Polymer Processing

Question 1

What is polymer extrusion?

Answer 1

Well suited for large scale processes (the backbone of thermoplastic polymer processing), versatile for:

• melting
• blending
• and conveying/transporting to downstream processes

Question 2

How does polymer extrusion work?

Answer 2

plastic pellets are fed through a hopper into a cylinder where it is moved with a screw. Heat is added to melt the polymer and the screw both transports it and mixes the polymer

*different screw types exist to maximize shear or elongation flow
**difficult to model due to mass flow and phase transitions

Question 3

What types of mixing are shear and elongation flow good for?

Answer 3

shear = distributive mixing
elongation = dispersive mixing

Question 4

How is compounding done in polymer extrusion?

Answer 4

• blending of polymers and additives to create specific formulation
• can be done inline (directly to feed) or in a downstream process,

*attempting to get well-blended polymers

Question 5

What is blown film extrusion?

Answer 5

• a high throughput technique used for manufacturing continuous thin films
• ideal for packaging, barrier applications, and membranes
• done by stretching films biaxially through a blowing process (like blowing bubble gum) - improves strength and durability
• requires knowledge and experience to maintain stability (don’t want bubble to collapse)
  *can create thin films as low as 20 micrometers

Question 6

What is cast film and sheet extrusion?

Answer 6

• ideal for continuous manufacturing of films and sheets (few microns to few mm in thickness)
• uses rollers to stretch film and to apply tension as polymer cools
• uses specialized dyes that allow polymer to relax before orifice of extruder and thus allow to be stretched into sheets

Question 7

What is profile extrusion?

Answer 7

• polymer is extruded directly into final product shape using specialized profile die
• commonly used for tubing and pipes but also for more complex geometries

*polymer contorts slightly after leaving mold (so molds are often made slightly larger)

Question 8

What is coextrusion?

Answer 8

• simultaneous extrusion of multiple polymers into a single combined flow. Very commonly used in packaging and barrier materials
• combination in melt state allows interdiffusion between layers and a cohesive interface - we want dissimilar layers to act like a single material, rather than 2 glued together

*need to match flow and viscoelastic properties to avoid flow instabilities at the boundary layer

Question 9

What is melt memory and die swell?

Answer 9

As polymer leaves extruder through the die, the polymer is compact and squeezed together. However, the chains do not like this, so when it exits the die, it will swell to a larger size

Question 10

How can die swell be reduced?

Answer 10

1. make die longer (gives more time for chain relaxation)
2. increase temperature (relaxation depends on temp)
3. slow down polymer movement through channel (more travel time + less stress applied to polymer)

Question 11

What happens to shear (and flow) for a newtonian fluid?

Answer 11

Assumes: no slip condition + well-developped uniaxial flow

• flow becomes a half circle, reaching its maximum at the center of the pipe
• shear is highest at the walls and lowest at the center of the pipe

Question 12

What happens to shear (and flow) for a shear thinning fluid?

Answer 12

shear thinning: viscosity decreases with higher shear, thus increasing speed

• Flow: no slip, then increases much faster than newtonian before becoming near flat as it crosses the pipe
• shear - similar shape as newtonian fluid, but curves a bit as change in viscosity causes shear to increase more drastically as it approaches the walls

Question 13

What are the processing condition strategies employed for multiphase flows in coextrusion?

Answer 13

at the junction we want introduction of two flows to occur as smoothly as possible. This is done by controlling channel design:

• want well-developed laminar flows
• minimal difference in layer velocities
• small angle between channels

*need to account for viscoelastic effects since we have normal forces perpendicular to flow (want to flow with this force) and polymers may have different viscoelastic effects (ex. different viscosities)

Question 14

What is the Kelvin-Helmholtz instability?

Answer 14

Turbulent flow is generated at the interface between 2 fluids moving at different speeds

*this can be seen in polymers (if the coextruders are moving at different speeds then will get undesired swirling motion of polymers into each other)

Question 15

What is viscous encapsulation?

Answer 15

A thermodynamically-driven phenomenon observed in layered polymer flows - lower viscosity polymer migrates to pipe walls and higher viscosity polymer moves to center of pipe, causing it to be encapsulated (low viscosity takes less E to flow on the outside so they will naturally thermodynamically move there - more shear at walls)

• minimize effect by having viscosity ratio close to 1 (at around 3.6 ratio start to get full encapsulation)

*this can be done by selecting a shear rate where the polymers viscosities are similar

Question 16

What is elastic layer rearrangement?

Answer 16

Recall: polymers experience normal forces perpendicular to flow

if the channel is square or rectangular, will be secondary flows due to unbalanced normal stresses, causing swirling forces at edges
- this causes layers to rearrange - which is a huge issue in packaging (or other applications) where want uniform predictable layers

Question 17

How can elastic layer rearrangement be mitigated?

Answer 17

• more difficult to mitigate (cylindrical pipe may not be possible - say for thin films)

Possibilities:
- use simulation to predict forces and attempt to design channel to minimize effects
- higher processing temperatures (minimizes elastic effects)
- lower Mw resins (lowers elastic effects)