Curled multilayer films may lead to problems during printing, lamination, thermoforming, etc. Curling occurs mainly in asymmetrical films. Some tools are shown to reduce this curling effect.

1. Curling in multilayer films
Ted Brink

2. Page  2
Curling in multilayer films
curling in multilayer films
In practice, curling occurs
only in asymmetrical films

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Contents
• pros and cons of asymmetrical films
• causes of curling
- multilayer film
- thick film
• dealing with curling
• conclusions
curling in multilayer films

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Asymmetrical films
• different materials used
- different Tm, Tg, crystallization behaviour
• often used for barrier films
- combinations of polyolefins and polyamide or EVOH
curling in multilayer films

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Advantages of asymmetrical films
• superiour thermoforming
• short sealing cycle
• good sealing
• excellent printing
• improved O2-barrier
curling in multilayer films

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Disadvantage of asymmetrical films: curling
Curled films may cause problems during:
• winding
• slitting
• printing
• lamination
• thermoforming/packaging
curling in multilayer films

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The main cause of curling
• curling may occur in asymmetrical films
• asymmetrical films have different polymers
• different polymers have different crystallization rate
curling in multilayer films

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The main cause of curling – model 2-layer PA/PE
curling in multilayer films
A
layer A and B liquid
B
layer B crystallizes
layer A follows
A
layer A crystallizes
layer B cannot follow
B
A
curling occurs when the film is cooled from the melt
A
B
2-layer model
B
layer polymer Tm Tcc
A PE 125 °C 115 °C
B PA6 220 °C 180 °C
c o o l i n g

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Factors influencing curling
• materials used
- difference in melting point
- difference in crystallization
- difference in shrinkage
- difference in moisture absorption
• processing conditions
- cast or blown film (cooling rate)
- cooling capacity
• film structure
curling in multilayer films

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Polyamide effects
• polyamide interacts with moisture
- high humidity: moisture absorption
- low humidity: moisture desorption
• Tg influenced by H2O
- postcrystallization may occur when Tg drops below ambient
conditions
curling in multilayer films

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Moisture induced crystallization of PA6
curling in multilayer films
Tg PA6
• dry ± 55 °C
• wet ± -10 °C
Effects of moisture on PA6 film:
• lower stiffness
• increased flexibility
• increased stretchability
• easier thermoforming
postcrystallization
occurs when Tg
drops below
ambient
temperature
O=C
N-H
|
O=C
N-H
|
O=C
N-H
|
O=C
N-H
|
O–H
H
H2O

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Remedies against curling
• use of a water bath
• film annealing
• relaxation and/or rewinding
• polyamide blending
curling in multilayer films

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Use of a water bath
• a water bath helps to reduce
curling
• PA absorbs water
- increase of dimensions
- increase of flexibility
- Tg drops, postcrystallization
may occur → decrease of
dimensions
curling in multilayer films
water temperature: 60 – 85 °C
residence time: 5 – 30 sec

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Curl reduction - water bath
0
50
100
150
200
250
1 2 3
curlfactor[m-1]
Reeks1
Reeks2
Reeks3
• residence time in water bath 5 sec
• 3-layer blown film of 100 μm
curling in multilayer films

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Curl reduction - film treatment
• film annealing
- in conditioned warehouse
- at elevated temperature
• film rewinding
- low speed → relaxation
- using heated rolls
curling in multilayer films

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Curl reduction – reduction crystallization rate
• reduce the crystallinity of the PA
- blend PA6 with PA6.66
- replace PA6 with PA6.66
• blend with amorphous polyamide
- PA6 with PA6I/6T
curling in multilayer films

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Curling in thick film
• films are mostly cooled one-sided, resulting in:
- density difference
- morphology difference
curling in multilayer films
film
cooled surface density crystallinity
• resulting in stresses and curling

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Conclusions thick film
To reduce curling in thick film:
• consider 2-sided cooling
- blown film: external cooling + IBC
- cast film → calendaring or extrusion into water bath
curling in multilayer films

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Conclusions asymmetrical films
• curling occurs predominately:
- in asymmetrical multilayer film
- in blown film
- with polyamide in the outer layer
• curling is reduced by:
- fast cooling of the melt
- use of water bath
- polyamide with lower crystallinity or melting point
- blending PA6 with PA6I/6T
- post treatment of film
curling in multilayer films

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More information and contact
Ted Brink
Email: ted.brink@extrusionist.com
Internet: www.extrusionist.com
Tel.: +31 651109899
Skype: ted.brink
curling in multilayer films