and acetylated furnish. Each predetermined mass of strands was based on the dimensions of the flatbed scanner used to generate the images of the strands. The scanned mass of strands was equivalent to a 4.2 percent layer in the experimental panel. This layer percentage value was deliberately selected because it is a multiple of the actual layers within the experimental panels (12.5% and 25%). 83

Figure 1. — Vertical density profile of the panel made with 100 percent untreated furnish (panel A) and the panel made from 100 percent acetylated furnish (panel B).

Essentially, the Visual Basic macro opens a number of scanned images, adjusts their brightness properties appropriately, randomly rotates them, and places them one on top of the other. When the images are placed on top of each other their intensities are added, so that the intensity of a given point is linearly related to the number of overlapping strands. The number of scanned images used is dependent on the mat weight being analyzed. The resulting image (after placing a number of scanned images on top of one another) is analyzed to determine the amount of area covered by 0, 1, 2, 3, or more strands. The macro is run a number of times and the resulting data are input into a spreadsheet where the averages, standard deviations, minima, and maxima for the data are calculated. The coverage of both the acetylated and the untreated furnish at different layer proportions of the experimental panels was compared using this technique.

Results and discussion Compression behavior of furnish during consolidation

Figure 2. — Pressure applied to the panels during the press cycle.

Strands were placed by hand on a flat bed scanner, with a scanning area measuring 224 by 367 mm, in such a way that no strands were overlapping. Strands were evenly distributed, and were randomly orientated as much as possible. For a given 4.2 percent layer mass of strands, this process was carried out a number of times until all of the strands had been scanned. Care was taken to scan the same number of images from each mass of strands, regardless of whether they were from the untreated or acetylated furnish. The strands were scanned with the scanner 84

lid open so that the background of the images was black. Scanning parameters were chosen in such a way that the resulting image was of black and white photographic quality, consisting of 256 shades of gray, with a high contrast between the strands and the background. Images were stored in a tiff format, and their size was 1,275 by 2,100 pixels. A commercial image analysis software package, SigmaScan Pro 5.0, was the environment in which the model operated. The model was written as a Visual Basic macro that utilized many of the functions available from SigmaScan.

Differences between the compressibility of the acetylated and untreated furnish were evident from two sources. When comparing the density profiles (Fig. 1) of the panel made from 100 percent untreated furnish (control) with the panel made from 100 percent acetylated furnish (panel B), there is a smaller difference between the core density and outer layer density in the acetylated panel than the untreated panel. Also, the final thickness of panel A (control) is less than that of panel B (acetylated), indicating that a greater degree of springback (after press opening) occurs for the panel made from 100 percent acetylated furnish. Also, towards the end of the hot-pressing cycle (platens closed and degassing period), the pressure applied to the mat when consolidating panel B was noticeably higher than that for panel A (Fig. 2). These two phenomena indicate that the acetylated furnish resists compression more than the untreated furnish. For the remaining panels, the measured values for consolidation pressure just before degassing and their final thickness were between those measured for panels A and B.

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Table 2. — Proportion of acetylated furnish and summary of thickness swell data after a 24-hour period of water exposure. Panel A

B

C

D

E

F

- - - - - - - - - - - - - - - - - - - - - - - - - (%) - - - - - - - - - - - - - - - - - - - - - - - - Acetylated furnish

0

100

50

50

25

25

TSa

24.9

2.4

12.9

20.1

21.9

23.5

Edge TSb

34.0

5.6

17.6

22.2

29.7

26.4

a b

Thickness swell measured 25 mm from the edge of the panel. Thickness swell measured at the panel edge.

Figure 3. — Edge layer thickness swell of the panel made with 100 percent untreated furnish (panel A) and the panel made from 100 percent acetylated furnish (panel B). Table 3. — Internal bond strength of panels A to F.a Panel A

B

C

D

E

F

- - - - - - - - - - - - - - - - - - - - - - - - - (kPa) - - - - - - - - - - - - - - - - - - - - - - - Mean

916

387

909

346

1025

752

SDb

294

198

130

218

118

132

a

An average of five test specimens per panel. b SD = standard deviation.

Figure 4. — Edge layer thickness swell of panel C (25% acetylated, 50% untreated, 25% acetylated). JANUARY 2003

Thickness swell Results of TS due to a 24-hour period of water exposure are summarized in Table 2. As expected, there is a decreasing amount of both TS and edge TS with an increasing amount of acetylated furnish. Examples of results from the optical edge layer TS measurement technique for panels A and B, C, D, and F are shown in Figures 3 through 6, respectively. The location of the layers of untreated and acetylated material was calculated from the vertical density profile data and is indicated on these figures. Each data point on these figures represents the average thickness swell of a discrete layer that was measured from the eight individual edges of two specimens. The board thickness shown on the x-axis represents the location of the center of each discrete layer. Figures 4 and 5 clearly show the influence of the acetylated material on the edge layer TS characteristics of these panels. Comparing the layer TS data from panels C and D (Figs. 4 and 5) with the 100 percent untreated panel (panel A in Fig. 3) shows that the behavior of the untreated layers in these panels is almost identical to the corresponding layers in panel A. In other words, the acetylated furnish did not appear to affect the TS behavior of the untreated furnish. Similarly, when the layer TS characteristics of the 100 percent acetylated panel (panel B in Fig. 3) are compared to panels C and D (Figs. 4 and 5), it appears that the untreated furnish did not alter the effectiveness of reducing TS by the acetylated furnish in the given layers. Considerably less material (12.5% rather than 25%) exists in each acetylated layer of panel F (Fig. 6). The reason that the impact of the acetylated furnish on the edge layer TS characteristics in panel F is not as clearly visible as for panels C and D is discussed in the strand coverage model section below. Figure 7 is a plot of TS versus time for a total period of 168 hours. With the data presented in this format, the role of not only the total amount of acetylated furnish, but also the location of acetylated furnish, becomes quite apparent. Understandably, panels A and B represent the highest and lowest measured TS, respectively. Panel C indicates a superior TS response compared to panel D, even though both consist of the same amount of acetylated furnish. A Wilcoxon 85

Figure 5. — Edge layer thickness swell of panel D (25% untreated, 50% acetylated, 25% untreated).

Figure 6. — Edge layer thickness swell of panel F (12.5% untreated, 12.5% acetylated, 50% untreated, 12.5% acetylated, 12.5% untreated).

Figure 7. — Thickness swell versus time of all the panels for a total period of 168 hours. Layers of untreated and acetylated material for each panel as indicated. 86

Rank-Sum Test (Milton and Arnold 1995) was used to validate whether the mean TS for panel C was in fact lower than that of panel D. When a p-value of 0.05 was used we could conclude that for the 2-, 8-, 24-, 48-, and 168-hour soak tests, TS of panel C was in fact lower than panel D. This finding represents the importance of the location of acetylated material within a panel to its overall TS behavior. Edge TS versus time (Fig. 8) shows a similar trend to TS (Fig. 7) with the exception of panels E and F. By comparing these two figures it appears that the TS for panel F is greater than that for panel E, while the edge TS for panel E is greater than that for panel F. However, a Wilcoxon Rank-Sum Test of the data showed that there is not a significant difference between panels E and F for either the TS or edge TS results. Internal bond strength Results from the IB tests are summarized in Table 3. From these results, two distinct levels of IB exist: 1) panels with acetylated furnish in the core; and 2) panels with untreated furnish in the core region. Panels with the acetylated furnish in the core have approximately a 50 percent lower IB than the other panels. Other investigators have concluded that this reduced IB is due to the decrease in wettability of the acetylated wood, which therefore prohibits penetration of the phenolic resin, hence reducing bond strength (Chowdhury and Humphrey 1999, Rowell et al. 1987, Rowell and Banks 1987). The most important finding of these results is that acetylated strands can be used in a panel, more specifically outside the core region, without significantly reducing the IB. Strand coverage model The strand coverage model was used to predict the coverage of layers equivalent to 12.5 and 25 percent of the actual laboratory panels, which were in fact the same weight as the untreated and acetylated layers in some of the panels. For each different scenario (furnish type and layer thickness), the model was run 50 times to build a sufficient pool of data. Average results from the strand coverage model are summarized in Figures 9 and 10. The standard deviation of the coverage areas predicted by the model was typically under 1 percent, and therefore no further discussion of statistical

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Figure 8. — Edge thickness swell versus time of all the panels for a total period of 168 hours. Layers of untreated and acetylated material for each panel as indicated.

Figure 9. — Area covered by different numbers of overlapping strands in a 12.5 percent layer of untreated or acetylated strands.

variation has been included. It is evident from these graphs that there is more coverage provided by the untreated furnish than the acetylated material. This is especially prominent in Figure 10 (25% layer), where the center of the “bell-curve” of the untreated material is further to the right (higher number of overlapping strands) than the acetylated material. The reason why the untreated furnish has more coverage than the acetylated furnish is because on an average basis, an untreated strand will have less mass than an acetylated strand with the same coverage area. Acetylated strands have a higher mass than untreated strands due to the chemical bulking. The relevance of this strand coverage information is that it provides a greater understanding of the potential effectiveness of a given treated layer size within a panel. When a layer consists of 12.5 percent acetylated furnish, as shown in Figure 9, the model predicts that in excess of 5 percent of the area will be uncovered, while 50 percent of the area is covered by only one to two strands. Due to the fact that such a high area is covered by only a minimal number of strands, it is not surprising that the effect of the acetylated material on the measured layer TS characteristics is not as highly pronounced for panel F (Fig. 6) as for panels C and D (Figs. 4 and 5). It should be noted that 20 percent of the area is covered by only a single strand of material which will be thinner than the resolution which the layer TS technique allows. Even with more than one strand present, it is likely that the optical layer TS measurement may measure a combination of both the acetylated furnish and the surrounding untreated material. As expected, coverage by a 25 percent layer of either untreated or acetylated furnish (Fig. 10) is superior to that of the 12.5 percent layer, and hence its influence is easily detected by the layer TS measurement technique.

Conclusion

Figure 10. — Area covered by different numbers of overlapping strands in a 25 percent layer of untreated or acetylated strands. JANUARY 2002

A clear improvement in the dimensional stability of flakeboard was observed when discrete layers of acetylated furnish were placed within the mat. The degree of improvement in dimensional stability provided by the addition of treated furnish appeared to be related to both the amount of acetylated material and its location. The greatest im87

provements in dimensional stability, using a given amount of acetylated furnish, have been shown to occur when this acetylated furnish is used in the surface layers. IB strength was reduced by approximately 50 percent when acetylated strands were placed in the core region of a panel, but was unaffected when acetylated strands were used in the outer layers only. The strand coverage model showed that a given weight of acetylated furnish has slightly less coverage than the same weight of untreated furnish. The strand coverage model also indicated an uncovered area in excess of 5 percent exists when a 12.5 percent layer of acetylated furnish is used. There was a minimal amount of coverage by a 12.5 percent layer of acetylated furnish and its impact on layer TS characteristics was less detectable than when a 25 percent layer of furnish was used.

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USDA Forest Serv., Forest Prod. Lab., Madison, WI. Wang, S. and P.M. Winistorfer. 2000. The effect of species and species distribution on the layer characteristics of OSB. Forest Prod. J. 50(4):37-44. __________ and __________. 2001. Flake compression behavior in a resinless mat as related to dimensional stability. Wood Sci. and Technology 35(5):379-393. __________ and __________. 2002. Process and system for determination of layer thickness swell of wood composites. U.S. Patent No. 6,396,590. __________ and __________. 2003. An optical technique for determination of layer thickness swell of MDF and OSB. Forest Prod. J. (in press). Winistorfer, P.M. and W. Xu. 1996. Layer water absorption of medium density fiberboard and oriented strandboard. Forest Prod. J. 46(6):69-72. Xu, W. and P.M. Winistorfer. 1995a. A procedure to determine thickness swell distribution in wood composite panels. Wood and Fiber Sci. 27(2):119-125. __________ and __________. 1995b. Layer thickness swell and layer internal bond of medium density fiberboard and oriented strandboard. Forest Prod. J. 45(10):67-71. Yoshida, Y., S. Kawai, Y. Imamura, K. Nishimoto, T. Satou, and M. Nakaji. 1986. Production technology for acetylated low-density particleboard. Part I. Mechanical properties and dimensional stability. Mokuzai Gakkaishi 32(12):965-971. Youngquist, J.A., A. Krzysik, and R.M. Rowell. 1986. Dimensional stability of acetylated aspen flakeboard. Wood and Fiber Sci. 18(1):90-98.

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