Application of Diene-BasedThermoplastic Polyurethanes

in Rubber Compounding

By Steven K. Henning and Herbert Chao

Cray Valley USA, LLCExton, Pennsylvania

USA

5726 01/10

Cray Valley USA, LLC • Oaklands Corporate Center • 468 Thomas Jones Way, Suite 100 • Exton, PA 19341 877-US1-CRAY (877-871-2729) • Web: www.crayvalley.com

3

ABSTRACTThermoplastic polymeric materials (TPEs) are widelyused in applications where the ability to transitionbetween effectively thermoset and plastic statesprovides a benefit in the manufacturing process. Globalexpansion of TPEs is predicted, led by increased useof styrenic-based products. However, polyolefin andpolyurethane TPEs enjoy wide application and offercompetition to styrenics for market share replacingnatural and synthetic rubbers.

Thermoplastic polyurethanes (TPUs) are typicallybased on polyether or polyester soft segments.However, a TPU based on polybutadiene canincrease the compatibility with rubber compounds bypresenting a new form with both rubber-like softsegments and urethane linkages. Polybutadiene-based TPUs are desired for their hydrophobicity,hydrolytic and chemical resistance, electrical insulationproperties, and low-temperature elasticity.

The present study introduces the technology associatedwith 7840 TPU resin, and the application of this newpolybutadiene-based TPU material in traditional rubbercompounding is explored. The use of diene-basedTPU as a co-curable adhesive tie layer between rubbercompounds and a polyurethane component will bedemonstrated. Additionally, the results from a studymixing diene-based TPU as an additive into traditionalrubber formulations are provided. Comparative data

using other commercially available TPUs and TPEs isincluded in order to assess the performance of the newpolybutadiene TPU.

INTRODUCTIONThermoplastic polyurethanes (TPUs) are typicallybased on polyether or polyester soft segments. Unlikestyrenic triblock thermoplastic elastomers (TPE)products, these TPUs are not widely utilized intraditional rubber compounding. The polar forms arenot very soluble in diene rubbers, or effectivelyvulcanized with sulfur curatives. However, a TPUbased on diene prepolymers can increase thecompatibility with rubber compounds by presenting anew form with both rubber-like soft segments andurethane linkages. Polybutadiene-based TPUs aredesired for their hydrophobicity, hydrolytic andchemical resistance, electrical insulation properties, andlow-temperature elasticity.1,2,3 Previously availablediene-based urethane materials were produced fromradical polymerized polybutadiene polyols, and werenot thermoplastic.4,5 New thermoplastic products aremade from anionically-prepared telechelicpolybutadiene diols which are linear and uniform inchain length. Reaction with diisocyanate and a chain-extending diol results in a thermoplastic material whichcan be characterized by chemical and physicalproperties intermediate to that of currently availableTPU and TPE grades. Figure 1 provides a schematiccomparing urethane and styrenic thermoplasticmaterials.

Figure 1. Typical styrenic triblock TPE (A) and multiblock TPU (B) macrostructure.

A.PS hard domains

elastomericsoft segment

B.PU hard domains

elastomericsoft segment

A.PS hard domains

elastomericsoft segment

B.PU hard domains

elastomericsoft segment

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7840 TPU resin (Cray Valley Company) is producedfrom Krasol® polybutadiene prepolymer by a reactiveextrusion process. 7840 TPU has been designed tobe compatible with rubber systems. The present studyintroduces the technology associated withpolybutadiene-based TPU materials. Physicalcharacteristics of the 7840 TPU product are presented.Possible applications of TPUs in traditional rubbercompounds are explored. The use of 7840 TPU as aco-curable adhesive tie layer between rubbercompounds and a polyurethane component will bedemonstrated. Additionally, the results from a studymixing 7840 TPU as an additive into traditional rubberformulations are provided. Comparative data usingother commercially available TPUs and TPEs isincluded in order to assess the performance of the newpolybutadiene TPU.

EXPERIMENTALIn addition to 7840 TPU, several other commerciallyavailable thermoplastic materials were included andused as received. Table I provides characterizationdata of 7840 TPU resin compared to other TPU andTPE products.

Table II details the formulation used as substrateswith virgin thermoplastic materials in adhesion studies.The formulation in Table III was used to preparecompounds in which the thermoplastic materials areused as a direct additive. A Banbury-style internalmixer was used to prepare the rubber compounds.Mixing was accomplished in two stages with the non-productive compound milled between stages andprior to curing. A thermoset polyurethane was usedalso as a substrate. Adiprene® L 100 (Uniroyal) is aTDI- terminated polyether based prepolymer thatproduces a vulcanizate with approximately 90 ShoreA hardness when cured with 4,4' - methylene-bis(2-chloroaniline).

Table I. Structural and physical properties of thermoplastic materials.

Typical Property 7840 TPU polyether TPU polyester TPU SBS SEBS Type TPU TPU TPU TPE TPE Soft Segment polybutadiene polyether polyester polybutadiene ethylene/butylene Hard Segment polyurethane polyurethane polyurethane polystyrene polystyrene Shore A Hardness 80 82 85 74 72 Tensile Strength (MPa) 14.0 36.6 44.8 9.3 26.8 Elongation (%) 550 670 550 785 630 100% Modulus (MPa) 5.6 5.2 5.5 3.2 2.9 Glass Transition Temp (C) -35 -49 -32 -61 -55 Vicat Softening Point (C) 52 70 85 ~100 ~100

Table II. Rubber substrate formulations. Table III. Model compound formulation.

Ingredient phr

Non-Productive ESBR (1502)a 100.0Carbon Black (N330) 60.0Process Oil (aromatic) 20.0Antioxidant (IPPD)b 2.0Antioxidant (TMQ)c 1.0Zinc Oxide 3.0Stearic Acid 2.0Thermoplastic Elastomer 0, 5.0, 10.0, 25.0

Productive Sulfur 2.0Accelerator (CBS)d 1.4aemulsion styrene-butadiene rubber, 23.5% styrenebN-isopropyl-N'-phenyl-p-phenylenediaminec2,2,4-trimethyl-1,2-hydroquinolinedN-cyclohexylbenzothiazole-2-sulfenamide

Ingredient phr

Non-Productive BR or IRa 100.0Carbon Black (N330) 50.0Process Oil (paraffinic) 10.0Antioxidant (TMQ)b 1.0Zinc Oxide 5.0Stearic Acid 2.0

Productive Sulfur 2.5Accelerator (TBBS)c 0.7

asolution cis -polybutadiene; solution cis-polyisopreneb2,2,4-trimethyl-1,2-hydroquinolinecN-t -butylbenzothiazole-2-sulfenamide

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RESULTS AND DISCUSSION

Adhesion StudyThe application of urethane materials in traditionalrubber compounding has been limited, primarily dueto poor compatibility between the systems. Urethanepolymers have very little solubility in diene-basedelastomers and are not readily cured by sulfur-basedvulcanization systems. As elastomeric components,urethane materials possess many desirable propertiessuch as excellent abrasion resistance, good flexuralproperties, low hysteresis, and good solventresistance.6 Benefits could be realized in manyengineered rubber product applications if a simplemeans of incorporating urethanes could be established.Today, adhesives and other applied methods arerequired to generate adhesion between urethane andrubber components in a laminate. The concept of usingdiene-based TPUs as a tie layer between unsaturatedrubber compounds and urethane materials has beenpromoted.7 However, previous work resulted in athermoset polyurethane in-situ using branchedpolybutadiene prepolymers with higher functionality(f > 2.0). Commercial development of 7840 TPU resinallows for improvement in such applications. Bycreating a material that possesses both unsaturated softsegments and hard segments of urethane linkages,diene-based TPUs can promote adhesion between thedissimilar components. In addition, the thermoplasticform allows for easy processing from the melt.

Adhesion properties of 7840 TPU were comparedto other thermoplastic materials. Adhesion to anuncured polybutadiene compound (Table II) was firstevaluated. Thin sheets of thermoplastic material (<2mm thickness) were placed against the uncured rubbercompound. The laminate was then compressionmolded in a press at 160ºC to the t90 cure time of therubber layer. Upon cooling, adhesion between thelayers was assessed qualitatively. In some cases,delamination between the layers was easily achieved(adhesive failure). Good adhesion resulted in cohesivefailure of the rubber or thermoplastic layer. Results areprovided in Table IV. As expected, those materialscapable of co-curing with the rubber compounddisplayed cohesive failure, while the saturated productsgave adhesive failure.

Adhesion between the different thermoplasticmaterials was also studied. Using an identicalprocedure, thermoplastic laminates were prepared.The results are given in Table V. 7840 TPU producedgood adhesion and cohesive failure against polyetheror polyester urethane substrates. Laminates of theTPUs and SBS TPE produced adhesive failure only.

Subsequently, the adhesion between the thermoplasticproducts and various substrates was assessedquantitatively. Peel adhesion testing, based on ASTMD1876-01, was performed at Akron RubberDevelopment Laboratory, Inc. The test was modifiedby restricting adhesion area to a 3” by 0.25” windowby masking with a nylon insert between the substrates.Figure 2 summarizes the peel adhesion results only forthe cured laminates which displayed cohesive failure(other laminates failed adhesively at the interface). Boththe cis-polybutadiene (BR) and cis-polyisoprene (IR)based rubber compounds were used as substrates.

Table IV. Adhesion testing (rubber).

Table V. Adhesion testing (thermoplastics).

Component A Component B Failure

7840 TPU polyether TPU cohesive7840 TPU polyester TPU cohesive7840 TPU SBS adhesiveSBS TPE polyether TPU adhesiveSBS TPE polyester TPU adhesive

Component A Component B Failure

7840 TPU polyether TPU cohesive7840 TPU polyester TPU cohesive7840 TPU SBS adhesiveSBS TPE polyether TPU adhesiveSBS TPE polyester TPU adhesive

Component A Component B Failure

rubber compound 7840 TPU cohesiverubber compound polyether TPU adhesiverubber compound polyester TPU adhesiverubber compound SBS cohesive

Component A Component B Failure

rubber compound 7840 TPU cohesiverubber compound polyether TPU adhesiverubber compound polyester TPU adhesiverubber compound SBS cohesive

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Auto-adhesion (self) of the cured rubber compoundsis shown. In comparison, the adhesive force betweenthe in-situ cured rubber compounds and the SBSthermoplastic and 7840 TPU is provided. Since thefailure mode was cohesive for all samples, actualadhesive force is also dependent upon the modulus ofthe failed component. 7840 TPU was the only TPUto produce an adhesive bond to the cured rubbersubstrates. By incorporating curatives into the TPUprior to laminate curing, the cohesive strength and,ultimately, the adhesive force may be increased.

Figure 3 outlines the data from peel adhesion testingbetween 7840 TPU and other polyurethanes. Boththermoplastic and thermoset polyether-basedpolyurethanes were used as substrates. With 7840 TPU,cohesive failure was seen in all cases (the SBS productproduced only adhesive failures). The highest adhesiveforce is achieved when the thermoset polyurethane wascast in-place against the diene-based TPU prior totesting.

Figure 2. Peel adhesion results for cured laminates resulting in cohesive failure.

Figure 3. Peel adhesion results for 7840 TPU and polyurethane substrates.

0

2040

60

80

100120

140

160

Self SBS 7840 TPU

Adh

esio

n Fo

rce

(lb/in

)

IR CompoundBR Compound

0

2040

60

80

100120

140

160

Self SBS 7840 TPU

Adh

esio

n Fo

rce

(lb/in

)

IR CompoundBR Compound

020406080

100120140160180

Polyether TPU Pre-molded Thermoset PU

Cast In-placeThermoset PU

Adh

esio

n Fo

rce

(lb/

in)

7840 TPU

020406080

100120140160180

Polyether TPU Pre-molded Thermoset PU

Cast In-placeThermoset PU

Adh

esio

n Fo

rce

(lb/

in)

7840 TPU

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Only the diene-based TPU effectively adhered thetwo dissimilar layers, producing cohesive failure ateach interface. The structure of 7840 TPU includesboth regions of unsaturated hydrocarbon capable ofco-curing to rubber compounds and hard segmentsthat promote interaction with similar structures in theurethane layer. Additionally, the diene-based TPU mayimprove adhesion between compounds based onelastomers with very different polarity, or as anadhesion aid to polar fabrics. The diene-based TPUmay provide utility as an intermediate tie-layer in multi-component articles including hose, belt, and tires.

Rubber CompoundingThe various thermoplastic materials were directlycompounded as additives in a rubber formulation inorder to determine how and to what extent thephysical properties are affected. 7840 TPU, polyetherTPU, SBS, and SEBS were mixed in the rubberformulation given (Table III). The polyester TPU didnot readily mix into the formulation and resulted in acompound with visible inhomogeneity which was nottested.

Results presented are normalized to the control. Thenormalized values coincide with the directionality of

the actual results. The effect of thermoplastic additiveson cure kinetics and state of cure were evaluated (TechPro MDpt, ASTM D 5289, 160ºC). Delta torquedecreases with loading of the additives, regardless oftype (Figure 4). It is important to realize that the torquevalue associated with the cure state may be attributableto reasons other than crosslink density derivedthrough the generation of sulfur linkages. At higherloadings a larger percentage of the apparent crosslinksare of the reversible type inherent to the thermoplasticadditive. Cure was measured above the softeningpoint of the TPU materials, and close to that of thetriblock TPEs. In addition, 7840 TPU has a lowermolecular weight (~ 30,000 g/mol, relative topolystyrene standard). It is common to lose some curedensity as a result of incorporating low molecularweight polydienes. An increase in the cure packageloading is typically recommended.

Trends in tensile properties of compounds containingincreasing loadings of 7840 TPU are summarized inFigure 7 (Thwing-Albert Materials Tester, ASTM D412 and D 624-C). Testing was conducted underambient conditions. Tensile strength, 300% modulusand tear strength increased, elongation decreased, and100% modulus remained constant with TPU loading.

Figure 4. Delta torque as a function of additive loading (control = 100).

Apparent Crosslink Density (Delta Torque)

0

20

40

60

80

100

120

5 10 25Additive phr

Nor

mal

ized

Val

ue

7840 TPU

Polyether TPU

SBS

SEBS

Apparent Crosslink Density (Delta Torque)

0

20

40

60

80

100

120

5 10 25Additive phr

Nor

mal

ized

Val

ue

7840 TPU

Polyether TPU

SBS

SEBS

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The cured compounds containing the thermoplasticadditives were submitted to a dynamic (tension)temperature sweep (TA Instruments DMA 2980, -100ºC to 100ºC at 11 Hz and 0.1% strain amplitude).Tangent delta was plotted as a function of temperatureto determine how the response of the base compound(control) is affected with additive loading (5, 10, and25 phr). Such a method is useful in ascertaining if grossphase morphology exists in these composites. Theresults for 7840 TPU and SBS are provided in Figures6 and 7.

According to the results in Figure 6, it appears that at5 and 10 phr of 7840 TPU, the TPU is soluble in thecompound but forms a discreet phase at 25 phr loading.A single peak (-30ºC) was seen at 5 and 10 phr,decreasing slightly in height with increased loading.However, at 25 phr an additional peak appeared atapproximately -10ºC. The difference in peaktemperatures (D 20ºC) correlates with that for theTgs of the individual polymers (-55ºC for 1502 ESBRand -35ºC for 7840 TPU). Only a single peak was

seen for the SBS material up to 25 phr loading (Figure6). The SBS appeared to be largely soluble in thesystem, however the Tg of the polystyrene blocksregister at > 100ºC.

The utility of phase separated morphology with apolyurethane-containing discreet phase was explored.DeMattia flex fatigue testing (ASTM D 813)wasperformed on the above compounds both below (10phr thermoplastic additive) and above (25 phrthermoplastic additive) the apparent compatibilitylimit of 7840 TPU in ESBR. Figures 8 and 9 providethe data, respectively. The compounds were cured tothe same 100% strain modulus values (2.0 MPa at 10phr additive, 2.4 MPa at 25 phr additive). At 10 phrloadings, each thermoplastic additive provided a slightimprovement in crack growth resistance. However, at25 phr loading, only the TPU additives provided thelarger reduction in fatigue resistance. It is evident thatthe flexural fatigue performance is enhanced by theaddition of a phase separated thermoplasticcomponent.

Figure 5. Tensile properties as a function of 7840 TPU loading.

Tensile Properties

5060708090

100110120130140150

0 5 10 25Additive phr

Nor

mal

ized

Val

ue Tensile Strength

% Elongation

100% Modulus

300% Modulus

Tear Strength

9

Figure 8. DeMattia flex fatigue results as a function of thermoplastic additive (10 phr).

00.10.20.30.40.50.60.70.80.9

-125 -100 -75 -50 -25 0 25 50 75 100 125Temperature (C)

Tang

ent D

elta

control5 phr10 phr25 phr

Figure 6. Temperature sweep for vulcanizates containing 0, 5, 10, and 25 phr 7840 TPU.

00.10.20.30.40.50.60.70.80.9

-125 -100 -75 -50 -25 0 25 50 75 100 125Temperature (C)

Tang

ent D

elta co

5 p1025

Figure 7. Temperature sweep for vulcanizates containing 0, 5, 10, and 25 phr SBS.

0 5

10 15 20 25

0 5000 10000 15000 20000 25000 30000 35000 Cycles

Crack Width (mm)

contr

7840

Poly

SBS

10

The phase-separated TPU can also enhance adhesionto other polyurethanes. Using the same modelformulation in Table II (cis-IR), 7840 TPU was addedat 5, 15, and 25 phr. DMA analysis showed a secondpeak corresponding to the TPU evolves between 5and 15 phr. Peel adhesion laminates were constructedagain, but no thermoplastic layer was used. Laminatedsamples were cured in-situ. A polyether TPU substratewas used for adhesion testing purposes. The resultsare provided in Figure 10. The adhesive force isconstant at loadings below phase separation, butincreases as a function of loading when a discreetTPU phase is formed.

Considering the structure-property relationships ofthe thermoplastic additives, the diene-based TPUprovides not only the ability to co-cure into theformulation (reducing hysteresis), but also has smallerhard segments unlike the very hysteretic styreneblocks associated with the triblock TPEs. Such acombination of structural properties is unique to 7840TPU, and may provide new opportunities informulating for molded tire applications.

0

5

10

15

20

25

0 5000 10000 15000 20000 25000 30000 35000

Cycles

Cra

ck W

idth

(m

m)

control

7840 TPU

Polyether TPU

SBS

0

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25

0 5000 10000 15000 20000 25000 30000 35000

Cycles

Cra

ck W

idth

(m

m)

control

7840 TPU

Polyether TPU

SBS

Figure 9. DeMattia flex fatigue results as a function of thermoplastic additive (25 phr).

Figure 10. Peel adhesion force as a function of 7840 TPU loading in cis-IR compound.

0.00.51.01.52.02.53.03.54.04.5

0 5 15 257840 TPU (phr)

Adh

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(kg/

cm)

0.00.51.01.52.02.53.03.54.04.5

0 5 15 257840 TPU (phr)

Adh

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rce

(kg/

cm)

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SUMMARY AND CONCLUSIONS7840 TPU resin is compatible with rubbercompounding, as the diene segments are capable ofco-curing with compounds employing sulfur orperoxide vulcanization. Linear urethane linkages allowfor not only thermoplastic quality but also the ability tobond with other urethane materials. The novel diene-TPU structure facilitates its use as an adhesive layerbetween rubber (uncured) and urethane compounds.

When used as an additive in rubber compounds, theperformance of 7840 TPU can be differentiated frompolyether TPUs or SBS TPEs. Unique processing andgreen strength properties as well as improved hysteresismay be explained in part by the evolution of phasemorphology with increased TPU loading. Results arepreliminary but suggest an interesting opportunity toadd urethane characteristics to traditional rubbercompounds, improving performance in a wide varietyof applications from molded and extruded articles suchas gaskets, shoe soles, cable sheathing and hose tomulti-component products such as tires, belts, andother automotive parts.

REFERENCES1 Poly bd, the Best of Two Worlds, Arco TechnicalBulletin, Arco Chemical Co., Division of AtlanticRichfield Co.2 J. Pytela, M. Sufcak, J. Cermak, and J. G. Drobny,Proceedings of the Polyurethanes EXPO98, Dallas,TX, 563 (1998).3 J. Pytela and M. Sufcak, Paper # 9, UTECH 2000Conference, The Hague, Netherlands, March 2000.4 Cray Valley Technical Bulletin No. 1560, Cray ValleyCo. (2004).5 N. Kebir, I. Campistron, A. Laguerre, J. F. Pilard,C. Bunel, J. P. Couvercelle, and C. Gondard,Polymer 46 (18), 6869 (2005).6 M. Szycher “Szycher’s Handbook of Polyurethanes,”CRC Press, Washington DC, 1999.7A. Schmidt, et al., US#4,774,142 (1988).

The information in this bulletin is believed to be accurate, but all recommendations are made without warranty since the conditions of use are beyond Cray Valley Company'scontrol. The listed properties are illustrative only, and not product specifications. Cray Valley Company disclaims any liability in connection with the use of the information,

and does not warrant against infringement by reason of the use of its products in combination with other material or in any process.