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Growing More Ductile Epoxies: An Essential Work of Fracture Study

Growing More Ductile Epoxies: An Essential Work of Fracture Study. Frederick A. Pfaff Stonhard A Division of the StonCor Group, Inc. Twofold Goal of This Study. Feasibility of Using Difunctional Amines as Chain Extenders in Amine-Cured Epoxy Formulations

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Growing More Ductile Epoxies: An Essential Work of Fracture Study

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  1. Growing More Ductile Epoxies:An Essential Work of Fracture Study Frederick A. Pfaff Stonhard A Division of the StonCor Group, Inc.

  2. Twofold Goal of This Study • Feasibility of Using Difunctional Amines as Chain Extenders in Amine-Cured Epoxy Formulations • Suitability of the Method of Essential Work to Determine Fracture Toughness of Thermoset Epoxy Compositions

  3. OUTLINE • Amine-Cured Epoxy Formulation Considerations • Fracture Toughness Test Methods • Experimental Details • Tensile Test Results • Fracture Toughness Results • Solvent Uptake • Conclusions

  4. Formulation Perspectives Amine – Cured Epoxies

  5. Formulation Trends • Lower VOC  Lower MWt. Resins • Liquid Epoxy • Amidoamine, Aliphatic & Cycloaliphatic Amine Hardeners • Diluents & Plasticizers • High Crosslink Density • Tend to be Relatively Brittle

  6. Cure & Glass Transition Temperature (Tg) • Two Components – Epoxy & Amine • Cures to Solid Crosslinked Mass • Tg of Reacting Mixture Increases as Cure Proceeds • Tg ≤ Cure Temperature + 30ºC • Vitrification Effect • Properties of Epoxies Used Within Approx. 30ºC of Tg Are Not Completely Glass-like

  7. Managing Formula Tg • Most Unmodified Amines Cure Liquid Epoxy To a High Tg • Ultimate Tg 80 - 150ºC or Greater • Will Not Cure to Completion Under Normal Ambient Conditions • Tg Reduced by Inclusion of Plasticizing Modifiers • Term Includes All Non-Fugitive Diluents & Plasticizers

  8. Plasticizing Modifiers • Reactive • Mono-epoxide Diluents • Stoichiometric Excess • Non-Reactive • Benzyl Alcohol, Ester & Hydrocarbon Plasticizers • Lower Tg • Reduce Viscosity • Important Impact on Cured Properties Proper Choice Critical to Performance!

  9. Side-Effects in Cured State • Stoichiometric Excess & Monofunctional Diluents • Dangling Chain Ends • Non-Reactive Plasticizers • Not Chemically Bound − Extractable • Plasticizer Migration • Sometimes Count as VOC • Can Result in Lower Strength & Poorer Chemical Resistance A Fresh Approach Could Be Helpful

  10. Chain Extension • Reactants With Functionality = 2 • Amines with Two Active Hydrogens Would be Chain Extenders • Mono-Primary Amines • Di-Secondary Amines • “Grow” Higher MWt. Simultaneous With Crosslinking Reaction

  11. Goals • Partial Replacement for Traditional Plasticizing Modifiers • Viscosity & Tg Reduction • Better Cured Property Balance • Reduced Brittleness • Minimal Negative Effect on Other Mechanical Properties & Chemical Resistance

  12. Fracture Toughness Testing

  13. Need For a New Method • Linear Elastic Fracture Method for Brittle Break Materials • Example: ASTM D5045 • ASTM E1820 for Ductile Materials • J-Integral Method • Successive Loadings & Unloadings • Careful Measurement of Crack Length After Each Loading Cycle Inconvenient For Most Industrial Coatings Labs

  14. Method of Essential Work • First Proposed by Cotterell & Reddel • Basically a Tensile Test on Notched Specimens Notches Ligament “DENT” Configuration

  15. Method of Essential Work, cont’d. Wf = We + Wi (1) wf = Wf / l*t = we + βwi*l (2)

  16. EWF Constraints • Dimensional Requirements • Full Yielding Before Failure • Self-Similarity of Load Curves at All Ligament Lengths • Plastic Zone Constraint l < 2rp

  17. Experimental Details

  18. Primary Materials IPDA BzOH Bis A Epoxy

  19. Epoxy Materials n Bisphenol A Epoxy C13 Epoxy Diluent

  20. Amine Chain Extenders N-Dodecyl Amine N-Amyl Amine N, N’-Dimethyl Ethylene Diamine

  21. Chain Extension Scenarios

  22. Experimental Formulations

  23. Parallel Glass Plate Casting Arrangement

  24. Solvent Uptake Testing • Weight Change After 6 Hour RT Immersion • Weight Change After 24 Hour 90ºC Immersion • Weight Change After Dry Bake • Final Weight Change %Δmf = 100 * [(mw – min(mi, md) ) / mi]

  25. Tensile Test Results

  26. Tensile Load Curves

  27. Tensile Yield Stress vs. Tg

  28. Essential Work of Fracture Test Results

  29. EWF Median Load Curves

  30. MOD 1 EWF Results

  31. MOD 2 EWF Results

  32. MOD 3 EWF Results

  33. MOD 4 EWF Results

  34. MOD 5 EWF Results

  35. MOD 6 EWF Results

  36. MOD 7 EWF Results

  37. MOD 8 EWF Results

  38. EWF vs. Yield Stress

  39. EWF vs. Tg

  40. Mechanical Properties Summary • Chain Extension Via Difunctional Amines Provides: • Reduction of Viscosity & Tg with Less Plasticizer • Tensile Strength Comparable to Strength Obtained With Higher MWT. Epoxy • Superior to Mono-Epoxide Modification • Superior Balance of Tensile Strength & Fracture Toughness Combined with Tg Reduction

  41. Solvent Uptake Test Results

  42. Solvent Exposure % Weight Changes

  43. Conclusions • Method of Essential Work Appears to be Applicable to Ductile Thermoset Epoxy Compositions • Plastic Zone Requirement Not Met • More Work Needed • Chain Extension Via Difunctional Amine Insertion Provides: • Superior Balance of Mechanical Properties • Little to No Adverse Effect on Solvent Uptake

  44. Conclusions • Solvent Uptake Excessive With Large Stoichiometric Excess of Epoxy • Best Overall Properties Obtained From Non-Pendant Chain Extender, DM-EDA

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