Curling in multilayer films
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The document discusses curling in multilayer films, focusing on the challenges presented by asymmetrical films composed of different polymers. It outlines the causes of curling, such as variations in melting points and crystallization rates, and presents potential remedies including the use of water baths and film annealing. The advantages and disadvantages of these films are also discussed, along with conclusions on reducing curling effects.
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Curl reduction - water bath
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• 3-layer blown film of 100 μm
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Curling in multilayer films
- 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
- 3. Page 3 Contents • pros and cons of asymmetrical films • causes of curling - multilayer film - thick film • dealing with curling • conclusions curling in multilayer films
- 4. Page 4 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
- 5. Page 5 Advantages of asymmetrical films • superiour thermoforming • short sealing cycle • good sealing • excellent printing • improved O2-barrier curling in multilayer films
- 6. Page 6 Disadvantage of asymmetrical films: curling Curled films may cause problems during: • winding • slitting • printing • lamination • thermoforming/packaging curling in multilayer films
- 7. Page 7 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
- 8. Page 8 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
- 9. Page 9 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
- 10. Page 10 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
- 11. Page 11 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
- 12. Page 12 Remedies against curling • use of a water bath • film annealing • relaxation and/or rewinding • polyamide blending curling in multilayer films
- 13. Page 13 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
- 14. Page 14 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
- 15. Page 15 Curl reduction - film treatment • film annealing - in conditioned warehouse - at elevated temperature • film rewinding - low speed → relaxation - using heated rolls curling in multilayer films
- 16. Page 16 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
- 17. Page 17 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
- 18. Page 18 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
- 19. Page 19 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
- 20. Page 20 More information and contact Ted Brink Email: [email protected] Internet: www.extrusionist.com Tel.: +31 651109899 Skype: ted.brink curling in multilayer films