Advanced Troubleshooting of Polyurethane Elastomers By Nick Skirkanich Sales Development Manager, COIM USA Inc.
Troubleshoo*ng Main Categories Raw Material Issues Processing Problems Appearance Problems Field Failures
Review Processing Procedures Raw Material Inventory and Storage Prepolymer/Curative Meltdown Prepolymer/Curative Transfer Prepolymer Degassing Mixing Prepolymer and Curative Mold Preparation Casting Cure Demold Postcure Finishing
Raw Material Issues
Raw Material Issues Prepolymer High Viscosity Prepolymer Difficult Degas Pot Life Changes Gel Time Changes
Prepolymer High Viscosity Causes: - Overheated prepolymer Ø Heat history Accumulative - Low prepolymer temperature - Use of inappropriate prepolymer
Prepolymer High Viscosity Solutions: - Adhere to melting guidelines - Log heat history of prepolymer - Thermocouple temperature check - Choose proper prepolymer - Manage package size to resin use.
Prepolymer Difficult Degas Causes: - High prepolymer viscosity - Insufficient degas vacuum - No degassing agent - Moisture contaminated prepolymer (no N2 cap),excessive degassing time
Prepolymer Difficult Degas Solutions: - Check prepolymer temperature and heat history - Check vacuum pump gauge - Add degassing agent - Change prepolymer
Pot Life Changes Causes: - Degraded prepolymer - Degraded curative - Improper catalyst addition - Incorrect temperature
Pot Life Changes Solutions: - Minimize heat history - Check catalyst addition - Monitor temperatures
Gel Time Changes Causes: - Degraded prepolymer - Degraded curative - Improper catalyst addition - Incorrect temperatures
Gel Time Changes Solutions: - Minimize heat history - Check catalyst addition - Monitor temperatures
Processing Problems
Processing Problems Poor Physical Properties Low Hardness
Poor Physical Proper*es Causes: - Incorrect stoichiometry - Overheated prepolymer - Incomplete cure
Poor Physical Proper*es Solutions: - Calibrate equipment regularly - Minimize heat history - Calculate curative on specific %NCO - Use elemental analyzer (Cl) - Follow supplier guidelines - Test sheets for daily production (ref)
Low Hardness Causes: - Overheated prepolymer - Incomplete cure (TDI/ triols) - Incorrect stoichiometry - Inadequate mixing
Low Hardness Solutions: - Minimize heat history - Calculate curative on specific %NCO - Mix adequately - Follow curing guidelines
Helpful Guidelines Raw Materials & Processing Complete melting of prepoymer - not just enough to use (MW range) Low NCO, (low duro) prepolymers more sensitive NCO content important tool, easy and quick to use Tracking heat history Production check list establishes references for YOUR process Add catayst to bulking agent to accurately meter
Helpful Guidelines Raw Materials & Processing II Vacuum trap for degassing (curative like HQEE) Check curative blend compatability / temp dependent (BDO/polyol) Temperature! Temperature! Temperature! v Measure oven temps with thermocouple (Oven hot spots) v Use temperature tape to confirm drum temps v Measure actual mold temperatures at pour. v Know your prepolymer temperature v Measure exotherms for your materials
Appearance Problems
Appearance Problems Large Voids in Parts Small Bubbles in Parts Excessive Shrinkage, Cracking or Warping Swirls/Streaks in Parts Surface cosmetics
Large Voids in Parts Causes: - Air entrapment during pour - Short pot life - Leaking mold
Large Voids in Parts Solutions: - Redesign mold/vents - Alter pour technique - Use lower viscosity prepolymer - Add degassing agent
Small Bubbles in Parts Causes: - Moisture contamination - Inadequate degassing - Poor pouring technique - Failed seal in machine - Mold leakage (during cure)
Small Bubbles in Parts Solutions: - Store products under dry N 2 - Degas all components pre- and post mixing - Mold properly heated - Maintain mix head seals
Excessive Shrink/Crack/ Warp Causes: - Incompatible resin and mold temperatures - Localized temperature variation - Low stoichiometry
Excessive Shrink/Crack/Warp Solutions: - Balance exotherm and mold temperatures - Heat molds adequately - Monitor raw material temperatures
Swirls/Streaks in Parts Causes: - Improper mixing - Unevenly dispersed filler - Knit lines
Swirls/Streaks in Parts Solutions: - Ensure mixing all components - Disperse fillers in plasticizer - Use smaller size filler particles - Use high viscosity materials - Use increased pot life systems
Surface Cosme*cs Causes - Cool mold temperatures ( lakeing ) surface shrinkage - Leak in mold seal ; drawing air onto surface - Excess mold release
Surface Cosme*cs Solutions - Insure mold temps are uniform - Improve seal at mold joints where air intake is located - Monitor/gauge mold release amount and frequency
Field Failures
Field Failures Part Softening in Use Part Hardening in Use Deformed Part Blowout
Field Failures Fatique (cracking/chunking) Polyurethane to Non- Polyurethane Bond Failure Swelling Abrasion Failure
Part SoOening in Use Causes: - Contact with solvents - Exposure to excessively high temperatures - Hydrolysis of Ester based resins
Part SoOening in Use Solutions: - Shield elastomer from solvents - Insulate elastomer from heat source - Choose proper prepolymer
Part Hardening in Use Causes: - Soft segment crystallization - Insufficient cure - Low stoichiometry - Plasticizer leaching
Part Hardening in Use Solutions: - Choose correct prepolymer - Use stoichiometry above 90% - Ensure plasticizer compatibility - Follow curing guidelines
Deformed Part Causes: - Softening by heat - Poor abrasion resistance - Tendency to be thermoplastic - High compression set
Deformed Part Solutions: - Reduce heat exposure - Higher abrasion resistance formula - Reduce load/redesign part - Process at correct stoichiometry
Blowout Causes: - Improper system choice - Wrong hardness elastomer - Incorrect stoichiometry - Uneven loading - Too great of a load/speed
Blowout Solutions: - Choose correct system - Choose correct hardness - Optimize stoichiometry - Load evenly - Redesign part/application
PU to Non- PU Bond Failures Causes: - Poor initial bond - Too much stress at bond line - Overheating - Water/solvent permeation
PU to Non- PU Bond Failures Solutions: - Use correct primer/adhesive - Apply adhesive properly (Clean room concept) - Design to minimize stress - Provide cooling - Need correct urethane system
Swelling Causes: - Exposure to harmful solvents
Swelling Solutions: - Choose proper elastomer - Shield elastomer from solvent - Test elastomer before exposure
Abrasion Failure Causes: - Incorrect stoichiometry - Excessive heat buildup - Incorrect system choice
Abrasion Failure Solutions: - Choose correct stoichiometry - Minimize heat buildup - Choose correct system
References
References Trouble Shooting Guide For Polyurethane Elastomers - Brochure Polyurethane Elastomers by Hepburn Castable Polyurethane Elastomers by Clemitson