Microstructure and Mechanical Properties of Carboxylated ...

Dec. 16, 2024

Microstructure and Mechanical Properties of Carboxylated Nitrile Butadiene Rubber Nanocomposites

Abstract

This study investigates the impact of varying amounts of carboxylated nitrile butadiene rubber (XNBR) functionalized halloysite nanotubes (XHNTs) on the curing characteristics, mechanical properties, and swelling behavior of XNBR/epoxy composites. The morphology of the synthesized XNBR/epoxy/XHNTs nanocomposites was analyzed using scanning electron microscopy (SEM). Furthermore, the effect of XNBR-grafted nanotubes on the damping factor was assessed through dynamic mechanical thermal analysis (DMTA). Characterization of curing behavior showed a decrease in scorch time alongside an increase in curing rate as XHNT loading increased in the XNBR/epoxy nanocomposites. SEM images of tensile fracture surfaces depicted a rougher surface combined with uniform dispersion of nanotubes within the polymer matrix. Stress-strain analysis revealed a significant enhancement in tensile strength—up to 40%—with a 7 wt % loading of XHNTs. Theoretical predictions of uniaxial tensile behavior using the Bergström-Boyce model indicated significant changes in some material parameters with XHNT incorporation. Additionally, the theoretical model successfully predicted the nonlinear large strain hyperelastic behavior of the nanocomposites.

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

1. Introduction

Recent years have witnessed increasing attention towards rubber composites and nanocomposites [1,2,3]. Numerous nanoparticles have been explored in various rubber matrices to enhance property performance [4,5,6]. Carboxylated nitrile butadiene rubber (XNBR) stands out as a specialized variant of nitrile butadiene rubber (NBR) enriched with carboxyl functional groups, promoting improved tear and abrasion resistance [7]. Notably, the rubber retains excellent resistance to oils and solvents [8]. The vulcanization of XNBR facilitates different types of chemical bonds, attributed to a variety of functional groups present in the polymer backbone, including nitrile, carboxylic, and alkene groups [9]. These carboxylic functional groups permit XNBR to interact with diverse materials such as metal oxides, amines, polyols, and epoxies during the curing process [10].

The functional carboxylic groups enhance the ability of XNBR to blend with fillers and polymers that exhibit sufficient interfacial interactions during mixing and curing, particularly with epoxy polymers. Epoxy resins are widely applied across various domains, including thermosetting composites [11], thermally conductive nanocomposites [12], and aerospace honeycomb sandwich panels [13]. Research by Chakraborty et al. [9] highlighted the attributes of XNBR/epoxy blends in conjunction with carbon black, revealing that 7.5 parts per hundred rubber (phr) of epoxy resin yielded optimal curing behavior and mechanical properties. Correspondingly, Laskowska et al. [14] demonstrated the effects of various magnesium aluminum layered double hydroxides (MgAl-LDH) on XNBR properties, emphasizing significant impacts on glass transition temperature (Tg), curing behavior, and mechanical properties. Sahoo et al. [15] examined the introduction of nano zinc oxide (ZnO) and validated superior cure characteristics and mechanical performances compared to conventional ZnO.

Halloysite nanotubes (HNTs) are naturally occurring nano-cylinders recently utilized in multiple polymer nanocomposites due to their remarkable thermal and mechanical properties [16,17,18]. We experimentally explored the effects of HNTs on the physical and mechanical properties across various polymer matrices. For example, polyamide 6 (PA6)/nitrile butadiene rubber (NBR) thermoplastic elastomers (TPEs) integrating different concentrations of pristine and silane-modified HNTs exhibited improved tensile strength and Young’s modulus due to the structural attributes of the nanotubes and their interactions with PA6 [19]. Subsequent studies focused on the influence of HNTs on crystallization [20] and degradation [21] behaviors within dynamically vulcanized PA6/NBR thermoplastic elastomer vulcanizates (TPVs), with results indicating enhanced thermal stability for nanocomposites incorporating higher HNT loadings. Previous work [22] also revealed that surface-modified HNTs with silane functional groups could be effectively grafted to XNBR, thus positioning XNBR-grafted HNTs as promising reinforcing agents in numerous polymer systems.

Our research is directed towards a comprehensive investigation of the curing behavior and mechanical characteristics of XNBR/epoxy nanocomposites with different concentrations of XNBR-grafted halloysite nanotubes (XHNTs). This study aims to evaluate both the theoretical and experimental dimensions of stiffness and stress-strain behavior in XNBR/epoxy/XHNTs nanocomposites utilizing appropriate large strain hyperelastic models. Comparisons between theoretical predictions of stiffness and uniaxial stress-strain behavior with experimental tensile test results for various XHNT loadings form a crucial part of this study.

2. Theoretical Background

The mechanical behavior of large strain rubber-like materials, including XNBR/epoxy/XHNTs nanocomposites, can be predicted through the Bergström-Boyce model as elaborated in our prior work [23]. The uniaxial stress-strain response of a polymer undergoing large strain deformation can be represented by two parallel networks exhibiting hyperelastic and time-dependent viscoelastic characteristics [24]. The Cauchy stress tensor for the hyperelastic response of a rubber-like material is expressed by the following equation [25]:

TA=μJλ*L₁(λ*/λL)L₁(1/λL)dev[B*]+κ[lnJ]I (1)

Conversely, the Cauchy stress tensor for time-dependent viscoelastic behavior is given as [25]:

TB=sμJBeλ*Be*L₁(λBe*/λL)L₁(1/λL)dev[BBe*]+κ[lnJBe]I (2)

In these equations, μ denotes shear modulus, κ represents bulk modulus, λL is the limiting chain stretch, I signifies the second-order identity tensor, and L₁(x) denotes the inverse Langevin function. The parameter J indicates the Jacobian, while λ* signifies the applied chain stretch. The ratio of shear modulus corresponding to viscoelastic response versus hyperelastic response is denoted as s in Equation (2), representing a dimensionless material parameter.

Several material parameters within the Bergström-Boyce model can be influenced by integrating XHNTs into the XNBR/epoxy matrix. These parameters can be calculated for each nanocomposite by aligning experimental tensile test data with theoretical models using an optimization method.

3. Experimental

3.1. Materials

Carboxylated nitrile butadiene rubber (XNBR) used was Krynac X160, provided by Lanxess Elastomers (Dormagen, Germany), containing 32.5% acrylonitrile and 1% carboxylic acid group by weight. The epoxy resin, diglycidyl ether of bisphenol A (DGEBA) type, KER828, with epoxy group content of xmmol/kg, was sourced from Kumho P&B Chemicals, Seoul, South Korea. The XNBR grafted halloysite nanotubes (XHNTs) were synthesized as previously described using ultrafine Halloysite nanotubes procured from Imerys Tableware Asia Limited (North Island, New Zealand). Other materials, including zinc oxide and acetic acid, were laboratory-grade reagents from Merck Co. (Darmstadt, Germany) and were utilized as received.

3.2. Nanocomposite Preparation

The preparation of XNBR/epoxy/XHNTs nanocomposites involved specific formulations per Table 1, carried out on two-roll mills at a speed ratio of 1:1.2 for 10 minutes at 40°C. The first step included masticating XNBR for one minute, followed by the addition of epoxy. After two minutes of blending, XHNTs were incorporated, and mixing continued for an additional five minutes. Finally, ZnO and stearic acid were added as curing agents and activators, with a further three minutes of mixing. As depicted in Scheme 1, ZnO reacts with the carboxylic group of XNBR to function as a curing agent. The resultant rubber compounds were compression-molded at 175°C, with a timeframe determined according to optimal cure times established using a 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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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 analyzed with a Vega II XMU scanning electron microscope (SEM) from Tescan Brno s.r.o., Brno, Czech Republic. SEM assays were performed on cryogenically fractured surfaces after gold sputtering.

The curing behaviors of the samples were assessed using a Monsanto Rheometer R-100, operating at 175°C and 3° arc for 30 minutes in accordance with ASTM D.

The uniaxial stress-strain behaviors of XNBR/epoxy nanocomposites with varying XHNT loadings were measured based on ASTM D412 using a universal tensile testing machine (Instron model, Instron Ltd., Norwood, MA, USA) at room temperature with a 500 mm/min extension speed and an initial gauge length of 25 mm.

The phase structure of the formed nanocomposites was evaluated through dynamic mechanical thermal analysis (DMTA) utilizing a Triton Technology Tritec DMA (Nottinghamshire, UK). Storage modulus and damping factors were evaluated in tension mode under a constant heating rate of 3 °C/min at a frequency of 1 Hz, within a strain of 0.02 mm across a temperature range from -100 °C to 100 °C.

Swelling analysis of rubber nanocomposites has been extensively studied [26,27]. The swelling behaviors of various XNBR/epoxy/XHNTs nanocomposites were examined in toluene according to ASTM D. Samples for swelling tests were cut from molded slabs and weighed in their dry state. Swollen weights of the samples immersed in solvent for 72 hours were documented to deduce swelling ratios and cross-link densities, employing Flory-Rehner equations [28]:

Qs=ws-wu/wu (3)

with the cross-link density (ρsw) determined as [29]:

ρsw=[ln(1-φ)+φ+χαφ2]Vs(φ13-φη2) (4)

where Qs denotes the swelling ratio, ws and wu signify the swollen and unswollen weights of the sample respectively. The parameter ρsw indicates cross-link density (mol/m3), χ reflects the polymer-solvent interaction parameter, Vs represents molar volume of the solvent (m3/mol), and φ indicates volume fraction of polymer in the swollen state calculated as follows [30]:

φ=wpdp/wpdp+wsdp (5)

where wp and ws refer to weight fractions of rubber and solvent respectively. Their corresponding densities are denoted as dp and ds. The polymer-solvent interaction parameter can be determined by the equation [31]:

χ=0.487+0.228φ (6)

Additionally, another method for calculating cross-link density in a cured rubber system employs Young's modulus as follows [32]:

ρe=E3RT (7)

where E represents Young’s modulus acquired from the slope of stress-strain data at the initial elongation phase, R is the universal gas constant (8.314 J/mol·K), and T is absolute temperature (K).

Moreover, cross-link density can also be inferred from the modulus at the rubbery plateau phase within the storage modulus versus temperature plot [32]:

ρst=Est6RT (8)

where Est represents storage modulus at the rubbery plateau.

5. Conclusions

XBR/epoxy nanocomposites with varying concentrations of XNBR grafted halloysite nanotubes (XHNTs) were successfully prepared utilizing two-roll mills. Results from the cure rheometer revealed that integrating XHNTs into the XNBR/epoxy matrix resulted in elevated maximum torque while reducing scorch and optimum cure times. Morphological investigations indicated a rougher fracture surface of nanocomposites, attributable to the unique dispersion of nanotubes within the rubber matrix. Higher nanotube loadings corresponded to increases in tensile strength and modulus at 300% elongation, although a decline in elongation at break was also noted. Theoretical analyses of uniaxial stress-strain behaviors indicated significant variations in material parameters within the Bergström-Boyce model influenced by XHNT loading. Overall, the model effectively predicted the large strain hyperelastic behavior of the XNBR/epoxy/XHNTs nanocomposites, though deviations were evident in stress-strain experimental values at ultimate elongation ranges. Dynamic mechanical analysis results illustrated enhanced storage modulus with greater XHNT loadings, although a reduction in damping factor was observed. Swelling tests indicated that nanotubes hindered solvent diffusion into the rubber matrix bulk. Overall, these findings favored the creation of rubber-based nanocomposites with superior mechanical properties, achievable via standard mixing equipment.

Author Contributions

Conceptualization was led by S.M.R.P. and M.R.S.; methodology was devised by S.M.R.P.; software-related tasks were handled by S.M.R.P.; validation was conducted by G.N.; formal analysis was done by E.M.; investigation was conducted by S.M.R.P. and H.M.; data curation was performed by K.F. and S.M.R.P.; the original draft was prepared by S.M.R.P.; the writing and review phases were collaboratively pursued by K.F. and M.R.S.; visualization efforts were undertaken by K.F. and M.R.S.; supervision was shared by S.M.R.P. and M.R.S.; all authors have contributed to and approved the final published version of the manuscript.