Film Formation of Waterborne Pressure-Sensitive Adhesives

Joseph Keddie

Department of Physics,

University of Surrey, Guildford

3 November, 2004

Pressure Sensitive Adhesives (PSAs)

• PSAs are aggressively and permanently tacky at room temperature, adhering under light pressure.

• Usually a polymer melt at room temperature (Tg< -30 °C)

• Used in medical applications• Used in tapes and labels • Used in graphic arts

Why are PSAs so sticky?

• With close contact (D ~ 0.2 nm) between surfaces, the van der Waals pressure become significant: P ~ A/(6D3) ~ 7000 atm!

• Usual polar or acid/base interactions between the PSA and the substrate, depending on chemistry. But there is no covalent bonding.

• Soft polymers can achieve intimate contact with a rough substrate, leading to mechanical interlocking.

V = 30 µm/s

Contact

tc = 1 sPc = 1 MPa

d

F vd = 0.1 - 500 µm/s

Energy Dissipation in PSA De-bonding

Lakrout, H.; Sergot, P.; Creton, C. J. Adhes. 1999, 69, 307-59.Lakrout, H.; Creton, C.; Ahn, D.; Shull, K. R. Macromolecules 2001, 34, 7448-58.

F

d

Key Challenges in PSAs

• Trend towards clear labels

• Trend towards waterborne, colloidal PSAs

Reduced VOCs

Environmentally-benign

Key Challenges in PSAs

Reduced VOCs

Environmentally-benign

• But the adhesion strength and water resistance of waterborne PSAs are poor!

Key Challenges in PSAs

• But the adhesion strength and water resistance of waterborne PSAs are poor!

After soaking in water for 10 min.:

Poor water resistance Good water resistance

Key Challenges in PSAs

Poor water resistance Good water resistance

Why?• There is a clear need to characterise PSA morphology and relate it to film formationmechanisms: Aim of our work

Polymer-in-water dispersion

Close-packing of particles

Water loss

Dodecahedral structure (honey-comb)

Deformation of particles

Idealised View of Latex Film Formation

Interdiffusion and coalescence

Homogenous Film

Typical Morphologies

Particles are deformed to fill all available space: rhombic dodecahedra

Y. Wang et al., Langmuir 8 (1992) 1435.

Example of Good Coalescence

Immediate film formation upon drying!

Hydrated film

J.L. Keddie et al., Macromolecules (1995) 28, 2673-82.

Tg of latex 5 °C; film-formed at RT

Environmental SEM

1 m

Experimental Evidence for Vertical Non-Uniformity during Drying

E. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10

Densely-packed particle layer

Cryogenic SEM

Theory: Peclet number for vertical drying

• Competition between Brownian diffusion that re-distributes particles and evaporation that causes particles to accumulate at the surface

H

E

Pe << 1

ODHE

Pe =R

RkT

Do 6= Dilute limit

Peclet number for vertical drying uniformity E

Pe >> 1

Simulations of the Vertical Distribution of Particles

Simulations by A.F. Routh

pol

Vertical Position

Pe = 0.2 Top

Close-packed

Simulations of the Vertical Distribution of Particles

pol

Vertical Position

Pe = 1Close-packed

Simulations of the Vertical Distribution of Particles

Vertical Position

pol

Pe = 10

A.F. Routh and W.B. Zimmerman, Chem. Eng. Sci., 59 (2004) 2961-68.

Driving Force for Particle Deformation

Energy “gained” by the reduction in surface area with particle deformation is “spent” on the deformation of particles:

Deformation is either elastic, viscous (i.e. flow) or viscoelastic (i.e. both).

For coalescence of 1 L of latex with a 200 nm particle diameter (50% solids), there are ~1017 particles and A = -1.3 x 104 m2. With = 3 x 10-2 J m-2, then G = - 390 J.

Particle Deformation Mechanisms

Skin Formation

Wet Sintering: pw

rP wa9.12

r

Dry Sintering: pa

Capillary Action: wa

Latex Film Formation Mechanismsand Vertical Homogeneity

HRE

wa

0

kTERH6

Pe

100

10000

1

Wet Sintering: pw

Capillary Deformation: wa

Receding Water Front

Dry Sintering: pa

1

0Skinning

Partial SkinningPSAs!

See A.F. Routh & W.B. Russel, Langmuir (1999) 15, 7762-73

• Very difficult because

(1) Polymer melt surface is very easily indented

(2) By definition, the surface is very sticky!

Atomic Force Microscopy (AFM) of PSAs

Ao : “free” amplitudeAsp : “setpoint” amplitudedsp : tip-surface distancezind : indentation depth

Asp=dsp+zindAo

(>Asp)

dsp/Ao = rsp < 1rsp : setpoint ratio

• Requires careful control and optimisation of

tapping parameters:

Discrete Particles Observed at PSA Surface!

acrylic latex

Tg = 20ºC

non-stickysurface

Ao=18nmdsp=15nm rsp=0.83

PSA latexTg = -33ºC(bimodal

particle size)

looptack on glass =512 N/m

Ao=163nmdsp=75nm rsp=0.46

Top views3m x 3m

scans

Slice views1m x 1m

scans

Vertical scale = 200nm Vertical scale = 50nm

Silicon tip, k = 48 N/m, fo = 360 kHz

Ao=163nm dsp=75nmrsp=0.46 Ra=6.9nm

Ao=123nm dsp=61nmrsp=0.49 Ra=5.8nm

Ao=98nm dsp=50nmrsp=0.51 Ra=4.7nm

Ao=72nm dsp=53nmrsp=0.73 Ra=2.6nm

Ao=38nm dsp=35nmrsp=0.92 Ra=1.2nm

Apparent Surface Topography is Sensitive to

Free Amplitude and Setpoint RatioSame Surface

Amplitude-distance curve obtained from

a PSA surface prone to

indentation, showing a

calculation of the indentation

depth.

-160 -120 -80 -40 0 400

20

40

60

80

100

120

140

160

180adhesion

Ao

tip sticks tothe surface

M

I zind

zind-max

contactpoint

Am

plitu

de (n

m)

Relative scanner displacement (nm)Bar et al., Surface Science, 457 : L404-L412 (2000).

Amplitude-distance curves are used to characterise the tip-sample interactions

Lessons:

• The surface is indented very deeply!

• Tip adheres to surface at tapping amplitudes < 35 nm.

-160 -120 -80 -40 00

20

40

60

80

100

120

140

160

180

Ao=123nm

Ao=98nm

Ao=72nm

Ao=38nm

Ao=163nm

Am

plitu

de (

nm)

Relative scanner displacement (nm)

Hard surface

Ao=163nm dsp=75nmzind=74nm

Ao=123nm dsp=61nmzind=44nm

Ao=98nm dsp=50nmzind=30nm

Ao=72nm dsp=53nm

zind=19nm

Ao=38nm dsp=35nmzind=3nm

Minimal indentation with a low amplitude and high setpoint ratio

If Ao < 35 nm, energy of tapping is low and tip sticks to surface!

• When the indentation depth is small, surface topography is less likely to be altered.

Indentation leads to artefacts

! (1m x 1m scans)height scale = 50nm

Ao=135nm dsp=115nm rsp=0.85

zind=18nm

Ao=135nm dsp=86nmrsp=0.63

zind=44nm

See Mallégol et al., Langmuir (2001) 17, 7022.

• Using optimised tapping conditions, cylindrical particles are observed, surrounded by a liquid-like medium.

Same PSA film after rinsing with water for 1 min.

Water

Water-soluble phase is likely to be surfactant and free polymer fragments.

Acrylic (EHA-BMA-MMA…) PSA film

formed at 60ºC (3 min)

1m

The second phase is water-soluble

Phase contrast image

100 200 300

10

100

1000

10000

100000

SK

Na

log

(cou

nts)

Channel

before rinsing with water after rinsing with water

RBS Evidence for Surfactant Excess at the Adhesive/Air Interface

0.08 at% Na

0.09 at% S

0.03 at% K

60 nm layer

< particle diam.

C O

Used a scanning beam with low current (5 nA) on a cryogenic stage

See Mallégol et al., Langmuir (2002) 18, p 4478.

Latex

Latex

Latex

Bilayer

Water

Stabilisation of the Latex Particles against Coalescence

Structure might be analogous to that of a biliquid foam, as has been observed in concentrated emulsions.

See Crowley T.L. et al. Langmuir (1992) 8, 2110 and Sonneville-Aubrun et al. Langmuir, (2000) 15, 1566

Effect of “Cleaning” Latex Serum

Image sizes: 5 m x 5 m; Height mode on left; phase mode on right

PSA film formed from a diluted bimodal dispersion

PSA film formed from a bimodal dispersion “cleaned” via dialysis

The Morphology of the Air Surface Differs Strongly from that at the Interface with the Substrate

Air Surface

Interface with Silicone Substrate

5 m x 5 m scan

Film formation at 60 °C

Particles are Stable under the Application of Shear Stress

Image of surface acquired between 4 and 11 min. after shearing

Acquired between 11 and 18 min. after shearing

Scan size: 5 m x 5 m

J. Mallégol et al., J. Adh. Sci. Tech. (2003)

How and why are the solids in the latex serum transported to the film surface?

Need for water concentration profiles during drying….

GARFIELDP. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

A low cost, permanent magnet with shaped pole pieces for the high resolution profiling of films.

GARField: A Magnet for Planar Samples

P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

GARField depth profiling magnet

Characteristics :• 0.7 T permanent magnet

(B0)• 17.5 T.m-1 gradient in the

vertical direction (Gy)

Abilities :• accommodates samples of 2 cm

by 2 cm area• achieves better than 10 m pixel

resolution!

B0

GyB1

Film Sample

Coverslip RF Coil

posi

tion

Signal intensity

Gravity

Gradient At Right-angles to the Field

Dependence of Water Concentration Profile on Pe

H = 255 m, E = 0.2 x 10-8 ms-1, D = 3.23 x 10-12 m2s-1

Uniform water concentration profiles

High humidity Pe 0.2

J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.

Dependence of Water Concentration Profile on Pe

H = 340 m, E = 15 x 10-8 ms-1, D = 3.23 x 10-12 m2s-1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

-50 0 50 100 150 200 250 300 350 400 450

Height (m)

Mag

netis

atio

n (A

rbitr

ary

Uni

ts)

2 minute

7 minutes

13 minutes

31 minutes

Flowing Air Pe 16

Non-uniform water concentration profilesJ.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.

Dependence of Water Concentration Profile on Pe

H = 420 m, E = 8 x 10-8 ms-1, D = 1.94 x 10-12 m2s-1

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

-50 0 50 100 150 200 250 300 350 400 450 500 550

Height (m)

Mag

netis

atio

n (A

rbitr

ary

Uni

ts)

12 minutes

32 minutes

40 minutes

72 minutes

Non-uniform water concentration profiles

Still air and higher viscosity Pe 16

J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.

Simulated Water Profiles with Various Types of Film Formation

Capillary deformation:

Water is always near the film surface

-40 0 40 80 120 160 200 24005

10152025303540455055

Wat

er c

once

ntra

tion

(vo

l.%)

Height (m)

-40 0 40 80 120 160 200 24005

10152025303540455055

Wat

er c

once

mtr

atio

n (v

ol.%

)

Height (m)

Dry Sintering:

Water recedes from the film surface

Drying Profiles in Other Waterborne Films

Low-Tg Alkyd Emulsion:

“Skin” formation

Height (m)

-50 0 50 100 150 200 2500.0

0.1

0.2

0.3

0.4

0.5

Rel

ativ

e in

tens

ity

Depth (m)

Acrylic Latex near Tg:

Uniform water recession from surface

Time

Height (m)

-25 0 25 50 75 100 125

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Air

Sub

stra

te

7'

5'

2'

Rel

ativ

e in

tens

ity

Height (m)

0 50 100 150 200 250

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250

0.0

0.1

0.2

0.3

0.4

0.5

0.6

Rel

ativ

e in

tens

ity

Height (m)

Rel

ativ

e in

tens

ity

Height (m)

MR Profiles of PSA Drying

Height (m)

Height (m)

Drying delayed by 14 min.

Drying delayed by 82 min.

• Linear water concentration gradients

• Surface always wet

• Pathway for surfactant and latex serum to be drawn to the film surface

Influence of Drying Rate on Morphology of Air Interfaces

5 m x 5 m scan

Very slow drying at 8 °C in high humidity: low Pe

Fast drying at 100 °C in a thicker film (400 m): high Pe

Influence of Drying Conditions on the Surface Excess of Surfactant

Slower drying More uniform water distributions Greater surface excess

100 200 300 400 500 600 700

1

10

100

1000

10000

S KNa

Cou

nts

Channels

UCB-C dried at 60oC UCB-C dried slowly

Tackifiers in PSAs

• “Tackifiers” are added to PSAs to increase tack.

• Tackifiers are typically a rosin ester or rosin-derivative with a relatively high Tg ( 20 °C).

• They function as “solid solvents” in acrylics.

• Their effect is to reduce the storage modulus (G’) at high temperature but to increase it at lower temperatures. Tackifiers also increase the Tg of PSAs.

• Polymer flow is enhanced and resistance to bond rupture is increased.

Effects of Tackifier on Film Morphology

a

e

c

d

b

Concentrations of Tackifier:

a = 0%

b = 5%

c = 10%

d = 25%

e = 50%

Particle identity is progressively lost!

Tacolyn® 3189 - Eastman Chemical

Effect of Tackifier on Water Loss Rate in PSA films

0 300 600 900 1200 1500 1800

0

20

40

60

80

100W

ater

per

cent

age

in f

inal

sta

ge

Normalised drying time (min)

0 % 10 % 25 % 50 % 75 % 100 %

The addition of tackifier strongly slows down drying.

MR Profiles of PSA/Tackifier Drying

0 50 100 150 2000.0

0.1

0.2

0.3

0.4 36'64'125'205'280'400'480'600'770'1050'

Relat

ive in

tensit

y

Height (m)

Evidence for “skin formation” with increasing amounts of tackifier

Tackifier concentrations:

a = 0%

b = 10%

c = 25%

d = 50%

e = 75%

f = 100%

Conclusions

• Particle coalescence does not occur near the surface of low-Tg waterborne acrylic PSAs.

• Surfactant excess near the surface, identified with Rutherford backscattering spectrometry (RBS), stabilises the particles against coalescence.

• Drying profiles, determined with MR profiling, are consistent with particle deformation under the action of capillary pressure.

• Tackifier alters the drying mechanism and promotes “skin” formation in PSAs.

• MR profiling is an ideal complementary technique to AFM and RBS.

Collaborators

• Dr Jacky Mallégol: all PSA experiments

• Dr Jean-Philippe Gorce: MR profiling of alkyd emulsions

• Dr Olivier Dupont (UCB Chemicals, Drogenbos): latex synthesis and complementary characterisation

• Professor Peter McDonald (University of Surrey): support and advice on MR profiling

• Dr Chris Jeynes (Surrey Ion Beam Centre): RBS

Funding

• UCB Chemicals, Drogenbos

(now “Surface Specialties”)

• “Pump-Priming” Grant for initial access to Surrey’s Ion Beam Facility

• UK Engineering and Physical Science Research Council for recent grant for access to the Surrey Ion Beam Facility

• ICI Paints, Slough

Tackified acrylic PSAs

0.01

0.1

1

10

-60 -40 -20 0 20 40 60 80 100T (°C)

tan

0.01

0.1

1

10

100

1000

10000

-60 -40 -20 0 20 40 60 80 100T (°C)

Sto

rag

e M

od

ulu

s (

MP

a)

UCBA -FUCBA

UCBA -FUCBA

• lower G’ @ T° » Tg (or low strain rate) polymer flow, bond formation• higher Tg, G’ @ T° ~ Tg ( higher strain rate) resistance to debonding• higher tan d @ T° ~ Tg energy dissipation upon debonding

DMA in Tensile mode

Ex: WB PSA (UCBA Tg~ -40°C (DSC)) with 25wt% (dry/dry) compatible stabilised rosin ester dispersion(Tacolyn®3189 softening point = 70°C)