Structure/Property Analysis of
Polyurethane Adhesives
Charles Frazier
Sustainable Biomaterials, Virginia Tech
Wood-Based Composites Center
Dakai Ren
(Macromolecular Science & Engineering, Virginia Tech)
Dow Chemical Company
Objectives
• Develop novel methods for structure/property analysis in
moisture-cure polyurethane adhesives (PUR).
• Determine the relative suitability of analyzing:
1. Neat PUR films, or
2. Composite wood/PUR specimens:
a. excised from real bondlines, or
b. failure surfaces from tested fracture specimens.
Materials
• Polymeric methylene(bisphenylisocyanate) (pMDI) NCO = 31%.
• Polypropylene glycol diols (PPG).
1. PPG2000: Mn = 2000 g/mol, polydispersity = 1.21.
2. PPG400: Mn = 400 g/mol, polydispersity = 1.16.
• Curing catalyst 4,4′-(Oxydi-2,1-ethanediyl)bismorpholine
JEFFCAT® DMDEE.
• Southern yellow pine (Pinus spp.),L × T ×R = 250 × 150 × 10 mm;
equilibrated RH= 65%, 25°C.
Synthetic Methods
• PUR prepolymers, mixture of PPG2000 and PPG400.
• NCO/OH = 5, 80°C (N2, 3 h) until complete by FTIR (2270 cm-1).
• Liquid prepolymers stored N2 / P2O5 until cure; < 15 days.
• Catalyst DMDEE (0.5%) mechanically mixed with prepolymer
under N2 immediately prior to use in films or bondlines.
PUR
Prepolymer
PPG2000/PPG400
mass ratio
Hard segment
content (%)
Theoretical
NCO (%)
Measured
NCO (%)
PU8020 80/20 53.5 13.28 13.35 ± 0.04
PU5050 50/50 65.4 16.23 16.10 ± 0.02
PU2080 20/80 72.5 17.97 17.85 ± 0.09
Analytical Methods
• Liquid PUR prepolymers.
1. Size exclusion chromatography (SEC) in THF; universal
calibration w/ polystyrene standards.
2. Differential scanning calorimetry (DSC); 20 mg equilibrated
−90°C 5 min, heat to 20°C at 10°C/min.
3. Viscosity, 25 mm dia. parallel-plate geometry (gap=400 µm),
steady-state flow, shear rate= 1–100 s−1, under N2.
Analytical Methods
• Cured neat films.
• PUR prepolymer cast onto dry polyethylene, 152 µm-gap film
applicator; placed in RH = 65% chamber; RT cure 2 weeks.
1. Dynamic mechanical analysis (DMA), compressive-torsion,
8 mm dia. SS parallel-plates, 10-15 N normal force, 3°C/min,
1 Hz, strain 0.1% within linear response, dry specimens or
immersed in H2O.
2. Infrared spectroscopy (FTIR), attenuated total reflection of
bulk film immediately after sectioning.
3. Water-vapor sorption; 10-mL vial W/ distilled H2O, rubber
septum w/ copper wire passing through holding 10 mm2 film;
measure weight gain.
4. Atomic force microscopy (AFM), embedded in epoxy,
cryo-ultramicrotome, phase images analyzed w/ image
analysis software.
Analytical Methods
• Bonded specimens.
• Flat-sawn pine, 250 g/m2 applied to one side, cold-press 0.69 Mpa
(100 psi), 24 h, ripped into double cantilever beams.
1. Mode-I fracture similar to Gagliano & Frazier
(Wood & Fiber, 2001, 33(3):377–85).
2. Fluorescence microscopy, 30 µm sections, stained 0.5%
Safranin O, A Zeiss Axioskop microscope, mercury lamp, filter
360-nm excitation, 400-nm dichromatic mirror, 420-nm
emission filter.
3. Infrared spectroscopy (FTIR), attenuated total reflection of
failure surfaces immediately after fracture testing.
4. AFM as above.
Analytical Methods
• Bondlines excised from bonded fracture specimens; wood trimmed
from bondline yielding 650 μm thick specimens.
• DMA as described, dry specimens or immersed in H2O.
Adhesive Adhesive layer
thickness (µm)
Penetration to
each side (µm)
Adhesive layer
fraction (v %)
Penetration
fraction (v%)
Total bondline
fraction (v%)
PU8020 57 128 9 20 29
PU5050 81 112 12 19 31
PU2080 133 34 20 5 25
Results: Liquid Prepolymers
-80 -60 -40 -20 0
-0.4
-0.3
-0.2
-0.1
0.0
Heat
Flo
w (
w/g
)
Temperature (C )
PPG 400
PPG 2000
PU8020
PU5050
PU2080
40
60
80
100A
pMDI
PPG2000
PPG400
B
11 12 13 14 15 16 17 18
40
60
80
PU8020
PU5050
PU2080
RI
Inte
ns
ity
(m
V)
Retention Volume (mL)
While PU2080 had the
lowest Mol. Wt. …
…higher hard segment
content resulted in highest Tg,
and highest viscosity due to
greater intermolecular interactions. 1 10 100
105
106
PU8020
PU5050
PU2080
Vis
co
sit
y (
mP
as)
Shear Rate (1/s)
PUR8020: Neat Films & Bonded DCB Fracture Specimens
0-100 101-500 > 500
20
25
30
35
40
45
50
Re
lati
ve
Oc
cu
ren
ce
(%
)
Soft Domain Size (nm2)
PUR Film
PUR/Wood Composite
Error bars = ± 1 stan.dev.; n = 6,
2 images x 3 separate specimens.
AFM phase images:
A. PUR8020 bulk neat-film;
B. PUR8020 within a wood-cell
lumen, excised from bondline.
• Wood alters size distribution of the dispersed soft phase.
• All three PUR’s have a continuous hard phase.
Bonded DCB Fracture Specimens
PU8020 PU5050 PU20800
20
40
60
80
100
120
140
160
Bo
nd
lin
e T
hic
kn
ess (m
)
PU8020 PU5050 PU20800
50
100
150
200
250
300
Eff
ecti
ve P
en
etr
ati
on
(m
)
PU2080
100 µm
PU2080PU2080
100 µm
Clear differences in bondline thickness and
adhesives penetration as related to viscosity.
Mode-I Fracture Performance
PU8020 PU5050 PU20800
50
100
150
200
250
300
350
400 Control
Weathered
Cri
tical F
ractu
re E
nerg
y (
J/m
2)
Accelerated weathering:
1. 25°C vacuum water soak
(10 mmHg, 1 hr);
2. Continue soaking (1 atm, 1 hr);
3. Dry (65°C, 22 h);
4. Steam (105°C, 1.2 bar, 2 hr)
5. repeat step 1–3.
Error bars = ± 1 stan.dev.; n ~ 60 for each mean.
• Unweathered control exhibits little dependence on PUR composition.
• Surprising; perhaps indicative of continuous hard phase.
• Weather durability dependent upon PUR composition.
Infrared Spectroscopy of Carbonyl Region
1750 1725 1700 1675 1650 1625
0.0
0.2
0.4
0.6
0.8
1.0
PU8020_Film
PU5050_Film
PU2080_Film
No
rma
lize
d A
bs
orb
an
ce
Wavenumber (cm-1
)
Free
Urethane
H-bonded
Urethane
Free
Urea
Monodentate
Urea
Bidentate
Urea
1750 1725 1700 1675 1650 1625
0.0
0.2
0.4
0.6
0.8
1.0
1.2
PU2080
PU5050
PU8020
No
rmalized
Ab
so
rban
ce
Wavenumber (cm-1)
Free
Urethane
H-bonded
Urethane
Free
Urea
Monodentate
H-bonded Urea
Bidentate
H-bonded Urea
Neat films, interior PUR/wood failure surface
Average spectra; error bars = ±1 stan. dev. (n=6 films; n=18 composites); normalized
by phenylene (1594 cm−1 not shown) with intensities of 1.0, 1.22, and 1.36 based on
hard phase contents of PU8020, PU5050, and PU2080, respectively.
Infrared Spectroscopy of Carbonyl Region
1750 1725 1700 1675 1650 1625
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Wavenumber (cm-1)
PU2080 Control
PU2080 Weathered
PU5050 Control
PU5050 Weathered
PU8020 Control
PU8020 Weathered
Ab
so
rba
nc
e
Wavenumber (cm-1)
Free
Urethane
H-bonded
Urethane
Free
UreaMonodentate
H-bonded Urea
Bidentate
H-bonded Urea
PUR/wood failure surface
Average spectra; error bars = ±1 stan. dev. (n=18); normalized by phenylene
(1594 cm−1 not shown) with intensities of 1.0, 1.22, and 1.36 based on hard phase
contents of PU8020, PU5050, and PU2080, respectively.
Infrared Spectroscopy of Carbonyl Region
• Infrared distinguishes neat films from bondline failure surfaces.
Demonstrates wood/adhesive interaction.
• Impact of weathering detected on failure surfaces.
• No simple correlation of infrared to PUR weather durability.
There is perhaps more useful information in this data
but we did not analyze the subtleties.
Water Vapor Absorption in Neat Films
0 5 10 15 200
5
10
15
PU8020
PU5050
PU2080
H2O
Ab
so
rpti
on
(%
mass in
cre
ase)
Time (days)
• Greater hard phase content
results in lower water absorption.
• Correlates to greater weather
durability in fracture testing.
1st Heat DMA Neat Films
Average DMA 1st heat PUR films (3°C/min, 1 Hz);
error bars = ±1 standard deviation (n=3).
25 50 75 100 125 150 17510
5
106
107
PU8020 Film
PU5050 Film
PU2080 FilmSto
rag
e m
od
ulu
s (
Pa
)
Temperature (C)25 50 75 100 125 150 175
0.1
0.2
0.3
0.4
0.5
tan
25 35 45 55 65 75 85 9510
5
106
107
PU8020 Film/water
PU5050 Film/water
PU2080 Film/water
Sto
rag
e m
od
ulu
s (
Pa)
Temperature (C)
25 35 45 55 65 75 85 95
0.1
0.2
0.3
0.4
0.5
tan
Dry Films Films Saturated/Immersed in H2O
1st Heat DMA Neat Films
Average DMA 1st heat PUR films (3°C/min, 1 Hz);
error bars = ±1 standard deviation (n=3).
Dry Films
25 35 45 55 65 75 85 9510
5
106
107
PU8020 Film/water
PU5050 Film/water
PU2080 Film/water
Sto
rag
e m
od
ulu
s (
Pa
)
Temperature (C)
25 50 75 100 125 150 17510
5
106
107
PU8020 Film
PU5050 Film
PU2080 FilmSto
rag
e m
od
ulu
s (
Pa)
Temperature (C)
Films Saturated/Immersed in H2O
1st Heat DMA Neat Films
Average DMA 1st heat PUR films (3°C/min, 1 Hz);
error bars = ±1 standard deviation (n=3).
Dry Films
25 35 45 55 65 75 85 95
0.1
0.2
0.3
0.4
0.5
Temperature (C)
PU8020 Film/water
PU5050 Film/water
PU2080 Film/water
ta
n
25 50 75 100 125 150 175
0.1
0.2
0.3
0.4
0.5
Temperature (C)
PU8020
PU5050
PU2080
ta
n
Films Saturated/Immersed in H2O
1st Heat DMA Neat Films
• Water immersion DMA reveals reduced stiffness in all PUR’s,
consistent with water vapor absorption.
• Lowest hard-segment content (PU8020):
Dramatically plasticized.
Steaming temp. exceeds hard-phase softening temp.
• High hard-segment content (PU5050, PU2080):
Remains stiff during steam treatment.
PU5050, mechanical damping increased near steaming temp.
1st Heat DMA of Weathered & Unweathered Specimens
0 30 60 90 120 150 18010
6
107
108
PU8020 Control
PU8020 Weathered
Sto
rag
e M
od
ulu
s (
Pa
)
Temperature (C)
0 30 60 90 120 150 1800.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
ta
n
A)
0 40 80 120 160
2E6
4E6
6E6
8E6
1E7 A)
PU8020 Film Control
PU8020 Film Weathered
Sto
rag
e M
od
ulu
s (
Pa)
Temperature (C)0 40 80 120 160
0.0
0.1
0.2
0.3
0.4
ta
n
Neat Films Excised Bondlines
• Weathering caused minor damping change in neat films.
• Weathering caused much greater damping change in bondline specimens.
Average dry-DMA 1st heating scans of control and weathered PUR films (left) &
bondlines (right) (3°C/min, 1 Hz.); error bars = ±1 standard deviation (n=3).
1st Heat DMA of Weathered & Unweathered Specimens
Neat Films Excised Bondlines
• Weathering caused little/no damping change in neat films.
• Weathering caused minor damping change in bondline specimens.
Average dry-DMA 1st heating scans of control and weathered PUR films (left) &
bondlines (right) (3°C/min, 1 Hz.); error bars = ±1 standard deviation (n=3).
0 40 80 120 160
2E6
4E6
6E6
8E6
1E7 B) PU5050 Film Control
PU5050 Film Weathered
Sto
rag
e M
od
ulu
s (
Pa
)
Temperature (C)0 40 80 120 160
0.0
0.1
0.2
0.3
0.4
tan
0 30 60 90 120 150 18010
6
107
108
A)
PU5050 Control
PU5050 Weathered
Sto
rag
e M
od
ulu
s (
Pa)
Temperature (C)
0 30 60 90 120 150 1800.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
tan
1st Heat DMA of Weathered & Unweathered Specimens
• For lowest hard-segment content (PU8020):
Films: weathering caused little change in mechanical damping.
Bondlines: weathering caused large reduction in damping.
• For high hard segment content (PU5050, PU2080):
• Films: weathering caused little change in mechanical damping.
• Bondlines: weathering caused little change in damping.
Summary
• AFM & infrared spectroscopy demonstrate significant wood/PUR
interaction, suggesting superiority of composite specimens.
• Water vapor absorption of neat films correlated to fracture durability.
• Water-submersion DMA of neat films correlated to fracture durability.
• DMA of weathered vs unweathered specimens suggests that bondlines
specimens may be superior to neat films.
• Conclude that both neat films and bondline specimens are valuable for
PUR structure/property analysis.
• Do these correlations have real meaning? More analysis required to
determine causality.
Acknowledgements
• The Wood-Based Composites Center, a National Science
Foundation Industry/University Cooperative Research Center.
• The Virginia Tech Graduate School.