bentonite acrylonitrile-butadiene study of rheological, physico- … · 2018. 2. 9. · dokki,...

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Bentonite acrylonitrile-butadiene rubber composites physico-mechani-cal properties

The rheometric characteristics and phy-sico-mechanical properties of bentonite (Bt)/ and modified bentonite (organo-Bt)/acrylonitrile-butadiene rubber (NBR) were investigated. Scanning electron mi-croscopy (SEM) showed that the rubber chains may be confined within the inter-particle space and the Bt particles pre-sented a physical dispersion in NBR mat-rix. Bt was modified with tetra butyl phosphonium bromide (TBP) in order to produce organo-Bt. Results showed that the maximum torque of organo-Bt/NBR composite increases at high Bt loading. The scorch time (tS2) and cure time (tC90) of the organo-Bt/NBR composites decre-ased simultaneously. The prepared com-posite exhibited significant improve-ment in mechanical properties.

Untersuchung der rheologi-schen, physikalisch-mechani-schen und morphologischen Eigenschaften von mit Orga-no-Bentonit gefülltem Nitril-butadienkautschuk Bentonit Acrylonitrilbutadienkaut-schuk Komposite Physikalisch- mechanische Eigenschaften

Es wurden die rheometrischen Daten und physikalisch-mechanischen Eigen-schaften von Bentonit (Bt)- bzw. modifi-zierten Bentonit (Organo Bt)-Nitrilbuta-dienkautschuken untersucht. Die Ras-ter-Elektronenmikroskopie zeigte, dass die Polymerketten den Partikelzwi-schenraum abgrenzen und dass Bt-Par-tikel eine physikalische Dispersion in der NBR-Matrix zeigen. Bt wurde mit Tetrabutylphosphoniumbromid (TBP) zu Organo-Bt modifiziert. Die Ergebnisse zeigen, dass bei Organo-Bt/NBR-Kom-positen das maximale Drehmoment bei hohen Bt-Füllgraden ansteigt. Die An-vulkanisationszeit (tS2) und die Vulkani-sationszeit (tC90) der Organo-Bt/NBR Komposite nehmen gleichzeitig ab. Die hergestellten Komposite weisen eine si-gnifikante Verbesserung der mechani-schen Eigenschaften auf.

Figures and Tables:By a kind approval of the authors.

IntroductionPolymer clay composites have gotten much attention in industries due to their promising improvement in the mechani-cal and physical properties compared to the conventionally filled composites [1]. Also, these composites have the ability to be a low cost for the marketing applica-tions in both the automotive and pack-aging industries. As a consequence, in-corporation of filler into a polymer ma-trix significantly enhances the physico-mechanical properties [2]. The degree of enhancement depends on the filler/ma-trix interfacial bonding and the disper-sion of the filler throughout the matrix [1, 2]. The filler is usually added into rub-ber matrix to either reducing the cost of production and/or reinforce the proper-ties of rubber. Among all the filler used, carbon black is more known due to its high reinforcement capability. But, its low variety of colour usage, besides po-tential to pollute the environment, mo-tive the researchers to find out other more friendly white filler to be used, which among them is layer silicate clay such as bentonite [3]. Bentonite, a highly useful clay mineral, is suitable for use as reinforcing filler to modify the properties of rubber and another polymer because of its whiteness, layered structure, fine particle size, and low heat [4, 5]. This clay is inexpensive filler and can improve the properties of some composite materials [5]. The clay usually needs to be organi-cally modified. This aims to enhance the compatibility of polymer and organoclay layers. The sodium ions are in the inter-layer space of clay which exchanged with an organic phosphonium salt to convert this material into a phosphonium treat-ed clay mineral [6]. The fine dispersibility and compatibility of modified bentonite can significantly improve the physico-mechanical properties of organo-Bt/NBR composite.

The aim of this work is to investigate the effects of the presence of Bt and or-

gano-Bt content on the curing, physico-mechanical and morphological proper-ties of clay/NBR composite.

Experimental

Materials ■ Acrylonitrile-butadiene rubber (NBR)

is produced by Bayer AG, Germany with acrylonitrile content 34 ± 1%, density 0.99g.cm-3, and Moony viscosity ML(1+4) is 65± 7 at 100°C.

■ The materials used sodium bentonite with 78.42mEquiv/100g cation exchange capacity (CEC) which was obtained from Alfa Aesar GmbH and Co. (Karlsruhe, Ger-many).

■ Tetrabutyl phosphonium bromide (TBP) (C16H36BrP, MW: 339.33 g/mol, 98% purity), used as modifier was purchased from Aldrich Company, Germany.

■ Accelerators: N-cyclohexyl-2-benzo-thiazole sulphenamide (CBS). Trade-name: Rhenogran® CBS-80, Vulkacit® CZ was obtained from Rheinehemie Ger-many.

■ Antioxidants: polymerized 2, 2, 4-tri-methyl-1, 2-dihydroquinoline (TMQ). Trade name: PILnox ® TDQ was pur-chased from NOCIL LIMITED Navi Mum-bai, India.

Study of rheological, physico-mechanical and morphological Properties of Nitrile Butadiene Rubber loaded with Organo Bentonite

AuthorsDoaa S. Mahmoud, Nivin M. Ahmed, Salwa H. El-Sabbagh,Dokki, Cairo, Egypt Corresponding author:Prof. Dr. Salwa H. El-SabbaghDept. of Polymers & PigmentNational Research CenterDokki, 12311, Cairo-EgyptFax 202-3355146Email: [email protected]


48 KGK · 01-2 2018 www.kgk-rubberpoint.de

■ Activators: Stearic acid with specific gravity 0.9-0.97 at 15ºC, and zinc oxide (ZnO) with density 5.5-5.61g.cm-3 at 15ºC were supplied by Aldrich Company, Ger-many.

■ Curing agent: sulfur with density is 2.04-2.06g.cm-3 at room temperature (25°C ± 1) and was supplied by Aldrich Company, Germany.

■ Solvent: Toluene was used in pure grade.

Preparation of organoclaysThe Cation exchange capacity (CEC) was determined for the raw clay sample by saturation with 1 N solution of sodium acetate trihydrate (CH3COONa .3H2O) for a while at pH 8.2, then washing for sev-eral times by ethanol 95% to get rid of the excess sodium ion. The reacted sodi-um (Na+) with the clay was extracted by reaction with 1 N ammonium acetate solution followed by determination of the sodium using flame photometer in the extracted solution [7], CEC can be calculated from the following equation;

CEC (meq /100g) = meq /L Na x A/Wt x 100/1000 (1)

Where A is the total volume of extract

(ml) and Wt. is the weight ofair dry sample (g).

■ 5g of Na-bentonite was first dispersed in 500ml of deionized water with mecha-nical stirring for about 24h.

■ A pre-dissolved stoichiometric amount of TBP solution was slowly ad-ded to the bentonite suspension at 80°C.

■ Concentrations of used TBP are 0.5, 1 and 2CEC of the bentonite, respectively.

■ The reaction mixtures were stirred for 6h at 80°C using mechanical stirring.

■ All products were washed several times with deionized water until no bro-mide anions were detected.

■ The presence of bromide anions was tested using 0.1N AgNO3 solution.

■ The products were dried at 90°C, ground and sieved through 230 meshes then stored in vacuum desiccators [7, 8].

Preparation of NBR rubber compositesRubber composites were prepared on a two-roll mill at room temperature. The rolls operated at a speed ratio of 1:1.4. The vulcanization additives (convention-al cure system) were added to the elasto-mer prior to the incorporation of the fill-er. The recipes used in this work are de-scribed in Table 1. Rubber compounds

were vulcanized at 152 ± 1°C and pres-sure of 4MPa in a hydraulic press then it was mixed with ingredients including organo-Bt with different loadings (3, 6, 9, and 12phr) under careful control of tem-perature [9]. The sample codes used in this article are showed in Table 1.

The rubber composites were tested according to standard methods, namely:

■ The cure characteristics of the rubber composites were determined using a Monsanto Rheometer (model 100) at 152 ± 1°C according to ASTM D2084.

■ The vulcanized sheets prepared for mechanical tests were cut into five indi-vidual dumb-bell shaped specimens by steel moulder of constant width (4 mm). The thickness of the tested specimen was determined by gauge calibrated in hundredths of a millimeter. A working part of size 15mm was chosen for each test specimen. The mechanical proper-ties (such as tensile strength and elonga-tion at break (%)) of the rubber com-pounds were determined according to ASTM D412.

■ Hardness (Shore A) of the vulcanized samples was measured using the Shore A durometer according to ASTM D 2240 by pressing circular sheet with radius 12mm and thickness of 6mm, in a hot press at temperature 152±1°C with res-pect to the investigated optimum cure time.

■ Thermal oxidative aging was measu-red according to ASTM D 573.

■ Swelling was determined according to ASTM D3616.In this test, the specimens were cut from the sheets and weighed using an electric balance, followed by immersion in tolu-ene for 24 hours at room temperature to extract any uncrosslinked components and low molecular weight substances from the samples and then they were dried until constant weight. After the conditioning period, these samples (3-5 per test) were first weighed to give the initial weight (W0) and then immersed in toluene until reaching equilibrium swelling. The equation of Lorenz and Parks has been applied to study this rub-ber-filler interaction.

QfQg� = ae

−Z + b (2)

Where Q is defined as grams of solvent per gram of rubber hydrocarbon, f and g refer to filler and gum mixes, Z is the weight fraction of filler in the vulcani-zates, a and b are characteristic con-stants of the system. The swelling per-

1 Formulations and rheological characteristics of NBR (acrylonitril-butadiene rubber) loaded with Bt and organo-Bt

Sample numbers

Filler content, phr

ML (dNm)

MH (dNm)

Tc90 (min)

Ts2 (min)

Cure rate index (min-1)

αf

NBR loaded with BtBt0 ‒ 4.75 58 14.5 2.25 7.9 ‒Bt1 3 3.75 62 14.5 2.25 7.9 3.2

Bt2 6 4 65 14 2.25 8.5 3Bt3 9 4 68 14.5 2.25 8.2 2.9Bt4 12 3.5 70 14 2 8.3 2.6

NBR loaded with 50% organo-BtBt5 3 6.5 80 14.25 2.3 8.9 6.6Bt6 6 6.8 90 11.75 2.3 10.6 6.3Bt7 9 7 91 9.2 2.12 14 3.3Bt8 12 7.5 100 9 2 14 2.2

NBR loaded with 100% organo-BtBt9 3 10 74 13.5 1.5 8.3 4.5Bt10 6 20 84 12.25 1 8.8 4.2Bt11 9 28 92 11.7 1 9.3 3.8Bt12 12 25 99 11.5 0.75 9.3 3.41

NBR loaded with 200% organo-BtBt13 3 15 76 13.5 1.25 8 4.8Bt14 6 15.5 63 7.8 0.87 14.4 3.7Bt15 9 19.5 81 10 1.25 11.2 3.6Bt16 12 22 91 10 0.87 10.38 2.4

Notes Base recipe in phr: Base recipe in phr: NBR (Acrylonitrile butadiene rubber) 100; stearic acid 2; zinc oxide 5; CBS (N-cyclohexyl-2-benzothiazole sulfenamide)1; DOP (dioctyl phthalate); TMQ (polymerized 2, 2, 4-trimethyl-1, 2-dihydro-quinoline ) 1; sulfur 2; ML is minimum torque; MH is maximum torque, tS2is scorch time; tC90 optimum cure time; CRI is cure rate index, αf is specific constant for reinforcement of fillers, where phr is part per hundred parts of rubber.



centage (Q%) was calculated using Eq. (3).

Q% = �(W − W′

W′�X 100 (3) (3)

Where W is the weight of the swollen sample, and W‘ is the weight of the dry sample. The crosslink density was measured by applying the Flory Rehner equation [10] as given in Eqs. (3-5); MC =

− ρP VS Vr1

3�

ln(1 − Vr) + Vr + χ Vr2 (4) (4)

Vr = 1

1 + Qm (5) (5)

VC = 1

2𝑀𝑀𝐶𝐶 (6)

(6)

Where, Mc is the molecular weight be-tween crosslinks, ρP is the density of NBR, Vs is the molar volume of toluene (106.35cm3mol-1), Vr is the volume frac-tion of swollen rubber, Qm is the swelling mass of NBR filled with CEBs in toluene while χ is interaction coefficient between rubber network and toluene (0.390), and Vc is the degree of crosslink density [8].

Methods of instrumental analysisX-ray diffraction (XRD)

X-ray powder diffraction patterns were obtained at room temperature using a Philips diffractometer (type PW1390) by employing Ni-filtered CuKα radiation source (k = 1.5404 Å), Japan. The diffrac-tion angle, 2θ, was scanned at a rate of 5°/min.

Scanning electron microscope (SEM) Scanning electron microscopy (SEM) was performed using JEOL (JXA-840A) elec-tron probe micro-analyzer, Japan. This technique is used to determine the shape of clay particles and to determine the distribution of clay particles in the rub-ber matrix. The second part is done by breaking rubber specimen in liquid nitro-gen and covering the cross-section with thin layer of gold.

Results and discussion

Characterization of organo-Bt

X-ray diffractionThe X-ray diffraction organo-Bt based on various concentrations of surfactant TBP (0.5, 1 and 2CEC of Bt) is shown in Fig. 1. The XRD pattern of Bt exhibits a charac-teristic peak at (2θ = 6.71, d001=12.67Å).

Fig. 1: XRD chart for basal spacing of Bt and different concentrations of organo-Bt

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Fig. 2: Variation of tensile strength and elongation at break for NBR and NBR composites loaded with 12 phr different concentrations organo-Bt

Platzhalter - bitte in gleicher

Qualität anfordern wie restliche Abbildungen

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After addition of TBP surfactant, the characteristic peak of the organo-Bt was shifted to lower angles and increase in d-spacing of silicate layer from12.67Å to 17.718 confirms the penetration of TBP chains into the gap between clay plate-lets and the development of an interca-lated structure [11].

Curing characteristicsThe results given in Table 1 show that minimum torque ML increased with an incorporation of organo-Bt content. This is as known indicated for a decrease in molecular movement of rubber that was resulted from the increasing amount of organo-Bt content in rubber [12]. Maxi-mum torque values MH can be seen to be increasing which may be attributed to the increased of organo-Bt content in-creased the stiffness of NBR, conse-quently increased the torque value re-corded [13]. Scorch time (ts2) which can evaluate the safety of rubber curing. Cure time which can indicate the effi-ciency of rubber curing. ts2 and tc90 of the organo-Bt/NBR composite significantly decreased compared with those of neat NBR and gradually decreased with de-creasing organo-Bt loading [3, 13]. This was probably due to high surface area of Bt that promotes better heat distribution in a rubber matrix. Also, the cure rate in-dex increase for all composites contain-ing organo-Bt. In the present work, the 0.5CEC organo-Bt/NBR composite showed the best comprehensive perfor-mance. Where αƒ is a factor representing the reinforcement factor and the rubber-filler interaction. Table 1 represents that αƒ is decreased with increasing organo-Bt loading which clearly reveals that or-gano-Bt are well dispersed in NBR ma-trix. The above data indicated that the cure properties of organo-Bt/NBR com-posites improved compared with those of neat NBR [7]. Decreased Bt particle size and increased organo-Bt content both can shorten the tS2 and tC90 of orga-no-Bt/NBR composite. The results may be due to the incorporation of rubber chains between the bentonite silicate layers, and the physical interactions of Bt and other rubber auxiliaries during cur-ing [14, 15].

Mechanical characteristicsThe mechanical characteristics of NBR composites containing organo-Bt are shown in Fig. 2. The results show en-hancements in the mechanical proper-ties of the NBR composites loading with

Fig. 3: Variation of modulus at 100% and 200% elongation for NBR and NBR composites loaded with different concentrations of organo-Bt

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Fig. 4: Variation of hardness for NBR and NBR composites loaded with different concentra-tions of organo-Bt

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2 Swelling characteristics of NBR (acrylonitril-butadiene rubber) loaded with Bt and organo-BtSample

numbersQf/Qg Q,% Molecular weight between

crosslinks (Mc) (g.mole-1)Crosslinking density x 10-4,

mol/ccNBR loaded with Na-bentonite

Bt0 ‒ 172 1271 3.9Bt1 0.98 165 1244 4.02

Bt2 0.97 156 1211 4.1Bt3 0.9 146 1082 4.6Bt4 0.86 142 987 5

NBR loaded with 50% organo-BtBt5 0.87 125 1014 5.1Bt6 0.79 113 859 5.4Bt7 0.72 103 740 5.7Bt8 0.67 96 662 5.5



organo-Bt compared to that containing Bt alone. Thus, there are increases in ten-sile strength and elongation at break and reached maximum values at a different loading of organo-Bt [12]. Such findings could be due to the fine dispersion of or-gano-Bt particles in the rubber matrix and the strong interactions between modified bentonite particles and rubber chains, which forced the motion of the rubber chains. Also, it can be shown that the value of modulus at 100% elongation and modulus at 200% elongation of or-gano-Bt/NBR composites are more than that of neat NBR composites [16]. This increase can be due to improvement in the interfacial bonding between organo-Bt and the matrix (NBR), where modified bentonite particles have a higher modu-lus and therefore act as reinforcing filler Fig. 3.

The rubber chains trapped between the organo-Bt particles forming the bon-ded rubber possessed the rigidity to the composites. Therefore, increased organo-Bt content in the NBR composite could carry much more stress and increased the hardness Fig. 4 [17, 18]. The organi-cally modified bentonite particles could interact with rubber molecules by for-ming chemical bonds, these interactions could restrict the motion of rubber chains and increase the volume of the filler in the composite system. Therefore, the mechanical properties of organo-Bt/NBR composite were significantly impro-ved especially composites containing 0.5CEC organo-Bt [18].

The NBR composites were subjected to thermal oxidative aging in an oven at 90°C up to seven days. The retained valu-es of each property are shown in Figs. 5 and 6. It could be concluded that bento-nite and organo-Bt helped in protecting NBR composites against thermal aging. In addition, the decrease of retained va-lues of tensile strength and elongation at break prove the tendency of NBR compo-sites containing organo-Bt to form more crosslinks which cause an increase in the aging time [19].

Swelling characteristicsThe results of the solvent uptake mea-surement are obtained in Table 2. One can see, as the equilibrium swelling values decrease, the degree of crosslink-ing increase. It is clear also that the ad-dition of organo-Bt to NBR matrix no-ticeably decreases the values of equilib-rium swelling. This can be explained by the intercalation of chains between clay

Fig. 5: Retained values of tensile strength of NBR loaded with different concentrations of; (a) Na-B, (b) 50% organo-Bt, (c) 100% organo-Bt and (d) 200% organo-Bt

5



platelets [20]. This intercalation leads to the formation of a bound rubber in close proximity to the reinforcing or-ganoclay which restricts the solvent up-take. However the increase of equilibri-um swelling values as the organo-Bt content becomes greater at 1CEC and 2CEC organo-Bt loadings, this can be explained by the aggregation of the or-gano-Bt particles which facilitate the penetration of the solvent and as a con-sequence increase its uptake effect. The molecular weight between crosslinks decreases systematically until reached a maximum value, 12phr loading of 0.5CEC organo-Bt. Due to the polarity matching between NBR and, organo-Bt it could work as a physical crosslinker and the good interaction between orga-no-Bt particles and NBR matrix was an-other reason [21].

In order to study the effect of organo-Bt on the crosslink density of the NBR, considering the organo-Bt as additional filler. It can be seen that the increase of crosslinking density in the presence of organo-Bt is due to modified bentonite particles having large surface area and

better compatibility with the polymer matrix enhanced the crosslink density, beside the additional physical and che-mical crosslinks determined by swelling tests [23]. The well dispersed organo-Bt particles could accommodate a few sol-vent molecules. At the loading of 0.5CEC organo-Bt, the capacity of organo-Bt ac-commodated solvent molecules reached the maximum [16, 22].

Morphological propertiesIt can be noted from Fig. 7 that neat NBR shows large agglomerates dispersed in the NBR matrix while the composites filled with Bt and organo-Bt show uni-formly dispersed clay throughout the rubber matrix. Bentonite surface was actually consisting of voids and pores, these voids and pores are believed able to reinforce properties and increase the interaction between rubber and filler [23]. The white dots correspond to the X-ray signals of silicon elements present in the bentonite particles. The Bt/NBR (Fig. 7a) and 1CEC organo-Bt /NBR (Fig. 6c) each has an agglomeration of the sili-cate particles dispersed in the NBR ma-

trix. This might be attributed to insuffi-cient shear and extensional stresses dur-ing the compounding process that did not achieved in separating the clay par-ticles and platelets. Here, the organo-clays with larger interlayer spacings (2CEC organo-Bt) show the better disper-sion of the filler with intercalated and exfoliated structures [23, 24]. Further-more, the organoclays with smaller in-terlayer spacing (0.5CEC organo-Bt) typi-cally show conventional composite structures. Fig. 7b shows that, when 0.5CEC of organo- Bt is added to NBR composites, it shows the good dispersion of organo-Bt in the NBR matrix; however, some aggregated silicate was still ob-served. This might be attributed to par-tially exfoliated and intercalated struc-tures [25].

ConclusionsA series of NBR composite filled with modified bentonite were prepared to in-vestigate the effects of organo-Bt con-tent on the curing and physico-mechani-cal properties of organo-Bt/NBR com-posite. Tetrabutyl phosphonium bromide (TBP) was used as an organomodifier for bentonite. The ML and MH of the compos-ites increased relative to that of pure NBR, which increased with increasing filler content. The tS2 and tC90 of the com-posite are shortened by different de-grees. The organo-Bt progressively rein-forced the mechanical properties of NBR composite, even at high 0.5CEC organo-Bt loading (12phr). The tensile strength of the composites (0.5CEC organo-Bt) reached 7.8MPa at 12phr 0.5CEC organo-Bt loading. Higher values of crosslink density of the composites containing ogano-Bt suggest a high degree of rub-ber-rubber and rubber-filler interaction involving intercalation of rubber chains between the platelets of bentonite and physical interactions between the filler and rubber. Finally, the surface of Bt/NBR shown by SEM proved that there was a good adhesion of NBR to organo-Bt with increasing modified clay loading. The ag-glomeration that caused by the pull out of the embedded agglomerates were a presence at 12phr Bt loading. The im-provement of mechanical properties of NBR composite is attributed to the fine dispersion of Bt particles in the rubber matrix and the strong interactions be-tween Bt particles and rubber chains, which trapped rubber particles, and con-sequently restricted the motion of the rubber chains.

Fig. 6: Retained values of elongation at break of NBR loaded with different concentrations of; (a) Na-B, (b) 50% organo-Bt, (c) 100% organo-Bt and (d) 200% organo-Bt

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Fig. 7: SEM images of NBR and NBR composites loaded with bentonite and organo-Bt at magnification 1500X

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