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