SZYCHERS HANDBOOK OFPOLYURETHANESSecond EditionMaterials ScienceSecond EditionSzycherISBN 978-1-4398-3958-4978143983958490000K11811Apracticalhandbookratherthanmerelyachemistryreference,SzychersHandbook ofPolyurethanes,SecondEditionofersaneasy-to-followcompilationofcrucial newinformationonpolyurethanetechnology,whichisirreplaceableinawiderangeof applications.Tisneweditionofabestsellerisaninvaluablereferencefortechnologists, marketers,suppliers,andacademicianswhorequirecutting-edge,commerciallyvaluable data on the most advanced uses for polyurethane, one of the most important and complex specialty polymers.Internationally recognized expert Dr. Michael Szycher updates his bestselling industry bible Withsevenentirelynewchaptersandfvethatarerevisedandupdated,thisbook summarizes vital contents from U.S. patent literatureone of the most comprehensive sourcesofup-to-datetechnicalinformation.Tesepatentsrepresentthemostuseful technologydiscoveredbycorporations,universities,andindependentinventors.Tis handbook features the full text for many carefully selected patents, because they contain a wealth of information and best illustrate the complex principles involved in polyurethane chemistry and technology.Features of this landmark reference include Hundreds of practical formulations Discussion of the polyurethane history, key terms, and commercial importance An in-depth survey of patent literature Useful stoichiometric calculations Te latest green chemistry applications A complete assessment of medical-grade polyurethane technologyNotbiasedtowardanyonesuppliersexpertise,thisspecialreferenceusesasimplifed language and layout and provides extensive study questions after each chapter. It presents rich technical and historical descriptions of all major polyurethanes and updated sections onmedicalandbiologicalapplications.Tesefeatureshelpreadersbetterunderstand developmental, chemical, application, and commercial aspects of the subject.K11811_COVER_final.indd 1 6/13/12 4:41 PMSZYCHERS HANDBOOK OFPOLYURETHANESSecond EditionMichael Szycher, Ph.D.SZYCHERS HANDBOOK OFPOLYURETHANESSecond EditionCRC PressTaylor & Francis Group6000 Broken Sound Parkway NW, Suite 300Boca Raton, FL 33487-2742 2013 by Taylor & Francis Group, LLCCRC Press is an imprint of Taylor & Francis Group, an Informa businessNo claim to original U.S. Government worksVersion Date: 20120312International Standard Book Number-13: 978-1-4398-6313-8 (eBook - PDF)Thisbookcontainsinformationobtainedfromauthenticandhighlyregardedsources.Reasonableeffortshavebeenmadeto publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material repro-duced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint.Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers.For permission to photocopy or use material electronically from this work, please access www.copyright.com (http://www.copy-right.com/)orcontacttheCopyrightClearanceCenter,Inc.(CCC),222RosewoodDrive,Danvers,MA01923,978-750-8400. CCC is a not-for-profit organization that provides licenses and registration for a variety of users. For organizations that have been granted a photocopy license by the CCC, a separate system of payment has been arranged.Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identifica-tion and explanation without intent to infringe.Visit the Taylor & Francis Web site athttp://www.taylorandfrancis.comand the CRC Press Web site athttp://www.crcpress.comThis handbook is dedicated to my wife, Laurie, whose unwavering support and dedication made this work possible.viiContentsPreface ..............................................................................................................................................xiAcknowledgment ........................................................................................................................ xiiiEditor...............................................................................................................................................xvContributors ................................................................................................................................ xvii1.Introduction ............................................................................................................................. 1Michael Szycher2.Basic Concepts in Polyurethane Chemistry andTechnology ..................................... 13Michael Szycher3.StructureProperty Relations in Polyurethanes ............................................................ 37Michael Szycher4.Isocyanate Chemistry .......................................................................................................... 87Michael Szycher5.Polyols ................................................................................................................................... 135Michael Szycher6.Chain Extenders .................................................................................................................. 155Michael Szycher7.Flexible and Semifexible Foams ..................................................................................... 181Michael Szycher8.Rigid Polyurethane Foams ............................................................................................... 257Michael Szycher9.Polyurethane Foam Surfactants ...................................................................................... 309Michael Szycher 10.Catalysis of Isocyanate Reactions ................................................................................... 319Michael Szycher 11.Elastomers ............................................................................................................................ 345Michael Szycher 12.Reaction Injection Molding .............................................................................................. 373Michael Szycher 13.Polyurethane Adhesives ................................................................................................... 393Michael Szycherviii Contents 14.Waterborne Polyurethanes ............................................................................................... 417Michael Szycher 15.Health and Safety ............................................................................................................... 449Michael Szycher 16.Radiation-Curable Adhesives and Coatings ................................................................. 495Michael Szycher 17.Processing Methods ........................................................................................................... 523Michael Szycher 18.Compounding Ingredients ............................................................................................... 547Michael Szycher 19.Copolymers and Polyblends ............................................................................................. 587Michael Szycher 20.Polyurethane Coatings ...................................................................................................... 597Michael Szycher 21.Castables, Sealants, and Caulking Compounds .......................................................... 613Michael Szycher 22.Medical Applications ......................................................................................................... 633Michael Szycher 23.Resorbable Polyurethanes ................................................................................................ 671David K. Dempsey, Hugh A. Benhardt, and Elizabeth M. Cosgriff-Hernandez 24.Biodurable Polyurethanes................................................................................................. 711Michael Szycher 25.Antimicrobial Polyurethanes ........................................................................................... 739Michael Szycher 26.Hydrophilic Polyurethanes .............................................................................................. 771Michael Szycher 27.Biocompatibility Testing ................................................................................................... 805Michael Szycher, Laurence Lister, and John Iannone 28.Eco-Friendly Polyurethanes ............................................................................................. 835Michael Szycher 29.Processing Thermoplastics Urethanes via Twin Screw Extrusion ........................... 873Charlie Martinix Contents 30.Infrared Analysis of Medical-Grade Polyurethane Elastomers: CanDurometerHardness Be Determined by IR Analysis? ...................................... 897Michael Szycher and Jay R. PowellAppendix A: Polyurethane Suppliers andManufacturers ............................................... 905Appendix B: Urethane Processing Systems ....................................................................... 1041Appendix C: Glossary ............................................................................................................. 1057Appendix D: Conversions and Formulas ............................................................................ 1087xiPrefaceA handbook is a compilation of data from many sources presented in a logical and easy-to-follow sequence. The data must be compiled from disparate sources; the sources may be strictlyacademic,commercial,orpromotional.Itmaycomefromtechnicalpublications, seminars, or the patent literature.The U.S. patent literature is one of the most comprehensive sources of technical informa-tion. The technical information provided in patents is exhaustive, current, and represents the most valuable technology discovered by corporations, universities, and independent inventors. Because of the wealth of information contained in patents, this handbook fea-tures many full-text patents. These patents have been carefully selected by the authors to best illustrate the complex principles involved in polyurethane chemistry and technology.This handbook is not composed of original articles; instead, it is based on hundreds of publishedreferences.Theauthorshavetriedtocreditoriginalsourcesbyprovidingan extensive bibliography. The reader is encouraged to refer to the original sources for more complete information and insight.Polyurethanes are arguably the most complex family of polymers. Polyurethanes range from soft elastomeric polymers to hard elastoplastics that rival metals. Polyurethanes are used as structural materials, coatings, adhesives, and sealants. Polyurethanes can be syn-thesizedasthermoplastics,thermosets,andcurablecompositionsbyeitherheatorUV energyand all by molecular design, as opposed to compounding by the addition of plas-ticizers or other modifers.Dr. Michael SzycherxiiiAcknowledgmentThis handbook summarizes the published work of many polyurethane chemists, lectur-ers, researchers, and technologists. The contributions of these outstanding authors grace the pages of this handbook.xvEditorMichael Szycher, PhD, is chairman and chief executive offcer of CardioTech International, Inc., a manufacturer of medical products based on specialized polyurethanes. He holds a PhD from Boston University School of Medicine and an MBA from Suffolk University.He is a recognized international authority on polyurethanes and blood-compatiblepoly-mers. Author of more than 100 research articles and a pivotal force in the creation of the Medical Plastics Division of the Society of Plastics Engineers (SPE), he is the editor of sev-eral other books:Biocompatible Polymers, Metals and CompositesSynthetic Biomedical PolymersBlood Compatible Materials and Devices: Perspectives towards the 21st CenturyHighPerformanceBiomaterials:AComprehensiveGuidetoMedical/Pharmaceutical ApplicationsSzychers Dictionary of Biomaterials and Medical DevicesSzychers Dictionary of Medical DevicesHe is also founding editor of the quarterly Journal of Biomaterials Applications.xviiContributorsHugh A. BenhardtDepartment of Biomedical EngineeringTexas A&M UniversityCollege Station, TexasElizabeth M. Cosgriff-HernandezDepartment of Biomedical EngineeringTexas A&M UniversityCollege Station, Texas David K. DempseyDepartment of Biomedical EngineeringTexas A&M UniversityCollege Station, TexasJohn IannoneToxikon Corporation Bedford, MassachusettsLaurence Lister Toxikon Corporation Bedford, MassachusettsCharlie MartinLeistritzNuremburg, GermanyJay R. PowellAnalytical Answers, Inc.Woburn, MassachusettsMichael SzycherSterling Biomedical, Inc.Lynnfeld, Massachusetts11IntroductionMichael Szycher1.1HistoricalThe year 1987 marked the 50th anniversary of the introduction of polyurethanes. Professor Otto Bayer was synthesizing polymer fbers to compete with nylon when he developed the frst fber-forming polyurethane in 1937. His invention ranks among the major breakthroughs in polymer chemistry, but the polymer was dismissed as impractical by his superiors at I.G. Farbenindustrie.Formorethan20years,Germanyhadbeenattheforefrontofsynthetic fber technology, beginning with the introduction of polyvinyl chloride (PVC) fbers in 1913.Germanyremainedpreeminentinthefberfelduntil1935,whenCarothersinthe United States discovered the nylons; E. I. DuPont in America introduced and began mar-keting nylon fbers, protected by a barrage of patents that proved impossible to overcome. Nothingasversatileandpracticalasthepolyamideswasavailable,promptingBayerto investigate similar polymers not covered by the impenetrable DuPont patents.AttheendofJanuary1938,Rinkeandcollaboratorsweresuccessfulinreactingan aliphatic1,8-octanediisocyanatewith1,4-butanedioltoformalow-viscositymeltfrom which they were able to draw fbers. These early efforts resulted in what are now known as polyurethanes: the esters of carbamic acid. These polyurethanes could be spun from the melt; yarns and monoflaments that could be made from their new polymer were of high quality. Rinke and Associates were awarded the frst U.S. patent on polyurethanes in 1938.1Like many other developments, polymer chemistry, which began as a small specialized branchoforganicchemistry,begantogrowrapidlyandadoptedanewnomenclature, muchasbiochemistryhaddonebefore.Table1.1presentstherecognizednamesoftwo important linkages found in polymers, comparing classical organic chemistry nomencla-ture to that used in polymer chemistry and biochemistry.ThefrstI.G.Farbenindustriepolyurethanehadameltingpointof185Candbecame available under the trade names Igamid U for synthetic fabrics, and Perlon U for producing artifcial silk or bristles. A softer version was also available under the trade name Igamid CONTENTS1.1Historical ................................................................................................................................. 11.2Polyurethanes ......................................................................................................................... 21.3Overview of Polyurethane Markets .................................................................................... 61.4Flexible Foams ........................................................................................................................ 71.5Rigid Foams ............................................................................................................................ 81.6Elastomers ............................................................................................................................... 9References ....................................................................................................................................... 122 Szychers Handbook of PolyurethanesUL. Foams were also produced by adding water to isocyanates in the presence of hydroxyl-terminatedpolyesterstoformcarbonamidesandreleasecarbondioxideastheblowing agent. These foams, named Troporit M, were used to produce aircraft propeller blades and rigid, foam-flled landing faps and skis.DuPont2 and ICI3 recognized the elastomeric properties of the polyurethanes which led to production on an industrial scale in the 1940s.4 Water was used as the chain extender, and the diisocyanate was naphthalene-1,5-diisocyanate (NDI).DuPont surged to the forefront of polyurethane technology in the United States, receiv-ingpatentsin1942coveringthefar-reachingreactionsofdiisocyanateswithglycol, diamines, polyesters, and certain other active hydrogen-containing chemicals. From these humble beginnings emerged the polyurethanes, one of the most versatile polymers in the modern plastics armamentarium.1.2PolyurethanesPolyurethanes are among the most important class of specialty polymers. But, ironically, the term polyurethane leads to a great deal of confusion. The term is more of convenience thanofaccuracybecausepolyurethanesarenotderivedfrompolymerizingamethane monomer,noraretheypolymerscontainingprimarilyurethanegroups.Thepolyure-thanes include those polymers containing a plurality of urethane groups in the molecular backbone, regardless of the chemical composition of the rest of the chain. Thus, a typical polyurethanemaycontain,inadditiontotheurethanelinkages,aliphaticandaromatic hydrocarbons, esters, ethers, amides, urea, and isocyanurate groups.Polyurethanes are used in a surprising array of commercial applications. Figure 1.1 pres-entstheuniverseofpolyurethaneapplications.Forconvenience,wehavedividedthe applicationsintosevenmajorgroups:fexibleslab,fexiblemoldedfoams,rigidfoams, solidelastomers,reactioninjectionmolding(RIM),carpetbacking,andtwo-component formulations.The chemistry of urethanes makes use of the reactions of organic isocyanates with com-pounds containing active hydrogens. When polyfunctional isocyanates and intermediates containing at least two active hydrogens per mole are reacted at proper ratios, a polymer results that can produce rigid or fexible foams, elastomers, coatings, adhesives, and sealants. Anisocyanategroupreactswiththehydroxylgroupsofapolyoltoformtherepeating urethane linkage, as shown in Reaction 1.1.The isocyanates also react with amines to form substituted urea linkages; they will react with water to form carbamic acid, which is an unstable intermediate, and it decomposes readily to evolve carbon dioxide and an amine. This amine, in turn, reacts with additional isocyanatetoformdisubstitutedurea.Inaddition,anumberofcross-linkingreactions may take place, depending on the reaction conditions such as temperature, the presence ofcatalysts,andthestructureoftheisocyanate,alcohols,andaminesinvolved.These TABLE 1.1Names of Some Important Nitrogen-Containing Polymer LinkagesLinkage Organic Chemistry Polymer Chemistry BiochemistryNHCO Amide Nylon PeptideNHCOO Carbamate Urethane Not applicable3 Introductionreactions form linkages of allophanate (reaction between urethane and isocyanate), biuret (reactionbetweensubstitutedureaandisocyanate),andisocyanurate(trimerizationof isocyanategroups).Isocyanatescanalsobepolymerizedtoformdimers(uretidine diones), carbodiimide, and 1-nylon.Urethaneapplications Attached cushion Unitary Casting Encapsulation Sealants Automotive Mechanical Elastomers Costings Adhesives Medical Insulation-boardstockfoam-in-place Appliances Automotive Automotiveseating Bedding Furniture Bedding Automotive Carpet under-layment Carrier mediaFlexible slabFlexible moldedRigid foamSolidRIMCarpetTwo componentsFIGURE 1.1The polyurethane universe.REACTION 1.1Classical Urethane Linkage ReactionN=C=O+ HONHCOOIsocyanate group Hydroxyl groupUrethane linkageTrade Names(Medical Grade)ManufacturerBionate The Polymer Technology Group (now DSM Inc.)ChronoFlex AdvanceSource BiomaterialsChronoThane AdvanceSource BiomaterialsEstane Lubrizol, Inc.Isoplast Lubrizol, Inc.Pellethane Lubrizol, Inc.Tecofex Lubrizol, Inc.Tecothane Lubrizol, Inc.Texin Bayer Inc.General DescriptionPolyurethanesareproducedbythecondensationreactionofanisocyanateandamaterialwithahydroxyl functionality,suchasapolyol.Polyurethanecanhavethechemicalstructureofeitherathermoplasticor thermosetandcanhavethephysicalstructureofarigidsolid,asoftelastomer,orafoam.Thechemical composition of polyurethane can also vary widely, depending on the specifc polyol- and isocyanate-bearing specieswhicharereactedtoformthepolyurethane.Themanydifferentchemicalstructuresandphysical forms possible for polyurethane make it a versatile, widely used polymer. Specialty grades available include fameretardant,clay,silica,andglass-flled.In1994,thepriceofpolyurethanerangedapproximatelyfrom $2.50 to $6.50 per pound at truckload quantities. 4 Szychers Handbook of PolyurethanesREACTION 1.1 (continued)Classical Urethane Linkage ReactionGeneral PropertiesThe major benefts offered by polyurethane are that it retains its high impact strength at low temperatures, it is readilyfoamable,anditisresistanttoabrasion,tearpropagation,ozone,oxidation,fungus,andhumidity. Although thermoplastic polyurethane is attacked by steam, fuels, ketones, esters, and strong acids and bases, it is resistant to aliphatic hydrocarbons and dilute acids and bases. The highest recommended use temperature ofthermoplasticpolyurethaneisapproximately220F(104C),renderingitinappropriateformosthigh-temperatureapplications. Aromaticthermoplasticpolyurethanehaspoorweatherabilitystemmingfromits poorresistancetoUVdegradation.Sincepolyurethanecanbepaintedwithfexiblepolyurethanepaints without pretreatment, it has found use in many automotive exterior parts.Typical Properties of PolyurethaneAmerican Engineering SIProcessing temperature 385450F 196232CLinear mold shrinkage 0.0040.014 in/in 0.0040.014 cm/cmMailing point 400450F 204232CDensity 69.977.4 lb/ft31.121.24 g/cm3Tensile strength, yield 4.935.0 lb/in2 1033.424.6 kg/cm3 103Tensile strength, break 4.935.0 lb/in2 1053.424.6 kg/cm3 102Elongation, break 100.0500.0% 100.0500.0%Tensile modulus 0.645.0 lb/in2 1050.431.6 kg/cm3 104Flexural strength, yield 6.060.0 lb/in2 1034.242.2 kg/cm3 102Flexural modulus 0.10.4 lb/in2 1050.00.2 kg/cm3 104 Compressive strength 1.229.5 lb/in2 1030.820.7 kg/cm2 102Izod notched, R.T. 1.5 ft-lb/in-no break 8.1 kg/cm/cmHardness A55-A95 Rockwell A55A95 RockwellThermal conductivity 1.72.3 BTU-in/h-ft2-F 0.250.33 W/m-KLinear thermal expansion 1.88.4 in/in-F 1033.215.1 cm/cm-C 105Defection temperature at 264 psi100330F 38166CDefation temperature at 66 psi115370F 46188CContinuous service temperature180220F 82104CDielectric strength 430730 V/103 in 1.72.9 V/mm 104Dielectric constant at 1 MHz4.45.1 4.451Dissipation factor at 1 MHz 0.0600.100 0.0600.100Water absorption, 24 h 0.100.60% 0.100.60%Typical Applications AutomotiveFacias, padding, seats, gaskets, body panels, bumpers Medical Implantable devices, catheters, blood bags, dialysis membranes, heart-assist devices MachineryBearings, nuts, wheels, seals, tubing ConsumerFurniture padding, mattress goods, roller skate wheels, athletic shoes Apparel Fine polyurethane treads, combined with nylon monoflaments produce stretchable, lightweight, and comfortable fabrics (Lycra)5 IntroductionThe repeating urethane linkage is the basis for the generic name polyurethane. However, the use of the generic term polyurethane is deceiving in that all useful polyurethane poly-mers contain a minority of urethane functional groups. Thus, polyurethane is more a term of convenience rather than accuracy, since these polymers are not derived by polymerizing amonomericurethanereactant,noraretheypolymerscontainingprimarilyurethane linkages.Infact,othergroupssuchasethers,amides,biurets,andallophanatesarethe majoritylinkagesinthemolecularchain.Urethanelinkagesrepresenttheminorityof functional groups as long as the polymers contain a signifcant number of urethane link-ages. The name polyurethane may be correctly ascribed to these polymers.Polyurethanesandthecloselyrelatedpolyureasaretheproductsofthereactionof isocyanates(NCO)withtheactivehydrogencompounds(ROH)or(RNH2).An alternative chemistry to the isocyanate reactions was explored by Hoff and Wicker5 as they developed the chemistry to prepare polyurea from bis chloroformate and diamines.The polyurethanes are a heterogeneous family of polymers unlike PVC, polyethylene, or polystyrene.Thepolyurethanescompriseanarrayofdifferentproducts,rangingfrom rigid foams to soft, millable gums.Figure1.2presentsasummaryofthestructurepropertyrelationshipforpolyure-thanes;branching/cross-linkingisplottedontheordinateandintermolecularforceson theabscissa.Undertheseconditions,wecanencompassallthecommerciallyavailable polyurethanesanddefnewhichelastomericproductsareofthegreatestimportancein current commercial applications. At the right corner, representing extreme branching and chainstiffness,aretherigidurethanefoams.Therefore,atthisextreme,thethermoset rigid foams occupy the highest rank of cross-linking and chain stiffness.Table 1.2 summarizes some of the most important events in the historical development of the polyurethanes.Degree ofbranching orcross-linkingTermo-plasticelastomersMillableelastomersSpandexbersPlasticsChain stiness, interchain attraction, crystallinityFilmsPoromericsTextile coatingsCastelastomersRigidfoamsSemirigid foamsSurface coatings FlexiblefoamsFIGURE 1.2Structureproperty relationships in polyurethanes.6 Szychers Handbook of Polyurethanes1.3Overview of Polyurethane MarketsThe worldwide demand for polyurethanes is estimated to approach 17 billion pounds in 2010, or about 5% of total world consumption of plastics. Polyurethanes, although consid-ered to be a specialty, are behind only such large-volume commodity plastics as polyeth-ylene, PVC, polypropylene, and polystyrene in overall volume.TheU.S.marketforpolyurethanesin2010isestimatedatabout5billionpounds andincreasingatarateofcloseto4%peryear.Figure1.3presentsthebreakdownof 865 million lbsCASEFlexible foamsRigid foamsCASECASE = Coatings, adhesives, sealants, and elastomers1090 million lbsrigid foams1790 million lbsexible foamFIGURE 1.3U.S. markets for polyurethanes (1993 consumption, estimated 3.6 billion pounds).TABLE 1.2Summary of Events in the Historical Development of the PolyurethanesYear Event Reference1849 Isocyanate reaction with an alcohol Wurtz, A. Ann. 71,326 (1849)1937 I.G. Farbenindustrie applies for frst polyurethane patent German Patent 728,9811938 First U.S. patent awarded to Rinke etal. U.S. Patent 2,511,5441942 DuPont receives patents for reaction of polyisocyanates with glycols, diamines, and polyesters1942 Introduction of Igamid U, Perlon U, and Igamid UL in Germany1943 Vulkollan polyester-based elastomers introduced in Europe1945 Allies reorganize German industry and create Farbenfabriken Bayer A.G.1954 Patent on Lycra spandex elastomeric fber awarded U.S. Patent 2,692,8931954 Bayer and Monsanto Co. form Mobay Chemical Co.1955 Patent on Estane thermoplastic elastomers awarded U.S. Patent 2,871,2181955 Union Carbide develops frst one-shot foam; Dow Chemical introduces polyether polyols; Wyandotte introduces polyfunctional polyether polyols for rigid foams1956 Teracol 30-PTMEG introduced Bulletin HR-11 DuPont1959 First use of chlorofuorocarbons as blowing agents1971 Patent on medical-grade polyurethanesilicone elastomer U.S. Patent 3,562,3521993 Patent on aliphatic biostable polyurethane elastomer U.S. Patent 5,254,6627 Introductionpolyurethane usage; as the fgure indicates, the majority of polyurethanes are used in the production of fexible foams, followed by rigid foams and elastomers. A surprising portion of polyurethanes (14.1%) is used for specialty applications, such as protective and decorative coat-ings, adhesives, caulks, sealants, and so on. Protective and decorative polyurethane coatings are used in a wide variety of substrates, including wood, plastic, metal, leather, and textiles.Table 1.3 presents a breakdown of polyurethane pattern of consumption in the United States. The polyurethane market may also be considered in light of a world market, since manyproductsbothrawmaterialsandendusearetradedaroundtheworld.While statistics are not available, we can safely assume that the North American market repre-sents about 28% of the world polyurethane consumption.1.4Flexible FoamsThe fexible foam market is composed primarily of the following applications:Automotive seating and crash padsCarpet underlay and cushionsBeddingFurnitureTABLE 1.3Polyurethane Pattern of Consumption (United States)Markets Millions of PoundsFlexible foam:Bedding 240Furniture 742Carpet underlay 194Transportation 401Other 182Total 1759Rigid foam:Building insulation 519Refrigeration 195Industrial insulation 92Packaging 88Transportation 57Other 77Total 1028RIM elastomers:Transportation 134Other 76Elastomers 71Others (sealants, adhesives, coatings) 553Total 834Grand total 36218 Szychers Handbook of PolyurethanesOne of the large outlets for fexible polyurethane foam is for furniture cushions. Lighter weight, greater strength, and ease of fabrication as compared to latex foam are some of the decidingfactorsinitssuccess.Inaddition,thefabricationandapplicationofslabstock foam is easier and faster than the use of animal hair, bird feathers, or other flling materi-als.Improvedmoldingtechniquesoffexiblefoamareresponsibleforitsacceptancein furniture with unusual shapes. Molded rigid foam has made great inroads into the furni-ture industry. Shortages of select hardwood and a scarcity of skilled wood workers encour-agedthefurnitureindustrytolookforreplacementmaterials.Amongothercandidate materials,rigidpolyurethanefoamhasfoundacceptanceindecorativeparts,minor frames, chair shells, and the like.MillionsofpolyurethanefoammattressesarebeingproducedintheUnitedStates yearly.Theyhavefoundacceptancebecauseoftheirsuperiordurability;freedomfrom odor; nonallergenic properties; ease of cleaning; resistance to dry cleaning solvents, oils, andperspiration;andthefactthattheyareonlyone-fourththeweightofacomparable innerspring and one-half to one-third the weight of latex foam.Another important use of fexible foam is in the automotive industry for seat cushioning, instrument panel trim, safety pads, arm rests, foor mats, sunvisors, underlays, roof insula-tion, weather stripping, air flters, and so on. It has been estimated that the 1995 model cars will use an average of 30 lb of foam per automobile. One of the frst uses of fexible poly-urethane foam was in seating for aircraft where its light weight and fame retardance were of special importance.The use of fexible foam as a bonding material for fabric primarily started in 1961, when the apparel industry began to employ it. Polyester foam is used for this application, wherein thefoamisbondedtothefabricbythefame-laminationtechnique.Inthisprocess,the surface of the foam is heated until the surface layer fuses and becomes soft and tacky. It is thenbondedunderpressuretothefabric.Adhesivescanalsobeusedforbondingthe foam to the fabric. A foam lining for the garments makes the fabric dimensionally stable andprovideshighinsulatingqualities.Otheradvantagesincludeexcellenthanddrape and outstanding crease and wrinkle resistance.Anotherimportantareaforfexiblepolyurethanefoamsisincarpetunderlay,where theyprovidecushioning,arenonskidding,anddonotmatdown.Theyalsoimparta luxurious feel, even to low-cost carpeting.1.5Rigid FoamsOneofthemajorusesofrigidpolyurethanefoamisinhomerefrigerators.Mostmajor manufacturers are currently using rigid urethane foam as insulation in their lines. Because of the superior insulating characteristics of the fuorocarbon-blown foams, manufacturers build refrigerators with thinner insulation and, therefore, larger inside capacity. All types of refrigerated trucks such as milk trucks, ice cream trucks, and trailers are insulated with rigid polyurethane foams. Besides having good insulating properties, rigid foams contrib-ute to the structural strength of the bodies of the trucks, have low moisture pickup, and can withstand gasoline and temperatures up to 100C.Potentially, the biggest market for rigid polyurethane foam is in the building industry. Theareasofutilizationinthisfeldencompasscurtain-wallconstruction,preformed rigid panels, spray-applied wall construction, and roofng insulation, either sprayed or in 9 Introductionpreformed panels. This market includes residential homes and commercial and industrial buildingssuchaslargerefrigeratedwarehouses.Rigidpolyurethaneandpolyisocyan-uratefoamsarethemostpopular,energy-effcient,andversatileinsulatingmaterialsin the construction industry.The use of rigid polyurethane foam in marine applications such as fotation equipment is growing. Many modern boats utilize rigid foam to help support the boat in the water. Larger ships have used rigid polyurethane foam as void fllers, and it is also used in life-boats and refrigerator ships.The major applications for rigid foams areBuilding insulationAppliance insulationPackaging1.6ElastomersThe dictionary defnes elastomer as a material which at room temperature can be stretched repeatedly,anduponimmediatereleasewillreturntoitsapproximateoriginallength. Sincenaturalrubberwastheoriginalelastomer,inpolymericnomenclature,synthetic materialsthatapproximateorexceedcurednaturalrubberinphysicalpropertiesare called elastomeric. Because natural rubber exhibits such an excellent combination of physi-calproperties(i.e.,tensilestrengthor4000 psi,400600%ultimateelongation),theterm elastomeric grade is frequently used to characterize and describe those synthetic materials with the highest physical performance.Elastomers exhibit initial elastic moduli in the range of 0.10.4 lb/in2, and instantaneous and nearly complete reversible extensibility. As the temperature is lowered, the extensibil-ity decreases signifcantly, and below its glass transition temperature (Tg), the elastomers become brittle. Other properties of polyurethane elastomers, along with some trade names, are shown in Figure 1.4.TheU.S.polyurethaneelastomersmarketisdividedprincipallybetweenthecastable resins and the thermoplastic polyurethane elastomers (TPUs). Figure 1.5 presents the esti-mated 2010 usage for elastomers in the United States. Elastomers represent a heterogeneous class of polyurethanes. Among the most typical applications, we can cite the following:1.Solid and microcellular elastomersFootwearAutomotive and transportationMaterial handlingIndustrialMedical devices2.Cast elastomersPump and pipe liningsChute liners10 Szychers Handbook of PolyurethanesIndustrial tiresSeals and o-ringsCasters and wheelsSonar windowsBearing and ski padsVibration and shock mountsRowling ballsSki bootsBuoys and dock fendersTypical Properties of PolyurethaneAmerican Engineering SIProcessing temperature 385450F 196232CLinear mold shrinkage 0.0040.014 in/in 0.0040.014 cm/cmMailing point 400450F 204232CDensity 69.977.4 lb/ft31.121.24 g/cm3Tensile strength, yield 4.935.0 lb/in2 1033.424.6 kg/cm3 103Tensile strength, break 4.935.0 lb/in2 1053.424.6 kg/cm3 102Elongation, break 100.0500.0% 100.0500.0%Tensile modulus 0.645.0 lb/in2 1050.431.6 kg/cm3 104Flexural strength, yield 6.060.0 lb/in2 1034.242.2 kg/cm3 102Flexural modulus 0.10.4 lb/in2 1050.00.2 kg/cm3 104Compressive strength 1.229.5 lb/in2 1030.820.7 kg/cm2 102Izod notched, R.T. 1.5 ft-lb/in-no break 8.10.0 kg cm/cmHardness A55A95 Rockwell A55A95 RockwellThermal conductivity 1.72.3 BTU-in/h-ft2-F 0.250.33 W/m-KLinear thermal expansion 1.88.4 in/in-F 1033.215.1 cm/cm-C 105Defection temperature at 264 psi 100330F 38166CDefation temperature at 66 psi 115370F 46188CContinuous service temperature 180220F 82104CDielectric strength 430730 V/103 in 1.72.9 V/mm 104Dielectric constant at 1 MHz 4.45.1 4.451Dissipation factor at 1 MHz 0.0600.100 0.0600.100Water absorption, 24 h 0.100.60% 0.100.60%Typical Applications AutomotiveFacias, padding, seats, gaskets, body panels, bumpers Medical Implantable devices, catheters, blood bags, dialysis membranes, heart-assist devices MachineryBearings, nuts, wheels, seals, tubing ConsumerFurniture padding, mattress goods, roller skate wheels, athletic shoes Apparel Fine polyurethane treads, combined with nylon monoflaments produce stretchable, lightweight, and comfortable fabrics (Lycra)FIGURE 1.4Thermoplastic polyurethane (TPU) manufacturers and trade names.11 IntroductionAmong the others classifcation, we include adhesives and sealants as well as protec-tive, decorative coatings, and fnishes as shown below:1.Adhesives and sealantsFoundry sandRebonded carpet underlayConstructionTransportationFootwearTextileAutomotive caulks and glazing compounds2.Protective/decorative coatings and fnishesTextileAutomotive and transportation fnishesArchitecturalIndustrialAnticorrosive metal fnishesIn terms of production capacity, the isocyanates and the polyols are the most important becauseoftheirvolume.Theisocyanateusageisdominatedbythetwoworkhorses, namelydiphenylmethane-4,4-diisocyanate(MDI)andtoluenediisocyanate(TDI).The worldwideproductionofMDIisestimatedatabout4.6millionpoundsin2010,while theworldwide production of TDI is estimated at approximately 3.5 million pounds for the same year.Closeto40%oftheproductioncapacityofMDIislocatedinWesternEurope,25%in North America, 25% in the Far East, and the remaining 10% in the rest of the world. Close Castable elastomersTermoplastic resinsFIGURE 1.5Polyurethane elastomer consumption (total U.S. market, estimated 80 million pounds, 2010).12 Szychers Handbook of Polyurethanesto30%oftheproductioncapacityofTDIislocatedinWesternEurope,20%inNorth America,30%intheFarEast,andtheremaining20%fairlyevenlydistributedbetween Eastern Europe, Latin America, and the rest of the world.Polyether polyols constitute the lions share of polyurethane raw materials production. The2010productionofpolyetherpolyolsisestimatedtobecloseto10millionpounds, with25%ofproductionfacilitiesfoundinWesternEurope,25%inNorthAmerica,and 50% in the Far East.The production capacity of polyester polyols is estimated at about 1.2 million pounds in 2010, with Western Europe, North America, Indonesia, and China representing the largest producers.Many other raw materials are not included in this market survey, such as catalysts, chain extenders, blowing agents, surfactants, fame retardants, stabilizers, lubricants, and so on because their production capacity is small compared to the isocyanates and polyols already mentioned.References1.Rinke, H., Schild, H., and Siefken, W. (I.G. Farben). U.S. Patent 2,511,544, 1938.2.Christ, A.E. (DuPont). U.S. Patent 2,333,639, 1943.3.British Patents (ICI) 580,524 (1941) and 574,134, 1942.4.Pinten, P. (Dynamit A.G.). German Patent 932,633, 1943.5.Hoff, G.P. and Wicker, D.B. (I.G. Farben). P.B. Report 1122, September 12, 1945.132Basic Concepts in Polyurethane Chemistry andTechnologyMichael Szycher2.1OverviewPolyurethanesareextremelylargeandcomplexmoleculesproducedbycombininga largenumberofsimplermoleculescalledmonomers.Monomersarecompoundswhose properties (molecular weight, boiling point, melting point, crystallinity, etc.) are discrete. Polyurethanes, on the other hand, typically do not have discrete properties but have aver-agepropertiesthatrepresentarangeofmoleculeswithdifferingmolecularweightand oftenslightlydifferingstructure.Table2.1showsthecomparisonandcontrastofthe properties of typical small molecules with those of a polyurethane polymer.Themolecularweightofpolyurethanescangreatlyaffectthephysicalpropertiesofa polymer.1 This is shown graphically in Figure 2.1. Molecular weight distribution can also haveasignifcanteffectuponpolyurethanecharacteristics,especiallyprocessingand rheological characteristics.The polymer chains have a spatial architecture. They may be linear, branched, or networked. Polyurethanesdisplaystereomicrostructure.Polyurethanesexistashomopolymersand copolymers. Copolymers may be random, alternating, segmented, block, or graft types.Polyurethanescanbecrystallinesolids,segmentedsolids,amorphousglasses,or viscoelasticsolids.Withrespecttomechanicalproperties,polyurethanesarenonideal solids.Themechanicalpropertiesofpolyurethanesaretimedependent.Forevery CONTENTS2.1Overview ............................................................................................................................... 132.1.1Traditional Polymer Classifcation ........................................................................ 142.1.1.1Step-Addition Polymers ........................................................................... 152.2Kinetics of Polyurethane Polymerization ........................................................................ 172.3Polyurethane Depolymerization Mechanisms ................................................................ 182.3.1Polyurethane Thermostability ............................................................................... 202.3.2Polyurethane Oxidation .......................................................................................... 232.3.2.1Thermooxidation ....................................................................................... 232.3.2.2Photooxidation .......................................................................................... 242.4Molecular Forces and Chemical Bonding ........................................................................ 262.5Amorphous Polymers ......................................................................................................... 282.6Crystalline Polymers ........................................................................................................... 282.7Segmented Polymers ........................................................................................................... 32References ....................................................................................................................................... 3614 Szychers Handbook of Polyurethanesexcitation, there are two responses: a viscous response and an elastic response; that is, a time-dependent and a nontime-dependent response. The properties of the linear polyure-thanesarealsoverymuchtemperatureandmoisturedependent.Figure2.2showsthe inverserelationshipbetweenmodulusofelasticityandfrequencyinatypicalpolyure-thane. Figure 2.3 shows a similar inverse relationship between modulus of elasticity and increasing temperature. Note that in both instances there is a sharp decrease in modulus at a critical frequency or temperature.2.1.1Traditional Polymer ClassificationPolymers were traditionally classifed according to their polymerization method as either addition-typeorcondensation-typepolymer.Recently,thedistinctionbetweenpolymer types has been drawn along the lines of the major differences in the kinetics of polymeriza-tion. In this handbook, the terms chain polymerization and step-growth polymerization replace the terms addition polymerization and condensation polymerization, as shown in Table 2.2.There are major differences between the kinetics of chain and step polymerizations. The chainpolymerizationandthehighpolymerisformedveryearlyinthepolymerization. The locus of the polymerization is only on those few chains containing an active propagat-ingcenter.Insteppolymerization,allspeciespresentcanreactwithallotherspecies TABLE 2.1Properties of Small Molecules versus Polyurethane PolymersSmall Molecules Polyurethane PolymersThree Discrete States: Three States:Solid, liquid, gas Liquid, quasisolid, solidDiscrete formula Average formulaDiscrete molecular weight Average molecular weightDiscrete Properties: Range of Properties:Melting point, Melting range, densitydensity, crystalline/glassy range, microcrystallinePropertiesMolecular weightHardnessCompression setElongationTensileMelt viscosityFIGURE 2.1Effect of polyurethane molecular weight on properties. (Adapted from Szycher, M. Opportunities in Polyurethane Elastomers: Industrial and Medical Applications, Technomic Seminar, Boston, MA, June 13, 1989.)15 Basic Concepts in Polyurethane Chemistry andTechnologypresent.Polymerizationproceedsasdimer,trimer,tetramer,pentamer,andsoon.High molecularweightpolymerdoesnotresultuntillateinthereaction.Incontrast,ionic polymerizations proceed in a nearly linear fashion. Most polyurethane reactions proceed to polymerization via step-addition reactions, as depicted in Figure 2.4.2.1.1.1Step-Addition PolymersIn step polymerization, monomers are typically difunctional or trifunctional. In a typical step polymerization, a single reaction and the propagation reaction is responsible for the formation of the polymer.Step polymerization typically occurs in more difunctional or trifunctional monomers that bear two distinctly different functionalities. A single chemical mechanism is responsible Frequency1 (or t)E*FIGURE 2.2Modulus versus frequency (t = constant).TABLE 2.2Modern Classifcation SystemBy Mode of Polymerization By Kinetics of PolymerizationAddition polymers Chain-addition polymersCondensation polymers Step-addition polymersE*TemperatureFIGURE 2.3Modulus versus temperature (t = constant).16 Szychers Handbook of Polyurethanesfor the formation of the polymer. Chain transfer and termination reactions are inherently absent in step polymerizations, although side reactions and reactions with contaminants can have similar effects as transfer or termination.In a step system, all monomer units and growing polymer chain ends participate in the propagationreactionthroughoutthepolymerizationprocess.Initially,monomersreact withmonomerstoformdimers(dimerswithmonomersordimerswithdimerstoform trimers or tetramers, etc.). Monomers are depleted very early in the polymerization; how-ever, high polymers are not formed until late in the reaction. Table 2.3 provides examples of some typical step-addition polymerizations.Condensationpolymerizationsaresteppolymerizationswheretherepeatunitofthe fnished polymer does not contain the same structures as the monomers from which it was prepared. Certain polymers such as polycaprolactones, polycaprolactams, polyurethanes, ChainAvg. M.W. of polymer fractionPercent conversionStepIonic (living)FIGURE 2.4Chain addition versus step addition.TABLE 2.3Step-Addition PolymersPolymer Type Group A Group B ExampleCondensation ROH RCOO PolyesterROH RCOCl PolyesterROH RCOOR PolyesterROH COCl2PolyesterR-OH RNCO PolyurethaneRNH RNCO PolyureaRNH RCOH PolyamideRNH RCOCl PolyamideSiR2Cl2H2O SiliconeArOH COH2Phenolic resinArOH RCHOCH2Epoxy resin17 Basic Concepts in Polyurethane Chemistry andTechnologyandpolyureasarealsoclassifedascondensationpolymers,eventhoughthedefnition does not ft the routes commonly used to prepare them.Condensation polymerization may well be thought of as a functional group polymeriza-tion.Thus,functionalgroupsoneachendofmonomermolecules(e.g.,NCO)reactwith functional groups on the ends of other monomers (e.g., OH) to form functional group link-ages(e.g.,urethanes).Monomersforthecondensationpolymerizationmayconsistofthe structure AB where molecules of AB react with other AB molecules to form the polymer.Therearenoinherentinitiationorchaintransferreactions(asinchainaddition reactions)onlypropagation.AnymonomeroffunctionalityAmayreactwithany monomer of functionality B from the outset. Polymerization thus proceeds by successive monomer,dimer,trimer,andtetramerformationsuntilitreachesahighconversion,a molecularweightpolyurethanepolymerisproduced.Table2.4summarizesthechar-acteristics of polyurethane reactions.2.2Kinetics of Polyurethane PolymerizationThe extent of reaction of a given polyurethane system (p) is given by pN NN=00 (2.1)whereN0 = initialnumberofmonomermolecules,N = remainingnumberofmonomer molecules, or N = N0 (1 p) so that, at 100% conversion (i.e., N = 0), p = 1.The degree of polymerization (DP) is defned as the ratio of the initial number of mole-cules of monomer to the remaining number of molecules, or DP or DP = =NN p011 (2.2)Now consider a reaction pushed to 90% conversion, or 90% yield. DP ==11 0 910.TABLE 2.4Characteristics of Polyurethane Polymerizations1.A single reaction (propagation) is responsible for polymer formation.2.Monomer system contains A, B, or C functional groups, any one of which may react at random.3.Monomers disappear early in the polymerization.4.Molecular weight rises slowly. High molecular weights achieved only at high conversions.5.Molecular weight distribution is very broad throughout the polymerization until the very end, when all oligomers have reacted.6.Precise stoichiometric ratios are required.7.High-purity monomers are required.8.Reactions are often reversible; equilibrium driven to high polymer by oligomer depletion.18 Szychers Handbook of PolyurethanesTherefore, it requires a 98% conversion to reach a DP = 50, which is approximately the threshold value for minimal mechanical properties in polyurethanes. The effects of a stoi-chiometric imbalance (whether by impurity, weighing error, or design) is given by DP ==+ 11 2rr rp (2.3)wherer = NA/NBforAA/BBsystem,orr = NA/NB = 2NBforAA/BBsystem,with monofunctional monomer B or AB system with BB added. From the foregoing discus-sion, we can calculate that, at 98% conversion, a 2% excess of monomer BB will reduce DP from 50 to 33. Or with a 2% excess of BB, it would require a conversion of just over 99% to restoreDPto50.Oneshouldreadilyrecognizethat,whilemostorganicchemistsmay consider a 95% conversion as a very good yield, the polyurethane chemist at the same con-version ratio would end up with only a sticky mess.2.3Polyurethane Depolymerization MechanismsAll polymers can be depolymerized; polyurethanes are no exception. Polyurethanes can be depolymerized chemically2 in eight ways (see Table 2.5).Hydrolysisisdefnedasachemicalreactioninwhichwaterreactswithanothermoleculeto form two or more substances. Thermolysis reactions are those that occur due to heat. Oxidation is a reaction in which oxygen combines chemically with another substance. Oxidation can beinitiatedwithheat(thermooxidation)orbylight(photooxidation).Photolysisisthe decomposition of a chemical compound into smaller molecular weight units caused by the interaction with light. Pyrolysis is the transformation of a substance into other substances by heat alone, that is, without oxidation. Chemical depolymerizations caused by the attack of microorganisms are called microbial degradations. Attack on polyurethanes by solvents, for example, alcohols, can cause a surface degradation referred to as solvolysis. Biologically induced environmental stress cracking is a special surface degradation caused by expos-ing polyurethanes to enzymes secreted by certain infammatory cells when the polyure-thane is implanted within a living system for prolonged periods of time.Polyurethanes,likeallmajorplastics,aretheobjectsofconsiderablerecyclingefforts. Several industry groups have been formed to develop recycling technology based on depo-lymerizationreactions.ThesegroupsincludethePolyurethaneRecyclingandRecovery CouncilandtheAmericanPlasticsCouncil.Currentrecyclingtechnologygenerallyfalls into three basic categories: reuse of scrap, chemical depolymerization, and incineration.At present, the most impressive story in recycling of polyurethane scrap lies with fexi-ble foams, since much of the fexible-foam scrap in the United States is reused in carpet underlay. Polyurethane scrap is compounded into rebond foam (the type of foam used TABLE 2.5Depolymerization Reactions of PolyurethanesHydrolysis Thermolysis OxidationPhotolysis Pyrolysis MicrobialSolvolysis Biologically induced environmental stress cracking19 Basic Concepts in Polyurethane Chemistry andTechnologyin carpet underlay) by chopping the scrap foam with a toluene diisocyanate (TDI) binder and polyol and then compressing the mass in a mold.Another recycling approach is to hydrolyze (or glycolize) the polyurethane back into the precursor raw materials. Developmental methods use an alcohol under high-temperature conditions to depolymerize the polyurethane and obtain a somewhat modifed version of the original polyol. The reconstituted polyol usually has a lower molecular weight distri-bution and cannot be used at 100% concentrations in the polyurethane reaction. However, it is possible to use reconstituted polyols as a 1015% blend with virgin polyols and pro-duce good parts.3First, let us examine the bonds involved in some depolymerization reactions (Figure 2.5). The three bonds most susceptible to hydrolytic degradation are the ester, urea, and ure-thane (Figure 2.6). The ester reverts to the precursor acid and alcohol; this precursor acid furthercatalyzesesterhydrolysis,andthusthereactionbecomesautocatalytic.Because ofthe autocatalytic nature of ester hydrolysis, this is the most prevalent hydrolytic degra-dation reaction. The urea bond can hydrolyze to form a carbamic acid and an amine. The carbamic acid normally is unstable and typically undergoes further reaction. The urethane linkage, although somewhat less susceptible, may undergo hydrolysis to yield a carbamic acid and the precursor alcohol.Prepolymer+H2NR NH2Prepolymer+HOR OHEsterDiisocyanate PolyolEtherOOORNHCNHRRNHCORUrethaneUreaPolyurethane prepolymerDiamineDiolRRRRRRR RCCC OOH HOOOCNOCNNCONCOONHCOFIGURE 2.5Chemical reactions involved in polyurethane preparation.EsterOOUreaUrethaneAmine Carbamic acidCarbamic acidAlcoholRCOH + HORRNHCOH + H2NRRNHCOH + HORAlcoholAcidRNHCOR + H2ORNHCNHR + H2ORCOR +H2OOOOOFIGURE 2.6Bonds susceptible to hydrolytic attack.20 Szychers Handbook of PolyurethanesComparingvariouspolyurethanesystems,Figure2.7showsthatpolyethyleneadipate glycol/methylenebis(diphenyl)diisocyanate(MDI)/butanediol(BD)systemshydrolyze quiterapidly,twotimesfasterthanpolybutyleneadipateglycol/MDI/BDsystems. Conversely, using the more stable polycaprolactone glycol, there is a signifcant increase in hydrolyticstability.Thegreatesthydrolyticresistanceisobtainedbytheuseofpolytetra methylene ether glycol (PTMEG). Although not shown in this date, the choice of isocyanate also infuences hydrolytic stability. Thus, a polyester/MDIBD system will display greater hydrolysis resistance than that of a polyester/TDIBD system.Another important environmental infuence on hydrolysis is temperature. At 50C, the tensile half-life of a polyester/TDI/diamine system may be 4 or 5 months, while that of a polyether/TDI/amine appears to be almost 2 years. At 70C, however, these half-lives fall to2weeksand5weeks,respectively.Andat100C,theester-basedpolymerscanbe expected to degrade in a matter of days.Ingeneral,polyestersdonotfarewellinhydrolysisresistance.Ifapolyestermust beusedinawetenvironment,carbodiimideshelpinprolongingtheirlongevity. Carbodiimides act as acid scavengers. As seen in Figure 2.8, the acid and carbodiimide reacts to form an intermediate, which rearranges to give an N-acyl urea. This consumes the acid thereby preventing further hydrolytic autocatalysis.Figure 2.9 shows the marked increase in life span of a polyester with a 2% addition of polycarbodiimide. It should be remembered, however, that the carbodiimide is being con-sumed and eventually will be totally depleted resulting in the onset of hydrolysis.2.3.1Polyurethane ThermostabilityHeat can cause the degradation of polyurethanes. The onset of allophanate disassociation isaround100120C.Thedissociationtemperatureofthebiuretlinkageisaround 115125C. These reactions are dissociations and somewhat reversible. They revert to the urethane or urea from which they were formed. The aromatic-based urethane bond begins 1000 MW PolyglycolPolybutylene adipate glycol (PBAG)Weeks3020304050607080901004 5% Retention of tensile strengthPolyethylene adipate glycol(PEAG)Polycaprolactone glycol (PCLG)Polytetramethylene ether glycol (PTMEG)10FIGURE 2.7Polyurethane hydrolytic performance, humidity aging at 80C/95% RH, tested 1 week after removal.21 Basic Concepts in Polyurethane Chemistry andTechnologyits thermal dissociation around 180C, which is prior to the urea linkage which is about 160200C. The urethane can dissociate into the isocyanate and polyol from which it was formed. This reaction is reversible as long as the isocyanate is not lost to a side reaction. The second reaction produces a primary amine and an olefn. The third reaction produces a secondary amine. Since these latter reactions generate CO2, which is lost as a gas, they are irreversible. These thermal dissociation relationships are summarized in Table 2.6.Referring to the urethane bond depicted in Figure 2.10, it is clear that the urethane link-agemayundergothreeseparatetypesofthermaldegradation:(1)theformationofthe precursorisocyanateandtheprecursoralcohol;(2)cleavageoftheoxygenofthe-CH2 groupandassociationofonehydrogenonthesecondCH2groupwouldleadtothe carbamic acid and an olefn with subsequent carbamic acid decomposition to give a pri-maryamineandCO2depictedinFigure2.11;and(3)theformationofaurethaneanda secondary amine, as seen in Figure 2.12.Figure 2.12, or mechanism B, refers to writing the urethane structure to include the frst CH2groupofthepolyol,thusvisualizinghowthesecondaryaminewouldbeformed. Cleavage of the oxygen CH2 bond with association of the CH2 hydrogen to the NH group would force cleavage of the nitrogen carbonyl carbon bond splitting out CO2. This would result in a secondary amine. Which of these thermodegradation reactions take place, and IntermediateOCR R C OH + OH R RCOH + RN = C = NRRNHCNRRNHCNRCRN-acyl ureaCarbodiimideAcid EsterRCOR + H2OOOOOOOFIGURE 2.8Carbodiimide stabilization of polyester-based polyurethanes.00204060801001 2 3 4 5 6 7 8 9BATime weeksTensile retention (%)A-Polyester polyurethaneB-Polyester polyurethane +2% polycarbodiimideImmersion 70CFIGURE 2.9Effect of carbodiimide on a polyester-based polyurethane.22 Szychers Handbook of PolyurethanesRNHR + CO2RNH2 + CO2 + olefin [A]RNCO + HORRNHCOROUrea > urethane > biuret > allophanate[B]FIGURE 2.10Thermal degradation of urethane linkages.O OOCHRC OH +CUrethaneRNH2CO2CHRRNH RNHCH2CH2RNHCOCH2CH2ROHFIGURE 2.11Thermal degradation of urethane linkages leading to an olefn and CO2.RROCH2RNH + CO2CH2O2 AmineUrethaneUrethaneRNHCRNHCOCH2ROFIGURE 2.12Thermal degradation of urethane linkages leading to a urethane and a secondary amine.TABLE 2.6Thermal Dissociation Temperatures of Linkages Found in PolyurethanesOnset of DissociationLinkage C FAliphatic allophanate 85105 185220Aromatic allophanate 100120 212250Aliphatic biuret 100110 212230Aromatic biuret 115125 240260Aliphatic urea 140180 285355Aromatic urea 160200 320355Aliphatic urethane 160180 320355Aromatic urethane 180200 355395Disubstituted urea 235250 45548023 Basic Concepts in Polyurethane Chemistry andTechnologyto what extent, depend on the structure of the urethane, the reacting conditions, and the environment.2.3.2Polyurethane OxidationOxidationissimplydegradationthatoccursduetothereactionwithoxygen.Oxidation reactions may be heat initiated or light initiated. Heat-initiated oxidation is called thermo-oxidation, and light-initiated oxidation is called photooxidation. Thermooxidation and pho-tooxidationwillbediscussedseparately.Thediscussionofphotooxidationwillinclude photolysis reactions since they are closely related.2.3.2.1ThermooxidationPreviously, we demonstrated the ester to be the weak link in hydrolysis. Now it is the ether that is the weak link in thermooxidation (see Figure 2.13). Thermooxidation proceeds via a free radical mechanism. Heat causes a hydrogen extraction at a carbon to the ether link-age. This radical is subject to oxygen addition and forms a peroxide radical. The peroxide radical then extracts another hydrogen from the backbone to form a hydroperoxide. The hydroperoxideradicalthendecomposestoformanoxideradicalandthehydroxylfree radical. The order of thermooxidation stability is ester > urea > urethane ether.The oxide radical will cleave at either of two places (see Figure 2.14). One, it may cleave atthecarbonbondadjacenttotheoxideradical.Ifso,formatesareformed.Two,ifthe cleavageisatthecarbonoxygenbond,aldehydesareformed.Theorderofstabilityof + RHOO OOHORCHOR + OHRCHOR + R RCHORRCHOR+ O2ORRCH2Cleaves via 2 mechanismsEther thermooxidationEster > urea > urethane >> etherFIGURE 2.13Thermooxidation.PTMEG > PEG > PPGAldehydeFormateO OO OR + CHORRCH + OR 2. RCHOR1. R CHORFIGURE 2.14Thermooxidation-oxide cleavage.24 Szychers Handbook of Polyurethanespolyethers to thermooxidation is PTMEG is more stable than poly(ethylene oxide) glycols, that are, in turn, more stable than poly(propylene oxide) glycols.2.3.2.2PhotooxidationThe exact mechanism of photolytic degradations is controversial. Photooxidation is believed to take place in MDI and TDI aromatic urethane via a quinoid route. The urethane bridge oxidizes to the quinoneimide structure as seen for MDI in Figure 2.15. This structure is a strong chromophore resulting in the yellowing of urethanes. Further oxidation produces thediquinoneimidestructurethatisamberincolorandisresponsibleinpartforthe browningofurethanes.Topreventthediscoloration,nonquinoidstructuresshouldbe used.A second scheme in the photolysis of polyurethane is the scission of the urethane bond. Therearetwopossiblebondssusceptibletoscission,asseeninFigure2.16.Whenthe nitrogen to carbon bond breaks, it results in an amino radical and a formate radical. The formate radical will liberate CO2, and an alkyl radical will result. If the carbon to oxygen bond cleaves, a carbamyl radical and an alkoxy radical will be formed. The carbamyl radi-OROCNHROCNH= C =Diquinone-imideMonoquinone-imideO2O2hhCH =ROC N =CH2 NHCOR= N COR= N COROOOOOOOO ===FIGURE 2.15Photolysis scheme 1: photooxidation.OO OCO2 + R (c)(a)(a)RNHCORRNHC + ORRNHC + COR NH + CORh h(b)FIGURE 2.16Photolysis scheme 2: urethane scission.25 Basic Concepts in Polyurethane Chemistry andTechnologycal decomposes to generate the amino radical and CO2. The net result of these urethane scissionreactionsisthreeradicals(amino,alkyl,andalkoxy)thatmayundergofurther reactions.Those further reactions are seen in Figure 2.17. In reaction 1, two amino radicals react to form an intermediate that, in turn, reacts with the alkoxy radical to form diazo products. Theseareagainchromophoricmaterialsthatareresponsibleforpolyurethanesturning browninsunlight.Thesecondreactiondemonstrateshowolefnsareformedinthese processes. The third reaction is an oxidation process wherein aldehydes are produced. In reaction4,thealkoxyradicalundergoesscissiontoproduceformaldehydeandanother alkyl radical which may then be used in reaction 2 or 3.Athirdphoto-degradativeprocessisknownasarearrangement.Asequenceofthis reactionforaTDI-basedurethanecanbeseeninFigure2.18.Thisrearrangedproduct undergoes additional degradation to colored azo-containing products.These radical-producing processes can be prevented or ameliorated by the use of stabi-lizers. Certain families of compounds known as antioxidants and UV stabilizers have been effectiveininhibitingdegradationinpolyurethanes.Hinderedphenolsandaromatic amine compounds act as radical chain terminators. Thioethers and phosphites are per-oxidedecomposers.Anyofthesecompoundswilldisruptthedegradationprocess. Benzotriazoles, along with certain hindered amines and benzophenones, will absorb the UV light and use its energy in a nondestructive sequence.Certain combinations of these compounds act synergistically. Typically a UV stabilizer such as benzotriazole, used in conjunction with an antioxidant such as a hindered phenol, will inhibit discoloring and property loss for longer periods of time than either one used alone.+( CH2CH2R)R RNHNHR CH2 R CH2 O 4. R O ( CH2CH2R)(O CH2 CH2 R )CHO CH2 R + OH+ O23. R RNH2 + CH2 = CH2RRN = NR OR + 2 + RNH 2. RNH1. RNH OOCH2CH2R FIGURE 2.17Photolysis scheme 2B: reactions of radicals.hNHNHCORC ORColoredAZOproductshOONHCORNHCOROOOCH3CH3OFIGURE 2.18Photolysis scheme 3: rearrangement.26 Szychers Handbook of Polyurethanes2.4Molecular Forces and Chemical BondingPolyurethanes are characterized by the forces at work within and between molecules. Of these, covalent bonds are the strongest and most signifcant. The energy needed to break covalentbondsenergyofdissociationhelpspredicthowpolymerswillreacttoheat degradation.Alistofcovalentbondscommonlyfoundinpolymers,alongwiththeir dissociation energies (kilocalories per mole), is shown in Table 2.7 in order of decreasing strength. The carboncarbon bond in polyethylene, for example, will break frst upon over-heating,whereasittakesanadditional16kilocalories/moletoundocarbonhydrogen bonds.Thefrstindicationofdegradationisareductioninmolecularweight,thatis, smaller polymer chains through cleavage. In polyvinyl chloride (PVC), the carbonchlorine bond will succumb frst to dissociation, which in turn, affects the nearby hydrogen atom bondedtothesamecarbonatom.EvidenceofPVCdegradationisthegenerationand evolution of chlorine or chlorine gas.Togainafullerunderstandingofthenatureofpolyurethanes,wemustaccountfor secondarybondingforcesthatactbetweenindividualpolymermolecules.Although muchweakerthancovalentbonds,theynonethelessdirectlyaffectamaterialsphysical properties,suchasviscosity,surfacetension,frictionalforces,miscibility,volatility,and solubility. In order of increasing strength, these secondary forces are classifed as van der Waalsforces,dipoleinteraction,hydrogenbonding,andionicbonding,asshownin Figure 2.19.VanderWaalsforcesareresponsiblefortheshort-rangenaturalattractionofsimilar molecules.Whentheyareovercomedbyheating,softeningandmeltingfollow.Slightly stronger dipole forces are generated by polar groups in the backbone (COC) or in side chains (CN,CCCNH2). Polar groups arise from the development of slightly positive and negative charges due to unequal sharing of electrons in covalent bonds. The slightly charged atoms attract oppositely charged atoms on adjacent molecules.TABLE 2.7Strength of Common Covalent BondsCovalent Bond Dissociation Energy (kcal/mol)aCN 213C=O 174C=C 146CF2103123OH 111CH 99NH 93CO 86CC 83CCI 81SH 81CN 73CS 62OO 35aDissociation energy increases as additional fuorine atoms are substituted on the same carbon atom.27 Basic Concepts in Polyurethane Chemistry andTechnologyHydrogen bonding, often considered a strong form of dipole interaction, is a third cate-goryofsecondarybondingforces.Hydrogenbondingisassociatedwiththegroupin backbones and the OH or NH2 groups in the side chains found in nylons, polyurethanes, polyvinyl alcohols, and butadiene-acrylonitrile copolymers. As in dipole interaction, oxy-gen and nitrogen atoms attract positively charged hydrogens of other molecules. Polymers withhydrogenbondingusuallyarecompatiblewithsmallmoleculessuchasthosethat constitute plasticizers, solvents, and water.Ionic bonds, the strongest of all, are attractive forces between positively and negatively charged ions. Because of their atomic arrangement, certain atoms tend to accept or release electrons to reach a more stable electron confgurationmuch like inert gases, and become positively or negatively charged in the process. Ionic bonds between polymers actually arehalf-polarinnature(iondipolebonds).Apositiveion(Zn2+)ispositionedbetween negatively charged polar groups (O) of two molecules, linking them together.CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2NNO O HHCCH +2 + Zn Na++OO OOOOOOC C C C C C C C C C C C C C C C C C C C C 50 %ionized100 %ionizedCCCCC CC10 to 203 to 71.5 to 30.5 to 2CH2CH2Dissociation energy(kilocalories per mole)Example TypeIonicbondingHydrogenbondingDipoleinteractionvan der WaalsforcesFIGURE 2.19Secondary bonding.28 Szychers Handbook of PolyurethanesWeaker than covalent bonds, secondary bonds will yield before covalent bonds under heat. Furthermore, there is evidence that secondary forces dissipate in groups and not in sequence like a zipper. Thus, polymers with strong secondary forces exhibit high viscosities and are morediffculttoprocess.Polytetrafuoroethylene(Tefon)isagoodexample.Oneofthe most chemically inert of all plastics is a linear polymer that is melt-processible with a ram extruder; however, complex parts can be made only by machining or sintering. When covalent bondingoccursbetweenpolymermolecules,anetwork(thermoset)moleculeisproduced. Once these cross-linking bonds have formed, they cannot be resoftened by heating.Until now, we have assumed that all synthetic polymers are plastics, but that is not true. Polymersdonotexhibitthestrengthassociatedwithplasticsuntilacertainnumberof repeatingunitshavepolymerizedineffect,theDP.Suchpolymersaresometimes referred as high polymers to distinguish them from polymers below the critical chain length, but in the plastics industry they are simply polymers. Polymers with network bonding or signifcant secondary forces have much lower DP values than simple linear polymers.Typically, polyamides (nylon) start gaining strength at a DP of 40, cellulosics at 60, and vinyls at 100. Polyamides have very strong hydrogen bonds, whereas vinyl molecules are secondary bonded by much weaker van der Waals forces. High polymers (plastics) usually have DP values around 600. Above the DP value, mechanical properties increase rapidly and eventually plateau. Polymer chain length greatly infuences viscosity. Flow resistance rises greatly at a DP of 1000, presumably because of increased molecular entanglements and secondary forces. The point at which viscosity behavior changes rapidly is also used to defne the minimum DP for a high polymer.2.5Amorphous PolymersCompletelyamorphouspolymerssuchasatacticpolystyreneexistaslong,randomly coiled, interpenetrating chains that are capable of forming stable, fow-restricting entan-glements at suffciently high molecular weight. In the melt state, thermal energy is suffcient for long segments of each polymer chain to move in random micro-Brownian motions. As the melt is cooled, a temperature eventually is reached at which all long-range segmental motions cease. This characteristic temperature is called the glass-transition temperature, or Tg, which varies widely with polymer structure (shown for several typically amorphous polymers in Table 2.8).In the glassy state (below Tg), the only molecular motions that can occur are short-range motionsofseveralcontiguouschainsegmentsormotionsofsubstituentgroups.These processesarecalledsecondarytransitions.Examplesofmain-chainsecondaryrelaxation processesaretwoproposedversionsofacrankshaft-rotationmodelwherebyseveral contiguous bonds are rotated around the main-chain axis, as illustrated in Figure 2.20.2.6Crystalline PolymersNopolymeriscompletelycrystalline.Eventhemostcrystallinepolymers,suchas high-density polyethylene, have lattice-defect regions that contain unordered, amorphous 29 Basic Concepts in Polyurethane Chemistry andTechnologyTABLE 2.8Glass-Transition Temperatures of Amorphous PolymersGlass-TransitionTemperature (Tg), C Repeat StructurePolymerPoly(2,6dimethyl1,4phenylene oxide)PolysulfonePolycarbonatePoly(methyl metha crylate)PolystyrenePoly(vinyl acetate)Cis1,4polyisoprenePolydimethylsiloxane100190220150105 72 73123CH3CH3CH2CH2CH2CH2CH3CH3CH3CH3CH3CH3CH3CH3nnnnC = OCHCHnnSi O [ ]OOOOO OOOOO CC = OCCSnCH3CH3[CH2 C = CH CH2]n[[[[[[]]]]]]7655443 32211FIGURE 2.20Proposed crankshaft secondary-relaxation motions of the polymer chain.30 Szychers Handbook of Polyurethanesmaterial.CrystallinepolymersmaythereforeexhibitbothaTgcorrespondingtolong-rangesegmentalmotionsintheamorphousregionsandacrystallinemelttemperature, Tm,atwhichlamellaearedestroyedandanamorphous,disorderedmeltisformed.For many polymers, Tg is approximately one-half to two-thirds of Tm (in K). Typical values of Tg and Tm for crystalline polymers are given in Table 2.9.Foragivenpolymer,theextentofcrystallizationattainedduringmeltprocessing depends on the rate of crystallization and the time during which melt temperatures are maintained. Above Tm, some polymers such as poly(ethylene terephthalate) and polycap-rolactoneexhibitingslowratesofcrystallizationcanbequenchedrapidlytoanamor-phousstate.Otherpolymershavingmuchmorerapidratesofcrystallization,suchas polyethylene, cannot be quenched quickly enough to prevent crystallization.Polymersdifferintheirrateofcrystallization.Inpolyurethanes,therateofcrystal-lizationdependsonthecrystallizationtemperature,asillustratedinFigure2.21.At Tm, crystallinelamellaearedestroyedasfastastheyareformedfromthemelt,and therefore the rate of crystallization is zero. Since, at Tg all large-scale segmental mobility requiredforchainfoldingcease,thecrystallizationrateisagainzero.Atintermediate temperature,Tmaxreachesanoptimumbalancebetweenchainmobilityandlamellar growth.The measurement of Tm and Tg can be performed in a differential scanning calorimeter (DSC). This method uses individual heaters to maintain identical temperatures for a sam-pleandareference.Thedifferentialpowerneededtomaintainbothsampleandrefer-ence isrecordedasafunctionoftemperature.Figure2.22showsanidealizedDSC thermogram.TABLE 2.9Thermal Transitions of Crystalline PolymersMeltingTemperature (Tm), CGlass-TransitionTemperature (Tg), C Repeat StructureOOOOOOOCOC CCCnnnnnnFFnnOCH2CH2OCCH2CH2CH2O(CH2)5CHOHCF2CF2(CH2)4 NH(CH2)6NH[854961135172195258265265327 none69 85 45125 60PolymerPoly(hexamethylene adipamide)PolytetrafuorpethylenePoly(ethylene terephthalate)Poly(vinyl alcohol)Poly(oxymethylene)Poly(vinylidene fuoride)Polyethylene (high density)Polycoprolactone[[[[[[[ ]]]]]]]]CH2CH231 Basic Concepts in Polyurethane Chemistry andTechnologyRelativeglass-transitiontemperature (Tg)Relative crystalline growth rateRelativemelttemperature (Tm)FIGURE 2.21Crystalline growth rate depends on crystallization temperature.Step change inspecic heatCpSlopeSlopeFusion peakCrystallizationpeakExothermHeat-flow rateEndothermMelttemperature (Tm)Glass-transitiontemperature (Tg)FIGURE 2.22Idealized DSC thermogram of a semicrystalline polymer.32 Szychers Handbook of Polyurethanes2.7Segmented PolymersOnereasonfortheexcellentphysicalpropertiesdisplayedbypolyurethaneelastomers istheirtendencytopackthemselvesintotight,stereoregularmolecularchainsa phenomenon referred to as crystallinity. A crystalline polymer is usually the opposite of an amorphous polymer; however, it is now known that polyurethane elastomers consist of a mixture of crystalline and amorphous domains, a state described as segmentation.Polymersciencecharacterizesintermolecularorderasthegeometricalrelationship between adjacent polymer molecules and their arrangement in the total mass of polymeric material. At present, we recognize three distinctly different states of intermolecular order: crystalline, amorphous, and segments, as shown in Figure 2.23.Whenpolymermoleculesarearrangedincompletelyrandom,intertwinedcoils,the unorderedstructureisknownastheamorphousstate.Whenpolymermoleculesareso neatlyarrangedthateachatomfallsintoprecisepositioninatightlypackedrepeating regularstructure,thishighlyorientedstructureisdescribedasthecrystallinestate. Polyurethane elastomers are a two-phase structure, where the hard segments separate to form discrete domains in a matrix of soft segments. This arrangement is termed segmented. Polyurethane elastomers are segmented polymers. The rigid segments act as bridges, and as fller particles, reinforcing the soft segment matrix.Anexampleofthistypeofstructureisinthesegmentedpolyurethaneelastomers obtainedfrompolyether[poly(oxytetramethylene)]andpolyester(acopolyester)-based elastomers incorporating MDI and extended with ethylene diamine. In the relaxed state, spatially separated hard and soft segments can be shown (by x-ray diffraction) to exist in the material. The hard segments are considered held together in discrete domains through theactionofvanderWaalsforcesandhydrogenbondinginteractions.Thisconceptis demonstrated in Figure 2.24, where the crystallization of soft segments is accomplished by elongating the polymer by 200%. Even greater crystallization is observed at 500% elonga-tion as shown in Figure 2.25.Segmented AmorphousCrystallineFIGURE 2.23States of intermolecular order.33 Basic Concepts in Polyurethane Chemistry andTechnologyOncecrystallinityhasbeenachieved,anadditionalphenomenonoccurswithinthe polyurethane chains: hydrogen bonding. Polyurethanes contain basic electronegative ions with semiavailable unshared pairs of valence electrons, such as nitrogen and oxygen ions. Nitrogenandoxygendonatethesevalenceelectronstothehydrogenatomsofadjacent molecules to produce hydrogen bonding between the two molecules. Hydrogen bonding RelaxedHard segmentFIGURE 2.24Strain-induced elongation of polyether soft segments in a segmented polyurethane elastomer by elongating to 200% elongation.StressedSoft segmentFIGURE 2.25Segmented polyurethane elastomer at 500% extension and placed in warm water at 80C.34 Szychers Handbook of Polyurethanesbetweenadjacentpolymerchainssignifcantlyincreasesthephysicalpropertiesofpoly-urethaneelastomers.Thisgivesrisetoathree-dimensionalvirtuallycross-linked molecular domain structure.Interchain attractive forces between rigid segments are far greater than those present inthesoftsegments,duetothehighconcentrationofpolargroupsandthepossibil-ity ofextensivehydrogenbonding.Hardsegmentssignifcantlyaffectmechanical properties,particularlymodulus,hardness,andtearstrength.Theperformanceof elastomersatelevatedtemperaturesisverydependentonthestructureoftherigid segmentsand their ability to remain associated at these temperatures. Rigid segments areconsideredtoresultfromcontributionsofthediisocyanateandchainextender components.The lateral effect of all the foregoing states and forces, particularly crystallinity and hydro-gen bonding, is to tie together or virtually cross-link the linear primary polyurethane chains. That is, the primary polyurethane chains are cross-linked in effect, but not in fact. Concurrently,ofcourse,thevirtuallinkagesalsolengthentheprimarypolyurethane chains.Theoverallconsequenceisalabileinfnitenetworkofpolymerchainsthatdis-plays the superfcial properties of a strong rubbery vulcanizate over a practical range of temperatures used.Virtualcross-linkingisaphenomenonthatisreversiblewithheatand,dependingon polymercomposition,withsolvation,thatoffersmanyattractiveprocessingalternatives forthermoplasticpolyurethanes.Thermalenergygreatenoughto(reversibly)break virtualcross-links,buttoolowtoappreciablydisruptthestrongercovalentchemical bonds that link the atoms in the primary polymer chains, can be used to extrude or mold thepolymers.Thesolventthatsolvatesthepolymerchains,reversiblyinsulatingthe virtualcross-links,carriestheprimarypolymer chainsintosolutionseparateand intact for such application as coatings or adhesives.Present views on the morphology and structure of segmented polyurethane polymers are as follows:Because the hard and soft blocks are partly incompatible with each other, the elas-tomers show a two-phase morphology, although there is signifcant level of mix-ing of the hard and soft blocks.Thesoftsegmentscontainingthemacroglycolformanamorphousmatrixin which the hard segments are dispersed.The hard domains containing the chain extender act as multifunctional cross-link sites or virtual cross-links, resulting in elastomeric behavior. Hydrogen bonding canoccurbetweenhardandsoftblocksalthoughtheextenttowhichthisis responsible for physical properties is not certain.Hydrogen bonding occurs between individual hard blocks giving rise to a three-dimensional molecular domain structure.Thesedomainsmaythemselvesbeinalarger,orderedarrangementincluding both soft and hard blocks, the hard blocks being built up in a transverse orienta-tion to their molecular axis leading, in cases, to the appearance of spherulites in the polymer.The morphology is unstable with respect to temperature and is dependent on both the chemical constitution and thermal history of the polymer.35 Basic Concepts in Polyurethane Chemistry andTechnologyIn summary, crystallinity refers to any highly ordered arrangement of atoms. Polymers can be visualized as long meandering chains of atoms that occasionally fnd themselves in patterns of highly oriented, close proximity.Thesesmallregionsofhighorderarereferredtoasspherulitesorcrystallites.Regions containingrandomlyorientedandwidelyspacedchainsaredescribedasbeingamor-phous. The degree of crystallinity is a measure of the frequency of these highly ordered crystallites. It is important to emphasize that no polymer is totally crystalline, and that an amorphous material can exhibit crystalline characteristics. In practice, polymers are clas-sifed as being amorphous, segmented, or crystalline according to their various degrees of crystallinity and their tendencies to form crystallites, as shown in Tables 2.10 and 2.11.Theprobabilitythatagivenpolymerwillexhibitcrystallinestructureisdetermined primarily by the chemical nature of the polymer chains. Polymer chains of low molecular weight,orthatpossesshighfexibility,favorcrystallinity;forexample,polyphenylene sulfdeiscomposedofmanyfexiblesulfdelinkagesaddingtoitstendencytoward crystallinity. Other fexible units include ether and ester linkages. Polyurethane elastomers are mixtures of crystalline and amorphous regions.TABLE 2.11Selected Characteristics of Crystalline and Amorphous PolymersCrystalline AmorphousHigh strength More pronounced glass-transition temperatureIncreased stiffness TransparencyIncreased density Reduced mold shrinkage (0.8 0.4 vs. 2.0 1.0)Resistance to organic solvents More uniform mold shrinkageOpacity Decreased dimensional response to temperature gradientsResistance to dynamic fatigue Low densityIncreased temperature range with reinforcement Good impact strengthPronounced melting point Melting rangeLow viscosity meltChemical resistanceTABLE 2.10 Common Polymers Classifed as Either CrystallineorAmorphousCrystalline AmorphousPolyacetal ABSPolyamide PolyamidePolybutylene terephthalate PolyacrylatePolyethylene terephthalate PolycarbonatePolyetherether ketone PolyetherimidePolyphenylene sulfde Polyphenylene oxidePolyethylene PolysulfonePolybutylene terephthalatePolypropyleneNylon 6/636 Szychers Handbook of PolyurethanesIfthepolymeriscapableofformingintermolecularbonds,andifthesebondsare advantageouslydistributedalongthepolymerchain,crystallinityismorelikely.These forces include hydrogen bonding, as in polyurethane. Homopolymers present more ideal conditions for crystalline structure than random copolymers, whose chemistry will result in an uneven distribution of intramolecular forces.Because crystallites consist of closely packed chains, it is correct to assume that polymer chains containing bulky side groups (as in polystyrene), or branching (as in low-density polyethylene), would inhibit close packing and interfere with the formation of crystallites. This is also known as steric hindrance.References1.Szycher,M.OpportunitiesinPolyurethaneElastomers:IndustrialandMedicalApplications, Technomic Seminar, Boston, MA, June 13, 1989.2.Gajewski, V. Chemical degradation of polyurethane, Rubber World, 1518, 202(6), September, 1990.3.Biermann, T.F. and Markovs, R.A. Modern Plastics Encyclopedia, 1995, B-46, 1995.373StructureProperty Relations in PolyurethanesMichael SzycherCONTENTS3.1Monomers, Reactions, and Structures .............................................................................. 413.1.1Polyols ........................................................................................................................ 473.1.1.1Polyethers ................................................................................................... 483.1.1.2Polyesters .................................................................................................... 483.1.1.3Hydrocarbons ............................................................................................ 503.1.1.4Triols and Higher-Functionality Polyols ............................................... 503.1.2Isocyanates ................................................................................................................ 503.1.2.1Prepolymers ............................................................................................... 513.1.2.2Quasiprepolymers ..................................................................................... 523.1.3Chain Extenders and Cross-Linking Agents ....................................................... 523.1.3.1Indigenous Cross-Linking Reactions ..................................................... 523.1.4Heteroblock Copolymers ........................................................................................ 533.1.5Submolecular Structure: Atoms and Functional Groups .................................. 553.1.5.1Processibility .............................................................................................. 553.1.5.2Mechanical Properties .............................................................................. 563.1.5.3Thermal Properties ................................................................................... 613.1.5.4Electrical Properties .................................................................................. 623.1.5.5Optical Stability ......................................................................................... 633.1.5.6Chemical Properties ................................................................................. 633.1.6Molecular Weight ..................................................................................................... 643.1.6.1Processibility .............................................................................................. 643.1.6.2Mechanical Properties .............................................................................. 653.1.6.3Thermal Stability ....................................................................................... 653.1.6.4Solubility .................................................................................................... 653.1.7Molecular Flexibility ............................................................................................... 653.1.7.1Effect of Structure on Molecular Flexibility .......................................... 663.1.7.2Processibility .............................................................................................. 673.1.7.3Mechanical Properties .............................................................................. 673.1.7.4Thermal Properties ................................................................................... 683.1.7.5Electrical Properties .................................................................................. 683.1.7.6Infrared Spectroscopy .............................................................................. 693.1.8Intermolecular Order: Crystallinity ...................................................................... 703.1.8.1Factors Affecting Crystallinity of Polyurethanes ................................ 703.1.8.2Effects of Crystallinity on Properties ..................................................... 713.1.9Intermolecular Attraction ....................................................................................... 723.1.9.1Total Intermolecular Attraction .............................................................. 7238 Szychers Handbook of PolyurethanesThe most specifc defnition of a polyurethane is any polymer with a plurality of carbamate (urethane) linkages. The terms polyurethane and polyisocyanates are general names for the segment of the plastics industry that manufactures use. The name polyurethane is almost universally applied to the fnal (polymeric) products of that segment of the industry.1Polyurethanes,alsocalledurethanes,areusedtoformabroadrangeofproducts, including fexible and rigid foams, elastomers, coatings, and adhesives. For our purposes, we will defne a polyurethane as a large family of polymers, based on the reaction products of an organic isocyanate with compounds containing a hydroxyl group.Polyurethanecharacteristicsarecontrolledbythemolecularstructureandinclude degrees of fexibility/rigidity, density (foamed or solid), cellular structure, hydrophilicity or hydrophobicity, processing characteristics, and end-use properties. The processing charac-teristic is controlled by their basic plastic nature, that is, whether the material is thermoplas-tic (linear molecular structure) or thermoset (cross-linked molecular structure). Figure 3.1 presents the universe of structureproperty relationships in polyurethane polymers.Thegeneralprinciplesofthestructurepropertyrelationshipcanbesummarizedas follows:1.Molecular weight. As the molecular weight increases, some properties such as ten-silestrength,meltingpoint,elongation,elasticity,glasstransitiontemperature, and so on increase up to a limiting value and then remain constant.2.Intermolecular forces. The weaker bonds such as hydrogen bonding, polarizability, dipole moments, and van der Waals forces may form, in addition to the primary chemical bonds, and these weaker bonds are affected by temperature and stress. If there is repulsion between like charges or bulky chains, or if there is high cross-link density, the effect of intermolecular forces will be reduced. Figure 3.2 shows the effect of hydrogen bonding in the hard segments of polyurethanes. Figure 3.1 is an idealized three-dimensional representation of hydrogen bonds in an aromatic polyurethane.3.Stiffnessofchain.Presenceofaromaticringsstiffensthepolymerchainsand causes a high melting point, hardness, and a decrease in elasticity. On the other 3.1.10Cross-Linking .......................................................................................