High prepolymer viscosity can lead to problems in processing and final productquality.

Overheated Prepolymer

Because prepolymers are generally heated for processing, it is possibleto degrade them by applying too much heat. Normally, the isocyanate(NCO) groups within a prepolymer react with a curative to form anelastomer. When the prepolymer has been overheated, however, theseNCO groups react with each other instead, leading to an increase inprepolymer viscosity. In severe cases, the prepolymer can actually gelto a solid mass. If overheated prepolymer is used in processing, softparts will result—even if the prepolymer has not degraded to the pointof gelation.

Low Processing Temperature

The viscosity of any liquid is a function of temperature; the higher thetemperature, the lower the viscosity. All prepolymers haverecommended processing temperatures which differ from one productto another. When the prepolymer is at too low a temperature, it will bedifficult or impossible to process. Polyester prepolymers are especiallylikely to be under-heated because they require higher processingtemperatures than do polyether prepolymers of similar hardness.

Use of Inappropriate Prepolymer Type

Polyether prepolymers are typically lower in viscosity than polyesterprepolymers. Similarly, different prepolymer production methods yieldproducts with different viscosity characteristics.

Adhere to Melting Guidelines

To avoid overheating a prepolymer, adhere to the melting time andtemperature guidelines provided by the prepolymer supplier. Althoughprepolymers degrade at different rates upon heating, minimizing heatexposure is always recommended. To monitor this, keep a log of theprepolymer heat history. Use of a band heater is not recommendedbecause heating is too localized. Also, if possible, the prepolymerdrum should be rolled while being heated.

Check the Prepolymer Temperature with a Thermocouple

To avoid low processing temperatures, check the temperature of theprepolymer with a reliable thermocouple to ensure that the preheatoven is working properly. Do not assume that the prepolymer is at thesame temperature as the setting of the oven. There may be localizedhot or cold spots in the oven, or the prepolymer may not have been inthe oven long enough.

Choose the Most Appropriate Prepolymer Type

To obtain lower viscosity, choose polyether prepolymers overpolyester prepolymers, if the application allows. Airthane®prepolymers have lower viscosities than conventionally madeprepolymers with similar percent NCOs. Airthane prepolymers alsoprovide longer pot life. Discuss your requirements with your supplierto determine the best prepolymer to meet your needs.

All prepolymers should be degassed prior to being mixed with the curative, and iftime permits, the prepolymer/curative mixture should be degassed again prior to beingpoured into the mold. This will help prevent potential problems caused by smallamounts of nitrogen which may have become entrained in the prepolymer duringproduction and drum filling.

High Prepolymer Viscosity

It is more difficult to degas a high-viscosity prepolymer than a low-viscosity prepolymer. High prepolymer viscosity can be a function ofthe specific prepolymer in use, or it may be because the prepolymerhas been degraded. See the High Prepolymer Viscosity Problem forfurther details on the causes of high viscosity.

Insufficient Vacuum

If degassing seems to be progressing at a slow rate for an extendedperiod of time, the vacuum system may be faulty. With insufficientvacuum, the pressure may not be low enough to allow entrained air toescape. Insufficient vacuum may be caused by a weak or worn vacuumpump or a leak in the vacuum lines.

No Degassing Agent

Degassing aids are additives which lower the surface tension of theprepolymer, thereby allowing for easier degassing. Without the use ofa degassing aid, degassing problems could be noted.

Check the Prepolymer Temperature and Heat History

The higher the prepolymer temperature, the easier it is to degas. Ensurethat the prepolymer is at the correct processing temperature whendegassing. Also check that the prepolymer has not been previouslyoverheated, which will result in increased viscosity. See the HighPrepolymer Viscosity Problem for further details on high prepolymerviscosity.

Check the Gauge on the Vacuum Pump

In order to properly degas raw materials, the vacuum gauge on thedegassing system should read at least 27 inches mercury (Hg). Withless vacuum than this, it will take an excessively long time to degas.

Add a Degassing Agent

Several commercial degassing aids are available from varioussuppliers. Two degassing aids used commonly are SAG-47 fromWitco, and X-Air from Ciba-Geigy. Use levels are typicallyrecommended at 0.2 parts per hundred parts prepolymer.

All prepolymer/curative combinations have a specific pot life and gel time. Pot life isgenerally defined as the amount of time available after mixing to work with thesystem before it becomes too thick to pour. Gel time is typically defined as the time ittakes for the system to cease exhibiting any liquid-like characteristics. Generally, thehigher the percent NCO of the prepolymer, the shorter the pot life and gel time. Potlife and gel time are also both influenced by temperature.

Degraded Prepolymer

A degraded prepolymer will usually exhibit a reduction in percentNCO. This reduction will shorten the apparent pot life and possiblylengthen the demold time.

Degraded Curative

Some curatives, such as Mboca, can be degraded with exposure to toomuch heat. When Mboca begins to decompose, a significantly fasterreaction with the prepolymer results.

Improper Catalyst Addition

Catalysts are often added to decrease the pot life and gel time of asystem to achieve increased productivity. Typical catalysts used forthis purpose are amine, acid or tin based. (See Recommended CatalystUse Levels for a list of recommended catalysts and their use levels.)Catalysts are added at very small use levels, and it can be very difficultto monitor an exact amount. In addition, since they are used at suchlow levels, a small amount of catalyst will last a long time. Somecatalysts, especially tin-based products, can become less active overtime and can lose their effectiveness.

Incorrect Temperatures

Higher reactant temperatures will cause an increased reactivity ratebetween the prepolymer and curative, thereby reducing the system potlife and gel time. The converse is also true. With smaller parts, moldtemperatures that are significantly different from the mix temperaturecan cause reactivity problems by quickly adding or removing too muchheat from the system.

Minimize the System Heat History Before Processing

To eliminate degradation of the prepolymer and/or curative due toexcessive exposure to heat, follow the supplier's processingtemperature guidelines for each product.

Check the Catalyst Addition

Catalysts, if they are used, can have a significant effect on thereactivity of the system. Some catalysts, especially tin-based products,can become less active over time. If reduced catalyst activity issuspected, a new lot of catalyst should be tried. To assist in monitoringcatalyst addition amounts, consider mixing the catalyst in a curative orplasticizer, such as Benzoflex.

Try a New Lot of Catalyst

Monitor Batch Temperatures Closely

Changes in batch temperature and mold temperature will affectreactivity. Monitor these temperatures carefully. Also, do not relysolely on oven temperature gauges—always have a secondary methodto determine temperatures.

Recommended Catalysts and Their Use Levels

Catalyst Type System RecommendationTypical Use Level

(Parts per Hundred Curative

Organic AcidsAzealic AcidOxalic Acid

Used with diamines 0.3–0.6 parts

Tertiary AminesDabco 33-LV® catalystDabco® S-25 catalyst

Used with diamines or diols 0.3–0.6 parts

TinsDabco T-12 catalystDabco 131 catalyst

Used with diols 0.1–0.5 parts

Upon demolding, there may be large voids either within the part or on the surface ofthe part. These voids are visible with the naked eye, and they usually occur at thin-walled sections, corners and other transition areas.

Air Entrapment During Pour

As the batch is poured into the mold, it displaces air from the mold. Ifthe air becomes trapped within various cavities and has no mechanismto escape, it will form a void. This can happen in thin-walled areas ifthe prepolymer viscosity is too high.

Short Pot Life

As the batch is poured into the mold and it displaces the air in themold, some air will become trapped there temporarily. This air willthen begin to work its way to the upper surface of the part. Thisprocess takes a finite amount of time, and if the pot life of the system istoo short, the air bubbles will not have a sufficient amount of time toescape.

Leaking Mold

If some of the material poured into the mold leaks out, a void will format the leaking area. Leaking is more likely to occur in systems with along pot life and low viscosity.

Redesign the Mold/Vents

A proper mold design with adequate venting will generally preventproblems with large voids.

Use Lower Viscosity Prepolymers

Using a lower viscosity prepolymer will make it easier for the air toescape from the mold. Using a polyether prepolymer instead of apolyester prepolymer will also decrease voids, if the application allowsfor this substitution. In addition, using a system with a lower viscosityand longer pot life, such as a system containing an Airthane®prepolymer, will help reduce void formation.

Tilt Mold and Pour Down Its Side to Minimize Splashing

Often, tilting the mold upon filling, especially in the case of largerollers, will allow air to escape more easily. Also, pouring down theside of the mold will decrease splashing, another source for air bubblesin the system.

Add a Degassing Agent

The addition of a degassing aid will reduce the surface tension of thesystem. With higher viscosity materials, this is often enough additionalhelp to reduce the formation of voids. Typical degassing aids are SAG-47 by Witco, and X-Air by Ciba-Geigy. Use levels are typicallyrecommended at 0.2 parts per hundred parts prepolymer.

Sometimes small bubbles are apparent throughout parts. These bubbles may or maynot be localized.

Moisture Contamination of Components

Moisture (H2O) will react with the isocyanate group in a prepolymer toproduce carbon dioxide gas (CO2). CO2 bubbles will manifestthemselves as small bubbles in the part. Moisture can come from manysources, including the prepolymer, curative, catalyst, adhesive, primeror additional additives. Of primary concern are polyol curatives (suchas 1,4-butanediol, or BDO) and pigments which are hygroscopic,meaning they readily absorb atmospheric moisture. With moisturecontamination of the raw materials, the bubbles are seen throughoutthe part, and when you have moisture contamination of the adhesive orprimer, the bubbles are generally seen only at the bond line.

Moisture Contamination of the Mold

Molds which have not been heated for a sufficient time above theboiling point of water could have atmospheric moisture on theirsurface. Molds may also have water condensation in humidenvironments if they are allowed to cool down and are not completelyreheated. Also, the improper application of water-based mold releaseagents can lead to water being on the mold. With water contaminationof the mold, bubbles usually appear on the surface of the part,especially on the lower horizontal surfaces.

Inadequate Degassing

If the prepolymer is not degassed prior to mixing, and the batch is notdegassed prior to pouring, small entrapped bubbles will be the result.

Poor Pouring Technique

Considerable splashing during pouring will introduce bubbles into thebatch. This is especially an issue with low-viscosity systems wheresplashing is more pronounced.

Failed Seal in the Processing Machine

When using a meter-mix dispensing machine, degassing usually occursin the component storage tanks. With this system, materials are

pumped from the storage tanks directly to the mixing head, where theyare mixed, so there is little opportunity for incorporation of air into themixture. However, seals in the mix head can fail, which can introduceair into the stream, and thereby negate the effect of the originaldegassing.

Store All Products Under Dry Nitrogen

Storing as many materials under dry nitrogen as possible will reducethe likelihood of the absorption of atmospheric moisture. With someproducts, there is also the added benefit of potentially reducingoxidation due to the presence of air.

Degas All Components Before and After Mixing

Degassing all components prior to and after mixing will remove anyentrained air in the components and the mixed system. Also, degassingraw materials will distill out any water in the material, which willnegate the NCO/H2O reaction which forms CO2.

Ensure that the Mold is Properly Heated

Heating the mold above the boiling point of water [212°F (100°C)]will remove moisture that has condensed on the mold, as well as anyremnants of water-based mold release left on the mold.

Maintain the Mix Head Seals

A regular maintenance program for machinery is always a wisepractice. Consult your machine supplier for their maintenancerecommendations.

Molds usually allow for the shrinkage which occurs in an elastomer when theprepolymer and curative react. Typical shrinkage rates are 1-2% under normalprocessing conditions. Excessive shrinkage, however, will cause warping or unnaturalstresses in the part. Cracking, which can also result, will vary from small internalcracks, to more obvious surface cracks.

Incompatible Resin and Mold Temperatures

As a prepolymer/curative mixture begins to react, it generates heatwhich increases the temperature of the mixture. If there is a substantialdifference between the reaction temperature and the mold temperature,there will be stresses placed on the system as it tries to shrink orexpand during the reaction. The critical time for these twotemperatures to be approximately in balance is when the mixture goesthrough its "green stage," which is when it changes from a liquid to asolid. Too low of a mold temperature relative to the mixturetemperature will cause the gelling reactants to shrink excessively,sometimes to the point of cracking. Too high of a mold temperature isgenerally less of an issue, though this could lead to excessive warping.

Localized Temperature Variation in the Mold

Localized hot and cold spots within a mold can also cause internalstresses which may lead to shrinkage, cracking and warping.

Low Stoichiometry

Low stoichiometry leaves unreacted isocyanate which will increasecrosslinking of the system. This additional crosslinking will tighten theelastomer and lead to increased cracking and shrinkage.

Balance the Exotherm and Mold Temperatures

Every polyurethane system will have a different peak exotherm, whichis the highest reaction temperature reached by the mixture. For a givensystem, this is a function of the individual components as well as thereaction mass. Larger mass mixtures will have a higher peak exothermthan smaller masses. The ideal case is to balance the mold temperatureto within ±5°C of the peak exotherm.

Heat the Mold Adequately

Ensure that the oven is circulating air properly, so that there is auniform temperature throughout the oven. Also, the mold should stayin the oven long enough to come to a constant, uniform temperature.

Monitor Raw Material Temperatures

Raw materials can lose heat as they are degassed and mixed, or whilethey sit before mixing, and therefore, they may need to be reheated inorder to maintain the proper temperature. If the temperature of thesystem is too high, cracking may result. Lower system temperaturesand higher mold temperatures are generally recommended.

Several processing conditions can cause surface irregularities in cast parts. This is aconcern since many applications for polyurethane parts require aesthetically pleasingsurfaces.

Improper Mold Release Application

Using either too much or not enough mold release will lead to poorpart surface characteristics. Many processors try to push their releaseagent to get several releases with one application. This will eventuallyresult in parts sticking in the mold and ruined part surfaces. It may alsobe tempting to try to use a heavy coat of mold release to get severalturns from the mold. However, this too can lead to surface qualityproblems. Not all mold releases work with all systems; wax-basedmold releases, for example, may work for MDI systems, but not forTDI systems.

Dirty Mold

Buildup of elastomer material on the mold will cause poor surfacequality on the molded parts. This buildup generally occurs because ofinadequate mold release, which in turn continues to allow elastomermaterial to build up on the mold surface. The molded partssubsequently take the shape of the mold, which now has deposits ofpolyurethane on it.

Excessively Cold Mold

Cold molds can lead to shrink marks on the surface of parts. This isparticularly true with MDI prepolymer systems. The long "green state"of these systems, combined with excessively cold molds, will lead toshrinkage marks.

Excessive ShrinkageApply the Proper Amount of Mold Release Evenly

Following the manufacturer's directions for mold release applicationwill solve most surface quality problems. Also make sure that the moldrelease is applied after the appropriate number of turns.

Clean Molds Regularly

Sandblasting the mold is one method of cleaning molds whenever abuildup is noticed and part surface imperfections have begun to appear.

Heat the Mold Thoroughly

Thorough heating of the mold will reduce the likelihood of cold moldmarks and will help avoid several other potential problems. The moldshould also be removed from the oven as close to pour time aspossible.

Choose the Best Mold Material

Steel molds retain heat better than aluminum; therefore, it is easier tokeep the temperature of steel molds uniform versus aluminum molds.

Occasionally, swirls and streaks will show up in a finished part. This generallymanifests itself as multicolored swirls throughout the part in a fairly random pattern.

Improper Mixing

Even without pigment, the prepolymer and curative are generallydifferent colors, or different shades of the same color. If these two (ormore) materials are not completely mixed together, prepolymer-rich orcurative-rich pockets will remain in the elastomer. This will lead topoor physical properties since the pockets will alter the stoichiometryof the elastomer. This phenomenon is usually fairly easy to spot whena pigment is in one stream of the mixture. The drastic differencebetween the colors in the streams is an excellent indicator of poormixing.

Unevenly Dispersed Filler

Many softer polyurethane formulations contain fillers which in someinstances can be very difficult to mix into the formulation. Insufficientmixing of fillers will cause swirls and streaks in the resulting part.

Knit Lines

In the production of some parts, most notably vertically cast rollers,one can observe knit lines. These lines occur when material flowsaround a core or insert and the material is partially cured when it meetsat the opposite side of the insert. Depending upon the degree of cure atthe point of contact, this knit line area can be a weak point in theelastomer.

Ensure Sufficient Mixing of All Components

Use of a pigment will help highlight areas where materials are mixedinadequately.

Disperse Fillers in Plasticizers

It is often more effective to dissolve or suspend fillers in a plasticizerbefore adding to the prepolymer.

Use Smaller Size Particles for Fillers

Smaller particles are generally more difficult to disperse in themixture. However, if the problem appears to be particles settling out ofthe mixture, then going to a smaller particle may help.

Use Materials with High Viscosity

Again, if settling appears to be the issue, the use of prepolymers withhigher viscosities could solve the problem.

Use Systems with Increased Pot Life

The general way to minimize knit lines is to either pour material intothe mold at a higher flow rate or to use a system which has a longer potlife, such as an Airthane prepolymer.

Inferior elastomer physical properties will generally show up in field performance,though testing is possible to confirm abnormal behavior before the part gets to acustomer. Repetitive errors, such as incorrect stoichiometry and incomplete cure, areeasy to determine through testing. Random errors, such as an overheated drum ofprepolymer, are more difficult to spot.

Incorrect Stoichiometry

Maintaining the correct ratio of prepolymer to curative, which is calledstoichiometry, is critical in producing consistent, high-qualityelastomers. The typical ratio used to obtain the best combination ofproperties is 95%. The table below outlines the effect of stoichiometryon elastomer physical properties.

Effect of Percent Stoichiometry on Elastomer Physical PropertiesPolyurethane ElastomerPhysical Properties Percent Stoichiometry EffectsHardness Stable; minor change over the range of 85-100%Modulus Stable; minor change over the range of 85-100%;

decrease outside this rangeBreak Tensile Maximizes at 90-95%; slight decrease outside this

rangeTear Strength Maximizes at 100-105%; significant decrease below

this rangeElongation Maximizes at 100-105%; minor decrease below this

rangeCompression Set Low percent stoichiometry results in low compression

setAbrasion Resistance High percent stoichiometry favors good abrasion

resistance; 100-105% is optimum rangeHysteresis, DynamicMechanical

Low percent stoichiometry is preferred; 90-95% isoptimum

Flex Life High percent stoichiometry favors good flexperformance; 100-105% is optimum range

Resilience Maximizes at 85-90%; slight decrease above thisrange

Overheated Prepolymer

Use of an overheated prepolymer will have the same ultimate effect onthe elastomer as processing at an incorrect stoichiometry. You will beable to tell that a prepolymer has become overheated because it willhave a lower percent NCO than what is indicated on the prepolymercontainer.

Incomplete Cure

Post-curing is used to relieve internal stresses and to allow properalignment of the polymer chains.

Calibrate Balances Regularly

Balances must be calibrated regularly. In a production environment,balances can become covered with prepolymer, curative and othermaterials. If these materials are on the weighing pan, they can force thebalance out of calibration.

Calculate the Amount of Curative Based on the NCO of Each Lot of Prepolymer

Many processors will use an average percent NCO for determining theamount of curative to use with a prepolymer. However, with someprepolymers, this can amount to a stoichiometry error of up to 5%,which can affect properties significantly.

Each drum of prepolymer has a specific percent NCO. It is importantto use the percent NCO of the particular prepolymer lot that you areusing. The amount of curative to use can then be determined with thefollowing equation:

Minimize the System Heat History

Follow the manufacturer's guidelines for exposure of prepolymer andcurative to heat.

Follow Supplier Curing Guidelines

Prepolymer suppliers have standard recommendations for the curing oftheir materials. A typical guideline is to cure the prepolymer for 16hours at 100°C, though please verify this recommendation for eachsystem. Some prepolymer/curative combinations require differentconditions.

Use an Elemental Analyzer During Production

For Mboca, Lonzacure® MCDEA curative or Ethacure® 300 curative-based elastomers, an analyzer with an X-ray source can be used todetermine the stoichiometry during production.

Hardness measurements are considered statistically the same if they are ±3 pointsfrom the average of several measurements. If the hardness is outside this range, thenthere could be a hardness variation concern. For further information on elastomerhardness testing, see Hardness Test Procedure

Overheated Prepolymer

Use of an overheated prepolymer will have the same ultimate effect onthe elastomer as processing at an incorrect stoichiometry. You will beable to tell that a prepolymer has become overheated because it willhave a lower percent NCO than what is indicated on the prepolymercontainer.

Incomplete Cure

Post-cure is used to relieve internal stresses and to allow properalignment of the polymer chains.

Incorrect Stoichiometry

With gross errors in stoichiometry, the hardness of the elastomer maydecrease.

Inadequate Mixing

Inadequate mixing will leave areas in the elastomer that are rich inprepolymer or curative, which will lead to incorrect stoichiometry, andin turn, will result in problems with part physical properties.

Minimize the Heat History

Follow the manufacturer's guidelines for exposure of the prepolymerand curative to heat.

Follow the Supplier Curing Guidelines

Prepolymer suppliers have standard recommendations for the curing oftheir materials. A typical guideline is to cure the prepolymer for 16hours at 100°C, though please verify this recommendation for eachsystem. Some prepolymer/curative combinations require differentconditions.

Calculate the Amount of Curative Based on the NCO of Each Lot of Prepolymer

Many processors will use an average percent NCO for determining theamount of curative to use with a prepolymer. However, with someprepolymers, this can amount to a stoichiometry error of up to 5%,which can affect properties significantly.

Each drum of prepolymer has a specific percent NCO. It is importantto use the percent NCO of the particular prepolymer lot that you areusing. The amount of curative to use can then be determined with thefollowing equation:

Ensure Sufficient Mixing of All Components

Use of a pigment will help highlight areas where materials are mixedinadequately. See Swirls/Streaks in Parts for further detail.

Occasionally, the hardness of an elastomer will be acceptable at the time ofmanufacture, but it will have softened considerably by a later date. This softeningmay have occurred during storage or field usage.

Contact with Solvents

Continued contact with solvents will generally soften a polyurethaneelastomer. Most petrochemical-based solvents cause significantsoftening of most polyether-based elastomers. In addition, water alsohas a great softening effect on most polyester elastomers aftersignificant contact. It has even been documented that polyester-basedelastomers which have been stored at elevated warehouse temperatureswith high humidity have softened considerably.

Exposure to Excessively High Temperatures

Continued exposure to elevated temperatures will begin to break downthe urethane bonds within the elastomer, which will cause softening. Atypical recommended high temperature for continuous use of commonelastomers is 180-200°F. This upper use temperature varies withformulation and application.

Incorrect Stoichiometry

Shield Elastomer from Solvent

In some cases, it is possible to engineer the process such that theelastomer does not come into contact with the solvents.

Insulate the Elastomer From the Heat Source

In some applications, it is possible to insulate the elastomer from theheat source to minimize its exposure to heat.

Choose the Best Prepolymer

Polyether and polyester-based elastomers react very differently tosolvents. Be sure to choose the correct backbone for the application.Generally, choose polyethers for water environments and polyestersfor most organic solvents.

Calculate the Amount of Curative Based on the NCO of Each Lot of Prepolymer

Many processors will use an average percent NCO for determining theamount of curative to use with a prepolymer. However, with someprepolymers, this can amount to a stoichiometry error of up to 5%,which can affect properties significantly.

Each drum of prepolymer has a specific percent NCO. It is importantto use the percent NCO of the particular prepolymer lot that you areusing. The amount of curative to use can then be determined with thefollowing equation:

Occasionally, the hardness of an elastomer will be acceptable at the time ofmanufacture, but it will have hardened considerably by a later date. This hardeningmay have occurred during storage or field usage.

Soft Segment Crystallization

Soft segment crystallization is a phenomenon seen frequently with softparts (<70 Shore A). The backbone in the polyol in these types ofsystems is generally a high molecular weight. With a very highmolecular weight, the concentration of isocyanate and curative in theelastomer is very low, and the polyester forms concentrated domains inthe elastomer. These concentrated domains then tend to solidify orcrystallize, which increases the hardness of the elastomer.

Insufficient Cure

If a part is not sufficiently cured prior to testing, it will continue toincrease in hardness as it cures.

Low Stoichiometry

With a low stoichiometry, the relatively fast prepolymer/curativereaction will occur and hardness will build in the prepolymer ratherquickly. The subsequent high concentration of unreacted NCO groupswill then react with each other or with moisture, resulting in increasedelastomer cross-linking. This will gradually increase the hardness ofthe system.

Plasticizer Leaching

Plasticizers do not react in the system. Over time, they can leach out ofthe elastomer, leading to increased elastomer hardness.

Choose a Mixed Backbone Prepolymer

Using a prepolymer with a mixed polyester backbone will disrupt thecrystallinity of the elastomer enough to reduce soft segmentcrystallization. Prepolymers such as the Versathane® QM varietyshould be used for these systems.

To test for soft segment crystallization place a softened elastomer

sample in an oven at 100°C for several hours. If the sample returns toits original hardness during heating, the cause of the softening willmost likely be soft segment crystallization. When this is the case, thepart will generally harden again over time.

Follow the Supplier Curing Guidelines

Prepolymer suppliers have standard recommendations for the curing oftheir materials. A typical guideline is to cure the prepolymer for 16hours at 100°C, though please verify this recommendation for eachsystem. Some prepolymer/curative combinations require differentconditions.

Use Stoichiometry Above 90%

Using a higher stoichiometry will ensure that there are no unreactedNCO groups left after curing which could increase the hardness overtime.

Ensure Plasticizer Compatibility

Not all plasticizers are compatible with every system. Benzoflex988SG is the most universally compatible plasticizer. Be sure to usethe SG grade of this product, which was designed specifically forpolyurethane applications.

Some parts, when improperly processed or used, can take on a permanentdeformation.

Softened by Heat

Prolonged exposure to heat will have a deleterious effect onelastomers, which can cause them to soften and become distorted bythe forces applied to them when in use.

Poor Abrasion Resistance

In extremely high-wear applications, sections of a part may wear awaycompletely and the part will then appear to be deformed.

Elastomer Tendency to be Thermoplastic

Some elastomer formulations have a tendency to be slightlythermoplastic. This means that they will actually distort and take on anew shape with the application of heat. In some cases this is a desiredproperty, though in general it is not desired.

High Compression Set

Some elastomers that have a high compression set will deform becauseof the forces being continually applied to them. High compression setmay be a function of the formulation or it may be the result of poorprocessing.

Reduce Exposure to Heat

Reduction of exposure to heat can sometimes be accomplished throughengineering controls.

Formulate for Higher Abrasion Resistance

Some elastomers are better at withstanding abrasion than others.Consult your prepolymer supplier for prepolymer recommendationsbased upon the specific application. Additionally, certain additives canbe formulated into an elastomer to help improve abrasion resistance.

Reduce the Load/Redesign the Part

Through proper part design, it is often possible to reduce the stress onthe part to a manageable level.

Process at the Correct Stoichiometry

Processing at an incorrect stoichiometry will lead to high compressionset. This in turn could lead to permanent deformation of the part.Lower stoichiometries are recommended to reduce compression set.

Blowout occurs with parts used in dynamic applications, including wheels, tires androllers. When polyurethane elastomers undergo cyclic loading and unloading,mechanical energy is transformed into heat energy and the elastomer will heat up. Ifthis heat buildup is too great, then the polyurethane could melt and the part couldexperience blowout.

Improper Choice of System

Some polyurethane elastomers have inherently better dynamicperformance than others. High-performance polyether systems aregenerally preferred for optimum dynamic performance. Within thehigh-performance class of materials, however, some systems performbetter than others.

Wrong Hardness Elastomer

With dynamic applications, the greater the cyclic deflection of theelastomer, the greater the heat buildup. If the deflection is higher than5-10%, then the part has a greater chance for blowout. Therefore,specifying a harder elastomer is usually recommended to minimizedeflection. However, there are typically other performance attributesthat also need to be considered, such as ride comfort or traction, whereuse of a softer formulation may be desired.

Incorrect Stoichiometry

The dynamic performance of an elastomer is optimized in the 90-95%stoichiometry range. Performance will decline as a formulation movessignificantly away from this stoichiometry.

Uneven Loading

Even when the correct formulation has been used, the loading on awheel, tire or roller can be uneven, leading to localized meltdown andblowout in the region of maximum load. This happens frequently withrollers when the ends are tightened down to increase the nip pressure.It also happens with tires and wheels that have an improper crown.

Too Great of a Load/Speed

Dynamics are a function of load and speed. If either is too great, thenthe part may not be able to dissipate the heat fast enough to avoid ablowout. This can happen even with a properly designed part.

Choose a System with Improved Dynamics

Generally, one will want to use a high-performance polyether-basedelastomer. Many applications use TDI/Mboca/PTMEG elastomers withgreat success. Within that class of materials, some perform better thanothers. Products such as Airthane® prepolymers usually provide thehighest performance materials in this class. In addition, alternativecuratives have been developed which can offer dynamic improvementsover conventional curatives. Consult your supplier forrecommendations on the appropriate system.

Choose the Correct Hardness

Generally, an in-use deflection of 5-10% is acceptable, although adeflection of under 5% is desirable. The table below provides theformula to calculate the deflection. Choose a system that will providethis level of deflection or less, within the other parameters which mayconstrain your choice of hardness.

Percent Deflection Formula

WhereD = deflection in inchesL = loading in poundsIR = inside radius of polyurethane in inchesOR = outside radius of polyurethane in inchesE = compression modulus, psiW = actual width, inches

Modulus (psi) Shore A Shore D750 52 —1000 62 —2200 78 —4400 90 409000 95 5028000 — 6036000 — 70

Optimize Stoichiometry

With most systems, a stoichiometry of 90-95% is ideal for optimizingdynamic performance, with 90-92% being preferred. Obviously, this issystem-dependent.

Load Evenly

Proper installation of the part into service is critical. Loading across theface of the part should be as uniform as possible. Crowning of thewheel or roller may be required.

Redesign the Part/Application to Conduct Heat Away

Sometimes with the existing design, the combination of load and speedmay be too great for the required polyurethane. In this case, it may benecessary to redesign the application. This can be accomplished byredesigning the wheel/roller to have a larger diameter, effectivelyreducing the speed. The part can also be made wider to distribute theload over a wider area. Also, additional wheels/tires can be added tospread the load. In some instances, the part could be redesigned tomore readily dissipate heat away from the polyurethane.

In many applications a polyurethane is bonded to a steel or aluminum insert, hub orcore. This can be difficult to accomplish, as many variables come into play to ensure astrong bond. Bonding is usually accomplished by the use of an adhesive applied to theinsert prior to the casting of the polyurethane system.

Poor Initial Bond

In parts which require bonding, precautions must be taken during theapplication of the mold release. Contamination of the bonding surfacewith mold release can weaken the bond line.

Too Much Stress at the Bond Line

Bond strength is dependent on many factors. At times, due to thedesign of the part, stresses are localized at the bond line, causing thebond to break.

Overheating

The adhesives generally used in these applications have an upper usetemperature limit in the range of 200°C to 300°C. If the systemtemperatures exceed these levels, the bond strength may becompromised.

Water/Solvent Permeation

In some roller applications where the part is immersed in water oranother solvent, the water or solvent way eventually migrate throughthe polyurethane and collect at the bond interface. This may causedegradation of the adhesive, or it may cause enough stress at the bondline to rupture the bond.

Choose the Correct Primer or Adhesive

There are many different primers and adhesives that can be used fortypical polyurethane applications. Choosing the correct adhesive towithstand the operating environment is critical. Consult your adhesivessupplier for the proper recommendations.

Prepare the Surface Properly

Be sure the surface of the substrate is properly roughened to helpimprove the mechanical bonding of the system. Also ensure that thesubstrate has been properly cleaned after blasting and that all greaseand oils have been removed. In addition, make sure that the moldrelease does not come into contact with the properly prepared substratesurface. Contact your adhesives supplier for additionalrecommendations.

Design the Part to Minimize Stress

When designing parts, there are many techniques that can be used tominimize the stress at the bond interface. We recommend thefollowing: avoid sharp corners; design-in fillets and overhangs; andequalize stress profiles. See Elastomer Design for further details.

Provide Cooling

In cases where overheating of the bond line may be an issue, providingcooling to either the substrate or the polyurethane would beadvantageous.

Choose the Correct System to Minimize Permeation

The correct choice of polyurethane system to minimize heat buildup isessential. See Blowout for further information on how to minimizeheat buildup.

Polyurethane elastomers will swell when exposed to certain solvents. This swellingcan range from a few percentages in mild cases to several hundred percentages insevere cases.

Exposure to Harmful Solvents

The overwhelming cause of swelling is exposure to solvents, materialswhich are incompatible with the polyurethane system. In addition,solvents used at elevated temperatures further accelerate the problem.The proper specification of polyurethane for the specific solvent iscritical.

Choose the Proper Elastomer

Generally, polyether-based elastomers are the proper choice for highhumidity and water applications. Polyesters, on the other hand, aregenerally used for applications where there is the potential forexposure to oil-based solvents.

Shield the Elastomer from the Solvent

In cases where the elastomer is being splashed by the solvent, it maybe possible to mechanically shield the part from the solvent.

Test the Elastomer Before Field Exposure

Testing the elastomer with the correct solvent before field applicationis recommended. It is a simple test—simply immerse a known weightof elastomer into a container filled with the desired solvent at thedesired temperature for an extended period of time. At the conclusionof the test, reweigh the elastomer to determine the uptake of solvent. Ifplaques of the proper thickness are used, standard ASTM tensile andtear tests can also be performed.

Performing laboratory tests to predict the abrasion performance of elastomers isdifficult. Usually field tests are the only reliable means of testing parts. Polyurethaneelastomers are generally very resistant to impingement abrasion and slightly lessresistant to sliding abrasion.

Incorrect Stoichiometry

Tear strength is related to abrasion resistance. In order to maximizeabrasion resistance, the tear strength of the elastomer should bemaximized.

Excessive Heat Buildup

Abrasion between the medium and elastomer can generate significantamounts of heat. This excessive heat generation can destroy thepolyurethane. Also, as an elastomer increases in temperature, the tearresistance decreases. Since tear resistance and abrasion resistance aredirectly related, the abrasion resistance also generally decreases as theelastomer temperature increases.

Incorrect System Choice

As with many other elastomer problems, the correct choice ofpolyether versus polyester prepolymer will have a large impact on theperformance of the finished part.

Choose the Correct Stoichiometry

The tear strength of most elastomers is maximized around 100%stoichiometry. Producing elastomers at this stoichiometry shouldimprove the abrasion resistance of the part.

Minimize Heat Buildup

Heat buildup can be dissipated with the use of a slurry, as is frequentlyused in mining applications.

Choose the Correct System

First and foremost, the formulation should be chosen based upon theexposure variables in the application. Polyether formulations are often

chosen for systems exposed to impingement abrasion due to theirhigher rebound capability. For systems exposed to sliding abrasion,polyester-based formulations should be chosen. This is becausepolyester formulations typically have better tear resistance. Additivessuch as Primax® surface-modified particles can also be used to lowerthe coefficient of friction, which results in better sliding abrasionresistance.