Pamela Rebolledo-Valenzuela, PhD student
Alain Cloutier, PhD
Martin-Claude Yemele, PhD
Centre de recherche sur les matériaux renouvelables
Wyoming – U.S.A
June 11, 2015
Gas Permeability and Porosity
of Fiberboard Mats as a
Function of Density
1
Outline
1. Background
2. Objectives
3. Material and methods
4. Results
5. Conclusions
6. Future work
7. Acknowledgements
2
• Medium density fiberboard performance and quality are determined
by hot-pressing
• Hot-pressing simulation can help to improve this process
• Thermo-hygromechanical behavior of mats is necessary for simulation
• Interaction of heat and mass transfer mechanisms are determined by
mat properties
Background
from Bolton and Humphrey, 1988
3
Background
𝑘𝑔∗ =
𝑄𝐿𝑃
𝐴∆𝑃 𝑃
• Ease of flow across a porous material under a pressure gradient
• Applicable to composite materials
• Permeability related to porous structure
• Mats permeability is not isotropic (particle size and density profile)
• Permeability is involved in heat and mass transfer during hot-
pressing
4
kg* : apparent gas permeability (m3
gas m-1
wood s-1 Pa-1)
Q : volumetric gas flow rate (m3gas s
-1)
L : length in the flow direction (mwood)
P : pressure at which Q is measured (Pa)
A : cross-sectional area of the specimen (m2wood)
ΔP : pressure differential across the specimen (Pa) P : average pressure across the specimen (Pa)
Wood Permeability (Siau 1984)
K= 𝑘𝑔 ∗ 𝜇K : intrinsic permeability (m2)
: viscosity (Pa s)
kg : gas permeability corrected for slip flow (mgas3 mwood
−1 s−1Pa−1)
Background
• Void inside wood in relation to total volume
• Wood-adhesive-void system (microstructure)
• Void spaces inter-fiber and intra-fiber
• Porosity is dependent of particle size and densification level
• Porosity impacts heat and mass transfer during hot-pressing
𝑉𝑎 = 1 − 𝐺1
𝐺0𝑤 +
0,01𝑀
𝐺𝑔
5
Va: wood porosity (m3void m-3
wood)
G : density
G0w: anhydrous cell wall density
M : moisture content (%)
Gg: density wood bound water
Wood Porosity (Siau 1984)
Background
Density and its relation to permeability and porosity
• Density increases during the hot-pressing process
• Density affects mat properties
• Permeability controls pressure inside the mat
• Porosity determines heat and mass transfer inside the mat
K = e
1
a+bρ+c
Lnρ
Permeability-density relationship Porosity-density relationship
(Thömen and Humphrey 2006; von
Haas 1998)(Kavazović et al. 2012)
6
K: intrinsic permeability (m2)
a, b, c: regression coefficients
ρ: density (kg/m3)
Φ: porosityr: oven-dry density (kg/m3)
Objectives
• To measure gas permeability of fiberboards at different
densities
• To measure porosity of fiberboards at different
densities
• To establish the relationship between these properties
and density for hot-pressing process simulation
7
Materials and methods
Materials
• Fibers : 2-3% MC
• Urea formaldehyde content: 14%
• Wax content: 1%
Panels fabrication
• Blending
• Mat forming
• Hot pressing
0.00
200.00
400.00
600.00
800.00
1000.00
0 2 4 6 8 10 12 14 16
den
sity
(kg
/m3 )
distance from surface (mm)
Homogeneous
density profile
8
Sampling
Samples for porosity
measurement
Samples for permeability measurement
• cube 1 cm3
• 50 mm diameter
• 16 mm thickness
Materials and methods
9
Materials and methods
Microtomography
• X-ray absorption
• 8.75 m resolution
• Cube sample
Porosity test
Mercury porosimetry
• Sample impregnated
• Mercury application by pressure
• Cube sample
10
• Microtomed thin layer from sample
• Microphotography
• WinCELL software
• Size layer area: 2.83 x 3.44 mm
• Thickness microtomed-sample: 3 m
• Picture size: 0.80 x 1.09 m
Materials and methods
Porosity measurement
11
Materials and methods
Permeability measurement
• Air flow through the sample
• Reading of flow by rotameters connected to chamber outlet
• 50, 100, 150 and 200 kPa
Flow
12
Results
Porosity
Statistics Microtomography Mercury
porosimetry
Image
analysis
Average porosity 55% 31% 50%
Standard deviation 0.5% 2% 4%
Density: 850 kg/m3
13
Results
Image analysis technique
Density: 850 kg/m3
Porosity: 51% Porosity: 48%
14
Captured image Analyzed image Captured image Analyzed image
ResultsPermeability
Slip flow
Standard
deviation
Density: 850 kg/m3
Average intrinsic
permeability
1.35 x 10-14 m2
5.10 x 10-14 m2
15
0.00E+00
1.00E-14
2.00E-14
3.00E-14
4.00E-14
5.00E-14
6.00E-14
7.00E-14
8.00E-14
800.00 820.00 840.00 860.00 880.00 900.00 920.00
K V/S DENSITY
Density (kg/m3)
K (m2)
y = 2E-4x + 2E-09
0.0E+00
1.0E-09
2.0E-09
3.0E-09
4.0E-09
5.0E-09
0.0E+00 2.0E-06 4.0E-06 6.0E-06 8.0E-06
kg*
(m3
gasm
-1w
oo
ds-
1P
a-1
)
1/ 𝑃 𝑃𝑎−1
Conclusions
• Porosimetry by X-ray microtomography and image analysis
gave similar results, about 55% for a density of 850 kg/m3
• The mercury porosity was 31% in panels with density 850
kg/m3
• For gas permeability measurement, slip flow was observed
• The average intrinsic permeability was 5.10 x 10-14 m2 at 850
kg/m3
16
Future work
• To complete the measurement of porosity and gas
permeability as a function of density
• To consider fiber morphology as a factor
17
Acknowledgements
The authors are grateful to the Natural Sciences and
Research Council of Canada (NSERC) for funding this
research under Discovery Grant #121954-2012.
18
Thank you!
UNIVERSITÉ LAVAL, Québec, Canada