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Curling in multilayer films
Ted Brink
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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
HH2O
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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, postcrystallizationmay 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
curl
facto
r [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
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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
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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: [email protected]
Internet: www.extrusionist.com
Tel.: +31 651109899
Skype: ted.brink
curling in multilayer films
