Rheological Properties

Question 1

What are the 3 types of deformation in polymers?

Answer 1

Simple shear - Occurs during processing of polymer melts

Elongation flow - Important in film formation, fibre pulling, blow moulding and vacuum forming

Bulk deformation - Important in injection moulding where liquid flow is generated by hydrostatic pressures

Question 2

What is a Bingham plastic?

Answer 2

Do not deform below a certain yield stress. Above that, they show Newtonian behaviour. t = t0 + ny

Eg. Plasticine, clay slurries

Question 3

What is a dilatant material?

Answer 3

Increase in shear viscosity with increasing shear rate (shear thickening)

Eg. Whipped cream lol

Question 4

What is a pseudo plastic?

Answer 4

Decreasing shear viscosity with increasing shear rate (shear thinning)

Eg. Most polymer melts

Question 5

Describe a stress strain graph for Bingham, pseudo plastic, dilatant and Newtonian.

Answer 5

Bingham is a straight gradient starting from a certain yield stress

Newtonian is a straight gradient starting from zero

Dilatant is increasing stress more steeply as strain increases - upwards curve

Pseudo plastic is increasing stress as strain increases but gradient decreases - levelling off curve

Question 6

What is the actual flow of polymers like at increasing shear rates?

Answer 6

Polymers are entangled like spaghetti.

When sheared, they start to disentangle and align which makes the viscosity drop. The degree of disentaglement depends on shear rate.

At high shear rates, polymer will fully align so viscosity will be independent of shear rate - will be Newtonian

At very low shear rates, entanglement doesn’t impede the shear flow so same applied

At infinite slow shear, we have the zero shear rate viscosity n0

Between these regions, power law model applies

Question 7

What does a graph of log apparent viscosity against log shear rate look like?

Answer 7

Newtonian region at low and high shear rates which are flat as in those regions, viscosity is independent of shear rate

Middle is a slope between the two Newtonian regions (flat slope area is the power law region)

Question 8

What is the power law model in normal and logarithmic form?

Answer 8

Shear stress = K(T) (shear strain rate dy/dt)^n

Log (stress) = log (K) + n log (dy/dt)

Question 9

What should a log log plot of shear stress v strain look like for a pseudo plastic?

Answer 9

Straight line

Question 10

In the power law equation, what will n be is Newtonian, thinning or thickening?

Answer 10

Newtonian - n=1
Thinning - n<1
Thickening - n>1

Question 11

What is apparent viscosity?

Answer 11

Shear stress / shear rate

For Newtonian, viscosity isn’t dependent on shear rate so apparent viscosity is constant.

Question 12

What is the yield stress needed for a Bingham plastic to start showing normal flow behaviour?

Answer 12

Critical yield stress

Question 13

When do you use the Bingham and Herschel-Bulkley model?

Answer 13

Bingham is for Bingham fluids with a constant viscosity above the critical yield stress

Herschel-Bulkley is for a shear thinning behaviour of a plastic fluid

Question 14

What are the factors affecting melt viscosity?

Answer 14

Temperature
Molecular Weight - lower are Newtonian, higher are more entangled
MWD
Branching - more branching disrupts entanglement so lower viscosity
Fillers - adding fillers increases melt viscosity and decreases die swell
Blends

Question 15

What is the viscosity equations for compatible and incompatible polymer blends?

Answer 15

Compatible: n = V1n1 + V2n2

Incompatible: 1/n = V1/n1 + V2/n2

V = volume fraction
n = viscosity

Question 16

When do we use the WLF equation?

Answer 16

When necessary to estimate how a change of temp affects the viscosity of a polymer - ng should be calculated with existing conditions then use that to work out new viscosity in same equation with new temps

Question 17

In capillary rheometry equations, what are R, P, L and Q?

Answer 17

R - radius of capillary
P - pressure
L - length of capillary
Q - volumetric output

Question 18

What is Bagley correction and why is it applied?

Answer 18

In capillary rheometry, pressure is measured above the die.

In reality, there is an additional pressure drop at the entrance of the die where the material goes from a wide reservoir to a narrow capillary.

It is possible to correct this and estimate true pressure drop using Bagley correction.

Bagley correction gives a true shear stress and viscosity - true viscosity is a property of the material so wont change under different testing conditions

Question 19

Why is Bagley correction recommended when using data for design, simulation and studies?

Answer 19

It gives a true value of viscosity and shear stress - this is a property of the material and wont change across instruments

Apparent viscosity can only be compared with samples tested in same instrument and die - usually used for quality control

Question 20

What is required to calculate Bagley correction?

Answer 20

At least two sets of data obtained with the same:

• sample
• temperature
• barrel
• capillary dia and diff lengths

Question 21

What would you do to the wall stress equation for capillary with Bagley correction?

Answer 21

Change L to Leffective which is L + eR

Question 22

Graphically, how is Bagley correction factor found?

Answer 22

Distance from 0 to where the data sets cross the x axis on a pressure drop vs. L/R graph

Question 23

When do we use the Rabinowitsch correction factor and what does it look like?

Answer 23

To factor in the pseudo plastic nature of the melt which means the velocity profile in the die is more plug like than parabolic.

Equation starting with y true

Question 24

When should you use the Rabinowitsch correction factor?

Answer 24

Factor wont affect comparability of data I of you are trying to compare

Factor only needs to be used where data is used as fundamental data, in practicality, the error difference is fine for quality control

Question 25

What is Melt Flow Index?

Answer 25

Simplified version of capillary flow experiment

Polymer is heated in a barrel and extruded through a standard die with a standard weight.

The weight in grams extruded after 10 minutes is the Melt Flow Index (MFI)

Question 26

How is melt elasticity shown when extruded through a die?

Answer 26

When the melt is deformed through the die, the chains become orientated in the direction of flow. When the melt exits the die, the stress is removed and the chains relax causing some elastic recovery (die swell). Not fully recovered though because of some viscous element which causes chain slippage.

The polymer will remember the large cross section in the barrel and tries to return to it by swelling.

Question 27

What affects die swell?

Answer 27

Increasing output rate increases the die swell.

Low temperatures increase the die swell.

Higher molecular weights increase the die swell.

Increasing the melt viscosity increases the die swell.

Increasing the difference between reservoir diameter and die damaged increases the die swell.

Decreasing the value of L/D increases the die swell (less time to forget the original chain state)

Question 28

What change in velocity profile happens at the exit of a die?

Answer 28

Goes from a parabolic to a plug

Question 29

Why must die swell be considered in design?

Answer 29

If you want a certain extrudate thickness, you need to calculate how much it is going to swell.

Question 30

What is melt fracture and when does it occur?

Answer 30

Occurs during high capillary flow rates - melt is elongated under too high a stress

Extrudate becomes irregular and distorted when applied stress exceeds tensile stress of the melt.

Question 31

How does die entrance angle affect melt fracture?

Answer 31

Increasing the die entrance angle to make it more sloped, increases the stress that can be applied before melt fracture.

If the entrance is flat (90*), then melt fracture happens at much lower stresses than if you slope the entrance.

Question 32

What factors reduce melt fracture?

Answer 32

Increasing the angle of die entrance

Decrease the flow output rate

Increase the temperature (less viscosity so higher critical shear rate)

Local heating of die during extrusion

Decrease molecular weight of polymer

Question 33

What is a cone and plate rheometer?

Answer 33

Polymer is placed between a heated cone and plate. Apex of cone is in contact with stationary plate and polymer fills gap. Angle of cone is usually small like 5*. Cone is rotated and Torque is measured over a range of rotational rates (w). Either constant speed or constant torque can be used.

Torque = 2/3 * pi * R^3 * shear stress

Question 34

What are the pros and cons of a cone and plate rheometer?

Answer 34

Pros:

Flow conditions are precisely defined
Shear rate is nearly the same everywhere in the fluid as the gap angle is small

Cons:

Shear rate range is limited
At high shear rates, flow pattern breaks up at free boundary making non-steady state conditions
Shear strain rates are limited

Question 35

What is a concentric cylinder viscometer?

Answer 35

Sample is sheared between two heated concentric cylinders, one of which is rotating at constant speed. Torque is measured over a range of speeds so viscosity and strain rates can be calculated.

Y = 2piR*N / H

n = TH/4pi2R3LN

N = revolution rate

Apparatus is limited to relatively low strain rates.

Question 36

What is the Weissenberg effect?

Answer 36

Phenomenon when placing a spinning rod into a polymer.

Instead of being thrown outward, entanglements cause polymer chains to be drawn towards the rod.

Strain tensor of the turning motion produces a non-zero difference between the normal components of the resulting stress tensor so there is a force up or down.