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 oC
Most efficient use of blowing agent, but beads become sensitive to shrinkage and heat oC
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. Good Distribution of Blowing Agent
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 Rely on Time & Temperature 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 Rely on time, temperature &/or volume
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. Expansion - Behind the Scenes
Beads Cool
Blowing agent(s), air and steam condense creating a vacuum
Beads are susceptible to
collapse from thermal shock or from over expansion
crushing due to internal vacuum
shrinkage due to rapid loss of blowing agent

27. Fluid Bed Dryers Description Purpose
Warm air is blown up through a screen and over freshly expanded beads
Purpose
Removal of surface moisture
Enable true density reading
Prepuff becomes free-flowing for easy transfer
Decreases heat sensitivity
Prevents agglomeration of prepuff in aging silo
Starts stabilization of prepuff
Breaks up soft lumps

28. 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.

29. Density Check Problems
Wet Prepuff
May appear heavy due to moisture weight
May appear light because the surface tension won’t allow beads to pack or flow
Too Dry Prepuff
May appear light because static won’t let it flow or pack

30. Conveyance of Prepuff Bead are not fully stabilized and delicate
Guidelines
Keep the number of bends in piping down to a minimum
Make sure piping size is adequate
Do Not pass beads through impeller blades

31. 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)

32. 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

33. Volatile Content on Aging

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

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

36. 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

37. 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

38. 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

39. 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)

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

41. 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

42. 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)

43. 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

44. 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