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University of Surrey, Guildford

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1 University of Surrey, Guildford
Film Formation of Waterborne Pressure-Sensitive Adhesives Joseph Keddie Department of Physics, University of Surrey, Guildford 3 November, 2004

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

3 Why are PSAs so sticky? • Soft polymers can achieve intimate contact with a rough substrate, leading to mechanical interlocking. • With close contact (D ~ 0.2 nm) between surfaces, the van der Waals pressure become significant: P ~ A/(6pD3) ~ 7000 atm! • Usual polar or acid/base interactions between the PSA and the substrate, depending on chemistry. But there is no covalent bonding.

4 Energy Dissipation in PSA De-bonding
vd = µm/s F F V = 30 µm/s d Contact t c = 1 s P c = 1 MPa d Lakrout, H.; Sergot, P.; Creton, C. J. Adhes. 1999, 69, Lakrout, H.; Creton, C.; Ahn, D.; Shull, K. R. Macromolecules 2001, 34,

5 Environmentally-benign
Key Challenges in PSAs • Trend towards waterborne, colloidal PSAs Reduced VOCs Environmentally-benign • Trend towards clear labels

6 Environmentally-benign
Key Challenges in PSAs • But the adhesion strength and water resistance of waterborne PSAs are poor! Reduced VOCs Environmentally-benign

7 Key Challenges in PSAs After soaking in water for 10 min.:
• 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

8 Why? Key Challenges in PSAs
• There is a clear need to characterise PSA morphology and relate it to film formation mechanisms: Aim of our work Poor water resistance Good water resistance

9 Dodecahedral structure
Idealised View of Latex Film Formation Polymer-in-water dispersion Close-packing of particles Water loss Dodecahedral structure (honey-comb) Deformation of particles Interdiffusion and coalescence Homogenous Film

10 Typical Morphologies Particles are deformed to fill all available space: rhombic dodecahedra Y. Wang et al., Langmuir 8 (1992) 1435.

11 Example of Good Coalescence
Environmental SEM Immediate film formation upon drying! Tg of latex  5 °C; film-formed at RT 1 mm Hydrated film J.L. Keddie et al., Macromolecules (1995) 28,

12 Experimental Evidence for Vertical Non-Uniformity during Drying
Densely-packed particle layer Cryogenic SEM E. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10

13 Theory: Peclet number for vertical drying
Competition between Brownian diffusion that re-distributes particles and evaporation that causes particles to accumulate at the surface

14 Peclet number for vertical drying uniformity
H R Dilute limit Pe << 1

15 Simulations of the Vertical Distribution of Particles
pol Vertical Position Pe = 0.2 Close-packed Simulations by A.F. Routh Top

16 Simulations of the Vertical Distribution of Particles
pol Vertical Position Pe = 1 Close-packed

17 Simulations of the Vertical Distribution of Particles
Vertical Position pol Pe = 10 A.F. Routh and W.B. Zimmerman, Chem. Eng. Sci., 59 (2004)

18 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 DA = -1.3 x 104 m2. With g = 3 x 10-2 J m-2, then DG = J.

19 Particle Deformation Mechanisms
Dry Sintering: gpa Wet Sintering: gpw Capillary Action: gwa Skin Formation r

20 Latex Film Formation Mechanisms and Vertical Homogeneity
Dry Sintering: pa 10000 Receding Water Front 100 Capillary Deformation: wa Partial Skinning PSAs! 1 Wet Sintering: pw Skinning 1 See A.F. Routh & W.B. Russel, Langmuir (1999) 15,

21 Atomic Force Microscopy (AFM) of PSAs
• 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” amplitude Asp : “setpoint” amplitude dsp : tip-surface distance zind : indentation depth Asp=dsp+zind Ao (>Asp) dsp/Ao = rsp < 1 rsp : setpoint ratio • Requires careful control and optimisation of tapping parameters:

22 Discrete Particles Observed at PSA Surface!
Silicon tip, k = 48 N/m, fo = 360 kHz PSA latex Tg = -33ºC (bimodal particle size) looptack on glass =512 N/m Ao=163nm dsp=75nm rsp=0.46 acrylic latex Tg = 20ºC non-sticky surface Ao=18nm dsp=15nm rsp=0.83 Top views 3mm x 3mm scans Vertical scale = 200nm Vertical scale = 50nm Slice views 1mm x 1mm scans

23 Apparent Surface Topography is Sensitive to
Free Amplitude and Setpoint Ratio Same Surface Ao=163nm dsp=75nm rsp= Ra=6.9nm Ao=123nm dsp=61nm rsp= Ra=5.8nm Ao=98nm dsp=50nm rsp= Ra=4.7nm Ao=72nm dsp=53nm rsp=0.73 Ra=2.6nm Ao=38nm dsp=35nm rsp=0.92 Ra=1.2nm

24 Amplitude-distance curve obtained from
a PSA surface prone to indentation, showing a calculation of the indentation depth. Bar et al., Surface Science, 457 : L404-L412 (2000).

25 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. Hard surface

26 Minimal indentation with a low amplitude and high setpoint ratio
Ao=163nm dsp=75nm zind=74nm Ao=123nm dsp=61nm zind=44nm Ao=98nm dsp=50nm zind=30nm Ao=72nm dsp=53nm zind=19nm Ao=38nm dsp=35nm zind=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!

27 Ao=135nm dsp=115nm rsp=0.85 zind=18nm
Indentation leads to artefacts ! (1mm x 1mm scans) height scale = 50nm Ao=135nm dsp=86nm rsp=0.63 zind=44nm • When the indentation depth is small, surface topography is less likely to be altered. • Using optimised tapping conditions, cylindrical particles are observed, surrounded by a liquid-like medium. See Mallégol et al., Langmuir (2001) 17, 7022.

28 The second phase is water-soluble
Acrylic (EHA-BMA-MMA…) PSA film formed at 60ºC (3 min) Same PSA film after rinsing with water for 1 min. Water Phase contrast image 1mm Water-soluble phase is likely to be surfactant and free polymer fragments.

29 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. Used a scanning mbeam with low current (5 nA) on a cryogenic stage C O See Mallégol et al., Langmuir (2002) 18, p 4478.

30 Stabilisation of the Latex Particles against Coalescence
Water 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

31 Effect of “Cleaning” Latex Serum
PSA film formed from a diluted bimodal dispersion PSA film formed from a bimodal dispersion “cleaned” via dialysis Image sizes: 5 mm x 5 mm; Height mode on left; phase mode on right

32 Interface with Silicone Substrate
The Morphology of the Air Surface Differs Strongly from that at the Interface with the Substrate Air Surface Film formation at 60 °C Interface with Silicone Substrate 5 mm x 5 mm scan

33 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 mm x 5 mm J. Mallégol et al., J. Adh. Sci. Tech. (2003)

34 P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.
How and why are the solids in the latex serum transported to the film surface? Need for water concentration profiles during drying…. GARFIELD P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

35 GARField: A Magnet for Planar Samples
A low cost, permanent magnet with shaped pole pieces for the high resolution profiling of films. P. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.

36 GARField depth profiling magnet
Gradient At Right-angles to the Field 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 Gy B1 Film Sample Coverslip RF Coil position Gravity Signal intensity

37 J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe High humidity Pe  0.2 H = 255 m, E = 0.2 x 10-8 ms-1, D = 3.23 x m2s-1 Uniform water concentration profiles J.-P. Gorce et al., Eur Phys J E, 8 (2002)

38 J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe Flowing Air Pe  16 H = 340 m, E = 15 x 10-8 ms-1, D = 3.23 x m2s-1 Non-uniform water concentration profiles J.-P. Gorce et al., Eur Phys J E, 8 (2002)

39 J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29.
Dependence of Water Concentration Profile on Pe Still air and higher viscosity Pe  16 H = 420 m, E = 8 x 10-8 ms-1, D = 1.94 x m2s-1 Non-uniform water concentration profiles J.-P. Gorce et al., Eur Phys J E, 8 (2002)

40 Simulated Water Profiles with Various Types of Film Formation
Dry Sintering: Water recedes from the film surface Capillary deformation: Water is always near the film surface

41 Drying Profiles in Other Waterborne Films
Acrylic Latex near Tg: Uniform water recession from surface Time Height (mm) Low-Tg Alkyd Emulsion: “Skin” formation Height (mm)

42 MR Profiles of PSA Drying
• Linear water concentration gradients • Surface always wet Height (mm) Drying delayed by 14 min. Drying delayed by 82 min. • Pathway for surfactant and latex serum to be drawn to the film surface Height (mm)

43 Influence of Drying Rate on Morphology of Air Interfaces
Very slow drying at 8 °C in high humidity: low Pe Fast drying at 100 °C in a thicker film (400 mm): high Pe 5 mm x 5 mm scan

44 Influence of Drying Conditions on the Surface Excess of Surfactant
Slower drying  More uniform water distributions  Greater surface excess

45 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.

46 Tacolyn® 3189 - Eastman Chemical
Effects of Tackifier on Film Morphology Concentrations of Tackifier: a = 0% b = 5% c = 10% d = 25% e = 50% Tacolyn® Eastman Chemical Particle identity is progressively lost!

47 Effect of Tackifier on Water Loss Rate in PSA films
The addition of tackifier strongly slows down drying.

48 Tackifier concentrations:
MR Profiles of PSA/Tackifier Drying Evidence for “skin formation” with increasing amounts of tackifier Tackifier concentrations: a = 0% b = 10% c = 25% d = 50% e = 75% f = 100%

49 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.

50 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

51 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

52 Tackified acrylic PSAs
Ex: WB PSA (UCBA Tg~ -40°C (DSC)) with 25wt% (dry/dry) compatible stabilised rosin ester dispersion (Tacolyn®3189 softening point = 70°C) DMA in Tensile mode 0.01 0.1 1 10 -60 -40 -20 20 40 60 80 100 T (°C) tan d 1000 10000 Storage Modulus (MPa) UCBA -F UCBA lower T° » Tg (or low strain rate)  polymer flow, bond formation higher Tg, T° ~ Tg ( higher strain rate)  resistance to debonding higher tan T° ~ Tg  energy dissipation upon debonding

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