Dynamic mechanical properties of nanoclay filled TPU/PP blends with compatibiliser

Abstract

Blends of thermoplastic polyurethane (TPU) and polypropylene (PP) are highly incompatible because of large differences in polarities and high interfacial tensions. On one hand, PP is added to TPU to improve TPU's thermal stability, chemical properties, mechanical properties (modulus, strength and hardness) and processing performance and to reduce TPU's cost. On the other hand, TPU is blended with PP to improve PP's properties (e.g. abrasion, flexibility, tear strength, shock absorbing capabilities, impact strength, adhesion and paintability/printability). Earlier works in polyurethane/organoclay nanocomposites, PP/organoclay nanocomposites and TPU/PP blends were studied. In our experimental work, both ester and ether based TPU nanocomposites were prepared by melt blending using 3 wt-% Cloisite 10A (organically modified montmorillonite clay) as the nanoscale reinforcement and blended with PP with/without PP-graft-maleic anhydride as the compatibiliser. Blends of nanoclay filled TPU/PP were evaluated for dynamic mechanical properties such as storage modulus E′, loss modulus E″ and dissipation factor tanδ.

Keywords

Polymer blends Dynamic mechanical properties Nanocomposites TPU/PP blend Compatibiliser

Introduction

Dynamic mechanical analysis has proved to be an effective tool especially for characterising the dual viscous and elastic nature of polymeric materials. In an oscillatory field, the viscoelastic material shows a phase lag between the imposed dynamic strain and the dynamic stress response or vice versa. Since the stress and strain are generally not in phase, modulus and phase angle can be determined. Dynamic mechanical results are generally given in terms of complex modulus E*, which can be represented as where E′ is the real part of the modulus, and E″ is the imaginary part of the modulus. E′ is the storage modulus due to the stored elastic energy in the material, and E″ is the loss modulus due to viscous dissipation. The angle that reflects the time lag between the applied stress and strain is δ and is defined by tan δ = E″/E′.

The main purpose of the present study is to analyse the dynamic mechanical properties of nanoclay filled thermoplastic polyurethane (TPU)/polypropylene (PP) blends. The properties including storage modulus E′, loss modulus E″ and dissipation factor tanδ were investigated as a function of temperature and frequency. The area under the loss curve is evaluated to investigate the damping behaviour. The parameters of interest in this study are the respective loss factor peak heights and the loss factor peak locations to assess the blend miscibility and phase continuity. The dynamic mechanical properties like E′, E″ and tanδ of polymer blends are sensitive not only to different molecular motions but also to various transitions, relaxation process, structural heterogeneity, extent of crosslinking and morphology of multiphase systems. The damping in the system or energy loss per cycle can be measured from the tangent of the phase angle.

The aim of this paper is to develop nanoclay filled TPU/PP blend with compatibiliser to get overall best performance, using both blending and nanocomposite technologies. Nanoclay reinforcement, besides giving substantial increase in modulus and tensile strength, also functions as a surface modifier for TPU hard segments. The reduction in surface energy of hard segments is confirmed by AFM tests and contact angle measurements. The reduced interfacial tensions between the TPU and the PP due to the incorporation of nanoclay gives better compatible blends. Cloisite 10A, which is a more hydrophobic nanoclay, when premixed with TPU, can function as a compatibiliser for TPU/PP blends because of its affinity for the non-polar PP. Compatibilisation can be further improved by introducing functionalised PP (MA-g-PP) in the nanoclay containing blends. The strategy of reactive compatibilisation is supposed to be fast reaction between carbonyl functional groups of TPU, hydroxyl group of nanoclay silicate layers and anhydride moiety of MA.

Potschke et al.^1–3 have studied the surface tension and interfacial tension of different polyurethane and polyolefil (PO). The reported surface tension value for TPU hard segment is 46 mN m^− 1 and for the soft segment is 29.4 mN m^− 1 (ester based) and 22.8 mN m^− 1 (ether based). Values depend on molecular weight of polyols and temperature. Potschke et al.^{4, 5} have carried out PO/TPU blend study without compatibilisers. They reported that at similar viscosity ratios (μ_d/μ_m), blends with polyether based TPU produced a finer morphology than blends with polyester based TPU, due to low interfacial tension between polyether based TPU soft segment and PO. They further concluded that blends with PP show a smaller drop in properties, compared to blends with polyethylene (PE). This may be due to the fact that PE is much more viscous and elastic than PP. Another possibility to improve the dispersity is given by compatibilisation. In compatibilised blends, the interfacial free energy is reduced, phase adhesion is induced and the morphology is stabilised. Macosko et al. have studied TPU/PP based blend in the ratio of 70/30 with compatibiliser like Maleic anhydride (MA)-g-PP and primary/secondary amine-g-PP.^6–8 Song et al. investigated the phase morphology of nanoclay polyurethane nanocomposite by small angle X-ray scattering and atomic force microscopy (AFM).⁹ It was observed that with increase in clay content, the surface energy of polyurethane hard segment decreased (reduced from 46 to 32 mN m^− 1). This result suggested that organic modification of nanoclay had a surface activation function to some extent.

Experimental

Ester-TPU (385S) and ether-TPU (KU2-8600E) were supplied by Bayer (TPU) India Ltd, Chennai. The melt flow index (MFI) value of 385S and KU2-8600E is 10 g/10 min and 11 g/10 min respectively (190°C/2.16 kg). The hardness value of 385S and KU2-8600E is Shore A 85 and 82 respectively. The PP (MA 1100) was supplied by Reliance Industries Ltd, Jamnagar, India. The MFI value of PP (MA 1100) is 11 g/10 min (230°C/2.16 kg). MA-g-PP compatibiliser was purchased from Pluss Polymers, New Delhi. The compatibiliser is a MA functionalised PP (MA-g-PP), containing 1 wt-% of MA. The MFI value of MA-g-PP compatibiliser is 12 g/10 min (190°C/2.16 kg). The organoclay used in this study (Cloisite 10A) is obtained from Southern Clay Products, USA. It is a Na⁺ montmorillonite, chemically modified with dimethyl benzyl hydrogenated tallow quaternary ammonium ions (N⁺2 MBHT), where N⁺ denotes quaternary ammonium ions, and HT denotes hydrogenated tallow. The HT is made of ∼ 65% C₁₈H₃₇, 30% C₁₆H₃₃ and 5% C₁₄H₂₉. Cation exchange capacity is 125 meq/100 g clay.

Ester-TPU/C10A and ether-TPU/C10A nanocomposites were prepared and subsequently blended with PP for various studies. Three weight per cent of the nanoclay was used. The ester- and ether-TPU pellets were dried at 100°C for 4 h. The nanoclay was dried at 100°C for 12 h in vacuum oven. The dried pellets were fed into a co-rotating twin screw extruder (Berstrof ZE 25), and the temperature of the die zone was maintained at 190°C. The extrudate was received as strands, which were cut into small granules for melt blending with PP. The granules of nanocomposites, along with pellets of PP and MA-g-PP, were predried at 100°C for 4 h in a vacuum oven. Blending was performed in the co-rotating twin screw extruder (Berstrof ZE 25) at a die zone temperature of 190°C, and the extrudate was again obtained in the form of strands.^10–12 Nanoclay filled blends of two compositions each for ester- and ether-TPU (Table 1) were prepared by the method described above. Neat ether- and ester-TPU/PP blends were prepared in the ratio of 70/30.

Table 1

Compositions of blend nanocomposites

Material	TPU(C10A) (wt %)	PP (wt %)	MA-g-PP (wt %)
Ester-TPU(C10A)/PP	70	30	0
Ester-TPU(C10A)/PP/MA	70	25	5
Ether-TPU(C10A)/PP	70	30	0
Ether-TPU(C10A)/PP/MA	70	25	5

The strands obtained after blending were cut into small granules for injection moulding. The prepared blend nanocomposites were moulded in an injection moulding machine (Ferromatik Milacron) to produce specimens for tensile test according to ASTM D-638. Moulding was performed after predrying at 90°C for 4 h. The various processing parameters were maintained as follows: nozzle temperature 200°C, barrel temperature 190°C, injection speed 110 mm s^− 1 and fill pressure 145 bar.

The change in gallery spacing of silicate layers in the blend nanocomposites was determined on an X-ray diffractometer (Bruker AXS D-8 Advance) using Cu (λ = 1.5406Å) as the radiation source. The samples were scanned at a rate of 3° min^− 1 (the increment step was 0.01°, and the step time was 0.2 s) at room temperature for 2-theta values starting from 1° to 50°. The d spacings were calculated using Bragg's equation (nλ = 2d sinθ).

Dynamic mechanical measurements were performed for the prepared blend nanocomposites (60 mm × 13 mm × 3.5 mm) using a NETZSCH DMA 242 instrument provided with a dual cantilever. Analysis was performed in the temperature range − 100 to 200°C at a frequency of 10 Hz and a heating rate of 5°C min^− 1. Tensile tests were carried out as per ASTM D 638 on a universal testing machine (International Equipments, Mumbai) at a crosshead speed of 200 mm min^− 1. A maximum of 400% elongation was allowed, and the stress required for 20, 100 and 200% elongation was recorded.

The injection moulded sample was dissolved in xylene at 105°C for 4 h to remove PP portion in the TPU/PP blend. Morphological studies were performed on the chemically etched and gold sputtered samples using JOEL (JSM-5800) scanning electron microscope.

The samples for TEM analysis were prepared by ultracryomicrotomy with a Leica Ultracut UCT (Leica Mikrosystems GmbH, Vienna, Austria). Freshly sharpened glass knives with cutting edges of 45°C were used to obtain cryosections of 100–120 nm thickness. Because these samples were elastomeric in nature, the sample and glass knife temperatures during ultracryomicrotomy were kept constant at − 75 and − 85°C respectively [these temperatures were well below the glass transition temperature (T_g‘s) of PUs]. The cryosections were collected individually in a sucrose solution and directly supported on a copper grid of 300 meshes in size. Microscopy was performed with a JEOL JEM 2000 TEM instrument (Japan), operating at an accelerating voltage of 120 kV.

Results and discussion

Dynamic mechanical analysis

The dynamic mechanical parameters (storage modulus and loss modulus) for the various blend nanocomposites are plotted as a function of temperature, at a constant frequency of 10 Hz (Figs. 1 and 2).^13–14 The storage and loss moduli at − 95°C are shown in Table 2. The storage modulus at − 95°C of ester-TPU/PP blend, without nanoclay and compatibilised with the same wt-% of MA-g-PP, prepared under identical conditions, is reported to be 7.90 GPa.¹⁵ Thus, a 3 wt-% C10A reinforcement increased the storage modulus of the blend by ∼70%. At room temperature, the storage modulus value of the ester-TPU blend nanocomposite is more than that of the ether-TPU blend nanocomposite. It is possible that well dispersed clay platelets resulted in an increase in the storage modulus.

Table 2

Storage, Loss moduli at -95°C/25°C and peak positions in the tan δ/E″ curve

Material	Storage Modulus E′ (GPa) ( − 95°C)	Storage Modulus E′ (GPa) (25°C)	Loss Modulus E″ (GPa) ( − 95°C)	Loss Modulus E″ (GPa) (25°C)	T_g from E″ peak (°C)	T_g from Tan δ peak (°C)
Ester-TPU(C10A)/PP	9.97	1.5	0.36	0.20	− 25	− 10.0
Ester-TPU(C10A)/PP/MA	13.30	2.32	0.67	0.25	− 20	− 8.0
Ether-TPU(C10A)/PP	8.97	0.66	0.24	0.052	− 35	− 18.5
Ether-TPU(C10A)/PP/MA	9.52	0.70	0.29	0.066	− 32	− 18.0

The dissipation factor (tanδ) of the materials based on ester- and ether-TPU is presented as a function of temperature in Fig. 3. The tanδ peak is associated with the soft segment glass transition temperature, and the peak positions are given in Table 2. The temperature corresponding to the tanδ peak and loss modulus peak E″ increases in the order – ether(C10A)/PP < ether-TPU(C10A)/PP/MA < ester-TPU(C10A)/PP < ester-TPU(C10A)/PP/MA. This is attributed to the increasing order of miscibility for the blends. The same trend was observed in the plot of loss modulus versus temperature for the blends, thereby confirming the higher miscibility in compatibilised ester-TPU(C10A)/PP/MA blends. Compatibilised ester-TPU(C10A)/PP showed a storage modulus of the order of approximately GPa in the sub T_g region, while uncompatibilised blends and compatibilised ether-TPU (C10A)/PP had storage moduli of ∼8.97 and 9.52 GPa respectively. Dynamic mechanical analysis showed a T_g (from E″) shift of ∼5°C in the higher side (for ester from − 25 to − 20°C).

The damping is low below T_g because thermal energy is insufficient to cause rotational and translational motions of the segments. As a result of this, the chain segments are frozen. Below T_g, the deformations are thus mainly elastic, and molecular slip resulting in viscous flow is low. As temperature increases, damping goes through a maximum, near T_g, in the transition region and then a minimum in the rubbery region. Above T_g, where rubbery region exists, the damping is also low because molecular segments are very free to move about and there is only very little resistance for flow. Thus, when the segments are either frozen or free to move, damping is low.

X-ray diffraction analysis

The diffraction characteristics of uncompatibilised and compatibilised ester- and ether- TPU(C10A)/PP blends are shown in Figs. 4 and 5. In ester-TPU(C10A)/PP, a peak appears at 2θ∼4.4° (d₀₀₁ = 1.59 nm), indicating a slight increase in gallery space as compared to neat nanoclay.^16–18 This is an indication that the non-polar PP chains have been incorporated in the gallery space because of their affinity for the nanoclay. The peak is absent in ester-TPU (C10A)/PP/MA indicating good dispersion. It appears that the compatibiliser MA-g-PP aids the dispersion of nanoclay in the blend. In the case of ether-TPU (C10A)/PP blend, peak appears at 2θ∼4.42°. However, the peak reduces in intensity for the compatibilised blend, and peaks appears at 2θ∼2.8° and 2θ∼4.2°. Ether based blend system shows two diffraction peaks; this may be due to higher percentage of clay loadings for the given blending conditions. Thus, it is confirmed that the compatibiliser makes the nanoclay better dispersed with the polymers, irrespective of ester or ether based TPU. However, the extent of dispersion of nanoclay is better in ester-TPU (C10A)/PP/MA.

X-ray diffraction (XRD) results are often reported to be misleading in terms of clay dispersion.¹⁶ Therefore, solely with the XRD analysis of these nanocomposites blends, it is not possible to conclude that these nanocomposites show the intercalated or the exfoliated clay structures. In this regard, SEM/TEM images were taken, and the results show similar trends as XRD results. In SEM pictures, effect of nanoclay dispersion is discussed with blend morphology of these blends.

Tensile test

The tensile test data of the nanocomposites and blends are summarised in Table 3. Stress values at 20, 100 and 200% elongations are shown in Fig. 6. Tensile test indicates that ester-TPU materials have better tensile properties than ether-TPU materials. In these materials, when elongation exceeded 800–900%, slippage of specimen from grips was observed; hence, elongation at break is not reported. Stress at 20, 100 and 200% elongations increased as a result of blending with PP. Compatibilised ester- and ether-TPU blend nanocomposites exhibited higher stresses at the respective elongations than the uncompatibilised blends.^{16, 17} The tensile test results substantiate the compatibilisation offered by MA-g-PP as seen earlier in the dynamic mechanical analysis (DMA) results. Ester-TPU (C10A)/PP/MA exhibited the best tensile properties as shown above.

Table 3

Summary of tensile properties

	Stress at % elongation (MPa)
Material	20%	100%	200%
Ether-TPU(C10A)/ PP	3.6	4.0	4.4
Ether-TPU(C10A)/ PP/MA	5.3	5.8	6.1
Ester-TPU(C10A)/ PP	5.5	6.6	6.8
Ester-TPU(C10A)/PP/MA	7.9	10.0	10.4

Scanning electron microscopy analysis

Images (SEM) of chemically etched (PP material was removed by xylene at 105°C) surfaces of the ester-TPU (C20A)/PP, ester-TPU (C10A)/PP/MA, ether-TPU (C10A)/PP and ether-TPU (C10A)/PP/MA blends are shown in Fig. 7. It is observed that the size of dispersed PP particle is considerably reduced in the ester-TPU (C10A)/PP/MA (Fig. 7d) system.^19–21

As mentioned in the introductory chapter, the high polarity difference between TPU and PP limits the miscibility of their blends. Nanoclay was used to reduce the surface energy of the TPU hard segments and makes them more compatible with the non-polar PP. More miscible blends have been obtained using MA-g-PP as the compatibiliser. Better dispersion of organoclay may be attributed to two reasons. First, MA forms hydrogen bonds with the hydroxyl groups of the silicate layers. Second, there is a possible chemical reaction between MA and the urethane linkages in the TPU hard segments. Compared to the ether-TPU based blend nanocomposites, the ester-TPU blends show better miscibility as confirmed by XRD, DMA, tensile strength, TEM and SEM analyses. The clear difference between ester-TPU and ether-TPU is that ester-TPU has carbonyl groups both in the polyol segments as well as in the TPU hard segments. This may lead to more extensive hydrogen bonding in the blend system thus improving the miscibility.

Transmission electron microscopy analysis

Influence of nanoclay addition on uncompatibilised blend morphology

To confirm the XRD results and clearly see the dispersion state of clays in the nanocomposites, the TEM images were taken and discussed. The TEM images of the ether-TPU (C10A)/PP and ester-TPU (C10A)/PP are shown in Fig. 8a and b. The TEM images presented in those figures shoe evidence that efficient dispersion and exfoliation of clay particles did not occur in this composition. The dispersion of nanoclay particles was found to be poor in this composition.

Clay is naturally a hydrophilic material, which makes it difficult to exfoliate in a polymer matrix. Therefore, the surface treatment of silicate layers is necessary to render its surface more hydrophobic, which facilitates exfoliation. Generally, this can be performed by ion exchange reactions with cationic surfactants, including primary, secondary, tertiary and quaternary alkylammonium cations. The first step in achieving nanoscale dispersion of clays in polymers is to open the galleries and to match the polarity of the polymer so that it will intercalate between the layers. This is performed by exchanging an organic cation for the inorganic cation. The larger organic cations swell the layers and increase the hydrophobic properties of the clay, resulting in an organically modified clay. Cloisite 10A is a natural montmorillonite modified with quarternary ammonium salt, and the alkyl groups in organic cations make montmorillonite hydrophobic. For polymer containing polar functional groups, an alkylammonium surfactant is adequate to promote the nanocomposite formation. However, in the case of PP, it is frequently necessary to use a compatibiliser, such as MA modified PP (MA-g-PP). There are two important factors to achieve the exfoliation of the clay layer silicates: (i) the compatibiliser should be miscible with the PP matrix, and (ii) it should include a certain amount of polar functional groups in a molecule. Generally, the PPs modified with MA fulfill the two requirements and are frequently used as compatibiliser for PP nanocomposites.

Influence of nanoclay addition on compatibilised blend morphology

Wang and coworkers^22–26 and other authors showed that there are two important factors to achieve the exfoliation of the clay layer silicates: (i) the compatibiliser should be miscible with PP matrix, and (ii) it should include a certain amount of polar functional groups in a molecule to react with TPU. Generally, the MA grafted PP (MA-g-PP) fulfills the two requirements and are used as compatibiliser for this blend system. When MA-g-PP is not introduced into the system, only intercalated structure is obtained, and many larger aggregates exist in the matrix. When the compatibiliser is introduced in the system, the nanoclay dispersion improves.

The TEM images of nanocomposite of ether-TPUs shown in Fig. 9a, where the degree of dispersion of clay is observed to be poorer than ester-TPU nanocomposites. The TEM image in Fig. 9b shows exfoliated and well dispersed nanoclay particles. In view of this, a true nanocomposites was produced in the case of ester-TPU(C10A)/PP/MA-g-PP and 3 wt-% nanoclay loading. The TEM image of the composite supports this and shows that individual clay layers were well dispersed in the polymer. These results are well correlated with the XRD results where almost no peaks were observed for ester-TPU(C10A)/PP/MA-g-PP nanocomposites. This is in agreement with the XRD result that some changes in clay layers were expected.

Conclusions

Both ester- and ether-TPU based nanocomposites were prepared and blended with PP using MA-grafted-PP as the compatibiliser. The compounds were characterised by DMA, tensile tests, XRD, SEM and TEM. Comparisons have been made between ester- and ether-TPU (C10A)/PP blend nanocomposites in terms of dispersibility of nanoclay, miscibility of blend components and effect of compatibiliser. Compatibilised ester-TPU (C10A)/PP showed a storage modulus of the order of 2.32 GPa in the room temperature (25°C) region, while uncompatibilised ester blends and compatibilised ether-TPU(C10A)/PP had storage moduli of ∼1.5 and 0.7 GPa respectively. Blend miscibility was better in compatibilised ester-TPU(C10A)/PP blend nanocomposites. Dynamic mechanical analysis showed a T_g (from E″) shift of ∼5°C in the higher side (for ester from − 25 to − 20°C). Stresses at 20, 100 and 200% elongations were highest for ester-TPU(C10A)/PP/MA (ester category: 6.8 to 10.4 MPa for 200% elongation). MA-g-PP was found to be an effective compatibiliser for TPU (C10A)/PP blends. Ester-TPU (C10A)/PP/MA blend nanocomposite was found to have very good overall performance and is a potential candidate for cost effective, high strength applications of TPU. This is because MA undergoes chemical reaction with the urethane groups in the TPU hard segments and also forms hydrogen bonds with the silicate layers of C10A. Ester-TPU with carbonyl groups both in the polyol segments and urethane hard segments has more extensive hydrogen bonding than ether-TPU.

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