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