The Fundamentals of Rheology
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This document discusses the fundamentals of rheology and how rheological tests can help with polymer processing and development. It describes different types of rheometers including capillary, rotational, and extensional rheometers. Capillary rheology provides information about how materials behave when melted and correlates flow parameters to mechanical properties. Capillary rheology can determine optimal processing parameters and investigate issues. The document also discusses how rheological properties relate to molecular weight and processing techniques like extrusion, injection molding, and blow molding that can be simulated using a capillary rheometer.
The Fundamentals of Rheology
- 2. Rheological tests can be very helpful tools for polymer processing and development. This presentation is designed to be an informative introduction and guide to rheological tests, and finding correlations between equipment and processing techniques.
- 3. CAPILLARY RHEOLOGY • Provides more information than melt flow testing • How does the material behave when melted? • What are the correlations between flow parameters and mechanical properties? • Polymers are non-Newtonian materials, consequently their flow is not proportional to the pressure applied “the flow and shear properties of materials up to high pressures”
- 4. WHY CAPILLARY RHEOLOGY? • Determine the optimal working parameters for materials processing (injection molding, blow molding, extrusion, etc.) • Investigate processing issues in a faster and non-disruptive manner • Find which materials will work best for complex parts or long flow lengths • Replicate manufacturing parameters for design, troubleshooting, and simulations
- 5. TYPES OF RHEOMETERS Extensional CapillaryRotational ROTATIONAL RHEOMETERS For viscoelastic properties Rotary motion Plate geometry: most common for thermoplastic melts EXTENSIONAL RHEOMETERS For elongational viscosity (high viscosity materials) Rotating drum Extensional flows: very sensitive to crystallinity & polymer long-chain branching CAPILLARY RHEOMETERS For viscous properties Capillary action Capillary flow: Flow through a narrow space. Different piston speeds (shear rates) applied. Viscosity changes tracked relative to shear rates.
- 6. SHEAR FLOW Flow between two parallel plates of area A Moving with constant velocity V θ s A D F
- 7. SHEAR FLOW Flow between two parallel plates of area A Moving with constant velocity V Shear Rate Shear Stress Viscosity
- 8. POLYMER RHEOLOGICAL BEHAVIOR WATER POLYMERSP1 1 P2 1 2 P3 1 2 3 Pressure Flow WATER POLYMERS Polymer flow is not proportional to the applied pressure Flow curve
- 9. MECHANICAL & RHEOLOGICAL PROPERTIES Vs. MOLECULAR WEIGHT A polymer’s structure influences all its mechanical, chemical, and rheological properties MOLECULAR WEIGHT LOW MEDIUM HIGH ULTRA-HIGH Young’s Modulus Impact Strength Melt Viscoscity
- 10. -2.000 -1.000 0.000 1.000 2.000 3.000 4.000 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 log(Viscosity) log (shear rate) Viscosity Vs. Shear Rate M = 50000 M = 75000 M = 100000 RHEOLOGICAL PROPERTIES Vs. MOLECULAR WEIGHT -2.000 -1.000 0.000 1.000 2.000 3.000 4.000 -2.00 -1.00 0.00 1.00 2.00 3.00 4.00 5.00 log(viscosity) log (shear rate) Viscosity Vs. Shear rate M = 75000 MWD broad M = 75000 MWD narrow Rheological curve at different MW (monodisperse polymers) Rheological curve at different MWD (monodisperse and polydisperse polymer) With the same average MW, polydisperse polymers can be processed better than monodisperse polymers
- 11. High-molecular weight leads to: 1) Higher Strength: due to higher inter-chain forces, more entanglements 2) Higher Impact Strength: due to lower degree of crystallization at higher chain length, more entanglements 3) Higher Chemical Resistance: due to higher inter-chain forces 4) Reduction of the “fluidity” (inverse of viscosity) of the polymer in the melted status: due to the presence of more entanglements MECHANICAL & RHEOLOGICAL PROPERTIES Vs. MOLECULAR WEIGHT
- 12. POLYMERS & PROCESSES MATERIAL Polycarbonate (PC) Polyethylene Terephthalate (PET) Polyvinyl chloride (PVC) Process • Injection molding • Blow molding • Extrusion Application (Examples) • Spotlights • Plastic bottles • Electrical wires Advantages • Transparency • High-optical clarity • Acts as a barrier • High impact resistance • Chemically resistant • Recyclable • Insulator • Lightweight • Durable • Mechanical damage resistance
- 13. CAPILLARY RHEOLOGY • Many polymer processing techniques can be simulated using a capillary rheometer • This allows users to experiment with new parameters for various polymer processing equipment without having to stop operation and waste a batch of material • Plastic extrusion, injection molding, blow molding, film blowing, co-extrusion...
- 14. PLASTIC EXTRUSION • Pellets are added into the feeder • A constant temperature is maintained • Screw is rotating continuously • Polymer melts at a constant temperature • Polymer is pushed through breaker plate into the die Polymer Granules Feeder Heaters Polymer MeltExtrudate Die Screw/Barrel Water tank Pull Roller
- 15. Feeder Heater Polymer Granules Extrudate Die Screw/Barrel Mold Cavity Molded Part PLASTIC INJECTION MOLDING (IM) • Pellets are added into the feeder • Screw is rotating (not continuously) • Different temperature for different zones • Screw moves along the barrel as a piston • Polymer is injected into a mold • The part is molded and ejected
- 16. Extrudate drops Extrudate fits to the mold Mold closes & air blows Air Hose Residue is trimmed Cutter PLASTIC BLOW MOLDING • Extrusion or injection blow molding • Molten material (parison) drops in the mold • Mold is closed • Air is blown through an air hose • Molded part is ejected Polymer Melt Extrudate Mold opens, part drops
- 17. PLASTIC FILM BLOWING • Molten material is extruded through a circular die (usually vertically) • Air is introduced in the center of the die • “Bubble–like” expansion • The tube of film passes through nip rolls Extrudate Air Nip Rolls Product
- 18. • Two or more materials fed into co-extrusion dies • Constant temperature is maintained in the die • Film is extruded • Layer thickness controlled by relative speeds and sizes of extruders Example Application: food packaging PLASTIC CO-EXTRUSION Adhesive Polymer Resin Polymer Resin Co-Extrusion Dies Rollers Co-Extruded Tape Winding Feeder 1 Feeder 2
- 19. )(Log )(Log-1 0 1 2 3 4 5 f (T, P, material) INJECTION MOLDINGEXTRUSION MELT FLOW CAPILLARY RHEOMETRY ROTATIONAL RHEOMETRY PROCESSING & FLOW CURVE OF POLYMERS
- 20. CAPILLARY RHEOMETERS MEASURE LOAD OR PRESSURE SET DIE GEOMETRY AND PISTON SPEED Shear Rate= Speed of Deformation Viscosity = Resistance to the Flow Shear Stress Pressure Transducer Motor-Driven Piston h V A F Capillary Die (L/D) Force (Load Cell)
- 21. RAW DATA: Constant shear rate steps with pressure reaching the equilibrium after a transient stage RHEOLOGICAL CURVES: Viscosity (Pa· s) as a function of shear rates (s-1) Shear stress (Pa) vs shear rate (s-1) RHEOLOGICAL DATA
- 22. PP @ 230°C Filled PP @ 230°C RHEOLOGICAL DATA Virgin PP: Non–Newtonian shear thinning behavior η = 241 – 34 Pa·s max P ≅ 8 MPa Filled PP (50% wt flax): Non–Newtonian shear thinning behavior η = 1061 – 81 Pa·s max P ≅ 20 MPa
- 23. EXTRUSION/IM & THE CAPILLARY RHEOMETER Polymer Granules Feeding Extrudate Die Barrel
- 24. CO-EXTRUSION & SQC ANALYSIS
- 25. Die Swell Accessory EXTRUSION & SR DIE SWELL ACCESSORY Extrudate Die Swelling of the Polymer
- 26. EXTRUSION/IM & MELT FRACTURE Unstable flow Polymer GranulesBarrel Production Rate Output Smooth Shark Skin Spurt Fracture Unstable flow Direction of flow Increasing Flow rate Melt Instability
- 27. IM MOLD FILLING & SR PVT ACCESSORY Partial Filling Complete Filling Simulations for Mold Filling Phase PVT Test Mold Cavity Molded Part Complete Mold Filling is Critical to the Process
- 28. IM MOLD FILLING& SR TC ACCESSORY TC Test Mold Cavity Molded Part Heat Conduction through the material is critical to get a perfectly molded part
- 29. BLOW MOLDING/FILM BLOWING & SR STRETCHING UNIT Polymer Melt Extrudate Air Hose Stretching
- 30. THANK YOU FOR YOUR TIME!