Presentation on theme: "Film Formation of Waterborne Pressure-Sensitive Adhesives Joseph Keddie Department of Physics, University of Surrey, Guildford 3 November, 2004."— Presentation transcript:
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 (T g < -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/(6 D 3 ) ~ 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 t c = 1 s P c = 1 MPa d F v d = 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 resistanceGood water resistance
Key Challenges in PSAs Poor water resistanceGood water resistance Why? There is a clear need to characterise PSA morphology and relate it to film formation mechanisms: 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. T g 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 R 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 = 1 Close- 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 ~10 17 particles and A = -1.3 x 10 4 m 2. With = 3 x 10 -2 J m -2, then G = - 390 J.
Particle Deformation Mechanisms Skin Formation Wet Sintering: pw r Dry Sintering: pa Capillary Action: wa
Latex Film Formation Mechanisms and Vertical Homogeneity 100 10000 1 Wet Sintering: pw Capillary Deformation: wa Receding Water Front Dry Sintering: pa 1 0 Skinning Partial Skinning PSAs! 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 A o : “free” amplitude A sp : “setpoint” amplitude d sp : tip-surface distance z ind : indentation depth A sp =d sp +z in d A o (>A sp ) d sp /A o = r sp < 1 r sp : setpoint ratio Requires careful control and optimisation of tapping parameters:
Discrete Particles Observed at PSA Surface! acrylic latex T g = 20ºC non-sticky surface A o =18nm d sp =15nm r sp =0.83 PSA latex T g = -33ºC (bimodal particle size) looptack on glass =512 N/m A o =163nm d sp =75nm r sp =0.46 Top views 3 m x 3 m scans Slice views 1 m x 1 m scans Vertical scale = 200nm Vertical scale = 50nm Silicon tip, k = 48 N/m, f o = 360 kHz
A o =163nm d sp =75nm r sp =0.46 R a =6.9nm A o =123nm d sp =61nm r sp =0.49 R a =5.8nm A o =98nm d sp =50nm r sp =0.51 R a =4.7nm A o =72nm d sp =53nm r sp =0.73 R a =2.6nm A o =38nm d sp =35nm r sp =0.92 R a =1.2nm Apparent Surface Topography is Sensitive to Free Amplitude and Setpoint Ratio Same Surface
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).
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
A o =163nm d sp =75nm z ind =74nm A o =123nm d sp =61nm z ind =44nm A o =98nm d sp =50nm z ind =30nm A o =72nm d sp =53nm z ind =19nm A o =38nm d sp =35nm z ind =3nm Minimal indentation with a low amplitude and high setpoint ratio If A o < 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 ! (1 m x 1 m scans) height scale = 50nm A o =135nm d sp =115nm r sp =0.85 z ind =18nm A o =135nm d sp =86nm r sp =0.63 z ind =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) 1m1m The second phase is water-soluble Phase contrast image
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. CO Used a scanning beam with low current (5 nA) on a cryogenic stage See Mallégol et al., Langmuir (2002) 18, p 4478.
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…. GARFIELD P. 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 (B 0 ) 17.5 T.m -1 gradient in the vertical direction (G y ) Abilities : accommodates samples of 2 cm by 2 cm area achieves better than 10 m pixel resolution! B0B0 GyGy B1B1 Film Sample Coverslip RF Coil position Signal intensity Gravity G radient A t R ight-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 m 2 s -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 m 2 s -1 Flowing Air Pe 16 Non-uniform water concentration profiles J.-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 m 2 s -1 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 Dry Sintering: Water recedes from the film surface
Drying Profiles in Other Waterborne Films Low-T g Alkyd Emulsion: “Skin” formation Height ( m) Acrylic Latex near T g : Uniform water recession from surface Time Height ( m)
MR Profiles of PSA Drying 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
Tackifiers in PSAs “Tackifiers” are added to PSAs to increase tack. Tackifiers are typically a rosin ester or rosin-derivative with a relatively high T g ( 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 T g of PSAs. Polymer flow is enhanced and resistance to bond rupture is increased.
Effects of Tackifier on Film Morphology 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 The addition of tackifier strongly slows down drying.
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%
Conclusions Particle coalescence does not occur near the surface of low-T g 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 -60-40-20020406080100 T (°C) tan 0.01 0.1 1 10 100 1000 10000 -60-40-20020406080100 T (°C) Storage Modulus (MPa) UCBA -F UCBAUCBA UCBAUCBA 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)