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