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-pentanelti-pentaneltc-pentane)

4
Expansion - Behind the Scenes

• Beads Expand
• Polymer/blowing agent matrix reaches its 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

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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 /gt 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
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Primary Expansion Controls

• Temperature
• Time

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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

• lt100oC
• Expansion can be sluggish due to stiff polymer
• gt120oC
• 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 dont 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

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Blowing Agent

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

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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 lt i-pentane lt c-pentane

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Blowing Agent

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

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Blowing Agent
Poor Distribution of Blowing Agent
Good Distribution of Blowing Agent
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Bead Size

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

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Expander Equipment

• Continuous
• Batch
• Wet
• Dry

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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

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Continuous Expanders

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

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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

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Batch Expanders

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

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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

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Fluid Bed Dryers

• Description
• 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

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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.

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Density Check Problems

• Wet Prepuff
• May appear heavy due to moisture weight
• May appear light because the surface tension
  wont allow beads to pack or flow
• Too Dry Prepuff
• May appear light because static wont let it flow
  or pack

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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

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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-pentanegti-pentanegtc-pentane)

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Why Age Prepuff?

• Foam becomes more resilient after its 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

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Volatile Content on Aging
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Aging

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

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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

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, youll know your steam quality.
• Insufficient steam times
• Too high a throughput through expander
  (continuous) gives raw material carry-over
• Wet material

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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

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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

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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)

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Expansion - Troubleshooting

• Density Fluctuations (continued)
• Purge valve sticking
• Vacuum problems
• Inconsistent water inject volume
• Poor additive distribution (block shape grade
  EPS)

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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

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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)

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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