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Mechanical and sorption behaviour of organo-modified montmorillonite nanocomposites based on EPDM – NBR Blends
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 5 (2018) 16132–16140
www.materialstoday.com/proceedings
SCICON 2016
Mechanical and sorption behaviour of organo-modified montmorillonite nanocomposites based on EPDM – NBR Blends Neelesh Ashoka,b, David Webertc , P V Suneeshd , Meera Balachandran a,b* a
Centre of Excellence in Advanced Materials & Green Technologies (CoE-AMGT), bDepartment of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, India – 641112 c École nationale supérieure d'ingénieurs de Caen & Centre de Recherche (ENSICAEN), France d Department of Sciences, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, India – 641112
Abstract Nanocomposites of blends based on ethylene propylene diene monomer (EPDM) rubber and acrylonitrile butadiene copolymer (nitrile rubber or NBR) rubber was prepared by melt compounding in an internal mixer using maleic anhydride as compatibilizer. The blends were reinforced with varying amount (0, 2, 4, 6 and 8 phr (parts per 100 parts rubber)) of organomodified montmorillonite (nanoclay). The nanocomposites were vulcanized at 170C for the optimum cure time in hydraulic compression press to obtain approximately 2mm thick sheets. Morphological analysis using X-ray diffraction and Atomic Force Microscopy confirmed intercalation and exfoliation of nanoclay at lower concentration and slight agglomeration at higher concentration. Consequently, owing to the large surface area and strong matrix – reinforcement interaction, the mechanical properties improved with increasing nanofiller content. The barrier properties, characterized by diffusion, sorption and permeation coefficients also improved with increasing nanoclay content. However, at higher concentration of nanoclay, both mechanical and barrier properties showed slight decline due to formation of agglomerates. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advanced Materials (SCICON ‘16). Keywords: EPDM; Blend; Nanocomposite; Mechanical properties; Sorption behaviour
* Corresponding author. Tel.: +91(422) 2685000; fax: +91(422) 2656274. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advanced Materials (SCICON ‘16).
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Nomenclature EPDM NBR PNC phr
Ethylene Propylene Diene Monomer Acrylo Nitrile Butadiene Rubber Polymer Nanocomposites parts per 100 parts of rubber
1. Introduction Elastomers, an important class of polymers, have several applications including in specialty applications like nuclear technology, aerospace, underwater and other defense areas. In nuclear applications, elastomeric components find utility in O-rings, gaskets, seals, booting materials, belts and gautlets for use in hot cells, glove box and belt drives. These components have to withstand aggressive environments such as radiation, chemicals and corrosion. Among elastomers, EPDM has the highest resistance to radiation and hence it is the material of choice in the above mentioned nuclear applications due to its fully saturated backbone structure and also having enough number of hydrogen bonds on it. In addition, EPDM rubbers have high resistance to heat, ozone, cold temperature and moisture. Acrylonitrile butadiene Copolymer or Nitrile Rubber (NBR) is a special purpose rubber that has outstanding oil, fuel and solvent resistance properties and its properties can be tailor made by choosing appropriate compounding ingredients. The acrylonitrile content in NBR provides fast cure characteristics of short induction time and faster cure rate [1]. Nitrile rubber also has high thermal stability, low permeability, apart from its resistance to fats, oils, hydraulic fluids and solvents. But compared to EPDM, NBR has inferior radiation resistance. Blending of two or more polymers enhances the mechanical properties, ageing resistance and processing characteristics. The drawback of one polymer can be overcome by the benefits of the other polymer. Addition of compatibilizing agents reduces the interfacial tension between the two phases and aids in developing network structure in the phases and the interphase.Though the radiation resistance of EPDM rubbers is superior to other elastomers they do not exhibit solvent resistance, which is a requisite for nuclear fuel reprocessing facilities. This drawback can be overcome without compromising the radiation resistance by blending EPDM with NBR by which the mechanical properties, ageing resistance and processing characteristics are enhanced. In the last two decades, several literature have reported on the enhancement in mechanical, thermal and barrier properties of elastomers on addition of nanofillers. The polymer materials with nanocomposites as fillers or reinforcements have attained a wide area of applications which specialises on less weight, controlled properties and better physic-mechanical properties. Recently, polymer nanocomposites (PNCs) have evoked considerable interest of researchers and technologists. PNCs are polymeric materials comprising particles at least one dimension in the nanosize range [1-100 nm]. The nanosize of the fillers cultivates a dramatic increase in interfacial area which leads to novel and better properties of the composites upon adding small quantities of filler. Historically proposed twenty years ago, these materials have attained a place today for the enormous interest in academic research and industrial applications. The use of nanoparticles as filler makes it possible to produce fundamentally new materials with a range of unusual mechanical and physical properties that is attributed to the enormous increase in interaction between the polymer and the nanofiller. They have improved gas barrier properties, abrasion resistance, reduced shrinkage and residual stress. Several studies have shown that the incorporation of nanosized fillers remarkably improves the solvent resistance of polymers. Within various types of nanocomposites invented so far, the layeres silicate nanocomposites known as nanoclay are the most compromising polymer based nanocomposites because of its peculiar layered structure of the filler, thickness being 1nm for each layer. Betterment in the properties of the polymer with the incorporation of nanofiller results basically from reinforcing action of inorganic particles and also due to filler matrix interfacial interaction. Incorporation of nanoclay in elastomers like nitrile rubber, EPDM etc. have resulted in tremendous enhancement in solvent resistance as the nanoparticles introduce a tortuous path for the permeation of solvent through the polymer. The type of filler and polymer type embedded, the bonding at the interfaces / interaction between the filler and polymer matrix and the state and degree of filler dispersion in the polymer matrix plays a vital role on the magnitude of improvement of specific property. Many review articles have
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been reported focusing on preparation and properties of polymer layered silicate nanocomposites. Improvement of radiation resistance, solvent permeability and mechanical properties are achieved by incorporation of nanofillers[2]. In the work reported here, the EPDM-NBR blend based nanocomposites have been prepared for different filler concentrations, i.e., 2,4,6 and 8 phr (parts per 100 parts of rubber). The effect of nanoclay content on the cure haracteristics, morphology, mechanical properties, solvent sorption characteristics and thermal degradation was studied. 2. Experimental 2.1. Materials and Preparation EPDM rubber (43% ethylene, 14.2% ENB as the diene and Mooney viscositiy [ML (1+4) at 125°C] of 47.8). NBR (medium acrylonitrile content with Mooney viscosity [ML 1+4 at 100C] of 47). Nanoclay montmorillonite surface modified containing 25-30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium. The preparation of the blend involved a two stage process, primarily the rubber-nanofiller masterbach was prepared on the internal mixer at 50rpm for 15minutes. In the secondary stage, compounding of EPDM and NBR along with compatibliser maleic anhydride, nanoclay master batch and other compounding ingredients like activators, vulcanising agent and accelerators and was carried out on a lab scale two-roll compounding mill. The samples were designated ExNRyNCz, where x is the percentage EPDM, y is the percentage of NBR in the blend and z is the phr of NanoClay in the blend. Thus, E80NBR20NC2represents a blend having 80% EPDM and 20% NBR and 2 phr of nanoclay. 2.2. Characterization The cure characteristics of the different blends prepared were evaluated and are given in table 1 using TechPro Rheotech ODR (ASTM D-2084). Vulcanization of compounds was done at 170C for the optimum cure time in hydraulic compression press at 200 MPa to obtain approximately 2mm thick sheets. Differential Scanning Calorimetry (DSC) was employed to determine the miscibility of blends. Evaluation of cure characteristics was carried out on Oscillating Disc Rheometer (ODR). Differential Scanning Calorimetry (DSC) was employed to determine the miscibility of blends using DSC Q20 V24.10 Build 122 in nitrogen atmosphere at 10Cmin-1 heating rate in the temperature range -80C to 420C. Since, the interaction at filler matrix interface drastically alters the composite morphology, characterizations were carried out to study the dispersion state by wide angle X-Ray diffraction studies followed by analysis using Atomic Force Microscopy. Morphological analysis was used to corroborate the observed changes in mechanical strength and identify optimal configuration of filler content. Comprehensive information regarding intercalation and exfoliation of the nanofiller has been provided by XRD and AFM technique. For evaluating the tensile properties of the blends, dumbbell specimens of 3mm width and 40mm gauge length were punched out from a compression molded sheet using a dumb bell shaped die along the mill direction. Modulus, Tensile strength and percentage of elongation at break were evaluated on a Universal Testing Machine (Tinius Olsen UTM) at a cross head speed of 500 mm/min and at room temperature according to ASTM D412 standards. For each blend, the average value obtained for five specimens is reported[3]. The effect of nanoclay content on thermal degradation was analyzed by Thermogravimetric Analysis (TGA) and Differential Thermal Gradient (DTG) analysis were performed on SDT Q600 V20.9 from 30C to 700C with a heating rate of 20Cmin-1 with nitrogen flow at 100mLmin-1. For diffusion studies, uniformly shaped round cut samples of 2cm in diameter was utilized. The round shape is made to avoid stress concentration at the edges. Weight and thickness of the samples before immersion in solvent were measured in an electronic balance and dial gauge respectively. The experiment was performed at room temperature.
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The cured samples were completely immersed in sample bottles with covers containing toluene used as solvent. At specific time intervals, the samples were removed from the solvent, excess solvent at the surface removed using filter paper and the weight measured. The samples were immersed back in the solvent immediately after weighing. The experiment was repeated till equilibrium swelling reached. Transport coefficients such as crosslink density, swelling coefficient, permeability and diffusivity were tabulated. 3. Results and Discussion 3.1. Cure Characteristics The cure time (t90) for nitrile rubber blended with EPDM is found to be lesser than that of pure EPDM due to presence of polar nitrile group content in it[1]. In nanocomposites, cure time decreased with increasing nanoclay content. This phenomenon is attributed to the accelerating effect of the amine group in organomodifier of the nanoclay[4]. The zinc-sulphur accelerator complex “extracts” the amine intercalant of the organosilicates during curing. At higher nanoclay content, there was the reduction in rubber – nanofiller interface due to slight agglomeration, consequently reducing the amine functional group interaction in the cure reaction and hence a slight increase in cure time was observed at 8 phr nanoclay. Table 1. Cure characteristics and Mechanical properties of EPDM-NBR Nanocomposites Sample
E80NBR20
E80NBR20NC2
E80NBR20NC4
E80NBR20NC6
E80NBR20NC8
t90 (min)
15.23
15.23
7.68
9.33
10.1
Tensile Strength (MPa)
1.48
1.81
1.94
2.04
1.83
Elongation at break (%)
263
279
404
444
926
Modulus at 100% (MPa)
0.69
1.03
1.19
1.39
0.82
3.2. Morphological analysis of Nanoclay based nanocomposites of EPDM-NBR blends by X-Ray Diffraction
Fig. 1. XRD pattern of (a) Nanoclay and EPDM-NBR Nanocomposites (b) EPDM – NBR Nanocomposites
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X-Ray Diffraction (XRD) was employed to analyze the states of dispersion of nanoclay in the elastomer matrix. Analysis was done for various nanoclay variations on EPDM-NBR blends. Intercalation due to lager d spacing is correlated to a deviation in the basal reflection to 2θ. XRD graphs of the EPDM-NBR based nanocomposites and the organomodified nanofiller are shown in Figure (1). The peak seen at 4.387° in XRD pattern is of the nanoclay, is analogy to the spacing between the layers of the layered silicate structure. In nanocomposites, the prominent peak found in nanoclay was absent indicating the dispersin of the clay platelets in the matrix. From Figure 1 (b), it can be observed that negligible peaks are found for the nanocomposites with lower concentration of nanoclay content (2 and 4 phr). This pointed out that the insertion of polymer chains separateds the galleriesof nanoclay. The slight peak observed at 2θ = 4.88° for nanocomposites containing 8 phr nanoclay denoted slight aggregates of clay in the matrix. The slight shift in peak at higher nanoclay content and the decrease in intensity suggest that there was a decrease in interlayer distance between the layers of the nanoclay. The participation of the alkyl groups in organoclay during vulcanization that increases the cure rate attributes to the decrease in interlayer distance of the layered silicate and re-aggregation of the nanoclay. A zincsulphur accelerator complex “extracts” the amine intercalant of the organosilicates during curing. 3.3. Image Topography by Atomic Force Microscopy Atomic Force Microscopy (AFM) is frequently used for imaging topography analysis and phase morphology of multi-component system. AFM has become attractive tool for morphological characterization owing to its low cost on purchasing, running and maintenance cost and straight forward method as compared with electron microscopy.AFM is facilitated as a tool for microscopic morphology of the polymer nanocomposites complementary to electron microscopy. The structural characterization of polymer based nanaocomposites is being demonstrated as shown in the Figure 2. AFM technique is not only used for interpreting the morphology of the composite materials but also could be utilized for imaging with nanometer resolution and also to gain insight knowledge onto their nanomechanical properties. With reference to polymer layered silicate nanocomposites, the AFM can be used as a versatile technique to correlate the structure-properties of polymer nanocomposites [5]. A straightforward characterization on the surface of the polymers which enables additional insight into the structure and properties of homopolymers, blends and composites. Atomic force microscopy was used to analyze the dispersion and corroborate the results obtained from XRD. The images also display two different colors of the matrix. It can be noted from the different colored peaks corresponding to different heights. In Fig 2(a), the distribution of color is uniform implying that nanoclay was exfoliated in E80NBR20NC4. In Fig 2 (b), E80NBR20NC8, it can be noted that the nanoparticles are agglomerated as evident from the colour distribution pattern that is not as even as fig 2(a).
Fig. 2. AFM images EPDM-NBR Nanocomposites
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3.4. Mechanical Properties Mechanical properties of the nanocomposites evaluated by ASTM D412 standards are reported in Table 2. With the increase in nanofiller content, the tensile strength and modulus of EPDM- NBR nanocomposites was found increased due to the dispersion of layered silicates in the rubber matrix, stiffness of the nanoclay and interface attachment between matrix and the filler. The larger interfacial region is due to the high aspect ratio of the nanoclay and thereby does not accumulate stress concentration across the interfaces. The poorer dispersion of the filler in the matrix at highest nanoclay content of 8 phr by the evidence from XRD and AFM micrographs is due to the formation of aggregates when compared with 2 , 4 and 6 phr clay loadings. This results in decrease in interfacial interaction and slight reduction in tensile strength for nanocomposites containing 8 phr nanoclay. It has been reported that the organo-modifier in the nanoclay has plasticizing effect on the nanocomposites and this contributes to the increase in elongation at break increasing nanoclay content. 3.5. Sorption Behavior Transport properties of the polymeric blend can be explained by sorption, diffusion and permeation phenomena. The diffusion through a polymeric membrane occurs due to random vibration of individual molecule. An effective tool for better understanding of the interfacial interaction and morphology of the system is the study of its transport process. The permeation of solvent depends on the concentration of available space in the polymer that is large enough to accommodate the penetrate molecule, size of the penetrant, temperature, polymer segment mobility, reaction between solvent and the matrix, etc. Therefore the studies on transport process can be emphasized to understand the interfacial interaction and morphology of the system. The mole percent uptake of the solvent, Qt ,at time t was determined using the formula Qt=(M2/M0)/MW x 100 (1) where M2 is the mass of solvent absorbed after time t of immersion, M0 the initial mass of the sample and MW is the molecular weight of the solvent. The sorption isotherms were plotted with Qt as a function of square root of time for the blends. The average value obtained for three experiments is reported here. The sorption isotherms (mole percent uptake of solvent as a function of time ½) for various blend compositions at 30°C calculated from experimental values using equation (1) are plotted in Figure 3. In cross linked polymers, the polymer segments between the cross-links take up the solvent, leading to swelling of the polymer. The initial solvent uptake for all compositions of the blend is high due to high concentration gradient for diffusion of the solvent. Consequently as the gradient decreases the solvent absorbance decreases and equilibrium was attained. During solvent sorption, the free energy of mixing is responsible for the penetration and consequent dilution of the polymer, resulting in increase in entropy. As the solvent uptake increases, the polymer segments elongate under swelling resulting in an elastic force that opposes the deformation caused by swelling. At equilibrium, the two forces balance and a steady state was attained. The solvent uptake for unfilled EPDM-NBR blend was found to be the highest. The solvent uptake of the blends progressively decreased with increasing nanoclay. The layered laminate structure of the nanoclay introduces tortuous path for the transport of solvent, thereby increasing the diffusion path length and decreasing the transport coefficients. Table 2. Transport properties of EPDM-NBR Nanocomposites Sample Swelling Coefficients 9
2
E80NB R20
E80NBR20NC2
E80NBR20NC4
E80NBR20NC6
E80NBR20NC8
2.92
2.60
2.23
2.33
2.27
Diffusivity , D (*10 ) m /s
2.77
2.13
1.71
1.58
1.55
Permeability ,P (*109)m2/s
8.088
5.54
3.82
3.69
3.53
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The swelling behaviour of composites can be quantified from sorption data in terms of swelling coefficient (β), cross link density (υ) and molecular weight of the polymer between the cross-links calculated using the equations. Sorption coefficient (S) is the ratio of solvent uptake at equilibrium to initial mass of sample. Permeability or permeation coefficient (P) is the product of diffusion and sorption coefficients (P = D x S). Table 2 summarizes the transport coefficients for the various blends. The coefficients of diffusion, sorption and permeation decreased as a function of increasing nanofiller content [6].
Fig. 3. Sorption Isotherms of EPDM-NBR Nanocomposites
With the addition of nanoclay, the swelling coefficient is getting decreased, as the nanoclay introduces torturous path for the solvent to transmit through the medium.
Fig. 4. Tortuous path of layered silicates
3.6. Thermal properties Differential Scanning Calorimetry was employed to evaluate the glass transition temperature of the EPDM-NBR based nanocomposites. Analysis of thermograms show that for the prepared compounds the glass transitions are 33°C for EPDM-NBR blend, -36.8°C for EPDM-NBRNC2, -34.9°C for EPDM-NBRNC4, -34°C for EPDMNBRNC6 and -32.6°C for EPDM-NBRNC8. A representative DSC thermogram of EPDM-NBR Nanocomposite is depicted in Figure 5. For the nanocomposites, the glass transition temperature lies between the same ranges. Along with this observation, the absence of two significant peaks confirms that the nanocomposite formed is completely miscible. The crosslinking temperatures for the nanocomposites are given in table 3.
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Table 3. Thermal properties of EPDM-NBR Nanocomposites Sample
EPDM-NB R
EPDMNBRNC2
EPDMNBRNC4
EPDMNBRNC6
EPDMNBRNC8
Cross-Linking Temp C
179.2
169.8
159.4
159.4
159.4
Peak degradation temp C
470.2
470.56
470.02
470.56
469.47
Fig. 5. DSC Thermograms of EPDM-NBR nanocomposites
Thermogravimetric Analysis (TGA) observes the weight loss and rate of variation of mass with respect to time or temperature. It is primarily used to determine the thermal and or oxidative stabilities of the material. This technique has the ability to analyze the material that exhibit either weight gain or loss due to decomposition, oxidation or loss of volatiles like moisture content. The exact peak degradation temperature value is given by Differential Thermal Gradient curve as shown in Figure 6 below. Temperature at onset of degradation starts at around 190C.
Fig. 6. TGA plot of EPDM-NBR nanocomposites
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4. Conclusions This paper presents detailed investigations on the mechanical, thermal, characterization using XRD and AFM, sorption properties of EPDM- Nitrile rubber (NR) blend based Nanocomposites with focus on application in radiation and hydrocarbon environment. These composites are produced in various compositions (2,4,6 and 8 phr) \ and the results are summarized below. The blends of EPDM-NBR nanocomposites have been prepared by two stage process. XRD and AFM confirmed the exfoliation and intercalation of nanoclay in polymer. Cure Time of blend is lesser than pure EPDM. In nanocomposites, cure time decreased due to plasticizing effect of organomodifier . The mechanical properties increased with increasing nanoclay content owing to the enhanced stress distribution at the polymer nanoclay interface. The higher interfacial interaction is attributed to large surface area of nanoclay. However, at larger concentrations, slight decline in properties was due to agglomeration. Solvent resistance characteristics of nanocomposites were superior to unfilled blends. Acknowledgements The authors would like to acknowledge the support of Ministry of Human Resources Development (MHRD) extended to CoE-AMGT, Amrita University through their fast grant, for the conduct of this research and Sophisticated Test and Instrumentation Centre at Kochi, India for XRD analysis. We also thank IGCAR-UGC-DAE, for funding this work. We would also like to thank the Department of Sciences, Amrita School of Engineering, Amrita University, India for allocating AFM instrument. References [1].Choi, S., J. Kim, C. Woo, bulletin-korean chemical society, 27(2006) 936. [2].Balachandran, M.,S.S. Bhagawan, J. Comp Mat. 2011 0021998311399484. [3].Balachandran, M.,S.S. Bhagawan, J.Polym Res. 19(2012) 1-10. [4].Balachandran, M.,S.S. Bhagawan, J. appl polm sci.126( 2012) 1983-1992. [5].Adhikari, R. Macromolecular Symposia. (2013) Wiley Online Library. [6] Brydson, J. A. Plastics materials, Butterworth-Heinemann: Oxford, (1999) Chapter 3,45-56.
ScienceDirect Materials Today: Proceedings 5 (2018) 16132–16140
www.materialstoday.com/proceedings
SCICON 2016
Mechanical and sorption behaviour of organo-modified montmorillonite nanocomposites based on EPDM – NBR Blends Neelesh Ashoka,b, David Webertc , P V Suneeshd , Meera Balachandran a,b* a
Centre of Excellence in Advanced Materials & Green Technologies (CoE-AMGT), bDepartment of Chemical Engineering and Materials Science, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, India – 641112 c École nationale supérieure d'ingénieurs de Caen & Centre de Recherche (ENSICAEN), France d Department of Sciences, Amrita School of Engineering, Coimbatore, Amrita Vishwa Vidyapeetham, Amrita University, India – 641112
Abstract Nanocomposites of blends based on ethylene propylene diene monomer (EPDM) rubber and acrylonitrile butadiene copolymer (nitrile rubber or NBR) rubber was prepared by melt compounding in an internal mixer using maleic anhydride as compatibilizer. The blends were reinforced with varying amount (0, 2, 4, 6 and 8 phr (parts per 100 parts rubber)) of organomodified montmorillonite (nanoclay). The nanocomposites were vulcanized at 170C for the optimum cure time in hydraulic compression press to obtain approximately 2mm thick sheets. Morphological analysis using X-ray diffraction and Atomic Force Microscopy confirmed intercalation and exfoliation of nanoclay at lower concentration and slight agglomeration at higher concentration. Consequently, owing to the large surface area and strong matrix – reinforcement interaction, the mechanical properties improved with increasing nanofiller content. The barrier properties, characterized by diffusion, sorption and permeation coefficients also improved with increasing nanoclay content. However, at higher concentration of nanoclay, both mechanical and barrier properties showed slight decline due to formation of agglomerates. © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advanced Materials (SCICON ‘16). Keywords: EPDM; Blend; Nanocomposite; Mechanical properties; Sorption behaviour
* Corresponding author. Tel.: +91(422) 2685000; fax: +91(422) 2656274. E-mail address: [email protected] 2214-7853 © 2017 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of International Conference on Advanced Materials (SCICON ‘16).
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Nomenclature EPDM NBR PNC phr
Ethylene Propylene Diene Monomer Acrylo Nitrile Butadiene Rubber Polymer Nanocomposites parts per 100 parts of rubber
1. Introduction Elastomers, an important class of polymers, have several applications including in specialty applications like nuclear technology, aerospace, underwater and other defense areas. In nuclear applications, elastomeric components find utility in O-rings, gaskets, seals, booting materials, belts and gautlets for use in hot cells, glove box and belt drives. These components have to withstand aggressive environments such as radiation, chemicals and corrosion. Among elastomers, EPDM has the highest resistance to radiation and hence it is the material of choice in the above mentioned nuclear applications due to its fully saturated backbone structure and also having enough number of hydrogen bonds on it. In addition, EPDM rubbers have high resistance to heat, ozone, cold temperature and moisture. Acrylonitrile butadiene Copolymer or Nitrile Rubber (NBR) is a special purpose rubber that has outstanding oil, fuel and solvent resistance properties and its properties can be tailor made by choosing appropriate compounding ingredients. The acrylonitrile content in NBR provides fast cure characteristics of short induction time and faster cure rate [1]. Nitrile rubber also has high thermal stability, low permeability, apart from its resistance to fats, oils, hydraulic fluids and solvents. But compared to EPDM, NBR has inferior radiation resistance. Blending of two or more polymers enhances the mechanical properties, ageing resistance and processing characteristics. The drawback of one polymer can be overcome by the benefits of the other polymer. Addition of compatibilizing agents reduces the interfacial tension between the two phases and aids in developing network structure in the phases and the interphase.Though the radiation resistance of EPDM rubbers is superior to other elastomers they do not exhibit solvent resistance, which is a requisite for nuclear fuel reprocessing facilities. This drawback can be overcome without compromising the radiation resistance by blending EPDM with NBR by which the mechanical properties, ageing resistance and processing characteristics are enhanced. In the last two decades, several literature have reported on the enhancement in mechanical, thermal and barrier properties of elastomers on addition of nanofillers. The polymer materials with nanocomposites as fillers or reinforcements have attained a wide area of applications which specialises on less weight, controlled properties and better physic-mechanical properties. Recently, polymer nanocomposites (PNCs) have evoked considerable interest of researchers and technologists. PNCs are polymeric materials comprising particles at least one dimension in the nanosize range [1-100 nm]. The nanosize of the fillers cultivates a dramatic increase in interfacial area which leads to novel and better properties of the composites upon adding small quantities of filler. Historically proposed twenty years ago, these materials have attained a place today for the enormous interest in academic research and industrial applications. The use of nanoparticles as filler makes it possible to produce fundamentally new materials with a range of unusual mechanical and physical properties that is attributed to the enormous increase in interaction between the polymer and the nanofiller. They have improved gas barrier properties, abrasion resistance, reduced shrinkage and residual stress. Several studies have shown that the incorporation of nanosized fillers remarkably improves the solvent resistance of polymers. Within various types of nanocomposites invented so far, the layeres silicate nanocomposites known as nanoclay are the most compromising polymer based nanocomposites because of its peculiar layered structure of the filler, thickness being 1nm for each layer. Betterment in the properties of the polymer with the incorporation of nanofiller results basically from reinforcing action of inorganic particles and also due to filler matrix interfacial interaction. Incorporation of nanoclay in elastomers like nitrile rubber, EPDM etc. have resulted in tremendous enhancement in solvent resistance as the nanoparticles introduce a tortuous path for the permeation of solvent through the polymer. The type of filler and polymer type embedded, the bonding at the interfaces / interaction between the filler and polymer matrix and the state and degree of filler dispersion in the polymer matrix plays a vital role on the magnitude of improvement of specific property. Many review articles have
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been reported focusing on preparation and properties of polymer layered silicate nanocomposites. Improvement of radiation resistance, solvent permeability and mechanical properties are achieved by incorporation of nanofillers[2]. In the work reported here, the EPDM-NBR blend based nanocomposites have been prepared for different filler concentrations, i.e., 2,4,6 and 8 phr (parts per 100 parts of rubber). The effect of nanoclay content on the cure haracteristics, morphology, mechanical properties, solvent sorption characteristics and thermal degradation was studied. 2. Experimental 2.1. Materials and Preparation EPDM rubber (43% ethylene, 14.2% ENB as the diene and Mooney viscositiy [ML (1+4) at 125°C] of 47.8). NBR (medium acrylonitrile content with Mooney viscosity [ML 1+4 at 100C] of 47). Nanoclay montmorillonite surface modified containing 25-30 wt % methyl dihydroxyethyl hydrogenated tallow ammonium. The preparation of the blend involved a two stage process, primarily the rubber-nanofiller masterbach was prepared on the internal mixer at 50rpm for 15minutes. In the secondary stage, compounding of EPDM and NBR along with compatibliser maleic anhydride, nanoclay master batch and other compounding ingredients like activators, vulcanising agent and accelerators and was carried out on a lab scale two-roll compounding mill. The samples were designated ExNRyNCz, where x is the percentage EPDM, y is the percentage of NBR in the blend and z is the phr of NanoClay in the blend. Thus, E80NBR20NC2represents a blend having 80% EPDM and 20% NBR and 2 phr of nanoclay. 2.2. Characterization The cure characteristics of the different blends prepared were evaluated and are given in table 1 using TechPro Rheotech ODR (ASTM D-2084). Vulcanization of compounds was done at 170C for the optimum cure time in hydraulic compression press at 200 MPa to obtain approximately 2mm thick sheets. Differential Scanning Calorimetry (DSC) was employed to determine the miscibility of blends. Evaluation of cure characteristics was carried out on Oscillating Disc Rheometer (ODR). Differential Scanning Calorimetry (DSC) was employed to determine the miscibility of blends using DSC Q20 V24.10 Build 122 in nitrogen atmosphere at 10Cmin-1 heating rate in the temperature range -80C to 420C. Since, the interaction at filler matrix interface drastically alters the composite morphology, characterizations were carried out to study the dispersion state by wide angle X-Ray diffraction studies followed by analysis using Atomic Force Microscopy. Morphological analysis was used to corroborate the observed changes in mechanical strength and identify optimal configuration of filler content. Comprehensive information regarding intercalation and exfoliation of the nanofiller has been provided by XRD and AFM technique. For evaluating the tensile properties of the blends, dumbbell specimens of 3mm width and 40mm gauge length were punched out from a compression molded sheet using a dumb bell shaped die along the mill direction. Modulus, Tensile strength and percentage of elongation at break were evaluated on a Universal Testing Machine (Tinius Olsen UTM) at a cross head speed of 500 mm/min and at room temperature according to ASTM D412 standards. For each blend, the average value obtained for five specimens is reported[3]. The effect of nanoclay content on thermal degradation was analyzed by Thermogravimetric Analysis (TGA) and Differential Thermal Gradient (DTG) analysis were performed on SDT Q600 V20.9 from 30C to 700C with a heating rate of 20Cmin-1 with nitrogen flow at 100mLmin-1. For diffusion studies, uniformly shaped round cut samples of 2cm in diameter was utilized. The round shape is made to avoid stress concentration at the edges. Weight and thickness of the samples before immersion in solvent were measured in an electronic balance and dial gauge respectively. The experiment was performed at room temperature.
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The cured samples were completely immersed in sample bottles with covers containing toluene used as solvent. At specific time intervals, the samples were removed from the solvent, excess solvent at the surface removed using filter paper and the weight measured. The samples were immersed back in the solvent immediately after weighing. The experiment was repeated till equilibrium swelling reached. Transport coefficients such as crosslink density, swelling coefficient, permeability and diffusivity were tabulated. 3. Results and Discussion 3.1. Cure Characteristics The cure time (t90) for nitrile rubber blended with EPDM is found to be lesser than that of pure EPDM due to presence of polar nitrile group content in it[1]. In nanocomposites, cure time decreased with increasing nanoclay content. This phenomenon is attributed to the accelerating effect of the amine group in organomodifier of the nanoclay[4]. The zinc-sulphur accelerator complex “extracts” the amine intercalant of the organosilicates during curing. At higher nanoclay content, there was the reduction in rubber – nanofiller interface due to slight agglomeration, consequently reducing the amine functional group interaction in the cure reaction and hence a slight increase in cure time was observed at 8 phr nanoclay. Table 1. Cure characteristics and Mechanical properties of EPDM-NBR Nanocomposites Sample
E80NBR20
E80NBR20NC2
E80NBR20NC4
E80NBR20NC6
E80NBR20NC8
t90 (min)
15.23
15.23
7.68
9.33
10.1
Tensile Strength (MPa)
1.48
1.81
1.94
2.04
1.83
Elongation at break (%)
263
279
404
444
926
Modulus at 100% (MPa)
0.69
1.03
1.19
1.39
0.82
3.2. Morphological analysis of Nanoclay based nanocomposites of EPDM-NBR blends by X-Ray Diffraction
Fig. 1. XRD pattern of (a) Nanoclay and EPDM-NBR Nanocomposites (b) EPDM – NBR Nanocomposites
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X-Ray Diffraction (XRD) was employed to analyze the states of dispersion of nanoclay in the elastomer matrix. Analysis was done for various nanoclay variations on EPDM-NBR blends. Intercalation due to lager d spacing is correlated to a deviation in the basal reflection to 2θ. XRD graphs of the EPDM-NBR based nanocomposites and the organomodified nanofiller are shown in Figure (1). The peak seen at 4.387° in XRD pattern is of the nanoclay, is analogy to the spacing between the layers of the layered silicate structure. In nanocomposites, the prominent peak found in nanoclay was absent indicating the dispersin of the clay platelets in the matrix. From Figure 1 (b), it can be observed that negligible peaks are found for the nanocomposites with lower concentration of nanoclay content (2 and 4 phr). This pointed out that the insertion of polymer chains separateds the galleriesof nanoclay. The slight peak observed at 2θ = 4.88° for nanocomposites containing 8 phr nanoclay denoted slight aggregates of clay in the matrix. The slight shift in peak at higher nanoclay content and the decrease in intensity suggest that there was a decrease in interlayer distance between the layers of the nanoclay. The participation of the alkyl groups in organoclay during vulcanization that increases the cure rate attributes to the decrease in interlayer distance of the layered silicate and re-aggregation of the nanoclay. A zincsulphur accelerator complex “extracts” the amine intercalant of the organosilicates during curing. 3.3. Image Topography by Atomic Force Microscopy Atomic Force Microscopy (AFM) is frequently used for imaging topography analysis and phase morphology of multi-component system. AFM has become attractive tool for morphological characterization owing to its low cost on purchasing, running and maintenance cost and straight forward method as compared with electron microscopy.AFM is facilitated as a tool for microscopic morphology of the polymer nanocomposites complementary to electron microscopy. The structural characterization of polymer based nanaocomposites is being demonstrated as shown in the Figure 2. AFM technique is not only used for interpreting the morphology of the composite materials but also could be utilized for imaging with nanometer resolution and also to gain insight knowledge onto their nanomechanical properties. With reference to polymer layered silicate nanocomposites, the AFM can be used as a versatile technique to correlate the structure-properties of polymer nanocomposites [5]. A straightforward characterization on the surface of the polymers which enables additional insight into the structure and properties of homopolymers, blends and composites. Atomic force microscopy was used to analyze the dispersion and corroborate the results obtained from XRD. The images also display two different colors of the matrix. It can be noted from the different colored peaks corresponding to different heights. In Fig 2(a), the distribution of color is uniform implying that nanoclay was exfoliated in E80NBR20NC4. In Fig 2 (b), E80NBR20NC8, it can be noted that the nanoparticles are agglomerated as evident from the colour distribution pattern that is not as even as fig 2(a).
Fig. 2. AFM images EPDM-NBR Nanocomposites
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3.4. Mechanical Properties Mechanical properties of the nanocomposites evaluated by ASTM D412 standards are reported in Table 2. With the increase in nanofiller content, the tensile strength and modulus of EPDM- NBR nanocomposites was found increased due to the dispersion of layered silicates in the rubber matrix, stiffness of the nanoclay and interface attachment between matrix and the filler. The larger interfacial region is due to the high aspect ratio of the nanoclay and thereby does not accumulate stress concentration across the interfaces. The poorer dispersion of the filler in the matrix at highest nanoclay content of 8 phr by the evidence from XRD and AFM micrographs is due to the formation of aggregates when compared with 2 , 4 and 6 phr clay loadings. This results in decrease in interfacial interaction and slight reduction in tensile strength for nanocomposites containing 8 phr nanoclay. It has been reported that the organo-modifier in the nanoclay has plasticizing effect on the nanocomposites and this contributes to the increase in elongation at break increasing nanoclay content. 3.5. Sorption Behavior Transport properties of the polymeric blend can be explained by sorption, diffusion and permeation phenomena. The diffusion through a polymeric membrane occurs due to random vibration of individual molecule. An effective tool for better understanding of the interfacial interaction and morphology of the system is the study of its transport process. The permeation of solvent depends on the concentration of available space in the polymer that is large enough to accommodate the penetrate molecule, size of the penetrant, temperature, polymer segment mobility, reaction between solvent and the matrix, etc. Therefore the studies on transport process can be emphasized to understand the interfacial interaction and morphology of the system. The mole percent uptake of the solvent, Qt ,at time t was determined using the formula Qt=(M2/M0)/MW x 100 (1) where M2 is the mass of solvent absorbed after time t of immersion, M0 the initial mass of the sample and MW is the molecular weight of the solvent. The sorption isotherms were plotted with Qt as a function of square root of time for the blends. The average value obtained for three experiments is reported here. The sorption isotherms (mole percent uptake of solvent as a function of time ½) for various blend compositions at 30°C calculated from experimental values using equation (1) are plotted in Figure 3. In cross linked polymers, the polymer segments between the cross-links take up the solvent, leading to swelling of the polymer. The initial solvent uptake for all compositions of the blend is high due to high concentration gradient for diffusion of the solvent. Consequently as the gradient decreases the solvent absorbance decreases and equilibrium was attained. During solvent sorption, the free energy of mixing is responsible for the penetration and consequent dilution of the polymer, resulting in increase in entropy. As the solvent uptake increases, the polymer segments elongate under swelling resulting in an elastic force that opposes the deformation caused by swelling. At equilibrium, the two forces balance and a steady state was attained. The solvent uptake for unfilled EPDM-NBR blend was found to be the highest. The solvent uptake of the blends progressively decreased with increasing nanoclay. The layered laminate structure of the nanoclay introduces tortuous path for the transport of solvent, thereby increasing the diffusion path length and decreasing the transport coefficients. Table 2. Transport properties of EPDM-NBR Nanocomposites Sample Swelling Coefficients 9
2
E80NB R20
E80NBR20NC2
E80NBR20NC4
E80NBR20NC6
E80NBR20NC8
2.92
2.60
2.23
2.33
2.27
Diffusivity , D (*10 ) m /s
2.77
2.13
1.71
1.58
1.55
Permeability ,P (*109)m2/s
8.088
5.54
3.82
3.69
3.53
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The swelling behaviour of composites can be quantified from sorption data in terms of swelling coefficient (β), cross link density (υ) and molecular weight of the polymer between the cross-links calculated using the equations. Sorption coefficient (S) is the ratio of solvent uptake at equilibrium to initial mass of sample. Permeability or permeation coefficient (P) is the product of diffusion and sorption coefficients (P = D x S). Table 2 summarizes the transport coefficients for the various blends. The coefficients of diffusion, sorption and permeation decreased as a function of increasing nanofiller content [6].
Fig. 3. Sorption Isotherms of EPDM-NBR Nanocomposites
With the addition of nanoclay, the swelling coefficient is getting decreased, as the nanoclay introduces torturous path for the solvent to transmit through the medium.
Fig. 4. Tortuous path of layered silicates
3.6. Thermal properties Differential Scanning Calorimetry was employed to evaluate the glass transition temperature of the EPDM-NBR based nanocomposites. Analysis of thermograms show that for the prepared compounds the glass transitions are 33°C for EPDM-NBR blend, -36.8°C for EPDM-NBRNC2, -34.9°C for EPDM-NBRNC4, -34°C for EPDMNBRNC6 and -32.6°C for EPDM-NBRNC8. A representative DSC thermogram of EPDM-NBR Nanocomposite is depicted in Figure 5. For the nanocomposites, the glass transition temperature lies between the same ranges. Along with this observation, the absence of two significant peaks confirms that the nanocomposite formed is completely miscible. The crosslinking temperatures for the nanocomposites are given in table 3.
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Table 3. Thermal properties of EPDM-NBR Nanocomposites Sample
EPDM-NB R
EPDMNBRNC2
EPDMNBRNC4
EPDMNBRNC6
EPDMNBRNC8
Cross-Linking Temp C
179.2
169.8
159.4
159.4
159.4
Peak degradation temp C
470.2
470.56
470.02
470.56
469.47
Fig. 5. DSC Thermograms of EPDM-NBR nanocomposites
Thermogravimetric Analysis (TGA) observes the weight loss and rate of variation of mass with respect to time or temperature. It is primarily used to determine the thermal and or oxidative stabilities of the material. This technique has the ability to analyze the material that exhibit either weight gain or loss due to decomposition, oxidation or loss of volatiles like moisture content. The exact peak degradation temperature value is given by Differential Thermal Gradient curve as shown in Figure 6 below. Temperature at onset of degradation starts at around 190C.
Fig. 6. TGA plot of EPDM-NBR nanocomposites
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4. Conclusions This paper presents detailed investigations on the mechanical, thermal, characterization using XRD and AFM, sorption properties of EPDM- Nitrile rubber (NR) blend based Nanocomposites with focus on application in radiation and hydrocarbon environment. These composites are produced in various compositions (2,4,6 and 8 phr) \ and the results are summarized below. The blends of EPDM-NBR nanocomposites have been prepared by two stage process. XRD and AFM confirmed the exfoliation and intercalation of nanoclay in polymer. Cure Time of blend is lesser than pure EPDM. In nanocomposites, cure time decreased due to plasticizing effect of organomodifier . The mechanical properties increased with increasing nanoclay content owing to the enhanced stress distribution at the polymer nanoclay interface. The higher interfacial interaction is attributed to large surface area of nanoclay. However, at larger concentrations, slight decline in properties was due to agglomeration. Solvent resistance characteristics of nanocomposites were superior to unfilled blends. Acknowledgements The authors would like to acknowledge the support of Ministry of Human Resources Development (MHRD) extended to CoE-AMGT, Amrita University through their fast grant, for the conduct of this research and Sophisticated Test and Instrumentation Centre at Kochi, India for XRD analysis. We also thank IGCAR-UGC-DAE, for funding this work. We would also like to thank the Department of Sciences, Amrita School of Engineering, Amrita University, India for allocating AFM instrument. References [1].Choi, S., J. Kim, C. Woo, bulletin-korean chemical society, 27(2006) 936. [2].Balachandran, M.,S.S. Bhagawan, J. Comp Mat. 2011 0021998311399484. [3].Balachandran, M.,S.S. Bhagawan, J.Polym Res. 19(2012) 1-10. [4].Balachandran, M.,S.S. Bhagawan, J. appl polm sci.126( 2012) 1983-1992. [5].Adhikari, R. Macromolecular Symposia. (2013) Wiley Online Library. [6] Brydson, J. A. Plastics materials, Butterworth-Heinemann: Oxford, (1999) Chapter 3,45-56.