The document discusses the complex viscoelastic behavior affecting coating properties, including viscosity, elasticity, and their time-dependent nature. It emphasizes the significance of various models and conditions that impact coating defects and solutions to mitigate issues like ribbing and voids. Ultimately, it concludes that understanding viscoelasticity is crucial for optimizing coating processes.

1. THE EFFECTS OF VISCOELASTIC
BEHAVIOR ON COATING

2. VISCOELASTICITY
• Viscosity (shear rate versus viscosity)
• Elasticity (stress and velocity interaction)

3. VISCOSITY
• (complex) Rheology = a curve,
not a point
• Storage and Loss Modulus (G’
& G”) = spring constant (PE &
KE)
• Molecular Weight (distribution)

4. VISCOSITY
• (MORE complex) Rheology = hysteresis
• Stress-relaxation
o Snap back (lack of self-leveling)
o Expansions and Contractions (manifold design)
• Film split

5. VISCOSITY
• tan δ = G”/G’ (balance of viscoelastic
behavior)

6. ELASTICITY
• No modulus dependence on time
• Elongational testing (rough or elegant)

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. COATING
• Recognized phenomenon
• Defect analysis
• Crossroads of liquid coating and polymer
extrusion

9. COATING
• Viscoelasticity dominates, then time
dependent stress
• Time is greater at lower temperatures

10. COATING
• Film split
o Misting
o Roll spatter (high MW = more splatter)

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. COATING
• Pre-metered flow control
Distribution
chamber
Feed
port
Slot gap
Lip land

13. • 2D non-Newtonian flow models (internal flow)
o Casson (short time model)
o Maxwell (long time model)
COATING

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. COATING
• Defects
o Coating while stressed (ribbing,
neck-in and edge bead/die
swell)
o Curing while stressed (wrinkle,
curl and voids)

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. 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