Expansion of Expandable Polystyrene (EPS)

1. Expansion of Expandable Polystyrene (EPS)

2. Expansion Basics • Heat Is Applied • Beads Expand • Beads Cool • Beads Age

3. Expansion - Behind the Scenes • Heat Is Applied • Blowing agent(s) vaporizes (28oC iso-pentane, 35oC normal-pentane, 49oC cyclo-pentane) • Blowing agent(s) permeate through the polymer (n-pentane<i-pentane<c-pentane)

4. Expansion - Behind the Scenes • Beads Expand • Polymer/blowing agent matrix reaches it’s glass transition temperature (Tg) (about 85oC, varies according to Mw & BA type) • Polymer chains become fluid • Internal pressure created by blowing agent vaporization push apart [unfold] polymer chains, creating cells

5. Expansion - Behind the Scenes

6. Expansion - Behind the Scenes • Beads Expand (cont.) • Air & steam permeate into the beads • As expansion continues, cell walls become thin and subjection to high heat can cause them to break and rupture

7. Expansion - Behind the Scenes • Beads Expand (cont.) • Throughout expansion, blowing agent(s) continue(s) to permeate out of the bead at an increasing rate [depending on temperature and thickness of cell walls] (When permeation rate =/> vaporization rate, expansion ceases)

8. Blowing agents begin to vaporize Polymer/blowing agent matrix reaches its Tg Cell walls begin to rupture Expansion begins after reaching Tg and proceeds rapidly Loss of blowing agent becomes more rapid as temperature increases

9. Primary Expansion Controls • Temperature • Time

10. Temperature Affects • Greater the temperature • Softer the polymer • Increased expansion rate • May result in uneven expansion due to inconsistent • pentane content • bead size • cell structure • raw material “carry over” [extreme cases] • Increased permeation rate of blowing agent

11. Expansion verses Temperature • <100oC • Expansion can be sluggish due to stiff polymer • >120oC • Polymer is too soft, blowing agent loss is too rapid • 110-120oC • Most efficient use of blowing agent, but beads become sensitive to shrinkage and heat • 100-110oC • Best compromise

12. Steam Quality • Key to Expansion • Consistent utilities are crucial to achieve consistent densities with consistent volatile content

13. Secondary Expansion Controls • Volume of Expander • Molecular Weight • Blowing Agent • Bead Size

14. Expander Volume Affects the total heat available to each bead • Things that can change it • Drop/charge weight • Lumps in expander that don’t discharge • Build up on walls or stir blades

15. Molecular Weight • The lower the molecular weight, • Increased expandability • Increased heat sensitivity • Increased permeation rate of blowing agent • Increased shrinkage • Increased collapse • Structural strength

16. Blowing Agent • Amount • Higher percentages give greater expandability (to a point) • Too high a percentage causes rapid permeation thus shrinkage, collapse and heat sensitivity

17. Blowing Agent • Type • The longer the blowing agent stays in the bead, • Increased expandability • Reduced shrinkage rate • Increased prepuff life (from expansion to mold) • Relative retention n-pentane < i-pentane < c-pentane

18. Blowing Agent • Degree of Distribution (has a direct affect on cell size) • Expandability • Heat sensitivity • Structural strength • R-Factor (insulation properties) • Permeation rate

19. Poor Distribution of Blowing Agent Blowing Agent Good Distribution of Blowing Agent

20. Bead Size • The larger the bead, the easier it is to achieve low densities • Less surface area for blowing agent to permeate out of

21. Expander Equipment • Continuous • Batch • Wet • Dry

22. Continuous Expanders • Description • Material is fed into the bottom of the expansion chamber where it is subjected to steam under agitation, material expands and as density decreases, material rises to the top and out the exit chute. • Rely on Time & Temperature

23. Continuous Expanders • Main Controls • Feed rate • Steam pressure (temperature) • Agitation rate • Outlet height • Fresh air introduction (temperature)

24. Batch Expanders • Description • A pre-weighed quantity of material is dropped (or charged) into the expansion chamber where either the expander walls are jacketed with steam (dry) &/or steam is injected into the chamber (wet). An agitator keeps material moving. Vacuum, purge or water inject may be used to stop the expansion. • Rely on time, temperature &/or volume

25. Batch Expanders • Main Controls • Steam pressure (temperature) • Volume or height • Time • Charge weight • Vacuum or purge time • Water inject

26. Density Check • Consistency is the Key • Procedure • Prepuff is overfilled into a known volume (pre-tared) container. The container is vibrated or tapped (vibration is more consistent). A straight edge is used to strike the top level with the canister. The canister is reweighed and the density calculated.

27. Expansion - Behind the Scenes • Beads Age, a.k.a. Maturing or Stabilizing • Internal moisture [from condensed steam] permeates out of the bead • Air permeates into the bead until internal and external pressures equal • Blowing agent(s) continue(s) to permeate out of the bead (n-pentane>i-pentane>c-pentane)

28. Why Age Prepuff? • Foam becomes more resilient after it’s stabilized • Internal vacuum makes beads susceptible to deformation • Reduces blowing agent levels • Too high a blowing agent level leads to excessive cool times and heat sensitivity during molding • Dry prepuff • Improves ease of transportation

29. Volatile Content on Aging

30. Aging • Key • A consistent environment is important to provide prepuff to mold with a consistent volatile content

31. Aging Time Controls • Environment • Air flow Time • Temperature Time •  Density Time •  Bead Size Time •  Blowing Agent Type(boiling point & molecular complexity) Time •  Polymer Mw Time

32. Expansion - Troubleshooting • High Density • Insufficient steam pressure/temperature • check traps, valves, accumulator pressure • steam flow restricted Note: by monitoring both steam pressure and steam temperature, you’ll know your steam quality. • Insufficient steam times • Too high a throughput through expander (continuous) gives raw material carry-over • Wet material

33. Expansion - Troubleshooting • High Density (continued) • Collapsed or over expanded prepuff • Low blowing agent content in raw EPS • Additive problem (block and shape EPS grades) • Increased drop weight • Electric eye level too low

34. Expansion - Troubleshooting • Low Density • High steam pressure/temperature • Longer steam times • Reduced drop/charge weight • Wet material • Over dried material • High blowing agent content • Surface additives (block & shape grade EPS) • Electric eye level too high

35. Expansion - Troubleshooting • Density Fluctuations • Inconsistent steam pressure/temperature • Inconsistent steam time • Erratic drop/charge weights • Inconsistent measuring techniques • Inconsistent blowing agent content • High static (affects electric eye)

36. Expansion - Troubleshooting • Density Fluctuations (continued) • Purge valve sticking • Vacuum problems • Inconsistent water inject volume • Poor additive distribution (block & shape grade EPS)

37. Expansion - Troubleshooting • Wet Prepuff • Common on expander start up • Wet steam • Purge valve or vacuum not working • Poor air flow through fluid bed dryer • Too much material in the fluid bed dryer

38. Expansion - Troubleshooting • Bead Collapse • Over-expanded • Excessive steam pressure/temperature • Excessive steam time • Too high an expansion rate • Thermal shock after expansion • Blades too close to walls or bottom of expansion chamber • Wrong additive package (block and shape grade EPS)

39. Expansion - Troubleshooting • Lumping • Too much moisture (condensate) • Inadequate stirring • Excessive steam pressure/temperature • Anti-lumping agent level too low • Hot spots in expander • Excessive steam time

40. Expansion - Troubleshooting • Irregular Prepuff (size/appearance) • Poor or irregular steam flow • Hot spots in expander • Insufficient time in expander • Contamination of prepuff in raw EPS (double pass) • Irregular raw EPS