design
Agricultural fibres can be pressed or extruded to plate material. The design that is elaborated here exists of kenaf particles extruded to a tube.
Kenaf (Hibiscus cannabinus) is a fast growing fiber crop related to cotton, okra, and hibiscus. The plants, which reach heights of 2,4 to 6 meters, are harvested for their stalks from which the fiber is extracted. The fiber is used in the manufacture of industrial textiles, ropes, and twines. Kenaf is among the most widely utilized of the bast fibers. (CES EDUPACk 2009)
Harvesting of Kenaf, source: http://bridgemail.bigbridge.com.au
Panasonic has set up a plant in Malaysia to manufacture kenaf core fibre boards and export them to Japan. Kenaf core fibres are comparable to hard wood. (http://en.wikipedia.org/wiki/Hibiscus_cannabinus)
source: http://www.stramit-int.com/index.html
variables
Input
Fibre type (kenaf core)
holocellulose content (71.24%)
lignin content (23.22%)
ash content (5.93%)
Kenaf core is a product from the flax plant. The properties of resin free fibreboard are superior to wheat straw, reed, palm or meadow (Jianying Xu 2006).
Fibre length (5,5 +- 2,49mm) 1,6//8 cm
Fibre length balances between modulus of elasticity and modulus of rupture.
Fibre diameter (284 +- 136 µm)fiber width (0.82–1.73 mm) cell wall thickness (3.36–5.25 lm) lumen diameter (5.82–10.39 lm)
Resin type (none)
Most fibre boards contain UF or MDI glues. Sometimes the natural lignin in the fibres can provide the necessary bonding.
Resin content (0 %)
Steam-injection during pressure -> yes
Steam injection gives better properties to the board (widyorini2005)
Pressure (0,6 MPa) 0,6/0,8
This results in a density (500 kg/m3). 300
Time of cooking (10min) 20/30
Cooking of fibres before bonding gives higher internal bonding and less thickness swelling.
Test conditions
Relative humidity (10-90%)
Outcomes
Modulus of Elasticity MOE (2,3 ± 0,1 MPa)
Modulus of Rupture MOR (19.4 ± 2,0 MPa)
Internal Bond IB (0,24 ± 0,04 MPa)
Thickness Swelling TS (18 ± 1 %)
design strength performance
height 350 mm
width 1200 mm
flange thickness 28 mm
web thickness 28 mm
Elastic deformation
MOE of 2,3 MPa results in a vertical deformation of the roof of 5/384*1,7*9000^4/ (2400*2*28*1200*175^2)= 29 mm. This is within the tolerance of 45 mm.
Long term deformation
Long term deformation caused by creep is not investigated yet. MDF shows a a comparable creep behaviour that could be studied. Conclusions from this study (Fernandez1998) are that the stress should remain below 20% of MOR to avoid rupture. Also a high relative humidity should be avoided.
Stressed Skin Panel tests with wood webs and OSB flanges showed that tests on deformation and Modulus of Elasticity showed similar outcomes (Kliger 1995).
bending strength
With a MOR of 17,4 MPa and a bending resistance of 2,1 *10^9 mm4 is a bending moment of 204 kNm possible. The designed load of 21 kNm is below this.
compression strength
The roof leans on the walls. The weight per half roof element is 70*4,5*1,2=380 kg. The surface of a wall column is 2 sides*260 depth*28 thickness=. This means that the compression force is 380*9,81/14560=0,26 N/mm2. This is below the maximum compression force of 17,4 N/mm2.
design moisture performance
Moisture and temperature conditions are calculated for worse case scenario.
inside temperature 20 oC
inside relative humidity 60%
outside temperature -10 oC
outside relative humidity 50%
To control relative humidity during high outside relative humidity, a vapour barrier around the construction could be helpful. Food industry uses bee wax coatings to improve the freshness duration.
The same wax layer on the inside and outside will trap the moisture with condensation as result.
A solution is to apply a thicker wax layer on the inside to lower the vapour pressure in the construction and insulation.
Rtot=0,04+0,06+0,06+5,00+0,06+0,10+0,13=5,45 m2K/W
Q=30/5,45=5,4 W/ m2
Tis= 20-0,13*5,5=19,3 oC -> Pio.max=2240 Ps
Tloam= 20-(0,13+0,1)*5,5=18,7 oC -> Ploam.max=2175 Ps
Twax= 20-(0,13+0,1+0)*5,5=18,7 oC -> Pwax.max=2175 Ps
Tflange= 20-(0,13+0,1+0,06)*5,5=18,4 oC -> Pweb.max=2117 Ps
Tstraw= 20-(0,13+0,1+0,06+5)*5,5= -9,1 oC -> Pweb.max=281 Ps
Tupperflange= -10+(0,04+0,06+0)*5,5= -9,5 oC -> Pweb.max=271 Ps
Tes= -10+0,04*5,5= -9,8 oC -> Pweb.max=264 Ps
Figure, Glazer diagram of vapour pressure in construction.
references
Jianying Xu. Development of binderless fiberboard from kenaf core, Journal of Wood Science, springer: Japan, 2006.
Kliger, IR, Pellicane, PJ. Prediction of Creep Properties of Chipboard Used in Stressed-Skin Panels Research scientist, Journal of Testing and Evaluation
Volume 23, Issue 6 , Chalmers University of Technology, 1995.
Ragil Widyorini, Jianying Xu, Takashi Watanabe and Shuichi Kawai. Chemical changes in steam-pressed kenaf core binderless particleboard, Journal of Wood Science. Springer: Japan 2005.
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