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Rheological Characterization of Wood & Wood Composites
Chip FrazierProfessor
Wood Science & Forest Products
DirectorWood-Based Composites Center
Rheological Characterization of Wood & Wood Composites
Outline
• Wood rheology
• Force mode
• Instrumentation
• Sample preparation
• Moisture control
• Linear viscoelastic response (LVR)
• Example Data
Wood Rheology
Rheology: Study of the flow and deformation of materials.
Reflecting chemical structure, molecularand supramolecular ordering
Static & Dynamic Force ModesA
pplie
d S
tress
Res
ulta
nt S
train
time
Oscillatory perturbation Static perturbationA
pplie
d S
tress
Res
ulta
nt S
train
Time
Instrumentation
Instrumentation
Modern instrumentation requires a choice between:
• Strain controlled machines
• Stress controlled machines
Strain Control: Apply displacement, anduse a load cell to measure the stress.
Stress Control: Apply a force, and use an interferometer to measure the displacement.
Sample Preparation
1. Machine a geometry that suits your needs.
Grain orientation
Specimen size
Clamp modes & clamp sizes
2. Hygro-thermal history (Solvent extraction?) is extremely important.
3-point bending cantilever bending parallel plate
Sample Preparation
Moisture Control
As always, moisture control during analysis is critical.
We have 3 choices:
1. Completely dry
Simple
Reveals weak, secondary relaxations
Thermal decomposition may be a concern
2. Completely saturated
Somewhat simple to dificult
Reveals strong, primary relaxations
Thermal decomposition of less concern
3. RH control
Difficult and very expensive
Moisture Control
-150 -100 -50 0 50 100 150
108
109
0.04
0.08
0.12
0.16
0.20
Temperature (°C)
Black: 1st scanBlue: 2nd scan
Sto
rage
Mod
ulus
(Pa)
ethylene glycol
tan
δ
-150 -100 -50 0 50 100 150
108
109
0.04
0.08
0.12
0.16
0.20
Sto
rage
Mod
ulus
(Pa)
Temperature (oC)
2nd scan 3rd scan
tan
δ
Completely dry pine in single cantilever bending
Saturated yellow-poplar in parallel-plate torsion
Solvent Submersion
0 10 20 30 40 50 60 70 80 90 100
4E7
8E7
1.2E8
1.6E8
0.04
0.08
0.12
0.16
0.20
0.24
tan
δ
RT_1 RT_2 RT_3
Stor
age
mod
ulus
, G' (
Pa)
Temperature (0C)
pH = 7
108
109
Black: 1st scanBlue: 2nd scan
glycerine 0.04
0.08
0.12
0.16
0.20
0.24
0 40 80 120 160
108
109
0.04
0.08
0.12
0.16
0.20
0.24Black: 1st scanBlue: 2nd scan
Temperature ( oC)
dimethylformamide
108
109
0.04
0.08
0.12
0.16
0.20
0.24Black: 1st scanBlue: 2nd scan
ethylene glycol
Sto
rage
mod
ulus
(Pa)
tan
δ
0 10 20 30 40 50 60 70 80 90 100
4E7
8E7
1.2E8
1.6E8
0.04
0.08
0.12
0.16
0.20
0.24
tan
δ
RT_1 RT_2 RT_3
Stor
age
mod
ulus
, G' (
Pa)
Temperature (0C)
pH = 10
Linear Viscoelastic Response (LVR)
• The response (e.g. strain) is directly proportional to the mechanical input (stress).
• Within the LVR, polymer packing is not altered and the response is independent of the input level.
• Data within the LVR is easiest to describe mathematically.
• Experiments are generally performed at the highest stress/strain within the LVR, optimizing signal/noise ratio without causing an erroneous response.
Linear Viscoelastic Response (LVR)
10000 100000
4x107
6x107
8x107
108
1.2x108
0 75000 150000 225000
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7 S
tora
ge M
odul
us (P
a)
Oscillation Stress (Pa)
Temperature 80oC
Stra
in (%
)
R-T T-R X-L
Oscillation Stress (Pa)
Yellow-poplar submerged in DMF under parallel-plate torsion
Example Data
N C ONCO
NC
O
pMDI analysis
Phenolic latex additives
Species Dependence of pMDI Performance
10 100 1000 10000
1.0
1.2
1.4
1.6
1.8
1.0
1.2
1.4
1.6
1.8
120oC
Time (sec)
Poplar
120oC
Control Resin-treated
Nor
mal
ized
Com
plia
nce
Pine
Open-cell vacuum impregnation:
• Anhydrous acetone/pMDI solutions.
• Cure: 120°C, 2 hrs, (3 – 5 % weight gain after cure).
• Heat control samples in the same fashion.
Creep response of dry wood
Now… Ethylene Glycol (EG) Plasticized Wood
• pMDI-treated and control specimens (impregnations as before but resin concentration 13% for poplar, 20% for pine).
• Post-cure resin content 5.5% for poplar, 5.3% for pine.
• Saturated in EG and subjected to 3 hour creep at 50°C (initially heated 90°C for 30 min., cooled (20°C/min) to 50°C; held at 50°C for 30 min).
• 5 replications each.
1.0
1.5
2.0
2.5
3.0
10 100 1000 10000
1.0
1.5
2.0
2.5n = 0.782 ± 0.006n = 0.785 ± 0.011
Control PMDI impreg
Control PMDI impreg
Nor
mal
ized
Com
plia
nce
Pine
n = 0.763 ± 0.006n = 0.765 ± 0.006
Time (sec)
Poplar
( ) ( )
−−×−+=
−n
urutDDDtD
1
exp1τ
Time/Temperature Superposition w/ EG Plasticized Wood
10-5 10-3 10-1 101 103101 102 103
5
10
15
20
25
30
Time (aT*s)
Com
plia
nce
(10-9
Pa-1
)
Time (s)
Now 10 decades of time!
Time/Temperature Superposition w/ EG Plasticized Wood
10-5 10-3 10-1 101 103101 102 103
5
10
15
20
25
30
Time (aT*s)
Com
plia
nce
(10-9
Pa-1
)
Time (s)
20 40 60 80-2
0
2
4
6
8
20 40 60 80 100
Control PMDI-Impreg
log
a T
Temperature (oC)
Pine
Poplar
Composite Specimens with Latex Adhesive
100x
Adhesive
Wood
Adhesive
Wood
Influence of Resol Fortifiers on Latex Durability
0
200
400
600
800
1000
Com
plet
e Sa
mpl
e Fa
ilure
Accel-WeatherControl
PVAc-AlCl3-PF
Frac
ture
Tou
ghne
ss (J
/m2 )
Glc Gla
PVAc-AlCl3
Control Accel-Weather
Influence of Resol Fortifiers on Latex Durability
20 40 60 80 100 120 140 160
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
106
107
108
109
Tan
δ
Temperature (oC)
COMPOSITES
PVAc-AlCl3 PVAc-AlCl3-PF Weathered PVAc-AlCl3 Weathered PVAc-AlCl3-PF
Sto
rage
Mod
ulus
(Pa)
20 40 60 80 100 120 140 160
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
106
107
108
109
Tan
δ
Temperature (oC)
FILMS
PVAc-AlCl3 PVAc-AlCl3-PF Weathered PVAc-AlCl3 Weathered PVAc-AlCl3-PF
Sto
rage
Mod
ulus
(Pa)
Influence of Resol Fortifiers on Latex Durability
10-2 101 104 107 1010 1013 1016 1019 1022
10-8
10-7
10-6
10-5
20 40 60 80 100 120 140
-16
-12
-8
-4
0
Time (s)
Com
plia
nce
(Pa-1
)Films PVAc-AlCl3
PVAc-AlCl3-PF W-PVAc-AlCl3 W-PVAc-AlCl3-PF
Log
aT
Temperature (oC)
Creep master curves from time/temperature superposition
Influence of Resol Fortifiers on Latex Durability
Weathered PVAc-AlCl3-50k x Weathered PVAc-AlCl3-PF-50k x
Summary
• Wood rheology deals with amorphous wood polymer relaxations.
• These relaxations reflect chemical structure, and morphology, as well as changes caused by chemical, thermal and other treatments.
• Such methods require a significant investment of funds and time, but they’re worth it.
Acknowledgements