Microstructure and Mechanical Properties of Carboxylated ...

Abstract

The effect of various amounts of carboxylated nitrile butadiene rubber (XNBR) functionalized halloysite nanotubes (XHNTs) on the cure characteristics, mechanical and swelling behavior of XNBR/epoxy compounds was experimentally and theoretically investigated. The morphology of the prepared XNBR/epoxy/XHNTs nanocomposites was imaged using scanning electron microscopy (SEM). The effects of various XNBR-grafted nanotubes on the damping factor of nanocomposites were evaluated by dynamic mechanical thermal analysis (DMTA). The cure behavior characterization indicated a fall in the scorch time, but a rise in the cure rate with higher loading of XHNTs into the XNBR/epoxy nanocomposites. SEM micrographs of tensile fracture surfaces were indicative of a rougher fracture surface with a uniform dispersion state of nanotubes into the polymer matrix in the XNBR/epoxy/XHNTs nanocomposites. The stress&#;strain behavior studies of XNBR/epoxy/XHNTs nanocomposites showed a higher tensile strength up to 40% with 7 wt % XHNTs loading. The theoretical predictions of uniaxial tensile behavior of nanocomposites using Bergström&#;Boyce model revealed that some of the material parameters were considerably changed with the XHNTs loading. Furthermore, the used theoretical model precisely predicted the nonlinear large strain hyperelastic behavior of nanocomposites.

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Keywords: carboxylated nitrile butadiene rubber, epoxy, halloysite nanotubes, mechanical behavior, Bergström&#;Boyce model, swelling

1. Introduction

Rubber composites and nanocomposites have received continued attention in recent years [1,2,3]. A wide variety of nanoparticles were examined in detail in various rubber matrices to achieve higher properties [4,5,6]. Carboxylated nitrile butadiene rubber (XNBR) is a special type of nitrile butadiene rubber (NBR) containing carboxyl functional groups which cause to exhibit enhanced tear and abrasion resistance [7]. Whereas the oil and solvent resistance properties of the rubber will retain at the excellent level [8]. The vulcanization of XNBR can form different types of chemical bonds due to the presence of variety of functional groups in the polymer chain backbone containing nitrile, carboxylic and alkene groups [9]. The carboxylic functional group in the chemical structure of XNBR can react with several materials such as metal oxides, amines, polyols and epoxies in the curing process [10].

The carboxylic functional group creates a potential for XNBR to mix with various fillers and polymers with sufficient interfacial interactions in the mixing and curing processes such as epoxy polymers. Epoxy resins are widely used in various applications such as thermosetting composites [11], thermal conductive nanocomposites [12] and honeycomb sandwich panels for aerospace applications [13]. Chakraborty et al. [9] studied the properties of XNBR/epoxy blends in presence of carbon black and found that 7.5 parts per hundred rubber (phr) of epoxy resin leads to a compound with optimum cure behavior and mechanical properties. Laskowska et al. [14] investigated the effect of various magnesium aluminum layered double hydroxide (MgAl&#;LDH) on the properties of XNBR and represented that the incorporation of LDH into the XNBR matrix had a significant impact on the glass transition temperature (Tg), cure behavior and mechanical properties. Sahoo et al. [15] studied the effect of introduction of nano zinc oxide (ZnO) on the cure characteristics and mechanical properties of XNBR and confirmed a better state of cure and higher mechanical propertied in comparison with the conventional ZnO.

Halloysite nanotubes (HNTs) are naturally occurs nano cylinders which recently used in various polymer nanocomposites for their high thermal and mechanical properties [16,17,18]. We have experimentally studied the effect of HNTs on the physical and mechanical properties of various polymer matrices. The nanocomposites of polyamide 6 (PA6)/nitrile butadiene rubber (NBR) thermoplastic elastomers (TPEs) containing various concentrations of pristine and silane modified HNTs were investigated and found that the introduction of silane modified HNTs into the PA6/NBR TPEs cause a rise in the tensile strength and Young&#;s modulus of polymer matrix due to the physical structure of the nanotubes and their interactions with PA6 [19]. Our following researches focused on the effect of HNTs on the crystallization [20] and degradation [21] behavior of dynamically vulcanized PA6/NBR thermoplastic elastomer vulcanizates (TPVs). The results indicated a higher thermal stability for nanocomposites containing higher HNTs loading. In our previous investigations [22], we stated that the surface modified HNTs with silane functional groups could be grafted with XNBR. The resulted XNBR grafted HNTs has a great potential as a reinforcing agent in many polymer systems.

Our findings suggested a detailed study of the cure behavior and mechanical characteristics of the XNBR/epoxy nanocomposites containing various concentrations of XNBR grafted halloysite nanotubes (XHNTs). The main objective of this research focused on the theoretical and experimental evaluations of the stiffness and stress&#;strain behavior of the XNBR/epoxy/XHNTs nanocomposites through using appropriate large strain hyperelastic theoretical models. The predictions on the stiffness analysis and uniaxial stress&#;strain behavior of nanocomposites were evaluated in comparison with the experimental results of tensile experiments of XNBR/epoxy/XHNTs containing various nanotube loadings.

2. Theoretical Background

The mechanical behavior of large strain rubber like materials such as XNBR/epoxy/XHNTs nanocomposites could be predicted through using Bergström&#;Boyce model which was discussed in details in our previous work [23]. The uniaxial stress&#;strain behavior of a polymer with large strain deformation could be represented in two parallel networks with a hyperelastic behavior and time dependent viscoelastic response [24]. The Cauchy stress tensor for hyperelastic response of a rubber like material is given by the following equation [25]:

TA=μJλ¯*L&#;1(λ¯*/λL)L&#;1(1/λL)dev[B*]+κ[lnJ]I (1)

In contrast, the Cauchy stress tensor for time dependent viscoelastic behavior could be represented as [25]:

TB=sμJBeλ¯Be*L&#;1(λ¯Be*/λL)L&#;1(1/λL)dev[BBe*]+κ[lnJBe]I (2)

where, μ and κ are the shear and bulk moduli, λL is defined as the limiting chain stretch, I is the second-order identity tensor and L&#;1(x) represented as the inverse Langevin function. The parameter J is the Jacobian and λ¯* is the applied chain stretch. The ratio of shear modulus of viscoelastic response to the shear modulus of hyperelastic response of material represented as s in Equation (2) which is a dimensionless material parameter.

There are some of the material parameters in the Bergström&#;Boyce model which could be affected by incorporation of XHNTs into the XNBR/epoxy matrix. These material parameters could be separately calculated for each nanocomposite by matching the experimental data of tensile tests with the theoretical model through using an optimization method.

3. Experimental

3.1. Materials

Carboxylated nitrile butadiene rubber (XNBR) was Krynac X160, supplied by Lanxess Elastomers (Dormagen, Germany) which contains 32.5% by weight of acrylonitrile and 1% by weight of carboxylic acid group. The diglycidyl ether of bisphenol A (DGEBA) type epoxy resin, KER828, with epoxy group content of &#; mmol/kg, was obtained from Kumho P&B Chemicals, Seoul, South Korea. XNBR grafted halloysite nanotubes (XHNTs) synthesized in accordance with our previous works through using halloysite nanotubes (HNTs), ultrafine grade, was procured from Imerys Tableware Asia Limited (North Island, New Zealand). Other ingredients such as zinc oxide and acetic acid were laboratory reagent grades which prepared from Merck Co. (Darmstadt, Germany) and used as received.

3.2. Nanocomposite Preparation

The XNBR/epoxy/XHNTs nanocomposites with various formulations in accordance with Table 1, were prepared on a laboratory open two roll mills, running at speed ratio of 1:1.2 for 10 min at 40 °C. At first stage, The XNBR was masticated for 1 min and then the epoxy resin was added to the rubber. The XHNTs was incorporated into the rubber mixture after 2 min of mixing process and the mixing was continued for 5 min. Finally, the ZnO and acid stearic were added to the nanocomposite as a curing agent and activator which mixed to the rubber for 3 min. As indicated in Scheme 1, the ZnO can react with carboxylic group of XNBR and acts as a curing agent for this rubber. The prepared rubber compounds were compression molded in a compression molding machine at 175 °C and time needed to reach the required optimum cure according to the optimum cure time obtained from Monsanto Oscillating Disc Rheometer R-100 (Monsanto Company, St. Louis, MO, USA).

Table 1.

Formulations of various carboxylated nitrile butadiene rubber (XNBR)/epoxy/ XNBR grafted halloysite nanotube (XHNT) nanocomposites (in parts per hundred rubber (phr)).

Sample Code XNBR/Epoxy (70/30) XHNT ZnO Stearic Acid XE15 100 0 6 2 XE15H3 100 3 6 2 XE15H5 100 5 6 2 XE15H7 100 7 6 2 Open in a new tab

Scheme 1.

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Possible reactions between ZnO and carboxylic groups in XNBR.

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3.3. Characterization

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The morphology of XNBR/epoxy/XHNTs nanocomposites was determined by Vega II XMU scanning electron microscope (SEM) from Tescan Brno s.r.o., Brno, Czech Republic. The SEM analysis was carried out on the cryogenically fractured surfaces after coating with gold powders by sputtering technique.

The cure behavior of prepared samples was studied by using a Monsanto Rheometer R-100 testing instrument operated at 175 °C with 3° arc at a period of 30 min in accordance with ASTM D.

The uniaxial stress&#;strain behavior of XNBR/epoxy nanocomposites containing various XHNTs loading was monitored according to ASTM D412 through using Universal tensile testing machine, Instron model (Instron Ltd., Norwood, MA, USA) operated at room temperature at an extension speed of 500 mm/min with an initial gauge length of 25 mm.

The phase structure of the prepared nanocomposites was evaluated from dynamic mechanical thermal analysis (DMTA) using Triton Technology Tritec DMA (Nottinghamshire, UK). The storage modulus and damping factor of prepared samples were recognized in tension mode at a constant heating rate of 3 °C/min and a frequency of 1 Hz in a strain of 0.02 mm from &#;100 °C to 100 °C.

Swelling of rubber nanocomposites was extensively studied [26,27] Swelling of the various XNBR/epoxy/XHNTs nanocomposites was investigated in toluene solvent in accordance with ASTM D. The needed samples for swelling test were cut from the molded slabs and weighted in dry state. The swollen weights of the samples immersed in the solvent for 72 h were recorded to determine the swelling ratio and cross-link density through using Flory&#;Rehner equations [28]:

Qs=ws&#;wuwu (3) υsw=&#;[ln(1&#;υ)+υ+χυ2]Vs(υ13&#;υ2) (4)

where Qs defined as the swelling ratio, ws and wu are the swollen and unswollen weights of the sample, respectively. The parameter, υsw, is the cross-link density (mol/m3), χ is the polymer-solvent interaction parameter, Vs is molar volume of the solvent (m3/mol) and υ is the volume fraction of polymer in swollen state which could be calculated from the following equation [29]:

υ=wpdpwpdp+wsdp (5)

where wp and ws are the weight fractions of rubber and solvent in the swollen sample, respectively. The parameters dp and ds are defined as the densities of polymer and solvent, respectively. The polymer-solvent interaction parameter could be calculated using following equation [30]:

χ=0.487+0.228υ (6)

Another approach to determine the cross-link density of a cured rubber system is the following equation which uses a hypothesis that the cross-link density has a direct relation with the Young&#;s modulus of the rubber [31]:

υe=E3RT (7)

where E is the Young&#;s modulus which determined from the slope of stress&#;strain cure at the initial region of elongation, R is universal gas constant (8.314 J/mol·K) and T is the absolute temperature (K).

Furthermore, the cross-link density parameter of a cured rubber could be calculated using the modulus at the rubbery plateau region in the plot of storage modulus versus temperature [31]:

υst=Est6RT (8)

where Est is the storage modulus at the rubbery plateau region.

5. Conclusions

The XNBR/epoxy nanocomposites containing various concentrations of XNBR grafted halloysite nanotubes (XHNTs) were prepared through using a two roll mills. The results of cure rheometer show that the introduction of XHNTs into the XNBR/epoxy matrix cause a rise in the maximum torque while decreases the scorch and optimum cure times. The morphology investigations of nanocomposites revealed a rougher fracture surface of nanocomposites with a unique dispersion of nanotubes into the rubber matrix. Tensile strength and modulus at 300% elongations increased with higher nanotubes loadings while there are some decreases in the values of elongation at break. The theoretical evaluations for uniaxial stress&#;strain behavior of XNBR/epoxy/XHNTs nanocomposites cleared that some of material parameters of used Bergström&#;Boyce model significantly varied with the XHNTs concentrations. Nevertheless, the model could precisely predict the large strain hyperelastic behavior of XNBR/epoxy/XHNTs nanocomposites. However, there is some deviations from experimental values of stress&#;strain data at the regions of ultimate elongations. Furthermore, the results of theoretical investigations show a more deviation from experimental values with higher loadings of XNBR grafted nanotubes. The results of dynamic mechanical analysis demonstrated a higher values of storage modulus with higher loadings of XHNTs, while there is some diminution of damping factor. Swelling studies of nanocomposites suggested that the nanotubes act as obstacle to reduce the diffusion of the used solvent into the bulk of rubber matrix. The results preferred to make a rubber based nanocomposite with higher mechanical behavior which could be prepared through using general mixing equipment.

Author Contributions

Conceptualization, S.M.R.P. and M.R.S.; methodology, S.M.R.P.; software, S.M.R.P.; validation, G.N.; formal analysis, E.M.; investigation, S.M.R.P. and H.M.; data curation, K.F. and S.M.R.P.; writing&#;original draft preparation, S.M.R.P.; writing&#;review and editing, K.F. and M.R.S.; visualization, K.F. and M.R.S.; supervision, S.M.R.P. & M.R.S.; All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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