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Curling in multilayer films

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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.

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Curling in multilayer films

  1. 1. Curling in multilayer films Ted Brink
  2. 2. Page  2 Curling in multilayer films curling in multilayer films In practice, curling occurs only in asymmetrical films
  3. 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. 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. 5. Page  5 Advantages of asymmetrical films • superiour thermoforming • short sealing cycle • good sealing • excellent printing • improved O2-barrier curling in multilayer films
  6. 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. 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. 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. 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. 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. 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. 12. Page  12 Remedies against curling • use of a water bath • film annealing • relaxation and/or rewinding • polyamide blending curling in multilayer films
  13. 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. 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. 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. 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. 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. 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. 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. 20. Page  20 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
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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.

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