The Fundamentals of Rheology

THE FUNDAMENTALS OF

RHEOLOGY

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.

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”

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

TYPES OF RHEOMETERS

Extensional Capillary Rotational

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.

SHEAR FLOW

Flow between two parallel plates of area A

Moving with constant velocity V

θ

s

A

D

F

SHEAR FLOW

Flow between two parallel plates of area A

Moving with constant velocity V

Shear Rate

Shear Stress

Viscosity

POLYMER RHEOLOGICAL BEHAVIOR

WATER

POLYMERS P1

1

P2

1 2

P3

1 2 3

Pressure

Flow

WATER

POLYMERS

Polymer flow is not proportional to the applied pressure Flow curve

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

-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(

Vis

co

sit

y)

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 (

vis

co

sit

y)

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

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

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

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...

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 Melt Extrudate Die Screw/Barrel

Water

tank

Pull

Roller

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

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

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

• 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

)(Log

)(Log-1 0 1 2 3 4 5

f (T, P, material)

INJECTION MOLDING EXTRUSION

MELT FLOW

CAPILLARY

RHEOMETRY

ROTATIONAL RHEOMETRY

PROCESSING & FLOW CURVE OF POLYMERS

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)

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

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

EXTRUSION/IM & THE CAPILLARY RHEOMETER

Polymer Granules

Feeding

Extrudate Die Barrel

CO-EXTRUSION & SQC ANALYSIS

Die Swell Accessory

EXTRUSION & SR DIE SWELL ACCESSORY

Extrudate Die

Swelling of the Polymer

EXTRUSION/IM & MELT FRACTURE

Unstable flow Polymer Granules Barrel

Production Rate

Ou

tpu

t

Smooth Shark

Skin

Spurt Fracture

Unstable flow

Direction

of flow

Increasing

Flow rate

Melt

Instability

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

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

BLOW MOLDING/FILM BLOWING & SR STRETCHING UNIT

Polymer Melt Extrudate

Air Hose

Stretching

THANK YOU FOR YOUR TIME!