Polyurea Spray-Applied Systems for Concrete Spray... · Polyurea spray coatings technology is a...

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2019/6/11 Polyurea Spray-Applied Systems for Concrete Protection

https://www.pcimag.com/articles/83510-polyurea-spray-applied-systems-for-concrete-protection 1/13

September1,2003

PolyureaSpray-AppliedSystemsforConcreteProtection

Polyureaspraycoatingstechnologyisarecentdevelopmentinthe

polyurethanecoatingsindustry.Polyurethanechemistryhasbeenin

existenceforapproximately60years,whileelastomericurethanecoatings

havebeenavailablesincethe1970s.Polyureaelastomertechnologywasintroducedsome10yearslater.

ThetwomainapplicationareasareReactionInjectionMoulding(RIM)andsprayablecoatings.

Polyureacoatingscombineextremeapplicationpropertiessuchasrapidcure(evenattemperatures

wellbelow0øC),andinsensitivitytohumidity,withexceptionalphysicalpropertiessuchashigh

hardness,flexibility,tearandtensilestrength,andchemicalandwaterresistance.Thisresultsingood

weatheringandabrasionresistance.Thesystemsare100%solids,makingthemcompliantwiththe

strictestVOCregulations.Duetoitsspecificcuringprofileandexceptionalfilmproperties,the

polyureaspraycoatingtechniquehasbeenintroducedintomanyareas,includingcorrosionprotection,

containment,membranes,liningsandcaulks.

Figure1/IsocyanateChemistry

DefinitionsTheterm'polyurea'hasbeenwronglyusedinthepast.Urethanecoatingschemistrycanbedividedinto

threesegments:polyurethanecoatings;polyureacoatings;andhybridpolyurethane/polyureacoatings.

Allthreearelinkedtodifferentisocyanatereactions(Figure1).Eachofthesesegmentsdealswith

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systemsthatcanbearomatic,aliphatic,orablendofbotharomaticandaliphatic.Pigments,fillers,

solventsand/oradditivescanbeintroducedtoallofthem.

Apolyurethanecoatingistheresultofareactionbetweenanisocyanatecomponentandaresin

blendmadewithonlyhydroxyl-containingresins.Thefinalcoatingfilmwillcontainnointentional

ureagroups.Apolyurethanesystemwillmostprobablycontainoneormorecatalysts.

Apolyureacoatingistheresultofaone-stepreactionbetweenanisocyanatecomponentanda

resin-blendcomponent.Theisocyanatecanbemonomerbased,aprepolymer,apolymerorablend.

Fortheprepolymer,amine-and/orhydroxyl-terminatedresinscanbeused.Ontheotherhand,the

resinblendshouldonlycontainamine-terminatedresinsand/orchainextendersandnotanyhydroxyl

reactivepolymercomponents.Allthepolyureacoatingsmentionedinthepapercomplywiththis

requirement.

Apolyurethane/polyureahybridcoatinghasacompositionthatisacombinationoftheabove-

mentionedtwo-coatingsystems.Theisocyanatecomponentcanbethesameasthe"pure"polyurea

systems.Theresinblendisablendofamine-terminatedandhydroxyl-terminatedpolymerresins

and/orchainextenders.Theresinblendmayalsocontainadditives,ornon-primarycomponents.To

bringthereactivityofthehydroxyl-containingresinstothesamelevelofreactivityastheamine-

terminatedresins,theadditionofoneormorecatalystsisnecessary.

Thewater/isocyanatereactionalsoproducesurea-groupsattheendoftheprocess.However,this

reactionshouldnotbeconsideredtobeapolyureareactionasthemechanismisatwo-stepprocess,

whichiscontrolledbythemuchslowerisocyanatewaterreactionandproducescarbondioxide.

Figure2/ApplicabilityoftheDifferentPUChemistries

ThePolyurethaneLandscapeThechoicebetweenthedifferentpolyurethane(PU)technologiesisbasedupondifferentparameters

(Figure2).Polyurethanepresentsthebestcompromisebetweencostandquality,butislimitedbythe

applicationperformance.Thepolyurethanesystemissusceptibletoblisteringwhenthesubstrate

containsmorethan5%humidity.Thisisduetocompetitionbetweenhydroxyl-polyolsandwaterfor

thereactionwithanisocyanategroup.Thehumiditycontentoftheenvironmentandtheapplication

temperaturearelimitingfactorsforpolyurethanesandotherchemicallyreactingsystems.

Hybridsystemsalreadyhavealargerscopeforapplicationconditions,butthepresenceofcatalystsin

hybridsmakesthemmoresensitivetohumiditythan"pure"polyureasystems.Moreover,becausethe

catalyzedpolyol/isocyanatereactionbehavesdifferentlyfromtheamine/isocyanatereactionto

changingapplicationtemperatures,thesystembecomeslessrobust.

Polyureacanbeusedinextremeconditions.Whenitisusedonsubstratesalmostsaturatedwithwater,

polyureawillnotprovokeblisteringnorwillblisteringoccurwhentheaircontainshighamountsof

humidity.Evenatverylowtemperatures(aslowas-20degC)thepolyureacoatingwillstillcure.

Polyureacoatingscombinehighflexibilitywithhardness.Theyarethemostsuitablecoatingswhenthe

followingisrequired:

highcuringspeed;

applicationunderhighhumidityand/oratlowtemperatures;

extremeabrasionresistance;

impermeablemembranes;

highthicknessbuildup;

chemicalresistance.

ApplicationsforPolyureaCoatingsAgoodunderstandingofthepropertiesofpolyureaspraycoatingsisrequiredtospecifytheright

application.Table1providesageneraloverviewofthephysicalandchemicalpropertiesthatcanbe

expectedofpolyureasprayproducts.Polyureasystemsareknowntobeverytough.Theycombinehigh

elasticitywithhighsurfacehardness,resultinginverygoodabrasionresistance.

ThemarketdevelopmentstartedintheUnitedStates,followedbyAsia,withverystronggrowthduring

thesecondhalfofthe1990s.Inthefirststageofdevelopment,polyureawasusedasaprotectivelayer

overpolyurethaneinsulationfoamforroofingapplications.InEurope,thepolyureaspraycoatings

marketonlystartedtodevelopinthelastfewyears.

Thebroadwindowofapplicationconditions,withahightoleranceforhumidity,bothfromthe

environmentandfromthesubstrateandtemperature,makespolyureaaverysuitablecoatingfor

concreteinconstructionapplicationssuchasroofrepair,containmentliners,membranes,carpark

decks,bridgesandoffshore.Thehighabrasionresistanceleadstoitsapplicationinlinersfortruck,

bulktransportwagons,freightersandconveyorbelts.Table2presentsanoverviewoftheapplication

fieldswherepolyureaischosenbasedononeormoreofitsuniqueapplicationand/orfilmproperties.

RawMaterialsApolyureaspraycoatingformulationconsistsoffivedifferentelements:theisocyanatecomponent;

(reactive)diluent;polyetheramines;chainextenders;andadditives,fillersandpigments.

IsocyanateSincethemostcommonlyusedisocyanateisdiphenylmethanediisocyanate(MDI),thispaperfocuses

onMDI-basedproducts.AliphaticsystemscanbeusedwhereUVstabilityisanissue.

StandardpolyureaspraycoatingsuseMDIprepolymerswithanNCOcontentof15to16%.InthisNCO

range,agoodcompromisebetweenviscosityofthematerialandthereactivityofthesystemis

obtained.Lower-NCOprepolymershaveahigherviscosity,butgivehigherelasticityandslower

reactivity.Higher-NCOprepolymersarelowerinviscosity,whichprovidesaneffectivemixtureofthe

twocomponents.However,theybecomemuchmorereactive,withtheriskofbuildingupmoreinternal

stress.Higher-NCOprepolymerswillbeusedifhighersurfacehardnessisneeded.Table3providesan

overviewofthemainpropertiesoftheMDIprepolymersusedforpolyureaspraycoatingsinEurope.

Diluent

Figure3/CarbamateStructure

JEFFSOL(r)PC,orpropylenecarbonate,isareactivediluentforpolyurea.Propylenecarbonatehasa

highflashpoint,lowtoxicityandshouldnotbeconsideredasavolatileorganiccompound(VOC).The

mainadvantagesofusingpropylenecarbonateare:

improvedshelflifeoftheisocyanate-prepolymer;

acompatibilizerforthemixingofthetwocomponentsinthemixingchamberofthespraygun;

aviscosityreducerforisocyanate-prepolymers;

improvedlevelingoftheappliedfilm.

Propylenecarbonatereactswithanaminetogiveacarbamatestructurecontainingasecondary

hydroxylgroup.Duetothequickreactionbetweenisocyanateandamine,thesecondaryhydroxyldoes

nothavetheopportunitytoreactwithanisocyanategroup.Thepropylenecarbonatemoleculeshould,

therefore,beconsideredasamono-functionalmolecule(Figure3).

Inapplicationswherecontactwithwatercannotbeavoided,theuseofpropylenecarbonateshouldalso

belimited,aspropylenecarbonateiscompletelymisciblewithwater,andunreactedpropylene

carbonatecouldbeextracted,increasingthewaterpermeabilityofthefilm.

HuntsmanPetrochemicalCorp.ownscertainpatentsrelatingtotheuseofpropylenecarbonatein

polyureaelastomers.Othersolventsorviscosityreducerscanbeusediftheyarecompatiblewiththe

isocyanatecomponent.TheymaybeconsideredasaVOC.However,theywillincreasetheshrinkage

effect.

PolyetheraminesTheamineblendusedinpolyureaspraycoatingsisamixtureofpolyetheraminesandchainextenders.

Themaincomponentoftheresinblendisamixtureofamine-terminatedethyleneoxideand/or

propyleneoxidepolyetherwithmolecularweightsvaryingfrom200to5000g/mole.Theprimaryamine

groupsprovideaveryfastandreliablereactionwiththeNCOgroupsoftheisocyanatecomponent.

Table4presentsthepropertiesofthepolyetheraminescommonlyusedinpolyurea.

ChainExtendersDiethyl-toluenediamine,orDETDA,isthestandardchainextenderusedinaromaticpolyureaspray

coatings.DETDAcontributestothehardblockandimprovestheheatresistanceofthecuredfilm.Itis

themostreactiveamineintheresinblendbut,becauseofthephaseseparationduringthecuring,it

controlsthereactionmechanismandmakesitpossibletosprayapolyureafilm.

Otherchainextenderslikedimethylthio-toluenediamine(DMTDA),N,N'-di(sec.butyl)-amino-biphenyl

methane(DBMDA)or4,4'-methylenebis-(3-chloro,2,6-diethyl)-aniline(MCDEA)slowdownthe

reactionsignificantly.Table5listsvariouschainextendersandtheircharacteristics.Significantly

slowingdownthereactionalsomeansthatthecompetitionwiththewaterreactionbecomesmore

importantandprecautionsneedtobetaken.

Additives,Fillers,Pigments

Dependingontheapplication,solvents,additives,pigmentsand/orfillersareintroducedtothe

formulation.Adhesionpromoterslikesilanesareusedtoenhancetheadhesiononsteelandconcrete.

UVabsorbersareusedtoslowdowntheyellowingeffectofaromaticpolyureasystems.Fillersareadded

tolowertherawmaterialcostand/orimprovethephysicalpropertiesofthecoating.Theadditionof

pigmentand/orfillersislimitedbecausetheviscosityofthetwocomponentsattheapplication

temperaturehastobekeptundercontrol.Higheramountsoffillersandreinforcementfillerscanbe

addedtothesystemasathirdcomponent.

Product-ApplicationSpecificsThemostimportantelementofhandlingpolyureacoatingsisthemixing.Goodmixingwillbeobtained

inasuitablemixingmodulebyimpingementwithmechanicalpurge.Theoperationalpressureand

temperatureoftheproductswillalsohelptooptimizethemixingefficiency.

Duetothehighcurespeedofpolyureaandtheshortmixingtime,theproductsaremixedby

impingementathighpressure.Indeed,forfieldapplicationsitispreferabletoformulatetheproducts

onafixed1:1volume-mixingratio.Thepressureusedinthefieldwillvarybetween150and250bar.

Theviscosityoftheproductsatapplicationtemperatureideallyneedstobelowerthan100mPa.s,and

theviscosityofthetwocomponentsneedstobeatthesamelevel.Thepropertiesoftheseprepolymers

canbefoundinTable3.Theviscosityoftheresinblendat25degCisapproximately900mPa.s,

droppingbelow100mPa.satapplicationtemperature.

Experimentsprovethatpolyureafilmsproducedat65degC,70degCand80degChavedifferent

properties,andthesepropertiesimprovewithincreasingtemperatures.Thesprayingequipmenthas

improvedsignificantly.Thenewsprayequipmentallowsdifferenttemperaturesettingsforthetwo

components,ensuringanoptimummixinginthesprayhead.Otherfeaturesareeasiervariableratio

settings;easyoutputcontrol;andeasymonitoringofapplicationparameters.

Theindexofapolyureasystemistypicallykeptataslightover-indexoftheisocyanateintherangeof

1.05-1.10.Astheisocyanategroupreactstohumidity,theexcessisocyanatecompensatesforthe'loss'

ofisocyanategroupsduringstorageand/orapplication.Thefilmpropertiesofthe1:1volumeratio

sprayedsystemweremeasuredforanindexvariationbetween0.90and1.15.Thetestresultsindicate

thatthefilmperformsbestatanindexof1.05andhigher.Belowanindexof1.05theresultscanvary

significantlyandbecomeunpredictable,evenforsmallindexshifts.

AspectsofSprayPolyureaTechnologyPolyureaapplicationhadsomeproblemsduringtheinitialstart-upphase,whichareattheoriginofthe

still-existingmisconceptionsaboutpolyureatechnology.Theseproblemscanbeattributedpartlyto

thelackofexperienceatthetimeofthetechnologyintroduction,partlyduetothelackofadequate

applicationequipment,andpartlytothefactthatthisnewtechnologycouldnotbeappliedinthesame

wayasthecurrentcoatingssystems.

Initially,polyureaspraycoatingslookedtooeasytoapply.Polyureaisveryfast-thecoatingcanbeput

intoserviceimmediatelyaftertheapplication,andthefinalpropertiesofthecoatingareobtainedonly

afewhoursafterwards.Polyureaisnotwater-ortemperature-sensitive,andiseasytoformulateand

produce.Thefirstsystemsonthemarketwereindeedveryfast,withageltimeoflessthantwo

seconds,andinitiallyanumberofproblemswerelinkedtothereactivityofthesystems.

Atfirst,substratewettingwasaproblem.Thisproblemwaslinkedtothedevelopmentphaseof

polyureawiththeuseofextremelyfastspraysystems.Developmentprogramsfocusingonadhesionon

concrete,withpolyureasystemspresentinggeltimesofthreetofourseconds,resultedincohesive

adhesionfailureintheconcrete.Inpractice,tolimittherisksundervariablefieldconditions,amulti-

layersystemisapplied,madeofaprimerandatopcoat.

Asecondproblemnotedinthefieldwasthelackofintercoatadhesion.Labtests,withtimesbetween

coatsofseveralweeks,haveshownthatintercoatadhesionisverygood.Whenproblemsoccurwith

intercoatadhesion,mostofthetimetheycanberelatedbacktoproblemswiththerawmaterials,the

manufacturingofthesystemsorthesprayequipment.Sprayequipmentproblems,oradisturbanceof

thefeedingofoneorbothcomponentstowardsthemixingmodule,cancausepoormixing.Adapting

themachinesettingsofthespraycansolvethis.

Duetothehighreactivityofthesystems,thesurfacequalityofthesprayedfilmwasinitiallyverypoor.

Fine-tuningthesprayingequipmentwasafirststeptowardssolvingtheproblem.Theuseofnon-VOC

reactivediluentsandthedevelopmentofnewMDIprepolymerswithhigher2,4'-isomercontent

resultedinperfectsurfacequalitywithoutcompromisingonworkingtime.

Thecostofpolyureaspraycoatingstechnologyisseenasabarriertoentry."Pure"polyureasystemsare

moreexpensive,whenconsideringrawmaterialscostalone,butcanbeappliedinareaswhereallother

systemswillfailorwheretheyarenotsuitable.Alsotheinitialinvestmentinequipmentisrather

costly.

However,whenestimatingthecapitalcostforaproject,polyureaismorecompetitivewhenboththe

processingtimeandthewaitingperiodbeforethecoatedsubstrateisputbackintoserviceare

included.

Asdiscussedabove,thesuccessoftheprojectisveryequipment-andapplicator-dependent,andwe

believethatthehighentrybarriercanonlyguaranteequalityservicesfromspecializedandskilled

operators.

Construction-RelatedAspectsPolyureaspraycoatingscanbeusedunderdifficultweatherconditionsbecausethechemistryisvery

fastandtheydonotexperiencenegativesideeffectscausedbythepresenceofhumidity.Theyalsocure

attemperaturesbelow0degC.However,whenusingpolyureacoatings,anumberofprecautionsstill

havetobetaken.

Ifprimersareconsidered,agoodapproachistostartwiththeevaluationofexistingprimerswith

knownperformanceonthesubstrate.Itisimportanttodeterminetheadhesionperformanceofthe

polyureacoatingontheprimersandtocheckwhethertheapplicationconditionsoftheprimerandthe

re-coatingconditionsfortheprimerstillworkforasystemwithpolyureaasafinish.

ConcreteSurfaceDefectsandSurfacePreparationThelowcost,highstrengthandstructuralpropertiesofconcretemakeitthematerialofchoiceforthe

constructionindustry.Someofthetypicalpropertiesofconcretelikethelimitedchemicalresistance,

dustreleaseandporosityorpermeabilitymakeitnecessarytoputaprotectiveand/ordecorativelayer

ontothesurface.

Thepreparationofthesurfaceisextremelyimportant.Dependingonthesurfacequalityofthe

concrete,oneormoreofthefollowingactionsneedstobetaken:

waterjetand/orsolventcleaning;

gritblasting;

bugholeandcrackfilling;

repairlayerofconcrete;

priming.

SteelSurfaceDefectsandSurfacePreparationThelifecycleofsteelconstructioncoatingsdependslargelyupontheprotectivesystemputinplace.

Thelifeoftheprotectivecoatingitselfisstronglydependentonthesurfaceconditionpriortothe

applicationofthecoating.Theprotectionofthesubstrateismainlyobtainedbyensuringagood

adhesion.Twoadhesionmechanismsarepossible:

molecularattractionoftheinterfacialforcesfromboththecoatingandthesubstrate,

mechanicalbondingoranchoringofthecoatingonthesubstrate.

Dependingontheconditionofthesurface,oneormoreofthefollowingpre-treatmentswillbe

necessary:cleaninganddegreasingwithsolvents,waterjetordetergents;handorpower-toolcleaning;

gritblasting.

Onfreshlygrit-blasted,dust-freesteelwithasurfaceroughnessSA21/2toSA2accordingtothe

specificationISO8501-1,veryhighadhesionvaluescanbeobtainedforpolyurea,evenwithouttheuse

ofprimers.

Figure4/MixingEfficiencyInfluenceonthePhysicalPropertiesofaPolyureaCoating

MixingEfficiencyThemixingefficiencyoftheapplicationequipmentisofvitalimportance.Whenformulatingasystem

ormodifyinganexistingsystem,itisnecessarytoverifythemixingefficiencyconstantly.Figure4

showstheinfluenceofchangingthemixingonthephysicalpropertiesofaformulatedproduct.The

tensilestrengthalmostdoublesfrom14N/mm2to23N/mm2,theangletearincreasesfrom75N/mm

to85N/mmandtheelongationincreasesfrom390to430percent.Inthiscase,theinfluenceonthe

otherphysicalpropertiesislimited.

Figure5/PhysicalPropertyVariationsatVaryingSystemIndex

SystemIndexInfluenceonFinalFilmPropertiesEarlierexperimentsrevealthatapolyureacoatingneedstobeformulatedatanindexabove1.00,

meaningwithaslightlyhigheramountofisocyanate-groupsthanamine-groups.Atanindexof1.00or

lower,thephysicalpropertiesofthecoatingbecomeunreliable.Figure5demonstratesthatmost

propertieshaveverygoodvaluesatindexesfrom1.10to1.30.Aboveanindexof1.30theperformance

tendstodropagain.

Takingintoaccountthat,inpractice,slightvariationsmightoccurintheapplicationparameters,

relatedtotheprecisionofthesprayequipmentandvariationsinapplicationconditions,itissaferto

workataminimumindexof1.10to1.15.

Figure6/PhysicalPropertyDevelopmentasaFunctionofFillerContent

FillerInfluenceonPhysicalPropertiesAddingfillerstoapolyureasystemcanbeusefulfordifferentreasons,suchasareductionintheraw

materialcostorimprovementinphysicalproperties.Inorganicfillershaveadifferenthardnessand

somewillabradepartsofthesprayequipmentmorethanothers.Themostsensitivepartsarethe

mixingchamberandthenozzleofthespraygun.Thefilledsystemsneedtobecarefullyfilteredbefore

packaging.

Dependingontheperformanceofthesprayinstallation,thedosingoffillercanvary.Weaddedupto

40%offillertotheresinblend.Themainlimitingfactorfortheprocessingistheincreaseinviscosity

ofthefilledcomponent.Thiscanresultindifficultieswiththepumpingunitand/orpoormixingdueto

bigdifferencesinviscosityforbothcomponents.

Addingfillersimprovesthesurfacehardness,theangletearandflexuralbendingmodulusbuthasa

negativeinfluenceontheelongationandthetearpropagationortrousertear(Figure6).

Figure7/WaterAbsorptionforDifferentSUPRASECPrepolymers

WaterAbsorptionForcorrosionprotection,themainconsiderationforgoodperformanceisadhesiontothesubstrate.

Furthertestingprovedthat,evenwithverygoodadhesion,theresistancetocathodicdisbondmentfor

somesystemsfails.Sincecathodicdisbondmentoccursoveralongertermandcertainlyisnotasimple

testmethod,wemeasuredthewaterabsorptionoveraperiodof10daysat80degCandadded3%

sodiumchloridetothewater.

AscanbeseeninFigure7,theunmodifiedstandardpolyureaspraycoatingsgiveonlyalimited

protectionagainstcorrosion.ThewaterabsorptiondropssignificantlywithincreasingtheNCO-

contentfortheprepolymer.Furtherfine-tuningofthepre-polymerresultedinwaterabsorptionbeing

wellbelow0.5%after10days.Cathodicdisbondmenttestingonthissystemgaveverysatisfyingtest

results.

Anti-SkidPerformanceFlooring,carparksandsportsfloorsarekeyapplicationsforpolyureaduetoitshighabrasion

resistance,goodmechanicalpropertiesandinsensitivitytoblisteringduringthecuringprocessin

humidconditions.

Oneexampleistheoutdoor,impact-absorbingplaygroundflooring.Thesystemtestedisbasedon

SUPRASEC2049,formulatedtobeappliedina1to1volumeratio.Thefilmpropertiesare70ShoreA

hardness,600%elongationand13N/mm2tensilestrength.TheresultsinTable6showthatitis

perfectlypossibletoformulateasystem,whichcomplieswiththeantiskidneedsofaflooringsystem,

inthiscaseforaflexiblesubstrate.

ConclusionsPolyureaspraycoatingtechnologyisdifferentfromothercoatingchemistriesandcanexpandthe

applicationrangeofcoatingstoareasandconditionswhereothercoatingsystemswillfail.

Polyureasareverysuitableforconstructionapplications.Thefastcuringmakesitpossibletousewhen

onlyveryshortdisturbanceperiodsareallowed.Thefactthattheisocyanate/waterreactionisnot

affectingthephysicalpropertiesoftheappliedfilm,expandstheuseofpolyureatohighrelative

humidityconditionsanddoesnotsetsuchstringentlimitsonthewatercontentofsubstrateslike

concrete.Althoughtheyslowdownatcoldertemperatures,polyureacoatingsstillcureattemperatures

whereotherchemistriesfail.

Theformulationofpolyureaspraycoatingshastobeapproachedsimilarlytoanyothercoatingsystem.

Carefulselectionofrawmaterialsforfine-tuningoftheformulationandevaluationofthesystem,in

thesometimesdifficultconditionswherethecoatingistobeapplied,isstillnecessary.

Polyureaspraycoatingtechnologymeanshandlingreactivechemicals.Whilehandlingthechemicals

duringtheirmanufacturing,packagingandapplication,thecorrectprotectiveclothingshouldbeworn

atalltimes.

AcknowledgementsTheauthorwouldliketothankStefanPriemenandDomienBerdenfortheapplicationandtestingof

thepolyureasamples,WesleyVerbekeforthesupportwiththetestingofthesystemsandallother

Huntsmanstaffwhohelpedtorealizethispaper.

Formoreinformation,e-mailMarcBroekaertatmarc_broekaert@huntsman.com.

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