May, 01/Forest Products Journal / -- In conventional approaches to reducing the press time of wood composites, high press temperatures and/or an external catalyst is often used to accelerate the resin cure rate to obtain acceptable press times. This paper explored the concept of combining a thermally conductive carbon filler material (e.g., synthetic graphite) with wood particles and flakes as a means to promote resin cure. The experimental results indicate that synthetic graphite does not act as a catalyst. Rather, graphite allows a faster resin cure by accelerating the heat (higher temperatures) into wood composites due to its higher thermal conductivity (600 Wm^sup -1^ K^sup -1^) as compared to the low thermal conductivity of the wood (0.2 Wm^sup - 1^K^sup -1^).

Wood-based composite boards are panels manufactured from comminuted wood materials (e.g., particles, flakes, fibers, etc.) combined typically with a thermoset resin and bonded under heat and pressure in a hot-press (Maloney 1993). In such a manufacturing process, the productivity of a plant and the quality of the board depend on several factors, such as resin cure time, wood geometry, wood moisture content (MC), wood orientation, board density, wax content, etc. Among these factors, resin cure may be the most important because the physical integrity and durability ofthe panel relate very strongly to resin cure during the hot-pressing process.

Generally, the efficiency of the press is crucial to the quality of the boards and the output of the plant (Park et al. 1999). Since synthetic adhesives used in wood composite panel manufacture are heat reactive, the efficiency of the hot-press decreases with increasing thickness of the mat; this is due to the poor heat conductivity of dry wood. The pressing time is strongly related to the heat transfer, which affects the curing rate of the resin. In other words, the transmission of heat from the mat faces to the core is a critical factor in determining the final press time. Because of the poor heat conductivity of wood, the cure of the resin takes place first in the faces when the hot platens touch the mat during pressing, while the core of the board is still cold. Consequently, longer time will be needed for sufficient heat to reach the core of the board, which will allow the middle (core) of the board to cure.

Several methods, used alone or in combinations, have been developed to accelerate the resin curing. These include pressing the panels at higher platen temperatures, use of different catalysts, additives, and modified resin formulations (Barry and Corneau 1999, Park et al. 1999, Sellers 1985). However, the internal or external catalyst is generally justified only if it will produce shorter press times at lower concentration in the range of 0.25 to 1 percent by weight of the resin (Maloney 1993). On the other hand, higher pressing temperatures are generally avoided because of the increased energy cost combined with the production of volatile organic compounds (VOCs) (Barry et al. 2001, 2000). Press times have also been reduced through manipulation of MC in the mat (Maloney 1993). High-frequency electrical heating has also been tried, in place of contact heating, due to its principle of curing the complete mat virtually simultaneously instead of heat penetrating only from the surface (Maloney 1993). Although this technique can cure the resin significantly faster, there are many practical difficulties, particularly involving the insulation of multi-platen presses. In addition, the density profile through the board will be affected and the generally desirable high-density surface layers (for higher bending strength and stiffness) prove difficult to attain with this method of resin curing (Maloney 1993). Although resin curing has been shortened dramatically through the approaches just mentioned, there is still need to develop cost-effective and environmentally friendly approaches to accelerate resin curing (or reduce the press times).

One approach to improving thermal conductivity of a wood composite is through the addition of a thermally conductive filler material, such as synthetic carbon (Matuana and King 2001 a, 2001b). Typical thermal conductivity values for some common materials are 0.2 for wood, 1 for carbon black, 10 for carbonized polyacrylonitrile (PAN) based carbon fibers, 234 for aluminum, 400 for copper, and 600 for graphite (all values in Wm^sup -1^K^sup - 1^) (Torrey 2001). Thus, carbon filters may act as a heat transfer medium by facilitating the transfer of heat from the faces to the core of the panel. This faster heat transfer may shorten the press time during wood composite panels manufacturing. Increased wood thermal conductivity will also promote lignin flow during pressing, which will help the pressed board to remain at the pressed dimensions. This will reduce internal stresses within the board and consequently lead to more dimensionally stable panels. The main goal of this work was to explore the concept of combining carbon fillers with wood flakes/particles as a means to promote resin cure and/or improve the dimensional stability of the panels.

Methodology

The carbon in this work was Thermocarb^sup R^, a high-purity synthetic graphite (99.9% carbon) manufactured by Conoco, Inc. (Houston, Texas). The properties of Thermocarb synthetic graphite were as follows: density = 2.24 g/cm^sup 3^; particle size = 48 percent volume of -180 g(mu)m/+75 p(mu)m (-80/+200 Tyler mesh); aspect ratio = 2.0; thermal conductivity at 23 deg C = 600 Wm^sup - 1^K^sup -1^ on a 6.34-mm particle; electrical resistivity = -10^sup 4^ ohm-cm. The wood particles in this study were produced from aspen flakes via grinding in a hammermill with 6.34 mm mesh screen. The liquid phenol-formaldehyde (PF) resin was Georgia Pacific GP' 3121 ResiStran^sup R^ surface resin (pH@25 deg C = 9.8 to 10.2; viscosity @25 deg C = 100 to 225 cP; and non-volatile solids = 53% to 55%).

Particleboard panels were manufactured as follows. Dried wood particles (5% MC) were placed in a rotating-drum blender and sprayed with 5 percent liquid PF resin (based on ovendry weight of the furnish). After this blending operation, wood particles were placed in a 20-L high-intensity mixer (Papenmeier, Type TGAHK20). Synthetic graphite was added and mixed with PF-coated wood furnish for 5 minutes. The concentration of synthetic graphite was fixed at 30 percent based on PF weight (approximately 1.7% of the ovendry weight of wood particles).

The blended materials were manually placed in a 305-mm by 305-mm forming mat box and hot-pressed in a laboratory press using the following press cycle: press closing time = 30 seconds to press stops; pressing times at stops = 120, 240, and 360 seconds; decompression time = ~30 seconds; platen temperature = 191 deg C (375 deg F). The panel thickness was 11 mm (7/16 in.) to give a targeted density of 0.64 g/cm^sup 3^(-40 pct.

Test specimens for property characterization were cut from the panels and conditioned to a constant weight in a walk-in temperature/ humidity-controlled room, set at 12 percent equilibrium MC. The density, internal bond strength (IB), and thickness swelling after 24-hour immersion in cold water were determined in accordance with the procedure outlined in ASTM D1037-99 (ASTM 1999).

Thermal analysis of pure liquid PF and liquid PF/synthetic graphite resins was also conducted to study the curing kinetics of these resins. Differential scanning calorimetry (DSC) tests were performed using a Mettler Toledo STAR System at a heating rate of 10 deg C per minute. The energy of activation (Ea) to start the reaction, nth order kinetics, maximum peak temperature, and the total thermal energy of reaction or enthalpy of reaction (AH) were the kinetic parameters evaluated.

The effect of synthetic graphite on the heat transfer was also evaluated by recording the temperature profile at the center of the panels during hot-pressing. The internal mat temperature was recorded as the board was pressed using J-type thermocouples. Flakeboard panels (18 mm thick and 0.64 g/cm^sup 3^) were manufactured using commercial aspen flakes from Louisiana Pacific Corporation (Sagola, Michigan) and liquid PF face resin from Borden Chemical, Inc. (Cascophen OS707) (Torrey 2001 ).

Results and discussion

The particleboard test results show that densities are similar at all three press times for both the control and synthetic graphite- containing boards (Table 1), with increasing density with increasing press times. The synthetic graphite did not appear to affect the board density.

With face PF resin alone in the board, a 120-second press time was too short to bond the wood particles, i.e., not enough time was given for the resin to cure during processing (Table 1). Interestingly, when the particleboards were pressed for 180 seconds and 240 seconds, the IB values of the synthetic graphite-containing particleboards were significantly higher (a 99% and 38% increase, respectively) than the panels made only with PF resin. This trend indicates that the curing of the resin was more advanced when the synthetic graphite was added into the PF resin.

At 180- and 240-second press times, the thickness swelling was lower (a 32% and 41% decrease, respectively) for the particleboard containing synthetic graphite as compared to the control (Table 1). These lower thickness swell values may be due to the higher IB values of the gra\phite bonded panels.

The DSC test results indicate that synthetic graphite did not change the curing kinetics of the PF resin. The results imply that synthetic graphite does not act as a catalyst. It is believed that synthetic graphite must be allowing a faster cure by faster heat penetration into the wood composite. This faster heat penetration is due to the high thermal conductivity of Thermocarb synthetic graphite (600 Wm^sup -1^K^sup -1^) as compared to the low thermal conductivity of the wood (0.2 Wm^sup-1^K^sup -1^).

The temperature profile during hot-pressing was recorded during pressing and the results (Fig. 1) clearly show the potential for synthetic graphite to enhance the heat conductivity of wood during hot pressing of the board. The temperature at the core of the panel reached 121 deg C (250 deg F) after only 225 seconds when synthetic graphite was incorporated into the PF resin compared to 270 seconds for panels without synthetic graphite.

The following conclusions were drawn from the experimental results from the data of the physical and mechanical properties of flakeboards made with Borden Chemical face PF resin (Cascophen OS707) (Table 3):

* Synthetic graphite did not statistically degrade the flexural strength of the flakeboard panels but the flexural moduli of the boards were statistically different.
* Synthetic graphite still had a real heat transfer effect since its incorporation into the board improved the lB strength of flakeboard as compared to the control.
* Unlike in particleboard, synthetic graphite did not statistically improve the dimensional stability (thickness swell) of flakeboard panels.

Literature cited

American Society for Testing and Materials (ASTM). 1999. Standard test methods for evaluating properties of wood-base fiber and particle panel materials. ASTM D1037-99. ASTM, West Conshohocken, PA.

Barry, A.O. and D. Comeau. 1999. Volatile organic chemicals emissions from OSB as a function of processing parameters. Holzforschung 53(4):441-446. and R. Lovell. 2000.

Press volatile organic compound emissions as a function of wood particleboard processing parameters. Forest Prod. J. 50(10):35-42.

R. Lepine, R. Lovell, and S. Raymond. 2001. Response surface methodology study of VOCs in plywood press emissions. Forest Prod. J. 51(l):65-73.

Maloney, T.M. 1993. Modem particleboard and dry-process fiberboard manufacturing. Updated edition. Miller Freeman Publications, Inc., San Francisco, CA. 681 pp.

Matuana, L.M. and J.A. King. 2001a. Veneer-based product and method of manufacture. U.S. Patent filed July 13, 2001 with the U.S. Patent and Trademark Office.

and .200 lb. Wood-based composite board and method of manufacture. U.S. Patent filed July 13, 2001 with the U.S. Patent and Trademark Office.

Park, B.D., B. Riedl, E.W. Hsu, and J. Shields 1999. Differential scanning calorimetry of phenol-formaldehyde resins cure-accelerated by carbonates. Polymer 40:1689-1699.

Sellers, T. Jr. 1985. Plywood and Adhesive Technology, Marcel Dekker, Inc., New York. 661 pp.

Torrey, K.S. 2001. Influence of thermally conductive fillers on the physical properties of wafer board. MS thesis. Michigan Tech. Univ., Houghton, Mt. 298 pp.

Laurent M. Matuana*

The author is an Assistant Professor, Dept. of Forestry, College of Agriculture and Natural Resources, Michigan State Univ., East Lansing, MI 48824-1222. Partial funding for this project was provided by the School of Forestry and Wood Products, Michigan Technological University. The author gratefully acknowledges the help of Peter DeJong (undergraduate student) and Dr. Fatih Mengeloglu (former Ph.D. Student) in conducting this research. The author gratefully acknowledges the reviewers for their thoughtful and constructive comments on the paper. This paper was received for publication in June 2002. Article No. 9507.

*Forest Products Society Member.