oil thermoplastic elastomers. Part 1. Physical, mechanical and thermal properties

Applied Clay Science 109–110 (2015) 151–156

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Effect of commercial clays on the properties of SEBS/PP/oil thermoplastic elastomers. Part 1. Physical, mechanical and thermal properties Hugo M. Tiggemann a,b, Vanda F. Ribeiro c, Fabricio Celso d, Sônia M.B. Nachtigall a,⁎ a

Federal University of Rio Grande do Sul, 9500 Bento Gonçalves Avenue, Porto Alegre, RS 91.501-970, Brazil Integrated Regional University, Frederico Westphalen, RS, Brazil Softer Brasil Company, Campo Bom, RS, Brazil d Feevale University, Novo Hamburgo, RS, Brazil b c

a r t i c l e

i n f o

Article history: Received 22 September 2014 Received in revised form 6 February 2015 Accepted 8 February 2015 Available online 14 March 2015 Keywords: Flexible nanocomposites Clay polymer nanocomposites (CPN) Mechanical properties Thermal properties Thermogravimetric analysis (TGA)

a b s t r a c t Thermoplastic elastomers based on SEBS/PP/oil blends show good flexibility at low temperatures, allowing complex shapes molding with good finishing. In this study, three different commercially available clays were used as fillers in these blends through melt mixing processes to produce clay polymer nanocomposites (CPN) with improved thermal and mechanical properties. The physicomechanical properties, melt flow index, density and thermal stability of the materials were studied. It was found that optimum concentration was 2 phr for all fillers. At this concentration, the mechanical properties reached a maximum while higher filler loading proved to be less beneficial. CPN prepared with Novaclay, a Brazilian clay organically modified with silane, showed the best properties, reaching 60% increase of mechanical resistance in comparison to the non-modified TPE matrix. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Thermoplastic elastomers (TPE) are polymers that show a behavior similar to conventional elastomers and can be processed as thermoplastics using traditional methods such as extrusion, injection, blow molding, etc. (Holden et al., 1996; Ohlsson et al., 1996). This class of polymers is growing and expanding to new applications since they allow mass production of complex parts and multilayered artifacts. The properties of TPE are due to its biphasic structure that contains a flexible phase of elastomer (which provides a rubber-like response in solid state) and a rigid phase of a polymer with high glass transition or a semicrystalline thermoplastic (which ensures mechanical strength). Di- and triblock styrene-based elastomers are typical TPE which are widely used in special areas despite their comparatively high price (Grein et al., 2012). The triblock copolymer poly (styrene–ethyleneco-butylene-styrene) (SEBS) is a commercial TPE which has been primarily used as a compatibilizer for binary blend systems and as toughening agent for brittle polymers (Brostow et al., 2008). However, several researchers have reported TPE blends of SEBS and polypropylene (PP) that exhibited improved properties and enhanced performance (Sengupta and Noordermeer, 2005; Deniz et al., 2009; Ahmad ⁎ Corresponding author at: Chemistry Institute, 9500 Bento Gonçalves Avenue, Porto Alegre, 91.501-970 Brazil. Tel.: +55 51 3308 7208. E-mail address: [email protected] (S.M.B. Nachtigall).

http://dx.doi.org/10.1016/j.clay.2015.02.028 0169-1317/© 2015 Elsevier B.V. All rights reserved.

et al., 2010; Grein et al., 2012). These blends appear as soft materials when PP/elastomer ratio is low. Their modulus is usually below 700 MPa. Because of this, they find application mostly in the area of films (Grein et al., 2012). SEBS/PP/oil blends have been commercialized since the early 1990s for soft-touch applications, such as grips on tools, sports goods and in automotive and medical domains owing to the fact that the properties of the rubber and the PP phase can be easily combined in a single product (Sengupta and Noordermeer, 2004, 2005). The hydrogenated styrene-butadiene triblock systems are most commonly used since they can be designed in a wide range of compatibility and viscosity by varying the molar mass and individual block lengths (Grein et al., 2012). This allows a variety of combinations of density and glass transition as well as good compatibility between the elastomer blocks and PP (Deniz et al., 2009). The main drawback of TPE is related to its low thermal stability, particularly during processing. The addition of fillers can greatly improve their processability as well as mechanical properties, reducing material costs at the same time. Thus, many applications may profit from filler loading. Nowadays, focus has shifted toward nanotechnology applications of TPE due to the ability of these materials to form self-assembled small domains whose size, shape and periodicity can be easily controlled by altering simple molecular parameters (Park et al., 2003). Moreover, the addition of clay mineral with nanometric dimensions has brought about a great improvement in thermal stability and mechanical

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performance of some TPE even at very low filler content (Su and Huang, 2009; Martin et al., 2010). The microstructure of a clay–polymer nanocomposite (CPN) depends on the type of polymer matrix, the nature of nanoclay used and the interactions between them. Due to the nonpolar nature of SEBS, its compatibility with hydrophilic clays needs to be improved, which may be attained through organic modification of the clays. In this work, three types of commercial organically modified clays were used for preparation of CPN based on SEBS/PP/oil blends. The thermal and mechanical properties of the obtained TPE were discussed in this study attempting to relate them to the structure of the clay used.

Table 2 Formulation of the compounds (in mass percentage). Sample designation

SEBS, %

PP, %

Oil, %

Clay, %

Clay/polymer, g/100 g

TPE matrix Cloisite 2 phr Cloisite 5 phr Cloisite 7 phr Nanofil 2 phr Nanofil 5 phr Nanofil 7 phr Novaclay 2 phr Novaclay 5 phr Novaclay 7 phr

34.11 33.88 33.53 33.31 33.88 33.53 33.31 33.88 33.53 33.31

12.11 12.03 11.90 11.83 12.03 11.90 11.83 12.03 11.90 11.83

53.79 53.43 52.88 52.53 53.43 52.88 52.53 53.43 52.88 52.53

0 0.68 1.68 2.33 0.68 1.68 2.33 0.68 1.68 2.33

0 1.47 3.68 5.15 1.47 3.68 5.15 1.47 3.68 5.15

Remark: 0.25% of antioxidant was added in all systems (with respect to the total mass).

2. Experimental 2.1. Materials The technical features of the clays used in this study are shown in Table 1. Other materials where as follows: PP homopolymer from Braskem S.A. (MFI = 1.5 g/10 min at 230 °C), SEBS T6151 from TSRC Group (styrene content 31-34% in mass), oil NYPAR 303 from Nynas AB and antioxidant Sunox 1010 from Sunny Chemical Corporation. 2.2. Samples preparation Compounding was carried out at fixed SEBS/PP/oil ratio. The composition of the systems is shown in Table 2. First, the dry components were premixed at room temperature and kept for plasticizer absorption overnight. Melt blending was performed in a Haake melt mixer Rheomix 600 OS, equipped with roller type rotors and controlled by Rheo-Drive 7 PolyLab, for 10 min, at 180 °C, 100 rpm. After compounding, the products were hot pressed at 90 psi, 180 °C, for 60 s into 2 mm thick slabs using a mold.

2.3.4. Melt flow index Melt flow indices were determined in a. Melt flow index determinations were performed using a CEAST Modular Melt Flow Indexer with 5.0 kg load at 200 °C, according to ASTM D1238-95. The average values of three samples were shown. 2.3.5. Specific gravity A hydrostatic method based on ASTM D 792-91 was used to determine the density of the materials. The samples were submersed in water, and their masses were measured at constant temperature. The density was calculated according to Eq. (1). Results were an average of three specimens. ρ¼

Massair ρ Massair  Masssolvent solvent

1

3. Results and discussion 3.1. Characterization of the clays

2.3. Characterization 2.3.1. Particle size distribution of nanoclays Particle size distribution was determined by laser diffraction. The equipment used was a CILAS 1180 particle size analyzer, and scanning ranged from 0.04 μm to 2500 μm. Particles were pre-dispersed in deionized water using ultrasound (60 s). 2.3.2. Tensile tests of CPN Tensile tests were performed at room temperature in a Universal Mechanical Tester Machine, EMIC DL 2.000, with clamp separation speed of 200 mm⋅min−1, load cell of 1.000 N and using an extensometer suitable for elastomers, according to DIN 53.504. Dumbbell-shaped specimens were cut out by blade. 2.3.3. Thermogravimetry General TGA analyses were performed using a T.A. Analyzer QA50, under nitrogen from 20 °C to 1000 °C for the nanoclays and from 20 °C to 700 °C for the CPN. Heating rate was 20 °C/min, amount of sample was ~12 mg.

3.1.1. Particle size and size distribution The granulometric analysis allows evaluating the particle size distribution of materials. The size of the clay mineral aggregates that have to be exfoliated during preparation of CPN can be estimated by measuring their dimensions through dispersion in an aqueous medium. An inconvenience of this analysis is the tendency of increasing aggregation of organophilic clays by water induction. Novaclay sample has a bimodal particle size distribution, whereas Nanofil exhibits a shoulder in the region of larger particle size (Fig. 1). Values of D10, D50 and D90 determined by these analyses as well as D50 provided by manufacturers are shown in Table 3. Obtained values of D50 for Cloisite 15A and Nanofil were slightly higher than those reported by manufacturers, which may indicate some agglomeration of these nanoclays in water. Comparison of clays makes apparent that Cloisite has both average diameter and D90 smaller than the other clays, which can be beneficial for its dispersion in the polymer matrix. 3.1.2. Thermal degradation behavior NovaClay 028 and Cloisite 15A are typical montmorillonites, whereas Nanofil SE 3000 is a bentonite, where the principal clay mineral constituent is montmorillonite (Bergaya and Lagaly, 2006; Cojocariu et al.,

Table 1 Commercial organically modified clays. Trade name Cloisite 15A Nanofil SE 3000 Novaclay 028

Type Montmorillonite Bentonite Montmorillonite

Organic modifier +

(CH3)2(HT)2 N (CH3)2(HT)2 N+ AMS - 32

d001 (Å)

Particle size at D50 (μm)

Manufacturer

31.5 24.2 28.8

6 10 9

Southern Clay Rockwood Clay Ioto International

HT = hydrogenated tallow (C18 ≅ 65%; C16 ≅ 30%; C14 ≅ 5%); counter-ion: Cl−. AMS-32 = Trade name of an organic modifier free of ammonium salt produced by Ioto International (Preparation described in Pending Patent DEPR 015100000646).

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Fig. 1. Size distribution curves of the clays: (a) Cloisite; (b) Nanofil; (c) Novaclay.

2012). Thermogravimetric curves (Fig. 2) show similar profiles for Nanofil and Cloisite, which are modified with alkylammonium salts. Both clay minerals degrade in three main steps. The first one (below 150 °C) can be attributed to the loss of adsorbed water (Nowicki et al., 2007). This step corresponds to about 1% mass loss, indicating low water content and low hydrophilicity of these clay minerals. Within the same range of temperatures, Novaclay showed higher mass loss (around 5%), indicating higher hydrophilicity. The second degradation step for Cloisite and Nanofil, ranging from 150 °C to ~380 °C, reflects the loss of the organic modifier (Monticelli et al., 2007). It can be noticed that mass loss profiles for these clays closely resemble each other, confirming the presence of similar organic modifiers. The mass loss in this step indicates that Cloisite has a slightly higher content of modifier (~ 34%) in comparison to Nanofil (~ 30%). Small differences in the degradation rate during this step can be due

to chain length differences in the modifiers, which can affect the speed of degradation process. The last step of decomposition for Nanofil and Cloisite occurs between 550 °C and 850 °C. Some authors have associated degradation

Table 3 Particle diameters determined by laser diffraction. Nanoclay

D10, μm

D50, μm

D90, μm

Average diameter, μm

D50 1, μm

Cloisite Nanofil Novaclay

2 2 2

9 14 10

20 38 35

10 17 14

6 10 9

1

Manufacturer information

Fig. 2. TG and DTG curves of the clays.

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within this temperature range to loss of structural water (dehydroxylation) (Monticelli et al., 2007). According to the manufacturer, Novaclay contains an organic modifier with no ammonium salt. The TGA curves of Novaclay show multiple overlapping degradation steps over a wide temperature range, probably related to this modifier. The mass loss associated with the organic modifier amounts to ca. 36%. 3.1.3. X-ray diffraction analyses X-ray diffractometry is widely used to determine the distance between layers in clay minerals. X-ray diffraction curves of the clay minerals used in this work were recorded in the range of angles 2θ from 1° to 10° (Fig. 3). The three clay minerals have different basal spacings reflecting their different structures. The basal spacings d001 and d002 of the nanoclays were calculated using Bragg's law: d = λ/2sinθ, where λ = 0.070 nm for Mo Kα irradiation used in the measurements. The basal spacings in Nanofil are larger than that in Cloisite by 0.4 nm (Table 4). Most probably this may be explained by higher exchange degree of the modifier in the interlayer space of Nanofil, despite the fact that this clay contains less amount of modifier (as determined by TGA) and has larger size of aggregates (as determined by laser diffraction). The difference between Cloisite and Nanofil can be related then to the different treatment used in manufacturing these nanoclays. The XRD diffraction pattern of Novaclay differs from the others having a sharp reflection at 2θ = 1.80°, which corresponds to the interplanar distanced001 = 4.90 nm. According to the producer, Novaclay does not contain alkylammonium salt as organic modifier but it contains another one, identified as AMS-32. As follows from TGA analyses the content of modifier in Novaclay is lower than in Nanofil and Cloisite. However, the interlayer space associated with the presence of modifier in Novaclay is bigger. This result may be accounted for the higher efficiency of the Novaclay modifier. At the same time, the second reflection (002 reflection) of Novaclay has greater intensity and width as compared to Nanofil and Cloisite, which may indicate that part of the clay was not expanded by the modifier. Cloisite showed the smaller basal spacing of all three nanoclays, with d001 equals to 3.35 nm. 3.2. CPN characterization and properties 3.2.1. Mechanical properties Improvement of the mechanical properties in CPN is closely related to adequate filler dispersion. In the case of clays, this depends on the exfoliation of clay minerals, producing particles of nanometric dimensions and thereby resulting in real CPN (Kausar et al., 2007). In Fig. 4, the stress × strain curves for the TPE matrix and the CPN containing 2 phr

Fig. 3. X-ray diffractograms of the clays.

Table 4 Particle diameter determined by X-ray diffraction. Nanoclay

2θ 1st reflection (o)

2θ 2nd reflection (o)

d001 (nm)

d002 (nm)

Cloisite 15A Nanofil SE 3000 Novaclay 028

2.50 2.35 1.80

7.15 6.95 5.45

3.35 3.75 4.90

1.23 1.27 1.61

of clays are shown. The elastic behavior in these curves—where large deformations occur at low tension—is typical of elastomers. It may be noticed that the tensile strength increased and the elasticity decreased when filler was embedded in the elastomeric matrix transforming it into a CPN, as expected. However, the elasticity of CPN remained high, even characterizing them as elastomers. Results from Fig. 5 confirm that all compositions showed noticeable increase in ultimate tensile strength in comparison to the pure TPE matrix. The highest values were found for CPN containing Novaclay, at all concentrations. At 2 phr Novaclay, tensile strength was 61% higher than for the TPE matrix. This was the highest value obtained. Lower values for 5 and 7 phr Novaclay imply that particle exfoliation was less efficient at higher clay content. Similar trends were found for Cloisite and Nanofill with maximum value also at 2 phr and subsequent decrease at 5 and 7 phr. However, the tensile strength values of CPN containing Cloisite and Nanofill were lower than those of Novaclay compositions. These may indicate that Novaclay has better effect on matrix/ clay interaction due to its higher basal spacing, which facilitates polymer penetration into the clay aggregates. Despite the fact that Cloisite and Nanofil showed similar trend, the effect was less important most probably due to smaller interlayer space. Elongation at break results indicates that these values decrease with increasing nanoclay content (Fig. 6). Elasticity reduction is typical for all CPN, although it does not limit their use in common applications. 3.2.2. Specific gravity The incorporation of mineral fillers with higher specific gravity raises the density of TPE-based nanocomposites. However, the materials prepared in this work have shown a negligible increase in density due to the low content of added clays. The specific gravity of CPN ranged from 0.89 to 0.91 g/cm3 (Table 5). 3.2.3. Shore hardness The hardness of a TPE is often regarded as one of the main criteria to be considered. It is defined as a material's resistance to indentation when a static load is applied. Hardness is well related to tensile and

Fig. 4. Stress × strain curves of the TPE matrix and the nanocomposites containing 2 phr of each clay.

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Table 5 Specific gravity, hardness and MFI of the materials. Sample

Density (g⋅cm−3)

Hardness (Shore A) (g × 10 min−1)

MFI

TPE matrix Cloisite 2 phr Cloisite 5 phr Cloisite 7 phr Nanofil 2 phr Nanofil 5 phr Nanofil 7 phr Novaclay 2 phr Novaclay 5 phr Novaclay 7 phr

0.89 0.90 0.90 0.91 0.89 0.89 0.89 0.90 0.90 0.90

37.2 39.0 39.4 40.2 39.8 38.5 38.8 37.8 38.4 38.3

29.7 27.9 24.6 26.8 25.5 20.8 21.6 24.1 30.8 33.0

4. Conclusions

Fig. 5. Ultimate tensile strength of nanocomposites according to clay concentration.

flexure modulus and in many TPE applications it reflects how the product feels to the touch. The hardness values of the CPN and the TPE matrix measured using Shore A Hardness scale are shown in Table 5. From these data, it follows that hardness is higher in the CPN in comparison with initial TPE matrix. Despite the low clay load, ranging from 2 to 7 phr, an increase from 37.2 (TPE matrix) up to 40.2 Shore A (for 7 phr Cloisite) was produced.

3.2.4. Melt flow index The melt flow index of polymers corresponds to the amount of melt polymer in grams that flows through a capillary tube of known geometry during a specific time. The value of melt flow índex may be used for estimation of material processability. The presence of a nanofiller in a polymer matrix may affect the melt flow index, depending on several parameters, such as nanofiller content, size, shape, aspect ratio and tendency to clustering of nanoparticles. The interaction between filler and polymer is also very important and it may be tuned up by treatment of the solid with a modifier (Xanthos, 2005). Generally, the incorporation of filler tends to reduce melt flow index of TPE-based materials. Results in this work showed that most CPN followed this tendency (Table 5). However, in the case of Novaclay, the CPN containing 5 and 7 phr exhibited melt flow indices even higher than that of initial TPE matrix. It can be supposed that this behavior is related to the presence of large amounts of the organic modifier, provoking a plasticizing effect in the systems.

With the aim of improving the thermal and mechanical properties of a thermoplastic elastomer compound based on SEBS/PP/oil three organically modified clays were embedded in this matrix via intercalation in molten state. Filler were clay minerals of montmorillonite type (Cloisite and Novaclay) and of bentonite (Nanofil). The fillers were characterized by laser scattering, thermogravimetry and X-ray diffraction. It was found that Novaclay, which is organically modified with a silane modifier, contained the highest percentage of modifier and had the largest interlayer distance. The studied CPN had enhanced ultimate tensile strength confirming the reinforcing effect of filler incorporation. All CPN exhibited the best mechanical properties at 2 phr content of solids. The mechanical resistance of the CPN containing 2 phr Novaclay was 60% higher than that of non-modified TPE matrix. Most mineral fillers have specific gravities and hardness considerably higher than thermoplastics. Despite of this, the specific gravity of the CPN remained almost unchanged and their hardness only insignificantly increased. TGA results showed that CPN were more thermally stable than the TPE matrix. Filler incorporation in TPE matrices generally increases viscosity reducing melt flow index. However, CPN containing 5 and 7 phr of Novaclay showed increase in melt flow index, which makes them easier to be processed. This may be explained by a plasticizing effect of the organic modifier used for treatment of Novaclay. Acknowledgments Authors are grateful to Dr. S.D. Mikhailenko for helpful discussions, to FINEP for financial support (grant number CT 186/0-2009), to CAPES for scholarship and to Softer Brazil Company and Federal University of Rio Grande do Sul for the infrastructure provided. References

Fig. 6. Elongation at break of nanocomposites according to clay concentration.

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