Expansion of Expandable Polystyrene (EPS)
Expansion Basics Heat Is Applied Beads Expand Beads Cool Beads Age
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)
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
Expansion - Behind the Scenes
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
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)
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
Primary Expansion Controls Temperature Time
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
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
Steam Quality Key to Expansion Consistent utilities are crucial to achieve consistent densities with consistent volatile content
Secondary Expansion Controls Volume of Expander Molecular Weight Blowing Agent Bead Size
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
Molecular Weight The lower the molecular weight, Increased expandability Increased heat sensitivity Increased permeation rate of blowing agent Increased shrinkage Increased collapse Structural strength
Blowing Agent Amount Higher percentages give greater expandability (to a point) Too high a percentage causes rapid permeation thus shrinkage, collapse and heat sensitivity
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
Blowing Agent Degree of Distribution (has a direct affect on cell size) Expandability Heat sensitivity Structural strength R-Factor (insulation properties) Permeation rate
Good Distribution of Blowing Agent Poor Distribution of Blowing Agent Blowing Agent Good Distribution of Blowing Agent
Bead Size The larger the bead, the easier it is to achieve low densities Less surface area for blowing agent to permeate out of
Expander Equipment Continuous Batch Wet Dry
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
Continuous Expanders Main Controls Feed rate Steam pressure (temperature) Agitation rate Outlet height Fresh air introduction (temperature)
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
Batch Expanders Main Controls Steam pressure (temperature) Volume or height Time Charge weight Vacuum or purge time Water inject
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
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
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.
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
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
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)
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
Volatile Content on Aging
Aging Key A consistent environment is important to provide prepuff to mold with a consistent volatile content
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
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
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
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
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)
Expansion - Troubleshooting Density Fluctuations (continued) Purge valve sticking Vacuum problems Inconsistent water inject volume Poor additive distribution (block & shape grade EPS)
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
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)
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
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