Methods for the Recycling of Polyurethane andPolyurethane CompositesMarcel P. Neuray

Hennecke MachineryPolyurethanes DivisionBayer Corporation100 Bayer RoadPittsburgh, PA 15205-9741USA

H. Michael SulzbachHennecke GmbHBirlinghovener Straße 30D-53754 Sankt Augustin – BirlinghovenGERMANY

Jürgen WirthHennecke GmbHBirlinghovener Straße 30D-53754 Sankt Augustin – BirlinghovenGERMANY

ABSTRACT

Product specific applications for polyurethane havebecome so diverse in recent years that the wide range ofproducts being manufactured from this extremelyversatile raw material is simply overwhelming. Thechemical cross-linking that occurs at the micro cellularlevel during the formation of polyurethane will in mostcases generate certain strategic advantages and superiormaterial properties over the vast majority of availablethermoplastics and other materials. These sameadvantages, which are to some extent based on thechemical cross-linking, have traditionally presentedobstacles in terms of the materials’ environmentalfriendliness and have challenged the potential to recyclepolyurethane effectively. Several new developments inrecent years have lead to more viable recyclingtechnologies for a wide range of polyurethane types andproducts. Aside from any such methods that involve therecycling of polyurethane through chemical means,several technologies based on the mechanical recycling ofpolyurethane and polyurethane composites have madeconsiderable advances in recent years. These newtechnologies, among with some of the more traditionalmethods, are described in further detail in this technicalpaper.

INTRODUCTION

The first step necessary for the potential recycling ofpolyurethane involves one of a number of processes

required to reduce the particle size of the original foamproduct to a level that will allow the material to bereprocessed in a secondary manufacturing process. Thevarious types of polyurethane waste products, consistingof either old recycled parts or production waste (e.g.trimmings, scrap parts, etc.), are generally reduced to amore usable form, such as flakes, powder or pellets,depending on the particular type of polyurethane that isbeing recycled. In most cases, this can be accomplishedby either shredding or grinding the polyurethane in orderto provide the necessary media for recycling. This wasteis usually a direct result of the trimming of the slabstockbuns and the subsequent fabrication into the finishedproduct. For molded foam operations, the waste istypically generated during the manufacturing process as aresult of the flashing at the mold closing edges and theventing holes, which in extreme cases may amount to atotal of 6% of material waste, depending on the size of theoverall part. In the case of slabstock or flexible foam, upto 12% of cut-off waste is to be expected, depending onthe particular manufacturing process.

THE REBOND PROCESS

The recycling of flexible foam cut-off waste, which isgenerated during the trimming of the slabs or buns and thesubsequent fabrication into the finished product, has beena widely used practice for several decades and easilyrepresents the oldest form of recycling with polyurethane.Most flexible foam waste generated throughout the worldis reused in one form or another to make a variety ofrecycled products, the most common application

Figure 1 - Various recycling methods for polyurethane raw materials

being carpet underlay or padding. Flexible foam waste istypically processed in flake mills, which act as largeshredders to reduce the size of the foam parts. The sizeand shape of the resulting foam flakes will varydepending on the application for which they areultimately intended. Some of these shredded foam gradesare used to fill cushions or toys, while the overwhelmingmajority are utilized for a process called rebonded foamproduction, or simply the “rebond” process.

In the rebond process, recycled foam flakes originatingfrom flexible slabstock foam production waste are usually

blown from storage silos into a mixer that consists of afixed drum with rotating blades or agitators, where thefoam flakes are sprayed with an adhesive mixture. Thetotal quantity of adhesive will typically representapproximately 10 to 25% of the overall weight of thefoam flakes and is dispensed from a spray gun ormetering unit equipped with a spray head. Depending onthe size of the mixer, this spraying process will last forabout 5 to 10 minutes, while the blades are continuouslyrotating to ensure a uniform distribution of the adhesivethroughout the entire batch of foam flakes.

Figure 2 - Rebonded foam: homogeneous distribution of the flake-binder mixture (cross sectional view)

Shredding Grinding Milling

Recycling Processes

CompressionMolding

InjectionMolding

PowderProcessing

“GrindFlex”

RebondProcessing

“RemoTec”

“ReboTec”

This mixture of foam flakes covered with polyurethane-based adhesive is then introduced into a box mold that issituated below the mixing unit, where it is thencompressed to about ½ to ⅛ of its original volume.Typically, that the walls of the box mold includeperforations to allow saturated steam to pass through themold walls and to improve ventilation. The water vaporacts as an accelerator for the curing step, whichessentially bonds together the entire block of recycledfoam. The actual curing time of the rebonded foam blockwill depend on the type of adhesive and on the curingmethod that is being used, but in most cases will takeanywhere from 30 minutes to 6 hours. The resultingblock of rebonded material is stored for a minimum of 12hours. This time period provides sufficient time for thematerial to finish the curing process, before it can beutilized for further post processing.

The rebonded foam blocks can be produced in densitiesranging from 3.75 pcf to 18.7 pcf (60 kg/m³ to 300kg/m³), which are much higher than commerciallyavailable grades of slabstock foam. In general, rebondedfoam manufactured from recycled flexible foam wastepresents a first class, semi-elastic material that isprimarily used for a wide variety of padding applications.Ideal applications for rebonded foam end products includecarpet underlay or padding, automotive interior paneling,gym mats padding material and edge stiffeners forfurniture cushions.

What originated many years ago as a desire to utilizeproduction scrap and waste material from the processingof flexible slabstock foam, has in the past twenty yearsdeveloped into a comprehensive and rather complex

technology. The rebond process incorporates both asurprising amount of flexibility and a wide variability inthe mechanical properties of the final product. Theenormous potential associated with the rebond process isbased on the establishment of a first class, independentgroup of materials, which can take advantage of thetraditional properties of slabstock foam and take these toanother level with the higher densities that can be offeredwith rebonded material.

Modern rebond plant concepts have provided thenecessary means to fulfill these requirements and arecapable of consistently producing top quality moldedparts from rebonded material. As a result of thetremendous advancement in this technology segment, therebonded material molded from leftover foam flakes hasestablished a separate class of materials, which is able toprovide material properties and processing advantagesthat are quite unique.

REBOTEC™ (MODERN REBOND TECHNOLOGY)

In an effort to further advance the traditional rebondprocess technology described previously for the modernmanufacture of rebonded foam blocks, Hennecke took theinitiative to develop a new technology that would takethis process to a next higher level. The resulting state-of-the-art technology, named ReboTec, was developed tomanufacture top quality rebonded blocks in either a semi-automatic or a fully-automatic flake-binder mixingprocess. Newly developed compression systems enablethe manufacture of both round and rectangular blocks,with very precise material properties that are consistentfrom block to block.

Figure 3 - Modern rebond plant, e.g. Hennecke ReboTec™, (top view)

Figure 4 - Modern rebond plant, e.g. Hennecke ReboTec™, (side view)

The plant schematic shown in Figure 3 represents atypical ReboTec plant layout with the following systemcomponents:

Shredding of foaming waste into flakesFlake millsFlake storage silosPrepolymer blend system and metering unitPrepolymer (polyurethane-based adhesive) storage tankScale and mixing stationCompression system

Characteristics of a state-of-the-art rebond plant:▪ Fully automatic processing with quick overall cycle

times

▪ Spray technology for the dispensing of the adhesive,including:- optimum distribution of the prepolymer- short mixing cycle

▪ Mixing station with maintenance-friendly accessibility▪ Fully automatic changeover for different product grades

(e.g. density, etc.)▪ Pigment additive system for liquid pigments tied into

adhesive metering unit to identify the various productgrades

▪ Homogeneous density distribution obtained throughoptimum feed system of the flake-binder mixture

▪ State-of-the-art compression system designed toachieve both uniform flake-binder distribution and amore homogeneous density distribution throughoutthe block

Figure 5 - Traditional compression system

Figure 6 - Modern compression system with floor space saving design

REMOTEC™ (REBOND MOLDINGTECHNOLOGY)

The single largest percentage of rebonded foam isutilized as carpet underlay or padding, but many other endproducts can certainly be manufactured. Unfortunately,the market for these products has been somewhat limited,mainly due to the additional cutting and trimming stepsthat are required to convert the rebonded blocks into afinished shape. These steps present additional costs andthe regeneration of new foam waste from already recycledrebond material. Also, the design options for theseproducts are based on the existing cutting technologies,placing additional limitations on the overallmanufacturing process.

The development of a more economical manufacturingprocess was necessary that would eliminate these cuttingand trimming steps by molding these parts directly fromthe flake-binder mixture. These requirements lead to anew processing technology called RemoTec (RebondMolding Technology), which presents an alternativesolution to the already existing recycling process.RemoTec enables the molding of rather complexpolyurethane shapes, which are produced from rebondedmaterial in a fully automatic process. The RemoTecprocess precisely determines the exact amount of flake-binder mixture that is injected into each individual moldin order to keep cutting waste to an absolute minimum.

Figure 7 - Schematic of Hennecke RemoTec plant

The plant schematic shown in Figure 7 represents atypical RemoTec plant layout, with the following systemcomponents:

Flake mill (shredder)Storage silo for polyurethane flakesMixing stationBinder metering unit & additive streamBatch siloMetering station with load cellTurntable Molds

The processing sequence for the RemoTec plant beginswhen flexible foam waste is introduced into a flake mill,where the foam is shredded into a predetermined size andstored in a storage silo. The polyurethane flakes are thentransported to a mixing station in batches ofpredetermined amounts based on weight. At this point,the system introduces a binder, preferably a prepolymer,at an automatically controlled mixing ratio and agitatesthe flake-binder mixture thoroughly. A batch silo thencalls for a certain amount of flake-binder mixture, whichis level-controlled through an auto-fill system. Themetering station is connected to a load cell that is PLCcontrolled to provide extremely accurate amounts offlake-binder mixture for each mold. This mixture isinjected to the individual molds by means of a proprietary

filling technique. In addition, the metering of the flake-binder mixture is continuously monitored to ensure thatthe various batches are consistent in quality.

The overall production process for the RemoTecprocess is fully automated. This high degree ofautomation provides relatively low manpowerrequirements for the operation of this complete plantconcept. In addition, the traditionally low raw materialcosts that are required to keep this plant operating providethe basis for an economical and convincing overallproduction concept.

Hennecke’s RemoTec technology has already beensuccessfully introduced for the mass production of soundabsorbing and shock absorbing automotive components.In general, RemoTec can produce molded foam partsfrom recycled polyurethane materials with typicaldensities between 3.75 pcf and 25 pcf (60 and 400 kg/m³).The ability to mold these unusually high densitiesprovides the opportunity to open new potential marketsfor flexible molded polyurethane parts. Someapplications that could prove ideal for RemoTec includesound absorbing parts in certain automotive markets, aswell as some energy absorbing or small molded parts (e.g.head rests) for various automotive applications.

Figure 8 - RemoTec manufactured part

GRINDFLEX™ (POLYURETHANE POWDERPROCESSING TECHNOLOGY)

The in-line metering of ground polyurethane powder(or “regrind”) into the polyol stream before mixing thepolyol/regrind mixture with isocyanate to form new foam,has proven very viable method to recycle polyurethane inrecent years. In order for the regrind material to beintroduced as a filler in the preparation of new foam, itmust be in the form of a very fine powder. This istypically accomplished by mechanically grinding thefoam waste material in a shearing process utilizing aroller mill until a particle size of 200 microns (0.2 mm)has been reached. The GrindFlex process, a jointdevelopment between Metzeler Schaum GmbH andHennecke GmbH, has already been successfully proven ina full-scale production application. This process wasmade possible through a reliable processing technologythat is applicable to most polyurethane types and canreadily be implemented at the production level.

Implementation of the regrind powder processingtechnology requires the following main production steps:▪ Generation of powder▪ Metering of powder▪ In-line mixing of powder and polyol▪ Foaming new foam▪ Adjusting the chemical formulation

Generation of Powder

The preferred process to generate the required finepolyurethane powder, or “regrind”, from flexible

polyurethane usually incorporates the use of a roller mill.This process, developed jointly by Bayer AG andHennecke GmbH, provides the ability to obtain a veryfine powder from flexible foam trimmings byincorporating a strainer to ensure that only very finegrains of powder are permitted to be passed on to the nextmanufacturing step.

The Hennecke roller mill system includes acontinuously recirculating return loop for all particles thathave not passed through the strainer. The recommendedparticle size or powder grain size should be on the orderof 100 µm, which will enable the powder to form part ofthe cell structure in the newly formed foam.

Processing New Flexible Slabstock Foam with RegrindFiller

The actual processing of the recycled finely groundpolyurethane powder as a filler during the production ofnew flexible slabstock foam is a topic that can bethoroughly discussed, based on thousands of pages of dataand test results, which is outside of the scope of thistechnical paper. It would be beneficial, however, to atleast briefly skim the surface of this issue, in order topromote the recycling technology that has already beenproven in production applications.

Hennecke’s GrindFlex technology provides the abilityto introduce a rather high percentage of regrind into thenew foam; values of 10 to 20% have been achievedwithout significant changes in the mechanical propertiesof the foam.

Figure 9 - Schematic of roller mill system

FoamMaterial

Shredder

Grinder

Strainer

Powder

ReturnLoop

CoolingSystem

Figure 10 - Hennecke GrindFlex roller mill plant

The plant schematic shown in Figure 12 represents atypical GrindFlex plant layout, with the following systemcomponents:

FlakesRoller millStrainer or sieveStorage siloPolyolPremixerPolyol/Regrind Main mixerIsocyanateAdditives

Summary of recommended GrindFlex plant parameters:

• Regrind (powder) production rate: 300 – 400 kg/hr• Particle size of polyurethane grains: < 200 µm• Concentration of particle size: 85% < 100 µm• Capacity of regrind storage silo: 20 m³• In-line blending flowrate of regrind: 20 – 100 kg/min• Mixing ratio (polyol/regrind): 100:5 to 100:30

COMPRESSION MOLDING TECHNOLOGY

The pelletizing of rigid polyurethane foam representsthe starting point for all forms of recycling involving thecompression molding process. This method of recyclingpolyurethane materials is typically achieved by means ofreprocessing the material that has already been reduced topellets (or granules), whose particle size is on the order of0.5 to 3.0 mm. Most polyurethane components withrather hard material properties, such as automotivebumpers and interior door panel substrates, as well asrigid insulating foam used in most refrigerationapplications (e.g. refrigerators, construction board, etc.),can typically be reprocessed to pellet form by utilizing theroller mill principle. Even polyurethane compositeproducts (e.g. automotive instrument panels, pick-uptruck boxes, etc.), which are made from a variety ofpolyurethane types and also entirely from polyurethane,can be introduced into this recycling process without anytype of separating or pre-work necessary.

Figure 11 - Typical cell structure of flexible foam with 15% loading of regrind

Figure 12 - Schematic of Hennecke GrindFlex plant

During the compression molding process, thepolyurethane pellets are molded into a new shape underboth high pressure and high temperature. This representsthe only processing technology currently available, whereclose to a 100% recycling efficiency can be achievedwithout the utilization of virgin material, additives orother bonding agents. Both production waste products,such as trimmings or flash, and already used foammaterial can be used for the compression moldingprocess. RIM and RRIM polyurethane materials can beused in a variety of conditions: painted or unpainted,filled or unfilled and with both IMR and EMRformulations. In most cases, the end product of thecompression molding process will exhibit the samephysical and mechanical properties with partsmanufactured from still “new” production waste materialor from “old” and already used foam material.

The processing sequence for the compression moldingof polyurethane components is detailed in Figure 13.

Both the production waste material and used foammaterial are ground into pellets. These pellets, whichrange in size from 0.5 to 3 mm, are heated to atemperature between 250 to 320°F (120 to 160°C) beforebeing introduced into the press. The compressionmolding process requires a high mold temperature of 350to 375°F (180 to 190°C) and a press capable of providinginterior mold pressures that are at least 4,350 psi (300bar). The required in-mold time will depend on thegeometry of the specific part and the required surfacecharacteristics, but will typically range anywhere from 30seconds to 4 minutes. During this process, the pressedpart will acquire a certain structure and form that is basedon the physical characteristics of the mold cavity, withoutthe pellets actually going into a liquid form. Uponcompletion of the in-mold cycle time, the part is removedfrom the mold in a “hot” state and is allowed to cooldown outside of the press.

Figure 13 - Schematic of Compression Molding Process scenario

)

AutomotiveBumper

Pelletizer

Pellets (0.2 – 1.0 mm)

AutomotivePart(e.g. Battery Tray)

Press(350 bar, 80 – 180 °c)

Preheat Oven(approx. 150°c)

Figure 14 - Transition from RIM-pellets (top), to loosely pressed RIM-pellets (middle), to compression molded part (bottom)

A sample part molded from Bayflex® 110 is shown as itpasses through the various phases of the compressionmolding process. Initially, the preheated pellets areintroduced into the mold, as shown in the top section ofFigure 14. Upon the initial closing of the mold, shown inthe middle section of Figure 14, the pellets are portrayedin a more compressed state and are beginning to take theshape of the mold cavity. The final step of the process,shown in the bottom section of Figure 14, shows the partafter the completion of the compression molding cycle. Itcan be clearly identified that the final part consists ofrecycled polyurethane that has been compression molded,

while the part is nearly homogeneous in nature with onlyvery few outlines of the individual pellets still visible.

These types of polyurethane pellets, which have alreadychemically bonded during their initial polymerizationreaction between the polyol and isocyanate, can only beprocessed with conventional injection molding machinesunder very limited circumstances. The fact that uniformparts can be produced with the compression moldingprocess, is most likely the result of a repolymerization ofthe urethane. This reaction takes place at the highpressure and high temperature inside the mold, whichremains well below the temperature at which the materialwill degrade under normal conditions.

Figure 15 - Headlamp bucket compression molded with amine bonded RIM urethane

Figure 16 - Seat backs compression molded with amine bonded RIM urethane

Sample Parts

The sample parts shown in Figures 15, 16 and 17,represent a selection of several parts that werecompression molded with amine bonded RIM urethane.The headlamp bucket, shown in Figure 15, consist of acomplex, three-dimensional shape, which requires therecycled material to cover vertical walls withapproximately 15 cm in height. The picture furthermoreprovides proof that compression molding is capable ofproviding excellent surface characteristics of the finishedpart. The seat backs in Figure 16 illustrate the ability of

the process to manufacture parts with a relatively highwall thickness and a large surface area. Field tests haveshown that these seat trays have no tendency to deform,even after several months of testing. Air intake ducts, asshown in Figure 17, are frequently used in transportationapplications and present prime candidates to becompression molded from recycled polyurethanes. Theseparts clearly show the ability of the technology to mold athin-walled, structural component. In addition, the goodflowability of the recycled RIM urethane material duringthe compression molding process is clearly demonstratedby this rather complex part.

Conclusion of Compression Molding Process

Based on the ongoing research and developmentthroughout the industry with compression molding, thetechnology has reached the point where it can besuccessfully implemented in a production environment.

The economic feasibility and overall viability of thisprocess depends entirely on the equipment technologythat is available. Current processors of thermoplasticswith SMC technology are generally in a more favorableposition to implement this technology on a short-termbasis.

Figure 17 - Air intake ducts compression molded with amine bonded RIM urethane

Compression molding presents a unique opportunity torecycle a wide variety of cross-linked polyurethane at arecycling efficiency of practically 100%. Depending onthe type of recycled material and an optimization of theoverall process, it may be possible to reach a comparablelevel of mechanical properties as the originalpolyurethane had for certain applications. Potential futureapplications for compression molded parts in theautomotive area include cover trays, battery cases, airducts and most load bearing parts where a Class A surfacefinish is not absolutely necessary.

SUMMARY AND CONCLUSION

The various mechanical recycling technologiespresented as part of this technical paper have greatlycontributed to improve the overall image regarding therecyclability of polyurethanes in recent years. In the past,polyurethane had traditionally been viewed as a non-recyclable raw material, a statement that is no longerjustified due to the recent advances in modern technology.

In order for the practical application of any of theserecycling methods to be successful, it should be stressedthat the secondary products resulting from these processtechnologies should by all means still exhibit the uniqueproperties of polyurethane. Aside from the recyclingapplications involving the SPT process (Same PartTechnology), where specific grades of recycledpolyurethane are reused in the identical process that wasoriginally used for the donor material, there remainnumerous opportunities to further improve the recyclingcapabilities of polyurethane in new market areas. It iscritical that the future mechanical recycling ofpolyurethane is taken into consideration during theproduct design phase of a new product, making it moreprobable to be recycled at the end of its product life cycle.All of these efforts to improve recycling technologiestogether with the implementation of new design conceptswill undoubtedly lead to the increased recycling ofpolyurethane in the new millenium.

REFERENCES

1. Sulzbach, H. M. and J. Wirth, (1999), “MechanischesRecycling von PUR und PUR Kompositen,”Hennecke GmbH, Germany.

2. Dodge, J. (1999), “Polyurethane Chemistry - SecondEdition,” Bayer Corporation, Pittsburgh, PA.

3. Bayer AG (1979), Polyurethane Application ResearchDepartment, “Bayer - Polyurethanes,” EditionJanuary 1979, Leverkusen, Germany.

4. Kirschner, R. L., K. V. Lamb, and H. M. Sulzbach,(1996), “Innovative Process Directly ProducesMolded Foam Parts from Shredded Flexible Foam

Trimmings and Production Scrap,” SPIPolyurethanes Expo 1996, Las Vegas, NV.

5. Hennecke GmbH, Brochure No. 54, “SPT RecyclingUsing PUR Regrind,” Sankt Augustin –Birlinghoven, Germany.

6. Hennecke GmbH, Brochure No. Pi 113 Information,“GrindFlex,” Sankt Augustin – Birlinghoven,Germany.

7. Hennecke GmbH, Brochure No. Pi 114 Information,“RemoTec”, Sankt Augustin – Birlinghoven,Germany.

8. Hennecke GmbH, Innovations 503, “Fully AutomaticMould Injection with Molded Rebond,” SanktAugustin – Birlinghoven, Germany.

9. Hennecke GmbH, Innovations 503, “In-Line Meteringof Recyclate Powder for Primary PUR Components,”Sankt Augustin – Birlinghoven, Germany.

BIOGRAPHIES

Marcel P. Neuray

Marcel P. Neuray received his B.S. degree in MechanicalEngineering from Washington University in St. Louis,Missouri, in 1993. In September of that year, Marceljoined Hennecke Machinery, a business unit of thePolyurethanes Division of Bayer Corporation, inPittsburgh, Pennsylvania, to pursue a career in Bayer’spolyurethane processing equipment business. Marcel hasheld various positions at Hennecke Machinery, includingTechnical Service Representative in Pittsburgh, andTechnical Sales & Service Representative for HenneckeMéxico, from 1994 to 1997, in Mexico City. His currentresponsibilities as Sales Engineer in the Sales &Marketing Group in Pittsburgh enable him to focus hisefforts on the North American appliance market.

H. Michael Sulzbach

Hans Michael Sulzbach receivedhis Mechanical Engineering degreefrom the Institute for Engineeringin Bad Honnef, Germany. Since1962 he has worked at HenneckeGmbH in Sankt Augustin. Michaelis the General Technical Managerof Research and EngineeringDepartments.

Jürgen Wirth

Jürgen Wirth received hismechanical engineering degreewith a concentration incompression engines and pumps atthe Fachhochschule Köln inCologne, in 1981. He joinedHennecke GmbH in 1986 and hassince been working in thecompany’s research anddevelopment group as an

applications engineer. Currently, Jürgen is responsiblefor the development of new polyurethane processingtechnologies.