MATERIALS INFORMATION SERVICE

The Materials Information Service helps those

interested in improving their knowledge of

engineering materials and highlights the

national network of materials expertise.

This Profile is one of a series produced by the

Materials Information Service.

For advice relating to your particular materials

problem, you can contact the MIS at:

The Materials Information Service

The Institute of Materials, Minerals and Mining

Danum House, South Parade

Doncaster DN1 2DY

Tel: 01302 320 486

Fax: 01302 380 900

MIS Profiles are produced by IOM Communications Ltd, a wholly owned subsidiary of the Institute of Materials, Minerals & Mining

Ref: 3/97

Introduction

Polyurethanes, often known as PUs, are a very large and varied family of incrediblyversatile and useful engineering materials.

So what are PUs? Well they mean different things to different people.

To a chemist they are polymers which contain urethane groups (-NH-CO-O-)produced by the reaction of a polyol with an isocyanate, of which a little more later.

To an engineer they are materials which offer a combination of unique propertieswhich allow products to meet a range of demanding applications.

To an accountant they are materials which are very cost effective both to processand in use.

A PU may be either thermosetting or thermoplastic. These different characteristics,and their extraordinary physical properties, are explained by the different chemicalstructures which can exist in PUs.

PUs can be processed into elastomers, coatings, adhesives, binders, sealants andfoams for a host of applications in, amongst others, the shoe, clothing, furniture,domestic appliance, building, marine, electrical engineering and transport industries.Indeed PUs can be formulated for a bewildering array of applications for anincreasing number of industries.

PUs are perhaps the most versatile polymeric materials.

Whilst their major use is still in foams; utilising their strength, insulation, resilienceand comfort properties, PUs also fulfil the need when a component needs to exhibitboth the strength of plastics and the flexibility of rubbers, see Tables I and 2 below.

Since their discovery over 60 years ago PU chemistry has been developed andadapted to make the diverse range of products available today. The versatility ofPUs means that they can be formulated into a very hard solid or a soft elastomer, or

Brian Lees, RAPRA Technologies Ltd

Table 1 Table 2

something in-between, which allows the material to be custom-tailored to its’ enduse. Figure I shows the hardness spread, i.e. from 10¡ Shore A (softer than aneraser) to 95¡ Shore D (harder than a golf ball or bone). The Shore A equates, for allpractical purposes to IHRD (International Rubber Hardness Degrees), see Figure 1

Fig.1: Hardness Spread of Polyurethanes

Uniquely PUs can be tailored, by both chemistry and processing, to yield products ina wide variety of forms and which allow the processor to control the nature and theproperties of the end product. For example thermoplastic polyurethanes (TPUs) areformulated for use in applications ranging through hose and cable sheathing, films

and sheet, catheters, gears, bushes and bearings and such specialised areas asmining cables and optical fibres.

Why use PolyurethanesPUs are used because they combine unique engineering properties with costeffectiveness.

Although PUs may initially be more expensive than other polymers, a partmanufactured from PU could last two, four or even ten times longer than a partmade from another material.

A further area of cost effectiveness is tooling, since moulds for PUs can be madefrom almost anything which does not retain moisture and can tolerate the modestheat and pressure of the moulding operation. Tool materials employed include therelatively inexpensive epoxies, glass reinforced plastics (GRP) and aluminium.

All PUs share high abrasion resistance, toughness, high impact and tear resistance,and high load bearing ability

The exceptional thermal insulation and structural properties of PU rigid foams offerparticular advantages for domestic appliances, homes and other buildings.

The strength and wear resistant properties of PU flexible foams have resulted in theirdominance of the upholstery and mattress markets.

The outstanding properties of PU adhesives have resulted in widespread use forrigorous engineering applications, particularly in the automotive sector where highstrength/weight ratios and reliability are important.

PUs can form tough, hard, chemical resistant coatings which have a high degree ofenvironmental resistance to oxygen and ozone attack.

PU elastomers can be formulated to far outwear any other rubber or elastomer.Generally PU elastomers will retain constant properties over a wider temperaturerange than other elastomers.

The Thermoplastic Polyurethanes (TPUs) offer excellent tensile strength andelongation characteristics. TPUs are also known for their excellent abrasionresistance, high resilience, good tear resistance, flexibility at low temperatures andlow compression.

PUs do have their disadvantages. The polyester types are susceptible to hydrolyticattack above ambient temperatures. The susceptibility to hydrolysis is even moreapparent in both acid and alkaline solutions and may ultimately result indisintegration. Solvent resistance is mixed, poor in polar organic solvents, but verygood with aliphatic hydrocarbons, fuels and oils.Some TPUs have a limited hardness range and have been known to stress crack incable jacketing applications when in contact with water at ambient temperatures.

The Making of a PolyurethaneA PU is made by mixing together the ingredient chemicals (isocyanate and polyolsee later) in pre-determined proportions, which then react to form the polymer.Uniquely, PUs utilise simultaneous polymerisation and shaping of the part.

The production of consistent end products depends on mixing, in precise ratio, theingredient chemicals and maintenance of the appropriate processing temperatures.As the liquid isocyanate and polyol react to form the PU, the liquid mix becomesincreasingly viscous eventually forming a solid mass. The reaction is exothermic andtherefore heat is involved.

Other ingredients will be included in the polyol blend, for example the catalyst whichcontrols the rate at which the liquid mixture reacts to become solid.

There are no hard and fast rules for obtaining the optimum PU end product, successis due to good formulation selection with well chosen and appropriate processingparameters and mould geometry. The process by which liquid polymers areconverted to elastomeric or glassy solids is fundamental to the manufacture of PUproducts.

Selection of a PolyurethaneThere are a number of steps:

Consider the requirements which the application will demand of the PU with respectto chemical and physical properties.

Based upon an understanding of what controls these properties select a fewcandidate PU systems. The properties of a PU are largely controlled by the chemicalnature of the system and how it is processed so it is prudent to consult specialistsuppliers and processors at this stage.

Establish that the converter can process the proposed system on existing plant. Theimportant processing characteristics of the system will include viscosity, pot life,reactive mix ratio control, demould time and process temperature.

Undertake preliminary tests, make prototypes, conduct field trials and obtaincustomer approval.

Raw Materials

IsocyanatesMany commercial grades of isocyanates used for making PUs are aromatic innature. Each isocyanate will give different properties to the end product, requiringdifferent curing systems and, in most cases, different processing systems. Animportant property of an isocyanate is its functionality, i.e. the number of isocyanategroups (-NCO) per molecule. For cross linked PU applications the averagefunctionality of the isocyanate is usually a little over two. The higher functionalityisocyanates are used for special applications. When a di-functional isocyanate is

used with a di-functional polyol a long linear PU molecule for elastomericapplications is formed.

The common isocynates used to make PUs are shown in Figure 2.

Fig.2: Typical Isocyanates

Many PU products, such as flexible foams, are made with toluene diisocyanate(TDI).

The other main isocyanate used is methylene diphenyl diisocyanate (MDI), the mostwidely used MDI product is ’Crude MDI’ with a functionality of about 2.8.

A monomeric derivative of MDI, called ’Pure MDI’, with a functionality of 2 can bedistilled from Crude MDI. ’Pure MDI’ is a solid at ambient temperatures and is usuallymodified to a liquid form for ease of handling.

The modified isocyanates and isocyanate pre-polymers with special reactivitycharacteristics are used when it is impractical to use the more conventionalisocyanates. Such derivatives are formed from the reaction of the isocyanate withcompounds such as amines, diols or triols

PolyolsThere are two main types of polyols used in the PU industry, polyetheis andpolyesters.

PolyethersThe more widely used polyethers have a relatively low molecular weight in the rangeof 500 to 3000 and are manufactured from propylene oxide (PO) and ethylene oxide(EO).

PO is the major constituent of the polyol, whereas EO is only included in smallamounts to modify the properties of the polyol.

The functionality of the polyether polyol (number of active hydroxyl groups permolecule) can be varied and is normally 2 for elastomers, approximately 3 for flexiblefoams and up to 6 or more for rigid foams.

PolyestersThe polyester polyols are typically produced by the condensation reaction of a diolsuch as ethylene glycol with a dicarboxylic acid.

Polyester polyols tend to be more expensive, are usually more viscous and difficultto handle but develop PUs with superior tensile, abrasion, flexing and oil resistanceproperties. Consequently they are used to make PUs for more demandingapplications.

A disadvantage of polyester based Pus is their lower hydrolysis resistance.

Typical polyols used are shown in Figure 3.

Fig.3: Typical Polyols

Pre polymersIn a pre polymer system, the polyol and isocyanate (either a polyester or a polyether)are reacted to give a pre polymer which may be either a liquid or a waxy solid.

The reactant ratios used ensure the pre polymer contains isocyanate groups at thechain ends. The pre polymer can, when required, be chain extended to give a highmolecular weight cross linked product.

Common Additives

Catalysts have a key role in PU production being required to maintain a balancebetween the reaction of the isocyanate and polyol.The combination of very complex PU chemistry and diverse processing andmoulding conditions make great demands of the catalyst. Its main function is toexploit the diverse reactions to create a product with the desired properties.

There are two main classes of catalyst used in PU production.

Organometallics are used to accelerate the reaction and formation of urethanelinkages and hence promote rapid curing.

The most popular organometallic catalysts are tinbutyltin dilaurate and stannousoctoate. Tin catalysts are used to catalyse micro cellular elastomers and reactioninjected moulded (RIM) systems.

Amines are the other major class of catalysts widely used in the making of PUfoams. Some amine catalysts promote crosslinking whilst others assist in controllingthe foam’s cell structure.

Chain extenders are reactive low molecular weight di-functional compounds suchas hydroxyl amines, glycols or diamines and are used to influence the end propertiesof the PU.

The chain-extender reacts with the isocyanate to affect the hard/soft segmentrelationship and therefore the modulus and glass transition temperature (Tg) of thepolymer. The Tg provides a measure of the polymer’s softening point and someindication of the safe upper limit of its working temperature range.

Blowing Agents. Cellular or foamed Pus are manufactured by using blowing agentsto form gas bubbles in the reaction mixture as it polymerises. They are usually lowboiling point liquids which are volatilised by the heat generated by the exothermicreaction between the isocyanate and polyol.

Rigid foams yield sufficient exothermic heat from the reaction to allow foamexpansion in association with the blowing agent.

Flexible PU foams are usually blown by the 002 generated by the reaction of waterand isocyanate (or in association with methylene chloride). Blowing of the foam canalso be accomplished by the direct injection of air or gas into the foam.Chloroflurocarbons (CFCs) have been used as blowing agents but their effectson the ozone layer have led to restrictions of their use and they are being replacedby more environmentally acceptable alternatives such as pentane.

Flame retardants. Certain end-use sectors now take greater account of possible’worst scenarios’ in materials selection.

These considerations will include the effects of smoke and toxic decompositionproducts on people, property and equipment. PU foams used in furniture are anexample which spring to mind. Fire retardancy can be achieved by the addition offluorine, chlorine, bromine or iodine compounds to the polyol. Solid compounds suchas melamine and aluminium trihydrate are also important flame retardants.

Materials and products are continuously evolving and developing and the trends arenow to lower smoke and fume generation, and in the much longer term lowertoxicity’. There is an increasing commitment to tougher requirements and in certain

sectors of the PU industry this has led to the development of low or halogen freesystems.

Pigments. Many PUs tend to yellow in the light albeit without any adverse affect onthe physical properties. To produce coloured PU pigmented pastes are added to thepolyol formulation. The pigments, both inorganic and organic, improve the lightstability of PU products.

Fillers. As with other polymers the use of fillers in PUs will yield products withmodified performance. Calcium carbonate and glass fibres are most commonly used.The former primarily to make cheaper formulations, the latter are of growing interestin reaction injection moulding (RIM) technology (see later).

Basic Polyurethane Chemistry

The simplest PU is linear in which the hydroxyl compound and the nitrogencompound each have a functionality of two. This can be represented by thefollowing:-

The isocyanate can react with different chemical groups, so the final properties ofthe polymer will vary according to the reaction route taken.

Therefore the formulation of a PU must take into account every possible reactiveconstituent. PUs may have a very widely varying structure depending on the typeof isocyanate and the type of reactive hydrogen components present in theformulation.

The presence or otherwise of the various groups along the urethane linkage willcontrol the end properties of the polymer.

The curing of a PU can be regarded as the formation of a network, also calledcrosslinking, the extent or degree of cure is often expressed as the crosslink density.

The extent of cross linking may vary and will be reflected in the final properties of thePU, ranging from longer, linear chains of flexible elastomers and foams to the rigid,heavily cross linked polymers. Thermoplastic polyurethanes (TPUs) are a particularcase. They are effectively co-polymers of a hard PU and a very flexible PU in whichmicrophase segregation of the hard phase occurs, see Figure 4.

Isocyanate+Polyol = Polyurethane

Fig.4: Microphase separation of the hard segment

The clusters of hard PU, shown as thicker lines, act as ’pseudo-cross links’ and allowthe material to behave as an elastomer. When the temperature is raised the clustersdisassociate and the material can be made to flow, when subsequently cooled theclusters reform and the material again exhibits elastomeric properties. Thus thesematerials show elastomeric behaviour at room temperature, but can be processed asthermoplastics. Hence the name of the material class — thermoplastic urethaneelastomer.

Processing Polyurethanes

Thermoset PUsThere are two main processes for thermoset PUs.

Many PUs, particularly the foamed products, utilise a one step (one-shot)

process; a two stage process, based on the pre-polymer system, is used in themanufacture of elastomers, paints, adhesives and sealants.

One step processIn this system the polyurethane is manufactured from liquid isocyanate and polyolusing a purpose designed machine. The blend instantly reacts as the mixture isinjected into the mould or, for coatings, sprayed onto the product. The high reactivityof the system causes a high exotherm and consequently rapid curing.

A dispensing machine monitors and controls the temperature of the constituents asrequired, accurately meters them at selected ratios into a mixing chamber wherethey are intimately blended and a pre-selected weight of liquid mix is dispensed tothe mould or spray head.

The output rate of the machine can be adjusted with respect to the weight of mixedliquid dispensed per second. The dispensing machine must be accurately calibratedand the processing conditions and temperatures must be kept within defined limits.

The process makes particular demands of the operators, who must be trained andexpert both in the setting up and use of machinery and the correct storage andhandling of the chemicals.

For example once a container of polyol or isocyanate is opened care must be takento prevent moisture pick-up. The recommended practice is to purge the containerwith nitrogen each time it is opened and resealed.

Two step process (pre polymer system)The PU is prepared from a pre polymer, where the polyol blend is reacted with anexcess of an isocyanate so that full polymerisation cannot occur, the reaction beingcompleted when the pre polymer reacts with an additional polyol and chain extendersee Figure 5.

Fig.5: Processing of a Prepolymer

The mixing can be done by hand, in low pressure mixer/dispensers, and in reactioninjection moulding (RIM) machines. In the latter operation the degassing operation isnot required.

Production of PU products

FoamsPU foams fall into two distinct categories flexible and rigid.

There are two techniques for the production of flexible foam depending on theproduct

Moulded Foam products such as car seats, furniture and packaging are produced ina one step process by the reaction of the raw materials in a mould using high or lowpressure dispensing equipment. Two types of process are used: hot cure foam andcold cure foam.

In the hot cure process external heat is applied to the mould after filling so that thereis sufficient cure of the foam moulding to allow early release from the mould. For thecold cure process lower operating temperatures are used.

Although the cycle times are slower there is greater freedom in the choice ofmaterials for mould construction.

Continuous slabstock is again a one step process using high capacity mixing anddispensing equipment. The handling of the large quantities of foam requiresappropriate automated cutting and conveying systems.

Rigid foam production. The continuous production of rigid foam uses equipmentvery similar to that for producing flexible slabstock foam.

Large quantities of block rigid foam can be manufactured by the proportioning of theingredients to a mixing head and then dispensing onto a moving conveyor belt. Therigid foam is then cut to the desired shape and size.

This method is also used for the continuous production of laminated or sandwich-panels where the reaction mixture is dispensed onto the lower surface of the panelfacing and rises to the upper surface. The panel thickness is controlled by the settingof the distance between the lower and upper conveyor.

For constructional panels of complicated design and varying dimensions thediscontinuous production route is used. The foam is injected into a cavity mould inwhich the panel facings have been placed. The reaction mixture must be preciselydispensed so that the cavity is fully, but not over filled, and, in order to withstand thefoam pressure, mould clamps are used.

A similar processing principle is applied when PU foam is used as insulation forrefrigerators and freezers. The reactive mixture is injected into the cavity of a pre-assembled outer casting and inner liner of a cabinet supported in a jig.

Much insulation work is carried out by the use of two component spray equipment.Cosmetically, sprayed rigid PU foam is not generally very good, but the process isvery useful when insulating difficult and intricate surfaces. Cavity wall insulation iscarried out by the reactive mixture being pumped in to the cavity to be filled. Thefoam adheres to the surfaces as it reacts and cures.

Reaction injection moulding (RIM) uses high pressure dispensing machines and isreally a specialised form of casting. It involves the high pressure mixing of the twoliquid components which are then injected into a mould see Figure 6.

Fig.6: The RIM process

The RIM process allows moulded articles to be produced at rates comparable toconventional injection moulding. The process produces a range of self skinnedmouldings from both flexible and rigid Pus.

Depending on the reactivity of the PU system and part dimensions, the part can beremoved from the mould within I to 10 minutes. The moulding pressures are low,typically less than 100 psi, which means lower tooling costs.

RIM machinery can allow fillers to be mixed into the reacting system, yieldingproducts with improved physical properties called Reinforced Reaction InjectionMoulding (RRIM).

Cast PU Elastomers are processed in liquid form and, for best results, should bedegassed to remove any moisture or air. This is best done by heating to theprocessing temperature and applying a vacuum.

Mixing may be either batch or continuous followed immediately by pouring ordispensing into a cold or heated moulds. The moulds are left at the appropriatetemperature until the part cures to a solid end product which is discharged andtrimmed as necessary.

The flow of the reactive chemical exerts very little pressure on the mould so they canbe made from inexpensive materials e.g. glass reinforced plastic (GRP). Usuallyafter 3-8 minutes the moulding can be removed. However a post moulding cure isoften required to develop optimum levels of physical properties. A typical cycle is40-50 minutes at 110¡C.

If a cast PU is not sufficiently cured, lower impact, flexural strength and heatdistortion temperature may result.

The casting of PUs is a batch process, small and large numbers can be readilyaccommodated and the castings from one gram to one hundred kilos may be madeby the same process. The solid end products can range from about a 10 Shore A to90 Shore D hardness.

PU millable gums are processed like conventional rubbers using a Banbury mixerand mill and are cured with peroxide or sulphur.

Both polyether and polyester derived PUs are available, their products are formed bythe usual elastomer techniques of extrusion, compression and transfer moulding.

ThermoplastIc polyurethanes – TPUs are supplied in granules or pellets forprocessing by all of the usual thermoplastic processing techniques such as injectionmoulding, extrusion (for tubing, film and profiles) and vacuum forming.

TPUs are usually supplied in a sealed moisture-proof bag since they are, to somedegree, hygroscopic and will deteriorate if left exposed to the atmosphere. Ideally,dried TPUs should have a maximum level of 0.02 wt% water, and must be kept

below 0.05 wt% water, at which level the reaction with the water starts. TPUs thathave been processed whilst wet are total scrap and cannot be recovered.

Of the two major types of TPUs available, polyesters and polyethers, a polyetherbased TPU is more sensitive to overheating during processing. Typical injection andextrusion temperatures are between 180-230¡C.

Health and Safety and Polyurethanes

In addition to the usual care associated with the use of any production machineryparticular precautions must be taken in storage, handling and processing of PUchemicals.

Isocyanates present a range of potential hazards

InhalationIsocyanates will irritate the entire respiratory tract and excessive concentrationscould cause difficulty in breathing. It is therefore important to avoid breathing thevapour of any isocyanate and control limits and threshold limit values (TLVs) shouldbe adhered to at all times. The level of hazard varies with different isocyanates,according to their vapour pressure and the temperature at which they are handled,the hazard increasing as the temperature rises. People suffering from asthma andbronchitis, or from an allergic reaction should not work with, or near, isocyanates.

Eye irritationIsocyanate vapour also causes discomfort to the eyes. Splashes of liquid isocyanateto the eyes will cause mild to severe irritation and should be treated immediatelyusing the contents of a sterile eye wash bottle.

Skin effectsAbsorption by the skin is low and slow, but will cause irritation and may causetanning. Soap and water are usually sufficient to remove small amounts and sincethe water will react with the isocyanate immediately, the irritant will be neutralised.

With pre polymers the isocyanate vapour pressure is reduced by the partial reactionwith the polyol, and therefore they are less hazardous.

It follows that the storage processing and handling of isocyanates and pre polymersmust be conducted under proper conditions.

Processing areas must be fitted with ventilation systems and personnel should betrained in handling drums of isocyanate, particularly when drums need to be heatedin order to melt the contents. For bulk storage, isocyanates should not be handled inopen vessels. Although isocyanates are not particularly flammable, it isrecommended that bulk storage should be in a well ventilated area and separatedfrom the work place.

The hazards to health from skin contact may be avoided by the use of proper safetyequipment, masks, protective clothing, eye protection and impermeable gloves.

Where to go for advice

Brian Lees,Polyurethanes Business ManagerRapra Technology LimitedShawbury, Shrewsbury,Shropshire SY44NRTel: 01939 250383 Fax: 01939 251118e mail: info¤ rapra.net

Material SuppliersBaxenden Chemicals LtdParagon WorksAccrington, Lancashire BB5 2SLTel: 01254872278 Fax: 01254871247

Bayer pieBayer HouseStrawberry HillNewbury, Berkshire RG14 IJATel: 01635 39000 Fax: 01635 563393

Brian Jones & Associates LtdFluorocarbon BuildingCaxton HouseHertford SG137NHTel: 01992 553065 Fax: 01992 551873

Dow Chemical Co. LtdLakeside HouseStockley ParkUxbridge, Middlesex UBII IBETel: 0181 848 8688 Fax: 0181 848 5400

Elastogran UK LtdAlfreton Trading EstateWinsey Way, SomercotesDerby DE55 4NLTel: 01773 607161 Fax: 01773 602089

Hyperlast LtdStation RoadBirch ValeStockport, Cheshire SK12 5BR

Tel: 01663746518 Fax: 01663 746605

ICI PolyurethanesHitchen LaneShepton Mallet, Somerset BA4 5TZTel: 01749 340217 Fax: 01749 344221

Polymed LtdUnit I, Clos MenterExcelsior Industrial EstateWestern Avenue, Cardiff CF4 3ATTel: 01222 521234 Fax: 01222 521221

Equipment SuppliersBeamech Group LtdTenax RoadTrafford Park, Manchester M17 IJTTel: 0161 843 0316 Fax: 0161 873 7718

Cannon VikingHilton HouseChester RoadStockport, Cheshire SK7 5NUTel: 0161 456 0095 Fax: 0161 456 2001

Edulan A/S UK40A Osbourne StreetBredburyStockport, Cheshire SK6 2BTTel: 0161 406 7278 Fax: 0161 406 7281

ESU CannonBrookfieldGlossopDerbyshire SK13 9LBTel: 01457 863511 Fax: 01457 867820

Liquid Controls LtdStewarts RoadWellingboroughNorthamptonshire NN84RJTel: 01933 277571 Fax: 01933 440273

OMS (UK) LtdOmnitech HouseBamford Business ParkWhitehill Industrial EstateStockport, Cheshire SK4 IPLTel: 0161 480 8100 Fax: 0161 480 0080

Recommended Books and Directories

The ICI Polyurethanes Bookby G Woods, WileyISBN 0-471-926582

Polyurethane Handbook 2nd Edition -by Gunter Oertel,Hanser/Gardner Publications IncISBN 3-446-17198-3

A Code of Practice for Polyurethanes Flexible Foam Manufacture Toxicity and SafeHandling of Di-isocy anates and Ancillary Chemicals, BRMAISBN 0-903110-29-6

Rubbicana - A Directory and Buyers Guide to Europe's Rubber and PolyurethanesIndustriesRapra Technology LtdTel: 01939 250383 Fax: 01939 251118e mail: info¤ rapra.net

Global Polyurethanes Industry HandbookGrain Communications LtdTel: 0171 457 1400 Fax: 0171 457 1440

Courses

Rapra Technology LtdTel: 01939 250383 Fax: 01939 251118website: http://www.rapra.net

IPMELoughborough UniversityLoughborough, Leics, LEII 3TIJTel: 01509223340 Fax: 01509 223949