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Page 1: Bubble stability in blown film

Blown film & bubble stability

Ted Brink

Page 2: Bubble stability in blown film

Page 2

WHATdetermines

bubble stability?Bubble stability in blown film

Page 3: Bubble stability in blown film

Page 3 Bubble stability in blown film

Overview of discussion topics

• blown film• bubble cooling• die construction• melt viscosity• melt strength• temperature influence

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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

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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

Page 6: Bubble stability in blown film

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

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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

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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

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Page 9 Bubble stability in blown film

Melt strength

Melt strength is influenced by:• molecular weight• molecular weight distribution• chain entanglements• molecular branches

determined byresin supplier

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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 byresin supplier

determined byfilm producer

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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 byresin supplier

determined byfilm producer

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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

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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

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Page 14 Bubble stability in blown film

Example extrusion temperature control for PA6

1 2 3 4 Tpol230

250

270

290set actual

barrel zone

tem

pera

ture

[°C]

1 2 3 4230

250

270

290set actual

barrel zone

tem

pera

ture

[°C]

flat temperature profile reversed temperature profile

poor control of polymer temperature

good control of polymer temperature

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Page 15 Bubble stability in blown film

Bubble cooling

• external bubble cooling- chilled air

• internal bubble cooling (IBC)

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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 timeSpiral dies• short residence times

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Page 17 Bubble stability in blown film

Measurement of melt strength

• Göttfert Rheotens

• oscillatory rheometer

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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

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Contact

Ted BrinkEmail: [email protected]: www.extrusionist.comTel.: +31 651109899

Skype: ted.brink

Bubble stability in blown film