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Some performance characteristics of aspen-kenaf composite boards.

Abstract

This study investigated some of the important properties of aspen-kenaf boards made from a combination of commercial liquid and powder phenol-formaldehyde adhesives with commercial oriented strandboard (OSB). The specific gravity of these boards varied from 0.68 to 0.75. Amount of kenaf, direction of flakes, resin content, and density significantly affected the modulus of rupture (MOR) and the modulus of elasticity (MOE) values of aspen-kenaf boards. Boards with 25 percent kenaf and 75 percent aspen produced MOR and MOE values comparable to commercial OSB. Percentage of kenaf and resin levels were significant factors influencing the internal bond (IB) strength and surface hardness when compared to commercial OSB. The 25 percent kenaf and 75 percent aspen boards produced IB values that could meet the required CSA standard. Aspen-kenaf boards obtained lower values for linear expansion. Lower percentage of kenaf flakes and higher resin content controlled thickness swelling in the 24-hour water-soak test. The 100 percent kenaf boards showed higher thickness swelling; however, boards with 25 percent kenaf and 75 percent aspen flakes recorded thickness swelling of 15 percent or less.

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The preservation of forests and increasing environmental awareness has focused research on the exploration of new renewable fibrous resources. Kenaf has been identified in many previous studies as a viable non-wood plant source of industrial fiber for the pulp and paper industry (Bagby et al. 1975). More recent studies have focused on the potential of kenaf for the manufacture of composition boards (Maldas and Kokta 1995). Kenaf is a non-wood fiber plant. It grows to a height of 12 to 18 feet and attains 1 to 1.5 inches in diameter in 120 to 150 days. On ovendry basis, the outer bast fibers account for about one-third of the stem mass and pithy core accounts for the remainder. Various kenaf-based products like thermoplastic composites for automobiles, packaging, and construction material are currently being tested in the market. Kenaf core particles have been found to be feasible raw materials for the manufacture of low-density insulation board panels similar to acoustic composite boards (Sellers et al. 1994). In the last two decades, research on kenaf has resulted in limited production of hardboard, medium density board, and particleboard (Chow et al. 1993, English et al. 1996). Results indicate that particle boards of various thickness and density can be made using kenaf or by mixing kenaf with other materials.

This study explores the potential suitability of kenaf as a non-wood material for waferboard/oriented strandboard (OSB). Kenaf flakes were mixed with traditional aspen flakes at different rations to make OSB. Both physical and mechanical properties of these boards were measured in the laboratory to find the appropriate ratio of kenaf flakes that can replace aspen flakes without compromising quality and strength of these boards.

Materials and methods

The general laboratory manufacturing process of aspen-kenaf board is shown in Figure 1. It is closely related to the procedure used in manufacturing commercial OSB. Commercial-grade aspen flakes were mixed with laboratory-manufactured kenaf flakes to produce the experimental test boards. Air-dried kenaf stem material was cut into 3-inch-long pieces and soaked in water for 18 hours. This helped the raw materials to attain the optimal softness required to flake them to a thickness that matched aspen flakes. The flaker used was a disc-type flaking machine. Kenaf flakes plugged the flaker several times. Later these flakes were dried to a moisture content (MC) of 5 to 7 percent and passed through a sieve to remove any impurities or small particles. The average length and thickness of kenaf flakes was 2.633 inches and 0.039 inches, respectively. The aspen average flake dimensions were 3.646 inches in length and 0.0278 inches in thickness. The ratio of bast to core fiber in the kenaf stem was around 35:65 on a dry weight basis.

[FIGURE 1 OMITTED]

The experimental design for the production of composition panels from aspen and kenaf flakes is shown in Table 1. There were five different ratios of aspen to kenaf flakes: 100:0, 75:25, 50:50, 25:75 and 0:100. Board composition was either homogenous with no orientation of flakes in the boards or 3-layered OSB. The 3-layered boards included parallel and perpendicular directions based on flake orientation on the surface. Two resin levels of 4 and 6 percent were applied. Resin type included equal amounts of both powdered and liquid phenol-formaldehyde. Target density levels were 42 pcf and 46 pcf.

Resin and wax were applied in a rotating drum blender fitted with a pneumatic spray gun. After the application of 2 percent wax, liquid resin was applied and then powdered resin was spread on these flakes uniformly. Two different kinds of liquid resins were used for core and face layers in the composition boards. Two laboratory flake alignment devices were constructed using a series of parallel metal fins to keep the flakes in alignment until they were aligned and deposited on the mat. The mat for the OSB was obtained by rotating the device at 90 degrees for each layer. The final MC of the mat after resin application varied from 8 percent for pure aspen boards to 12 percent in pure kenaf boards. An oil-heated hydraulic hot-press was used for pressing the mat. The mat was laid on aluminum platens. The maximum press pressure was 522 psi to 555 psi (3.6 MPa to 3.8 MPa), depending on board density. The press temperature was maintained at about 400[degrees]F (204[degrees]C). The press closure time (compaction time) was between 90 and 120 seconds depending on density and the amount of kenaf in the boards. The total press time varied from 10 minutes for 4 percent resin content to 12 minutes for 6 percent resin content. All panels were pressed to a nominal thickness of 0.435 inch (11.1 mm). Standard test methods for evaluating properties of wood-based fibers and particle panels materials were used (ASTM 1996). Conditioned samples were cut into specimens for mechanical and physical property testing in accordance with methods described in ASTM D1037-96a. A factorial analysis was conducted through the use of split-plot design to determine the effects of the treatment combinations and their interactions on properties of boards. There were six replications for each treatment. The properties included modulus of rupture (MOR), modulus of elasticity (MOE), internal bond (IB), thickness swell (TS), linear expansion (LE), and hardness.

Results

Results of some physical properties of the boards are shown in Table 2. MC during the board manufacturing process played a key role in determining press time. Initially boards made of 100 percent kenaf showed blisters in the board center so the press time was increased to 12 minutes to overcome this problem. The higher the kenaf content, the longer the press time, because kenaf flakes picked up moisture quickly from the humid environment. MC of the conditioned boards after pressing varied from 11.96 in 100 percent kenaf boards to 6.52 in 100 percent aspen boards. The results discussed in this paper are for aspen-kenaf composition boards made with 6 percent resin content and a target density of 46 pcf. Boards made with 4 percent resin content and 42 pcf density were weaker in strength properties and demonstrated higher TS and LE. Table 3 shows the factorial analysis for the various properties measured in this study.

[FIGURE 2 OMITTED]

Bending MOR and MOE

MOR and MOE values are shown in Figures 2 and 3, respectively. The results of the analysis of variance conducted on the effects of the different factors and their interactions showed that the amount of kenaf, the resin content, and the type of board significantly affected both MOR and MOE values. The interaction effects did not significantly contribute to the bending properties. Further, it was observed that values for MOR and MOE increased as the content of aspen flakes increased in the boards. Overall, 3-layered boards with a parallel direction of flakes on the surface showed better MOR and MOE values as compared to the homogenous type. Interestingly, both 3-layered with a parallel direction of flakes on the surface and homogenous boards with 50 percent or less kenaf gave an acceptable MOR value for building materials standards (SBA 1998). The amount of kenaf, board type, and the direction of flakes were highly significant in influencing the resultant value of MOE. Resin content did not show much impact on the MOE values. MOE values of boards increased with the increase in the amount of aspen flakes. The boards with 25 percent kenaf and 75 percent aspen produced MOE values comparable to 100 percent aspen; overall, the 3-layered boards exhibited higher MOE values than homogenous boards. The 3-layered boards with 75 percent kenaf and 25 percent aspen with flakes oriented in a parallel direction to the length produced MOE values higher than standards as specified by SBA (1998).

[FIGURE 3 OMITTED]

Internal bond

The main factors, kenaf and resin content, significantly affected the IB strength. Kenaf content levels showed a linear relationship to IB strength values. Figure 4 shows the mean and coefficient of variation values for IB. The IB strength increased as the content of aspen flakes increased in the boards. Overall, 3-layered boards obtained slightly higher average IB strength values as compared to the homogenous type. However, 100 percent kenaf homogenous boards produced higher IB values than kenaf 3-layered boards. The 6 percent resin content produced higher IB values than the 4 percent resin. Boards with 50 to 75 percent kenaf flakes exhibited IB values of 30 to 45.5 psi, which is below the OSB minimum standard. Only 3-layered boards with 25 percent kenaf, 75 percent aspen, 6 percent resin, and 46 pcf density gave satisfactory results that met the standard requirements (SBA 1998).

[FIGURE 4 OMITTED]

Thickness swell

Analysis of variance showed that both main factors, kenaf and resin content, controlled the TS and water absorption properties in aspen-kenaf boards. The amount of kenaf was highly significant for TS in the boards. Figure 5 shows the mean and coefficient of variation values for TS by aspen-kenaf boards. As the kenaf ratio dropped, so did the TS. Boards made with 6 percent resin, 46 pcf, and 25 to 50 percent kenaf and aspen flakes experienced lower values for TS. The TS values for boards made with 75 percent, 50 percent, and 25 percent kenaf were far lower than the required standard value for OSB (SBA 1998).

Linear expansion

In this study, no main factors and their interactions contributed to LE; however, kenaf, resin, and board density controlled LE values. The LE of aspen-kenaf boards ranged from 0.03 to 0.13 percent, which was substantially below the recommended standards set for OSBs. Higher average values for LE were noticed in 3-layered 100 percent kenaf. LE values decreased as the amount of kenaf decreased and aspen increased. On the contrary, homogenous boards showed lower LE than 3-layered boards. Kenaf flakes consisting of bark and a considerable core region might have influenced LE.

[FIGURE 5 OMITTED]

Face hardness

The hardness of aspen-kenaf boards was affected by the amount of kenaf, density, and their interaction. Hardness of these boards increased as the content of aspen flakes increased in the boards. Resin content and board types did not show any effect on hardness. The hardness values for 100 percent kenaf and 100 percent homogenous boards was 515 and 1,004 pounds, respectively. Boards with 25 to 75 percent kenaf flakes exhibited lower hardness values. The 3-layered and homogenous boards with 75 percent aspen, 25 percent kenaf, 46 pcf density, and 6 percent resin showed hardness close to commercial OSB.

[FIGURE 6 OMITTED]

Conclusions

Kenaf fibers carry strong potential as an alternative fibrous material for making fiberboards, composition panels, and also as reinforcing fibers in wood-plastic composites. Kenaf fibers show strong affinity to moisture, therefore, either fiber treatment or a long press time is required. Bending properties such as MOR and MOE improve as the kenaf content decreases. Properties like IB. TS, and hardness of the boards improved with lower levels of kenaf flakes. Overall, boards made with 25 percent kenaf and 75 percent aspen flakes and 6 percent resin exhibited board property values comparable to 100 percent aspen boards and also met or exceeded SBA standards.

[FIGURE 7 OMITTED]
Table 1.--Experimental design for production of composition panels
from aspen and kenaf flakes. (a)

Aspen-kenaf ratios (5)

Aspen Kenaf Type(2) Resin(2) Density(2)
--------(%)-------- (%) (pcf)
0 100 Homogenous 4 42
25 75 OSB 6 46
50 50 -- -- --
75 25 -- -- --
100 0 -- -- --

(a) Total treatments = 40.

Table 2.--Physical properties of aspen-kenaf boards.

Type of board Density Thickness Specific gravity

 (pcf) (in.)
100% kenaf 42 0.428 0.68
 46 0.449 0.73
75% kenaf, 25% aspen 42 0.436 0.73
 46 0.431 0.75
50% kenaf, 50% aspen 42 0.421 0.74
 46 0.449 0.75
25% kenaf, 75% aspen 42 0.427 0.71
 46 0.439 0.75
100% aspen 42 0.426 0.72
 46 0.440 0.74

Type of board Moisture content
 (%)
100% kenaf 11.96
 10.91
75% kenaf, 25% aspen 7.54
 6.55
50% kenaf, 50% aspen 6.61
 6.64
25% kenaf, 75% aspen 7.29
 6.40
100% aspen 6.61
 6.52

Figure 2.--Modulus of rupture.

MOR (psi)

 Homogenous OSB(Per) OSB (Par)

100% kenaf (29.4) (9.4) (11.8)

75% kenaf+25%Aspen (16.8) (27.1) (7.6)

50% kenaf+50%Aspen (24.1) (5.4) (11.1)

25% kenaf+75%Aspen (21.1) (9.3) (6.0)

100%Aspen (9.2) (13.2) (9.3)

Per = Flakes Oriented in perpendicular direction to length to test
sample.
Par = Flakes Oriented in parallel direction to length of test sample.
C.V.(%) = Values in parenthesis are coefficient of variation.
Minimum OSB value of MOR required by CSA 0437 Standard is 4200 Psi
(Parallel) and 1800 Psi (Perpendicular).

Note: Table made from bar graph.

Table 3.--Factorial analysis of aspen-kenaf composition boards. (a)

Independent variables MOR MOE IB TS LE

Kenaf content S** S** S** S** NS
Resin content NS NS S* S* NS
Kenaf x resin NS NS -- NS NS
Density NS NS NS NS NS
Board type S* S NS NS NS
Face direction (type) S* S -- -- NS

 (a) S** = highly significant at 1 percent level: S* = significant at
5 percent level: NS = not significant at 5 percent level.

Figure 3.--Modulus of elasticity.

MOE (x1,000 psi)

 HoMogenous OSB (per) OSB (par)

100% Kenaf (14.2) (13.9) (24.0)

75% Kenaf+25%Aspen (9.0) (16.7) (11.4)

50% Kenaf+50%Aspen (21.3) (14.2) (0.1)

25% Kenaf+75%Aspen (34.2) (24.5) (16.2)

100% Aspen (8.7) (15.7) (7.2)

Minimum OSB value of MOE required by CSA 0437 standard is 800,000 psi
(parallel) and 225,000 psi (perpendicular).

Note: Table made from bar graph.

Figure 4.--Internal bond strength.

Internal Bond (psi)

 Homogenous OSB

100% Kenaf (9.7) (21.13)

75% Kenaf+25%Aspen (11.8) (4.7)

50% Kenaf+50%Aspen (18.0) (12.6)

25% Kenaf+75%Aspen (25.0) (14.1)

100%Aspen (13.2) (18.5)

All Values are average of two samples

Minimum OSB value for internal bond required by CSA 0437 standard is 50
psi.

Note: Table made from bar graph.

Figure 5.--Thickness Swell.

Thickness Swelling(%)

 Homogenous OSB

100% Kenaf (30.1) (3.7)

75% Kenaf+25%Aspen (31.4) (22.7)

50% Kenaf+75%Aspen (27.2) (8.7)

25% Kenaf+75%Aspen (11.7) (23.6)

100%Aspen (9.3) (11.3)

Minimum OSB value for internal bond required by CSA 0437 standard is
15%.

Note: Table made from bar graph.

Figure 6.--Linear expansion.

LE(%)

 Homogenous OSB

100% Kenaf (0.0) (14.6)

75% Kenaf+25%Aspen (10.0) (5.2)

50% Kenaf+50%Aspen (28.3) (15.1)

25% Kenaf+75%Aspen (26.0) (13.6)

100%Aspen (24.7) (13.7)

Minimum OSB value for internal bond required by CSA 0437 standard is
0.35% (parallel).

Note: Table made from bar graph.

Figure 7.--Hardness.

Hardness (lbs)

 Homogenous OSB

100% Kenaf (22.5) (7.8)

75% Kenaf+25%Aspen (26.8) (17.1)

50% Kenaf+50%Aspen (14.9) (13.8)

25% Kenaf+75%Aspen (17.2) (9.1)

100%Aspen (28.4) (1.8)

Note: Table made from bar graph.


Literature cited

American Society of Testing Materials (ASTM), 1996. Standard test methods for evaluating properties of wood-base fiber and particle panel materials ASTM D 1037-96a, ASTM. West Conshohocken, PA.

Bagby, M.O., R.L. Cunningham, and T.F. Clark, 1975, Kenaf pulp - soda vs. sulphate, Tappi J. 58(7):121-123.

Chow, P., M.O. Bagby, and J.A. Youngquist, 1993. Furniture panels made from kenaf stalks, wood waste, and selected crop fiber residues. In: Proc. of the 5th Inter. Kenaf Conference. California State Univ. at Fresno, Fresno, CA.

Maldas, D. and B.V. Kokta, 1995. Composite molded products based on recycled thermoplastic and waste cellulosics, II. Kenaf fiber-recycled PE composites, J. of Reinforced Plastics and Composites 14(5):458-470.

English, B., P. Chow, and D.S. Bajwa, 1996. Processing of plant fibers into composites. In: Paper and Composites from Agro-Based Resources. Chapter 8, CRC. Lewis Publishers. CRC Press, Inc., Boca Raton, FL. pp. 269-299.

Sellers, T., D.J. Miller, and M.J. Fuller, 1994. Kenaf core as a board raw material, Bull, 1011. Mississippi Agri, and Forestry Experiment Sta., Starkville, MS. pp. 28-29.

Structural Board Association (SBA), 1998. OSB Performance by Design. Table 1, CSA 0437 and PS-2-92. SBA, Toronto, ON, Canada, 6 pp.

The authors are, respectively, Research Scientist, Masonite International Corp., Coates Technical Center, W. Chicago, IL 60185; and Professor, Natural Resource and Environmental Sciences, Univ. of Illinois, Urbana-Champaign, IL 61801. This material is based upon work supported by the USDA Cooperative State Research Education and Extension Service under Project 1-6-54092 and Hatch 65-389 at the University of Illinois; and the Illinois Council on Food and Agricultural Research. State of Illinois Project 00E-052-1. Acknowledgement should also be given to Georgia Pacific Resin Company for providing resin and wax in this research. This paper was received for publication in December 2001. Article No. 9412.

Dilpreet S. Bajwa *

Poo Chow *

* Forest Products Society Member.

[C]Forest Products Society 2003.

Forest Prod. J. 53(10):30-35.
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Author:Bajwa, Dilpreet S.; Chow, Poo
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Date:Oct 1, 2003
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