Understanding bubble stability in blown film extrusion

1. Blown film & bubble stability
Ted Brink

2. Page  2
WHAT
determines
bubble stability?
Bubble stability in blown film

3. Page  3 Bubble stability in blown film
Overview of discussion topics
• blown film
• bubble cooling
• die construction
• melt viscosity
• melt strength
• temperature influence

4. Page  4
frost line
below frost line:
melt phase
above frost line:
solid phase
the bubble is:
• pulled up by nip rolls
• supported by melt strength
Bubble stability in blown film

5. Page  5 Bubble stability in blown film
Bubble stability
Bubble stability is a combination of:
• integrity of the molten film
- melt strength
- melt viscosity
- shear rate
• integrity of the solid film
- thickness
- material (stiffness)
melt phase
solid phase

6. Page  6 Bubble stability in blown film
Integrity of the solid film
Film characteristics:
• film thickness
• materials used (stiffness, crystallization rate, Mw or MFI)
Machine characteristics:
• output
• die gap
• cooling efficiency
• bubble height

7. Page  7 Bubble stability in blown film
Polymer rheology
• high zero shear viscosity positive for bubble stability
• for multilayer extrusion
- flow simulation is useful tool
- most layers consist of more than one component

8. Page  8 Bubble stability in blown film
Melt strength
• melt strength determines
- bubble stability
- stretching behaviour
• melt strength = resistance to drawing
• melt strength is related to entanglements
- Mw
- Mw/Mn
- branching

9. Page  9 Bubble stability in blown film
Melt strength
Melt strength is influenced by:
• molecular weight
• molecular weight distribution
• chain entanglements
• molecular branches
determined by
resin supplier

10. Page  10 Bubble stability in blown film
Melt viscosity
Melt viscosity is influenced by:
• molecular weight
• polydispersity MWD
• polymer temperature
• bubble cooling
• shear rate
determined by
resin supplier
determined by
film producer

11. Page  11 Bubble stability in blown film
Melt strength and viscosity are influenced by:
• molecular weight
• molecular weight distribution
• chain entanglements
• molecular branches
• polymer temperature
• bubble cooling
• shear rate
determined by
resin supplier
determined by
film producer

12. Page  12 Bubble stability in blown film
Extrusion temperature - 1
A low polymer temperature:
• improves bubble stability
• reduces thermal polymer breakdown
But:
• increases shear
- risk of uncontrolled polymer temperature
- polymer temperature influenced by output

13. Page  13 Bubble stability in blown film
Extrusion temperature - 2
Common practice:
• flat temperature profile on the extruder barrel
Why?
• adequate for low shear 3-zone screws
• operation simplicity
• reduce chance for mistakes

14. Page  14 Bubble stability in blown film
Example extrusion temperature control for PA6
230
250
270
290
1 2 3 4 Tpol
temperature[°C]
barrel zone
set actual
230
250
270
290
1 2 3 4 5
temperature[°C]
barrel zone
set actual
flat temperature profile reversed temperature profile
poor control of
polymer temperature
good control of
polymer temperature

15. Page  15 Bubble stability in blown film
Bubble cooling
• external bubble cooling
- chilled air
• internal bubble cooling (IBC)

16. Page  16 Bubble stability in blown film
Pancake dies versus spiral dies
Pancake dies
• temperature isolation of the layers
- nylon at 230 – 250 °C, EVOH at 175 – 205 °C
• relative long residence time
Spiral dies
• short residence times

17. Page  17 Bubble stability in blown film
Measurement of melt strength
• Göttfert Rheotens
• oscillatory rheometer

18. Page  18 Bubble stability in blown film
Summary
Improvement of bubble stability can be realised by:
• high melt viscosity of the polymer in the bubble
- high Mw
- branched polymer
- low polymer temperature
- efficient bubble cooling
- lower frost line

19. Page  19
Contact
Ted Brink
Email: ted.brink@extrusionist.com
Internet: www.extrusionist.com
Tel.: +31 651109899
Skype: ted.brink
Bubble stability in blown film