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The Effects of Viscoelastic Behavior on Coating


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Particularly useful in adhesive coating, understanding of Viscoelastic Behavior in slot die coating technology can help you avoid defects and design a process to work for your fluid and substrate attributes.

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The Effects of Viscoelastic Behavior on Coating

  2. 2. VISCOELASTICITY • Viscosity (shear rate versus viscosity) • Elasticity (stress and velocity interaction)
  3. 3. VISCOSITY • (complex) Rheology = a curve, not a point • Storage and Loss Modulus (G’ & G”) = spring constant (PE & KE) • Molecular Weight (distribution)
  4. 4. VISCOSITY • (MORE complex) Rheology = hysteresis • Stress-relaxation o Snap back (lack of self-leveling) o Expansions and Contractions (manifold design) • Film split
  5. 5. VISCOSITY • tan δ = G”/G’ (balance of viscoelastic behavior)
  6. 6. ELASTICITY • No modulus dependence on time • Elongational testing (rough or elegant)
  7. 7. RHEOLOGY & COATING • Reynolds • Capillary • Stokes • Elasticity • Deborah Re = ρVL/µ Ca = µV/σ St = ρgL2/µV El = µV/EL De = λV/L (inertial forces) / (net viscous force) (viscosity induced pressure gradient) / (capillary pressure) (gravity force) / (net viscous force) (viscous stress) / (elastic stress in boundary) (elastic stress within liquid) / (viscous stress)
  8. 8. COATING • Recognized phenomenon • Defect analysis • Crossroads of liquid coating and polymer extrusion
  9. 9. COATING • Viscoelasticity dominates, then time dependent stress • Time is greater at lower temperatures
  10. 10. COATING • Film split o Misting o Roll spatter (high MW = more splatter)
  11. 11. COATING • Pre-metered flow control o Forced flow (no self-leveling) o Die swell (edge bead) o Neck-in (speed effects) o Retraction (wrinkling, curl or voids)
  12. 12. COATING • Pre-metered flow control Distribution chamber Feed port Slot gap Lip land
  13. 13. • 2D non-Newtonian flow models (internal flow) o Casson (short time model) o Maxwell (long time model) COATING
  14. 14. COATING • 3D Finite Element Analysis (defined boundary conditions) o Stress Relaxation o Time-Temperature and Boltzmann Superposition principles o Creep (long time frames)
  15. 15. COATING • Defects o Coating while stressed (ribbing, neck-in and edge bead/die swell) o Curing while stressed (wrinkle, curl and voids)
  16. 16. COATING • Solutions o Ribbing = positional adjustment o Edge bead = slot to substrate gap o Neck-in = speed related o Wrinkle/Curl = reduce stress at coating or during curing o Voids = limit of process
  17. 17. SUMMARY • Viscoelasticy is complex – test appropriately • Reduce stress to improve coating • Implement mathematical understanding to coating process development • Coating window is fundamentally reduced because of viscoelastic behavior