177

Macromolecular Research, Vol. 18, No. 2, pp 177-184 (2010) www.springer.com/13233

The Polymer Society of Korea

Thermoplastic Polyurethane Elastomer/Thermoplastic Polyolefin Elastomer BlendsCompatibilized with a Polyolefinic Segment in TPU

Tae Kyoon KimResearch Center, Hwaseung T&C Corp., Gyoungnam 626-220, Korea

Byung Kyu Kim*Department of Polymer Science and Engineering, Pusan National University, Busan 609-735, Korea

Sang Yun Lee and Yang Lae ChoUlsan R&D Center, Hosung Chemex Co. Ltd., Ulsan 689-821, Korea

Min Seok Kim and Han Mo Jeong*Department of Chemistry, University of Ulsan, Ulsan 680-749, Korea

Received July 28, 2009; Revised September 11, 2009; Accepted September 15, 2009

Abstract: The effect of incorporating a hydrogenated polybutadiene segment (HPB) in a thermoplastic polyure-thane elastomer (TPU) on the compatibility of TPU with a thermoplastic polyolefin elastomer (TPO) was examined.Scanning electron microscopy showed that the particle size of the dispersed phase in the TPU/TPO blends decreasedwith increasing HPB concentration in TPUs due to the compatibilizing effect of HPB. The crystallization behavior,tensile properties, and rheological properties of the blend also showed that the HPB present in TPUs enhanced thecompatibility between TPU and TPO.

Keywords: TPU, TPO, blend, hydrogenated polybutadiene, compatibility.

Introduction

Polymer modification through blending is an efficientmethod to satisfy critical performance requirements that arenot possible with the use of homopolymers alone. There-fore, a significant number of commercial polymer blendshave become available, and continued effort to create newmaterials with enhanced chemical or mechanical perfor-mance is expected. The majority of commercial polymerblends are comprised of a phase-separated mixture, wherethe components reside in separate domains due to positiveenthalpy change and a small increase in entropy upon mix-ing. Because properties of these phase-separated polymerblends depend on the morphology of the blends, factorswhich determine phase-separated morphology, such as theinterfacial tension, viscosities of constituent polymers, andshear rate should be controlled to produce the optimum sizeand shape of constituent phases in polymer blends.1-7

Thermoplastic polyurethane elastomer (TPU) is a linear,

segmented copolymer composed of a rubbery, soft segmentand a rigid, polar hard segment. TPU is one of the most ver-satile engineering thermoplastics with elastomeric proper-ties because a wide range of monomeric materials arecommercially available, and tailor-made properties can beobtained from well-designed combinations of monomericmaterials. TPU possesses a high tensile modulus comparedto rubber. In addition, TPU has high abrasion resistance,high wear and tear resistance, is resistant to oil and manysolvents, and is also easily painted.8-10

Thermoplastic polyolefin elastomer (TPO), which is adynamically cured blend of polypropylene and polyolefinelastomers, is one of the most popular commodity thermo-plastics due to its high impact strength, light weight, ease ofprocessing and recycling, low cost, and good chemical resis-tance. However, low surface energy and lack of reactivesites causes difficulties in applications requiring good sur-face/interface properties, such as when bonding and paint-ing.11,12 The polarity of TPO can be increased by the introductionof polar groups into the backbone of the polyolefin. Surfacetreatment with plasma or corona discharge, or the graftingof unsaturated molecules bearing functional groups such asmaleic anhydride, are typical methods to enhance the sur-

*Corresponding Authors. E-mails: bkkim@pnu.edu orhmjeong@mail.ulsan.ac.kr

DOI 10.1007/s13233-009-0165-1

T. K. Kim et al.

178 Macromol. Res., Vol. 18, No. 2, 2010

face tension of TPO. Blending polar polymers such as male-ated polypropylene (PP) or polyetheramines is a recentapproach to modifying TPO.11 TPU is another candidate forthe modification of TPO because TPU has superior physicalproperties and its molecular structure can be properlydesigned for blending. Previously published work concerningTPU/polyolefin blends utilized a compatibilizer or a modifiedpolyolefin to improve the compatibility of TPU and TPO.9,10,13-16

However, to the author’s knowledge, no work has focusedon modifying the molecular structure of TPU to enhancecompatibility.

In this study, nonpolar hydrogenated polybutadiene seg-ments were introduced in TPU molecules to increase thecompatibility with nonpolar TPO, and the effects of this molec-ular design on the morphology and physical properties ofTPU/TPO blends were examined.

Experimental

Materials. Poly(butylene adipate) diol (PBA, molecularweight: 2,000 g/mol) was obtained from Union Chemical.Hydrogenated polybutadiene polyol (HPB, Polytail H, molec-ular weight: 2,800 g/mol, functionality: 2.3) was a productof Mitsubishi Chemical. Liquid methylene diphenyl diiso-cyanate (MDI) containing 50% para,para’-MDI and 50%ortho,para’-MDI was obtained from BASF Corporation.The TPO used in this study was a commercially availableproduct (KEYFLEX 1175A) of LG Chemical.

Blend Preparation. A 1 L round-bottomed, four-neckedflask equipped with a mechanical stirrer, thermometer,nitrogen gas inlet, and a condenser with a drying tube wasused as the reaction vessel for the synthesis of TPUs. Thereaction temperature was controlled using an oil bath. Thereference TPU, which contained no HPB (H0U100), wasprepared as follows; PBA (89.59 g, 44.80 mmol) was addedinto the reactor, melted, and maintained at 90 οC. Subse-quently, liquid MDI (10.41 g, 41.64 mmol) at 60 οC was fedinto the reactor and allowed to react with the PBA for 3~6min at 90 οC to obtain TPU. Modified TPUs containingincreased amounts of HPB (1 g: H1U100, 3 g: H3U100, 5 g:H5U100) were prepared using the same procedure asdescribed above with the varying amounts of HPB fed intothe reactor with the PBA. When the amount of HPB fed intothe reactor was more than 5 g, it was not easy to control thepolymerization reaction reproducibly, because the compati-bility of HPB with other polyols was poor and because itcaused gelling due to its high functionality larger than 2.The sample designation codes shown in Table I provideinformation about the kind of TPU used in blending and theweight ratio of the TPU/TPO blend. For example, H3U75was prepared with H3U100 by the H3U100/TPO weightratio of 75:25.

Before processing, the TPU and TPO were dried in a vacuumat 100 οC for 3 h. TPU and TPO pellets were compounded

with a 25 mm twin-screw extruder (L/D=40, Berstoff) at200 οC. The screw configuration was assembled appropri-ately for the TPU/TPO blend system. The output was 10 kg/hand residence time was about 50 sec. The specimens fortesting were compression molded at a pressure of 100 bar at180 οC for 5 min. The molds employed were Teflon-coatedto provide a non-adhesive surface.

Morphology. Morphology of the cryogenically fracturedsurface was observed with a field emission scanning elec-tron microscope (FE-SEM, JEOL JSM-6500F). The fracturedsurfaces were coated with gold to make them electricallyconductive and to prevent charging.

Thermal Properties. Thermal properties were examinedwith a differential scanning calorimeter (DSC, TA Instru-ment Model Q10). The sample was loaded into the DSC at30 οC and then rapidly cooled to -80 οC at a cooling rate of240 οC/min. After 2 min at -80 οC, the first heating scan wasperformed up to 240 οC at a heating rate of 15 οC/min. Fol-lowing 2 min at 240 οC, a cooling scan to -80 οC was per-formed. After maintaining temperature at -80 οC for 2 min,the second heating scan was performed.

Rheological Properties. Dynamic rheological propertiesof the TPUs were examined with a parallel plate rheometer(Physica, MCR 301) at 200 οC with a 0.5% strain level; theupper limit where the linear viscoelastic behavior was main-tained. The diameter of the plate was 50 mm.

Mechanical Properties. Tensile tests were performedwith a tensile tester (Instron 3365) according to ASTM D882.The dumbbell-shaped micro-tensile specimen had the fol-lowing dimensions: length, 100 mm; width, 10 mm; and thick-ness, 0.2 mm. The specimen was elongated at a rate of 200mm/min.

Results and Discussion

Morphology. Figure 1 shows the morphology of cryo-genically fractured surfaces of TPU/TPO (25/75 by weight)blends as observed by SEM. The morphology of H0U25shows discrete domains of TPU with a diameter of about 1µm dispersed in the TPO matrix and the holes show thatTPU domains were pulled out during fracturing. This dem-onstrates that the compatibility between TPU and TPO, andthe interfacial adhesion at the phase boundary, are poor. Asthe content of HPB in the TPU was increased, the dispersedTPU domain size became finer, and the interface becameobscure. In the polymer blends, the number average particlesize of dispersed phase, an can be described by the follow-ing eq. (1),2,17,18 where G is the shear rate, γ the interfacialtension, ηm the matrix viscosity, and ηd the viscosity of dis-persed phase. Therefore, the results of Figure 1 suggest thatHPB, which has the structure of polyolefin, lies at the inter-face of TPU phase and TPO phase, and consequently reducesthe interfacial tension and improves compatibility betweenTPU and TPO.

TPU/TPO Blends Compatibilized with a Polyolefinic Segment

Macromol. Res., Vol. 18, No. 2, 2010 179

(1)The morphology of 75/25 TPU/TPO blends show that the

particle size of the dispersed TPO droplet is reduced as thean4γ

Gηm---------- ηd

ηm------⎝ ⎠⎛ ⎞

0.84

∝

Figure 1. SEM micrographs of (a) H0U25, (b) H1U25, (c) H3U25, and (d) H5U25.

Figure 2. SEM micrographs of (a) H0U75, (b) H1U75, (c) H3U75, and (d) H5U75.

T. K. Kim et al.

180 Macromol. Res., Vol. 18, No. 2, 2010

content of HPB in TPU is increased, due to the compatibi-lizing effect of HPB (Figure 2). However, the disperseddroplet size of TPO in Figure 2 is relatively larger than thedispersed droplet size of TPU shown in Figure 1 when theblends prepared with the same kind of TPU are compared.That is, the dispersed droplet size of Figure 2(a) is largerthan that of Figure 1(a), and that of Figure 2(b) is larger thanthat of Figure 1(b), and so on. Eq. (1) suggests that dis-persed particle size increases as ηm is decreased and ηd isincreased. Therefore, the size differential seen between Fig-ure 1 and Figure 2 can be explained by this rheological fac-tor. Because TPO has a higher melt viscosity than TPU (seeFigure 8), it can be estimated from eq. (1) that the size of theTPO droplet in the TPU matrix will be larger than that ofthe TPU droplet in TPO matrix, as seen in our experimentalresults.

Thermal Properties. DSC thermograms of the H0U100/TPO blends obtained from the first cooling scan and fromthe subsequent heating scan are shown in Figure 3 and Fig-ure 4, respectively. The thermal properties obtained fromthese thermograms are summarized in Table I.

The glass transition temperature (Tg) of H0U100 was-55.2 οC and the broad melting endothermic peak tempera-ture (Tm) was 152.1 oC. TPO also had Tg and Tm at similartemperatures, -54.8 οC and 150.8 οC, respectively. As expected,the Tg and Tm of blends were near these temperatures (Fig-ure 4).

The exothermic crystallization peak (Tc) of TPO was 112.7 oC,and H0U100 had a crystallization peak at the much lowertemperature of 67.4 oC (Figure 3). Two blends, H0U75 and

Figure 3. DSC thermograms obtained on cooling of (a) H0U100,(b) H0U75, (c) H0U50, (d) H0U25, and (e) TPO.

Figure 4. DSC thermograms obtained on heating of (a) H0U100,(b) H0U75, (c) H0U50, (d) H0U25, and (e) TPO.

Table I. Thermal Properties of TPU/TPO Blends

Sample Tc of TPU/TPO(oC)

∆Hc of TPU/TPO(J/g-TPU/J/g-TPO)

Series 0 -

H0U100 67.4 / - 4.8 / -

H0U75 65.4 / 110.2 4.4 / 14.6

H0U50 65.0 / 110.6 2.5 / 21.4

H0U25 - / 110.9 - / 22.9

TPO - /112.7 - / 25.2

Series 1

H1U100 56.9 / - 4.7 / -

H1U75 63.1 / 109.4 2.8 / 15.3

H1U50 62.7 / 110.3 1.6 / 20.0

H1U25 - / 111.0 - / 23.5

Series 3

H3U100 55.8 / - 4.0 / -

H3U75 51.2 / 109.7 2.3 / 14.0

H3U50 - / 110.8 - / 21.3

H3U25 - / 111.0 - / 24.3

Series 5

H5U100 38.9 / - 4.1 / -

H5U75 35.0 / 110.0 0.9 / 17.2

H5U50 - / 110.9 - / 22.6

H5U25 - / 111.3 - / 24.9

TPU/TPO Blends Compatibilized with a Polyolefinic Segment

Macromol. Res., Vol. 18, No. 2, 2010 181

H0U50 had two separate Tc's. Figure 3 and Table I show thatin Series 0, the crystallization temperatures of H0U75 andH0U50 shifted to a lower temperature in the blends fromthose of TPU or TPO as blend content decreased. Table Ialso shows that in Series 0, the heat of crystallization (∆Hc)at Tc decreased as blend content was reduced, and no Tc peakof TPU was evident in H0U25 where the TPU content ofthe blend was 25 wt%. These results suggest that the crys-tallization of TPU or TPO in blends was hindered by theother component, which shows that a partial mixing at themolecular level exists between these two components.

Series 3 and Series 5 in Table I show similar results tothose seen in Series 0, however the reductions in Tc and ∆Hc

of TPUs, as the content TPU in blend was decreased, aremore evident, suggesting that the partial miscibility betweenTPU and TPO was improved as the HPB content in TPUwas increased, confirming the compatibilizing effect of HPB.

In Series 1, the Tc of TPU was increased in blends com-pared to that of TPU itself (Table I), which demonstratesthat the dispersed TPO, which crystallized at a higher tem-perature during cooling, can accelerate the crystallizationrate of TPU by a nucleating effect,19 even though the ∆Hc ofTPU was reduced compared to Series 0.

Mechanical Properties. Figure 5 shows the variation ofmodulus as the TPU/TPO blend weight ratio was varied.Modulus of blends are expressed by a modified rule ofmixture:

Eb = w1E1 + w2E2 + α12w1w2 (2)

where E is the modulus, w is weight fraction, and α12 is an

Figure 5. The modulus of (a) H0U100/TPO, (b) H1U100/TPO, (c) H3U100/TPO, and (d) H5U100/TPO blends.

Table II. α 12 and β 12 Values of TPU/TPO Blends

Sample α12 (MPa) β12 (Pa·s)

Series 0

H0U75 -5.6 -3,532

H0U50 -2.8 -3,054

H0U25 -5.1 -4,649

Series 1

H1U75 -8.9 -3,751

H1U50 -4.2 -2,414

H1U25 -5.5 -4,703

Series 3

H3U75 -4.8 -1,537

H3U50 -2.0 -2,566

H3U25 -3.7 1,303

Series 5

H5U75 -1.2 -692

H5U50 -1.0 -946

H5U25 -2.5 2,116

T. K. Kim et al.

182 Macromol. Res., Vol. 18, No. 2, 2010

empirical parameter. Subscripts b, 1, and 2 indicate blend,component 1, and component 2, respectively. The interac-

tion term (α12) expresses the magnitude of deviation fromlinearity, and can be used as a relative measure of compati-

Figure 6. Tensile strength of (a) H0U100/TPO, (b) H1U100/TPO, (c) H3U100/TPO, and (d) H5U100/TPO blends.

Figure 7. Elongation at break of (a) H0U100/TPO, (b) H1U100/TPO, (c) H3U100/TPO, and (d) H5U100/TPO blends.

TPU/TPO Blends Compatibilized with a Polyolefinic Segment

Macromol. Res., Vol. 18, No. 2, 2010 183

bility.19,20 The α12 values calculated using eq. (2) are pre-sented in Table II. All the α12 values are negative, showingthat the compatibility between TPU and TPO is marginal.However, the values generally increased as the content ofHPB in TPU was increased when compared to the blendswith the same TPU/TPO ratio. Only Series 1 (Figure 5(b))deviates from this trend. These data suggest that compatibil-ity between TPU and TPO was improved by the addition ofHPB in TPUs.

Tensile strengths also show similar trends (Figure 6) inthat the negative deviation was reduced as the content ofHPB in TPU was increased, although Figure 6(b) deviatesfrom this trend.

As can be see in Figure 7, elongation at break values mostlydeviated positively from the simple additive rule. The defor-mation of thin specimens can be approximated by the planestress state whereas thick specimens are mostly in a planestrain state. The stress at which a material yields is lower inthe plane stress state than in the plane strain state. Thus in aplane stress state, higher degree of plasticity and consequentlya higher elongation break can be developed.21,22 Therefore,poor adhesion at the TPU/TPO interface seems to induce areduced thickness effect at the microscopic level and promotesplastic deformation for a higher elongation break value.

Rheological Properties. Figure 8 shows that TPO hadmuch higher complex viscosity, η*, relative to H0U100, andblends had intermediate η* values which varied consistentlywith composition.

Figure 9(a) shows that η* values of blends measured at0.1 rad/sec deviated negatively from the simple additiverule, indicating incompatibility within the blend.23,24 How-ever, the negative deviation was reduced and became posi-tive as the content of HPB in TPU was increased. Thedegrees of deviation for all TPU/TPO blend systemsshown in Figure 9 were analyzed with the following eq.(3):

Figure 9. Complex viscosity of (a) H0U100/TPO, (b) H1U100/TPO, (c) H3U100/TPO, and (d) H5U100/TPO blends.

Figure 8. Complex viscosity versus frequency for (○) H0U100, (▽)H0U75, ( □ ) H0U50, (◇ ) H0U25, and (△ ) TPO.

T. K. Kim et al.

184 Macromol. Res., Vol. 18, No. 2, 2010

η*b = w1η*

1 + w2η*2 + β12w1w2 (3)

where η* is the complex viscosity, w is weight fraction, andβ12 is an empirical parameter. Subscripts b, 1, and 2 indicateblend, component 1, and component 2, respectively. Theinteraction term (β12) expresses the magnitude of deviationfrom linearity, and can be used as a relative measure ofcompatibility. The results of these calculations are listed inTable II, which show that the β12 value generally increasedas HPB content in the TPU increased, although there existsome exceptions. These results suggest that compatibilitybetween TPU and TPO was enhanced by the compatibiliz-ing effect of HPB in TPU.

Conclusions

Our experimental results showed that compatibility betweenTPU and TPO could be improved by the incorporation ofpolyolefinic segment, HPB in TPU. As the content of HPBin TPU was increased, the particle size of the dispersedphase in the matrix was reduced, the tensile properties of theblends were improved, and the negative deviation of η*

from the simple additive rule was reduced. These resultssuggest that HPB segment lies at the interface of TPU phaseand TPO phase to do its role as a compatibilizer whichreduces the interfacial tension. So, one can conclude that HPBis an effective TPU modifier to prepare TPU/TPO blendswith balanced properties, except that its price is expensive,about 40,000 won/kg.

Acknowledgement. This study was supported by the regionalindustry-leading technology development program of TheMinistry of Knowledge Economy of Korea Government (MKE);contract grant number: 70004208. BKK is also indebted tothe PNU-IFAM JRC organized at PNU.

References

(1) P. Pötschke, J. Pionteck, and H. Stutz, Polymer, 43, 6965(2002).

(2) S. J. Ahn, K. H. Lee, B. K. Kim, and H. M. Jeong, J. Appl.

Polym. Sci., 73, 935 (1999).(3) J.-K. Yun, H.-J. Yoo, and H.-D. Kim, Macromol. Res., 15, 22

(2007).(4) B. Y. Shin, G. S. Jo, K. S. Kang, T. J. Lee, B. S. Kim, S. I.

Lee, and J. S. Song, Macromol. Res., 15, 291 (2007).(5) S.-K. Na, B.-G. Kong, C. Choi, M.-K. Jang, J.-W. Nah, J.-G.

Kim, and B.-W. Jo, Macromol. Res., 13, 88 (2005).(6) S. Park, C. Yim, B. H. Lee, and S. Choe, Macromol. Res., 13,

243 (2005).(7) J. K. Lee, J. E. Im, and K. H. Lee, Macromol. Res., 12, 172

(2004).(8) H. M. Jeong, B. K. Kim, and Y. J. Choi, Polymer, 41, 1849

(2000).(9) X. Wang and X. Luo, Eur. Polym. J., 40, 2391 (2004).

(10) J. Yang, X. Chen, R. Fu, M. Zhang, H. Chen, and J. Wang, J.Appl. Polym. Sci., 109, 3452 (2008).

(11) M. Hailat, H. X. Xiao, and K. C. Frisch, J. Elastom. Plast.,32, 194 (2000).

(12) M. Hailat, H. X. Xiao, K. C. Frish, and D. Klempner, J. Elas-tom. Plast., 33, 154 (2001).

(13) Y. Di, M. Kang, Y. Zhao, S. Yan, and X. Wang, J. Appl.Polym. Sci., 99, 875 (2006).

(14) H. Stutz, W. Heckmann, P. Pötschke, and K. Wallheinke, J.Appl. Polym. Sci., 83, 2901 (2002).

(15) S. Paul, S. Verenich, and B. Pourdeyhimi, Polym. Int., 57, 975(2008).

(16) C. P. Papadopoulou and N. K. Kalfoglou, Polymer, 40, 905(1999).

(17) S. Wu, Polym. Eng. Sci., 27, 335 (1987).(18) S. J. Anh, K. H. Lee, B. K. Kim, and H. M. Jeong, J. Appl.

Polym. Sci., 78, 1861 (2000).(19) Y. K. Lee, Y. T. Jeong, K. J. Kim, H. M. Jeong, and B. K.

Kim, Polym. Eng. Sci., 31, 944 (1991).(20) I. Mondragon and J. Nazabal, Polym. Eng. Sci., 25, 178 (1985).(21) A. J. Kinloch and R. J. Young, Fracture Behavior of Poly-

mers, Elsevier, London, 1985.(22) D. W. Jin, K. H. Shon, H. M. Jeong, and B. K. Kim, J. Appl.

Polym. Sci., 69, 533 (1998).(23) P. Robson, G. J. Sandilands, and J. White, J. Appl. Polym.

Sci., 26, 3515 (1981).(24) S. J. Park, B. K. Kim, and H. M. Jeong, Eur. Polym. J., 26,

131 (1990).