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.