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1University of Surrey, Guildford Film Formation of Waterborne Pressure-Sensitive AdhesivesJoseph KeddieDepartment of Physics,University of Surrey, Guildford3 November, 2004
2Pressure Sensitive Adhesives (PSAs) • PSAs are aggressively and permanently tacky atroom 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
3Why 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.
5Environmentally-benign Key Challenges in PSAs• Trend towards waterborne,colloidal PSAsReduced VOCsEnvironmentally-benign• Trend towards clear labels
6Environmentally-benign Key Challenges in PSAs• But the adhesion strength andwater resistance of waterbornePSAs are poor!Reduced VOCsEnvironmentally-benign
7Key Challenges in PSAs After soaking in water for 10 min.: • But the adhesion strength andwater resistance of waterbornePSAs are poor!After soaking in water for 10 min.:Poor water resistanceGood water resistance
8Why? Key Challenges in PSAs • There is a clear need to characterise PSAmorphology and relate it to film formationmechanisms: Aim of our workPoor water resistanceGood water resistance
9Dodecahedral structure Idealised View of Latex Film FormationPolymer-in-water dispersionClose-packing of particlesWater lossDodecahedral structure(honey-comb)Deformationof particlesInterdiffusionand coalescenceHomogenous Film
10Typical MorphologiesParticles are deformed to fill all available space: rhombic dodecahedraY. Wang et al., Langmuir 8 (1992) 1435.
11Example of Good Coalescence Environmental SEMImmediate film formation upon drying!Tg of latex 5 °C; film-formed at RT1 mmHydrated filmJ.L. Keddie et al., Macromolecules (1995) 28,
12Experimental Evidence for Vertical Non-Uniformity during Drying Densely-packed particle layerCryogenic SEME. Sutanto et al., in Film Formation in Coatings, ACS Symposium Series 790 (2001) Ch. 10
13Theory: Peclet number for vertical drying Competition between Brownian diffusion that re-distributes particles and evaporation that causes particles to accumulate at the surface
14Peclet number for vertical drying uniformity HRDilute limitPe << 1
15Simulations of the Vertical Distribution of Particles polVertical PositionPe = 0.2Close-packedSimulations by A.F. RouthTop
16Simulations of the Vertical Distribution of Particles polVertical PositionPe = 1Close-packed
17Simulations of the Vertical Distribution of Particles Vertical PositionpolPe = 10A.F. Routh and W.B. Zimmerman, Chem. Eng. Sci., 59 (2004)
18Driving 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.
20Latex Film Formation Mechanisms and Vertical Homogeneity Dry Sintering: pa10000Receding Water Front100Capillary Deformation: waPartial SkinningPSAs!1Wet Sintering: pwSkinning1See A.F. Routh & W.B. Russel, Langmuir (1999) 15,
21Atomic 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 PSAsAo : “free” amplitudeAsp : “setpoint” amplitudedsp : tip-surface distancezind : indentation depthAsp=dsp+zindAo(>Asp)dsp/Ao = rsp < 1rsp : setpoint ratio• Requires careful control and optimisation oftapping parameters:
22Discrete Particles Observed at PSA Surface! Silicon tip, k = 48 N/m, fo = 360 kHzPSA latexTg = -33ºC(bimodal particle size)looptack on glass=512 N/mAo=163nmdsp=75nmrsp=0.46acryliclatexTg = 20ºCnon-stickysurfaceAo=18nmdsp=15nmrsp=0.83Top views3mm x 3mmscansVertical scale = 200nmVertical scale = 50nmSlice views1mm x 1mmscans
23Apparent Surface Topography is Sensitive to Free Amplitude and Setpoint RatioSame SurfaceAo=163nm dsp=75nmrsp= Ra=6.9nmAo=123nm dsp=61nmrsp= Ra=5.8nmAo=98nm dsp=50nmrsp= Ra=4.7nmAo=72nmdsp=53nmrsp=0.73Ra=2.6nmAo=38nmdsp=35nmrsp=0.92Ra=1.2nm
24Amplitude-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).
25Amplitude-distance curves are used to characterise the tip-sample interactionsLessons:• The surface is indented very deeply!• Tip adheres to surface at tapping amplitudes < 35 nm.Hard surface
26Minimal indentation with a low amplitude and high setpoint ratio Ao=163nm dsp=75nmzind=74nmAo=123nm dsp=61nmzind=44nmAo=98nm dsp=50nmzind=30nmAo=72nmdsp=53nmzind=19nmAo=38nmdsp=35nmzind=3nmMinimal indentation with a low amplitudeand high setpoint ratioIf Ao < 35 nm, energy of tapping is low and tip sticks to surface!
27Ao=135nm dsp=115nm rsp=0.85 zind=18nm Indentationleads to artefacts!(1mm x 1mm scans)height scale = 50nmAo=135nm dsp=86nmrsp=0.63 zind=44nm• When the indentation depth is small, surface topography is less likely to be altered.• Using optimised tapping conditions, cylindrical particlesare observed, surrounded by a liquid-like medium.See Mallégol et al., Langmuir (2001) 17, 7022.
28The 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.WaterPhase contrast image1mmWater-soluble phase is likely to be surfactant and free polymer fragments.
29RBS Evidence for Surfactant Excess at the Adhesive/Air Interface 0.08 at% Na0.09 at% S0.03 at% K60 nm layer< particle diam.Used a scanning mbeam with low current (5 nA) on a cryogenic stageCOSee Mallégol et al., Langmuir (2002) 18, p 4478.
30Stabilisation of the Latex Particles against Coalescence WaterStructure 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
31Effect of “Cleaning” Latex Serum PSA film formed from a diluted bimodal dispersionPSA film formed from a bimodal dispersion “cleaned” via dialysisImage sizes: 5 mm x 5 mm; Height mode on left; phase mode on right
32Interface with Silicone Substrate The Morphology of the Air Surface Differs Strongly from that at the Interface with the SubstrateAir SurfaceFilm formation at 60 °CInterface with Silicone Substrate5 mm x 5 mm scan
33Particles are Stable under the Application of Shear Stress Image of surface acquired between 4 and 11 min. after shearingAcquired between 11 and 18 min. after shearingScan size: 5 mm x 5 mmJ. Mallégol et al., J. Adh. Sci. Tech. (2003)
34P. 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….GARFIELDP. M. Glover, et al., J. Magn. Reson. (1999) 139, 90.
35GARField: 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.
36GARField depth profiling magnet Gradient At Right-angles to the FieldCharacteristics :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 areaachieves better than 10 m pixel resolution!B0GyB1Film SampleCoverslipRF CoilpositionGravitySignal intensity
37J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29. Dependence of Water Concentration Profile on PeHigh humidity Pe 0.2H = 255 m, E = 0.2 x 10-8 ms-1, D = 3.23 x m2s-1Uniform water concentration profilesJ.-P. Gorce et al., Eur Phys J E, 8 (2002)
38J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29. Dependence of Water Concentration Profile on PeFlowing Air Pe 16H = 340 m, E = 15 x 10-8 ms-1, D = 3.23 x m2s-1Non-uniform water concentration profilesJ.-P. Gorce et al., Eur Phys J E, 8 (2002)
39J.-P. Gorce et al., Eur Phys J E, 8 (2002) 421-29. Dependence of Water Concentration Profile on PeStill air and higher viscosity Pe 16H = 420 m, E = 8 x 10-8 ms-1, D = 1.94 x m2s-1Non-uniform water concentration profilesJ.-P. Gorce et al., Eur Phys J E, 8 (2002)
40Simulated Water Profiles with Various Types of Film Formation Dry Sintering:Water recedes from the film surfaceCapillary deformation:Water is always near the film surface
41Drying Profiles in Other Waterborne Films Acrylic Latex near Tg:Uniform water recession from surfaceTimeHeight (mm)Low-Tg Alkyd Emulsion:“Skin” formationHeight (mm)
42MR Profiles of PSA Drying • Linear water concentration gradients• Surface always wetHeight (mm)Drying delayed by 14 min.Drying delayed by 82 min.• Pathway for surfactant and latex serum to be drawn to the film surfaceHeight (mm)
43Influence of Drying Rate on Morphology of Air Interfaces Very slow drying at 8 °C in high humidity: low PeFast drying at 100 °C in a thicker film (400 mm): high Pe5 mm x 5 mm scan
44Influence of Drying Conditions on the Surface Excess of Surfactant Slower drying More uniform water distributions Greater surface excess
45Tackifiers 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.
46Tacolyn® 3189 - Eastman Chemical Effects of Tackifier on Film MorphologyConcentrations of Tackifier:a = 0%b = 5%c = 10%d = 25%e = 50%Tacolyn® Eastman ChemicalParticle identity is progressively lost!
47Effect of Tackifier on Water Loss Rate in PSA films The addition of tackifier strongly slows down drying.
48Tackifier concentrations: MR Profiles of PSA/Tackifier DryingEvidence for “skin formation” with increasing amounts of tackifierTackifier concentrations:a = 0%b = 10%c = 25%d = 50%e = 75%f = 100%
49Conclusions• 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.
50Collaborators • 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
51Funding • 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
52Tackified 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 mode0.010.1110-60-40-2020406080100T (°C)tand100010000Storage Modulus (MPa)UCBA -FUCBAlower T° » Tg (or low strain rate) polymer flow, bond formationhigher Tg, T° ~ Tg ( higher strain rate) resistance to debondinghigher tan T° ~ Tg energy dissipation upon debonding