Introduction to Polyurethane! Chemistry and Structure-Property Relationships
05/07/16
Monomers and Polymers
• Monomer(mono–one;mer–part):Smallmolecules• Polymer–Cons8tutesmanymonomers.• Polymeriza8on–Processofcovalently(chemically)bondingmanymonomers
together.Canbefewhundredstothousandsmonomerunits
Polymer types
• Polymers can be broadly classified into • Thermosets–Cannotbemeltedonceformed
• Elastomers(NaturalRubber,PU,SBRetc)• RigidThermosets(Epoxy,Vinylesteretc)
• Thermoplas8cs-Canberecycled• EngineeredPlas8cs(PC,Nylon,AlloyssuchasXenoyetc)• CommodityPlas8cs(HDPE,PPetc)
• Choice of Polymer depends on • Hardnessandotherphysicalpropertyrequirements• Environmentalfactors(Exposuretooil,water,otherfluids,Temperatureetc)• Toolingcost(Injec8onmoldsvsopencastvscompressionmolds)• Modeoffailure(Fa8gue,cutandtear,wear,compressiveloadsetc
What Are Polyurethanes?
• Polyurethanes are organic polymers that contain the urethane group in the structure
• Typically, polyurethanes are formed via the reaction of a Polyol (-OH group) with a Isocyanate (-NCO group)
• Polyureas contain the urea group in the structure
• Compositions may be contain just urethane group or a combination of urea and urethane groups.
R
NH O
O
R R
NH
O
NH
R
UrethaneGroup UreaGroup
Polyurethane classifications
- Linear polyurethanes - Castable polyurethanes - Millable polyurethanes - Thermoplastic polyurethanes - Cellular polyurethanes - Sprayable polyurethanes - Porometric polyurethanes - Spandex fibers
Castable Polyurethanes
• Represent only a small portion of the overall polyurethane industry • Made by mixing several ingredients, introducing into a mold, and heat curing at
temperatures (100-130°C)
• The curing process operates by extending chains of a prepolymer made from a macro diol and a diisocyanate
• The full mechanical properties of the urethane are not realized until a posturing operation is completed
• Castable urethanes can be cured with amine or diol curatives. Special formulations may use a combination of diols and amines as curatives.
POLYURETHANES
RUBBERS PLASTICS
RubberBand
CarTireTread
Men’sShoeHeel
MaterialHardnessComparison
GolfBall
BowlingBall
Fluo
rocarbon
s
Polyprop
ylen
e
Polystyren
e
Nylon
s
Acetals
Ulte
m
Acrylics
Phen
olics
4555657585
SHOREDDUROMETER507090100110120130140150
ROCKWELLR
203040506070809095
SHOREADUROMETER
Polyurethane (A versatile compound)
CuredPolyurethane
DiisocyanateType1)TDI2)MDI3)PPDI
4)H12MDI5)HDI6)TODI
PolyolType1)Polyether2)Polyester
3)Polycaprolactone4)Polycarbonate
CuraVveType1)MBOCA
2)Butanediol3)TMP4)TIPA
5)DiethyltolueneDiamne
PartA,Prepolymeror“Resin”
PartB,Cura8veor“Poly”
Diisocyanates used in Cast Urethanes
Aromatic • Toluene diisocynate (TDI) 2,4 & 2,6
• 4,4’ diphenylmethane diisocyanate (MDI)
• Paraphenylene diisocyanate (PPDI)
• 1,5-naphthalene diisocyanate (NDI)
• Reactivity: NDI> MDI> TDI
TDI
MDI
PPDI
NDI
Diisocyanates (cont’d.)
Aliphatic • H12MDI
• 1,6 Hexamethylene diisocyanate (HDI)
• Lower reactivity
• Non-yellowing due to lack of double bonds.
• Lower volume in hot cast market
H12MDI
HDI
Polyols used in Hot Cast Urethanes
Polyethers • PTMEG or Poly THF(C4): Excellent mechanical properties, hydrolysis resistance and very low abrasion
loss. • PPG(C3)-cheaper with lower performance. More susceptible to oxidation
Polyester • Compounds formed by polymeric reaction of an acid (adipic acid) with a glycol (ethylene glycol). Water is
the by product • Better tear, abrasion and oil resistance but lower hydrolytic stability Polycaprolactone (C6) • Subgroup of polyesters. More expensive • Hydrolysis resistance: Polyester< Polycaprolactone< PTMEG
Polycarbonate • Superior hydrolysis resistance • Excellent High Temperature properties • High viscosity polymers. Difficult processing.
Polyols - Polyethers O
CH2 - CH - CH3
Propylene Oxide
HO - (CH - CH2 - O)n - CH2 - CH - (O - CH2 - CH)n - OH
CH3 CH3 CH3
H2C CH2
H2C CH3
O
Tetrahydrofuran
HO - (CH2 - CH2 - CH2 - CH2 - O )n - CH2 - CH2 - CH2 - CH2 - OH
Poly(oxytetramethylene) glycol (PTMG)
Polypropylene Glycol (PPG)
Polyols - Polyesters
HO - CH2 - CH2 - OH + HO - C - CH2 - CH2 - CH2 - CH2 - C - OHEthylene Glycol Adipic Acid
HO - CH2 - CH2 - (O - C - CH2 - CH2 - CH2 - CH2 - C - O - CH2 - CH2)n - OH
Polyethylene Adipate Glycol
-H2O
O
O O
O
H2N CH2 NH2
MBCA (4,4'-Methylene bis (2-chloroaniline)
ClCl
1,4 -Butanediol
HO - CH2 - CH 2 - CH2 - CH 2 - OH
Hydroquinone bis (beta hydroxyethyl) ether (HQEE, XA)
HO - CH2 - CH 2 - O O - CH2 - CH 2 - OH
Trimethylolpropane (TMP)CH3 - CH2 - C - CH2 OH
CH2 OH
CH2 OH
MBOCA
BDO, 1,4-BD
HQEE
TMP
Curatives
Reactivity of Isocyanate group
• Highly reactive, affected by electronic structure
• “R” group effects the reactivity of the NCO group
• Also reactivity is driven by the compound reacting with isocyanate group
• Amines>Hydroxyls• Alipha8cAmines>Aroma8camines• PrimaryHydroxyls>SecondaryHydroxyls
R"N=C=O
!OH!NH
H"O"H
Cast Polyurethane Types
CastPolyurethanes
Prepolymers
QuasiPrepolymers
OneShotSystems
• BestPerformance• Lessforgivingonra8o• Mostexpensive
• Highermonomercontentthatprepolymer
• Widerangeofhardnessfrom3components
• Processingcanbechallenging
• Cheapest• Lowestperformance• EH&Sissuesduetomonomer
handling• Exothermneedstobe
managed
Formation of Prepolymer
OHHO R NCOOCN
OO
+
RNH NCOC
OR N
HOCN CO
POLYOL DIISOCYANATE
URETHANE PREPOLYMER
FurtherchainextensioncanoccurashydroxylsitesreactwithterminalNCOgroups.
Prepolymer Curing with Chain Extender
ChainextensionwithUrethanegroup:Polyurethane
ChainextensionwithUreagroup:Poly(Urea-Urethane)
Prepolymer Curative
Polyurethane Prepolymer Processing
Dispensing
Demold
Curing
Finishing
Post-Cure
(Melting),Warming &Degassing
Melting orWarming
(Degassing)
Metering
Molding
Mixing
Part A Part B
Choice of Polyurethane
• Application specific performance • Ethershavebeeerhydoly8cstabilitythanesters• Estershavebeeeroilresistancethanethers• Estershavebeeercutandtearresistancethanethers• Aminecuredurethaneshavebeeerhightemperatureproper8esthandiolcured
urethanes.
• Processing • TDIsystemsareeasiertoprocessthanMDIsystems• Diols(suchas1,4BDareliquidatroomtemperature)whereascertainamines(suchas
MBOCA)needtobemelted
• Cost • MDIsystemsaregenerallycheaperthanTDIsystems• Estersaretypicallycheaperthanethers• QuasisystemsarecheaperthanPrepolymers
Schematic of Polyurethane elastomer
• SofSegment• Hardsegment• Hydrogenbonding
• SelfreinforcingstructureofPU
• Performancecanbeachievedbychoosingrightrawmaterials
• Performanceenhancingaddi8vesrarelyneeded
1. CONCENTRATION OF HARD SEGMENT
2. COMPOSITION OF SOFT SEGMENT
3. COMPOSITION OF HARD SEGMENT
4. SEGREGATION OF HARD SEGMENT
SoftSegment
HardSegment
What controls properties?
Phase segregation and effect on properties
• Phasesegrega8onrequirementsareapplica8onspecific
• Aprocessorcancontrolphasesegrega8onbychoosing• Chemistry(Iso/Polyol/cura8vepackage)
• TDI/PTMEG/MOCA=Good.TDI/PTMEG/BD=Bad
• Ra8oofcura8ve• >100%theory=morelinear=beeerfa8gueresistance.• Toomuchexcesscura8ve(>120%)disruptsphasesegrega8on
• Processingcondi8ons• Highermoldtemperatures(belowdegrada8on)leadstosmallercrystalsize=lowerhardness
• Typically suppliers are able to provide application specific chemistry but a general knowledge of urethane chemistry helps in understanding why certain choices are made
• Good phase segregation leads to • Hightemperatureproper8es• Beeerdynamics(lessheatbuildunderdynamicloads)• Fa8gueproper8es• Cutandtearresistance
• Certain applications may also require “not to great” phase segregation • Deadblowhammer–cura8vepackagesinten8onallydisruptnaturalphasesegrega8on.
Structure property relationships
ApplicaVonrequirements
Chemistry Comments
Highcutandtear Polyesterbackone Hydrogenbondingleadstocutgrowthresistance
Highrebound PTMEGbackbone Highlevelsofphasesegrega8on.Beeerrebound
Sofcompounds Triolcures Triolcuresdisruptphasesegrega8onleadingtolowerhardness
Clearcompounds Alipha8c Lowerphasesegrega8on.Saturateddoublebonds–nofreeradicalgenera8on
Morein“WhatUrethaneWhere”presenta8on
• Reac8onsofisocyanatesareallexothermic• LiquidPrepolymerscanReactWith:
-HydroxyCompounds-Amines-Water-UreasandUrethanes-AndcanDimerize(isocyanatepolymeriza8on)
Reactions of Prepolymers
Reaction with Water
H2N-R-NH2 + 2CO2 2 H2O+ OCN-R-NCO
• The reaction with water provides the primary source of gas for blowing in the manufacture of low density flexible foams.
• Can be an issue in hot cast where bubbles/foam is not needed • Water can be introduced while
• Drum handling • Curative handling • Processing
R’-NCO+OCN-R’
CCO
O
R’-NN-R
Uretidinedione (dimer)
Isocyanate Polymerization Reactions
• Dimerreac8onismorecommonwithMDIprepolymerswithhighmonomercontent
• Happensatalltemperaturebutmoreprevalentathigherorlowertemperatures• Shelflifecri8calinmanycompoundswithhighmonomercontent
~ R C O R ~ + H2O ~ R C OH + HO R ~O O
~ R NH C NH R ~ + H2O ~ NH C OH + H2N R ~O O
~ R NH C O R ~ + H2O ~ NH C OH + HO R ~O O
Ester Acid Alcohol
Urea Carbamic Acid Amine
Urethane Carbamic Acid Alcohol
Hydrolysis Reactions
• EsterhydrolysiscommonissuewhenusingPolyesterbasedPUinhotmoistenvironments(Example:TDI/Ester/AminecureinAsia)
• Diolcureslesssuscep8blethanaminecures
Additives Plasticizers:
– Non-Reactive Diluents (Do Not Change Ratio of Curative/Prepolymer) – Benzoflex 9-88SG: 10pph Lowers Hardness 5-6 Points With Esters – Usually Preblend with Prepolymer; With Curative Sometimes – Typically used to soften or reduce cost
Fillers – No Reinforcing Action in Urethanes – Must Be Dry – Cost reduction in Conjunction with Plasticizers – 10 pph Increases Hardness About 2 Points – Not common to use fillers
Catalysts and Other Additives
Catalysts – Amine Cures (e.g., MBCA, E 300)
• Azelaic, Adipic, Oleic Acids – 0.2% Azelaic in MBCA
Reduces Pot Life and Demold by Factor of 2
– Diol/Triol Cures (e.g., 1,4-BD, HQEE, TMP)
• Niax A-33, Fomrez C-2, SUL-4, UL-32
– Highly Active in MDI Systems
Other Additives – Degassing Aids – SAG-47
– Internal Lubricants – Graphite, MoS2 , Silicones, etc.
– Antistatic Agents – Catafor, Carbon Blacks
– Stabilizers – Stabaxol for Esters, Antioxidants, UV Stabilizers
Ques8ons