Article pubs.acs.org/IECR
Microcellular Foam from Ethylene Vinyl Acetate/Polybutadiene Rubber (EVA/BR) Based Thermoplastic Elastomers for Footwear Applications Madhuchhanda Maiti*,† and Raksh Vir Jasra† †
Reliance Technology Group, Vadodara Manufacturing Division, Reliance Industries Ltd., Vadodara-391346, Gujarat, India
Suraj K. Kusum‡ and T. K. Chaki‡ ‡
Rubber Technology Centre, Indian Institute of Technology, Kharagpur-721302, India S Supporting Information *
ABSTRACT: Thermoplastic elastomer (TPE) from ethylene vinyl acetate (EVA) and synthetic general purpose rubber, polybutadiene rubber (BR), which can improve set, wear, and tear properties of virgin EVA without any coloring problem, was developed. EVA-BR blend ratio and processing parameters were optimized based on physicomechanical properties. It was found that EVA:BR 80:20 blend forms TPE of high strength (20 MPa) and low hardness (71 Shore A). This TPE shows 58 and 40% improvement in tension and compression set compared to virgin EVA. Tear strength and abrasion resistance of TPE was also improved by 9 and 35% respectively compared to EVA. The observed properties of TPE were explained in terms of morphology and interaction parameter. Microcellular foam of this TPE was developed using organic and inorganic blowing agents. Dinitrosopentamethylene tetramine at 3 phr loading forms closed cell microcellular foams with an average cell diameter of 100 μm. EVA:BR microcellular foam showed overall properties comparable with commercial EVA:NR microcellular foam. EVA finds its application as a matrix for sole compounds in the footwear industry. However, virgin EVA shows poor set, wear, and tear properties reducing the durability of its products. To overcome these limitations, EVA is blended with NR for commercial use. However, the coloring problem of the EVANR blend and the scanty availability of natural rubber are the rising concerns of the industries. Polybutadiene rubber (BR) is expected to enhance the set properties and abrasion resistance of EVA like NR, without a coloring problem. However, to the best of our knowledge, there is no literature report about EVABR blends. Hence, in the present work, we have studied EVA-BR blends and investigated their performance as thermoplastic elastomers. Moreover, microcellular foams, used in the footwear industry, were made out of these blends. The properties of these foams were compared with commercial EVA:NR based foam to evaluate their practical applicability.
1. INTRODUCTION During the last four decades, the development and growth of thermoplastic elastomers (TPE) have reached a high level of significance, and they have become an important segment of polymer science and technology. In the simplest way, TPE can be defined as class of polymers, which combine the properties of elastomers with the processing ease of thermoplastics.1 Blending an elastomer with a thermoplastic polymer can result in TPE, where the elastomeric part is the soft segment and the thermoplastic is the hard-segment.1,2 TPE have merits and demerits in their practical uses over conventional vulcanized rubbers, which have been discussed in detail in the existing literature.1−9 The use of TPE has significantly increased since these were first produced. Global demand for TPE is forecast to rise 6.3% annually to 5.6 million metric tons in 2015.10 Ethylene vinyl acetate (EVA) was used as both soft-segment as well as hard-segment. EVA was used as soft-phase with different polyolefins such as various grades of polyethylene, polypropylene mainly for wire and cable applications.11−18 EVA was blended with polyphenylene ether to have a TPE of superior heat aging performance.19 EVA, as a soft-phase, was also blended with polystyrene.20 It was also used to develop TPE based propellant systems.21 Blends and foams of EVA with rubbers such as natural rubber (NR),22−25 recycled ground tire rubber,26 epoxidized natural rubber,27 acrylonitrile-butadiene rubber (NBR),28−30 styrenebutadiene rubber (SBR),31,32 EPDM,33,34 polychloroprene,35 and acrylic rubber36 have been reported. However, there are a few claims on TPE using EVA as a hard-phase and different rubbers as soft-phase.22,28,29 © 2012 American Chemical Society
2. EXPERIMENTAL SECTION 2.1. Materials. EVA with 18% vinyl acetate content (meltflow index = 0.2 g/10 min at 2.16 kg load) and BR (Cisamer 1220, Mooney viscosity, ML1+4 at 100 °C = 45) were generously supplied by Reliance Industries Limited, India. Dicumyl peroxide (DCP; 99% purity) was procured from Sigma-Aldrich Co., USA. Sodium bicarbonate (NaHCO3; Received: Revised: Accepted: Published: 10607
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constant frequency of 1 Hz, at 0.01% strain, and within the temperature range of −120 to 80 °C at a heating rate of 3 °C min−1. The loss tangent (tan δ) was measured as a function of temperature for the representative samples under identical conditions. The temperature corresponding to tan δ peak was taken as glass transition temperature (Tg).
99.5% purity) and calcium carbonate (CaCO3; 98.5% purity) were procured from Merck Limited, India. Azodicarbonamides (ADC) and dinitrosopentamethylene tetramine (DNPT) were obtained from Uniroyal Chemical, USA. Antioxidant (2,2,4trimethyl-1,2-dihydroquinoline, polymerized) was supplied by Merchem Limited, India. NR (ISNR 5) was obtained from Rubber Research Institute, India. 2.2. Preparation of Blends. All the blends were prepared by batch process in a Brabender plasticorder (PLE 330 Duisburg, Germany) of 70 cm3 capacity. The blending was performed at different rotor speeds (40, 50, and 60 rpm) and temperatures (100, 110, and 120 °C). EVA:BR blends of different ratios, viz., 60:40, 70:30, and 80:20 were prepared without adding any additives. First the rubber was softened and then EVA was charged. The mixing was continued for 5 min and dumped. It was passed through a two-roll mill immediately. The sheet was then cut into small pieces and was remixed for 2 min. The blend was removed from the mixer and, while still molten, passed once through a cold two-roll mill to achieve a sheet about 2 mm thick. The sheet was molded in a compression-molding machine (Moore press, Birmingham, UK) at 160 °C and 5 MPa pressure. Aluminum foils were placed between the mold platens. The sheet was then cooled down to room temperature under the same pressure. The test specimens were die-cut from the compression molded sheet and used for testing. The samples were recycled thrice at the above-mentioned conditions. The microcellular formulations were mixed at 100 °C temperature for 15 min. Molding was done for optimum cure time (25 min) at 160 °C using compression-molding machine. All types of blowing agents used in the study have decomposition temperature lower than the curing temperature. 2.3. Test Methods. 2.3.1. Physicomechanical Properties. Tensile test of the samples was carried out according to ASTM D 412-98a on dumbbell shaped specimen using a Hounsfield H25KS universal testing machine at ambient temperature at a crosshead speed of 500 mm/min. Tear strength of the samples were measured using the same universal testing machine according to ASTM D 624-00. An average of five samples is reported here. The tension set was determined with dumbbell specimens as per ASTM D 412-98a. Shore A hardness of each composition was tested according to ASTM D 2240-97. Abrasion loss was measured using DuPont abrader as per ASTM D394-59. Compression set was done as per ASTM D 395-03. 2.3.2. Morphology. Morphology studies of different blends were done by using scanning electron microscope (SEM; JEOL JSM-5800, Japan) at 20 kV of acceleration voltage at room temperature. The elastomeric component of the blends was etched out using toluene as solvent for 24 h at room temperature. Samples were sputter coated with thin gold layer prior to scanning. 2.3.3. Differential Scanning Calorimetry (DSC). Differential scanning calorimetric analysis was done using DSC 2910 modulated DSC (TA Instruments, USA). The samples were heated from ambient temperature to 180 °C (at 10 °C/min heating rate) in nitrogen atmosphere. 2.3.4. Dynamic Mechanical Thermal Analysis (DMTA). The dynamic mechanical thermal analysis of the specimens was carried out by using a dynamic mechanical thermal analyzer (DMA 2980, TA Instruments, USA). The sample specimens were analyzed (22 × 6 × 0.75 mm) in tension mode at a
3. RESULTS AND DISCUSSION 3.1. Optimization of Process Parameters for Development of EVA/BR Thermoplastic Elastomers. 3.1.1. Effect of Rotor Speed on the Mechanical Properties. To start with a blend ratio of 70:30 was taken, keeping footwear application in mind. Mixing was done at three different rotor speeds of 40, 50, and 60 rpm at 100 °C. The physicomechanical properties of the blends are given in the Supporting Information (Table S1). Tensile properties are observed to be best at 60 rpm. With increasing rotor speed the higher shear force generated has helped in better mixing. After the third recycling, there is little detrimental change in the physicomechanical properties especially in the case when the rotor speed is 60 rpm. So, the rotor speed was optimized to 60 rpm. Henceforth, all the mixing were done at this optimized rotor speed. 3.1.2. Effect of Temperature on the Mechanical Properties. Effect of mixing temperature on the physicomechanical properties of 70:30 blend is reported in Table S2 (given in the Supporting Information). From the table, it is clear that for the blends, prepared freshly in the first cycle, there is no significant detrimental effect in the physicomechanical properties with increasing temperature. But after third recycling there are significant detrimental effects on the physicomechanical properties of the blends prepared at 110 and 120 °C temperature. The prolonged exposure to higher temperature may degrade the polybutadiene rubber chains which are more prone to thermo-oxidative degradation due to the presence of unsaturation in the structure, as observed in the case of other diene rubbers.37 This degradation leads to inferior properties. Therefore, 100 °C was chosen as the mixing temperature to have better energy efficiency of the process and consistency of properties even after recycling. Afterward, all blends were prepared at 100 °C and 60 rpm. 3.1.3. Optimization of Blend Ratio. The EVA:BR blend ratio was optimized at the already optimized process conditions. EVA:BR ratios of 60:40, 70:30, and 80:20 were chosen keeping the application in mind. Table S3 (provided in the Supporting Information) reports the physicomechanical properties of these blends. It can be seen from Table S3, that EVA:BR 80:20 blend forms soft TPE, as the hardness is 71 Shore A and the tension set is below 20%. In the case of the other two blends the tension set values are also near 20%. Tensile strength increases with increasing EVA ratios, i.e., increasing the amount of hard phase. Tear strength also follows the same trend. The best physicomechanical properties can be observed at an 80:20 ratio. At this ratio it forms a low hardness (in Shore A range) and high strength (20 MPa) TPE. It is observed that the physicomechanical properties of the EVA:BR 80:20 blend is even better as compared to virgin EVA. The tension set improves in the EVA:BR 80:20 blend by 58% over virgin EVA. The compression set of EVA:BR 80:20 is 1.2% while that of virgin EVA is 2.0%. It means that both the tension and the compression set improve in the case of TPE compared to EVA alone. As, it is blended with BR which has very good rebound resilience properties, the set properties are improved in the blends. Tear strength and abrasion resistance, which are 10608
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discussed earlier. The EVA:BR 80:20 blend (Figure 1d) again exhibits a two-phase morphology, like 60:40 blend, made of a continuous EVA matrix with dispersed BR nodules whose size varies from 300 nm to 2.5 μm. A smaller size of BR particles with homogeneous dispersion in an EVA matrix of an 80:20 blend may be responsible for providing the best properties in this blend compared to the other ones. The experimental data for viscoelastic parameters are shown in Figure 2. Based on the tan δ peak value, the Tg of BR was
important properties for footwear applications, are also better in TPE over the virgin EVA. The tear strength of virgin EVA is 55 N/mm while that of TPE is 60 N/mm. Abrasion loss in TPE is 0.56 wt % compared to 0.86 wt % in the case of virgin EVA. Similarly, better tear and abrasion properties are results of incorporation of the rubbery phase, BR. Morphology of the blends plays a significant role in the properties which has been explained later. Lower tension set, higher tear strength, and higher abrasion resistance along with low hardness make this TPE appropriate for footwear application. Table S3 also registers the percentage change in physicomechanical properties of all blends after a third recycling. It is seen that there is a drop in mechanical properties of each blend. During recycling, the samples are subjected to heat and shear. At such conditions, diene rubbers are susceptible to degradation, which leads to lowering of properties.37 Hence, the property-drop is minimum in the EVA:BR 80:20 blend because of higher quantity of saturated polymer, EVA. Hardness values remain almost constant after recycling. 3.2. Characterization of the Blends. Morphology of 70:30 blends prepared at 40 and 60 rpm rotor speeds are shown in Figure 1a and Figure 1b respectively. As it can be seen
Figure 2. Temperature dependence of tangent delta of the EVA:BR 80:20 blend and virgin components.
found to be around −105 °C and that of EVA was at −5 °C. The blend exhibits two glass transition temperatures corresponding to each of the components. In the blend, Tg of BR shifts to a higher temperature (−102 °C) and that of EVA moves toward a lower temperature (−7 °C). Maxima of both the peaks of the blend are lower than the virgin components. These indicate that the components of the blend are thermodynamically immiscible but technologically compatible to each other. It again proves that the EVA:BR 80:20 blend forms a TPE. The performance of EVA-BR blends can be explained with the help of interaction parameter. Solubility parameter (δ) at 25 °C of EVA and BR is calculated to be 8.96 and 8.20 cal1/2 cm−3/2 respectively.37 The close solubility parameters indicate better compatibility between the blend components. The difference between solubility parameters of EVA and BR is low (δEVA − δBR = 0.76). The lower the difference between solubility parameter, the higher will be the thickness of the interface. The adhesion between the interface and the polymer matrix is very important in the case of rubber-plastic blends, as often the failure occurs in such blends because of poor interfacial adhesion.38 Systems having a thin interface usually show complete phase separation. The interfacial regions (the white region surrounding the black hole of BR) of 100−250 nm can be seen in the EVA:BR 80:20 blend (Figure 1d). Hence, a higher thickness of interface is responsible for all the improved properties observed in this case. The Flory−Huggins parameter or interaction parameter of the blend at 25, 100, and 160 °C is 0.13, 0.10, and 0.09 respectively. A lower interaction parameter of these two components also explains the reason behind the excellent properties obtained.38 3.3. Effect of Concentration of Blowing Agents on Foam Formation. After optimizing the process parameters, different doses [3, 4, and 5 parts per 100 g of rubber (phr)] of
Figure 1. Morphology of 70:30 blend at (a) 40 rpm and (b) 60 rpm. Morphology of (c) 60:40 and (d) 80:20 blends.
in Figure 1, all blends show a continuous EVA matrix with a dispersed BR phase (seen as dark holes or patches as a result of etching of BR phase with toluene). Figure 1a shows a twophase morphology made of a continuous EVA matrix with micronic and submicronic BR nodules whose size varies from 500 nm to 2.5 μm, while Figure 1b shows a lamellae-like cocontinuous structure. This morphological difference could be due to the difference in shear force at different rotor speeds. The EVA:BR 60:40 blend (Figure 1c) exhibits a two-phase morphology made of a continuous EVA matrix with micronic BR nodules whose size varies from 1 to 5 μm. The EVA:BR 70:30 (Figure 1b) blend shows a lamellae-like structure as 10609
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DNPT were used in the EVA:BR 80:20 blend to develop the microcellular thermoplastic elastomeric foams. The 5 phr DNPT containing sample was completely blown out, and testing could not be performed. The tensile strength of the 3 and 4 phr DNPT containing samples were also not encouraging (4.85 and 4.01 MPa respectively). The cells formed can be observed from SEM studies of the blends (Figure 3a-c). The 3
phr) are reported in Table S5 (given in the Supporting Information). All the products fulfill the requirements of the specifications given by the footwear industry, specifically for inner sole application, i.e. tear strength is 25−50 N/mm and compression set at room temperature should be less than 20%. SEM photographs of different microcellular blends are shown in Figure 4a-c. All types of blowing agents show closed-cell
Figure 3. SEM photographs of (a) 3 phr, (b) 4 phr, and (c) 5 phr DNPT containing blends.
Figure 4. SEM photographs of (a) DNPT, (b) ADC, and (c) NaHCO3 containing samples.
and 4 phr DNPT loaded samples show a closed-cell structure, whereas a 5 phr loaded sample shows a mixed morphology of open and closed-cell structures (open cells are shown inside the white circles). Formation of too many cells of diameters ranging from 200 to 500 μm in the case of 3 and 4 phr DNPT loaded samples may be the reason for decreased strength. Hence, further formulations were developed based on this TPE to have restricted blowing which can find practical applications. 3.4. Effect of Different Blowing Agents on the Properties of Microcellular Foams. Formulations were developed using various blowing agents, keeping practical application in mind. The formulation is given in Table 1.
structures. Better blowing can be observed in the case of a DNPT containing sample (Figure 4a) compared to ADC (Figure 4b) and NaHCO3 (Figure 4c) containing samples. At this particular loading, ADC and NaHCO3 cause insufficient and inhomogeneous blowing. The effect of different combinations of organic (DNPT) and inorganic (NaHCO3) blowing agents on the properties of the blends was also studied, Table S5 (Supporting Information). Though the microcellular blends fulfill the criteria of tear strength (25−50 N/mm) and compression set at room temperature (less than 20%), no remarkable synergistic effect can be seen in these two types of blowing agents. 3.5. Comparison of EVA:BR and EVA:NR Blends. After considering all the above results, 3 phr DNPT has been optimized. It was then used in EVA:BR 80:20 and EVA:NR 80:20 blends. The rest of the formulation was the same as in Table 1. The performance of these blends was also compared with only EVA based formulation. The properties are reported in Table 2. All the microcellular compounds are of the same
Table 1. Formulation of Microcellular Sole Based on the EVA-BR Blend formulation
phr
ethylene vinyl acetate (EVA) polybutadiene rubber (BR) dicumyl peroxide (DCP) blowing agent ethylene glycol calcium carbonate (CaCO3) antioxidant
80 20 1 3 or 4 or 5 2 40 1
Table 2. Comparison of Different Microcellular Blends
Different blowing agents of organic and inorganic nature, e.g., DNPT, ADC, and NaHCO3, were used in this standard footwear formulation. Different combinations of organic (DNPT) and inorganic (NaHCO3) blowing agents were also tried. Effect of blowing agent concentration on this filled and cured EVA:BR TPE is reported in Table S4 (provided in the Supporting Information). The samples were prepared under restricted blowing conditions. Similar to the unfilled and uncured TPE, the properties optimize at 3 phr loading. With increasing blowing agent concentration (until 4 phr) properties deteriorate slowly. At higher concentration (5 phr), the sample may be overblown and thus causing maximum deterioration of properties. Density of the compound decreased gradually with increasing level of cell formation with higher blowing agent doses. The physicomechanical properties of different microcellular blends containing various blowing agents at optimum dose (3
properties
EVA
tensile strength (MPa) 300% modulus (MPa) elongation at break (%) tear strength (N/mm) abrasion (volume loss, cm3 per 10 min.) compression set (%) hardness (Shore A) density (g cm−3) a
EVA:BR
EVA:NR
7.88 ± 0.42 4.16 ± 0.21 610 ± 20 42 ± 1.92 0.13 ± 0.007
9.25 ± 0.25 5.98 ± 0.26 520 ± 20 48 ± 0.87 0.06 ± 0.002
8.47 ± 0.13 4.68 ± 0.31 570 ± 30 46 ± 0.64 0.10 ± 0.005
3.05 65 0.98
3.83 72 0.98
4.48 73 0.98
a
Standard deviation.
density. Both the blends show almost similar results. However, the only EVA based compound shows poorer properties in terms of tensile and tear strength and abrasion resistance compared with both the blends. SEM micrographs of both the blends show a similar type of microcellular morphology (Figure 5 a-b). The microcells of EVA:NR are closed-cell in nature, while EVA:BR shows both open and closed cell morphology. The average cell diameter is 70 and 80 μm respectively. 10610
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MPa) at low hardness (∼71 Shore A). The morphology of the blends and interaction parameter of EVA-BR are the key parameters, which basically govern the final properties of the blends. Microcellular foams of this TPE were made using different organic and inorganic blowing agents at various loadings. Among all, dinitrosopentamethylene tetramine (DNPT) at 3 phr loading was observed to be the best blowing agent. A formulation was developed for shoe applications based on optimized conditions. The product meets the requirements of the footwear industry. EVA:BR based formulation also showed similar properties to that of commercial EVA:NR based formulation.
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Figure 5. SEM photographs of (a) EVA:NR and (b) EVA:BR microcellular blends.
ASSOCIATED CONTENT
S Supporting Information *
Formation of microcellular foams has apparent effects on thermal properties of the blend. The DSC plots of virgin EVA and other EVA based blends are shown in Figure 6. The
Physicomechanical properties of various blends at different processing conditions and blend ratios. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Fax/Phone: +91-265-6693934. E-mail: madhuchhanda.maiti@ ril.com. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank Mr. S. Chatterjee, Mr. H. M. Desai, Ms. R. Dave, Mr. N. F. Patel, and Mr. B. P. Sahu for their support. The authors are grateful to Reliance Industries Ltd. for their consent to publish this work.
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Figure 6. DSC plot of different blends including the microcellular one.
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Figure 7. tan δ vs temperature plots of TPE and microcellular foam.
4. CONCLUSIONS In the present work, EVA:BR blends were prepared at different ratios. The EVA:BR 80:20 blend, prepared at optimized temperature and rotor speed, exhibits thermoplastic elastomeric nature, as evident from physicomechanical and dynamic mechanical properties. It forms a TPE of high strength (20 10611
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